Invited: Hydrogen Oxidation and Evolution Reaction (HOR/HER) on Pt Electrodes in Acid vs. Alkaline Electrolytes: Mechanism, Activity and Particle Size Effects
Authors: Durst Julien , Simon Christoph , Siebel Armin , Rheinländer Philipp Jan , Schuler Tobias , Hanzlik Marianne , Herranz Juan , Hasché Frédéric , Gasteiger Hubert A. 
Published: February 2020

Fuel cells and electrolyzers based on proton-exchange membranes (PEMs), operating at low pH (pH≈0), offer high power densities, but require large amounts of noble metal for the oxygen reduction reaction (ORR) in fuel cells and the oxygen evolution reaction (OER) in electrolyzers. For the hydrogen oxidation/evolution reaction (HOR/HER) only very small amounts of Pt is required due to its high activity for the HOR/HER.1 In alkaline electrolyte, non-noble metal catalysts are very active for the ORR and for the OER. Therefore, a replacement of the noble-metal intensive PEM technology by alkaline membrane technology seems promising. Unfortunately, for yet unclear reasons, the HOR/HER kinetics on Pt are much slower in alkaline than in acid electrolytes, and more active catalysts are needed to catalyze the HOR/HER in alkaline environment.2 It is therefore critical to elucidate the reasons for the poor HOR/HER activity of Pt in alkaline electrolytes, and this will be possible only by having a clear idea about the reaction mechanism and the rate determining step.

Historically, the HOR/HER equilibrium has been described by invoking over potential deposited hydrogen, H-OPD, as the reactive intermediate species.
3
In contrast with adsorbed H intermediate present on noble metal surfaces at potentials positive to the reversible H+/H2 potentials in the process of the so-called underpotential-deposition (UPD), the believed reacting H-OPD species would form at potentials negative to the reversible H+/H2 potentials. So far, no clear evidence of the difference in the physical nature of H-OPD and H-UPD species has ever been reported. Recently, computational studies have addressed the HOR/HER rates by considering one of the three microscopic steps (Tafel, Volmer, Heyrovsky) as rate determining and only one state of adsorbed hydrogen species as reaction intermediate.
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However, the link between this reacting adsorbed hydrogen intermediate and the H-UPD has never been explicitly stated.

By comparing HOR/HER exchange current densities of carbon supported nanoparticles (Fig. 1) with the H-UPD charge transfer resistance (Fig. 2) which was determined by electrochemical impedance spectroscopy on polycrystalline surfaces (following refs.5
,
6) and, for the first time, on carbon supported nanoparticles, we will show that the H-UPD charge transfer rates closely match the HOR/HER exchange current densities in acid and alkaline electrolytes. Therefore, no H-OPD species has to be invoked to describe the two decades HOR/HER activity decrease from acid to base. This fundamental result gives credence to a HOR/HER rate controlled by the Volmer reaction.

Moving away from the platinum system, iridium being an oxophilic surface is an interesting electrocatalyst system as it probes the hypothesis advanced by Strmcnik et al. that a more oxophilic surface might be effective in catalyzing the interaction with H2O/OH-, which they proposed to be the governing reaction in base.
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However, our findings demonstrate explicitly that Ir does not have a higher activity than Pt in base (nor in acid). This leads to the conclusion that water dissociation is not the rate determining step in base, and, therefore, points toward identical microscopic HOR/HER reaction steps in acid and base.

In conclusion, these novel insights confirm the H-binding energy as a relevant HOR/HER activity descriptor.8 Future studies will have now to address the pH effect on this binding energy and the synthesis of electrocatalysts with tuned H-binding energies.

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