## Researchers convert atmospheric CO2 to carbon nanofibers and nanotubes for use as anodes in Li-ion and Na-ion batteries

##### 03 March 2016

Researchers from George Washington University and Vanderbilt University have demonstrated the conversion of atmospheric CO2 into carbon nanofibers (CNFs) and carbon nanotubes (CNTs) for use as high-performance anodes in both lithium-ion and sodium-ion batteries. As described in an open-access paper in the journal ACS Central Science, optimized storage capacities were more than 370 mAh g-1 (lithium) and 130 mAh g-1 (sodium) with no capacity fade under durability tests up to 200 and 600 cycles, respectively.

The conversion process builds upon the solar thermal electro-chemical process (STEP) introduced by GWU Professor Stuart Licht and his colleagues in 2009. (Earlier post.) STEP is an efficient solar chemical process, based on a synergy of solar thermal and endothermic electrolyses, designed to convert greenhouse gas carbon dioxide into a useful carbon commodity. In short, STEP uses solar thermal energy to increase the system temperature to decrease electrolysis potentials.

Our climate change solution is two fold: To transform the greenhouse gas carbon dioxide into valuable products and to provide greenhouse gas emission-free alternatives to today’s industrial and transportation fossil fuel processes. In addition to better batteries other applications for the carbon nanotubes include carbon composites for strong, lightweight construction materials, sports equipment and car, truck and airplane bodies.

—Stuart Licht

STEP uses inexpensive electrode materials (galvanized steel cathode and a nickel anode) and molten carbonate electrolytes that are heated and powered using concentrated photovoltaic (CPV) cells that convert sunlight into electricity at 39% efficiency. STEP has been shown to function effectively with or without solar powered operation to electrolytically split water, carbon dioxide, or metal oxides, produce STEP carbon, produce STEP ammonia and STEP organic, and produce STEP iron or cement.

Here we show that this process can be used as a sustainable synthetic pathway for defect-controlled CNT and CNF materials, which exhibit excellent performance in the context of lithium-ion and sodium-ion battery anode materials. This presents a sustainable route to convert carbon dioxide into materials relevant to both grid-scale and portable storage systems.

—Licht et al.

The team used DC electrolysis with CO2 dissolved in 750 °C molten Li2CO3 with, or without, added Li2O. A nickel crucible served as both container and (O2 generating) anode, with galvanized steel as the cathode. Two types of nanostructures were generated: straight CNTs that are grown in electrolyte without added Li2O, and tangled CNTs that are grown with the addition of Li2O to the electrolyte.

The control of diffusion conditions during electrolytic splitting of CO2 in molten lithium carbonate leads to either filled CNF or hollow CNT nanostructures, and control of oxide and transition metal concentration leads to tangled or straight fibers. This level of control on the synthesized carbon nanostructures is critical for battery applications, the team said.

The straight and tangled materials were developed into electrodes, combined into half-cells along with electrolyte and a separator and pressed into coin cells for electrochemical testing. In lithium-ion batteries, the nanotubes replace the carbon anode used in commercial batteries. The team demonstrated that the carbon nanotubes gave a small boost to the performance, which was amplified when the battery was charged quickly. In sodium-ion batteries, the researchers found that small defects in the carbon, which can be tuned by STEP, can unlock stable storage performance over 3.5 times above that of sodium-ion batteries with graphite electrodes. Most importantly, both carbon-nanotube batteries were exposed to about 2.5 months of continuous charging and discharging and showed no sign of fatigue.

The researchers estimate that with a battery cost of $325 per kWh (the average cost of lithium-ion batteries reported by the Department of Energy in 2013), a kilogram of carbon dioxide has a value of about$18 as a battery material—six times more than when it is converted to methanol—a number that only increases when moving from large batteries used in electric vehicles to the smaller batteries used in electronics.

Licht also proposes a modified flue system for combustion plants that incorporates this process that could be self-sustaining, as exemplified by a new natural gas power plant with zero carbon dioxide emissions. That’s because the side product of the process is pure oxygen, which plants could then use for further combustion. The calculated total cost per metric tonne of CNTs would be much less expensive than current synthetic methods.

This approach not only produces better batteries but it also establishes a value for carbon dioxide recovered from the atmosphere that is associated with the end-user battery cost unlike most efforts to reuse CO2 that are aimed at low-valued fuels, like methanol, that cannot justify the cost required to produce them.Vanderbilt assistant professor of mechanical engineering Cary Pint

The research was partially supported by National Science Foundation grants 1230732 and 1505830 and NSF Graduate Research Fellowship grant 1445197.

Resources

• Stuart Licht, Anna Douglas, Jiawen Ren, Rachel Carter, Matthew Lefler, and Cary L. Pint (2016) “Carbon Nanotubes Produced from Ambient Carbon Dioxide for Environmentally Sustainable Lithium-Ion and Sodium-Ion Battery Anodes” ACS Central Science doi: 10.1021/acscentsci.5b00400

Done on a very large scale, with a $100/ton subsidy (paid by$100/ton carbon tax), this process could be one of the 101 ways to reduce CO2 and GHG while producing useful materials?