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Plasma Enabled and Graphene Allowed Synthesis of Unique nano Structures

Periodic Reporting for period 2 - PEGASUS (Plasma Enabled and Graphene Allowed Synthesis of Unique nano Structures)

Reporting period: 2018-11-01 to 2020-04-30

PEGASUS primary objective is the design and construction of an operational proof-of-concept machine for large-scale production (e.g. 10 mg/min) of nitrogen-doped graphene (N-graphene) and N-graphene based nanocomposites. To this end, a controllable, single-step, catalyst-free and environmentally benign plasma-enabled novel method for assembling of free-standing N-graphene sheets with prescribed structural qualities, is being developed. The subsequent goal is to develop a plasma enabled manufacturing method for N-graphene based nanocomposites comprising metal oxides (MO) as well as metal sulphides through a single-step at atmospheric pressure conditions that avoids harsh chemistry and cumbersome batch processes; along with developing a procedure to cover the produced nanocomposites with conductive polymers. Considering the extension of N-graphene properties in the third dimension, the development of graphene-based 3D architectures, such as unique vertical N-graphene arrays grown on metal substrates and their hybrids, is also pursued. The targeted electrochemical performance of the considered structures will allow their assessment as electrode constituents in a proof-of-concept supercapacitor device.
Free-standing N-graphene sheets and derivatives:
• Production of free-standing N-graphene (NG) sheets with controllable doping level (up to 8 at % N), residual amounts (~1%) of oxygen, relatively high yield (~0.5 mg/min) and selectable nitrogen doping configurations is achieved by using a small-scale proof-of-concept device based on a novel, single-step plasma driven process at atmospheric pressure conditions. The type and level of doping is controlled by the position where N-precursor is injected into plasma environment. The N-graphene synthesized in the framework of PEGASUS project has demonstrated superior quality when compared with high-quality N-graphene produced by a commercial reference supplier, while being nearly one order of magnitude cheaper.
• The overall optimization of the manufactured PEGASUS plasma machine for large-scale N-graphene fabrication, along with completion of patent portfolio, is in progress. After the first trials, the rate of N-graphene synthesis is approaching the 10 mg/minute mark. The current estimated production cost for 1gr N- graphene is 50 €/g.
• A novel method for a single-step, microwave plasma driven controllable assembly of nanocomposites comprising N-graphene sheets decorated with metal oxide and metal sulphides nanoparticles at atmospheric pressure was created. A pending patent was issued.
• Printing of N-graphene on dielectric surfaces was accomplished.
Vertically aligned graphene nanostructures at metal substrates
• Vertically aligned N-graphene structures were fabricated using low-pressure RFICP cold nitrogen plasma treatment, and a high doping level (~8-12 at %N) was attained.
• A novel fast and facile method was developed for fabricating polymer/MO/vertical N-graphene hybrid composite.
• A novel fast and facile method for the controlled formation of the Ni3S2 on the Ni-anchored vertical graphene nanostructure was developed by simple thermal annealing in the H2S environment. Vertical graphene/N-graphene nanostructures and Ni3S2/Ni@VCN hybrid nanostructure were tested as a binder-free electrode for lithium-ion batteries (LIBs). The Ni3S2/Ni@VCN hybrid electrode attained one of the highest specific capacity among the binder-free electrodes for LIBs.
• Vertically oriented carbon nanostructures were successfully grown on Ni-foam in a very short time (~ 8 min) at atmospheric pressure conditions by using a hybrid microwave-DC plasma reactor.
Electrochemical performance
• The test regarding high frequency response of N-graphene has demonstrated a maximum specific capacitance of 300 µF and phase angle of -80˚ at 100 Hz.
• The test of N-graphene as conductive agent in activated carbon slurry has demonstrated an improvement in specific capacitance and rate capability performance. Its asymmetric assembly with iron cobalt sulphide delivered an impressive energy density of 94 W h kg-1 at power density of 975 W kg-1.
• An electrode based on vertical carbon nanostructures grown on Ni and decorated with Ni3S2 have demonstrated an excellent electrochemical performance with a specific capacitance of 400 C g-1 at a current density 2 A g-1 after 450 cycles. The electrode retained 84% (337.6 C g -1) of its initial value at a high current density of 10 A g -1.
• Proof of concept using N-graphene as conductive agent in activated carbon slurry was established. In that, symmetric assembly of two electrodes were tested which delivered specific capacitance of 1.15 F at 0.01 A and stability up to 77 % after 5000 cycles
• Prototype using graphene as conductive agent “cell-foam-graphene” was assembled which demonstrated a very good charge storage property with a high coulombic efficiency of 98%.
Contrasting with other graphene & derivatives production methods available, PEGASUS created a novel disruptive technology that ensures the continuous production of consistent batches of high-quality N-graphene&derivatives (nanocomposites comprising N-graphene sheets decorated with metal oxide and metal sulphides nanoparticles) for incorporation into quality-demanding applications. Our solution is simple to use, versatile and scalable, meeting the requirements of the most exigent graphene-enabled product developers.
The first step towards a fast and controllable (~8 min) microwave/RF plasma-assisted method for the synthesis of vertically aligned carbon nanowalls (multilayer graphenes) on different substrates was performed, applying various approaches at low and atmospheric presure conditions. Both in-situ and ex-situ N-doping approaches were performed on vertical graphenes to obtain different N-configurations. The processes were upgraded to design hybrid graphene structures, which could provide a novel platform for further development of targeted energy-related devices.
The energy-related applications potential of N-graphene was tested at three fronts: in high frequency response application, in energy storage applications and in improving the conductivity of activated carbon slurry. In this regard, an electrospray technique was developed to make uniform and thin deposits of N-graphene on the current collector. Aiming an excellent energy storage performance, several composites based on metal compounds and N-graphene have been synthesized and tested. N graphene was used and tested as conductive agent by replacing commercial conductive agent in activated carbon slurry to make electrodes. The optimum ratio of activated carbon and graphene reduced the response time to 4 s compared to 25.1 s obtained using commercial conductive agent.
RFCC plasma for the production of vertically aligned graphene structures
Pegasus supercapacitor cell used in the project as a working prototype.
SEM image of grown 3D vertically aligned graphenes/N-graphenes on substrate.
Scanning Electron Microscopy image of N-graphene sheets
1 gram of N-graphene sheets as synthesized
The illumination of the RF plasma with a laser sheet reveals the clouds of levitating nanoparticles.
Analyzing the samples at BESSY II, Berlin
Microwave plasma delivering free-standing N-graphene sheets