Control of evolution of porous copper-based metal–organic materials for electroreduction of CO2 to multi-carbon products

Electrochemcial reduction of CO2 to multi-carbon (C2+) products is an important but challenging task. Here, we report the control of structural evolution of two porous Cu(ii)-based materials (HKUST-1 and CuMOP, MOP = metal–organic polyhedra) under electrochemical conditions by adsorption of 7,7,8,8-tetracyanoquinodimethane (TNCQ) as an additional electron acceptor. The formation of Cu(i) and Cu(0) species during the structural evolution has been confirmed and analysed by powder X-ray diffraction, and by EPR, Raman, XPS, IR and UV-vis spectroscopies. An electrode decorated with evolved TCNQ@CuMOP shows a selectivity of 68% for C2+ products with a total current density of 268 mA cm−2 and faradaic efficiency of 37% for electrochemcial reduction of CO2 in 1 M aqueous KOH electrolyte at −2.27 V vs. RHE (reversible hydrogen electrode). In situ electron paramagnetic resonance spectroscopy reveals the presence of carbon-centred radicals as key reaction intermediates. This study demonstrates the positive impact of additional electron acceptors on the structural evolution of Cu(ii)-based porous materials to promote the electroreduction of CO2 to C2+ products.

Theoretical modelling of the spectra was performed with the EasySpin toolbox within Matlab. 2 The BET surface areas were obtained from N 2 adsorption/desorption isotherms recorded on a Micromeritics 3-Flex instrument at 77 K. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Kratos Axis Ultra Hybrid spectrometer with monochromatized Al Kα X-ray source, using 20 eV energy pass for core levels spectra. C 1s electron at binding energy of 284.8 eV was used as a standard reference to calibrate the photoelectron energy shift. All the data analysis was performed on the Casa XPS software (version: 2.3.22PR1.0). Peak deconvolution was performed with Tougaard type background and LA peak shape. The morphologies of the materials were measured by scanning electron microscopy (SEM) on a Quanta FEG 650.
Electrochemical study. All electrochemical experiments were carried out on a CHI 660E, USA electrochemical workstation with a flow cell. Carbon paper (CP) was used as the substrate for preparing working electrodes. The working electrodes HKUST-1/CP, TCNQ@HKUST-1/CP, CuMOP/CP and TCNQ@CuMOP/CP were prepared using the following procedure: 10 mg of HKUST-1, TCNQ@HKUST-1, CuMOP or TCNQ@CuMOP was suspended in 750 μL isopropanol and 250 μL H 2 O containing 100 μL Nafion D-521 dispersion (5 wt%). This was treated with ultrasound for 30 mins to form a homogeneous ink. 100 μL of the ink was spread onto the CP (1×1 cm 2 ) surface and dried at room temperature. To increase the hydrophobicity of the working electrode, PTFE was placed on the gas chamber side, just behind the catalyst/CP working electrode, to prevent the catholyte from entering the gas chamber.
The flow cell (Supplementary Figure 4) contains a hydrophobic and porous cathodic working electrode separating the gas and catholyte chambers. The cathode and anode are separated by an anion exchange membrane, and 1.0 M KOH solution was used as both catholyte and anolyte, and passed through the cathode and anode chambers separately. Ag/AgCl (in saturated KCl) was used as the reference electrode and Pt was the counter electrode. CO 2 (50 sccm) was passed over the porous working electrode, and then reduced over the catalyst into the cathodic section. After electrolysis, the liquid products were studied by 1 H NMR spectroscopy and gas products were collected in a gasbag and analysed by Bruker Matrix MG5 FTIR spectrometer.
Electrochemical impedance spectroscopy (EIS) was recorded at −0.174 V vs RHE with an amplitude of 5.0 mV (10 -1 to 10 6 Hz). The value for the resistance of charge transfer (R ct ) was obtained by fitting the EIS spectra using the Zview software (Version 3.5f, Scribner Associates, Inc). Linear sweep voltammetry (LSV) scans were conducted in CO 2 and Ar saturated catholyte.
Quantitative analysis of products in liquid and gas phase. All liquid products were quantified using 1 H NMR spectroscopy. DMSO (1.0g) was dissolved in H 2 O (25 mL) and this solution used as the reference. After the CO 2 RR, 100 μL of the as-prepared reference solution of DMSO in H 2 O was injected into the catholyte. Then 0.9 mL of catholyte was mixed with 0.1 mL D 2 O, and around 0.7 mL of this solution was subsequently transferred into an NMR tube for measurement. Gas products were quantified by Bruker Matrix MG5 FTIR spectrometer The value of FE was calculated using the equation: where n product is the amount of product (mol) from Bruker Matrix MG5 FTIR spectrometer or 1 H NMR spectroscopy (formic acid), n electrons is electron transfer number (both the production of H 2 and formate are two-electron processes), F is the Faraday constant (96485 C mol −1 ), and Q is the total charge passed during the CO 2 RR.  Table 2. Based on the electrochemical CO 2 reduction results, most of the products are multi-carbon products, denoted DMPO-C  above. The observed EPR simulated parameters for DMPO carbon-centred radicals are different to those derived from •CO 2 and •COOH radicals. In addition, using DMPO as a spin trap does not differentiate between various carbon-centred radicals.