Evolution of bismuth-based metal–organic frameworks for efficient electroreduction of CO2

Understanding the structural and chemical changes that reactive metal–organic frameworks (MOFs) undergo is crucial for the development of new efficient catalysts for electrochemical reduction of CO2. Here, we describe three Bi(iii) materials, MFM-220, MFM-221 and MFM-222, which are constructed from the same ligand (biphenyl-3,3′,5,5′-tetracarboxylic acid) but which show distinct porosity with solvent-accessible voids of 49.6%, 33.6% and 0%, respectively. We report the first study of the impact of porosity of MOFs on their evolution as electrocatalysts. A Faradaic efficiency of 90.4% at −1.1 V vs. RHE (reversible hydrogen electrode) is observed for formate production over an electrode decorated with MFM-220-p, formed from MFM-220 on application of an external potential in the presence of 0.1 M KHCO3 electrolyte. In situ electron paramagnetic resonance spectroscopy confirms the presence of ·COOH radicals as a reaction intermediate, with an observed stable and consistent Faradaic efficiency and current density for production of formate by electrolysis over 5 h. This study emphasises the significant role of porosity of MOFs as they react and evolve during electroreduction of CO2 to generate value-added chemicals.

Synthesis of single crystals of MFM-220, MFM-221 and MFM-222. The reaction conditions are shown in Figure S1 and the crystallographic data are summarised in Table S1.   An H-type cell with a three electrodes configuration was used for the electrochemical study ( Figure S9). This cell is consists of a working electrode, a platinum gauze as the counter electrode, and Ag/AgCl (submerged in saturated KCl) as the reference electrode. Both the catholyte and anolyte were 0.1 M KHCO 3 , which was separated by the Nafion-117 membrane. CO 2 was bubbled into the catholyte before the experiments for 30 minutes, and CO 2 was continually bubbled into the catholyte during the electrolysis. After the electrolysis, the liquid products were measured by 1 H NMR spectroscopy and the gas products were collected using a gasbag and analysed by GC and a Bruker Matrix MG5 FTIR spectrometer.
The Electrochemical impedance spectroscopy (EIS) was recorded at −0.5 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 by 1 where n product is the amount of product (mol) from GC, 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 twoelectron processes), F is the Faraday constant (96485 C mol −1 ), and Q is the total charge passed during the The potential versus Ag/AgCl was converted to the potential versus RHE (RHE = reversible hydrogen electrode) using the following Equation: E (V RHE ) = E (V Ag/AgCl ) + 0.197 +0.059 * pH EPR measurement. CW electron paramagnetic resonance (EPR) spectroscopy was carried out at X-band (9.85 GHz) on a Bruker Micro EPR spectrometer at room temperature with a microwave power of 6.325 mW.
EPR spectra were collected with a modulation amplitude of 1 G. DMPO (200 mmol/L) was dissolved in Ar-degassed deionised water as a spin trap. Electrolysis was carried out at -1. the position of the EPR tube, the volume of measured solvent were controlled to achieve accurate quantitative measurement of any generated radicals. Strong pitch (g = 2.0028) was used as a reference sample when measuring X-band EPR spectra. Theoretical modelling of EPR spectra was performed using EasySpin toolbox (Version 6.0.0-dev.34) 2 for Matlab.

Crystallographic section
Data Collection. X-Ray data for compounds MFM-221 and MFM-222 were collected at 100 K using Cu-kα radiation on a Rigaku FR-X rotating anode with a Hypix 6000HE detector.
Crystal structure determinations and refinements. X-Ray data were processed and reduced using                            Table S2).