6.2 Experimental investigation of Au-MnOx catalysts 107
Au(50%)-
MnOx
1MKOH.20mV/s
1600RPM
1,21,41,61,8
20
15
10
5
0
Au(30%)-
MnOx
Mn3O4
jgeo/mA.cm-2
U-iR/V(vs.RHE)
Figure 6.10: First anodic scan of Mn3O4 (black), Au(30%)-MnOx (blue) and
Au(50%)-MnOx (red) lms. Measurements were done in 1M KOH, with 1600 RPM
and at 20 mV/s. The potential scale has been corrected for Ohmic drop. The inset
shows current density of the lms at an overpotential of 400 mV as function of the Au
concentration. The activity of a gold polycrystalline disk is also shown in teal. Error
bars are 1 standard deviation from three independent measurements.
The inset of gure 6.10 shows a comparison of the current density at 400 mV
overpotential for the lms. Both Au concentrations lead to an increase in current
density. However, 30 % Au lead to a 1.9 times improvement in current density
while at 50 % the improvement is a factor of 5.5 over the pure Mn3O4. Another
metric often used in comparing OER catalysts is the overpotential needed to
reach 10 mA/cm2. For the 50 % Au lm the overpotential for reaching that
current density is 65 mV lower than for the pure Mn3O4. Since the SEM images
indicated a slight dierence in porosity for the lms, with the more active sample
looking less densely packed, it is interesting to analyse the electrochemically
active surface area. Double layer capacitance has been reported to be proportional
to this area 109. For these lms the double layer capacitance could be
approximated at 1.3 VRHE where no other electrochemical process is expected
to occur. The positive and negative currents measured for the second cycle at
1.3 VRHE were therefore used as an evaluation of the pseudo capacitance. With
this method the capacitances for the Au modied lms were found to be slightly