Hierarchical nanocomposites of nickel/iron-layered double hydroxide ultrathin nanosheets strong-coupled with nanocarbon networks for enhanced oxygen evolution reaction

Oxygen Evolution Enhancement of Oxalate-Based Nickel-Iron MOF through Bipyridine Coordinated Strategy

The catalytic performance of oxalate-based Ni-Fe metal-organic frameworks (MOFs) in the oxygen evolution reaction (OER) was investigated via a coordination strategy. The bidentate chelating ligand 2,2'-bpy (2,2'-bipyridine), was utilized to improve the catalytic kinetics under ambient conditions. The results revealed that a MOF-to-MOF transformation including the formation of [M(2,2'-bpy)n]2/3+ (M = Ni/Fe, n = 1-3) could boost alkaline OER, giving an impressive ultralow overpotential of 220 mV at a current density of 10 mA/cm2 in a 1 M KOH solution, surpassing the performance of control group activity of oxalate-based Ni-Fe MOF. However, excessive addition of the ligand had a negative effect, leading to decreased activity. Further investigation revealed the double role of 2,2'-bpy: Both promote and suppress catalytic reactions. The catalytic mechanism was then discussed, highlighting the potential of secondary ligands to effectively fine-tune the catalytic behavior of these materials.

Advances in Stability of NiFe-Based Anodes toward Oxygen Evolution Reaction for Alkaline Water Electrolysis

Alkaline electrolysis plays a crucial role in sustainable energy solutions by utilizing electrolytic cells to produce hydrogen gas, providing a clean and efficient method for energy storage and conversion. Efficient, stable, and low-cost electrocatalysts for the oxygen evolution reaction (OER) are essential to facilitate alkaline water electrolysis on a commercial scale. Nickel-iron-based (NiFe-based) transition metal electrocatalysts are considered the most promising non-precious metal catalysts for alkaline OER due to their low cost, abundance, and tunable catalytic properties. Nevertheless, the majority of existing NiFe-based catalysts suffer from limited activity and poor stability, posing a significant challenge in meeting industrial applications. This also highlights a common situation where the emphasis on material activity receives significant attention, while the equally critical stability aspect is often underemphasized. Initiating with a comprehensive exploration of the stability of NiFe-based OER materials, this article first summarizes the debate surrounding the determination of active sites in NiFe-based OER electrocatalysts. Subsequently, the degradation mechanisms of recently reported NiFe-based electrocatalysts are outlined, encompassing assessments of both chemical and mechanical endurance, along with essential approaches for enhancing their stability. Finally, suggestions are put forth regarding the essential considerations for the design of NiFe-based OER electrocatalysts, with a focus on heightened stability.

Interface Synergistic Effect of NiFe-LDH/3D GA Composites on Efficient Electrocatalytic Water Oxidation

Currently, NiFe-LDH exhibits an excellent oxygen evolution reaction (OER) due to the interaction of the two metal elements on the layered double hydroxide (LDH) platform. However, such interaction is still insufficient to compensate for its poor electrical conductivity, limited number of active sites and sluggish dynamics. Herein, a feasible two-step hydrothermal strategy that involves coupling low-conductivity NiFe-LDH with 3D porous graphene aerogel (GA) is proposed. The optimized NiFe-LDH/GA (1:1) produced possesses a 257 mV (10 mA cm-2) overpotential and could operate stably for 56 h in an OER. Our investigation demonstrates that the NiFe-LDH/GA has a three-dimensional mesoporous structure, and that there is synergistic interaction between LDH and GA and interfacial reconstruction of NiOOH. Such an interface synergistic coupling effect promotes fast mass transfer and facilitates OER kinetics, and this work offers new insights into designing efficient and stable GA-based electrocatalysts.

Controllable synthesis of a hybrid mesoporous sheets-like Fe0.5NiS2@ P, N-doped carbon electrocatalyst for alkaline oxygen evolution reaction

Owing to the high cost of precious metal catalysts for the oxygen evolution reaction (OER), the production of highly efficient and affordable electrocatalysts is important for generating pollution-free and renewable energy via electrochemical processes. A facile hydrothermal approach was employed to synthesize hybrid mesoporous iron-nickel bimetallic sulfides @ P, N-doped carbon for the OER. The prepared Fe0.5NiS2@C exhibited an overpotential (η) of 250 mV at 10 mA/cm2. This exceeded the overpotentials recently reported for surface-modified P, N-doped carbon-based catalysts for the OER in a 1 M KOH medium. Moreover, the Fe0.5NiS2@C catalyst showed a notable Tafel slope of 90.5 mV/dec with long-dated stability even after 24 h at 10 mA/cm2. The superior OER performance of the Fe0.5NiS2@C catalysts may be due to their large surface area, sheet-like morphology with abundant active sites, fast transfer of mass and electrons, control of the electronic structure by co-treatment with heteroatoms (e.g., P and N), and the synergistic effect of bimetallic sulfides, making them favorable catalysts for the oxygen evolution reaction. Density functional theory (DFT) calculations showed that the Fe0.5NiS2@C catalyst exhibited strong H2O-adsorption energy. The enhanced OER activity of Fe0.5NiS2@C was attributed to its higher surface area, favorable H2O adsorption energy, improved electron transfer efficiency, and lower Gibbs free energy compared to those of the other proposed catalysts.

Synthesis of NiFe-layered double hydroxides using triethanolamine-complexed precursors as oxygen evolution reaction catalysts: effects of Fe valence

The synthesis of highly efficient NiFe-layered double hydroxides (NiFe-LDHs) to catalyze the oxygen evolution reaction (OER) is urgent and challenging. Herein, NiFe-FeCl3-x and NiFe-FeCl2-x samples (where FeCl3 and FeCl2 represent the Fe sources and x represents the imposed reaction time: 6, 12, and 24 h) were prepared via one-pot hydrothermal synthesis using Fe sources characterized by Fe(III) or Fe(II) valence states. In the presence of triethanolamine, when FeCl3 was used as the Fe source, pure NiFe-LDH was obtained, whose crystallinity increased with increasing hydrothermal treatment time. In contrast, when FeCl2 was used as the Fe source, a mixture of NiFe-LDH, Fe2O3, and trace amounts of Fe3O4 was obtained. The content of NiFe-LDH in the mixture increased under longer hydrothermal treatment and NiFe-FeCl3-x catalysts exhibited better OER performance than NiFe-FeCl2-x catalysts. Specifically, NiFe-FeCl3-6 afforded the highest OER performance with an overpotential of 246.8 mV at 10 mA cm-2 and a Tafel slope of 46.1 mV dec-1. Herein, we investigated the effects of the valence state of Fe precursors on the structures and OER activities of the prepared catalysts; the mechanism of NiFe-LDH formation via hydrothermal synthesis in the presence of triethanolamine was also proposed.

Synergetic Nanostructure Engineering and Electronic Modulation of a 3D Hollow Heterostructured NiCo2O4@NiFe-LDH Self-Supporting Electrode for Rechargeable Zn-Air Batteries

Developing electrocatalysts that integrate the merits of the hollow structure and heterojunction is an attractive but still challenging strategy for addressing the sluggish kinetics of oxygen evolution reaction (OER) in many renewable energy technologies. Herein, a 3D hierarchically flexible self-supporting electrode with a hollow heterostructure is intentionally constructed by assembling thin NiFe layered double hydroxide (LDH) nanosheets on the surface of metal-organic framework-derived hollow NiCo₂O₄ nanoflake arrays (NiCo₂O₄@NiFe-LDH) for rechargeable Zn-air batteries (ZABs). Theoretical calculations demonstrate that the interfacial electron transfer from NiFe-LDH to NiCo₂O₄ induces the electronic modulation, improves the conductivity, and lowers the reaction energy barriers during OER, ensuring high catalytic activity. Meanwhile, the 3D hierarchically hollow nanoarray architecture can afford plentiful catalytic active sites and short mass-/charge-transfer pathways. As a result, the obtained catalyst exhibits remarkable OER electrocatalytic performance, showing low overpotentials (only 231 mV at 10 mA cm^-2, 300 mV at 50 mA cm^-2) and robust stability. When assembling liquid and flexible solid-state ZABs with NiCo₂O₄@NiFe-LDH as the OER catalyst, the ZABs achieve excellent power density, high specific capacity, superior cycle durability, and good bending flexibility, exceeding the RuO₂ + Pt/C benchmarks and other previously reported self-supporting catalysts. This work not only constructs an advanced hollow heterostructured catalyst for sustainable energy systems and wearable electronic devices but also provides insights into the role of interfacial electron modulation in catalytic performance enhancement.

Triethanolamine-assisted synthesis of NiFe layered double hydroxide ultrathin nanosheets for efficient oxygen evolution reaction

Water electrolysis is a promising technique for producing high-quality hydrogen, the application of which is impeded by the sluggish oxygen evolution reaction (OER) process. In this study, ultrathin nickel-iron layered double hydroxide (NiFe LDH) nanosheets were successfully synthesized through a facile hydrothermal reaction with the assistance of triethanolamine (TEA). Morphological and structural characterizations revealed that the presence of TEA modified the morphology of NiFe LDH, facilitated the synthesis of high-purity NiFe LDH, improved the crystallinity of NiFe LDH and resulted in a slight decrease in specific surface area. X-ray photoelectron spectroscopy (XPS) analysis demonstrated the modulation of the electronic structure engendered by the addition of TEA, with nickel and iron appearing in high valence state in the resulting NiFe LDH nanosheets. The as-prepared NiFe LDH nanosheets possessed outstanding OER activity with fast kinetics, exhibiting a low overpotential of 261 mV to achieve a current density of 10 mA cm-2 and a small Tafel slope of 32.5 mV dec-1 in 1 M KOH. The excellent OER performance and rapid OER kinetics are mainly attributed to the high-valence Ni and Fe rather than the modification in the morphology and microstructure.