Porous N-Doped Carbon-Encapsulated CoNi Alloy Nanoparticles Derived from MOFs as Efficient Bifunctional Oxygen Electrocatalysts

Highly efficient selective hydrogenation of adiponitrile to hexamethylene diamine over barium and melamine formaldehyde resin-modified nickel-cobalt-based zeolitic imidazolate framework-derived catalyst

In the present study, the catalyst modified with alkaline oxide can enhance the selectivity to primary amines. However, the addition of alkaline oxide inevitably reduces catalytic activity. In this study, NiCo-NC@BaO-MFC catalyst derived from zeolitic imidazolate framework-67, Ba(CH3COO)2, and melamine formaldehyde (MF) resin was prepared and used for the hydrogenation of adiponitrile (ADN) to hexamethylene diamine (HDMA). The carbon layer obtained from the MF resin effectively prevents the interaction between barium (Ba) and the active center, thus improving target product selectivity without decreasing catalytic activity. The results of the density functional theory (DFT) calculation and characterization indicated that the effect of synergy between nickel (Ni) and cobalt (Co) bimetals induces an electron density growth on the Ni surface, bringing the d-band center toward the Fermi surface. Meanwhile, the high electron density of the active center compensates for the electron-deficient state of the carbon atom in -CN, thus improving the catalytic activity. Furthermore, it was found that the introduction of Ba promotes the formation of nucleophilic hydrogen anions, which facilitates the hydrogenation of 6-aminohexylimine (AHIM) to HDMA and inhibits the intramolecular condensation of AHIM, hence improving the selectivity to HDMA. The NiCo-NC@BaO-MFC catalyst gives 98.6 % ADN conversion and 97.2 % selectivity to HDMA in an alkali-free system.

Recent Advances in Pristine Iron Triad Metal-Organic Framework Cathodes for Alkali Metal-Ion Batteries

Pristine iron triad metal-organic frameworks (MOFs), i.e., Fe-MOFs, Co-MOFs, Ni-MOFs, and heterometallic iron triad MOFs, are utilized as versatile and promising cathodes for alkali metal-ion batteries, owing to their distinctive structure characteristics, including modifiable and designable composition, multi-electron redox-active sites, exceptional porosity, and stable construction facilitating rapid ion diffusion. Notably, pristine iron triad MOFs cathodes have recently achieved significant milestones in electrochemical energy storage due to their exceptional electrochemical properties. Here, the recent advances in pristine iron triad MOFs cathodes for alkali metal-ion batteries are summarized. The redox reaction mechanisms and essential strategies to boost the electrochemical behaviors in associated electrochemical energy storage devices are also explored. Furthermore, insights into the future prospects related to pristine iron triad MOFs cathodes for lithium-ion, sodium-ion, and potassium-ion batteries are also delivered.

F-regulated Ni2P-F3 nanosheets as efficient electrocatalysts for full-water-splitting and urea oxidation

Heteroatomic anion doping represents a powerful approach for manipulating the electronic configuration of the active metal locus in electrocatalysts, resulting in enhanced multifunctional electrocatalytic properties in hydrogen/oxygen evolution reactions (HER/OER). Here, fluorine-tailored Ni2P-F3 nanosheets were synthesized and evaluated as a robust multifunctional electrocatalyst for HER, OER, and UOR. Our comprehensive experimental and theoretical investigations reveal that the anionic F effectively tailored the electronic states of the Ni2P-F3 nanosheets, resulting in an elevated d-band center and optimizing the sorption capacity of intermediates. In addition to thermodynamically and kinetically favoured redox reactions, F doping facilitates the reconstruction and generation of active γ-NiOOH. Resulting from the optimized electronic configuration and nanosheet architecture, outstanding catalytic activities are demonstrated by Ni2P-F3 with low overpotentials to reach 100 mA cm-2 for HER (177 mV) and OER (293 mV), surpassing Ni2P by 234 and 205 mV, respectively. Notably, 1.618 V is required for full-water-diversion to reach 10 mA cm-2, while 1.414 V is required with urea oxidation for 100 mA cm-2.

Iron Triad-Based Bimetallic M-N-C Nanomaterials as Highly Active Bifunctional Oxygen Electrocatalysts

The use of precious metal electrocatalysts in clean electrochemical energy conversion and storage applications is widespread, but the sustainability of these materials, in terms of their availability and cost, is constrained. In this research, iron triad-based bimetallic nitrogen-doped carbon (M-N-C) materials were investigated as potential bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The synthesis of bimetallic FeCo-N-C, CoNi-N-C, and FeNi-N-C catalysts involved a precisely optimized carbonization process of their respective metal-organic precursors. Comprehensive structural analysis was undertaken to elucidate the morphology of the prepared M-N-C materials, while their electrocatalytic performance was assessed through cyclic voltammetry and rotating disk electrode measurements in a 0.1 M KOH solution. All bimetallic catalyst materials demonstrated impressive bifunctional electrocatalytic performance in both the ORR and the OER. However, the FeNi-N-C catalyst proved notably more stable, particularly in the OER conditions. Employed as a bifunctional catalyst for ORR/OER within a customized zinc-air battery, FeNi-N-C exhibited a remarkable discharge-charge voltage gap of only 0.86 V, alongside a peak power density of 60 mW cm-2. The outstanding stability of FeNi-N-C, operational for about 55 h at 2 mA cm-2, highlights its robustness for prolonged application.

Transforming electrochemical hydrogen Production: Tannic Acid-Boosted CoNi alloy integration with Multi-Walled carbon nanotubes for advanced bifunctional catalysis

The dimensions of alloy nanoparticles or nanosheets have emerged as a critical determinant for their prowess as outstanding electrocatalysts in water decomposition. Remarkably, the reduction in nanoparticle size results in an expanded active specific surface area, elevating reaction kinetics and showcasing groundbreaking potential. In a significant leap towards innovation, we introduced tannic acid (TA) to modify multi-walled carbon nanotubes (MWCNTs) and CoNi alloys. This ingenious strategy not only finely tuned the size of CoNi alloys but also securely anchored them to the MWCNTs substrate. The resulting synergistic "carbon transportation network" accelerated electron transfer during the reaction, markedly enhancing efficiency. Furthermore, the exceptional synergy of Co and Ni elements establishes Co0.84Ni1.69/MWCNTs as highly efficient electrocatalysts. Experimental findings unequivocally demonstrate that TA-Co0.84Ni1.69/MWCNTs require minimal overpotentials of 171 and 294 mV to achieve a current density of ± 10 mA cm-2. Serving as both anode and cathode for overall water splitting, TA-Co0.84Ni1.69/MWCNTs demand a low voltage of 1.66 V at 10 mA cm-2, maintaining structural integrity throughout extensive cyclic stability testing. These results propel TA-Co0.84Ni1.69/MWCNTs as promising candidates for future electrocatalytic advancements.

Scalable One-Pot Fabrication of Carbon-Nanofiber-Supported Noble-Metal-Free Nanocrystals for Synergetic-Dependent Green Hydrogen Production: Unraveling Electrolyte and Support Effects

Electrocatalytic hydrogen evolution reactions (HER) are envisaged as the most promising sustainable approach for green hydrogen production. However, the considerably high cost often associated with such reactions, particularly upon scale-up, poses a daunting challenge. Herein, a facile, effective, and environmentally benign one-pot scalable approach is developed to fabricate MnM (M═Co, Cu, Ni, and Fe) nanocrystals supported over in situ formed carbon nanofibers (MnM/C) as efficient noble-metal-free electrocatalysts for HER. The formation of carbon nanofibers entails impregnating cellulose in an aqueous solution of metal precursors, followed by annealing the mixture at 550 °C. During the impregnation process, cellulose acts as a reactor for inducing the in situ reductions of MnM salts with the assistance of ether and hydroxyl groups to drive the mass production (several grams) of ultralong (5 ± 1 μM) carbon nanofibers ornamented with MnM nanoparticles (10-14 nm in size) at an average loading of 2.87 wt %. For better electrocatalytic HER benchmarking, the fabricated catalysts were tested over different working electrodes, i.e., carbon paper, carbon foam, and glassy carbon, in the presence of different electrolytes. All the fabricated MnM/C catalysts have demonstrated an appealing synergetic-effect-dependent HER activity, with MnCo/C exhibiting the best performance over carbon foam, close to that of the state-of-the-art commercial Pt/C (10 wt % Pt), with an overpotential of 11 mV at 10 mA cm-2, a hydrogen production rate of 2448 mol g-1 h-1, and a prolonged stability of 2 weeks. The HER performance attained by MnCo/C nanofibers is among the highest reported for Pt-free electrocatalysts, thanks to the mutual alloying effect, higher synergism, large surface area, and active interfacial interactions over the nanofibers. The presented findings underline the potential of our approach for the large-scale production of cost-effective electrocatalysts for practical HER.

Nanoflower-Like High-Entropy Co-Fe-Cr-Mo-Mn Spinel for Oxygen Evolution

Oxygen evolution reaction (OER) is the key anode reaction of electrolytic water. To improve the slow OER kinetics, we synthesize nanoflower-like Co-Fe-Cr-Mo-Mn high-entropy spinel (HES) nanosheets on nickel foam (NF) by one-step solvothermal method, which exhibit an overpotential (η10) of only 188 mV at 10 mA cm-2, much lower than bimetallic CoFeOx/NF (233 mV), trimetallic CoFeCrOx/NF (211 mV), and tetrametallic CoFeCrMoOx/NF (200 mV). The OER overpotential decreases with the increase of the number of metals, indicating that the formation of HES has a positive effect on the improvement of electrocatalytic performance, since the synergistic effect between different metals enhances the charge transfer rate and decreases reaction barrier. In-situ Raman spectra demonstrate that the formation of γ-NiOOH on the HES surface is a crucial active species for the OER. This work demonstrates a simple and efficient synthesis method to prepare nanoflower-like high-entropy electrocatalysts for efficient OER electrocatalysis.

Metal Alloys-Structured Electrocatalysts: Metal-Metal Interactions, Coordination Microenvironments, and Structural Property-Reactivity Relationships

Metal alloys-structured electrocatalysts (MAECs) have made essential contributions to accelerating the practical applications of electrocatalytic devices in renewable energy systems. However, due to the complex atomic structures, varied electronic states, and abundant supports, precisely decoding the metal-metal interactions and structure-activity relationships of MAECs still confronts great challenges, which is critical to direct the future engineering and optimization of MAECs. Here, this timely review comprehensively summarizes the latest advances in creating the MAECs, including the metal-metal interactions, coordination microenvironments, and structure-activity relationships. First, the fundamental classification, design, characterization, and structural reconstruction of MAECs are outlined. Then, the electrocatalytic merits and modulation strategies of recent breakthroughs for noble and non-noble metal-structured MAECs are thoroughly discussed, such as solid solution alloys, intermetallic alloys, and single-atom alloys. Particularly, unique insights into the bond interactions, theoretical understanding, and operando techniques for mechanism disclosure are given. Thereafter, the current states of diverse MAECs with a unique focus on structural property-reactivity relationships, reaction pathways, and performance comparisons are discussed. Finally, the future challenges and perspectives for MAECs are systematically discussed. It is believed that this comprehensive review can offer a substantial impact on stimulating the widespread utilization of metal alloys-structured materials in electrocatalysis.

A multi-layer core-shell structure CoFe2O4@Fe3C@NiO composite with high broadband electromagnetic wave-absorption performance

Enhancing the absorption strength of electromagnetic waves and broadening the absorption band are constant goals in designing and preparing absorbing materials. The use of composites has shown to be a very efficient method for acquiring broadband-absorbing materials, while the construction of a core-shell structure has demonstrated a significant enhancement in absorption capability. In this paper, the nanocomposite metal-organic framework (MOF) derivative CoFe2O3@C with a double core-shell structure and the nanocomposite MOF derivative CoFe2O4@Fe3C@NiO with a three-layered core-shell structure have been prepared using a chemical compound. The multi-layer structure provides more active sites for the multiple reflection and scattering of electromagnetic waves, effectively improving the attenuation capability. The effective absorption band (EAB) (reflection loss (RL) ≤ -5 dB) of both CoFe2O3@C and CoFe2O4@Fe3C@NiO are broadened compared to that of the ZIF-67 derivative. In particular, the minimum reflection loss (RLmin) of CoFe2O3@C was -52.7 dB at 13.3 GHz and 2.04 mm, and the EAB (RL ≤ -5 dB) is as wide as 9.35 GHz. Compared with the ZIF-67 derivative, the EAB exhibits a twofold rise, accompanied by a corresponding thickness increase of just 0.24 mm. At a matched thickness of 2.2 mm, the EAB of CoFe2O4@Fe3C@NiO can even reach 11.9 GHz.

Nitrogen-doped porous carbon encapsulates multivalent cobalt-nickel as oxygen reduction reaction catalyst for zinc-air battery

In this study, we present a bimetallic ion coexistence encapsulation strategy employing hexadecyl trimethyl ammonium bromide (CTAB) as a mediator to anchor cobalt-nickel (CoNi) bimetals in nitrogen-doped porous carbon cubic nanoboxes (CoNi@NC). The fully encapsulated and uniformly dispersed CoNi nanoparticles with the improved density of active sites help to accelerate the oxygen reduction reaction (ORR) kinetics and provide an efficient charge/mass transport environment. Zinc-air battery (ZAB) equipped CoNi@NC as cathode exhibits an open-circuit voltage of 1.45 V, a specific capacity of 870.0 mAh g^-1, and a power density of 168.8 mW cm^-2. Moreover, the two CoNi@NC-based ZABs in series display a stable discharge specific capacity of 783.0 mAh g^-1, as well as a large peak power density of 387.9 mW cm^-2. This work provides an effective way to tune the dispersion of nanoparticles to boost active sites in nitrogen-doped carbon structure, and enhance the ORR activity of bimetallic catalysts.

Hierarchical N-doped carbon nanofiber-loaded NiCo alloy nanocrystals with enhanced methanol electrooxidation for alkaline direct methanol fuel cells

The high catalytic activity of non-precious metals in alkaline media opens a new direction for the development of alkaline direct methanol fuel cell (ADMFC) electrocatalysts. Herein, a highly dispersed N-doped carbon nanofibers (CNFs) -loaded NiCo non-precious metal alloy electrocatalyst based on metal-organic frameworks (MOFs) was prepared, which conferred excellent methanol oxidation activity and resistance to carbon monoxide (CO) poisoning through a surface electronic structure modulation strategy. The porous electrospun polyacrylonitrile (PAN) nanofibers and the P-electron conjugated structure of polyaniline chains provide fast charge transfer channels, enabling electrocatalysts with abundant active sites and efficient electron transfer. The optimized NiCo/N-CNFs@800 was tested as an anode catalyst for ADMFC single cell and exhibited a power density of 29.15 mW cm^-2. Due to the fast charge transfer and mass transfer brought by its one-dimensional porous structure and the synergistic effect between NiCo alloy, NiCo/N-CNFs@800 is expected to be an economical, efficient and CO-resistant methanol oxidation reaction (MOR) electrocatalyst.

Catalytic advancements in carbonaceous materials for bio-energy generation in microbial fuel cells: a review

Microbial fuel cells (MFCs) are a sustainable alternative for wastewater treatment and clean energy generation. The efficiency of the technology is dependent on the cathodic oxygen reduction reaction, where the sluggish reaction kinetics hampers its propensity. Carbonaceous materials with high electrical conductivity have been widely explored for oxygen reduction reaction (ORR) catalysts. Here, incorporating transition metal (TM) and heteroatom into carbon could further enhance the ORR activity and power generation in MFCs. Nitrogen (N)-doped carbons have also been a popular research hotspot due to abundant active sites formed, resulting in superior conductivity, stability, and catalytic activity over carbons. This review summarizes the progress in the carbon-based materials (primary focus on the cathode) for ORR and their utilization in MFCs. Furthermore, we discussed the conceptualization of MFCs and carbonaceous materials to instigate the ORR kinetics and power generation in MFC. Furthermore, prospects of carbon-based materials for actual application in bio-energy generation have been discussed. Carbonaceous catalysts and biomass-derived carbons exhibit good potential to replace precious Pt catalysts for ORR. M-N-C catalysts were found to be the most suitable catalysts. Electrocatalysts with MN_x sites are able to achieve excellent activity and high-power output by taking advantage of the active site exposure and rapid mass transfer rate. Moreover, the use of biomass-derived carbons/self-doped carbons could further reduce the overall cost of catalysts. It is anticipated that the research gaps and future perspectives discussed will show new avenues to develop excellent electrocatalysts for better performance and transformation of technology to industrial applications.

Electronic Tuning of Core-Shell CoNi Nanoalloy/N-Doped Few-Layer Graphene for Efficient Oxygen Electrocatalysis in Rechargeable Zinc-Air Batteries

The discovery of highly efficient, durable, and affordable bifunctional ORR/OER electrocatalysts is of great significance for the commercialization of rechargeable metal-air batteries. Herein, we synthesized uniformly sized CoNi alloy nanoparticles encapsulated with N-doped few-layer graphene (N-FLG) sheets via pyrolysis of a CoNi dual metal-organic framework precursor. The developed CoNi/N-FLG catalyst exhibited excellent oxygen reduction activity (comparable to a commercial 20 wt % Pt/C catalyst) and outstanding oxygen evolution activity (superior to a commercial 20 wt % IrO2/C catalyst), thus enabling efficient bifunctional oxygen electrocatalysis and stability when applied in prototype rechargeable zinc-air batteries. The remarkable electrochemical properties of CoNi/N-FLG originate from its unique core-shell structure and favorable electron penetration effects, thereby optimizing the adsorption/desorption strengths of intermediates formed during the oxygen reduction and oxygen evolution reactions.

Random Occupation of Multimetal Sites in Transition Metal-Organic Frameworks for Boosting the Oxygen Evolution Reaction

Developing robust oxygen evolution reaction (OER) electrocatalysts with excellent performance is essential for the conversion of renewable electricity to clean fuel. Herein, we present a facile concept for the synthesis of efficient high-entropy metal-organic frameworks (HEMOFs) as electrocatalysts in a one-step solvothermal synthesis. This strategy allows control of the microstructure and corresponding lattice distortion by tuning the metal ion composition. As a result, the OER activity was improved by optimizing the coordination environment of the metal catalytic center. The optimized Co-rich HEMOFs exhibited a low overpotential of 310 mV at a current density of 10 mA cm^-2 , better than a RuO₂ catalyst tested under the same conditions. The finding of lattice distortion of the HEMOFs provides a new strategy for developing high-performance electrocatalysts for energy conversion.

Size-controlled, hollow and hierarchically porous Co2Ni2 alloy nanocubes for efficient oxygen reduction in microbial fuel cells

Schematic illustration of the fabrication of CoxNiy alloy nanocubes (ANCs) and hollow CoxNiy alloy nanocubes (HANCs).

Emerging carbon shell-encapsulated metal nanocatalysts for fuel cells and water electrolysis

The development of low-cost, high-efficiency electrocatalysts is of primary importance for hydrogen energy technology. Noble metal-based catalysts have been extensively studied for decades; however, activity and durability issues still remain a challenge. In recent years, carbon shell-encapsulated metal (M@C) catalysts have drawn great attention as novel materials for water electrolysis and fuel cell applications. These electrochemical reactions are governed mainly by interfacial charge transfer between the core metal and the outer carbon shell, which alters the electronic structure of the catalyst surface. Furthermore, the rationally designed and fine-tuned carbon shell plays a very interesting role as a protective layer or molecular sieve layer to improve the performance and durability of energy conversion systems. Herein, we review recent advances in the use of M@C type nanocatalysts for extensive applications in fuel cells and water electrolysis with a focus on the structural design and electronic structure modulation of carbon shell-encapsulated metal/alloys. Finally, we highlight the current challenges and future perspectives of these catalytic materials and related technologies in this field.

Advanced Oxygen Electrocatalyst for Air-Breathing Electrode in Zn-Air Batteries

The electrochemical reduction of oxygen to water and the evolution of oxygen from water are two important electrode reactions extensively studied for the development of electrochemical energy conversion and storage technologies based on oxygen electrocatalysis. The development of an inexpensive, highly active, and durable nonprecious-metal-based oxygen electrocatalyst is indispensable for emerging energy technologies, including anion exchange membrane fuel cells, metal-air batteries (MABs), water electrolyzers, etc. The activity of an oxygen electrocatalyst largely decides the overall energy storage performance of these devices. Although the catalytic activities of Pt and Ru/Ir-based catalysts toward an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER) are known, the high cost and lack of durability limit their extensive use for practical applications. This review article highlights the oxygen electrocatalytic activity of the emerging non-Pt and non-Ru/Ir oxygen electrocatalysts including transition-metal-based random alloys, intermetallics, metal-coordinated nitrogen-doped carbon (M-N-C), and transition metal phosphides, nitrides, etc., for the development of an air-breathing electrode for aqueous primary and secondary zinc-air batteries (ZABs). Rational surface and chemical engineering of these electrocatalysts is required to achieve the desired oxygen electrocatalytic activity. The surface engineering increases the number of active sites, whereas the chemical engineering enhances the intrinsic activity of the catalyst. The encapsulation or integration of the active catalyst with undoped or heteroatom-doped carbon nanostructures affords an enhanced durability to the active catalyst. In many cases, the synergistic effect between the heteroatom-doped carbon matrix and the active catalyst plays an important role in controlling the catalytic activity. The ORR activity of these catalysts is evaluated in terms of onset potential, number of electrons transferred, limiting current density, and durability. The bifunctional oxygen electrocatalytic activity and ZAB performance, on the other hand, are measured in terms of potential gap between the ORR and OER, ΔE = E_j10^OER - E_1/2^ORR, specific capacity, peak power density, open circuit voltage, voltaic efficiency, and charge-discharge cycling stability. The nonprecious metal electrocatalyst-based ZABs are very promising and they deliver high power density, specific capacity, and round-trip efficiency. The active site for oxygen electrocatalysis and challenges associated with carbon support is briefly addressed. Despite the considerable progress made with the emerging electrocatalysts in recent years, several issues are yet to be addressed to achieve the commercial potential of rechargeable ZAB for practical applications.

Metal-Organic Framework-Derived Trimetallic Nanocomposites as Efficient Bifunctional Oxygen Catalysts for Zinc-Air Batteries

Transition-metal-based multifunctional catalysts have attracted increasing attention owing to high possibilities of substituting the expensive noble-metal-based catalysts in various scenarios. Multivariate metal-organic frameworks (MTV-MOFs) are ideal precursors to prepare multimetallic nanocomposites with high catalytic activity since the uniform distribution and precise regulation of mixed metal centers, as well as the consequent strong synergistic effect, could be readily achieved. Herein, a Mn/Co/Ni trimetallic catalyst (MnCoNi-C-D) with a hollow rhombic dodecahedron shape was synthesized via pyrolysis of the corresponding trimetallic-based MTV-MOF. The catalyst shows outstanding electrochemical activity toward the oxygen reduction reaction including a half-wave potential of 0.82 V and superior tolerance against methanol as well as high stability in an alkaline medium, and its oxygen evolution reaction activity also surpasses a RuO₂ catalyst. Moreover, primary and rechargeable zinc-air batteries based on MnCoNi-C-D delivered preferable performances compared with commercial Pt/C-RuO₂, including higher peak power density (116.4 mW cm^-2), higher specific capacity (841.3 mAh g^-1), higher open-circuit potential (OCV) (1.46 V), and better stability for more than 180 h. A comprehensive comparison was also conducted to prove the necessity of employing the MTV-MOF as the precursor and investigate the intrinsic superiority of the catalyst.

Hierarchical Hollow MOF-Derived Bamboo-like N-doped Carbon Nanotube-Encapsulated Co0.25Ni0.75 Alloy: An Efficient Bifunctional Oxygen Electrocatalyst for Zinc-Air Battery

The synthesis of nonprecious electrocatalysts for oxygen electrocatalysis is of considerable interest for the development of electrochemical energy devices. Herein, we demonstrate a facile approach for the synthesis of bamboo-like nitrogen-doped carbon nanotube-encapsulated Co_0.25Ni_0.75 alloy electrocatalyst (Co_0.25Ni_0.75@NCNT) and its bifunctional oxygen electrocatalytic performance toward oxygen reduction and oxygen evolution reactions. The Co_0.25Ni_0.75 alloy wrapped with NCNT is obtained by a one-step carbothermal reduction approach using dicyandiamide and NiCo-MOF precursors. Dicyandiamide acts as a nitrogen source, and the in situ generated Co_0.25Ni_0.75 alloy nanoparticles catalyze the growth of bamboo-like NCNTs. The hollow NiCo-MOF plays a sacrificial role in providing a suitable environment for the controlled growth of Co_0.25Ni_0.75 alloy and NCNT. Co_0.25Ni_0.75@NCNT efficiently catalyzes both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) at a favorable overpotential. It shows a low potential gap (ΔE) of ∼0.8 V between the two reactions, and it qualifies for the development of air cathode in metal-air batteries. The enhanced bifunctional activity and excellent durability stem from the chemical composition and the synergistic effect between Co_0.25Ni_0.75 alloy and encapsulating NCNT. The original phase and morphology of the catalyst is preserved after an extensive durability test. Aqueous rechargeable Zn-air battery (ZAB) is fabricated using a Co_0.25Ni_0.75@NCNT-based air cathode. The battery has high open-circuit voltage (1.53 V) and a maximum peak power density of 167 mW cm^-2 with only 1.6% loss in the voltaic efficiency after 36 h charge-discharge cycles. As a proof-of-concept demonstration, the as-fabricated ZAB is successfully used for the electrochemical water splitting in alkaline solution.

Engineering Two-Phase Bifunctional Oxygen Electrocatalysts with Tunable and Synergetic Components for Flexible Zn-Air Batteries

Metal-air batteries, like Zn-air batteries (ZABs) are usually suffered from low energy conversion efficiency and poor cyclability caused by the sluggish OER and ORR at the air cathode. Herein, a novel bimetallic Co/CoFe nanomaterial supported on nanoflower-like N-doped graphitic carbon (NC) was prepared through a strategy of coordination construction-cation exchange-pyrolysis and used as a highly efficient bifunctional oxygen electrocatalyst. Experimental characterizations and density functional theory calculations reveal the formation of Co/CoFe heterostructure and synergistic effect between metal layer and NC support, leading to improved electric conductivity, accelerated reaction kinetics, and optimized adsorption energy for intermediates of ORR and OER. The Co/CoFe@NC exhibits high bifunctional activities with a remarkably small potential gap of 0.70 V between the half-wave potential (E_1/2) of ORR and the potential at 10 mA cm^‒2 (E_j=10) of OER. The aqueous ZAB constructed using this air electrode exhibits a slight voltage loss of only 60 mV after 550-cycle test (360 h, 15 days). A sodium polyacrylate (PANa)-based hydrogel electrolyte was synthesized with strong water-retention capability and high ionic conductivity. The quasi-solid-state ZAB by integrating the Co/CoFe@NC air electrode and PANa hydrogel electrolyte demonstrates excellent mechanical stability and cyclability under different bending states.

Designing MOF Nanoarchitectures for Electrochemical Water Splitting

Electrochemical water splitting has attracted significant attention as a key pathway for the development of renewable energy systems. Fabricating efficient electrocatalysts for these processes is intensely desired to reduce their overpotentials and facilitate practical applications. Recently, metal-organic framework (MOF) nanoarchitectures featuring ultrahigh surface areas, tunable nanostructures, and excellent porosities have emerged as promising materials for the development of highly active catalysts for electrochemical water splitting. Herein, the most pivotal advances in recent research on engineering MOF nanoarchitectures for efficient electrochemical water splitting are presented. First, the design of catalytic centers for MOF-based/derived electrocatalysts is summarized and compared from the aspects of chemical composition optimization and structural functionalization at the atomic and molecular levels. Subsequently, the fast-growing breakthroughs in catalytic activities, identification of highly active sites, and fundamental mechanisms are thoroughly discussed. Finally, a comprehensive commentary on the current primary challenges and future perspectives in water splitting and its commercialization for hydrogen production is provided. Hereby, new insights into the synthetic principles and electrocatalysis for designing MOF nanoarchitectures for the practical utilization of water splitting are offered, thus further promoting their future prosperity for a wide range of applications.

Ultrasensitive simultaneous voltammetric determination of 4-aminophenol and acetaminophen based on bimetallic MOF-derived nitrogen-doped carbon coated CoNi alloy

Simultaneous electrochemical determination of 4-aminophenol (4-AP) and acetaminophen (ACOP) is crucial due to their high toxicity when are overused. Herein, a novel electrocatalyst of nitrogen-doped carbon coated CoNi alloy (CoNi@CN) is derived from bimetallic CoNi(BDC)₂(DABCO) for the first time. A series of characterizations demonstrate that composite has been successfully synthesized, and all elements are evenly distributed in the catalyst. The optimal sensor based on Co₁Ni₁@CN-700 exhibits two wide linear responses for 4-AP (0.05-60 μM and 60-250 μM) and ACOP (0.05-40 μM and 40-150 μM) with the lowest detection limit of 5.2 nM and 3.8 nM compared with current known reports. Moreover, the sensor has superior reproducibility, selectivity and stability. In addition, the wonderful recoveries also are obtained when sensor is used to detect 4-AP and ACOP in real samples, illustrating that electrochemical sensor has great prospect in the clinical application.

Advanced non-noble materials in bifunctional catalysts for ORR and OER toward aqueous metal-air batteries

The catalyst in the oxygen electrode is the core component of the aqueous metal-air battery, which plays a vital role in the determination of the open circuit potential, energy density, and cycle life of the battery. For rechargeable aqueous metal-air batteries, the catalyst should have both good oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic performance. Compared with precious metal catalysts, non-precious metal materials have more advantages in terms of abundant resource reserves and low prices. Over the past few years, great efforts have been made in the development of non-precious metal bifunctional catalysts. This review selectively evaluates the advantages, disadvantages and development status of recent advanced materials including pure carbon materials, carbon-based metal materials and carbon-free materials as bifunctional oxygen catalysts. Preliminary improvement strategies are formulated to make up for the deficiency of each material. The development prospects and challenges facing bifunctional catalysts in the future are also discussed.

Inner Sphere Electron Transfer Promotion on Homogeneously Dispersed Fe-Nx Centers for Energy-Efficient Oxygen Reduction Reaction

The study reports the optimized incorporation of pyridinic nitrogen in nitrogen-doped carbon nanotubes (CNTs) to realize effective Fe-N_x centers throughout the framework. The study unveils nitrogen as a valuable asset to promote the homogeneous dispersion of Fe moieties throughout the CNT framework, which is a necessary component to institute uniform Fe-N_x centers. In addition, pyridinic nitrogen causes disruption in strongly delocalized π-electrons, which impart electron-withdrawing nature in the carbon matrix, resulting in an anodic shift in oxygen reduction reaction (ORR) onset potential (E_onset). The direct interaction of Fe-N_x with O₂, as evidenced by poisoning and computational studies, ensures the preferential inner sphere electron transfer mechanism. Despite the alkaline medium, the outer sphere electron transfer mechanism was muted, with suppressed HO₂^- generation, preferential 4e^- reduction pathways, and excellent cyclic stability. The study indicates the dependency of ORR half-wave potential on the electron transfer mechanism. The poisoning study unveils the direct involvement of Fe-N_x electroactive centers in facilitating ORR in alkaline medium. It further indicates a noticable increase (up to ∼25%) in peroxide generation-an unwanted ORR intermediate-and concomitant reduction in average electron transfer no. per oxygen molecule.

Bifunctional Catalysts for Reversible Oxygen Evolution Reaction and Oxygen Reduction Reaction

Metal-air batteries (MABs) and reversible fuel cells (RFCs) rely on the bifunctional oxygen catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Finding efficient bifunctional oxygen catalysts is the ultimate goal and it has attracted a great deal of attention. The dilemma is that a good ORR catalyst is not necessarily efficient for OER, and vice versa. Thus, the development of a new type of bifunctional oxygen catalysts should ensure that the catalysts exhibit high activity for both OER and ORR. Composites with multicomponents for active centers supported on highly conductive matrices could be able to meet the challenges and offering new opportunities. In this Review, the evolution of bifunctional catalysts is summarized and discussed aiming to deliver high-performance bifunctional catalysts with low overpotentials.

MOF-derived PdNiCo alloys encapsulated in nitrogen-doped graphene for robust hydrogen evolution reactions

The synergistic effect of PdNiCo nanoalloys and nitrogen-doped graphene for robust hydrogen evolution reactions.

All-around coating of CoNi nanoalloy using a hierarchically porous carbon derived from bimetallic MOFs for highly efficient hydrolytic dehydrogenation of ammonia-borane

In situ synthesis of core–shell carbon enclosed CoNi alloys achieves efficient heterogeneous catalysis.

Trimetallic FeCoNi@C Nanocomposite Hollow Spheres Derived from Metal-Organic Frameworks with Superior Electromagnetic Wave Absorption Ability

Organic ligands and metal ions in the metal-organic frameworks (MOFs, a type of porous magnetic metal/carbon nanocomposites obtained through high-temperature carbonization) have caused widespread concerns in the field of microwave absorption because of the existence of various microwave loss mechanisms in these materials. However, MOF-driven microwave absorbing materials with high absorption intensity and wide absorption band still require further research and development. In this work, hollow sphere trimetallic FeCoNi@C microwave absorbing materials via high-temperature carbonization were obtained using FeCoNi-based MOF-74 (FeCoNi-MOF) as the precursor. The effects of different carbonization conditions on the microwave absorption properties of the materials were studied. FeCoNi-MOF-74 annealed at 700 °C showed superior microwave absorption capacity, where the RL value reached -64.75 dB at 15.44 GHz corresponding to the actual application thickness of the absorber (only 2.1 mm), and the minimum RL values reached -69.03 dB at 5.52 GHz. Furthermore, the as-prepared sample can fully cover the Ku band and X band at only 2.1 and 3.1 mm, respectively. The maximum EAB reached 8.08 GHz (9.92-18 GHz) when the thickness of the absorber was 2.47 mm. Such remarkable absorption performance is attributed to the synergetic effects between the multiple loss mechanisms of the FeCoNi@C, and the improved impedance matching characteristic came from the hollow sphere morphology.

Enhanced Electrocatalytic Performance through Body Enrichment of Co-Based Bimetallic Nanoparticles In Situ Embedded Porous N-Doped Carbon Spheres

Extending available body space loading active species and controllably tailoring the d-band center to Fermi level of catalysts are of paramount importance but extremely challenging for the enhancement of electrocatalytic performance. Herein, a melamine-bridged self-construction strategy is proposed to in situ embed Co-based bimetallic nanoparticles in the body of N-doped porous carbon spheres (CoM-e-PNC), and achieve the controllable tailoring of the d-band center position by alloying of Co and another transition metal M (M = Ni, Fe, Mn, and Cu). The enrichment and exposure of the active sites in the body interior of porous carbon spheres, and the best balance between the adsorption of OH species and the desorption of O₂ induced by optimizing the d-band center position, collectively enhance the oxygen evolution reaction (OER) performance. Meanwhile, the relationship of d-band center position and OER activity is found to exhibit the volcano curve rule, where the CoNi-e-PNC catalyst shows optimal OER performance with an overpotential of 0.24 V at 10 mA cm^-2 in alkaline media, outperforming those of the ever-reported CoNi-based catalysts. Besides, CoNi-e-PNC catalyst also demonstrates high OER stability with slight current decrease after 100 h.