Synthesis and structural characterization of QMOF. Previous investigation has indicated that the crystallization of the MOFs undergoes Ostwald ripening process, during which unsaturated metal sites (UMS) may be observed.31 With this in mind, retarding the crystal ripening process can generate more UMS in MOFs, so as to obtain more open metal sites for electrocatalytic reaction. The synthetic procedures of catalysts are illustrated in Figure 1a and b, and the experimental details are summarized in Supplementary Information. Cu(OH)2 nanowires (NWs) with smooth surface were first grown on Cu foil based on a reported chemical oxidation method.32 Subsequently, Cu(OH)2 sample was reacted with atomized 1,3,5-benzenetricarboxylic acid (trimesic acid, H3BTC) solution for 60 s to form uniform quasi-MOF (QMOF) clusters on Cu(OH)2 NWs surface, as illustrated in Figure 1a. The electrochemical reconstruction was undertaken by electrochemically reducing QMOF in 0.1 M KHCO3 electrolyte at -0.4 V vs reversible hydrogen electrode (RHE) to form CU-CPWC catalyst. For comparison, Cu(OH)2 sample was reacted with sufficient H3BTC solution to form complete MOF crystal (CMOF), as illustrated in Figure 1b.
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images confirmed the synthesis of a smooth-surface nanowire structure (Supplementary Figure S1 and S2). X-ray diffraction (XRD) patterns verified the presence of a crystalline Cu(OH)2 phase (Figure 1c), which was further evidenced by Raman spectroscopy (Figure 1d). Note that no diffraction peaks for HKUST-1 were detected in QMOF (Figure 1c), suggesting an amorphous structure may be formed.
In order to investigate the surface structure of samples, we performed Raman and Fourier transform infrared (FT-IR) analysis. The high-frequency region ranging from 700~ 1200 cm-1 is dominated by bands associated with the organic part of HKUST-1 (Figure 1d).33-35 The band at 1006 cm−1 corresponds to C=C symmetric stretching of benzene ring. At 827 and 748 cm-1, the out-of-plane ring (C-H) bending vibrations are recognized in QMOF and CMOF.33, 35 QMOF exhibits weaker bands in high-frequency region than CMOF, suggesting UMS of QMOF. In the low-frequency region, the doublet at 458 and 505 cm−1 is related to Cu-O stretching modes involving oxygen atoms of carboxylate bridges, which is consistent with previous reports, suggesting the metal−organic coordination in the samples.7, 35 Vibrational properties of the samples were further investigated by FT-IR spectroscopy. As shown in Figure 1e, the QMOF and CMOF exhibit the significant peaks ranging from 1100 to 1800 cm−1 compared with Cu(OH)2 NWs. Characteristic peaks assigned to vibrations of the carboxylate group in HKUST-1 are observed: 1370 and 1419 cm-1 corresponding to the symmetric vibrations of -COO groups and 1450, 1592 and 1646 cm-1 attributed to the asymmetric vibrations of -COO groups.8, 34, 36 The peak at 1114 cm-1 is assigned to C-O vibration in the HKUST-1. In addition, the peaks at 732 and 762 cm−1 are observed in CMOF, which can be ascribed to the bonding of Cu with BTC linker molecules (Figure 1e).7 A weak peak at 1545 cm−1 is also found in CMOF, which is due to water coordinated with Cu in the MOF.34 Meanwhile, for the QMOF, two features of the FT-IR spectrum are worth noting. Firstly, notable shifts to lower wavenumber are observed for the peaks of asymmetric vibrations of -COO groups, suggesting an elongation of the associated bonds. Secondly, the peaks at 1545 cm-1 (water coordinated with Cu in the MOF), 1419 cm-1 (one of the symmetric vibrations of -COO groups) and 732 cm-1 (one of the Cu with BTC linker molecules) in QMOF are undetectable, which indicates that a defective HKUST-1 structure with UMS or distorted metal nodes may be formed in QMOF.
Notably, SEM and TEM images of QMOF reveal that the uniform nanowire structure with many small clusters on the surface (Figure 2a−e). As shown in Figure 2d and e, small clusters are clearly observed in the high-resolution TEM (HR-TEM) images. Figure 2f−j show the scanning TEM (STEM) and corresponding energy dispersive X-ray spectroscopy (EDS) elemental mapping images of QMOF, confirming the existence and uniform distribution of elemental Cu, O and C in the nanowires.
Moreover, the structural evolution of the Cu(OH)2 NWs in the presence of the atomized H3BTC was explored. As shown in Supplementary Figure S3, the controllable structural evolution from QMOF clusters to complete MOF crystal (CMOF) is achieved through controlling the reaction time with “atomized H3BTC”. In order to prove the superiority of “atomized H3BTC” method, common “immersed method” was performed, that is, Cu(OH)2 NWs precursor was submersed into liquid H3BTC solution to synthesize HKUST-1. As a contrast, only CMOF is obtained by the ordinary “immersed method” even in a short time of 10 s (Supplementary Figure S3d). SEM image of CMOF reveals a typical octahedral HKUST-1 morphology (Supplementary Figure S3c), which is consistent with previous reports.37-39 High
magnification SEM and TEM images identify Cu(OH)2 NWs trace in CMOF (Supplementary Figure S4), further revealing the structural evolution from Cu(OH)2 NWs to CMOF.
The surface hydrophilicity and CO2 capture capability of the samples were characterized by contact angle and CO2-temperature programmed desorption (TPD) measurements (Supplementary Figure S5 and S6). Contact angle measurements illustrated that Cu(OH)2 surface is the most hydrophilic. The surface clusters of QMOF decrease the hydrophilicity of Cu(OH)2, but increase the CO2 capture capability.
Electrochemical reconstruction of QMOF and CMOF. Attention was then paid to the structural changes of QMOF and CMOF during the electrochemical reconstruction process. X-ray photoelectron spectroscopy (XPS) tests under ex-situ (Figure 3a-c) and quasi-in situ (Figure 3d-e) conditions were first performed to reveal the electronic configuration of each catalyst before and after electrochemical reconstruction.
The ex-situ XPS spectra of C 1s for QMOF and CMOF are shown in Figure 3a, which can be divided into four peaks, including sp2 carbon in benzene ring (284 eV), sp3 carbon (C-C, 284.8 eV), C-O (286 eV) and C=O (289 eV).40-43 The metal node [-Cu2(COO)4] defect of QMOF is further confirmed by the lower binding energy shift of C-O peak compared with that of CMOF. Note that an additional O 1s peak at 533.2 eV is clearly observed in QMOF, which is related to H2O molecule (Figure 3b).44, 45 This result has been confirmed by FT-IR peak at 1545 cm−1, corresponding to water coordinated with Cu in the MOF, as described above. O 1s spectra of QMOF and CMOF both display two peak regions at approximately 530.5 and 531.8 eV, which can be assigned to the Cu-O and -COO groups.43 As expected, the QMOF sample exhibits the lower binding energy of Cu-O and -COO than CMOF for the defect of metal nodes [-Cu2(COO)4] in QMOF. XPS data for the Cu 2p3/2 exhibit Cu2+ and Cu1+ characteristic peak (Supplementary Figure S7).41, 46 Previous report suggests the conversion of Cu2+ to Cu1+ in CMOF is induced by the exposure of MOF to X-ray source and charge neutralizer during XPS data collection.46 Note that Cu Auger (L3M45M45) region shows that a reduced oxidation state for the Cu species in QMOF, which may be attributed to the lower coordinatively unsaturated Cu atoms (Figure 3c).47
Furthermore, the electronic structures and chemical bonding of Cu atoms in their local environment were investigated by X-ray absorption spectroscopy (XAS), including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The QMOF sample exhibits lower-energy edge position as compared with CMOF, indicating a reduced oxidation state for the Cu species (Supplementary Figure S8), which is consistent with XPS auger spectrometry (Figure 3c). The dominant coordination peaks in EXAFS can be ascribed to Cu-O (EXAFS, Supplementary Figure S9). Overall, a comprehensive study of ex-situ XPS, XAS, FT-IR, and Raman provides clear evidence that quasi MOF with UMS has been formed in QMOF sample.
Significant changes were observed in the XPS spectra for CU-CPWC and CPW under quasi-in situ condition, of which the detail is shown in Experimental Section. C 1s spectra in Figure 3d shows that sp2 carbon in benzene ring (284 eV) is not detected in the samples, indicating the loss of organic coordinating ligands (linkers) during electrochemical reconstruction. But the peaks corresponding to C-O and C=O in the metal nodes [-Cu2(COO)4] still show up (Figure 3d). Figure 3e displays that the -COO groups peaks (532.0 and 532.2 eV) in CU-CPWC and CPW have shifted towards higher binding energy compared with those in QMOF and CMOF, suggesting that the lost organic coordinating ligands may be replaced by hydrogen during electrochemical reconstruction. The peaks at 536.2 ~ 537.0 eV assigned to CO2 molecule are also observed (Figure 3e), due to the superior CO2 adsorption capacity.44 The presence of Cu0 and Cu1+ characteristic peak indicates a reduced oxidation state for the Cu species (Figure 3f). Based on these findings, it is suggested that [Cu2(HCOO)4] cage and defective low-coordinated [Cu2(HCOO)4-x] (0 < x < 4) cage may be formed in CMOF and QMOF respectively after electrochemical reconstruction.
In order to further gain insight into the dynamic electronic structures and chemical state of the Cu active site during CO2RR, in situ XAS were investigated under real operating conditions at −0.4 V vs RHE in aqueous CO2-saturated 0.1 M KHCO3. Figure 4a presents the schematic of XAFS set-up and recording processes for the in situ spectro-electrochemistry experiment. Figure 4b shows the Cu K-edge XANES spectra with electron transitions from 1s to 4p states, including 1s → 4pz transition (A), and the 1s → 4px,y transition (B, C).25, 48-51 The higher absorption edge position and the lower 1s → 4pz transition intensity indicate a gradually oxidized Cu species.50-52 Furthermore, FT-EXAFS spectra in Figure 4c reveal detailed
information regarding the local coordination environment of Cu. The dominant peak at approximately 2.2 Å, which is assigned to the coordination of Cu-Cu, is clearly observed in Cu foil, ER-Cu(OH)2 and CU-CPWC. Note that the existence of Cu-O coordination peak suggests the oxygen-bearing copper clusters may be formed in CU-CPWC. Meanwhile, the Cu-O coordination (approximately 1.5 Å) is the main peak in CPW (Figure 4c). A straight comparison of XAS data before and after electrochemical reconstruction is shown in Supplementary Figure S10, highlighting the changes of coordination environment in the samples.
An additional analysis of Morlet wavelet-transform (WT) was performed to better understand the local structure of Cu atoms. As shown in Figure 4d, ER-Cu(OH)2 exhibits an intensity maximum at approximately 7.2 Å−1, which is very close to that in the reference Cu foil. Note that a slightly higher WT maximum at about 8.1 Å−1 is observed in QMOF, indicating the influence of oxygen on the local coordination environment of copper. Furthermore, two intensity maximums at 5.2 and 9.8 Å−1, corresponding to Cu−O and Cu−Cu shells, is visible in CU-CPWC.
To better understand the stepwise structure evolution process of HKUST-1, more in-depth localized observations were performed by in situ Raman and in situ FT-IR. Figure 4e shows in situ Raman spectra measured at −0.4 V vs RHE in aqueous CO2-saturated 0.1 M KHCO3 and a time resolution of about 30 s. The nethermost spectrum is the first collected data. In situ Raman results show that the peak attributed to C-H and C=C peaks disappeared, indicating the loss of organic ligands during electrochemical reconstruction. In pursuit of a molecular-level understanding of electrochemical reconstruction for catalyst, advanced in situ ATR FT-IR was employed to dynamically monitor the structure evolution at the fixed potential of -0.4 V (Supplementary Figure S11 and S12). As shown in Supplementary Figure S12, the peak at 1370 cm-1 corresponding to the symmetric vibrations of -COO groups in [Cu2(HCOO)4-x] cage becomes stronger as the reaction progresses. This further confirms the loss of organic ligands and [Cu2(HCOO)4-x] cage is exposed during electrochemical reconstruction.
Based on the above results, we can reasonably speculate about the structure of the [Cu2(HCOO)4-x] cage in CU-CPWC and perform quantitative EXAFS curve-fitting analysis to reveal the quantitative coordination configuration of Cu atoms. A previous investigation has shown that Cu easily forms paddle-wheel dinuclear Cu(ii) carboxylate coordination compounds.53 A good fitting of the experimental FT-EXAFS data is achieved by the simulated EXAFS based on tricoordinated [Cu2(HCOO)3] cage model for CU-CPWC (Supplementary Figure S13c). The typical tetracoordinate copper paddle wheel [Cu2(HCOO)4] cage model fits well with experimental FT-EXAFS data of CPW (Supplementary Figure S13d).
In order to clarify the details of the adsorption and activation of CO2 on CU-CPWC, in situ XAS data were recorded in aqueous CO2 and Ar-saturated 0.1 M KHCO3, respectively. It can be found that the Cu K-edge of CU-CPWC shows the lower 1s → 4pz transition intensity in CO2-saturated KHCO3 solution under -0.4 V compared with that in Ar-saturated KHCO3 solution (Figure. 4f), which suggests an increase in the Cu oxidation state as a result of the charge transfer from Cu to the carbon 2p orbital in CO2 to form a CO2δ− species.54, 55 The Cu-Cu coordination peak slightly decreases in CO2-saturated KHCO3 solution (Figure. 4g), which further confirms the increase in the Cu oxidation state compared with that in Ar-saturated KHCO3 solution.
The SEM image of CPW shows that the octahedral morphology has collapsed into irregular nanoparticles after electrochemical reconstruction (Supplementary Figure S14). These changes indicate the organic links are removed from HKUST-1 frames during reconstruction, resulting in large cleavages and structural collapse. The morphological change is also shown for CU-CPWC, as revealed in Supplementary Figure S15.
On the basis of these results from quasi-in situ XPS, in situ XAS, in situ Raman and in situ FTIR, it is concluded that the CU-CPWC sample surface has been modified by oxygen-bearing copper clusters based on coordinatively unsaturated [Cu2(HCOO)4-x] cage. In order to verify whether the defect oxygen-bearing copper clusters promotes electrochemical CO2 reduction, we carried out CO2RR assessments on target catalyst CU-CPWC and the control samples including Cu foil, ER-Cu(OH)2 and CPW.
Electrochemical activity of catalysts in CO2 reduction. We evaluated the electrochemical CO2 reduction activity using a gas-tight H-cell system filled with 0.1 M KHCO3 solution. Prior to CO2RR testing at a given potential, all of the samples were subjected to a potential of –0.4 V vs RHE in CO2-saturated KHCO3 solution for activation. As indicated by the linear sweep voltammetry (LSV) curves shown in Figure 5a, CU-CPWC shows the lower overpotentials at fixed current densities, suggesting the improved activity. The CO2 reduction product distribution of the four catalysts was investigated under various potentials (Supplementary Figure S16). As shown in Figure 5b and c, the CU-CPWC exhibits dramatically increased CH4 and C2H4 Faradaic efficiency among the four catalysts, benefiting from the superior CO2 capture and activation capability. Significantly, the total Faraday efficiency of CO2 reduction to CH4 and C2H4 has reached approximately 80.5 % at -0.90 V.
Electrochemical impedance spectroscopy (EIS) tests were performed to investigate the kinetics of electrochemical processes. CU-CPWC exhibits the smaller Nyquist semicircle diameter than the control samples (Supplementary Figure S17), suggesting the fast charge-transfer kinetics in CRR. Cyclic voltammetry (CV) measurements were then performed to determine the double-layer capacitances (Cdl) (Supplementary Figure S18 and S19), which is related to electrochemical active surface area (ECSA). Not surprisingly, CU-CPWC exhibits the higher Cdl than the control samples, indicating the larger ECSA, which is also an important contributor to the enhanced activity. In addition, LSV tests of the four samples in Ar-saturated and CO2-saturated KHCO3 electrolyte were performed as well (Supplementary Figure S20). The higher current density in CO2-saturated KHCO3 electrolyte demonstrated that the carbon source for the evolved hydrocarbons comes from CO2, which provides a direct evidence for the reduction of CO2.
For clarifying the origin CO2RR mechanisms of CU-CPWC and CPW, the-first principles-based density functional theory (DFT) calculations were performed. The CU-CPWC and CPW models have been built and optimized, the detail of which is shown in Supporting Information. The spin-orbit projected 3d density of states of CU-CPWC and CPW are shown in Figure 5d. From this figure, the CU-CPWC presents obvious spin properties with asymmetrical spin distribution around the Fermi level. The d band center of CU-CPWC and CPW is -1.16 and -1.96 eV respectively from the calculations, which indicates that the level change of d band center can be the intrinsic origins of the different CO2RR performance.56 In addition, there are more states around the Fermi level for Cu-CPWC model with CO2 adsorption (Figure 5e) than those of CU-CPW model as shown in Figure 5f. It means that the CU-CPWC model has better electron transport behavior than that of CPW model, which can help to accelerate the reaction process. The data shown in Supplementary Figure S21, further support the statement.
CU-CPWC catalyst also exhibits good stability for CH4 and C2H4 generation under continuous CO2 electroreduction, as shown in Supplementary Figure S22. The Cu K-edge XANES spectra before and after stability testing reveal that the valence state shows negligible change after the constant-potential electrolysis (Supplementary Figure S23a), which is further confirmed by the first-derivative spectra of XANES (Supplementary Figure S23b). In addition, FT-EXAFS spectra show that the distinct Cu-O peak is still present after stability testing, indicating the structure stability of the CU-CPWC (Supplementary Figure S24).