Search

This work reports a novel approach for the synthesis of FeCo alloy nanoparticles (NPs) embedded in the N,P‐codoped carbon coated nitrogen‐doped carbon nanotubes (NPC/FeCo@NCNTs). Specifically, the synthesis of NCNT is achieved by the calcination of graphene oxide‐coated polystyrene spheres with Fe3+, Co2+ and melamine adsorbed, during which graphene oxide is transformed into carbon nanotubes and simultaneously nitrogen is doped into the graphitic structure. The NPC/FeCo@NCNT is demonstrated to be an efficient and durable bifunctional catalyst for oxygen evolution (OER) and oxygen reduction reaction (ORR). It only needs an overpotential of 339.5 mV to deliver 10 mA cm−2 for OER and an onset potential of 0.92 V to drive ORR. Its bifunctional catalytic activities outperform those of the composite catalyst Pt/C + RuO2 and most bifunctional catalysts reported. The experimental results and density functional theory calculations have demonstrated that the interplay between FeCo NPs and NCNT and the presence of N,P‐codoped carbon structure play important roles in increasing the catalytic activities of the NPC/FeCo@NCNT. More impressively, the NPC/FeCo@NCNT can be used as the air‐electrode catalyst, improving the performance of rechargeable liquid and flexible all‐solid‐state zinc–air batteries. [ABSTRACT FROM AUTHOR]

A simple and cost‐effective process is developed for the synthesis of a high‐performance anode consisting of porous CoO nanowires intertwined with reduced graphene oxide sheets (CoO−NWs/rGO). When tested in a lithium‐ion battery, the CoO−NWs/rGO exhibits high initial reversible capacity (∼960 mAh g−1) and excellent rate capability. More importantly, its performance is further enhanced when CoO−NWs/rGO is discharged/charged for >100 cycles. For example, the specific capacity of the CoO−NWs/rGO can reach up to 1195 mAh g−1 after ∼130 cycles. This enhancement of the electrochemical performance of the CoO−NWs/rGO is attributed to two effects during the repeated discharge/charge cycling: a gradual evolution in morphology and crystal structure of electrode materials and the transformation of CoO onto the surface of rGO sheets. While the initial porous structure and the wire‐shaped morphology of CoO NWs may help to alleviate the strains/stresses induced by the volume change associated with cycling, they do not seem to be the primary attributes to the observed performance enhancement. The implication of this finding is very important to the rational design of high‐performance electrode materials: the equilibrium structure and morphology of electrode materials under repeated cycling conditions are far more important than the initial structure and morphology in designing nanostructured electrodes with desired performance and durability. Let's do the twist: the morphology evolution of reduced graphene oxide integrated CoO nanowires occurs when used in a lithium‐ion battery, leading enhanced performance after the repeated discharge/charge cycles. The results indicate that the equilibrium structure and morphology of electrode materials under repeated cycling conditions are far more important than the initial structure and morphology in designing nanostructured. [ABSTRACT FROM AUTHOR]

Nitrogen-doped graphene hollow microspheres (NGHSs), synthesized through a template sacrificing method, have been employed as the anode material for lithium ion batteries (LIBs). The electrochemical results show that the NGHSs can exhibit an initial discharge capacity of 2716.4 mAh g −1 , which is significantly higher than the theoretical discharge capacity of graphite (372.0 mAh g −1 ). Further analysis shows that the NGHSs can exhibit significantly high rate capacity, good reversibility, and excellent cycling stability, which clearly demonstrate the potential uses of the NGHSs as the anode material to boost both energy and power densities of LIBs. Their excellent electrochemical performance could be attributed to their specific microspherical hollow structure which consists of nitrogen-doped graphene. The nitrogen doping can greatly increase the reactivity and electrical conductivity of graphene-based carbonaceous materials and create more sites that are active for the Li + ion adsorption, while the hollow microspherical structure allows a ready access of the electrolyte to the electrode surface, easier electron and ion transfer, and faster diffusion of electrolytes in and out of the electrode. In addition, the nitrogen-doped graphene and the specific hollow microspherical structure also make the NGHSs highly stable and can buffer against the local volume change during the Li + detercalation and intercalation processes, which give the corresponding LIBs with better cycling stability. [ABSTRACT FROM AUTHOR]

A 3-dimensional porous graphene material (PGM) has been synthesized using a simple two-step process: hydrothermal reaction and calcination. Hydrothermal reaction of graphene oxide (GO) in the presence of resorcinol and glutaraldehyde leads to covalent grafting of partially reduced GO with glutaraldehyde and the deposition of phenolic resin. Subsequent calcination of the composite consisting of phenolic resin deposited on partially reduced GO in the presence of KOH produces structurally stable, highly porous graphene material with a specific surface area of ∼1,066 ± 2 m 2 g −1 . When used as an active electrode material in a lithium battery, the PGM exhibits an initial discharge capacity of ∼1,538 mAh g −1 , which is significantly higher than those of graphite and other carbonaceous materials reported previously. More importantly, when cycled at higher discharge/charge rates, the PGM-based electrodes still deliver large capacities and excellent cycling performance, demonstrating great potential for high-performance lithium-ion batteries. The attractive electrochemical performance of the PGM is attributed to its unique porous structure with large specific surface area and the presence of more disordered carbon atoms produced by the KOH activation. [ABSTRACT FROM AUTHOR]

A facile template-free Ostwald ripening method is developed for the preparation of the reduced graphene oxide supported splode-like NiCo 2 O 4 hollow microsphere (SNHM/rGO). The graphene oxide used in the reaction mixture is found to play a crucial role in the formation of the SNHM/rGO. It promotes the formation of the NiCo-glycerol microspheres suitable for the Ostwald ripening to form the reduced graphene oxide supported hollow NiCo-glycerol microspheres, which is important for the subsequent calcination to form the SNHM/rGO. The obtained SNHM/rGO shows a great promise as the anode for lithium-ion batteries and can deliver a stable reversible capacity of 1048.1 mA h g −1 at the current density of 100 mA g −1 . The performance of the SNHM/rGO is much higher than that of most NiCo 2 O 4 -based materials reported previously, strongly suggesting that the SNHM/rGO could be used as the anode for practical applications. This is well supported by the higher performance of the LiCoO 2 //SNHM−rGO full cell. The excellent electrochemical performance can be attributed to the specific structure of the SNHM/rGO, which comprises the splode-like hollow NiCo 2 O 4 microspheres with the reduced graphene oxide integrated. [ABSTRACT FROM AUTHOR]

High specific capacitance, large surface area, good flexibility and super durability are recognized as advantageous characteristics to acquire ideal supercapacitor, but it is a great challenge to realize them at the same time. Here, a novel cobalt-nickel layered double hydroxide nanoflakes grow on carbon nitride coated N-doped graphene hollow spheres is successfully assembled by a facile chemical bath deposition method. The carbon nitride coated N-doped graphene hollow spheres scaffold not only possesses large surface area for increasing loading of electroactive material but also has hollow structure for accelerating electron and ion transport. Owing to synergistic contributions, the resulting Co 1 Ni 1 layered double hydroxide nanoflakes on carbon nitride coated N-doped graphene hollow spheres with large surface area of 268 m 2 g −1 exhibited enhanced specific capacitance (1815 F g −1 at 1 A g −1 ) and excellent cycling stability (82.1% retention after 4,000 cycles even at 20 A g −1 ). In addition, the capacitance retention still keeps 90.7% after rolling-up about 360° due to wonderful flexibility. Finally, an asymmetric supercapacitor is assembled to further investigate its practical application, which exhibited high energy density of 28.9 Wh kg −1 at an average power density of 1875 W kg −1 and outstanding capacitance retention (no any losses of initial specific capacitance after 10,000 cycles). These advantageous characteristics demonstrate that the new electrode gives a comprehensive application prospect in miniaturized and flexible energy storage. [ABSTRACT FROM AUTHOR]

The sandwiched membrane which consists of the sulfonated holey graphene oxide paper with the deposition of the sulfonated poly(ether ether ketone) (SPEEK/SHGO/SPEEK) membrane on its both sides has been prepared. This membrane is reported to have higher proton conductivity (155.6 ± 1.6 mS cm −1 ), but lower methanol permeability (7.05 ± 0.21 × 10 −6 cm 2 s −1 ) than Nafion ® 112 at 80 °C, whose respective values are 138.6 ± 1.4 mS cm −1 and 15.40 ± 0.46 × 10 −6 cm 2 s −1 , respectively. The specific structure of the SHGO comprising the sulfonated and holey graphitic plane plays an important role in the high performance of the SPEEK/SHGO/SPEEK membrane. First, the sulfonated structure could increase the overall density of the sulfonic acid groups in the membranes, which increases the number of proton exchange groups participating in the proton transport. Second, the holey structure of the SHGO provides an additional path for the proton transport, which well avoids the blockage of the proton transport by the high aspect ratio graphitic planes. Third, the sulfonated and holey structure could also increase the interlayer spacing of the graphitic planes in the SHGO paper, which would promote the proton transport as well. Finally, the presence of the SHGO paper with the layer-by-layer stacked graphitic plane would effectively retard the diffusion of methanol through the membrane. When evaluated in the semi-passive direct methanol fuel cell (DMFC), the cell with the SPEEK/SHGO/SPEEK membrane shows higher stability and exhibits a maximum power density 61.1% higher than with Nafion ® 112, which strongly suggests that the SPEEK/SHGO/SPEEK membrane could be used as the proton exchange membrane for DMFCs. [ABSTRACT FROM AUTHOR]

Sulfonated poly(ether ether ketone) (SPEEK) membranes have been deposited on the both sides of a sulfonic acid functionalized graphene oxide (SGO) paper to form a proton exchange membrane (PEM) with a sandwiched structure. The obtained SPEEK/SGO/SPEEK membrane could exhibit proton conductivity close to Nafion ® 112 and lower methanol permeability. The use of this SPEEK/SGO/SPEEK membrane greatly improves the performance of the semi-passive direct methanol fuel cell (DMFC). The semi-passive DMFC with the SPEEK/SGO/SPEEK membrane is found to be capable of delivering the peak power density 60% higher than that with the commercial Nafion ® 112. This, along with its comparable durability to Nafion ® 112, strongly suggests the great promise of using the SPEEK/SGO/SPEEK membrane as the PEM. [ABSTRACT FROM AUTHOR]