**1. Introduction**

160 Electrochemical Cells – New Advances in Fundamental Researches and Applications

Yu, H. F. & Liao, W.H. (1998). Evaporation of solution droplets in spray pyrolysis.

Φstergård, M.J.L. (1995). Manganite-zirconia composite cathodes for SOFC: Influence of

*International Journal of Heat and Mass Transfer.* Vol. 41, pp. 993.

structure and composition. *Eletrochimica Acta.* Vol. 40, pp. 1971.

Direct methanol fuel cells (DMFCs) possess obvious advantages over traditional hydrogen fuel cells in terms of hydrogen storage, transportation, and the utilization of existing infrastructure. However, the commercialization of this fuel cell technology based on the use of proton-conductive polymer membranes has been largely hindered by its low power density owing to the sluggish kinetics of both anode and cathode reactions in acidic media and high cost owing to the use of noble metal catalysts. These could be potentially addressed by the development of alkaline methanol fuel cells (AMFCs). In alkaline media, the polarization characteristics of the methanol electrooxidation and oxygen electroreduction are far superior to those in acidic media (Yu et al., 2003; Prabhuram & Manoharan, 1998). Another obvious advantage of using alkaline media is less-limitations of electrode materials. The replacement of Pt catalysts with non-Pt catalysts will significantly decrease the cost of catalysts. Recently, the AMFCs have received increased attention (Dillon et al., 2004). However, these fuel cells are normally operated at temperature lower than 80 °C. In this low temperature range, both methanol electrooxidation and oxygen electroreduction reactions are not sufficiently facile for the development of high performance AMFCs. Considerable undergoing efforts are now focused on the development of highly active catalysts for accelerated electrode reactions.

Alternatively, increasing temperature has been proven as an effective way to accelerate electrode reactions. The changes of the reaction rates with increasing temperature are strongly determined by the values of activation energy, as described by the Arrhenius equation. More obvious changes are expected for the methanol electrooxidation in alkaline media than in acidic media since reported values of the activation energy are higher in alkaline media (Cohen et al, 2007). Additionally, increasing temperature may decrease concentration polarization, Ohmic polarization, and CO poisoning of the catalysts. All these advantages can contribute to the performance improvement of the AMFCs.

Further finding of increasing temperature for the methanol oxidation is that methanol can be efficiently converted with water in the aqueous phase over appropriate heterogeneous catalysts at temperatures near 200 °C to produce primarily H2 and CO2 (Huber et al., 2003; Cortright et al., 2002). The aqueous-phase reforming (APR) process eliminates the need to vaporize both water and the oxygenated hydrocarbon, which reduces the energy requirements for producing hydrogen. Moreover, the formation of CO could be minimized since the APR occurs at temperatures and pressures where the water-gas shift reaction is favorable. The APR of methanol on supported Pt over 200~265 °C results in the production of H2 at a usually high selectivity of around 99% as follows (Davda et al., 2005):

$$\text{CH}\_3\text{OH} + \text{H}\_2\text{O} \rightarrow \text{CO}\_2 + \text{3H}\_2\tag{1}$$

Investigations of Intermediate-Temperature Alkaline Methanol

reported were referred to the RHE unless otherwise stated.

(C).

performance.

**2.2 Instruments and materials** 

Fuel Cell Electrocatalysis Using a Pressurized Electrochemical Cell 163

coupled plasma–atomic emission spectroscopy (ICP-AES) spectra. A 15 cm length Pt wire of 0.5 mm in diameter was used as the counter electrode. Three kinds of Ag-based reference electrodes were evaluated as the internal reference electrode under our conditions by monitoring the changes of both hydrogen- and oxygen-electrochemistry on measured voltammograms for a Pt electrode in 0.5 mol dm-3 potassium hydroxide (KOH) at varying temperature. It was found that the Ag/AgCl reference provides the most reproducible results while both Ag wire quasi-reference electrode and Ag/Ag2O electrode were inapplicable. The Ag/AgCl reference electrode was introduced into a glass tube containing 0.1 mol dm-3 HCl which was separated from KOH solution in the working electrode chamber by a microporous ceramic pellet fixed at the top end of the glass tube. The potentials of the Ag/AgCl were measured versus a reversible hydrogen electrode (RHE) at varying temperature in a hydrogen atmosphere. In the following sections, all potentials

Fig. 1. A pressurized electrochemical cell based on a modified Parr autoclave. Schematic diagram of the cell (A); Image of the cell (B); and Schematic structure of a working electrode

Both voltammetric and chronoamperometric measurements were performed using Autolab general purpose electrochemical system (Ecochemie, Netherland). A lab-constructed singlecell system with temperature control, gas flow rate and pressure control, and liquid flow rate and pressure control constructed was used for the measurements of fuel cell

All chemicals and materials were used as received without further purification. All solutions were prepared from methanol (>99.9%, Aldrich) or KOH (>85%, Aldrich) with deionized water (18.2 M cm). Unsupported electrocatalysts were used in voltammetric and chronoamperometric measurements, including Pt black (Alfa, S.A. typically 27 m2 g-1), Pd black (Alfa, S.A. typically 20 m2 g-1) and Ag nanopowder (Alfa, 20-40 nm). Carbon supported Pt (Fuel Cell Store, 60 wt% Pt/C) and carbon supported Pd (Sigma-Aldrich, 30

During the methanol APR, trace amount of methane is the side product and the use of more basic/neutral catalyst favors H2 production. The methanol APR indicates that sluggish methanol oxidation reaction which has plagued low temperature AMFCs, could become highly facile in alkaline/neutral media at temperatures close to 200 °C where the APR of methanol is triggered. Substantially accelerated electrooxidation of methanol would make it possible to achieve low anode overpotentials. Therefore, the investigations of the electrooxidation of methanol in an intermediate-temperature range over the methanolboiling temperature (around 80 °C) and the triggering temperature of the methanol APR (about 200 °C) would be of academic and practical importance for the development of high performance AMFC technology. This intermediate temperature range has been rarely used because of the limitation of boiling points of both methanol and water.

Our research efforts have been focused on the investigations of fuel cell electrocatalysis in alkaline media in the intermediate-temperature range of 80 to 200 °C for accelerated anode and cathode reaction kinetics, and the development of high performance AMFCs. In this work, we have successfully developed a pressurized electrochemical cell by modifying a Parr autoclave which can be operated at pressure up to 2000 psi and at temperature up to 200 °C. An Ag/AgCl electrode has been identified as a suitable internal reference electrode with good stability in this intermediate temperature range. It has been found that the methanol oxidation and oxygen reduction reactions can be significantly accelerated in aqueous alkaline media with increasing temperature. The former is characterized by an onset overpotential of less than 0.1 V at 150 °C for substantial methanol electrooxidation at Pt. Furthermore, highly facile methanol oxidation and oxygen reduction reactions have been also achieved at non-Pt electrodes. This accelerated kinetics of both the methanol oxidation and oxygen reduction reactions provides fundamental support for the development of novel methanol fuel cells. Accordingly, high performance intermediate-temperature alkaline methanol fuel cells using Pt and non-Pt electrocatalysts have been successfully demonstrated.
