News Article | October 23, 2015
By Michael Klein, The University of Texas at Austin, Electrochemical Energy Laboratory and MRS Student Chapter Amid the ubiquitous search for energy savings across all fields of modern society, reducing the power cost of computation looks to play an outsize role in limiting the expansion of energy demands into the foreseeable future. In the August issue of the MRS Bulletin, Lionel Kimerling, Dim-Lee Kwong, and Kazumi Wada look at the prospect of integrating microscale photonics into computer chips. As the authors point out, limitations imposed by transistors’ power draw has already shifted the direction of processor development from increasing clock speeds to increasing parallelism (the number of processing cores per chip). This shift imposes two distinct problems for traditional chip architectures: the cores must be interconnected in an efficient, low-latency manner, and software must explicitly code for the appropriate inter-processor connections to take advantage of the increased parallelism. Both problems become exponentially worse with increasing numbers of processors. The authors propose that the use of integrated photonics can solve both issues. By utilizing a technique referred to as all-to-all computing (ATAC), each processor could communicate to every other processor on its own unique wavelength of light through multichannel photonic waveguides, rather than making individual connections from one processor to another. This architecture would decrease the energy demand of parallel processing by drastically reducing the consumption associated with core-to-core networking, possibly by a factor of several hundred. Concurrently, ATAC schemes dramatically simplify effective software utilization of parallel processing. The article goes on to discuss the numerous challenges facing the realization of this vision. While all of necessary microscale optical devices have been developed in some form, the prospect of integrating high-volume manufacture with traditional microelectronic circuits at the requisite tolerances is daunting. For instance, in order for the ATAC scheme to work, waveguides with single-nanometer dimensional tolerances would need to be fabricated at a chip-scale. Promisingly, the authors discuss the success of the transformation of a 180-nm CMOS manufacturing line to fabricate microphotonics by the Institute of Microelectronics and GlobalFoundries in Singapore. [Figure: Schematic representation of the complexity of scaling traditional inter-processor networking with a photonic ATAC approach. Courtesy of Cambridge University Press: [MRS Bulletin] L. Kimerling, et al., MRS Bull. 39 (8), 687-695, copyright 2014]
News Article | October 23, 2015
By Michael Klein, The University of Texas at Austin, Electrochemical Energy Laboratory and MRS Student Chapter Water desalination lies at a critical junction of the energy and water economies. It is a vital necessity as populations grow in arid regions around the world and climate, water, and land use changes drive desertification of populated regions. However, existing commercial desalination technologies are inefficient consumers of energy, both thermal distillation techniques and existing reverse osmosis (RO) technologies. Membrane technologies hold promise, however, as next-generation membranes with high-permeability and excellent salt rejection could greatly reduce the energy requirements and capital costs of RO water desalination. In a paper in a recent issue of Nano Letters, David Cohen-Tanugi and Jeffrey Grossman of MIT perform molecular dynamics simulations to evaluate the mechanical robustness of one of these candidate membranes, nanoporous graphene (NPG). The authors use 5 MPa as a benchmark osmotic pressure needed to be withstood by any RO membrane used to desalinate seawater and examine how a number of factors influence NPG’s ability to withstand these pressures. Among the factors examined are the pore radius, pore separation, and porosity of the NPG; the pore radius and porosity of the substrate; and the effect of wetting and grain boundaries in the NPG. Additionally the authors simulate the effect of pressure on the desalination performance of the NPG membrane. The examination of the effect of substrate pore properties is particularly interesting as it illustrates how important design of the full membrane is—an NPG membrane could be designed nearly ideally to perform desalination, but would still fail if laid down on a substrate with pores in excess of 10 microns. The authors point out the significance of this going forward: existing porous substrates, particularly thin-film polysulfone, can have large fluctuations in pore sizes, raising the likelihood that NPG membranes on these substrates could ultimately fail in a desalination application. The results of these simulations remain to be tested by experiment, but should provide useful direction for the design of experimental NPG membranes. [Figure: diagrams showing the effect of substrate porous radius for two NPG pore configurations (left) and the combined effects of substrate pore radius and porosity (righ) on the maximum pressure the NPG/substrate membranes can withstand. Reprinted with permission from D. Cohen-Tanugi and J.C. Grossman, Nano Lett. 14(11), 6171-6178 (2014). Copyright 2014 American Chemical Society.]
News Article | October 23, 2015
By Michael Klein, The University of Texas at Austin, Electrochemical Energy Laboratory and MRS Student Chapter The formation of a solid electrolyte interphase (SEI) plays a critical role in the operation and cyclability of lithium-ion batteries (LIBs). Without a stable SEI, the lithiated graphite anode will continually attack the electrolyte and lithium dendrite formation can be exacerbated. A good SEI layer enables the favorable kinetics (high rate performance) of LIBs while providing safety and cyclability to the system. Characterizing the SEI layer remains problematic as it forms during battery cycling and its stability should ideally be characterized in situ in the relevant electrolyte solution. Scanning electrochemical microscopy (SECM) is a potential panacea for this problem, as it provides local electrochemical and topological characterization in an appropriate chemical environment. Its utility is hampered by the considerable experimental difficulty required to successfully apply the technique on a battery electrode surface. Heinz Bülter of Professor Gunther Wittstock’s group at the University of Oldenburg and coworkers have successfully applied SECM to garner some insight into the stability of the SEI formed on composite graphite anodes after cycling using DBDMB as a redox mediator. Their work, published this year in the Angewandte Chemie International Edition showed evidence of spontaneous changes in the SEI, with the authors suggesting the possibility of damage from volume changes, re-dissolution of parts of the SEI, or even gas bubble formation and evolution. The difficulty in analyzing the simultaneous influence of the surface morphology, local electrochemistry, and other possible changes (i.e. gas formation) is readily apparent in the research. However, this approach is admirable, and continued work with this technique should afford currently unattainable insight into the operation of lithium-ion batteries.
News Article | October 23, 2015
By Michael Klein, The University of Texas at Austin, Electrochemical Energy Laboratory and MRS Student Chapter Research into hydride materials for energy applications typically focuses on maximizing gravimetric storage density and ion transport. However, the requirements for stationary applications such as fuel cells can be significantly different and amenable to a broader class of potential materials. In work recently published online in Nature Materials, Maarten Verbraeken of John Irvine’s research group at the University of Saint Andrews, and coworkers, characterize the transport properties and phase behavior of barium hydride at high temperature, and report very high hydride conductivity. The authors look at the sequence of alkaline earth hydrides and report a first-order, reversible phase transition in BaH above 500 oC. Neutron diffraction studies reveal that hydrogen occupancy splits from a higher-symmetry 2d site to a lower-symmetry 4f site upon going from the low-temperature to high-temperature phase. This results in a doubling of charge carriers in the (002) plane (though not an anticipated decrease in the activation energy for hydride transport due to not-positively-identified competing effects). The net result is a material with charge-carrying properties defined purely by ionic conduction and at a level an order of magnitude (0.2 S cm-1) better than competing oxide and perovskite structures around 600 oC. These materials could thus have promising applications as separator membranes, hydrogen fuel cell components, and in other electrochemical uses. [The figure shows the splitting of one of the hydrogen (deuterium) sites (identified in green as D1) (d) into two sites (f). Reprinted by permission from Macmillan Publishers Ltd: [Nature Materials] M. C. Verbraeken, et al., Nat. Mater., advance online publication, 8 December 2014, DOI: 10.1038/NMAT4136 copyright 2014]
News Article | October 23, 2015
By Michael Klein, The University of Texas at Austin, Electrochemical Energy Laboratory and MRS Student Chapter In the November 5th issue of Advanced Materials, Zhibin Yang and coworkers from Professor Huisheng Peng’s group at Fudan University report the fabrication of a flexible microfiber integrating a dye-sensitized TiO photoelectric energy-converting shell surrounding a carbon nanotube electrochemical capacitor. After stable charging under AM1.5 simulated solar illumination, the full device discharged for over 40 seconds at a very respectable 100 mA/g rate. In verifying the functional flexibility of the integrated device, the microfibers was subjected to bending as well as stretching up to almost 30% with minimal degradation in photovoltage and discharge performance. The intricately assembled architecture starts with two aligned CNT sheets separated by a gel electrolyte wrapped around an elastic fiber electrode. This electrochemical capacitor core is then inserted into a plastic tube coated with another aligned CNT sheet. Then, a helical Ti fiber with perpendicularly-grown N719 dye-sensitized TiO2 nanotubes is wrapped around this structure to act as the photoanode. Finally, this entire structure is inserted into another tube and filled with an I-/I3- electrolyte to serve as the redox couple for the solar cell. While the complexity of this assembly process is daunting from a fabrication perspective, the applicability and promise of this scheme is undeniable. As the authors highlight, wearable electronics (as well as microscale bio-devices) are currently severely limited by the need for on-board energy storage. Integrated generation is key; but most existing solutions are neither as elegant nor as flexible as the scheme presented in this paper.
Suntivich J.,Massachusetts Institute of Technology |
Suntivich J.,Electrochemical Energy Laboratory |
Suntivich J.,Harvard University |
Perry E.E.,Massachusetts Institute of Technology |
And 5 more authors.
Electrocatalysis | Year: 2013
Understanding the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) mechanisms is critical to the design of future electrocatalysts for fuel cells, electrolyzers, and metal-air batteries. As parts of the effort to elucidate the reaction mechanisms, we report the influence of the cationic species on the ORR/OER activity of select transition metal oxide catalysts in alkaline solutions. Specifically, we use Li+, Na+, and K+-containing electrolytes to assess the role of the cation on the ORR activity of Pt nanoparticles and LaMnO3+δ, as well as the OER activity of rutile IrO2 and Ba0. 5Sr0. 5Co0. 8Fe0. 2O3-δ. We found that all these benchmark electrocatalysts share the same cation trends, where the presence of the smaller cation (Li+) always leads to lower activity. We argue that this finding represents the possible cation influence on the ORR/OER intermediate stabilization. © 2012 Springer Science+Business Media New York.
Sajjad S.D.,Electrochemical Energy Laboratory |
Liu D.,Electrochemical Energy Laboratory |
Wei Z.,Electrochemical Energy Laboratory |
Sakri S.,Electrochemical Energy Laboratory |
And 3 more authors.
Journal of Power Sources | Year: 2015
Guanidinium based blend anion exchange membranes (AEMs) for direct methanol alkaline fuel cells have been fabricated and studied. The guanidinium prepolymer is first synthesized through a simple polycondensation process with the ion exchange moieties incorporated directly into the polymer backbone, and then is used to make guanidinium - chitosan (Gu-Chi) blend membranes. Besides, a lipophilic guanidinium prepolymer, synthesized by means of a precipitation reaction between sodium stearate and guanidinium salt, is adopted to tune solubility and mechanical properties of the blend AEMs. Results show that both ionic conductivity and methanol permeability of the AEMs can be tuned by blend composition and chemistry of the guanidinium based prepolymer. The selectivity (ratio of ionic conductivity to methanol permeability) of the fabricated membranes is superior to that of commercial membranes. Under fuel cell tests using 3 M methanol, the open circuit voltage (OCV) value for the blend AEM with 72 wt% of the guanidinium polymer (0.69 V) is much higher than that of the commercial Tokuyama A201 (0.47 V) at room temperature, while the blend AEMs with 50 wt% guanidinium content still show comparable values. Overall, the developed membranes demonstrate superior performance and therefore pose great promise for direct methanol anion exchange fuel cell (DMAFC) applications. © 2015 Elsevier B.V. All rights reserved.