Multi-dimensional hierarchical CoS2@MXene as trifunctional electrocatalysts for zinc-air batteries and overall water splitting

The demanding all-in-one electrocatalyst system for oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in zinc-air batteries or water splitting requires elaborate material manufacturing, which is usually complicated and time-consuming. Efficient interface engineering between MXene and highly active electrocatalytic species (CoS2) is, herein, achieved by an in situ hydrothermal growth and facile sulfurization process. The CoS2@MXene electrocatalyst is composed by one-dimensional CoS2 nanowires and two-dimensional MXene nanosheets, which lead to a hierarchical structure (large specific surface area and abundant active sites), a spatial electron redistribution (high intrinsic activity), and high anchoring strength (superior performance stability). Therefore, the electrocatalyst achieves enhanced catalytic activity and long-time stability for ORR (a half-wave potential of 0.80 V), OER (an overpotential of 270 mV at 10 mA cm−2, i.e., η10 = 270 mV) and HER (η10 = 175 mV). Furthermore, the asymmetry water splitting system based on the CoS2@MXene composites delivers a low overall voltage of 1.63 V at 10 mA cm−2. The solid-state zinc-air batteries using CoS2@MXene as the air cathode display a small charge-discharge voltage gap (0.53 V at 1 mA cm−2) and superior stability (60 circles and 20-h continuous test). The energy interconversion between the chemical energy and electricity can be achieved by a self-powered system via integrating the water splitting system and quasi-solid-state zinc-air batteries. Supported by in situ Raman analyses, the formation of cobalt oxyhydroxide species provides the active sites for water oxidation. This study paves a promising avenue for the design and application of multifunctional nanocatalysts.