Chemically and thermally reduced graphene oxide supported Pt catalysts prepared by supercritical deposition

Reduced graphene oxide (RGO) is used in many energy applications, especially in Polymer Electrolyte Membrane (PEM) fuel cells, as carbon sourced catalyst support materials. In this study, thermally (T-RGO) and chemically (C-RGO) reduced GO support materials were synthesized for utilization in PEM fuel cells. Pt catalysts were synthesized using supercritical carbon dioxide (SCCO₂) deposition technique over synthesized support materials. Physical (BET, SEM-EDX, FTIR, RAMAN, XRD, TEM, ICP-MS and optical tensiometer) and electrochemical (CV, PEM fuel cell test) characterizations of synthesized support materials and corresponding Pt catalysts were performed. The differences between the structures of thermally and chemically reduced graphene oxide supports and their Pt catalysts were investigated. The ECSA values of the Pt/T-RGO and Pt/C-RGO catalysts are 19.86 m² g^?1 and 6.31 m² g^?1, respectively. The current and power density values of the Pt/T-RGO and Pt/C-RGO catalysts at 0.6 V are 84 mA cm^?2, 80 mA cm^?2 and 50 mW cm^?2, 45 mW cm^?2, respectively. Pt/T-RGO and Pt/C-RGO catalysts showed similar trend in PEMFC environment.     

Monodisperse rutheniumcopper alloy nanoparticles decorated on reduced graphene oxide for dehydrogenation of DMAB

In this study, we report a superior dehydrogenation catalyst for dimethylamine borane, which exhibited one of the best catalytic activities. The newly formed catalyst system contains well dispersed ruthenium-copper nanomaterials on reduced graphene oxide (3.86 ± 0.47 nm), which was prepared by using the ultrasonic double reduction technique. The characterization of monodisperse ruthenium-copper alloy nanoparticles was performed using some advanced analytical methods such as TEM, HRTEM, XPS, Raman spectroscopic analysis. The experiments results revealed that the monodisperse ruthenium-copper alloy catalyst (RuCu@rGO) has one of the highest catalytic activity compared to previous studies, having a high turnover frequency value (256.70 h⁻¹). The detailed kinetic parameters such as activation energy, enthalpy, and entropy values were also calculated for the dehydrogenation of dimethylamine borane at room temperature. Also, the results showed that the monodisperse RuCu@rGO catalyst has high durability and reusability as retained its 81% initial catalytic activity even after 4th runs for the dehydrogenation of dimethylamine borane.     

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.     

Metal chalcogenide based photocatalysts decorated with heteroatom doped reduced graphene oxide for photocatalytic and photoelectrochemical hydrogen production

Heteroatom (N, B and P) doped reduced graphene oxide (RGO)-metal chalcogenide nanocomposites (RGO-Cd_0.60Zn_0.40S) were prepared by the solvothermal method, and then they were characterized with X-ray diffraction, Raman spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, UV–Vis diffuse reflectance spectroscopy and photoluminescence techniques. Doping of RGO with heteroatoms of N, B and P increased charge-transfer capability of nanocomposites and thus, improved both photocatalytic and photoelectrochemical hydrogen production activities of them. N-doped RGO-Cd_0.60Zn_0.40S photocatalyst exhibited the highest photocatalytic hydrogen production rate (1114 μmolh⁻¹ g⁻¹) in photocatalytic (PC) system amongst other and it was 1.5 times higher than that of RGO-Cd_0.60Zn_0.40S photocatalyst. Having a current density of 0.92 mAcm⁻², photoelectrochemical hydrogen production activity of N-RGO-Cd_0.60Zn_0.40S electrode was found to be 3 times higher than RGO-Cd_0.60Zn_0.40S photoelectrode without any applied bias potential under visible light irradiation in photoelectrochemical system. In general, these results clearly showed that heteroatom doping of RGO led to promising materials for renewable hydrogen production in the photocatalytic and photoelectrochemical systems.     

Hydrogen sorption studies of palladium decorated graphene nanoplatelets and carbon samples

With the increasing population of the world, the need for energy resources is increasing rapidly due to the development of the industry. 88% of the world's energy needs are met from fossil fuels. Since there is a decrease in fossil fuel reserves and the fact that these fuels cause environmental pollution, there is an increase in the number of studies aimed to develop alternative energy sources nowadays. Hydrogen is considered to be a very important alternative energy source due to its some specific properties such as being abundant in nature, high calorific value and producing only water as waste when burned. An important problem with the use of hydrogen as an energy source is its safe storage. Therefore, method development is extremely important for efficient and safe storage of hydrogen. Surface area, surface characteristics and pore size distribution are important parameters in determining the adsorption capacity, and it is needed to develop new adsorbents with optimum parameters providing high hydrogen adsorption capacity. Until recently, several porous adsorbents have been investigated extensively for hydrogen storage. In this study, it was aimed to develop and compare novel Pd/carbon, Pd/multiwalled carbon nanotube, and Pd/graphene composites for hydrogen sorption. All the palladium/carbon composites were characterized by t-plot, BJH desorption pore size distributions, N₂ adsorption/desorption isotherms, and SEM techniques. The maximum hydrogen storage of 2.25 wt.% at −196 °C was achieved for Pd/KAC composite sample. It has been observed that the spillover effect of palladium increases the hydrogen sorption capacity.     

Studies on the electrochemical reduction of oxygen catalyzed by reduced graphene sheets in neutral media

Reduced graphene sheets (RGSs) were prepared via chemical reduction of graphite oxide and their morphology was characterized by atomic force microscopy. The electrochemical reduction of oxygen (O₂) with RGSs was studied by cyclic, rotating disk electrode, and rotating ring-disk electrode voltammetry using the RGSs-modified glassy carbon (RGSs/GC) electrode in 3.5% NaCl solution. The results show that O₂ reduction undergoes three steps at the RGSs/GC electrode: electrochemical reduction of O₂ to H₂O₂ mediated by quinone-like groups on the RGSs surface, a direct 2-electron reduction of O₂, and reduction of the H₂O₂ produced to H₂O. The modification of RGSs results in an obvious positive shift of the peak potential and a larger current density. The kinetics study shows that the number of electrons transferred for O₂ reduction can reach to 3.0 at potentials of the first reduction step, indicating RGSs can effectively catalyze the disproportionation of H₂O₂. Such catalytic activity of RGSs enables a 4-electron reduction of O₂ at a relatively low overpotential in neutral media. RGSs are a potential electrode material for microbial fuel cells.     

Significant enhancement of the electrochemical hydrogen uptake of reduced graphene oxide via boron-doping and decoration with Pd nanoparticles

Development of advanced hydrogen storage materials with high capacity and stability is vital to achieve an envisaged hydrogen economy. Here, we report a uniformly dispersed Pd nanoparticles on the boron-doped reduced graphene oxide (Pd/B-rGO) as a novel nanocomposite for efficient hydrogen storage. The effects of the incorporation of Pd NPs and the substitution of boron atoms into the graphene-based nanomaterial matrix on the electrochemical hydrogen up-taking and releasing were comparatively studied using electrochemical techniques, and duly supported by density functional theory (DFT) calculations. The discharge capacities of the Pd-rGO and Pd/B-rGO nanocomposites were determined to be over 45 and 128 times higher than that of the Pd NPs, respectively, showing that the B doping and the rGO support played significant roles in the enhancement of the hydrogen storage capability. Moreover, the galvanostatic charging and discharging cycling tests demonstrated a high stability and efficient kinetics of the Pd/B-rGO nanocomposite in the H₂SO₄ electrolyte for hydrogen up-taking and release.     

Surface structural alteration of multi-walled carbon nanotubes decorated by nickel nanoparticles based on laser ablation/chemical reduction methods to enhance hydrogen storage properties

The catalytic effect of nickel is addressed to decorate the multi-walled carbon nanotubes for the purpose of hydrogen storage. The hydrogen sorption/desorption are investigated using the volumetric technique. Nickel nanoparticles are distributed on the surface of nanotubes using the laser ablation/chemical reduction treatments. The hydrogen uptake is elevated at higher nickel population up to a certain value and then experiences a significant drop for larger nickel content. The laser treatment is accompanied by the induced pores around nanotubes. This gives rise to the creation of the larger pores at higher laser doses leading to decrease the hydrogen trapping. Despite the pore size distribution strongly alters during both synthesis methods, however the abundance of small pore size in laser treatments is relatively higher than the that of the other technique. In comparison, the laser ablation demonstrates a relatively smaller desorption temperature against chemical one, mainly owing to the formation of larger pore size/volume. Generally, the hydrogen trapping efficiently takes place in the laser treated samples against chemical reduction method. The highest value of hydrogen storage ∼1% (0.6% weight) is corresponding to 12.3% (13% weight) of nickel loading via the laser ablation (chemical reduction).     

Influence of doping level on the electrocatalytic properties for oxygen reduction reaction of N-doped reduced graphene oxide

Reduced Graphene Oxide (rGO) doped with different nitrogen (N) concentrations (5, 10, and 15 parts in weight) were successfully synthesized via hydrothermal conditions, from graphene oxide (GO) and 3-amino-1,2,4-triazole (amitrole) to obtain N₅-rGO, N₁₀-rGO, and N₁₅-rGO. These N-doped materials were characterized and evaluated for the first time as catalysts for the oxygen reduction reaction (ORR). The physicochemical characteristics confirmed the simultaneous N-doping and reduction processes with a turbostratic re-stacking of graphene layers. Nitrogen was successfully introduced as a mix of pyridine-N, amine like-N, pyrrolic-N, and quaternary bonding species into the carbon lattice. The N content were 8.1, 9.9, and 10.5 at.% for N₅-rGO, N₁₀-rGO, and N₁₅-rGO. High catalytic activity was demonstrated in alkaline media with an onset potential of ~0.88 V and high current density (3.9 mA cm^?2) for N₁₅-rGO, leading the ORR via the 4-electron transfer pathway. The results demonstrated that the amine like-N species enhance the ORR catalytic activity in addition to pyridinic and quaternary.     

The influence of nitrogen doping on reduced graphene oxide as highly cyclable Li-ion battery anode with enhanced performance

A straightforward, one-step route has been established to fabricate reduced- (rGO) and nitrogen-doped reduced graphene oxide (NrGO) with remarkable lithium-ion storage properties. The graphene oxide (GO) was synthesized as starting material by improved Hummers’ method. Thereafter, thermally annealing GO with NH₃ at elevated temperature to synthesize NrGO was yielded a more open structure with nitrogen sites suitable for enhanced Li intercalation. NrGO exhibited a reversible capacity of 240 mAhg^?1 at 10 Ag^-1 after 500 cycles with 90% capacity retention, which is the best result achieved among graphene oxide-based anodes at this current density. In contrast to rGO, NrGO cells exhibited a gradually increasing capacity profile, reaching up to 114% of the initial capacity at 0.1, 2, and 10 Ag^-1 current densities. Results showed that high occupancy of pyridinic N within NrGO enhanced battery performance and cell kinetics upon cycling which offers long-time operability at high current density.     

Investigation of electrocatalytic activity on a N-doped reduced graphene oxide surface for the oxygen reduction reaction in an alkaline medium

Today the search for new energy resources is a crucial topic for materials science. The development of new effective catalysts for the oxygen reduction reaction can significantly improve the performance of fuel cells as well as electrocatalytic hydrogen production. This study presents the scalable synthesis of nitrogen-doped graphene oxide for the oxygen reduction reaction. The combination of an ab initio theoretical investigation of the oxygen reduction reaction (ORR) mechanism and detailed electrochemical characterization allowed the identification of electrocatalytically active nitrogen functionalities. The dominant effect on electrocatalytic activity is the presence of graphitic and pyridinic nitrogen and also N-oxide functionalities. The overpotential of ORR for nitrogen-doped graphene oxide prepared by microwave-assisted synthesis outperformed the metal-doped graphene materials.     

S,N co-doped graphene quantum dots decorated TiO2 and supported with carbon for oxygen reduction reaction catalysis

Sluggish kinetics and catalyst instability in oxygen reduction reaction are the central issues in fuel cell and metal-air battery technologies. For that, highly active, stable, and low-cost non-platinum based electrocatalysts for oxygen reduction reaction are an immediate requirement in fuel cell and metal-air battery technologies. A new composite (S,N-GQD/TiO₂/C-800) is synthesized, made of sulfur (S) and nitrogen (N) co-doped graphene quantum dot (GQD) with TiO₂. This composite is supported on carbon on heating at 800 °C under N₂ atmosphere and is explored for oxygen reduction reaction (ORR) catalyst. The synthesized composite S,N-GQD/TiO₂/C-800, shows outstanding catalytic activity with an onset potential of 0.91 V and a half-wave potential of 0.82 V vs. RHE, an alkaline medium. The Tafel slope of the catalyst is 61 mV dec^?1. The catalyst is an excellent methanol tolerant and shows good stability in an alkaline medium. The excellent ORR activity of S,N-GQD/TiO₂/C-800 is ascribed to well-built interactivity between the S,N-GQD/TiO₂, and the carbon support. The unique structure offers advantages, with outstanding electrical conductivity, high surface area, and excellent charge transfer kinetics between the doped GQD and TiO₂ interface and subsequently from the carbon surface to the S,N-GQD/TiO_2.     

Soybean milk derived carbon intercalated with reduced graphene oxide as high efficient electrocatalysts for oxygen reduction reaction

Exploring high-performance and low-cost metal-free oxygen reduction reaction (ORR) catalysts from biomass-derived materials is vital to the development of novel energy conversion devices such as fuel cells, etc. Herein, nitrogen-enriched soybean milk derived carbon (BDC/rGO-HT-NH₃) intercalated with reduced graphene oxide (rGO) electrocatalyst is prepared via one-pot hydrothermal synthesis method followed with nitridation by NH₃. The resultant catalyst with high surface area, good conductivity and high content of N (9.4 at.%) shows high electrocatalytic activity towards ORR in alkaline medium, which mainly happens through the direct 4-electron pathway. The onset potential of BDC/rGO-HT-NH₃ catalyzed ORR is 0.96 V vs RHE, which is only 0.11 V lower than that of the commercial Pt/C (20 wt%) catalyst. In addition, the BDC/rGO-HT-NH₃ catalyst shows superior long-term running durability. The desirable catalytic performances enable the facile synthesis approach of BDC/rGO-HT-NH₃ to be a promising methodology for transforming other biomass materials to N-enriched carbon based materials as low-cost and environmental friendly catalysts for ORR.     

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.     

Synthesis and lithium storage property of high-performance N-doped reduced graphene oxide

石墨烯是一种具有高比表面积、高导电性和良好化学稳定性的新型二维碳材料,在电化学储能领域具有广阔的应用前景。氮原子掺杂可以制造结构缺陷并改变电荷分布,有利于增强其电化学储能性能。本工作以尿素为氮源,与氧化石墨烯混合冻干,经过高温热还原制备出氮掺杂石墨烯材料,研究了热还原温度对其化学组成、形貌结构以及电化学储锂性能的影响。研究结果表明,随着热还原温度的升高,材料的氮含量下降,石墨化程度升高,电导率提高。将其作为负极材料组装成锂离子半电池进行测试,样品N-rGO-800在0.05 A/g的电流密度下表现出高达876 mA·h/g的稳定比容量,优于目前文献报道的比容量。在1 A/g的大电流密度下,其依然具有584 mA·h/g的比容量,经过850圈的长循环,容量保持稳定,显示出该材料优异的循环和倍率性能。     

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.     

The role of Pt loading on reduced graphene oxide support in the polyol synthesis of catalysts for oxygen reduction reaction

Common carbon-blacks have shown insufficient stability as cathodic catalyst supports for proton exchange membrane fuel cells (PEMFCs). In this regard, alternative supports have been proposed and, specifically graphene or reduced graphene oxide (rGO), have attracted special attention. Herein, a set of electrocatalysts using reduced graphene oxide (rGO) as support is synthetized by a modified polyol method. The influence of Pt loading on the support is studied and compared with conventional supports, considering Pt particle morphologies and oxygen reduction reaction (ORR) performance in rotating disk electrode (RDE). Despite Pt average particle size typically increases with the Pt loading, 30 wt% of Pt on rGO is the optimal Pt loading, yielding the highest ORR activity among the rGO-supported electrocatalysts. These results show that both Pt loading and type of support greatly impact on the morphology and electrochemical performance of Pt nanoparticles.     

Enhancement of the electrocatalytic activity for the oxygen reduction reaction of boron-doped reduced graphene oxide via ultrasonic treatment

Commercial polymer electrolyte membrane fuel cells have relied on scares Platinum to catalyse the kinetically sluggish oxygen reduction reaction occurring at their anodes. Over the last decade organic materials, frequently based on graphitic structures have been demonstrated as promising alternative electrocatalysts to the noble metals. Researchers typically utilize ultrasonic treatment as part of the synthesis procedure to achieve homogeneous dispersion of graphitic carbon prior to. Herein we investigate the implications of the structural and compositional changes induced by the ultrasonication treatment on boron-doped reduced graphene oxide for oxygen reduction reaction. It is shown that ultrasonication pre-treatment prior to the boron doping and reduction of graphene oxide via hydrothermal process step leads to the increase of both substitutional B and electrocatalytic surface area, with associated reduction of average pore size diameter, leading to a significant improvement in the oxygen reduction reaction performance, with respect to the non-ultrasonicated material. It is proposed that the higher degree of substitutional doping of boron is a result of formation of the additional epoxy functionalities on graphitic planes, which act as a doping site for boric acid.     

Facile synthesis of reduced graphene oxide supported PtAg nanoflowers and their enhanced electrocatalytic activity

In this work, a simple and facile method is developed in the synthesis of well-dispersed PtAg nanoflowers on reduced graphene oxide nanosheets (PtAg/RGOs) under solvothermal conditions, using ethylene glycol as a reducing agent and hexadecyl trimethyl ammonium bromide (CTAB) as capping and stabilizing agents. The as-prepared nanocomposites show a superior electrocatalytic activity, good tolerance, and better stability toward the oxidation of formic acid and ethylene glycol in alkaline media, compared with the commercial Pt/C (10 wt%) catalyst. For the oxidation of formic acid, the PtAg nanoflowers own thirty times higher of the catalytic currents than those of the commercial Pt/C catalyst. Meanwhile, for the oxidation of ethylene glycol, the ratio of forward current (j_F) to reverse current (j_R) is high up to 8.4, which is almost four times higher than that of the commercial Pt/C catalyst. This strategy provides a promising platform for direct formic acid and ethylene glycol fuel cells.