2D Undulated Metal Hydrogen-Bonded Organic Frameworks with Self-Adaption Interlayered Sites for Highly Efficient C–C Coupling in the Electrocatalytic CO2 Reduction

Abstract

The hydrogen-bonded organic frameworks (HOFs) as a new type of porous framework materials have been widely studied in various areas. However, the lack of appropriate active sites, low intrinsic conductivity, and poor stability limited their performance in the field of electrocatalysis. Herein, we designed two 2D metal hydrogen-bonded organic frameworks (2D–M–HOF, M = Cu2+ or Ni2+) with coordination compounds based on 2,3,6,7,14,15-hexahydroxyl cyclotricatechylene and transition metal ions (Cu2+ and Ni2+), respectively. The crystal structure of 2D–Cu–HOF is determined by continuous rotation electron diffraction, indicating an undulated 2D hydrogen-bond network with interlayered π-π stacking. The flexible structure of 2D–M–HOF leads to the formation of self-adaption interlayered sites, resulting in superior activity and selectivity in the electrocatalytic conversion of CO2 to C2 products, achieving a total Faradaic efficiency exceeding 80% due to the high-efficiency C–C coupling. The experimental results and density functional calculations verify that the undulated 2D–M–HOF enables the energetically favorable formation of *OCCHO intermediate. This work provides a promising strategy for designing HOF catalysts in electrocatalysis and related processes.

1. Introduction

Hydrogen-bonded organic frameworks (HOFs) are a type of molecular-based porous crystalline materials that are self-assembled via intermolecular hydrogen-bonding and/or van der Waals interaction [1–7]. Unlike the well-established metal–organic framework and covalent-organic framework that are connected by strong coordinated or covalent bonds, HOFs constructed with hydrogen-bonding networks possess high flexibility, which endows them with a great potential to build unique and efficient active sites [8–12]. However, the number of reported HOFs with permanent porosity is limited and most of the structural units are rigid fused aromatic rings, representative examples are porphyrin [13, 14], pyrene [15, 16], and triptycene [2, 17]. Meanwhile, the low electrical conductivity and stability severely encumber their applications in electrochemistry [18–20].

The introduction of building blocks containing metal coordinated centers to form 2D metal HOFs not only provides monodisperse metal active sites but also promotes electrical conductivity within the framework via the d-π conjugation and π-π stacking [21–23]. Moreover, the geometric interlocking π–π stacking of organic ligands effectively strengthens the chemical and thermal stability of HOFs with permanent porosity [24, 25]. These unique properties make metal HOFs promising candidates for applications in electrocatalysis, such as oxygen evolution reaction [26], oxygen reduction reaction [22], and CO2 reduction reaction (CO2RR) [27]. During these electrocatalytic processes, the adsorption and activation of multiple intermediates are quite important [28–31]. Adsorption and activation of reactants or intermediates on the catalyst require dedicated designed active sites with suitable distance and favored low energy barrier pathways during the reaction [32–34]. Therefore, the flexibility of HOFs is essential for ultrahigh catalytic performance but has been scarcely explored, which mainly lacks suitable ligands.

Hexahydroxyl cyclotricatechylene (HHCC) has a unique flexible structure with a bowl-shaped shallow cavity, which has found important applications in host–guest chemistry, liquid crystal, and others [35–37]. HHCC units form a 2D plane with different linkages that exhibit an undulated flexible feature, which has not been observed in other rigid polyaromatic organic ligands [38, 39]. More importantly, the HHCC would be integrated into metal HOFs to construct the desired flexible framework.