Enhanced Electrocatalytic CO 2 Reduction of Bismuth Nanosheets with Introducing Surface Bismuth Subcarbonate

: The electrocatalytic CO 2 reduction reaction (CO 2 RR) into hydrocarbon products is one of the most promising approaches for CO 2 utilization in modern society. However, the application of CO 2 RR requires optimizing state-of-the-art catalysts as well as elucidating the catalytic interface formation mechanism. In this study, a ﬂower-like nano-structured Bi catalyst is prepared by a facile pulse current electrodeposition method wherein the morphologies could be accurately controlled. Interestingly, nano-structured Bi is inclined to generate Bi 2 O 2 CO 3 in the air and form a stable Bi 2 O 2 CO 3 @Bi interface, which could enhance the CO 2 adsorption and conversion. In-situ Raman spectroscopy analysis also proves the existence of Bi 2 O 2 CO 3 on the electrode surface. In a practical CO 2 reduction test by a ﬂow-cell reactor, the Bi 2 O 2 CO 3 @Bi electrode delivers a high faradaic efﬁciency of the CO 2 to formate/formic acid (~90%) at − 1.07 V vs. reversible hydrogen electrode (RHE) with no obvious decay during more than a 10 h continuous test. The introducing surface Bi 2 O 2 CO 3 in nano-structured Bi supports a promising strategy as well as facile access to prepare improved CO 2 RR electrocatalysts.


Introduction
Carbon dioxide (CO 2 ) conversion through a physicochemical process has been considered as a promising means to halt greenhouse gas emissions and global warming [1,2]. Moreover, extraterrestrial CO 2 utilization is the only effective method for human beings to survive in outer space for a long-term [3]. The electrochemical CO 2 reduction reaction (CO 2 RR) has advantages, including facile operation, controllable product, and scalable application, as well as when it is coupled with renewable resources such as solar and potential energy [4,5]. A variety of CO 2 RR products, such as methane (CH 4 ), ethylene (C 2 H 4 ), formate/formic acid (HCOOH), and methanol (CH 3 OH) could be achieved and used as fuels or chemical raw materials [6,7]. Among the above hydrocarbons, HCOOH is a desired product with high economic value and broad applications, and could be produced with high Faradaic efficiency (FE) at low over-potential due to its relatively simple two-electron transfer reductive procedure [8,9].
Although Bi-based catalysts show remarkable FE HCOOH selectivity on electrochemical CO 2 RR, the current density is still low in the CO 2 concentration aqueous reaction and inapplicable for large-scale industrial production. It is noteworthy that previous studies of Bi metal electrocatalysts did not take notice of the generated Bi-based compound during CO 2 RR and its influence on electrochemical performance, especially in the nanoscale electrodes with a high CO 2 chemisorption activity [32][33][34]. Therefore, it is necessary to investigate the catalytic active site governing the CO 2 RR in Bi metal catalyst and elucidate the formation mechanism of Bi-based compounds from Bi metal. Recent research indicated that the Bi-O and [Bi 2 O 2 ] 2+ bonds played a key role in adsorbing CO 2 as well as forming the *OOCH intermediate, thus improving the FE HCOOH product and inhibiting HER [35][36][37][38][39][40]. As a result, Bi-based compounds that contained Bi-O bonds, such as bismuth oxides (Bi 2 O 3 , BiO x ) [27,36,37,[40][41][42][43] and bismuth subcarbonate (Bi 2 O 2 CO 3 ) [17,35,38,[44][45][46][47], have been prepared and disclosed with improved CO 2 [38]. These complexes showed improved Faradaic formate selectivity and a wide operating potential range. It could be concluded that the existence of Bi 3+ -O 2− pairs, rather than Bi 0 in a Bi-based catalyst, enhanced the CO 2 adsorption and activation as well as stabilized the *OOCH intermediate [48][49][50].
In this study, a facile route to synthesize a flower-like Bi catalyst by electrodeposition on carbon paper was carried out. By employing a pulse current electrodeposit (PC) method in ethylene glycol (EG), the morphologies of the Bi electrode could be accurately controlled. Compared with bulk-structured Bi metal, the nano-structured Bi catalyst was inclined to generate Bi 2 O 2 CO 3 on its surface and the in-situ Bi 3+ -O 2− pairs formation could enhance the CO 2 adsorption and formate conversion. In situ Raman spectroscopy was employed to observe the formation of Bi 2 O 2 CO 3 on the Bi metal surface and the achieved bismuth subcarbonate covered Bi metal (Bi 2 O 2 CO 3 @Bi) has been carefully characterized. The introduction of Bi 2 O 2 CO 3 to a Bi metal catalyst significantly improved the CO 2 RR performance, which delivered a high FE HCOOH of~90% at −0.95 V vs. RHE in a three-electrode flow-cell reactor with no obvious decay during more than a 10 h continuous test. On the other hand, the bulk-structured Bi metal catalyst showed a low FE HCOOH and HER side reaction in a range of applied potential.

Sample Preparation
Flower-like Bi was directly prepared on a selected substrate by the PC method with a two-electrode system. Carbon paper (CP, TGP-H-060, Toray, Japan) was used as the substrate for product deposition and washed several times by deionized water and ethanol to remove the impurities and then dried under 50°C for 12 h before use. In brief, 0.5 mmol Bi(NO 3 ) 3 ·5H 2 O (Tianjin Yongda Chemical Reagent Company Limited, Tianjin, China) was slowly dissolved into 50 ml EG (Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China), then 5 ml of anhydrous dimethyl sulfoxide (DMSO, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) was added and stirred for 1 h at 40°C to form a homogeneous solution. Subsequently, the pretreated carbon paper was immerged into the solution and the synthetic process was carried out at room temperature, which was Coatings 2022, 12, 233 3 of 12 controlled by the PC method at 2 s duty time and 4 s pause time (total cycles = 600, duty cycle = 0.33). The potential range between the anode and cathode was applied from 1.6 to 2.5 V and the corresponding current was~0.4, 0.8, 1.2, and 1.6 mA, respectively. A Pt mesh (1.00 cm × 2.00 cm) was employed as the auxiliary electrode. The achieved bismuth nanosheets were dried at room temperature for 60 h to introduce the bismuth subcarbonate composite on the surface of the Bi/CP electrode.

Sample Characterizations
The surface morphologies of the samples were observed by a Helios G4 CX Field emission scanning electron microscope (FESEM, Thermo Fisher, Waltham, MS, USA) at an accelerating voltage of 5 kV. X-ray diffraction (XRD) patterns were recorded on a X' pert Pro MPD diffractometer with Cu Kα radiation (Philips, Amsterdam, Netherlands). Transmission electron microscopy (TEM) images and high-resolution TEM (HR-TEM) images were obtained on a JEM-2011 electron microscope operated at 200 kV (JEOL, Tokyo, Japan). The binding energies of Bi, O, and C were carried out on an Axis Ultra X-ray photoelectron spectrometer (XPS, Shimadzu, Kioto, Japan) with an Mg Kα = 1253.6 eV excitation source. All the binding energies were corrected with C1s level at 284.8 eV as an internal standard. In situ Raman spectra were studied using a laser confocal Raman spectrometer (LabRAM-010, Horiba, Loos, France) with a 633 nm laser source.

Electrochemical Measurements
All the electrochemical measurements were performed by using a Biologic VMP3 electrochemical workstation with a three-electrode electrochemical cell. The prepared Bi 2 O 2 CO 3 @Bi/CP (2.00 cm 2 ), Ag/AgCl electrode filled with 3 M KCl solution, and an IrO 2 /CP (2.00 cm 2 ) were used as the working, reference, and counter electrode, respectively. The electrochemical properties were evaluated using a linear sweep voltammetry (LSV) range from −0.3 to −1.2 V vs. the RHE and cyclic voltammetry (CV) range from 0.4 to −1.2 V vs. The RHE at a scan rate of 20 mV s −1 under N 2− or CO 2− saturated 0.5 M KHCO 3 . The pH of the CO 2− and N 2− saturated electrolytes are 7.2 and 8.8, respectively. The durability test was carried out by chronoamperometry in a commercialized flow cell reactor with CO 2 saturated by a 0.5 M KHCO 3 electrolyte. More details can be seen in the supporting information. The conversion equation for the Ag/AgCl reference electrode to RHE is described as follows: E RHE = E Ag/AgCl + 0.197 + 0.0591 pH

Results and Discussion
Scheme 1 reveals the electrodeposition reaction and surface conversion of flower-like bismuth into Bi 2 O 2 CO 3 . During the electrodeposition process, Bi 3+ ions were reduced into Bi 0 and underwent nucleation and continuous growth on the carbon fiber substrate to form a multilayer flower-like bismuth film after 60 min. The Bi 2 O 2 CO 3 composite was generated slowly on the surface of the obtained Bi/CP sample after exposing it to air. Indeed, O 2 , H 2 O, or CO 2 molecules adsorbed and interacted with the exposed Bi, and the oxide or hydroxide intermediate would be produced in the form of a thin layer [32,38]. As shown in Scheme 1, Bi 2 O 2 CO 3 was eventually generated on the surface of the electrode after a series of physicochemical reactions. The in-situ conversion processes from Bi to Bi 2 O 2 CO 3 are inferred as follows: 2Bi(s) + CO 2 (g) + 3 2 O 2 (g) = Bi 2 O 2 CO 3 (s) (6) ate would form and react with CO2 to transform into Bi2O2CO3 [32,51]. It is worth notin that the conversion of the nano Bi into Bi2O2CO3 was faster than the bulk Bi [33]. The nan size Bi is a very efficient material for capturing CO2 [33]. For the in-situ conversion, a larg number of Bi-O bonds and grain boundaries formed and were exposed on the surfac Figure S1 shows the color of Bi/CP changed from black to silver, revealing the formatio of Bi2O2CO3 after the drying treatment. In addition, the nano-structured Bi could tran form into Bi2O2CO3 more easily than the dendrite bulk due to its reactive surface chara teristics, which indicates the morphologies of Bi played a key role in the subsequen Bi2O2CO3 conversion processes. Scheme 1. Schematic reaction mechanism of Bi 3+ reduction and formation of flower-lik Bi2O2CO3@Bi/CP. Figure 1a,b displays the original carbon fiber with graining and the clusters of bi muth, which both appeared after electrodeposition, respectively. A multilayer flower-lik bismuth with a diameter of about 1000 nm could be observed in Figure 1c. It can be ex plored that a longer deposition time led to the formation of a thicker Bi layer (Figure 1c,d and a high current was inclined to form a Bi dendrite cluster with a large size, and thu covered the surface of the CP substrate ( Figure S2). It should be noted that the electrodep osition time also has an impact on the morphology of the catalysts. Figure S3 showed th micrographs of an electrode under ~90 min deposition. A longer time of deposition le the nanosheets growing continuously, which further formed the multilayered Bi catalys with obvious particle aggregation and damaged the flower-like structure. More detai are shown in Figure S4, in which the flower-like Bi formed a thin layer on the surface o Scheme 1. Schematic reaction mechanism of Bi 3+ reduction and formation of flower-like Bi 2 O 2 CO 3 @Bi/CP. First, the Bi nanosheet reacted with the absorbed O 2 and formed a surface oxide layer as depicted in Equation (2). At the same time, the absorbed CO 2 and H 2 O involved reactions that produced buffers on the Bi surface (Equations (3) and (4)). Finally, the oxidized Bi 3+ layers combined with the buffer ions CO 3 2− as shown in Equation (5). The total reaction could be described in Equation (6). CO 2 and O 2 absorbed onto the surface of the Bi nanosheets and then converted into Bi 2 O 2 CO 3 . Moreover, the unstable Bi(OH) 3 intermediate would form and react with CO 2 to transform into Bi 2 O 2 CO 3 [32,51]. It is worth noting that the conversion of the nano Bi into Bi 2 O 2 CO 3 was faster than the bulk Bi [33]. The nano size Bi is a very efficient material for capturing CO 2 [33]. For the in-situ conversion, a large number of Bi-O bonds and grain boundaries formed and were exposed on the surface. Figure S1 shows the color of Bi/CP changed from black to silver, revealing the formation of Bi 2 O 2 CO 3 after the drying treatment. In addition, the nano-structured Bi could transform into Bi 2 O 2 CO 3 more easily than the dendrite bulk due to its reactive surface characteristics, which indicates the morphologies of Bi played a key role in the subsequent Bi 2 O 2 CO 3 conversion processes. Figure 1a,b displays the original carbon fiber with graining and the clusters of bismuth, which both appeared after electrodeposition, respectively. A multilayer flower-like bismuth with a diameter of about 1000 nm could be observed in Figure 1c. It can be explored that a longer deposition time led to the formation of a thicker Bi layer (Figure 1c,d) and a high current was inclined to form a Bi dendrite cluster with a large size, and thus covered the surface of the CP substrate ( Figure S2). It should be noted that the electrodeposition time also has an impact on the morphology of the catalysts. Figure S3 showed the micrographs of an electrode under~90 min deposition. A longer time of deposition led the nanosheets growing continuously, which further formed the multilayered Bi catalysts with obvious particle aggregation and damaged the flower-like structure. More details are shown in Figure S4, in which the flower-like Bi formed a thin layer on the surface of the carbon fiber uniformly. Compared to the nano-structured Bi generated at a low electrodeposition current density, the scree-structured Bi with a micro scale was obtained at a current density greater Coatings 2022, 12, x FOR PEER REVIEW the carbon fiber uniformly. Compared to the nano-structured Bi generated at a l trodeposition current density, the scree-structured Bi with a micro scale was obt a current density greater than 1.2 mA cm −2 . The desired flower-like bismuth c achieved at an appropriate current density range of 0.8 to 1 A cm −2 . The morphology of Bi2O2CO3@Bi/CP was similar to the multilayer flower-lik shown in Figure S3 and S5, some fuzzy streaks can clearly be seen on the SEM im Bi2O2CO3@/Bi/CP, which is attributed to the decrease in conductivity. The redu electrical conductivity could be ascribed to introducing Bi2O2CO3 into Bi/CP. X-ray tion (XRD) patterns are provided in Figure 2a to reveal the crystal types and ch the samples. The (002) (10 and (202) planes of the rhombohedral Bi (JCPDS No. 85-1329), respectively, and impurity peaks are observed. The intensive peak ~26.83°, which is attributed to ferred orientation of the (012) plane, manifests the growth of flower-like Bi upon tion. The spectrum of the sample exposed to the air presents the peaks at 23.83, 29. and 45.22°, which corresponds to the (021), (023), (110) and (024) planes of B (JCPDS No. 411488). In addition, Figure 2a shows that a wide weak peak appea XRD patterns of the Bi2O2CO3@Bi/CP, implying the composite has converted to a phous phase. It could be demonstrated that a partial flower-like Bi was conver Bi2O2CO3 after exposing it to the air. According to previous studies, O2 and C freely chemi-absorbed on the bismuth surface in the air and led to the conversion of Bi2O2CO3 [33]. To obtain further details on the delicate structure of the Bi2O2CO a high-resolution TEM image (Figure 2b) is shown and proves that there is a frag the lattice and the interface of Bi and Bi2O2CO3. The apparent lattice fringes in F are measured at ~0.295 and ~0.328 nm, corresponding to the (012) plane of Bi2O2 the (013) plane of Bi, respectively. Combined with the aforementioned SEM and sults, the existence of the transformed Bi2O2CO3 and primal Bi could be verifie Bi2O2CO3@Bi/CP, implying the conversion process of the Bi nanosheets to Bi2O2CO interfaces.
The surface chemical states of the Bi2O2CO3@Bi/CP were further investigated The morphology of Bi 2 O 2 CO 3 @Bi/CP was similar to the multilayer flower-like Bi. As shown in Figure S3 and S5, some fuzzy streaks can clearly be seen on the SEM images of Bi 2 O 2 CO 3 @/Bi/CP, which is attributed to the decrease in conductivity. The reduction in electrical conductivity could be ascribed to introducing Bi 2 O 2 CO 3 into Bi/CP. X-ray diffraction (XRD) patterns are provided in Figure 2a  In addition, Figure 2a shows that a wide weak peak appears in the XRD patterns of the Bi 2 O 2 CO 3 @Bi/CP, implying the composite has converted to an amorphous phase. It could be demonstrated that a partial flower-like Bi was converted into Bi 2 O 2 CO 3 after exposing it to the air. According to previous studies, O 2 and CO 2 were freely chemi-absorbed on the bismuth surface in the air and led to the conversion reaction of Bi 2 O 2 CO 3 [33]. To obtain further details on the delicate structure of the Bi 2 O 2 CO 3 @Bi/CP, a high-resolution TEM image (Figure 2b) is shown and proves that there is a fragment of the lattice and the interface of Bi and Bi 2 O 2 CO 3 . The apparent lattice fringes in Figure 2b are measured at~0.295 and~0.328 nm, corresponding to the (012) plane of Bi 2 O 2 CO 3 and the (013) plane of Bi, respectively. Combined with the aforementioned SEM and XRD results, the existence of the transformed Bi 2 O 2 CO 3 and primal Bi could be verified in the Bi 2 O 2 CO 3 @Bi/CP, implying the conversion process of the Bi nanosheets to Bi 2 O 2 CO 3 on the interfaces. and were assigned to the Bi−O bond and CO3 2− species in Bi2O2CO3, respectively. Figure  2e shows two intensive symmetrical peaks located at 159.33 and 164.64 eV, implying the characteristic spin orbit splitting of the Bi 3+ 4f peaks and corresponding to Bi 3+ 4f7/2 and Bi 3+ 4f5/2, respectively. Otherwise, two subordinate peaks at 157.24 and 162.59 eV are ascribed to the presence of Bi 0 . The XPS results are consistent with the above characterizations and further validate the surface transformation from nano-structured Bi to Bi2O2CO3 in air. An in-situ Raman spectroscopy study was performed to explore the surface transformation mechanism of the Bi2O2CO3@Bi/CP electrode. The measurement was carried out through an electrochemical reduction procedure at −0.9 V vs. RHE in the CO2 saturated 0.5 M KHCO3 electrolyte. The Raman spectra (Figure 3) indicate the peaks at 92 cm −1 are related to the Bi-Bi bond that appeared at the beginning. The intensity of the peak at 154 cm −1 (belonged to the Bi-O bond) decreased and almost disappeared after a six min reduction, which is attributed to the reduction of surface Bi2O2CO3 into Bi. The broad external vibration peak at 264.05 cm −1 [26,40], as well as a weak peak around 502.94 cm −1 attributed to the Bi=O bond, are assigned to Bi2O2CO3 [52], which did not vanish even after a continuous reduction process ( Figure S6). In this study, flower-like Bi nanosheets could provide plenty of reaction sites to regenerate Bi2O2CO3 on its surface in a CO2 rich environment and the composite would not reduce, even in a relative negative potential (~−0.90 V vs. RHE) during the heterogeneous electrocatalysis process. The surface chemical states of the Bi 2 O 2 CO 3 @Bi/CP were further investigated by XPS measurement. Figure 2c-e displays the high-resolution C 1s, O 1s, and Bi 4f spectrum, respectively, thereby confirming the formation of Bi 2 O 3 CO 3 . As shown in Figure 2c, three peaks at 284.79, 285.68, and 288.29 eV, which were attributed to the C-C, C-O, and O-C=O groups, respectively, are found in C1s spectrum, indicating the existence of CO 3 2− . The O1s peak (Figure 2d) could be subdivided into two peaks of 530.17 and 531.46 eV, and were assigned to the Bi−O bond and CO 3 2− species in Bi 2 O 2 CO 3 , respectively. Figure 2e shows two intensive symmetrical peaks located at 159.33 and 164.64 eV, implying the characteristic spin orbit splitting of the Bi 3+ 4f peaks and corresponding to Bi 3+ 4f 7/2 and Bi 3+ 4f 5/2 , respectively. Otherwise, two subordinate peaks at 157.24 and 162.59 eV are ascribed to the presence of Bi 0 . The XPS results are consistent with the above characterizations and further validate the surface transformation from nano-structured Bi to Bi 2 O 2 CO 3 in air.
An in-situ Raman spectroscopy study was performed to explore the surface transformation mechanism of the Bi 2 O 2 CO 3 @Bi/CP electrode. The measurement was carried out through an electrochemical reduction procedure at −0.9 V vs. RHE in the CO 2 saturated 0.5 M KHCO 3 electrolyte. The Raman spectra (Figure 3) indicate the peaks at 92 cm −1 are related to the Bi-Bi bond that appeared at the beginning. The intensity of the peak at 154 cm −1 (belonged to the Bi-O bond) decreased and almost disappeared after a six min reduction, which is attributed to the reduction of surface Bi 2 O 2 CO 3 into Bi. The broad external vibration peak at 264.05 cm −1 [26,40], as well as a weak peak around 502.94 cm −1 attributed to the Bi=O bond, are assigned to Bi 2 O 2 CO 3 [52], which did not vanish even after a continuous reduction process ( Figure S6). In this study, flower-like Bi nanosheets could provide plenty of reaction sites to regenerate Bi 2 O 2 CO 3 on its surface in a CO 2 rich environment and the composite would not reduce, even in a relative negative potential (~−0.90 V vs. RHE) during the heterogeneous electrocatalysis process. To study the electrochemical CO2RR performance of the obtained ca sweep voltammetry (LSV) measurements were conducted from −0.3 to −1.2 the CO2 saturated 0.50 M KHCO3 electrolyte. As shown in Figure 4a, the CO onset potential is observed at −0.56 and −0.76 V vs. RHE for the Bi2O2CO3@ Bi/CP electrode in the CO2 saturated 0.50 M KHCO3 electrolyte, respectivel the onset potential of HER on carbon paper is −0.84 V vs. RHE. The lower o of Bi2O2CO3@Bi/CP manifests a higher CO2 reduction activity, indicating th Bi2O2CO3 into Bi-based catalysts is favorable to CO2RR. The total current d the Bi2O2CO3@Bi/CP electrode is ~20 mA cm −2 at −1.07 V vs. RHE, which is than those in the N2− saturated condition and sufficiently demonstrates the C ( Figure S7). The cyclic voltammetry curves in Figure 4b and S8 exhibit an o at ~0.31 V and a reduction peak at ~0.12 V, which can be attributed to the process in the Bi2O2CO3@Bi/CP electrode, and another reduction peak at a m potential (−0.90 V) is assigned to the electrochemical CO2 reduction or comp lution. The as-prepared Bi2O2CO3@Bi/CP electrode revealed a high interfa leading to a relatively low current density during the initial CV cycles. After cycles, the adsorbed impurities, such as inert gas on the surface of the electro removed and the catalytic interface became dynamically stable, and thus t density reached the maximum. After that, the Bi2O2CO3 composite in the e ually reduced during the negative potential and the current density decreas peaks still remained after 50 cycles, which indicates the stability of the Bi2O faces and is in accordance with the previous reports [53,54]. Meanwhile, positive shift in the reduction peak was recorded on the CVs curves, which w To study the electrochemical CO 2 RR performance of the obtained catalysts, linear sweep voltammetry (LSV) measurements were conducted from −0.3 to −1.2 V vs. RHE in the CO 2 saturated 0.50 M KHCO 3 electrolyte. As shown in Figure 4a, the CO 2 RR Faradaic onset potential is observed at −0.56 and −0.76 V vs. RHE for the Bi 2 O 2 CO 3 @Bi/CP and the Bi/CP electrode in the CO 2 saturated 0.50 M KHCO 3 electrolyte, respectively. In addition, the onset potential of HER on carbon paper is −0.84 V vs. RHE. The lower onset potential of Bi 2 O 2 CO 3 @Bi/CP manifests a higher CO 2 reduction activity, indicating that introducing Bi 2 O 2 CO 3 into Bi-based catalysts is favorable to CO 2 RR. The total current density (J tot ) of the Bi 2 O 2 CO 3 @Bi/CP electrode is~20 mA cm −2 at −1.07 V vs. RHE, which is much higher than those in the N 2− saturated condition and sufficiently demonstrates the CO 2 RR activity ( Figure S7). The cyclic voltammetry curves in Figure 4b and S8 exhibit an oxidation peak at~0.31 V and a reduction peak at~0.12 V, which can be attributed to the Bi 3+ /Bi 0 redox process in the Bi 2 O 2 CO 3 @Bi/CP electrode, and another reduction peak at a more negative potential (−0.90 V) is assigned to the electrochemical CO 2 reduction or competitive H 2 evolution. The as-prepared Bi 2 O 2 CO 3 @Bi/CP electrode revealed a high interface resistance, leading to a relatively low current density during the initial CV cycles. After several redox cycles, the adsorbed impurities, such as inert gas on the surface of the electrode, have been removed and the catalytic interface became dynamically stable, and thus the CV current density reached the maximum. After that, the Bi 2 O 2 CO 3 composite in the electrode gradually reduced during the negative potential and the current density decreased. The redox peaks still remained after 50 cycles, which indicates the stability of the Bi 2 O 2 CO 3 @Bi interfaces and is in accordance with the previous reports [53,54]. Meanwhile, an observable positive shift in the reduction peak was recorded on the CVs curves, which was attributed to the conversion of Bi-O or Bi=O to Bi-Bi bonds [47].  To practically evaluate the catalytic performance of the Bi2O2CO3@Bi/CP electrode, we performed CO2RR tests in a three-electrode flow-cell reactor ( Figure S10). The Bi2O2CO3@Bi/CP and IrO2/CP were employed as the work and counter electrode, respectively. More details can be found in the Supplementary Data. Figure 4d shows the product distribution ratio results obtained at a wide potential ranging (−0.67 to −1.17 V). The highest FE of HCOOH (~82%) was reached on the Bi/CP electrode at −0.89 V vs. RHE as shown in Figure S11, and the Bi2O2CO3@Bi/CP electrode conveyed a maximum FEHCOOH of 89%, which was achieved at −1.07 V vs. RHE. The enhancement of the CO2RR performance by introducing Bi2O2CO3 into the Bi/CP electrode could be attributed to the large amounts of Bi-O bonds and grain boundaries, which lowers the energy barrier for the formation of the CO2 −· intermediate and promotes the production of formate [39,40]. The residual Bi2O2CO3 in the catalyst was expected to form a surface environment to improve the CO2 adsorption process and is of great benefit to CO2 reduction activation.
In addition, to evaluate the long-term durability of the Bi2O2CO3@Bi/CP electrode, the CO2RR tests were performed at −1.07 V vs. RHE in the same flow-cell reactor. As shown in Figure 5a,b, a constant current density is achieved at ~18.6 mA cm −2 with no remarkable decline, and the FEHCOOH value is found to be basically consistent with the former result and within the range of 84-89% during a 10 h continuous test. As a result, the Bi2O2CO3@Bi/CP electrode exhibits outstanding stability and high selectivity in a practical CO2 catalytic measurement. The electrochemical surface area (ECSA) of the electrodes was evaluated by CV measurements at a potential range from 0 to −0.2 V vs. RHE with a variety of scanning rates ( Figure S9). As shown in Figure 4c, the current densities of the electrodes are proportional to the scanning rates and the Bi 2 O 2 CO 3 @Bi/CP (1890 µFcm −2 ) delivers a four times greater double layer capacitance than the Bi/CP (423 µF cm −2 ). The higher ECSA of the Bi 2 O 2 CO 3 @ Bi/CP confirms the electrode could provide more catalytic active sites, which could benefit the CO 2 RR performance.
To practically evaluate the catalytic performance of the Bi 2 O 2 CO 3 @Bi/CP electrode, we performed CO 2 RR tests in a three-electrode flow-cell reactor ( Figure S10). The Bi 2 O 2 CO 3 @Bi/CP and IrO 2 /CP were employed as the work and counter electrode, respectively. More details can be found in the Supplementary Data. Figure 4d shows the product distribution ratio results obtained at a wide potential ranging (−0.67 to −1.17 V). The highest FE of HCOOH (~82%) was reached on the Bi/CP electrode at −0.89 V vs. RHE as shown in Figure S11, and the Bi 2 O 2 CO 3 @Bi/CP electrode conveyed a maximum FE HCOOH of 89%, which was achieved at −1.07 V vs. RHE. The enhancement of the CO 2 RR performance by introducing Bi 2 O 2 CO 3 into the Bi/CP electrode could be attributed to the large amounts of Bi-O bonds and grain boundaries, which lowers the energy barrier for the formation of the CO 2 − intermediate and promotes the production of formate [39,40]. The residual Bi 2 O 2 CO 3 in the catalyst was expected to form a surface environment to improve the CO 2 adsorption process and is of great benefit to CO 2 reduction activation.
In addition, to evaluate the long-term durability of the Bi 2 O 2 CO 3 @Bi/CP electrode, the CO 2 RR tests were performed at −1.07 V vs. RHE in the same flow-cell reactor. As shown in Figure 5a,b, a constant current density is achieved at~18.6 mA cm −2 with no remarkable decline, and the FE HCOOH value is found to be basically consistent with the former result and within the range of 84-89% during a 10 h continuous test. As a result, the Bi 2 O 2 CO 3 @Bi/CP electrode exhibits outstanding stability and high selectivity in a practical CO 2 catalytic measurement.

Conclusions
In summary, a flower-like Bi nanosheet was fabricated by pulse current electrodepo sition, and the morphologies could be accurately and facilely controlled by the curren density. The achieved flower-like Bi nanosheets could form stable Bi2O2CO3@Bi interface after being exposed to air, which was also confirmed by the in-situ Raman spectroscopy The Bi2O2CO3@Bi interfaces exhibited an improved CO2 reduction activity than the bulk Bi electrode. The electrochemical test demonstrated that the Bi2O2CO3@Bi/CP electrode conveyed an excellent FEHCOOH of 89% at −1.07 V vs. RHE with a current density of ~16 mA cm −2 in a flow-cell reactor. In addition, the catalyst showed an outstanding stability and no obvious degradation after a continuous test of over 10 h. This composite catalyst sup ports a rational strategy for designing Bi-based CO2RR catalysts and can also be applied to various electrochemical energy conversion and storage fields.
Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Figure S1. The sample photo of original carbon paper, dendrite Bi/CP, and Bi2O2CO3@Bi/CP. Figure Figure S3. SEM image of electrode at 90 min electrodeposition with a current density of 0.8 mA cm −2 . Figure S4. SEM photographs at different magnifications. Figure S5. SEM characterizations of a flower-like Bi sample exposed in ai for a long time (>60 h). Figure S6. The particular Raman spectra of Bi2O2CO3@Bi/CP: (a) the Bi-Bi bond peak and Bi-O bond peak; (b) the broad external vibration peaks on Bi2O2CO3; and (c) the Bi=O bond peak. Figure S7. LSV curves of Carbon paper, Bi/CP, and Bi2O2CO3@Bi/CP in the N2 saturated 0.5 M KHCO3. Figure S8. The typical CV curve of a pristine Bi2O2CO3@Bi/CP at scanning rate of 20 mV s -1 from −1.2 to 0.4 V vs. RHE. Figure S9. CV curves of different samples: (a) the CV curves result of original carbon paper; (b) the CV curves test of Bi/CP; (c) the CV curves of Bi2O2CO3@Bi/CP. Figure S10. A flow cell rector for CO2RR with auxiliary equipment, including peristaltic pump, mass flow meter, resource gas, and electrochemical station. Figure S11.

Conclusions
In summary, a flower-like Bi nanosheet was fabricated by pulse current electrodeposition, and the morphologies could be accurately and facilely controlled by the current density. The achieved flower-like Bi nanosheets could form stable Bi 2 O 2 CO 3 @Bi interfaces after being exposed to air, which was also confirmed by the in-situ Raman spectroscopy. The Bi 2 O 2 CO 3 @Bi interfaces exhibited an improved CO 2 reduction activity than the bulk Bi electrode. The electrochemical test demonstrated that the Bi 2 O 2 CO 3 @Bi/CP electrode conveyed an excellent FE HCOOH of 89% at −1.07 V vs. RHE with a current density of 16 mA cm −2 in a flow-cell reactor. In addition, the catalyst showed an outstanding stability and no obvious degradation after a continuous test of over 10 h. This composite catalyst supports a rational strategy for designing Bi-based CO 2 RR catalysts and can also be applied to various electrochemical energy conversion and storage fields.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/coatings12020233/s1, Figure S1. The sample photo of original carbon paper, dendrite Bi/CP, and Bi 2 O 2 CO 3 @Bi/CP. Figure S2. SEM images of samples on different pulse current: (a) 0.4 mA cm −2 , (b) 0.8 mA cm −2 , (c) 1.2 mA cm −2 , and (d) 1.6 mA cm −2 . Figure S3. SEM image of electrode at 90 min electrodeposition with a current density of 0.8 mA cm −2 . Figure S4. SEM photographs at different magnifications. Figure S5. SEM characterizations of a flower-like Bi sample exposed in air for a long time (>60 h). Figure S6. The particular Raman spectra of Bi 2 O 2 CO 3 @Bi/CP: (a) the Bi-Bi bond peak and Bi-O bond peak; (b) the broad external vibration peaks on Bi 2 O 2 CO 3 ; and (c) the Bi=O bond peak. Figure S7. LSV curves of Carbon paper, Bi/CP, and Bi 2 O 2 CO 3 @Bi/CP in the N 2 saturated 0.5 M KHCO 3. Figure S8. The typical CV curve of a pristine Bi 2 O 2 CO 3 @Bi/CP at scanning rate of 20 mV s -1 from −1.2 to 0.4 V vs. RHE. Figure S9. CV curves of different samples: (a) the CV curves result of original carbon paper; (b) the CV curves test of Bi/CP; (c) the CV curves of Bi 2 O 2 CO 3 @Bi/CP. Figure S10. A flow cell rector for CO 2 RR with auxiliary equipment, including peristaltic pump, mass flow meter, resource gas, and electrochemical station. Figure S11. FE of CO 2 RR products at various potentials in CO 2 -saturated 0.5 M KHCO 3 electrolytes on Bi/CP.