Towards Medium-Temperature Hydrogen Fuel Cells with Glassy Proton-Conductive Membranes—Part I: Fundamentals and Single-Anion Matrices

The accelerated deployment of hydrogen technologies is widely discussed as a pathway to mitigate climate change and reduce environmental pollution associated with fossil fuel use. In this context, intermediate-temperature proton-exchange membranes that operate in the 120–200 °C window, similar to the one characterizing liquid-acid PAFC systems (much larger in their power range), are sought as a bridge between low-temperature PFSA-based PEMFCs and low-temperature PCFs, thus combining reduced sensitivity to external humidification with solid-electrolyte handling. This Part I review surveys phosphate- and silicate-based glassy proton conductors as single-anion baseline matrices and organizes the literature around a mechanistic screening framework that links processing fingerprints—particularly sol–gel hydrolysis/condensation conditions, aging, drying, and thermal treatment—to pore architecture, hydration state, and the dominant proton-transport regime. Across both families, conductivity is governed by coupled variables: network chemistry (acidic site density and connectivity), water activity (RH), and microstructure-controlled percolation and retention. Reported σ values can arise from fundamentally different regimes, ranging from hopping-dominated transport supported by dense hydrogen-bond networks and proton-bearing groups to carrier-assisted, water-mediated transport in connected porosity, with distinct humidity dependence and stability implications. Accordingly, the review treats σ(T,RH) and activation energy together with hydration/porosity indicators as primary screening metrics, and it records missing durability and device-level information—chemical stability (hydrolysis and leaching/acid migration), mechanical robustness and cycling response, and current/power density where available—as explicit knowledge gaps. While substantial progress has been achieved within single-anion phosphate and silicate glasses, particularly through engineered acidity and microstructural control, most systems remain limited by hydration drift under gradients, thermal/humidity cycling stability, and electrode/electrolyte interfacial constraints when evaluated against intermediate-temperature membrane requirements. These conclusions establish a quantitative baseline and comparison rules for Part II, which will assess mixed-network, composite, and hybrid strategies designed to decouple conductivity from water-retention and durability trade-offs.