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 H₂ 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 Zn₂₀Al₈₀ 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 H₂ and Zn­(OH)₂ at ambient temperature and pressure and continuously produce H₂ 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 H₂ applications, during which H₂ 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 H₂ 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 Zn₂₀Al₈₀ 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 H₂ and Zn­(OH)₂ at ambient temperature and pressure and continuously produce H₂ 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 H₂ applications, during which H₂ 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 H₂ 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 Zn₂₀Al₈₀ 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 H₂ and Zn­(OH)₂ at ambient temperature and pressure and continuously produce H₂ 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 H₂ applications, during which H₂ 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 Mg₂Si intermediate, and this results in samples with larger pores (diameter ≈ 90 nm), modest total porosity (34%), and modest specific surface area (50 m² g^–1). 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 m² g^–1). The material with smaller pores outperforms the one with larger pores, delivering a capacity of 1121 mAh g^–1 at 10 A g^–1 and retains 1292 mAh g^–1 at 5 A g^–1 after 500 cycles. For comparison, the sample with larger pores delivers a capacity of 731 mAh g^–1 at 10 A g^–1 and retains 845 mAh g^–1 at 5 A g^–1 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