Determination of Oxygen and Hydrogen Mass Transfer Coefficients in PEMFC GDE and Their Separation into Gas and Electrolyte Contributions
Abstract
Electrochemical power systems, such as proton exchange membrane fuel cells (PEMFCs), direct methanol fuel cells, zinc/air and lithium/air batteries should operate at high current to maximize power density and decrease size in applications with rigorous volume requirements . Under these conditions, perfonnance is limited by the finite transport rates of reactants and products. Specifically for PEMFCs, transport takes place within gas, solid, liquid phases and at the interface between them within a gas diffusion electrode, consisting of a gas diffusion layer and a catalyst layer. This issue is a significant concern for a PEMFC cathode because ambient ai r, the most econoniical and convenient oxidant, only contains 21 % of oxygen. At the anode, a low hydrogen stoichiometry accompanied by recirculation is beneficial to maximize fuel efficiency, but hydrogen dilution occurs as a result of nitrogen and water crossover from the cathode. A better understanding of transport limitations is necessary to increase performance, improve membrane/electrode assembly design, reduce fuel cell size in applications with volume restrictions and support cost reduction efforts, because a decrease in platinum-containing catalyst loading causes mass transport issues due to the appearance of an interfacial transport resistance of unknown origin. The project focuses on validation and further development of a recently proposed method to determine and separate mass transfer coefficients and study fundamental aspects of transport mechanisms at the PEMFC cathode and anode. The method is based on two key elements: the use of a limiting current density distribution mathematical model and different diluent gases with varying molecular_weights for the oxygen or hydrogen reactant streams. A limiting current density distribution is measured with a segmented cell system operated with a low reactant concentration and is fitted to the analytic model to extract the overall mass transfer coefficient. The change in reactant diluent molecular weight only affects mass transport in the gas phase. The resulting linear relationship between the inverse overall mass transfer coefficient and the diluent molecular weight yields the combined reactant mass transfer coefficient through ionomer/water films and fine pores in catalyst and microporous layers (Knudsen diffusion). The gas phase mass transfer coefficient (molecular diffusion) is calculated using the additive relation between the overall mass transfer coefficient and the individual contributions. The project has two main tasks: method validation and method development. Method validation ¥ Verify measurement reproducibility. ¥ Identify mass transport processes responsible for the impact of the microporous layer (gas phase transport) on the oxygen mass transfer coefficient originally assigned to the ionomer (solid phase transport). ¥ Compare oxygen mass transfer coefficients to values derived from impedance spectroscopy models and data. Method development ¥ Simplify the measurement procedure by decreasing the number of gaseous dilents. á Extend the applicability range of the method to lower current densities by modifying the current distribution model with a mixed kinetic and mass transfer control dependent local current. ¥ Investigate the impact of operating conditions (pressure, gas humidity) on mass transport mechamisms at the PEMFC electrodes. ¥ Study the effect of ionomer loading and thickness on the oxygen mass transfer coefficients to clarify differences between nano structure thin films and ionomer inks catalyst layers. á Apply the method to the anode and determi ne hydrogen mass transport processes.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jan 12, 2017
- Source ID
- W911NF1510188
Entities
People
- Tatyana V. Reshetenko
Organizations
- Army Contracting Command
- United States Army
- University of Hawaiʻi System