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SCIENTIFIC ADVANCES IN FUEL CELL SYSTEMS REPORTED IN MUNICH
Source: Platinum Reveiw
"Scientific Advances in Fuel Cell Systems" (1) was the theme
of the second in a series of biannual European meetings on fuel cells,
that are held in alternate years to the Grove Symposia (2). The conference
reported here was held on the 6th and 7th October 2004 at the Hilton Munich
Park Hotel, Munich, following the first meeting in Amsterdam in 2002 (3).
Organised by Elsevier, these conferences provide a balance between the
scientific and the more commercial aspects of the technology. Authors
from around the world had submitted oral papers and posters for this technically-orientated
programme. The meeting attracted 320 delegates from 39 countries including
Germany [76], Japan [47], the U.K. [35], Italy [22] and the U.S.A. [13].
These represented universities, research organisations, and fuel cell
component and system manufacturers.
Besides almost 60 oral papers, there were 170 high quality poster presentations,
and many of these will be published in a special edition of the Journal
of Power Sources in May 2005. The symposium consisted of eight sessions,
on membrane science, fuel processing, materials science, electrochemistry
and catalysis, cell and stack technology, and systems and applications.
For this review, only work that involves the platinum group metals (pgms)
is being reported.
Performance Targets for Transport
The conference began with a Plenary Session and a keynote talk. In this,
Frank Preli of UTC Fuel Cells, U.S.A., outlined some of the performance
targets for transport applications, compared to what is currently achievable
with fuel cells. These can be summarised by five interrelated characteristics:
power density, operability (including factors such as cold starting and
the range of acceptable ambient conditions), efficiency, durability and
cost. Small passenger vehicles are likely to need a power output of around
85 kW, while buses will need 100-200 kW. To compete successfully with
the internal combustion engine, stack power density will need to be over
1.6 kW litre-1, while the complete system will need to provide more than
0.5 kW litre-1 to minimise intrusion into the passenger space. High overall
thermal efficiency implies a cell voltage in excess of 0.750 volts, with
a decay rate of less than 2 mV per 1000 hours for adequate lifetimes.
Performance will need to be maintained over 17,000 start-stop cycles despite
adverse operating conditions, such as less than 60% relative humidity
at the air inlet. In addition, stacks will need to have the ability to
start up quickly from the frozen state for up to 1000 times without damage.
PEMFCs to Achieve these Goals
Proton exchange membrane fuel cells (PEMFCs) operating at relatively low
temperatures and catalysed by pgm catalysts offer the best chance of achieving
these goals. The cost objective for wide-scale applications in transport
is set at $35 kW-1. Currently, the cell stack plates and membrane electrode
assemblies (MEAs) account for 50% of the cost of the whole system, and
various means to reduce this proportion are being considered. These include
using injection moulding techniques for the inter-cell plates. For high
stack power density, the internal resistance of individual cells needs
to be reduced, to provide power densities upwards of 0.7 volts per cell
at 0.6 A cm-2. Water management within the membrane has been identified
as one of the principal limiting factors to cell performance.
For comparison, the stationary 200 kW PC25 phosphoric acid fuel cell
system made by UTC costs $4,300 kW-1. Several hundred of these highly
developed units have accumulated over 6 million operating hours. Laboratory
trials indicate that marginally reducing the operating temperature of
the fuel cell stack has a marked effect in reducing sintering of the platinum
catalyst, enabling stack lifetimes to be doubled to over 80,000 hours
or 10 years. In the past few years the life of PEMFCs has also been extended
from a few hundreds of hours to over 10,000 hours and laboratory tests
indicate that these can be extended to over 20,000 hours.
Progress in PEM Technology
In a later keynote talk, Charles Stone, of Ballard Power Systems, Canada,
also discussed the technical challenges and progress made in PEM technology.
The performance and durability of PEMFCs are already established, and
products being evaluated by the public include 30 buses in the Clean Urban
Transport for Europe (CUTE) programme, with 9 more in California, Australia
and China. Hydrogen has evolved as the fuel of choice, and a supply infrastructure
is being developed in several parts of the world. Power densities over
2.2 kW litre-1 are achievable for stacks operating at 0.6 volts per cell.
However, component costs need to be reduced, while technology needs to
be developed for the rapid start of frozen cell stacks, for water management
and for resistance to impurities in the reactant gas. In particular, air
cathodes that can operate at over 1 A cm-2 need to be developed.
In the longer term, modelling and measurement tools should enable more
rapid progress in optimising systems. Membranes with increased durability
have been developed; these have resistance to peroxy radical attack and
reduced weight loss with time. Degradation of the anode catalyst carbon
support has been greatly reduced, thanks to a programme jointly carried
out by Ballard and Johnson Matthey, and new catalysts have demonstrated
up to 500-fold greater stability.
For cathode catalysts, graphitised Vulcan carbon supports provide greatly
improved stability and durability. Considerable progress has been made
in reducing the pgm content of PEMFCs. Platinum loadings have been reduced
to around 1.0 mg cm-2 for electrodes made using screen printing, compared
to loadings of 8-10 mg cm-2 Pt that were common ten years ago. These are
projected to fall further to 0.3-0.5 mg cm-2 using roll coating techniques,
and ultimately to 0.1 mg cm-2 with chemical vapour deposition methods.
Improved current collector plates and MEAs with new gas diffusion layers
have helped to improve water management and hence performance.
Stone emphasised the need to carry forward improvements to all the interrelated
aspects of the technology. Several of the subsequent papers highlighted
the efforts being devoted to understanding and developing models of various
aspects of PEMFC operation including water management, low temperature
start-up, and the quest for improved catalyst and membrane materials.
Membrane Science
Direct oxidation of methanol in fuel cells has made considerable advances
in recent years to the point where several are on the verge of being manufactured
commercially. In a talk entitled '0.5 W/cm2 PCM-based methanol-air fuel
cell - recent progress at Tel Aviv University', A. Aharon provided details
of a novel and inexpensive ($4 m-2) nanoporous proton conducting membrane,
consisting of a non-conducting ceramic powder mixed with a polymer binder
and an acid. This is extremely permeable to water penetration during cell
operation.
One of the problems of direct methanol fuel cells (DMFCs) is the undesirable
migration of solvated water through the membrane, associated with protons,
with up to 18 molecules of water transferred for each molecule of methanol
oxidised. This has been identified as a major reason for cathode flooding
and performance loss at high current densities. The high porosity of the
Tel Aviv University membrane enables surplus water to permeate back through
the membrane to the anode compartment. Substituting trifluoro-methane
sulfonic (triflic) acid for sulfuric acid yields performances of 0.5 volts
at 0.8 A cm-2 with a platinum loading of 4 mg cm-2. These performances
have been demonstrated on small scale cells which are being increased
in size to 50 cm2 bipolar cells, and a 12 W cell is being built which
will occupy 900 cm3.
Water transfer across the membrane is also accompanied by loss of methanol
into the cathode compartment by migration. Another approach to the problem
of methanol and water diffusion in DMFCs was presented by L. Pitol-Filho,
of the Universitat Rovira I Virgili, Spain. Composite membranes were made
from mixtures of polysulfone (PSf) and poly(ethylene glycol) (PEG) and
used to study the rates of transfer of reactants. PEG contains OH- hydrophilic
groups which combine with hydrated protons, and experimental data confirmed
that higher PEG concentrations assisted proton transport, with a plateau
of about 50% PEG. At this ratio, the ratio of H+: methanol was about 8.8
: 1 compared to 4.0 : 1 for membranes containing 20% PEG.
Fuel Processing
A paper by Q. Li of the Technical University of Denmark, 'Integration
of high temperature PEMFC with a methanol reformer' emphasised the need
for high PEMFC stack operating temperatures to facilitate integrating
the reformer into a system. Newly-developed thermally stable polymer membranes
such as acid-doped polybenzimidazole membranes allow PEMFCs to operate
at up to 200ºC. Using platinum/ruthenium alloy anode catalysts, raising
the operating temperature from 80 to 200ºC increases the tolerance
to carbon monoxide in the fuel gas from 100 ppm to over 30,000 ppm (that
is, 3%). The CO impurity content in the hydrogen from a methanol reformer
is typically less than 1% by volume, and hence reformate can be directly
used to fuel the PEMFC. So far, small (10 cm × 10 cm) fuel cell
stacks working at 170ºC combined with methanol reformers operating
at 210ºC have demonstrated performances of up to 50 A at 750 mV per
cell.
Electrochemistry and Catalysis
Work by the Energy Research Centre of the Netherlands ECN, has confirmed
that carbon dioxide, which is present in reformate fuel in concentrations
of up to 25%, can have a detrimental effect on fuel cell performance that
goes beyond the dilution effects associated with an inert gas. In his
talk 'Carbon dioxide poisoning on proton-exchange-membrane fuel cell anodes',
G. J. M. Janssen explained that these poisoning effects arise from the
reverse of the water gas shift reaction: CO2 is reduced by hydrogen to
a reduced form (most likely carbon monoxide) which is adsorbed preferentially
on the catalyst. Carbon monoxide is a well-known poison for pure platinum
catalysts at low temperatures, the catalyst becoming inactive for hydrogen
dissociation. From kinetic data, it is evident that some bimetallic catalysts
also catalyse the oxidation of the adsorbed species to CO2. Hence catalyst
poisoning can be mitigated by using bimetallic alloy catalysts such as
supported Pt/Ru, which has a high CO electrooxidation rate constant.
In a talk entitled 'Novel high performance platinum and alloy catalysts
for PEMFC & DMFC', Y. Tsou of the E-TEK Division of De Nora North
America Inc., reviewed the status of their products, including catalysts,
gas diffusion electrodes, and membrane electrode assemblies (MEAs). In
a departure from their traditional platinum sulfite route for catalyst
preparation, E-TEK have developed new platinum chemistry to provide materials
with a more homogeneous particle size and higher surface area. This enables
increased metal loadings on carbon to be achieved while maintaining high
metal surface areas. Using thermal treatments, true bimetallic alloy catalysts
such as Pt/Ru can be produced with up to 50% metal loading on carbon.
When used in PEMFCs, these exhibit resistance to CO poisoning. Alloy catalysts
with a ratio of 80% Pt to 20% Ru have been found to provide optimum performance
for DMFCs, while other carbon-supported alloy catalysts such as Pt/Mo,
Pt/W and Pt/Sn can be prepared.
In his talk 'Ultra-low Pt loading anode for DMFC application', A. S.
Aricò of CNR-ITAE Institute, Italy, described work undertaken to
reduce the pgm requirements for DMFCs. A preparation procedure allowing
the surface decoration of unsupported Ru catalysts by Pt nanoparticles
has been developed. These have been examined by electrochemical stripping
voltammetry to compare their electrocatalytic activity to state-of-the-art
carbon supported Pt-Ru (1 : 1) alloys and bare unsupported Ru catalysts.
Suitable performances have been achieved with ultra-low Pt loadings on
DMFC anodes at temperatures of 80-130ºC. Reducing the anode Pt loading
by a factor of twenty produces a loss of power density of about 35%. Catalysts
containing small amounts of Pt nanoparticles on the surface of a less
expensive metal, such as Ru, may prove a useful route to reducing catalyst
costs for DMFC devices.
However, Aricò emphasised that improvements to catalytic activity
are dwarfed by the effects of temperature in increasing the reaction rate.
With an anode loading of 0.1 mg cm-2 of platinum, and 1 molar aqueous
methanol solution, raising the operating pressure and the temperature
to 130ºC could increase the current density from less than 0.6 A
cm-2 to almost 4.0 A cm-2 at 0.4 volts per cell.
Stack and Cell Technology
In view of the current activity in commercialising micro fuel cells for
electronic and consumer applications, a paper entitled 'Miniaturised proton
exchange fuel cell in micromachined silicon surface' by G. D'Arrigo of
CNR-IMM, Italy, was of particular interest. In this work, PEMFCs were
fabricated using technology developed for ultra-large scale integrated
(ULSI) microchips. The fuel cells consist of two symmetrical structures
fabricated on 2.2 cm × 1.9 cm porous silicon wafers. Miniature rhomboidal
microchannels, several microns below the surface, are formed by surface
micromachining and etching processes. The rhomboidal trenches thus formed
are closed up and formed into microchannels by depositing a surface layer
of silicon using chemical vapour deposition. These microchannels distribute
fuel and oxidant across each electrode. A patterned gold layer is used
to define permeable porous sector areas and to collect the current.
Metallic clusters of Pt or Ru catalysts are deposited inside the porous
silicon skeleton by electrodeposition, while the proton exchange membrane
is deposited on the patterned porous membrane by a spinning process. The
structures represent a novel method of miniature cell construction and
possibly a new application for the electronics industry.
Poster Exhibition
The large number of posters concerned with pgm catalysts reflects the
intense interest in direct methanol and direct ethanol fuel cells, and
CO-tolerant catalysts for PEMFCs. A wide variety of catalysts are being
investigated, including alloys of platinum with ruthenium, rare-earths,
selenium and tin. These are supported on materials ranging from titanium
mesh to carbon nanotubes, mesoporous carbon and acetylene carbon black.
In their poster 'Towards shape selective PEMFC/DMFC catalysts: Dependence
of CO oxidation on Pt nanoparticle shape', S. Kinge et al. (Max-Planck-Institut
für Kohlenforschung, Germany) describe how a "seeding method"
has been developed to produce shape-specific nanocrystallites. This was
used to deposit 4 nm truncated octahedral platinum nanoparticles on Vulcan
XC72 carbon (20 wt.% Pt). Examination of these catalysts by cyclic voltammetry
in a rotating disc electrode method shows two peaks corresponding to two
different sites of CO oxidation, with peaks assigned to the {111} (0.72
V NHE) and {100} (0.83 V) crystallographic planes of the platinum particles.
A poster entitled 'A new direct methanol fuel cell by a membrane electrode
assembly zigzag folded down' is self explanatory. This novel design for
a micro DMFC is proposed by M. Shibasaki et al. of Tokyo University of
Science. A membrane is folded into a 3-dimensional zigzag shape with electrodes
inserted into the folds. Methanol is fed from one side of the assembly,
and air diffuses into the cathode side via a porous insulator layer. Overlapping
cathode supports provide inter-cell connectors to enable multiples of
cells to be connected in series. A cell with an active area of 16 cm2
× 2 has demonstrated 3 mW cm-2 power density, operating on 2 M methanol
and at ambient temperature.
Poster Prize Awards
Of the 170 posters presented at the conference, twelve were highly commended
and, after a short presentation to the final selection panel, four of
these were chosen to receive a prize. In the Low Temperature Fuel Cell
category, W. Y. Lee et al. (Korea Institute of Energy Research, Korea,
(ROK)) received a prize for the poster 'Effect of micro-layers in gas
diffusion layer on the performance of PEMFCs'.
In the Systems and Applications category, M. Oszcipok (Fraunhofer Institute
for Solar Energy Systems, Germany) was awarded a prize for the poster
'Statistic analysis of operational influences on the cold start behaviour
of PEM fuel cells'.
In the High Temperature Fuel Cells category, K. Sugiura (Osaka Prefectural
College of Technology, Japan) gained a prize for the poster 'Evaluation
of volatile behaviour and the volatilization volume of molten salt in
DIR-MCFC by using the image measurement technique'.
Finally, in the Fuel Processing and Storage category, G. O. Alptekin
(TDA Research Inc., U.S.A.) was awarded a prize for the poster 'Selective
sorbents for natural gas desulfurization'.
Conclusions
The increased attendance at the conference compared to the one held in
2002, particularly by the academic community and research students indicates
a higher level of interest in the technical aspects of fuel cells. The
tremendous variety of papers and posters provides ample evidence that
new ideas and innovative designs continue to advance fuel cell technology,
and are rapidly overcoming the remaining barriers to producing commercial
devices. Perhaps most significantly, development of modelling and other
experimental techniques will make it possible to study the complex interrelated
characteristics of cells, stacks and systems to enable even more rapid
strides to be made in future.
References
1 http://www.fuelcelladvances.com/oral.htm
2 http://www.grovefuelcell.com/; D. S. Cameron, Platinum Metals Rev.,
2004, 48, (1), 32, (8th); D. S. Cameron, Platinum Metals Rev., 2001, 45,
(4), 146, (7th); D. S. Cameron, Platinum Metals Rev., 1999, 43, (4), 149,
(6th); D. S. Cameron, Platinum Metals Rev., 1997, 41, (4), 171, (5th);
G. A. Hards, Platinum Metals Rev., 1995, 39, (4), 160, (4th); D. G. Lovering,
Platinum Metals Rev., 1993, 37, (4), 197, (3rd); D. G. Lovering, Platinum
Metals Rev., 1991, 35, (4), 209 , (2nd); D. G. Lovering, Platinum Metals
Rev., 1989, 33, (4), 169, (1st)
3 D. S. Cameron, Platinum Metals Rev., 2003, 47, (1), 28
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