Heterogeneous Electron Transfer Kinetics Research Articles

Efficient Electrocatalytic H2 Production by Immobilized Co(III)‐myoglobin

AbstractThe thermodynamics and kinetics of heterogeneous electron transfer (ET) for Co‐substituted horse myoglobin (Co−Mb) and its derivatives with ammonia and imidazole as heme axial ligands were studied with cyclic voltammetry on a pyrolytic graphite electrode along with their ability to mediate the electro‐catalytic production of H2. All the proteins experience a non‐diffusive electrochemical regime as electrode‐bound species. The adsorbed Co−Mb construct carries out the electro‐catalytic reduction of water protons to H2 with a good efficiency under anaerobic conditions thus yielding a simple and tunable system for H2 production. Replacement of H2O as Co axial ligand by ammonia and imidazole significantly lowers the catalytic currents for H3O+/H2O reduction to H2. The E°’ values of the Co(III)/Co(II) redox couple for all species are mainly determined by the enthalpic contribution. Differences were found in the kinetics of ET for the different protein adducts due to changes in the activation enthalpies. However, all species share the same distance of about 14 Å from the electrode surface to the Co(III)/Co(II) center determined using the Marcus model, consistent with a non‐denaturing adsorption of the protein.

Regulation of heterogeneous electron transfer reactivity by defect engineering through electrochemically induced brominating addition.

Enhancing the electrochemical activity of graphene holds great significance for expanding its applications in various electrochemistry fields. In this study, we have demonstrated a facile and quantitative approach for modulating the defect density of single-layer graphene (SLG) via an electrochemically induced bromination process facilitated by cyclic voltammetry. This controlled defect engineering directly impacts the heterogeneous electron transfer (HET) rate of SLG. By utilizing Raman spectroscopy and scanning electrochemical microscopy (SECM), we have established a correlation between the HET kinetics and both the defect density (nD) and mean distance between defects (LD) of SLG. The variation of the HET rate (k0) with the defect density manifested a distinctive three-stage behavior. Initially, k0 increased slightly with the increasing nD, and then it experienced a rapid increase as nD further increased. However, once the defect density surpassed a critical value of about 1.8 × 1012 cm-2 (LD < 4.2 nm), k0 decreased rapidly. Notably, the results revealed a remarkable 35-fold enhancement of k0 under the optimal defect density conditions compared to pristine SLG. This research paves the way for controllable defect engineering as a powerful strategy to enhance the electrochemical activity of graphene, opening up new possibilities for its utilization in a wide range of electrochemical applications.

Pencil Graphite Electrocatalytic Sensors Modified by Pyrene Coated Reduced Graphene Oxide Decorated with Molybdenum Disulfide Nanoroses for Hydrazine and 4-Nitrophenol Detection in Real Water Samples.

Novel nanostructured platforms based on Pencil Graphite Electrodes (PGEs), modified with pyrene carboxylic acid (PCA) functionalized Reduced Graphene Oxide (rGO), and then decorated by chronoamperometry electrodeposition of MoS2 nanoroses (NRs) (MoS2NRs/PCA-rGO/PGEs) were manufactured for the electrocatalytic detection of hydrazine (N2H4) and 4-nitrophenol, pollutants highly hazardous for environment and human health. The surface morphology and chemistry of the MoS2NRs/PCA-rGO/PGEs were characterized by scanning electron microscopy (SEM), Raman, and X-ray photoelectron spectroscopy (XPS), assessing the coating of the PCA-rGO/PGEs by dense multilayers of NRs. N2H4 and 4-nitrophenol have been monitored by Differential Pulse Voltammetry (DPV), and the MoS2NRs/PCA-rGO/PGEs electroanalytical properties have been compared to the PGEs, as neat and modified by PCA-rGO. The MoS2NRs/PCA-rGO/PGEs demonstrated a higher electrochemical and electrocatalytic activity, due to their high surface area and conductivity, and very fast heterogeneous electron transfer kinetics at the interphase with the electrolyte. LODs lower than the U.S. EPA recommended concentration values in drinking water, namely 9.3 nM and 13.3 nM, were estimated for N2H4 and 4-nitrophenol, respectively and the MoS2NRs/PCA-rGO/PGEs showed good repeatability, reproducibility, storage stability, and selectivity. The effectiveness of the nanoplatforms for monitoring N2H4 and 4-nitrophenol in tap, river, and wastewater was addressed.

Boron-doped {113}, {115} and {118}-oriented single-crystal diamond electrodes: Effect of surface pre-treatment

In this study, the effect of vicinal crystal orientation ({113}, {115}, and {118}), boron doping level, and surface termination induced by various pre-treatments on electrical and electrochemical properties of single crystal boron-doped diamond (SC-BDD) electrodes was carefully investigated. van de Pauw measurements confirmed Hall concentration to increase with increasing vicinal angle, {118} < {115} < {113}. As-deposited H-terminated surfaces were converted to predominantly O-terminated surfaces (confirmed by X-ray photoelectron spectroscopy) using hot acid, ozone and anodic pre-treatments, or by spontaneous oxidation in air. This conversion was recognized by inhibited heterogeneous electron transfer kinetics of inner-sphere redox markers ([Fe(CN)6]3−/4− and dopamine), by an increase in double layer capacitance and charge transfer resistance values. The most pronounced surface oxidation was recognized for electrodes with the lowest boron content as a result of blocked boron atoms by oxygen functionalities and loss of superficial hydrogen on the surface. Reverse conversion from oxidized to H-terminated surfaces using cathodic pre-treatment led mainly to restoration of electrochemical parameters. Moreover, spontaneous oxidation of electrode surfaces in ambient atmosphere was confirmed, with the fastest being within the first five days. The results show the key role of boron content, and hence the crystal orientation and superficial hydrogen in affecting conductivity and performance of SC-BDD electrodes, which is demonstrated by changes in heterogenous electron transfer kinetics of surface sensitive redox systems.

Theory for Heterogeneous Electron Transfer Kinetics on Nanocorrugated Atomic Stepped Metal Electrodes

We develop a novel mean-field theoretical approach for the outer-sphere heterogeneous electron transfer (OS-HET) rate constant at an atomically stepped nanocorrugated metal surface. Theory accounts for the contributions from the nature of the metal, local curvature of the nanocorrugated atomic step, the density of atomic steps and kinks, dipolar solvents, and the frontier molecular orbital of electroactive molecules. Our theoretical approach develops a novel model for the free energy of activation obtained using the alignment of the Fermi energy level in the metal with one of the frontier molecular orbitals of the electroactive species. The activation free energy is obtained as a product of a modified work function (WF) and the fractional electronic charge of activation for the alignment of levels. Metal surface local curvatures at atomic steps are modeled as a hyperbolic tangent functional with a random nanocorrugated step edge. The influence of step density and edge fluctuations (or kinks) on the activation free energy is included through the surface average mean curvature and the mean-square gradient dependent modified WF. The theoretical results highlight the step density-dependent nonmonotonic variation in WF, fractional electronic charge, and activation free energy. The increase in step density shifts the Fermi energy level toward LUMO and away from HOMO energy levels. This causes the reduced (for the LUMO) and enhanced (for the HOMO) activation free energy for ET kinetics compared to the basal plane. The dichotomy in the OS-HET kinetics at the atomically stepped metal results in anomalous enhancement and suppression for the LUMO and HOMO, respectively. The strong influence of kinks along the step edge on the HET rate constant is predicted up to 6 decades (compared to the basal surface of Pt metal). Finally, theory shows agreement with the reported experimental data of WF and exchange current density for single crystal Pt electrodes.

Reorganization energy in a polybromide ionic liquid measured by scanning electrochemical cell microscopy.

Room temperature ionic liquids (RT-ILs) are promising electrolytes for electrocatalysis. Understanding the effects of the electrode-electrolyte interface structure on electrocatalysis in RT-ILs is important. Ultrafast mass transport of redox species in N-methyl-N-ethyl-pyrrolidinium polybromide (MEPBr2n+1) enabled evaluation of the reorganization energy (λ), which reflects the solvation structure in the inner Helmholtz plane (IHP). λ was achieved by fitting the electron transfer rate-limited voltammogram at a Pt ultramicroelectrode (UME) to the Marcus-Hush-Chidsey model for heterogeneous electron transfer kinetics. However, it is time-consuming or even impossible to prepare electrode materials, including alloys of numerous compositions in the form of UME, for each experiment. Herein, we report a method to evaluate the λ of MEPBr2n+1 by scanning electrochemical cell microscopy (SECCM), which allows high throughput electrochemical measurements using a single electrode with high spatial resolution. Fast mass transport in the nanosized SECCM tip is critical for achieving heterogeneous electron transfer-limited voltammograms. Furthermore, investigating λ on a high-entropy alloy materials library composed of Pt, Pd, Ru, Ir, and Ag suggests a negative correlation between λ and the work function. Given that the potential of zero charge correlates with the work function of electrodes, this can be attributed to the surface-charge sensitive ionic structure in the IHP of MEPBr2n+1, modulating the solvation energy of the redox-active species in the IHP.

Semi-microscopic Theory for the Current Rectification Phenomenon in Nanogap Molecular Devices.

A semi-microscopic theory is developed for heterogeneous electron transfer (HET) kinetics based on the energy level alignment approach at self-assembled monolayer (SAM) covered metal electrodes. Theory provides the electronic and molecular property-dependent equations for the HET rate constant (k0) and the transfer coefficient (α) for potential. k0 is formulated using the activation free energy as a product of the SAM covered metal work function (WF) and fractional electronic charge exchanged at the transition state (attained through the alignment of the frontier molecular orbital (FMO) energy level of the electroactive group with the WF of metal). k0 is a function of the metal jellium electronic screening length and dielectric and of the molecular self-assembly (through its dipole moment, size, and packing density) and the FMO energies of electroactive groups. The operative potential at the transition state is governed by α, which is a function of molecular spacer length and characteristic electronic-dipolar coupling length. The current rectification phenomenon in nanogap molecular devices is theoretically analyzed using equations for k0 and α for SAM covered source and drain electrodes. Theory unravels the LUMO or HOMO dichotomy for a given metal: (i) for the HOMO assisted ET, the metal with a high WF has a high current rectification ratio (RR), while (ii) for the LUMO assisted ET, the metal with a low WF has a high current RR in asymmetrical devices. Theory predicts the reversal in current rectification by altering the dipole moment of the anchoring molecule, the HOMO/LUMO energy of the electroactive groups, and the nature of the metal. Finally, theory shows qualitative and quantitative coherence with the reported experimental current-potential response of molecular device.

Redox mediators for oxygen reduction reactions in lithium–oxygen batteries: governing kinetics and its implications

Intrinsic properties of quinones such as steric hindrance and heterogeneous electron transfer kinetics that follows Marcus theory concurrently govern their performance as redox mediators for oxygen reduction reactions in lithium–oxygen batteries.

Electrochemical response of carbon paste electrodes modified with carbon nanotubes: Effects of temperature of nitrogen doping and oxygen functionalization

Carbon paste electrodes (CPE) were modified with nitrogen-doped carbon nanotubes (N-CNTs) synthesized at 750 °C (N-CNT750) and 850 °C (N-CNT850), carboxyl functionalized carbon nanotubes (MWCNTCOOH), and pristine carbon nanotubes (MWCNT). Their electrochemical properties were investigated and compared towards a common redox probe, potassium ferricyanide (K3[Fe(CN)6]). The electrochemical responses were probed using cyclic voltammetry, and the results unveiled that modification with carbon nanotubes enhanced anodic peak currents (Ipa) for the electrodes CPE/MWCNTCOOH, CPE/N-CNT750 and CPE/N-CNT850 (Ipa, p < 0.05 in relation to pure CPE). In particular, CPE modified with N-CNTs synthesized at 850 °C (CPE/N-CNT850) exhibited better electrochemical response, with lower peak-to-peak potential separation (ΔEp, p < 0.05 in relation to pure CPE). Complementary studies were conducted to investigate the electroactive area (A) and heterogeneous electron-transfer kinetics (k0). CPE/N-CNT850 (0.034 ± 0.005 cm2) and CPE/MWCNTCOOH (0.057 ± 0.002 cm2) presented more satisfactory results for the anodic and cathodic electroactive area (A), respectively. Kinetic studies revealed a higher k0 (1.03 × 10−3 cm.s−1) for the CPE/N-CNT850 electrode. Differential pulse voltammetry was employed for the electrochemical detection of Acetaminophen (AC). The response was linear for an AC concentration range from 66.2 to 259.7 μmol L−1, with the lowest detection limit (LOD) of 0.37 μmol L−1 and the highest sensitivity of 0.244 ± 0.002 μA L μmol−1 for the respective electrodes CPE/N-CNT850 and CPE/MWCNTCOOH. The CPE/N-CNT850 and CPE/MWCNTCOOH electrodes show the best correlation of results, the presence of polar groups on the surface of MWCNTCOOH and a higher amount of graphitic nitrogen bond of N-CNT850 suggest that these CNTs are particularly promising for applications in electrochemical fields.

Mechanistic Approach to Reveal Interaction of Uranyl Ions in Alkyltriphenylphosphonium Bromide-Based Deep Eutectic Solvent.

Speciation is known to control fundamental aspects of metal processing and electrochemical behavior such as solubility and redox potentials. Deep eutectic solvents (DESs) are an emerging class of green, low-cost and designer solvents and are being explored as alternatives for recycling nuclear fuel and critical materials. However, there is a lack of knowledge about the behavior of metals in them. Here, for the first time, we synthesized three new DESs based on alkyltriphenylphosphonium bromide (CnPPh3Br), with varied alkyl chain lengths (n), as the hydrogen-bond acceptor along with decanoic acid (DA) as the hydrogen-bond donor and explored the redox speciation of uranyl nitrate. The changes in the Fourier transform infrared and NMR spectra helped elucidate the formation of hydrogen bonds in DES. The absorption maxima of uranyl in DES was red-shifted by 10 nm compared to the free uranyl, with concomitant increase in intensity and luminescence lifetime, which suggested a strong interaction of uranyl nitrate with DES. Cyclic voltammetry was probed to understand the redox thermodynamics, transport properties, and heterogeneous electron transfer kinetics of the irreversible electron transfer of uranyl ions in the three DESs. Electrochemical and spectroscopic techniques together with density functional theory calculations unlocked microscopic insights into the solvation and speciation of UO22+ ions in three DESs and also the associated unusual trends observed in the physical properties of the DESs. The hydrogen-bonded structure of DES plays a crucial role in the redox behavior of the UO22+ ion due to its strong potent complexation with its components. The basic findings of the present work can have far-reaching consequences for the extraction, electrochemical separation, and future development of redox-based separation processes in the nuclear fuel cycle.

Revealing the Influence of Diverse Secondary Metal Cations on Redox-Active Palladium Complexes.

Incorporation of redox-inactive metals into redox-active complexes and catalysts attracts attention for engendering new reactivity modes, but this strategy has not been extensively investigated beyond the first-row of the transition metals. Here, the isolation and characterization of the first series of heterobimetallic complexes of palladium with mono-, di-, and tri-valent redox-inactive metal ions are reported. A Reinhoudt-type heteroditopic ligand with a salen-derived [N2 ,O2 ] binding site for Pd and a crown-ether-derived [O6 ] site has been used to prepare isolable adducts of the Lewis acidic redox-inactive metal ions (Mn+ ). Comprehensive data from single-crystal X-ray diffraction analysis reveal distinctive trends in the structural properties of the heterobimetallic species, including an uncommon dependence of the Pd⋅⋅⋅M distance on Lewis acidity. The reorganization energy associated with reduction of the heterobimetallic species is strongly modulated by Lewis acidity, with the slowest heterogeneous electron transfer kinetics associated with the strongest incorporated Lewis acids. This hitherto unexplored reorganization energy penalty for electron transfer contrasts with prior thermodynamic studies, revealing that kinetic parameters should be considered in studies of reactivity involving heterobimetallic species.

Enzymeless glucose sensor based on disposable Ecoflex®/graphite thermoplastic composite substrate modified with Au@GQDs

There is an increasing demand for robust, affordable, and sensitive analytical devices for clinical diagnostics. Herein, we describe a simple route to produce biodegradable and disposable thermoplastic composite electrodes using the commercial polymer (Ecoflex®) and graphite microparticles. The surface of the carbon composite electrode was modified with gold nanoparticles conjugated with graphene quantum dots (Au@GQDs) for sensitive and non-enzymatic glucose sensing. The morphological and structural characterizations of the modified electrochemical device revealed a hydrophilic rough composite surface, with high dispersion of graphite in the polymer matrix and confirmed the successful superficial modification of the working electrode with spherical Au@GQDs nanoparticles. Cyclic voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) were used to electrochemically characterize the modified and nonmodified thermoplastic sensor, demonstrating a 4.5-fold enhancement of the electroactive area, higher heterogeneous electron transfer kinetics, and electrocatalytic properties for glucose sensing when the sensor was modified with Au@GQDs. The electrochemical determination of the glucose was carried out using an amperometric method applying -0.15 V in alkaline conditions (0.1 mol L−1 NaOH). Under optimized conditions, the proposed method presented a high sensitivity (4.85 A L mol−1) and a low limit of detection (LOD) of 9.12 µmol L−1. The Ecoflex:graphite/Au@GQDs sensor was applied for glucose sensing in synthetic saliva samples with recovery values ranging from 97.84% to 105.20%, highlighting the excellent accuracy of the proposed method. Finally, the electrode was subjected to thermal and biological degradation studies and presented disposable and eco-friendly characteristics. Based on these results, we believe that this new thermoplastic composite sensor modified with Au@GQDs can be an excellent candidate for a disposable, low-cost, and enzymeless electrochemical glucose sensor.

Electrochemistry of the inclusion complexes of metallocene derivatives with cucurbit[7]uril on glassy carbon and mercury electrodes

• The electrochemical behavior of ferricenium and cobalticenium cations and supramolecular complexes formed by such cations and ferrocene with a cucurbit[7]uril was studied on GC and Hg electrodes. • Revealed significant differences in the kinetics of heterogeneous electron transfer for these systems due to the principally different adsorption behavior of supramolecular complexes of these metallocenes on GC and Hg. The aim of this work is to compare the electrochemical behavior of ferricenium and cobalticenium cations and supramolecular complexes formed by these cations and ferrocene with a cavitand – cucurbit[7]uril on glassy carbon and mercury electrodes. On the Hg electrode the adsorption properties of these supramolecular complexes were also studied. The substantial differences in the processes of heterogeneous electron transfer for these systems are explained by the principally different adsorption behavior of supramolecular complexes of these metallocene derivatives on GC and Hg. It was found that the mechanism of reactions of these supramolecular complexes on the Hg electrode is determined by the conditions of formation of their adlayers in the potential region preceding the beginning of electrochemical processes. For complexes formed by ferricenium cation and cobaltocene with cucurbit[7]uril we have determined the more precise values for the constants of complex formation as compared to the literature data (7 × 10 9 M −1 and 1 × 10 8 M −1 , respectively).

Enhanced heterogeneous electron transfer kinetics in Graphene Oxide produced from mechanically milled Graphite

Controlled structural and chemical defects were induced on to graphene oxide (GO) by mechanical milling of the precursor graphitic sheets for different times followed by their chemical oxidation. Raman, XRD, FTIR, XPS and AFM measurements was carried out as an evidence to study the structural and chemical modifications. Milling induces in plane and edge plane defects on graphitic sheets which influence oxidation degree, sp3/sp2 hybridization and functional groups constitution on produced GO. The influence of these structural variations towards electrochemical heterogenous electron transfer (HET) kinetics was explored using redox probe potassium ferricyanide and through non-enzymatic electrochemical detection of Hydrogen Peroxide (H2O2). The increased in-plane and edge plane functionalities actively enhanced the HET kinetics on GO and therefore it's the sensing ability significantly.

Electrochemical Characterization of Aromatic Molecules with 1,4-Diaza Groups for Flow Battery Applications

The aqueous redox flow battery is a promising technology for large-scale low cost energy storage. The rich possibilities for the tailoring of organic molecules and the possibility to discover active materials of lower cost and decreased environmental impact continue to drive research and development of organic compounds suitable for redox flow battery applications. In this work, we focus on the characterization of aromatic molecules with 1,4-diaza groups for flow battery applications. We examine the influence of electron-withdrawing and electron-donating substituents and the effect of the relative position of the substituent(s) on the molecule. We found that electron-withdrawing substituents increased the potential, while electron-donating decreased it, in agreement with expectations. The number of carboxy-groups on the pyrazinic ring was found to have a strong impact on the heterogeneous electron transfer kinetics, with the slowest kinetics observed for pyrazine-2,3,5,6-tetracarboxylic acid. The stability of quinoxaline was investigated by cyclic voltammetry and in a flow cell configuration. Substitution at the 2,3-positions in quinoxaline was found to decrease the capacity fade rate significantly. Furthermore, we demonstrated how molecular aggregation reduces the effective number of electrons involved in the redox process for quinoxalines. This translates to a significant reduction of the achievable volumetric capacity at higher concentrations, yielding values significantly lower than the theoretical capacity. Finally, we demonstrate that such capacity-limiting molecular aggregation may be reduced by introducing flexible side chains with bulky charged groups in order to increase electrostatic repulsion and steric hindrance.

Functionalized 2D Germanene and Silicene Enzymatic System

AbstractIn recent years, 2D Group 14 graphane analogs, such as germanane, methyl germanane, and siloxene, have taken materials scientists by storm due to their facile synthesis procedures and the numerous attractive properties proffered. Due to their fascinating properties, these emerging Group 14 graphane analogs are studied for varied applications including supercapacitors, photocatalysts, and sensors. Although several groups have reported the viability of using Group 14 graphane analogs for the construction of biosensors, they are mainly based upon computational studies, and few experimental studies are conducted. This paper aims to experimentally investigate the feasibility of using germanane and siloxene‐based materials as 2D functional support for enzymatic systems. The heterogeneous electron transfer kinetics and the glucose sensing response of the as‐synthesized germanane, methyl germanane, and siloxene are examined, and the most outstanding material, germanane, is employed for further construction of electrochemical glucose biosensor. The fabricated biosensing platform delivers excellent analytical performances, displaying good linearity over various magnitudes of glucose concentrations, and possesses a low detection limit. The findings reported herein showcase the potential of applying these 2D Group 14 graphane analogs for future developments of highly selective and sensitive biosensors for biomedical, environmental monitoring, and food sampling applications.

Investigation of localized electrochemical reactivity on a β-PbO2 electrode using scanning electrochemical microscopy

Most of the PbO 2 electrode reactivity studies have been mainly focused on the macro level, while the PbO 2 electrode electrochemical reactivity is less studied in the micro range. In this study, scanning electrochemical microscopy (SECM) was employed, and [Fe(CN) 6 ] 3− /[Fe(CN) 6 ] 4− was used as a redox mediator. Cyclic voltammetry (CV), probe approach curve (PAC), constant-height scanning and electrochemical impedance spectroscopy (EIS) were conducted to study the localized electrochemical reactivity and interfacial reaction kinetics of the β-PbO 2 electrode. The results show that the β-PbO 2 electrode can support the oxidation reaction of [Fe(CN) 6 ] 4− under the open-circuit potential, at microscopically different positions, [Fe(CN) 6 ] 4− has different chemical reaction rates, and hence significantly different electrochemical reactivity. The effective heterogeneous charge transfer rate constant k eff is computed to be 0.0253 cm/s by fitting the positive feedback curve. The largest [Fe(CN) 6 ] 4− oxidation reaction rate on the localized β-PbO 2 electrode emerges when the substrate potential is maintained at 0.5 V, achieving the best resolution of the constant-height image; furthermore, the EIS results macroscopically strengthen this conclusion by demonstrating that the charge transfer process of [Fe(CN) 6 ] 4− is the easiest. This study provides a novel way to study the electrochemical reactivity of PbO 2 electrodes from a micro perspective. • Localized electrochemical reactivity of β-PbO 2 electrodes were imaged using SECM. • Heterogeneous electron transfer kinetics was computed through approach curve. • Macroscopic and microscopic electrochemical reactivity were compared by EIS.

Electrochemical Characterization of Aromatic Molecules with 1,4-Diaza Groups for Flow Battery Applications

Redox flow batteries based on organic active materials can provide a cost effective, biosustainable method for storing energy [4]. Quinones have been the primary focus for RFB applications with numerous reports on the properties and effect of structural modification by substitution with different functional groups [2, 3, 5]. The present study was focused on aromatic diaza compounds for RFB applications, namely pyrazine and quinoxaline. Both compounds undergo two electron transfer on the pyrazinic ring [6, 7]. Their small size and high solubility makes them attractive for flow cell application as it can maximize energy density [3]. In this work we examine the electrochemical properties as a function of pH and substitution. An initial screening using cyclic voltammetry in alkaline environment was performed on pristine pyrazine, and on pyrazine substituted with 1-4 methyl or carboxylic acid groups. The electron-donating methyl groups reduced the average redox potential about 55 mV per methyl group, while the electron-withdrawing carboxylic groups increased it. Additionally, the degree of substitution influenced the heterogeneous electron transfer kinetics significantly, with the greatest negative effect observed in the carboxylic pyrazines. Quinoxalines in the same alkaline environment featured similar results in different extend. The position of the functional group in regards to the active pyrazinic ring altered the degree of the effect, allowing for fine tuning of properties. Quinoxaline and quinoxaline-2,3-diyldimethanesulfonate (2,3-DMSQUI) were further examined in different pH under buffered and unbuffered solutions. It revealed that different processes take place depending on the availability of protons. Similar phenomena have been observed in quinone systems as well [2]. In the unbuffered solution, quinoxaline’s average potential had been stable up around pH = 11 where a slope of 44 mV/pH appeared. The cathodic and anodic potential had different slopes (25 and 63 mV/pH) indicating an uneven transfer coefficient, different rate limiting step, unequal amount of protons or a combination of the previous. The sharper voltammograms appeared in the high pH range. In contrast, cyclic voltammetry of 2,3-DMSQUI revealed high heterogeneous rate constant at pH 2-3. While at higher pH the kinetics appeared to be sluggish. The slopes of average, anodic and cathodic peak potential of quinoxaline in buffered solution were the same in the pH range of 2 to 12. Assuming that in buffered solution protons are always available, the difference of the slopes in the unbuffered case was most likely to an uneven amount of protons involved in the oxidation and reduction processes resulting from different acid dissociation constants. Moreover, in the acidic region, a peak at higher potential appeared which it has been attributed to an irreversible oxidative addition of a hydroxyl group [1]. Whereas, the same irreversible process was not present in 2,3-DMSQUI which meant substituting the 2 and 3 position hindered this degradation. No difference of 2,3-DSMQUI in buffered solution was found with its unbuffered counterpart, besides that the most reversible voltammogram appeared at a slightly more acidic environment ( pH = 4). Thus 2,3-DSMQUI is a good candidate for an acidic flow battery. Lastly, kinetic parameters determined by steady state voltammetry with rotating disk electrode. Diffusion coefficient and heterogeneous rate constant for quinoxaline were 7.93(±0.06)∗10-6 cm2/s and 9.6∗10-5 cm/s respectively, and 2,3-DSMQUI are 4.02(±0.06)∗10-6 cm2/s and 10.8∗10-4 cm/s.[1] M. Aleksic, J. Pantic, and V. Kapetanovic. Evaluation of kinetic parameters and redox mechanism of quinoxaline at glassy carbon electrode. Facta universitatis - series: Physics, Chemistry and Technology, 12(1):55–63, 2014.[2] C. Costentin. Electrochemical approach to the mechanistic study of proton-coupled electron transfer. Chemical Reviews, 108:2145–2179, 2008.[3] Y. Ding, C. Zhang, L. Zhang, Y. Zhou, and G. Yu. Molecular engineering of organic electroactive materials for redox flow batteries. Chemical Society Reviews, 47(1):69–103, 2018.[4] J. D. Hofmann, S. Schmalisch, S. Schwan, L. Hong, H. A. Wegner, D. Mollenhauer, J. Janek and D. Schröder. Tailoring Dihydroxyphthalazines to Enable Their Stable and Efficient Use in the Catholyte of Aqueous Redox Flow Batteries. Chemistry of Materials, 2020.[5] Y. Ji, M. A. Goulet, D. A. Pollack, D. G. Kwabi, S. Jin, D. De Porcellinis, E. F. Kerr, R. G. Gordon, and M. J. Aziz. A Phosphonate-Functionalized Quinone Redox Flow Battery at Near-Neutral pH with Record Capacity Retention Rate. Advanced Energy Materials, 9(12):1–7, 2019.[6] J. D. Milshtein, L. Su, C. Liou, A. F. Badel, and F. R. Brushett. Voltammetry study of quinoxaline in aqueous electrolytes. Electrochimica Acta,180:695–704, 2015.[7] E. D. Moorhead and D. Britton. Evidence for the Reversible Stepwise Polarographic Reduction of Pyrazine from Strong Acid Media. Analytical Letters, 1 (9):541–549, jan 1968.

Identification of Mechanistic Subtleties that Apply to Voltammetric Studies at Boron-Doped Diamond Electrodes

The α-[S2W18O62]4–/5– and α-[S2W18O62]5–/6– polyoxometalate (POM) reduction processes at boron-doped diamond (BDD) disk electrodes in acetonitrile (0.50 M [n-Bu4N][PF6]) have been investigated using large-amplitude Fourier transformed alternating current (FTAC) voltammetry. The origins of subtle differences in experimental data and simulated data modeled assuming the Butler–Volmer relationship with ion-paring implicitly included, planar diffusion and a uniform surface are considered. Parameters estimated are the apparent heterogeneous electron-transfer kinetics (kapp0′), apparent formal reversible potential (Eapp0′), and apparent charge transfer coefficient (αapp′). The electrode kinetic parameters, in contrast to theoretical predictions of the model employed, are dependent on frequency, POM concentration, and the data analysis method. Reasons for nonconformance to the model include the adsorption of POM on graphite-like sp2-bonded carbon impurities, limitations in the availability of charge carriers in BDD, and the method of incorporating ion pairing when slow electrode kinetics apply. Masking of the sp2 carbon-rich (edge) region of the BDD disk provided FTAC voltammetric data that complied much more closely with the simulated data. Nevertheless, data analysis still produces a concentration dependence in the estimated kapp0′ values, which is considered in terms of the assumption of an infinite number of charge carriers. Conclusions derived from this study are likely to be generally applicable to electrode kinetic investigations at BDD electrodes.

Nitrogen-doped carbon coated TiC nanofiber arrays deposited on Ti-6Al-4V for selective and sensitive electrochemical detection of dopamine

Core-shell titanium carbide/carbon (TiC@C) and titanium carbide/nitrogen-doped carbon (TiC@CNx) quasi-aligned nanofiber arrays (NFAs) are fabricated on biomedical Ti-6Al-4V by a simple thermal chemical process in the presence of benzene and pyridine at 800 °C, respectively. The morphology, structure, and composition of the TiC@C and TiC@CNx NFAs are characterized by X-ray diffraction, electron microscopy, energy-dispersive X-ray spectrometry, and Raman scattering spectroscopy. The TiC@C and TiC@CNx nanofibers consisting of highly conducting single-crystalline TiC cores and carbon shells or N-doped carbon shells have a large amount of defects and N doping endows the TiC@CNx NFAs with excellent electrochemical properties. The TiC@CNx NFAs electrode exhibits fast heterogeneous electron transfer (HET) kinetics in the Fe(CN)63−/4− redox couple showing an ideal peak-to-peak potential separation (59 mV) at a high scan rate of 7000 mV s−1. As a result, the TiC@CNx NFAs electrode has high activity in the simultaneous detection of dopamine (DA), ascorbic acid (AA), uric acid (UA), and serotonin (5-hydroxytryptamine, 5-HT) as well as high sensitivity and selectivity in the determination of DA in the presence of UA, AA, and 5-HT. The linear range for the determination of DA is 0.5–180 μM and detection limit is 8 nM (S/N = 3).