Molecule-Based Proton–Electron Mixed Conductor with the Highest Ambipolar Conductivity

Abstract

Proton–electron mixed conductors (PEMCs) are an essential component for potential applications in hydrogen separation and energy conversion devices. However, the exploration of PEMCs with excellent mixed conduction, which is quantified by the ambipolar conductivity, σ_amb = σ_eσ_H/(σ_e + σ_H) (σ_e: electronic conductivity; σ_H: proton conductivity), is still a great challenge, largely due to the lack of structural characterization of both conducting mechanisms. In this study, we prepared a molecule-based proton–electron mixed-conducting cation radical salt, (ET)₄[Pt₂(pop)₂(Hpop)₂]·PhCN (ET: bis(ethylenedithio)tetrathiafulvalene, pop^2–: P₂H₂O₅^2–), by electrocrystallization. The salt shows metallic electronic conduction, which arises from the (ET)₂^•+ layers, with a high σ_e value (1–2 S cm^–1 at room temperature). The metallic state was corroborated by magnetic susceptibility measurement and band structure calculation. The salt also shows superprotonic conduction (σ_H = 2.1 × 10^–2 S cm^–1 at room temperature under dried conditions), which relies on the one-dimensional (1D) hydrogen-bonding network of protonated paddlewheel-type Pt-dimer complex anions, [Pt₂(pop)₂(Hpop)₂]^2–. Crystallographic and computational studies revealed the presence of infinite intra- and intermolecular O–H···O hydrogen bonds, which show a double-well-like potential energy curve with a negligible energy barrier in the 1D chain, facilitating Grotthuss-type proton hopping via Lewis basic sites. Among the structurally defined PEMCs, the present salt displays the highest room temperature σ_amb value of 2.1 × 10^–2 S cm^–1 even under dried conditions. The σ_amb value has the same order as those of reported perovskites-type metal oxides at high temperatures and achieves a level that is beneficial for the practical applications.