Design Rules for Efficient Charge Transfer in Metal-Organic Framework Films: The Pore Size Effect

Area of Science:

• Materials Science
• Electrochemistry
• Nanotechnology

Background:

• Redox-active metal-organic frameworks (MOFs) facilitate charge transfer via redox hopping, a mechanism involving electron hopping and ion diffusion for electroneutrality.
• Understanding the relationship between MOF structure and charge transport dynamics is essential for designing advanced energy storage and electronic devices.

Purpose of the Study:

• To investigate the correlation between the pore size of ferrocene-doped MOFs (Fc-MOFs) and the rates of electron and ion diffusion.
• To quantify the independent electron and ion diffusion coefficients and transfer rate constants in Fc-MOFs with varying pore sizes.

Main Methods:

• Preparation of three Fc-MOF films with distinct pore sizes (15-47 Å) immobilized on conductive substrates.
• Application of a theoretical model to chronoamperometric responses to independently determine diffusion coefficients (D_e, D_i) and transfer rate constants (k_e-hop, k_i-hop).

Main Results:

• Electron diffusion coefficients (D_e) ranged from approximately 10^-12 to 10^-7 cm^2 s^-1, and ion diffusion coefficients (D_i) ranged from 10^-16 to 10^-12 cm^2 s^-1.
• Electron transfer rate constants (k_e-hop) were between 10^3 and 10^7 s^-1, while ion transfer rate constants (k_i-hop) were between 10^-3 and 10^1 s^-1.
• Increasing MOF pore size correlated with an increase in k_i-hop and a decrease in k_e-hop.

Conclusions:

• MOF pore size significantly impacts the kinetics of both electron and ion transport.
• Larger MOF pore sizes enhance ion hopping and the overall charge-transfer rate (k_hop), demonstrating a pathway for optimizing redox-active MOF performance.