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Increasing the energy density of carbon-based supercapacitors operating in aqueous electrolytes
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Abstract(s)
Nesta tese, usaram-se duas estratégias para tentar aumentar a densidade de energia de supercondensadores constituídos por elétrodos de carbono e um eletrólito aquoso. Primeiro, eletrodepositou-se polióxido de fenileno nos elétrodos, para tentar suprimir as reações parasitas que limitam a sua janela de estabilidade. Embora as correntes catódicas não tenham sido afetadas pelo polímero, este diminuiu as correntes parasitas anódicas. Comparando as curvas de polarização obtidas em Na2SO4(aq) para elétrodos polimerizados e não polimerizados, os primeiros demonstraram uma densidade de corrente de troca anódica menor (3.4 μA/cm2 em vez de 10.1 μA/cm2 ). Isto sugere que potenciais mais elevados podem ser aplicados sem acelerar a degradação (e que, para uma mesma janela, a degradação deve progredir mais lentamente na presença do revestimento passivante). No entanto, medições preliminares de estabilidade ainda não confirmaram isto. Mais repetições são necessárias para compensar a variabilidade entre as células pertencentes a um mesmo grupo. Na segunda parte desta tese, usou-se um laser para perfurar canais em elétrodos espessos de carbono, para fazer com que estes conseguissem manter uma capacidade elevada, mesmo quando rapidamente descarregados (em Na2SO4(aq)). Usaram-se diferentes combinações de intensidade do laser e de distâncias entre os canais por ele criados, e os elétrodos foram preparados por dois métodos distintos: serigrafia e impressão flexográfica manual. Apesar da macroporosidade pré-existente no primeiro conjunto de elétrodos, o seu desempenho a elevadas velocidades de (des)carregamento melhorou: o declive do gráfico de Ragone diminuiu até 50%. Este declive diminuiu apenas 20% para os melhores elementos do segundo conjunto. Apesar da melhoria na difusão iónica, a capacidade específica diminuiu, em ambos os conjuntos, com o processamento a laser. Isto deveu-se possivelmente à redução dos grupos funcionais oxigenados. No entanto, num eletrólito orgânico, a capacidade específica a 2 A/g aumentou cerca de 66% após o processamento a laser.
Two strategies were employed to try and extend the energy density of carbon-based supercapacitors operating in an aqueous electrolyte. Firstly, poly(phenylene oxide) was electrodeposited on the electrodes, as an attempt to suppress the parasitic reactions that limit their stability window. Although the cathodic currents were unaffected by the polymer, the coating decreased the anodic parasitic currents. Comparing the polarisation curves obtained in Na2SO4(aq) for coated and uncoated electrodes, the former demonstrated a lower anodic exchange current density (3.4 μA/cm2 instead of 10.1 μA/cm2 ). This suggests that higher potentials can be applied without accelerating degradation (and that, for a same window, degradation should progress slower in the presence of the passivating coating). However, preliminary cyclability measurements have not yet confirmed this. More repeats are necessary to compensate for the variability of the coin cells within a same group. Secondly, channels were laser-drilled on high mass loading carbon-based electrodes, in an attempt to improve their rate capability in Na2SO4(aq). Different combinations of pulse energies and channel spacings were employed and electrodes were prepared by two distinct methods: screen-printing and K-bar coating. Despite the pre-existing macroporosity of the first set of electrodes, their rate capability increased: the Ragone plot’s slope decreased up to 50%. This slope only decreased ca. 20% for the best elements of the second set. Despite the enhanced ionic diffusion, specific capacitance decreased, in both sets, upon laser-processing. This was possibly caused by the reduction of oxygen functional groups. However, in an organic electrolyte, the specific capacitance at 2 A/g was up to 66% higher for laser processed electrodes.
Two strategies were employed to try and extend the energy density of carbon-based supercapacitors operating in an aqueous electrolyte. Firstly, poly(phenylene oxide) was electrodeposited on the electrodes, as an attempt to suppress the parasitic reactions that limit their stability window. Although the cathodic currents were unaffected by the polymer, the coating decreased the anodic parasitic currents. Comparing the polarisation curves obtained in Na2SO4(aq) for coated and uncoated electrodes, the former demonstrated a lower anodic exchange current density (3.4 μA/cm2 instead of 10.1 μA/cm2 ). This suggests that higher potentials can be applied without accelerating degradation (and that, for a same window, degradation should progress slower in the presence of the passivating coating). However, preliminary cyclability measurements have not yet confirmed this. More repeats are necessary to compensate for the variability of the coin cells within a same group. Secondly, channels were laser-drilled on high mass loading carbon-based electrodes, in an attempt to improve their rate capability in Na2SO4(aq). Different combinations of pulse energies and channel spacings were employed and electrodes were prepared by two distinct methods: screen-printing and K-bar coating. Despite the pre-existing macroporosity of the first set of electrodes, their rate capability increased: the Ragone plot’s slope decreased up to 50%. This slope only decreased ca. 20% for the best elements of the second set. Despite the enhanced ionic diffusion, specific capacitance decreased, in both sets, upon laser-processing. This was possibly caused by the reduction of oxygen functional groups. However, in an organic electrolyte, the specific capacitance at 2 A/g was up to 66% higher for laser processed electrodes.
Description
Keywords
supercondensador densidade de energia eletrólito aquoso passivação anódica processamento a laser supercapacitor energy density aqueous electrolyte anodic passivation laser processing