(Invited) Electrocatalysis in Low Temperature Fuel Cells
Authors: Lamy Claude , Guenot Benoît 
Published: February 2020

Electrocatalysis, i.e. the heterogeneous catalysis of electrochemical reactions occurring at the electrode-electrolyte interface, has mainly concerned technological developments related either to energy storage (e.g. water electrolysis) or to energy conversion (e.g. fuel cells) [l, 2]. However, except for the hydrogen electrode which is now well known, and the oxygen electrode which has been extensively studied, other catalytic electrodes for the direct oxidation of soluble fuels, need much more investigations.
Among them primary alcohols are particularly interesting as fuels in a Direct Oxidation Fuel Cell (DOFC), because of several favorable features, such as a high theoretical energy density (4 to 9 kWh kg-1 compared to 33 kWh kg-1 for molecular di-hydrogen) and a great facility of handling. Moreover alcohols, which may be produced from the biomass, are very convenient fuels due to a lot of advantages: high solubility in aqueous electrolytes, relatively high reactivity, ease of storage and supply, small toxicity (except for methanol). They can be directly electro-oxidized in a Direct Alcohol Fuel Cell (DAFC) working at low temperature [3]. This explains why many fundamental investigations were undertaken in the last three decades on the electrochemical oxidation of several alcohols: methanol [4, 5], ethanol [3, 6, 7], glycerol [8], etc. and also on related compound, such as carbon monoxide [9], etc. Until now, the most promising and most studied fuels for application in a DOFC, with the direct oxidation of the organic molecule, are methanol and ethanol.
However, a lot of electrocatalytic problems still arise due to the relative complexity of the reaction mechanisms. These include the effect of the molecular structure of the organic compound, the nature and structure of the adsorbed intermediates, the nature and structure of the electrode material, the pH and the anions of the supporting electrolyte, and the role of the water adsorption residues.
Furthermore, the catalyst structure, such as particle size, composition and degree of alloying, are also of great importance, since most of the alcohol oxidation reactions are structure sensitive. On the other hand the synthesis of plurimetallic catalysts is one way to reduce the platinum group metal loading in order to reduce the cost of a DAFC system.
Thus the key research topics for practical applications are the following:
● the investigation of reaction mechanisms on new anode electrocatalysts, by combination of several catalytic materials (including platinum), each of them being able to activate a given path of the mechanism ;
● the optimization of the electrode catalysts in terms of composition, particle size, metal loading and accessibility to the reactants.
Detailed oxidation mechanisms of several alcohols (methanol, ethanol, and glycerol) will be discussed and will be correlated to the nature and structure of the electrocatalysts.
References
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[2] W. Vielstich, A. Lamm, H. Gasteiger (Eds.), Handbook of Fuel Cells: Fundamentals and Survey of Systems, Vol. 1, Wiley, Chichester, 2003.
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