Comparing copper and tin-based medium-entropy alloy oxide for producing hydrogen linking to waste polyethylene terephthalate (PET) upcycling.

Electrocatalytic water splitting for hydrogen production presents a promising solution to the global energy crisis. The high-value recycling and utilization of waste polyethylene terephthalate (PET) presents an environmental-friendly solution to address the "white pollution" caused by plastics. How to link the two reactions? Significantly, in a PET hydrolysate solution, the hydrogen evolution reaction (HER) occurs at the cathode, while the ethylene glycol oxidative reaction (EGOR) occurs at the anode, producing hydrogen and formic acid (FA), respectively. The design of electrocatalyst is the key point. Herein, we synthesised and evaluated three copper (Cu) and tin (Sn)-based medium-entropy alloy oxides (MEAOs): Cu0.5Co0.5SnO3.17, Cu0.5Ga0.5SnO3.25 and Cu0.5Ni0.5SnO3. Cu0.5Co0.5SnO3.17 showed the most favourable electrochemical performance, with an HER overpotential of 181 mV at 10 mA cm-2 and a low cell voltage of 1.26 V. Its electrochemical performance was better than that of the commercial RuO2 + Pt/C system. Besides, Cu0.5Co0.5SnO3.17 efficiently converts EG to FA, achieving a Faradaic efficiency (FE) of 97.7 % at 1.6 V, slightly surpassing the performances of Cu0.5Ni0.5SnO3 and Cu0.5Ga0.5SnO3.25 MEAOs. Density functional theory (DFT) reveals that the Cu0.5Co0.5SnO3.17 possesses a d-band center that is closer to the Fermi level, and the Co 3d orbit has the most contribution to the density of state (DOS), reflecting more synergetic effect in the Cu0.5Co0.5SnO3.17.