Cyclic Voltammetry Analysis of Electrocatalytic Films

Contemporary energy challenges require the catalytic activation of small molecules such as H₂O, H⁺, O₂, and CO₂ in view of their electrochemical reduction or oxidation. Mesoporous films containing the catalyst, conductive of electron or holes and permeable by the substrate appearance, when coated onto the electrode surface, as a convenient means of carrying out such reactions. Cyclic voltammetry then offers a suitable way of investigating mechanistically the interplay between catalytic reaction, mass, and charge transport, forming the basis of rational strategies for optimization of the film performances and for benchmarking catalysts. Systematic analysis of the cyclic voltammetric responses of catalytic films reflecting the various mechanistic scenarios has been lacking so far. It is provided here, starting with simple reaction schemes, which provides the occasion of introducing the basic concepts and relationships that will serve to the future resolution of more complex cases. Appropriate normalizations and dimensionless formulations allow the definition of actual governing parameters. The use of kinetic zone diagrams provides a precious tool for understanding the functioning of the catalytic film

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

Catalysis of Electrochemical Reactions by Surface-Active Sites: Analyzing the Occurrence and Significance of Volcano Plots by Cyclic Voltammetry

Cyclic voltammetry (CV) of heterogeneous electrocatalysts offers a convenient means to critically assess the occurrence of “volcano plots” rendered popular by acid reduction on metal electrodes. The equations relevant to Volmer–Heyrovsky-type reactions shows that the adsorption free energy of the surface-bound intermediate is one of the rate-controlling parameters, which, plotted against the exchange current, could lead to a volcano-looking curve if other rate-controlling factors such as the rate ratio of the two successive electron transfer steps would remain constant upon changing electrocatalyst. This is not necessarily the case in practice, thus blurring the occurrence of volcano plots. Therefore, careful recording and analysis of the CV responses should be a preferred strategy, leading additionally to catalytic Tafel plots for rational electrocatalyst benchmarking. The alternative Volmer–Tafel mechanism gives remarkably rise to S-shaped current–potential responses and to a volcano upon plotting the exchange current against the adsorption standard free energy of the primary intermediate. Again, a wealth of kinetic information results from the characteristics of the current–potential responses

Homogeneous Catalysis of Electrochemical Reactions: The Steady-State and Nonsteady-State Statuses of Intermediates

In the context of modern energy challenges, there is an increasing need to decipher the mechanism of complex, multistep catalytic processes, as a basis of their optimization and improvement. Cyclic voltammetry (CV) is one of the most popular electrochemical techniques in this purpose. Mechanistic complexities often trigger a quest for simplification in the treatment of data, such as the application of the steady-state approximation to intermediates. The validity of such assumptions actually needs justification. This is the object of the present work, which examines the question for five homogeneous catalytic reaction schemes of practical interest, which can also serve as tutorial examples for the analysis of further schemes. The analysis is simplified by the consideration of pure kinetic conditions and constancy of substrate concentration. These conditions can be achieved, in practice, by appropriate manipulation of the scan rate and concentrations. The current–potential responses are consequently S-shaped and independent of the scan rate. The CV responses are then dependent upon only two dimensionless parameters that group the experimental intrinsic and operational parameters. Limiting the subcases reached for the extreme values of these parameters is worth considering in terms of mechanism diagnosis and kinetic characterization. They are conveniently represented by kinetic zone diagrams. The present work not only provides the tools required to check the correctness of the kinetic analysis but also to gauge the possibility of characterizing transient intermediates by structurally informative techniques (e.g., spectroscopic)

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

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(<i>N</i>-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(<i>N</i>-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

Molecular Catalysis of H₂ Evolution: Diagnosing Heterolytic versus Homolytic Pathways

Molecular catalysis of H₂ production from the electrochemical reduction of acids by transition-metal complexes is one of the key issues of modern energy challenges. The question of whether it proceeds heterolytically (through reaction of an initially formed metal hydride with the acid) or homolytically (through symmetrical coupling of two molecules of hydride) has to date received inconclusive answers for a quite simple reason: the theoretical bases for criteria allowing the distinction between homolytic and heterolytic pathways in nondestructive methods such as cyclic voltammetry have been lacking heretofore. They are provided here, allowing the distinction between the two pathways. The theoretical predictions and the diagnosing strategy are illustrated by catalysis of the reduction of phenol, acetic acid, and protonated triethylamine by electrogenerated iron(0) tetraphenylporphyrin. Rather than being an intrinsic property of the catalytic system, the occurrence of either a heterolytic or a homolytic pathway results from their competition as a function of the concentrations of acid and catalyst and the rate constants for hydride formation and H₂ evolution by hydride protonation or dimerization

Benchmarking of Homogeneous Electrocatalysts: Overpotential, Turnover Frequency, Limiting Turnover Number

In relation to contemporary energy challenges, a number of molecular catalysts for the activation of small molecules, mainly based on transition metal complexes, have been developed. The time has thus come to develop tools allowing the benchmarking of these numerous catalysts. Two main factors of merit are addressed. One involves their intrinsic catalytic performances through the comparison of “catalytic Tafel plots” relating the turnover frequency to the overpotential independently of the characteristics of the electrochemical cell. The other examines the effect of deactivation of the catalyst during the course of electrolysis. It introduces the notion of the limiting turnover number as a second key element of catalyst benchmarking. How these two factors combine with one another to control the course of electrolysis is analyzed in detail, leading to procedures that allow their separate estimation from measurements of the current, the charge passed, and the decay of the catalyst concentration. Illustrative examples from literature data are discussed

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 <i>v</i> or to <i>v</i>^1/2, whereas the latter displays a quasi-rectangular response proportional to the scan rate. On the contrary, the notion of <i>pseudocapacitance</i>, 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

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

Cyclic voltammetry of phosphate cobalt oxide (CoP_i) 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. CoP_i 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