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.

Interplay of Homogeneous Reactions, Mass Transport, and Kinetics in Determining Selectivity of the Reduction of CO2 on Gold Electrodes

Gold electrocatalysts have been a research focus due to their ability to reduce CO₂ into CO, a feedstock for further conversion. Many methods have been employed to modulate CO₂ reduction (CDR) vs hydrogen evolution reaction (HER) selectivity on gold electrodes such as nano-/mesostructuring and crystal faceting control. Herein we show that gold surfaces with very different morphologies (planar, leaves, and wires) lead to similar bell-shaped CO faradaic efficiency as a function of applied potential. At low overpotential (E > -0.85 V vs standard hydrogen electrode (SHE)), HER is dominant via a potential quasi-independent rate that we attribute to a rate limiting process of surface dissociation of competent proton donors. As overpotential is increased, CO faradaic efficiency reaches a maximal value (near 90%) because CO production is controlled by an electron transfer rate that increases with potential, whereas HER remains almost potential independent. At high overpotential (E < -1.2 V vs SHE), CO faradaic efficiency decreases due to the concurrent rise of HER via bicarbonate direct reduction and leveling off of CDR as CO₂ replenishment at the catalyst surface is limited by mass transport and homogeneous coupled reactions. Importantly, the analysis shows that recent attempts to overcome mass transport limitations with gas diffusion electrodes confront low carbon mass balance owing to the prominence of homogeneous reactions coupled to CDR. The comprehensive kinetics analysis of the factors defining CDR vs HER on gold electrodes developed here provides an activation-driving force relationship over a large potential window and informs on the design of conditions to achieve desirable high current densities for CO₂ to CO conversion while maintaining high selectivity.

Energy storage: pseudocapacitance in prospect

This question and its implications are discussed in detail.

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.

Oxygen activation at a dicobalt centre of a dipyridylethane naphthyridine complex

The mechanism of oxygen activation at a dicobalt bis-μ-hydroxo core is probed by the implementation of synthetic methods to isolate reaction intermediates. Reduction of a dicobalt(iii,iii) core ligated by the polypyridyl ligand dipyridylethane naphthyridine (DPEN) by two electrons and subsequent protonation result in the release of one water moiety to furnish a dicobalt(ii,ii) center with an open binding site. This reduced core may be independently isolated by chemical reduction. Variable-temperature 1H NMR and SQUID magnetometry reveal the reduced dicobalt(ii,ii) intermediate to consist of two low spin Co(ii) centers coupled antiferromagnetically. Binding of O2 to the open coordination site of the dicobalt(ii,ii) core results in the production of an oxygen adduct, which is proposed to be a dicobalt(iii,iii) peroxo. Electrochemical studies show that the addition of two electrons results in cleavage of the O-O bond.

Properties of Site-Specifically Incorporated 3-Aminotyrosine in Proteins To Study Redox-Active Tyrosines: Escherichia coli Ribonucleotide Reductase as a Paradigm

3-Aminotyrosine (NH₂Y) has been a useful probe to study the role of redox active tyrosines in enzymes. This report describes properties of NH₂Y of key importance for its application in mechanistic studies. By combining the tRNA/NH₂Y-RS suppression technology with a model protein tailored for amino acid redox studies (α₃X, X = NH₂Y), the formal reduction potential of NH₂Y₃₂(O^•/OH) ( E°' = 395 ± 7 mV at pH 7.08 ± 0.05) could be determined using protein film voltammetry. We find that the Δ E°' between NH₂Y₃₂(O^•/OH) and Y₃₂(O^•/OH) when measured under reversible conditions is ∼300-400 mV larger than earlier estimates based on irreversible voltammograms obtained on aqueous NH₂Y and Y. We have also generated D₆-NH₂Y₇₃₁-α2 of ribonucleotide reductase (RNR), which when incubated with β2/CDP/ATP generates the D₆-NH₂Y₇₃₁^•-α2/β2 complex. By multifrequency electron paramagnetic resonance (35, 94, and 263 GHz) and 34 GHz ¹H ENDOR spectroscopies, we determined the hyperfine coupling (hfc) constants of the amino protons that establish RNH₂^• planarity and thus minimal perturbation of the reduction potential by the protein environment. The amount of Y in the isolated NH₂Y-RNR incorporated by infidelity of the tRNA/NH₂Y-RS pair was determined by a generally useful LC-MS method. This information is essential to the utility of this NH₂Y probe to study any protein of interest and is employed to address our previously reported activity associated with NH₂Y-substituted RNRs.

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.

Cyclic voltammetry modeling of proton transport effects on redox charge storage in conductive materials: application to a TiO2 mesoporous film

Cyclic voltammetry is a particularly useful tool for characterizing charge accumulation in conductive materials. A simple model is presented to evaluate proton transport effects on charge storage in conductive materials associated with a redox process coupled with proton insertion in the bulk material from an aqueous buffered solution, a situation frequently encountered in metal oxide materials. The interplay between proton transport inside and outside the materials is described using a formulation of the problem through introduction of dimensionless variables that allows defining the minimum number of parameters governing the cyclic voltammetry response with consideration of a simple description of the system geometry. This approach is illustrated by analysis of proton insertion in a mesoporous TiO₂ 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.

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 v^1/2, 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.

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.

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.