Modulating the Surface Electronic Structure of Active Ni Sites by Engineering Hierarchical NiFe-LDH/CuS over Cu Foam as an Efficient Electrocatalyst for Water Splitting.

Water electrolysis encounters a challenging problem in designing a highly efficient, long durable, non-noble metal-free electrocatalyst for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, in our work, a two-step hydrothermal reaction was performed to construct a hierarchal NiFe-layer double hydroxide (LDH)/CuS over copper foam for the overall water splitting reaction. While employed the same as an anode material, the designed heterostructure electrode NiFe-LDH/CuS/Cu exhibits excellent OER performance and it demands 249 mV overpotential to reach a current density of 50 mA cm -2 with a lower Tafel slope value of 81.84 mV dec -1 . While as a cathode material, the NiFe-LDH/CuS/Cu shows superior HER performance and it demands just 28 mV of overpotential value to reach a current density of 10 mA cm -2 and a lower Tafel slope value of 95.98 mV dec -1 . Hence, the NiFe-LDH/CuS/Cu outperforms the commercial Pt/C and RuO 2 in terms of activity in HER and OER, respectively. Moreover, when serving as both the cathode and anode catalysts in an electrolyzer for total water splitting, the synthesized electrode only needs a cell potential of 1.55 V versus RHE to reach a current density of 20 mA cm -2 and long-term durability for 25 h in alkaline media. To study the interfacial electron transfer, Mott-Schottky experiments were performed, representing that the electron is transferred from n-type NiFe-LDH to p-type CuS as a result of creating the p-n junction in NiFe-LDH/CuS/Cu. The formation of this p-n junction allows the LDH layer to be more active toward the OH- adsorption and thereby could allow the OER or HER with a less energy input. This work affords another route to a cost effective, highly efficient catalyst toward producing clean energy across the globe.