I. INTRODUCTION
Human activities now generate a broad range of organic pollutants. Many of these compounds are not biodegradable and therefore require physical-chemical treatments for complete degradation to CO2 and H2O, or for conversion to organic compounds that are biodegradable. 1,Reference Appenzeller and Tsatsakis2 Commonly, physical chemical processes for treatment of effluents involve the generation of hydroxyl radicals (HO˙). Reference Maldonado, Passarinho, Oller, Gernjak, Fernandez, Blanco and Malato3,Reference Wojnárovits and Takács4 In the case of the electrochemical oxidation of organic pollutants, the radicals are generated on the surface of an electroactive anode that enables the generation of weakly adsorbed HO˙ able to oxidize organic matter. Reference Kapałka, Fóti and Comninellis5 Among the anodes that provide excellent results in terms of electro-oxidation of organic molecules are those that have a high overpotential for the oxygen evolution reaction (OER). Boron-doped diamond (BDD), SnO2–Sb, and PbO2 anodes are suitable for this purpose. Reference Panizza and Cerisola6 However, although the BDD electrode is chemically stable and shows excellent results in the degradation of organic compounds, Reference Iniesta, Michaud, Panizza, Cerisola, Aldaz and Comninellis7–Reference Aquino, Rodrigo, Rocha-Filho, Sáez and Cañizares9 it is expensive, while the SnO2–Sb electrode becomes deactivated after a period of use. Reference Montilla, Morallo, De Battisti, Benedetti, Yamashita and Vázques10 On the other hand, the PbO2 electrode can be easily prepared by electrodeposition, is inexpensive to manufacture, and shows good electrochemical activity. It is stable at high potentials as well as in media with different pH values. Reference Panizza and Cerisola6,Reference Li, Pletcher and Walsh11,Reference Costa and Da Silva12
The electrochemical oxidation of organic pollutants offers a versatile alternative for the treatment of aqueous effluents; however, to make it viable for use in industrial oxidation of organic pollutants, it needs to aim at reducing capital and energy costs by optimizing the efficiency of degradation, while at the same time minimizing energy consumption. Reference Jüttner, Galla and Schmieder13,Reference Sirés, Brillas, Oturan, Rodrigo and Panizza14 Despite the advantages of electrochemical technology, its main disadvantage concerns the high consumption of energy resulting from low current efficiencies, Reference Kapałka, Fóti and Comninellis5,Reference Rajkumar, Guk Kim and Palanivelu15 which in turn are a direct consequence of mass transport restrictions caused by the low concentrations of organic compounds in effluents. Reference Polcaro, Vacca, Mascia, Palmas, Pompei and Laconi16–Reference Aquino, Rocha-Filho, Ruotolo, Bocchi and Biaggio18 In applications involving the electrochemical removal of metal ions present in dilute aqueous solutions, this problem can be circumvented by the use of three-dimensional cathodes. Reference Pletcher, Whyte, Walsh and Millington19,Reference Britto-Costa and Ruotolo20 In addition to high specific surface area, these cathodes also provide high interstitial velocities that generate turbulence and significantly increase the mass transfer coefficient. However, in the case of electrochemical systems for the treatment of organic compounds, the use of three-dimensional electrodes is often limited by the choice of porous substrate material to be used, which should be resistant to anodic corrosion and be easily coated by the deposition of electroactive material onto its surface.
Recio et al. Reference Recio, Herrasti, Sirés, Kulak, Bavykin, Ponce-de-León and Walsh21 reported the preparation of reticulated vitreous carbon (RVC) electrodes coated with PbO2 films, opening new perspectives for the application of three-dimensional anodes in effluent treatment. RVC is a highly suitable material for use in three-dimensional electrodes, and can serve as an excellent substrate for the electrodeposition of electroactive films, because its surface area can be as high as 6600 m2/m3 and it provides negligible pressure drop. It also offers excellent levels of chemical inertness and electrical conductivity. Reference Wang22,Reference Friedrich, Ponce-de-León, Reade and Walsh23
In previous work, Farinos et al. Reference Farinos, Zornitta and Ruotolo24 studied the electrodeposition of PbO2 films onto a 45 ppi RVC substrate using an electrochemical flow reactor. It was found that strict control of the synthesis conditions (current density and electrolysis time) was required to achieve a uniform adherent coating of β-PbO2. The use of an electrochemical flow reactor enables improved control of the current distribution in the porous electrode, as well as renewal of the solution containing lead ions within the reactor. As far as we know, there have been no reports in the literature concerning the use of an electrochemical flow reactor for the production of tridimensional RVC/PbO2 anodes. Given the substantial increases in the kinetics and the efficiency of the electro-oxidation process, compared to a flat electrode, as found in RB19 dye decolorization experiments, this study investigates the possibility of producing RVC/PbO2 electrodes on 60 and 80 ppi substrates. In addition to providing a greater specific surface area, the smaller pore sizes of these electrodes increase turbulence and the interstitial velocity, hence causing an even greater increase in the mass transfer coefficient and improving the kinetics and efficiency of the electro-oxidation process. The electrodes were prepared using different current densities and electrolysis times, and were characterized in terms of their structure, morphology, and coating uniformity of the substrate by the PbO2 film. The electrochemical characterization was carried out by linear scanning voltammetry and determination of the electroactive area. The electrocatalytic activity of the electrode was evaluated using the RB19 dye decolorization reaction.
II. EXPERIMENTAL
A. Materials
The PbO2 film on the RVC substrate was produced using an electrolyte containing 0.1 mol/L Pb(NO3)2 (Sigma-Aldrich, St. Louis, Missouri), 0.1 mol/L HNO3 (Synth), and 0.5 mg/L SLS surfactant. Reference Andrade, Ruotolo, Rocha-Filho, Bocchi, Biaggio, Iniesta, García-García and Montiel25 The temperature used was 65 °C. All solutions were prepared with deionized water.
The decoloration procedure used a solution containing 30 mg/L Reactive Blue 19 dye (RB 19) (Sigma-Aldrich) and 0.5 mol/L Na2SO4 (Qhemis) as supporting electrolyte.
The three-dimensional substrates used for deposition of the PbO2 film was RVC with porosities of 45, 60, and 80 ppi and dimensions of 0.5 cm (thickness) × 2.5 cm (width) × 2.5 cm (length) (Fig. S1). For both the production of the PbO2 films and for the experiments involving the decoloration of RB 19, a dimensionally stable Ti/Ti0.7Ru0.3O2 anode (De Nora do Brasil) was used as the current collector and an AISI 316 stainless steel plate was used as the counter electrode. Both electrodes had dimensions of 2.5 × 2.5 cm.
B. Experimental setup
The electrodeposition of the β-PbO2 film on RVC was performed using the experimental system shown in Fig. S2. The electrolyte circulated through the electrochemical reactor in a closed system, driven by a peristaltic pump (BP 200D, Tecnopon, Piracicaba, Brazil). A current source (3003D, Minipa, São Paulo, Brazil) was used to supply a constant electric current to the reactor. A multimeter was connected to the reactor to read the cell potential, and an ultra-thermostatic bath (SL-152, Solab, Piracicaba, Brazil) maintained a constant electrolyte temperature. A magnetic stirrer was used to ensure that the electrolyte solution remained homogeneous during the experiment.
The same electrochemical reactor was used in the study of decoloration of the RB 19 dye, but in this case, the RVC/PbO2 electrode had dimensions of 2.5 × 1.0 × 0.5 cm. The experimental device was composed of the components shown in Fig. S3. The solution containing the dye circulated through a flow cuvette coupled to a UV/Vis spectrophotometer (Ultrospec 2100 Pro, Amersham Pharmacia, Buckinghamshire, United Kingdom), and absorbance measurements were recorded in real time.
C. Electrochemical reactor
Figure S4 shows a detailed view of the flow reactor used for preparation of the RVC/PbO2, which consisted of juxtaposed rectangular acrylic plates, sealed with coatings of silicone and assembled using nuts and bolts. The current collector was built into the left-hand plate shown in Fig. S4. The counter electrode was covered with a polyethylene mesh coated with polyamide fabric (5) to avoid short circuit of the system. The counter electrode was built into the right-hand plate (6). The electrochemical reactor used for decoloration of the RB 19 dye had the same characteristics, but the central plate had a smaller volume to accommodate an electrode with dimensions of 2.5 × 1.0 × 0.5 cm.
D. Experimental procedures
The RVC substrates were first immersed for 10 min in a 3:1 (v/v) solution of sulfuric acid and hydrogen peroxide, washed with deionized water and dried at room temperature. This pretreatment was used to increase the hydrophilicity of the RVC surface. Reference Dalmolin, Biaggio, Rocha-Filho and Bocchi26
The PbO2 films were produced under different experimental conditions of current density (j), calculated based on the specific surface areas for the RVC of 45, 60, and 80 ppi (28.5, 38.6, and 52.1 m2/m3, respectively) provided by the manufacturer (Electrosynthesis Company, USA). The electrodeposition was performed using the system and flow reactor shown in Figs. S2 and S4, respectively. The experimental procedure involved the galvanostatic electrodeposition of PbO2, according to the experimental conditions: 0.7 and 3.5 mA/cm2 for RVC 45 ppi; 0.7, 3.5, and 5.0 mA/cm2 for RVC 60 and 80 ppi.
The temperature and flow rate of the electrolyte were kept at 65 °C and 25 mL/min, respectively. The volume of electrolyte was 80 mL. The electrolyte was pumped by the peristaltic pump into the base of the reactor and then returned to the reservoir. After the pre-established electrodeposition time, the position of the RVC in the reactor was inverted and submitted to another 30 min of electrodeposition to try to ensure that both sides of the electrode were covered. Therefore, the total electrodeposition time was 60 min.
The electrocatalytic activities of the electrodes were evaluated in the decoloration of RB 19, performed in the same flow reactor shown in Fig. S4. The flow rate, current density (i), and temperature were kept constant at 25 mL/min, 10 mA/cm2, and 30 °C, respectively. The experimental procedure consisted of circulating the electrolyte containing the dye through the electrochemical reactor and applying the desired current density. The electrolyte exited the reactor and then passed through a flow cuvette, where the absorbance of the solution (at 600 nm) was measured every 30 s. The experimental setup used in the decoloration experiments is shown in Fig. S3.
The plots of the normalized absorbance (A/A 0) as a function of time t were fitted using a pseudo-first order kinetic model [Eq. (1)], and the values of the kinetic constants (k d) were determined by means of exponential regression using the Levenberg–Marquardt method.
$$\ln \left( {{A \over {{A_0}}}} \right) = - {k_{\rm{d}}}t$$
E. Characterization of the RVC/PbO2 electrode
Analysis of the coating of the RVC substrates achieved by electrodeposition of PbO2 was performed using a scanning electron microscope (XL30 FEG, Philips, Tustin, California). A schematic illustration of the surface of the electrode used in the microscopy analyses is shown in Fig. S5. The thickness of the PbO2 film was measured in a distance of 0.5 mm from the substrate edge.
Characterization of the structures of the films produced was performed by triturating the RVC/PbO2 electrodes followed by analysis using a Siemens D5005 X-ray diffractometer (Bruker Corporation, Madison, Wisconsin). The scan rate was 2°/min in the range 20° ≤ 2θ ≤ 80°. The diffractograms obtained were compared to JCPDS card standards.
The electrochemical characterization of the RVC/PbO2 electrodes was performed in Na2SO4 0.1 mol/L electrolyte at 25 °C. Linear sweeping voltammetries were recorded at 50 mV/s using a potentiostat Autolab® PGSTAT 30 (Metrohm Autolab, Utrecht, The Netherlands). The reference electrode was Ag/AgCl KCl 3.0 mol/L.
The electrochemically active area was determined using Cottrell's equation [Eq. (2)], which describes the behavior of the electric current when a potential step is applied to the electrode in the region where the reaction is controlled by mass transport. Reference Bard and Faulkner27
$${I_t} = {{nF{\rm{EAA}}{C^*}\sqrt {{D_0}} } \over {\sqrt {\pi t} }} = B\sqrt t \quad .$$
In Eq. (2), I t is the current (in A) at a given time t, n is the number of electrons exchanged in the oxidation or reduction reaction, F is the Faraday constant (96,485 As), EAA is the electrochemically active area of the electrode (in cm2), C* is the concentration of the electroactive species ([Fe3(CN)6]3− was used at a concentration of 1.0 × 10−6 mol/cm3), and D 0 is the diffusion coefficient of the oxidized species (3.93 × 10−5 cm2/s).
The EAA values of the 45, 60, and 80 ppi RVC uncovered and covered by PbO2 films were measured by immersing a 0.5 cm3 (1.0 × 1.0 × 0.5 cm) of the electrode in a solution containing 0.1 mol/L of potassium ferricyanide and potassium chloride, adjusted to pH 3. A platinum plate (2.5 cm2) was used as the counter electrode. For each electrode, chronoamperometry was performed for 50 s. During the first 15 s of this analysis, application of a potential of 0 V versus Ag/AgCl allowed formation of the electrical double layer. Subsequently, a potential of 1.0 V versus Ag/AgCl was applied, which was monitored until the end of the established period. This procedure was repeated five times.
The results showed an exponential decay of the current as a function of time. These results were then linearized based on Eq. (2). The EAA values were then determined from the angular coefficients (B) of the plots of I t as a function of t 1/2.
III. RESULTS AND DISCUSSION
A. Influence of current density on coating of the substrate
A systematic procedure was followed to identify the best current density for formation of a uniform film of β-PbO2 on the RVC substrates with different porosities, using different current densities while maintaining constant conditions of flow rate (30 mL/min) and temperature (65 °C). The electrodeposition time was kept at 30 min (each side), and a current density of 0.7 mA/cm2 was initially applied, based on the results obtained previously for a 45 ppi electrode, since this was the value of j that provided the best coatings. Reference Farinos, Zornitta and Ruotolo24
Figure 1 shows SEM micrographs of the electrodes obtained after deposition of the PbO2 film onto the substrates with porosities of 45, 60, and 80 ppi. It was found that application of a current density of 0.7 mA/cm2 was not sufficient for complete coating of the 60 and 80 ppi substrates, with the least efficient coating for the 80 ppi electrode. There was a change in the morphology of the film when lower porosity electrodes were used, with the PbO2 film formed on the 45 ppi substrate having a compact morphology and a thickness of ∼10 μm [Fig. 1(b)], while the film deposited on the 60 ppi substrate was thinner (∼1.4 μm) with larger crystallite [Fig. 1(d)]. The 80 ppi electrode showed no film formation, with only a few small particles of PbO2 present in the center of the electrode [Fig. 1(e)], while at its extremities there was the formation of agglomerates with needle type morphology [Fig. 1(f)], due to the effect of the uneven current distribution. Reference Gaunand and Coeuret28–Reference Ruotolo and Gubulin32
SEM images of the PbO2 films electrodeposited onto the RVC substrates: 45 ppi (a and b), 60 ppi (c and d), and 80 ppi (e and f). j = 0.7 mA/cm2 (60 min).
Since the 60 and 80 ppi substrates were not fully coated with the PbO2 film, the value of j was increased to 2.5 mA/cm2. However, although there was an improvement in the coating of the 60 ppi electrode, this current density was not sufficient for a complete covering of either the 60 ppi or the 80 ppi substrate. The value of j was therefore further increased to 3.5 mA/cm2. The results are shown in Fig. 2. For comparison purposes, electrodeposition was also performed onto the 45 ppi substrate, and it can be seen that there was deposition of a thicker film at the extremities of the electrode, but without complete coverage of the substrate in the central region [Fig. 2(a)]. On the other hand, application of 3.5 mA/cm2 was sufficient for complete coverage of the 60 ppi substrate [Fig. 2(c)], while the 80 ppi substrate showed almost no formation of PbO2 film, with only a few nuclei present in the center of the electrode [Fig. 2(f)], together with agglomerates in regions close to the counter electrode [Fig. 2(e)], similar to the results obtained using applied current densities of 0.7 and 2.5 mA/cm2.
SEM images of the PbO2 films electrodeposited onto the RVC substrates: 45 ppi (a and b), 60 ppi (c and d), and 80 ppi (e and f). j = 3.5 mA/cm2 (60 min). Inset of (b): photograph of the electrode surface.
These results showed that in all cases it was possible to electrodeposit PbO2. However, to obtain films that were compact, homogeneous, and uniformly distributed on the surface of the substrate, it was necessary to optimize the applied current density. Nonuniform PbO2 film thickness and incomplete coating of the substrate could be explained by the effect of electric field penetration, because porous electrodes show uneven distributions of current and potential in their interiors. Reference Gaunand and Coeuret28–Reference Martins, Britto-Costa and Ruotolo34
Since a current density of 3.5 mA/cm2 provided the best conditions for coating the 60 ppi substrate, the film formed was characterized using x-ray analysis. The x-ray diffraction pattern obtained is shown in Fig. S6(a) as compared with that obtained for the PbO2 film deposited onto the 45 ppi substrate [Fig. S6(b)]. For the 60 ppi electrode, peaks only occurred at 2θ values attributed to the β-PbO2 phase, according to the JCPDS 41–1492 crystallographic data card. This phase is desirable because it is more electroactive in the degradation of organic compounds. Reference Li, Pletcher and Walsh11 These results are in agreement with the findings of Bemelmans et al., Reference Bemelmans, Okeef and Cole35 who showed that the addition of sodium lauryl sulfate (SLS) to the acidic electrolyte used in preparation of the PbO2 films inhibited growth of the α phase and favored formation of the β phase. On the other hand, the x-ray diffraction pattern of the 45 ppi electrode revealed the presence of β-PbO peaks, indicating that the electrodeposition using a very high current density could lead to the formation of undesirable oxides. In fact, after the synthesis process, it could be seen that a yellowish film had been formed on the RVC surface close to the current collector [inset of Fig. 2(b)]. A possible explanation for the PbO formation is that the PbO2 formed on the outer surface close to the counter electrode was reduced to PbO when the position of the electrode was inverted (see Sec. II. D). In this case, due to the very irregular potential distribution in three-dimensional electrodes, the potentials close to the current collector would provide the conditions for the PbO2 reduction, but it is important to highlight that the mechanism of PbO formation is still not clear and it has been investigated in our group.
To identify the optimal condition for the electrodeposition of β-PbO2 onto the 80 ppi substrate, electrolyses were performed applying 5.0 mA/cm2. For comparison purpose, the electrodeposition onto the 60 ppi substrate was also performed using this value of j. Photographs of the electrodes obtained (Fig. S7) revealed that in the case of the 60 ppi RVC/PbO2 electrode, there was the formation of a small quantity of deposit, similar to that observed for the 45 ppi RVC/PbO2 electrode obtained by applying 3.5 mA/cm2. On the other hand, the formation of the yellowish electrodeposit occurred with even greater intensity when the 80 ppi substrate was used [Fig. S7(b)]. The formation of a new compound was also evidenced by a change in morphology, as can be seen in the micrograph of Fig. 3(f). This compound was again identified by XRD (not shown) as being β-PbO.
SEM images of the PbO2 film electrodeposited onto the RVC substrates: 60 ppi (a and b) and 80 ppi (c–f). j = 5.0 mA/cm2 (60 min).
Figure 3(a) shows a cross-section micrograph of the 60 ppi electrode. The electrodeposit had a thickness of ∼22 µm, as estimated from Fig. 3(b). There appeared to be a complete covering of the substrate, although due to its thickness, the PbO2 film was easily detached from the substrate surface when the electrode was cut to place it in the sample holder. Regions of the substrate can be seen in Fig. 3(a) not due to lack of a coating, but due to breakage and detachment of the film. Detachment of the film also occurred during the course of the electrochemical characterization; after the voltammetry, pieces of PbO2 film were present at the base of the electrochemical cell. Hence, the application of 5.0 mA/cm2 was not suitable for deposition of PbO2 onto the 60 ppi substrate. In the case of the 80 ppi substrate, there was incomplete coverage of the substrate surface, as well as deterioration of the RVC, as shown in Fig. 3(e), where cracks can be seen in the substrate surface. Furthermore, the 80 ppi electrode crumbled when it was removed from the reactor. There was also a change in the morphology of the electrodeposit, which could be attributed to the formation of PbO, as discussed previously.
In a final attempt to obtain a uniform deposit of PbO2 on the 80 ppi substrate, the applied current density was lowered to 4.0 mA/cm2, but the results obtained were similar to those shown in Fig. 2. Another option to achieve coating of the 80 ppi substrate would be to increase the electrodeposition time, with application of 3.5 mA/cm2. However, given the findings of the previous experiments, although the increased time might result in complete coating, there would also be an excessive increase in film thickness at the extremities of the substrate, which could cause mechanical instability of the film, resulting in its detachment during the electrolysis.
Analysis of all the results for the electrodeposition of PbO2 films onto the RVC substrates with porosities of 45, 60, and 80 ppi suggested that decreasing the porosity of the electrode by increasing the number of pores per inch (ppi), caused a substantial change in the charge distribution within the porous electrode. In light of the present findings, and considering the available literature on porous electrodes, Reference Gaunand and Coeuret28–Reference Martins, Britto-Costa and Ruotolo34 an electrode potential distribution model is proposed, as shown schematically in Fig. 4. According to the model, a decrease in porosity (ε) leads to increasingly anodic potentials at the extremity of the electrode close to the counter electrode, as well as more pronounced potential and current profiles. The SEM images corroborated this model, because when the current density used for the 45 ppi substrate was increased from 0.7 to 3.5 mA/cm2, a much thicker film was formed at the extremity of the electrode, while the center remained without the coating that was achieved when a lower current density was applied. Reference Farinos, Zornitta and Ruotolo24 The same was observed for the 60 ppi electrode when the j value was increased from 3.5 to 5.0 mA/cm2. The proposed model is also corroborated by results obtained for the 80 ppi electrode where increase, of j from 3.5 to 5.0 mA/cm2 not only caused the formation of other compounds close to the counter electrode after electrode inversion, but also resulted in deterioration of the substrate, without full coverage of the substrate, which confirmed the existence of high anodic current and potential in the region near the counter electrode.
Schematic representation of the proposed model for the electrode potential distribution as a function of the applied current and the electrode porosity.
In summary, the optimal current density values for formation of uniform adherent β-PbO2 films on the 45 and 60 ppi substrates were 0.7 and 3.5 mA/cm2, respectively. No electrodeposition condition was identified that enabled the formation of a uniform adherent film on the 80 ppi substrate.
B. Electrochemical characterization
In voltammetry performed using the flat electrodes (Fig. S8), the presence of the PbO2 film caused a shift in the OER potential from approximately 1.6 to 1.1 V. Despite showing the highest OER potential, the vitreous carbon electrode was not a good electrocatalyst for the degradation of organic compounds, as will be shown below. Comparison of the voltammetric data for VC and RVC (Fig. S8) revealed that there was an increase of i due to the increased specific surface area provided by the porous matrix. In the case of the RVC, the OER potential was slightly shifted to a less positive value (∼1.2 V), without the presence of the capacitive current observed for VC. This behavior could be attributed to the uneven potential and current distributions of the three-dimensional electrode. Since the reference electrode was located near the current collector, the potential recorded in the voltammogram corresponded to the potential in this region. However, as the potential was not uniform, the potentials in the counter electrode region were indeed more positive, Reference Ruotolo and Gubulin33 which would then explain why the OER occurred at potentials lower than those obtained using the flat electrode.
It was observed that for all the values of j applied, there was an increase in the current and a shift of the OER potential after deposition of the PbO2 film onto the different porosity RVC substrates (Fig. 5), which could be considered an indirect measure of the coating efficiency and the electrocatalytic activity of the electrode. The results of the voltammetry measurements were in agreement with the coatings observed in the SEM images. In the case of the electrodes produced by applying 0.7 mA/cm2, the 45 ppi RVC/PbO2 electrode showed i values that were substantially higher than those obtained using the 60 and 80 ppi electrodes, for which the electroactive film coverage was much lower (Fig. 1). When the value of j was increased to 3.5 mA/cm2 [Fig. 5(b)], there was a reduction of the current for the 45 ppi RVC/PbO2 electrode, which could be attributed to the decreased substrate coating and the formation of PbO, as shown in Figs. 3(a) and 5(b), respectively. In this case, as there was better coating of the 60 ppi substrate, there were also increase in the i values for this electrode. These values were slightly lower than those observed for the 80 ppi electrode, which despite having an incomplete coating, presented a greater surface area. Finally, when 5.0 mA/cm2 was applied [Fig. 5(c)], there were large decreases in the electrocatalytic activities of the 60 and 80 ppi electrodes, which could be mainly attributed to the formation of PbO and loss of physical integrity of the substrate [Figs. 3(e) and 3(f)].
Voltammograms obtained using RVC/PbO2 electrodes produced by applying different current densities: (a) 0.7 mA/cm2; (b) 3.5 mA/cm2; (c) 5.0 mA/cm2. Conditions: 0.1 mol/L Na2SO4; ν = 50 mV/s.
Table I provides the values of the electrochemically active areas (EAA) of the 45 and 60 ppi RVC electrodes with and without PbO2 film coatings, for the cases where the substrates were completely covered. For both electrodes, the presence of the film resulted in an increase in the EAA, although the increase was approximately 50% smaller in the case of the 60 ppi electrode. In terms of EAA, the 45 and 60 ppi RVC/PbO2 electrodes were almost identical. However, an additional consideration is that in applications such as flow electrolysis, where the mass transfer coefficient has a substantial influence on the kinetics and current efficiency of the process, the hydrodynamic turbulence resulting from use of the electrode with porosity of 60 ppi could lead to a large increase in process efficiency. In fact, as will be shown in Sec. III. C, there was a clear improvement in decolorization of the RB 19 dye when the 60 ppi electrode was used.
EAA values for the RVC and RVC/PbO2 electrodes with different porosities.
C. Decolorization of RB 19 dye
After synthesis of the β-PbO2 films on the substrates with different porosities, the electrocatalytic activities of the electrodes with the best coatings were evaluated in decolorization experiments employing RB 19 dye. The decolorization results, expressed in terms of the normalized absorbance as a function of time, are shown in Fig. 6, where it can be seen that coating with the PbO2 film resulted in a significant increase in the reaction kinetics, compared to the use of RVC alone. In all cases, the reaction followed pseudo-first order kinetics, so the decolorization rate constants, k d, [Fig. 6(b)] were determined by means of an exponential regression.
(a) Normalized absorbance as a function of time for the decolorization of RB19 dye using the RVC and RVC/PbO2 electrodes; (b) pseudo first order constant of the decolorization for the VC and RVC electrodes with and without PbO2 film coatings. i = 10 mA/cm2.
As expected, the performance of the tridimensional RVC/PbO2 electrode was superior to that of the flat electrode, once again showing that the tridimensional matrix not only provided a higher specific surface area, but also promoted an increase in mass transfer, due to the greater hydrodynamic turbulence. The increase in the electrode pore number from 45 to 60 ppi (corresponding to a decrease in porosity) led to a significant increase (∼70%) in the kinetics, which could be attributed to the greater surface area as well as a decrease in the hydrodynamic boundary layer due to greater turbulence and the higher interstitial velocity resulting from the narrower pores of the 60 ppi electrode. Meanwhile, comparison of the 60 and 80 ppi RVC/PbO2 electrodes showed that the k d value was lower for the 80 ppi electrode, which can be attributed to the uneven coating of the substrate, which probably resulted in a smaller EAA (not measured) than found for the 60 ppi RVC/PbO2 electrode, although the EAA was still higher than the value obtained for the 45 ppi RVC/PbO2 electrode. It could therefore be concluded that among the electrodes studied, the 60 ppi RVC/PbO2 electrode prepared by applying 3.5 mA/cm2 presented the best results in terms of the coating, the EAA, and the electrocatalytic activity. Furthermore, based on the results for the electrodeposition of the PbO2 film, the voltammetry measurements, and the decolorization of RB 19, there was clear evidence of the effect on the EAA of the distribution of potential and current within the porous electrode. Considering that the most electrochemically active region was concentrated in the region near the counter electrode, it can be proposed to replace the electrochemical reactor configuration used in this work [Fig. S9(a)] by the configuration shown in Fig. S9(b). In the latter case, the use of two counter electrodes would provide better utilization of the surface area of the tridimensional electrode, since it would enable a better distribution of potential and current, with values of E d higher than the minimum necessary for the reaction of interest, across the entire electrode.
IV. CONCLUSIONS
The electrodeposition of PbO2 films onto RVC substrates with low porosities was only possible after optimization of the applied current density. The potential distribution, which was typical of porous electrodes, led to a deposit with nonuniform film thickness and uneven coating of the substrate. The current density used for a particular substrate must be adjusted to ensure complete coating of the surface, while avoiding excessively thick films at the extremities of the electrode near to the counter electrode, which could cause the film to become fragile and be lost to the solution. The use of very high j values should also be avoided, because this could lead to deterioration of the vitreous carbon substrate. The optimal j values were 0.7 and 3.5 mA/cm2 for the 45 and 60 ppi substrates, respectively. In the case of the 80 ppi substrate, it was not possible to identify suitable conditions for production of an electrode with the desired characteristics. The use of the RVC/PbO2 electrode enabled an average 2.5-fold increase in the kinetics of decolorization of RB 19 dye, compared to the use of RVC alone. On the other hand, using the 45 and 60 ppi electrodes, a decrease in pore size resulted in a 1.6-fold increase in the decolorization kinetics, demonstrating that the increases in specific surface area and the turbulence generated by the porous matrix led to improved mass transfer and greater efficiency in the electrooxidation of organic compounds present at low concentrations.
Supplementary Material
To view supplementary material for this article, please visit http://dx.doi.org/10.1557/jmr.2016.354.
ACKNOWLEDGMENTS
The authors are grateful to FAPESP, Process 2012/04168-0, for financial support. R.M.F. thanks CNPq for the PhD fellowship. The authors are indebted to Rafael L. Zornitta for the EAA measurements.
