26 Electrocatalysis and the splitting of water
by thermal decomposition of metallic precursors on a titanium substrate, resulting
in conductive mixed oxides. The active components of the oxides were
Ru and Ir and they were mixed with Sn, Ta and Ti for better stability and
selectivity. These electrodes were, as their name suggests, more stable under
reaction conditions compared to the graphite anodes originally used for chlorine
evolution. DSAsr also found use as OER electrodes, due to the high activity of
Ru and Ir, and are still considered the state of the art for electrolysis in acidic
environment 75, 85, 86.
2.5.1 Theoretical model for Oxygen evolution
These early reports have greatly inspired modern research eorts in describing
the OER activity of metals and oxides. On the theoretical side developments
in computational power and methodology have greatly aided the understanding
of catalysis in general 49, 87. For oxygen electrocatalysis a large step towards
understanding oxygen reduction was reported by Nørskov and co-workers in
2004 88. They used a simple linear free energy model to relate the overpotential
to the binding energies of intermediates. This model was later used to describe
the OER on both metallic and oxide surfaces by Rossmeisl et al. 89, 90. At
this point it is useful to look into the reaction mechanism of OER. Several
reaction mechanisms have been proposed but here we will consider one of the
most common, which is also the one used in the free energy models. In acidic
solution it can be written in the following way 89. First a water molecule is
dissociated into an adsorbed hydroxide and a solvated proton.
H2O+ ! HO + H+ + e (2.14)
The hydroxide is further oxidised to adsorbed atomic oxygen.
HO + H+ + e ! O + 2(H+ + e) (2.15)
From the adsorbed oxygen atom and a water molecule a superoxide, *OOH, can
H2O + O + 2(H+ + e) ! HOO + 3(H+ + e) (2.16)
Finally, molecular oxygen is formed and leaves the surface.
HOO ! O2(gas) + 4(H+ + e)+ (2.17)
In these equations denotes an active surface site. In alkaline environment the
reaction mechanism looks slightly dierent but the intermediates are the same.
4OH ! OH + 3OH + e (2.18)
OH + 3OH + e ! O + H2O + 2OH + 2e (2.19)