Removal of Adsorbed CO on Pt Electrode By Reaction with Alcohols
Authors: Iida Keisuke , Mukouyama Yoshiharu 
Published: February 2020

Direct alcohol fuel cells (DAFCs), which mostly use low-molecular weight alcohols such as methanol and ethanol as fuels, have attracted considerable interest in recent years. Methanol is recognized as a promising fuel for DAFCs, and thus the anode reaction in direct methanol fuel cells (DMFCs), namely, the electrooxidation of methanol, has been extensively studied in order to improve performance of DMFCs and to achieve cost reduction. Among various electrocatalysts for the oxidation, Pt-based anodes show excellent electrocatalytic activities. Complete electrooxidation of a methanol molecule, which is necessary for efficient oxidation, gives six electrons:
CH3OH + H2O → CO2 + 6H+ + 6e-. (1)
It is well accepted that the oxidation reaction proceeds on Pt electrodes 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.
Water produces oxygenated spices such as hydroxide and oxide, and it plays an important role in removal of COad. Thus, it seems that the oxidation does not proceed without water. However, as we have reported previously [1, 2], the methanol oxidation produces current without water (panel a1 in Figure 1), and the current is of the same order of magnitude as that observed in the presence of water, e.g., 0.1 and 1 M methanol aqueous solutions. A detailed study using surface-enhanced infrared absorption spectroscopy (SEIRAS) and high-performance liquid chromatography (HPLC) has revealed that COad reacts with methanol to form methyl formate (HCOOCH3):
COad + CH3OH → HCOOCH3, (2)
and also that the current producing reactions without water is partly the formation of formaldehyde (HCHO) and mainly that of methyl formate (HCOOCH3) via non-CO pathway:
CH3OH → HCHO + 2H+ + 2e- (3)
and 2CH3OH → HCOOCH3 + 4H+ + 4e-. (4)
Methanol is toxic for human which is one of the disadvantages of using methanol as fuel, and most methanol is produced from natural gas. On the other hand, ethanol is non-toxic, and it can be sustainably produced on a large scale from biomass. Thus, in these regards, direct ethanol fuel cells (DEFCs) have a strong advantage over DMFCs. Moreover, ethanol has a higher energy density than methanol, that is, complete electrooxidation of ethanol with water gives twelve electrons:
C2H5OH + 3H2O → 2CO2 + 12H+ + 12e-. (5)
The cleavage of the C–C bond, which is involved in the complete oxidation, requires a high activation energy. Thus, even when Pt is used as the anode, most ethanol is incompletely oxidized to acetic acid which is furthr oxizied to acetaldehyde:
C2H5OH → CH3CHO + 2H+ + 2e-. (6)
CH3CHO + H2O →CH3COOH + 2H+ + 2e-. (7)
We have recently found that ethanol is also oxidized at Pt electrode in the absent of water (panel b1). From the analogy of the reaction between COad and methanol (reaction 2), it can be deduced that COad reacts with ethanol to form ethyl formate (HCOOC2H5):
COad + C2H5OH → HCOOC2H5. (8)
The coverage of COad gradually decreases and the current increases as the electrode potential (E) increases in the potential region above 0.4 V vs. SHE. It is thus likely that the number of vacant sites on Pt surface increases gradually due to reaction 8 and reaction 6 proceeds at vacant sites at E > 0.4 V.
SEIRA measurements show that the removal of COad in the 24 M methanol solution is much faster than that in the 17 M ethanol solution (panels a2 and b2). This indicates that COad reacts easily with methanol whereas it hardly reacts with ethanol, which we will be disucssed in the presentation.

REFERENCES

[1] H. Okamoto, T. Gojuki, N. Okano, T. Kuge, M. Morita, A. Maruyama, Y. Mukouyama, Electrochim
.
Acta, 136 (2014) 385.
[2] Y. Mukouyama, S. Yamaguchi, K. Iida, T. Kuge, M. Kikuchi, S. Nakanishi, ECS Trans., 80 (2017) 1471.

FIGURE CAPTION

Figure 1 (top) Cyclic voltammograms for the oxidation of (a1) methanol and (b1) ethanol at a sweep rate of 0.1 Vs-1. The concentration of methanol was 24 M, and that of ethanol was 17 M. The high-concentration alcohol solutions contained 0.5 M H2SO4 as an electrolyte. (bottom) The dependence of the coverage of COad on E for (a2) the 24 M metanol oxidation and (b2) the 17 M ethanol oxidation, evaluated from SEIRA measurements.

Figure 1