J Electroceram DOI 10.1007/s10832-014-9966-5
A novel, effective and low cost catalyst for formaldehyde electrooxidation based on nickel ions dispersed onto chitosan-modified carbon paste electrode for fuel cell Seyed Karim Hassaninejad–Darzi
Received: 17 March 2014 / Accepted: 31 August 2014 # Springer Science+Business Media New York 2014
Abstract In this study, a novel modified carbon paste electrode (CPE) was fabricated by chitosan and then Ni2+ ions incorporated to this electrode by immersion of the modified electrode in a 0.5 M nickel chloride solution. The values of charge-transfer rate constant and electrode surface coverage for Ni(II)/Ni(III) redox couple and redox sites of modified carbon paste electrode (Ni-CHIT/CPE) were found to be 0.174 s−1 and 7.38×10−8 mol cm−2, respectively. The electrochemical behavior of the Ni-CHIT/CPE electrode towards oxidation of formaldehyde was evaluated by cyclic voltammetry technique as well as chronoamperometry method. It has been observed that chitosan at the surface of CPE can improve catalytic efficiency of the dispersed nickel ions toward oxidation of formaldehyde. The values of electron transfer coefficient, diffusion coefficient and the mean value of catalytic rate constant for formaldehyde and redox sites were obtained to be 0.47, 2.68×10−6 cm2 s−1 and 2.06×105 cm3 mol−1 s−1, respectively. Obtained results from cyclic voltammetry and chronoamperometric techniques specified that the electrode reaction is a diffusion-controlled process. The good catalytic activity, high sensitivity, good stability and easy in preparation rendered the Ni-CHIT/CPE to be a capable electrode for electrooxidation of formaldehyde.
Keywords Chitosan . Modified CPE . Formaldehyde . Electrocatalytic oxidation, Fuel cell
S. K. Hassaninejad–Darzi (*) Department of Chemistry, Faculty of Science, Babol University of Technology, Babol, Mazandaran, IranP.O.Box: 47148–71167 e-mail:
[email protected]
1 Introduction Electrocatalytic oxidation of small organic molecules such as CH3OH, C2H5OH, HCHO and HCOOH, on the surface of different modified electrodes have received special attention, due to their great potential for utilization as electron donors in fuel cells and generation of high power density [1]. Although formaldehyde is toxic and not very suitable for fuel cells, study of its electrochemical oxidation is important for a full understanding of methanol oxidation. This component is one of the intermediate products in methanol oxidation process [2]. Many efforts have been tried for the electrooxidation of formaldehyde including platinum and platinum alloys electrodes [3–9], copper and copper alloys electrodes [10, 11], gold electrode [12], palladium nanoparticle electrodes [13–15] and nickel based electrodes [16–20]. It is important to develop a novel electrode that has high sensitivity and stability for the electrooxidation of formaldehyde as anode. Modified electrodes provide an excellent way to accelerate charge transfer processes, decrease the over potentials as well as to increase the intensity of the corresponding voltammetric responses [21]. Chitosan (CHIT) is a converted polysaccharide and is generated commercially by deacetylation of chitin, which is the structural element in the exoskeleton of crustaceans (such as crabs and shrimps) [22]. It has a number of commercial and possible biomedical applications [23]. It has also been defined as a promising material for modification of the electrode surface, due to its attractive properties, such as good stability, high permeability and strong adherence to the electrode surface, no toxicity, and low cost material. Recently, much attention has been paid to the study of the application of chitosan in electroanalytical chemistry [24–27], for as a new modifier, chitosan is harmless to the environment. Some works were focused on the application of CHIT in the modification of electrode such as Pd-MWCNTs into CHIT-glassy carbon electrode (GCE) [24], MWCNTs-
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CHIT/GCE [25], Gold-chitosan nanocomposite [26] and chitosan scaffold electrode for fuel cell applications [27]. The chitosan structure contains free hydroxyl and amino functional groups, which can form chemical bonds to other biomolecules and can act as a sorbent of Ni(II) or other catione. Several electrochemical sensors have been fabricated based on the interaction between chitosan and analytes [28–40]. In respect of literature survey, no chitosan modified carbon paste electrode was employed hitherto towards electrocatalytic oxidation of formaldehyde. The aim of this work presented here is to develop a new chitosan modified carbon paste electrode (CPE) to form complex with Ni2+, which provided a simple, sensitive, rapid and low-cost sensor for electrocatalytic oxidation of formaldehyde in the alkaline medium.
2 Experimental 2.1 Reagents and materials Sodium hydroxide, formaldehyde, NiCl2.6H2O, potassium hexacyanoferrate (K4Fe(CN)6) and potassium chloride were purchased from Merck company that were of analytical reagent grade. Chitosan (75–85 %, low molecular weight) was prepared from Aldrich company. Graphite powder and paraffin oil (d=0.88 g cm−3) as the binding agent (both from Daejung company) were used for preparing the pastes. All materials were used without any further purification. Also, all solutions were prepared with deionized water. 2.2 Apparatus The electrochemical experiments were performed at room temperature using potentiostat/galvanostat electrochemical analyzer (Ivium, Netherlands, V11100) with a voltammetry cell in a three electrodes configuration. The platinum wire and Ag|AgCl|KCl (3 M) were used as auxiliary and reference electrodes, respectively. The bare CPE and chitosan modified carbon paste electrodes (CHIT/CPE) were used as working electrodes. 2.3 Preparation of working electrode In a typical electrode preparation, diethyl ether was added to a mixture of 0.02 g chitosan 0.18 g graphite powder. After hand mixing with a mortar and pestle, the solvent was evaporated with stirring. Then, paraffin oil (35 % wt) was blended with the mixture in a mortar by hand mixing for 25 min until a uniformly wetted paste was obtained. This paste was packed into the end of a glass tube (ca. 0.35 cm i.d. and 10 cm long) and the copper wire was utilized for electrical contact. A new surface was achieved by pushing an excess of the paste out of the tube and polishing with a weighing paper. For comparison,
unmodified CPE (bare CPE) was also prepared in the same mentioned method.
3 Results and discussion 3.1 Electrochemistry of fabricated electrodes Cyclic voltammetry (CV) was used for investigation of electrochemical properties of fabricated bare CPE and chitosan modified CPE (CHIT/CPE) in potassium ferricyanide (K4Fe(CN)6) solution. Figure 1 illustrates the CVs of the electrochemical oxidation of K4Fe(CN)6 at the surface of the bare CPE and CHIT/CPE electrodes in the 10 mM of K4Fe(CN)6 plus 0.1 M of KCl solution. As can be seen in Fig. 1, the anodic and cathodic peak currents for CHIT/CPE is higher than that at bare CPE. The experimental results show reproducible anodic and cathodic peaks ascribed to Fe(CN)63 − /Fe(CN)64− redox couple at slow scan rates at the surface of CHIT/CPE. This is a quasi-reversible system because the peak separation potential, ΔEp (Epa −Epc), is equal to 135 mV (0.310−0.175) and is greater than 59 mV that expected for a reversible system. The ΔEp at the surface of bare CPE was obtained to be 180 mV and is greater than that at CHIT/CPE. The obtained result from CVof CHIT/CPE in various buffered solutions with different pH value does not show any shift in the anodic peak potentials for oxidation of K4Fe(CN)6. It can be deduced that the electrochemical behavior of the Fe(CN)63 − /Fe(CN)64− redox couple in the CHIT/CPE electrode is not dependent on the pH of solution. The active surface areas of the fabricated electrodes were calculated by the CV method at various scan rates using K4Fe(CN)6 as a probe. For this approach, the following Randles–Sevcik formula can be used [41, 42]: Ipa ¼ 0:4463 A C0
3 . 1 = 3 1 = 1 2 F n =2 DR 2 ν =2 RT
ð1Þ
where Ipa (A) refers to the anodic peak current, A (cm2) is the active surface area of the electrode, C0 (mol cm−3) is the concentration of K 4 Fe(CN) 6 , F is Faraday’s constant (96,485 C mol −1 ), R is the universal gas constant (8.314 J mol−1 K−1), T is the absolute temperature (298 K), n is the electron transfer number, D (cm2 s−1) is diffusion coefficient and υ (V s−1) is the scan rate. For 1.0 mM K4Fe(CN)6 in 0.1 M KCl as supporting electrolyte the amount of n and DR is 1 and 7.6×10−6 cm2 s−1, respectively [41, 42]. Figure 2 illustrates CVs for the oxidation of K4Fe(CN)6 at various scan rates and plot of Ipa versus υ1/2 at the surface of bare CPE and CHIT/CPE. In this experiment, the slopes were 6.1579×10−5 A (V−1 s−1)1/2 and 8.5885×10−5 A (V−1 s−1)1/2 for bare CPE and CHIT/CPE, respectively. Therefore, the active microscopic
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Fig. 1 The CVs of a bare CPE and b CHIT/CPE in the presence of 10 mM K4Fe(CN)6 solution at a scan rate of 20 mV s−1 and pH of 7.0 in 0.1 M KCl as supporting electrolyte
surface areas were found to be 0.083, 0.116 cm2 for bare CPE and CHIT/CPE electrodes, respectively. These results indicated that modification of CPE with chitosan causes to increase the active surface area of the electrode. 3.2 Electrochemical behavior of fabricated electrodes in alkaline solution Inset in Fig. 3 shows CVs of bare CPE and CHIT/CPE in 0.1 M NaOH solution at the potential range from 0.2 to 0.7 V vs. Ag|AgCl|KCl (3 M) and potential sweep rate of 20 mV s−1. The obtained results showed that no current can be obtained with these electrodes and the background current for CHIT/CPE is much larger than that at bare CPE. In order to incorporate Ni(II) ions at the surface of the CPE and CHIT/CPE electrodes, these electrodes was placed in a well-stirred aqueous solution of 0.5 M NiCl2 at an open circuit for 10 min at 150 rpm and then washed completely with distilled water to remove the surface adsorbed species. Fig. 2 The CVs of bare CPE (a) and CHIT/CPE (b) at various scan rates of 10, 25, 50, 75, 100, 125, 150, 175, 200, 250, 300, 350, 400 and 450 mV s−1 for the oxidation of 1 mM K4Fe(CN)6 in 0.1 M KCl as supporting electrolyte, and c The plot of Ipa and Ipc versus υ1/2 at the surface of bare CPE and CHIT/CPE
Figure 3a and c illustrates CVs of Ni-CPE and Ni-CHIT/ CPE electrodes (CPE and CHIT/CPE that immersed in the 0.5 M NiCl2) in 0.1 M NaOH solution and potential sweep rate of 20 mV s−1. It can be deduced that the electrochemical behavior of Ni-CHIT/CPE modified electrode in alkaline solution is similar to that of Ni anode [21, 43, 44]. These redox waves are ascribed to the oxidation of Ni(OH)2 at the chitosan/ electrolyte interface to NiOOH and reduction of NiOOH to Ni(OH)2 with a peak potential of 0.49 and 0.38 V vs. Ag|AgCl|KCl (3 M), respectively [41, 43]. By comparison of Fig. 3a and c, it can be indicated that the presence of chitosan in the electrode construction performed a significant effect to accumulation of nickel species on the surface of electrode. Because, the chitosan structure contains free hydroxyl and amino functional groups, it can act as a sorbent of Ni2+ cation [28–30]. After stabilization of nickel species on the surface of the electrode, results indicated that the anodic and cathodic peak currents for Ni-CHIT/CPE were much greater than those of Ni-CPE. The peak-to-peak potential separation (ΔEp) of Ni(OH)2 to NiOOH and reverse conversion is found to be 110 mV at the surface of Ni-CHIT/CPE modified electrode at the scan rates of 20 mV s−1 (see Fig. 3c). The electrochemical oxidation of the formaldehyde was investigated at the surface of Ni/CPE and Ni-CHIT/CPE in 0.1 M NaOH solution. Figure 3b and d were displayed the CV for electrocatalytic oxidation of the formaldehyde in the surface of Ni-CPE and Ni-CHIT/CPE electrodes in 0.1 M NaOH and 0.015 M formaldehyde at scan rate of 20 mV s−1. In the presence of formaldehyde, an increase in current was observed at the surface of Ni-CHIT/CPE (Fig. 3d) but no significant change in the current intensity was observed at the surface of Ni-CPE (Fig. 3b). Comparison of curves b and d in Fig. 3 demonstrates that incorporation of CHIT onto a carbon paste electrode enhances the electrochemical signal of formaldehyde oxidation. Furthermore, the presence of free
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amino and hydroxyl groups in CHIT can be responsible for an electrostatic interaction with the nickel cation. The CHIT could act as a scavenger toward the target molecule (i. e. formaldehyde), which once close to nickel can easily react and promote the electrocatalytic oxidation of formaldehyde [26–28]. The CHIT in the construction of modified electrode can enhance the repeatability of the modified electrode by its antifouling effect [25]. Also, these Figs. were revealed that anodic peak current for formaldehyde oxidation is 682.6 μA, meanwhile anodic current is 319.7 μA for Ni(OH)2/NiOOH conversion in the same scan rate (i. e. 20 mV s−1) at the surface of Ni-CHIT/CPE. It can be observed in Fig. 3d that the oxidation of formaldehyde gives rise to a typical electrocatalytic response, with an increase in the anodic peak current and a decrease in the cathodic peak current. The oxidation potential of formaldehyde is observed at ca. 0.56 V that is positive than the potential observed for Ni2+ to Ni3+ transition at the surface of Ni-CHIT/CPE in the absence of formaldehyde (i.e. 0.49 V). This matter suggests an interaction between the formaldehyde and the redox sites of modified electrode surface. The electrochemical behavior of Ni-CHIT/CPE was investigated in 0.1 M NaOH at various scan rates. Figure 4a displays the CVs of the Ni-CHIT/CPE electrode in 0.1 M NaOH at various scan rates. Obviously, the anodic and cathodic currents were enhanced with increasing of scan rate and a potential moved to positive values. The positive shift may be due to the kinetic limitation in diffusion layer which created at high current density. The ΔEp was increased with scan rate that indicated a limitation in the charge transfer kinetics due to the chemical interactions between the electrolyte ions and the modified electrode. This observation is possible according to a theory defined by Laviron for the linear potential sweep voltammetric response in the case of surface confined electroactive species at the small concentrations [45]. The
Fig. 3 The CVs of the Ni/CPE and Ni-CHIT/CPE in the absence (a and c) and presence (b and d) of 0.015 M formaldehyde in 0.1 M NaOH at scan rate of 20 mV s−1, respectively, and Inset shows CVs of the CPE and CHIT/CPE before immersion in 0.5 M NiCl2 solution
expressions for peak-to-peak separation of ΔEp >200/n mV, where n is the number of exchanged electrons, can be written as: 1−α Epa ¼ E0 þ Xln ð2Þ m
hαi Epc ¼ E0 þ Yln m
logks ¼ αlogð1−αÞ
ð3Þ
αð1−αÞnFΔEp RT þ ð1−αÞlogα−log − ð4Þ nFν 2:30RT
where X=RT/(1−α)nF, Y=RT/αnF, m=(RT/F) (ks/nυ), Epa and Epc are the anodic and cathodic peak potentials, respectively. Also, α, ks, and υ are the electron-transfer coefficient, apparent charge-transfer rate constant and scan rate, respectively. From these expressions, the α can be determined by measuring the variation of the peak potential with respect to the scan rate and ks can be determined for electron transfer between the electrode and the surface layer by measuring the Ep values. Figure 4b is depicted plot of the Ep versus logarithm υ in the ranges of 0.010–0.400 V s−1 for both anodic and cathodic peaks that data extracted from CVs of Ni-CHIT/CPE in 0.1 M NaOH solution. It can be observed that Ep is proportional to log υ at υ>0.100 V s−1 demonstrated by Laviron [45]. From Fig. 4b and Eqs. (2) and (3), the value of anodic electron transfer coefficient (α) is obtained to be 0.73. This results show that the rate limiting steps for cathodic and anodic might not be the same step [46]. According to the Eq. 3, the mean value of charge-transfer rate constant (ks) is found to be 0.174 s−1. Figure 4c presents plots of anodic and cathodic peak currents for oxidation–reduction of the NiOOH/Ni(OH)2 redox couple versus scan rate at low values from 0.010 to 0.075 V s−1. This dependence is probably due to electrochemical activity of immobilized redox species at the surface of modified electrode. The electrode surface coverage (Γ*) can be calculated from the linear part of the plot and using the following equation which correspond to reversible process with adsorbed species [47]. Ip ¼
n2 F2 Γ* Aν 4RT
ð5Þ
J Electroceram Fig. 4 a The CVs of the NiCHIT/CPE in 0.1 M NaOH at various scan rates from inner to outer: 0.010, 0.015, 0.025, 0.040, 0.050, 0.075, 0.100, 0.125, 0.150, 0.200, 0.250, 0.300, 0.350 and 0.400 V s−1, b The plot of Ep vs. log υ for CVs showed in the a for anodic peaks a and cathodic peaks b, c The dependency of Ipa a and Ipc b on lower values of υ (0.01–0.060 V s−1), and d The plot of Ipa a and Ipc b on υ1/2 at higher values of υ (υ> 0.100 V s−1)
where Ip, n, Γ* and A are the peak current, the number of electrons involved in the reaction (n = 1), the surface coverage of the redox species and the surface area of the electrode (0.0962 cm2), respectively. The total surface coverage of the immobilized active species is found to be about 7.38×10−8 mol cm−2, considering the mean of both anodic and cathodic currents. At scan rates larger than 0.10 V s−1, both the anodic and cathodic peak currents depend on root mean square of scan rate (υ1/2) that express a diffusion controlled process dominates with increasing the scan rates (see Fig. 4d). This limiting-diffusion process can be related to the charge neutralization of the electrode surface during the oxidation/reduction process [48, 49]. 3.3 Effects of formaldehyde concentration and scan rate As can be seen in Fig. 3c and d, the anodic peak around 0.49 V corresponding to the Ni(OH)2/NiOOH convertion was disappeared by addition of formaldehyde; meanwhile, one enhanced oxidation peak appears at around 0.56 V vs. Ag/AgCl/KCl (3 M). Also, cathodic peak current around 0.38 V decreased after h
addition of formaldehyde. It can be deduced that the over potential for electrooxidation of formaldehyde was decreased in the surface of Ni-CHIT/CPE electrode. This result indicated that the applied modifier (i.e. chitosan) in this process participates directly in the electrocatalytic oxidation of formaldehyde. It can be specified that the formaldehyde molecule is completely hydrated and converted to the methylene glycol (CH2(OH)2) with an equilibrium constant on the order of 2.28 ×103 in aqueous solution [50]. The methylene glycol exists predominantly in its ionized form (CH2(OH)O−) in 0.1 M NaOH solution due to its pKa of ca. 12.8. When CH2(OH)O− diffuses from the bulk solution to the electrode surface and is quickly oxidized to CH 2 (O)O − by the NiOOH species on the surface of Ni-CHIT/CPE electrode. Therefore, the amount of NiOOH species decreases due to its chemical reaction with CH2(OH)O−. In the overall reaction, formaldehyde can be converted to the CH2(O)O− and generated one electron (see Fig. 5). Simply, this behavior is a typical observation expected from the mediated oxidation (EC′ mechanism), illustrated in the following equations [18, 51–53]:
. i h . i NiðOHÞ2 −CHIT CPE þ OH− ⇄ NiOOH−CHIT CPE þ H2 O þ e− ðEÞ h . i h . i NiOOH−CHIT CPE þ CH2 ðOHÞO− ⇄ NiðOHÞ2 −CHIT CPE þ CH2 ðOÞO− Ć
J Electroceram Fig. 5 Representative schematic for mechanism of formaldehyde electrooxidation at the surface of Ni-CHTT/CPE
Voltammetric responses of the five different modified electrodes with chitosan ratio of 6, 8, 10, 12 and 14 % (w/w) with respect to the graphite powder were studied by CV technique at 0.1 M of NaOH solution. The CVs of Ni(OH)2/NiOOH oxidation on different modified electrodes showed that higher anodic current is obtained in 10 % of chitosan with respect to the graphite powder (see Fig. 6a). It is suggested that at low ratio of CHIT, the amount of CHIT is so low in the surface of modified electrodes that the available site for Ni2+ insertion can be decreased. Also, with increasing the CHIT over than 10 % in the modified electrodes, the peak shape was distorted and anodic current was decreased [54]. The amount of CHIT in the modified electrodes had a significant effect on the anodic current of formaldehyde oxidation. The voltammetric signal of above modified carbon paste electrodes were investigated under identical conditions in 0.012 M formaldehyde at 0.1 M of NaOH solution (see Fig. 6b). As can be seen in Fig. 6b, maximum current was obtained when CHIT amount in the paste was 10 % and it was used throughout as an optimum value for fabrication of modified electrode. It is suggested that at low ratio of CHIT, the amount of produced NiOOH is so low in the modified electrodes that can be Fig. 6 a The variation of anodic peak for Ni(OH)2/NiOOH oxidation vs. CHIT percentage in the preparation of modified electrodes, and b The variation of anodic peak current vs. CHIT percentage for electrocatalytic oxidation of 0.012 M formaldehyde
decreased anodic current of formaldehyde oxidation. When the amount of CHIT was higher than 10 % (w/w) with respect to the graphite powder, the capacitive current was greater, with the peak becoming broad and the peak currents decreased. This could be related to a decrease in the graphite value of the paste and, therefore, reduction of the conductive electrode area hindering the electron transfer at the electrode surface and increasing ohmic losses [55]. This amount of 10 % (w/w) CHIT with respect to the graphite powder was select as an optimum value for fabrication of modified electrode in the following experiment. Figure 7 illustrates the influence of formaldehyde concentration on its electrooxidation current onto Ni-CHIT/CPE at scan rate of 20 mV s−1. It is clearly observed that the anodic peak current increased with increasing of formaldehyde up to the concentration of 0.09 M. In the concentrations above 0.09 M, no remarkable increase was observed in the anodic peak current. It can be specified that this effect may be due to the saturation of active sites and/or poisoning the electrode surface with adsorbed intermediates. Thus, 0.09 M of formaldehyde represented the optimum concentration after which the adsorption of the oxidation products at the surface of electrode
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Fig. 7 The CVs of the Ni-CHIT/CPE in 0.1 M NaOH solution with various concentrations of formaldehyde: a 0.0, b 0.003, c 0.005, d 0.01, e 0.015, f 0.02, g 0.025, h 0.03, k 0.035, l 0.045, m 0.055, n 0.065, o 0.075,
p 0.09 and q 0.1 M, at scan rate of 20 mV s−1, and Inset shows plot of electocatalytic current versus formaldehyde concentration
may cause the stoppage of further oxidation. Also, it can be noted that at high concentration of formaldehyde, there
appears to be an obvious deviation from the linearity response, due most probably to the kinetic limitation.
Fig. 8 (a) CVs of the Ni-CHIT/ CPE in the presence of 0.012 M formaldehyde in 0.1 M NaOH at various scan rates from inner to outer: 0.010, 0.020, 0.040, 0.065, 0.080, 0.100, 0.125, 0.150, 0.175, 0.200, 0.250, 0.300, 0.350 and 0.400 V s−1, Variation of b Ipa vs. υ1/2 and c Ipa vs. υ for CVs showed in the a for anodic peaks
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Figure 8a displays CVs of Ni-CHIT/CPE in the presence of 0.012 M formaldehyde in 0.1 M NaOH at scan rates between 10 and 400 mV s−1. By an increase in the scan rate, the anodic peak currents (Ipa) were increased and the anodic peak potentials shifted to more positive directions. It can be proposed that a kinetic limitation was existed in the reaction between the redox sites of Ni-CHIT/CPE and formaldehyde molecule. Instead, simpler explanation of this shift might be due to ohmic drop. The increase in the peak current with the scan rate can be considered an adsorption or diffusion control of the process. Figure 8b displays the plot of Ipa versus square root of scan rate (υ1/2) meanwhile Fig. 8c shows plot of Ipa versus scan rate (υ) obtained from CVs of Ni-CHIT/CPE in 0.012 M formaldehyde and 0.1 M NaOH solution. A plot of Ipa versus the scan rate (υ) in the range of 10–400 mV s−1 did not show an linear curve (Fig. 8c) meanwhile, a plot of Ipa versus the square root of scan rate (υ1/2) was found to be linear with equation of Ipa (μA)=92.616 υ1/2 (mVs−1)1/2 +123.02 and R2 =0.9973 (Fig. 8b). This observation indicated that this process is diffusion-controlled process rather than surfacecontrolled process. Also, Fig. 8b displays that the oxidation current for formaldehyde increased linearly with the square
root of the scan rate, indicating that the reaction is mass transfer controlled [47]. From theoretical point, a slope of 0.5 or 1.0 is expected for the plot of log Ipa vs. log υ under diffusion or adsorption control, respectively [43]. A linear dependence is observed between log Ipa and log υ at the surface of Ni-CHIT/CPE in the oxidation of formaldehyde (Fig. 9a). From linear section, the slope of 0.4337 is found that is near to the theoretically predicted value of 0.5 for a purely diffusion-controlled current. However, the contribution of a kinetic limitation to the overall process cause to the small alteration with theoretical value [56]. Also, Fig. 9b displays a plot of scan rate normalized current (Ipa/υ1/2) vs. logarithm scan rate (log υ). This Fig. shows the characteristic shape of an EC′ process which express that the electrode reaction is coupled with an irreversible follow up chemical step [57, 58]. Also, at higher potential scan rates, the cathodic peak current relating to Ni(III) reduction to Ni(II) appears which confirms EC′ mechanism [59]. In order to obtain information about the rate determining step, a Tafel plot was depicted for Ni-CHIT/CPE using the data derived from the raising part of the current–voltage curve. Figure 9c displays CVof the Ni-CHIT/CPE in the presence of
Fig. 9 a The plot of log Ipa vs. log υ and b Ipa/υ1/2 vs. υ on the Ni-CHIT/ CPE in the presence of 0.012 M formaldehyde in 0.1 M NaOH at various scan rates, c CV of the Ni-CHIT/CPE in the presence of 0.012 M
formaldehyde at scan rate of 20 mV s−1, and d The plot of log I vs. Ep (Tafel plot) for CV showed in the C
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As can be seen in the inset of Fig. 7, no significant increase in the anodic peak current was observed in the concentrations above 0.09 M of formaldehyde. In fact, oxidation process seems to be limited by the catalytic process, and its rate depends on the reaction between formaldehyde and NiOOH species in the film. In order to support our conclusion, we investigated the effect of scan rate on the ratio of anodic to cathodic current of Ni-CHIT/CPE in the absence and the presence of 0.012 M formaldehyde (see Fig. 10). This Fig. exhibits that the ratio of Ipa/Ipc in the presence of formaldehyde was decreased significantly by increasing in the scan rate. Therefore, reduction in the time window for formaldehyde oxidation at higher scan rates avoided the facile electron transfer between NiOOH species and formaldehyde [53]. Fig. 10 Variation of the Ipa/Ipc ratio of Ni-CHIT/CPE with scan rate in the absence a and presence of 0.012 M formaldehyde b in 0.1 M NaOH solution
3.4 Chronoamperometric studies
0.012 M formaldehyde at scan rate of 20 mV s−1 in 0.1 M NaOH solution and Fig. 9d shows corresponding plot of log Ip vs. Ep. The Tafel slope was found to be 8.9539 V, which indicates that transfer coefficient (α) for mediated electrooxidation of formaldehyde is about 0.47.
To evaluate the electrocatalytic performance of the Ni-CHIT/ CPE for the formaldehyde oxidation, we performed some chronoamperometric measurements. Figure 11a illustrates double step chronoamperometric measurements at the surface of the Ni-CHIT/CPE with several concentrations of
Fig. 11 a The double step chronoamperograms of Ni-CHIT/CPE in 0.1 M NaOH solution in the absence a and presence of b 0.003, c 0.015, d 0.02 and e 0.025 of formaldehyde (The potential steps were 0.65 and 0.350 V vs. Ag/AgCl/ KCl (3 M)), b Charge-time curves in the
absence a/ and presence of 0.003 M of formaldehyde b/, c Dependence of Icat/IL on t1/2, derived from the data of chronoamperograms a and c, and d Dependence of I on t−1/2, derived from the data of chronoamperogram d
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formaldehyde (i.e. 0.0, 0.003, 0.015, 0.02 and 0.025 M). The applied potential steps were 0.65 (in the first step) and 0.35 V (in the second step) vs. Ag|AgCl|KCl (3 M) determined based on peak potential of redox process at various concentrations of formaldehyde. It was found that the observed current from chronoamperograms was in good agreement with the observed current from CV experiments and the current increases as the formaldehyde concentration increases (curves b–e). This result supports our conclusion about the catalytic role of NiOOH for oxidation of formaldehyde that its oxidation starts directly after the formation of the first amount of NiOOH on the surface of electrode [60]. The forward and backward potential step chronoamperometry of the Ni-CHIT/ CPE electrode in the blank solution showed an almost symmetrical chronoamperogram, which shows that almost equivalent charges were consumed for the oxidation and reduction of surface confined Ni(OH)2/NiOOH sites. However, in the presence of formaldehyde, the charge value associated with the forward chronoamperometry is greater than that observed for the backward chronoamperometry (see Fig. 11b). Chronoamperometry can be applied for the evaluation of the catalytic rate constant (kcat) of the electrocatalytic oxidation of formaldehyde on the active sites of the modified electrode according to the following equation [46]: 1 1 1 1 I cat ¼ γ =2 π =2 ¼ π =2 ðk cat co t Þ =2 IL
ð7Þ
where, Icat and IL are the currents in the presence and absence of formaldehyde, respectively. The symbol kcat is the catalytic rate constant (cm3 mol−1 s−1), c0 is the bulk concentration of formaldehyde (mol cm−3) and t is the elapsed
time (s). Figure 11c displays plot of Icat/IL versus t1/2 derived from the data of chronoamperograms in the absence of formaldehyde (a) and in the presence of 0.015 M formaldehyde (c). From the slopes of the Icat/IL versus t1/2 for all concentrations, the mean value of k cat was calculated to be 2.06 × 105 cm3 mol−1 s−1. Comparison of the estimated kcat with other kcat values in the literature is performed and presented in Table 1 [10, 17–19, 53]. An exponential behavior of I–t curves shows that a diffusion-controlled process has occurred according to Cottrell equation. From the chronoamperometric study, the diffusion coefficient of formaldehyde (D) was determined in aqueous solution by Cottrell equation [49, 61]: −1= 1 −1 2 I ¼ nFACD =2 ⋅π =2 ⋅t
ð8Þ
where n is the number of electron (i.e. 1), F is the faraday number (96,485 C mol−1), A is the area of the electrode (0.0962 cm2), C is the known concentration of formaldehyde and D is the apparent diffusion coefficient. Figure 11d demonstrates experimental plots of I vs. t−1/2 for 0.02 M of formaldehyde at the surface of Ni-CHIT/CPE. The same curves were plotted for all concentrations and then the slopes of the resulting straight lines were plotted versus the formaldehyde concentration. From the slope of the resulting plots and using the Eq. 8, the mean value of the D was obtained to be 2.68×10−6 cm2 s−1. The current is negligible when potential is stepped down to 0.35 V, indicating the irreversibility of formaldehyde oxidation process. A linear relationship in the plot of I vs. t−1/2 indicated that the electrode reaction is a diffusion-controlled process and this result is in good
Table 1 Comparison of the electrocatalytic behavior of Ni-CHIT/CPE for oxidation of formaldehyde with some of the previously reported electrodes Electrode
Electrolyte
Saturation limit of formaldehyde (M)
Ep/V vs. (Ag/ AgCl)
Current density (mA cm−2)
kcat (cm3 mol−1 s−1)
Ref.
Pt/SWCNT/PANI
0.5 M HClO4 0.5 M H2SO4 0.1 M H2SO4 0.1 M NaOH 0.2 M NaOH 0.1 M NaOH 0.1 M NaOH 0.1 M NaOH 0.1 M NaOH 0.1 M NaOH
0.50
0.66
90.0
–
[6]
0.50
0.45
7.32
–
[7]
0.75
0.85
31.40
–
[8]
0.30
0.15
9.40
–
[14]
0.17
0.63
25.56
7.16×106
[10]
0.17
0.80
7.6
2.0×106
[17]
0.1
0.6
16.5
1.58×106
[53]
0.048
0.70
12.70
1.1×105
[18]
0.07
0.74
4.10
8.96×104
[19]
0.09
0.56
7.09
2.06×105
This work
Pt/PAANI/MWNTs/GCE Pt/Carbon-Ceramic Pd–CILE Cu/P(2ADPA)/MCNTPE Ni/P(1,5-DAN)/MCPE Ni/IL/CPE Ni(OH)2/POT (TX-100)/ MCNTPE Ni/P(NMA)/MCPE Ni-CHIT/CPE
J Electroceram
agreement with cyclic voltammetric experiments (see previous section). 3.5 Reproducibility and stability of the Ni-CHIT/CPE The reproducibility and stability of the Ni-CHIT/CPE were evaluated via the comparison of the currents of different electrodes using CV technique. The anodic current of five modified electrodes (Ni-CHIT/CPE) to electrocatalytic oxidation of 0.012 M formaldehyde was tested independently, and the relative standard deviation (RSD) is found to be 3.84 %. A reproducible current response with a RSD of 2.85 % was observed for 5 successive assays of 0.012 M formaldehyde. The long-term stability was explored by measuring a formaldehyde solution alternatingly, and the fabricated electrode was stored at room temperature when it is not in use. After 1 and 3 months, the electrode response to electrocatalytic oxidation of formaldehyde retains 92 and 85 % of initial value, respectively. In comparison with some other previous works [6–8, 10, 14, 17–19, 53], it seems clearly that nickel hydroxide in the Ni-CHIT/CPE can act as a comparable catalyst in the oxidation of formaldehyde. Besides, the surface modification of the electrode is very simple and reproducible compared that other modified electrodes and this novel catalyst can be utilized in fuel cell system.
4 Conclusions In this research, the role of chitosan in electrocatalytic process of formaldehyde oxidation was investigated with fabrication of Ni-CHIT/CPE. This process was investigated using cyclic voltammetry and chronoamperometry techniques. It was seen that in the presence of formaldehyde, the anodic oxidation current increases whereas the cathodic current decreases at the surface of Ni-CHIT/CPE. The modified electrode can enhance the oxidation of formaldehyde through a decrease in over potential of formaldehyde oxidation and overcome the low kinetic of reaction in alkaline solution. This modified electrode showed good electrocatalytic activity toward formaldehyde versus Ni-CPE and comparable to many of the previously reported electrodes. The free hydroxyl and amino functional groups in chitosan structure provide a framework for Ni2+ uptake which convert to Ni(OH)2 and NiOOH during the anodic oxidation in alkaline solution and collaborate in electrocatalytic oxidation of formaldehyde. Also, the CHIT in the modified electrode can enhance the repeatability of the modified electrode by its antifouling effect. It can be specified from obtained results that this non-noble catalyst had some advantages such as low cost and stability, ease of preparation and regeneration, stable response and very low ohmic resistance.
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