Donor–Acceptor Couples of Metal and Metal Oxides with Enriched Ni³⁺ Active Sites for Oxygen Evolution

Abstract

Exploiting precious-metal-free and high-activity oxygen evolution reaction (OER) electrocatalysts has been in great demands toward many energy storage and conversion processes, for example, carbon dioxide reduction, metal−air batteries, and water splitting. In this study, the simple solid-state method is employed for coupling Ni (electron donors) with lower-Fermi-level MoO 2 or WO x (electron acceptors) into donor− acceptor ensembles with well-designed interfaces as robust electrocatalysts for OER. The resulting Ni/MoO 2 and Ni/WO x electrocatalysts exhibit smaller overpotentials of 287 and 333 mV at 10 mA cm −2 as well as smaller Tafel slopes of 51 and 65 mV/ dec, respectively, with respect to the single Ni, MoO 2 , WO x , and even the benchmark RuO 2 in 1 M KOH. Specially, on account of a higher Fermi level of Ni in comparison with MoO 2 and WO x , their strong electronic interaction results in directional interfacial electron transfer and increases the hole density over Ni, dramatically enriching the population of high-valence Ni 3+ active sites and decreasing the Fermi level of Ni. The existence of Ni 3+ can strengthen the chemisorption of OH − , and the downshift of the Ni Fermi level can significantly expedite migration of electrons toward the surface of catalysts during OER, thus synergistically boosting the OER catalytic performance. Furthermore, the inner Ni/MoO 2 and Ni/WO x heterostructures and the electrochemically induced surface layers of oxides/hydroxides collectively boost the OER kinetics. This study highlights the importance of designing highly efficient OER electrocatalysts with high-valence active species (Ni 3+ ) and better matched energy levels induced by the work function difference through interfacial engineering.

Enhanced Oxygen Evolution Catalytic Activity of Ni3Mo3N-MoO2-NiO Nanoparticles via Synergistic Effect

“…In the Ni 2p XPS spectrum (Figure b), the peaks at around 852.7 and 870.6 eV are consistent with Ni 2p 3/2 and Ni 2p 1/2 of Ni + , which shift to higher binding energies relative to Ni 3 Mo 3 N, whereas Ni 2p 3/2 and Ni 2p 1/2 of Ni–O are at approximately 856.0 and 873.5 eV. In addition, the peaks center at 862.0 and 880.2 eV are specified as satellite peaks. − Other than the above, the peaks of Ni 3+ near 858.0 and 875.6 eV certify the occurrence of surface oxidation, and the Ni 3+ /Ni 2+ ratio is 46% . In the Mo 3d region (Figure c), the two peaks located at 227.8 and 230.5 eV are qualified as Mo 3d 5/2 and Mo 3d 3/2 of Mo + , respectively, which shift to higher binding energies compared with Ni 3 Mo 3 N. , Besides, the diffraction peaks at 229.1 and 232.6 eV correspond to Mo 4+ .…”

“…37−39 Other than the above, the peaks of Ni 3+ near 858.0 and 875.6 eV certify the occurrence of surface oxidation, and the Ni 3+ /Ni 2+ ratio is 46%. 40 In the Mo 3d region (Figure 2c), the two peaks located at 227.8 and 230.5 eV are qualified as Mo 3d 5/2 and Mo 3d 3/2 of Mo + , respectively, which shift to higher binding energies compared with Ni 3 Mo 3 N. 41,42 Besides, the diffraction peaks at 229.1 and 232.6 eV correspond to Mo 4+ . The peaks of Mo 5+ are 231.0 and 233.7 eV, and the peaks at 232.2 and 235.3 eV belong to Mo 6+ , which originates from MoO 2 and the surface oxidation of the material.…”

Promoting nickel oxidation state transitions in single-layer NiFeB hydroxide nanosheets for efficient oxygen evolution

“…When a Ni-based catalyst was conjugated to plasmonic nanoparticles, positively charged “holes” were produced in the nanoparticles under visible light irradiation, which readily enhanced oxidation of nearby Ni 2+ into high-oxidation-state species, showing improved OER activity 29 , 30 . Conjugation of Ni oxide catalysts with other metal oxides, such as MoO 2 and WO x , and nonmetals, such as phosphate, was also found to be effective in forming high-oxidation-state Ni for the OER 28 , 31 . As a result, OER activities can be greatly enhanced with these catalysts.…”

Solution combustion synthesis of Ni-based hybrid metal oxides for oxygen evolution reaction in alkaline medium

“…Several reports demonstrated that the presence of a high-valence state ion like Ni 3+ is considered as real active sites of OER. 66,67 In addition, coupling Ni to the higher oxidation states of other metal oxides i.e. , Co 3+ , Fe 3+ , Mo 6+ , and Cr 3+ enhanced the OER performance of hybrid materials.…”