Advanced Supercapacitor based on Combination of Graphene Hybrid Materials and Redox-Electrolytes
- Author(s): Hwang, Jee Youn
- Advisor(s): Kaner, Richard B
- et al.
Electrochemical capacitors, also known as supercapacitors, are attractive energy storage devices with the ability to recharge in seconds instead of hours for traditional batteries. They also can be used for up to a million charge/discharge cycles. Because of their enabling features, supercapacitors are replacing batteries and capacitors in an increasing number of applications including back-up power, cold starting, flash cameras, and regenerative braking. However, the low energy density of carbon electrodes is the main impediment to realizing the full potential of this technology. State-of-the-art supercapacitors feature activated carbon electrodes impregnated with a non-aqueous electrolyte (typically acetonitrile) that operate at voltages between 2.2-2.7 V. Unfortunately, activated carbons have low specific capacitance in organic electrolytes which severely limits the energy density of commercial systems. In addition, organic solvents are often flammable leading to safety and environmental concerns. Aqueous electrolytes, on the other hand, are safer, cheaper and have higher ionic conductivity, promising higher capacitance electrodes. This has triggered tremendous research efforts in order to develop new hybrid electrode materials that are capable of providing a huge amount of energy in an aqueous electrolyte. We have used a commercial grade LightScribe DVD burner for the direct synthesis and processing of graphene-metal oxide (RuO2, Fe3O4, MnO2, V2O5, etc.) hybrid electrodes in a single step. By anchoring metal oxide nanoparticles directly onto graphene, the 3-dimensional, highly porous graphene surfaces serve as excellent conductors for fast electron transfer, while metal oxide nanoparticles provide a large electrochemically active surface area for fast and reversible Faradaic reactions.
Although metal oxide and carbon composite active materials are able to improve the capacitance, the energy density cannot be improved significantly if one only relies on the solid active materials comprising the electrode. Another important deciding factor for the performance of a supercapacitor is the electrolyte. Adding redox-additives into a traditional aqueous electrolyte has proven to be an efficient route to enhance supercapacitor performance because redox additives can contribute to the capacitance and enhance the energy densities via redox reactions between the electrode and the electrolyte. While carbon hybrid electrodes can only operate at 1.0 V, the addition of a redox mediator extends the decomposition voltage of the electrolyte, resulting in a symmetric supercapacitor with an ultrahigh voltage of 2 V. This represents a conceptual advance in the field of aqueous supercapacitors and may enable a new generation of eco-friendly energy storage devices. As both electrode and electrolyte contribute to charge storage simultaneously, an ultrahigh specific capacitance can be obtained. In addition, single-step direct laser writing of hybrid micro-supercapacitors shows great potential for miniaturized electronics. Thus, the current work provides an effective strategy for designing and fabricating aqueous supercapacitors and micro-supercapacitors that hold promise for a sustainable energy future.