Huai Qin Fu, Junxian Liu, Nicholas M. Bedford, Yun Wang, Joshua Wright,Peng Fei Liu, Chun Fang Wen, Liang Wang, Huajie Yin, Dongchen Qi,Porun Liu?, Hua Gui Yang, Huijun Zhao?

ABSTRACT Bismuth?based materials (e.g., metallic, oxides and subcarbonate) are emerged as promising electrocatalysts for convert?ing CO2 to formate. However, Bio?based electrocatalysts possess high overpotentials, while bismuth oxides and subcarbonate encounter sta?bility issues. This work is designated to exemplify that the operando synthesis can be an effective means to enhance the stability of electro?catalysts under operando CO2RR conditions. A synthetic approach is developed to electrochemically convert BiOCl into Cl?containing sub?carbonate (Bi2O2(CO3)xCly) under operando CO2RR conditions. The systematic operando spectroscopic studies depict that BiOCl is converted to Bi2O2(CO3)xCly via a cathodic potential?promoted anion?exchange process. The operando synthesized Bi2O2(CO3)xCly can tolerate - 1.0 V versus RHE, while for the wet?chemistry synthesized pure Bi2O2CO3,the formation of metallic Bio occurs at - 0.6 V versus RHE. At - 0.8 V versus RHE, Bi2O2(CO3)xCly can readily attain a FEHCOO? 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)xCly has proceeded via a *COOH intermediate step that only requires low energy input of 2.56 eV.

KEYWORDS Carbon dioxide reduction; Chloride?containing bismuth subcarbonate; Cathodic potential?promoted anion?exchange; Stability

1 Intro duction

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 mon?oxide, formate, methane and methanol), C2and C2+(e.g.,ethylene, ethanol, acetylene, acetate, acetaldehyde, oxalic acid andn?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, con?verting CO2to HCOO-/HCOOH is more desirable because HCOO-/HCOOH are more valuable commodity chemicals[4, 5]. To date, the reported high?performance electrocata?lysts for CO2reduction to HCOO-/HCOOH are almost exclusively made ofp?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 Bio, oxides and subcarbonate (Table S1) have been employed to electrocat?alytically convert CO2to HCOO-/HCOOH. As shown in Table S1, in general, the metallic Bio?based ones perform better than other forms of bismuth?containing electrocata?lysts. Nevertheless, the metallic Bio?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(FEHCOO?/FEHCOOH) [12]. The bismuth oxides?based electrocatalysts were also reported (Table S1). Notice?ably, such electrocatalysts often encounter stability issues because the bismuth oxides in these electrocatalysts can be easily converted to metallic Biounder CO2RR conditions[13]. For example, Deng et al. reported a Bi2O3electro?catalyst with the optimal performance at - 0.9 V (vs RHE)to achieve a FEHCOO? of 91% with a partial HCOO-cur?rent density (JHCOO?) of ～ 8 mA cm-2[14]. However, the as?synthesized Bi2O3is found to be partially converted to metallic Biounder the CO2RR conditions at - 0.9 V vs RHE. In fact, the reported bismuth oxides electrocatalysts require cathodic potentials ≥ - 0.9 (vs RHE) to concur?rently achieve FEHCOO? ＞ 90% withJHCOO? ≥ 15 mA cm-2[13, 15, 16]. Under such CO2RR conditions, the bismuth oxides in these electrocatalysts are either partially or com?pletely converted to metallic Bio. Other than metallic Bioand bismuth oxides, Zhang’s group reported the use of ultrathin bismuth subcarbonate (Bi2O2CO3) nanosheets to catalyze CO2reduction to HCOO-[17]. Their Bi2O2CO3electrocatalyst exhibits a very low overpotential of 610 mV and can achieve a FEHCOO? of 85% with aJHCOO?of ～ 11 mA cm-2at - 0.7 V (vs HRE), however, partial conversion of Bi2O2CO3to the metallic Biooccurs within 30 min under - 0.65 V (vs RHE).

As reviewed above, under the required cathodic poten?tials to concurrently achieve high FEHCOO? andJHCOO?, the reported bismuth oxide and subcarbonate electrocatalysts are unavoidably reduced to metallic Bio, leading to the structural and compositional changes underoperandoCO2RR condi?tions. Critically, suchoperandostructural 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 struc?tural transformation under theoperandoCO2RR conditions.If this is true, the severeoperandostability issues might be effectively mitigated by employing identical synthesis and application conditions.

In this contribution, we report an approach to electro?chemically convert bismuth oxychloride (BiOCl) into chlo?ride?containing bismuth subcarbonate (Bi2O2(CO3)xCly)underoperandoCO2RR conditions (at - 0.8 V vs RHE in CO2?saturated 0.5 M KHCO3solution) and use it to exem?plify that theoperandosynthesis can be an effective means to enhance theoperandoelectrochemical stability of electro?catalysts. Systematicoperandospectroscopic studies were conducted to depict the conversion mechanism and electro?chemical stability. BiOCl is converted to Bi2O2(CO3)xClyvia the cathodic potential?promoted anion?exchange process. The obtained Bi2O2(CO3)xClycan tolerate - 1.0 V versus RHE,while for the wet?chemistry synthesized pure tetragonal phased Bi2O2CO3, the formation of metallic Biooccurs at - 0.6 V versus RHE, signifying a markedly improved electrochemical stability. No notable structural change and performance decay are observed when Bi2O2(CO3)xClyis subjected to the stabil?ity test at - 0.8 V versus RHE over a 20 h period. At - 0.8 V versus RHE, Bi2O2(CO3)xClycan readily attain a FEHCOO? of 97.9%, much higher than that of Bi2O2CO3(81.3%). The den?sity functional theory (DFT) calculations indicate that dif?fering 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)xClyhas proceeded via a*COOH intermediate step that requires a notably reduced energy input of 2.56 eV.

2 Experimental and Calculation

2.1 Materials and Chemicals

Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, 99%), potas?sium chloride (KCl, 99.5%), ethanol (C2H5OH, 99%) and eth?ylene 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 HNO3at 100 °C for 12 h, thoroughly washed with the deionized water and ethanol and dried in air.

2.2 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.

2.3 Synthesis of Bi2O2(CO3)xCly

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 car?bon fiber paper substrate with an exposed area of 1 × 1 cm2(2 mg cm-2of BiOCl?NSs). The carbon fiber paper with loaded BiOCl?NSs was used as the working electrode and sub?jected to - 0.8 V (vs RHE) in CO2?saturated 0.5 M KHCO3solution for 2 h to electrochemically transform BiOCl?NSs to Bi2O2(CO3)xCly.

2.4 Synthesis of Bi2O2CO3

For comparative purpose, pure Bi2O2CO3was 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 Bi2O2CO3was adequately washed with deionized water and ethanol and dried in a vacuum oven of 60 °C for 6 h.

2.5 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)xClyworking electrode (1 × 1 cm2) was fabricated by operando electro?chemical transformation of the immobilized BiOCl?NSs on carbon fiber paper, while the Bi2O2CO3working electrode was prepared by immobilizing 2 mg cm-2of Bi2O2CO3on carbon fiber paper (1 × 1 cm2). For all electrochemical meas?urements, an Ag/AgCl (3.5 M KCl) reference electrode, a Pt mesh counter electrode and CO2?saturated 0.5 M KHCO3electrolyte (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 CO2at a flow rate of 5 mL min-1controlled by a universal flow meter(Alicat Scientific, LK2). All reported potentials were con?verted to the reversible hydrogen electrode (RHE) in accord?ance withERHE=EAg/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., H2and CO or other gaseous hydrocarbons). The CO and H2Faradaic efficiency were calculated as below:

2.6 Characterizations

The morphologies and structures of the samples were char?acterized 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 diffractom?eter. TheoperandoXRD patterns were recorded using a Bruker D8 diffractometer and a home?made three-electrode electrochemical cell. Raman spectra were taken by a REN?ISHAW mVia Raman Microscope using a 532 nm excita?tion laser. TheoperandoRaman studies were performed on a RENISHAW mVia Raman Microscope equipped with a microscopic lens immersed under the electrolyte to cap?ture Raman signals and a home?made three-electrode elec?trochemical cell consisting of a BiOCl?NSs or Bi2O2CO3working 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 Bi2O2CO3on a commercial Si substrate (1 × 1 cm2). XPS spectra were recorded by Kratos Axis ULTRA using the C1sat 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 meas?urements were performed at the 10?ID?B beamline of the Advanced Photon Source (APS), Argonne National Laboratory (ANL). Data reduction, processing and subse?quent modeling were performed using the Demeter XAS software package [18]. Modeling of the EXAFS data of Bi2O2(CO3)xClywas performed using Bi—O, Bi—C and Bi—Bi backscattering paths from the crystal structure of Bi2O2CO3[19], while the Bi—Cl contributions were gener?ated from the optimized structure generated from the DFT calculations. All EXAFS fitting was performed using an S02value 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 fac?tors were optimized in initial rounds of EXAFS fitting and then held constant.

2.7 DFT Calculations

All computation studies were performed using density func?tional 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 gen?eralized 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, 3s23p5and 1s1treated as valence electrons of Bi, C, O, Cl and H, respectively. The plane?wave cutoff of 520 eV was set for all the computa?tions. The (1 × 2) clean {001} faceted Bi2O2CO3was mod?eled by a 14?atomic layer slab separated by a vacuum layer of 20 ? in this study. When geometries of all structures were optimized, top seven layers of the surfaces includ?ing adsorbate were relaxed, while the bottom seven layers were fixed. The gamma?centered Monkhorst—Packk?point meshes with a reciprocal space resolution of 2π× 0.04 ?-1were utilized for structural optimization. For the calculations on CO2and formic acid molecules, a (20 × 20 × 20) ?3unit cell and a Γ?onlyk?point grid were used. All atoms were allowed to relax until the Hellmann—Feynman forces were smaller than 0.01 eV ?-1, and the convergence criterion for the electronic self?consistent loop was set to 10—5eV.The adsorption energy of each adsorbate [ΔE(eV/n)] was calculated as follows:

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

where theEelecis the electronic energy obtained from DFT calculation;EZPEis the zero?point vibrational energy esti?mated by harmonic approximation; ∫CpdTis the enthalpic correction andTSis the entropy. Here, reported values ofEZPE, ∫CpdTandTSare adopted [24]. The solvation effect has been considered for *COOH by stabilizing 0.25 eV [24].

3 Results and Discussion

3.1 Synthesis and Characterization of Bi2O2(CO3)xCly

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 KHCO3solution to electrochemically convert the loaded BiOCl?NSs into Bi2O2(CO3)xCly. The XRD pattern (Fig. 1b) of the resultant Bi2O2(CO3)xClycan 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 Bi2O2CO3crystal and theν1mode of the intercalated CO32-between the (BiO)22+planes [28, 29]. Raman peak at 182 cm-1could be assigned to theA1gmode of the intercalated Cl-in the interlayer [29,30]. The FE?SEM and AFM images (Figs. 1d and S5) unveil that Bi2O2(CO3)xClypossesses a sheeted structure with lat?eral sizes of 600—800 nm and thicknesses of 130—140 nm.The TEM image (Fig. 1e) shows that Bi2O2(CO3)xClyis 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 cor?responding EDX elemental mapping (Fig. 1h) unveil the homogenously distributed Bi, O, C and Cl. The EDX esti?mated Bi/Cl atomic ratio in Bi2O2(CO3)xClyis 17.7:1 (Fig.S6), significantly higher than that of BiOCl (1.2:1), confirm?ing the presence of Cl in Bi2O2(CO3)xCly.

Fig. 1 a Schematic illustrating electrochemical conversion of BiOCl to Bi2O2(CO3)xCly (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)xCly 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 X?ray photoelectron spectroscopy (XPS) analy?sis 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)xClyshow a negative shift of 0.26 eV, con?sistent with that of reported Bi2O2CO3[32]. Figure 2b shows the high?resolution XPS O 1sspectra of BiOCl and Bi2O2(CO3)xCly. 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)xClyshifted to lower energies due to the substitu?tion of chloride by carbonate. The two binding energy peaks at 199.2 and 200.8 eV assignable to Cl 2p3/2and Cl 2p1/2can be deconvoluted from the high?resolution XPS Cl 2pspectra of both BiOCl and Bi2O2(CO3)xCly(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 2pspectrum of Bi2O2(CO3)xCly(Fig. S7), confirming the presence of chemi?cally bonded Cl on the surface of Bi2O2(CO3)xCly.

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)xClyand reference samples are shown in Fig. 2d.The observed binding energy peaks at 532.0 and 537.0 eV from Bi2O2(CO3)xClyare assignable to the hybridization of O 2pwith Bi 6sorbitals [38, 39], while the binding energy peak at 534 eV corresponds to theπ*C=O transi?tion, indicating the presence of lattice carbonyl oxygen spe?cies [40]. The displayed binding energy peaks at 539.4 and 543.2 eV in the spectrum of Bi2O2(CO3)xClyare ascribed to the non?equivalentσ*C—O bonds in the carboxylic group originated from the adsorbed carbonate [41]. Based on the C K?edge NEXAFS spectra of Bi2O2(CO3)xClyand reference samples (Fig. 2e), the binding energy peak at 289.5 eV can be attributed to theσ*states of C—O [41], while the peaks at 297.2 and 299.8 eV are assignable to theσ*C = O reso?nances associated with the presence of carbonate species[42]. It is to note that the O K?edge and C K?edge NEX?AFS spectra obtained from Bi2O2CO3and Bi2O2(CO3)xClyexhibit very similar characteristics, implying that the crystal structure of Bi2O2CO3in Bi2O2(CO3)xClyis 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,Bi2O2CO3and Bi2O2(CO3)xCly. Figure 2g shows thek2?weighted Fourier transformed Bi L3?edge extended X?ray absorption fine structure (k2?weighted FT?EXAFS) spectra of Bi2O2CO3and Bi2O2(CO3)xCly. The peaks at 1.74, 2.33 and 3.58 ? assignable to the Bi—O and Bi—Bi bonds in the(BiO)22+abplane are observed from both Bi2O2CO3and Bi2O2(CO3)xCly, indicating an identical (BiO)22+abplane in both Bi2O2CO3and Bi2O2(CO3)xCly. The spectrum of Bi2O2CO3shows a peak at 2.93 ?, corresponding to the interactions of Bi in (BiO)22+abplane with the intercalated CO32-between the (BiO)22+abplane layers (the interlayer).Notably, for Bi2O2(CO3)xCly, the peak is shifted to 3.01 ?,implying the differences in the interactions of Bi in (BiO)22+abplane with the interlayer anions due to the presence of the intercalated Cl-in the interlayer of Bi2O2(CO3)xCly. The fittings of the Bi L3?edgek2?weighted FT?EXAFS spectra of Bi2O2(CO3)xClyand Bi2O2CO3inRspace andkspace were then performed (Figs. 2h, S8 and Table S2) [43]. It unveils that the spectrum of Bi2O2(CO3)xClyfits well with Bi2O2CO3and 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)xClyand Bi2O2CO3are 2.34 at 2.22 ? and 2.27 at 2.24 ?, respec?tively, further confirming an almost unchanged (BiO)22+abplane coordination environment. For Bi2O2CO3, a Bi—C CN of 1.75 at 3.36 ? represents the interactions between the Bi atoms in the (BiO)22+abplane and the intercalated CO32-in the interlayer. For Bi2O2(CO3)xCly, the measured Bi—C CN of 1.51 at 3.38 ? indicates a reduction in the Bi—C coordination. Notably, the fitting of the EXAFS spectrum of Bi2O2(CO3)xClyusing 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+abplane layers in Bi2O2(CO3)xCly.A likely Bi2O2(CO3)xClystructure is shown in Fig. 2i. The above results confirm that under - 0.8 V (vs RHE) cathodic potential in CO2?saturated 0.5 M KHCO3solution for 2 h,the BiOCl?NSs are electrochemically

3.2 Operando Converting BiOCl into Bi2O2(CO3)xCly

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)xCly. d O K?edge spectra, e C K?edge spectra and f Bi L3?edge spectra of BiOCl, Bi2O2CO3, Bi2O2(CO3)xCly and referenced samples. g Bi L3?edge k2?weighted FT?EXAFS spectra of Bi2O2(CO3)xCly and Bi2O2CO3 in R space. h Fitting analysis of Bi2O2(CO3)xCly using Bi—O, Bi—C and Bi—Cl paths. i Proposed geometric configuration of Bi2O2(CO3)xCly

It is known that both of the tetragonal BiOCl and Bi2O2CO3crystals (Fig. S9) belong to the Sillén crystal family, featuring a matlockite?type positively charged(BiO)22+abplane layer structure stacking between the negatively charged bichloride and “standing?on?end”CO32-anions slabs, respectively. Comparing to thedspac?ing of {001} faceted BiOCl along thecaxis (7.83 ?), thed spacingof {002} faceted Bi2O2CO3is markedly reduced to 6.84 ?. Therefore, under apt cathodic potentials, due to the layer structure similarity, and the apparently decreaseddspacing of {002} faceted Bi2O2CO3, the transformation of BiOCl to Bi2O2CO3could occur via the intercalative substitution of the interlayer Cl-with CO32-through a glide of the neighboring (BiO)22+abplanes along [100]and [010] directions with the translational distances of?aand ?b, respectively [17]. To depict the structural evolution processes under theoperandoCO2RR condi?tions, the BiOCl?NSs immobilized on the carbon fiber paper were subjected to different cathodic potentials(EApp) in CO2?saturated 0.5 M KHCO3solution, and the XRD patterns wereoperandorecorded. The XRD pat?terns recorded under the open circuit potential (OCP) andEApp≤ - 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)xClyoccurs atEApp= - 0.3 V as indicated by the observed diffraction peak at 56.9° cor?responding to {123} faceted Bi2O2CO3. WhenEAppis 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)xClyis evidenced by the progressively increased intensities of Bi2O2CO3diffraction peaks and the accompanied decrease in the intensities of BiOCl diffrac?tion peaks. WithEApp= - 0.8 V, all recorded diffraction peaks belong to Bi2O2CO3(PDF No. 41?1488), signifying the complete conversion of BiOCl to Bi2O2(CO3)xCly. The structural evolution and the time required to completely convert BiOCl to Bi2O2(CO3)xClyunderEApp= - 0.8 V were subsequently investigated (Fig. S10). As can be seen, BiOCl is fully covered to Bi2O2(CO3)xClywithin 60 min underEApp= - 0.8 V. As disclosed in Fig. 3a,Bi2O2(CO3)xClyremains as the sole product when- 0.8 V ≤EApp≤ - 1.0 V. WithEApp= - 1.1 V, the bis?muth in Bi2O2(CO3)xClyis partially reduced to the metallic phased Bioas evidenced by the appearance of the diffrac?tion peaks assignable to rhombohedral phased Bio(PDF No. 05—0519). WithEApp= - 1.2 V, all of the recorded diffraction peaks belong to the rhombohedral phased Bio,confirming the ultimate conversion of Bi2O2(CO3)xClyto the metallic Biophase. As shown in Fig. S11, the BiOCl?derived BioatEApp= - 1.2 V is formed by the aggre?gated BioNSs with the exposed {001} facets. Notably,the required cathodic potential to convert Bi2O2(CO3)xClyto Biois more negative than those reported potentials to reduce Bi2O2CO3to Bio[44], {Lv, 2017 #1765}inferring a superior electrochemical stability of Bi2O2(CO3)xClyover Bi2O2CO3, which might be attributed to the presence of Cl-in Bi2O2(CO3)xCly. To confirm this, the pure tetrago?nal phased Bi2O2CO3nanosheets (Figs. S12—S14) were synthesized by a wet?chemistry method [43] and subjected to different cathodic potentials in CO2?saturated 0.5 M KHCO3solution. Figure 3b shows theoperandorecorded XRD patterns. The formation Biooccurs atEApp= - 0.6 V,while the Bi2O2CO3is fully converted to the rhombohedral phased metallic Bio(PDF No. 05?0519) atEApp= - 0.8 V,confirming that the presence of Cl-in Bi2O2(CO3)xClyis responsible for the improved electrochemical stability.It is noteworthy that compared to the characteristic dif?fraction peaks of Bi2O2CO3, all of the recorded charac?teristic diffraction peaks from Bi2O2(CO3)xClyare shifted slightly toward lower angles (Fig. S15), indicating an expendeddspacing in Bi2O2(CO3)xClydue to the presence of Cl-in the interlayer. The aboveoperandoXRD stud?ies unveil that the electrochemical conversion of BiOCl to Bi2O2(CO3)xClyis realized by the cathodic potential?promoted anion?exchange in the interlayer between the(BiO)22+abplanes.atEApp= - 1.2 V due to the formation of metallic Bio, con?sistent with theoperandoXRD observations. For compara?tive purpose, theoperandopotential?dependent Raman spec?tra of the pure tetragonal phased Bi2O2CO3nanosheets were obtained (Fig. 3d). WhenEApp≤ - 0.5 V, the peak at 182 cm-1associating with theA1gmode of the intercalated Cl-in the CO32-slab is absent, while the characteristic Raman peaks of Bi2O2CO3at 163 and 1068 cm-1are appar?ent, however, rapidly extinct whenEApp≥- 0.6 V due to the formation of metallic Bio, consistent with theoperandoXRD observations shown in Fig. 3b. This further confirms that compared to the chloride?free Bi2O2CO3, the reduction of Bi2O2(CO3)xClyto metallic Biorequires a much higher cathodic potential due to presence of the intercalated Cl-in the CO32-slab, signifying a noticeably improved electro?chemical stability. TheseoperandoRoman studies further suggest that the conversion of BiOCl to Bi2O2(CO3)xClyis achieved by the cathodic potential?promoted anion?exchange in the interlayer.

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

3.3 CO2RR Performance

All electrochemical measurements were performed using a three?electrode electrochemical system with Bi2O2(CO3)xClyor Bi2O2CO3working electrode in CO2? or Ar?satu?rated 0.5 M KHCO3solution. WhenEApp＞ - 0.5 V (vs EHE), the linear sweep voltammetry (LSV) responses of Bi2O2(CO3)xClyin CO2?saturated solution display higher cathodic current densities than that obtained from the Ar?saturated solution (Fig. S16), indicating a superior elec?trocatalytic activity of Bi2O2(CO3)xClytoward CO2RR.The potentiostatic experiments were then performed under different cathodic potentials to examine the electrocata?lytic CO2RR activity and selectivity. Figure 4a shows the chronoamperometric curves of Bi2O2(CO3)xClyfrom the CO2?saturated 0.5 M KHCO3solution. The reaction prod?ucts in gaseous and aqueous phases were qualitatively iden?tified and quantitatively determined by the gas chromatog?raphy (GC) and nuclear magnetic resonance (NMR). For all cases investigated, H2is 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 FEHCOO? andJHCOO? values.Figure 4b shows the plot ofJHCOO? againstEApp. For both Bi2O2(CO3)xClyand Bi2O2CO3, an increase inEAppleads to an increase inJHCOO?. For a givenEApp, the observedJHCOO? from Bi2O2(CO3)xClyis higher than that observed from Bi2O2CO3, implying a superior CO2RR activity of Bi2O2(CO3)xClyover Bi2O2CO3. AtEApp= - 0.8 V, theJHCOO? attained by Bi2O2(CO3)xClyis 18.4 mA cm-2, higher than that of Bi2O2CO3(14.2 mA cm-2). Figure 4c shows the plot of FEHCOO? (derived from Fig. 4a) againstEApp. For Bi2O2(CO3)xCly, an increase inEAppfrom - 0.4 to - 0.6 V leads to a rapidly increased FEHCOO? from 79.2 to 96.2%,and further increasingEAppto - 0.8 V leads to an increased FEHCOO? to 97.9%. FEHCOO? remains almost unchanged whenEAppis further increased to - 1.0 V, while the correspond?ingJHCOO? is increased to 40.5 mA cm-2(Fig. 4b). Based on theoperandoXRD and Raman observations (Fig. 3a, c),the formation of the metallic phased Biowill not occur withEApp≤ - 1.0 V, therefore, the observed changes in FEHCOO?from the potential range of - 0.4 V ≤EApp≤ - 1.0 V reflect the influence of potential on CO2RR selectivity of Bi2O2(CO3)xCly. Although Bi2O2(CO3)xClyis partially con?verted to metallic Bi—NSs within - 1.0 V ≤EApp≤ - 1.2 V(Fig. 3a, c), the high FEHCOO? can still be attained due to the electrocatalytic activity of Biotoward CO2RR under high cathodic potentials [17]. WhenEApp＞ - 1.2 V, the observed decrease in FEHCOO? is due to the intensified com?petition from HER [7]. Interestingly, for pure Bi2O2CO3, a rapidly increased FEHCOO? from 65.5 to 77.5% is observed whenEAppis increased from - 0.4 to - 0.6 V and reached a maxima FEHCOO? of 83.0% atEApp= - 0.7 V, where Bi2O2CO3is partially converted to the metallic Bio. With- 0.8 V ≤EApp≤ - 1.2 V, Bi2O2CO3is fully converted to the metallic Bioand the slightly decreased FEHCOO? reflects the influence of potential on CO2RR selectivity of metal?lic Biorather than that of Bi2O2CO3. WhenEApp＞ - 1.2 V,FEHCOO? is rapidly decreased due to the intensified competi?tion from HER.

The chronoamperometric stability of Bi2O2(CO3)xCly(Fig. 4d) was evaluated over a 20 h period in CO2?saturated 0.5 M KHCO3solution atEApp= - 0.8 V vs RHE. While FEHCOO? of ～ 95% is well retained, a 12.1% increase in the cathodic current density is observed, indicating an increasedJHCOO?. Interestingly, when the used electrolyte is replaced by the fresh one, an almost identical chrono?amperometric curve is obtainable with ～ 12% increase in the cathodic current density at 20 h, indicating an excellent long?term stability of Bi2O2(CO3)xCly[46]. The excellent electrocatalytic stability of Bi2O2(CO3)xClycan be attrib?uted 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)xClyafter the chronoamperometric stability test. The chronoamperometric stability of Bi2O2CO3was also evaluated atEApp= - 0.8 V vs RHE (Fig. S22). Over a 20 h testing period, FEHCOO? 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 chronoampero?metric stability test, the pure tetragonal phased Bi2O2CO3is fully converted to the metallic phased Bio(Figs. S23—S26).The stability test result confirms that Bi2O2(CO3)xClyfabri?cated underoperandoCO2RR conditions possesses excellent stability.

Fig. 4 a Chronoamperometric curves of Bi2O2(CO3)xCly recorded from CO2?saturated 0.5 M KHCO3 solution under different cathodic poten?tials. b, c Plots of HCOOH partial current density and Faradic efficiency against catholic potential for Bi2O2(CO3)xCly? and Bi2O2CO3?catalyzed CO2RR. d Chronoamperometric curves and FEHCOOH of Bi2O2(CO3)xCly at - 0.8 V versus RHE. e Free energy diagrams of Bi2O2(CO3)xCly? and Bi2O2CO3?catalyzed CO2 reduction to HCOOH. f PDOSs plots of Bi2O2(CO3)xCly? and Bi2O2CO3?catalyzed CO2 reduction to HCOOH

3.4 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 calcula?tions were therefore carried out to determine the preferential intermediates of the pure tetragonal phased Bi2O2CO3? and Bi2O2(CO3)xCly?catalyzed CO2reduction to HCOOH. Our DFT calculations unveil that*OCHO intermediate can pref?erentially adsorb on the {001} faceted Cl?free Bi2O2CO3surface [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 Bi2O2CO3can be obtained. These results imply that the pure tetragonal phased Bi2O2CO3?catalyzed CO2reduction to HCOOH has proceeded via a*OCHO intermediate pathway. In contrast,our initial DFT calculations are failed to obtain a stable struc?ture of*OCHO intermediate adsorbed on Bi2O2(CO3)xClysurface. Nonetheless, further DFT calculations unveil that the*COOH intermediate is apt to absorb to the Bi2O2(CO3)xClysurface with a ΔG*COOHof - 1.25 eV (Fig. S27b), inferring that the Bi2O2(CO3)xCly?catalyzed CO2reduction to HCOOH has proceeded via a*COOH intermediate pathway. Fig?ure 4e illustrates the free energy diagrams of Bi2O2CO3? and Bi2O2(CO3)xCly?catalyzed CO2reduction to HCOOH. During the 1st PCET step, the formation of*OCHO on Bi2O2CO3surface and*COOH on Bi2O2(CO3)xClysurface is exother?mic. During the 2nd PCET step, the formation of*HCOOH on Bi2O2(CO3)xClyis exothermic, while on Bi2O2CO3is endothermic. The desorption of*HCOOH from Bi2O2CO3and Bi2O2(CO3)xClyto form HCOOH are energetically uphill.However, the desorption of*HCOOH from Bi2O2(CO3)xClyrequires 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)xClyover the Cl?free Bi2O2CO3. The poor kinetic activity of Bi2O2CO3could be resulted from the excessively high adsorption energy of*OCHO impeded active sites turno?ver. Figure 4f shows the projected density of states (PDOS)of thepbands of Bi sites in Bi2O2(CO3)xClyand the {001}faceted Bi2O2CO3. As can be seen, the PDOS of the {001}faceted Bi2O2CO3near Fermi level is dominated by thep?orbital electron states with much higher electronic densi?ties than that of Bi2O2(CO3)xCly, indicating a higher reactiv?ity for*OCHO intermediate adsorption, hence an impeded active sites regeneration [50, 51]. In addition, the high PDOS density of the {001} faceted Bi2O2CO3near Fermi level indi?cates high densities of the unoccupiedp?orbital states of Bi in the {001} faceted Bi2O2CO3with an estimated lowest unoc?cupied 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)xCly, thep?orbital electron states can only be observed at 2.3 eV above the Fermi level with low densities, indicating low unoccupiedp?orbital states of Bi in Bi2O2(CO3)xCly, 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 injec?tion. As illustrated in Fig. S28, compared to Bi2O2CO3,the higher LUMO potential of Bi2O2(CO3)xClyinfers that a higher cathodic potential is required to convert Bi3+in Bi2O2(CO3)xClyto metallic Bio, which explains the superior electrochemical stability of Bi2O2(CO3)xClyover Bi2O2CO3under CO2RR conditions.

4 Conclusions

In summary, we reported an approach to electrochemi?cally convert bismuth oxychloride (BiOCl) into chloride?containing bismuth subcarbonate (Bi2O2(CO3)xCly) underoperandoCO2RR conditions. We demonstrated that theoperandosynthesis is an effective strategy to enhance the electrochemical stability of bismuth?based electro?catalysts. The exemplified approach in this work could be widely applicable to enhance the electrochemical stabili?ties of other electrocatalysts for other reactions.

AcknowledgementsThis work was financially supported by Australian Research Council Discovery Project (DP200100965).Bi L3?edge XAS measurements were performed at the 10?ID?B beamline of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE?AC02?06CH11357. Operations at 10?ID?B are further supported by members of the Materials Research Collabora?tive Access Team. C and O K?edge measurements were performed at the SXR beamline of the Australian Synchrotron, part of the Australian Nuclear Science and Technology Organisation.

FundingOpen access funding provided by Shanghai Jiao Tong University.

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Supplementary InformationThe online version contains supplementary material available at https:// doi. org/ 10. 1007/s40820? 022? 00862?0.