Enhanced CO Tolerance of PEFC Anode Catalyst by Optimized Degree of Activation of Resorcinol-Formaldehyde Carbon Support

1. Introduction

Carbon monoxide contaminations in hydrogen reforming from hydrocarbon can cause a serious problem for PEFC performance [1]. Even a commercial platinum–ruthenium alloy catalyst which well-know as the highest CO tolerance cannot accept H₂ with CO contamination higher than 300 ppm. The highly dispersed and high-alloy degree Pt₂Ru₃/C was found to be much higher CO tolerance than that of commercial Pt₂Ru₃/C [2]; however, effect of carbon support was still unclear. Resorcinol-formaldehyde Carbon gel is attracting more attention for its high electrical conductivity, high surface area and mesoporosity; moreover, pore characters are adjustable by vary of synthesis condition [3]. In this work, effect of porosity of carbon support on CO tolerance was investigated.

2. Experiment

Resorcinol-formaldehyde carbon was prepared at a fixed R/C ratio of 1400, and degrees of activation were 0%(no activation), 42% and 58%, Rc1400 ac0, ac37 and ac58, respectively. Pt₂Ru₃ was deposited on RC1400 by rapid quenching method (30wt%Pt-25wt%Ru/C) [2]. The catalysts were characterized by XRD, BET surface area, STEM. The anodes were tested in 5 cm² membrane electrode assemblies (MEA). The CO tolerance experiments were performed at constant current density mode (0.2 A/cm²) First, pure H₂ flow for one hour and then introduce 100 ppm of CO in H₂for 2 hours, followed by 500 ppm, 1000 ppm, and 2000 ppm in every 2 hours.

3. Results

Rc1400 ac42 and ac58 have meso-micro pore structure with bimodal pore size distribution. In Table 1 , it showed that an increase in degrees of activation from ac0 to ac42, only mesopore increases with maintaining regular mesopore structure of Rc1000, and not much micropore increased (compare with later result) resulting in increase in BET surface area. But form ac42 to ac 58, degree of activation increased with micropore volume from 0.471 to 1.576 cm³g^-1. Crystallite size of metal particle also depend on density of micropore, Pt₂Ru₃/RC1400ac0 showed biggest crystallite size at 4.2 nm. Following by commercial Pt₂Ru₃/C but other two catalysts was 2.3-2.5 nm. It can be seeing that surface area have direct relation to crystallite size but after some point there are not much effect. As in case of Pt₂Ru₃/RC1400ac42 and Pt₂Ru₃/RC1400ac58, even surface area was very different (1215 m²g^-1) but the crystallite size was not much diffent (0.2 nm) 

As shown in Fig. 1, cell voltage at 0.2 A/cm² drops from 0.76 to 0.59 V (24.4%) for Pt₂Ru₃/Rc1400ac58 while the voltage drops from 0.75 to 0.4 (46.7%) for commercial Pt₂Ru₃/C. Pt₂Ru₃/RC1400ac37 catalysts have superior CO tolerance to commercial Pt₂Ru₃/C. Pt₂Ru₃/Rc1000ac42 and Pt₂Ru₃/Rc1000ac0 has lower efficiency and CO tolerance than commercial Pt₂Ru₃/C. These results indicate that small Pt₂Ru₃particles and higher density of the mesopores is important for CO tolerance. In the other hand, surface area and micropore density does not much influent CO tolerance of PEFC anode catalyst.

These studies could lead to significant knowledge of metal support interaction, and Resorcinol-Formaldehyde carbon gel can be a promising support for future development of CO tolerance catalyst for PEFC.  

 Table 1 Physical properties of Pt₂Ru₃/Rc1400 catalysts compare with commercial Pt₂Ru₃/C catalyst

Pt2Ru3/RC 1400	Surface area of support (m2g-1)	Micro- pore Volume (CM3g-1)	Mesopore Volume (CM3g-1)	Crystallite size by XRD  (nm)
Ac 0 Ac 42 Ac 58	619 1933 2748	0.231 0.471 1.576	0.175 0.467 0.467	4.2 2.5 2.3
Commercial Catalyst	827	-	-	3.4

 

Figure 1

 

Figure 1 Effect of CO concentration on cell voltage at 0.2 A/cm². Cell temp.: 70^oC; Electrolyte: Nafion® NRE 212; Cathode: Pt/C (0.5 mg/cm² ); O₂ humidified at 70^oC; Flow rate: 80 mL/min; Anode: Pt₂Ru₃/C (0.5 mg-PtRu/cm² ); H₂ containing 0-2000 ppm CO humidified at 70^oC; Flow rate: 80 mL/min.

References

1.   M. Watanabe, S. Motoo, J. Electroanal. Chem., 60 (1975), p. 275-283

2.   T. Takeguchi, T. Yamanaka, K. Asakura, E. N. Muhamad, K. Uosaki and W. Ueda, J. Am. Chem. Soc. 134, (2012),14508−14512

3.   K. Kraiwattanawong, H. Tamon, P. Praserthdam, Microporous Mesoporous Mater, 138 (2011), p. 8–16

 

Acknowledgments

This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) Japan.