effect of gas diffusion layer characteristics and

Chemical Engineering Communications

ISSN: 0098-6445 (Print) 1563-5201 (Online) Journal homepage: http://www.tandfonline.com/loi/gcec20

EFFECT OF GAS DIFFUSION LAYER CHARACTERISTICS AND ADDITION OF POREFORMING AGENTS ON THE PERFORMANCE OF POLYMER ELECTROLYTE MEMBRANE FUEL CELLS Erce Şengül , Serdar Erkan , İnc[idot] Eroğlu & Nurcan Baç To cite this article: Erce Şengül , Serdar Erkan , İnc[idot] Eroğlu & Nurcan Baç (2008) EFFECT OF GAS DIFFUSION LAYER CHARACTERISTICS AND ADDITION OF PORE-FORMING AGENTS ON THE PERFORMANCE OF POLYMER ELECTROLYTE MEMBRANE FUEL CELLS, Chemical Engineering Communications, 196:1-2, 161-170, DOI: 10.1080/00986440802293130 To link to this article: http://dx.doi.org/10.1080/00986440802293130

Published online: 20 Oct 2008.

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Date: 09 October 2015, At: 06:40

Chem. Eng. Comm., 196:161–170, 2008 Copyright # Taylor & Francis Group, LLC ISSN: 0098-6445 print/1563-5201 online DOI: 10.1080/00986440802293130

Effect of Gas Diffusion Layer Characteristics and Addition of Pore-Forming Agents on the Performance of Polymer Electrolyte Membrane Fuel Cells ˘ LU,1 ¨ L,1 SERDAR ERKAN,1 I˙NCI˙ EROG ERCE S¸ ENGU AND NURCAN BAC ¸2 Downloaded by [Ataturk University] at 06:40 09 October 2015

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Department of Chemical Engineering, Middle East Technical University, Ankara, Turkey 2 Department of Chemical Engineering, Yeditepe University, Kayisdag, Istanbul, Turkey The performance of five layer membrane electrode assemblies having different characteristics of gas diffusion layers was determined at 70C cell temperature and ambient pressure. The maximum power density at 0.6 V was 0.36 W=cm2 for the membrane electrode assembly prepared with the gas diffusion layer having minimum thickness (SGL BC 30 type). On the other hand, the maximum power density at 0.5 V was 0.44 W=cm2 for the membrane electrode assembly prepared with SGL BC 34 type gas diffusion layer. It was found that resistance of a membrane electrode assembly is strongly dependent on gas diffusion layer thickness. Moreover, membrane electrode assemblies prepared with carbon paper gas diffusion layers resulted in higher performance than the assembly prepared with carbon cloth gas diffusion layer. Addition of pore-forming agents, which were ammonium carbonate, ammonium bicarbonate, ammonium sulfonate, and ammonium oxalate, to the catalyst ink lowered the performance. Keywords GDL; MEA preparation; PEMFC; Pore-forming agent

Introduction Polymer electrolyte membrane fuel cells (PEMFCs) have been seen as one of the most promising energy sources for the future due to their ability to produce highly efficient and largely pollution-free energy. The PEMFC, which is fed with hydrogen at the anode side and oxygen or air at the cathode side, is particularly attractive owing to low operating temperature (50–80C) and pressure (1–3 atm). The electrochemical reaction occurs in the membrane electrode assembly (MEA), which is stated to be the heart of PEMFC (Barbir, 2005). Figure 1 illustrates the working principle of a PEMFC. When hydrogen gas comes to the anode side of the cell, it separates into its protons and electrons. The protons are conducted through the membrane (electrolyte) whereas the free electrons produced at the anode travel through an external circuit to the cathode. At the cathode side, oxygen gas combines with those electrons and protons. The final products of such a cell are electric power, water, and heat (Larminie and Dicks, 2003). Address correspondence to Nurcan Bac¸, Department of Chemical Engineering, Yeditepe University, Kayisdag, Kadikoy 34755 Istanbul, Turkey. E-mail: [email protected]

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E. S¸ engu¨l et al.

Figure 1. Schematic representation of working principle of PEMFC.

The main components of single a PEMFC are shown in Figure 2. The polymer membrane is located in the middle. On both sides of the membrane there is a porous electrode, which is composed of active catalyst layer (the side facing the membrane) and gas diffusion layer (GDL). The combination of membrane and electrodes is termed an MEA. The last component of the PEMFC is a flow field plate or end plates that enclose the MEA (Barbir, 2005). In order to minimize all transport resistances and increase the performance, it is required to use qualified GDLs: good electrical conductivity, high gas permeability, and low resistance. Kong et al. (2002) investigated the effect of pore size distribution on the performance of PEMFCs. They changed the morphology of electrodes by heat treatment and applying a pore former (Li2CO3). It was suggested that there is an optimum amount of macropore volume for performance enhancement and the pore-size distribution in GDL is more important than its total porosity

Figure 2. Main components of PEMFC.


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Figure 3. Linearization of a fuel cell polarization curve.

for effective management of water and gas transport. Yoon et al. (2003) fabricated three kinds of catalytic layers to investigate the effect of pore structure of catalyst layer on the performance of a PEMFC. The first ones were fabricated by changing droplet size from fine to coarse mode during the spraying of electrocatalyst slurry onto the teflonized carbon cloth, used as a gas diffusion media backing. All the modes gave approximately the same result, in the current range of 0–900 mA=cm2. However, at high current densities, coarse droplets showed better results, which was an unexpected result, because, in general, finer droplet size makes a higher portion of grain boundary area and secondary pores between the grains, thereby enhancing the gas transport through the catalytic layer. It was seen that when the thermoplastic agent TBAOH was added performance increased due to the increased structural stability of the catalytic layer. However, when the pore-forming agent ethylene glycol was added no further increase in the performance was observed. The possible reason for this may be the fact that pure O2 was fed to cell. Lee et al. (2004) investigated various GDL fabrication methods. They suggested that spraying and screen printing methods are good for the construction of gas diffusion layers since they provide good distribution of macro- and micropores. Song et al. (2005) used ammonium bicarbonate as a pore former in the structure of a catalyst layer. It was reported that 20 wt.% of ammonium bicarbonate in the platinum black is optimum. Zhao et al. (2007) investigated the effect of pore-forming substances on the performance of PEMFCs. Three pore-forming agents, ammonium bicarbonate, ammonium oxalate, and ammonium sulfate, were used in order to increase the porosity of the MEA. The MEAs were prepared by the standard hot-press process and the pore formers were introduced to the MEA during the catalyst ink preparation step. It was found that addition of ammonium bicarbonate is an effective way to improve the performance of MEAs, reducing the cost of PEMFC. With the addition of ammonium bicarbonate in the ratio of 1:2 Pt:NH4HCO3 the catalyst loading can be decreased from 0.4 to 0.2 mgPt=cm2. Gamburzev and Appleby (2002) stated that for H2=air fuel cells addition of pore formers to the structure of the

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cathode gas diffusion layer greatly enhanced the performance. It was reported that 33 wt.% pore former gave the best result for the cathode side. Barbir (2005) stated that for most fuel cells and their practical operating range, a linear approximation of a polarization curve is considered to be very good fit. This is shown in Figure 3. A linear polarization curve has the following form: Vcell ¼ V0 ki where V0 is the intercept (actual open circuit voltage (OCV) is always higher) and k is the slope of the curve. The value of k is not the actual sum of ohmic resistances but it gives an idea about the ohmic losses in the MEA.


Experimental Section MEA Preparation The GDL spraying technique was used for the preparation of MEAs. In the first step, catalyst ink, which is comprised of 20 wt.% Pt on Vulcan XC-72 catalyst (E-Tek), 5 wt.% Nafion1 solution (Ion Power Inc.), distilled water, and 2-propanol, was prepared and mixed via ultrasonics for 2 h. In addition to these substances, pore-forming agents were added to the catalyst ink. For 1 g of Pt-C catalyst, 0.86 g Nafion solution, 6 mL distilled water, 12 mL 2-propanol, and 0.2–2 g poreforming agents were used. In order to clean and increase the proton conductivity of the Nafion1 membrane it is conditioned. The conditioning was done as described by Qi and Kaufman (2002). First, the membrane was boiled in 3% H2O2 solution. Then it was boiled in distilled water. The third step was boiling in 0.5 M H2SO4 solution. Then it was boiled in distilled water again. In the fifth step, it was washed with distilled water several times in order to ensure that there was no H2SO4. The last step was drying. It was realized that drying the membrane by placing it between two filter papers was very suitable to prevent swelling or surface deformation. All the boiling steps took about 1 h and were carried out at 80C. In order to coat the GDLs with a catalyst layer, the anode and cathode side GDLs were fixed on a paper frame. An air heating gun blowing air at 150C was located 45 cm above the paper frame for the vaporization of 2-propanol and water in the catalyst layer. The catalyst ink was sprayed until the desired catalyst loading (0.4 mgPt=cm2 for both anode and cathode sides) was achieved. The catalyst loading was controlled by just weighing the GDLs at different times. After the GDLs were loaded with catalyst, they were kept in an oven at 80C for 1 h in order to completely remove the liquid components of catalyst ink (when the pore-forming agent was added, the GDLs were heat treated at 100C in a vacuum oven for 2 h in order to decompose the pore-forming agent). Then it was weighed again. To complete the MEA, the GDLs were hot pressed to the membrane at 130C and 250 psig for 3 min.

Performance Test The performance of fabricated MEAs was measured via the PEMFC test station built at the METU Fuel Cell Technology Laboratory. A schematic representation of the test station is given in Figure 4. The main elements of the test station are gas cylinders, mass flow controllers, cathode and anode side humidifiers,


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Figure 4. Schematic representation of PEMFC test station.

temperature controllers, electronic load, and the computer, which has special data logging software. A single-cell PEMFC (Electrochem FC05–01SP-REF) having 5 cm2 active area was used in the experiments. The test cell was surrounded with heating bands to raise the temperature to the desired value. In order to see the gas flow, the exhausts of the anode and cathode were purged by means of passing from the gas bubblers. The external load (Fideris1) was applied by means of an electronic load, which can be controlled either manually or from the computer. The current and voltage of the cell were monitored and logged throughout the operation of the cell by the software (FCPower v. 2.1.102 Fideris1) of the electronic load. The fabricated MEA was placed in the test cell and the bolts were tightened at a torque of 15–20 lb-in for each bolt. The cell temperature was set to 70C and the temperatures of the humidifiers and gas transfer lines were set 10C above the cell temperature. Before passing the reactant gases, the anode side was purged with nitrogen in order to ensure that no oxygen was present in the anode side or there was no leak in the cell. In case of any leak, combustion occurs immediately. This may cause very high temperature zones or even membrane puncture. After the cell passed the required level of safety and set temperatures were achieved, hydrogen and oxygen were supplied to the cell at a rate of 0.1 slpm. The cell was operated at 0.5 V until it came to steady state, which took about 5–6 h. However, in case of membrane drying or flooding conditions, the voltage was adjusted. That is, if drying occurred, the voltage was decreased to produce more water. On the other hand, if there was flooding, the reverse measure was taken. After steady state was achieved, starting from the OCV value, the current-voltage data were logged by changing the load.

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Table I. Properties of tested GDLs GDL type Thickness (mm) Areal weight (g=m2) Bending stiffness Air permeability

30 BC

31 BC

34 BC

310 >1.0 trans >4.0 long 2.0 11.0

330 >1.0 trans >4.0 long 2.5 11.0

35 BC

Carbon cloth

315 —

325 —

350 —

0.70