Mechanistic analysis of coupled humidification, backpressure, and cathode stoichiometry effects in HT-PEMFCs

• Coupled λ air , backpressure and RH effects mapped via EIS-DRT. • Backpressure lowers mass transport resistance by up to 59% at 0.4 A/cm². • 5% RH raises mass transport resistance by up to 63% compared to 0% RH. • Humidified pressurization amplifies instability × 13.5. • UOI ranks dry moderate-pressure and λ air =2.5 operation as optimal. High-temperature proton exchange membrane fuel cells (HT-PEMFCs) based on phosphoric-acid-doped polybenzimidazole membranes are promising for reformate-fed systems, yet their performance is highly sensitive to system operating parameters. This study evaluates the influence of cathode stoichiometry (λ air =1.8–2.5), reactant backpressure (0.0–2.0 barg at the anode and 0.0–1.8 barg at the cathode), and 5% inlet humidification at 160°C using polarization analysis combined with electrochemical impedance spectroscopy (EIS), distribution of relaxation times (DRT), Kramers–Kronig validation, and Monte-Carlo uncertainty quantification. Relative to dry atmospheric operation (0.0 barg, 0% RH), increasing backpressure reduces transport-related resistance by up to 43% at λ air =1.8 and 59% at λ air =2.5 (0.4 A/cm²). In contrast, introducing 5% RH at atmospheric pressure increases mass transport resistance by approximately 36% (λ air =1.8) and 63% (λ air =2.5), while KK residuals increase by up to 48% and 24%, respectively. A Unified Operating Index (UOI) integrating voltage, resistance, and stability was developed to rank operating regimes. At 0.8 A/cm², λ air =1.8 and 2.0-1.8 barg, humidification lowers the UOI by 75.8% relative to dry operation (amplification factor ≈13.5). Increasing stoichiometry to λ air =2.5 lowers this effect by ∼50% under identical conditions but does not eliminate instability. Dry operation with higher stoichiometry and moderate backpressure provides the most suitable operating window.