Effect of Ni/NiO Heterostructure on Hydrogen Evolution Performance in AEM Water Electrolysis: In-Operando Raman Study

The effect of the presence of NiO in Ni-based catalysts (Ni/NiO heterostructures) on the kinetics of the hydrogen evolution reaction (HER) has been studied ex-situ and simulated by kinetic models provided by Oshchepkov et al. 1 , 2 A tenfold increase in the activity for the HER has been observed. The kinetic model also shows a shift from the Heyrovsky–Volmer mechanism to the Tafel–Volmer mechanism due to the presence of NiO. 1 , 2 Danilovic et al. provided a four times increase of the HER activity of Ni/ Ni(OH) 2 in an alkaline solution. 3 Another result by Bates et al. 4 showed enhanced HER at adjacent Ni/NiO x sites. In these catalysts metallic Ni behaves as a basin for the H ads intermediate of the HER, while the oxide behaves as a basin for OH ads , thus increasing reaction rates of the HER via water reduction. 4 NiO or β-Ni(OH) 2 is expected to partially cover the Ni surface. The oxidation state for Ni cannot be determined without additional in-situ studies. The precise identification of the catalytic active sites and elucidation of the reaction mechanisms in these systems remain a grand challenge. Our results confirm that Ni/NiO heterostructure has a higher HER activity compared to bare Ni or NiO. Our Ni/NiO heterostructure achieves -10 mA/cm 2 at low overpotential (226 mV) compared to bare Ni (354 mV) or NiO (372 mV) in 1 mol dm -3 KOH. In-situ confocal Raman spectroscopy with cyclic voltammetry (CV) can provide useful information about which surface species are being produced and reduced during cycling and the form in which they exist in-situ, which will help explaining the reasons for the superior performance of Ni/NiO heterostructure. In this work, we have performed in-situ Raman studies of the electrochemical activity of Ni, NiO and Ni/NiO heterostructures for the HER under alkaline conditions (0.1 and 1 mol dm -3 KOH) and deionized water only. Intermediate formation and structural changes have been evaluated during linear sweep voltammetry and cyclic voltammetry and their relation to the kinetic enhancement for the HER in Ni-NiO heterostructures. In-situ Raman was used to analyze the degradation of Ni/NiO heterostructures under chronoamperometric measurements at different applied overpotentials. References Oshchepkov, A. G., Bonnefont, A., Parmon, V. N. & Savinova, E. R. On the effect of temperature and surface oxidation on the kinetics of hydrogen electrode reactions on nickel in alkaline media. Electrochim. Acta (2018). doi:10.1016/j.electacta.2018.02.106 Oshchepkov, A. G. et al. Exploring the Influence of the Nickel Oxide Species on the Kinetics of Hydrogen Electrode Reactions in Alkaline Media. Top. Catal. 59, 1319–1331 (2016). Danilovic, N. et al. Enhancing the Alkaline Hydrogen Evolution Reaction Activity through the Bifunctionality of Ni(OH) 2 /Metal Catalysts. Angew. Chemie 124, 12663–12666 (2012). Bates, M. K., Jia, Q., Ramaswamy, N., Allen, R. J. & Mukerjee, S. Composite Ni/NiO-Cr 2 O 3 catalyst for alkaline hydrogen evolution reaction. J. Phys. Chem. C 119, 5467–5477 (2015).