Tailor-designed nanowire-structured iron and nickel oxides on platinum catalyst for formic acid electro-oxidation

This investigation is concerned with designing efficient catalysts for direct formic acid fuel cells. A ternary catalyst containing iron (nano-FeOx) and nickel (nano-NiOx) nanowire oxides assembled sequentially onto a bare platinum (bare-Pt) substrate was recommended for the formic acid electro-oxidation reaction (FAOR). While nano-NiOx appeared as fibrillar nanowire bundles (ca. 82 nm and 4.2 μm average diameter and length, respectively), nano-FeOx was deposited as intersecting nanowires (ca. 74 nm and 400 nm average diameter and length, respectively). The electrocatalytic activity of the catalyst toward the FAOR depended on its composition and loading sequence. The FeOx/NiOx/Pt catalyst exhibited ca. 4.8 and 1.6 times increases in the catalytic activity and tolerance against CO poisoning, respectively, during the FAOR, relative to the bare-Pt catalyst. Interestingly, with a simple activation of the FeOx/NiOx/Pt catalyst at -0.5 V vs. Ag/AgCl/KCl (sat.) in 0.2 mol L^-1 NaOH, a favorable Fe²⁺/Fe³⁺ transformation succeeded in mitigating the permanent CO poisoning of the Pt-based catalysts. Interestingly, this activated a-FeOx/NiOx/Pt catalyst had an activity 7 times higher than that of bare-Pt with an ca. -122 mV shift in the onset potential of the FAOR. The presence of nano-FeOx and nano-NiOx enriched the catalyst surface with extra oxygen moieties that counteracted the CO poisoning of the Pt substrate and electronically facilitated the kinetics of the FAOR, as revealed from CO stripping and impedance spectra.