Elsevier

Journal of Power Sources

Volume 105, Issue 2, 20 March 2002, Pages 283-296
Journal of Power Sources

Recent advances in the development of direct alcohol fuel cells (DAFC)

https://doi.org/10.1016/S0378-7753(01)00954-5Get rights and content

Abstract

Recent developments into technology of proton exchange membrane fuel cells (PEMFC) now allow serious consideration to be given to a direct alcohol fuel cell (DAFC) based on a PEMFC, in which alcohol is used directly as the fuel. This is particularly advantageous for mobile applications, since this will avoid the use of a bulky and expensive reformer. However, the relatively complex reaction mechanism, leading to a low electroreactivity of most alcohols, even methanol, needs the investigation of new platinum-based electrocatalysts, particularly active for breaking the CC bond when alcohols other than methanol are to be used. Moreover, in order to overcome the deleterious effect of alcohol crossover through the proton exchange membrane, it is necessary to develop new oxygen reduction electrocatalysts insensitive to the presence of alcohols.

Introduction

During the past 10 years, there has been an increasing interest in the development of direct alcohol/air proton exchange membrane fuel cells (PEMFC), particularly for applications to the electric vehicle [1]. Among the different possible alcohols, methanol is the most promising organic fuel because its use as a fuel has several advantages in comparison to hydrogen: high solubility in aqueous electrolytes, liquid fuel available at low cost, easily handled, transported and stored, high theoretical density of energy (6 kWh/kg) comparable to that of gasoline (10–11 kWh/kg). Other alcohols, such as ethanol, ethylene glycol, propanol, etc. have also been considered for use in a fuel cell, but until now very few direct alcohol fuel cells (DAFC) have been demonstrated, the most advanced system being the direct methanol fuel cell (DMFC) [2].

The electro-oxidation of methanol in a DMFC has been studied for more than three decades [2], [3], [4]. But little progress was made in the past, due to fundamental problems inherent in the fuel cell design (liquid electrolyte, such as sulphuric acid, to reject CO2, the reaction product of total oxidation, and conventional carbon supported platinum-based metal catalysts). Twenty years ago, Shell Research Center in Great Britain [5] and Hitachi Research Laboratories [6] in Japan built relatively large stacks (up to 5 kW for Hitachi), but the performances were low, e.g. 20–30 mW/cm2 for a relatively large platinum loading (≈10 mg/cm2, which corresponds to 2–3 W/g of platinum catalysts). Only recently has the concept of PEMFC been applied to the DMFC leading to a great improvement in the electrical characteristics: 200–400 mA/cm2 at 0.5 V, i.e. 100–200 mW/cm2 depending on the working conditions of temperature and pressure [7], [8]. But these performances are still limited because of several problems: (i) the low activity of the state-of-the-art electrocatalysts, which can only be enhanced by increasing the operating temperature, (ii) anode poisoning by strongly adsorbed intermediates (mainly CO) formed during methanol oxidation, and (iii) the high extent of methanol cross-over through the Nafion® type membranes, which depolarizes the air cathode.

Moreover, methanol has some particular disadvantages, e.g. it is relatively toxic, inflammable with a low boiling point (65 °C), and it is not a primary fuel, nor a renewable fuel. Therefore, other alcohols, particularly those coming from biomass resources, begin to be considered as alternative fuels. Ethanol is an attractive fuel for the electric vehicle, since it can be easily produced in great quantity by the fermentation of sugar-containing raw materials from agriculture. In addition, in some countries like Brazil, ethanol is already distributed through the gas station network to fuel the thermal engine cars. Other alcohols have also been considered as interesting fuels for fuel cells, some of them having led to prototype production, such as ethylene-glycol [9], or glycerol [10].

This paper aims to review the more recent advances made in the development of the DAFC, using the PEMFC configuration. In particular, we will present recent progress made in the understanding of the mechanisms of the oxidation reaction, leading to the conception and realization of plurimetallic catalysts for alcohol oxidation. Moreover, new oxygen reduction catalysts insensitive to the presence of alcohol will be discussed as a convenient way to avoid the cathode depolarization which arises as a consequence of alcohol crossover through the polymeric membrane (usually Nafion® membranes).

Section snippets

Principle of a direct alcohol fuel cell (DAFC)

The direct anodic oxidation of an alcohol fuel other than methanol would allow building compact power sources (with no heavy and bulky fuel reformer), fed with a convenient electrochemically reactive and relatively non-toxic and cheap liquid fuel. Recent progress in the PEMFC and the DMFC allow us to conceive a DAFC based on a PEMFC. In the case of ethanol, taken as a typical example (Fig. 1), an anode mixture of ethanol dissolved in water (a few percent in weight to make a 1–2 M ethanol

Investigation of the reaction mechanism of alcohol oxidation

Since an alcoholic group only contains one oxygen atom, the complete oxidation of a (primary) alcohol to CO2 needs an extra oxygen atom. This atom must be provided by water, or by water adsorbed residue (adsorbed OH). Thus, the general overall electro-oxidation reaction of a primary alcohol can be written as follows:CnH2n+1OH+(2n−1)H2O→nCO2+6nH++6ne

It turns out that the reaction mechanisms of alcohol oxidation always involve the participation of water, or of its adsorption residue, so that a

Electrocatalytic materials for the oxidation of alcohols

The nature and the structure of the electrode material play a key role in the adsorption and electro-oxidation of most organic fuels, particularly aliphatic alcohols.

On the other hand the use of a proton exchange membrane as acid electrolyte induces pH limitations, and therefore, in such a strong acid environment, the only stable and active catalyst for the electroxidation of alcohols is platinum. Even if some attempts have been made to develop other electrocatalysts, such as tungsten

Direct methanol fuel cell (DMFC)

Due to the large progresses made in the development of electrocatalysts and proton exchange membranes, different DMFC systems have been built by several research laboratories and industrial companies (IFC, JPL, LANL, etc. in the USA, and Siemens, University of Newcastle, University of Poitiers, ITAE Messina, etc. in Europe). Typically, the anode catalyst used consists of Pt/Ru particles deposited on a carbon powder (e.g. Vulcan XC72), and is hot pressed on the surface of a proton exchange

Conclusions

The current density vs. cell potential characteristics of a PEMFC fed with methanol or ethanol are rather similar at high temperatures (170 °C), so that ethanol appears as an alternative fuel to methanol. Moreover, ethanol is much less toxic than methanol, and its mass production from agriculture may decrease its price, so that it may compete economically with methanol. However, the electro-oxidation reaction stops at intermediates steps (production mainly of acetaldehyde and acetic acid),

Acknowledgements

This work was carried out under the framework of different research agencies (DGXII of the Commission of the European Union, Agence de l’Environnement et de la Maı̂trise de l’Energie and CNRS) to which we are greatly indebted. The authors are also grateful to the Province Council “Région Poitou-Charentes” for its support under Grants no. 94/RPC-R-108, 96/RPC-R-225 and 94.2.87.1408.

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