Quinary Defect-Rich Ultrathin Bimetal Hydroxide Nanosheets for Water Oxidation

Edge-Induced Synergy of Ni-Ni Defects in NiFe Layered-Double-Hydroxide for Electrocatalytic Water Oxidation Reaction

Defect engineering is widely regarded as a promising strategy to enhance the performance of electrocatalysts for water splitting. In this work, defective NiFe layered double hydroxide (NiFe LDH) with a high density of edge sites (edge-rich NiFe LDH) is synthesized via a simple reduction process during the early stages of nucleation. The introduction of edges into oxygen evolution reaction (OER) catalysts modulates the electronic structure of the active sites. X-ray absorption spectroscopy (XAS) analyses revealed that the edges facilitated the formation of unsaturated Ni-Ni coordination, which is crucial for promoting the deprotonation of the OH* intermediate. Consequently, the edge-rich NiFe LDH exhibited a significantly lower overpotential of 228 mV to achieve a current density of 10 mA cm⁻2, compared to 275 mV for pristine NiFe LDH. The assembled membrane electrode can reach a current density of 1000 mA cm⁻2 at a cell voltage of 2.5 V. This study highlights the role of edge effects in defect engineering to enhance OER activity and provides valuable theoretical insights for the design of efficient electrocatalysts.

Layered Double Hydroxide Nanosheets: Synthesis Strategies and Applications in the Field of Energy Conversion

In recent years, layered double hydroxides (LDH) nanosheets have garnered substantial attention as intriguing inorganic anionic layered clay materials. These nanosheets have captured the attention of researchers due to their unique physicochemical properties. This review aims to showcase the latest advancements in laboratory research concerning LDH nanosheets, with a specific emphasis on their methods of preparation. This review provides detailed insights into the factors influencing the anionic conductivity of LDH, along with delineating the applications of LDH nanosheets in the realm of energy conversion. Notably, the review highlights the crucial role of LDH nanosheets in the oxygen evolution reaction (OER), a vital process in water splitting and diverse electrochemical applications. The review emphasizes the significant potential of LDH nanosheets in enhancing supercapacitor technology, owing to their high surface area and exceptional charge storage capacity. Additionally, it elucidates the prospective application of LDH nanosheets as anion exchange membranes in anion exchange membrane fuel cells, potentially revolutionizing fuel cell performance through improved efficiency and stability facilitated by enhanced ion transport properties.

Transition Metal-Based 2D Layered Double Hydroxide Nanosheets: Design Strategies and Applications in Oxygen Evolution Reaction

Water splitting driven by renewable energy sources is considered a sustainable way of hydrogen production, an ideal fuel to overcome the energy issue and its environmental challenges. The rational design of electrocatalysts serves as a critical point to achieve efficient water splitting. Layered double hydroxides (LDHs) with two-dimensionally (2D) layered structures hold great potential in electrocatalysis owing to their ease of preparation, structural flexibility, and tenability. However, their application in catalysis is limited due to their low activity attributed to structural stacking with irrational electronic structures, and their sluggish mass transfers. To overcome this challenge, attempts have been made toward adjusting the morphological and electronic structure using appropriate design strategies. This review highlights the current progress made on design strategies of transition metal-based LDHs (TM-LDHs) and their application as novel catalysts for oxygen evolution reactions (OERs) in alkaline conditions. We describe various strategies employed to regulate the electronic structure and composition of TM-LDHs and we discuss their influence on OER performance. Finally, significant challenges and potential research directions are put forward to promote the possible future development of these novel TM-LDHs catalysts.

Turning Waste into Treasure: Regulating the Oxygen Corrosion on Fe Foam for Efficient Electrocatalysis

Iron corrosion causes a great damage to the economy due to the function attenuation of iron-based devices. However, the corrosion products can be used as active materials for some electrocatalytic reactions, such as oxygen evolution reaction (OER). Herein, the oxygen corrosion on Fe foams (FF) to synthesize effective self-supporting electrocatalysts for OER, leading to "turning waste into treasure," is regulated. A dual chloride aqueous system of "NaCl-NiCl₂ " is employed to tailor the structures and OER properties of corrosion layers. The corrosion behaviors identify that Cl^- anions serve as accelerators for oxygen corrosion, while Ni²⁺ cations guarantee the uniform growth of corrosion layers owing to the appeared chemical plating. The synergistic effect of "NaCl-NiCl₂ " generates one of the highest OER activities that only an overpotential of 212 mV is required to achieve 100 mA cm^-2 in 1.0 m KOH solution. The as-prepared catalyst also exhibits excellent durability over 168 h (one week) at 100 mA cm^-2 and promising application for overall water splitting. Specially, a large self-supporting electrode (9 × 10 cm² ) is successfully synthesized via this cost-effective and easily scale-up approach. By combining with corrosion science, this work provides a significant stepping stone in exploring high-performance OER electrocatalysts.

Defect Engineering for Fuel-Cell Electrocatalysts

The commercialization of fuel cells, such as proton exchange membrane fuel cells and direct methanol/formic acid fuel cells, is hampered by their poor stability, high cost, fuel crossover, and the sluggish kinetics of platinum (Pt) and Pt-based electrocatalysts for both the cathodic oxygen reduction reaction (ORR) and the anodic hydrogen oxidation reaction (HOR) or small molecule oxidation reaction (SMOR). Thus far, the exploitation of active and stable electrocatalysts has been the most promising strategy to improve the performance of fuel cells. Accordingly, increasing attention is being devoted to modulating the surface/interface electronic structure of electrocatalysts and optimizing the adsorption energy of intermediate species by defect engineering to enhance their catalytic performance. Defect engineering is introduced in terms of defect definition, classification, characterization, construction, and understanding. Subsequently, the latest advances in defective electrocatalysts for ORR and HOR/SMOR in fuel cells are scientifically and systematically summarized. Furthermore, the structure-activity relationships between defect engineering and electrocatalytic ability are further illustrated by coupling experimental results and theoretical calculations. With a deeper understanding of these complex relationships, the integration of defective electrocatalysts into single fuel-cell systems is also discussed. Finally, the potential challenges and prospects of defective electrocatalysts are further proposed, covering controllable preparation, in situ characterization, and commercial applications.

Ni-Based Nanoparticle-Embedded N-Doped Carbon Nanohorns Derived from Double Core-Shell CNH@PDA@NiMOFs for Oxygen Electrocatalysis

The development of highly efficient electrocatalysts for the oxygen evolution reaction (OER) plays a crucial role in many regenerative electrochemical energy-conversion systems. Herein, we report a novel double core-shell-structured CNH@PDA@NiMOF (CNH-D-NiMOF) composite based on the support of carbon nanohorns (CNHs) and the direction of polydopamine (PDA) on the synthesis of metal-organic frameworks (MOFs). It is found that this unique structure improves the electrocatalytic performance and stability of the composites. Furthermore, a controlled partial pyrolysis strategy was proposed to construct the Ni-based nanoparticle-embedded N-doped CNHs. The partial pyrolysis method preserves the framework structure of MOFs for effective substrate diffusion while producing highly active nanoparticles. This leads to the result that the Ni-based nanoparticle-embedded N-doped CNHs possess higher stability and significantly improved electrocatalytic properties. Among these derivatives, the sample prepared at a pyrolysis temperature of 400 °C (named as CNH-D-NiMOF-400) outperforms most of the reported unprecious-metal catalysts. At current densities of 20 and 100 mA·cm^-2, the overpotentials of CNH-D-NiMOF-400 are 270 and 340 mV for the OER on a carbon fiber paper (CFP), respectively. The outstanding electrocatalytic properties above suggest that this composite is an excellent candidate for the substitution of noble metal-based catalysts for OER.