High‐Yield Electrosynthesis of Formic Acid from CO2 Reduction on Single‐Bismuth Catalyst Loaded on N‐Doped Hollow Carbon Nanospheress

Electroreduction of CO2 into formic acid (HCOOH) is of great economical value and potential for industrialization. However, it is still a substantial challenge due to the lack of efficient catalysts with simultaneously high activity, selectivity, and durability. Herein, a single‐atom bismuth loaded on N‐doped hollow carbon sphere (Bi–SA/NHCS) catalyst is reported and its catalytic activity and selectivity are modulated by changing the coordination structure of Bi center. The obtained Bi–SA/NHCS with a Bi–N3 site exhibits significantly enhanced electrocatalytic activity and selectivity of HCOOH synthesis from CO2 reduction. The HCOOH production rate achieves 16.2 mmol L−1 h−1 cm−2 at a current density of 20 mA cm−2, and its Faradaic efficiency remains 100% during a long‐term reaction. The HCOOH production rate normalized by catalyst loading is at a molar level of nearly 1.5 mol h−1 gcat−1. The production rate and Faradaic efficiency of HCOOH electrosynthesis on Bi–SA/NHCS are significantly boosted as compared with other catalysts reported in the literature. Experimental and density‐functional theory results demonstrate that the boosted activity and selectivity of HCOOH synthesis owe to the electronic structure modulation to the Bi center via threefold coordinated N‐ligands, leading to a proper binding energy of HOCO* intermediates.


Introduction
The renewable electricity-driven reduction of carbon dioxide (CO 2 ) is a promising approach for large-scale generating sustainable chemicals from CO 2 . [1]CO 2 electroreduction requires a multi-electron-transfer process and generates various reduction products based on the number of transferred electrons, including carbon monoxide (CO), carboxylic acid or carboxylate (e.g., formic acid [HCOOH] or formate [HCOO À ]), alcohols (e.g., methanol [CH 3 OH] or ethanol [C 2 H 5 OH]), and hydrocarbon (e.g., methane [CH 4 ] or ethylene [C 2 H 4 ]). [2]owever, the multi-electron-transfer process generally creates more sites for the subsequent hydrogenation step, which associates with a complicated mechanism, resulting in a low production rate and poor Faradaic efficiency of products.Of the various chemicals, HCOOH or HCOO À is a desired reduction product as its formation only needs two-electron transfer, which targets high selectivity toward CO 2 reduction.More importantly, HCOOH is one of the most commercial value materials.The price of HCOOH is around 1099 U.S. dollars per ton, and the HCOO À is estimated to be 444-1123 U.S. dollars per ton. [3]It is not only used as a feedstock in the chemical, textile, and pharmaceutical industries but also can be used as liquid fuel and an efficient carrier of hydrogen (53 g L À1 of hydrogen). [4]In 2021, the global market of HCOOH is about 710 000 tonnes, and it is expected to Electroreduction of CO 2 into formic acid (HCOOH) is of great economical value and potential for industrialization.However, it is still a substantial challenge due to the lack of efficient catalysts with simultaneously high activity, selectivity, and durability.Herein, a single-atom bismuth loaded on N-doped hollow carbon sphere (Bi-SA/NHCS) catalyst is reported and its catalytic activity and selectivity are modulated by changing the coordination structure of Bi center.The obtained Bi-SA/NHCS with a Bi-N 3 site exhibits significantly enhanced electrocatalytic activity and selectivity of HCOOH synthesis from CO 2 reduction.The HCOOH production rate achieves 16.2 mmol L À1 h À1 cm À2 at a current density of 20 mA cm À2 , and its Faradaic efficiency remains 100% during a long-term reaction.The HCOOH production rate normalized by catalyst loading is at a molar level of nearly 1.5 mol h À1 g cat

À1
. The production rate and Faradaic efficiency of HCOOH electrosynthesis on Bi-SA/NHCS are significantly boosted as compared with other catalysts reported in the literature.Experimental and density-functional theory results demonstrate that the boosted activity and selectivity of HCOOH synthesis owe to the electronic structure modulation to the Bi center via threefold coordinated N-ligands, leading to a proper binding energy of HOCO* intermediates.
be 1 300 000 tonnes by 2035. [3]Due to the high market price, vast industry consumption, and simplest reaction process, HCOOH and HCOO À have been regarded as the most economically viable and atom-economic target products for large-scale electroreduction of CO 2 . [5]lectroreduction of CO 2 to HCOOH through a two-electron coupled proton-transfer process.The HOCO* intermediate, which serves as the key intermediate for HCOOH generation, is formed through the first electron step and combines proton to form HCOO À anion without C-O bond cleavage. [6]The HCOOH selectivity is related to the adsorption energy of HOCO* intermediate, which can be affected by regulating the electronic structure of the catalyst. [7]Recently, numerous materials have been exploited for the electrochemical conversion of CO 2 to HCOOH, including metal-based catalysts, [8] carbon-based catalysts, [9] and metal-organic frameworks (MOFs). [10]Among these electrocatalysts, p-block metal (e.g., Pb, In, Sn) materials generally exhibit high selectivity for HCOOH production due to their high O affinity and low H affinity. [11] However, most of these p-block metals suffer from either high toxicity or low activity. [12]10b,14] Unfortunately, these catalysts for reducing CO 2 to HCOOH have not achieved the required combination of high selectivity, high reaction activity, high energy efficiency, and excellent long-term stability.
Single-atom catalysts (SACs) with maximum atomic utilization, fully exposed active sites, and controllable coordination structure have been regarded as attractive electrocatalysts for CO 2 reduction. [15]For instance, partially oxidized single-atom Co converts CO 2 -to-HCOOH with a Faradaic efficiency of 90%. [16]Recently, Bi SACs have been reported for CO 2 electroreduction.The Bi single-atoms-anchored carbon black (Bi SAs/C) can convert CO 2 to HCOOH and its Faradaic efficiency is 83.6%, while the Bi single atoms on N-doped carbon networks are reported to convert CO 2 into CO. [17]It concludes that the single-atom sites enhance the activity of CO 2 reduction, but the selectivity toward products is still unclear.In addition to further improving activity, selectivity, and long-term stability for converting CO 2 to HCOOH, elucidating different selectivity of products on SACs is also a significant issue for rational design of efficient catalysts.
In this work, the single-atom-Bi-anchored N-doped hollow carbon sphere (Bi-SA/NHCS) is designed for highperformance HCOOH synthesis from CO 2 electroreduction.The electronic properties of Bi-SA/NHCS are adjusted by changing the mass and local coordination environment of atomically distributed Bi species.The obtained Bi-SA/NHCS exhibits remarkable activity and selectivity for HCOOH formation.The active sites and mechanisms of HCOOH formation production from CO 2 reduction on Bi-SA/NHCS are revealed by experimental results and density-functional theory (DFT) calculations.

Results and Discussion
Synthesis and Characterization of Bi-SA/NHCS.The Bi-SA/ NHCS was prepared by a template method via stirring, carbonizing, and etching process as illustrated in Figure 1a (details in Supporting Information).NHCS, Bi/NHCS, and Bi-SA/NHCS catalysts exhibit a uniform spherical structure with a diameter of 150-400 nm (Figure 1b and S1, Supporting Information).The diameter of the carbon sphere enlarged with increasing the content of metallic Bi in the precursor.The diameter of Bi-SA/NHCS is about 300 nm and it retains the spherical morphology after the alkali etching while displaying a hollow structure (Figure 1c and S2, Supporting Information).Highmagnification TEM images reveal the existence of pores on Bi-SA/NHCS, the pore structure is further analyzed by N 2 -sorption isotherms (Figure S3, Supporting Information).Meanwhile, no metal nanoparticles are observed on Bi-SA/ NHCS.The atomic-resolution high-angle annular dark-field scanning TEM (HAADF-STEM) image shows isolated bright dots on Bi-SA/NHCS, which might be attributed to the atomically distributed Bi species (Figure 1d,e).The size of the bright dots is about 0.1 nm, demonstrating Bi dispersion at an atomic level.The energy-dispersive spectrometer (EDS) images reveal the uniform distribution of C, O, N, and Bi elements on the carbon framework (Figure 1f-j).
The crystal structure of NHCS, Bi/NHCS, and Bi-SA/NHCS was investigated by X-Ray diffraction measurement (Figure 2a).All the obtained catalysts show a broad diffraction peak located at 23°, which corresponds to the (002) plane of carbon.Three additional diffraction peaks at 27.2°, 37.9°and 39.6°are observed on Bi/NHCS, corresponding to the (012), (104), and (110) plane of metal Bi (JCPDS No. 85-1329), respectively. [18]This result indicates the formation of metallic Bi-doped carbon composite on Bi/NHCS.No Bi-related crystal phase is observed on Bi-SA/NHCS, which is ascribed to the low loading amount of Bi element on Bi-SA/NHCS (0.18 wt%) measured by inductively coupled plasma (ICP) test (Table S1, Supporting Information), while the content of Bi is measured to be 0.78 wt% on Bi/ NHCS.As shown in X-ray photoelectron spectroscopy (XPS) spectra (Figure 2b), the NHCS catalyst presents peaks associated with the C, N, and O elements, while the Bi/NHCS and Bi-SA/ NHCS show a small peak of metal Bi except for the peaks of C, N, and O.The presence of Bi element in Bi/NHCS and Bi-SA/NHCS is further verified by the enlarged XPS spectra (Figure S4, Supporting Information).The content of metallic Bi is increased with the mass increment of Bi-contained precursor, which is consistent with ICP results.15b] Except for pyridinic N, pyrrolic N, graphite N, and oxidized N, the peak related to metal N at 398.9 eV is observed in the high-resolution N 1s spectra of Bi/NHCS and Bi-SA/NHCS (Figure S6, Supporting Information), suggesting the formation metal-N bond on Bi/NHCS and Bi-SA/NHCS.
The atomic structural information of Bi-SA/NHCS was analyzed using X-Ray absorption near-edge structure and the extended X-Ray absorption fine structure (EXAFS).17b] A main peak at %1.8 Å is observed on FT-EXAFS spectra of Bi-SA/NHCS, which might be associated with the either Bi-O or Bi-N bond.To investigate the coordination environment of atomic Bi, the wavelet transform spectra and EXAFS fitting were conducted.According to the EXAFS fitting results, the single-atom Bi is coordinated with the N atom.The coordination number of atomic Bi is around 3.2 with an error range of 0.6, and the distance of Bi and N is fitted to be 2.2 Å (Figure 2e, S7, and S8 and Table S2, Supporting Information).As shown in wavelet transform spectra, the k value peak of Bi-SA is lower than that of Bi 2 O 3 , suggesting that the atomic Bi is bonded with N rather than O element (Figure 2f and S9, Supporting Information), which further certifies the single-atom Bi combined with N atom.Based on these phenomena, the Bi species are atomically distributed and threefold coordinated with N atoms in Bi-SA/NHCS.Evaluation of CO 2 Electroreduction Activity.The electrochemical activity of NHCS, Bi/NHCS, and Bi-SA/NHCS toward CO 2 reduction was investigated by cyclic voltammetry and LSV test in phosphate solution (pH 6.8) saturated with CO 2 or Ar (Figure 3a and S10, Supporting Information).The phosphate solution was used to eliminate the effects of pH variation by bubbling the CO 2 and Ar into KHCO 3 solution.NHCS, Bi/NHCS, and Bi-SA/NHCS catalysts show a higher current density in the CO 2 -saturated electrolyte than that in the Ar-saturated electrolyte, suggesting that the prepared catalysts are active for CO 2 reduction.Of all the obtained catalysts, Bi-SA/NHCS exhibits the highest current density for CO 2 reduction with a value of 2.5 mA cm À2 at the potential of À0.9 V versus reversible hydrogen electrode (RHE), which is 2.8 and 2.5 times higher than that of NHCS (0.9 mA cm À2 ) and Bi/NHCS (1.0 mA cm À2 ) at the same potential, respectively.The onset potential (CO 2 reduction current density achieved 0.1 mA cm À2 ) for CO 2 reduction on Bi-SA/NHCS is À0.01 V, which is more positive than that on NHCS (À0.45 V) and Bi/NHCS (À0.49V).These results demonstrate that the Bi-SA/NHCS is more active for CO 2 reduction reaction than both NHCS and Bi/NHCS.Electrochemical impedance spectroscopy was performed to investigate the electrontransfer capability (Figure 3b).Obviously, the Bi-SA/NHCS exhibits a lower charge-transfer resistance than NHCS and Bi/NHCS, suggesting a more favorable electron transfer during the CO 2 reduction process.Such fast electron transfer promotes the formation of CO 2 •À intermediate from adsorbed CO 2 molecules through the first electron-transfer process, [19] thus leading to a higher current density and more positive overpotential of Bi-SA/NHCS toward CO 2 reduction.These results demonstrate that single-atom Bi-doping into a carbon framework can boost its CO 2 reduction activity.
To identify the reduction products, potentiostatic electrolysis of CO 2 was measured in an H-type cell reactor with a threeelectrode system.HCOOH is the only liquid product for all the tested catalysts in the potential range from À0.8 to À1.3 V versus RHE (Figure 3c and S11a, Supporting Information).
No gaseous products are detected on Bi-SA/NHCS (Figure S12, Supporting Information), while a small amount of CO and H 2 is observed on the NHCS and Bi/NHCS catalyst.The electrolysis of CO 2 is further performed in an Ar-saturated electrolyte, and no signal of HCOOH is detected on the 1 H nuclear magnetic resonance (NMR) spectrum (Figure S11b, Supporting Information).The isotope experiment using 13 CO 2 gas as a carbon source was performed to analyze the source of HCOOH.As depicted in the 13 C NMR spectrum, only one peak at a chemical shift of 160 ppm related to the H 13 COOH is observed (Figure S13, Supporting Information). [20]These phenomena demonstrate that the liquid product HCOOH is ascribed to CO 2 reduction.As depicted in Figure 3c, all the catalysts present a volcanic curve with HCOOH production rate and applied potential.Among the tested catalysts, Bi-SA/NHCS exhibits the highest yield of HCOOH production, which is much higher than that of NHCS and Bi/NHCS, indicating that the incorporation of single-atom Bi into carbon framework significantly enhanced its CO 2 reduction activity and HCOOH selectivity.The HCOOH production rate increases from 100.9 to 1123.6 μmol L À1 h À1 cm À2 with the potential negative shifted from À0.8 to À1.1 V, while the value decreases to 677.9 μmol L À1 h À1 cm À2 with the potential further shifted to À1.3 V.It is worth noting that the HCOOH forms at the potential of À0.8 V, corresponding to an overpotential of 190 mV for generation of HCOOH from CO 2 .The HCOOH production rate normalized by catalyst loading is 1513.5-11824.5 μmol h À1 g cat À1 on Bi-SA/NHCS (Figure 3d).The maximum production rate of CO 2 -to-HCOOH achieves 11 824.5 μmol h À1 g cat À1 on Bi-SA/NHCS at 1.1 V versus RHE and its Faradic efficiency reaches 95.1%, which greatly outperformed NHCS and Bi/NHCS catalyst.
The HCOOH production rate is limited by the low solubility of CO 2 in aqueous solution (33 mmol L À1 in 0.1 mol L À1 KHCO 3 ) and the large mass-transfer resistance of CO 2 diffusion. [21]To address these challenges, CO 2 reduction experiments were performed in a flow-through electrochemical cell with a gas diffusion electrode (GDE).In the flow-through cell configuration, CO 2 reduction experiments are driven with a galvanostatic system, and the cell potential is monitored by the electrochemical workstation.With such GDE setting, the yield of HCOOH production is markedly enhanced for all the tested catalysts.It is worth noting that the yield of HCOOH electrosynthesis linearly increases with the current density increasing from 5 to 20 mA cm À2 for all tested catalysts (Figure 4a and S14, Supporting Information).Especially, the Bi-SA/NHCS exhibits the highest HCOOH production rate and is capable of continuously producing HCOOH with increasing concentration from 2.5 to 16.2 mmol L À1 h À1 cm À2 in the current range of 5-20 mA cm À2 .As depicted in Figure 4a, Bi-SA/NHCS shows the maximum HCOOH generation rate of 16.2 mmol L À1 h À1 cm À2 , which is 16.2 and 2.5 times higher than that of NHCS (1.0 mmol L À1 h À1 cm À2 ) and Bi/NHCS (6.5 mmol L À1 h À1 cm À2 ), respectively, highlighting the promotion effect brought by atomic Bi doped into carbon framework, while further increasing the current density to 30 mA cm À2 results in the decrement of HCOOH production rate to 7.9 mmol L À1 h À1 cm À2 , which might be caused by the HER.The HER that occurs on BiSA/NHCS at high current density is further certified by its Faradaic efficiency and gaseous products analysis.The Faradaic efficiency of HCOOH production is calculated to be 42.1% at a current density of 30 mA cm À2 , and slight H 2 is detected by gas chromatography (Figure 4b and S15, Supporting Information).Importantly, the Faradaic efficiency of HCOOH production on Bi-SA/NHCS is nearly 100% at the current density of 10 and 20 mA cm À2 , demonstrating the superior selectivity of HCOOH generation from CO 2 reduction.The HCOOH production rate normalized by catalyst loading is calculated to be 215.8-1458.0mmol h À1 g cat À1 on Bi-SA/NHCS.At the current density of 20 mA cm À2 , the production rate of CO 2 -to-HCOOH is at a molar level of nearly 1.5 mol h À1 g cat À1 .
According to the literature, the Bi-related catalysts generally show a high Faradaic efficiency.This phenomenon is ascribed to its high O affinity and low H affinity, which leads to a preferable HOCO* adsorption energy, thus benefitting HCOOH production.4b,10,14a,b,21,22] It is interesting to see that the yield of HCOOH production depends on the Bi-SA/NHCS catalyst loading amount on GDE. Figure 4d displays the production rate and Faradic efficiency of HCOOH synthesis versus catalyst dosage.The HCOOH production rate achieves 3.9, 16.2, 6.8, and 3.4 mmol L À1 h À1 cm À2 when the catalyst loading amount of 0.3, 0.5, 0.75, and 1.0 mg cm À2 (Figure S16, Supporting Information).The yield of HCOOH production normalized by per mass unit of catalyst is calculated to be 585.0,1,458.0,409.8, and 153.5 mmol h À1 g cat À1 at catalyst loading amounts of 0.3, 0.5, 0.75, and 1.0 mg cm À2 .4b,10,14a,b,21,22] The production rate of HCOOH decreases when further increasing catalysts dosage from 0.5 to 1.0 mg cm À2 , which might be resulted from the shedding of catalysts during the flow-through reaction.It is worth noting that the HCOOH concentration shows a volcanic relationship with the concentrated KOH electrolyte (Figure S17, Supporting Information).The HCOOH concentration largely increases from 0.43 to 16.2 mmol L À1 h À1 cm À2 with the electrolyte concentration increasing from 0.5 to 1.0 mol L À1 .The low production rate and correspondingly poor Faradaic efficiency of HCOOH formation might results from the HER process, as concentrated KOH could suppress HER during CO 2 reduction. [23]Although the HCOOH concentration slightly decreases to 14.1 mmol L À1 h À1 cm À2 with a further increase in the electrolyte concentration to 1.5 mol L À1 , the Faradaic efficiency of HCOOH electrosynthesis remains 100%.The Bi-SA/NHCS is capable of producing a higher concentration of HCOOH by changing the electrolyte flow rate (Figure S18, Supporting Information).The HCOOH formation rate achieves 15.9-16.2mmol L À1 h À1 cm À2 at an electrolyte flow rate in the range of 21-60 mL h À1 at the current density of 20 mA cm À2 , and the Faradaic efficiency of HCOOH production is kept at 100% at the investigated range (Figure S18, Supporting Information).Although the concentration of accumulated HCOOH slightly decreases to 13.5 mmol L À1 h À1 cm À2 , the yield of HCOOH production is further increased to 1619.0 mmol h À1 g cat À1 .These results indicate that the concentrated HCOOH electrosynthesis with nearly 100% Faradaic efficiency is successfully realized on Bi-SA/NHCS catalyst.Moreover, the production rate of HCOOH synthesis can be further promoted by the scale-up of the device.
The stability of HCOOH electrosynthesis on Bi-SA/NHCS was evaluated through the five successive cycles and the longterm operation reaction.After five cycles, the accumulated HCOOH concentration is almost undiminished compared with the fresh Bi-SA/NHCS catalyst, revealing good repeatability of Bi-SA/NHCS (Figure S19, Supporting Information).The long-term operational stability is measured at a current density of 20 mA cm À2 .The activity and selectivity of HCOOH synthesis are maintained at a constant during the long-term testing (%10 h) with generated HCOOH concentration of 16.2 mmol L À1 h À1 cm À2 and Faradaic efficiency of 100% (Figure 4e and S20, Supporting Information).The spherical morphology of the post-catalysts is retained, and the XPS spectra show a similar structure to the fresh Bi-SA/NHCS before the reaction (Figure S21 and S22, Supporting Information).These results demonstrate the good physical and chemical structure stability of Bi-SA/NHCS and good repeatability and long-term stability toward electrosynthesis of HCOOH from CO 2 reduction.The good stability might be caused by the low level of Bi loading content and negative formation energy of Bi-N 3 structure (À0.09 eV) (Figure S23, Supporting Information). [24]

Mechanisms Analysis
The DFT calculations are conducted to elucidate the mechanisms for enhancing CO 2 -to-HCOOH on Bi-SA/NHCS.According to the EXAFS results, the metallic Bi is atomically distributed and coordinated with three N atoms in Bi-SA/NHCS.Four catalyst models were constructed, involving a pristine graphite sheet (Gr), Bi-cluster-doped graphite sheet (Bi x -Gr), single-atom Bi-doped graphite sheet with three N atoms (Bi-N 3 ), and four N atoms (Bi-N 4 ).The distances of Bi-N in these models are about 2.3 Å, which is close to the bond length of Bi-N obtained by EXAFS results (Figure S23 and Table S4, Supporting Information).Generally, the electrocatalytic CO 2 reduction to HCOOH is a two-electron-transfer process with activation of CO 2 to form HOCO*, reduction of HOCO* to generate HCOOH*, and subsequently desorption of *HCOOH to produce HCOOH.Therefore, the Gibbs free energy of HOCO*, HCOOH*, and HCOOH formation is calculated and the lowest energy pathway of HCOOH generation from CO 2 reduction on Gr, Bi x -Gr, Bi-N 3 , and Bi-N 4 is illustrated in Figure 5a, S24, and S25, Supporting Information.
The free energy change for the overall process of HCOOH synthesis on Gr and Bi x -Gr is positive, while that on Bi-N 3 , and Bi-N 4 is negative, confirming the catalytic capacity associated with single-atom Bi.Activation of CO 2 molecule through single-electron transfer is the first step for CO 2 reduction, which is generally served as the rate-limiting step.It is worth to note that the incorporation of metallic Bi clusters and single-atom Bi into the carbon framework could reduce the energy barrier for CO 2 activation to HOCO*.Comparing to the metallic Bi, single-atom Bi is more active for CO 2 activation and thus convert CO 2 -to-HCOOH as the ΔG value for HOCO* intermediate generation on Bi-N 3 and Bi-N 4 sites is more negative.These results are consistent with the HCOOH production rate that doping of single-atom Bi significantly boosts the activity toward CO 2 -to-HCOOH.Based on the current DFT results, the ΔG value of *HOCO formation calculated at À0.8 V for HOCO* generation on Bi-N 3 and Bi-N 4 is negative, indicating that the activation of CO 2 can occur readily on both Bi-N 3 and Bi-N 4 sites (Figure 5a,b).Therefore, we further calculate the density of states (DOS) and charge density calculation of Bi-N 3 and Bi-N 4 structure (Figure 5c, S26, and S27, Supporting Information).According to DOS profiles, CO 2 and Bi-N 3 and Bi-N 4 possess overlapped states around the Fermi level, indicating the potential covalent bonding between Bi-N 3 or Bi-N 4 and CO 2 .The charge density suggests that the electrons are favorable to transfer from Bi-N 3 or Bi-N 4 to CO 2 molecules, as confirmed by calculated Bader charge (see Figure S27, Supporting Information).Under such case, initial CO 2 reduction is downhill in free energies on both Bi-N 3 and Bi-N 4 , which is beneficial for CO 2 reduction.It is worth noting that the ΔG value of HOCO* formation on Bi-N 3 is calculated to be À1.36 eV, being lower than that on Bi-N 4 , demonstrating that CO 2 activation to form HOCO* is more facile on Bi-N 3 site.The subsequent reduction of HOCO* to HCOOH* is downhill on both Bi-N 3 and Bi-N 4 , which is beneficial for adsorption of proton to form HCOOH*. Notably, the ΔG for converting HOCO* to HCOOH* on Bi-N 3 is À1.77 eV, which is still more negative than that on Bi-N 4 .These calculations indicated that Bi-N 3 is more active for HCOOH production than Bi-N 4 .This is not surprising because CO 2 can be effectively activated with electron injection into its antibonding π*; therefore, the activity of Bi-site is essentially affected by its reduction capacity.Further calculations of charge density reveal that the electrons are favorable to inject into CO 2 molecules on the Bi-N 3 site.As indicated by calculated Bader charges δ (see Figure S27, Supporting Information), threecoordinated Bi shows lower oxidation degree (δ = 0.77e) than that in Bi-N 4 (δ = 1.14e), partially keeping its metallic nature.As a result, Bi-N 3 presents stronger capacity to adsorb and activate CO 2 via Bi!CO 2 electron injection, as supported by CO 2 -Bi distances and local charge on adsorbed CO 2 .Similar catalytic performance has been reported for low valence state of metals, such as copper [1c] and zinc. [25]lectrocatalytic CO 2 reduces to HCOOH through a twoelectron-transfer process, which generally competes with CO formation.For CO formation, it goes through CO 2 !COOH*!CO* pathway, and the energetic pathway for CO formation on Bi-N 3 is calculated (Figure 4b and S28, Supporting Information).The selectivity-determining step for HCOOH versus CO formation is the activation of CO 2 to produce the HOCO* step.At the potential of À0.8 V versus RHE, the ΔG for initial CO 2 reduction to produce HOCO* is À1.36 eV, which is more negative than that for COOH* formation (ΔG = À0.12 eV).This result demonstrates that it is a favor for HOCO* formation from CO 2 activation, which accounts for the high selectivity of HCOOH production on the Bi-N 3 site.

Conclusion
In summary, an efficient Bi-SA/NHCS catalyst with a Bi-N 3 site has been designed for active and selective electroreduction of CO 2 to HCOOH.The production rate of HCOOH remains 16.2 mmol L À1 h À1 cm À2 and the Faradaic efficiency of HCOOH production is achieved 100% during a long-term reaction.The HCOOH production rate normalized by catalyst loading is calculated to be 215.8-1458.0mmol h À1 g cat À1 on Bi-SA/NHCS, which significantly boosts as compared with other electrocatalysts reported in the literature.Based on the experimental results and DFT calculations, the high activity and selectivity of Bi-SA/NHCS for HCOOH production are attributed to the atomic dispersion of Bi species and its threefold coordination of N atoms, which reduces the energies barrier for CO 2 activation and shows a downhill in free energy change for HCOOH formation.We anticipate the strategy for improving CO 2 -to-HCOOH by constructing the coordination structure of the single-atom metal will provide new insights for the rational design of efficient electrocatalysts for converting CO 2 to high-value products.

Figure 4 .
Figure 4. a) Production rate of CO 2 -to-HCOOH in the flow-through reactor.b) Faradic efficiency and normalized production rate of CO 2 -to-HCOOH on Bi-SA/NHCS in the flow-through reactor.c) Comparison of Faradic efficiency and production rate of CO 2 -to-HCOOH with previously reported catalysts.d) Effect of catalysts dosage on Faradic efficiency and normalized production rate.e) Various potential and production rates at the long-term reaction.

Figure 5 .
Figure 5. Density-functional theory (DFT) calculations.a) Free energy diagrams calculated at a potential of À0.8 V for CO 2 reduction to HCOOH.b) Free energy for activation of CO 2 to form HCOOH* and CO* at À0.8 V. c) Density of states of CO 2 adsorption on Bi-N 3 .d) Optimized structures of all reaction intermediates involved in the pathways of CO 2 -to-HCOOH on the Bi-N 3 site (golden: C; white: N; purple: Bi; red: O; and pink: H).