Fe in the Alkaline Electrolyte Causes Poisoning of the Methanol Oxidation Reaction at Ni-Oxyhydroxide Unrelated to the Competing Oxygen Evolution Reaction

Ni-oxo/hydroxide (NiOxHy) is a catalyst for the methanol oxidation reaction (MOR), and Fe-doped NiOxHy is a catalyst for the oxygen evolution reaction (OER) in an alkaline electrolyte. The spontaneous inclusion of Fe in NiOxHy from traces in alkaline medium as low-coordination surface sites (Fesurface) facilitates OER, and, in this work, we examined the effect of these sites on MOR through electrochemical measurements comparing MOR catalysis at thin, amorphous NiOxHy that are mostly surface in alkaline electrolyte with Fe traces (KOH) or purified of Fe (Fe-free KOH). The presence of Fe traces was found to reduce the MOR current stability and formate yield during hours of operation beyond the known role in facilitating the competing redox-mediated OER. MOR catalysis was markedly more stable in alkaline electrolyte stripped of Fe; a NiOxHy film retained 99% of the initial MOR rate after 24 h at OER onset bias in Fe-free KOH. Formate production was indicated by 13C and 1H NMR and by ion chromatography and was ca.1.5-fold greater in Fe-free KOH at OER onset and with greater faradaic efficiency. The yield was also higher in Fe-free KOH prior to the onset of the OER with similar faradaic efficiencies. We hypothesize, to reconcile electrochemical observations, including electrochemical impedance spectroscopy, that Fesurface in NiOOH renders the electrocatalyst prone to poisoning by a reaction intermediate, while slower kinetics are indicated only at low anodic bias in the presence of dual Ni(OH)2/NiOOH sites with Fesurface inclusion. Sustainable MOR catalysis at NiOxHy necessitates that the electrochemical cell be stripped of Fe.