Compact Direct Methanol Fuel Cells for Portable ...

International Journal of Chemical Reactor Engineering Volume 

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Article A

Compact Direct Methanol Fuel Cells for Portable Applications: A Modeling Study

∗ Politecnico

di di ‡ Politecnico di ∗∗ Politecnico di † Politecnico

Stefania Specchia∗

Ugo A. Icardi†

Vito Specchia‡

Guido Saracco∗∗

Torino, Torino, Torino, Torino,

[email protected] [email protected] [email protected] [email protected]

ISSN 1542-6580 c Copyright 2005 by the authors. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher, bepress, which has been given certain exclusive rights by the author.

Compact Direct Methanol Fuel Cells for Portable Applications: A Modeling Study Stefania Specchia, Ugo A. Icardi, Vito Specchia, and Guido Saracco

Abstract Consumers demand for portable audio/video/ICT devices has driven the development of advanced power technologies in recent years. Fuel cells are a clean technology with low emissions levels, able to work with renewable fuels and capable, in a next future, to replace conventional power systems meeting the targets of the Kyoto Protocol for a society based on sustainable energy systems. Within such a perspective, the objective of the European project MOREPOWER (Compact direct methanol fuel cells for portable applications) is to develop a low cost, low temperature, portable Direct Methanol Fuel Cell (DMFC; nominal power 250 W) of compact construction and modular design for the potential market area of weather stations, medical devices, signal units, gas sensors and security cameras. This investigation is focused on a conceptual study of the DMFC system carried out in the Matlab/Simulink platform. Two different DMFC configurations were devised in which alternative components arrangements along the methanol recycle line are considered. Based on a number of simulations, the system configuration characterized by a gas-liquid separator for carbon dioxide removal placed upstream the radiator for heat removal shows the most promising results, thanks to an easier design and operability of the radiator itself. KEYWORDS: Direct methanol fuel cell, portable devices, gas-liquid separator, radiator

Specchia et al.: Compact DMFC for Portable Applications

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1. INTRODUCTION A sustainable high quality of life cannot be kept apart from the worldwide supply of a clean, safe, reliable and secure energy, taking into account that the energy demand is growing more and more. The European “World Energy Technology and Climate Policy Outlook” (WETO) predicts, for primary energy, an average growth rate worldwide of 1.8% per year for the period 2000-2030 (EU Commission, 2003). Consumers’ demand for portable audio/video ICT products has driven the development of advanced power technologies in recent years. The global market for portable power supplies is now served with batteries and electric generators based on small internal combustion engines, with problems linked to the environment as noxious emissions, noises, treatment and disposal of exhaust batteries. Today, some 400 million phones are sold annually, compared to about 15 million portable digital assistants (PDAs) and 25 million to 35 million notebooks. No one yet knows the exact power requirements of new emerging devices. Batteries, already a $ 6 billion annual market, are in rising demand due to the proliferation of portable electronic devices. Power needs keep growing, but batteries seldom yield lifetimes meeting customers’ satisfaction or can be purchased at costs that are acceptable. The introduction of portable fuel cell in the market will bring considerable advance in this power-supply sector (Dillon et al., 2004). Fuel cells are ideal devices to generate electricity from either fossil or renewable fuels: they are a clean and efficient energy supply system, with low emissions levels, capable, in a next future, to replace conventional power systems thereby meeting the targets of the Kyoto Protocol on the reduction of EU green house gases by 8% in 2008 and even beyond the Kyoto deadline of 2010 (Kyoto Protocol, 1997). Within such a perspective, a specific project MOREPOWER (Compact direct methanol fuel cells for portable applications) has been funded by the European Community. Its goal is to develop a low cost, low temperature, portable direct methanol fuel cell device of compact construction and modular design for the potential market area of weather stations, medical devices, signal units, gas sensors and security cameras. The present work deals with the development and the optimization of a suitable model for both the stack design and the entire system (water and heat management, fuel feed distribution, components integration, …), focused on advanced concepts for a maximum compactness and miniaturization. In terms of power density the performance of a Direct Methanol Fuel Cell (DMFC) is still significantly lower (about 30% efficiency at 80°C) than that of the Polymer Electrolyte Membrane Fuel Cells (PEMFC) (Cacciola et al., 2001). However, the current component developments carried out world-wide is leading to significant performance improvements. For instance, DMFC systems with a power density up to 200 mW/cm2 at 120°C and 120 mW/cm2 at 80°C were recently developed by using Pt catalyst and pressurized air at the cathode side (Aricò et al., 2004).

2. THE MOREPOWER PROJECT The main objective of the MOREPOWER project is to develop a low cost, low temperature, portable DMFC device. It will also offer limited operation on ethanol fuel and will be of compact construction and modular design. The areas of development include: novel proton exchange membranes, anode and cathode electrocatalysts and fully optimized multilayer membrane electrode assemblies. New low-cost proton exchange membranes are under development to reduce the fuel crossover rate through the electrolyte to levels significantly lower than that of currently available materials, e.g. Nafion® (Ponce et al., 2004; Silva et al., 2005). New electrocatalyst materials are also under development to enhance the low temperature methanol (and ethanol) electro-oxidation activity of the anode (Acres et al., 1997; Oliveira Neto et al., 2003). Catalyst development for the cathode is focusing on enhancing the oxygen reduction activity of platinum electrocatalyst and increasing its selectivity to enhance methanol tolerance (Baglio et al., 2005). The structure of the electrocatalyst and electrode layers will be optimized to promote efficient operation at low temperatures with practical flows and pressures. •

The main target specifications of the MOREPOWER DMFC are: nominal power: 250 W (min/max acceptable peaks: 50-500 W);

Produced by The Berkeley Electronic Press, 2005

International Journal of Chemical Reactor Engineering

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• • • • •

Vol. 3 [2005], Article A24

electrical characteristics: 40 A, 12.5 V (single cells operating at 0.5 V at 0.2 A/cm2 at 30-60°C in atmospheric pressure air); energy density: 800 Wh/kg; max operating time at nominal power: 8 h; max operating time at peak power: 2 h; operation temperature: 30-60°C.

The final goal is the delivery of an optimized, low cost, very compact and highly efficient DMFC prototype. Conversely, the main objective of the present study is the development and the optimization of a suitable model for both the stack design and the entire system (water and heat management, fuel feed distribution, components integration), focused on advanced concepts for a maximum compactness and miniaturization. In particular, this model will be employed for discriminating different systems architectures on the grounds of technical feasibility.

3. MODEL DESCRIPTION The objective of system modeling is the estimation of heat and mass fluxes and pressure drops, for the integration and optimization of the DMFC components of a complete Auxiliary Power Unit (APU). The main components are the fuel cell, the radiator to cool the fuel solution, the gas-liquid separator to dump the produced CO2, the catalytic burner to burn the residual CH3OH vapor, the pump to feed the fuel solution, the methanol sensor to control the solution concentration, the cold start heater to heat up the solution during the start up, the methanol cartridge to feed the fresh methanol solution through the piezo-valve, the heat humidity exchanger to cool and to dry the exhausted air and the blower to feed the fresh air. Two different systems were modeled with the software Matlab/Simulink® based on the schemes in Figures 1 and 2: in the scheme number 1, the gas-liquid separator is placed upstream the radiator, whether in the scheme number 2 it is present downstream the radiator. Both configurations were thoroughly simulated to choose the most performing scheme.

Figure 1. DMFC system scheme number 1.

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Figure 2. DMFC system scheme number 2. Apart from electrochemical reactions, the DMFC system also hosts combustion reaction, because part of methanol permeates the membrane without reacting on the anode and it burns on the cathode (i.e. the so-called CH3OH crossover). The main reactions are electrochemical reactions at the anode, where CH3OH methanol is electrochemically oxidised to carbon dioxide according to (1), and at the cathode, where oxygen is reduced to water according to (2), and a cross-over combustion reaction at the cathode, where the CH3OH is oxidized according to (3): CH3OH + H2O → CO2 + 6H+ + 6e-

(1)

6H+ + 3/2O2 + 6e- → 3H2O

(2)

CH3OH + 3/2O2 → CO2 + 2H2O

(3)

According to the huge experimental evidence, methanol crossing over is virtually totally oxidized at the cathode catalyst layer: the parasitic methanol oxidation reduces the cathode potential as can be explained as short circuit in the cathode catalyst layer (Wang and Wang, 2003). The addition of fresh solution from a methanol cartridge to the exhaust solution is controlled via a methanol sensor; the obtained feed solution is then pumped into the fuel cell, where the overall chemical reactions between the fuel and air produce power and heat. The needed air, heated-up and humidified with the heat/humidity exchanger on the exhaust air stream, is fed to the fuel cell through the blower. The direct use of methanol in fuel cells is considerably attractive from the point of view of system design simplicity and hence cost (Shukla et al., 2002). In the scheme number 1 (Figure 1) the exhausted fuel solution from the fuel cell directly goes to the gasliquid separator, where the CO2 leaves the reacted solution. Small quantities of CH3OH vapor and H2O steam get also lost the gas phase. Besides, some CO2 remains dissolved in the solution. The separated gas flow is thus burnt in a catalytic burner to remove the CH3OH vapors. Conversely, the liquid flow from gas-liquid separator is cooled in the radiator and afterwards added to the fresh solution as described above. According to scheme number 2 (Figure 2) the exhaust fuel solution from fuel cell is at first cooled by the radiator prior to the gas-liquid separator. As a result, in the first configuration the loss of vapor CH3OH is larger than in the second one since the gasliquid separator inlet temperature is higher; however, the high temperature reduces quantity of CO2 dissolved in the



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solution, which results lower in the first configuration than in the second one. This last occurrence should favor the anode performance by reducing the amount of CO2 released at this location. The equations for the electric current (4), the electric power (5), the overall energy balances (6 and 7) and the liquid-vapor balance at the gas-liquid separator (8) were implemented into the mathematical model:

I = z ⋅ F ⋅ G reacted

(4)

P = I⋅V

(5)

(G out ⋅ H out ) − (G in ⋅ H in ) +Q w = G reacted ⋅ ΔH reaction

(6)

(G out ⋅ H out ) − (G in ⋅ H in ) + Q w = 0

(7)

p i ⋅ X i = p tot ⋅ Yi

(8)

The ΔHreaction were calculated from the heats and free energies of formation of the chemical species involved in the reactions (Perry and Green, 1997). Equation (6) was used where a chemical reaction is involved (in the stack and in the afterburner) whereas equation (7) was used when no chemical reactions are involved (in the heat+humidity exchanger, in the radiator+fan and in the gas-liquid separator). The model of the stack consists of 40 cells, 240 cm2 each: forty energy balance blocks of the model based on equation (6) estimate the anode and cathode temperatures by means of 2nd grade equations system. The energy and mass balances were estimated under steady state conditions. The power of the fuel cell stack is a function of the current drawn from the stack and the resulting stack voltage. The cell voltage is a function of the stack current, reactant partial pressure inside each cell, cell temperature and crossover effect. The law of mass conservation is used to track the concentration profiles of species in each cell. The electrochemical reaction at the membranes is assumed to occur instantaneously. The fuel cell stack block (Figure 3) contains two interacting sub-models, the anode and cathode flow blocks (Pukrushpan et al., 2004), able to estimate the outlet flows of all the species involved, including the CH3OH crossover reaction. Moreover, the anode flow block contains an equation to calculate the voltage based on the experimental data of the polarization curves, as a function of the anodic pressure (Figure 4), given by a MOREPOWER project’s partner.

Figure 3. Fuel cell stack reaction block diagram (anode and cathode sub-models).



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Figure 4. Experimental polarization curves (60°C, 1M). Courtesy of CNR-ITAE (Messina, Italy). The simulations were done for three different powers: the lowest (50 W), the maximum (500 W) and the nominal power (250 W). The voltage producing electric power is taken from the polarization curve of Figure 4, while the remain fraction voltage compared with open circuit (1.21 V) was assumed to produce thermal power. Furthermore, suitable CO2 solubility and methanol saturation partial pressure databases were implemented (Perry and Green, 1997) in order to better quantify the differences between the two system configurations. The CH3OH concentration and the solution flow rates fed to the FC were used as variable parameters.

4. SIMULATION RESULTS AND DISCUSSION The simulations were performed by using the following conditions: • • • • • • • • •

constant inlet flow temperatures (solution and air) of 60°C at the stack; a 10 times stoichiometric excess of methanol solution in the stack (i.e. 1 part gets reacted, 9 parts recycled); a 2 times stoichiometric excess of air in the stack; a stoichiometric amount of air fed to the afterburner; an ambient temperature of 25°C; an anode pressure gauge of 4/3.5/3/2.5/2/1.5 bar, respectively; a cathode pressure gauge of 1 bar; a gas-liquid separator pressure gauge of 3/2.5/2/1.5/1/0.5 bar, respectively; an amount of methanol crossover through the membranes as depicted in Figure 5, given by a MOREPOWER project’s partner. The main simulation results are represented in the Figures 6-12 listed hereafter.


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Figure 5. CH3OH crossover waste flow through the membrane at 60°C. Courtesy of CNR-ITAE (Messina, Italy). Figure 6 represents the converted methanol flow necessary to obtain the requested power at the stack: by increasing the anodic pressure the flow remains practically constant and it is independent of the system configuration as it depends on the anodic reaction.

Figure 6. Converted CH3OH flow necessary to obtain the requested power (CH3OH feed concentration 1 M). Figure 7 represents the methanol flow evaporated in the gas-liquid separator and burnt into the catalytic afterburner; of course this represents a loss of the DMFC. Such loss is less with the second configuration due to the fact that the total flow from the fuel cell is first cooled, then flashed at lower temperature in the gas-liquid evaporator.



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Figure 7. CH3OH vapor flow lost in the gas phase into the gas-liquid separator and burnt in the afterburner (CH3OH feed concentration 1 M; scheme nr 1: solid-line; scheme nr 2: dashed-line). Figure 8 represents the total methanol flow to be fed to the system (as sum of the methanol flow rates presented in Figures 5, 6 and 7, i.e. the converted flow to obtain the requested power, the flow lost due to the methanol crossover and the methanol lost due to the flash in the gas-liquid separator). The differences between the two considered configurations are slight and become negligible by increasing the anodic pressure.

Figure 8. Overall methanol flow converted at the anode and lost by crossover and by evaporation in the gas-liquid separator (CH3OH feed concentration 1 M; scheme nr 1: solid-line; scheme nr 2: dashed-line). Figure 9 shows the flow of CO2 dissolved in the solution at the outlet of the gas-liquid separator: by flashing the flow at higher temperatures (as in scheme number 1), a lower amount of CO2 can be kept in the flow to the stack (i.e. a major amount of CO2 is removed, but also a major CH3OH is lost, as shown in Figure 7). This is obviously an


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advantage to avoid corrosion problems, to allow a better anode operation and to guarantee an adequate response of the methanol sensor.

Figure 9. Flow of CO2 dissolved in the solution downstream the gas-liquid separator vs the anode operating pressure (CH3OH feed concentration 1 M; scheme nr 1: solid-line; scheme nr 2: dashed-line). Conversely, Figure 10 shows the total heat to be removed from the whole system, to maintain the operative temperature at the constant value of 60°C: it is the sum of the heat from the radiator+fan, from the catalytic afterburner and from the heat-humidity exchanger.

Figure 10: Total heat produced and to be removed vs the anode operating pressure (CH3OH feed concentration 1 M; scheme nr 1: solid-line; scheme nr 2: dashed-line). Finally, the water balance calculations pointed out that at low pressure and low power (50 W), a certain loss of water by evaporation is obtained for scheme 1: this entails that the lost water is to be replaced in order to maintain



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constant the solution concentration. Figures 11 and 12 show such phenomenon, mostly due to water evaporation at the cathode side and at the gas-liquid separator, which actually represents an operating problem since water refilling is not amenable for portable applications.

Figure 11. Fresh water flow rate to be fed to the systems in order to keep the overall water balance neutral for a 1 M feed methanol concentration (scheme nr 1).

Figure 12. Fresh water to be fed to the systems in order to keep the overall water balance neutral for a 2 M feed methanol concentration (scheme nr 1). The great difference in H2O flow curves represented in Figures 11 and 12 is easily explained taking into account that, independently of the CH3OH concentration (1M or more), the CH3OH flow remains constant for a fixed power to be generated. This means that by increasing the molarity of the solution fed to the stack, the overall feed flow rate of water is reduced (1 M corresponds to about 3% CH3OH whereas 2 M corresponds to about 6% CH3OH in weight): the heat developed by the electro-oxidation of CH3OH warms thus a much lower quantity of mass, which induces an anode temperature increase, as shown also in Table 1. The increase of the anode temperature provokes a



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more significant water evaporation. As a consequence, a major water make-up is thus necessary to maintain the water balance constant. Moreover, the crossover effect, enhanced with the increase of the molarity (see Figure 5), also contributes to the anode temperature increase and the consequent water make-up. Other important calculated data such as the anode and cathode temperatures and the total flow rate to be fed to the stack (as sum of the overall CH3OH and water flow rates) are listed in Table 1. These values are independent of any particular system configuration. Table 1. Data calculated for the DMFC systems fed with an air excess of 2 and a methanol excess of 10. Solution Power [W] 50

250

500

concentration: 1 M Solution concentration: 2 M p tot Total flow T anode out T cathode out Power p tot Total flow T anode out T cathode out [bar] [ml/min] [K] [K] [W] [bar] [ml/min] [K] [K] 4.0 127.6 393.2 328.4 50 4.0 93.5 415.9 327.7 3.5 123.9 391.1 328.1 3.5 90.2 411.2 327.2 3.0 120.3 388.8 327.9 3.0 86.8 405.8 326.7 2.5 116.6 386.4 327.6 2.5 83.5 399.7 326.2 2.0 113.0 383.5 327.3 2.0 80.1 392.5 325.6 4.0 558.0 355.5 332.3 250 4.0 308.8 401.5 332.7 3.5 551.4 354.8 332.3 3.5 303.9 398.7 332.5 3.0 544.7 354.0 332.2 3.0 299.0 395.5 332.3 2.5 538.0 353.2 332.1 2.5 294.2 391.6 332.1 2.0 531.3 352.4 332.1 2.0 289.3 386.5 331.8 4.0 1280.0 349.6 333.5 500 4.0 669.6 379.0 334.0 3.5 1262.0 349.3 333.5 3.5 659.4 377.7 333.9 3.0 1247.0 348.9 333.4 3.0 650.4 376.3 333.8 2.5 1237.0 348.5 333.4 2.5 643.8 374.8 333.7 2.0 1262.0 348.1 333.4 2.0 637.1 372.9 333.5

5. SCHEME SELECTION The two different configurations show two main issues for a comparative evaluation. The first one is related to the loss of CH3OH in the gas-liquid separator as vapor to be burnt in the afterburner; the second one is the amount of CO2 not removed by the gas-liquid separator and dissolved in the solution to the stack, which may possibly lead to corrosion problems or hinder the anodic reaction (i.e. reduction of the catalytic active sites and shift of the reaction equilibrium). The loss of CH3OH as vapor in the gas-liquid separator (Figure 7) is nearly the same in the two schemes for medium-high pressures in the gas-liquid separator; for lower pressures the difference is more significant. Moreover, if the concentration of the solution is higher (2 M or more), such effect is enhanced due to the higher temperature of the system (see Table 1) causing a major evaporation of the solution. Taking into account that the CH3OH loss represents a negative issue for the DMFC operating conditions, on this point of view the second scheme is preferable; therefore, working with low concentrations is favorable. The amount of dissolved CO2 is always higher for the configuration with the gas-liquid separator downstream the radiator (scheme number 2), due to its lower operating temperature. Larger differences are obtained in the cases of high pressure, while for low pressure the differences are smaller (see Figure 9). If the system works above the atmospheric pressure, the loss of CH3OH by evaporation is nearly the same in the two compared schemes, while the CO2 dissolved in the solution is larger for the system with the radiator upstream the gas-liquid separator (scheme number 1). The lower the dissolved CO2 in the total flow, the better the system operability: on this point of view the first scheme is preferable. Another important issue is the total heat to be removed to maintain at a constant value the DMFC operative temperature; this is nearly the same for both the schemes (see Figure 10), but it increases if the methanol



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concentration of the feed solution is increased (a 2 M concentration provokes higher anode temperatures respect 1 M concentration, see Table 1). Once more, working at low CH3OH concentration is favorable. Besides, it is important to take into account the difficulty to design and build a two-phase (vapor and liquid) heat exchanger (i.e. the radiator), entailed by the scheme number 2. Conversely, the scheme number 1 requires a onephase (liquid) heat exchanger, much simpler to design and build. A last but not least issue to consider is the necessity, fortunately only in a few cases (low pressure, low power and high methanol concentration, see Figures 11 and 12), to feed fresh water in the recycling flow in the case with the gas-liquid separator upstream the radiator (scheme number 1). Taking into account all these issues, with the DMFC working at the nominal power of 250 W, with a solution 1 M and with a pressure of at least 2 bars at the anode side, the scheme number 1, i.e. the system with the gas-liquid separator upstream the radiator, seems to be preferable because of its much simpler radiator design and operation, notwithstanding a slight higher methanol loss in the gas-liquid separator. Of course CO2 solubility plays an important role in the present study. This is mainly due to the presence of fuel cells manufacturers in the MOREPOWER project consortium, who focused their attention also on practical problems of prototype manufacturing and overall efficiency obtainable, which drive the consortium conclusions on the modeling study on more practical application (some compromises are then necessary). In the MOREPOWER project, some partners are directly involved in the development of new alternative membranes able to reduce the methanol loss due to crossover, in order to partially compensate the methanol loss into the gas-liquid separator.

6. CONCLUSIONS A DMFC will be developed, built an tested within the European Project MOREPOWER. The purpose is to provide electricity for audio/video/communications/medical devices by replacing the actual batteries. In this study, two possible process schemes were compared. Based on a number of conceptual design simulations, the scheme with the gas-liquid separator upstream the radiator was chosen, thanks to an easier design and operability of the heat exchanger. As these conclusions might be significantly affected by the membranes performance (all the carried out calculations are based on the behavior of a reference membrane fed at 60°C; see Figures 4 and 5), a sensitivity analysis is currently in progress in order to assess the effect of different feed temperatures, membrane permeabilities (crossover) and polarization curves.

ACKNOWLEDGMENTS The Authors gratefully acknowledge the financial support of the European Commission for the MOREPOWER project (Compact direct methanol fuel cells for portable applications, project nr. SES6-CT-2003-502652) and all the involved partners: GKSS Forschungszentrum (D), Centro Ricerche Fiat (I), Solvay SA (B), Johnson Matthey (UK), Consiglio Nazionale delle Ricerche – Istituto di Tecnologie Avanzate per l’Energia “Nicola Giordano” (I), Institut für Mikrotechnik Mainz (D), Nedstack Fuel Cell Technology (NL) and Nedstack Components (NL).

NOMENCLATURE F G H I P p Q V X

Faraday constant, 9.6458·104 C/mol flow, mmol/ s enthalpy, J/mol current, A power, W pressure, bar gauge heat, W voltage, V liquid molar fraction



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Y z


vapor molar fraction number of electrons

Subscripts i in out reacted tot w

species involved in the chemical reactions entering the fuel cell leaving the fuel cell reacted in the fuel cell total waste

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