Three-Dimensional Hierarchical Porous Nanotubes Derived from Metal-Organic Frameworks for Highly Efficient Overall Water Splitting

Cooperation between holey N-doped carbon and Ni nanoparticles as an efficient electrocatalyst for the hydrogen evolution reaction

Exploring non-noble and high-performance metal catalysts to replace platinum-based catalysts for the hydrogen evolution reaction (HER) via electrochemical water splitting significantly alleviates environmental pollution and the energy crisis. However, the synthetic approaches of such electrocatalysts are generally complex and challenging for large-scale production. Herein, a facile and green solid-state synthesis of Ni nanoparticles decorated with N-doped porous carbon is presented. These materials are derived from chitosan as carbon, nitrogen sources, and nickel acetate as a nickel source with NaCl as a template. The synthesis procedure is simple to scale up without an organic solvent. Benefiting from its porous structure, splendid conductivity, and the synergistic effect of Ni nanoparticles and holey N-doped carbon, the as-prepared Ni@CN exhibits superior HER performance in 1 M KOH with a low potential of 121 mV at 10 mA cm-2. These findings indicate that the convenient and environmentally friendly synthesis approach provides a novel method for large-scale synthesis of HER electrocatalysts for industrial electrolytic water splitting applications.

Core-bishell NiFe@NC@MoS2 for boosting electrocatalytic activity towards ultra-efficient oxygen evolution reaction

Designing and developing suitable oxygen evolution reaction (OER) catalysts with high activity and stability remain challenging in electrolytic water splitting. Hence, NiFe@NC@MoS2 core-bishell composites wrapped by molybdenum disulphide (MoS2) and nitrogen-doped graphene (NC) were prepared using hydrothermal synthesis in this research. NiFe@NC@MoS2 composite exhibits excellent performance with an overpotential of 288 mV and a Tafel slope of 53.2 mV·dec-1 at a current density of 10 mA·cm-2 in 1 M KOH solution, which is superior to commercial RuO2. NC and MoS2 bishells create profuse edge active sites that enhance the adsorption ability of OOH* while lowering the overall overpotential of the product and improving its oxygen precipitation performance. The density function theory(DFT) analysis confirms that the layered MoS2 in NiFe@NC@MoS2 provides additional edge active sites and enhances electron transfer, thus increasing the intrinsic catalytic activity. This research paves a novel way for developing OER electrocatalysts with excellent catalytic performance.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Organic compound-based nanozymes for agricultural herbicide detection

Nanozymes are increasingly being used for agricultural applications, but their adoption is limited as they are generally considered toxic, have low cost-effectiveness, and pose complexity of fabrication. In this study, an organic compound-based, peroxidase-like nanozyme (OC nanozyme) was developed for use in the agricultural environment. This nanozyme was synthesized through a self-assembled one-pot particle synthesis process, interacting with urea and the metal ion to form a homogenous nanoparticle containing partially mimicked cofactors (Fe-N) of the natural enzyme. The OC nanozyme exhibited decent kinetic properties (H2O2/Km:0.056 mM and Vmax:2.19 μM s-1) and pH stability. The OC nanozyme was successfully used to detect glyphosate via integrated colorimetric assay, with a good limit of detection (LOD) of at least 0.001 ng mL-1. The authors envision that this agricultural-friendly OC nanozyme holds great potential for a wide range of agricultural applications.

Nickel -supported PdM (M = Au and Ag) nanodendrites as formate oxidation (electro)catalytic anodes for direct fuel cells and hydrogen generation at room temperature

The study provides a proof of concept for the first time that unique palladium-gold (PdAu) and palladium-silver (PdAg) nanodendrites are bifunctional catalytic active sites for formate oxidation reactions (FORs) and formate dehydrogenation reactions (FDRs). The unique nanodendritic structure was developed via a simple galvanic displacement reaction for the direct growth of PdAu and PdAg nanodendrites on a nickel foam (PdAu/NiNF and PdAg/NiNF). These PdAu/NiNF and PdAg/NiNF electrodes exhibited 2.32 and 1.59 times higher specific activity than that of the commercial Pd/C electrode and promising stability toward FORs. Moreover, the PdAu/NiNF and PdAg/NiNF nanodendrites were also highly active and selective catalysts for hydrogen generation from a formate solution with turnover frequency (TOF) values of 311 h^-1 and 287 h^-1 respectively. Impressively, a passive air-breathing formate fuel cell with PdAu/NiNF used as an anode can yield an open-circuit voltage of 1.12 V and a peak power density of 21.7 mW cm^-2, which outperforms most others reported in the literature. PdAu and PdAg nanodendritic catalysts supported on a nickel foam demonstrate an open structure and uniform catalyst distribution and offer a promising nanoalloy for air-breathing formate fuel cells and on-site chemical hydrogen production systems.

Self-Supporting Metal-Organic Framework-Based Nanoarrays for Electrocatalysis

The replacement of powdery catalysts with self-supporting alternatives for catalyzing various electrochemical reactions is extremely important for the large-scale commercial application of renewable energy storage and conversion technologies. Metal-organic framework (MOF)-based nanoarrays possess tunable compositions, well-defined structure, abundant active sites, effective mass and electron transport, etc., which enable them to exhibit superior electrocatalytic performance in multiple electrochemical reactions. This review presents the latest research progress in developing MOF-based nanoarrays for electrocatalysis. We first highlight the structural features and electrocatalytic advantages of MOF-based nanoarrays, followed by a detailed summary of the design and synthesis strategies of MOF-based nanoarrays, and then describe the recent progress of their application in various electrocatalytic reactions. Finally, the challenges and perspectives are discussed, where further exploration into MOF-based nanoarrays will facilitate the development of electrochemical energy conversion technologies.

MOF-derived nanoarrays as advanced electrocatalysts for water splitting

Developing efficient, nanostructured electrocatalysts with the desired compositions and structures is of great significance for improving the efficiency of water splitting toward hydrogen production. In this regard, metal-organic framework (MOF) derived nanoarrays have attracted great attention as promising electrocatalysts because of their diverse compositions and adjustable structures. In this review, the recent progress in MOF-derived nanoarrays for electrochemical water splitting is summarized, highlighting the structural design of the MOF-derived nanoarrays and the electrocatalytic performance of the derived composite carbon materials, oxides, hydroxides, sulfides, and phosphides. In particular, the structure-performance relationships of the MOF-derived nanoarrays and the modulation strategies toward enhanced catalytic activity for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are discussed, providing insights into the development of advanced catalysts for the HER and OER. The challenges and prospects in this promising field for future industrial applications are also addressed.

Metal nitride-based nanostructures for electrochemical and photocatalytic hydrogen production

The over-dependence on fossil fuels is one of the critical issues to be addressed for combating greenhouse gas emissions. Hydrogen, one of the promising alternatives to fossil fuels, is renewable, carbon-free, and non-polluting gas. The complete utilization of hydrogen in every sector ranging from small to large scale could hugely benefit in mitigating climate change. One of the key aspects of the hydrogen sector is its production via cost-effective and safe ways. Electrolysis and photocatalysis are well-known processes for hydrogen production and their efficiency relies on electrocatalysts, which are generally noble metals. The usage of noble metals as catalysts makes these processes costly and their scarcity is also a limiting factor. Metal nitrides and their porous counterparts have drawn considerable attention from researchers due to their good promise for hydrogen production. Their properties such as active metal centres, nitrogen functionalities, and porous features such as surface area, pore-volume, and tunable pore size could play an important role in electrochemical and photocatalytic hydrogen production. This review focuses on the recent developments in metal nitrides from their synthesis methods point of view. Much attention is given to the emergence of new synthesis techniques, methods, and processes of synthesizing the metal nitride nanostructures. The applications of electrochemical and photocatalytic hydrogen production are summarized. Overall, this review will provide useful information to researchers working in the field of metal nitrides and their application for hydrogen production.

Construction of Fe-doped CoP with hybrid nanostructures as a bifunctional catalyst for overall water splitting

Due to their open skeleton structures, adjustable active sites and homogeneous catalytic centers, PBA-based materials have promising applications in electrochemical water splitting. Herein, we report a PBA derived Fe_0.25-CoP electrocatalyst with a hybrid nanostructure, which offered a large specific surface area and active sites for the HER and OER, respectively. The as-synthesized Fe_0.25-CoP catalyst exhibits remarkable catalytic performance and durability at overpotentials of 262 mV for the OER and 111 mV for the HER, requiring a voltage of merely 1.57 V to achieve a current density of 10 mA cm^-2 for the electrocatalytic water splitting process. The preeminent activity of Fe_0.25-CoP was mainly ascribed to the framework structures of Co-PBA and appropriate doping of Fe³⁺ which regulated the electronic structures and morphology of Fe_0.25-CoP. In addition, the partial phosphating strategy retained the active centers for the OER in Co-PBA, which were further enhanced by the catalysis of Fe³⁺. In short, the rational design and regulation of catalyst structures and compositions is a promising approach for the development of highly efficient water splitting catalysts.

Synergistically coupling of Fe-doped CoP nanocubes with CoP nanosheet arrays towards enhanced and robust oxygen evolution electrocatalysis

The rational design of high-performance and low-cost oxygen evolution reaction (OER) electrocatalysts for water splitting is of vital importance for development of renewable hydrogen energy. Herein, we demonstrate an interfacial engineering strategy to prepare Fe-doped CoP nanocubes/CoP nanosheet arrays heterostructure supported on carbon cloth (denoted as CoFeP/CoP/CC). The resultant CoFeP/CoP/CC heterostructure catalyst possesses abundant heterogeneous interfaces, which enables the exposure of reaction active sites and possibly modulation of electronic structure of the catalyst. Furthermore, this strong interfacial coupling of CoFeP and CoP as well as the integration structure on the carbon cloth guarantee high electronic conductivity and enhanced mechanical stability. Benefiting from these advantages, the CoFeP/CoP/CC-heterostructure exhibits high electrocatalytic OER performance with a low overpotential of 240 mV for reaching a current density of 10 mA cm^-2, which outperforms the commercial noble metal RuO₂ (255 mV) and many reported TMPs-based electrocatalysts. Moreover, this CoFeP/CoP/CC catalyst shows a remarkable OER catalytic stability over 100 h. This work provides an effective avenue for the design of the high-performance OER catalyst by interfacial engineering strategy.

Metal–organic frameworks and their derivatives as electrocatalysts for the oxygen evolution reaction

This review summarizes the recent progress on MOFs and their derivatives used for OER electrocatalysis in terms of their morphology, composition and structure–performance relationship.

Hierarchically porous metal-organic frameworks: synthesis strategies, structure(s), and emerging applications in decontamination

Metal-organic frameworks (MOFs) with high porosity have received much attention as promising materials for many applications owing to their unique properties. However, to date, most of the reported MOFs have microporous structures, which slow down diffusion/mass transfer and limit the accessibility of bulky molecules to its internal surface. Thus, it is crucial to develop an efficient way to create larger pores (mesoporous and/or macroporous) into microporous MOFs to form hierarchical porous metal-organic frameworks (HP-MOFs), which facilitate the diffusion and mass transfer of guest molecules. HP-MOFs are excellent and promising candidates for environmental applications under the background of environmental contaminations. In this review paper, we are primarily focusing on the latest progress in the preparation of HP-MOFs by employing template-assisted and template-free synthetic approaches for environmental cleaning applications. Particularly, the adsorptive purification of the most common toxic substances, including gases, dyes, heavy metal ions, and antibiotics from the environment using HP-MOFs as adsorbents is briefly discussed. The overall results clearly showed that the superiority of HP-MOFs compared with conventional microporous MOFs. Finally, we summarize the remaining challenges and provide personal perspectives on possible future development of HP-MOFs.

Anode-Driven Controlled Release of Cathodic Fuel via pH Response for Smart Enzymatic Biofuel Cell

Enzymatic biofuel cells (EBFCs) with or without a membrane to separate the anodic and cathodic compartments generally suffered from high internal resistance or interactive interference, both of which restricted the improvement of their performance. Herein, a smart membrane-less EBFC was engineered based on anode-driven controlled release of cathodic acceptor via pH-responsive metal-organic framework ([Fe(CN)6]3-@ZIF-8) nanocarriers. The glucose anodic oxidation would produce gluconic acid accompanied by the change in pH value from neutral to the acidic case, which could drive the degradation of [Fe(CN)6]3-@ZIF-8 nanocarriers and further realize the controlled release of cathodic acceptor [Fe(CN)6]3-. More importantly, compared with controlled EBFC with or without membrane, the power output of the as-proposed EBFC enhanced at least 700 times due to the seamless electronic communication. Therefore, the ingenious strategy not only realized the successful engineering of the membrane-less EBFC but also provided an appealing idea for constructing smart devices.

Hydrogen Evolution Reaction-From Single Crystal to Single Atom Catalysts

Hydrogen evolution reaction (HER) is one of the most important reactions in electrochemistry. This is not only because it is the simplest way to produce high purity hydrogen and the fact that it is the side reaction in many other technologies. HER actually shaped current electrochemistry because it was in focus of active research for so many years (and it still is). The number of catalysts investigated for HER is immense, and it is not possible to overview them all. In fact, it seems that the complexity of the field overcomes the complexity of HER. The aim of this review is to point out some of the latest developments in HER catalysis, current directions and some of the missing links between a single crystal, nanosized supported catalysts and recently emerging, single-atom catalysts for HER.