Yinghua Niu

An Energy Investigation into 1D/2D Oriented‐Attachment Assemblies of 1D Ag Nanocrystals

In the field of oriented-attachment crystal growth, one-dimensional nanocrystals are frequently employed as building blocks to synthesize two-dimensional or large-aspect-ratio one-dimensional nanocrystals. Despite recent extensive experimental advances, the underlying inter-particle interaction in the synthesis still remains elusive. In this report, using Ag as a platform, we investigate the van der Waals interactions associated with the side-by-side and end-to-end assemblies of one-dimensional nanorods. The size, aspect ratio, and inter-particle separation of the Ag precursor nanorods are found to have dramatically different impacts on the van der Waals interactions in the two types of assemblies. Our work facilitates the fundamental understanding of the oriented-attachment assembling mechanism based on one-dimensional nanocrystals.

Oriented-attachment dimensionality build-up via van der Waals interaction

In this study, molecular static calculation is carried out to evaluate the van der Waals interaction (vdW) associated with different oriented attachment (OA) growth systems involving 1D nanorods (NRs), 2D nanoplates (NPts) and 3D nanostructures (NSts) for the first time. Our results show that the vdW is, to a large extent, determined by the attaching area at all OA growth stages of nanocrystals. The vdW increases significantly as the OA growth varies from 1D NR–NP, end-to-end NR–NR assemblies to 2D side-by-side NR–NR/3D NPt–NPt assemblies. Our study reveals the fundamental details in vdW, one of the governing inter-particle interactions involved in OA growth of NCs, and facilitates the analytical understanding of the OA growth thermodynamics.

Space matters: Li+ conduction versus strain effect at FePO4/LiFePO4 interface

FePO4/LiFePO4 (FP/LFP) interfacial strain, giving rise to substantial variation in interfacial energy and lattice volume, is inevitable in the (de)lithiation process of LiFePO4, a prototype of Li ion batterycathodes. Extensive theoretical and experimental research has been focused on the effect of lattice strain energy on FP/LFP interface propagation orientation and cyclic stability of the electrode. However, the essential effect of strain induced lattice distortion on Li+ transport at the FP/LFP interface is typically overlooked. In this report, a coherent interface model is derived to evaluate quantitatively the correlation between FP/LFP lattice distortion and Li+conduction. The results illustrate that the effect of lattice strain on Li+conduction depends strongly on FP/LFP interface orientations. Lattice strain induces a 90% decrease of Li+conductivity in ac-plane oriented (de)lithiation at room temperature. The opposite effect of lattice strain on delithiation and lithiation for ab- and bc-orientations is elucidated. In addition, the effect of lattice strain tends to be more pronounced at a lower working temperature. This study provides an efficient platform to comprehend and manipulate Li+conduction in the charge and discharge of lithium ion batteries, the large-scale application of which is frequently challenged by limited in-cell ion conduction.

Assembly of anisotropic one dimensional Ag nanostructures through orientated attachment: on-axis or off-axis growth?

Recently it has been found that one dimensional (1D) Ag nano-structures can be synthesized through oriented attachment (OA) growth. In OA growth of 1D structures, nanoparticles (NPs) attach to the growing nanorods (NRs) via either on-axis or off-axis attachment. However, the thermodynamic basis for understanding the preference of each growth mode has remained unexplored until now. In this paper, molecular static calculations are performed to investigate the van der Waals interactions (vdW) in both on-axis and off-axis attachments of 1D Ag nano-structures. The correlation of parameters including the size, aspect ratio (AR), crystalline orientation of NR, the inter-particle separation and the off-axis approaching angle, with both OA attachments is investigated in detail. The results show that off-axis attachment is thermodynamically favorable compared to on-axis attachment in a typical OA growth, and straight on-axis OA growth are typically realized by tuning the other aspects of an OA growth system. Interestingly, it is found that the off-axis growth is both precursor-size dependent and crystalline-orientation dependent.

Gas leak diffusion induced polarization in submicro/nanoscale non-tight electrolytes of solid oxide fuel cells

Solid oxide fuel cells with submicro/nanoscale electrolytes (μSOFCs) are attracting increasing attention since the ohmic energy loss arising from an ion-resistive electrolyte decreases significantly with decreasing thickness of the electrolyte interlayers. However, gas leak diffusion can be induced due to increasing microstructural flaws such as cracks and pinholes in thin electrolytes. Evaluation of the effects of gas leak diffusion through electrolyte on cell performance is thus an urgent demand. In this work, the effect of gas leak diffusion on concentration polarizations (CPs) is investigated quantitatively for both anodes and cathodes of SOFCs under various operating conditions. The results show that gas leak diffusion through electrolyte typically induces dominant cathode CP. The direct reaction of leaked H2 and O2 correlates has a large impact on both anode and cathode CP induced by gas leak diffusion. Lowering the operating temperature decreases CP induced by gas leak diffusion. Our work provides a quantitative model to evaluate the impact of gas leak diffusion in electrolytes on SOFC performance and facilitates the rational design of high performance μSOFCs.

Initial-stage oriented-attachment one-dimensional assembly of nanocrystals: fundamental insight with a collision–recrystallization model

Oriented attachment (OA) growth has been a promising method for the synthesis of one dimensional (1D) anisotropic nanocrystals (NCs). An unresolved fundamental issue is to understand the growth mechanism at the initial stage of an OA nanorod (NR) growth. In this report, a collision–recrystallization model is proposed to investigate the initial OA growth of NRs. The repulsive electrical double layer (EDL) interaction and the attractive van der Waals (vdW) interaction at the initial OA stage are derived by the accurate surface element integration (SEI) technique and the classical Hamaker equation, respectively. Our results show that the self-recrystallization of nanochains increases the collision activation energy of NPs dramatically as their surface potentials and Hamaker constants increase. Under a specific electrolyte concentration, the collision activation energy reaches the maximum, indicating that the growth rate of OA can be controlled by adjusting the electrolyte concentration.

Reduced electrochemical performances of proton exchange membrane fuel cells due to gaseous diffusion in electrolytes

In this report, we derive a ‘gas leak mold’ model to investigate gas diffusion in the electrolyte membranes of proton exchange membrane fuel cells. The electrochemical parameters of proton exchange membrane fuel cells, including limiting current density and concentration polarization are studied systematically. In particular, we investigate the correlation between fuel cell performance and diffusion-induced gas leak of proton exchange membranes. This work is expected to facilitate the development of highly-efficient proton exchange membrane fuel cells.

Oriented-Attachment Nanocrystals in Lithium Ion Batteries

The world has entered an era with an increasing demand for new energy, including wind, tide, solar, and geothermal energy, due to the prompt reduction of fossil-based energy. But these new energies are subject to the restrictions of space and time. To address these issues, renewable energy storage devices have been developed, such as the lithium ion batteries (LIBs).

Oriented-Attachment Nanocrystals in Supercapacitors

Supercapacitors are the energy storage devices with high power density (1–10 kW kg−1), long lifetime (500,000–1,000,000 cycles), fast charging (with seconds), a wide range of operation temperatures (−40 to 70 °C), and high safety.

Oriented-Attachment Nanocrystals in Solar Cells

Solar cells convert solar energy into electricity directly. The first-generation solar cells, which are silicon-based photovoltaic devices are efficient but costly. Although second-generation solar cells are cost-effective, the conversion efficiency is not desirable. Now, people are focusing on the third-generation solar cells with low cost, highly efficiency, and non-toxicity.