Exploring the Electrode-Electrolyte Interface of Electrically Conductive Layered Metal-Organic Frameworks with Nuclear Magnetic Resonance

Supercapacitors are high-power energy storage devices which are imperative to a green-energy transition. However, their energy density is currently limiting. An understanding of supercapacitor performance at the molecular level is essential to enable design of higher energy-density electrodes to meet increasing demands for renewable energy from the grid and electric vehicles. Conductive layered metal-organic frameworks (MOFs) have recently demonstrated promising electrochemical performances as supercapacitor electrode materials. The well-defined chemical structures of these crystalline porous electrodes facilitate structure-performance studies of a model electrode material. Despite this, there is a fundamental lack in the molecular-level understanding of charge storage mechanisms in conductive layered MOFs. In this work, we address this by employing solid-state nuclear magnetic resonance (NMR) spectroscopy to study ion adsorption and the charging mechanisms of these materials. Further, to address a lack of feasibility studies for deployment of MOFs in supercapacitor electrodes, we conduct a Life-Cycle Assessment (LCA) against benchmark activated carbon electrodes. 19F NMR spectroscopy was initially used to study the layered MOF nickel 2,3,6,7,10,11-hexaiminotriphenylene, Ni3(HITP)2 soaked with organic electrolyte. In this system, separate resonances are observed for the MOF’s in-pore and ex-pore ions. The chemical shift of the in-pore electrolyte ions is found to be influenced by specific chemical interactions with the MOF functional groups, with this result supported by dynamic quantum-mechanics/molecular-mechanics (QM/MM) simulations and density functional theory (DFT) calculations. Quantification of the in-pore electrolyte environments by NMR was also found to provide a proxy for electrochemical performance and MOF porosity, which could facilitate the rapid screening of synthesised MOF samples. The assignment of the positively shifted in-pore electrolyte environment revealed by NMR spectroscopy was found to be applicable to other related layered MOF-organic electrolyte systems, and other potential factors influencing the in-pore chemical shift were investigated. Aromatic ring-currents were found likely to play a role in the consistent positive shifts observed. MOFs with paramagnetic metal nodes were found to have increased in-pore T2 relaxation rates and peak-widths but only had significant paramagnetic chemical shifts when accompanied by specific interactions with the electrolyte species. The MOF particle morphology was also found to strongly affect the appearance of the NMR spectra, with rod-like morphologies leading to slower exchange between environments and thus better resolved peaks. Having identified the electrolyte environments and factors influencing the NMR spectra for the family of layered MOFs, the charge storage mechanism for Ni3(HITP)2 with organic electrolyte was explored using a combination of ex-situ NMR and operando electrochemical quartz-crystal microbalance (EQCM) experiments. These measurements revealed that cations are the dominant contributor to charge storage in Ni3(HITP)2, with anions only marginally contributing to the charge storage. Despite this, NMR revealed significant in-pore anion rearrangements during charging. Given the dominant contribution of cations to the MOF’s charge storage mechanism, 7Li ex-situ NMR spectroscopy was used to test the existence of specific interactions previously hypothesised to result in a capacitance enhancement of copper 2,3,6,7,10,11-hexahydroxytriphenylene, Cu3(HHTP)2, with a Li+ cation electrolyte. The presence of paramagnetic chemical shifts in the charged electrodes, identified through variable temperature NMR and paramagnetic NMR calculations, was supportive of these specific interactions and their link to improved performance. Finally, cradle-to-gate LCAs of Ni3(HITP)2 and Cu3(HHTP)2 were conducted to compare MOFs Ni3(HITP)2 and Cu3(HHTP)2 to benchmark activated carbon electrode materials. The calculated impacts associated with MOFs were found to be three to six orders of magnitude greater than benchmark activated carbon electrode materials. Hotspot and sensitivity analysis were used to investigate ways to reduce the impacts of synthesising these conductive layered MOFs but could not achieve reductions of an equivalent magnitude. These results suggest that new types of conductive layered MOF syntheses, or significant improvements in performance, are required before these materials should be employed in commercial supercapacitor devices. Overall, this work establishes the methods for studying MOF-electrolyte interactions in supercapacitors and their charging mechanisms via NMR spectroscopy. The insights made accessible with NMR, into how the presence and type of interactions influence the charging storage mechanism and performance, will aid the design of new electrode materials to optimise the performance of supercapacitors, as well as other electrochemical devices including electrocatalysts and sensors.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Balhatchet, Chloe

Advisor dc:contributor.advisor

• Forse, Alexander

• supercapacitor
• NMR
• metal-organic framework
• MOF
• energy
• nuclear magnetic resonance

Licence

• https://www.repository.cam.ac.uk/bitstreams/7710013c-6fd6-43d9-a901-0e3b7d96d02e/download
• https://creativecommons.org/licenses/by/4.0/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.121306

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/389410