High performing Y 3 H 5 P 6 O 22 and Er 3 H 5 P 6 O 22 proton conductors: Preparation and Conductivity Study

Abstract

Two novel trivalent rare-earth mixed-anion phosphates, Y3H5P6O22 (YHPO) and Er3H5P6O22 (EHPO), were synthesised using the thin layer technique (TLT) and processed into dense ceramic electrolytes via the cold sintering process (CSP). Structural characterisation by powder X-ray diffraction confirmed tetragonal symmetry for both compositions, with slight variations in lattice parameters consistent with ionic radius differences between Y 3+ and Er 3+ . Chemical analysis and infrared spectroscopy verified the coexistence of orthophosphate (PO4 3-) and pyrophosphate (P2O7 4-) groups in a 1:1 molar ratio, forming hydrogen-bonded networks relevant to proton transport. The cold-sintered pellets exhibited high relative densities (>88 %) and preserved stoichiometry. Electrochemical impedance spectroscopy performed between 40 °C -140 °C under controlled wet (pH2O = 0.033 atm) and low humidity (pH2O ~10 -5 atm) nitrogen atmospheres revealed humidity-dependent proton conduction.Under wet conditions, two transport regimes were identified: a low-temperature vehicular mechanism associated with surface-adsorbed water and a higher-temperature thermally activated regime consistent with Grotthuss-type proton hopping. EHPO exhibited superior conductivity, reaching 1.47 x 10 -4 S cm -1 at 140 °C, nearly one order of magnitude higher than YHPO. Under low humidity, both materials showed reduced conductivity and single Arrhenius-type behaviour with activation energies in the range of 50 kJ mol -1 -60 kJ mol -1 , indicating dominant structural proton diffusion. The enhanced performance of EHPO is attributed to its slightly more compact lattice, promoting favourable hydrogen-bond connectivity. These findings demonstrate the potential of trivalent rare-earth mixed phosphates as promising intermediate-temperature proton-conducting electrolytes.