Flexible Supercapacitors Utilising the Multifunctional Rôle of Ionic Liquids

  • Marta Lorenzo Fernandez

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

The study on flexible, ultrathin and safe energy storage devices such as supercapacitors or batteries is an emerging area to “power-up” the next-generation of portable and flexible electronics such as mobile phones, computers, displays, wearable and implantable biomedical devices. In general, a supercapacitor is composed of two electrodes, electrolyte, separator and current collectors and it can store and deliver charge at relatively high rates. The challenge to design such supercapacitors to be flexible lies in the development of flexible electrodes and leak-proof electrolytes, as well as, the retention of the electrochemical characteristics of high power density, long cycle life and high efficiency under considerable physical deformation.

An innovative one-pot synthesis to fabricate electronically conducting polymer-biopolymer composites films such as polypyrrole-cellulose composites that are intrinsically conducting and flexible is presented. The method consisted of an in situ polymerisation of pyrrole in a solution of cellulose in the ionic liquid, 1-butyl-3-methylimidazolium chloride. The resulting polypyrrole-cellulose composite film was chemically blended, and it showed flexible polymer properties while retaining the electronic properties of the polypyrrole. Addition of a hydrophobic ionic liquid, trihexyl(tetradecyl)phosphonium bis{(trifluoromethyl) sulfonyl}amide and graphite powder enhanced the flexibility and conductivity of the composite films, respectively.

The composites films obtained were applied as electrodes in flexible supercapacitors using a simple scalable method to design flexible, ultrathin and safe supercapacitors. Three devices were fabricated, (i) electrical double-layer supercapacitors, (ii) electrochemical supercapacitors and (iii) hybrid supercapacitors. The multifunctional rôle of ionic liquids as solvent, electrolyte and plasticiser was exploited to fabricate these novel flexible supercapacitors which showed an excellent cycle life of 15000 cycles with nearly 100 % of capacitance retention, an operational voltage between 1.6 V and 3.2 V and a maximum energy and power density of 0.008 μW h cm-2 and 1.78 μW cm-2, respectively. Moreover, the design nature of these electrodes, chemical stability and feasibility to use biocompatible components will enable the fabrication of task-specific flexible supercapacitors with durable cycle life and chemical stability.
Date of Award30 May 2018
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SupervisorCristina Lagunas-Castedo (Supervisor) & Geetha Srinivasan (Supervisor)

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