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 Ni0.8Co0.1Fe0.1O x H y 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 Ni0.9Co0.1O x H y 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 Ni0.8Co0.1Fe0.1O x H y as the anode and Ni0.9Co0.1O x H y 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.