The Role of Theory in the Development of Electrocatalysts: Case Study on Pt-Skin/Pt Alloy Nanoparticles for Hydrogen Oxidation and Evolution
Authors: Tryk Donald A. , Shi Guoyu , Yano Hiroshi , Uchida Hiroyuki , Iiyama Akihiro 
Published: February 2020

A well-known principle in chemistry states that “Theory guides, experiment decides.”1Theorists and experimentalists can debate which comes first, but our guiding principle has been that both must work together, hand in hand. Recently, we have been investigating the hydrogen oxidation reaction (HOR)2-5for the polymer electrolyte fuel cell (PEFC) and the hydrogen evolution reaction (HER) for the polymer electrolyte water electrolysis cell (PEWEC). Initially, we focused on making the HOR less vulnerable to CO poisoning by using Pt alloys on which CO binds less strongly, but we found, surprisingly, that the HOR itself became more active.2,3We proposed that this was due to weakened H bonding at both steps and (111) terraces, according to density functional theory (DFT) calculations. Weakening of H adsorption results in its easier removal to produce hydrated protons. Among several Pt-M alloys, the highest CO tolerance and highest HOR activity were found for Pt skin-covered Pt-Fe alloy nanoparticles, upon which the CO adsorption (steps and terraces) and H adsorption (terraces) both were calculated to be the weakest.3The weakening of H adsorption may be even more important for the development of improved alloy catalysts for the HOR in anion exchange membrane fuel cells (AEMFCs), since H adsorption at steps on Pt becomes stronger in base.6Theory will most likely continue play an important role in the further development of HOR catalysts, as we approach the point at which the weakening of H adsorption becomes excessive, and the activity starts to decrease.
Recently, we have extended these approaches to the HER. It is well recognized that the HOR and HER are complementary, as illustrated in Fig. 1. The dissociative adsorption of H2molecules at (110) steps is the first step in the HOR,7and the associative desorption of 2H is the last step in the HER. At 0.0 V vs. RHE, the energy of H2and 2H+(aq.) should be the same, so if the energy of 2H(ad) is higher, as in the alloy case pictured at the right, the activation energies for both reactions should be smaller.
This work was supported by funds for the ‘‘Superlative, Stable, and Scalable Performance Fuel Cell (SPer-FC)’’ project and “High Performance Fuel Cell (HiPer-FC)” project from NEDO of Japan.

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Fig. 1. Atomic models of (left) Pt(221) and (right) Pt/PtFe(221) surfaces, showing the (forward direction) dissociative adsorption of H2to produce 2H at (110) steps on (221) stepped surfaces, the first step in the HOR, and (backward direction) the associative desorption of 2H to produce H2, the last step and proposed rate-determining step in the HER.

Figure 1