Potential Oscillations in Galvanostatic Electrooxidation of Formic Acid on Platinum:  A Time-Resolved Surface-Enhanced Infrared Study

Abstract

The mechanism of temporal potential oscillations that occur during galvanostatic formic acid oxidation on a Pt electrode has been investigated by time-resolved surface-enhanced infrared absorption spectroscopy (SEIRAS). Carbon monoxide (CO) and formate were found to adsorb on the surface and change their coverages synchronously with the temporal potential oscillations. Isotopic solution exchange (from H13COOH to H12COOH) and potential step experiments revealed that the oxidation of formic acid proceeds dominantly through adsorbed formate and the decomposition of formate to CO2 is the rate-determining step of the reaction. Adsorbed CO blocks the adsorption of formate and also suppresses the decomposition of formate to CO2, which raises the potential to maintain the applied current. The oxidative removal of CO at a high limiting potential increases the coverage of formate and accelerates the decomposition of formate, resulting in a potential drop and leading to the formation of CO. This cycle repeats itself to give the sustained temporal potential oscillations. The oscillatory dynamics can be explained by using a nonlinear rate equation originally proposed to explain the decomposition of formate and acetate on transition metal surfaces in UHV.

Oscillatory instabilities during the electrocatalytic oxidation of methanol on platinum

“…For the reference experiment, first the working electrode was polarized at 50 mV and the full CO ad monolayer was built up during CO bubbling for 10 min; afterwards the solution was purged with nitrogen for 30 min in order to remove the dissolved CO; finally, the adsorbed CO was oxidized in a potential step to 900 mV. The surface saturation by carbon monoxide was confirmed by the suppression of hydrogen adsorption and it was assumed CO-L + CO-B = 0.85 ML and CO-L + 2 CO-B = 1 ML, and is in agreement to that reported by Osawa and co-workers 48 under similar conditions. Figure 4b shows the carbon monoxide coverage as a function of time.…”

“…A remarkable smaller mean CO coverage ( CO ) of about 0.25-0.30 ML was obtained in this case. Osawa and co-workers 47,48 studied the potential oscillations in the electrooxidatioon of formic acid under similar conditions, but using Surface Enhanced Infrared Absorption Spectroscopy (SEIRAS) in a Kretschmann-type attenuated total reflection (ATR) mode, and found similar values for the mean CO . Furthermore, Osawa's set up allowed at following the changes in CO during the oscillatory electrooxidation of formic acid.…”

“…In addition, the spectroscopic determination of intermediate species known to be formed during methanol electrooxidation under regular conditions, but in oscillatory regime is a challenging task. Therefore, further studies on the Pt|H 3 COH system under oscillatory conditions using ATR-SEIRAS 47,48 would be of help to uncover the detailed mechanism responsible for the temporal instabilities discussed here.…”

“…Besides the already mentioned richer dynamic behavior, formic acid was chosen because of the considerable amount of spectroscopic data available for this system under oscillatory conditions. [46][47][48] Figure 5 shows potential oscillations obtained under galvanostatic control for the electrooxidation of formic acid on platinum and in sulfuric acid aqueous media. As observed, the potential oscillations are of relaxation-like type and about 5-6 times slower than the ones showed so far for methanol.…”

Voltage Inversion Caused by Self-organized Oscillations in a Direct Formic Acid Fuel Cell

“…These species react with CO ad via the Langmuir-Hinshelwood mechanism, releasing free sites which causes the anodic overpotential to decrease and the new cycle begins again [1] [9]. The emergence of oscillatory kinetics during the electro-oxidation of formic acid has been extensively studied with the aid of techniques such as infrared spectroscopy [12] and numerical modeling [13] [14]. Mota et al [15] have showed kinetic instabilities in a DFAFC using H 2 on the cathode instead of O 2 .…”