Unique Proton Transportation Pathway in a Robust Inorganic Coordination Polymer Leading to Intrinsically High and Sustainable Anhydrous Proton Conductivity

Although
comprehensive progress has been made in the area of coordination
polymer (CP)/metal–organic framework (MOF)-based proton-conducting
materials over the past decade, searching for a CP/MOF with stable,
intrinsic, high anhydrous proton conductivity that can be directly
used as a practical electrolyte in an intermediate-temperature proton-exchange
membrane fuel cell assembly for durable power generation remains a
substantial challenge. Here, we introduce a new proton-conducting
CP, (NH4)3[Zr­(H2/3PO4)3] (ZrP), which consists of one-dimensional zirconium
phosphate anionic chains and fully ordered charge-balancing NH4+ cations. X-ray crystallography, neutron powder
diffraction, and variable-temperature solid-state NMR spectroscopy
suggest that protons are disordered within an inherent hydrogen-bonded
infinite chain of acid–base pairs (N–H···O–P),
leading to a stable anhydrous proton conductivity of 1.45 × 10–3 S·cm–1 at 180 °C, one
of the highest values among reported intermediate-temperature proton-conducting
materials. First-principles and quantum molecular dynamics simulations
were used to directly visualize the unique proton transport pathway
involving very efficient proton exchange between NH4+ and phosphate pairs, which is distinct from the common guest
encapsulation/dehydration/superprotonic transition mechanisms. ZrP as the electrolyte was further assembled into a H2/O2 fuel cell, which showed a record-high electrical
power density of 12 mW·cm–2 at 180 °C
among reported cells assembled from crystalline solid electrolytes,
as well as a direct methanol fuel cell for the first time to demonstrate
real applications. These cells were tested for over 15 h without notable
power loss.