A wafer-scale 1 nm Ni(OH)2 nanosheet with superior electrocatalytic activity for the oxygen evolution reaction

Ultrathin, large area β-Ni(OH)2 crystalline nanosheet as bifunctional electrode material for charge storage and oxygen evolution reaction

Bifunctional electrode materials are highly desirable for meeting increasing global energy demands and mitigating environmental impact. However, improving the atom-efficiency, scalability, and cost-effectiveness of storage systems, as well as optimizing conversion processes to enhance overall energy utilization and sustainability, remains a significant challenge for their application. Herein, we devised an optimized, facile, economic, and scalable synthesis of large area (cm2), ultrathin (∼2.9 ± 0.3 nm) electroactive nanosheet of β-Ni(OH)2, which acted as bifunctional electrode material for charge storage and oxygen evolution reaction (OER). The β-Ni(OH)2 nanosheet electrode shows the volumetric capacity of 2.82 Ah.cm-3(0.82 µAh.cm-2) at the current density of 0.2 mA.cm-2. The device shows a high capacity of 820 mAh.cm-3 with an ultrahigh volumetric energy density of 0.33 Wh.cm-3 at 275.86 W.cm-3 along with promising stability (30,000 cycles). Furthermore, the OER activity of ultrathin β-Ni(OH)2 exhibits an overpotential (η10) of 308 mV and a Tafel value of 42 mV dec-1 suggesting fast reaction kinetics. The mechanistic studies are enlightened through density functional theory (DFT), which reveals that additional electronic states near the Fermi level enhance activity for both capacitance and OER.

Nucleation Kinetics and Structure Evolution of Quasi-Two-Dimensional ZnO at the Air-Water Interface: An In Situ Time-Resolved Grazing Incidence X-ray Scattering Study

The design and synthesis of high-quality two-dimensional (2D) materials with desired morphology are essential for property control. One critical challenge that impedes the understanding and control of 2D crystal nucleation and growth is the inability of direct observation of the nanocrystal evolution process with high enough time resolution. Here, we demonstrated an in situ X-ray scattering approach that directly reveals 2D wurtzite ZnO nanosheet growth at the air-water interface. The time-resolved grazing incidence X-ray diffraction (GID) and grazing incidence X-ray off-specular scattering (GIXOS) results uncovered a lateral to vertical growth kinetics switch phenomenon in the ZnO nanosheet growth. This switch represents the 2D to three-dimensional (3D) crystal structure evolution, which governs the size and thickness of nanosheets, respectively. This phenomenon can guide 2D nanocrystal synthesis with rationally controlled size and thickness. Our work opens a new pathway toward the understanding of 2D nanomaterial growth kinetics based on time-resolved liquid surface grazing incidence X-ray techniques.

Hexagonal NiMoO4-MoS2 nanosheet heterostructure as a bifunctional electrocatalyst for urea oxidation assisted overall water electrolysis

A hexagonal NiMoO4-MoS2 nanosheet heterostructure on nickel foam (NiMoO4-MoS2/NF) was synthesized by simple hydrothermal and annealing treatment.

Ethylene glycol-mediated one-pot synthesis of Fe incorporated α-Ni(OH)2 nanosheets with enhanced intrinsic electrocatalytic activity and long-term stability for alkaline water oxidation

Sustainable electrocatalytic water splitting stipulates the development of cheap, efficient and stable electrocatalysts to promote comparatively sluggish oxygen evolution reaction. We have synthesized iron-incorporated pure phase α-nickel hydroxide, Ni0.8Fe0.2(OH)2 electrocatalyst utilizing N,N,N',N'-Tetramethylethane-1,2-diamine (TMEDA) and ethylene glycol (EG) following a simple one-pot synthesis process. PXRD and FTIR data suggest that the intercalation of EG in the interlayer spacing promotes amorphousness of the material. FESEM and TEM analyses suggest that the catalyst possesses hierarchical sheet-like morphology and BET measurements indicated the surface area of 50 m2 g-1 with high mesoporosity. Electrochemical studies suggest that Ni0.8Fe0.2(OH)2 prepared using water-EG mixture is the most efficient electrocatalyst for OER activity as it requires only 258 mV overpotential (considering backward LSV) on a glassy carbon electrode to achieve the benchmark current density of 10 mA cm-2geo. Additionally, the catalyst shows remarkable long-term stability for up to 7 days. The efficiency of Ni0.8Fe0.2(OH)2 electrocatalyst is reflected in its low Tafel slope (43 mV dec-1) and high OER faradaic efficiency (93%). The enhanced activity is attributed to the increase in the interlayer spacing due to the intercalation of EG into the material, which facilitates the transport of ions during the OER process. The overall improved catalytic property is due to the enhanced ionic mobility, controllable textural property, higher per-site activity and increased conductivity for the Ni0.8Fe0.2(OH)2 catalytic network.

Controlling the Size and Pattern Pitch of Ni(OH)2 Nanoclusters Using Dip-Pen Nanolithography to Improve Water Oxidation

We use dip-pen nanolithography to accurately pattern Ni(OH)₂ nanoclusters on a metachemical surface with an exceptionally large surface area. The distance between the nanoclusters can be manipulated to control the oxygen-evolution reaction current and overpotential, thereby improving the efficiency of the water-splitting process while using minute amounts of the catalyst.

Trace Fe Incorporation into Ni-(oxy)hydroxide Stabilizes Ni3+ Sites for Anodic Oxygen Evolution: A Double Thin-Layer Study

Iron incorporation is essential for the record activity of NiFe-(oxy)hydroxides to oxygen evolution reaction (OER), but the details of how Fe affects catalysis remain under active investigation. In this work, we present a double thin-layer strategy for finding unique and solid evidence for the role of Fe in the OER mechanism. A thin-layer catalyst of a few nanometers of thickness was deposited on a Ni substrate and a thin-layer electrolyte of 0.1 mm thickness was created using a thin-layer spectroelectrochemical cell. The OER activity, the catalyst composition, and the electrolyte species were investigated together as a function of the Fe deposition time. The results show that trace Fe incorporation favors the formation of β-NiOOH in the thin-layer catalyst and effectively suppresses the dissolution of NiOOH into the electrolyte. The results of double-potential step chronoabsorptometry and cyclic voltabsorptometry demonstrate the potential-dependent formation of a Ni³⁺ intermediate in the electrolyte and, more importantly, the dissolution suppression effect due to Fe incorporation. These findings link the role of Fe in OER catalysis to the increased insolubility of Ni³⁺ active sites and highlight the importance of paying close attention to the active-site stability of an electrocatalyst impaired by the electrolyte at a reaction potential.

Nanoparticle-Decorated Ultrathin La2O3 Nanosheets as an Efficient Electrocatalysis for Oxygen Evolution Reactions

Electrochemical catalysts for oxygen evolution reaction are a critical component for many renewable energy applications. To improve their catalytic kinetics and mass activity are essential for sustainable industrial applications. Here, we report a rare-earth metal-based oxide electrocatalyst comprised of ultrathin amorphous La₂O₃ nanosheets hybridized with uniform La₂O₃ nanoparticles (La₂O₃@NP-NS). Significantly improved OER performance is observed from the nanosheets with a nanometer-scale thickness. The as-synthesized 2.27-nm La₂O₃@NP-NS exhibits excellent catalytic kinetics with an overpotential of 310 mV at 10 mA cm^-2, a small Tafel slope of 43.1 mV dec^-1, and electrochemical impedance of 38 Ω. More importantly, due to the ultrasmall thickness, its mass activity, and turnover frequency reach as high as 6666.7 A g^-1 and 5.79 s^-1, respectively, at an overpotential of 310 mV. Such a high mass activity is more than three orders of magnitude higher than benchmark OER electrocatalysts, such as IrO₂ and RuO₂. This work presents a sustainable approach toward the development of highly efficient electrocatalysts with largely reduced mass loading of precious elements.

Improved oxygen evolution activity of IrO2 by in situ engineering of an ultra-small Ir sphere shell utilizing a pulsed laser

Noble metal-based catalysts are vital electrocatalysts for the oxygen evolution reaction (OER), which is a half reaction among multiple renewable energy-related reactions. To fully exploit their potential as efficient OER catalysts, we developed a fast one-step strategy to engineer a unique nanostructure for the benchmark catalyst IrO2 utilizing an ultra-fast pulse laser, through which a shell of ultra-small Ir spheres with a diameter of ca. 2 nm is in situ engineered around the IrO2 core. The creation of the Ir sphere shell not only increases the electrochemical surface area, but also improves the electrical conductivity of the electrocatalyst. The as-engineered IrO2@Ir architecture exhibits extremely high electrocatalytic activity towards the OER, revealing an overpotential of 255 mV at 10 mA cm-2 and Tafel slope of 45 mV dec-1. These values are much lower than those observed for the unmodified structure. Furthermore, the catalytic performance is the best among all the noble metal-based OER catalysts. This work may open a new avenue to efficiently improve the catalytic activity of noble metal-based catalysts and significantly advance the development in the energy industry.

Recent Progress on Nickel-Based Oxide/(Oxy)Hydroxide Electrocatalysts for the Oxygen Evolution Reaction

Developing clean and sustainable energies as alternatives to fossil fuels is in strong demand within modern society. The oxygen evolution reaction (OER) is the efficiency-limiting process in plenty of key renewable energy systems, such as electrochemical water splitting and rechargeable metal-air batteries. In this regard, ongoing efforts have been devoted to seeking high-performance electrocatalysts for enhanced energy conversion efficiency. Apart from traditional precious-metal-based catalysts, nickel-based compounds are the most promising earth-abundant OER catalysts, attracting ever-increasing interest due to high activity and stability. In this review, the recent progress on nickel-based oxide and (oxy)hydroxide composites for water oxidation catalysis in terms of materials design/synthesis and electrochemical performance is summarized. Some underlying mechanisms to profoundly understand the catalytic active sites are also highlighted. In addition, the future research trends and perspectives on the development of Ni-based OER electrocatalysts are discussed.

Assembling Ni-Co phosphides/carbon hollow nanocages and nanosheets with carbon nanotubes into a hierarchical necklace-like nanohybrid for electrocatalytic oxygen evolution reaction

Currently, designing and developing high-performance, cost-effective yet stable electrocatalysts for oxygen evolution reaction (OER) is a challenging task owing to the existing high overpotential and sluggish OER kinetics. Herein, we successfully fabricated a hierarchical necklace-like nanohybrid via assembling Ni-Co mixed metal phosphides/carbon (NiCoP/C) hollow nanocages and nanosheets with carbon nanotubes (CNTs). It was revealed that Ni-Co-layered double hydroxide/carbon hollow nanocages and nanosheets derived from Co-based MOF polyhedrons (ZIF-67) could pass through CNTs and perpendicularly anchor onto the surface of CNTs, respectively, thus finally constructing a hierarchical CNTs@NiCoP/C nanohybrid via further low-temperature phosphorization. The as-assembled CNTs@NiCoP/C electrocatalyst exhibited promising electrocatalytic performance towards OER with a low overpotential of about 0.297 V for reaching a current density of 10.0 mA cm-2 and a low Tafel slope of 57.35 mV dec-1 in 1.0 M KOH owing to the advantages of its unique superstructure and multiple composition. Furthermore, the present CNTs@NiCoP/C also possessed good long-term operation stability. This synthesis strategy involving the combination of transition metal phosphides and CNTs to form hierarchical nanostructures holds promising potential for designing highly OER-active electrocatalysts.