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 because liquid fuels are suitable for portable applications compared to hydrogen gas, that is, they can be easy handled, stored, and transported. Methanol is recognized as a promising fuel for DAFCs, and the anode reaction in direct methanol fuel cells (DMFCs) or the electrooxidation of methanol has been extensively studied in order to improve performance of DMFCs and to achieve cost reduction. However, 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.

Among the anode reaction in DAFCs, the electrooxidation of methanol on platinum has been studied the most because Pt is an excellent electrocatalyst for the oxidation. The oxidation occurs at the anode in the presence of water:

CH₃OH + H₂O → CO₂ + 6H⁺ + 6e^-. (1)

It is well accepted that the oxidation proceeds via a dual path mechanism consisting of indirect and direct paths. The indirect path involves adsorbed carbon monoxide (CO_ad) 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 CO_ad. 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, 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 CO_ad reacts with methanol to form methyl formate (HCOOCH₃):

CO_ad + CH₃OH → HCOOCH₃, (2)

and also that the main current producing reaction without water is the formation of methyl formate (HCOOCH₃) via non-CO pathway:

2CH₃OH → HCOOCH₃ + 4H⁺ + 4e^-. (3)

Pt is also known to be active for the electrooxidation of ethanol in aqueous solution. Complete oxidation of ethanol to CO₂ is expressed as follows:

C₂H₅OH + 3H₂O → 2CO₂ + 12H⁺ + 12e^-. (4)

The cleavage of the C–C bond, which is involved in the complete oxidation, requires a high activation energy, and thus most ethanol is incompletely oxidized to acetic acid or acetaldehyde in acid media:

C₂H₅OH + H₂O →CH₃COOH + 4H⁺ + 4e^-, (5)

C₂H₅OH → CH₃CHO + 2H⁺ + 2e^-. (6)

We have recently found that ethanol is also oxidized at Pt electrode in the absent of water (CVs shown in Figure 1). From the analogy of the reaction between CO_ad and methanol (reaction 2), it can be deduced that CO_ad reacts with ethanol to form ethyl formate (HCOOC₂H₅):

CO_ad + C₂H₅OH → HCOOC₂H₅. (7)

However, CO_ad is found to be hardly removed in the absent of water (SEIRA spectra shown in Figure 1). In spite of this, the current increases monotonically as the electrode potential (E) increases in the potential region above ca. 0.4 V vs. SHE, indicating that the electrode surface is not completely covered with CO_ad. It is thus possible that the formation of acetaldehyde (reaction 6) proceeds at vacant sites at E > 0.4 V, which is in a good agreement with literature data [3]. In the presentation, we will discuss how ethanol is oxidized without water.

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.

[3] O. Guillén-Villafuerte, G. García, M. C. Arévalo, J. L. Rodríguez, E. Pastor, Electrochem. Comm., 63 (2016) 48.

FIGURE CAPTION

Figure 1 Cyclic voltammograms for the oxidation of ethanol at a sweep rate of 0.1 V/s and simultaneously-measured SEIRA spectra. The concentration of ethanol is 1 and 17 M. The reference spectrum was taken with 0.5 M H₂SO₄ in the absence of ethanol at 0.05 V.