Lithium-coupled electron transfer reactions of nano-confined WO_x within Zr-based metal-organic framework

Interfacial charge transfer reactions involving cations and electrons are fundamental to (photo/electro) catalysis, energy storage, and beyond. Lithium-coupled electron transfer (LCET) at the electrode-electrolyte interfaces of lithium-ion batteries (LIBs) is a preeminent example to highlight the importance of charge transfer in modern-day society. The thermodynamics of LCET reactions define the minimal energy for charge/discharge of LIBs, and yet, these parameters are rarely available in the literature. Here, we demonstrate the successful incorporation of tungsten oxides (WO_x) within a chemically stable Zr-based metal-organic framework (MOF), MOF-808. Cyclic voltammograms (CVs) of the composite, WO_x@MOF-808, in Li⁺-containing acetonitrile (MeCN)-based electrolytes showed an irreversible, cathodic Faradaic feature that shifted in a Nernstian fashion with respect to the Li⁺ concentration, i.e., ∼59 mV/log [(Li⁺)]. The Nernstian dependence established 1:1 stoichiometry of Li⁺ and e^-. Using the standard redox potential of Li^+/0, the apparent free energy of lithiation of WO_x@MOF-808 (ΔG_app,Li) was calculated to be -36 ± 1 kcal mol^-1. ΔG_app,Li is an intrinsic parameter of WO_x@MOF-808, and thus by deriving the similar reaction free energies of other metal oxides, their direct comparisons can be achieved. Implications of the reported measurements will be further contrasted to proton-coupled electron transfer (PCET) reactions on metal oxides.