Search

Author(s): Xu, J; Ling, M; Terborg, L; Zhao, H; Qiu, F; Urban, JJ; Kostecki, R; Liu, G; Tong, W | Abstract: Si-Sn-C nanocomposites were synthesized via a facile mechanical milling method. Phase composition and morphologies of the asproduced Si-Sn-C nanocomposites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. Both XRD and Raman studies revealed the amorphization of Si after mechanical milling process, while Sn remained as the crystalline phase in both Si-Sn and Si-Sn-C nanocomposites. Meanwhile, the particle size was significantly reduced, but Si tended to agglomerate during the milling and it was alleviated through the addition of carbon. The galvanostatic charge/discharge measurements were carried out to evaluate the electrochemical performance. Compared to milled Si, Si-Sn nanocomposites, Si-Sn-C nanocomposites exhibited a higher initial capacity of ∼1000 mAh/g, and its capacity was retained at ∼80% after 50 cycles. The possible buffering effect of Sn and carbon at different operating potentials during the lithiation/delithiation process was discussed.

In this talk, we present research that aims to identify the specific pathways of heat and charge transport in complex hybrid polymer-nanowire thermoelectric systems. These materials have demonstrated remarkable performance due to interfacial charge transport dynamics that have proven difficult to untangle. Here I present the work of a few recent papers to disaggregate the role of the organic and inorganic layer in the transport of charge and heat, as well as the mechanism that causes an enhancement in thermoelectric performance. We also assess the role of specific features of the polymer and nanowire themselves, and how those contribute to transport. We do this at both the single nanowire level, and also at the film level, enabling meaningful comparisons and conclusions. Through combining thermoelectric property measurements using a custom-built suspended microbridge setup for single nanowires, the Kang-Snyder charge transport model as well as gate modulated electrical conductivity measurements, we conclude that the thin organic PEDOT:PSS shell is the predominant channel for charge carrier transport in this hybrid material, and the large electrical conductivity is attributed to the highly ordered PEDOT chains on the Te nanowire surface brought about by a templating effect. We also present complementary research on charge transport characteristics of modified PEDOT:PSS derivatives as well. This talk will summarize the work of our 4 most recent manuscripts from the last few months.

Heat and charge are fundamental energy carrying modes relevant to nearly all dynamic properties of interest in engineering, physics, and modern energy technologies. Here, I will discuss the design and understanding of transport properties in complex nanoscale systems where the dimensional constraints are changed systematically to reveal fundamental principles of heat and charge transport. The first part of this talk will focus on heat and charge transport in polymeric and hybrid nanomaterials, revealing the physical origins of scaling laws for charge transport in these materials. The second portion of my talk will discuss heat and charge transport in a different class of hybrid materials, the ever-popular perovskites. Here, I will briefly discuss possible origins for the ultra-low thermal conductivity observed in nanoscale samples, and new measurement approaches for resistive bulk perovskites. The final portion of my talk will briefly highlight strong deviations from the Wiedemann-Franz law in materials with electronic phase transitions, and possible applications of this phenomenology for novel devices exhibiting, for example, thermal rectification and thermal memristance. Selected references: 1. Organic electronics: one model to rule them all, Nature Materials (2017) 2. Polymer morphology dominates over energy-dependent scattering in inorganic-organic hybrid thermoelectrics, Nature Communications (2018) 3. Ultralow thermal conductivity in all inorganic halide perovskites, PNAS (2017) 4. Phonon dispersion lifetimes of organic-inorganic hybrid perovskite CH3NH3PbI3 from Inelastic X-ray scattering (2018, in review). 5. Anomalously low electronic thermal conductivity in metallic vanadium dioxide, Science (2017) 6. Solid-state thermal rectification and negative differential thermal resistance using junctions of phase transition materials, Phys. Rev. Applied (2018)

New methods for scalable water desalination has been identified as the most impactful and necessary scientific technology for human prosperity in a 2015 report by the Institute for Globally Transformative Technologies, surpassing malaria vaccines, synthetic fertilizers, and improved TB drugs. Due to the climate change, rapidly expanding global population, and worldwide water scarcity, the demand for purification of non-traditional sources such as brackish and seawater with low environmental impacts is extensively growing. There is an increasingly urgent need for low-cost methods for water desalination that can effectively couple to, or piggyback on, renewable energy sources. In this talk I will describe our efforts in developing new materials to enhance the efficacy and utility of forward osmosis using new plasmonically-enhanced hybrid materials. Classical membrane-based water separation and purification processes such as forward osmosis (FO) is gaining renewed attention as a potentially low-energy alternative technique to energy intensive methods such as distillation, reverse osmosis (RO), multiple effect distillation (MED), etc. Unlike RO, the industry standard for desalination, where energy intensive hydraulic/external pressure drives water flow, here an internal pressure (osmotic) drives flow. It has the characteristics of no/low pressure operation compared to standard RO processes. Consequently, membrane fouling is potentially much less severe and thinner membranes can be used. These advantages make FO extensively applied to a variety of fields to date, which could potentially relieve the stress of freshwater scarcity. As the draw solution is one of the main contributing factors to the FO process, the lack of suitable draw agent has become the obstacle jeopardizing the real application of FO in desalination process. In general, an ideal draw agent needs to provide high osmotic pressure and expel water from itself with minimal energy input. Present FO technology relies on either non-reusable draw agents or draw agents that require complicated process for regeneration. This talk will discuss development of new classes of inorganic/organic hybrid draw agents, which exhibit high osmolalities and easy regeneration process by utilizing industrial waste heat or naturally abundant (solar, geothermal) sources. Results on the design, performance, and integration of these new draw agents will be discussed.

Converting CO2 into hydrocarbons or other liquid fuels and leveraging solar energy to do so are of great importance as an alternative energy source and as a potential pathway to slow climate change. However, CO2 reduction reactions suffer from high overpotentials and poor selectivity. Recently, localized surface plasmon resonance (LSPR) was suggested as a way to overcome these challenging barriers [1-3]. Nanostructured catalysts absorbing visible light at their plasmon frequency can excite hot carriers which can be transferred directly or indirectly into the molecular orbitals of adsorbates. This unique charge injection mechanism along with the ability to tune the plasmon frequency over a wide range of energies may allow plasmonic photocatalysts to unlock a new, unexplored CO2 reduction pathway resulting in increased selectivity and lower overpotentials. Silver (Ag) is a strong plasmonic material and well-known for selectively producing CO [4,5]. In the hopes of taking advantage of the plasmonic properties to make more reduced products on Ag, we have developed Ag nanopyramid arrays, resulting in strong plasmonic and catalytic behaviors. Photocurrent measurements were performed on the plasmonic nanopyramid catalysts in aqueousNaClO4 saturated with either CO2 or Ar. The photocurrent is greatly enhanced in the presence of CO2, and shows ‘resonant-like’ behavior at a specific potential (-1.1 V vs. RHE). In Ar-saturated electrolyte, the photocurrent was drastically reduced and no characteristic feature of photocurrent was observed. This result indicates that the hot carriers created by LSPR react specifically with CO2 molecules or other CO2 reduction intermediates. By measuring incident photon to current efficiency (IPCE), we find that the hot-carrier transfer mechanism can be modulated as a function of applied potentials. Further, we discuss the differences in product distributions measured with a gas chromatograph (GC) and high performance liquid chromatography (HPLC) under light illumination and in the dark. References [1] A. O. Govorov, et. al., Nano Today 9, 85 (2014). [2] R. Sundararaman, et.al., Nature Comm. 5, 5788 (2014). [3] S. Mukherjee, et.al., J. Am. Chem. Soc. 136, 64 (2014). [4] R. Kostecki, et. al., J. Appl. Electrochem. 23, 567 (1993). [5] T. Hatsukade, et. al., Phys. Chem. Chem. Phys. 16, 13814 (2014).

Energy storage systems are confronted by a paradox: high-energy density materials are, by definition, metastable and able to release energy. However, technological advances require robust, reliable systems to store and deliver energy which mitigates safety concerns. In this talk, I focus on solving this dual challenge of storage vs. stability in a prototype system - nanoscale magnesium nanocrystals embedded in a graphene matrix for high density, stable, hydrogen storage for fuel cell systems. This work (reference below) was recently published in Nature Comms. Storage of hydrogen on board vehicles is one of the critical enabling technologies for creating hydrogen- fueled transportation systems that can reduce oil dependency and mitigate the long-term effects of fossil fuels on climate change. Stakeholders in developing hydrogen infrastructure (e.g., state governments, automotive OEMs, and industrial gas suppliers) are currently focused on high-pressure storage at 350 bar and 700 bar, in part because no viable solid-phase storage material has emerged. Nevertheless, solid-state materials remain of interest because of their unique potential to meet all DOE/FCTO targets and deliver hydrogen at lower pressures and higher on-board densities. A successful solution would significantly reduce costs and ensure the economic viability of a U.S. hydrogen infrastructure. In this talk, I present work highlighting advances in the performance and viability of nanoscale metal hydrides (Cho et al. Nature Communications, 2016) by using graphene-based materials as both hydrogen-specific barrier materials and catalytic functional groups to enhance the performance of nanoscale magnesium hydride. Historically, hydrides have largely been abandoned because of oxidative instability and sluggish kinetics. In this work, we report a new, environmentally stable hydrogen storage material constructed of Mg nanocrystals encapsulated by atomically thin and gas-selective reduced graphene oxide (rGO) sheets. This material, protected from oxygen and moisture by the rGO layers, exhibits exceptionally dense hydrogen storage surpassing that of compressed hydrogen (we report 6.5wt% and 0.105kg H2 per litre in the total composite). As rGO is atomically thin, this approach minimizes inactive mass in the composite, while also providing a kinetic enhancement to hydrogen sorption performance. These multilaminates of rGO-Mg are able to deliver exceptionally dense hydrogen storage and provide a material platform for harnessing the attributes of sensitive nanomaterials in demanding environments and have implications for other classes of energy storage devices. I will also discuss future and ongoing work using chemically synthesized versions of graphene nanobelts to co-localize dopant moieties on the surfaces of metal hydrides using a related materials architecture. Reference: “Graphene oxide/metal nanocrystal laminates: the atomic limit for safe, selective hydrogen storage”, Eun Seon Cho, Anne M. Ruminski, Shaul Aloni, and *Jeffrey J. Urban , Nature Communications (2016)

Using inputs of only sunlight, electricity, carbon dioxide, and water, photocatalytic CO2 reduction could mitigate the effects of climate change caused by the burning of fossil fuels and produce valuable fuels or chemical feedstocks. However, current CO2 reduction technologies suffer from high overpotentials and low selectivity, producing a mixture of carbon monoxide, methane, ethylene, formate, methanol, and others. Plasmon-assisted photocatalytic CO2 reduction leads to greater selectivity and lower overpotentials by unlocking unique mechanistic pathways. It has been well-documented that the strongly localized near fields at the surface of plasmonic nanoparticles promote electron-hole pair generation in nearby semiconductors.1–3 Plasmon decay generates an excited “hot” electron which can be transferred to a surface molecule for direct reduction or injected into an adjacent wide band gap catalyst, effectively limiting carrier recombination through charge separation and expanding the usable portion of the solar spectrum.4–7 The electron dynamics in an irradiated plasmonic nanoparticle can alter the electronic coupling with surface adsorbed CO2 and reaction intermediates, thereby changing the binding energy of these species and the catalytic properties of the plasmonic metals. The wide tunability of the plasmon resonance frequency with shape, size, and material confers fine control over these catalytic mechanisms, allowing for optimization of the photocatalytic performance. To take advantage of plasmonic catalysis, nano-sized spheres, cubes, and wires of gold, silver, and copper have been synthesized as these materials exhibit strong plasmonic and electrocatalytic behavior. For the semiconducting component of the photoelectrodes, nanostructured TiO2 co-catalysts have also been prepared. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to determine the shape and size distribution of the nanoparticles. X-ray diffraction (XRD) and optical spectroscopy were employed to characterize the electrodes. The CO2 reduction ability of the plasmonic metal nanoparticles with and without a semiconductor co-catalyst has been tested in a custom glass photoelectrochemical cell coupled to a light source and gas chromatograph-mass spectrometer (GC-MS) that allows detection and quantification of gaseous products. Ex-situ high performance liquid chromatography (HPLC) was used to quantify liquid products. (1) Thomann, I.; Pinaud, B. A.; Chen, Z.; Clemens, B. M.; Jaramillo, T. F.; Brongersma, M. L. Nano Lett. 2011, 11, 3440–3446. (2) Ingram, D. B.; Linic, S. J. Am. Chem. Soc. 2011, 133, 5202–5205. (3) Gao, H.; Liu, C.; Jeong, H. E.; Yang, P. ACS Nano 2012, 6, 234–240. (4) Mukherjee, S.; Zhou, L.; Goodman, A. M.; Large, N.; Ayala-Orozco, C.; Zhang, Y.; Nordlander, P.; Halas, N. J. J. Am. Chem. Soc. 2014, 136, 64–67. (5) Mubeen, S.; Lee, J.; Singh, N.; Krämer, S.; Stucky, G. D.; Moskovits, M. Nat. Nanotechnol. 2013, 8, 247–251. (6) Naya, S.; Teranishi, M.; Isobe, T.; Tada, H. Chem. Commun. 2010, 46, 815–817. (7) Lee, J.; Mubeen, S.; Ji, X.; Stucky, G. D.; Moskovits, M. Nano Lett. 2012, 12, 5014–5019.

not Available.