32 Electrocatalysis and the splitting of water
therefore compelling to investigate the activity of well dened nanoparticles.
Compared to the opposite reaction ORR where there is a large amount of work
in this area 123129, there are only few studies available for OER 105, 130.
From those experiments it was suggested that changing the size from 45 to 15 nm
lead to increased current densities for OER. Investigations of particle size eects
are related to which facets or sites that are active, and a decrease in particle
size generally leads to a higher fraction of undercoordinated sites vs. at terrace
sites. According to DFT calculations the binding to intermediates on at terrace
sites results in activity trends that match experimental values 90, 94, 131, 132.
The binding on undercoordinated sites are instead expected to be radically different
133 and would not be expected to play a dominant role in the reaction.
From such considerations it is expected that smaller particles exhibit lower speci
c activity, but possibly higher mass activity 40. Another related research
eort is focused on substituting the noble metal core with a more abundant
element, while keeping the activity constant. This has been done successfully
for IrNi nanoparticles by Strasser and co-workers 134136. IrNi3:3 particles
exhibited a mass activity 10 times higher than rutile IrO2 particles. The recent
eorts for nding better OER catalysts in acid is summarized in gure
2.9, where the current is normalised to geometric surface area. It is clear from
this plot that only noble metal oxides are considered and currently no other
materials are being reported as alternatives.