9 research outputs found
Origin of the Volume Contraction during Nanoporous Gold Formation by Dealloying for High-Performance Electrochemical Applications
Nanoporous metals
used in various electrochemical applications
including electrochemical actuators, electrocatalysts, supercapacitors,
and batteries exhibit an irreversible volume shrinkage during their
formation by dealloying, the origin of which remains obscure. Here
we use dilatometry techniques to measure the irreversible shrinkage
in nanoporous Au <i>in situ</i> during electrochemical dealloying.
A linear contraction up to ∼9% was recorded. To identify the
origin of this dimensional change, we borrow the time-dependent isothermal
shrinkage model from sintering theory, which we use to fit the dimensional
changes measured in our nanoporous Au during dealloying. This shrinkage
model suggests that bulk transport through plastic flow is the primary
mass transport mechanism responsible for the material contraction
in dealloying. Based on the current understanding of the mechanism
of porosity formation in dealloying, mass transport through surface
diffusion of undissolved materials is critical in the process. The
present work sheds new light in the sense that bulk transport through
plastic flow seems also to play an important role in dealloying
Fundamental Mechanisms of Solvent Decomposition Involved in Solid-Electrolyte Interphase Formation in Sodium Ion Batteries
Prolonged decomposition
of electrolytes forming a thick and unstable
solid-electrolyte interphase (SEI) continues to be a major bottleneck
in designing sodium-ion batteries (SIBs). We have carried out quantum
chemistry simulations to investigate the fundamental mechanisms of
reduction-induced decomposition of electrolyte solvents in the vicinity
of a sodium ion. Kinetics and thermodynamics of several reaction pathways
for one- and two-electron reduction of ethylene carbonate (EC) have
been examined. Our calculations indicate that the high reduction potential
and low barrier for the ring opening of EC is the main cause for the
continuous growth of SEI observed in SIBs. The impact of two well-known
electrolyte additives, vinyl carbonate (VC) and fluoroethylene carbonate
(FEC), on SEI composition was evaluated by studying decomposition
pathways of (1) VC and FEC molecules in the bulk EC solvent and (2)
an EC molecule in a supermolecular cluster comprising an EC and the
additive molecule. The additive molecules have significantly low barriers
for decomposition and therefore decompose first. Additionally, the
presence of an additive molecule was also shown to increase the barrier
for decomposition of EC. Another observation suggests that the preferred
reduction state of an EC molecule changes when it forms a dimer with
additive molecules, and these reduction states have different decomposition
pathways which leads to formation of different SEI compounds. On the
basis of these observations, we predict that not only do the additive
molecules protect solvent molecules from reductive decomposition but
also they can promote alternate pathways for the decomposition, leading
to qualitatively different and potentially stable SEI products
Using X‑ray Microscopy To Understand How Nanoporous Materials Can Be Used To Reduce the Large Volume Change in Alloy Anodes
Tin metal is an attractive
negative
electrode material to replace graphite in Li-ion batteries due to
its high energy density. However, tin undergoes a large volume change
upon alloying with Li, which pulverizes the particles, and ultimately
leads to short cycling lifetimes. Nevertheless, nanoporous materials
have been shown to extend battery life well past what is observed
in nonporous material. Despite the exciting potential of porous alloying
anodes to significantly increase the energy density in Li-ion batteries,
the fundamental physics of how nanoscale architectures accommodate
the electrochemically induced volume changes are poorly understood.
Here, operando transmission X-ray microscopy has been used to develop
an understanding of the mechanisms that govern the enhanced cycling
stability in nanoporous tin. We found that in comparison to dense
tin, nanoporous tin undergoes a 6-fold smaller areal expansion after
lithiation, as a result of the internal porosity and unique nanoscale
architecture. The expansion is also more gradual in nanoporous tin
compared to the dense material. The nanoscale resolution of the microscope
used in this study is ∼30 nm, which allowed us to directly
observe the pore structure during lithiation and delithiation. We
found that nanoporous tin remains porous during the first insertion
and desinsertion cycle. This observation is key, as fully closed pores
could lead to mechanical instability, electrolyte inaccessibility,
and short lifetimes. While tin was chosen for this study because of
its high X-ray contrast, the results of this work should be general
to other alloy-type systems, such as Si, that also suffer from volume
change based cycling degradation
Enhanced Strain in Functional Nanoporous Gold with a Dual Microscopic Length Scale Structure
We have synthesized nanoporous Au with a dual microscopic length scale by exploiting the crystal structure of the alloy precursor. The synthesized mesoscopic material is characterized by stacked Au layers of submicrometer thickness. In addition, each layer displays nanoporosity through the entire bulk. It is shown that the thickness of these layers can be tailored <i>via</i> the grain size of the alloy precursor. The two-length-scale structure enhances the functional properties of nanoporous gold, leading to charge-induced strains of amplitude up to 6%, which are roughly 2 orders of magnitude larger than in nanoporous Au with the standard one-length-scale porous morphology. A model is presented to describe these phenomena
pH-Controlled Dealloying Route to Hierarchical Bulk Nanoporous Zn Derived from Metastable Alloy for Hydrogen Generation by Hydrolysis of Zn in Neutral Water
Dealloyed
nanoporous metals made of very-reactive elements have rarely been
reported. Instead, reactive materials are used as sacrificial components
in dealloying. The high chemical reactivity of nonprecious nanostructured
metals makes them suitable for a broad range of applications such
as splitting water into H<sub>2</sub> gas and metal hydroxide. On
the other hand, the same high chemical reactivity hinders the synthesis
of nanostructured metals. Here we use a pH-controlled dealloying strategy
to fabricate bulk nanoporous Zn with bulk dimensions in the centimeter
range via the selective removal of Al from metastable face-centered
cubic bulk Zn<sub>20</sub>Al<sub>80</sub> at. % parent alloys. The
corresponding bulk nanoporous Zn exhibits a hierarchical ligament/pore
architecture characterized by primary ligaments and pores with an
average feature size in the submicrometer range. These primary structures
are made of ultrafine secondary ligaments and pores with a characteristic
feature size in the range of 10–20 nm. Our bulk nanoporous
Zn can split water into H<sub>2</sub> and ZnÂ(OH)<sub>2</sub> at ambient
temperature and pressure and continuously produce H<sub>2</sub> at
a constant rate of 0.08 mL/min per gram of Zn over 8 h. We anticipate
that in this hierarchical bulk architecture, the macropores facilitate
the flow of water in the bulk of the material, while the mesopores
and ultrafine ligaments provide a high surface area for the reaction
of water with Zn. The bulk nanoporous Zn/water system can be used
for on-board or on-demand H<sub>2</sub> applications, during which
H<sub>2</sub> is produced when needed, without prior storage of this
gas compressed in cylinders as it is currently the case
pH-Controlled Dealloying Route to Hierarchical Bulk Nanoporous Zn Derived from Metastable Alloy for Hydrogen Generation by Hydrolysis of Zn in Neutral Water
Dealloyed
nanoporous metals made of very-reactive elements have rarely been
reported. Instead, reactive materials are used as sacrificial components
in dealloying. The high chemical reactivity of nonprecious nanostructured
metals makes them suitable for a broad range of applications such
as splitting water into H<sub>2</sub> gas and metal hydroxide. On
the other hand, the same high chemical reactivity hinders the synthesis
of nanostructured metals. Here we use a pH-controlled dealloying strategy
to fabricate bulk nanoporous Zn with bulk dimensions in the centimeter
range via the selective removal of Al from metastable face-centered
cubic bulk Zn<sub>20</sub>Al<sub>80</sub> at. % parent alloys. The
corresponding bulk nanoporous Zn exhibits a hierarchical ligament/pore
architecture characterized by primary ligaments and pores with an
average feature size in the submicrometer range. These primary structures
are made of ultrafine secondary ligaments and pores with a characteristic
feature size in the range of 10–20 nm. Our bulk nanoporous
Zn can split water into H<sub>2</sub> and ZnÂ(OH)<sub>2</sub> at ambient
temperature and pressure and continuously produce H<sub>2</sub> at
a constant rate of 0.08 mL/min per gram of Zn over 8 h. We anticipate
that in this hierarchical bulk architecture, the macropores facilitate
the flow of water in the bulk of the material, while the mesopores
and ultrafine ligaments provide a high surface area for the reaction
of water with Zn. The bulk nanoporous Zn/water system can be used
for on-board or on-demand H<sub>2</sub> applications, during which
H<sub>2</sub> is produced when needed, without prior storage of this
gas compressed in cylinders as it is currently the case
pH-Controlled Dealloying Route to Hierarchical Bulk Nanoporous Zn Derived from Metastable Alloy for Hydrogen Generation by Hydrolysis of Zn in Neutral Water
Dealloyed
nanoporous metals made of very-reactive elements have rarely been
reported. Instead, reactive materials are used as sacrificial components
in dealloying. The high chemical reactivity of nonprecious nanostructured
metals makes them suitable for a broad range of applications such
as splitting water into H<sub>2</sub> gas and metal hydroxide. On
the other hand, the same high chemical reactivity hinders the synthesis
of nanostructured metals. Here we use a pH-controlled dealloying strategy
to fabricate bulk nanoporous Zn with bulk dimensions in the centimeter
range via the selective removal of Al from metastable face-centered
cubic bulk Zn<sub>20</sub>Al<sub>80</sub> at. % parent alloys. The
corresponding bulk nanoporous Zn exhibits a hierarchical ligament/pore
architecture characterized by primary ligaments and pores with an
average feature size in the submicrometer range. These primary structures
are made of ultrafine secondary ligaments and pores with a characteristic
feature size in the range of 10–20 nm. Our bulk nanoporous
Zn can split water into H<sub>2</sub> and ZnÂ(OH)<sub>2</sub> at ambient
temperature and pressure and continuously produce H<sub>2</sub> at
a constant rate of 0.08 mL/min per gram of Zn over 8 h. We anticipate
that in this hierarchical bulk architecture, the macropores facilitate
the flow of water in the bulk of the material, while the mesopores
and ultrafine ligaments provide a high surface area for the reaction
of water with Zn. The bulk nanoporous Zn/water system can be used
for on-board or on-demand H<sub>2</sub> applications, during which
H<sub>2</sub> is produced when needed, without prior storage of this
gas compressed in cylinders as it is currently the case
Tuning Porosity and Surface Area in Mesoporous Silicon for Application in Li-Ion Battery Electrodes
This
work aims to improve the poor cycle lifetime of silicon-based
anodes for Li-ion batteries by tuning microstructural parameters such
as pore size, pore volume, and specific surface area in chemically
synthesized mesoporous silicon. Here we have specifically produced
two different mesoporous silicon samples from the magnesiothermic
reduction of ordered mesoporous silica in either argon or forming
gas. In situ X-ray diffraction studies indicate that samples made
in Ar proceed through a Mg<sub>2</sub>Si intermediate, and this results
in samples with larger pores (diameter ≈ 90 nm), modest total
porosity (34%), and modest specific surface area (50 m<sup>2</sup> g<sup>–1</sup>). Reduction in forming gas, by contrast, results
in direct conversion of silica to silicon, and this produces samples
with smaller pores (diameter ≈ 40 nm), higher porosity (41%),
and a larger specific surface area (70 m<sup>2</sup> g<sup>–1</sup>). The material with smaller pores outperforms the one with larger
pores, delivering a capacity of 1121 mAh g<sup>–1</sup> at
10 A g<sup>–1</sup> and retains 1292 mAh g<sup>–1</sup> at 5 A g<sup>–1</sup> after 500 cycles. For comparison, the
sample with larger pores delivers a capacity of 731 mAh g<sup>–1</sup> at 10 A g<sup>–1</sup> and retains 845 mAh g<sup>–1</sup> at 5 A g<sup>–1</sup> after 500 cycles. The dependence of
capacity retention and charge storage kinetics on the nanoscale architecture
clearly suggests that these microstructural parameters significantly
impact the performance of mesoporous alloy type anodes. Our work is
therefore expected to contribute to the design and synthesis of optimal
mesoporous architectures for advanced Li-ion battery anodes
Nanoporous Tin with a Granular Hierarchical Ligament Morphology as a Highly Stable Li-Ion Battery Anode
Next generation Li-ion
batteries will require negative electrode materials with energy densities
many-fold higher than that found in the graphitic carbon currently
used in commercial Li-ion batteries. While various nanostructured
alloying-type anode materials may satisfy that requirement, such materials
do not always exhibit long cycle lifetimes and/or their processing
routes are not always suitable for large-scale synthesis. Here, we
report on a high-performance anode material for next generation Li-ion
batteries made of nanoporous Sn powders with hierarchical ligament
morphology. This material system combines both long cycle lifetimes
(more than 72% capacity retention after 350 cycles), high capacity
(693 mAh/g, nearly twice that of commercial graphitic carbon), good
charging/discharging capabilities (545 mAh/g at 1 A/g, 1.5C), and
a scalable processing route that involves selective alloy corrosion.
The good cycling performance of this system is attributed to its nanoporous
architecture and its unique hierarchical ligament morphology, which
accommodates the large volume changes taking place during lithiation,
as confirmed by synchrotron-based ex-situ X-ray 3D tomography analysis.
Our findings are an important step for the development of high-performance
Li-ion batteries