Investigating Redox States and Reaction Dynamics of Ni-based Nano-Catalysts for Alkaline Water Splitting

• Design and synthesis of highly active and cost-effective electrocatalysts for hydrogen and oxygen generation by water electrolysis can be of paramount importance, as hydrogen has been considered as one of the most promising energy alternatives to traditional fossil fuel-based energy because of its high specific energy density and potentially clean production. Here, we investigate the interfacial cause-effect-relationships in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) for Ni-based nano-catalysts by modifying the structure and  performing in-situ characterization during the reaction. Based on different pH condition during synthesis of NiO nanoflakes (NFs), we show that the crystal growth direction and morphology of the NiO nanostructure can be altered according to the different surface charges, which control the exposure of the surface active sites, resulting in much improvedcatalytic performance. Additionally, by incorporating a redox active dopant (Fe), NiO can be tuned into higher catalytic efficiency and becomes bi-functional with respect to the OER and the HER processes under alkaline conditions. Exploring the 3D synergistic NiFe nano-layered double hydroxide, we find improved catalytic performance after prolonged use, where the current density increases from 9.3 mA cm-2 to 12.7 mA cm-2 during 100 h running at 1.7 V without iR compensation in a 2-electrode system. In order to understand the function and precise mechanism of metal doping and the synergistic effect to improve the catalytic property after prolonged use, we utilize in situ Raman spectroscopic and in situ electrochemical impedance spectroscopy (EIS) to monitor the interfacial redox state and reaction dynamics. As we all know, the material structure plays a vital role on improving the electrocatalytic property and stability. The structural and physical characterization on 3D ultrathin NiFe nano-layered double hydroxide were also investigated with XRD, SEM, TEM and XPS before and after 100 h electrolysis in a two electrode configuration in 1 M KOH at room temperature. The structure changes after the oxygen evolution reaction are minor, while, after the hydrogen evolution reaction, the catalyst undergoes recrystallization as observed by selected area electron diffraction (SAED), with markedly improved electrocatalytic activity. The successful identifications of the underlying reasons for the electrocatalytic improvement open the possibilities for a rational design of Ni-based nano-catalysts, and possibly also for other material systems for use as efficient electrocatalysts for practical alkaline HER and OER processes.

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