Structural reconstruction of BiPbO₂Br nanosheets for electrochemical CO₂ reduction to formate

Abstract

The electrochemical reduction of CO₂ to value-added products represents a promising strategy to store renewable energy and realize global carbon neutrality. Bi-based materials have attracted extensive attention for formate production from CO₂ reduction. Herein, we employ BiPbO₂Br nanosheets as the bimetallic oxyhalide precursor to prepare a reconstructed Bi–Pb composite catalyst by electroreduction. The resultant catalyst exhibits impressive performance for formate production, including a high partial current density (40 mA cm⁻² at −1.0 V), a high faradaic efficiency (96.6% at −0.9 V), a high energy conversion efficiency (>50% from −0.8 to −1.1 V), and good stability. As compared with the monometallic Bi and Pb counterparts, the Bi–Pb composite catalyst demonstrates superior intrinsic performance, which is attributed to the multiphase composites and abundant heterogeneous interfaces derived from electroreduction induced structural reconstruction of BiPbO₂Br. This work offers an attractive strategy to further improve the performance of Bi catalysts for CO₂ reduction to formate.

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1. Introduction

The worldwide energy demand is largely covered by fossil fuels, which brings about massive carbon dioxide (CO₂) emission in the atmosphere, resulting in the ever-growing global warming and environmental load.^1–3 The electrochemical CO₂ reduction reaction (CO₂RR) using sustainable energies provides a promising strategy for not only recycling CO₂ into valuable chemicals or liquid fuels, but also storing electricity from renewable energy sources such as solar and wind.^4,5 Over the past decade, it has been demonstrated that the CO₂RR can undergo multiple proton-coupled electron transfer processes, leading to the formation of diversified products such as carbon monoxide (CO), formic acid (HCOOH), methane (CH₄), ethylene (C₂H₄), methanol (CH₃OH), ethanol (C₂H₅OH), and so on.^6–9 Among them, regarding the energy input and market price of the product, the CO₂RR to formic acid (or formate) is more economically profitable.^10–12 Besides, formate is widely used in the chemical industry as a raw material, as well as energy sectors as a liquid hydrogen storage carrier or a high energy density fuel.^13–15 Given these, the scientific community has put a lot of efforts to explore efficient electrocatalysts for selective formate production via the CO₂RR.

So far, numerous metallic catalysts, such as Pd, Bi, In, Pb, and Sn, have shown the capability to electrochemically convert CO₂ to formate.^16–18 As per the reported results, Bi-based catalysts stand out due to the weak affinity to key *OCHO intermediates and poor activity for the undesired hydrogen evolution reaction (HER), enabling high faradaic efficiency (FE) for formate production.^19–22 To meet the requirements of practical application, however, it is still imperative to explore effective strategies to optimize the performance of Bi electrocatalysts, that is, achieving large formate partial current density (j_formate) at low overpotential. Morphology engineering has been proved to be one of the effective means, which can be achieved using nanostructured Bi-based compounds (such as oxides, sulfides, and oxyhalides) through in situ electroreduction.^23–31 Beyond this, the introduction of another element to form a Bi-containing binary component may create multiphase composites and heterogeneous interfaces during the electroreduction induced structural reconstruction, which is expected to regulate the active sites and optimize the adsorption and stabilization of the reaction intermediates. As a result, recent studies on binary composite catalysts, such as Bi–Cu, Bi–Sn, Bi–Ag, Bi–Ce, and so on,^32–38 have demonstrated favorable formate production. Besides, Pb doped Zn or Cu can also facilitate the CO₂RR to formate with impressive performance.^39,40 Nevertheless, the combination of Bi and Pb has been rarely reported for the CO₂RR.

Bearing these aspects in mind, therefore, we herein employ BiPbO₂Br nanosheets (NSs) as the bimetallic oxyhalide precursor to prepare a reconstructed Bi–Pb composite catalyst via electroreduction. When evaluated in an H-type reaction cell, the resultant catalyst exhibits high activity and selectivity towards formate production. The j_formate reaches up to 40 mA cm⁻² at −1.0 V versus the reversible hydrogen electrode (RHE) with an FE of 93.1%. The catalyst also achieves impressive energy efficiency (EE) over a wide potential window and good long-term stability. Compared with monometallic Bi and Pb counterparts, the Bi–Pb catalyst demonstrates superior intrinsic performance for the CO₂RR to formate, which is associated with the abundant heterogeneous interfaces created by electroreduction induced structural reconstruction.

2. Experimental section

2.1. Chemicals and materials

All the chemical reagents, including bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O, Aladdin, AR, ≥99%), lead acetate trihydrate (C₂H₄O₃Pb·3H₂O, Sinopharm, AR, ≥99.5%), hexadecyl trimethyl ammonium bromide (CTAB, Macklin, ≥99%), ethanol (Macklin, AR, ≥99.7%), sodium bromide (NaBr, Macklin, AR, ≥99%), mannitol (Aladdin, HPLC, ≥99%), ammonium hydroxide solution (NH₃·H₂O, Macklin, ≥28% NH₃ in H₂O), sodium carbonate (Na₂CO₃, Aladdin, 99.99%), sodium formate (Aladdin, 99.99%), deuterium oxide (D₂O, Macklin, 99.9% D), dimethyl sulfoxide (DMSO, Aladdin, ≥99.9%), Nafion proton exchange membrane (N117, Dupont), Nafion solution (5 wt% in ethanol, Dupont), Vulcan XC-72 C (Cabot), carbon paper (TORAY), CO₂ gas (99.999%) and Ar gas (99.99%), were used as purchased without further purification. Deionized water was used in all experiments with a specific resistance of 18.2 MΩ cm.

2.2. Synthesis of BiPbO₂Br NSs

The BiPbO₂Br NSs were synthesized following a previously reported method.⁴¹ Briefly, 0.5 mmol of CTAB was added into 20 mL of ethanol containing Bi(NO₃)₃·5H₂O (0.5 mmol) and C₂H₄O₃Pb·3H₂O (0.5 mmol), which was continuously stirred until complete dissolution. After adding 5 ml of NH₃·H₂O and stirring for 1 h, the mixture solution was transferred into a 50 ml Teflon-lined stainless autoclave and heated at 190 °C for 12 h. Followed by cooling to room temperature, light yellow precipitates were collected by filtration and washing with water and ethanol and freeze-dried to obtain BiPbO₂Br NSs.

2.3. Synthesis of BiOBr NSs

In a typical synthesis of BiOBr NSs,⁴² 1.5 mmol of Bi(NO₃)₃·5H₂O was dissolved in 30 ml of 0.1 M mannitol solution with stirring to obtain a transparent solution. After that, 1.5 mmol of NaBr was added into the above solution. The mixture solution was stirred for 0.5 h and then transferred into a 50 ml Teflon-lined stainless autoclave and heated at 160 °C for 3 h. After cooling to room temperature, white precipitates were collected by filtration and washing with water and ethanol and freeze-dried to obtain BiOBr NSs.

2.4. Synthesis of PbO_xBr_y

The PbO_xBr_y was synthesized using a similar procedure to that of BiPbO₂Br without the addition of Bi(NO₃)₃·5H₂O.

2.5. Characterization

The crystalline structure of the samples was analyzed by X-ray diffraction (XRD) using an ARL-X’TRA diffractometer with Cu Kα radiation (40 kV and 35 mA). Field emission scanning electron microscopy (FE-SEM) was performed to detect the morphology of the samples by using JSM-5900. Transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDS) were performed with an FEI Talos F200X G2 electron microscope. X-Ray photoelectron spectroscopy (XPS) was performed using a Thermo Fisher Scientific ESCALAB 250Xi with 200 W monochromated Al Kα radiation. Raman spectra were recorded on LabRAM HR Evolution with a diode laser emitting 532 nm.

2.6. Preparation of the working electrode

The catalyst loaded onto carbon paper (1 × 1 cm²) was used as the working electrode. The catalyst ink was prepared by dispersing 8.3 mg of the as-prepared catalyst and 2.8 mg of XC-72 C in a mixture of 1.2 mL of ethanol and 0.4 mL of H₂O, and then 35 μL of Nafion was added and sonicated for 2 h to produce a homogenous suspension. Afterwards, the catalyst ink was airbrushed on to carbon paper and dried in air at room temperature. The total loading on carbon paper was controlled to be 1 mg cm⁻², indicating the effective catalyst loading of 0.75 mg cm⁻².

2.7. Electrochemical measurements

All the electrochemical measurements were performed using a CHI660E potentiostat, with a gas-tight two-compartment H-type electrochemical cell separated by a piece of Nafion 117 membrane. A piece of Pt gauze and Ag/AgCl/sat. KCl were used as the counter electrode and reference electrode, respectively. The electrolyte, 0.5 M NaHCO₃ (pH = 7.3), was prepared by purging CO₂ into 0.25 M Na₂CO₃ aqueous solution. Linear sweep voltammetry (LSV) was performed at a rate of 10 mV s⁻¹. The CO₂RR activity of the electrode was evaluated in a CO₂-saturated electrolyte using controlled potential electrolysis for 1 h at room temperature. During each electrolysis, the electrolyte was continuously bubbled with CO₂ at a flow rate of 10 sccm. All potentials were 85% iR-compensated and converted to RHE scale via the equation: E_RHE = E_Ag/AgCl + 0.0591 × pH + 0.197 V. The reported current density was normalized to the geometric surface area of the carbon paper electrode unless otherwise specified.

2.8. Product analysis

The gas products, such as CO and H₂, were analyzed by online gas chromatography (GC, 9790Plus, Zhejiang Fuli) with Ar as the carrier gas. The liquid product was determined by ¹H nuclear magnetic resonance (¹H NMR, Bruker Avance III 400 MHz) with the water suppression method. The NMR sample was prepared by mixing 0.5 mL of electrolyte after electrolysis with 0.1 mL of D₂O. To quantify the yield of formate in the electrolyte, a calibration curve was built using standard sodium formate solution with a known concentration (Fig. S1, ESI†).

2.9. Calculations

The FE of formate was calculated based on the charge consumed for formate production and the total charge passed through the electrode:
in which A is the ¹H NMR peak area of formate, S is the coefficient (10 014.18 mM⁻¹) of the calibration curve in Fig. S1 (ESI†), V is the volume (25 mL) of the electrolyte in the cathodic compartment, n is the number of electrons transferred per formate (n = 2), F is the Faraday constant (96 485 C mol⁻¹), and Q is the total charge consumed during the electrolysis.

The electrochemical active surface area (ECSA) was determined by the following equation:

in which S is the geometric area of the work electrode (1 cm²), C_dl is the double-layer capacitance, and C_s is the specific capacitance (40 μF cm⁻²).^33,38

The EE for the CO₂RR to formate was estimated according to the following equation:³⁴

where 1.23 V is the thermodynamic equilibrium potential for the anode OER reaction, E_eq,formate is the thermodynamic equilibrium potential for the cathode reaction to formate (E_eq,formate = −0.03 V), and η_cathode is the cathode overpotential.

3. Results and discussion

The synthesis of BiPbO₂Br NSs were achieved via a one-step hydrothermal method, as depicted in Scheme 1. Briefly, the metallic precursors of lead acetate and bismuth nitrate were stoichiometrically mixed with an aqueous solution containing ammonium hydroxide and CTAB in a Teflon-lined stainless autoclave, followed by hydrothermal treatment. Of note, CTAB possesses a dual functionality of both the surfactant and the bromine source. The monometallic BiOBr and PbO_xBr_y were also synthesized and used for control experiments.

Scheme 1 Schematic illustration of the synthesis of BiPbO₂Br NSs.

The crystal structure of the as-prepared samples was first investigated by XRD. As shown in Fig. S2 (ESI†), the XRD pattern of BiOBr NSs well matches with the layered tetragonal matlockite structure (PDF# 09-0393), which is alternatively composed of the [Bi₂O₂]²⁺ layer and Br⁻ ion layer.⁴³ After the introduction of Pb²⁺, the BiPbO₂Br NSs still show an isostructure of the tetragonal phase BiOBr, wherein the Bi³⁺ in the pristine [Bi₂O₂]²⁺ layer is randomly substituted with Pb²⁺ with a Bi/Pb molar ratio of 1. Fig. 1(a) shows that the XRD pattern of the sample can be readily indexed to tetragonal BiPbO₂Br (PDF# 38-1008). Specifically, the observed (00l) peak in the 2θ range of <20° indicates an ordered stacking along the c-axis.^44–46 TEM images (Fig. 1(b) and (c)) show the irregular sheet-like morphology of BiPbO₂Br with the lateral size ranging from tens to hundreds of nanometers and the average thickness of around 18 nm. As shown in Fig. 1(d), the layered stacking feature can be clearly identified from the high-resolution TEM (HRTEM) image, in which the lattice fringe distance is measured to be 0.642 nm, corresponding to the (002) plane of the tetragonal BiPbO₂Br. Further analysis of the surface of BiPbO₂Br NSs (Fig. 1(e)) also reveals the (103) plane with a lattice spacing of 0.291 nm. Besides, the SAED pattern shows the (103), (211) and (109) planes of BiPbO₂Br (Fig. 1(f)), consistent with the XRD results. Moreover, the high-angle annular dark-field scanning TEM (HAADF-STEM) and the corresponding EDS element mapping images (Fig. 1(g)–(k)) confirm the homogenous distribution of Bi, Pb, O and Br elements over BiPbO₂Br NSs. Comparatively, the as-prepared tetragonal phase BiOBr is in a quasi-square sheet morphology (Fig. S3, ESI†), in agreement with the previous report.⁴² Similar to PbO_xBr_y, it mainly exists as biphasic bulk particles composed of Pb₈O₇Br₂ and Pb₄O₃Br₂ (Fig. S4 and S5, ESI†).

Fig. 1 Structural characterization of BiPbO₂Br NSs. XRD pattern (a), TEM (b) (c) and HRTEM (d) and (e) images of BiPbO₂Br NSs. Inset (c): The thickness distribution. (f) Shows the SAED pattern. (g) HAADF-STEM image and the (h)–(k) corresponding EDS elemental mapping of Bi, Pb, O, Br in BiPbO₂Br NSs.

To discover the chemical state, XPS measurements were performed on the as-prepared samples. The survey spectrum verifies the existence of Bi, Pb, O and Br elements on the surface of BiPbO₂Br (Fig. S6, ESI†). In the high resolution XPS spectra of Bi 4f (Fig. 2(a)), the binding energies of 158.9 and 164.2 eV are observed for BiPbO₂Br, corresponding to 4f_7/2 and 4f_5/2 of Bi³⁺, which are lower than those for BiOBr. In contrast, two Pb 4f peaks of BiPbO₂Br at 137.9 and 142.8 eV shift to higher energies compared to those of PbO_xBr_y (Fig. 2(b)). These observations suggest the electron flow from Pb to Bi in BiPbO₂Br, which could be ascribed to the smaller ionic radius of Bi³⁺ together with the structural distortion induced by Pb introduction.⁴⁶ Undoubtedly, the results mentioned above evidence the successful synthesis of BiPbO₂Br NSs in this work.

Fig. 2 (a) Bi 4f XPS spectra of BiPbO₂Br and BiOBr. (b) Pb 4f XPS spectra of BiPbO₂Br and PbO_xBr_y.

To evaluate the CO₂RR performance, the catalytic reactions were investigated in a gas-tight H-type reaction cell using 0.5 M NaHCO₃ as the electrolyte. The obtained BiPbO₂Br, BiOBr and PbO_xBr_y were mixed with conductive carbon and spray-coated on a carbon paper electrode, which were then electrochemically reduced at −0.9 V before the measurements. The catalytic activities were first examined by linear sweep voltammetry (LSV). As shown in Fig. 3(a), the cathodic current density of BiPbO₂Br in the Ar-saturated electrolyte is quite low, which mostly originates from the HER. However, when the electrolyte is saturated with CO₂, the current density dramatically increases below −0.6 V and approaches −77.5 mA cm⁻² at −1.2 V, suggesting the substantial activity of BiPbO₂Br for the CO₂RR. Comparatively, a much lower current density of PbO_xBr_y indicates its unfavorable CO₂RR activity, while BiOBr shows a slightly higher current density than BiPbO₂Br in the same potential region, which could be contributed by its higher surface area and HER partial activity (vide infra). To systematically investigate the product distribution from the CO₂RR, the controlled electrolysis at different cathodic potentials was performed on the three catalytic electrodes (Fig. S7, ESI†). The gaseous and liquid products were separately quantified using online gas chromatography (GC) and nuclear magnetic resonance (NMR) spectroscopy, respectively. It is found that formate is the exclusive liquid product generated over BiPbO₂Br throughout all potentials, together with a small amount of gaseous products including H₂ and CO (Fig. 3(b) and Fig. S8, S9, ESI†). At the initial potential (−0.7 V), the CO₂RR does not prevail over the HER completely due to its higher activation barrier and inferior reaction kinetics, resulting in the formate FE of 74.9%. Nevertheless, the FEs are increased and maintained at more than 90% across a wide potential region from −0.8 to −1.1 V, with the maximum of 96.6% obtained at −0.9 V. As the potential shifts to a more negative value (−1.2 V), the FE of formate reduces to 89.3%, which may suffer from CO₂ mass-transport limitation in the aqueous electrolyte and H-type reaction cell.³² For the control sample of BiOBr, a similar tendency on the potential-dependent formate FE is observed. Although the formate generation dominated in the product distribution, the obtained FEs of BiOBr are lower than those of BiPbO₂Br over all measured potentials. Besides, different from BiPbO₂Br and BiOBr, PbO_xBr_y shows a much more favorable selectivity for the HER, exhibiting the highest formate FEs of <20%. These comparisons demonstrate that the Bi catalyst possesses high selectivity for electrochemical conversion of CO₂ to formate, while Pb introduction is facilitated for the further suppression of the competitive HER.

Fig. 3 (a) LSV curves of BiPbO₂Br, BiOBr, and PbO_xBr_y in CO₂- or Ar-saturated 0.5 M NaHCO₃ with a scanning rate of 10 mV s⁻¹. (b) FE and (c) j_formate in CO₂-saturated 0.5 M NaHCO₃ at various potentials over BiPbO₂Br, BiOBr, and PbO_xBr_y. (d) Long-term chronoamperometric test and the corresponding FE of BiPbO₂Br at −0.9 V in CO₂-saturated 0.5 M NaHCO₃. Error bars denote the standard deviation of data from three tests.

Furthermore, the according j_formate and production rate of formate are calculated as shown in Fig. 3(c). Apparently, the j_formate is found to increase gradually with the increase of overpotential for both BiPbO₂Br and BiOBr. It is observed that BiPbO₂Br exhibits lower j_formate as compared to BiOBr in the low potential region (−0.7 to −0.9 V), which is largely due to its inferior total current density (Fig. S7, ESI†). However, this trend is totally reversed in the high potential region (−1.0 to −1.2 V), attributing to the reduced difference of the total current density and the superior formate FE (Fig. 3(b)). Typically, the j_formate and formate production rate can reach up to 26 mA cm⁻² (FE of 96.6%) and 654 mmol h⁻¹ g⁻¹ at −0.9 V over BiPbO₂Br, which are further increased to 40 mA cm⁻² (FE of 93.1%) and 993 mmol h⁻¹ g⁻¹ at −1.0 V. From the perspective of practical application, achieving more formate production at a large overpotential is highly desired, highlighting the critical role of Pb in BiPbO₂Br. Regarding PbO_xBr_y, the combination of the inferior total current density and formate FE results in the negligible j_formate. Of note, negligible formate is produced when using the Ar-saturated electrolyte instead, confirming the formate production exclusively from the electroreduction of CO₂ input (Fig. S10, ESI†). Finally, the stability of BiPbO₂Br was accessed through a long-term electrolysis at −0.9 V. As shown in Fig. 3(d), the current density gradually increases from 21 to 30.5 mA cm⁻² after more than 10 h of electrolysis, which is mainly ascribed to the decreased contact resistance of the electrode during the dynamic structural evolution of the catalyst for the CO₂RR (Fig. S11, ESI†). Nevertheless, high formate FEs of approximately 95% are still maintained at different sampling time intervals. It should be noted that the superior activity and selectivity of BiPbO₂Br toward formate production for the CO₂RR are comparable or even better than those of many materials ever measured in an H-type cell (Table S1, ESI†).

To figure out if the activities of BiPbO₂Br and BiOBr were dependent on their ECSAs, the C_dl was determined by cyclic voltammetry measurement (Fig. S12, ESI†). Fig. 4(a) reveals that BiOBr possesses larger C_dl and thus ECSA than BiPbO₂Br, which exactly explains their difference in current density (Fig. 3(a) and Fig. S7, ESI†). Subsequently, to reflect the intrinsic activity, the j_formate is normalized with respect to the calculated ECSA of BiPbO₂Br (75.75 cm²) and BiOBr (102.25 cm²). As shown in Fig. 4(b), BiPbO₂Br exhibits higher normalized current density than BiOBr throughout all potentials, indicating its intrinsically superior activity for selective conversion of CO₂ to formate. Besides, the Tafel slope of BiPbO₂Br is measured to be 92 mV dec⁻¹, smaller than the value of 99 mV dec⁻¹ for BiOBr (Fig. 4(c)), verifying that the presence of Pb is favorable for improving the reaction kinetics of formate production. Moreover, EE is another essential metric of the electrocatalyst for the practical application of the CO₂RR. Benefiting from the combination of high FE and low overpotential for formate production, BiPbO₂Br shows a high EE of over 50% in a wide potential region ranging from −0.8 to −1.1 V, with the maximum value of 58.7% obtained at −0.8 V (Fig. 4(d)).

Fig. 4 (a) Charging current density plotted against scan rates on BiPbO₂Br, BiOBr, and PbO_xBr_y for estimating C_dl and ECSA. (b) ECSA normalized j_formate, (c) Tafel slope, and (d) EE of BiPbO₂Br and BiOBr.

It can be expected that the electroreduction process of the catalyst will give rise to the evolution of structure and chemical states, forming a self-regulation state, which has a significant influence on the exhibited performance for the CO₂RR.^32–38 To unveil the origin of the superior performance, the self-regulation state of BiPbO₂Br was analyzed after electroreduction (denoted as R-BiPbO₂Br). First, the time dependent structural reconstruction is validated by XRD (Fig. 5(a)). Obviously, the electroreduction leads to a gradual disappearance of BiPbO₂Br peaks, and the emergence of diffraction peaks of metallic Bi, Bi₂O₂CO₃, and PbCO₃. Due to the high oxophilicity, metallic Bi shows a relatively weak diffraction signal, readily forming Bi₂O₂CO₃ in the bicarbonate electrolyte.²⁰ Additionally, the absence of metallic Pb signals may be attributed to the occurrence of the spontaneous galvanic replacement reaction and primary cell reaction given the difference of redox potential between Pb (vs. SHE) and Bi (vs. SHE). This assumption can be indirectly affirmed by the observation of metallic Bi and Pb signals from monometallic BiOBr and PbO_xBr_y after electroreduction, respectively (Fig. S13, ESI†). Fig. 5(b) reveals the Raman spectrum of R-BiPbO₂Br, and the observed peaks at 65 and 91 cm⁻¹ are ascribed to the E_g and A_1g bands of metallic Bi. In contrast to Bi powders, the Raman blue shift of R-BiPbO₂Br may be due to the decreased grain size.⁴⁷ Subsequently, TEM measurements reveal that R-BiPbO₂Br well inherits its pristine nanosheet-like morphology (Fig. 5(c)). From the HRTEM image, the lattice fringe distances of 0.391, 0.166, 0.348 nm are identified in different areas (Fig. 5(d)), which can be assigned to the planes of Bi (003), Bi₂O₂CO₃ (024), and PbCO₃ (021), respectively, indicating the formation of heterogeneous interfaces. The XPS measurements show free bromine on the surface of R-BiPbO₂Br (Fig. S14, ESI†), confirming the complete structural reconstruction during electroreduction. The presence of a small amount of metallic Bi in addition to Bi³⁺ and Pb²⁺ is also observed (Fig. 5(e) and (f)), in agreement with the XRD and TEM results.

Fig. 5 (a) XRD pattern of R-BiPbO₂Br with different reduction times. (b) Raman spectra of R-BiPbO₂Br and Bi powders. (c) TEM and (d) HRTEM images of R-BiPbO₂Br. (e) Bi 4f and (f) Pb 4f XPS spectra of R-BiPbO₂Br.

Previous studies have demonstrated that the conversion of CO₂ to formate on Bi generally adopts an *OCHO intermediate pathway with oxygen atom binding to the Bi sites.^19–22 The construction of heterogeneous interfaces will promote the electron delocalization of interfacial Bi sites, facilitating the strong interaction with the *OCHO intermediate and thereby the production of formate.^33–38 In this work, therefore, the presence of Pb in BiPbO₂Br is beneficial for forming and stabilizing the multiphase composites during the structural reconstruction induced by electroreduction, thus facilitating the formation of abundant heterogeneous interfaces, which play the critical role in enhancing the activity and selectivity of interfacial Bi sites for the CO₂RR to formate.

4. Conclusions

In summary, we have developed a Bi–Pb composite catalyst through the electroreduction of BiPbO₂Br nanosheets for the CO₂RR, which exhibits impressive performance for formate production, including a high activity (j_formate of 40 mA cm⁻² at −1.0 V), a high selectivity (FE of 96.6% at −0.9 V), and a high energy conversion efficiency (EE of >50% from −0.8 to −1.1 V). The catalyst also shows good stability with no degradation of current density and a formate FE of around 95% is maintained over 10 h. More importantly, the Bi–Pb composite catalyst demonstrates superior intrinsic performance for the CO₂RR to formate, as compared with its monometallic Bi and Pb counterparts. The detailed investigation indicates that the structural reconstruction of BiPbO₂Br during electroreduction is favorable for the formation and stabilization of the multiphase composites, creating abundant heterogeneous interfaces and thus enhancing the intrinsic performance. This work highlights the role of Pb introduction in promoting the Bi catalyst for the CO₂RR to formate, which may offer guidance for the rational design of other efficient bimetallic catalysts with heterogeneous interfaces.

Author contributions

G. Sun, C. Zou, Y. Zhu, and J. Wang conceived the project. G. Sun, C. Zou, Y. Fang and W. Sun: methodology, investigation and writing. S. He, Y. Liu, and J. Zhang: visualization, investigation, and formal analysis. Y. Zhu and J. Wang: funding acquisition and writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by Jiangsu Specially Appointed Professorship, the National Natural Science Foundation of China (52202252), the Natural Science Foundation of Jiangsu Province (BK20210553), the Nanjing Science and Technology Innovation Project for Overseas Scholar, and the Startup Fund from Nanjing Tech University.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qm00262d

‡ These authors contributed equally to this work.

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