Electrokinetics of CO₂ Reduction in Imidazole Medium Using RuO₂.SnO₂-Immobilized Glassy Carbon Electrode

Abstract

The pursuit of electrochemical carbon dioxide reduction reaction (CO₂RR) as a means of energy generation and mitigation of global warming is of considerable interest. In this study, a novel RuO₂-incorporated SnO₂-fabricated glassy carbon electrode (GCE) with a Nafion binder was used for the electrochemical reduction of CO₂ in an aqueous alkaline imidazole medium. The electrode fabrication process involved the drop-casting method, where RuO₂.SnO₂ was incorporated onto the surface of the GCE. Electrochemical studies demonstrated that the GCE-RuO₂.SnO₂ electrode facilitated CO₂ reduction at −0.58 V vs. the reversible hydrogen electrode (RHE) via a diffusion-controlled pathway with the transfer of two electrons. Importantly, the first electron transfer step was identified as the rate-determining step (RDS). A Tafel slope of 144 mV dec⁻¹ confirmed the association of two-electron transfer kinetics with CO₂RR. Moreover, the standard rate constant (k_o) and formal potential (E°′) were evaluated as 2.89 × 10⁻⁵ cm s⁻¹ and 0.0998 V vs. RHE, respectively. Kinetic investigations also reveal that the deprotonation and electron release steps took place simultaneously in the CO₂RR. Based on the reported results, the GCE-RuO₂.SnO₂ electrode could be a promising candidate for CO₂ reduction, applicable in renewable energy generation.

1. Introduction

The cumulative rise of CO₂ in the atmosphere has been identified as one of the key drivers of the greenhouse effect and global warming [1,2,3]. Due to widespread reliance on fossil fuels, atmospheric CO₂ levels are rising, leading to elevated global temperatures. Alarmingly, CO₂ accounts for 76% of the total greenhouse gas emissions [3]. Consequently, researchers are actively seeking ways to mitigate CO₂ levels by exploring its use as a renewable energy source and developing technologies for its capture and storage [4]. By converting CO₂ into useful fuels and utilizing it as a chemical feedstock, the electrochemical or photoelectrochemical reduction of CO₂ may provide a desirable answer to this climate-related challenge.

In recent decades, diverse techniques, such as electrochemical, photochemical, adsorption, and photoelectrochemical methods, have been applied to convert CO₂ into valuable chemicals [5]. These techniques for CO₂ mitigation offer distinct advantages when viewed from various perspectives. Among these techniques, the electrochemical CO₂ reduction reaction (CO₂RR) exhibits considerable potential for energy conversion due to its applicability to large-scale electricity generation and feasibility under ambient temperature and pressure [6,7]. Furthermore, electrocatalysts provide a greater number of active sites, a high surface area, and high porosity, all of which significantly enhance the CO₂RR performance [8]. Given these benefits, the electrochemical reduction of CO₂ into high-value-added chemicals offers a practically feasible approach, characterized by its simple and rapid operational process. Electrochemical CO₂RR involves the reduction of CO₂ to generate CO, CH₃OH, HCOO⁻, HCOOH, HCHO, C₂H₄, and other value-added chemicals [9,10,11,12,13,14,15,16,17,18]. Especially, the conversion of CO₂ into non-toxic liquid formate holds significant appeal since the conversion has substantial market value and reduces the electricity production costs [19]. Additionally, it offers potential applications in direct fuel cells as efficient hydrogen carrier systems [20,21]. Therefore, the focus of ongoing global research lies in developing durable electrocatalysts capable of selectively converting CO₂ into formate, overcoming the considerable energy barriers associated with the CO₂RR [22].

Notably, CO₂ is a highly stable chemical whose reduction needs a significant amount of energy [23,24]. The low efficiency and poor selectivity of the reaction are the biggest obstacles in CO₂RR [25]. Also, the generated products frequently combine, which are difficult to separate, resulting in low efficiency. To overcome these obstacles, researchers have investigated a variety of approaches, such as the tailoring of catalysts, customization of cell designs, and selection of solution parameters [26,27,28,29]. Pertinently, catalysts play a crucial role in CO₂RR because they can boost the reaction rate and selectivity. Earlier investigations have revealed that transition and p-block metal electrodes, such as Au, Ag, Cu, Pd, Pt, In, Sn, Bi, Hg, and Pb, are fascinating candidates for electrochemical CO₂ reduction reactions [30,31,32,33]. Among them, Au is the most active electrocatalyst for the reduction of CO₂ to CO. Nevertheless, the high activation energy of CO₂ rupture still restricts the electrocatalytic activity of Au during reduction [34]. Cu is the only metal catalyst that has been reported to generate considerable C1–C3 hydrocarbon products [35]. Sn is cost-effective with no toxicity like Bi- and Pb-based materials. Furthermore, oxides of Sn provide oxygen vacancy, grain boundaries, and low coordinated, facile sites that absorb CO₂ and accelerate the transfer of electrons to form formic acid [36]. For instance, Kayan et al. noticed a significantly greater CO₂ reduction capability in tin/tin oxide electrodes compared to tin foil [37]. Rende et al. also studied the CO₂ reduction reaction with Sn/SnO₂ that showed a Faradic efficiency of 74.7% for formate production [38]. Meanwhile, the oxides of Ru provide a lower free energy pathway as well as a high oxygen vacancy to adsorb CO₂ [39]. Remarkably, dopants like Cu and Ru have the capability to increase the oxygen vacancy in SnO₂ [40]. Peng et al. studied CO₂RR with Cu- and Sn-deposited nitrogen-doped carbon cloth electrocatalysts that generated formate with 90.24% Faradic efficiency and 15.56 mA cm⁻² current density at −0.97 V against the reversible hydrogen electrode (RHE) [41]. Similarly, other bimetallic and/or bimetallic oxide catalysts could be tailored to attain selective CO₂RR to formate. For example, the Ru–Ru bridge sites aid in lowering the overpotential for the formation of formate, according to research by Atrak et al. that used density functional theory to assess the CO₂ reduction reaction on TiO₂/RuO₂ alloy [42]. Consequently, there is a pressing need to design a bimetallic catalyst based on Sn and Ru that will display facile activation of CO₂ and its subsequent conversion to harmless as well as profitable reduction products.

An additional noteworthy obstacle in the CO₂ reduction process is the difficulty in amassing a reaction medium with an adequate amount of CO₂ gas. In this context, amine compounds have been recognized for their efficacy in adsorbing CO₂ at room temperature and ambient pressure [43]. In an aqueous environment, imidazole (C₃N₂H₄) is capable of capturing CO₂ gas and delivering a suitable amount of CO₂ to the surface of the electrode for reduction. Additionally, amino groups in imidazole undergo protonation, leading to the formation of >NH₂⁺ groups in the aqueous environment. These >NH₂⁺ groups act as Lewis acids and exhibit a strong interaction with CO₂, which acts as a Lewis base. This interaction enhances the solubility of CO₂ [44]. The presence of active >NH₂⁺ sites in the structure of imidazole enables improved catalytic performance during CO₂ reduction [45].

Thus, this study aims to design a RuO₂-incorporated SnO₂ catalyst fabricated over glassy carbon electrode (GCE) surfaces with the assistance of Nafion. The catalytic performance and kinetics of the CO₂ reduction reaction have been investigated. To the best of our knowledge, no such kinetic investigation pertaining to the electrochemical reduction reaction of CO₂ was reported in any previous literature using a RuO₂.SnO₂ catalyst.

2. Results and Discussion

2.1. Surface Characterization

The phase compositions of the synthesized catalyst were analyzed with the help of an X-ray diffraction (XRD) study; the observed XRD pattern is presented in Figure 1A. It is seen that the crystalline phase of the pure SnO₂ is the tetragonal rutile phase, which is also the major bulk phase found in the synthesized RuO₂.SnO₂ catalyst. However, peaks belonging to RuO₂ were not found in the case of the composite catalyst, suggesting a probable formation of a solid solution through the insertion of Ru⁴⁺ cations into the crystal lattice of SnO₂ [46,47,48]. The structural feature of both the oxides is almost identical, that is, tetrahedral, and both of the metallic ions have similar ionic radii, which could make the oxides prone to developing a solid solution with a defined lattice capacity [46,47,48]. Given the usage of a small amount of RuO₂ in the synthesis process, it is conjectured that the lattice capacity of Ru is too small to detect the Ru–Sn–O solid solution by means of XRD analysis [46,47,48]. Consequently, all of the ruthenium-related species could be incorporated into the SnO₂ lattice matrix.

In order to assess the surface morphology of the synthesized catalyst, the scanning electron microscopy technique was employed, and the resultant images are illustrated in Figure 1C,D. It is apparent from the images that the surface of the catalyst is porous and has sponge-like morphology with significant roughness and complexity. Furthermore, SEM images reveal that the particles of the synthesized catalyst are of irregular shape due to the formation of clusters of small crystals. This observation indicates substantial enhancement of surface area, which might be beneficial for electrochemical application. The enhancement of surface area as a result of the incorporation of RuO₂ into SnO₂ matrix was proved by means of Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) techniques, as described in our previous study [49].

The transmission emission microscopic (TEM) analysis was then carried out to extract the information about the distribution of RuO₂ particles on SnO₂ phases. TEM images of pure SnO₂ particles, as illustrated in Figure S1A–D in the Supporting Information file, obtained at various magnifications, demonstrate that the structures are interconnected with each other. In addition, the crystalline nature of the pure SnO₂ particles is evident from the lattice fringes found in the high-resolution TEM images. On the other hand, RuO₂.SnO₂ retains a sphere-shaped structural feature similar to the pure SnO₂ particles, but the surface of the composite is rougher compared to the SnO₂, as presented in Figure 2A,B.

The elemental composition of RuO₂.SnO₂ was then determined by performing the energy-dispersive X-ray (EDX), and the resultant plot is illustrated in Figure 3. From the spectrum, it is seen that Ru, Sn, and O elements are present with a composition of 5.15 wt%, 74.44 wt %, and 20.41 wt %, respectively. These findings validate the successful formation of the RuO₂.SnO₂ composite. Furthermore, EDX mapping was also carried out to analyze the spatial distribution of elements, as portrayed in Figure S2. It is clearly seen that the Ru element was almost homogeneously distributed on the SnO₂ matrix.

At this point, the X-ray photoelectron spectroscopic (XPS) experiments were performed to investigate the surface properties of the RuO₂.SnO₂ catalyst. Figure 4A,B illustrates the XPS spectra of Sn 3d of pure SnO₂ and the synthesized RuO₂.SnO₂, respectively. The peaks belonging to the binding energy of 487.19 eV and 495.62 eV in Figure 4A might be attributed to the existence of Sn 3d_5/2 and Sn 3d_3/2 of Sn(IV) species in pure SnO₂ particles, respectively [50]. In the case of RuO₂.SnO₂, these two peak maxima shifted to relatively lower binding energies, appearing at 486.97 and 495.39 eV for Sn 3d_5/2 and Sn 3d_3/2 (see Figure 4B and

Table 1. XPS peak position of the metal ions in pristine SnO₂, RuO₂, and the synthesized RuO₂.SnO₂ catalysts.

Catalyst	Sn 3d5/2 (Sn2+)/eV	Sn 3d3/2 (Sn2+)/eV	Sn 3d5/2 (Sn4+)/eV	Sn 3d3/2 (Sn4+)/eV	Ru 3d5/2 (Ru4+)/eV	Ru 3d3/2 (Ru4+)/eV	Ru 3p3/2 (Ru4+)/eV
SnO2	485.05	493.67	487.19	495.62	-	-	-
RuO2	-	-	-	-	280.61	284.83	462.56
RuO2.SnO2	484.84	493.57	486.97	495.39	280.86	285.06	462.75

), respectively. A similar shift to low binding was also noticed in the case of the peaks that correspond to the Sn²⁺ cation, as shown in Figure 4A,B and Table 1. This lower shifting of the binding energy indicates the mobilization of electrons from the Ru to Sn species while the Sn–Ru–O solid solution was formed [49,50]. The phenomenon of electron transfer between Ru and Sn was further validated from the XPS spectra of Ru 3d and 3p. The XPS analysis of pristine RuO₂ showed the appearance of peaks in the region of 280–290 eV, as presented in Figure 4C, in which the peaks at 280.61 and 284.83 eV after deconvolution can be attributed to Ru 3d_5/2 and Ru 3d_3/2 of Ru⁴⁺ cations, respectively [51,52]. These two peaks of Ru⁴⁺ ion in the case of RuO₂.SnO₂ shifted to the higher binding energy, as illustrated in Figure 4D, confirming the electronic transition from Ru to Sn metallic species. The shifting of peaks to higher binding energy was also observed in the case of Ru 3p, which is evident from the comparison of the peak positions of Ru 3p in RuO₂ and RuO₂.SnO₂, as depicted in Figure 4E,F. A comparison of the major XPS peaks for Sn and Ru is presented in Table 1 for a better understanding in support of the spectra in Figure 4. Since the composite is formed by the combination of two oxide materials, it is necessary to examine the surface oxygen properties of the catalysts. In this regard, the XPS spectra of O 1s of pristine SnO₂ and RuO₂.SnO₂ were analyzed.

From Figure 5A,B, it is apparent that two peaks at 531 eV and 532.5 eV are present in both catalysts, and these peaks can be attributed to lattice oxygen (O_lat) and adsorbed oxygen (O_ads), respectively [50]. The molar ratio of these two different types of oxygen, that is, O_ads/O_lat, in the case of both SnO₂ and RuO₂.SnO₂ was evaluated to be ca. 0.132 and 0.174, respectively. The increased molar ratio in the case of composite RuO₂.SnO₂, however, supports the development of the Sn–Ru–O bond in the solid-state material. Almost similar findings were also reported in our previous work, where this composite was synthesized for HER application [49]. However, due to the formation of solid solutions, lattice distortion may result in defect formation, which may enhance the generation of surface mobile oxygen species [53,54,55].

2.2. Electrochemical Characterization

The electrochemical characterization of the GCE-RuO₂.SnO₂ electrode was accomplished through linear polarization and electrochemical impedance spectroscopy (EIS) measurements. Open circuit potential (OCP) was determined by linear polarization to characterize the electrode material where the potential at zero current is measured. Figure 6A shows the polarization curves for bare GCE and GCE-RuO₂.SnO₂ in 0.05 M imidazole. As the GCE-RuO₂.SnO₂ electrode was expected to be catalytically viable, the nature of the electronic charge developed was assessed when the GCE-RuO₂.SnO₂ interface interacted with CO₂ at open circuit conditions in 0.05 M imidazole. The OCP value examined with a bare GCE was found to appear at 0.56 V vs. RHE. Conversely, when a GCE-modified RuO₂.SnO₂ electrode was employed, the OCP appeared relatively at a more negative potential (0.32 V vs. RHE) in reference to a bare GCE. This observation confirms that the fabricated GCE-RuO₂.SnO₂ electrode acquired additional negative charges i.e., a more reducing environment on its surface, indicating a provable enhancement in the adsorption process of CO₂.

To perceive the provable charge transfer properties of GCE and GCE-RuO₂.SnO₂, the EIS was recorded in CO₂-saturated imidazole by applying a potential below the OCP value, e.g., −0.58 V vs. RHE, as an excitation potential. Figure 6B demonstrates the typical appearance of Nyquist plots at bare GCE and GCE-RuO₂.SnO₂ electrodes in the presence of CO₂ in 0.05 M imidazole. In EIS analysis, the size of the semicircle observed in the Nyquist plot corresponds to the charge transfer resistance (R_ct) of the electrode surface, and a higher R_ct value suggests that the reaction kinetics are slower [56,57,58]. In this research, the GCE-RuO₂.SnO₂ composite unveiled a smaller semicircle diameter with an R_ct of 3.19 kΩ, while at bare GCE, the R_ct value was observed to be 20.9 kΩ. The lower polarization resistance at the GCE-RuO₂.SnO₂ electrode compared to a bare GCE indicates that the CO₂RR reduction activity is more convenient at GCE-RuO₂.SnO₂ in comparison to a bare GCE, since this decrease in polarizable character indicates the formation of catalytic sites at the electrode surface. The equivalent circuit is shown as the inset of Figure 6B, while the relative EIS parameters of the electrode processes are reported in

Table 2. EIS parameters of the bare GCE and GCE-RuO₂.SnO₂ electrodes recorded in CO₂-saturated 0.05 M Imidazole solution.

Electrode	Rs (Ω)	Rct (kΩ)	CPE (µMho)
GCE	669	20.9	64.9
GCE-RuO2.SnO2	790	3.19	3.36

R_s = solution resistance; R_ct = charge transfer resistance; CPE = constant phase element.

.

2.3. Cyclic Voltammetry

As outlined in the previous section, the EIS investigation revealed that the GCE-RuO₂.SnO₂ electrode potentially produces catalytic sites for CO₂ reduction. Consequently, to assess the catalytic performance, CV analysis was conducted at both the GCE and GCE-RuO₂.SnO₂ electrodes using a CO₂-saturated imidazole solution employing a 0.1 V s⁻¹ scan rate. Figure 7A shows diffusive currents resulting from the reduction of CO₂ with a sharp peak at −0.58 V vs. RHE at the GCE-RuO₂.SnO₂ electrode, while bare GCE exhibits no peak in 0.05 M imidazole solution in the presence of CO₂. The peak at −0.58 V vs. RHE for the reduction of CO₂ further clarified when the reaction was carried out at the GC-RuO₂.SnO₂ surface between the potential regions of 0.67 V and −1.13 V vs. RHE with and without CO₂, as shown in Figure 7B. Note that the GCE modified with RuO₂.SnO₂ showed no peak in the absence of CO₂ but while the medium was saturated with CO₂, a sharp peak appeared, which confirms the reduction of CO₂ at the electrode surface. By contrast, at the pristine GCE electrode, a potential-dependent kinetic current was observed. This comparable observation implies that under the experimental conditions, the GCE-RuO₂.SnO₂ electrode possesses more active catalytic sites than a pristine GCE electrode to execute CO₂ reduction. Briefly, when RuO₂.SnO₂ nanocomposites are immobilized on the GCE surface, a robust catalytic effect is generated pertinent to a quicker electron transfer rate with an onset potential (E_i) of 0.26 V vs. RHE and a peak potential (E_p) of −0.58 V vs. RHE. It is worthwhile to note that no significant competition from the HER was observed in the working potential range as depicted by the dashed-line CV obtained without CO₂ (blank) in the N₂-saturated imidazole solution in Figure 7B. Furthermore, we have achieved one of the highest current densities as well as lower peak potentials in a well-defined diffusive CV, indicating that the electrode’s behavior is activation-controlled for CO₂RR and is favorable to investigating peak-related kinetics [59]. While numerous studies have been conducted on the electrochemical CO₂ reduction, only a few have managed to achieve a diffusive nature of the CV in CO₂ reduction [60].

Table 3. Comparison of CV parameters for different electrodes regarding CO₂RR.

Electrodes	−Ep/V vs. RHE	jp/mA cm−2	υ/Vs−1	[CO2]/ mM	Solvent	Ref.
GCE	1.60	0.25	0.01	5	BMP TFSI	[61]
Cu	1.93		0.1	454	BMP TFSI	[62]
Ag	1.38	0.1	0.5	45	[BMIM] [TFSI]	[63]
[CoII(bipy)3](BF4)2/GCE	1.31	0.38	0.1		TBAPF6 + acetonitrile	[64]
SnS|PTFE|Pt	0.43	1.52	0.05	18	imidazole	[12]
[(Mn(TPP)Cl)]/VCE	0.73	6.0	0.1	230	acetonitrile	[65]
[Re(BPEBP)(CO)3Cl]/Pt	1.03	2.6	0.1	280	acetonitrile	[66]
Co(TPP)Cl/VCE	0.83	0.0176	0.1		[Bu4N][BF4]-acetonitrile + DMF	[67]
GCE-RuO2.SnO2	0.58	10.49	0.1	18	imidazole	This work

E_p: peak potential; j_p: peak current density; and υ: scan rate.

provides a comparison between various electrochemical CO₂RR parameters reported in our findings and several previously reported studies.

2.4. Kinetics

2.4.1. Tafel Analysis

The Tafel analysis is a valuable tool for studying the electrochemical process because it provides insights into reaction kinetics, mechanism, and catalyst performance. By analyzing the Tafel slope under various experimental conditions, it is possible to gain a deeper understanding of the CO₂ reduction pathways and develop more efficient and selective processes for converting CO₂ into valuable products (such as carbon monoxide, formate, or methane). Therefore, to delve deeper into the electrochemical CO₂RR pathway, Tafel analysis was performed at the kinetic domain of the voltammogram using the following Equation (1) [68,69,70]:

$$\log \left(j\right)=\log (j_k)+b_T(E-{E°}^{\prime })$$

(1)

Herein, $(E-{E°}^{\prime })$ represents overpotential, and $b_T$ is the Tafel slope. The value of $b_T$ was calculated as 144 mV dec⁻¹ from the correlation of log(j) and E vs. RHE, as shown in Figure 7C. Numerous research groups have previously analyzed the Tafel slopes of CO₂RR to determine the reaction pathway. The Tafel slope in this study (144 mV dec⁻¹) is consistent with the mechanism of CO₂ reduction to formate, which involves the addition of one electron to the adsorbed CO₂ to generate carbon dioxide radical anion as the initial RDS (rate-determining step), according to earlier studies [37,68,71,72,73]. The findings reveal that CO₂RR at the GCE-RuO₂.SnO₂ electrode surface involves the ${2e}^{-}$ transfer mechanism producing formate, where the conversion of CO₂ to CO radical anion is the RDS.

2.4.2. Scan Rate Effect

Determination of the kinetic parameters of electron transport (ET) relies significantly on the scan rate. To uncover the kinetic parameters of CO₂RR at the GCE-RuO₂.SnO₂ electrode, CVs were recorded at various scan rates ranging from 0.010 to 0.2 V s⁻¹ (Figure 8A). It is seen that the CO₂RR current increased with the increase in scan rate; afterward, the corresponding slope (log j_p vs. log υ) was found to be 0.32 (Figure 8B), which suggests that the CO₂RR at the GCE-RuO₂.SnO₂ electrode follows mass transfer (diffusion-controlled) kinetics [68]. The above phenomenon is also correlated to the underlying character of the diffusion-controlled irreversible charge transfer process. This result indicates that the electron transfer step is striking upon the adsorption step on the surface at potentials more negative than the onset potential (0.0998 V vs. RHE). The typical characteristics of the CVs regarding E_i, E_p, and j_p observed due to the increased scan rate ensure that the GCE-RuO₂.SnO₂ electrode is stable and reproducible for CO₂RR.

Nonetheless, the transfer coefficient (α) distinguishes between the various categories of ET routes concerned in an irreversible reaction. Equations (2) and (3) show how α of an irreversible process is correlated to the activation-free energy (∆G^‡(E)) and reaction-free energy (∆G^o) [68]. According to the formulas, if α varies linearly with potential (E), the ET kinetics could be described by the Butler–Volmer (B–V) model:

$$\mathsf{\alpha }={\displaystyle \frac{{∆G}^{‡}(\mathrm{E} )}{{∆G}^o}}={\displaystyle \frac{{∆G}^{‡}(\mathrm{E} )}{F(E-E^o)}}$$

(2)

$$\mathsf{\alpha }={\displaystyle \frac{{∆G}^{‡}\left(\mathrm{E}\right)}{{∆G}^o}}=0.5+{\displaystyle \frac{F(E-E^o))}{{{∆G}^{‡}}_o}}$$

(3)

where ∆G^‡_o represents the activation-free energy while E = E^o. A theoretical analysis of the B–V kinetics suggests that the peak potential (E_p) increases linearly with the logarithm of the scan rate (υ). Furthermore, α is related to both the peak potential and half-peak width potential (E_p/2) at 298 K, according to the following Equations (4) and (5) [68]:

$${\displaystyle \frac{{\partial E }_p}{\partial \mathrm{l}\mathrm{o}\mathrm{g}\left(v\right)}}={\displaystyle \frac{29.6}{\mathsf{\alpha }}} \mathrm{m}\mathrm{V}$$

(4)

$$E_p-E_{{\displaystyle \frac{\mathrm{p}}{2}}}={\displaystyle \frac{47.7}{\mathsf{\alpha }}}\mathrm{m}\mathrm{V}$$

(5)

The dependence of (E-−E_P/2) on the scan rate is plotted to determine the reaction pathway as shown in Figure 8C. When the scan rate was increased from 0.010 to 0.20 V s⁻¹, a significant variation in both E_p–E_P/2 (shifted from 0.202 to 0.341 V) and the corresponding α (altered from 0.24 to 0.14) was observed (see Figure 8C,D). This observed variation suggests that the B–V method is not suitable for explaining the ET kinetics of the CO₂RR at the GCE-RuO₂.SnO₂ surface [68]. It is worthwhile to note that the B–V model is only applicable to the kinetic region of the voltammogram.

2.4.3. Convolution Study

The Butler–Volmer (B–V) model is restricted in its applicability to the kinetic region and requires the transfer coefficient (α) to remain constant within this range. The earlier scan-rate-dependent analysis reveals that the transfer coefficient of CO₂RR on GCE-RuO₂.SnO₂ is not invariant and changes with the scan rate. In light of this limitation, Convolution Potential Sweep Voltammetry (CPSV) emerges as a more robust and precise method for determining the transfer coefficient (α). Unlike the B–V model, CPSV enables accurate estimations not only within the kinetic region but also across the entire voltammogram, ensuring a comprehensive analysis. The convolution of voltammetric current generates a classic sigmoid curve including a plateau. According to Equation (6), the peak corresponds to the limiting convolution current (I_l) as follows [68,74,75]:

I_l = nFAC√D_o

(6)

Here, n is the number of electron transfers, F is Faraday’s constant, A stands for the surface area of the electrocatalyst, C represents the concentration of the electroactive species, and D_o is the diffusion coefficient.

This limiting current is self-reliant on scan rate and can be employed to calculate ‘n’ or ‘D_o’ almost precisely if the capacitive current is correctly extracted. As per a previous report, the diffusion coefficient (D_o) of carbon dioxide is 1.71 × 10⁻⁵ cm² s⁻¹ [76]. By applying this value of D_o, the number of electrons involved in CO₂RR was calculated to be 1.92 (≈2) using Equation (6). The proposed reaction pathways corresponding to a 2e⁻ transfer CO₂ reduction reaction are presented as Scheme 1.

Scheme 1. Provable CO₂ reduction pathways on GCE-RuO₂.SnO₂ electrocatalytic surface. Replicated from [77] and accessible under a CC-BY 4.0 license. Copyright 2019, Zhao et al.

Scheme 1. Provable CO₂ reduction pathways on GCE-RuO₂.SnO₂ electrocatalytic surface. Replicated from [77] and accessible under a CC-BY 4.0 license. Copyright 2019, Zhao et al.

The production of formate and CO can occur via three different pathways, as depicted in Scheme 1. However, the choice of pathway largely relies on whether the initial proton coupling occurs at the carbon or oxygen atom of the adsorbed CO₂ radical anion. If protonation takes place at the carbon atom, it forms an HCOO· intermediate, leading to pathway 1, where subsequent electron transfer and protonation result in HCOOH formation. Pathway 2 involves an additional step where HCOO⁻ is converted to the ·OCHO intermediate via electron transfer, followed by protonation for HCOOH formation. Pathway 3 initiates with protonation at the oxygen atom, forming a ·COOH intermediate, which then undergoes electron transfer and protonation to yield either HCOOH or CO, releasing water. To unveil more on kinetics, the transfer coefficient was next evaluated by exploiting CPSV.

Figure 9A displays the CPSV currents measured at 0.1 V s⁻¹, and using the value of the limiting current, the heterogeneous rate constant (k_het) can be calculated for an irreversible ET process as per Equation (7) [68], where I_(l), i_(t), and I_(t) stand for the limiting convolution current, cyclic voltammetric current, and time-dependent convolution current, respectively. Applying Equation (8) within a limited potential range, the apparent transfer coefficient (α_app) was finally calculated [68,78] as follows:

$$\ln k_{het}=ln\sqrt{D}-\ln {\displaystyle \frac{I_{\left(l\right)}-I_{\left(t\right)}}{i_{\left(t\right)}}}$$

(7)

$${\mathsf{\alpha }}_{\mathrm{a}\mathrm{p}\mathrm{p}}={\displaystyle \frac{RT}{F}}{\displaystyle \frac{dlnk}{dE}}$$

(8)

Figure 9B,C display the plots of lnk_het vs. E and α vs. E, respectively, in the case of two different scan rates. The curving behavior observed in Figure 9B suggests that the ET kinetics of CO₂RR at the GCE-RuO₂.SnO₂ surface was potential-dependent [68,78]. From Figure 9C, it is evident that the α value was ca. 0.5 near the onset potential (0.0998 V vs. RHE) and then declines to 0.23 around −0.2202 V vs. RHE. This decrease in α (α < 0.5) within the potential range (0.0998 V to −0.2202 V vs. RHE) suggests that the protonation and electron release steps occur simultaneously [68,72]. Furthermore, α_app becomes equal to 0.5 at E = E°′, which indicates a nonlinear electron transfer process [79]. Thus, by adjusting α_app = 0.5 in Figure 9C, the formal potential (E°′) value regarding CO₂RR was calculated to be 0.0998 V vs. RHE, which is consistent with the OCP value. Finally, the standard rate constant (k^o) was estimated as 2.89 × 10⁻⁵ cm s⁻¹. Therefore, at this point, it can be concluded that the CO₂RR mechanism at the GCE-RuO₂.SnO₂ electrode surface is highly dependent on the applied potential, where a potential of 0.0998 V vs. RHE represents the breakthrough point.

An α value less than 0.5 indicates the presence of a slow step, in which radical species (COO•−) are formed following the initial ET [80]. This observation further suggests a two-step ET process, with the first step being the slowest, followed by a faster heterogeneous electron transfer. Consequently, the rate of radical anion formation on the catalytic surface is slower compared to the rate of the second ET [81]. The initial ET step, becoming the RDS in CO₂RR over the developed GCE-RuO₂.SnO₂ electrode, strongly suggests the formation of formate anion via a two-electron transfer step, as illustrated in Scheme 2 [82].

The stability of the GCE-RuO₂.SnO₂ electrode was then investigated by conducting 500 CV cycling with the electrode in CO₂-saturated imidazole solution. However, no significant change in the onset potential (E_i), peak potential (E_p), and peak current (I_p) was observed after 500 cycles of CV runs, boasting an impressive stability of the proposed electrode, as shown in Figure 10.

3. Experimental

3.1. Chemicals and Instruments

All the chemicals used in this research were of analytical grade and used without further purification. The major chemicals, such as SnCl₄ and RuCl₃, were purchased from Wako (Japan). Meanwhile, urea was collected from Shahjalal fertilizer industry (Sylhet, Bangladesh). In all cases, solutions were prepared with ultrapure deionized water having resistance nearly 18.1 MΩcm.

The pure CO₂ gas was obtained in high-pressure liquid form and stored in a cylinder for dissolution into an aqueous imidazole medium. It is important to note that employing high-pressure/supercritical CO₂ rather than 99.99% pure CO₂ gas has some benefits, such as being more affordable, leaving no unwanted compounds behind, and being conveniently easy to store [83]. Potentiostat PGSTAT 128N (Metrohm Autolab BV, Kanaalweg 29G, 3526 KM Utrecht, The Netherlands), CHI602E electrochemical workstation (CH Instruments Inc., 3700 Tennison Hill Dr, Bee Cave, TX 78738, USA), and Wavedrive 20 (Pine Research Instrumentation, Inc. 2741 Campus Walk Avenue, Building 100, Durham, NC 27705, USA) were used to perform all the electrochemical experiments in combination with the globally acknowledged three-electrode system.

3.2. Synthetic Method of RuO₂.SnO₂ Catalyst

At first, SnO₂ was synthesized following homogeneous precipitation method. Shortly, 5 g SnCl₄ was mixed with 20 g urea in a beaker, and then 100 mL of deionized (DI) water was added to it. The resulting mixture was then stirred at 200 rpm at 90 °C in a magnetic bath for 4 h until it started to reflux. The refluxing process was continued until a white precipitate was formed. The as-developed precipitate was then centrifuged and washed with required amount of DI water prior to drying at 120 °C. The dried material as developed was pulverized with a mortar until fine powder was obtained. The as-prepared material was heated at 500 °C for 3 h under aerated conditions in a furnace, and powdered SnO₂ material was obtained by pulverizing again in a mortar. Next, 650 mg SnO₂ and 65 mg RuCl₃ were mixed in a beaker containing 50 mL of DI water. The mixture was magnetically agitated for ca. 4 h at 200 rpm. The blend was immediately exposed to heating at 160 °C until all the water was removed via evaporation. After removal of water, the resultant solid material was left in an oven at 120 °C overnight. The composite material obtained in this way was crushed to fine powder using a mortar, which again was heated at 500 °C for 3 h in a muffle furnace. Finally, a homogeneous RuO₂.SnO₂ composite was obtained by pulverizing the material after cooling it to room temperature.

3.3. Morphological and Chemical Characterization

A powder X-ray diffractometer (Rigaku D/MAX RINT-2000) operating at 40 kV and 40 mA with Cu Kα radiation was used to determine the crystal structures of the synthesized catalysts. Diffraction patterns were recorded in continuous scan mode over a 2θ range of 10° to 80°, with a sampling pitch of 0.1° and a scan rate of 2° min⁻¹. X-ray photoelectron spectroscopy (XPS) was performed with Al Kα monochromatic radiation (12 keV) using a K-Alpha spectrometer (Thermo Fisher Scientific) to assess the valence states and chemical bonding characteristics. Surface charge effects were corrected using the C 1s binding energy at 285 eV as a reference, and all spectra were normalized to Al 2p for quantitative analysis.

3.4. Electrode Preparation

For cyclic voltammetric measurements, Nafion-stabilized RuO₂.SnO₂-modified glassy carbon electrode (GCE-RuO₂.SnO₂; geometric area of 0.07 cm²) has been used as the working electrode (WE). At first, the GCE was polished with 0.3 μm alumina slurries until a smooth, glossy surface was observed. After polishing, the electrode was exposed to ultrasonication in the presence of methanol and acetone for a duration of 20 min at 25 °C to remove abrasive particles from the electrode surface. Following the ultrasonic treatment, the GCE was cleaned in 0.1 M H₂SO₄ by maintaining the potential between $-$1.2 and 0.5 V at 0.1 V s⁻¹ for 100 cycles until a repeatable cyclic voltammogram (CV) corresponding to the characteristic behavior of GC was achieved. After electrochemical cleaning, the GCE surface was capped with the as-developed RuO₂.SnO₂ catalyst following drop-casting method. To prepare catalyst ink, exactly 0.5 mg of RuO₂.SnO₂ catalyst was suspended in a homogeneous solution of 75 µL ethanol and 25 µL of Nafion (5 wt%), and the mixture was continuously stirred for 10 min under ultrasound conditions. Then, 5 µL of the catalyst ink (RuO₂.SnO₂) was pasted on the cleaned GCE disk surface and left overnight in open air, which resulted in a GCE-RuO₂.SnO₂ electrode. The electrode surface was finally dried for 4 h at room condition. Herein, Nafion polymer acted as a binder and a supportive layer by stabilizing the drop-casted RuO₂.SnO₂ catalyst over the GCE surface.

3.5. Electrochemical Measurements

An internationally recognized three-electrode configuration was used for the electrochemical investigations of CO₂RR in a 0.05 M imidazole electrolyte solution. The WE was GCE-RuO₂.SnO₂, while Pt and Ag/AgCl (saturated in KCl) were used as the counter and reference electrode, respectively. To elucidate the electronic properties of the GCE-RuO₂.SnO₂ electrode, linear polarization curves were recorded in a 0.05 M imidazole solution. Next, to perceive the provable charge transfer properties of the catalyst, the EIS of the electrode was recorded in CO₂-saturated imidazole by applying a potential below the OCP value, e.g., −0.43 V vs. RHE, as an excitation potential. To assess the catalytic performance, Cyclic voltammetric (CV) analysis was conducted with and without CO₂-saturated imidazole solution, employing 0.1 V s⁻¹ scan rate. The mass transfer effect was studied under saturated conditions of CO₂ by altering the scan rate between 0.01 and 0.2 V s ⁻¹. Convolution potential sweep voltammetry analysis was carried out using the CHI660 electrochemical workstation by subtracting the background current. Note that all potentials were adjusted to the reversible hydrogen electrode (RHE) scale with 80% iR compensation applied using the following Equation (9):

$$E_{RHE}=E_{Ag/AgCl(saturated KCl)}+0.197+\left(0.0591\times pH\right)-iR$$

(9)

4. Conclusions

An RuO₂.SnO₂-covered GCE was used for the CO₂ reduction reaction in an alkaline imidazole medium. From the voltammetric responses, it was deduced that the catalyst selectively produces formate at moderate overpotential. The convolution study indicated the involvement of an RDS among the two-step electron transfer kinetics that served as proof for the production of formate over CO during CO₂ reduction at the GCE-RuO₂.SnO₂ electrode surface. A scan rate-dependent CV analysis uncovered that the reaction is diffusion-controlled, whereas the convolution study suggested that the deprotonation and electron release steps occur simultaneously during the CO₂RR. Overall, these findings provide a new dimension for the use of Ru–Sn-based energy materials in CO₂ reduction reaction, thus offering numerous valuable insights for further understanding of the CO₂ reduction kinetics toward formate production.