1.4 Thesis Outline 15
It is evident that while both can deliver high eciency (>80 %), the PEM
electrolyzers can do so at currents approximately four times higher. Other
advantages are a simple gas separation due to the solid electrolyte and purity
of hydrogen and oxygen produced 39. The high current densities in PEM cells
represent a big advantage over the alkaline systems, since very compact systems
can handle a large power range without compromising the eciency. With
more compact systems the cost of other system components becomes lower. A
disadvantage of PEM cells is that the acidic environment limits the number of
stable materials that can be used. Currently only noble metal based materials
are used as electrocatalysts. Not only does this increase the price of the overall
system, it signicantly limits the scalability due to the scarcity of these elements.
As an example of that, the IEA estimated that 8 % storage capacity should be
installed by 2050, as mentioned above. For 12.5 TW of power generation this
means 1 TW storage. For 1 TW of PEM electrolysis approximately 10 years
of annual production of iridium would be needed and half a year of annual
platinum production just for the electrocatalysts, based on state of the art
electrolyzers reported by Debe et al. 40, 41. These estimations show that a
search for alternative and abundant catalysts could have a huge impact on the
commercialisation of the technology.
Solid alkaline membranes have emerged as a possible alternative to PEM,
which exhibit some of the advantages from the acidic counterpart 42. They
can be fabricated very thin and therefore allow for the same type of cell design.
At the same time the alkaline environment means that the selection of electrode
materials are less constrained and abundant elements could be utilized 42.
However, they conduct OH�� with a larger resistivity than PEM and currently
the stability and performance in the long term are not satisfactory. Finally,
solid oxide cells operating at high temperature (higher than 600 oC) have been
proposed for large scale electrolysis oering high eciencies 43, 44. This technology
is still in its early stage and it is dicult to assess the potential. The
high temperature leads longer time for start up and greater fabrication costs,
however, there are advantages in fuel diversity and higher tolerance towards fuel
impurities.
1.4 Thesis Outline
The overall focus of this thesis is to investigate alternative catalysts for the
oxygen evolution reaction. Here I will present and discuss the main ndings of
the project.
Chapter 2 will be used for setting the scene in terms of explaining concepts
of catalysis and water splitting. Naturally this thesis is build upon a massive
amount of research done in the eld over decades and this is useful to keep
in mind. The most important ndings from the literature will therefore be
highlighted and discussed.