MOF-derived nanocarbon materials for electrochemical catalysis and their advanced characterization

Abstract

Because of the demand for clean and sustainable energy sources, nanocarbons, modified carbons and their composite materials derived from metal-organic frameworks (MOFs) are emerging as distinct catalysts for electrocatalytic energy conversion. These materials not only inherit the advantages of MOFs, like customizable dopants and structural diversity, but also effectively prevent the aggregation of nanoparticles of metals and metal oxides during pyrolysis. Consequently, they increase the electrocatalytic efficiency, improve electrical conductivity, and may play a pivotal role in green energy technologies such as fuel cells and metal-air batteries. This review first explores the carbonization mechanism of the MOF-derived carbon-based materials, and then considers 3 key aspects: intrinsic carbon defects, metal and non-metal atom doping, and the synthesis strategies for these materials. We also provide a comprehensive introduction to advanced characterization techniques to better understand the basic electrochemical catalysis processes, including mapping techniques for detecting localized active sites on electrocatalyst surfaces at the micro- to nano-scale and in-situ spectroscopy. Finally, we offer insights into future research concerning their use as electrocatalysts. Our primary objective is to provide a clearer perspective on the current status of MOF-derived carbon-based electrocatalysts and encourage the development of more efficient materials.

1. Introduction

Metal-organic frameworks (MOFs) are crystalline materials composed of self-assembled metal ions or clusters with organic ligands[1–2]. In recent years, MOFs have found widespread applications in various fields such as gas adsorption and separation, catalysis, chemical sensing, energy storage and conversion due to their periodic crystal structure, structural flexibility, tunable pore topology, high surface area, and tailor-able properties[3–9]. Notably, Zheng et al. have discovered that the in-depth exploration, precise design, and efficient synthesis of MOFs can now be achieved through the collaboration of GPT-4 chemists and human researchers, enhancing the feasibility and efficiency of research activities, thus accelerating the progress in MOF materials[10]. Furthermore, BASF, as a groundbreaking development, has announced its position as the first global producer of MOFs on a scale of several hundred tons per year. These MOFs, particularly zinc-triazole-oxalate-based MOF (CALF-20, developed from University of Calgary), have been designed for carbon dioxide storage and can also adsorb greenhouse gas methane, making industrial-level carbon capture possible. This significant achievement, as published in the journal Science, signifies the true industrialization of MOFs and the successful enhancement of economic benefits[6].

However, the presence of weak coordination between metal nodes and organic ligands in the majority of MOFs leads to issues such as low catalytic activity and challenges in catalyst recovery under demanding reaction conditions. These conditions include organic/water solvents, acidic or alkaline environments, and high temperatures, which have limited the practical application of MOFs in the field of electrocatalysis. On another note, high conductivity is another critical requirement for MOFs when used as electrocatalysts. Nonetheless, due to the presence of typical organic linkers surrounding redox-active sites, MOFs often exhibit poor conductivity, rendering them insulating materials. Additionally, the electronic interactions between metal nodes and organic linkers further affect the conductivity of MOFs[11]. Carbon-based catalysts exhibit distinctive traits, displaying superior catalytic activity in electrochemical reduction reaction, chemical stability, cost-effectiveness and environmental compatibility in research applications. From economic and environmental perspectives, carbon-based nanomaterials offer certain advantages due to their abundant resources and eco-friendly nature compared to metal-based catalytic materials. These attributes have garnered significant attention for potential applications across various domains for a multitude of carbon-based materials. The carbon allotrope family primarily constitutes of a vast, periodically arranged sp2 lattice, forming an extensive π-conjugated system that offers enhanced thermal and electrical conductivity. Through the efforts of numerous scholars, modifications to the lattice structure of carbon-based materials have been explored.