Preparation of proton conducting composites by mechanical milling for phosphorus-containing solid acids

Solid acids such as AlH2P3O10·2H2O, Zr(HPO4)2·H2O, amorphous-AlPO4, and AlH3(PO4)2·3H2O showed low conductivities of 10− 8–10− 7 S cm− 1 at 130 °C, whereas mechanical milling for these solid acids was found to improve the conductivities by 2 to 4 orders of magnitude. On the other hand, a large amount of phosphorus was leached out during a durability test after milling. XRD showed the solid acids were transformed to amorphous or crystal–amorphous composites by mechanical milling. 31P MAS NMR spectra indicated that the intensities of the bands due to Q3 and Q4 units decreased and that of the peak due to Q0 units increased. The increase in conductivity and the decrease of chemical durability of the mechanically milled solid acids are probably caused by the formation Q0 units, i.e., orthophosphoric acid, in the resultant composites due to the disruption of inorganic structures.