Parametric investigation to enhance the performance of a PBI-based high-temperature PEMFC

Energy Conversion and Management 78 (2014) 431–437

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Parametric investigation to enhance the performance of a PBI-based high-temperature PEMFC Y.M. Ferng a,⇑, A. Su b, J. Hou b a b

Department of Engineering and System Science, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Rd., Hsingchu 30013, Taiwan, ROC Fuel Cell Center, Department of Mechanical Engineering, Yuan Ze University, 135 Yuan-Tung Rd., Nei-Li, Chung-Li 32026, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 2 September 2013 Accepted 28 October 2013 Available online 28 November 2013 Keywords: High-temperature PBI-based PEMFC Cell performance PBI loading Acid doping level Electrode thickness and porosity

a b s t r a c t With the advantages of simpler heat and water management, lower CO poisoning, and higher reaction kinetics, the high-temperature polybenzimidazole (PBI)-based proton exchange membrane fuel cell (PEMFC) can be considered as one of the commercialized energy generators in the near future. This paper experimentally and analytically investigates different design and operating parameters to enhance the performance of a PBI-based PEMFC, an in-house cell prepared in the Fuel Cell Center of Yuan Ze University. These parameters studied include PBI loading, operating temperature, gas flowrate, electrode thickness and porosity, and acid doping level. Experiments are performed to study the effects of PBI loading, operating temperature, and gas flowrate on the cell performance. Validated against the measured data of polarization and power curves, a simplified two-dimensional model for this PBI-based PEMFC is also developed to help the experiments to investigate other parameters. Based on the experimental data and the model predictions, the cell performance can be enhanced as the PBI loading is reduced, the operating temperature is elevated. Thinner electrode thickness, smaller porosity, and higher acid doping level are also predicted to benefit to the performance of the PBI-based PEMFC. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Proton exchange membrane fuel cells (PEMFCs) have been widely investigated since they can be one of the candidates for commercialized energy generators in the near future. Conventional Nafion-based PEMFCs can only be operated at the temperature below 100 °C due to the need of liquid water for the Nafion membrane to maintain its good proton conductivity. Besides water and heat management, carbon monoxide (CO) poisoning and slower electrochemical kinetics challenge widespread commercialization of the Nafion-based PEMFCs. Based on the previous researches [1–3], high-temperature operation (>100 °C) for a PEMFC can simplify heat and water management, lower CO poisoning, as well as elevate reaction kinetics, which enhances the cell performance. The polybenzimidazole (PBI)-based PEMFC is one type of fuel cells that can be reliably operated at the high temperature up to 200 °C [4]. PBI-based PEMFCs had been investigated in the literatures [5–37]. Several researches had focused on enhancing the cell performance through experiments [11,15,20,21,23,24,28] and simulations [12,16,26,34–36]. Seland et al. [11] tested different designs of membrane–electrode-assembly (MEA) by varying the

⇑ Corresponding author. E-mail address: [email protected] (Y.M. Ferng). 0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2013.10.069

Pt content and the PBI loading in terms of cell performance. They found that MEA design with a high Pt content and a thin catalyst layer gave the overall best performance. As clearly seen in the Lobato et al. [15] work, the temperature greatly influences the cell performances. The catalyst stability was also investigated by a cyclic voltammetry (CV) method. In Mamlouk and Scott paper [20], effect of electrode parameters in a phosphoric acid-doped PBI membrane fuel cell was reported. According to their study, high levels of doping with phosphoric acid in the anode catalyst layer were beneficial to the cell performance. Through performance and electrochemical impedance spectroscopy (EIS) tests, Chen and Lai [21] showed that increase in the temperature significantly improved the PBI/H3PO4 cell performance and no improvement was observed with the increasing gas humidity. Lobato et al. [23] studied influence of the PBI amount on the PBI-based PEMFC performance. Their test results indicated that the electrochemical active surface area decreases and the mass transfer limitations increases if a large amount of PBI loading was used in the electrodes. The best amount of PBI loading corresponded to a carbon/PBI weight ratio of 20, based on their studies. The benefits of the micro-porous layer (MPL) inclusion in the electrode structure was also observed in the other tests of Lobato et al. [24]. Araya et al. [28] experimentally investigated the effects of methanol and water vapor on the performance of a PBI-based PEMFC. The test results showed that 5% and 8% of vapor in the anode feed clearly degraded the cell performance.

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Nomenclature a C D D⁄ F i j jr k⁄ M n P Pt/C R rp S T To V wPBI X

effective surface area of Pt/C pellets in the PBI membrane concentration, mol/m3 diffusion coefficient, m2/s effective diffusion coefficient, m2/s Faraday constant, 96,485 C/mol current density, A/m2 transfer current density, A/m3 reference current density, A/m3 effective thermal conductivity, W/m K molar concentration of H3PO4 number of transfer electrons pressure, N/m2 platinum and carbon gas constant, 8.314 J/K mol radius of Pt/C pellet source term temperature, K fuel gas temperature, K voltage, V PBI weight percentage acid-doping level

Greek symbol a transfer coefficient uel electrical potential of electrolyte phase, V usol electrical potential of solid phase, V

Cheddie and Munroe [12] developed one-dimensional (1-D) mathematical model to predict the polarization performance for a PBI-based PEMFC. Their results indicated that the cell performance can be greatly elevated by improving the membrane conductivity and the membrane-catalyst interface. Based on an electrochemical AC impedance technique, Hu et al. [16] developed a diffusion–convection/electrochemical mathematical model for a high-temperature PBI fuel cell. The effects of oxygen diffusivity and GDL thickness on cell performance were studied. Lobato et al. [26] also showed that the neural network tool can be applied in investigating influence of the gas diffusion layer designs on the cell performance of a PBI-based PEMFC, including Teflon content, air permeability, porosity, mean pore size. Grigoriev et al. [34] developed 2-D model to propose optimum values of design requirements to optimize the efficiency for the PBI-based PEMFCs. The key cell components considered in their work included flow-field channels, current transfer ribs of bipolar plates, gas diffusion electrodes. A gas crossover model for a PBI-based PEMFC is developed by Chippar and Ju [35]. They investigated the effects of gas crossover on the cell performance by varying three critical parameters, including operating current density, operating temperature and gas crossover diffusivity. The numerical results indicated that the effect of gas crossover on the cell performance is insignificant for a fresh membrane. Kim et al. [36] numerically investigated the effects of operating conditions on the performance degradation for the PBI-based PEMFC through a 1-D model. The cell lifetimes had been predicted according to different operating conditions. Their results showed that the MEA with the lower doping level gives the shorter lifetime. Majority of this paper investigates the parameters that may enhance the performance of a PBI-based PEMFC. This in-house PBI-based PEMFC is prepared in the Fuel Cell Center of Yuan Ze University. The PBI loading, operating temperature, gas flowrate, electrode thickness and porosity, and acid doping level are consid-

re rp

d (U)|n

effective electron conductivity, S/m effective proton conductivity, S/m membrane ionic conductivity, S/m overpotential, V volume fraction thickness normal derivative for U

Subscript a c CL e el H2 in n m O2 p PA Pt sol T WW /

anode cathode catalyst layer electron electrolyte hydrogen inlet normal direction membrane oxygen proton H3PO4 platinum solid temperature water vapor species

rm g e

ered. The experiments related to effects of PBI loading, operating temperature, gas flowrate on the cell performance are conducted. A simplified 2-D model [33] for this PBI-based PEMFC is adopted to assist the experimental work. Effects of acid doping level, electrode thickness and porosity on the cell performance are simulated. An optima design and management of a PBI-based PEMFC can be achieved through a series of parametric investigation.

Fig. 1. Photograph of test PEMFC (a) and schematic of 2-D solution domain (b).

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All of the source terms in above equations are listed in Table 2.

2. Experimental and modeling

 Correlations for X, ePA, and rm.

2.1. Experimental descriptions

The acid-doping level can be estimated using the H3PO4 concentration (M).

Fig. 1(a) and (b) shows the photograph of in-housed PBI-based PEMFC and the schematic of 2-D solution domain, respectively. Basic dimensions of this PBI-based PEMFC are listed in Table 1. The PBI solution used in the experiments is prepared by mixing with LiCl in a DMAc (dimethylacetamide) solvent. This solution is cast onto a glass plat and dried in a vacuum oven to remove the solvent. The PBI membrane is then immersed in distilled water that is used to remove LiCl. The residual water is also removed to obtain the dry membrane that is subsequently doped with 85 wt% H3PO4 aqueous solution to prepare the PBI membrane. The carbon supported Pt catalyst and the carbon paper are used as the catalyst layer (CL) and the gas diffusion layer (GDL), respectively. The catalyst solution with Pt–C/PBI/LiCl/DMAc is coated onto two pieces of carbon paper using an ultrasonic sprayer and the solvent in the coated CL is evaporated. Then, the catalyst electrodes with PBI + Pt–C are prepared, which are also doped with phosphoric acid by immersing in a 10 wt% phosphoric acid aqueous solution. Detailed descriptions about syntheses and characterizations of PBI membrane and cell preparation can be conferred to the works of Su et al. [33] and Lin et al. [37]. Two catalyst electrodes are pressed with a pressure of 50 N/cm on both sides of a PBI membrane to prepare a membrane–electrode-assembly (MEA). Four sets of MEAs with PBI/[PBI + Pt–C] weight ratios of 5, 10, 20, and 30 wt% in the CLs are adopted in the experiments. The cell performance of a in-house PBI-based PEMFC is tested using a FC5100 fuel cell testing system. The nonhumidified H2/O2 input flow rates are used as the fuel gases. The polarization curves are obtained through measuring the current density at the constant voltage.

X ¼ 0:7119 þ 1:2363M þ 0:2111M2 þ 0:012M 3

1 ½1 þ 4:81=ðX  2Þ

rm ¼

100  exp½8:0219  ð2605:6  70:1XÞ=T T

jc ¼

½6

ð7Þ

½22

ð8Þ

P O2 4F þ nk1g HPBI O2

ð9Þ

2ð1  eCL Þð1  hÞ ðr p þ dPBI Þ

ð10Þ

dPBI aDPBI O 2

where

a¼

h ¼ membrane blockage factor ¼ 1:714g  0:893g2 n ¼ effective reaction factor ¼

½3w cothð3wÞ  1 3w2

ð11Þ ð12Þ

!1=2 kg rp w ¼ Thiele modulus for spherical pellet ¼ 3 DIO ð1  eCL Þ 2 ð13Þ kg ¼ reaction rate constant ¼

    1 r aF j exp  g RT c 4FC rO2 o;c

PA 1:945 HRBI ½5:79ð1  e1:8 O2 ¼ PBI solubility in oxygen ¼ ePA PA Þ þ H O2 

ð14Þ

ð15Þ

2 lnð10HPA O2 Þ ¼ ð178:45 þ 431:08mPA þ 257:13mPA Þ  ð64; 288

 Species concentration equation.

 15; 6646mPA þ 93; 500m2PA Þ=T

rC / Þ ¼ S/

D/ ¼ D/ e1:5

ePA ¼

 Correlations for current density. The two-phase agglomerate approach proposed by Ye and Nguyen [40] is used to estimate the cathode current density (jc).

A simplified 2-D model is used in this paper to assist the experimental work in investigating effects of different parameters on the cell performance. Fig. 1(b) schematically shows the simulation domain, including the GDLs, CLs, and a PBI membrane. The flow channel is not considered herein. Thus, half of the flow channel and rib in the cathode and anode compartments are used as the simulation boundaries. All the equations adopted in the simulations are briefly described as follows.

r

ð6Þ

And then, ePA and rm can be described as

2.2. Model descriptions

ðD/

½39

ð1Þ

½38

DIO2 ¼ oxygen diffusion coefficient at PBI=catalyst pellets interface

ð2Þ

PBI ¼ DPBI O2 ½ð1  eCL Þepell 

where / = O2, H2, WV for oxygen, hydrogen, and water vapor, respectively.  Potential equation for electron.

rðre rusol Þ ¼ Se

ð3Þ

The anode current density (ja) can be evaluated using the Butler– Volmer equation.

ð4Þ

     aga F ag F r  exp  c ja ¼ ja j9:869  106 Pj0:5 exp RT RT

ð5Þ

The values of the parameters used in the above correlations are listed in Table 3. In addition, the boundary conditions used in the simulations are summarized in Table 4.

 Temperature equation. 

rðk rTÞ ¼ ST

1:5

ð17Þ

 Potential equation for proton.

rðrp ruel Þ ¼ Sp

ð16Þ

Table 1 Dimensions of PBI-based PEMFC. Channel width (mm)

Rib width (mm)

GDL thickness (mm)

CL thickness (mm)

Membrane thickness (lm)

Total reaction area (cm2)

0.5

0.5

0.25

0.02

0.01

5

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Table 2 Source terms in the governing equations. SO2 (mol/m3 s)

SH2 (mol/m3 s)

SWV (mol/m3 s)

Se (A/m3)

Sp (A/m3)

Ja

Ja

Jc

Jc

Anode GDL

ST (W/m3) i2 =reff e

Anode CL

Ja/2F

2 eff jjjjgj þ i2e =reff e þ ip =rp

PBI Cathode CL

Jc/4F

Ja/2F

i2 =reff p   2 eff DS þ i2e =reff jjj jgj  TnF e þ ip =rp

Cathode GDL

i2 =reff e

3. Results and discussion Effects of the PBI loading and the operating temperature on the polarization curves for an in-house PBI-based PEMFC had been presented in the previous paper [33]. However, these two parameters are further discussed herein. The experimental data are presented using the power curves in order to completely investigate the parametric effects to enhance the performance of PBI-based PEMFC. In addition, the test data related to the flowrate effect and the simulation results about the CL thickness and porosity effects as well as the acid doping effect on the cell performance are also added in this paper. Fig. 2 shows the PBI loading effect on the performance of PBIbased PEMFC with the operating temperature of 160 °C. Four different PBI loadings, namely, 5, 10, 20, 30 wt% are considered. In this figure, dots are experimental measurements and lines are the corresponding predictions. The solid and dash lines present the polarization curve (to left longitude) and the power curve (to right longitude), respectively. PBI can improve the protonic transport [11] and suppress the PA adsorption [6]. However, excessive PBI content would block gas transport and decrease the Pt volume fraction, which lowers the performance of PBI-based PEMFCs. Both measurements and predictions show the trend that the lower PBI loading can enhance the cell performance. The peaking values of power density for this in-house PBI-based PEMFC with the PBI loading of 30, 20, 10, 5 wt% are measured to be 7.79  102 W/m2, 2.03  103 W/m2, 3.07  103 W/m2 and 3.45  103 W/m2, respectively. The corresponding predicted values are 8.45  102 W/m2, 2.06  103 W/m2, 3.16  103 W/m2, and 3.57  103 W/m2. Increase in the temperature would increase the chemical reaction rate and the fuel gas transfer rate, and lower the cell resistance, which can enhance the cell performance. The temperature effect is investigated with experiments and simulations that are conducted under the PEMFC with the PBI loading of 5 wt% and the operating temperatures of 160 and 200 °C. The beneficial effect of temperature on the performance of PBI-based PEMFC is clearly revealed in the measurements and the predictions that are shown in Fig. 3. The peaking values of power density for the PBI-based PEMFC with the operating temperatures of 160 and 200 °C are measured to be 3.45  103 W/m2 and 5.34  103 W/m2. The corresponding predicted values are 3.57  103 W/m2 and 5.38  103 W/ m2. In addition, based on the agreement between the measured and the predicted curves revealed in Figs. 2 and 3, the present simplified 2-D model can assist the experiments in investigating the parameters to enhance the cell performance. Fig. 4 experimentally compares the performance of in-house PBI-based PEMFC under different values of H2/O2 flowrate, namely 0.05, 0.1, 0.2, 0.3 L/min. The operating temperature is 160 °C and

the PBI loading is 10 wt%. As shown in the measured results, the flowrate has little effect on the cell performance. The result implies that the test minimum flowrate of 0.05 L/min is enough to support this PBI-based PEMFC operated in the activation and ohmic polarization regions. However, there is still a lower degradation in the cell performance in the concentration polarization region, which is resulted from the insufficient supply of fuel at this low value of flowate. CL thickness has significant effect on the PEMFC performance since the sluggish oxidation reduction reaction (ORR), activation and ohmic polarization are strongly related to the CL thickness [41]. The electrode activation resistance and ohmic resistance decrease as the CL thickness decreases, which enhances the cell performance. This positive effect can be predicted by the present model, as revealed in Fig. 5. Three values of CL thickness are considered in this figure, including 1, 10, 100 lm, respectively. The temperature is set to be 160 °C and the PBI loading is 5 wt% for all the cases. As clearly shown in the polarization and power curves, the cell performance is greatly enhanced as the CL thickness is reduced from 100 lm to 1 lm. The peaking values of power density for the PBI-based PEMFC with the CL thickness of 100, 10, 1 lm are predicted to be 2.94  103 W/m2, 3.57  103 W/m2 and 4.30  103 W/m2, respectively. Fig. 6 shows effect of the acid doping level (X) on the performance of PBI-based PEMFC. The acid-doping level is defined as the number of phosphoric acid absorbed per PBI. The operating temperature is 160 °C and the PBI loading is 10 wt%. The activation area of CL increases as the acid doping level increases, which reduces the cell internal resistance and subsequently enhances the cell performance [42]. The beneficial effect of acid doping level on the cell resistance can be modeled using the following equation [10]. The membrane ionic conductivity (rm) decreases with the increasing value of X.

rm ¼

   100 2605:6  70:1X exp 8:0219  T T

ð18Þ

The peaking values of power density for the PBI-based PEMFC with the X value of 4, 6.8 and 10 are predicted to be 2.39  103 W/m2, 3.16  103 W/m2 and 3.63  103 W/m2, respectively. The predicted effect of CL porosity on the performance of the test PBI-based PEMFC is revealed in Fig. 7. The values of CL porosity considered in this paper are 0.2, 0.4 and 0.6. The temperature is set to be 160 °C and the PBI loading is 5 wt% for all the cases. The electronic conductivity increases with the decreasing in the CL porosity. However, smaller porosity may block the transport of fuel gas. Based on the comparisons of polarization and power curves, the cell performance is enhanced as the CL porosity decreases. These results imply that the increasing effect of electronic conductivity

Table 3 Input values for the simulations. r

km (W/m K)

kGDL (W/m K)

re,GDL

re,CL (S/m)

dGDL (lm)

dACL (lm)

dCCL (lm)

rp (lm)

a

(S/m)

ja

C O2 ;in (mol/m3)

C H2 ;in (mol/m3)

DPBI O2 (m/s)

40

20

250

500

250

20

10

10

1.5

2.1  108

121.32

121.32

8  1010

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Y.M. Ferng et al. / Energy Conversion and Management 78 (2014) 431–437 Table 4 Boundary conditions used in the simulations. Boundary location

Concentration eq.

Temperature eq.

Electron potential eq.

O2 side H2 side Rib wall at the cathode side Rib wall at the anode side

C O2 ¼ C O2 ;in C O2 ¼ C H2 ;in ðrC O2 Þjn ¼ 0 ðrC H2 Þjn ¼ 0

T = T0 T = T0 Thermal insulation Thermal insulation

(rusol)|n = 0 (rusol)|n = 0 Preset usol usol = 0

Fig. 2. PBI loading effects on polarization and power curves based on measurements and predictions.

Fig. 3. Temperature effects on polarization and power curves based on measurements and predictions.

would surpass the decreasing effect of fuel supply for the in-house PBI-based PEMFC with these CL porosity designs. The peaking values of power density for the PBI-based PEMFC with the CL porosity of 0.6, 0.4 and 0.2 are predicted to be 3.15  103 W/m2, 3.57  103 W/m2 and 4.00  103 W/m2, respectively.

4. Conclusions Based on the experimental and analytical results, the change trends of different parameters to enhance the performance of a

Fig. 4. Flowrate measurements.

effects

on

polarization

curves

based

on

experimental

Fig. 5. CL thickness effects on polarization and power curves based on model predictions.

PBI-based PEMFC are summarized in Table 5. Several important conclusions can also be drawn from the present investigation.  The cell performance is enhanced with the decrease in the PBI loading, as shown in both the measurements and predictions. Advantages of PBI-based electrode include good protonic transport, excellent thermal and chemical stability, low volatility, and high CO tolerance. However, higher PBI loading would reduce the electrocatalytic activity and limit the mass transfer

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Y.M. Ferng et al. / Energy Conversion and Management 78 (2014) 431–437

 Appropriate fabrication of the CL can enhance the PEMFC performance. Under consideration of cell structure integrity, the thinner the CL thickness and the smaller the CL porosity are, the higher the cell performance is. This performance enhancement can be precisely captured by the present simulation model.  The acid doping level in a PBI solution also has a significant effect on the performance of a PBI-based PEMFC. As the acid doping level is elevated, the cell internal resistance is decreased due to the increasing activation area of CL, which enhances the cell performance. The beneficial effect of acid doping level on the cell performance can also be predicted in this paper.

References

Fig. 6. Acid doping effects on polarization and power curves based on model predictions.

Fig. 7. CL porosity effects on polarization and power curves based on model predictions.

Table 5 Parametric trends for the performance enhancement of PBI-based PEMFC. Parameter

Trend

PBI loading Temperature Electrode thickness Electrode porosity Acid doping level

Decrease Increase Decrease Decrease Increase

process, which lowers the cell performance. This inferior effect of high PBI loading on the performance of PBI-based PEMFC is revealed in both the experiments and simulations.  The cell performance is enhanced as the operating temperature is elevated due to the increase in the chemical reaction rate and the mass transfer rate. This beneficial effect on the cell performance is clearly shown in the measured and the predicted polarization and power curves.

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