Electrochemically activated NiOOH/NiFeV-LDH@CC for a highly efficient oxygen evolution reaction

Abstract

The surface reconstruction of NiFe-based layered double hydroxide (LDH) electrocatalysts has been widely studied. The reconstructed NiOOH phase plays a critical role in improving the oxygen evolution reaction (OER) performance of NiFe-based LDHs, but observing the NiOOH phase is difficult because of its instability and exploring the functional mechanism of NiOOH in NiFe-based LDHs remains a great challenge. A simple electrochemical activation was used to synthesize a NiOOH/NiFeV-LDH@CC catalyst consisting of an array of V-doped NiFe-LDH nanosheets on carbon cloth (CC), in which the reconstructed NiOOH phase is the active species. During electrochemical activation, the release of doped V leads to the formation of abundant vanadium vacancy (VV) and oxygen vacancy (VO) species, and thus the surface of the NiFe-LDH nanosheets is reconstructed to form NiOOH. Because of the improved intrinsic activity from the NiOOH active phase, and the increased electrical conductivity produced by the abundant VO, NiOOH/NiFeV-LDH@CC has an excellent OER performance in an alkaline solution, with low overpotentials of 209 mV and 241 mV at 20 mA cm−2 and 100 mA cm−2, respectively. It also has a long-term stability of 80,000 s at a constant current density of 10 mA cm−2. Using NiOOH/NiFeV-LDH@CC as the anode, an assembled over water splitting (OWS) battery can drive a current density of 20 mA cm−2 (without iR compensation) at a much lower voltage of 1.597 V. At the same time, the electrolytic cell can deliver a current density of 10 mA cm−2 at ~1.55V for more than 80,000 s without significant loss. This electrochemical activation method can be used in future designs of electrocatalysts for OER.

1. Introduction

The reliance on fossil fuels since the Industrial age has resulted in the emission of carbon dioxide, which causes global warming. With the objective of achieving Net Zero, hydrogen is considered an ideal substitute for its high specific energy density, regenerative, and pollution-free properties[1]. Hydrogen production from water electrolysis is the most promising method due to its abundant source of reactants, relatively mature process and environmental friendliness. During electrolysis, oxygen evolution reaction (OER) at the anode suffers from poor efficiency and high kinetic barrier due to having a four-electron coupled process and complicated intermediate reaction[2–4].

The difficulties can be overcome using high-performance electrocatalysts. At present, Ir- and Ru-based electrocatalysts such as IrO2 and RuO2 show good OER performance, but their high price and scarcity in the earth’s crust hinder the adoption of hydrogen to replace fossil fuels[5–8]. Therefore, the discovery and research for inexpensive and easily prepared non-noble metal electrocatalysts for efficient and stable OER is becoming increasingly important[9].

Transition metal-based compounds such as transition metal oxides[1,10−11], phosphides[12–15], sulphides[16–18] and hydroxides[19–22] are considered as candidates for OER electrocatalysts. Among these, layered double hydroxides (LDHs) are a type of transition metal-based electrocatalyst that consists of layers of divalent and trivalent metal cations coupled to hydroxide anions, with the guest anion wedged between the layers[19−20,23−24]. NiFe-based LDHs have efficient OER performance in alkaline solution[25]. The valence of Ni increases from 2+ to 3+ under an applied electric field, leading to the surface reconstruction of NiFe-based LDHs during OER. The formation of NiOOH phase indicates the true active phase of NiFe-based LDHs[26]. The impact of surface reconstruction on the OER performance of NiFe-based LDHs electrocatalysts has been verified by researchers. Zhai et al. studied ways to promote the generation of ...