Effect of mixed alkali metal ions in highly proton conductive K/Cs-hydrogen sulfate-phosphotungstic acid composites prepared by mechanical milling
Abstract Fast proton transport materials are of high interest for their application in diverse fields and devices like electrochemical sensors, fuel cells, capacitors, electrochromic devices and electrochemical reactors. This work shows inorganic solid proton conducting composites prepared with phosphotungstic (WPA) heteropolyacid base material and MHSO 4 (MHS (M K and Cs)) inorganic solid acids via mechanical planetary ball milling. Very fast proton conducting irreversible new phase composites were obtained, with 90 (50KHS-50CHS)-10WPA (mol %) mixed metal ions composite exhibiting the highest performance with conductivity from 4.9 × 10 −2 to 1.4 × 10 −3  S cm −1 at 160 to 40 °C, respectively; compared to those of the precursor materials (CHS and MHS) on the order of 10 −3 to 10 −8 under the same temperature regime. The XRD, FT-Raman, 1 H-MAS-NMR and HR-TEM results of the composites revealed the formation of an amorphous nano-interface proton-conducting pathway shell around an M-substituted WPA crystalline core, resulting in a nanoionics effect leading to the high conductivities observed; in addition to preferred sites of alkali metal ions substitution into the WPA with different stabilities and conductivities with respect to type of ion. These results will be of significant interest to the applicable fields and inspire further research.