What are batteries, fuel cells, and supercapacitors

What Are Batteries, Fuel Cells, and Supercapacitors?

Contents1. Introduction 4245

1.1. Batteries versus Fuel Cells versusElectrochemical Capacitors

4245

1.2. Definitions 42471.3. Thermodynamics 42481.4. Kinetics 42491.5. Experimental Techniques 42501.6. Current Distribution and Porous Electrodes 4251

2. Batteries 42522.1. Introduction and Market Aspects 42522.2. Battery Operations 42532.3. Characteristics of Common Battery Systems 42542.4. Primary Batteries 42542.5. Rechargeable Batteries 42572.6. Selection Criteria for Commercial Battery

Systems4258

3. Fuel Cells 42593.1. Introduction and Market Aspects 42593.2. Fuel Cell Operation 42613.3. Characteristics of Various Types of Fuel

Cells4264

4. Electrochemical Capacitors (ECs) 42664.1. Introduction and Market Aspects 42664.2. Characteristics of the Electrical Double Layer 42674.3. EC Operation 4267

5. Summary 4269

1. Introduction

1.1. Batteries versus Fuel Cells versusElectrochemical Capacitors

Energy consumption/production that rely on thecombustion of fossil fuels is forecast to have a severefuture impact on world economics and ecology. Elec-trochemical energy production is under serious con-sideration as an alternative energy/power source, aslong as this energy consumption is designed to bemore sustainable and more environmentally friendly.Systems for electrochemical energy storage andconversion include batteries, fuel cells, and electro-chemical capacitors (ECs). Although the energy stor-age and conversion mechanisms are different, thereare “electrochemical similarities” of these three sys-tems. Common features are that the energy-providingprocesses take place at the phase boundary of theelectrode/electrolyte interface and that electron andion transport are separated. Figures 1 and 2 showthe basic operation mechanisms of the three systems.Note that batteries, fuel cells, and supercapacitorsall consist of two electrodes in contact with anelectrolyte solution. The requirements on electronand ion conduction in electrodes and the electrolyteare given in Figure 1 and are valid for all threesystems.

In batteries and fuel cells, electrical energy isgenerated by conversion of chemical energy via redoxreactions at the anode and cathode. As reactions atthe anode usually take place at lower electrodepotentials than at the cathode, the terms negativeand positive electrode (indicated as minus and pluspoles) are used. The more negative electrode isdesignated the anode, whereas the cathode is themore positive one. The difference between batteriesand fuel cells is related to the locations of energystorage and conversion. Batteries are closed systems,with the anode and cathode being the charge-transfermedium and taking an active role in the redox

Dr. Martin Winter is currently University Professor for Applied InorganicChemistry and Electrochemistry at the Institute for Chemistry andTechnology of Inorganic Materials, Graz University of Technology (Austria).His fields of specialization are applied electrochemistry, chemical technol-ogy and solid state electrochemistry with special emphasis on thedevelopment and characterization of novel materials for rechargeablelithium batteries.

Dr. Ralph J. Brodd is President of Broddarp of Nevada. He has over 40years of experience in the technology and market aspects of theelectrochemical energy conversion business. His experience includes allmajor battery systems, fuel cells, and electrochemical capacitors. He is aPast President of the Electrochemical Society and was elected HonoraryMember in 1987. He served as Vice President and National Secretary ofthe International Society of Electrochemistry as well as on technicaladvisory committees for the National Research Council, the InternationalElectrotechnic Commission, and NEMA and on program review committeesfor the Department of Energy and NASA.

4245Chem. Rev. 2004, 104, 4245−4269

10.1021/cr020730k CCC: $48.50 © 2004 American Chemical SocietyPublished on Web 09/28/2004

reaction as “active masses”. In other words, energystorage and conversion occur in the same compart-ment. Fuel cells are open systems where the anodeand cathode are just charge-transfer media and theactive masses undergoing the redox reaction aredelivered from outside the cell, either from theenvironment, for example, oxygen from air, or from

a tank, for example, fuels such as hydrogen andhydrocarbons. Energy storage (in the tank) andenergy conversion (in the fuel cell) are thus locallyseparated.1

In electrochemical capacitors (or supercapacitors),energy may not be delivered via redox reactions and,thus the use of the terms anode and cathode may notbe appropriate but are in common usage. By orienta-tion of electrolyte ions at the electrolyte/electrolyteinterface, so-called electrical double layers (EDLs) areformed and released, which results in a parallelmovement of electrons in the external wire, that is,in the energy-delivering process.

In comparison to supercapacitors and fuel cells,batteries have found by far the most applicationmarkets and have an established market position.Whereas supercapacitors have found niche marketsas memory protection in several electronic devices,fuel cells are basically still in the development stageand are searching to find a “killer application” thatallows their penetration into the market. Fuel cellsestablished their usefulness in space applicationswith the advent of the Gemini and Apollo spaceprograms. The most promising future markets forfuel cells and supercapacitors are in the same ap-plication sector as batteries. In other words, super-capacitor and fuel cell development aim to competewith, or even to replace, batteries in several applica-tion areas. Thus, fuel cells, which originally wereintended to replace combustion engines and combus-tion power sources due to possible higher energyconversion efficiencies and lower environmental im-pacts, are now under development to replace batter-ies to power cellular telephones and notebook com-puters and for stationary energy storage. The moti-vation for fuel cells to enter the battery market issimple. Fuel cells cannot compete today with com-bustion engines and gas/steam turbines because ofmuch higher costs, inferior power and energy per-formance, and insufficient durability and lifetime.With operation times of typically <3000 h and, atleast to an order of magnitude, similar costs, batteriesare less strong competitors for fuel cells.

The terms “specific energy” [expressed in watt-hours per kilogram (Wh/kg)] and “energy density” [inwatt-hours per liter (Wh/L)] are used to compare theenergy contents of a system, whereas the rate capa-bility is expressed as “specific power” (in W/kg) and“power density” (in W/L). Alternatively, the attributes“gravimetric” (per kilogram) and “volumetric” (perliter) are used. To compare the power and energycapabilities, a representation known as the Ragoneplot or diagram has been developed. A simplifiedRagone plot (Figure 3) discloses that fuel cells canbe considered to be high-energy systems, whereassupercapacitors are considered to be high-powersystems. Batteries have intermediate power andenergy characteristics. There is some overlap inenergy and power of supercapacitors, or fuel cells,with batteries. Indeed, batteries with thin film

1 Strictly speaking, a single electrochemical power system is denoteda cell or element, whereas a series or parallel connection of cells isnamed a battery. The literature is confusing, as the terms fuel CELLand BATTERY are used independent of the number of cells described.

Figure 1. Representation of a battery (Daniell cell)showing the key features of battery operation and therequirements on electron and ion conduction.

Figure 2. Representation of (A, top) an electrochemicalcapacitor (supercapacitor), illustrating the energy storagein the electric double layers at the electrode-electrolyteinterfaces, and (B, bottom) a fuel cell showing the continu-ous supply of reactants (hydrogen at the anode and oxygenat the cathode) and redox reactions in the cell.

4246 Chemical Reviews, 2004, Vol. 104, No. 10 Editorial

electrodes exhibit power characteristics similar tothose of supercapacitors. Moreover, there are alsohybrids such as metal/air batteries (or, in otherwords, metal/air fuel cells), which contain a batteryelectrode (metal anode) and a fuel cell electrode (aircathode). Finally, Figure 3 also shows that no singleelectrochemical power source can match the charac-teristics of the internal combustion engine. Highpower and high energy (and thus a competitivebehavior in comparison to combustion engines andturbines) can best be achieved when the availableelectrochemical power systems are combined. In suchhybrid electrochemical power schemes, batteries and/or supercapacitors would provide high power and thefuel cells would deliver high energy.

Figure 4 shows the theoretical specific energies[(kW h)/t] and energy densities [(kW h)/m3)] ofvarious rechargeable battery systems in comparisonto fuels, such as gasoline, natural gas, and hydrogen.The inferiority of batteries is evident. Figure 5,showing driving ranges of battery-powered cars incomparison to a cars powered by a modern combus-tion engine, gives an impressive example of why fuel

cells, and not batteries, are considered for replace-ment of combustion engines. The theoretical valuesin Figure 4 are an indication for the maximum energycontent of certain chemistries. However, the practicalvalues differ and are significantly lower than thetheoretical values. As a rule of thumb, the practicalenergy content of a rechargeable battery is 25% ofits theoretical value, whereas a primary batterysystem can yield >50% of its theoretical value indelivered energy. In the future, fuel cells might beable to convert the used fuels into electrical energywith efficiencies of >70%. The difference between thetheoretical and practical energy storage capabilitiesis related to several factors, including (1) inert partsof the system such as conductive diluents, currentcollectors, containers, etc., that are necessary for itsoperation, (2) internal resistances within the elec-trodes and electrolyte and between other cell/batterycomponents, resulting in internal losses, and (3)limited utilization of the active masses, as, forexample, parts of the fuel in a fuel cell leave the cellwithout reaction or as, for example, passivation ofelectrodes makes them (partially) electrochemicallyinactive. However, as batteries and fuel cells are notsubject to the Carnot cycle limitations, they mayoperate with much higher efficiencies than combus-tion engines and related devices.

1.2. DefinitionsThe following definitions are used during the

course of discussions on batteries, fuel cells, andelectrochemical capacitors.

A battery is one or more electrically connectedelectrochemical cells having terminals/contacts tosupply electrical energy.

A primary battery is a cell, or group of cells, forthe generation of electrical energy intended to beused until exhausted and then discarded. Primarybatteries are assembled in the charged state; dis-charge is the primary process during operation.

A secondary battery is a cell or group of cells forthe generation of electrical energy in which the cell,after being discharged, may be restored to its originalcharged condition by an electric current flowing inthe direction opposite to the flow of current when thecell was discharged. Other terms for this type ofbattery are rechargeable battery or accumulator. Assecondary batteries are ususally assembled in the

Figure 3. Simplified Ragone plot of the energy storagedomains for the various electrochemical energy conversionsystems compared to an internal combustion engine andturbines and conventional capacitors.

Figure 4. Theoretical specific energies [(kW h)/tonne] andenergy densities [(kW h)/m3] of various rechargeable bat-tery systems compared to fuels, such as gasoline, naturalgas, and hydrogen.

Figure 5. Comparison of the driving ranges for a vehiclepowered by various battery systems or a gasoline-poweredcombustion engine.

Editorial Chemical Reviews, 2004, Vol. 104, No. 10 4247

discharged state, they have to be charged first beforethey can undergo discharge in a secondary process.

A specialty battery is a primary battery that is inlimited production for a specific end-use. In this paperspecialty batteries will not be particularly addressed.

The anode is the negative electrode of a cellassociated with oxidative chemical reactions thatrelease electrons into the external circuit.

The cathode is the positive electrode of a cellassociated with reductive chemical reactions thatgain electrons from the external circuit.

Active mass is the material that generates electricalcurrent by means of a chemical reaction within thebattery.

An electrolyte is a material that provides pure ionicconductivity between the positive and negative elec-trodes of a cell.

A separator is a physical barrier between thepositive and negative electrodes incorporated intomost cell designs to prevent electrical shorting. Theseparator can be a gelled electrolyte or a microporousplastic film or other porous inert material filled withelectrolyte. Separators must be permeable to the ionsand inert in the battery environment.

A fuel cell is an electrochemical conversion devicethat has a continuous supply of fuel such as hydro-gen, natural gas, or methanol and an oxidant suchas oxygen, air, or hydrogen peroxide. It can haveauxiliary parts to feed the device with reactants aswell as a battery to supply energy for start-up.

An electrochemical capacitor is a device that storeselectrical energy in the electrical double layer thatforms at the interface between an electrolytic solutionand an electronic conductor. The term applies tocharged carbon-carbon systems as well as carbon-battery electrode and conducting polymer electrodecombinations sometimes called ultracapacitors, super-capacitors, or hybrid capacitors.

Open-circuit voltage is the voltage across theterminals of a cell or battery when no externalcurrent flows. It is usually close to the thermody-namic voltage for the system.

Closed-circuit voltage is the voltage of a cell orbattery when the battery is producing current intothe external circuit.

Discharge is an operation in which a batterydelivers electrical energy to an external load.

Charge is an operation in which the battery isrestored to its original charged condition by reversalof the current flow.

Internal resistance or impedance is the resistanceor impedance that a battery or cell offers to currentflow.

The Faraday constant, F, is the amount of chargethat transfers when one equivalent weight of activemass reacts, 96 485.3 C/g-equiv, 26.8015 Ah/g-equiv.

Thermal runaway is an event that occurs when thebattery electrode’s reaction with the electrolyte be-comes self-sustaining and the reactions enter anautocatalytic mode. This situation is responsible formany safety incidents and fires associated withbattery operations.

1.3. ThermodynamicsThe energy storage and power characteristics of

electrochemical energy conversion systems followdirectly from the thermodynamic and kinetic formu-lations for chemical reactions as adapted to electro-chemical reactions. First, the basic thermodynamicconsiderations are treated. The basic thermodynamicequations for a reversible electrochemical transfor-mation are given as

and

where ∆G is the Gibbs free energy, or the energy ofa reaction available () free) for useful work, ∆H isthe enthalpy, or the energy released by the reaction,∆S is the entropt, and T is the absolute temperature,with T∆S being the heat associated with the or-ganization/disorganization of materials. The terms∆G, ∆H, and ∆S are state functions and depend onlyon the identity of the materials and the initial andfinal states of the reaction. The degree symbol is usedto indicate that the value of the function is for thematerial in the standard state at 25 °C and unitactivity.

Because ∆G represents the net useful energyavailable from a given reaction, in electrical terms,the net available electrical energy from a reaction ina cell is given by

and

where n is the number of electrons transferred permole of reactants, F is the Faraday constant, beingequal to the charge of 1 equiv of electrons, and E isthe voltage of the cell with the specific chemicalreaction; in other words, E is the electromotive force(emf) of the cell reaction. The voltage of the cell isunique for each reaction couple. The amount ofelectricity produced, nF, is determined by the totalamount of materials available for reaction and canbe thought of as a capacity factor; the cell voltagecan be considered to be an intensity factor. The usualthermodynamic calculations on the effect of temper-ature, pressure, etc., apply directly to electrochemicalreactions. Spontaneous processes have a negative freeenergy and a positive emf with the reaction writtenin a reversible fashion, which goes in the forwarddirection. The van’t Hoff isotherm identifies the freeenergy relationship for bulk chemical reactions as

where R is the gas constant, T the absolute temper-ature, AP the activity product of the products and ARthe activity product of the reactants. Combining eqs4 and 5 with the van’t Hoff isotherm, we have the

∆G ) ∆H - T∆S (1)

∆G° ) ∆H° - T∆S° (2)

∆G ) -nFE (3)

∆G° ) -nFE° (4)

∆G ) ∆G° + RT ln(AP/AR) (5)


Nernst equation for electrochemical reactions:

Faraday’s laws, as summarized in eq 7, give thedirect relationship between the amount of reactionand the current flow. There are no known exceptionsto Faraday’s laws.

g is the grams of material transformed, I is thecurrent flow (amps), t is the time of current flow(seconds, hours), MW is the molecular or atomicweight of the material being transformed, and n isthe number of electrons in the reaction.

Assuming thermodynamic reversibility2 of the cellreaction and with the help of eqs 1 and 3, we canobtain the reversible heat effect.

By measuring the cell voltage as a function oftemperature, the various thermodynamic quantitiesfor the materials in an electrode reaction can bedetermined experimentally. If dE/dT is positive, thecells will heat on charge and cool on discharge. Leadacid is an example of a negative dE/dT, where thecells cool on charge and heat on discharge. Ni-Cd isan example of a positive dE/dT, where the cells heaton charge and cool on discharge. Heating and coolingof the cell can proceed with heat exchange with theenvironment. In general, the entropic heat is negli-gibly small compared to the irreversible heat re-leased, q, when a cell is in operation. Equation 10describes total heat release, including the reversiblethermodynamic heat release along with the irrevers-ible joule heat from operation of the cell in anirreversible manner, during charge or discharge atfinite current/rate. Irreversible behavior manifestsitself as a departure from the equilibrium or thermo-dynamic voltage. In this situation, the heat, q, givenoff by the system is expressed by an equation inwhich ET is the practical cell terminal voltage andEOCV is the voltage of the cell on open circuit.

The total heat released during cell discharge is thesum of the thermodynamic entropy contribution plusthe irreversible contribution. This heat is releasedinside the battery at the reaction site on the surfaceof the electrode structures. Heat release is not a

problem for low-rate applications; however, high-ratebatteries must make provisions for heat dissipation.Failure to accommodate/dissipate heat properly canlead to thermal runaway and other catastrophicsituations.

1.4. Kinetics

Thermodynamics describe reactions at equilibriumand the maximum energy release for a given reaction.Compared to the equilibrium voltage () open ciruitvoltage, EOCV), the voltage drops off () “electrodepolarization” or “overvoltage”) when current is drawnfrom the battery because of kinetic limitations ofreactions and of other processes must occur toproduce current flow during operation. Electrochemi-cal reaction kinetics follow the same general consid-erations as those for bulk chemical reactions. How-ever, electrode kinetics differs from chemical kineticsin two important aspects: (1) the influence of thepotential drop in the electrical double layer at anelectrode interface as it directly affects the activatedcomples and (2) the fact that reactions at electrodeinterfaces proceed in a two-dimensional, not three-dimensional, manner. The detailed mechanism ofbattery electrode reactions often involves a series ofphysical, chemical, and electrochemical steps, includ-ing charge-transfer and charge transport reactions.The rates of these individual steps determine thekinetics of the electrode and, thus, of the cell/battery.Basically, three different kinetics effects for polariza-tion have to be considered: (1) activation polarizationis related to the kinetics of the electrochemical redox(or charge-transfer) reactions taking place at theelectrode/electrolyte interfaces of anode and cathode;(2) ohmic polarization is interconnected to the re-sistance of individual cell components and to theresistance due to contact problems between the cellcomponents; (3) concentration polarization is due tomass transport limitations during cell operation. Thepolarization, η, is given by

where EOCV is the voltage of the cell at open circuitand ET is the terminal cell voltage with current, I,flowing.

Activation polarization arises from kinetics hin-drances of the charge-transfer reaction taking placeat the electrode/electrolyte interface. This type ofkinetics is best understood using the absolute reac-tion rate theory or the transition state theory. Inthese treatments, the path followed by the reactionproceeds by a route involving an activated complex,where the rate-limiting step is the dissociation of theactivated complex. The rate, current flow, i (I ) I/Aand Io ) Io/A, where A is the electrode surface area),of a charge-transfer-controlled battery reaction canbe given by the Butler-Volmer equation as

where the exchange current density, io ) koFA is theexchange current density (ko is the reaction rate

2 A process is thermodynamically reversible when an infinitesimalreversal in a driving force causes the process to reverse its direction.Since all actual processes occur at finite rates, they cannot proceedwith strict thermodynamic reversibility and thus additional nonrevers-ible effects have to be regarded. In this case, under practical operationconditions, voltage losses at internal resistances in the cell (thesekinetic effects are discussed below) lead to the irreversible heatproduction (so-called Joule heat) in addition to the thermodynamicreversible heat effect.

E ) E° + (RT/nF) ln(AP/AR) (6)

g )It(MW)

nF(7)

∆G ) -nFE ) ∆H - T∆S (8)

) ∆H - nFT(dE/dT) (9)

q ) T∆S + I(EOCV - ET) (10)

q ) heat given off by the system (11)

η ) EOCV - ET (12)

i ) io exp(RFη/RT) - exp((1 - R)Fη)/RT (13)


constant for the electrode reaction, and A is theactivity product of the reactants), η is the polarizationor departure (overpotential) from equilibrium (η )EOCV - ET), and R is the transfer coefficient, whichis best considered as the fraction of the change ofoverpotential that leads to a change in the rateconstant for charge-transfer reaction. The exchangecurrent density is directly related to the reaction rateconstant, to the activities of reactants and products,and to the potential drop across the double layer.Reactions with larger io are more reversible and havelower polarization for a given current flow. Electrodereactions having high exchange currents (io in therange of 10-2 A/cm2) at room temperature are favoredfor use in battery applications. The buildup and decayof the activation polarization are fast and can beidentified by the voltage change on current interrup-tion in a time frame of 10-2-10-4 s.

The activation polarization follows the Tafel equa-tion derived from eq 13

where a and b are constants.Ohmic polarization arises from the resistance of the

electrolyte, the conductive diluent, and materials ofconstruction of the electrodes, current collectors,terminals, and contact between particles of the activemass and conductive diluent or from a resistive filmon the surface of the electrode. Ohmic polarizationappears and disappears instantaneously (e10-6 s)when current flows and ceases. Under the effect ofohmic resistance, R, there is a linear Ohm’s Lawrelationship between I and η.

As the redox reactions proceed, the availability ofthe active species at the electrode/electrolyte interfacechanges. Concentration polarization arises from lim-ited mass transport capabilities, for example, limiteddiffusion of active species to and from the electrodesurface to replace the reacted material to sustain thereaction. Diffusion limitations are relatively slow,and the buildup and decay take g10-2 s to appear.For limited diffusion the electrolyte solution, theconcentration polarization, can be expressed as

where C is the concentration at the electrode surfaceand Co is the concentration in the bulk of the solution.The movement or transport of reactants from thebulk solution to the reaction site at the electrodeinterface and vice versa is a common feature of allelectrode reactions. Most battery electrodes are po-rous structures in which an interconnected matrixof small solid particles, consisting of both noncon-ductive and electronically conductive materials, isfilled with electrolyte. Porous electrode structures areused to extend the available surface area and lowerthe current density for more efficient operation.

1.5. Experimental Techniques

In practical batteries and fuel cells, the influenceof the current rate on the cell voltage is controlled

by all three types of polarization. A variety ofexperimental techniques are used to study electro-chemical and battery reactions. The most commonare the direct measurement of the instantaneouscurrent-voltage characteristics on discharge curveshown in Figure 6. This curve can be used to deter-mine the cell capacity, the effect of the dischage-charge rate, and temperature and information on thestate of health of the battery.

The impedance behavior of a battery is anothercommon technique that can reveal a significantamount of information about battery operation char-acteristics. The impedance of an electrode or batteryis given by

where X ) ωL - 1/(ωC), j ) x-1, and ω is theangular frequency (2πf); L is the inductance, and Cis the capacitance. A schematic of a battery circuitand the corresponding Argand diagram, illustratingthe behavior of the simple electrode processes, areshown in Figure 7a. Activation processes exhibit asemicircular behavior with frequency that is char-acteristic of relaxation processes; concentrationprocesses exhibit a 45° behavior characteristic ofdiffusion processes often referred to as Warburgbehavior; ohmic components are independent offrequency.

Each electrode reaction has a distinctive, charac-teristic impedance signature. A schematic of a batterycircuit and the corresponding Argand diagram, il-lustrating the behavior of the simple processes, areshown in Figure 7b. In ideal behavior, activationprocesses exhibit a semicircular behavior with fre-quency that is characteristic of relaxation processes;concentration processes exhibit a 45° behavior char-acteristic of diffusion processes, and ohmic polariza-tions have no capacitive character and are independ-ent of frequency. The frequency of the maximum, fm,of the semicircle gives the relaxation time, where τ) 1/fm ) RC. Here R is related to the exchangecurrent for the reaction and C is called the polariza-tion capacitance, CP. Typically, the CP is of the orderof 200 µF/cm, ∼10 times larger than the capacitanceof the EDL. Some electrochemical capacitors takeadvantage of this capacitance to improve their per-formance of the supercapacitors. Battery electrodeshave large surface areas and, therefore, exhibit large

η ) a - b log(I/Io) (14)

η ) IR (15)

η ) (RT/n) ln(C/Co) (16)

Figure 6. Typical discharge curve of a battery, showingthe influence of the various types of polarization.

Z ) R + jωX (17)


capacitances. It is common for cells to have a capaci-tance of farads and a resistance of milliohms.

The experimental techniques described above ofcharge-discharge and impedance are nondestructive.“Tear-down” analysis or disassembly of spent cellsand an examination of the various components usingexperimental techniques such as Raman microscopy,atomic force microscopy, NMR spectroscopy, trans-mission electron microscopy, XAS, and the like canbe carried out on materials-spent battery electrodesto better understand the phenomena that lead todegradation during use. These techniques providediagnostic techniques that identify materials proper-ties and materials interactions that limit lifetime,performance, and thermal stabiity. The acceleratedrate calorimeter finds use in identifying safety-related situations that lead to thermal runaway anddestruction of the battery.

1.6. Current Distribution and Porous Electrodes

Most practical electrodes are a complex compositeof powders composed of particles of the active mate-rial, a conductive diluent (usually carbon or metalpowder), and a polymer binder to hold the mixtogether and bond the mix to a conductive currentcollector. Typically, a composite battery electrode has∼30% porosity with a complex surface extendingthroughout the volume of the porous electrode. Thisyields a much greater surface area for reaction thanthe geometric area and lowers polarization. The poresof the electrode structures are filled with electrolyte.

Although the matrix may have a well-definedplanar surface, there is a complex reaction surfaceextending throughout the volume of the porouselectrode, and the effective active surface may bemany times the geometric surface area. Ideally, whena battery produces current, the sites of currentproduction extend uniformly throughout the electrodestructure. A nonuniform current distribution intro-duces an inefficiency and lowers the expected per-formance from a battery system. In some cases thenegative electrode is a metallic element, such as zincor lithium metal, of sufficient conductivity to requireonly minimal supporting conductive structures.

Two types of current distribution, primary andsecondary, can be distinguished. The primary distri-bution is controlled by cell geometry. The placing ofthe current collectors strongly influences primarycurrent distribution on the geometric surface area ofthe electrodes. The monopolar construction is mostcommon. The differences in current distribution fortop connections and opposite end current collectionare shown in Figure 8A,B. With opposite end con-nections the current distribution is more uniform andresults in a more efficient use of the active material.The bipolar construction depicted in Figure 8C givesuniform current distribution wherein the anodeterminal or collector of one cell serves as the currentcollector and cathode of the next cell in pile config-uration.

Secondary current distribution is related to currentproduction sites inside the porous electrode itself. The

Figure 7. (A, top) Simple battery circuit diagram, whereCDL represents the capacitance of the electrical double layerat the electrode-solution interface (cf. discussion of super-capacitors below), W depicts the Warburg impedance fordiffusion processes, Ri is the internal resistance, and Zanodeand Zcathode are the impedances of the electrode reactions.These are sometimes represented as a series resistancecapacitance network with values derived from the Arganddiagram. This reaction capacitance can be 10 times the sizeof the double-layer capacitance. The reaction resistancecomponent of Z is related to the exchange current for thekinetics of the reaction. (B, bottom) Corresponding Arganddiagram of the behavior of impedance with frequency, f,for an idealized battery system, where the characteristicbehaviors of ohmic, activation, and diffusion or concentra-tion polarizations are depicted.

Figure 8. Primary current distribution on the frontsurface of the electrodes based on Kirkhof’s law calculationfor three different cell constructions: (A) Both connectionsto the cell are at the top. The higher resistance path atthe bottom sections of the electrode reduces the currentflow and results in a nonuniform current distribution. (B)All paths have equal resistance, and a uniform currentdistribution results. (C) The bipolar construction has equalresistance from one end to the other.


incorporation of porous electrode structures increasesthe surface area and shortens diffusion path lengthsto the reaction site. Current-producing reactions canpenetrate into a porous electrode structure to con-siderable depth below the surface of the electrodesas noted in Figure 9. The location of the reaction siteinside a porous electrode is strongly dependent onthe characteristics of the electrode structure andreactions themselves. The key parameters include theconductivity of the electrode matrix, electrolyte con-ductivity, the exchange current, the diffusion char-acteristics of reactants and products, and the totalcurrent flow. In addition, the porosity, pore size, andtortuosity of the electrode play a role. The effective-ness of a porous electrode can be estimated from theactive surface area, S, in cm2/cm3, and the penetra-tion depth LP of the reaction process into the porouselectrode. Factors that influence the secondary cur-rent distribution are the conductivity of the electro-lyte and electrode matrix, the exchange current ofthe reactions, and the thickness of the porous layer.Sophisticated mathematical models to describe andpredict porous electrode performance of practicalsystems have been developed. These formulationsbased on models of primary and secondary batterysystems permit rapid optimization in the design ofnew battery configurations. The high-rate perfor-mance of the present SLI automotive batteries hasevolved directly from coupling current collector de-signs with the porous electrode compositions identi-fied from modeling studies.

Modeling has become an important tool in develop-ing new battery technology as well as for improvingthe performance of existing commercial systems.Models based on engineering principles of currentdistribution and fundamental electrochemical reac-tion parameters can predict the behavior of porous

electrode structures from the older lead acid automo-tive technology to the newest lithium ion (Li ion)technology.

2. Batteries

2.1. Introduction and Market Aspects

Batteries are self-contained units that store chemi-cal energy and, on demand, convert it directly intoelectrical energy to power a variety of applications.Batteries are divided into three general classes:primary batteries that are discharged once anddiscarded; secondary, rechargeable batteries that canbe discharged and then restored to their originalcondition by reversing the current flow through thecell; and specialty batteries that are designed to fulfilla specific purpose. The latter are mainly military andmedical batteries that do not find wide commercialuse for various reasons of cost, environmental issues,and limited market application. They generally donot require time to start-up. At low drains, up to 95%of the energy is available to do useful work.

Success in the battery market depends largely onfour factors, noted in Figure 10. The market forbatteries in Table 1 is directly related to the applica-tions they serve, such as automobiles, cellular phones,notebook computers, and other portable electronicdevices. The growth in any particular segment fol-lows closely the introduction of new devices poweredby batteries. The introduction of new materials withhigher performance parameters gives the variousdesigners freedom to incorporate new functionalityin present products or to create new products to

Figure 9. Schematic porous electrode structure: (A)Electrons from the external circuit flow in the currentcollector which has contact to the conductive matrix in theelectrode structure. The redox reaction at the electrodeproduces electrons that enter the external circuit and flowthrough the load to the cathode, where the reductionreaction at the cathode accepts the electron from theexternal circuit and the reduction reaction. The ions in theelectrolyte carry the current through the device. (B) Thereaction distribution in the porous electrode is shown forthe case where the conductivity of the electrode matrix ishigher than the conductivity of the electrolyte.

advantages disadvantages

operate over a widetemperature range

low energy content comparedto gasoline and other fuels

choice of chemical systemand voltage

expensive compared to coaland gasoline

operate in any orientation no single general purposedo not require pumps,

filters, etc.system

variable in sizecommonality of cell sizes,

worldwidecan deliver high current pulsescan choose best battery for a

specific purpose (portable,mobile, and stationaryapplications)

Figure 10. How batteries are judged by users and thefactors that control these criteria.


expand the market scope. Batteries for notebookcomputers have experienced double-digit growth,whereas the automobile SLI market segment hasgrown with the gross national product. Batteries canrange in size from aspirin tablet (and even smaller)with a few tens of mAh, for in-the-ear hearing aids,to a building with 40 MWh for energy storage andemergency power.

2.2. Battery OperationsFigure 11 depicts the basic elements of a battery.

Figure 12 illustrates the operation of a battery,showing the energy levels at the anode (negative) andcathode (positive) poles and the electrolyte expressedin electronvolts. The negative electrode is a goodreducing agent (electron donor) such as lithium, zinc,or lead. The positive electrode is an electron acceptorsuch as lithium cobalt oxide, manganese dioxide, orlead oxide. The electrolyte is a pure ionic conductorthat physically separates the anode from the cathode.

In practice, a porous electrically insulating materialcontaining the electrolyte is often placed between theanode and cathode to prevent the anode from directlycontacting the cathode. Should the anode and cathodephysically touch, the battery will be shorted and itsfull energy released as heat inside the battery.Electrical conduction in electrolytic solutions followsOhm’s law: E ) IR.

Battery electrolytes are usually liquid solvent-based and can be subdivided into aqueous, nonaque-ous, and solid electrolytes. Aqueous electrolytes aregenerally salts of strong acids and bases and arecompletely dissociated in solution into positive andnegative ions. The electrolyte provides an ionicconduction path as well as a physical separation ofthe positive and negative electrodes needed forelectrochemical cell operation. Each electrolyte isstable only within certain voltage ranges. Exceedingthe electrochemical stability window results in itsdecomposition. The voltage stability range dependson the electrolyte composition and its purity level.In aqueous systems, conductivities of the order of 1S/cm are common. The high conductivity of aqueoussolvent-based electrolytes is due to their dielectricconstants, which favor stable ionic species, and thehigh solvating power, which favors formation of hy-drogen bridge bonds and allows the unique Grotthusconductivity mechanism for protons. Thermodynami-cally, aqueous electrolytes show an electrochemicalstability window of 1.23 V. Kinetic effects mayexpand the stability limit to ∼2 V.

In the nonaqueous organic solvent-based systemsused for lithium batteries, the conductivities are ofthe order of 10-2-10-3 S/cm-1. Compared to water,most organic solvents have a lower solvating powerand a lower dielectric constant. This favors ion pairformation, even at low salt concentration. Ion pairformation lowers the conductivity as the ions are nolonger free and bound to each other. Organic elec-trolytes show lower conductivities and much higher

Table 1. Estimated Battery Market in 2003 ($ Millionsof Dollars)

system market size

primarycarbon-zinc 6500alkaline 10000lithium, military, medical, etc. 3400

subtotal 19900

secondarylead acid 18400small sealed rechargeable cells

lithium ion 3500nickel metal hydride 1800nickel cadmium 1500othera 3100

subtotal 28300

total battery market 48200a Large vented and sealed Ni-Cd, Ni-Fe, Ag-Zn, etc.

Figure 11. Block diagram of a cell or battery powering adevice. If a battery is recharged, the load is replaced withan energy source that imposes a reverse voltage that islarger than the battery voltage and the flow of electrons isreversed.

Figure 12. Voltage levels in the various sections of theunit cell of a battery, fuel cell, or electrochemical capacitor.The structure and composition of the electrical double layerdiffer at the anode and cathode.


viscosities than aqueous electrolytes. Organic solvent-based electrolytes (again with the help of kinetics)are limited to ∼4.6 V. Exceeding the voltage limit inthe organic electrolytes results in polymerization ordecomposition of the solvent system. Solid electrolytebatteries have found limited use as the power sourcefor heart pacemakers and for use in military applica-tions. The basic principles described above apply tofuel cells and electrochemical capacitors as well asto batteries.

2.3. Characteristics of Common Battery Systems

A list of common commercial systems is found inTable 2. A graphical representation of the energystorage capability of common types of primary andsecondary batteries is shown in Figures 13 and 14.It is beyond the scope of this paper to discuss allsystems in detail. Instead, we want to review themost common electrode mechanisms for dischargeand charge depicted in Figure 15.

2.4. Primary BatteriesFigure 15A shows the discharge reaction of a CuS

electrode in a Li-CuS cell. During the cell reaction,Cu is displaced by Li and segregates into a distinctsolid phase in the cathode. The products of thisdisplacement type of reaction, Li2S and Cu, arestable, and the reaction cannot be easily reversed.Hence, the electrode reactions cannot be rechargedand the cell is considered to be a primary cell, as thedischarge reaction is not reversible. The Li electrodein Figure 15B is discharged by oxidation. The formedLi+ cation is going into solution. The reaction isreversible by redeposition of the lithium. However,like many other metals in batteries, the redepositionof the Li is not smooth, but rough, mossy, anddendritic, which may result in serious safety prob-lems. This is in contrast to the situation with a leadelectrode in Figure 15C, which shows a similarsolution electrode. Here, the formed Pb2+ cation isonly slightly soluble in sulfuric acid solution, andPbSO4 precipitates at the reaction site on the elec-

Table 2. Common Commercial Battery Systems

common name nominal voltage anode cathode electrolyte

primaryLeclanche (carbon-zinc) 1.5 zinc foil MnO2 (natural) aq ZnCl2-NH4Clzinc chloride (carbon-zinc) 1.5 zinc foil electrolytic MnO2 aq ZnCl2alkaline 1.5 zinc powder electrolytic MnO2 aq KOHzinc-air 1.2 zinc powder carbon (air) aq KOHsilver-zinc 1.6 zinc powder Ag2O aq KOHlithium-manganese dioxide 3.0 lithium foil treated MnO2 LiCF3SO3 or LiClO4

a

lithium-carbon monofluoride 3.0 lithium foil CFx LiCF3SO3 or LiClO4a

lithium-iron sulfide 1.6 lithium foil FeS2 LiCF3SO3 and/or LiClO4a

rechargeablelead acid 2.0 lead PbO2 aq H2SO4nickel-cadmium 1.2 cadmium NiOOH aq KOHnickel-metal hydride 1.2 MH NiOOH aq KOHlithium ion 4.0 Li(C) LiCoO2 LiPF6 in nonaqueous solventsa

specialtynickel-hydrogen 1.2 H2 (Pt) NiOOH aq KOHlithium-iodine 2.7 Li I2 LiIlithium-silver-vanadium oxide 3.2 Li Ag2V4O11 LiAsFa

lithium-sulfur dioxide 2.8 Li SO2 (C) SO2-LiBrlithium-thionyl chloride 3.6 Li SOCl2 (C) SOCl2-LiAlCl4lithium-iron sulfide (thermal) 1.6 Li FeS2 LiCl-LiBr-LiFmagnesium-silver chloride 1.6 Mg AgCl seawater

a In nonaqueous solvents. Exact composition depends on the manufacturer, usually propylene carbonate-dimethyl ether forprimary lithium batteries and ethylene carbonate with linear organic carbonates such as dimethyl carbonate, diethyl carbonate,and ethylmethyl carbonate for lithium ion cells.

Figure 13. Energy storage capability of common com-mercial primary battery systems. Figure 14. Energy storage capability of common recharge-

able battery systems.


trode surface. This solution-precipitation mechanismis also working during the charge reaction, whenPbSO4 dissolves and is retransformed into metallicPb. Figure 15D shows a typical electrochemicalinsertion reaction. The term “electrochemical inser-tion” refers to a solid-state redox reaction involvingelectrochemical charge transfer, coupled with inser-tion of mobile guest ions (in this case Li+ cations)from an electrolyte into the structure of a solid host,which is a mixed, that is, electronic and ionic,conductor (in this case graphite). Unlike displace-ment type electrodes (Figure 15A) and solution typeelectrodes (Figure 15B), the insertion electrodes(Figure 15D) have the capability for high reversibil-ity, due to a beneficial combination of structure andshape stability. Many secondary batteries rely oninsertion electrodes for the anode and cathode. Aprerequisite for a good insertion electrode is elec-tronic and ionic conductivity. However, in thosematerials with poor electronic conductivity, suchas MnO2, good battery operation is possible. In thiscase, highly conductive additives such as carbonare incorporated in the electrode matrix, as inFigure 15E. The utilization of the MnO2 starts at thesurface, which is in contact with the conductiveadditive and continues from this site throughout thebulk of the MnO2 particle. Most electrodes in batter-ies follow one of the basic mechanisms discussed inFigure 15.

Zinc manganese batteries consist of MnO2, a protoninsertion cathode (cf. Figure 15E), and a Zn anode ofthe solution type. Depending on the pH of theelectrolyte solution, the Zn2+ cations dissolve in theelectrolyte (similar to the mechanism shown inFigure 15B) or precipitate as Zn(OH)2 (cf. mechanismin Figure 15C).

The discharge reaction of the MnO2 electrodeproceeds in two one-electron reduction steps as shownin the discharge curve (Figure 16). Starting at cellvoltages of 1.5 V, a coupled one-electron transfer andproton insertion reaction takes place. The transfor-mation of MnO2 into MnOOH is a one-phase reaction.Further reduction leads to a phase change as thesolid MnOOH turns into Mn2+ soluble in the solution,that is, a two-phase reaction. This is consistent withthe Gibbs phase rule that predicts the shape of thedischarge curve for one- and two-phase reactions(Figure 16). When the number of phases, P, is equalto the number of components, C, taking part in thereaction as in the case of a two-phase reaction, thenumber of degrees of freedom F () number ofthermodynamic parameters that have to be specifiedto define the system) is 2. If the values of twoparameters, usually pressure, p, and temperature,T, are specified, there is no degree of freedom left andother parameters of the system such as voltage haveto be constant. Hence, the cell voltage stays constantfor a two-phase discharge reaction. If there is adegree of freedom left, as in the case of a one-phasereaction, the cell voltage can be a variable andchanges (slopes-off) during discharge.

Zinc-manganese batteries dominate the primarybattery market segment. Leclanche invented theoriginal carbon-zinc cell in 1860. He used a naturalmanganese dioxide-carbon black core cathode withaqueous zinc chloride-ammonium chloride electro-lyte, contained in a zinc can. An alternative versionemploys a zinc chloride electrolyte and a syntheticelectrolytic manganese dioxide and has better per-formance than the original Leclanche cell. The car-bon-zinc with zinc chloride electrolyte gives aboutthe same performance, on lower radio-type drains,as the alkaline cell and is strong in the Japanese andEuropean markets. The carbon-zinc cell still finds

Figure 15. Schematics showing various discharge andcharge mechanisms of battery electrodes, which serve asexamples of the battery electrode charge/discharge mech-anisms discussed in the text.

Figure 16. Two-step discharge curve of a MnO2 electrodein aqueous solution showing the influence of one- and two-phase discharge reaction mechanisms on the shape of thedischarge curve. The different shapes of the dischargecurves can be explained with the help of the Gibbs phaserule.


wide use, and in 2003, worldwide it outsold thealkaline cell about 30 billion to 12 billion cells.

In the Unites States, the alkaline electrolyte (KOH)version accounts for ∼80% of sales. The detaileddischarge reaction mechanism is shown in Figure 17.The influence of the pH and the change of the pH ofthe electrolyte solution during discharge on theformation/solubility of various zincate compounds[e.g., Zn(OH)4

2+, etc.] to a change from a one phaseto a two phase reaction at the point where Zn(OH)2begins to precipitate. It should be noted that localpH changes occur also during discharge of the MnO2electrode (Figure 15E).

Compared to the carbon-zinc cell, the alkaline cellis more reliable, has better performance, and is bestfor higher rate applications required for advancedportable electronic devices. The current version of thealkaline cell is mercury free. Instead, it uses acombination of alloying agents and corrosion inhibi-tors to lower the hydrogen gas generation fromcorrosion of the zinc anode and to compensate for thecorrosion protection originally provided by the mer-cury. A synthetic gel holds the zinc powder anodetogether. The cathode is composed of an electrolyticmanganese dioxide-graphite mixture with criticalimpurities controlled to a e1 ppm level.

The zinc-air battery system has the highest en-ergy density of all aqueous batteries and equals thatof the lithium thionyl chloride battery (which is thehighest energy density lithium battery). The highenergy density results from the cell design, as onlythe zinc powder anode is contained in the cell. Theother reactant, oxygen, is available from the sur-rounding air. The air electrode is a polymer-bondedcarbon, sometimes catalyzed with manganese di-oxide. The electrode has a construction similar to thatof fuel cell electrodes (see section 3). The zinc-air

system has captured the hearing aid market. Cellsare available in sizes smaller than an aspirin tabletthat fit into the ear to power the hearing aid.

The main applications of Zn-Ag2O cells are buttoncells for watches, pocket calculators, and similardevices. The cell operates with an alkaline electrolyte.The Zn electrode operates as discussed, whereas theAg2O electrode follows a displacement reaction path(cf. Figure 15A).

Primary lithium cells use a lithium metal anode,the discharge reaction of which is depicted in Figure15B. Due to the strong negative potential of metalliclithium, cell voltages of 3.7 V or higher are possible.As the lithium metal is very reactive, the key tobattery chemistry is the identification of a solventsystem that spontaneously forms a very thin protec-tive layer on the surface of metallic lithium, calledthe solid electrolyte interphase (SEI) layer. Thiselectronically insulating film selectively allows lithiumion transport. Lithium batteries show higher energydensity than the alkaline cells but have a lower ratecapability because of the lower conductivity of thenonaqueous electrolyte and the low lithium cationtransport rate through the SEI.

Commercial lithium primary batteries use solidand liquid cathodes. Solid cathodes include carbonmonofluoride, CFx, manganese dioxide (MnO2), FeS2,and CuS. Chemical (CMD) or electrolytic manganesedioxide (EMD) is used as the cathode with high-temperature treatment to form a water-free activematerial. The CFx is made from the elements, andits cost is somewhat higher than that of the manga-nese dioxide cathode material. These cells are de-signed for relatively low-rate applications. Bothchemistries are very stable, and cells can deliverg80% of their rated capacity after 10 years of storage.The lithium iron sulfide (FeS2) system has beendeveloped for high-rate applications and gives supe-rior high-rate performance compared to the alkalinezinc-manganese cells. Typically, the electrolyte ispropylene carbonate-dimethyl ether (PC-DME) withlithium triflate (LiSO3CF3) or lithium perchlorate(LiClO4) salt.

The lithium thionyl chloride system employs asoluble cathode construction. The thionyl chlorideacts as the solvent for the electrolyte and the cathodeactive material. It has the highest energy density ofany lithium cell and is equal to that of the zinc-airaqueous cell. The reaction mechanism of the cell isexplained in Figure 18. In the inorganic electrolyte(LiAlCl4 dissolved in SOCl2) the lithium metal andthe electrolyte react chemically and form a SEImainly consisting of LiCl and S. LiCl and S are alsothe products of the electrochemical discharge reactionat the carbon positive electrode, where the liquidcathode SOCl2 is reduced. The cell discharge stopswhen the electronically insulating discharge productsblock the carbon electrode.

Lithium-sulfur dioxide cells also use a liquidcathode construction. The SO2 is dissolved in anorganic solvent such as PC or acetonitrile. Alterna-tively, SO2 is pressurized at several bars to use it inthe liquid state. The cell reaction is similar to thatdepicted in Figure 18, with electronically insulating

Figure 17. Discharge mechanism of a Zn-MnO2 cell.From top to bottom, various stages of the discharge reactionare depicted. On the Zn side, the local change of the pHalters the composition of the discharge product.


Li2S2O4 being the SEI component at the Li anode andthe solid discharge product at the carbon cathode.The Li-SOCl2 and Li-SO2 systems have excellentoperational characteristics in a temperature rangefrom -40 to 60 °C (SOCl2) or 80 °C (SO2). Typicalapplications are military, security, transponder, andcar electronics. Primary lithium cells have alsovarious medical uses. The lithium-silver-vanadiumoxide system finds application in heart defibrillators.The lithium-iodine system with a lithium iodidesolid electrolyte is the preferred pacemaker cell.

2.5. Rechargeable BatteriesRechargeable cells generally have lower energy

storage capability than primary cells. The additionalrequirements for rechargeability and long operationlimit the choice of chemical systems and construc-tions to those that are more robust than for primarybatteries. The lead acid battery dominates the re-chargeable market. Both the Pb and PbO2 electrodereaction mechanisms follow the solution-precipita-tion mechanism as depicted in Figure 15C and thecell reaction shown in Figure 19. In addition to thelead and lead oxide electrodes, sufficient amounts of

sulfuric acid and water have to be provided for thecell reaction and formation of the battery electrolyte.For ionic conductivity in the charged and dischargedstates, an excess of acid is necessary. Considering thelimited mass utilization and the necessity of inactivecomponents such as grids, separators, cell containers,etc., the practical value of specific energy (Wh/kg) isonly ∼25% of the theoretical one (Figure 19) forrechargeable batteries. Due to the heavy electrodeand electrolyte components used, the specific energyis low.3 Nevertheless, the lead acid system serves avariety of applications from automotive SLI andmotive power for forklift trucks and the like tostationary energy storage for uninterruptible powersupplies. Its low cost and established recycling pro-cesses make it one of the “greenest” battery systems.According to Battery Council International, Inc.,∼98% of the lead acid batteries in the United Statesare recycled.

Nickel-cadmium (Ni-Cd) was the first smallsealed rechargeable cell. In alkaline (KOH) electro-lyte, the Cd negative electrode functions reversibly,according to a solution-precipitation mechanism (cf.Figure 15C) with Cd(OH)2 being the discharge prod-uct. The Ni positive electrode is actually a Ni(OH)2electrode, which is able to reversibly de-insert/insertprotons during discharge/charge. It has excellent low-temperature and high-rate capabilities. For a longtime, it was the only battery available for power tools.It powered the early cellular phones and portablecomputers. The availability of stable hydrogen stor-age alloys provided the impetus for the creation ofthe nickel-metal hydride (Ni-MH) cell. The hydro-gen storage alloy is a proton-inserting negativeelectrode material that replaced the environmentallythreatened cadmium negative electrode in the Ni-Cd. The positive electrode and the electrolyte stayedthe same. Ni-MH quickly replaced the Ni-Cd forelectronic applications because of its significantlyhigher energy storage capability and somewhat lighterweight. The Ni-MH has poor low-temperature ca-pability and limited high-rate capability, but itshigher energy density served to spur the developmentof the portable electronic device market. Today, it isthe battery of choice for hybrid gasoline-electricvehicles and is beginning to challenge the Ni-Cd forpower tool applications.

The Li ion battery, with significantly higher energydensity and lighter weight, replaced the Ni-MH assoon as production capability was available. It is nowthe battery of choice for portable electronic devicesand is challenging the Ni-MH for the hybrid vehicleapplication. The Li ion cell has a carbon/graphiteanode, a lithium-cobalt oxide cathode, and an or-ganic electrolyte of lithium hexafluorophosphate(LiPF6) salt with ethylene carbonate-organic solventmixture. As in the Ni-MH battery, both the anodeand cathode in the Li ion cell follow an insertionmechanism; however, instead of protons, lithiumcations are inserted and de-inserted (cf. Figure 15D).4

3 A specific energy of 30 Wh/kg literally means that 1 kg of a leadacid battery is able to power a 60 W lamp for only 0.5 h.

4 For layered host materials as used in the lithium ion cell, the term“intercalation” is used for the insertion of guests into the host structure.

Figure 18. Discharge mechanism of a Li-SOCl2 cell. Thecell can operate until the surface of the carbon cathode isfully covered by electronically insulating LiCl and Sdischarge products. The Li-SO2 cell is also a solublecathode system with a cell construction similar to that ofthe Li-SOCl2 cell. It follows a similar discharge reactionwhere the reaction product is LiS2O4.

Figure 19. Depiction of the components of a lead acidbattery showing the differences between theoretical andpractical energy density of a lead acid battery and sourceof the differences.


Sony introduced the Li ion in 1991. Since its intro-duction, it has more than doubled in capacity inresponse to the demand for higher performanceportable electronic devices, such as cellular phonesand notebook computers. The construction of the Liion cells is shown in Figure 20. Cells are availablein both liquid electrolyte and plasticized polymerelectrolyte configurations. New anode and cathodematerials hold promise to double the present perfor-mance over the next 10 years. The Li ion market ispoised to segment into the higher performance,higher cost segment, which continues the increasein energy density, and a segment with lower cost

materials but with high-rate performance for hybridelectric vehicle and power tool applications. The newmaterials that make possible this improvement inperformance are discussed elsewhere and are beyondthe scope of this overview.

Lithium metal rechargeable cells would have thehighest energy of all battery systems. Unfortunately,on recharge, the lithium has a strong tendency toform mossy deposits and dendrites in the usual liquidorganic solvents (cf. Figure 15B). This limits the cyclelife to ∼100-150 cycles (considerably lower that the300 cycles required for a commercial cell), as well asincreasing the risk of a safety incident. Rechargeablelithium metal-vanadium oxide cells (Li-VOx) withpoly(ethylene oxide) polymer electrolytes have beendeveloped for stationary energy storage applications.

Only a few of the thousands of proprosed batterysystems have been commercialized. A set of criteriacan be established to characterize reactions suitablefor use in selecting chemical systems for commercialbattery development. Very few combinations canmeet all of the criteria for a general purpose powersupply. The fact that two of the major batterysystems introduced more than 100 years ago, leadacid (rechargeable) and zinc-manganese dioxide(primary), are still the major systems in their cat-egory is indicative of the selection process for chemi-cal reactions that can serve the battery marketplace.

2.6. Selection Criteria for Commercial BatterySystems

A set of criteria that illustrate the characteristicsof the materials and reactions for a commercialbattery system follow.

1. Mechanical and Chemical Stability. The materi-als must maintain their mechanical properties andtheir chemical structure, composition, and surfaceover the course of time and temperature as much aspossible. This characteristic relates to the essentialreliability characteristic of energy on demand. Ini-tially, commercial systems were derived from materi-als as they are found in nature. Today, syntheticmaterials can be produced with long life and excellentstability. When placed in a battery, the reactants oractive masses and cell components must be stableover time in the operating environment. In thisrespect it should be noted that, typically, batteriesreach the consumer ∼9 months after their originalassembly. Mechanical and chemical stability limita-tions arise from reaction with the electrolyte, ir-reversible phase changes and corrosion, isolation ofactive materials, and local, poor conductivity ofmaterials in the discharged state, etc.

2. Energy Storage Capability. The reactants musthave sufficient energy content to provide a usefulvoltage and current level, measured in Wh/L or Wh/kg. In addition, the reactants must be capable ofdelivering useful rates of electricity, measured interms of W/L or W/kg. This implies that the kineticsof the cell reaction are fast and without significantkinetics hindrances. The carbon-zinc and Ni-Cdsystems set the lower limit of storage and releasecapability for primary and rechargeable batteries,respectively.

Figure 20. Construction of (A) cylindrical, (B) prismatic,and (C) polymer Li ion cells. (Reprinted with permissionfrom a brochure by Sony Corporation).


3. Temperature Range of Operation. For militaryapplications, the operational temperature range isfrom -50 to 85 °C. Essentially the same temperaturerange applies to automotive applications. For ageneral purpose consumer battery, the operatingtemperature range is ∼0-40 °C, and the storagetemperatures range from -20 to 85 °C. These tem-peratures are encountered when using automobilesand hand-held devices in the winter in northernareas and in the hot summer sun in southern areas.

4. Self-Discharge. Self-discharge is the loss ofperformance when a battery is not in use. An accept-able rate of loss of energy in a battery dependssomewhat on the application and the chemistry ofthe system. People expect a battery to perform itsintended task on demand. Li-MnO2 primary cellswill deliver 90% of their energy even after 8 yearson the shelf; that is, their self-discharge is low. Somemilitary batteries have a 20-year storage life and stilldeliver their rated capacity. On the other hand,rechargeable batteries can be electrically restored totheir operating condition and generally have morerapid loss of capacity on storage. The rechargeableNi-MH cell, for instance, will lose up to 30% of itscapacity in a month. Usually, self-discharge increaseswith temperature.

5. Shape of the Discharge Curve. The issue of asloping versus a flat discharge depends on theintended use. For operation of an electronic device,a flat, unchanging, discharge voltage is preferred. Asloping discharge is preferred for applications whendetermining the state-of-charge is important. Thismay be modified somewhat by the impact of cost.Although a constant brightness is preferred in aflashlight, the user may select carbon-zinc with asloping discharge for its lower cost. The influence ofone- or two-phase reactions on the shape of thedischarge curve was discussed previously (cf. Figure16).

6. Cost. The cost of the battery is determined bythe materials used in its fabrication and the manu-facturing process. The manufacturer must be able tomake a profit on the sale to the customer. The sellingprice must be in keeping with its perceived value(tradeoff of the ability of the user to pay the priceand the performance of the battery). Alkaline pri-mary Zn-MnO2 is perceived to be the best value inthe United States. However, in Europe and Japanthe zinc chloride battery still has a significant mar-ket share. In developing countries, the lower costLeclanche carbon-zinc is preferred. Likewise, leadacid batteries are preferred for automobile SLI overNi-Cd with superior low-temperature performancebut with a 10 times higher cost.

7. Safety. All consumer and commercial batteriesmust be safe in the normal operating environmentand not present any hazard under mild abuse condi-tions. The cell or battery should not leak, venthazardous materials, or explode.

Added criteria for rechargeable batteries are asfollows:

8. Ability To Recharge and Deliver Power. Therechargeable battery systems place a severe addedrequirement. The active materials must be capable

of being restored exactly to their original condition(crystal structure, chemical composition, etc.) onreversal of the current flow (charging). After beingrecharged by current reversal, the electrode materialsmust be able to deliver the same rate of dischargewhile maintaining their voltage level. Very fewchemical systems exhibit this characteristic.

9. Cycle Life. It is not enough for a chemical systemto be recharged and deliver power to qualify as acommercial rechargeable system. A commercial cellmust be capable of completely discharging its energyand then fully recharging a minimum of 300 timesand not lose >20% of its capacity. This requires avery robust system and reversible electrode reactions.There can be no side reactions that result in the lossof the active materials during the charge-dischargecycle.

10. Charge Time. The time it takes to recharge abattery completely relates to the use. For conven-ience, recharging in 15 min is accepted for manyconsumer applications. However, fast charging placesa stress on the robustness of the electrode reactionsand may result in shortened cycle life. Most batteriesrequire 3-8 h to recharge completely and maintaintheir required cycle life. This slower charge rateallows time for the atoms and molecules to find theircorrect positions in the charged material.

11. Overcharge/Overdischarge Protection. When abattery is forced outside its thermodynamic voltagelevels, the reaction path becomes unstable; irrevers-ible new reactions can occur, and new compounds canform. These events harm the active material andeither reduce the capacity or render the systeminoperable. In addition, unsafe battery conditionsmay occur under overcharge/overdischarge condi-tions. The Ni-Cd, Ni-MH, and lead acid have abuilt-in overcharge and overdischarge characteristicbased on an oxygen recombination mechanism. Celldesigns often use the ratio of the capacities of eachelectrode (cell balance) to accomplish protection of thebattery system. It is also possible to use electroniccontrols to control the charge and discharge voltagelimits within safe limits. The lithium-cobalt oxidecathode in the Li ion system is protected fromovervoltage and overdischarge by electronic means.Voltage excursions outside its operating range cancause irreversible changes in its crystal structure anddamage cell operations.

3. Fuel Cells


The chemical energy stored in hydrogen and sev-eral hydrocarbon fuels is significantly higher thanthat found in common battery materials. This factprovides the impetus to develop fuel cells for a varietyof applications. Fuel cells are an ideal primary energyconversion device for remote site locations and findapplication where an assured electrical supply isrequired for power generation, distributed power,remote, and uninterruptible power. Figure 21 depictsthe operation of typical fuel cells. The various com-ponents of a functioning fuel cell are shown inFigures 22 and 23. Operating fuel cells are complex


chemical plants that require sophisticated manufac-turing techniques and control circuitry.

Some companies, hospitals, and buildings arechoosing to install fuel cell power in order to be freefrom the outages experienced in utility supply lines.Previously, emphasis had been placed on the devel-opment of large fuel cells in the 200-300 kW rangefor these applications. Several demonstration projectsof 1 MW and larger fuel cells have been undertaken,usually composed of units of 250 kW output. Smallerfuel cells in the range of 50-75 kW are now underintense development for use in automobile and buspropulsion, where their low emission characteristicfinds favor. The direct conversion methanol fuel cellsin the range of 5-25 W are proposed for portableelectronics as a replacement for Li ion and Ni-MHbatteries. The more promising commercial applica-tions of fuel cells appear to be as a stationary powersource for central and dispersed power stations(megawatts) and as mobile power for portable elec-tronic devices and automobiles.

Although several fuel cell technologies are reachingtechnical maturity, the economics of a fuel cell arenot clear. The commercial potential of fuel cells willdepend on the ability to reduce catalyst and otherexpensive materials costs and to manufacture theunits at a competitive cost. Many uses of fuel cellsplace a premium on specific performance character-istics. The relatively simple alkaline fuel cells (AFC)

will continue to be used in space applications, wherehigh energy densities and cryogenic storage of hy-drogen and oxygen represent a cost savings bylowering the weight of the launch vehicle. They arealso used for longer space missions, when the highreliability and the production of drinking water areimportant considerations. The high cost of qualifyinga new technology is a strong impediment for replac-ing the AFC in space applications.

Fuel cells offer the cleanest power generationpossible. They are quiet in operation and can belocated close to the application. They produce muchless greenhouse emissions and can be more efficientin conversion of the energy in a fuel into power thangasoline engines or utility thermal power plants. Fuelcells are best utilized as a steady energy source andnot as a power source to supply dynamic demands.For applications that require varying power de-mands, such as automotive propulsion, the use of thefuel cell in a hybrid configuration with a battery orEC will be required. The fuel cell provides steadypower demand while the battery or EC handles thesurge for regenerative breaking and acceleration aswell as initial start-up.

Figure 21. Summary of the reactions and processes thatoccur in the various fuel cell systems.

advantages disadvantages

efficient energy conversion complex to operatemodular construction best as primary energy sourcenonpolluting impurities in gas stream shorten lifelow maintenance pulse demands shorten cell lifesilent expensivesafe limited availabilityhigh energy density low durability

low power density per volume

Figure 22. Block diagram of the component parts of afunctioning fuel cell.

Figure 23. Depiction of the components of a complete fuelcell system including the re-former and power conditioningunit.


Fuel cells have been identified as a nearly idealsolution to power the requirements for motor vehiclemanufacturers, utility and nonutility generators, andportable electronic devices. Each segment has sig-nificant incentives to develop alternate power sources.For automakers, the incentive is environmentallyrelated, coupled with strong support from govern-ment programs designed to move away from fossilhydrocarbon fuels. For the utilities, increasing envi-ronmental pressure and power demand, coupled withlimited generation capability, have created a need fordistributed power generation and storage. In a simi-lar vein, those who need an uninterruptible powersupply, or freedom from the utility grid, find fuel cellsto be an attractive option. Direct methanol fuel cellsfor portable electronic devices such as notebookcomputers seem close to commercial reality and willcompete with batteries for this market. The keychallenge for each will be to meet the cost-perfor-

mance barrier in a small size as well as governmentalregulations.

It is estimated that the fuel cell market for dis-tributed power and demonstration projects and con-tracts amounted to about $100 million for 2003.Research and development contracts to develop fuelcells for automotive propulsion and stationary energystorage are an order of magnitude larger.

3.2. Fuel Cell Operation

Fuel cells, like batteries, convert the chemicalenergy residing in a fuel into electrical energy ondemand. As in batteries and other electrochemicalcells, fuel cells consist of an anode, where oxidationoccurs, a cathode, where reduction occurs, and anelectrolyte, where ions carry the current between theelectrodes. Fuel cells differ from batteries in that thefuel and oxidant are not contained within the fuel

Table 3. Types of Fuel Cells

advantages disadvantages comments

Alkaline (AFC)a

mechanically rechargeable limited activated life original development >30 years agolow-cost KOH electrolyte intolerant of impurities in gas streams Apollo fuel cell

CO2 and CO operates at room temp to 80 °Cpure H2 only suitable fuel demo in vehicles in the 1970s

Polymer Electrolyte Membrane Fuel Cell (PEMFC)nonvolatile electrolyte expensive catalysts required operates best at 60-90 °Cfew materials problems CO a strong poison originally developed for space by GECO2 rejecting electrolyte H2O management essential hydrogen fuel (re-formed hydrocarbons, pure H2,pressure differential between high-cost electrolyte MH storage)

anode and cathode pure H2 only suitable fuel main development efforts for automotive andpolymer electrolyte oxygen kinetics are slow stationary applications

intolerant of impuritieslimited lifewater management essential

Direct Methanol Fuel Cell (DMFC)direct fuel conversion stable reaction intermediates operates best at 60 to 90 °C)slow electrode kinetics high catalyst loadings same construction as PEMFCimproved wt and vol water management essential methanol fuel eliminates reformerpolymer electrolyte low overall efficiency lower current capability

methanol hazardous methanol crossover reduces efficiencyneeds new membrane, higher efficiencyhigh catalyst loadingsmain effort for portable electronic devices

Phosphoric Acid Fuel Cell (PAFC)CO2 rejecting electrolyte H2 only suitable fuel operates best at ∼200 °Chigh fuel efficiency anode CO catalyst poison stationary energy storage (nominal units, 250 kW)

O2 kinetics hindered available commerciallylow conductivity electrolytehigh-cost catalystslimited life

Molten Carbonate Fuel Cell (MCFC)fast electrode kinetics materials problems and life operates best at 550 °Chigh efficiency low sulfur tolerance nickel catalysts, ceramic separator membraneCO/CH4 usable fuel high ohmic electrolyte hydrocarbon fuels re-formed in situdirect reforming feasible low tolerance to sulfur several large demonstration unitshigh-grade heat available need to recycle CO2 significant government support

limited life

Solid Oxide Fuel Cell (SOFC)high grade heat available high fabrication costs operates at 900 °Cfast electrode kinetics severe materials constraints conducting ceramic oxide electrodesin situ reforming feasible high electrolyte conductivity hydrocarbon fuels re-formed in situno electrolyte management least sensitive to sulfur, etc.high system efficiencytolerant of impurities

a Metal air batteries with replaceable anodes are often considered to be a fuel cell but are not considered here.


cell compartment but supplied continuously from anexternal source. In a real sense, fuel cells are likeinternal combustion engines, as they operate only solong as the fuel is supplied. Fuel cells are notelectrically recharged, but after use, the tank isrefilled with fuel. From an operational point of view,the fuel of choice is hydrogen gas, with the exhaustgas being water. Other fuels and hydrocarbons mustbe converted to hydrogen for use in a fuel cell. Thedirect conversion of fuels such as CH3OH and CH4is possible under certain conditions. Each type of fuelcell has a unique set of processes and reactions todescribe its operation. A summary of the character-istics for the various fuel cell systems is given inTables 3 and 4.

The basic reaction of a H2-O2 fuel cell is

For this reaction ∆G° ) -235.76 kJ/mol and ∆H°) -285.15 kJ/mol. Fuel cells follow the thermody-namics, kinetics, and operational characteristics forelectrochemical systems outlined in sections 1 and2. The chemical energy present in the combinationof hydrogen and oxygen is converted into electricalenergy by controlled electrochemical reactions at eachof the electrodes in the cell.

Fuel cells can be roughly divided into low-temper-ature (ca. <200 °C) and high-temperature (ca. >450°C) fuel cells. Low-temperature fuel cells typically usealkaline or acidic electrolytes. In acidic electrolytesthe electrode reactions are

Oxygen undergoes a two-step indirect reductionreaction. The stable H2O2 intermediate is undesir-able, as it lowers the cell voltage and H2O2 attacksand corrodes the carbonaceous electrode materialcommonly used. Better catalysts are needed to speedthe decomposition of H2O2 to reduce its impact on theoverall reaction. Similarly, a catalyst can enhance theH2 dissociation rate at the anode. Platinum orplatinum alloys are usually employed as catalysts tospeed the reactions on both electrodes in low-tem-perature fuel cells. The hydrogen reaction kineticsare fairly fast and do not require as high platinumloading to deliver high currents. To suppress the two-step oxygen reduction, significantly more catalyst isrequired. Due to the very high price of nanostruc-tured Pt, the Pt may be distributed on a porous highsurface area carbon. Typical catalyst loadings are∼0.1 mg/cm2 of platinum or platinum alloy catalyston the anode and ∼0.5 mg/cm2 catalyst on thecathode. However, the catalyst loading varies, de-pending on the type of fuel cell and its application.A major thrust in research on low-temperature fuelcells is to reduce the catalyst loading, to improve COtolerance, and to identify lower cost catalysts. As the

H2 + 1/2O2 ) H2O E° ) 1.229 V (18)

anode: H2 - 2e- ) 2H+ (19)

cathode: O2 + 2H+ + 2e- ) H2O2 (20)

H2O2 + 2H+ + 2e- ) 2H2O (21)

overall: O2 + 4H+ + 4e- ) 2H2O (22)

Tab

le4.

Typ

ical

Ch

arac

teri

stic

sof

Var

iou

sF

uel

Cel

lS

yste

ms

type

anod

efe

edan

ode

com

posi

tion

cath

ode

feed

cath

ode

com

posi

tion

elec

trol

yte

oper

atin

gte

mp,

°C

alka

lin

e(A

FC

)pu

reh

ydro

gen

carb

on/p

lati

nu

mca

taly

stox

ygen

(or

air)

carb

on/p

lati

nu

mca

taly

stan

dal

tern

ativ

esaq

KO

Ham

bien

t-90

PE

M(P

EM

FC

)pu

reh

ydro

gen

carb

on/p

lati

nu

mca

taly

stox

ygen

orai

rca

rbon

/pla

tin

um

cata

lyst

acid

icpo

lym

eram

bien

t-90

dire

ctm

eth

anol

(DM

FC

)m

eth

anol

orm

eth

anol

-w

ater

carb

on/p

lati

nu

mca

taly

stox

ygen

orai

rca

rbon

/pla

tin

um

cata

lyst

acid

icpo

lym

er60

-90

phos

phor

icac

id(P

AF

C)

pure

hyd

roge

nca

rbon

/pla

tin

um

cata

lyst

oxyg

enor

air

carb

on/p

lati

nu

mca

taly

stph

osph

oric

acid

inS

iCm

atri

x20

0m

olte

nca

rbon

ate

(MC

FC

)h

ydro

gen

orn

atu

ralg

aspo

rou

sN

iox

ygen

orai

rpo

rou

sN

iOm

olte

nL

i 2C

O3

inL

iAlO

2-55

0so

lid

oxid

e(S

OF

C)

gaso

lin

eor

nat

ura

lgas

poro

us

cerm

etof

Nio

rC

oan

dyt

tria

-zi

rcon

iaox

ygen

orai

rst

ron

tia-

dope

dla

nth

anu

m-

man

gan

ite

Per

ovsk

ite

yttr

ia-s

tabi

lize

dor

yttr

ia/c

alci

a-st

abil

ized

zirc

onia

supp

ort

900a

aM

g-or

Sr-

dope

dla

nth

anu

mch

rom

ate

asce

llin

terc

onn

ects

.


O2 and H2 dissociation kinetics are better at highertemperatures (>400 °C), low-cost electrode structuresof high surface area Ni and oxides such as spinels orperovskites to replace the very effective, but costly,Pt catalysts have been sought.

Fuel cell electrodes are more complex structuresthan battery electrodes. They serve three functions:(1) to ensure a stable interface between the reactantgas and the electrolyte, (2) to catalyze the electrodereactions, and (3) to conduct the electrons from or tothe reaction sites. A significant problem is the controlof the interface at the juncture of the reactant (gas)phase, the electrolyte medium, and the catalyzedconducting electrode, the so-called “three-phase bound-ary”, where the electrolyte, electrode, and gas allcome together. A stable three-phase boundary iscritical to good performance and long operation.Therefore, the porosity and the wetting behavior withelectrolyte and the electrode surface must be pre-cisely adjusted.

Fuel cells can operate with very high electricalefficiencies approaching 60-70%. If the waste heatof the fuel cell is also used, fuel efficiencies of 90%are possible. The performance of the fuel cell isjudged by the voltage-current curve depicted inFigure 24. The operating voltage is significantly lessthan the theoretical 1.23 V as the actual cell reactioninvolves H2, O2, and H2O and also the peroxideintermediate (cf. above), the latter influencing theNernst voltage and the OCV. The activation, ohmic,and diffusion polarization characteristics determinethe shape of the fuel cell discharge curve as they dofor batteries (cf. Figure 7).

As for the other electrochemical storage/conversiondevices, the fuel cell electrolyte must be a pure ionicconductor to prevent an internal short circuit of thecell. It may have an inert matrix that serves tophysically separate the two electrodes. Fuel cells maycontain all kinds of electrolytes including liquid,polymer, molten salt, or ceramic.

Hydrogen gas is the preferred fuel for low-temper-ature fuel cells. The main obstacle in the use ofhydrogen as energy carrier is that hydrogen is not areadily available fuel. In high-temperature fuel cells,a hydrocarbon fuel, for example, methane or gasoline,can be fed directly into the cell. To use hydrocarbons

in low-temperature fuel cells, the hydrocarbon fuelsmust be converted into hydrogen through a separatere-forming step placed between the tank and the fuelcell. Any trace amounts of CO, CO2, and H2S foundin the hydrogen-rich re-formate gas must be removedbefore using the gas, as they irreversibly block thePt catalyst and thus “poison” the electrode. To reducedependence on hydrocarbon fuels, the use of off-peakelectricity from a series of new nuclear plants hasbeen proposed, to produce the hydrogen for use inautomotive applications.

Apart from hydrocarbons and gasoline, other pos-sible fuels include hydrazine, ammonia, and metha-nol, to mention just a few. Fuel cells powered bydirect conversion of liquid methanol have promise asa possible alternative to batteries for portable elec-tronic devices (cf. below). These considerations al-ready indicate that fuel cells are not stand-alonedevices, but need many supporting accessories, whichconsume current produced by the cell and thus lowerthe overall electrical efficiencies. The schematic of themajor components of a so-called “fuel cell system” isshown in Figure 22. Fuel cell systems require so-phisticated control systems to provide accurate me-tering of the fuel and air and to exhaust the reactionproducts. Important operational factors include stoi-chiometry of the reactants, pressure balance acrossthe separator membrane, and freedom from impuri-ties that shorten life (i.e., poison the catalysts).Depending on the application, a power-conditioningunit may be added to convert the direct current fromthe fuel cell into alternating current.

Figure 24. Typical power curve for a fuel cell. The voltagedrops quickly from the OCV due to the formation of theperoxide intermediate. Operation of the fuel cell at the kneeof the curve where concentration is limiting performancecan damage the electrodes and lead to rapid deteriorationof cell operation.

Figure 25. (A) Comparison of the energy storage capabil-ity of fuel cells and batteries. Only after several refuelingoperations are fuel cells more efficient energy storagedevices on a Wh/L and Wh/kg basis. (B) Fuel cells have aset volume and weight for the fuel cell stack and peripher-als to supply the reactants to the stack. The smallincremental fuel volume to continue operation supplyingenergy makes them more efficient for longer operations.


Fuel cell systems are not very volume efficient. Theancillary equipment to operate the device can be twoto three times larger than the fuel cells themselves,where energy conversion actually occurs. Everythingconsidered, the fuel cell stack5 is much less than 50%of the total volume of the unit. In addition, fuel cellstacks themselves have a low energy density in Wh/Land Wh/kg compared to batteries as the gas distribu-tion system requires significant extra volume andweight. Only by taking into account the continuoussupply of fuel to the system during long-term opera-tion is the energy density of the fuel cell greater thanthat of advanced battery systems, as shown in Figure25. However, due to the slow kinetics, especially atthe oxygen cathode side, the power capability of fuelcells is lower than that of most batteries and inferiorto combustion engines and gas turbines (cf. Figure3). The detrimental impact of the weight and volumeof the fuel cell may be hidden by giving the “powercapability” in A/cm2 and not in W/L or W/kg.

3.3. Characteristics of Various Types of FuelCells

Fuel cells are typically classified by the type ofelectrolyte. Apart from certain specialty types, thefive major types of fuel cells are alkaline fuel cell(AFC), polymer electrolyte fuel cell (PEMFC), phos-phoric acid fuel cell (PAFC), molten carbonate fuelcell (MCFC), and solid oxide fuel cell (SOFC).

The AFC is one of the oldest fuel cell types. Thecell reactions are as follows (the existence of theperoxide intermediate HO2

- has been already dis-cussed):

The AFC was first developed for the Apollo mis-sions. An updated version has been developed and isstill in use to provide electrical power for shuttlemissions. These power plants reach efficiencies of60% in space applications. The electrolyte is potas-sium hydroxide based. Noble metal catalysts are veryactive in the AFC for both the hydrogen and oxygenelectrodes. The hydrogen and oxygen kinetics aremore facile in alkaline than acid electrolytes, thusresulting in higher cell voltages.6 This permits theuse of non-noble metal catalysts, such as Raneynickel, for the fuel electrode. Silver and spinel-typeoxides along with iron phthalocyanines and otherporphyrins are good catalysts for the oxygen (air)electrodes. These catalysts cannot be used in acidic

electrolytes, as they are soluble in acidic media. TheAFC is susceptible to CO2 contamination of theelectrolyte (clogs the pores) when air is used as wellas to poisoning of the Pt and Ni catalyst by sulfideand CO impurities in the feedstock. In other words,the AFC requires pure hydrogen and oxygen asreactants. Because of the requirement of pure fuelsand elimination of CO2 for long life, terrestrialapplications are limited. Both electrodes are fabri-cated with an active layer of platinum catalyst oncarbon support and binder that is backed by a wet-proofed Teflon [PTFE, poly(tetrafluoroethylene)]bonded carbon layer to control the wetting of theelectrodes by the electrolyte and thus the location ofthe three-phase boundary. The carbons are generallytreated to remove active entities on the surface beforebeing catalyzed. Graphite bipolar endplates containthe gas flow channels and serve to provide waste heatremoval. On shutdown, it is common to have theelectrolyte empty into a sump on earthbound ap-plications, as the cathode needs to be protected fromboth absorption of CO2 by the electrolyte and corro-sion by wet O2. The AFC operates at up to 1 A/cm2

at 0.7 V.The PEMFC was first developed for the Gemini

space vehicle. The electrodes are formed on a thinlayer on each side of a proton-conducting polymermembrane, used as electrolyte. In a sense, theelectrolyte is composed similarly to plasticized elec-trolyte in a Li ion cell, where a liquid electrolytecomponent is immobilized in a polymer matrix. Itconsists of a solid polymer PTFE backbone with aperfluorinated side chain that is terminated with asulfonic acid group. Hydration of the membraneyields dissociation and solvation of the proton of theacid group. The solvated protons are mobile withinthe polymer and provide electrolyte conductivity. Themembrane has low permeability to oxygen andhydrogen (crossover) for high coulombic efficiency. Atypical PEMFC is depicted in Figure 26.

A graphite (or metal) plate serves as the plenumfor the gas supply and for heat removal. A catalyzedcarbon layer is applied to the membrane surface. Athicker gas diffusion layer or porous carbon paper

5 A fuel stack is a series connection of fuel cells. Strictly speaking,it is fuel cell battery (cf. footnote 1). The composition and design ofthe fuel cell stack differ for the implementation of each type of cell.

6 The higher voltage is not only due to better kinetics but also dueto the fact that oxygen reaction via the intermediate peroxide (HO2

-

in alkaline electrolytes) is more facile.

anode: H2 + 2OH- - 2e- ) 2H2O (23)

cathode: O2 + H2O + 2e- ) HO2- + OH- (24)

HO2- + H2O + 2e- ) 3OH- (25)

or: O2 + 2H2O + 4e- ) 4OH- (26)

Figure 26. Schematic of a polymer electrolyte membrane(PEM) fuel cell. The fuel cell stacks operate at 30-180 °Cwith 30-60% efficiency. Fuel options include pure hydro-gen, methanol, natural gas, and gasoline.


provides gas transport to the reaction zone. Thecomposition and amount of catalyst differ for eachelectrode. The anode has a lower catalyst loadingthan the cathode. The platinum-based catalysts aresensitive to H2S and CO impurities among others sothey must be eliminated from the feedstock for longoperation. The catalyst content of the anode is ∼0.1mg/cm2, and that of the cathode is ∼0.5 mg/cm2.

Water management in the membrane is critical tolong-term performance. The proton transport carrieswater along (water drag) while carrying the current.The water concentration gradient results in back-diffusion. However, for operation at high current, theanode side of the membrane must be humidified orit will dehydrate. By the same token, product wateris removed from the cathode side in the air streamto prevent flooding of the active layer. For reliableoperation, a water content of 30-60% in the mem-brane is preferred for reliable operation.

For higher temperature operation, a polybenzimid-azole-based polymer electrolyte may be preferred.The PEMFC structures have good mechanical integ-rity under compression and expansion from dif-ferential temperature and pressure gradients thatoccur during operation. This system has minimalmaterials problems, except for the cost and operationcharacteristics of the membrane. The PEMFC oper-ates at ∼1 A/cm2 at 0.7 V. The electrode reactions inacidic media have been discussed above.

The DMFC uses the same basic cell constructionas for the PEMFC. It has the advantage of a liquidfuel in that is easy to store and transport. There isno need for the re-former to convert the hydrocarbonfuel into hydrogen gas. The anode feedstock is amethanol and water mixture or neat methanol,depending on cell configuration. The DMFC is underdevelopment as a power source for portable electronicdevices such as notebook computers and cellularphones. The pure methanol or a methanol-watermixture would be stored in a cartridge similar to thatused for fountain pens. Refueling would involve thequick replacement of the cartridge. The platinum-ruthenium catalyst loadings for the anode are higherthan for the PEMFC and are in the range of 1-3 mg/cm2. Without the ruthenium, that is, with neat Pt,the anode reaction produces a stable formic acidintermediate. The reaction for the direct conversionof methanol has a similar voltage as for hydrogen.The overall cell reaction is

The reaction at the anode is

and

Due to the chemical similarity of water and metha-nol, the methanol has considerable solubility in thepolymer membrane, leading to significant crossover

from the anode side to the cathode side of the cell.On reaching the cathode, the methanol is oxidized.This significantly lowers the cathode voltage and theoverall efficiency of cell operation. The typical DMFCyields ∼0.5 V at 400 mA/cm2 at 60 °C.

The PAFC is another fuel cell operating in acidicmedia. It has been frequently used in energy storageapplications. The cell operates at ∼200 °C. Below 150°C, its conductivity is reduced, and above 220 °C, thephosphoric acid is too volatile and tends to decom-pose. A SiC matrix separator holds the acid. The acidrejects the water reaction product. Above 150 °C,some CO can be tolerated in the anode feedstock. Thekinetic hindrance at the oxygen cathode is the majorsource of losses. The active layer of platinum catalyston carbon black support and polymer binder isbacked by a carbon paper with 90% porosity, reducedsomewhat by PTFE binder. The active layer consistsof pores in the range of 3-50 µm with micropores inthe range of 0.0035 µm. The anode operates at nearlyreversible voltage with ∼0.1 mg/cm2 catalyst loading.The cathode requires a higher catalyst loading of ∼1mg/cm2 of catalyst. Graphite bipolar endplates con-tain the gas flow channels and serve to provide heatremoval by liquid flow. On shutdown, the cathodeneeds to be protected from corrosion by wet O2. Theelectrode reactions are

The hot H3PO4 electrolyte rejects water, the reactionproduct. The high temperature favors H2O2 decom-position, and peroxide buildup is less pronouncedthan for the aqueous electrolyte systems.

The MCFC finds application in energy storageapplications. It operates best at ∼560 °C, and thewaste heat can be used in cogeneration. The systemdoes not use any noble metal catalysts and has ahigher efficiency than the PEMFC and the PAFC.The separator is a LiAlO2 ceramic tile separator filledwith molten carbonates to prevent crossover of thereactants and aid in CO3

-2 transport. Lithium-richcarbonate electrolytes have higher conductivity butlower gas solubility and higher corrosion rates. Atthe operating temperature, problem areas includeNiO dissolution and structural stability of the anodesand cathodes, changes in pore size distribution, anddistortion of the electrode structures. Alternate LiFeO2cathode materials have low conductivity, and Ni-Cr anode materials have creep and stability issues.Control of the pore diameter is critical in the separa-tor tiles. Increased pressure raises voltage but in-creases gas solubility and cathode dissolution andlowers operating life. The performance is limited bythe electrolyte resistivity and the removal of the heatgenerated by the losses in electrode polarization. Thecells are sensitive to sulfur contamination becauseof its effect on the nickel electrode materials andcatalyst. Other problem areas include seal stabilityduring thermal cycling and electrolyte creep. Typicaloperating parameters are 150 mA/cm2 at 0.8 V at 600°C. The anode reactions using the fuel methane (or

CH3OH + 3/2O2 ) CO2 + 2H2O E ) 1.186 V(27)

anode: CH3OH + H2O - 6e- ) 6H+ + CO2 (28)

cathode: O2 + 4H+ + 4e- ) 2 H2O (29)

anode: H2 - 2e- ) 2H+ (30)

cathode: O2 + 4H+ + 4e- ) 2H2O (31)


CO and H2 stemming from CH4 conversion) are

The SOFC operates at ∼800-1000 °C with O2-

conduction in the solid phase. Limitations and prob-lems arise from the high operating temperatures andplace severe restrictions on the choice of materials.The materials must have very similar coefficients ofexpansion and be chemically stable in oxidizing andreducing conditions. At the operating temperatures,the hydrocarbon fuels (plus water, if needed) arequickly and completely re-formed in situ. The systemoperates at close to 96% thermodynamic efficiencyand is tolerant to most impurities. It can deliver high-quality heat for cogeneration applications. The SOFCcan operate on most any hydrocarbon or hydrogenfuel and is tolerant of short circuiting and to over-loads. No noble metal catalysts are required. Theanode consists of a porous cermet of Ni or Co catalyston yttria-stabilized zirconia. The zirconia acts toinhibit grain growth of the catalyst particles of nickelor cobalt and protects against thermal expansion. Theelectrolyte itself consists of yttria-stabilized zirconia,which can be additionally deposited onto a calcia-stabilized zirconia. Yttria and calcia doping provideoxygen defects for better conductivity. The cathodeis generally a strontia-doped lanthanum-manganiteperovskite. The Sr dopant provides for oxygen trans-fer to the cathode-electrolyte interface. A Mg- or Sr-doped lanthanum chromate is used for the currentcollector and the intercell connection. It is imperviousto the fuel and oxygen gases and is chemically andstructurally stable in thin, dense, layered configura-tions. The cells operate at ∼1 A/cm2 at 0.7 V. Cellsare constructed in cylindrical form or in the flat plateformat shown in Figure 27. The cell reactions are

4. Electrochemical Capacitors (ECs)


ECs are sometimes called supercapacitors, ultra-capacitors, or hybrid capacitors. The term ultra-capacitor or supercapacitor is usually used to describe

an energy storage device based on the charge storagein the electrical double layer (EDL) of a high-surface-area carbon in aqueous electrolytes. The marketfor these devices used for memory protection inelectronic circuitry is about $150-$200 million an-nually. New potential applications for ECs includethe portable electronic device market, the powerquality market, due particularly to distributed gen-eration, and low-emission hybrid cars, buses, andtrucks.

There is no commonly accepted nomenclature forECs except that the definitions of anode, cathode,etc., carry over from battery and fuel cell usage, suchas the anode as the negative terminal and cathodeas the positive terminal. In general, ultracapacitorshave referred to capacitors with two high-surface-area carbon electrodes for the anode and cathode.This arrangement where both electrodes have thesame configuration will be referred to as a symmetriccapacitor. The term supercapacitor has also beenused to refer to the symmetric combination of twocarbon electrodes that are catalyzed with rutheniumdioxide (RuO2). The RuO2 introduces a redox couplebetween two valence states of ruthenium, to resultin higher capacitance for the carbon electrodes, butwith a penalty in slower time constant for reactingto a pulse demand. A second type of EC combines abattery or redox electrode with a carbon electrode,such as nickel hydroxide cathode with a carbonanode. These supercapacitor or hybrid capacitors willbe referred to as an asymmetric EC.

anode: CH4 + 2H2O ) CO2 + 4H2 or (32)

CH4 + H2O ) CO + 3H2 (33)

then: CO + H2O ) CO2 + H2 (34)

then: H2 + CO32- - 2e- ) CO2 + H2O

(principal reaction) or (35)

CO + CO32- ) 2CO2

(minor reaction) (36)

cathode: O2 + 2CO2 + 4e- ) 2CO32- (37)

anode: H2 + O2- - 2e- ) H2O or (38)

CO + O-2 - 2e- ) CO2 or (39)

CH4 + 4O-2 - 8e- ) 2H2O + CO2 (40)

cathode: O2 + 4e- ) 2O-2 (41)

Figure 27. Schematic view of SOFC cylindrical and flatplate cell constructions.


4.2. Characteristics of the Electrical Double Layer

When an electrode, that is, an electronic conductor,is immersed into an electrolyte solution, that is, anionic conductor, there is a spontaneous organizationof charges at the surface of the electrode and in theelectrolyte facing the electrode. This EDL forms atthe electrode-electrolyte interface with one layer atthe surface inside the conductor and the other layerin the electrolyte as depicted in Figure 28. The twocharged layers are considered to behave as a physicalcapacitor, with the charges in the solution and in theconductor separated by a distance of the order ofmolecular dimensions. The characteristics of the EDLdepend on the electrode surface structure, the com-position of the electrolyte, and the potential fieldbetween the charges at the interface. Depending onthe surface charge of the electrode materials, positiveor negative ions from the electrolyte form the solutionpart of the EDL at the interface between the elec-trode and the electrolyte. A simplified structure isshown in Figure 28A for the case of a negativelycharged electrode surface. According to this simpleHelmholtz model, the charges are concentrated oneach side of the electrode surface. A more complexmodel of the EDL structure in Figure 28B takes intoaccount the different sizes of the ions and their

reactivity with the surface. The outer Helmholtzplane (OHP) refers to the distance of closest approachof nonspecifically adsorbed ions (generally cations)in solution. Cations that populate the OHP areusually solvated and thus are generally larger thanthe less solvated anions. The interactions of the ionsof the OHP with the surface charge have the char-acter of coulombic interactions. The inner Helmholtzplane (IHP) refers to the distance of closest approachof specifically adsorbed ions (generally anions) and/or adsorbed solvent molecules to the electrode sur-face. These adsorption processes are determined bychemical affinities of the ions to the electrode surfaceand the field strength in the EDL. In practice, thestructure of the EDL is much more complex than themodels discussed above.

The double layer at the electrode surface forms andrelaxes almost instantaneously. It has a time con-stant, or time of formation, of ∼10-8 s. Therefore, thestructure of the double layer has the capability torespond rapidly to potential changes in the same timeframe. The process involves only a charge rearrange-ment, not a chemical reaction. This rapid responseto change is in contrast to the situation with theredox electrode reactions in batteries and fuel cells.The time constant for the redox reactions is muchslower and in the range of 10-2-10-4 s related to theimpedance of the reaction. The redox reactionscontribute to “polarization capacitance” associatedwith the electrode reactions. The other main differ-ence between supercapacitors and batteries and fuelcells is the reversibility (short time constant) of theEDL process compared to the longer time constantof the redox reactions and the stress from detrimentalside reactions, which reduce the cycle life and long-term stability of the device. Whereas cycle life andstability of the double layer electrochemical capacitorcan easily exceed 1 million cycles, battery electrodescan reach this level only if charged and dischargedat a low depth of discharge.

When carbons are placed in an electrolyte, theygenerally assume a voltage near the zero point ofcharge of the EDL. In aqueous solutions, this is near0 V versus hydrogen. By applying an external volt-age, many more additional ions and electrons can bemoved to the double layer, increasing the capacitanceC () charge per applied voltage, C ) Q/U). As a ruleof thumb, carbons and metals typically have a double-layer capacitance in the range of 10-40 µF/cm2. Theexact values depend mainly on the voltage and theextent of participation of the IHP in the electricaldouble layer. A high-surface-area carbon electrodecan yield a capacitance of ∼4 F/g.

4.3. EC OperationAs noted above, electrochemical capacitors are close

cousins to batteries. The simple circuit shown il-lustrates their basic operation.

Here, Ca and Cc are the double-layer capacitances ofthe anode and cathode, respectively. Ri is the internal

Figure 28. (A, top) Simple Helmholtz model of theelectrical double layer. It is essentially a picture of aconventional capacitor. (B, bottom) Depiction of the electri-cal double layer at the surface of the negative electrodeshowing the outer Helmholtz plane (OHP) and the innerHelmholtz plane (IHP). The inner Helmholtz plane (IHP)refers to the distance of closest approach of specificallyadsorbed ions and solvent molecules to the electrodesurface. The outer Helmholtz plane (OHP) refers to thedistance of ions, which are oriented at the interface bycoulomb forces.


resistance of the cell. For capacitors in series

If Ca ) Cc, as would be expected for an ultracapacitor,then

High-surface-area carbon is the material of choice,as it combines a large surface area wetted by theelectrolyte, high electronic conductivity, and chemicaland electrochemical stabilities with low cost. Thecapacitance of these devices can be orders of magni-tude larger than those of conventional dry andelectrolytic capacitors. The voltage for electrochemi-cal capacitors with aqueous electrolytes is ∼1 V,limited by the voltage stability of the electrolyte. Byswitching to an organic-based electrolyte, voltages ofup to 2.7 V can be found in practice. However, theorganic electrolytes have lower double-layer capaci-tance and poorer conductivity. Because the energystorage is given by energy ) 1/2QV2, the highervoltage permitted by an organic electrolyte signifi-cantly increases the energy storage capability of theEC. Because the resistivity is ∼100 times larger thanfor aqueous electrolytes, the time constant for re-sponse to a large pulse is slower for the nonaqueouselectrolyte-based ECs.

The charge-discharge of a symmetric EC com-posed of two carbon electrodes with approximatelythe same mass immersed in an aqueous or nonaque-ous electrolyte is shown in Figure 29. With zeroapplied charge Q, both electrodes of a cell are at thesame voltage. The potential of the electrodes in-creases in opposite directions during charge, as eachhas approximately the same capacitance. Maximumcell operating voltage is reached when one of theelectrodes reaches the stability limit of the electro-lyte.

The asymmetric type of EC incorporates a batteryelectrode as one of the electrodes. The battery elec-trode has a capacitance associated with the redox

battery reaction of ∼10 times the capacitance of theelectrical double layer. For instance, if the nickelbattery cathode is substituted for the cathode in asymmetric capacitor, for example, NiOOH for carbon,then substituting Cc ) 10Ca in eq 42, the capacitanceof the EC is essentially doubled.

This asymmetric type of EC is often termed a“hybrid” capacitor. The typical discharge curve forthis hybrid EC is shown in Figure 30. Because thebattery electrode has a capacity of 3-10 times thatof the double-layer electrode, it remains at an invari-ant voltage during charge and discharge. As a result,the discharge voltage of the hybrid capacitor fallsmore slowly than that of the carbon-carbon EDLcapacitor.

In some cases, the kinetics of the redox charge-discharge reactions can proceed almost as quicklyand reversibly as EDL charging. Thin film redoxelectrodes, based on the lithium intercalation/inser-tion principle such as Li4Ti5O12, exhibit high revers-ibility and fast kinetics. The RuO2 materials depos-ited on carbon show “pseudo-capacitive” charge-

Figure 29. Depiction of the charging process of a symmetric capacitor.

1/C ) 1/Ca + 1/Cc (42)

C ) 1/2Ca (43)

Figure 30. Operation principle of an EC in the dischargedstate, during charging, and in the charged state: (A) for asymmetric construction and (B) for an asymmetric con-struction.

1/C ) 1/Ca + 1/10Ca (44)

C ≈ Ca (45)


discharge behavior as do polymeric materials suchas polyaniline, polypyrrole, and polydiaminoanthra-quinone (DAAQ). These have facile kinetics and haveshown high capacitance and long life. The insertionof anions and cations into their structure can yieldcapacitances of up to 200 µF/cm2 and, moreover, theycan be easily fabricated as thin films.

5. Summary

Electrochemical energy conversion devices arepervasive in our daily lives. Batteries, fuel cells andsupercapacitors belong to the same family of energyconversion devices. They are all based on the funda-mentals of electrochemical thermodynamics and ki-netics. All three are needed to service the wide energyrequirements of various devices and systems. Neither

batteries, fuel cells nor electrochemical capacitors, bythemselves, can serve all applications.

Martin WinterInstitute for Chemistry and Technology of

Inorganic Materials, Graz University ofTechnology, Stremayrgasse 16,

A-8010 Graz, Austria

Ralph J. Brodd*Broddarp of Nevada, Inc.,

2161 Fountain Springs Drive,Henderson, Nevada 89074

CR020730K
