Ag-Sn Intermetallic Compounds Synthesized via Mechanical Alloying as Electrocatalysts for CO₂ Reduction Reaction

Abstract

Ag-Sn bimetallic alloys were synthesized via mechanical alloying using a ball-milling process as electrocatalysts for the carbon dioxide (CO₂) reduction reaction. Single-phase intermetallic compounds or solid solutions of bimetallic Ag-Sn alloy were successfully synthesized. The main reaction product for the CO₂ reduction reaction was formate over the synthesized Ag-Sn alloy catalysts, and the catalyst with an intermetallic phase exhibited the highest activity toward formate generation, especially at high current density. This study demonstrated that mechanical alloying is a potential approach for the development of CO₂ reduction reaction electrocatalysts.

The electrochemical carbon dioxide (CO₂) reduction reaction (CO2RR) is attracting attention as a key strategy for anthropogenic CO₂ reduction to value-added products using renewable energy-derived electricity.^1–5 Several reaction products are obtained in the CO2RR, such as carbon monoxide (CO), formic acid (HCOOH or formate (HCOO⁻)), and hydrocarbons (e.g., methane (CH₄) and ethylene (C₂H₄)).^5–7 It is therefore important to obtain the desired product with high selectivity in the CO2RR. The design and development of appropriate CO2RR catalysts is one effective approach to obtain desired CO2RR products with high efficiency.

Various CO2RR catalysts with metal active centers have been developed in previous studies.^8–10 Metal alloys have been extensively studied because the catalytic activity could be tuned by the sophisticated choice of the constituent elements and their compositions.^11,12 For example, tin (Sn) based alloys are known to generate HCOO⁻ as the main CO2RR products with high efficiency.^13–15 These reports claimed that Sn-based alloys with an appropriate Sn composition show the highest efficiencies for the generation of HCOO⁻.

Intermetallic compounds, a group of metal alloys, possess unique surface atomic arrangements and electronic states due to their unusual composition and crystal structure compared to the pure constituent metal elements.^16,17 The electronic state and coordination structure of metal active centers are important determining factors for catalytic activity; therefore, intermetallic compounds are attractive as electrocatalysts for various reactions including the CO2RR.^15,17,18 Ag₃Sn and Ag₄Sn have been reported as intermetallic compound phases for Ag and Sn alloys.¹⁹ Luc et al. synthesized Ag₃Sn nanoparticles using a solution process in organic solvents and reported the synthesized catalysts functioned as efficient CO2RR catalysts for HCOO⁻ generation.¹⁵ They reported that Ag₃Sn showed the highest efficiency for the generation of HCOO⁻ among the catalysts with various Ag and Sn ratios, and that Ag-Sn catalysts with appropriate compositions are promising catalysts for the CO2RR toward HCOO⁻ generation.

Based on this background, we have aimed to develop the novel synthesis method for CO2RR catalysts composed of Ag and Sn intermetallic compounds for the highly efficient production of HCOO⁻. A mechanical alloying process using ball-milling was employed as the synthesis method. This process has attracted attention as a method for the synthesis of electrode and alloy materials for secondary battery applications.^20–24 Furthermore, mechanochemical synthesis using ball milling offers significant advantages over traditional solution-based synthesis methods, as ball milling is a solvent-free, environmentally friendly, cost-effective, and scalable process.²⁵ The CO2RR activity of the Ag-Sn electrocatalysts prepared by mechanical alloying was evaluated using a gas diffusion electrode system.

In the present study, Ag and Sn alloy catalysts (Ag/Sn) were synthesized via a mechanical alloying process using Ag and Sn powders with desired ratios as starting materials and a planetary ball mill (for details, see Supporting Information). Ag/Sn with a fixed Ag and Sn ratio was first synthesized by mechanical alloying with different synthesis times. The Ag and Sn ratio was set to 4 : 1 because Ag₄Sn is known as an intermetallic compound (ζ phase). Powder X-ray diffraction (XRD) patterns for Ag₄Sn with different milling times (20, 30, and 40 h) are shown in Fig. 1a. The main peaks that correspond to the ζ phase were observed for all samples at diffraction angles of 34.9°, 37.7°, and 39.8°, which indicates that Ag₄Sn was formed during the mechanical alloying process.^26,27 When the synthesis time was set to 20 h, peaks due to pure Ag (e.g., 38.1° and 44.3°) were observed in addition to the ζ phase, and peaks due to pure Sn and Ag were observed for the sample synthesized for 40 h. Only peaks that corresponded to the ζ phase were observed for the sample ball-milled for 30 h; therefore, ball-milling for 30 h was considered to be the appropriate time for synthesis.

Figure 1.

Powder XRD patterns for synthesized materials. (a) Ag : Sn = 4 : 1 with different ball-milling times. (b) Wide and (c) narrow range XRD patterns for Ag/Sn with different Ag to Sn ratios. Pure Sn and pure Ag represent the starting materials for the synthesis.

Material synthesis was then performed by changing the composition of Ag/Sn with the synthesis time fixed at 30 h. Table 1 summarizes the compositions and sample names for the materials synthesized in this study. The catalysts were denoted as Ag/Sn-x, in which x corresponds to the x : 1 ratio of Ag to Sn as the starting material. The synthesized catalysts were then characterized by XRD (Figs. 1b and 1c). The peaks assignable to single phase Ag and Sn were observed in addition to the ζ phase for Ag/Sn-3. For Ag/Sn-5 and Ag/Sn-6, peaks that corresponded to the ζ phase and peaks similar to that for pure Ag were observed. The latter peaks are inferred to be associated with Sn-doped Ag, as they were slightly shifted to lower angles compared to the Ag precursor. For Ag/Sn-7, no Ag, Sn, or ζ phases were observed, and only single-phase peaks for Sn-doped Ag were evident. These results suggest that an appropriate Ag to Sn ratio in the precursor is crucial for synthesizing single-phase intermetallic or alloy compounds.

Table 1. Sample names and Ag : Sn compositions.

Samples’ name	Ag : Sn ratio
Ag/Sn-3	3 : 1
Ag/Sn-4	4 : 1
Ag/Sn-5	5 : 1
Ag/Sn-6	6 : 1
Ag/Sn-7	7 : 1

Ag/Sn-4 exhibits a peak associated with a single-phase intermetallic compound, as shown in Fig. 1; therefore, further structural analyses were performed for Ag/Sn-4. A scanning electron microscopy (SEM) image of Ag/Sn-4 and a magnified image are shown in Figs. 2a and 2b, respectively. The size of secondary particles agglomerated from primary particles is considered to be approximately 1 to 20 µm. In addition, the transmission electron microscopy (TEM) image of Ag/Sn-4 shown in Fig. S1 shows that the primary particle size for Ag/Sn-4 could be as small as 15 to 30 nm. These results and the broad XRD pattern compared to the starting materials suggest that the primary particles in Ag/Sn-4 form intermetallic compound nanoparticles and are in an aggregated form with a size of 1–20 µm. Further microscopic analyses of Ag/Sn-4 by SEM-energy dispersive X-ray spectroscopy (SEM-EDX) are summarized in Figs. 2c–2e. Ag and Sn are uniformly dispersed throughout the Ag/Sn-4 particles at this scale. The elemental ratios determined by SEM-EDX are shown in Table S1, and the Ag and Sn content is almost the same as that for the starting materials. Previous studies have shown that extending the ball milling time leads to the formation of crystalline phases, including alloy phases from supersaturated solid solutions.²⁸ In this study, we speculate that as the ball milling time is prolonged, either the crystallization of the amorphous phase (such as oxidized Sn or oxidized Sn species doped with Ag segregated on the surface) into pure Sn or Ag phases, or the dealloying of the ζ phase occurs. The detailed analysis of the surface of the catalysts was conducted using X-ray photoelectron spectroscopy (XPS) measurements and is discussed in the latter part.

Figure 2.

(a) SEM image, (b) magnified SEM image, (c) SEM-EDX image for Ag/Sn-4, and (d) Ag and (e) Sn EDX mapping images.

Further analysis of the catalyst surface was performed by XPS measurements. A depth profile analysis of the catalyst was performed by Ar⁺ sputtering. Figures 3a, 3b, and 3c show the Ag 3d, Sn 3d, and O 1s XPS spectra of Ag/Sn-4, respectively. The peak intensity for Ag 3d increased with sputtering depth (towards the interior of the particle) with peaks observed at 368.4 eV and 374.4 eV, which are assignable to Ag⁰, regardless of sputtering depth.^29,30 Peaks around 486.6 to 495.1 eV in the Sn 3d spectra of the sample without sputtering (0 nm) are assignable to Sn²⁺,³¹ and the negatively-shifted shoulders around 485.0 eV and 493.4 eV are possibly assignable to Sn⁰. The relative peak intensity for Sn⁰ increased with increasing sputtering depth, compared to those for Sn²⁺, which suggests that Sn at the catalyst surface is oxidized, whereas Sn⁰ is the primary species in the particle interior. This result also shows good agreement with the decrease in the O 1s intensity (Fig. 3c) as the sputtering depth increases. The atomic ratio of Ag, Sn, and O species obtained by depth profile analyses is summarized in Fig. 3d. Although the Sn content was almost constant, the Ag ratio increased, and the O ratio decreased as the sputtering depth increased, which suggests that oxidized Sn species such as SnO (or Ag-doped Sn oxide species) are segregated on the surface of the catalyst particles. The tendency is also confirmed by the fitted Sn 3d_5/2 XPS spectra by Sn⁰ and Sn²⁺ species and the depth profile of atomic ratio of Sn⁰ and Sn²⁺, which are summarized in Fig. S2. Ag/Sn-4 was further analyzed by X-ray adsorption structure (XAS) measurements.³² The X-ray absorption near edge structure (XANES) spectra for the Ag-K edge and Sn-K edge revealed that the adsorption edges of Ag and Sn in Ag/Sn-4 were similar to those of Ag and Sn foils, respectively (Figs. S3a and S3b). However, the XANES spectra of Ag and Sn species in Ag/Sn-4 were distinct from those of the Ag and Sn foils. These results indicates that Ag⁰ and Sn⁰ are dominant for the Ag and Sn species in Ag/Sn-4, and that their electronic states are different from those in pure Ag and Sn possibly due to alloying effects. The extended X-ray absorption fine structure (EXAFS) spectra of the Ag-K and Sn-K edges for Ag/Sn-4 are different from those of reference samples (metal foils and oxides, Figs. S3c and S3d). The main peak for Ag/Sn-4 was slightly shifted from that for Sn-foil, especially at the Sn-K edge. These results indicate that the electronic and coordination structure of the Ag/Sn-4 particles are different from those of the Ag and Sn foils and oxides. This difference is suggested to be due to changes in the electronic and coordination structures by the formation of the intermetallic phase, although the surface of the catalyst is oxidized as indicated by XPS measurements.

Figure 3.

(a) Ag 3d, (b) Sn 3d and (c) O 1s XPS spectra for Ag/Sn-4. The values in the legends represent the sputtering depth by Ar⁺. (d) Elemental atomic ratio of Ag, Sn and O calculated from XPS spectra with different sputtering depths.

The CO2RR activity of the synthesized catalysts was evaluated using CO₂-saturated 1 mol L⁻¹ (1 M) KHCO₃ solution under constant current conditions (galvanostatic conditions). The cross-section and top view backscattered electron (BSE) SEM images were acquired for the structural characterization of the catalyst-loaded GDEs (Fig. S4 and a related note). The SEM images confirmed that the catalyst particles were homogeneously distributed in the catalyst layer. The photograph of the electrochemical cell employed in this study is shown in Fig. S5. First, the CO2RR activities of Ag/Sn-7 and Ag/Sn-4 were evaluated under 100 mA (53.0 mA cm⁻²), given that pure Sn-doped Ag and intermetallic phase were observed for these two samples (Fig. 4a). Pure Ag and Sn, used as starting materials, were also evaluated as control samples. HCOO⁻ and CO were detected as the CO2RR products, and hydrogen (H₂) was also detected as a side reaction product. Pure Sn generated HCOO⁻ with a faradaic efficiency of 77.3 %, while pure Ag generated CO at 89.7 % as the main CO2RR products. Ag/Sn-4 and Ag/Sn-7 generated HCOO⁻ as the major CO2RR product at 84.7 % and 92.3 %, respectively. CO2RR activity was further evaluated under different current conditions for Ag/Sn-4, Ag/Sn-7 and pure Sn (Figs. 4b, 4c, and 4d respectively). At all current densities, HCOO⁻ was the major CO2RR product, with faradaic efficiencies for Ag/Sn-4 and Ag/Sn-7 exceeding 80 %, compared to 66 to 83 % for pure Sn. In addition, the HCOO⁻ selectivity for Ag/Sn-4 was higher than that for Ag/Sn-7 especially at higher current density conditions. Although the XPS analysis inferred that the surface of Ag/Sn-4 is a segregated oxidized Sn species, the selectivity of the CO2RR toward HCOO⁻ generation of Ag/Sn-4 was higher than that for pure Sn. Therefore, we speculate that the segregated oxidized Sn species formed on the surface of intermetallic compounds could exhibit higher catalytic activity for HCOO⁻ generation than the pure Sn. This is possibly due to the electronic interaction between the intermetallic compound phase and the segregated Sn oxide species on the surface, as well as the enhancement of electrical conductivity due to the thin oxidized Sn layer on the surface, as discussed in a previous report.¹⁵ Further optimization of the synthesis conditions detailed control of the particle sizes and surface species would further enhance the selectivity of the CO2RR.

Figure 4.

CO2RR activity for synthesized sample under different constant current conditions. (a) Ag/Sn-7 and Ag/Sn-4 at 100 mA. Pure Ag and Sn nanoparticles were also measured as references. Current density dependence of CO2RR activity for (b) Ag/Sn-4, (c) Ag/Sn-7 and (d) pure Sn nanoparticles. The error bar represents the standard deviation from three experimental trial.

In conclusion, mechanochemical synthesis of Ag/Sn intermetallic compounds as CO2RR catalysts was attempted in the present study. Single phase of Sn-doped Ag and Ag-Sn intermetallic compounds was successfully synthesized by selecting appropriate compositions and synthesis conditions. The Ag/Sn-4 (intermetallic compound phase) generated HCOO⁻ as the main CO2RR product and showed higher HCOO⁻ selectivity under a higher current density condition than pure Sn and Sn-doped Ag (Ag/Sn-7). As far as we know, this is the first report on the synthesis of CO2RR catalysts using the mechanical alloying process, specifically for the Ag-Sn system. This work confirmed that Ag/Sn intermetallic compounds prepared using mechanical alloying function as a CO2RR catalyst. The mechanical alloying method could also be applied to other element systems as CO2RR catalysts.

Acknowledgment

This work was supported by a KAKENHI Grant-in-Aid (No. 22H02175) from the Japan Society for the Promotion of Science (JSPS), the PRESTO program, a grant (No. JPMJPR2371) from the Japan Science and Technology Agency (JST), and the TOBE MAKI Scholarship Foundation, Japan. Synchrotron radiation experiments were performed using the BL01B1 beamline of SPring-8 (Proposal Nos. 2022B0566, 2023A1690 and 2023A1782).

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.25981411.

• 1) Ag-Sn bimetallic alloys were synthesized via mechanical alloying using a ball-milling process as electrocatalysts for the carbon dioxide (CO₂) reduction reaction. Single-phase intermetallic compounds or solid solutions of bimetallic Ag-Sn alloy were successfully synthesized. The main reaction product for the CO₂ reduction reaction was formate over the synthesized Ag-Sn alloy catalysts, and the catalyst with an intermetallic phase exhibited the highest activity toward formate generation, especially at high current density. This study demonstrated that mechanical alloying is a potential approach for the development of CO₂ reduction reaction electrocatalysts.

CRediT Authorship Contribution Statement

Kazuyuki Iwase: Conceptualization (Lead), Funding acquisition (Lead), Methodology (Equal), Project administration (Lead), Writing – original draft (Lead), Writing – review & editing (Lead)

Takeyuki Kamimura: Investigation (Lead), Methodology (Equal), Writing – review & editing (Supporting)

Itaru Honma: Supervision (Lead), Writing – review & editing (Lead)

Conflict of Interest

The authors declare no competing financial interest.

Funding

Japan Society for the Promotion of Science: 22H02175

Precursory Research for Embryonic Science and Technology: JPMJPR2371

TOBE MAKI Scholarship Foundation

Footnotes

T. Kamimura: ECSJ Student Member

K. Iwase and I. Honma: ECSJ Active Members

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