Bo Ouyang

Rapid Synthesis of Cobalt Nitride Nanowires: Highly Efficient and Low‐Cost Catalysts for Oxygen Evolution

Electrochemical splitting of water to produce hydrogen and oxygen is an important process for many energy storage and conversion devices. Developing efficient, durable, low-cost, and earth-abundant electrocatalysts for the oxygen evolution reaction (OER) is of great urgency. To achieve the rapid synthesis of transition-metal nitride nanostructures and improve their electrocatalytic performance, a new strategy has been developed to convert cobalt oxide precursors into cobalt nitride nanowires through N2 radio frequency plasma treatment. This method requires significantly shorter reaction times (about 1 min) at room temperature compared to conventional high-temperature NH3 annealing which requires a few hours. The plasma treatment significantly enhances the OER activity, as evidenced by a low overpotential of 290 mV to reach a current density of 10 mA cm(-2) , a small Tafel slope, and long-term durability in an alkaline electrolyte.

Catalyst-Free Plasma Enhanced Growth of Graphene from Sustainable Sources

Details of a fast and sustainable bottom-up process to grow large area high quality graphene films without the aid of any catalyst are reported in this paper. We used Melaleuca alternifolia, a volatile natural extract from tea tree plant as the precursor. The as-fabricated graphene films yielded a stable contact angle of 135°, indicating their potential application in very high hydrophobic coatings. The electronic devices formed by sandwiching pentacene between graphene and aluminum films demonstrated memristive behavior, and hence, these graphene films could find use in nonvolatile memory devices also.

Plasma surface functionalization induces nanostructuring and nitrogen-doping in carbon cloth with enhanced energy storage performance

A facile, one-step and environmentally-friendly strategy for the preparation of hierarchical nitrogen-doped carbon cloth (hNCC) is presented via nitrogen plasma processing of commercial carbon cloth. In addition to N-doping, the RF plasma treatment induces nanostructuring, thus significantly increasing the surface area and liquid electrolyte wettability. The untreated carbon cloth (CC) has negligible Li-ion and supercapacitive storage capacities. However, after plasma treatment, the obtained hNCC delivers dramatically enlarged capacities. Specifically, the enhancement is by three times for Li-ion storage (150 mA h g−1versus 50 mA h g−1 at 100 mA g−1), and by three orders of magnitude for pseudocapacitance (391 mF cm−2versus 0.12 mF cm−2 at 4 mA cm−2). The effect of power-dependent plasma treatment for optimized performance is also investigated. We propose a plausible mechanism for achieving a hNCC architecture with highly enhanced ion/charge storage properties. Our research provides an approach to fabricate N-doped carbon materials with a controllable surface morphology and electrochemical properties. This deterministic and plasma-based method of preparing hNCC may offer new opportunities in the design and fabrication of high-performance carbon-based electrodes for energy storage devices.

Ultrathin CNTs@FeOOH nanoflake core/shell networks as efficient electrocatalysts for the oxygen evolution reaction

Transition metal (oxy)hydroxides are a class of promising non-noble metal based electrocatalysts utilized for the water oxidation reaction but suffer from poor electrical conductivity. Herein, we report on CNTs@ultrathin FeOOH nanoflake core/shell networks on a carbon cloth (CNTs@FeOOH/CC) for the oxygen evolution reaction (OER). With the assistance of a layer of ZnO formed via atomic layer deposition (ALD), ultrathin FeOOH nanoflakes are uniformly grown on CNTs. The CNT cores serve as highly conductive channels to facilitate the transfer of electrons, which effectively enhances the electrical conductivity of FeOOH. Furthermore, the interwoven network structure increases the mass loading and utilization of FeOOH. As a result, the CNTs@FeOOH/CC catalyst exhibits a high OER performance, with features such as a low onset overpotential, large anodic current density, small Tafel slope and excellent long-term electrolysis durability, which are highly desirable for a promising OER electrocatalyst.

Nitrogen-Plasma-Activated Hierarchical Nickel Nitride Nanocorals for Energy Applications

Developing transition metal nitrides with unique nanomorphology is important for many energy storage and conversion processes. Here, a facile and novel one-step approach of growing 3D hierarchical nickel nitride (hNi3 N) on Ni foam via nitrogen plasma is reported. Different from most conventional chemical synthesis, the hNi3 N is obtained in much shorter growth duration (≤15 min) without any hazardous or reactive sources and oxide precursors at a moderate reaction zone temperature of ≤450 °C. Among possible multifunctionalities of the obtained nanocoral hNi3 N, herein the performance in reversible lithium ion storage and electrocatalytic oxygen evolution reaction (OER) is demonstrated. The as-obtained hNi3 N delivers a considerable cycling performance and rate stability as a lithium ion battery anode, and its property can be further enhanced by coating the hNi3 N surface with graphene quantum dots. The hNi3 N also serves as an active OER catalyst with high activity and stability. Additionally, on the basis of controlled growth under different nitrogen plasma treatment time, the formation mechanism of the nanocoralline hNi3 N is outlined for further extension to other materials. The results on time- and energy-efficient nitrogen-plasma-based preparation of hNi3 N pave the way for the development of high-performance metal nitride electrodes for energy storage and conversion.

Green synthesis of vertical graphene nanosheets and their application in high-performance supercapacitors

Vertical standing graphene sheets are highly desirable in energy storage applications because without π–π stacking their surface can be fully utilized. In this work, vertical graphene nanosheets (VGS) are successfully synthesized on nickel foam via a simple plasma enhanced chemical vapor deposition (PECVD) technique. Instead of hazardous and costly hydrocarbon gases, we adopt a green approach by using a low-cost, non-toxic, sustainable and environmentally-friendly natural organic material, M. alternifolia essential oil (containing a hydrocarbon monomer), as the precursor. The 4 minute deposition duration results in multilayered horizontal graphene (h-GS) with sparsely distributed vertical graphene while 16 minute deposition leads to fully covered vertical graphene nanosheets (f-VGS). To demonstrate their application as a conductive and high-surface-area substrate in energy storage, MnO2 thin films are hydrothermal grown to form MnO2@f-VGS core–shell structure and MnO2@h-GS. The core@shell electrode of MnO2@f-VGS demonstrates a significantly higher specific capacitance of 203 F g−1 at a current density of 10 A g−1 compared to that of 82 F g−1 at 10 A g−1 shown by MnO2@h-GS. Moreover, the assembled full supercapacitors containing MnO2@f-VGS‖active carbon as electrodes can deliver a reasonably high specific capacitance of 250 F g−1 at 2 A g−1. Such f-VGS may have application also in battery and fuel cell electrodes.

Prereduction of Metal Oxides via Carbon Plasma Treatment for Efficient and Stable Electrocatalytic Hydrogen Evolution

Prereduction of transition metal oxides is a feasible and efficient strategy to enhance their catalytic activity for hydrogen evolution. Unfortunately, the prereduction via the common H2 annealing method is unstable for nanomaterials during the hydrogen evolution process. Here, using NiMoO4 nanowire arrays as the example, it is demonstrated that carbon plasma (C-plasma) treatment can greatly enhance both the catalytic activity and the long-term stability of transition metal oxides for hydrogen evolution. The C-plasma treatment has two functions at the same time: it induces partial surface reduction of the NiMoO4 nanowire to form Ni4 Mo nanoclusters, and simultaneously deposits a thin graphitic carbon shell. As a result, the C-plasma treated NiMoO4 can maintain its array morphology, chemical composition, and catalytic activity during long-term intermittent hydrogen evolution process. This work may pave a new way for simultaneous activation and stabilization of transition metal oxide-based electrocatalysts.

C-Plasma of Hierarchical Graphene Survives SnS Bundles for Ultrastable and High Volumetric Na-Ion Storage

Tin and its derivatives have provoked tremendous progress of high-capacity sodium-ion anode materials. However, achieving high areal and volumetric capability with maintained long-term stability in a single electrode remains challenging. Here, an elegant and versatile strategy is developed to significantly extend the lifespan and rate capability of tin sulfide nanobelt electrodes while maintaining high areal and volumetric capacities. In this strategy, in situ bundles of robust hierarchical graphene (hG) are grown uniformly on tin sulfide nanobelt networks through a rapid (5 min) carbon-plasma method with sustainable oil as the carbon source and the partially reduced Sn as the catalyst. The nucleation of graphene, CN (with size N ranging from 1 to 24), on the Sn(111) surface is systematically explored using density functional theory calculations. It is demonstrated that this chemical-bonded hG strategy is powerful in enhancing overall electrochemical performance.

Carbon-Based Nanomaterials Using Low-Temperature Plasmas for Energy Storage Application

The increasing demands for energy serve as the essential factor of current research and development toward innovative energy storage systems. Hence, lithium-ion batteries (LIBs) and supercapacitors (SCs) have received considerable interests, in which carbon-based materials have been tremendously investigated as active material as well as in nano-composite form. Among various strategies, plasma is a novel approach due to its unique environment, high efficiency, shorter duration, and large scalability. This chapter provides an overview of carbon-based architectures as electrode materials for LIBs and SCs via plasma-based approaches, especially carbon nanotubes (CNTs) and vertical graphene nanosheets (VGNSs). The chapter starts with a general introduction of two energy storage devices, specific active materials for different types of devices as well as advantages and challenges of these systems. Subsequently, plasma-based approaches for synthesizing CNTs and VGNSs are reviewed. The utilization of carbon-based composites for enhanced electrochemical performance is also presented. To conclude, the main aim of this chapter is to provide an outline of recent progresses and perspectives on carbon-based architectures as electrodes for LIBs and SCs.

Green plasma route for nitrogen functionalized vertical graphene synthesis using sustainable resources

Summary form only given. In this paper we report the synthesis of nitrogen functionalized vertical graphene (N-VG) on nickel foam and silicon substrates using radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) system. Two different environmental friendly green plasma approaches were adopted where we tried to achieve: (i) direct synthesis of N-VG by simultaneous use of nitrogen and hydrogen gas along with Pelargonium Graveolens (geranium) oil as carbon source in RF-PECVD system and (ii) in-direct synthesis of N-VG, the vertical graphene (VG) samples were first synthesized by using Pelargonium Graveolens oil and hydrogen gas in RF-PECVD system and then exposed to nitrogen plasma at different RF power, discharge duration and nitrogen flow rate. Pelargonium Graveolens essential oil is used for the first time as a carbon precursor to make our approach environmentally friendly and sustainable. We have earlier reported the use of another essential oil, tea tree oil, as a sustainable resource for graphene synthesis1. The direct synthesis of N-VG and nitrogen plasma exposure of pre-synthesized VG was tried under the various deposition parameters like the RF-PECVD reactor zone temperature, nitrogen gas flow rate, and power and duration of RF plasma discharge. SEM and Raman spectroscopy results shows that the direct synthesis of N-VG failed as no VG was observed in SEM image and Raman results also did not show characteristic Raman peaks for VG or carbon in general. It was found that nitrogen ions have strong etching effect on the deposited material and did not allow the growth of carbon nanostructures or VG. The second method of processing the pre-deposited VG by nitrogen plasma, at lower RF power, however allowed VG to survive after nitrogen plasma exposure if the plasma duration is kept low. The XPS analysis confirms the formation of nitrogen functionalized VGs for nitrogen plasma processed pre-deposited VGs.