The opening sessions brought to light the considerable increase in collaboration and consolidation of commercial and research activities in SOFCs. Collaboration was most clearly demonstrated by the formation in the USA of SECA (Solid State Energy Conversion Alliance) to coordinate US SOFC activities, the continuing work in Japan of NEDO (New Energy and Industrial Technology Development Organisation), and in Europe the latest stage of the European Commission programme Framework V.
Also of great interest was the strong presence of the BMW-Delphi project to develop an SOFC APU (Auxiliary Power Unit) for use in trucks and luxury vehicles. This is one of the first serious attempts to bring SOFCs into the transport market and a clever one as it bypasses direct competition with internal combustion engines. In such a scheme the SOFC is not used for propulsion, instead it is used for electricity generation for auxiliary systems such as refrigeration, fuel injection and air conditioning. As well as significant efficiency gains (20 per cent) the elimination of the alternator (used to generate electrical power from the engine) means that auxiliary systems can be operated whilst the engine is off. Further efficiency gains can be made if SOFC exhaust is fed into the internal combustion engine.
Electrolyte Materials, Processing and Performance
The driving force behind many of the electrolyte presentations was the quest for lower operating temperatures. Low temperature operation is limited by decreasing electrolyte ionic conductivity as temperature is reduced. The state of the art electrolyte - Yttria Stabilised Zirconia (YSZ) - has many benefits, such as long term stability and compatability with cell operating conditions and electrode materials, but exhibits low ionic conductivity. Solutions to this are thinner electrodes or alternative materials.
Electrolyte thickness has seen a 2-3 order of magnitude reduction from the several hundred micron electrolyte-supported early designs to the few-micron electrolytes widely used today (with submicron electrolytes under development). Reducing thickness to this level whilst maintaining gas tightness requires excellent adhesion to electrodes and very high & homogenous densification. The use of nanoscale powders was reported by several groups and gives benefits such as higher ionic conductivity, improved activity and greater homogeneity.
Among alternative electrolytes Gadolinium doped Ceria (CGO) received most attention, the focus being understanding and inhibiting the (undesirable) electronic conduction and expansion seen under certain cell conditions. Also widely reported was use of Lanthanum Gallate which boasts high ionic conductivity without significant electronic conductivity, but is prone to interact with nickel anodes. These and many other materials under investigation require considerably more development before they can compete with YSZ.
Cathode Materials, Processing and Performance
As in the case of electrolytes, many papers were motivated by reducing operating temperatures. It was widely noted that in thin electrolyte cells, even at lower temperatures (perhaps 600 degrees C), cathode losses dominate and thus reduction of cathode overpotential is complementary and/or an alternative to changing electrolyte material.
The state of the art cathode material is Strontium-doped Lanthanum Manganite (LSM) and much attention was devoted to processing and optimisation of cathodes of this type, particularly on forming good Triple Phase Boundaries (TPB) at the electrolyte interface.
The primary variant on the state of the art is the development of cathodes which take advantage of ionic conduction (of oxygen ions) in order to generate extended TPBs. This can be achieved through the use of composite cathodes combining ionic conducting YSZ with electronic conducting LSM, or through the use of Mixed Ionic & Electronic Conductors (MIEC) cathodes such as LSCF (based on cobalt and iron) or nickelites. These materials have their own problems such as the formation (when using LSCF) of lanthanum and strontium zirconate at the YSZ-LSCF boundary.
A very interesting paper presented in the Cathode session concentrated not on SOFC technology but on lab equipment. Electrochemical Impedance Spectroscopy (EIS) is invariably used to attempt to separate losses associated with different cathode processes such as gas transport, charge transfer and solid state diffusion. The paper outlined a technique under development to utilise the information contained not only in the first but in the higher harmonics of the spectrum thus giving better resolution, particularly for nonlinear phenomena.
Anode Materials, Processing and Performance
The anode session was dominated by papers on the inhibition of carbon deposition, particularly in relation to direct oxidation/internal reforming of methane. The state of the art Nickel-YSZ cermet anode is not well suited to this purpose due to the high activity of nickel in forming carbon (or coke). A number of papers investigated the possibility of improving this performance through microstructural changes and additional components. Ongoing work on redox mechanisms and the influence of microstructure (down to the nanoscale) was also reported.
Alternative anode materials based on ceria are under investigation due to its high activity in oxidising hydrocarbons without carbon formation, but it was noted that another material such as copper must be added to give adequate electrical conductivity.
Interconnection, seal and SOFC metallic materials and processing
This session covering the remaining components of the SOFC stack the interconnect, current collectors, supports and seals was once again divided between high and low temperature operation. At high temperature ceramic interconnects based on lanthanum chromite are typically used. Issues considered in relation to these materials are the migration of dopants such as calcium and stresses generated by expansion during redox cycling.
For low temperature operation, indeed one of the key arguments supporting low temperature operation, "cheap" austenitic and ferritic stainless steels can be used as interconnects. These however suffer from formation of poorly conducting chromium oxides and in some cases excessive evaporation of chromium resulting in cathode poisoning. Attempts to address these issues include coating with other materials such as lanthanum and manganese to generate conductive spinel or perovskite phases which inhibit undesired oxide formation. Work is also underway to develop steels specifically for SOFC components.
In sealing technology we see a move away from rigid glass ceramic seals in favour of potentially more robust metallic, mica and ceramic fibre sheet seals. These systems require a permanent sealing load but may be more amenable to relative movement.
Cell Design Fabrication and Performance
This session gave established groups a chance to showcase their latest high performance cells and also gave a multitude of small and/or new groups the chance to demonstrate their capability to produce state of the art and "novel" SOFC assemblies.
For established groups, major preoccupations were achieving long term performance and redox capability and improving production. To this end, commercial cell output has been ramped up, yield is improving and cost dropping due to the adoption of cheaper fabrication techniques such as screenprinting and spray deposition and the reduction in firing steps, temperature and time through improved raw materials, processing and understanding of component interactions. At the extreme vacuum plasma spraying was suggested as a method of eliminating all firing steps.
This session also saw a number of papers describing further anode, cathode and electrolyte combinations such as the use of scandia in place of yttria for zirconia stabilisation giving increased conductivity, and the use of complex mixed ionic and electronic conductors (MIEC) systems for low temperature (300-650 degree C) SOFCs.
SOFC Fuels
Papers at this session ranged from calculation of equilibrium composition of various fuel air mixtures at SOFC conditions to practical implementation of steam and dry partial oxidation of methane, diesel, gasoline, alcohols and propane fuels. Further studies included suggestions for synthetic fuels such as dimethyl ether.
Motivation for this work was typically either the commercialisation of SOFCs requiring the ability to use standard fuels or a more strategic recognition of the poor properties of hydrogen as a large scale energy carrier seeking "the carrier" which will bring about the transition to an "electrochemical" rather than "hydrogen economy".
Cell, Stack and System Modelling
The modelling session saw a wide range of modelling scales, techniques and degrees of sophistication. The most striking presentation came from the SECA modelling group who presented work in progress on a range of models covering everything from microstructural parameters to full system models and use of massive parallel processor facilities. Ambitious plans were also presented by Forschungszentrum Juelich which has taken a strategic approach to modelling by simultaneously developing in parallel efficient in-house codes alongside 3-D stack models using a commercial CFD package.
Somewhat removed from the physics an alternative modelling approach presented was a control theory based model of stack dynamic performance. This model contained very little physics yet by characterising key parameters and applying fuzzy logic to incorporate "expert knowledge" gave potentially very useful results.
Finally, papers on the mechanical integrity of cells considered the effects of residual stresses due to thermal expansion mismatch of components, thermally induced stresses and creep.
Conclusions
SOFC VIII was a great success and illuminated many trends in the field:
- SOFC interest continues to grow with many new research groups contributing to the massive 1,500 page proceedings of this conference.
- Increasing collaboration in Europe, Japan and the USA is bringing more rapid and coherent progress.
- The significant investment made recently by BMW and Delphi in SOFC APUs promises to bring SOFCs into the "consumer product" automotive market.
- Ever thinner YSZ electrolytes and the maturing development of alternative materials is greatly improving high temperature performance and making possible intermediate temperature operation.
- Understanding of LSM cathodes is allowing high performance at high temperatures and the development of mixed ionic and electronic conductor cathodes promise excellent low temperature performance.
The author is currently studying for a Ph.D. in solid oxide fuel cell technology at Cambridge University in the UK. He also runs the Fuel Cell Knowledge website.