Synthesis of strontium chromate-nitrogen and sulfur co-doped graphene and its potential for electrochemical hydrogen storage

In the present study, monazite-type strontium chromate (SrCrO₄) as a ternary metal oxide was prepared by the sol-gel method. Nitrogen and sulfur co-doped graphene decorated with SrCrO₄ nanocrystals was synthesized successfully, and the electrochemical hydrogen storage performance of the SrCrO₄ and its relative nanocomposites also were investigated by chronopotentiometry (CHP) technique. The effect of doped graphene as a substrate of the SrCrO₄ sample on the improvement of the electrochemical hydrogen storage performance was considered as well. The SrCrO₄-nitrogen and sulfur co-doped graphene (SrCrO₄/NSG) displayed the highest discharge capacity in comparison to SrCrO₄-reduced graphene oxide (SrCrO₄/rGO), SrCrO₄ calcined at 1000 °C (SrCrO₄ (1000)) and SrCrO₄ calcined at 800 °C (SrCrO₄ (800)). Also, increasing the hydrogen storage capacity of the samples by repeating the cycles indicated the excellent cycle stability of the nanoparticles. In monazite-type structures, oxygen vacancies can be created by thermal treatment. Creating oxygen vacancies can improve redox reactions, which increase the conductivity of the samples and hydrogen storage capacity.     

Facile synthesis of triangular shaped palladium nanoparticles decorated nitrogen doped graphene and their catalytic study for renewable energy applications

We report a novel method for the synthesis of triangular shaped palladium nanoparticles (Pd NPs) decorated nitrogen doped graphene. Nitrogen doped graphene (N-G) is synthesized by uniform coating of polyelectrolyte modified graphene surface with a nitrogen containing polymer followed by their pyrolysis. The triangular shaped Pd NPs are decorated over nitrogen doped graphene (Pd/N-G) by kinetically controlling the polyol reduction process. The kinetic control of the growth of the nanoparticles and nitrogen doping of the supporting material leads to the formation of highly dispersed anisotropic nanoparticles over the graphene support. Hydrogen storage study of N-G and Pd/N-G give a storage capacity of 1.1 wt% and 1.9 wt%, respectively at 25 °C and 2 MPa hydrogen equilibrium pressure. Electrocatalytic study of Pd/N-G shows that it is a very good electrocatalyst for oxygen reduction reaction and highly stable in acidic media due to the strong binding between Pd NPs and graphene support as a result of nitrogen doping besides has high methanol tolerance in acidic media. The present synthesis procedure highlights a new pathway for the highly dispersed and different morphological metal nanoparticles decorated graphene composites for energy related applications.     

Energy storage of thermally reduced graphene oxide

The energy-storage capacity of reduced graphene oxide (rGO) is investigated in this study. The rGO used here was prepared by thermal annealing under a nitrogen atmosphere at various temperatures (300, 400, 500 and 600 °C). We measured high-pressure H₂ isotherms at 77 K and the electrochemical performance of four rGO samples as anode materials in Li-ion batteries (LIBs). A maximum H₂ storage capacity of ∼5.0 wt% and a reversible charge/discharge capacity of 1220 mAh/g at a current density of 30 mA/g were achieved with rGO annealed at 400 °C with a pore size of approximately 6.7 Å. Thus, an optimal pore size exists for hydrogen and lithium storage, which is similar to the optimum interlayer distance (6.5 Å) of graphene oxide for hydrogen storage applications.     

Nitrogen/sulfur co-doped disordered porous biocarbon as high performance anode materials of lithium/sodium ion batteries

Nitrogen/sulfur co-doped disordered porous biocarbon was facilely synthesized and applied as anode materials for lithium/sodium ion batteries. Benefiting from high nitrogen (3.38 wt%) and sulfur (9.75 wt%) doping, NS_1-1 as anode materials showed a high reversible capacity of 1010.4 mA h g⁻¹ at 0.1 A g⁻¹ in lithium ion batteries. In addition, it also exhibited excellent cycling stability, which can maintain at 412 mAh g^-1 after 1000 cycles at 5 A g⁻¹. As anode materials of sodium ion batteries, NS_1-1 can still reach 745.2 mA h g⁻¹ at 100 mAg^-1 after 100 cycles. At a high current density (5 A g^-1), the reversible capacity is 272.5 mA h g⁻¹ after 1000 cycles, which exhibits excellent electrochemical performance and cycle stability. The preeminent electrochemical performance can be attributed to three effects: (1) the high level of sulfur and nitrogen; (2) the synergic effect of dual-doping heteroatoms; (3) the large quantity of edge defects and abundant micropores and mesopores, providing extra Li/Na storage regions. This disordered porous biocarbon co-doped with nitrogen/sulfur exhibits unique features, which is very suitable for anode materials of lithium/sodium ion batteries.     

Improved room-temperature hydrogen storage performance of lithium-doped MIL-100(Fe)/graphene oxide (GO) composite

Li⁺ doping is regarded as an effective strategy to enhance the room-temperature hydrogen storage of metal-organic frameworks (MOFs). In this work, Li⁺ is doped into both MIL-100(Fe) and MIL-100(Fe)/graphene oxide (GO) composite, and it is demonstrated that the hydrogen uptake of Li⁺ doped MIL-100(Fe)/GO (2.02 wt%) is improved by 135% compared with Li⁺ doped MIL-100(Fe) (0.86 wt%) at 298 K and 50 bar, which is ascribed to its higher isosteric heat of adsorption (7.33 kJ/mol) resulting from its more accessible adsorption sites provided by doped Li⁺ ions and ultramicropores. Grand canonical Monte Carlo (GCMC) simulation reveals that Li⁺ ions distributing in the interface between MIL-100(Fe) and GO within MIL-100(Fe)/GO composite is favorable for hydrogen adsorption owing to the increased number of adsorption sites, thus contributing to the enhanced hydrogen storage capacity. These findings demonstrate that MIL-100(Fe)/GO is a more promising Li⁺ doping substrate than MIL-100(Fe).     

Sulfur encapsulated in nitrogen-doped graphene aerogel as a cathode material for high performance lithium-sulfur batteries

Lithium-sulfur batteries are considered to be an ideal high-performance rechargeable lithium battery. However, some problems have seriously hindered the practical application of lithium-sulfur batteries. A simple one-step hydrothermal method has been applied to design nitrogen-doped graphene aerogel (N-GA) with three different nitrogen sources. Subsequently, sulfur was encapsulated in N-GA by chemical deposition method to synthesize sulfur encapsulated in N-GA (N-GA/S) composites. Among them, the N-GA1/S composite with sulfur content reaching 75.5 wt% has a discharge specific capacity of 723.9 mAh g^?1 after 100 cycles at 0.7 C, and the capacity retention rate is up to 87.4% while the coulombic efficiency still remains 98%. The outstanding electrochemical performance is owing to the good coating of sulfur by nitrogen-doped graphene aerogel, the improvement of the conductivity of the graphene skeleton by nitrogen doping, and the strong adsorption capacity of the doped nitrogen atom to lithium polysulfide. The graphene skeleton also helps to reduce the volume effect while charging and discharging. Furthermore, the proportion of pyridinic-N in N-GA1/S composites is higher than that in the two other composites, and it has a better adsorption capacity for lithium polysulfide.     

Fe3+-Clinoptilolite/graphene oxide and layered MoS2@Nitrogen doped graphene as novel graphene based nanocomposites for DMFC

Fe³⁺ doped in a natural zeolite (Fe³⁺-Clinoptilolite) hybridized with graphene oxide (GO) was used as an electro-catalyst for methanol oxidation in direct methanol fuel cells (DMFC). Furthermore, thin layered molybdenum disulfide (MoS₂) composited with nitrogen doped graphene (NG) was used for oxygen reduction. Successful synthesis of these nanomaterials was confirmed by X-ray diffraction (XRD), X-ray florescence (XRF), Fourier transform infrared (FTIR), energy-dispersive X-ray (EDX), Raman spectroscopy, Field Emission Scanning Electron Microscopy (FESEM) and transmission electron microscopy (TEM) images. In the following, by using the cyclic voltammetry (CV) technique the electrochemical behaviors of the glassy carbon electrodes modified with the mentioned composites were investigated. The results of methanol oxidation and oxygen reduction showed sufficient electro-catalytic effects as well as significant diffusion currents in presence of the non-precious synthetic materials. Obtained exchange currents (i₀) from Tafel plots showed increasment up to 6.02 × 10^?6 and 1.47 × 10^?5 μA for anode and cathode respectively. Also, thermodynamic potential of the DMFC was estimated about 1.1 V in alkaline media that was very close to reported value for theoretical potential in DMFC.     

Nitrogen and sulfur co-doped graphene supported PdW alloys as highly active electrocatalysts for oxygen reduction reaction

Nitrogen and sulfur co-doped graphene (NSG) is prepared by a facile microwave irradiation method and palladium-tungsten (PdW) alloy nanoparticles are supported on the NSG substrate. Several techniques, including X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry and scanning electrochemical microscopy etc. are used to characterize the physical and electrochemical properties of the as-prepared samples. It is found that the PdW alloy nanoparticles are uniformly dispersed on the surface of NSG and the electrochemical performance of PdW/NSG is much better than those of Pd/NSG and Pd/G. The reason for the improved electrochemistry performance of PdW/NSG is considered to be the strong interactions and synergetic effects between PdW nanoparticles and NSG.     

Nitrogen/oxygen co-doped graphene bonded nickel particles composite for dehydrogenation of formic acid at near room temperature

Low-cost non-noble metal catalyst for dehydrogenation of formic acid to hydrogen at near room temperature is considered as a key to promoting commercial technology for clean energy. We have constructed reduced graphene oxide (RGO) self-assembly bonded nickel particles for synthesis of graphene nanosheets embedded with nickel nanoparticles architecture Ni@xRGO. Nitrogen and oxygen co-doped graphene facilitates the adsorption of hydrogen protons from formic acid. Electron transfer ability of Ni@xRGO with active sites is enhanced via Ni–C bond in the interface between the RGO nanosheets and nickel particles, which promoted the C–H bond breaking for dehydrogenation of formic acid. The Ni@0.20RGO has excellent catalytic performance for hydrogen production from formic acid at near room temperature (the yield of hydrogen, 240.0 mL g^?1 h^?1 at 50 °C), comparable to the most active non-noble metal catalysts.     

Hydrogen storage in Ni-B nanoalloy-doped 2D graphene

Two-dimensional graphene material is doped with Ni-B nanoalloys via a chemical reduction method, and shows that the optimal graphene doped with Ni (0.14 wt.%) and B (0.63 wt.%) has a hydrogen capacity of 2.81 wt.% at 77 K and 106 kPa, which is more than twice of that of the pristine graphene. The measured adsorption isotherms of hydrogen and nitrogen suggest that the Ni-B nanoalloys function as catalytic centers to induce the dissociative adsorption of hydrogen (spillover) on the graphene. The Ni-B nanoalloys without using any noble metal may be a promising catalyst for hydrogen storage application.     

Scope of doped mesoporous (<10 nm) surfactant‐modified alumina templated carbons for hydrogen storage applications

Metal (Ni/Pd) and nitrogen codoped mesoporous templated carbons were synthesized using low‐cost surfactant‐modified mesoporous alumina as a hard template via chemical vapor deposition for hydrogen storage application. Initially, high surface area (1508 m²/g) nitrogen‐doped templated carbon was successfully prepared. Pore volume was also significant (1.64 cm³/g). The codoping with metals (Ni or Pd) reduced both the area and pore volume. All the codoped carbons were mesoporous (2‐8 nm). Aggregated morphology was observed for nitrogen‐doped carbon; tubular or noodle shape appeared on codoping with metals. The dispersion of Pd metal within the carbon framework was highest. The 2 wt% Pd codoped carbon showed the highest hydrogen uptake of 5 wt% (?196°C; 25 bar). This may be attributed to its most number of active sites corresponding to the highest metal dispersion and amount of nitrogen present. The cyclic stability of the samples was also good with only 3% to 5% loss in storage capacity up to 10 cycles.     

Preparation and electroactivity of polymer-functionalized graphene oxide-supported platinum nanoparticles catalysts

Polymer-functionalized graphene oxide or pristine graphene oxide supported platinum nanoparticles (Pt NPs) was prepared to study the surface modification effects. The catalysts were characterized by transmission electron microscopy, energy dispersive spectrometry, X-ray diffraction and thermogravimetric analysis. The electrochemical activities of Pt NPs were measured by cyclic voltammograms. The poly(diallyldimethylammonium chloride) (PDDA) was used as a modifier agent which formed a functionalized layer on graphene oxide (GO) sheets. As a result, the electrochemical active surface area (ESA) of PDDA functionalized GO supported Pt (Pt/PDDA–graphene) was shown to 66 m²/g that indicated higher hydrogen adsorption amount than 55 m²/g of the pristine Pt/graphene. In addition, an average particle size of Pt/PDDA–graphene NPs was measured to 1.8 nm slightly smaller than 2.0 nm of pristine Pt/graphene NPs.     

Insight into effects of graphene and zinc oxide in Li4Ti5O12 as anode materials for Li-ion full-cell battery

Programmable design of nanocomposites of Li₄Ti₅O₁₂ (LTO) conducted through hydrothermal route in the presence of ethylenediamine as basic and capping agent. In this work, effect of ZnO and Graphene on the Li₄Ti₅O₁₂ based nanocomposites as anode materials investigated for Li-Ion battery performances. The full cells battery assembled with LTO based nanocomposites on Cu foil as the anode electrode and commercial LMO (LiMn₂O₄) on aluminum foil as cathode electrode. X-Ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FT-IR), along with Field Emission Scanning Electron Microscopy (FE-SEM) and Transmission electron microscopy (TEM) images was applied for study the composition and structure of as-prepared samples. The electrochemical lithium storage capacity of prepared nanocomposites was compared with pristine LTO via chronopotentiometry charge-discharge techniques at 1.5–4.0 V and current rate of 100 mA/g. As a result, the electrode which is provided by LTO/TiO₂/ZnO and LTO/TiO₂/graphene nanocomposites provided 765 and 670 mAh/g discharge capacity compared with pristine LTO/TiO₂ (550 mAh/g) after 15 cycles. Based on the obtained results, fabricated nanocomposites can be promising compounds to improve the electrochemical performance of lithium storage.     

Assessment of the effect of electrophoretic deposition parameters on hydrogen storage performance of graphene oxide layer applied on nickel foam

In this study, the hydrogen storage capacity of the graphene oxide layer was studied electrochemically. The graphene oxide was synthesized by modified Hummers' method and applied on the nickel foam by electrophoretic deposition (EPD) method at different potentials (20 and 60 V) and times (20 and 60 min) to determine the effect of applied potential and time of deposition on the hydrogen adsorption performance. The hydrogen adsorption tests including charge-discharge test, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were conducted in 6 M KOH solution and at room temperature. Based on the achieved CV curves, the graphene oxide (GO) layer achieved at 60 V within 20 min has a higher electrochemical hydrogen adsorption capability compared to other obtained samples. The calculated hydrogen storage capacity is obtained 50.9 mA. h. g^?1. The rosette flower like morphology of the obtained GO layers at optimum condition, has an impressive effect on the improving electrochemical hydrogen adsorption based on morphology study by field emission scanning electron microscopy.     

Catalytic performance of Samarium-modified Ni catalysts over Al2O3–CaO support for dry reforming of methane

Mesoporous calcina-modified alumina (Al₂O₃–CaO) support was produced through the simple and economical co-precipitation method, then nickel (Ni, 10 wt%) and samarium (Sm, 3 wt%) ions loaded by two-solvent impregnation and one-pot strategies. The unpromoted/samarium-promoted catalysts were evaluated using X-ray Diffraction (XRD), High-Resolution Transmission Electron Microscopy (HR-TEM), nitrogen adsorption-desorption, Temperature Programmed Oxidation/Reduction (TPR/TPO), and Field Emission Electron Scanning Microscopy (FE-SEM) methods, then investigated in methane dry reforming. The results revealed that with adding samarium to Ni catalyst through impregnation method, the average Ni crystallite size and specific surface area decreased from 11.5 to 5.75 nm and from 76.08 to 30.9 m²/g, respectively; as a result, the catalytic activity increased from about 50% to 68% at 700 °C. Furthermore, the TPO and FE-SEM tests indicated the formation of carbon with nanotube nature on the catalyst surface.     

Fabrication of highly dispersed Pd nanoparticles supported on reduced graphene oxide for solid phase catalytic hydrogenation of 1,4-bis(phenylethynyl) benzene

In this work, nanopalladium catalysts supported on the surface of reduced graphene oxide (rGO/Pd) with different palladium loadings have been prepared by one-step reduction in aqueous phase. They were mixed with 1,4-bis(phenylethynyl)benzene (DEB) to form rGO/Pd-DEB composites according to a mass ratio of 1:3. It was shown that nanopalladium particles with particle size of about 2–6 nm were disperse uniformly on the surface of rGO when the Pd loadings were in the range of 3.97–10.60 wt%. The maximum hydrogen uptake capacity of rGO/Pd-DEB composites at 25 °C determined according to PCT method was about 182.5 ml/g after reacted with hydrogen for about 20 h, which was some lower than that of the common Pd/C-DEB pallet getter (216 ml/g) but significantly higher than alkynyl modified polyvinyl alcohol supported palladium hydrogen absorbing materials (0.32 ml/g), indicating that rGO/Pd could be used in solid phase catalytic hydrogenation due to the high dispersion of palladium nanoparticles and the physical proximity of rGO/Pd catalyst with DEB organic molecules. This provides a good potential technical way for perparing the moldable carbon aerogel hydrogen absorption materials.     

Electrocatalytic hydrogen evolution reaction on sulfur-deficient MoS2 nanostructures

Molybdenum disulfide (MoS₂) is a 2D layered structured material with a Mo:S of 1:2 and is a great attention seeker for hydrogen production through water-splitting. In the present work, we prepared nanostructured MoS_x with different sulfur molar concentrations (x = 2, 1, 0.5) through a one-step hydrothermal method. The decrease in sulfur concentration resulted in a new phase that is MoO₃ with a Mo:S of 1:0.5. The structural, morphological, and optical properties of all the samples were studied through X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared spectroscopy (FTIR), and Ultraviolet–Visible (UV–Vis) spectroscopy, respectively. Moreover, the electrochemical behavior was studied using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and Tafel slope. Optimum properties were observed for Mo:S (1:1) with an onset potential of 96 mV, an overpotential of 130 mV for hydrogen evolution reaction (HER) coupled with a specific capacitance of 889 F/g and low charge transfer resistance of 43 Ω. Further, it was noted that the electrocatalytic activity of MoS₁ was better than that of the composite and bare MoO₃. It is proposed that the excellent electrochemical activity arises from sulfur vacancies which provide active sites for HER and a free path for ions to flow through the material.     

First principles study on hydrogen storage in yttrium doped graphyne: Role of acetylene linkage in enhancing hydrogen storage

Through Density Functional Theory Simulations we predict that a Ytrrium atom attached on graphyne surface can adsorb up to a maximum of 9 molecular hydrogens (H₂), with a uniform binding energy of ∼0.3 eV/H₂ and an average desorption temperature of around 400 K (ideal for fuel cell applications), leading to 10 wt% of hydrogen, substantially higher than the requirement by DoE. The higher hydrogen wt% in Y doped graphyne compared to Y doped Single Walled Carbon Nanotubes (SWNT) and graphene is due to the presence of sp hybridized C atoms (in the acetylene linkage) supplying additional in-plane p_x-p_y orbitals leading to π (π*) bonding (antibonding) states. Charge transfer from metal to carbon nanostructure results in a redistribution of s, p, d orbitals of the metal leading to a non - spin polarized ground state in Y doped graphyne, due to the presence of the acetylene linkage, whereas Y doped SWNT and graphene remain magnetic like the isolated metal atom. In the non-magnetic graphyne + Y system, the net charge transfer from Y to successive H₂ molecules is less than in magnetic Y + graphene and Y + SWNT systems, enabling Y + graphyne to store a larger number of H₂ molecules. Furthermore, our ab initio MD simulations show that the system is stable even at room temperature and there is no dissociation of H₂ molecules, enabling the system to achieve 100% desorption. So Y doped graphyne is found to be a promising hydrogen storage device with high wt%, 100% recyclability and desirable desorption temperature.     

Supercritical fluid processing of N-doped graphene and its application in high energy symmetric supercapacitor

A simple one-pot methodology is developed for the synthesis of nitrogen doped graphene via supercritical fluid (SCF) processing using glycine as a nitrogen precursor. The presence of various N-containing functional groups was determined by FT-IR and the amount of N-doping in the graphene was found to be 4.5 wt% using the elemental analysis and X-ray photoelectron spectroscopy. The electrochemical capacitance measurements are performed using cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The nitrogen doped graphene exhibited enhanced capacitive performance with a maximum specific capacitance of 270 F/g at 0.5 A/g current density with high specific capacitance retention of 90% over 10,000 cycles at 10 A/g current density. The fabricated symmetric supercapacitor cell showed a high energy density of 4.1 and 36 Wh/kg in aqueous and ionic liquid electrolyte, respectively. The high energy density obtained in ionic liquid is promising for their potential application in electrochemical energy system.     

Effects of graphene oxide and sacrificial reagent for highly efficient hydrogen production with the costless Zn(O,S) photocatalyst

Searching for noble metal-free co-catalyst is still a strenuous part in photocatalytic hydrogen evolution reaction (HER), as most of the great catalysts contain noble metals like the expensive platinum. The present work demonstrates a feasible synthesis method of Zn(O,S)/GO nanocomposite with graphene oxide (GO) to serve as an inexpensive co-catalyst. Raman spectra and transmission electron microscopy (TEM) images clearly verified that GO was successfully loaded on the surface of Zn(O,S). This GO layer could effectively decrease the charge transfer resistance and promote the charge carrier separation for enhancing hydrogen production rate. By optimizing the GO content, the best hydrogen production rate of 2840 μg h⁻¹ was achieved with Zn(O,S)/0.5 wt% GO catalyst under 16 W UV lamp with illumination light at a wavelength of 352 nm, which showed about two times higher for GO-free Zn(O,S). The effect of sacrificial reagent on the hydrogen production rate of Zn(O,S)/0.5 wt% GO catalyst was also evaluated. The sacrificial reagent showed the efficiency with the following trend: ethanol > methanol > isopropanol > ethylene glycol. The mechanism for enhancing hydrogen production rate is elucidated in this paper. We consider the simple synthesis method and its low cost to make Zn(O,S)/GO a great potential for practical application.