Following are the most common fuel cell types
- Alkaline Fuel Cell (AFC)
- Phosphoric Acid Fuel Cell (PAFC)
- Molten Carbonate Fuel Cells (MCFC)
- Solid Oxide Fuel Cells (SOFC)
- Proton/polymer Exchange Membrane Fuel Cells (PEMFC)
Fuel Cell Types
Important fuel cell types are described here except the alkaline fuel cell (AFC) as it is discussed earlier.
Phosphoric Acid Fuel Cells (PAFC)
A schematic diagram of phosphoric acid fuel cell is shown in Figure. It was developed in 1980s. It uses either pure hydrogen or rich hydrogen gas as fuel and oxygen or air as oxidant.
The concentrated phosphoric acid (H3PO4) is used as electrolyte. It is a medium temperature type of fuel cell. It has two electrodes of porous conducting material, usually of nickel material to collect charge.
The electrochemical reaction is normally very slow, therefore a catalyst is needed in the electrodes to accelerate the reaction. Finely powdered nickel/platinum/silver coating is provided on the outer surface of electrodes which act as catalyst. Nickel is used since it is comparatively cheaper.
Anode: H2 → 2H+ + 2e
Cathode: ½O2 + 2H+ + 2e → H2O
Overall reaction: H2 + ½O2 → H2O + heat + power
If air is used as oxidant instead of O2, the insert gas N2 is discharged along with spent O2 and water vapor formed.
The operating temperatures for this type of fuel cell are in the range of 150oC – 220oC. As present these are most economical since they have lowest cost per kW. These are used commercially having the plant capacity in the range of 50 KW to 200 KW.
Molten Carbonate Fuel Cell (MCFC)
It is a high temperature fuel cell. The fuel, oxidant, electrodes and electrolytes used are as follows:
Fuel: Mixture of H2 and CO.
Oxidant: O2 or air.
Electrodes: Porous nickel electrodes.
Electrolyte: Mixture of carbonate salts either lithium carbonate and potassium carbonate or lithium carbonate and sodium carbonate.
Since it uses the carbonates of alkali metals as electrolytes in molten (liquid) phase, operating temperatures must be in the range of (650°C — 700°C). The electrolytes are held is sponge like ceramic matrix. When the mixtures of carbonate salts are heated they become conducive to CO3– – ions.
The carbonate ions so produced flow from cathode to anode where they combine with hydrogen to produce CO2, water (H20) and electrons and while combining with CO it produces CO2, and electrons. The electrons flow from anode to cathode to generate electrical power and heat.
The EMF produced by each cell is theoretically 1V and actual EMF of 0.8 V at 700°C. The expected efficiency is about 60%.
Advantages of Molten Carbonate Fuel Cell
It uses comparatively cheaper fuels like H2, and CO. It can utilize even fossil fuel like coal for producing the H2, and CO by its gasification.
Discharges are mainly water vapor, CO, and N2 (if air is used as oxidant) temperature exceeding 550°C. These hot gases can be used for generation of steam power generation or as process heat for industries.
Efficiency of cell is high in the range of 60%. It’s overall efficiency can be fur increased by utilizing the heat of the products in co-generation plant.
Polymer Electrolyte Membrane Fuel Cell (PEMFC)
Solid Polymer Fuel Cell (SPFC)
In this type of fuel cell, the electrolyte is a solid polymer membrane of an organic material such as polystyrene sulphonic acid. This polymer is permeable to H+ ions to pass through it when it is saturated with water but it does not conduct electrons.
Other properties include that this polymer has high resistance to dehydration and oxidation coupled with high ionic conductivity. The membrane is coated on its both sides with finely powdered platinum which acts as catalyst.
The fuel used if H2 and oxidant as O2. The electro-chemical reactions are:
At anode: H2 → 2H+ + 2e
At cathode: ½O2 + 2H+ + 2e → H2O
Overall Reaction: H2 + ½O2 → H2O + heat
Advantages of PEMFC or SPFC
- It can be started quickly.
- Polymer membrane retains only limited quantity of water and rejects excess H2O vapor.
- It is comparatively cheap.
- It has no corrosion problems due to use of polymer.
- It has an ideal EMF of 1.23V and actual EMF of about 1V at 25o
- It operates at low temperature of 40oC – 60oC
Properties of Ideal Ion-exchange Membrane
- Zero electronic conductivity.
- Low permeability of fuel and oxidant.
- High ionic conductivity.
- High resistance to dehydration and hydrolysis.
- Mechanical stability.
Solid Oxide Fuel Cells (SOFC)
Certain solid ceramic oxides (e.g. zirconium dioxide) are able to conduct electricity at high temperatures, therefore such ceramic oxides can be used as electrolytes in such fuel cells.
This material is able to conduct O– – ions at high temperatures. Therefore these cells operate at high temperatures in the range of 650°C to 1000°C. Due to high temperature operation no catalyst is required. The negative electrode (anode) is made of porous nickel and positive electrode (cathode) uses metal oxide.
At the cathode the ½O2, molecules are split into oxygen ions with addition of two electrons. These O ions migrate to anode through electrolyte and combine with one hydrogen molecules with formations of H2O and two electrons. Thus the chemical reactions are:
At anode: H2 + O– – → H2O + 2e
At cathode: ½O2 + 2e → O– –
Overall reaction: H2 + ½O2 → H2O + heat
The electrons move through the external circuit to produce power.
The output voltage is about 0.65V at about 800°C. A tubular type of SOFC system has been developed which operates at high temperature of about 900°C — 1000°C.
Like molten carbonate fuel cell, the heat of discharge can be utilized for process heat or additional power generation.
Other Fuels for SOFC System
Apart from H2, other sources of fuel for SOFC system could be methanol, ammonia and hydrazine.
Methanol can be catalytically reformed with steam at 200°C to yield the mixture of H2 and CO2 gas.
Liquid NH3 — air fuel cell can be catalytically converted into H2 and N2. The electrolyte used is KOH solution.
Another source of fuel as hydrazine (N2H4) and air as oxidant can be used. Disadvantage of N2H4 is that it is toxic and highly reactive. This uses porous nickel as anode and silver deposit on porous material as cathode. The chemical reactions are:
At anode: N2H4 + 4OH → N2 + 4H2O + 4e
At cathode: O2 + 2H2O + 4e → 4OH–
Overall reaction: N2H4 + O2 → N2 + 4H2O
Thus in this cell nitrogen, water vapor and heat is produced as by-product with generation of power.
Selection of Fuel Cells
The selection of a fuel cell is mainly based on the following factors:
- Availability of fuel and its heat
- Size/volume of cell
Hydrocarbons like methane, ethane, and propane gases are not used since they are less reactive and their oxidation in fuel cell is difficult. Therefore, these fuels are not suitable for use in fuel cells.
Thus, most of the fuels which can be used in fuel cells are H2, NH3, CH3OH (Methanol), N2H4 (Hydrazine), CO etc. Out of these fuels, hydrogen is the best fuel since its heat of reaction per kg of fuel is highest amongst all the fuels. Thus, fuel cells based on hydrogen — O2/air are best suited for applications like spacecraft, submarines etc.
Fuels like ammonia, methanol and hydrazine are reasonably reactive and can be conveniently used in fuel cells. Ammonia is toxic. Hydrazine is toxic, poisonous and does not require any catalyst.
Though the ammonia is cheaper than hydrazine but it is comparatively less reactive. Therefore, hydrazine is quite suitable for use in fuel cells particularly for military applications.
On the other hand, NH3 being cheap and easily available, it is suitable for remote area and low power applications.
Thanks for reading about “fuel cell types”.