ABSTRACT
In a hydrogen fuel cell, hydrogen and oxygen atoms combine to yield water molecules, that is, the reversed reaction of water splitting is used. This is a downhill reaction accompanied with negative changes of Gibbs free energy, that is, ΔG < 0 and for this reason it runs facilely. The thermodynamic functions of this reaction are known from the water splitting discussed in Chapter 7. A stream of hydrogen is delivered to the anode side of the membrane electrode assembly (MEA). At the anode side it is catalytically split into protons and electrons. This oxidation half-cell reaction is represented by: H2 Æ 2H+ + 2e-, E°= 0 VSHE (9.1) The reduction half-cell reaction at the cathode is represented by 0.5 O2 + 2H+ + 2e-Æ H2O, E°= 1.23 VSHE (9.2) The overall reaction is as follows: H2+ 0.5 O2 Æ H2O, E°= 1.23 VSHE (9.3) 9.2 Design of a Fuel CellThere are plenty of various fuel cells; however, they all have general common features. They are made up of three main functional units that are sandwiched together: the anode, the electrolyte, and the cathode (Fig. 9.1). Two chemical reactions occur at the interfaces of the three different domains resulting in the consumption of fuel, creation of water or carbon dioxide, and generation of electric current, which can be used to power electrical devices, normally referred to as the load [352]. At the anode a specially selected catalyst oxidizes the fuel, usually hydrogen, turning it into a positively charged ion and a negatively charged electron. The electrolyte is a substance specifically designed so that it is permeable to the ions, but not to the electrons. The released electrons travel through a wire creating the electric current. The ions flow through the electrolyte to the cathode. On reaching the cathode, the ions are recombined with the electrons and the two react with a third chemical, usually oxygen, to create water or carbon dioxide [352]. So, again we see the membrane architecture for the energy conversion process where hydrogen plays the main role. The ability of hydrogen to break out easily into the charged carrier is employed.
The electrolyte performs the function of charge separation similar to that for the biological membranes in metabolic processes considered above. Again Mitchell’s “proticity” is created via splitting of hydrogen atoms into electrons and hydrogen proton, which drives the vectorial flow of the protons to the cathode.
