Estudo e desenvolvimento de conjuntos membrana-eletrodos (MEA) para célula a combustível de eletrólito polimérico condutor de prótons (PEMFC) com eletrocatalisadores à base de paládio

Abstract

PEMFC systems are capable of generating electricity with high efficiency and low or no emissions, but durability and cost issues prevent its large commercialization. In this work MEA with palladium based catalysts were developed, Pd/C, Pt/C and alloys PdPt/C catalysts with different ratios between metals and carbon were synthesized and characterized. A study of the ratio between catalyst and Nafion Ionomer for formation of high performance triple-phase reaction was carried out, a mathematical model to implement this adjustment to catalysts with different relations between metal and support taking into account the volumetric aspects of the catalyst layer was developed and then a study of the catalyst layer thickness was performed. X-ray diffraction, Transmission and Scanning Electron Microscopy, X-ray Energy Dispersive, Gas Pycnometry, Mercury Intrusion Porosimetry, Gas adsorption according to the BET and BJH equations, and Thermo Gravimetric Analysis techniques were used for characterization and particle size, specific surface areas and lattice parameters determinations were also carried out. All catalysts were used on MEAs preparation and evaluated in 5 cm 2 single cell from 25 to 100 °C at 1 atm and the best composition was also evaluated at 3 atm. In the study of metals for reactions, to reduce the platinum applied to the electrodes without performance losses, Pd/C and PdPt/C 1:1 were selected for anodes and cathodes, respectively. The developed MEA structure used 0,25 mgPt.cm-2 , showing power densities up to 550 mW.cm-2 and power of 2.2 kW net per gram of platinum. The estimated costs showed that there was a reduction of up to 64.5 %, compared to the MEA structures previously known. Depending on the temperature and operating pressure, values from US$ 1,475.30 to prepare MEAs for each installed kilowatt were obtained. Taking into account recent studies, it was concluded that the cost of the developed MEA is compatible with PEMFC stationary application. FIGURA 11-Curvas de polarização de um MEA com Pd/C 20 % Mm. Estrutura com Membranas Nafion 115, GDLs MF15, ECs com 20 % Mm , eletrodos de 5 cm 2 com 0,4 mgPd.cm-2 e 35 % NM. Avaliações com variação da temperatura do H 2 entre 25 e 100 ºC e com as temperaturas do O 2 e da célula unitária sendo estudadas entre 5 e 15 °C abaixo dessa, conforme TAB. 8. H 2 com pureza 5.0, suprido com vazão de 160 mL.min-1 a 1 atm e O 2 com pureza 4.0, suprido com vazão de 80 mL.min-1 a 1 atm, ambos saturados com água pura ....... FIGURA 12-Densidades de corrente a 500, 600, 700 e 800 mV de um mesmo MEA com Pd/C 20 % Mm em ambos os eletrodos. Estrutura com Membrana Nafion 115, GDLs MF15, eletrodos de 5 cm 2 com 0,4 mgPd.cm-2 e 35 % NM. Avaliações com variação da temperatura do H 2 entre 25 e 100 ºC e com as temperaturas do O 2 e da célula unitária sendo estudadas entre 5 e 15 °C abaixo dessa, conforme TAB. 8. H 2 com pureza 5.0, suprido com vazão de 160 mL.min-1 a 1 atm e O 2 com pureza 4.0, suprido com vazão de 80 mL.min-1 a 1 atm, ambos saturados com água pura ..