Homogeneous redox catalysis of electrochemical reactions

Molecular Unravelling of the Structural Effect of Quinone Redox Mediators on Oxygen Reduction Reaction in Aprotic Lithium-Oxygen Batteries

Redox mediators (RMs) provide tantalizing solutions to unlock the energy capabilities of aprotic lithium-oxygen (Li-O2) batteries by driving solution-mediated Li2O2 growth. However, the structural effect of RMs on the catalytic efficiency of the oxygen reduction reaction remains incompletely understood. Herein, we present the interplay between the structure of RMs and their discharge capabilities by a comparative study of model quinone (Q)-based RMs. Specifically, at low current densities, incorporating electron-withdrawing groups onto the Q ring can positively move the discharge potential and deliver larger discharge capacity by extending the lifespan of the LiQO2 intermediate and allowing for Li2O2 growth into deeper electrolyte regions. Conversely, at high current densities, the absence of electron-withdrawing groups facilitates homogeneous reaction kinetics from LiQ to regenerate Q (i.e., decreased lifespan of LiQO2), mitigating electrode potential polarization and preserving catalytic activity of Q for higher discharge capacity. The work establishes structure-property relationships that guide the rational design of RMs toward next-generation Li-O2 batteries.

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

Combined Effects of Hemicolligation and Ion Pairing on Reduction Potentials of Biphenyl Radical Cations

Formal reduction potentials of highly oxidizing and short-lived radical cations of substituted biphenyls generated by pulse radiolysis in 1,2-dichloroethane (DCE) were measured using a redox equilibrium ladder method. The effect of halide ion-radical interactions on reduction potentials of biphenyls was examined by utilizing the ability of DCE to release Cl- in the vicinity of the radical cation. The Hammett correlation of measured potentials across a range of over 700 mV shows saturation at high Hammett sigma values. This effect has been explained by both ion-pairing and hemicolligation interactions between biphenyl radical cations and Cl- and appears to modulate reduction potentials by as much as 400 mV. This finding offers a convenient way to manipulate the energetics of electron transfer involving organic redox species.

Triarylmethyl cation redox mediators enhance Li-O2 battery discharge capacities

A major impediment to Li-O2 battery commercialization is the low discharge capacities resulting from electronically insulating Li2O2 film growth on carbon electrodes. Redox mediation offers an effective strategy to drive oxygen chemistry into solution, avoiding surface-mediated Li2O2 film growth and extending discharge lifetimes. As such, the exploration of diverse redox mediator classes can aid the development of molecular design criteria. Here we report a class of triarylmethyl cations that are effective at enhancing discharge capacities up to 35-fold. Surprisingly, we observe that redox mediators with more positive reduction potentials lead to larger discharge capacities because of their improved ability to suppress the surface-mediated reduction pathway. This result provides important structure-property relationships for future improvements in redox-mediated O2/Li2O2 discharge capacities. Furthermore, we applied a chronopotentiometry model to investigate the zones of redox mediator standard reduction potentials and the concentrations needed to achieve efficient redox mediation at a given current density. We expect this analysis to guide future redox mediator exploration.

The Solvent Effect on H2 O2 Generation at Room Temperature Ionic Liquid|Water Interface

H₂ O₂ is a versatile chemical and can be generated by the oxygen reduction reaction (ORR) in proton donor solution in molecular solvents or room temperature ionic liquids (IL). We investigated this reaction at interfaces formed by eleven hydrophobic ILs and acidic aqueous solution as a proton source with decamethylferrocene (DMFc) as an electron donor. H₂ O₂ is generated in colorimetrically detectable amounts in biphasic systems formed by alkyl imidazolium hexafluorophosphate or tetraalkylammonium bis(trifluoromethylsulfonyl)imide ionic liquids. H₂ O₂ fluxes were estimated close to liquid|liquid interface by scanning electrochemical microscopy (SECM). Contrary to the interfaces formed by hydrophobic electrolyte solution in a molecular solvent, H₂ O₂ generation is followed by cation expulsion to the aqueous phase. Weak correlation between the H₂ O₂ flux and the difference between DMFc/DMFc⁺ redox potential and 2 electron ORR standard potential indicates kinetic control of the reaction.

Electrochemistry for Atom Transfer Radical Polymerization

Atom Transfer Radical Polymerization (ATRP) is the most powerful and most employed technology of Controlled Radical Polymerization (CRP) to produce polymers with well-defined architecture, that is, composition, topology, and functionality. Several hundreds of papers are published every year on ATRP processes, mainly based on empiric experimental procedures. Electrochemistry powerfully entered in the field of ATRP about 10 years ago, providing important contributions both to the further development of the process and to a better understanding of its mechanism. Five main issues took advantage of electrochemistry and/or its synergism with ATRP: i) understanding the mechanism of ATRP activation; ii) determination of thermodynamic parameters; iii) determination of activation and deactivation rate constants; iv) the SARA ATRP vs SET-LRP dispute: the role of Cu⁰ ; v) electrochemically-mediated ATRP.

Interplay between Structure and Mechanism in Reductive Dissociative Electron Transfers to α,β -Epoxyketones

The electrochemical reduction of several α,β -epoxyketones was studied using cyclic (linear sweep) voltammetry, convolution voltammetry, and homogeneous redox catalysis. The results were reconciled to pertinent theories of electron transfer. α,β -Epoxyketones undergo dissociative electron-transfer reactions with C-O bond cleavage, via both stepwise and concerted mechanisms, depending on their structure. For aliphatic ketones, the preferred mechanism of reduction is consistent with the "sticky" concerted model for dissociative electron transfer. Bond cleavage occurs simultaneously with electron transfer, and there is a residual, electrostatic interaction in the ring-opened (distonic) radical anion. In contrast, for aromatic ketones, because the ring-closed radical anions are resonance-stabilized and exist at energy minima, a stepwise mechanism operates (electron transfer and bond cleavage occur in discrete steps). The rate constants for ring opening are on the order of 10⁸  s^-1 , and not significantly affected by substituents on the 3-membered ring (consistent with C-O bond cleavage). These results and conclusions were fully supported and augmented by molecular orbital calculations.

Analysis of multi-electron, multi-step homogeneous catalysis by rotating disc electrode voltammetry: theory, application, and obstacles

Rotating disc electrode (RDE) voltammetry has been widely adopted for the study of heterogenized molecular electrocatalysts for multi-step fuel-forming reactions but this tool has never been comprehensively applied to their homogeneous analogues. Here, the utility and limitations of RDE techniques for mechanistic and kinetic analysis of homogeneous molecular catalysts that mediate multi-electron, multi-substrate redox transformations are explored. Using the ECEC' reaction mechanism as a case study, two theoretical models are derived based on the Nernst diffusion layer model and the Hale transformation. Current-potential curves generated by these computational strategies are compared under a variety of limiting conditions to identify conditions under which the more minimalist Nernst Diffusion Layer approach can be applied. Based on this theoretical treatment, strategies for extracting kinetic information from the plateau current and the foot of the catalytic wave are derived. RDEV is applied to a cobaloxime hydrogen evolution reaction (HER) catalyst under non-aqueous conditions in order to experimentally validate this theoretical framework and explore the feasibility of RDE as a tool for studying homogeneous catalysts. Crucially, analysis of the foot-of-the-wave via this theoretical framework provides rate constants for elementary reaction steps that agree with those extracted from stationary voltammetric methods, supporting the application of RDE to study homogeneous fuel-forming catalysts. Finally, obstacles encountered during the kinetic analysis of cobaloxime, along with the voltammetric signatures used to diagnose this reactivity, are discussed with the goal of guiding groups working to improve RDE set-ups and help researchers avoid misinterpretation of RDE data.

CO2-based hydrogen storage – hydrogen liberation from methanol/water mixtures and from anhydrous methanol

New strategies for the reforming of methanol under mild conditions on the basis of heterogeneous and molecular catalysts have raised the hopes and expectations on this fuel. This contribution will focus on the progress achieved in the production of hydrogen from aqueous and anhydrous methanol with molecular and heterogeneous catalysts. The report entails thermal approaches, as well as light-triggered dehydrogenation reactions. A comparison of the efficiency and mechanistic aspects will be made and principles of catalytic pathways operating in biological systems will be also addressed.

Mechanism of Supplemental Activator and Reducing Agent Atom Transfer Radical Polymerization Mediated by Inorganic Sulfites: Experimental Measurements and Kinetic Simulations

The mechanism of atom transfer radical polymerization (ATRP) mediated by sodium dithionite (Na2S2O4), with CuIIBr2/Me6TREN as catalyst (Me6TREN: tris[2-(dimethylamino)ethyl]amine)) in ethanol/water mixtures, was investigated experimentally and by kinetic simulations. A kinetic model was proposed and the rate coefficients of the relevant reactions were measured. The kinetic model was validated by the agreement between experimental and simulated results. The results indicated that the polymerization followed the SARA ATRP mechanism, with a SO2•- radical anion derived from Na2S2O4, acting as both supplemental activator (SA) of alkyl halides and reducing agent (RA) for CuII/L to regenerate the main activator CuI/L. This is similar to the reversible-deactivation radical polymerization (RDRP) procedure conducted in the presence of Cu0. The electron transfer from SO2•-, to either CuIIBr2/Me6TREN or R-Br initiator, appears to follow an outer sphere electron transfer (OSET) process. The developed kinetic model was used to study the influence of targeted degree of polymerization, concentration of CuIIBr2/Me6TREN and solubility of Na2S2O4 on the level of polymerization control. The presence of small amounts of water in the polymerization mixtures slightly increased the reactivity of the CuI/L complex, but markedly increased the reactivity of sulfites.

Faster Dissociation: Measured Rates and Computed Effects on Barriers in Aryl Halide Radical Anions

Carbon-halogen bond dissociation rates for a series of aryl halide radical anions (ArX-: X = Cl, Br) in NMP were measured at room temperature by pulse radiolysis with 10-11 s time resolution. To obtain accurate dissociation rates, care was taken to measure and correct for competing decay channels. The observed rates correlated well with activation energies computed in the gas phase by density functional (DFT) calculations. The rates did not correlate well with electron affinities or dissociation energies obtained by the same computational methods, although such correlations are reported in the literature and are expected on the basis of simple models. The calculations also found that the transition state structures had bent carbon-halogen bonds. Bending enables large reductions of the activation energies by an electronic effect involving mixing of phi* and sigma* states. This bending-induced mixing is computed to increase the dissociation rates by a few orders of magnitude and is thus essential to understanding these reactions.

Reaction of Aromatic Radical Cations with RuCl3·3H2O

Interactions of RuCl3·3H2O with radical cations of aromatic ethers, 1,4-di-tert-butyl- 2,5-dimethoxybenzene, 3,5-di-tert-butyl-1,2-dimethoxybenzene and 2-tert-butyl-1,4-di- methoxybenzene, and with aromatic amines, 2,4,6-tri-tert-butylaniline and N,N,N',N'-tetra- methyl-1,4-phenylenediamine, were observed by voltammetry. ESR and UV spectroscopies were used for the study of the the first two ethers. The effect of RuCl3·3H2O was also examined by controlled potential electrolysis.