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Solid Oxidegathers pace as Europeboostsfuel cells race

Existing fossil fuel power genera-

tion sources are notoriously ineffi-

cient in converting primary hydro-

carbons into electrical energy.

They first have to use combustion to convert

the fossil fuel from chemical energy into

heat, which is then converted into mechani-

cal, and finally into electrical energy.

At the same time such power plants are

emitting huge amounts of CO2 greenhouse

gases. It would be far more beneficial to

directly convert the primary energy carriers

such as hydrogen, as well as hydrocarbons

such as methane, into electrical energy. This

can be done by solid oxide fuel cells

(SOFCs), but as commercialisation app-

roaches the usual market cost equations are

having to be worked through, with materials

costs making a significant contribution.

Power plants using SOFCs are already

being used at the pre-commercialisation

stage for stationary power units capable of

around 1 kW and even larger units capable

of 250 kW to 30 MW are being considered

by various utility companies around the

world.

The European Commission recently

announced that the development of

hydrogen-based energy and fuel cells is

being put on the "QuickStart" list of

research and development projects with

the aim of transferring Europe from a fos-

sil-fuel based to a hydrogen-based econo-

my. Other global regions are taking a sim-

ilar line.

Professor Detlev Stöver, director of the

Forschungszentrum Jülich in Germany,

presented a fascinating insight at the

Hagen PM symposium at the end of

November into the advances that have

been made to develop materials and

processes to produce SOFCs making use of

both PM and ceramic technology.

Simply put, he said, SOFCs create elec-

tricity by combining hydrogen with oxygen

(2H2 + O2 --> 2H2O). Only water is pro-

duced as a by-product. However, to pre-

vent a direct reaction between these two

gases and the corresponding partial reac-

tions - the oxidation of hydrogen, and the

reduction of oxygen, the fuel cell mem-

brane must separate them.

Solid Oxide Fuel Cells are attracting interest onboth sides of the Atlantic with their promise ofenabling movement towards cleaner power generating technology. It's another example ofan exciting cutting-edge technology wheremetal powders can provide a vital contribution…

22 MPR March 2004 0026-0657/04 ©2004 Elsevier Ltd. All rights reserved.

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The HagenSymposium

The unit. Section through one SOFC unit. Several can be grouped to form a stack.

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metal-powder.net March 2004 MPR 23

Special feature

These mem-

branes, he said, consist of a

gas-tight electrolyte capable of conducting

oxygen ions. In one gas chamber the oxi-

dant (air) is supplied. The air diffuses

through the air electrode (cathode) to the

electrolyte and takes up two electrons per

oxygen atom and then migrates as an oxy-

gen ion through the electrolyte. In the fuel

gas chamber the hydrogen is oxidised at the

fuel gas electrode (anode) - electrolyte inter-

face. The electrons then move along an out-

er circuit to the current collecting cathode.

In addition to being highly efficient,

even under partial load conditions, SOFCs

require no moving parts, and thus have no

wear issues, no mechanical stress, and no

noise. Professor Stöver said that they

could easily be adapted to supply electrici-

ty on demand (Watt to megawatt region),

and any waste heat can be used to further

increase overall efficiency, e.g. by the use of

turbines. However, there are still many

practical problems to be resolved relating

to materials and process technology, and

not least the high cost of high-temperature

alloys and ceramics Professor Stöver out-

lined the different routes to producing

multiplanar fuel cells as shown in Figure 1,

where the layer thicknesses may be only a

few microns. Here the fuel cell comprises

a porous anode functional layer where the

The stack. This consists of several fuel cell elementsconnected in series. They generate 1 kW of electricalpower, which covers the basic power requirements ofa single family home (operated parallel to the grid).

Image courtesy Sulzer Hexis

Staying cooland drivingdown costsONE OF the limiting frontiers of SOFC

technology is the high temperature that

most trial cells need to function. The fine

dimensions of the membranes and other

components mean that only a few materi-

als are able to withstand the thermal stress

of the heating and cooling cycles experi-

enced during operation.

However, work in the United States has

produced SOFCs that operate successfully

at intermediate temperatures, with the

attendant benefits of longer life and lower

material costs. NanoDynamics based in

Buffalo, New York, has operated cells at

less than 550ºC while producing power

equivalent to cells operating at 800ºC to

1000ºC. The intermediate-temperature

cells also produce power in a significantly

shorter time from start-up.

The company says it hopes to be mar-

keting its own compact portable fuel cell

this year. Technology Director Caine

Finnerty said: "The 900ºC to 1000ºC typi-

cal operating temperatures of convention-

al solid oxide fuel cells limit the types of

materials that can be employed, as well as

creating sealing and longevity problems

associated with thermal stresses due to

heating and cooling of the cell."

• Although the US is striving to produce

intermediate-temperature SOFCs,

European development at present seems

firmly fixed in the direction of high-tem-

perature products. Late last year an

agreement was announced between H C

Starck, a Bayer subsidiary, and Webasto

to focus on the development of high-

temperature SOFCs for automotive

applications. The companies say that the

co-operative venture should form the

basis for later mass production.

They see close collaboration and the

combination of their respective techno-

logical competences and market positions

as an ideal foundation on which to com-

mercialise SOFC technology.

The partners visualise SOFC stacks

being used in auxiliary power units (APU)

to generate electricity on board vehicles,

independently from the main engine. They

operate at high efficiency and generate

power even when the engine is turned off.

Solid oxide fuel cells could be an answer to

the growing demand for on-board power

in modern motor vehicles.

Webasto has been developing an APU

system for some time that uses liquid fuels.

H C Starck is the majority shareholder in

InDEC, which produces SOFC compo-

nents and materials.

24 MPR March 2004 metal-powder.net

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electrochemical reaction takes place, an

electrolyte, a cathode functional layer

where the oxygen is reduced, and a cath-

ode that acts as a current collector. The

metallic parts in the fuel cell allow gas

supply and electrically interconnect adja-

cent fuel cell membranes. Contact layers

are necessary in order to optimise the

electrical contact between fuel cell and

interconnector. Figure 2 shows the three

phases that have to be present to allow

anode partial reactions: the hydrogen gas,

an electron-conducting phase, and an ion-

conducting phase. The boundaries where

these three phases meet are marked with

small circles.

Some of the powder methods described

by Professor Stöver to produce substrates

include warm pressing where a powder

mixture of NiO-YSZ is coated with a

binder and compacted at around 120ºC.

During cooling the binder hardens leaving

a green body that can be handled and sin-

tered to produce a solid porous substrate.

Another industrial method is tape casting

a powder slurry onto a supporting tape

and controlling the height of the tape with

a Doctor blade. The tape is then dried and

sintered. Professor Stöver said that the

anode-functional layer on top of the

porous substrate is produced by "Vacuum-

Slip-Casting". Here the porous substrate

acts as a filter sucking through the solvent

in the functional layer suspension, thereby

An early example of a portable medium-tem-perature solid oxide fuel cell manufactured inthe United States. Image courtesy NanoDynamics

At the cathode, the atmospheric oxygen takes up electrons to form oxygen ions. During thetransport of these ions through the electrolyte, charge exchange occurs. The chemical reactionproduces not only electricity but heat as well. This heat is fed to a hot water storage tank andused for heating and hot water purposes. An auxiliary heater cuts in automatically if required.Image courtesy Sulzer Hexis

metal-powder.net March 2004 MPR 25

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leaving NiO-YSZ as a filter cake on top. Vacuum-Slip-Casting can

also be used to produce an electrolyte layer. Technically simple

processes such as "Wet Powder Spraying" or screen printing can be

used to process cathodes.

Professor Stöver stated that one of the main drawbacks of SOFCs

is the cost per kW of power density. The challenge, he said, will be

to make them cheaper by industry-compatible manufacturing

processes such as tape casting or screen printing, or to introduce

new and novel materials giving improved performance when meet-

ing all of the requirements of SOFC technology. One PM company

which has been actively involved in developing powders for metallic

interconnectors is Plansee AG in Reutte, Austria. Plansee supplies a

chromium-based powder, designated Ducrolloy Cr5Fe-1Y2O3 ,

which has a thermal expansion coefficient compatible with sta-

bilised zirconia (YSZ) electrolytes. The powder is used in Sulzer-

Hexis SFOC stacks.

Bernard Williams

The components and the finished article. SOFCs could find applicationsin small industrial and domestic power systems. Images courtesy Sulzer Hexis