Experimental and model based analysis of the steady state behavior of DMFCs.

The final output voltage obtained from a direct methanol fuel cell is influenced by various physio-chemical processes in the system.  Most of the processes such as evolution of carbon-dioxide, crossover of methanol, cathode flooding, and mixed potential on the cathode catalyst layer are intimately coupled. Therefore, understanding these processes is necessary to optimize the fuel cell material and operating parameters for maximum performance and improved efficiency. A mathematical model to describe the fuel cell operation is developed with emphasis on the mechanism of mixed potential on the cathode catalyst layer and mass transfer of oxygen through the cathode gas diffusion layer (GDL).  It is often cited that the mixed potential is due to the electrochemical oxidation of methanol (potential dependent) on the cathode catalyst layer. However a few electrochemical studies and impedance studies have revealed that in parallel to electrochemical oxidation (potential dependent), methanol also reacts with oxygen chemically (potential independent).

We have developed an accurate mathematical model of the cathodic reaction mechanism with intermediate steps to evaluate the cathode potentials and surface coverage on the cathode catalyst layer. Our results show that the absolute value of the cathode overpotential is slightly lower when both the mechanisms are included. We also found that with reduction in oxygen mass transport there is a higher drop in cathode overpotential in the chemical reaction than in the electrochemical reaction. This suggests the necessity to investigate the role of the operating parameters on the mechanisms involved.  Model discrimination and estimation of appropriate kinetic parameters is explored with the help of experiments which should help separate the anode and cathode contributions.          

The mass transfer of oxygen through the cathode GDL depends on the current density, which influences the reaction rates of oxygen reduction and methanol oxidation. The water crossover from the anode to the cathode together with the produced water from the cathode reaction could potentially flood the cathode GDL pores. A suitable cathode mass transfer model is formulated to study the effect of cathode reaction kinetics with and without the presence of water in the GDL.  This would improve our understanding of the variation of cathode overpotential with oxygen mass transfer.

 

Figure 1.  Mixed Methanol oxidation and oxygen reduction mechanism on the cathode catalyst layer