Hydrogen: somewhere over the rainbow?
electrons. This allows the electrons to travel through a wire to the cathode, creating electricity. Each fuel cell typically produces only 0.9V of direct current (DC) electricity. But they are small, so can be stacked for industrial use creating higher voltages at power plants (Song, 2002).
Types of fuel cells
There are five major types of hydrogen fuel cell, all of which have an array of different variations. These major types are distinguished by their electrolytes and subsequently at which electrode water will form. In a proton exchange membrane fuel cell (PEMFC) the electrolyte is usually an ionomer, a polymer with ionic properties, such as Nafion. 1 This has a fluorocarbon backbone attached to acid groups, which do not allow the movement of molecules but does allow hydrogen ions, which are protons, to travel across the membrane to produce water at the cathode (O'Hayre, et al., 2016). On the other hand, a solid oxide fuel cell (SOFC) uses a ceramic electrolyte allowing oxide ions to travel to the anode so water is generated here. Alkaline fuel cells (AFC) use aqueous potassium hydroxide as their electrolyte. At the cathode the oxygen reacts not only with the electrons from the circuit, but also with water vapour to produce hydroxyl ions. These can then travel across to the anode to react with hydrogen molecules (not ions), reproducing the water and electrons. The AFC is the most electrically efficient fuel cell, as the oxygen reduction kinetics are much quicker in alkaline conditions (Carrette, et al., 2000). This makes it particularly useful in electricity production in space as resources are limited and valuable there. However, it is more challenging to implement into most industries compared to other categories of fuel cell, due to its initial demand for very pure gases and difficulty to remove water from the anode, so its future is limited. Both the phosphoric acid fuel cell (PAFC) and the molten carbonate fuel cell (MCFC) have liquid membranes like the AFC. The PAFC works in a similar way to the PEMFC but the phosphoric acid electrolyte in a silicon carbide matrix requires an expensive platinum catalyst. The MCFC incorporates carbon dioxide into the reaction at the cathode, similar to how the AFC uses water. This is to produce carbonate ions to travel to the anode and react with hydrogen.
Advantages and disadvantages of fuel cells
The greatest advantage of this alternative way of making energy is that the only by-product is water, making it sustainable. Furthermore, the hydrogen fuel cell can be seen as a hybrid of the benefits of both batteries and combustion engines. They are as efficient as batteries because the chemical energy of the fuel is directly generated into electrical energy. By contrast, the combustion engine has to sandwich the thermal energy store in between, meaning a lot of energy is lost as heat. Fuel cells are also chemically unchanged when used, unlike combustion engines. Batteries, conversely, have a short, fixed lifetime. Moreover, the PEMFC and SOFC are the only two fuel cells all in solid state. This makes them mechanically ideal, highly reliable, long-lasting and less noise polluting, whilst also removing the possibility of corrosion which has safety fears.
However, fuel cells have their own limitations and complications. The general low power density and low durability under start-stop cycling of fuel cells makes them difficult to integrate into the automotive industry at the moment. Temperatures between 200-1000 degrees Celsius are needed, with
1 Nafion is a trademark of DuPont.
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