Ni³⁺-Induced Hole States Enhance the Oxygen Evolution Reaction Activity of Ni_xCo_3–xO₄ Electrocatalysts

Abstract

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.

The Effect of Heteroatom Doping on Nickel Cobalt Oxide Electrocatalysts for Oxygen Evolution and Reduction Reactions

“…All the samples display the same diffraction lines with four apparent characteristic peaks observed at 2θ about 22, 36.4, 42.8, which could be assigned to (111), (220), (311), and (400) plane, in a good agreement with the literature (JCPDS card no 73-1702). [31,32] Compared to the others published works, a small peaks shift to lower angles was observed suggesting change in the lattice parameters. The sample prepared using Cl precursor displays an additional weak diffraction peak at 50.5°indicating the co-existence of a small amount of CoO (PDF 43-1004).…”

“…While for SÀ NiCoO the main oxidation states are Ni 3 + and Co 2 + suggestion a value of x bellow 0.6 as well as the presence of sulfate. [32] The generated materials electrocatalytic activities toward the ORR were investigated. The LSV curves, on the RRDE disk electrode, were performed in O 2 -saturated KOH (0.1 mol L À 1 ) solution at 1600 rpm by scanning potential from 1 V to À 0.4 V at 10 mV s À 1 scan rate (Figure 5a).…”

“…This metric value is reached for an over-potential value of 0.340 V, which is comparable to the best performance of other nickel cobalt oxides reported in the literature. [32,40,60] Recently, heteroatoms (S, N, P) doped transition metal have been reported to be an efficient approach for enhancing the catalytic activities. [61,62] The pronounced OER performance is linked to the main Ni 3 + oxidation state of Ni on SÀ NiCoO and also to the presence of sulfide which increase the number of active sites, change the chemical as well as the coordination of Ni/Co.…”

Manganese‐Doped Hollow Layered Double (Ni, Co) Hydroxide Microcuboids as an Efficient Electrocatalyst for the Oxygen Evolution Reaction

“…[16,17] Compared with Ni 3 + , Ni 2 + shows poor activity. [18][19][20][21][22] So if we could convert the Ni 2 + LDH into Ni 3 + , it will greatly improve the OER activity of the material. [19,[22][23][24] Since Mn has similar ionic radius to those of Co cation, it is easy substitution of Co by Mn.…”

“…[18][19][20][21][22] So if we could convert the Ni 2 + LDH into Ni 3 + , it will greatly improve the OER activity of the material. [19,[22][23][24] Since Mn has similar ionic radius to those of Co cation, it is easy substitution of Co by Mn. Besides, due to the ionic radius of Mn 3 + cation (0.0645 nm) slightly larger than that of the Co 3 + cation (0.0545 nm), the lattice volumes are expected to be expanded, leading to more lattice distortions.…”