Two-Dimensional Redox-Active Cerium-Based Metal–Organic Frameworks Coordinated on Carbon Nanotubes as Materials for Supercapacitors
Authors: Wu Kuan-Chu , Tsai Meng-Dian , Wu Chung-Huan , Yang Tzu-Hsien , Chen You-Liang , Kung Chung-Wei 
Published: December 2024

Cerium oxide has emerged as a promising active material for supercapacitors, owing to its abundance, eco-friendliness, and impressive redox properties. Nonetheless, the low external surface area of bulk cerium oxide strongly limits its specific capacitance, which becomes a hurdle for its broad adoption in energy storage systems. As a result, cerium-based metal–organic frameworks (MOFs) have attracted great attention for the use in supercapacitors due to their ultrahigh specific surface areas and tunable porosity. With the hexa-cerium clusters as the building blocks, Ce(IV)-based MOFs are highly stable in neutral aqueous electrolytes. In addition, the redox activity of a part of cerium atoms present in their hexa-cerium nodes makes Ce-MOFs redox-active with an obvious pseudocapacitive behavior. However, even though charges can be transported through the redox-hopping process between cerium-based nodes in the framework during the electrochemical operation, the low electrical conductivity of the pristine cerium-based MOFs still limits the performance of such MOFs in electrochemical energy storage.
In this work, nanosheets of a redox-active, two-dimensional (2D), and highly dispersible MOF constructed from hexa-cerium(IV) clusters, CeBTB (BTB = 1,3,5-benzenetribenzoate), which is the only reported 2D Ce(IV)-based MOF in the literature, were directly coordinated onto the surface of carboxylic acid-functionalized multi-walled carbon nanotubes (CNTs) by utilizing a post-synthetically grafting approach; see Fig. 1. Nanocomposites with various CeBTB-CNT ratios were synthesized. Crystallinity, porosity, and morphology of CeBTB and all nanocomposites were first investigated. Powder X-ray diffraction confirms that the CeBTB present in all nanocomposites are still crystalline. Although the majority of nanocomposites with various MOF-to-CNT ratios possess similar specific surface areas compared to the pristine CeBTB and CNTs, the expansion of space between carbon nanotubes or the slight increase in the microporosity of CeBTB can be observed when a minority of CeBTB or a minority of CNTs was incorporated into the nanocomposite, respectively. Electron microscopic images reveal that carbon nanotubes embedded in aggregates of CeBTB sheets can be observed in nanocomposites. In addition, infrared spectra confirm the formation of coordination bonds between the hexa-cerium clusters of CeBTB and carboxylate groups of CNTs during the grafting process. The bulk electrical conductivity of the CeBTB and its nanocomposite is highly tunable; according to the results of two-probe measurements, it ranges from 10-13 S/cm to 10-3 S/cm depending on the ratio between CeBTB and CNTs in the material.
Thin films of materials with the same mass loading were then fabricated to measure their capacitive performances in neutral aqueous solutions of Na2SO4. With the electrical conductivity provided by carbon nanotubes and the redox activity of CeBTB between Ce(IV) and Ce(III), the optimal nanocomposite, CeBTB-CNT-2:1, can show a significant enhancement of capacitive performance compared to the pristine CeBTB, pristine CNTs, and the physical mixture of both of them with the same MOF-to-CNT ratio.
With such a simple post-synthetically grafting approach that can precisely control the ratio between the 2D MOF and CNTs, the findings here open opportunities for designing numerous composites composed of redox-active 2D-MOFs and nanocarbons with tunable electrical conductivity, redox activity, and functionality; such chemically robust MOF-based composites are considered as potential candidates for a range of applications.

Figure 1