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Article

Effect of the Metal of a Metallic Ionic Liquid (-butyl-methylimidazolium tetrachloroferrate) on the Oxidation of Hydrazine

by
Marcela Brockmann
1,2,
Freddy Navarro
3,
José Ibarra
2,4,
Constanza León
1,2,
Francisco Armijo
2,4,
María Jesús Aguirre
2,3,*,
Galo Ramírez
2,4,* and
Roxana Arce
1,2,*
1
Departamento de Ciencias Químicas, Facultad de Ciencias Exactas, Universidad Andrés Bello, Av. República 275, Santiago 8370146, Chile
2
Millennium Institute on Green Ammonia as Energy Vector (MIGA), Av. Vicuña Mackenna 4860, Macul, Santiago 7820436, Chile
3
Departamento de Química de Los Materiales, Facultad de Química y Biología, Universidad de Santiago de Chile, USACH, Av. L.B. O’Higgins 3363, Santiago 9170022, Chile
4
Departamento de Química Inorgánica, Facultad de Química y de Farmacia, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago 7820436, Chile
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(6), 359; https://doi.org/10.3390/catal14060359
Submission received: 27 March 2024 / Revised: 25 May 2024 / Accepted: 26 May 2024 / Published: 31 May 2024
(This article belongs to the Section Catalytic Materials)
Browse Figures
Versions Notes

Abstract

:
This work investigates the electrocatalytic properties of carbon paste electrodes (CPEs) modified with ionic liquids (IL) and metallic ionic liquid (ILFe) for the hydrazine oxidation reaction (HzOR). The results indicate that ILFe significantly enhances the catalytic activity of the electrode, exhibiting catalysis towards hydrazine oxidation, reducing overpotential, and increasing reaction current. It is determined that the HzOR on the MWCNT/MO/ILFe electrode involves the transfer of four electrons, with high selectivity for nitrogen formation. Additionally, ILFe is observed to improve the wettability of the electrode surface, increasing its capacitance and reaction efficiency. This study highlights the advantages of ILFe-modified CPEs in terms of simplicity, cost-effectiveness, and improved performance for electrochemical applications, demonstrating how the ionic liquid catalyzes hydrazine oxidation despite its lower conductivity.

Graphical Abstract

1. Introduction

Growing energy demand and the environmental crisis caused by fossil energy use have necessitated the development of sustainable and clean energy technologies. In this context, it highlights the use of hydrazine (Hz) for application in polymer electrolyte membrane fuel cells (PEMFC) [1]. The direct hydrazine fuel cell (DHzFC) offers advantages such as zero greenhouse gas emission and high open circuit potential (OCP) over 1.56 V [2]. In this context, it is of interest to develop catalysts that promote this oxidation and are environmentally friendly. In this respect, ionic liquids stand out for their low vapor pressure and because they are recoverable [3,4,5,6] and they have been used for modifying electrodes showing interesting catalytic properties [7,8]. In the oxidation of hydrazine, the adsorption/desorption of analytes and intermediates plays an important role because of the multistage electronic processes involving proton transfer [9]. In relation to the electrode substrates used for electrochemical hydrazine oxidation, electrodes with a favorable potential window, high electrical conductivity, stability and robustness, mechanical properties, cost, availability, and low toxicity are of interest. Electrocatalysts based on transition metals, such as Ni, Fe, or Co [10,11,12,13], are reported in the literature. However, this type of metal electrocatalysts can suffer corrosion, resulting in a loss of electrocatalytic activity. Moreover, metals, such as Pt, Au, and Ag, which are very active in the hydrazine oxidation reaction (HzOR), are too expensive for practical applications and, unfortunately, hydrazine has a large overpotential oxidation in common carbon electrodes such as glassy carbon, so they are not very suitable for oxidation through conventional electrochemical approaches. However, electrodes based on carbon nanomaterials allow a promising approach to minimize overpotential effects, modulate the electronic environment, and activate the adsorption/desorption of reagents [14,15,16,17,18,19,20,21,22]. Carbon paste electrodes (CPEs) have attracted attention for their outstanding activity in many fields such as energy and environmental fields [23,24,25,26,27,28,29,30,31,32,33,34,35]. These electrodes consist of a carbonous material and a binder, usually silicone or mineral oil (MO). If the carbonous material is a nanomaterial, its conductive and catalytic properties are increased. In addition, they are of low cost, have remarkable conductivity, have a wide range of potential, are portable, and have renewable surfaces [36,37,38,39,40]. The most commonly used nanomaterials are graphene, fullerene, and carbon nanotubes (CNTs) [41]. The CNTs [42] exhibit several unique electrical, geometric, and mechanical properties, and can be used to promote electron transfer reactions when used as electrode material in electrochemical devices. Moreover, the CPEs have a recoverable and renewable area [43]. To bind CPEs, they can be used together or instead of silicone or mineral oil, employing an ionic liquid (IL). The ILs are salts, usually with melting points below 100 °C, formed by an organic cation such as imidazolium, pyridinium, or phosphonium and an anion that can be relatively small, such as a chloride or bromide atom or a more complex anion such as ethyl sulfate or hexafluorophosphate [44,45,46]. The advantages described by ILs include good chemical and thermal stability, low vapor pressure, good ionic conductivity, and low volatility [44,45]. The ILs also show high viscosity and solvation capacity, so they are good dispersers of carbon materials, facilitating the generation of uniform and homogeneous carbon pastes [46]. Hayyan M. et al. [47] have shown that the ILs used to modify electrodes contribute to obtaining wide windows of potential [48,49,50,51,52,53,54,55,56,57].
On the other hand, metallic ionic liquids or magnetic ionic liquids (MILs) have been found to have been used as recoverable catalysts in organic reactions such as Grignard cross-coupling with primary and secondary halides [58], in Friedel–Crafts acylation [59] or as solvents in biphasic liquid–liquid catalysis [60], but there is no information in the literature on the application of MILs as binders in carbon paste or as electrode substrates, specifically on carbon paste electrodes, in order to study new materials or active electrode surfaces for different analytes of energy or environmental interest. The study was aimed at developing CPE electrocatalysts with MILs as binders and determining if there is a possible effect of the metal as a catalyst in carbon paste electrodes and its application in electrocatalysis. On the other hand, testing whether the CPEs with MILs as binders exhibit good performance for the detection of hydrazine [33,34,35], thereby establishing its application as a sensor from the electrochemical approach. In this case, CNTs and two ionic liquids (see Figure 1) were used as binders combined with mineral oil. One of the ILs was 1-butyl-3-methylimidazolium chloride ([BMIM+] [Cl−), and the other, a metallic IL (ILFe), 1-butyl-3-methylimidazolium tetrachloroferrate ([BMIM+] [FeCl4−]). They were mixed with CNTs and mineral oil to make different electrodes to determine the effect of the ionic liquid and the metal of the ionic liquid towards the oxidation of hydrazine (HzOR) compared to a CPE agglutinated with mineral oil alone.

2. Results

2.1. Metallic Ionic Liquid Characterization

Once the metallic ionic liquid, MIL, [BMIM+] [FeCl4−] was synthesized, a Raman spectrum was performed to determine the presence of cation structure [BMIM+] and Fe-associated bands. The use of near-infrared excitation allows the collection of Raman spectra with high resolution for dark-colored liquids. Figure 2 shows the Raman spectrum of the MIL obtained. The spectrum shows strong bands at 113.24 cm−1 and 330.20 cm−1. Those peaks were reported and assigned to the symmetric Fe–Cl bond stretch vibrations of [FeCl4]− in the literature [61]. Since the chloride anion does not have Raman bands, all of the other Raman bands observed in Figure 2 are aligned with the vibrations of the [BMIM+] cation, the ring symmetric stretching and bending modes are found in the range of 1000 to 1600 cm−1. Finally, the weakest peaks between 2700 and 3200 cm−1 correspond to the terminal CH3 groups, the ring CH3, and the butyl chains (symmetric and asymmetric H−C−H stretches). Additionally, other weaker peaks (some of which overlap) are observed, corresponding to the N−C(H)−N stretching modes (C−H of the ring) and a mixture of the ring HC=CH symmetric stretch and ring in-plane symmetric stretching modes, and show good agreement with a previous report [61].

2.2. Morphological Characterization of Carbon Paste Electrodes

Once the CPEs had been prepared using the three different binders. The results are summarized in Table 1 and confirm the presence of the ionic liquid [BMIM+] [Cl−] for the MWCNT/MO/IL electrode and the metallic ionic liquid, [BMIM+] [FeCl4−] for the electrode on the surface of the MWCNT/MO/ILFe electrode (FESEM images in Supplementary Materials, Figure S1). Surfaces bound with ionic liquids show a low percentage of chlorine, and no chlorine was detected for the MWCNT/MO, as expected.

2.3. Electrochemical Impedance Spectroscopy Comparison between MWCNT/MO/IL and MWCNT/MO/ILFe

To understand the difference between the two electrodes prepared with IL, an EIS comparison was made using the potassium ferrocyanide/potassium ferricyanide redox couple at the OCP of each electrode.
It is interesting to note that the comparison was made before and after stabilization/activation, obtaining the first interesting result concerning the stability of the electrodes. In fact, if the process of stabilization and activation is not performed, the system MWCNT/MO/IL shows visible signals of degradability and requires various hours to acquire a stable open circuit potential value. This effect is not visible for the electrode containing Fe, which shows that the hydrophilicity of this system is higher than that of the IL without Fe. The system MWCNT/MO/ILFe shows negligible changes in its OCP when immersed in the aqueous solution for a few seconds or during 20 cycles, indicating its high wettability. When the stabilization/activation is carried out, the electrode MWCNT/MO/IL is much more stable and requires less time (ca. one hour) to reach a stable open circuit potential value.
Figure 3 shows the Nyquist graphs of the systems with and without activation.
From that figure, the drastic change of the MWCNT/MO/IL before and after activation is clear. The first part of the graph, at high frequencies, corresponds to the left side of the graph showing the apparition of a semicircle corresponding to the charge transfer resistance where the semicircle would intercept the x-axis. In order to compare the electrical parameters of the electrochemical impedance response, a data adjustment was made with the Zview program. For that, different equivalent circuits were proved, and the Kramer–Kronig relations [62] were measured in order to check the adjustment.
According to that, all of the electrodes except activated MWCNT/MO/IL are well represented by the equivalent circuit (Randles circuit) shown in Figure 4a. The activated MWCNT/MO/IL responds to the equivalent circuit shown in Figure 4b, indicating that after cycling that system in an aqueous solution, it presents electrical changes modifying the equivalent circuit to a codified Randles equivalent circuit.
The electrical data obtained using each equivalent circuit are presented in Table 2.
Capacitance (C) was calculated from the CPE data using the following formula (see Equation (1)) [63]
C = Q 1 α R s R e t R s + R e t 1 − α α
where Q (the pseudo capacitance) and α (fractional exponent) are the parameters defining the CPE; and Rs and Ret are ohmic resistance and electron transfer resistance, respectively.

2.4. Contact Angle Measurements

In order to compare the surface hydrophobicity of each modified electrode before and after the activation, the contact angles of sessile drops were obtained and are shown in Figure 5.
The MWCNT/MO/IL electrode before activation exhibited a more hydrophobic surface with an average contact angle (θ) of 134.65° (Figure 5a), which drastically decreased by about 25° after activation (Figure 5b). In contrast, a more hydrophobic surface is observed with the incorporation of ILFe before activation, causing an increase in the average contact angle compared to the MWCNT/MO/IL electrode surface (Figure 5c). After activation, the surface containing ILFe shows a decrease of 19.5°. These changes indicate that when the solvent and ions of the electrolyte are incorporated into the mixture of carbon and ionic liquid, they generate permanent and stable interactions that modify the surface, stabilize it (in the case of MWCNT/MI/IL) and change the surface polarity making it more hydrophilic, compared to the same systems before activation. It is interesting that the system MWCNT/MO/ILFe is more hydrophobic than MWCNT/MO/IL before and after activation and despite that, it is a better electrocatalyzer for hydrazine oxidation in aqueous solutions as will be seen later. To elucidate this effect, a theoretical calculation route could be proposed to determine what type of Interactions with hydrazine are generated by the presence of Fe in the mixture in order to understand the catalytic effect of this metallic ionic liquid.

2.5. Electrochemical Characterization

The electrocatalytic properties of CPEs with and without ionic liquids for HzOR are evaluated using cyclic voltammetry. Cyclic voltammograms (CVs) (Figure 6) are recorded using a 0.100 mol L−1 hydrazine solution in PBS (pH 7.4).
In Figure 6, the catalytic effect of both modified electrodes compared to MWCNT/MO is clear. In this figure, there is an increase in current and a shift in oxidation potential towards lower values. This effect is larger for the metal-containing system, which shows greater current and lower onset potential than the other two systems, realizing the catalytic effect that is obtained in the presence of the ionic liquid and especially if the ionic liquid contains the Fe element. Square wave voltammetry is used to verify the current and potential values of the electrode systems toward hydrazine oxidation (Figure S2). Data from Figure S2 is shown in Table 3, which depicts the values obtained for Vonset, Vp, and Ip for HzOR using the three electrodes.
To determine the number of electrons involved in the reaction using the MWCNT/MO/ILFe electrode, the scan rate effect (from 20 mV s−1 to 200 mV s−1) was studied in a potential window of −0.4 to 1.4 V in PBS at pH 7.4. Figure 7a shows the shift of the curve peaks towards more positive potentials and an increase in current as the scan rate increases. This behavior indicates that the electrocatalytic reaction is kinetically limited and that the electrocatalysis of hydrazine oxidation is being controlled by the diffusion of this analyte to the surface of the electrode [37].
From Figure 7b, a good linear correlation between the current peak (Ip) and the square root of the scan rate (v1/2) is observed, with a correlation coefficient of 0.9978 (Equation (2)).
y = 0.0375 x − 0.0102         R 2 = 0.9978
As this is an irreversible and diffusion-controlled reaction, the number of electrons transferred can be calculated using the Randles–Sevcik equation for irreversible reactions (Equation (3)):
I p = 2.99 × 10 5 n 1 − α n α 1 2 A   C   D 1 2 v 1 2
where A is the geometric area of the electrode (0.0314 cm2), D is the diffusion coefficient of hydrazine (9.2 × 10−6 cm2 s−1 extracted from [53]), C is the concentration of the analyte (1.0 × 10−4 mol cm−3), α is the electron transfer coefficient, and n and nα are the number of electrons transferred in the overall reaction and in the slow step of the reaction.
Also, considering Equation (4)
( 1 − α ) n α = 0.0477 V E p − E p / 2
and the values of Ep and Ep/2 from the voltammogram at a scan rate of 100 mVs−1 (Equation (5)),
( 1 − α ) n α = 0.0477 V 0.7420 − 0.3770 V = 0.1225
then, the transferred electrons n are calculated from the slope of the graph in Figure 7b.
n = 3.73~4 electrons
According to this, the HzOR taking place on the surface of MWCNT/MO/ILFe goes through a reaction involving four electrons, generating N2 as a product.

2.6. Electroanalytical Measurement to Evaluate MWCNT/MO/ILFe as a Possible Hydrazine Sensor

MWCNT/MO/ILFe modified electrode shows good electrocatalytic activity towards hydrazine oxidation. Figure 8 shows the square wave voltamperometric profiles for hydrazine oxidation on the MWCNT/MO/ILFe at Hz concentrations of 0.04 mmol L−1, 0.05 mmol L−1, 0.07 mmol L−1, 0.08 mmol L−1, and 0.10 mmol L−1 in PBS pH 7.4 in a potential window from −0.4 to 1.4 V. In the voltammograms, the peak potentials appear between 0.40 V and 0.55 V. Well-defined peak potentials can be observed and proportional to the increase in peak concentration, allowing the construction of a calibration curve graph. The calibration curve for hydrazine detection has a linear range (LR) of 0.04–0.10 mol L−1 with a regression equation Ipa (mA) = 6430.7 [Hz] (mmol L−1) + 15.327 and a correlation coefficient of R2 = 0.9918. In addition, the experimental detection limit (LOD) (S/N) equal to 0.008 mol L−1 is obtained.

3. Discussion

Data obtained from Figure 2 and Table 1 clearly show the MIL containing Fe was obtained. This MIL mixed with mineral oil is capable of binding MWCNT to obtain a stable and robust electrode with a renewable surface. From Figure 3 and using the Randles equivalent circuit and a modified Randles circuit (Figure 4a,b), it is interesting to observe Table 2 where the electric values of the elements of both equivalent circuits are shown. It is noticeable that Rs is abnormally high in the case of the MWCNT/MO/IL, showing a hydrophobic behavior that changes after activation, in accordance with the difficulty of this system in achieving stable OCP and the signs of degradation mentioned in the Results section. The results are consistent with the changes easily observed in the integrity of the MWCNT/MO/IL electrode when it is activated. They reflect a noticeable change in the ohmic resistance before and after activation related to the change in the chemical nature of the surface. On the other hand, previous reports mentioned changes in the ohmic resistance of the solution when the hydrophobicity of the system changes. In this sense, Chen et al. [64] reported changes in the ohmic resistance of the solution when they add Teflon over the surface of the air electrodes for lithium/air batteries. Also, Figueroa at al. [65] explain that the sealing of an anodized aluminum with a hydrophobic material changes the ohmic resistance of the solution. Then, a drastic change in the resistance of the solution may be related to changes in the hydrophobicity of a surface after a change of its nature.
On the other hand, it is interesting to note that the presence of Fe in the electrode does not decrease the resistance of the charge transfer, indicating that the electrode is not more conductive than the others. On the contrary, its resistance to charge transfer increases. This effect is very interesting for the discussion above because, as will be shown later, the electrode containing Fe is best for the HzOR but its efficiency is not due to a better conductivity. Finally, it is interesting to note that capacitance for the MWCNT/MO/ILFe system is almost twice that of the other systems, indicating that the wettability of the system generates a wide double layer which is consistent with the short time it took for this system to achieve a stable OCP value and the negligible variation in the OCP value observed before and after activation for this system [66]. This capacitance indicates that a hydrophobic surface manages to generate a homogeneous interaction involving all of the electrode material and that it is not drastically modified when the electrode is activated. This fact accounts for the role of iron. That is, despite the hydrophobicity, the interaction is rapid and homogeneous and clearly attributable to the effect of Fe on the ionic liquid. These results are reflected in the contact angle values (Figure 5) because the changes in the contact angles are more important for the MWCNT/MO/IL before and after the activation (variation of ca. 18%) compared to the changes in MWCNT/MO/ILFe (variation of 13.4%). However, it is noticeable that the MWCNT/MO/ILFe system is always more hydrophobic than the system that does not contain Fe. Then, the wettability mentioned above could correspond to the capacity of the surface to “react with water”, having the capacity to form a uniform double layer interaction with the solution, which is an advantage for electrocatalyzing hydrazine in aqueous media.
CVs shown in Figure 6 showed an irreversible oxidation process, indicating that all CPEs can electrocatalyze HzOR. However, their onset potentials (Vonset) for the reaction are different; in the MWCNT/MO electrode it is close to 0.20 V vs. Ag/AgCl, while in both electrodes with ILs, the value is shifted towards less positive potentials, close to 0.0 V, clearly showing that the presence of IL in the mixture generates a catalytic effect in this reaction. In addition, a shift towards less positive values in the peak potential is observed when the CPEs are agglutinated with IL, with a lower value for MWCNT/MO/ILFe compared to MWCNT/MO/IL and MWCNT/MO and an increase in the current for the MWCNT/MO/ILFe electrode, considering all three electrodes have equal geometric area. This agrees with the high capacitance shown for the electrode containing Fe according to data in Table 2. It is interesting to point out any electrode that presents signs of passivation or poisoning because the response of each one is identical before or after the renovation of the surface once the electrode has been activated.
Using of the data in Table 3, it is clear that the MWCNT/MO/ILFe electrode has a displacement greater than 100 mV towards negative potentials, generating a higher current than MWCNT/MO. It can also be seen from Figure 6 that the presence of IL without Fe favors the oxidation of hydrazine without increasing the current and that the presence of Fe in the IL anion further decreases the overpotential, generating higher currents and indicating a clear effect of the metal in this reaction, generating N2 as the main product according to the transferred electrons calculated. In this way, we obtain a promising electrocatalyst that is easy to prepare, robust, stable, and cheap and that can be used to perform the oxidation of hydrazine without poisoning. It is clear that even this electrode does not aim to be scalable but it is a first starting point for large-scale production and its use in hydrazine fuel cells and other devices that require the ability to decrease the overpotential of this reaction through a low-cost electrode system.
Finally, when the system MWCNT/MO/ILFe is used as a hydrazine sensor, increasing current signals are observed when the concentration of hydrazine increases. Clearly, this behavior indicates that the surface is not poisoned by the products of intermediates of the reaction. The comparison of the detection limit obtained in this work and those of other carbon or nanoparticle-based electrodes used for HzOR is shown in Table 4.
As can be seen, the detection limit of the system studied in this research is higher compared to the other electrodes reported in the literature. However, it highlights the simplicity of the hydrazine detection method, the easy and cheap electrode surface, and the powerful application that can have the use of metal ionic liquids for species that represent an alternative route towards the detection or production of green ammonia, such as hydrazine. It is also interesting to note that the MWCNT/MO/ILFe electrode does not suffer poisoning or passivation because the response is identical with or without surface renewal. The MWCNT/MO/ILFe electrode has a low sensitivity to detect hydrazine compared to other reported studies. It has the advantage that its surface is not poisoned and that it is very stable, but it must be modified to use it in the future as a sensor capable of detecting hydrazine with the limits required for its use, for example, in water analysis. In this sense, the easily obtained electrode and its carbon paste nature allow other nanomaterials to be incorporated in the future and an increase in the sensitivity of the electrode to lower its detection limits and make it a low-cost instrument for the detection of hydrazine in water.
On the other hand, the electrode was stored for 30 days at room temperature and exposed to air to measure the maximum oxidation current of Hz at a fixed concentration (0.100 mol L−1) to measure its stability. The results showed a decrease in the electrochemical response for Hz of around 5.0% (I = 0.60 mA; Vonset = 0.010 V vs. Ag/AgCl, at day 30) compared to the initial current response (I = 0.63 mA; Vonset = 0.010 V vs. Ag/AgCl). For reproducibility measurement, five MWCNT/MO/ILFe electrodes were prepared under the same conditions for Hz detection (0.100 mol L−1), and the results exhibited a relative standard deviation (RSD) of 3.12% (see Table S1 in Supplementary Materials).

4. Materials and Methods

4.1. Reagents

Diethyl ether (DE) Emsure® Merck (Boston, MA, USA) was employed as received. Chemicals materials were used as obtained from the suppliers: 1-butyl-3-methylimidazolium chloride (99%), Iron (III) chloride hexahidrate (98%), multi-walled nanotube carbon (99%), mineral oil (98%), and hydrazine (98%) were purchased from Sigma-Aldrich (Saint Louis, MO, USA). Potassium hexacyanoferrate (III) (98%), potassium dihydrogen phosphate (99%), di-sodium hydrogen phosphate (99%), potassium chloride (98%), and sodium chloride (98%) were obtained from Merck.
Ultrapure water (>18 MΩ-cm) was obtained from a Direct-Q® water purification system (Model ZRQSVP3WW) and argon gas (99.99% pure) was purchased from AIR LIQUID (Santiago, Chile).

4.2. Synthesis

Butylmethylimidazolium tetrachloroferrate [BMIM+] [FeCl4−]: 1.46 g of 1-butyl-3-methylimidazolium chloride (Sigma-Aldrich, 99%) and 2.31 g of FeCl3 × 6H2O (Sigma-Aldrich, 99%) were mixed in a dry box with N2 atmosphere, and a dark brown liquid was obtained as a result of an endothermic solid-state reaction [73,74,75]. In order to spectroscopically characterize the metallic ionic liquid, we first measured the Raman spectra to obtain information about the structure.

4.3. Carbon Paste Electrode Preparation

The 70:30 (w/w) ratio of carbon materials and binder was used after experimentally checking that the electrodes lose their mechanical and structural consistency at different binder ratios (both higher and lower). For the manufacture of the carbon paste electrode, the bare carbon paste electrode was made by mixing multi-walled carbon nanotubes (MWCNT) and mineral oil (MO) in a 70:30 (MWCNT/MO) weight-to-weight ratio [76,77,78]. On the other hand, to manufacture the carbon paste electrodes with MWCNT and ionic liquid, a 70:20:10 (w/w) ratio of MWCNT, mineral oil, and ionic liquid were used, respectively. In the case of the [BMIM+] [Cl−], the electrode was labeled as MWCNT/MO/IL, and in the case of [BMIM+] [FeCl4−], the electrode was labeled as MWCNT/MO/ILFe. The mixtures were homogenized in an agate mortar by adding diethyl ether. After homogenization, the diethyl ether was evaporated until a carbon paste was obtained. The generated paste was used to fill hollow Teflon electrodes. After compacting by hand, the electrodes were left at 90 °C in an oven. After 3 h, the electrodes were cooled to room temperature and polished to a smooth surface with weighing paper. The electrodes obtained were electrochemically stabilized and activated by cycling the potential from −0.4 to 1.4 V in phosphate-buffered saline (PBS) at pH 7.4 for twenty cycles. The obtained electrodes were characterized by their voltammetric response using the potassium ferrocyanide/potassium ferricyanide redox couple, showing that they are very reproducible and stable systems.

4.4. Characterization Measurements

Electrochemical studies were performed in a conventional three-electrode electrochemical cell, which contained the following: an Ag/AgCl (KCl (3 M)) reference electrode (RE), a platinum wire counter electrode (CE), and a hollow Teflon working electrode (WE) of 2 mm diameter, which is filled with the carbon pastes. The solutions were maintained before and during the experiments in an inert atmosphere of Ar. Cyclic voltammetry (CV), square wave voltammetry (SWV), and electrochemical impedance spectroscopy (EIS) were performed at room temperature, 21 ± 1 °C. EIS measurements were taken at open circuit potential (OCP) and with 2 mM [Fe(CN)6]3−/4− in PBS buffer at pH 7.4. A sinusoidal voltage perturbation with an amplitude of 5 mV was applied, scanning the frequency range from 100 kHz to 10 mHz with 12 points per decade. The impedance data were analyzed and fitted to electrical equivalent circuits using ZView 4.0h software and validated using Kramers–Kronig relations. All experiments were repeated at least three times. The χ2 tests for the fitted EIS data were less than 0.05. The contact angle measurements were made using the sessile drop technique with ultrapure water, similarly to a procedure described in the literature [66]. The measurements of contact angle (θ) were realized for 2 min (total of 120 values) for a total of 10 drops for MWCNT/MO/IL and MWCNT/MO/ILFe electrodes without/with activation, and average values of θ were determined.

4.5. Equipment

CV and SWV analyses were performed on a 620E and 750D workstation potentiostat, while EIS analyses were performed on a 604C potentiostat (CH Instruments, Austin, TX, USA). To determine the morphology and composition of the carbonaceous pastes, a field emission scanning electron microscopy (FE-SEM) with Energy dispersive X-ray spectroscopy (EDX) (QUANTA FEG250, Thermo Fisher Scientific Co., Ltd., Waltham, MA, USA) was used. The samples were supported on vitreous carbon plates and analyzed before the electrochemical study. RAMAN spectroscopy was carried out using an excitation wavelength of a 785 nm laser to avoid fluorescence from impurities on an Alpha 300-RA near-infrared multichannel Raman spectrometer, WITec, Ulm, Germany. The contact angle measurements were made with a goniometer by Ossila (Sheffield, UK).

5. Conclusions

The present research elucidates the advantages associated with the utilization of a carbon paste electrode, highlighting its simplicity, cost-effectiveness, and renewable sur-face. Notably, the study explores the novel aspect of investigating the influence of metal ionic liquid on hydrazine oxidation. Conclusive findings reveal a clear catalytic effect of the Fe because the system containing Fe is less conductive than the others, as indicated by its high resistance to the charge transfer measured by EIS. In spite of this behavior, the system containing Fe shows a low overpotential and an increased current for the oxidation of hydrazine. On the other hand, this system catalyzes the oxidation of hydrazine to nitrogen, becoming a very good and simple alternative for its use in an electrolyzer of Hz. Also, the metal significantly affects the wettability of the surface, which is not high in a system containing the IL without the metal. This wettability permits all of the electrodes to take part in the reaction, explaining its high capacitance and the higher currents obtained during the oxidation. Finally, this paper explains the low stability of the system containing IL without Fe in aqueous solution due to its poor wettability. This paper contributes valuable insights into the enhanced performance and potential applications of the carbon paste electrode in electrochemical processes.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14060359/s1, Figure S1: FESEM images, (a) MWCNT/MO/IL (60000X), (b) MWCNT/MO/ILFe (60000X). Figure S2: Comparative square wave voltammograms of MWCNT/MO, MWCNT/MO/IL y MWCNT/MO/ILFe in presence of 0.100 mol L−1 Hz at ambient temperature, under Ar atmosphere. Increment 0.015 V, amplitude de 0.01 V, frequency 10 Hz. Table S1: Dataset for current (mA) of MWCNT/MO/ILFe. The reproducibility of the electrode construction was examined by the determination of 0.100 mol·L−1 hydrazine using three electrodes prepared from the different carbon paste and hydrazine solution.

Author Contributions

M.B.: Investigation. F.N.: Investigation, Writing—original draft, Writing—review and editing, Visualization. J.I.: Investigation. C.L.: Investigation, Writing—review & editing, Visualization. F.A.: Investigation, Writing—review and editing, Visualization. M.J.A.: Conceptualization, Investigation, Resources, writing—original draft, Writing—review and editing, Visualization, Funding acquisition. G.R.: Conceptualization, Investigation, Resources, writing—original draft, Writing—review and editing, Visualization, Funding acquisition. R.A.: Conceptualization, Investigation, Resources, writing—original draft, Writing—review and editing, Visualization, Project administration, Funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Regular-UNAB project N° DI-010-22/REG, ANID FOND-ECYT N° 1220107, FONDEQUIP EQM 190016, Dicyt-Usach and ANID/Millennium Science Initiative Program/ICN2021_023.

Data Availability Statement

Data are contained within this article and Supplementary Materials.

Acknowledgments

The authors acknowledge financial support from Regular-UNAB project N° DI-010-22/REG (R.A.), National Agency for Research and Development (ANID) through grants FONDECYT N° 1220107 (G.R.). M.J.A. acknowledges the support of FONDEQUIP EQM 190016, Dicyt-Usach and ANID/Millennium Science Initiative Program/ICN2021_023 (J.I., F.A., M.J.A., G.R., and R.A.).

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. (a) Structure of [BMIM+] [FeCl4−]. (b) Structure of [BMIM+] [Cl−].
Figure 1. (a) Structure of [BMIM+] [FeCl4−]. (b) Structure of [BMIM+] [Cl−].
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Figure 2. Raman spectrum of [BMIM+] [FeCl4−].
Figure 2. Raman spectrum of [BMIM+] [FeCl4−].
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Figure 3. Nyquist graphs of the systems containing IL with and without activation. Measurements were done using a potassium ferrocyanide/potassium ferricyanide redox couple at open circuit potential of each electrode, under Ar atm. Electrochemical impedance spectroscopy data for modified carbon paste electrodes both before (MWCNT/MO/IL ■ and MWCNT/MO/ILFe ●) and after (MWCNT/MO/IL □ and MWCNT/MO/ILFe ○) activation.
Figure 3. Nyquist graphs of the systems containing IL with and without activation. Measurements were done using a potassium ferrocyanide/potassium ferricyanide redox couple at open circuit potential of each electrode, under Ar atm. Electrochemical impedance spectroscopy data for modified carbon paste electrodes both before (MWCNT/MO/IL ■ and MWCNT/MO/ILFe ●) and after (MWCNT/MO/IL □ and MWCNT/MO/ILFe ○) activation.
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Figure 4. Equivalent circuits obtained from experimental data fitted with the Zview program according to Kramer–Kronig relations. (a) A Randles circuit, and (b) a modified Randles circuit. Rs is the ohmic resistance; Ret is the electron transfer resistance; CPEdl is a constant phase element representing the capacitance of the double layer; W is the Warburg element that explains diffusion. CPE1 and R1 are elements added to explain the behavior of electrode MWCNT/MO/IL after activation.
Figure 4. Equivalent circuits obtained from experimental data fitted with the Zview program according to Kramer–Kronig relations. (a) A Randles circuit, and (b) a modified Randles circuit. Rs is the ohmic resistance; Ret is the electron transfer resistance; CPEdl is a constant phase element representing the capacitance of the double layer; W is the Warburg element that explains diffusion. CPE1 and R1 are elements added to explain the behavior of electrode MWCNT/MO/IL after activation.
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Figure 5. Photographs of the contact angle at 298 K for ultrapure water on (a) MWCNT/MO/IL before activation, (b) MWCNT/MO/IL after activation, (c) MWCNT/MO/ILFe before activation, and (d) MWCNT/MO/ILFe after activation.
Figure 5. Photographs of the contact angle at 298 K for ultrapure water on (a) MWCNT/MO/IL before activation, (b) MWCNT/MO/IL after activation, (c) MWCNT/MO/ILFe before activation, and (d) MWCNT/MO/ILFe after activation.
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Figure 6. Comparative voltammograms of the CPEs manufactured with MWCNT/MO (black line), MWCNT/MO/IL (red line), and MWCNT/MO/ILFe (green line) in the presence of 0.100 mol L−1 Hz at room temperature. Sweep rate 0.10 V s− 1 under Ar atmosphere.
Figure 6. Comparative voltammograms of the CPEs manufactured with MWCNT/MO (black line), MWCNT/MO/IL (red line), and MWCNT/MO/ILFe (green line) in the presence of 0.100 mol L−1 Hz at room temperature. Sweep rate 0.10 V s− 1 under Ar atmosphere.
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Figure 7. (a) Cyclic voltammetry of the MWCNT/MO/ILFe electrode at different scan rates for the oxidation of hydrazine 0.100 mol L−1 in PBS (pH 7.4) under Ar atm. (b) Graph of Ip vs. v1/2.
Figure 7. (a) Cyclic voltammetry of the MWCNT/MO/ILFe electrode at different scan rates for the oxidation of hydrazine 0.100 mol L−1 in PBS (pH 7.4) under Ar atm. (b) Graph of Ip vs. v1/2.
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Figure 8. Square wave voltammetry of MWCNT/MO/ILFe electrodes at different concentrations of hydrazine in PBS (pH 7.4). Increment 0.015 V, amplitude of 0.01 V. Inset: Linear regression obtained by plotting I (mA) vs. hydrazine concentration (mol L−1).
Figure 8. Square wave voltammetry of MWCNT/MO/ILFe electrodes at different concentrations of hydrazine in PBS (pH 7.4). Increment 0.015 V, amplitude of 0.01 V. Inset: Linear regression obtained by plotting I (mA) vs. hydrazine concentration (mol L−1).
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Table 1. Percentage values by weight for MWCNT/MO, MWCNT/MO/IL, and MWCNT/MO/ILFe electrodes.
Table 1. Percentage values by weight for MWCNT/MO, MWCNT/MO/IL, and MWCNT/MO/ILFe electrodes.
Electrode%C%O%Fe
MWCNT/MO96.81 ± 0.213.19 ± 0.26--
MWCNT/MO/IL94.42 ± 3.824.08 ± 2.50--
MWCNT/MO/ILFe86.22 ± 1.352.45 ± 1.654.19 ± 1.42
Table 2. Experimental electrical data obtained for MWCNT/MO/IL, and MWCNT/MO/ILFe before and after activation.
Table 2. Experimental electrical data obtained for MWCNT/MO/IL, and MWCNT/MO/ILFe before and after activation.
ElementBefore ActivationAfter Activation
MWCNT/MO/ILMWCNT/MO/ILFeMWCNT/MO/ILMWCNT/MO/ILFe
Rs (Ω)481140223171
CPEdl (µF)1.61.01.73.5
Ret (Ω)83457373692912,547
CPE1 (µF)--229-
R1 (Ω)--10,675-
Table 3. Experimental voltammetric data were obtained for HzOR using MWCNT/MO, MWCNT/MO/IL, and MWCNT/MO/ILFe.
Table 3. Experimental voltammetric data were obtained for HzOR using MWCNT/MO, MWCNT/MO/IL, and MWCNT/MO/ILFe.
ElectrodeVonset/V vs. Ag/AgClVp/V vs. Ag/AgClIp/mA
MWCNT/MO0.160.610.56
MWCNT/MO/IL0.140.540.56
MWCNT/MO/ILFe0.0100.490.63
Table 4. Electroanalytical comparison of different electrodes used for the oxidation of hydrazine.
Table 4. Electroanalytical comparison of different electrodes used for the oxidation of hydrazine.
Electrochemical SensorElectrochemical MethodLinear Range, LR (μ mol L−1)Limit of Detection, LOD (μ mol L−1)Ref.
Polythiophene (PTh)/GCEAmperometry0.5−480.207[67]
CuS-ordered mesoporous carbon/GCEAmperometry0.25−400.10[68]
CuO NPs/IL/CPEDifferential pulse voltammetry0.05–1500.03[56]
Nitrogen-doped graphene-polyvinylpyrrolidone-Au nanoparticles (NPs)/SPCESquare wave voltammetry2−3000.07[69]
Polyaniline (PANI)-doped mesoporous SrTiO3 nanocomposite/GCELinear sweep voltammetry/Amperometry200−3560
16−58		1.09
0.95		[70]
β-nickel hydroxide nanoplatelets/carbon paste electrode (CPE)Amperometry1−13000.28[71]
CdSe quantum dots (QDs)@Ni hexacyanoferrate (NiHCF) core−shell NPs modified electrodeChronoamperometry1.6−13000.5[72]
MWCNT/MO/ILFeSquare wave voltammetry40,000–100,0008000This work
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Brockmann, M.; Navarro, F.; Ibarra, J.; León, C.; Armijo, F.; Aguirre, M.J.; Ramírez, G.; Arce, R. Effect of the Metal of a Metallic Ionic Liquid (-butyl-methylimidazolium tetrachloroferrate) on the Oxidation of Hydrazine. Catalysts 2024, 14, 359. https://doi.org/10.3390/catal14060359

AMA Style

Brockmann M, Navarro F, Ibarra J, León C, Armijo F, Aguirre MJ, Ramírez G, Arce R. Effect of the Metal of a Metallic Ionic Liquid (-butyl-methylimidazolium tetrachloroferrate) on the Oxidation of Hydrazine. Catalysts. 2024; 14(6):359. https://doi.org/10.3390/catal14060359

Chicago/Turabian Style

Brockmann, Marcela, Freddy Navarro, José Ibarra, Constanza León, Francisco Armijo, María Jesús Aguirre, Galo Ramírez, and Roxana Arce. 2024. "Effect of the Metal of a Metallic Ionic Liquid (-butyl-methylimidazolium tetrachloroferrate) on the Oxidation of Hydrazine" Catalysts 14, no. 6: 359. https://doi.org/10.3390/catal14060359

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