The environmental of solid oxide he1 impact

cell ma ndacturing By Nigel Hart (Brunel University, and Rolls-Royce, UK), Nigel Brandon (Rolls-Roycesd Imperial ( College, London, UK),

and J&e Shemilt (Brunel University, UK)

S Solid oxide fuel cells (SOFCs) offer many advantages over conventional method

of electrical generation, including higher efficiencies and lower emissions. Thl

environmental benefits of the use phase of SOFCs have been extensivel: reported, but it is important that the total environmental burden is evaluated

This article compares the environmental impact of six fabrication methods whicl have the potential to be used within the SOFC manufacturing process: screen

printing, slurry spraying, tape casting, calendering, electrochemical vapou

deposition and physical vapour deposition.

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Fuel cell systems are recognised for their potential

environmental benefits, including high

efftciencies and low emission levels. However, for

fuel cells to be a truly “clean” power generation

technology, the total environmental impact must

be small, including both the manufacture and

disposal of plant. This work considers the

environmental impact associated with the

manufacture of one fuel cell variant, the solid

oxide fuel cell (SOFC), and focuses on processes

suited to the fabrication of the ceramic thick filn

used in these devices. Six such processes hav

been evaluated using a life cycle analysis (LCA

approach. The processes considered were scree

printing, [I1 slurry spraying, tape casting,”

calendering,131 electrochemical vapour depositio

(EVD) [*, 51 and physical vapour depositio

(PVD).[61 The application of each process to th

manufacture of a fuel cell stack is predominant1

governed by the nature of the fuel cell desigr

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Most designs use more than one technique, for

example screen-printed anode and electrolyte

with a sprayed cathode,“] or a tape-cast

electrolyte with sprayed electrodes.[*l

To ensure a common basis for comparison,

each manufacturing process was analysed on the

basis that it was producing two million 10 cm x

10 cm x 50 pm thick yttria-stabilised zirconia

(YSZ) electrolyte thick films per year. Thus, if

these films comprised the electrolyte of a cell

producing a power density of 0.5 W/cm2, then

the manufacturing plant would be producing

sufficient thick-film electrolyte to manufacture

100 MWe of SOFCs per year. No account was

taken of the production of the other functional

or structural materials needed to produce an

SOFC stack. Further simplifying assumptions

were that each of the six manufacturing routes

was capable of producing 50 pm YSZ thick

films, and that each also produced the same

quality of electrolyte, i.e. that cell performance

was independent of the manufacturing process.

These assumptions again enabled the processes

to be compared on a common basis.

Flowsheets were drawn up for each thick-film

manufacturing step, including all the unit

processes necessary to produce a dense YSZ

thick film. The analysis was carried out on the

basis of a plant working 240 days per year, and

operating one 8 hour shift per day.

Boundaries of the study The fabrication routes investigated here can be

separated into two categories, wet routes and gas

phase routes, as shown in Table 1.

The wet ceramic processes had a closed

boundary of study drawn around them as shown

in Figure 1. Both the starting powders and the

final sintering step were considered constant. All

wet route processes were assumed to require a

clean, air-conditioned environment. The energy

Fuel Cek Bulletin No. 15

and costs associated with this were based on floor

area, and are described as “building services” in

the analysis.

The gas-phase routes differ in precursor

materials, namely chlorides for EVD, and a solid

ceramic target for PVD, and in that they do not

require a sintering step. This study did not

include an assessment of the environmental

impact associated with the production of

precursor materials such as YSZ powders, or

zirconium and yttrium chlorides.

Results of the analysis The four wet ceramic manufacturing routes

assessed (screen printing, slurry spraying,

calendering and tape casting) do not use any

complex equipment requiring energy-intensive

manufacturing techniques or exotic materials.

The majority of the equipment could be reused

for other applications, the materiah recycled or

disposed of in landfill. EVD and PVD use

complex apparatus, including large high-vacuum

chambers and power sources, which are likely to

require energy-intensive manufacturing and

involve rare and expensive materials. This is

supported by the high capital cost of the

equipment. The reaction chambers are likely to

be contaminated and unsuitable for use in other

applications, although it is possible that

individual components could be reused.

The total energy requirements for each of the

six manufacturing processes considered in this

study will now be compared.

Screen printing A flexible fabrication process which is used in

many different applications, screen printing

relies on flowing ink through a screen under

pressure.1’1 The screen defines the area to be

printed. In this case, the ink contains fine YSZ

powder, together with suitable binders and

solvent. The manufacturing scheme assumes that

the YSZ powder is mixed with auxiliary

materials (i.e. binders, solvent and dispersant),

and then triple roll-milled to produce an ink.

The ink is screen-printed onto a substrate. The

film is dried to remove the solvents, and then

fired at high temperature to drive off the binder

and form a dense electrolyte film.

Slurry spraying During spraying the slurry is forced through a

small nozzle using compressed air. This forms an

aerosol which is projected onto the substrate. In

the manufacturing process YSZ powder is mixed

with the auxiliary materials, and then ball-milled

to form the spraying slurry. The slurry is sprayed

onto the substrate and dried before high-

temperature sintering.

Tape casting In tape-casting a flat surface, often a glass bed, is

covered with a carrier film onto which the layer

is deposited using a doctor blade.r21 The

manufacturing scheme assumes that YSZ

powder is milled with auxiliary materials to form

the tape-casting slip. This slip is then tape-cast

and the solvent removed by drying, resulting in a

“leather”-like layer. Films are cut by stamping,

before being sintered.

Tape calendering This process involves squeezing a softened

thermoplastic ceramic mix between two rollers to

produce a continuous sheet of material.[31 In the

assumed manufacturing process YSZ powder is

blended with the binder and plasticiser

components of the solvent system, and viscous

mixing used to remove agglomerates. The solvent

component is added, and an extrusion process used

to form a homogenous plastic mass. This is then

passed through rollers during the calendering stage

and the solvent is evaporated off. Films are cut by

stamping, before being sintered.

Electrochemical vapour deposition

(EW EVD involves growing a dense oxide layer on a

substrate. The growth takes place over two steps

- a pore closure stage and a scale growth stage. If

a dense substrate is used, only the second step is

required. A full description of the process can be

found in References 4 and 5. The EVD process

essentially consists of one reactor, although such

a reactor is not yet commercially available. The

design is based on vacuum furnace technology.

Based on the results of Ippommatsu et al., [91 the

energy requirement for producing dense thick-

film YSZ in a large-scale EVD reactor was

estimated to be to 100 kWh/m2 of electrolyte.

Physical vapour deposition (PVD) A PVD fabrication method is one in which the

source of the film forming material is a solid

(target) that needs to be vaporised and

transported to the substrate. This study focused

on sputtering techniques in which a potential

difference is applied across the material to be

deposited and the substrate. Inert gas ions are

generated and accelerated onto the target. These

impacting ions dislodge atoms of the target

material, which are then projected to, and

deposited on, the substrate.161 The

manufacturing flowsheet for PVD essentially

consists of one reactor, although again large-scale

PVD reactors are not yet commercially available.

The energy consumption of such a reactor was

estimated to be 6.25 kWhlm2 of electrolyte.

Table 2 summarises the energy requirements for

each of the six thick-film processes.

The EVD process has the highest energy

consumption at 100 kWh/m’ of electrolyte,

more than five times higher than for the other

processes. The energy requirements of the wet

route processes are very similar, at 14-17

kWh/m2. This is because the sintering stage

dominates energy use for all the wet route

processes, being around 75% of the total. The

energy requirement for PVD is the smallest at 6

kWh/m’. This reflects the fact that PVD does

not require a large mass of material to be heated

to a high temperature during electrolyte thick-

film fabrication. However, further work on the

energy required to produce the precursor

materials is still required before a full energy

assessment can be made.

Fuel Cells Bulletin No. 15

Cathode - Sprayed t Electrolyte - Screen Printed

Anode - Tape Cast -

Energy assessment of a 200 kWe CHP system The environmental burdens presented by the

manufacturing phase of the fuel cell were

compared with the potential benefits arising

from the use phase. A 200 kWe combined heat

and power (CHP) system, such as could find

application in a hotel or hospital, was used as the

basis for this comparison. This follows the work

of Hart eta~,[‘Ol who have assessed the use phase

of both fuel cell and conventional systems for

this application. The system provides both

electricity and heating, with a heat-to-power

ratio of 1.85. Hart etn~l’~l compared the SOFC

unit to conventional systems where the

equivalent electrical energy is obtained from the

national grid, and the heat output is obtained

from an on-site gas boiler.

The results are summarised in Table 3, which

shows that the SOFC system reduced the

emissions of NOx, SOx and CO by more than

one order of magnitude. The non-methane

hydrocarbons (NMHCs) were reduced to 84%,

CO, to 93%, methane to 75%, and particulate

matter (PM) to 93%, of the values produced by

the conventional technology. The reduction in

CO, output is a consequence of the increased

energy efficiency of the SOFC, as the same fuel

(natural gas) is used in all cases.

To enable the manufacturing phase to be

compared with the use phase, it is necessary to

consider a specific fuel cell design, since this will

clearly influence the manufacturing process. For

this study a supported fuel cell design was

considered, representative of a number of fuel

cell designs in which the electrolyte (YSZ) is

deposited onto a supporting nickel-YSZ cermet

anode.‘“, 121 The cell was based on a tape-cast

Ni-YSZ cermet anode of 500 pm thickness, a

screen-printed YSZ electrolyte of 25 pm

thickness, and a strontium-doped lanthanum

manganite cathode of thickness 50 pm slurry-

sprayed onto the electrolyte. It was assumed that

each layer was fired separately. The final

structure is illustrated in Figure 2. Assuming a

power density for the SOFC cells of 0.5 W/cm2

(assumed throughout this study), then a total

cell area of 40 m2 would be required to produce

200 kWe.

The energy and materials inputs required to

manufacture the cell components for a 200 kWe

solid oxide fuel cell stack were evaluated, taking

into account the thickness of the layers. The

analysis only considered the anode/

electrolyte/cathode structure of the SOFC. It

excluded other essential stack components such

as the interconnects, structural materials,

thermal insulation and casing, and auxiliary

equipment such as the pumps and heac-

exchangers required for a fully operational

system. Analysis of these components will be the

subject of further studies.

Table 4 summarises the energy required to

produce the films making up the SOFC structure,

and demonstrates that it would require an energy

input of 2.076 MWh to manufacture 40 m2 of

fuel cells of the design considered here. If this

energy were obtained from the national grid then

this will result in the generation of emissions.

Following the same methodology as that used to

analyse the use phase, 1101 the emissions associated

with the energy used during the manufacturing

phase are summarised in Table 5. Also shown is

the number of operating hours required to repay

the burden of these emissions, when comparing

the use phase of the SOFC system with

conventional technology. The power output of

the unit will vary with respect to the required

load; therefore a continuous output of 100 kWe

from the 200 kWe unit was assumed as a

representative case.

Table 5 demonstrates that the longest

repayment time was 73.5 h for non-methane

hydrocarbons, with 49 h for CO, and 47 h for

methane. Other burdens were repaid in shorter

times of around 20 h. All the burdens associated

with cell manufacture were therefore repaid in

just over three days’ use, and-after this the unit

showed a net environmental benefit. Future

work will extend the analysis of SOFC

manufacture to cover the impact of other stack

components, such as interconnects and

structural components, as well as considering the

sensitivity of the analysis to stack design.

Conclusions The manufacturing plant associated with the four

wet ceramic routes assessed (screen printing, slurry

spraying, calendering and tape casting) for SOFC

production does not present a significant burden

on the environment during either its construction

or disposal. EVD and PVD plants have the

potential for higher environmental burdens, but

when considered against the total life cycle of the

SOFC system, these will not be significant. Five of

the production methods have good material

utilisation, with only EVD reported to have a poor

materials utilisation during manufacture. While

this work has identified the quantity and nature of

the materials input to these processes, further work

is required to assess the impact of the emissions

arising from these inputs.

Fuel Cells Bulletin No. 15

An analysis of the energy required to

manufacture the anode, electrolyte and cathode

layers for a 200 kWe SOFC CHP unit has shown

that the environmental burden associated with

manufacturing is repaid within three days of

operation of the unit, when its use phase is

compared to conventional technology. Further

work is required to look at the burdens associated

with the manufacture of other essential stack

components such as interconnects, structural

ceramics, insulation, and stack casing.

Acknowledgments N. Hart is supported by an EPSRC EngD

Studentship. The authors would like to thank

Rolls-Royce, Gaz de France, IRD (Denmark),

EPFL (Switzerland), Lenton Thermal Design

(UK), SM Tech (UK), CERAM Research (UK),

Binks-Bullows (UK) and IMI Air Conditioning

(UK) for help in supplying information and data

for the study. This work was carried out with

financial support from the European

Commission through the “Industrial and

Materials Technologies” programme under

contract number BRPR-CT97-0413.

This short paper is drawn from a forthcoming

publication, which contains further details of the

analysis.l131

References 1. P.J. Holmes, R.G. Loasby: “Handbook of

thick film technology” (Electrochemical

Publications, 1976), 14.

2. H. Hellerbrand: “Processing of ceramics, Part

I”, in: R.W. Cahn, I? Haasen, E.J. Kramer

(Eds.): “Materials science and technology, vol.

17A” (Wiley-VCH, Weinheim, I995), 190.

3. K. Kordesch, G. Simader: “Fuel cells and their

applications” (Wiley-VCH, Weinheim, 1996), 146.

4. N.Q. Minh, T. Takahashi: “Science and

technology of ceramic fuel cells” (Elsevier

Science, 1995), 247.

5. R.A. George, N.F. Bessette: “Reducing the

manufacturing cost of tubular solid oxide fuel

cell technology’: J Power Sources 71(1/2)

(March 1998) 131-137 (5th Grove Fuel Cell

Symposium, London, UK, September 1997).

6. PK. Sivastava, T. Quach, Y.Y. Duan, R.

Donelson, SE Jiang, ET Ciacchi, S.l?S. Badwal: in:

B. Thorstensen (Ed.): Proceedings of 2nd European

Solid Oxide Fuel Cell Forum (European Fuel Cell

Forum, Oberrohrdorf, Switzerland, 1996), 761.

7. “Integrated planar solid oxide fuel cell (IP-

SOFC): Evaluation phase summary report”.

ETSU report F/OI/00070/REP 1996.

8. C. Bagger, M. Juhl, I? Hendriksen, I! Larsen,

M. Mogensen, J. Larson, S. Pehrson: in: B.

Thorstensen (Ed.): Proceedings of 2nd

European Solid Oxide Fuel Cell Forum

(European Fuel Cell Forum, Oberrohrdorf,

Switzerland, 1996), 175.

9. M. Ippommatsu, H. Sasaki, S. Otoshi:

“Evaluation of the cost performance of the

Stop Press News

MIT1 guiding fuel cells

The Ministry of International Trade & Industry

in Japau will set up a joint publicfprivate body

before the end of the year, to coordinate the

development of fuel cell technology, ministry

sources have told the N&on hhkzi Shimbun.

The ministry aims to improve the

competitiveness of Japanese companies by setting

standards that become accepted globally in key

aspects of me1 cell production. Methods of storing

and supplying hydrogen for fuel cells involve

methanol, gasoline and special metal alloys. The

new research body will evaluate the best storage

and supply methods to help companies develop

these key technologies efficiently.

The ministry believes that Japanese companies

excel in important areas like the removal of

electrons from hydrogen molecules and the

development of catalysts. In fields where Japanese

firms have the edge over foreign competition, the

ministry will help them advance their

development efforts. The ministry will earmark

Y2 billion (US$17 million) for the project in its

request for funds from the secondary

supplementary budget for the current fiscal year.

Demo project in Montana

The Big Shy Economic Development Authority

in Biigs, Montana has received US$350,000

to conduct a hydrogen fuel cell feasibility study

and demonstration project, The feasibility study

phase could be completed by August 2000, and

its bndings published the following month.

The Technical Advisory Team has

communicated with several major fuel cell

manufacturing companies, three of which have

expressed interest in Billings as a manufacturing

location and/or made a preliminary visit.

The Advisory Team is exploring three

technology areas: to improve air quality and

address environmental concerns, to provide cost-

effective alternative power for remote applications,

and to provide reliable back-up power.

The local Stillwater Mining Company is one

of the worlds leading producers of platinum

group metals, and could provide a proprietary

precious metal fuel cell catalyst.

SOFC cell in the market”, ht. J of Hydrogen

Energy21(2) (February 1996) 129-135.

10. D. Hart, G. Hormandinger: “Initial

assessment of the environmental charact-

eristics of fuel cells and competing tech-

nologies, vol. 1”. ETSU report F/02/001 1 I/

REP/l, 1997.

11. M. Cassidy, K. Kendall, G. Lindsay: in: B.

Thorstensen (Ed.): Proceedings of 2nd

European Solid Oxide Fuel Cell Forum

(European Fuel Cell Forum, Oberrohrdorf,

Switzerland, 1996), 667.

12. H.I? Buchkremer, U. Diekmann, D. Stover:

in: B. Thorstensen (Ed.): Proceedings of 2nd

European Solid Oxide Fuel Cell Forum

(European Fuel Cell Forum, Oberrohrdorf,

Switzerland, 1996), 221. \

13. N.T. Hart, N.l? Brandon, J. Shemilt:

“Environmental evaluation of thick film ceramic

fabrication techniques for solid oxide fuel cells”,

Materials & Manufacturing Processes 15(2)

(2000, to be published).

For more information, contact: Dr Nigel Brandon,

Senior Lecturer in Electrochemical Engineering,

Imperial College of Science, Technology & Medicine, T H

Huxley School of Environment, Earth Sciences &

Engineering, Royal School of Mines Building, Prince

Consort Road, London SW7 2BP, UK.

Tel: +44 171 594 7326,

Fax: +44 171 594 7444,

Email: n.brandon@ic.ac.uk

Automotive hydrogen storage

Impco Technologies Inc is claiming a

breakthrough in hydrogen storage

technology for automotive applications. The

company says that its proprietary, advanced

hydrogen storage cylinder technology

removes the barriers to more rapid

commercialisation of hydrogen-powered

internal combustion engines and fuel cells.

The ultra-lightweight, low-cost hydrogen

storage technology addresses problems relating to

permeability, diffusion of hydrogen through

storage cylinder walls, and the effects of hydrogen

embrittlement on metal storage cylinders.

Developed at the company’s Advanced

Technology Center in Irvine, California, the

advanced composite hydrogen storage cylinder

provides a dramatic weight reduction, increased

fuel storage capacity, and the capability to produce

a wide range of sizes. With a 5000 psi (350 bar)

operating pressure and a designed hydrostatic

failure pressure of 14,000 psi (980 bar), the storage

cylinder technology offers an unparalleled degree

of safety.

Fuel Cells Bulletin No. 15 0