8 Prakt. Met. Sonderband 47 (2015)
4.2 Applications in energy technology
One of the most urgent problems that have to be solved in the near future is the energy problem and
the environmental impact resulting from fossil fuels. Electrochemical reactions in “electrochemical
devices” which describe the products of a group of technologies including fuel cells, batteries, elec-
trolysers and super-capacitors, are supported by porous materials, which need to combine a range of
functions to achieve effective performance - for example, supporting electrocatalysis, diffusion and
charge transfer. The physical nature of these highly complex electrode geometries will affect all of
these processes, and therefore, there is a direct link between material microstructure and device per-
formance. With improvements in X-ray microscopy and X-ray tomography techniques, direct rela-
tionships between the microscopic structure of electrode materials and their macroscopic perfor-
mance at the device level can be established [13].
A promising secondary energy carrier is hydrogen which can be used in different types of fuel cells.
Several storage technologies for hydrogen that rely on new materials are under investigation, like
metal hydrides or porous materials (metal organic frameworks, carbon nanotubes). Another possible
approach is based on a cyclic redox reaction, having water and highly reactive nano-sized iron
powder in one half cycle, and magnetite powder and hydrogen in the other half cycle (3 Fe +4 H20
«> Fe304 + 4 Ho) — see Figure 6 [14]. One issue that has to be solved is the decrease of storage ca-
pacity after repeated storage cycles. High temperatures lead to particle coarsening which results in
deactivation and loss of storage capacity and kinetics. Metal oxides like aluminium oxide as addi-
tive in the reduced iron oxide can preserve the reactivity of the Fe/Fe3O4 powder throughout cy-
cling. The morphology of such iron/iron oxide particles and of particle agglomerates before and
after cyclic hydrogen storage was studied using a laboratory X-ray microscope under atmospheric
pressure and at elevated temperature. Positioning a micro reaction chamber into the beam path of
the X-ray microscope allows to image the morphology change of powder particle agglomerates dur-
ing the half reaction of the steam iron process directly [15].
Hydrogen storage = 3Fe + 41,0 Hydrogen production
(chemically bound) (H, recovery)
Endothermic Cyclic Redox Exothermic
Reaction
Energy excess side pe = £139 kJ Energy demand side
{Solar/Wind, Electrolysis® (no wind, night ...)
5 Some energy is used >=» Fe;04 => Recovery process self
to power the process + Ha sustained, power
> Burn hydrogen compressor ctc.
Fig. 6: Scheme of the steam-iron-process as cyclic driven redox reaction for hydrogen storage.
In [6] we used electron and X-ray microscopy to characterize the morphology of such iron/iron ox-
ide particles and of particle agglomerates before and after cyclic hydrogen storage. The size of the
powder particles is in the range of few 10 nm to some 100 nm. It is possible to image larger powder
particle agglomerates under atmospheric pressure and at elevated temperatures using X-ray micros-
