Investigating Layered Electrically Conductive Metal-Organic Frameworks For Supercapacitor Applications

Supercapacitors are high-power energy storage devices that will play an important role in the transition to a low-carbon society. In recent years, layered electrically conductive metal-organic frameworks (MOFs) have emerged as one of the most promising electrode materials for next-generation supercapacitors. Their crystalline and tuneable structures facilitate structure-performance studies, which are challenging to conduct with traditional porous carbon electrodes. In this work, the electrochemical performances of layered conductive MOFs in supercapacitors are investigated to both improve our understanding of these materials and to develop structure-performance relationships. Having demonstrated that the layered conductive MOF Cu₃(HHTP)₂ (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) exhibits good performance in supercapacitors, measurements on samples with different particle morphologies reveal that ‘flake’ particles, with small length-to-width aspect ratios, are optimal for these devices. This is due to improved ion accessibility and dynamics through the short pores of the ‘flake’ particles, resulting in a higher power performance compared to particle morphologies with longer pores. Electrochemical quartz crystal microbalance (EQCM) and three-electrode experiments are then performed with Cu₃(HHTP)₂ and a series of electrolytes with different cation sizes to investigate both the charging mechanism of this MOF and how electrolyte ion size impacts electrochemical performance. It is shown that cations are the dominant charge carriers in Cu₃(HHTP)₂, with co-ion desorption occurring upon positive charging and counterion adsorption during negative charging. Large ions lead to porosity saturation in MOF electrodes, reducing charge storage and forcing solvent molecules to participate in the charge storage mechanism. The impact of modifying MOF-electrolyte interactions on the electrochemical capacity of layered MOF supercapacitors is then investigated by altering both the electrolyte cation and the MOF electrode functionality. These experiments allow for the systematic probing of the influence of different functional groups on supercapacitor performance, and reveal that MOFs with hydroxy ligating groups, together with Li⁺ electrolytes, constitute the best electrode-electrolyte combination for maximising capacitive performance. Finally, an interlaboratory study is conducted to assess the variability in the reporting of performance metrics across different laboratories. Overall, this work provides unique insights into the performances of layered conductive MOFs for supercapacitor applications, and will guide the design of improved electrode materials for next-generation supercapacitors.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Gittins, James

Advisor dc:contributor.advisor

• Forse, Alexander

• Energy Storage
• Metal-Organic Frameworks
• Supercapacitors

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/22b6ec4d-532a-44fc-b641-093b2effca4f/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

Author Identifier

0000-0002-9106-8910

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/375455