Transport Limitations in Polymer Electrolyte Membrane Fuel Cell Electrodes

Kurzfassung

Polymer electrolyte membrane fuel cell (PEMFC) can convert the chemical energy within an electrochemical reaction to electrical energy with high efficiency. This technology enables powering electric automotive, portable, and stationary devices without producing any harmful emissions to the environment. There has been an effort to adapt it to the high-power density and durability standards of the automotive sector while keeping it commercially marketable and competitive. One step into making it cost-efficient is reducing the platinum loading of the membrane electrode assembly (MEA) used on the energy conversion. Moreover, the cell’s performance at high currents densities (HCD) suffers from significant mass transport losses that shorten the PEMFC’s lifetime and limits its maximum power output, making it unfit for automotive purposes. Amid the recent progress achieved, the requirement of operating at HCD to fit the spatial limitations of a conventional automobile is the main shortcoming of this technology. The study herein tested the performance of a commercial MEA with 25 cm² produced by Gore Fuel Cell Technologies with the operating parameters validated by the automotive sector for PEMFC usage while assessing the impact of oxygen concentration (10 %, 15 %, 20.95 %, and 30 %) and cathodic stoichiometry (1.6, 1.8, 2.0, and 2.2) changes in the reactant feed stream. Mass transport resistance always affected the performance at 0.8 A/cm² when coupling 10 % oxygen and cathode’s stoichiometry of 1.6. At the low current density (LCD) region, switching to 30 % oxygen concentration instead of 10 % oxygen concentration only granted a 4 % increase in the power output. At high current density HCD (2.0 A/cm²), this difference grew to 22 % (now comparing 15 % oxygen concentration to 30 % due to some limitations in the installation). Electrochemical impedance spectroscopy (EIS) was employed to diagnose the impact the chosen variables had on the mass transport limitations of the cell. An equivalent electric circuit (EEC) was selected to replicate the cell’s electrical behavior. The mathematical output of this fitting allowed the quantification of the primary resistances contributing to the performance loss. The mass transport was the most significant resistance at HCD and ranged from about 0.1 to 0.3 Ω∙cm², and the lowest assessed loss at LCD was going from about 0.01 to 0.03 Ω∙cm². The EIS was also useful to identify water management issues at 0.5 A/cm² due to the configuration of the flow field multi-serpentine pattern. This issue was not detected during this thesis when using a flow field with a single-serpentine gas channel (GC). Consequently, the characterization setup could be significantly improved for further research.