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The study of proton conductivity processes has gained intensive attention in the past decades due to their potential applications in chemical sensors, electrochemical devices, and energy generation. The scientific community has focused its efforts on the development of high‐performing polymeric membranes as proton exchange membranes (PEMs) for fuel cell (FC) applications. In particular, high conductivity at different humidity and temperature and enhanced chemical and mechanical stability under operative conditions are considered the main goals to be reached. The design of mixed‐matrix membranes (MMMs) based on conductive polymers and inorganic fillers is an approach commonly used for achieving materials with improved conductive and mechanical properties. In the last five years, the use of metal‐organic frameworks (MOFs) as fillers for conductive MMMs has rapidly grown for their intrinsic stability and structural versatility. The recent progress around the proton conductivity of MOF based composite membranes on PEMs for FC applications is critically reviewed.

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Mixed membrane matrices (MMMs) made up with Nafion and nanocrystals of zirconium metal-organic framework (MOF) UiO-66 or the analogous sulfonated SOH-UiO-66 were prepared by varying the filler loading and the size of the crystals. The combined effects of size and loading, together with the presence of sulfonic groups covalently linked to the MOFs, were studied with regard to the conductivity and mechanical properties of the obtained composite matrices. A large screening of membranes was preliminarily made and, on the most promising samples, an accurate conductivity study at different relative humidities and temperatures was also carried out. The results showed that membranes containing large crystals (200 nm average size) in low amounts (around 2%) displayed the best results in terms of proton conductivity values, reaching values by 30% higher than those of pure Nafion, while leaving the mechanical properties substantially unchanged. On the contrary, MMMs containing MOFs of small size (20 nm average size) did not show any conductivity improvements if compared to pure Nafion membranes. The effect of MOF sulfonation was negligible at low filler loading whereas it became important at loading values around 10%. Finally, membranes with a high filler loading (up to 60 wt %) of sulfonated UiO-66 showed a slight reduction of conductivity in comparison with membranes loaded at 20% of nonsulfonated ones.

An important problem for medium temperature polymer electrolyte fuel cells (MT PEMFCs) operating in the temperature range 90–140 °C is the short time‐life of proton conducting membranes. To shed some light on the empirical annealing treatments used for increasing the membrane durability, a systematic research on the effects of thermal treatments of Nafion 117 membranes was undertaken with the hope that the information obtained could be useful for a better understanding of the real limits for MT PEMFCs. Kinetic experiments showed that, for each couple of T–RH values, the water taken up from the membrane reaches a constant value only after long times of equilibration (≥200 h). Taking into account that the enlargements provoked by the water‐uptake remain as permanent deformations when the samples are cooled, it was found that the evolution of the deformations provoked by changes in temperature and RH can be conveniently estimated at 20 °C by determining the water taken up after equilibration in liquid water. By relating the counter‐elastic index of the matrix (nc(m)) to the extent of these deformations, a set of equations were obtained which allowed us to predict their evolution with changes of temperature and relative humidity. A good agreement with experimental values was found. The importance of this discovery for the development of MT PEMFCs is discussed.

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