Jong Kahk Keum

PS‐b‐P3HT Copolymers as P3HT/PCBM Interfacial Compatibilizers for High Efficiency Photovoltaics

A conducting diblock copolymer of PS-b-P3HT was added to serve as a compatibilizer in a P3HT/PCBM blend, which improved the power-conversion efficiency from 3.3% to 4.1% due to the enhanced crystallinity, morphology, interface interaction, and depth profile of PCBM.

Li2OHCl crystalline electrolyte for stable metallic lithium anodes

In a classic example of stability from instability, we show that Li2OHCl solid electrolyte forms a stable solid electrolyte interphase (SEI) layer with a metallic lithium anode. The Li2OHCl solid electrolyte can be readily achieved through simple mixing of LiOH and LiCl precursors at a mild processing temperature <400 °C. Additionally, we show that continuous, dense Li2OHCl membranes can be fabricated at temperatures <400 °C, standing in great contrast to current processing temperatures of >1600 °C for most oxide-based solid electrolytes. The ionic conductivity and Arrhenius activation energy were explored for the LiOH-LiCl system of crystalline solid electrolytes, where Li2OHCl with increased crystal defects was found to have the highest ionic conductivity and reasonable Arrhenius activation energy. The Li2OHCl solid electrolyte displays stability against metallic lithium, even in extreme conditions past the melting point of lithium metal. To understand this excellent stability, we show that SEI formation is critical in stabilizing the interface between metallic lithium and the Li2OHCl solid electrolyte.

An Air‐Stable Na3SbS4 Superionic Conductor Prepared by a Rapid and Economic Synthetic Procedure

All-solid-state sodium batteries, using solid electrolyte and abundant sodium resources, show great promise for safe, low-cost, and large-scale energy storage applications. The exploration of novel solid electrolytes is critical for the room temperature operation of all-solid-state Na batteries. An ideal solid electrolyte must have high ionic conductivity, hold outstanding chemical and electrochemical stability, and employ low-cost synthetic methods. Achieving the combination of these properties is a grand challenge for the synthesis of sulfide-based solid electrolytes. Design of the solid electrolyte Na3 SbS4 is described, realizing excellent air stability and an economic synthesis based on hard and soft acid and base (HSAB) theory. This new solid electrolyte also exhibits a remarkably high ionic conductivity of 1u2005mSu2009cm(-1) at 25u2009°C and ideal compatibility with a metallic sodium anode.

High conducting oxide--sulfide composite lithium superionic conductor

A solid electrolyte for a lithium-sulfur battery includes particles of a lithium ion conducting oxide composition embedded within a lithium ion conducting sulfide composition. The lithium ion conducting oxide composition can be Li7La3Zr2O12 (LLZO). The lithium ion conducting sulfide composition can be β-Li3PS4 (LPS). A lithium ion battery and a method of making a solid electrolyte for a lithium ion battery are also disclosed.

Strong and Electrically Conductive Graphene-Based Composite Fibers and Laminates

Graphene is an ideal candidate for lightweight, high-strength composite materials given its superior mechanical properties (specific strength of 130 GPa and stiffness of 1 TPa). To date, easily scalable graphene-like materials in a form of separated flakes (exfoliated graphene, graphene oxide, and reduced graphene oxide) have been investigated as candidates for large-scale applications such as material reinforcement. These graphene-like materials do not fully exhibit all the capabilities of graphene in composite materials. In the current study, we show that macro (2 inch × 2 inch) graphene laminates and fibers can be produced using large continuous sheets of single-layer graphene grown by chemical vapor deposition. The resulting composite structures have potential to outperform the current state-of-the-art composite materials in both mechanical properties and electrical conductivities (>8 S/cm with only 0.13% volumetric graphene loading and 5 × 10(3) S/cm for pure graphene fibers) with estimated graphene contributions of >10 GPa in strength and 1 TPa in stiffness.

Morphological origin for the stratification of P3HT:PCBM blend film studied by neutron reflectometry

Understanding the origin for the film stratification of electron donor/acceptor blend is crucial for high efficiency organic photovoltaic cell. In this study, P3HT:PCBM blend is deposited onto hydrophilic and hydrophobic substrate to examine the film stratifications. The neutron reflectivity results show that, on the different surfaces, PCBM diffuses toward the two interfacial regions in an identical fashion during thermal annealing. This evidences that the film stratification is not affected by the substrates. Instead, since P3HT remains more amorphous in the interfacial regions and PCBM is miscible with amorphous P3HT, PCBM preferentially diffuses to the interfacial regions, resulting in the stratification.

One-Step Synthesis of Nb2O5/C/Nb2C (MXene) Composites and Their Use as Photocatalysts for Hydrogen Evolution

Hydrogen production through facile photocatalytic water splitting is regarded as a promising strategy to solve global energy problems. Transition-metal carbides (MXenes) have recently drawn attention as potential co-catalyst candidates for photocatalysts. Here, we report niobium pentoxide/carbon/niobium carbide (MXene) hybrid materials (Nb2 O5 /C/Nb2 C) as photocatalysts for hydrogen evolution from water splitting. The Nb2 O5 /C/Nb2 C composites were synthesized by one-step CO2 oxidation of Nb2 CTx . Nb2 O5 grew homogeneously on Nb2 C after mild oxidation, during which some amorphous carbon was also formed. With an optimized oxidation time of 1.0u2005h, Nb2 O5 /C/Nb2 C showed the highest hydrogen generation rate (7.81u2005μmolu2009h-1 u2009gcat-1 ), a value that was four times higher than that of pure Nb2 O5 . The enhanced performance of Nb2 O5 /C/Nb2 C was attributed to intimate contact between Nb2 O5 and conductive Nb2 C and the separation of photogenerated charge carriers at the Nb2 O5 /Nb2 C interface; the results presented herein show that transition-metal carbide are promising co-catalysts for photocatalytic hydrogen production.

Exploring Anomalous Polarization Dynamics in Organometallic Halide Perovskites

Organometallic halide perovskites (OMHPs) have attracted broad attention as prospective materials for optoelectronic applications. Among the many anomalous properties of these materials, of special interest are the ferroelectric properties including both classical and relaxor-like components, as a potential origin of slow dynamics, field enhancement, and anomalous mobilities. Here, ferroelectric properties of the three representative OMHPs are explored, including FAPbx Sn1-x I3 (x = 0, x = 0.85) and FA0.85 MA0.15 PbI3 using band excitation piezoresponse force microscopy and contact mode Kelvin probe force microscopy, providing insight into long- and short-range dipole and charge dynamics in these materials and probing ferroelectric density of states. Furthermore, second-harmonic generation in thin films of OMHPs is observed, providing a direct information on the noncentrosymmetric polarization in such materials. Overall, the data provide strong evidence for the presence of ferroelectric domains in these systems; however, the domain dynamics is suppressed by fast ion dynamics. These materials hence present the limit of ferroelectric materials with spontaneous polarization dynamically screened by ionic and electronic carriers.

Nanostructure-Driven Ion Transport in PCBM-Based Polymer Electrolytes

Nanostructure-Driven Ion Transport in PCBM-Based Polymer Electrolytes Che-Nan Sun1, Thomas A. Zawodzinski1,2, Fei Ren3, Jong Kahk Keum1 and Jihua Chen1, (1)Oak Ridge National Laboratory, (2)The University of Tennessee, (3)Temple University Polyethylene oxide or PEO is an extensively-examined candidate for solid polymer electrolyte materials of lithium ion batteries, and its composite electrolytes has promising ion conductivities.[1-3] Oxide nanoparticles with sizes of 5-10 nm are often introduced into these polymer-based composite electrolytes in order to suppress their room-temperature crystallite formation.1-9 The size, geometry and surface functionality of the added particles were known to largely affect the structure and performance of the blended electrolytes.5,10 In this study, we examined a functionalized-fullerene-based composite electrolytes, providing details in their self-assembled nanostructures, modulus, hardness, as well as temperature-dependent ion-conducting behaviors. To the best of our knowledge, no fullerene-based, lithium conducting, composite electrolyte has been reported previously. Herein we used a bench-mark fullerene derivative, phenyl-C61-butyric acid methyl ester (PCBM) as a model fullerene compound and performed impedance spectroscopy, equivalent circuit modeling, nanoscale elemental mapping (in transmission electron microscope), wide-angle X-ray diffraction, as well as nanoindentation to shed light on a 6-fold enhancement in low temperature (less than 50oC) ion conductivity of PEO - lithium bis(trifluoromethanesulfonyl) imide (LiTFSI)-PCBMmorexa0» electrolytes, along with the underlying changes in nanomorphology , mechanical properties, and crystal structures. Based on a previous density functional theory (DFT) calculation, 11 the interaction energies Ei among PEO polymers is estimated to be 2.58 kcal mol-1 per monomer, the Ei between PCBM and PEO is 3.50 kcal mol-1 per monomer (PCBM is taken as 1 repeat unit), and the Ei among PCBMs themselves is 6.01 kcal mol-1 per monomer. This explains that at very low PCBM weight percentage, without sufficient PCBM-PCBM contacts, it is more energetically favorable for fullerenes to disperse into PEO matrix. However, with higher PCBM concentration, the fullerenes will efficiently pack with each other into domains with gradually increased dimensions. Quantification of PCBM domains is performed by line scan analysis of energy filtered TEM (EFTEM) images. Upon the addition of PCBM, the average domain sizes gradually increase from 3.4 1 nm ( 0% PCBM), to 4.6 1 nm (10% PCBM) and 4.9 2 nm (20% PCBM), and finally to 7.5 5 nm (40% PCBM ). (A precise determination of PCBM domain dimension is not possible when the domain size is less than 3 nm, due to the lack of EFTEM contrast in these samples). We attribute the observed ion conductivity improvement to those nanomorphological variation in PCBM-PEO-LiTFSi systems.«xa0less