Potential Oscillations during Oxidation of Methanol on Pt Electrode in the Absence of Water

Abstract

The electrochemical oxidation of methanol on Pt has been extensively studied in order to develop direct methanol fuel cells. On the anode of the fuel cells, the oxidation reaction occurs in the presence of water: CH3OH + H2O → CO2 + 6H+ + 6e-. (1) It is well accepted that the oxidation reaction proceeds via a dual path mechanism consisting of indirect and direct paths. The indirect path involves adsorbed carbon monoxide (COad) as a poisoning intermediate, and the direct path involves a non-CO intermediate as a reactive one. Although water, which adsorbs and produces hydroxide and oxide on Pt, plays an important role in oxidative removal of COad, little studies have been carried out with respect to the methanol oxidation in the absence of water. We, therefore, have studied if the methanol oxidation proceeds without water. As reported in our previous study [1], the oxidation produced current in the absence of water, and the current was larger than that in the presence of water, e.g., 0.1 and 1 M methanol aqueous solutions (see Figure 1 for 0.1 M solution). The ability of producing current for the methanol oxidation without water is due to desorption of COad, which has been directly confirmed by surface-enhanced infrared absorption spectroscopy. Accordingly, we have proposed that the CO desorption is via the reaction: COad + CH3OH → HCOOCH3, (2) and also that the current producing reactions is: CH3OH → HCHO + 2H+ + 2e- (3) or 2CH3OH → HCOOCH3 + 4H+ + 4e-. (4) It is well known that the electrode potential spontaneously oscillates when the methanol oxidation in aqueous solutions is performed under galvanostatic conditions. In general, an N-shaped negative differential resistance (N-NDR) plays an important role in an electrochemical oscillation, because it causes a positive feedback mechanism. On the analogy of an N-NDR that is observed for the oxidation of formic acid in aqueous solutions [2], it is most likely that an N-NDR, which is involved in the direct path of the methanol oxidation in aqueous solutions, is attributed to the adsorption of water on the electrode surface, that is, the methanol oxidation is suppressed by the water adsorption. Surprisingly, the potential oscillation has been found to occur in the absence of water [1]. This clearly indicates that there exists an N-NDR that is not attributed to the water adsorption; hence we have proposed that the adsorbed methanol acts as a site blocker at the same time as a reactant. In this work, to verify this, the methanol oxidation without water will be studied in detail. REFERENCES [1] H. Okamoto, T. Gojuki, N. Okano, T. Kuge, M. Morita, A. Maruyama, Y. Mukouyama, Electrochim . Acta, 136, 385-395 (2014). [2] T. Gojuki, Y. Numata, Y. Mukouyama, H. Okamoto, Electrochimica Acta, 129, 142-151 (2014). This work was partially supported by Research Institute for Science and Technology of Tokyo Denki University under Grant Q16K-02. FIGURE CAPTION Figure 1 (left) Voltammograms for the oxidation of methanol at a sweep rate of 0.1 V/s, and (right) waveforms of potential oscillations observed at (top) 0.3 and (bottom) 0.8 mA. The concentration of methanol is (top) 0.1 and (bottom) 25 M. Figure 1