Porously Reduced 2‐Dimensional Bi2O2CO3 Petals for Strain‐Mediated Electrochemical CO2 Reduction to HCOOH

Here we introduce bismuth‐based catalysts for the efficient electrochemical reduction of CO2 to formic acid (HCOOH), which are composed of petal‐shaped Bi2O2CO3 (BOC) that spontaneously formed from Bi thin film in aqueous carbonate solution at room temperature. During the electrochemical reduction process, the BOC petals transform to reduced BOC (R‐BOC) consisting of individual BOC and Bi domains. Lattice mismatch between both domains induces biaxial strain at the interfaces. Density functional theory calculations suggest that the tensile strain on the Bi domain stabilizes the *OCHO intermediate, reducing the thermodynamic barrier toward CO2 conversion to HCOOH. Together with the thermodynamic benefit and the unique nanoporous petal‐shaped morphology, R‐BOC petals have a superior Faradaic efficiency of 95.9% at −0.8 VRHE for the electrochemical conversion of CO2 to HCOOH. This work demonstrates that the spontaneously formed binary phases with desirable lattice strain can increase the activity of bismuth catalysts to the CO2 reduction reaction; such a strategy can be applicable in design of various electrocatalysts.


Introduction
[3][4] Among various byproducts of CO 2 RR, formic acid (HCOOH) has tremendous market potential owing to its high energy density in the application of hydrogen carriers in fuel cells. [5]8][9][10][11][12][13][14] Among them, Bi catalysts have been widely investigated due to their high Faradaic efficiency of HCOOH (FE HCOOH ) > 90%, low cost, and environmental benignity. [15,16]Nevertheless, they still suffer from low current density and narrow potential windows for practical applications.Such problems can be mitigated by understanding the mechanism that affects the catalytic activity during CO 2 RR.
[19][20][21] Since the electrochemical CO 2 RR is conducted in aqueous media, the metallic electrocatalysts can be partially deformed into oxides, hydroxides, or carbonates depending on the type of electrolyte.Therefore, the coexistence of these metal-based compounds can produce numerous heterogeneous interfaces, which contain two or more different functional units, at the surface of catalysts.The heterogeneous interfaces would have different electronic structures compared to individual compounds, and they can regulate the charge transfer at the electrode/electrolyte interface, controlling the formation of intermediates and their corresponding final products in the catalytic reactions.Several groups reported the Bi-based heterogeneous catalysts with various types such as metal/ metal, [22,23] metal/oxide composite, [24,25] and oxide composite/ oxide composite.The hierarchical mesoporous Bi/Bi 2 O 3 interface could exhibit over 90% FE HCOOH (at −0.9 V RHE ) due to the different local electronic states induced by local dislocation of Bi/ Bi 2 O 3 , which stabilized the CO 2 to *OCHO intermediate. [26]i 2 S 3 -Bi 2 O 3 nanosheets showed higher efficiency of 90% FE H- DOI: 10.1002/eem2.12490 Here we introduce bismuth-based catalysts for the efficient electrochemical reduction of CO 2 to formic acid (HCOOH), which are composed of petalshaped Bi 2 O 2 CO 3 (BOC) that spontaneously formed from Bi thin film in aqueous carbonate solution at room temperature.During the electrochemical reduction process, the BOC petals transform to reduced BOC (R-BOC) consisting of individual BOC and Bi domains.Lattice mismatch between both domains induces biaxial strain at the interfaces.Density functional theory calculations suggest that the tensile strain on the Bi domain stabilizes the *OCHO intermediate, reducing the thermodynamic barrier toward CO 2 conversion to HCOOH.Together with the thermodynamic benefit and the unique nanoporous petal-shaped morphology, R-BOC petals have a superior Faradaic efficiency of 95.9% at −0.8 V RHE for the electrochemical conversion of CO 2 to HCOOH.This work demonstrates that the spontaneously formed binary phases with desirable lattice strain can increase the activity of bismuth catalysts to the CO 2 reduction reaction; such a strategy can be applicable in design of various electrocatalysts.
results, the interfacial strain induces the charge transfer at the heterostructure interface, leading to electron redistribution and optimizing the binding energies of CO 2 to intermediates, thereby controlling the catalytic pathway.Nevertheless, no studies have been conducted on interfacial strain within hetero-interfaces of Bi-based catalyst, which is produced by deformation in the aqueous media.
According to the Pourbaix diagram, Bi 2 O 2 CO 3 (BOC) could be spontaneously formed due to the dissolution of BiO + ions when Bi film is immersed into an aqueous carbonate electrolyte [36] for CO 2 RR.[39] Thus, nano-sized oxide surrounded by reduced metals could be distributed during CO 2 RR.Herein, we demonstrated a petal-shaped BOC to reveal the effect of interfacial strain within the hetero-interface of Bi/BOC after electrochemical reduction (ECR).The BOC petals formed spontaneously by immersing a planar Bi thin film into the aqueous carbonate solution (0.1 M KHCO 3 solution) at room temperature.The BOC petals are composed of randomly stacked 2-dimensional (2D) nanosheets.During the CO 2 RR, it was found that the BOC petals were partially reduced to metallic Bi, resulting in nanoporous reduced BOC (R-BOC) petals.Transmission electron microscopy (TEM) showed that the 2D BOC domain is surrounded by metallic Bi, leading to producing the extrinsic stacking faults.Thus, mechanical stress accumulated at the Bi/BOC interface due to the lattice mismatch, confirmed by Raman spectroscopy.Theoretical calculations support that the tensile strain at the Bi/BOC interface stabilizes the *OCHO intermediate and facilitates CO 2 conversion to HCOOH.

Morphology and Microstructure of Petal-Shaped Bi Catalysts
The BOC petals were simply fabricated by depositing 30 nm-thick Bi thin film on Cu foil, followed by immersion in the CO 2 -saturated 0.1 M KHCO 3 for 30 min (Figure 1a).Based on the Pourbaix diagram, metallic Bi tends to be oxidized to BiO + at pH = 6.77(Figure S1, Supporting Information). [36]Hence, the oxidation reaction of Bi thin film occurs spontaneously in CO 2 -saturated 0.1 M KHCO 3 as follows. [40] þ In the oxidation reaction, metallic Bi film reacts with water (H 2 O) molecules, releasing BiO + ions, H + ions, and electrons.Then, the BiO + ions readily react with CO 2À 3 ions in the carbonate solution, resulting in the formation of BOC on the surface.During this spontaneous oxidation reaction, the planar morphology of the Bi film (Figure 1b) transforms into BOC petals composed of 2D nanostructures (Figure 1c).Since 2D nanostructures stand on the substrate by edge-on configuration, both front and back sides of petals are exposed, revealing remarkable increase in surface area (Figure S2, Supporting Information).When the BOC petals were subjected to ECR by applying a negative potential of −0.8 V RHE , R-BOC petals developed (Figure 1d).The shape and the size of R-BOC petals were the same as those of BOC petals, but the morphology became porous as the duration of the ECR reaction increased (Figure S3, Supporting Information).The microstructures of Bi film, BOC petals, and R-BOC petals were characterized using X-ray diffraction (XRD) (Figure 1e).The Bi film mainly showed the (003) peak of metallic Bi with small peaks of (100) and (104) (JCPDS #44-1246).In contrast, the BOC petals showed the peaks of the ( 002

Characterization of Petal-Shaped Bi Catalysts
Atomic compositions were identified using X-ray photoelectron spectroscopy (XPS).The doublet of Bi 4f XPS spectra was deconvoluted into Bi 0 (161.7 and 156.5 eV), Bi 3+ in Bi 2 O 3 (163.8[43] The Bi film showed the peaks of Bi 0 and Bi 3+ due to the formation of native oxide on the surface under ambient air during the preparation sample for XPS analysis.The BOC petals exhibited the unique characteristic peaks of Bi 3+ in BOC.After the ECR, the intensity of Bi 3+ in BOC peaks decreased meanwhile metallic Bi 0 peaks appeared.This result means that some regions of BOC were retained even after ECR, so the surface of R-BOC petals consisted of a mixture of BOC and Bi phases.O 1s XPS spectra of Bi film could be fitted by O-Bi (529.6 eV) bonds in oxide and OH-Bi (531.4 eV) bonds in hydroxide (Figure 2b).Microstructures of BOC petals and R-BOC petals were characterized using a high-resolution transmission electron microscope (HR-TEM) and electron energy loss spectroscopy (EELS) elemental mapping.The BOC petals showed 2D nanosheet morphology with sharp edges and corners (Figure 3a).The presence of clear signals of Bi, O, and C elements confirmed the ternary composition of the BOC petals.However, R-BOC petals had a porous 2D nanostructure composed of small nanoparticles (Figure 3b).Bi signal was uniformly distributed over the entire surface, whereas O and C signals were non-uniform and relatively weak.This difference indicates that the ECR did not completely reduce the BOC to metallic Bi.Some portion of the BOC remained on the petals during the CO 2 RR.BOC and Bi grains could be observed in HR-TEM images of the BOC petal (Figure 3c) and the R-BOC petal (Figure 3d).In the BOC petal, the HR-TEM image and electron diffraction patterns showed a lattice spacing a = 0.295 nm which corresponds to BOC (013), and a = 0.342 nm, which corresponds to BOC (004).In contrast, R-BOC petals had two distinct planes: Bi (012) with a = 0.328 nm and BOC (004) with a = 0.395 nm.Interestingly, the BOC (004) plane aligned with the Bi (012) plane at the interface of the Bi and BOC grains.The large lattice mismatch of 0.014 nm between them leads to interfacial strain; the presence of misfit dislocation is evidence of strain accumulation.The effect of interfacial strain on catalytic activity will be discussed later.

Electrochemical CO 2 Reduction
To study the catalytic activity, linear sweep voltammetry curves of Bi film and R-BOC petals were measured in N 2 -saturated and CO 2saturated 0.1 M KHCO 3 electrolytes at a scan rate of 50 mV s −1 (Figure 4a).Both the Bi film and the R-BOC petals showed more-negative current density under the CO 2 -saturated electrolyte than under the N 2saturated one.This reveals that both catalysts have higher activity toward CO 2 RR than toward the hydrogen evolution reaction.Moreover, it is noteworthy that the reductive current density of the R-BOC petals (22 mA cm −2 ) was ~2.2 times higher than Bi film (10 mA cm −2 ) at −1.2 V RHE .
The kinetics of the electron-transfer process were quantified using electrochemical impedance spectroscopy (EIS) (Figure 4b).The Nyquist impedance plots were obtained in a frequency range from 2 MHz to 1 Hz with an amplitude of 30 mV at a reduction potential of −0.8 V RHE .The semicircle in the Nyquist plot at high frequency represents the solution resistance, and the semicircle at low frequency represents the charge-transfer resistance during the CO 2 RR. [44]The second semicircle was smaller for R-BOC petals than for the Bi film; this difference indicates that R-BOC petals facilitated charge transfer for CO 2 conversion.Electrochemical active surface area (ECSA) of the Bi film and the R-BOC petals were determined by charge/discharge curves measured at different scan rates in the non-Faradaic region (0.28-0.38 V RHE ) (Figure S5, Supporting Information), and then, the current densities at 0.33 V RHE were plotted as a function of scan rate (Figure 4c).The slope of the current density scan rate curve means a double-layer capacitance that is linearly correlated with ECSA.The double-layer capacitance of R-BOC petal (0.85 mF cm −2 ) was ~6.5 times larger than that of Bi film (0.13 mF cm −2 ).From these results, the increased reductive-current density and decreased charge-transfer resistance  Energy Environ.Mater.2024, 7, e12490 could be explained by the unique 2D morphology of nanoporous R-BOC petal, which has a larger ECSA and more catalytic active sites than Bi film.FE HCOOH of the Bi film and the R-BOC petals was measured at different potentials from −0.4 to −1.2 V RHE (Figure 5a; Table S1, Supporting Information).At −0.4 V RHE , the R-BOC petals had a higher FE HCOOH = 43.4% than the Bi film (FE HCOOH = 18.3%).Interestingly, the maximum FE HCOOH = 95.9% at −0.8 V RHE was achieved from the R-BOC petal, whereas Bi film showed a lower FE HCOOH = 79.7% at the same potential.Owing to the highly increased electrochemical properties and catalytic selectivity, the current density of HCOOH (J HCOOH ) was higher in R-BOC petals than in Bi film over the entire potential region (Figure 5b).We also measured the catalytic properties varying the thickness of Bi film (t Bi ) (Figure S6 and Table S2, Supporting Information).When t Bi ≥ 30 nm, the radius of the second semicircle in the Nyquist plot was almost the same, indicating facile charge transfer.Also, the FE and J of byproducts from CO 2 RR were constant when t Bi ≥ 30 nm.Therefore, the high catalytic activity of the R-BOC petals was reproducible regardless of the thickness of the Bi thin film when thickness ≥30 nm was prepared.
For evaluating the feasibility of the R-BOC petals for practical application, their stability was measured at −0.8 V RHE for 30 h in CO 2saturated 0.1 M KHCO 3 (Figure 5c).For the total current density measurement, pure CO 2 gas was kept bubbled in the electrolyte to prevent the changing of the pH value and CO 2 concentration during the CO 2 RR.J HCOOH of the R-BOC petal remained nearly constant (~3 mA cm −2 ) but the FE HCOOH slightly decreased from 95.9% to 79.1% after reaction for 30 h.The decrease in the selectivity is due to a change in the shape of the R-BOC petal.The sharp edges or corners of the R-BOC petal, which are known as catalytic active sites, collapsed after the long-term operation (Figure S7, Supporting Information).This observation implies that the morphology of the Bi catalyst changes continuously during immersion in the aqueous carbonate electrolyte as well as during the electrochemical CO 2 RR.0][51]

Raman Spectroscopy and Density Functional Theory Calculation
We measured the Raman spectra of BOC petals with different ECR time to investigate the lattice-mismatch-induced interfacial strain, (Figure 6a).Spectra of the BOC petals showed the Bi-Bi vibration peak at 98.2 cm −1 and the external vibration peak of CO 2À 3 at 160.5 cm −1 . [52,53]As the ECR time increased, the Bi-Bi vibration peak  was negatively shifted to 95.36 cm −1 and the external vibration peak of CO 2À 3 was positively shifted to 163.82 cm −1 with 30 min ECR process (R-BOC petals).The shift of peak positions revealed the presence of tensile stress at Bi and of compressive stress at BOC during ECR process, and the strain is increased as the ECR time increased.
To understand the strain effect on catalytic activity of R-BOC petals, the change of Gibbs-free energy (Δ) of CO 2 conversion to HCOOH via *OCHO intermediate was calculated using first-principles density functional theory (Figure 6b,c).On the BOC surface, the potentialdetermining step (PDS) was the conversion from *OCHO to HCOOH (Δ = 2.29 eV).When the compressive stress was induced on BOC slab, the reaction barrier was reduced to 1.76 eV.On the Bi surface, the PDS was the hydrogenation of *CO 2 to *OCHO (Δ = 0.24 eV).This reaction barrier is much lower than in BOC, so the CO 2 RR would mainly occur on Bi surface.With tensile stress on the Bi slab, the PDS changed to the conversion from *OCHO to HCOOH (Δ = 0.16 eV).It is remarkable that the tensile stress on Bi surface greatly stabilized the *OCHO intermediate and decreased the thermodynamic energy barrier to HCOOH production.Therefore, the R-BOC petals could achieve lower overpotential and higher selectivity for production of HCOOH compared to the planar Bi film.

Conclusion
In summary, we elucidated the mechanisms of spontaneous redox reactions of Bi catalysts in aqueous carbonate solution and investigated the effect of strain at the binary interface in the R-BOC petals on catalytic activity.The R-BOC petals were fabricated by immersion of planar Bi film in aqueous 0.1 M KHCO 3 solution and subsequent ECR.The residual BOC grains in R-BOC petals induced tensile strain at the Bi surface; this strain promotes the catalytic CO 2 RR to HCOOH by stabilizing the *OCHO intermediate.The synergy of nanoporous morphology and reduced thermodynamic reaction barrier resulted in excellent electrochemical properties as well as high selectivity toward HCOOH (FE HCOOH = 95.9% at −0.8 V RHE ).The facile fabrication of BOC nanostructures without an expensive procedure provides a promising route to achieve low-cost, highly efficient CO 2 RR.Moreover, the finding of lattice-mismatch-induced strain effect may guide development of methods to increase the activity of catalysts by using carefully designed heterogeneous interfaces.

Experimental Section
Fabrication of BOC and R-BOC petal: Thirty nanometer-thick Bi thin-film was deposited on a Cu foil (purchased by Nilaco Co.) by a thermal deposition process under a pressure of 1 × 10 −6 torr.Then, the Bi film was immersed into 0.1 M KHCO 3 electrolyte for 30 min under ambient pressure at room temperature.The electrolyte was prepared by dissolving the KHCO 3 salt (Sigma-Aldrich; 99.95%) in deionized (DI) water and then purging with CO 2 gas.After the immersion, the BOC petals were washed with DI water and then dried by N 2 blowing.To convert the BOC petals into the R-BOC petals, ECR reaction was conducted in a threeelectrode system with a working electrode of the BOC petals, a reference electrode of an Ag/AgCl, and a counter electrode of a Pt mesh.The ECR reaction was done for 30 min by applying −0.8 V RHE in CO 2 -saturated 0.1 M KHCO 3 electrolyte, followed by cleaning and drying by DI water and N 2 blowing, respectively.
Characterization: Field-emission scanning electron microscope (FE-SEM) (PHILIPS, XL 30S FEG) was performed at accelerating voltage of 5 kV and working distance of 6 mm.High-resolution transmission electron microscopy equipped with EELS was analyzed at 200 kV with Cs-corrector using JEOL JEM 2200FS.Xray diffraction patterns of the catalysts were characterized using Rigaku, D/MAX-2500-PC.X-ray photoelectron spectroscopy was performed using a Thermo ESCA-LAB250i X-ray photoelectron spectrometer at a base pressure of 1 × 10 −9 torr.The Raman spectra were obtained using a WITECH Alpha 300R Raman spectroscope equipped with a Nd:YAG laser.The excitation wavelength was 532 nm.
Electrochemical measurement: Linear sweep voltammetry, electrochemical ECSA, and the EIS measurements were performed in an H-type cell separated by Nafion membrane with three-electrode system.The three-electrode system consists of working electrode, reference electrode (Ag/AgCl filled with 3 M KCl), and counter electrode (Pt mesh).Electrochemical measurement was performed in an aqueous solution of 0.1 M KHCO 3 saturated with N 2 or CO 2 gas.The measured potential (V Ag/AgCl ) (V) was converted to the reversible hydrogen electrode (V RHE ) (V) by using the Nernst equation: V RHE = V Ag/AgCl + 0.21 + 0.0592 × pH (pH of CO 2 -saturated 0.1 M KHCO 3 has a value of 6.77).All measurements were conducted at an ambient pressure and room temperature.The electrochemical measurement was performed using a multichannel electrochemical analyzer (IVIUM TECH).
Product analysis: Gas products were analyzed using a gas chromatography (Inficon; 3000 Micro GC) equipped with two thermal conductivity detectors (TCD) connected to Molsieve column and Plot U column.Before beginning the electrochemical CO 2 RR, the apparatus was purged with CO 2 gas for 3 min to remove other gases in the H-type cell.During the electrocatalysis, gas was circulated within the reactor at a flow rate of 50 mL min −1 .After the reaction, gas products were automatically pumped into the gas chromatography for analysis.Liquid product of HCOOH was analyzed using a 500 MHz 1H 1D liquid NMR spectrometer (Bruker) with water suppression method at 25 °C.The standard solution was 5 mM N,N-dimethylformamide (Sigma-Aldrich) in D 2 O (Sigma-Aldrich) solvent.The amount of HCOOH product was calculated by integrating areas of the product with that of N,N-dimethylformamide standard solution.
Density functional theory calculations: Vienna ab initio simulation package [54,55] was used to perform calculations with the Perdew-Burke-Ernzerhof [56] exchange-correlation functional.To replace the core electrons with pseudopotentials, the projector-augmented wave [57,58] method is implemented while the valence electrons are described by using a plane-wave basis set with the cut-off energy of 400 eV.Atomic and electronic structure of the system are relaxed until the Hellman-Feynman forces were <0.01 eV Å−1 and electronic convergence of Energy Environ.Mater.2024, 7, e12490 <10 −5 , respectively.The influence of van der Waals interaction is corrected by Grimme's method (D3).For the asymmetric slab cell, dipole correction along the c-axis is applied.All the slab cells of Bi and BOC are constructed with fully relaxed bulk unit cells.From the crystallographic direction and surface energy of materials, a surface vacuum slab with (001) facet is fabricated. [59]The thickness of the Bi and BOC slabs is controlled to have at least 15 Å.The bottom layer of the slab with a thickness of ~5 Å is fixed for the bulk-like region of the slab.The vacuum region of the slab cell is set to have a thickness of 15 Å to avoid the artificial interaction between periodic structures.In the case of strained slab cells, a tensile and compressive strain of 5% along the a-axis is applied for Bi and BOC, respectively.For both Bi and BOC slab cells, a 3 × 3 × 1 k-point grid of Monkhorst-Pack was used for the Brillouin zone integration.
), (004), and (013) planes of Bi 2 O 2 CO 3 (JCPDS #41-1488); this result means that the polycrystalline BOC petals formed spontaneously from the Bi film.During the ECR of the BOC petals at −0.8 V RHE , the diffraction patterns of the Bi 2 O 2 CO 3 vanished from R-BOC within 20 s (Figure S4, Supporting Information).

Figure 1 .
Figure 1.a) Schematic fabrication process of petal-shaped Bi catalyst.Scanning electron microscope image of b) Bi film, c) BOC petals, and d) R-BOC petals (Left images: top view and right images: 5°tilted cross-sectional view).e) Glancing-angle synchrotron X-ray diffraction patterns of Bi film, BOC petals, and R-BOC petals.Lines are offset for clarity.
BOC petals showed the peaks of three bonding states: O-Bi (529.8 eV), OH-Bi (531.8 eV), and O-C (531.0 eV).The O-C bond might originate from CO 2À 3 anions sandwiched by BiO + layers in the BOC petals.After the ECR, the intensity of the O-C peak decreased to one-fifth due to the release of CO 2À 3 from the BOC petals; this change is in good agreement with the formation of metallic Bi in the Bi 4f spectra.

Figure 2 .
Figure 2. XPS spectra of a) Bi 4f and b) O 1s for Bi film, BOC petals, and R-BOC petals.

Figure 3 .
Figure 3. Transmission electron microscope (TEM) images and electron energy loss spectroscopy (EELS) elemental maps of a) BOC petal and b) R-BOC petal.High-resolution TEM images and electron diffraction patterns of c) BOC petal and d) R-BOC petal.

Figure 4 .
Figure 4. a) LSV curves of Bi film and R-BOC petals measured in N 2 -and CO 2 -saturated 0.1 M KHCO 3 electrolytes at a scan rate of 50 mV s −1 .b) Nyquist impedance plots in a frequency range from 2 MHz to 1 Hz and amplitude of 30 mV at −0.8 V RHE .c) Current-density plots at various scan rates.The current densities were obtained from the double-layer charge/discharge curves at 0.33 V RHE .

Figure 5 .
Figure 5. a) Faradaic efficiency and b) current density of HCOOH for Bi film and R-BOC petals.c) Stability tests of Bi film and R-BOC petals at −0.8 V RHE for 30 h.

Figure 6 .
Figure 6.a) Raman spectra of BOC catalysts with different electrochemical reduction (ECR) time (Inset: Variation of the peak position of Bi-Bi vibration and CO 2À 3 with different ECR time).The free energy diagrams of CO 2 reduction to HCOOH for b) BOC and c) R-BOC samples with or without strain.