Líquidos iônicos em interfaces: estudo computacional empregando modelos polarizáveis
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Data
2019
Autores
Pereira, Guilherme Ferreira Lemos [UNIFESP]
Orientadores
Siqueira, Leonardo José Amaral de [UNIFESP]
Tipo
Dissertação de mestrado
Título da Revista
ISSN da Revista
Título de Volume
Resumo
Líquidos iônicos contendo cátions fosfônio com cadeias alquílicas curtas apresentam
baixa viscosidade além de alta estabilidade eletroquímica. Estas propriedades
fazem deles ótimos candidatos para eletrólitos para capacitores de dupla camada
elétrica (CDCE). Neste trabalho realizou-se simulações de dinâmica molecular
(DM) do líquido iônico formado pelo cátion 2-(metoxi)-etil-trietil-fosfônio [P222,2O1]
e pelo ânion [bis(trifluorometanossulfonil)]imidato [NTf2] confinado em eletrodos de
grafeno planos e nanoporosos. Os eletrodos foram simulados utilizando um modelo
de potencial constante, que permite que as cargas dos átomos de carbono do
eletrodo possam flutuar. Apesar da função éter na cadeia mais longa de fosfônio, os
íons se organizam em camadas de carga alternada perto da superfície dos eletrodos
planares. A capacitância diferencial no eletrodo negativo é menor do que no eletrodo
positivo, o que reflete o maior tamanho dos cátions fosfônio. Dentro dos poros de
8,2 Å, os íons se organizam em uma monocamada, enquanto que nos poros maiores
(12 Å) há uma camada de átomos de nitrogênio do ânion e duas camadas de átomos
de fósforo do cátions fosfônio. Em ambos os eletrodos porosos, os íons de mesma
carga são adsorvido principalmente um na frente do outro entre folhas de grafeno
devido à alta carga imagem dos átomos de carbono do eletrodo geradas pelos íons.
No eletrodo de poro mais estreito, a capacitância varia com a diferença de potencial
aplicada, o que afeta a densidade de energia global do eletrodo.O que resulta em
uma melhor performance do eletrodo com poro maior em voltagens acima de 3,4 V.
Phosphonium-based ionic liquids with short alkyl chains present low viscosity besides their relative high electrochemical stability. These properties make them good candidates for electrolytes of electrochemical double-layer capacitors (EDLC). We performed molecular dynamics (MD) simulations of 2-(methoxy)- ethyl-triethylphosphonium [P222,2O1] bis(trifluoro-methanesulfonyl)imide [NTf2] ionic liquid confined in planar and nanoporous graphene electrode. The electrodes were simulated with a constant potential model, which allow the carbon charges to fluctuate. In spite of the ether function in the longer chain of phosphonium, the ions are organized in layers of alternated charge close to the surface of planar electrodes. The differential capacitance on the negative electrode is lower than in the positive electrode, which reflects the larger size of phosphonium cations. In nanoporous carbons, inside the pores of 8.2 Å, there is a monolayer of ions, whereas in larger pores (12 Å) there are one layer of N atom of anion and two layers of P atom of phosphonium cations. With both porous electrode the ions of the same charge are mostly adsorbed in front of each other across the graphene plane due to high image charges of carbon atoms of the electrode in between the ions. In the electrode of narrower pore, the capacitance varies with the applied voltage, which impacts the overall energy density of the electrode. This results in a better performance of the electrode with larger pore at voltages above 3.4 V.
Phosphonium-based ionic liquids with short alkyl chains present low viscosity besides their relative high electrochemical stability. These properties make them good candidates for electrolytes of electrochemical double-layer capacitors (EDLC). We performed molecular dynamics (MD) simulations of 2-(methoxy)- ethyl-triethylphosphonium [P222,2O1] bis(trifluoro-methanesulfonyl)imide [NTf2] ionic liquid confined in planar and nanoporous graphene electrode. The electrodes were simulated with a constant potential model, which allow the carbon charges to fluctuate. In spite of the ether function in the longer chain of phosphonium, the ions are organized in layers of alternated charge close to the surface of planar electrodes. The differential capacitance on the negative electrode is lower than in the positive electrode, which reflects the larger size of phosphonium cations. In nanoporous carbons, inside the pores of 8.2 Å, there is a monolayer of ions, whereas in larger pores (12 Å) there are one layer of N atom of anion and two layers of P atom of phosphonium cations. With both porous electrode the ions of the same charge are mostly adsorbed in front of each other across the graphene plane due to high image charges of carbon atoms of the electrode in between the ions. In the electrode of narrower pore, the capacitance varies with the applied voltage, which impacts the overall energy density of the electrode. This results in a better performance of the electrode with larger pore at voltages above 3.4 V.