Preparation and characterization of osmium hexacyanoferrate films and their electrocatalytic properties

Osmium hexacyanoferrate films have been prepared using repeated cyclic voltammetry, and the deposition process and the films’ electrocatalytic properties in electrolytes containing various cations have been investigated. The cyclic voltammograms recorded the deposition of osmium hexacyanoferrate films directly from the mixing of Os³⁺ and Fe(CN)₆³⁻ ions from solutions containing various cations. An electrochemical quartz crystal microbalance, cyclic voltammetry, and UV-visible spectroscopy were used to study the growth mechanism of the osmium hexacyanoferrate films. The osmium hexacyanoferrate films showed a single redox couple, and the redox reactions included “electron transfer” and “proton transfer” with a formal potential that demonstrates a proton effect in acidic solutions up to a 12 M aqueous HCl solution. The electrochemical and electrochemical quartz crystal microbalance results indicate that the redox process was confined to the immobilized osmium hexacyanoferrate film. The electrocatalytic reduction of dopamine, epinephrine, norepinephrine, S₂O₃²⁻, and SO₅²⁻ by the osmium hexacyanoferrate films was performed. The preparation and electrochemical properties of co-deposited osmium(III) hexacyanoferrate and copper(II) hexacyanoferrate films were determined, and their two redox couples showed formal potentials that demonstrated a proton effect and an alkaline cation effect, respectively. Electrocatalytic reactions on the hybrid films were also investigated.     

Mass changes accompanying the pseudocapacitance of hydrous RuO2 under different experimental conditions

Pseudocapacitance reaction of hydrous ruthenium oxide was investigated by cyclic voltammetry combined with electrochemical quartz-crystal nanobalance (EQCN) in sulfuric acid as well as in neutral solutions of Na₂SO₄ and K₂SO₄. The ruthenium oxide electrode was prepared by attaching the ruthenium oxide particles on gold covered quartz electrode. The results show that there are different types of charge taking place simultaneously during the redox reaction of ruthenium oxide electrode. Their contribution to the overall charge depends on the experimental conditions. Depending on the potential and electrolyte used the redox reaction of ruthenium oxide is accompanied either by mass loss or by mass gain. The average molar masses of the species exchanged between the solid phase and the electrolyte solution depend on the potential and scan rate. The effect of Nafion™ top layer was also investigated. It has been found that it does not affect significantly the overall specific capacitance of ruthenium oxide electrode but the apparent molar masses of exchanged species decrease in comparison with the uncovered electrodes.     

Anodically formed oxide films and oxygen reduction on electrodeposited ruthenium in acid solution

The impedance of the anodically formed hydrous Ru oxide in the system Ru|oxide film|1 M HClO₄ solution has been studied in the range of potentials where the electrode process occurs by a double electron and proton exchange between the oxide film and the solution. The results allowed us to clearly distinguish between the surface process at higher frequency and the bulk process at lower frequency. The high-frequency charging is found to be coupled to Faradaic charging at the film/solution interface. Evaluation of the impedance data at lower frequency, using diffusion equations for the finite boundary conditions, yields an effective proton diffusion coefficient to be 10⁻¹⁰ to 10⁻¹¹ cm² s⁻¹.Oxygen reduction on the spontaneously oxidized ruthenium electrode was discussed on the basis of a rotating ring-disk voltammetry.     

Electrochemical sensing based on graphene oxide/Prussian blue hybrid film modified electrode

A novel graphene oxide (GO)/Prussian blue (PB) hybrid film was constructed by electropolymerizing Prussian blue onto the GO modified glassy carbon electrode, and its electrochemical behaviors were studied. Raman spectra were used to investigate the successful formation of the GO/PB hybrid film. Electrochemical experiments showed that the graphene oxide greatly enhanced electrochemical reactivity of the PB. Moreover, a much higher Prussian blue (PB) loading (6.388 × 10⁻⁸ mol cm⁻²) is obtained as compared to the bare glass carbon surface (3.204 × 10⁻⁹ mol cm⁻²). The GO/PB hybrid film modified electrode was used for the sensitive detection of hydrogen peroxide. The sensor exhibited a wide linearity range from 5.0 × 10⁻⁶ to 1.2 × 10⁻³ M with a detection limit of 1.22 × 10⁻⁷ M (S/N = 3), high sensitivity of 408.7 μA mM⁻¹ cm⁻² and good reproducibility. Furthermore, with glucose oxidase (GOD) as a model, the GO/PB/GOD/chitosan composite-modified electrode was also constructed.The resulting biosensor exhibited good amperometric response to glucose with linear range from 0.1 to 13.5 mM at 0.1 V, good reproducibility and detection limit of 3.43 × 10⁻⁷ M (S/N = 3). In addition, the biosensor presented high selectivity and long-term stability. Therefore, the PB/GO hybrid films-based modified electrode may hold great promise for electrochemical sensing and biosensing applications.     

Microwave-polyol synthesis of nanocrystalline ruthenium oxide nanoparticles on carbon nanotubes for electrochemical capacitors

The ruthenium oxide nanoparticles dispersed on multi-wall carbon nanotubes (CNTs) were successfully synthesized via microwave-polyol process combined with forced hydrolysis without additional thermal oxidation or electrochemical oxidation treatment. The HRTEM, Raman spectra and TGA curve indicate that CNTs were uniformly coated with crystalline and partially hydrous RuO₂·0.64H₂O nanoparticles of 2 nm diameter and the loading amount of ruthenium oxide in the composite could be controlled up to 70 wt.%. The specific capacitance was 450 Fg⁻¹ of ruthenium oxide/CNT composite electrode with 70 wt.% ruthenium oxide at the potential scan rate of 10 mV s⁻¹ and it decreased to 362 Fg⁻¹ by 18% at 500 mV s⁻¹. The specific capacitance of ruthenium oxide in the composite was 620 Fg⁻¹ of ruthenium oxide at 10 mV s⁻¹. The ruthenium oxide nanoparticles in ruthenium oxide/CNT nanocomposite electrode had a high ratio of outer charge to total charge of 0.81, which confirmed its high-rate capability of the composite through the preparation of the nano-sized ruthenium oxide particles on the external surface of CNTs.     

The characterization and bioelectrocatalytic properties of hemoglobin by direct electrochemistry of DDAB film modified electrodes

Direct electrochemistry of hemoglobin can be performed in acidic and basic aqueous solutions in the pH range 1-13, using stable, electrochemically active films deposited on a didodecyldimethylammonium bromide (DDAB) modified glassy carbon electrode. Films can also be produced on gold, platinum, and transparent semiconductor tin oxide electrodes. Hemoglobin/DDAB films exhibit one, two, and three redox couples when transferred to strong acidic, weak acidic and weak basic, and strong basic aqueous solutions, respectively. These redox couples, and their formal potentials, were found to be pH dependent. An electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ deposition of DDAB on gold disc electrodes and hemoglobin deposition on DDAB film modified electrodes. A hemoglobin/DDAB/GC modified electrode is electrocatalytically reduction active for oxygen and H₂O₂, and electrocatalytically oxidation active for S₂O₄²⁻ through the Fe(III)/Fe(II) redox couple. In the electrocatalytic reduction of S₄O₆²⁻, S₂O₄²⁻, and SO₃²⁻, and the dithio compounds of 2,2′-dithiosalicylic acid and 1,2-dithiolane-3-pentanoic acid, the electrocatalytic current develops from the cathodic peak of the redox couple at a potential of about −0.9 V (from the Fe(II)/Fe(I) redox couple) in neutral and weakly basic aqueous solutions. Hemoglobin/DDAB/GC modified electrodes are electrocatalytically reduction active for trichloroacetic acid in strong acidic buffered aqueous solutions through the Fe(III)/Fe(II) redox couple. However, the electrocatalytic current developed from the cathodic peak of the redox couple at a potential of about −0.9 V (from the Fe(II)/Fe(I) redox couple) in weak acidic and basic aqueous solutions. The electrocatalytic properties were investigated using the rotating ring-disk electrode method.     

Zinc hexacyanoferrate film as an effective protecting layer in two-step and one-step electropolymerization of pyrrole on zinc substrate

The two-step and one-step electrosynthesis processes of polypyrrole (PPy) films on the zinc substrate are described. The two-step process includes (i) the zinc surface pretreatment with hexacyanoferrate ion in the aqueous medium in order to form a zinc hexacyanoferrate (ZnHCF) film non-blocking passive layer on the surface and with the view to prevent its reactivity and (ii) electropolymerization of pyrrole on the ZnHCF|Zn-modified electrode in aqueous pyrrole solution. In this context, both the non-electrolytic and electrolytic procedures were adapted, and the effect of some experimental conditions such as supporting electrolyte, pH and temperature of the solution at the zinc surface pretreatment step as well as pyrrole concentration and electrochemical techniques at the polymerization step was investigated. By optimizing the experimental conditions in both steps, we have obtained a homogeneous and strongly adherent PPy films on the zinc substrate.The one-step process is based on the use of an aqueous medium containing Fe(CN)₆⁴⁻ and pyrrole. The ferrocyanide ion passivates the substrate by formation of ZnHCF film during the electropolymerization process of pyrrole and therefore makes it possible to obtain strongly adherent PPy films, with controlled thickness, either by cyclic voltammetry or by electrolysis at constant current or constant potential without any previously treatment of the zinc electrode surface. The polypyrrole films deposited on the zinc electrode were characterized by cyclic voltammetry and scanning electron microscopic (SEM) measurement.     

Passivity and passivity breakdown of a zinc electrode in aerated neutral sodium nitrate solutions

The passivation and pitting corrosion behaviour of a zinc electrode in aerated neutral sodium nitrate solutions was investigated by cyclic voltammetry and chronopotentiometry techniques, complemented by ex situ scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray (EDX) examinations of the electrode surface. Measurements were conducted under different experimental conditions. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of ZnO on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film. SEM images confirmed the existence of pits on the electrode surface. The breakdown potential decreases with an increase in NO₃⁻ concentration and temperature, but increases with increasing potential scan rate. Addition of SO₄²⁻ ions to the nitrate solution accelerates pitting corrosion, while addition of WO₄²⁻ and MoO₄²⁻ ions inhibits pitting corrosion. The chronopotentiometry measurements show that the incubation time for pitting initiation decreases with increasing NO₃⁻ concentration, temperature and applied anodic current density. Addition of SO₄²⁻ ions decreases the rate of passive film growth and the incubation time, while the reverse changes produced by addition of either WO₄²⁻ or MoO₄²⁻ ions.     

Development of new nanocomposite based on nanosized-manganese oxide and carbon nanotubes for high performance electrochemical capacitors

We report the synthesis of a new composite electrode based on nanosized-manganese oxide and carbon nanotubes (CNTs) by electrophoretic deposition of CNTs on a stainless steel (SS) substrate followed by direct spontaneous reduction of MnO₄⁻ ions to MnO₂ to form the multi-scaled SS-CNT-MnO₂ electrode. The resulting material was characterized by scanning electron microscopy, energy dispersive X-ray analysis, cyclic voltammetry and galvanostatic charge-discharge in a 0.65 M K₂SO₄ aqueous solution. The binderless SS-CNT-MnO₂ nanocomposite electrode shows a very high specific capacitance of 869 F/g of CNT-MnO₂ and good stability during long galvanostatic charge-discharge cycling. To the best of our knowledge, this is one of the highest capacitance for manganese oxide electrode ever reported. In addition to its applicability in electrochemical capacitors, this methodology could be extended to develop other high performance nanocomposite material electrodes based on carbon nanotubes and metal oxide for the future generation of electrochemical power sources.     

Graphite–graphite oxide composite electrode for vanadium redox flow battery

A graphite/graphite oxide (GO) composite electrode for vanadium redox battery (VRB) was prepared successfully in this paper. The materials were characterized with X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The specific surface area was measured by the Brunauer–Emmett–Teller method. The redox reactions of [VO₂]⁺/[VO]²⁺ and V³⁺/V²⁺ were studied with cyclic voltammetry and electrochemical impedance spectroscopy. The results indicated that the electrochemical performances of the electrode were improved greatly when 3 wt% GO was added into graphite electrode. The redox peak currents of [VO₂]⁺/[VO]²⁺ and V³⁺/V²⁺ couples on the composite electrode were increased nearly twice as large as that on the graphite electrode, and the charge transfer resistances of the redox pairs on the composite electrode are also reduced. The enhanced electrochemical activity could be ascribed to the presence of plentiful oxygen functional groups on the basal planes and sheet edges of the GO and large specific surface areas introduced by the GO.     

Electrochemical and spectroscopic characterization of a tungsten electrode as a sensitive amperometric sensor of small inorganic ions

Cyclic voltammetry was used to investigate the electrochemical behaviour of the tungsten oxide films toward the electroreduction of BrO₃⁻, ClO₂⁻ and NO₂⁻ ions in acidic medium. The effects of the temperature, scan rate, pH, chemical composition of the electrolytic solutions, were investigated and the reduction mechanism was critically discussed.The reduction currents, evaluated in cyclic voltammetry and measured at −0.250 V versus SCE, increased linearly on increasing analyte concentration up to 20, 55 and 45 mM for nitrite, chlorite and bromate ions, respectively. The detection limits, evaluated in cyclic voltammetry, were 0.1, 0.4 and 0.7 mM for BrO₃⁻, ClO₂⁻ and NO₂⁻, respectively.The tungsten oxide film was successfully characterized as an amperometric sensor for the analytical determination of BrO₃⁻, ClO₂⁻ and NO₂⁻ ions in flowing stream. Operating under constant applied potential of −0.3 V versus Ag/AgCl the good reproducibility of the peak height and background current level during consecutive injections, indicates the absence of fouling effects and the potential applicability of the amperometric sensor for the routine analytical determination of the investigated inorganic ions. Considering the low values of the background currents (ca. 1.1 ± 0.1 μA) obtained in acidic and not deoxygenated carrier electrolyte, the tungsten sensing electrode seems to compete favourably with other common sensors for the amperometric determination of electroactive molecules under cathodic conditions.The X-ray photoelectron spectroscopy technique (XPS) was used in order to evaluate the chemical composition of the tungsten film upon electrochemical treatment in 0.1 M H₂SO₄ solution. Independently of the electrochemical treatment in acid solution, the tungsten surface electrode is generally composed by 50-60% of W⁰, 35-40% of W⁶⁺ and traces of W²⁺ oxide species.     

Study of a gold electrode modified by trans-[Ru(NH3)4(Ist)SO4] to produce an electrochemical sensor for nitric oxide

The modification of a gold electrode surface by electropolymerization of trans-[Ru(NH₃)₄(Ist)SO₄]⁺ to produce an electrochemical sensor for nitric oxide was investigated. The influence of dopamine, serotonin and nitrite as interferents for NO detection was also examined using square-wave voltammetry (SWV). The characterization of the modified electrode was carried out by cyclic voltammetry, electrochemical quartz crystal microbalance (EQCM) and SERS techniques. The gold electrode was successfully modified by the trans-[Ru(NH₃)₄(Ist)SO₄]⁺ complex ion using cyclic voltammetry. The experiments show that a monolayer of the film is achieved after ten voltammetric cycles, that NO in solution can coordinate to the metal present in the layer, that dopamine, serotonin and nitrite are interferents for the detection of NO, and that the response for the nitrite is much less significant than the responses for dopamine and serotonin. The proposed modified electrode has the potential to be applied as a sensor for NO.     

Effect of electrolyte on the supercapacitive behaviour of copper oxide/reduced graphene oxide nanocomposite

Cupric oxide/reduced graphene oxide (CuO/rGO) nanocomposites were synthesized through a chemical reduction method using hydrazine hydrate as the reducing agent. The morphology, elemental composition, and bonding network of the CuO/rGOnanocomposites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy respectively. The XRD results reveal lattice spacing and lattice strain from 3.371 to 3.428 Å and 1.05 × 10⁻³to 5.44 × 10⁻³ respectively, with the increasing ratio of rGO: CuO from 1:1 to 1:5. The cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS)and galvanostatic charge-discharge (GCD) studyofCuO/rGOas the electrode material showed excellent super-capacitive behavior in H₂SO₄ over Na₂SO₄ electrolytes. Moreover CuO/rGO nanocomposites exhibited better capacitance retention in H₂SO₄(75.69%) compared to Na₂SO₄(12.06%).     

Voltammetric and electrochemical impedance spectroscopy characterization of a cathodic and anodic pre-treated boron doped diamond electrode

The effect of boron doped diamond (BDD) surface termination, immediately after cathodic and anodic electrochemical pre-treatments, on the electrochemical response of a BDD electrode in aqueous media and the influence of the different supporting electrolytes utilized in these pre-treatments on the final surface termination was investigated with [Fe(CN)₆]^4−/3−, as redox probe, by cyclic and differential pulse voltammetry and electrochemical impedance spectroscopy. The cyclic voltammetry results indicate that the electrochemical behavior for the redox couple [Fe(CN)₆]^4−/3− is very dependent on the state of the BDD surface, and a reversible response was observed after the cathodic electrochemical pre-treatment, whereas a quasi-reversible response occurred after anodic electrochemical pre-treatment. Differential pulse voltammetry in acetate buffer also showed that the potential window is very much influenced by the electrochemical pre-treatment of the BDD surface. Electroactivity of non-diamond carbon surface species (sp² inclusions) incorporated into the diamond structure was observed after cathodic and anodic pre-treatments. Electrochemical impedance spectroscopy confirmed the cyclic voltammetry results and indicates that the BDD surface resistance and capacitance vary significantly with the electrolyte and with the electrochemical pre-treatment, caused by different surface terminations of the BDD electrode surface.     

Electrocatalytic oxidation of deoxyguanosine on a glassy carbon electrode modified with a ruthenium oxide hexacyanoferrate film

The electrocatalytic oxidation of deoxyguanosine on a ruthenium hexacyanoferrate (RuOHCF) glassy carbon (GC) modified electrode was investigated in acid medium by using rotating disc electrode (RDE) voltammetry. Chronoamperometric experiments allowed information on the charge transport rate through the RuOHCF film and at a very short time window a diffusion-like behavior was observed with a D_ct value of 2.7 × 10⁻¹¹ cm² s⁻¹ for a film with Γ = 4.47 × 10⁻⁹ mol cm⁻². The influence of systematic variation of rotation rate, film thickness and the electrode potential indicates that the rate of cross-chemical reaction between Ru(IV) centers immobilized into the film and deoxyguanosine controls the overall electrodic process and the value of the rate constant was found to be 3.2 × 10⁶ mol⁻¹ L¹ s⁻¹. The relatively high rate constant of the cross-reaction, the facile penetration of the substrate through the film and the fast transport of electrons suggest that the electrocatalytic process occurs throughout the film layer.     

Layer-by-layer construction of multi-walled carbon nanotubes, zinc oxide, and gold nanoparticles integrated composite electrode for nitrite detection

A novel composite electrode of Au/ZnO/MWCNTs/GC has been constructed for the electrochemical detection of nitrite, where ZnO thin film and Au nanoparticles are electrodeposited through layer-by-layer onto MWCNTs/GC substrate. The resulting electrode is characterized by scanning electron microscopy and energy-dispersed X-ray spectroscopy. Its electrocatalytic activity toward the electro-oxidation of nitrite has been examined and compared to various modified electrodes, including MWCNTs/GC, Au/ZnO/GC, Au/MWCNTs/GC, and ZnO/MWCNTs/GC via cyclic voltammetry. The electrodeposition time for ZnO and the Au loading amount together with the solution pH are investigated to achieve optimal conditions for the electrode fabrication and nitrite detection. Linear relationship between current response and nitrite concentration is observed in the range from 7.8 × 10⁻⁷ to 4.0 × 10⁻⁴ M and the limit of the detection is 4.0 × 10⁻⁷ M (S/N = 3). The influence of various anions and cations on the nitrite detection has been studied. The proposed method is also employed for the determination of nitrite in real samples.     

Preparation of ZnO/Eu mixed films by electrochemical precipitation

The electrochemical preparation of europium doped zinc oxide and europium oxide/hydroxide as thin films is investigated. First, a thermodynamic study of the Eu-Cl-H₂O system has been carried out at 25 and 70 °C in order to predict the electrochemical behaviour of Eu(III) dissolved in aqueous solution containing chloride ions. A comparison of the Eu-Cl-H₂O and Zn-Cl-H₂O systems indicates the possible coprecipitation of ZnO and Eu(OH)₃ from deposition solutions containing well-adjusted Eu(III)/Zn(II) concentrations ratio. The thermodynamic predictions have been confirmed experimentally by the electrochemical co-deposition of ZnO/Eu thin films on conducting electrode substrates at −1.4 V versus MSE. The presence of europium in the film is detected for Eu(III)/Zn(II) concentration ratio at (0.6 mM/5 mM) which is lower than the predicted value. Increasing Eu(III) concentration leads to the rapid appearance of two phases: dispersed zinc oxide nanorods and, at the bottom of the rods, a covering layer containing Eu(OH)₃ and zinc. The density of ZnO rods decreases and the rod size increases with increasing Eu(III) concentration in the bath. Above 1 mM EuCl₃, a dramatic fall in the current density is observed with the formation of a less conducting ZnO/Eu mixed deposit.     

Electrodeposited PbO2 thin film as positive electrode in PbO2/AC hybrid capacitor

Lead dioxide (PbO₂) thin films were prepared on Ti/SnO₂ substrates by means of electrodeposition method. Galvanostatic technique was applied in PbO₂ film formation process, and the effect of deposition current on morphology and crystalline form of the PbO₂ thin films was studied by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The energy storage capacity of the prepared PbO₂ electrode was investigated by means of cyclic voltammetry (CV) and charge/discharge cycles, and a rough surface structure PbO₂ film was selected as positive electrode in the construction of PbO₂/AC hybrid capacitor in a 1.28 g cm⁻³ H₂SO₄ solution. The electrochemical performance was determined by charge/discharge tests and electrochemical impedance spectroscopy (EIS). The results showed that the PbO₂/AC hybrid capacitor exhibited high capacitance, good cycling stability and long cycle life. In the voltage range of 1.8-0.8 V during discharge process, considering the weight of all components of the hybrid capacitor, including the two electrodes, current collectors, H₂SO₄ electrolyte and separator, the specific energy and power of the device were 11.7 Wh kg⁻¹ and 22 W kg⁻¹ at 0.75 mA cm⁻², and 7.8 Wh kg⁻¹ and 258 W kg⁻¹ at 10 mA cm⁻² discharge currents, respectively. The capacity retains 83% of its initial value after 3000 deep cycles at the 4 C rate of charge/discharge.     

Direct electrochemistry of horseradish peroxidase on graphene-modified electrode for electrocatalytic reduction towards H2O2

Graphene was synthesized by a chemical method to reduce graphite oxide and well characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD) and Fourier transform infrared (FTIR) spectra. Horseradish peroxidase (HRP) immobilized on a graphene film glassy carbon electrode was found to undergo direct electron transfer and exhibited a fast electron transfer rate constant of 4.63 s⁻¹. The HRP-immobilized electrode was investigated by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The CV results showed that the modified electrode gave rise to well-defined peaks in phosphate buffer, corresponding to the electrochemical redox reaction between HRP–Fe(III) and HRP–Fe(II). The obtained electrode also displayed an electrocatalytic reduction behavior towards H₂O₂. The new H₂O₂ sensor shows a linear range of 0.33–14.0 μM (R² = 0.9987) with a calculated detection limit of 0.11 μM (S/N = 3). Furthermore, the biosensor exhibits both good operational storage and storage stability.     

Effects of 1,3-dialkylimidazolium cations with different lengths of alkyl chains on the Pt electrode/electrolyte interface in dye-sensitized solar cells

The electrochemical behaviors of the tri-iodide (I₃⁻)/iodide (I⁻) redox couple of symmetric cells were investigated by cyclic voltammetry, steady-state polarization, chronocoulometry, and electrochemical impedance spectroscopy. 1,3-Dialkylimidazolium cations affected the characteristics of the Pt electrode/electrolyte interface by adsorbing on the Pt electrode, as a result of electrostatic interactions, and further affected the reduction of I₃⁻ on the Pt electrode. Capacitance of the double layers of the Pt electrode/electrolyte interface was chiefly determined by capacitance of the compact layer according to the Helmoholtz theory.