Effect of Ionomer on Electrode Performance Durability Christina Johnston, Zhongfen Ding, Baeck Choi, and Yu Seung Kim Los Alamos National Laboratory MS D429, MPA-11 Los Alamos, NM 87545 Modern proton-exchange membrane fuel cells require stable performance during long-term operation [1]. The insufficient durability of Pt-based electrodes remains a major barrier to commercialization. Although most strategies to increase electrode durability focus on preventing the loss of electrochemically-active surface area (ECSA), we demonstrated recently that performance loss after potential cycling could be reduced by changing the dispersing solvent used to prepare the electrode [2, 3]. Because the ECSA did not vary much between electrodes, the improvement in durability was interpreted as an increase in the robustness of the ionomer and/or ionomer interfaces. As a continuation of the strategy of manipulating the ionomer to improve durability, we report the effect of varying the ionomer side chain structure. The cathode electrode for each membrane electrode assembly (MEA) was prepared from commercial or house-made polymer dispersion, and a carbon-supported Pt catalyst (20 wt.%, ETEK). LANL-standard Pt-based electrode [4] and Nafion ® 212 were used for the anode electrode and proton exchange membrane, respectively. Two types of ionomer were compared: short side chain (SSC) perfluorinated sulfonic acid (PFSA) (Aquivion™, Solvay Solexis) and long side chain (LSC) PFSA (Nafion ® , Ion Power). MEAs were fabricated by LANL standard decal method [4]. Initial fuel cell performance and cell durability during potential cycling [5] were measured. Figure 1 compares H 2 /air polarization curves of initial and after 30,000 potential cycles under H 2 /N 2 conditions for SSC and LSC ionomer bonded electrodes. Electrodes prepared from SSC ionomer exhibited slightly better initial fuel cell performance in both kinetic and ohmic region than the electrode prepared from LSC ionomer, likely because the higher proton conductivity of low EW SSC ionomer improves the rate of ORR and lowers resistive losses. After 30,000 potential cycles, fuel cell performance decreased in both SSC and LSC-bonded electrodes. The SSC-bonded electrode exhibited greater ECSA loss than the LSC-bonded electrode (particularly during first 1000 potential cycles) which resulted in slightly better kinetic performance for LSC. Nevertheless, the performance loss using SSC-ionomer bonded electrode was much smaller than that using LSC ionomer- bonded electrode at lower voltages than 0.80 V. Increased resistance of the aqueous-dispersed LSC-bonded electrode indicates that disintegration of electrodes occurred. Further performance durability improvement of electrode using LSC and SSC ionomers could be obtained by changing other variables such as EW or dispersion solvents, and these factors will be discussed at the meeting. Combining specific observations from this data and previous work, especially AC impedance, XRD, and microscopy measurements, some general concepts regarding the effect of potential cycling on the ionomer structure, the ionomer/catalyst interface, and the electrode/membrane interface will be further developed. Figure 1. H 2 /air polarization curves of MEAs prepared from aqueous SSC and LSC dispersion before and after 30000 potential cycling. (H 2 /N 2 potential cycling condition [5]: cycling potential: 0.6-1.0 V, rate: 50 mV/sec at 80 o C under fully humidified conditions; H 2 /air fuel cell test condition: at 80 o C under fully humidified conditions, anode/cathode pressure: 30 psig). Table 1. ECSA and fuel cell performance change of LSC and SSC PFSA bonded electrode. LSC PFSA SSC PFSA SSC PFSA* % ECSA loss after 30k 71.3 75.0 66.3 Current density at 0.9V (A/cm 2 ) Initial 0.042 0.044 0.046 After 30k 0.010 0.005 0.020 Current density at 0.4V (A/cm 2 ) initial 1.64 1.74 1.77 After 30k 1.28 1.53 1.70 * House-made dispersion Acknowledgment The authors thank Drs. C.F. Welch, Andrea Labouriau, M. Hawley, R.P. Hjelm, and E.B. Orler (Los Alamos National Laboratory) for instrumental characterization. We also thank US DOE Fuel Cell Technologies Program, Technology Development Manager Dr. Nancy Garland, for financial support. References [1] R. Borup, J. Meyers, B. Pivovar, Y. S. Kim, R. Mukundan, N. Garland et al., Chem. Review, 107 (10), 3904-3951 (2007). [2] C. M. Johnston, K. Lee, Z. Ding, T. Rockward et al. Manuscript submitted. (2010). [3] C. M. Johnston, K. Lee, T. Rockward, A. Labouriau, N. Mack, and Y. S. Kim, ECS Trans. 25 (1), 1617 (2009) [4] M. Wilson, S. Gottesfeld, J. Electrochem. Soc. 139, L28-L30 (1992). [5] U.S. DOE, Cell Component Accelerated Stress Test Protocols for PEM Fuel Cells, 2007. Initial performance Current density (A cm -2 ) 0.0 0.5 1.0 1.5 2.0 cell voltage (V) 0.0 0.2 0.4 0.6 0.8 1.0 HFR(Ohm cm 2 ) 0.0 0.1 0.2 0.3 0.4 0.5 SSC electrode (EW=830) LSC electrode (EW=1000) After 30,000 cycles Current density (A cm -2 ) 0.0 0.5 1.0 1.5 2.0 cell voltage (V) 0.0 0.2 0.4 0.6 0.8 1.0 HFR(Ohm cm 2 ) 0.0 0.1 0.2 0.3 0.4 0.5 SSC electrode (EW=830) LSC electrode (EW=1000) Abstract #736, 218th ECS Meeting, © 2010 The Electrochemical Society