| Type | Invented | Operating Temp | Efficiency | Applications |
|---|---|---|---|---|
| Alkaline | 1839 | 70°c | 55-60% | Domestic, Industrial, Stand-by and Marine |
| Molten Carbonate | 1953 | 650-750°c | 40-55% | Mainly power stations |
| Solid Oxide | 1928 | 400-1200°c | 35-45% | Mainly domestic |
| PEM | 1992 | 90°c | 35-40% | Mainly vehicle power |
| Direct Methanol | 1944 | 120°c | 30-35% | Mainly vehicle power |
Alkaline Fuel Cells (AFC’s)
Read more about why AFC Energy chose alkaline fuel cell technology
This is the type of fuel cell being developed by AFC Energy. The electrolyte is potassium hydroxide aka KOH. The gas used at the anode is hydrogen and the gas used at the cathode is air containing oxygen. The operating temperature is below 80°C and so is classed as a low temperature fuel cell. Alkaline fuel cells typically have an efficiency of 50-70% - one of the highest efficiencies of all types of fuel cells. The AFC fuel cell system is capable of being operated in CHP mode.
The first major application of alkaline fuel cells was by NASA in the space shuttle program, and they were also utilised to power the Gemini missions. An advantage of alkaline fuel cells is that they produce water as a by-product and do not produce any hazardous chemicals.
The main ‘disadvantage’ that is commonly believed to be true, is that carbon dioxide will react with the hydroxide and thus poison the fuel cell or its catalyst. Typically, PEM cells are irreversibly damaged by carbon monoxide, whereas alkaline fuel cells electrolyte is contaminated by CO2. however, replacing the electrolyte (just like an oil change on a car) restores full efficiency. Read more about this issue.
Polymer Electrolyte Membrane Fuel Cells (PEM’s)
PEM’s are also known as proton exchange membrane fuel cells (PEMFC’s). The electrolyte is a solid polymer membrane. The gas used at the anode is hydrogen and the gas used at the cathode air/atmospheric oxygen. The operating temperature is approximately 75°C, making it a low temperature fuel cell. The efficiency of this type of fuel cell is between 35-55%.
The advantage this has over AFC’s is that they do not require a corrosive electrolyte. Another advantage is that they have high power density, and have a low weight and volume compared to other fuel cells. A disadvantage with these types of cells is that they are subject to CO2 poisoning of the catalysts, and this is terminal. The other big disadvantage is that the cell material must be kept moist at all times to avoid the permanent destruction of the electrode, to do this, there needs to be a humidifier and a dryer in the system which is much more costly, and reduces the efficiencies of the system.
Direct Methanol Fuel Cells (DMFC’S)
The electrolyte for this type of fuel cell is a solid polymer membrane. The gas at the anode is a methanol solution in water and the gas at the cathode is atmospheric oxygen. The operating temperature is 75°C and is a low temperature fuel cell. The efficiency of this fuel cell is typically 35-40%.
One of the major advantages of using this type of fuel cell is that there is no need for the costly storage of hydrogen as methanol is used instead. Methanol is much easier to transport. DMFC’s are a relatively new type of fuel cell so more research is needed to understand the limitations more.
Phosphoric Acid Fuel Cells (PAFC’s)
The electrolyte used contains some form of phosphoric acid. The anode gas is hydrogen and the cathode gas is atmospheric oxygen. The operating temperature is around 210°C. This is a mid-temperature fuel cell. The efficiency of this type is 35-50%.
An advantage of this type is that they are more tolerant towards impurities found in the gases used. The major disadvantage is that they are large and heavy. They also need to use a platinum catalyst which raises the cost.
Molten Carbonate Fuel Cells (MCFC’s)
The electrolyte used is an alkaline carbonate solution. The gas at the anode is either hydrogen or methane and the gas at the cathode is atmospheric oxygen. The operating temperature is 650°C and is a mid/high temperature fuel cell. The efficiency is 40-55%.
One of the advantages of MCFC’s is that it is not prone to carbon dioxide poisoning – they can use carbon oxides as a source of fuel. There is interest in using these cells using gases produced by burning coal and other fossil fuels. The main disadvantage is the durability of the cells.
Solid Oxide Fuel Cells (SOFC’s)
The electrolyte used is a non-porous ceramic oxide. The anode gas is hydrogen or methane while the gas at the cathode is atmospheric oxygen. The operating temperature for this type of fuel cell is 800-1000°C. This is a high temperature fuel cell. The efficiency is normally 45-60%.
The major advantage of this type of fuel cell is that it can readily be used in a CHP Combined Heat and Power configuration making them very suitable for domestic applications.
Due to the high temperatures used there is no need for an expensive catalyst such as platinum. Also they are the most sulphur resistant fuel cells to date so tend to be used in combination with the gases produced by coal.
The disadvantage is that the start-up time is long due to the time needed to heat up the fuel cell. The other disadvantage is that they are not capable of rapid thermal cycling as this reduces the life of the electrode. Even the (later introduced) so-called medium temperature SOFC’s still operate at around 450-600°C.