In Situ Reconstructing NiFe Oxalate Toward Overall Water Splitting

Surface reconstruction plays an essential role in electrochemical catalysis. The structures, compositions, and functionalities of the real catalytic species and sites generated by reconstruction, however, are yet to be clearly understood, for the metastable or transit state of most reconstructed structures. Herein, a series of NiFe oxalates (Ni_xFe_{1- x}C₂O₄, x = 1, 0.9, 0.7, 0.6, 0.5, and 0) are synthesized for overall water splitting electrocatalysis. Whilst Ni_xFe_1-xC₂O₄ shows great hydrogen evolution reaction (HER) activity, the in situ reconstructed Ni_xFe_1-xOOH exhibits outstanding oxygen evolution reaction (OER) activity. As identified by the in situ Raman spectroscopy and quasi-in situ X-ray absorption spectroscopy (XAS) techniques, reconstructions from Ni_xFe_1-xC₂O₄ into defective Ni_xFe_1-xOOH and finally amorphous Ni_xFe_1-xOOH active species (R-Ni_xFe_1-xOOH) are confirmed upon cyclic voltammetry processes. Specifically, the fully reconstructed R-Ni_0.6Fe_0.4OOH demonstrates the best OER activity (179 mV to reach 10 mA cm^-2), originating from its abundant real active sites and optimal d-band center. Benefiting from the reconstruction, an alkaline electrolyzer composed of a Ni_0.6Fe_0.4C₂O₄ cathode and an in situ reconstructed R-Ni_0.6Fe_0.4OOH anode achieves a superb overall water splitting performance (1.52 V@10 mA cm^-2). This work provides an in-depth structure-property relationship understanding on the reconstruction of catalysts and offers a new pathway to designing novel catalyst.