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Novel Lead dioxide-Graphite-Polymer composite anode for electrochemical chlorine generation
Electrochimica Acta 169 (2015) 109–116
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Novel Lead dioxide-Graphite-Polymer composite anode for electrochemical chlorine generation Nitin Gedam a,b , Nageswara Rao Neti b, * , Martin Kormunda c , Jan Subrt d, Snejana Bakardjieva d a
School of Environmental Sciences, Jawaharlal Nehru University, New Delhi-110067, India Wastewater Technology Division, CSIR-National Environmental Engineering ResearchInstitute, Nagpur-440020, Maharashtra, India Department of Physics, Faculty of Science, J. E. Purkyne University, Ceske mladeze 8, 40096 Usti nad Labem, Czech Republic d Institute of Inorganic Chemistry of the ASCR, v.v.i., 250 68 Rez, Czech Republic b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 January 2015 Received in revised form 9 April 2015 Accepted 9 April 2015 Available online 11 April 2015
Lead dioxide coated graphite powder (G-PbO2) was synthesized using in-situ wet chemical synthesis method. Phase identification by X-ray diffraction (XRD) revealed the successful synthesis of G-PbO2 powder, containing b-PbO2. This powder was mixed with poly-methyl methacrylate (PMMA) and molded into circular discs for use as electrodes conveniently. The surface morphology and composition of the polymer composite (G-PbO2-PMMA) electrodes was characterized using SEM, EDXA and XPS. Electron transfer dynamics at the G-PbO2-PMMA electrode were examined using standard ferroferricyanide redox couple, Fe(CN)63/4, which displayed peak-to-peak separation of 71 mV. The electrochemical evolution of chlorine at G-PbO2-PMMA anode was also studied which showed favorableshift in the value of oxidation peak potential by 116 mV relative to Pt electrode. The concentration of total chlorine in solution was determined as a function of number of cyclic voltammetric scans at different scan rates. The observed concentration of the dissolved Cl2 was 23 mg L1 (G-PbO2PMMA, 5 mVs1, 50CV cycles) and 15 mg L1(Pt, 5 mVs1, 50CV cycles). The performance of G-PbO2PMMA with respect to chlorine evolution was found to be better compared with that of Pt electrode. The electron transfer at the lead dioxide coated graphite is found to be facile and the G-PbO2-PMMA is inferred to be good anode material for efficient Cl2 evolution. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: b-Lead dioxide Graphite Polymer composite anode Chlorine generation Cyclic voltammetry
1. Introduction Search for anode materials having profound catalytic influence on the electrochemical production of chlorine is important because of two major reasons: i) bulk production of chlorine, and ii) Cl2/OCl mediated indirect oxidation of organic pollutants in wastewater. The highly promising electrocatalytic property of platinum group metal oxide anodes (Dimensionally Stable Anode, DSA) has been a great success in the past [1,2]. Despite their high cost, DSA anodes have been commercially used worldwide in chlor-alkali processes [3]. Nevertheless, research targeting low cost anode materials with comparable Cl2 evolution rate and long
* Corresponding author at: Wastewater Technology Division, CSIR-National Environmental Engineering Research Institute, Nehru Marg, Nagpur-110020, Maharashtra State, India. Tel.: + 91 712 2249885 88/2249970 72; fax: +91 712 2249900. E-mail address: [email protected] (N.R. Neti). http://dx.doi.org/10.1016/j.electacta.2015.04.058 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
term stability as replacement of costly DSA anodes is intensively pursued. Recent research reported a number of successful mixed metal oxide (MMO) anodes useful in Cl2 evolution [4–9]. Lead dioxide has prominently featured as an excellent electrode material and is widely used in several electrochemical applications as it is easily available low cost electrode material with good stability. The tetragonal b-PbO2 form is more electro active than orthorhombic a-PbO2. b-PbO2 is easily accessible through electro deposition from acidic electrolyte media of lead nitrate [10]. High overpotential for oxygen evolution on this anode makes it more preferred over DSA and MMO type noble metal oxide coated anodes. The electrodeposited lead dioxide on titanium substrate was successfully demonstrated for the oxidation of model organic pollutants such as phenols [11–13] in aqueous medium. In the process of electro oxidation of pollutants in wastewater, oxygen and chlorine generated at the anode surface lead to oxidation of the pollutants by direct or indirect degradation pathways [14]. Available various DSA for electrolysis processes are well studied for their catalytic properties of chlorine evolution [15–17]. The
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PbO2 was reported in the application of potentiometric determination of acids and bases [18], organic and inorganic acid determinations [19], lead acid batteries [20], electrical capacitor [21], electro oxidation of organic pollutant [22,23], ammonia removal [24,25], oxygen [26] and ozone [27] generation. Graphite deposited PbO2 polymer (epoxy) composite was used as electrochemical sensor for ammonia, nitrite and phenols [28]. Our group has been contributing to research on electrochemical degradation of organic pollutants, where in special emphasis on indirect oxidation mediated by in situ generated Cl2/OCl- redox couple is laid [29–33]. The major driving factor for this research is the need for low cost electro active materials that exhibit favorable kinetics of electro chemical generation of chlorine. The aim of the present study was to synthesize b-PbO2 coated graphite powder and prepare polymer based composite electrodes for use in
understanding the Cl2 evolution reaction towards ultimate goal of using in ‘indirect oxidation’ of pollutants. The composite anode was characterized and applied for testing electro catalytic activity towards standard redox reaction couple as well as chlorine evolution. 2. Materials and methods 2.1. Chemicals All the chemicals including Pb3O4, HNO3(69-70%), NaHPO4, Na2HPO4,K3[Fe(CN)6], KNO3 are analytical reagent grade chemicals (Merck, India), NaCl (AR, Fisher scientific), Poly Methyl Methacrylate (PMMA, M/s Pyrax polymars, Roorkee, India) and Monomer Methyl Methacrylate (Stabilized 99%, Merck, India) were used as
Fig. 1. Flow chart for wet chemical synthesis of PbO2 impregnated graphite powder and polymer composite anode.
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received. De-ionized water (Millipore, 18.2 MV cm) was used for the preparation of all the solutions. Graphite plate was obtained from Vadodara, India. This was cleaned, washed, dried and ground into fine powder (200 mesh size, 0.074 mm). 2.2. Wet chemical synthesis and deposition of b-PbO2 on graphite powder The wet chemical synthesis of b-PbO2 and deposition on graphite powder was performed. The process flow sheet of wet chemical synthesis of b-PbO2, its deposition on graphite powder and preparation of polymer composite electrode is shown in the Fig. 1. Lead tetroxide (Pb3O4, 3 g) weighed in the clean dry glass beaker of 250 cm3 capacity was mixed with 30 cm3 of dilute nitric acid solution (2:3 v/v, concentrated HNO3: distilled water). The solution was stirred for 30 min on a magnetic stirrer. Graphite powder (15 g, 200 Mesh Size) was added to the above mixture and stirred gently for 60 min. The slurry was then sonicated for 30 min. (20 KHz; Micro Clean-103, OSCAR) and subsequently filtered using a membrane filter (Millipore, 0.45 mm). Washing was done with excess of de-ionized water to remove the water soluble salt i.e., Pb (NO3)2 from the residue of PbO2 coated graphite powder. The GPbO2 powder was dried in oven (overnight, 150 C).
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(staircase normal) program was employed. Platinum (Metrohm, Netherlands) was employed as an auxiliary electrode and a Silver/ Silver Chloride (Metrohm, Netherlands) with 3 M KCl as a reference electrode. The inter electrode distance of 1 cm was maintained throughout in the three electrode cell in all the electrochemical experiments. The G-PbO2-PMMA and platinum was employed as a working electrode with the area of the exposed surface being 0.07 cm2. The redox reaction was conducted in 25 cm3 electrolyte of 10 mM K3Fe(CN)6 prepared in 0.1 M KNO3 as supporting electrolyte in the scan range of 0–1.0 V at a scan rate of 10 mV s1. Chlorine evolution experiments were performed in 100 cm3 of NaCl (0.1 M) electrolyte prepared in sodium phosphate buffer (pH 7), scan range of 0-2.0 V and scan rate of 5-30 mV sec1 was used. Degassing of the electrolyte by purging with pure nitrogen gas was performed prior to the electrolysis experiments. All the experiments were carried out at 25 4 C. The concentration of total chlorine (dissolved) was measured by taking aliquot of electrolyte from electrochemical cell after interval of every 10th scan in each experiment with corresponding scan rate (5, 10, 20, 30 mV s1) and corresponding anode. N, N Diethyl-p- phenylenediamine (DPD) method was used for Cl2 determination with Pocket ColorimeterTM II (HACH, USA; Estimated Detection Limit, 0.1 mg L1; Precision, 0.2 mg L1) and corrected concentration for the entire dilution factor reported in mgL1 (USEPA Method 330.5).
2.3. G-PbO2-Polymer composite electrode 2.5. Material characterization G-PbO2 powder was mixed with PMMA by adding 5 cm3 of methyl methacrylate solvent. The G-PbO2 powder to polymer weight ratio was kept constant throughout the study (1:0.2). The resulting semisolid (paste) was placed in a circular die (dia. 1 cm; depth, 3 mm). This was left open overnight at ambient temperature (20-25 C). The PbO2-graphite-polymer composite disc (G-PbO2PMMA) was dried at 100 C in hot air oven (12 h). The dried disc was polished to glossy finish using fine quality buffing cloth. 2.4. Electrochemical characterization The performance of G-PbO2-PMMA electrodes towards redox behavior of Fe (CN)63/4 and Cl2 evolution was studied. All the electrochemical experiments were performed using a three electrode cell (250 cm3 capacity) set-up with an Autolab PGSTAT-20 a computer-controlled potentiostat/galvanostat (EcoChemie, Netherlands) and the dedicated software package GPES (General Purpose Electrochemical System). The cyclic voltammetry
MiniFlex IIX- ray diffractometer (Rigaku, Japan) was used to record X-ray diffraction (XRD) of PbO2 as well as PbO2 coated graphite powder (Cu Ka, 1.54060 Å, 45 mA and 40 kV; 2u range of 5–80 ). BET surface area was determined using Autosorb iQ Station 1 and Quantachrome1 ASiQwinTM-Automated Gas Sorption data Acquisition and Reduction Software. XPS apparatus was equipped with SPECS X-Ray XR50 (Al cathode 1486.6 eV) and SPECS PHOIBOS 100Hemispheric Analyzer with 5-channels detector. A background pressure in XPS during the measurements was under 2 108 mbar. XPS survey-scan spectra were made at pass energy of 40 eV; the energy resolution was set to0.5 eV. While individual highresolution spectra were taken at pass energy of 10 eV with 0.05 eV energy steps. A software tool CasaXPS was used to fit highresolution multi components peaks. The proper surface charge compensation was done by fitting C-C, C-H component of C 1 s peak to reference binding energy 284.5 eV. The atomic concentration of compounds was evaluated with relative sensitivity factors (RSF)
Fig. 2. XPS survey spectra of graphite, G-PbO2 and G-PbO2-PMMA; (inset) Pb 4f core level XPS peaks in the G-PbO2 and G-PbO2-PMMA.
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defined in standard table of CasaXPS software. A scanning electron microscope Philips XL 30CP (30 kV, W cathode) equipped with EDX, SE, BSE and Robinson detectors was used to obtain atomic percentages of elements on the surface of G-PbO2-PMMA. The sample was directly mounted onto a scanning electron microscope (SEM) specimen holder. Surface morphology of G-PbO2-PMMA was observed under Zeiss make SEM instrument at 10 kV. 3. Results and Discussion 3.1. Material characterization The G-PbO2 powder and G-PbO2-PMMA discs were characterized using, BETSA, XRD, XPS and EDXA techniques. The BET surface area of G-PbO2-PMMA was determined to be 15.6 m2/g. XRD pattern of the b-PbO2 and G-PbO2 powders are shown in Fig. S1. The characteristic peaks are designated as diffraction planes with respect to the 2u values; 25.5 (11 0), 32.0 (1 0 1), 36.3 (2 0 0), 49.1 (2 11), 52.1 (2 2 0), 59.0 (3 1 0), 62.5 (3 0 1), 66.8 (2 0 2) and 74.4 (3 2 1) [34,35]. This confirms the formation of the tetragonal b-PbO2 through the wet chemical synthesis route. The diffraction peaks at 2u values ca. 26.5 (0 0 2), 44.5 (1 0 1), 50.7 (1 0 2), 54.63 (0 0 4), 59.85 (1 0 3), 77.5 (11 0) are the characteristics of the graphite planes [36]. Due to these high intensity diffraction peaks of graphite the peaks of PbO2 are markedly suppressed, however the peaks at 2u values of 32.0 , 36.3 and 49.1 can be attributed to PbO2 in graphite. While the low intensity peaks of PbO2 signify its relatively lower concentration in graphite matrix, the peaks confirm successful coating of PbO2 on to graphite through the wet chemical synthesis route. XPS survey spectrum of the graphite powder, G-PbO2 powder and G-PbO2-PMMA disc are shown in Fig. 2. It shows sharp peaks at binding energy 284.5 and 533 eV which can be attributed to C1s and O1s, respectively. The Pb4f core level XPS peak present in the G-PbO2 powder and G-PbO2-PMMA spectrum clearly display two well defined symmetric peaks (inset, Fig 2) corresponding to Pb 4f5/2 and Pb4f7/2 at 143 and 138 eV respectively [12]. The peak separation of 5 eV (inset in Fig.2) can be assigned to Pb(IV) state [37]. The two symmetric peaks at binding energy 413 and 435 eV in the G-PbO2 powder correspond to the Pb 4d5/2 and 4d3/2 core level spectral values of PbO2 [38]. However, these peaks have very low intensity in the G-PbO2-PMMA surface. The atomic % of the C1s, O1s, Pb 4f in the G-PbO2 powder and G-PbO2-PMMA was calculated from XPS peak data and values are given in Table 1. The elemental composition (atomic %) of the graphite powder used in this study is 90.06% C and 9.94% O. This implies that some carbon is in the oxidized form. The atomic % of C1s, O1s, and Pb4f in GPbO2 are 91.99, 6.58 and 1.43, respectively. Similarly the atomic % of C1s, O1s, and Pb4f in G-PbO2-PMMA are 94.00, 5.89 and 0.11, respectively, for G-PbO2-PMMA. The atomic% correlates well with EDXA data also (Table 1). On the basis of the available Pb4f concentration in G-PbO2 and G-PbO2-PMMA and stoichiometry between Pb and O in PbO2 (86.62 and 13.28) the fraction of O1s associated with Pb was calculated in each case. The calculated data
suggest that 0.016 atomic % of O in the G-PbO2-PMMA is bound to Pb (as PbO2) whereas remaining O (5.87%) is contributed from graphitic PMMA polymer. In the G-PbO2 powder, 0.22 atomic % of O binds with Pb and 6.36 atomic % O is associated with graphite. SEM/EDX and EDS mapping studies were performed on G-PbO2PMMA sample to understand morphology, composition and dispersion of PbO2 on the surface (Fig. 3). The EDXA spectrum clearly shows 1.28 wt% (0.08 atomic %) Pb; both C and O elements associated with the electrode matrix were also found. While the polished surface is fairly smooth and uniform, (photomicrograph ‘a’), the random packing of dense graphitic particles in the bulk is also visible (photomicrograph ‘b’). The elemental mapping for Pb shows uniform distribution of Pb on the surface. The EDX result together with elemental mapping of Pb element (‘c’), confirms presence of PbO2 electrocatalyst on the surface for the G-PbO2PMMA. 3.2. Electrochemical redox behavior of G-PbO2-PMMA Redox behaviour of G-PbO2-PMMA anode was compared with platinum electrode using standard redox couple of 10 mM solution of potassium ferricyanide prepared in the 1 M potassium nitrate as supporting electrolyte. Fig. 4 illustrates the CV of the redox reaction with platinum and G-PbO2-PMMA electrodes, respectively. Both anodes support well defined reversible single-electron redox behavior. The standard redox peak for the FeII(CN)6/FeIII(CN)6 with the peak potential of anodic (Epa) and cathodic (Epc) are
Table 1 XPS peak quantification data for PbO2 impregnated graphite and the polymer composite. Peak
Position (eV)
Atomic % G-PbO2-PMM
G-PbO2
Graphite
C1s O1s Pb4f
284 532 138.5
94.00 (93.22)* 5.89 (6.60) 0.11 (0.08)
91.99 6.58 1.43
90.06 9.94 –
*
values in parenthesis are atomic % derived from EDXA.
Fig. 3. EDXA, SEM and EDS mapping of Pb on the surface of G-PbO2-PMMA.
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Fig. 4. Cyclic voltammogram of 10 mM K3Fe(CN)6 in 0.1 M KNO3 as supporting electrolyte versus Ag/AgCl (3 M KCl) at scan rate of 10 mV s1 at Pt and G-PbO2PMMA electrode.
observed at 0.272 and 0.141 V respectively at Pt anode and the peak separation (DEp = Epa-Epc) was 0.131 V. In the case of G-PbO2PMMA the experimentally observed values of Epa and Epc are 0.242 and 0.171 V respectively, and DEp was 0.071 V. The closer Epa and Epc values at G-PbO2-PMMA anode relative to Pt anode suggest that the redox process at G-PbO2-PMMA is more facile. The smallest values of the DEp correspond to the best performances (facile electron exchange), whereas a large value indicates a slow FeII(CN)6/FeIII(CN)6 kinetics. The DEp is correlated with the electron transfer process and a value 0.059 V at 25 C signifies singleelectron reaction. Surface characteristics can significantly influence this value. The corresponding anodic and cathodic peak current values are given in Table 2 for both the anodes. The redox potential (E0) at both Pt and G-PbO2-PMMA electrodes is the same, 0.207 V vs Ag/AgCl. The corresponding cathodic and anodic currents were somewhat higher on Pt anode. 3.3. Electrochemical chlorine evolution Electrochemical chlorine evolution using both G-PbO2-PMMA and Pt anodes was examined at various scan rates and the corresponding CV are given in Fig. 5(a & b). The oxidation of Cl and reduction of Cl2 at platinum anode can be inferred from Fig. 5(a) with a peak potential for Cl2 evolution at ca. 1.385 V for the experiment with low scan rate, 5 mV s1. The oxidation peaks are not prominently observed at higher scan rates but a sharp increase in the anodic peak current occurs thereafter. It is plausible that at higher scan rates the potential is swept past the Cl2 evolution potential quickly. The corresponding reduction peak in the reverse scan due to reduction of the dissolved chlorine is at 1.062 V. A small increase in the value of anodic peak current was also observed with the corresponding increase in the number of cycles (data not shown). Similar observations are noted in the case of G-PbO2PMMA which showed sharp increase in current >1.39 V due to chloride oxidation as shown in the Fig. 5(b). The scan rate dependence of anodic/cathodic (peak) currents reveals stability of the electrochemically generated product. This is illustrated for the case of G-PbO2-PMMA in Fig. 5(b), inset. The observed current at +1.50 V was considered for forward scan, while the reduction peak current at 0.875 V was used for plotting the I vs y1/2 graphs. It can be seen that anodic current (ia) increases more steeply (than
Table 2 Electrochemical parameter of the anodes in 10 mM K3Fe(CN)6 with 0.1 M KNO3 as supporting electrolyte versus Ag/AgCl (3 M KCl) at scan rate of 10 mV s1. Anode/Electrode
Epa
Epc
D Ep
E = (Epa + Epc)/2
ipa(mA)
Platinum G-PbO2-PMM
0.272 0.242
0.141 0.171
0.131 0.071
0.207 0.207
72 58.4
ipc (mA)
112 91.9
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cathodic current, ic) with increase in the scan rate. This may be due to rapid diffusion of the electrochemically generated Cl2 away from electrode/solution interface, thereby causing only small cathodic reduction currents. The measured total chlorine concentrations in the three electrode cell with different scan rates (5, 10, 20, 30 mV s1) are shown for platinum and for G-PbO2-PMMA anode in Fig. 6(a & b, respectively. The trend in chlorine concentration is markedly influenced by the scan rate; with the increase in the scan rate the Cl2 concentration is decreased. On the other hand, it increases slowly with increasing number of scans at each corresponding scan rate. Evolution of more Cl2 at slow scan rates and for more number of scans is understandable because the anode is held for more time at Cl2 evolving potential at slow scan rates. Thus, the observed concentration of the dissolved Cl2 was 23 mg L1 (G-PbO2-PMMA, 5 mV s1, 50CV cycles, 566 min.) and 15 mg L1 (Pt, 5 mV s1, 50CV cycles, 566 min.). On the contrary, only 0.7 and 0.8 mg L1 Cl2 was observed at Pt and G-PbO2-PMMA respectively at 30 mV s1 at which the time for completion of 50 cycles was only 94.0 min. The performance of G-PbO2-PMMA (Fig. 6b) with respect to chlorine evolution was better compared to Pt (Fig. 6(a). Repetitive experiments over a period of time with wide scan rates from 5 to 30 mV s1 shows no wear and tear of the polymer composite anode and consistent results of measured chlorine concentration were obtained. The method of imposing the anode at a particular potential in the Cl2 evolution region and examining the current or Cl2 evolution behaviour as a function of time reveals important information about the Cl2 evolution reaction. In this study, the anode was held at three different potentials (1.20, 1.40 and 1.5 V vs Ag/AgCl) and both evolved Cl2 and current trend were examined over 1-h electrolysis period. The corresponding data is given in Fig. S2(a and b) and Fig. S3(a and b) and obtained at G-PbO2-PMMA and Ptanodes, respectively. It can be seen that higher is the applied potential greater is the amount of Cl2 evolved. Accordingly, applied potential >1.40 V vs Ag/AgCl well above the thermodynamic potential (+1.17 V vs Ag/AgCl) for Cl2 generation is required to obtain substantial build up of Cl2. Thus, at the end of 1-h electrolysis period, the measured Cl2 was 3.0 mg/L and 0.07 mg/L using G-PbO2-PMMA and Pt anodes, respectively. This is in agreement with the observed equilibrium current which is 15 times greater at G-PbO2-PMMA anode ( 0.50 103 A) at 1.50 V vs Ag/AgCl. Chlorine evolution experiments were also performed in 100 cm3 of 0.1 M NaCl electrolyte prepared in sodium phosphate buffer (pH 7, with and without added NaCl), scan range 0f 0-2.0 V and scan rate of 5 mV s1 was applied. Both G-PbO2-PMMA and Pt anodes were used. The comparison is illustrated in Fig. 7. In the absence of NaCl, only O2 evolution reaction can be expected on the electrodes with the corresponding onset potentials being 1.27 V (G-PbO2-PMMA) and 1.33 V (Pt). It can be seen that in the presence of NaCl, each electrode displayed significant depolarization, i.e. the onset potential shifted to less positive values (see inset, Fig. 7), 0.14 V and 0.12 V, respectively at G-PbO2-PMMA and Pt anodes. This depolarization can be attributed to facile Cl oxidation on these anodes and it is more facile on polymer composite anode. The chlorine evolution potentials (CEP) on different anode materials are compared in Table. 3. It can be seen that CEP is relatively more positive by 0.20-0.40 V on Pt and Ti/RuO2 [39]; ternary oxide based Ti anode [40]; DSA [41] and Ti/Sb-SnO2/Pb3O4 [42] compared to the G-PbO2-PMMA anode in this study. The polymer composite anode displays a CEP value comparable with that of many DSA type mixed metal oxide coated titanium anodes. In the experimental conditions, though oxygen evolution reaction is thermodynamically favoured it is probably suppressed by fast kinetics of Cl2 evolution reaction. In this study, this result is further
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Fig. 5. Cyclic voltammograms illustrating electrochemical chlorine evolution (a) at Pt anode, and (b) at G-PbO2-PMMA anode in 0.1 M sodium chloride prepared in phosphate buffer of pH 7 versus Ag/AgCl (3 M KCl) at different scan rates.
corroborated by measuring total chlorine concentration as a function of scan rate and number scans (Fig. 6). Similar behaviour was reported for Cl2 evolution on DSA in brine [15]. Enhanced rates of electrochemical dye [23] and glucose [43] degradation was reported with the increase in concentration of salt containing chloride ion when the PbO2 deposited anode on various substrates was employed. In the cathodic half cycle corresponding NaCl containing buffer solutions, a reduction peak (Epc) at 1.062 Vat the Pt anode and ca. 0.885 V at the G-PbO2-PMMA anode (Fig. 7) was observed. This may be attributed to the reduction of chlorine species. However, this reduction is relatively unfavourable on G-PbO2-PMMA. This also implies build-up of more Cl2 when G-PbO2-PMMA anode is used. This feature is particularly beneficial when the polymer composite electrodes are intended for application in wastewater treatment wherein the higher Cl2 concentration leads to greater degradation of organic pollutants. The above results confirm the better performance of the GPbO2-PMMA anode for the Cl2 evolution from NaCl containing solutions. The mechanism of Cl oxidation reaction may be similar to the previous reports which suggest Cl2 is formed according to the Volmer/Heyrovsky mechanism [44]. This consists of i) Volmer reaction Cl ! Clads + e and ii) Heyrovsky reaction Cl+ Clads ! Cl2 + e. It is interesting to see that relatively low concentration of electro catalyst, 1-1.5 wt.% b-PbO2 present in electrically conducting graphite-polymer matrix is able to cause remarkable facilitation of chlorine oxidation reaction. Further, as expensive noble metals are not included in G-PbO2-PMMA anode preparation, and it has more favourable CEP compared to DSA –type anodes discussed above it can be considered to be promising anode material for indirect oxidation of pollutants in industrial effluents.
Fig. 6. Comparison of measured amounts of free chlorine during electrochemical chlorine evolution (a) at Pt anode and (b) at G-PbO2-PMMA anode in 0.1 M sodium chloride prepared in sodium phosphate buffer of pH 7 versus Ag/AgCl (3 M KCl) at different scan rates and scanning cycles.
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Fig. 7. Cyclic voltammogram in different electrolyte (A) 0.1 M sodium chloride in phosphate buffer of pH 7 at G-PbO2-PMMA (B) phosphate buffer of pH 7 at G-PbO2-PMMA (C) 0.1 M sodium chloride in phosphate buffer of pH 7 at Pt (D) phosphate buffer of pH 7 at Pt versus Ag/AgCl (3 M KCl) at scan rate of 5 mV s1. (Inset) Onset potentials and magnitude of depolarization (DEp). Table 3 Comparison of chlorine evolution potentials (CEP) for different electrode materials. Electrode
Experimental Conditions
CEP* /V vs Ag/AgCl
Ref.
Graphite
Brine: 25% at 363 K; Current density: 3 kA m3
+1.80
[39]
Platinum RuO2/Ti Ti/RuO2-IrO2-SnO2-Sb2O5 Platinum Glassy Carbon DSA Ti/Sb–SnO2/Pb3O4 G-PbO2-PMMA *
Seawater, scan rate = 0.001 Vs1 at 25 C. 0.01 ; HCl in acetonitrile
0.5 mol L1 Na2SO4 and 0.5 mol L1 NaCl, Scan rate of 0.05 V s1 0.1 M NaCl in Phosphate Buffer (pH 7)
+1.60 +1.45 +1.47 +1.53 +1.81 +1.45 +1.20 +1.20
[40] [41]
[42] This study
all the reported values were converted into potential vs. Ag/AgCl. The theoretical potential value of chlorine evolution is 1.17 V (vs Ag/AgCl).
4. Conclusions Our results demonstrate successful synthesis of tetragonal
b-PbO2 coated graphite powder using wet chemical route. The
powder can be molded into polymer composite disc (G-PbO2PMMA) which in turn can be fabricated as electrodes. The G-PbO2PMMA electrodes containing catalytic amounts of b-PbO2 on surface are remarkably electro active towards standard redox reactions as well as chloride oxidation. The redox process at GPbO2-PMMA is more facile (compared to Pt). The performance of G-PbO2-PMMA with respect to chlorine evolution was better due to the greater depolarization of the Cl oxidation reaction on G -PbO2-PMMA anode as well as unfavorable corresponding reduction reaction leading to more dissolved Cl2 concentration. Thus, we conclude that the G-PbO2-PMMA electrode is a promising anode for efficient chlorine generation; and it can be an effective and low cost anode material for indirect oxidation of organic pollutants in aqueous media. Acknowledgements The authors wish to thank Director, CSIR-NEERI for encouragement. NG thanks CSIR, New Delhi for Senior Research Fellowship. The authors thank Dr. R. P. Pant, CSIR-NPL, New Delhi, for providing assistance on BETSA and SEM analyses. A part of this work was carried out under the CSIR-CAS Bilateral project between CSIRNEERI Nagpur India and Institute of Inorganic Chemistry, CAS, Rez Czech Republic (Ind 2012/16Cz-46).
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Novel Lead dioxide-Graphite-Polymer composite anode for electrochemical chlorine generation Nitin Gedam a,b , Nageswara Rao Neti b, * , Martin Kormunda c , Jan Subrt d, Snejana Bakardjieva d a
School of Environmental Sciences, Jawaharlal Nehru University, New Delhi-110067, India Wastewater Technology Division, CSIR-National Environmental Engineering ResearchInstitute, Nagpur-440020, Maharashtra, India Department of Physics, Faculty of Science, J. E. Purkyne University, Ceske mladeze 8, 40096 Usti nad Labem, Czech Republic d Institute of Inorganic Chemistry of the ASCR, v.v.i., 250 68 Rez, Czech Republic b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 January 2015 Received in revised form 9 April 2015 Accepted 9 April 2015 Available online 11 April 2015
Lead dioxide coated graphite powder (G-PbO2) was synthesized using in-situ wet chemical synthesis method. Phase identification by X-ray diffraction (XRD) revealed the successful synthesis of G-PbO2 powder, containing b-PbO2. This powder was mixed with poly-methyl methacrylate (PMMA) and molded into circular discs for use as electrodes conveniently. The surface morphology and composition of the polymer composite (G-PbO2-PMMA) electrodes was characterized using SEM, EDXA and XPS. Electron transfer dynamics at the G-PbO2-PMMA electrode were examined using standard ferroferricyanide redox couple, Fe(CN)63/4, which displayed peak-to-peak separation of 71 mV. The electrochemical evolution of chlorine at G-PbO2-PMMA anode was also studied which showed favorableshift in the value of oxidation peak potential by 116 mV relative to Pt electrode. The concentration of total chlorine in solution was determined as a function of number of cyclic voltammetric scans at different scan rates. The observed concentration of the dissolved Cl2 was 23 mg L1 (G-PbO2PMMA, 5 mVs1, 50CV cycles) and 15 mg L1(Pt, 5 mVs1, 50CV cycles). The performance of G-PbO2PMMA with respect to chlorine evolution was found to be better compared with that of Pt electrode. The electron transfer at the lead dioxide coated graphite is found to be facile and the G-PbO2-PMMA is inferred to be good anode material for efficient Cl2 evolution. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: b-Lead dioxide Graphite Polymer composite anode Chlorine generation Cyclic voltammetry
1. Introduction Search for anode materials having profound catalytic influence on the electrochemical production of chlorine is important because of two major reasons: i) bulk production of chlorine, and ii) Cl2/OCl mediated indirect oxidation of organic pollutants in wastewater. The highly promising electrocatalytic property of platinum group metal oxide anodes (Dimensionally Stable Anode, DSA) has been a great success in the past [1,2]. Despite their high cost, DSA anodes have been commercially used worldwide in chlor-alkali processes [3]. Nevertheless, research targeting low cost anode materials with comparable Cl2 evolution rate and long
* Corresponding author at: Wastewater Technology Division, CSIR-National Environmental Engineering Research Institute, Nehru Marg, Nagpur-110020, Maharashtra State, India. Tel.: + 91 712 2249885 88/2249970 72; fax: +91 712 2249900. E-mail address: [email protected] (N.R. Neti). http://dx.doi.org/10.1016/j.electacta.2015.04.058 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
term stability as replacement of costly DSA anodes is intensively pursued. Recent research reported a number of successful mixed metal oxide (MMO) anodes useful in Cl2 evolution [4–9]. Lead dioxide has prominently featured as an excellent electrode material and is widely used in several electrochemical applications as it is easily available low cost electrode material with good stability. The tetragonal b-PbO2 form is more electro active than orthorhombic a-PbO2. b-PbO2 is easily accessible through electro deposition from acidic electrolyte media of lead nitrate [10]. High overpotential for oxygen evolution on this anode makes it more preferred over DSA and MMO type noble metal oxide coated anodes. The electrodeposited lead dioxide on titanium substrate was successfully demonstrated for the oxidation of model organic pollutants such as phenols [11–13] in aqueous medium. In the process of electro oxidation of pollutants in wastewater, oxygen and chlorine generated at the anode surface lead to oxidation of the pollutants by direct or indirect degradation pathways [14]. Available various DSA for electrolysis processes are well studied for their catalytic properties of chlorine evolution [15–17]. The
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PbO2 was reported in the application of potentiometric determination of acids and bases [18], organic and inorganic acid determinations [19], lead acid batteries [20], electrical capacitor [21], electro oxidation of organic pollutant [22,23], ammonia removal [24,25], oxygen [26] and ozone [27] generation. Graphite deposited PbO2 polymer (epoxy) composite was used as electrochemical sensor for ammonia, nitrite and phenols [28]. Our group has been contributing to research on electrochemical degradation of organic pollutants, where in special emphasis on indirect oxidation mediated by in situ generated Cl2/OCl- redox couple is laid [29–33]. The major driving factor for this research is the need for low cost electro active materials that exhibit favorable kinetics of electro chemical generation of chlorine. The aim of the present study was to synthesize b-PbO2 coated graphite powder and prepare polymer based composite electrodes for use in
understanding the Cl2 evolution reaction towards ultimate goal of using in ‘indirect oxidation’ of pollutants. The composite anode was characterized and applied for testing electro catalytic activity towards standard redox reaction couple as well as chlorine evolution. 2. Materials and methods 2.1. Chemicals All the chemicals including Pb3O4, HNO3(69-70%), NaHPO4, Na2HPO4,K3[Fe(CN)6], KNO3 are analytical reagent grade chemicals (Merck, India), NaCl (AR, Fisher scientific), Poly Methyl Methacrylate (PMMA, M/s Pyrax polymars, Roorkee, India) and Monomer Methyl Methacrylate (Stabilized 99%, Merck, India) were used as
Fig. 1. Flow chart for wet chemical synthesis of PbO2 impregnated graphite powder and polymer composite anode.
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received. De-ionized water (Millipore, 18.2 MV cm) was used for the preparation of all the solutions. Graphite plate was obtained from Vadodara, India. This was cleaned, washed, dried and ground into fine powder (200 mesh size, 0.074 mm). 2.2. Wet chemical synthesis and deposition of b-PbO2 on graphite powder The wet chemical synthesis of b-PbO2 and deposition on graphite powder was performed. The process flow sheet of wet chemical synthesis of b-PbO2, its deposition on graphite powder and preparation of polymer composite electrode is shown in the Fig. 1. Lead tetroxide (Pb3O4, 3 g) weighed in the clean dry glass beaker of 250 cm3 capacity was mixed with 30 cm3 of dilute nitric acid solution (2:3 v/v, concentrated HNO3: distilled water). The solution was stirred for 30 min on a magnetic stirrer. Graphite powder (15 g, 200 Mesh Size) was added to the above mixture and stirred gently for 60 min. The slurry was then sonicated for 30 min. (20 KHz; Micro Clean-103, OSCAR) and subsequently filtered using a membrane filter (Millipore, 0.45 mm). Washing was done with excess of de-ionized water to remove the water soluble salt i.e., Pb (NO3)2 from the residue of PbO2 coated graphite powder. The GPbO2 powder was dried in oven (overnight, 150 C).
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(staircase normal) program was employed. Platinum (Metrohm, Netherlands) was employed as an auxiliary electrode and a Silver/ Silver Chloride (Metrohm, Netherlands) with 3 M KCl as a reference electrode. The inter electrode distance of 1 cm was maintained throughout in the three electrode cell in all the electrochemical experiments. The G-PbO2-PMMA and platinum was employed as a working electrode with the area of the exposed surface being 0.07 cm2. The redox reaction was conducted in 25 cm3 electrolyte of 10 mM K3Fe(CN)6 prepared in 0.1 M KNO3 as supporting electrolyte in the scan range of 0–1.0 V at a scan rate of 10 mV s1. Chlorine evolution experiments were performed in 100 cm3 of NaCl (0.1 M) electrolyte prepared in sodium phosphate buffer (pH 7), scan range of 0-2.0 V and scan rate of 5-30 mV sec1 was used. Degassing of the electrolyte by purging with pure nitrogen gas was performed prior to the electrolysis experiments. All the experiments were carried out at 25 4 C. The concentration of total chlorine (dissolved) was measured by taking aliquot of electrolyte from electrochemical cell after interval of every 10th scan in each experiment with corresponding scan rate (5, 10, 20, 30 mV s1) and corresponding anode. N, N Diethyl-p- phenylenediamine (DPD) method was used for Cl2 determination with Pocket ColorimeterTM II (HACH, USA; Estimated Detection Limit, 0.1 mg L1; Precision, 0.2 mg L1) and corrected concentration for the entire dilution factor reported in mgL1 (USEPA Method 330.5).
2.3. G-PbO2-Polymer composite electrode 2.5. Material characterization G-PbO2 powder was mixed with PMMA by adding 5 cm3 of methyl methacrylate solvent. The G-PbO2 powder to polymer weight ratio was kept constant throughout the study (1:0.2). The resulting semisolid (paste) was placed in a circular die (dia. 1 cm; depth, 3 mm). This was left open overnight at ambient temperature (20-25 C). The PbO2-graphite-polymer composite disc (G-PbO2PMMA) was dried at 100 C in hot air oven (12 h). The dried disc was polished to glossy finish using fine quality buffing cloth. 2.4. Electrochemical characterization The performance of G-PbO2-PMMA electrodes towards redox behavior of Fe (CN)63/4 and Cl2 evolution was studied. All the electrochemical experiments were performed using a three electrode cell (250 cm3 capacity) set-up with an Autolab PGSTAT-20 a computer-controlled potentiostat/galvanostat (EcoChemie, Netherlands) and the dedicated software package GPES (General Purpose Electrochemical System). The cyclic voltammetry
MiniFlex IIX- ray diffractometer (Rigaku, Japan) was used to record X-ray diffraction (XRD) of PbO2 as well as PbO2 coated graphite powder (Cu Ka, 1.54060 Å, 45 mA and 40 kV; 2u range of 5–80 ). BET surface area was determined using Autosorb iQ Station 1 and Quantachrome1 ASiQwinTM-Automated Gas Sorption data Acquisition and Reduction Software. XPS apparatus was equipped with SPECS X-Ray XR50 (Al cathode 1486.6 eV) and SPECS PHOIBOS 100Hemispheric Analyzer with 5-channels detector. A background pressure in XPS during the measurements was under 2 108 mbar. XPS survey-scan spectra were made at pass energy of 40 eV; the energy resolution was set to0.5 eV. While individual highresolution spectra were taken at pass energy of 10 eV with 0.05 eV energy steps. A software tool CasaXPS was used to fit highresolution multi components peaks. The proper surface charge compensation was done by fitting C-C, C-H component of C 1 s peak to reference binding energy 284.5 eV. The atomic concentration of compounds was evaluated with relative sensitivity factors (RSF)
Fig. 2. XPS survey spectra of graphite, G-PbO2 and G-PbO2-PMMA; (inset) Pb 4f core level XPS peaks in the G-PbO2 and G-PbO2-PMMA.
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defined in standard table of CasaXPS software. A scanning electron microscope Philips XL 30CP (30 kV, W cathode) equipped with EDX, SE, BSE and Robinson detectors was used to obtain atomic percentages of elements on the surface of G-PbO2-PMMA. The sample was directly mounted onto a scanning electron microscope (SEM) specimen holder. Surface morphology of G-PbO2-PMMA was observed under Zeiss make SEM instrument at 10 kV. 3. Results and Discussion 3.1. Material characterization The G-PbO2 powder and G-PbO2-PMMA discs were characterized using, BETSA, XRD, XPS and EDXA techniques. The BET surface area of G-PbO2-PMMA was determined to be 15.6 m2/g. XRD pattern of the b-PbO2 and G-PbO2 powders are shown in Fig. S1. The characteristic peaks are designated as diffraction planes with respect to the 2u values; 25.5 (11 0), 32.0 (1 0 1), 36.3 (2 0 0), 49.1 (2 11), 52.1 (2 2 0), 59.0 (3 1 0), 62.5 (3 0 1), 66.8 (2 0 2) and 74.4 (3 2 1) [34,35]. This confirms the formation of the tetragonal b-PbO2 through the wet chemical synthesis route. The diffraction peaks at 2u values ca. 26.5 (0 0 2), 44.5 (1 0 1), 50.7 (1 0 2), 54.63 (0 0 4), 59.85 (1 0 3), 77.5 (11 0) are the characteristics of the graphite planes [36]. Due to these high intensity diffraction peaks of graphite the peaks of PbO2 are markedly suppressed, however the peaks at 2u values of 32.0 , 36.3 and 49.1 can be attributed to PbO2 in graphite. While the low intensity peaks of PbO2 signify its relatively lower concentration in graphite matrix, the peaks confirm successful coating of PbO2 on to graphite through the wet chemical synthesis route. XPS survey spectrum of the graphite powder, G-PbO2 powder and G-PbO2-PMMA disc are shown in Fig. 2. It shows sharp peaks at binding energy 284.5 and 533 eV which can be attributed to C1s and O1s, respectively. The Pb4f core level XPS peak present in the G-PbO2 powder and G-PbO2-PMMA spectrum clearly display two well defined symmetric peaks (inset, Fig 2) corresponding to Pb 4f5/2 and Pb4f7/2 at 143 and 138 eV respectively [12]. The peak separation of 5 eV (inset in Fig.2) can be assigned to Pb(IV) state [37]. The two symmetric peaks at binding energy 413 and 435 eV in the G-PbO2 powder correspond to the Pb 4d5/2 and 4d3/2 core level spectral values of PbO2 [38]. However, these peaks have very low intensity in the G-PbO2-PMMA surface. The atomic % of the C1s, O1s, Pb 4f in the G-PbO2 powder and G-PbO2-PMMA was calculated from XPS peak data and values are given in Table 1. The elemental composition (atomic %) of the graphite powder used in this study is 90.06% C and 9.94% O. This implies that some carbon is in the oxidized form. The atomic % of C1s, O1s, and Pb4f in GPbO2 are 91.99, 6.58 and 1.43, respectively. Similarly the atomic % of C1s, O1s, and Pb4f in G-PbO2-PMMA are 94.00, 5.89 and 0.11, respectively, for G-PbO2-PMMA. The atomic% correlates well with EDXA data also (Table 1). On the basis of the available Pb4f concentration in G-PbO2 and G-PbO2-PMMA and stoichiometry between Pb and O in PbO2 (86.62 and 13.28) the fraction of O1s associated with Pb was calculated in each case. The calculated data
suggest that 0.016 atomic % of O in the G-PbO2-PMMA is bound to Pb (as PbO2) whereas remaining O (5.87%) is contributed from graphitic PMMA polymer. In the G-PbO2 powder, 0.22 atomic % of O binds with Pb and 6.36 atomic % O is associated with graphite. SEM/EDX and EDS mapping studies were performed on G-PbO2PMMA sample to understand morphology, composition and dispersion of PbO2 on the surface (Fig. 3). The EDXA spectrum clearly shows 1.28 wt% (0.08 atomic %) Pb; both C and O elements associated with the electrode matrix were also found. While the polished surface is fairly smooth and uniform, (photomicrograph ‘a’), the random packing of dense graphitic particles in the bulk is also visible (photomicrograph ‘b’). The elemental mapping for Pb shows uniform distribution of Pb on the surface. The EDX result together with elemental mapping of Pb element (‘c’), confirms presence of PbO2 electrocatalyst on the surface for the G-PbO2PMMA. 3.2. Electrochemical redox behavior of G-PbO2-PMMA Redox behaviour of G-PbO2-PMMA anode was compared with platinum electrode using standard redox couple of 10 mM solution of potassium ferricyanide prepared in the 1 M potassium nitrate as supporting electrolyte. Fig. 4 illustrates the CV of the redox reaction with platinum and G-PbO2-PMMA electrodes, respectively. Both anodes support well defined reversible single-electron redox behavior. The standard redox peak for the FeII(CN)6/FeIII(CN)6 with the peak potential of anodic (Epa) and cathodic (Epc) are
Table 1 XPS peak quantification data for PbO2 impregnated graphite and the polymer composite. Peak
Position (eV)
Atomic % G-PbO2-PMM
G-PbO2
Graphite
C1s O1s Pb4f
284 532 138.5
94.00 (93.22)* 5.89 (6.60) 0.11 (0.08)
91.99 6.58 1.43
90.06 9.94 –
*
values in parenthesis are atomic % derived from EDXA.
Fig. 3. EDXA, SEM and EDS mapping of Pb on the surface of G-PbO2-PMMA.
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Fig. 4. Cyclic voltammogram of 10 mM K3Fe(CN)6 in 0.1 M KNO3 as supporting electrolyte versus Ag/AgCl (3 M KCl) at scan rate of 10 mV s1 at Pt and G-PbO2PMMA electrode.
observed at 0.272 and 0.141 V respectively at Pt anode and the peak separation (DEp = Epa-Epc) was 0.131 V. In the case of G-PbO2PMMA the experimentally observed values of Epa and Epc are 0.242 and 0.171 V respectively, and DEp was 0.071 V. The closer Epa and Epc values at G-PbO2-PMMA anode relative to Pt anode suggest that the redox process at G-PbO2-PMMA is more facile. The smallest values of the DEp correspond to the best performances (facile electron exchange), whereas a large value indicates a slow FeII(CN)6/FeIII(CN)6 kinetics. The DEp is correlated with the electron transfer process and a value 0.059 V at 25 C signifies singleelectron reaction. Surface characteristics can significantly influence this value. The corresponding anodic and cathodic peak current values are given in Table 2 for both the anodes. The redox potential (E0) at both Pt and G-PbO2-PMMA electrodes is the same, 0.207 V vs Ag/AgCl. The corresponding cathodic and anodic currents were somewhat higher on Pt anode. 3.3. Electrochemical chlorine evolution Electrochemical chlorine evolution using both G-PbO2-PMMA and Pt anodes was examined at various scan rates and the corresponding CV are given in Fig. 5(a & b). The oxidation of Cl and reduction of Cl2 at platinum anode can be inferred from Fig. 5(a) with a peak potential for Cl2 evolution at ca. 1.385 V for the experiment with low scan rate, 5 mV s1. The oxidation peaks are not prominently observed at higher scan rates but a sharp increase in the anodic peak current occurs thereafter. It is plausible that at higher scan rates the potential is swept past the Cl2 evolution potential quickly. The corresponding reduction peak in the reverse scan due to reduction of the dissolved chlorine is at 1.062 V. A small increase in the value of anodic peak current was also observed with the corresponding increase in the number of cycles (data not shown). Similar observations are noted in the case of G-PbO2PMMA which showed sharp increase in current >1.39 V due to chloride oxidation as shown in the Fig. 5(b). The scan rate dependence of anodic/cathodic (peak) currents reveals stability of the electrochemically generated product. This is illustrated for the case of G-PbO2-PMMA in Fig. 5(b), inset. The observed current at +1.50 V was considered for forward scan, while the reduction peak current at 0.875 V was used for plotting the I vs y1/2 graphs. It can be seen that anodic current (ia) increases more steeply (than
Table 2 Electrochemical parameter of the anodes in 10 mM K3Fe(CN)6 with 0.1 M KNO3 as supporting electrolyte versus Ag/AgCl (3 M KCl) at scan rate of 10 mV s1. Anode/Electrode
Epa
Epc
D Ep
E = (Epa + Epc)/2
ipa(mA)
Platinum G-PbO2-PMM
0.272 0.242
0.141 0.171
0.131 0.071
0.207 0.207
72 58.4
ipc (mA)
112 91.9
113
cathodic current, ic) with increase in the scan rate. This may be due to rapid diffusion of the electrochemically generated Cl2 away from electrode/solution interface, thereby causing only small cathodic reduction currents. The measured total chlorine concentrations in the three electrode cell with different scan rates (5, 10, 20, 30 mV s1) are shown for platinum and for G-PbO2-PMMA anode in Fig. 6(a & b, respectively. The trend in chlorine concentration is markedly influenced by the scan rate; with the increase in the scan rate the Cl2 concentration is decreased. On the other hand, it increases slowly with increasing number of scans at each corresponding scan rate. Evolution of more Cl2 at slow scan rates and for more number of scans is understandable because the anode is held for more time at Cl2 evolving potential at slow scan rates. Thus, the observed concentration of the dissolved Cl2 was 23 mg L1 (G-PbO2-PMMA, 5 mV s1, 50CV cycles, 566 min.) and 15 mg L1 (Pt, 5 mV s1, 50CV cycles, 566 min.). On the contrary, only 0.7 and 0.8 mg L1 Cl2 was observed at Pt and G-PbO2-PMMA respectively at 30 mV s1 at which the time for completion of 50 cycles was only 94.0 min. The performance of G-PbO2-PMMA (Fig. 6b) with respect to chlorine evolution was better compared to Pt (Fig. 6(a). Repetitive experiments over a period of time with wide scan rates from 5 to 30 mV s1 shows no wear and tear of the polymer composite anode and consistent results of measured chlorine concentration were obtained. The method of imposing the anode at a particular potential in the Cl2 evolution region and examining the current or Cl2 evolution behaviour as a function of time reveals important information about the Cl2 evolution reaction. In this study, the anode was held at three different potentials (1.20, 1.40 and 1.5 V vs Ag/AgCl) and both evolved Cl2 and current trend were examined over 1-h electrolysis period. The corresponding data is given in Fig. S2(a and b) and Fig. S3(a and b) and obtained at G-PbO2-PMMA and Ptanodes, respectively. It can be seen that higher is the applied potential greater is the amount of Cl2 evolved. Accordingly, applied potential >1.40 V vs Ag/AgCl well above the thermodynamic potential (+1.17 V vs Ag/AgCl) for Cl2 generation is required to obtain substantial build up of Cl2. Thus, at the end of 1-h electrolysis period, the measured Cl2 was 3.0 mg/L and 0.07 mg/L using G-PbO2-PMMA and Pt anodes, respectively. This is in agreement with the observed equilibrium current which is 15 times greater at G-PbO2-PMMA anode ( 0.50 103 A) at 1.50 V vs Ag/AgCl. Chlorine evolution experiments were also performed in 100 cm3 of 0.1 M NaCl electrolyte prepared in sodium phosphate buffer (pH 7, with and without added NaCl), scan range 0f 0-2.0 V and scan rate of 5 mV s1 was applied. Both G-PbO2-PMMA and Pt anodes were used. The comparison is illustrated in Fig. 7. In the absence of NaCl, only O2 evolution reaction can be expected on the electrodes with the corresponding onset potentials being 1.27 V (G-PbO2-PMMA) and 1.33 V (Pt). It can be seen that in the presence of NaCl, each electrode displayed significant depolarization, i.e. the onset potential shifted to less positive values (see inset, Fig. 7), 0.14 V and 0.12 V, respectively at G-PbO2-PMMA and Pt anodes. This depolarization can be attributed to facile Cl oxidation on these anodes and it is more facile on polymer composite anode. The chlorine evolution potentials (CEP) on different anode materials are compared in Table. 3. It can be seen that CEP is relatively more positive by 0.20-0.40 V on Pt and Ti/RuO2 [39]; ternary oxide based Ti anode [40]; DSA [41] and Ti/Sb-SnO2/Pb3O4 [42] compared to the G-PbO2-PMMA anode in this study. The polymer composite anode displays a CEP value comparable with that of many DSA type mixed metal oxide coated titanium anodes. In the experimental conditions, though oxygen evolution reaction is thermodynamically favoured it is probably suppressed by fast kinetics of Cl2 evolution reaction. In this study, this result is further
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Fig. 5. Cyclic voltammograms illustrating electrochemical chlorine evolution (a) at Pt anode, and (b) at G-PbO2-PMMA anode in 0.1 M sodium chloride prepared in phosphate buffer of pH 7 versus Ag/AgCl (3 M KCl) at different scan rates.
corroborated by measuring total chlorine concentration as a function of scan rate and number scans (Fig. 6). Similar behaviour was reported for Cl2 evolution on DSA in brine [15]. Enhanced rates of electrochemical dye [23] and glucose [43] degradation was reported with the increase in concentration of salt containing chloride ion when the PbO2 deposited anode on various substrates was employed. In the cathodic half cycle corresponding NaCl containing buffer solutions, a reduction peak (Epc) at 1.062 Vat the Pt anode and ca. 0.885 V at the G-PbO2-PMMA anode (Fig. 7) was observed. This may be attributed to the reduction of chlorine species. However, this reduction is relatively unfavourable on G-PbO2-PMMA. This also implies build-up of more Cl2 when G-PbO2-PMMA anode is used. This feature is particularly beneficial when the polymer composite electrodes are intended for application in wastewater treatment wherein the higher Cl2 concentration leads to greater degradation of organic pollutants. The above results confirm the better performance of the GPbO2-PMMA anode for the Cl2 evolution from NaCl containing solutions. The mechanism of Cl oxidation reaction may be similar to the previous reports which suggest Cl2 is formed according to the Volmer/Heyrovsky mechanism [44]. This consists of i) Volmer reaction Cl ! Clads + e and ii) Heyrovsky reaction Cl+ Clads ! Cl2 + e. It is interesting to see that relatively low concentration of electro catalyst, 1-1.5 wt.% b-PbO2 present in electrically conducting graphite-polymer matrix is able to cause remarkable facilitation of chlorine oxidation reaction. Further, as expensive noble metals are not included in G-PbO2-PMMA anode preparation, and it has more favourable CEP compared to DSA –type anodes discussed above it can be considered to be promising anode material for indirect oxidation of pollutants in industrial effluents.
Fig. 6. Comparison of measured amounts of free chlorine during electrochemical chlorine evolution (a) at Pt anode and (b) at G-PbO2-PMMA anode in 0.1 M sodium chloride prepared in sodium phosphate buffer of pH 7 versus Ag/AgCl (3 M KCl) at different scan rates and scanning cycles.
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Fig. 7. Cyclic voltammogram in different electrolyte (A) 0.1 M sodium chloride in phosphate buffer of pH 7 at G-PbO2-PMMA (B) phosphate buffer of pH 7 at G-PbO2-PMMA (C) 0.1 M sodium chloride in phosphate buffer of pH 7 at Pt (D) phosphate buffer of pH 7 at Pt versus Ag/AgCl (3 M KCl) at scan rate of 5 mV s1. (Inset) Onset potentials and magnitude of depolarization (DEp). Table 3 Comparison of chlorine evolution potentials (CEP) for different electrode materials. Electrode
Experimental Conditions
CEP* /V vs Ag/AgCl
Ref.
Graphite
Brine: 25% at 363 K; Current density: 3 kA m3
+1.80
[39]
Platinum RuO2/Ti Ti/RuO2-IrO2-SnO2-Sb2O5 Platinum Glassy Carbon DSA Ti/Sb–SnO2/Pb3O4 G-PbO2-PMMA *
Seawater, scan rate = 0.001 Vs1 at 25 C. 0.01 ; HCl in acetonitrile
0.5 mol L1 Na2SO4 and 0.5 mol L1 NaCl, Scan rate of 0.05 V s1 0.1 M NaCl in Phosphate Buffer (pH 7)
+1.60 +1.45 +1.47 +1.53 +1.81 +1.45 +1.20 +1.20
[40] [41]
[42] This study
all the reported values were converted into potential vs. Ag/AgCl. The theoretical potential value of chlorine evolution is 1.17 V (vs Ag/AgCl).
4. Conclusions Our results demonstrate successful synthesis of tetragonal
b-PbO2 coated graphite powder using wet chemical route. The
powder can be molded into polymer composite disc (G-PbO2PMMA) which in turn can be fabricated as electrodes. The G-PbO2PMMA electrodes containing catalytic amounts of b-PbO2 on surface are remarkably electro active towards standard redox reactions as well as chloride oxidation. The redox process at GPbO2-PMMA is more facile (compared to Pt). The performance of G-PbO2-PMMA with respect to chlorine evolution was better due to the greater depolarization of the Cl oxidation reaction on G -PbO2-PMMA anode as well as unfavorable corresponding reduction reaction leading to more dissolved Cl2 concentration. Thus, we conclude that the G-PbO2-PMMA electrode is a promising anode for efficient chlorine generation; and it can be an effective and low cost anode material for indirect oxidation of organic pollutants in aqueous media. Acknowledgements The authors wish to thank Director, CSIR-NEERI for encouragement. NG thanks CSIR, New Delhi for Senior Research Fellowship. The authors thank Dr. R. P. Pant, CSIR-NPL, New Delhi, for providing assistance on BETSA and SEM analyses. A part of this work was carried out under the CSIR-CAS Bilateral project between CSIRNEERI Nagpur India and Institute of Inorganic Chemistry, CAS, Rez Czech Republic (Ind 2012/16Cz-46).
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