From layered zirconium phosphates and phosphonates to nanofillers for ionomeric membranes

This paper aims at giving a historical perspective on the work made at Perugia University in the field of proton conducting materials based on layered phosphates, phosphonates and organophosphates, and at commemorating Professor Giulio Alberti, one of the most authoritative scientists who pioneered this research field. Due to high chemical inertia and good thermal stability, layered zirconium phosphate (ZP) was investigated as a proton conductor since mid 80s, but it was soon clear that the conductivity was too low for application in electrochemical devices working at high current densities. Attempts to enhance the ZP conductivity led first to the development of intercalation compounds with aliphatic or aromatic amines and then to the synthesis of ZP organic derivatives (zirconium phosphonates) where acidic functions, such as carboxylic or sulfonic groups, were covalently anchored to the ZP layers through alkyl or aryl groups. Although with both types of materials it was possible to enhance to a great extent the conductivity of ZP, low thermal or hydrolytic stability prevented their use as fillers of ionomeric membranes for fuel cell application. On the other hand, ZP based nanofillers, bearing hydrophobic groups anchored to the ZP layers, turned out to be suitable to improve both mechanical properties and, surprisingly, conductivity of perfluorosulfonic acid ionomers (PFSA). Moreover, the combined use of such hydrophobic materials and hydrophilic ZP as PFSA fillers led to more effective mechanical reinforcement and to better fuel cell performance than that achieved by single filler membranes.