Strain Controlling Catalytic Efficiency …
7
Table 2 Free energies and overpotentials for Fe-doped NiOOH (8%) with or without strain at pH
= 14 and V = 1 V
Reaction
−10%
−5%
0
5%
10%
A to B
−0.79
−0.81
−0.41
−0.47
−0.57
B to C
0.76
0.25
−0.35
−1.73
−1.23
C to D
−0.82
−0.92
−0.69
−0.71
−0.59
D to A
−1.96
−1.33
−1.36
−0.90
−0.42
Over potential
1.48
0.97
0.37
0.25
0.30
Units are eV
Fig. 4 Free energies for water oxidation reaction for Fe-doped NiOOH (8%) as a function of %
contraction/expansion
site of 8% Fe-doped NiOOH (calculated magnetization for intermediate A is 3.6
Bohr magneton) [18]. The calculated shorter bond distances are in good agreement
with previous experiment [10]. And evidence for the presence of Fe
4+ and Fe
3+ has
been reported in various studies [44]. The deprotonation reaction is affected by the
bond distances and determines the overpotential. Hence, the key to the overpotential
decrease upon Fe doping is the similarity of the free energies of reactions (1) and (2).
Our test bed of applying strain helps to explain that the overpotential lowers upon
doping since Fe decreases the bond distances near the active site and contributes
to less compression needed to equalize the overpotentials of reactions (1) and (2).
Moreover, the positive effect of reducing the overpotential of pure NiOOH upon Fe
doping is generally maintained also under strain (compare overpotentials in Tables 1
and 2). Furthermore, the overpotential decreases by an additional 0.12 eV upon the
application of 5% strain (Table 2).
The overpotential further decreases with the increase of Fe content till it reaches
an optimal efficiency at 33% Fe concentration (see Table 3; Fig. 5). The overpotential
7
Table 2 Free energies and overpotentials for Fe-doped NiOOH (8%) with or without strain at pH
= 14 and V = 1 V
Reaction
−10%
−5%
0
5%
10%
A to B
−0.79
−0.81
−0.41
−0.47
−0.57
B to C
0.76
0.25
−0.35
−1.73
−1.23
C to D
−0.82
−0.92
−0.69
−0.71
−0.59
D to A
−1.96
−1.33
−1.36
−0.90
−0.42
Over potential
1.48
0.97
0.37
0.25
0.30
Units are eV
Fig. 4 Free energies for water oxidation reaction for Fe-doped NiOOH (8%) as a function of %
contraction/expansion
site of 8% Fe-doped NiOOH (calculated magnetization for intermediate A is 3.6
Bohr magneton) [18]. The calculated shorter bond distances are in good agreement
with previous experiment [10]. And evidence for the presence of Fe
4+ and Fe
3+ has
been reported in various studies [44]. The deprotonation reaction is affected by the
bond distances and determines the overpotential. Hence, the key to the overpotential
decrease upon Fe doping is the similarity of the free energies of reactions (1) and (2).
Our test bed of applying strain helps to explain that the overpotential lowers upon
doping since Fe decreases the bond distances near the active site and contributes
to less compression needed to equalize the overpotentials of reactions (1) and (2).
Moreover, the positive effect of reducing the overpotential of pure NiOOH upon Fe
doping is generally maintained also under strain (compare overpotentials in Tables 1
and 2). Furthermore, the overpotential decreases by an additional 0.12 eV upon the
application of 5% strain (Table 2).
The overpotential further decreases with the increase of Fe content till it reaches
an optimal efficiency at 33% Fe concentration (see Table 3; Fig. 5). The overpotential