3 Electrochemical behavior of nanodiamond electrode deposited on porous activated carbon pellet Jing ZHANG and Xi-cheng WEI School of Materials Science and Engineering, Shanghai University, Shanghai, 200072, China Email: superjing@126.com Jing ZHANG, Rong-biQ LI and Xin WANG School of Mechanical Engineering, Shanghai Dianji University, Shanghai, 200245, China Activated carbon pellets as substrates for the deposition of diamond films were prepared using chemical vapor deposition (CVD). The ballas diamond morphology and crystal structure of the as-deposited films were analyzed using scanning electron microscopy and Raman spectroscopy, respectively. The electrochemical behavior of a boron-doped diamond-film electrode prepared on the porous activated carbon substrates was also studied, by examining the cyclic voltammetry and AC impedance. The diamond films exhibited a ballas morphology and contained microcrystallites. The measured results indicated that the electrode had a very wide potential window and very low background current, while the potential windows in acidic, neutral, or alkaline media were 4.4, 4.0, and 3.0 [V], respectively. The background current was as low as -8 10-6 -5 10-7 [A]. In electrolytes, including ferri/ferrocyanide, the electrode surface maintained good activity, and the electrochemical reaction occurring on the surface was a diffusion-controlled reaction with good quasi-reversibility. Compared with platinum and graphite electrodes, diamond electrodes can effectively oxidize compounds (phenol), and the oxidization process is very simple and complete. Keywords: Activated Carbon Pellet; Diamond Film Electrode; Electrochemical Behavior. 1. Introduction Diamond is an attractive material since it possesses extreme hardness, the highest thermal conductivity of any material, high chemical and electrochemical inertness, low background current, and a wide working potential window for the majority of solvents and electrolytes [1-3]. Diamond film can be produced by chemical vapor deposition (CVD) at low pressure and at moderate temperature on graphite, silicon, and titanium substrates. Further, the conductivity of diamond film may guarantee an effective electrode, fabricated using boron-doped diamond (BDD), through control of the boron doping level. As a
4 result of these excellent properties, conducting BDD appears to be a promising electrode material. Therefore, it is of interest as regards the use of diamond electrodes in electrochemical processes [4-5], such as in the treatment of water containing organic pollutants [6-8]. However, despite the numerous advantages of diamond electrodes, this material has not yet achieved its full potential; one of the responsible factors being that the substrate material is costly, making industrial application difficult. In order to expand the applications of CVD diamond with regard to diamond-film electrodes, it is necessary to develop a new substrate material that is both easy to fabricate and low-cost, which is very important for economic large-scale manufacture. Bearing this in mind, we are attempting to discover a cost-effective substrate. One approach to achieving this aim is the use of sintered porous activated carbon pellets as a substrate, which are composed of self-made carbonaceous mesophase and fillers. The main purpose of this study is the preparation and characterization of a CVD BDD electrode grown on activated carbon pellet substrates. The physical and electrochemical characteristics of the electrode and its electrochemical response to oxidized compounds are also investigated. 2. Materials and Methods The initial materials used in this experiment are mainly self-made biomass-derived carbonaceous mesophase (BCM) and other fillers. The fillers include apple-derived activated carbon, carbon fiber, graphite, and apple castoff. The BCM and the filler were mill-mixed together in appropriate proportions for 10 min. The resultant mixture was then pelletized into small pellets at a pressure of approximately 300 MPa, which was maintained for 1 min. Finally, the pellets were sintered in a vacuum furnace at 1000 C for 60 min and then slowly cooled within the furnace. Diamond nucleation and growth were undertaken in a microwave plasma CVD (MPCVD) reactor using acetone diluted in hydrogen gas. The total pressure and total flow rate were set to 30 [Mbar] and 100 [SCCM], respectively. The deposition temperature, which was measured by a thermocouple on the backside of the substrate, was set to 1020 [K]. Cyclic voltammetry (CV) experiments were conducted using a model CH1660A voltammetric analyzer using a single-compartment, three-electrode cell with a Ag/AgCl (3.0 [mol/l] NaCl) reference and Pt-wire auxiliary electrodes. The anodic and cathodic charges were evaluated from CV curves obtained in the range of 20-200 [mv/s].
5 3. Results and Discussion During vacuum sintering, both the carbonaceous mesophase and the fillers are pyrolyzed to produce different types of carbon, and they therefore expel gases. After vacuum sintering of the pellets, all the samples exhibit weight loss and bulk shrinkage, and also become microporous. Figure 1 shows the room-temperature Raman spectrum of the diamond deposited on the porous carbon substrate. In the interests of comparison, the Raman spectra of samples treated with hydrogen plasma only were also recorded. As can be seen from Figure 1, the Raman spectra exhibit graphitic carbon peaks at approximately 1580 cm -1 and disordered carbon peaks at approximately 1320 cm -1 following treatment with the hydrogen plasma; this is the result of vacuum sintering. However, after MPCVD, the Raman spectra of the sample displays the diamond peak in the region of 1332 cm -1. Note that a relative sharp peak at approximately 1332 cm -1 in the spectra corresponds to c-c sp 3 bonds, which is a characteristic of the diamond structure. Further, a wide band exists in the region of 1580 cm -1 in Figure 1. Figure 1. Raman spectra of sample before/after diamond deposition. Figure 2 is a SEM micrograph of diamond films on activated carbon substrate after 6 hours deposition. The sample is covered with isolated diamond grains, both on the surface and in the pores, with the grains tending to congregate in the pores (Figure 2, inset). The grains on the activated carbon substrates are ball-shaped, and the morphology of the deposited diamond is
6 similar to that reported in another study [9], in which the authors have suggested that the ball-shaped diamond is polycrystalline (ballas). We believe that the main source of this morphology is the activated carbon substrate, which tends to act as a carbon source and, therefore, increases the carbon concentration. Thus, it may change the hydrocarbon/hydrogen ratio in the CVD chamber. Figure 2. SEM micrographs of diamond deposited on porous carbon substrate. The CV behavior was observed for BDD electrodes, Pt, and graphite electrodes in H 2 SO 4, and is shown in Figure 3. In these cases, a broad and relatively flat potential window appeared between the electrode anodic and cathodic limits, with the potential range exhibited by the BDD extending from approximately +2.5 to 1.5 [V] (vs. Ag/AgCl). The BDD electrodes had a significantly wider potential window than those of the Pt and graphite electrodes. Further, the working potential window of BDD was significantly larger (approximately 4.0 [V]). On the basis of these results, we can speculate that the majority of organic compounds can be oxidized using BDD electrodes. An important property of these electrodes is their low background current (two orders of magnitude lower than those of Pt and graphite electrodes). The values of the potential windows and background currents for the BDD, Pt, and graphite electrodes in 1.0-[mol/L] H 2 SO 4 are given in Table 1.
7 Figure 3. Cyclic voltammograms of three electrodes in 1.0-[mol/L] H 2SO 4: (1) BDD, (2) Pt, and (3) graphite electrodes. Table 1. Potential windows and background current values for BDD, Pt, and graphite electrodes in 1.0-[mol/L] H 2SO 4. Electrodes Anodic Potential/[V] Cathodic Potential /[V] Potential Window /[V] Background current /[A] BDD +2.5-1.5 4.0-8 10-6 5 10-7 Platinum +1.5-0.3 1.8-1 10-5 1 10-5 Graphite +1.0-1.0 2.0-6 10-4 7 10-4 Figure 4 shows the CVs of the BDD electrodes in different electrolytes varying from acidic, neutral, to alkali substances. It can be seen that the anodic potential was the lowest in the alkali medium and the highest in the acidic electrolyte; the values of the anodic potential in the acidic, neutral, and alkali media were +2.4, +2.1, and +1.0 [V], respectively. This implies that the possibility of organic pollutants being oxidized in the acidic electrolyte was significantly greater than that for the alkali electrolyte.
8 Figure 4. Cyclic voltammograms for BDD electrode in 1.0-[mol/L] acidic, neutral, and alkaline media: (1) H 2SO 4, (2) Na 2SO 4, and (3) NaOH. The electrochemical response of the BDD electrode surface was investigated using a ferro/ferrocyanide redox couple. Figure 5 shows that the current peak intensity (I p ) of the BDD electrodes increases with an increase in sweeping rate (ν), and that the cathodic peak potential (E pc ) shifts to negative values with increasing ν. These results confirm the system s quasi-reversible behavior, and also that the electrochemical reaction occurring on the surface was a diffusion-controlled reaction. Figure 5. Cyclic voltammograms for diamond electrode in 5.0-[mmol/L] [Fe(CN)6] 4- /[Fe(CN)6] 3- + 1.0-[mol/L] KCl.
Figure 6 shows the impedance spectra of the BDD electrodes in a Fe(CN) 3-/4-6 solution. A typical complex plane plot of the interfacial impedance 3-/4- of the BDD electrodes in the Fe(CN) 6 solution, measured at +1.8 [V], is presented in Figure 6(a). At high over-potential, the response exhibits an RC-semicircle peak of 45 at high frequency. The resistance, which is the measured casual current at the electrode/electrolyte interface in the supporting electrolyte solution, was approximately 30 [Ω], and the double layer capacitance was in the region of 2.5 [μf.cm -2 ]. The phase angle of the electrochemical impedance against frequency in the Fe(CN) 3-/4-6 solution is shown in Figure 6 (b). More interestingly, only one time constant was observed for the electrolyte solution, suggesting that the BDD electrode surface maintained good activity. 9 Figure 6. Electrochemical impedance spectroscopy of boron-doped diamond in 5.0-[mmol/L] [Fe(CN) 6] 4- /[Fe(CN) 6] 3- + 1.0-[mol/L] KCl solution: (a) Complex impedance plane plot and (b) Bode plot. The electrocatalytic activity of the BDD electrodes in acidic solution was compared with that of other electrodes (Pt and graphite), regarding the oxidation of phenol. This activity was measured using a cycle voltammogram, as shown in Figure 7. It can be seen that a large number of redox peaks were observed for the graphite electrodes; in contrast, only a single oxidation peak was measured for the BDD electrode, at a high anodic peak current and with a large peak area. These results imply that BDD electrodes can oxidize phenol effectively, and that the oxidation process was simple and complete compared to that of the Pt and graphite electrodes.
10 Figure 7. Cyclic voltammograms of three electrodes in 400.0-[mg/L] phenol +1.0-[mol/L] H 2SO 4: (1) BDD, (2) graphite, and (3) Pt electrode. 4. Conclusions Ballas boron-doped diamond was deposited onto porous activated carbon substrates. After deposition, the sample was primarily in the diamond phase, with very low amorphous-phase and graphite content. The diamond electrode exhibited a very wide potential window and very low background current, and the potential windows in acidic, neutral, and alkaline media were 4.3, 4.0, and 3.0 [V], respectively. The background current was as low as -8 10-6 -5 10-7 [A], which is considerably smaller than that of the platinum electrode. In a ferri/ferrocyanide electrolyte, the BDD electrode surface maintained good activity, and the electrode response occurring on the surface was a diffusion-controlled reaction with good quasi-reversibility. Further, BDD electrodes can oxidize pollutants such as phenol very easily, effectively, and completely. 5. Acknowledgments This work was supported by the Innovation Program of Shanghai Municipal Education Commission (13ZZ143, 2015Z11458002). References 1. H.B. Martia, A. Argoitia, J.C. Augus, U. Lanau, Voltammetry studies of single-crystal and polycrystalline diamond electrodes, J. Electrochem. Soc. 146, 2959 (1999).
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