Platinum deposition on functionalised graphene for corrosion resistant oxygen reduction electrodes.

Graphene-related materials are promising supports for electrocatalysts due to their stability and high surface area. Their innate surface chemistries can be controlled and tuned via functionalisation to improve the stability of both the carbon support and the metal catalyst. Functionalised graphenes were prepared using either aryl diazonium functionalisation or non-destructive chemical reduction, to provide groups adapted for platinum deposition. XPS and TGA-MS measurements confirmed the presence of polyethyleneglycol and sulfur-containing functional groups, and provided consistent values for the extent of the reactions. The deposited platinum nanoparticles obtained were consistently around 2 nm via reductive chemistry and around 4 nm via the diazonium route. Although these graphene-supported electrocatalysts provided a lower electrochemical surface area (ECSA), functionalised samples showed enhanced specific activity compared to a commercial platinum/carbon black system. Accelerated stress testing (AST) showed improved durability for the functionalised graphenes compared to the non-functionalised materials, attributed to edge passivation and catalyst particle anchoring.

Real-time mechanistic study of carbon nanotube anion functionalisation through open circuit voltammetry.

The mechanism of the functionalisation of reduced single walled carbon nanotubes with organobromides was monitored by open circuit voltammetry throughout the reaction and further elucidated through a series of comparative reactions. The degree of functionalisation was mapped against the reagent reduction potential, degree of electron donation of substituents (Hammett parameter), and energies calculated, ab initio, for dissociation and heterolytic cleavage of the C-Br bond. In contrast to the previously assumed reduction/homolytic cleavage mechanism, the reaction was shown to consist of a rapid association of carbon-halide bond to the reduced nanotube as a complex, displacing surface-condensed countercations, leading to an initial increase in the net nanotube surface negative charge. The complex subsequently slowly degrades through charge transfer from the reduced single-walled carbon nanotube to the organobromide, utilizing charge, and the carbon-halide bond breaks heterolytically. Electron density on the C-Br bond in the initial reagent is the best predictor for degree of functionalisation, with more electron donating substituents increasing the degree of functionalisation. Both the mechanism and the new application of OCV to study such reactions are potentially relevant to a wide range of related systems.

Hybrid Energy Storage of Ni(OH)2-coated N-doped Graphene Aerogel//N-doped Graphene Aerogel for the Replacement of NiCd and NiMH Batteries.

Although Nickel-Cadmium (NiCd) and Nickel-metal hydride (NiMH) batteries have been widely used, their drawbacks including toxic Cd and expensive La alloy at the negative electrodes, low energy density (40-60 Wh/kg for NiCd and 140-300 Wh/L for NiMH), low power density (150 W/kg for NiCd and 1000 W/kg for NiMH), and low working potential (1.2 V) limit their applications. In this work, Cd and La alloy were replaced with N-doped reduced graphene oxide aerogel (N-rGOae) providing a hybrid energy storage (HES) having the battery and supercapacitor effects. The HES of Ni(OH)2-coated N-rGOae//N-rGOae provides 1.5 V, a specific energy of 146 Wh/kg, a maximum specific power of 7705 W/kg, and high capacity retention over 84.6% after 5000 cycles. The mass change at the positive electrode during charging/discharging is 8.5 µg cm-2 owing to the insertion/desertion of solvated OH- into the α-Ni(OH)2-coated N-rGOae. At the negative electrode, the mass change of the solvated K+, physically adsorbed/desorbed to the N-rGOae, is 7.5 µg cm-2. In situ X-ray absorption spectroscopy (XAS) shows highly reversible redox reaction of α-Ni(OH)2. The as-fabricated device without using toxic Cd and expensive La alloy has a potential as a candidate of NiCd and NiMH.

High-Performance Asymmetric Supercapacitors of MnCo2O4 Nanofibers and N-Doped Reduced Graphene Oxide Aerogel.

The working potential of symmetric supercapacitors is not so wide because one type of material used for the supercapacitor electrodes prefers either positive or negative charge to both charges. To address this problem, a novel asymmetrical supercapacitor (ASC) of battery-type MnCo2O4 nanofibers (NFs)//N-doped reduced graphene oxide aerogel (N-rGOAE) was fabricated in this work. The MnCo2O4 NFs at the positive electrode store the negative charges, i.e., solvated OH-, while the N-rGOAE at the negative electrode stores the positive charges, i.e., solvated K+. An as-fabricated aqueous-based MnCo2O4//N-rGOAE ASC device can provide a wide operating potential of 1.8 V and high energy density and power density at 54 W h kg-1 and 9851 W kg-1, respectively, with 85.2% capacity retention over 3000 cycles. To understand the charge storage reaction mechanism of the MnCo2O4, the synchrotron-based X-ray absorption spectroscopy (XAS) technique was also used to determine the oxidation states of Co and Mn at the MnCo2O4 electrode after being electrochemically tested. The oxidation number of Co is oxidized from +2.76 to +2.85 after charging and reduced back to +2.75 after discharging. On the other hand, the oxidation state of Mn is reduced from +3.62 to +3.44 after charging and oxidized to +3.58 after discharging. Understanding in the oxidation states of Co and Mn at the MnCo2O4 electrode here leads to the awareness of the uncertain charge storage mechanism of the spinel-type oxide materials. High-performance ASC here in this work may be practically used in high-power applications.