Ultramicroelectrode Voltammetry Research Articles

Mechanistic Study on Oxygen Reduction Reaction in High-Concentrated Electrolytes for Aprotic Lithium-Oxygen Batteries.

Highly concentrated electrolytes (HCEs) have energized the development of high-energy-density lithium metal batteries by facilitating the formation of robust inorganic-derived solid electrolyte interfaces on the lithium anode. However, the oxygen reduction reaction (ORR) occurring on the cathode side remains ambiguous in HCE-based lithium-oxygen (Li-O2) batteries. Herein, we investigate the ORR mechanism in a highly concentrated LiTFSI-CH3CN electrolyte using ultra-microelectrode voltammetry coupled with in situ spectroscopies. It is found that, compared to the dilute electrolyte, the HCE prolongs the lifespan of superoxide intermediates and decelerates their migration rate to the bulk solution, resulting in a change in growth mode for the discharge product of Li2O2 from traditional two-dimensional film growth to surface three-dimensional expansion growth. This alteration reduces the cathode passivation and thus delivers the enhanced discharge capacity. Additionally, the HCE also increases the reaction energy barrier between superoxide and solvent molecules, thereby minimizing parasitic reactions and improving the cycle performance of Li-O2 batteries. Our study reveals the intricate interplay between electrolytes and oxygen intermediates and provides important insights into electrolyte chemistries for better Li-O2 batteries.

The Effects of Excess Protic Species on Oxygen Reduction in Protic Ionic Liquids and Implications for the Use of Ionic-Liquid Electrolytes in Fuel Cells

The conventional polymer-electrolyte membranes used in low-temperature fuel cells are limited to operating temperatures below about 120 ºC, as they must be fully hydrated to facilitate proton transport. Protic ionic liquids (PILs) are ionic liquids formed by transferring protons from Brønsted acids to Brønsted bases, and it has recently been shown that some ammonium-based PILs inherently exhibit high proton conductivities. Consequently, PILs have been proposed for use as electrolytes in non-humidified fuel cells that can operate above 120 ºC (at intermediate temperatures). However, while they nominally consist entirely of ions, PILs can often contain a significant quantity of neutral species (either molecules or ion clusters) that can affect the physicochemical properties of the liquids. In this contribution, we first describe an electroanalytical method for detecting and quantifying residual Brønsted acids in a series of ammonium-based PILs. Ultramicroelectrode voltammetry reveals that some of the accepted methods for synthesizing PILs can readily result in the formation of nonstoichiometric PILs containing up to 230 mmol dm−3 residual acid. We will then show that residual acid in PILs can have a drastic effect on the electrocatalytic oxygen reduction reaction (ORR; O2 + 4e−+ 4H+→ 2H2O) in the PILs. For example, the potential at which the ORR occurs at Pt in the PIL diethylmethylammonium trifluoromethanesulfonate, [dema][TfO], decreases linearly as the strength of the proton donor in the liquid decreases. In “pure” [dema][TfO], in which the proton donors during the ORR are the [dema]+ cations of the PIL (pK a = 10), the onset potential of the ORR is the same as that of the hydrogen oxidation reaction (HOR) in the PIL. These observations have significant implications for the use of PILs as electrolytes in fuel cells and indicate that the best PILs are highly "acidic" liquids that can support oxygen reduction at high potentials.

Electroanalysis of Neutral Precursors in Protic Ionic Liquids and Synthesis of High-Ionicity Ionic Liquids.

Protic ionic liquids (PILs) are ionic liquids that are formed by transferring protons from Brønsted acids to Brønsted bases. While they nominally consist entirely of ions, PILs can often behave as though they contain a significant amount of neutral species (either molecules or ion clusters), and there is currently a lot of interest in determining the degree of "ionicity" of PILs. In this contribution, we describe a simple electroanalytical method for detecting and quantifying residual excess acids in a series of ammonium-based PILs (diethylmethylammonium triflate [dema][TfO], dimethylethylammonium triflate [dmea][TfO], triethylammonium trifluoroacetate [tea][TfAc], and dimethylbutylammonium triflate [dmba][TfO]). Ultra-microelectrode voltammetry reveals that some of the accepted methods for synthesizing PILs can readily result in the formation of nonstoichiometric PILs containing up to 230 mM excess acid. In addition, vacuum purification of PILs is of limited use in cases where nonstoichiometric PILs are formed. Although excess bases can be readily removed from PILs under ambient conditions, excess acids cannot be removed, even under high vacuum. The effects of excess acid on the electrocatalytic oxygen reduction reaction (ORR) in PILs have been studied, and the onset potential of the ORR in [dema][TfO] increases by 0.8 V upon addition of acid to PIL. On the basis of the results of our analyses, we provide some recommendations for the synthesis of highly ionic PILs.

Redox Active Polymers for Non-Aqueous Redox Flow Batteries: Validation of the Size-Exclusion Approach

Non-aqueous redox flow batteries (NRFBs) are emerging technologies that promise higher energy densities than aqueous counterparts. Unfortunately, cell resistance and redox component crossover observed when using ion-exchange membranes (IEMs) hinders NRFB development. The size exclusion approach for polymer-based NRFBs addresses these issues by using macromolecular design to mitigate crossover. Here, we highlight the benefits of this approach using inexpensive nano-porous separators (PS) (Celgard and Daramic). We evaluated these along with an IEM (Fumasep) in a flow cell configuration using a classical redox couple of viologen and ferrocene, both in monomer and polymer forms. High Coulombic efficiencies above 98% and access up to 80% of capacity were observed for the polymer cells. These displayed better performance with PS than with the IEM, which exhibited lower energy efficiencies from higher overpotentials. The monomer equivalent cells with PS resulted in lower efficiencies and rapid decrease in depth of discharge. Post-cycling analysis by ultramicroelectrode voltammetry showed that the small molecules freely crossed PS and to a lesser degree through the IEM. Therefore, here we demonstrate that the combination of redox active polymers and simple PS enables a potential next-generation NRFB system that provides a competitive alternative to the use of IEMs in NRFBs.

Impact of redox-active polymer molecular weight on the electrochemical properties and transport across porous separators in nonaqueous solvents.

Enhancing the ionic conductivity across the electrolyte separator in nonaqueous redox flow batteries (NRFBs) is essential for improving their performance and enabling their widespread utilization. Separating redox-active species by size exclusion without greatly impeding the transport of supporting electrolyte is a potentially powerful alternative to the use of poorly performing ion-exchange membranes. However, this strategy has not been explored possibly due to the lack of suitable redox-active species that are easily varied in size, remain highly soluble, and exhibit good electrochemical properties. Here we report the synthesis, electrochemical characterization, and transport properties of redox-active poly(vinylbenzyl ethylviologen) (RAPs) with molecular weights between 21 and 318 kDa. The RAPs reported here show very good solubility (up to at least 2.0 M) in acetonitrile and propylene carbonate. Ultramicroelectrode voltammetry reveals facile electron transfer with E1/2 ∼ -0.7 V vs Ag/Ag(+)(0.1 M) for the viologen 2+/+ reduction at concentrations as high as 1.0 M in acetonitrile. Controlled potential bulk electrolysis indicates that 94-99% of the nominal charge on different RAPs is accessible and that the electrolysis products are stable upon cycling. The dependence of the diffusion coefficient on molecular weight suggests the adequacy of the Stokes-Einstein formalism to describe RAPs. The size-selective transport properties of LiBF4 and RAPs across commercial off-the-shelf (COTS) separators such as Celgard 2400 and Celgard 2325 were tested. COTS porous separators show ca. 70 times higher selectivity for charge balancing ions (Li(+)BF4(-)) compared to high molecular weight RAPs. RAPs rejection across these separators showed a strong dependence on polymer molecular weight as well as the pore size; the rejection increased with both increasing polymer molecular weight and reduction in pore size. Significant rejection was observed even for rpoly/rpore (polymer solvodynamic size relative to pore size) values as low as 0.3. The high concentration attainable (>2.0 M) for RAPs in common nonaqueous battery solvents, their electrochemical and chemical reversibility, and their hindered transport across porous separators make them attractive materials for nonaqueous redox flow batteries based on the enabling concept of size-selectivity.

Oxygen and hydrogen peroxide reduction by 1,2-diferrocenylethane at a liquid/liquid interface

Molecular oxygen and hydrogen peroxide reduction by 1,2-diferrocenylethane (DFcE) was investigated at a polarized water/1,2-dichloroethane (W/DCE) interface. The overall reaction points to a proton-coupled electron transfer (PCET) mechanism, where the first step consists of the protonation of DFcE to form the DFcE–H+ in DCE phase, either by DFcE facilitated proton transfer across the liquid–liquid interface or by the homogeneous protonation of DFcE in the presence of protons extracted in the oil phase by tetrakis(pentafluorophenyl)borate. The formation of DFcE–H+ is followed up by the O2 reduction to hydrogen peroxide and further reduction to water. The final products of DFcE oxidation, namely DFcE+ or DFcE2+, were investigated by ion transfer voltammetry, ultramicroelectrode voltammetry and UV/visible spectroscopy. These results show that mostly DFcE+ is produced, although DFcE+ can also reduce oxygen at longer time scales. Hydrogen peroxide reduction is actually faster than oxygen reduction, but both reactions are slow due to relatively low thermodynamic driving force.

Ultramicroelectrode voltammetry and scanning electrochemical microscopy in room-temperature ionic liquid electrolytes

The high viscosity and unusual properties of room temperature ionic liquids (RTILs) present a number of challenges when performing steady-state voltammetry and scanning electrochemical microscopy in RTILs. These include difficulties in recording steady-state currents at ultramicroelectrode surfaces due to low diffusion coefficients of redox species and problems associated with unequal diffusion coefficients of oxidised and reduced species in RTILs. In this tutorial review, we highlight the recent progress in the use of RTILs as electrolytes for ultramicroelectrode voltammetry and SECM. We describe the basic principles of ultramicroelectrode voltammetry and SECM and, using examples from the recent literature, we discuss the conditions that must be met to perform steady-state voltammetry and SECM measurements in RTILs. Finally, we briefly discuss the electrochemical insights that can be obtained from such measurements.

Study on the interaction between dihydroxybenzenes and DTAB by ultramicroelectrode voltammetry

The diffusion behavior of dihydroxybenzenes which can indicate their interactions with DTAB has been investigated by measuring the diffusion coefficients of dihydroxybenzenes in DTAB solutions. From the diffusion coefficient we can see that the position of the substituted hydroxyl groups can affect the interaction mechanism. Catechol mainly interacts with DTAB micelles by hydrophobic forces. However, hydroquinone mainly interacts with DTAB monomers rather than DTAB micelles through electrostatic forces. On the other hand, the interaction mechanism is also dependent on the hydrocarbon chain length. Shorter hydrocarbon chains interact much easier with dihydroxybenzenes. According to the above mechanism, the partition coefficients of dihydroxybenzenes in the surfactant solutions are calculated. The surface tension of the system was also measured to confirm the voltammetry results. The results obtained from surface tension measurement are well consistent with the voltammetry results.

Study on the first-step adsorption of dodecyltrimethylammonium bromide solutions on silica wafer surfaces by ultramicroelectrode voltammetry

Ultramicroelectrode (UME) voltammetry is introduced to study the first-step adsorption of dodecyltrimethylammonium bromide (DTAB) solutions on silica wafer surfaces. This method is based on the exchange reaction of the surfactant molecules with hydrogen ions (H +) on the surfaces. In the first-step adsorption process, when a surfactant molecule is adsorbed to the hydroxylated silica surfaces, a H + will be displaced. Therefore, H + concentration will change with the adsorption process until it reaches saturation of the first-step adsorption. The molar adsorption amount of DTAB (mol m −2) before critical micelle concentration (CMC) can be calculated from the change in H + concentration. The following adsorption process at higher surfactant concentrations is dominated by hydrophobic forces. Consequently, the H + concentrations do not change with the adsorption process any more, which makes the measurement uninfluenced by the following hydrophobic adsorption process. The adsorption isotherms of DTAB on silica wafer surfaces under different pH are measured with this method. It is found that all the adsorption isotherms exhibit asymptote (L) shape and the equilibrium molar adsorption amounts increase with increasing the pH of the solution. These results indicate that H + not only change the surface charge but also compete with surfactant for adsorption at higher proton concentrations.

Electrochemical, Pulsed‐Field‐Gradient Spin‐Echo NMR Spectroscopic, and ESR Spectroscopic Study of the Diffusivity of Molecular Probes inside Gel‐Type Cross‐Linked Polymers

The permeability of five gel-type synthetic resins, obtained by polymerization of 1-vinylpyrrolidin-2-one cross-linked with N,N'-methylenebisacrylamide (1, 2, 3, 4, and 5 wt %) and swollen by N,N-dimethylformamide (DMF), has been analyzed. The diffusion of 2,2,6,6-tetramethyl-4-oxo-1-piperidinyloxyl (TEMPONE) was studied by ultramicroelectrode voltammetry. Similar measurements were performed for solutions of non-cross-linked poly(vinylpyrrolidone) in DMF. To provide information on the rotational mobility of TEMPONE and the translational mobility of DMF, electron spin resonance (ESR) spectroscopic and pulsed-field-gradient spin-echo nuclear magnetic resonance (PGSE-NMR) spectroscopic experiments, respectively, were carried out. Comparative analysis of the results obtained by electrochemical, ESR spectroscopic, and PGSE-NMR spectroscopic measurements showed that diffusivity inside the polymer framework is significantly affected by the extent of cross-linking, the size of the diffusing probe, and the presence of electrolytes.

A Simple Method Based on NMR Spectroscopy and Ultramicroelectrode Voltammetry for the Determination of the Number of Electrons in Faradaic Processes

A new, convenient method to determine the number of electrons (n) involved in a faradaic process was demonstrated using a series of four compounds containing two, three, four, and eight equivalent ferrocene centers. The method takes advantage of pulse gradient stimulated echo (PGSE) NMR spectroscopy to determine the diffusion coefficient (Do) of the electroactive species. The value of n is subsequently determined from the steady-state limiting current (iL) measured on a disk ultramicroelectrode.

Oxidation of 4-Methylanisole by Aqueous Cerium(IV) in a Two−Phase Immiscible Liquid/Liquid System: Interfacial versus Homogeneous Control

The kinetics of the oxidation of 4-methylanisole (MA) by Ce(IV) have been measured in both two-phase (MA/aqueous sulfuric acid) and single-phase (aqueous sulfuric acid) arrangements by using microelectrochemical measurements at expanding droplets (MEMED) and UV−visible spectrophotometry, respectively. In the two-phase arrangement, the electron transfer (ET) reaction appeared to occur interfacially, rather than by dissolution of MA followed by reaction in solution as previously considered. The effective first-order rate constant for the consumption of Ce(IV) at the interface is 2.0 × 10-4 cm s-1. The homogeneous reaction is first-order in both Ce(IV) and MA, with a rate constant for the loss of Ce(IV) of 1.0 mol-1 dm3 s-1. The lifetime of the 4-methylanisole radical cation in aqueous sulfuric acid, formed in the first step of the oxidation process, has been measured independently as ∼10 ms using steady-state ultramicroelectrode voltammetry. The factors leading to the different energetics of the one phase a...

Effect of electrolyte concentration on axial anion ligation in manganese(III) meso-tetraphenylporphyrin chlorides

The ionic strength dependence of axial ligation in manganese(III) 5,10,15,20-tetraarylporphyrin chlorides has been investigated and found to be much more important than has hitherto been recognized. In this study tetrahydrofuran solutions of the metalloporphyrin, 0.0-0.5 M in the electrolyte tetrabutylammonium tetrafluoroborate, were examined by electronic absorption and resonance Raman spectroscopies as well as ultramicroelectrode voltammetry. The metalloporphyrin complexes were found to be significantly dissociated even at moderate electrolytc concentrations.

Ultramicroelectrode voltammetry in a drop of solution: a new approach to the measurement of adsorption isotherms at the solid-liquid interface

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTUltramicroelectrode voltammetry in a drop of solution: a new approach to the measurement of adsorption isotherms at the solid-liquid interfacePatrick R. Unwin and Allen J. BardCite this: Anal. Chem. 1992, 64, 2, 113–119Publication Date (Print):January 15, 1992Publication History Published online1 May 2002Published inissue 15 January 1992https://doi.org/10.1021/ac00026a003RIGHTS & PERMISSIONSArticle Views261Altmetric-Citations38LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (3 MB) Get e-Alerts Get e-Alerts

Anodic oxidation of some tertiary amines

In our investigations we focused our attention on aromatic amines leading to the formation of a benzidine structure where the loss of a proton occurs at the level of the phenyl group. This study was performed in acetonitrile with five amines using linear sweep voltammetry and ultramicroelectrode voltammetry

Electrochemical and photoelectronic spectral study of compounds with high ionization potentials: Anodic oxidation of vinyl triflates in aprotic solvents

AbstractMicroelectrodes can be used to measure redox half‐wave potentials in aprotic solvents containing no purposely added supporting electrolyte. By employing an electrode of sufficiently small size, the accessible potential range in solution is considerably extended. The electrochemical oxidation of vinyl (enol) triflates, which are oxidized at high electrode potentials, can therefore be studied using an ultramicroelectrode. Oxidation and ionization potentials, determined by ultramicroelectrode voltammetry and He I photoelectron spectroscopy, respectively, of 2‐methylprop‐1‐enyl, cyclohexenyl, cyclopentenyl, 1,1‐diphenylethenyl and prop‐2‐enyl triflate are reported. The results from electrochemical measurements and photoelectron spectra were compared.