Introduction

Global energy consumption is highly dependent upon fossil fuels, and climate models have shown that CO2 emissions are inducing climate change due to the greenhouse effect [1, 2]. The conversion of CO2 into valuable chemicals and fuels by means of electrochemical reduction could solve both the environmental and energy crises [2,3,4]. This process carries several advantages, such as low-temperature operation, that it can be run at ambient pressure, and the required energy input can be supplied from renewable energy sources (i.e., solar or wind), creating a net-zero CO2 emission condition in certain energy business cases [5,6,7]. Moreover, the performance and selectivity of such an electrochemical reaction can be tuned, and the scale-up of this process becomes simpler than other such as: photochemical and thermochemical process [8,9,10].

The CO2 reduction reaction (CO2RR) is a multielectron process that may proceed via different reaction pathways, yielding diverse reduction products, such as CO and HCOOH (2 electrons), CH3OH (6 electrons), CH4 (8 electrons), C2H4 (12 electrons) depending on the electrocatalysts and experimental conditions [11]. Formic acid has been receiving significant attention as an CO2RR product due to its stability, remarkably high volumetric capacity, and its versatile potential use in various applications (e.g., direct formic acid fuel cells, and the leather, textile, food, and chemical industries) [12, 13]. The economic viability of various chemicals from the CO2RR demonstrated that formic acid has a great business value, which is one of the most desired products [10]. However, the inertness of CO2 due to its high chemical stability results in a process with high overpotential, sluggish kinetics, and broad distribution of products [3, 14, 15]. The physical–chemical properties of an electrocatalyst, such as morphology [16], chemical state [17,18,19], and surface features strongly depend on its CO2RR performance. Therefore, rationalizing the effect of each catalyst’s properties on CO2 reduction is especially important in the development of new kind of materials to overcome the major challenges in this field [20].

Among the materials for converting CO2 into formic acid, tin (Sn)-based ones exhibit remarkable features, such as good selectivity, low costs, and nontoxicity [21,22,23,24]. However, bare Sn planar electrodes present a current density of − 5 mA cm−2 with 80% Faradaic efficiency and an overpotential of almost 0.90 V, which is too low for practical applications [25]. The role played by metal semiconducting oxide, oxidation state and morphology whether as catalysts for the formation of oxygenates, still remains unclear [18, 26, 27]. Some researchers have shown that a layer of metal oxide on the catalyst surface can decrease the reaction overpotential and increase the performance of the CO2RR [18, 26,27,28,29]. Additionally, Kanan et al. [28] demonstrated that the removal of the SnOx native layer from an Sn electrode results in near exclusive H2 evolution activity. It was also demonstrated that CO2RR performance of SnOx-based electrocatalyst decreased after Sn4+ reduction to Sn0, they verified the occurrence of three processes: CO2 reduction into formic acid, hydrogen evolution reaction and the reduction of the SnO2 catalyst which yields tin species of lower oxidation number (that is, Sn0 and probably Sn2+ species as well) [27]. Additionally, at moderately cathodic potentials, SnO2 exhibited high selectivity for the production of formate, while at very negative potentials it was observed the reduction of oxide to Sn, and the efficiency of formate production was significantly decreased [27]. Therefore, Sn-based catalysts showed low stability during long periods of operation, as the reduction of SnOx species in Sn metallic decreases its activity [28, 30]. However, it has also been found that the deposition of electrocatalysts on carbon paper can increase its performance and stability [31, 32]. Nevertheless, SnOx-based nanoparticles deposited on carbon paper have thus far been little explored for CO2 reduction applications.

This manuscript reports on a series of evaluations of the effect of tin oxidation states, as well as the particle size of SnOx deposited on carbon fiber on CO2RR activity and selectivity. Additionally, the electrocatalyst’s stability was also assessed by means of physicochemical characterization of the electrodes before and after the CO2RR experiments. Finally, the influence of the reduction potential was evaluated in terms of CO2RR performance and the stability of SnOx-based materials.

Results and discussion

XRD patterns of the SnOx-based electrodes (Fig. 1) were obtained to confirm the presence of SnOx material and crystalline phase on carbon paper electrode, for comparison purposes the XRD patterns of the pristine materials were also obtained (Figure S1). All of the electrodes exhibited an identical crystalline phase before and after deposition onto carbon fiber, indicating that the method proposed was effective for impregnating the SnOx materials. All of the electrodes exhibited a broad peak at about 26° that could be related to the carbon fiber [13]. SnO (PDF2: 01-172-1012) exhibited small peaks due to the small amount in the carbon fiber (18 mg); however, its characteristic peaks related to the plane (101) could be observed. Microsized SnO2 exhibited all peaks in a well-defined way; on the other hand, as expected, nanometric SnO2 exhibited small and broad peaks due to its small particle size [33]. In addition, the main peak from SnO2, i.e., the (101) plane was overlapped by a carbon fiber peak of about 26º. The material’s crystallite size was estimated from the XRD pattern using Scherrer equation (Table SI). It can be seen that SnO and SnO2 microsized exhibited a crystallite size value of ca. 43 and 45 nm, respectively. On the other hand, the SnO2 nanosized exhibited a crystallite size of ca. 5 nm. The results are in agreement with the material’s morphology. Therefore, the XRD results confirmed that the electrode fabrication method was efficient to obtain the SnOx electrodes without crystalline phase modification.

Figure 1
figure 1

XRD patterns of SnOx (microsized and nanosized) deposited onto carbon fiber with all reflection peaks indexed.

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SEM images of the electrodes revealed that SnOx particles were successfully deposited on the carbon fiber, as can be seen in Figure S2. The formation of agglomerated spherical particles attached to the carbon fibers can also be observed, such a feature becomes deleterious in catalytical applications [34, 35]. However, it can be seen that the carbon fiber was not entirely covered by the SnOx particles, and this feature could aid in CO2 reduction processes using oxide-based materials. It was demonstrated that a complete recovery of carbon fiber or electrode by a semiconductor can be deleterious for electrocatalytic performance, as it increases the electrode resistance, thus hampering the charge transfer processes [13]. Nanosized SnO2 electrode seems more homogeneous than the microsized SnO2 and SnO, it is likely related to the lower particle size and higher specific surface area that will be easier spread on the carbon fiber. Additionally, the specific surface area of the pristine materials was determined applying the BET model to N2 physisorption data. SnO2 nanosized material exhibited a SSA value at least 30 times higher than the SnOx microsized materials, as can be seen in the Table SI. This result is in agreement with the materials morphologies.

Cyclic voltammetry was performed on the SnOx-based electrodes to identify the reduction potential of the CO2 reduction reaction, the hydrogen evolution reaction, and for SnOx redox (Fig. 2). All of the electrodes exhibited two coinciding anodic peak in the potential region between 0.1 and − 0.1 V, and this was associated with the oxidation of Sn metallic to Sn2+ and Sn+2 to Sn+4, even in SnO2 electrodes, which is likely due to in situ reduction of Sn4+ to Sn2+ and Sn metallic during cathodic sweeps. The enlarged cyclic voltammetry of microsized SnO2 exhibited two anodic peaks at − 0.13 V and + 0.01 V (Figure S3a), while the microsized SnO exhibited two peaks at − 0.06 V and + 0.05 V (Figure S3b). These peaks can be attributed to two different oxidation steps, the first one could be related to the oxidation of Sn to Sn2+, while the second one could be related to the oxidation of Sn2+ to Sn4+ [21, 22, 36, 37].

Figure 2
figure 2

(a) Cyclic voltammetry in the electrolytic solution (0.5 M KHCO3) performed with SnOx-coated carbon fiber electrodes.

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The micrometric SnO2 exhibited the highest current density at − 1.0 V, followed by the SnO and nanometric SnO2 electrode, respectively. It was unexpected, because the material with a lower particle size and higher specific surface area should exhibit the higher electrochemical active surface area and, consequently, a higher current density. The reasons of the low activity of nanosized SnO2 was investigated by X-ray photoelectron spectroscopy (Figure S4). The survey spectrum confirms the presence of Sn, O, C, and Cl. Note that, the presence of the elements Sn and O is in accordance with the composition and chemical state of the SnO2 nanosized. The peaks related to carbon are from adventitious contamination commonly used as a charge reference for XPS spectra. The peak at 199.0 eV can be attributed to Cl 2p, which is an impurity derived from the precursor salt (SnCl2.2H2O) employed to synthesize the SnO2. Additionally, the FTIR spectrum (Figure S5) demonstrated the presence of some impurities related to carbonic groups (-CH). Therefore, the low performance of nanosized SnO2 can in principle be attributed to the presence of these impurities. On the other hand, the sample with the highest cathodic current does not necessarily achieve greater efficiency in CO2 reduction, as the CO2RR competes with the H2 evolution reaction and electrode redox reactions.

To confirm the CO2RR performance of the SnOx-based electrodes, electrolysis experiments were carried out under galvanostatic conditions (Fig. 3), applying − 50 mA (or − 6.25 mA/cm2) for 30 min. The microsized SnO2 electrocatalyst required the lower potential to maintain a constant current (of ca. − 0.7 V, followed by SnO (of ca. − 1.0 V) and nanosized SnO2 (of ca. − 1.4 V) in the electrocatalysts. Therefore, nanometric SnO2 requires much more power consumption to keep the current constant, which is likely due to the presence of impurities on its surface as demonstrated by FTIR and XPS analyses (Figure S4 and S5).

Figure 3
figure 3

Electrolysis under galvanostatic conditions of − 50 mA during 30 min of SnOx-based electrocatalysts.

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The HPLC results showed that formic acid was the only product in the liquid phase (Table 1). The SnO electrode exhibited the highest performance in the HCOO− formation, followed by nanosized SnO2 and microsized SnO2. The uncoated fibers did not exhibit any catalytic activity in this reaction, indicating that SnOx was the catalytically active site for CO2RR. The different performances achieved by the SnOx electrocatalysts indicate that the oxidation state of Sn, as well as the particle size, were both responsible for achieving significant CO2RR performance profiles. The results suggest that the lower performance of SnO2 electrodes compared to those of SnO can be related to the Sn4+ to Sn2+ competition reaction, as observed by CV analysis (Fig. 2). Nevertheless, the nanosized SnO2 requires a reduction potential of approximately − 1.4 V in order to keep the current constant, whereas the microsized SnO2 requires a reduction potential of about − 0.8 V. Therefore, the faradaic efficiency for nanosized SnO2 could be higher due to the increase in the reduction potential, and not only because of the particle size, since it is well known that reduction potential influences directly on CO2 reduction activity and selectivity. The Latimer diagram of Sn species around pH 7 showed that Sn4+ oxide should be reduced into Sn2+ oxide in a reduction potential around − 0.17 V vs RHE, while the Sn2+ oxide should be reduced into Sn metallic in a reduction potential around − 0.47 V vs RHE [27]. However, Dutta et al. [27] described the SnOx’s species stability under different electrochemical conditions, which was based on Pourbaix Diagram of Sn species and in operando Raman spectroscopy. They showed that the conversion of SnO2 to metallic Sn requires more negative potentials than what could be predicted based on thermodynamic data, because the reduction of the SnO2 is kinetically hindered. Therefore, both microsized SnO2 and SnO are stable under galvanostatic conditions studied; however, the nanosized SnO2 required a very high potential around − 1.4 V vs RHE to keep the − 50 mA. This catalyst should be partially converted into Sn2+ oxide and Sn metallic, it explains why the nanosized SnO2 exhibited a higher faradaic efficiency than the microsized material.

TABLE 1 HCOO− concentration outputs and the FE (η) of the CO2 reduction performed with SnOx-based electrocatalysts under galvanostatic conditions (− 50 mA during 30 min).
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To further understand the effects of particle size and reduction potential, we evaluated CO2RR performance in potentiostatic conditions, i.e., by applying − 0.8 V over 3 h for nanosized and microsized SnO2. It was observed that the microsized SnO2 produced 789 ppm of formic acid, with a faradaic efficiency of 56%, whereas the nanosized SnO2 achieved 21 ppm and an FE of 22%. Furthermore, microsized SnO2 exhibited superior activity and selectivity for formic acid production. Again, this could reflect the impurities on the surface of nanosized SnO2 electrocatalysts. This finding confirms the last experiment observation (galvanostatic), the reduction potential difference was the key to boosting the performance of the nanosized SnO2.

In this step, the effect of the oxidation state of tin on the CO2RR performance was evaluated. To attain this objective, we carried out a pre-reduction step in the SnO2 electrodes by applying − 50 mA for 30 min under Ar purge in the same electrochemical cell. Then, the CO2RR performance was evaluated under potentiostatic conditions, applying − 1.0 V for 2.5 h (Fig. 4). The obtained results were then compared with the performance of SnO2 electrodes without any pre-reduction treatment (Table 2). It can be observed that the performance of SnO2 electrocatalysts was enhanced following the pre-reduction step, and therefore this finding confirmed that the reduction of Sn4+ can compete with the CO2 reduction and decrease the FE of formic acid formation. Additionally, we can confirm that Sn4+ is not the active site of the CO2 reduction reaction. This corresponds to previously published data, for example, Lee et al. [38] showed that, under neutral conditions, metallic Sn is most likely the active site of CO2RR. On the other hand, some authors have found that the Sn electrode possesses an oxide layer and shows excellent catalytic activity for CO2 reduction, and that the Sn electrode removed its oxide layer, resulting in poor catalytic activity for CO2 reduction, but with the hydrogen evolution reaction accelerating [23]. The results reported by Baruch et al. suggest that the active species for catalysis is an Sn2+ species rather than an Sn4+ one, as the first species can react with CO2 to form the surface-bound carbonate, which is the intermediate products in the reduction of CO2 to formate (Scheme S1). The first step of the reduction of CO2 is preceded by a two-electron reduction of the electrode from a native SnO2 to a Sn2+ oxyhydroxide, as described. Then CO2 can react with this specie and forms the surface-bound carbonate, in the third step two electrons and a proton are transferred to the tin carbonate to form formate. Finally, the formate is quickly desorbed to return the surface to the Sn2+ oxyhydroxide surface [22]. It was demonstrated that CO2 reduction overpotential is decreased by lowering the free energy of formation of carbonate [39]. These finding were confirmed by electrochemical experiments and DFT calculations [40]. Figure 4 shows a microsized SnO2 electrocatalyst with a higher current density, production, and FE for formic acid formation than nanometric SnO2.

Figure 4
figure 4

CO2 reduction in 0.5 mol L−1 of KHCO3 aqueous solution under potentiostatic conditions (− 1.0 V during 2.5 h) with microsized and nanosized SnO2 electrodes following the pre-reduction step.

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TABLE 2 HCOO− concentration outputs and FE (η) of CO2 reduction performed with SnO2 electrocatalysts with and without the pre-reduction (PR) step in potentiostatic conditions (− 1.0 V during 3 h).
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The CO2RR performance was evaluated at four different reduction potentials (− 0.2, − 0.6, − 0.8 and − 1.0 V), the polarization curves were obtained by average the current over a time period of 9000 s to each point, as is displayed in Fig. 5. It can be observed that when the reduction potential increased, the FE for formic acid formation was also enhanced. The increase in the reduction potential not only enhanced the current density/amount of formic acid formed but also increased the selectivity of the microsized SnO2 electrocatalyst. This behavior is due to the H2 evolution reaction, which exhibited a low reduction potential compared to that of the CO2RR to formic acid [25].

Figure 5
figure 5

CO2 reduction in 0.5 mol L−1 of KHCO3 aqueous solution under potentiostatic conditions (− 0.6, − 0.8 and − 1.0 V for 2.5 h) with a micrometric SnO2 electrodes after the pre-reduction step.

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The stability of the SnOx-based electrodes was evaluated by means of two cycles of reuse under galvanostatic conditions, applying − 50 mA during 30 min (Table 1). It was verified for all SnOx-based electrodes that, after the first cycle, the CO2RR performance was increased. It can be observed that the increase in performance of the SnO2 samples after the first cycle was more pronounced than for the SnO sample, likely due to the competition of the Sn4+/Sn2+ reduction with the CO2RR in the first cycle, this reaction may lead to a loss of the selectivity for formate production. This result corresponds to those observed in the pre-reduction experiment, i.e., after partial reduction of the SnO2 electrodes, the catalytic activity was increased. To confirm the presence of the SnO2 sample in the carbon fiber surface, even after two cycles of the CO2RR experiment, an XRD and SEM analysis were performed on the micrometric SnO2 (Fig. 6). The former showed that the crystalline SnO2 phase did not change during the CO2RR experiment; however, it was clear that the SnO2 peaks decreased after the CO2RR experiment. This peak decrease can be related to the Sn4+ reduction and/or its leaching into the solution. The SEM images showed that the morphology of microsized SnO2 did not change during the CO2RR; however, it could be verified that the SnO2 particles became less agglomerated and more uniformly dispersed on the carbon fibers, which could relate to the surface reconstruction during CO2RR operation, as observed in other studies [41,42,43]. Therefore, the enhanced CO2RR performance after the first cycle could also be related to the better distribution of the SnO2 particles across the carbon fiber surface.

Figure 6
figure 6

SEM images of the micrometric SnO2 electrode, (a) before and (b) after CO2RR. (c) XRD pattern of the micrometric SnO2 electrode before and after CO2RR.

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Conclusions

A simple, easy, and scalable method for impregnating SnOx in carbon fibers was proposed in this study. All SnOx electrodes were capable of converting CO2 into formic acid, with a maximum FE of almost 70% for the SnO2 catalyst. On the other hand, carbon fiber without tin oxide was not effective for the CO2 reduction reaction. It was demonstrated that both the particle size and oxidation state played an important role in the CO2RR performance. The SnO electrocatalyst exhibited high activity for CO2RR compared to SnO2 electrocatalysts, likely due to the requirement of reducing Sn4+. Nanosized SnO2 exhibited a low CO2 reduction performance due to the impurities on this material surface, as demonstrated by FTIR and XPS analysis. The pre-reduction step for both SnO2 electrodes increased their CO2RR performance, indicating that Sn2+ is the active site under potentiostatic conditions, and the microsized SnO2 electrode showed greater efficiency in the generation of formic acid. The CO2 reduction mechanism on the SnOx surface was proposed based on our results and in the literature. The increase in the reduction potential increased the CO2RR conversion of formic acid selectivity of the micrometric SnO2 electrocatalyst. The results showed that the oxide remained on the surface, and therefore it constitutes a stable surface for reactions.

Methodology

Preparation and characterization of SnOx-based electrodes

A general procedure was used to deposit the SnOx catalyst ink on the carbon fibers. To obtain the catalyst ink, 18 mg of SnOx with Nafion (5% w/w) was dispersed in 2 mL of isopropanol, using sonication. The catalyst/Nafion ratio was 90:10 (wt:wt), respectively. The mixture was dried in a fume hood and then deposited to the carbon fiber electrode (area of 8 cm2) by means of brushing. The electrodes were then cured at 80 °C for 1 h. Three different SnOx-based materials were evaluated: (i) microsized SnO2 (99.9%, Sigma-Aldrich); (ii) microsized SnO (99.99%, Sigma-Aldrich); and (iii) nanosized SnO2 that was prepared by means of hydrothermal treatment, as described elsewhere [33]. In detail, 0.564 g of SnCl2.2H2O (Sigma-Aldrich) was dissolved in 100 mL of ethanol anhydrous (99.5% purity, Dinâmica) and then under vigorous stirring, 22.5 mL of distilled water was added dropwise at room temperature for 12 h. After that, the suspension was cleaned by dialysis in 5 L of distilled water, often renewed, until removing all chloride ions, which was confirmed by tests with AgNO3 solution (0.1 mol L−1). Then, the solution inside the dialysis bag was dried at 70 °C for 12 h, ground with an agate mortar. After that, the as-prepared sample was treated hydrothermally up to 200 °C for 4 h. The concentrations of the SnOx investigated (2.2 mg cm−2) were chosen on the basis of previous reports [26, 44, 45]. The samples were characterized by X-ray diffraction (XRD) at 2θ = 10° to 70°, using a monochromatized X-ray beam from nickel-filtered Cu kα radiation (λ = 0.15406 nm, 30 kV, 30 mA—Rigaku Multiflex-Ultima IV). The morphological properties of the samples were characterized using a scanning electron microscope (SEM, JEOL JSM 670F) operating at 5 kV (secondary electron detector) at different magnifications. A Fourier Transform Infrared spectrometer (FTIR) (Bruker VERTEX 70) was used to investigate surface changes using nanosized SnO2-containing KBr disks with 64 scans and 4 cm−1 resolution in the 4000–400 cm−1. X-ray photoelectron spectroscopy (XPS) analyses were performed on a Scienta Omicron, model ESCA+ spectrometer using monochromatic AlKα (1486.6 eV) radiation. Peak decomposition was performed using a Gaussian–Lorentzian line shape with a Shirley nonlinear sigmoid-type baseline. The binding energies were corrected for charging effects by assigning a value of 284.8 eV to the adventitious C 1s line. The data were analyzed using CasaXPS software (Casa Software Ltd., U.K.). N2 adsorption–desorption isotherms were recorded on a Micromeritics ASAP 2020 analyzer at 77 K. Samples were previously degassed at 80 °C under vacuum until a degassing pressure, 10 μmHg. The Brunauer–Emmett–Teller (BET) method was used to calculate the SSA (SBET).

Evaluation of CO2 reduction performance

The CO2 reduction reaction experiments were studied in a two-chamber electrochemical cell, separated by a proton exchange membrane (Nafion 117) in an aqueous KHCO3 (0.5 mol L−1) solution. The electrolyte was saturated with CO2 (pH 6.9) for 30 min prior to each measurement. The SnOx-coated carbon fiber electrode was used as the working electrode, and a Pt mesh and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. All of the potentials presented herein were converted to the RHE reference scale using the following equation: E (vs RHE) = E (vs Ag/AgCl) + 0.210 V + 0.0591*pH. The electrochemical measurements were carried out using a Potentiostat/Galvanostat (Model 273, Princeton Applied Research).

These cyclic voltammetric measurements were performed from 1.1 to − 1.0 V. The measurements were conducted in order to verify the regions where the reactions of interest occurred. The performance of the SnOx-based catalysts was evaluated under galvanostatic conditions, applying a current of − 0.05 A for 30 min. The stability of the catalysts was evaluated by two reuse cycles of galvanostatic experiments. The species produced by the reduction of CO2 in the liquid phase were analyzed by high-performance liquid chromatography (HPLC) in order to calculate the FE (%) of the process. The sample was collected in the cathode compartment of the electrochemical cell following the CO2 reduction reaction and injected into the chromatograph (HPLC-LC-20AD, Shimadzu) with an Aminex HPX-87H column (300 × 7.8 mm) capable of analyzing carboxylic acids and alcohols. A diluted solution of H2SO4 (3.3 mmol L−1) was used as the mobile phase with a flow rate of 0.6 mL min−1. A sample volume of 20 μL was injected into the column loop. The column and detectors were kept at 40 °C. The chromatograph was equipped with a differential refractive index detector (RID-10A) that is suitable for the detection of alcohols, and a UV–Vis detector (SPD-20A, deuterium lamp, λ = 210 nm), which is suitable for the analysis of carboxylic acids. Data obtained by HPLC were used to calculate the Faradaic efficiency (FE) of the CO2 electrochemical reduction in relation to formation of formate (HCOO−). The Faradaic efficiency (FE) of the CO2 electrochemical reduction of products was calculated as

$$FE= \frac{n\times F\times {n}_{HCOO-}}{Q}\times 100\%$$

where n represents the number of electrons transferred from CO2 for the production of one molecule of formate, F is the Faraday constant, nHCOO- is the number of moles of the formate, and Q is the total charge. In this case, n = 2; F is Faraday’s constant (96 485 C mol−1 of electrons); nHCOO- is calculated by HPLC measurements.

To evaluate the effect of Sn oxidation state, it was performed a pre-reduction step in the same electrochemical cell in both SnO2 nanosized and microsized, i.e., a current of − 0.05 A was applied for 30 min under an Ar purge in order to reduce the species of Sn4+; then, an experiment was performed under potentiostatic conditions, with − 0.8 V applied for 2.5 h. The performance of both electrodes was compared with and without this pre-treatment, to investigate the Sn site active. Additionally, the effect of the reduction potential (− 0.2 to − 1.0 V) on the FE of the microsized SnO2 was also analyzed after the same pre-reduction step.