Hexagonal nanoplates of NiO/CoO/Fe2O3 composite acting as an efficient photocatalytic and electrocatalytic water oxidation catalyst

Matched Kinetics Process Over Fe2O3-Co/NiO Heterostructure Enables Highly Efficient Nitrate Electroreduction to Ammonia

Tandem nitrate electroreduction reaction (NO3 -RR) is a promising method for green ammonia (NH3) synthesis. However, the mismatched kinetics processes between NO3 --to-NO2 - and NO2 --to-NH3 results in poor selectivity for NH3 and excess NO2 - evolution in electrolyte solution. Herein, a Ni2+ substitution strategy for developing oxide heterostructure in Co/Fe layered double oxides (LDOs) was designed and employed as tandem electrocataltysts for NO3 -RR. (Co0.83Ni0.16)2Fe exhibited a high NH3 yield rate of 50.4 mg ⋅ cm-2 ⋅ h-1 with a Faradaic efficiency of 97.8 % at -0.42 V vs. reversible hydrogen electrode (RHE) in a pulsed electrolysis test. By combining with in situ/operando characterization technologies and theoretical calculations, we observed the strong selectivity of NH3 evolution over (Co0.83Ni0.16)2Fe, with Ni playing a dual role in NO3 -RR by i) modifying the electronic behavior of Co, and ii) serving as complementary site for active hydrogen (*H) supply. Therefore, the adsorption capacity of *NO2 and its subsequent hydrogenation on the Co sites became more thermodynamically feasible. This study shows that Ni substitution promotes the kinetics of the NO3 -RR and provides insights into the design of tandem electrocatalysts for NH3 evolution.

Interfacial engineering and chemical reconstruction of Mo/Mo2C@CoO@NC heterostructure for promoting oxygen evolution reaction

Chemical reorganization and interfacial engineering in hybrid nanomaterials are promising strategies for enhancing electrocatalytic performance. Herein, MoO₃@zeolitic imidazolate framework-67 (ZIF-67) heterogeneous nanoribbons are designed through coordination assembly. By following heat treatment, a Mo/Mo₂C@CoO@NC heterostructure with nitrogen-doped carbon-encapsulated CoO hexagons (CoO@NC) anchored on the Mo/Mo₂C jag matrix was fabricated. Notably, through controllable experimental optimization, the as-prepared Mo/Mo₂C@CoO@NC heterostructure exhibits numerous active centers (e.g. Mo, Mo₂C, CoO, and NC), fully exposed active sites (numerous pores and jagged structures), and abundant heterointerfaces (Mo/Mo₂C, Mo₂C/CoO@NC, Mo₂C/amorphous, and CoO@NC/amorphous), and exhibits good conductivity (localized single-crystal behavior, graphitized carbon). As a result, the as-developed Mo/Mo₂C@CoO@NC heterostructures inherit impressive oxygen evolution reaction (OER) performance with an overpotential of only 215 mV at 10 mA cm^-2. Furthermore, Mo/Mo₂C@CoO@NC heterostructures exhibit excellent stability with a current density retention of 98.4% after 20 h chronoamperometry. This work provides deep insights into chemical reconstructions and tuning heterointerfaces to efficiently enhance the OER activity of heterostructure-based electrocatalysts.

Boosting molecular diffusion following the generalized Murray's Law by constructing hierarchical zeolites for maximized catalytic activity

Diffusion is an extremely critical step in zeolite catalysis that determines the catalytic performance, in particular for the conversion of bulky molecules. Introducing interconnected mesopores and macropores into a single microporous zeolite with the rationalized pore size at each level is an effective strategy to suppress the diffusion limitations, but remains highly challenging due to the lack of rational design principles. Herein, we demonstrate the first example of boosting molecular diffusion by constructing hierarchical Murray zeolites with a highly ordered and fully interconnected macro-meso-microporous structure on the basis of the generalized Murray's Law. Such a hierarchical Murray zeolite with a refined quantitative relationship between the pore size at each length scale exhibited 9 and 5 times higher effective diffusion rates, leading to 2.5 and 1.5 times higher catalytic performance in the bulky 1,3,5-triisopropylbenzene cracking reaction than those of microporous ZSM-5 and ZSM-5 nanocrystals, respectively. The concept of hierarchical Murray zeolites with optimized structural features and their design principles could be applied to other catalytic reactions for maximized performance.

Fabrication of Dandelion-like p-p Type Heterostructure of Ag2O@CoO for Bifunctional Photoelectrocatalytic Performance

A novel three-dimensional purple dandelion-like hierarchical Ag₂O@CoO heterojunction with an appropriate redox potential was constructed by chemical precipitation of Ag₂O nanoparticle on flower-like CoO. By feat of this hierarchical structure, the Ag₂O@CoO photocathode showed significantly high photoelectroreduction activities toward p-nitrophenol (p-NP) and Cr(VI). The high performance of Ag₂O@CoO was mainly attributed to the specific structural characteristics and synergistic effect of each chemical component. This hierarchical structure could effectively increase the specific surface area, provide more exposed active edges, and be beneficial for multiple light reflection/scattering channels and light utilization efficiency. The introduction of Ag₂O optimized the composition and further improved the band structure, resulting in an improved separation of photogenerated electrons and holes. The unique photocathode achieves a removal efficiency of 86% for photoelectrocatalytic p-NP degradation after 120 min and 95% for Cr(VI) after 40 min under visible light irradiation with excellent stability. This research provided a simple way for the synthesis of photoelectrocatalytic material with potential applications in the field of environmental governance with visible light illumination.

An exceptionally stable octacobalt-cluster-based metal-organic framework for enhanced water oxidation catalysis

Extensive efforts have been devoted to developing efficient and durable catalysts for water oxidation. Herein, we report a highly stable metal-organic framework that shows high catalytic activity and durability for electrically driven (an overpotential of 430 mV at 10 mA cm-2 in neutral aqueous solution) and photodriven (a turnover frequency of 16 s-1 and 12 000 cycles) water oxidation, representing the best catalyst for water oxidation reported to date. Computational simulation and isotope tracing experiments showed that the μ4-OH group of the {Co8(μ4-OH)6} unit participates in the water oxidation reaction to offer an oxygen vacancy site with near-optimal OH- adsorption energy.

An electron trapping protocol of FePW12O40 microflowers with dual catalytic properties: visible light photodegradation of amphetamine and electrocatalytic oxygen evolution

We have demonstrated the development of a novel polyxometalate compound, FePW12O40, for photocatalytic degradation of amphetamine and electrocatalytic oxygen evolution. It showed robust optoelectronic properties, which can be attributed to numerous electron trapping sites created by atomic level defects in the microflower-like architecture.

Recent Advances in the Development of Water Oxidation Electrocatalysts at Mild pH

Developing anodic oxygen evolution reaction (OER) electrocatalysts with high catalytic activities is of great importance for effective water splitting. Compared with the water-oxidation electrocatalysts that are commonly utilized in alkaline conditions, the ones operating efficiently under neutral or near neutral conditions are more environmentally friendly with less corrosion issues. This review starts with a brief introduction of OER, the importance of OER in mild-pH media, as well as the fundamentals and performance parameters of OER electrocatalysts. Then, recent progress of the rational design of electrocatalysts for OER in mild-pH conditions is discussed. The chemical structures or components, synthetic approaches, and catalytic performances of the OER catalysts will be reviewed. Some interesting insights into the catalytic mechanism are also included and discussed. It concludes with a brief outlook on the possible remaining challenges and future trends of neutral or near-neutral OER electrocatalysts. It hopefully provides the readers with a distinct perspective of the history, present, and future of OER electrocatalysts at mild conditions.

Ionic-Liquid-Assisted One-Step Synthesis of CoO Nanosheets as Electrocatalysts for Oxygen Evolution Reaction

The sluggish oxygen evolution reaction (OER) hinders the development of electrocatalytic water splitting for energy conversion and storage. Therefore, it is imperative to explore the cost-effective and highly efficient noble-metal-free electrocatalysts for OER. Herein, we are introducing such OER electrocatalyst based on Co, fabricated through an ionic-liquid-assisted one-step synthesis, where ionic liquid played a dual role as solvent cum structure-directing agent. Besides possessing large-accessible surface area and numerous active sites, the as-prepared stable CoO nanosheets exhibited excellent electrochemical activity through establishing an extensive contact with the electrolyte. Under alkaline conditions, the overpotential to achieve a current density of 10 mA cm^-2 is only 320 mV, and the Tafel slope is as small as 70 mV dec^-1. Thus, our work provides a new pathway for designing and engineering the highly efficient non-noble metal OER electrocatalysts by using ionic liquids.

Space-Confined Earth-Abundant Bifunctional Electrocatalyst for High-Efficiency Water Splitting

Hydrogen generation from water splitting could be an alternative way to meet increasing energy demands while also balancing the impact of energy being supplied by fossil-based fuels. The efficacy of water splitting strongly depends on the performance of electrocatalysts. Herein, we report a unique space-confined earth-abundant electrocatalyst having the bifunctionality of simultaneous hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), leading to high-efficiency water splitting. Outperforming Pt/C or RuO₂ catalysts, this mesoscopic, space-confined, bifunctional configuration is constructed from a monolithic zeolitic imidazolate framework@layered double hydroxide (ZIF@LDH) precursor on Ni foam. Such a confinement leads to a high dispersion of ultrafine Co₃O₄ nanoparticles within the N-doped carbon matrix by temperature-dependent calcination of the ZIF@LDH. We demonstrate that the OER has an overpotential of 318 mV at a current density of 10 mA cm^-2, while that of HER is -106 mV @ -10 mA cm^-2. The voltage applied to a two-electrode cell for overall water splitting is 1.59 V to achieve a stable current density of 10 mA cm^-2 while using the monolithic catalyst as both the anode and the cathode. It is anticipated that our space-confined method, which focuses on earth-abundant elements with structural integrity, may provide a novel and economically sound strategy for practical energy conversion applications.

Degradation of phenol by a heterogeneous photo-Fenton process using Fe/Cu/Al catalysts

Cu²⁺ and Fe³⁺ can react with H₂O₂, producing Cu⁺, Fe²⁺, and ˙OH. Then ˙OH can react with phenol directly. The degradation of phenol leads to the formation of the mixed byproducts, such as catechol, benzoquinone, resorcinol and hydroquinone.

Facile synthesis of mesoporous hierarchical ZnS@β-Ni(OH)2 microspheres for flexible solid state hybrid supercapacitors

Mesoporous hierarchical ZnS@β-Ni(OH)₂ microspheres have been successfully synthesized via a facile route and exhibited good performance as electrode materials for supercapacitors.