Enhanced Proton Conductivity via Lithium-Ion Synergism in Li₂MnO₃ Electrolyte for Protonic Ceramic Fuel Cells

This study introduces a novel mechanism for enhancing proton conductivity in protonic ceramic fuel cells (PCFCs) through lithium-ion synergy, addressing the limitations posed by structural defects in traditional oxide proton conductors and improving the cell performance at lower temperatures. We repurposed Li₂MnO₃ (LMO), originally a cathode material for lithium-ion batteries, into a highly effective ionic conducting electrolyte during PCFC operation. The dual conduction mechanism of protons and lithium-ions in LMO was thoroughly investigated using Raman spectroscopy, proton filtering fuel cell tests, the hydrogen isotopic effect on impedance spectroscopy, and distribution of relaxation times (DRT) analysis. Operational tests using H₂/air revealed that LMO electrolyte achieved a peak power density of 701 mW·cm^-2 at 520 °C and exhibited an ionic conductivity of 0.15 S·cm^-1. Critically, the coupling of lithium-ions with protons during the proton embedding process enhanced proton mobility via a new interlayer lithium vacancy-interstitial migration pathway, markedly boosting the proton conductivity and significantly improving fuel cell performance.