Measurement of Oxygen Gas Transport Resistance in Cathode Catalyst Layers of PEFC

Introduction

  Polymer electrolyte fuel cells (PEFCs) are a promising power sources for automotive use. For the commercialization, cost reduction is one of the most important issues. In order to reduce cost, the Pt-loading of a membrane electrode assembly (MAE) should be reduced. Many studies for this purpose have been done. As a part of such studies, in-situ analytical methods for evaluating oxygen transport properties have been developed using limiting current measurements¹⁾²⁾. It is said that oxygen gas transport resistance in catalyst layers CLs (R_CL) is mainly consisted of two parts, diffusion resistance in micro pore (R_pore) and diffusion resistance around Pt particles (R_Pt).  And it has been found   that the diffusion resistance R_CL was significantly increased when Pt loading in CLs was reduced²⁾. In this study, the oxygen diffusion resistances in CLs of two types of Pt/carbon catalysts with different kind of carbon support are measured by changing the ionomer carbon ratio (I/C) and the relation between I/C and R_Pt, R_porewas investigated.

 Experimental

Table 1 shows the specifications of the MEAs used in this study. The MEA samples with an active area of 1cm² were fabricated by coating CLs consisting of catalysis powders (Pt/C) and Nafion^R  ionomer onto the perfluorosulfonated polymer membrane (Nafion^R NR212). Pt-loadings of the samples for the working electrode were 0.05 -0.6 mg cm^-2 respectively. In order to evaluate the oxygen gas transport resistance in the CLs, limiting currents were measured in nitrogen balance gases. RCL was measured by the method of Mashio et al¹⁾. R_pore and R_Pt were determined by the method we described before³⁾.

 Results and discussions

    In order to estimate R_pore and R_Pt,  an Equation  (1) was introduced with the analogy of porous electrode model and value was determined from R_CL with different I/C.    A imaginary limiting current inversely proportional to R_CL was also introduced (eq.(2)).   In Fig.1, each dot shows measured R_CL, full line shows the calculation curves of R_CL with eq.(1) and dotted line shows imaginary limiting  current calculated by the eq.(2) with using the obtained values of R_pt and R_pore.  Table2 shows R_pt and R_porevalues obtained.

     R_pore increased very much at I/C was 1.3 for both catalysts.This seems to be cause of the closure or isoration of pore by ionomer.R_Pt shows minimum values at some I/C which depend on catalyst types. From this result, R_Pt seems to be affected with the state of three-phase interface⁴⁾and oxygen gas permeability through ionomer.

Acknowledgement

   This research was performed under a grant from the Cell Evaluation Project of the New Energy and Industrial Technology Development Organization (NEDO).

References

1. T.Mashio, A.Ohma, S.Yamamoto and K.Shinohara, ECS Transactions, 11(1) 529 (2007).
2. K.Sakai, K.Sato, T.Mashio, A.Ohma, K.Yamaguchi and K.Shinohara, ECS Transactions, 25(1) 1193 (2009).
3. H.Yasuda, K.Kobayashi, A.Daimaru and M.Hori., ECS Transactions, 50(2) 261 (2012)
4. M. Lee, M. Uchida, H. Yano, D.A. Tryk, H. Uchida and M.Watanabe, Elecrochim. Acta  55 8504 (2010)

 Table 1   MEA specification

	Counter Electrode	Working Electrode
Catalyst	Pt/C (CB)   (TEC10E50E, TKK) Pt/C (GCB)  (TEC10EA50E, TKK)
Ionomer	NafionR (D2020, Dupont)
Pt loading /mgcm-2	0.05 – 0.6
Ionomer Carbon Ratio (I/C)	0.4 – 1.3
Active Area /cm2	1.0
Membrane	NafionR (NR212, Dupont)
GDL	TGP H060,  Toray (without MPL)

 Table 2    R_Pt and R_pore with different I/C

I/C	TEC10E50E		TEC10EA50E
	RPt	Rpore	RPt	Rpore
0.4	4.3	19	4.3	10
0.5	-	-	2.8	29
0.7	3.5	13	4.0	12
1.0	2.8	46	4.8	7.8
1.3	3.5	88	5.2	69