Microporous 2D NiCoFe phosphate nanosheets supported on Ni foam for efficient overall water splitting in alkaline media.

The development of high-performance non-precious electrocatalysts for both H2 and O2 evolution reactions (HER and OER activities) and overall water splitting is highly desirable but remains a grand challenge. Herein, we report a facile method to synthesize ultrathin, amorphous, porous, oxygen and defect enriched NiCoFe phosphate nanosheets (NSs). Owing to their microporous confinement in a 2D orientation, which can reduce the ion transport resistance during electrochemical processes, and defect enriched structure with higher electrochemically active surface area, these NiCoFe phosphate porous nanosheets supported on nickel foam (NiCoFe phosphate NSs/NF) facilitate the diffusion of gaseous products (H2 and O2) and exhibit remarkable catalytic performance and outstanding stability for both HER, OER and overall water splitting in an alkaline electrolyte (1.0 M KOH). For the OER electrocatalyst, 2D NiCoFe phosphate NSs/NF was oxidized to NiCoFe oxides/hydroxides on the catalyst surface and exhibited remarkable OER activity with a low overpotential of only 240 mV needed to reach a current density of 10 mA cm-2. For HER, 2D NiCoFe phosphate NSs/NF afforded a current density of 10 mA cm-2 at a low overpotential of only -231 mV. Furthermore, employing 2D NiCoFe phosphate NSs/NF as the electrocatalyst for both the anode and the cathode, a water splitting electrolyzer was able to reach 10 mA cm-2 at a cell voltage of 1.52 V with robust durability. Various characterization techniques indicated that the long term stability and the activity for overall water splitting are due to the porosity, the electrochemically active constituents, and synergistic effects. This work could be inspiring in the design of Earth abundant and highly efficient electrocatalysts for overall water splitting, especially for OER.