Researchers working at UC Santa Cruz and the LLNL (Lawrence Livermore National Laboratory) have achieved substantial improvements to supercap performance by using a new type of electrode design. The electrodes are made using a printable graphene aerogel to construct a porous 3D scaffold structure loaded with a pseudocapacitive material.
 
Laboratory tests indicate this new electrode structure produces a supercap with the greatest areal capacitance (stored electric charge per unit of electrode surface area) ever achieved. The research findings were published recently in the journal Joule.
 
 
Scanning electron microscpe image of the 3D
printed graphene aerogel scaffold.
Image: Bin Yao / UCSC
As a component used to store electrical energy supercaps have the advantage over batteries that they can be charged very quickly (in a matter of seconds or minutes) and their properties remain fairly stable over time, providing the same level of storage capacity over tens of thousands of charge cycles. They are often used to store energy generated in vehicular regenerative braking systems. However, compared to batteries, they require a greater volume to store the same level of charge and also the stored charge leaks away more quickly. With improvements in the technology we could soon find that supercaps become a serious competitor to batteries. To build this novel electrode described in the paper researchers used a new type of graphene aerogel to create a porous scaffold structure coated with manganese oxide pseudocapacitive material.
Unlike a conventional supercap which stores energy in an electrostatic field, a pseudocapacitor stores energy via a reaction at the electrode surface, giving it more battery-like properties than ordinary supercaps. The problem with such pseudocapacitors, however, is that as the electrode thickness is increased, the value of capacitance decreases rapidly due to slower ion diffusion occurring through the material bulk. This new type of electrode shows a significant improvement in balancing the pseudocapacitor mass loading and capacitance. Researchers were able to increase the mass loading to more than 100 mg / cm2 of manganese oxide without sacrificing performance. More importantly, the areal capacitance was found to increase linearly with the mass loading of manganese oxide and thickness of the electrode, while the capacitance remained almost unchanged relative to weight. This shows that electrode performance is not compromised by ion diffusion.
 
Scanning electron microscpe image of the 3D
printed graphene aerogel scaffold coated with
manganese oxide.
Image: Bin Yao / UCSC
 
To build a traditional commercial supercap a thin metal sheet is coated with electrode material to act as the current collector. Increasing the layer thickness degrades performance so multiple stacked sheets are generally used to produce higher values of capacitance, but this make the capacitor heavier and more costly. The new electrode design does not need stacked structures. The researchers were able to increase the thickness of their electrode up to 4 millimeters without loss of performance.
The resulting supercaps managed to retain 90% of their initial storage capacity after more than 20,000 charge/discharge cycles. The 3D printed novel electrodes also facilitate tremendous flexibility in equipment design because their outline can be configured specifically to fit into a device.