Nucleophilicity and electrophilicity dictate the reactivity of polar organic reactions. In the past decades, Mayr et al. established a quantitative scale for nucleophilicity (N) and electrophilicity (E), which proved to be useful tools for the rat...
Temperature Ramp Strategy for Regulating Oxygen Vacancies in NiCo2O4 Nanoneedles Towards Enhanced Electrocatalytic Water Splitting
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Effect of structural tuning on the acidic HER credentials of NiCo2O4 nanoneedles is achieved through the temperature ramp method. The catalyst prepared at a temperature ramp of 10 °C/min (NCO-10) depicts lower overpotential relative to catalysts prepared at 5 °C/min (NCO-5) and 2 °C/min (NCO-2) for acidic HER.
Oxide-based systems often suffer from higher overpotentials compared to transition metal sulfides and phosphides for the electrochemical hydrogen evolution reaction (HER). Interestingly, the generation of oxygen vacancy/defect has been seen as the strategy for further activating transition metal oxides (NiCo2O4 as a model system) for an electrochemical water-splitting process. Herein, we employ the temperature ramp strategy (ambient air calcination) for the generation of oxygen vacancies in NiCo2O4 (NCO) towards the tuning of electrocatalytic enhancements. The NiCo2O4 synthesized at temperature ramp rates of 2 °C/min (NCO-2), 5 °C/min (NCO-5), and 10 °C/ min (NCO-10) depicts contrasting structural features and varying Ni : Co : O surface composition. The decrease in the crystallite size and converse trend in the particle strain were observed from NCO-2 to NCO-10. Interestingly, the surface Ni : Co : O ratios of 1 : 0.78 : 3.6, 1 : 0.81 : 3.3, and 1 : 0.69 : 2.8 for NCO-2, NCO-5, and NCO-10, respectively, were observed. The reduced relative oxygen ratio in the latter implies the generation of an ample amount of oxygen vacancy defects. HER performance depicts a consistent trend with enhanced oxygen defect concentration with the overpotential requirement of 700, 647, and 597 mV for NCO-2, NCO-5, and NCO-10, respectively, for the generation of a cathodic current of 25 mA cm−2. The same trend in an electrocatalytic enhancement is observed for other cathodic currents.Zum Volltext
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