Regulating the hydrophobic microenvironment of SnS₂ to facilitate the interfacial CO₂/H₂O ratio towards pH-universal electrocatalytic CO₂ reduction

Abstract

Electrocatalytic CO₂ reduction to formic acid is a promising strategy to obtain value-added chemicals and achieve the carbon cycle. However, its practical application is generally impeded by the limited accessibility of CO₂ to the catalyst's surface and the lack of an efficient universal catalyst across different pH levels. Herein, we report a new catalyst, i.e., SnS₂ decorated with a hydrophobic polymer polyvinylidene fluoride (SnS₂ + PVDF), for the effective electrocatalytic CO₂ reduction to formate/formic acid across a wide pH range in a flow cell. This catalytic system accomplishes a remarkable faradaic efficiency for the formic acid production in alkaline (98%), neutral (86%), and acidic (93%) electrolytes. Also, the single-pass carbon efficiency reaches up to 72.77% in acidic electrolytes. Water contact angle measurements in association with in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy results indicate that the inclusion of PVDF creates a hydrophobic microenvironment which increases the CO₂/H₂O ratio near the surface of SnS₂ particles. As a consequence, SnS₂ particles enjoy the enhanced CO₂ concentration around their surface to form many three-phase (solid–liquid–gas) boundaries. In situ Raman spectra combined with electrocatalytic studies reveal that SnS₂ undergoes reconstitution to form catalytically active Sn/SnS₂ during the reaction. These findings ensure and expand the generality of a hydrophobic microenvironment regulation strategy in promoting electrocatalytic CO₂ reduction to formic acid.

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Green foundation

1. The electrocatalytic conversion of CO₂ into formic acid presents an appealing means for generating valuable chemicals using renewable energy while contributing to the carbon cycle, which is of vital significance for alleviating energy and environmental challenges.

2. This study reports that the SnS₂ + PVDF 50 electrode elevates the local CO₂/H₂O ratio on the catalyst surface and constructs more gas–electrolyte–liquid triple-phase boundaries. The catalyst exhibits excellent performance across a wide range of pH for electrocatalytic CO₂ reduction to formate/formic acid. This enhances the reaction's practicability in diverse environmental settings, further contributing to the global effort to reduce carbon emissions.

3. The in-depth investigation of the optimal CO₂/H₂O ratio in different catalytic systems and the specific catalyst-reconstitution mechanism during electrochemical CO₂ reduction need further exploration in the future.

Introduction

The continuous increase of atmospheric CO₂ concentration due to fossil fuel combustion, deforestation, and industrial activities has led to severe environmental impact.^1–3 The electrocatalytic CO₂ reduction reaction (ECO₂RR) using renewable energy offers a potential pathway to mitigate CO₂ concentration by converting CO₂ into value-added fuels and chemicals.^4–6 Among the produced value-added fuels, formic acid has drawn considerable attention due to its wide applications as an organic chemical intermediate and a hydrogen storage carrier.⁷ Notably, Sn-based catalysts have emerged as promising candidates for selective formic acid production due to their optimal adsorption strength for the key *OCOH intermediate. Besides, Sn is a low-toxic and naturally abundant metal. These inherent characteristics in addition to their unique electronic configuration underline Sn-based materials as promising candidates for the ECO₂RR.^8–10 However, the phenomenon of ‘water flooding’ results in insufficient interfacial CO₂ on the catalyst surface in the flow cell during the ECO₂RR, which in turn favors the hydrogen evolution reaction (HER).^11–14 Hence, developing a strategy to optimize the CO₂/H₂O ratio on the catalyst surface is a prerequisite for the successful ECO₂RR.

Recently, regulating the hydrophobic microenvironment has been proven to be effective in enriching CO₂ concentration on the catalyst surface during the ECO₂RR.^15–17 This strategy significantly enhances the selectivity of C₁ and C₂₊ products by controlling the local CO₂/H₂O ratio and suppressing the competing HER. For instance, Zheng et al. modified the Cu electrode using quaternary ammonium functionalized polynorbornene ionomers and confirmed that the strong hydrophobicity extended the residence of the *CO intermediate on Cu surfaces, thus improving the rate of C–C coupling and boosting the selectivity of C₂₊ products. Similarly, a thin polymer coating has been used to modulate the ratio of CO₂/H₂O on the Cu surface, which could improve the selectivity of C₂₊ products and offer considerable energy efficiency at nearly industrialized current densities.¹⁸ Qiu et al. constructed a special CO₂-philic and hydrophobic interface by employing a defective carbon to modify a Bi nanoparticle catalyst.¹⁹ This catalyst with a stable spatial structure accelerated the mass transfer of CO₂ and inhibited the competitive HER, significantly improving the selectivity of formate. Wang et al. further demonstrated the effectiveness of this strategy by decorating an In₂O₃ electrode with fluoropolymers, which increased the local CO₂ concentration near the catalyst surface and boosted the production of formate.¹⁷ Few reports are available in the literature on regulating the hydrophobic microenvironment of Cu, Bi, and In-based catalysts. However, such reports on Sn-based catalysts are very rare and are not reported so far to the best of our knowledge. In this context, utilizing a hydrophobic polymer to enhance the ECO₂RR performance and uncover the underlying mechanisms seems crucial for preparing high-performance Sn-based ECO₂RR catalysts.

The ECO₂RR in alkaline and neutral electrolytes has been proven to be an effective method to alleviate the competing HER. However, it inevitably leads to the formation of carbonates or bicarbonates that diminish the electrolyte's conductivity and the efficiency of CO₂ utilization.^20–22 Employing an acidic electrolyte seems an excellent idea to solve this issue but the acidic electrolytes promote the competitive HER and thus hinder the effective CO₂ conversion.^23–25 Therefore, the pH-universal ECO₂RR has gained interest in picking an appropriate reaction environment according to practical demand. Also, the ability to operate across a wide pH range enhances the technology's applicability in diverse environmental settings.^26,27 So, developing an efficient ECO₂RR catalyst to form formic acid across a full pH range is of greater importance.

Based on the above facts, in this work, we have decorated SnS₂ nanoflowers with a hydrophobic polymer (polyvinylidene fluoride, PVDF) using a facile method. The obtained electrode SnS₂ + PVDF 50 displays an excellent performance: 98% formate faradaic efficiency (FE) at −0.8 V vs. reversible hydrogen electrode (RHE) (all the potentials are referred to RHE in this work) in 1 M KOH, 93% HCOOH FE at −1.9 V in 0.05 M H₂SO₄ + 0.5 M K₂SO₄ and 86% formate FE at −1.2 V in 0.5 M KHCO₃. It is important to note that the SnS₂ + PVDF 50 electrode exhibits robust stability (>30 h) in the alkaline electrolyte under a current density of 150 mA cm⁻². In situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) reveals that the introduction of PVDF increases the local ratio of CO₂/H₂O near the surface of SnS₂ and thus enhances the accessibility of CO₂ to the catalyst while suppressing the HER. Furthermore, in situ Raman studies along with electrocatalytic studies indicate the reconstruction of SnS₂ to the Sn/SnS₂ composite (the real catalytically active phase).

Results and discussion

Characterization of SnS₂

SnS₂ nanoflowers were synthesized using a hydrothermal method.²⁸ The powder X-ray diffraction (XRD) pattern (Fig. 1a) supports the successful preparation of SnS₂ in line with JCPDS No. 83–1705. As shown in the scanning electron microscopy (SEM) image (Fig. 1b), the prepared SnS₂ particles exhibit a nanoflower morphology. X-ray photoelectron spectroscopy (XPS) and Raman spectra of SnS₂ on carbon paper (CP) further confirm the formation of SnS₂ (Fig. S1 of the ESI†). SnS₂ was modified using a hydrophobic polymer (PVDF) and a hydrophilic polymer (polyvinyl pyrrolidone, PVP).²⁹ As indicated by the XRD pattern (Fig. 1a), the crystal structure of the SnS₂ + PVP electrode remains unchanged as compared to that of the pure SnS₂ electrode. For SnS₂ + PVDF 50, an additional diffraction peak appears at 20.2° due to the (110) plane of PVDF.³⁰ PVDF particles cover the surface of the SnS₂ layer in the SnS₂ + PVDF electrode (Fig. 1c). This type of coverage is not observed in the SnS₂ + PVP electrode (Fig. 1d) except in a block-shaped morphology. Furthermore, the introduction of PVDF and PVP is confirmed by XPS. In the XPS spectrum of the SnS₂ + PVDF electrode, the peak at 688.97 eV for the F 1s high-resolution spectrum and the peaks at 290.88 and 286.09 eV for high-resolution C 1s spectrum are characteristic peaks of fluorinated hydrocarbons (Fig. S2†).³¹ In the XPS spectrum of the SnS₂ + PVP electrode, the strong N 1s peak at 400.11 eV is ascribed to PVP (Fig. S3†).³² These results indicate the incorporation of PVDF and PVP into the SnS₂ electrode. To investigate the impact of polymer modification on the hydrophobicity of SnS₂, the water contact angle measurement was conducted. As illustrated in (Fig. 1b, c, and d), water contact angles for SnS₂ + PVDF 50, SnS₂, and SnS₂ + PVP were 123.2°, 102.8°, and 61.7°, respectively. This underlines that PVDF enhances the hydrophobicity of the SnS₂ electrode, while PVP enhances its hydrophilicity.

Fig. 1 Physical characterization of the as-synthesized catalysts. (a) XRD patterns. The SEM images and water contact angles of (b) SnS₂ + PVDF 50, (c) SnS₂, and (d) SnS₂ + PVP.

ECO₂RR performance

To elucidate the impact of various microenvironments on the electrocatalytic performance of SnS₂, we initially conducted ECO₂RR experiments in the flow cell (more details in Fig. S4†) in 1.0 M KOH.^33–35 Evaluation of the FE at varying potentials reveals that formate/formic acid is the sole liquid-phase product (Fig. S5†). To assess the activity of three distinct electrodes during the ECO₂RR (SnS₂, SnS₂ + PVDF 50, and SnS₂ + PVP), linear sweep voltammetry (LSV) curves were initially recorded. As shown in Fig. S6,† the SnS₂ + PVDF 50 electrode exhibits a significantly enhanced current density along with a more positive onset potential compared to both pure SnS₂ and SnS₂ + PVP electrodes. This emphasizes its superior activity for the ECO₂RR. The standard curve for calculating formate FE is shown in Fig. S7.† According to Fig. 2a, the SnS₂ + PVDF 50 electrode demonstrates a high FE for formate across an extensive potential range from −0.8 V to −1.1 V (>90%).

Fig. 2 ECO₂RR performance in a flow cell. The formate FE and formate partial current density of the SnS₂ + PVDF 50 electrode in (a) 1 M KOH, (c) 0.05 M H₂SO₄ + 0.5 M K₂SO₄, and (d) 0.5 M KHCO₃. (b) The ECO₂RR stability of the SnS₂ + PVDF 50 electrode under −150 mA cm⁻² in 1 M KOH. (e) SPCE and formate FE comparison of the SnS₂ + PVDF 50 electrode under different CO₂ flow rates.

Compared to both SnS₂ and SnS₂ + PVP, the SnS₂ + PVDF 50 electrode displays the highest formate FE and the highest formate production rate in 1 M KOH (Fig. S8†). Conversely, the SnS₂ + PVP electrode exhibits the highest H₂ FE. To find out the impact of the SnS₂/PVDF ratio on the ECO₂RR performance, a few SnS₂ + PVDF electrodes varying in the quantity of PVDF were prepared. As illustrated in Fig. S9,† the SnS₂ + PVDF 50 electrode exhibits optimal activity and selectivity. Meanwhile, the single-pass carbon efficiency (SPCE) was also determined to evaluate the practical viability of the catalyst. As shown in Fig. S10,† when the CO₂ flow rates decrease from 15 to 5 sccm, SPCE reaches its peak value of 31.63% at −1.2 V. Next, the long-term stability of the SnS₂ + PVDF 50 electrode in 1 M KOH was examined. As depicted in Fig. 2b, the SnS₂ + PVDF 50 electrode is significantly durable over time. On average, formate FE remains above 80% for more than thirty hours at 150 mA cm⁻² in 1 M KOH, ultimately yielding a 2.67 mol L⁻¹ formate solution.

The performance of the ECO₂RR was also evaluated in a neutral electrolyte 0.5 M KHCO₃. According to the LSV curves, the SnS₂ + PVDF 50 electrode exhibits superior performance compared to both the SnS₂ and SnS₂ + PVP electrodes (Fig. S11†). Meanwhile, the SnS₂ + PVDF 50 electrode demonstrates a high formate FE over a broad potential range from −0.9 V to −1.4 V, exceeding 80% (Fig. 2c). Compared to the SnS₂ and SnS₂ + PVP electrodes in the 0.5 M KHCO₃ electrolyte, the SnS₂ + PVDF 50 electrode reaches the highest formate FE (Fig. S12a†). Furthermore, the SnS₂ + PVDF 50 electrode exhibits the highest rate of formate production at 1.8 mmol mg⁻¹ h⁻¹ along with an SPCE of 4.88% (Fig. S12c and d†).

To evaluate the pH-universal feasibility of the ECO₂RR, its performance was assessed in an acidic electrolyte (0.05 M H₂SO₄ + 0.5 M K₂SO₄). The SnS₂ + PVDF 50 electrode reveals a markedly improved current density and a more positive onset potential (Fig. S13†), along with a high FE for HCOOH across an extensive potential range from −0.8 V to −1.1 V (Fig. 2d). Meanwhile, the SnS₂ + PVDF 50 electrode achieves the highest HCOOH FE compared with SnS₂ and SnS₂ + PVP electrodes, as depicted in Fig. S14.† Also, the SnS₂ + PVDF 50 electrode attains an exceptional HCOOH production rate of 4.82 mmol mg⁻¹ h⁻¹. As seen in Fig. 2e, SPCE reaches an optimal value of 72.77% at −1.9 V at a CO₂ flow rate of 2 sccm, which is better than some advanced catalysts (Table S1†). The SnS₂ + PVDF 50 electrode displays a superior performance invariably in alkaline, acidic, and neutral electrolytes compared with some advanced catalysts (Fig. S15, Tables S2 and 3†). Intriguingly, the SnS₂ + PVDF 50 electrode shows a better performance than a few advanced SnS₂-based catalysts (Table S4†).

Mechanism of performance enhancement

It is clear that the SnS₂ + PVDF 50 electrode significantly suppresses the HER while enhancing both the activity and selectivity of the CO₂RR. In contrast, the SnS₂ + PVP electrode severely inhibits both activity and selectivity in the ECO₂RR. To further elucidate the mechanism behind the impact of polymer modification on the catalyst performance of SnS₂ in the ECO₂RR, the electrochemically active surface area (ECSA) of the electrolytes was calculated. The ECSA is proportional to the electrochemical double-layer capacitance (C_dl) measured using CV curves (Fig. S16†).³⁶ The ECSA refers to the portion of an electrode that is wetted and accessible to the electrolyte.¹⁵ It is hypothesized that the increased gas volume within the catalyst layer reduces its ECSA due to a decreased contact area with the electrolyte. As shown in Fig. 3a, the SnS₂ + PVDF 50 electrode exhibits an ECSA value of 0.86 mF cm⁻², which is lower than that of the unmodified electrode (0.96 mF cm⁻²). The largest ECSA was recorded for the SnS₂ + PVP electrodes at 2.98 mF cm⁻² while comparing with both SnS₂ and SnS₂ + PVDF 50 electrodes. This indicates that the increased likelihood of gas bubble formation around the SnS₂ + PVDF 50 electrode creates a more favorable environment for the production of formate during the CO₂RR. In contrast, the largest ECSA of the SnS₂ + PVP electrode contributes to the enhanced presence of electrolytes surrounding the SnS₂ + PVP electrode, which is more conducive to H₂ production.¹⁵ Then, the electrochemical impedance spectroscopy (EIS) results for all three electrodes under the ECO₂RR conditions are shown in Fig. 3b. According to the EIS results, the SnS₂ + PVDF 50 electrode exhibits the lowest charge transfer resistance (R_ct) while the SnS₂ + PVP electrode displays the highest R_ct. To quantitatively elucidate the dynamic evolution of Sn species in SnS₂ + PVDF 50 during the CO₂RR, CO₂ electrolysis was conducted at −1.2 V in a saturated 0.5 mol L⁻¹ KHCO₃ solution. The results in Fig. S17a† reveal that SnS₂ gradually reduced to Sn which became the dominant phase after 30 min. A small amount of SnS₂ remained was observed even after 4 h of electrolysis. The corresponding results of electrolytic experiments carried out for different reaction time indicate the gradual increase of FE_formate with the increase of electrolysis time (Fig. S17b†), suggesting that metallic Sn is the primary active phase. In the XPS spectra (Fig. 3c and d), the two characteristic peaks at 494.94 and 486.52 eV of the original Sn 3d spectrum are attributed to the predominate existence of the Sn⁴⁺ state.³⁷ After 4 h of electrolysis, the two peaks at 493.65 and 485.26 eV are attributed to the metallic Sn⁰ species.³⁸ The above results suggest that the co-existence of a small amount of SnS₂ and dominated metallic Sn is crucial for the ECO₂RR as they (Sn/SnS₂) seem to be catalytically active materials.

Fig. 3 (a) Linear relationship between the current density and scan rate for the SnS₂ + PVP, SnS₂, and SnS₂ + PVDF 50 electrodes for the estimation of ECSA. (b) EIS plots. The in situ reduction results of SnS₂ + PVDF 50 (c) Sn 3d XPS spectrum without electrolysis and (d) Sn 3d XPS spectrum after 4 h of electrolysis.

The evolution of SnS₂-based catalysts during the ECO₂RR was investigated using in situ Raman spectroscopy. As depicted in Fig. 4a, the band observed at 315 cm⁻¹ corresponds to the A_1g mode of SnS₂.³⁹ For both SnS₂ + PVDF and SnS₂ electrodes, intensities of A_1g mode grow weak with the decrease of the applied potential. However, with PVDF introduction, this weakening occurs at a slower rate, which may be due to its hydrophobic nature. In the ATR-SEIRAS spectra (Fig. 4c and d), a broad peak observed between 3000 and 3700 cm⁻¹ is attributed to the O–H stretching vibrations (ν_O–H),⁴⁰ while the peak at 1650 cm⁻¹ corresponds to the O–H bending vibrations (δ_O–H).⁴¹ Additionally, the peak at 1520 cm⁻¹ is associated with carbonate ions (CO₃²⁻), and the peak at 1370 cm⁻¹ is linked to HCOO*.⁴² Two distinct peaks located at 2365 and 2340 cm⁻¹ are attributed to gaseous CO₂.⁴³ As illustrated by the ATR-SEIRAS spectra for SnS₂ + PVDF 50, an increase of the ν_O–H peak intensity coupled with a blue shift is noted at more negative applied potentials. This indicates a significant decrease in isolated water molecules. In contrast, the ν_O–H of the SnS₂ spectra shows only a modest increase along with a red shift (Fig. S18a†). This behavior may be assigned to the enhanced hydrogen bonding interactions resulting from hydrolysis that occurs at the electrified interface,⁴⁴ which is consistent with the intensification of the HER. Thus, it is obvious that the SnS₂ + PVDF 50 electrode creates a hydrophobic microenvironment at the gas–liquid–solid interface. In addition, to elucidate how interfacial concentrations of CO₂ and H₂O influence both reaction pathways and selectivity towards HCOOH, in situ ATR-SEIRAS was employed. As shown in Fig. 4d, the CO₂ absorption peak of the SnS₂ + PVDF 50 electrode gradually increases as the applied potential becomes more negative. In contrast, the CO₂ absorption peak of SnS₂ reaches the maximum at −0.8 V and subsequently decreases as the applied potential continues to become more negative (Fig. S18b†). Then, by comparing the ratio of CO₂ to H₂O over the SnS₂ + PVDF 50 and SnS₂ electrodes (Fig. S19†), a higher value is observed for SnS₂ + PVDF 50 across −0.4 V to −1.4 V (Fig. 4e). This indicates that the incorporation of hydrophobic PVDF results in higher concentrations of CO₂ around the electrode.⁴⁵ Simultaneously, an HCOO* peak appears at −0.8 V for SnS₂ + PVDF 50, which proves the formation of formate. However, there is nearly no change observed for SnS₂ itself. Above all, the in situ ATR-SEIRAS analysis reveals that the SnS₂ + PVDF 50 electrode creates a hydrophobic microenvironment, which enhances the ratio of CO₂ to H₂O across three-phase interfaces, and the ECO₂RR performance gets boosted as a consequence. Moreover, the δ_O–H peak (1650 cm⁻¹) for SnS₂ + PVDF 50 exhibits an increasing trend due to the reorientation and adsorption of water that is oriented downwards on the electrode surface to facilitate coupling with CO₂ reduction.⁴⁶

Fig. 4 The in situ Raman spectra of (a) SnS₂ + PVDF 50 and (b) SnS₂ with different applied potentials. The in situ ATR-SEIRAS spectra of SnS₂ + PVDF 50 in a range of different wavelength segments (c) 4000–3000 cm⁻¹ and 2000–1200 cm⁻¹, and (d) 2200–2400 cm⁻¹. (e) CO₂/H₂O ratio of SnS₂ + PVDF 50 and SnS₂ at different applied potentials.

Based on the aforementioned analysis, in the three-phase set-up (gas–liquid–solid interface), an effective CO₂RR-to-formate system is generated due to the combination of the SnS₂ catalyst and hydrophobic PVDF particles (Fig. 5). The integration of the hydrophobic polymer and modified SnS₂ catalyst creates a hydrophobic microenvironment that enhances the local ratio of CO₂ to H₂O and constructs more three-phase (solid–liquid–gas) boundaries. The enhanced CO₂ concentration could promote the CO₂ mass transfer, thus hindering the HER and promoting the CO₂ conversion.

Fig. 5 Illustration of the gas–liquid–solid boundaries before and after modification of the SnS₂ catalyst with PVDF particles.

Conclusions

In summary, a simple method has been adapted to decorate SnS₂ with polymer PVDF to achieve the pH-universal ECO₂RR to obtain formic acid. The PVDF-modified SnS₂ exhibits a higher formate FE of 98% in 1 M KOH than pure SnS₂. Furthermore, the SnS₂ + PVDF 50 electrode offers excellent stability (>30 h) in 1 M KOH under a current density of 150 mA cm⁻² with a negligible decrease in FE, yielding a 2.67 mol L⁻¹ formate solution. SnS₂ + PVDF 50 could achieve 93% HCOOH FE in 0.05 M H₂SO₄ + 0.5 M K₂SO₄. In particular, its SPCE could reach 72.77% in acidic electrolyte, which is twice more than that under alkaline conditions. The introduction of the hydrophobic polymer PVDF could enhance the local ratio of CO₂/H₂O and generate rich gas–liquid–solid interfaces, as studied using in situ ATR-SEIRAS spectra. Furthermore, the increased local ratio of CO₂/H₂O could boost the accessibility of CO₂ to the catalyst surface, thus improving the catalytic activity and inhibiting the HER. In situ Raman spectra, post-reaction structure analysis, and performance evaluation point out that SnS₂ undergoes reconstruction to form catalytically active Sn/SnS₂ composites during the ECO₂RR. This work presents a facial and green method to achieve efficient CO₂ conversion to formic acid using a pH-universal catalyst.

Experimental

Materials

Tin(IV) chloride pentahydrate (Adamas-beta, SnCl₄·5H₂O, 98%, AR), thioacetamide (Macklin C₂H₅NS, 99%, AR), ethanol (DM, CH₃CH₂OH, 99.5% AR), polyvinylidene fluoride (Aladdin, (CH₂CF₂)_n), polyvinylpyrrolidone K30 (OKA, (C₆H₉NO)_n), potassium bicarbonate (Aladdin, KHCO₃, 99.7%, AR), potassium hydroxide (Aladdin, KOH, AR), Nafion perfluorinated resin (Adamas, 5 wt% in the mixture of lower aliphatic alcohols and water, contains 45% water), formic acid solution (Aladdin, HCOOH, 1% w/v), and sulfuric acid (H₂SO₄, 96%, AR) were used without further purification. Ultrapure Millipore water (18.25 MΩ) was used in all experiments. An Ag/AgCl electrode (GaossUnion, R1038), a GDE (SGL, 28 BC), an anion-exchange membrane (DM, Sustanion X37–50), a proton exchange membrane (DuPont, Nafion 117), and Nickel foam (Youveim®) were used.

Synthesis of SnS₂

SnS₂ nanoflowers were synthesized using a modified method based on a previous report.²⁸ 211.2 mg of SnCl₄·5H₂O and 135.2 mg of CH₄N₂S were dissolved in 25 mL of ethanol and then poured into a 50 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed properly and heated at 150 °C for 12 h. After the reaction, the resulting sample was collected by centrifugation at 9500 rpm. The collected sample was washed with ethanol and then dried under vacuum at 70 °C overnight to obtain SnS₂ nanoflowers.

Electrode preparation

4 mg of SnS₂ powder and 1 mg of carbon black (XC-72) were dispersed in 720 μL of isopropanol, 240 μL of ultrapure water, and 40 μL of 5% Nafion to form the SnS₂ ink. A total of 200 μL of the prepared SnS₂ catalyst slurry was dropped onto carbon paper to prepare the SnS₂ electrode with an appropriate catalyst loading (1 mg cm⁻²). The prepared SnS₂ ink was uniformly mixed using ultrasonication for 1 h. Meanwhile, 5 mg of PVDF was dispersed in 500 μL of DMF and mixed evenly with ultrasonication for 1 h. Next, the two mixtures were mixed and ultrasonicated for an additional hour. Subsequently, 300 μL of the prepared catalyst slurry was dropped onto carbon paper to create the SnS₂ + PVDF 50 electrode. The loading amounts of SnS₂ and PVDF were both 1 mg cm⁻². Similarly, SnS₂ + PVDF electrodes with varying mass ratios (10, 20, 30, 40, 60, and 70) were prepared by adjusting the amount of PVDF (0.55 mg, 1.25 mg, 2.14 mg, 3.33 mg, 5 mg, 7.5 mg, and 11.67 mg) in DMF. For the SnS₂ + PVP electrode, 5 mg of PVP was directly added to the as-prepared SnS₂ ink, followed by 1 hour of ultrasonication. Then, 200 μL of the prepared catalyst slurry was dropped onto carbon paper to fabricate the SnS₂ + PVP electrode. The loading amounts of SnS₂ and PVP were both 1 mg cm⁻².

Characterization

XRD patterns of catalysts loaded on CP were analyzed using an X-ray diffractometer (Japan Rigaku), operating at 4° min⁻¹ over a range of 10°–90°. Morphological features were observed using a TESCAN MIRA3 LMH Field Emission Scanning Electron Microscope, which was operated at 10 kV. The XPS spectra were obtained using a Thermo Scientific ESCALAB 250Xi spectrometer, with measurements conducted via a Kratos Analytical AXIS Supra spectrometer utilizing Al Kα radiation. The binding energy scale was calibrated using the C 1s peak, fixed at 284.8 eV. Water contact angle measurements were performed using a contact angle goniometer from KRUSS (Germany).

Electrochemical measurements

The ECO₂RR was conducted in a typical flow cell system using circulating electrolytes: alkaline (1 M KOH), neutral (0.5 M KHCO₃), and acidic (0.05 M H₂SO₄ + 0.5 M K₂SO₄). The CO₂RR electrolysis was performed using a CHI 760e electrochemical workstation. The prepared SnS₂ + PVDF 50, SnS₂ + PVP, and SnS₂ electrodes were used as the working electrodes. An Ag/AgCl electrode with a saturated 3 M KCl solution served as the reference electrode. Foamed nickel was employed as the counter electrode in alkaline and neutral electrolytes, while IrO₂ supported on Ti mesh was used as the counter electrode in the acidic electrolyte. In the ECO₂RR process in alkaline and neutral electrolytes, both the anolyte and catholyte consisted of 30 mL of either 1 M KOH or 0.5 M KHCO₃ respectively. An anion-exchange membrane (Sustanion) was employed to separate the anodic and cathodic compartments. For processes employing acidic catholytes, both the anolyte and catholyte comprised 30 mL of electrolytes (0.05 M H₂SO₄ + 0.5 M K₂SO₄), with a Nafion-N117 membrane utilized to divide the anodic and cathodic compartments. The catholyte was circulated through a peristaltic pump at a flow rate of 5 mL min⁻¹, while the anolyte was cycled through a gas–liquid mixed flow pump. CO₂ gas was delivered past the GDLs of the cathode at a flow rate of 15 sccm. The inlet gas flow rate was measured using a mass flow meter (HPI S4933/MT), and the outlet gas flow rate was recorded with a mass flow meter (Sevenstar D07). All measured potentials were converted to values relative to the RHE using the following equation, without IR compensation.

E_{(V vs. RHE)} = E_{(V vs. Ag/AgCl)} + 0.0591 × pH + 0.197(V) (1)

EIS tests were conducted at open circuit potential with an amplitude of 5 mV over a frequency range from 0.01 Hz to 100 kHz in 0.5 M KHCO₃. LSV scanning was conducted at a scan rate of 10 mV s⁻¹. C_dl was determined through various CV measurements across different scan rates in 0.5 M KHCO₃: specifically at intervals of 20, 40, 60, 80, and 100 mV s⁻¹.

Electrocatalytic CO₂RR product analysis

Gas products were analyzed by GC using an Agilent 8890 GC system. H₂ was detected using thermal conductivity detectors (TCD), while CO was detected using a flame ionization detector (FID). The FE of the gas products was calculated as follows:

FE_gas(%)= Q_gas/Q_total = N × F × n_gas/Q (2)

where F represents the faradaic constant, N denotes the number of electrons transferred from CO₂ to the products, n_gas indicates the total amount of gas products (in moles), and Q signifies the total charge (in coulombs). Liquid products were analyzed using a HPLC system equipped with diode-array detectors (DAD). An H₂SO₄ solution with a pH of 2.5 was utilized as the mobile phase, and areas with varying concentrations of HCOOH solution (200–4000 ppm) were employed to quantify the concentration of liquid products. The FE of the gas products was calculated as follows:

FE_HCOOH(%) = Q_liquid/Q_total = n_liquid × N × F/Q × 100% (3)

where n_liquid is the amount of liquid products produced within a specific time (t) frame, v is the flow rate of CO₂ and t is the duration of the electrolysis process. The SPCE was calculated as follows,

SPCE = n_liquid/(v × t/24.5) (4)

where n_liquid is the amount of liquid products within a certain time (t) frame, v is the flow rate of CO₂, and t is defined as above.

In situ ATR-SEIRAS

In situ ATR-SEIRAS was conducted using a Bruker Tensor 27 ATR-FTIR Spectrometer equipped with a liquid nitrogen-cooled MCT detector and an electrochemistry in situ ATR infrared reaction cell (EC-ATR-H) from Beijing Scistar Technology Co., Ltd. An Au electrode with coated catalysts was installed on top of a silicon crystal and served as the working electrode. A Pt wire and an Ag/AgCl electrode were employed as the counter and reference electrodes, respectively. The detector was cooled with liquid nitrogen for at least 30 minutes before detection to ensure signal stability. The electrolyte (0.5 M KHCO₃) was purged with CO₂ for no less than 30 minutes to achieve saturation before testing and continued to be purged with CO₂ throughout the experiment. All spectra were collected by incrementally varying the potential from −0.4 V to −1.4 V, while the spectrum obtained at an open circuit voltage served as the baseline for initial comparison.

In situ Raman spectroscopy

In situ Raman spectroscopy was conducted utilizing a Renishaw inVia Reflex micro-Raman spectrometer that employed a 532 nm laser as its excitation source. Before testing, the electrolyte (0.5 M KHCO₃) underwent CO₂ purging for at least 30 minutes; subsequently, this saturated electrolyte was used for further analysis. During in situ Raman testing, all spectra were sequentially collected by adjusting the potential from −0.4 V to −1.4 V.

Abbreviations

Nomenclature

ATR-SEIRAS	Attenuated total reflectance surface-enhanced infrared absorption spectroscopy
Cdl	Double-layer capacitance
CP	Carbon paper
CV	Cyclic voltammetry
DAD	Diode-array detector
DMF	Dimethyl formamide
ECSA	Electrochemical active surface area
ECO2RR	Electrocatalytic CO2 reduction reaction
EIS	Electrochemical impedance spectroscopy
FE	Faradaic efficiency
FID	Flame ionization detector
GC	Gas chromatography
GDE	Gas-diffusion electrode
HER	Hydrogen evolution reaction
HPLC	High-performance liquid chromatography
LSV	Linear sweep voltammetry
PVDF	Polyvinylidene fluoride
PVP	Polyvinyl pyrrolidone
R ct	Charge transfer resistance
RHE	Reversible hydrogen electrode
SEM	Scanning electron microscopy
SPCE	Single-pass carbon efficiency
TCD	Thermal conductivity detector
XPS	X-ray photoelectron spectroscopy
XRD	X-ray powder diffraction

Italics

E	Applied potential (V)
f	The frequency of EIS tests (Hz)
N	Transferred electrons from CO2 to products
n gas	Total amount of gas products (mol)
n liquid	Total amount of liquid products (mol)
Q	The amount of total charge (C)
R	faradaic constant (C mol^−1)
t	Reaction time (s)
T	Temperature (°C)
v	The flow rate of inlet and outlet gas (sccm)

Author contributions

Z. W. Dong: data curation, investigation, formal analysis, and writing – original draft; Z. Y. Wang: validation; Y. L. Jia: conceptualization and writing – review & editing; A. Rajendran: supervision and writing – review & editing; W.-Y. Li: funding acquisition, methodology, resources, supervision, and writing – review & editing.

Data availability

The data supporting this article have been included as part of the ESI.†

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.

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (2022YFE0208400), the Fundamental Research Project of Shanxi Province (202303021212031), and the Fundamental Research Funds for the Central Universities (2022ZFJH004).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5gc00635j

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