Elsevier

Journal of Power Sources

Volume 187, Issue 2, 15 February 2009, Pages 387-392
Journal of Power Sources

Performance of alkaline electrolyte-membrane-based direct ethanol fuel cells

https://doi.org/10.1016/j.jpowsour.2008.10.132Get rights and content

Abstract

A single alkaline direct ethanol fuel cell (alkaline DEFC) with an anion-exchange membrane and non-platinum (non-Pt) catalysts is designed, fabricated, and tested. Particular attention is paid to investigating the effects of different operating parameters, including the cell operating temperature, concentrations of both ethanol and the added electrolyte (KOH) solution, as well as the mass flow rates of the reactants. The alkaline DEFC yields a maximum power density of 60 mW cm−2, a limiting current density of about 550 mA cm−2, and an open-circuit voltage of about 900 mV at 40 °C. The experimental results show that the cell performance is improved on increasing the operating temperature, but there exists an optimum ethanol concentration under which the fuel cell has the best performance. In addition, cell performance increases monotonically with increasing KOH concentration in the region of low current density, while in the region of high current density, there exists an optimum KOH concentration in terms of cell performance. The effect of flow rate of the fuel solution is negligible when the ethanol concentration is higher than 1.0 M, although the cell performance improves on increasing the oxygen flow rate.

Introduction

Direct alcohol fuel cells (DAFCs) are electrochemical devices that directly convert the chemical energy stored in liquid alcohol (e.g., methanol and ethanol) into electricity. DAFCs have many advantages compared with hydrogen-feed fuel cells, including higher energy densities, facile liquid storage, and simpler system structures [1], [2], [3], [4], [5], [6], [7], [8], [9]. These benefits suggest that this type of fuel cell is a promising power source for portable and other mobile applications. Over the past decade, special attention has been paid to fuel cells that use methanol as the fuel, namely direct methanol fuel cells (DMFCs), mainly because methanol is the simplest alcohol and potentially has better electrode kinetics than other alcohol fuels. However, in addition to other technical problems encountered in the development of this type of fuel cell, the toxicity of methanol is another issue that limits the wide application of DMFCs. By comparison, ethanol is more environmentally friendly and can be easily produced in large quantities from agricultural products or biomass. Hence, direct ethanol fuel cells (DEFCs) have recently received increased attention.

Based on the electrolyte membrane used, DEFCs can be divided into two types: acid- and alkaline-membrane DEFCs. A considerable amount of effort has been devoted to acid DEFCs; as a result, significant progress has been made in their development [1], [2], [3]. For example, Xin and co-workers [1], [2] developed a highly active PtSn catalyst for the ethanol oxidation reaction (EOR) in an acid medium; the application of this electrocatalyst to the anode of the DEFC resulted in a maximum power density of 60 mW cm−2 at 90 °C, the highest performance reported in the open literature [1]. Although the performance seems appealing, the commercialization of acid DEFCs has been hindered by several issues. First, the slow kinetics of the EOR in acid media leads to a serious activation polarization loss, thereby diminishing cell performance. Second, the electrocatalyst (e.g., PtSn) suffers from corrosion in acid DEFCs, which results in poor durability of the fuel cell. Another critical obstacle that limits the wide application of acid DEFCs is the cost: acid electrolyte membranes (typically Nafion® material) are expensive; and a considerable amount of precious Pt is needed to achieve decent performance in acid DEFCs. All these issues can be alleviated when acid membranes are replaced by alkaline membranes. The most striking feature of alkaline DEFCs is their quicker kinetics of the oxygen reduction reaction (ORR) in alkaline media, even with low-cost non-platinum metals as the electrocatalyst. Another important feature of alkaline DEFCs is the use of a non-Pt electrocatalyst on the cathode eliminates the oxidation of the fuel that may be transported from the anode, which makes the cathode potential much higher than in acid DEFCs. Because of these important features, alkaline DEFCs have recently attracted increasing attention [4], [5], [6], [7], [8], [9]. So far, effort has mainly been concentrated on the synthesis of alkaline membranes and electrocatalysts for the EOR and ORR in alkaline media [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Stoica and co-worker [15] prepared an alkaline membrane that was composed of two cyclic diamines and demonstrated that this membrane was good in terms of ionic conductivity and thermal stability up to 220 °C. Recently, Slade and co-worker [16] prepared a series of FEP-based alkaline membranes via a radiation-grafting method and showed that the OH conductivity could be as high as 0.023 S cm−1 at 50 °C. On the development of electrocatalysts for the ORR and EOR, Ogumi and co-workers [17] prepared a carbon-supported La1−xSrxMnO3 (LSM/C) cathode catalyst and studied its catalytic activities for the ORR under the existence of ethylene glycol (EG) by using a rotating disk electrode. The experiment results indicated that LSM/C can serve as a cathode catalyst in alkaline direct EG fuel cells with no mixed potential problem. Shen and co-worker [18] investigated the addition of tungsten carbide nanocrystals to the Ag based electrocatalysts for the ORR and demonstrated that the composite catalyst yielded a unique selectivity for the ORR in alcohol-containing solutions and was immune to methanol, ethanol, isopropanol and glycerol. More recently, Shen and co-worker [19] synthesized a hexagonal tungsten carbide single nanocrystal-supported Pd electrocatalyst and demonstrated that this catalyst had extremely high electrocatalytic activity for the EOR as a result of the synergistic interaction between Pd and WC.

Our literature review indicates that past efforts in developing alkaline DEFCs have been devoted mainly to the development of alkaline membranes and electrocatalysts, whereas system design and development of DEFCs have not yet been addressed. The objective of this work is to develop a single alkaline DEFC with commercial alkaline membranes from Tokuyama and HYPERMEC™ catalysts from Acta. Particular attention is paid to a systematic investigation of the effects of various operating parameters on cell performance, namely, cell operating temperature, concentrations of ethanol and KOH solutions, and flow rates of reactants.

Section snippets

Membrane electrode assembly

The membrane electrode assembly (MEA), with an active area of 2.0 cm × 2.0 cm, consisted of two single-sided electrodes and an anion-exchange membrane. Both the anode and cathode electrodes with non-platinum HYPERMEC™ catalysts were provided by Acta. The catalyst loadings in the anode and cathode were 2.0 and 1.0 mg cm−2, respectively. While the membrane (A201), with a thickness of 28 μm, was provided by Tokuyama. The anode and cathode backing layers were made of nickel foam (Hohsen Corp., Japan) and

General performance

Fig. 1 shows the polarization and power-density curves of the alkaline DEFC with non-Pt catalysts both at the anode and cathode, and an anion-exchange membrane. The experiment was performed at 40 °C with an aqueous solution of 3.0 M ethanol mixed with 7.0 M KOH pumped to the anode at a rate of 2.0 ml min−1 and with dry pure oxygen at a flow rate of 100 standard cubic centimeters per minute (sccm) fed to the cathode. A maximum power density of 60 mW cm−2 is achieved at a current density of 250 mA cm−2,

Conclusions

A single alkaline DEFC with an anion-exchange membrane and non-Pt catalysts has been developed and tested under different operating conditions. The experimental results show that the alkaline DEFC can yield a maximum power density of 60 mW cm−2 at 40 °C, a maximum current density of about 550 mA cm−2, and an open-circuit voltage of about 900 mV. These results suggest that the alkaline DEFC is superior to all other similar DAFCs in terms of performance. The cell performance improves with increasing

Acknowledgements

The work was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 622807) and by the Joint Research Fund for Hong Kong and Macao Young Scholars (Project No. 50629601). The material support from Acta and Tokuyama is greatly acknowledged.

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