Ultra-cold temperatures in outer space require an energy-consuming heater to allow batteries to power exploration devices such as a Mars rover. Scientists have now developed a model for a new supercapacitor that can operate at extremely low temperatures and reduces heating needs, potentially revolutionizing future space missions.
In a paper published March 10 in Nano Letters, a group of researchers developed a energy storage system using carbon aerogels, a type of synthetic material, that can function in the very low temperatures of Mars without heating units, which add weight and energy requirements to machinery, better than existing ultra-cold systems. The work was partially funded by NASA and the U.S. Department of Energy.
"The capability to rapidly store and deliver electricity at ultralow temperatures is critical for human exploration of the moon and for human missions to Mars, as well as human activities in the polar areas," the authors said in the paper. The average temperature on Mars is about -65 degrees Celsius (-85 degrees Fahrenheit), though temperatures can drop as low as -125 degrees Celsius (-195 degrees Fahrenheit).
Yat Li, a chemistry professor at the University of California, Santa Cruz and a lead author of the paper, has been studying energy storage devices for several years, with a particular focus on supercapacitors. These devices enable ultra-fast charging compared to devices such as batteries. A battery turns chemical energy into electricity and may take an hour or longer to fully charge, while a supercapacitor stores electrical energy in an electric field and can charge in seconds.
The paper's proposed device model addresses the need for an energy storage system that can operate at extreme temperatures. The lowest working temperatures of commercial lithium ion batteries and supercapacitors currently range from −20 to −40 degrees Celsius (-4 to -40 degrees Fahrenheit) because they are limited by the freezing points of their electrolytes.
Electrolyte solutions facilitate the diffusion of ions between chambers of the energy storage system when it is being used and recharged. In the past decade, electrolytes with improved low-temperature properties have been developed to enhance energy storage devices' low-temperature performance, the authors said.
But the team hypothesized that a 3D-printed lattice electrode interspersed with nano- and micropores, which create channels formed by carbon ligaments, could substantially reduce ion diffusion resistance and distance, allowing devices to function in ultra-cold climates. The pores serve as electrolyte reservoirs that can significantly shorten the ion diffusion length during fast charging in a supercapacitor.
Low-temperature devices have been of interest to NASA for many years, but Li told The Academic Times that most previous studies on the subject have focused on developing new electrolyte systems to support energy storage. The current paper proposes a different approach by modifying the actual electrode architecture in order to improve overall performance, which Li said is an under-explored area.
Li and Jennifer Lu, a lead author of the paper and director of the NASA-funded MACES, a center for nanomaterials-based research and education at the University of California, Merced, explained that they achieved their 3D-printed multiscale porous carbon aerogel, known as the 3D-MCA, by using a unique combination of chemical methods and direct ink writing, one of the 3D printing methods.
Aerogel is a lightweight, synthetic and porous material that is more than 99% air and is derived from gels. It has a low density and low thermal conductivity, and can be made from different materials, including silica and carbon. Carbon aerogel is commonly manufactured as composite paper, which is useful for electrodes in capacitors, and is also used to create supercapacitors due to the aerogel's high surface area.
Extremely cold temperatures cause conventional energy storage systems to quickly deteriorate in performance as ion diffusion slows down, eventually becoming immobile. But the 3D-MCA structure is built with multiple levels of different-sized pores that are connected in a lattice-like structure, which mitigates the requirement for fast ion diffusion, Lu said.
Through tests run on the new structure, the researchers found that the multiscale porous network was able to preserve adequate ion diffusion and charge transfer through an electrode at -70 degrees Celsius (-94 degrees Fahrenheit).
The results rank among the best reported for other low-temperature supercapacitors, according to the paper, and highlight "the essential role of open porous structures for preserving capacitive performance at ultralow temperatures."
Lu and Li noted that these preliminary results are promising, but said a commercial product is not in development yet. The technology is still in the research stage, and they hope to continue working closely with NASA to make it more successful and reliable.
The porous structure of the model can likely be further adjusted to improve its capacitive performance, expanding the temperature range to accommodate even more extreme conditions. And reducing the diameter of the 3D printed ligaments and increasing the porosity could also decrease the ion diffusion length and resistance, the authors said.
"The capability to fabricate complex porous carbon structures via harnessing additive manufacturing with unique ink formulation together with bottom up chemical etching opens new opportunities for low temperature energy storage systems," said the authors, who worked together on the effort under a collaboration through MACES, which receives funding principally from NASA.
The study, "Printing Porous Carbon Aerogels for Low Temperature Supercapacitors," published March 10 in the Nano Letters journal, was authored by Yat Li, Bin Yao, Huarong Peng, Junzhe Kang, Gerardo Delgado, Dana Byrne, Soren Faulkner and Megan Freyman, the University of California, Santa Cruz; Haozhe Zhang and Xihong Lu, Sun Yat-Sen University; Cheng Zhu and Marcus A. Worsley, Lawrence Livermore National Laboratory; and Jennifer Lu, the University of California, Merced.