Molecular Catalysis Of Electrochemical Reactions Research Articles

Redox Behavior and Kinetics of Hydroxo Ligand Exchange on Iron Tetraphenylporphyrin: Comparison with Chloro Exchange and Consequences for Its Role in Self-Modulation of Molecular Catalysis of Electrochemical Reactions.

Thermodynamics and kinetics of hydroxide ion binding to iron tetraphenylporphyrin (TPPFe) at different redox states is investigated by electrochemistry and UV-vis spectroscopy. The reduction of initial TPPFe(III) drastically decreases the binding affinity of hydroxide ions. An activation-driving force correlation is revealed showing that the strongest the binding affinity, the largest the association rate constant and vice versa. Comparison with chloride ions shows that hydroxide ions are stronger ligands for iron tetraphenylporphyrin. However, kinetic data indicate that coordination and decoordination of chloride ions is intrinsically faster than coordination and decoordination of hydroxide ions. Finally, the consequence of hydroxide ion binding dynamics when TPPFe is used as a molecular catalyst for electrochemical reactions liberating hydroxides is discussed in the framework of self-modulation of catalytic processes.

Importance of Ligand Exchange in the Modulation of Molecular Catalysis: Mechanism of the Electrochemical Reduction of Nitrous Oxide with Rhenium Bipyridyl Carbonyl Complexes

Molecular catalysis of electrochemical reactions involving transition-metal complexes as catalysts requires getting a free metal coordination site to bind the substrate. It implies that the generation of a strong coordinating ligand as a product or coproduct of the reaction might be detrimental for an efficient catalysis because it can bind the metal center and block or slow down the catalytic process. This self-modulation phenomenon is revealed and illustrated via a thorough spectro-electrochemical investigation of the mechanism of the electrochemical reduction of nitrous oxide with rhenium bipyridyl triscarbonyl complexes [Re(bpy)(CO)3X]n+ (X = CH3CN, Cl–, n = 0 or 1) as a catalyst. We show that the bi-reduced [Re0(bpy•–)(CO)3]−, electrogenerated from [Re(bpy)(CO)3X]n+, readily reacts with N2O and produces the hydroxo complex [ReI(bpy)(CO)3(OH)]. Because hydroxide, a product of the reaction, is a stronger coordinating ligand than acetonitrile or chloride, catalysis does not occur significantly at a potential where [Re0(bpy•–)(CO)3]− is generated from [Re(bpy)(CO)3X]n+. Substantial catalysis is only triggered at a potential corresponding to the second reduction of [ReI(bpy)(CO)3(OH)]. Nonetheless, we show that a slower innersphere reduction of N2O by the mono-reduced [ReI(bpy•–)(CO)3X](n−1)+ (X = CH3CN, Cl–) occurs due to the lability of acetonitrile and chloride in these species. Because hydroxide is less labile and cannot be displaced to create an open coordination position for N2O, only an even slower outersphere reduction of N2O by the mono-reduced [ReI(bpy•–)(CO)3(OH)]− takes place. However, we finally show that an excess of free chlorides is able to displace hydroxide and then open the way for a faster innersphere process. This remarkable example emphasizes the critical role of ligand exchange in modulating molecular catalysis of electrochemical reactions with transition-metal complexes as catalysts, a likely general phenomenon.

Square wave voltammetry as a powerful tool for studying multi-electron molecular catalysts

• SWV of homogeneous two-electron molecular catalysis is theoretically investigated. • The rigorous theory applies to wide mechanistic diversity and SWV parameters. • SWV signal can enable the determination of catalytic pathways and rate constants. • SWV components are remarkably valuable to elucidate the catalyst active form. • Accurate guidelines and data analysis protocols are given. A comprehensive theoretical investigation of the square wave voltammetry (SWV) of the homogeneous two-electron molecular catalysis of electrochemical reactions is carried out. The SWV signal is studied as a function of the pulse time, the SW amplitude and the mechanistic features (catalytic rate constants, substrate concentration, formal potentials), also paying attention to the SW components that are demonstrated to be remarkably helpful in the identification of the catalyst active form. Thus, the ability of the SWV technique for the elucidation of the catalytic species and pathways, and for the determination of the corresponding rate constants is assessed, establishing accurate and valuable diagnosis criteria and procedures for quantitative data analysis based on mathematical expressions that are rigorous and easy-to-implement, and that consider the influence of diffusion on the response.

Cyclic voltammetric response of homogeneous catalysis of electrochemical reaction. Part 3: A theoretical and numerical approach for one-electron two-step reaction scheme

• Homogeneous molecular catalysis of electrochemical reactions for the one-electron two-step reaction scheme is discussed. • The exact expression of species concentrations, current-voltage and turnover frequency is derived. • The effect of the intrinsic and operational parameters on current and turnover frequency is analysed. A theoretical model for homogeneous redox catalysis of electrochemical reactions for the one-electron two-step reaction scheme is discussed. This model is described by the system of nonlinear time dependent partial differential equations for which the nonlinear term is related to a homogeneous reaction. The species concentration, current-voltage and turnover frequency curves for the transient condition have been calculated by solving the equations using a new analytical method. The current response and turnover frequency predicted under steady-state conditions when t → ∞ are in good agreement with earlier results, proving the validity of the mathematical analysis. The effect of two rate constants and initial concentration of substrate on current and turn over frequency is discussed. Our analytical results for concentration are compared with numerical (MATLAB) results, and good agreement is noted.

Molecular Catalysis of Electrochemical Reactions: Competition between Reduction of the Substrate and Deactivation of the Catalyst by a Cosubstrate Application to N2O Reduction

AbstractIn the context of molecular catalysis of electrochemical reactions, the competition between reduction of the substrate and deactivation of the catalyst by a cosubstrate is investigated. It is a frequent situation because proton donors are ubiquitous cosubstrates in reductive electrochemical reactions and molecular catalysts, either transition metal complexes or organic aromatic molecules, and are often prone to electrohydrogenation. We provide a formal kinetic analysis in the framework of cyclic voltammetry, and we show that the response is governed by two parameters and that the competition does not depend on the scan rate. From this analysis a methodology is proposed to analyze such systems and then illustrated via the study of N2O to N2 electroreduction catalyzed by 4‐cyanopyridine in acetonitrile electrolyte with water as proton donor. Incidentally, new insights into the mechanism of 4‐cyanopyridine radical anion protonation are revealed.

In Memoriam of Jean‐Michel Savéant (1933–2020)

Jean-Michel Savéant, Emeritus Professor at Université de Paris, France, and former Research Director at the Centre national de la recherche scientifique (CNRS), passed away on August 16th, 2020, at the age of 86. He was a giant of electrochemistry, combining a strong personality with extreme rigor and elegance in his work, deep creativity, and a communicative enthusiasm for ceaselessly exploring the frontiers of knowledge and chemical sciences. Born in 1933, Savéant received his undergraduate degree from the Ecole Normale Supérieure, Agrégation des Sciences Physiques (1954–1958), France, then did pre-doctoral work at l'Istituto di Chimica Fisica dell'Università di Padova (1959), Italy. After performing his military service (1960–1962), Savéant completed his Doctorat d'Etat ès Sciences at the Ecole Normale Supérieure (1966) where he then served as Vice-Director of the Chemistry Department. In 1971, he became Professor at the Université Paris VII–Denis Diderot, France. Savéant was appointed Research Director in Centre National de la Recherche Scientifique in 1985. From 1988–1989, Savéant was a distinguished Fairchild Scholar at the California Institute of Technology. Among the many honors bestowed on Savéant are the Bruno Breyer Award of the Royal Australian Chemistry Institute (2005), ECS Organic and Biological Electrochemistry Division Manuel M. Baizer Award (2002), the Galvani Medal of the Italian Chemical Society (1997), the ECS Olin Palladium Award (1993), and Faraday Medal of the Royal Chemical Society (1983). He became a member of the Académie des sciences (French Academy of Sciences) in 2000, and a Foreign Associate of the National Academy of Sciences, U.S., in 2001. Author of over 400 peer-reviewed articles, J.-M. Savéant applied for and/or received at least 10 patents. The author (with Cyrille Costentin) of “Elements of Molecular and Biomolecular Electrochemistry”, Savéant was Oskar K. Rice Distinguished Lecturer at the University of North Carolina, Chapel Hill (1995); Nelson Leonard Distinguished Lecturer at the University of Illinois (1999); and George Fisher Baker Lecturer at Cornell University (2002). Savéant presented frequently at ISE and ECS meetings, delivering invited lectures when he received the Olin Palladium and Manuel M. Baizer Awards. Jean-Michel Savéant Savéant's scientific career is deeply intertwined with the founding and development of modern molecular electrochemistry. He pioneered and developed most aspects of the field, providing it with rigorous approaches and views, from instrumentation to theoretical aspects and practical applications. In a conceptual and practical effort to solve contemporary challenges, notably energy related ones, J.-M. Savéant pioneered an enormous body of knowledge, tools, and models to develop highly original and key contributions towards various sub-fields of chemistry and biochemistry, including electron and proton transfer chemistry, modelling of chemical reactivity, chemistry of free radicals, chemistry of metal complexes, photochemistry, physical chemistry of solids, enzymology, and catalytic activation of small molecules such as CO2, O2 and H2O. Table 1 shows a selection of Savéant's publications. The complete list is given in the Supporting Information (SI). 1. Savéant, J.-M. Proton Relays in Molecular Catalysis of Electrochemical Reactions : Origin and Limitations of the Boosting Effect. Angew. Chem. Int. Ed., 2019, 58, 2125–2128. 2. Costentin, C.; Savéant, J.-M. Multielectron, multistep molecular catalysis of electrochemical reactions: benchmarking of homogeneous catalysts. ChemElectroChem, 2014, 1, 1226–1236 3. Costentin, C.; Drouet, S.; Robert, M.; Savéant, J.-M. A local proton source enhances CO2 electroreduction to CO by a molecular Fe catalyst. Science, 2012, 338, 90–94 4. Costentin, C.; Robert, M.; Savéant, J.-M. Electrochemical concerted proton and electron transfers. Potential-dependent rate constant, reorganization factors, proton tunneling and isotope effects. J. Electroanal. Chem., 2006, 588, 197–206. 5. Limoges, B.; Moiroux, J.; Savéant, J.-M. Kinetic control by the substrate and/or the cosubstrate in electrochemically monitored redox enzymatic homogeneous systems. Catalytic responses in cyclic voltammetry. J. Electroanal. Chem., 2002, 521, 1–7. 6. Robert, M.; Savéant, J.-M. Photoinduced dissociative electron transfer: is the quantum yield theoretically predicted to equal unity? J. Am. Chem. Soc., 2000, 122, 514–517. 7. Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Savéant, J.-M. Covalent modification of carbon surfaces by aryl radicals generated from the electrochemical reduction of diazonium salts. J. Am. Chem. Soc., 1997, 119, 201–207. 8. Bhugun, I.; Lexa, D.; Savéant, J.-M. Catalysis of the electrochemical reduction of carbon dioxide by iron(0) porphyrins: synergystic effect of weak Brönsted acids. J. Am. Chem. Soc., 1996, 118, 1769–1776. 9. Andrieux, C. P.; Hapiot, P.; Savéant, J.-M. Fast kinetics by means of direct and indirect electrochemical techniques. Chem. Rev., 1990, 90, 723–738. 10. Savéant, J.-M. A simple model for the kinetics of dissociative electron transfer in polar solvents. Application to the homogeneous and heterogeneous reduction of alkyl halides. J. Am. Chem. Soc., 1987, 109, 6788–6795. 11. Amatore, C.; Savéant, J.-M.; Tessier, D. Charge transfer at partially blocked surfaces: a model for the case of microscopic active and inactive sites. J. Electroanal. Chem. Interfacial Electrochem., 1983, 147, 39–51. 12. Anson, F. C.; Savéant, J.-M.; Shigehara, K. Kinetics of mediated electrochemical reactions at electrodes coated with polymer films. J. Electroanal. Chem. Interfacial Electrochem., 1983, 145, 423–430. 13. Lexa, D.; Savéant, J.-M. The electrochemistry of vitamin B12. Acc. Chem. Res., 1983, 16, 235–243. 14. Amatore, C.; Savéant, J.-M. ECE and disproportionation. VI. General resolution. Application to potential step chronoamperometry. J. Electroanal. Chem. Interfacial Electrochem., 1979, 102, 21–40. 15. Andrieux, C. P.; Dumas-Bouchiat, J.-M.; Savéant, J.-M. Homogeneous redox catalysis of electrochemical reactions. I. Introduction. J. Electroanal. Chem. Interfacial Electrochem., 1978, 87, 39–53. 16. Savéant, J.-M.; Tessier, D. Potential dependence of the electrochemical transfer coefficient. Reduction of some nitro compounds in aprotic media. J. Phys. Chem., 1977, 81, 2192–2197. 17. Nadjo, L.; Savéant, J.-M. Linear sweep voltammetry. Kinetic control by charge transfer and/or secondary chemical reactions. I. Formal kinetics. J. Electroanal. Chem. Interfacial Electrochem., 1973, 48, 113–145. 18. Garreau, D.; Savéant, J.-M. Linear sweep voltammetry. Compensation of cell resistance and stability. Determination of the residual uncompensated resistance. J. Electroanal. Chem. Interfacial Electrochem., 1972, 35, 309–331. 19. Savéant, J.-M.; Vianello, E. Potential-sweep voltammetry: general theory of chemical polarization. Electrochim. Acta, 1967, 12, 629–646. 20. Savéant, J.-M.; Vianello, E.: Recherches sur les courants catalytiques en polarographie oscillographique à balayage linéaire de tension. Étude théorique. In Advances in Polarography; 1 ed.; Longmuir, I. S., Ed.; Pergamon Press: Cambridge, U. K., 1959; Vol. 1; pp 367–374 In this special joint issue, three journals, ChemElectroChem, ChemPhysChem, and Electroanalysis have exceptionally accepted to team up to honour the memory of Prof. Jean-Michel Savéant in order to cover the three main facets of his contributions. The collection of articles illustrates the vast impact of Savéant's work in a variety of domains with contributions investigating many and diverse phenomena in electrochemistry, molecular organic and inorganic electrochemistry, molecular catalysis and electrocatalysis, bioelectrochemistry, electroanalysis, modified electrodes, fundamental aspects of electron transfers (both under electrochemical and photochemical conditions), energy storage and conversion… It is remarkable that, despite the variety of their topics, ranging from fundamental to applications, every contribution can be related to some aspects of Savéant's work, highlighting his enormous scientific legacy and its remarkable impact in current scientific challenges. We would like to thank all the authors for contributing to this special issue. Their contributions show that the future of electrochemistry, physical chemistry and electroanalysis is bright and full of promises. We hope that the readers will find in this collection a lot of inspiration, thus following the spirit of Jean-Michel Savéant who belonged to a glorious era of electrochemistry when science was guided by a curiosity for understanding the world better, and scientists were colorful personalities. Christian Amatore (Doctorat d'Etat ès Sciences: 1974–1979)1 Cyrille Costentin (Ph.D.: 1997–2000)1 Marc Robert (Ph.D.: 1992–1995)1 As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Molecular Catalysis of Electrochemical Reactions. Overpotential and Turnover Frequency: Unidirectional and Bidirectional Systems

Molecular catalysis of electrochemical reactions is taking on an increasing importance in modern energy challenges involving the activation of small molecules. The import of the notion of overpotential from the domain of electrocatalysis is a source of confusion. The consequences are particularly deleterious for the establishment of kinetic vs thermodynamic correlations with the aim to design the “best catalyst” for a targeted reaction. Herein we propose a clarification of the notion of overpotential in the context of molecular catalysis of electrochemical reactions, examining both unidirectional and bidirectional systems. In the latter case, the turnover frequencies are defined for both monoelectronic and bielectronic systems and we show that the space-dependent departure from the equilibrium in the diffusion-reaction layer has to be considered for a correct extraction of relevant kinetic parameters from electrochemical data.

Transport Phenomena: Challenges and Opportunities for Molecular Catalysis in Metal-Organic Frameworks.

Metal–organic frameworks (MOFs) are appealing heterogeneous support matrices that can stabilize molecular catalysts for the electrochemical conversion of small molecules. However, moving from a homogeneous environment to a porous film necessitates the transport of both charge and substrate to the catalytic sites in an efficient manner. This presents a significant challenge in the application of such materials at scale, since these two transport phenomena (charge and mass transport) would need to operate faster than the intrinsic catalytic rate in order for the system to function efficiently. Thus, understanding the fundamental kinetics of MOF-based molecular catalysis of electrochemical reactions is of crucial importance. In this Perspective, we quantitatively dissect the interplay between the two transport phenomena and the catalytic reaction rate by applying models from closely related fields to MOF-based catalysis. The identification of the limiting process provides opportunities for optimization that are uniquely suited to MOFs due to their tunable molecular structure. This will help guide the rational design of efficient and high-performing catalytic MOF films with incorporated molecular catalyst for electrochemical energy conversion.

Molecular approach to catalysis of electrochemical reaction in porous films

Abstract Current attempts to bridge the fields of what is conventionally called ‘electrocatalysis’ and of molecular catalysis of electrochemical reactions are surveyed and discussed. It amounts in many cases to ‘heterogenize’ molecular catalytic systems. Information on the meso- to nanostructures of the resulting catalytic films forms the basis of the understanding of new modes of transport of the reactants (catalysts, substrates and cosubstrates, products) that may govern the mechanistic competitions and consequently selectivity. Efforts to adapt benchmarking procedures developed in homogeneous molecular catalysis (catalytic Tafel plots) should be encouraged, taking into account, as additional factors, the transport of electrons and reacting species (including gases) through the catalytic system.

Proton Relays in Molecular Catalysis of Electrochemical Reactions: Origin and Limitations of the Boosting Effect

AbstractHomogeneous catalysis of electrochemical reactions, related to contemporary energy challenges, often involves proton‐coupled electron transfer sequences. The idea rapidly emerged that installing the proton donor (for reductions, or acceptor for oxidations) inside the catalyst molecule should be beneficial in terms of efficiency, as it would then be closer to the nerve center of the process (usually the metal in the case of transition metal complex catalysts). If this proton relay has indeed done the job, it has lost its proton at the end of each catalytic loop, and must therefore be reprotonated (for reductions, or deprotonated for oxidations) from acid (or base) from the solution before a new catalytic loop can start. The impression may thus be that there is a zero‐sum game. The conditions under which this is not the case may entail, in contrast, a considerable boosting of catalysis. This will also allow explain why the proton is such a specifically appropriate agent for this task.

Proton Relays in Molecular Catalysis of Electrochemical Reactions: Origin and Limitations of the Boosting Effect.

Homogeneous catalysis of electrochemical reactions, related to contemporary energy challenges, often involves proton-coupled electron transfer sequences. The idea rapidly emerged that installing the proton donor (for reductions, or acceptor for oxidations) inside the catalyst molecule should be beneficial in terms of efficiency, as it would then be closer to the nerve center of the process (usually the metal in the case of transition metal complex catalysts). If this proton relay has indeed done the job, it has lost its proton at the end of each catalytic loop, and must therefore be reprotonated (for reductions, or deprotonated for oxidations) from acid (or base) from the solution before a new catalytic loop can start. The impression may thus be that there is a zero-sum game. The conditions under which this is not the case may entail, in contrast, a considerable boosting of catalysis. This will also allow explain why the proton is such a specifically appropriate agent for this task.

Homogeneous Molecular Catalysis of Electrochemical Reactions: Manipulating Intrinsic and Operational Factors for Catalyst Improvement.

Benchmarking and optimization of molecular catalysts for electrochemical reactions have become central issues in the efforts to match contemporary renewable energy challenges. In view of some confusion in the field, we precisely define the notions and parameters (potentials, overpotentials, turnover frequencies) involved in the accomplishment of these objectives and examine the correlations that may link them, thermodynamically and/or kinetically to each other (catalytic Tafel plots, scaling relationships, "iron laws"). To develop this tutorial section, we have picked as the model catalytic reaction scheme a moderately complex mechanism, general enough to illustrate the essential issues to be encountered and sufficiently simple to avoid the algebraic nightmare that a systematic study of all possible pathways would entail. The notion of scaling relations will be the object of particular attention, having notably in mind the delimitation of their domain of applicability. At this occasion, emphasis will be put on the necessity of clearly separating what is relevant to intrinsic characteristics (through standard quantities) to what deals with the effect of varying the reactant concentrations. It will be also stressed that the occurrence of such scaling relations, otherwise named "iron laws", is not a general phenomenon but rather concerns families of catalysts. Likewise, the search of a general correlation between the maximal turnover frequency and the equilibrium free energy of the electrochemical reaction appears as irrelevant and misleading. This general analysis will then be illustrated by experimental data previously obtained with the O2-to-H2O conversion catalyzed by ironIII/II porphyrins in N, N'-dimethylformamide in the presence of Brönsted acids.

Molecular Catalysis of Electrochemical Reactions. Cyclic Voltammetry of Systems Approaching Reversibility

Efforts to design catalytic schemes approaching reversibility in which the catalyst is active for both the oxidation and reduction processes are attracting active attention boosted by recent successes. Kinetic analysis of such systems, by electrochemical techniques such as cyclic voltammetry (CV), would contribute to establish reliable mechanisms. So far, the relationships required to achieve this task have been restrained to irreversible catalytic schemes. The purpose of the present communication is to fill this gap. As a preliminary contribution, the analysis is limited to simple on-electron-one step schemes so as to set out the main features of the competition between catalytic reaction and diffusion transport of catalyst and substrates. Emphasis is put on S-shaped CV responses, which offer the best opportunities to accessing the detailed kinetic information forming the basis for determination of mechanisms.

Multielectron, multisubstrate molecular catalysis of electrochemical reactions: Formal kinetic analysis in the total catalysis regime

Cyclic voltammetry responses are derived for two-electron, two-step homogeneous electrocatalytic reactions in the total catalysis regime. The models developed provide a framework for extracting kinetic information from cyclic voltammograms (CVs) obtained in conditions under which the substrate or cosubstrate is consumed in a multielectron redox process, as is particularly prevalent for very active catalysts that promote energy conversion reactions. Such determination of rate constants in the total catalysis regime is a prerequisite for the rational benchmarking of molecular electrocatalysts that promote multielectron conversions of small-molecule reactants. The present analysis is illustrated with experimental systems encompassing various limiting behaviors.

Identifying and Breaking Scaling Relations in Molecular Catalysis of Electrochemical Reactions.

Improving molecular catalysis for important electrochemical proton-coupled electron transfer (PCET) reactions, such as the interconversions of H+/H2, O2/H2O, CO2/CO, and N2/NH3, is an ongoing challenge. Synthetic modifications to the molecular catalysts are valuable but often show trade-offs between turnover frequency (TOF) and the effective overpotential required to initiate catalysis (ηeff). Herein, we derive a new approach for improving efficiencies-higher TOF at lower ηeff-by changing the concentrations and properties of the reactants and products, rather than by modifying the catalyst. The dependence of TOF on ηeff is shown to be quite different upon changing, for instance, the pKa of the acid HA versus the concentration or partial pressure of a reactant or product. Using the electrochemical reduction of dioxygen catalyzed by iron porphyrins in DMF as an example, decreasing [HA] 10-fold lowers ηeff by 59 mV and decreases the TOF by a factor of 10. Alternatively, a 10-fold decrease in Ka(HA) also lowers ηeff by 59 mV but only decreases the TOF by a factor of 2. This approach has been used to improve a catalytic TOF by 104 vs the previously reported scaling relationship developed via synthetic modifications to the catalyst. The analysis has the potential to predict improved efficiency and product selectivity of any molecular PCET catalyst, based on its mechanism and rate law.

Homogeneous Molecular Catalysis of Electrochemical Reactions: Catalyst Benchmarking and Optimization Strategies.

Modern energy challenges currently trigger an intense interest in catalysis of redox reactions-electrochemical and photochemical-particularly those involving small molecules such as water, hydrogen, oxygen, proton, carbon dioxide. A continuously increasing number of molecular catalysts of these reactions, mostly transition metal complexes, have been proposed, rendering necessary procedures for their rational benchmarking and fueling the quest for leading principles that could inspire the design of improved catalysts. The search of "volcano plots" correlating catalysis kinetics to the stability of the key intermediate is a popular approach to the question in catalysis by surface-active sites, with as foremost example the electrochemical reduction of aqueous proton on metal surfaces. We discussed here for the first time, on theoretical and experimental grounds, the pertinence of such an approach in the field of molecular catalysis. This is the occasion to insist on the virtue of careful mechanism assignments. Particular emphasis is put on the interest of expressing the catalysts' intrinsic kinetic properties by means of catalytic Tafel plots, which relate kinetics and overpotential. We also underscore that the principle and strategies put forward for the catalytic activation of the above-mentioned small molecules are general as illustrated by catalytic applications out of this particular field.

Heterogeneous Molecular Catalysis of Electrochemical Reactions: Volcano Plots and Catalytic Tafel Plots.

We analyze here, in the framework of heterogeneous molecular catalysis, the reasons for the occurrence or nonoccurrence of volcanoes upon plotting the kinetics of the catalytic reaction versus the stabilization free energy of the primary intermediate of the catalytic process. As in the case of homogeneous molecular catalysis or catalysis by surface-active metallic sites, a strong motivation of such studies relates to modern energy challenges, particularly those involving small molecules, such as water, hydrogen, oxygen, proton, and carbon dioxide. This motivation is particularly pertinent for what concerns heterogeneous molecular catalysis, since it is commonly preferred to homogeneous molecular catalysis by the same molecules if only for chemical separation purposes and electrolytic cell architecture. As with the two other catalysis modes, the main drawback of the volcano plot approach is the basic assumption that the kinetic responses depend on a single descriptor, viz., the stabilization free energy of the primary intermediate. More comprehensive approaches, investigating the responses to the maximal number of experimental factors, and conveniently expressed as catalytic Tafel plots, should clearly be preferred. This is more so in the case of heterogeneous molecular catalysis in that additional transport factors in the supporting film may additionally affect the current-potential responses. This is attested by the noteworthy presence of maxima in catalytic Tafel plots as well as their dependence upon the cyclic voltammetric scan rate.

Molecular catalysis of electrochemical reactions

Molecular catalysis of electrochemical reactions currently attracts a lot of attention, notably concerning the transformation of small molecules in response to issues raised by modern energy challenges. This review summarizes recent advances in the mechanism analysis of multi-electrons–multi-steps processes involved in homogenous molecular catalysis of such electrochemical reactions. It also describes strategies for a rational catalyst benchmarking, through the establishment of “catalytic Tafel plots”. We show that careful analysis of through-structure and through-space substituent effects within a given family of catalyst is a powerful tool for intelligent design of new catalysts. Challenges raised by the “heterogenization” of molecular catalysts are discussed in the final section.

Electrochemistry with Impact

ChemElectroChem is in its third year and is now established as one of the top electrochemistry journals worldwide. The journal has received its first (partial) impact factor of 3.506 from Thomson Reuters's Web of Knowledge, which is an excellent result. The ten most cited articles published so far are listed in Table 1. Authors Title Vol. Page Nos. X. Zhou, Y.-G. Guo Highly Disordered Carbon as a Superior Anode Material for Room-Temperature Sodium-Ion Batteries 1 83–86 Q. Wang , J. Xu, X. Wang, B. Liu, X. Hou, G. Yu, P. Wang, D. Chen, G. Shen Core–Shell CuCo2O4@MnO2 Nanowires on Carbon Fabrics as High-Performance Materials for Flexible, All-Solid-State, Electrochemical Capacitors 1 559–564 C. Costentin, J.-M. Savéant Multielectron, Multistep Molecular Catalysis of Electrochemical Reactions: Benchmarking of Homogeneous Catalysts 1 1226–1236 X. Hou, X. Wang, B. Liu, Q. Wang, Z. Wang, D. Chen, G. Shen SnO2@TiO2 Heterojunction Nanostructures for Lithium-Ion Batteries and Self-Powered UV Photodetectors with Improved Performances 1 108–115 I. Valov Redox-Based Resistive Switching Memories (ReRAMs): Electrochemical Systems at the Atomic Scale 1 26–36 H. Kim, K.-Y. Park, M.-Y. Cho, M.-H. Kim, J. Hong, S.-K. Jung, K. C. Roh, K. Kang High-Performance Hybrid Supercapacitor Based on Graphene-Wrapped Li4Ti5O12 and Activated Carbon 1 125–130 A. Chiappone, F. Bella, J. R. Nair, G. Meligrana, R. Bongiovanni, C. Gerbaldi Structure–Performance Correlation of Nanocellulose-Based Polymer Electrolytes for Efficient Quasi-solid DSSCs 1 1350–1358 R. Frydendal, E. A. Paoli, B. P. Knudsen, B. Wickman, P. Malacrida, I. E. L. Stephens, I. Chorkendorff Benchmarking the Stability of Oxygen Evolution Reaction Catalysts: The Importance of Monitoring Mass Losses 1 2075–2081 U. B. Nasini, V. G. Bairi, S. K. Ramasahayam, S. E. Bourdo, T. Viswanathan, A. U. Shaikh Oxygen Reduction Reaction Studies of Phosphorus and Nitrogen Co-Doped Mesoporous Carbon Synthesized via Microwave Technique 1 573–579 N. Yabuuchi, Y. Matsuura, T. Ishikawa, S. Kuze, J.-Y. Son, Y.-T. Cui, H. Oji, S. Komaba Phosphorus Electrodes in Sodium Cells: Small Volume Expansion by Sodiation and the Surface-Stabilization Mechanism in Aprotic Solvent 1 580–589 Of course, ChemElectroChem would not be successful without the support from you, our readers, authors and reviewers, as well as our society partners. So, many thanks to all of you. Here is to a successful 2017!

Multielectron, Multistep Molecular Catalysis of Electrochemical Reactions: Benchmarking of Homogeneous Catalysts

AbstractCurrent–potential responses in cyclic voltammetry and in preparative‐scale electrolysis are established for two‐electron two‐step homogeneous molecular catalysis after systematic categorization of the various possible reaction schemes. They allow the derivation of catalytic Tafel plots, reflecting the intrinsic properties of the catalysts independent of contingent electrolysis parameters. They serve as a rational basis for benchmarking catalysts of the same electrochemical reaction within the same log(turnover frequency) versus overpotential plane of merit.