Operating Alkaline Alcohol Fuel Cell (ITAAFC) in Intermediate-Temperature Range

Direct alcohol fuel cells (DAFCs) 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 alcohol fuel cells (AAFCs). In alkaline media, the polarization characteristics of alcohol electrooxidation and oxygen electroreduction are far superior to those in acidic media. 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 AAFCs have received increased attention. 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 AAFCs. 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 alcohol electrooxidation in alkaline media than in acidic media since reported values of the activation energy are higher in alkaline media. 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 AAFCs.

In an intermediate temperature range of 80 to 200 ^oC, CO is likely to spontaneously react with OH^- to give formate as follows [1]:

    (1)

Because formate produced via Reaction I is soluble and more electrochemically active than CO on Pt, surface CO formed during alcohol oxidation would be readily removed, resulting in considerable decrease in the CO-poisoning of Pt-based catalysts. Moreover, the kinetics of the alcohol oxidation in alkaline media on Pt has been suggested to be largely determined by the reaction between the surface CO and surface oxygen-containing species (OH), following a Langmuir-Hinshinwood mechanism. Accordingly, accelerated electrooxidation of CO intermediate would increase the rate of the alcohol oxidation.

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 ^oC to produce primarily H₂ and CO₂ [2].

                           (2)

During the methanol liquid-phase reforming, trace amount of methane is the side product and the use of more basic catalyst favors H₂ production. The methanol reforming indicates that sluggish methanol oxidation reaction which has plagued low temperature DAFCs, could become highly facile in alkaline media at temperatures close to 200 ^oC where the reforming of methanol is triggered. Substantially accelerated electrooxidation of methanol would make it possible to achieve low anode overpotentials. Therefore, driving the electrooxidation of methanol in an intermediate-temperature range over the methanol-boiling temperature (around 80 ^oC) and the triggering temperature of the methanol reforming (about 200 ^oC) would be of academic and practical importance for the development of high performance a technology.

We have investigated systematically the electrocatalysis associated with the ITAAFC and demonstrated high performance of this promising fuel cell technology. Our status-of-the-art ITAAFC operating with methanol at 120 ^oC demonstrated a peak power density of around 250 mW cm^-2 at around 700 mA cm^-2.

References

1. J. van Trump, S. Miller, Earth Planet. Sci. Lett. 20, 145 (1973).

2. R. Davda, W. Shabaker, W. Huber, D. Cortright, and A. Dumesic, Applied Catalysis B: Environmental 56, 171 (2005).