The effect of temperature on the output characteristics of micro direct methanol fuel cell

The effect of temperature on the output characteristics of micro direct methanol fuel cell

Journal of Power Sources 285 (2015) 318e324 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/lo...

2MB Sizes 0 Downloads 3 Views

Journal of Power Sources 285 (2015) 318e324

Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

The effect of temperature on the output characteristics of micro direct methanol fuel cell Zhenyu Yuan*, Jie Yang* College of Information Science and Engineering, Northeastern University, Shenyang 110819, China

h i g h l i g h t s  A novel two-dimensional, multi-physics model is established.  A 0.64 cm2 metal-based mDMFC is fabricated by micro-stamping technology.  The experimental validation with high power density is conducted.  The experimental results are in good agreement with the simulation.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 November 2014 Received in revised form 7 March 2015 Accepted 14 March 2015 Available online 16 March 2015

In this paper, the effects of operating temperature on mass transport and micro direct methanol fuel cell (mDMFC) performance are presented. Furthermore, a whole two-dimensional model coupled with mass/ momentum transports and temperature characteristic is established. Simulation results show that the temperature has significant effects on methanol concentration/CO2 distributions, crossover current density, and the polarization curve. The metal-based mDMFC with the effective area of 0.64 cm2 is fabricated using micro-stamping technology, and the detailed experimental validation is conducted. The results reveal that when the cell is supplied with a relatively low aqueous methanol flow rate, the peak power density exhibits a trend of initially going up, reaching the peak value of 85.3 mW cm2 at 60  C, and then dropping off. At the higher flow rate, however, a proportional relationship between the power density and temperature is obtained. The experimental results are in good agreement with the simulation. © 2015 Elsevier B.V. All rights reserved.

Keywords: Micro direct methanol fuel cell Temperature effect Mass transport High performance

1. Introduction Conventional batteries have the disadvantage of serious environmental impact. Therefore, it is urgent to find a clean and highenergy power source for portable electronics [1e4]. Meanwhile, the micro direct methanol fuel cell (mDMFC) has been considered as a prime candidate due to the advantages of high-efficiency, lowemission, silent-operation and simplicity [5,6]. Presently, one of the most challenges for mDMFC application is the low power density. To improve the power density of mDMFC, many studies have been conducted to explore the mechanisms of methanol oxidation kinetics, methanol crossover and the mass transport inside the cell [7e11]. It is widely acknowledged that

* Corresponding authors. E-mail addresses: [email protected] (Z. Yuan), [email protected] (J. Yang). http://dx.doi.org/10.1016/j.jpowsour.2015.03.094 0378-7753/© 2015 Elsevier B.V. All rights reserved.

temperature has dramatic effects upon above aspects and the performance of mDMFC [12,13]. Therefore, significant attentions have been devoted to the effect of operating temperature on mDMFC performance recently. Alizadeh et al. [14] analyzed the performances of the direct methanol single cell at various cell temperatures. The results indicated that the cell performance improved with an increase of temperature in a certain range because the conductivity of the membrane and the reaction kinetics at both the anode and cathode were increased. Chen et al. [15] also investigated the effects of methanol concentration, methanol flow rate, oxygen flow rate and cell temperature on DMFC performance, and concluded that the DMFC performance increased significantly with an increase in cell temperature. In previous studies, however, only experimental investigations on the temperature effect were engaged briefly without in-depth theoretical analysis. Given the importance of temperature on cell performance, it is essential to conduct a comprehensive study on both

Z. Yuan, J. Yang / Journal of Power Sources 285 (2015) 318e324

simulation and experiment to fully understand the relationship between the temperature and the inner transport characteristics and cell performance. Based on this understanding, a novel two-dimensional cell model coupled with mass/momentum transports and temperature effect was established. In this model, the methanol solution transport, CO2 distribution and crossover current density of different operating temperatures were numerically defined. In addition, a 0.64 cm2 stainless-based mDMFC was fabricated using micro-stamping technology, at which the effects of operating temperature on cell performance were experimentally investigated. The results from a series of experiments including polarization curve and Electrochemical Impedance Spectroscopy (EIS) showed that the mDMFC behaviors are influenced by the temperature in a complex manner. At low methanol flow rate, the power density of the cell exhibits a non-monotonic relationship with the temperature; while at higher methanol flow rate, the cell performance monotonously increased with the temperature. 2. The model analysis A two-dimensional model was established to investigate the methanol/CO2 transports and the temperature effect. Fig. 1 shows the calculation domain of the model. The mDMFC is assumed to be under steady-state conditions, and the diffusion layer is defined as homogeneous porous electrode. The mass transport can be described using the ConvectioneDiffusion equation as follows:

  eff V$  Di;l VCi;l þ Ci;l ul ¼ Si;l

(1)

  VCi;g þ Ci;g ug ¼ Si;g V$  Deff i;g

(2)

where subscripts l/g represent liquid/gas substance,Ci is the liquid/ gas concentration.Deff is the effective diffusion coefficient, which i can be modified as: eff

(3)

eff

(4)

Di;l ¼ Di;l ε1:5 s1:5 Di;g ¼ Di;g ε1:5 ð1  sÞ1:5

319

According to NaviereStokes equation, the anode momentum transport suitable for methanol solution/CO2 is given by:

vðfrl ul Þ þ V$ðfrl ul ul Þ ¼ Vpl þ V$ðfml Vul Þ þ frl g vt

(5)

  v ð1  fÞrg þ V$ ð1  fÞrg ug ¼ 0 vt

(6)

where f represents the liquid volume fraction.pl is the liquid pressure in channel. rand u denote the average density and the average velocity, respectively. The relationship between the gas and the liquid is defined as:

ug ¼ ul þ uslip

(7)

where,uslip is the slip velocity. Based on the ideal gas law, the relationship between CO2 density and pressure can be described as:

rCO2 ¼

 pl þ pref MCO2 RT

(8)

where MCO2 , R and T are molar mass, ideal gas constant and the operating temperature, respectively. The continuity equation suitable for the porous region can be described as:

V$ðrl ul Þ ¼ Sl  V$ rg ug ¼ Sg

(9) (10)

where S represents the source term. The momentum transport in diffusion layer is defined by Darcy law:

ul ¼

Kkrl Vpl ml

(11)

ug ¼

Kkrg Vpg mg

(12)

where K and k represent the absolute permeability and the relative permeability and m represents liquid/gas phase viscosity, k in Equations (11) and (12) can be further modified as:

krl ¼ s3

(13)

krg ¼ ð1  sÞ3

(14)

where s is the liquid saturation. Similarly, the cathode momentum transport can also be solved according to the NaviereStokes equation:

  v ð1  fÞrg ug þ V$ ð1  fÞrg ug ug vt  ¼ Vpg þ V$ ð1  fÞmg Vug þ ð1  fÞrg g   v frH2 O þ V$ frH2 O ul ¼ 0 vt Fig. 1. Simulation domains of the model.

The oxygen diffusion and convection can be described as:

(15)

(16)

320

Z. Yuan, J. Yang / Journal of Power Sources 285 (2015) 318e324

  V$  Deff O2 ;cc VCO2 þ CO2 ug ¼ 0

(17) eff

where, CO2 represents the molar concentration, DO2 ;cc is the effective diffusion coefficient, which can be modified as: 1:5 Deff O2 ;cc ¼ DO2 ð1  fÞ

(18)

The methanol crossover and the water transport in proton exchange membrane region can be illustrated as: cross NMeOH

¼

;mem mem Deff MeOH VCMeOH



 pl;c  pl;a I þ nMeOH d F ml lmem

mem Kmem CMeOH

(19)  rH2 O Kmem pl;c  pl;a I cross 2O NH ¼  þ nH d F 2O MH2 O ml lmem

(20)

Methanol mass transfer flux includes diffusion, convection and electro-osmosis, while, the water transport flux includes pressure difference and electro-osmosis. Also, in the above equations,nMeOH d represents the electro-osmosis coefficient, lmem is the membrane thickness. 3. Simulation results and discussion The above equations were couple-solved by the finite element method. The parameters used during the calculation are listed in Table 1. The structure of the fabricated mDMFC was identical to the calculated domain, and the boundary setting was also consistent with experimental condition. The simulation results were compared with the experimental polarization curves to verify the accuracy of this model. As shown

Table 1 Parameters used in the 2D mDMFC model calculations. Property

Value 1

Gas constant R (J$(mol K) ) Standard pressure Patm (Pa) Faraday constant F (C mol1) Molar mass of water MH2 O (kg mol1) Molar mass of methanol MMeOH (kg mol1) Molar mass of CO2 MCO2 (kg mol1) Molar mass of O2 MO2 (kg mol1) Water density rH2 O (kg m3) Methanol density rMeOH (kg m3) Liquid dynamic viscosity ml (kg$(m s)1)

Gas dynamic viscosity mg (kg$(m s)1) Methanol diffusivity DMeOH(m2 s1) O2 diffusivity DO2 (m2 s1) Surface tension s (N m1) ref Anode referential current im (A m2) 2 Cathode referential current iref O2 (A m )

Permeability of gas diffusion layer (m2) Permeability of catalyst layer (m2) Porosity of gas diffusion layer Porosity of catalyst layer Anode inlet velocity VMeOH,in (m s1) Inlet concentration CMeOH,in (mol m3) cathode inlet velocity VO2 ;in (m s1) Inlet concentration CO2 ;in (mol m3)

8.314 1.013  105 96,485 0.018 0.032 0.044 0.032 1000 791.7 0.458509  5.30474  103  T þ 2.31231  105  T2  4.49161  108  T3 þ 3.27681  1011  T4 2.03  105 105.4163999.778/T 1.775  105  (T/273)1.823 0.0625 94.25exp(35,570  (1/353  1/T)) 0.04222exp(73,200/R  (1/353  1/T)) 1.2  1012 1.5  1014 0.7 0.4 0.045 1500 0.45 Patm/RT

Fig. 2. Comparison of the modeling results and experimental results.

in Fig. 2, the simulation results demonstrate the typical mDMFC polarization trend and are well fitted with the experimental data under different methanol concentrations, indicating the validity of this model. At the low current region, the simulation results are slightly higher than the experimental values due to the model assumptions. Fig. 3(a) is the model simulation result of the methanol concentration distribution on the anode catalyst layer. In this figure, the x-axis represented the channel from inlet to outlet. The data illustrates that the methanol concentration on the anode catalyst layer is increased when the temperature is higher. It is well known that the increase of temperature enhances the methanol diffusion coefficient, and then accelerates the mass transport. Meanwhile, the electrochemical reaction rate and the methanol consumption rate are also increased with temperature increase. The figure reveals that the effect of the methanol mass transport suppresses the effect of methanol consumption. At higher temperature, the adequate fuel supply can ensure the progress of the violent electrochemical reaction. The trends illustrated in Fig. 3(a) are supported by the findings illustrated in Fig. 3(b). As can be seen from the figure, the methanol crossover phenomenon was increased with the increase of temperature. Besides the higher methanol concentration on the surface of the anode catalyst layer, the methanol transfer coefficient through Proton Exchange Membrane (PEM) was also increased with the increase of temperature. To further explore the inner transport characteristics, the CO2 volume fraction distributions at different temperatures were simulated, as illustrated in Fig. 3(c). When the temperature was increased from 293.15 K to 333.15 K, the average total fraction at the outlet was increased by 13.3%. According to the state equation of ideal gas, when the temperature is higher, the CO2 density becomes smaller and the volume will be bigger. Therefore, the gas content was slightly elevated. Based on above investigations, the polarization curves at different temperatures were simulated in Fig. 3(d). The effect of temperature on cell performance became more obvious when the operating current is higher. Despite the negative effects of methanol crossover and CO2, the increase of temperature could effectively accelerate the transfer rate and internal electrochemical reaction, which is the dominant factor.

Z. Yuan, J. Yang / Journal of Power Sources 285 (2015) 318e324

321

Fig. 3. Simulation results: (a)Distributions of methanol concentrations on the anode catalyst layer; (b)Crossover current density; (c)Distributions of CO2 gas volume fractions in the anode channel; (d)Polarization curves.

4. Experimental validation

4.2. The determination of the optimal operating parameter

4.1. The design and fabrication of mDMFC

mDMFC performances with different methanol concentrations (0.5, 1.0, 1.5 and 2.0 mol L1) are depicted in Fig. 5(a). The operating temperature was 40  C and the cathode oxygen flow rate was 10 ml min1. The results indicate that the best performance (65.7 mW cm2) was achieved when the cell was supplied with the concentration of 1.5 mol L1 and the current density was 320.31 mA cm2. While, the lowest performance, on the other hand, was achieved with the concentration of 0.5 mol L1. Under this condition, an obvious concentration polarization phenomenon can be observed: the peak power density of 41.6 mW cm2 was reached when the discharge current density was increased to 193.8 mA cm2, and then decreased sharply thereafter. At the concentration level of 1.5 mol L1 and the temperature of  40 C, mDMFC performances with different flow rate (0.25, 0.5, 1.0, 2.5 and 5.0 ml min1) are shown in Fig. 5(b), The figure revealed that the maximum cell performance of 65.7 mW cm2 and minimum cell performance of 43.2 mW cm2 were achieved when the cell was supplied with the methanol flow rate of 1.0 ml min1 and 0.25 ml min1, respectively.

A metal-based mDMFC was designed and fabricated with the basic structure shown in Fig. 4(a). The mDMFC is constructed by current collectors, end plates, assembly gaskets and a membrane electrode assembly with the effective area of 0.64 cm2. The current collectors were fabricated from 304 L stainless steel using micro-stamping technology, as shown in Fig. 4(c). The local topography of the current collector, in which the widths of the flow channel and the rib are 1000 mm and 400 mm, respectively. The inlet and outlet were fabricated by laser-cut technology. The SEM images of the surface of the channels are also proposed. It can be seen that a 500 nm-thickness TiN film was deposited on the surface of the current collector through Magnetron Sputtering Ion Plating technology to prevent the electrochemical corrosion. The Nafion117 membrane was adopted as the PEM, whose electrode assembly is composed of 4.0 mg cm2 Pt/Ru on the anode catalyst layer and 2.0 mg cm2 Pt on the cathode catalyst layer. The Membrane Electrode Assembly (MEA) was prepared by hotpressing at 135  C and 10 MPa for 180 s. Finally, the mDMFC assembly was finished with the auxiliary components of gaskets and stainless-steel mesh, as shown in Fig. 4(b).

4.3. The effect of temperature on output characteristics After the optimal concentration and flow rate were determined, the effects of temperature on cell output were studied and the

322

Z. Yuan, J. Yang / Journal of Power Sources 285 (2015) 318e324

Fig. 4. (a) Schematic of the structure of the metal-based mDMFC; (b) Photograph of the assembled mDMFC; (c) Photograph of current collectors, Confocal Laser Scanning Microscope image, and the SEM images of the channel surface.

Fig. 5. The determination of the optimal operating parameter: (a) methanol concentration; (b) anode flow rate.

results are summarized in Fig. 6(a). According to the simulation, the best performance should be emerged at 80  C. However, during the experimental verification, a complex relationship between the operating temperature and the cell performance was discovered. It is interesting to notice that if the current density is lower than 260.92 mA cm2, a better cell performance could be maintained at 80  C; otherwise, the cell exhibits the best performance at 60  C when the discharge current become larger. The overall maximum

power density is 85.3 mW$cm2 at 60  C. Operating temperature is a key factor affecting mDMFC performance, which can be explained as follows:(1) the increase of catalyst activity and the acceleration of the electrochemical reaction rate; (2) the improvement of mass transport and methanol diffusion coefficient; (3) the decrease of proton exchange membrane resistance, which can be verified by Electrochemical Impedance Spectroscopy(EIS) test. The high-frequency resistance

Z. Yuan, J. Yang / Journal of Power Sources 285 (2015) 318e324

323

Fig. 6. (a) Effect of the operating temperature on mDMFC performance with the flow rate of 1.0 ml min1; (b) Effect of the operating temperature on the internal resistance.

RHF is equal to the cell internal resistance Ri paralleled with the load resistor R. Therefore, Ri can be calculated by the following equation:

Ri ¼ RRHF =ðR  RHF Þ

(21)

The EIS test was conducted by controlling the mDMFC operated at the voltages of 200, 300, 400 and 500 mV, and the relevant results were shown in Fig. 6(b). The data indicated that Nafion membrane resistance decreased with the increase of temperature. The difference of the optimal temperature between simulation and experiment result was mainly due to the following reasons. Firstly, the electrochemical reaction rate on the surface of catalyst layer increased with the increase of temperature, and intense reaction could not be maintained at the methanol flow rate of 1.0 ml min1. Secondly, the CO2 production increased because of the violent reaction at high temperature and the decreasing CO2 solubility in methanol solution, therefore, CO2 in the channel could not be removed efficiently under low flow rate. However, too high operating temperature will bring up the water-loss phenomenon in the PEM region, which can dramatically influence the proton conductivity.

Two sets of experiments with the flow rates of 2.5 and 5.0 ml min1 were conducted to prove the above theories. Fig. 7 shows the cell performance curves. It revealed that when the feeding flow rates was increased to 2.5 and 5.0 ml min1, the maximum power densities of mDMFC were improved significantly to 106.4 mW cm2 and 115.1 mW cm2 at 80  C, which validated the model analysis. Experiments over 80  C could not be continued because the PEM would eventually dehydrate, resulting in an unrecoverable damage.

5. Conclusions In this paper, the effects of temperature on mass transport and output characteristics were conducted by simulation and experimental investigations. Firstly, a multi-physics model coupled with mass/momentum transports and temperature effect was established, the simulation results were well fitted with the experimental data. The methanol concentration, crossover current density, polarization curve and the distribution of CO2 were positively correlated to the temperature. Secondly, a metal-based

Fig. 7. Effect of the operating temperature on mDMFC performance with different anode flow rates (a)Flow rate ¼ 2.5 ml min1 (b) Flow rate ¼ 5.0 ml min1.

324

Z. Yuan, J. Yang / Journal of Power Sources 285 (2015) 318e324

mDMFC was designed and fabricated using micro-stamping technology, while characterization of local topography and SEM were used to prove the fabrication accuracy. Thirdly, the polarization test and EIS test were conducted to prove the model accuracy. Experimental results showed that temperature had significant and complex impacts on cell performance. With the flow rate of 1.0 ml min1, the maximum power density was 85.3 mW cm2 at 60  C. When the flow rates increased to 2.5 and 5.0 ml min1, the maximum power densities were 106.4 mW cm2 and 115.1 mW cm2 at 80  C, respectively. Although the effect of temperature is beneficial upon the observed power density in this paper, under certain conditions there may be increased methanol crossover, which will lead to decreased fuel utilization and fuel cell efficiency. The investigation on temperature is practically critical for the portable application of mDMFC. Acknowledgements The work described in this paper was supported by the National Natural Science Foundation of China (No. 61372015), Research Fund for the Doctoral Program of Higher Education (No. 20130042120023) and Fundamental Research Funds for the Central

Universities in China (No. N140403001 and N120204001).

References [1] S.K. Kamarudin, F. Achmad, W.R.W. Daud, Int. J. Hydrogen Energy 34 (2009) 6902. [2] Y.H. Lu, R.G. Reddy, Int. J. Hydrogen Energy 36 (2011) 822. [3] H. Yang, T.S. Zhao, Q. Ye, J. Power Sources 139 (2005) 79. [4] A. Kundu, J. Power Sources 170 (2007) 67. [5] H. Yang, T.S. Zhao, Electrochim. Acta 50 (2005) 3243. [6] A. Kamitani, S. Morishita, H. Kotaki, S. Arscott, J. Micromech. Microeng 18 (2008), 125019. [7] A. Kamitani, S. Morishita, H. Kotaki, S. Arscott, J. Power Sources 187 (2009) 148. [8] Z.G. Shao, F.Y. Zhu, W.F. Lin, P.A. Christensen, H.M. Zhang, J. Power Sources 161 (2006) 813. [9] M. Tafaoli-Masoule, A. Bahrami, E.M. Elsayed, Energy 70 (2014) 643. [10] Y.S. Na, J.M. Kwon, H. Kim, Energy 50 (2013) 406. [11] Valter Bruno Silva, Abel Rouboa, Fuel 93 (2012) 677. [12] P.O. Mojtaba, M. Farhadi, K. Sedighi, J. Fuel Cell Sci. Technol. 8 (2011) 061004e061011. [13] K. Scott, S. Pilditch, M. Mamlouk, J. Fuel Cell Sci. Technol. 10 (2013) 051003e051011. [14] E. Alizadeh, M. Farhadi, K. Sedighi, J. Fuel Cell Sci. Technol. 10 (2010) 031002e031011. [15] S.Z. Chen, F. Ye, W.M. Lin, Int. J. Hydrogen Energy 35 (2010) 8225.