chemistry

BATTERY TECHNOLOGY

Commercial Cells.

Galvanic cells used as source of electric energy for various consumer, industrial and military applications.

Classification:

Primary Cells. Eg.: Dry cell

Secondary Cells. Eg.: Lead acid cell.

Objectives: Describe the major features of commercial

cells.Know the two major types of batteries.Distinguish between primary & secondary

battery types.Know the various applications of Dry cell,

Nicad cell, Lead acid cell, H2-O2 fuel cell and CH3OH – O2 fuel cell.

Basic Requirements Of Primary Cell.

Compactness and lightweight. Fabricated from easily available raw materials. Economically priced. High energy density and constant voltage. Benign environmental properties Longer shelf life and discharge period. Leak proof containers and variety of design

options.

Basic Requirements Of Secondary Cell.

Long shelf-life and cycle life.

High power to weight ratio

Short time for recharging

Tolerance to service condition.

High voltage & high energy density.

Primary Cells.

Produce electricity from chemicals that are sealed into it.

Cannot be recharged as the cell reaction cannot be reversed efficiently by recharging. The cell must be discarded after discharging.

e.g. Zinc - manganese dioxide cell (Dry cell)

Mercuric oxide – Zinc cell.

Silver oxide – zinc cell.

Secondary Cells

Generation of electric energy, that can be restored to its original charged condition after its discharge by passing current flowing in the opposite direction. These cells have a large number of cycles of discharging and charging. They are known as rechargeable cells, storage cells, or accumulators.

e.g. Lead storage cell.Nickel- cadmium cell.Lithium- ion batteries.

Differences

Primary Batteries Secondary Batteries Cell reaction is irreversible Cell reaction is reversible.

Must be discarded after use. May be recharged Have relatively short shelf life Have long shelf life. Function only as galvanic Functions both galvanic cells . Cell & as electrolytic cell. They cannot be used as They can be used as energy

storage devices storage devices (e.g. solar/ thermal energy

converted to electrical energy) They cannot be recharged They can be recharged.

e.g. Dry cell. Li-MnO2battery.Lead acid, Ni-Cd battery.

DRY CELL(LECLANCHE CELL)

• Anode: Zinc metal container.

• Cathode: MnO2 + Carbon (powdered graphite)

• Electrolyte: Aqueous paste of NH4Cl and

ZnCl2

• Cell Scheme:

Zn(s)/ ZnCl2(aq),NH4Cl(aq),MnO2(s)/C

• O.C.V. = 1.5 V

Working.

Primary Electrode Reactions:

Anode: Zn(s)→Zn2+ (aq)+ 2e-

Cathode: 2MnO2(s)+H2O(l) + 2e- → Mn2O3(s) + 2OH-

(aq)

Net Reaction: Zn(s)+2MnO2(s)+ H2O(l) → Zn2+

(aq)+Mn2O3(s)+2OH-(aq)

Secondary Reactions:

NH4+

(aq)+OH-(aq) → NH3(g)+H2O(l)

Zn2+(aq)+2NH3(s)+2Cl - → [Zn(NH3)2 Cl2]

Zn + 2MnO2 + 2NH4Cl →[Zn(NH3)2Cl2]+ H2O+

Mn2O3

Applications:

In small portable appliances where

small amount of current is needed.

In consumer electronic devices-

quartz wall clocks, walkman etc.

Advantages.

Dry cell is cheap.

Normally works without leaking (leak proof cells).

Has a high energy density.

It is not toxic

It contains no liquid electrolytes.

Disadvantages.

Voltage drops due to build up of reaction products around the electrodes when current is drawn rapidly from it .It has limited shelf life because the zinc is corroded by the faintly acidic,ammonium chloride. The shelf life of dry cell is 6-8 months.They cannot be used once they get discharged.Its emf decreases during use as the material is consumed.

Lead-acid battery:

LEAD STORAGE BATTERY.

• Anode: Spongy lead on lead grid.

• Cathode: Porous PbO2.

• Electrolyte: H2SO4(aq)( 20 %)

(density 1.21-1.30g/ml)

• Cell Scheme:

Pb/PbSO4;H2SO4(aq);PbSO4;PbO2/Pb

O.C.V. = 2V (Pair of plates)

Reactions during discharging.

• Anode: Pb (s) → Pb2+ (aq) + 2e-

Pb2+(aq) + SO4

2-(aq) → PbSO4(s)

Pb(s)+ SO42-

(aq) → PbSO4(aq) + 2e-

• Cathode:PbO2(s)+ 4H+(aq)+2e- →Pb2+

(aq)+ 2H2O(l)

Pb2+(aq)+SO4

2-(aq)→PbSO4(s)

PbO2(s)+4H+(aq)+SO4

2-(aq)+2e- → PbSO4(s)+

2H2O(l)

• Overall: Pb (s)+PbO2 (s)+4H+(aq)+ 2SO4

2-(aq) →

2PbSO4 (s)+2H2O(l)

Charging the Lead-acid battery:

Charging reactions

• Cathode:

PbSO4(s)+2H2O(l)→PbO2(s)+ SO42-

(aq)+4H+(aq)

+2e-

• Anode :

PbSO4(s) + 2e- → Pb(s)+ SO42- (aq)

• Net:2PbSO4 (s)+ 2H2O(aq) → Pb(s)+ PbO2(s) +2H2SO4

Limitations.• Self discharge: They are subject to self discharge

with H2 evolution at negative plates and O2 evolution at positive plates.

Pb +H2SO4 PbSO4 + H2

PbO2 + H2SO4 PbSO4 +H2O +1/2 O2

Loss of Water: Due to evaporation, self discharge and electrolysis of water while charging. Hence water content must be regularly checked and distilled water must be added.

• Sulfation: If left in uncharged state, for a prolonged

period, or operated at too high temperatures or at too

high acid concentrations, transformation of porous

PbSO4 into dense and coarse grained form by re

crystallization.

* This results in passivation of negative plates

inhibiting their charge acceptance.

• Corrosion of Grid: Can occur due to

overcharging when grid metal gets exposed to

the electrolyte. This weakens the grid and

increases the internal resistance of the battery.

• Effectiveness of battery is reduced at low

temperature due to increase in the viscosity of

electrolyte.

• Recent years have seen the introduction of “maintenance – free batteries” without a gas – release vent. Here the gassing is controlled by careful choice of the composition of the lead alloys used i.e. by using a Pb-Ca (0.1 % ) as the anode which inhibits the electrolysis of water

• Alternatively, some modern batteries contain a catalyst (e.g. a mixture of 98% ceria (cerium oxide) & 2% platinum, heated to 1000o C) that combines the hydrogen and oxygen produced during discharge back into water. Thus the battery retains its potency and requires no maintenance. Such batteries are sealed as there is no need to add water and this sealing prevents leakage of cell materials.

Applications.

*Automative: For starting, lighting and

ignition of IC engine driven vehicles.

*Consumer Applications: Emergency

lighting, security alarm system.

*Heavy duty Application: Trains, lift

trucks, mining machines etc.

Advantages:

A lead storage battery is highly efficient. The voltage efficiency of the cell is defined as follows.

Voltage efficiency = average voltage during discharge

average voltage during charge

The voltage efficiency of the lead – acid cell is about 80 %.

The near reversibility is a consequence of the faster rate of the chemical reactions in the cell i.e. anode oxidizes easily and cathode reduces easily leading to an overall reaction with a high negative free energy change.

A lead – acid battery provides a good service for several years. Its larger versions can last 20 to 30 years, if carefully attended (i.e. longer design life)

It can be recharged. The number of recharges possible range from 300 to 1500, depending on the battery’s design and conditions. The sealed lead-acid batteries can withstand upto 2000 – rechargings. Generally the most costly, largest, heaviest cells are the longest–lived.

The battery’s own internal self – discharging is low.

The length of time that is generally required for re-charging process is less i.e. recharge time is 2-8 hours depending on the status of battery.

Low environmental impact of constituent materials is an added advantage

It has sensitivity to rough handling and good safety characteristics.

Ease of servicing as indicated by several local battery service points.

It is a low- cost battery with facilities for manufacture throughout the world using cheap materials.

NICKLE- CADMIUM CELL

Anode: Porous cadmium powder

compressed to cylindrical pellets.

Cathode: Ni(OH)3 or NiO(OH) mixed with 20%

graphite powder

Electrolyte: 20-28% Aq. KOH jelled with a

jelling agent.

Cell Scheme:

Cd/Cd(OH)2,KOH,Ni(OH)2, Ni(OH)3/Ni

O.C.V. = 1.25V

Reactions during discharging.

Anode:

Cd(s)+2OH-(aq)→Cd(OH)2(s)+ 2e-

Cathode:

2Ni(OH)3(s)+2e- → 2Ni(OH)2(s)+2OH-(aq)

• Net Reaction:

Cd(s)+2Ni(OH)3(s)→ 2Ni(OH)2(s)+ Cd(OH)2(s)

Charging reactions:Anode:Cd(OH)2(s)+2e-→ Cd(s) +2OH-

(aq)

Cathode: 2Ni(OH)2(s) +2OH-

(aq)→2Ni(OH)3(s)+2e-

Net:

2Ni(OH)2(s)+Cd(OH)2(s)→2Ni(OH)3(s)+ Cd(s)

Discharging reaction:

Anode: Cd(s)+2OH-(aq) → Cd(OH)2(s) +

2e-

Cathode: 2NiO (OH) (s) + 2 H2O + 2 e- →

2Ni (OH)2(s) + 2OH-(aq)

Net Reaction: Cd(s) + 2NiO (OH) (s) + 2H2O → 2 Ni(OH)2 (s) + Cd(OH)2(s)

Charging reactions:

-ve pole: Cd(OH)2 (s) + 2e-→ Cd(s) + 2OH-

(aq)+ve pole: 2 Ni(OH)2(s) + 2OH-(aq) → 2

NiO(OH) (s) + 2H2O+2e-

Overall reaction: 2 Ni(OH)2 (s) + Cd(OH)2(s) →

2 NiO(OH) (s) + Cd(s) +2H2O(l)

Applications.

In flash lights, photoflash units and portable electronic equipments.

In emergency lighting systems, alarm systems.

In air crafts and space satellite power systems.

For starting large diesel engines and gas turbines etc.,

Advantages.

Can be recharged many times.They maintain nearly constant voltage level through out their discharge. There is no change in the electrolyte composition during the operation.It can be left unused for long periods of time at any state of charge without any appreciable damage (i.e. long shelf life).It can be encased as a sealed unit like the dry cell because gassing will not occur during nominal discharging or recharging.They exhibit good performance ability at low temperatures.

They can be used to produce large instantaneous

currents as high as 1000-8000 A for one second.

It is a compact rechargeable cell available in three

basic configurations – button, cylindrical and

rectangular.

They have low internal resistance.

Disadvantages.

It poses an environmental pollution hazard due to higher toxicity of metallic cadmium than lead.

Cadmium is a heavy metal and its use increases the weight of batteries, particularly in larger versions.

Cost of cadmium metal is high and hence the cost of construction of NiCad batteries is high.

The KOH electrolyte used is a corrosive hazardous chemical.

Lithium cells/ batteries

Lithium is a theoretically active material for negative electrode of the electrochemical cells owing to its least noble nature and low specific gravity.

1. Primary cells with metallic lithium electrodes and non-aqueous electrolytes were successfully introduced into the market with outstanding features like high voltage, high energy density, low self-discharging rate and wide range of operation etc.

2. The secondary lithium negative electrodes have attracted much attention with high energy density however they are facing many more practical problems such as poor cycle life, need for long charging time, poor safety characteristics etc.,

Primary lithium cells

Lithium primary cells can be classified into several categories, based on the type of electrolyte and cathode material that is used.

• Soluble cathode cells:- Use liquid or gaseous cathode materials, such as SO2,

SO2Cl2 (sulfuryl dichloride) , SOCl2 etc that dissolve in the electrolyte or in the electrolyte solvent. Results in the formation of protective thin film on the lithium anode

• Solid cathode materials:- Uses solid for the cathode. Ex. Li/MnO2 cell

 Ex: 1) LITHIUM/COPPER SULFIDE (Li/CuS) 2) LITHIUM/COPPER OXIDE (Li/CuO) CELLS:-• Solid Electrolyte cells:- Extremely long storage life

LITHIUM/COPPER SULFIDE (Li/CuS)

Construction

• Anode: Lithium

• Cathode: Copper sulphide.

• Electrolyte: mixture of 1,2-dimethoxyethane, 1,3-dioxolane

and 3,5-dimethylisoxazole as a stabilizer with LiClO4 solute.

OR

Tetrahydrofuran (THF) and dimethoxyethane (DME) binary

solvent with LiClO4 for the solute

O.C.V. = 1.7 V

Cell reaction

• Anode reaction: x Li x Li+ + x e-

• Cathode reaction: CuS + xLi+ + xe- LixCuS

On continued discharge, the second step of the cathode discharge occurs:

2LixCuS + 4e- 2LixCu + 2S2-

An electrolyte reservoir is placed between the cathode and can to obtain the most efficient use of the cathode. This reservoir retains electrolyte at the cathode surface next to the can and improves higher discharge rate performance.

disadvantages

• Cells can withstand short-circuit but should not be forced-discharged or exposed to temperatures as high as 180oC,the melting point of lithium.

• New designs, replacing the LiClO4 which is very reactive, appear effective in Improving the safety of the cell under abusive conditions.

Lithium ion cells:-

The essential feature of the Lithium ion battery is that at no stage in the charge-discharge cycle should there be any Lithium metal present.

Rather, Lithium ions are intercalated into the positive electrode in the discharged state and into the negative electrode in the charged state and move from one to the other across the electrolyte.

Lithium-ion batteries thus operate based on what is sometimes called the "rocking chair" or "swing" effect. This involves the transfer of lithium ions back and forth between the two electrodes. Hence called lithium rocking chair or swing batteries.

Construction Cathode: a layered graphite crystal into which

lithiated metal oxides are inserted.

Anode: made up of graphite, coated on copper foil 14.

Separators: polyolefin’s using 3-8- μm layers with 50% porosity.

Electrolyte: The electrolyte is usually a 1-molar solution of a lithium salt in an organic solvent.

Ex: 1)Lithium hexafluorophosphate (LiPF6) in the solvent

propylene carbonate 2)Lithium tetrafluoroborate(LiBF4 ) in the solvent

ethylene carbonate

Discharge reaction:- The main principle is based on the movement of

lithium ions between anode and cathode through the electrolyte occurs during charge and discharge process.

Anode reaction: Li ( C ) Li+ + e-

Cathode: Li+ + e- +CoO2 LiCoO2

The overall cell reaction is as follows:

Overall: CoO2 + Li ( C ) LiCoO2

Applications: Lithium-ion batteries are most commonly used in mobile telephones and mobile computing devices, where the battery needs to be a particular shape, laptops, cellular phones, camcorders.

Cathodes:-

Cathode material Practical Theoretical Capacity

LiCoO2 140 275

LiNiO2 (or mixed) 190-200 274

LiMn2O4 120 148

Advantages:- (b) Smaller, lighter and provide more energy. (c) Operated in a wide temperature range.

Disadvantages: (a) Poor charge retention. (b) High self discharge rate. (c) Highly expensive.

Lithium –ion battery characteristics

Type Secondary Chemical reaction Varies, depending on electrolyte Operating temperature 4oF to 140oF (-20oC to 60oC) Recommended for Cellular telephones, mobile

computing devices. Initial voltage 3.6 & 7.2 Capacity Varies (generally up to twice the

capacity of a Ni-Cd cellular battery) Discharge rate Flat Recharge life 300-400 cycles Charging temperature 32oF to 140oF (0oC to 60oC) Storage life Losses less than 0.1% per month Storage temperature -4oF to 140oF (-20oC to 60oC)

Discharge-charge cycle

Serial no Cobalt Manganese

Energy density

(Wh/kg)

140 120

Safety On overcharge, the cobalt electrode

provides extra lithium, which can form

into metallic lithium, causing a

potential safety risk if not protected by

a safety circuit.

On overcharge, the manganese electrode

runs out of lithium, causing the cell only

to get warm. Safety circuits can be

eliminated for small one- and two-cell

packs

Temperature Wide temperature range Capacity loss above 40 degrees C

Aging Short-term storage is possible,

impedance increases with age newer

versions offer longer storage

Slightly less aging than cobalt, impedance

changes little over the life of the cell, and

due to continuous improvements, the

storage time is difficult to predict.

Life expectancy Minimum 300, 50 percent at 500 cycles May be shorter than cobalt

Cost Raw materials is relatively high,

protection circuits adds to costs

Raw materials are 30 percent lower than

cobalt. Cost advantage on less circuitry

LiMn2O4:-

(1) Lower charge-discharge efficiency at the first cycle than LiCoO2.(2) Highest stability among the three candidates at high temperatures.(3) Low cost.(4) Abundance of manganese resources.(5) Poor cyclability.(6) A little higher operating voltage than LiCoO2.

Fuel Cells.

A fuel cell is a galvanic cell in which chemical energy of a fuel – oxidant system is converted directly into electrical energy in a continuous electrochemical process.

• Cell Schematic Representation:

Fuel;electrode/electrolyte/electrode/oxidant.

e.g. H2-O2; CH3OH-O2

• The reactants (i.e. fuel + oxidant) are constantly supplied

from outside and the products are removed at the same

rate as they are formed.

• Anode:

Fuel+ oxygen → Oxidation products+ ne-

• Cathode:

Oxidant + ne- → Reduction products.

Advantages:High fuel to electricity conversion efficiency of 70-75% is observed in fuel cells. Typically, a thermal power plant converts only 35-40% of the chemical energy of coal to electricity. Therefore, the efficiency of a fuel cell is about twice that of a conventional thermal power plant, thus saving natural fuel resources.Fuel cell producers do not cause pollution problems such as noise pollution, chemical pollution & thermal pollution normally associated with conventional power plants.

A fuel cell will produce a steady electric current as long as fresh reactants are available.

A small instrument contain fuel cell could be used to detect ‘drinking & driving’by a roadside test by police.

Fuel cells operate at near constant efficiency independent of size. So fuel cell power plants can be configured in a wide range of electrical levels from a few watts to hundreds of MW.

High reliability and low maintenance

The amount of CO2 released into the atmosphere is less per MW of electricity than other electricity generating processes, which is very important for environmental reasons.

Disadvantages

Degradation or malfunction of components limits the practical operating life of working fuel cells on a large scale.

They are sensitive to fuel contaminants such as CO,H2S, NH3 & halides, depending on the type of fuel cell. These contaminants must be minimized in the fuels to enhance the cells’ efficiency

High initial cost because of the expensive noble metals required in the construction of certain fuel cells. At the moment, cells with relatively inexpensive fuels require expensive catalysts, whereas those with relatively inexpensive catalysts require expensive fuels.

Classification of fuel cells:-

Alkaline fuel cell

Phosphoric acid fuel cell

Proton exchange membrane fuel cell

Direct methanol fuel cell

Molten carbonate fuel cell

Solid oxide fuel cell

Alkaline fuel cell

One of the first fuel cell technologies developed

First type widely used in the U.S. space program to produce electrical energy and water onboard spacecraft

• Anode: Porous graphite electrode/ porous nickel

electrode impregnated with finely divided Pt/Pd.

• Cathode: Porous graphite electrode/porous nickel

electrode impregnated with finely divided Pt/Pd.

• Electrolyte: 35-50% KOH held in asbestos matrix.

• Operating Temperature: 90oC.

• O.C.V. =1.20V

• Anode :

2H2(g) +4OH- (aq)→ 4H2O(l)+4e-

• Cathode:

O2(g)+2H2O(l)+4e- →4OH (aq)

• Net Reaction:

2H2(g)+O2(g)→2H2O(l).

*Water should be removed from the cell.

*O2should be free from impurities.

Applications.

Used as energy source in space shuttles e.g. Apollo spacecraft.

Used in small- scale applications in submarines and other military vehicles.

Suitable in places where, environmental pollution and noise are objectionable.

Limitations and contemporary advancements

• Method of preparation of the electrodes.

• Costs of the electrode, stacks and fuel cell systems.

• Life time of the electrode.

• Diaphragm made of asbestos.

• CO2 – contaminated fuel gases (carbonating of electrolyte and electrodes).

Phosphoric acid fuel cell (PAFC)

• Construction:-• Anode: - PTFE-bonded Pt/C Vulcan XC-72 0.10 mg

Pt/cm2

• Cathode: - PTFE-bonded Pt black Vulcan XC-72 0.25 mg/cm2

• Electrode support: - Carbon paper• Electrolyte support:- PTFE-bonded SIC (ball milling)

• Electrolyte:- Liquid H3PO4 99% (wt) with additives like imidazole and 1-methyl imidazole

• Interconnect:- Graphite• Catalyst:- Platinum• Operating temperature:-150- 205oC. • Charge carrier:- H+

first generation” of modern fuel cells.

typically used for stationary power generation

Cell reactions:- At anode:- 2H2 4H+ + 4e- (E0 = 0.0 V Vs. SHE)

At cathode:-

O2 + 2H2 2H2O (Ecell = 1.23 V)

O2 + 4H+ + 4e- 2H2O (E0 = 1.23 Vs. SHE)

Net reaction:-

Advantages

• PAFCs are much less sensitive to CO than PEFCs and AFCs.

• The operating temperature is low enough to allow use of common construction materials, the operating temperature also provides considerable design flexibility for thermal management.

• PAFCs have demonstrated system efficiencies of 37 to 42 percent.

• The expelled water can be converted to steam for space and water heating. In this combined heat and power application, overall efficiencies can approach 80

Disadvantages

• Cathode-side oxygen reduction is slower than in AFC, and requires the use of a platinum catalyst.

• PAFCs still require extensive fuel processing, including typically a water gas shift reactor to achieve good performance .

• The highly corrosive nature of phosphoric acid requires the use of expensive materials in the stack.

Applications

1. Extensive use in stationary power industry where on site, high quality and reliable power is needed

2. Used in hotels, hospitals, office buildings and large vehicles( buses) in USA , Japan etc.

Proton exchange membrane cell

Construction• Electrolyte:- Ion exchange polymeric membranes. • Electrodes:- Typical gas diffusion electrodes,

made up of porous C impregnated with Pt catalyst.

• Fuel:- Hydrogen• Oxidant:- Air• Catalyst:- Platinum• Interconnect:- Carbon or metal• Operating temperature:- 40 – 80oC. • Charge-carrier:- H+

Perflourinated membrane by DuPont.

C

F

F F

F

C C

F

F

C

F

F

C

F

C

F

F

C

F

F

C

F

F

C

F

FO

C FF

F FC

O

F FC

F FC

O=S=O

O-

H+

PERFLUOROSULFONIC ACID MEMBRANE

Single cell structure of representative PEFC

Membrane-electrode assembly (MEA)

• PEMFC electrode • Anode reaction:

• H2 2H+ + 2e-

• Cathode reaction:

• O2 + 2H+ + 2e- H2O

• Overall reaction:-

• H2 + O2 H2O

Advantages

• Solid electrolyte provides excellent resistance to gas crossover.

• Low operating temperature allows rapid start-up

• capable of high current densities

Disadvantages:-• The low and narrow operating

temperature range makes thermal management difficult

• Dehydration of the membrane reduces the proton conductivity and excess water can lead to the flooding of the electrolyte. Both the conditions leading to poor performance.

• Perflourinated membranes have high cost

• quite sensitive to poisoning by trace levels of contaminants including CO, sulfur species and ammonia.

Direct methanol fuel cell

Construction• Electrodes: porous nickel plates impregnated with

finally- divided platinum.

• Fuel: Methyl alcohol

• Oxidant: Pure oxygen / air

• Electrolyte: Concentrated phosphoric acid

• Temperature: 150-200oC

Working

Working:-

Anode:- CH3OH + H2O CO2 + 6H+ +6e-

Cathode:- O2 + 6H+ + 6e- 3 H2O

Net reaction:- CH3OH + O2 CO2 + 2H2O

Advantages

• MeOH can be easily transported stored & dispensed

• The fuel is very cheap and available in large quantities.

• only products of combustion are CO2 and H2O which can be removed easily.

• no production of NOx gases as the operating temperature is about 150oC.

• Methanol is stable in contact with the acidic membrane.

Disadvantages:-• anode reaction has poor

electrode kinetics, particularly at lower temperatures.

• The reduction of oxygen at the cathode is slow.

• permeability of the current perfluorosulfonic acid membranes to methanol, allowing considerable crossover of fuel.

• Methanol is toxic to humans,

Molten carbonate fuel cell

• Developed for natural gas and coal-based power plants for electrical utility, industrial and military applications.

Construction

• Electrolyte: Molten carbonate salt mixture. The composition of electrolyte varies, but usually consists of lithium carbonate and potassium carbonate.

• Electrodes:- Anode is porous sintered Ni powder, alloyed with Cr The cathode is a porous nickel oxide material doped with Li.

• Fuel: Hydrogen or CO• Oxidant: Oxygen• Catalyst:- Electrode material• Interconnect:- Stainless steel or nickel• Operating temperature:- 650oC

• Charge-carrier:- CO32-

Cell reaction

• Anode reaction:

H2 + CO32- H2O +CO2 + 2e-

• Cathode reaction: ½ O2 + CO2 + 2e- CO3

2-

• Overall reaction:

H2 + 1/2O2 H2O

Advantages:-

• Relatively high operating temperature.• Noble metal catalysts are not required. • Very high efficiency.• An external reformer to convert more energy-dense

fuels to hydrogen not required.• Not prone to carbon monoxide or carbon dioxide

"poisoning.“• More power is available at higher fuel efficiency

from MCFC.• Produce excess heat at a temperature, which is high

enough to yield high-pressure steam, which may be fed to a turbine to generate additional electricity.

• It could operate directly on gaseous hydrocarbon fuels.

Disadvantages

• Very corrosive and mobile electrolyte, which requires use of nickel and high-grade stainless steel as the cell hardware.

• Higher temperatures promote material problems, impacting mechanical stability and stack life.

• source of CO2 is required at the cathode.

• High contact resistances and cathode resistance limit power densities to around 100 – 200 mW/cm2 at practical operating voltages.

Solid Oxide Fuel Cells

Construction• Electrolyte:- Perovskites (Ceramics)• Electrodes:- Anode constructed from an

electronically conducting Co-ZrO2 or Ni-ZrO2 (ceria/Nickel cement) and cathode is Sr doped LaMnO3 (La, Sr, Co, Fe oxides)

• Catalyst:- Electrode material• Interconnect:- Nickel, ceramic or steel• Operating temperature:- 600-1000oC• Charge-carrier:- O2-

• Fuel: Hydrogen, CO• Oxidant: oxygen

Cell reaction:-

Anode reaction: H2(g) + O2- H2O(g) + 2e-

CO(g) + O2- CO2(g) + 2e-

Cathode reaction: O2 + 4e- 2O2-

Overall reaction: H2 + O2 + CO H2O + CO2

Limitations Of Fuel Cells.

• Cost of power is high as a result of the cost of electrodes.

• Fuels in the form of gases and O2 need to be stored in tanks under high pressure.

• Power output is moderate.

• They are sensitive to fuel contaminants such as CO,H2S, NH3 & halides, depending on the type of fuel cell.

Differences.• Fuel Cell Galvanic Cell

*Do not store chemical Stores chemical energy energy *Reactants are fed from The reactants form an

outside continuously. integral part of it. *Need expensive noble These conditions are metal catalysts. not required*No need of charging Get-discharged when stored – up energy is exhausted.

*Never become dead Limited life span in use *Useful for long-term Useful as portable power services

electricity generation.

CH3OH-O2 Fuel Cell

• Both electrodes: Made of porous nickel plates impregnated with finely- divided

Platinum.

• Fuel: Methyl alcohol.

• Oxidant: Pure oxygen / air.

• Electrolyte: Conc.Phosphoric acid/Aq.KOH

• Operating Temperature: 150-200oC.

• At anode:

CH3OH + 6OH- →CO2 + 5H2O + 6e-

• At cathode:

3/2 O2 +3H2O + 6e →6OH-

Net Reaction:

CH3OH +3/2O2 →CO2 + 2H2O.

It is used in military applications and in large scale

power production. It has been used to power

television relay stations.

If Acid Electrolyte used:

• Anode:CH3OH + H2O → CO2(g) + 6H+(aq) + 6e-

• Cathode:3/2O2(g) + 6H+(aq) + 6e- →

3 H2O(l)• Net reaction: CH3OH + 3/2 O2(g)

→CO2(g) + 2H2O(l)

Advantages:

1) MeOH can be easily Transported stored & dispensed within the current fuel network because it is a liquid fuel.

2) The fuel is very cheap and available in large quantities.

3) The only products of combustion are CO2 & H2O which can be removed easily.

4) There is no production of NOx gases as the operating temperature is about 150o C.

5) Methanol is stable in contact with the acidic membrane.

Disadvantages:

1) The anode reaction has poor electrode kinetics, particularly at lower temperatures.

2) The reduction of oxygen at the cathode is also slow.

3) Methanol is toxic to humans, causing blindness

in low doses and death in larger amounts. So there are concerns about their proper handling.

• The emf of the cell is 1.20 V at 25oC.

• MeOH is one of the most electro active organic fuels in

the low temperature range as

*It has a low carbon content

*It posseses a readily oxidizable OH group

*It is miscible in all proportions in

aqueous electrolytes.

Numerical Problems Q. 1) What standard cell potential can be expected

from a fuel cell that consumer’s hydrogen  and

oxygen gases and produces water vapor? (ΔGo =

-237.2KJ/ml)

Solution: The overall reaction is

H2(g) + ½ O2(g) → H2O(l)

ΔGo = - ήFEo

E o = - ΔG / ήF (+ 237.2 KJmol -1) (1000 J/KJ /

2(96,500 C/mol) = 1.14V.

2) Calculate the standard emf of the H2 – O2 fuel cell if the Eo values are – 0.40V & 0.83 V for the hydrogen and oxygen half cells respectively.

Eo cell = Eoc – EoA = 0.83 – (-0.40) =

+1.23 V.

3) The standard free energy change ΔGo for the net reaction in the lead acid cell at room temperature is – 393.9 KJ mol -1. Calculate the thermodynamic cell voltage.

ΔGo = - ήEoF

Eo cell = - ΔGo/ήF

= 393.9 x 1000/ 2 x 96500      = 1.9 V

4.Calculate the standard cell potential from the standard reduction potentials of -0.356V   & 1.685 V for the anode and cathode half sections of a lead –acid cell Eocell = EoC – EoA = Eo PbO2/PbSO4 –

Eo PbSO4/Pb

= 1.685 –(-0.356) = 2.041 V