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• The accelerating consumption of fossil fuels inevitably causes serious CO₂ emissions all over the world, which brings out environmental and climate deterioration. At the same time, the proportion of renewable energy such as wind and solar energies has been increasing significantly among the energy resources^{[1, 2]}. However, efficient technologies are urgently needed to store and convert the renewable energy due to their local and intermittent properties. Electrochemical CO₂ reduction reaction (CO₂RR) can produce chemicals and liquid fuels using renewable electricity input, which has been considered as a feasible avenue for simultaneous conversion of renewable energy and CO₂^[1–4].

  In spite of continuous achievements in CO₂RR, there are still some challenges on developing efficient catalysts to both accelerate the sluggish kinetics of CO₂RR and inhibit the competitive hydrogen evolution reaction (HER) at industrial current densities^[5]. At present, economic and technical analysis suggests that the production of valuable C₁ chemicals such as formic acid and CO is the most economically practicable route for CO₂RR^[6]. Although some noble metal catalysts (e.g. Pd, Au and Ag) have shown considerable CO₂RR performances^{[7, 8]}, it is more meaningful and practical to exploit non-noble metal catalysts with high faradaic efficiency, industrial current density and considerable stability at low overpotentials. Among the widely-investigated catalysts, Sn-based catalysts have been identified as the most promising one for catalyzing CO₂RR to C₁ products with high performance^{[9, 10]}.

  The preparation method affects the composition and structure of Sn-based catalysts, and thereby determines the catalytic performance of CO₂RR. Although electrodeposition technology has been applied to catalyst preparation for CO₂RR in its infancy, it has been attracting much interest due to several advantages: (1) The composition and structure of catalysts can be facilely controlled by adjusting electrodeposition parameters such as electrolyte, current density, conductive substrate, deposition time and temperature, etc. (2) Gas diffusion electrode can be prepared in one step by electrodepositing metal or oxide catalysts on gas diffusion media like Cu foam or carbon paper without the use of organic binders, thus improving the catalyst utilization efficiency. (3) The fabrication process is simple, eco-friendly and easy to be scaled up for further practical applications of CO₂RR^{[11, 12]}.

  In this perspective, a brief survey of Sn-based catalysts including metal, alloy and their oxide for catalyzing CO₂RR to produce C₁ chemicals as well as catalytic mechanisms on their surfaces and interfaces are carried out^[13–23]. The composition and structure of Sn-based catalysts can be conveniently controlled by the electrodeposition parameters, thus affecting the CO₂RR performance. Further fundamental understanding and industrial applications of electrodeposited Sn-based catalysts in CO₂ electrolyzer device are also discussed.

  Electrodeposition is actually electrochemically reducing metal cations to their corresponding metal and alloy on conductive substrates in electrolyte solutions, which is the basis of electroplating, electroforming, electrolytic refining and metal electrolytic smelting^[12]. Recently, electrodeposition technology has been explored as a useful method to prepare Sn-based catalysts for CO₂RR. Wang et al.^[13] electrodeposited various Sn films on Cu film in the 0.1 M SnCl₂ and 0.4 M K₄P₂O₇ aqueous electrolyte with adding 0.1 g·L^‒1 sodium dodecyl benzene sulfonate, 0.2 g·L^‒1 quinol, 0.5 g·L^‒1 gelatin and 0.05 M tartaric acid as additives. The thickness of electrodeposited Sn films was affected by current density from 5 to 20 mA·cm^‒2 for the constant time of 10 min. The thickness of Sn film reached 22.9 μm at 20 mA·cm^‒2 (Fig. 1a). The electrodeposited Sn film showed the formate faradaic efficiency of 91.0% at ‒1.4 V vs. SCE. Wang et al.^[14] utilized an electrodeposition method to prepare porous Sn film on 0.1 mm-thick Cu foil in the aqueous solution of 0.018 M SnCl₂ and 0.05 M sodium citrate at the constant current of 2.4 mA for the deposition time of 5~35 min. The thickness of porous Sn films was influenced by the electrodeposition time. The porous Sn film with a thickness of 325 nm electrodeposited for 15 min demonstrated the maximum formate faradaic efficiency of 91.5% at ‒1.8 V vs. Ag/AgCl. Scanning electron microscopy (SEM) image of porous Sn film showed no change after CO₂RR, indicative of a high stability (Fig. 1b). Shen et al.^[15] synthesized the bimetallic Sn-Cu nanoparticles by a two-step electrodeposition process. Sn nanoparticles were firstly potentiostatically electrodeposited on Cu foil in the aqueous solution of 10 mM SnCl₂ and 0.1 M KCl for 7 min at −0.6 V vs. Ag/AgCl, and then Cu species were electrodeposited on Sn nanoparticles in 10 mM CuCl₂ and 0.1 M KCl solution to obtain bimetallic Sn-Cu nanoparticles. The Sn-Cu nanoparticles displayed a formate faradaic efficiency of 92.0% at ‒0.95 V vs. RHE in 0.1 M KHCO₃ electrolyte and remained stable for 12 h. In situ Raman spectroscopy results and density functional theory (DFT) calculations indicate that the reaction pathway has a two-step course from CO₂ to CO₂^•‒ and CO₂^•‒ to HCOO^•. Cui et al.^[16] coelectrodeposited CuSn₃ alloy on carbon paper substrate in 0.01 M SnCl₂, 0.01 M CuCl₂, 0.06 M (NH₄)₂C₂O₄ and 50 ppm PEG2000 solution at –0.6 V vs. Ag/AgCl for 20 h (Fig. 1c). High-resolution transmission electron microscopy (HRTEM) image revealed some crystals in the amorphous CuSn₃ alloy (Fig. 1d). CuSn₃ alloy showed a current density of 33.0 mA·cm⁻² with formate faradaic efficiency of 95.0% (Fig. 1e) at −0.50 V vs. RHE for 50 h. DFT calculations indicated that the alloying of Sn and Cu could facilitate formate production while suppress the competitive HER. In situ X-ray absorption spectroscopy (XAS) results suggested that the valence state of Sn species maintained positive in CuSn₃ alloy during the CO₂RR. Haruyama et al.^[17] prepared Cu₃Sn alloy on Cu substrates by electrodeposition in 0.04 M SnSO₄, 0.16 M CuSO₄, 0.05 M C₆H₁₄N₂O₇ and 0.5 M K₄P₂O₇ solution at 3.2 mA·cm⁻² for 5 min. Cu₃Sn alloy displayed CO as a major product with faradaic efficiency above 35.0% at the potential between −0.69 V and −1.09 V vs. RHE. DFT calculations indicated that Cu sites had a more contribution to the stabilization of H*, CO* and COOH* than Sn sites in Cu₃Sn (002).

  Figure 1

  

  Figure 1.  (a) SEM image of the cross-section of Sn/Cu electrode deposited at 20 mA·cm^–2. (b) SEM image of the Sn/Cu electrode after CO₂RR at –1.8 V vs. Ag/AgCl. (c) Schematic of the preparation of Sn-Cu alloy. (d) High-resolution transmission electron microscopy (HRTEM) image of CuSn₃ electrode. (e) Potential-dependent faradaic efficiency of Sn, CuSn, CuSn₃ and Cu electrodes for CO₂RR. Reproduced from references [13, 14] and [16]

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  In contrast to mild current density and applied potential in the above electrodeposition process, there is an alternative electrodeposition route that requires a high current density or applied potential to generate hydrogen bubbles as a dynamic template. Generally, proton in aqueous solution can be electroreduced to hydrogen gas at high overpotentials. The formed hydrogen bubbles are able to exclude the grown deposits as dynamic hydrogen bubble template (DHBT) during electrodeposition^[12]. DHBT-assistant electrodeposition technology can efficiently acquire a porous or dendritic morphology without extra additive templates, enhancing the specific surface area of catalysts. Woo et al.^[18] prepared dendritic Sn catalysts on Sn foil by electrodeposition in 0.05 M SnCl₂ and 1 M HCl solution at −2.0 V vs. SCE for 60 s and then heat treated at 180 ^oC for 3 h in air. The heat-treated Sn catalysts presented a formate yield of 228.6 μmol·h⁻¹·cm⁻² at −1.36 V vs. RHE for 18 h. SEM image of the heat-treated Sn catalyst displayed that the catalyst maintained its morphology after CO₂RR (Fig. 2a). Cheng et al.^[19] employed a two-step electrodeposition to fabricate the spiky Cu@Sn nanocones on Cu foam. Firstly, the Cu nanocones were electrodeposited on Cu foam in 0.03 M CuSO₄, 0.0024 M NiSO₄, 0.05 M Na₃C₆H₅O₇, 0.24 M NaH₂PO₂, 0.5 M H₃BO₃ and 6 g·L⁻¹ polyethylene glycol solution at −1.12 V vs. Ag/AgCl at 75 ^oC for 20 min and then immersed in 0.1 M H₂SO₄ to remove Ni species. Secondly, Sn was electrodeposited in 0.05 M SnSO₄, 2 M NaOH and 0.1 M Na₃C₆H₅O₇ on Cu nanocones at 3.3 mA·cm⁻² for 2100 s to prepare spiky Cu@Sn nanocones (Fig. 2b). These nanocones showed a formate faradaic efficiency of 90.4% and current density of 57.7 mA·cm⁻² at ‒1.1 V vs. RHE. The high performance is ascribed to the three-dimensional (3D) porous structure with conical characters that favored the growth of active sites and accelerated mass transfer. Qiao et al.^[20] also electrodeposited 3D porous Cu@Sn electrodes in two steps. Porous Cu was electrodeposited on Cu foil using DHBT in 0.2 M CuSO₄ and 0.7 M H₂SO₄ solution at 1.0 A·cm⁻² for 10 s. Then Sn was coated on the Cu electrode in 0.6 M SnCl₂, 0.03 mM C₁₈H₂₉NaO₃S and 0.15 M Na₃C₆H₅O₇ solution at 20 mA·cm⁻² for 1 min to prepare 3D porous Cu@Sn (Fig. 2c). The 3D porous Cu@Sn electrode reached a formate faradaic efficiency close to 100% and current density of 16.5 mA·cm⁻² at −0.93 V vs. RHE for 15 h. Wang et al.^[21] fabricated A-Cu/SnO₂ catalyst by an electrodeposition-annealing-electroreduction method. Firstly, Cu foam was electrodeposited on the surface of Cu foil with DHBT in 0.2 M CuSO₄ and 1.5 M H₂SO₄ solution at 3.0 A·cm⁻² for 15 s. Secondly, SnO₂ was deposited in 0.02 M SnCl₄, 0.075 M HNO₃ and 0.1 M NaNO₃ solution on Cu foam at −0.3 V vs. SCE for 30 min, followed by annealing at 200 ^oC in air for 6 h. Finally, the as-prepared electrode was electrochemically reduced in 0.1 M KHCO₃ solution at −0.5 V vs. RHE for 1 h to acquire A-Cu/SnO₂ catalyst (Fig. 2d). The A-Cu/SnO₂ catalyst exhibited a CO faradaic efficiency of ~75.0% with the suppression of formate production and HER at −1.0 V vs. RHE for 10 h.

  Even though the above Sn-based catalysts prepared by electrodeposition technology have a favorable formate faradaic efficiency, they are still limited by inferior mass activity. In order to obtain synergistic effects on Sn-Cu alloy, Ye et al.^[22] presented a strategy to improve CO₂RR mass activity through a decorated electrodeposition method. Sn-Cu alloy showed partial mass activity and current density of 1490.6±7.5 mA·mg⁻¹ and 79.0±0.4 mA·cm⁻² with a faradaic efficiency of 82.3±2.1% towards formate production at −1.14 V vs. RHE. DFT calculations manifested that Sn-Cu alloy was able to selectively facilitate CO₂RR to formate by producing a key intermediate of HCOO* and suppress the competitive HER.

  In order to further improve the Sn-based catalysts with higher C₁ faradaic efficiency, current density and stability for CO₂RR at low potentials, the rational preparation approach should be urgently designed. Ye et al.^[23] demonstrated a 3D hierarchical Sn-Cu catalyst via co-electrodeposition with DHBT (Fig. 3a). It had a thin amorphous SnO_x shell and metallic hierarchical Sn-Cu core (Fig. 3b). In situ XAS and ex situ structural characterizations revealed that the outer shell could regenerate in situ Sn/SnO_x interfaces during the CO₂RR due to sufficient Sn species in the core (Fig. 3c). The regenerative Sn_2.7Cu catalyst showed the optimal CO₂RR performance (Fig. 3d and e), achieving a C₁ faradaic efficiency of 98.0±0.9% with 406.7±14.4 mA·cm^–2 (Fig. 3d) at –0.70 V vs. RHE, as well as high stability. DFT calculations indicated that regenerative Sn/SnO_x interfaces suppressed the competitive HER and promoted CO₂RR to formate with the optimal binding for HCOO* intermediate, leading to a high performance of CO₂RR.

  Figure 2

  

  Figure 2.  (a) Field-emission scanning electron microscopy (FESEM) image of the heat-treated Sn electrode after CO₂RR. (b) SEM image of Cu@Sn nanocones. (c) Schematic of the preparation of Cu@Sn for CO₂RR. (d) Schematic of the preparation of Cu/SnO₂and A-Cu/SnO₂. Reproduced from references [18, 19, 20] and [21]

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  In summary, various electrodeposited Sn-based catalysts have been investigated to catalyze the CO₂RR and shown promising practical applications. Alloying Sn with Cu is an efficient strategy to decrease the reaction barrier, and thus obtain a low overpotential and high mass activity of CO₂RR. The Sn-enriched hierarchical Sn-Cu core drives the in situ reconstruction of SnO_x shell to Sn/SnO_x interface during the CO₂RR. The binding of HCOO* intermediate is optimized over the Sn/SnO_x interface, which promotes the formate production with high faradaic efficiency, industrial current density and considerable stability at low overpotentials. However, some challenges still remain in fundamental and application aspects. Although the in situ XAS results show that the SnO_x is highly reduced and Sn species keep a positive valence state during the CO₂RR, the oxidation state of Sn species needs to be accurately determined for providing more deep insights into the active site structure and reaction mechanism. Mössbauer spectroscopy is a powerful and unique technique to monitor the dynamic evolution of oxidation state of all Sn species during the CO₂RR^[24]. Developing in situ/operando Mössbauer spectroscopy technique is vitally important to investigate the relationship between the coordination structural features of Sn species in the catalysts and CO₂RR performance, thus identifying the active site. The combination of in situ/operando spectroscopy techniques and DFT calculations would help to clarify the CO₂RR mechanisms and guide rational design of more efficient Sn-based catalysts. For future industrial applications, CO₂ electrolyzer device needs to simultaneously meet the requirements of high current density, energy efficiency and stability. Developing polymer electrolyte membranebased CO₂ electrolyzer device is considered to be a feasible strategy for further practical applications due to its low resistance and highly compact structure^[25]. Many efforts should be devoted to optimizing the gas diffusion electrode and membrane assembly for obtaining abundant and robust electrochemical interfaces as well as facilitating the mass transport of reactants and products for industrial applications of CO₂RR.

  Figure 3

  

  Figure 3.  (a) Schematic of the preparation of 3D hierarchical Sn-Cu catalyst. HRTEM images of the Sn_2.7Cu gas diffusion electrode (GDE) before (b) and after CO₂RR (c) in flow cell. Geometrical total current density (d) and potential-dependent faradaic efficiency (e) on Sn, Sn_2.7Cu, SnCu_1.1 and SnCu_4.0 GDEs in flow cell. Reproduced from reference [23]

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  Figure 1  (a) SEM image of the cross-section of Sn/Cu electrode deposited at 20 mA·cm^–2. (b) SEM image of the Sn/Cu electrode after CO₂RR at –1.8 V vs. Ag/AgCl. (c) Schematic of the preparation of Sn-Cu alloy. (d) High-resolution transmission electron microscopy (HRTEM) image of CuSn₃ electrode. (e) Potential-dependent faradaic efficiency of Sn, CuSn, CuSn₃ and Cu electrodes for CO₂RR. Reproduced from references [13, 14] and [16]

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  Figure 2  (a) Field-emission scanning electron microscopy (FESEM) image of the heat-treated Sn electrode after CO₂RR. (b) SEM image of Cu@Sn nanocones. (c) Schematic of the preparation of Cu@Sn for CO₂RR. (d) Schematic of the preparation of Cu/SnO₂and A-Cu/SnO₂. Reproduced from references [18, 19, 20] and [21]

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  Figure 3  (a) Schematic of the preparation of 3D hierarchical Sn-Cu catalyst. HRTEM images of the Sn_2.7Cu gas diffusion electrode (GDE) before (b) and after CO₂RR (c) in flow cell. Geometrical total current density (d) and potential-dependent faradaic efficiency (e) on Sn, Sn_2.7Cu, SnCu_1.1 and SnCu_4.0 GDEs in flow cell. Reproduced from reference [23]

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