Open-Framework Chalcogenide (H₃O)KCu₆Ge₂S₈· nH₂O Exhibiting High Mixed Proton-Electron Conduction

Mixed ion-electron conductors have a wide range of important applications in devices relying on mixed ion-electron transport, mainly including conducting polymer composites and ceramics. Herein, we present a new type of mixed ion-electron conductor, an open-framework chalcogenide, (H3O)KCu6Ge2S8·nH2O (1), with a three-dimensional (3D) framework and two types of one-dimensional (1D) channels, occupied by hydronium ions and potassium ions, respectively. Thermogravimetric (TG) and powder X-ray diffraction (PXRD) measurements suggest that the hydrated proton species in the channels of 1 are in the form of H2n+1On+ (n > 1). The impedance measurements reveal that 1 is an intrinsic mixed proton-electron conductor. In the N2 atmosphere, the conductivity (σ) of 1 increases with release of water molecules and increasing temperature with σ = 0.71 S cm-1 at 123 K and 1.61 S cm-1 at 473 K in the first cooling run, and in this case, the electron conduction is much higher than the ion conduction. At 98% relative humidity (RH), however, the situation is opposite, and the proton conductivity of 1 is higher than its electron conduction with an ion conductivity (σi)/electron conductivity (σe), σi/σe, of 1.99 × 10-3/6.31 × 10-4 S cm-1 at 298 K and 2.49 × 10-2/2.52 × 10-3 S cm-1 at 343 K. To the best of our knowledge, 1 is the first example of an open-framework material with mixed proton-electron conduction, and this study demonstrates that open-framework materials, including chalcogenides, MOFs, and COFs, are good candidates for mixed proton-electron conductors.