Tailoring the Pt Surface Oxophilicity Via Single-Atom Rh Doping for Boosting Hydrogen Oxidation/Evolution Reaction in Alkaline Electrolyte
Authors: Wan Chengzhang , Huang Yu , Duan Xiangfeng 
Published: July 2021

Recently, the rapid development of the anion exchange membrane (AEM) shows a promising future in the commercialization of low cost and long-term stable AEM based fuel cells and water electrolyzers. AEM features an alkaline electrolyte, where non-precious metal ORR catalysts possess high ORR activity and can replace Pt catalysts. However, Pt is still the best selection for HOR catalysts in the alkaline medium. The HOR activity of Pt is highly pH-dependent and decreases over two orders of magnitude when pH increases from 1 to 14. Thus, the sluggish anodic HOR reaction becomes the most critical issue in the development of AEMFCs. The major reason for the slow HER/HOR process in the alkaline electrolyte is ascribed to the positive shift of potential of zero free charge (PZFC) while pH value goes up from pH = 1 to 14. As PZFC shifts to a more positive value, it becomes more difficult for OHads to get adsorbed on the Pt surface to facilitate the water dissociation or the removal of Hads. Tailoring Pt surface with a second transition metal hydroxides has been proved to be an effective method to enhance the HOR activity on Pt, but at the sacrifice of tremendous active surface Pt sites (~50%), leading to the waste of electrochemical surface area (ECSA) and the decrease of the maximum mass activity (MA). Herein, taking advantage of the single-atom tailoring strategy, we have achieved single Rh atom tailoring of Pt nanowires for the optimization of surface oxophilicity for OH adsorption to boost the HER/HOR activity in the alkaline condition.

The HOR activity of PtRh nanowires is highly sensitive to the oxophilicity of Pt surface and the structure of surface Rh species. Pt surface with higher oxophilicity usually shows a higher HER/HOR rate in alkaline conditions because the sluggish Volmer step can be accelerated by the early adsorption of OH in the Hupd potential region. However, both the HOR and HER activity of the PtRh nanowires reach the maximum value only when the Rh is in the single-atom form and the Pt surface is under the optimum oxophilicity.

It was revealed by the XANES spectra that the single Rh atoms under 0 V vs. RHE are positively charged. The in-situ and ex-situ XANES spectra of Rh L edge show that Rh atoms are redox-active and electroactive. The positive shift of the edge position from Rh foil to Rh single atoms under 0 V vs. RHE indicates that the single Rh atoms are slightly oxidized and coordinated with eight Pt and one O, even under 0 V vs. RHE. When the applied potential increases, Rh-O bond intensity increases, and Rh-Pt bond intensity decreases, as expected from the oxidation from slightly charged single atom Rh to single Rh(OH)x species. We also noted that the Rh-O peak shift positively when potentials shifts from 0.54 V to 0 V vs. RHE, indicating an elongated Rh-O bond at a lower overpotential, which indicates that the adsorbed oxygen species under 0 V are not the OH from Rh(OH)x and should be weakly adsorbed OH or H2O↓. The fitting of in-situ EXAFS of Rh L edge on the SARh-PtNWs also confirms that the Rh atoms are bonded with eight Pt atoms and one O, and no Rh-Rh interaction is found throughout the whole HOR range.

The existence of single atom Rh-OHads or Rh-H2O↓ under 0 V vs. RHE significantly facilitates the sluggish Volmer step at the minimal sacrifice of the active Pt sites where H2 dissociation and H adsorption takes place. When the surface Rh concentration surpasses a critical point, singly dispersed Rh atoms start to aggregate to a continuously distributed Rh layer which significantly increases the surface oxophilicity for earlier OH adsorption, but at the sacrificial of highly active Pt site for H2 dissociation and high Hads surface coverage. HOR/HER requires an equilibrium of H adsorption and OH/H2O adsorption on the Pt surface to reach the maximum oxidation/production rate of H2. However, both the Tafel step and the Volmer step on the Rh rich Pt surface are far away from the optimum condition, Also, the excess Rh and adsorbed water may form a rigid water layer that covers the Pt surface and prevent it from accessing the H2, results in the decreased overall HOR activity.

In short, we have synthesized single atom Rh tailored Pt nanowires and used in-situ EXAFS and XANES to prove the existence of the surface Rh-OHads or Rh-H2O↓ which significantly facilitate the HER and HOR process.

Figure 1