Understanding the Influence of Particle Morphology on the Capacitive Performance of Conductive Layered Metal-Organic Frameworks

Two-dimensional layered metal-organic frameworks (MOFs), characterised by their combined high intrinsic porosities and electrical conductivities, have emerged as one of the most promising electrode materials for next-generation energy storage devices, particularly supercapacitors. Several such MOFs have displayed encouraging performances in supercapacitors with a wide range of electrolytes, and have exhibited specific and areal capacitances on par with or exceeding state-of-the-art carbon materials. 1-3 This has raised the prospect of using these materials in commercial devices, and their well-defined structures make them promising model electrode materials for supercapacitor structure-property investigations. However, layered MOFs can be synthesised with a range of particle morphologies, and hence 3D pore structures, as well as different degrees of particle agglomeration. Minimal work has been performed to understand how these factors impact the capacitive performances of these frameworks, and existing literature has struggled with small differences in observed morphology and low control over the samples. 4 This has cast doubts over reported results, and has hindered both the development of MOF-based supercapacitors and their implementation as model electrodes. In this work, a greater variation in particle morphology was obtained than observed in previous work on this topic. This was achieved by synthesising samples of the layered conductive MOF Cu 3 (HHTP) 2 , (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with different modulating agents, which influence the equilibrium determining the self-assembly process of a MOF and regulate the rate of framework growth. Using this approach, we present a detailed study into the influence of particle morphology and agglomeration on the capacitive performance of the layered MOF Cu 3 (HHTP) 2 in symmetric supercapacitors with both organic and ionic liquid electrolytes. These studies reveal that ‘flake-like’ particle morphologies with very small length-to-width aspect ratios (<< 1) and little agglomeration are optimal for layered MOF supercapacitors. This sample displayed greater rate capabilities with organic electrolytes and significantly higher specific capacitances with ionic liquid electrolytes compared to ‘rod-like’ particles, with much larger length-to-width aspect ratios, and heavily agglomerated samples. These findings will pave the way for the next generation of supercapacitor devices using conductive layered MOF electrodes. Figure 1