2.5 Oxygen Evolution 23
range of active catalysts available for HER can be found in gure 2.3a and b for
both acid and alkaline electrolytes.
IrO2filmonTi
RuO2filmonTi
0,00,10,20,30,40,5
1000
100
10
1
0,1
0,01
0,00,10,20,30,40,5
1
10
0,1
Ni2P
MoSx
onCNTsVwire
|jgeo|/mAcm-2
HER/V
Rhwire
Mo3S13-2
cluster
MoS2
nanowire
Auwire
Pt/CinMEA
Ptpc
Nicrystal
Cowire
Alwire
Mowire
Acid
CoP
Mo2C
CNTs
Ni2P
|jgeo|/mAcm-2
HER/V
Pt/C
Pt3Ni/C
nanoframes/Ni(OH)2
Pdpc
Nipc
Fepc
Copc
WpcCupc
Aupc
Agpc
NiMo
nanopowders
CoNiMo
Alkaline
Figure 2.3: An overview of catalyst materials for HER. The logarithmic of the
current density is plotted as function of overpotential. a) Catalysts tested in acidic
solutions. Pt pc is from 36. Pt/C in MEA is from 58. IrO2 and RuO2 are lms
from 59. CoP is from 60. MoSx on CNTs is from 61. Ni2P is from 62. Mo2C
CNTs is from 63. Mo3S13��2 cluster is from 64. MoS2 nanowire is from 65. Ni
crystal is from 66. Rh, Mo and Au wires are from 67. Co, Cr and V wires are
from 68. Al wire is from 69. b) Catalysts tested in alkaline solutions. CoNiMo is
from 70. NiMo nanopowders is from 71. Pt/C and Pt3Ni/C nanoframes/Ni(OH)2
are from 72. Ni2P is from 62. Pd pc, Ni pc, Fe pc, Co pc, W pc, Au pc, Ag pc and
Cu pc are from 73.
2.5 Oxygen Evolution
Compared to HER the oxygen evolution reaction, OER, is more complex and involves
several intermediates. As discussed in the previous section, the hydrogen
electrode in an electrolyzer can run at appreciable current densities with lower
than 50 mV overpotential using a very low amount of platinum. This is far
from the situation at the oxygen electrode which imposes a large overpotential,
more than 300 mV, at signicant currents with a high loading of even the best
catalysts. While 1.23 VRHE is already highly oxidising, the substantial overpotential
makes matters worse so that the selection of stable materials available
for this electrode is rather narrow. For these reasons it has long been recognised
as a great challenge in electrochemistry to nd new active and stable catalysts
for this reaction 74, 75. In fact, a number of electrochemical processes besides
water electrolysis depends on the oxygen evolution reaction including CO2 and
CO reduction, photoelectrochemical water splitting, electrowinning of metals
and metal-air batteries. In this section an overview of the developments in the
eld of fundamental research for OER will be given.