Fuel cells provide continuous electrical energy, which have many applications, including portable electric devices and electric cars. As long as continuous fuel and oxidant supplies are provided, oxidation and reduction take place spontaneously at the anode and the cathode respectively. As the result of the spontaneous oxidation and reduction, electricity is generated as electrons flow from the anode to the cathode through an external conducting wire.
Figure. (A) Schematic of a fuel cell. (B) Details of the anode. Not all components are present in all fuel cells. 1 – Solution to carry the ions at the anode; 2 – Anode; 3 – Polymer electrolyte membrane or other types of electrolyte; 4 – Cathode; 5 – Solution to carry the ions at the cathode; 2a – Gas-diffusing electrode made of metal or other materials; 2b – Catalytic layer (i.e. a support layer with catalysts); 2c – Blocking layer.
In a common PEMFC, the fuel is H2 and the oxidant is O2. H2 is oxidized at the anode and O2 is reduced at the cathode. The two half-cell reactions are shown below:
Half-cell reaction on anode (oxidation): H2 ----> 2H+ + 2e-
Half-cell reaction on cathode (reduction): O2 + 4H+ + 4e- ----> 2H2O
Overall:2H2 + O2 ----> 2H2O
Under standard conditions, the potential of the anode, Ea0 is 0.000 V vs. NHE (normal hydrogen electrode) and that of the cathode, Ec0 is 1.229 V vs. NHE. (If not mentioned otherwise, all electrode potentials are vs. NHE in this proposal.) The standard electromotive force (emf) is the Ecell0 of the overall reaction, which is the difference between Ec0 and Ea0. In this case, the standard emf is 1.229 V. In the research of fuel cells, Ea, Ec and emf are all important parameters and need to be considered scrupulously.
Both oxidation and reduction require proper catalysts. Technically, the catalysts are usually incorporated into a support layer that is attached to the electrode surface. As shown in Figure, layer 2b is the catalytic layer and the catalysts are incorporated in it. In a PEMFC, the catalytic layer does not contact the fuel or oxidant gas directly, requiring the electrode (2a in Figure) to be gas-diffusing.
Layer 3 in Figure is the electrolyte. It transports H+ ions in a PEMFC. That is why PEMFC (polymer-electrolyte membrane fuel cell) is also called proton-exchange membrane fuel cell. Protons are generated on the anode as the fuel H2 is oxidized. The protons then transport through the electrolyte and react with O2 on the cathode.
Fuel and oxidant should be separated totally in an efficient fuel cell, and they undergo half-cell reactions at the anode and the cathode, respectively. Crossover is an adverse effect. It happens when the fuel penetrates the electrolyte and mixes with the oxidant directly. Crossover reduces the efficiency of a fuel cell and also reduces emf in many cases. To prevent crossover, a blocking layer (2c in Figure) is always applied to the anode of a PEMFC. With proper pressure on the fuel H2, the blocking layer allows the penetration of protons but not H2 molecules. For the same reason, the cathode is also coated with a blocking layer to prevent penetration of the oxidant.