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

Abstract

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, <b>PCM-2</b> {[Co(bpy)(H₂O)₂(NO₃)₂]·H₂O}_<i>n</i> and <b>PCM-3</b> {[Co₂(bpy)₂(SO₄)₂(H₂O)₆].4H₂O}_<i>n</i>, with distinct alignments in coordinated water and coordinated oxo-anions (nitrate and sulfate, respectively). The acidity of the metal-bound water molecules in <b>PCM-2</b> 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 <b>PCM-3</b> (closed loops) resulted in a lesser conductivity of 5.87 × 10^-5 S cm^-1 (95% RH, 85 °C). <b>PCM-2</b> represents the first example of SSPC exhibiting conductivity in the order 10^-1 S cm^-1 at ambient temperature (25 °C) with excellent recyclability.