Advancing Alkaline Water Electrolysis Using Electrochemical Reconstruction‐Driven Nickel Oxalate/Iron Oxide Heterostructure to Ni/Fe Hydroxide‐(Oxy)Hydroxide Electrocatalyst

The electrochemical reconstruction of heterostructures emerged as a striking strategy for crafting high-performance electrode materials to boost the alkaline oxygen evolution reaction (OER). This work reports the synthesis of nickel oxalate/hematite (NiOX/Fe₂O₃) heterostructure, exhibiting enhanced porosity, relocation of the electronic distribution, optimal hydrophilicity, and abundant Ni-Fe heterojunctions. Upon electrochemical reconstruction, the heterostructure transforms into Ni/Fe hydroxide-(oxy) hydroxide (named NiOX/Fe₂O₃-AP (after precondition)), demonstrating a significantly low overpotential of 234 ± 3 mV @10 mA/cm² _geo compared to individual activated counterparts. In addition, the NiOX/Fe₂O₃-AP shows swift reaction kinetics and 100 h stability across 10-100 mA/cm² _geo on redox-inactive carbon paper. The comprehensive electrochemical studies of NiOX/Fe₂O₃-AP reveal that improved active site density, higher intrinsic activity, and facile charge migration play a key role in the noted better activity. Furthermore, NiOX/Fe₂O₃-AP displayed 90 ± 4% Faradaic efficiency and improved geometric competency compared to the physical mixture, emphasizing the importance of Ni/Fe synergy. Moreover, the inferior prowess of NiFe-LDH and FeOOH compared to NiOX/Fe₂O₃-AP highlights the significance of the primitive Ni-Fe heterojunction and their subsequent electrochemical reconstruction to Ni/Fe hydroxide-(oxy) hydroxide. Finally, the electrolyzer was designed using NiOX/Fe₂O₃-AP as an anode and Pt/C as a cathode, which achieved 1.518 ± 0.006 V cell potential at 10 mA/cm² _geo, sustaining 80 h across 10-100 mA/cm² _geo.