Glassy Carbon Disk Electrode Research Articles

Voltammetry in Microemulsion Formed by Electron Donor Solution in Organic Solvent|Ionic Liquid Microdroplets in Aqueous Electrolyte

Oxygen reduction reaction (ORR)1 or hydrogen evolution reaction (HER)2 at liquid|liquid interface provide the possibility of separation of their products: hydrogen peroxide or hydrogen.3,4 Typically, these reactions are studied at single interface formed between immiscible electrolyte solutions in quiescent conditions. In such geometry the frequency of successful reaction events when highly hydrophobic electron donor (i.e. decamethylferrocene (DMFc) or excited form of decamethylruthenocene (DMRc)) dissolved in organic phase meet proton from aqueous phase is rather limited. Importantly, electrochemical recycling of electron donor at electrode positioned close to the liquid|liquid interface increases efficiency of this reaction.5,6 The use of microemulsion is promising higher efficiency alternative, because of extended interfacial area of numerous organic droplets. As efficient electron exchange between oxidized form of electron donor and the electrode is crucial for biphasic ORR and HER one has to focus on electrochemical studies of DMFc or DMRc in emulsions.are important. Voltammetric signal of various redox probes as ferrocene or ferrocene surfactants in quiescent emulsions is similar to that in single phase with some variation of peak density currents or redox potential.7-11 Other studies focused on chronoamperometric experiments to detect collisions of redox probe (e.g. ferrocene12) containing droplets with the electrode surface. It was found that the addition of an hydrophobic ionic liquid, typically trihexyltetradecyl-phosphonium bis(trifluoro-methylsulfonyl) amide to the organic phase as stabiliser and electrolyte, does not influence the magnitude of the electrochemical signal.13 Here we will report results of electrochemical studies of emulsion prepared from aqueous HClO4 and DMFc or DMRc and trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl) imide (IL-PI) solution in toluene. As aqueous phase acid solution was selected, because in biphasic ORR or HER it serves as a source of protons. During the measurements the microemulsion was stirred to achieve faster droplets transport towards the electrode surface. Results will be compared with these obtained in a single organic phase.Voltammograms obtained with glassy carbon disc electrode indicate reversible oxidation/reduction of the redox probe similar to the seen with an electrode immersed in single organic phase. During continuous scanning redox only small change of peak currents is seen. Importantly, when after experiment in emulsion working electrode was transferred to aqueous HClO4, the voltammetric signal is still seen, but the peak currents are significantly smaller. This may indicate the redox reaction in liquid film formed on the electrode surface. Formation of liquid organic film is confirmed by the results of chronoamperometric experiments. When the electrode is set at potential corresponding to the oxidation of the redox probe, of the current is seen after c.a. 100 s drop, what may indicate the gradient of redox probe concentration. As DMFc is insoluble in water this points out on gradient of organic phase close to the glassy carbon surface.14 In turn voltammetric current related to electroreduction of highly hydrophilic hexacyanoferrate(III) is significantly diminished in emulsion as compared to pure aqueous electrolyte indicating partial coverage of the GC surface by hydrophobic film.Other effects related with electrochemical behavior of studied emulsions will be also presented and discussed.

(Poster Award - 2nd Place) Enhancement of ORR Activity of Pt Catalyst for PEFCs By Modification with Hydrogen-Bonded Organic Framework Composed of Melamine and Cyanuric Acid

Introduction Improvement in ORR activity of Pt and Pt-based catalysts used in PEFCs for FCEVs is crucial for cost reduction and widespread commercialization [1]. We have reported that surface modification of Pt and Pt-based catalysts with melamine and its derivatives enhance ORR activity [2, 3]. However, it is concerned that melamine is easily washed out by water generated at the cathode in PEFCs because melamine is soluble in water (3.1 g/L). In this study, a hydrogen-bonded organic framework (HOF) composed of melamine (MEL) and cyanuric acid (CA) [4] was prepared to suppress the washing-out problem and investigated as a modifier of Pt catalyst loaded on mesoporous carbon (MPC) support. Experimental MPC (CNovel MH-18, SBET 1,334 m2/g, TOYO TANSO) was used as a carbon support for Pt catalyst. Pt/MPC (50 wt%) was prepared by an impregnation-thermal decomposition method using Pt(NO2)2(NH3)2 as a Pt source. Pt/MPC was modified as the following procedure. First MEL was impregnated within the Pt/MPC by ultrasonically dispersing 100 mg Pt/MPC catalyst and 10 mg MEL (10 wt.% to the catalyst) in 100 mL pure water, followed by drying using a rotary evaporator (MEL-Pt/MPC). Then MEL-Pt/MPC (110 mg) was dispersed in 100 mL water containing 10 mg CA and dried to obtain HOF-Pt/MPC. The catalyst was loaded on a glassy carbon (GC) RDE. CV of the catalyst was recorded in Ar-saturated 0.1 M HClO4 at 25oC at a scan rate of 50 mV/s and ORR activity was evaluated by LSV performed at 25oC in O2-saturated 0.1 M HClO4 at a scan rate of 10 mV/s on the positive sweep while rotating the GC disk electrode at 1,600 rpm. To investigate the stability of MEL and HOF on the electrode, the GC electrode after the initial measurements was immersed in 0.1 M HClO4 at 80oC for 1 h, followed by washing with pure water, and CV and LSV were measured in the same manner at 25oC. Results and Discussion Figure 1 shows the molecular structure of MEL-CA HOF [4]. Figures 2 and 3 show CVs and LSVs, respectively, for bare, MEL-, and HOF-Pt/MPC before the stability tests. For bare Pt/MPC in Fig. 2, peaks were observed around 0.05−0.4 V and 0.6−1.0 V, which are assigned to the adsorption/desorption of underpotential deposited hydrogen (Hupd) and Pt-oxide species formation/reduction, respectively. For MEL- and HOF-Pt/MPC, the HUPD peak area decreased slightly, while the onset potential for Pt-oxide formation was shifted positively in Fig. 2. As a result, ORR activity of MEL- and HOF-Pt/MPC was improved remarkably as shown in Fig. 3 (mass activity: 894 and 829 A/g, respectively, at 0.9 V). These results indicate that HOF has an enhancement effect for ORR activity similar to that of MEL modification. Figure 4 shows CVs for bare, MEL-, and HOF-Pt/MPC after the stability tests at 80oC for 1 h. In Fig. 4, the waves of Hupd and Pt-oxide of MEL-Pt/MPC became similar to those of bare Pt/MPC after the stability test, which indicated that MEL was washed out in the electrolyte solution. On the contrary, the HOF-Pt/MPC catalyst substantially retained the waves shapes before the stability test shown in Fig. 2. These results indicate that the HOF has higher stability against washing-out than MEL owing to a much lower solubility at 80°C. Figure 5 shows LSVs after the stability tests. The loss of activity of HOF-Pt/MPC (639 A/g) was smaller than MEL-Pt/MPC (553 A/g) owing to the suppression of washing-out, which is consistent with the results in Fig. 4. The results of single cell tests using bare Pt/MPC, MEL-Pt/MPC and HOF-Pt/MPC cathode catalysts will be presented at the meeting.This study was partly supported by NEDO, Japan.

Demonstration of Feasibility Using Ionic Liquid as Electrolyte for Electrochemical Deposition and Recovery of Silver from Silver Oxide

Silver oxide (Ag₂O) is soluble in the hydrophobic protic amide-type ionic liquid (IL), protonated-betaine bis((trifluoromethyl)sulfonyl)amide ([Hbet][TFSA]). Cyclic voltammetric behavior of Ag(I) showed a single redox couple, and NMR spectra implied that Ag⁺ might be coordinated with the [TFSA] anions in more extent than with the deprotonated [Hbet] ions (i.e. [bet]; a Zwitterion). By changing the scan rates and cycle numbers of potential scan utilized for the cyclic voltammetric electrodeposition, different particle sizes and distribution densities of silver nanoparticles (AgNPs) could be electrodeposited on glassy carbon disc electrode (GCE), which showed activities towards the electrochemical reduction of nitrate and oxidation of hydrazine, respectively, in alkaline solutions. The high faradic efficiencies (F.E.) of Ag electrodeposition using the contents of Ag₂O coin batteries as the Ag source implied that it may be possible to develop a process for recovering Ag from spent Ag₂O coin batteries based on the electrochemical system reported here.

Differences in the electrochemical behavior of Vulcan XC-72 carbon black and glassy carbon after prolonged potential cycling

The electrochemical characteristics of a carbon carrier play an important role in catalyst investigations. The evolution of the electrochemical characteristics of Vulcan XC-72 carbon black and a glassy carbon disk electrode was studied under prolonged electrochemical action. The electrochemical action was performed by repeated application of a sawtooth periodic potential to the electrodes in the range from 0.05 to 1 V vs. RHE and back at a scan rate of 50 mV/s. The EDL charging current, the cyclic current–voltage curves, the equilibrium electrode potential, and the dissolved oxygen reduction current in potentiostatic mode at 0.05 V vs. RHE were recorded periodically. Carbon black was characterized by helium pycnometry, SEM, TEM, X-ray diffraction, and Raman spectroscopy. The currents on the carbon black were several orders of magnitude higher than those on the glassy carbon. The differences in the dynamics of electrochemical characteristics are discussed.

Enhanced alkaline hydrogen oxidation reaction using electrodeposited Ni-Ir alloy catalysts

The development of high-performance non/less-precious metal-based electrocatalysts for the hydrogen oxidation reaction (HOR) in anion exchange membrane fuel cells remains a challenge. Here, we report a bimetallic Ni-Ir catalyst fabricated using electrodeposition and optimized to improve its HOR performance in an alkaline environment. Bimetallic Ni-Ir catalysts of various compositions are electrochemically deposited on a glassy carbon (GC) disk electrode and their HOR activity in 0.1 M KOH is examined. It is found that Ni47Ir53 has the highest HOR activity of 2.09 and 2.28 mA cm−2 at 0.05 and 0.10 VRHE, respectively, outperforming a commercial Pt/C electrode. This improved performance is ascribed to the larger electrochemical surface area and the modification to the electronic structure via alloying. Electrochemical analysis revealed a low Tafel slope and a large exchange current density for Ni47Ir53, suggesting the presence of the Tafel-Volmer mechanism and a rapid Volmer reaction. The balanced H binding and OH binding with Ni47Ir53 is a significant contributor to the higher HOR activity. During accelerated durability testing, the oxidation state and electronic structure of the catalyst become more unfavorable for the HOR due to Ni dissolution, which supports the strategy of employing a dissolution-resistive alloying metal with Ir for alkaline HOR catalysts.

(Digital Presentation) Electrochemical Analysis of Oxygen Reduction in Phosphonium Ionic Liquids Based on Rotating Ring-Disk Electrode Method

Ionic liquids (ILs) have characteristic physicochemical properties such as high thermal stability, low volatility, low melting point, and flame resistance. Moreover, ILs exhibit also excellent electrochemical properties represented by a wide potential window.[1] Hence, they have a great potential for electrochemical applications such as electrolytes for batteries, capacitors, fuel-cells and so on. Phosphonium ILs have unique physical characters such as lower viscosity and higher ionic conductivity compared with corresponding structured ammonium ILs.[2] Recently, we have evaluated the diffusion coefficient and the solubility of oxygen molecules in phosphonium ILs and have clarified that such oxygen behaviors strongly depends on the chemical structure of cation rather than that of anions. In the present study, we demonstrated the proton effect for the ORR in phosphonium ILs using rotating ring disk electrode (RRDE) technique.[3] Tetraalkyl-phosphonium(PXXXY+ and bis(trifluoromethane sulfone) imide (TFSA-)) ILs were used for electrolytes.Cyclic voltammetry (CV) and convective voltammogram were performed using a three electrode cell at 25oC. The RRDE constructed a glassy carbon (GC) or platinum (Pt) disk, and Pt ring electrodes were used as the working electrodes. The ring current was measured at a potential of 1.0 V vs. Ag/Ag+ reference electrode, where the intermediate oxidation reaction enough attached to diffusion limit.In the non-protonic media, ORR currents were observed from –0.6 V on GC disk electrode and –0.5 V on Pt disk electrode respectively, which were corresponding to the one-electron reduction reaction in ORR. On the other hand, the on-set potential of ORR in protonic media was higher than that in non-protonic media on both electrodes. Such potential shifts were strongly appeared on the Pt disk electrode than that on the GC disk electrode. Moreover, the diffusion limiting current was observed on Pt disk electrode at higher potential region in protonic media, and the current increased with increasing H-TFSA content. Therefore, the limiting current originated from not the transport of oxygen molecules, but that of proton, while the ring current oppositely decreased with increasing H-TFSA content. The ring current originated from intermediate formed on Pt disk electrode was smaller than that on GC disk electrode. These results indicated that the number of electrons in ORR increased by the existence of proton, and the effect of proton was promoted on Pt surface better than the GC surface.[1] A. Khan, C. Zhao, ACS sustainable Chem. Eng. 4, 506 (2016)[2] K. Tsunashima, and M. Sugiya, Electrochemistry Communications, 9, 2353- 2358(2007)[3] M. Hildera, et al, Electrochimica Acta, 202, 100-109(2016).

Electrodeposition of Metal Alloy Carbon Composites for the Electrochemical Detection of Methylene Blue and Pyocyanin

Electrodeposited nickel-iron nanoclusters and nickel-iron-graphene oxide onto a carbon electrode was fabricated to enhance charge transfer while at the same time permitting adsorption of organic molecules to the carbon surface. Ni–Fe and Ni–Fe–graphene oxide were prepared from a sulfate-boric acid and sulfate-boric acid-graphene oxide water dispersion electrolyte at a pH of 3 by a pulse galvanostatic method onto a glassy carbon disk electrode. Electrochemical impedance spectroscopy (EIS) was conducted on the Ni–Fe/glassy carbon and Ni–Fe–graphene oxide/glassy carbon composite electrodes in a phosphate buffer solution (PBS) containing various concentrations of methylene blue (MB) or pyocyanin (PYO). MB is used as a dye and is a pollutant, and can serve as a comparable model for PYO due to similar structure and redox potential. PYO is a metabolite of the Pseudomonas aeruginosa, a gram-negative bacterium, associated with disease in humans responsible for chronic infection in people with cystic fibrosis and in burn patients; thus its detection is of interest. The RsC parameters determined from EIS equivalent circuit models for the interfacial region of the Ni–Fe/glassy carbon electrode with an aqueous electrolyte containing MB or PYO had a larger signal as the analyte concentration decreased, in contrast to conventional voltammetry techniques. The fractional adsorption with concentration for MB followed a Langmuir adsorption isotherm, the adsorption energy determined, and compared with PYO. The electrode fabricated with graphene oxide in the electrolyte increased the RsC value under PYO detection.

(Digital Presentation) Rotating Ring-Disk Electrodes Studies on Oxygen Reduction Reaction at Hydrophobic Ionic Liquids Layer Modified Pt/C Catalyst

Polymer electrolyte fuel cells (PEFCs) are expected as one of the clean, highly effective energy conversion systems, and have been actively studied. Pt nanoparticles on carbon supports (Pt/C) are commonly used as a cathode catalyst for PEFCs, because of the high activity on oxygen reduction reaction (ORR). However, its durability in long term driving and high mass activity for ORR have not sufficiently fulfilled at the present. More improvements of Pt/C catalyst for ORR activity and the durability are required for the spread of PEFCs To improve the activity and durability of Pt/C, ionic liquids (ILs) modified catalysts, called Pt/C-Solid catalysis with an IL layer (SCILL) catalysts, have recently attracted much attention [1]. ILs are liquid salts around room temperature, which have high ionic conductivity and electrochemical stability. We focused on hydrophobic ionic liquids with quaternary phosphonium cations [2]. Quaternary phosphonium cations ionic liquids have higher ionic conductivity and hydrophobicity than quaternary ammonium ionic liquids with the same structure and the same counter ions. Because of interference in the oxygen reduction reaction on platinum by water, increasing the hydrophobicity of the surface is expected to improve the activity on Pt/C catalysis. Hence, by using the quaternary phosphonium cations ILs for Pt/C-SCILL catalysts, it promises to improve the ORR activities for the catalyst. On the other hand, proton conduction and oxygen permeation rate are also important factors for ORR activity enhancement. In this study, we report the ORR activity for Pt/C-SCILL catalysts with ILs composed of highly hydrophobic cations and hydrophobic anions. Quaternary phosphonium based ILs (PXXXY+TFSA-[alkyl chain X= 4 Y= 1, 12, 16]) were used for Pt/C-SCILL modified electrodes. Commercially available 20 wt% or 30 wt% Pt/C (Cabot, Vulcan XC-72R\U0001f12c) catalyst was used for bare Pt/C modified electrodes. The ultrasonicated Pt/C catalyst suspension was dropped on the mirror polished glassy carbon disk electrodes. Pt/C-SCILL modified electrodes were prepared by recasting the ILs-2-propanol solution on the bare Pt/C modified electrodes or adding ILs to the catalyst suspension. Moreover, the mixture of the ILs and bis(trifluoromethanesulfonyl)imide (HTFSA) were also examined. The thickness of the ILs layer was calculated relative to the BET surface area of the carbon support. A platinum wire and a reversible hydrogen electrode (RHE) were used as the counter and reference electrodes, respectively. CV and LSV were carried out in 0.1 M HClO4 solution under Ar (CV) or O2 (LSV) atmosphere, respectively. The Pt/C-SCILL catalyst was prepared by recasting (thin layers, ~0.4 nm) or mixing (thick layers, 1nm~) methods. In the case of the thin layer (~0.4 nm) of ILs, the ORR activity increased as the size of the cations increased. However, for the thicker (1 nm ~) ILs layers, the ORR activity was lower than for the bare Pt/C catalysts. It seems that the aprotic ILs interfered with the proton supply, making the four-electron reduction reaction of oxygen less likely to occur. Therefore, we attempted to supply protons to the thick ILs layer by mixing an ionic liquid with a proton source. HTFSA was dissolved in P444(12)TFSA, which showed relatively good ORR activity in thin film studies. The Pt/C-SCILL catalyst was prepared by adding the mixture with a catalyst dispersion. This proton mixing Pt/C-SCILL catalyst showed a significant improvement in ORR activity compared to bare Pt/C catalyst.

Development of a facile time-resolved spectroelectrochemical method: An application to determine the rate constant of protonation for anions of 9,10-diphenylanthracene, biphenyl, and p-quaterphenyl

• A simple time-resolved spectroelectrochmeical method was used. • Rate constant of protonation for organic radical anions and dianion was determined. • Two or three ethanol molecules are involved in the protonation reactions. • Comproportionation reaction between anion and dianion was directly measured. Using a conventional three-electrode system and electrochemical cell, we developed a simple spectroelectrochemical method that can be used to determine the rate constant of protonation reaction for several reduced species. A glassy carbon (GC) disk electrode as a working electrode and halogen lamp as a light source were employed. The reflected light on the GC electrode surface through the diffusion layer of electrochemically generated species was recorded by an electron multiplying charge coupled device camera equipped with a spectrometer. With this system, the absorbance change by the generation of the radical anion of 9,10-diphenylanthracene (DPA), biphenyl, and p -quaterphenyl was monitored. The molar extinction coefficient ( ε ) of the radical ions was calculated with the absorbance and charge during potential steps. The absorption spectrum of electrochemically generated dianion of p -quaterphenyl was also monitored, and the ε value was determined. The comproportionation reaction between p -quaterphenyl 2− and neutral p -quaterphenyl was also clearly observed on the spectroelectrochemical data. The absorbance change during the protonation at different concentration of ethanol ( c EtOH ) for DPA •− , biphenyl •− , p -quaterphenyl •− , and p -quaterphenyl 2− and the ε values were used to determine the first-order rate constant of the protonation reaction. The first-order rate constant increased linearly against c EtOH 2 for DPA •− , p -quaterphenyl •− , and p -quaterphenyl 2− , and against c EtOH 3 for biphenyl •− . From the slopes, the reaction rate was determined to be 0.45, 4.2×10 2 , and 4.3×10 4 M −2 s −1 , for DPA •− , p -quaterphenyl •− , and p -quaterphenyl 2− , respectively, and for biphenyl •− , 6.8×10 4 M −3 s −1 .

Reactive Sputtered Ir1−yNiyOx Electrocatalysts For The Oxygen Evolution Reaction in Alkaline Media

One drawback of alkaline water electrolysis (AWE) is the sluggish oxygen evolution reaction (OER) kinetics at the anode of the electrolyser and improvement of the anode material is still a necessity. Various Ir1−yNiyOx electrocatalyst combinations were prepared by reactive sputtering and subjected to high-throughput electrochemical screening using a Au/SiO2 wafer. The best performing electrocatalysts were identified and subjected to single electrode electrochemical and physical characterisation using glassy carbon disk electrodes as the substrate. Rotating disk electrode (RDE) analysis included linear sweep voltammetry (LSV) before and after short term chronopotentiometry (CP). Energy dispersive X-ray spectroscopy and X-ray photo-electron spectroscopy (XPS) confirmed the controlled synthesis of thin film electrocatalysts by utilising reactive sputtering, and confirmed Ir dissolution which occurred during CP measurements. Overall, the Ir1−yNiyOx electrocatalyst combinations containing higher amounts of Ir (Ir92Ni8Ox, Ir68Ni32Ox and Ir62Ni38Ox) performed the best of the evaluated mixed metal oxide electrocatalysts. However, evident from this study was that excellent mass-specific activity (current per mass of noble metal) was not exhibited by the mixed metal oxides when compared to Ni.

Characterisation of carbon paste electrodes bulk-modified with surfactants for measurements in nonaqueous media

This article deals with carbon paste electrodes bulk-modified with surfactants for measurements in pure organic solvents where common carbon paste-based mixtures undergo disintegration. The electrodes of interest were prepared by mixing of selected carbon materials (namely: two types of graphite powder, glassy carbon powder, raw shungite powder, and multi-wall carbon nanotubes, with various binders (mineral oil, silicone oil, paraffin wax, vaseline, atactic polypropylene, and tricresyl phosphate), together with surfactants of choice (sodium dodecyl sulfate, cetylpyridinium chloride, benzethonium chloride, cetyltrimethylammonium bromide, didodecyldimethylammonium bromide, Triton X-100, and 1,3‒didecyl‒2‒methylimidazolium chloride). The corresponding studies were focused on the optimal composition of the individual pastes and their characterisation in organic solvents, when using cyclic voltammetry with model redox pairs (ferrocenium/ferrocene and quinone/hydroquinone). The results obtained suggest one that amount of the individual components depends predominantly on the type of carbon used. Key factors for selection of the proper surfactant are its polarity, chemical inertness, and electrochemical inactivity. For comparison, glassy carbon disc electrode was also used.

Reference electrodes with polymeric membranes containing ionic liquids of various physicochemical properties

In this paper, the development of reference electrodes with polyurethane membranes containing ionic liquids is described. In particular, the influence of physicochemical properties of ionic liquids on the performance of polymeric membrane reference electrodes was elucidated. It was found that the potential stability of tested electrodes strongly depends on the differences in partition coefficient values between cation and anion of the ionic liquid. Among tested ionic liquids, 1-(2-hydroxyethyl)− 3-imidazolium bis{(trifluoromethyl)sulfonyl}imide induced the best potential stability of reference electrodes. It was also shown that the amount of the ionic liquid in the polymeric membrane does not influence significantly the performance of resulting reference electrodes. Moreover, results demonstrate that the addition of the plasticizer to the polyurethane membrane is not necessary to prepare reference electrodes that show sufficiently stable EMF values in solutions of various ion concentrations. Membranes of optimal composition were used to prepare All-Solid-State reference electrodes on glassy carbon disc electrodes with acceptable potential stability.

Electrodeposited Ni-Fe onto Glassy Carbon for the Detection of Methylene Blue

A composite electrode composed of electrodeposited, nickel-iron nanostructured clusters onto a glassy carbon (GC) disk electrode was used as a working electrode to detect methylene blue at concentrations below 10 μM. The Ni-Fe clusters were prepared by pulse electrodeposition and a lateral composition variation was observed reflective of a local pH change across the Ni-Fe feature. The applied potential for the detection of MB at a pH of 4 was determined through voltammetry, and demonstrated using chronoamperometry and electrochemical impedance spectroscopy (EIS). The adsorption of MB influenced both the capacitance, C, and ohmic resistance, R s . A peak present in vs t chronoamperometry plots decreased with lower MB bulk concentration, while in contrast, the RsC parameters determined from equivalent circuit models of EIS had the opposite behavior having a larger signal with lower MB concentration, and hence providing a way to increase the detection signal at lower MB concentration.

Electrochemical Impedance Spectroscopy Investigation of Organic Redox Reactions (p-Benzoquinone/Hydroquinone Couple)─Implications to Organic Redox Flow Batteries

Redox reactions of organic redox couples are investigated on Nafion-free and Nafion-containing carbon (Vulcan XC-72)-modified glassy carbon disk (GCD) electrodes. The model system chosen is the p-benzoquinone (BQ)/hydroquinone (BQH2) couple because BQ derivatives are common to most of the organic redox couples. The electrochemical impedance spectroscopy patterns show an unexpected rise in real resistance with time and with rotation speed (rpm). This increase in resistance is remarkable with Nafion-containing GCDs. It seems that the sulfonate group on the polymer chain attacks the redox couple to bind the species to the electrode surface. Moreover, the internal and external mass transfer features are better resolved than their nonbinding inorganic counterparts (V4+/V5+ couple). These findings have enormous implications to the development of organic redox flow batteries, organic synthesis, supercapacitors, sensors, and so on.

Kinetics of V5+/V4+ Redox Reaction—Butler-Volmer and Marcus Models

Kinetics of the V5+/V4+ redox reaction on Vulcan XC-72 modified glassy carbon disk electrode is investigated in a three-electrode configuration. Cyclic voltammograms of V5+/V4+ redox couple suggest that the overpotential range for the kinetic analysis is limited to ±300 mV, after excluding V4+/V3+ redox reaction at the negative overpotential and the oxygen evolution reaction at the positive overpotential. The linear sweep-voltammograms (LSVs) are corrected for potential drop due to solution resistance (iR s ), mass-transfer resistance, and most importantly, for the back reaction current. These corrections are imperative to estimate the Tafel slope in the limited range of overpotential for V5+/V4+ redox reaction. The charge-transfer coefficient (α) estimated from the Tafel slope deviates significantly from the expected value of 0.5 for the single electron-transfer reaction. Moreover, the instantaneous slope of the Tafel plot suggests that the α is overpotential dependent. Therefore, Marcus theory of electrochemical kinetics is applied to estimate the α. The reorganization energy (λ) calculated from the Arrhenius plots is in the range of values reported in the literature for the other redox couples.

EIS Investigation of Redox Reactions (Quinone/Hydroquinone Couple) - Relevant to Organic Redox Flow Batteries

The evolution of internal mass-transfer feature and the effect of Nafion on the transport of inorganic redox species (VO2+/VO2 +, V2+/V3+, Ti3+/Ti4+, and Fe2+/Fe3+) across the porous thin-film electrode/electrolyte interface is reported in the literature [1, 2]. Redox reactions of organic redox couple are investigated on Nafion-free and Nafion-containing porous carbon (Vulcan XC-72) modified glassy carbon disk electrodes (GCD) in acidic medium. The model system chosen is the p-benzoquinone (BQ)/hydroquinone (BQH2) couple, since BQ derivatives are common to most of the organic redox couples. The electrochemical impedance spectroscopy (EIS) patterns show an unexpected rise in mass-transport resistance with time, rotation speed (rpm), and Nafion content in the porous electrode. The increase in resistance is remarkable with Nafion-containing GCDs. It is observed that one of the redox species of the organic redox couple strongly interact with cation/anion exchange membrane and may tremendously impact the ion exchange membrane conductivity. Moreover, the internal and external mass-transfer features are better resolved, unlike that with the metal-ion based inorganic counterparts. These findings have enormous implications to the development of ORFBs, organic synthesis, supercapacitors, and sensors. Figure caption :Effect of rotation speed of electrode on the impedance of equimolar solution of 5 mM BQ/BQH2 (1:1) in 1 M H2SO4 on carbon-modified GCD for various Nafion concentration. References P. Leuaa, D. Priyadarshani, A. K. Tripathi, and M. Neergat, J. Phys. Chem. C, 123, 21440–21447 (2019).P. Leuaa, D. Priyadarshani, D. Choudhury, R. Maurya, and M. Neergat, RSC Advances, 10, 30887–30895 (2020). Figure 1

Effect of Electrolyte Contaminants and Surface Roughness on the Constant Phase Element

The origin of constant phase element (CPE) to explain electrode/electrolyte interface is subject of debate in the literature. CPE behavior is generally attributed to the presence of reactivity, surface inhomogeneity, various crystal planes and atomic scale defects in the crystal; all these may lead to dispersion of local capacitance at the electrode/electrolyte interface. Therefore, electrode/electrolyte interface of polycrystalline Pt (Pt-PC) and smooth glassy carbon disk electrode (GCE) are investigated in ultra-pure H2SO4 (0.5 M) electrolyte[1]. The surface of GCE is roughened mechanically to test the effect of roughness. The effect of intentional and unintentional contaminants are investigated using halide (Cl˗, Br-, I-) ions[2]. It is found that the CPE is affected by the concentration and nature of anion in the electrolyte. Unintentional impurities and cations do not affect the CPE behavior.[1] Singh, R. K., Devivaraprasad, R., Kar, T., Chakraborty, A., & Neergat, M. (2015). Electrochemical impedance spectroscopy of oxygen reduction reaction (ORR) in a rotating disk electrode configuration: effect of ionomer content and carbon-support. Journal of The Electrochemical Society, 162(6), F489.[2] Devivaraprasad, R., Ramesh, R., Naresh, N., Kar, T., Singh, R. K., & Neergat, M. (2014). Oxygen reduction reaction and peroxide generation on shape-controlled and polycrystalline platinum nanoparticles in acidic and alkaline electrolytes. Langmuir, 30(29), 8995-9006. Figure 1

Influence of experimental conditions on the electrochemical window. Case study on bis(trifluoromethylsulfonyl)imide-based ionic liquids

Ionic liquids (ILs) are often selected as electrolytes in various devices, such as batteries, capacitors and sensors, because of their chemical stability, non-flammability, low volatility, high electrical conductivity, low melting point, high thermal stability, and wide electrochemical window (EW). The width of the EW of an IL depends on the cation type, the water content, the electrode material, and the chosen specific current density used to measure it. In this work, the EWs of four ILs containing the common and stable bis(trifluoromethylsulfonyl)imide anion and triethylsulfonium, 1-(2-hydroxyethyl)-3-methylimidazolium, 1-methyl-1-propylpiperidinium and 1-butyl-1-methylpiperidinium cations were investigated by linear sweep voltammetry in order to analyze their electrochemical stability toward reduction and oxidation at polycrystalline platinum, gold and glassy carbon disc electrodes under anhydrous conditions. Specific current densities of 0.1 mA·cm−2 or 1 mA·cm−2 or the linear fitting method were used for the systematic comparison of the EWs obtained from each dataset. The results indicate the dependence of the EW width of each IL on the electrode material and, more importantly, on the chosen specific current density. In this paper, we present new insight into some of the difficulties encountered during determination of the EW for ILs.

Theory-Driven Design of Electrocatalysts for the Two-Electron Oxygen Reduction Reaction Based on Dispersed Metal Phthalocyanines

Theory-Driven Design of Electrocatalysts for the Two-Electron Oxygen Reduction Reaction Based on Dispersed Metal Phthalocyanines

Eliminating the need for craftsmanship: Facile and precise determination of oxygen reduction reaction activity by spraying catalyst ink on rotating disk electrode

Fabricating uniform catalyst thin films on rotating disk electrodes (RDEs) is crucial for the accurate and reproducible evaluation of the oxygen reduction reaction (ORR) activity of high surface area catalysts via the thin film (TF)-RDE method. For the conventional methods, a catalyst thin film is fabricated by drying one droplet of the catalyst ink on the surface of a glassy carbon (GC) disk electrode under the optimized conditions. However, the optimization of the drying conditions is time-consuming or sometimes impossible depending on the catalyst samples. In this study, we proposed a facile method of fabricating highly-uniform catalyst thin films using a commercial ultrasonic atomizer, which can forcibly deposit the catalyst particles evenly on the GC electrode. Although the catalyst loading is not precisely controlled by the atomizer, it can be determined using the total electrochemical surface area (ECSA) of Pt and a calibration line. By employing the developed method for two different commercial Pt/C catalysts, the ORR activities, which are comparable to the highest values reported in the previous literature, were obtained. Moreover, the developed method was also successfully applied to the ORR activity evaluation of a homemade Pt/NbB2 catalyst, which could not be properly tested by a conventional ink deposition method because of the difficulty of preparing a uniform catalyst thin film. Thus, the developed method eliminates the need for setting a specific film fabrication condition for each catalyst sample and enables the easy and precise ORR activity determination of any kind of high surface area catalyst as long as well-dispersed catalyst inks are available.