Modification d x 2 - y 2 ${{d}_{{{x}^2} - {{y}^2}}}$ Orbital Electronic States in Nickel-Based Hydroxides Via Cobalt/Iron Co-Doping for High-Efficiency Methanol Electrooxidation.

The nickel hydroxide-based (Ni(OH)2) methanol-to-formate electrooxidation reaction (MOR) performance is greatly related to the d x 2 - y 2 ${{d}_{{{x}^2} - {{y}^2}}}$ orbital electronic states. Hence, optimizing the d x 2 - y 2 ${{d}_{{{x}^2} - {{y}^2}}}$ orbital electronic states to achieve enhanced MOR activities are highly desired. Here, cobalt (Co) and iron (Fe) doping are used to modify the d x 2 - y 2 ${{d}_{{{x}^2} - {{y}^2}}}$ orbital electronic states. Although both dopants can broaden the d x 2 - y 2 ${{d}_{{{x}^2} - {{y}^2}}}$ orbital; however, Co doping leads to an elevation in the energy level of d x 2 - y 2 ${{d}_{{{x}^2} - {{y}^2}}}$ highest occupied crystal orbital (HOCO), whereas Fe doping results in its reduction. Such a discrepancy in the regulation of d x 2 - y 2 ${{d}_{{{x}^2} - {{y}^2}}}$ orbital electronic states stems from the disparate partial electron transfer mechanisms amongst these transition metal ions, which possess distinct energy level and occupancy of d orbitals. Motivated by this finding, the NiCoFe hydroxide is prepared and exhibited an excellent MOR performance. The results showed that the Co dopants effectively suppress the partial electron transfer from Ni to Fe, combined with the d x 2 - y 2 ${{d}_{{{x}^2} - {{y}^2}}}$ orbital broadening induced by NiO6 octahedra distortion, endowing NiCoFe hydroxide with high d x 2 - y 2 ${{d}_{{{x}^2} - {{y}^2}}}$ HOCO and broad d x 2 - y 2 ${{d}_{{{x}^2} - {{y}^2}}}$ orbital. It is believed that the work gives an in-depth understanding on d x 2 - y 2 ${{d}_{{{x}^2} - {{y}^2}}}$ orbital electronic states regulation in Ni(OH)2, which is beneficial for designing Ni(OH)2-based catalysts with high MOR performance.