Nanosheet-structured NiCoO2/carbon nanotubes hybrid composite as a novel bifunctional oxygen electrocatalyst

Controlled three-dimensional leaf-like NiCoO2@NiCo layered double hydroxide heterostructures for oxygen evolution electrocatalysts in rechargeable Zn-air batteries

Rechargeable zinc-air batteries (ZABs) have garnered attention as a viable choice for large-scale energy storage due to their advantageous characteristics, such as high energy density and cost-effectiveness. Strategies aimed at improving the kinetics of the oxygen evolution reaction (OER) through advanced electrocatalytic materials or structural designs can significantly enhance the efficiency and longevity of ZABs. In this study, we introduce a three-dimensional (3D) leaf-vein system heterojunction architecture. In this structure, NiCoO2 nanowire arrays form the central vein, surrounded by an outer leaf composed of NiCo layered double hydroxide (LDH) nanosheets. All these components are integrated onto a substrate made of Ni foam. Notably, when tested in an alkaline environment, the NiCoO2@NiCo LDH exhibited an overpotential of 272 mV at a current density of 10 mA cm-2, and extended durability evaluations over 12 h underscored its robustness at 99.76 %. The rechargeable ZABs achieved a peak power density of 149 mW cm-2. Furthermore, the NiCoO2@NiCo LDH demonstrated stability by maintaining high round-trip efficiencies throughout more than 680 cycles (equivalent to 340 h) under galvanostatic charge-discharge cycling at 5 mA cm-2. The leaf-vein system heterojunction significantly increased the active sites of the catalysts, facilitating charge transport, improving electronic conductivity, and enhancing overall stability.

Surface state engineering of carbon dot/carbon nanotube heterojunctions for boosting oxygen reduction performance

Platinum-based (Pt) catalysts are the most common commercial catalysts for oxygen reduction reactions (ORR). Unfortunately, their high price, scarcity and poor durability hinder their further development. Therefore, the development of effective and economical ORR electrocatalysts has received increasing attention. Here, carbon dots (CDs) enriched in amino functional groups were successfully loaded onto carbon nanotubes (CNTs) with a large surface area and helical structure through a surface state engineering strategy. The resulting composites (CD/CNTs) are 0D/1D nano heterojunction structures. The CD/CNTs showed superior ORR activity compared with CNTs and CDs (E_oneset = 0.95 V, E_1/2 = 0.81 V and limiting current density = 4.74 mA cm^-2). In addition, the stability of CD/CNTs in an alkaline medium was up to 30000 s. The excellent ORR performance of CD/CNTs can be attributed to the dominant role of amino-N, the synergistic effect of heterojunctions formed by CDs and CNTs, and the high Lewis basicity. The composite electrocatalyst synthesized by the CD-regulated CNT strategy is expected to be a reliable cathode candidate for future energy conversion devices.

Recent advances of noble-metal-free bifunctional oxygen reduction and evolution electrocatalysts

Bifunctional oxygen reduction and evolution constitute the core processes for sustainable energy storage. The advances on noble-metal-free bifunctional oxygen electrocatalysts are reviewed.

Local probe investigation of electrocatalytic activity

As the world energy crisis remains a long-term challenge, development and access to renewable energy sources are crucial for a sustainable modern society. Electrochemical energy conversion devices are a promising option for green energy supply, although the challenge associated with electrocatalysis have caused increasing complexity in the materials and systems, demanding further research and insights. In this field, scanning probe microscopy (SPM) represents a specific source of knowledge and understanding. Thus, our aim is to present recent findings on electrocatalysts for electrolysers and fuel cells, acquired mainly through scanning electrochemical microscopy (SECM) and other related scanning probe techniques. This review begins with an introduction to the principles of several SPM techniques and then proceeds to the research done on various energy-related reactions, by emphasizing the progress on non-noble electrocatalytic materials.

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

The synthesis of nickel cobalt oxide materials and their electrocatalytic performance toward the oxygen reduction and evolution reactions are reported. Nickel cobalt oxides were synthesized in a sol-gel process with different precursors, namely nitrate, sulfate, and chloride. Structural analyses show that the structures have mesoporous morphologies and indicate the formation of nickel cobalt oxide spinel structures with a size ranging from 35 to 65 nm. Furthermore, the physicochemical properties differ depending on the nature of the selected precursors, including the materials' morphology and the chemical composition. Electrocatalytic investigations demonstrate that the catalytic activity toward the oxygen reduction reaction (ORR) could be modulated between two- and four-electron pathways, depending on the precursors used. The Cl-NiCoO sample displays a selective two-electron reduction of O₂ , with H₂ O₂ production higher than 90 %. The sample prepared using sulfate displays the highest performance toward the oxygen evolution reaction (OER), with a low overpotential value (0.34 V) to drive a current density of 10 mA.cm^-1 . Overall, these results confirm that the chemical composition of the precursor used during the nanomaterials synthesis can be used to tune the electrocatalytic performances toward ORR and OER.

Single-Step Electrospun Ir/IrO2 Nanofibrous Structures Decorated with Au Nanoparticles for Highly Catalytic Oxygen Evolution Reaction

Nanocomposites of gold (Au) and iridium (Ir) oxide with various compositions (denoted as Au _xIr_{1- x}O _y, x = 0.05, 0.10, or 0.33, Au precursor molar ratio to Ir precursor) were synthesized via electrospinning and subsequent calcination method with two different solvent composition ratios of ethanol to N, N-dimethylformamide (DMF) in the electrospinning solution (ethanol/DMF = 70:30 or 50:50% v/v). Simple single-step electrospinning successfully fabricated a hierarchical nanostructure having Au nanoparticles formed on fibrous main frames of Ir/IrO₂. Different solvent composition in the electrospinning solution induced the formation of main frames with distinct nanostructures; nanoribbons (Au _xIr_{1- x}O _y-70) with ethanol/DMF = 70:30; and nanofibers (Au _xIr_{1- x}O _y-50) with ethanol/DMF = 50:50. The pure Ir or Au counterparts (IrO _y and Au) were also prepared by the same synthetic procedure as Au _xIr_{1- x}O _y. Oxygen evolution reaction (OER) activities of as-synthesized Au _xIr_{1- x}O _y were investigated in 0.5 M H₂SO₄ and compared to those of IrO _y, Au, and commercial iridium (Ir/C, 20% Ir loading on Vulcan carbon). Among them, Au_0.10Ir_0.90O _y-50 exhibited the best OER activity, even better than previously reported catalysts containing both Ir and Au. The high OER activity of Au_0.10Ir_0.90O _y-50 was mainly attributed to the fiber frame structure and the optimal interfacial areas between Au and Ir/IrO₂, which are electrophilic OER active sites. The stability of Au_0.10Ir_0.90O _y-50 was also evaluated to be much higher than that of Ir/C during OER. The current study suggests that the presence of Au on the Ir/IrO₂ surface improves the OER activity of Ir/IrO₂.

Controllable synthesis and electrochemical capacitor performance of MOF-derived MnOx/N-doped carbon/MnO2 composites

Different amount of carbon and nitrogen, for MOF-derived nitrogen-doped carbon/Mn₃O₄ composites, can result in the discrepancies of synergistic effect which plays an important role in final electrochemical performance.

A sulfonated cobalt phthalocyanine/carbon nanotube hybrid as a bifunctional oxygen electrocatalyst

With the sulfur modified CoN₄ sites and the conductive CNT, the CoPc-SO₃H/CNT hybrid exhibits ORR/OER bifunctional activity.