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

Nowadays, a variety of multifunctional electrocatalysts have been developed while there is still a lack of proper design to fully realize their multifunctionalities. Here, we propose a general, simple, two-component design of the 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 proper 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 reaction before HER consumes the residual O2 left in the electrolyser, guarantees 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 up to 94.3 % can be obtained for as-assembled electrolyser at a current density of 10mA/cm2. When combined with a commercial Si PV module with efficiency of 14.4 %, as-designed PV-electrolysis system showed a solar-to-hydrogen conversion efficiency up to 10.4 %. Utilization of trifunctional electrocatalyst 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 in the utilization of multifunctional electrocatalysts in other devices, such as metal-air batteries and fuel cells.