This work reports
a systematical study on the relationship of electronic
structure to oxygen evolution reaction (OER) activity of Ni
x
Co3–x
O4 (x = 0–1) mixed oxides. The specific OER
activity is substantially increased by 16 times from 0.02 mA cm–2
BET for pure Co3O4 to 0.32 mA cm–2
BET for x = 1 at an overpotential of 0.4 V and exhibits a strong correlation
with the amount of Ni ions in the +3 oxidation state. X-ray spectroscopic
study reveals that inclusion of Ni3+ ions upshifts the
occupied valence band maximum (VBM) by 0.27 eV toward the Fermi level
(E
F), and creates a new hole (unoccupied)
state located ∼1 eV above the E
F. Such electronic features favor the adsorption of OH surface intermediates
on Ni
x
Co3–x
O4, resulting in enhanced OER. Furthermore, the
emerging hole state effectively reduces the energy barrier for electron
transfer from 1.19 to 0.39 eV, and thereby improves the kinetics for
OER. The electronic structure features that lead to a higher OER in
Ni
x
Co3–x
O4 can be extended to other transition metal oxides for
rational design of highly active catalysts.
Double perovskite oxides are one of the most promising bifunctional electrocatalysts for efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) due to their adjustable electronic structures via doping with different metal cations or engineering of defects. Herein, we report a systematic study on the tuning of the electronic structure of La 2−x Sr x NiMnO 6 with 0 ≤ x ≤ 1.0 to promote the bifunctional OER/ORR activity. The bifunctional index (ΔE) is substantially reduced with increasing of Sr contents and achieves an optimal value of 0.85 V for La 1.4 Sr 0.6 NiMnO 6 , exceeding that of widely studied LaNiO 3 . Our study on electronic structures reveals that the enhancement of the ORR and OER activities strongly correlates with the appearance of Ni 3+ oxidation states and the upshift of the O 2p-band center promoted by Sr doping. Furthermore, an increase of hole states, derived from Ni 3+ states, reduces the energy barrier for the electron transfer from 0.44 to 0.12 eV and hence improves the intrinsic OER activities. The tuning of the electronic structure that leads to higher OER and ORR activities in La 2−x Sr x NiMnO 6 can be extended to other materials for the design of active bifunctional electrocatalysts.
The Fe0.1Ni0.9S2 catalyst can maintain its own metallic phase as a conductive channel for fast electron transfer and a thin layer of Fe0.1Ni0.9OOH serves as an active catalytic phase for the OER.
Perovskite oxides have emerged as promising candidates for the oxygen evolution reaction (OER) electrocatalyst due to their flexible lattice structure, tunable electronic structure, superior stability, and costeffectiveness. Recent research studies have mostly focused on the traditional methods to tune the OER performance, such as cation/anion doping, A-/B-site ordering, epitaxial strain, oxygen vacancy, and so forth, leading to reasonable yet still limited activity enhancement. Here, we report a novel strategy for promoting the OER activity for perovskite LaNiO 3 by crystal phase engineering, which is realized by breaking long-range chemical bonding through amorphization. We provide the first and direct evidence that perovskite oxides with an amorphous structure can induce the self-adaptive process, which helps enhance the OER performance. This is evidenced by the fact that an amorphous LaNiO 3 film on glassy carbon shows a 9-fold increase in the current density compared to that of an epitaxial LaNiO 3 single crystalline film. The obtained current density of 1038 μΑ cm −2 (@ 1.6 vs RHE) is the largest value among the literature reported values. Our work thus offers a new protocol to boost the OER performance for perovskite oxides for future clean energy applications.
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