Coordinated water molecule-induced solid-state superprotonic conduction by a highly scalable and pH-stable coordination polymer (CP)

Abstract

The effective design of new efficient crystalline solid-state proton conductors (SSPCs) is relevant to the advancement of clean energy applications. Although it is well known that metal ions enhance the acidity of their coordinated water molecules, proton-conducting coordination polymers (CPs) and metal–organic frameworks (MOFs) in which coordinated water molecules act as sole intrinsic proton sources are largely unexplored. Considering such a design principle, we present a highly scalable and highly robust (stable over a wide pH range of 2–10, in open air for six months, water and boiling water) CP, IITKGP-101, in which Cu₄(μ₃-OH)₄ SBUs are coordinated with ample water molecules and display superprotonic conductivity in both the single crystal and pellet forms. The 1D chains with hydrophilic interlayer spaces within the framework housing abundant coordinated water as the sole proton source, which make an extended H-bonding pathway for efficient proton migration and thus exhibit a conductivity value of 5.2 × 10⁻⁴ S cm⁻¹ in single crystal form and 2.45 × 10⁻⁴ S cm⁻¹ in pellet form at 80 °C and 98% RH. The easy scalability in gram scale in an aqueous medium and highly robust nature make this framework a promising candidate as an SSPC.