Among all energy storage devices, supercapacitors (SC) have garnered substantial attention in recent years due to their ultra-fast charge and discharge rate, excellent stability, long cycle life, and very high power density. These characteristics are desirable for many applications including electric vehicles (EVs) and portable electronics. However, SCs may only serve as standalone power sources in systems that require power delivery for a short duration (<10 sec). This is due to their relatively lower energy density in comparison to other types of energy storage devices such as batteries and fuel cells. In often cases SCs are used as part of a hybrid system with other high energy storage devices in real applications. Therefore, boosting the energy density of SCs has become one of the most promising methods for the development of future high energy and high power density energy storage devices.

… There are two types of electrochemical capacitors: (i) electrochemical double layer capacitors (EDLCs), which are generally based on pure graphitic nanostructures including CNTs, graphene, carbon onions/spheres, template derived carbons, activated carbon, etc. and (ii) pseudocapacitors which are based on pseudocapacitive materials like V₂O₅, RuO₂, MnO₂, Co₂O₃, Co₃O₄, In₂O₃, NiO/Ni(OH)₂, binary Ni-Co hydroxide, etc. which introduce fast surface redox reactions. … Among all pseudocapacitive materials, RuO₂ appears to be the most promising material for SCs with the following advantages: i) simple and scalable synthesis, ii) high capacitance, and iii) rapid charging-discharging.

—Wang et al.

The UC Riverside team had previously developed high-quality graphene and carbon nanotube (CNT) hybrid hierarchical nanostructures—3D graphene foam conformally covered with densely packed CNT networks. In this latest study, the researchers anchored the RuO₂ nanoparticles to the hybrid nanostructures.

The porous GM foam not only provides a large surface area for the loading of RuO– nano-particles but also facilitates electrolyte infiltration, they noted. The embedded CNTs in the CNT-RuO₂ network layer work as a conductive framework.

Our innovative RGM architecture offers seamless connections at the graphene and CNT interface and enhances the interfacial integrity of major components (active materials and current collector) which improves the conductivity of the whole electrode compared with other reported hybrid RuO₂ systems. Good electrolyte access, enhanced conductivity, and improved charge transport further lead to very high active material utilization, smaller internal resistance (ESR ~ 1V), superior rate capability, and cycling stability. In addition to a very high specific capacitance compared with other previously reported pseudocapacitive systems, symmetric ECs based on this innovative RGM hybrid foam architecture could be cycled reversibly in a operational voltage window of 1.5 V, which is much larger than the majority of aqueous electrolyte supercapacitors (~1.0 V). The high specific capacitance and extended operational voltage window lead to an impressive maximum energy density of 39.28 Wh kg^-1 and power density of 128.01 kW kg^-1.

—Wang et al.

The foam electrode was successfully cycled more than 8,000 times with no fading in performance.

Resources

• Wei Wang, Shirui Guo, Ilkeun Lee, Kazi Ahmed, Jiebin Zhong, Zachary Favors, Francisco Zaera, Mihrimah Ozkan & Cengiz S. Ozkan (2014) “Hydrous Ruthenium Oxide Nanoparticles Anchored to Graphene and Carbon Nanotube Hybrid Foam for Supercapacitors,” Scientific Reports 4, Article number: 4452 doi: 10.1038/srep04452