There is still an ongoing effort to search for sustainable, clean and highly efficient energy generation to satisfy the energy needs of modern society. Among various advanced technologies, electrocatalysis for the oxygen evolution reaction (OER) plays a key role and numerous new electrocatalysts have been developed to improve the efficiency of gas evolution. Along the way, enormous effort has been devoted to finding high-performance electrocatalysts, which has also stimulated the invention of new techniques to investigate the properties of materials or the fundamental mechanism of the OER. This accumulated knowledge not only establishes the foundation of the mechanism of the OER, but also points out the important criteria for a good electrocatalyst based on a variety of studies. Even though it may be difficult to include all cases, the aim of this review is to inspect the current progress and offer a comprehensive insight toward the OER. This review begins with examining the theoretical principles of electrode kinetics and some measurement criteria for achieving a fair evaluation among the catalysts. The second part of this review acquaints some materials for performing OER activity, in which the metal oxide materials build the basis of OER mechanism while non-oxide materials exhibit greatly promising performance toward overall water-splitting. Attention of this review is also paid to in situ approaches to electrocatalytic behavior during OER, and this information is crucial and can provide efficient strategies to design perfect electrocatalysts for OER. Finally, the OER mechanism from the perspective of both recent experimental and theoretical investigations is discussed, as well as probable strategies for improving OER performance with regards to future developments.

Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives

Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications

Over the past few decades, researchers have established artificial enzymes as highly stable and low-cost alternatives to natural enzymes in a wide range of applications. A variety of materials including cyclodextrins, metal complexes, porphyrins, polymers, dendrimers and biomolecules have been extensively explored to mimic the structures and functions of naturally occurring enzymes. Recently, some nanomaterials have been found to exhibit unexpected enzyme-like activities, and great advances have been made in this area due to the tremendous progress in nano-research and the unique characteristics of nanomaterials. To highlight the progress in the field of nanomaterial-based artificial enzymes (nanozymes), this review discusses various nanomaterials that have been explored to mimic different kinds of enzymes. We cover their kinetics, mechanisms and applications in numerous fields, from biosensing and immunoassays, to stem cell growth and pollutant removal. We also summarize several approaches to tune the activities of nanozymes. Finally, we make comparisons between nanozymes and other catalytic materials (other artificial enzymes, natural enzymes, organic catalysts and nanomaterial-based catalysts) and address the current challenges and future directions (302 references).

Nanomaterials with enzyme-like characteristics (nanozymes): next-generation artificial enzymes

The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Increasing demand for finding eco-friendly and everlasting energy sources is now totally depending on fuel cell technology. Though it is an eco-friendly way of producing energy for the urgent requirements, it needs to be improved to make it cheaper and more eco-friendly. Although there are several types of fuel cells, the hydrogen (H2) and oxygen (O2) fuel cell is the one with zero carbon emission and water as the only byproduct. However, supplying fuels in the purest form (at least the H2) is essential to ensure higher life cycles and less decay in cell efficiency. The current large-scale H2 production is largely dependent on steam reforming of fossil fuels, which generates CO2 along with H2 and the source of which is going to be depleted. As an alternate, electrolysis of water has been given greater attention than the steam reforming. The reasons are as follows: the very high purity of the H2 produced, the abundant source, no need for high-temperature, high-pressure reactors, and so on. In earlier days,...

Recent Trends and Perspectives in Electrochemical Water Splitting with an Emphasis on Sulfide, Selenide, and Phosphide Catalysts of Fe, Co, and Ni: A Review

Cobalt sulfide materials have attracted enormous interest as low-cost alternatives to noble-metal catalysts capable of catalyzing both oxygen reduction and oxygen evolution reactions. Although recent advances have been achieved in the development of various cobalt sulfide composites to expedite their oxygen reduction reaction properties, to improve their poor oxygen evolution reaction (OER) activity is still challenging, which significantly limits their utilization. Here, the synthesis of Fe3O4-decorated Co9S8 nanoparticles in situ grown on a reduced graphene oxide surface (Fe3O4@Co9S8/rGO) and the use of it as a remarkably active and stable OER catalyst are first reported. Loading of Fe3O4 on cobalt sulfide induces the formation of pure phase Co9S8 and highly improves the catalytic activity for OER. The composite exhibits superior OER performance with a small overpotential of 0.34 V at the current density of 10 mA cm−2 and high stability. It is believed that the electron transfer trend from Fe species to Co9S8 promotes the breaking of the Co–O bond in the stable configuration (Co–O–O superoxo group), attributing to the excellent catalytic activity. This development offers a new and effective cobalt sulfide-based oxygen evolution electrocatalysts to replace the expensive commercial catalysts such as RuO2 or IrO2.

Fe3O4‐Decorated Co9S8 Nanoparticles In Situ Grown on Reduced Graphene Oxide: A New and Efficient Electrocatalyst for Oxygen Evolution Reaction

One-pot solvothermal preparation of magnetic reduced graphene oxide-ferrite hybrids for organic dye removal

Graphene, which possesses unique nanostructure and excellent properties, is considered a low cost alternative to carbon nanotubes in nanocomposites. In this paper, we demonstrate a facile in situreduction approach for the synthesis of reduced graphene oxide/Ni (RGO/Ni) nanocomposites with different morphologies. The concentration of nickel ions has a great influence on the morphology of the RGO/Ni nanocomposites and an interesting RGO-wrapped nanostructure was obtained. Magnetic studies reveal a room-temperature ferromagnetic behavior of the RGO/Ni nanocomposites. The catalytic activities of the RGO/Ni nanocomposites were investigated for the reduction of p-nitrophenol by NaBH4. It was found that the nanocomposites show higher catalytic activity compared with the unsupported Ni nanoparticles. The catalytic performance of the RGO/Ni nanocomposites was even better than the RANEY® Ni catalyst. Moreover, after completion of the reaction the nanocomposite catalyst can be easily re-collected from the reaction system by a magnet. Thus, the RGO/Ni nanocomposites obtained in this work may find applications in catalysis, data storage, targeted drug transportation and magnetic resonance imaging technologies.

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Reduced graphene oxide/nickel nanocomposites: facile synthesis, magnetic and catalytic properties

In this work, an all solid Z-scheme g-C3N4/Ag3PO4/NCDs photocatalyst has been prepared through decorating the direct Z-scheme g-C3N4/Ag3PO4 photocatalyst with nitrogen-doped carbon dots (NCDs). The g-C3N4/Ag3PO4/NCDs photocatalyst exhibits excellent photocatalytic activity for the degradation of methylene blue (MB), rhodamine B (RhB) and phenol under visible light irradiation. The solutions of MB (10 mg L−1) and RhB (10 mg L−1) can be efficiently degraded within 20 min and 15 min, respectively, and the phenol (50 mg L−1) can be degraded to 36% within 80 min, which are much better than those of Ag3PO4 and g-C3N4/Ag3PO4, indicating that the introduction of NCDs into g-C3N4/Ag3PO4 can effectively improve the photocatalytic activity. Moreover, the photocatalytic performance of g-C3N4/Ag3PO4/NCDs shows just a slight decrease after four degradation cycles, indicating a high stability of the g-C3N4/Ag3PO4/NCDs photocatalyst. A possible photocatalytic mechanism based on the experimental results is proposed. It is revealed that NCDs on the ternary g-C3N4/Ag3PO4/NCDs can enhance the light harvesting capacity and molecular oxygen activation ability of the photocatalyst, and serve as excellent electronic transmission medium to promote the transfer and separation of photo-generated electron-hole pairs. This study demonstrates that NCDs decorated photocatalysts are very promising for environment and related applications.

Nitrogen-doped carbon dots decorated on g-C3N4/Ag3PO4 photocatalyst with improved visible light photocatalytic activity and mechanism insight

A novel composite with CoP nanoparticles uniformly deposited on reduced graphene oxide (RGO) sheets is prepared through a facile two-step approach. The as-prepared CoP/RGO composite is investigated as an electrocatalyst for the hydrogen evolution reaction (HER). It is found that the CoP/RGO composite shows an enhanced catalytic activity with a smaller Tafel slope (104.8 mV per decade), a much larger exchange current density (4.0 × 10−5 A cm−2) and lower estimated HER activation energy (41.4 kJ mol−1) than pure CoP. Besides, the CoP/RGO composite exhibits good stability in acidic solution, the HER catalytic activity of which shows no obvious degradation after 500 cycles. Such enhanced catalytic activity stems from the abundance of active catalytic sites, the increased electrochemically accessible surface area and significantly improved electrochemical conductivity of the CoP/RGO composite. The good catalytic activity demonstrates that the CoP/RGO composite could be a promising electrocatalyst in hydrogen production.

Guoxing Zhu

Papers

Fe3O4‐Decorated Co9S8 Nanoparticles In Situ Grown on Reduced Graphene Oxide: A New and Efficient Electrocatalyst for Oxygen Evolution Reaction

One-pot solvothermal preparation of magnetic reduced graphene oxide-ferrite hybrids for organic dye removal

Reduced graphene oxide/nickel nanocomposites: facile synthesis, magnetic and catalytic properties

Nitrogen-doped carbon dots decorated on g-C3N4/Ag3PO4 photocatalyst with improved visible light photocatalytic activity and mechanism insight

CoP nanoparticles deposited on reduced graphene oxide sheets as an active electrocatalyst for the hydrogen evolution reaction