(Invited) A Kinetics Analysis of Methanol Oxidation under Electrolysis/Fuel Cell Working Conditions

Instead of using a DMFC hardware as an elementary Direct Methanol Fuel Cell, it is possible to use it as an electrolyzer to produce clean hydrogen at the Pt/C electrode negatively polarized by applying a positive electrode potential at the Pt-Ru anode. In this way the DMFC hardware works as an elementary electrolysis cell with hydrogen generation at the Pt/C cathode by electro-reduction of the protons coming from methanol oxidation at the Pt-Ru anode and crossing over the protonic membrane. The elementary electrochemical reactions involved are:

CH₃OH + H₂O  →  CO₂ + 6 H⁺ + 6 e^-       anode reaction            (1)

6 H⁺ + 6 e^-      →  3 H₂                             cathode reaction         (2)

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CH₃OH + H₂O  →  CO₂ + 3 H₂                  overall reaction         (3)

with the thermodynamic data ΔH^o = + 131.5 kJ/mole and ΔG^o = + 9.3 kJ/mole for the overall reaction (3). Thus the minimum energy needed to generate one mole of hydrogen is equal to ΔH^o/3 ≈ + 44 kJ/mole, which is much lower than that used in water electrolysis (ΔH^o = + 286 kJ/mole under standard conditions). Moreover the anode potential is E_a⁺ = + ΔG^o/6F » 0.016 V/SHE and the cell voltage under standard conditions is U^o_cell = E_a⁺ - E_c- ≈ 0.016 V where E_c- = 0 V is the cathode potential vs. the Standard Hydrogen Electrode (SHE).

Thus the electrochemical decomposition of methanol according to reaction (3), using a Proton Exchange Membrane Electrolysis Cell (PEMEC), is a more convenient and efficient process [1-2] to produce high-purity hydrogen than water electrolysis [3], since the theoretical cell voltage under standard conditions is much lower (U^o_cell = 0.016 V) than that of water decomposition (U^o_cell= 1.23 V).

The DMFC hardware, provided by ElectroChem (ref.: EFC-05-02-DM), consists of a Pt/C electrode and a Pt-Ru (1/1)/C electrode of 5 cm² surface area separated by a Nafion^®117 membrane. The cell was polarized at a constant current density, j (from 1 to 100 mA cm^-2), using a potentiostat and the methanol anode potential was deduced from the cell voltage since the hydrogen evolution cathode can be used as a hydrogen reference electrode. In that way it was possible to obtain the methanol oxidation electric characteristics E_meth= f(j) at several methanol concentrations (from 0.1 M to 10 M) and several working temperatures (from 25°C to 85°C). These methanol oxidation characteristics, corrected from ohmic losses due to membrane and interface resistances, were then analysed leading to the transfer coefficient, α, the reaction order vs. methanol, p, and the heat of activation, ΔH*. The results obtained (α ≈ 0.5 to 0.6, p ≈ 0.3 and ΔH* ≈ 50 to 60 kJ/mole) are in good agreement with those obtained in a 3-electrode electrochemical cell [4-7].

References

[1] Z. Hu, M. Wu, Z. Wei, S. Song, P.K. Shen, J. Power Sources, 166 (2007) 458.

[2] S.R. Narayanan, W. Chun, B. Jeffries-Nakamura, T. I. Valdez, US Patent 6533919, March 18, 2003.

[3] P. Millet, F. Andolfatto, R. Durand, Int. J. Hydrogen Energy, 21 (1996) 87–93.

[4] F. Kadirgan, B. Beden, J.M. Léger and C. Lamy, J. Electroanal. Chem, 125 (1981) 89-103.

[5] H.A. Gasteiger, N. Markovic, P.N. Ross and E.J. Cairns, J. Electrochem., Soc. 141 (1994) 1795.

[6] L. Dubau, C. Coutanceau, E. Garnier, J.M. Léger, C. Lamy, J. Appl. Electrochem., 33 (2003) 419-429.

[7] E.A. Batista, H. Hoster 1, T. Iwasita, J. Electroanal. Chem, 554-555 (2003) 265-271.

 

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[1]  This communication is dedicated  to our close friend Hubert Gasteiger on the occasion of his Grahame award

See more of: Grahame Award Session
See more of: L04: Electrocatalysis 7
See more of: Physical and Analytical Electrochemistry, Electrocatalysis, and Photoelectrochemistry

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