(Invited) Electrocatalytic Oxidation of Ethanol in a PEM Device: A Way to Produce Either Energy in a DEFC or Clean Hydrogen in a Proton Exchange Membrane Electrolysis Cell
Authors: Lamy Claude , Baranton Stève , Coutanceau Christophe 
Published: May 2020

Introduction

Ethanol, which is easily produced by the fermentation of biomass products (sugar cane, corn, grain, beetroot, ligno-cellulosic wastes, etc.) is an alternative fuel to feed a fuel cell [1]. Its liquid state (which allows an easy storage) with a low toxicity and its large availability in many countries (Brazil, USA, Spain, France, etc.), together with its high energy density for complete oxidation to CO2 (8 kWh kg-1 compared to 11 kWh kg-1 for gasoline) are important positive points for its use as a fuel [2].

Can we use ethanol directly in a Direct Ethanol Fuel Cell (DEFC) or do we have to convert it into pure hydrogen by electrochemical reforming in a Proton Exchange Membrane Electrolysis Cell (PEMEC)???

Direct Ethanol Fuel Cell

The oxidation of ethanol is more difficult to perform than that of methanol with the necessity of breaking the C-C bond to obtain its complete oxidation to CO2. The electrocatalytic reaction has been the subject of numerous studies on several Pt-based electrodes, including Pt-X alloys (with X = Ru, Sn, Mo, etc.) and dispersed nanoparticules [3]. From these studies it appears that breaking the C-C bond at relatively low temperatures (80 to 120°C) is still a challenge, since the main reaction products are acetaldehyde and acetic acid together with a small amount of CO2 [4]. It turns out that a Direct Ethanol Fuel Cell (DEFC) gives electrical performance one order of magnitude lower (Pmax < 0.1 W cm-2) than that obtained with a Proton Exchange Membrane Fuel Cell (PEMFC) fed with pure hydrogen (Pmax ≈ 1 W cm-2).

Production of hydrogen by electrochemical processes

The principle of the electrochemical decomposition of ethanol is similar to that of water electrolysis using a Proton Exchange Membrane Electrolysis Cell (PEMEC) [5, 6]. Ethanol is fed to the anodic compartment where it can be completely oxidized in the presence of water, producing carbon dioxide and protons, i.e.:

C2H5OH + 3 H2O → 2 CO2 + 12 H+ + 12 e- (1)

After crossing-over the proton exchange membrane the protons reach the cathode compartment where they are reduced to hydrogen according to reaction (2):

2 H+ + 2 e- → H2 (2)

This corresponds to the electrochemical reforming of ethanol into hydrogen and carbon dioxide, according to the overall reaction:

C2H5OH + 3 H2O → 6 H2 + 2 CO2 (3)

with the thermodynamic data under standard conditions:

ΔH = + 348 kJ (mole ethanol)-1 and ΔG = + 97 kJ (mole ethanol)-1.

corresponding to a reversible cell voltage UEtOH = ΔG/nF = 0.084V, instead of UH2O = 1.229V for water electrolysis.

In this communication the principle of the electrocatalytic oxidation of ethanol either in a DEFC or in a PEMEC will be discussed. The same electrocatalysts are used in both cases since the same electrochemical reaction is involved and the right electrocatalysts are similar, i.e. Pt-based nanoparticles dispersed on a carbon powder such as PtxSn1-x/C and PtxSnyRuz/C catalysts. The current density vs. anode potential curves has been recorded and displays the same behaviour.

When comparing the energy efficiency coefficient ε of both processes we arrived at the conclusion that the energy efficiency of the DEFC (εDEFC ≈ 15%) is a little bit greater than that of a PEMFC using hydrogen produced in a PEMEC (ε = εPEMFC x εPEMEC ≈ 0.55 x 0.18 ≈ 10%) when the oxidation process of ethanol stops at the acetic acid stage. But the most important point is that the electrical energy used is much lower, i.e. ≤ 2.2 kWh (Nm3)-1, since U
cell ≤ 1.0 V, compared to 4-4.5 kWh (Nm3)-1 for water electrolysis at 1.8-2.0 V.

References

[1] C. Lamy, C. Coutanceau, J.-M. Léger, in Catalysis for Sustainable Energy Production, P. Barbaro and C. Bianchini (Eds.), Wiley-VCH, Weinheim, 2009, Chap.1, pp. 3-46.

[2] C. Lamy, E.M. Belgsir, in Handbook of Fuel Cells: Fundamentals and Survey of Systems, W. Vielstich, A. Lamm and H. Gasteiger (Eds.), Wiley, Chichester, 2003, Vol. 1, Chap. 19, pp. 323-334.

[3] F. Vigier, S. Rousseau, C. Coutanceau, J.-M. Léger, C. Lamy, Topics in Catalysis 40 (2006) 111.

[4] S. Rousseau,C. Coutanceau, C. Lamy, J.M. Léger, J Power Sources (2006) 158: 18–24

[5] C. Lamy, T. Jaubert, S. Baranton, C. Coutanceau, J Power Sources 245 (2014) 927–936.

[6] C. Coutanceau, S. Baranton, Electrochemical Conversion of alcohols for hydrogen production: a short review, Wire Energy Environ 5 (2016) 388–400.