Deciphering Oxygen Diffusion Resistance in the PEM Fuel Cell Cathode

Quantifying gas diffusion resistance in proton exchange membrane fuel cells (PEMFCs), particularly on the cathode side, is crucial for performance optimization. Oxygen must diffuse from the gas channel through porous transport layers to reach the active sites in the cathode catalyst layer, where the oxygen reduction reaction (ORR) occurs. The multi-layered structure of the membrane electrode assembly (MEA) creates a complex mass transport environment governed by both pressure-dependent molecular diffusion and pressure-independent Knudsen diffusion, which are determined by the molecular mean free path ( λ ) and the pore size ( d p ) of the layers [1]. Consequently, the total O 2 transport resistance ( R d ) consists of both a pressure-dependent component ( R P ) and a pressure-independent component ( R NP ) [2], [3]. Most existing theoretical and experimental studies focus on oxygen transport under low oxygen concentrations (c O 2 ) and low current densities ( i ) to minimize liquid water formation, often assuming ideal water management [4]. However, these conditions do not reflect real PEMFC operation, where air serves as the cathode reactant and liquid water accumulates at high current densities. Few studies assess oxygen transport under both dry and wet conditions [5]. Moreover, the widely used limiting current density method (LCM) is time-consuming, requiring extensive data collection and relying on subjective user interpretation when it comes to determining the exact value of the limiting current density. In this work, we systematically evaluate oxygen transport resistance at the PEMFC cathode under standard operating conditions using Greenlight Innovation’s novel polarization data acquisition methodology [6]. This approach employs closed-form solutions to extract polarization parameters from a limited set of fuel cell polarization data while validating the reliability of both the data and the derived parameters. Unlike conventional LCM, this method provides a closed-form solution for calculating R d across any current density regime under conditions that closely resemble real-world PEMFC operation, as illustrated in Fig. 1. This work begins with the physical characterization of the MEA to measure the thickness and pore size distribution of the diffusion media and cathode catalyst layer. These measurements allow us to calculate the Knudsen number, Kn d (λ ,d p ), which determines the dominant oxygen diffusion mechanism in each layer. Next, polarization curves are obtained at different pressures, enabling us to analyze R d as a function of i to visualize how oxygen transport resistance varies between dry and wet operating conditions. Additionally, plotting R d against pressure ( P ) allows us to distinguish between R P ( i ) and R NP ( i ) components. Our findings reveal a dynamic shift in dominant R d with i , highlighting a transition from gas-phase diffusion limitations to water-induced transport obstacles. This refined characterization provides deeper insight into the interplay between diffusion mechanisms within the gas diffusion media and catalyst layers, offering a more comprehensive understanding of mass transport phenomena in PEMFCs Figure 1

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