Towards a Deeper Understanding of the Ionic Charging in Naphthalenediimide-Based N-Type Conjugated Polymer Electrodes

Hang Yu^a, Anna A. Szumska^a, Iuliana P. Maria^b^,^c, Nicholas Siemons^a, Sachetan M. Tuladhar^a, Alexander Giovannitti^a^,^d, Jenny Nelson^a

^aDepartment of Physics, Imperial College London, London SW7 2AZ, UK

^bDepartment of Chemistry, Imperial College London, London W12 0BZ, UK

^cDepartment of Chemistry, University of Oxford, UK, Mansfield Rd, Oxford, United Kingdom

^dDepartment of Materials Science and Engineering, Stanford University, Stanford, CA, USA;, United States

Materials for Sustainable Development Conference (MATSUS)
Proceedings of MATSUS Fall 2023 Conference (MATSUSFall23)

#OMIECs - Fundamentals of mixed ionic-electronic transport in polymers

Torremolinos, Spain, 2023 October 16th - 20th

Organizer: Simone Fabiano

Oral, Hang Yu, presentation 180
DOI: https://doi.org/10.29363/nanoge.matsus.2023.180
Publication date: 18th July 2023

Conjugated polymers (CPs) with hydrophilic side chains are capable of simultaneously conducting electronic and ionic charges, making them interesting materials for applications in bioelectronics, electrochromics, electrocatalysis and energy storage. Whilst numerous p-type mixed-conducting polymers have shown excellent stability and performance, n-type mixed-conducting polymers usually suffer from poor stability in aqueous environments and in air. To date, most reported solution processable n-type mixed conducting polymers that show good stability, high electron mobility and high specific capacity, involve the naphthalenediimide (NDI) unit. [1-3] However, the electrochemical stability of NDI-based mixed-conducting polymers varies with the specific chemical structure and choice of electrolyte. The underlying mechanisms by which ionic charging/discharging deteriorates the electrochemical stability are not properly understood. In this work, we study several NDI-based n-type mixed conducting polymers with different side chain and backbone structures operating in aqueous electrolytes of systematically varying concentrations to control the electrochemical stabilities. We observe that increasing electrolyte concentration and replacing a small fraction of hydrophilic side chains with hydrophobic ones largely improve the electrochemical stability of the NDI-based n-type mixed conducting polymers. To understand the phenomena, we use spectroelectrochemical and electrogravimetric characterisation tools and molecular dynamic (MD) simulations to explore the impact of side chains and electrolytes on the electrochemical stabilities of the polymers. We show that the improvement in stability can be tentatively attributed to a reduction in swelling, which avoids excessive electrolyte uptake, thereby improving the environment for ion transport and maintaining the intermolecular connections. We propose design rules for both material and electrolyte to maximise reversible multi-electron charging of n-type mixed conducting polymers.

References:

[1] Moia, D.; Giovannitti, A.; Szumska, A. A.; et al. Design and Evaluation of Conjugated Polymers with Polar Side Chains as Electrode Materials for Electrochemical Energy Storage in Aqueous Electrolytes. Energy Environ. Sci. 2019, 12, 1349-1357.

[2] Maria, I. P.; Paulsen, B. D.; Savva, A.; et al. The Effect of Alkyl Spacers on the Mixed Ionic-Electronic Conduction Properties of N-Type Polymers. Adv. Func. Mater. 2021, 31(14), 2008718.

[3] Szumska, A. A.; Maria, I. P.; Flagg, L. Q.; et al. Reversible Electrochemical Charging of n-Type Conjugated Polymer Electrodes in Aqueous Electrolytes. J. Am. Chem. Soc. 2021, 143(36), 14795–14805.

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