Publications by authors named "Alan M Bond"

213 Publications

Advanced Spatiotemporal Voltammetric Techniques for Kinetic Analysis and Active Site Determination in the Electrochemical Reduction of CO.

Acc Chem Res 2022 Jan 12. Epub 2022 Jan 12.

Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K.

ConspectusElectrochemical reduction of the greenhouse gas CO offers prospects for the sustainable generation of fuels and industrially useful chemicals when powered by renewable electricity. However, this electrochemical process requires the use of highly stable, selective, and active catalysts. The development of such catalysts should be based on a detailed kinetic and mechanistic understanding of the electrochemical CO reduction reaction (eCORR), ideally through the resolution of active catalytic sites in both time (i.e., temporally) and space (i.e., spatially). In this Account, we highlight two advanced spatiotemporal voltammetric techniques for electrocatalytic studies and describe the considerable insights they provide on the eCORR. First, Fourier transformed large-amplitude alternating current voltammetry (FT ac voltammetry), as applied by the Monash Electrochemistry Group, enables the resolution of rapid underlying electron-transfer processes in complex reactions, free from competing processes, such as the background double-layer charging current, slow catalytic reactions, and solvent/electrolyte electrolysis, which often mask conventional voltammetric measurements of the eCORR. Crucially, FT ac voltammetry allows details of the catalytically active sites or the rate-determining step to be revealed under catalytic turnover conditions. This is well illustrated in investigations of the eCORR catalyzed by Bi where formate is the main product. Second, developments in scanning electrochemical cell microscopy (SECCM) by the Warwick Electrochemistry and Interfaces Group provide powerful methods for obtaining high-resolution activity maps and potentiodynamic movies of the heterogeneous surface of a catalyst. For example, by coupling SECCM data with colocated microscopy from electron backscatter diffraction (EBSD) or atomic force microscopy, it is possible to develop compelling correlations of (precatalyst) structure-activity at the nanoscale level. This correlative electrochemical multimicroscopy strategy allows the catalytically more active region of a catalyst, such as the edge plane of two-dimensional materials and the grain boundaries between facets in a polycrystalline metal, to be highlighted. The attributes of SECCM-EBSD are well-illustrated by detailed studies of the eCORR on polycrystalline gold, where carbon monoxide is the main product. Comparing SECCM maps and movies with EBSD images of the same region reveals unambiguously that the eCORR is enhanced at surface-terminating dislocations, which accumulate at grain boundaries and slip bands. Both FT ac voltammetry and SECCM techniques greatly enhance our understanding of the eCORR, significantly boosting the electrochemical toolbox and the information available for the development and testing of theoretical models and rational catalyst design. In the future, it may be possible to further enhance insights provided by both techniques through their integration with and spectroscopy and microscopy methods.
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http://dx.doi.org/10.1021/acs.accounts.1c00617DOI Listing
January 2022

Unveiling the Impact of the Cations and Anions in Ionic Liquid/Glyme Hybrid Electrolytes for Na-O Batteries.

ACS Appl Mater Interfaces 2022 Jan 12;14(3):4022-4034. Epub 2022 Jan 12.

ARC Centre of Excellence for Electromaterials Science, Institute for Frontier Materials, Deakin University, Geelong, Victoria 3200, Australia.

A series of hybrid electrolytes composed of diglyme and ionic liquids (ILs) have been investigated for Na-O batteries, as a strategy to control the growth and purity of the discharge products during battery operation. The dependence of chemical composition of the ILs on the size, purity, and distribution of the discharge products has been evaluated using a wide range of experimental and spectroscopic techniques. The morphology and composition of the discharge products found in the Na-O cells have a complex dependence on the physicochemical properties of the electrolyte as well as the speciation of the Na and superoxide radical anion. All of these factors control the nucleation and growth phenomena as well as electrolyte stability. Smaller discharge particle sizes and largely homogeneous (2.7 ± 0.5 μm) sodium superoxide (NaO) crystals with only 9% of side products were found in the hybrid electrolyte containing the pyrrolidinium IL with a linear alkyl chain. The long-term cyclability of Na-O batteries with high Coulombic efficiency (>90%) was obtained for this electrolyte with fewer side products (20 cycles at 0.5 mA h cm). In contrast, rapid failure was observed with the use of the phosphonium-based electrolyte, which strongly stabilizes the superoxide anion. A high discharge capacity (4.46 mA h cm) was obtained for the hybrid electrolyte containing the pyrrolidinium-based IL bearing a linear alkyl chain with a slightly lower value (3.11 mA h cm) being obtained when the hybrid electrolyte contained similar pyrrolidinium-based IL bearing an alkoxy chain.
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http://dx.doi.org/10.1021/acsami.1c20257DOI Listing
January 2022

Inclusion of multiple cycling of potential in the deep neural network classification of voltammetric reaction mechanisms.

Faraday Discuss 2021 Dec 13. Epub 2021 Dec 13.

School of Chemistry, Monash University, Clayton, Vic. 3800, Australia.

The use of deep neural networks (DNNs) for the classification of electrochemical mechanisms using simulated voltammograms with one cycle of potential for training has previously been reported. In this paper, it is shown how valuable additional patterns for mechanism distinction become available when a new DNN is trained simultaneously on images obtained from three cycles of potential using tensor inputs. Significant improvements, relative to the single cycle training, in achieving the correct classification of E, EC and EC mechanisms (E = electron transfer step and C and C are first and second order follow up chemical reactions, respectively) are demonstrated with noisy simulated data for conditions where all mechanisms are close to chemically reversible and hence difficult to distinguish, even by an experienced electrochemist. Challenges anticipated in applying the new DNN to the classification of experimental data are highlighted. Directions for future development are also discussed.
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http://dx.doi.org/10.1039/d1fd00050kDOI Listing
December 2021

Electron Delocalization in Spectroelectrochemically and Computationally Characterized [Pt{(-BrCF)NCH═C(Cl)NEt}Cl(py)] Formed by Electrochemical Oxidation of [Pt{(-BrCF)NCH═C(Cl)NEt}Cl(py)].

Inorg Chem 2021 Dec 1;60(24):18899-18911. Epub 2021 Dec 1.

School of Chemistry, Monash University, Clayton, VIC 3800, Australia.

[Pt{(-BrCF)NCH═C(Cl)NEt}Cl(py)] () is the product of the hydrogen peroxide oxidation of the Pt anticancer agent [Pt{(-BrCF)NCHCHNEt}Cl(py)] (). Insights into electron delocalization and bonding in [Pt{(-BrCF)NCH═C(Cl)NEt}Cl(py)] () obtained by electrochemical oxidation of have been gained by spectroscopic and computational studies. The process is chemically and electrochemically reversible on the short time scale of voltammetry in dichloromethane (0.10 M [BuN][PF]). Substantial stability is retained on longer time scales enabling a high yield of to be generated by bulk electrolysis. IR and visible spectroelectrochemical studies on the oxidation of to and the reduction of back to confirm the long-term chemical reversibility. DFT calculations indicate only a minor contribution to the electron density (13%) resides on the Pt metal center in , indicating that the oxidation process is extensively ligand-based. Published X-ray crystallographic data show that is present in only one structural form, while NMR data on the dissolved crystals revealed the presence of two closely related structural forms in an almost equimolar ratio. Solution-phase EPR spectra of are consistent with two closely related structural forms in a ratio of about 90:10. The average value for the frozen solution spectra (2.0567 for the major species) is significantly greater than the 2.0023 expected for a free radical. Crystal field analysis of the EPR spectra leads to an estimate of the 5d() character of around 10% in . Analysis of X-ray absorption fine structure derived from also supports the presence of a delocalized singly occupied metal molecular orbital with a spin density of approximately 17% on Pt. Accordingly, the considerably larger electron density distribution on the ligand framework (diminished Pt character) is proposed to contribute to the increased stability of compared to that of .
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http://dx.doi.org/10.1021/acs.inorgchem.1c02682DOI Listing
December 2021

A Voltammetric Perspective of Multi-Electron and Proton Transfer in Protein Redox Chemistry: Insights From Computational Analysis of HypD Fourier Transformed Alternating Current Voltammetry.

Front Chem 2021 14;9:672831. Epub 2021 Jun 14.

Department of Chemistry, University of York, Heslington, United Kingdom.

This paper explores the impact of pH on the mechanism of reversible disulfide bond (CysS-SCys) reductive breaking and oxidative formation in hydrogenase maturation factor HypD, a protein which forms a highly stable adsorbed film on a graphite electrode. To achieve this, low frequency (8.96 Hz) Fourier transformed alternating current voltammetric (FTACV) experimental data was used in combination with modelling approaches based on Butler-Volmer theory with a dual polynomial capacitance model, utilizing an automated two-step fitting process conducted within a Bayesian framework. We previously showed that at pH 6.0 the protein data is best modelled by a redox reaction of two separate, stepwise one-electron, one-proton transfers with slightly "crossed" apparent reduction potentials that incorporate electron and proton transfer terms ( > ). Remarkably, rather than collapsing to a concerted two-electron redox reaction at more extreme pH, the same two-stepwise one-electron transfer model with > continues to provide the best fit to FTACV data measured across a proton concentration range from pH 4.0 to pH 9.0. A similar, small level of crossover in reversible potentials is also displayed in overall two-electron transitions in other proteins and enzymes, and this provides access to a small but finite amount of the one electron reduced intermediate state.
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http://dx.doi.org/10.3389/fchem.2021.672831DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8238118PMC
June 2021

CdS-Enhanced Ethanol Selectivity in Electrocatalytic CO Reduction at Sulfide-Derived Cu-Cd.

ChemSusChem 2021 Jul 16;14(14):2924-2934. Epub 2021 Jun 16.

School of Chemistry, Monash University, Clayton, 3800, Victoria, Australia.

The development of Cu-based catalysts for the electrochemical CO reduction reaction (eCO RR) is of major interest for generating commercially important C liquid products such as ethanol. Cu is exclusive among the eCO RR metallic catalysts in that it facilitates the formation of a range of highly reduced C products, with a reasonable total faradaic efficiency but poor product selectivity. Here, a series of new sulfide-derived copper-cadmium catalysts (SD-Cu Cd ) was developed. An excellent faradaic efficiency of around 32 % but with a relatively low current density of 0.6 mA cm for ethanol was obtained using the SD-CuCd catalyst at the relatively low overpotential of 0.89 V in a CO -saturated aqueous 0.10 m KHCO solution with an H-cell. The current density increased by an order of magnitude under similar conditions using a flow cell where the mass transport rate for CO was greatly enhanced. Ex situ spectroscopic and microscopic, and voltammetric investigations pointed to the role of abundant phase boundaries between CdS and Cu /Cu sites in the SD-CuCd catalyst in enhancing the selectivity and efficiency of ethanol formation at low potentials.
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http://dx.doi.org/10.1002/cssc.202100903DOI Listing
July 2021

Diverse and unexpected outcomes from oxidation of the platinum(II) anticancer agent [Pt{(p-BrCF)NCHCHNEt}Cl(py)] by hydrogen peroxide.

J Inorg Biochem 2021 05 5;218:111360. Epub 2021 Feb 5.

School of Chemistry, Monash University, Clayton 3800, VIC, Australia. Electronic address:

Oxidation of the anti-tumour agent [Pt{(p-BrCF)NCHCHNEt}Cl(py)], 1 (py = pyridine) with hydrogen peroxide under a variety of conditions yields a range of organoenamineamidoplatinum(II) compounds [Pt{(p-BrCF)NCH=C(X)NEt}Cl(py)] (X = H, Cl, Br) as well as species with shared occupancy involving H, Cl and Br. Thus, oxidation of the -CH-CH- backbone (dehydrogenation) occurs, often accompanied by substitution. Oxidation of 1 with HO in acetone yielded 1:1 co-crystallized [Pt{(p-BrCF)NCH=CHNEt}Cl(py)], 1H and [Pt{(p-BrCF)NCH=C(Cl)NEt}Cl(py)], 1Cl. The former was obtained pure in low yield from the oxidation of 1 with (NH)[Ce(NO)] in acetone, and the latter was obtained from 1 and HO in CHCl at near reflux. From the latter reaction under vigorous refluxing [Pt{(p-BrCF)NCH=C(Br)NEt}Cl(py)], 1Br was isolated. In refluxing acetonitrile, oxidation of 1 with HO yielded [Pt{(p-BrCF)NCH=C(H.Br)NEt}Cl(py)], 1HBr, in which the alkene is mainly substituted by Br in a dual occupancy. Treatment of 1 with HO and tetrabutylammonium hydroxide in acetone at room temperature formed [Pt{(p-HCF)NCHCHNEt}Cl(py)], 2. Oxidation of [Pt{(p-HCF)NCHCHNEt}Br(py)], 3 with HO in boiling acetonitrile gave the ligand oxidation product [Pt{(p-HCF)NCH=C(Br)NEt}Br(py)], 3Br. All major products were identified by X-ray crystallography as well as by H and F NMR spectra. In cases of mixed crystals or dual occupancy compounds, the F and H NMR spectra showed dissociation into the components in the solution in the same proportions as in isolated crystalline material.
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http://dx.doi.org/10.1016/j.jinorgbio.2021.111360DOI Listing
May 2021

Recent advances and future perspectives for automated parameterisation, Bayesian inference and machine learning in voltammetry.

Chem Commun (Camb) 2021 Feb;57(15):1855-1870

School of Chemistry, Monash University, Clayton, Vic. 3800, Australia.

Advanced data analysis tools such as mathematical optimisation, Bayesian inference and machine learning have the capability to revolutionise the field of quantitative voltammetry. Nowadays such approaches can be implemented routinely with widely available, user-friendly modern computing languages, algorithms and high speed computing to provide accurate and robust methods for quantitative comparison of experimental data with extensive simulated data sets derived from models proposed to describe complex electrochemical reactions. While the methodology is generic to all forms of dynamic electrochemistry, including the widely used direct current cyclic voltammetry, this review highlights advances achievable in the parameterisation of large amplitude alternating current voltammetry. One significant advantage this technique offers in terms of data analysis is that Fourier transformation provides access to the higher order harmonics that are almost devoid of background current. Perspectives on the technical advances needed to develop intelligent data analysis strategies and make them generally available to users of voltammetry are provided.
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http://dx.doi.org/10.1039/d0cc07549cDOI Listing
February 2021

Using Purely Sinusoidal Voltammetry for Rapid Inference of Surface-Confined Electrochemical Reaction Parameters.

Anal Chem 2021 Feb 8;93(4):2062-2071. Epub 2021 Jan 8.

Department of Computer Science, University of Oxford, Wolfson Building, Parks Road, Oxford, OX1 3QD United Kingdom.

Alternating current (AC) voltammetric techniques are experimentally powerful as they enable Faradaic current to be isolated from non-Faradaic contributions. Finding the best global fit between experimental voltammetric data and simulations based on reaction models requires searching a substantial parameter space at high resolution. In this paper, we estimate parameters from purely sinusoidal voltammetry (PSV) experiments, investigating the redox reactions of a surface-confined ferrocene derivative. The advantage of PSV is that a complete experiment can be simulated relatively rapidly, compared to other AC voltammetric techniques. In one example involving thermodynamic dispersion, a PSV parameter inference effort requiring 7,500,000 simulations was completed in 7 h, whereas the same process for our previously used technique, ramped Fourier transform AC voltammetry (ramped FTACV), would have taken 4 days. Using both synthetic and experimental data with a surface confined diazonium substituted ferrocene derivative, it is shown that the PSV technique can be used to recover the key chemical and physical parameters. By applying techniques from Bayesian inference and Markov chain Monte Carlo methods, the confidence, distribution, and degree of correlation of the recovered parameters was visualized and quantified.
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http://dx.doi.org/10.1021/acs.analchem.0c03774DOI Listing
February 2021

Impact of the Lithium Cation on the Voltammetry and Spectroscopy of [XVMO] (X = P, As ( = 4), S ( = 3); M = Mo, W): Influence of Charge and Addenda and Hetero Atoms.

Inorg Chem 2020 Aug 17;59(15):10522-10531. Epub 2020 Jun 17.

Department of Marine Resources Science, Faculty of Agriculture and Marine Science, Kochi University, Nankoku, Kochi 783-8502, Japan.

Polyoxometalates (POMs) have been proposed as electromaterials for lithium-based batteries because they provide access to multiple electron transfer reactions coupled to fast lithium ion transport processes and high stability over many redox cycles. Consequently, knowledge of reversible potentials and Li cation-POM anion interactions provides a strategic basis for their further development. In this study, detailed cyclic voltammetric studies of a series of [XVMO] (XVM) POMs (where X (heteroatom) = P ( = 4), As ( = 4), and S ( = 3) and M (addenda atom) = Mo, W) have been undertaken in CHCN in the presence of LiClO, with -BuNPF also present when required to keep the ionic strength close to constant value of 0.1 M. An analysis of the data has allowed the impact of the POM charge, and addenda and hetero atoms on the reversible potentials and the interaction between Li and the oxidized XVM and reduced XVM forms of the V redox couple to be determined. The SVM process is independent of the Li concentration, implying the absence of the association of this cation with either SVM or SVM redox levels. However, lithium-ion association constants for both V and V redox levels were obtained from a comparison of simulated and experimental cyclic voltammograms for the reduction of the more negatively charged XVM (X = P, As; M = Mo, W), since the Li interaction with these more negatively charged POMs is much stronger. The interaction between Li and the oxidized, XVM, and reduced, XVM, forms was also investigated by V NMR and EPR spectroscopy, respectively, and it was confirmed that, due to their lower charge density, SVM and SVM interact significantly less strongly with the lithium ion than XVM and XVM (X = P, As). The lithium-POM association constants are substantially smaller than the corresponding proton association constants reported previously, which is attributed to a smaller surface charge density. The much stronger impact of Li on the W- and Mo-based reductions that occur at more negative potentials than the V process also has been qualitatively evaluated.
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http://dx.doi.org/10.1021/acs.inorgchem.0c00876DOI Listing
August 2020

The Origin of the Electrocatalytic Activity for CO Reduction Associated with Metal-Organic Frameworks.

ChemSusChem 2020 May 7;13(10):2552-2556. Epub 2020 Apr 7.

School of Chemistry, Monash University, Wellington Road, Clayton, 3800, VIC, Australia.

There has been a rapid growth in the use of metal-organic framework (MOF) materials as electrocatalysts. However, simple anodic stripping analysis reveals that some well-known previously reported stable MOFs are in fact unstable at the negative potentials used to catalytically reduce CO in aqueous electrolyte media. Thus, it is the resulting metal nanoparticles derived from reduction of the MOFs rather than the MOFs themselves that are the electrocatalysts. The results reported herein therefore suggest that stability data and the origin of the activity for MOF electrocatalysts may need careful re-evaluation and that suitable strategies are needed to ensure that stable MOF electrocatalysts have been synthesized. The use of the readily accessible stripping analysis method provides a powerful tool to assess MOF stability under turnover conditions.
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http://dx.doi.org/10.1002/cssc.202000639DOI Listing
May 2020

Unprecedented Formation of a Binuclear Au(II)-Au(II) Complex through Redox State Cycling: Electrochemical Interconversion of Au(I)-Au(I), Au(II)-Au(II), and Au(I)-Au(III) in Binuclear Complexes Containing the Carbanionic Ligand CFPPh.

Inorg Chem 2019 Oct 1;58(20):13999-14004. Epub 2019 Oct 1.

School of Science , RMIT University , GPO Box 2476, Melbourne , Victoria 3001 , Australia.

The rational design of binuclear Au(I)-Au(I), Au(II)-Au(II), and Au(I)-Au(III) complexes requires an understanding of how the redox states interconvert. Herein, the electrochemical interconversion of the three oxidation states I, II, and III is reported on the voltammetric (cyclic and rotating disk electrode) time scales for binuclear gold complexes containing CFPPh as a ligand, to demonstrate for the first time formation of a binuclear Au(II)-Au(II) from a Au(I)-Au(III) complex. Results are supported by bulk electrolysis and coulometry with reaction products being identified by P NMR and UV-vis spectroscopy. All electrochemical processes involve an overall two-electron charge-transfer process with no one-electron intermediate being detected. Importantly, the kinetically rather than thermodynamically favored isomer [AuX(μ-2-CFPPh)] is formed on redox cycling of [XAu(μ-2-CFPPh)(κ-2-CFPPh)AuX] (X = Cl, ONO). Finally, a mechanism is proposed to explain the simultaneous change of coordination of the chelating carbanionic ligand to bridging mode and interconversion of oxidation states in binuclear gold complexes.
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http://dx.doi.org/10.1021/acs.inorgchem.9b01983DOI Listing
October 2019

Automatically Identifying Electrode Reaction Mechanisms Using Deep Neural Networks.

Anal Chem 2019 10 10;91(19):12220-12227. Epub 2019 Sep 10.

School of Chemistry , Monash University , Clayton , Victoria 3800 , Australia.

At present, electrochemical mechanisms are most commonly identified subjectively based on the experience of the researcher. This subjectivity is reflected in bias to particular mechanisms as well as lack of quantifiable confidence in the chosen mechanism compared to potential alternative mechanisms. In this paper we demonstrate that a deep neural network trained to recognize dc cyclic voltammograms for three commonly encountered mechanisms provides correct classifications within 5 ms without the problem of subjectivity. To mimic experimental data, the impact of noise, uncompensated resistance, and dependence on scan rate, factors that are relevant to practical studies, has also been investigated. Outcomes with two experimental data sets are also presented.
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http://dx.doi.org/10.1021/acs.analchem.9b01891DOI Listing
October 2019

Two-Dimensional Electrocatalysts for Efficient Reduction of Carbon Dioxide.

ChemSusChem 2020 Jan 10;13(1):59-77. Epub 2019 Oct 10.

School of Chemistry, Monash University, Clayton, VIC, 3800, Australia.

Two-dimensional (2D) materials are attractive catalysts for the electrochemical reduction of carbon dioxide reaction (eCO RR) by virtue of their tunable atomic structures, abundant active sites, enhanced conductivity, suitable binding affinity to carbon dioxide and/or reaction intermediates, and intrinsic scalability. Herein, recent advances in 2D catalysts for the eCO RR are reviewed. Structural features and properties of 2D materials that contribute to their advanced electrocatalytic properties are summarized, and strategies for enhancing their activity and selectivity for the eCO RR are reviewed. Prospects and challenges of applications of 2D catalysts for the eCO RR on an industrial scale are highlighted.
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http://dx.doi.org/10.1002/cssc.201901794DOI Listing
January 2020

Electrohydrogenation of Carbon Dioxide using a Ternary Pd/Cu O-Cu Catalyst.

ChemSusChem 2019 Oct 3;12(19):4471-4479. Epub 2019 Sep 3.

School of Chemistry, Monash University, Clayton, Victoria, 3800, Australia.

A simple one-pot method has been developed to synthesize a palladium/cuprous oxide-copper (Pd/Cu O-Cu) material with a well-defined structure, by modification of Cu O-Cu with Pd through a galvanic replacement reaction. Compared with the well-known copper/cuprous oxide (Cu/Cu O) catalysts, the Pd/Cu O-Cu material can catalyze the electroreduction of CO into C products with much higher faradaic efficiencies at lower overpotentials in a CO -saturated 0.5 m NaHCO solution. In particular, the highest faradaic efficiencies of 92 % for formate and 30 % for methane were achieved at -0.25 and -0.65 V (vs. the reversible hydrogen electrode), respectively. The improvement is suggested to be the result of a synergistic effect between PdH and the catalytically active copper sites during electrochemical CO reduction.
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http://dx.doi.org/10.1002/cssc.201901636DOI Listing
October 2019

Microcavity-Supported Lipid Bilayers; Evaluation of Drug-Lipid Membrane Interactions by Electrochemical Impedance and Fluorescence Correlation Spectroscopy.

Langmuir 2019 06 5;35(24):8095-8109. Epub 2019 Jun 5.

School of Chemical Sciences and National Centre for Sensor Research , Dublin City University , Dublin 9 , Ireland.

Many drugs have intracellular or membrane-associated targets, thus understanding their interaction with the cell membrane is of value in drug development. Cell-free tools used to predict membrane interactions should replicate the molecular organization of the membrane. Microcavity array-supported lipid bilayer (MSLB) platforms are versatile biophysical models of the cell membrane that combine liposome-like membrane fluidity with stability and addressability. We used an MSLB herein to interrogate drug-membrane interactions across seven drugs from different classes, including nonsteroidal anti-inflammatories: ibuprofen (Ibu) and diclofenac (Dic); antibiotics: rifampicin (Rif), levofloxacin (Levo), and pefloxacin (Pef); and bisphosphonates: alendronate (Ale) and clodronate (Clo). Fluorescence lifetime correlation spectroscopy (FLCS) and electrochemical impedance spectroscopy (EIS) were used to evaluate the impact of drug on 1,2-dioleyl- sn-glycerophosphocholine and binary bilayers over physiologically relevant drug concentrations. Although FLCS data revealed Ibu, Levo, Pef, Ale, and Clo had no impact on lipid lateral mobility, EIS, which is more sensitive to membrane structural change, indicated modest but significant decreases to membrane resistivity consistent with adsorption but weak penetration of drugs at the membrane. Ale and Clo, evaluated at pH 5.25, did not impact the impedance of the membrane except at concentrations exceeding 4 mM. Conversely, Dic and Rif dramatically altered bilayer fluidity, suggesting their translocation through the bilayer, and EIS data showed that resistivity of the membrane decreased substantially with increasing drug concentration. Capacitance changes to the bilayer in most cases were insignificant. Using a Langmuir-Freundlich model to fit the EIS data, we propose R as an empirical value that reflects permeation. Overall, the data indicate that Ibu, Levo, and Pef adsorb at the interface of the lipid membrane but Dic and Rif interact strongly, permeating the membrane core modifying the water/ion permeability of the bilayer structure. These observations are discussed in the context of previously reported data on drug permeability and log P.
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http://dx.doi.org/10.1021/acs.langmuir.9b01028DOI Listing
June 2019

Application of Bayesian Inference in Fourier-Transformed Alternating Current Voltammetry for Electrode Kinetic Mechanism Distinction.

Anal Chem 2019 04 1;91(8):5303-5309. Epub 2019 Apr 1.

School of Chemistry , Monash University , Victoria 3800 , Australia.

Estimation of parameters of interest in dynamic electrochemical (voltammetric) studies is usually undertaken via heuristic or data optimization comparison of the experimental results with theory based on a model chosen to mimic the experiment. Typically, only single point parameter values are obtained via either of these strategies without error estimates. In this article, Bayesian inference is introduced to Fourier-transformed alternating current voltammetry (FTACV) data analysis to distinguish electrode kinetic mechanisms (reversible or quasi-reversible, Butler-Volmer or Marcus-Hush models) and quantify the errors. Comparisons between experimental and simulated data were conducted across all harmonics using public domain freeware (MECSim).
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http://dx.doi.org/10.1021/acs.analchem.9b00129DOI Listing
April 2019

Separating the Effects of Experimental Noise from Inherent System Variability in Voltammetry: The [Fe(CN)] Process.

Anal Chem 2019 Feb 15;91(3):1944-1953. Epub 2019 Jan 15.

Department of Computer Science , University of Oxford , Wolfson Building, Parks Road , Oxford OX1 3QD , United Kingdom.

Recently, we introduced the use of techniques drawn from Bayesian statistics to recover kinetic and thermodynamic parameters from voltammetric data and were able to show that the technique of large amplitude ac voltammetry yielded significantly more accurate parameter values than the equivalent dc approach. In this paper, we build on this work to show that this approach allows us, for the first time, to separate the effects of random experimental noise and inherent system variability in voltammetric experiments. We analyze ten repeated experimental data sets for the [Fe(CN)] process, again using large-amplitude ac cyclic voltammetry. In each of the ten cases, we obtain an extremely good fit to the experimental data and obtain very narrow distributions of the recovered parameters governing both the faradaic (the reversible formal potential, E, the standard heterogeneous charge transfer rate constant, k, and the charge transfer coefficient, α) and nonfaradaic terms (uncompensated resistance, R, and double layer capacitance, C). We then employ hierarchical Bayesian methods to recover the underlying "hyperdistribution" of the faradaic and nonfaradaic parameters, showing that in general the variation between the experimental data sets is significantly greater than suggested by individual experiments, except for α where the interexperiment variation was relatively minor. Correlations between pairs of parameters are provided, and for example, reveal a weak link between k and C (surface activity of a glassy carbon electrode surface). Finally, we discuss the implications of our findings for voltammetric experiments more generally.
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http://dx.doi.org/10.1021/acs.analchem.8b04238DOI Listing
February 2019

Controllable Synthesis of Few-Layer Bismuth Subcarbonate by Electrochemical Exfoliation for Enhanced CO Reduction Performance.

Angew Chem Int Ed Engl 2018 Oct 3;57(40):13283-13287. Epub 2018 Sep 3.

School of Chemistry, Monash University, Wellington Road, Clayton, 3800, VIC, Australia.

Two-dimensional (2D) engineering of materials has been recently explored to enhance the performance of electrocatalysts by reducing their dimensionality and introducing more catalytically active ones. In this work, controllable synthesis of few-layer bismuth subcarbonate nanosheets has been achieved via an electrochemical exfoliation method. These nanosheets catalyse CO reduction to formate with high faradaic efficiency and high current density at a low overpotential owing to the 2D structure and co-existence of bismuth subcarbonate and bismuth metal under catalytic turnover conditions. Two underlying fast electron transfer processes revealed by Fourier-transformed alternating current voltammetry (FTacV) are attributed to CO reduction at bismuth subcarbonate and bismuth metal. FTacV results also suggest that protonation of CO is the rate determining step for bismuth catalysed CO reduction.
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http://dx.doi.org/10.1002/anie.201807466DOI Listing
October 2018

Use of the TCNQF Dianion in the Spontaneous Redox Formation of [Fe (L ) ][TCNQF ].

Chempluschem 2018 Jul 9;83(7):658-668. Epub 2018 Mar 9.

School of Chemistry, Monash University, Clayton, VIC, 3800, Australia.

The reaction of [Fe (L ) ](BF ) with Li TCNQF results in the formation of [Fe (L ) ][TCNQF ] (1) where L is the radical ligand, 4,4-dimethyl-2,2-di(2-pyridyl)oxazolidine-N-oxide and TCNQF is 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane. This has been characterised by X-ray diffraction, Raman and Fourier transform infrared (FTIR) spectroscopy, variable-temperature magnetic susceptibility, Mössbauer spectroscopy and electrochemistry. X-ray diffraction studies, magnetic susceptibility measurements and Raman and FTIR spectroscopy suggest the presence of low-spin Fe ions, the anionic form (L ) of the ligand and the anionic radical form of TCNQF ; viz. TCNQF . Li TCNQF reduces the [Fe (L ) ] dication, which undergoes a reductively induced oxidation to form the [Fe (L ) ] monocation resulting in the formation of [Fe (L ) ][TCNQF ] (1), the electrochemistry of which revealed four well-separated, diffusion-controlled, one-electron, reversible processes. Mössbauer spectroscopy and electrochemical measurements suggest the presence of a minor second species, likely to be [Fe (L ) ][TCNQF ].
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http://dx.doi.org/10.1002/cplu.201800010DOI Listing
July 2018

Macrocycles Bearing Ferrocenyl Pendants and their Electrochemical Properties upon Binding to Divalent Transition Metal Cations.

Chempluschem 2018 Jul 13;83(7):728-738. Epub 2018 Mar 13.

School of Life and Environmental Sciences, Deakin University, Burwood, VIC, 3125, Australia.

Metal complexes of Cu , Co , Cd , Zn , and Ni formed with the ligands [Fc(cyclen)] (1) and [Fc(cyclen) ] (2) (Fc=ferrocene, cyclen=1,4,7,10-tetraazacyclododecane) are synthesised and characterised. The X-ray structure of the Cu complex of 2, Fc([Cu(cyclen)(CH CN)] (ClO ) , is reported, and shows that the two positively charged Cu -cyclen units have a coordination number of five, adopting a distorted trigonal-bipyramidal configuration. The Cu -cyclen units are arranged in a trans-like configuration with respect to the Fc group, presumably to minimise electrostatic repulsion. The voltammetric oxidation of the free ligands 1 and 2 in a CH Cl /CH CN (1:4) solvent mixture yields two closely spaced oxidation processes. Both electron-transfer steps are associated with the ferrocenyl moiety, implying strong communication between the cyclen nitrogen atoms and the ferrocenyl group. In contrast, cyclic voltammograms display only a simple reversible one-electron process if 1 and 2 are complexed with Cd , Cu , Zn , Ni , or Co . Binding of these metal ions produces a significant shift in the reversible midpoint potential (E ). Except for Ni , E is linearly proportional to the charge density of the transition metal ion, demonstrating that 1 and 2 may undergo redox switching. The diffusion coefficients of Fc, DmFc, 1 and 2, and their metal ion complexes correlate well with their molecular weights.
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http://dx.doi.org/10.1002/cplu.201700550DOI Listing
July 2018

Voltammetric Perspectives on the Acidity Scale and H/H Process in Ionic Liquid Media.

Annu Rev Anal Chem (Palo Alto Calif) 2018 06 19;11(1):397-419. Epub 2018 Mar 19.

School of Chemistry and Australian Research Council Centre of Excellence for Electromaterials Science, Monash University, Clayton, Victoria 3800, Australia; email: ,

Nonhaloaluminate ionic liquids (ILs) have received considerable attention as alternatives to molecular solvents in diverse applications spanning the fields of physical, chemical, and biological science. One important and often overlooked aspect of the implementation of these designer solvents is how the properties of the IL formulation affect (electro)chemical reactivity. This aspect is emphasized herein, where recent (voltammetric) studies on the energetics of proton (H) transfer and electrode reaction mechanisms of the H/H process in IL media are highlighted and discussed. The energetics of proton transfer, quantified using the p K (minus logarithm of acidity equilibrium constant, K) formalism, is strongly governed by the constituent IL anion, and to a lesser extent, the IL cation. The H/H process, a model inner-sphere reaction, also displays electrochemical characteristics that are strongly IL-dependent. Overall, these studies highlight the need to carry out systematic investigations to resolve IL structure and function relationships in order to realize the potential of these diverse and versatile solvents.
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http://dx.doi.org/10.1146/annurev-anchem-061417-010022DOI Listing
June 2018

Identification of a new substrate effect that enhances the electrocatalytic activity of dendritic tin in CO reduction.

Phys Chem Chem Phys 2018 Feb;20(8):5936-5941

School of Chemistry, Monash University, Wellington Road, Clayton 3800, VIC, Australia.

In this study, a selective Sn based catalytic system that electrochemically reduces CO to formate has been developed based on a new substrate effect. Dendritic Sn catalysts were synthesised by hydrogen gas bubble assisted electrodeposition on Pt, Cu, Sn or In substrates and applied to electrocatalytic reduction of CO. The four substrates exhibit a hydrogen evolution activity that follows the order Pt > Cu > Sn > In. However, the Cu supported dendritic Sn catalyst provides the best selectivity towards formate formation (67.3% at -0.95 V vs. RHE). A substrate induced local pH change is proposed as the origin of formate selectivity. This was confirmed by the bulk electrolysis results obtained from two electrolyte solutions with different buffer capacities. The high buffer capacity phosphate buffer electrolyte solution provides minimal local pH change while an electrolyte with a low buffer capacity such as NaHCO maximises this effect to enhance the selectivity towards formate. The strategy reported here does not only focus on the catalyst, but also takes into consideration the local chemical environment. Hence, this work provides an optimal approach to improving the catalytic performance of electrocatalysts for electrochemical CO reduction.
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http://dx.doi.org/10.1039/c7cp07723hDOI Listing
February 2018

Solvent-, Cation- and Anion-Induced Structure Variations in Manganese-Based TCNQF Complexes: Synthesis, Crystal Structures, Electrochemistry and Their Catalytic Properties.

Chempluschem 2018 Jan 12;83(1):24-34. Epub 2017 Dec 12.

School of Chemistry, Monash University, Clayton, VIC, 3800, Australia.

The reaction of Mn(BF ) ⋅x H O with (Pr N) TCNQF (TCNQF =2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) in a mixture of CH OH/CH Cl gives a 2:3 stoichiometric complex of (Pr N) [Mn (TCNQF ) (CH OH) ] (1). If the solvent system used for the crystallisation of 1 is changed to CH OH/DMF, then a different product, [Mn(TCNQF )(DMF) ]⋅(CH OH) (2), is obtained. The use of Li TCNQF instead of (Pr N) TCNQF leads to the generation of [Mn (TCNQF ) (DMF) ]⋅3 DMF (3). An unexpected mixed oxidation state network with a composition of [Mn Mn O (OH) (OCH ) (TCNQF ) ](NO ) ⋅24 CH OH (4), is formed if Mn(NO ) ⋅x H O is used in place of Mn(BF ) ⋅x H O in the reaction that leads to the formation of 3. Compounds 1-3 have been characterised by X-ray crystallography; FTIR, Raman and UV/Vis spectroscopy; and electrochemistry. Compound 4 has only been analysed by X-ray crystallography and vibrational spectroscopy (Raman, FTIR), owing to rapid deterioration of the compound upon exposure to air. These results indicate that relatively minor changes in reaction conditions have the potential to yield products with vastly different structures. Compound 1 adopts an anionic 2D network with unusual π-stacked dimers of the TCNQF dianion, whereas 2 and 3 are composed of similar neutral sheets of [Mn(TCNQF )(DMF) ]. Interestingly, the solvent has a significant influence on the stacking of the sheets in the structures of 2 and 3. In compound 4, clusters with a composition of [Mn Mn O (OH) (OCH ) (CH OH) ] serve as eight-connecting nodes, whereas TCNQF ligands act as four-connecting nodes in a 3D network that has the same topology as fluorite. Compound 3 exhibits an exceptionally high super-catalytic activity for the electron-transfer reaction between ferricyanide and thiosulfate ions in aqueous media.
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http://dx.doi.org/10.1002/cplu.201700421DOI Listing
January 2018

Stabilization of Low-Valent Iron(I) in a High-Valent Vanadium(V) Oxide Cluster.

Angew Chem Int Ed Engl 2017 11 9;56(46):14749-14752. Epub 2017 Oct 9.

Institute of Inorganic Chemistry I, Ulm University, Albert-Einstein-Allee 11, 89081, Ulm, Germany.

Low-valent iron centers are critical intermediates in chemical and bio-chemical processes. Herein, we show the first example of a low-valent Fe center stabilized in a high-valent polyoxometalate framework. Electrochemical studies show that the Fe -functionalized molecular vanadium(V) oxide (DMA)[Fe ClV O Cl] (DMA=dimethylammonium) features two well-defined, reversible, iron-based electrochemical reductions which cleanly yield the Fe species (DMA)[Fe ClV O Cl] . Experimental and theoretical studies including electron paramagnetic resonance spectroscopy and density functional theory computations verify the formation of the Fe species. The study presents the first example for the seemingly paradoxical embedding of low-valent metal species in high-valent metal oxide anions and opens new avenues for reductive electron transfer catalysis by polyoxometalates.
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http://dx.doi.org/10.1002/anie.201706828DOI Listing
November 2017

Probing biological redox chemistry with large amplitude Fourier transformed ac voltammetry.

Chem Commun (Camb) 2017 Aug;53(69):9519-9533

Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK.

Biological electron-exchange reactions are fundamental to life on earth. Redox reactions underpin respiration, photosynthesis, molecular biosynthesis, cell signalling and protein folding. Chemical, biomedical and future energy technology developments are also inspired by these natural electron transfer processes. Further developments in techniques and data analysis are required to gain a deeper understanding of the redox biochemistry processes that power Nature. This review outlines the new insights gained from developing Fourier transformed ac voltammetry as a tool for protein film electrochemistry.
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http://dx.doi.org/10.1039/c7cc03870dDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5708363PMC
August 2017

Electrochemical Reduction of Carbon Dioxide in a Monoethanolamine Capture Medium.

ChemSusChem 2017 10 19;10(20):4109-4118. Epub 2017 Sep 19.

School of Chemistry, Monash University, Clayton, Victoria, 3800, Australia.

The electrocatalytic reduction of CO in a 30 % (w/w) monoethanolamine (MEA) aqueous solution was undertaken at In, Sn, Bi, Pb, Pd, Ag, Cu and Zn metal electrodes. Upon the dissolution of CO , the non-conducting MEA solution is transformed into a conducting one, as is required for the electrochemical reduction of CO . Both an increase in the electrode surface porosity and the addition of the surfactant cetyltrimethylammonium bromide (CTAB) suppress the competing hydrogen evolution reaction; the latter has a significantly stronger impact. The combination of a porous metal electrode and the addition of 0.1 % (w/w) CTAB results in the reduction of molecular CO to CO and formate ions, and the product distribution is highly dependent on the identity of the metal electrode used. At a potential of -0.8 V versus the reversible hydrogen electrode (RHE) with an indium electrode with a coralline-like structure, the faradaic efficiencies for the generation of CO and [HCOO] ions are 22.8 and 54.5 %, respectively compared to efficiencies of 2.9 and 60.8 % with a porous lead electrode and 38.2 and 2.4 % with a porous silver electrode. Extensive data for the other five electrodes are also provided. The optimal conditions for CO reduction are identified, and mechanistic details for the reaction pathways are proposed in this proof-of-concept electrochemical study in a CO capture medium. The conditions and features needed to achieve industrially and commercially viable CO reduction in an amine-based capture medium are considered.
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http://dx.doi.org/10.1002/cssc.201701075DOI Listing
October 2017

Untangling Complex Redox Chemistry in Zeolitic Imidazolate Frameworks Using Fourier Transformed Alternating Current Voltammetry.

Anal Chem 2017 10 18;89(19):10181-10187. Epub 2017 Sep 18.

School of Chemistry, The University of Sydney , Sydney, New South Wales, Australia 2006.

Two zeolitic imidazolate frameworks, ZIF-67 and ZIF-8, were interrogated for their redox properties using Fourier transformed alternating current voltammetry, which revealed that the 2-methylimidazolate ligand is responsible for multiple redox transformations. Further insight was gained by employing discrete tetrahedral complexes, [M(DMIM)] (DMIM = 1,2-dimethylimidazole, M = Co or Zn) which have similar structural motifs to ZIFs. In this work we demonstrate a multidirectional approach that enables the complex electrochemical behavior of ZIFs to be unraveled.
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http://dx.doi.org/10.1021/acs.analchem.7b01224DOI Listing
October 2017

Retuning the Catalytic Bias and Overpotential of a [NiFe]-Hydrogenase via a Single Amino Acid Exchange at the Electron Entry/Exit Site.

J Am Chem Soc 2017 08 26;139(31):10677-10686. Epub 2017 Jul 26.

Department of Chemistry, University of York , Heslington, York YO10 5DD, U.K.

The redox chemistry of the electron entry/exit site in Escherichia coli hydrogenase-1 is shown to play a vital role in tuning biocatalysis. Inspired by nature, we generate a HyaA-R193L variant to disrupt a proposed Arg-His cation-π interaction in the secondary coordination sphere of the outermost, "distal", iron-sulfur cluster. This rewires the enzyme, enhancing the relative rate of H production and the thermodynamic efficiency of H oxidation catalysis. On the basis of Fourier transformed alternating current voltammetry measurements, we relate these changes in catalysis to a shift in the distal [FeS] redox potential, a previously experimentally inaccessible parameter. Thus, metalloenzyme chemistry is shown to be tuned by the second coordination sphere of an electron transfer site distant from the catalytic center.
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http://dx.doi.org/10.1021/jacs.7b03611DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5562392PMC
August 2017
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