Membrane-free two-step water splitting enabled by heterostructured Ni(OH)2-CoMoO4·0.75H2O charge-buffering electrode

Abstract

Membrane-free, decoupled water splitting is essential for cost-effective, large-scale hydrogen production but is hindered by challenges such as limited charge storage capacity and slow charge transfer during electrochemical reactions. Herein, we introduced a facile heterostructure engineering approach to synthesize Ni(OH)₂@CoMoO₄·0.75H₂O nanosheets on nickel foam as a highly efficient charge-buffering electrode. This design effectively decoupled the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), enabling continuous hydrogen production at the cathode for 1500 seconds under 8.33 mA/cm² with a driving potential of 1.55 V. Simultaneously, the anode underwent oxidation to produce an oxidized mediator, which triggered O₂ generation in the second OER step at 0.49 V, ensuring that the oxygen production time matched the duration of HER. Notably, pairing this oxidized mediator with zinc foil eliminated the need for a second OER stage, allowing sustained hydrogen production without external power. Raman spectroscopy revealed the oxidation-reduction pathways of the buffering electrode during cycling, while work function analysis showed that the heterostructure induced charge redistribution and formed an interfacial electric field, boosting electron transport, affording rich active sites, and reducing ion diffusion barriers. By spatially and temporally decoupling HER and OER, this approach offers a scalable solution for renewable hydrogen production.