Mechanistic analysis of coupled humidification, backpressure, and cathode stoichiometry effects in HT-PEMFCsTodorovski, Filip (Avtor) Todorovski, Emilija (Avtor) Kravos, Andraž (Avtor) Katrašnik, Tomaž (Avtor) Sekavčnik, Mihael (Avtor) Mori, Mitja (Avtor) Lotrič, Andrej (Avtor) high-temperature PEM fuel celldistribution of relaxation timescathode stoichiometryreactants backpressurereactants relative humidityHigh-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.20262026-04-29 12:34:04Članek v reviji182193UDK: 621.352.6:539.125.4:662.611.2ISSN pri članku: 0013-4686DOI: 10.1016/j.electacta.2026.148933COBISS_ID: 276710659sl