Electrocatalysis of Formic Acid Electro-Oxidation at Platinum Nanoparticles Modified Surfaces with Nickel and Cobalt Oxides Nanostructures

Abstract

The present study proposes a novel promising binary catalyst for formic acid electro-oxidation (FAO); the main anodic reaction in direct formic acid fuel cells (DFAFCs). The catalyst is basically composed of two metal oxides of nickel and cobalt nanostructures (i.e., NiOx and CoOx) assembled onto a platinum nanoparticles (PtNPs)−modified glassy carbon (Pt/GC) electrode. Actually, FAO proceeds at bare Pt surfaces in two parallel routes; one of them is desirable (called direct or hydrogenation) and occurred at a low potential domain (I ^d_p ). While, the other (undesirable) involves the dehydration of formic acid (FA) at low potential domain to produce a poisoning intermediate (CO), which next be oxidized (indirect, I ^ind_p ) at a higher potential domain after the platinum surface becomes hydroxylated. Unfortunately, the peak current ratio (I ^d_p /I ^ind_p ) of the two oxidation routes, which monitors the degree of the catalytic enhancement and the poisoning level, stands for bare Pt surfaces at a low value (less than 0.2). Interestingly, this ratio increased significantly as a result of the further modification of the Pt/GC electrode with NiOx $\left({I}_{\mathrm{p}}^{\mathrm{d}}/{I}_{\mathrm{p}}^{\mathrm{ind}}=3\right)$, CoOx $\left({I}_{\mathrm{p}}^{\mathrm{d}}/{I}_{\mathrm{p}}^{\mathrm{ind}}=4\right)$ and a binary mixture of both $\left({I}_{\mathrm{p}}^{\mathrm{d}}/{I}_{\mathrm{p}}^{\mathrm{ind}}=15\right)$. This highlights the essential role of these in promoting the direct FAO, presumably via a mediation process that ultimately improved the oxidation kinetics or through a catalytic enhancement for the oxidation of the poisoning CO at the low potential domain of the direct FAO. The effect of the deposition order of NiOx and CoOx on the catalytic activity was addressed and fount influencing. The addition of CoOx to the catalyst was really important, particularly in improving the catalytic stability of the catalyst towards a long-term continuous electrolysis experiment, which actually imitates the real industrial applications.