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.     

Photoinducedly electrochemical preparation of Prussian blue film and electrochemical modification of the film with cetyltrimethylammonium cation

This work presents a photoinducedly electrochemical preparation of Prussian blue from a single sodium nitroprusside and insertion of cetyltrimethylammonium cations into Prussian blue as counter ions. The product of photoinducedly electrochemical reactions has a couple of voltammetric peaks at E° = 0.266 V in 0.2 mol l⁻¹ KCl solution, the measurements of X-ray powder diffraction and FT-IR spectroscopy show that it is Prussian blue (PB). The formation mechanism of a pre-photochemical reaction and subsequent electrochemical reaction is suggested. The cyclic voltammetric treatment of the freshly as-prepared PB film in 1.0 mmol l⁻¹ cetyltrimethylammonium (CTA) bromide solution leads to the insertion of cetyltrimethylammonium cations into the channels of Prussian blue, which substitutes for potassium ions as counter ions in Prussian blue. The Prussian blue containing CTA counter ions shows two couples of voltammetric peaks at E° = −0.106 V and E° = 0.249 V in 0.2 mol l⁻¹ KCl solution containing 1.0 mmol l⁻¹ cetyltrimethylammonium bromide. Compared with the electrochemical behaviors of KFeFe(CN)₆ in 0.1 mol l⁻¹ KOH alkali solution, CTAFeFe(CN)₆ shows relatively durable voltammetric currents due to the hydrophobic effects of cetyltrimethylammonium. The diffusion coefficients for CTA and potassium cations were estimated to be D_CTA 1.25 × 10⁻¹² cm² s⁻¹, D_K 2.59 × 10⁻¹² cm² s⁻¹, respectively. The peak current of electro-catalytic oxidization on hydrogen peroxide showed a linear dependence from 6.59 × 10⁻⁶ to 2.20 × 10⁻⁴ mol l⁻¹ with R = 0.99947 (n = 8). The linear regression equation was I_p (mA) = 0.82949 + 0.00594C (μmol l⁻¹) with errors of ±7.92833 × 10⁻⁵ for the slope and ±0.01085 for the intercept with the detection limit of 1.46 × 10⁻⁶ mol l⁻¹. Thus, it is expected to find its application in neutral or weak alkali medium for biosensors.     

Effect of zwitterionic salt on the electrochemical properties of a solid polymer electrolyte with high temperature stability for lithium ion batteries

In this study, we prepare a kind of solid polymer electrolyte (SPE) based on N-ethyl-N′-methyl imidazolium tetrafluoroborate (EMIBF₄), LiBF₄ and poly(vinylidene difluoride-co-hexafluoropropylene) [P(VdF-HFP)] copolymer. The resultant SPE displays high thermal stability above 300 °C and high room temperature ionic conductivity near to 10⁻³ S cm⁻¹. Its electrochemical properties are improved with incorporation of a zwitterionic salt 1-(1-methyl-3-imidazolium)propane-3-sulfonate (MIm3S). When the SPE contains 1.0 wt% of the MIm3S, it has a high ionic conductivity of 1.57 × 10⁻³ S cm⁻¹ at room temperature, the maximum lithium ions transference number of 0.36 and the minimum apparent activation energy for ions transportation of 30.9 kJ mol⁻¹. The charge-discharge performance of a Li₄Ti₅O₁₂/SPE/LiCoO₂ cell indicates the potential application of the as-prepared SPE in lithium ion batteries.     

The physical–chemical properties and electrocatalytic performance of iridium oxide in oxygen evolution

An IrO₂ anode catalyst was prepared by using the Adams method for the application of a solid polymer electrolyte (SPE) water electrolyzer. The effect of calcination temperature on the physical–chemical properties and the electrochemical performance of IrO₂ were examined to obtain a low loading and a high catalytic activity of oxygen evolution at the electrode. The physical–chemical properties were studied via thermogravimetry–differential scanning calorimetry (TG–DSC), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The electrochemical activity was investigated by using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronopotentiometry in 0.1 mol L⁻¹ H₂SO₄ at room temperature. The optimum condition was found to be at the calcination temperature of 500 °C, where the total polarization reached a minimum at high current densities (>200 mA cm⁻²). The optimized catalyst was also applied to a membrane electrode assembly (MEA) and stationary current–potential relationships were investigated. With an optimized catalytic IrO₂ loading of 1.5 mg cm⁻² and a 40% Pt/C loading of 0.5 mg cm⁻², the terminal applied potential difference was 1.72 V at 2 A cm⁻² and 80 °C in a SPE water electrolysis cell.     

Acetic acid-doped poly(ethylene oxide)-modified poly(methacrylate): a new proton conducting polymeric gel electrolyte

A proton conducting polymeric gel membrane was first developed from poly(ethylene oxide)-modified poly(methacrylate) (PEO-PMA) containing poly(ethylene glycol) dimethylether (PEGDE). Acetic acid (HAc) was doped by immersing the polymeric film directly in the aqueous solution of HAc. Characterization by FT-IR, XRD and AC conductivity measurements were carried out on the film electrolytes consisting of different gel compositions. The ionic conductivity of the membrane showed a sensitive variation with the immersion time and concentration of the acid in the doping solution through the changes in the contents of acid and water in the gel. The ionic conductivity also depended on the PEGDE content in the polymer. The proton conductivity was 6.2×10⁻⁴ S cm⁻¹ at 20 °C and 1.0×10⁻³ S cm⁻¹ at 80 °C for the gel prepared from HAc concentration of 3.0 mol l⁻¹. The temperature dependence of the conductivity was found to be consistent with Arrhenius-type relationship at a temperature range from 20 to 80 °C, except for the films with low PEGDE contents. The apparent activation energy for the proton conduction was in the range of 5-30 kJ mol⁻¹, depending on the HAc concentration and the polymer matrix composition. The FT-IR spectra of the polymeric membranes showed that HAc does not protonate the carbonyl or ester groups of the polymer matrix, but interacts with them by the hydrogen bonding interaction or weak molecular interactions.     

Electrogeneration of hydrogen peroxide on a novel highly effective acetylene black-PTFE cathode with PTFE film

A novel acetylene black-PTFE cathode for electrogeneration of hydrogen peroxide (H₂O₂) was fabricated using acetylene black powder (ABP) as catalyst, PTFE as binder. The effect of mass ratio of ABP to PTFE, solution pH (3–11), applied current density (5–20 mA cm⁻²) and surface hydrophobic property on the activity and useful life of cathode was investigated using a two-electrode undivided cell fed with air at 2 L min⁻¹ in Na₂SO₄ solution of 0.05 M, and the characteristics of cathode were examined by FTIR spectra, BET measurement, SEM image and CV study. The experimental results showed that: (1) so far the activity of cathode is the highest for the electrogeneration of H₂O₂ with air feed, at pH 3 and the current density of 20 mA cm⁻² the average H₂O₂ generation rate reaching 58.9 mg L⁻¹ h⁻¹ cm⁻² during 150 min of electrolysis and the current efficiency being 92.7%; (2) a proper and stable surface hydrophobic property could significantly increase the useful life of cathode; (3) the cathode had a remarkable restoration and regeneration performance.     

Derivatization of SWCNTs with cobalt phthalocyanine residues and applications in screen printed electrodes for electrochemical detection of thiocholine

Single-walled carbon nanotubes (SWCNTs) derivatized with cobalt phthalocyanine (CoPh) were applied onto screen-printed graphite electrodes (SPEs) to be used for the low-potential electrochemical oxidation of thiocholine (TCh). Covalent attachment of CoPh to SWCNTs via stable sulfonamide bonds was confirmed by Raman/FT-IR spectroscopy and thermogravimetric analysis (TGA) coupled with FT-IR detection. The resulting modified SPE surfaces (CoPh-SWCNT-SPEs) were characterized by cyclic voltammetry and electrochemical impedance spectroscopy (EIS) with the redox probe [F₃(CN)₆]^3−/4−. Detection of TCh was accomplished using cyclic voltammetry and amperometry; a lower overpotential (100 mV vs. Ag/AgCl pseudoreference electrode) was obtained using CoPh-SWCNT-SPEs as compared to unmodified SPEs and SPEs modified with non-functionalized SWCNTs (SWCNT-SPEs). The linear range for TCh detection was 0.077–0.45 mM, with a sensitivity of 5.11 × 10⁻¹ μA mM⁻¹ and a limit of detection of 0.038 mM according to the 3 s/m definition.     

SiO2/C/Cu(II)phthalocyanine as a biomimetic catalyst for dopamine monooxygenase in the development of an amperometric sensor

A mesoporous carbon ceramic SiO₂/50 wt% C (S_BET = 170 m² g⁻¹), where C is graphite, was prepared by the sol–gel method. This material was used as matrix to support copper phthalocyanine (CuPc), prepared in situ on their surface, to assure homogeneous dispersion of the electrocatalyst complex in the pores of the matrix. Pressed disk electrodes made with SiO₂/C/CuPc was tested as amperometric sensors for dopamine. Under optimized conditions, at −20 mV vs SCE in 0.08 mol dm⁻³ Britton–Robinson buffer (BRB) solution (pH = 6.0) containing 100 μmol dm⁻³ of H₂O₂, a linear response range for dopamine from 10 up to 140 μmol dm⁻³ was obtained with a sensitivity of 0.63 (±0.006) nA dm³ μmol⁻¹ cm⁻² and the limit of detection LOD was 0.6 μmol dm⁻³. The sensors presented stable response during successive determinations. The repeatability, evaluated in terms of relative standard deviation of 1.37% for n = 10 and 10 μmol dm⁻³ dopamine. The response time was 1 s and lifetime 9 months. Finally, the sensor was tested to determine dopamine in the sample, and gives very good results for its determination. The presence of other phenols like catechol and resorcinol did not show any interference in the detection of dopamine on this electrode, even in the same concentration with the dopamine.     

Discharge performance of a primary alkaline CuO cathode material prepared via a novel non-aqueous precipitation method

An amorphous copper oxide material with a large BET surface area (191.9 m² g⁻¹) was prepared via the room temperature conversion of a Cu(OH)₂ intermediate, itself formed via a novel base precipitation technique from an ethylene glycol solvent. The electrochemical discharge rate performance as a primary alkaline cathode material was examined galvanostatically (50 − 2000 mA g⁻¹) in 9.0 mol dm⁻³ KOH electrolyte, and compared to the incumbent primary alkaline cathode material, electrolytic manganese dioxide (EMD). The prepared CuO material performs favourably compared to EMD under high rate discharge conditions (>600 mA g⁻¹), where the material discharges 154 and 135 mA h g⁻¹ capacity at a discharge rate of 1000 and 2000 mA g⁻¹, respectively, compared to 132 and 83 mA h g⁻¹ for EMD under the same conditions.     

Galvanostatic cycling of graphite intercalation electrodes with anions in aqueous acids

Natural graphite flakes (80 wt%), with polypropylene (20 wt%) as a binder, constitute a practical and non-expensive graphite electrode of high crystallinity CPP. Galvanostatic cycling of these electrodes with current densities in the range 0.3–30 mA cm^–2 (charging time 5–120 min) has been investigated in aqueous acids (12, 20 and 36 mol dm^–3 HF, 6 and 12 mol dm^–3 H₂SO₄, 4 mol dm^–3 HClO₄). The anion of the acid is anodically intercalated and cathodically de-intercalated. In spite of the high water concentrations, quantitative current efficiencies have been obtained. From variation of the rest times after charging, a corrosion current density of less than 0.03 mA cm^–2 (j _ch=3 mA cm^–2) has been derived. The overvoltage during charge and discharge is typically about 0.1 V. The potential at the start of the charging process coincides with the intercalation potential defined previously. A strong electrode formation effect is observed upon cycling. The electrode is initially smooth and non-porous; it acquires a high surface roughness after a few cycles, which is then stable. The initial charging curves increase witht ^1/2, while the charging curve after electrode formation is linear. Both clearly indicate a linear relationship between surface concentration of intercalated anions and potential. This agrees with our previous finding of linear dependence with respect to acid concentration in the solution.     

Composite polyether electrolytes with a poly(styrenesulfonate) lithium salt and Lewis acid type additive

A poly(styrenesulfonate) lithium salt was tested as a single-ion conductor in a poly(ethylene oxide)/poly(ethylene glycol) dimethyl ether matrix. Impedance spectroscopy and voltage step polarization were used to characterize the composite electrolytes. A specific conductivity of about 7×10⁻⁸ S cm⁻¹ was evaluated at 70 °C. The very low conductivity was attributed to the poor solubility of Li⁺ in the polymer matrix. AlCl₃ was added to the polymer electrolyte to increase the salt dissolution. The addition of the Lewis acid strongly increases the conductivity and a specific conductivity of about 4×10⁻⁶ S cm⁻¹ was measured at 20 °C. For temperature lower than 60 °C, the specific conductivity dependence with increasing temperature follows an Arrhenius-type behavior. An activation energy of about 55.6 kJ mol⁻¹ was calculated. A very similar activation energy (60.3 kJ mol⁻¹) was found for the charge transfer resistance. The transport properties of the polymer electrolyte were tested by applying a d.c. voltage step to a symmetrical lithium cell. The current at the applied voltage decreased with time. The decrease was related to an increase in the cell resistance due to the continuous growth of a passivation layer on the lithium surface.     

Capillary electrophoretic separation and post-column electrochemical detection of mercury and methyl mercury and applications to coal samples

A study of the electrochemistry of mercury and organo mercury complexes with cysteine indicated the formation of stable complexes which can be utilized for the determination of the species in environmental matrices. A technique based on capillary electrophoresis and amperometric detection (CE-AD) has been developed for the speciation of mercury with a newly developed cross hair design electrochemical detector. This technique has the capability to detect mercury species that are electrochemically active. For CE-AD the detection limits were 0.005 ± 0.002 μg L⁻¹ for Hg²⁺ and 0.4 ± 0.05 μg L⁻¹ for MeHg⁺. These detection sensitivities are very attractive for environmental monitoring and the method was utilized for the determination of mercury in coal.     

Tris (2,2′-bipyridil) copper (II) chloride complex: a biomimetic tyrosinase catalyst in the amperometric sensor construction

The use of tris (2,2′-bipyridil) copper (II) chloride complex, [Cu(bipy)₃]Cl₂·6H₂O, as a biomimetic catalyst, is reported in the construction of an amperometric sensor for dopamine. The sensor was prepared modifying a glassy carbon electrode with a Nafion® membrane doped with the complex. The optimized conditions for the sensor response were obtained in 0.25 mol dm⁻³ Pipes buffer (pH 7.0) containing 150 μmol dm⁻³ H₂O₂, with an applied potential of −50 mV versus saturated calomel electrode (SCE). In these conditions, a linear response range between 9 and 230 μmol dm⁻³, with a sensitivity of 1.43±0.01 nA dm³ μmol⁻¹ cm⁻² and a detection limit of 4.8 μmol dm⁻³ were observed for dopamine. The response time for this sensor was about 1 s, presenting the same response for at least 150 successive measurements, with a good repeatability (4.8%) expressed as relative standard deviation for n=13. After its construction, this sensor can be used after 180 days without loss of sensitivity, kept at room temperature. The difference of the sensor response between four preparations was 4.2%. A detailed investigation about the sensor response for other eighteen phenolic compounds and five interfering species was performed. The sensor was applied for dopamine determination in pharmaceutical preparation with success.     

Differential capacitance and frequency dispersion on solid and liquid mercury in liquid ammonia

The behaviour of solid and liquid mercury electrodes in 0·1 M ammonium nitrate solutions of liquid ammonia is investigated. The differential-capacitance/potential curves for the solid (at −40°C) and the liquid (at −38°C) are U-shaped and have a minimum of 11·2 μF/cm² at 0·45 V (vs Cd/satd. Cd(NO₃)₂ in liquid NH₃) for the solid of and 10·25 μF/cm² at 0·4 V for the liquid. The curves have no anodic hump. The values for the solid are about 10% higher than that for the liquid but this is still within the error of the surface area measurement. The frequency dispersion of the interfacial impedance was measured between 400 and 20,000 Hz at 0·60, 0·40, 0·10, and −0·25 V. Assuming de Levie's model of parallel grooves of depth l and aperture 2 β to be applicable, the parameter l/sin β was calculated from the data. Values obtained for the liquid mercury electrodes are 5·5–6·8 × 10⁻² cm and are attributed to shielding and solution creepage. For the solid electrodes l/sin β is 12–23 × 10⁻²cm. Although these numbers are too large, the increase in l/sin β by a factor of 3 when going from a liquid to a solid surface is parallel to an increase in roughness factor to about 3 expected for this case. This suggests that l/sin β may be a useful way to measure surface roughness and/or surface roughness changes.     

Nucleation and surface morphology of aluminum-lanthanum alloy electrodepsited in a LaCl3-saturated AlCl3-EtMeImCl room temperature molten salt

The cathodic polarization curve on a tungsten disk electrode was measured in a LaCl₃-saturated AlCl₃-EtMeImCl [1-ethyl-3-methylimidazolium chloride] melt (N=0.667: N is molar fraction of AlCl₃) at 298 K. The deposition overpotential of aluminum increases compared with the curve obtained before adding LaCl₃. It was found that the nucleation/growth process is instantaneous nucleation from chronoamperometric data. When galvanostatic electrolysis was performed in the LaCl₃-saturated melt, the strong orientation of (200) for the electrodeposits is observed at low current densities (≤7.5 mA cm⁻²). On the other hand, the normalized integrated intensity of XRD for (200) and (220) reflections has similar strength at high current densities (≥10.0 mA cm⁻²). The electodeposits become denser than those obtained in the original melt. In particular, very smooth surface is obtained in the case of 15.0 mA cm⁻² with stirring the bath.     

A nano-structured Ni(II)–ACDA modified gold nanoparticle self-assembled electrode for electrocatalytic oxidation and determination of tryptophan

A nano-structured Ni (II)/ACDA (2-amino-1-cyclopentene-1-dithiocarboxylic acid) film was electrodeposited on a gold nanoparticle–cysteine–gold electrode. The formation of Ni (II)/ACDA film and electrocatalytic oxidation of tryptophan on the surface of the modified electrode were investigated with cyclic voltammetric and chronoamperometric techniques. The hydrodynamic amperometry at rotating modified electrode was used for determination of tryptophan in the range of 0.085–43.0 μmol l⁻¹. The detection limit was found to be 23 nmol l⁻¹. The rate constant, transfer coefficient for the catalytic reaction and the diffusion coefficient of tryptophan in the solution were found to be 9.1 × 10² M⁻¹ S⁻¹, 0.52 and 1.09 × 10⁻⁵ cm² s⁻¹ respectively. It is worth noting that the as formed matrix in our work possesses a 3D porous network structure with a large effective surface area and high catalytic activity and behaves like microelectrode ensembles. The modified electrode indicated reproducible behavior and a high level stability during the experiments, making it particularly suitable for the analytical purposes.     

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.     

Removal of Pb(II) from simulated wastewaters using a stainless-steel wool cathode in a flow-through cell

The investigation of an electrolytic process to remove Pb(II) from simulated wastewaters using a stainless-steel wool (SSW) cathode in a flow-through cell under potentiostatic condition is reported. Voltammetry under hydrodynamic conditions was used to estimate the diffusion coefficient, which was found to be 1.4× 10⁻⁵ cm² s⁻¹ in the supporting electrolyte (0.10 mol l⁻¹ NaNO₃ and 0.10 mol l⁻¹ H₃BO₃, pH 4.8). The performance of the flow-through cell was evaluated for three potentials: −0.70, −0.80 and −0.90 V vs. saturated calomel electrode (SCE). At −0.70 V, the reaction was found not to be completely controlled by mass transfer, while at −0.80 V and −0.90 V the Pb(II) concentration decayed exponentially. At −0.90 V, using a flow rate of 250 l h⁻¹, after a 90-min electrolysis, the Pb(II) concentration decayed from 50 ppm to only 1 ppm, corresponding to a 98% removal.     

A glassy carbon supported bilayer lipid-like membrane of 5,5-ditetradecyl-2-(2-trimethyl-ammonioethyl)-1,3-dioxane bromide for electrochemical sensing of epinephrine

A self-assembled bilayer lipid-like membrane (BLM) supported on glassy carbon electrode (GCE) was fabricated using 5,5-ditetradecyl-2-(2-trimethyl-ammonioethyl)-1,3-dioxane bromide (DTDB) for epinephrine (EP) determination in the presence of ascorbic acid (AA). This modified electrode (DTDB/GCE) has strong membrane adsorption accumulation and electrocatalytic ability toward EP and AA. The oxidation of EP was controlled by double step adsorption accumulation process of the DTDB-BLM. The parameters of fitted Langmuir isotherm Γ_max, B_ADS, and ΔG_ADS values were determined as 1.0×10⁻¹¹ mol cm⁻², 2.04×10⁶ dm³ mol⁻¹, and −45.17 kJ mol⁻¹ for the fist step for EP concentration less than 1 mM, and 4.92×10⁻¹¹ mol cm⁻², 7.35×10⁴ dm³ mol⁻¹, and −37.1 kJ mol⁻¹ for the second step for EP concentration higher than 1 μM. The DPV peaks for EP and AA oxidations were appeared at 0.220 and 0.085 V versus SCE, respectively, allowing the determination of EP in the presence of high concentration of AA. The advantage of DTDB-BLM was demonstrated experimentally in comparison with other three BLMs, and attributed to the dioxane group as well as the suitable length of the carbon chain of DTDB molecule. The current response of the DTDB/GCE was fast and reproducible, suitable for the electrochemical sensing in flow-injection systems. A linear range of 1×10⁻⁸ to 1×10⁻⁴ M EP was preliminary obtained using a simple setup.     

In situ immobilization of cobalt phthalocyanine on the mesoporous carbon ceramic SiO2/C prepared by the sol–gel process. Evaluation as an electrochemical sensor for oxalic acid

The mesoporous carbon ceramics SiO₂/20 wt% C (S_BET = 160 m² g⁻¹) and SiO₂/50 wt% C (S_BET = 170 m² g⁻¹), where C is graphite, were prepared by the sol–gel method. Scanning electron microscopy images and the respective element mapping showed that, within the magnification used, no phase segregation was detectable. The materials containing 20 and 50 wt% of C presented electric conductivities of 9.2 × 10⁻⁵ and 0.49 S cm⁻¹, respectively. These materials were used as matrices to support cobalt phthalocyanine (CoPc), prepared in situ on their surfaces, to assure homogeneous dispersion of the electroactive complex in the pores of both matrices. The surface densities of cobalt phthalocyanine on both matrix surfaces were 0.014 mol cm⁻² and 0.015 mol cm⁻² for materials containing 20 and 50 wt% of C, respectively. Pressed disk electrodes made with SiO₂/50 wt% C/CoPc and SiO₂/20 wt% C/CoPc were tested as sensors for oxalic acid. The electrode was chemically very stable and presented very high sensitivity for this analyte, with a limit of detection, LOD = 5.8 × 10⁻⁷ mol L⁻¹.