Effect of Electrospray-Generated Ionomer Morphology on Polymer Electrolyte Fuel Cell Performance

Abstract

Oxygen transport resistance in the catalyst layer is a dominant factor in limiting polymer electrolyte fuel cell (PEFC) performance, regardless of current density. The oxygen transport resistance is made up of three components. Two small pore diffusion resistances originate from the microporous layer (molecular diffusion resistance: RMD) and the catalyst layer (Knudsen diffusion resistance: RKnud). The third resistance originates from ionomer film diffusion on the Pt surface (RFilm). The RFilm significantly influences the catalyst activity in overall PEFC performance.[1] In order to decrease RFilm, various studies have been conducted, for example, by introducing a high oxygen permeability ionomer[2] and modifying the Pt surface.[3] In the present study, without modification of either the ionomer or Pt surface, RFilm was successfully mitigated via electrospray (ES) catalyst layer preparation. We prepared the catalyst layer according to two different ionization methods (positive (ESP), and negative (ESN) ionization) and ionomer/carbon ratios (I/C 0.3 and 0.7). The ionomer morphology revealed differences according to the ionization method, as analyzed by scanning transmission electron microscopy (STEM). Spherical and dendritic ionomer morphologies were observed resulting from the ESP and the ESN, respectively. With ESP, it is noteworthy that separate ionomer fragments were observed that were not well integrated with the catalyst. In order to quantitatively measure the RFilm, the limiting current test can be used. As a result, the separate ionomer fragments increased the thickness of the ionomer film on the surface of a catalyst, and the RMD values were approximately 15 s/m, regardless of I/C or catalyst layer preparation method. Consequently, the catalyst layer prepared by ESN exhibited the lowest RFilm value compared to that for the conventional pulse-swirl-spray spray (PSS) method and ESP (I/C 0.3 PSS: 61, ESP: 94, ESN: 40 s/m, and I/C 0.7 PSS: 68, ESP: 66, ESN: 46 s/m). Moreover, the difference in the ionomer morphology influenced not only the mass activity in the low current density region but also the specific power in the high current density region. The catalyst layer prepared by ESN exhibited better PEFC overall performance, regardless of I/C (I/C 0.7 mass activity under O2 conditions, at 0.9 V PSS: 22, ESP: 24, ESN: 46 A/gPt, specific power at 1 A/cm2 under O2 conditions, PSS: 4.7, ESP: 4.7, ESN: 5.6 W/mgPt). Therefore, it was confirmed that the ionomer thinly deposited on the surface of the catalyst by ESN decreased RFilm, by a factor of 1.5, and, as a result, improved the overall PEFC performance by the same factor.