Metal-organic frameworks derived spinel NiCo2O4/graphene nanosheets composite as a bi-functional cathode for high energy density Li–O2 battery applications

Electrode materials having a combined heterostructure morphology can boost the electrochemical performance of energy conversion and storage applications. In this paper, we prepared three-dimensional (3D) porous NiCo₂O₄ dodecahedron nanosheets (NCO) from a metal–organic framework template (ZIF-67) and incorporated them with two-dimensional (2D) multilayer graphene nanosheets (GNS) through a simple and rapid ultrasonication process. The combination of these 3D/2D nanostructures created effective interfaces between the NCO and GNS components that enhanced the intrinsic electronic properties and increased the number of active catalytic sites on the NCO@GNS surfaces. Accordingly, the NCO@GNS electrocatalyst displayed superior kinetics for both the oxygen reduction and evolution reactions in both aqueous and non-aqueous electrolytes and could be fabricated into an air-cathode for Li–O₂ battery applications. The NCO@GNS air-cathode delivered a specific storage capacity (7201 mA h g^?1) higher than those of the NCO and commercial carbon black electrodes. We tested the durability of the Li–O₂ battery featuring the NCO@GNS cathode in a new PAT-cell configuration; it exhibited long-term cyclability for 200 cycles with a limited capacity of 500 mA h g^?1 at a current density of 100 mA g^?1. This cathode design featuring meso- and micropores shortened the pathways for Li⁺ ion diffusion and ensured rapid electron and oxygen transfer, thereby increasing the lifetime of its corresponding Li–O₂ battery.     

Intermittent microwave synthesis of nanostructured Pt/TiN–graphene with high catalytic activity for methanol oxidation

A one-step and fast microwave technique was developed to synthesize graphene-supported TiN nanoparticles (TiN–G) directly from graphene and dihydroxybis (ammonium lactato) titanium (IV). During the synthesis graphene served as a reductant and template to reduce the Ti-precursor into TiN and then uniformly disperse TiN nanoparticles on it. Pt/TiN–G catalyst was also successfully prepared with the portion of Pt nanoparticles was anchored at the interface of TiN and graphene. Electrochemical measurements showed that the Pt/TiN–G catalyst exhibited improved catalytic activity for methanol oxidation and enhanced CO tolerance than those of Pt/G catalyst, attributed to the formation of –OH groups on the surface of TiN. And the –OH attached TiN assisted the conversion of CO into CO₂.     

Pulse-reverse electrodeposition of Ni–Mo–S nanosheets for energy saving electrochemical hydrogen production assisted by urea oxidation

Electrochemical hydrogen production from water splitting is one of the effective methods for hydrogen production that has recently attracted particular attention. One of the limitations of the electrochemical water splitting method is the slow oxygen evolution reaction (OER), which leads to an increase in overpotential and a decrease in hydrogen production efficiency. Here, Ni–Mo–S ultra-thin nanosheets were synthesized using the pulse reverse electrochemical deposition technique, and then this electrode was used as an electrode material for accelerating hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). Remarkably, the optimized electrode needs only 74 mV to attain the 10 mA cm⁻² current density in HER and require only 1.3 V vs RHE potential in the UOR process. Also, results showed that the replacement of the UOR with the OER process resulted in a significant improvement in the electrochemical production of hydrogen in which for delivering the current density of 10 mA cm⁻² in overall urea electrolysis, only 1.384 V is needed. In addition, outstanding catalytic stability was obtained, after 50 h electrolysis, the voltage variation was negligible. Such outstanding catalytic activity and stability was due to 3-D ultrathin nanosheets, the synergistic effect between elements, and the superhydrophilic/superaerophobic nature of fabricated electrode.     

Improved uniformity of Fe3O4 nanoparticles on Fe–N–C nanosheets derived from a 2D covalent organic polymer for oxygen reduction

Developing high-performance non-precious metal-based electrocatalysts for oxygen reduction reaction (ORR) is an urgent demand. In this study, we prepared highly efficient Fe₃O₄-nanoparticles-decorated Fe–N–C nanosheets (Fe₃O₄/CCOP_HM-F127) from 2D covalent organic polymer (COP) for ORR via a facile impregnation method. The introduced anionic surfactant, i.e. F127, can effectively improve the distribution and size uniformity of Fe₃O₄ nanoparticles. The onset potential of Fe₃O₄/COP_HM-F127 is 1.0 V, which is equivalent to that of commercial Pt/C. Moreover, the half-wave potential of Fe₃O₄/COP_HM-F127 can reach up to 0.87 V, which is better than that of commercial Pt/C. Fe₃O₄/COP_HM-F127 also exhibits excellent durability and methanol tolerance during the electrocatalytic process.     

Solid-state synthesis method for the preparation of cobalt doped Ni–Al2O3 mesoporous catalysts for CO2 methanation

In this study, a simple solid-state synthesis method was employed for the preparation of the Ni–Co–Al₂O₃ catalysts with various Co loadings, and the prepared catalysts were used in CO₂ methanation reaction. The results demonstrated that the incorporation of cobalt in nickel-based catalysts enhanced the activity of the catalyst. The results showed that the 15 wt%Ni-12.5 wt%Co–Al₂O₃ sample with a specific surface area of 129.96 m²/g possessed the highest catalytic performance in CO₂ methanation (76.2% CO₂ conversion and 96.39% CH₄ selectivity at 400 °C) and this catalyst presented high stability over 10 h time-on-stream. Also, CO methanation was investigated and the results showed a complete CO conversion at 300 °C.     

Controlled synthesis of N–CuCo2S4@Ni3S2 nanoarrays as promising urea oxidation electrocatalyst

One of the most attractive means for mitigation of environmental pollution is to produce hydrogen by electrocatalytic urea splitting. In the paper, the heterogeneous interfacial rich N–CuCo₂S₄@Ni₃S₂ material was in situ synthesized on nickel foam through a typical hydrothermal and sulfuration process. This N–CuCo₂S₄@Ni₃S₂ electrode displays excellent urea oxidation performance (potential of 1.38 V@50 mA cm⁻²), which is one of the best reactivity reported so far. Experimental results show that the superior catalytic activity is attributed to the rapid charge transfer, more reaction center exposed and superior electrical conductivity. Density functional theory shows that this Ni₃S₂ material accelerates the reaction rate in the catalytic process, the introduction of this N–CuCo₂S₄ material improves the conductivity of the material, and the synergistic catalysis of the N–CuCo₂S₄ and Ni₃S₂ makes this N–CuCo₂S₄@Ni₃S₂ material exhibit superior urea oxidation activity. Notably, long-term tests have shown a decrease in catalytic activity, which suggests that the surface of the sulfide is in situ generated to oxides or hydroxides, which are truly active species. This work provides a new idea for the development of efficient and stable urea oxidation catalysts for sulfides.     

Engineering of N,P,S-Triple doped 3-dimensional graphene architecture: Catalyst-support for “surface-clean” Pd nanoparticles to boost the electrocatalysis of ethanol oxidation reaction

The engineering of robust electrocatalysts for the ethanol oxidation reaction (EOR) with cost-natural, superior electrocatalytic activity, and stability, is crucial for the scaled-up applications of direct ethanol fuel cells. Herein, a facile bottom-up hydrothermal strategy has been implemented to synthesize N,P,S triple-doped 3-dimensional (3D) graphene architectures (N,P,S-3DG) with interconnected, hierarchical porous structure, followed by Pd nanoparticles were uniformly decorated onto the N,P,S-3DG via solvothermal approach. As fabricated hybrid nanocatalyst, labeled as Pd@N,P,S-3DG, is of charming physicochemical characteristics including large electrochemically active specific surface area, interconnected hierarchical pore network, a satisfactory percentage of heteroatom dopants, uniform distribution of Pd nanoparticles, as well as superior electrocatalytic performance metrics such as high catalytic activity, long-term stability, and tolerance to poisoning. The characterizations have confirmed the strong electrostatic interaction between the Pd nanoparticles and carbonaceous support material, thereby leading to homogeneously anchoring Pd nanoparticles onto 3D architecture and forming of novel active sites as well as synergistically boosting the EOR catalytic activity. The Pd@N,P,S-3DG has offered an enlarged electrochemically active surface area (50.3 m² g^?1), an enhanced catalytic current density of 1784 mA mg^?1_Pd, and outstanding long-term stability, thereby distinctly transcending those of commercial carbonaceous material-supported Pd catalysts. The work is of great importance since it may pave the way for the rational design of low-cost high-performance carbonaceous-based nano-electrocatalysts to be utilized in large-scale energy applications.     

Nickel–cobalt layered double hydroxide/NiCo2S4/g-C3N4 nanohybrid for high performance asymmetric supercapacitor

Graphitic carbon nitride (g-C₃N₄) with semiconducting nature can be considered for energy storage system by modifying its electrical conductivity and structural properties through formation of hybrid with materials such as bimetallic metal sulfide and nickel-cobalt layered double hydroxide (LDH). g-C₃N₄ as a N-rich compound with basic surface sites can change the surface properties of nanohybrid and impress the charge transfer. In this study, a nanohybrid based on nickel-cobalt LDH and sulfide and graphitic carbon nitride (NiCo LDH/NiCo₂S₄/g-C₃N₄) was synthesized through a three-step method. At first, Ni doped ZIF-67 was formed at the surface of g-C₃N₄ nanosheets and then the product was calcined in a furnace to form NiCo₂O₄/g-C₃N₄. At next step, the sample was hydrothermally converted to NiCo₂S₄/g-C₃N₄ using thioacetamide and finally modified with NiCo LDH nanoplates to form porous structure with high surface area. The NiCo LDH/NiCo₂S₄/g-C₃N₄ nanohybrid showed high specific capacitance of 1610 F g^?1 at current density of 1 A g^?1 and also excellent stability of 108.8% after 5000 cycles at potential scan rate of 50 mV s^?1, which makes it promising candidate for energy storage. An asymmetric system was prepared using nickel foams modified with NiCo LDH/NiCo₂S₄/g-C₃N₄ and g-C₃N₄ as positive and negative electrodes, respectively. The specific capacitance of 246.0 F g^?1 was obtained at 1 A g^?1 in 6 M KOH solution and system maintained 90.8% cyclic stability after 5000 cycles at potential scan rate of 50 mV s^?1. The maximum energy density and power density of the system were calculated as 82.0 Wh kg^?1 and 12,000 W kg^?1, respectively, which demonstrate its capability for energy storage.     

Controlled synthesis of Mn3O4/Co9S8–Ni3S2 on nickel foam as efficient electrocatalyst for oxygen evolution reaction

Advances in electrochemical interfaces have greatly facilitated the development of new energy systems that can replace traditional fossil fuels. Oxygen evolution reaction (OER) is the core reaction in the new energy conversion system to produce hydrogen. Here, nanorods structure of Mn₃O₄/Co₉S₈–Ni₃S₂/NF-4 was designed and assembled. The Mn₃O₄ has served as an appropriate matrix to build a composite structure with Co₉S₈–Ni₃S₂ to enhance the stability of catalyst. And the introduction of Mn regulated the electronic structure of Ni and Co, which increased the OER activity of matericals. Further characterization and electrochemical testing have suggested that between polymetallic can effectively optimize conductivity and enhance reaction kinetics. Mn₃O₄/Co₉S₈–Ni₃S₂/NF-4 can achieve overpotential of 188 mV at the current density of 10 mA cm^?2 in alkaline solution, with small Tafel slope of 43.2 mV dec^?1 and satisfactory stability of 30 h at 10 mA cm^?2. This work may show a feasible reference in the design of high-efficient OER catalysts.     

Scalable synthesis of nitrogen and nitrogen–silicon co-doped graphene: SiC4 and SiN1C3 as new active centers for boosting ORR performance

Innovations in energy storage and conversion technologies are closely dependent on developing superior materials that can be used in this field. Here, we present an industrially scalable and low-cost solvothermal method for synthesizing Si-N-co-doped (Si-N-GN) and N-doped graphene (N-GN) with a high specific surface area of 523 m² g⁻¹ and 1289 m² g⁻¹, respectively. Silicon atoms were successfully incorporated into the 2D graphene at a doping rate of 2.28 at.% via Si-C, Si-N, and Si-O bonds, thanks to the decarbonylation of N,N-dimethyl formamide (DMF) into dimethylamine and highly reactive carbonyl at the solvothermal conditions. The Si-N-GN exhibited an average electron transfer number of 3.83 e⁻ per mole of O₂ in a wide potential range with similar on-set potential (0.988 V vs. 1.012 V), greater methanol tolerance capability, and higher diffusion limiting current density (7.2 vs. 6.5 mA cm⁻² at 0.4 V) for oxygen reduction reaction (ORR) in alkaline electrolytes compared to the commercial Pt/C catalyst. The improved ORR performance of Si-N-GN was attributed to the effectively decreased adsorption energy of O₂ on SiC₄ and SiN₁C₃ type bondings supported by the density functional theory (DFT) calculations based on the model created according to the XPS results. The promising electrocatalytic activity of Si-N-GN for ORR could also be enlarged to other electrochemical applications, including metal-air batteries.     

Controlled synthesis of W–Co3S4@Co3O4 as an environmentally friendly and low cost electrocatalyst for overall water splitting

Electrochemistry splitting of water is considered to be one of the most fascinating methods to replace traditional chemical fuels. Here, we design a new method to exploit W–Co₃S₄@Co₃O₄ heterostructures. The W–Co₃S₄@Co₃O₄ material was first prepared and grown in situ on nickel foam by a typical hydrothermal and calcination approach. Based on the principle of electronic regulation, the synergistic effect of W and Co metal ions can increase the charge transfer of the electrode, thus significantly prompting the catalytic activity of the electrode. The W–Co₃S₄@Co₃O₄ material present superior catalytic performance for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), and the overpotential at 10 mA cm⁻² is 260 mV and 140 mV, respectively. Notably, W–Co₃S₄@Co₃O₄ catalyst showed excellent water splitting performance under alkaline conditions (cell voltage of 1.63V @10 mA cm⁻²). Density functional theory calculation shows that the existence of the Co₃O₄ material accelerates the rate of hydrogen production reaction, and the existence of the W–Co₃S₄ material promotes the conductivity of the W–Co₃S₄@Co₃O₄ electrode. The synergistic effect of W–Co₃S₄ and Co₃O₄ materials is beneficial to the improvement of the catalytic activity of the electrode. This study provides a novel view for the development of electrodes synthesis and a novel paradigm for the development of robust, better and relatively non-toxic bifunctional catalysts.     

A more economical choice for the cathode material of Ni-MH batteries with high electrochemical performances: 3D flower-like Ni–Fe LDHs

A series of 3D flower-like Ni–Fe layered double hydroxides (LDHs) were synthesized successfully and used as the cathode materials for nickel-metal hydride battery (Ni-MH battery). The 4Ni–Fe LDH electrode (Ni/Fe molar ratio = 4:1) displays the highest high-rate discharge property and the most excellent cycling performance. The discharge capacity of the 4Ni–Fe LDH electrode can reach up to 291.3 mAh/g at a discharge rate of 200 mA/g and which delivers a high capacity retention of 98.9% over 200 cycles. In contrast, the pure Ni(OH)₂ electrode only has a capacity of 243 mAh/g, and after 100 cycles the capacity retention is just 73.4%. The above improvement can be ascribed to the formation of Ni–Fe LDHs which can consolidate the stability of α-Ni(OH)₂ in the KOH solution. In addition, the unique flower-like morphology and the enlarged interlayer spacing also paly important role to promote ion transmission and charge transfer. Considering the competitive price of Fe, 3D flower-like Ni–Fe LDH may be a more economical choice for the cathode material of Ni-MH batteries.     

Fabrication of 0D/1D/2D ZnS–CuS nanodots/GNRs/g-C3N4 heterojunction photocatalyst for efficient photocatalytic overall water splitting

Photocatalytic water splitting has become a significant challenge in modern chemistry. In this process, the rate-determining step is the hydrogen evolution reaction (HER). In the present work, a surface modification approach for graphitic carbon nitride (g-C₃N₄) was applied to improve its photocatalytic HER. 0D ZnS–CuS nanodots were synthesized with the hydrothermal method as a co-catalyst to enhance the capability and stability of water splitting in the presence of visible light irradiation. Also, graphene nanoribbons were synthesized from CNTs unzipping to reduce the energy barrier of HER, improve the HE kinetic, and enhance the catalytic performance of the g-C₃N₄. By using ZnS–CuS/GNRs₍₂₎/g-C₃N₄ photocatalyst, a low onset potential of 130 mV, slight Tafel slope of 41 mV dec^?1, as well as excellent stability of 2000 s was obtained in acidic media. This efficient performance is attributed to the increased visible light absorption level in the proposed photocatalyst and the high stability in electron-hole pairs.     

In-situ growth of Zn–AgIn5S8 quantum dots on g-C3N4 towards 0D/2D heterostructured photocatalysts with enhanced hydrogen production

Photocatalytic water splitting for hydrogen production represents an ideal pathway for solar energy harvesting and conversion, for which narrow bandgap multinary sulfides play an important role. Here, series of Zn–AgIn₅S₈/g-C3N4 0D/2D nanocomposites were prepared by in-situ growth of the Zn–AgIn₅S₈ quantum dots (QDs) on g-C3N4 nanosheets for improved charge separation. To our surprise, rather than a photoactive component, here g-C3N4 nanosheets act as a charge transfer mediator, where only a relatively low ratio is required. The as-fabricated Zn–AgIn₅S₈/g-C3N4 nanocomposites were systematically studied. When the mass ratio of g-C3N4 was 10%, the hydrogen production rate was maximized, which was 1.39 times higher than pure Zn–AgIn₅S₈ QDs and 138.6 times higher than g-C3N4. The enhanced photocatalytic activity of the Zn–AgIn₅S₈/g-C3N4 nanocomposites is attributed to the intimate interface contact, which results in the effective separation and transfer of the photogenerated charge carriers as proved by the PL lifetime, transient photocurrent and electrochemical impedance spectra measurements. The Zn–AgIn₅S₈/g-C3N4 nanocomposites also exhibit excellent cycle stability. A plausible mechanism was proposed for the 0D/2D Zn–AgIn₅S₈/g-C3N4 composite photocatalysts. This work provides a relatively simple method for constructing high-quality 0D/2D heterostructure of QDs/nanosheets, as well as new insight for the efficiency improvement of narrow-bandgap sulfide photocatalysts.