(Invited) Investigation of Hydrogen Oxidation and Evolution Reaction Activity on Iridium Metal in Alkaline Electrolyte

One of the challenges that currently impedes the commercialization of anion exchange membrane fuel cells (AEMFCs) is the development of suitable catalysts for the hydrogen oxidation reaction (HOR, H₂ + 2 OH^- → 2 H₂O + 2 e^-).(1) Because this challenge is unique to the alkaline environment of AEMFCs and is not present in the acidic environment of the proton exchange membrane fuel cells (PEMFCs), there has been renewed interest in quantifying hydrogen oxidation and evolution (HOR/HER) reaction kinetics and understanding the differences in activities of catalysts in acidic and alkaline environments. In these studies, a disagreement has emerged concerning the HOR/HER activity of iridium (Ir) metal in base, with researchers reporting that it is both higher(2, 3) and lower(4) than the corresponding activity of platinum (Pt) metal. In our study, we re‑examine the electrochemical behavior of polycrystalline Ir in alkaline environment and find that the HOR activity of Ir depends on the electrochemical characterization procedure used in its quantification. In particular, we determine that subjecting the electrode to increasingly more cathodic potentials leads to a decrease in the HOR activity. Although this phenomenon could be argued to correspond to the adsorption of contaminants onto the catalyst surface, we demonstrate that the underlying reason for the drop in activity is likely not a surface contamination effect, but rather absorption of hydrogen into the sub‑surface of the Ir electrode. Specifically, we find that exposure of the Ir surface to potentials that are lower than 0.10 V vs. the reversible hydrogen electrode (RHE) in H₂‑saturated environment leads to an appearance of an oxidation peak (Fig. 1) in a potential region in which surface contaminants are not expected to be oxidized (0.15-0.20 V vs. RHE), and that the size of this peak scales inversely with the HOR activity of the surface. Our results have important implications towards understanding of the HOR/HER kinetics and the development of improved catalysts for these reactions.

References

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