Jean-Michel Savéant

Using the remarkable study of C–H oxidation of substituted fluorenyl-benzoates as an example, we have shown that a mechanism crossover takes place upon decreasing the driving force, from a CPET pathway to a PTET pathway.

The transition between the insulator state and the band-conducting state is investigated by means of cyclic voltammetry in cobalt oxide porous film electrodes in phosphate-buffered solutions. It is shown that a proton-coupled faradaic oxidative process starting in the insulator region eventually builds an ohmic conduction mode upon anodic polarization. This model allows one to understand the origin of the authentic capacitive behavior of conductive metal oxide films rather than the so-called “pseudocapacitive” behavior. The particular example of cobalt oxide serves to illustrate the way in which, more generally, the behavior of “pseudocapacitors”, long ascribed to the superposition of faradaic reactions, is in fact that of true capacitors, once band-conduction has been established upon oxidation of the material.

Significance: It seems opportune to accompany the present revival of synthetic organic electrochemistry by drawing attention of synthetic chemists to the heterogeneous nature of electrochemical reactions. Quantitative analysis of the successive layers in between working electrode and solution––electrical double layer, forced convection–diffusion layer, and reaction–diffusion layers––allows the mastering of the relationships between mechanism and product selectivity. The multiple connections between catalysis and electrochemical reactions are emphasized and illustrated by typical examples. Advantage is taken of the recent advances in activation of small molecules in response to modern energy and environment challenges with extension to synthetic organic electrochemical reactions.

The two main types of charge storage devices – batteries and double layer charging capacitors – can be unambiguously distinguished from one another by the shape and scan rate dependence of their cyclic voltammetric current–potential (CV) responses. This is not the case with “pseudocapacitors” and with the notion of “pseudocapacitance”, as originally put forward by Conway et al. After insisting on the necessity of precisely defining “pseudocapacitance” as involving faradaic processes and having, at the same time, a capacitive signature, we discuss the modelling of “pseudocapacitive” responses, revisiting Conway's derivations and analysing critically the other contributions to the subject, leading unmistakably to the conclusion that “pseudocapacitors” are actually true capacitors and that “pseudocapacitance” is a basically incorrect notion. Taking cobalt oxide films as a tutorial example, we describe the way in which a (true) electrical double layer is built upon oxidation of the film in its insulating state up to an ohmic conducting state. The lessons drawn at this occasion are used to re-examine the classical oxides, RuO₂, MnO₂, TiO₂, Nb₂O₅ and other examples of putative “pseudocapacitive” materials. Addressing the dynamics of charge storage—a key issue in the practice of power of the energy storage device—it is shown that ohmic potential drop in the pores is the governing factor rather than counter-ion diffusion as often asserted, based on incorrect diagnosis by means of scan rate variations in CV studies.

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 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.

Benchmarking and optimization of molecular catalysts of electrochemical reactions have become a central issue in the efforts to match contemporary renewable energy challenges. We first attempt to precisely define the notions and the parameters (potentials, turnover frequency) involved in the accomplish-ment of these objectives as well as the experimental conditions in which they can be effected. The notion of scaling relation will then be tackled, having particularly in mind the delimita-tion of their domain of applicability as well as the necessity of clearly separating what is relevant to intrinsic characteristics to what deals with the effect of varying the substrate and co-substrate concentrations. It will be emphasized that the occur-rence of such scaling relations, otherwise named “iron laws”, is not a general phenomenon but rather concerns families of cata-lysts. 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 mislead-ing. The discussion is illustrated by experimental data recently acquired in the O2-to-H2O conversion catalyzed by ironIII/II porphyrins in N,N’ dimethylformamide in the presence of a Brönsted acid.

Significance: Coordination complexes are constantly sought as catalysts in the transformation of small molecules involved in contemporary energy and environmental challenges. Insistent emphasis on the well-established notion of ligand “noninnocence” may blur the essential role of the metal center when kinetic, mechanistic, and selectivity issues are targeted rather than the detailed electronic structure of the starting catalyst molecule. With the help of modern concepts and techniques of molecular electrochemistry, several examples are described in which extremely efficient catalysis takes place at the metal center despite an a priori discouraging large delocalization of charge and electron density on the noninnocent ligand.