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In-situ nanodiamond to carbon onion transformation in metal matrix composites
In the present study, nickel matrix composites reinforced with a fine distribution of nanodiamonds (6.5 vol%) as reinforcement phase are annealed in vacuum at different temperatures ranging from 750 °C to 1300 °C. This is carried out to evaluate the in-situ transformation of nanodiamonds to carbon onions within a previously densified composite. The resulting materials are thoroughly analyzed by complementary analytical methods, including Raman spectroscopy, transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The proposed in-situ transformation method presents two main benefits. On one hand, since the particle distribution of a nanodiamond-reinforced composite is significantly more homogenous than in case of the carbon onions, it is expected that the transformed particles will preserve the initial distribution features of nanodiamonds. On the other hand, the proposed process allows for the tuning of the sp3/sp2 carbon ratio by applying a single straightforward post-processing step.
Vacuum or flowing argon: What is the best synthesis atmosphere for nanodiamond-derived carbon onions for supercapacitor electrodes?
We present a comprehensive study on the influence of the synthesis atmosphere on the structure and properties of nanodiamond-derived carbon onions. Carbon onions were synthesized at 1300 and 1700 °C in high vacuum or argon flow, using rapid dynamic heating and cooling. High vacuum annealing yielded carbon onions with nearly perfect spherical shape. An increase in surface area was caused by a decrease in particle density when transitioning from sp3 to sp2 hybridization and negligible amounts of disordered carbon were produced. In contrast, carbon onions from annealing nanodiamonds in flowing argon are highly interconnected by few-layer graphene nanoribbons. The presence of the latter improves the electrical conductivity, which is reflected by an enhanced power handling ability of supercapacitor electrodes operated in an organic electrolyte (1 M tetraethylammonium tetrafluoroborate in acetonitrile). Carbon onions synthesized in argon flow at 1700 °C show a specific capacitance of 20 F/g at 20 A/g current density and 2.7 V cell voltage which is an improvement of more than 40% compared to vacuum annealing. The same effect was measured for a synthesis temperature of 1300 °C, with a 140% higher capacitance at 20 A/g for argon flow compared to vacuum annealing.
Performance evaluation of conductive additives for activated carbon supercapacitors in organic electrolyte
In this study, we investigate two different activated carbons and four conductive additive materials, all produced in industrial scale from commercial suppliers. The two activated carbons differed in porosity: one with a narrow microporous pore size distribution, the other showed a broader micro-mesoporous pore structure. Electrochemical benchmarking was done in one molar tetraethylammonium tetrafluoroborate in acetonitrile. Comprehensive structural, chemical, and electrical characterization was carried out by varied techniques. This way, we correlate the electrochemical performance with composite electrode properties, such as surface area, pore volume, electrical conductivity, and mass loading for different admixtures of conductive additives to activated carbon. The electrochemical rate handling (from 0.1 A g−1 to 10 A g−1) and long-time stability testing via voltage floating (100 h at 2.7 V cell voltage) show the influence of functional surface groups on carbon materials and the role of percolation of additive particles.