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▪ Abstract  The main obstacles to greater commercialization of polymer electrolyte fuel cells are mostly related to the low-proton conductivity at low-relative humidity of the known ionomeric membranes, to their high methanol permeability and poor mechanical properties above ∼130°C. A possible solution for these problems has been found in the development of composite membranes, where particles of suitable fillers are dispersed in the ionomer matrix. The preparation methods for obtaining composite membranes are described, and recent work dealing with composite ionomeric membranes containing silica, heteropolyacids, layered metal phosphates, and phosphonates is reviewed. Finally, new strategies for the preparation of nano-composite membranes and for the filling of porous polymeric membranes with highly conductive zirconium phosphonates are described. The expected influence of size and orientation of these particles on membrane properties, such as conductivity and permeability to methanol, is also discussed.

Proton conductors, catalysts, and solar energy storage materials are some of the potential uses of various crystalline acid salts of tetravalent metals. Results obtained on a large number of metal(IV) phosphates and phosphonates are reviewed, the α‐ and β‐structured layered metal(IV) phosphates and the organic derivatives of zirconium phosphates being emphasized. Methods of preparation and the structures determined—especially layered and pillared—are described.

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