2D MOFs and their derivatives for electrocatalytic applications: Recent advances and new challenges

As a class of porous materials with crystal lattices, metal–organic frameworks (MOFs), featuring outstanding specific surface area, tunable functionality, and versatile structures, have attracted huge attention in the past two decades. Since the first conductive MOF is successfully synthesized in 2009, considerable progress has been achieved for the development of conductive MOFs, allowing their use in diverse applications for electrochemical energy storage. Among those applications, supercapacitors have received great interest because of their high power density, fast charging ability, and excellent cycling stability. Here, the efforts hitherto devoted to the synthesis and design of conductive MOFs and their auspicious capacitive performance are summarized. Using conductive MOFs as a unique platform medium, the electronic and molecular aspects of the energy storage mechanism in supercapacitors with MOF electrodes are discussed, highlighting the advantages and limitations to inspire new ideas for the development of conductive MOFs for supercapacitors.

Conductive Metal–Organic Frameworks for Supercapacitors

The electrochemical CO2 reduction to high-value-added chemicals is one of the most promising and challenging research in the energy conversion field. An efficient ECR catalyst based on a Cu-based conductive metal-organic framework (Cu-DBC) is dedicated to producing CH4 with superior activity and selectivity, showing a Faradaic efficiency of CH4 as high as ~80% and a large current density of −203 mA cm−2 at −0.9 V vs. RHE. The further investigation based on theoretical calculations and experimental results indicates the Cu-DBC with oxygen-coordinated Cu sites exhibits higher selectivity and activity over the other two crystalline ECR catalysts with nitrogen-coordinated Cu sites due to the lower energy barriers of Cu-O4 sites during ECR process. This work unravels the strong dependence of ECR selectivity on the Cu site coordination environment in crystalline porous catalysts, and provides a platform for constructing highly selective ECR catalysts. Crystalline porous catalysts with single Cu sites are dedicated to exploring the dependence of CO2 electroreduction selectivity on the coordination environment of catalytic sites. The conductive MOF Cu-DBC with oxygen-coordinated Cu sites shows a high Faradaic efficiency ~80% of CO2-to-CH4.

Coordination environment dependent selectivity of single-site-Cu enriched crystalline porous catalysts in CO2 reduction to CH4.

A highly effective electrocatalyst is the central component of advanced electrochemical energy conversion. Recently, two-dimensional conjugated metal-organic frameworks (2D c-MOFs) have emerged as a class of promising electrocatalysts because of their advantages including 2D layered structure with high in-plane conjugation, intrinsic electrical conductivity, permanent pores, large surface area, chemical stability, and structural diversity. In this Review, we summarize the recent advances of 2D c-MOF electrocatalysts for electrochemical energy conversion. First, we introduce the chemical design principles and synthetic strategies of the reported 2D c-MOFs, as well as the functional design for the electrocatalysis. Subsequently, we present the representative 2D c-MOF electrocatalysts in various electrochemical reactions, such as hydrogen/oxygen evolution, and reduction reactions of oxygen, carbon dioxide, and nitrogen. We highlight the strategies for the structural design and property tuning of 2D c-MOF electrocatalysts to boost the catalytic performance, and we offer our perspectives in regard to the challenges to be overcome.

Two-Dimensional Conjugated Metal-Organic Frameworks for Electrocatalysis: Opportunities and Challenges.

The electrocatalytic CO2 reduction reaction (ECO2RR) is considered one of the approaches with the most potential to achieve lower carbon emissions in the future, but a huge gap still exists between the current ECO2RR technology and industrial applications. Therefore, the design and preparation of catalysts with satisfactory activity, selectivity and stability for the ECO2RR have attracted extensive attention. As a classic type of functional porous framework, crystalline porous materials (e.g., metal organic frameworks (MOFs) and covalent organic frameworks (COFs)) and derived porous materials (e.g., MOF/COF composites and pyrolysates) have been regarded as superior catalysts for the ECO2RR due to their advantages such as designable porosity, modifiable skeleton, flexible active site structure, regulable charge transfer pathway and controllable morphology. Meanwhile, with the rapid development of nano-characterization and theoretical calculation technologies, the structure-activity relationships of functional porous frameworks have been comprehensively considered, i.e., metallic element type, local coordination environment, and microstructure, corresponding to selectivity, activity and mass transfer efficiency for the ECO2RR, respectively. In this review, the rational design strategy for functional porous frameworks is briefly but precisely generalized based on three key factors including metallic element type, local coordination environment, and microstructure. Then, details about the structure-activity relationships for functional porous frameworks are illustrated in the order of MOFs, COFs, composites and pyrolysates to analyze the effect of the above-mentioned three factors on their ECO2RR performance. Finally, the challenges and perspectives of functional porous frameworks for the further development of the ECO2RR are reasonably proposed, aiming to offer insights for future studies in this intriguing and significant research field.

A rational design of functional porous frameworks for electrocatalytic CO2 reduction reaction.

2D conductive metal-organic frameworks (2D c-MOFs) are promising candidates for efficient electrocatalysts for the CO2 reduction reaction (CO2RR). A nitrogen-rich tricycloquinazoline (TQ) based multitopic catechol ligand was used to coordinate with transition-metal ions (Cu2+ and Ni2+), which formed 2D graphene-like porous sheets: M-3(HHTQ)(2) (M=Cu, Ni; HHTQ=2,3,7,8,12,13-Hexahydroxytricycloquinazoline). M-3(HHTQ)(2) can be regarded as a single-atom catalyst where Cu or Ni centers are uniformly distributed in the hexagonal lattices. Cu-3(HHTQ)(2) exhibited superior catalytic activity towards CO2RR in which CH3OH is the sole product. The Faradic efficiency of CH3OH reached up to 53.6 % at a small over-potential of -0.4 V. Cu-3(HHTQ)(2) exhibited larger CO2 adsorption energies and higher activities over the isostructural Ni-3(HHTQ)(2) and the reported archetypical Cu-3(HHTP)(2). There is a strong dependence of both metal centers and the N-rich ligands on the electrocatalytic performance.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/ange.202105331

Tricycloquinazoline-Based 2D Conductive Metal-Organic Frameworks as Promising Electrocatalysts for CO2 Reduction

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Tricycloquinazoline-Based 2D Conductive Metal-Organic Frameworks as Promising Electrocatalysts for CO2 Reduction