Enhanced Anhydrous\nProton Conductivity in Azole Phosphonic\nAcid Mixtures

Azole molecules are investigated as potential candidates\nfor proton\nconductors under anhydrous conditions. Since 1,2,3-triazole has the\nlowest melting point (<i>T</i><sub>m</sub> = 17 °C),\nit was blended with three phosphonic acid-containing molecules (small\nmolecules with one and two phosphonic acids per molecule and a phosphonic\nacid polymer) to provide a source of excess protons to enhance the\nproton conductivity of the blends. We study a wide range of compositions\nin each system to find that these three mixtures show a maximum proton\nconductivity at moderate doping compositions, approximately 5–10\nazole molecules per phosphonic acid group. Using NMR diffusometry,\nwe show that the protons bonded to nitrogen move faster than the protons\nbonded to carbons of 1,2,3-triazole, suggesting proton hopping between\nazole proton carriers. Given the high proton conductivity at 90 °C\nof the best mixtures, in the range of 20–60 mS/cm, this work\nprovides a path forward for future work in anhydrous proton-conducting\npolymer membranes. Additionally, Raman spectroscopy was used to accurately\ndetermine the molar percentage of protonated 1,2,3-triazole. Combining\nthat with the proton diffusion results, we find that the phosphonic\nacid polymer shows the most proton hopping at low acid content.