MOF‐Mediated Fabrication of a Porous 3D Superstructure of Carbon Nanosheets Decorated with Ultrafine Cobalt Phosphide Nanoparticles for Efficient Electrocatalysis and Zinc–Air Batteries

Ruthenium Complex of sp2 Carbon-Conjugated Covalent Organic Frameworks as an Efficient Electrocatalyst for Hydrogen Evolution

It is still a great challenge to explore hydrogen evolution reaction (HER) electrocatalysts with both lower overpotential and higher stability in acidic electrolytes. In this work, an efficient HER catalyst, Ru@COF-1, is prepared by complexation of triazine-cored sp² carbon-conjugated covalent organic frameworks (COFs) with ruthenium ion. Ru@COF-1 possesses high crystallinity and porosity, which are beneficial for electrocatalysis. The large specific surface area and regular porous channels of Ru@COF-1 facilitate full contact between reactants and catalytic sites. The nitrogen atoms of triazines are protonated in the acidic media, which greatly improve the conductivity of Ru@COF-1. This synergistic effect makes the overpotential of Ru@COF-1 about 200 mV at 10 mA cm^-2 , which is lower than other reported COFs-based electrocatalysts. Moreover, Ru@COF-1 exhibits exceptionally electrocatalytic durability in the acidic electrolytes. It is particularly stable and remains highly active after 1000 cyclic voltammetry cycles. Density functional theory calculations demonstrate that tetracoordinated Ru-N₂ Cl₂ moieties are the major contributors to the outstanding HER performance. This work provides a new idea for developing protonated HER electrocatalysts in acidic media.

MOF-Derived Ultrathin Cobalt Molybdenum Phosphide Nanosheets for Efficient Electrochemical Overall Water Splitting

The development of high-performance and cost-effective earth-abundant transition metal-based electrocatalysts is of major interest for several key energy technologies, including water splitting. Herein, we report the synthesis of ultrathin CoMoP nanosheets through a simple ion etching and phosphorization method. The obtained catalyst exhibits outstanding electrocatalytic activity and stability towards oxygen and hydrogen evolution reactions (OER and HER), with overpotentials down to 273 and 89 mV at 10 mA cm⁻², respectively. The produced CoMoP nanosheets are also characterized by very small Tafel slopes, 54.9 and 69.7 mV dec⁻¹ for OER and HER, respectively. When used as both cathode and anode electrocatalyst in the overall water splitting reaction, CoMoP-based cells require just 1.56 V to reach 10 mA cm⁻² in alkaline media. This outstanding performance is attributed to the proper composition, weak crystallinity and two-dimensional nanosheet structure of the electrocatalyst.

Electrochemical Hydrogenation of Furfural in Aqueous Acetic Acid Media with Enhanced 2-Methylfuran Selectivity Using CuPd Bimetallic Catalysts

Herein, we report a series of CuPd catalysts for electrochemical hydrogenation (ECH) of furfural to 2-methylfuran (MF or FurCH₃ where Fur=furyl) in aqueous 0.1 M acetic acid (pH 2.9). The highest faradaic efficiency (FE) for MF reached 75 % at -0.58 V vs. reversible hydrogen electrode with an average partial current density of 4.5 mA cm^-2 . In situ surface-enhanced Raman spectroscopic and kinetic isotopic experiments suggested that electrogenerated adsorbed hydrogen (H_ads ) was involved in the reaction and incorporation of Pd enhanced the surface coverage of H_ads and optimized the adsorption pattern of furfural, leading to a higher FE for MF. Density functional theory calculations revealed that Pd incorporation reduced the energy barrier for the hydrogenation of FurCH₂ * to FurCH₃ *. Our study demonstrates that catalyst surface structure/composition plays a crucial role in determining the selectivity in ECH and provides a new strategy for designing advanced catalysts for ECH of bio-derived oxygenates.

Two-Dimensional Cobalt Sulfide/Iron-Nitrogen-Carbon Holey Sheets with Improved Durability for Oxygen Electrocatalysis

Transition-metal sulfide as a promising bifunctional oxygen electrocatalyst alternative to scarce platinum-group metals has attracted much attention, but it suffers activity loss over time owing to poor structural/compositional stability during catalysis. Herein, we report a self-template method for preparing a two-dimensional cobalt sulfide holey sheet superstructure with hierarchical porosity followed by the encapsulation of thin iron-nitrogen-carbon as a protective layer. The iron-nitrogen-carbon layer to some degree precludes the phase transition of cobalt sulfide underneath and preserves the structural integrity during catalysis, therefore rendering an exceptional durability in terms of no obvious activity loss after 10,000 cycles of the accelerated durability test. It also noticeably enhances the intrinsic activity of cobalt sulfide and does not influence its exposure into the electrolyte, resulting in showing an extraordinary electrochemical performance in terms of a potential difference of 0.69 V for the overall oxygen redox. A rechargeable zinc-air battery assembled by a cobalt sulfide/iron-nitrogen-carbon air cathode delivers approximately 4.2 times higher power density than that without an iron-nitrogen-carbon layer and stably operates for 300 h with a high voltaic efficiency. This work gives a facile and effective strategy for improving the long-term durability of transition-metal sulfide electrocatalysts.

Combining Multivariate Electrospinning with Surface MOF Functionalization to Construct Tunable Active Sites toward Trifunctional Electrocatalysis

The development of high-performance multifunctional electrocatalysts operating in the same electrolyte is key to reduce the material and process costs of renewable energy conversion and storage devices. Herein, the fabrication of freestanding integral electrodes by combining multivariate electrospinning with surface metal organic framework functionalization to arrest pyrolytic emissions from fiber interior is reported, resulting in the expression of rich active sites with controlled composition, for example, the tunable Co-P coordination. The as-fabricated electrode of CoP@CF-900, when used as both the cathode and anode for overall water splitting, is able to deliver 200 mA cm^-2 at a cell voltage of 1.89 V, significantly outshining the Pt/C‖RuO₂ couple; when used as the air cathode for a zinc-air battery, is able to operate more than 150 h at 10 mA cm^-2 with a nearly constant round-trip energy efficiency of ≈60%, also outperforming the Pt/C+RuO₂ benchmark. The activity and kinetics origin of the superb multi-functionality is further elucidated through extensive electroanalytical, post-mortem, and operando characterizations, which underscore the construction of robust integral electrodes through synergistic structure and composition engineering.

Electronic and architecture engineering of hammer-shaped Ir–NiMoO4-ZIF for effective oxygen evolution

Electronic and architecture engineering is realized via doping an ultralow amount of Ir into NiMoO4-ZIF hammers to achieve outstanding electrocatalytic OER performance.

Popcorn-like Co3O4 Nanoparticles Confined in Three-Dimensional Hierarchical N-doped carbon nanotubes Networks as Highly Efficient Trifunctional Electrocatalyst for Zinc-Air Batteries and Water Splitting Device

A novel unique popcorn-like three-dimensional (3D) hierarchical structural electrocatalyst is synthesized by the pyrolysis of ZIF-8/ZIF-67 and polyacrylonitrile fibers composites, where popcorn-like Co3O4 nanoparticles coated with nitrogen-doped amorphous carbon anchor...

Atomically Dispersed, Low-Coordinate Co-N Sites on Carbon Nanotubes as Inexpensive and Efficient Electrocatalysts for Hydrogen Evolution

Hydrogen produced using renewable electricity is considered the key to achieving a low-carbon energy economy. However, the large-scale application of electrochemical water splitting for hydrogen evolution currently requires expensive platinum-based catalysts. Therefore, it is important to develop efficient and stable catalysts based on the rich reserves of transition metals as alternatives. In this study, the authors prepare a carbon-nanotube material enriched with atomically dispersed CoN sites having uniquely low coordination numbers via the simple mixing, pyrolysis, and leaching of inexpensive precursors. These atomically dispersed low-coordinate CoN sites provide an overpotential of only 82 mV at 10 mA cm^-2 for the hydrogen evolution reaction (HER) under challenging acidic conditions and show excellent durability in accelerated stability tests. Theoretical simulations also confirm that these unique, low-coordinate CoN₂ sites have lower energy barriers in catalyzing the HER than Fe/NiN₂ sites and commonly reported CoN₃ /N₄ sites. Therefore, the method provides a new concept for the design of single-atom catalytic sites with low coordination numbers. It also serves to reduce the cost of hydrogen production in the future owing to the high catalytic activity, low cost, and scalable production process.

Different nanostructured CoP microcubes derived from metal formate frameworks with enhanced oxygen evolution reaction performance

Different nanostructured CoP microcubes derived from metal formate frameworks with enhanced oxygen evolution reaction performance.

MOF-on-MOF Strategy to Construct a Nitrogen-Doped Carbon-Incorporated CoP@Fe-CoP Core-Shelled Heterostructure for High-Performance Overall Water Splitting

The design and preparation of efficient and low-cost catalysts for water electrolysis are crucial and highly desirable to produce eco-friendly and sustainable hydrogen fuel. Herein, we prepared nitrogen-doped carbon-incorporated CoP@Fe-CoP core-shelled nanorod arrays grown on Ni foam (CoP@Fe-CoP/NC/NF) through phosphorization of ZIF-67@Co-Fe Prussian blue analogue (ZIF-67@CoFe-PBA). The hierarchical nanorod arrays combined with the core-shelled structure offer favorable mass/electron transport capacity and maximize the active sites, thus enhancing the electrochemically active surface area. The synergistic effect of the bimetallic components and the nitrogen-doped carbon matrix endow the composite with an optimized electronic structure. Benefiting from the above superiorities of morphological and chemical compositions, this self-supported CoP@Fe-CoP/NC/NF heterostructure can drive alkaline hydrogen evolution reaction and oxygen evolution reaction with overpotentials of 97 and 270 mV to yield 100 mA cm^-2, respectively. The two-electrode alkaline electrolyzer constructed by this heterostructure shows a low cell voltage of 1.58 V to yield 10 mA cm^-2, superior to the precious-metal-based electrocatalyst apparatus (IrO₂∥Pt/C). This study offers a feasible and facile approach to develop efficient electrocatalysts for water electrolysis, which applies to other electrochemical energy conversion and storage applications.

Strategies for Developing Transition Metal Phosphides in Electrochemical Water Splitting

Electrochemical water splitting involving hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is a greatly promising technology to generate sustainable and renewable energy resources, which relies on the exploration regarding the design of electrocatalysts with high efficiency, high stability, and low cost. Transition metal phosphides (TMPs), as nonprecious metallic electrocatalysts, have been extensively investigated and proved to be high-efficient electrocatalysts in both HER and OER. In this minireview, a general overview of recent progress in developing high-performance TMP electrocatalysts for electrochemical water splitting has been presented. Design strategies including composition engineering by element doping, hybridization, and tuning the molar ratio, structure engineering by porous structures, nanoarray structures, and amorphous structures, and surface/interface engineering by tuning surface wetting states, facet control, and novel substrate are summarized. Key scientific problems and prospective research directions are also briefly discussed.

Pyrimidine-Functionalized Covalent Organic Framework and its Cobalt Complex as an Efficient Electrocatalyst for Oxygen Evolution Reaction

A pyrimidine-modified covalent organic framework (COF-Pyr) was designed to be synthesized via the Povarov reaction. The nitrogen atom on the pyrimidine showed excellent coordination ability to metal ions. Their stable metal composite material (Co@COF-Pyr) exhibited remarkable performance for electrocatalytic oxygen evolution reaction (OER) in 1.0 m KOH aqueous solution. The overpotential was 450 mV at 10 mA cm^-2 . The Co@COF-Pyr with large specific surface area (392 m²  g^-1 ) and regular crystal structure provided free passage for H₂ O to move and make them fully contact with the uniformly dispersed cobalt ions on the surface. Thus, the turnover frequency of Co@COF-Pyr was 0.1 s^-1 at the overpotential of 370 mV, which was higher than most reported OER catalysts. This work provided a new way to design and prepare nitrogen-containing heterocyclic functionalized COFs. They can be combined with metal ions to expand the application of COFs in the field of electrocatalysis.

Metal-Organic Frameworks-Derived Self-Supported Carbon-Based Composites for Electrocatalytic Water Splitting

Electrocatalytic water splitting has been considered as a promising strategy for the sustainable evolution of hydrogen energy and storage of intermittent electric energy. Efficient catalysts for electrocatalytic water splitting are urgently demanded to decrease the overpotentials and promote the sluggish reaction kinetics. Carbon-based composites, including heteroatom-doped carbon materials, metals/alloys@carbon composites, metal compounds@carbon composites, and atomically dispersed metal sites@carbon composites have been widely used as the catalysts due to their fascinating properties. However, these electrocatalysts are almost powdery form, and should be cast on the current collector by using the polymeric binder, which would result in the unsatisfied electrocatalytic performance. In comparison, a self-supported electrode architecture is highly attractive. Recently, self-supported metal-organic frameworks (MOFs) constructed by coordination of metal centers and organic ligands have been considered as suitable templates/precursors to construct free-standing carbon-based composites grown on conductive substrate. MOFs-derived carbon-based composites have various merits, such as the well-aligned array architecture and evenly distributed active sites, and easy functionalization with other species, which make them suitable alternatives to non-noble metal-included electrocatalysts. In this review, we intend to show the research progresses by employment of MOFs as precursors to prepare self-supported carbon-based composites. Focusing on these MOFs-derived carbon-based nanomaterials, the latest advances in their controllable synthesis, composition regulation, electrocatalytic performances in hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting (OWS) are presented. Finally, the challenges and perspectives are showed for the further developments of MOFs-derived self-supported carbon-based nanomaterials in electrocatalytic reactions.

Superstructured Macroporous Carbon Rods Composed of Defective Graphitic Nanosheets for Efficient Oxygen Reduction Reaction

Rationally designed carbon materials with superstructures are promising candidates in applications such as electrocatalysis. However, the synthesis of highly porous carbon superstructures with macropores and carbon defects from a simple crystalline solid remains challenging. In this work, superstructured macroporous carbon rods composed of defective graphitic nanosheets are synthesized by direct carbonization of crystalline poly tannic acid (PTA) rods as precursors. During carbonization, PTA rods with a highly ordered lamellar structure induce a spatially confined two-step localized contraction that takes place in different dimensions and directions in each step. The unexpected contraction behavior results in the sponge-like macroporous carbon superstructure with large surface area, high porosity, and abundant defects, thus showing a superior electrocatalytic performance with high activity and selectivity for oxygen reduction reaction. The study provides new understandings in the design of functional carbon materials with distinctive structures and applications.

Wood-Derived Bimetallic and Heteroatomic Hierarchically Porous Carbon Aerogel for Rechargeable Flow Zn-Air Batteries

It is necessary to correctly research and synthesize efficient and inexpensive catalysts to achieve reversible oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which is also a prerequisite for zinc-air batteries (ZABs). However, it is still a huge challenge to manufacture electrocatalysts with durable and high electrocatalytic performance from biomass. Here, a convenient method of delignification was used to transform natural balsa wood into a layered porous carbon material, FeCo alloy supported on a N, S-doped wood-based carbon aerogel (FeCo@NS-CA) as the cathode in rechargeable flow ZAB. The obtained FeCo@NS-CA with the porous lamellar architecture exhibits superior bifunctional electrocatalysis, including excellent electrochemical activities and superior stabilities. For ORR, relative to the reversible hydrogen electrode, the onset potential of FeCo@NS-CA is 0.97 V, and the half-wave potential is 0.85 V, which is consistent with the potential of commercial Pt/C. For OER, FeCo@NS-CA obtained an overpotential of 450 mV, which is very similar to the overpotential of the benchmark RuO₂. The superior performance could be owing to the alloy carrier interaction between the FeCo alloy and the wood-based carbon aerogel co-doped with N and S. Moreover, the bifunctional air cathode in a flow ZAB assembled with the FeCo@NS-CA catalyst at a current density of 10 mA cm^-2; the power density is 140 mW cm^-2, and the specific capacitance is 760 mA h g_Zn^-1, with a remarkable long-term stability of 400 h better than ZAB of benchmark Pt/C + RuO₂. This research lays the foundation for transforming abundant biomass resources into high environmental protection materials for energy-related applications.

Co/MoC Nanoparticles Embedded in Carbon Nanoboxes as Robust Trifunctional Electrocatalysts for a Zn-Air Battery and Water Electrocatalysis

To meet the application needs of rechargeable Zn-air battery and electrocatalytic overall water splitting (EOWS), developing high-efficiency, cost-effective, and durable trifunctional catalysts for the hydrogen evolution reaction (HER), oxygen evolution, and reduction reaction (OER and ORR) is extremely paramount yet challenging. Herein, the interface engineering concept and nanoscale hollowing design were proposed to fabricate N-doping carbon nanoboxes confined with Co/MoC nanoparticles. Uniform zeolitic imidazolate framework nanocube was employed as the starting material to construct the trifunctional electrocatalyst through the conformal polydopamine-Mo layer coating and the subsequent pyrolysis treatment. The Co@IC/MoC@PC catalyst displayed superior electrochemical ORR performances with a positive half-wave potential of 0.875 V and a high limiting current density of 5.89 mA/cm². When practically employed as an electrocatalyst in regenerative Zn-air battery, a high specific capacity of 728 mAh/g, a large peak power density of 221 mW/cm², a high open-circuit voltage of 1.482 V, and a low charge/discharge voltage gap of 0.41 V were obtained. Moreover, its practicability was further exploited by overall water splitting, affording low overpotentials of 277 and 68 mV at 10 mA/cm² for the OER and HER in 1 M KOH solution, respectively, and a decent operating potential of 1.57 V for EOWS. Ultraviolet photoelectron spectroscopy and density functional theory calculation revealed that the Co/MoC interface synergistically facilitated the charge-transfer, thereby contributing to the enhancements of electrocatalytic ORR/OER/HER processes. More importantly, this catalyst design concept can offer some interesting prospects for the construction of outstanding trifunctional catalysts toward various energy conversion and storage devices.

The controllable synthesis of urchin-shaped hierarchical superstructure MOFs with high catalytic activity and stability

A novel dissolution-crystallization-attachment strategy was developed to synthesize urchin-shaped superstructure metal-organic frameworks (MOFs) with self-assembled one-dimensional nanorods. The superstructure MOFs not only inherited the high activity of nanosized MOFs but also displayed the high stability of microsized MOFs.

Hydrangea-like Superstructured Micro/Nanoreactor of Topotactically Converted Ultrathin Bismuth Nanosheets for Highly Active CO2 Electroreduction to Formate

An electrocatalytic carbon dioxide reduction reaction (CO₂RR) is an appealing route to obtain the value-added feedstocks and alleviate the energy crisis. However, how to achieve high-performance electrocatalysts for CO₂ reduction to formate is challenging owing to the poor intrinsic activity, insufficient conductivity, and low surface density of active sites. Herein, we fabricated an extremely active and selective hydrangea-like superstructured micro/nanoreactor of ultrathin bismuth nanosheets through an in situ electrochemical topotactic transformation of hierarchical bismuth oxide formate (BiOCOOH). The resulted bismuth nanosheet superstructure is in the form of three-dimensional intercrossed networks of ultrathin nanosheets, forming an ordered open porous structure through self-assembly, which can be used as a micro/nanoreactor to enable a large electrochemically active surface area as well as high atomic utilization. Such a distinctive nanostructure endows the material with high electrocatalytic performances for CO₂ reduction to formate with near-unity Faradaic selectivity (>95%) in a wide potential window from -0.78 to -1.18 V. Furthermore, this micro/nanoreactor can give the high current densities over 300 mA cm^-2 at low applied potentials without compromising selectivity in a flow cell reactor. Density functional theory (DFT) and in situ attenuated total reflection-infrared spectroscopy (in situ ATR-IR) were further conducted to interpret the CO₂RR mechanisms.

Optimization of metal–organic framework derived transition metal hydroxide hierarchical arrays for high performance hybrid supercapacitors and alkaline Zn-ion batteries

A Setaria viridis-like hierarchical A-Co(OH)₂@NiCo-LDH nanoarray has been fabricated and it possesses the advantages of large surface area and stable microstructure, leading to excellent energy storage performances of HSC and AZIB.

Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis

Metal phosphides (MPs) with unique and desirable physicochemical properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, especially for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Additionally, different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.