Electrocatalyst development for PEM water electrolysis and DMFC: towards the methanol economy

Abstract

In this thesis, the hydrogen obtained in a PEM water electrolizer (PEMWE) as a reactant to produce methanol when combined with the CO2 captured from the combustion of fossil fuels is proposed. Methanol is easy to manage as a fuel for DMFCs and this would help to recycle the CO2 responsible for the climate change. PEMWEs have several advantages in comparison with the alkaline electrolysis such as ecological cleanness, low power consumption, small mass, and high purity of the evolved gases. TiO2 nanoparticles and nanotubes as supports for electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were developed. TiO2 and Nb-doped TiO2 with different Nb contents (3-10 at.% Nb vs. Ti) were synthesized via sol-gel method, whereas TiO2 nanotubes (TNT) and Nb-doped TiO2 nanotubes (Nb-TNT) were prepared by the hydrothermal method. The specific surface areas were in the range of 80-100 m2 g-1 for nanoparticles and in the range 150 – 300 m2 g-1 for nanotubes. XPS measurements showed a local increase of the electron density on Pt when supported onto Nb-TNT, thus indicating a strong metal-support interaction. According to the electrochemical testing, the highest activity towards HER corresponded to Pt supported on 3 at. % Nb-TNT, obtaining better results than those reported in the literature using other materials. IrO2 and IrRuOx (atomic ratio Ir:Ru equal to 60:40) as OER catalysts were synthesized via the hydrolysis method. From the electrochemical experiments, the highest OER activity of IrO2/Nb-TNT, due to the better dispersion of IrO2 onto the support, was shown. The catalysts supported onto Nb-doped TNT presented the lowest overpotentials for OER. MEAs 5 cm2 in section were prepared using a new low temperature decal method. IrO2, IrRuOx and 50 wt. % IrO2/Nb-TNT were applied as the anode electrocatalysts with a catalyst loading optimized to 2.0 mgoxide cm-2. Pt loading on the cathode was optimized to 0.5 mgPt cm-2 (Pt black and 20 wt. % Pt/Vulcan XC72 were used). The best performance at 80 °C corresponded to current densities of 0.100 and 0.500 A cm-2 at 1.430 and 1.494 V, respectively, with 50 wt.% IrO2/Nb-TNT on the anode and 20 wt. % Pt/Vulcan XC72 on the cathode. The increase in cost of the MEA with respect to the use of unsupported IrO2 was not significant. Different solvents (n-butyl acetate (NBA) and 2-propanol (IPA)) having different polarity were used to prepare the catalyst inks of the DMFC electrodes. The catalysts were commercially available Pt and PtRu blacks. The light scattering experiments indicated that the PtRu-Nafion® aggregates in the inks prepared with NBA were larger. The SEM and porosimetry measurements of the catalyst layers showed that the secondary pore volume between the agglomerates was larger for NBA. The linear sweep voltammetry and electrochemical impedance spectroscopy (EIS) results for the methanol electrooxidation in the three-electrode cell denoted the higher active surface area for NBA. The transport limitation was more apparent for IPA because the corresponding size and porosity of the agglomerates formed by the ionomer and the catalyst nanoparticles were smaller than for NBA. The polarization curves of MEAs in which the anode catalyst layers were formulated with NBA and IPA were recorded in single DMFCs with 2 mol dm-3 CH3OH aqueous solutions at 60 °C. The cathode feed was dry synthetic air at atmospheric pressure. The power density given by the MEA prepared with NBA was about 74 % greater when compared to that prepared with IPA. The interpretation of the EIS results indicated that the proton resistance for NBA was significantly lower than for IPA, thus confirming the greater number of accessible active sites for methanol oxidation in the former.

La economía del metanol contempla el uso de dicho alcohol como combustible, obtenido a partir de hidrógeno y CO2 capturado de la combustión de combustibles fósiles, ayudando a mitigar el cambio climático. Para ello se han preparado nanopartículas y nanotubos de TiO2 y de TiO2 dopados con Nb como soportes de catalizadores para electrolizadores de agua PEM. El Nb permitió aumentar la superficie específica de los soportes hasta 300 m2 g-1 (nanotubos). Mediante XPS se demostró un aumento local de la densidad electrónica sobre el Pt soportado sobre TiO2 dopado con Nb, resultando el de contenido del 3 at. % en Nb el de mejores prestaciones para la reducción del hidrógeno, con valores superiores a los descritos en la literatura. Para el desprendimiento de oxígeno se sintetizaron los catalizadores IrO2 e IrRuOx (Ir: Ru de 60:40 at. %), también aplicados sobre nanotubos de TiO2. Se encontró una mejor actividad para IrO2 soportado sobre nanotubos de TiO2 dopados con Nb debido a una mejor dispersión del catalizador sobre el soporte. Se prepararon MEAs con los mejores electrodos para un electrolizador PEM mediante un nuevo método de calcomanía de baja temperatura. El mejor rendimiento correspondió al IrO2 (50 % en peso) soportado sobre nanotubos de TiO2 dopados con Nb en el ánodo, con escaso impacto económico con respecto al uso del IrO2 sin soportar. En cuanto a la pila de combustible DMFC, se prepararon electrodos de PtRu sin soportar, empleando tintas con Nafion y dos disolventes diferentes, con distinta polaridad, acetato de n-butilo (NBA) y 2-propanol (IPA). El tamaño de los agregados y la porosidad fue superior en NBA debido a su menor polaridad, obteniéndose también en este caso una mayor superficie activa. Las curvas de polarización en CH3OH 2 mol dm-3 y aire a 60 °C de los MEAs formulados con NBA, catalizados mediante negro de PtRu y negro de Pt en ánodo y cátodo, respectivamente, indicaron también mejores prestaciones cuando los MEAs se formularon con NBA en el ánodo en lugar de IPA. La densidad de corriente límite con NBA fue unas tres veces mayor y la densidad de potencia un 75% superior.