Synthesis and application of gas diffusion cathodes in an advanced type of undivided electrochemical cell

This paper reports the oxidation of Remazol black B dye by employing iron ions catalyst based gas diffusion cathodes, (GDCs). A GDC was synthesized by using a layer of carbon black and iron ions catalyst for oxygen reduction to hydrogen peroxide. The results demonstrated around 97% decolorization of Remazol black-B dye for 50 min by iron ions catalyst based GDC. The degradation study was performed under electrogenerated hydrogen peroxide at a constant voltage of − 0.6 V vs Hg/HgSO4 in which the rate of degradation was correlated with hydrogen peroxide production. Overall, the GDC’s found to be effective method to degrade the dyes via electro-Fenton.

Synthesis of gas diffusion electrodes. The improved gas diffusion cathodes (GDCs) were made by adopting method previously reported in the literature 34 . Similarly, GDC comprised of catalytic carbon and Ni mesh for current collector. A paste for catalyst layer was produced from 25 mg carbon black and 125 mg FeCl 2 . Ultrasonic bath (XUBA3, Grant Instruments, UK) was used to sonicate the mixture of carbon black and FeCl 2 for 10 min in 5 wt% Nafion ® from Alfa Aesar, UK (D-520 dispersion). The resulting sonicated paste of Nafion, carbon black, and FeCl 2 was rolled over 40 mm × 40 mm × 0.4 mm on a piece of carbon cloth pre-treated with 25 wt% PTFE as shown in Fig. 1. The catalyst loading was found around 5 mg cm −2 on the electrode. Finally, electrode hot-pressed by using Carver 3851-0 Hydraulic Press (Cole Palmer, UK) for 2 min at 140 °C under a pressure of 5 MPa by arranging a 0.02 mm thick nickel mesh (current collector) as a top cover for the layered carbon cloth with carbon black and FeCl 2 layer. The GDC electrodes were recovered from the hydraulic press and allowed to spontaneously cool down in air for 10 min.
Characterization of GDC. The characterization of the as prepared GDC electrodes was conducted by employing JEOL JSM-6500F field emission scanning electron microscope (FESEM) at an accelerating voltage of 10-15 kV. The surface morphology of the carbon black and iron ions catalyst layer carbon cloth and nickel mesh layer were observed by using FESEM.
Electrochemical study. The electrochemical experiments were carried out as our previous studies 34 in an undivided advanced type of three electrode electrochemical cell (Fig. 1) by using high-end potentiostat/ galvanostat PGSTAT302 N (Metrohm Autolab B.V., Utrecht, Netherlands) and analysed by Nova 1.11. A 0.80 cm 2 working electrode made from iron ions modified GDC was deployed. Meanwhile, Pt mesh was employed as counter electrode with SCE (i.e., Hg/HgSO 4 ) as reference electrode. A 0.15 L solution containing 17 ppm of Remazol black-B dye was made in 0.5 mol L 3 of Na 2 SO 4 solution as background electrolyte. The electrolysis was conducted at − 0.6 V and − 0.7 V vs. Hg/HgSO 4 in an undivided electrochemical batch reactor with oxygen supply (flow rate = 0.1 dm 3 min −1 ) across the gas diffusion electrodes.
A UV-Vis spectrophotometer Hitachi U3010 (Abingdon, UK) was used to measure the absorbance at 597 nm as the peak of Remazol black-B dye. Beer-Lambert law was applied for calibration and calculation of the Remazol black-B concentration. Further, demineralization studies of the Remazol black-B dye were performed using Shimadzu TOC-V CPH TOC detector. Using the standard nonpurgeable organic carbon (NPOC) analysis, TOC values were measured.

Results and discussion
Surface characterization of GDC electrodes. Figure 2A-D exhibits the representative images of the gas diffusion electrodes taken by using FESEM. Figure 2A shows the carbon and catalyst layer, nickel mesh and carbon cloth. Figure 2B depicts the side view of the carbon cloth fibers. Figure 2C shows the top view of the GDC electrodes across the nickel mesh which shows the carbon layer and catalyst layer over the surface of carbon cloth. Figure 2D represents the top view of the carbon and catalyst paste over the carbon cloth. It can be seen that the layer of carbon black and iron ions catalyst were successfully adhered over the carbon cloth surface. The  Polarization curve studies for the ORR. The GDC electrodes were mounted in an advanced type of electrochemical batch cell ( Fig. 1) to conduct the polarization studies for oxygen reduction reaction (ORR). The experiments were conducted in a 0.5 mol dm −3 Na 2 SO 4 at pH 3 by feeding pure oxygen at 0.1 dm 3 min −1 across gas diffusion electrodes at scan rate of 10 mVs −1 in range of − 0.1 to − 1.0 V vs. Hg/HgSO 4 . The dissolved oxygen supplied across the GDC electrodes was electrochemically transformed into hydrogen peroxide via the (ORR) as shown in Eq. (3). However, it has been reported in the literature studies related to ORR that in this process, two side competing reactions also occurs as a secondary reaction 37 . These two side reactions are shown in Eq. (4) i.e. reduction of hydrogen peroxide to water and hydrogen gas evolution as depicted in Eq. (5): Figure 3 showed when the potential was sweep at a scan rate of 10 mVs −1 , a steep rise in cathodic current was observed after − 0.5 V vs. Hg/HgSO 4 . The cathodic polarization curve indicates that the ORR takes place in potential range of − 0.3 to − 0.6 V vs. Hg/HgSO 4 , this corresponding region for limiting current indicates the optimal cathodic region for the oxygen reduction to hydrogen peroxide. When the potential was sweep and continues to rise above − 0.6 V vs. Hg/HgSO 4 , the current increases steadily which representing the region of the competing side reactions as indicated by Eqs. (4) and (5).
Hydrogen peroxide concentration accumulation during electrolysis. The generation of hydrogen peroxide during ORR is the most important step during the electroFenton process. The constant potential studies were conducted to measure the accumulation of hydrogen peroxide. The studies were carried out in the  Fig. 4. It can be seen that the concentration of hydrogen peroxide was increased as the cathode potential rise from − 0.3 to − 0.6 V vs. Hg/HgSO 4 . The results seem to be in agreement with the polarization curve as shown in Fig. 3. So, for the decolorization studies of the Remazol black-B dye the electrolysis at − 0.6 V vs. Hg/ HgSO 4 seems to be desirable for electro Fenton process.

Decolorization of Remazol black-B by using iron ions based GDC electrodes.
After the successfully generating hydrogen peroxide at − 0.6 V vs. Hg/HgSO 4 , the decolorization of Remazol black-B dye was performed by applying constant potential electrolysis at − 0.6 V vs. Hg/HgSO 4 . The oxidation of Reactive Black 5 dye was assisted by powerful oxidizing agent hydroxyl radicals created from the breakup of electrogenerated H 2 O 2 in the presence of Fe 3+ and propagated by Fe 2+ ions over the surface of GDC electrodes. The generation of hydroxyl radical mechanism at the Fe 2+ and Fe 3+ based GDC electrodes is depicted in Eqs. (6) and (7) 38 ;   Figure 5 shows the spectrum obtained from UV spectrophotometer which displayed the absorbance vs. wavelength of the samples withdrawn during electrolysis studies for the decolorization of Remazol black-B dye at a constant potential of − 0.7 V vs. Hg/HgSO 4 . Initially, the concentration of dye was 17 mg dm −3 . The dye spectra at this point was represented by continuous line having 2 absorption peaks, which is the visible spectrum region at 597 nm and the UV spectrum region at 311 nm. The absorbance at a wavelength of 597 nm is related with the azo group (N=N) of Remazol black-B while at a wavelength of 311 nm is related with the aromatic groups 34 . It can be seen in that after 40 min of electrolysis was sufficient to eliminate the peaks related to azo group and aromatic groups in the spectrum of absorbance vs. wavelength.
In Fig. 6, the effect of temperature and pH on the hydrogen peroxide production was recorded at − 0.7 V vs. Hg/HgSO 4 to find the optimum condition. It was found that the production of hydrogen peroxide was optimum at 20 °C in pH 3 solution. High temperature, 40 °C, and high pH (pH > 5) may degrade the hydrogen peroxide hence less concentration was detected 39,40 . The hydrogen peroxide decomposed into oxygen and hydrogen in which the reaction was highly influenced by temperature, pH, and the use of catalyst. It was also reported that most of the commercial product was kept below pH 5 to preserve the hydrogen peroxide 41 . Figure 7 displayed the impact of the electrode potential on the decolorization of Remazol black B during the constant potential electrolysis of electrolyte containing 17 mg dm −3 of Remazol black B and 0.5 mol dm −3 Na 2 SO 4 at pH 3. The results showed that the decolorization studies at two potentials − 0.6 and − 0.7 V vs. Hg/ HgSO 4 . The impact of the applied potential on the degradation of Remazol black B is due to the oxygen reduction of hydrogen peroxide species in which the iron ions assist the well-known electro Fenton reaction 34,37 by promoting powerful · OH radical as represented in Eq. (4).
The Fig. 7 depicted that the dye oxidation seems to be 97% after 50 min at the electrode potential of − 0.6 V vs. Hg/HgSO 4 . This behaviour is in the agreement with polarization curves results presented in Fig. 3 which shows the region of highest hydrogen peroxide at − 0.6 V vs. Hg/HgSO 4 . When the decolorization studies were carried out at − 0.7 V vs. Hg/HgSO 4 , it was observed that the decolorization decreased which obviously means the evolution of side reactions which competes with the oxygen reduction reaction. Since, the electrode was coated with the immobilized iron ions which initiate the classical electro Fenton reaction producing the · OH radical which attacks the Remazol black-B to breakdown into intermediates. The comparison of the rate constant at different electrode potential displayed that using − 0.6 V vs. Hg/HgSO 4 oxidized 97% of the Remazol black-B dye. Some studies related to photo electro Fenton studies exhibited 70% conversion of RB-5 dye 42 which seems to be quite lower in comparison to these studies as represented in Table1. Notice that the colour removal of the dye decreased by increasing the applied voltage to − 0.7 V vs. Hg/HgSO 4 which caused by the competing secondary reactions occurred over the electrode surface.

Demineralization of Remazol black-B by using iron ions based GDC electrodes.
The TOC abatement studies of Remazol black B dye was conducted to account for the demineralization of dye. For this study, constant potentials at − 0.6 and − 0.7 V vs. Hg/HgSO 4 were opted for electro Fenton demineralization of Remazol black B dye. The results are presented in the Fig. 8, it can be observed that degradation of Remazol black-B dye Hg/HgSO 4 as seen in Fig. 8. Demineralization was decreased while applying potentials at − 0.6 V vs. Hg/HgSO 4 . This represented that the application of greater constant potential (− 0.7 V vs. Hg/HgSO 4 ) demineralized about 88% of the total organic carbon content present in the solution of Remazol black-B dye as shown in Fig. 8.
The proposed route for the degradation pathway of Remazol black-B is shown in Fig. 9. The proposed route was associated with the breaking of the aromatic heterocyclic chain (azo bond) of Remazol black-B producing transitory intermediates. The intermediates were further converted into benzene amino sulphonic acid in the presence of the · OH radical by removing R = (CH 2 ) 2 OSO 3 Na 38 . The addition of the · OH radical to amino benzene sulphonic acid resulted in the generation of phenol molecule by removing the NH 2 group. Finally, Remazol black-B dye was transformed into aliphatic acidic compounds, water and carbon dioxide. Current efficiency and energy consumption for the demineralization of Remazol black-B dye using iron ions based GDC electrodes. The ratio of electric energy employed for electrochemical decomposition reaction to total current passed through the electrochemical process is known as current efficiency.  The current efficiency is influenced by TOC abatement analysis, time of electrochemical reaction and total current utilized. Figure 10 shows the current efficiency versus electrolysis time. At − 0.6 V vs. Hg/HgSO 4 , the current efficiency obtained after 30 min of electrolysis reaction was about 60% and gradually decreased with time to 34% as calculated by using Eq. (8). At − 0.7 V vs. Hg/HgSO 4 , the highest current efficiency after 30 min of constant potential electrolysis was about 17% and gradually decrease to 3% at the end of electrolysis. The decreasing trends in CE during the electrolysis was due to the difficulty to degrade more aromatic compounds into inorganic compounds, which ultimately produce CO 2 . However, the appearance of side reactions like hydrogen evolution may lead to less current efficiency at higher applied potential.
The energy consumption, EC, (kWh kg −1 TOC) for the electrolysis process was calculated by the equation given below 34 ; where E (V) is the potential at which electrolysis reaction takes place, I o (A) is the current detected during electrolysis reaction. Meanwhile, time of electrolysis is denoted as ∆t (h). (TOC 0 ) and (TOC f ) are the TOC (kg dm −3 )  Fig. 11. Note that the energy consumption for decomposition of Remazol black B dye decreased as applied constant potential decreased to − 0.6 V vs. Hg/HgSO 4 .  www.nature.com/scientificreports/

Conclusions
In this work an improved GDC electrodes has been produced by rolling a layer of C black with iron ion species for the oxidation of Remazol black B dye. The polarization studies showed − 0.6 V vs. Hg/HgSO 4 founds to be feasible for the hydrogen peroxide production. It was demonstrated that 25 mmol dm −3 hydrogen peroxide was produced by ORR in an advanced type of undivided cell by applying an electrode potential of − 0.6 V vs. Hg/HgSO 4 . GDC containing iron ions can effectively oxidize the Remazol black-B dye with 99% decolorization and demineralization of 88%. The current efficiency (CE) was found to be 17% after 30 min of electrolysis at − 0.7 V vs. Hg/HgSO 4 . The energy consumption (EC) was 157 kWh kg −1 TOC by applying constant potential difference of − 0.7 V vs.
Hg/HgSO 4. The results suggested the significance of immobilized iron ions over GDC electrodes for the removal of toxic Remazol black-B dye in an advanced type of 3 electrode electrochemical batch cell.