We present an update and revision to our 2010 review on the topic of protoncoupled electron transfer (PCET) reagent thermochemistry. Over the past decade, the data and thermochemical formalisms presented in that review have been of value to multiple fields. Concurrently, there have been advances in the thermochemical cycles and experimental methods used to measure these values. This Review (i) summarizes those advancements, (ii) corrects systematic errors in our prior review that shifted many of the absolute values in the tabulated data, (iii) provides updated tables of thermochemical values, and (iv) discusses new conclusions and opportunities from the assembled data and associated techniques. We advocate for updated thermochemical cycles that provide greater clarity and reduce experimental barriers to the calculation and measurement of Gibbs free energies for the conversion of X to XH n in PCET reactions. In particular, we demonstrate the utility and generality of reporting potentials of hydrogenation, E°(V vs H 2 ), in almost any solvent and how these values are connected to more widely reported bond dissociation free energies (BDFEs). The tabulated data demonstrate that E°(V vs H 2 ) and BDFEs are generally insensitive to the nature of the solvent and, in some cases, even to the phase (gas versus solution). This Review also presents introductions to several emerging fields in PCET thermochemistry to give readers windows into the diversity of research being performed. Some of the next frontiers in this rapidly growing field are coordination-induced bond weakening, PCET in novel solvent environments, and reactions at material interfaces.

Proton-coupled electron transfer (PCET) reactions are increasingly being studied in nonaqueous conditions, where the thermochemistry of PCET substrates is largely unknown. Herein, we report a method to obtain electrochemical standard potentials and calculate the corresponding bond dissociation free energies (BDFEs) of stable PCET reagents in nonaqueous solvents, using open-circuit potential (OCP) measurements. With this method, we measure PCET thermochemistry in acetonitrile and tetrahydrofuran for substrates with O–H and N–H bonds that undergo 1e –/1H+ and 2e –/2H+ redox processes. We also report corrected thermochemical values for the 1/2H2(g)/H• 1M and H+/H• (C G) couples in several organic solvents. For 2e –/2H+ couples, OCP measurements provide the multielectron/multiproton standard potential and the average of the two X–H BDFEs. In contrast to traditional approaches for calculating BDFEs from electrochemical measurements, the OCP method directly measures the overall PCET reaction thermodynamics and avoids the need for a pK a scale in the solvent of interest. Consequently, the OCP approach yields more accurate thermochemical values and should be general to any solvent mixture compatible with electrochemical measurements. The longer time scale of OCP measurements enables accurate thermochemical measurements for redox couples with irreversible or distorted electrochemical responses by cyclic voltammetry, provided the PCET reaction is chemically reversible. Recommendations for successful OCP measurements and limitations of the approach are discussed, including the current inability to measure processes involving C–H bonds. As a straightforward and robust technique to determine nonaqueous PCET thermochemistry, these OCP measurements will be broadly valuable, with applications ranging from fundamental reactivity studies to device development.

A novel equilibrium strategy for measuring the hydrogen atom affinity of colloidal metal oxide nanoparticles is presented. Reactions between oleate-capped cerium oxide nanoparticle colloids (nanoceria) and organic protoncoupled electron transfer (PCET) reagents are used as a model system. Nanoceria redox changes, or hydrogen loadings, and overall reaction stoichiometries were followed by both 1 H NMR and X-ray absorption near-edge spectroscopies. These investigations revealed that, in many cases, reactions between nanoceria and PCET reagents reach equilibrium states with good mass balance. Each equilibrium state is a direct measure of the bond strength, or bond dissociation free energy (BDFE), between nanoceria and hydrogen. Further studies, including those with larger nanoceria, indicated that the relevant bond is a surface O−H. Thus, we have measured surface O−H BDFEs for nanoceriathe first experimental BDFEs for any nanoscale metal oxide. Remarkably, the measured CeO−H BDFEs span 13 kcal mol −1 (0.56 eV) with changes in the average redox state of the nanoceria colloid. Possible chemical models for this strong dependence are discussed. We propose that the tunability of ceria BDFEs may be important in explaining its effectiveness in catalysis. More generally, metal oxide BDFEs have been used as predictors of catalyst efficacy that, traditionally, have only been accessible by computational methods. These results provide important experimental benchmarks for metal oxide BDFEs and demonstrate that the concepts of molecular bond strength thermochemistry can be applied to nanoscale materials.

Although the oxygen reduction reaction (ORR) involves multiple proton-coupled electron transfer (PCET) processes, early studies reported the absence of kinetic isotope effects (KIEs) on polycrystalline Pt, likely due to the use of unpurified heavy water (D2O). Here, we developed a methodology to prepare ultrapure D2O, which is indispensable to reliably investigate extremely surface-sensitive Pt single crystals. Pt(111) exhibits significantly higher ORR activity in D2O than in H2O with potential-dependent inverse KIEs of ~0.5, whereas Pt(100) and Pt(110) exhibit potential-independent inverse KIEs of ~0.8. Such inverse KIEs are closely correlated to the lower *OD coverage and weakened *OD binding strength, relative to *OH, which, based on theoretical calculations, are attributed to the differences in their zero-point energies. This study suggests that the competing adsorption between *OH/*OD and *O2 likely plays an instrumental role in the ORR rate-determining steps involving a chemical step preceding an electrochemical step (CE mechanism). This strategy of combining ultrapure D2O and single-crystal electrodes can be applied beyond the ORR to many other electrochemical reactions.

Metal oxide (MO x ) materials are effective catalysts or cocatalysts in a range of electrochemical reactions and energy systems. A key component of their redox chemistry is protoncoupled electron transfer (PCET). Reported here are studies of isolated cerium oxide nanoparticles deposited on fluorine-doped tin oxide (FTO) electrodes with a Langmuir−Blodgett trough and calcined. Cyclic voltammograms of these films showed welldefined, quasi-reversible waves. The E 1/2 values moved with pH by −64 ± 4 mV/pH, close to the ideal Nernstian −59 mV/pH for a 1e − /1H + couple. These results imply that the electroactive CeO− H bonds had an average bond dissociation free energy (BDFE) of 78.6 ± 1.5 kcal mol −1 . Integration of the faradaic currents indicates that 0.15 ± 0.04 electrons can be added per cerium atom in the nanoparticle film or ∼20% of the surface cerium atoms. This study shows how electrochemical investigations can elucidate the stoichiometry and thermochemistry of PCET processes of MO x nanoparticle films. Comparison of these results with those for related ceria nanoparticles shows the remarkable range of properties of this useful material.