Strain Controlling Catalytic Efficiency …
9
Fig. 6 Density of states for the Ni 1−x Fe x OOH alloy at a x = 8%, b x = 25%, c 33%, and d 42%
Table 4, all of the bonds around the active site are the longest at 33% with an average
length of 1.97 Å, which is longer than in the pure NiOOH case and corresponds to
the expected ordering of Fe and Ni elements in the periodic table of elements.
As a side note, we remark that not all bonds react to the contraction or expansion
in a trivial way, that is, for example, bonds do not necessarily elongate as a result of
5% expansion (as seen in Table 4). Another important location where bond distances
are central to the catalytic efficiency is at the location where oxygen penetrates (see
Table 18; Fig. 12). There, expect for some exceptions, the bond distances vary with
expansion or contraction but do not change significantly as a result of Fe content
increase, probably due to similar chemical environment around this site.
At high iron contents of 42%, the atomic location of Fe atoms close to the active site
results in a low overpotential and is less stable (see higher energy of intermediate A
in Table 12). Indeed, the creation of inactive FeOOH aggregates has been observed
experimentally [10]. Hence, the preferred (low energy) configuration has less Fe
atoms near the surface and Fe
4+ at the active site (as seen in Table 19, the atomic
magnetization of Fe is 3.6 Bohr magneton at intermediate A for 8, 25, and 42% of
Fe content) and a higher overpotential compared to 33% iron content (see Fig. 5).
The best performance is achieved when applying 5% expansion strain to NiOOH
with 33% Fe metallic content. As seen in Fig. 8, strain has a significant effect on
the overpotential for all percentages of Fe in NiOOH. In all cases, expansion is
desired in order to reach optimal hybridization of all atomic orbitals, enabling further
delocalization and easier extraction of charge during deprotonation.
9
Fig. 6 Density of states for the Ni 1−x Fe x OOH alloy at a x = 8%, b x = 25%, c 33%, and d 42%
Table 4, all of the bonds around the active site are the longest at 33% with an average
length of 1.97 Å, which is longer than in the pure NiOOH case and corresponds to
the expected ordering of Fe and Ni elements in the periodic table of elements.
As a side note, we remark that not all bonds react to the contraction or expansion
in a trivial way, that is, for example, bonds do not necessarily elongate as a result of
5% expansion (as seen in Table 4). Another important location where bond distances
are central to the catalytic efficiency is at the location where oxygen penetrates (see
Table 18; Fig. 12). There, expect for some exceptions, the bond distances vary with
expansion or contraction but do not change significantly as a result of Fe content
increase, probably due to similar chemical environment around this site.
At high iron contents of 42%, the atomic location of Fe atoms close to the active site
results in a low overpotential and is less stable (see higher energy of intermediate A
in Table 12). Indeed, the creation of inactive FeOOH aggregates has been observed
experimentally [10]. Hence, the preferred (low energy) configuration has less Fe
atoms near the surface and Fe
4+ at the active site (as seen in Table 19, the atomic
magnetization of Fe is 3.6 Bohr magneton at intermediate A for 8, 25, and 42% of
Fe content) and a higher overpotential compared to 33% iron content (see Fig. 5).
The best performance is achieved when applying 5% expansion strain to NiOOH
with 33% Fe metallic content. As seen in Fig. 8, strain has a significant effect on
the overpotential for all percentages of Fe in NiOOH. In all cases, expansion is
desired in order to reach optimal hybridization of all atomic orbitals, enabling further
delocalization and easier extraction of charge during deprotonation.