Impedance modeling of PEM fuel cells incorporating catalyst-layer heterogeneity

Voltage losses in proton exchange membrane fuel cells stem from electrochemical reactions and tortuous mass transport arising from structural heterogeneity. Although physics-based impedance modeling is effective for analyzing losses, homogeneous models have been widely employed owing to their straightforward implementation and low computational cost; however, their applicability to real electrode structures is limited. Therefore, this study aims to develop an impedance modeling approach that incorporates heterogeneous catalyst layer (CL) structures. A heterogeneous continuum model incorporating the local properties of the constructed 3D CL structure was employed. Impedance spectra were computed by solving steady-state nonlinear equations and frequency-domain linearized equations for small perturbation amplitudes. The simulated impedance spectra were compared with experimental data using the distribution of relaxation times (DRT), and the cell-voltage dependence of DRT peaks and their frequencies were consistent, confirming model validity. The effect of current density, oxygen partial pressure, and relative humidity on impedance spectra and reaction–transport resistances was systematically investigated. At high current density or low oxygen partial pressure, not only oxygen-diffusion resistance but also proton-conduction resistance became significant, and the increase was reflected in the low-to-mid frequency range. The model enables the extraction of conductance distributions encompassing various reaction–transport contributions, facilitating microscopic analysis. A homogeneous model with the same macroscopic properties as the heterogeneous model yielded nearly identical DRT peak frequencies. However, the heterogeneous model showed greater polarization resistance and peak magnitudes, demonstrating the limitations of homogeneous models. The presented heterogeneous model enables advanced CL designs through macroscopic and microscopic impedance analyses. • Development of heterogeneous continuum model incorporating catalyst-layer structure. • Impedance derivation from steady-state and frequency-domain calculations. • Model validation with experimental data via distribution of relaxation times. • Analyses of reaction–transport resistances and conductance distributions. • Comparison of impedance between heterogeneous and homogeneous catalyst-layer models.