Direct methanol fuel cells (DMFCs) are one of the most promising alternative power sources for portable applications because of methanol's low cost, easy storage and high energy density. Despite extensive research, the development of DMFC technology is hindered by fuel crossover which limits the operation of direct liquid fuel cells to dilute feeds [1]. Here we will focus on the design and characterization of an air-breathing vapor feed methanol fuel cell (VFMFC) with a flowing electrolyte stream. In contrast to liquid feed DMFCs, passive vapor feed DMFCs can simultaneously maintain a low methanol concentration, minimizing fuel crossover, but increase overall system energy density, utilizing a highly concentrated liquid methanol reservoir [2]. Under ambient conditions, the high vapor pressure of methanol causes substantial evaporation, enabling cell operation with no ancillary power losses [3]. We have systemically designed the cell to balance mass transport from the liquid fuel reservoir to anode catalytic surface with the removal of both carbon dioxide and unreacted methanol.

Additionally, as opposed to vapor feed DMFCs with conventional polymer electrolyte membranes, our VFMFC utilizes a flowing electrolyte stream located between the anode and cathode gas diffusion electrodes. A constantly refreshing electrolyte stream eliminates water management issues, limits fuel crossover and enables media flexibility, acidic or alkaline [4]. Key advantages of operating the VFMFC in alkaline media are superior reaction kinetics for both methanol oxidation and oxygen reduction. In addition to significantly improving cell performance, the facile kinetics enable the use of cheaper non-precious metal catalysts such as Ag and Pt₃Co cathode catalysts in lieu of Pt catalysts [5]. Moreover, any carbonates formed at the anode-electrolyte interface are removed by the flowing alkaline stream.

In this presentation, we will illustrate the design and characterization of a media flexible VFMFC. We will investigate cell performance as a function of methanol concentration, electrolyte type, flow rate and concentration. We will also present on the effects of various passive barrier layers on cell performance.

References

[1] Z. Guo and A. Faghiri, J. Power Sources, 167, 2007, 378; A. Heinzel, V.M. Barragán, J. Power Sources, 84, 1999, 70; J.G. Lui, T.S. Zhao, R. Chen, C.W. Wong, Electrochem. Commun., 7, 2005, 288.

[2] M. Hogarth, P. Christensen, A. Hamnett, A. Shukla, J. Power Sources, 69, 1997, 125; H. Fukunaga, T. Ishida, N. Teranishi, C. Arai, K. Yamada, Electrochim. Acta, 49, 2004, 2123.

[3] H. Kim, J. Power Sources, 162, 2006, 1232.

[4] R.S. Jayashree, M. Mitchell, D. Natarajan, L.J. Markoski, and P.J.A. Kenis, Langmuir, 2007, 6871.

[5] W.P. Zhou, R.S. Jayashree, F.R. Brushett and P.J.A. Kenis, J. Electrochemical Society, manuscript in preparation.