A direct methanol fuel cell (DMFC) that does not require an external water feed or a powered water-recovery system has potential for wide application in portable electronic devices. This paper provides experimental data for the water recovery rate in a novel DMFC design featuring a hydrophobic cathode gas-diffusion layer that allows for passive water recovery. Water and methanol crossover rates were experimentally characterized by measuring the water vapor and carbon dioxide concentration at the cathode exit with an infrared sensor. The result showed that the mass-transport parameter of water vapor, increases linearly with increasing cell temperature and remains invariant with respect to the cell current density. Water neutrality of the DMFC stack was achieved while the cell operated close to 50 °C and 150 mA/cm2 with a 1M methanol solution. A comprehensive empirical equation based on the experimental results is presented, along with system-level insights into the controllability of water management in designing an open cathode DMFC system.
A constructive critique and a suite of proposed improvements for a recent one-dimensional semianalytical model of a direct methanol fuel cell are presented for the purpose of improving the predictive ability of the modeling approach. The model produces a polarization curve for a fuel cell system comprised of a single membrane-electrode assembly, based on a semianalytical one-dimensional solution of the steady-state methanol concentration profile across relevant layers of the membrane electrode assembly. The first improvement proposed is a more precise numerical solution method for an implicit equation that describes the overall current density, leading to better convergence properties. A second improvement is a new technique for identifying the maximum achievable current density, an important piece of information necessary to avoid divergence of the implicit-equation solver. Third, a modeling improvement is introduced through the adoption of a linear ion-conductivity model that enhances the ability to better match experimental polarization-curve data at high current densities. Fourth, a systematic method is advanced for extracting anodic and cathodic transfer-coefficient parameters from experimental data via a least-squares regression procedure, eliminating a potentially significant parameter estimation error. Finally, this study determines that the methanol concentration boundary condition imposed on the membrane side of the membrane-cathode interface plays a critical role in the model’s ability to predict the limiting current density. Furthermore, the study argues for the need to carry out additional experimental work to identify more meaningful boundary concentration values realized by the cell.
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