Hydrogen Fuel CellsHydrogen fuel cells are both one of the oldest and also most promising of cells. It consumes Hydrogen gas and Oxygen gas and produces only pure water. The water can be produced in gas or liquid form depending on the design of the cell. Claim To FameThis fuel cell has a claim to fame. A version of it was used in the Gemini and Apollo space missions for both electricity and the production of liquid water in the space ships. There were three batteries of these fuel cells in the service module of each mission. The electricity generated was used to operate all the on board electrics.The most common type is the alkaline fuel cell. Cell OperationThis cell is also called the alkaline fuel cell and the hydrogen - oxygen fuel cell. These names come from the nature of the electrolyte and the reactants used. A basic view of a single fuel cell unit is shown below:
If we compare this unit to the general fuel cell we see some differences. First there are no "oxidant" gases leaving the cell and the fuel enters the left side of the cell from the top, not the bottom. This is so that the water vapor, which is heavier than the Hydrogen gas, drops to the bottom. It also allows any water that condenses to leave the cell without causing any problems. Cells Into BatteriesOne fuel cell by itself does not produce a great deal of electricity, so for useable amounts of electricity to be generated the cells need to be arranged into batteries of cells. These are not referred to as batteries as there would be confusion between them and the batteries used in electrical devices. A battery of fuel cells is just referred to as a fuel cell. The ReactionsThe chemical reactions occurring in this cell are quite simple as far as reactions go. The Anode Reaction: The reaction at the anode is the combining of hydrogen gas (the fuel) with the electrolyte to form water and release electrons:
At the cathode oxygen gas consumes electrons and so reacts with water in the electrolyte. This produces more of the hydroxide ions that the electrolyte is made of. In this way the electrolyte is never used up, but is replenished at the same rate it is consumed. This allows continuous operation of the hydrogen fuel cell.
Because the hydroxide ions are produced at the cathode and consumed at the anode, they are shown on the cell diagram above to be migrating from left to right. The Overall Reaction:Both the reactions given above are half reactions, meaning one half is occurring at each electrode. To get the overall picture, all we need to do is put the two half reactions together, being careful to keep the correct parts on the correct side of the arrow. The arrow is like an equals sign; we can cross off things that are the same on each side. If we do this, we get the final reaction:
Only in this last stage does it become apparent that the hydrogen fuel cell is doing the same thing as simply burning Hydrogen gas in air. The key difference here is that the fuel cell harnesses the movement of the electrons directly, rather than just using the heat of the reaction as a combustion engine does. Return from Hydrogen Fuel Cells to Fuel Cells or return to the Green Planet home page for Solar Power Facts.
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