Browsing by Author "Oliveira, V. B."
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- A direct methanol fuel cell with low methanol crossover and high methanol concentrations :modelling and experimenal studiesPublication . Oliveira, V. B.; Rangel, C. M.; Pinto, A. M. F. R.The direct methanol fuel cell (DMFC) with proton exchange membrane (PEM) as electrolyte and liquid methanol/water as the energy carrier is a promising power source for micro and various portable electronic devices (mobile phones, PDA’s, laptops and multimedia equipment). However a number of issues need to be resolved before DMFC can be commercially viable such as the methanol crossover and water crossover which must be minimised in portable DMFC’s. In the present work, a detailed experimental study on the performance of an «in-house» developed DMFC with 25cm2 of active membrane area, working near ambient conditions (ambient temperature and pressure) is described. Tailored MEAS (membrane electrode assemblies), with different structures and combinations of gas diffusion layers (GDL), were designed and tested in order to select optimal working conditions at relatively high methanol concentration levels without sacrificing performance. The experimental polarization curves were successfully compared with the predictions of a steady state, one-dimensional model accounting for coupled heat and mass transfer, along with the electrochemical reactions occurring in the DMFC recently developed by the same authors. The influence of the anode gas diffusion layer media, the membrane thickness and the MEA properties on the cell performance is explained under the light of the predicted methanol crossover rate across the membrane
- Experimental and modeling studies of a micro direct methanol fuel cellPublication . Falcão, D. S.; Oliveira, V. B.; Rangel, C. M.; Pinto, A. M. F. R.The Direct Methanol Fuel Cell (DMFC) has attracted much attention due to its potential applications as a power source for transportation and portable electronic devices. Based on the advantages of the scaling laws, miniaturization promises higher efficiency and performance of power generating devices and the MicroDMFC is therefore an emergent technology. In this work, a set of experiments with a MicroDMFC of 2.25 cm2 active area are performed in order to investigate the effect of important operating parameters. Maximum power density achieved was 32 mW/cm2 using a 4 M methanol concentration at room temperature. Polarization curves are compared with mathematical model simulations in order to achieve a better understanding of how parameters affect performance. The one-dimensional model used in this work takes in account coupled heat and mass transfer, along with the electrochemical reactions occurring in a direct methanol fuel cell and was already developed and validated for DMFC in previous work by Oliveira et al. [1–3]. The model is also used to predict some important parameters to analyze fuel cell performance, such as water transport coefficient and methanol crossover. This easy to implement simplified model is suitable for use in real-time MicroDMFC simulations. More experimental data are also reported bearing in mind the insufficient experimental data available in literature at room temperature, a goal condition to use this technology in portable applications.
- Heat and mass transfer effects in a passive direct methanol fuel cell: 1D modelPublication . Oliveira, V. B.; Rangel, C. M.; Pinto, A. M. F. R.
- Heat and mass transfer effects in direct methanol fuel cell: 1D modelPublication . Pinto, A. M. F. R.; Falcão, D. S.; Oliveira, V. B.; Rangel, C. M.Models play an important role in fuel cell development since they facilitate a better understanding of parameters affecting the performance of fuel cells and fuel cells systems. In this work, a steady state, one-dimensional model accounting for coupled heat and mass transfer, along with the electrochemical reactions occurring in the DMFC is presented. The model accounts for the kinetics of the multi-step methanol oxidation at the anode while the kinetics of the cathodic oxygen reduction is modelled using the Tafel equation. Two-phase flow effects are neglected. The anode and cathode flow channels are treated using the continuous stirred tank reactor (CSTR) approach. The cell voltage expression incorporates the anodic and cathodic overpotentials as well as the ohmic losses across the membrane. The mixed potential of the cathode due to methanol crossover is also included. The reactions in the catalyst layers are considered homogeneous. Pressure gradients across the layers are assumed as negligible. Methanol and water transport through the membrane is assumed to be due to the combined effect of the concentration gradient and electro-osmotic force. Mass transport in the diffusion layers and membrane is described using effective Fick models. Local equilibrium at interfaces is represented by partition functions. The methanol flux in the cathode catalyst layer is considered as well as methanol crossover. The transport of heat through the gas diffusion layers is assumed to be a conduction-dominated process. The thermal conductivity for all the materials is assumed to be constant. Heat generation is considered in the catalyst layers. The analytical solutions for concentration and temperature across the cell are compared with recently data existing in literature and with in-house obtained results, for a wide range of operating conditions. The model shows very good agreement. This easily implemented simplified model is suitable for use in real-time DMFC simulations
- Modeling and simulation of micro direct methanol Fuel CellsPublication . Falcão, D. S.; Oliveira, V. B.; Oliveira, M. S. N.; Rangel, C. M.Fuel cells have unique technological attributes: efficiency, absence of moving parts and low emissions. The Direct Methanol Fuel Cell (DMFC) has attracted much attention due to its potential applications as a power source for transportation and portable electronic devices. With the advance of micromachining technologies, miniaturization of power sources became one of the trends of evolution of research in this area. Based on the advantages of the scaling laws, miniaturization promises higher efficiency and performance of power generating devices, so, MicroDMFC is an emergent technology. Models play an important role in fuel cell development since they facilitate a better understanding of parameters affecting the performance of fuel cells. In this work, a steady state, one-dimensional model accounting for coupled heat and mass transfer, along with the electrochemical reactions occurring in a fuel cell, already developed and validated for DMFC in [1-3], is used to predict Micro DMFC performance. The model takes in account all relevant phenomena occurring in a DMFC. Polarization curves predicted by the model are compared with experimental data existing in literature and the model shows good agreement, mainly for lower current densities. The model is used to predict some important parameters to analyze fuel cell performance, such as water transport coefficient and leakage current density. This easily to implement simplified model is suitable for use in real-time MicroDMFC simulations.
- One-dimensional and non-isothermal model for a passive DMFCPublication . Oliveira, V. B.; Rangel, C. M.; Pinto, A. M. F. R.Passive direct methanol fuel cells (DMFCs) are promising energy sources for portable electronic devices. Different from DMFCs with active fuel feeding systems, passive DMFCs with nearly stagnant fuel and air tend to bear comparatively less power densities. A steady state, one-dimensional, multi-component and thermal model is described and applied to simulate the operation of a passive direct methanol fuel cell. The model takes into consideration the thermal and mass transfer effects, along with the electrochemical reactions occurring in the passive DMFC. The model can be used to predict the methanol, oxygen and water concentration profiles in the anode, cathode and membrane as well as to estimate the methanol and water crossover and the temperature profile across the cell. Polarization curves are numerically simulated and successfully compared with experiments for different methanol feed concentrations. The model predicts with accuracy the influence of the methanol feed concentration on the cell performance and the correct trends of the current density and methanol feed concentration, on methanol and water crossover. The model is rapidly implemented and is therefore suitable for inclusion in real-time system level DMFC calculations. Due to its simplicity the model can be used to help seek for possibilities of optimizing the cell performance of a passive DMFC by studying impacts from variations of the design parameters such as membrane thickness, catalyst loading, diffusion layers type and thicknesses.
- Review on micro-direct methanol fuel cellsPublication . Falcão, D. S.; Oliveira, V. B.; Rangel, C. M.; Pinto, A. M. F. R.Fuel cells have unique technological attributes: efficiency, minimization of moving parts and low emissions. The Direct Methanol Fuel Cell (DMFC) has attracted much attention due to its potential applications as a power source for transportation and portable electronic devices. With the advance of micromachining technologies, miniaturization of power sources became one of the trends of evolution of research in this area. Based on the advantages of the scaling laws, miniaturization promises higher efficiency and performance of power generating devices, therefore, Micro-DMFC is an emergent technology. There has been a growing interest in the development of this type of micro cells in the last years, resulting both in experimental studies (operating conditions, cell design and new materials) and in modeling studies. Despite the increase in the knowledge acquired, many challenges are still to be reached. This book provides a detailed comprehensive review both on fundamental and technological aspects of Micro-DMFC. Special attention is devoted to systematization of published results on experimental area and also to a special section dedicated to modeling studies.
- Water management in a passive direct methanol fuel cellPublication . Oliveira, V. B.; Falcão, D. S.; Rangel, C. M.; Pinto, A. M. F. R.Passive direct methanol fuel cells (DMFCs) are under development for use in portable applications because of their enhanced energy density in comparison with other fuel cell types. The most significant obstacles for DMFC development are methanol and water crossover because methanol diffuses through the membrane generating heat but no power. The presence of a large amount of water floods the cathode and reduces cell performance. The present study was carried out to understand the performance of passive DMFCs, focused on the water crossover through the membrane from the anode to the cathode side. The water crossover behaviour in passive DMFCs was studied analytically with the results of a developed model for passive DMFCs. The model was validated with an in-house designed passive DMFC. The effect of methanol concentration, membrane thickness, gas diffusion layer material and thickness and catalyst loading on fuel cell performance and water crossover is presented. Water crossover was lowered with reduction on methanol concentration, reduction of membrane thickness and increase on anode diffusion layer thickness and anode and cathode catalyst layer thickness. It was found that these conditions also reduced methanol crossover rate. A membrane electrode assembly was proposed to achieve low methanol and water crossover and high power density, operating at high methanol concentrations. The results presented provide very useful and actual information for future passive DMFC systems using high concentration or pure methanol.
- Water management in direct methanol fuel cellsPublication . Oliveira, V. B.; Rangel, C. M.; Pinto, A. M. F. R.Direct methanol fuel cell (DMFC) are a promising power source for micro and portable applications due to their high energy density and inherent simplicity of operation with methanol as the liquid fuel. Present state-of-the-art optimised operating conditions are elevated cell temperatures to improve the anode reaction, high air stoichiometries to prevent cathode flooding and dilute methanol solutions to mitigate methanol crossover. These very dilute fuel solutions require the presence of a high amount of water leading to a reduction of the energy density of the system. More concentrated methanol solutions would be preferable in order to achieve energy densities needed for portable power applications. However, the possibility of using highly concentrated methanol solutions at the anode is limited by the significant water loss from the anode to cathode occurring in the DMFC due to electro-osmotic drag and molecular diffusion through the membrane. So, low crossover of both methanol and water through a polymer membrane in a DMFC is essential for using high concentration methanol in portable power applications. In this work, the results of a simulation study using a previous developed model for DMFCś are presented. Particular attention is paid to the water distribution across the cell. The influence of different parameters (such as methanol concentration, membrane thickness and gas diffusion media) over the water transport and on the cell performance is studied. The model used to predict the water transport was validated with recent published data.
- Water transport through a PEM Fuel Cell: a one-dimensional model with heat transfer effectsPublication . Pinto, A. M. F. R.; Oliveira, V. B.; Falcão, D. S.; Pinho, C.; Rangel, C. M.One of the critical problems and design issues of PEM fuel cells is the water management because the membrane’s hydration determines the performance and durability of the cell. In this work, a steady state, one-dimensional model accounting for coupled heat and mass transfer in a single PEM fuel cell is presented. Two-phase flow effects are neglected. The anode and cathode flow channels are treated using the continuous stirred tank reactor (CSTR) approach. The cell voltage expression incorporates the anodic and cathodic overpotentials as well as the ohmic losses across the membrane. The reactions in the catalyst layers are considered as homogeneous. The kinetics of the cathodic oxygen reduction is modelled using the Tafel equation while a modified Tafel expression is used to describe the anode losses. Pressure gradients across the layers are assumed as negligible. Mass transport in the diffusion layers and membrane is described using effective Fick models. Local equilibrium at interfaces is represented by partition functions. Water transport through the membrane is assumed to be a combined effect of diffusion and electro-osmotic drag. It is assumed that the membrane proton conductivity and water diffusivity are a function of , the number of water molecules per ionic group. The heat transport through the gas diffusion layers is assumed as a conduction-dominated process. The thermal conductivity for all the materials is assumed as constant. Heat generation or consumption is considered in the catalyst layers. The analytical solutions for concentration and temperature across the cell are computed. Particular attention is paid to the water distribution across the membrane. The influence of different parameters (such as the current density and the level of humidification of inlet gases) over the water transport and on the cell performance is studied. The model is validated with recent published data and with experimental results obtained with an in-house designed PEMFC (25cm2 of active area). This easily implemented simplified model is suitable to define the optimal hydration conditions of the membrane