# Electrolyte-Dependent, “Microscopically Irreversible” H‑Atom Transfer Kinetics of Ce-Based Metal–Organic Framework, Ce-MOF-808

## TL;DR

This paper shows how changing the electrolyte can greatly affect hydrogen atom transfer reactions in a cerium-based metal-organic framework, leading to faster and more efficient redox reactions.

## Contribution

The study reveals that electrolyte composition can dramatically alter PCET kinetics in Ce-MOF-808, defying microscopic reversibility expectations.

## Key findings

- Changing the buffer species and proton activity in the electrolyte can alter PCET hopping kinetics by orders of magnitude.
- Reductive reactions in Ce-MOF-808 are 3–10 times faster than oxidative reactions under the same electrochemical driving force.
- ITC and simulations show that buffer-node binding thermodynamics differ based on buffer chemistry and node oxidation state.

## Abstract

Redox reactions at
the interface of metal oxides and protic electrolytes
almost always involve protons and electrons in equal amounts. Given
the stoichiometry, these proton-coupled electron transfer (PCET) reactions
are thermochemically equivalent to net H-atom transfer (HAT) reactions.
The correlation between the chemical nature of solid catalysts and
HAT kinetics has been employed for decades as the design principle
for energy-relevant reactions (e.g., reactions of 2H+/H2). More recently, chemists have experimentally determined
that a change in liquid electrolytes that alters the microenvironment
at the redox-active sites has an equally profound impact on electrocatalysis
involving PCET/HAT. Yet, precise correlations between the chemical
nature of electrolytes and the PCET kinetics are, to date, rare in
the literature. Herein, we report our findings using the Ce-based
metal–organic framework, Ce-MOF-808, as a model system. Each
Ce6(μ3–O)4(μ3–OH)4(OH)6(H2O)6 node of this MOF undergoes a 1H+/1e– redox reaction. Using chronoamperometry and the Cottrell analysis,
we have determined that the PCET hopping kinetics within the pores
of Ce-MOF-808 can change by orders of magnitude by altering the buffer
species and the proton activity of the electrolyte. Furthermore, in
all buffers, reductive reactions were ∼3–10 times faster
in kinetics than the reverse oxidative reaction with the same electrochemical
driving force, suggesting that the system, at first glance, violates
the principle of microscopic reversibility. Isothermal titration calorimetry
(ITC) and computational simulations corroborated that the buffer-node
binding thermodynamics are quite distinct, depending on the chemical
nature of the buffer and the oxidation state of the node. Together,
these results suggest that the substrate and the product during the
oxidative vs reductive reaction of Ce-MOF-808 are chemically different
species, which explains the apparent ‘microscopic irreversibility.’
Thus, the rational modulation of electrolytes can dramatically enhance
PCET kinetics, even though the solid electrodes remain identical.
Implications of these findings are contrasted with the electrochemical/electrocatalytic
behavior of other redox-active MOFs, heterogeneous catalysts, and
enzymatic systems at the solid–liquid interface.