Robust and highly efficient electrocatalyst based on ZIF-67 and Ni2+ dimers for oxygen evolution reaction: In situ mechanistic insight

Abstract

Electrochemical water splitting is a straightforward process that involves two distinct reactions: the oxygen evolution reaction (OER) which produces oxygen (O2) and the hydrogen evolution reaction (HER) which generates hydrogen (H2). However, in the whole process, the OER is a bottleneck as it requires more energy than a four-electron reaction involving critical raw materials (such as RuO2 or IrO2) as electrocatalysts. Therefore, here, we address the challenge of erratic kinetics/limited durability of OER in water electrolysis. In this paper, we demonstrate that the deposition of ultrasmall amounts of nickel(II) nitrate in zeolitic imidazolate framework-67 (ZIF-67) can be used as a general approach to enhance the electrocatalytic performance of the framework. We investigated the influence of Ni(NO3)2·x6H2O loading on ZIF-67 (from 0.1 to 0.0001 M) and found that ZIF-67 enriched with only 0.001 M of Ni(NO3)2·x6H2O (ZIF-67 0.001Ni) exhibited massive promotion in OER. The ZIF-67 0.001Ni showed a large specific surface area of 2577 m2 g−1, a low overpotential of 299 mV, a lower Tafel slope of 94.1 mA dec−1, and an outstanding overpotential retention of 99.8% (at 50 mA cm−2). By conducting electron paramagnetic resonance (EPR) measurements, we also discovered that the 0.001 M of Ni(NO3)2·x6H2O loading in ZIF-67 introduces Ni2+ dimers, which contribute to the enhanced electroactivity of the modified ZIF-67. This phenomenon was further revealed during density-functional theory (DFT) calculations, which allowed us to identify different possible forms of Ni2+ dimers and modeling of active centers. Along with in situ experiments, we provide mechanistic insight into the OER mechanism under alkaline conditions and found that it follows the lattice oxygen mechanism (LOM). Our study proposes a facile and efficient room-temperature route to boost the electrochemical performance of ZIF-67 in OER. For the first time, we demonstrate that modifying ZIF-67 with an ultrasmall amount of different nickel(II) salts opens a general route to enhance its electroactivity during water-splitting reactions.