Water chemistry at pure and Ni-modified cobalt oxyhydroxide (CoOOH, Ni-CoOOH) nanowires was investigated using ambient-pressure photoelectron spectroscopy (APPES) under different water vapor pressures and temperatures. This study was motivated by the large number of theoretical calculations and empirical results on transition metal (oxy)hydroxide materials demonstrating excellent activity for the oxygen evolution reaction (OER). The promoting effect of Ni for the OER activity of CoOOH was previously explained in terms of improved stability of a surface reaction intermediate by DFT calculations and impedance analysis of electrochemical data. However, no direct evidence was available to correlate the stability of surface bound species such as adsorbed hydroxyl groups (OHads) with the presence of Ni. Using APPES, the dissociation of H2O on CoOOH and Ni-CoOOH, and the thermal stability of surface species, were monitored for the first time on this important class of OER catalysts. With an increase in the H2O pressure, dissociation of H2O on both catalysts was observed, characterized by the increase in the OHads (Co-OH, or Ni-OH) and lattice OH (Olattice-H) components in the O 1s core-level spectra. Interestingly, the onset of surface hydroxylation for Ni-CoOOH occurred at lower H2O vapor pressure compared to the condition required for hydroxylation on CoOOH. Isobaric experiments performed at 1 torr H2O vapor pressure revealed that a significant fraction of OHads and Olattice-H groups were lost at 220 °C and the complete transformation of CoOOH to Co3O4 occurred by 320 °C. In stark contrast, OHads on Ni-CoOOH exhibited much greater stability and a substantial fraction still remained on the nanowire surfaces at 220 °C. Furthermore, divalent Co ions (Co2+) were the dominant species upon heating Ni-CoOOH to 320 °C, which can be contrasted with Co3O4, and suggests the formation of a CoO-like structure.

Interface chemistry of H2O on pure and Ni-modified CoOOH nanowires probed by ambient-pressure x-ray photoelectron spectroscopy

254th National Meeting (2017)

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