Supercapacitors are energy storage devices that charge very rapidly and can retain their storage capacity through tens of thousands of charge cycles. Their applications include regenerative braking systems in electric vehicles. They hold less energy in the same amount of space as batteries, and they don’t hold a charge for quite as long – but advances in supercapacitor technology could make them competitive with batteries in a wider range of applications. In a study entitled “Efficient 3D Printed Pseudocapacitive Electrodes with Ultrahigh MnO2 Loading,” a group of researchers at UC Santa Cruz and Lawrence Livermore National Laboratory have achieved unprecedented performance from a supercapacitor electrode. The electrode was fabricated from a 3D printable graphene aerogel, which was used to build a porous 3D scaffold loaded with pseudocapacative material.
In tests, the electrodes achieved the highest areal capacitance ever reported for a supercapacitor. In an earlier study, the researchers achieved extremely fast supercapacitor electrodes 3D printed from graphene aerogel. This time, they used an improved graphene aerogel to build a porous scaffold which was loaded with manganese oxide.
A pseudocapacitor is a type of supercapacitor that stores energy through a reaction at the electrode surface, giving it more battery-like performance than supercapacitors that store energy primarily through an electrostatic mechanism (called electric double-layer capacitance, or EDLC).
“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,” said UC Santa Cruz Professor of Chemistry and Biochemistry Yat Li. “So the challenge is to increase the mass loading of pseudocapacitor material without sacrificing its energy storage capacity per unit mass or volume.”
The study demonstrates a breakthrough in balancing mass loading and capacitance in a pseudocapacitor. The researchers increased mass loading to record levels of more than 100 milligrams of manganese oxide per square centimeter without compromising performance, a major increase compared to commercial devices, which have levels of about 10 milligrams per square centimeter.
The areal capacitance also increased linearly with mass loading of manganese oxide and electrode thickness, while the capacitance per gram (gravimetric capacitance) remained almost unchanged. This indicates that the electrode’s performance is not limited by ion diffusion even at such a high mass loading.
In the traditional fabrication of supercapacitors, according to graduate student Bin Lao, a thin coating of electrode material is applied to a thin metal sheet that serves as a current collector. Increasing the thickness of the coating causes performance to decline, so multiple sheets are stacked to build capacitance, increasing weight and material cost.
“With our approach, we don’t need stacking because we can increase capacitance by making the electrode thicker without sacrificing performance,” Yao said.
The researchers managed to increase the thickness of the electrodes to four millimeters without sacrificing performance. The electrodes were designed with a periodic pore structure that allows for both uniform deposition of the material and efficient ion distribution for charging and discharging. The printed structure itself is a lattice made from cylindrical porous rods of the graphene aerogel. Manganese oxide is then deposited onto the lattice.
“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 said. “These findings validate a new approach to fabricating energy storage devices using 3D printing.”
Supercapacitor devices made with the electrodes showed good cycling stability, retaining more than 90 percent of initial capacitance after 20,000 cycles of charging and discharging. The 3D printed electrodes allow for a large amount of design flexibility, and the graphene-based inks offer ultrahigh surface area, lightweight properties, elasticity, and superior electrical conductivity.
Authors of the paper include Bin Yao, Swetha Chandrasekaran, Jing Zhang, Wang Xiao, Fang Qian, Cheng Zhu, Eric B. Duoss, Christopher M. Spadaccini, Marcus A. Worsley and Yat Li.
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