High Energy-Density Hybrid Supercapacitors: Combining High-Voltage Window Ionic Liquids with MnO2-Nanomaterials

Abstract

Hybrid supercapacitors, utilizing faradaic as well as electrochemical reactions for energy storage, have attracted considerable attention during the last years as the need for fast charging and reliable energy storage is ever increasing in today’s society. However, issues still remain that prevents utilizing the full potential of hybrid supercapacitors. The highest hurdle is that the energy density (gravimetric and volumetric) of hybrid supercapacitors is still too low compared to batteries[1]. Additionally, hybrid supercapacitors suffer from poor cycle lifetime, compared to traditional supercapacitors. A promising approach to increase the energy density of hybrid supercapacitors is the use of electrolytes, which are stable in a larger potential window compared to aqueous electrolytes. This enables higher total operating voltages, which is especially important, since the energy density has a quadratic dependency on the voltage. However, so far hybrid supercapacitors, using MnO2 or RuO2 as electrochemically active component, have mostly been designed using aqueous electrolytes[1], thereby missing the possibility to benefit from the higher operating voltage of non-aqueous electrolytes. In our work, we aim at increasing both the operating voltage and the capacitance of hybrid supercapacitors, which together would enable higher energy density. We investigate how different solvents, mainly based on ionic liquids interact with carbon based electrodes containing MnO2 nanoparticles[2] as electrochemically active species. We reveal how different proton characteristics (protic and aprotic) of the ionic liquids influence the performance. For instance, we compare the performance of the protic ionic liquid 1-ethylimidazolium bis(trifluoromethanesulfonyl) (EI-TFSI) and the aprotic 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) (EMI-TFSI). To further investigate the role of protons and to increase their availability, we add different additives, such as water to the ionic liquid electrolytes. We discuss the fundamental mechanisms, i.e. the role of protons, the role of additional cations, and the influence of the voltage window on the performance of the system (capacitance, energy density, and stability). Our study provides insight that can help to achieve a better understanding of the requirements on the electrolyte for hybrid supercapacitors. It shows that the MnO2 nanomaterial requires protons to work efficiently. Using protic electrolytes, such as water and EI-TFSI, a contribution from MnO2 can be observed, whereas there is no additional electrochemical contribution in the aprotic electrolyte. However, by adding water, and thereby available protons, to the aprotic ionic liquid we are able to increase the proton concentration and activity of the MnO2 material. [1]Chem. Soc. Rev., 2015, 44, 7484—7539 [2]J. Phys. Chem. C 2011, 115, 5413–5421