(349i) Morphological Tuning of Conductive Bismuth-Based Coordination Polymer for Selective CO2 Electroreduction to Formate

The overwhelming combustion of fossil fuels and industrial production lead to CO₂ emissions in excess of capacity and climate change of global concern.^1-3 Electrochemical CO₂ reduction reaction has been regarded as a promising approach to transform CO₂ to valuable chemicals and reduce the CO₂ concentration in atmosphere due to its high efficiency, mild conditions, and the ability of coupling with intermittent renewable energy sources.⁴ Formic acid/formate (HCOOH/HCOO^-) is an important chemical intermediate for industrial application (e.g., leather and textile manufacturing, livestock feed, fuel cells, and H₂ storage).⁵ However, the direct conversion of CO₂ to formate at an electrode is an energy intensive process and suffers from low selectivity.^5-7

Bismuth (Bi) has been demonstrated as an efficient catalyst to drive the conversion of CO₂ to formate.^5,6 The majority of Bi-based electrocatalysts consisting of dense clusters of Bi flakes or large Bi particles have low utilization of Bi, since only the surface/edge Bi atoms are regarded as accessible and electrochemically active.⁸ Taking the relatively low natural abundance of Bi into consideration, the electrocatalysts with large Bi clusters are unfavorable. Bi-based coordination polymers and metal-organic frameworks (MOFs) have drawn much attention for electrocatalysis owing to the Bi atom economy and single-atom dispersion. As one of the heterogeneous molecular catalysts, coordination polymers and MOFs incorporate the advantages of homogeneous catalysts and heterogeneous catalysts, not only showing high utility and activity, but also making it able to tune the intrinsic activity and selectivity at molecular level by changing the ligands.^9-11 Despite the intrinsic merits of heterogeneous molecular catalysts, the poor electrical conductivity and structural stability impede the application of coordination polymers as electrocatalysts.¹² In recent years, studies on Bi-based coordination polymers and MOFs as electrocatalysts have mainly focused on applying them as pre-catalysts to construct Bi metals/metal oxides particles through thermal emission and electroreduction based on the instability of coordination polymer and redox nature of Bi(III).^13-16 Constructing conductive coordination polymer with robust nature and employing it as heterogeneous molecular catalyst for CO₂ electroreduction shows much promise to advance the knowledge of intrinsic catalytic activity and increase the utilization of metal atoms.

In this work, we employ a two-dimensional (2D) coordination polymer nanosheet where Bi³⁺ coordinate with 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) ligands for CO₂ electroreduction (Figure 1).¹⁷ The large Ï€-conjugated system and planar geometry enable efficient through-bond charge delocalization of HHTP are beneficial to electron transfer and migration; the robust nature of HHTP enhance the structural stability of Bi-based coordination polymers during the electroreduction process. Moreover, a surfactant-assisted method enables the morphology tuning of coordination polymer for 2D nanosheets and exposure of specific facet. The 2D Bi(HHTP) nanosheets exhibited high selectivity toward formate from CO₂ reduction, large current density, desirable mass activity and high turnover frequency in aqueous media. It showed over 80% faradaic efficiency toward formate in near-neutral media at a wide potential range from -0.9 to -1.2 V vs. RHE with no obvious degradation after 10 h of continuous electrolysis. Its mass activity with respect to Bi exceeded 200 A g^–1. Experimental methods including in-situ ATR-SEIRAS and in-situ Raman spectroscopies and DFT calculations were used to figure out the intermediates, clarify the mechanism of CO₂ electroreduction to formate, and specify the effects of exposure of facets on electrocatalytic performance of the Bi(HHTP).

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