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

Area of Science:

• Materials Science
• Electrochemistry
• Physical Chemistry

Background:

• Proton-coupled electron transfer (PCET) is crucial for molecular reactivity but its role in long-range charge transport remains unclear.
• Ion-coupled charge transport (ICCT) impacts devices like batteries and electrocatalysts, yet its experimental signatures and governing factors are poorly understood.
• High-surface-area materials, like battery electrodes, are prime candidates for observing ion-electron coupling due to proximity of electrons and electrolytes.

Purpose of the Study:

• To investigate the effect of ion-electron coupling on electronic and ionic conductivity using metal-organic frameworks (MOFs) as a model system.
• To understand the experimental signatures and microscopic factors governing ion-coupled charge transport (ICCT).

Main Methods:

• Electrochemical investigation of Ti-containing MOFs.
• Photochemical doping to generate proton-electron (e^--H⁺) pairs.
• Introduction of solvent guest molecules to modulate conductivity.
• Direct current (DC) and alternating current (AC) conductivity measurements.

Main Results:

• Ti-MOFs transitioned from electronic 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) upon doping and solvent inclusion.
• DC and AC techniques confirmed the existence of proton-electron coupling.
• Enhanced ionic conductivity was found to directly improve electronic conductivity.

Conclusions:

• This study provides direct evidence that proton-coupled electron transfer (PCET) facilitates long-range charge transport in materials.
• The findings offer generalized electrochemical tools and synthetic strategies for studying ion-electron coupling in diverse materials.
• The results have implications for designing advanced materials for energy storage and catalysis.