Hydrogen Oxidation on Ni-Based Electrocatalysts: The Role of the Synthetic Method

Abstract

Hydrogen electrooxidation is one of the key reactions underlying the fuel cell technology for electromobility and stationary power supply, the critical segments of the hydrogen economy of the future. Progress in alkaline anion exchange membranes (AAEMs) may lead to fuel cells employing catalysts that are less costly than those of the platinum-group metals required by the PEM technology1 - 6. To this end Ni appears to be a well suited metal, and development and implementation of affordable Ni-based electrocatalysts as a replacement for the cost-intensive platinum group metals would significantly facilitate the progress in AAEM fuel cells. However, the low catalytic activity of Ni for the hydrogen oxidation reaction (HOR) and inevitable (electro)chemical passivation of the metallic surface impede the advancement of the technology. In this work, we have synthesized two series of binary Ni3M/C (M=Fe, Cu, Co) catalysts using two different synthesis techniques, viz. low-temperature chemical reduction (CR) and high temperature solvothermal reduction (STR). We have undertaken a comprehensive experimental study using transmission electron microscopy (TEM), scannin-transmission electron microscopy (STEM), Energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR), cyclic voltammetry (CV), and methods based on rotating disc electrodes (RDEs) to understand the role of the synthetic methods in determining the structural (the formation of true alloys or mechanical mixtures), (electro)chemical, and catalytic properties of Ni-based catalysts in the HOR. We show that neither the degree of alloying nor the degree of phase crystallinity is critical in determining the HOR catalytic activity. The chemical and electrochemical oxidation of the surface on the other hand, are critical to the catalytic activity, and these are in turn dependent on the chemical nature of the dopant and on its surface concentration, as well as the synthetic technique used. Due to the oxophilic properties, the Ni-M surface promotes the bifunctional HOR catalysis better than the monometallic Ni surface. However, for the same reason, doping of nickel by Co, Cu and Fe also promotes the electrochemical oxidation of the Ni surface and thus has an adverse effect on the overpotential range over which the catalyst will be active.Due to the formation of NiOH and Ni(OH)2 and possibly other species at the catalyst surface, establishing credible Tafel plots are difficult from experimental data. We therefore interpreted voltammograms recorded in hydrogen-saturated solutions of KOH in terms of dynamic microkinetic models for the HOR that also includes odxidation of the Ni surface7, 8. The figure shows the results of such fits to experimental data for a numerical model that includes the Tafel, Heyrovsky, and Volmer steps for the HOR and steps for the formation of NiOH and Ni(OH)2 and a simplified, approximate model for the forward sweep.