| Summary: |
Direct methanol fuel cells (DMFC) have a great potential, especially as power supply for small electronic devices. To become a competitive technology, however, DMFC has to resolve two problems: the methanol crossover and the low power density. This project aims to contribute for resolving the above-mentioned drawbacks.
It comprehends two approaches: i) a new strategy towards proton exchange membrane (PEM) doping for decreasing the methanol crossover and a new strategy for supporting the electrochemical catalyst and ii) a challenging approach where it is proposed the use of a very recently described new steam reforming (SR) catalyst for upgrading the fuel from methanol to H2 inside the anode chamber. This catalyst is able to perform the SR of methanol at temperatures as low as 150 °C with 100 % conversion and no formation of CO. The new catalyst is made of La2CuO4 nanofibers and can easily be inserted in the anode gas diffusion layer (GDL). This catalyst will upgrade the fuel from methanol to hydrogen, which shows higher power densities, in the range of 1 Wcm-2, and energy efficiencies, over 50 %. Moreover, because the SR reaction is endothermic and the fuel cell operation is exothermic, there is a synergetic effect when following this approach. Additionally, both the methanol and H2 electro-oxidation are catalyzed by the electrochemical PtRu catalyst.
In the most widely used PEM, Nafion, protons are transported through nanosized channels using liquid water as vehicle. Since methanol and water are miscible, methanol crosses the membrane driven by the different concentrations in both sides of the membrane. Operational temperatures of more than 100 °C are detrimental for DMFC performance using Nafion due to increased methanol crossover. Additionally, the content of liquid water inside Nafion nanopores becomes an important issue. Alternatively, polybenzimidazole (PBI) doped with phosphoric acid (PA) membranes is dense and less permeable to methanol and has shown t  |
Summary
Direct methanol fuel cells (DMFC) have a great potential, especially as power supply for small electronic devices. To become a competitive technology, however, DMFC has to resolve two problems: the methanol crossover and the low power density. This project aims to contribute for resolving the above-mentioned drawbacks.
It comprehends two approaches: i) a new strategy towards proton exchange membrane (PEM) doping for decreasing the methanol crossover and a new strategy for supporting the electrochemical catalyst and ii) a challenging approach where it is proposed the use of a very recently described new steam reforming (SR) catalyst for upgrading the fuel from methanol to H2 inside the anode chamber. This catalyst is able to perform the SR of methanol at temperatures as low as 150 °C with 100 % conversion and no formation of CO. The new catalyst is made of La2CuO4 nanofibers and can easily be inserted in the anode gas diffusion layer (GDL). This catalyst will upgrade the fuel from methanol to hydrogen, which shows higher power densities, in the range of 1 Wcm-2, and energy efficiencies, over 50 %. Moreover, because the SR reaction is endothermic and the fuel cell operation is exothermic, there is a synergetic effect when following this approach. Additionally, both the methanol and H2 electro-oxidation are catalyzed by the electrochemical PtRu catalyst.
In the most widely used PEM, Nafion, protons are transported through nanosized channels using liquid water as vehicle. Since methanol and water are miscible, methanol crosses the membrane driven by the different concentrations in both sides of the membrane. Operational temperatures of more than 100 °C are detrimental for DMFC performance using Nafion due to increased methanol crossover. Additionally, the content of liquid water inside Nafion nanopores becomes an important issue. Alternatively, polybenzimidazole (PBI) doped with phosphoric acid (PA) membranes is dense and less permeable to methanol and has shown to be stable for high temperature PEM fuel cells (150 ºC - 230 ºC) operating at low hydration levels. However the performance reduces with time due to leaking of PA. In the framework of this project, it is proposed the synthesis of PEMs able to work at temperatures higher than 100 ºC with reduced methanol crossover. Composite polymeric membranes will be prepared by incorporating hydrophilic proton conducting nanoparticles of acid derivatives of zirconia (ZrO2) and yttria stabilized zirconia (YSZ) in the polymeric matrix. The hydrophilicity is very important, mainly in Nafion, because dehydration is prevented. Also, the use of proton conducting nanoparticles having strong acid groups will not decrease proton conductivity. Actually, the nanometric and narrow size distribution of the particles will enhance the proton conduction properties due to a better grain to grain transfer and a higher proton exchange surface.
The electrochemical activity will be improved by increasing the operational temperature and by using high surface area and electric conductive single wall carbon nanohorns (SWNH) as support for the catalyst nanoparticles. SWNH have a similar graphitic structure as carbon nanotubes, ca. 2-3nm in diameter and ca. 50 nm in length, and self-assemble in aggregates of about 100 nm. This feature increases the surface area (ca. 300 m2g-1) while maintaining the electrical a high conductivity due to a closer contact between SWNH. Moreover, the auto assembled aggregates allows that gas and liquid easily permeate across, gaining straightforward access to the catalyst when used as support. |