34 research outputs found
Sustainable Capture and Conversion of Carbon Dioxide into Valuable Multiwalled Carbon Nanotubes Using Metal Scrap Materials
Increasing
amounts of nondegradable waste and rising levels of
atmospheric carbon dioxide (CO<sub>2</sub>) similarly threaten a sustainable
future, leaving routes to address these issues at the forefront of
ongoing research efforts. Here, we demonstrate a route where electrochemical
processing of scrap metals extracts catalytic species to the surface
that actively convert CO<sub>2</sub> scavenged from the atmosphere
to form multiwalled carbon nanotubes (MWCNTs). Our findings demonstrate
two distinct pathways for this technique that is generalizable to
a broad range of scrap metals. First is the case where the catalytic
elements are the primary constituents of the material (e.g., Fe in
stainless steel) and the reaction with CO<sub>2</sub> consumes the
material. Second is the case where the catalytic elements are impurities
(e.g., Fe in brass) where reaction with CO<sub>2</sub> leads to impurity
consumption. Our results demonstrate facile growth of MWCNTs directly
from irregular scraps, such as shavings and pipes. Overall, this study
presents a route where input sources of atmospheric CO<sub>2</sub> and low-value metal scraps can be transformed to higher valued purified
metals and high valued MWCNTs with the promise of an overall carbon
negative capture and conversion strategy
Isothermal Sulfur Condensation into Carbon Scaffolds: Improved Loading, Performance, and Scalability for Lithium–Sulfur Battery Cathodes
Here
we demonstrate an isothermal technique that enables rapid
vapor infiltration of sulfur into carbon templates to overcome scalability
and performance bottlenecks associated with common melt infiltration
techniques. Building on straightforward thermodynamic principles of
capillary condensation, self-limited sulfur loadings up to 82 wt %
can be achieved in as little as 10 min at temperatures between 155
and 175 °C. We demonstrate a broad range of device performance
criteria using a carbon black–single-walled carbon nanotube
binder-free cathode framework, including a side-by-side comparison
to melt infiltrated electrodes with 74 wt % loading that shows improved
capacity (1015 mAh/g vs 768 mAh/g), ∼92% capacity retention
after 200 cycles at 0.5 C, and ∼98% Coulombic efficiency as
a result of enhanced uniformity and conductivity. Further, we demonstrate
this technique over a range of different electrodes (1) electrodes
with high sulfur loading (82 wt %) with high initial discharge capacity
of 1340 mAh/g, (2) electrodes with high areal loading of 8 mg/cm<sup>2</sup> sulfur with >6.5 mAh/cm<sup>2</sup> areal capacity, and
(3)
electrodes based on carbons with microporous confining pores. Most
importantly, this vapor infiltration approach requires over 5×
less energy input and enables over 60× greater throughput than
standard melt infiltration, enabling integration into roll-to-roll
rapid processing schemes without compromising device performance.
This technique liberates cost and manufacturing barriers to commercialization
of Li–S batteries at larger scales while opening new avenues
to infiltrate preformed cathode assemblies with sulfur for assessment
at lab scales
Size-Dependent Phononic Properties of PdO Nanocrystals Probed by Nanoscale Optical Thermometry
With the advent of novel nanoscale
devices, fast and reliable thermal
mapping with high spatiotemporal resolution is imperative for probing
the characteristics of phonons and evaluating the local temperature
at the nanoscale. In this work, Raman spectroscopy is employed as
a rapid and noncontact optical thermometry technique to investigate
phononic properties of macroscopic assemblies of monodisperse palladium
oxide (PdO) nanocrystals. PdO has been extensively employed in high
temperature catalytic devices; however, the phonon behavior which
determines the thermal stability of PdO remains unexplored thus far.
Our study focuses on homogeneous, large-scale assemblies of monodisperse
4 and 10 nm nanocrystals synthesized using colloidal chemistry to
understand size-dependent effects on the measured thermal properties.
By monitoring the Raman peak shifts, peak broadening, and alterations
in peak intensities as a function of laser power and particle concentration,
a size-dependent trend is observed attributable to confinement of
optical phonons within nanocrystal grain boundaries and laser-induced
heating, both influenced by nanocrystal size. This study correlates
size-dependent single-particle heating effects with size-dependent
interparticle heat transfer under laser irradiation and is enabled
by controlled nanocrystal synthesis
Toward Small-Diameter Carbon Nanotubes Synthesized from Captured Carbon Dioxide: Critical Role of Catalyst Coarsening
Small-diameter
carbon nanotubes (CNTs) often require increased
sophistication and control in synthesis processes, but exhibit improved
physical properties and greater economic value over their larger-diameter
counterparts. Here, we study mechanisms controlling the electrochemical
synthesis of CNTs from the capture and conversion of ambient CO<sub>2</sub> in molten salts and leverage this understanding to achieve
the smallest-diameter CNTs ever reported in the literature from sustainable
electrochemical synthesis routes, including some few-walled CNTs.
Here, Fe catalyst layers are deposited at different thicknesses onto
stainless steel to produce cathodes, and atomic layer deposition of
Al<sub>2</sub>O<sub>3</sub> is performed on Ni to produce a corrosion-resistant
anode. Our findings indicate a correlation between the CNT diameter
and Fe metal layer thickness following electrochemical catalyst reduction
at the cathode-molten salt interface. Further, catalyst coarsening
during long duration synthesis experiments leads to a 2× increase
in average diameters from 3 to 60 min durations, with CNTs produced
after 3 min exhibiting a tight diameter distribution centered near
∼10 nm. Energy consumption analysis for the conversion of CO<sub>2</sub> into CNTs demonstrates energy input costs much lower than
the value of CNTsa concept that strictly requires and motivates
small-diameter CNTsand is more favorable compared to other
costly CO<sub>2</sub> conversion techniques that produce lower-value
materials and products
Role of Catalyst Oxidation State in the Growth of Vertically Aligned Carbon Nanotubes
The impact of gas-phase pretreatment of supported iron-oxide
catalyst
utilized in aligned carbon nanotube (CNT) growth is studied to understand
the correlation between the catalyst oxidation state and the growth
characteristics of the aligned CNT forests. By varying the pretreatment
conditions from a reducing to an oxidizing environment, notable changes
are observed in both the collective CNT array growth behavior and
the individual CNT characteristics. Although the greatest catalytic
activity was observed following a full reduction to the zerovalent
(metallic) Fe catalyst, growth is also observed from a catalyst composed
of both Fe<sub>2</sub>O<sub>3</sub> and Fe<sub>3</sub>O<sub>4</sub> particles. XPS core-level analysis, following pretreatment of the
catalyst, emphasizes the critical nature of the combined catalyst–underlayer
interaction to achieve optimal catalyst activity during growth and
hence the most efficient catalyst reduction process. Additionally,
CNT diameters during growth were strongly affected by the pretreatment
process. Overall, this work gives a collective picture of how the
catalyst oxidation state affects the CNT growth based on the catalyst
pretreatment environment and the nature of the catalyst–underlayer
interactions. Such concepts are critical for the rational design of
alternative catalyst–underlayer systems for efficient CNT synthetic
processes
Sulfur Vapor-Infiltrated 3D Carbon Nanotube Foam for Binder-Free High Areal Capacity Lithium–Sulfur Battery Composite Cathodes
Here,
we demonstrate a strategy to produce high areal loading and
areal capacity sulfur cathodes by using vapor-phase infiltration of
low-density carbon nanotube (CNT) foams preformed by solution processing
and freeze-drying. Vapor-phase capillary infiltration of sulfur into
preformed and binder-free low-density CNT foams leads to a mass loading
of ∼79 wt % arising from interior filling and coating of CNTs
with sulfur while preserving conductive CNT–CNT junctions that
sustain electrical accessibility through the thick foam. Sulfur cathodes
are then produced by mechanically compressing these foams into dense
composites (ρ > 0.2 g/cm<sup>3</sup>), revealing specific
capacity
of 1039 mAh/g<sub>S</sub> at 0.1 C, high sulfur areal loading of 19.1
mg/cm<sup>2</sup>, and high areal capacity of 19.3 mAh/cm<sup>2</sup>. This work highlights a technique broadly adaptable to a diverse
group of nanostructured building blocks where preformed low-density
materials can be vapor infiltrated with sulfur, mechanically compressed,
and exhibit simultaneous high areal and gravimetric storage properties.
This provides a route for scalable, low-cost, and high-energy density
sulfur cathodes based on conventional solid electrode processing routes
Roll-to-Roll Nanomanufacturing of Hybrid Nanostructures for Energy Storage Device Design
A key
limitation to the practical incorporation of nanostructured materials
into emerging applications is the challenge of achieving low-cost,
high throughput, and highly replicable scalable nanomanufacturing
techniques to produce functional materials. Here, we report a benchtop
roll-to-roll technique that builds upon the use of binary solutions
of nanomaterials and liquid electrophoretic assembly to rapidly construct
hybrid materials for battery design applications. We demonstrate surfactant-free
hybrid mixtures of carbon nanotubes, silicon nanoparticles, MoS<sub>2</sub> nanosheets, carbon nanohorns, and graphene nanoplatelets.
Roll-to-roll electrophoretic assembly from these solutions enables
the controlled fabrication of homogeneous coatings of these nanostructures
that maintain chemical and physical properties defined by the synergistic
combination of nanomaterials utilized without adverse effects of surfactants
or impurities that typically limit liquid nanomanufacturing routes.
To demonstrate the utility of this nanomanufacturing approach, we
employed roll-to-roll electrophoretic processing to fabricate both
positive and negative electrodes for lithium ion batteries in less
than 30 s. The optimized full-cell battery, containing active materials
of prelithiated silicon nanoparticles and MoS<sub>2</sub> nanosheets,
was assessed to exhibit energy densities of 167 Wh/kgcell<sup>–1</sup> and power densities of 9.6 kW/kgcell<sup>–1</sup>
Ultrafast Solvent-Assisted Sodium Ion Intercalation into Highly Crystalline Few-Layered Graphene
A maximum sodium capacity of ∼35
mAh/g has hampered the use of crystalline carbon nanostructures for
sodium ion battery anodes. We demonstrate that a diglyme solvent shell
encapsulating a sodium ion acts as a “nonstick” coating
to facilitate rapid ion insertion into crystalline few-layer graphene
and bypass slow desolvation kinetics. This yields storage capacities
above 150 mAh/g, cycling performance with negligible capacity fade
over 8000 cycles, and ∼100 mAh/g capacities maintained at currents
of 30 A/g (∼12 s charge). Raman spectroscopy elucidates the
ordered, but nondestructive cointercalation mechanism that differs
from desolvated ion intercalation processes. In situ Raman measurements
identify the Na<sup>+</sup> staging sequence and isolates Fermi energies
for the first and second stage ternary intercalation compounds at
∼0.8 eV and ∼1.2 eV
Anode-Free Sodium Battery through in Situ Plating of Sodium Metal
Sodium-ion
batteries (SIBs) have been pursued as a more cost-effective and more
sustainable alternative to lithium-ion batteries (LIBs), but these
advantages come at the expense of energy density. In this work, we
demonstrate that the challenge of energy density for sodium chemistries
can be overcome through an anode-free architecture enabled by the
use of a nanocarbon nucleation layer formed on Al current collectors.
Electrochemical studies show this configuration to provide highly
stable and efficient plating and stripping of sodium metal over a
range of currents up to 4 mA/cm<sup>2</sup>, sodium loading up to
12 mAh/cm<sup>2</sup>, and with long-term durability exceeding 1000
cycles at a current of 0.5 mA/cm<sup>2</sup>. Building upon this anode-free
architecture, we demonstrate a full cell using a presodiated pyrite
cathode to achieve energy densities of ∼400 Wh/kg, far surpassing
recent reports on SIBs and even the theoretical maximum for LIB technology
(387 Wh/kg for LiCoO<sub>2</sub>/graphite cells) while still relying
on naturally abundant raw materials and cost-effective aqueous processing
Role of Nitrogen-Doped Graphene for Improved High-Capacity Potassium Ion Battery Anodes
Potassium
is an earth abundant alternative to lithium for rechargeable
batteries, but a critical limitation in potassium ion battery anodes
is the low capacity of KC<sub>8</sub> graphite intercalation compounds
in comparison to conventional LiC<sub>6</sub>. Here we demonstrate
that nitrogen doping of few-layered graphene can increase the storage
capacity of potassium from a theoretical maximum of 278 mAh/g in graphite
to over 350 mAh/g, competitive with anode capacity in commercial lithium
ion batteries and the highest reported anode capacity so far for potassium
ion batteries. Control studies distinguish the importance of nitrogen
dopant sites as opposed to sp<sup>3</sup> carbon defect sites to achieve
the improved performance, which also enables >6× increase
in
rate performance of doped <i>vs</i> undoped materials. Finally, <i>in situ</i> Raman spectroscopy studies elucidate the staging
sequence for doped and undoped materials and demonstrate the mechanism
of the observed capacity enhancement to be correlated with distributed
storage at local nitrogen sites in a staged KC<sub>8</sub> compound.
This study demonstrates a pathway to overcome the limitations of graphitic
carbons for anodes in potassium ion batteries by atomically precise
engineering of nanomaterials