Polysulfide functionalized reduced graphene oxide for electrocatalytic hydrogen evolution reaction and supercapacitor applications

Functionalized carbon based 2D materials are promising candidates for low cost and environment friendly electrocatalyst for hydrogen evolution reaction (HER) and supercapacitor applications. To overcome the limitations posed by the noble metals and transition metal based composites, we have successfully synthesized metal free polysulfide functionalized reduced graphene oxide (GPS) in a simple chemical route. Structure and morphology of the material are characterized via XRD, FTIR, Raman, TEM, XPS measurements. The material behaves as an efficient HER electrocatalyst in acidic medium as well as energy storage device. It shows an onset potential of 97 mV and overpotential of 254 mV to reach a high current density of 10 mA/cm². DFT calculations are carried out to understand the structural stability and identification of active sites of the material. Boosting catalytic activity via increasing the number of active sites is an elegant approach. In this material we have used the S atoms of polysulfide polymer to facilitate hydrogen adsorption and desorption, thus improving the hydrogen evolution ability. The supecapacitor attains the high specific capacitance 347 F/g at the current density of 1 A/g. The origin of such performances is due to synergistic effect of both the graphene network and the polysulfide functionalizations.     

Highly efficient and durable phosphine reduced iron-doped tungsten oxide/reduced graphene oxide nanocomposites for the hydrogen evolution reaction

The design and development of inexpensive and highly efficient electrocatalysts for hydrogen production from water splitting are highly crucial for green energy and the hydrogen economy. Herein, we report phosphine reduced an iron-doped tungsten oxide nanoplate/reduced graphene oxide nanocomposite (Fe-WOxP/rGO) as an excellent electrocatalyst for the hydrogen evolution reaction. This electrocatalyst was synthesized using a hydrothermal method, followed by reduction with phosphine (PH₃), which was generated from sodium hypophosphite. The catalyst onset potential, Tafel slope, and stability were investigated. Accordingly, Fe-WOxP/rGO exhibited impressively high electrocatalytic activity with a low overpotential of 54.60 mV, which is required to achieve a current density of 10 mAcm^?2. The Tafel slope of 41.99 mV dec^?1and the linear sweep voltammetry curve is almost the same as 2000 cycles and electrolysis under static overpotential (54.60 mV) is remain for more than 24 h in 0.5 M H₂SO₄. The catalytic activity and conductivity of Fe-WOxP/rGO were higher than WO_XP, Fe-WOxP and WOxP/rGO. Such an outstanding performance of the Fe-WOxP/rGO nanocomposite is attributed to the coupled synergic effect between high oxygen vacancies formation on tungsten oxide in the nanoplate-like structure of Fe-WOxP and rGO nanosheet, making it as an excellent electrocatalyst for hydrogen evolution reaction.     

Tungsten-molybdenum oxide nanowires/reduced graphene oxide nanocomposite with enhanced and durable performance for electrocatalytic hydrogen evolution reaction

Hydrogen has attracted huge interest globally as a durable, environmentally safe and renewable fuel. Electrocatalytic hydrogen evolution reaction (HER) is one of the most promising methods for large scale hydrogen production, but the high cost of Pt-based materials which exhibit the highest activity for HER forced researchers to find alternative electro-catalyst. In this study, we report noble metal free a 3D hybrid composite of tungsten-molybdenum oxide and reduced graphene oxide (GO) prepared by a simple one step hydrothermal method for HER. Benefitting from the synergistic effect between tungsten-molybdenum oxide nanowires and reduced graphene oxide, the obtained W-Mo-O/rGO nanocomposite showed excellent electro-catalytic activity for HER with onset potential 50 mV, a Tafel slope of 46 mV decade^?1 and a large cathodic current, while the tungsten-molybdenum oxide nanowires itself is not as efficient HER catalyst. Additionally, W-Mo-O/rGO composite also demonstrated good durability up to 2000 cycles in acidic medium. The enhanced and durable hydrogen evolution reaction activity stemmed from the synergistic effect broadens noble metal free catalysts for HER and provides an insight into the design and synthesis of low-cost and environment friendly catalysts in electrochemical hydrogen production.     

Fabrication of robust CoP/Ni2P/CC composite for efficient hydrogen evolution reaction and the reduction of graphene oxide

Developing the novel catalysts with an excellent performance of hydrogen generation is essential to facilitate the application of hydrogen evolution reaction (HER). Herein, a heterostructured cobalt phosphide/nickel phosphide/carbon cloth (CoP/Ni₂P/CC) composite was fabricated via an interfacial engineering strategy to achieve the modification of CoP nanoleaf on Ni₂P nanosheet skeleton supported by carbon cloth. By virtue of the unique heterostructure, abundant exposing active sites and the synergistic coupling effect of CoP and Ni₂P nanoparticles, the elaborated CoP/Ni₂P/CC composite exhibits a robust catalytic property. Among fabricated composites, the optimal CoP/Ni₂P/CC-4 catalyst behaves an excellent HER performance at a wide pH range (overpotentials of 67, 71 and 95 mV to afford 10 mA cm^?2 in 0.5 M H₂SO₄, 1 M KOH and 1 M PBS, respectively). The HER current density of this composite shows a negligible degradation after continuous test for 24 h. Charmingly, the HER process of this catalyst was innovatively applied to reduce graphene oxide, and thus exploiting the fabrication route of reduced graphene oxide (rGO). We are sure that this work will provide a firm guideline for the exploitation of pH-universal HER catalysts and the exploration of HER application.     

Bimetallic AuPt alloy nanodendrites/reduced graphene oxide: One-pot ionic liquid-assisted synthesis and excellent electrocatalysis towards hydrogen evolution and methanol oxidation reactions

Herein, reduced graphene oxide supported well-dispersed bimetallic AuPt alloy nanodendrites (AuPt ANDs/rGO) were fabricated by a one-pot coreduction approach using ionic liquid (1-aminopropyl-3-methylimidazolium bromide, [APMIm]Br) as the stabilizer and capping agent. There is no any other polymer or seed involved. Characterized measurements include transmission electron microscopy (TEM), high angle annular dark-field scanning TEM (HAADF-STEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The typical samples displayed excellent electrocatalytic activity and durability towards hydrogen evolution reaction (HER) and methanol oxidation reaction (MOR) in contrast with Pt nanocrystals/rGO and commercial Pt/C (50%) catalysts, which make it promising for practical catalysis in energy conversion and storage.     

Near-infrared responsive reduced graphene oxide supported CuS: Enhanced electrocatalytic hydrogen evolution performance

Various reduced graphene oxide supported CuS (RGO-CuS) composites were obtained via a hydrothermal way in this work. The as-obtained RGO-CuS with a low bandgap of ~1.24 eV showed significant light absorption in near-infrared (NIR) region. In acidic media, the optimized 0.1RGO-CuS needed an overpotential of 179 mV (at 10 mA cm⁻²) to trigger the hydrogen evolution reaction under NIR light. And the lowest Tafel slope of 0.1RGO-CuS (61 mV dec⁻¹), suggesting that the photoelectrocatalytic hydrogen evolution was performed under the Volmer-Heyrovsky mechanism. The 0.1RGO-CuS displayed a good durability after 5000 cycles in acid. According to the results of EIS measurement, both NIR irradiation and RGO modification could effectively lower charge transfer resistance and improve charge transport rate of the photoelectrochemical process. The introduction of RGO could improve the electron and hole separation ability of the photoelectrochemical process, which resulted in an enhanced photoelectrocatalytic HER performance.     

Synergistic coupling of CoFe-layered double hydroxide nanosheet arrays with reduced graphene oxide modified Ni foam for highly efficient oxygen evolution reaction and hydrogen evolution reaction

Novel CoFe-LDH (layered double hydroxide) nanosheet arrays in situ grown on rGO (reduced graphene oxide) uniformly modified Ni foam were synthesized by a citric acid-assisted aqueous phase coprecipitation strategy. Systematic characterizations indicates that the series of Co_xFe₁-LDH/rGO/NF (x = 4, 3, 2) all show Co_xFe₁-LDH nanosheets (150–180 × 15 nm) grown vertically on the surface of rGO/NF. Especially, the Co₃Fe₁-LDH/rGO/NF exhibits the best performance with overpotentials of 250 and 110 mV at 10 mA cm^?2 in 1 M KOH for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. When it is used as cathode and anode simultaneously for overall water splitting, they require 1.65 and 1.84 V at 10 and 100 mA cm^?2, respectively. Excellent performance of Co₃Fe₁-LDH/rGO/NF is due to the nanosheet arrays structure with open channels, synergistic coupling between Co₃Fe₁-LDH and rGO enhancing electrical conductivity, and in-situ growth of Co₃Fe₁-LDH on rGO/NF enhancing stability.     

Reduced graphene oxide-supported ruthenium nanocatalysts for highly efficient electrocatalytic hydrogen evolution reaction

Hydrogen energy has received great attention because of its advantages such as large energy density and not producing carbon dioxide, and it is currently considered to be one of the most valuable green energy sources. Therefore, the development of efficiently hydrogen production is of great importance. Hydrogen production from water electrolysis has large application prospects due to its cleanliness and no pollution. However, how to prepare an efficient, stable and low-cost electrocatalyst for this process is still challenging. Here, we develop a reduced graphene oxide-supported ruthenium (Ru) nanoparticle electrocatalyst synthesized by a simple method. The ruthenium precursors are encapsulated and isolated with N,N-dimethylformamide (DMF) (Ru³⁺-DMF), which effectively inhibits the further agglomeration growth of ruthenium. After Ru³⁺-DMF being loaded on graphene oxide, Ru is supported on reduced graphene oxide (Ru/rGO) by the liquid phase chemical reduction method and the remaining organic solvent could be removed by calcination to form a well-dispersed Ru-based electrocatalyst. Ru/rGO shows excellent electrocatalytic activity and long-term stability for hydrogen evolution reaction (HER). In a solution of 1.0 M KOH, the overpotential of 3.0 wt%Ru/rGO for the HER at 100 mA cm^?2 is only 111.7 mV, and the Tafel slope is 31.5 mV dec^?1. It exhibits better HER performance compared to commercial Pt/C and other Ru/rGO catalysts with different Ru loadings. The work could give a new strategy for the synthesis of efficient electrocatalysts.     

Hexagonal CoSe2 nanosheets stabilized by nitrogen-doped reduced graphene oxide for efficient hydrogen evolution reaction

CoSe₂ is considered as a promising candidate among non-noble metal electrocatalysts for the hydrogen evolution reaction (HER) due to its intrinsic metallicity and low Gibbs free energy for hydrogen adsorption. Recently, the hexagonal CoSe₂ becoming increasingly popular owing to its chemically favorable basal plane, which provides more active sites, but remains limited by the poor stability. In this study, we design a small-molecule-amine-assisted hydrothermal method to in situ anchor the hexagonal CoSe₂ nanosheets (NSs) on nitrogen-doped reduced graphene oxides (RGO) as an advanced electrode material for HER. Due to the existence of abundant functional groups and high specific surface area of RGO, the hexagonal CoSe₂ NSs could be stably formed on RGO. As a result, only a small overpotential of 172 mV is needed for the optimized sample to drive a current density of 10 mA cm⁻² in 0.5 M H₂SO₄ and the Tafel slope is 35.2 mV dec⁻¹, which is comparable with the state-of-the-art Pt catalyst (32.3 mV dec⁻¹). Therefore, the facile and low-cost method for synthesizing hexagonal TMDs with robust electrical and chemical coupling developed in this work is promising in promoting the large-scale application of non-precious electrocatalysts.     

A novel co-electrodeposited Co/MoSe2/reduced graphene oxide nanocomposite as electrocatalyst for hydrogen evolution

In this study, a simple and fast electrochemical method was employed to synthesis molybdenum diselenide thin film. The morphology, structure and chemical composition of the nanocomposites were investigated by field emission scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The progressive effects of transition metal ions including Ni, Cu, and Co were surveyed on the hydrogen evolution activity of MoSe₂ thin films. Co/MoSe₂ nanocomposite thin films has significant electrocatalytic activity as compared to other samples, In order to achieve higher performance, preparing Co/MoSe₂/RGO nanocomposite thin film, two strategies including layer by layer electrodeposition and co-electrodeposition has been employed. The presence of reduced graphene oxide leading to the onset potential shifts to more positive values and increase the current density. Also, results showed that the Co/MoSe₂/RGO nanocomposite prepared by co-electrodeposition exhibits the best electrochemical hydrogen evolution at onset potential of −0.18 with an overpotential of −0.45 V.     

Co(OH)2 nanoparticles deposited on reduced graphene oxide nanoflake as a suitable electrode material for supercapacitor and oxygen evolution reaction in alkaline media

In this work, cobalt hydroxide nanoparticles are simply synthesized (size is about 50 nm) and deposited on the reduced graphene oxide nanoflake by the hydrothermal method. Then, the ability of glassy carbon electrode modified with this low-cost nanocomposite is examined as a supercapacitor and oxygen evolution electrocatalysts in 2.0 mol L^?1 KOH by a three-electrode system. The modified electrode as a pseudocapacitor with potential windows of 0.35 V, exhibits a powerful specific capacitance (235.20 F g^?1 at 0.1 A g^?1 current density), energy density, stability (about 90% of the initial capacitance value maintain after 2000 cycles at 1.0 A g^?1) and fast charge/discharge ability. Furthermore, the modified electrode displays a good electrocatalytic activity for oxygen evolution reaction with a current density of 10.0 mA cm^?2 at 1.647 V, small Tafel slope of 56.5 mV dec^?1, good onset potential of 1.521 V vs. RHE and suitable durability.     

Flower-like CoS2/MoS2 nanocomposite with enhanced electrocatalytic activity for hydrogen evolution reaction

Molybdenum sulfide (MoS₂) has received tremendous attracts for its promising performance in the aspects of hydrogen evolution reaction (HER). To improve the HER activity of MoS₂, we designed a flower-shaped CoS₂/MoS₂ nanocomposite with enhanced HER electroactivity compared with MoS₂ nanosheets by a simple one-step hydrothermal method. The facile approach brings about distinct transformation of the morphology from nanosheets to nanoflower structures. The introduction of Co element into MoS₂ results in the larger active surface area, more edge-terminated structures, and higher conductivity of the CoS₂/MoS₂ nanocomposite, which are good for improving the HER electroactivity. In brief, the optimized catalyst exhibits the low overpotential of 154 mV at 10 mA cm^?2, small Tafel slope of 61 mV dec^?1, and excellent stability in acidic solution.     

CoS2 with carbon shell for efficient hydrogen evolution reaction

In this study, we demonstrated the active electrocatalysts of CoS₂ coated by N-doped carbon microspheres, CoS₂@NHCs-x (x = 600, 700, 800, and 900; x is pyrolysis temperature). Results show that the obtained electrocatalyst has good catalytic activity and cyclic stability for the reaction of hydrogen evolution (HER) when the pyrolysis temperature is 800 °C. At a current density of 10 mA cm⁻², the overpotential of CoS₂@NHCs-800 was only 98 mV in 0.5 M H₂SO₄, and 118 mV in 1 M KOH, respectively. In addition, CoS₂@NHCs-800 also revealed excellent electrochemical stability, with only 32.7% performance degradation after continuous reaction in 0.5 M H₂SO₄ for 20 h, and the later current density almost no longer deceased with time as the reaction process stabilized. The excellent HER catalytic performance of CoS₂@NHCs-800 is mainly attributed to the rich active sites of CoS₂, the unique porous core-shell structure, and the enhanced conductivity of the carbon carrier caused by N and S co-doping. This work opens up an opportunity for advanced CoS₂-based electrocatalysts for HER.     

Cobalt-molybdenum disulfide supported on nitrogen-doped graphene towards an efficient hydrogen evolution reaction

Herein, the cobalt-molybdenum bimetallic sulfide catalysts supported on nitrogen-doped graphene (CoMoS₂/NGO) were facilely synthesized by a one-pot hydrothermal route. These composites exhibit various special nanostructures, rich in abundant edge site exposure and defects, which play an important role in providing active sites for catalyzing hydrogen evolution reaction (HER). When hydrogen peroxide (H₂O₂) was used as an additive in hydrothermal process, the as-fabricated composite exhibited more efficiency towards HER, showing as low onset overpotential (η_on) as −54 mV in 0.5 M H₂SO₄. Typical H₂O₂-assisted composite realized a remarkable cathodic current density of 30 mA cm⁻² at an overpotential η = −137 mV and it possessed a small Tafel slope of 34.13 mV dec⁻¹. Moreover, it exhibited an excellent cycling stability and superior electronic exchange rate. The results prove that CoMoS₂/NGO catalysts have great potential for electrochemical HER.     

Simple routes for the improvement of hydrogen evolution activity of Ni-Mo catalysts: From sol-gel derived powder catalysts to graphene supported co-electrodeposits

Development of noble metal-free catalysts for hydrogen production is one of the cores of the sustainable energy economy. Here we present results of systematic analysis of catalytic activity of Ni-Mo alloy powders in alkaline media towards hydrogen evolution reaction (HER). Catalysts were prepared in a wide concentration range (from Ni_0.2Mo_0.8 to Ni_0.9Mo_0.1), and resulted with a volcano shaped activity-composition relationship, with maximum catalytic activity achieved for the powder with nominal composition Ni_0.6Mo_0.4. Improved HER activity is ascribed to reduced deactivation by hydride formation and adequate hydrogen-surface energetics on Ni-Mo catalysts. In the second part, we demonstrate a novel method for electrochemical formation of NiMo@rGO composites. Prepared composite electrodes show improved electrocatalytic activity compared to both pure Ni and Ni@rGO electrodes. Activity was observed to depend on the deposition time and is contributed by two factors: (i) formation of Ni-Mo system and (ii) formation of an interfacial region with rGO. We expect that the provided activity-composition relationship in combination with novel electrochemical NiMo-rGO composite formation procedure will provide a route for the development of new highly efficient noble metal-free HER electrocatalysts.     

MoS2 supported CoS2 on carbon cloth as a high-performance electrode for hydrogen evolution reaction

Here we report a strategy to prepare electrocatalysts for hydrogen generation based on MoS₂ grown on highly conductive CoS₂ decorated carbon cloth (MoS₂/CoS₂/CC) through a two-step method. The rational design of sandwich structure of MoS₂/CoS₂/CC electrode was significant for homogenously dispersing MoS₂ on the carbon substrate, tuning the properties of the MoS₂ and improving long-term durability. Benefiting from the sandwich structure with highly exposed edges, fast electron transport and additional active sites brought by interfacial sulfur of MoSCo, the resulting MoS₂/CoS₂/CC electrode exhibited superior HER activity and excellent stability in acid solution including an overpotential of 118 and 159 mV at current density of 10 and 100 mA/cm², respectively, a Tafel slope of 37 mV per decade and excellent cycling stability in acid solution. The high catalytic performance and simplicity of the preparation method suggest applicability of the MoS₂/CoS₂/CC electrode for large-scale practical applications.     

CoS2/MoS2 decorated with nitrogen doped reduced graphene oxide and multiwalled carbon nanotube 3D hybrid as efficient electrocatalyst for hydrogen evolution reaction

Designing an efficient and stable electrocatalyst made of earth abundant elements to take over expensive noble metal based for Hydrogen Evolution Reaction (HER) have been focused. Cobalt disulfide-molybdenum disulfide nanocomposite supported by nitrogen doped reduced graphene oxide and multiwalled carbon nanotubes (CoS₂/MoS₂@NrGO-MWCNT) is reported as an efficient electrocatalyst for HER. CoS₂/MoS₂@N-rGO-MWCNT and ternary hybrids composed of CoS₂, MoS₂ and N-rGO/MWCNT have been investigated. The catalysts were prepared by facile hydrothermal method, and the optimal doping ratio referred to date cobalt to molybdenum as 2:1 was chosen. It is found that co-existence of CoS₂, MoS₂ brings abundant active sites and incorporation of MWCNT offered stability. Good dispersion of CoS₂ nanoparticles on graphene and MoS₂ sheets is observed. Additionally nitrogen doping on rGO sheets has been carried out to boost up the electronegativity of the catalyst as a support to enhance the catalytic activity of CoS₂/MoS₂ for refine structure and better electrical conductance. Precisely, CoS2/MoS2@N-rGO-MWCNT exhibited smaller tafel slope 73 mV dec^?1 at overpotential 281 mV for current density 10 mA cm^?2 and the substantial stability of 14 h is recorded in 0.5 M H₂SO₄ medium, results suggest that catalyst is viable alternate for HER.     

Enhanced oxygen evolution reaction activity of NiFe layered double hydroxide on nickel foam- reduced graphene oxide interfaces

Herein, we prepared a novel nickel iron-layered double hydroxide/reduced graphene oxide/nickel foam (NiFe-LDH/RGO/NF) electrodes by two step electrodeposition processes for oxygen evolution reaction (OER). The modification of NF by RGO increased the interface conductivity and electrochemical active surface areas (ECSA) of the electrode. The NiFe-LDH/RGO/NF electrode has shown higher catalytic activity with a lower overpotential of 150 mV at the current density of 10 mA cm⁻². The NiFe-LDH/RGO/NF electrode has also shown a small Tafel slope of 35 mV per decade due to the synergy effect between the larger ECSA and the conductive RGO interface. Furthermore, the electrodes exhibits almost 10 h stability under a general current density of 10 mA cm⁻².     

Thermally reduced graphite oxide/carbon nanotubes supported molybdenum disulfide as catalysts for hydrogen evolution reaction

We report an efficient molybdenum disulfide (MoS₂) supported by thermally reduced graphite oxide and carbon nanotubes (TRGO-CNT) for hydrogen evolution reaction. The TRGO-CNT-MoS₂ composite is successfully prepared by a simple sonication process, exhibiting excellent catalytic activity of the hydrogen evolution reaction (HER) with a low overpotential of −0.14 V, which is much lower compared to that of MoS₂, CNT-MoS₂ and TRGO-MoS₂, respectively. TRGO-CNT-MoS₂ also exhibits high stability even after 1000 cycles and strong durability after 48 h. The high HER performance of TRGO-CNT-MoS₂ attributes to a synergic effect of thermal reduced GO and CNT that support MoS₂ due to significant decrease of electrochemical impedance and reliable supporting material for the efficient HER.     

Enhanced photocatalytic activity and stability of the reduced graphene oxide loaded potassium niobate microspheres for hydrogen production from water reduction

A facile approach to synthesize reduced graphene oxide (RGO) loaded potassium niobate microspheres was reported. The composition, microstructure and electron-transfer properties of the obtained product were characterized. Compared to pure potassium niobate microspheres and commercial P25 TiO₂, the as-prepared potassium niobate microspheres/RGO composite showed much higher photocatalytic activity for generating hydrogen under UV irradiation. It was ascribed to the enhanced separation efficiency of electron/hole pairs as testified by electrochemical impedance spectrum and fluorescence spectrum. Importantly, the composite photocatalyst was stable and easy to recycle, and the amount of hydrogen evolution did not decrease after six recycles. The results are potentially applicable to a range of semiconductors useful in water reduction as well as other areas of heterogeneous photocatalysis.