Energy storage: pseudocapacitance in prospect.

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.

Nature of Electronic Conduction in "Pseudocapacitive" Films: Transition from the Insulator State to Band-Conduction.

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.

Concepts and tools for mechanism and selectivity analysis in synthetic organic electrochemistry.

As an accompaniment to the current renaissance of synthetic organic electrochemistry, the heterogeneous and space-dependent nature of electrochemical reactions is analyzed in detail. The reactions that follow the initial electron transfer step and yield the products are intimately coupled with reactant transport. Depiction of the ensuing reactions profiles is the key to the mechanism and selectivity parameters. Analysis is eased by the steady state resulting from coupling of diffusion with convection forced by solution stirring or circulation. Homogeneous molecular catalysis of organic electrochemical reactions of the redox or chemical type may be treated in the same manner. The same benchmarking procedures recently developed for the activation of small molecules in the context of modern energy challenges lead to the establishment and comparison of the catalytic Tafel plots. At the very opposite, redox-neutral chemical reactions may be catalyzed by injection (or removal) of an electron from the electrode. This class of reactions has currently few, but very thoroughly analyzed, examples. It is likely that new cases will emerge in the near future.

Hydrogen and proton exchange at carbon. Imbalanced transition state and mechanism crossover.

A recent remarkable study of the C-H oxidation of substituted fluorenyl-benzoates together with the transfer of a proton to an internal receiving group by means of electron transfer outer-sphere oxidants, in the noteworthy absence of hydrogen-bonding interactions, is taken as an example to uncover the existence of a mechanism crossover, making the reaction pass from a CPET pathway to a PTET pathway as the driving force of the global reaction decreases. This was also the occasion to stress that considerations based on "imbalanced" or "asynchronous" transition states cannot replace activation/driving force models based on the quantum mechanical treatment of both electrons and transferring protons.

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.

Ligand "noninnocence" in coordination complexes vs. kinetic, mechanistic, and selectivity issues in electrochemical catalysis.

The world of coordination complexes is currently stimulated by the quest for efficient catalysts for the electrochemical reactions underlying modern energy and environmental challenges. Even in the case of a multielectron-multistep process, catalysis starts with uptake or removal of one electron from the resting state of the catalyst. If this first step is an outer-sphere electron transfer (triggering a "redox catalysis" process), the electron distribution over the metal and the ligand is of minor importance. This is no longer the case with "chemical catalysis," in which the active catalyst reacts with the substrate in an inner-sphere manner, often involving the transient formation of a catalyst-substrate adduct. The fact that, in most cases, the ligand is "noninnocent," in the sense that the electron density and charge gained (or removed) from the resting state of the catalyst are shared between the metal and the ligand, has become common-place knowledge over the last half-century. Insistent focus on a large degree of noninnocence of the ligand in the resting state of the catalyst, even robustly validated by spectroscopic techniques, may lead to undermining the essential role of the metal when such essential issues as kinetics, mechanisms, and product selectivity are dealt with. These points are general in scope, but their discussion is eased by adequately documented examples. This is the case for reactions involving metalloporphyrins as well as vitamin B12 derivatives and similar cobalt complexes for which a wealth of experimental data is available.

Catalysis and Inhibition in the Electrochemical Reduction of CO2 on Platinum in the Presence of Protonated Pyridine. New Insights into Mechanisms and Products.

In the framework of modern energy challenges, the reduction of CO2 into fuels calls for electrogenerated low-valent transition metal complexes catalysts designed with considerable ingenuity and sophistication. For this reason, the report that a molecule as simple as protonated pyridine (PyH+) could catalyze the formation of methanol from the reduction of CO2 on a platinum electrode triggered great interest and excitement. Further investigations revealed that no methanol is produced. It appears that CO2 is not really reduced but rather participates, on the basis of its aquation into carbonic acid, in hydrogen evolution. Actually, the situation is not that straightforward, as revealed by scrutinizing what happens at the platinum electrode surface. The present study confirms the lack of methanol formation upon bulk electrolysis of PyH+ solutions at Pt and provides a detailed account of the Faradaic yield for H2 production as a function of the electrode potential, but the main finding is that CO2 reduction is accompanied by a strong inhibition of the electrode process taking place when it is carried out in the presence of acids such as PyH+ and AcOH. Cyclic voltammetry and in situ infrared spectroscopy were closely combined to investigate and understand the nature and consequences of the inhibition process. Constant comparison between the two acids was required to decipher the course of the reaction owing to the fact that the IR responses are perturbed by PyH+ adsorption. It finally appears that inhibition is caused by the reduction of CO2 into CO, whose high affinity with platinum triggers the formation of a Pt-CO film that prevents the reaction process. Thus, a paradoxical situation develops in which the high affinity of Pt for CO helps to decrease the overpotential for the reduction of CO2 and therefore blocks the electrode, preventing the reaction process.

Nanodiffusion in electrocatalytic films.

In the active interest aroused by electrochemical reactions' catalysis, related to modern energy challenges, films deposited on electrodes are often preferred to homogeneous catalysts. A particularly promising variety of such films, in terms of efficiency and selectivity, is offered by sprinkling catalytic nanoparticles onto a conductive network. Coupled with the catalytic reaction, the competitive occurrence of various modes of substrate diffusion-diffusion toward nanoparticles ('nanodiffusion') against film linear diffusion and solution linear diffusion-is analysed theoretically. It is governed by a dimensionless parameter that contains all the experimental factors, thus allowing one to single out the conditions in which nanodiffusion is the dominant mode of mass transport. These theoretical predictions are illustrated experimentally by proton reduction on a mixture of platinum nanoparticles and carbon dispersed in a Nafion film deposited on a glassy carbon electrode. The density of nanoparticles and the scan rate are used as experimental variables to test the theory.

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.

How Do Pseudocapacitors Store Energy? Theoretical Analysis and Experimental Illustration.

Batteries and electrochemical double layer charging capacitors are two classical means of storing electrical energy. These two types of charge storage can be unambiguously distinguished from one another by the shape and scan-rate dependence of their cyclic voltammetric (CV) current-potential responses. The former shows peak-shaped current-potential responses, proportional to the scan rate v or to v1/2, whereas the latter displays a quasi-rectangular response proportional to the scan rate. On the contrary, the notion of pseudocapacitance, popularized in the 1980s and 1990s for metal oxide systems, has been used to describe a charge storage process that is faradaic in nature yet displays capacitive CV signatures. It has been speculated that a quasi-rectangular CV response resembling that of a truly capacitive response arises from a series of faradaic redox couples with a distribution of potentials, yet this idea has never been justified theoretically. We address this problem by first showing theoretically that this distribution-of-potentials approach is closely equivalent to the more physically meaningful consideration of concentration-dependent activity coefficients resulting from interactions between reactants. The result of the ensuing analysis is that, in either case, the CV responses never yield a quasi-rectangular response ∝ ν, identical to that of double layer charging. Instead, broadened peak-shaped responses are obtained. It follows that whenever a quasi-rectangular CV response proportional to scan rate is observed, such reputed pseudocapacitive behaviors should in fact be ascribed to truly capacitive double layer charging. We compare these results qualitatively with pseudocapacitor reports taken from the literature, including the classic RuO2 and MnO2 examples, and we present a quantitative analysis with phosphate cobalt oxide films. Our conclusions do not invalidate the numerous experimental studies carried out under the pseudocapacitance banner but rather provide a correct framework for their interpretation, allowing the dissection and optimization of charging rates on sound bases.

Through-Space Charge Interaction Substituent Effects in Molecular Catalysis Leading to the Design of the Most Efficient Catalyst of CO2-to-CO Electrochemical Conversion.

The starting point of this study of through-space substituent effects on the catalysis of the electrochemical CO2-to-CO conversion by iron(0) tetraphenylporphyrins is the linear free energy correlation between through-structure electronic effects and the iron(I/0) standard potential that we established separately. The introduction of four positively charged trimethylanilinium groups at the para positions of the tetraphenylporphyrin (TPP) phenyls results in an important positive deviation from the correlation and a parallel improvement of the catalytic Tafel plot. The assignment of this catalysis boosting effect to the Coulombic interaction of these positive charges with the negative charge borne by the initial Fe0-CO2 adduct is confirmed by the negative deviation observed when the four positive charges are replaced by four negative charges borne by sulfonate groups also installed in the para positions of the TPP phenyls. The climax of this strategy of catalysis boosting by means of Coulombic stabilization of the initial Fe0-CO2 adduct is reached when four positively charged trimethylanilinium groups are introduced at the ortho positions of the TPP phenyls. The addition of a large concentration of a weak acid-phenol-helps by cleaving one of the C-O bonds of CO2. The efficiency of the resulting catalyst is unprecedented, as can be judged by the catalytic Tafel plot benchmarking with all presently available catalysts of the electrochemical CO2-to-CO conversion. The maximal turnover frequency (TOF) is as high as 106 s-1 and is reached at an overpotential of only 220 mV; the extrapolated TOF at zero overpotential is larger than 300 s-1. This catalyst leads to a highly selective formation of CO (practically 100%) in spite of the presence of a high concentration of phenol, which could have favored H2 evolution. It is also very stable, showing no significant alteration after more than 80 h of electrolysis.

Catalysis at the nanoscale may change selectivity.

Among the many virtues ascribed to catalytic nanoparticles, the prospect that the passage from the macro- to the nanoscale may change product selectivity attracts increasing attention. To date, why such effects may exist lacks explanation. Guided by recent experimental reports, we propose that the effects may result from the coupling between the chemical steps in which the reactant, intermediates, and products are involved and transport of these species toward the catalytic surface. Considering as a thought experiment the competitive formation of hydrogen and formate upon reduction of hydrogenocarbonate ions on metals like palladium or platinum, a model is developed that allows one to identify the governing parameters and predict the effect of nanoscaling on selectivity. The model leads to a master equation relating product selectivity and thickness of the diffusion layer. The latter parameter varies considerably upon passing from the macro- to the nanoscale, thus predicting considerable variations of product selectivity. These are subtle effects in the sense that the same mechanism might exhibit a reverse variation of the selectivity if the set of parameter values were different. An expression is given that allows one to predict the direction of the effect. There has been a tendency to assign the catalytic effects of nanoscaling to chemical reactivity changes of the active surface. Such factors might be important in some circumstances. We, however, insist on the likely role of short-distance transport on product selectivity, which could have been thought, at first sight, as the exclusive domain of chemical factors.

Efficient electrolyzer for CO2 splitting in neutral water using earth-abundant materials.

Low-cost, efficient CO2-to-CO+O2 electrochemical splitting is a key step for liquid-fuel production for renewable energy storage and use of CO2 as a feedstock for chemicals. Heterogeneous catalysts for cathodic CO2-to-CO associated with an O2-evolving anodic reaction in high-energy-efficiency cells are not yet available. An iron porphyrin immobilized into a conductive Nafion/carbon powder layer is a stable cathode producing CO in pH neutral water with 90% faradaic efficiency. It is coupled with a water oxidation phosphate cobalt oxide anode in a home-made electrolyzer by means of a Nafion membrane. Current densities of approximately 1 mA/cm(2) over 30-h electrolysis are achieved at a 2.5-V cell voltage, splitting CO2 and H2O into CO and O2 with a 50% energy efficiency. Remarkably, CO2 reduction outweighs the concurrent water reduction. The setup does not prevent high-efficiency proton transport through the Nafion membrane separator: The ohmic drop loss is only 0.1 V and the pH remains stable. These results demonstrate the possibility to set up an efficient, low-voltage, electrochemical cell that converts CO2 into CO and O2 by associating a cathodic-supported molecular catalyst based on an abundant transition metal with a cheap, easy-to-prepare anodic catalyst oxidizing water into O2.

Conduction and Reactivity in Heterogeneous-Molecular Catalysis: New Insights in Water Oxidation Catalysis by Phosphate Cobalt Oxide Films.

Cyclic voltammetry of phosphate cobalt oxide (CoPi) films catalyzing O2-evolution from water oxidation as a function of scan rate, phosphate concentration and film thickness allowed for new insights into the coupling between charge transport and catalysis. At pH = 7 and low buffer concentrations, the film is insulating below 0.8 (V vs SHE) but becomes conductive above 0.9 (V vs SHE). Between 1.0 to 1.3 (V vs SHE), the mesoporous structure of the film gives rise to a large thickness-dependent capacitance. At higher buffer concentrations, two reversible proton-coupled redox couples appear over the capacitive response with 0.94 and 1.19 (V vs SHE) pH = 7 standard potentials. The latter is, at most, very weakly catalytic and not responsible for the large catalytic current observed at higher potentials. CV-response analysis showed that the amount of redox-active cobalt-species in the film is small, less than 10% of total. The catalytic process involves a further proton-coupled-electron-transfer and is so fast that it is controlled by diffusion of phosphate, the catalyst cofactor. CV-analysis with newly derived relationships led to a combination of the catalyst standard potential with the catalytic rate constant and a lower-limit estimation of these parameters. The large currents resulting from the fast catalytic reaction result in significant potential losses related to charge transport through the film. CoPi films appear to combine molecular catalysis with semiconductor-type charge transport. This mode of heterogeneous molecular catalysis is likely to occur in many other catalytic films.

Attempts To Catalyze the Electrochemical CO2-to-Methanol Conversion by Biomimetic 2e(-) + 2H(+) Transferring Molecules.

In the context of the electrochemical and photochemical conversion of CO2 to liquid fuels, one of the most important issues of contemporary energy and environmental issues, the possibility of pushing the reduction beyond the CO and formate level and catalytically generate products such as methanol is particularly attractive. Biomimetic 2e(-) + 2H(+) is often viewed as a potential hydride donor. This has been the object of a recent interesting attempt (J. Am. Chem. Soc. 2014, 136, 14007) in which 6,7-dimethyl-4-hydroxy-2-mercaptopteridine was reported as a catalyst of the electrochemical conversion of CO2 to methanol and formate, based on cyclic voltammetric, (13)C NMR, IR, and GC analyses. After checking electrolysis at the reported potential and at a more negative potential to speed up the reaction, it appears, on (1)H NMR and gas chromatographic grounds, that there is neither catalysis nor methanol and nor formate production. (1)H NMR (with H2O presaturation) brings about an unambiguous answer to the eventual production of methanol and formate, much more so than (13)C NMR, which can even be misleading when no internal standard is used as in the above-mentioned paper. IR analysis is even less conclusive. Use of a GC technique with sufficient sensitivity confirmed the lack of methanol formation. The direct or indirect hydride transfer electrochemical reduction of CO2 to formate and to methanol remains an open question. Original ideas and efforts such as those discussed here are certainly worth tempting. However, in view of the importance of the stakes, it appears necessary to carefully check reports in this area.

Current Issues in Molecular Catalysis Illustrated by Iron Porphyrins as Catalysts of the CO2-to-CO Electrochemical Conversion.

Recent attention aroused by the reduction of carbon dioxide has as main objective the production of useful products, the "solar fuels", in which solar energy would be stored. One route to this goal is the design of photochemical schemes that would operate this conversion using directly sun light energy. An indirect approach consists in first converting sunlight energy into electricity then using it to reduce CO2 electrochemically. Conversion of carbon dioxide into carbon monoxide is thus a key step through the classical dihydrogen-reductive Fischer-Tropsch chemistry. Direct and catalytic electrochemical CO2 reduction already aroused active interest during the 1980-1990 period. The new wave of interest for these matters that has been growing since 2012 is in direct conjunction with modern energy issues. Among molecular catalysts, electrogenerated Fe(0) porphyrins have proved to be particularly efficient and robust. Recent progress in this field has closely associated the search of more and more efficient catalysts in the iron porphyrin family with an unprecedentedly rigorous deciphering of mechanisms. Accordingly, the coupling of proton transfer with electron transfer and breaking of one of the two C-O bonds of CO2 have been the subjects of relentless scrutiny and mechanistic analysis with systematic investigation of the degree of concertedness of these three events. Catalysis of the electrochemical CO2-to-CO conversion has thus been a good testing ground for the mechanism diagnostic strategies and the all concerted reactivity model proposed then. The role of added Brönsted acids, both as H-bond providers and proton donors, has been elucidated. These efforts have been a preliminary to the inclusion of the acid functionalities within the catalyst molecule, giving rise to considerable increase of the catalytic efficiency. The design of more and more efficient catalysts made it necessary to propose "catalytic Tafel plots" relating the turnover frequency to the overpotential as a rational way of benchmarking the catalysts within iron porphyrins and among all available molecular catalysts, independently of the characteristics of the electrolytic cell in use. To be reliable, such assignments of the intrinsic characteristics of catalysts are grounded in the accurate elucidation of mechanisms. Without forgetting the importance of large scale electrolysis, not only mobilization of all resources of nondestructive techniques such as cyclic voltammetry was necessary to achieve this challenge, but also new approaches, such as foot-of-the-wave analysis combined with raising of scan rate, had to be applied. The latest improvement in catalyst design was to render it water-soluble while preserving, or even augmenting, its catalytic efficiency. The replacement of the nonaqueous solvents so far used by water makes the CO2-to-CO half-cell reaction much more attractive for applications, allowing its association with a water-oxidation anode through a proton-exchange membrane. Manipulation of pH and buffering then allow CO2-to-CO conversions from those involving complete CO-selectivity to ones with prescribed CO-H2 mixtures. Overall, it appears that not only are iron porphyrins the most efficient catalysts of the CO2-to-CO electrochemical conversion but also they can serve to illustrate general issues concerning the field of molecular catalysis as a whole, including other reductive or oxidative processes.

Molecular Catalysis of O2 Reduction by Iron Porphyrins in Water: Heterogeneous versus Homogeneous Pathways.

Despite decades of active attention, important problems remain pending in the catalysis of dioxygen reduction by iron porphyrins in water in terms of selectivity and mechanisms. This is what happens, for example, for the distinction between heterogeneous and homogeneous catalysis for soluble porphyrins, for the estimation of H2O2/H2O product selectivity, and for the determination of the reaction mechanism in the two situations. With water-soluble iron tetrakis(N-methyl-4-pyridyl)porphyrin as an example, procedures are described that allow one to operate this distinction and determine the H2O2/H2O product ratio in each case separately. It is noteworthy that, despite the weak adsorption of the iron(II) porphyrin on the glassy carbon electrode, the contribution of the adsorbed complex to catalysis rivals that of its solution counterpart. Depending on the electrode potential, two successive catalytic pathways have been identified and characterized in terms of current-potential responses and H2O2/H2O selectivity. These observations are interpreted in the framework of the commonly accepted mechanism for catalytic reduction of dioxygen by iron porphyrins, after checking its compatibility with a change of oxygen concentration and pH. The difference in intrinsic catalytic reactivity between the catalyst in the adsorbed state and in solution is also discussed. The role of heterogeneous catalysis with iron tetrakis(N-methyl-4-pyridyl)porphyrin has been overlooked in previous studies because of its water solubility. The main objective of the present contribution is therefore to call attention, by means of this emblematic example, to such possibilities to reach a correct identification of the catalyst, its performances, and reaction mechanism. This is a question of general interest, so that reduction of dioxygen remains a topic of high importance in the context of contemporary energy challenges.

Cyclic voltammetry of fast conducting electrocatalytic films.

In the framework of contemporary energy challenges, cyclic voltammetry is a particularly useful tool for deciphering the kinetics of catalytic films. The case of fast conducting films is analyzed, whether conduction is of the ohmic type or proceeds through rapid electron hopping. The rate-limiting factors are then the diffusion of the substrate in solution and through the film as well as the catalytic reaction itself. The dimensionless combination of the characteristics of these factors allows reducing the number of actual parameters to a maximum of two. The kinetics of the system may then be fully analyzed with the help of a kinetic zone diagram. Observing the variations of the current-potential responses with operational parameters such as film thickness, the potential scan rate and substrate concentration allows a precise assessment of the interplay between these factors and of the values of the rate controlling factors. A series of thought experiments is described in order to render the kinetic analysis more palpable.