Electrochemical and Computational Study of Oxygen Reduction Reaction on Non-Precious Transition Metal/Nitrogen Doped Carbon Materials

An important challenge in low temperature fuel cell development is to find high performance non-precious catalysts replacing expensive platinum cathode catalysts for oxygen reduction reaction (ORR). The recently developed transition metal/nitrogen doped carbon-based materials (TM-N-C) exhibited catalytic activity close to that of platinum, but there are still many questions under debate. In this thesis, electrochemical and computational methods were employed to reveal the ORR mechanism on TM-N-C catalysts. In the experimental aspect, a cost-effective two-step process, electrospinning and pyrolysis, was developed to produce the TM-N-C catalysts. The electrochemical techniques were applied to the catalytic property characterization of the synthesized catalysts, showing that the Fe-N-C catalyst possesses higher activity than the Co-N-C catalyst and could promote 4e- pathway, while O2 electroreduction was found to proceed mainly with 2e- pathway yielding hydrogen peroxide on the Co-N-C catalyst. In the computational aspect, the first-principles density functional theory (DFT) was employed to calculate the adsorption energies and activation energies, elucidating the ORR mechanism on TM-N4 type active sites. Based upon the calculation results, the linear correlation between the O2 adsorption energy and the non-bonding d orbitals center in the transition metal macrocyclic complexes was established. The free energy diagram extracted from the calculation results uncovered that the ORR could happen through 4e- associative pathway on the FeN4 site, whereas might end with a 2e- pathway on the CoN4 site due to high energy barrier for O-O bond splitting, supporting the experimental observations. The superior activity of TM-N-C catalysts in alkaline than in acid was well-explained by the activity loss of the coexisted metal-free active sites induced by pyridinic nitrogen protonation in acid. Moreover, the microkinetic analysis was demonstrated to interpret DFT calculated energy parameters as polarization curves. With this tool, the study of the local carbon structures in the Fe-N-C catalysts directly revealed that the introduction of micropores could enhance their catalytic activity through facilitating the formation of FeN4-C8 active sites with high specific activity

Modulating Porous Carbon Electrocatalyst for Efficient Water Splitting

The increasing public issues about the energy crisis urge the development of sustainable energy as alternatives to replace the fossil fuels. Considering the energy regeneration and environment friendly, hydrogen possesses the potential to meet the criterion of renewable and clean energies. H2 can be produced in an electrochemical water electrolyser by cathodic hydrogen evolution reaction (HER), coupled with anodic oxygen evolution reaction (OER). The kinetic barrier of both reactions require efficient electrocatalysts. However, the benchmarking electrocatalysts for HER or OER are based on precious metals, such as Pt or Ir, their high cost greatly hinders the practical H2 production from water electrolyser in an economic manner. Thus, the search of in-expensive but efficient HER and OER electrocatalysts is imminent. Among various candidature materials, low cost and high conductive carbon based nanomaterials have attracted intensive attention. Through heteroatom doping, the inert carbon nanomaterials can be activated to show promising catalytic activity. In this study, nitrogen doped nanoporous carbon electrocatalysts were obtained from thermal pyrolysis of a zinc based metal-organic framework. Cathodic treatment is successfully applied to achieve systematic modulation of the type and surface functionalities. The modulated electrocatalysts show high activity and good stability towards hydrogen and oxygen evolution in various electrolyte. Strong correlation between catalytic performance and surface chemical properties of these carbon electrocatalysts has been found. My work here paves a new way to design metal free carbon electrocatalysts for future green energy applications

Multifunctional Carbon Fibre Reinforced Hierarchical Composites based on Vertical Graphene

Carbon fibre reinforced polymer (CFRP) is becoming increasingly popular in various areas owing to its high strength to weight ratio and high resistance to corrosion compared with metal rivals. However, there are also some drawbacks, such as poor resistance to crack propagation, low electrical and thermal conductivity in thickness direction, which are dominated by the inherent nature of interface and matrix in composites. These drawbacks limited the wider application of CFRP, while developing multifunctional composite is a good solution to these issues. The additive nature of the manufacturing processes of CFRP makes it possible to introduce other functionalities, such as improved strength, improved electrical conductivity, and energy storage capability. This thesis aims to develop carbon fibre (CF) reinforced multifunctional composites by combining CFs with vertical graphene (VG). Such a combination can improve the mechanical and electrical properties of CFRP and create a platform for the introduction of other nanomaterials, such as silver nanowires (AgNWs) and manganese dioxides (MnO2) nanoflowers, to develop multifunctional composites. Firstly, VGs were deposited onto CFs through a plasma enhanced chemical vapor deposition. Multiple characterizations were then conducted on the resulted VG modified CFs. These results showed that VGs grafted on the CF dramatically increased the surface roughness and surface wettability. The interfacial shear strength between CF and epoxy matrix was enhanced without significant tensile strength degradation on CF. Next, conductive AgNWs were added into epoxy matrix and combined with VG modified CFs to improve the electrical conductivity of composites, which has potential application in the lightning strike protection of composite structures. Then, MnO2 nanoflowers were introduced onto the VG modified CFs through an electrochemical deposition process and enabled the CFs with energy storage capability. A structural supercapacitor was developed as an example of multifunctional composites with VG&MnO2 modified CF electrodes and polymer electrolyte. Lastly, aiming at the poor cycling stability of MnO2 modified CF electrode, a multilayer electrode structure was developed to achieve the long-term cycling stability. In achieving these, important scientific contributions are made to the developing methods of multifunctional CF reinforced composites, which broad the application of CFRP and enable the further lightweight design of CFRP in the future

Noise and Gain Characterization of Interband Cascade Infrared Photodetectors

Infrared (IR) detectors are an enabling technology for a broad and growing list of applications including gas detection, night vision, and space-based missile warning. There are ongoing efforts in IR detector research to explore the potential of new material systems and energy band structures in addition to continuously improving their sensitivity through increasing their quantum efficiency and lowering their dark current and noise. This dissertation examines an emerging class of IR detectors known as Interband Cascade Infrared Photodetectors (ICIPs). ICIPs contain multiple regions to facilitate the collection of photogenerated electrons and to limit unwanted dark current. Theory regarding their performance also indicates that multi-stage ICIPs may have lower noise than single-stage ICIPs and may provide improved detectivity in cases where the absorption coefficient of a material system is small and/or where the diffusion length in the material is short or degraded. In this work, four long-wavelength infrared ICIP devices with one, four, six, and eight stages were characterized at varying temperatures from 80 to 300 K and at biases up to one volt in both forward and reverse polarities. Noise spectra were collected on the four devices and show significant 1/f noise that prevented direct measurement of the ICIP noise gain. The 1/f noise in the ICIPs was correlated to generation-recombination current. The devices were found to cause circuit instability when operated in bias regions with negative differential conductance (NDC) due to bias-dependent resonant tunneling. Additionally, bias-dependent photocurrent gain was observed using illumination of the devices with 632 nm and 1550 nm lasers which peaked near the NDC regions. This photocurrent gain was experimentally shown to be linked to current-mismatch between device stages, supporting theories regarding its origin

Development of Electro-active Graphene Nanoplatelets and Composites for Application as Electrodes within Supercapacitors

The mounting concern for renewable energies from ecologically conscious alternatives is growing in parallel with the demand for portable energy storage devices, fuelling research in the fields of electrochemical energy storage technologies. The supercapacitor, also known as electrochemical capacitor, is an energy storage device possessing a near infinite life-cycle and high power density recognized to store energy in an electrostatic double-layer, or through a pseudocapacitance mechanism as a result of an applied potential. The power density of supercapacitors far exceeds that of batteries with an ability to charge and discharge stored energy within seconds. Supercapacitors compliment this characteristic very well with a cycle life in excess of 106 cycles of deep discharge within a wide operational temperature range, and generally require no further maintenance upon integration. Conscientious of environmental standards, these devices are also recyclable. Electrochemical capacitors are currently a promising candidate to assist in addressing energy storage concerns, particularly in hybridized energy storage systems where batteries and supercapacitors compliment each other’s strengths; however specific challenges must be addressed to realize their potential. In order to further build upon the range of supercapacitors for future market applications, advancements made in nanomaterial research and design are expected to continue the materials development trend with a goal to improve the energy density through the development of a cost-efficient and correspondingly plentiful material. However, it is important to note that the characteristic power performance and exceptional life-cycle should be preserved alongside these efforts to maintain their niche as a power device, and not simply develop an alternative to the average battery. It is with this clear objective that this thesis presents research on an emerging carbon material derived from an abundant precursor, where the investigations focus on its potential to achieve high energy and power density, stability and integration with other electroactive materials. Activated carbons have been the dominant carbon material used in electric double-layer capacitors since their inception in the early 1970s. Despite a wide range of carbon precursors and activation methods available for the generation of high surface area carbons, difficulties remain in controlling the pore size distribution, pore shape and an interconnected pore structure to achieve a high energy density. These factors have restricted the market growth for supercapacitors in terms of the price per unit of energy storage. Activation procedures and subsequent processes for these materials can also be energy intensive (i.e. high temperatures) or environmentally unfriendly, thus the challenge remains in fabricating an inexpensive high surface-area electroactive material with favourable physical properties from a source available in abundance. Double-layer capacitive materials researched to replace active carbons generally require properties that include: high, accessible surface-area; good electrical conductivity; a pore size distribution that includes mesopore and micropore; structural stability; and possibly functional groups that lend to energy storage through pseudocapacitive mechanisms. Templated, fibrous and aerogel carbons offer an alternative to activated carbons; however the drawbacks to these materials can include difficult preparation procedures or deficient physical properties with respect to those listed above. In recent years nanostructured carbon materials possessing favourable properties have also contributed to the field. Graphene nanoplatelet (GNP) and carbon nanotube (CNT) are nanostructured materials that are being progressively explored for suitable development as supercapacitor electrodes. As carbon lattice structured materials either in the form of a 2-dimensional sheet or rolled into a cylinder both of these materials possess unique properties desirable in for electrode development. In the proceeding report, GNPs are investigated as a primary material for the synthesis of electrodes in both a pure and composite form. Three projects are presented herein that emphasize the suitability of GNP as a singular carbon electrode material as well as a structural substrate for additional electroactive materials. Investigation in these projects focuses on the electrochemical activity of the materials for supercapacitor devices, and elucidation of the physical factors which contribute towards the observed capacitance. An initial study of the GNPs investigates their distinct capacitive ability as an electric double-layer material for thin-film applications. The high electrically conductivity and sheet-like structure of GNPs supported the fabrication of flexible and transparent films with a thickness ranging from 25 to 100 nm. The thinnest film fabricated (25 nm) yielded a high specific capacitance from preliminary evaluation with a notable high energy and power density. Furthermore, fast charging capabilities were observed from the GNP thin film electrodes. The second study examines the use of CNT entanglements dispersed between GNP to increase the active surface area and reduce contact resistances with thin-film electrodes. Through the use MWNT/GNP and SWNT/GNP composites it was determined that tube aspect ratio influences the resulting capacitive performance, with the formation of micropores in SWNT/GNP yielding favourable results as a composite EDLC. The third study utilizes electrically conducting polypyrrole (PPy) deposited onto a GNP film through pulse electrodeposition for use as a supercapacitor electrode. Total pulse deposition times were evaluated in terms of their corresponding improvements to the specific capacitance, where an optimal deposition time was discovered. A significant increase to the total specific capacitance was observed through the integration PPy, with the majority charge storage being developed via psuedocapacitive redox mechanisms. A summary of the studies presented here centers on the development of GNP electrodes for application in high power supercapacitor devices. The potential use for GNP in both pure and composite electrode films is explored for electrochemical activity and capacitive capabilities, with corresponding physical characterization techniques performed to examine influential factors which contribute to the final results. The work emphasizes the suitability of GNP material for future investigations into their application as carbon or carbon composite electrodes in supercapacitor devices

East and Southeast Asian energy transition and politics

Talking about the energy transition in East Asia is tantamount to placing it at the threshold of both a historical and transitional relationship with energy needs, production and consumption in the East and Southeast Asian context. For this reason, each of the articles in this Special Issue replaces, in their own way, the question of energy policies in the historical evolution experienced by the countries of the region or by regional inter-governmental bodies (Mekong Commission, the Association of Southeast Asian Nations (ASEAN), ASEAN + 2, +3, +4). Despite the endless vows to maintain a collegial and cooperative spirit amongst partners and neighbors of the region and despite international pressures/agreements, these bodies are constantly struggling in achieving their original mission. In varying degrees, diversity characterizes not only the methods of managing this energy transition, but also the strategies adopted to respond to energy and de facto environmental challenges. Furthermore, it needs to be recognized that these environmental challenges are being approached more from a domestic policy platform than from an international or global one. The three chapters below highlight the complexity of the energy policies adopted in East and Southeast Asia, which is the subject of political and economic arbitrations being played out on several fronts (national, regional, international) in institutionalized forms (ASEAN, COP21, etc.) or otherwise (bilateral inter-ministerial decisions). The great range of actions and challenges, the insufficient coordination of energy policies and the competition between different governmental and institutional actors, have, until now, negated the possibility of a common, unified, and unidirectional Asian or Southeast Asian policy.Parler de la transition énergétique en Asie de l'Est revient à la placer au seuil d'une relation à la fois historique et transitoire avec les besoins, la production et la consommation d'énergie dans le contexte de l'Asie de l'Est et du Sud-Est. Pour cette raison, chacun des articles de ce numéro spécial remplace, à sa manière, la question des politiques énergétiques dans l'évolution historique vécue par les pays de la région ou par les instances intergouvernementales régionales (Commission du Mékong, Association of Southeast Nations asiatiques (ASEAN), ASEAN + 2, +3, +4). Malgré les vœux sans fin de maintenir un esprit collégial et coopératif entre les partenaires et voisins de la région et malgré les pressions / accords internationaux, ces organismes ont constamment du mal à réaliser leur mission initiale. À des degrés divers, la diversité caractérise non seulement les modes de gestion de cette transition énergétique, mais aussi les stratégies adoptées pour répondre aux enjeux énergétiques et environnementaux de facto. En outre, ces défis environnementaux sont davantage abordés à partir d'un point de vue de politique intérieure plutôt qu’internationale ou mondiale. Les trois chapitres ci-dessous mettent en évidence la complexité des politiques énergétiques adoptées en Asie de l'Est et du Sud-Est, qui fait l'objet d'arbitrages politiques et économiques se déroulant sur plusieurs fronts (national, régional, international) sous des formes institutionnalisées (ASEAN, COP21, etc.) ou autre (décisions interministérielles bilatérales). Le large éventail d'actions et de défis, la coordination insuffisante des politiques énergétiques et la concurrence entre les différents acteurs gouvernementaux et institutionnels ont jusqu'à présent écarté la possibilité d'une politique commune, unifiée et unidirectionnelle en Asie, de l’Est ou du Sud-Est