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Electrochemistry, ion adsorption and dynamics in the double layer: a study of NaCl(aq) on graphite

Graphite and related sp(2) carbons are ubiquitous electrode materials with particular promise for use in e.g., energy storage and desalination devices, but very little is known about the properties of the carbon–electrolyte double layer at technologically relevant concentrations. Here, the (electrif...

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Autores principales: Finney, Aaron R., McPherson, Ian J., Unwin, Patrick R., Salvalaglio, Matteo
Formato: Online Artículo Texto
Lenguaje:English
Publicado: The Royal Society of Chemistry 2021
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8386640/
https://www.ncbi.nlm.nih.gov/pubmed/34522314
http://dx.doi.org/10.1039/d1sc02289j
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author Finney, Aaron R.
McPherson, Ian J.
Unwin, Patrick R.
Salvalaglio, Matteo
author_facet Finney, Aaron R.
McPherson, Ian J.
Unwin, Patrick R.
Salvalaglio, Matteo
author_sort Finney, Aaron R.
collection PubMed
description Graphite and related sp(2) carbons are ubiquitous electrode materials with particular promise for use in e.g., energy storage and desalination devices, but very little is known about the properties of the carbon–electrolyte double layer at technologically relevant concentrations. Here, the (electrified) graphite–NaCl(aq) interface was examined using constant chemical potential molecular dynamics (CμMD) simulations; this approach avoids ion depletion (due to surface adsorption) and maintains a constant concentration, electroneutral bulk solution beyond the surface. Specific Na(+) adsorption at the graphite basal surface causes charging of the interface in the absence of an applied potential. At moderate bulk concentrations, this leads to accumulation of counter-ions in a diffuse layer to balance the effective surface charge, consistent with established models of the electrical double layer. Beyond ∼0.6 M, however, a combination of over-screening and ion crowding in the double layer results in alternating compact layers of charge density perpendicular to the interface. The transition to this regime is marked by an increasing double layer size and anomalous negative shifts to the potential of zero charge with incremental changes to the bulk concentration. Our observations are supported by changes to the position of the differential capacitance minimum measured by electrochemical impedance spectroscopy, and are explained in terms of the screening behaviour and asymmetric ion adsorption. Furthermore, a striking level of agreement between the differential capacitance from solution evaluated in simulations and measured in experiments allows us to critically assess electrochemical capacitance measurements which have previously been considered to report simply on the density of states of the graphite material at the potential of zero charge. Our work shows that the solution side of the double layer provides the more dominant contribution to the overall measured capacitance. Finally, ion crowding at the highest concentrations (beyond ∼5 M) leads to the formation of liquid-like NaCl clusters confined to highly non-ideal regions of the double layer, where ion diffusion is up to five times slower than in the bulk. The implications of changes to the speciation of ions on reactive events in the double layer are discussed.
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spelling pubmed-83866402021-09-13 Electrochemistry, ion adsorption and dynamics in the double layer: a study of NaCl(aq) on graphite Finney, Aaron R. McPherson, Ian J. Unwin, Patrick R. Salvalaglio, Matteo Chem Sci Chemistry Graphite and related sp(2) carbons are ubiquitous electrode materials with particular promise for use in e.g., energy storage and desalination devices, but very little is known about the properties of the carbon–electrolyte double layer at technologically relevant concentrations. Here, the (electrified) graphite–NaCl(aq) interface was examined using constant chemical potential molecular dynamics (CμMD) simulations; this approach avoids ion depletion (due to surface adsorption) and maintains a constant concentration, electroneutral bulk solution beyond the surface. Specific Na(+) adsorption at the graphite basal surface causes charging of the interface in the absence of an applied potential. At moderate bulk concentrations, this leads to accumulation of counter-ions in a diffuse layer to balance the effective surface charge, consistent with established models of the electrical double layer. Beyond ∼0.6 M, however, a combination of over-screening and ion crowding in the double layer results in alternating compact layers of charge density perpendicular to the interface. The transition to this regime is marked by an increasing double layer size and anomalous negative shifts to the potential of zero charge with incremental changes to the bulk concentration. Our observations are supported by changes to the position of the differential capacitance minimum measured by electrochemical impedance spectroscopy, and are explained in terms of the screening behaviour and asymmetric ion adsorption. Furthermore, a striking level of agreement between the differential capacitance from solution evaluated in simulations and measured in experiments allows us to critically assess electrochemical capacitance measurements which have previously been considered to report simply on the density of states of the graphite material at the potential of zero charge. Our work shows that the solution side of the double layer provides the more dominant contribution to the overall measured capacitance. Finally, ion crowding at the highest concentrations (beyond ∼5 M) leads to the formation of liquid-like NaCl clusters confined to highly non-ideal regions of the double layer, where ion diffusion is up to five times slower than in the bulk. The implications of changes to the speciation of ions on reactive events in the double layer are discussed. The Royal Society of Chemistry 2021-07-14 /pmc/articles/PMC8386640/ /pubmed/34522314 http://dx.doi.org/10.1039/d1sc02289j Text en This journal is © The Royal Society of Chemistry https://creativecommons.org/licenses/by/3.0/
spellingShingle Chemistry
Finney, Aaron R.
McPherson, Ian J.
Unwin, Patrick R.
Salvalaglio, Matteo
Electrochemistry, ion adsorption and dynamics in the double layer: a study of NaCl(aq) on graphite
title Electrochemistry, ion adsorption and dynamics in the double layer: a study of NaCl(aq) on graphite
title_full Electrochemistry, ion adsorption and dynamics in the double layer: a study of NaCl(aq) on graphite
title_fullStr Electrochemistry, ion adsorption and dynamics in the double layer: a study of NaCl(aq) on graphite
title_full_unstemmed Electrochemistry, ion adsorption and dynamics in the double layer: a study of NaCl(aq) on graphite
title_short Electrochemistry, ion adsorption and dynamics in the double layer: a study of NaCl(aq) on graphite
title_sort electrochemistry, ion adsorption and dynamics in the double layer: a study of nacl(aq) on graphite
topic Chemistry
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8386640/
https://www.ncbi.nlm.nih.gov/pubmed/34522314
http://dx.doi.org/10.1039/d1sc02289j
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