Efficient medium entropy alloy thin films as bifunctional electrodes for electrocatalytic water splitting

The development of high-performance, robust, and economical bifunctional electrode materials for efficient H₂ and O₂ evolution reactions (HER and OER) in water splitting is one of the fundamental aspects of the ever-growing H₂ research. In this regard, high and medium entropy alloy systems exhibit great potential in cost-to-performance trade-off as bifunctional electrodes. In particular, the richness in mixing various elements with different hydrogen bonding abilities brings the possibility to tune the catalytic properties that aim for high HER/OER efficiency. In this regard, CoNiCr and CoNiV systems are highly promising model systems with only 3 elements and low hydrogen diffusivity. Also, excellent corrosion resistance in acidic/alkaline environments and high conductivity, and simple crystal structure make these materials very auspicious candidates for HER/OER studies. Here, we have evaluated the electrocatalytic performance of the medium entropy alloy system NiCo(Cr/V) for water splitting. For our investigations, thin films of three compositions namely, NiCoCr, NiCoV, and NiCo(Cr/V) were fabricated using magnetron sputtering. In particular, the NiCo(CrV) (thickness: 1 μm) demonstrates excellent performance with overpotentials of 87 and 320 mV at 10 mA/cm² and Tafel slopes of 96 and 69 mV/dec towards HER and OER, respectively in 1 M KOH solution. Our findings suggest that the higher HER catalytic activity originates from the synergistic effects of the multicomponent alloy system in single-phase, while the higher OER activity comes from the multicomponent functional oxides of NiCo(CrV) generated on the surface during CV activation. An alkaline electrolysis cell with NiCo(CrV) as bifunctional electrode requires only 1.58 V to maintain 10 mA/cm², with outstanding electrochemical stability with no compositional reorganization after long-term durability tests.