Nanoflower-like NiCo2O4 Composite Graphene Oxide as a Bifunctional Catalyst for Zinc-Air Battery Cathode

Developing efficient bifunctional catalysts for nonprecious metal-based oxygen reduction (ORR) and oxygen evolution (OER) is crucial to enhance the practical application of zinc-air batteries. The study harnessed electrostatic forces to anchor the nanoflower-like NiCo2O4 onto graphene oxide, mitigating the poor inherent conductivity in NiCo2O4 as a transition metal oxide and preventing excessive agglomeration of the nanoflower-like structures during catalysis. Consequently, the resulting composite, NiCo2O4-GO/C, exhibited notably superior ORR and OER catalytic performance compared to pure nanoflower-like NiCo2O4. Notably, it excelled in OER catalytic activity of the OER relative to the precious metal RuO2. As a bifunctional catalyst for ORR and OER, NiCo2O4-GO/C displayed a potential difference of 0.88 V between the ORR half-wave potential and the OER potential at 10 mA·cm-2, significantly lower than the 1.08 V observed for pure flower-like NiCo2O4 and comparable to the 0.88 V exhibited by precious metal catalysts Pt/C + RuO2. The NiCo2O4-GO/C-based zinc-air battery demonstrated a discharge capacity of 817.3 mA h·g-1, surpassing that of precious metal-based zinc-air batteries. Moreover, charge-discharge cycling tests indicated the superior stability of the NiCo2O4-GO/C-based zinc-air battery compared to its precious metal-based counterparts.

Porous spinel-type transition metal oxide nanostructures as emergent electrocatalysts for oxygen reduction reactions

Porous spinel-type transition metal oxide (PS-TMO) nanocatalysts comprising two kinds of metal (denoted as A_xB_3-xO₄, where A, B = Co, Ni, Zn, Mn, Fe, V, Sm, Li, and Zn) have emerged as promising electrocatalysts for oxygen reduction reactions (ORRs) in energy conversion and storage systems (ECSS). This is due to the unique catalytic merits of PS-TMOs (such as p-type conductivity, optical transparency, semiconductivity, multiple valence states of their oxides, and rich active sites) and porous morphologies with great surface area, low density, abundant transportation paths for intermediate species, maximized atom utilization and quick charge mobility. In addition, PS-TMOs nanocatalysts are easily prepared in high yield from Earth-abundant and inexpensive metal precursors that meet sustainability requirements and practical applications. Owing to the continued developments in the rational synthesis of PS-TMOs nanocatalysts for ORRs, it is utterly imperative to provide timely updates and highlight new advances in this research area. This review emphasizes recent research advances in engineering the morphologies and compositions of PS-TMOs nanocatalysts in addition to their mechanisms, to decipher their structure-activity relationships. Also, the ORR mechanisms and fundamentals are discussed, along with the current barriers and future outlook for developing the next generation of PS-TMOs nanocatalysts for large-scale ECSS.

Defective NiFe2 O4 Nanoparticles for Efficient Urea Electro-oxidation

Urea is an important organic pollutants in sewage and needs to be removed for environmental protection. Here, we report defective NiFe₂ O₄ (NFO) nanoparticles with excellent performance for urea electro-oxidation. The results show that defects can be effectively implanted at the surface of NFO nanoparticles by a facile and versatile lithium reduction method without affecting its main crystal structure and grain size. The defective NFO-5Li nanoparticles displayed a significantly improved urea electro-oxidation performance compared with NFO-Pristine nanoparticles. Particularly, the NFO-Pristine and NFO-5Li show a potential of 1.398 and 1.361 V at the current density of 10 mA cm^-2 and Tafel slope of 37.3 and 31.4 mV dec^-1 , respectively. In addition, the NFO-5Li nanoparticles also revealed outstanding electrocatalytic stability. The superior performance can be attributed to the designed tunable surface defect engineering. Furthermore, the defect engineering strategy as well as the defective NFO nanoparticles hold great potential for applications in other materials and areas.

Study of the Oxygen Evolution Reaction Catalytic Behavior of CoxNi1-xFe2O4 in Alkaline Medium

Catalysts for the oxygen evolution reaction (OER) play an important role in the conversion of solar energy to fuel of earth-abundant water into H₂ and O₂ through splitting/electrolysis. Heterogeneous electrocatalysts for hydrogen and oxygen evolution reactions (HER and OER) exhibit catalytic activity that depends on the electronic properties, oxidation states, and local surface structure. Spinel ferrites (MFe₂O₄; M = Ni and Co) based materials have been attractive for the catalytic water oxidation due to their well-known stability in alkaline medium, easy synthesis, existence of metal cations with various oxidation states, low cost, and tunable properties by the desired metal substitution. To understand the better catalytic activity of MFe₂O₄ in detail the role of Ni and Co was studied through M_xNi_1-xFe₂O₄ (M = Co; 0 < x < 1), which was prepared by the sol-gel method. The results showed that bare NiFe₂O₄ has better catalytic activity (η = 381 mV at 10 mA cm^-2 and Tafel slope of 46.4 mV dec^-1) compared to Co-containing M_xNi_1-xFe₂O₄ (η = 450-470 mV at 10 mA cm^-2 and Tafel slope of 50-73 mV dec^-1) in alkaline medium, and the substitution of Co is found to suppress the catalytic activity of NiFe₂O₄. The degradation of catalytic activity with an increase in Co content was accounted for in further detailed investigations.

NiMnO3/NiMn2O4 Oxides Synthesized via the Aid of Pollen: Ilmenite/Spinel Hybrid Nanoparticles for Highly Efficient Bifunctional Oxygen Electrocatalysis

This work develops the NiMnO₃/NiMn₂O₄ ilmenite/spinel hybrid oxides as highly efficient bifunctional catalysts toward both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). They are prepared with the aid of pollen, using a two-step annealing method. The interaction between NiMnO₃ and NiMn₂O₄ nanoparticles results in an enhanced charge transfer between nanoparticles and active species in electrolytes, which is favorable for their electrocatalytic activities. The surface oxidation states of Ni and Mn for the hybrid oxides can be tuned by pollen, which greatly influences the OER and ORR activities and the overall bifunctional activity. The surface Ni³⁺ facilitates OER activity, while the surface Mn³⁺ with a small amount of Mn⁴⁺ favors ORR processes. Through optimization, 0.61NiMnO₃/NiMn₂O₄ shows the highest OER activity, while 1.57NiMnO₃/NiMn₂O₄ can outperform the others in promoting ORR processes. Between them, 0.61NiMnO₃/NiMn₂O₄ exhibits the higher overall bifunctional activity. Furthermore, both of the optimized hybrid oxides show excellent durability during both OER and ORR processes. They can be considered as promising bifunctional catalysts for OER/ORR.

Magnetic memory effect in self-assembled nickel ferrite nanoparticles having mesoscopic void spaces

We report a novel approach for fabricating nanocrystalline and mesoporous nickel ferrite nanoparticles of ca. 5–9 nm size and it showed interesting memory effect as a consequence of interparticle interaction of self-assembled nanoparticles.

The direct growth of highly dispersed CoO nanoparticles on mesoporous carbon as a high-performance electrocatalyst for the oxygen reduction reaction

Highly dispersed CoO nanoparticles on mesoporous carbon show the excellent activity and stability toward the electrocatalytic oxygen reduction reaction with a four-electron reaction path, compared to commercial Pt/C.

Spinel nickel ferrite nanoparticles strongly cross-linked with multiwalled carbon nanotubes as a bi-efficient electrocatalyst for oxygen reduction and oxygen evolution

NiFe₂O₄ nanoparticles successfully cross-linked with the outer walls of MWCNTs demonstrate excellent catalytic activities and stabilities for both the ORR and OER compared to commercial Pt/C, owing to the strong coupling and synergistic effects.