Carbon Corrosion Induced by Surface Defects Accelerates Degradation of Platinum/Graphene Catalysts in Oxygen Reduction Reaction.

Graphene supported electrocatalysts have demonstrated remarkable catalytic performance for oxygen reduction reaction (ORR). However, their durability and cycling performance are greatly limited by Oswald ripening of platinum (Pt) and graphene support corrosion. Moreover, comprehensive studies on the mechanisms of catalysts degradation under 0.6-1.6 V versus RHE (Reversible Hydrogen Electrode) is still lacking. Herein, degradation mechanisms triggered by different defects on graphene supports are investigated by two cycling protocols. In the start-up/shutdown cycling (1.0-1.6 V vs. RHE), carbon oxidation reaction (COR) leads to shedding or swarm-like aggregation of Pt nanoparticles (NPs). Theoretical simulation results show that the expansion of vacancy defects promotes reaction kinetics of the decisive step in COR, reducing its reaction overpotential. While under the load cycling (0.6-1.0 V vs. RHE), oxygen containing defects lead to an elevated content of Pt in its oxidation state which intensifies Oswald ripening of Pt. The presence of vacancy defects can enhance the transfer of electrons from graphene to the Pt surface, reducing the d-band center of Pt and making it more difficult for the oxidation state of platinum to form in the cycling. This work will provide comprehensive understanding on Pt/Graphene catalysts degradation mechanisms.

Regulating the Pore Structure of Activated Carbon by Pitch for High-Performance Sodium-Ion Storage.

The pore structure of carbon anodes plays a crucial role in enhancing the sodium storage capacity. Designing more confined pores in carbon anodes is accepted as an effective strategy. However, current design strategies for confined pores in carbon anodes fail to achieve both high capacity and initial Coulombic efficiency (ICE) simultaneously. Herein, we develop a strategy for utilizing the repeated impregnation and precarbonization method of liquid pitch to regulate the pore structure of the activated carbon (AC) material. Driven by capillary coalescence, the pitch is impregnated into the pores of AC, which reduces the specific surface area of the material. During the carbonization process, numerous pores with diameters less than 1 nm are formed, resulting in a high capacity and improved ICE of the carbon anode. Moreover, the ordered carbon layers derived from the liquid pitch also enhance the electrical conductivity, thereby improving the rate capability of as-obtained carbon anodes. This enables the fabricated material (XA-4T-1300) to have a high ICE of 91.1% and a capacity of 383.0 mA h g-1 at 30 mA g-1. The capacity retention is 95.5% after 300 cycles at 1 A g-1. This study proposes a practical approach to adjust the microcrystalline and pore structures to enhance the performance of sodium-ion storage in materials.

Activated Carbon Based Supercapacitors with a Reduced Graphene Oxide Additive: Preparation and Properties.

We have successfully enhanced the performance of commercial supercapacitors that use Japan Kuraray 80F activated carbon and Super-P conductive carbon black as the conductive agent with reduced graphene oxide (rGO) additive. The ratios of conductive carbon black to rGO studied are 3:1, 5:1, 10:1, 15:1 and 1:0. The enhancement is most pronounced at 15:1, and the specific capacitance being 137.5 F g-1, which is a 23.8% improvement over the 1:0 control. The specific capacitance retention is 70.1% after 10000 cycles. The impedance resistance is also reduced to 1.5 Ω, which is 3.3 times lower than the 1:0 control. Additionally, the rGO additive does not alter the favorable pore size distribution of the primary matrix and successfully preserves its small mesoporous structure, which facilitates facile transport of electrolyte.

Unusual high oxygen reduction performance in all-carbon electrocatalysts.

Carbon-based electrocatalysts are more durable and cost-effective than noble materials for the oxygen reduction reaction (ORR), which is an important process in energy conversion technologies. Heteroatoms are considered responsible for the excellent ORR performance in many carbon-based electrocatalysts. But whether an all-carbon electrocatalyst can effectively reduce oxygen is unknown. We subtly engineered the interfaces between planar graphene sheets and curved carbon nanotubes (G-CNT) and gained a remarkable activity/selectivity for ORR (larger current, and n = 3.86, ~93% hydroxide + ~7% peroxide). This performance is close to that of Pt; and the durability is much better than Pt. We further demonstrate the application of this G-CNT hybrid as an all-carbon cathode catalyst for lithium oxygen batteries.We speculate that the high ORR activity of this G-CNT hybrid stems from the localized charge separation at the interface of the graphene and carbon nanotube, which results from the tunneling electron transfer due to the Fermi level mismatch on the planar and curved sp(2) surfaces. Our result represents a conceptual breakthrough and pioneers the new avenues towards practical all-carbon electrocatalysis.

A graphene-based nanostructure with expanded ion transport channels for high rate Li-ion batteries.

A graphene-based nanostructure with expanded Li(+) transport channels is reported, which is characterized by high capacity and excellent rate performance as an anode material for Li-ion batteries. The expanded structure is obtained by employing linear polymers as the spacers in the stacking process of graphene nanosheets.

Graphene oxide hydrogel at solid/liquid interface.

A strong solid/liquid interfacial interaction is found between porous alumina and graphene oxide (GO) aqueous dispersion, which promotes a fast enrichment of GO on the alumina surface and results in the formation of a GO hydrogel.