Estudio del comportamiento de una celda combustible conformada por un nuevo sistema de conducción protónica de polivinil difluoruro (PVDF) + ácido fosfórico (H3PO4) trabajando hasta 60 °C

Abstract

Se prepararon membranas de intercambio protónico compuestas por fluoruro de polivinilideno (PVDF) y ácido fosfórico (H3PO4) en diversas concentraciones (Ácido/Polímero): 0.08, 0.10, 0.15, 0.20 y 0.60. Los resultados en calorimetría diferencial de barrido (DSC) arrojan tres anomalías térmicas, una alrededor de los 160 °C atribuida a la temperatura de reblandamiento ó Tm de la membrana polimérica pura la cual está conformada con PVDF y disolvente tetrahidrofurano (THF), la segunda alrededor de los 300 °C asociada a la salida de Oxirano y la última asociada a la degradación del sistema polimérico. Se observan cambios no relevantes en las diferentes membranas con ácido para la Tm. No se observaron anomalías térmicas asociadas a la temperatura de transición vítrea (Tg), esto debido posiblemente al rango de medida en temperaturas realizado sobre las muestras que comprende entre los 25 °C y 500 °C. Por otro lado, se realizaron medidas de impedancia compleja en barridos de frecuencia desde 42 Hz a 5 MHz, en un rango de temperaturas entre los 25 °C y 70 °C sobre las diversas muestras arrojando resultados entre los 102 Ω y 104 Ω en la impedancia real y cuyo valor se atribuye posiblemente al contenido de agua en la membrana polimérica ya que, en presencia del ácido fosfórico, este sistema se torna higroscópico. Las muestras sin humedad presentan valores de impedancia real del orden de 106 Ω. Luego de realizar el ajuste y análisis de datos se logró determinar un valor medio en la energía de activación atribuido a los radicales H+, alrededor de 0.54 eV. Los resultados en la parte imaginaria del módulo eléctrico vs. frecuencia (M" vs. ) muestran un solo comportamiento en forma de pico indicando un solo tipo de dinámicas del ion hacia el bulto del material.

Proton exchange membranes composed of polyvinylidene fluoride (PVDF) and phosphoric acid (H3PO4) were prepared in various concentrations (Acid/Polymer): 0.08, 0.10, 0.15, 0.20 and 0.60. The differential scanning calorimetry (DSC) results show three thermal anomalies, one around 160 C attributed to the softening temperature or Tm of the pure polymeric membrane, the second around 300 associated with the release of Oxirane and the last associated with the degradation of the polymer system. Non-relevant changes are observed in the different membranes with acid for Tm. No thermal anomalies associated with the glass transition temperature (Tg) were observed, possibly due to the range of temperature measurements carried out on the samples, which ranges between 30 C and 500 C. On the other hand, complex impedance measurements were carried out in frequency sweeps from 42 Hz to 5 MHz, in a temperature range between 25 C and 70 C on the various samples, yielding results between 102 Ω and 104 Ω and whose value is possibly attributed to the water content in the polymeric membrane and that, in the presence of phosphoric acid, this system becomes hygroscopic. The samples without humidity have impedance values of the order of 106 Ω. After performing the adjustment and data analysis, it was possible to determine an average value in the activation energy attributed to the H+ radicals, around 0.54 eV. The results in the imaginary part of the electrical module vs. frequency (M" vs.) show a single peak-shaped behavior indicating a single type of ion dynamics in the bulk of the material. Morphological characterization measurements were also carried out through the use of the scanning electron microscopy (SEM) technique, which allowed the identification of smaller pores (0.5μm and 1.0μm) with the increase in the concentration of acid on the membranes, favoring ionic mobility. All these studies are carried out with the aim of an application in fuel cells given the limitation in the current density that Commercial cells based on Nafion 1110 have working at more than 35 C, which is why after all the characterizations of the different membranes of the polymeric system, each of them was implemented in a fuel cell prototype, there the behavior could be verified. of the same with the temperature and which membrane gave the best results, in this case the maximum initial voltage obtained was 951mV, and a maximum current of 20 mA with a load resistance, functioning correctly up to 60C and providing the voltage and current necessary to operate a vibrating motor with a resistance of 39 Ω, the best polymeric system for the operation of the cell with the temperature was the one with concentration X=0.60