Carbon electrode for the oxygen reduction reaction

This PhD thesis presents work on developing freestanding carbon electrodes for the oxygen reduction reaction application cost-effectively and sustainably. Within different alternatives to the high-cost Pt catalysts, heteroatoms and transitional metals modified carbon electrocatalysts have shown great promise to reduce the use of Pt. Meanwhile, synthesising freestanding catalysts has drawn interest due to the advantages of being binder-free, fewer manufacturing steps, and high recyclability. The first part of this thesis focuses on synthesising a freestanding carbon electrode with a hierarchical porosity and abundant nitrogen-doped sites. The carbon electrodes were synthesised through hydrothermal carbonization, followed by a pelleting process and further carbonization. Uniformly dispersed nitrogen sites and high specific surface area were obtained for the carbon electrodes. The electrochemical activity showed high stability in the freestanding configuration, and I found only the surface of electrode was reducing oxygen. The second part focuses on improving the carbon electrode's catalytic performance via post functionalization of the as-obtained nitrogen-doped carbon electrodes. Functionalization was carried out by immersing the carbon electrode into Fe solutions and followed by carbonization. The Fe was found to exist mainly as single sites. The electrochemical performance showed doubled current density compared to without Fe, and 100,000 s (27.77 h) stability was observed at 0.5 V. Through ex-situ X-ray absorption spectroscopy and electron paramagnetic resonance studies, Fe sites were found responsible for reducing oxygen. The third part focuses on the scalable synthesis of a low-cost iron, nitrogen co-doped carbon. Powdered iron, nitrogen co-doped carbon catalysts was prepared by hydrothermal carbonization and high-temperature post carbonization. FeN4 was found to be the main iron existing form in the obtained catalysts. Two different precursors containing Fe2+ and Fe3+ are compared. Both chemical and structural differences have been observed in catalysts starting from Fe2+ and Fe3+ precursors. Furthermore, this catalyst is studied in an anion exchange membrane fuel cell.Open Acces

SYNTHESIS AND CHARACTERIZATIONS OF OXIDIZERS FOR BIOCIDAL NANOENERGETIC APPLICATIONS

Nanoenergetic materials have been shown to have reactive properties superior to traditional energetic materials since the nanoscale enables more intimate mixing of fuel and oxidizer to reduce the heat and mass transport limitations. The growing threat of biological weapons has prompted research efforts into new energetic materials with biocidal capabilities. Most notably the biocidal nanothermite involves the aluminum fuel with a strong oxidizer, which releases the biocide. In this thesis two kinds of attempted synthesis and characterizations were covered for this biocidal nanoenergetic topic: (1) the highly hygroscopic strong oxidizer I2O5 was successfully passivated into the Fe2O3 shell, which exhibited excellent combustion performance with biocidal capabilities when formulated into aluminum based nanothermite reaction; (2) the copper iodate and iron iodate particles were prepared by co-precipitation and demonstrated to be good candidates for the potential biocidal energetic applications

Preference-based spectrum pricing in dynamic spectrum access networks

With market-driven secondary spectrum trading, licensed users can receive benefits in terms of monetary rewards or various transmission services, thus setting a fair pricing structure by suitably defining spectrum quality characteristics and accurately addressing participant’s requirement is a key issue. In this paper, we investigate the pricing-based spectrum access by casting the problem of spectrum pricing into a Hotelling game model according to spectrum quality diversity. Particularly, we first build a pricing system model where unused spectrum from primary systems with different qualities forms a spectrum pool and can be divided into a number of uniform channels. A secondary user purchases a channel for usage according to its selection preference which is closely related to the channel quality and spectrum evaluation. The secondary user not only needs to consider the channel’s quality and price, but also the interference cost on primary system. Detailed analysis on the policy preference of both primary system and secondary buyer are provided. By forming a game problem of spectrum pricing between primary and secondary users, we apply the Hotelling game model to handle the interaction between the participants. Specifically, by fixing Nash equilibrium of the game, an iterative algorithm for spectrum pricing is proposed based on the distribution characteristics of secondary user’s preference. Essential analysis for the existence and uniqueness of the Nash equilibrium along with algorithm’s convergence conditions are provided. Numerical results are also supplemented to show the effectiveness of the proposed algorithm in ensuring spectrum owner’s profit

Simulation of the calcination of a core-in-shell CuO/CaCO 3 particle for Ca–Cu chemical looping

The internal heat balance through heat generation due to CuO reduction and its consumption by CaCO3 decomposition makes calcination a critical step in a novel Ca–Cu chemical looping process (CaL–CLC). Thus, the calcination behaviour of composite Ca/Cu particles needs to be well understood, especially taking into account that mismatching of heat generation and consumption in the particles can lead to local superheating, agglomeration and loss of activity due to enhanced sintering. In this work, a composite particle model was developed to study the calcination behaviour within a spherical core-in-shell type of particle containing grains of CuO and CaCO3. Simulation results showed that ambient temperature, shell porosity, particle size, and CaCO3 grain size significantly affected the CuO and CaCO3 reaction processes, while the impact of initial particle temperature and CuO grain size can be ignored in the range of parameters considered in the study. By comparison of different types of particles, it was concluded that the core-in-shell pattern was more advantageous if such particles are being applied in CaL–CLC cycles due to better matching in reaction kinetics resulting in more stable and uniform particle temperature distribution during the calcination stage

3-D motion recovery via low rank matrix analysis

Skeleton tracking is a useful and popular application of Kinect. However, it cannot provide accurate reconstructions for complex motions, especially in the presence of occlusion. This paper proposes a new 3-D motion recovery method based on lowrank matrix analysis to correct invalid or corrupted motions. We address this problem by representing a motion sequence as a matrix, and introducing a convex low-rank matrix recovery model, which fixes erroneous entries and finds the correct low-rank matrix by minimizing nuclear norm and `1-norm of constituent clean motion and error matrices. Experimental results show that our method recovers the corrupted skeleton joints, achieving accurate and smooth reconstructions even for complicated motions

Lattice strain effects on the optical properties of MoS2 nanosheets.

"Strain engineering" in functional materials has been widely explored to tailor the physical properties of electronic materials and improve their electrical and/or optical properties. Here, we exploit both in plane and out of plane uniaxial tensile strains in MoS2 to modulate its band gap and engineer its optical properties. We utilize X-ray diffraction and cross-sectional transmission electron microscopy to quantify the strains in the as-synthesized MoS2 nanosheets and apply measured shifts of Raman-active modes to confirm lattice strain modification of both the out-of-plane and in-plane phonon vibrations of the MoS2 nanosheets. The induced band gap evolution due to in-plane and out-of-plane tensile stresses is validated by photoluminescence (PL) measurements, promising a potential route for unprecedented manipulation of the physical, electrical and optical properties of MoS2

3-D motion recovery via low rank matrix restoration on articulation graphs

This paper addresses the challenge of 3-D skeleton recovery by exploiting the spatio-temporal correlations of corrupted 3D skeleton sequences. A skeleton sequence is represented as a matrix. We propose a novel low-rank solution that effectively integrates both a low-rank model for robust skeleton recovery based on temporal coherence, and an articulation-graph-based isometric constraint for spatial coherence, namely consistency of bone lengths. The proposed model is formulated as a constrained optimization problem, which is efficiently solved by the Augmented Lagrangian Method with a Gauss-Newton solver for the subproblem of isometric optimization. Experimental results on the CMU motion capture dataset and a Kinect dataset show that the proposed approach achieves better recovery accuracy over a state-of-the-art method. The proposed method has wide applicability for skeleton tracking devices, such as the Kinect, because these devices cannot provide accurate reconstructions of complex motions, especially in the presence of occlusion