Scientists from the University of California Santa Cruz and the Lawrence Livermore National Laboratory (LLNL) have broken performance records using a 3D printed supercapacitor electrode. The electrodes reportedly achieved the highest areal capacitance—the electric charge stored per unit of surface area—of any supercapacitor ever.
The research project, recently published in the journal Joule, consisted of 3D printing the electrodes out of a graphene aerogel, which were printed into a porous scaffold structure loaded with pseudocapacitive material.
Supercapacitors are energy storage devices which offer certain advantages. For instance, they can be charged in seconds or minutes and can retain their storage capacity over tens of thousands of charge cycles. In terms of applications, supercapacitors are used in electric vehicles for regenerative braking systems. However, there are still some limitations to using supercapacitors, as they hold less energy than batteries and do not hold their charge for as long.
Advances in supercapacitors, such as the one achieved by the UC Santa Cruz and LLNL scientists, could make supercapacitors more competitive with batteries and make them suitable for broader applications.
The joint research team has already demonstrated its ability to 3D print ultrafast supercapacitor electrodes using a graphen aerogel in a previous study. In this project, they used an updated graphene aerogel to print a porous scaffold which was then loaded with manganese oxide, a common pseudocapacitive material. (A Pseudocapcitor, for its part, is a type of supercapacitor that stores energy through a reaction at the electrode surface, giving it a better storing capacity than regular supercapacitors.)
“The problem for pseudocapacitors is that when you increase the thickness of the electrode, the capacitance decreases rapidly because of sluggish ion diffusion in bulk structure,” explained Yat Li, professor of chemistry and biochemistry at UC Santa Cruz. “So the challenge is to increase the mass loading of pseudocapacitor material without sacrificing its energy storage capacity per unit mass or volume.”
In the recent study, the team took a significant step forward in balancing mass loading and capacitance within the 3D printed pseudocapacitor. The team also marked a breakthrough by linearly increasing the areal capacitance of the supercapacitor.
The 3D printed construction of the supercapacitor was part of the achievement as it enabled the team to overcome challenges imposed using traditional techniques. As Bin Yao, first author of the study and a graduate student at UC Santa Cruz, explained, in commercial fabrication of supercapacitors, a thin coating of electrode material is applied to a thing metal sheet which collects the current.
The challenge with this is that it is not possible to thicken the electrode coating while maintaining performance, so the solution is to layer multiple sheets of metal and the coating, which adds weight and increases material costs. “With our approach, we don’t need stacking because we can increase capacitance by making the electrode thicker without sacrificing performance,” Yao added.
In the study, the researchers report that they were able to increase the thickness of the electrodes to 4 mm without sacrificing any performance. This was achieved by integrating a periodic pore structure that “enables both uniform deposition of the material and efficient ion diffusion for charging and discharging.”
The 3D printed supercapacitor performed well in tests and demonstrated good cycling capacity. For instance, it retained over 90% of initial capacitance after 20,000 cycles of charging and discharging. Moreover, by 3D printing the structure of the supercapacitor, the scientists say they benefit from design flexibility. In other words, the printable graphene electrodes can be printed into any shape or size to fit a specific device.
“The key innovation in this study is the use of 3D printing to fabricate a rationally designed structure providing a carbon scaffold to support the pseudocapacitive material,” Li added. “These findings validate a new approach to fabricating energy storage devices using 3D printing.”