(690b) Understanding Mass Transfer Limitations in the Anode Porous Catalyst Layer of Direct Formic Acid Fuel Cell

Direct Formic Acid Fuel Cells (DFAFC) are a promising alternative to hydrogen and alcohol based fuel cells and are attractive alternatives for small portable applications for example laptops, cell phones, etc. Among the available types of fuel cells, formic acid (HCOOH) has emerged as a promising choice due to better oxidation kinetics, high theoretical open circuit potential, limited crossover of fuel from anode to cathode through the Nafion membrane and reasonable power density at room temperature. The anode porous catalyst layer must provide efficient transport of HCOOH from the Gas Diffusion Layer (GDL) to the surface of the catalyst and at the same time the products of HCOOH oxidation reaction (water and carbon dioxide) should be efficiently removed from the catalyst layer since product accumulation in the system can lower the efficiency and performance of the cell by blocking surface sites. Also, the performance of the anode catalyst depends on the parameters of the intrinsic chemical kinetics of formic acid oxidation and physical parameters such as electrode thickness, surface area, effective diffusion, concentration of formic acid and temperature.

In this contribution, a diffusive-kinetic-based model is developed for the porous anode catalyst layer of DFAFCs to study mass transfer limitations that may affect the distribution of fuel and removal of products. An effectiveness factor approach is introduced to find the effects of mass transfer and kinetic parameters on Pt-Ru catalyst utilization. This model is developed in stages from simple physical situations that capture kinetics and pore geometry to more complex and realistic ones. The analysis is used to provide guidelines for experimental conditions that help the transport of reactants and products in the layer. The need and effect of pore formers to facilitate effective transfer of reactants and products through the Catalyst Layer will also be analyzed and discussed. Details about the formulation and preliminary results will be discussed. Pore size, concentrations, diffusion, and pore shape are among the key parameters studied. Since the effectiveness factor is a "macro-scale" approach, comparisons with more rigorous microscopic models will also be presented as validation.