A conductive catecholate-based framework coordinated with unsaturated bismuth boosts CO2 electroreduction to formate

Abstract

Bismuth-based metal-organic frameworks (Bi-MOFs) have received attention in electrochemical CO₂-to-formate conversion. However, the low conductivity and saturated coordination of Bi-MOFs usually lead to poor performance, which severely limits their widespread application. Herein, a conductive catecholate-based framework with Bi-enriched sites (HHTP, 2,3,6,7,10,11-hexahydroxytriphenylene) is constructed and the zigzagging corrugated topology of Bi-HHTP is first unraveled via single-crystal X-ray diffraction. Bi-HHTP possesses excellent electrical conductivity (1.65 S m^-1) and unsaturated coordination Bi sites are confirmed by electron paramagnetic resonance spectroscopy. Bi-HHTP exhibited an outstanding performance for selective formate production of 95% with a maximum turnover frequency of 576 h^-1 in a flow cell, which surpassed most of the previously reported Bi-MOFs. Significantly, the structure of Bi-HHTP could be well maintained after catalysis. In situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) confirms that the key intermediate is *COOH species. Density functional theory (DFT) calculations reveal that the rate-determining step is *COOH species generation, which is consistent with the in situ ATR-FTIR results. DFT calculations confirmed that the unsaturated coordination Bi sites acted as active sites for electrochemical CO₂-to-formate conversion. This work provides new insights into the rational design of conductive, stable, and active Bi-MOFs to improve their performance towards electrochemical CO₂ reduction.

Fig. 1. (a) The general process for the synthesis of Bi–HHTP. (b–d) The views in a different dimension. (e) Chelation of HHTP toward Bi³⁺ ions. (f) The coordination environment of Bi³⁺ ions (distorted tetragonal pyramid).

Fig. 2. (a) XRD patterns of Bi–HHTP and HHTP. (b) Current–voltage characteristic of Bi–HHTP using the two-contact probe method. (c) The compassion of conductivity of different MOFs. (d) EPR spectrum of Bi–HHTP. (e) TEM image, (f) HAADF-STEM image and the corresponding EDS elemental mappings of Bi–HHTP. (g) UV-vis spectra of Bi–HHTP and HHTP. (h) FTIR and (i) in situ Raman spectra of Bi–HHTP.

Fig. 3. (a) The illustration scheme of a liquid-phase flow cell device. (GDL, CE, RE, and AEM represent the working electrode, counter electrode, reference electrode, and anion exchange membrane, respectively) (b) LSV curves of Bi–HHTP in CO₂ and N₂ atmospheres without correction. (c) Potential-dependent formate FEs and current density of Bi–HHTP. (d) Stability test of Bi–HHTP at −0.7 V vs. RHE. (e) Formate rate and TOF of Bi–HHTP. (f) Comparison of our work with previously reported literature.

Fig. 4. (a) In situ ATR-FTIR spectra in CO₂-saturated 0.5 M KHCO₃ at applied potentials. (b) The active site Bi coordinative unsaturation with 4 oxygen atoms and one H₂O molecule bonded. (c) Four proposed reaction pathways on the active site to different products corresponding to the CO₂RR and HER. (d) The energy profiles in eV for the four pathways proposed in (c) with active compositions. (e) The charge difference of every active species (*COO, *COOH, CO + H₂O, HCOOH, *OCO, *OCHO, *HCOOH, and *H) on the catalyst surface. The yellow part indicates the charge accumulation and the cyan part illustrates the charge depletion.