Tese de mestrado em Química Tecnológica, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2012; No desenvolvimento deste trabalho foram caracterizados eléctrodos, utilizando como electrocatalisador o óxido do tipo perovskite LaNiO3, sintetizado através de uma técnica de autocombustão. Também foi testada, como electrocatalisador, uma mistura do óxido LaNiO3 e 5% em massa de nanopartículas de Pt-Ru dispersas em carbono. Como substrato utilizou-se espuma de níquel e papel de carbono.
O pó foi caracterizado por DRX de pós, SEM e BET. A fase cristalina LaNiO3 foi sintetizada com sucesso. As imagens de SEM mostraram que o pó preparado apresenta-se compacto e aglomerado sendo visíveis partículas esféricas e placas. Os estudos de BET para o LaNiO3, LaNiO3 + Pt-Ru e LaNiO3 + C revelaram que as áreas superficiais são 10,7; 6,9 e 11,6 m2 g-1.
Os eléctrodos foram preparados através da técnica de brush painting e foram caracterizados morfologicamente através de técnicas de microscopia: MO, SEM e AFM. As imagens mostraram que filme fino do catalisador possui uma superfície homogénea e compacta apresentando alguma rugosidade.
A caracterização electroquímica foi feita através de medidas de potencial em circuito aberto e voltametria cíclica. Foi possível determinar que o par redox envolvido nos processos que ocorrem na interface eléctrodo/KOH 1M é Ni2+/Ni3+. A determinação dos factores de rugosidade e de morfologia mostrou que os eléctrodos possuem elevada rugosidade e baixa porosidade...
Perovskite-type oxides are potential catalysts for next generation of regenerative fuel cells. In particular, LaNiO3 has been recognised as one of the most promising oxygen electrodes. In this work LaNiO3 perovskite-type oxides, prepared by a self-combustion method [1, 2], have been used for the preparation of porous gas-diffusion electrodes (GDE). Electrodes were prepared on Toray carbon paper (CP) substrates, consisting of a diffusion layer, a catalyst layer and a Nafion® layer. The
gas diffusion layers were prepared using Vulcan XC-72R. The catalyst ink was prepared by suspending the material in isopropanol, stirring the mixture in an ultrasonic bath to thoroughly disperse it. Ink slurries were also pasted onto glassy carbon discs and used as working electrodes for full kinetic studies at potential domains for the oxygen reduction (ORR) and oxygen evolution (OER) reactions. A systematic study on the effect of the oxide loading (OL) on the electrodes surface area was done by cyclic voltammetry. It was found a quasi linear variation between the electrodes surface area and the oxide loading. Roughness values varying from 106±3 to 307±6 were obtained for OL between 1 and 5 mg cm-2 respectively. The results show that the peak current density increases with the increasing on oxide loading as shown in Fig. 1. Higher current densities for ORR were obtained for the electrodes prepared using LaNiO3-based perovskite with partial substitution of Ni by Cu. Stability studies of the GDEs...
In this work LaNiO3 perovskite-type oxide, prepared by a self-combustion method, was optimized for activity and stability as an anode material for water electrolysis. A full electrochemical study was conducted in order to kinetically characterize electrodes prepared using carbon paper as a base for porous gas-diffusion electrodes in alkaline media, regarding water oxidation and oxygen reduction reactions at room temperature. An electrode stability study was performed by potential cycling and at constant current density, using cyclic voltammetry and electrochemical impedance spectroscopy to check on stability after cycling with complementary scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDS) analysis of fresh and degraded electrodes. Comparison was made using nickel foam as a support for LaNiO3 deposition. Carbon instability in the potential region of interest contrasted with the lower contact resistance between the oxide and support of the Ni foam. Higher metal oxide loadings and dimensional stability were also possible.
Perovskites are of great interest when searching replacements for precious metals as catalyst for bifunctional oxygen electrodes involving the oxygen evolution(OER) and oxygen reduction reaction (ORR) as is the case of regenerative fuel cells. In this work a full electrochemical study on the electrochemical properties of gas diffusion electrodes (GDEs) using LaNiO3-based catalysts, conducted in alkaline media, led to a study of cyclability and durability. The incorporation of GDEs in a low power electrolyzer/fuel cell prototype was also attempted. The stability of the electrodes was assessed by potential cycling and at constant current density with good
Resetting pluripotency through nuclear reprogramming and redirecting stem cells into defined lineages underscores remarkable cell fate plasticity. Acquisition of and departure from stemness are governed by genetic and epigenetic controllers. Modulation of energy metabolism and associated signaling is increasingly implicated in cell identity determination. Transition from oxidative metabolism, typical of somatic tissues, into glycolysis is a prerequisite to fuel proficient reprogramming directing a differentiated cytotype back to the pluripotent state. The glycolytic metabotype supports the anabolic and catabolic requirements of pluripotent cell homeostasis. Conversely, redirection of pluripotency into defined lineages requires mitochondrial biogenesis and maturation of efficient oxidative ATP generation and distribution networks to match the evolving bioenergetic demands. The vital function of bioenergetics in regulating stemness and lineage specification implicates a broader role for metabolic reprogramming in cell fate decision and determination of tissue regenerative potential.
Fonte: American Chemical SocietyPublicador: American Chemical Society
Tipo: Artigo de Revista Científica
Publicado em //2014Português
Relevância na Pesquisa
Hybrid porous nanowire arrays composed of strongly interacting Co₃O₄ and carbon were prepared by a facile carbonization of the metal-organic framework grown on Cu foil. The resulting material, possessing a high surface area of 251 m² g⁻¹ and a large carbon content of 52.1 wt %, can be directly used as the working electrode for oxygen evolution reaction without employing extra substrates or binders. This novel oxygen evolution electrode can smoothly operate in alkaline solutions (e.g., 0.1 and 1.0 M KOH), affording a low onset potential of 1.47 V (vs reversible hydrogen electrode) and a stable current density of 10.0 mA cm⁻² at 1.52 V in 0.1 M KOH solution for at least 30 h, associated with a high Faradaic efficiency of 99.3%. The achieved ultrahigh oxygen evolution activity and strong durability, with superior performance in comparison to the state-of-the-art noble-metal/transition-metal and nonmetal catalysts, originate from the unique nanowire array electrode configuration and in situ carbon incorporation, which lead to the large active surface area, enhanced mass/charge transport capability, easy release of oxygen gas bubbles, and strong structural stability. Furthermore, the hybrid Co₃O₄-carbon porous nanowire arrays can also efficiently catalyze oxygen reduction reaction...
The discovery of neural stem cells (NSC) within the adult mammalian brain continues to fuel optimism regarding the ability of potential regenerative medicine applications to provide enhanced functional recovery from brain injuries. The adult NSC population is maintained within a complex microenvironment, referred to as the niche, where a unique cellular and extracellular environment maintains and regulates the NSC population and their progeny, enabling ongoing neurogenesis throughout adulthood. Characterization of how NSC interact with the extracellular environment and other cell subpopulations is an active area of research that will generate fundamental design parameters for biomaterial and tissue engineering strategies for neural tissue repair. A major obstacle to further progress is the lack of access to purified populations of primary NSC, a challenge which became the focus of this thesis. To address this obstacle, experimental methods were developed and optimized for isolating neural stem and progenitor cells (NSPC) from the adult NSC niche with fluorescence activated cell sorting (FACS). These methods were enhanced by the incorporation of a fluorescent reporter mouse driven by the gene Sox2, a neural stem cell associated transcription factor...
La situación energética actual está dominada por los combustibles fósiles, especialmente por el petróleo. Esta dependencia se está conviertiendo en arriesgada debido a las decrecientes reservas, a la incertidumbre de los recursos de petróleo y a las consecuencias económicas y políticas de una concentración de reservas en países de Oriente Medio. El sector transportes tiene una particular dependencia del petróleo como combustible y es una fuente de contaminación debido a su combustión. De cara a reducir esta dependencia se están introduciendo energías renovables como fuentes alternativas de energía. Las más comunes son la eólica y la fotovoltaica, aunque existen otros tipos, como las energías mareomotriz, de gradiente térmico, biomasa o undomotriz. El principal inconveniente de estas energías es su carácter no programable, que fomenta el uso de sistemas de almacenamiento de energía. Las energías renovables mencionadas no son aplicables a vehículos, por lo que para el sector transportes es más interesantes el uso de vectores energéticos tales como el hidrógeno. El hidrógeno puede ser un sustituto del petróleo para aplicaciones vehiculares, pero primero se han de resolver ciertos problemas, como son el elevado coste...
Cycles of reversible hydrogenation reactions are important for at least two different energy-related applications: reversible chemical hydrogen storage and thermally regenerative fuel cells. Hydrogen fuel is a green alternative to conventional hydrocarbon fuels for transportation applications. This is because the combustion product of hydrogen is simply water, which is non-toxic and ubiquitous. Hydrogen is also an attractive fuel because of its high energy content; however, because it is a gas it has poor volumetric energy density. In Chapter 2, ionic liquids consisting of both cations and anions that can undergo reversible dehydrogenative aromatization were used to chemically store hydrogen. Cations investigated included pyridinium ions, which were easily hydrogenated but could not be regenerated through the dehydrogenation of piperidinium ions; and carbazole containing ammonium (whose synthesis failed) and imidazolium (which failed to hydrogenate) cations. The anions studied were heterocyclic carboxylates and sulfonates, these ions were observed to undergo both hydrogenation and dehydrogenation to various degrees when reacted in solution. However, as components of ionic liquids, they fail to react at a significant rate. The viscosity of the fluids was suspected to be hindering the diffusion of either hydrogen or the ions to or from the catalyst surface.
In addition to using hydrogen as the primary source of energy in a vehicle...
A test system for the performance analysis of a novel thermally regenerative fuel cell (TRFC) using propiophenone and hydrogen as the oxidant and fuel respectively was designed and built. The test system is capable of either hydrogen-air or hydrogen-propiophenone operation.
Membrane electrode assemblies (MEAs) were made using commercial phosphoric acid-doped polybenzimidazole (PBI) membranes and commercial electrodes. Using Pt/carbon paper electrodes with a catalyst loading of 1mg/cm2 and a membrane with an acid doping level of 10.2 mol acid/mol of polymer repeat unit, a maximum performance of 212 mW/cm2 at a current density of 575 mA/cm2 was achieved for baseline hydrogen-air testing at 110°C. Problems were encountered, however, in achieving consistent, reproducible performance for in-house fabricated MEAs. Furthermore, ex-situ electrochemical impedance spectrometry (EIS) showed that the phosphoric acid-doped PBI was unstable in the propiophenone and that acid-leaching was occurring.
In order to have MEAs with consistent characteristics for verifying the test system performance, commercial phosphoric acid-doped PBI membrane electrode assemblies were used. At a temperature of 160°C and atmospheric pressure with hydrogen and air flowrates of 150 mL/min and 900 mL/min respectively a maximum power density of 387 mW/cm2 at a current density of 1.1 A/cm2 was achieved. This performance was consistent with the manufacturer’s specifications and these MEAs were subsequently used to verify the performance of TRFC test system despite the EIS results that indicated that acid-leaching would probably occur.
The Pt catalyzed commercial MEAs achieved very limited performance for the hydrogenation of the ketone. However...
Fonte: Fondo Editorial EIAPublicador: Fondo Editorial EIA
Tipo: Artigo de Revista Científica
Publicado em 07/11/2013Português
Relevância na Pesquisa
This paper proposes a power system for fuel cell applications able to transfer energy from the power source to the load, and to charge an auxiliary storage device using regenerative power flows generated by the load. The solution is based on a closed loop bidirectional DC/DC converter, where additional devices have been also designed to experimentally test the solution in a safe and realistic environment: a fuel cell emulator and an electronic load.; Este artículo propone un sistema de potencia para aplicaciones de pilas de combustible capaz de transferir energía de la fuente de potencia a la carga y de cargar un sistema de almacenamiento con flujos regenerativos de potencia desde la carga. La solución está basada en un convertidor bidireccional DC/DC en lazo cerrado. Además, se presentan dispositivos auxiliares diseñados para evaluar experimentalmente la solución en un entorno seguro y realista: un emulador de pila de combustible y una carga electrónica.
The discrete regenerative fuel cell is being developed as a residential power control that synchronizes with a renewables load which fluctuates significantly with the time and weather. The power of proton exchange membrane fuel cells can be scaled-up adjustably to meet the residential power demand. As a result, scale-ups from a basic unit cell with a 25 cm2 active area create a serpentine flow-field on an active area of 100 cm2 and take into account the excessive current and the remaining power obtained by stacking single cells. Operating a fuel cell utilising oxygen produced by the electrolyser instead of air improves the electrochemical reaction and the water balance. Furthermore, the performance test results with oxygen instead of air show almost no hysteresis, which results in the very stable operation of the proton exchange membrane fuel cell as well as the sustainable cycle of water by hydrogen and oxygen mediums.
ABSTRACT: Regenerative fuel cells (RFCs) can provide very high energy storage at minimal weight in a dual mode system, by combining an electrolyzer and a fuel cell. Although RFCs are an appealing technology their development is still at an early stage. One key issue is the search for highly active electrocatalysts for both oxygen reduction and water oxidation. Presently, platinum is the best electrocatalyst for the oxygen reduction reaction (ORR), but has a poor oxygen evolution (OER) performance while metal oxides catalyze the OER but not the ORR. Yet, the search for the development of bi-functional oxygen electrodes is also associated to structurally stable gas diffusion layers - they must be capable of withstanding high potentials when cells are operated in the electrolyzer mode and in addition, mass transport limitations when used as a cathode in fuel cell mode. A novel approach is used in this work to tackle the issue, focussing on the development of stable gas diffusion electrodes for the oxygen reactions, having as a base high surface area LaNiO3. Previous work by the authors has optimised the synthesis of the mentioned perovskite-type oxide, prepared by a self-combustion method. The high electrochemical surface area and low porosity of the oxide has been indicated by electrochemical impedance spectroscopy (EIS) and BET measurements. A full characterization has been the subject of recent publications [1...
One key issue in the development of Regenerative fuel cells (RFCs) is the availability of cheap, highly active electrocatalysts for both oxygen reduction and water oxidation. Perovskite-type oxides, with the general formula ABO3, are potential catalysts for next generation of regenerative fuel cells. In particular, LaNiO3 has been recognised as one of the most promising oxygen electrodes. In this work LaNiO3 perovskite-type oxides, prepared by a self-combustion method [1, 2], have been simultaneously optimized for activity and stability as an anode and cathode material for water oxidation and oxygen reduction reaction (ORR), respectively. Extremely high surface area has been measured by BET analysis with matching electrochemical activity estimated by cyclic voltammetry and electrochemical impedance spectroscopy. A full electrochemical study has been conducted in order to kinetically characterize the prepared electrodes in alkaline media, using a Ni foam and carbon paper as support material for the electrodes. For LaNiO3 deposits on Ni foam, low contact resistance between the oxide and support, possibility of high metal oxide loadings and dimensional stability were accomplished with remarkable stability in the region of oxygen evolution. For LaNiO3 deposits on carbon paper...