Implication Of Electrocatalysis Research Articles

How to Assess and Predict Electrical Double Layer Properties. Implications for Electrocatalysis.

The electrical double layer (EDL) plays a central role in electrochemical energy systems, impacting charge transfer mechanisms and reaction rates. The fundamental importance of the EDL in interfacial electrochemistry has motivated researchers to develop theoretical and experimental approaches to assess EDL properties. In this contribution, we review recent progress in evaluating EDL characteristics such as the double-layer capacitance, highlighting some discrepancies between theory and experiment and discussing strategies for their reconciliation. We further discuss the merits and challenges of various experimental techniques and theoretical approaches having important implications for aqueous electrocatalysis. A strong emphasis is placed on the substantial impact of the electrode composition and structure and the electrolyte chemistry on the double-layer properties. In addition, we review the effects of temperature and pressure and compare solid-liquid interfaces to solid-solid interfaces.

First-Principles Study on Nb2C–X (X = S, Cl, F)/Graphene Heterostructures: Assessing Aqueous Stability and Implications for Electrocatalysis

First-Principles Study on Nb₂C–X (X = S, Cl, F)/Graphene Heterostructures: Assessing Aqueous Stability and Implications for Electrocatalysis

Nanogap-Resolved Adsorption-Coupled Electron Transfer by Scanning Electrochemical Microscopy: Implications for Electrocatalysis.

Here, we demonstrate for the first time that the mechanism of adsorption-coupled electron-transfer (ACET) reactions can be identified experimentally. The electron transfer (ET) and specific adsorption of redox-active molecules are coupled in many electrode reactions with practical importance and fundamental interest. ACET reactions are often represented by a concerted mechanism. In reductive adsorption, an oxidant is simultaneously reduced and adsorbed as a reductant on the electrode surface through the ACET step. Alternatively, the non-concerted mechanism mediates outer-sphere reduction and adsorption separately when the reductant adsorption is reversible. In electrocatalysis, reversibly adsorbed reductants are ubiquitous and crucial intermediates. Moreover, electrocatalysis is complicated by the mixed mechanism based on simultaneous ACET and outer-sphere ET steps. In this work, we reveal the non-concerted mechanism for ferrocene derivatives adsorbed at highly oriented pyrolytic graphite as simple models. We enable the transient voltammetric mode of nanoscale scanning electrochemical microscopy (SECM) to kinetically control the adsorption step, which is required for the discrimination of non-concerted, concerted, and mixed mechanisms. Experimental voltammograms are compared with each mechanism by employing finite element simulation. The non-concerted mechanism is supported to indicate that the ACET step is intrinsically slower than its outer-sphere counterpart by at least four orders of magnitude. This finding implies that an ACET step is facilitated thermodynamically but may not be necessarily accelerated or catalyzed by the adsorption of the reductant. SECM-based transient voltammetry will become a powerful tool to resolve and understand electrocatalytic ACET reactions at the elementary level.

Dynamics and Surface Propensity of H+ and OH- within Rigid Interfacial Water: Implications for Electrocatalysis.

Facile solvent reorganization promoting ion transfer across the solid–liquid interface is considered a prerequisite for efficient electrocatalysis. We provide first-principles insight into this notion by examining water self-ion dynamics at a highly rigid NaCl(100)–water interface. Through extensive density functional theory molecular dynamics simulations, we demonstrate for both acidic and alkaline solutions that Grotthuss dynamics is not impeded by a rigid water structure. Conversely, decreased proton transfer barriers and a striking propensity of H3O+ and OH– for stationary interfacial water are found. Differences in the ideal hydration structure of the ions, however, distinguish their behavior at the water contact layer. While hydronium can maintain its optimal solvation, the preferentially hypercoordinated hydroxide is repelled from the immediate vicinity of the surface due to interfacial coordination reduction. This has implications for alkaline hydrogen electrosorption in which the formation of undercoordinated OH– at the surface is proposed to contribute to the observed sluggish kinetics.

Seed-Mediated Growth of Pt on High-Index Faceted Au Nanocrystals: The Ag Lining and Implications for Electrocatalysis

Seed-Mediated Growth of Pt on High-Index Faceted Au Nanocrystals: The Ag Lining and Implications for Electrocatalysis

Cation Co-Adsorption and Island Formation in the Electrochemical Environment

The electrode/electrolyte interface is complex, and its composition, structure, and dynamics affect the rate and mechanism of electrocatalytic reactions. The presence of spectator cations near the electrode surface, for example, is well known to affect the rate and selectivity of carbon dioxide electroreduction, where changing from a lithium to a cesium cation can increase the selectivity of ethylene production on copper from 2% to 31% [1]. While many explanations of the mechanism of the effect of these alkali metal cations has been proposed, no one theory has been well accepted. In previous work, we have shown that alkali metal cations can sit close to an electrode surface, in particular at high pH and low potentials; further, once close to (“adsorbed” on) the surface, these spectator cations can then interact with and alter the adsorption energy of reactive intermediates, such as those present during CO2 electroreduction [2], or adsorbed hydroxide (OH*) on platinum (an intermediate in many electrochemical reactions) [3]. In this work, we find a cation which when present in an alkaline electrolyte has a large effect on the binding strength of hydroxide (larger than that of the alkali metal cations) on a Pt(111) electrode and causes a significant hysteresis in hydroxide adsorption/desorption. Using a combination of detailed experiments on a single crystal Pt(111) electrode and density functional theory simulations, we show that this hysteresis results from the co-adsorption of the cation and of hydroxide, with an interaction between the cation and hydroxide which is so strong that it drives island formation and a first order phase transition in the OH* adlayer. This provides direct evidence for cation (co)adsorption. Implications for electrocatalysis and electrolyte design will be discussed.[1] M.R. Singh, Y. Kwon, Y. Lum, J.W. Ager III, A.T. Bell, J. Am. Chem. Soc., 138, 13006-13012 (2016).[2] S.A. Akhade, I.T. McCrum, M.J. Janik, J. Electrochem. Soc., 163, F477 (2016).[3] I.T. McCrum, M.J. Janik, J. Phys. Chem. C, 120, 1, 457-471 (2016).

Structure–property relationships describing the buried interface between silicon oxide overlayers and electrocatalytic platinum thin films

This article identifies Pt-oxide interlayers at the buried interface of Si-oxide/Pt thin films and explores their implications for electrocatalysis.

The Non-Monotonic Surface Charging Behavior of Platinum: Implications for Electrocatalysis in PEFC

In electrochemical systems, metal surface charging phenomena dictate the strength of electrostatic interactions between the electrified electrode and ions in solution. Historically, the potential of zero charge (pzc) of the metal has been employed to parameterize the surface charging relation [1][2]. However, for a Pt electrode, classical radiotracer experiments of Frumkin and Petrii revealed a non-monotonic charging behavior upon increasing the metal phase potential, with consecutive transitions from a negative to a positive surface charging region and further to another negative surface charging region [3]. Garcia-Araez et al. later reported a similar ‘negative-positive-negative’ charging relation derived from laser-pulsed experiments [4]. This alternating charging behavior defies the role of the pzc as a sufficient descriptor of electrode charging phenomena. The situation calls for an overhaul of the charging relation that must be derived from first principles. This study presents a refined structural model for electrified interfaces, as shown on the left side of Figure 1. The model accounts explicitly for charging effects caused by the formation of a surface oxide layer and a layer of oriented water molecules. An analytical expression for the charging relation is derived. The charging relation is parameterized using experimental data for a Pt surface as well as DFT calculations. It exhibits the three different charging regions observed in the experiments cited above. To illustrate the impact of the modified charging relation, it is employed to study the oxygen reduction reaction in nanoporous electrodes of polymer electrolyte fuel cells.

Anodic Oxidation of Nitrotriarylamines and Its Implications for Electrocatalysis

In 1980 Steckhan and Schmidt introduced the use of 4,4’,4”-tribromotriphenylamine (1a) and several more highly halogenated derivatives for the electrocatalytic anodic oxidation of organic substrates,1 and a sizeable number of such transformations have been reported.2 The relatively modest oxidation potential of 1 (+0.78 vs Ag/0.1 M AgNO3 reference) imposes restrictions, however, on the range of substrates that can treated in this manner. For this reason, we synthesized a series of triarylamines bearing several electron-withdrawing groups with the expectation that these would be useful for effecting catalytic oxidation of substrates that high oxidation potentials.3 We had found earlier that such substances can be difficult to oxidize anodically.4We have had some success using these more electronegatively substituted triarylamines.5 However, the situation is not as simple as it may appear. For example, the generalization is often made that mediated oxidation can be effected at potentials up to 500-600 mv positive of the oxidation potential of the substrate, but the latter depends, inter alia, upon the rate of further chemical reaction of the oxidized form of the substrate. Other assumptions are that electronegative substituents increase the oxidative power of the triarylamine catalyst, but not any of its other chemical properties and that the amine cation radical is stable on the time scale of the electrolysis. As we will report here, we find that both of these assumptions are often unwarranted for triarylamines. We recently carried out a series of controlled potential electrolyses of a 10:1 mole ratio of trans-stilbene and 4,4’,4”-trimethyl-2,2’,2”-trinitrotriphenylamine (1b) in aqueous acetonitrile. Under these conditions it was found that the initially produced dark blue solution of 1b+ (the cation radical of 1b) quickly turns first violet and then red and much unreacted stilbene remains. The color change demonstrated that 1 + is unstable under these conditions. A subsequent electrolysis of 1 in the absence of stilbene, followed by flash chromatography of the electrolysis mixture afforded a red solid whose formula (from exact mass measurement by mass spectrometry) is C21H16N4O8. We assign structure 2 (a substituted N-phenylphenoxazine) to this substance because a reasonable mechanistic path to it can be written involving three consecutive ECE processes involving nucleophilic attack by water upon 1 +to introduce the oxygen atoms onto the rings.One might conclude from this that the presence of three highly withdrawing nitro groups in 1 + is responsible for its instability by increasing its electrophilicity and that a less highly nitrated derivative would not take this path. This is not so; more recently, we have found that anodic oxidation of the mononitro triphenylamine 1c in aqueous acetonitrile under the same conditions also affords a red substance to which we again assign a phenoxazine structure. Although 1b and 1c both exhibit reversible cyclic voltammograms, this is misleading. In fact, the cation radicals from both 1b and 1c both undergo rather efficient nucleophilic attack by water in experiments of longer time scale. Phenoxazine formation is apparently efficient from 1c even though its oxidation potential is much lower than that of 1b. Decomposition of triphenylamine cation radicals may be a general pattern; the tribromide 1a has also been observed to decompose during preparative scale electrolysis.6 Finally, we note that instability of cation radicals, with its concomitant undesirable impact on electrosynthetic applications, may prove to be problematical in other organic electrocatalytic systems, particularly when they bear one or more strongly electron-withdrawing groups.

(Invited) Electrochemistry on 4d and 5d Transition Metal-Oxide Films Epitaxially Grown on Single Crystal Substrates: Implications for Electrocatalysis

We present our electrochemical studies of epitaxial ruthenate and iridate films grown on single crystal substrates. Iridates and ruthenates are among the most studied oxygen evolution reaction (OER) electrocatalysts for water splitting applications. In the past decade, advances in deposition technologies and substrate availabilities have enabled the growth of single-crystal transition-metal oxides with high structural perfection. We use these advances, specifically Molecular-Beam Epitaxy (MBE), to prepare epitaxial ruthenate and iridate films on single crystal substrates. The use of epitaxial, single-crystal films allow us to link the features from our electrochemical characterizations to the physical adsorptions, which we will compare to the first-principle calculations. We further discuss the alloying effect on the surface adsorption process. We use this information to reveal insights into how to tune the surface physicochemical properties on the ruthenates and iridates and discuss the implications on the OER electrocatalysis.

Electrocatalytic cyclization of 3-(5-hydroxy-3-methylpyrazol-4-yl)-3-arylpropionitriles: ‘one-pot’ simple fast and efficient way to substituted spirocyclopropylpyrazolones

Electrolysis of 3-(5-hydroxy-3-methylpyrazol-4-yl)-3-arylpropionitriles in alcohol in an undivided cell in the presence of sodium bromide as mediator results in fast and efficient cyclization with the formation of the substituted spirocyclopropylpyrazolones in 60–90% substance yield. The fast (35min) electrocatalytic reaction smoothly proceeds under neutral and mild conditions. The implication of electrocatalysis in cascade cyclization reaction is an efficient approach to medicinally relevant spirocyclopropylpyrazolones and is beneficial from the viewpoint of ‘green chemistry’ and diversity-oriented large-scale processes.

Chemical and electrocatalytic cascade cyclization of salicylaldehyde with three molecules of malononitrile: ‘one-pot’ simple and efficient way to the chromeno[2,3-b]pyridine scaffold

А new type of catalytic cascade cyclization was found: the direct ‘one-pot’ simple and efficient transformation of salicylaldehyde and three molecules of malononitrile into the chromeno[2,3-b]pyridine systems. Chemical and electrochemical methods result in direct ‘one-pot’ formation of chromeno[2,3-b]pyridines in 60–90% yields. The implication of electrocatalysis in complex cascade cyclization reaction is an efficient approach to medicinally relevant chromeno[2,3-b]pyridines—the promising small-molecule ligands with pronounced antihistamic, antirheumatic and antiasthmatic activities. The electrocatalytic method is beneficial from the viewpoint of diversity-oriented large-scale processes and represents fast efficient and environmentally benign synthetic concept for cascade reactions strategy.

ChemInform Abstract: Theory of Multiple Proton—Electron Transfer Reactions and Its Implications for Electrocatalysis

AbstractReview: 96 refs.

Theory of multiple proton–electron transfer reactions and its implications for electrocatalysis

This perspective article outlines a simple but general theoretical analysis for multiple proton–electron transfer reactions, based on the microscopic theory of proton-coupled electron transfer reactions, recent developments in the thermodynamic theory of multi-step electron transfer reactions, and the experimental realization that many multiple proton-coupled electron transfer reactions feature decoupled proton–electron steps in their mechanism. It is shown that decoupling of proton and electron transfer leads to a strong pH dependence of the overall catalytic reaction, implying an optimal pH for high catalytic turnover, and an associated optimal catalyst at the optimal pH. When more than one catalytic intermediate is involved, scaling relationships between intermediates may dictate the optimal catalyst and limit the extent of reversibility that may be achievable for a multiple proton–electron transfer reaction. The theory is discussed in relation to the experimental results for a number of redox reactions that are of importance for sustainable energy conversion, primarily focusing on their pH dependence.

Electrocatalytic and chemical methods in MHIRC reactions: the first example of the multicomponent assembly of medicinally relevant spirocyclopropylbarbiturates from three different molecules

Electrolysis of aldehydes, barbituric acids, and malononitrile in alcohol in an undivided cell in the presence of sodium bromide results in efficient MHIRC formation of the corresponding spirocyclopropylbarbiturates in 50–65% substance yield. The electrocatalytic reaction smoothly proceeds under neutral and mild conditions with aromatic aldehydes bearing both electron-donating and electron-withdrawing groups. The implication of electrocatalysis in MHIRC reaction is an efficient approach to medicinally relevant spirocyclopropylbarbiturates avoiding the inconvenient direct use of molecular halogen or halogenated substrates.

Electrocatalysis in MIRC reaction strategy: facile stereoselective approach to medicinally relevant spirocyclopropylbarbiturates from barbituric acids and activated olefins

The combined electrolysis of barbituric acids and benzylidenemalononitriles or benzylidenecyanoacetates in methanol in an undivided cell in the presence of sodium bromide results in efficient MIRC (Michael-initiated ring-closure) formation of the corresponding spirocyclopropylbarbiturates in 45–93% yield. The electrocatalytic reaction proceeds smoothly under neutral and mild conditions with benzylidenemalononitriles or benzylidenecyanoacetates bearing both electron-donating and electron-withdrawing groups. NMR and single X-ray diffraction studies indicate that the electrocatalytic MIRC transformation of barbituric acids and benzylidenecyanoacetates results in the stereoselective formation of spirocyclopropanes with an (E)-configuration of aryl and alkoxycarboxylate substituents. The implication of electrocatalysis in the MIRC reaction strategy allows the combination of the synthetic virtues of both methods and accounts for an efficient approach to medicinally relevant spirocyclopropylbarbiturates avoiding inconvenient direct use of molecular halogen or halogenated substrates in accordance with the concepts of modern green chemistry.

Large Amplitude Fourier Transformed AC Voltammetric Investigation of the Active State Electrochemistry of a Copper/Aqueous Base Interface and Implications for Electrocatalysis

The higher harmonic components available from large-amplitude Fourier-transformed alternating current (FT-ac) voltammetry enable the surface active state of a copper electrode in basic media to be probed in much more detail than possible with previously used dc methods. In particular, the absence of capacitance background current allows low-level Faradaic current contributions of fast electron-transfer processes to be detected; these are usually completely undetectable under conditions of dc cyclic voltammetry. Under high harmonic FT-ac voltammetric conditions, copper electrodes exhibit well-defined and reversible premonolayer oxidation responses at potentials within the double layer region in basic 1.0 M NaOH media. This process is attributed to oxidation of copper adatoms (Cu*) of low bulk metal lattice coordination numbers to surface-bonded, reactive hydrated oxide species. Of further interest is the observation that cathodic polarization in 1.0 M NaOH significantly enhances the current detected in each of the fundamental to sixth FT-ac harmonic components in the Cu*/Cu hydrous oxide electron-transfer process which enables the underlying electron transfer processes in the higher harmonics to be studied under conditions where the dc capacitance response is suppressed; the results support the incipient hydrous oxide adatom mediator (IHOAM) model of electrocatalysis. The underlying quasi-reversible interfacial Cu*/Cu hydrous oxide process present under these conditions is shown to mediate the reduction of nitrate at a copper electrode, while the mediator for the hydrazine oxidation reaction appears to involve a different mediator or active state redox couple. Use of FT-ac voltammetry offers prospects for new insights into the nature of active sites and electrocatalysis at the electrode/solution interface of Group 11 metals in aqueous media.

Thermodynamic theory of multi-electron transfer reactions: Implications for electrocatalysis

This paper summarizes the thermodynamic theory of multi-electron transfer reactions and its implications for electrocatalysis. We discuss the fundamental differences between catalyzing reactions involving the transfer of one electron or no catalytic intermediates, two electron transfers with one catalytic intermediate, two electron transfer with two catalytic intermediates, and more than two electron transfers with more than one intermediate. These different classes of reactions imply different optimization problems for finding the best catalyst, dictated primarily by the thermodynamics of binding of the catalytic intermediates. The application of this theory to hydrogen evolution and oxidation, oxygen evolution and reduction, and carbon dioxide reduction, is discussed.

The Implication of Electrocatalysis in MCR Strategy: Electrocatalytic Multicomponent Transformation of Cyclic 1,3-Diketones, Aldehydes and Malononitrile into Substituted 5,6,7,8-Tetrahydro-4H-Chromenes

not Available.

The Implication of Electrocatalysis in MCR Strategy: Electrocatalytic Multicomponent Transformation of Cyclic 1,3‐Diketones, Aldehydes and Malononitrile into Substituted 5,6,7,8‐Tetrahydro‐4H‐chromenes.

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