Room-Temperature Superprotonic Conductivity beyond 10–1 S cm–1 in a Co(II) Coordination Polymer

An efficient design of crystalline solid-state proton conductors (SSPCs) is crucial for the progress of clean energy applications. Developing such materials to make them work at room temperature with a conductivity of ≥10 –1 S cm –1 is of significant interest in terms of technical and commercial aspects. Utilizing the recently highlighted “coordinated-water-driven proton conduction” approach, herein, we have rationally synthesized two highly stable and scalable 1D Co(II) coordination polymers (CPs) as SSPCs, PCM-2 {[Co(bpy)(H 2 O) 2 (NO 3 ) 2 ]·H 2 O} n and PCM-3 {[Co 2 (bpy) 2 (SO 4 ) 2 (H 2 O) 6 ].4H 2 O} n, with distinct alignments in coordinated water and coordinated oxo-anions (nitrate and sulfate, respectively). The acidity of the metal-bound water molecules in PCM-2 is further enhanced through cooperative long-range continuous H bonds with coordinated Brønsted basic nitrates (proton acceptors), leading to ultrahigh superprotonic conductivities even at 25 °C (1.03 × 10 –1 S cm –1 under 95% RH), and reached a maximum of 2.99 × 10 –1 S cm –1 at 85 °C (95% RH). The conductivity at 25 °C is even higher than that of commercial Nafion 117 (6.74 × 10 –2 S cm –1 at 100% RH). The absence of such an H-bonding interaction in PCM-3 (closed loops) resulted in a lesser conductivity of 5.87 × 10 –5 S cm –1 (95% RH, 85 °C). PCM-2 represents the first example of SSPC exhibiting conductivity in the order 10 –1 S cm –1 at ambient temperature (25 °C) with excellent recyclability.