Turn-On Conductivity with Proton-Coupled Electron Transport in Metal-Organic Frameworks

Proton-coupled electron transfer (PCET) has been studied for decades in the context of molecular reactivity, but its impact on long-range electron transport is barely understood. When defined broadly as ion-coupled charge transport (ICCT), relevant systems include lithium-ion battery electrodes, electrochromic coatings, and myriad electrocatalysts. Despite ample evidence that ion-electron coupling enhances or diminishes the performance of these devices, little is known about the experimental signatures of ICCT and the microscopic factors that govern its mechanism. We expect that ion-electron coupling becomes especially relevant in high-surface-area materials, such as the layered electrodes of intercalation batteries, due to the close proximity of itinerant electrons and electrolytes. Here, we report an electrochemical investigation into a family of metal-organic frameworks (MOFs) that serves as a well-defined platform for understanding the effect of ICCT on both electronic and ionic conductivity. Through photochemical doping of e^--H⁺ pairs and the introduction of solvent guest molecules, the Ti-containing MOFs convert from electronic-only insulators (σ_e ≈ 10^-12 S cm^-1) to mixed ion-electron semiconductors (σ_e ≈ 10^-7 S cm^-1, σ_ion ≈ 10^-5 S cm^-1). Direct current and alternating current techniques support the existence of proton-electron coupling and, critically, demonstrate that improved ionic conductivity enhances electronic conductivity. Taken together, these results provide direct evidence that PCET enables long-range charge transport and generalized electrochemical tools and synthetic methods for studying ion-electron coupling in materials broadly.