Iron(III) octaethylporphyrin chloride supported on glassy carbon as an electrocatalyst for oxygen reduction

Oxygen reduction was investigated at iron(III) octaethylporphyrin chloride adsorbed on a glassy carbon electrode. The title porphyrin was adsorbed irreversibly and strongly on the surface of a glassy carbon electrode. The electrochemical behavior and stability of the modified electrode were investigated using cyclic voltammetry, chronoamperometry and rotating disk electrode methods. The modified electrode showed clear but modest electrocatalytic activity for the reduction of oxygen to a mixture of water and hydrogen peroxide in buffered solutions on both the acid and basic sides of neutral with the domination of an overpotential of about 690 mV and an increase in peak current. The heterogeneous rate constant for the reduction of O₂ at the surface of the modified electrode and the diffusion coefficient of oxygen were determined by rotation disk electrode voltammetry using the Koutecký–Levich plots. In addition, iron(III) octaethylporphyrin chloride exhibited strong catalytic activity toward the reduction of H₂O₂.     

Kinetics of oxygen reduction on gold nanoparticle/multi-walled carbon nanotube hybrid electrodes in acid media

In this paper, the electrochemical reduction of oxygen has been studied on gold nanoparticle/multi-walled carbon nanotube (AuNP/MWCNT) modified glassy carbon (GC) electrodes in 0.5 M H₂SO₄ using the rotating disk electrode (RDE) method. The AuNP/MWCNT catalysts were prepared by chemical deposition of AuNPs onto MWCNTs spontaneously grafted with 4-nitrophenyl groups. The composite electrode was characterised by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). The oxygen reduction behaviour of these electrodes was compared with that of a bulk gold electrode. The AuNP/MWCNT catalyst showed a pronounced electrocatalytic activity towards O₂ reduction in acid media. The half-wave potential of O₂ reduction on the AuNP/MWCNT catalyst shifted ca 80 mV to more positive potentials as compared to that of a polished Au electrode. The kinetic parameters of oxygen reduction were determined and the specific activity of the hybrid electrode was slightly higher than that of the bulk Au electrode.     

Effects of carbon electrode surface states for the electro-reduction of (hmc)CoOOH2+ which is an intermediate during the catalytic reduction of O2

The reduction of the cobalt(III) complex with a macrocyclic ligand C-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (hmc) dissolved in solution or adsorbed on a graphite electrode in the presence of O₂ showed two cathodic peaks. As discussed in earlier reports, an intermediate (hmc)CoOOH²⁺ produced by the first two-electron reduction of (hmc)Co³⁺ in the presence of O₂ was further reduced to (hmc)Co²⁺ and HOOH. This process appeared as a second cathodic wave and represents a barrier in the overall reduction of O₂. In relation to these studies, it was found that the second cathodic reduction was greatly affected by the surface states of the carbon electrode. The reduction potential of the (hmc)CoOOH²⁺ intermediate when it was adsorbed on a pyrolytic edge plane graphite electrode (EPG) surface was more positive than the corresponding value of the dissolved species measured at a glassy carbon (GC) electrode polished with alumina. It was also shown that the reduction potential shifted in a positive direction when the solution pH was lowered or the surface of the glassy carbon electrode was heavily electro-oxidized. It is proposed that a proton transfer is involved in the electro-reduction of the (hmc)CoOOH²⁺ complex. The EPG surface which has more surface functional groups has a more acidic environment than the GC electrode and the reduction of (hmc)CoOOH²⁺ was facilitated. The oxidation of the glassy carbon electrode gives the same effect.     

Effect of loading level in platinum-dispersed carbon black electrocatalysts on oxygen reduction activity evaluated by rotating disk electrode

The dependence of oxygen reduction reaction (ORR) activity on the loading level of Pt electrocatalysts highly dispersed on carbon black (Pt/CB) has been investigated. We present a standard method for the evaluation of ORR activity at Pt/CB by a rotating disk electrode (RDE), in which Pt/CB powder was attached on a glassy carbon (GC) disk uniformly, followed by coating a thin Nafion film. We describe the optimized protocols for preparing a suspension of the Pt/CB in an ethanol aqueous solution, controlling vapor pressure during drying process, and coating very thin (0.05 μm) Nafion film. The area-specific activity for the ORR in air-saturated 0.1 M HClO₄ electrolyte solution was found not to be influenced by the Pt-loading level on CB from 19.2 to 63.2 wt%, when the amount of Pt/CB on GC and the dispersion state were optimized.     

The effect of pretreatment of Vulcan XC-72R carbon on morphology and electrochemical oxygen reduction kinetics of supported Pd nano-particle in acidic electrolyte

The influence of chemical pretreatment of carbon support for oxygen reduction on palladium nano-particles in acidic electrolyte was studied. Vulcan XC-72R carbon as catalyst support for palladium nano-particles was pretreated with 5% HNO₃, 0.07 M H₃PO₄, 0.2 M KOH and 10% H₂O₂. The effect of treatment on the properties of the carbon support was studied by N₂ adsorption and X-ray photoelectron spectroscopy (XPS). It was found that chemical treatment significantly changed the surface chemical properties and surface area of the carbon support. The surface area and pore volume of 5% HNO₃ and 10% H₂O₂ treated carbon supports were drastically decreased due to the oxidative nature of treatment. Ethylene glycol (EG) reduction method was used to synthesise 20% Pd on pr-treated and un-treated carbon supports. Differences in catalyst morphology were characterized using X-ray diffraction, energy dispersive X-ray analysis and transmission electron microscope techniques. It was observed that by using a mild reducing agent, namely EG, well-dispersed and nano-size Pd particles could be achieved during catalyst synthesis. The electrocatalytic activity of different Pd/C catalysts towards the oxygen reduction reaction (ORR) was examined by cyclic voltammetry (CV) on a rotating ring-disc electrode (RRDE) and compared with E-Tek 20% Pd/C catalyst under identical experimental conditions. The kinetics of ORR on these electrocatalysts predominantly involved a four-electron step reduction with the first electron transfer being the rate-determining step. However, the observed specific activity, mass activity and amount of hydrogen peroxide produced during ORR were greatly influenced by the pretreatment employed for carbon support.     

Oxygen reduction reaction kinetics and mechanism on platinum nanoparticles inside Nafion®

The aim of this work is to study kinetic parameters and the mechanism of the oxygen reduction reaction (orr) on platinum nanoparticles supported on carbon, inside Nafion^® (i.e. in PEMFC cathode conditions). Stationary and electrochemical impedance spectroscopy techniques were used to measure exchange current densities, Tafel slopes, and reaction orders with respect to O₂ pressure and H⁺ activity. The platinum nanoparticle size effect was confirmed. A specific low frequency inductive behaviour of the cathode impedance was observed. The latter demonstrates the presence of (at least) two electrochemical steps in the orr mechanism. Secondly, dc and ac modelling of the reaction in a gas diffusion electrode is proposed in order to simulate current–potential curves and impedance spectra. This paper reveals that an ECE mechanism for oxygen reduction proposed by Damjanovic and coworkers on bulk platinum in acidic medium is also valid for that on Pt nanoparticles.     

Electrode reactions and accumulation of hydrogen at carbon paste electrodes in the presence of tetrachloropalladate

The electrochemical behaviour of the unmodified carbon paste electrode (CPE) in the presence of [PdCl₄]^2? in chloride solutions of different pH (in the range from 3 to 7) is studied by cyclic voltammetry (CV). During cathodic treatment or cycling, Pd(0) is deposited in situ on the electrode surface. Results obtained at negative potentials as a function of the concentrations of Pd(II) and protons in the solution are presented. It is concluded that hydrogen ions are reduced in different ways: first, the reduction at deposited Pd(0) or the simultaneous reduction of H⁺ and [PdCl₄]^2?, respectively, which occurs during the whole cathodic cycle; second, the reduction in the potential range from about ?0.5 to ?1.0 V versus SCE, where surface groups are reduced at the unmodified CPE. The second pathway is the prevailing reduction at lower pH, which occurs at the carbon surface probably via protonated chloropalladate complexes adsorbed at the electrode surface. Potentiostatic pretreatment in the voltammetric range between 0 and +0.2 V leads in solutions of pH 5 to a pronounced increase of the H⁺ reduction current caused by an increase of the sorption capacity of the electrode or by the accumulation of protons at the electrode surface. Protons could be accumulated at the electrode surface by formation of protonated chloropalladate complexes at the CPE.     

In situ atomic force microscopy study of polypyrrole synthesis and the volume changes induced by oxidation and reduction of the polymer

The morphology changes of polypyrrole (PPy) films on polycrystalline platinum, glassy carbon and gold electrodes during synthesis and oxidation–reduction processes have been investigated using in-situ electrochemical AFM. It is found that the morphology of the film at the first stages of synthesis depends on the nature of the electrode and it is different from that of the thick film morphology. Remarkable changes in the morphology of PPy films were observed following synthesis suggesting that PPy films have an extraordinary capacity to alter their morphology over time. Upon reduction (undoping) in aqueous solution of NaClO₄, a perchlorate-doped PPy undergoes initially rapid swelling and subsequent continuous slow shrinking during the first reduction potential step from 0.2 to ?0.8 V, but during subsequent redox potential steps no remarkable changes of volume are observed. In contrast, in an aqueous solution of sodium p-toluenesulfonate, the polymer swells on reduction and shrinks on oxidation during continuous redox potential steps between 0.2 and ?0.8 V.     

Oxygen reduction on a high-surface area Pt/Vulcan carbon catalyst: a thin-film rotating ring-disk electrode study

We describe the adaptation of the recently developed thin-film rotating disk electrode method and its application in a rotating ring disk configuration (RRDE) to the investigation of the oxygen reduction reaction (orr) on a supported catalyst powder (Pt/Vulcan XC 72 carbon). This allows the determination of kinetic data, such as reaction orders or apparent activation energies, for the orr directly without mathematical modeling. Collection experiments reveal a potential and rotation rate independent collection efficiency. RRDE measurements allow, for the first time, the direct determination of the fraction of peroxide production during oxygen reduction on supported catalysts. Finally, comparison of measurements in 0.5 M H₂SO₄ and 0.5 M HClO₄, respectively, reveals a significant effect of (bi)sulfate adsorption on the orr activity. On the basis of the present results, predictions are made on the kinetic limit of the orr in polymer electrolyte fuel cells, in the absence of ohmic and mass transport resistances at 100% utilization.     

Electrodeposited indigotetrasulfonate film onto glutaraldehyde-cross-linked poly-l-lysine modified glassy carbon electrode for detection of dissolved oxygen

The nano composited film of indigotetrasulfonate (ITS) electrodeposited onto poly-l-lysine (PLL)–glutaraldehyde (GA) (ITS/PLL–GA) was modified on glassy carbon electrode (GCE) by multiple scan cyclic voltammetry. Composited of the proposed film was characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), electrochemical quartz crystal microbalance (EQCM), electrochemical impedance spectroscopy (EIS), and UV–vis spectrum for the absorption at λ_max at 566 nm. For the electrocatalytic reduction of dissolved oxygen, ITS/PLL–GA film modified electrodes was determined in 0.1 M acetate buffer solution (pH 5.6) by cyclic voltammetry and rotating disk electrode voltammetry. This dissolved oxygen electrochemical sensor exhibited a linear response range (from 0 to 178.4 μM, R² = 0.9949), lowest detection limit (2.2 μM), lowest overpotential at −0.09 V, high sensitivity (906 μA mM⁻¹) and relative standard deviation (RSD) for determining dissolved oxygen (n = 3) was 4.2%. In addition, the ITS/PLL–GA/GCE was advantageous in terms of its simple preparation, specificity, stability and the ability of regeneration.     

The pzc of Au(111) and Pt(111) in a perchloric acid solution: an ex situ approach to the immersion technique

An electrochemically activated glassy carbon electrode was employed for fabrication of an H₂O₂ sensor through incorporation of horseradish peroxidase (HRP) into the activated surface layer of the electrode during the electrochemical deposition of phenol. The immobilized HRP showed excellent stability and biocatalytic activity for the reduction of H₂O₂. The enzyme electrode exhibited linear responses to H₂O₂ in a concentration range of 5 × 10^?8 to 1 × 10^?5 M and its response time was less than 5 s. At a moderate operating potential of 0V vs. SCE, the direct responses of H₂O₂ and other electroactive interferants, such as ascorbate and urate, on the activated glassy carbon electrode were minimal. The interference of ascorbate and urate was further reduced by the insulating polyphenol film.     

Electrochemical characteristics of a cobalt pentacyanonitrosylferrate film on a modified glassy carbon electrode and its catalytic effect on the electrooxidation of hydrazine

Electrochemically prepared thin films of cobalt pentacyanonitrosylferrate (CoPCNF) were used as surface modifiers for glassy carbon electrodes. The electrochemical behavior of a CoPCNF-modified glassy carbon electrode was studied by cyclic voltammetry; the modified electrode shows one pair of peaks with a surface-confined characteristic in 0.5 M KNO₃ as supporting electrolyte. The effect of different alkali metal cations in the supporting electrolyte on the behavior of the modified electrode was studied and the transfer coefficient (α) and charge transfer rate constant (k_s) for the electron transfer between the electrode and modifier layer were calculated. The experimental results show that the peak potential and peak current vary with different alkali metal cations, but anions such as Cl^?, NO₃^?, CH₃COO^?, H₂PO₄^?/HPO₄^2? and SO₄^2? at 0.5 M concentration have no effect on the peak potential and peak current. An extensive study showed that the response of the modified electrode is not affected within a pH range of 2–8. The CoPCNF films on glassy carbon electrodes show excellent electrocatalytic activity toward the oxidation of hydrazine in 0.5 M KNO₃. The kinetics of the catalytic reaction were investigated by using cyclic voltammetry, rotating disk electrode (RDE) voltammetry and chronoamperometry. The average value of the rate constant for the catalytic reaction and the diffusion coefficient were evaluated by different approaches for hydrazine.     

Comparison of the direct electrochemistry of myoglobin and hemoglobin films and their bioelectrocatalytic properties

The electrocatalytic properties of two related proteins, myoglobin (Mb) and hemoglobin (Hb), immobilized in a surfactant film (didodecyldimethylammonium bromide, DDAB) on an electrode were investigated. The direct electrochemistry of the myoglobin/DDAB and hemoglobin/DDAB films was compared and showed one redox couple, two redox couples, and three redox couples, when transferred to strong acidic, weak acidic and basic, and pH 13 aqueous solutions, respectively. The redox couples and their formal potentials are pH dependent. An electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry were used to study the in situ growth of both DDAB deposition on gold disk electrodes and myoglobin deposition on DDAB film-modified electrodes. The electrocatalytic properties were investigated and showed that the myoglobin/DDAB and hemoglobin/DDAB film modified electrodes are both electrocatalytically active for oxygen reduction, and more H₂O₂ was produced during electrocatalytic reduction using a myoglobin/DDAB film than when using a hemoglobin/DDAB film. The electrocatalysis that was active for l-cystine, N₂O, and 2,2′-dithiosalicylic acid reductions showed an electrocatalytic current developed from the cathodic peak of the redox couple at about ?0.9 V (Fe(II)/Fe(I) redox couple) in neutral and weak basic aqueous solutions. The electrocatalysis that was active for l-cysteine oxidation, showed an electrocatalytic current developed from the cathodic peak of the redox couple at about +0.2 V (Fe(III)/Fe(IV) redox couple). Mb/DDAB and Hb/DDAB film modified electrodes are electrocatalytically active for trichloroacetic acid reduction in weak acidic and basic buffered aqueous solutions through the Fe(II)/Fe(I) redox couple. However, the electrocatalytic current developed from the cathodic peak of the redox couple at a potential of about ?0.1 V (from the Fe(III)/Fe(II) redox couple) in strong acidic aqueous solutions occurs only with higher concentrations of reactant.     

Direct electrochemistry of cholesterol oxidase on MWCNTs

The direct electrochemistry of cholesterol oxidase (ChOx) was achieved on the surface of graphite electrode by immobilizing positively charged ChOx and negatively charged multi walled carbon nanotubes through electrostatic interaction using layer-by-layer technique. Two sets of well defined redox peaks were observed in cyclic voltammogram of the modified graphite electrode in phosphate buffer solution of pH 7. These peaks are corresponding to direct electron transfer of FAD/FADH₂ of ChOx and carboxylic groups of multi walled nanotubes. The modified electrode is characterized using cyclic voltammetry and electrochemical impedance spectroscopy. ChOx modified electrode was used for the determination of cholesterol. The reduction current of oxygen at the modified electrode decreases linearly with the addition of cholesterol in the concentration range of 0.2–1 mM with the lower detection limit of 30 × 10⁻⁶ M. Some common interferents like glucose, ascorbic acid, uric acid and acetaminophen did not cause any interference due to the use of a low operating potential.     

Polynuclear nickel hexacyanoferrates: monitoring of film growth and hydrated counter-cation flux/storage during redox reactions

An electrochemical quartz crystal microbalance (EQCM) was employed to monitor directly the growth of nickel(II) hexacyanoferrate(III) (NiHCNFe) films on gold substrates during electrodeposition as well as a result of sol-gel aggregation in colloidal nickel ferricyanide solutions used for modification. Frequency changes due to mass changes of the gold/crystal working electrode were correlated with cyclic voltammetric (CV) data. Evidence is also provided for the sorption of counter-cations (Li⁺, Na⁺ and K⁺), and associated water molecules, during redox reactions of the film. There is a strict relationship between the amount of alkali metal ions incorporated into the film during reduction, or excluded from the film during oxidation, and the frequency changes during EQCM measurements. The amount of solvent (H₂O) transferred and sorbed in the NiHCNFe film reflects the degree of hydration of the investigated counter-ions. Anions also seem to participate in NiHCNFe electrochemistry, but their role is much less pronounced.     

Diamond-like carbon electrodes in electrochemical microgravimetry

Conducting diamond-like carbon films on quartz crystal electrodes were prepared by laser ablation of graphite. They were stable in many solvents including water, dichloromethane, and acetonitrile. Their potential window is much wider than those of the metal electrodes used usually in quartz oscillators and the electron transfer rate at the electrode surface is fast enough to perform electrochemistry. In dichloromethane with 0.1 M (n-Bu)₄NClO₄, DLC electrodes could scan the potential range between +2.0 and ?2.0 V to record cyclic voltammetric (CV) curves of [Ru(bpy)₃]^3+/2+. We also showed that the DLC works satisfactorily as a useful electrode material for electrochemical microgravimetry by examining the deposition–dissolution of [Ru(bpy)₃]³⁺ in dichloromethane solutions. It was found that the oxidized [Ru(bpy)₃]³⁺ have two different fates, that is, some of them make deposits on the DLC electrode and the rest diffuse out of the electrode surface instead of depositing. Based on the frequency data representing the amount of deposit only and the oxidation charge data reflecting the amount of oxidized species, deposits were found to have the average composition of [Ru(bpy)₃](ClO₄)₃?·?1.7(n-Bu)₄NClO₄.     

Clay-modified electrodes as studied by the quartz crystal microbalance: Redox processes of ruthenium and iron complexes

The electrochemical quartz crystal microbalance (EQCM) has been employed to investigate the mass transport processes on a clay-modified electrode. The systems investigated were the redox couples of [Ru(bpy)₃]²?₆ (bpy = 2,2′-bipyridine), [Ru(NH₃)]₆^2+/3+ and [Fe(CN)₆]^{4 ?/3?}. A clay-modified electrode was prepared by depositing synthetic saponite onto a gold coated quartz crystal. An electrode was allowed to swell for more than 50 h in 0.01 M Na₂SO₄ or NaCl or NaClO₄ prior to the adsorption and electrochemical measurements. The EQCM results revealed that the charge balancing during a redox reaction was accomplished by leaching or incorporating mobile ions in the clay film. For the [Ru(bpy)₃]^2+/3 couple, one [Ru(bpy)₃]^3? molecule was eliminated from the clay film when three [Ru(bpy)₃]²⁺ ions were oxidized. For the [Ru(NH₃)₆]^{2+ 3+} couple, one SO₄² ion, which was co-adsorbed with the ruthenium complex, was removed from the clay film when two [Ru(NH₃)₆]³⁺ molecules were reduced. For the [Fe(CN)₆]^4?/3? couple, a part of the excess charge generated by the initial oxidation of [Fe(CN)₄]^4? was canceled out by the elimination of a sodium ion bound by a clay layer. No mass transfer was detected during the oxidation at the later stage. This was probably because sodium ions in the aqueous medium within a clay film carried excess charge. The results are discussed in relation to the reported electrochemical behavior of these metal complexes.     

Temperature-dependence of oxygen reduction activity at a platinum electrode in an acidic electrolyte solution investigated with a channel flow double electrode

The temperature dependence of the oxygen reduction reaction (orr) activity at a platinum electrode in 0.1 M HClO₄ electrolyte solution was investigated with a channel flow double electrode at 20–90 °C. We demonstrate that an apparent rate constant (k_app) for the orr can be evaluated from the hydrodynamic voltammograms by correcting the oxygen concentration in the electrolyte solution. The apparent activation energy for the k_app was found to be 38 kJ/mol at ?0.525 V vs. E⁰ (0.760 V vs. RHE at 30 °C). Production of H₂O₂ was not detected at potentials more positive than 0.3 V in the temperature region examined. One capacitive semicircle was observed in electrochemical impedance spectroscopy for the orr, suggesting the fast complete reduction of intermediates to H₂O following the rate determining step of the first electron-transfer.     

A monomer–dimer equilibrium in self-assembled monolayers of N-ethyl-N′-octadecylviologen on the electrode surface. The influence of water content and hexafluorophosphate ion

This paper describes the dimerization of self-assembled monolayers (SAMs) of N-ethyl-N′-octadecylviologen (1) on GC and Au electrode surfaces in the presence of 0.1 M NH₄PF₆ aqueous solutions. The ‘wet’ and ‘dry’ SAMs of 1 showed multiple redox peaks for the first reduction of 1 in the presence of NH₄PF₆, in contrast to the case of other supporting electrolytes (typically KCl, NaNO₃, Na₂SO₄ and NaClO₄) where both wet and dry SAMs of 1 exhibited a single redox wave for the first reduction. The dry SAM showed a well defined reduction peak at ?0.57 V along with a shoulder reduction peak at ?0.50 V and two oxidation peaks at ?0.50 and ?0.42 V. On the contrary, the wet SAM gave a very sharp reduction peak at ?0.50 V and a small shoulder peak at ?0.57 V in addition to two oxidation peaks like those observed for the dry SAM. The reduction peak of ?0.50 V was ascribed to the reduction of strongly hydrated dications of 1, while the reduction peak at more negative potential (?0.57 V) was attributed to the reduction of the dehydrated dications of 1. The two oxidation peaks at ?0.50 and ?0.42 V were ascribable to the oxidation of the usual radical cation monomer and the radical cation dimer, respectively. In the case of the wet SAM, upon continuous potential cycling, the sharp reduction peak of ?0.50 V clearly decreased, whereas the more negative reduction peak of ?0.57 V was highly stable. In this case, in the oxidation process, the monomer peak of ?0.50 V increased, while the dimer peak of ?0.42 V decreased. Thus it is reasonably assumed that in the wet SAM, initially the radical cations of 1 feel an aqueous environment in the monolayer where the dimerization is highly favored and at subsequent potential cycles, due to the entry of hydrophobic anions of PF₆^? into the monolayer, the pre-existent water molecules are expelled from the monolayer and under this circumstance the radical cations of 1 may feel the environment very similar to non-aqueous media where the dimerization is totally suppressed. The adsorption tendency of 1 on the electrode surface was also studied using the SAMs prepared by dissolving 1 in water+ethanol mixtures of different ratios. The appearance of multiple peaks was found to depend significantly on the alkyl chain length of asymmetric viologen. The inclusion/expulsion of solvents and anions into/from the SAM during the redox reaction were studied by the electrochemical quartz crystal microbalance (EQCM). It was found that in the presence of SO₄^2? ions ca. 17 water molecules per one SO₄^2? ion were transported to the SAM of 1 during the oxidation, whereas ca. five water molecules were transported in the presence of PF₆^? ions.     

Mass transport accompanied with electron transfer between the gold electrode modified with 11-ferrocenylundecanethiol monolayer and redox species in solution — an electrochemical quartz crystal microbalance study

An electrochemical quartz crystal microbalance (EQCM) was employed to investigate mass transport during the redox reaction of the ferrocenylundecanethiolate (FcC₁₁S(H)) monolayer modified gold electrode in solution containing other redox species. The FcC₁₁S-monolayer on gold acts as a barrier for the electron transfer between a gold electrode and Fe(CN)₆^4?/3? in solution and as a mediator for the reduction of Fe³⁺ in solution. In both cases, electrochemical current responses were complicated because the observed currents were due to the redox of both the ferrocenyl group immobilized on gold and others in electrolyte solutions. The frequency change, i.e. interfacial mass change on the gold electrode surface, was observed only during the redox of ferrocenyl groups. The complex current response was deconvoluted into the current components of the redox reaction of ferrocene and that of other redox species in solution by comparing cyclic voltammograms with the current calculated from frequency changes.