Effect of Operating Conditions on the Acidity of H 2 /Air PEM Fuel Cells’ Water A. M. Abdullah a, b , T. Okajima b , F. Kitamura b , and T. Ohsaka b a Department of Chemistry, Faculty of Science, University of Cairo, Giza 12613, Egypt b Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 – G1-5 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Abstract The pH of the water drained out from the cathode and the anode sides was measured using a flow pH meter connected to the exits of the anode and cathode compartments and was found to depend mainly on the cell’s operating conditions and the MEA’s aging condition. Aged MEA operating at low temperature – low RH condition led to a significant increase in the acidity of the water drained out from the cathode side (pH 1.8). On the other hand, the water drained out from the anode side has a pH value of 4.8 on average. Reactions that might affect the pH of the water drained out from the PEM fuel cell were studied. Membrane degradation via hydroxyl radicals ( · OH) was found to be the most influencing reaction that lowers the pH of water within PEM fuel cells. Introduction Durability is one of the main challenges for commercializing PEM fuel cells. The decrease of the PEM fuel cells’ performance with time is attributed either to the degradation of one or more of the cell components (1-8) and/or to the decrease of the catalytic activity (9, 10). The local environmental factors within a fuel cell e.g. temperature, humidity, pH, water distribution, membrane resistance have direct effects on the kinetics of the electrode reactions, since under the operating conditions it is reported that the distribution of these parameters is non-uniform (11-15). In our previous work, the local temperature distribution within a segmented fuel cell was discussed (16). It is worthy to mention that enough attention is not being paid to study the pH change of the water drained out from the fuel cell. In fact, measuring the pH might give an indication about the performance of the fuel cell under operation and help in estimating the life time of the MEA. In the absence of sulfur and nitrogen gaseous pollutants, a decrease in the pH can be expected as a result of the following reactions 2Rp – CF 2 COOH + 4 · OH → Rp – CF 2 OH + Rp – COOH + 2CO 2 + H 2 O + 2HF [1] CO 2 + H 2 O → H + + HCO 3 - [2] Equation 1 shows the membrane’s polymer degradation by the reaction between the Nafion membrane and the · OH radicals. The latter species are formed as a result of the decomposition of hydrogen peroxide molecules through their reaction with the Pt metal ions dissolved in the membrane (17, 18). Equation 2 is the dissolution of CO 2 gas in water. ECS Transactions, 16 (2) 543-550 (2008) 10.1149/1.2981889 ©The Electrochemical Society 543 Downloaded 31 Mar 2010 to 41.235.143.130. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp