Hybrids as City Runabouts, Natural Gas Fueled Cars for the Country

Practically-based comparisons in the laboratory

During the study, Empa engineer Robert Alvarez and colleagues compared the fuel consumption of three different hybrid cars. The fuel usage characteristics were measured on a dynamometer, both for the standard driving cycle as well as for"real world" driving profiles, which better simulate everyday driving under inner city, rural and motorway conditions. In addition, the researchers measured the amount of energy returned to the storage batteries during regenerative braking (known as recuperation) and the current supplied by the batteries to deliver extra torque to the engine when necessary.

The comparison with conventional gasoline engined cars showed that hybrids achieve up to twice the efficiency in city driving, which naturally has a very positive effect on their fuel consumption and CO₂emission levels. The repeated strong acceleration and braking phases combined with the modest speeds characteristic of urban"stop-and-go" driving particularly favor hybrid drive systems. Full hybrids, which can use purely electrical propulsion for short distances, achieved even better values under these conditions than mild hybrids, which do not have this ability, or the luxury-class hybrid evaluated in the test. Because of their weight, vehicles belonging to the latter category are generally equipped with a large internal combustion engine and a comparatively smaller electric motor.

On the other hand, during rural driving hybrids show little savings in terms of fuel consumption or CO₂emissions and on the motorway none at all compared to gasoline engined vehicles. Because of the power required to propel the vehicle at country-road or motorway speeds the electric motor is hardly able to offer any additional support to the internal combustion engine. In summary, hybrid vehicles are therefore ideal as city runabouts.

Natural gas fuelled vehicles are another alternative

In terms of CO₂emission reduction, natural gas fuelled vehicles represent another alternative, with the further advantage of significant additional economy. Technically, they are practically identical to gasoline fuelled vehicles, but they generate less carbon dioxide because natural gas contains less carbon than gasoline. Their level of CO₂emission lies about 20 to 25 per cent below that of an equivalent vehicle fuelled with petrol, but above that of a full hybrid. During rural driving conditions natural gas fuelled cars and hybrids are equally"clean," and on the motorway natural gas fuelled vehicles actually emit less CO₂than hybrid cars. Taken over all three driving profiles, the total CO₂emission levels of natural gas powered automobiles are therefore quite comparable to those of hybrid vehicles, and when rural and motorway driving predominate then they are in fact better.

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Fuel Cells in Operation: A Closer Look

Now a team of scientists from the University of Maryland, the U.S. Department of Energy's Sandia National Laboratories, and DOE's Lawrence Berkeley National Laboratory has used a new kind of XPS, called ambient-pressure XPS (APXPS), to examine every feature of a working solid oxide electrochemical cell. The tests were made while the sample cell operated in an atmosphere of hydrogen and water vapor at one millibar pressure (about one-thousandth atmospheric pressure) and at very high temperatures, up to 750° Celsius (1,382 degrees Fahrenheit).

"Our team, led by Bryan Eichhorn of the Department of Chemistry and Biochemistry at the University of Maryland, combined the expertise in fuel cells at U Maryland, the experience of our Sandia Lab colleagues in collecting electrochemical data, and Berkeley Lab's own development of a method for doing x-ray photoelectron spectroscopy in situ," says Zahid Hussain of Berkeley Lab's Advanced Light Source (ALS)."Together we were able to measure the fundamental properties of a solid oxide fuel cell under realistic operating conditions."

The researchers report their results in the November 2010 issue ofNature Materials, in an article now published online.

How a solid oxide fuel cell works

Like a battery, a fuel cell is a device that uses chemical reactions to produce electricity. Unlike a battery, a fuel cell won't run down as long as it's supplied with fuel and oxidant from outside. The main components are two electrodes, an anode and a cathode, separated by an electrolyte.

In a solid oxide cell (SOC) the cycle begins at the cathode, which ionizes oxygen (usually from air) by adding free electrons. These oxygen ions then flow through the solid oxide electrolyte (from which the SOC gets its name), often a material known as yttria-stabilized zirconia. High temperature is needed to maintain good conduction of oxygen ions through the electrolyte.

The oxygen ions travel through the electrolyte to reach the anode, where they oxidize the fuel. (The fuel may be pure hydrogen gas or a hydrocarbon.) Electrons freed by oxidation form the current in the device's electrical circuit and eventually return to the cathode. Unused fuel or other compounds, plus water formed from the positive hydrogen ions and negative oxygen ions, exits the fuel cell.

For the APXPS experiment, the University of Maryland collaborators built a model fuel cell that combined the essential elements of an SOC in a special miniaturized design less than two millimeters in length. Except for the electrolyte of yttria-stabilized zirconia, which formed the base of the device, the various components were thin films measuring from 30 nanometers (billionths of a meter) up to 300 nanometers thick.

Says the University of Maryland's Eichhorn,"We designed and fabricated solid oxide electrochemical cells that provided precise dimensional control of all the components, while providing full optical access to the entire cell from anode to cathode."

Instead of stacking the components as in a real fuel cell, the sample's arrangement was a planar design that placed all the components on the same side of the electrolyte, so the x-ray beam from the ALS could reach them. This allowed direct measurement of local chemical states and electric potentials at surfaces and interfaces during the cell's operation.

Introducing ambient-pressure x-ray photoelectron spectroscopy

Photoemission occurs when light ejects electrons from a material. By collecting the emitted electrons and analyzing their energies and trajectories, photoelectron spectroscopy establishes exactly what elements are in the material and their chemical and electronic states within narrow regions. At the Advanced Light Source, intense x-ray light is used to explore what happens at or near the surface of materials: the only photoelectrons that can escape are from atoms near the surface.

The APXPS system begins by shining the x-ray beam on the sample fuel cell inside a chamber at the ambient pressure of the gas needed for it to operate. The emitted electrons then travel through chambers pumped to lower pressure, finally entering the high-vacuum chamber of the detector. By itself this arrangement would lose emitted electrons at every stage because of their spreading trajectories, leaving a signal too weak to be useful. So Berkeley Lab researchers developed a system of"lenses" -- not made of glass but of electric fields -- to capture and refocus the emitted electrons at each stage, preventing excessive loss.

"This is what allows us to find out what's happening within small regions on the surface of a sample in the presence of a gas," says Hendrik Bluhm of Berkeley Lab's Chemical Sciences Division, one of the inventors of APXPS, which was awarded a coveted R&D 100 Award in 2010."Using the APXPS instruments at the ALS's molecular environmental science beamline, 11.0.2., and the chemical and materials science beamline, 9.3.2, we can spatially correlate the catalytic activity with the electrical electrical potentials across the different components of the model fuel cells."

Says Zhi Liu of the ALS,"At first we weren't sure we could use this technique with an operating fuel cell, because we had to bring it to 750° C -- an extreme temperature for such ambient pressure experiments. Few people have done it before. Now we're able to perform this kind of analysis routinely."

Michael Grass of the ALS says,"What you need to know to improve any kind of fuel cell is where the inefficiencies are -- places where energy is being lost compared to what theoretically should be possible. By scanning across the surface of the cell while it was operating, we could directly measure both the inefficiencies and the chemical states associated with them."

A new way to study electrochemistry in action

With their model SOC, the Maryland-Sandia-Berkeley Lab team saw details never seen before in an operating fuel cell. Where an overall measurement gave only the fuel cell's total losses in potential energy, the APXPS measurements found the local potential losses associated with the interfaces of electrode and electrolyte, as well as with charge transport within the ceria electrode. The sum of the losses was equal to the cell's total loss, or inefficiency.

"The in situ XPS experiments at 750 C allowed us to pinpoint the electroactive regions, measure length scales of electron transport through mixed ionic-electronic conductors, and map out potential losses across the entire cell," Eichhorn says."Others have suggested similar experiments in the past, but it was the remarkable facilities and scientific expertise at the ALS that facilitated these challenging measurements for the first time."

APXPS can provide this kind of fundamental information to solid oxide fuel cell designers, information not available using any other technique. New fuel cell designs are already taking advantage of this new way to study fuel cells in operation.

"Measuring fundamental properties in operating solid oxide electrochemical cells by using in situ x-ray photoelectron spectroscopy," by Chunjuan Zhang, Michael Grass, Anthony McDaniel, Steven DeCaluwe, Farid El Gabaly, Zhi Liu, Kevin McCarty, Roger Farrow, Mark Linne, Zahid Hussain, Gregory Jackson, Hendrik Bluhm, and Bryan Eichhorn, appears in the November, 2010 issue ofNature Materialsand is available to subscribers in advance online publication. Zhang, DeCaluwe, Jackson, and Eichhorn are with the University of Maryland. McDaniel, Gabaly, McCarty, Farrow, and Linne are with Sandia National Laboratories. Grass, Liu, Hussain, and Bluhm are with Berkeley Lab.

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Future of Electric Cars? Running Fuel Cells on Biodiesel

The power supply unit can run on biodiesel as well as regular diesel.

This combination of two advanced technologies is now undergoing testing, thanks to funding under the Research Council's RENERGI programme. In trials, a 200-W solid-acid fuel cell ran on both pure hydrogen and on hydrogen produced from diesel by the unit's reformer -- with only an insignificant difference in performance.

Low CO₂emissions

The reformer converts hydrocarbons into hydrogen, CO₂and heat. Due to the unit's high efficiency, CO₂emissions are substantially lower than in conventional combustion engines, and no other demonstrable exhaust is discharged -- meaning that diesel particulates, black carbon soot, nitrous oxide (NOx) and carbon monoxide (CO) are elimi¬nated. An added plus is that the reformer emits no smoke or odour.

The silent electric generator is being developed and produced by the Norwegian company Nordic Power Systems (NPS). The California firm SAFCell Inc. is developing and will deliver the new type of fuel cell. Also on the team is the California Institute of Technology (Caltech). DagØvrebø, Technical Director of NPS, has many years' experience with fuel cells and has been working closely with Caltech on this new technologyGerman conversion technology

It all began in Germany. In 2006 the NPS founders came across an interesting conversion technology developed at RWTH Aachen University in the late 1990s. NPS acquired the licensing rights, envisioning a clear market potential for an electric power supply unit based on a fuel cell that is not dependent on hydrogen filling stations, and that can run on regular, easily available fuel without surrendering the environmental benefits of fuel cells.

In 2009 NPS secured usage rights to the new US solid-acid technology for use with various fuel types such as diesel and biofuels.

Tor-Geir Engebretsen, Managing Director and co-founder of NPS, is very pleased with this summer's tests."Now we have demonstrated that the solid-acid technology works. The next step is to test a larger unit of 1 200 W."

Armed Forces first user

Engebretsen points out that since the technology is scalable, it is well suited for future generators in electric vehicles. But NPS is taking the development in stages. The company's first market is power supply for the defence industry; NPS has a technology development agreement with the Royal Norwegian Armed Forces. In addition, NPS has a product development agreement with Marshall Land Systems, of the UK, with the aim of supplying silent-running generators for the British Armed Forces.

If all goes according to plan, the unit being developed with Marshall will be ready for market launch by mid-2011, while the solid-acid fuel cell will be phased in somewhat later. An assembly plant in Høyanger, Norway, is scheduled to open in early 2012 with Industrial Development Corporation of Norway (SIVA) as contractor.

Nordic Power Systems (NPS)

NPS currently has seven employees in Norway, and six in the USA through a contract with SAFCell in California.

• So far NOK 50 million has been spent on development and market preparations.
• Financing has come from a score of private investors and from: o Research Council of Norway o Innovation Norway o Royal Norwegian Armed Forces o Høyanger Næringsutvikling AS

The project

• Name:Development of cutting-edge fuel cell technology, integration and testing of NPS' proprietary fuel cell generator for industrialisation in Norway
• Project manager:Nordic Power Systems/DagØvrebø
• Partners:SAFCell, Caltech, Nordic Power Systems
• Overall budget:NOK 11.8 million. Funding under the RENERGI programme: NOK 5.9 million

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Methane-Powered Laptops? Materials Scientists Unveil Tiny, Low-Temperature Methane Fuel Cells

With advances in nanostructured devices, lower operating temperatures, and the use of an abundant fuel source and cheaper materials, a group of researchers led by Shriram Ramanathan at the Harvard School of Engineering and Applied Sciences (SEAS) are increasingly optimistic about the commercial viability of the technology.

Ramanathan, an expert and innovator in the development of solid-oxide fuel cells (SOFCs), says they may, in fact, soon become the go-to technology for those on the go.

Electrochemical fuel cells have long been viewed as a potential eco-friendly alternative to fossil fuels -- especially as most SOFCs leave behind little more than water as waste.

The obstacles to using SOFCs to charge laptops and phones or drive the next generation of cars and trucks have remained reliability, temperature, and cost.

Fuel cells operate by converting chemical energy (from hydrogen or a hydrocarbon fuel such as methane) into an electric current. Oxygen ions travel from the cathode through the electrolyte toward the anode, where they oxidize the fuel to produce a current of electrons back toward the cathode.

That may seem simple enough in principle, but until now, SOFCs have been more suited for the laboratory rather than the office or garage. In two studies appearing in theJournal of Power Sourcesthis month, Ramanathan's team reported several critical advances in SOFC technology that may quicken their pace to market.

In the first paper, Ramanathan's group demonstrated stable and functional all-ceramic thin-film SOFCs that do not contain any platinum.

In thin-film SOFCs, the electrolyte is reduced to a hundredth or even a thousandth of its usual scale, using densely packed layers of special ceramic films, each just nanometers in thickness. These micro-SOFCs usually incorporate platinum electrodes, but they can be expensive and unreliable.

"If you use porous metal electrodes," explains Ramanathan,"they tend to be inherently unstable over long periods of time. They start to agglomerate and create open circuits in the fuel cells."

Ramanathan's platinum-free micro-SOFC eliminates this problem, resulting in a win-win: lower cost and higher reliability.

In a second paper published this month, the team demonstrated a methane-fueled micro-SOFC operating at less than 500° Celsius, a feat that is relatively rare in the field.

Traditional SOFCs have been operating at about 800-1000°C, but such high temperatures are only practical for stationary power generation. In short, using them to power up a smartphone mid-commute is not feasible.

In recent years, materials scientists have been working to reduce the required operating temperature to about 300-500°C, a range Ramanathan calls the"sweet spot."

Moreover, when fuel cells operate at lower temperatures, material reliability is less critical -- allowing, for example, the use of less expensive ceramics and metallic interconnects -- and the start-up time can be shorter.

"Low temperature is a holy grail in this field," says Ramanathan."If you can realize high-performance solid-oxide fuel cells that operate in the 300-500°C range, you can use them in transportation vehicles and portable electronics, and with different types of fuels."

The use of methane, an abundant and cheap natural gas, in the team's SOFC was also of note. Until recently, hydrogen has been the primary fuel for SOFCs. Pure hydrogen, however, requires a greater amount of processing.

"It's expensive to make pure hydrogen," says Ramanathan,"and that severely limits the range of applications."

As methane begins to take over as the fuel of choice, the advances in temperature, reliability, and affordability should continue to reinforce each other.

"Future research at SEAS will explore new types of catalysts for methane SOFCs, with the goal of identifying affordable, earth-abundant materials that can help lower the operating temperature even further," adds Ramanathan.

Fuel cell research at SEAS is funded by the same NSF grant that enabled the"Robobees" project led by Robert J. Wood, Assistant Professor of Electrical Engineering. Wood and Ramanathan hope that micro-SOFCs will provide the tiny power source necessary to get the flying robots off the ground.

Ramanathan's co-authors on the papers were Bo Kuai Lai, a Research Associate at SEAS, and Ph.D. candidate Kian Kerman '14.

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New Highly Stable Fuel-Cell Catalyst Gets Strength from Its Nano Core

Now, scientists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory have developed a new electrocatalyst that uses a single layer of platinum and minimizes its wear and tear while maintaining high levels of reactivity during tests that mimic stop-and-go driving. The research -- described online inAngewandte Chemie, International Edition -- may greatly enhance the practicality of fuel-cell vehicles and may also be applicable for improving the performance of other metallic catalysts.

The newly designed catalysts are composed of a single layer of platinum over a palladium (or palladium-gold alloy) nanoparticle core. Their structural characterization was performed at Brookhaven's Center for Functional Nanomaterials and the National Synchrotron Light Source.

"Our studies of the structure and activity of this catalyst -- and comparisons with platinum-carbon catalysts currently in use -- illustrate that the palladium core 'protects' the fine layer of platinum surrounding the particles, enabling it to maintain reactivity for a much longer period of time," explained Brookhaven Lab chemist Radoslav Adzic, who leads the research team.

In conventional fuel-cell catalysts, the oxidation and reduction cycling -- triggered by changes in voltage that occur during stop-and-go driving -- damages the platinum. Over time, the platinum dissolves, causing irreversible damage to the fuel cell.

In the new catalyst, palladium from the core is more reactive than platinum in these oxidation and reduction reactions. Stability tests simulating fuel cell voltage cycling revealed that, after 100,000 potential cycles, a significant amount of palladium had been oxidized, dissolved, and migrated away from the cathode. In the membrane between the cathode and anode, the dissolved palladium ions were reduced by hydrogen diffusing from the anode to form a"band," or dots.

In contrast, platinum was almost unaffected, except for a small contraction of the platinum monolayer."This contraction of the platinum lattice makes the catalyst more active and the stability of the particles increases," Adzic said.

Reactivity of the platinum monolayer/palladium core catalyst also remained extremely high. It was reduced by merely 37 percent after 100,000 cycles.

Building on earlier work that illustrated how small amounts of gold can enhance catalytic activity, the scientists also developed a form of the platinum monolayer catalyst with a palladium-gold alloy core. The addition of gold further increased the stability of the electrocatalyst, which retained nearly 70 percent of reactivity after 200,000 cycles of testing.

"This indicates the excellent durability of this electrocatalyst, especially when compared with simpler platinum-carbon catalysts, which lose nearly 70 percent of their reactivity after much shorter cycling times. This level of activity and stability indicates that this is a practical catalyst. It exceeds the goal set by DOE for 2010-2015 and it can be used for automotive applications," Adzic said.

He noted that fuel cells made using the new catalyst would require only about 10 grams of platinum per car -- and less than 20 grams of palladium. Currently, in catalytic convertors used to treat exhaust gases, 5 to 10 grams of platinum is used. Since fuel-cell-powered cars would emit no exhaust gases, there would be no need for such catalytic converters, and therefore no net increase in the amount of platinum used.

"In addition to developing electrocatalysts for automotive fuel cell applications, these findings indicate the broad applicability of platinum monolayer catalysts and the possibility of extending this concept to catalysts based on other noble metals," Adzic said.

The fundamental science leading to the development of the new electrocatalyst and early scale-up work was funded by the DOE Office of Science. Additional funding came from the Toyota Motor Corporation.

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