Chemical Engineering Journal 442 (2022) 136129 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej A novel π-d conjugated cobalt tetraaza[14]annulene based atomically dispersed electrocatalyst for efficient CO2 reduction Zhifu Liang a, c, 1, Ting Zhang a, c, 1, Pengfei Cao b, g, *, Takefumi Yoshida d, Weiqiang Tang e, *, Xiang Wang c, Yong Zuo c, Pengyi Tang f, Marc Heggen g, Rafal E. Dunin-Borkowski g, Joan Ramon Morante c, Andreu Cabot c, j, *, Masahiro Yamashita h, i, Jordi Arbiol a, j, * a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain b School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China c Catalonia Institute for Energy Research – IREC, Sant Adrià de Besòs, Barcelona 08930, Catalonia, Spain d Innovation Research Center for Fuel Cells, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan e State Key Laboratory of Chemical Engineering and School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China f State Key Laboratory of Information Functional Materials, 2020 X-Lab, ShangHai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China g Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute Forschungszentrum Jülich GmbH, 52425 Jülich, Germany h Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramaki-Aza-Aoba, Aoba-Ku, Sendai 980-8578, Japan i School of Materials Science and Engineering, Nankai University, Tianjin 300350, China j ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain A R T I C L E I N F O A B S T R A C T Keywords: Tetraaza[14]annulenes (TAA) are synthetic macrocycles which are analogue to porphyrins. However, there are Tetraaza[14]annulene almost no reports about the synthesis of polymers based on TAA and neither on their use as electrocatalysts. The π-d Conjugated study of new catalysts to promote an efficient electrochemical conversion of carbon dioxide to valuable chem­ Atomically dispersed Co catalyst icals is a promising approach to relieve the pressure of carbon emissions and realize the carbon cycle. Herein, we Electrocatalytic CO2 reduction first report the synthesis of a novel tetraaza[14]annulene (TAA) based organic polymeric metal complex (PMC) by a non-template method. This PMC is used as ligand to construct a π-d conjugated cobalt coordination polymer (Poly-TAA-Co) with CoN4 structure which is supported on multi-wall carbon nanotubes (CNTs) to work as an atomically dispersed efficient electrocatalyst for the CO2 reduction reaction (CO2RR). The resulting catalyst (Poly-TAA-Co-CNT) exhibits excellent performance, with a 90% CO faradaic efficiency, a low overpotential (390 mV) and good stability in 0.5 M KHCO3 aqueous solution. Density functional theory calculations confirmed that the cobalt tetra[14]annulene is an excellent active site for electrocatalytic CO2RR. This work not only inspires the design of novel TAA based macromolecules, but also paves the way to the development and application of new molecular-based catalysts for electrocatalytic CO2RR. 1. Introduction metal coordination structures. Metal complexes based on TAA have been used in field-effect transistors [5], CO electrochemical oxidation [31], as Tetraaza[14]annulenes (TAA) are a type of synthetic macrocycles single molecular magnets [6], catalysts [7] and dye-sensitized solar cells analogue to porphyrins, which have been much less studied [1–4]. They [8]. Most of the previous studies used the metal ion as a template for the contain four nitrogen atoms in a central cavity that can be easily synthesis of the TAA complexes. However, this type of macrocycles have deprotonated and coordinated with metal ions to form tetra[14]anulene been rarely used to build polymeric metal complexes (PMCs), because * Corresponding authors at: Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain (J. Arbiol); State Key Laboratory of Chemical Engineering and School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China (W. Tang); Catalonia Institute for Energy Research - IREC, Sant Adrià de Besòs, Barcelona 08930, Catalonia, Spain (A. Cabot); School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China (P. Cao). E-mail addresses: p.cao@fz-juelich.de (P. Cao), wqtang@ecust.edu.cn (W. Tang), acabot@irec.cat (A. Cabot), arbiol@icrea.cat (J. Arbiol). 1 These authors Zhifu Liang and Ting Zhang contributed equally to this work. https://doi.org/10.1016/j.cej.2022.136129 Received 18 January 2022; Received in revised form 27 March 2022; Accepted 28 March 2022 Available online 29 March 2022 1385-8947/© 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Z. Liang et al. Chemical Engineering Journal 442 (2022) 136129 the suitable organic precursors are too little to work as linkage ligand 2. Experimental section between metal atoms. In addition, most of the tetraaza[14]annulene complexes use the template synthesis method, namely, the metal ion is 2.1. Materials working as template which makes it more difficult and limited to be developed. Recently, Y. Jiang and co-workers reported the first two- Methanol (99%), N,N-dimethylformamide (DMF) (98%), hydrazine dimensional (2D) Ni(II)-based TAA-linked metal organic frameworks hydrate (98%) and carbon paper were purchased from Alfa Aesar. (MOFs) by a template method, and characterized its electrical conduc­ 1,2,4,5-benzenetetraamine tetrahydrochloride, potassium thiocyanate tivity and magnetic properties [9]. We believe that these novel struc­ (99%), and Nafion (10%) were purchased from Sigma-Aldrich. Cobalt tures are excellent platforms for electrocatalytic applications, but a more acetate tetrahydrate (99%), potassium hydrogen carbonate (99.7%), 1- in-depth exploration of these novel TAA-based PMCs with atomically butanol (99%), and 2-(4-pyridyl)malondialdehyde (95%) were bought dispersed metal atoms and the study of their relative catalytic perfor­ from Acros Organics. All solutions were prepared with Milli-Q water (DI- mance are needed. H2O, Ricca Chemical, ASTM Type I). Nafion membreanes (N-117 The utilization of renewable energy to drive the electrocatalytic membrane, 0.18 mm thick) was purchased from Alfa Aesar and kept in reduction of carbon dioxide to valuable chemicals is a potentially cost- 0.5 M NaOH solution. All chemicals were used without further purifi­ effective strategy to promote the carbon cycle. In this direction, the cation. Carbon Nanotubes (CNTs) was ordered from Sailed Technology. search for electrocatalysts [10,67] and photocatalysts [63] able to effi­ ciently activate the CO2 reduction reaction (CO2RR) and explore other 2.2. Synthesis of Poly-TAA green energy sources [64] have attracted a strong interest during the past years. Most electrocatalytic materials reported for CO2RR are 2-(4-Pyridyl)malondialdehyde (149 mg, 1 mmol), 1,2,4,5-benzene­ nanomaterials [11,12,39–41,45,46,52,53]. Even though these nano­ tetraamine tetrahydrochloride (142 mg, 0.5 mmol) and 6 mL 1-butanol materials have displayed efficient catalytic activity toward electro­ were added to a 25 mL three-neck round bottom flask. Then a few drops chemical CO2 reduction, [42] most are not easy to be synthesized in of acetic acid were added. The resulting solution was sonicated for half large quantities while controlling the active sites under a mild and easy an hour to obtain a homogenous mixture. The mixture was heated to controllable condition and the metal atoms are not 100% utilized. 110 ◦ C with stirring under argon for 48 h and afterward cooled to room Recently, atomically-dispersed metal-based catalysts, especially those temperature. The resulting brown precipitate was collected by vacuum with a coordination structure of metal-N4, have been successfully filtration and washed with ethanol, then Soxhlet extracted with meth­ applied to high-efficiency electrocatalytic carbon dioxide reduction due anol for 24 h, then vacuum dried at 60 ◦ C for 24 h to give a brown to their high catalytic activity and 100% atom utilization [32,43]. powder with ~ 63% yield. However, due to that most are prepared from the general high- temperature pyrolysis method, the active site structure and the ligand 2.3. Synthesis of Poly-TAA-Co structure are not easy to be determined and controlled [10,24]. In contrast, molecular catalysts also work as a type of atom dispersed Poly-TAA (60 mg), cobalt(II) chloride hexahydrate (60 mg) and 6 mL catalysts, which can be easily prepared under mild and controllable DMF were added to a 15 mL three-neck round bottom flask. The conditions and provide a well-defined platform that facilitates the un­ resulting mixture was maintained at 130 ◦ C with stirring under argon for derstanding of the structure-performance relationships, thus enabling a 48 h and then cooled to room temperature. The resulting brown pre­ rational catalyst design and optimization. Combining the merits of these cipitate was collected by vacuum filtration and washed with ethanol, two worlds, the development of stable and high-performance hetero­ then vacuum dried at 60 ◦ C for 24 h to give a brown-black powder with geneous molecular catalysts is raising increasing interest [13,14]. ~ 56% yield. In recent years, molecular catalysts based on metalloporphyrin and metallophthalocyanine supported on multi-wall carbon nanotubes 2.4. Synthesis of Poly-TAA-Co/CNT composites (CNTs) have shown low overpotentials, high Faradaic efficiencies and excellent selectivity for electrocatalytic CO2RR [15–19]. However, small The obtained Poly-TAA-Co and pre-oxidized carbon nanotubes molecular catalysts are not easily to work for heterogeneous reduction (CNTs) were put into a 15 mL glass vial. Then 3 mL DMF were added. reactions, which limits their application [20,21]. Therefore, it is chal­ The resulting solution was sonicated for half an hour, then stirred at lenging to develop novel materials and molecular engineering strategies 100 ◦ C for 12 h. The obtained black composites were filtrated and to realize the highly efficient and stable molecular electrocatalysts that washed with ethanol, then vacuum dried at 60 ◦ C for 24 h to get the an efficient CO2RR would require. Alternatively, π-d conjugated PMCs, composite. with a hybrid interaction between the π orbital of the ligand with the d orbital of the transition metal, have aroused widespread interest in 2.5. Electrochemical measurements recent years [34–38]. This hybrid interaction is similar to that presented in the π-π conjugated organic polymer, with the advantages such as their 5 mg of the Poly-TAA-Co-CNT and 100 µL 5 wt% Nafion solutions functionalization flexibility through ligand engineering design. How­ were dissolved in ethanol (1 mL) and ultrasonicated for 30 min to form ever, the π-d conjugated PMCs have been less studied for electrocatalytic an evenly suspension for the electrochemical experiments. To prepare CO2 reduction. In the present work we combine the π-d conjugated the working electrode, 400 µL above as-prepared inks were dropped structure and the Tetraaza[14]annulenes macrocyclic structure to onto the two sides of the carbon paper electrode with 1 × 1 cm2 and then design a novel atomically dispersed cobalt atom catalyst as efficient dried at room temperature for a few minutes, resulting in a catalyst electrocatalysts for CO2 reduction. loading mass of ~ 2 mg/cm2. Herein, a non-template synthesis method was used to construct a The electrocatalytic performance of different catalysts were new type of Tetra[14]annulene based π-d conjugated metal organic measured at room temperature by using a gas-tight H-cell with two polymer, namely, Poly-TAA-Co with CoN4 structure. This macromolec­ compartments separated by a cation exchange membrane (Nafion N-117 ular complex is loaded on a conductive support, namely multi-wall membrane) with a continuously Ar or CO2 gas injection. Each CNTs, and applied for the efficient electrochemical CO2RR. The ob­ compartment contained 70 mL electrolyte (0.5 M KHCO3 made from de- tained results have been rationalized by means of density functional ionized water). In a typical experiment, a standard three-electrode setup theory (DFT) calculations. in 0.5 M KHCO3 solution was assembled: an Ag/AgCl electrode as reference electrode, a Pt wire as auxiliary electrode and a carbon paper coated with the different samples as working electrode (surface area = 1 2 Z. Liang et al. Chemical Engineering Journal 442 (2022) 136129 Scheme 1. Scheme of the synthesis of Poly-TAA-Co. cm2). The potentials were measured versus Ag/AgCl and converted to 2.6. Characterizations the reversible hydrogen electrode (RHE) according to the following equation: ERHE = E0Ag/AgCl + EAg/AgCl + 0.059 × pH, pH = 7 [44] All The crystal structure information was characterized by means of electrochemical results were recorded without iR-compensation by powder X-ray diffraction (XRD) measured in a Bruker AXS D8 Advance using a computer-controlled BioLogic VMP3 electrochemical worksta­ X-ray diffractometer. (Cu-Kα radiation, λ = 1.5106 Å, 40 kV and 40 mA; tion. Meanwhile, the I-t curves were obtained to reach a stable state at Bruker, Germany). High-resolution transmission electron microscopy − 0.30 V vs. RHE in Ar-saturated 0.5 M KHCO3 (pH = 8.5) as supporting (HRTEM) were conducted in a FEI Tecnai F20 microscope at an oper­ electrolyte. The CV and LSV curves were performed at 20 mV s− 1. ating voltage of 200 keV equipped with an embedded Quantum Gatan Moreover, electrochemical impedance spectroscopy (EIS) of different Image Filter for electron energy loss spectroscopy (EELS) analyses. High samples were carried out in a frequency range from 100 kHz to 100 angle annular dark-field (HAADF)-scanning transmission electron mi­ mHz. croscopy (STEM) images and Energy Dispersive X-Ray Spectroscopy Before the electrochemical CO2 reduction experiments, an average (STEM-EDS) analysis were obtained in a spherical aberration corrected rate of 20 mL min− 1 Ar was injected into cathodic electrolyte in order to transmission electron microscopy FEI Titan G2 80–200 (ChemiSTEM). It form an Ar-saturated solution. During the electrochemical CO2 reduc­ is equiped with four EDX detectors and operated at 200 kV. High- tion experiments, the CO2 gas was delivered at an average rate of 20 mL resolution transmission electron microscopy (HRTEM) and high angle min− 1 at room temperature and ambient pressure, and we measured the annular dark-field (HAADF)-scanning transmission electron microscopy downstream by a volumetric digital flowmeter. The gas-phase compo­ (STEM) images were obtained in a FEI Tecnai F20 microscope at an sition was analyzed by gas chromatography (GC) during potentiostatic operating voltage of 200 keV equipped with an embedded Quantum measurements every 20 min. Gatan Image Filter for electron energy loss spectroscopy (EELS) ana­ Details concerning the calculation of Faradaic Efficiency (FE) are lyses. Images had been analyzed by means of Gatan Digital Micrograph shown below.[47,48]. software. Thermogravimetric analyses (TGA) were performed under air The partial current density for a given gas product was calculated as at a heating rate of 5 ◦ C/min using a Thermogravimetric Analyzer Q200. below: X-ray photoelectron spectroscopy (XPS) data was obtained by using 150 W and a Phoibos 150 MCD-9 detector. The X-ray absorption fine struc­ ni FP0 ji = xi × V × ×(electrode area)− 1 (1) ture (XAFS) measurements of the pellet of powder sample were carried RT out at the BL12C beamline of the Photon Factory, the High Energy where xi is the volume fraction of certain product determined by online Accelerator Research Organization (KEK), under proposal no. GC referenced to calibration curves from three standard gas samples, v is 2019G117, 2021G129. Co foil was used for the calibration of Co K-edge. the flow rate, ni is the number of electrons involved, P0 = 101.3 kPa, F is Structural analysis was performed using the Demeter software platform. the Faraday constant, and R is the gas constant. The corresponding FE at The reported crystal data were used for calculating Feff. The samples each potential is calculated by. were pressed onto a double-sided carbon tape and the data was recorded in the X-ray fluorescence mode. The XAFS data was processed with the ji FE = × 100% (2) ATHENA program.[49] The EXAFS was analyzed using the IFEFFIT j package [50] and the EXAFS fitting was performed with FEFF6L.[51]. 3 Z. Liang et al. Chemical Engineering Journal 442 (2022) 136129 Fig. 1. (a)-(c) HAADF-STEM images of Poly-TAA-Co-CNT (3:7) displaying atomically dispersed cobalt atoms. (d) Low magnification HAADF-STEM image and EDS elemental maps for Co (red), C (light blue), N (green) and O (yellow). 2.7. DFT calculation details thicknesses were higher than 15 Å. The whole process of CO2 electrochemical reduction to CO mainly The lowest unoccupied molecular orbital (LUMO) and highest includes the following three steps: occupied molecular orbital (HOMO) of cobalt phthalocyanine (CoPc) and Co(II) tetraaza[14]annulenes (CoTAA) structure unit were calcu­ CO2(g) + * + H+ + e- ↔ COOH* (Activation process)(1) lated by using DFT with the hybrid B3LYP functional and the 6–311++G COOH* + H+ + e- ↔ CO* + H2O(l) (Surface reaction)(2) (d,p) basis set.[57–59] For Co atoms the Stuttgart Dresden (SDD) triple zeta effective-core potential (ECP) basis set was used.[60] D3 dispersion CO* ↔ CO(g) + * (Desorption process)(3) correction developed by Grimme is included for weak interactions.[61] where the *, COOH* and CO* represent free sites, the adsorption state of These calculations were carried out using the Gaussian16 program COOH and CO, respectively. The (g) represent the gas phase. The re­ package.[62]. The visualization of the HOMO and LUMO plots is carried action free energies of each steps were calculated by the following out using the Multiwfn 3.8 program and Visual Molecular Dynamics formula: software [65,66]. The spin-polarized density functional theory (DFT) calculations with G = EDFT + EZPE − TS + Esol (4) projector augmented wave (PAW) method were performed using the Vienna Ab initio Simulation Package code.[54,55] The generalized where EDFT is the DFT calculated energy, EZPE is the zero-point energy, T gradient approximation of Perdew-Burke-Ernzerhof (PBE) with van der (=298.15 K) is temperture, S is the entropy, and Esol is the solvation Waals correlation was employed to optimize the geometric structures. correction, which for CO* was stabilized by 0.1 eV and for COOH* by [56] The convergence criteria was 0.03 eV/ Å in force and 1 × 10− 5 eV 0.25 eV. in energy, while the plane wave cutoff was 500 eV. The Monkhorst–Pack mesh k-point grids was 4 × 1 × 1 for all models. All the vacuum 4 Z. Liang et al. Chemical Engineering Journal 442 (2022) 136129 Fig. 2. (a) XRD patterns of CNT, Poly-TAA-Co and Poly-TAA-Co-CNT. (b) N 1 s and (c) Co 2p XPS spectra of Poly-TAA-Co powder. (d) Co K-edge XANES spectrum. (e) Fourier transformed EXAFS spectrum and fitted curve. (f) Co K-edge EXAFS oscillations in k-space (k3χ(k)) of Poly-TAA-Co and its fitted spectrum. 3. Results and discussions coordination with cobalt ion (Fig. 2b) [9,30]. The high resolution Co 2p XPS spectrum of Poly-TAA-Co (Fig. 2c) showed two main peaks at 796 3.1. Structural characterization eV (2p1/2) and 780.3 eV (2p3/2), that are assigned to a Co2+ chemical state. The respective satellite peaks were also observed [29]. Ther­ The Poly-TAA-Co conjugated polymer was prepared in two steps mogravimetric analyses (TGA) were performed under air to analyze the (Scheme 1). First, the Poly-TAA was synthesized by using a non-template Cobalt content of Poly-TAA-Co (Figure S6). The small mass lost at about method based on the Schiff base reaction between 2-(4-pyridyl)malon­ 100 ◦ C corresponds to the solvent (mainly water). The poly-TAA-Co start dialdehyde and 1,2,4,5-benzenetetramine tetrahydrochloride in 1- to decompose at about 350 ◦ C. Finally, after about 650 ◦ C, Poly-TAA-Co butanol with a few drops of acetic acid. The obtained brown precipi­ fully decomposes to cobalt oxide. From the TGA analysis, we can tate was Soxhlet extracted with methanol to remove the small impurity calculate that the Co content of the Poly-TAA-Co is about 12.6% (the molecules and dried at 60 ◦ C under vacuum. The brown color of the theoretical value was 14%). After loading the poly-TAA-Co on the CNTs, product was an indication of the formation of a conjugated organic where the CNTs corresponding mass was 70%, the Co mass content in polymer. In a second step, Poly-TAA was reacted with cobalt chloride in the Poly-TAA-Co-CNT (3:7) is 3.78%. dimethylformamide (DMF) to yield metal organic polymer Poly-TAA- The coordination structure of cobalt in Poly-TAA-Co was further Co. analyzed by means of X-ray absorption fine structure (XAFS) at the Co K- Fourier-transform infrared spectroscopy (FTIR) analysis of Poly- edge (Fig. 2d-f). The Co K-edge X-ray absorption near-edge structure TAA-Co displayed the footprint of C– – N at 1606 cm− 1, suggesting the (XANES) spectrum of Poly-TAA-Co exhibits the fingerprint of CoN4 at formation of Co(II) tetraaza[14]annulene (Figure S1) [22]. The 7709.4 eV assigned to the dipole forbidden 1s to 3d transition [16]. In morphology of Poly-TAA-Co is displayed in Figures S2 and S3. addition, we observed the XANES absorption onset of Poly-TAA-Co at To improve the electrical conductivity of Poly-TAA-Co organic higher energy than that of cobalt foil (Fig. 2d). Besides, the main ab­ polymer toward their use as efficient electrocatalysts for CO2RR, they sorption peak of Poly-TAA-Co has a stronger intensity than that of Co were grafted through π-π interaction to the surface of pre-oxidized multi- foil. These features demonstrate the valence state of Co in Poly-TAA-Co wall CNTs by just mixing both in DMF (Scheme 1). Isolated Co atoms to be higher than that of metallic cobalt, consistently with XPS results. sites were characterized via high angle annular dark-field (HAADF)- The fitting of the Fourier transformed extended X-ray absorption fine aberration-corrected scanning transmission electron microscopy (STEM) structure (EXAFS) spectrum suggests that the Co is coordinated with 4 analysis (Fig. 1a-d). The Energy Dispersive X-Ray Spectroscopy (EDS) nitrogen atoms with a bond length of 2.05 Å (Fig. 2e and Table S1). analysis revealed the homogeneous distribution of Poly-TAA-Co on the Overall, XAFS analysis confirmed the formation of the CoN4 structure surface of CNTs, and the uniform atomic distribution of Co within Poly- units, thus demonstrating our synthetic route to yield Poly-TAA-Co TAA-Co (Fig. 1e). The powder X-ray diffraction (PXRD) pattern of Poly- (Scheme 1). The surface area of Poly-TAA, Poly-TAA-Co and Poly- TAA-Co displayed no diffraction peak (Fig. 2a), which exposed the TAA-Co-CNT, evaluated from N2 adsorption–desorption isotherms at amorphous nature of the material and discarded the presence of any 77 K using the Brunauer-Emmett-Teller equation was 4.1, 124.9 and crystalline Co-based phase. The PXRD pattern of the obtained composite 208.2 m2 g− 1, respectively (Figure S7). (Poly-TAA-Co-CNT) showed a unique peak at about 26◦ , which was assigned to the overlap between the CNT and the organic polymer structures. 3.2. Electrochemical CO2 reduction performance X-ray photoelectron spectroscopy (XPS) analyses displayed the presence of C, N, O, Cl of Poly-TAA and Poly-TAA-Co (Figure S4 and The electrocatalytic activity toward CO2RR of the Poly-TAA-Co Figure S5). Compared with Poly-TAA, the high-resolution N 1 s XPS metal organic polymer and it blended with two main different spectrum of Poly-TAA-Co is shifted to lower binding energies, which is amounts of CNTs (Poly-TAA-Co:CNT = 3:7 and 1:1) were investigated attributed to the deprotonation of the nitrogen in the TAA cores upon using a three-electrode H-cell including an anion exchange membrane and a 0.5 M KHCO3 electrolyte. Gas-phase products were periodically 5 Z. Liang et al. Chemical Engineering Journal 442 (2022) 136129 (a) (b) Current Density (mA cm-2) Current Density (mA cm-2) 0 0 -10 -6 -20 -12 -30 -18 -40 -24 Poly-TAA-Co-Ar Poly-TAA-Co-CNT(3:7)-Ar Poly-TAA-Co-CO2 -50 -30 0 2 4 6 8 10 12 14 -0.8 -0.6 -0.4 -0.2 0.0 (c) Time (min) (d) Potential (V vs. RHE) Current Density (mA cm-2) Current Density (mA cm-2) 0 0 -10 -10 -20 -20 -30 -30 Poly-TAA-Co-CNT(1:1)-Ar -40 Poly-TAA-Co-CNT(3:7)-Ar -40 Poly-TAA-Co-CNT(1:1)-CO2 -50 Poly-TAA-Co-CNT(3:7)-CO2 -0.8 -0.6 -0.4 -0.2 0.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 Potential (V vs. RHE) Potential (V vs. RHE) Fig. 3. (a) Electrode current recorded during reduction of Poly-TAA-Co-CNT(3:7) at − 0.30 V vs. RHE in 0.5 M KHCO3 purged with Ar gas. Linear sweep voltammetry (LSV) comparison for (b) Poly-TAA-Co, (c) Poly-TAA-Co-CNT(1:1) and (d) Poly-TAA-Co-CNT(3:7) samples in Ar- and CO2-saturated 0.5 M KHCO3 solution. (a) (b) (c) 100 0 0 Poly-TAA-Co-CNT (3:7) Faradic Efficient (CO) 80 Poly-TAA-Co-CNT (1:1) -4 JTotal (mA cm-2) -2 JCO (mA cm-2) 60 -8 -4 40 1D-TAA-Co -12 Poly-TAA-Co-CNT (3:7) -6 20 Poly-TAA-Co-CNT (1:1) Poly-TAA-Co CNT -16 Poly-TAA-Co-CNT (3:7) -8 Poly-TAA-Co-CNT (1:1) 0 -10 -0.7 -0.6 -0.5 -0.4 -0.7 -0.6 -0.5 -0.4 -0.7 -0.6 -0.5 -0.4 (d) Potential (V vs. RHE) (e) Potential (V vs. RHE) (f) Potential (vs. RHE) 0 100 5 100 Faradaic Efficiency of CO (%) Faradic Efficiency of CO (%) -2 80 0 80 JH2 (mA cm-2) JTotal (mA cm-2) -4 60 -5 60 -6 40 -10 40 Poly-TAA-Co-CNT (3:7) Co-TAA-CNT(3:7) -8 Poly-TAA-Co-CNT (1:1) 20 CoPc-CNT(3:7) -15 20 Poly-TAA-Co-CNT(3:7) -10 0 -20 0 -0.7 -0.6 -0.5 -0.4 -0.7 -0.6 -0.5 -0.4 0 2 4 6 8 10 Potential (V vs. RHE) Potential (V vs. RHE) Time (h) Fig. 4. (a) Total current density, (b) FE of CO at various potentials, (c) current density for CO production, (d) FE of H2 at various potentials, (e) FE of CO at various potentials on Co-TAA-CNT(3:7), CoPc-CNT(3:7) and Poly-TAA-Co-CNT(3:7). (f) Stability test of Poly-TAA-Co-CNT(3:7) at − 0.50 V vs. RHE. quantified using gas chromatography (GC). Online GC results showed Figure S8a), indicating a faster reaction rate in the presence of pre- CO and H2 to be the main gas products for all catalysts. Electrodes were oxidized CNTs. prepared by coating the catalysts on carbon paper (1 cm × 1 cm) with a The Faradaic efficiency (FE) for CO2 conversion to CO (FECO) and H2 mass loading of ~ 2.0 mg cm− 2. Electrodes were pretreated at a constant (FEH2), and the related current density were determined in the potential potential of − 0.30 V vs. RHE for 15 min until a stable current was range from − 0.40 to − 0.70 V vs. RHE. Fig. 4b and Figure S8b shows the reached (Fig. 3a). Then, linear sweep voltammetry (LSV) curves were FECO on these of the Poly-TAA-Co-based cathodes. The FE of bare CNTs measured (Fig. 3b-d). The three samples displayed a clear increase of the can also be found in Fig. 4b. For Poly-TAA-Co-CNT(3:7) samples, the current density in the presence of CO2 compared to values obtained in an CO2RR to CO started at a potential as low as − 0.40 V vs. RHE with a Ar-saturated electrolyte. Besides, the total current densities significantly maximum FECO of 90 % at − 0.50 V vs. RHE. This FECO is clearly above increased with the loading of Poly-TAA-Co on CNTs (Fig. 4a and those obtained from Poly-TAA-Co-CNT(1:1), Poly-TAA-Co and CNT 6 Z. Liang et al. Chemical Engineering Journal 442 (2022) 136129 (a) (b) (c) 3 Current Density / (mA/cm2) 25 mV/s 0.6 1D-TAA-Co-CNT (1:1) 0.06 Current Density / (mA/cm2) 50 mV/s 1D-TAA-Co-CNT (3:7) 75 mV/s 2 Overpotential / V 100 mV/s 0.03 125 mV/s 1 0.5 150 mV/s 0 0.00 -1 c 0.4 V /d e -2 25 mV/s 5m / d ec -0.03 50 mV/s 2 mV -3 75 mV/s 21 -4 100 mV/s 0.3 -0.06 125 mV/s 150 mV/s -5 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.60 0.65 0.70 0.75 0.80 Log (Current density of CO (mA/cm2)) Potential / V (vs. RHE) Potential / V (vs. RHE) (d) (e) (f) 4 Current Density / (mA/cm2) 25 mV/s 0.10 Poly-TAA-Co 3 mF/cm2 6 Poly-TAA-Co-CNT(3:7) 3 50 mV/s J1-J2 (mA cm-2) 75 mV/s 5 19 mF/cm2 J1-J2 (mA cm-2) 2 0.08 1 4 0 0.06 3 -1 0.04 -2 100 mV/s 2 Poly-TAA-Co-CNT(1:1) 125 mV/s 0.02 -3 150 mV/s 1 14 mF/cm2 0.80 0.85 0.90 0.95 1.00 20 40 60 80 100 120 140 160 20 40 60 80 100 120 140 160 Potential / V (vs. RHE) Scan Rate (mV/s) Scan Rate (mV/s) Fig. 5. (a) Tafel plots of Poly-TAA-Co-CNT (1:1), Poly-TAA-Co-CNT (3:7). Cyclic voltammograms curves for (b) Poly-TAA-Co, (c) Poly-TAA-Co-CNT(1:1) and (d) Poly-TAA-Co-CNT(3:7). Plots of the current density vs. scan rate for (e) Poly-TAA-Co, (f) Poly-TAA-Co-CNT(1:1) and Poly-TAA-Co-CNT(3:7) electrodes. samples. Poly-TAA-Co-CNT(3:7) exhibits the highest current densities Co-CNT(1:1) and Poly-TAA-Co-CNT(3:7) gradually decreased and the for CO formation among all the studied electrodes and in the whole competitive HER became the dominant reaction, as evidenced in the range of applied potentials, with a maximum partial current density of FEH2 and the partial H2 current densities shown in Fig. 4d and − 5.8 mA cm− 2 at − 0.50 V vs. RHE (Fig. 4c). It is attributed to higher Figure S9, respectively. Tafel plots were obtained by calculating the conductivity with higher ratio of CNTs. As the potential changed to more logarithm of current density [log(jCO)] vs. the overpotential (η) to negative values (from − 0.55 to − 0.70 V vs. RHE), the FECO of Poly-TAA- reveal the possible reaction pathway of electrochemical CO2 reduction Fig. 6. (a) Poly-TAA-Co-CNT(1:1) and (b) Poly-TAA-Co-CNT(3:7) with and without 0.05 M KSCN. (c) Current density and FE of Poly-TAA-Co-CNT(3:7) at different KHCO3 concentrations at a constant potential (− 0.50 V vs. RHE). (d) Partial CO current density of Poly-TAA-Co-CNT(3:7) vs. KHCO3 concentration at − 0.50 V vs. RHE. 7 Z. Liang et al. Chemical Engineering Journal 442 (2022) 136129 Fig. 7. EDX mapping of Poly-TAA-Co-CNT (3:7) after electrocatalytic CO2RR. (Fig. 5a). We obtained the plot in the linear part of low overpotentials activity of Poly-TAA-Co, Poly-TAA-Co-CNT(1:1) and Poly-TAA-Co-CNT ranging from − 0.3 to − 0.43 V and − 0.28 to − 0.38 V for Poly-TAA-Co- (3:7) was observed. This phenomenon is attributed to the poisoning of CNT(1:1) and Poly-TAA-Co-CNT(3:7), respectively. The Tafel slopes tetra[14]anulene cobalt active sites by SCN− . were calculated to be 25 mV dec− 1 and 21 mV dec− 1 for Poly-TAA-Co- The current density at different HCO3− concentrations was measured CNT(1:1) and Poly-TAA-Co-CNT(3:7), respectively, indicating that the at a constant applied potential of − 0.50 V vs. RHE to probe the role of possible rate-determining step is a single electron transfer to CO2 to HCO3− within the reaction. As revealed in Figure 6c-d, a plot of log generate a CO2•− intermediate, which is consistent with the previous (jCO) versus log ([HCO3− ]) showed a slope of 0.72, suggesting that the report [16,33]. The EIS spectrum in Figure S10 also confirmed a faster concentration of HCO3− plays a considerable role, influencing the effi­ electron transfer when the Poly-TAA-Co was loaded on the surface of ciency of the conversion reaction of CO2 to CO. As observed previously CNTs [23–26]. that the HCO3− not only acts as a pH buffer and proton donor, but also The electrochemical active surface area (ECSA) was determined from increases the concentration of CO2 near the electrode surface the electrochemical double-layer capacitance (Cdl) of the active mate­ [25,27,28]. rials (see supporting information for details). The Cdl of Poly-TAA-Co, The CO2RR performance of Poly-TAA-Co-CNT(3:7) was also Poly-TAA-Co-CNT(1:1) and Poly-TAA-Co-CNT (3:7) samples were ob­ compared with that of commercial cobalt phthalocyanine (CoPc) sup­ tained by CV at different scan rates, from the slope of ΔJ = Ja-Jc against ported on CNTs (CoPc-CNT) at the same weight ratio (3:7) and with the scan rate (Fig. 5b-d), the slope which is twice of Cdl could be ob­ catalysts based non-polymerized Co-TAA molecules supported on CNTs tained. The Cdl of Poly-TAA-Co-CNT(1:1) and Poly-TAA-Co-CNT(3:7) (Co-TAA-CNT(3:7)). As shown in Fig. 4e and Figure S12, the maximum samples are 14 mF cm− 2 and 19 mF cm− 2, respectively, which are FECO obtained from Poly-TAA-Co-CNT(3:7) was significantly higher significantly larger values than the 3 mF cm− 2 obtained from Poly-TAA- than those obtained from CoPc-CNT(3:7) and Co-TAA-CNT(3:7). These Co (Fig. 5e-f). Such different Cdl and thus of the related ECSA is a further results demonstrated the higher activity of the Tetra[14]anulene cobat indication of the higher CO2RR catalytic activity of Poly-TAA-Co-CNT active site in Poly-TAA-Co-CNT, and also the important role of the samples, and particularly of Poly-TAA-Co-CNT (3:7). In addition, the conjugated polymer structure, which was proposed to favor the charge surface roughness factor (Rf) is calculated by taking the estimated ECSA transfer during CO2RR compared with non-polymeric Co-TAA. and dividing by the geometric area of the electrode (1 cm2). Generally, a The stability of the Poly-TAA-Co-CNT(3:7) electrocatalyst was eval­ constant capacitance is used in the same solution, therefore, Rf is linear uated through a 10 h durability test at a constant − 0.50 V vs. RHE with Cdl. The higher Rf obtained on the Poly-TAA-Co-CNT samples could cathode potential. Outlet gases were analyzed every 30 min by GC to significantly reduce the adhesion force between the electrode surface determine the FECO. The current density of Poly-TAA-Co-CNT(3:7) and gas bubbles, favoring the CO2RR [25]. maintained a steady value of approximately − 5.8 mA cm− 2 with no To further demonstrate the key role of tetra[14]anulene cobalt significant decay (Fig. 4f) during the 10 h test. Besides, the corre­ complex in the CO2RR, we tested the catalytic activity of the samples sponding FECO just slightly decreased from 90 % to 83 % after the 10 h after inactivating these sites by adding SCN− ions which is with high stability test. From the measurement of the XRD pattern of the electro­ affinity to Co into the solution. As shown in Figure S11 and Fig. 6a-b, catalysts before and after the CO2RR stability test, no additional upon addition of 0.05 M KSCN, a significant depression of the catalytic diffraction peak was observed, indicating no cobalt aggregating under 8 Z. Liang et al. Chemical Engineering Journal 442 (2022) 136129 Fig. 8. DFT calculation results. (a). Calculated energy diagrams for CO2 to CO conversion on CoPc and Co-TAA molecule, respectively. (b-c). Calculated electron density distribution of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of CoPc and Co-TAA. CO2RR conditions (Figure S13). The HAADF-STEM and HRTEM images Declaration of Competing Interest (Figure S14) also confirm that no Co-based nanoparticles or clusters appear after the test and no significant change in the morphology of the The authors declare the following financial interests/personal re­ studied catalysts. Furthermore, the EELS (Figure S15) and EDX (Fig. 7) lationships which may be considered as potential competing interests: elemental maps also confirm that the remaining Co atoms maintain a Jordi Arbiol reports financial support was provided by Spanish MINECO. uniform dispersion. The inductively coupled plasma optical emission Jordi Arbiol reports financial support was provided by Government of spectrometry (ICP-OES) was further used to analyzed the electrolyte Catalonia. Jordi Arbiol reports financial support was provided by Ho­ during CO2RR stability test, the Co signal was found, indicating that rizon 2020. Jordi Arbiol reports financial support was provided by there is leaching of Co during the electrochemical CO2RR process. MCIN. Jordi Llorca reports financial support was provided by European To further understand the high catalytic activity of the cobalt active Regional Development Fund. Xiang Wang reports financial support was site in Poly-TAA-Co for CO2RR, DFT calculations were conducted. We provided by China Scholarship Council. PengYi Tang reports financial particularly analyzed the Kohn-Sham molecular orbital of CoPc and the support was provided by Humboldt Research Fellowship. Lijia Liu re­ structure unit namely Co-TAA of Poly-TAA-Co (Fig. 8a-b). The energy of ports financial support was provided by Natural Sciences and Engi­ the lowest unoccupied molecular orbital (LUMO) of Co-TAA is − 5.97 eV, neering Research Council Canada. Mohsen Shakouri reports financial which is below that of the CoPc molecule, − 4.95 eV. The lower reduc­ support was provided by Canada Foundation for Innovation. Mohsen tion potential of Co-TAA indicates that it is easier for Co-TAA to accept Shakouri reports financial support was provided by National Research the charge transfer from the electrode to promote the CO2RR. Council. Mohsen Shakouri reports financial support was provided by The Gibbs free energy diagram of the CO2RR for CoPc and Co-TAA Government of Saskatchewan. Weiqiang Tang reports financial support are shown in Fig. 8c. The rate-determining step (RDS) for CO2RR is was provided by Chinese Postdoctoral Science Foundation. the adsorption of CO2 on the cobalt center to form *COOH by a proton- couple electron transfer reaction. At 0 V, the free energy of RDS for Co- Acknowledgements TAA is 0.6 eV, which is lower than the 0.91 eV obtained for CoPc. Furthermore, under − 0.5 V (Figure S16), the free energy of RDS for Co- The present work is supported by the projects SEHTOP, ENE2016- TAA (0.1 eV) is still significantly lower than that of CoPc (0.41 eV). 77798-C4-3-R, COMBENERGY (PID2019-105490RB-C32) and NANO­ These DFT results are in good agreement with the experimental data. GEN (PID2020-116093RB-C43), funded by MCIN/ AEI/10.13039/ 501100011033/ and by “ERDF A way of making Europe”, by the “Eu­ 4. Conclusions ropean Union”. This study was supported by MCIN with funding from European Union NextGenerationEU (PRTR-C17.I1) and the Government We have reported for the first time a synthetic route to obtain new of Catalonia. ICN2 is supported by the Severo Ochoa program from π-d conjugated cobalt building blocks based on Tetraaza[14]annulene Spanish MINECO (Grant No. SEV-2017-0706) and ICN2 and IREC are organic polymer by a non-template method. Upon loading the cobalt- funded by the CERCA Programme /Generalitat de Catalunya. Part of the based macromolecule complex on the conductive carbon nanotube, present work has been performed in the of the Universitat Autònoma de the catalyst showed an excellent performance toward electrocatalytic Barcelona Materials Science PhD program. Z.F Liang acknowledges CO2 reduction, with a FECO of 90% at a low overpotential (390 mV) and funding from MINECO SO-FPT PhD grant (SEV-2013-0295-17-1). X. a stable performance (>10 h). DFT simulations revealed the Co-TAA Wang and T. Zhang thank the China Scholarship Council, China, for the active sites to have the lowest activation energy for CO2RR, well scholarship support. W. Tang thanks China Postdoctoral Science Foun­ below that of commercial CoPc. Our results not only demonstrate that dation (Nos.2021M691008). Authors acknowledge funding from Gen­ the non-template synthesis method can be used to synthesize tetra[14] eralitat de Catalunya 2017SGR327 and 2017SGR1246. This research annulene organic polymer, but also provide a new strategy to design was supported by JSPS KAKENHI Grant Numbers JP20K15293 (TY). M. tetra[14]annulene metal organic polymer based atomically dispersed Yamashita thanks the support from the 111 project (B18030) from metal atom catalyst for efficient CO2RR and opens the way to use this China. The X-ray absorption fine structure was collected on the BL-12C type of materials for other applications. beamline in the Photon Factory of the High Energy Accelerator Research Organization (KEK, proposal no. 2019G117, 2021G129). We thank Dr. 9 Z. Liang et al. Chemical Engineering Journal 442 (2022) 136129 Guillaume Sauthier and Dr. Albert Llorente Mola for the great help on [23] O.S. Bushuyev, P. De Luna, C.T. Dinh, L. Tao, G. Saur, J. van de Lagemaat, S. O. Kelley, E.H. Sargent, What should we make with CO2 and how can we make it? XPS and BET measurements, respectively. Joule 2 (5) (2018) 825–832. [24] W. Ren, X. Tan, W. Yang, C. 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