Ultrafine Co6Mo6C nanocrystals on reduced graphene oxide as efficient and highly stable electrocatalyst for hydrogen generation

Up to now, it is still a great challenge to develop active, durable and low-cost non-precious metal catalysts toward hydrogen evolution reaction (HER). In this paper, we synthesized ultrafine Co₆Mo₆C nanocrystals on reduced graphene oxide (RGO) support (Co₆Mo₆C/RGO). The Co₆Mo₆C/RGO shows Pt-like HER performance, which exhibits almost zero onset overpotential, and very small overpotential of 64 mV at 10 mA cm^?2. In addition, the Co₆Mo₆C/RGO has a very small Tafel slope of 44 mV dec^?1 and a high exchange current density of 0.402 mA cm^?2, suggesting fast reaction kinetics. Furthermore, the Co₆Mo₆C/RGO demonstrates superior durability in acid electrolyte. The distinguished HER performance makes Co₆Mo₆C/RGO the promising candidate as non-precious metal catalyst for HER in acid electrolyte.     

Three-dimensional graphene promoted by palladium nanoparticles,an efficient electrocatalyst for energy production and storage

Three-dimensional (3D) graphene was easily obtained by a simple hydrothermal method from two-dimensional (2D) graphene to create the interspace sites and active surface area. So, the fabrication of the 3D-graphene nanocomposite is promising for advanced energy production and storage application. The structure of the 3D-graphene nanocomposite was characterized by various techniques. Then, 3D-graphene was decorated with Pd nanoparticles. Morphological characterization shows the porous structure of 3D-Pd/rGO, so it has a high electroactive surface area. The function of the electrocatalyst toward the supercapacitor, hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) were investigated. The obtained results as a supercapacitor displayed that the supercapacitor on 3D-Pd/rGO has a high specific capacitance of 582.0 F g^?1, the high energy density of 180 (W h Kg^?1), high power density of 3750 (W Kg^?1), long potential window of 1.00 V and long life. The electrocatalyst shows the small onset potential of ?0.08 V (vs. RHE), Tafel slope of 29 mV dec^?1 and high durability. Also, in the electroanalytical application of the nanocompound as an electrocatalyst for ORR shows an excellent onset potential of 0.90 V (vs. RHE), slow drop in the current density (34% in the presence of MeOH) and the reduction process via a four electrons pathway.     

Preparation of carbon nanotube and graphene doped polyphenylene sulfide flexible film electrodes and the electrodeposition of Cu2O nanocrystals for hydrogen-generation

Through annealing and electrochemical reduction methods, we successfully fabricates reduced graphene oxide layer (RGOL) modified carbon nanotube and reduced graphene oxide (CNT + RGO) doped polyphenylene sulfide (PPS) flexible thin film electrodes. These composite structure films can not only overcome the brittle nature of PPS, but also make good use of the thermal stability of PPS. Furthermore, carbon nanotube and reduced graphene oxide enhance the electrical conductivity of the composite films. Truncated octahedral and cuboctahedral Cu₂O nanocrystals are synthesized on RGOL modified CNT + RGO doped PPS (RGOL@PPS/CNT + RGO) composite film by a facile electrodeposition method without using any surfactants or external heating. RGOL on the PPS/CNT + RGO substrate facilitates the formation of Cu₂O morphology. The obtained Cu₂O composite film shows a superior ability for the hydrogen evolution reaction (HER) compared with other Cu₂O electrocatalysts. The Cu₂O with a smaller loading less than 0.04 mg cm^?2 on the composite film exhibits excellent HER activities with a low onset potential of 0.05 V and large current densities. The results of the HER performance indicates that the RGOL@PPS/CNT + RGO composite film has a potential application in flexible hydrogen-producing devices.     

Efficient and stable HER electrocatalyst using Pt-nanoparticles@poly(3,4–ethylene dioxythiophene) modified sulfonated graphene nanocomposite

The green production of hydrogen by electrochemical water splitting has been recently paid attention. It is more focused to research about the preparation of efficient electrocatalysts, which catalyze hydrogen evolution reaction (HER) in acidic media at low overpotential. Platinum is known as an ideal option, but its rarity and high-cost limit its application in practical industrial plants. Hence, minimizing the level of it can be a solution. It can be achieved by the decoration of platinum nanoparticles (PtNPs) on the different composites such as poly(3,4–ethylene dioxythiophene, PEDOT) and sulfonated graphene nanosheets (SG) in this work. Accordingly, the successful preparation and HER electrocatalytic manner of this nanocomposite were main objectives in the present report. The related characterization and performance were monitored using various analytical and electrochemical techniques. The low charge transfer resistance (around 50 Ω), low overpotential (?0.040 V vs. RHE), and stable manner (until 500 cycles) resulted in this HER electrocatalyst. It was controlled by Tafel reaction with electrochemical adsorption-desorption because of kinetic factors including Tafel slope (28.4 mV dec^?1), charge-transfer coefficient of 2.0, and exchange current of 7.27 mA cm^?2.     

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.     

Glassy carbon electrode modified by Pd–Cu bimetallic nano-dendrites film decorated on the reduced graphene oxide using galvanic replacement as a low-cost anode in methanol fuel cells

Glassy carbon electrode (GCE) modified by reduced graphene oxide Cu–Pd nano-dendrimer (Pd-CuNDs-RGO/GCE) was prepared using electro-deposition and spontaneous displacement methods. Graphene oxide was put on the surface of GCE by drop-casting, then a thin film of reduced graphene oxide (RGO) was formed by electro-reduction at ?0.9 V. The copper nano-dendrimers (CuNDs) were electro-plated on RGO/GCE surface. Finally, Pd-CuNDs-RGO/GCE was prepared by the spontaneous replacement of CuNDs with palladium nanoparticles (PdNPs) in a dilute solution of palladium. The electrode surface was characterized using field-emission scanning electron microscopy (FESEM), X-ray energy diffraction (EDX) spectroscopy, and electrochemical techniques. The electrochemical behavior of the modified electrode in the oxidation of alkaline solution of methanol was investigated. The experimental conditions affecting the performance of the modified electrode in the methanol oxidation were studied and optimized. Finally, the proposed electrode has the onset potential of ?0.5 V and the ratio of i_f/i_b equal to 2.2, which confirms the high catalytic activity. The electrode has appropriate stability and shows about 86% of initial activity after 100 times testing.     

Enhanced photoelectrochemical water splitting in hierarchical porous ZnO/Reduced graphene oxide nanocomposite synthesized by sol-gel method

Today the utilization of solar energy to split water and its conversion to hydrogen and oxygen has been considered as a powerful way to solve the environmental crisis. Hierarchical porous nanostructured ZnO and ZnO/reduced graphene oxide (rGO) composite photoanodes are synthesized by innovated sol-gel method using triethylenetetramine (TETA) as a stabilizer. The hierarchical porous ZnO structure containing large agglomerates each consisting of tiny nanoparticles are formed. The X-ray diffraction analysis and Raman spectroscopy confirm the in-situ reduction of graphene oxide sheets during synthesis and formation of ZnO/rGO nanocomposite. Although the band gap and transmittance of the porous nanocomposites do not dramatically change by rGO addition, the main photoluminescence peak quenches entirely showing prolonging exciton lifetime. The ZnO/rGO porous structure achieved remarkably improved current density (1.02 mA cm^?2 at 1.5 V vs. Ag/AgCl) in 1 wt% rGO, up to 12 times higher compared to the bare ZnO (0.09 mA cm^?2 at 1.5 V vs. Ag/AgCl), which attributes to positive role of ZnO hierarchical porous structure and rGO electron separation/transportation. These findings provide new insights into the broad applicability of this methodology for promising future semiconductor/graphene composite in the field of photoelectrochemical water splitting.     

Synthesis and investigation of a high-activity catalyst: Au nanoparticles modified metalic Ti microrods for NaBH4 electrooxidation

A novel catalyst electrode of Au nanoparticles modified Ti microrods is synthesized through a route of hydrothermal etching and electrodeposition. As substrate, the metallic Ti microrods are in-situ etched from the Ti plate using hydrochloric acid as an etching reagent. After that, Au is prepared on the metallic Ti microrods in a form of nanoparticle by electrodeposition. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) are conducted to investigate the structure and phase composition of the Au nanoparticles modified Ti microrods (Au/Ti MRs) electrode. Besides, the electrocatalytic property of the Au/Ti MRs electrode for NaBH₄ oxidation is explored through chronoamperometry (CA) and cyclic voltammetry (CV). In alkaline solution, the Au/Ti MRs electrode displays excellent electrocatalytic property and good stability. At 0 V, there is a current density of 12.12 mA cm^?2 on the as-prepared electrode in 2 mol L^?1 NaOH and 0.1 mol L^?1 NaBH₄ that is twice as the current density on Au nanoparticles modified Ti plate electrode demonstrating huge potential for application in direct borohydride fuel cell.     

Preparation and evaluation of a new hybrid support based on exfoliation of graphite by ball milling for Ni nanoparticles in hydrogen evolution reaction

A mixture of graphene/graphite as new support was prepared by ball milling procedure and was used for Nickel nanoparticles supported that was employed as a cathode catalyst for hydrogen evolution reaction (HER) in the KOH solution. The structure and electrocatalytic activity of electrocatalyst were investigated by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, and electrochemical techniques. The results indicated that by raising the time of the ball milling from 0 to 270 min, the HER activity of electrocatalyst first increased, and then decreased according to the increase of active sites and then agglomeration of Ni nanoparticles. Ni nanoparticles supported on the mixture with 180 min ball milling exhibited the highest HER activity with a low overpotential (205 mV at 10 mA cm^?2), Tafel slope of 84 mV dec^?1, and remarkable durability.     

One-step synthesis of the 3D flower-like heterostructure MoS2/CuS nanohybrid for electrocatalytic hydrogen evolution

In this work, a facile one-step hydrothermal method was developed to fabricate three types different of nanomaterials: the two-dimension (2D) of MoS₂ nanosheets; 3D spherical CuS nanoparticles; and 3D flower-like heterostructure of MoS₂/CuS nanohybrid, respectively. The as-synthesized MoS₂, CuS and MoS₂/CuS were characterized by transmission electron microscopy (TEM), field emission scanning electron microscopy (SEM) and X-ray diffraction (XRD) etc. The morphology of the MoS₂/CuS nanohybrid is different from the MoS₂ nanosheets and CuS nanoparticles. The hydrogen evolution reaction (HER) activity of MoS₂ nanosheets, CuS nanoparticles and MoS₂/CuS nanohybrid, were investigated by the Linear Sweep Voltammetry (LSV) and Tafel slope. The HER activity of MoS₂/CuS nanohybrid is better than those of MoS₂ nanosheets and CuS nanoparticles, which can be attributed to the good electron-transport ability of CuS and the strong reduction ability of hydrogen ions by MoS₂. Thus, MoS₂/CuS nanohybrid exhibited excellent activity for HER with a small onset potential of 0.15 V, a low Tafel slope of 63 mV dec^?1, and relatively good stability. However, the MoS₂ nanosheets and CuS nanoparticles respectively shows a bigger onset potential of 0.25 V and 0.35 V, a higher Tafel slope of 165 and 185 mV dec^?1. This 3D flower-like heterostructure of MoS₂/CuS nanohybrid catalyst exhibits great potential for renewable energy applications.     

Three-dimensional structure of WS2/graphene/Ni as a binder-free electrocatalytic electrode for highly effective and stable hydrogen evolution reaction

Tungsten disulfide (WS₂) has attracted much attention as the promising electrocatalyst for hydrogen evolution reaction (HER). Herein, the three-dimensional (3D) structure electrode composed of WS₂ and graphene/Ni foam has been demonstrated as the binder-free electrode for highly effective and stable HER. The overpotential of 3D WS₂/graphene/Ni is 87 mV at 10 mA cm^?2, and the current density is 119.1 mA cm^?2 at 250 mV overpotential, indicating very high HER activity. Moreover, the current density of 3D WS₂/graphene/Ni at 250 mV only decreases from 119.1 to 110.1 mA cm^?2 even after 3000 cycles, indicating a good stability. The high HER performance of 3D WS₂/graphene/Ni binder-free electrode is superior than mostly previously reported WS₂-based catalysts, which is attributed to the unique graphene-based porous and conductive 3D structure, the high loading of WS₂ catalysts and the robust contact between WS₂ and 3D graphene/Ni backbones. This work is expected to be beneficial to the fundamental understanding of both the electrocatalytic mechanisms and, more significantly, the potential applications in hydrogen economy for WS₂.     

Benign synthesis of robust nickel thin films as stretchable electrodes for electrochemical hydrogen evolution reaction

In this paper, we fabricate the nickel thin film on copper and acrylonitrile-butadiene-styrene (ABS) polymer substrate by means of electroless deposition method and compare their hydrogen evolution reactions (HER). Ni deposited on both specimens reveals the preferred crystalline orientation of (111) plane and petal like topography. The HER activity and durability of the Ni electrode is investigated through linear sweep voltammetry and Tafel polarization studies. We measure the current density value of the first cycle at 350 mV vs. RHE and found that nickel deposited on the ABS polymer and a copper substrate exhibits the cathode current density of 29 and 4 mA cm^?2 respectively. After 1000 cycles, 61% of HER activity has been retained in nickel deposited on the ABS polymer whereas nickel plated on the copper strip retains only 20% of HER activity. A small Tafel slope of 82 and 144 mV dec^?1 is observed for nickel deposited on the ABS polymer and copper samples respectively.     

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.     

Vertically-heterostructured TiO2-Ag-rGO ternary nanocomposite constructed with {001} facetted TiO2 nanosheets for enhanced Pt-free hydrogen production

TiO₂ nanosheets with high ratio of {001} facets were coupled with reduced graphene oxide (rGO) nanosheets through the link of silver (Ag) nanoparticles, forming a novel ternary nanocomposite photocatalyst with a vertical heterostructure, TiO₂-Ag-rGO. The vertical anchoring of TiO₂-Ag nanosheets between rGO sheets was confirmed by transmission electron microscopy (TEM), Raman spectroscopy, energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Due to excellent separation of electron-hole pairs in the TiO₂ nanosheets, enhanced electron transfer to rGO via Ag nanoparticles, the TiO₂-Ag-rGO nanocomposite exhibited an outstanding performance in photocatalytic hydrogen production, with a hydrogen production rate of 593.56 μmol g^?1 h^?1. This study provides new insights to the development of Pt-free photocatalysts for hydrogen production.     

Porous CoS2 nanostructures based on ZIF-9 supported on reduced graphene oxide: Favourable electrocatalysis for hydrogen evolution reaction

The research and developments of porous, highly active non-noble metal cathode materials are the current hot spots. In our work, ZIF-9 (Zeolitic imidazolate framework-9) as a cobalt source provide porous structure, we have sulfurized the ZIF-9 into CoS₂ by a simple hydrothermal method. Ultimately, the porous CoS₂/RGO cathode material was obtained. Through a series of characterization analyses (powder X-ray diffraction, X-ray photoelectron spectroscopy), it is confirmed that the CoS₂/RGO composite was successfully formed. Furthermore, electrochemical tests demonstrated that the pursued catalyst exhibited remarkable hydrogen evolution reaction (HER) activities with a favorable overpotential (only 180 mV for 10 mA cm^?2 vs reversible hydrogen electrode), a low Tafel slopes (75 mV decade^?1) and high stability in acidic condition (more than 18 h).     

CoSe2 nanoparticles grown on carbon nanofibers derived from bacterial cellulose as an efficient electrocatalyst for hydrogen evolution reaction

Exploration of high-efficiency and inexpensive Pt-free electrochemical catalysts for hydrogen evolution reaction (HER) is highly significative for carbon dioxide free energy conversion systems. In this work, we described the development of CoSe₂ nanoparticles grown on the carbon nanofibers (CNFs) derived from bacterial cellulose (CNFs/CoSe₂) through a facile one-step hydrothermal preparation, which not only showed a three-dimensional (3D) porous network structure, but also possessed large surface area. This rationally designed architecture realizes the uniform distribution of CoSe₂ nanoparticles to provide with fully exposed active edges and the unique conductive interwoven carbon nanofibers facilitates the charge transportation in HER process, thus leading to remarkable HER activity. As expected, the CNFs/CoSe₂ shows a low onset overpotential of ?85 mV, low overpotential (η₁₀ = 119 mV) for reaching a current density of ?10 mA cm^?2 and smaller Tafel slope of 54 mV dec^?1 as well as good cycling stability in acidic electrolyte.     

Monodisperse Pd nanoparticles assembled on reduced graphene oxide-Fe3O4 nanocomposites as electrocatalysts for borohydride fuel cells

5 nm palladium nanoparticles (Pd NPs) are synthesized and assembled on reduced graphene oxide-iron oxide nanocomposite (rGO-Fe₃O₄) to be used in oxygen reduction reaction (ORR) and borohydride oxidation reaction (BOR) studies in alkaline media. The structure and morphology of the resulting Pd/rGO-Fe₃O₄ hybrid material are evaluated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS) analyses. The electrochemical behavior of Pd/rGO-Fe₃O₄ hybrid material for the ORR and BOR is investigated by voltammetry with rotating disk and rotating ring disk electrode and electrochemical impedance spectroscopy, enabling evaluation of the number of exchanged electrons, Tafel slope, exchange current density and activation energy. The results reveal that ORR at Pd/rGO-Fe₃O₄ proceeds as a 2-electron process with Tafel slope of 0.133 V dec^?1, while BOR proceeds as a 5.6-electron process with Tafel slope of 0.350 V dec^?1 and exchange current density of 1.38 mA cm^?2. The BOR activation energy was found to be 12.4 kJ mol^?1. Overall, this study demonstrates the good efficiency of Pd/rGO-Fe₃O₄ hybrid material for BOR.     

Nanoscale engineering MoP/Fe2P/RGO toward efficient electrocatalyst for hydrogen evolution reaction

The preparation of hydrogen evolution reaction (HER) electrocatalyst with high catalytic performance is a huge challenge. In this work, we develop a MoP/Fe₂P/RGO composite as a electrocatalyst for HER. The MoP/Fe₂P/RGO exhibits excellent electrocatalytic performance with a Tafel slope and an onset overpotential of 51 mV/dec and 105 mV, respectively. To drive 10 mA/cm², it only requires a small over-potential of 156 mV. The high electrocatalytic HER activity is mainly due to the synergistic effect of MoP and Fe₂P. In addition, the introduction of RGO not only prevents particle aggregation and coalescence during high temperature phosphating, but also improves the conductivity of the catalyst.     

Development of a ternerry condunting composite (PPy/Au/CNT@Fe3O4) immobilized FRT/GOD bioanode for glucose/oxygen biofuel cell applications

Enzymatic biofuel cells (EBFCs) are considered as a promising technology to sustain the requirements of miniaturized portable energy sources. Electrode materials being one of the substantial aspects of EBFCs have stimulated immense interest in research. A new platform made of nanocomposite, involving magnetic particles of iron oxide (Fe₃O₄), carbon nanotubes (CNT), gold nanoparticles (Au) and a conducting polymer polypyrrole (PPy), was used as the electrode support for the immobilization of glucose oxidase (GOD) which improves the bioelectrocatalysis of the enzyme towards oxidation of glucose. The structural and electrochemical characterization of the modified bioanode GCE/PPy/Au/CNT@Fe₃O₄/FRT/GOD was performed using Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and linear sweep voltammetry (LSV). The performance of the bioanode was evaluated with promising results showing the maximum current density of 6.01 mA cm^?2 (0.22 V vs. Ag/AgCl) in 40 mM of glucose concentration at 0.38 V open circuit potential (OCV). The results suggested that PPy/Au/CNT@Fe₃O₄ nanocomposite significantly improve the surface area of the electrode and served as a suitable environment for enzyme immobilization, and established good electrical communication by facilitating electron transfer between enzymes and electrode surface. The as-synthesized composite has been inferred as a promising platform for improved bioelectrocatalysis owing to the excellent combination of superparamagnetism of Fe₃O₄ with good electrocatalytic properties of CNT, PPy, and Au nanoparticles. Thus PPy/Au/CNT@Fe₃O₄ nanocomposite can be considered as a prospective electrode material for developing better electrochemical biosensors and biofuel cell anode.     

AuNi alloy nanoparticles supported on reduced graphene oxide as highly efficient electrocatalysts for hydrogen evolution and oxygen reduction reactions

Bimetallic nanoparticles of Au and Ni in the form of alloy nanostructures with varying Ni content are synthesized on reduced graphene oxide (rGO) sheets via a simple solution chemistry route and tested as electrocatalysts towards the hydrogen evolution (HE) and oxygen reduction (OR) reactions using polarization and impedance studies. The AuNi alloy NPs/rGO nanocomposites display excellent electrocatalytic activity which is found to improve with increasing Ni content in the AuNi/rGO alloy nanocomposites. For HER, the best AuNi alloy NPs/rGO electrocatalyst, the one with the highest Ni content, exhibits high activity with an onset overpotential approaching zero versus the reversible hydrogen electrode and an overpotential of only 37 mV at 10 mA cm^?2. Additionally, a low Tafel slope of 33 mV dec^?1 and a high exchange current density of 0.6 mA cm^?2 are measured which are very close to those of commercial Pt/C catalyst. Also, in the ORR tests, this electrocatalyst displays comparable activity to Pt/C. The Koutecky–Levich plots referred to a 4-electron mechanism for the reduction of dissolved O₂ on the AuNi alloy NPs/rGO catalyst. The electrocatalyst thus demonstrates excellent activity towards HER and ORR. Additionally, it exhibits outstanding operational durability and activation after 10,000^th cycles assuring its practical applicability.