Intrinsic and extrinsic proton conductivity in metal-organic frameworks

Abstract

Metal-organic frameworks (MOFs), a new class of solid-state materials, have recently been investigated as proton conductors, but little is known about their mechanisms. Since most of the conductivities reported so far were measured using powder samples, there is uncertainty as to whether they exhibit intrinsic proton transport through frameworks and/or micropores, or extrinsic transport through interparticle phases. Herein, we re-visit ferrous oxalate dihydrate [Fe(ox)(H₂O)₂] (ox = oxalate anion), which is a dense MOF and recognized as a model system for MOF-based proton conductors. By single-crystal measurements using microelectrodes, we show that protons do not transport through the crystals (<10⁻⁹ S cm⁻¹), but that the conductivity observed in powder samples originates from interparticle phases. This result raises a question as to how general is this phenomenon? We have comprehensively surveyed the literature on solid-state proton conductors and found that large numbers of MOFs, including [Fe(ox)(H₂O)₂], have a similar activation energy to those of gels and interparticle conductors in classical solid-state materials. This indicates a considerable contribution from interparticle phases towards proton conductivity in MOFs, and single crystal analysis or special methods for powder analysis are clearly necessary to confirm intrinsic conductivity.