Fuel Cell

FUEL CELLS

FUEL CELLSDONE BY:ARVIND SAN INTRODUCTION TO FUEL CELLSA fuel cell is a device that generates electricity by a chemical reaction. Each fuel cell has two electrodes, namely the anode(+) and the cathode(-), and an electrolyte.Hydrogen is the main component but they also need oxygen, which is supplied by oxidants. They cause very little pollution as its by-product, in most cases, is just water.Each fuel cell produces a small amount of DC current(An average emf of 1.66V) but when stacked together, they can produce a large potential difference.How fuel cells work:

There are many kinds of fuel cells but in general hydrogen enters at the anode where it loses electrons and becomes ions. It combines with oxygen at the cathode producing water, and generating DC current.Types of fuel cells: They can be broadly classified as:Alkali Molten carbonatePhosphoricPEMSolid oxide

Alkali fuel cells:They work on compressed hydrogen and oxygen in presence of KOH(conc.) as the electrolyte. The cell reactions are : AT ANODE: 2H + 4OH 4HO + 4 The electrons flow through the circuit and reach the cathode. AT CATHODE:O + 2HO + 4 4OH Overall reaction: 2H + O 2HO

This is a typical alkaline fuel cell where the hydrogen is oxidized and electrons flow through the load and combine with oxygen in presence of water to form hydroxyl ions, which flows back to form water.Advantages and Disadvantages: Advantages:Their efficiency can reach up to 70%They cause very little pollution. Hence they are environment friendly.They also produce potable water as a by-product. Disadvantages: They require very pure hydrogen. Its not always possible to get very pure hydrogen and thus some impurities interfere in the cell reaction.They also require pure platinum electrodes, making them costly.

Applications of alkaline fuel cellsThey have been chosen to power space shuttles as they can be engineered to produce good efficiencies.They also produce water which can be used as a coolant and also for drinking.NASA has been employing these fuel cells, including the Apollo missions as well.

Molten Carbonate fuel cells

The molten salt conducts carbonate ions from the cathode to the anode. At the anode, hydrogen reacts with the ions to produce water, carbon dioxide, and electrons. The electrons travel through an external circuit and return to the cathode. There, oxygen from air and carbon dioxide recycled from the anode react with the electrons to form CO3 ions that replenish the electrolyte and transfer current through the fuel cell.In these types the electrolyte is a carbonate salt. They are operated at temperatures >650CThe anode is made up of Ni and the cathode is made of NiO.The reactions are: Anode: 2H + 2CO 2HO + 2CO + 4 Cathode: O + 2CO + 4 2CO Overall reaction:2H + O 2HOAdvantages and disadvantages: Advantages:They operate at high temperatures which makes their reaction quotient high and thus fast.CO poisoning can be reduced.They can also operate on a variety of fuels as they can be extracted by a reformer(internally or externally). Disadvantages:Liquid electrolytes are to be used instead of solid electrolytes.Carbon dioxide has to be injected for the reaction to proceed.There are cases where the metallic parts gets corroded due to the high temperatures involved.

Applications :Since these fuel cells operate at high temperatures they can be used in stationary power plants. They can also be employed to power space shuttles. Phosphoric acid fuel cell

Phosphoric acid fuel cells use phosphoric acid as the electrolyte. Hydrogen atoms are oxidized(anode) and the electrons flow through the external circuit and recombines the the cathode to form water.It operates at a range of 100 to 200 C.Some noble metal catalyst has to be used, like Pt.The cell reactions are: Anode: 2H 4H + 4 Cathode : O + 4H + 4 2HO Overall Reaction: 2H + O 2HOAdvantages and disadvantages: Advantages:Commercially available.Lenient to fuels.Uses heat for co-generation. Disadvantages:Can get corroded easily due to the strong acidic conditions.Expensive catalystLimited service life. ApplicationsThey have been used in generators to produce output in the range of 100 to 400 kW.They have also made their way into the automotive industry to run buses and other public transport. Proton Exchange Membrane(PEM)

In this type of fuel cell, a polymer membrane selective to Hydrogen ions is used.The ionized hydrogen diffuses through and the electrons return from the external circuit. There it combines with oxygen forming water.The reactions are: Anode: 2H 4H + 4 Cathode: O + 4H + 4 2HO Overall reaction: 2H + O 2HO Each individual fuel cell of this type produces about 1.3V emf.Advantages and disadvantages: Advantages:Compact designLonger operating lifeFunctions at lower temperatures. Disadvantages:CostlyPure hydrogen requiredComplex heat and water management.Applications: The applications of these are :Usage in automobilesMedium and large scale portable power generation.Small scale portable power generation.Solid oxide fuel cell:

These use a hard ceramic electrolyte, i.e. zirconiumoxide and calcium oxide. At the high operating temperatures, oxygen ions travel through the lattice and oxidize the fuel at the anode. The electrons travel through the external circuit to the cathode.The reactions occurring are: Anode: 2H + 2O HO + 2 Cathode: O + 2 2O Overall reaction: 2H + O 2HOThey operate at 1000C and the fuel can directly oxidized.Advantages and disadvantages: Advantages:Lenient to fuels.No reformer needed.Can take in natural gas directly60% efficiency. Disadvantages:High temperatureOxidation issuesLower specific power.Applications:Medium to large scale power generationThey can be used as auxiliary power units for automobiles.Portable power generation and biogas conversionUsed in the production of synthetic fuel production.Benefits of using Fuel CellsLow-to-Zero EmissionsHigh EfficiencyFuel Flexibility

Lightweight and Long LastingA fuel cell operating on pure hydrogen emits zero emissions at the source.Some stationary fuel cells use natural gas or hydrocarbons as a hydrogen feedstock, but even these systems produce far fewer emissions.

High EfficiencyBecause fuel cells create energy electrochemically, and do not burn fuel, they are fundamentally more efficient than combustion systems.Fuel cell systems today achieve 40-50 percent fuel-to-electricity efficiency using hydrocarbon fuels such as natural gas.Fuel FlexibilityMost fuel cells run on hydrogen and all will continue to generate power as long as fuel is supplied. The source of the hydrogen does not matter in most fuel cells.