Direct methanol fuel cell stacks optimization and improvement of components for engineering applications: a design approach
- Autores: Óscar Santiago Carretero
- Directores de la Tesis: Emilio Navarro Arévalo (dir. tes.), Teresa Leo Mena (codir. tes.)
- Lectura: En la Universidad Politécnica de Madrid ( España ) en 2021
- Idioma: inglés
- Materias:
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- Resumen
Direct methanol fuel cells (DMFCs) show outstanding advantages over internal combustion engines in terms of simplicity, modularity, greenhouse emissions and pollutants, levels of vibrations and noise, reliability and efficiency. Despite all this, some issues linger, thereby limiting the wide commercialization of this technology for different purposes such as portable applications or in vehicles with low power demands such as unmanned aerial vehicles (UAVs) or autonomous underwater vehicles (AUVs). Among these aspects are included the high fuel consumption and the low power densities and specific power of the stacks, as well as the high cost, fundamentally associated with catalysts and membranes, and the high methanol crossover through the membrane that constrains the efficient methanol use and the general performance of this type of fuel cells. The aim of this doctoral dissertation is to address these issues from different design-oriented approaches, in order to improve the potential commercialization of DMFCs.
An insufficiently studied subject in the field of DMFC is the influence of the stack constructive parameters on their power density, their specific power and their methanol consumption. In the same manner, the optimization of these parameters in a systematic way from early design stages to enhance the general performance of the stacks has not been tackled in depth either. In this sense, in this doctoral dissertation a computer-aided automatic design system has been developed in order to optimize the constructive parameters of DMFC stacks from early stages of the design, demonstrating the substantial influence these parameters play on mass, volume and methanol consumption. Likewise, it is highlighted the importance of properly assigning the weights of the functions to be optimized based on the application to which the stack is intended. Thus, within the Pareto front of optimal solutions for the design of a 230 W stack, and associated only with the variation of constructive parameters, there may be variations greater than 10 %, 18 % and 6 % in terms of power density, specific power and methanol consumption, respectively.
One of the constructive parameters with the greatest influence on specific power and power density identified by the developed design system is the material of the bipolar plates (BPs). The reason is that this component may represent more than 80 % of the total stacks mass and practically all of its volume. In order to reduce the mass and general cost of the DMFC stacks, the selection of the most appropriate thermoplastic materials to be used as BPs of DMFC stacks with different applications has been carried out. In this context, 7 different polymers have been investigated, namely, ABS, CPE, CPE+, Nylon, PC, PLA y TPU 95A. To solve these material selection problems, four multi-criteria decision-making techniques have been implemented: TOPSIS, COPRAS, SAW and AHP. These methods are derived from decision-making theory widely used in material selection problems. However, these methods require as input information data adequately describing the performance of the corresponding materials in the application under study. For this reason, long-term experiments have been conducted with the cited polymers in a simulated environment that reproduces the conditions in a DMFC. According to these long-term experiments, PLA undergoes extreme degradation in a very short period of time, thereby preventing its use in DMFC. On the contrary, ABS and PC are, based on the decision-making methods, the preferred materials for the three proposed DMFC applications: stationary, small portable devices and UAVs.
Nevertheless, the two selected thermoplastic materials are not conductors of electricity, which is an essential functional requirement to be used as BPs. In order to provide this characteristic to the potential BPs, two methods of metal deposition have been analyzed: the electroless Ni deposition by hydrophobic interaction and the physical vapor deposition of Au by sputtering. By means of immersion in a simulated DMFC environment, the Au sputtering deposits have been found to be significantly more durable than electroless Ni deposits. In this way, different Au deposition times have been investigated in terms of contact angle, concluding that longer deposition times give rise to more hydrophobic surfaces with better results in the case of ABS.
Another DMFC component notably related to the aspects to be improved previously described are the electrodes. In this case, Pt electrodes have been developed by means of Pt-Cu pulsed electrodeposition with subsequent reoxidation of Cu so that a porous structure with a high electrochemical surface area (ECSA) is generated. Such a technique derives from the literature research conducted on glucose fuel cells where it is a widely used technique, particularly on the development of implantable glucose fuel cells. For the sake of optimizing the Pt deposition and increase the catalyst utilization factor, three parameters of the deposition process have been studied: the concentration of sulfuric acid, the stirring over deposition process and the reduction pulse length. Through the estimations associated with the hydrogen adsorption area of the cyclic voltammograms, it has been verified the great influence of the sulfuric acid concentration and the transport processes on the roughness factor, the mass of Pt deposited and the electrochemical surface area. Higher ECSAs were achieved when low sulfuric acid concentrations and an unstirred deposition solution were used.
Finally, the potential application in DMFCs of different sulfonated styrene-ethylene-butylene-styrene (sSEBS) membranes modified by the direct infiltration of a zirconia phosphosilicate gel has been studied. These membranes were prepared jointly by the ENAP and SGEG groups belonging to Spanish National Research Council. SEBS is a very affordable raw material, i.e. its use could reduce the total cost of DMFC stacks, but it is subjected to severe mechanical and dimensional limitations, hence the need to infiltrate said polymer to improve its characteristics, especially to reduce methanol crossover. To determine the potential of these membranes in DMFCs, polarization curve, limiting crossover current density and impedance tests have been carried out. The latter in order to figure out the conductivity of the membranes. These tests confirmed the higher current densities and lower crossover of the infiltrated membranes compared to the pure sSEBS membrane. In fact, longer infiltration times led to better results, being particularly noteworthy the performances obtained with the membrane infiltrated over 40 minutes. In addition, these hybrid membranes achieved lower limiting crossover current densities than commercial Nafion 112, highlighting its potential use in DMFCs.
The design tool developed, the polymeric materials studied with their corresponding metallic coating, the electrodes investigated and the hybrid membranes tested make possible to improve the characteristics of the DMFCs in terms of power density, specific power, fuel consumption and cost; all of which could help to achieve a wide commercialization of this type of fuel cells.
