| Sumario: | High-temperature proton exchange membrane fuel cells based on phosphoric acid-doped polybenzimidazole membranes are a technology characterized by simplified construction and operation along with possible integration with, e.g., methanol reformers. Significant progress has been achieved in terms of key materials, components and systems. This review is devoted to updating new insights into the fundamental understanding and technological deployment of this technology. Polymers are synthetically modified with basic functionalities, and membranes are improved through cross-linking and inorganic–organic hybridization. New insights into phosphoric acid along with its interactions with basic polymers, metal catalysts and carbon-based supports are recapped. Recognition of parasitic acid migration raises acid retention issues at high current densities. Acid loss via evaporation is estimated with respect to the acid inventory of membrane electrode assembly. Acid adsorption on platinum surfaces can be alleviated for platinum alloys and non-precious metal catalysts. Binders have been considered a key to the establishment of the triple-phase boundary, while recent development of binderless electrodes opens new avenues toward low Pt loadings. Often ignored microporous layers and water impacts are also discussed. Of special concern are durability issues including acid loss, platinum sintering and carbon corrosion, the latter being critical during start/stop cycling with mitigation measures proposed. Long-term durability has been demonstrated with a voltage degradation rate of less than 1 μV h−1 under steady-state tests at 160 °C, while challenges remain at higher temperatures, current densities or reactant stoichiometries, particularly during dynamic operation with thermal, load or start/stop cycling.
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