Electrolyte-Dependent, "Microscopically Irreversible" H-Atom Transfer Kinetics of Ce-Based Metal-Organic Framework,...

Area of Science:

• Electrochemistry
• Materials Science
• Catalysis

Background:

• Proton-coupled electron transfer (PCET) reactions are crucial in energy-relevant processes, often involving equal proton and electron transfer.
• While solid catalyst properties are well-studied, the influence of liquid electrolytes on PCET kinetics is less understood.
• Metal-organic frameworks (MOFs) offer tunable platforms for studying interfacial redox reactions.

Purpose of the Study:

• To investigate the impact of electrolyte composition on PCET kinetics using a Ce-based MOF (Ce-MOF-808) as a model system.
• To correlate electrolyte properties with PCET reaction rates and understand deviations from microscopic reversibility.

Main Methods:

• Utilized chronoamperometry and Cottrell analysis to measure PCET hopping kinetics within Ce-MOF-808.
• Varied buffer species and proton activity in electrolytes to probe microenvironmental effects.
• Employed isothermal titration calorimetry (ITC) and computational simulations to study buffer-node binding thermodynamics.

Main Results:

• PCET kinetics in Ce-MOF-808 varied by orders of magnitude with changes in electrolyte composition.
• Reductive reactions were consistently faster than oxidative reactions, suggesting apparent microscopic irreversibility.
• Distinct buffer-node binding thermodynamics were observed for different buffers and Ce oxidation states.

Conclusions:

• Rational modulation of electrolytes can dramatically enhance PCET kinetics without altering the solid catalyst.
• Apparent microscopic irreversibility is explained by chemically distinct substrate and product species during redox reactions.
• Findings have implications for designing efficient electrocatalysts, including MOFs, heterogeneous catalysts, and enzymes.