Operando Converting BiOCl into Bi₂O₂(CO₃)_xCl_y for Efficient Electrocatalytic Reduction of Carbon Dioxide to Formate

Highlights

• An operando synthetic approach was exemplified to enhance catalyst stability for efficient reduction of CO₂ to formate.

• A highly stable Bi₂O₂(CO₃)_xCl_y electrocatalyst was synthesized by direct electrochemical conversion of BiOCl via a cathodic potential-promoted anion-exchange process under operando CO₂RR conditions.

• The surface Cl⁻ in Bi₂O₂(CO₃)_xCl_y changes the p-orbital electron states to enhance the stability and alters the CO₂RR pathway to markedly reduce the energy barrier.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40820-022-00862-0.

Abstract

Bismuth-based materials (e.g., metallic, oxides and subcarbonate) are emerged as promising electrocatalysts for converting CO₂ to formate. However, Bi^o-based electrocatalysts possess high overpotentials, while bismuth oxides and subcarbonate encounter stability issues. This work is designated to exemplify that the operando synthesis can be an effective means to enhance the stability of electrocatalysts under operando CO₂RR conditions. A synthetic approach is developed to electrochemically convert BiOCl into Cl-containing subcarbonate (Bi₂O₂(CO₃)_xCl_y) under operando CO₂RR conditions. The systematic operando spectroscopic studies depict that BiOCl is converted to Bi₂O₂(CO₃)_xCl_y via a cathodic potential-promoted anion-exchange process. The operando synthesized Bi₂O₂(CO₃)_xCl_y can tolerate − 1.0 V versus RHE, while for the wet-chemistry synthesized pure Bi₂O₂CO₃, the formation of metallic Bi^o occurs at − 0.6 V versus RHE. At − 0.8 V versus RHE, Bi₂O₂(CO₃)_xCl_y can readily attain a FE_HCOO- of 97.9%, much higher than that of the pure Bi₂O₂CO₃ (81.3%). DFT calculations indicate that differing from the pure Bi₂O₂CO₃-catalyzed CO₂RR, where formate is formed via a ^*OCHO intermediate step that requires a high energy input energy of 2.69 eV to proceed, the formation of HCOO⁻ over Bi₂O₂(CO₃)_xCl_y has proceeded via a ^*COOH intermediate step that only requires low energy input of 2.56 eV.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40820-022-00862-0.

Introduction

The renewable electricity-powered electrocatalytic carbon dioxide reduction reaction (CO₂RR) to produce chemicals/fuels not only curbs greenhouse gas emissions but also reduces our reliance on the rapidly diminished petroleum resources [1]. In this regard, various C₁ (e.g., carbon monoxide, formate, methane and methanol), C₂ and C₂₊ (e.g., ethylene, ethanol, acetylene, acetate, acetaldehyde, oxalic acid and n-propanol) CO₂RR products have been obtained [2, 3]. Among them, CO and HCOO⁻/HCOOH are the most energy-efficient CO₂RR products as they can be formed by transferring two electrons to CO₂. Comparing to CO, converting CO₂ to HCOO⁻/HCOOH is more desirable because HCOO⁻/HCOOH are more valuable commodity chemicals [4, 5]. To date, the reported high-performance electrocatalysts for CO₂ reduction to HCOO⁻/HCOOH are almost exclusively made of p-block metals-based materials such as In, Pb, Sn, Sb and Bi [6, 7].

Owning to their low toxicity and high selectivity toward HCOO⁻/HCOOH, Bi-based CO₂RR electrocatalysts have attracted increasing attentions [8, 9]. Various Bi-based CO₂RR electrocatalysts such as metallic Bi^o, oxides and subcarbonate (Table S1) have been employed to electrocatalytically convert CO₂ to HCOO⁻/HCOOH. As shown in Table S1, in general, the metallic Bi^o-based ones perform better than other forms of bismuth-containing electrocatalysts. Nevertheless, the metallic Bi^o-based electrocatalysts usually require high overpotentials, consequently the high cathodic potentials, to achieve their optimal performances [10, 11], undesirable for energy efficiency. In addition, high cathodic potentials are favorable for the competing hydrogen evolution reaction (HER), which often leads to low Faradic efficiencies toward HCOO⁻/HCOOH (FE_HCOO-/FE_HCOOH) [12]. The bismuth oxides-based electrocatalysts were also reported (Table S1). Noticeably, such electrocatalysts often encounter stability issues because the bismuth oxides in these electrocatalysts can be easily converted to metallic Bi^o under CO₂RR conditions [13]. For example, Deng et al. reported a Bi₂O₃ electrocatalyst with the optimal performance at − 0.9 V (vs RHE) to achieve a FE_HCOO- of 91% with a partial HCOO⁻ current density (J_HCOO-) of ~ 8 mA cm⁻² [14]. However, the as-synthesized Bi₂O₃ is found to be partially converted to metallic Bi^o under the CO₂RR conditions at − 0.9 V vs RHE. In fact, the reported bismuth oxides electrocatalysts require cathodic potentials ≥ − 0.9 (vs RHE) to concurrently achieve FE_HCOO- > 90% with J_HCOO- ≥ 15 mA cm⁻² [13, 15, 16]. Under such CO₂RR conditions, the bismuth oxides in these electrocatalysts are either partially or completely converted to metallic Bi^o. Other than metallic Bi^o and bismuth oxides, Zhang’s group reported the use of ultrathin bismuth subcarbonate (Bi₂O₂CO₃) nanosheets to catalyze CO₂ reduction to HCOO⁻ [17]. Their Bi₂O₂CO₃ electrocatalyst exhibits a very low overpotential of 610 mV and can achieve a FE_HCOO- of 85% with a J_HCOO- of ~ 11 mA cm⁻² at − 0.7 V (vs HRE), however, partial conversion of Bi₂O₂CO₃ to the metallic Bi^o occurs within 30 min under − 0.65 V (vs RHE).

As reviewed above, under the required cathodic potentials to concurrently achieve high FE_HCOO- and J_HCOO-, the reported bismuth oxide and subcarbonate electrocatalysts are unavoidably reduced to metallic Bi^o, leading to the structural and compositional changes under operando CO₂RR conditions. Critically, such operando structural transformation processes are progressive and potential-dependent, leading to great difficulties to confirm the actual active sites, hence the catalysis mechanisms. Parenthetically, the synthetic conditions of the reported bismuth oxide and subcarbonate electrocatalysts are vastly different to their electrocatalytic application conditions, which could be a cause of their structural transformation under the operando CO₂RR conditions. If this is true, the severe operando stability issues might be effectively mitigated by employing identical synthesis and application conditions.

In this contribution, we report an approach to electrochemically convert bismuth oxychloride (BiOCl) into chloride-containing bismuth subcarbonate (Bi₂O₂(CO₃)_xCl_y) under operando CO₂RR conditions (at − 0.8 V vs RHE in CO₂-saturated 0.5 M KHCO₃ solution) and use it to exemplify that the operando synthesis can be an effective means to enhance the operando electrochemical stability of electrocatalysts. Systematic operando spectroscopic studies were conducted to depict the conversion mechanism and electrochemical stability. BiOCl is converted to Bi₂O₂(CO₃)_xCl_y via the cathodic potential-promoted anion-exchange process. The obtained Bi₂O₂(CO₃)_xCl_y can tolerate − 1.0 V versus RHE, while for the wet-chemistry synthesized pure tetragonal phased Bi₂O₂CO₃, the formation of metallic Bi^o occurs at − 0.6 V versus RHE, signifying a markedly improved electrochemical stability. No notable structural change and performance decay are observed when Bi₂O₂(CO₃)_xCl_y is subjected to the stability test at − 0.8 V versus RHE over a 20 h period. At − 0.8 V versus RHE, Bi₂O₂(CO₃)_xCl_y can readily attain a FE_HCOO- of 97.9%, much higher than that of Bi₂O₂CO₃ (81.3%). The density functional theory (DFT) calculations indicate that differing from Bi₂O₂CO₃-catalyzed CO₂RR, where HCOOH is formed via a ^*OCHO intermediate step that requires a high energy input energy of 2.69 eV to proceed, the formation of HCOOH over Bi₂O₂(CO₃)_xCl_y has proceeded via a ^*COOH intermediate step that requires a notably reduced energy input of 2.56 eV.

Experimental and Calculation

Materials and Chemicals

Bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O, 99%), potassium chloride (KCl, 99.5%), ethanol (C₂H₅OH, 99%) and ethylene glycol (C₂H₆O₂, 99.8%) were purchased from Chem-Supply. Urea (CH₄N₂O), Nafion (5 wt%) was purchased from Sigma-Aldrich. Carbon paper (TGP-H-060) and Nafion 115 proton exchange membrane were purchased from Alfa Aesar. The carbon paper was ultrasonically treated in deionized water and ethanol, followed by emerging in the concentrated HNO₃ at 100 °C for 12 h, thoroughly washed with the deionized water and ethanol and dried in air.

Synthesis of BiOCl-NSs

0.164 g of KCl and 0.868 g of Bi(NO₃)₃·5H₂O were dissolved in 70 mL H₂O and stirred for 1 h. The solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave and kept at 120 °C for 24 h. The obtained BiOCl-NSs was adequately washed with deionized water and ethanol and dried at 60 °C for 6 h in vacuum oven.

Synthesis of Bi₂O₂(CO₃)_xCl_y

Twenty milligrams of the as-synthesized BiOCl-NSs was mixed with 80 μL Nafion solution (5 wt%) and dispersed 0.92 mL isopropanol under sonication for 40 min to form the ink. 100 μL ink was then cast onto the pre-treated carbon fiber paper substrate with an exposed area of 1 × 1 cm² (2 mg cm⁻² of BiOCl-NSs). The carbon fiber paper with loaded BiOCl-NSs was used as the working electrode and subjected to − 0.8 V (vs RHE) in CO₂-saturated 0.5 M KHCO₃ solution for 2 h to electrochemically transform BiOCl-NSs to Bi₂O₂(CO₃)_xCl_y.

Synthesis of Bi₂O₂CO₃

For comparative purpose, pure Bi₂O₂CO₃ was synthesized. Under constant stirring, 0.234 g of Bi(NO₃)₃·5H₂O was dissolved into 10 mL H₂O, followed by adding 1.502 g of CH₄N₂O and 10 mL of C₂H₅OH. The resultant solution was then placed in the oil bath under 90 °C for 4 h. The obtained pure Bi₂O₂CO₃ was adequately washed with deionized water and ethanol and dried in a vacuum oven of 60 °C for 6 h.

Electrochemical Measurements

The electrochemical measurements were performed using a Nafion 115 proton exchange membrane separated two-compartment electrochemical cell consisting of a three-electrode system controlled by an electrochemical station (CHI 660E). For CO₂RR, the Bi₂O₂(CO₃)_xCl_y working electrode (1 × 1 cm²) was fabricated by operando electrochemical transformation of the immobilized BiOCl-NSs on carbon fiber paper, while the Bi₂O₂CO₃ working electrode was prepared by immobilizing 2 mg cm⁻² of Bi₂O₂CO₃ on carbon fiber paper (1 × 1 cm²). For all electrochemical measurements, an Ag/AgCl (3.5 M KCl) reference electrode, a Pt mesh counter electrode and CO₂-saturated 0.5 M KHCO₃ electrolyte (pH of 7.2) were employed. During CO₂RR, the electrolyte in the cathode compartment was constantly stirred at a rate of 800 rpm and bubbled with CO₂ at a flow rate of 5 mL min⁻¹ controlled by a universal flow meter (Alicat Scientific, LK2). All reported potentials were converted to the reversible hydrogen electrode (RHE) in accordance with E_RHE = E_Ag/AgCl + 0.059 × pH + 0.205. The gas chromatography (GC, RAMIN, GC2060) equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD) was used to qualitatively and quantitatively determine the gaseous products (e.g., H₂ and CO or other gaseous hydrocarbons). The CO and H₂ Faradaic efficiency were calculated as below:

$${\text{FE}}_{\text{CO}}=2x_{\text{CO}}p\text{GF}/\text{IRT}$$ 1

$${\text{FE}}_{\text{H2}}=2x_{\text{H2}}p\text{GF}/\text{IRT}$$ 2

where x_CO and x_H2 (vol%) are the volume fractions of CO and H₂ in the exhaust gas, I (A) is the steady-state current, G = 5 mL min⁻¹ is the CO₂ flow rate, p = 1.013 × 10⁵ Pa, T = 273.15 K, F = 96,485 C mol⁻¹, R = 8.3145 J mol⁻¹ K⁻¹.

¹H nuclear magnetic resonance (¹H‐NMR) was used to qualitatively and quantitatively determine the liquid phase products, including HCOOH. After reaction, 0.5 mL electrolyte from the cathode compartment was mixed with 0.1 mL D₂O containing 3-(trimethylsilyl)propanoic acid (TMSP) as the internal standard and subjected to NMR analysis. The Faradaic efficiency for formation of HCOO⁻ was calculated as below:

$$\text{FE}=2F\times n_{\text{HCOO}}-/\left(I\times t\right).$$ 3

Characterizations

The morphologies and structures of the samples were characterized by SEM (JEOL JSM-7100) and TEM (Tecnai F20, 200 kV). The STEM images were recorded using a probe corrected JEOL JEM-ARM200F instrument with at an acceleration voltage of 200 kV. AFM measurements were performed using a Bruker Dimension Icon system. XRD patterns were collected from a Bruker D8 diffractometer. The operando XRD patterns were recorded using a Bruker D8 diffractometer and a home-made three‐electrode electrochemical cell. Raman spectra were taken by a RENISHAW mVia Raman Microscope using a 532 nm excitation laser. The operando Raman studies were performed on a RENISHAW mVia Raman Microscope equipped with a microscopic lens immersed under the electrolyte to capture Raman signals and a home-made three‐electrode electrochemical cell consisting of a BiOCl-NSs or Bi₂O₂CO₃ working electrode, an Ag/AgCl (3.5 M KCl) reference electrode and a Pt mesh counter electrode. The working electrodes were prepared by immobilizing BiOCl-NSs or Bi₂O₂CO₃ on a commercial Si substrate (1 × 1 cm²). XPS spectra were recorded by Kratos Axis ULTRA using the C1 s at 284.8 eV as the internal standard. C and O K-edge XAS measurements were performed at the Soft X-ray spectroscopy beamline at Australian Synchrotron Facility, Australia's Nuclear Science and Technology Organisation (Clayton, Victoria, Australia). Bi L₃-edge XAS measurements were performed at the 10-ID-B beamline of the Advanced Photon Source (APS), Argonne National Laboratory (ANL). Data reduction, processing and subsequent modeling were performed using the Demeter XAS software package [18]. Modeling of the EXAFS data of Bi₂O₂(CO₃)_xCl_y was performed using Bi–O, Bi–C and Bi–Bi backscattering paths from the crystal structure of Bi₂O₂CO₃ [19], while the Bi–Cl contributions were generated from the optimized structure generated from the DFT calculations. All EXAFS fitting was performed using an S₀² value of 0.868, which were obtained by modeling the EXAFS of a reference Bi foil (L₃-edge at 13,419 eV). To minimize error in CN and NND values, Debye–Waller factors were optimized in initial rounds of EXAFS fitting and then held constant.

DFT Calculations

All computation studies were performed using density functional theory (DFT) implemented in the Vienna Ab-initio Simulation Package (VASP) code in this study [20, 21]. For the effects of electron–electron exchange and correlation, the Perdew–Burke–Ernzerhof (PBE) functional at the generalized gradient approximation (GGA) level was employed [22]. The projected augmented wave (PAW) potentials were used throughout for ion–electron interactions [23], with the 5d¹⁰6s²6p³, 2s²2p², 2s²2p⁴, 3s²3p⁵ and 1s¹ treated as valence electrons of Bi, C, O, Cl and H, respectively. The plane-wave cutoff of 520 eV was set for all the computations. The (1 × 2) clean {001} faceted Bi₂O₂CO₃ was modeled by a 14-atomic layer slab separated by a vacuum layer of 20 Å in this study. When geometries of all structures were optimized, top seven layers of the surfaces including adsorbate were relaxed, while the bottom seven layers were fixed. The gamma-centered Monkhorst–Pack k-point meshes with a reciprocal space resolution of 2π × 0.04 Å⁻¹ were utilized for structural optimization. For the calculations on CO₂ and formic acid molecules, a (20 × 20 × 20) ų unit cell and a Γ-only k-point grid were used. All atoms were allowed to relax until the Hellmann–Feynman forces were smaller than 0.01 eV Å⁻¹, and the convergence criterion for the electronic self-consistent loop was set to 10^–5 eV. The adsorption energy of each adsorbate [ΔE (eV/n)] was calculated as follows:

$$\mathrm{\Delta }E=\frac{1}{n}\left(E_{\text{ad/surf}}-E_{\text{surf}}-n{E}_{\text{ad}}\right)$$ 4

where E_ad, E_surf and E_ad/surf are the energies of an adsorbate, the clean {001} facet and the surface with adsorbates, respectively. And n is the number of adsorbates on the surface.

Based on computational hydrogen electrode (CHE) model [24, 25], each electrochemical reaction step can be regarded as a simultaneous transfer of the proton–electron pair as a function of the applied potential. The reaction mechanism of CO₂ reduction should consist of the following elementary reactions:

$${\text{CO}}_2\left(\text{g}\right)+\ast +{\text{H}}^{+}\left(\text{aq}\right)+{\text{e}}^{-}\to {}^{\ast }\text{COOH}$$ 5

$${}^{\ast }\text{COOH}+{\text{H}}^{+}\left(\text{aq}\right)+{\text{e}}^{-}\to \text{HCOOH}+\ast$$ 6

or

$${\text{CO}}_2\left(\text{g}\right)+\ast +{\text{H}}^{+}\left(\text{aq}\right)+{\text{e}}^{-}\to {}^{\ast }\text{OCHO}$$ 7

$${}^{\ast }\text{OCHO}+{\text{H}}^{+}\left(\text{aq}\right)+{\text{e}}^{-}\to \text{HCOOH}+\ast$$ 8

where * means the corresponding surface and adsorbed states. The free energy for all intermediate states and non-adsorbed gas-phase molecule is calculated as:

$$G=E{-}_{\text{elec}}+E_{\text{ZPE}}+\int C_p\text{d}T-TS$$ 9

where the E_elec is the electronic energy obtained from DFT calculation; E_ZPE is the zero-point vibrational energy estimated by harmonic approximation; ∫C_pdT is the enthalpic correction and TS is the entropy. Here, reported values of E_ZPE, ∫C_pdT and TS are adopted [24]. The solvation effect has been considered for *COOH by stabilizing 0.25 eV [24].

Results and Discussion

Synthesis and Characterization of Bi₂O₂(CO₃)_xCl_y

In this work, Bi₂O₂(CO₃)_xCl_y was synthesized by direct electrochemical conversion of the pre-synthesized BiOCl under operando CO₂RR conditions (Fig. 1a). The BiOCl nanosheets (BiOCl-NSs) were firstly synthesized as the precursor via a one-pot hydrothermal method [26]. The X-ray diffraction (XRD) pattern of the as-synthesized BiOCl-NSs (Fig. S1) can be indexed to the tetragonal BiOCl (PDF No. 06-0249). The Raman spectrum (Fig. S2) displays two strong peaks centered at 143 and 199 cm⁻¹, assignable to $A_{1g}^1$ (external) and $A_{1g}^2$ (internal) Bi–Cl vibration modes, respectively, while the weak peak at 400 cm⁻¹ can be attributed to $B_g^1$ mode [27]. The atomic force microscopy (AFM) and field-emission scanning electron microscopy (FE-SEM) images (Fig. S3a, b) disclose that the obtained BiOCl-NSs are octagonal shaped with sizes between 600 and 800 nm and thicknesses of ~ 150 nm. The high-resolution inverse fast Fourier transformation transmission electron microscopy (IFFT-TEM) image perpendicular to the nanosheet plane (Fig. S3c) reveals lattice spacings of 2.75 and 2.75 Å with an interplanar angle of 90°, corresponding to the (110) and (1͞10) facets of BiOCl. The selected area electron diffraction (SAED) pattern (Fig. S3d) coincides to the diffraction pattern of the single crystal BiOCl (k±l0, k = l = n) from [001] zone axis. The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and the corresponding energy-dispersive X-ray spectroscopy (EDX) elemental mapping images confirm the homogeneous distribution of Bi, O and Cl throughout the entire BiOCl-NSs (Fig. S3e).

Fig. 1.

a Schematic illustrating electrochemical conversion of BiOCl to Bi₂O₂(CO₃)_xCl_y (Bi: pink, O: red, C: brown, Cl: green). b XRD pattern, c Raman spectrum, d FE-SEM images, e TEM image and SAED pattern, f HRTEM image, g IFFT-HRTEM image and h HAADF-STEM image and corresponding EDX element mapping images of Bi₂O₂(CO₃)_xCl_y resulted from the electrochemical treatment of BiOCl-NSs under − 0.8 V versus RHE in CO₂-saturated 0.5 M KHCO₃ solution for 2 h. (Color figure online)

The as-synthesized BiOCl-NSs were then immobilized onto a conductive carbon paper substrate (1.0 × 1.0 cm²) with a loading density of 2.0 mg cm⁻² (Fig. S4) and subject to − 0.8 V (vs RHE) for 2 h in CO₂-saturated 0.5 M KHCO₃ solution to electrochemically convert the loaded BiOCl-NSs into Bi₂O₂(CO₃)_xCl_y. The XRD pattern (Fig. 1b) of the resultant Bi₂O₂(CO₃)_xCl_y can be assigned to the tetragonal phased Bi₂O₂CO₃ (PDF No. 41–1488). The Raman spectrum (Fig. 1c) displays two strong peaks at 163 and 1068 cm⁻¹, attributing to the external vibration of Bi₂O₂CO₃ crystal and the ν₁ mode of the intercalated CO₃²⁻ between the (BiO)₂²⁺ planes [28, 29]. Raman peak at 182 cm⁻¹ could be assigned to the A_1g mode of the intercalated Cl⁻ in the interlayer [29, 30]. The FE-SEM and AFM images (Figs. 1d and S5) unveil that Bi₂O₂(CO₃)_xCl_y possesses a sheeted structure with lateral sizes of 600–800 nm and thicknesses of 130–140 nm. The TEM image (Fig. 1e) shows that Bi₂O₂(CO₃)_xCl_y is formed by multiple thin-layer structures with “doughnuts-like” shape, resulting from the substitution of chloride by carbonate. The SAED pattern normal to the nanosheets (inset of Fig. 1e) manifests the reflections of Bi₂O₂CO₃ (k00 and 0l0, k = l = n) with [001] zone axis. The high-resolution TEM image (HRTEM, Fig. 1f) displays a lattice spacing of 0.273 nm, corresponding to Bi₂O₂CO₃ (110) plane, which is also confirmed by the high-resolution IFFT-HRTEM image (Fig. 1g). The HAADF-STEM image and the corresponding EDX elemental mapping (Fig. 1h) unveil the homogenously distributed Bi, O, C and Cl. The EDX estimated Bi/Cl atomic ratio in Bi₂O₂(CO₃)_xCl_y is 17.7:1 (Fig. S6), significantly higher than that of BiOCl (1.2:1), confirming the presence of Cl in Bi₂O₂(CO₃)_xCl_y.

The X-ray photoelectron spectroscopy (XPS) analysis was then carried out. The high-resolution XPS Bi 4f spectra (Fig. 2a) confirm the presence of Bi³⁺ and Bi–O bonds (160.0 and 165.3 eV) [31] in BiOCl. The Bi³⁺ peaks of Bi₂O₂(CO₃)_xCl_y show a negative shift of 0.26 eV, consistent with that of reported Bi₂O₂CO₃ [32]. Figure 2b shows the high-resolution XPS O 1s spectra of BiOCl and Bi₂O₂(CO₃)_xCl_y. The former could be deconvoluted into the binding energy peaks assignable to the Bi–O lattice O (530.8 eV), the surface adsorbed hydroxyl (~ 531.9 eV) and O species in Nafion (536.3, 533.3 and 531.9 eV) [33, 34], while the deconvoluted binding energy peaks at 530.2 and 531.0 eV from the later are ascribed to the Bi–O lattice O and C=O, respectively [35, 36]. The lattice O peaks in Bi₂O₂(CO₃)_xCl_y shifted to lower energies due to the substitution of chloride by carbonate. The two binding energy peaks at 199.2 and 200.8 eV assignable to Cl 2p_3/2 and Cl 2p_1/2 can be deconvoluted from the high-resolution XPS Cl 2p spectra of both BiOCl and Bi₂O₂(CO₃)_xCl_y (Fig. 2c), indicating the presence of the lattice Cl⁻ [37]. Notably, a Bi/Cl atomic ratio of 61.5:1 is determined from the XPS Cl 2p spectrum of Bi₂O₂(CO₃)_xCl_y (Fig. S7), confirming the presence of chemically bonded Cl on the surface of Bi₂O₂(CO₃)_xCl_y.

Fig. 2.

High-resolution XPS spectra of a Bi 4f, b O 1s and c Cl 1s obtained from the as-synthesized BiOCl-NSs and Bi₂O₂(CO₃)_xCl_y. d O K-edge spectra, e C K-edge spectra and f Bi L₃-edge spectra of BiOCl, Bi₂O₂CO₃, Bi₂O₂(CO₃)_xCl_y and referenced samples. g Bi L₃-edge k²-weighted FT-EXAFS spectra of Bi₂O₂(CO₃)_xCl_y and Bi₂O₂CO₃ in R space. h Fitting analysis of Bi₂O₂(CO₃)_xCl_y using Bi–O, Bi–C and Bi–Cl paths. i Proposed geometric configuration of Bi₂O₂(CO₃)_xCl_y

The X-ray absorption spectroscopy (XAS) measurements were then conducted to probe the electronic structure and local atomic environments. The O K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra of BiOCl, Bi₂O₂(CO₃)_xCl_y and reference samples are shown in Fig. 2d. The observed binding energy peaks at 532.0 and 537.0 eV from Bi₂O₂(CO₃)_xCl_y are assignable to the hybridization of O 2p with Bi 6 s orbitals [38, 39], while the binding energy peak at 534 eV corresponds to the π^* C=O transition, indicating the presence of lattice carbonyl oxygen species [40]. The displayed binding energy peaks at 539.4 and 543.2 eV in the spectrum of Bi₂O₂(CO₃)_xCl_y are ascribed to the non-equivalent σ^* C–O bonds in the carboxylic group originated from the adsorbed carbonate [41]. Based on the C K-edge NEXAFS spectra of Bi₂O₂(CO₃)_xCl_y and reference samples (Fig. 2e), the binding energy peak at 289.5 eV can be attributed to the σ^* states of C–O [41], while the peaks at 297.2 and 299.8 eV are assignable to the σ^* C = O resonances associated with the presence of carbonate species [42]. It is to note that the O K-edge and C K-edge NEXAFS spectra obtained from Bi₂O₂CO₃ and Bi₂O₂(CO₃)_xCl_y exhibit very similar characteristics, implying that the crystal structure of Bi₂O₂CO₃ in Bi₂O₂(CO₃)_xCl_y is not noticeably altered by the presence of Cl⁻. According to the Bi L₃-edge X-ray absorption near-edge structure (XANES) spectra (Fig. 2f), the same valence states of Bi³⁺ exist in BiOCl, Bi₂O₂CO₃ and Bi₂O₂(CO₃)_xCl_y. Figure 2g shows the k²-weighted Fourier transformed Bi L₃-edge extended X-ray absorption fine structure (k²-weighted FT-EXAFS) spectra of Bi₂O₂CO₃ and Bi₂O₂(CO₃)_xCl_y. The peaks at 1.74, 2.33 and 3.58 Å assignable to the Bi–O and Bi–Bi bonds in the (BiO)₂²⁺ ab plane are observed from both Bi₂O₂CO₃ and Bi₂O₂(CO₃)_xCl_y, indicating an identical (BiO)₂²⁺ ab plane in both Bi₂O₂CO₃ and Bi₂O₂(CO₃)_xCl_y. The spectrum of Bi₂O₂CO₃ shows a peak at 2.93 Å, corresponding to the interactions of Bi in (BiO)₂²⁺ ab plane with the intercalated CO₃²⁻ between the (BiO)₂²⁺ ab plane layers (the interlayer). Notably, for Bi₂O₂(CO₃)_xCl_y, the peak is shifted to 3.01 Å, implying the differences in the interactions of Bi in (BiO)₂²⁺ ab plane with the interlayer anions due to the presence of the intercalated Cl⁻ in the interlayer of Bi₂O₂(CO₃)_xCl_y. The fittings of the Bi L₃-edge k²-weighted FT-EXAFS spectra of Bi₂O₂(CO₃)_xCl_y and Bi₂O₂CO₃ in R space and k space were then performed (Figs. 2h, S8 and Table S2) [43]. It unveils that the spectrum of Bi₂O₂(CO₃)_xCl_y fits well with Bi₂O₂CO₃ and Bi–Cl path, confirming the presence of the intercalated Cl⁻ in the interlayer. The coordination numbers (CNs) of Bi–O in the first coordination sphere of Bi₂O₂(CO₃)_xCl_y and Bi₂O₂CO₃ are 2.34 at 2.22 Å and 2.27 at 2.24 Å, respectively, further confirming an almost unchanged (BiO)₂²⁺ ab plane coordination environment. For Bi₂O₂CO₃, a Bi–C CN of 1.75 at 3.36 Å represents the interactions between the Bi atoms in the (BiO)₂²⁺ ab plane and the intercalated CO₃²⁻ in the interlayer. For Bi₂O₂(CO₃)_xCl_y, the measured Bi–C CN of 1.51 at 3.38 Å indicates a reduction in the Bi–C coordination. Notably, the fitting of the EXAFS spectrum of Bi₂O₂(CO₃)_xCl_y using Bi–Cl backscattering path from the optimized structure corresponds to a Bi–Cl CN of 0.33, which is closely approximated to the dropped Bi–C CN, unambiguously confirming the presence of the intercalated Cl⁻ between (BiO)₂²⁺ ab plane layers in Bi₂O₂(CO₃)_xCl_y. A likely Bi₂O₂(CO₃)_xCl_y structure is shown in Fig. 2i. The above results confirm that under − 0.8 V (vs RHE) cathodic potential in CO₂-saturated 0.5 M KHCO₃ solution for 2 h, the BiOCl-NSs are electrochemically

Operando Converting BiOCl into Bi₂O₂(CO₃)_xCl_y

It is known that both of the tetragonal BiOCl and Bi₂O₂CO₃ crystals (Fig. S9) belong to the Sillén crystal family, featuring a matlockite-type positively charged (BiO)₂²⁺ ab plane layer structure stacking between the negatively charged bichloride and “standing-on-end” CO₃²⁻ anions slabs, respectively. Comparing to the d spacing of {001} faceted BiOCl along the c axis (7.83 Å), the d spacing of {002} faceted Bi₂O₂CO₃ is markedly reduced to 6.84 Å. Therefore, under apt cathodic potentials, due to the layer structure similarity, and the apparently decreased d spacing of {002} faceted Bi₂O₂CO₃, the transformation of BiOCl to Bi₂O₂CO₃ could occur via the intercalative substitution of the interlayer Cl⁻ with CO₃²⁻ through a glide of the neighboring (BiO)₂²⁺ ab planes along [100] and [010] directions with the translational distances of ½ a and ½ b, respectively [17]. To depict the structural evolution processes under the operando CO₂RR conditions, the BiOCl-NSs immobilized on the carbon fiber paper were subjected to different cathodic potentials (E_App) in CO₂-saturated 0.5 M KHCO₃ solution, and the XRD patterns were operando recorded. The XRD patterns recorded under the open circuit potential (OCP) and E_App ≤ − 0.2 V (Fig. 3a) are almost identical to that of the as-synthesized BiOCl (Fig. S1). The initial conversion of BiOCl to Bi₂O₂(CO₃)_xCl_y occurs at E_App = − 0.3 V as indicated by the observed diffraction peak at 56.9° corresponding to {123} faceted Bi₂O₂CO₃. When E_App is increased from − 0.3 to − 0.7 V, although the recorded XRD patterns are still dominated by the diffraction patterns of BiOCl, the progress of converting BiOCl to Bi₂O₂(CO₃)_xCl_y is evidenced by the progressively increased intensities of Bi₂O₂CO₃ diffraction peaks and the accompanied decrease in the intensities of BiOCl diffraction peaks. With E_App = − 0.8 V, all recorded diffraction peaks belong to Bi₂O₂CO₃ (PDF No. 41-1488), signifying the complete conversion of BiOCl to Bi₂O₂(CO₃)_xCl_y. The structural evolution and the time required to completely convert BiOCl to Bi₂O₂(CO₃)_xCl_y under E_App = − 0.8 V were subsequently investigated (Fig. S10). As can be seen, BiOCl is fully covered to Bi₂O₂(CO₃)_xCl_y within 60 min under E_App = − 0.8 V. As disclosed in Fig. 3a, Bi₂O₂(CO₃)_xCl_y remains as the sole product when − 0.8 V ≤ E_App ≤ − 1.0 V. With E_App = − 1.1 V, the bismuth in Bi₂O₂(CO₃)_xCl_y is partially reduced to the metallic phased Bi^o as evidenced by the appearance of the diffraction peaks assignable to rhombohedral phased Bi^o (PDF No. 05–0519). With E_App = − 1.2 V, all of the recorded diffraction peaks belong to the rhombohedral phased Bi^o, confirming the ultimate conversion of Bi₂O₂(CO₃)_xCl_y to the metallic Bi^o phase. As shown in Fig. S11, the BiOCl-derived Bi^o at E_App = − 1.2 V is formed by the aggregated Bi^o NSs with the exposed {001} facets. Notably, the required cathodic potential to convert Bi₂O₂(CO₃)_xCl_y to Bi^o is more negative than those reported potentials to reduce Bi₂O₂CO₃ to Bi^o [44], {Lv, 2017 #1765}inferring a superior electrochemical stability of Bi₂O₂(CO₃)_xCl_y over Bi₂O₂CO₃, which might be attributed to the presence of Cl⁻ in Bi₂O₂(CO₃)_xCl_y. To confirm this, the pure tetragonal phased Bi₂O₂CO₃ nanosheets (Figs. S12–S14) were synthesized by a wet-chemistry method [43] and subjected to different cathodic potentials in CO₂-saturated 0.5 M KHCO₃ solution. Figure 3b shows the operando recorded XRD patterns. The formation Bi^o occurs at E_App = − 0.6 V, while the Bi₂O₂CO₃ is fully converted to the rhombohedral phased metallic Bi^o (PDF No. 05-0519) at E_App = − 0.8 V, confirming that the presence of Cl⁻ in Bi₂O₂(CO₃)_xCl_y is responsible for the improved electrochemical stability. It is noteworthy that compared to the characteristic diffraction peaks of Bi₂O₂CO₃, all of the recorded characteristic diffraction peaks from Bi₂O₂(CO₃)_xCl_y are shifted slightly toward lower angles (Fig. S15), indicating an expended d spacing in Bi₂O₂(CO₃)_xCl_y due to the presence of Cl⁻ in the interlayer. The above operando XRD studies unveil that the electrochemical conversion of BiOCl to Bi₂O₂(CO₃)_xCl_y is realized by the cathodic potential-promoted anion-exchange in the interlayer between the (BiO)₂²⁺ ab planes.

Fig. 3.

a, b Operando XRD patterns of the as-synthesized BiOCl-NSs and Bi₂O₂CO₃ recorded from CO₂-saturated 0.5 M KHCO₃ solution under different cathodic potentials. c, d Operando Raman spectra of the as-synthesized BiOCl-NSs and Bi₂O₂CO₃ recorded from CO₂-saturated 0.5 M KHCO₃ solution under different cathodic potentials

To further elaborate the electrochemical conversion pathway, the operando potential-dependent Raman spectra of BiOCl immobilized on the carbon fiber paper were recorded under different cathodic potentials in CO₂-saturated 0.5 M KHCO₃ solution (Fig. 3c). Under the OCP and E_App ≤ − 0.3 V conditions, the recorded spectra are almost identical to that of the as-synthesized BiOCl-NSs (Fig. S2). With E_App = − 0.4 V, the characteristic peaks of BiOCl at 143 ($A_{1g}^1$) and 199 cm⁻¹ ($A_{1g}^2$) are markedly reduced and disappeared, respectively, which are accompanied by the appearance of a new peak at 182 cm⁻¹ that might be assigned to the A_1g mode of the intercalated Cl⁻ in the interlayer, although this could be complicated by the reduced crystal symmetry due to the disorder or free rotation of CO₃²⁻ in the interlayer [29, 30]. These observed changes in the Raman spectrum signify the initial conversion of BiOCl to Bi₂O₂(CO₃)_xCl_y at E_App = − 0.4 V, consistent with the operando XRD observation shown in Fig. 3a. At E_App = − 0.8 V, two new peaks at 163 and 1068 cm⁻¹ attributed to the external vibration of Bi₂O₂CO₃ crystal and the ν₁ mode of CO₃²⁻ anions between the (BiO)₂²⁺ planes appear, signifying the complete conversion of BiOCl to Bi₂O₂(CO₃)_xCl_y. Within − 0.8 V ≤ E_App ≤ − 1.0 V, the peak at 182 cm⁻¹ (A_1g mode) is rapidly decreased, while the peaks at 163 and 1068 cm⁻¹ are evolved and intensified, which are likely due to the changes in the local atomic symmetry rather than the crystal cell parameters because of the unchanged operando XRD patterns within the same potential range (Fig. 3a). It is known that the high-intensity Raman bands between 150 and 200 cm⁻¹ normally correspond to the out-of-plane lattice vibrations of the Bi atoms perpendicular to the (BiO)₂²⁺ layers [45]. Because the phonon frequencies are sensitive to the dopant-induced asymmetry and the interlayer thickness, therefore, the blueshifted phonons from 182 to 163 cm⁻¹ could be resulted from the suppressed vibrations of the (BiO)₂²⁺ layers along c direction due to the replaced Cl⁻ by CO₃²⁻ at relatively high cathodic potentials. The characteristic Raman peaks of Bi₂O₂CO₃ at 163 and 1068 cm⁻¹ are dramatically decreased under E_App = − 1.1 V and vanished at E_App = − 1.2 V due to the formation of metallic Bi^o, consistent with the operando XRD observations. For comparative purpose, the operando potential-dependent Raman spectra of the pure tetragonal phased Bi₂O₂CO₃ nanosheets were obtained (Fig. 3d). When E_App ≤ − 0.5 V, the peak at 182 cm⁻¹ associating with the A_1g mode of the intercalated Cl⁻ in the CO₃²⁻ slab is absent, while the characteristic Raman peaks of Bi₂O₂CO₃ at 163 and 1068 cm⁻¹ are apparent, however, rapidly extinct when E_App ≥− 0.6 V due to the formation of metallic Bi^o, consistent with the operando XRD observations shown in Fig. 3b. This further confirms that compared to the chloride-free Bi₂O₂CO₃, the reduction of Bi₂O₂(CO₃)_xCl_y to metallic Bi^o requires a much higher cathodic potential due to presence of the intercalated Cl⁻ in the CO₃²⁻ slab, signifying a noticeably improved electrochemical stability. These operando Roman studies further suggest that the conversion of BiOCl to Bi₂O₂(CO₃)_xCl_y is achieved by the cathodic potential-promoted anion-exchange in the interlayer.

CO₂RR Performance

All electrochemical measurements were performed using a three-electrode electrochemical system with Bi₂O₂(CO₃)_xCl_y or Bi₂O₂CO₃ working electrode in CO₂- or Ar-saturated 0.5 M KHCO₃ solution. When E_App > − 0.5 V (vs EHE), the linear sweep voltammetry (LSV) responses of Bi₂O₂(CO₃)_xCl_y in CO₂-saturated solution display higher cathodic current densities than that obtained from the Ar-saturated solution (Fig. S16), indicating a superior electrocatalytic activity of Bi₂O₂(CO₃)_xCl_y toward CO₂RR. The potentiostatic experiments were then performed under different cathodic potentials to examine the electrocatalytic CO₂RR activity and selectivity. Figure 4a shows the chronoamperometric curves of Bi₂O₂(CO₃)_xCl_y from the CO₂-saturated 0.5 M KHCO₃ solution. The reaction products in gaseous and aqueous phases were qualitatively identified and quantitatively determined by the gas chromatography (GC) and nuclear magnetic resonance (NMR). For all cases investigated, H₂ is identified as the sole product in the gaseous phase, while the formate is found to be the sole product in the aqueous phase. The NMR determined formate concentrations (Figs. S17 and S18) corresponding to the chronoamperometric curves shown in Fig. 4a were used to calculate the corresponding FE_HCOO- and J_HCOO- values. Figure 4b shows the plot of J_HCOO- against E_App. For both Bi₂O₂(CO₃)_xCl_y and Bi₂O₂CO₃, an increase in E_App leads to an increase in J_HCOO-. For a given E_App, the observed J_HCOO- from Bi₂O₂(CO₃)_xCl_y is higher than that observed from Bi₂O₂CO₃, implying a superior CO₂RR activity of Bi₂O₂(CO₃)_xCl_y over Bi₂O₂CO₃. At E_App = − 0.8 V, the J_HCOO- attained by Bi₂O₂(CO₃)_xCl_y is 18.4 mA cm⁻², higher than that of Bi₂O₂CO₃ (14.2 mA cm⁻²). Figure 4c shows the plot of FE_HCOO- (derived from Fig. 4a) against E_App. For Bi₂O₂(CO₃)_xCl_y, an increase in E_App from − 0.4 to − 0.6 V leads to a rapidly increased FE_HCOO- from 79.2 to 96.2%, and further increasing E_App to − 0.8 V leads to an increased FE_HCOO- to 97.9%. FE_HCOO- remains almost unchanged when E_App is further increased to − 1.0 V, while the corresponding J_HCOO- is increased to 40.5 mA cm⁻² (Fig. 4b). Based on the operando XRD and Raman observations (Fig. 3a, c), the formation of the metallic phased Bi^o will not occur with E_App ≤ − 1.0 V, therefore, the observed changes in FE_HCOO- from the potential range of − 0.4 V ≤ E_App ≤ − 1.0 V reflect the influence of potential on CO₂RR selectivity of Bi₂O₂(CO₃)_xCl_y. Although Bi₂O₂(CO₃)_xCl_y is partially converted to metallic Bi–NSs within − 1.0 V ≤ E_App ≤ − 1.2 V (Fig. 3a, c), the high FE_HCOO- can still be attained due to the electrocatalytic activity of Bi^o toward CO₂RR under high cathodic potentials [17]. When E_App > − 1.2 V, the observed decrease in FE_HCOO- is due to the intensified competition from HER [7]. Interestingly, for pure Bi₂O₂CO₃, a rapidly increased FE_HCOO- from 65.5 to 77.5% is observed when E_App is increased from − 0.4 to − 0.6 V and reached a maxima FE_HCOO- of 83.0% at E_App = − 0.7 V, where Bi₂O₂CO₃ is partially converted to the metallic Bi^o. With − 0.8 V ≤ E_App ≤ − 1.2 V, Bi₂O₂CO₃ is fully converted to the metallic Bi^o and the slightly decreased FE_HCOO- reflects the influence of potential on CO₂RR selectivity of metallic Bi^o rather than that of Bi₂O₂CO₃. When E_App > − 1.2 V, FE_HCOO- is rapidly decreased due to the intensified competition from HER.

Fig. 4.

a Chronoamperometric curves of Bi₂O₂(CO₃)_xCl_y recorded from CO₂-saturated 0.5 M KHCO₃ solution under different cathodic potentials. b, c Plots of HCOOH partial current density and Faradic efficiency against catholic potential for Bi₂O₂(CO₃)_xCl_y- and Bi₂O₂CO₃-catalyzed CO₂RR. d Chronoamperometric curves and FE_HCOOH of Bi₂O₂(CO₃)_xCl_y at − 0.8 V versus RHE. e Free energy diagrams of Bi₂O₂(CO₃)_xCl_y- and Bi₂O₂CO₃-catalyzed CO₂ reduction to HCOOH. f PDOSs plots of Bi₂O₂(CO₃)_xCl_y- and Bi₂O₂CO₃-catalyzed CO₂ reduction to HCOOH

The chronoamperometric stability of Bi₂O₂(CO₃)_xCl_y (Fig. 4d) was evaluated over a 20 h period in CO₂-saturated 0.5 M KHCO₃ solution at E_App = − 0.8 V vs RHE. While FE_HCOO- of ~ 95% is well retained, a 12.1% increase in the cathodic current density is observed, indicating an increased J_HCOO-. Interestingly, when the used electrolyte is replaced by the fresh one, an almost identical chronoamperometric curve is obtainable with ~ 12% increase in the cathodic current density at 20 h, indicating an excellent long-term stability of Bi₂O₂(CO₃)_xCl_y [46]. The excellent electrocatalytic stability of Bi₂O₂(CO₃)_xCl_y can be attributed to its excellent structural stability as evidenced by the almost unchanged XRD pattern (Fig. S19), Raman spectra (Fig. S20), as well as SEM and TEM images (Fig. S21) of Bi₂O₂(CO₃)_xCl_y after the chronoamperometric stability test. The chronoamperometric stability of Bi₂O₂CO₃ was also evaluated at E_App = − 0.8 V vs RHE (Fig. S22). Over a 20 h testing period, FE_HCOO- is decreased from 86.2 to 80.0%, while the cathodic current density is increased from 12.5 to 14.2 mA cm⁻². However, after the chronoamperometric stability test, the pure tetragonal phased Bi₂O₂CO₃ is fully converted to the metallic phased Bi^o (Figs. S23–S26). The stability test result confirms that Bi₂O₂(CO₃)_xCl_y fabricated under operando CO₂RR conditions possesses excellent stability.

DFT Calculations

It is known that electrocatalytic CO₂RR to HCOOH has normally proceeded via a proton-coupled electron transfer (PCET) step to form ^*COOH or ^*OCHO intermediates and followed by another PCET step to generate HCOOH [47]. It is also known that the CO₂RR pathway depends strongly on the adsorption energy of the intermediates [48]. DFT calculations were therefore carried out to determine the preferential intermediates of the pure tetragonal phased Bi₂O₂CO₃- and Bi₂O₂(CO₃)_xCl_y-catalyzed CO₂ reduction to HCOOH. Our DFT calculations unveil that ^*OCHO intermediate can preferentially adsorb on the {001} faceted Cl-free Bi₂O₂CO₃ surface [49] with an adsorption free energy (ΔG_*OCHO) of − 2.67 eV (Fig. S27a), while no stable structure of ^*COOH intermediate adsorbed on the {001} faceted Cl-free Bi₂O₂CO₃ can be obtained. These results imply that the pure tetragonal phased Bi₂O₂CO₃-catalyzed CO₂ reduction to HCOOH has proceeded via a ^*OCHO intermediate pathway. In contrast, our initial DFT calculations are failed to obtain a stable structure of ^*OCHO intermediate adsorbed on Bi₂O₂(CO₃)_xCl_y surface. Nonetheless, further DFT calculations unveil that the ^*COOH intermediate is apt to absorb to the Bi₂O₂(CO₃)_xCl_y surface with a ΔG_*COOH of − 1.25 eV (Fig. S27b), inferring that the Bi₂O₂(CO₃)_xCl_y-catalyzed CO₂ reduction to HCOOH has proceeded via a ^*COOH intermediate pathway. Figure 4e illustrates the free energy diagrams of Bi₂O₂CO₃- and Bi₂O₂(CO₃)_xCl_y-catalyzed CO₂ reduction to HCOOH. During the 1st PCET step, the formation of ^*OCHO on Bi₂O₂CO₃ surface and ^*COOH on Bi₂O₂(CO₃)_xCl_y surface is exothermic. During the 2nd PCET step, the formation of ^*HCOOH on Bi₂O₂(CO₃)_xCl_y is exothermic, while on Bi₂O₂CO₃ is endothermic. The desorption of ^*HCOOH from Bi₂O₂CO₃ and Bi₂O₂(CO₃)_xCl_y to form HCOOH are energetically uphill. However, the desorption of ^*HCOOH from Bi₂O₂(CO₃)_xCl_y requires 2.56 eV to proceed, which is 0.13 eV lower than that of Bi₂O₂CO₃ (2.69 eV), indicating a better kinetic activity of Bi₂O₂(CO₃)_xCl_y over the Cl-free Bi₂O₂CO₃. The poor kinetic activity of Bi₂O₂CO₃ could be resulted from the excessively high adsorption energy of ^*OCHO impeded active sites turnover. Figure 4f shows the projected density of states (PDOS) of the p bands of Bi sites in Bi₂O₂(CO₃)_xCl_y and the {001} faceted Bi₂O₂CO₃. As can be seen, the PDOS of the {001} faceted Bi₂O₂CO₃ near Fermi level is dominated by the p-orbital electron states with much higher electronic densities than that of Bi₂O₂(CO₃)_xCl_y, indicating a higher reactivity for ^*OCHO intermediate adsorption, hence an impeded active sites regeneration [50, 51]. In addition, the high PDOS density of the {001} faceted Bi₂O₂CO₃ near Fermi level indicates high densities of the unoccupied p-orbital states of Bi in the {001} faceted Bi₂O₂CO₃ with an estimated lowest unoccupied molecular orbital (LUMO) energy of 0.4 eV above Fermi level, corresponding to a LUMO potential of − 0.4 eV. In strong contrast, for Bi₂O₂(CO₃)_xCl_y, the p-orbital electron states can only be observed at 2.3 eV above the Fermi level with low densities, indicating low unoccupied p-orbital states of Bi in Bi₂O₂(CO₃)_xCl_y, corresponding to a LUMO potential of − 2.3 eV. It is known that for semiconductor electrodes, the reduction reaction takes place via the injection of electrons into LUMO. Therefore, the LUMO potential corresponds to the minimum required cathodic potential for electron injection. As illustrated in Fig. S28, compared to Bi₂O₂CO₃, the higher LUMO potential of Bi₂O₂(CO₃)_xCl_y infers that a higher cathodic potential is required to convert Bi³⁺ in Bi₂O₂(CO₃)_xCl_y to metallic Bi^o, which explains the superior electrochemical stability of Bi₂O₂(CO₃)_xCl_y over Bi₂O₂CO₃ under CO₂RR conditions.

Conclusions

In summary, we reported an approach to electrochemically convert bismuth oxychloride (BiOCl) into chloride-containing bismuth subcarbonate (Bi₂O₂(CO₃)_xCl_y) under operando CO₂RR conditions. We demonstrated that the operando synthesis is an effective strategy to enhance the electrochemical stability of bismuth-based electrocatalysts. The exemplified approach in this work could be widely applicable to enhance the electrochemical stabilities of other electrocatalysts for other reactions.

Supplementary Information

Below is the link to the electronic supplementary material.

Funding

Open access funding provided by Shanghai Jiao Tong University.