Evidencing Fast, Massive, and Reversible H + Insertion in Nanostructured TiO2 Electrodes at Neutral pH. Where Do Protons Come From?

International audienceOngoing developments of sustainable energy technologies based on high-surface-area semiconductive metal oxide electrodes operating under mild and safe aqueous conditions require deep understanding of proton and electron transfer/transport throughout their porous structure. To address this issue, we investigated the electrochemical reductive protonation of high surface area nanostructured amorphous TiO 2 electrodes (produced by glancing angle deposition) in both buffered and unbuffered aqueous solutions. Quantitative analysis of the two charge storage mechanisms was achieved, allowing proper deconvolution of the electrical double-layer capacitive charge storage from the reversible faradaic one resulting from the proton-coupled reduction of bulk TiO 2. We evidence that this latter process occurs reversibly and extensively (up to an intercalation ratio of 20%) not only under strongly acidic pH conditions but also, more interestingly, under neutral pH with the intercalated proton arising from the buffer rather than water. Moreover, we show that in comparison with reductive Li + intercalation the proton-coupled electron charge storage occurs more rapidly (in a few seconds). This important finding suggests that a high-rate and high-power charge storage device could potentially be achieved with the reversible H +-coupled charge/discharge process in TiO 2 at neutral pH, opening thus new opportunities to the development of eco-friendly batteries for electrical energy storage

Efficient and selective molecular catalyst for the CO 2 -to-CO electrochemical conversion in water

Substitution of the four paraphenyl hydrogens of iron tetraphenylporphyrin by trimethylammonio groups provides a watersoluble molecule able to catalyze the electrochemical conversion of carbon dioxide into carbon monoxide. The reaction, performed in pH-neutral water, forms quasi-exclusively carbon monoxide with very little production of hydrogen, despite partial equilibration of CO 2 with carbonic acid-a low pK a acid. This selective molecular catalyst is endowed with a good stability and a high turnover frequency. On this basis, prescribed composition of CO-H 2 mixtures can be obtained by adjusting the pH of the solution, optionally adding an electroinactive buffer. The development of these strategies will be greatly facilitated by the fact that one operates in water. The same applies for the association of the cathode compartment with a proton-producing anode by means of a suitable separator. CO 2 -to-CO conversion | contemporary energy challenges | electrochemistry | catalysis | solar fuels O ne of the most important issues of contemporary energy and environmental challenges consists of reducing carbon dioxide into fuels by means of sunlight (1-3). One route toward this ultimate goal is to first convert solar energy into electricity, which will then be used to reduce CO 2 electrochemically. Direct electrochemical injection of an electron into the CO 2 molecule, forming the corresponding anion radical CO 2 .− requires a very high energy [the standard potential of the CO 2 / CO 2 .− couple is indeed −1.97 V vs. normal hydrogen electrode (NHE) in N,N′dimethylformamide (DMF)] (4, 5). Electrochemical conversion of CO 2 to any reaction product thus requires catalytic schemes that preferably avoid this intermediate. Carbon monoxide may be an interesting step en route to the desired fuels because it can be used as feedstock for the synthesis of alkanes through the classic Fischer-Tropsch process. A number of molecular catalysts for the homogeneous electrochemical CO 2 -to-CO conversion have been proposed. They mainly derive from transition metal complexes by electrochemical generation of an appropriately reduced state, which is restored by the catalytic reaction. So far, nonaqueous aprotic solvents (mostly DMF and acetonitrile) have been used for this purpose (5-16). Brönsted acids have been shown to boost catalysis. However, they should not be too strong, at the risk of leading to H 2 formation at the expense of the CO. Trifluoroethanol and water (possibly in large amounts) have typically played the role of a weak acid in the purpose of boosting catalysis while avoiding hydrogen evolution. One of the most thoroughly investigated families of transitionmetal complex catalysts of CO 2 -to-CO conversion is that of iron porphyrins brought electrochemically to the oxidation degree 0. The importance of coupling electron transfer and introduction of CO 2 into the coordination sphere of iron with proton transfers required by the formation of CO, CO 2 + 2e − , appeared from the very beginning of these studies. Sustained formation of CO was indeed only achieved upon addition of weak and Lewis acids (20, The results thus obtained in nonaqueous or partially aqueous media enabled the discovery of remarkably efficient and selective catalysts of the CO 2 -to-CO conversion. They were also the occasion of notable advances in terms of mechanisms and theory of concerted bond-breaking proton-electron transfer (29). It must, however, be recognized that, from the point of view of practical applications, the use of nonaqueous solvents is not the most exciting aspect of these results. One would rather like to use water as the solvent, which would render more viable the CO 2 -to-CO half-cell reaction as well as its association with a water-oxidation anode through a proton-exchange membrane. Significance CO 2 -to-CO electrochemical conversion is a key step in the production of liquid fuels through dihydrogen-reductive FischerTropsch chemistry. Among molecular catalysts, iron porphyrins reduced electrochemically to the Fe(0) state are particularly efficient and led to a deeper understanding of mechanisms involving coupled bond-breaking proton-electron transfer processes. The replacement of nonaqueous solvents by water should make the CO 2 -to-CO half-cell reaction much more attractive for applications, particularly because it would allow association with a water-oxidation anode through a protonexchange membrane. Here it is demonstrated that electrochemical CO production catalyzed by a water-soluble iron porphyrin can occur with high catalytic efficiency. Manipulation of pH and buffering then allows conversions from those involving complete CO selectivity to ones with prescribed CO-H 2 mixtures

Efficient and selective molecular catalyst for the CO 2 -to-CO electrochemical conversion in water

Substitution of the four paraphenyl hydrogens of iron tetraphenylporphyrin by trimethylammonio groups provides a watersoluble molecule able to catalyze the electrochemical conversion of carbon dioxide into carbon monoxide. The reaction, performed in pH-neutral water, forms quasi-exclusively carbon monoxide with very little production of hydrogen, despite partial equilibration of CO 2 with carbonic acid-a low pK a acid. This selective molecular catalyst is endowed with a good stability and a high turnover frequency. On this basis, prescribed composition of CO-H 2 mixtures can be obtained by adjusting the pH of the solution, optionally adding an electroinactive buffer. The development of these strategies will be greatly facilitated by the fact that one operates in water. The same applies for the association of the cathode compartment with a proton-producing anode by means of a suitable separator. CO 2 -to-CO conversion | contemporary energy challenges | electrochemistry | catalysis | solar fuels O ne of the most important issues of contemporary energy and environmental challenges consists of reducing carbon dioxide into fuels by means of sunlight (1-3). One route toward this ultimate goal is to first convert solar energy into electricity, which will then be used to reduce CO 2 electrochemically. Direct electrochemical injection of an electron into the CO 2 molecule, forming the corresponding anion radical CO 2 .− requires a very high energy [the standard potential of the CO 2 / CO 2 .− couple is indeed −1.97 V vs. normal hydrogen electrode (NHE) in N,N′dimethylformamide (DMF)] (4, 5). Electrochemical conversion of CO 2 to any reaction product thus requires catalytic schemes that preferably avoid this intermediate. Carbon monoxide may be an interesting step en route to the desired fuels because it can be used as feedstock for the synthesis of alkanes through the classic Fischer-Tropsch process. A number of molecular catalysts for the homogeneous electrochemical CO 2 -to-CO conversion have been proposed. They mainly derive from transition metal complexes by electrochemical generation of an appropriately reduced state, which is restored by the catalytic reaction. So far, nonaqueous aprotic solvents (mostly DMF and acetonitrile) have been used for this purpose (5-16). Brönsted acids have been shown to boost catalysis. However, they should not be too strong, at the risk of leading to H 2 formation at the expense of the CO. Trifluoroethanol and water (possibly in large amounts) have typically played the role of a weak acid in the purpose of boosting catalysis while avoiding hydrogen evolution. One of the most thoroughly investigated families of transitionmetal complex catalysts of CO 2 -to-CO conversion is that of iron porphyrins brought electrochemically to the oxidation degree 0. The importance of coupling electron transfer and introduction of CO 2 into the coordination sphere of iron with proton transfers required by the formation of CO, CO 2 + 2e − , appeared from the very beginning of these studies. Sustained formation of CO was indeed only achieved upon addition of weak and Lewis acids (20, The results thus obtained in nonaqueous or partially aqueous media enabled the discovery of remarkably efficient and selective catalysts of the CO 2 -to-CO conversion. They were also the occasion of notable advances in terms of mechanisms and theory of concerted bond-breaking proton-electron transfer (29). It must, however, be recognized that, from the point of view of practical applications, the use of nonaqueous solvents is not the most exciting aspect of these results. One would rather like to use water as the solvent, which would render more viable the CO 2 -to-CO half-cell reaction as well as its association with a water-oxidation anode through a proton-exchange membrane. Significance CO 2 -to-CO electrochemical conversion is a key step in the production of liquid fuels through dihydrogen-reductive FischerTropsch chemistry. Among molecular catalysts, iron porphyrins reduced electrochemically to the Fe(0) state are particularly efficient and led to a deeper understanding of mechanisms involving coupled bond-breaking proton-electron transfer processes. The replacement of nonaqueous solvents by water should make the CO 2 -to-CO half-cell reaction much more attractive for applications, particularly because it would allow association with a water-oxidation anode through a protonexchange membrane. Here it is demonstrated that electrochemical CO production catalyzed by a water-soluble iron porphyrin can occur with high catalytic efficiency. Manipulation of pH and buffering then allows conversions from those involving complete CO selectivity to ones with prescribed CO-H 2 mixtures

REACTIONS EN CHAINE COMME OUTILS D'ANALYSE DE L'ASSOCIATION ENTRE TRANSFERT D'ELECTRON, COUPURE ET FORMATION DE LIAISON

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Deciphering Reversible Homogeneous Catalysis of the Electrochemical H2 Evolution and Oxidation: Role of Proton Relays and Local Concentration Effects

Nickel bisdiphosphine complexes bearing pendant amines form a unique series of catalysts (so-called DuBois' catalysts) capable of bidirectional/reversible electrocatalytic oxidation and production of dihydrogen. This unique behaviour is directly linked to the presence of proton relays installed close to the metal center. We report here for the arginine derivative [Ni((P2N2Arg)-N-Cy)(2)](6+) on a mechanistic model and its kinetic treatment that may apply to all DuBois' catalysts and show that it allows for a good fit of experimental data measured at different pH values, catalyst concentrations and partial hydrogen pressures. The bidirectionality of catalysis results from balanced equilibria related to hydrogen uptake/evolution on one side and (metal)-hydride installation/capture on the other side, both controlled by concentration effects resulting from the presence of proton relays and connected by two square schemes corresponding to proton-coupled electron transfer processes. We show that the catalytic bias is controlled by the kinetic of the H-2 uptake/evolution step. Reversibility does not require that the energy landscape be flat, with redox transitions occurring at potentials up to 250 mV away for the equilibrium potential, although such large deviations from a flat energy landscape can negatively impacts the rate of catalysis when coupled with slow interfacial electron transfer kinetics

Deciphering Reversible Homogeneous Catalysis of the Electrochemical H2 Evolution and Oxidation: Role of Proton Relays and Local Concentration Effects

Nickel bisdiphosphine complexes bearing pendant amines form a unique series of catalysts (so-called DuBois catalysts) capable of bidirectional or even reversible electrocatalytic oxidation and production of dihydrogen. While this unique behaviour is directly linked to the presence of proton relays installed within the molecular structure, close to its metal center, quantitative activity descriptors are still lacking to guide the rational design of molecular catalysts with enhanced activity. We report here for the arginine derivative [Ni(P2CyN2Arg)2]6+ on a detailed kinetic treatment based on a mechanistic model that applies to the whole DuBois catalyst series and show that, with a unique set of parameters, it allows, for a good fit of the experimental data measured in a wide range of pH values, catalyst concentrations and partial hydrogen pressures. The bidirectionality of catalysis results from the balanced equilibrium constant of the kinetically critical hydrogen uptake and evolution chemical step and the corresponding rate constants being large enough in both directions, as well as a very fast intramolecular proton transfer, both being likely due to concentration effects resulting from the presence of proton relays at the immediate vicinity of the catalysts. In that specific case, we show that hydrogen oxidation, kinetically limited by H2 insertion, has a larger turnover frequency than hydrogen evolution that is kinetically limited by H2 release. The reversibility of catalysis appears also to result from a subtle balance between the characteristics of two sequential proton-coupled electron transfer square schemes and the equilibrium constants as well as the kinetic constants of both chemical steps. We illustrate experimentally that reversibility does not required that the energy landscape be flat, with in the present case redox transitions occurring at potentials ~250 mV away for the equilibrium potential. Still, large deviations from a flat energy landscape requires interfacial electron transfers to occur far from their equilibrium potential, which impacts their kinetics and the overall rate of catalysis. At that point, the rate of catalysis may be limited by the efficiency of deprotonation/reprotonation of the relays, a concern that also holds for the design of improved monodirectional electrocatalysts

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

Catalyse moléculaire homogène de la réduction électrochimique de N2O en N2 : catalyse redox vs. catalyse chimique

International audienceHomogeneous electrochemical catalysis of N2O reduction to N2 is investigated with a series of organic catalysts and rhenium and manganese bipyridyl carbonyl complexes. An activation-driving force correlation is revealed with the organic species characteristic of a redox catalysis involving an outer-sphere electron transfer from the radical anions or dianions of the reduced catalyst to N2O. Taking into account the previously estimated reorganization energy required to form the N2O radical anions leads to an estimation of the N2O/N2O˙− standard potential in acetonitrile electrolyte. The direct reduction of N2O at a glassy carbon electrode follows the same quadratic activation driving force relationship. Our analysis reveals that the catalytic effect of the mediators is due to a smaller reorganization energy of the homogeneous electron transfer than that of the heterogeneous one. The physical effect of “spreading” electrons in the electrolyte is shown to be unfavorable for the homogeneous reduction. Importantly, we show that the reduction of N2O by low valent rhenium and manganese bipyridyl carbonyl complexes is of a chemical nature, with an initial one-electron reduction process associated with a chemical reaction more efficient than the simple outer-sphere electron transfer process. This points to an inner-sphere mechanism possibly involving partial charge transfer from the low valent metal to the binding N2O and emphasizes the differences between chemical and redox catalytic processes

Electrophotocatalysis: Cyclic Voltammetry as an Analytical Tool

International audienceElectrophotocatalysis (e-PC) is currently experiencing a renewed interest. By taking advantage of the highly oxidizing or reducing power of excited state of electrogenerated ion radicals, it allows thermodynamically difficult redox reactions to be performed. However, e-PC is facing various specific issues, such as its fundamentally heterogeneous nature, implying that mass transport is coupled to chemical reactions and light absorption; back electron transfer of the ion radical excited state with the electrode; and local heating near the electrode surface modifying mass transport conditions. Herein, we address these issues in the context of cyclic voltammetry as an analytical tool and we provide a rational framework for kinetic studies of electrophotocatalytic reactions under realistic conditions and hypothesis based on literature data. This approach may be beneficial to rationalize the design and the efficiency of present and future e-PC systems