Tuning Electronic Push/Pull of Ni-Based Hydroxides to Enhance Hydrogen and Oxygen Evolution Reactions for Water Splitting

Enhancing the efficiency of non-noble electrocatalysts in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) for water splitting remains a challenging task. Herein, we report an electronic push/pull effect of Co and Fe doping on the HER and OER performance of Ni-based hydroxides as revealed by thorough cyclic voltammetry and X-ray photoelectron spectroscopy analysis. A Fe dopant pulls partial electrons from nearby Ni/Co active sites resulting in a higher electron affinity at the Ni/Co sites to facilitate OH- adsorption and charge transfer from the adsorbed OH- for OER. In contrast, a Co dopant tends to push its partial electrons to nearby Ni sites and increase the number of lattice O2- as proton adsorption sites, which lead to faster charge transfer for HER. By adjusting the contents of Co/Fe dopants in Ni-based hydroxides, we were able to tune the electronic configuration of the catalyst and to optimize the OER and HER performance specifically. An optimized catalyst with a composition of Ni0.8Co0.1Fe0.1OxHy showed excellent OER performance with an overpotential (η) of 239 mV at 10 mA·cm-2 and a Tafel slope of 45.4 mV·dec-1. The HER performance was optimized with a catalyst composition of Ni0.9Co0.1OxHy and reached an η of 85 mV at 10 mA·cm-2 and a Tafel slope of 84.5 mV·dec-1. Overall water splitting with Ni0.8Co0.1Fe0.1OxHy as the anode and Ni0.9Co0.1OxHy as the cathode was demonstrated at a low potential of 1.58 V at 10 mA·cm-2. Utilizing the electronic push/pull effect by dopant elements provides a new pathway for the design and optimization of the transition hydroxides as HER and OER catalysts. © 2018 American Chemical Society.