Towards Medium-Temperature Hydrogen Fuel Cell with Glassy Proton-Conductive Membrane—Part II: Mixed-Anion Matrices, Composites and Hybrid Systems

With the rising interest in hydrogen technologies as a pathway toward lower-carbon energy systems, there is a growing need for proton exchange membranes that can operate reliably in the 120–200 °C window. This second part of the review examines mixed phosphate–silicate networks, composites, and hybrid membranes designed to move beyond the limitations of the single-anion glasses discussed in Part I. Rather than listing compositions only, the present analysis is organized around a comparative framework that links network chemistry, hydration management, pore-space morphology, interfacial proton transport, and durability under thermal/humidity cycling. Mixed-anion lattices, sol–gel-derived porous glasses, polymer-assisted interpenetrating networks, ionic-liquid-modified systems, fully inorganic composites, and mechanochemically prepared hybrids are evaluated with respect to conductivity, humidity tolerance, structural stability, and device relevance. Particular attention is paid to strategies that attempt to decouple proton conductivity from simple water uptake by combining acidic-site engineering with mesostructural control. The literature shows that recent progress is real but uneven. Conductivity gains are often achieved through better retention of hydrated proton pathways or acid-rich interphases, yet these benefits remain constrained by pore collapse, acid migration, gas crossover, interfacial losses, or insufficient long-term validation in membrane–electrode assemblies. The review, therefore, closes with a cross-class benchmarking matrix and a synthesis-oriented guide intended to support more critical comparison of future intermediate-temperature membrane designs.