Boosting the HER/OER bifunctional electrocatalytic activity of Ni–Cu alloy via water-containing deep eutectic solvent system†

Green hydrogen produced from water electrolysis is regarded as a promising alternative to fossil fuel. To achieve this, non-noble metal-based electrocatalysts with high hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performances are crucial. Addressing this need, a simple one-step electrodeposition method using a water-containing choline chloride/ethylene glycol deep eutectic solvent (ChCl/EG DES) medium is proposed to create Ni–Cu alloy/carbon fiber felt (CF) electrodes for water splitting. By controlling the water content within the DES, an appropriate ionic conductivity and viscosity can be achieved without disrupting the DES molecular structures. Importantly, the DES with 3 wt% water content provides an environment where formation of a moderate Ni(OH)₂ colloid membrane can be achieved, resulting in a uniform deposition structure and an appropriate Ni/Cu ratio of Ni–Cu alloy over the CF substrate. Thanks to these specific features, the optimized Ni–Cu/CF electrode (NC(3)) demonstrates exceptional HER and OER performances with low overpotentials of 53 mV at 10 mA cm⁻² and 390 mV at 50 mA cm⁻², respectively. The favorable bifunctional activity makes the NC(3) electrode-based electrolysis cell highly stable against water splitting, with a reduced cell voltage of 1.52 V at 10 mA cm⁻² and and 1.86 V at 50 mA cm⁻². Specifically for the HER, the composite electrode exhibits a high faradaic efficiency of over 99%, resulting in H₂ gas with a purity of approximately 99%. Furthermore, the improved electrode kinetics and stability of the NC(3) electrode are supported by its smaller Tafel slope, lower charge transfer impedance, increased electrochemical active surface area, and enhanced anti-corrosion capability. The promoted electronic effect between the Ni and Cu species of the Ni–Cu alloy electrocatalyst may play a key role in this regard.