An in situ Raman study of the intercalation of supercapacitor-type electrolyte into microcrystalline graphite

An initial Raman study on the effects of intercalation for aprotic electrolyte-based electrochemical double-layer capacitors (EDLCs) is reported. In situ Raman microscopy is employed in the study of the electrochemical intercalation of tetraethylammonium (Et₄N⁺) and tetrafluoroborate (BF₄⁻) into and out of microcrystalline graphite. During cyclic voltammetry experiments, the insertion of Et₄N⁺ into graphite for the negative electrode occurs at an onset potential of +1.0 V versus Li/Li⁺. For the positive electrode, BF₄⁻ was shown to intercalate above +4.3 V versus Li/Li⁺. The characteristic G-band doublet peak (E_2g2(i) (1578 cm⁻¹) and E_2g2(b) (1600 cm⁻¹)) showed that various staged compounds were formed in both cases and the return of the single G-band (1578 cm⁻¹) demonstrates that intercalation was fully reversible. The disappearance of the D-band (1329 cm⁻¹) in intercalated graphite is also noted and when the intercalant is removed a more intense D-band reappears, indicating possible lattice damage. For cation intercalation, such irreversible changes of the graphite structure are confirmed by scanning electron microscopy (SEM).     

“Deactivation of copper electrode” in electrochemical reduction of CO2

Metallic Cu electrode can electrochemically reduce CO₂ to CH₄, C₂H₄ and alcohols with high yields as revealed by the present authors. Many workers reported that formation of CH₄ and C₂H₄ rapidly diminishes during electrolysis of CO₂ reduction. This paper shows that such deactivation of Cu electrode is reproduced with electrolyte solutions prepared from reagents used by these workers. Deactivated Cu electrodes recovered the electrocatalytic activity for CO₂ reduction by anodic polarization at −0.05 V versus she in agreement with the previous reports. Features of the deactivation depend greatly on the individual chemical reagents. Purification of the electrolyte solution by preelectrolysis with a Pt black electrode effectively prevents the deactivation of Cu electrode. Anode stripping voltammetry of Cu electrodes, which were deactivated during electrolysis of CO₂ reduction, showed anodic oxidation peaks at ca. −0.1 or −0.56 V versus she. The severer the deactivation of the Cu electrode was, the higher electric charge of the anodic peak was observed. It is presumed that some impurity heavy metal, originally contained in the electrolyte, is deposited on the Cu electrode during the CO₂ reduction, poisoning the electrocatalytic activity. On the basis of the potential of the anodic peaks, Fe²⁺ and Zn²⁺ are assumed to be the major contaminants, which cause the deactivation of the Cu electrode. Deliberate addition of Fe²⁺ or Zn²⁺ to the electrolyte solutions purified by preelectrolysis exactly reproduced the deactivation of a Cu electrode in CO₂ reduction. The amount of the deposited Fe or Zn on the electrode was below the monolayer coverage. Electrothermal atomic absorption spectrometry (etaas) showed that Fe originally contained in the electrolyte solution is effectively removed by the preelectrolysis of the solution. Mechanistic difference is discussed between Fe and Zn in the deterioration of the electrocatalytic property of Cu electrode in the CO₂ reduction. The concentration of the impurity substances originally contained in the chemical reagents as Fe or Zn is estimated to be far below the standard of the impurity levels guaranteed by the manufacturers. Presence of trimethylamine in the electrolyte solution also severely poisons a Cu electrode in the CO₂ reduction. It was concluded that the deactivation of Cu electrode in CO₂ reduction is not caused by adsorption of the products or the intermediates produced in CO₂ reduction.     

Effects of phosphate species on localised corrosion of steel in NaHCO3 + NaCl electrolytes

Pitting corrosion of carbon steel electrodes in 0.1 M NaHCO₃ + 0.02 M NaCl solutions was induced by anodic polarisation. The evolution of the breakdown potential E_b with the phosphate concentration was investigated by linear voltammetry. E_b increased from −15 ± 5 mV/SCE for [HPO₄²⁻] = 0 to 180 ± 40 mV/SCE for [HPO₄²⁻] = 0.02 mol L⁻¹. During anodic polarisation (E = 50 mV/SCE), the behaviour of the whole electrode surface, followed by chronoamperometry, was compared to the behaviour of one single pit, followed via the scanning vibrating electrode technique (SVET). The addition of a Na₂HPO₄ solution after the beginning of the polarisation did not lead to the repassivation of pre-existing well-grown pits. The corrosion products forming in the pits were identified in situ by micro-Raman spectroscopy. They depended on the phosphate concentration. For [HPO₄²⁻] = 0.004 mol L⁻¹, siderite FeCO₃ was detected first. It was oxidised later into carbonated green rust GR(CO₃²⁻) by dissolved O₂. The beginning of the process is therefore similar to that observed in the absence of phosphate. Finally, GR(CO₃²⁻) was oxidised into ferrihydrite, the most poorly ordered form of Fe(III) oxides and oxyhydroxides. Phosphate species, adsorbing on the nuclei of FeOOH, inhibited their growth and crystallisation. For [HPO₄²⁻] = 0.02 mol L⁻¹, siderite was accompanied by an amorphous precursor of vivianite, Fe₂(PO₄)₃·8H₂O. This shows that, in any case, phosphate species interact strongly with the iron species produced by the dissolution of steel.     

Electropolymerization of aniline in acid media on the bare and chemically pre-treated aluminum electrodes: A comparative characterization of the polyaniline deposited electrodes

The electrosynthesis of polyaniline on the bare aluminum and pre-treated aluminum surface achieved in aqueous H₂PtCl₆ solution saturated with NaF for few seconds is described. The effect of some factors such as pre-treatment time, aniline and sulfuric acid concentrations on the electropolymerization process was investigated and optimum conditions were obtained. The stability of polyaniline film on the pre-treated aluminum electrode (Al-Pt) was studied as function of the potential imposed on the electrode. For applied electrode potentials of 0.1-0.7 V, the first-order degradation rate constant, k, of polyaniline film varies between 1 × 10⁻⁶ and 2 × 10⁻⁵ s⁻¹, and a relatively low slope (i.e. 2.1) was obtained for the plot of log k versus E. The coatings were characterized by scanning electron microscopy (SEM), and cyclic voltammetric behavior of the polyaniline-deposited Al electrode (Al/PANI) and polyaniline-deposited Al-Pt electrode (Al-Pt/PANI) in 0.1 H₂SO₄ solutions is described. The electrocatalytic activity of the Al-Pt/PANI electrode against para-benzoquinone/hydroquinone (Q/H₂Q) and Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox systems was investigated and the obtained results are compared with those obtained on Al/PANI and bulk Pt electrodes.     

Preparation and characterisation of visible light responsive iodine doped TiO2 electrodes

Characteristics are presented of new iodine doped TiO₂ (I-TiO₂) prepared via the hydrothermal method, where titania (IV) complexes with a ligand containing an iodine atom have been used as a precursor. The structure of samples has been examined by XPS, XRD, UV-vis and FT-IR-ATR techniques. These studies confirm that the obtained powder exhibits a decrease in the bandgap energy value (E_g = 2.8 eV). The report presents electrochemical studies of I-TiO₂ films on a Pt electrode, which allow determination of the flatband potential E_fb = −0.437 V vs. SCE (in 0.5 M Na₂SO₄). Cyclic voltammetry measurements show anodic and cathodic activities under Vis and UV-vis radiation. The photocurrent enhancement due to visible light radiation reached 30% of the whole photoacitivity exhibited under UV-vis illumination.     

The ultrasonic electropolymerization of an 5-[o-(4-bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) film electrode and its electrocatalytic properties to dopamine oxidation in aqueous solution

5-[o-(4-Bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) was electropolymerized on a glassy carbon electrode (GCE), and the electrocatalytic properties of the prepared film electrode response to dopamine (DA) oxidation were investigated. A stable o-BrPETPP film was formed on the GCE under ultrasonic irradiation through a potentiodynamic process in 0.1 M H₂SO₄ between −1.1 V and 2.2 V versus a saturated calomel electrode (SCE) at a scan rate of 0.1 V s⁻¹. The film electrode showed high selectivity for DA in the presence of ascorbic acid (AA) and uric acid (UA), and a 6-fold greater sensitivity to DA than that of the bare GCE. In the 0.05 mol L⁻¹ phosphate buffer (pH 6.0), there was a linear relationship between the oxidation current and the concentration of DA solution in the range of 5 × 10⁻⁷ mol L⁻¹ to 3 × 10⁻⁵ mol L⁻¹. The electrode had a detection limit of 6.0 × 10⁻⁸ mol L⁻¹(S/N = 3) when the differential pulse voltammetric (DPV) method was used. In addition, the charge transfer rate constant k = 0.0703 cm s⁻¹, the transfer coefficient α = 0.709, the electron number involved in the rate determining step n_α = 0.952, and the diffusion coefficient D_o = 3.54  10⁻⁵ cm² s⁻¹ were determined. The o-BrPETPP film electrode provides high stability, sensitivity, and selectivity for DA oxidation.     

Electrochemical study of the interaction between Eu and ciliate Euplotes octocarinatus centrin

A new species was formed when protein P₂₃ (one segment of ciliate Euplotes octocarinatus centrin) was added to a solution of Eu³⁺. The interaction between P₂₃ and Eu³⁺ was investigated by cyclic voltammetry, pulse voltammetry and electrochemical impedance spectroscopy in 10 mM N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid (HEPES) buffer (pH 7.4) using a pyrolytic graphite electrode. The formal potential (E^o′) of Eu³⁺ shifted from −0.61 to −0.84 V (versus saturated calomel electrode) after P₂₃ was added to the Eu³⁺ solution. The diffusion coefficient (D), the charge-transfer coefficient (α) and the electron transfer standard rate constant (k_s) were obtained in the absence and the presence of P₂₃. The affinity constant of Eu³⁺ and P₂₃ was determined to be (1.89 ± 0.51) × 10⁴ M⁻¹. The electrochemical investigation of europium bound to the protein provided useful data for the studies of calcium-binding proteins.     

Structural and optical properties of magnesium nitride formed by a novel electrochemical process

The electrochemical formation of magnesium nitride (Mg₃N₂) films in LiCl-KCl containing Li₃N at 723 K was investigated. From a thermodynamic point of view, a potential-pN³⁻ diagram was constructed for the Mg-N system in an analogous fashion to Pourbaix diagrams for aqueous solutions. As a result, the thermodynamically stable region of Mg₃N₂ in LiCl-KCl-Li₃N was identified. XRD analysis revealed that Mg₃N₂ film was obtained by potentiostatic electrolysis of a magnesium electrode between 0.4 and 0.8 V (versus Li⁺/Li), and the structure of obtained Mg₃N₂ was anti-bixbyite (a = 1.001 nm). Reflectance measurements clarified that assuming direct transition, the bandgap energy was 3.15 eV and assuming indirect transition, the bandgap energy was 2.85 eV.     

Direct electron-transfer and electrochemical catalysis of hemoglobin immobilized on mesoporous Al2O3

The direct electrochemistry of hemoglobin (Hb) has been achieved by immobilizing Hb on mesoporous Al₂O₃ (meso-Al₂O₃) film modified glassy carbon (GC) electrode. Meso-Al₂O₃ shows significant promotion to the direct electron-transfer of Hb, thus it exhibits a pair of well defined and quasi-reversible peaks with a formal potential of −0.345 V (vs. SCE). The electron-transfer rate constant (k_s) is estimated to be 3.17 s⁻¹. The immobilized Hb retains its biological activity well and shows high catalytic activity to the reduction of hydrogen peroxide (H₂O₂) and nitrite (NO₂⁻). Under the optimized experimental conditions, the catalytic currents are linear to the concentrations of H₂O₂ and NO₂⁻ in the ranges of 0.195-20.5 μM and 0.2-10 mM, respectively. The corresponding detection limits are 1.95 × 10⁻⁸ M and 3 × 10⁻⁵ M (S/N = 3). The resulting protein electrode has high thermal stability and good reproducibility due to the protection effect of meso-Al₂O₃. Ultraviolet visible (UV-vis) absorption spectra and reflection-absorption infrared (RAIR) spectra display that Hb keeps almost natural structure in the meso-Al₂O₃ film. The N₂ adsorption-desorption experiments show that the pore size of meso-Al₂O₃ is about 14.4 nm, suiting for the encapsulation of Hb (average size: 5.5 nm) well. Therefore, meso-Al₂O₃ is an alternative matrix for protein immobilization and biosensor preparation.     

Electrochemical reduction of high pressure CO2 at a Cu electrode in cold methanol

The electrochemical reduction of high pressure CO₂ with a Cu electrode in cold methanol was investigated. A high pressure stainless steel vessel, with a divided H-type glass cell, was employed. The main products from CO₂ by the electrochemical reduction were methane, ethylene, carbon monoxide and formic acid. In the electrolysis of high pressure CO₂ at low temperature, the reduction products were formed in the order of carbon monoxide, methane, formic acid and ethylene. The best current efficiency of methane was of 20% at −3.0 V. The maximum partial current density for CO₂ reduction was approximately 15 mA cm⁻². The partial current density ratio of CO₂ reduction and hydrogen evolution, i(CO₂)/i(H₂), was more than 2.6 at potentials more positive than −3.0 V. This work can contribute to the large-scale manufacturing of fuel gases from readily available and inexpensive raw materials, CO₂-saturated methanol from industrial absorbers (the Rectisol process).     

Comparison of AlPO4- and Co3(PO4)2-coated LiNi0.8Co0.2O2 cathode materials for Li-ion battery

Electrochemical and thermal properties of Co₃(PO₄)₂- and AlPO₄-coated LiNi_0.8Co_0.2O₂ cathode materials were compared. AlPO₄-coated LiNi_0.8Co_0.2O₂ cathodes exhibited an original specific capacity of 170.8 mAh g⁻¹ and had a capacity retention (89.1% of its initial capacity) between 4.35 and 3.0 V after 60 cycles at 150 mA g⁻¹. Co₃(PO₄)₂-coated LiNi_0.8Co_0.2O₂ cathodes exhibited an original specific capacity of 177.6 mAh g⁻¹ and excellent capacity retention (91.8% of its initial capacity), which was attributed to a lithium-reactive Co₃(PO₄)₂ coating. The Co₃(PO₄)₂ coating material could react with LiOH and Li₂CO₃ impurities during annealing to form an olivine Li_xCoPO₄ phase on the bulk surface, which minimized any side reactions with electrolytes and the dissolution of Ni⁴⁺ ions compared to the AlPO₄-coated cathode. Differential scanning calorimetry results showed Co₃(PO₄)₂-coated LiNi_0.8Co_0.2O₂ cathode material had a much improved onset temperature of the oxygen evolution of about 218 °C, and a much lower amount of exothermic-heat release compared to the AlPO₄-coated sample.     

Electrochemical copolymerization of aniline with m-aminophenol and novel electrical properties of the copolymer in the wide pH range

A copolymer, poly(aniline-co-m-aminophenol), has been synthesized using repeated potential cycling. The monomer concentration ratio, acid concentration and applied potential strongly affect the copolymerization rate and the properties of the copolymer. The optimum conditions for the copolymerization are that the scan potential range is controlled between −0.10 and 0.95 V (vs.SCE), and a solution consists of 0.34 M aniline, 0.012 M m-aminophenol and 2 M H₂SO₄. The IR spectra of the copolymers demonstrate that the m-aminophenol units are included in the copolymer chains. The cyclic voltammograms of the copolymers in 0.3 M Na₂SO₄ solution with various pH values were performed at the potential ranges from −0.20 to 0.80 V and at a scan rate of 60 mV s⁻¹. The results indicate that the copolymer still hold 41.7% of the electrochemical activity when the copolymer electrode was transferred from a solution of pH 5.0 to a solution of pH 11.0 in the potential range of −0.20 to 0.80 V. An impedance plot of the copolymer in a solution with pH 12.0 and at 0.40 V is constructed of a semicircle and a Warburg line with a slope of 1. This means that the electrode reaction of the copolymer at pH 12.0 is also under mass transfer control. The conductivity of the copolymer prepared under the optimum conditions is 1.42 S cm⁻¹, and slightly depends on the pH value. Thus, the pH dependence of the electrical properties of the copolymer is improved compared with poly(aniline-co-o-aminophenol), and is much better than that of the parent polyaniline.     

An unusual H2O2 electrochemical sensor based on Ni(OH)2 nanoplates grown on Cu substrate

In this work, Ni(OH)₂ nanoplates grown on the Cu substrate were synthesized and characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Then a novel Cu-Ni(OH)₂ modified glass carbon electrode (Cu-Ni(OH)₂/GCE) was fabricated and evaluated by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and typical amperometric response (i-t) method. Exhilaratingly, the Cu-Ni(OH)₂/GCE shows significant electrocatalytic activity toward the reduction of H₂O₂. At an applied potential of −0.1 V, the sensor produces an ultrahigh sensitivity of 408.1 μA mM⁻¹ with a low detection limit of 1.5 μM (S/N = 3). The response time of the proposed electrode was less than 5 s. What's more, the proposed sensor displays excellent selectivity, good stability, and satisfying repeatability.     

Co (II) porphyrin adsorbed on SiO2/SnO2/phosphate prepared by the sol-gel method: Application in electroreduction of dissolved dioxygen

This paper describes the immobilization procedure of 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21-H,23-H-porphyrin ion on SiO₂/SnO₂/phosphate, obtained by the sol-gel processing method. P 2p X-ray photoelectron and the ³¹P MAS NMR spectra revealed that dihydrogen phosphate is the species present on the surface. The porphyrin was adsorbed on the surface of the modified material and furthermore metallized in situ with Co (II) ion. The porphyrin metallation process was followed with UV-vis spectroscopy by inspecting the Q bands of the free and metallated porphyrin. The free porphyrin presented four Q bands associated to a D_2h local symmetry and the metallated one, two bands related to a D_4h local symmetry. The amount of electroactive species adsorbed on the material was estimated by integrating the area under the peak of Co (II) → Co (I) reduction by using the pulse differential voltammetric technique. The amount of the metallated porphyrin was 2.3 × 10⁻¹⁰ mol cm⁻². A carbon paste electrode of the modified material containing metallated porphyrin was used to study the electrocatalytic reduction of dissolved dioxygen by means of cyclic voltammetry, chronoamperometry and linear sweeping voltammetry. The modified electrode was very stable and exhibited the electrocatalytic reduction of dissolved dioxygen at −180 mV versus SCE by a two-electron mechanism, producing hydrogen peroxide at pH 5.4. The electroactive species was strongly retained on the material surface, presumably inside the pores of the material, since in a test of various oxidation-reduction cycles no significant decrease of the current densities was detected, indicating that it was not leached off during the experiment.     

Preparation of micro-dot electrodes of LiCoO2 and Li4Ti5O12 for lithium micro-batteries

Fabrications of micro-dot electrodes of LiCoO₂ and Li₄Ti₅O₁₂ on Au substrates were demonstrated using a sol-gel process combined with a micro-injection technology. A typical size of prepared dots was about 100 μm in diameter, and the dot population on the substrate was 2400 dots cm⁻². The prepared LiCoO₂ and Li₄Ti₅O₁₂ micro-dot electrodes were characterized with scanning electron microscopy, X-ray diffraction, micro-Raman spectroscopy, and cyclic voltammetry. The prepared LiCoO₂ and Li₄Ti₅O₁₂ micro-dot electrodes were evaluated in an organic electrolyte as cathode and anode for lithium micro-battery, respectively. The LiCoO₂ micro-dot electrode exhibited reversible electrochemical behavior in a potential range from 3.8 to 4.2 V versus Li/Li⁺, and the Li₄Ti₅O₁₂ micro-dot electrode showed sharp redox peaks at 1.5 V.     

Electrochemical and thermal studies of Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 (x = 1/12, 1/4, 5/12, and 1/2)

Samples of the layered cathode materials, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (x = 1/12, 1/4, 5/12, and 1/2), were synthesized at 900 °C. Electrodes of these samples were charged in Li-ion coin cells to remove lithium. The charged electrode materials were rinsed to remove the electrolyte salt and then added, along with EC/DEC solvent or 1 M LiPF₆ EC/DEC, to stainless steel accelerating rate calorimetry (ARC) sample holders that were then welded closed. The reactivity of the samples with electrolyte was probed at two states of charge. First, for samples charged to near 4.45 V and second, for samples charged to 4.8 V, corresponding to removal of all mobile lithium from the samples and also concomitant release of oxygen in a plateau near 4.5 V. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples with x = 1/4, 5/12 and 1/2 charged to 4.45 V do not react appreciably till 190 °C in EC/DEC. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples charged to 4.8 V versus Li, across the oxygen release plateau, start to significantly react with EC/DEC at about 130 °C. However, their high reactivity is similar to that of Li_0.5CoO₂ (4.2 V) with 1 μm particle size. Therefore, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples showing specific capacity of up to 225 mAh/g may be acceptable for replacing LiCoO₂ (145 mAh/g to 4.2 V) from a safety point of view, if their particle size is increased.     

High performance electrode for electrochemical oxygen generator cell based on solid electrolyte ion transport membrane

A double-layer composite electrode based on Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ + Sm_0.2Ce_0.8O_1.9 (BSCF + SDC) and BSCF + SDC + Ag was investigated to be a promising cathode and also anode for the electrochemical oxygen generator based on samaria doped ceria electrolyte. The Ag particles in the second layer were not only the current collector but also the improver for the oxygen adsorption at the electrode. a.c. impedance results indicated that the electrode polarization resistance, as low as 0.0058 Ω cm² was reached at 800 °C under air. In oxygen generator cell performance test, the electrode resistance dropped to half of the value at zero current density under an applied current density of 2.34 A cm⁻² at 700 °C, and on the same conditions the oxygen generator cell was continual working for more than 900 min with a Faradic efficiency of ∼100%.     

Study on the rapid synthesis of LiNi1−xCoxO2 cathode material for lithium secondary battery

LiNi_1−xCo_xO₂ (x = 0, 0.1, 0.2) cathode materials were successfully synthesized by a rheological phase reaction method with calcination time of 0.5 h at 800 °C. All obtained powders are pure phase with α-NaFeO₂ structure (R-3m space group). The samples deliver an initial discharge capacity of 182, 199 and 189 mAh g⁻¹ (25 mA g⁻¹, 4.35-3.0 V), respectively. The reaction mechanism was also discussed, which consists of a series of defect reactions. As a result of these defect reactions, the reaction of forming LiNi_1−xCo_xO₂ takes place in high speed.     

Development of a sensor based on tetracyanoethylenide (LiTCNE)/poly-l-lysine (PLL) for dopamine determination

The catalytic oxidation of dopamine (DA) at a LiTCNE (lithium tetracyanoethylenide) film modified electrode is studied by electrochemical approaches. The immobilization of LiTCNE was performed by a polymer (poly-l-lysine) to prepare this modified electrode and its application for dopamine (DA) determination is described in detail. The modified electrode showed a high activity for the electrooxidation of dopamine (DA) at E_p = 0.20 V versus SCE. The modified electrode presented a wide linear response range for DA from 0.01 up to 10 μmol l⁻¹ by differential pulse voltammetry (DPV) with a detection limit of 0.5 nmol l⁻¹. The repeatability of the proposed sensor evaluated in term of relative standard deviation was 3.2% for n = 10. The sensor was applied for the determination of dopamine in pharmaceutical formulations and the average recovery for these samples was 101.9 (±0.1)%.     

o-Phenylenediamine adsorbed onto silica gel modified with niobium oxide for electrocatalytic NADH oxidation

A study of a modified carbon paste electrode employing o-phenylenediamine (PDA) adsorbed onto silica gel modified with niobium oxide (SN) for electrocatalytic oxidation of nicotinamide adenine dinucleotide (NADH) is described. The species adsorbed on SN was used to prepare a modified carbon paste electrode to investigate its electrochemical properties. The formal potential (E^0′) of the adsorbed PDA was −140 mV vs. SCE (saturated calomel electrode). The electrochemical behavior of the adsorbed PDA, compared to that of PDA dissolved in aqueous solution, was completely different. In solution, pH between 3.0 and 8.0, E^0′ remained almost constant and the response was very stable. A linear response range for NADH between 4.0×10⁻⁵ and 8.0×10⁻⁴ mol l⁻¹, at pH 7.0, was observed for the electrode, with an applied potential of −50 mV vs. SCE. The formation of an intermediate charge transfer complex is proposed for the charge transfer reaction between NADH and adsorbed PDA. The heterogeneous electron transfer rate, k_obs, was 5480 mol⁻¹ l s⁻¹ and the apparent Michaelis-Menten constant, 1.04×10⁻⁴ mol l⁻¹ at pH 7.0, evaluated with rotating disk electrode (RDE) experiments with an electrode with a coverage of PDA of 5.7×10⁻⁹ mol cm⁻². The slight increase in the reaction rate with the solution pH was assigned to the thermodynamic driving force.