Structural characterizations of BiCuSeO Ns. Herein, ultrathin BiCuSeO nanosheets are synthesized by using polyvinylpyrrolidone (PVP) as the control agent via a mild hydrothermal route. X-ray diffraction pattern (XRD) and Raman spectrum, shown in Supplementary Fig. S1, all solidly index to the successful synthesis of the crystalline BiCuSeO phase with high purity. The atomic force microscopy (AFM) and transmission electron microscopy (TEM) characterizations in Fig. 1c, d and Supplementary Fig. S2 reveal sheet-like morphology with an average thickness of ~ 9.2 nm of obtained BiCuSeO. The high-resolution TEM (HRTEM) image shows two sets of mutually perpendicular lattice fringes with a spacing of ~ 0.28 nm (Fig. 1e), corresponding to the (110) and (1–10) planes of tetragonal BiCuSeO respectively. A single set of diffraction spots with a fourfold symmetry in the selected area electron diffraction (SAED) pattern (Fig. 1f) illustrates the high orientation along c axis of the as-obtained BiCuSeO Ns, which is consistent with HRTEM analysis. Additionally, the energy dispersive spectroscopy (EDS) mapping analysis points out the uniform distribution of Bi, Cu, Se and O elements (Fig. 1g). Consequently, all above structural characterization results evidently verify the successful synthesis of ultrathin BiCuSeO single-crystal nanosheets.
Electrochemical performances of BiCuSeO in 0.5 M KHCO 3 . The electrochemical performance of the as-fabricated BiCuSeO is evaluated using a three-electrode flow cell in CO2-saturated KHCO3 aqueous solution (Supplementary Fig. S3). To investigate the effect of Bi and Cu elements in BiCuSeO for CO2RR, the electrocatalytic activities of Bi2O3 and Cu2Se Ns (Supplementary Fig. S4 and S5) are also tested for comparisons. First, the linear sweep voltammetry (LSV) curves are conducted in a potential range of 0~-1.1 V (the reversible hydrogen electrode, RHE, all the potentials mentioned in the following are RHE). As shown in Fig. 2a, BiCuSeO reveals significantly improved current density and more positive onset potential than Cu2Se and Bi2O3, demonstrating its best electrocatalytic performance for CO2RR. To identify the products and their Faraday efficiency (FE) at different potentials, we conduct the electrolysis at a variety of constant potentials from − 0.4 to -1.1 V and collected samples for the further test. From the chronoamperometry curves (Fig. 2b), the current density is consistent with LSV curves and remains steady, suggesting a good electrochemical stability of BiCuSeO catalysts. Accordingly, the gaseous and liquid products are quantitatively analyzed by online gas chromatograph (GC) and 1H nuclear magnetic resonance (1H NMR) spectroscopy, respectively. The GC and NMR results display that formate is the predominant product, accompanied with minor amounts of H2 and CO gas. Apparently, the BiCuSeO shows a high selectivity towards formate production, and its FE (FEformate) is over 90% in a wide potential window ranging from − 0.4 to -1.1 V and a maximum value can reach to ~ 93.4% at -0.9 V, while the FE for CO and H2 gas are ~ 2.4% and ~ 2.4%, respectively (Fig. 2c). Noticeably, the overpotential for formate generation is as low as 190 mV, which is smaller than that of most other Bi-based catalysts11, 30, 31, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47. Moreover, the FEformate of for BiCuSeO are much higher than that of Cu2Se (the maximum FEformate is ~ 60%) and Bi2O3 (the maximum FEformate is ~ 85%) at the tested potentials, indicating that BiCuSeO Ns is more inclined to yield the formate product (Fig. 2d). Furthermore, the calculated formate partial current densities (jformate) of BiCuSeO are significantly larger than that of Cu2Se and Bi2O3 at same potentials, and the maximum value can reach ~ 47.5 mA cm− 2 at -1.1 V without IR compensation (Fig. 2e). This result underlies that the CO2RR on BiCuSeO Ns electrocatalysts is significantly more efficient and selective. Gratifyingly, compared with other state-of-the-art Bi-based catalysts11, 30, 31, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, BiCuSeO can hold the outstanding formate selectivity over a wide potential window (> 90% from − 0.4 to -1.1 V, Fig. 2f). In general, the CO2 reduction rate will increase obviously in the alkaline electrolyte. Therefore, the electrocatalytic performance of CO2 are also tested in 1 M KOH and exhibit similar outstanding formate selectivity over a wide potential window and impressive current density ~ 267 mA cm− 2 at -1.1 V (Supplementary Fig. S6). In addition, Tafel slope is determined by using the logarithm of formate partial current density against with the applied potentials to evaluate the reaction kinetic of CO2 RR3, 48. The calculated Tafel slope of BiCuSeO is about 568.6 mV dec− 1, which is much smaller than that of Cu2Se (1052.4 mV dec− 1) and Bi2O3 (674.4 mV dec− 1), suggesting its favorable kinetics of BiCuSeO for formate generation (Fig. 2g). Furthermore, electrochemical reduction reaction for CO2 at a fixed potential of -0.9 V is carried out over an extended period to evaluate the stability of the BiCuSeO. As shown in Fig. 2h, the total current density stabilizes at about 32 mA cm− 2 together with an average FEformate of ~ 93% over 10 h. Remarkably, by the SEM images, the morphology of BiCuSeO catalysts is substantially preserved (Supplementary Fig. S7). To sum up, all the above results indicate that the BiCuSeO exhibits splendid formate selectivity in a wide potential window for CO2RR.
Intermediates detection. To monitor the reaction process and the intermediate species of CO2RR15, 49, we carry out in situ electrochemical Raman spectroscopy tests (Fig. 3a). Two obvious Raman peaks at 1160 and 1540 cm− 1 are detected at -0.5 V or lower potentials in Fig. 3b, c. Specifically, the peaks at 1160 cm− 1 can be ascribed to the C = O stretching vibration of surface adsorbed carbonate (νsCO2˙−) during the CO2RR electrolysis process (Fig. 3b, c, Supplementary Fig. S8)50, 51, 52. Meanwhile, the peaks located at 1540 cm− 1 are vibrational fingerprints of asymmetric C–O stretching vibration modes of proton-trapped carboxylate *CO2˙− radicals (νasCO2˙−)50, 51, 52. Therefore, both peaks are attributed to the key intermediates HCOO* for formate product during electroreduction CO251, 52, 53, which also confirm that CO2 could be directly activated and reduced into formate with lower potential on BiCuSeO catalysts. Notably, their peak intensity gradually increases as the applied potentials and reached a high point at -0.7 V. This variation trend of characteristic peak intensity with potentials may depend on the trade-off between adsorption and transformation of intermediate products. Moreover, the time-count in situ Raman spectra at -0.6 and − 0.7 V show that the peaks intensity of the crucial intermediates for formate gradually strengthen with expanding time (Fig. 3d, e), resulting from a favorable adsorbing and proton-trapping capacity of formate intermediates. This indicates a step-by-step reaction process for formate generation from CO2. No obvious band associating with C–O stretching or C = O stretching of *COOH intermediate for CO product indicating that the formation of CO on BiCuSeO is almost suppressed50, 51, 52, 54, which is consistent with the above experimental FE results. To validate the binding affinity of CO2˙−, the adsorption of OH− as a surrogate for CO2˙− is detected by oxidative LSV scans in a N2-bubbled 0.1 M NaOH electrolyte. Figure 3f reveals that the potential for surface OH− adsorption on the BiCuSeO is lower than that for Bi2O3 and Cu2Se Ns. This result combined with the Raman analyses adequately illustrates that the BiCuSeO possesses a stronger adsorption affinity of OH−, and hence they could efficiently stabilize the CO2˙− intermediate, finally facilitating formate production.
Structural transformation, XAFS, XPS and TEM characterizations of BiCuSeO after CO 2 RR. Actually, metal oxides based electrocatalysts would inevitably undergo spontaneous reduction under the function of negative potential during CO2RR process16. XRD pattern and Raman spectrum in Supplementary Fig. S9a-c show that the crystalline BiCuSeO phase can still be mainly retained with partial Se escaping during CO2RR process. To further determine whether the structure of the Bi-O layer can be maintained as expected, the synchrotron radiation X-ray absorption fine structure (XAFS) spectroscopy were performed. Specifically, Bi L3-edge XAFS measurements are explored and presented in Fig. 4a. Both the absorption edge and white line peak of the XANES for BiCuSeO nearly overlaps with that of Bi2O3 reference, suggesting the Bi3+ species of Bi atoms in BiCuSeO. After CO2RR, the absorption edge of BiCuSeOR (the BiCuSeO after CO2RR is denoted as BiCuSeOR) only slightly shifts towards a lower energy, indicating that the oxidation state of Bi mainly retains. Furthermore, the Fourier transform (FT) of the EXAFS curve is resolved to evaluate the Bi local environment at atomic-level (Fig. 4b). The intense peak of BiCuSeOR is located 1.63 Å (without chemical shift), which is still consistent with the pristine BiCuSeO and can be attributed to the first Bi-O coordination shell. The relatively controlled peak weakening further explains that most of the Bi-O bonds of BiCuSeO are maintained after CO2RR. Meanwhile, wavelet transform (WT) is used to precisely analyze the Bi L3-edge extended X-ray absorption fine structure (EXAFS) oscillations. The WT contour plots of BiCuSeOR display only one intensity maximum at 1.6 Å, which is apparently corresponding to the Bi-O coordination with BiCuSeO rather than the Bi-Bi in Bi powders. Notably, combined with its crystal structure, the length of Bi-O bond in the BiCuSeO superlattice is significantly larger than that in the common Bi2O3, showing a new type of Bi-O coordination structure for CO2RR. Furthermore, to clearly illustrate the coordination states, the intense peak is finely fitted and the fitting result suggests that it consists of two peaks superimposed, named as Bi-O1 (2.13 Å) and Bi-O2 (2.27 Å) (Supplementary Fig. S10-12, Table S1). The Bi-O2 is attributed to the lattice bond in BiCuSeO, and the coordination number (CN) is calculated to be 2.5 (Supplementary Table S1), which is close to the value for BiCuSeO (CN = 4), further illustrating that only a small amount of reduced Bi3+ after CO2RR different from contrastive Bi2O3 Ns. The fitted coordination number of metal Bi-Bi bond (3.09 Å) for BiCuSeOR is only 0.3 (Supplementary Table S1), which also supports the above conclusion. Combined with in situ Raman results (Fig. 3), the appearance of the Bi-O1 can be reasonably attributed to chemisorbed *OH and HCOO* species, suggesting the effective adsorption of intermediates in CO2RR. Taken together, all these results clearly illustrate that Bi sites are still retained in the oxidation state under the negative potential during CO2RR as assumed, which certainly is conducive to drive the conversion of CO2 to formate (Fig. 4a, b, e, Supplementary Fig. S10-12, Table S1). By contrast, combined with XAFS, XRD and XPS, Bi2O3 does essentially transform the metal Bi as reported (Fig. 4a, b, e, Supplementary Fig. S16).
In addition to the Bi-O bond, it is noteworthy that the Bi-Se bond weakens dramatically, with the coordination number reduced from 8 to 0.8 (Fig. 4b, Supplementary Table S1), which results from the precipitation of Se. In general, metal selenides are very sensitive and may undergo structural evolution to form metal oxide catalysts during the electrochemical process. Meanwhile, Cu-Se sublayer as conductive layer undertake electron transport, leading to the self-reduction. Therefore, the local structure of Cu-Se sublayers are also investigated by Cu K-edge XAFS spectra (Fig. 4c). First, the absorption edge for Cu K-edge XANES spectrum of BiCuSeOR locates between Cu foil and BiCuSeO, implying that Cu+ species are partially reduced and the valence state of its Cu atoms is between 0 and + 1. Furthermore, EXAFS curves of Cu K-edge reveal that the main peak at 2.06 Å disappeared and two new characteristic peaks appeared at 1.93 Å and 2.58 Å for BiCuSeOR (Fig. 4d, Supplementary Fig. S10, S11, S13, Table S1), respectively. Based on the comparison, the emergent peaks can be attributed to Cu-O bonds and metallic Cu-Cu bond. Meanwhile, the calculated coordination number for Cu-Se bonds, Cu-O bonds and metallic Cu-Cu bond are 0.7, 0.8 and 4, respectively (Supplementary Fig. S10, S11, S13, Table S1). Based on the above structural characteristics, it is reasonable to assume that in the process of CO2 reduction, a large number of Se atoms overflow from the conductive Cu-Se lattice. To keep the structure stable, some of the space left are filled with ambient oxygen, while some of the copper atoms bonded directly (Fig. 5a). Similarly, the in situ XANES, EXAFS and XPS (Fig. 4f, Fig. 5a, Fig. 5e and Supplementary Fig. S14, S15, Table S1) conformably point to their structural features and further reinforce our inference. As a control, we also explore the structural evolution process of Cu2Se during CO2 reduction. Similar to other conventional selenides and Cu-Se layer of BiCuSeO, Se atoms are essentially escaped from the Cu2Se lattice and thus completely transform into Cu and Cu2O (Supplementary Fig. S17, S18).
To further confirm the structural features, X-ray photoelectron spectroscopy (XPS) are also carried out. From Fig. 5b, the intensity of two typical Se 3d characteristic peaks exhibit a sharp decrease for BiCuSeOR compared to that for BiCuSeO34, 55, suggesting that a large percentage of Se escape from the Cu-Se sublayers during the electrocatalytic reaction. Meanwhile, the indistinct Se distribution through the BiCuSeOR nanosheet in EDS mapping explain the loss of Se (Fig. 5g), which are identical with XAFS and XPS results (Fig. 4, Fig. 5a, Supplementary Fig. S10-S15). Furthermore, the high-resolution O 1s XPS spectrum of pristine BiCuSeO (Fig. 5c) can be split into three deconvolution peaks at circa 529.37, 530.87, and 533.03 eV, which belong to Bi-O, Bi-OH, and surface-adsorbed oxygen species, respectively30, 31. After CO2RR, besides the peaks at circa 530 eV corresponding to Bi-O and 531.92 eV corresponding to Bi-OH, an intense peak with higher binding energy at 535.29 eV, arising from surface adsorbed carbonate species56, 57, appear in the O 1s spectrum of BiCuSeOR. These observations are in line with the corresponding Raman results (Fig. 3) and further suggests that the CO2 molecules are stably adsorbed onto the surface of the BiCuSeO catalyst during the electrocatalytic CO2RR. As expected, the XPS Bi 4f core-level spectra of BiCuSeOR and BiCuSeO remain consistent (Fig. 5d), which confirms the existence of metallic Bi11, 47. Interestingly, in addition to changes in the local structure and chemical states, the structural framework of the BiCuSeO superlattice tends to be stable during CO2RR, which may be due to the mutual support of the sublayers (Fig. 5). In summary, XANES, EXAFS, XPS and HRTEM studies consistently confirm that the highly active Bi oxidation state can be stabilized by finely designing superlattices stacked with Bi-O layer and conductive Cu-Se layer (Fig. 4, Fig. 5, Supplementary S14, S15), which can undoubtedly contribute to high selective CO2 electroreduction performance at a wide potential window and explore its structure-activity relationship.
DFT calculations. To explore the intrinsic origin of high formate selectivity and activity over the [Bi2O2]2+ sublayers in BiCuSeO superlattice, DFT calculations are performed to investigate the electronic structure of catalyst as well as the strength of the interaction between catalytic interface and CO2 molecular species (Supplementary Fig. S19a). Firstly, to verify whether the [Cu2Se2]2− sublayer of BiCuSeOR alters and functions as conductive channel after the structural transformation, their electron charge density and density of states (DOS) are studied. Clearly, the typical positive charges feature localizes along [Cu2Se2]2− sublayers after structural transformation (Fig. 6a), suggesting [Cu2Se2]2− sublayers still maintains a good conductivity and can be employed as the conductive channel. Moreover, the total DOS (sum) of BiCuSeOR in the neighborhood below the Fermi level is mainly contributed by bonding hybridized Cu d and Se p states from Cu2Se2]2− sublayers (Fig. 6d), which is the origin endowing the conductive character. The sharp peak characteristic in the PDOS of Cu d orbital further illustrate a strong d electron localization, resulting in the localized charge distribution in [Cu2Se2]2− sublayers. More notably, different from pristine BiCuSeO reported previously33, the DOS of BiCuSeOR below Fermi level cross over Fermi surface, implying the typical semi-metallic/metallic nature and thus an enhanced conductivity for BiCuSeOR after structural evolution. The above results and analyses consistently indicate that [Cu2Se2]2− sublayers in superlattice still maintain fine and even enhanced conductivity. Meanwhile, PDOS of Bi p and Bi s orbitals overlap with that of O p orbitals to a great extent, indicating a strong interaction between Bi atoms and O atoms, and the oxide state of Bi in [Bi2O2]2+ sublayer is negligibly influenced by structural transformation and can be well retained (Fig. 6e). All of the above results show that [Cu2Se2]2+ sublayer still functions as a conductive channel while [Bi2O2]2+ sublayer maintain strong Bi-O coordination structure feature in BiCuSeO superlattices after structural transformation. To dive deep into the interaction process between catalytic interface and CO2 molecules, we further calculate the charge density difference and DOS of HCOO*intermediate adsorbed BiCuSeOR. The charge density difference (Fig. 6b, c, Supplementary Fig. S20) reveals that the charge transfers directly from Bi atoms in [Bi2O2]2+ sublayers to O atoms in HCOO* intermediate, which is beneficial to activate CO2 molecules as well as generate and stabilize formate intermediates HCOO* on BiCuSeOR surface. Even more, HCOO* absorbed BiCuSeOR displays a higher DOS value near the Fermi level than BiCuSeOR, further suggesting that the good charge transfer from BiCuSeOR towards formate intermediates HCOO* (Supplementary Fig. S21). Actually, large overlaps of Bi p states and O p states below the fermi level in the PDOS of HCOO* absorbed BiCuSeOR (Fig. 6f, Supplementary Fig. S22) further point out that Bi atoms in [Bi2O2]2+ layer of BiCuSeOR have a strong interaction effect with the O atoms in HCOO* intermediates, which contributes to the absorption and stabilization of HCOO* intermediate on BiCuSeOR surface. Taken together, new typical Bi-O oxide structure the [Bi2O2]2+ sublayer endows strong activation capacity of CO2 molecules and stabilization ability of intermediates HCOO*, resulting in a high activity for CO2RR.
To better elucidate the high selectivity of CO2RR, reaction Gibbs free energies (∆G) for CO2 electroreduction to formate on catalytic model (Supplementary Fig. S19) are calculated and presented in Fig. 6g. For comparison, Gibbs free energies for CO2 electroreduction to CO and competitive HER are also performed (Fig. 6h, i, Supplementary Fig. S19). The Gibbs free energies for HCOO* formation processes on four possible sites are all exergonic (∆GHCOO* on sites 1–4 are − 0.4551, -0.4957, -0.4827, and − 0.4575 eV respectively), indicating their CO2 activation and protonation processes are spontaneous. Whether CO2 electroredution to form *COOH or HER to form *H are endergonic (∆G*COOH = 0.8994, 0.8900, 0.8498, 0.8471eV; ∆G*H = 0.7165, 0.8614, 0.70244, 0.7035eV), suggesting a much lower energy barrier for the formation of CHOO*, the crucial intermediate for generating formate. Interestingly, it can be observed that the energy barrier for CHOO* intermediate transformation to formate and subsequent desorption process is still smaller than that for CO and that for H2. These results indicate that BiCuSeOR thermodynamically enables the activation of CO2 molecules to form HCOO* intermediate and thus further produce formate. Consistent of the predesigned scenario, the DFT calculation results solidly support that in natural BiCuSeO superlattices the [Cu2Se2]2− sublayers conducts electrons, while the [Bi2O2]2+ sublayer act as the active center for the activation of CO2 molecules and subsequent formation/stabilization of HCOO*intermediates, enabling highly selective CO2 electroreduction to formate.