Elaboration of carbon paste electrode containing pentadentate Nickel-(II) Schiff base complex: Application to electrochemical oxidation of thiosulfate in alkaline medium

2.1 Chemicals

All chemicals were obtained from commercial sources and were used as received without any further purification. All solvents, absolute ethanol and dichloromethane are obtained from Fluka while N, N’-bis (3-aminopropyl) amine and salicylaldehyde are purchased from Sigma Aldrich. Nickel acetate tetrahydrated and NaOH were obtained from Fluka and Na₂S₂O₃ in analytical grade from Merck.

2.2 Instrumentation

Electrochemical experiments were carried out using a 301/10 Potentiostat / Galvanostat type PGZ 301-Voltalab 10 Radiometer with PC work station and electrochemical interface IMT 301 with Volta Master 4 software. An electrochemical standard three-electrode cell of 10 ml was used. Potentials are referred to a (Hg/Hg₂Cl₂) saturated with KCl reference electrode. A Platinum wire was used as counter electrode. The different electrodes were examined using a Hitachi SU 8000 cold field emission scanning electron microscope.

2.3 Preparation of Ni ^II-L Nickel Schiff base complex

The Nickel-(II) [N,N’-bis(Salicylidenepropylenetriamine)] complex (Ni^II-L) was prepared according to the literature (Ourari et al., 2015; Charef et al., 2015) A mixture of the Schiff base (1 mmol) and the metal salt (1 mmol) [Ni (OAc) ₂·4H₂O], in 25 ml of ethanol was refluxed for 1–2 h. After cooling the mixture a greenish solid product was precipitated, collected by filtration, washed several times with hot ethanol and distilled water, then dried in air. The obtained Ni complex Ni^II-L is soluble in most organic solvents and insoluble in water.

2.4 Elaboration of the electrode

2.4.1

2.4.1 Carbon paste electrode

Carbon paste electrodes were prepared by mixing carbon powder with solid paraffin wax in the ratio 70/ 30 (w/w) respectively. Carbon powder was used as electric conductive support and paraffin wax as binder. These materials must be carefully mixed in an Agate mortar, to obtain a carbon paste sufficiently homogeneous for practical electrochemical uses. In our case, the composite material was filled in a syringe body of polyethylene (3 ml) in the side of needle (depth 3 mm, inner diam. 2 mm) and the electric contact is ensured by introducing a copper wire from the piston side as shown in Scheme 1.

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Scheme 1

Carbon paste electrode used for electrochemical studies and electrocatalysis applications.

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2.4.2

2.4.2 Modified carbon paste electrode (MCPE)

The modified carbon paste electrodes were prepared by the same manner than previously, masses of the Nickel Schiff base complex (Ni^II-L) equal to 2.5, 5, 7.5 and 10 mg were dissolved separately in 10 ml of dichlomethane each, then carbon powder was introduced to each solution as 97.5, 95, 92.5, and 90 mg respectively, the mixture was stirred for 1 h. After solvent evaporation the obtained solid was over night dried under vacuum to eliminate the residual solvent. In these formulations, the obtained modified carbon powders have the percentages of the modifier as 2.5, 5, 7.5, and 10% (w/w). For this, it was revealed that the most appropriate percentage for the electrochemical study was 5% of Ni^II-L. The MCPE was obtained by mixing the modified carbon powder with paraffin wax as binder in the ratio 70/ 30 (w/w) respectively for example we mix 70 mg of modified carbon powder with 30 mg of paraffin wax to obtain 100 mg of modified carbon paste ready for use(as shown in Scheme 2).

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Scheme 2

Ni (II)-Schiff base complex (Ni(II)L) mixed with carbon black and paraffin wax modified electrode used in electro-oxidation of thiosulfate.

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3.1 Characterization of new composite material

Fig. 1A shows the SEM micrographs of the modified carbon powder depicted at different magnitudes, presents a dark surface coated with well distributed clear particles with different shapes, having an average size less than 5 µm. Fig. 1 (B, C) shows the mapping of the different existing elements with EDX pattern, and confirms the presence of carbon with (95% w) which indicate that the most of the past constituted with carbon but it is less than expected, the presence of oxygen is not only from Schiff base but probably adsorbed from air on the carbon surface. Nickel presence is in good agreement with calculated amount; it should be around (0.75%w) it is present at (0.57% w) and well distributed on the surface. The elements analysis confirm the purity of carbon used in the preparation of the past, the small size of complex crystals and the good dispersion of nickel through carbon past, demonstrate the efficiency of the preparation protocol using CH₂Cl₂ as solvent.

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Fig. 1

(A) SEM image of modified carbon powder taken at different magnifications. (B) SEM image of modified carbon powder with at medium magnification from left to right, SEM image without mapping, mapping of carbon element C first in right mapping of Nickel element. (C) EDX pattern of modified carbon powder with element composition.

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3.2 Electrochemical behavior of the MCPE

Fig. 2 (A curves a and b) shows typical cyclic voltammograms (CV) of carbon paste electrode-CPE (without modifier) (curve a), and carbon paste electrode modified with Nickel Schiff base complex Ni^II-CPE (curve b) recorded in 0.1 M NaOH, between 0 and 650 mV/SCE at a scan rate of 15 mV.s⁻¹. In the absence of Ni^II complex no electrochemical signal is recorded. In contrast, in the presence of Ni^II complex a pair of redox peaks appears at 312 and 206 mV/SCE respectively, in anodic and cathodic regions.

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Fig. 2

(A) Cyclic voltammograms of (a) CPE, (b) Ni(II)L-CPE recorded in 0.1 M NaOH at a scan rate of 15 mVs⁻¹; (B) Cyclic voltammograms of MCPE in 0.1 M NaOH, at different scan rates: (a) 5, (b) 15, (c) 30, (d) 50, (e) 70, (f) 100, (g) 150, (h) 200, (i) 250, and (j) 300 mVs⁻¹; (C) Anodic and cathodic peak currents (Ipa/c) versus square root of the scan rate (ν^1/2); recorded in 0.1 M NaOH (D) Anodic and cathodic potentials (E_pa/c) versus Logν. recorded in 0.1 M NaOH. (F) The plot of anodic pic current Ipa in A vs square root of scan rate in Vs⁻¹ for the MCPE in 5 mM solution containing 1:1 ratio of K₃Fe(CN)₆/K₄Fe(CN)₆ + 0.1 M KCl, in the scan rate range from 15 to 100 mVs⁻¹.

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These two peaks correspond to the Ni^II/Ni^III redox couple (Di Girolamo et al., 2019).

The electrochemical reaction may be written according to Eq. (1):

During the forward scan, the anodic peak corresponds to the oxidation reaction of [Ni^II-L(OH)]⁻ (Ni^II) and leads [Ni^III-L(OH)] (Ni^III) species. Whereas during the reverse scan the cathodic peak is assigned to the reduction of [Ni^III-L(OH)] to [Ni^II-L(OH)]⁻ according to Eqs. (2) and (3) respectively.

The current values associated to this redox process are 45 µA for the anodic peak I_Pa and −36 µA for the cathodic one I_Pc, while the peak-to-peak separation ΔE_P is equal to 92 mV indicating that this redox system is not Nernstian but it is rather quasi-reversible (Godwin and Lyons, 2013; Neiva et al., 2016). By varying the scan rate in the range between 5 and 300 mV.s⁻¹ (Fig. 2B), the recorded voltammograms show a continuous increasing of both anodic and cathodic peak currents (i_pa, i_pc) accompanied with a shifting of anodic and cathodic peak potentials to a positive and negative values respectively, enlarging the peak-to-peak difference ΔE_P. The ipa/ipc ratios are systematically higher than unity. From these results, we can conclude that the redox couple Ni^II/Ni^III is in accordance with a slow electrochemical process. The linear dependency of anodic and cathodic peak currents (i_pa, i_pc) with the square root of the scan rate (v^1/2) Fig. 2 (curve C) can be attributed to an electrochemical activity of Niⁿ⁺ active species controlled by surface bound redox species (Abdel Hameed and Medany, 2017; Amani-Beni and Nezamzadeh-Ejhieh, 2018). Furthermore, the anodic and cathodic peak potentials (Epa, Epc) are proportional to the logarithm of the scan rate (Logν) as illustrated in Fig. 2 (curve D). These results are in accordance with those reported in the (Nosuhi and Nezamzadeh-Ejhieh, 2018; Tamiji and Nezamzadeh-Ejhieh, 2019) expressing probably an EC mechanism, due to the need of hydroxide ions for a charge compensation inducing a limitation of the charge transfer.

Our modified electrodes were fabricated from a composite material as mentioned, loading of the surface by electro-active species like Ni^II-L could not be homogeneous. The geometric surface area of the electrode surface is equal to 0.0314 cm² (Id = 1 mm) whereas the effective surface area was 0.0436, for its estimation CVs of MCPE at different scan rate were recorded in 5 mM solution containing 1:1 ratio of K₃Fe(CN)₆/ K₄Fe(CN)₆ + 0.1 M KCl, the diffusion coefficient (D) value for K₄Fe(CN)₆ in similar conditions has been reported about 6.56 10⁻⁶ cm² s⁻¹ (Konopka and McDuffie, 2002), based on the plot of anodic pic current versus square root of scan rate and the reported (D) value (Fig. 2E), the resulting line equation is y = 1.52 10⁻⁴ + 1.18 10⁻⁵, r² = 0.998, based on Randles-sevcik equation the effective surface area was calculated equal to 0.0436 cm². The surface coverage can be estimated from the equation Eq. (4)

The integration of charge under the oxidation wave at slow scan rate (15 mVs⁻¹) gives a quantity of charge equal to 0.24 mC.

With Q the quantity of charge calculate previously, n the number of exchanged electron equal to 1, F the Faraday constant and A the effective surface area equal to 0.0436 cm² after calculation the surface loading was estimated to 5.7 10⁻⁸ mol.cm⁻². By plotting the E_Pa vs Logv a linear behavior was shown for scan rate below 80 mVs⁻¹ with line equation of y = 0.455 + 0.048x, R² = 0.995. Basing on Laviron theory (Laviron, 1979; Laviron and Roullier, 1980) the transfer coefficient α was determined as given below:

The slope resulting from the line equation is presented in Fig. 2D:, for a scan rate ≥ 80 mV.s⁻¹, the slope is b/2 where b is $\frac{2,303RT}{\mathit{nF}(1-\alpha )}$ leading to a value of 0,048 V decade⁻¹.

k_s can be determined from Eq. (5):

3.3 Electro-catalytic oxidation of thiosulfate

The oxidation of thiosulfate was extensively studied in the literature using different modified electrodes. Formally, when this oxidation is occurred in alkaline medium using our modified electrode may be summarized by Eq.7:

However, the adsorbed intermediate species are not clearly known. The formation of tetrathionate S₄O₆²⁻ is more favored at low potential and high pH (Feng et al., 1995); Using CPE in 0,1M NaOH and in presence of 10 mM of thiosulfate Fig. 3 (A, curve a), the thiosulfate oxidation reaction takes place at around 1 V/SCE. By using MCPE in presence of thiosulfate its oxidation reaction was observed at 420 mV/SCE with a gain in potential value around 600 mV compared to CPE only. In addition, a significant current was recorded at slightly more positive potential compared to that obtained in the absence of TS Fig. 3 (A, curve band c), representing a strong catalytic activity of the MCPE toward thiosulfate oxidation reaction (Keivani et al., 2017). A full recovery of cathodic current i_pc was observed during the reverse scan. The effect of the concentration of thiosulfate on MCPE response was investigated. Fig. 3 (B, curve a) shows the voltammograms plotted at a scan rate of 15 mV.s⁻¹ at a concentration range varying from 0 to 50 mM. The anodic peak current increases by increasing the concentration of TS. The peak potential shifts slightly to positive directions, demonstrating the kinetic limitation of electro-oxidation process. In the studied concentration range the electro-catalytic peak current increases linearly with the concentration as shown in Fig. 3 (B, inset II). The cathodic current was fully recovered, which can be justified by the poor adsorption affinity of thiosulfate to the active sites. The thiosulfate radical anion S₂O₃^.– is probably formed on the electrode surface as clarified by the equations below:

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Fig. 3

(A) Electrochemical responses for (a) CPE, in alkaline solution 0.1 M NaOH and 15 mVs⁻¹ as scan rate in presence of 10 mM of thiosulfate; (b) Ni^IIL-CPE in alkaline solution 0.1 M NaOH and absence of thiosulfate; (c) Ni^IIL-CPE in alkaline solution 0.1 M NaOH and in presence of 10 mM of thiosulfate. (B): (I) Cyclovoltammograms obtained with modified electrode MCPE for electrocatalytic oxidation of thiosulfate in 0.1 M NaOH at scan rate 15 mVs⁻¹ using various concentrations of thiosulfate: (a) 0, (b) 5, (c) 15, (d) 20, (e) 40, and (f) 50 mM, (II) Inset: the variation of anodic peak currents versus Thiosulfate concentration.

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To understand the kinetic of the thiosulfate electro-catalytic oxidation in 0.1 M NaOH at MCPE surface, the effect of scan rate was investigated in the range between 15 and 100 mV.s⁻¹ at relatively low concentration 10 mM as illustrated in (Fig. 4 A curve I). The recorded curves show an increase of the peak current with a shifting of the peak potentials to more anodic value, the catalytic peak current have a linear dependency with the square root of scan rate which indicates the diffusion controlled process (Fig. 4A inset II). At a concentration of 50 mM as shown in (Fig. 4B) the shape of the voltammogram was significantly changed, the plot of the normalized anodic peak current versus scan rate shown in (Fig. 4C), the normalized current was decreased to reach a constant level at higher scan rates, indicating a kinetic controlled process with a regenerative mechanism of the Ni^III active species. The adsorbed thiosulfate at the electrode surface reacts chemically with Ni^III and leads to Ni^II. The Ni^II species obtained undergo further electro-oxidation at the applied potential and regenerate Ni^III active species again via an EC process as illustrated in the catalytic cycle (given in Scheme 3).

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Fig. 4

(A): (I) Cyclovoltammograms of modified electrode MCPE obtained in presence of 10 mM thiosulfate and 0.1 M NaOH at different scan rates ν: (a) 15, (b) 20, (c) 30, (d) 40, (e) 50 (f) 70, (g) 100, and (h)125 mVs⁻¹; (II) Inset: the variation of anodic peak currents versus square root of scan rate (B): (I) Cyclovoltammograms of modified electrode MCPE obtained in presence of 50 mM thiosulfate and 0.1 M NaOH at different scan rates ν: (a) 15, (b) 20, (c) 30, (d) 40, (e) 50 (f) 70, (g) 100, and (h) 125 mVs⁻¹. (C): The normalized anodic peak current versus scan rate.

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Scheme 3

Proposed catalytic cycles of Ni (II)-Schiff base complex (Ni(II)L) toward thiosulfate electro-oxidation reaction.

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In order to study the mass transfer kinetics and estimate the heterogeneous catalytic rate constant K_h, chronoamperometry was used and chronoamperograms were recorded by setting our MCPE potential at 600 mV/SCE in absence (Fig. 5A curves a) and in presence of TS in the range of 2 to 50 mM in 0.1 M NaOH, (Fig. 5A, curves b-h). The transient current increased upon increasing TS concentration. As shown in (Fig. 5B) by plotting net current with respect to the minus square root of time a linear dependency from line equation y = 182.6 x + 23.04, r² = 0.999 was observed, obeying Cottrell law By applying Cottrell’s equation. Eq. (10)

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Fig. 5

(A): Chronoamperogramm of MCPE in 0.1 M NaOH at 600 mV/SCE and different concentration of thiosulfate. (a) 0, (b) 2, (c) 5, (d) 15, (e) 20, (f) 30, (g) 40, and (h) 50 mM. (B): Dependency of the net current on the minus square root of time in 0.1 M NaOH and 10 mM TS. (C): Dependency of the I_c/I on the square root of time. In 0.1 M NaOH and in absence and presence 10 mM TS. (D): (I) Chronoamperogram of MCPE in 0.1 M NaOH at 600 mV/SCE and after successive addition of thiosulfate. (a) 0, (b) 2, (c) 5, (d) 15, (e) 20, (f) 30, (g) 40, (h) 50 mM. (II) Inset: Dependency of transient current on the thiosulfate concentration. (F): Chronoamperogram of MCPE in 0.1 M NaOH at 600 mV/SCE firstly in presence of 5 mM TS and after successive addition of 500 µM SCN^–, SO₃²⁻ and S^2– interferon’s respectively, followed by twice successive addition of 2.5 mM TS.

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To evaluate sensing capability of our MCPE, a typical amperometric signals recorded during successive additions of TS to 0.1 M NaOH solution at applied potential of 600 mV/SCE as shown in (Fig. 5D curves I). Under slow stirring the electrode responses were quite rapid and proportional to TS concentration. (Fig. 5D inset II) shows the calibration curve showing the amperometric current versus TS concentration, from the slope in the line equation

y = 7.12x + 1.34, r² = 0.999 our MCPE sensitivity equal to 7.12 µA.mM⁻¹ taking in account he geometric surface 0.0314 cm² the sensitivity is 226.75 µA mM⁻¹ cm⁻².

The interference studies

Under optimal conditions, the interference of some compounds such as NO₃^–, Cl^–, SO₄²⁻,S^2–, SCN–, SO₃²⁻, K⁺ on the oxidation of 5 mM TS in 0.1 M NaOH at 600 mV vs SCE has been evaluated. It can be found that most ions such as NO₃^–, Cl^–, SO₄²⁻, K⁺, did not show any interference effect during the determination of TS by using MCPE. However, 10-fold SCN^– and 10-fold SO₃²⁻ and S^2– 10-fold were found to have noticeable interference in TS determination on the MCPE, (Fig. 5E) shows amperometric responses of MCPE in 0.1 M NaOH in the presence of 5 mM TS after successive addition of 500 µM SCN^–, SO₃²⁻ and S^2–. These results suggest that MCPE can be successfully used to measure TS concentration in the presence of a variety of possible interfering substances.

3.4 Electrode stability

The modified carbon paste electrode (MCPE) studied in this work shows a good stability, after 7 month has been stored in ambient conditions, the electrode responds still remained. During cycling between 0 and 600 mV/SCE in 0.1 M NaOH, 10 mM TS at 15 mVs⁻¹ for 200 cycles (Fig. 6A), the catalytic pic current remain stable with slight decrease to 96.71% of its initial value. As shown (Fig. 6B). The chronoamperogram of TS at a concentration of 10 mM in the time span of 90 min shows after an initial decrease a stabilization of the current indicating a good stability toward TS oxidation, by switching the potential between 0 mV vs SCE for 10 s and 600 mV vs SCE for 30 s in 0.1 M NaOH and 15 mM TS the chronoamperometric response shows stable signal during 30 cycles (Fig. 6C). The above results indicate no poisoning of the catalyst by the oxidation product. This electrochemical behavior may be explained by the stability of Nickel Schiff base complex in the working conditions.

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Fig. 6

(A): 200 successive CVs cycles of modified electrode MCPE obtained in presence of 10 mM thiosulfate and 0.1 M NaOH at scan rate 15 mVs⁻¹. Inset: the 1st and 200th cycles. (B): Chronoamperogramm of MCPE in 0.1 M NaOH at 600 mV/SCE and 10 mM TS for 90 min electrolysis. (C): 30 successive chronoamperometric cycles of MCPE in 0.1 M NaOH and 15 mM TS by switching potential at 0 mV vs SCE for 10 s and at 600 mV vs SCE for 10 s.

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Our results compared to the literature seem to be very interesting as illustrated by Table 1.