Cobalt Sulfide Nanoparticles Grown on Nitrogen and Sulfur Codoped Graphene Oxide: An Efficient Electrocatalyst for Oxygen Reduction and Evolution Reactions

Atomic Layer Deposition of Ultrathin Nickel Sulfide Films and Preliminary Assessment of Their Performance as Hydrogen Evolution Catalysts

Transition metal sulfides show great promise for applications ranging from catalysis to electrocatalysis to photovoltaics due to their high stability and conductivity. Nickel sulfide, particularly known for its ability to electrochemically reduce protons to hydrogen gas nearly as efficiently as expensive noble metals, can be challenging to produce with certain surface site compositions or morphologies, e.g., conformal thin films. To this end, we employed atomic layer deposition (ALD), a preeminent method to fabricate uniform and conformal films, to construct thin films of nickel sulfide (NiS_x) using bis(N,N'-di-tert-butylacetamidinato)nickel(II) (Ni(amd)₂) vapor and hydrogen sulfide gas. Effects of experimental conditions such as pulse and purge times and temperature on the growth of NiS_x were investigated. These revealed a wide temperature range, 125-225 °C, over which self-limiting NiS_x growth can be observed. In situ quartz crystal microbalance (QCM) studies revealed conventional linear growth behavior for NiS_x films, with a growth rate of 9.3 ng/cm² per cycle being obtained. The ALD-synthesized films were characterized using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) methods. To assess the electrocatalyitic activity of NiS_x for evolution of molecular hydrogen, films were grown on conductive-glass supports. Overpotentials at a current density of 10 mA/cm² were recorded in both acidic and pH 7 phosphate buffer aqueous reaction media and found to be 440 and 576 mV, respectively, with very low NiS_x loading. These results hint at the promise of ALD-grown NiS_x materials as water-compatible electrocatalysts.

Preparation of Cobalt Sulfide Nanoparticle-Decorated Nitrogen and Sulfur Co-Doped Reduced Graphene Oxide Aerogel Used as a Highly Efficient Electrocatalyst for Oxygen Reduction Reaction

A novel 3D cobalt sulfide (CoS) nanoparticle-decorated nitrogen and sulfur co-doped reduced graphene oxide aerogel (NSGA), referred to as CoS/NSGA, is prepared via three sequential processes, i.e., freeze-drying, annealing, and sulfidization. The obtained CoS/NSGA exhibits excellent electrocatalytic performance in the alkaline solution.

Nanostructured Bifunctional Redox Electrocatalysts

Electrocatalysts are playing a prominent role in the design of renewable energy devices. Benefiting from a long and prosperous history of synthesizing individual hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) electrocatalysts, the development of bifunctional ORR/OER or HER/OER electrocatalysts has recently emerged as a new research hotspot. In this review, a brief account of recent developments of bifunctional electrocatalysts for ORR/OER and HER/OER are introduced, aiming to provide insights into theoretical understanding of these reactions through analysis and comparison of various bifunctional electrocatalysts. The related reaction mechanisms and the associated activity descriptors for aforementioned reactions in the recent literatures are also presented. Different series of bifunctional electrocatalysts with much improved performances are discussed in detail and their design principles are outlined. Finally, the existing challenges and the future effort directions for enhancing the performance of bifunctional electrocatalysts are proposed.

PdCo/Pd-Hexacyanocobaltate Hybrid Nanoflowers: Cyanogel-Bridged One-Pot Synthesis and Their Enhanced Catalytic Performance

Elaborate architectural manipulation of nanohybrids with multi-components into controllable 3D hierarchical structures is of great significance for both fundamental scientific interest and realization of various functionalities, yet remains a great challenge because different materials with distinct physical/chemical properties could hardly be incorporated simultaneously into the synthesis process. Here, we develop a novel one-pot cyanogel-bridged synthetic approach for the generation of 3D flower-like metal/Prussian blue analogue nanohybrids, namely PdCo/Pd-hexacyanocobaltate for the first time. The judicious introduction of polyethylene glycol (PEG) and the formation of cyanogel are prerequisite for the successful fabrication of such fascinating hierarchical nanostructures. Due to the unique 3D hierarchical structure and the synergistic effect between hybrid components, the as-prepared hybrid nanoflowers exhibit a remarkable catalytic activity and durability toward the reduction of Rhodamine B (RhB) by NaBH₄. We expect that the obtained hybrid nanoflowers may hold great promises in water remediation field and beyond. Furthermore, the facile synthetic strategy presented here for synthesizing functional hybrid materials can be extendable for the synthesis of various functional hybrid nanomaterials owing to its versatility and feasibility.

Metal-Organic Framework-Derived Honeycomb-Like Open Porous Nanostructures as Precious-Metal-Free Catalysts for Highly Efficient Oxygen Electroreduction

Honeycomb-like porous carbon nanostructures are rationally constructed from a metal-organic framework composite. The unique architecture with uniformly distributed high-density active sites significantly enhances the electrocatalytic performance by increasing the accessible active sites and enhancing mass transport of the gas and electrolyte, rendering the resulting catalyst adequate in reaching the desired catalytic performance afforded by Pt for the oxygen reduction reaction.

Novel CoS2 embedded carbon nanocages by direct sulfurizing metal-organic frameworks for dye-sensitized solar cells

Owing to its excellent electrocatalytic properties, cobalt disulfide (CoS2) is regarded as a promising counter electrode (CE) material for dye-sensitized solar cells (DSSCs). However, hindered by its relatively poor electrical conductivity and chemical instability, it remains a challenge to apply it into high-performance DSSCs. In this work, we have developed novel CoS2 embedded carbon nanocages as a CE in DSSCs, using ZIF-67 (zeolitic imidazolate framework 67, Co(mim)2, mim = 2-methylimidolate) as a template. The CoS2 samples sulfurized for different time lengths are prepared through a facile solution process. It is found that the sulfurization time can be optimized to maximize the DSSC efficiency and the DSSC based on the CoS2 embedded carbon nanocages sulfurized for 4 hours exhibits the highest photovoltaic conversion efficiency (PCE) of 8.20%, higher than those of DSSCs consisting of other CoS2 CEs and Pt-based DSSC (7.88%). The significantly improved DSSC PCE is contributed by the synergic effect of inner CoS2 nanoparticles and an amorphous carbon matrix, leading to a CE with high catalytic activity, good electrical conductivity and excellent durability. This study demonstrates that the CE based on inexpensive CoS2 embedded carbon nanocages is a prospective substitute to expensive platinum and provides a new approach for commercializing high-efficiency DSSCs.

Nitrogen and Sulfur Codoped Reduced Graphene Oxide as a General Platform for Rapid and Sensitive Fluorescent Detection of Biological Species

Nitrogen (N) and sulfur (S) codoped reduced graphene oxide (N,S-rGO) was synthesized through a facile solvothermal process. The introduction of N and S heteroatoms into GO effectively activated the sp(2)-hybridized carbon lattice and made the material an ideal electron/energy acceptor. Such unique properties enable this material to perform as a general platform for rapid and sensitive detection of various biological species through simple fluorescence quenching and recovering. When quantum dot (QD)-labeled HBV (human being disease-related gene hepatitis B virus DNA) and HIV (human being disease-related gene human immunodeficiency virus DNA) molecular beacon probes were mixed with N,S-rGO, QD fluorescence was quenched; when target HBV and HIV DNA were added, QD fluorescence was recovered. By the recovered fluorescence intensity, the target virus DNA detection limits were reduced to 2.4 nM for HBV and 3.0 nM for HIV with detection time of less than 5 min. It must be stressed out that different viruses in the same homogeneous aqueous media could be discriminated and quantified simultaneously through choosing diverse QD probes with different colors. Moreover, even one mismatched target DNA could be distinguished using this method. When altering the molecular beacon loop domain to protein aptamers, this sensing strategy was also able to detect thrombin and IgE in 5 min with detection limits of 0.17 ng mL(-1) and 0.19 ng mL(-1), respectively, which was far more rapid and sensitive than bare GO-based fluorescence detection strategy.

Exfoliated Graphene Oxide/MoO2 Composites as Anode Materials in Lithium-Ion Batteries: An Insight into Intercalation of Li and Conversion Mechanism of MoO2

Exfoliated graphene oxide (EG)/MoO2 composites are synthesized by a simple solid-state graphenothermal reduction method. Graphene oxide (GO) is used as a reducing agent to reduce MoO3 and as a source for EG. The formation of different submicron sized morphologies such as spheres, rods, flowers, etc., of monoclinic MoO2 on EG surfaces is confirmed by complementary characterization techniques. As-synthesized EG/MoO2 composite with a higher weight percentage of EG performed excellently as an anode material in lithium-ion batteries. The galvanostatic cycling studies aided with postcycling cyclic voltammetry and galvanostatic intermittent titrations followed by ex situ structural studies clearly indicate that Li intercalation into MoO2 is transformed into conversion upon aging at low current densities while intercalation mechanism is preferably taking place at higher current rates. The intercalation mechanism is found to be promising for steady-state capacity throughout the cycling because of excess graphene and higher current density even in the operating voltage window of 0.005-3.0 V in which MoO2 undergoes conversion below 0.8 V.

Unique Cobalt Sulfide/Reduced Graphene Oxide Composite as an Anode for Sodium-Ion Batteries with Superior Rate Capability and Long Cycling Stability

Exploitation of high-performance anode materials is essential but challenging to the development of sodium-ion batteries (SIBs). Among all proposed anode materials for SIBs, sulfides have been proved promising candidates due to their unique chemical and physical properties. In this work, a facile solvothermal method to in situ decorate cobalt sulfide (CoS) nanoplates on reduced graphene oxide (rGO) to build CoS@rGO composite is described. When evaluated as anode for SIBs, an impressive high specific capacity (540 mAh g(-1) at 1 A g(-1) ), excellent rate capability (636 mAh g(-1) at 0.1 A g(-1) and 306 mAh g(-1) at 10 A g(-1)), and extraordinarily cycle stability (420 mAh g(-1) at 1 A g(-1) after 1000 cycles) have been demonstrated by CoS@rGO composite for sodium storage. The synergetic effect between the CoS nanoplates and rGO matrix contributes to the enhanced electrochemical performance of the hybrid composite. The results provide a facile approach to fabricate promising anode materials for high-performance SIBs.

Noble-metal-free Co3S4-S/G porous hybrids as an efficient electrocatalyst for oxygen reduction reaction

A high-efficiency noble-metal-free oxygen reduction reaction (ORR) catalyst was obtained based on the pyrolysis of the inexpensive precursor cobalt dithiolene (an S₄-chelate complex) on simultaneously reduced graphene oxide (GO) as a support matrix.

Developing of a new noble-metal-free catalyst to replace Pt-based catalysts of the oxygen reduction reaction (ORR) both in alkaline and acidic conditions is extremely significant for the fuel cell. In this paper, based on the pyrolysis of an inexpensive precursor cobalt dithiolene (a S₄-chelate complex) on simultaneously reduced graphene oxide (GO) as a support matrix, a high-efficiency noble-metal-free hybrid for oxygen reduction reaction (ORR) consisting of Co₃S₄ nanoparticles encapsulated in porous sulfur doped graphene (referred as Co₃S₄–S/G) was fabricated. The catalyst obtained at 800 °C (Co₃S₄–S/G-800) manifests excellent oxygen reduction activity. Of note, the Co₃S₄–S/G-800 hybrids also exhibited prominent ORR activity with high selectivity (mainly 4e^– reaction process) and very low H₂O₂ yield in acidic electrolyte. The optimal Co₃S₄–S/G-800 hybrid displayed much greater tolerance to methanol and higher stability than that of Pt/C. These admirable performances endorse Co₃S₄–S/G-800 electrocatalyst holding great potential for fuel cells. Meanwhile, this work also provides a simple and practical method to fabricate cobalt chalcogenides by using the cost-effective and easily synthesized S₄-chelate complex.

One-pot synthesis of monodispersed porous CoFe2O4 nanospheres on graphene as an efficient electrocatalyst for oxygen reduction and evolution reactions

Monodispersed porous CoFe₂O₄ nanospheres grown on graphene with high ORR/OER activities are fabricated by a simple one-pot solvothermal method.

An enhanced oxygen electrode catalyst by incorporating CoO/SnO2 nanoparticles in crumpled nitrogen-doped graphene in alkaline media

An advanced and highly efficient oxygen electrode catalyst was fabricated by anchoring CoO/SnO₂ nanocrystals on nitrogen-doped graphene.

Two-dimensional nanostructures of non-layered ternary thiospinels and their bifunctional electrocatalytic properties for oxygen reduction and evolution: the case of CuCo2S4nanosheets

CuCo₂S₄nanosheets are synthesizedviaa “bottom-up” route, which show excellent bifunctional electrocatalytic properties toward oxygen reduction and evolution reactions.

Sulfur-mediated synthesis of N-doped carbon supported cobalt catalysts derived from cobalt porphyrin for ethylbenzene oxidation

Nitrogen-doped carbon supported cobalt catalysts are synthesized by a sulfur-mediated heat treatment.

A noble and single source precursor for the synthesis of metal-rich sulphides embedded in an N-doped carbon framework for highly active OER electrocatalysts

Metal-rich sulphide (Co₉S₈ and Ni₃S₂) embedded in N-doped carbon (NC) frameworks were synthesized from novel Tris(ethylenediamine) Metal (ii) Sulfate complex whereas counter sulphate (SO₄²⁻) ion is the source of S. Both the hybrids show superior OER activity compared to commercial RuO₂.

A simple way to fine tune the redox potentials of cobalt ions encapsulated in nitrogen doped graphene molecular catalysts for the oxygen evolution reaction

A simple approach to fine-tuning the redox potential of Co²⁺ ions encapsulated in nitrogen doped graphene (NG) has been proposed. We found that the redox potential determines the oxygen evolution reaction activity of the catalyst.

Porous carbon-coated cobalt sulfide nanocomposites derived from metal organic frameworks (MOFs) as an advanced oxygen reduction electrocatalyst

PC-CoS_1.097 NCs were synthesized from the ZIF-9 and sulfur powder and exhibit excellent performance for ORR in alkaline medium.

Co-supported catalysts on nitrogen and sulfur co-doped vertically-aligned carbon nanotubes for oxygen reduction reaction

A high-performance ORR catalyst of Co supported on nitrogen and sulfur co-doped VACNTs was fabricated by cobalt sputtering and subsequent annealing in decomposition atmosphere of NH₄SCN.

Cobalt sulfide/N,S codoped porous carbon core-shell nanocomposites as superior bifunctional electrocatalysts for oxygen reduction and evolution reactions

Exploring highly-efficient and low-cost bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reactions (OER) in the renewable energy area has gained momentum but still remains a significant challenge. Here we present a simple but efficient method that utilizes ZIF-67 as the precursor and template for the one-step generation of homogeneous dispersed cobalt sulfide/N,S-codoped porous carbon nanocomposites as high-performance electrocatalysts. Due to the favourable molecular-like structural features and uniform dispersed active sites in the precursor, the resulting nanocomposites, possessing a unique core-shell structure, high porosity, homogeneous dispersion of active components together with N and S-doping effects, not only show excellent electrocatalytic activity towards ORR with the high onset potential (around -0.04 V vs.-0.02 V for the benchmark Pt/C catalyst) and four-electron pathway and OER with a small overpotential of 0.47 V for 10 mA cm(-2) current density, but also exhibit superior stability (92%) to the commercial Pt/C catalyst (74%) in ORR and promising OER stability (80%) with good methanol tolerance. Our findings suggest that the transition metal sulfide-porous carbon nanocomposites derived from the one-step simultaneous sulfurization and carbonization of zeolitic imidazolate frameworks are excellent alternative bifunctional electrocatalysts towards ORR and OER in the next generation of energy storage and conversion technologies.

Recent advances in transition-metal dichalcogenide based nanomaterials for water splitting

The desire for sustainable and clean energy future continues to be the concern of the scientific community. Researchers are incessantly targeting the development of scalable and abundant electro- or photo-catalysts for water splitting. Owing to their suitable band-gap and excellent stability, an enormous amount of transition-metal dichalcogenides (TMDs) with hierarchical nanostructures have been extensively explored. Herein, we present an overview of the recent research progresses in the design, characterization and applications of the TMD-based electro- or photo-catalysts for hydrogen and oxygen evolution. Emphasis is given to the layered and pyrite-phase structured TMDs encompassing semiconducting and metallic nanomaterials. Illustrative results and the future prospects are pointed out. This review will provide the readers with insight into the state-of-the-art research progresses in TMD based nanomaterials for water splitting.

Vapor-Phase Atomic Layer Deposition of Co9S8 and Its Application for Supercapacitors

Atomic layer deposition (ALD) of cobalt sulfide (Co9S8) is reported. The deposition process uses bis(N,N'-diisopropylacetamidinato)cobalt(II) and H2S as the reactants and is able to produce high-quality Co9S8 films with an ideal layer-by-layer ALD growth behavior. The Co9S8 films can also be conformally deposited into deep narrow trenches with aspect ratio of 10:1, which demonstrates the high promise of this ALD process for conformally coating Co9S8 on high-aspect-ratio 3D nanostructures. As Co9S8 is a highly promising electrochemical active material for energy devices, we further explore its electrochemical performance by depositing Co9S8 on porous nickel foams for supercapacitor electrodes. Benefited from the merits of ALD for making high-quality uniform thin films, the ALD-prepared electrodes exhibit remarkable electrochemical performance, with high specific capacitance, great rate performance, and long-term cyclibility, which highlights the broad and promising applications of this ALD process for energy-related electrochemical devices, as well as for fabricating complex 3D nanodevices in general.

Formation of a Cu@RhRu core-shell concave nanooctahedron via Ru-assisted extraction of Rh from the Cu matrix and its excellent electrocatalytic activity toward the oxygen evolution reaction

A facile one step route has been developed for the synthesis of trimetallic Cu@RhRu core-shell concave nanooctahedra by co-decomposition of Ru, Rh and Cu precursors. A mechanistic study reveals that nanoparticles with a CuRh alloy core and a Ru shell are initially formed and a subsequent migration of Rh to the shell results in the Cu@RhRu core-shell concave nanooctahedron. The shell exhibits atomically mixed Ru and Rh phases with an fcc atomic structure, although the hcp atomic structure is commonly found for the bulk Ru. We also report an unusually high catalytic activity of the Cu@RhRu octahedral nanocrystals toward the oxygen evolution reaction in alkaline solution.