In the operation of a hydrogen fuel cell, one of the two principle reactions that takes place is the oxygen reduction reaction (ORR). It is the reaction by which oxygen gains electrons. It is a more complex reaction than the hydrogen oxidation reaction (HOR), with slower kinetics, making it the bottleneck in the overall redox reaction of the fuel cell. Because of this, ORR is the focus of much research concerning hydrogen fuel cells.
For each O2 molecule, there is a transfer of four electrons. This can proceed through one of two basic routes: the four-electron pathway and the two-electron pathway. In the four-electron pathway, the exchange of the four electrons occurs in a single step, whereas in the two-electron pathway, it occurs in a two-step sequence, with two electrons exchanged in each step, and involves the creation of an intermediate. In an alkaline system, the four-electron pathway goes as follow:
O2 + 2H2O +4e– → 4OH–
Whereas the two-electron pathway goes as:
(1) O2 + H2O + 2e– → HO2– + OH–
(2) HO2– + H2O + 2e– → 3OH– or 2HO2– → 2OH– + O2
Here is the four-electron pathway in an acidic system:
O2 + 4H+ + 4e– → 2H2O
While the two-electron pathway looks like this:
(1) O2 + 2H+ + 2e– → H2O2
(2) H2O2 + 2H+ + 2e– → 2H2O or 2H2O2 → 2H2O + O2
It can be seen that the two-electron pathway, in the case of alkaline systems, involves the formation of hydroperoxyl ion intermediates, whereas in acidic systems it involves the formation of hydrogen peroxide as an intermediate. For this reason, the two-electron pathway is sometimes referred to as the “peroxide pathway”.
Though the two-electron pathway ends up with the same net result as the four-electron pathway, there are important differences between the two. In proceeding through the two-electron pathway, not all of the intermediate products produced in the first step will go on to be used as reactants in the second step, or there may be a delay in their doing so, reducing the overall ORR efficiency. The intermediates can also go on to participate in undesirable side reactions. Now in the operation of any fuel cell, ORR will proceed through a combination of both the two- and four-electron pathways. Given the disadvantages associated with the two-electron pathway, it is desirable to promote the four-electron pathway over the two-electron pathway as much as possible.