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%.
Keywords: electrodeposition; hierarchical structure; layered double hydroxide; oxygen evolution reaction; solar-powered overall water splitting.