Recent research progress of bismuth-based electrocatalysts for electrochemical reduction of carbon dioxide

Single-atom catalysts

Recent studies have indicated that nanometallic materials have better competence in CO₂RR than bulk materials due to their higher specific surface area and size effects. Therefore, expanding their specific area through nanoscale structural engineering is very important to improve their catalytic performance. SAC, as an attractive heterogeneous catalyst, shows unique advantages in electrocatalysis. SAC has tremendous advantages as follows over the traditional nanocatalysts: i) Substantially higher atom utilization, even up to 100%: as the particle size of catalyst is reduced, the atom dispersion is continuously improved until it reaches complete dispersion, resulting in exposing more active catalytic sites and thus significantly boost catalytic performance; ii) Regulable coordination environment: the coordination number and coordination atoms of the metal catalytic sites dispersed on various carriers are often adjustable, so as to regulate unsaturated and supersaturated coordination structures to obtain unprecedented catalytic performance; iii) Metal-carrier interaction: in SAC, there is strong mutual anchoring between isolated catalytic atoms and carriers and this interaction is beneficial to electron transfer, which can not only structurally prevent metal atoms from flocking together into clusters, but also facilitate the formation of strong chemical bonds between metals and carriers^[52]. For CO₂RR, well-defined active centers in SAC have the potential to reduce the diversity of reaction pathways or weaken HER. Applying SAC in the field of CO₂RR to regulate product distribution makes sense, both in terms of productivity and cost savings. In most cases, due to the unsaturated coordination structure, the central atom that provides a suitable adsorption/desorption site for the reaction intermediate acts as the active center. Sometimes, the reconfiguration of SAC occurs during the catalytic process, resulting in a change in the active component SAC to the nanoparticle/cluster. The reasonable construction of the coordination structure gives SAC an incomparable advantage in electrocatalytic CO₂ reduction.

Zhang et al. used Bi-based metal-organic framework (MOF) precursors to obtain Bi SAC immobilized on porous carbon supports [Figure 3A]^[53]. They effectively used in situ environmental transmission electron microscopy (ETEM) to monitor the catalytic process of the catalyst. The Bi-based MOF precatalyst was reconstructed to single Bi atoms riveted on N-doped carbon networks (Bi SAC/NC). DFT calculations indicate that Bi-N₄ as the active center of the Bi SAC/NC catalyst optimized the catalytic activity by reducing the energy barrier for the formation of *COOH intermediate [Figure 3B]. The atomically distributed catalytic site changes the adsorption energy of *COOH and *CO, making CO more selective than formic acid.

Figure 3. (A) Diagram of the transformation to Bi SACs and TEM images of Bi-MOF decomposed at different reaction temperatures; (B) DFT diagrams for CO₂RR on different catalysts. Reprinted with permission from Ref.^[53]. Copyright 2019, American Chemical Society; (C) The EXAFS spectra from Bi L₃ edge XANES spectra of Bi@C samples annealed at different temperatures, along with Bi foil and Bi₂O₃ as references; (D) Electrochemical testing of CO₂RR products HCOOH and CO as a function of annealing temperatures for Bi@C catalysts. Reprinted with permission from Ref.^[54]. Copyright 2023, Wiley-VCH. SAC: Single-atom catalysts; TEM: transmission electron microscopy; MOF: metal-organic framework; DFT: density functional theory; EXAFS: extended X-ray adsorption fine structure; XANES: X-ray absorption near edge structure.

The Bi SAC in different coordination environments also shows different selectivity of CO₂RR products. Santra et al. synthesized atomically dispersed Bi SAC with carbon black as a carrier^[54]. Scanning transmission electron microscopy (STEM) and extended X-ray adsorption fine structure (EXAFS) studies have confirmed the success of monatomic morphology synthesis at low and high temperature annealing [Figure 3C]. The SAC coordination environment in the carbon-nitrogen framework was significantly changed by changing the synthesis parameters under high temperature and low temperature treatment. The low temperature treatment resulted in a high load of the Bi₂O₃ coordination center, while the high temperature treatment resulted in low coverage of the Bi₂N site [Figure 3D]. The spectra show that under the same electrochemical CO₂RR conditions, nitrogen-coordinated Bi SAC tends to produce CO, while oxygen-coordinated Bi SAC mainly generates HCOOH.

Furthermore, the single atom alloy (SAA) catalyst composed of single atoms of Bi and other isolated atoms of external metals has recently received extensive attention as a special SAC. Monatomic alloys have the following main advantages: i) The structure of the active center of SAA is determined, which facilitates the understanding of the relationship between the structure of catalyst and performance^[55,56]; ii) SAA catalysts may exhibit particular electronic structures and strong synergies among different metals, thus breaking the linear scaling used to promote heterogeneous catalytic activity and/or selectivity^[57,58]; The early development of SAA catalyst was mainly to optimize the performance during the process of multiphase thermal catalysis, and recently, the application field of SAA catalyst has been expanded to photocatalysis and electrocatalysis, demonstrating its rationality in adjusting electronic structure and various multiphase catalytic properties.

BiCu-SAA was synthesized in situ by electrochemistry using Bi-CuS precursors [Figure 4A]^[59]. The catalyst can promote C-C coupling in the catalytic process, and the addition of copper improves the selectivity of electrocatalytic CO₂RR to generate C₂₊ products, and 73.4% of high C₂ products can be obtained at -0.9 V_{vs RHE}. At a high current density of 400 mA·cm^-2, the developed BiCu-SAA catalyst also maintained its structural stability and showed good robustness [Figure 4B]. In situ Fourier-transform infrared spectroscopy (FTIR) based on Synchrotron radiation (SR-FTIRs) showed that the peak intensity of BiCu-SAA was significantly higher than that of Cu-nano-samples, indicating that the BiCu-SAA catalyst had better *CO generation ability [Figure 4C]. DFT calculations show that the position of the Bi state below the Fermi level in the projected density of states (PDOS) results proves that the Bi site acts as a promoter/modifier to change the electronic structure of the copper plate. The localized electrons can be transferred to the antibonding orbital of the CO₂ molecule. Through the analysis of the reconstruction process, unlike most series catalysts, this catalyst mainly reduces the energy of CO₂ adsorption/activation by optimizing the electronic structure of metal nanoparticles. BiCu-SAA catalyst can achieve rapid consumption of *CO intermediates, so as to achieve the purpose of optimization of intermediates [Figure 4D and E]. Moreover, with the single Bi-SAC and Cu-SAC, the optimization of catalytic products and the stability of catalytic process can be realized, and the purpose of one plus one is greater than two is realized. It is well known that SAC can be used as a model system to fundamentally understand the CO₂RR mechanism in terms of the active site as well as the interaction between atomically dispersed metal catalysts and supporting materials. However, this is necessarily accompanied by difficulties of the SAC itself, including scale-up synthesis and atomic-level characterization of high metal loading, identification of active sites, and understanding of basic mechanisms.

Figure 4. (A) The supposed reaction mechanism of CO₂RR on the BiCu (111)-SAA (left) and Cu (111)-Nano (right); (B) FE values of different products; (C) In situ SR-FTIR spectra of BiCu SAA and Cu nanocatalyst at different potentials; (D) DFT diagram for the CO₂RR to C₂H₄ on the surface of catalysts; (E) The free energy plots for different processes. Reprinted with permission from Ref.^[59]. Copyright 2023, Wiley-VCH. SAA: Single atom alloy; FE: faraday efficiency; FTIR: fourier-transform infrared spectroscopy; DFT: density functional theory.

1D nanostructures

One-dimensional nanomaterials, such as NW (including alloys and coaxial heterostructures) and NT^[60], are considered as active and stable electrode contact regions for electronic materials due to their distinct and homogeneous structure, easy electron transfer, shortened ion diffusion distance, increased electrolyte, and rigid durability corresponding to unidirectional stress and strain^[61]. Due to the strong boundary effect and the quantum binding principle, the abundant open Spaces and pores between adjacent one-dimensional nanostructures enable the rapid transfer and reaction of catalyst molecules on the electrode/catalyst surface. Therefore, one-dimensional nanomaterials are supposed to be an excellent electrochemical catalysis^[33].

Zhang et al. reported a strategy of lattice dislocation Bi NW (Cu@BiNW) on the surface of copper foam by in situ electrochemical preparation of Bi atoms^[62]. The electrochemical test data show that Cu@BiNW is a high efficiency electrocatalyst with low overpotential, a FE of 95%, and a formic acid current density of about 15 mA·cm^-2. Compared with RHE, the FE formate value remains above 93% in the voltage range of 0.69-0.99 V_{vs RHE}. Fourier transform alternating current (AC) voltammetry studies showed significant differences in the catalytic path and rate compared to other Bi-based materials. Due to the high porosity, the catalytic surface area is significantly improved. At the same time, due to the introduction of a considerable number of lattice dislocations in the Bi NW, the CO₂ reduction activity of the Cu@BiNW electrode is greatly enhanced.

2D nanosheets

Due to the excellent properties of layered graphene in many aspects, 2D single crystals are also currently being investigated. Bismuth is the largest atomic mass member of group VA in 2D elemental crystals and has a quasi-layered structure similar to that of rhombic antimony and arsenic. Ultra-thin Bi nanosheets are effective CO₂RR electrocatalysts with significant FE and high current density. Despite its many excellent physical and electrical properties, experimental realization of single-crystal 2D bismuth has been hindered.

Zhang et al. used the liquid phase dissolution method to dissolve powder Bi in isopropanol (IPA) solution of NaOH by ultrasonic, and prepared ultra-thin Bi nanosheets on a large scale [Figure 5A]^[63]. The successful synthesis of Bi nanosheets was confirmed by high-resolution transmission electron microscopy (HETEM), Raman and ultraviolet (UV)-visible spectroscopy. Bi nanosheets offer several advantages, including their ultra-thin structure, high electron transfer rate, good CO₂ adsorption rate, and high electrical conductivity. They also showed that Bi nanosheet edges with more catalytic sites can effectively promote the generation of *OCOH, a key intermediate for CO₂ selective production of formic acid. DFT calculations indicate that *OCOH intermediates tend to form at the margin positions of Bi nanosheets to reconstruct a more stable configuration. Via subsequent electron and proton transfer, the intermediate to generate HCOOH is formed and consumed rapidly, thus greatly improving the FE of formate. Therefore, Bi nanosheets have high FE, moderate overpotential, and long-term stability for formate production in aqueous media, thanks to the excellent surface wettability of Bi nanosheets, which can be in full contact with the electrolyte [Figure 5B and C]. Bi nanosheets can also improve CO₂ absorption and provide better electron transport capabilities, enabling rapid formation and transfer of intermediates, and ultimately improve the selectivity of formic acid production [Figure 5D].

Figure 5. (A) Diagram of the preparation of Cu-doped Bi SACs; (B, C) Contact angle of Bi NSs and bulk Bi in aqueous solution; (D) Schematic diagram of the mechanism on Bi nanosheet. Reprinted with permission from Ref.^[63]. Copyright 2018, Elsevier; (E) Schematic diagram of the growth mechanism of APCVD; (F) Corresponding Raman mapping of Bi A_1g peak intensity; (G) Electrochemical performance testing of bismuthene samples at different applied potentials (vs. RHE). Reprinted with permission from Ref.^[64]. Copyright 2023, American Chemical Society. SAC: Single-atom catalysts; APCVD: atmospheric chemical vapor deposition.

Hu et al. used hexagonal boron nitride (h-BN) layer encapsulation technology to epitaxial grow to-dimensional Bi nanosheets on copper foil by atmospheric chemical vapor deposition (APCVD)^[64]. First, principle calculations and experiments show that an intermediate layer between h-BN and copper foil can be successfully fabricated using 2D Bi nanosheets. Detailed characterization confirmed that the fabricated Bi nanosheets have typical hexagonal single-crystal characteristics and do not contain any detectable impurities [Figure 5E and F]. The additional charge supply of the h-BN layer can effectively suppress the structural distortion of bismuth on copper foil, which is beneficial to the epitaxial growth of hexagonal Bi nanosheets by APCVD method to a large extent. The synthesized Bi nanosheets have single-crystal characteristics and large lateral size. In addition, under the protection of the h-BN top layer, the Bi nanosheet showed excellent thermal stability, which was not oxidized even after annealing at 500 °C at room temperature, maintaining excellent stability. Finally, the electrocatalytic CO₂RR experiment confirmed that the Bi nanosheets involved in the CO₂RR process before the Cu region was not found had good performance in reducing CO₂ to formic acid, and had 96.3% ultra-high FE at -1.0 V_{vs RHE} and -1.0 V_{vs RHE} for a long time [Figure 5G]. During this process, Bi nanosheets are reconstructed and more active sites are generated. In Bi-based electrocatalyst, Bi nanosheets also maintains great long-term catalytic stability and excellent catalytic performance. DFT calculations show the charge transfer from the h-BN layer to the bismuth could compensate for the electron consumption and lead to a more stable structure. And the top h-BN layer is conducive to the formation and stability of 2D bismuth-ene. The sandwich epitaxial growth strategy provides favorable pathways for high surface energy materials in the range of h-BN encapsulation. In addition, the top layer can be replaced with another 2D material, thus expanding the fabrication range of 2D heterogeneous structures. As mentioned above, although 2D Bi-based nanomaterials series have effective CO₂RR catalytic activity, their catalytic performance still needs to be further improved. In particular, how 2D materials adjust the electronic structure and electron conduction path of the edge part and the middle part of the nanosheet is still worthy of further optimization to improve their catalytic performance of CO₂RR.

Heterostructure strategy

By combining with multifunctional structures, heterogeneous structure engineering is an important approach to modulate reaction pathways in numerous catalytic processes. Coupling heterogeneous structure interfaces to achieve synergistic catalytic effects is crucial to regulating electron transfer, geometry, and interface coordination environment, thereby lowering the energy barrier of key reaction intermediates.

Feng et al. prepared a new sheet Bi₂O₃/strip BiO₂ heterojunction by top-down selective alkali-assisted dealuminization method using Bi₂Al₄O₉ [Figure 6A], and confirmed the existence of BiO₂ phase by X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES) and other test methods^[65]. The Bi₂O₃/BiO₂ catalyst has the correct starting potential and high current density in the catalytic process, and the Tafel slope is also small, which indicates that the heterogeneous structure can reduce the reaction barrier of CO₂RR and benefit the response of CO₂RR [Figure 6B]. In situ tests showed that the Bi₂O₃ in the Bi₂O₃/BiO₂ heterojunction was completely reduced to metallic Bi under CO₂RR conditions [Figure 6C], and the catalysts have reconstructed from Bi₂O₃/BiO₂ to the new heterostructure Bi/BiO₂, thus continuing to act as an efficient catalytic active center. For the Bi/BiO₂ heterojunction, HER overpotential is 1.08 V, while the overpotential of CO₂RR to formate via the *OCHO pathway is only 0.19 V, and the overpotential of CO via the *COOH pathway is only 1.10 V_{vs RHE} [Figure 6D]. That is, among the three competing cathodic processes, the reduction of CO₂ formation of formic acid is the most favorable in terms of energy and can effectively inhibit the HER process.

Figure 6. (A) Schematic synthesis of the Bi₂O₃/BiO₂ heterojunction; (B) XRD patterns of the Bi₂O₃/BiO₂ heterojunction. Comparison of TOF, electrochemical testing plots; (C) In situ Raman spectra of the Bi₂O₃/BiO₂ heterojunction during electrocatalysis; (D) DOS of the Bi p orbital with different intermediations adsorbed. Reprinted with permission from Ref.^[65]. Copyright 2022, American Chemical Society; (E) HRTEM images of SnO₂/Bi₂O₃; (F, G) XPS spectra of SnO₂, Bi₂O₃, SnO₂/Bi₂O₃. Reprinted with permission from Ref.^[66]. Copyright 2021, Wiley-VCH. XRD: X-ray diffraction spectroscopy.

Tian et al. synthesized SnO₂/Bi₂O₃ heterostructures with SnO₂ clusters on Bi₂O₃ matrix^[66]. Due to their different work functions, once they come into contact in the form of a heterojunction, a built-in potential is formed at the interface, equalization of their Fermi levels [Figure 6E]. During the catalytic reaction, interfacial electron transfer was observed between SnO₂ and Bi₂O₃, making SnO₂ at the electron-rich stage. The firm interfacial interaction protects the active site of SnO₂ from electrolytic reduction and achieves long-term electrochemical stability [Figure 6F and G]. This interfacial electronic effect significantly improves the CO₂ adsorption performance and the *CO₂ intermediate stability at high potentials. She is effectively inhibited at high potentials. DFT calculations show that the Bi₂O₃ support can improve the catalytic activity of CO₂ to HCOOH conversion. In particular, the synthesized SnO₂/Bi₂O₃ has a FE of 90% for C₁ products, including HCOOH and CO, at 1.0 V/RHE. FE of HCOOH has a wide potential range of from 1.0 to 1.4 V_{vs RHE}, and its potential remains above 76%.

Defects strategy

The importing of structural perturbations or defects can effectively affect the local electronic state and produce discordant sites with particularly high activity. Although the certain mechanism is dimness, defects strategy has been commonly used in various electrocatalysts^[65,66].The defective Bi oxide was designed as a template for cathodic conversion into defect-rich metallic Bi nanostructures for electrocatalysis of CO₂RR. Gong et al. developed a simple solution method to synthesize defective β-Bi₂O₃ double-walled NT^[67]. Operational X-ray absorption spectroscopy (XAS) measurement displays that these oxide NT would be turned into defective Bi NT under cathode-polarization. When it was applied for CO₂RR, defective Bi NT enables highly reactive and selective formic acid production. DFT studies have shown that this robust activity and selectivity can be on account of numerous defective Bi sites in the stable *OCHO intermediate.

Controlled morphology strategy

Gong et al. introduced poly (vinyl pyrrolidone) (PVP) as a structure guiding agent and prepared defective β-Bi₂O₃ double-wall NT by the controlled aqueous solution method^[67]. The double-wall structure of the material and the surface coverage of highly defective fragments also provide a template for the cathodes to be converted into defective Bi metal nanostructures. Operational XAS measurements show that oxide NT can be converted into defective bismuth NT under cathode-polarization. DFT calculations also show that the presence of defects, whether 5-7 ring defects, single vacancies or double vacancies, facilitates the adsorption of stable *OCHO intermediates on the ideal Bi (001) surface, thereby facilitating the formation of formic acid. When applied for CO₂RR, it enables highly reactive and selective formic acid production.

Yang et al. studied the catalytic mechanism of Bi₂O₃^[68]. They investigated the in situ structural reconstruction process from Bi@Bi₂O₃ nanodendrites (Bi@Bi₂O₃-NDs) to Bi nanoflowers (Bi-NFs) composed of ultra-thin 2D Bi nanosheets [Figure 7A]. Bi@Bi₂O₃-NDs grew on Cu foil. Besides, the activated Bi-NFs consisting of 3D self-assembled ultra-thin Bi nanosheets were finally formed by the two-step transformation of Bi@Bi₂O₃-NDs in KHCO₃ solution with bismuth oxycarbonate (Bi₂O₂CO₃, BOC) as an intermediate through in situ electroreduction. The extensive surface remodeling of Bi@Bi₂O₃NDs enables customized Bi-NFs electrocatalyst. Thus, Bi-NF has a FE of 92.3% at -0.9 V_{vs RHE} and a high local current density of 28.5 mA·cm^-2 at -1.05 V. In addition, the potential interface-dependent processes and mechanisms of Bi-NFs electrocatalysts in CO₂RR were investigated using in situ Raman spectroscopy. It is supposed that *OCHO is crucial in the formation of HCOOH, and that the surface remodeling and activation process of the active site of Bi-NFs is crucial to achieve the ultra-high selectivity of HCOOH [Figure 7B].

Figure 7. (A) Schematic illustration of the preparation process of Bi-NFs in situ synthesis on Cu foil; (B) In situ Raman spectra characterization derived from Bi-NFs. Reprinted with permission from Ref.^[68]. Copyright 2023, Wiley-VCH; (C) SEM characterization of the Bi₂O₃; (D) FE formate plots as a function of different deposition times. Reprinted with permission from Ref.^[69]. Copyright 2019, Wiley-VCH; (E) Illustration of the formation process of the Bi₂O₃NSs@MCCM; (F) j_(HCOOH) of the Bi₂O₃NSs@MCCM and other comparison samples. Reprinted with permission from Ref.^[70]. Copyright 2019, Wiley-VCH. FE: Faraday efficiency; Bi-NFs: Bi nanoflowers.

Tran-Phu et al. proposed an effective strategy to produce a fractal Bi₂O₃ catalyst layer for CO₂RR to formate by one-step magnification on a porous fiber substrate [Figure 7C]^[69]. This approach has several advantages, including the possibility to increase the electron density by tuning the physicochemical properties of Bi₂O₃. Furthermore, the fractal morphology of thermos-aerosol self-assembled Bi₂O₃ provides an open and homogeneous morphology, enabling efficient utilization of the deposited catalyst with a mass and current density three times higher than Bi₂O₃ collected when the filter cast CO₂RR nanoparticles on the same substrate. With richer electrons, Bi₂O₃ comprising metastable β phase Bi₂O₃ and abundant edge sites is highly selective for CO₂RR formation and exhibits 87% FE of HCOO^- at -1.2 V_{vs RHE}, with an HCOO^- current density of about -20.9 mA·cm^-2 [Figure 7D].

Liu et al. successfully grew ultra-thin Bi₂O₃ nanosheets on conductive multi-channel carbon matrix (Bi₂O₃NSs@MCCM) by a simple hydrothermal method [Figure 7E]^[70]. This composite has several important advantages. Specifically, the nanostructured hollow mccm accelerated electron transfer, increased CO₂ adsorption, and increased the ratio of pyrrole-N to pyridine-N. Layered porous Bi₂O₃ nanosheets provide abundant active sites, reduce contact resistance and work function, and shorten the diffusion path of CO₂ saturated electrolyte. Importantly, Bi₂O₃NSs@MCCM can achieve relatively high current densities at moderate overpotential, allowing the conversion of CO₂RR to HCCOH to be selective, highly energy efficient, and with good long-term stability [Figure 7F]. As mentioned above, as a commonly used modification method, morphology control can not only play a crucial role in adjusting the catalytic site and catalytic activity, but also provide ideas for structural reconstruction. However, the choice of specific materials still needs to be made according to the material characteristics.

Bi-based core-shell electrocatalysts

On account of relatively low conductivity and selectivity, Bi-based oxides tend to exhibit poor activity at low and wide cathodic potential Windows. A feasible strategy to improve conductivity is to synthesize core-shell structured nanocatalysts with highly conductive metal cores and lamina metal oxide shells, which may optimize electronic properties and intermediate bond strengths^[71,72]. To further improve the activity of electrocatalysts, the design of novel Bi-based materials in combination with other components has become an effective strategy to regulate the adsorption, activation and desorption of reactants. Zhao et al. prepared a novel Sn-doped Bi/BiO_x NW with a highly conductive bimetallic core and an amorphous Sn-doped BiO_x shell by electrochemical dealloying [Figure 8A]^[73]. Sn atoms in the Bi₂O₃ and BiOx lattices are supposed to cooperate with oxygen atoms in an octahedral form. Bi/Bi(Sn)O_x NWs have a metallic binuclear structure [Figure 8B-D]. Compared with reversible hydrogen electrodes, these NWs showed excellent CO₂RR activity in a wide potential window, selectively generated formate, and achieved FE in the range of -0.5 to -0.9 V (vs. RHE). The maximum FE at -0.7 V_{vs RHE} gas diffusion cell is 98 ± 2% [Figure 8E]. DFT calculations reveal that the Bi/Bi(Sn)O_x catalyst showed a lower energy barrier (ΔG) for *OCHO formation, suggesting that the Sn-doped BiO_x shell can greatly promote *OCHO formation. The incorporation of Sn atoms into BiO_x modulates the electronic properties of Bi and allows the adsorption of *OCHO intermediates on the Bi(Sn)O_x surface, while increasing formic acid production by inhibiting the competing HER [Figure 8F]. In summary, Table 2 compares the best FE of bismuth oxide-based catalysts. It can be seen that the listed materials have good FE of bismuth oxide compounds and formic acid through different modification strategies, so the above modification strategies have good effects on bismuth oxide.

Figure 8. (A) Schematic diagram of the synthesis of Sn-doped Bi/BiO_x nanowires; (B-D) HAADF-STEM characterization diagrams; (E) Different products FEs of the Bi/Bi(Sn)O_xNWs; (F) P-orbital DOS plots of absorbed with *OCHO on Sn-decorated Bi/BiO_x nanowires and Bi/BiO_x nanowires. Reprinted with permission from Ref.^[73]. Copyright 2021, American Chemical Society. FE: Faraday efficiency.

Bismuth subcarbonate-based catalysts

Among Bi-based compounds, BOC is considered a valuable electrocatalyst for CO₂RR to formate on account of its significant advantages: (1) A large number of Bi-O bonds in BOC can promote the adsorption of CO₂ and production and transformation of intermediates during the CO₂RR process; (2) BOC species of carbonate enable to enhance the adsorption of initial intermediates^[74-76]. Therefore, it is attractive to ulteriorly develop electrocatalysts based on the advantages of BOC for efficient generation of CO₂RR within a wider potential window.

Fu et al. reported a method for electrochemical conversion of bismuth oxychloride (BiOCl) to basic bismuth carbonate containing chlorine [Bi₂O₂(CO₃)_xCl_y] under the condition of CO₂RR^[77]. It is proved that in situ synthesis is an effective method to improve the electrochemical stability of electrocatalyst in situ. In situ spectroscopic studies were systematically performed to describe the conversion mechanism and electrochemical stability. BiOCl is converted to BOC by anion exchange [Figure 9A]. The fabricated Bi₂O₂(CO₃)_xCl_y tolerates -1.0 V (relative to RHE), while the wet chemically synthesized pure BOC undergoes metal Bi⁰ formation at -0.6 V_{vs RHE}, implying a significant increase in electrochemical stability. No significant structural changes and performance degradation were observed when Bi₂O₂(CO₃)_xCl_y was stabilized at a voltage of -0.8 V relative to RHE for 20 h [Figure 9B]. At a voltage of -0.8 V_{vs RHE}, Bi₂O₂(CO₃)_xCl_y can easily achieve 97.9% FEHCOO^-, which is much higher than BOC (81.3%). DFT calculations show that, unlike the generation of HCOOH from CO₂RR catalyzed by BOC, the generation of HCOOH on Bi₂O₂(CO₃)_xCl_y through the *COOH intermediate step requires an energy input of 2.56 eV.

Figure 9. (A) 3D configuration of bismuth carbonate containing chlorine; (B) Plots of Faradic efficiency for CO₂RR at different potentials (vs. RHE). Reprinted with permission from Ref.^[77]. Copyright 2022, Springer; (C) Surface characterization of the BOC-In-0.1 NSs; (D) Projected density of states (PDOS) of d-band centers of Bi atoms, (E) the Bi site absorbed with *OCOH and (F) calculated free energy diagrams for the electroreduction of CO₂ to HCOOH on the (001) facets of BOC and BOC-In. from Ref.^[78]. Copyright 2022, Elsevier.

Wu et al. prepared In-doped BOC nanosheets (BOC-In-X NSs) via electrochemical operando deposition and electrochemical reduction [Figure 9C]^[78]. At a voltage of 0.9 V, the FE of formate of BOC-In-0.1NS is the highest with 98.3%, and it has good stability for 22 h. In addition, FE formate achieves an average of 93.5%. DFT calculations show that compared with BOC (-1.801 eV), the E_d of BoC-In (-1.780 eV) is closer to the Fermi level (E_f), and the E_p of BOC-In (-1.492 eV) moves more closely to the E_f, which illustrates that BOC-In is more conducive to the occurrence of catalytic reactions [Figure 9D and E]. The energy barrier on the BOC NS (010) surface (0.55 eV) is much higher than that of In-doped BOC NS (0.48 eV) [Figure 9F]. And the addition of BOC not only enhances the adsorption of CO₂, *CO₂- and *OCOH intermediates, but also reduces the energy barrier of *OCOH to form formic acid, which is conducive to the formation of formic acid.

Bi sulfide-based catalysts

Sulfur atom doping can activate bismuth-specific sites while passivating edge sites, thus promoting the production of highly selective and reactive formic acids, providing high catalytic activity to CO₂RR. From a thermodynamic point of view, sulfur atoms prefer edge sites of metal catalysts, and the addition of sulfur changes the electronic structure of adjacent sites, hence increasing the catalytic activity of metals. In addition, the binding of S to the metal edge site occupies the adsorption site of *H, thus inhibiting the main competitive reaction of HER. Therefore, the addition of S enables selective improvement of the performance of CO₂RR.

Lv et al. designed Bi nanosheets with numerous edge defect sites coordinated with sulfur atoms by electrochemical reconstruction of Bi₁₉Br₃S₂₇ NW (BBS) [Figure 10A]^[79]. The marginal sulfur bismuth catalyst increased the yield of formate and inhibited the precipitation of hydrogen. During the structural transition, the BBS precatalyst is reconstructed into the metal Bi with abundant defects, and the Br atom escapes completely as HBr. In situ EXAFS characterization shows that Bi atoms are reconstructed during the catalytic process, which has a significant effect on catalysis [Figure 10B]. DFT and characterizations display sulfur atoms tend to appear in the margin region of the defect, reducing the density of coordinated unsaturated Bi sites for *H adsorption, while adjusting the adjacent Bi sites in the center of the p-band to obtain better CO₂RR performance [Figure 10C]. Under alkaline conditions, formic acid exhibits high FE (~95%) [Figure 10D].

Figure 10. (A) Schematic illustration of the transition from BBS to Bi nanosheet; (B) In situ FT k₃χ(R) EXAFS of Bi L₃-edge EXAFS signals; (C) Calculated adsorption energy plot for the electroreduction of CO₂ to HCOOH through different intermediations on sample catalysts; (D) FE of formate of BBS sample and Bi sample. Reprinted with permission from Ref.^[79]. Copyright 2023, Wiley-VCH. EXAFS: Extended X-ray adsorption fine structure; FE: faraday efficiency; BBS: Bi₁₉Br₃S₂₇ NW.

Wang et al. developed an atom-dispersed N and S-coordinated bismuth-atom site catalyst (Bi-SAs-NS/C) for electrocatalytic CO₂ reduction through simultaneous cation and anion diffusion strategies^[80]. Bi is then trapped by abundant nitrogen-rich vacancies, and S binds to the carbon support at high temperatures to form the Bi coordination of N and S. On account of the simultaneous diffusion of Bi atoms and sulfur, nitrogen atoms with different electronegativity, the catalysts can effectively cooperate with Bi to form homogeneous Bi-N₃S /C sites. The prepared Bi-SAs-NS/C showed high selectivity for CO, with a FE of over 88% over a broad potential range and a maximum of 98.3% FE of CO at a current density of 10.24 mA·cm^-2 at -0.8 V_{vs RHE}. Within 24 h, the continuous electrolysis can be kept constant with negligible degradation. DFT calculations show that the catalytic performance of Bi-SAs-NS/C is significantly better than that of Bi-SAs-N/C, due to the substitution of the ligand N in the center of Bi-N₄C by the less electroactive S, which greatly reduces the energy barrier formed by the intermediate in the rate-limiting step and improves the reaction kinetics.

Bi halide-based catalysts

A number of recent studies have revealed possible effects of halides on CO₂RR activity. Surface modification of halides can be directly introduced into electrolyte or cathodic conversion of metal halides. They can affect the charge distribution on the surface of the metal catalyst and thus regulate the relative binding strength of key intermediates.

Luo et al. designed bismuth halide perovskite as a precatalyst according to the characteristics of halide anions and alkali metal cations, and used the metal-air battery cathode to prepare bismuth co-modified by halide and alkali metal as a working catalyst^[81]. They used a modified heat injection method to prepare hexagonal Cs₃Bi₂I₉ nanocrystals and introduced long-chain ammonium oil palm amI-I (OAMI-I) to slow down the nucleation and growth of perovskite nanocrystals [Figure 11A]. The results show that the modified Cs₃Bi₂I₉ catalyst provides an industrial-scale current density of 300 mA·cm^-2 and an 87% FE for formic acid in the flow cell [Figure 11B]. Moreover, after assembly in Al-CO₂ cells, Cs₃ Bi₂ I₉ exhibits a very high current density (69 mA·cm^-2) with a peak power density of about 7 mW·cm^-2.

Figure 11. (A) Structural model diagram of Cs₃Bi₂I₉; (B) Electrochemical properties of different products. Reprinted with permission from Ref.^[81]. Copyright 2023, Wiley-VCH; (C) AFM image of BiOX sample; (D) In-BOB, measured at -1.15 V_{vs RHE} and plotted as normalized intensity versus energy. Reprinted with permission from Ref.^[82]. Copyright 2023, Springer; (E) Schematic diagram of the preparation of bismuth halide and reaction process to Bi NSs; (F) Electrochemical property testing of sample. Reprinted with permission from Ref.^[83]. Copyright 2022, Elsevier. BOB: BiOBr.

Yang et al. synthesized layered oriented double halide oxide (BiOX, where X = Cl, Br or I) nanosheets [Figure 11C]^[82]. Operando XRD and Operando Raman spectroscopy measurements were performed to track the kinetics of the in situ conversion of the BiOX catalyst material to the active metallic Bi electrocatalyst [Figure 11D]. It was supposed that in situ activated dielectric catalysts expose specific crystal planes, depending on the choice of halide: Br^-promotes Bi (003) exposure, Cl^- leads to dominant Bi (012) crystal planes, and I^- produces mixtures. In addition, we relate surface exposure to catalytic properties. Compared with reversible hydrogen electrodes, BiOBr (BOB) showed HCOOH selectivity of up to 91% at a current density of 148 mA·cm^-2 at -1.05 V_{vs RHE}. BiOCl exhibits a HCOOH FE of 69% at a current density of 88 mA·cm^-2 and a potential of -1.09 V. BiOI (BOI) showed 76% selectivity for HCOOH at 95 mA·cm^-2 current density, while RHE was between BOB and BiOCl at -1.08 V current density. This suggests that the Bi (003) base surface formed in situ is more catalytic than the stepped Bi (012) site. Furthermore, in situ XAS measurements indicate that BiOCl restructures rapidly after the onset of reduction and exhibits transient transition states, while the transition states of BOB and BOI are longer lasting.

Wang et al. used bismuth halide (Bi₄O₅Cl₂, Bi₄O₅Br₂ and BiOI) as a precatalyst to study the effect of halogen on the in situ conversion of BiOI in CO₂RR [Figure 11E]^[83]. Among them, BiOI in situ transformed Bi NSs showed high CO₂RR selectivity. The FE of formate (vs. RHE) is greater than 90% in the range of -0.71 to -1.0 V (all potentials of RHE are as follows). FE formate was highest at -1.0 V, which was 91.33% ± 0.58%. The in situ FE formate of both Bi₄O₅Br₂ and Bi₄O₅Cl₂ was below 86% at the selected application potential [Figure 11F]. The in situ transformed bismuth oxide halide was analyzed by XPS and only type I residue was detected. In summary, Table 3 compares the optimal FE of other types of Bi-based materials, and it can be seen that by passing carbonate, bismuth sulfur-based catalyst pairs well with bismuth halide compounds, especially for formate formation.

In situ/operando raman spectroscopy

Raman spectroscopy can determine the vibrational modes of molecules and identify intermediate species. Molecules with different vibrational/rotational states can be detected by measuring the energy difference between the incident and inelastically scattered photons. Raman spectroscopy has a great advantage in identifying metastable catalysts and intermediates^[37].

Li et al. used in situ Raman to demonstrate that the prepared 2D Bi₂O₃ nanosheets were used as precursor templates to directly synthesize well-defined Bi nanoribbons^[84]. The activated Bi-O sites in high temperature treatment remained stable throughout the CO₂RR process. As a result, the resulting one-dimensional Bi heterostructures exhibit excellent CO₂RR properties with high formic FE over a wide and mild potential range (approximately 95%), as well as the impressive stability of 100 h of continuous operation under a variety of conditions without attenuation [Figure 12A]. Zhao et al. used the transfer of Raman peaks in situ Raman tests at different potentials to show that the Bi-O structure in BOC after reaction is more likely to be restored^[85]. Combined with the above in situ study under CO₂RR, we obtained a detailed hypothesis of the evolution of BOS to BOC after reaction structure; that is, BOS was first completely transformed into BOC, and then part of BOC was cathodically reduced to metallic Bi under CO₂RR conditions. To demonstrate the stability of the Bi-O substance in surface-oxygen-rich carbon-nanorod-supported bismuth nanoparticles (SOR Bi@C NP), Liu et al. performed in situ Raman spectroscopy to understand how the catalyst surface structure changes with the applied reduction potential^[86]. They observed the appearance and disappearance of Bi-O characteristic bands at different potentials, demonstrating the stability of the Bi-O substance. These ectopic and in situ characterization techniques indicate that oxygen-enriched SOR Bi@C NP is structurally stable during CO₂RR. The formation of *OCHO intermediates on Bi-O/Bi (110) is considered to be the rate-determining step, and the Bi-O/Bi (110) structure is the root of the excellent CO₂RR performance in SOR Bi@C NP. Liu et al. explored the surface transition mechanism of BOC@Bi/CP (carbon paper) electrodes by in situ Raman spectroscopy^[87]. The reduction of BOC on the surface to Bi by Raman spectroscopy proves that flower-like Bi nanosheets can provide sufficient reaction sites to regenerate bismuth oxide (BO) on the surface in a CO₂-rich environment during continuous reduction. Moreover, even under the relative negative potential (~0.90 V vs. RHE) during the polyphase electrocatalysis, the composite does not reduce^[88].

Figure 12. (A) In situ Raman spectra of two-dimensional Bi nanosheets. Reprinted with permission from Ref.^[84]. Copyright 2022, American Chemical Society; (B) In situ DRIFTS spectra of CO₂ adsorbed on BOC NFs surface. Reprinted with permission from Ref.^[89]. Copyright 2022, Wiley-VCH; (C) In situ XRD patterns of Bi catalysts. Reprinted with permission from Ref.^[91]. Copyright 2021, Elsevier; (D) Operando Bi L-edge XANES spectra of Bi₂O₃ nanotubes (NTs) at OCV and NTD-Bi; inset plot is the partially enlarged spectra. Reprinted with permission from Ref.^[67]. Copyright 2019, Springer; (E) In situ XPS measurements for O 1 s orbit. Reprinted with permission from Ref.^[92]. Copyright 2022, Elsevier; (F, G) Formation of Co-TMH clusters and nanocage around the nanobubble surface. Reprinted with permission from Ref.^[94]. Copyright 2022, Elsevier. XRD: X-ray diffraction spectroscopy; XANES: X-ray absorption near edge structure; OCV: open circuit voltage.

In situ/operando infrared spectrum

In situ FTIRs is a combination of electrochemical measurement methods and infrared spectroscopy technology, real-time monitoring of the catalytic reaction occurring at the gas-liquid-solid three-phase interface, at the molecular level to obtain reactants, target products, electrode surface bonding, intermediates and other information.

Cao et al. pioneered an atomically thin bismuthene (Bi-ene) through an in situ electrochemical transformation process of an ultra-thin bismuth group organic layer^[50]. In situ attenuated total reflection infrared (ATR-IR) and DFT results confirmed that the adsorbed HCO₃^- groups in the electrolyte played an important role in the CO₂RR process. In situ infrared spectroscopy can also be used to further analyze the catalytic local reaction microenvironment and the adsorption capacity of CO₂ in the three-phase interface, so as to further understand the gas adsorption process of the reaction. According to the spectral band analysis [Figure 12B], BOC NF (bismuth subcarbonate nanoflowers) and Bi have high physical adsorption capacity, but Bi has almost no chemical adsorption capacity. Compared with Bi₂O3, BOC NF also has a high adsorption capacity for carbon dioxide^[89].

In order to better understand the role of Te and Bi species in CO₂RR, Cui et al. used in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) to monitor adsorbed species on the electrochemical surface^[90]. The experiments show that the consumption rate of the reaction intermediate increases, which leads to a faster conversion to HCOOH, indicating that the Te-doped Bi catalyst can effectively accelerate the reaction. The peak intensity indicated that the intermediate accumulates at the interface, which may be related to the slower kinetics of the HCOOH reaction.

Other in situ/operando characterizations

Many other in situ testing techniques are gradually being applied to CO₂RR. Li et al. used in situ XRD analysis to reveal the structural evolution of Bi catalyst during CO₂RR processes [Figure 12C]^[91]. For the Bi catalyst under open circuit voltage (OCV) condition, the phase transition from Bi₂O₃ to metallic Bi was proved by the change in the position of the derived peak during the process, and the phase transition was completed within one hour after the reduction potential was applied. Gong et al. performed in situ XAS measurements at the Bi L edge^[67]. The Bi edges of XANES under OCV conditions are well aligned with Bi₂O₃ reference Bi edges [Figure 12D]. When the electric potential is reduced to -0.24 V, the feature of the Bi (III) edge at 13.423 keV gradually changes to a lower energy of 13.417 keV, which is aligned with the XANES edge of Bi metal foil. Zhang et al. used in situ XPS analysis of Bi4f spectra to reveal that the degree of Bi oxidation of BiCu with varying Bi layer thicknesses differed during exposure to CO₂ [Figure 12E]^[92]. This indicates coverage dependence of Bi oxidation at the Bi-Cu interface with exposure to CO₂. Since oxidation of Bi is caused by activation and dissociation of CO₂ on BiCu (111), dissociation of CO₂ and oxidation of Bi correspondingly become difficult or energy intensive as Bi coverage on Cu (111) increases, which, in turn, requires further enhanced annealing. Using quasi in situ XPS analysis, Jiang et al. showed that nanoclusters maintain their oxidation state (+3) during CO₂RR and BiO clusters are much more resistant to reduction under CO₂RR compared to bulk Bi oxide, where all Bi atoms are present on the surface and in low-coordinated clusters^[93]. In the surface studies, in situ electron microscopy is often used to monitor transient structures and measure the reaction kinetics of nanostructure transformation. In-situ transmission electron microscopy (TEM) can provide vital information about the reactions on modified surfaces as well as the catalytic reactions and the growth of thin films or nanocrystals. Experiments involve exposing the sample to the reaction environment while imaging under conditions sensitive to changes in the sample surface rather than inside the sample. The reaction environment can include high temperature, reaction gas flow, deposition flow, or even exposure to a liquid environment. Therefore, in situ TEM also has applications in the field of CO₂RR. Through in situ TEM analysis and diffusion theory derivation, Xiao et al. found that the diffusion rate of the reaction system is positively correlated with the temperature, and negatively correlated with the system viscosity, and the system viscosity is negatively correlated with the temperature^[94]. Porous MOF derivatives were synthesized ex situ at -80 °C [Figure 12F and G]. However, at room temperature, when the diffusion coefficient of the system is large, hollow MOF derivatives composed of nanosheets are formed. However, the porous derivatives have more surface unsaturated sites and reactive active sites. In order to understand the important role of the structure of the molecular catalyst in the CO₂ reduction process at the gas-liquid interface of the microdroplet. A series of mass peaks associated with Cu (1)-containing catalysts were observed by in situ mass spectrometry^[95]. The high abundance of the monovalent Cu species confirms its potential role as an electron reservoir/transfer agent within the microdroplet, which facilitates the subsequent chemical transformation.