• A systematic study of HT-PEMFC response to realistic methanol reformate compositions • Reformate gas compositions based on methanol steam reforming 0D process modeling. • Spatial mapping reveals impurity effects and how water vapor restores current distribution. • DRT analysis separates impurity effects on electrochemical processes. HT-PEMFCs are attractive for operation with methanol steam reforming systems, but their response to individual reformate components remains complex and spatially non-uniform. This study investigates the effects of carbon dioxide (CO₂), carbon monoxide (CO), and water vapor (H₂O) on HT-PEMFC performance under representative reformate compositions at 160°C and 170°C. Gas mixtures containing hydrogen with controlled CO₂ (10-30%) and CO (0.1-3%) concentrations were examined, while water vapor levels (1.0-2.5% RH) were selected based on reformer process modeling. A combined diagnostic approach using polarization curves, electrochemical impedance spectroscopy with distribution of relaxation times (DRT), and segmented current density mapping was applied. CO₂ induces performance losses primarily through hydrogen dilution and changes in fast anode processes, with higher overall degradation observed at 170°C due to increased ohmic resistance rather than kinetic. CO addition causes severe voltage losses, particularly at 160°C, driven by catalyst site blocking and strong kinetic limitations. Spatial analysis reveals that both CO₂ and CO preferentially suppress initially highly active MEA regions, leading to a more uniform-but overall reduced-current distribution. Introducing water vapor improves performance mainly by reducing the ohmic resistance associated with membrane hydration. This effect is spatially localized near the anode inlet and does not fully restore previously poisoned regions, indicating that hydration mitigates transport and conductivity losses but does not reverse kinetic deactivation. These findings highlight the importance of temperature-dependent ohmic effects and spatial diagnostics for realistic reformate-fed HT-PEMFC operation.

• Scalable electrodeposition yields Ni/MXene and Ni/Mo₂C composites with enhanced HER performance. • Carbides and MXenes reshape Ni into high-area, high-activity interfaces for alkaline electrolysis. • DEMS enable reliable HER onset detection and clarify the rate-determining step. • Composites outperform Ni cathodes in activity, charge transfer, and stability for low-cost alkaline electrolyzers. • Ni/Mo₂C shows the fastest charge-transfer kinetics (lowest R ct ), while Ni/Mo₂TiC₂ offers the best activity–stability balance. Nickel-based catalysts are promising alternatives to noble metals for hydrogen evolution in alkaline media; however, their long-term stability and scalability remain a challenge. In this paper, we report the synthesis of nickel composites with laboratory-synthesized molybdenum carbide (Mo₂C) and MXenes (Ti₃C₂ and Mo₂TiC₂) via electrodeposition, a cost-effective and industrially scalable method compatible with the fabrication of alkaline water electrolysis (AWE) electrodes. The catalysts were systematically characterized by SEM/EDX, DRX, EIS, cyclic voltammetry, chronoamperometry, and, notably, differential electrochemical mass spectrometry (DEMS). DEMS enabled real-time quantification of hydrogen evolution, providing mechanistic insights beyond conventional electrochemical techniques. The Ni/Mo₂C composite exhibited the lowest charge-transfer resistance and superior intrinsic activity, whereas Ni/MXene electrodes demonstrated higher electrochemically accessible surface areas and improved stability. Overall, this work highlights electrodeposition as a versatile strategy for preparing advanced Ni-based composites and underscores the value of DEMS in elucidating the hydrogen evolution mechanism in noble-metal-free systems

High entropy materials (HEMs) have emerged as a promising new class of electrocatalysts distinguished by their immense compositional flexibility and structural complexity, which can give rise to unprecedented catalytic properties and open new avenues for fundamental insight into catalysis. Realizing the potential of HEM electrocatalysts is dependent on building a deep understanding of structure-property relationships, based on nuanced characterization of both structural and electrochemical properties. This critical review navigates the current landscape of HEMs in electrocatalysis, exploring the evolving nomenclature and conceptual frameworks that shape how these materials are described and understood across the literature. We consider the diverse characterization techniques used to investigate the structure of HEMs, discussing the length scales and chemical contrasts they provide as well as the challenges associated with characterizing highly complex, multielement systems. Building on this foundation, we highlight key studies of HEMs in electrocatalysis, including those focused on screening the vast compositional space of HEMs to identify new electrocatalysts, or else examining the activity and stability of HEM electrocatalysts in detail, drawing insights into the factors that govern their performance. This critical review also emphasizes emerging research directions and strategies where advanced characterization and design approaches may help unlock the full potential of HEMs in electrocatalytic applications.

• Transport impedance with Current-modulated Hydrogen flow-rate Spectroscopy (CH2S). • Effective diffusivities and mass transport resistances of hydrogen measured with CH2S. • GDL substrate and MPL layers operando transport properties differentiated. • Transport properties compared for passive-PEMFC and conventional-PEMFC • Current density and active area effects on hydrogen transport in passive PEMFC. Current-modulated Hydrogen flow-rate Spectroscopy (CH2S) is the mass-transport impedance that relates cell current ( I ˜ ) and hydrogen flow rate ( Q ˜ H 2 ) in the anode of a proton exchange membrane fuel cell (PEMFC). The transfer function ( H = n F Q ˜ H 2 / I ˜ ) provides kinetic information about transport processes in the anode of a PEMFC under working conditions. CH2S can differentiate hydrogen transport in the gas diffusion layer substrate (GDLS) and in the microporous layer (MPL) of the anode. Using an analytical model and the distribution of relaxation times formalism, the effective, operando , hydrogen diffusivities and mass transport resistances of the GDLS and MPL are obtained. Here CH2S is used to study mass transport in a PEMFC anode fed with quasi-static hydrogen and air atmospheres, or passive PEMFC (p-PEMFC). The anode is a commercial gas diffusion electrode, with Pt/C based catalyst layer, a hydrophobic carbon black MPL, and a woven carbon cloth GDLS. The results show larger mass transport losses in the GDLS of the p-PEMFC compared with a convective PEMFC (c-PEMFC) fed with gases under forced convection. The MPL, however, presents same mass transport losses in both cell types, with operando water saturation close to 100%. Increasing current density, up to 180 mA cm -2 , has little impact on GDLS transport properties but improves them in the MPL attributed to thermal activation and drier conditions in the anode by water electroosmotic dragging. Larger active area size increases transport losses in p-PEMFC due to more difficult passive elimination of water from the cell by natural forces. Changes in the p-PEMFC design are proposed to mitigate mass transport losses and bring its efficiency closer to the c-PEMFC.