In recent years, electrochemical capacitors, or supercapacitors, have gained the most intense interest as an alternative to traditional energy storage devices such as batteries. The demands of the potential supercapacitor applications range from plug-in hybrid electric vehicles (PHEVs) to backup power sources. While the power density of supercapacitors surpasses that of most batteries, most commercially available batteries have a significantly higher specific energy density than supercapacitors. Electrode composite materials have been developed that combine graphene with a metal oxide nanocomposite of MnO2 and Co3O4.
A scalable, integrated material synthesis and device fabrication process is used to optimize the specific capacitance as well as cycling lifetime and device reliability. Both energy density and power density are high. The method by which the composite electrode is to be assembled is a two-step, low-cost, scalable solution process of reduced graphene oxide (rGO) and transition metal oxide nanostructures. First, a hybrid metal oxide composite layer of Co3O4 and MnO2 is deposited onto a current collecting substrate, followed by the electrophoretic deposition of a graphene oxide (GO) top layer. This top GO layer is then chemically reduced, allowing for significant conductivity while creating a porous, high-surface-area layer atop the metal oxide layer. Reduction typically is accomplished through the use of chemical agents like hydrazine and sodium borohydride, or by high-temperature treatment. The approach uses sodium borohydride as a supplementary reduction method.
The variety of methods by which the hybrid metal oxide nanocomposite layer may be deposited onto different current collecting substrates makes this arrangement extremely attractive. Metal oxide nanowire arrays can be deposited using a huge variety of hydrothermal and electrodeposition methods. This variety of methods by which the metal oxide component can be deposited onto a conductive substrate also allows for a high degree of flexibility in the choice of a current collecting substrate.
The high porosity of the rGO layer is of great importance to allow sufficient diffusion of ions into and out of the metal oxide composite layer. While faradic, redox reactions take place within the metal oxide composite, the rGO top layer allows for an electrical double layer (EDL) to form simultaneously. The high specific area of the rGO allows for a high capacitive contribution from the EDL, while the metal oxide layer provides for a significant increase to the overall energy density of the electrode.