Nanoscale Copper–Tin Dioxide Interfaces for Efficient CO₂ Electroreduction to Formic Acid and Formate at High Rates

Abstract

This study reports highly selective and stable electrocatalysts consisting of Cu and Sn for efficient reduction of CO₂ to formate (HCOO⁻) and formic acid (HCOOH). The copper–tin dioxide catalyst (CuSn) is prepared through a one‐pot microwave‐assisted solvothermal method. Different samples are obtained by varying Cu‐to‐Sn atomic ratios from 4 to 1. When Sn content is low, the CuSn sample shows a main crystalline phase of cuprous oxide (Cu₂O), with low amounts of surface CuO and SnO₂ species. An increase in Sn percentage leads to the appearance of the crystalline phase of metallic Cu and SnO₂, which becomes dominant for the sample with the highest Sn content. During CO₂ electrolysis, the optimal CuSn material shows a faradaic efficiency (FE) for HCOO⁻ greater than 90% at −200 mA cm⁻² in a flow cell with alkaline electrolyte. It also performs well under acidic condition (pH 3) to produce HCOOH with an FE of 70% at the same current density. A long‐term test is carried out in 1 M KOH at −100 mA cm⁻² for over 20 h, showing good retention in FE_HCOO−. This study highlights the potential of implementing low‐cost catalysts for CO₂ conversion at an industrial scale.

This work highlights the green synthesis of Cu–Sn nanocomposites for the efficient electrochemical reduction of CO₂ to formate and formic acid. It demonstrates a sustainable pathway for developing high‐performance, low‐cost, and noncritical raw‐material catalysts for industrial‐scale CO₂ conversion into valuable chemical products.

1. Introduction

In recent decades, the overexploitation and intense use of fossil fuels have led to rapid depletion of fuel availability and increased CO₂ emissions. Electrochemical conversion of CO₂ to produce high‐value fuels and chemicals is a promising pathway to mitigate the CO₂‐related greenhouse effect and promote economic sustainability.^{[ 1} , ² , ³ , ⁴ , ⁵ , ^{6 ]} Among many products, formic acid (HCOOH) is of high interest as a convenient hydrogen storage medium via reversible CO₂ hydrogenation.^{[ 7} , ^{8 ]}

At present, various metals such as tin (Sn),^{[ 9} , ^{10 ]} indium (In),^{[ 11 ]} bismuth (Bi),^{[ 12} , ^{13 ]} cobalt (Co),^{[ 14 ]} and palladium (Pd)^{[ 15 ]} have been investigated as catalysts to produce HCOOH or formate (HCOO⁻) from electrochemical CO₂ reduction reaction (CO₂RR). Among these metals, Sn electrocatalysts have attracted particular attention since it is a noncritically raw metal and have exhibited excellent selectivity for CO₂‐to‐HCOO⁻ (or HCOOH) conversion.^{[ 16} , ^{17 ]} However, industrial deployment remains limited by the high overpotentials required to sustain high current densities with good faradaic efficiencies, as well as by the low stability of Sn‐based catalysts.^{[ 18 ]} Alloying Sn with other metals could be a promising strategy. It is well known that Cu is effective in activating CO₂ molecule and uniquely converts it into hydrocarbon products,^{[ 19} , ²⁰ , ²¹ , ^{22 ]} while its selectivity for HCOO ⁻ or HCOOH is very limited. Combining Sn with Cu could take advantage of both Sn and Cu, maintaining excellent selectivity for HCOO ⁻ (or HCOOH) at relatively low overpotentials and improved stability. Some works have been reported to deliver an excellent CO₂‐to‐HCOO⁻ conversion by tuning the composition or structure of the CuSn bimetallic catalysts.^{[ 23} , ²⁴ , ²⁵ , ²⁶ , ²⁷ , ²⁸ , ^{29 ]} Wang et al.^{[ 27 ]} reported that the excellent activity in electrocatalytic reduction of CO₂ to HCOO⁻ is ascribed to the abundant Cu/SnO₂ interfaces in the heterostructure, by reducing the reaction free energies for the formation of HCOO⁻ species. Lei et al.^{[ 28 ]} achieved effective performance for the electrochemical CO₂ reduction to HCOO⁻ through reconstruction of CuS/SnO₂‐S nanoparticles to Cu/Sn/Cu_6.26Sn₅ nanowires. Gunji et al.^{[ 29 ]} reported that carbon‐black‐supported Cu₆Sn₅ core–shell nanoparticles give 65% faradaic efficiency for HCOO⁻ at ≈−0.6 V, superior to Cu or Sn alone, highlighting the importance of intermetallic‐core/oxide‐shell synergy. However, most of the previously reported electrocatalysts still have limitations for HCOO⁻ generation via CO₂RR, such as complicated preparation methods and limited working conditions such as low current densities and narrow potential windows.

In this regard, we present a facile one‐pot microwave‐assisted method for the synthesis of CuSn‐based nanocomposites for efficient CO₂RR. The optimal CuSn material exhibits excellent selectivity and good stability for HCOO⁻ and HCOOH production across a wide range of current densities in alkaline and acidic electrolytes, respectively.

2. Experimental Section

2.1. Chemicals

Copper (II) acetate hydrate [Cu (OAc)₂ · xH₂O], tin sulfate (SnSO₄), ethylene glycol, potassium hydroxide (KOH), and potassium sulfate (K₂SO₄) of analytical grade were purchased from Merck. All chemicals were used as received. Deionized water was supplied by MyMilli‐Q Lab Water Systems (18.2 MΩ cm, 25 °C).

2.2. Synthesis

The preparation of CuSn bimetallic catalyst is schematically represented in Scheme  1 , through a microwave‐assisted process. Initially, 40 mL ethylene glycol (EG) and 5 mL H₂O were mixed. 2.5 mmol Cu (OAc)₂ · xH₂O and 2.5 mmol SnSO₄ were added to the mixture and stirred for 1 h until the complete dissolution of the salts. The solution was then transferred into a Teflon vessel and irradiated for 2 min (T _max = 220 °C, Power_max = 900 W). After cooling down to 60 °C, the precipitates were washed twice with water and once with ethanol, each time centrifuged at 4900 rpm for 10 min, and then dried under vacuum. The obtained catalyst prepared was named Cu₅Sn₅. The nanocomposites with different atomic ratios of Cu‐to‐Sn were prepared by changing the ratio of Cu and Sn precursors (8:2 and 7:3), and the total mole of the metal salts was kept at 5 mmol for each synthesis. The synthesized catalyst with only Cu (OAc)₂ · xH₂O precursor was denoted as Cu₂O catalyst.

Scheme 1.

Illustration of the synthesis process of CuSn catalysts.

2.3. Physicochemical Characterization

X‐ray diffraction (XRD) was performed in Bragg–Brentano symmetric geometry by using a PANalytical X’Pert Pro instrument (CuKa radiation, 40 kV and 30 mA) equipped with an X’Celerator detector.

Field emission scanning electron microscopy (FESEM, ZEISS Auriga) was used to evaluate the morphology of the materials. For transmission electron microscopy (TEM) analysis, the sample was deposited on a commercial lacey carbon film suspended on an Au grid (Ted Pella Inc., USA) and analyzed in a Talos F200X TEM operated at its maximum acceleration voltage of 200 kV, equipped with an in‐column energy‐dispersive X‐ray (EDX) detector for elemental analysis.

The actual atomic ratios of Cu to Sn in the CuSn samples were determined by inductively coupled plasma–optical emission spectrometry (ICP–OES) analysis.

Raman spectra were acquired on solid powders deposited on a microscope slide by means of a Renishaw InVia Reflex micro‐Raman spectrometer, equipped with a cooled Charge Coupled Device (CCD) camera. The light source was a laser diode (λex = 514.5 nm) with power lower than 10 mW, and sample inspection occurred through a microscope objective (50×), in backscattering light collection conditions.

2.4. Preparation of Electrodes

Herein, a drop‐casting method was used to fabricate the electrodes. Typically, 7.5 mg catalyst, 450 μL isopropanol, and 80 μL Nafion117 solution were sonicated for 20 min until a uniform slurry was obtained. Then, the slurry was coated onto a carbon gas diffusion layer (GDL; SI‐GRACET 28BC, SGL Technologies) substrate. The obtained electrode was dried at room temperature overnight in the air. The mass loading of each catalyst was ≈1.67 mg cm⁻².

2.5. Electrochemical CO₂RR Tests

The CO₂RR experiments were performed on the Biologic potentiostat in a commercial flow cell (ElectroCell A/S) at room temperature. All tests were conducted in a chronopotentiometry (CP) mode. The cell configuration was composed of three chambers and three electrodes, as shown in Figure S1, Supporting Information. An anion exchange membrane (Sustainion X37‐50 Grade RT, Dioxide Materials) divided the cell into anodic and cathodic sides, and the cathode further divided the cathodic side into catholyte and CO₂ gas chambers. Unless specified, both anolyte and catholyte were 1 M KOH aqueous solution. The working electrode was a catalyst‐coated GDL with a geometric area of 1.5 cm². A mini‐Ag/AgCl (1 mm, leak‐free LF‐1) and IrO₂‐coated Titanium mesh were used as reference and counter electrodes, respectively. All reported potentials were rescaled to the reversible hydrogen electrode (RHE). The details of FE calculations for gaseous and liquid products are provided in the Supporting Information.

3. Results and Discussion

3.1. Physicochemical Characterization of the As‐Prepared Catalysts

The atomic ratios of Cu to Sn in the as‐prepared bimetallic catalysts were analyzed by ICP‐OES. As shown in Table S1, Supporting Information, the actual Cu:Sn ratios are 3.1, 1.9, and 0.53 for the Cu₈Sn₂, Cu₇Sn₃, and Cu₅Sn₅ samples, respectively. At lower Cu:Sn ratios, the actual values closely match the nominal ones, whereas at the highest ratio, the Cu content is significantly lower than expected.

FESEM was employed to investigate the morphological features of the as‐prepared materials (Figure  1a–d). The Cu₂O sample consists of micrometer‐sized cubic particles. Upon the addition of Sn, a significant morphological transformation is observed, independent of the Sn concentration. All CuSn catalysts exhibit similar morphology characterized by agglomerates of densely packed nanoparticles.

Figure 1.

FESEM images of as‐prepared catalysts. a) Cu₂O, b) Cu₈Sn₂, c) Cu₇Sn₃, and d) Cu₅Sn₅.

TEM analysis was performed on the selected Cu₅Sn₅ sample, confirming the agglomerations of very small nanoparticles with a broad distribution in size (about 2 to 7 nm in diameter). TEM images of the sample are shown in Figure  2a,c. Figure 2b reports the Fast Fourier transform (FFT) of the image in Figure 2a. These images demonstrate that the nanoparticles are crystalline, displaying lattice fringes with interplanar spacings predominantly of 0.33–0.34 nm, which can be indexed to the (110) planes of SnO₂. A minority of fringes with spacings of 0.25–0.26 nm are also observed, consistent with the (101) planes of SnO₂.^{[ 24 ]} Figure 2d,e shows an EDX map of the sample acquired in scanning TEM (STEM mode), where the local abundance of Sn and Cu is displayed along with the high‐angle annular dark‐field (HAADF) image of the sample portion. These analyses show the presence of Cu and Sn all over the sample with a distribution not always uniform. While the main part of the agglomerations shows a homogeneous ratio of Sn and Cu, there are also a certain number of nanoparticles showing a higher fraction of Cu. Such particles are likely to be preferentially located on the outer surfaces of agglomerations.

Figure 2.

High‐magnification TEM images of the compound Cu₅Sn₅. a) Zoomed view displaying the average size of particles in the agglomerates, b) FFT of the image reported in (a), c) magnified view showing the two lattice spacings which were found in the structure, d) HAADF‐STEM image of and e) the relative abundance of Cu and Sn obtained by EDX analysis in the same area.

Figure  3 presents the XRD patterns of all as‐prepared catalysts. The Cu₂O sample displays diffraction peaks corresponding to the crystalline structure of cuprous oxide (Cu₂O, PDF #01‐1142), with no detectable impurities. A similar pattern is observed for the Cu₈Sn₂ sample, showing no significant presence of crystalline Sn phases, as further confirmed by the higher‐resolution plotting (Figure S2, Supporting Information). Increasing the Sn content in the Cu₇Sn₃ sample leads to the appearance of crystalline SnO₂ (PDF #01‐0657) and metallic Cu (PDF #01‐01241). In the Cu₅Sn₅ sample, which contains the highest Sn concentration, only peaks corresponding to SnO₂ and metallic Cu are observed. The broad peaks are attributed to the small crystallite size of SnO₂.^{[ 24 ]}

Figure 3.

XRD patterns of synthesized catalysts.

Raman spectroscopy analysis was performed on selected samples, as displayed in Figure  4 . The Raman spectrum of Cu₂O reveals its preliminary oxidation states, with a clear signature of Cu₂O as evidenced by the most prominent peak at 217 cm⁻¹ and a further peak at 145 cm⁻¹. Weak signals from CuO are observed at 271 cm⁻¹, 332 cm⁻¹, and 611 cm⁻¹.^{[ 30} , ^{31 ]} The small amount of CuO is attributed to surface oxidation upon air exposure, typically forming in an amorphous state or in quantities too low to be detected by XRD.^{[ 32} , ^{33 ]} Raman spectrum of Cu₅Sn₅ sample shows a dominant feature at 611 cm⁻¹ and some additional weaker features at 499 cm⁻¹, 535 cm⁻¹, 675 cm⁻¹, and 745 cm⁻¹, which arise from the vibrations in SnO₂ nanocrystals with a shift related to the interaction of SnO₂ with CuO, in line with previously reported observations.^{[ 31} , ^{34 ]} Both Cu₈Sn₂ and Cu₇Sn₃ catalysts show a surface composed of Cu oxides and SnO₂ (Figure S3, Supporting Information).

Figure 4.

Raman spectra of synthesized Cu₂O and selected CuSn sample.

In summary, the CuSn samples exhibit similar morphologies to one another, but show notable differences compared to the Cu₂O sample. In contrast, their chemical composition is strongly influenced by the Cu‐to‐Sn ratio: increasing Sn content promotes the formation of SnO₂ and metallic Cu. Nonetheless, Cu and Sn are homogeneously distributed at the nanoscale, with only a slight enrichment of Cu at the edges.

3.2. Electrochemical Characterization

The catalytic performance toward the CO₂ RR of various CuSn electrodes was explored in 1 M KOH alkaline solution at a current density of −200 mA cm⁻². The Cu₂O electrode was also tested for comparison. As shown in Figure  5a, the Cu₂O electrode mainly produces CO and C₂H₄, with a scarce selectivity of 6% for formate. In contrast, all CuSn electrodes exhibit a significantly enhanced selectivity toward formate production. The Cu₈Sn₂ sample shows a FE_HCOO– of 55%, which further increases to 86% and 92% for Cu₇Sn₃ and Cu₅Sn₅, respectively. It is interesting to note that the potential follows a clear trend at the applied current density: Cu₅Sn₅ > Cu₇Sn₃ > Cu₈Sn₂ > Cu₂O, reflecting the same order in the catalytic activity of these electrodes. These results indicate that the addition of Sn plays a crucial role in shifting the selectivity of Cu toward formate production, and its concentration significantly influences the enhancement of formate formation. The enhanced performance observed in high Sn‐loading samples could be attributed to a cooperative effect between metallic Cu and SnO₂, where metallic Cu enhances electron transfer,^{[ 32 ]} while SnO₂‐derived sites favor formate formation.^{[ 35} , ^{36 ]}

Figure 5.

CO₂RR performance of various electrodes at −200 mA cm⁻² in 1 M KOH. a) FEs of different products and b) corresponding chronopotentiometry curves.

The Cu₅Sn₅ bimetallic catalyst was further evaluated at current densities of −100 and −300 mA cm⁻². As shown in Figure  6a, it maintains consistently high selectivity for formate across the tested current range, with faradaic efficiencies between 85% and 92%. Figure 6b shows that the potential remains nearly stable at each current density, with a gradual negative shift as the current increases, suggesting no significant mass transport limitations. The combination of high selectivity and relatively low overpotentials highlights the Cu₅Sn₅ catalyst as a highly promising candidate for CO₂‐to‐formate conversion.

Figure 6.

CO₂RR performance of Cu₅Sn₅ catalyst at different current densities in 1 M KOH. a) FEs of various products and b) corresponding chronopotentiometry curves.

The stability of the Cu₅Sn₅ catalyst was assessed through a long‐term test in a flow cell using 1 M KOH electrolyte. As shown in Figure  7 , the FE_HCOO– remains above 80% during the first 5 h, followed by a gradual decline of ≈10% over the subsequent 15 h. The potential stays stable for 12 h before experiencing a gradual negative shift of about 200 mV. The decline in performance is likely due to a combination of factors and cannot be attributed to a single cause. However, since no backpressure was applied to the gas or liquid lines during the test, gradual electrode flooding over time is a plausible explanation. In any case, the observed stability is consistent with the best‐performing systems reported in the literature under similar experimental conditions.^{[ 13} , ^{37 ]}

Figure 7.

Long‐term CO₂RR test on Cu₅Sn₅ electrode at −100 mA cm⁻².

In alkaline electrolytes, CO₂ reduction primarily yields formate rather than HCOOH, due to the high pH and the low pKa of HCOOH (3.75). As a result, an additional acidification step is required to convert formate into the more valuable HCOOH, increasing the overall process cost. Direct production of HCOOH requires acidic electrolytes (pH < 3). However, in such conditions, the hydrogen evolution reaction (HER) typically dominates due to the high proton concentration at the electrode/electrolyte interface, often suppressing CO₂RR.^{[ 38} , ^{39 ]} Thus, the catalyst's selectivity plays a more critical role under acidic conditions. To evaluate the potential of Cu₅Sn₅ for direct HCOOH production, CO₂RR tests were conducted in 0.5 M K₂SO₄ (pH 3) using a flow cell. The acidic electrolyte was prepared by adding concentrated H₂SO₄ to a 0.5 M K₂SO₄ solution, maintaining a total K⁺ concentration of ≈1.0 M. During testing, the catholyte was circulated in single‐pass mode. As shown in Figure  8 , the Cu₅Sn₅ electrode predominantly produces HCOOH across the tested current densities, reaching a peak FE of 70% at −200 mA cm⁻². Compared to alkaline conditions, the acidic setup requires a more negative potential to reach the same current density, indicating that the CuSn electrode exhibits higher activity in a basic environment. To understand this phenomenon, the electric double layer capacitance (C _dl) of Cu₅Sn₅ electrodes after tests in pH 3 and 14 electrolytes was assessed. As illustrated in Figure S4, Supporting Information, the C _dl at pH 14 is 2.5 times as high as that at pH 3, highlighting a significant enhancement in the electrochemically active surface area (ECSA) under alkaline conditions. A higher ECSA provides more active sites, thereby boosting the overall electrode activity in a basic electrolyte.

Figure 8.

CO₂RR performance of Cu₅Sn₅ catalyst in 0.5 M K₂SO₄ (pH 3). a) FEs of HCOOH at various current densities and b) corresponding CO₂RR chronopotentiometry curves.

3.3. Comparison of Synthesized CuSn Catalysts with Literature

The catalyst proposed in this study shows superior activity and selectivity for formate production in CO₂RR compared to previously reported Cu‐ and Sn‐based systems (Table  1 ). The Cu and Sn sources are low‐cost and abundantly available. Moreover, the microwave‐assisted one‐pot synthesis is energy‐efficient, rapid, and reproducible. These excellent performances highlight the strong potential of the proposed catalysts for practical applications.

Table 1.

Performance comparison of various electrocatalysts for CO₂RR to formate.

Catalyst	Electrolyte	j HCOO− [mA cm−2]	Potential (V versus RHE)	FEHCOO− [%]	Reference
Cu5Sn5	1 M KOH	−184	−1.1	92	This work
Cu5Sn5	0.5 M K2SO4 (pH 3)	−140	−2.1	70	This work
Cu‐SnS2	0.5 M KHCO3	−23.8	−1.0	90.5	[23]
5%Cu‐SnO2	0.5 M KHCO3	−22	−0.9	91	[24]
Cu3Sn/Cu6Sn5	1 M KOH	−18.9	−1.0	87	[25]
Hemicapsule Cu@SnO2	0.1 M KHCO3	−28	−1.35	67	[26]
CuSn NPs/C‐A	0.1 M KHCO3	−12.6	−1.0	71.5	[27]
CuS/SnO2‐S	0.5 M KHCO3	−18.8	−0.8	84.9	[28]
CuO/SnO2	1 M KOH	−3.19	−1.05	57.5	[40]
R‐CuSnO3	1 M KOH	−26	−0.9	93.4	[41]
CuO–SnO2 NFs	1 M KOH	−186	−0.7	74.3	[42]
CuSn HFGDE	0.5 M KHCO3	−68	−1.2	78	[43]
Cu–Sn aerogel	0.5 M KHCO3	−21.8	−1.1	90.8	[44]

4. Conclusion

This study presents CuSn bimetallic catalysts synthesized via a simple microwave‐assisted method for the efficient electrochemical conversion of CO₂ to formate/formic acid. The catalyst morphology remains largely unchanged across different Sn contents, from 25 at% to 65 at%. However, the phase composition varies significantly with Sn content: the low‐Sn sample predominantly exhibits crystalline Cu₂O, whereas the high‐Sn sample shows crystalline phases of metallic Cu and SnO₂. These compositional differences strongly influence the selectivity of the CO₂ reduction reaction, highlighting the critical role of Sn content in tuning catalyst performance. Specifically, the high‐Sn sample demonstrates excellent selectivity (92%) and good activity (−1.1 V versus RHE at −200 mA cm⁻²) for formate production in alkaline conditions. This remarkable performance could be attributed to the synergistic interaction between metallic Cu and SnO₂, where the former facilitates electron transfer and the SnO₂‐derived sites promote formate formation. Moreover, it maintains a promising selectivity of 70% for direct formic acid production in acidic electrolyte. This study offers a new pathway for developing high‐performance and low‐cost catalysts for industrial‐scale CO₂ conversion into valuable chemical products.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supplementary Material

Huang Lan, Di Costola Felicia, Allione Marco, Bianco Stefano, Sacco Adriano, Pirri Candido F., Zeng Juqin, ChemSusChem. 2025, 18, e202501686. 10.1002/cssc.202501686

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.