Measurements of Temperature and Relative Humidity Dependencies of Oxygen Reduction Reaction Rate in PEFC

Abstract

IntroductionFor widespread use of polymer electrolyte fuel cell (PEFC), it is necessary to reduce the amount of platinum by optimizing the cathode catalyst layer (CCL) structure. To establish the model of the CCL,[1] the intrinsic oxygen reduction reaction (ORR) rate which does not include mass transfer resistance in the through-plane direction of the CCL should be formulated. Neglecting the backward reaction, the ORR rate per unit mass of Pt, r mc, is proportional to oxygen partial pressure, p O [Pa], as follows: r mc = k mc p O [mol/(kg·s)][1] (1) k mc = k mc0 + exp(–E/RT – E c/b c) (2)where k mc is the reaction rate constant per unit mass of Pt [mol/(Pa·kg·s)], E c is the cathode electromotive force (emf) [V], E is the activation energy [J/mol], T is temperature [K] and b c is the Tafel slope [V].The objective of this study is to develop a measurement method of temperature and relative humidity dependencies of the intrinsic ORR rate constant using platinum-sputtered catalyst.ExperimentalA Japan Automobile Research Institute (JARI) standard cell whose active area was 50 mm × 50 mm was used in experiments. The membrane-anode assembly (MAA, Eiwa Corp.) was composed of a proton exchange membrane (PEM, DuPont Nafion® NR-212), an 8.3‒9.8 μm thick anode which was made of Pt/Vulcan® XC-72 catalyst (Pt/C weight ratio: 1) with a Nafion® ionomer (ionomer/carbon weight ratio: 0.74). Pt thin-layer cathode (ρs = 0.054 mg/cm2) was prepared by sputtering (AOV, SPAD-4240UM, Ar atmosphere, 5 Pa, 100 ℃, for 50 minutes) on the microporous layer (MPL) on a carbon paper (SIGRACET-GDL 29 BC, 235‒238 μm thick) to eliminate mass transfer resistance in the through-plane direction.[2] The MAA and Pt thin-layer cathode were stacked as shown in Fig. 1. A carbon current collector had a single serpentine flow channel, the width and depth of which were 1.0 mm and the width of the ribs between the channels was 1.0 mm. H2 and O2 were humidified at 75 oC. H2 and O2 flow rates were 600 and 300 cm3/min, respectively. The cell was operated at 80 oC. The pressure at the cell outlet was 1 atm.Results and DiscussionFig. 2 shows the polarization curves at different temperatures. The ORR rate constant per geometric area k gc (= ρs k mc) on the sputtered catalyst calculated from Fig. 2 is plotted with the ORR rate constant on the supported catalyst in Fig. 3. As shown in Fig. 2, current density at a fixed E cm , cathode emf vs. RHE at PEM–catalyst boundary, increased with raising temperature. The observed activation energy was 45 kJ/mol in case of the sputtered catalyst while the observed activation energy was 9–26 kJ/mol in case of the supported catalyst. This is because temperature dependency of mass transfer resistance is weaker than that of ORR, so temperature dependency of current density is not remarkable in case of supported catalyst, which has mass transfer resistance in the through-plane direction in CCL. In fact, the apparent activation energy on Pt-supported catalyst of 20–29 kJ/mol was reported.[3][4] Fig. 3 also shows that the observed activation energy on Pt-sputtered catalyst is almost independent of E cm . That indicates the effects of temperature and E cm on ORR rate constant was appropriately separated by using Pt-sputtered catalyst.Fig. 4 shows the experimental results at different RH. The current density at a fixed E cm increased by raising RH although the ORR rate constant should not depend on relative humidity. Since Pt-sputtered catalyst whose CCL thickness can be regarded as 0 does not have proton transfer resistance, this result means there are other factors affected by relative humidity. For example, as shown in Fig. 5, ECSA in working cells depends on relative humidity. It was also reported in literature.[5] In addition, RH dependency of current per ECSA in Fig. 6 was not remarkable. In fact, the current density at E cm = 0.5 V is 1.66 times higher at RH = 0.81 than 0.53 while the ratio is 1.22 in case of the current per ECSA.ConclusionsTemperature and relative humidity dependencies of ORR rate constant, k gc, were measured using platinum-sputtered catalyst. Experimental results at different temperatures showed k gc on the sputtered catalyst did not include through-plane mass transfer resistance and the effects of temperature and cathode emf on k gc was appropriately separated. Also, experimental results at different RH showed current density at a fixed E cm increased by raising RH. RH dependency of ORR rate was about 70 % reduced by eliminating the effect of ECSA. The RH dependency of ORR rate is mostly attributed to ECSA. Figure 1