Quantum chemical studies on elementary electrode reactions in a proton exchange membrane fuel cell were made using theoretical approaches developed in our lab.
Reversible potentials for the reaction
H2O(ads) ⇌ OH(ads) + H+(aq) + e− (1)
were evaluated for both Pt and Pt/Co electrode surfaces to understand the dissolutions of these electrodes. All calculated potentials are versus the standard hydrogen electrode (SHE). The reversible potential for reaction (1) on the Co atom of Pt3Co(111) surface is 0.50 V, significantly lower than the calculated 0.70 V for that on the Pt atom of Pt(111) surface. Co(OH)2 forms at 0.58 V on the Pt3Co(111) surface but Pt(OH)2will not form before 1.92 V on the Pt(111) surface. Our results suggest that Co dissolves into the solution before Pt from alloy surfaces leading to the formation of a Pt skin on the surfaces.
To understand the formation of OH on electrodes and its role in the oxygen reduction reaction, reaction (1) on Pt(111) was studied. Calculated reversible potentials for the formation of 1/6 ML OH(ads) by reaction (1) for initial H2O coverages of 1/6, 1/3, 1/2, and 2/3 ML are in the range of 0.59-0.71 V. Calculated reversible potentials for oxidizing the last 1/6 ML of H2O(ads) at 1/3, 1/2, and 2/3 ML initial H2O coverage are higher than 1.2 V. These high reversible potentials suggest that high coverage for OH(ads) beyond 1/3 ML can be considered to exist only at very high electrode potentials.
To understand the thermodynamics of the oxygen reduction reaction, reductions of O(ads) and O2(ads) and dissociation of OOH(ads) represented as:
O(ads) + H+(aq) + e– ⇌ OH(ads) (2)
O2(ads) + H+(aq) + e– ⇌ OOH(ads) (3)
OOH(ads) → OH(ads) + O(ads) (4)
on Pt(111) in acidic solution were studied. To reduce 1/6 ML O(ads) on Pt(111), the reversible potential was in the range of 0.6 ~ 1.1 V. All the reversible potentials calculated for OOH(ads) formations with water presented are in the range of 0.24-0.44 V. With 1/3 ML OH(ads) or O(ads), reversible potentials are 0.27 and 0.11 V, respectively. Adding 1/6 ML H2O increases reversible potentials from 0.27 and 0.11 V to 0.37 and 0.22 V, respectively. Dissociations of OOH(ads) are highly exergonic and contribute to the overpotential from the oxygen reduction reaction.