Complex Electrooxidation of Formic Acid on Palladium

Abstract

Herein, oscillatory formic acid (FA) electrooxidation on polycrystalline palladium is investigated and compared with the one on polycrystalline platinum; major differences between both are attributed to differences on the kinetics of sub-set chemical network as well as to preferential routes admitted on palladium surface. To presume the kinetic rate of poison accumulation on palladium, FA oxidation was accomplished in presence of occluded hydrogen and hydrogen-free electrodes. The preferential routes were presumed from the temporal pattern. Markedly, oscillations during FA electrooxidation have minimum potential at 0.2 V, which is linked to the fast rate of direct dehydrogenation; moreover, it has one of the largest induction period (ca. 60 min) and oscillatory period (20 min) observed in electrochemistry, since subsurface hydrogen slows down the rate of CO accumulation on the surface

Coupled Dynamics of Anode and Cathode in Proton‐Exchange Membrane Fuel Cells

“…A point that was never addressed is what happens to the cathode when the fuel cell is operated in an oscillatory regime. It is well known that the electro‐oxidation of small organic molecules is subject to oscillatory kinetics when the reaction current is kept constant (galvanostatic control) or under potentiostatic control when the resistance between the working electrode and the potentiostat exceeds a critical value . There are various studies on how autonomous oscillatory behavior occurs in fuel cells based on the electro‐oxidation of H 2 /CO and, recently, methanol and formic acid .…”

Dependência da temperatura e do pH da solução e de outros parâmetros na eletro-oxidação de moléculas orgânicas pequenas

“…Contrary to Pt, the voltammograms reveal one main oxidation peak with its maximum between 0.3 and 0.5 V and a shoulder at more positive potentials between 0.7 and 0.8 V in dependence on the formic acid concentration in good agreement to earlier studies [84,191,192]. Meanwhile the main peak may represent the active formic acid oxidation via the direct pathway, the side peak may be related to oxidation of trace CO ad , which suggests a minor activity of the indirect pathway.…”

“…Yet, by increasing the formic acid bulk concentration, periodic oscillations could also be found, which show a wide amplitude between 0.25 and 1 V and period 200-300 s. Both parameters are significantly higher than on Pt. The absence of any oscillation pattern at higher applied current densities and the oscillating behaviour are in good agreement to the literature and are sign again, that CO formation is not a highly active process like on Pt and the surface is faster covered by oxygenated species, leading to a fast increase to potentials in the oxygen evolution [84,192]. As it has been shown in this work and others, the oscillatory regime can allow another point of view on the electro-oxidation of small organic molecules and shed more light in the complex reaction mechanism and related processes.…”

“…This interpretation is supported by the slight hysteresis between anodic and cathodic scan, suggesting a highly reversible, almost uninhibited formic acid oxidation as it is found on Au electrodes [76]. This behaviour also is reported in literature and highlights the catalytic advantages of Pd over Pt [192]. The performed galvanostatic measurements for the oscillatory analysis of formic acid oxidation on Pd are plotted in Figure 71a) and b) for both applied concentrations, which confirm the interpretations from the steady state voltammetry.…”

“…Considering the higher availability and less costs in production, Pd electrodes may be the favoured, noble metal compared to Pt. Yet, to the best of my knowledge there are no studies on the electro-oxidation of small organic molecules on Pd in the oscillatory regime, except few for formic acid [84,191,192]. The steady state voltammograms of a Pd bulk electrode in 0.5 and 2 M HCOOH+ 0.5 H 2 SO 4 are plotted in Figure 70.…”