Valence-State Effect of Iridium Dopant in NiFe(OH)₂ Catalyst for Hydrogen Evolution Reaction

Engineering high-performance electrocatalysts is of great importance for energy conversion and storage. As an efficient strategy, element doping has long been adopted to improve catalytic activity, however, it has not been clarified how the valence state of dopant affects the catalytic mechanism and properties. Herein, it is reported that the valence state of a doping element plays a crucial role in improving catalytic performance. Specifically, in the case of iridium doped nickel-iron layer double hydroxide (NiFe-LDH), trivalent iridium ions (Ir³⁺ ) can boost hydrogen evolution reaction (HER) more efficiently than tetravalent iridium (Ir⁴⁺ ) ions. Ir³⁺ -doped NiFe-LDH delivers an ultralow overpotential (19 mV @ 10 mA cm^-2 ) for HER, which is superior to Ir⁴⁺ doped NiFe-LDH (44 mV@10 mA cm^-2 ) and even commercial Pt/C catalyst (40 mV@ 10 mA cm^-2 ), and reaches the highest level ever reported for NiFe-LDH-based catalysts. Theoretical and experimental analyses reveal that Ir³⁺ ions donate more electrons to their neighboring O atoms than Ir⁴⁺ ions, which facilitates the water dissociation and hydrogen desorption, eventually boosting HER. The same valence-state effect is found for Ru and Pt dopants in NiFe-LDH, implying that chemical valence state should be considered as a common factor in modulating catalytic performance.