Page 350 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

32 6 High Temperature Solid Oxide Fuel Cells: Fundnmentak, Design and Applications

self-consistent solution of the Schrodinger equation for a cluster (up to
about 100) of atoms and a set of (usually periodic) boundary conditions
[78-SO]. Ab initio calculations can be useful in furthering the
understanding of the electrode process. For example, the computed barrier
to desorption of H20ad on Ni may be used to see whether it is the rate-
determining step. Calculations can also be used to understand the reaction
of Had and Oad, Had and OH,d as well as adsorption and diffusion of H, OH,
and H20 on the YSZ. This could be very useful in finding new catalysts
capable of providing a current orders of magnitude larger than Ni as only a
very small fraction of Had on Ni participates in the oxidation reaction
[ 79,801. By effectively eliminating the need for the self-consistent iterative
process, accurate and robust ab initio molecular dynamics are now
available, and simulations for systems of more than 100 atoms can be
performed in a single CPU [ 8 1,s 21.
0 Hybrid techniques. These use, for example, quantum mechanical
techniques or their simplified variants to provide the effective potentials
needed for the simulation of interatomic forces [83]. The computational
efficiency and accuracy of the hybrid methods fall between the ab initio
methods and the empirical methods.

With the improvement in hardware and software tools, the ab initio electronic
structure calculations will gain importance because they can deal with
increasingly complex systems and yield higher precision in the result. Along
with this trend, the hybrid techniques will grow in relevance. It is expected that
the hybrid methods will play an important role in the molecular-level modelling
of SOFCs in the near future.

11.10 Summary

Modelling of SOFCs is advancing at a rapid rate, facilitating quick predictions of
SOFC performance at a number of levels, and aiding the design of SOFC systems.
Macroscopic flow and thermal models are the best known and have followed
from straightforward chemical engineering principles of mass and energy
balance. At the nanoscale of atoms and molecules, predictions of material
behaviour and of interface interactions are also becoming possible. Most
significant advances are now taking place in the understanding of complex
composite structures of electrodes and three phase boundaries. Ultimately these
should lead to predictions of cell behaviour which at present are measured
empirically and inserted into stack models. Stack modelling has advanced to
the point where acceptable start-up rates can be predicted and where
overall performance can be optimised. The integration of these stacks into
complete systems can also be predicted with some precision, leading to new
design possibilities for hybrid SOFCs. In the immediate future, it is anticipated
that models which combine the macroscopic and atomistic approaches will
develop rapidly.