Engineering Interfaces in Vertically Aligned Ni₃S₂/MnO₂ Heterojunction Nanoflakes for Efficient Overall Water...

Green hydrogen adoption demands intensive research efforts focusing on improving the performance and durability of electrodes used in water electrolyzers, enabling cheaper hydrogen production on a commercial scale. For catalyzing the oxygen evolution (OER) and hydrogen evolution (HER) electrode reactions in a water electrolyzer, the state-of-the-art electrocatalysts used are expensive and scarce, thus preventing their successful commercialization. There is a dire-need to replace those expensive catalysts with cheaper, earth-abundant non-platinum group of transition metals. Heterointerface engineering could be employed as an effective strategy to synthesize such kind of electrocatalysts to tune the electronic and catalytic properties of these environmentally friendly transition metal electrocatalysts. In this report, we have studied the heterointerface formation between Ni₃S₂ and MnO₂ phases using two synthesis approaches: sequential as well as simultaneous growth methods. Our studies show that sequential growth exhibits a critical impact on the chemical and electrocatalytic behavior of the as-synthesized vertically aligned nanoflakes. When Ni₃S₂ was grown over the MnO₂ phase, it resulted in the most superior bifunctional electrocatalytic activity. Along with the electrical impedance measurement, X-ray photoelectron spectroscopy and Raman spectroscopy reveal that the interfacial charge transfer due to heterointerface formation via sequential growth is more effective than the simultaneous method of heterojunction preparation. The best catalyst exhibits a lowering of OER overpotentials of 300 mV and HER onset overpotentials of 230 mV, surpassing the standard catalysts. DFT study has been performed to correlate the experimental and theoretical reaction kinetics over Ni₃S₂@MnO₂@NF heterointerfaces, which suggests a lower overpotential of 1.391 V when Ni₃S₂ is grown over MnO₂ for OER as compared with the MnO₂ (1.719 V) grown over Ni₃S₂. Ni₃S₂@MnO₂@NF electrodes registered a low cell voltage of 1.68 V at 10 mA cm^-2 current density in an alkaline water electrolysis prototype, performing better than the standard catalyst in terms of cell voltage and operation stability at higher current densities of up to 50 mA cm^-2. This study shows how strategic design of interfaces in heterojunction can control the overall catalytic performance.