Boosting Electrocatalytic Activity of 3d‐Block Metal (Hydro)oxides by Ligand‐Induced Conversion

Nickel molybdate/cobalt iron carbonate hydroxide heterojunction with oxygen vacancy enables interfacial synergism to trigger oxygen evolution reaction

The development of electrocatalysts with excellent performance toward oxygen evolution reaction (OER) for the production of hydrogen is of great significance to alleviate energy crisis and environmental pollution. Herein, the heterostructure (NMO/FCHC-0.4) was fabricated by the coupling growth of NiMoO4 (NMO) and cobalt iron carbonate hydroxide (FCHC) on nickel foam as an electrocatalyst for OER. The interfacial synergy on NMO/FCHC-0.4 heterojunction can promote the interfacial electron redistribution, affect the center position of d band, optimize the adsorption of intermediate, and improve the conductivity. Beyond, oxygen defect sites are conducive to the adsorption of intermediates, and increase the number of active sites. Real-time OER kinetic simulation revealed that the interfacial synergism and molybdate could reduce the adsorption of hydroxide, promote the deprotonation step of M-OH, and facilitate the formation of M-OOH (M represents the metal active site). As a result, NMO/FCHC-0.4 displays excellent OER electrocatalytic performance with an overpotential of 250/280 mV at the current density 100/200 mA cm-2 and robust stability at 100 mA cm-2 for 100 h. This work provides deep insights into the roles of interfacial electronic modulation and oxygen vacancy to design high-efficiency electrocatalysts for OER.

Controlling the electronic structure of Fe-MOF electrocatalyst for enhanced water splitting and urea oxidation: A plasma-assisted approach

The design of high-performance electrocatalysts for water splitting and urea oxidation reactions requires effective regulation of their electronic structure and electrochemical surface area (ECSA). In this study, we developed an in-situ grown Fe-MOF electrocatalyst on Fe foam (FF) by using a combination of easy hydrothermal synthesis and advanced plasma technology (Fe-MOF/FF). By varying the plasma treatment time, we could tailor the surface morphology and electronic structure of the Fe-MOF/FF microrods. Meanwhile, density functional theory (DFT) calculations investigated the catalytic mechanism, revealing that plasma-treated Fe-MOF/FF has a lower energy barrier for water splitting and H* adsorption during the HER process, and higher catalytic activity for UOR. Additionally, the electronic density of optimized Fe-MOF/FF is significantly expanded near the Fermi level. Remarkably, our catalysts achieved exceptional activity in both water splitting and urea electrolysis, requiring only 1.54 V and 1.472 V, respectively, at 10 mA cm-2, with excellent stability. Our findings highlight the potential of plasma technology as a powerful tool for developing multifunctional electrocatalysts for clean energy and industrial wastewater treatment applications.

Progress in Manipulating Dynamic Surface Reconstruction via Anion Modulation for Electrocatalytic Water Oxidation

The development of efficient and economical electrocatalysts for oxygen evolution reaction (OER) is of paramount importance for the sustainable production of renewable fuels and energy storage systems; however, the sluggish OER kinetics involving multistep four proton-coupled electron transfer hampers progress in these systems. Fortunately, surface reconstruction offers promising potential to improve OER catalyst design. Anion modulation plays a crucial role in controlling the extent of surface reconstruction and positively persuading the reconstructed species' performances. This review starts by providing a general explanation of how various types of anions can trigger dynamic surface reconstruction and create different combinations with pre-catalysts. Next, the influences of anion modulation on manipulating the surface dynamic reconstruction process are discussed based on the in situ advanced characterization techniques. Furthermore, various effects of survived anionic groups in reconstructed species on water oxidation activity are further discussed. Finally, the challenges and prospects for the future development directions of anion modulation for redirecting dynamic surface reconstruction to construct highly efficient and practical catalysts for water oxidation are proposed.

Sustainable zinc-air battery chemistry: advances, challenges and prospects

Sustainable zinc-air batteries (ZABs) are considered promising energy storage devices owing to their inherent safety, high energy density, wide operating temperature window, environmental friendliness, etc., showing great prospect for future large-scale applications. Thus, tremendous efforts have been devoted to addressing the critical challenges associated with sustainable ZABs, aiming to significantly improve their energy efficiency and prolong their operation lifespan. The growing interest in sustainable ZABs requires in-depth research on oxygen electrocatalysts, electrolytes, and Zn anodes, which have not been systematically reviewed to date. In this review, the fundamentals of ZABs, oxygen electrocatalysts for air cathodes, physicochemical properties of ZAB electrolytes, and issues and strategies for the stabilization of Zn anodes are systematically summarized from the perspective of fundamental characteristics and design principles. Meanwhile, significant advances in the in situ/operando characterization of ZABs are highlighted to provide insights into the reaction mechanism and dynamic evolution of the electrolyte|electrode interface. Finally, several critical thoughts and perspectives are provided regarding the challenges and opportunities for sustainable ZABs. Therefore, this review provides a thorough understanding of the advanced sustainable ZAB chemistry, hoping that this timely and comprehensive review can shed light on the upcoming research horizons of this prosperous area.

Amorphous TiOx Stabilized Intermetallic Pt3Ti Nanocatalyst for Methanol Oxidation Reaction

Intermetallic compounds, featuring atomically ordered structures, have emerged as a class of promising electrocatalysts for fuel cells. However, it remains a formidable challenge to controllably synthesize Pt-based intermetallics during the essential high-temperature annealing process as well as stabilize the nanoparticles (NPs) during the electrocatalytic process. Herein, we demonstrated a Ketjen black supported intermetallic Pt₃Ti nanocatalyst coupled with amorphous TiO_x species (Pt₃Ti-TiO_x/KB). The TiO_x can not only confine Pt₃Ti NPs during the synthesis and electrocatalytic process by a strong metal-oxide interaction but also promote the water dissociation for generating more OH species, thus facilitating the conversion of CO_ad. The Pt₃Ti-TiO_x/KB showed a significantly enhanced mass activity (2.15 A mg_Pt^-1) for the methanol oxidation reaction, compared with Pt₃Ti/KB and Pt/C, and presented an impressively high mass activity retention (∼71%) after the durability test. This work provides an effective strategy of coupling Pt-based intermetallics with functional oxides for developing highly performed electrocatalysts.

Fe-Incorporated Nickel-Based Bimetallic Metal-Organic Frameworks for Enhanced Electrochemical Oxygen Evolution

Developing cost-effective and high-efficiency catalysts for electrocatalytic oxygen evolution reaction (OER) is crucial for energy conversions. Herein, a series of bimetallic NiFe metal-organic frameworks (NiFe-BDC) were prepared by a simple solvothermal method for alkaline OER. The synergistic effect between Ni and Fe as well as the large specific surface area lead to a high exposure of Ni active sites during the OER. The optimized NiFe-BDC-0.5 exhibits superior OER performances with a small overpotential of 256 mV at a current density of 10 mA cm^-2 and a low Tafel slope of 45.4 mV dec^-1, which outperforms commercial RuO₂ and most of the reported MOF-based catalysts reported in the literature. This work provides a new insight into the design of bimetallic MOFs in the applications of electrolysis.

Self-Adaptive Electronic Structure of Amphoteric Conjugated Ligand-Modified 3 d Metal-C3 N4 Smart Electrocatalyst by pH Self-Response Realizing Electrocatalytic Self-Adjustment

Constructing pH-responsive smart material provides a new opportunity to address the problem that traditional electrocatalysts cannot achieve both alkaline oxygen evolution reaction (OER) and acidic hydrogen evolution reaction (HER) activities. In this study, amphoteric conjugated ligand (2-aminoterephthalic acid, BDC-NH₂ )-modified 3d metal-anchored graphitic carbon nitride (3d metal-C₃ N₄ ) smart electrocatalysts are constructed, and self-adaptation of the electronic structure is realized by self-response to pH stimulation, which results in self-adjustment of alkaline OER and acidic HER. Specifically, the amino and carboxyl functional groups in BDC-NH₂ undergo protonation and deprotonation respectively under different pH stimulation to adapt to environmental changes. Through DFT calculations, the increase or decrease of electron delocalization range brought by the self-response characteristic is found to lead to redistribution of the Bader charge around the modified active sites. The OER and HER activities are greatly promoted roughly 4.8 and 8.5 times over Co-C₃ N₄ after BDC-NH₂ -induced self-adaptive processes under different environments, arising from the reduced energy barrier of O* to OOH* and ΔG_H* . Impressively, the proposed BDC-NH₂ -induced smart regulation strategy is applicable to a series of 3d metal anchors for C₃ N₄ , including Co, Ni and Fe, providing a general structural upgrading method for constructing smart electrocatalytic systems.

Intelligent Chip-Controlled Smart Oxygen Electrodes for Constructing Rechargeable Zinc-Air Batteries with Excellent Energy Efficiency and Durability

High-performance rechargeable oxygen electrodes are key devices for realizing high-specific-energy batteries, including zinc-air and lithium-air batteries. However, these batteries have severe problems of premature decay in energy efficiency by serious corrosion, wide charge-discharge gap, and catalyst peeling off. Herein, we propose a "smart dual-oxygen electrode", which is composed of an intelligent switch control module + heterostructured Fe₁Ni₃-LDH/PNCNF OER catalysis electrode layer + ion conductive | electronic insulating membrane + Pt/C ORR catalysis electrode layer, where OER and ORR layers are automatically switched by the intelligent switch control module as required. This smart dual-oxygen electrode offers an ultralow energy efficiency decay rate of 0.0067% after 300 cycles during cycling, much lower than that of the commercial Pt/C electrode (1.82%). The assembled rechargeable zinc-air battery (RZAB) displays a super narrow voltage gap and achieves a high energy efficiency of 71.7%, far higher than that of the existing RZABs (about 50%). Therefore, this strategy provides a complete solution for designing various high-performance metal-air secondary batteries.

Tunable Heterogeneous FeCo Alloy-Mo0.82N Bifunctional Electrocatalysts for Temperature-Adapted Zn-Air Batteries

The practical applications of temperature-tolerant Zn-air batteries (ZABs) rely on highly active and stable bifunctional catalysts that accelerate cathodic oxygen reduction (ORR) and oxygen evolution (OER) reactions. Herein, we successfully integrated fascinating transition metal nitrides and FeCo alloys through a simple coordination assembly and pyrolysis process. Importantly, the alloy-to-nitride ratio in the heterogeneous catalyst can be carefully regulated through the subsequent etching process. Moreover, the composition-dependent ORR/OER performance of the FeCo-Mo_0.82N catalysts was revealed. Aqueous ZABs using the optimized FeCo-Mo_0.82N-60 as a cathode exhibit a high peak power density of 149.7 mW cm^-2 and an impressive stability of 600 h with a low charge-discharge voltage gap decay rate of 0.025 mV h^-1, which exceeds those of most of recent reports. Furthermore, the FeCo-Mo_0.82N-60-based flexible ZABs display a small specific capacity degradation (3%) from 40 to -10 °C, demonstrating excellent temperature tolerance.

Heterogeneous Ni-MoN nanosheet-assembled microspheres for urea-assisted hydrogen production

Electrocatalytic water splitting is a promising technology for sustainable hydrogen (H₂) production; however, it is restricted by the kinetically sluggish anodic oxygen evolution reaction (OER). Replacing OER with urea oxidation reaction (UOR) with low thermodynamic potential can simultaneously improve the energy efficiency of H₂ production and purify urea-containing wastewater. Here we report a facile assembly-calcination two-step method to synthesize heterogeneous Ni-MoN nanosheet-assembled microspheres (Ni-MoN NAMs). The nanosheet-assembled structure and the synergistic metallic Ni-MoN heterogeneous interface endow the Ni-MoN NAMs with good OER (1.52 V@10 mA cm^-2), UOR (1.28 V@10 mA cm^-2), and hydrogen evolution reaction (HER, 0.16 V@10 mA cm^-2) activity. The two-electrode urea electrolysis cell with Ni-MoN NAMs as both the cathode and anode requires an extremely low cell voltage of 1.41 V to afford 20 mA cm^-2, which is 0.3 V lower than that of the water electrolyzer, paving the way for energy-saving H₂ production.

Construction of NiFe-Layered Double Hydroxides Arrays as Robust Electrocatalyst for Oxygen Evolution Reaction

Electrochemical water splitting is considered to be an important method for efficient hydrogen production to alleviate energy shortage and environmental pollution, but its development is currently limited by the slow oxygen evolution reaction (OER). To solve the sluggish reaction kinetics of OER, the focus is on the exploration of low-cost and efficient electrocatalysts, which is quite significant for the development of electrochemical water splitting. Herein, a NiFe layered double hydroxides (LDH) electrocatalyst (denoted as FNH) is achieved by a simple one-step hydrothermal method. The experimental results show that due to the synergistic interaction of introduced Fe species, the FNH possesses a special three-dimensional (3D) vertical nanosheet array structure, which results in efficient ion access. More importantly, the strong electronic interaction between Fe and Ni sites results in the optimized electronic structure of the Ni sites, which not only generates abundant Ni3+ sites as optimized active sites for OER, but also decrease the charge transfer resistance. Thus, the FNH catalyst exhibits an extraordinary overpotential of 386.8 mV to deliver 100 mA cm−2, showing better activity than that of RuO2, and satisfactory cycling stability after continuous operation for 28 h. Our work provides an easy-to-implement method to obtain high-efficiency OER electrocatalysts.

Fabrication of highly efficient Rh-doped cobalt-nickel-layered double hydroxide/MXene-based electrocatalyst with rich oxygen vacancies for hydrogen evolution

The development of nonprecious metal catalysts for producing hydrogen from economical alkaline water electrolysis that is both stable and efficient is crucial but remains challenging. In this study, Rh-doped cobalt-nickel-layered double hydroxide (CoNi LDH) nanosheet arrays with abundant oxygen vacancies (Ov) in-situ grown on Ti₃C₂T_x MXene nanosheets (Rh-CoNi LDH/MXene) were successfully fabricated. The synthesized Rh-CoNi LDH/MXene exhibited excellent long-term stability and a low overpotential of 74.6 ± 0.4 mV at -10 mA cm^-2 for hydrogen evolution reaction (HER) owing to its optimized electronic structure. Experimental results and density functional theory calculations revealed that the incorporation of Rh dopant and Ov into CoNi LDH and the coupling interface between Rh-CoNi LDH and MXene optimized the hydrogen adsorption energy, which accelerated the hydrogen evolution kinetics, thereby accelerating the overall alkaline HER process. This work presents a promising strategy for designing and synthesizing highly efficient electrocatalysts for electrochemical energy conversion devices.

Edge-oriented phosphatizing engineering of 2D Ni-MOFs with a tailored d-band center for boosting catalytic activity

Metal-organic framework (MOF)-based heterostructures have aroused widespread interest owing to their extensive compositional tunability and interesting catalytic properties. However, the precise edge-oriented growth of transition metal compounds at the edges of 2D MOFs to construct edge mode heterostructures remains a great challenge due to their inherent thermodynamic instability. Here, edge-oriented growth of Ni₂P at the edges of a 2D Ni-MOF was achieved for the first time by precisely tuning the phosphorus source content and phosphating temperature. Owing to the formation of the edge mode Ni-MOF/Ni₂P heterostructure, the as-prepared heterostructure showed upregulated d-band center, more robust 4-nitrophenol (4-NP) adsorption capacity, lowered energy barrier of the rate-determining step (RDS), and higher specific surface area, resulting in the best performance of the hydrogenation reduction of 4-NP to 4-aminophenol (4-AP) in the presence of non-precious metal catalysts.

Self-sacrificial reconstruction of MoO42- intercalated NiFe LDH/Co2P heterostructures enabling interfacial synergies and oxygen vacancies for triggering oxygen evolution reaction

Exploring high-efficiency electrocatalysts for oxygen evolution reaction (OER) is one of the most important concerns to produce hydrogen in water electrolysis. Herein, the FNM/Co2P-0.4 heterostructure was designed as an electrocatalyst for the OER process by the combination of MoO42- intercalating NiFe LDH and Co2P on nickel foam (NF). The surface reconstruction and MoO42- leaching can induce the conversion of Co2P and NiFe LDH on FNM/Co2P-0.4 to generate Co/NiOOH with more oxygen vacancies. Beyond, CoOOH and NiOOH can also synergize to reduce the energy barrier of OER, optimize conductivity, and improve stability. The surface reconstruction and the formation of OOH⁎ were further unveiled by in-situ UV-vis absorption spectra and Fourier-transformed alternative current voltammetry (FTACV). The integration of interfacial synergies and oxygen vacancies can facilitate the adsorption/desorption of intermediates, regulate the d-band center, and expose more active sites. And as a result, FNM/Co2P-0.4 shows a significant low overpotential (240 mV) at 50 mA cm-2, a small Tafel (74 mV dec-1), low activation energy (Ea) and remarkable durability. This work provides a new pathway to improve the OER performance by using interfacial synergies and rich oxygen vacancies derived from the self-sacrificial reconstruction of heterostructured electrocatalysts.

Tailoring of Active Sites from Single to Dual Atom Sites for Highly Efficient Electrocatalysis

Single atom catalysts (SACs) have been attracting extensive attention in electrocatalysis because of their unusual structure and extreme atom utilization, but the low metal loading and unified single site induced scaling relations may limit their activity and practical application. Tailoring of active sites at the atomic level is a sensible approach to break the existing limits in SACs. In this review, SACs were first discussed regarding carbon or non-carbon supports. Then, five tailoring strategies were elaborated toward improving the electrocatalytic activity of SACs, namely strain engineering, spin-state tuning engineering, axial functionalization engineering, ligand engineering, and porosity engineering, so as to optimize the electronic state of active sites, tune d orbitals of transition metals, adjust adsorption strength of intermediates, enhance electron transfer, and elevate mass transport efficiency. Afterward, from the angle of inducing electron redistribution and optimizing the adsorption nature of active centers, the synergistic effect from adjacent atoms and recent advances in tailoring strategies on active sites with binuclear configuration which include simple, homonuclear, and heteronuclear dual atom catalysts (DACs) were summarized. Finally, a summary and some perspectives for achieving efficient and sustainable electrocatalysis were presented based on tailoring strategies, design of active sites, and in situ characterization.

Promoting Reversible Dissolution/Deposition of MnO2 for High-Energy-Density Zinc Batteries via Enhancing Cut-Off Voltage

Zn//MnO₂ batteries based on the MnO₂ /Mn²⁺ conversion reaction mechanism featuring high energy density, safety, and affordable cost are promising in large-scale energy storage application. Nonetheless, the continuous H⁺ intercalation at low potential reduces the average output voltage and the energy efficiency, impeding the development of the high-performance zinc battery. In this work, a strategy was proposed of enhancing the cut-off voltage from the perspective of electrochemical parameters, toward high energy efficiency and stable output voltage of the Zn//MnO₂ battery. This strategy was beneficial to promoting MnO₂ dissolution/deposition through the increase of acidity caused by the constant accumulation of MnO₂ and inhibiting H⁺ (de)intercalation during cycling process, thereby improving the energy efficiency (83.5 %) along with the stable average output voltage (1.88 V) under the cut-off voltage of 1.8 V. This work provides a new pathway to promote aqueous zinc batteries with high energy density and stable output voltage.

Construction of trifunctional electrode material based on Pt-Coordinated Ce-Based metal organic framework

The main challenge hindering the use of Pt nanoparticles (Pt NPs) for electrochemical applications is their high cost and agglomeration. Herein, a trifunctional electrode material based on a two-dimensional cerium-based metal organic framework (2D Ce-MOF) decorated with Pt NPs is constructed. The large specific surface area of the 2D Ce-MOF can effectively prevent the phenomenon of Pt NPs reaction. The strong synergy between Pt NPs and the 2D Ce-MOF not only significantly enhances electron transport efficiency, but also increases the number of electrochemically reaction reactive sites. As a result, the Ce-MOF@Pt presents excellent performance in the HER (Hydrogen Evolution Reaction), OER (Oxygen Evolution Reaction) and supercapacitor reactions. The Tafel slopes of OER and HER are 47.9 and 188.1 mV dec^-1, respectively. Meanwhile, Ce-MOF@Pt-0.05 shows a specific capacity of 1894F g^-1 at a current density of 1 A g^-1 and remains at 111.5% of the initial capacitance after 3000 cycles. In general, this study highlights the importance of Pt NPs in promoting the electrochemical performance of MOFs and reveals a new way to reduce electrocatalyst prices.

Quantum Dots Compete at the Acme of MXene Family for the Optimal Catalysis

It is well known that two-dimensional (2D) MXene-derived quantum dots (MQDs) inherit the excellent physicochemical properties of the parental MXenes, as a Chinese proverb says, "Indigo blue is extracted from the indigo plant, but is bluer than the plant it comes from." Therefore, 0D QDs harvest larger surface-to-volume ratio, outstanding optical properties, and vigorous quantum confinement effect. Currently, MQDs trigger enormous research enthusiasm as an emerging star of functional materials applied to physics, chemistry, biology, energy conversion, and storage. Since the surface properties of small-sized MQDs include the type of surface functional groups, the functionalized surface directly determines their performance. As the Nobel Laureate Wolfgang Pauli says, "God made the bulk, but the surface was invented by the devil," and it is just on the basis of the abundant surface functional groups, there is lots of space to be thereof excavated from MQDs. We are witnessing such excellence and even more promising to be expected. Nowadays, MQDs have been widely applied to catalysis, whereas the related reviews are rarely reported. Herein, we provide a state-of-the-art overview of MQDs in catalysis over the past five years, ranging from the origin and development of MQDs, synthetic routes of MQDs, and functionalized MQDs to advanced characterization techniques. To explore the diversity of catalytic application and perspectives of MQDs, our review will stimulate more efforts toward the synthesis of optimal MQDs and thereof designing high-performance MQDs-based catalysts.

Coupling LaNiO3 Nanorods with FeOOH Nanosheets for Oxygen Evolution Reaction

Perovskite-based electrocatalysts with compositional flexibility and tunable electronic structures have emerged as one of the promising non-noble metal candidates for oxygen evolution reaction (OER). Here, we propose a heterostructure comprising perovskite oxide (LaNiO3) nanorods and iron oxide hydroxide (FeOOH) nanosheets as an effective electrochemical catalyst for OER. The optimized 0.25Fe-LNO catalyst with an interesting 1D-2D hierarchical structure shows a low overpotential of 284 mV at 10 mA cm−2 and a small Tafel slope of 69 mV dec−1. The enhanced performance can be explained by the synergistic effect between LaNiO3 and FeOOH, resulting in an improved electrochemically active surface area, facilitated charge transfer and the optimized adsorption of OH intermediates.

Metal-Organic Framework-Based Materials for Aqueous Zinc-Ion Batteries: Energy Storage Mechanism and Function

Aqueous rechargeable zinc-ion batteries (ZIBs) featuring competitive performance, low cost and high safety hold great promise for applications in grid-scale energy storage and portable electronic devices. Metal-organic frameworks (MOFs), relying on their large framework structure and abundant active sites, have been identified as promising materials in ZIBs. This review comprehensively presents the current development of MOF-based materials including MOFs and their derivatives in ZIBs, which begins with Zn storage mechanism of MOFs, followed by introduction of various types of MOF-based cathode materials (PB and PBA, Mn-based MOF, V-based MOF, conductive MOF and their derivatives), and the regulation approaches for Zn deposition behavior. The key factors and optimization strategies of MOF-based materials that affect ZIBs performance are emphasized and discussed. Finally, the challenges and further research directions of MOF-based materials for advanced zinc-ion batteries are provided.

A Pyrolysis-Free Method Toward Large-Scale Synthesis of Ultra-Highly Efficient Bifunctional Oxygen Electrocatalyst for Zinc-Air Flow Batteries

The transition-metal nitrogen-carbon (M-N-C) catalysts, as one of the optimal bifunctional oxygen catalysts, are vital for cathodic oxygen electrode of Zn-based air flow batteries (ZAFBs). However, chemical complexity of M-N-C catalysts prepared via the traditional pyrolytic process increases the difficulties of precise control toward configuration and repeatability, especially in large-scale synthesis. Herein, a bifunctional oxygen catalyst via a pyrolysis-free approach based on closed π-conjugated covalent organic polymers (COPs, microwave synthesis) is developed, which inherits the advantage of the well-defined configuration in an atomic manner. Profited from distinct catalytic centers and strong electronic coupling at the interface between COP and layered double hydroxides, the as-synthesized catalyst not only more easily permits large quantity production (>1 kg per batch), but also maintains an ultrahigh bifunctional activity and a long cycle stability even after scale synthesis (ΔE [Ej₁₀ - E_1/2 ] = 591 mV; energy efficiency drops by only 2.02% after 1200 cycles), which overwhelmingly exceeds the benchmark Pt/C+IrO₂ and the state-of-the-art pyrolytic bifunctional M-N-C oxygen catalysts.

Competitive Coordination-Oriented Monodispersed Ruthenium Sites in Conductive MOF/LDH Hetero-Nanotree Catalysts for Efficient Overall Water Splitting in Alkaline Media

Rational exploration of efficient, inexpensive, and robust electrocatalysts is critical for the efficient water splitting. Conjugated conductive metal-organic frameworks (cMOFs) with multicomponent layered double hydroxides (LDHs) to construct bifunctional heterostructure catalysts are considered as an efficient but complicated strategy. Here, the fabrication of a cMOF/LDH hetero-nanotree array catalyst (CoNiRu-NT) coupled with monodispersed ruthenium (Ru) sites via a controllable grafted-growth strategy is reported. Rich-amino hexaiminotriphenylene linkers coordinate with the LDH nanotrunk to form cMOF nanobranches, providing numerous anchoring sites to precisely confine and stabilize RuN₄ sites. Moreover, monodispersed and reduced Ru moieties facilitate H₂ O adsorption and dissociation, and the heterointerface between the cMOF and the LDH further modifies the chemical and electronic structures. Optimized CoNiRu-NT displays a significant increase in electrochemical water-splitting properties in alkaline media, affording low overpotentials of 22 mV at 10 mA cm^-2 and 255 mV at 20 mA cm^-2 for the hydrogen evolution reaction and oxygen evolution reaction, respectively. In an actual electrochemical system, CoNiRu-NT drives an overall water splitting at a low cell voltage of 1.47 V to reach 10 mA cm^-2 . This performance is comparable to that of pure noble-metal-based materials and superior to most reported MOF-based catalysts.

Boosting the Electrocatalytic Activity of Nickel-Iron Layered Double Hydroxide for the Oxygen Evolution Reaction byTerephthalic Acid

The development of a new type of oxygen evolution reaction (OER) catalyst to reduce the energy loss in the process of water electrolysis is of great significance to the realization of the industrialization of hydrogen energy storage. Herein, we report the catalysts of NiFe double-layer hydroxide (NiFe-LDH) mixed with different equivalent terephthalic acid (TPA), synthesized by the hydrothermal method. The catalyst synthesized with the use of the precursor solution containing one equivalent of TPA shows the best performance with the current density of 2 mA cm−2 at an overpotential of 270 mV, the Tafel slope of 40 mV dec−1, and excellent stable electrocatalytic performance for OER. These catalysts were characterized in a variety of methods. X-ray diffraction (XRD), Fourier Transform Infrared Spectrometer (FTIR), and Raman spectrum proved the presence of TPA in the catalysts. The lamellar structure and the uniform distribution of Ni and Fe in the catalysts were observed by a scanning electron microscope (SEM) and a transmission electron microscope (TEM). In X-ray photoelectron spectroscopy (XPS) of NiFe-LDH with and without TPA, the changes in the peak positions of Ni and Fe spectra indicate strong electronic interactions between TPA and Ni and Fe atoms. These results suggest that a certain amount of TPA can boost catalytic activity.

Anion-exchange membrane water electrolyzers and fuel cells

The key components, working management, and operating techniques of anion-exchange membrane water electrolyzers and fuel cells are reviewed for the first time.

p–d hybridization in CoFe LDH nanoflowers for efficient oxygen evolution electrocatalysis

Organic ligands can induce p–d orbital hybridization at the interface of CoFe LDH, resulting in a downshift of the d-band center of CoFe LDH and the optimization of the binding strength of intermediates during the OER.

A polyaniline-modified electrode surface for boosting the electrocatalysis towards the hydrogen evolution reaction and ethanol oxidation reaction

Here, polyaniline (PANI) is reported loaded on carbon paper to modify the carbon paper-PANI-Pt electrode surface, tailoring the electrocatalytic capability towards the hydrogen evolution reaction and ethanol oxidation reaction. The reasons for the enhancement by the PANI layer are attributed to the hydrophilic electrode surface, uniform dispersion of Pt, and large electrochemical active surface.

Enhanced activity towards oxygen electrocatalysis for rechargeable Zn-air batteries by alloying Fe and Co in N-doped carbon

Large-scale application of rechargeable Zn-air batteries requires low-cost electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as alternatives to noble metals. Herein, FeCo nanoparticles embedded in N-doped carbon (FeCo/N-C) were prepared by a two-step pyrolysis route. FeCo/N-C exhibits excellent activities toward both the ORR (half-wave potential of 0.84 V) and OER (overpotential of 345 mV at 10 mA cm^-2), which are comparable to those of commercial Pt/C and RuO₂, and by far exceeding their counterparts Fe/N-C and Co/N-C. Furthermore, the FeCo/N-C catalyst was evaluated in a rechargeable Zn-air battery for the full-cell test. The FeCo/N-C based battery is more durable with a smaller round-trip overpotential after 800 cycles than the battery using an expensive Pt/C + RuO₂ mixture catalyst.

Unlocking active metal site of Ti-MOF for boosted heterogeneous catalysis via a facile coordinative reconstruction

Constructing sophisticated hollow structure and exposing more metal sites in metal-organic frameworks (MOFs) can not only enhance their catalytic performance but also endow them with new functions. Herein, we present a facile coordinative reconstruction strategy to transform Ti-MOF polyhedron into nanosheet-assembled hollow structure with a large amount of exposed metal sites. Importantly, the reconstruction process relies on the esterification reaction between the organic solvent, i.e. ethanol and the carboxylic acid ligand, allowing the conversion of MOF without the addition of any other modulators and/or surfactants. Moreover, the surface and internal structure of the reconstructed MOF can be well tuned via altering the conversion time. Impressively, the reconstructed MOF exhibits ∼5.1-fold rate constant compared to the pristine one in an important desulfurization reaction for clean fuels production, i.e. the oxidation of dibenzothiophene.

Ultrathin Co(OH)2 Nanosheets@Nitrogen-Doped Carbon Nanoflake Arrays as Efficient Air Cathodes for Rechargeable Zn-Air Batteries

Developing highly active, cost-effective, and durable bifunctional oxygen electrocatalysts is an important step for the advancement of rechargeable Zn-air batteries (ZABs). Herein, an efficient bifunctional oxygen electrocatalyst of ultrathin Co(OH)₂ nanosheets supported on nitrogen-doped carbon nanoflake arrays (named as Co(OH)₂ @NC), is reported, which yields excellent bifunctional activity, i.e., a low overpotential of 285 mV to reach 10 mA cm^-2 for oxygen evolution reaction (OER), a high half-wave potential (0.83 V) for oxygen reduction reaction (ORR), and a low potential gap (ΔE) of 0.69 V. The excellent bifunctional catalytic performance can be ascribed to the concerted efforts of cobalt hydroxide toward OER and nitrogen-doped carbon for ORR. The Co(OH)₂ @NC nanoflake arrays is further used as binder-free air cathodes for rechargeable Zn-air batteries, exhibiting a high specific capacity of 798.3 mAh g_Zn ^-1 , improved stability (a working life of >70 h at 5 mA cm^-2 ), as well as a reduced long-term charging voltage, which outperforms the counterparts of NC nanoflake arrays and Pt/C-based air cathodes. One step further, the Co(OH)₂ @NC nanoflake arrays on carbon cloth are directly used as binder-free air cathodes for flexible, solid-state ZABs, showing excellent performance under deformation as well.

Ultra-Fast and Scalable Saline Immersion Strategy Enabling Uniform Zn Nucleation and Deposition for High-Performance Zn-Ion Batteries

Although aqueous Zn-ion batteries (ZIBs) with low cost and high safety show great potential in large-scale energy storage system, metallic Zn anode still suffers from unsatisfactory cycle stability due to unregulated growth of Zn dendrites, corrosion, and formation of various side products during electrochemical reaction. Here, an ultrafast and simple method to achieve a stable Zn anode is developed. By simply immersing a Zn plate into an aqueous solution of CuSO₄ for only 10-60 s, a uniform and robust protective layer (Zn₄ SO₄ (OH)₆ ·5H₂ O/Cu₂ O) is formed on commercial Zn plate (Zn/ZCO), which enables uniform electric field distribution and controllable dendrite growth, leading to a long-term cycle life of over 1400 h and high average Coulombic efficiency (CE) of 99.2% at 2.0 mA cm^-2 and 2.0 mAh cm^-2 . These excellent characteristics of the prepared Zn anode show great potential in practical applications for high-performance aqueous Zn-ion batteries.

Bismuth Nanoparticle-Embedded Porous Carbon Frameworks as a High-Rate Chloride Storage Electrode for Water Desalination

Capacitive deionization (CDI) is a promising cost-effective and low energy consumption technology for water desalination. However, most of the previous works focus on only one side of the CDI system, i.e., Na⁺ ion capture, while the other side that stores chloride ions, which is equally important, receives very little attention. This is attributed to the limited Cl^- storage materials as well as their sluggish kinetics and poor stability. In this article, we demonstrate that a N-doped porous carbon framework is capable of suppressing the phase-transformation-induced performance decay of bismuth, affording an excellent Cl^- storage and showing potential for water desalination. The obtained Bi-carbon composite (Bi/N-PC) shows a capacity of up to 410.4 mAh g^-1 at 250 mA g^-1 and a high rate performance. As a demonstration for water desalination, a superior desalination capacity of 113.4 mg g^-1 is achieved at 100 mA g^-1 with excellent durability. Impressively, the CDI system exhibits fast ion capturing with a desalination rate as high as 0.392 mg g^-1 s^-1, outperforming most of the recently reported Cl^- capturing electrodes. This strategy is applicable to other Cl^- storage materials for next-generation capacitive deionization.