Intermediate Temperature Superprotonic Conductivity beyond 10–2 S cm–1 with Low Proton Transfer Energy Barrier in a Metal–Organic Framework

Abstract

Design of solid-state proton conductors (SSPCs) operating at low to intermediate temperatures (25–60 °C) is desirable to address the thermal-management issue in proton-exchange membrane fuel cells (PEMFCs). Among the various approaches in designing SSPCs, “orthophosphate coordination” is highly unexplored in MOFs. Herein, a new MOF: IITKGP-103 {[Ag(hmta)H₂PO₄]·2H₂O}_n with orthophosphate coordinated to Ag(I) center, is rationally designed. Synchronous alignments of two amphiprotic proton sources/carriers (H₂PO₄^– and water) resulted in highly extended H-bonded networks that eased the fastest protonic relay (σ = 2.92 × 10^–2 S cm^–1) at intermediate temperature (60 °C) with smaller proton transfer (PT) energy barriers for both the identified conducting channels (∼5.6 kcal mol^–1 for intralayer and ∼2.2 kcal mol^–1 for interlayer), making MOF:IITKGP-103 the best performing intermediate temperature SSPC material. In contrast, nonsynchronous alignments of proton carriers in another orthophosphate coordinated framework Ag-bpy (1) resulted in 100 times lower conductivity with higher barriers (∼6.2 and ∼10.9 kcal mol^–1) as validated through quantum-tunneling analysis.