Proton conductor NASICON-structure Li_1+xCd_x/2Zr_2−x/2(PO₄)₃ as solid electrolyte for intermediate-temperature fuel cells

Low ionic conductivity of solid electrolytes at intermediate temperatures hinders the commercialization process of solid fuel cell technology. A sodium superionic conductor (NASICON)-structure with a rigid three-dimensional network and an interconnected interstitial space is expected to be an ideal solid electrolyte for fuel cells. Based on the H⁺/Li⁺ exchange engineering strategy, here we report a NASICON-structure proton conductor Li_1+xCd_x/2Zr_2−x/2(PO₄)₃ (x = 0.5, 1, 1.5, 2) derived from CdZr₄(PO₄)₆ to construct a fuel cell device. Among all samples, the Li₃Cd₁Zr₁(PO₄)₃ cell device exhibits a high performance including peak power density 815 mW cm⁻², proton conductivity 0.165 S cm⁻¹ and activation energy 0.372 eV at 550 °C. Theoretical and experimental studies both suggest that the high proton conductivity benefits from the unique 3D interstitial space and rapid H⁺/Li⁺ exchange in the NASICON material. Under fuel cell operating conditions, the interstitial space of Li_1+xCd_x/2Zr_2−x/2(PO₄)₃ (x = 2) substitutes mobile Li⁺ with H⁺ enabling fast proton transport. The new transport mechanism and excellent proton conductivity suggest that Li_1+xCd_x/2Zr_2−x/2(PO₄)₃ provides new opportunities for enriching novel electrolyte materials in intermediate temperature protonic ceramic fuel cells (IT-PCFCs).