Spin-Enhanced C–C Coupling in CO 2 Electroreduction with Oxide-Derived Copper

Open AccessCCS ChemistryRESEARCH ARTICLES14 Nov 2022Spin-Enhanced C–C Coupling in CO2 Electroreduction with Oxide-Derived Copper Jinjie Hao, Shijie Xie, Qing Huang, Zijing Ding, Hua Sheng, Chuang Zhang and Jiannian Yao Jinjie Hao Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Shijie Xie Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Qing Huang Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Zijing Ding Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026 , Hua Sheng *Corresponding authors: E-mail Address: email protected E-mail Address: email protected Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Chuang Zhang *Corresponding authors: E-mail Address: email protected E-mail Address: email protected Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 and Jiannian Yao Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.022.202202263 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Electrocatalytic reduction of carbon dioxide (CO2) to multicarbon (C2+) products involves intricate multiple protons and electron transfer of C–C coupling, which is dictated by not only the intrinsic reactivity but also the spin states of electrons in the catalyst. Here, we observe spin-enhanced CO2 reduction (CO2RR) electrocatalytic activity on an oxide-derived copper (OD-Cu) catalyst due to the existence of a specific Cu* site that carried the magnetic moments. Due to the correlation of magnetic and catalytic properties in OD-Cu, the current density through the OD-Cu electrode increases by nearly 10% at 350 mT. The field strength and angle dependence of such magnetic field effect (MFE), together with the time-resolved measurements proved that it originated from the alignment of magnetic moments on Cu* sites. The MFE on the electrocatalytic process enabled an enhancement (up to 15%) of the CO2RR Faradaic efficiency using the OD-Cu catalyst. Importantly, the enhancement was attributed to the spin-antiparallel alignment of electrons to promote C–C coupling on asymmetric Cu*-Cu sites; consequently, the optimal bias was reduced by ∼0.2 V under the magnetic field for C2 products with Faradaic efficiency >30% and selectivity >75%. Our work uncovers a new paradigm for spin-enhanced catalysis applicable to a broad range of chemical reactions involving spin singlet products. Download figure Download PowerPoint Introduction The continuously increasing atmospheric level of carbon dioxide over the past few decades has led to serious climate problems and attracted worldwide attention. Electrochemical carbon dioxide reduction (CO2RR) to valuable fuels and chemical feedstocks is a potential strategy to reduce carbon emissions and achieve a carbon-neutral energy cycle.1–3 Copper is by far the only metal catalyst capable of producing significant amounts of C2+ hydrocarbons.4–7 However, the efficiency of C2+ hydrocarbon generation on the pure copper electrode is still low. One strategy to reduce the overpotential of CO2 reduction and steer the selectivity toward C2+ hydrocarbons is the employment of oxide-derived copper (OD-Cu),8–10 which is prepared for electrocatalysis from the thermal or electrochemical reduction of copper oxide. It is generally recognized that, compared with Cu atoms under perfect coordination in pristine metal, specific Cu sites (noted as Cu*), for example, under-coordinated atoms11–14 or residual Cu+ from the oxide,15–17 are formed during the subsequent oxidation and reduction of OD-Cu. These Cu* sites are considered to be more active in the CO2RR; however, the exact roles of Cu* sites in promoting the CO2RR are still under debate. Experimental approaches to explore the participation of Cu* sites in the electrochemical CO2RR are usually based on operando spectroscopy, such as infrared, Raman, and X-ray absorption spectroscopies.18,19 Owing to the very limited concentration of Cu* sites in the OD-Cu catalyst, employing the above spectroscopies confronts the difficulties that the signals of Cu* itself or the intermediates generated in situ on Cu* would be covered by the vast majority of background signals from ordinary Cu sites. Therefore, it is necessary to distinguish the trace amount of Cu* in OD-Cu and reveal the pivotal role of such specific sites in governing the activity and underlying mechanism of the CO2RR. Recently, applying a magnetic field to catalytic reactions has emerged as a promising strategy for improving catalytic activity, as well as an approach to investigate catalytic mechanisms.20–28 Magnetic fields can influence the electrochemical process in different manners, including thermal effects produced by high-frequency alternating fields and accelerated mass transportation by magnetic-field-induced Lorentz/Kelvin forces.29–32 More interestingly, the spins of 3d or 4f electrons in transition metal-based catalysts can be aligned when they are magnetized by external fields,33 which may transfer spin angular momentum to intermediate radicals generated during catalytic reactions. This leads to the observation of the magnetic field effect (MFE) on the oxygen evolution reaction (OER).34–36 The parallel alignment of electron spins in a ferromagnetic OER catalyst under an applied field facilitates the generation of O2, which is in a triplet ground state with frontier π* orbitals occupied with two electrons with parallel alignment. However, such a positive MFE can hardly be expanded to other catalytic reactions, such as CO2RR or hydrogen production via hydrogen evolution reaction (HER), which only involve products in the singlet ground state, as the parallelly aligned electrons under a magnetic field are unfavorable for chemical bonding. In particular, the CO2RR involves complicated electron/proton transfer and abundant radical intermediates,5 and the C–C coupling toward C2+ hydrocarbons should be correlated to the spin state of electrons on neighboring active sites, but the spin-related mechanism and spin-sensitive reaction kinetics remain unexplored. In this work, an OD-Cu catalyst is uncovered with a significant MFE on an electrochemical CO2RR, which led to spin-enhanced C–C coupling toward C2+ production under an applied field based on the asymmetric configuration of Cu*-Cu sites. The existence of Cu* in OD-Cu was revealed by measuring its magnetic moment, which determined the electrochemical activity of the OD-Cu catalyst, enabling an MFE of nearly 10% to be observed in a chronoamperometry experiment. Quantitative analysis of the steady-state and transient MFEs provided strong evidence that the alignment of the magnetic moment at Cu* sites accelerated the electron transfer from OD-Cu to CO2. For instance, the magnitude of MFE increased and saturated at a high field, and the descending of MFE when turning off the field took a longer time than its ascending. Magnetic enhancement (up to 15%) of the Faradaic efficiency of the CO2RR was achieved using the OD-Cu catalyst due to the magnetic property of the Cu* sites. Furthermore, the stray field of Cu* polarized the electron spin transferred from neighboring Cu, which facilitated C–C coupling on Cu*-Cu sites toward C2 products (e.g., methane and ethane) in the CO2RR; thus, the overpotential for C2 product generation was reduced under the magnetic field, and the bias to reach the maximal selectivity of C2 products positively shifted to ∼ 0.2 V compared with the absence of a magnetic field. At a bias of −1.3 V (vs. Ag/AgCl), the Faradaic efficiency of C2 hydrocarbons increased by 65% with the application of a magnetic field. Distinct from previous work to facilitate the production of triplet species (e.g., O2) through the employment of long-range magnetic ordering ferromagnetic catalysts, we prove that the manipulation of local magnetic moments enables spin-antiparallel electrons from active sites in nonferromagnetic catalysts, beneficial to chemical bonding for the production of more common singlet species. Experimental Details Chemicals and materials All chemicals, including copper foil (0.1 mm thick, 99.9%;), potassium bicarbonate (KHCO3, 99.995%), and phosphoric acid (85.0%–87.0%), were purchased from Alfa Aesar Chemical Co. Ltd (Shanghai, China) and were used without further purification. Deionized water was prepared with a Milli-Q purification system (MilliporeSigma, Burlington, MA) and used throughout all the experiments. Preparation of Cu foil The Cu foil was electropolished in 85% phosphoric acid using a potentiostat set at 2.0 V versus the counter electrode for 300 s. During this procedure, another copper foil was utilized as the counter electrode, and the solution was agitation with a magnetic stirrer. The copper foil was then rinsed copiously with deionized (DI) water and dried with a stream of nitrogen. Preparation of OD-Cu Preparation of OD-Cu via the thermal annealing process was performed according to the procedure of Kanan and coworkers.8,37 The electropolished copper foil was placed in a muffle furnace under an air atmosphere and heated to 500 °C. Once the temperature reached 500 °C, it was maintained for 3 h. The copper foil was then allowed to cool slowly to room temperature over several hours. Then the copper oxide layer was reduced electrochemically before applying electrolysis using a bias of −1.2 V versus Ag/AgCl until complete reduction was reached. Characterizations The morphology of Cu foil and OD-Cu was examined with scanning electron microscopy (SEM; Hitachi SU-8100, Hitachi,Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) was conducted on a PHI Quantera SXM X-ray photoelectron spectrometer (Thermo Fisher Scientific, Waltham, Massachusetts, United States) equipped with an Al X-ray excitation source (1486.6 eV; Thermo Fisher Scientific, Waltham, Massachusetts, United States). The binding energies (BEs) were corrected by the C1 s peak at 284.8 eV. X-ray diffraction (XRD) patterns of the samples were recorded on a Al X-ray excitation source (1486.6 eV; Thermo Fisher Scientific, Waltham, Massachusetts, United States) operated at 40 kV voltage and 40 mA current with Cu Kα radiation (λ = 1.5406 Å). The magnetic hysteresis (M–H) loops of Cu foil and OD-Cu were measured at room temperature with a vibrating sample magnetometer (VSM) in the physical property measurement system (Quantum Design DynaCool-9T; Quantum Design, San Diego, California, United States). Electroactive surface area measurements were performed according to procedures described by Nilsson and coworkers,38 and the same electrochemical cell as that for CO2RR 0.1 M KHCO3 was used as the electrolyte, then cyclic voltammetry was carried out in a region where Faradaic processes occurred. Product analysis Product analysis was performed according to procedures similar to those reported previously.9 The gaseous product was monitored by gas chromatography (Fuli GC-2060; Fuli Analytical Instrument, Wenling, Zhejiang, China) with a sampling (0.5 mL). For liquid phase products, ion chromatography (Thermal ICS-900, Dionex IonPac AS18; Thermo Fisher Scientific, Waltham, Massachusetts, United States) was used for detection. The hydrocarbon products were analyzed by GC (Fuli GC-2060; Fuli Analytical Instrument, Wenling, Zhejiang, China) with a flame ionization detector (FID) equipped with a 5 Å molecular column and a thermal conductive detector (TCD) with a packed column (TDX-01; Agilent, Santa Clara, California, United States) for H2. CO was converted to CH4 by a methanation reactor prior to detection by another FID. Electrochemical measurements under a magnetic field A CHI 660D potentiostat (CH Instruments, Shanghai, China), consisting of a three-electrode configuration was used in all electrochemical experiments, where OD-Cu, Pt wire, and Ag/AgCl electrodes were employed as the working, counter, and reference electrodes, respectively. The CO2-saturated 0.1 M KHCO3 was used as the electrolyte. To execute the electrocatalysis under a magnetic field, the electrochemical cell was placed between the two poles of a computer-controlled electromagnet (Beijing Chaoruirenda Technology CR1, China). The OD-Cu electrode was placed toward the counter electrode. With this configuration, the magnetic field was applied uniformly on the electrochemical cell and parallel to the charge transport direction. The μ-metal shield was covered on the electromagnet during the electrochemical measurements, and the detection limit for MFE on current density was typically below ∼0.2%. Results and Discussion MFE on the electrochemical reaction with the OD-Cu catalyst The OD-Cu catalyst was derived from copper oxide and showed superior electrochemical CO2RR activity compared with pristine Cu.8,36 During the subsequent oxidation and rereduction to prepare OD-Cu (see Supporting Information for details), we proposed that specific surface Cu sites (noted as Cu*) were formed, such as low-coordination sites in the step edges or adparticles resulting from increased roughness in OD-Cu or the polarized Cuδ+ induced by the residual oxygen, which exhibited a relatively lower electron density than ordinary metallic Cu0 and is generally regarded as the active site for the promoted CO2RR in OD-Cu.16,17 As shown in Scheme 1, it is well known that metallic Cu0 is diamagnetic because the unpaired electrons in the 4s orbitals are highly delocalized and form metallic bonds, while the 3d orbitals are fully filled and the net magnetic moment is zero. We showed that on the Cu* site in OD-Cu, the loss of electrons broke the full occupation of 3d orbitals, as well as the symmetry of spin-up and spin-down electrons. Therefore, the net magnetic moment from the electron spins was expected to appear on the Cu* site. In addition, the 3d unoccupied orbital of Cu* enabled the delocalization of 3d electrons to neighboring Cu sites, which also contributed to the electron conductivity of OD-Cu in addition to the itinerant 4s electrons. The participation of 3d electrons might influence the electron transfer for the CO2RR electrochemical activity of the CO2RR in OD-Cu, and the magnetic moment of Cu* provided the possibilities for the observation of MFE in the electrochemical reaction with the OD-Cu catalyst. We considered that the magnetic moment of the Cu* species might introduce the spin alignment of electrons on Cu*-Cu sites in OD-Cu, significantly distinct from previously reported mechanisms for electrocatalysis. Scheme 1 | Spin-related catalysis for CO2RR in OD-Cu catalyst containing Cu* species. For the convenience of description, the low-coordination surface Cu atom at the step edge is drawn to be represented as Cu*. Download figure Download PowerPoint Our modified OD-Cu electrodes were prepared by annealing Cu foil in air and subsequently, reducing it electrochemically.8,9 The structural morphology of the Cu foil and as-prepared OD-Cu was characterized by SEM, as shown in Figure 1a,b. The Cu foil had a relatively smooth surface; in comparison, the roughness of the OD-Cu electrode was significantly enhanced, exhibiting a porous and grain-aggregated structure ( Supporting Information Figure S1) with nanowire-like appendages protruding from the surface.9 The electrochemically active surface area (EASA) of the OD-Cu catalysts was determined by measuring the double-layer capacitance through cyclic voltammetry. Based on the results in Figure 1c,d, the roughness factor of OD-Cu was determined to be 345 times larger than that of the Cu foil. The numerous grain-aggregated structure on the very rough surface implied the presence of under-coordinated sites and atomic steps in OD-Cu. XRD in combination with high-resolution Cu 2p XPS was performed to study the composition and valence states of Cu species in the catalysts (Figure 1e,f). This result indicated that after aerobic annealing, diffraction peaks for copper oxide (CuO and Cu2O) emerged (see Supporting Information Figure S2 for further analysis of the XRD results). Consistent with the XRD results, the characteristic Cu2+ satellite peaks appeared in the XPS spectrum, both of which indicated the oxidation of copper foil under aerobic annealing. The high-resolution Cu 2p spectra showed the spin-orbit components Cu 2p3/2 and Cu 2p1/2, at ∼932.8 and 952.6 eV, respectively. After being electrochemically reduced ( Supporting Information Figure S3), the OD-Cu was obtained, showing only XRD peaks that corresponded to (111), (200), and (220) of metallic Cu0. The Cu 2p XPS spectrum was dominated by the presence of Cu0; it was difficult to determine the existence of residual oxidized Cu species, and very little change was observed in the XRD and XPS results for OD-Cu catalysts before and after the reaction. These observations are consistent with previously reported results.9,37 Figure 1 | Preparation and characterization of the OD-Cu catalyst. SEM images of (a) Cu foil and (b) as-prepared OD-Cu. Measurement of the electroactive active surface area (EASA): (c) Plot of the capacitance current versus applied bias at various scan rates and (d) plot of the capacitance current versus scan rate. (e) XRD patterns and (f) XPS spectra measured from Cu foil (red), annealed Cu (black), OD-Cu (blue), and OD-Cu after reaction (navy). Download figure Download PowerPoint We envisioned that the Cu* sites in OD-Cu might carry a magnetic moment of electrons, a VSM was used to characterize the magnetic properties of copper-based catalysts and reveal the magnetic moments contributed by 3d electrons. As shown in Figure 2a, the M–H loop of polycrystalline Cu foil shows a straight line with a negative magnetic susceptibility, indicating its diamagnetic nature of fully occupied 3d electrons. The diamagnetic property was solely produced by the orbital angular momentum of electrons, and no magnetic moment of electron spins was detected in Cu foil. Interestingly, the M–H loop of OD-Cu showed a small but significant "step" near the zero field on top of the diamagnetic background after we carefully checked and eliminated the possible interference ( Supporting Information Figure S4). The low field signal of OD-Cu after the subtraction of the linear background was the same as that measured from the Cu2O sample, in which the alignment of magnetic moments was achieved by the electron spins from Cu vacancies.39 Therefore, it provided direct evidence for the existence of magnetic Cu* species in the as-prepared OD-Cu catalyst where the 3d orbitals were no longer fully occupied. The unpaired spin in the 3d electrons of Cu* ( Supporting Information Figure S5) possibly originated from the residual Cu-O bond ( Supporting Information Figure S6) in the OD-Cu catalyst. The magnetic moment was further verified by density functional theory (DFT) calculations of the spin density-of-state ( Supporting Information Figure S7), and the concentration of Cu* sites in the as-prepared OD-Cu catalyst was estimated to be on the order of 10−3. Coincidently, the unoccupied 3d orbitals at Cu* sites also activated the 3d electrons among neighboring Cu sites (Scheme 1), which might have altered the catalytic properties of OD-Cu and recruited unprecedented MFE to the electrochemical reaction. Figure 2 | Magnetic and electrochemical characteristics of the OD-Cu catalyst under applied (a) M–H loops measured from to 5 of Cu and OD-Cu samples M–H loops for Cu2O and OD-Cu sample after a linear background (b) scanning for electrochemical reactions using Cu and OD-Cu The V showing in the current density under magnetic fields using (c) OD-Cu and (d) Cu catalysts in the electrochemical reactions. Download figure Download PowerPoint we the electrochemical CO2RR of Cu and OD-Cu catalysts under an applied magnetic field. The linear voltammetry of Cu and OD-Cu electrodes were recorded at a scan of in 0.1 M KHCO3 As shown in Figure the that the potential of the catalytic current on the OD-Cu electrode positively shifted by V compared with that of Cu foil. possible reactions, that the CO2RR and hydrogen evolution reaction (HER), were under that on OD-Cu, the potential under line in Figure was more negative than that under saturated while the for Cu foil under and CO2 are the Therefore, the employment of OD-Cu reduced the overpotential of the CO2RR than the and the superior electrocatalytic activity for the CO2RR on OD-Cu was MFEs on both Cu foil and OD-Cu were examined during chronoamperometry at V (vs. Ag/AgCl), when a external magnetic field the field was to the surface of the was applied in situ with a ( Supporting Information Figure As shown in Figure an of in current density was observed after the magnetic field on OD-Cu and when turning off the applied magnetic such and descending of the electrochemical current was by turning the magnetic field. As a the pristine Cu foil only exhibited a magnitude current under an applied field (Figure the in current the OD-Cu was still times that on pristine Cu foil. the CO2 atmosphere was to pure on OD-Cu, in which only had the in current was ( Supporting Information Figure indicating a more significant MFE on CO2RR with OD-Cu catalyst than and transient analysis of the MFE To further the MFE on the CO2RR we measured the electrochemical of the OD-Cu catalyst the application of magnetic fields with various and and the current under a magnetic field was employed to the magnitude of the As shown in Figure the MFE was observed at all field of and 350 while the effect more when the field was The field dependence plot in Figure (see Supporting Information Figure for a nearly linear in the low field and a at This result was consistent with the M–H loop observed in OD-Cu, that the alignment of the magnetic moment of Cu* sites was for the MFE on the CO2RR. In addition, the change of MFE magnitude the field strength was highly when the applied field to zero on the OD-Cu catalyst ( Supporting Information Figure the angle and between the of the applied field and the electrode surface (Figure MFE was observed at all and the between the at and at or was not significant (Figure Supporting Information Figure for The of MFE on OD-Cu was significantly different from that for Cu foil under an applied field ( Supporting Information Figure attributed to the that the on the mass transport of intermediates during the electrochemical reaction. that the effect on the electrochemical reaction increased with the surface roughness of the Cu foil ( Supporting Information Figure however, its magnitude was still lower than the observed MFE on the OD-Cu catalyst. The electrochemical spectra ( Supporting Information Figure further the electrochemical activity of OD-Cu compared with Cu, a catalytic mechanism to the magnetic moment of Cu* species. Therefore, the effect should be limited to MFE on the CO2RR with the OD-Cu the magnetic moment of Cu* species might have accelerated the electron transfer from OD-Cu to CO2 and increased the electrochemical current density through the OD-Cu electrode, under an applied magnetic field. Figure 3 | analysis of MFEs on CO2RR. (a) measured at V (vs. under magnetic fields with various The field is to the electrode (b) the magnitude of MFEs as a of field strength from to mT. (c) for the electrochemical reaction on the OD-Cu electrode measured at V (vs. under an applied magnetic field of 350 at various between the field and the electrode surface; (d) magnitude of MFEs as a of the in (e) measured at various under an applied field of 350 (f) magnitude of MFEs and current density change as a of applied bias (vs. Download figure Download PowerPoint The bias applied on the OD-Cu electrode is to the between the CO2RR and we characterized the magnetic-field-induced in current density under various applied to the of the CO2RR and to electrochemical As shown in Figure the MFEs on the electrochemical current through the OD-Cu electrode were measured at and V (vs. Ag/AgCl), respectively. The bias at V was the potential of OD-Cu and was used as a With an in bias to −1.3 and the current density indicating the of two electrochemical reactions, and CO2RR, above The MFE is in Figure (see Supporting Information Figure for in which the change in current density application of an external field increased continuously with increasing current density at Interestingly, the magnitude of MFE reached a at which implied that the field was for the CO2RR in its with the on the OD-Cu electrode. It is that the MFE was more in the CO2RR than in the and the CO2RR was expected to be relatively enhanced, to of the OD-Cu catalyst. The transient of MFE on the CO2RR are further analyzed to reveal the between the magnetic and catalytic properties of OD-Cu. As shown in Figure the change in current density was when turning magnetic but it took of to reach another state in current and the time for both the and be Cu the time for both the and descending transient of MFE were at s. In