Electronic Tuning of CO2 Interaction by Oriented Coordination of N‐Rich Auxiliary in Porphyrin Metal–Organic Frameworks for Light‐Assisted CO2 Electroreduction

Abstract The efficient CO2 electroreduction into high‐value products largely relies on the CO2 adsorption/activation or electron‐transfer of electrocatalysts, thus site‐specific functionalization methods that enable boosted related interactions of electrocatalysts are much desired. Here, an oriented coordination strategy is reported to introduce N‐rich auxiliary (i.e., hexamethylenetetramine, HMTA) into metalloporphyrin metal organic frameworks (MOFs) to synthesize a series of site‐specific functionalized electrocatalysts (HMTA@MOF‐545‐M, M = Fe, Co, and Ni) and they are successfully applied in light‐assisted CO2 electroreduction. Noteworthy, thus‐obtained HMTA@MOF‐545‐Co presents approximately two times enhanced CO2 adsorption‐enthalpy and electrochemical active surface‐area with largely decreased impedance‐value after modification, resulting in almost twice higher CO2 electroreduction performance than its unmodified counterpart. Besides, its CO2 electroreduction performance can be further improved under light‐illumination and displays superior FECO (≈100%), high CO generation rate (≈5.11 mol m−2 h−1 at −1.1 V) and energy efficiency (≈70% at −0.7 V). Theoretical calculations verify that the oriented coordination of HMTA can increase the charge density of active sites, almost doubly enhance the CO2 adsorption energy, and largely reduce the energy barrier of rate determining step for the boosted performance improvement. This work might promote the development of modifiable porous crystalline electrocatalysts in high‐efficiency CO2 electroreduction.


Characterizations and instruments
Powder X-ray diffraction (PXRD) experiments are recorded on Bruker D8 Advance (operating at 40 kV and 20 mA) with Ni-filtered Cu Ka radiation at 1.5406 (Å) with a speed of 5 o min -1 . SEM images is obtained from a FEI NOVA NANO 430 Field Emission Scanning Electron Microscope equipped with an Oxford Energy Dispersive X-ray spectroscopy. N2 and CO2 sorption measurements are carried out on a Micromeritics ASAP 2460 system at 77 K and 298 K, respectively, after the samples are first degassed at 120 o C for 8 h. Raman spectra are collected on a Renishaw in Via Raman Spectrometer. Fourier transform infrared (FTIR) spectra are acquired from a Nicolet 6700 spectrometer (Nicolet Instrument Co., USA). The thermo-gravimetric analysis (TGA) was performed on a Shimadzu DTG-60H thermogravimetric analyzer in N2 flow of 30 mL min -1 and the heating rate of 10 o C min -1 . X-ray Photoelectron Spectroscopy (XPS) was carried out on a Thermo ESCALAB 250XI multifunctional imaging electron spectrometer using the binding energy of C as the internal standard. TEM images and STEM-HAADF images coupled to EDS elemental mapping were collected on a JEOL JEM-2100 electron microscope at 200 kV equipped with an Oxford Energy Dispersive X-ray spectroscopy.
The preparation of working electrode 10 mg sample and 10 mg acetylene black were grinded for 10 min and dispersed in 1 mL 0.5% Nafion solution followed with sonication for 30 min to form uniform catalyst ink. The ink was dropped directly on a hydrophobic carbon paper (1 cm × 2 cm) to form a 1 × 1 cm 2 catalyst area with a catalyst loading of ~1 mg cm -2 (cathode) and ~2 mg cm -2 (anode). The deposited carbon paper was further dried at room temperature.
Electrolysis and analysis of CO2 reduction product All electrochemical tests are performed in a standard three-electrode configuration in 0.5 M KHCO3 solution using a CHI660-E electrochemical workstation. Carbon rod and Ag/AgCl are used as counter electrode and reference electrode, respectively, and modified carbon paper (1 cm ×1 cm) is used as work electrode. The electro-chemical CO2RR performance is carried out in an airtight electro-chemical H-type cell, in which, two compartments are separated by a Nafion®117 proton exchange membrane to prevent mixing of products from the two electrode chambers. The polarization curves are performed by The gaseous reduction products were monitored by a gas chromatography (Shimadzu-2010 plus) equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The liquid products (e.g. formate) were collected from the anode chambers after electrolysis and quantified by NMR. In this work, all reported values are average ones calculated from three or more independent measurements, and all errors are given as standard deviations.

Evaluation of CO2RR performance
For products, the Faradaic efficiency (FECO) was calculated according to the following equation: N: the number of electrons transferred for product formation (N = 2 for CO).
F: Faraday constant, 96485 C mol -1 ; : the moles of products; Q: the total charge obtained from chronoamperometry (C).
The turnover frequency (TOF, h −1 ) of CO was calculated by the equation: itotal: the total current (A);

Computational details
The first principle was used to perform all density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerh of (PBE) formulation. The projected augmented wave (PAW) potentials were chosen to describe the ionic cores and take valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 400 eV. Partial of 0.05 eV. The electronic energy was considered self-consistent when the energy change was smaller than 10−4 eV. A geometry optimization was considered convergent when the energy change was smaller than 0.04 eV Å −1 . In our structure, the U correction is used for Co atoms. The Brillouin zone integration is performed using 2×2×1 Monkhorst-Pack k-point sampling for a structure. Finally, the adsorption energies (Eads) were calculated as Eads= Ead/sub -Ead -Esub, where Ead/sub, Ead, and Esub are the total energies of the optimized adsorbate/substrate system, the adsorbate in the structure, and the clean substrate, respectively. The free energy was calculated using the equation:

G=Eads+ZPE-TS
where G, Eads, ZPE and TS are the free energy, total energy from DFT calculations, zero point energy and entropic contributions, respectively.