Propiedades de materiales para el conjunto membrana/electrodo de pilas de combustible PEM
2009
| Título | Propiedades de materiales para el conjunto membrana/electrodo de pilas de combustible PEM |
| Nombre | Nores Pondal, Federico Ingeniero en Materiales UNSAM Argentina Doctor en Ciencia y Tecnología, mención Materiales UNSAM |
| Directores | Dr. Corti Horacio Roberto. . . CNEA Departamento Física, Centro Atómico Constituyentes |
| Fecha Defensa | 2009 |
| Jurado | |
| Código | IT/TD-48/09 |
Resumen
Entre los componentes de la pila que presentan un desafío tecnológico y cuya optimización es deseable, se encuentran los catalizadores catódicos y la membrana de electrolito polimérico, objetos de este trabajo de tesis. Actualmente se desarrollan numerosos estudios de catalizadores tendientes a mejorar la cinética lenta de la reacción de reducción de oxígeno (RRO), basados por ejemplo en aleaciones bimetálicas de base Pt que han demostrado, por un lado, aumentar la eficiencia de la RRO, y por otro han permitido bajar los costos al disminuir la cantidad de metal precioso del catalizador. Los esfuerzos actuales se orientan también a encontrar una membrana que pueda reemplazar a la membrana Nafion®, utilizada en la gran mayoría de las pilas de combustibles a pesar de sus inconvenientes, como la imposibilidad de utilizarlas a temperaturas mayores de 100 oC o la alta permeabilidad al metanol en pilas PEM de metanol directo. El objetivo de esta tesis consistió por un lado en la caracterización térmica de membranas de ABPBI preparadas en nuestro grupo, como de membranas de polibenzoimidazol comercial (PBI) y Nafion®, equilibradas a diferentes actividades de agua y en mezclas de agua-metanol, por medio de la técnica calorimétrica diferencial de barrido (CDB). A bajas temperaturas, menores a -73 oC, se observaron transiciones débiles atribuibles a la transición vítrea del polímero en el caso del Nafion, y en el caso del ABPBI y PBI, a la transición vítrea del ácido fosfórico que dopa la membrana. En algunos casos se detectó la presencia de una cantidad considerable de agua que cristaliza dentro de las membranas, a unos pocos grados Celsius bajo cero, reduciendo su conductividad y dificultando el arranque de la pila, y que puede usarse como un criterio al momento de seleccionar la membrana para aplicaciones a baja temperatura.Un segundo objetivo fue la síntesis de catalizadores de Pt y de aleación PtCo, soportados sobre carbón Vulcan XC-72, utilizando cuatro métodos químicos de preparación diferentes. Los catalizadores fueron caracterizados estructuralmente:difracción de rayos X (DRX), microscopía electrónica de transmisión (MET) acoplada con espectroscopia dispersiva en energía (EDX), espectroscopia de fotoelectrones (XPS); químicamente: quimisorción de hidrógeno y deshidrogenación de ciclohexano; y electroquímicamente: voltametría cíclica en electrodo de carbono vítreo, semi-pila y pila de combustible. Los métodos químicos utilizados para la síntesis, probaron producir catalizadores comerciales; y se presentan como métodos simples de baja temperatura, fácil implementación a gran escala y bajo costo.
Complete Title
Abstract
Among the components of the fuel cell that present a technological challenge and for which optimization is desirable, can be mentioned the cathode catalysts and the proton exchange membrane (PEM); objectives of this thesis. Several studies are currently been carried out in order to improve the sluggish kinetics of the oxygen reduction reaction (ORR), using bimetallic Pt based alloys, that have shown an improved activity towards the ORR compared to pure Pt and allowed to lower the cost by diminishing the amount of precious metal. Current efforts focus also on finding a membrane that could replace Nafion®, used in most of the fuel cells even with all its inconvenient, i. e. the impossibility to use them over 100 °C or its high permeability to methanol in direct methanol PEM fuel cells.
One first objective of this thesis consisted in the thermal characterizations of ABPBI (polibenzoimidazol) membranes prepared in our group, together with commercial polibenzoimidazol (PBI) and Nafion® membranes, equilibrated at different water activities and in water-methanol solutions. Weak transitions were observed at temperatures lower than – 73 °C, which were attributed to the glass transition of the polymer in the case of Nafion, and to the glass transition of the phosphoric acid doping the membrane, in the case of ABPBI and PBI. In some cases, a considerable amount of water that crystallizes inside the membrane was observed, a few degrees below zero Celsius, reducing its conductivity and affecting the start up of the fuel cell. Freezable water could be used as a criteria for selecting the membrane material for low temperature application.
A second objective was the synthesis of Pt and PtCo alloy catalysts, supported on Vulcan XC-72 carbon, using four different chemical methods. The structure of the catalysts was characterized by XRD, TEM/EDX and XPS. A chemical characterization was performed by hydrogen chemisorptions and cyclehexane dehydrogenation; and electrochemically characterization by cyclic voltammetry on glassy carbon, half fuel cell and fuel cell. The chemical methods used have proved to produce catalysts with similar, and in some cases even higher, activities than commercial ones, and have the advantages of being simple, low temperature methods with good scalability properties and low cost.
One first objective of this thesis consisted in the thermal characterizations of ABPBI (polibenzoimidazol) membranes prepared in our group, together with commercial polibenzoimidazol (PBI) and Nafion® membranes, equilibrated at different water activities and in water-methanol solutions. Weak transitions were observed at temperatures lower than – 73 °C, which were attributed to the glass transition of the polymer in the case of Nafion, and to the glass transition of the phosphoric acid doping the membrane, in the case of ABPBI and PBI. In some cases, a considerable amount of water that crystallizes inside the membrane was observed, a few degrees below zero Celsius, reducing its conductivity and affecting the start up of the fuel cell. Freezable water could be used as a criteria for selecting the membrane material for low temperature application.
A second objective was the synthesis of Pt and PtCo alloy catalysts, supported on Vulcan XC-72 carbon, using four different chemical methods. The structure of the catalysts was characterized by XRD, TEM/EDX and XPS. A chemical characterization was performed by hydrogen chemisorptions and cyclehexane dehydrogenation; and electrochemically characterization by cyclic voltammetry on glassy carbon, half fuel cell and fuel cell. The chemical methods used have proved to produce catalysts with similar, and in some cases even higher, activities than commercial ones, and have the advantages of being simple, low temperature methods with good scalability properties and low cost.
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