The cost and practicality of greenhouse
gas removal processes,
which are critical for environmental sustainability, pivot on high-value
secondary applications derived from carbon capture and conversion
techniques. Using the solar thermal electrochemical process (STEP),
ambient CO2 captured in molten lithiated carbonates leads
to the production of carbon nanofibers (CNFs) and carbon nanotubes
(CNTs) at high yield through electrolysis using inexpensive steel
electrodes. These low-cost CO2-derived CNTs and CNFs are
demonstrated as high performance energy storage materials in both
lithium-ion and sodium-ion batteries. Owing to synthetic control of
sp3 content in the synthesized nanostructures, optimized
storage capacities are measured over 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.
This work demonstrates that ambient CO2, considered as
an environmental pollutant, can be attributed economic value in grid-scale
and portable energy storage systems with STEP scale-up practicality
in the context of combined cycle natural gas electric power generation.
One pathway to remove the greenhouse gas carbon dioxide to mitigate climate change is by dissolution and electrolysis in molten carbonate to produce stable, solid carbon. This study determines critical knowledge to minimize the required electrolysis energy, the reaction stoichiometry in which carbon and O 2 are the principal products, and that CO 2 can be electrolyzed inexpensively. Thermochemical and experimental results indicate that the principal carbon-deposition reaction in molten Li 2 CO 3 or Li 2 O/Li 2 CO 3 electrolytes at 750°C is Li 2 O + 2CO 2 → Li 2 CO 3 + C + O 2 . The reaction occurs at high Faradaic efficiency of the 4e − reduction of CO 2 to carbon and oxygen at an electrolysis voltage as low as <1 V. Electrolytes without lithium carbonate but containing calcium and/or barium carbonate can also be employed as reaction media for successful carbon deposition, e.g. in an Na/BaCO 3 melt. However, the electrolysis reduction in pure Na or K or Na/K carbonate eutectics at 1 atm of CO 2 forms metals and/or gases, i.e., CO.
Conspectus
Climate change represents one of the most important environmental
issues of our time. Due to high levels of anthropogenic CO2 emissions, atmospheric CO2 has for the first time ever
exceeded 415 ppm and has increased from 315 ppm in 1950. An annual
increase in atmospheric CO2 of ∼2 ppm is equal to
a net increase of ∼15.6 billion tons of CO2. The
combustion of fossil fuels for electricity and transportation is still
the main reason accounting for the CO2 accumulation. On
the top of that, fossil fuels are widely used in our modern industry
for the productions of indispensable social staples. For instance,
the millennia old thermal reduction of iron ore by charcoal or baked
coal (3C + 2Fe2O3 → 4Fe + 3CO2) continues as the main method for the production of iron. The artificial
fertilizer ammonia boosts the global population and is mainly produced
from the Haber–Bosch process, in which hydrogen is generated
via steam reforming of methane (CH4 + 2H2O →
4H2 + CO2). Sequestration and diminution of
CO2 require the development of a portfolio of technologies
on (1) efficient and long-term harvesting of renewable energy, that
is, solar, not only for electricity but also directly as the energy
force in vital chemical processes, wherever possible, (2) carbon-neutral
processes to replace current industrial processes that emit vast amounts
of CO2, such as iron and ammonia production, and (3) new,
low-cost technologies for CO2 capture and conversion with
particular interests in the exploration of CO2 as the feedstock
for fuels or other valuable chemicals and materials. To this end,
we conducted some studies on the sustainable synthesis of ammonia
and iron with net-zero CO2 emissions and large-scale CO2 capture and conversion into fuels and high value nanocarbon
products via electrolysis in molten salt(s) with the introduction
of the Solar Thermal Electrochemical Process (STEP).
In STEP,
solar UV–visible energy is focused on a photovoltaic
device that generates the electricity to drive the electrolysis, while
concurrently the solar thermal energy is focused on a second system
to generate heat for the electrolysis cell. The utilization of the
full spectrum of sunlight in STEP results in a higher solar energy
efficiency than other solar conversion processes. STEP has been applied
to conduct (1) CO2-free ammonia synthesis from nitrogen
and water with the aid of nano-Fe2O3 in a molten
hydroxide electrolyte, (2) CO2-free production of iron
via electrochemical reduction of iron ore in molten carbonate, (3)
CO2 capture and conversion into nanostructured carbon products
as well as fuels in molten or mixed molten electrolytes, and (4) organic
electrosynthesis of benzoic acid from benzene without overoxidizing
into CO2. In this Account, we highlight some recent achievements
in these topics and propose that using STEP is a highly efficient
strategy for saving energy and, consequently, the environment. STEP
is an ideal tool that can theoretically be applied to all endothermic
reactions.
This SEM, TEM and Raman Spectra and economic calculations data provides a benchmark for carbon nanotubes synthesized via molten electrolyte via the carbon dioxide to carbon nanotube (C2CNT) process useful for comparison to other data on longer length C2CNT wools; specifically: (I) C2CNT electrosynthesis with bare (uncoated) cathodes and without pre-electrolysis low current activation. (II) C2CNT Intermediate length CNTs with intermediate integrated electrolysis charge transfer. (III) C2CNT Admixing of sulfur, nitrogen and phosphorous (in addition to boron) to carbon nanotubes, and (IV) Scalability of the C2CNT process. This data presented in this article are related to the research article entitled “Carbon Nanotube Wools Made Directly from CO2 by Molten Electrolysis: Value Driven Pathways to Carbon Dioxide Greenhouse Gas Mitigation” (Johnson et al., 2017) [1].
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