A novel design of an electrolyser using a trifunctional (HER/OER/ORR) electrocatalyst for decoupled H2/O2 generation and solar to hydrogen conversion

Abstract

Nowadays, a variety of multifunctional electrocatalysts are being developed while there is still a lack of suitable design to fully realize their multifunctionalities. Here, we propose a general, simple, two-component design of an electrolyser to replace the traditional three-component design for decoupled water splitting. Trifunctional (OER, HER and ORR) electrocatalysts (such as nickel sulfide foams with surface grown N-doped carbon nanotube arrays) are used as the gas evolution electrode to replace both the cathode and the anode, while materials with suitable redox activities (NaTi₂(PO₄)₃ or commercial Ni(OH)₂) are used as the relay electrode. In such a design, the H₂/O₂ evolution can be switched by reversing the current polarity, and the ORR before the HER consumes the residual O₂ left in the electrolyser, guaranteeing the high purity (∼99.9%) of the as-obtained H₂. With NaTi₂(PO₄)₃ as the relay electrode and the nickel sulfide foam as the gas evolution electrode, owing to the high decoupling efficiency of the NaTi₂(PO₄)₃ relay (97%) and the low HER/OER overpotentials of the trifunctional nickel sulfide foam, an energy conversion efficiency of up to 94.3% can be obtained for the as-assembled electrolyser at a current density of 10 mA cm⁻². When combined with a commercial Si PV module with an efficiency of 14.4%, the as-designed PV-electrolysis system showed a solar-to-hydrogen conversion efficiency of up to 10.4%. Utilization of trifunctional electrocatalysts greatly reduces the complexity of the electrolyser and the overall cost for electrochemical H₂ production, and these electrolysers may potentially be used to construct highly competitive water splitting systems for continuous H₂ production and green energy harvesting. Our research may also bring new insights into the utilization of multifunctional electrocatalysts in other devices, such as metal–air batteries and fuel cells.