Fuel cells are electrolyzers in reverse: They generate electricity by oxidizing hydrogen into water. Today, both the fuel cells and the electrolyzers are bulky and expensive, but that can be changed by good process control. In the March issue (www.controlglobal.com/articles/2008/073.html) I have described how the size and weight of this combined device (the reversible or regenerative fuel cell–RFC) can be reduced, and how the two devices can be combined into a single unit that can fulfill both functions. Once this is accomplished, and the RHCs are mass-produced, their costs will drop, and our dependence on fossil fuels will be over.
Electrolysis is an endothermic reaction and requires energy to occur (blue arrow in Figure 3), while the fuel-cell reaction, which oxidizes hydrogen into water is exothermic (red arrow in Figure 3). Depositing one mole of any material at the cathode of an electrolytic cell requires 96,484 coulombs (Faraday’s constant). Therefore, it takes 237.13 kJ (224.79 BTU = 0.066 kwh) of electricity to synthesize one mole of hydrogen. When hydrogen is generated at ambient temperatures, the environment contributes 48.7 kJ of thermal energy for a total of 286 kJ.
The weight of one mole of hydrogen is 2 grams, and the energy content of one gallon of gasoline equals the energy content of one kilogram (about 500 moles) of hydrogen. Therefore the electricity needed to obtain the amount of hydrogen, which has the same energy content as one gallon of gasoline, is: 32.9 kWh. [(237.13 x 500 x 0.948)/3,413)]. Naturally if free solar energy generates the electricity, the cost of generation is only the cost of equipment depreciation and the emission is only oxygen. As shown in Figure 2,
H2O + (286 kJ of ENERGY) ↔ H2 + ½O2
Electrolysis (Blue arrow), Fuel Cell (Red arrow)
It takes the same amount of energy to split water into hydrogen and oxygen as the energy obtained by oxidizing hydrogen into water. The only difference between the two operations is that electrolysis increases the entropy and, therefore, not all the energy needs to be supplied in the form of electricity, as the environment also contributes 48.7 kJ in the form of thermal energy. Therefore, only 237.1 kJ of electrical energy is needed to make a mole of hydrogen from water. On the other hand, in case of the fuel cell operation, the oxidation of one mole of hydrogen will generate 237.1 kJ of electrical energy plus 48.7 kJ of heat. Therefore, when the RFC operates in the hydrogen- generating mode (when the sun is out), it cools, and when it operates in the electricity generating mode (at night), it heats its environment.
There are two types of water electrolyzers: alkaline and polymer (PEM). The alkaline designs can be unipolar or bipolar (Figure 4). In the unipolar design, the anode and the cathode are separated by a semi-permeable diaphragm membrane and are immersed in an alkaline solution. The diaphragm allows only salts to pass through and prevents the gases from mixing. The hydrogen and oxygen that is so produced is dried downstream of the electrolyzer to remove water vapor. These units operate at lower current densities and temperatures and must be under differential pressure control to protect the diaphragm.
In proton exchange membrane (PEM) or solid polymer electrolyte (SPE) electrolysis, the electrolyte is replaced by an ion-exchange resin. These units are compact, provide high current densities, but are more expensive and because of the corrosive nature of the electrolyte, require special materials of construction.