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Dec 20, 2012 - (6) The catalyst layer is usually porous to allow for transport of gases for the .... 3 potentiostat/galvanostat (USA, Princeton Applie...
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Membrane Electrode Assembly with Enhanced Membrane/Electrode Interface for Proton Exchange Membrane Fuel Cells Liang Wang,* Suresh G. Advani, and Ajay K. Prasad Center for Fuel Cell Research, Department of Mechanical Engineering, University of Delaware, Newark, Delaware 19716-3140, United States ABSTRACT: A novel structure for the membrane electrode assembly (MEA) of proton exchange membrane (PEM) fuel cells was developed by incorporating a porous PTFE matrix within the MEA. The highly porous structure of the PTFE matrix enhanced the membrane/electrode interface on the cathode side of the MEA by increasing the electrochemically active surface area (ESA) of the cathode catalyst. Moreover, the enhanced membrane/electrode interface is expected to improve the mechanical and electrical contact and create stronger bonding between the membrane and the electrode. Scanning electron microscope images confirmed the structure of the enhanced membrane/electrode interface. Cyclic voltammetry showed that the PTFE-enhanced interface resulted in a 3-fold increase of the cathode’s electrochemical surface area (ESA). Higher ESA resulted in higher catalyst activity, which improved the performance of the novel MEA by 20% in comparison with the traditional MEA.

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MEAs.10,11 However, no work to date has reported on the membrane/electrode interface. Catalyst layers are typically formed using nanostructured support elements bearing catalyst particles or thin films of catalytic material.12,13 The nanostructured catalyst electrodes may be incorporated into very thin layers to form a dense distribution of catalyst particles on the membrane surface. Effective MEA design strives to increase the interfacial contact area between the various layers of the MEA to effectively facilitate the transport of electrons, protons, reactants, and products through the MEA. Increased interfacial area provides an increased current carrying capacity and higher efficiency. In this work, a novel MEA with an enhanced membrane/ electrode interface is designed to improve the fuel cell performance by increasing the electrochemical surface area (ESA) and the electrochemical activity on the cathode side of the fuel cell. For identical catalyst loadings, an MEA with higher ESA achieves higher fuel cell performance due to the higher catalytic activity. Therefore, increasing the ESA can reduce the amount of platinum, and thereby reduce the cost of the fuel cell stack dramatically. Second, the enhanced interface microstructure could improve the electrical contact between the membrane and the electrode leading to reduced voltage losses and improved performance and efficiency. Finally, the enhanced interface microstructure could also increase the mechanical bonding between the membrane and the electrode, which may mitigate degradation mechanisms such as

INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) have attracted much interest as energy conversion devices due to their high efficiency and zero emission operation. The membrane electrode assembly (MEA) is the central component of a PEM fuel cell.1,2 The MEA consists of a PEM, catalyst layers, and gas diffusion layers (GDLs). These components are typically fabricated individually, and then pressed together at high pressure and temperature.3−5 The most widely studied and used electrocatalysts are based on the platinum (Pt)-group metals as they show superior electrocatalytic behavior.6 The catalyst layer is usually porous to allow for transport of gases for the reaction. High porosity increases the available surface area of the catalysts and improves the reaction rate.7 A high surfaceto-weight ratio of the platinum particles is necessary to maximize the surface area for the reaction. This is typically achieved by depositing small metal particles on a conducting support.8 The reduction of oxygen and oxidation of hydrogen are the most important and widely used electrochemical reactions,9 with Pt and its alloys being the best-known electrocatalysts for these reactions in acid fuel cell systems. Because of the high cost of Pt, it is necessary to maximize the surface area of the Pt catalyst and utilize it effectively. Furthermore, it is important to deposit the Pt on a support that allows for a good interaction with the ionomer (e.g., Nafion electrolyte), resulting in a triple-phase contact that permits protonic access at the Pt catalyst sites, while still encouraging diffusion of the gaseous species through the catalyst layer. In addition, the MEA must exhibit excellent bonding between the electrodes and the membrane to minimize electrical losses and promote durability. Several papers have reported the use of PTFE to improve the © 2012 American Chemical Society

Received: July 11, 2012 Revised: December 20, 2012 Published: December 20, 2012 945

dx.doi.org/10.1021/jp306887p | J. Phys. Chem. C 2013, 117, 945−948

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delamination due to hydrothermal cycling over hundreds of start−stop cycles.

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EXPERIMENTAL METHODS To prepare the novel membrane electrode assembly, 2 and 10 g of 5 wt % Nafion solution (E.I. DuPont) were prepared separately and dried at 60 °C to vaporize the solvent. The two Nafion resins were then dissolved in dimethylacetamide (DMAc) to form Nafion/DMAc solutions containing 0.1 and 0.5 g of Nafion resin, respectively. Each solution was stirred for 6 h. A thin (∼15 um) sheet of porous PTFE (manufactured by Shanghai DaGong New Materials Co Ltd., China, with a pore diameter of 0.3−0.5 um and porosity of around 85%) was stretched around an 11 cm × 10 cm stainless steel frame and heated to 80 °C. Next, the Nafion/DMAc solution with 0.1 g of Nafion resin was poured over the porous PTFE sheet, and the temperature was maintained at 80 °C for the first 12 h, and then raised to 120 °C for the next 12 h to form the PTFE/ Nafion membrane. The amount of Nafion resin added to the porous PTFE sheet was carefully controlled such that it was not sufficient to completely fill all of the pores; consequently, a large number of unfilled pores remained exposed on the upper surface. In contrast, the bottom surface of the PTFE/Nafion membrane was smooth and completely filled with 0.1 g of Nafion resin solution. This prepared PTFE/Nafion composite membrane was immersed in 0.5 M sulfuric acid for 2 h, and rinsed with DI water. Next, the composite membrane was dried overnight in an oven at 40 °C. The Nafion/DMAc solution with 0.5 g of Nafion resin was sprayed onto the bottom surface of the prepared PTFE/Nafion composite membrane, and dried at 120 °C for 2 h. Finally, catalyst ink with Pt/C, Nafion, and IPA was sprayed on both sides of the membrane. The catalyst loading was controlled at 0.4 mg Pt/cm2. The prepared catalystcoated membrane was hot-pressed with commercial gas diffusion layers at 130 °C for 2 min to fabricate the novel MEA. The side with the enhanced membrane/electrode interface was used as the cathode. The structure of the novel MEA is shown schematically in Figures 1 and 2. The MEA’s performance was then tested in a 10 cm2 fuel cell. A traditional MEA with homemade Nafion/PTFE membrane (25 um thickness) was also prepared with the same amount of Nafion resin and catalyst using a similar procedure. The microstructure of the composite membrane was examined with a JEOL 2000FX scanning electron microscope

Figure 2. Structure of the MEA with enhanced membrane/electrode interface.

(SEM) with an accelerating voltage of 200 kV. Cyclic voltammetry (CV) was conducted to measure the ESA of the cathode electrode using a VersaSTAT 3 potentiostat/ galvanostat (USA, Princeton Applied Research). Humidified hydrogen (200 mL/min) and nitrogen (400 mL/min) were fed to the anode and the cathode, respectively. The CV measurement was carried out between 0.1 and 0.9 V with a sweep rate of 50 mV/s. The polarization I−V evaluation of the fuel cell was conducted and controlled by a fuel cell test station from Arbin Instruments. The fuel cell was started by increasing its temperature to the set point of 70 °C and raising the saturation temperature of the reactant gas streams to their respective values. The H2 and O2 pipeline temperatures were maintained 5 °C higher than the saturation temperatures to prevent water condensation in the feed lines. The fuel and oxidant were fed in coflow (i.e., along the same direction in the flow channels) to the fuel cell. All fuel cell testing was conducted at ambient pressure. The fuel cell was conditioned for 2 h at a current density of 1 A/cm 2 with fully humidified H 2 /O 2 at stoichiometries of 1.5/2. Data were recorded after stable performance was obtained.

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RESULTS AND DISCUSSION The novel MEA structure was designed to increase the electrochemically active surface of the catalyst layer, and improve the mechanical and electrical bonding between the membrane and the catalyst layer, thereby improving the cell’s performance and durability. As shown in Figure 3, the top

Figure 3. Surface SEM of the porous PTFE substrate after partially filling with 0.1 g of Nafion solution.

surface of the Nafion/PTFE membrane with 0.1 g of Nafion shows a significant fraction of unfilled pores, which can be exploited to create the novel enhanced membrane/electrode interface. SEM was used next to examine the cross-section of the novel MEA. As shown in Figure 4, the enhanced membrane/electrode interface shows very good contact between the membrane and the cathode catalyst layer. A highly porous structure with PTFE fibers can be observed at the interface. Cyclic voltammetry plots for the novel and traditional MEAs confirmed the increase of the ESA resulting from the new

Figure 1. Schematic of the preparation method for the MEA with enhanced membrane/electrode interface. 946

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Because we used the same amount of the material to make both the novel MEA with enhanced membrane/electrode interface and the traditional MEA, the ECA is greatly enhanced in the novel MEA. A fuel cell performance test of the novel MEA was conducted and compared against a traditional MEA. The result is shown in Figure 6 and indicates that the novel MEA with enhanced Figure 4. SEM photomicrographs of the cross-section of the novel cathode catalyst layer.

preparation method as shown in Figure 5. To determine the ESA of the cathode, the charge due to adsorbed hydrogen on

Figure 6. Fuel cell performance of the traditional MEA and the novel MEA with enhanced membrane/electrode interface.

Figure 5. Cyclic voltammetry of the traditional MEA and the novel MEA with enhanced membrane/electrode interface.

membrane/electrode interface exhibits better fuel cell performance than the traditional MEA. The improvement at low current density can be attributed to the novel MEA’s higher catalytic activity due to its larger electrochemically active surface area as compared to the traditional MEA. Figure 6 also indicates that the novel MEA’s I−V curve exhibits a smaller slope for intermediate current densities due to improved electrical contact at the interface. Finally, the novel porous structure appears to enhance mass transport within the fuel cell at high current densities. While the voltage of the traditional MEA is seen to drop quickly at high current densities due to the mass transport limitation, this adverse behavior is avoided for the MEA with enhanced membrane/electrode interface.

the Pt catalyst was obtained by integration of the corresponding peak in the voltammogram with a double layer charging current as a baseline. The ESA of the Pt/C electrocatalyst in the cathode was calculated with the following equation:14 Se = 100Ad /cmv

Here, Se is the ESA, m2/g; Ad is the integral area of hydrogen oxidation desorption peak in the CV curve, AV; c is the electrical charge associated with monolayer adsorption of hydrogen on Pt surface, c = 0.21 mC/cm2; m is the mass of Pt in the cathode, mg; and v is the sweep rate of CV, V/s. The calculated areas show that the ESA of the novel MEA with the enhanced membrane/electrode interface is 98.7 m2/g of Pt, which is significantly higher than the ESA of the traditional MEA of 34.6 m2/g of Pt. The improvement of the ESA is due to the highly porous structure of the PTFE substrate at the membrane/electrode interface as seen from Figure 4. The increase in current density above 0 A as seen in Figure 5 for the traditional MEA is possibly due to substrate, electron, or proton transport limitations of the MEA.15,16 Reference 10 mentions that the ECA of the catalyst decreased after adding PTFE (1% to 7%) to the catalyst layer; this decrease in ECA could be due to the aggregation of the PTFE added to the catalyst layer, which reduced the available reaction area within the catalyst layer. In the current work, we used porous PTFE, which has an inherent porosity of about 85%. These pores are not closed, but are instead open or contiguous within the PTFE film. This unique structure transforms the conventional two-dimensional catalyst layer into a three-dimensional reaction interface.

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CONCLUSIONS A porous PTFE matrix was incorporated within the MEA of a PEM fuel cell to enhance the membrane/electrode interface on the cathode side of the MEA. Such an enhancement is expected to provide two benefits: (1) an increase of the ESA of the cathode catalyst, which should yield higher fuel cell performance, and (2) an improved bonding between the catalyst layer and the membrane, which should improve the mechanical integrity of the MEA. SEM images showed that the use of a porous PTFE matrix led to the formation of a highly porous cathode catalyst layer. The CV measurements confirmed that the novel MEA preparation method greatly increases the ESA. The fuel cell performance of the MEA with enhanced membrane/electrode interface at 100% RH showed around 20% improvement over conventional MEA at 1 A/cm2 due to the highly porous catalyst structure with increased ESA. 947

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Federal Transit Administration. REFERENCES

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