April 15, 2021. By Kolemann Lutz
California-based researchers 3D printed and tested porous carbon aerogel electrodes with outstanding capacitance of 148.6 Farads per gram when a voltage of 5 mV per sec was applied in a -70 C environment, which holds great potential to be used for supercapacitors in ultralow-temperature space environments.
Considering the average temperature on Mars is around -50 Celsius at mid-latitudes with warmer, shorter winters in the northern hemisphere, novel approaches to engineering will enable long term operations of materials and hardware. As heaters are typically required to keep rovers and machine’s battery systems from freezing, these external or self-heating components in energy storage devices add weight and constantly require power for instruments and machinery.
Lithium ion batteries and supercapacitors typically have the lowest operating temperatures of -20 C to -40 C, limited by freezing points of electrolytes, which are ion transport mechanisms between the cathode and anode of battery cells. A eutectic mixture of ionic liquids can dramatically extend the minimum temperature operating range of electrodes and electrical energy storage systems to achieve significantly lower freezing points than each individual electrolyte due to the suppressions of the crystallization process.
The structural engineering of multiscale channeled porous electrodes are highly effective in improving the rate capability and ion transfer and diffusion of supercapacitors. As the macro and mesopores become electrolyte reservoirs, which shortens ion diffusion length, the micropores significantly increased the electrode’s specific surface area to enable rapid charging. A 3D printed carbon lattice with aligned pores facilitates more efficient ion diffusion throughout several mm thick electrodes, similar to how 3D printed channeled electrodes significantly enhance gas bubbles in water electrolysis.
In a 2021 study funded by NASA MIRO Center, Jennifer Lu, Yat Li and colleagues from the University of California, Santa Cruz prepared and engineered 3D printed porous carbon aerogel electrodes with high surface area via the direct ink writing (DIW) method using cellulose nanocrystal (CNC) based ink, which are are nanoparticles from cellulose, one of the most abundant organic polymers found in the cell wall of many plants and algae. CNC’s are promising materials for 3D printing due to the accessibility, affordability, and unique properties in aqueous solutions.
After rich hydroxyl groups form strong hydrogen bonds on cellulose molecules, CNCs were used as a viscosifier to increase the viscosity of viscoelastic inks for printing, instead of a carbon precursor. Afterward CNCs were combined with silica (SiO2), the second most abundant compound on Mars which comprises around 60% of the crust. The Silica microsphere suspension formed a homogenous and highly viscous ink, which is beneficial to maintain structural imprints in direct ink writing with reduced dot area, liquid penetration and liquid surface coverage. CNC’s provide carbon ligaments while silica serves as a hard template for creating macropores around 500 um in diameter that are added layer by layer to form the electrode.
The 3D printing electrode process employs four key steps: (i) freeze drying, (ii) carbonization, (iii) SiO2 Removal, and (iv) KOH Activation. Since nanocellulose has a high young’s modulus of Ca 150 GPa, CNCs better enable cellulose to retain structures after freeze drying. After removal of silica microsphere templates, the macropores inside interconnected hollow carbon spheres become electrolyte reservoirs to shorten ion diffusion length and enable rapid charge accumulation for fast charging. Common activating agent potassium hydroxide (KOH) is utilized to create small pores and tailor the electrode surface area. The 3D printed multiscale porous carbon aerogel (3D-MCA) was tested in a home-built bath cooling system by tuning the ratio between ethanol and H2O and maintaining temperatures with dry ice (solid CO2) to obtain low temperatures between 0 and -118 Celsius.
The two 1 mm thick 3D-MCA electrodes demonstrated significantly higher absorbed nitrogen volume than the control 3D-CA electrode without KOH activation. 3D-MCA exhibited a BET specific surface area of about 1750 square meters / gram, which is considerably larger than the 3D-CA with 322.8 m2/g and other previously reported methods such as 3D printed graphene macropores, and foam. The low tortuosity(diffusion and fluid flow), high surface area, and superior electrolyte wetting of the 3D-MCA make it a promising electrode for the low temperature aerogel. The 3D-MCA displayed a high capacitance of 148.6 F g^-1 when a voltage of 5 mV/sec was applied and retained high capacitance of 71.4 Farads per gram at high scan rates of 200 mV / sec, which is 6.5X higher than non-3D printed porous aerogel and outperforms all other previously reported carbon-based electrodes.
To further improve capacitive performance of the electrodes, researchers allude to future work by reducing the diameter of 3D printed carbon ligaments as well as increasing porosity to decrease ion diffusion length and resistance. Additionally, the electrochemically active surface areas (ECSAs) can be improved by adjusting the size and distribution of silica templates and chemical etching methods.
3D printed aerogels are actively being utilised and developed for a wide range of application from energy storage and catalysis to insulation, desalination, and other industries with ultra-low temperature environments.
In hindsight, the multiscale DIW approach to manufacture porous carbon aerogels enable adequate ion diffusion and charge transfer through an electrode at -94 F, achieving higher energy storage capacitance than all previously reported low-temperature supercapacitors. 3D printed open porous structures enable electrodes and batteries to facilitate sufficient electron transfer and charge to sustain abundant energy in the space environment and on Mars.
B. Yao, H. Peng, H. Zhang, J. Kang, C. Zhu, G. Delgado, D. Byrne, S. Faulkner, M. Freyman, X. Lu, M. Worsley, J. Lu, and Y. Li. “Printing Porous Carbon Aerogels for Low Temperature Supercapacitors”. Nano Letters. March 15, 2021
