Hierarchical Heterogeneous NiFe Layered Double Hydroxides for Efficient Solar-Powered Water Oxidation

Highly active, stable, and low-cost oxygen evolution reaction (OER) electrocatalysts are urgently needed for the realization of large-scale industrial hydrogen production via water electrolysis. Layered double hydroxides (LDHs) stand out as one of the most promising nonprecious electrocatalysts worth pursuing. Here, a hierarchical heterogeneous Ni²⁺Fe³⁺@Ni²⁺Fe²⁺ LDH was successfully synthesized via a sequential electrodeposition technique using separate electrolytes containing iron precursors with different valence states (Fe²⁺, Fe³⁺). The underlying highly crystalline Ni²⁺Fe²⁺ LDH nanosheet array provides a large surface for the catalytically more active Ni²⁺Fe³⁺ LDH overlayer with low crystallinity. The resulting Ni²⁺Fe³⁺@Ni²⁺Fe²⁺ LDH demonstrates excellent OER activity with overpotentials of 218 and 265 mV to reach current densities of 10 and 100 mA cm^-2, respectively, as well as good long-term stability for 30 h even at a high current density of 500 mA cm^-2. In an overall water splitting, an electrolyzer using an electrocatalyst of Sn₄P₃/CoP₂ as a cathode requires only a cell voltage of 1.55 V at 10 mA cm^-2. Furthermore, the solar-powered overall water splitting system consisting of our electrolyzer and a perovskite/Si tandem solar cell exhibits a high solar-to-hydrogen conversion efficiency of 15.3%.