The oxygen evolution reaction (OER) is a kinetically sluggish and particularly efficiency-limiting reaction with four-electron redox process, which often requires a high overpotential to break O–H bond and form O–O bond. Some state-of-the-art precious-metal electrocatalysts and their oxides (IrO2 and RuO2) endowed with high OER activity are seriously restricted by their scarcity, high cost, and inferior stability leading to low market penetration of these catalysts. Transition metal based nanocomposites are regarded as cheap and stable OER electrocatalysts for mass production of oxygen. In particular, NiFe (oxy)hydroxide-based catalysts are among the best-performing nonprecious OER catalysts in alkaline media likely attributed to a strong interaction upon incorporating Fe into NiOOH. It's widely believed that the performance of one-component OER catalyst may be further boosted through being combined with another active component for OER benefiting from enhanced electron transfer efficiency and intense synergetic effects within the strong coupled interface of different components, which becomes one of the most important research targets in the field of OER electrocatalyst.
Herein, by choosing self-supported Ni-Co-P two-dimensional (2D) nanosheets fabricated on the commercial carbon cloth (NiCoP/CC) through a facile electro-deposition route integrated with succeeding in-situ phosphorization process, we present a novel architecture comprising NiCoP/CC followed by electro-depositing NiFe-LDH nanosheets (NiFe-LDH/NiCoP/CC) to form an open three-level hierarchy via the interaction of p-n heterojunction at the semiconductor-semiconductor interface of NiFe-LDH/NiCoP/CC hierarchical nanosheet structure, where NiCoP and NiFe-LDH manifest as p- and n-type semiconductor respectively. Based on the Tauc plots and Mott-Schottky plots, the positions of the conduction band (CB) and the valence band (VB) for NiFe-LDH/NiCoP/CC have been determined, which demonstrates the p-n heterojunction catalyst can be a great boost for the OER through smoothing and promoting electron transfer among NiFe-LDH, NiCoP, and CC. The performance test can further confirm that the catalyst with p-n heterojunction effect deliver extremely low overpotentials (η) of 216, 244, and 255 mV at current densities (j) of 10, 100, and 300 mA cm-2 respectively; a small Tafel slope of 35.7 mV dec-1, and great long-term durability in 1 M KOH electrolyte. Impressively, the j at 1.485 V (vs RHE) of NiFe-LDH/NiCoP/CC catalyst is ~10 and ~30 times as those of NiCoP/CC and NiFe-LDH/CC rather than a simple addition, to which the strong p-n heterojunction synergic effect is a key contribution. This proof-of-concept strategy of utilization of p-n heterojunction synergic effect enables the exploration of more efficient and economic OER electrocatalysts and exploits a promising avenue for functional nanocatalysts for use in clean energy technologies.