Single-Point Linkage Engineering in Conjugated Phthalocyanine-Based Covalent Organic Frameworks for Electrochemical CO2 Reduction

Abstract

The utilization of covalent organic frameworks (COFs) holds great potential for achieving tailorable tuning of catalytic performance through bottom-up modulation of the reticular structure. In this work, we show that a single-point structural alteration in the linkage within a nickel phthalocyanine (NiPc)-based series effectively modulates the catalytic performance of the COFs in electrochemical CO2 reduction reaction (CO2RR). A NiPc-based COF series with three members which possess the same NiPc unit but different linkages, including piperazine, dioxin, and dithiine, have been constructed by nucleophilic aromatic substitution reaction between octafluorophthalocyanine nickel and tetrasubstituted benzene linkers with different bridging groups. Among these COFs, the dioxin-linked COF showed the best activity of CO2RR with a current density of CO (jCO) = −27.99 mA cm−2 at −1.0 V (versus reversible hydrogen electrode, RHE), while the COF with piperazine linkage demonstrated an excellent selectivity of Faradaic efficiency for CO (FECO) up to 90.7% at a pretty low overpotential of 0.39 V. In addition, both a high FECO value close to 100% and a reasonable jCO of −8.20 mA cm–2 at the potential of −0.8 V (versus RHE) were obtained by the piperazine-linked COF, making it one of the most competitive candidates among COF-based materials. Mechanistic studies exhibited that single-point structural alteration could tailor the electron density in Ni sites and alter the interaction between the active sites and the key intermediates adsorbed and desorbed, thereby tuning the electrochemical performance during CO2RR process.

1. Introduction

Electrochemical carbon dioxide reduction reaction (CO2RR) represents an effective pathway to mitigate the increasing concentration of atmospheric CO2; meanwhile, it produces value-added products [1, 2]. Over the years, a variety of homogeneous and heterogeneous catalysts have been developed to improve the activity, selectivity, and energy efficiency of electrochemical CO2RR. Homogeneous catalysts, exemplified by a few metal coordination complexes, including metalloporphyrins (MPys) [3, 4], metallophthalocynines (MPcs) [5, 6], and their derivates [7, 8], often have highly versatile and tailorable structures to give high selectivity [9, 10], however, with low catalytic activities due to the hindered charge transfer. By contrast, heterogeneous materials [11], such as widely applied metal oxides [12], graphene [13], and polymers [14], usually demonstrate high efficiency and activity due to their inherent high conductivity [15]. However, these materials often have difficulty in offering precise atomic-level control over the activity and selectivity. The quest for novel catalysts that integrate high activity, high selectivity, and tunability within a system remains an ideal but challenging target [9].

Covalent organic frameworks (COFs), a class of crystalline porous materials constructed through covalent bonds of organic ligands, have emerged as a promising platform for electrocatalysts during CO2RR [16–18]. Catalytic sites embedded in building blocks could be readily introduced into the COF backbone by reasonably selecting metal complexes as the linkers. This bottom-up approach enables precise and rational control over the selectivity and activity of the catalysts. In addition, the metal centers and surroundings can be finely tuned to modulate their electronic structures and interactions with key reaction intermediates for optimal electrocatalytic performances compared with inorganic metal catalysts [19–21]. Furthermore, the intrinsic ultrahigh surface area and porosity of COFs allow optimal exposure of active sites for high selectivity and activity in electrocatalytic CO2RR. A few previous studies have demonstrated that different metal nodes introduced into the COFs as active sites influence the electronic properties of COFs, subsequently, the overall performance of COFs in electrochemical CO2RR [22]. In fact, the chemistry of linkages also indeed plays a crucial role in determining the properties of COFs by modulating the extent of π-conjugation, planarity, conductivity, redox property, and stability of the reticular framework [23], offering a key factor to alter the electronic structure.