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The average fuel-economy (EPA window-sticker) value of new light-duty vehicles sold in the US in November was 24.8 mpg, up 0.1 mpg from the revised October value, according to the monthly report from Michael Sivak and Brandon Schoettle at the University of Michigan Transportation Research Institute. Vehicle fuel economy is up 4.7 mpg or 23% since October 2007 (the first month of their monitoring).
The University of Michigan Eco-Driving Index (EDI)—an index that estimates the average monthly emissions of greenhouse gases generated by an individual US driver—stood at 0.80 in September (the lower the value the better). This value indicates an improvement of 20% since October 2007. The EDI takes into account both the fuel used per distance driven and the amount of driving (the latter relying on data that are published with a two-month lag).
New composite materials based on selenium (Se) sulfides used as the cathode in a rechargeable lithium-ion battery could increase Li-ion density five times, according to research carried out at the US Department of Energy’s Advanced Photon Source at Argonne National Laboratory. The work most recently reported in the Journal of the American Chemical Society by a team of researchers from Argonne and King Abdulaziz University (Saudi Arabia) advances their earlier work with selenium as a high energy density cathode material. (Earlier post.)
The researchers have focused on carbon-selenium sulfide composites as an alternative material to the conventional lithium transition metal oxide positive electrode material in standard Li-ion batteries. The new SeSx cathodes could also provide a way around the cycling challenges facing Li-air and Li-S, while delivering a comparable boost in energy density (and hence, for example, range in an electric vehicle.)
State-of-the-art rechargeable batteries are mainly based on conventional lithium intercalation chemistry, using lithium transition metal oxides as cathode material with typical capacities of 120−160 mA·h/g-1. The low energy density and/or high cost of these cathode materials have limited their large-scale production and application in Li ion batteries. Discovery of new cathode materials with higher energy density is, thus, a key to realizing more efficient energy storage systems. Recently, lithium−sulfur (Li/S) and lithium−oxygen (Li/O2) cells have been demonstrated to possess the potential to provide 2−5 times the energy density of conventional Li ion cells. However, both Li/S and Li/O2 cells suffer from poor cycling performance, which had impeded their commercial utilization.
For instance, the cyclability of Li/O2 cells is limited by severe electrolyte decomposition and large cell polarization under deep discharge/charge conditions, while Li/S cells suffer from the solubility of intermediate lithium polysulfide species during cycling, which causes the so-called redox shuttle effect and, thus, poor cyclability. —Cui et al.
Selenium has higher electrical conductivity compared to sulfur and high theoretical gravimetric capacity (678 mA·h g-1) and volumetric capacity (3,268 mA·h/cm3). In their 2012 paper they we reported that Se represented an attractive cathode material for not only rechargeable lithium ion batteries but also sodium batteries.
There are fundamental issues with the use of selenium that need to be clarified, they noted. For example, they said:
Charge and discharge voltages are evolving in the Li/Se system during the initial cycles in the carbonate-based electrolyte. Significant polarization occurs once the charge and discharge voltages are stabilized after five cycles, which leads to a low roundtrip efficiency.
The Coulombic efficiency is quite low during the first 20 cycles. The mechanisms or underlying reasons for this unsatisfactory performance are still not well understood due to inadequate characterization of the battery materials during electrochemical cycling.
The effects of organic electrolytes on the (de)lithiation process (i.e., both lithiation and delithiation) of the Li/SeSx cells, if any, are still not clear, nor are the effects of the sulfur content in SeSx.
In the study, they adopted an ether-based electrolyte in the Li/SeSx cell to investigate its effect; results showed significantly improved cell performance in terms of voltage profile and Coulombic efficiency.
The voltage profiles of these cells indicate that complete lithiation of selenium to Li2Se is occurring through the formation of intermediate phases, i.e., Li2Sen. This behavior differs from the single-phase transition in carbonate-based electrolyte, as reported earlier. This result clearly suggests that cell performance highly depends on the nature of the electrolyte.—Cui et al.
To examine electrochemical performance in the ether-based electrolyte, they tested active cathode materials containing Se, SeS2, and SeS7 combined with carbon against Li metal anodes between 0.8 and 4.0 V. A discharge capacity of 350, 571, and 833 mA·h g-1 for the Li/Se, Li/SeS2, and Li/SeS7 cells, respectively, was maintained for more than 50 cycles.
Capacity increased with increasing S content in the composites due to its contribution to the overall capacity. The capacity faded a little for all three cells tested. The Coulombic efficiency was nearly 100% for the Li/Se and Li/ SeS2 cells. The Li/SeS7 cell showed a relatively low Coulombic during the initial 20 cycles, although much higher capacity.
Using the X-ray Science Division (XSD) beamline 11-ID-C at the Advanced Photon Source, the team carried out in situ synchrotron high-energy x-ray diffraction (HEXRD) studies and complementary, selenium K-edge x-ray absorption near-edge structure (XANES) analysis to observe the chemical changes that take place in these novel electrode materials as they charge and discharge a battery.
These measurements, which were undertaken at more than 12-keV energy, were also done in transmission mode on the XSD bending-magnet beamlines 9-BM-C and 20-BM-B. This technique allowed the team to hone in on the changing chemistry of the selenium atoms in the electrode and how they shift between crystalline and non-crystalline phases as current and lithium ions flow through the experimental battery’s ether-based electrolyte. Raman microscopy at Argonne’s Center for Nanoscale Materials provided additional information about the Li2Se that was observed on the Li anode of the charged cells.
The team discovered that it is the chemical composition of the electrolyte that seems to have the most impact on the changes that take place. The researchers suggest it might be possible to tune the efficiency of a battery based on these new composites by optimizing the electrolyte and so improve battery performance still further.
The x-ray studies and analysis of the electrochemistry of the electrode as it operates also allowed the team to suggest a plausible chemical mechanism for the processes involved in discharging the battery.
The composite electrode is reduced to form lithium polyselenide with more than four selenium atoms per lithium atom; additional discharging to lower voltage leads to chemical species containing two lithium ions per selenium atom. Charging involves the reverse process. This mechanism is first proposed and experimentally proven by the team, and it is similar to that seen in experimental lithium-sulfur electrodes.
Yanjie Cui, Ali Abouimrane, Jun Lu, Trudy Bolin, Yang Ren, Wei Weng, Chengjun Sun, Victor A. Maroni, Steve M. Heald, and Khalil Amine (2013) “(De)Lithiation Mechanism of Li/SeSx (x = 0−7) Batteries Determined by in situ Synchrotron X‑ray Diffraction and X‑ray Absorption Spectroscopy,” J. Am. Chem. Soc. 135, 8047 doi: 10.1021/ja402597g
Ali Abouimrane, Damien Dambournet, Karena W. Chapman, Peter J. Chupas, Wei Weng, and Khalil Amine (2012) “A New Class of Lithium and Sodium Rechargeable Batteries Based on Selenium and Selenium–Sulfur as a Positive Electrode,” J. Am. Chem. Soc. doi: 10.1021/ja211766q
An Australian aviation biofuel industry is technically viable but significant obstacles remain, according to a report released by Qantas and Shell today. The study, conducted with the support of the Australian Government through the Australian Renewable Energy Agency (ARENA), is the most comprehensive investigation so far into the economic viability of producing biofuel on a commercial scale in Australia.
Identifying natural oils as a proven source material, the study modelled a plant capable of producing 1.1 billion liters (291 million gallons US) of renewable fuels, including jet fuel and diesel, per year using existing supply chain infrastructure.
It found that such a plant could be viable if three key priorities are addressed.
Feedstock. The volume of natural oil feedstocks available at a competitive price in Australia is not sufficient to power a commercial scale biofuel plant. This volume and price gap has potential to be filled by increased investment, research and development in production of emerging feedstocks such as algae and pongamia.
Infrastructure. While existing brownfield refining locations could be used for producing aviation biofuel, new infrastructure would be required, with associated capital costs.
Policy. Extending biodiesel production grants that currently apply to biodiesel to bio-jet fuel would go a long way to making a commercial plant viable.
The Qantas-Shell study looked only at certified biofuel production methods already approved for use in commercial aviation, and which meet strict operational and environmental criteria.
As well as natural oil-based fuel production pathways, Qantas worked directly with Solena Fuels to assess the opportunities around a waste-based pathway, which shows promise but also faces commercial challenges.
In addition to Qantas and Shell, a number of other companies with relevant expertise took part, including Sinclair Knight Merz, SkyNRG, AltAir Fuels, Solena Fuels and the Australian Research Council.
A rigorous emissions testing of modern heavy-duty diesel engines in the US has demonstrated a greater than 94% reduction in the levels of nitrogen dioxide (NO2 - an important contributor to ozone smog), and substantial reductions in all other pollutants, even when compared to engines first marketed to meet 2007 standards, according to a study released today by the Coordinating Research Council (CRC).
For a number of the most important pollutants, levels were substantially lower than required by regulations. The study, the Phase 2 Report of the comprehensive Advanced Collaborative Emissions Study (ACES) (earlier post), found that emissions of NO2 and other nitrogen oxides—which can have direct health effects and contribute to the formation of smog—were approximately 61% below the 2010 EPA standard and 99% lower than in 2004 engines. These reductions came while emissions of fine particles were also 92% lower than the 2010 standard 99% lower than 2004 emissions.
Emissions of carbon monoxide and hydrocarbons were also significantly below required 2010 levels: 97% and more than 99.9%, respectively.
The Phase 2 ACES study was conducted by the Southwest Research Institute in San Antonio, Texas, under the oversight of the CRC. Investigators tested heavy-duty diesel engines from the three major manufacturers of these engines (Cummins, Detroit Diesel, and Volvo), and subjected them to well-established federal test procedures, and to a much more rigorous 16-hour operation cycle designed especially for ACES.
All the engines were equipped with after-treatment devices to reduce the emissions of particulate matter as well as oxides of nitrogen. They were tested on multiple repeats of these cycles, and measurements of more than 300 regulated and unregulated air pollutants were made in accordance with the highest laboratory standards.
ACES is a multi-party five year initiative to test the emissions and health effects of new technology diesel engines to document the improvements that have been made and to ensure that there are no unintended emissions from this new technology. The study is being undertaken by the Health Effects Institute (HEI)3 and the CRC with support from a wide range of government and private sector sponsors, including the US Department of Energy, US Environmental Protection Agency, California Air Resources Board, Engine Manufacturers Association, American Petroleum Institute, and manufacturers of emission control equipment.
Overall design and management of ACES—and all laboratory testing of health effects—is being undertaken under the aegis of HEI. All emissions characterization for ACES is being overseen by CRC.
The Health Effects Institute is an independent, non-profit research institute funded jointly by government and industry to provide credible, high quality science on air pollution and health for air quality decisions.
Background. In 2010, EPA’s stringent NOx limit of 0.20 g/hp-hr became fully enforceable; the NOx emissions limit decreased from an average level of 1.2 g/hp-hr between 2007 and 2009 to a level less than or equal to 0.20 g/hp-hr in 2010.
To comply with the 2010 NOX limit, on-highway heavy-duty engine manufacturers utilized a urea-based selective catalytic reduction (SCR) catalyst in engine exhaust placed downstream of a diesel oxidation catalyst (DOC) and a catalyzed diesel particulate filter (DPF) used for particulate matter (PM) emissions control.
The engine manufacturers devoted substantial efforts to calibration of the urea dosing and mixing, SCR catalyst formulation, and engine control to achieve the desired NOx reduction while maintaining a controlled level of ammonia slip using an ammonia oxidation (AMOX) catalyst downstream of the SCR catalyst.
Improvements were also made in PM emissions control, eliminating the need for active regeneration (onboard cleaning via exhaust fuel injection upstream of DOC) of the DPF during ACES Phase 2 testing, compared to the several regeneration occurrences with the 2007 technology engines tested in ACES Phase 1.
CRC Report: ACES Phase 2. Phase 2 Of The Advanced Collaborative Emissions Study.