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Understanding Sorption of Aqueous Electrolytes in Porous Carbon by NMR Spectroscopy.

Accepted version
Peer-reviewed

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Abstract

Ion adsorption at solid-water interfaces is crucial for many electrochemical processes involving aqueous electrolytes including energy storage, electrochemical separations, and electrocatalysis. However, the impact of the hydronium (H3O+) and hydroxide (OH-) ions on the ion adsorption and surface charge distributions remains poorly understood. Many fundamental studies of supercapacitors focus on non-aqueous electrolytes to avoid addressing the role of functional groups and electrolyte pH in altering ion uptake. Achieving microscopic level characterization of interfacial mixed ion adsorption is particularly challenging due to the complex ion dynamics, disordered structures, and hierarchical porosity of the carbon electrodes. This work addresses these challenges starting with pH measurements to quantify the adsorbed H3O+ concentrations, which reveal the basic nature of the activated carbon YP-50F commonly used in supercapacitors. Solid-state NMR spectroscopy is used to study the uptake of lithium bis(trifluoromethanesulfonyl)-imide (LiTFSI) aqueous electrolyte in the YP-50F carbon across the full pH range. The NMR data analysis highlights the importance of including the fast ion-exchange processes for accurate quantification of the adsorbed ions. Under acidic conditions, more TFSI- ions are adsorbed in the carbon pores than Li+ ions, with charge compensation also occurring via H3O+ adsorption. Under neutral and basic conditions, when the carbon's surface charge is close to zero, the Li+ and TFSI- ions exhibit similar but lower affinities toward the carbon pores. Our experimental approach and evidence of H3O+ uptake in pores provide a methodology to relate the local structure to the function and performance in a wide range of materials for energy applications and beyond.

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Keywords

Journal Title

J Am Chem Soc

Conference Name

Journal ISSN

0002-7863
1520-5126

Volume Title

Publisher

American Chemical Society (ACS)
Sponsorship
Royal Society (RP/R1/180147)
Faraday Institution (FIRG024)
Engineering and Physical Sciences Research Council (EP/M009521/1)
Faraday Institution (Unknown)
Faraday Institution (FIRG001)
Faraday Institution (FIRG001)
Faraday Institution (FIRG001)
Faraday Institution (FIRG001)
Faraday Institution (Unknown)
D.L. acknowledges the Cambridge Trust Scholarship and the China Scholarship Council (CSC). D.L. thanks Richard Chen for the high field NMR measurements. K.M. was supported by the Faraday Institution Degradation Project (FIRG001and FIRG024). A. B. G. acknowledges funding from the Royal Society (RP/R1/180147) and EPSRC-EP/M009521/1. This work was also supported by a UKRI Future Leaders Fellowship to A. C. F. (MR/T043024/1). CPG acknowledges support from a Royal Society Research Professorship (grant no. RP/R1/180147).