Hiraoka, F.

Matsuzawa, K.

Mitsushima, S.

Ota, K.-I.

Shinkoda, K.

Juelich Shared Electronic Resources

Relationship between Degradation of Pt/C and Property ofElectrolyte for PEFCsS. Mitsushima, K. Shinkoda, F. Hiraoka, K. Matsuzawa, K.-I. OtaThis document appeared inDetlef Stolten, Thomas Grube (Eds.):18th World Hydrogen Energy Conference 2010 - WHEC 2010Parallel Sessions Book 1: Fuel Cell Basics / Fuel InfrastructuresProceedings of the WHEC, May 16.-21. 2010, EssenSchriften des Forschungszentrums Jülich / Energy & Environment, Vol. 78-1Institute of Energy Research - Fuel Cells (IEF-3)Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag, 2010ISBN: 978-3-89336-651-4Relationship between Degradation of Pt/C and Property of Electrolyte for PEFCs Shigenori Mitsushima, Kazuki Shinkoda, Fumiya Hiraoka, Koichi Matsuzawa, Ken-ichiro Ota, Yokohama National University(YNU), Japan 1 Introduction Polymer electrolyte fuel cells (PEFCs) have started commercialization for small size co-generation systems and fuel cell vehicles because of the high power density, capability of start up from ambient temperature and high energy conversion efficiency. In the present technology, platinum electrocatalyst is essential. However toward real commercialization, precious metal loading should be reduced by one tenth or more because of the limit of natural resources. To reduce precious metal loading with high catalytic activity, higher dispersion technology, alloying and core shell technology have been developed [1-3]. These materials were also required high stability during the system lifetime. Generally, nano-particle is unstable compared to larger particles, and rated load enhances the degradation of electrocatalyst. There are many reports for the degradation of electrocatalyst for PEFCs. For example, the decrease of electrochemical surface area under potential cycling with small size single cells or three – electrode conventional electrochemical cells, platinum consumption under potential cycling, and dissolution of platinum in acidic medium have been reported. The Ostwald ripening and the sintering model have been proposed as degradation mechanism. However, the degradation mechanism has not been clarified, yet [1, 2, 4-6]. In this study, the electrochemical surface area decrease with various pH electrolytes under various potential patterns has been investigated as a fundamental study to clarify the degradation mechanism and improvement of the durability for PEFCs. 2 Experimental Pt/C (Tanaka K. K., TEC10E50E) water dispersion was painted on a glassy carbon disk (Tokai carbon, GC-20SS) which was polished to mirror surface. Reference and counter electrodes were reversible hydrogen electrode (RHE) and platinum wire, respectively. Electrolyte was in the concentration range from 0.1 to 1 M (=mol dm-3) of sulfuric acid (Junsei, Analysis grade) or percholoric acid (Merck, Ultrapur). The potential cycle was applied between 0 to 1.2 V vs. RHE. As shown in Fig. 1, 4 types of potential cycling patterns which were triangle waves with 50 mV s-1 of sweep rate, rectangular waves with 12 s interval, and the combinations of potential step and sweep were applied for 5000-cycle in nitrogen or oxygen atmosphere. The electrochemical surface area was frequently determined by hydrogen desorption wave of the cyclic voltammogram between 0.05 to 1.2 V vs. RHE during potential cycling under nitrogen atmosphere. After potential cycling, the Pt/C was observed by TEM. The particle diameter was determined from a projection area of a particle to the diameter of a circle. The number of the particle was more than 300. Proceedings WHEC2010 191.2 V0 V12sa) Triangular b) Rectangular50 mV s-11.2 V0 V12sc) Step to sweep d) Sweep to step  Figure 1: Potential cycling patterns. 3 Results and Discussion During potential cycling, the shapes of cyclic voltammograms showed normal poly-crystal with small peaks of carbon black support for quinone / hydroquinone, and hydrogen adsorption / desorption and surface oxidation / reduction peaks decreased with potential cycling for all potential patterns. Therefore, the platinum surface did not change to a specific face, and electrochemical surface area was determined by hydrogen desorption charge with 210 μC cm-2. Figure 2 shows the TEM images Pt/C before and after 5000 triangular potential cycles in various concentration of H2SO4 at 25oC. The platinum particle of the initial Pt/C was uniform diameter, and the particle grew during potential cycling. The particle seems lager diameter and distribute for higher concentration. Figure 3 shows the platinum particle diameter distribution in various concentration of H2SO4 under nitrogen atmosphere. The particle size distribution was log-normal distribution, and the medium diameters were 2.4, 4.7, 4.7, and 5.6 nm before and after potential cycling in 0.1, 0.5, and 1M, respectively. Figure 4 shows the medium diameter of platinum particle as a function of pH. The diameter decreased with the increase of pH for all conditions. Figure 4 a) is for triangular pattern in H2SO4 and HClO4 under N2 or O2. The difference between N2 and O2 atmosphere was not significant. The particle diameter in H2SO4 was larger than that in HClO4 at the same pH, and this behaviour was same to the dissolution of Pt in acidic media. Therefore, acidity and solubility would be one of important factor in particle growth process. Figure 4 b) is for rectangular and triangular potential cycling pattern in H2SO4 under N2. The particle diameter of rectangular was ca. 20 % larger than that of triangular, therefore dissolution kinetics would be taken into account for the analysis of the particle growth. 20 Proceedings WHEC2010Before 0.1 M1 M25 nm0.5 M25 nm25 nm25 nm0204060801000 0.5 1.0 1.5Log (d / nm)Frequency / -Before0.1 M0.5 M1 MFigure 2: TEM images of Pt/C before and after 5000 triangular potential cycles in various concentration of H2SO4 at 25°C. Figure 3: Platinum particle diameter distributions before and after 5000 triangular potential cycles in various concentration of H2SO4 at 25°C. pH / -d/ nma)H2SO4 in N2H2SO4 in O2HClO4 in N2As received 2.4 nm23456780 0.5 1.0 1.5 0 0.5 1.0 1.5pH / -d/ nmb)TriangularRectangularAs received 2.4 nm2345678 a) Triangular wave in H2SO4 and HClO4 under O2 or N2. b) Rectangular and triangular potential pattern in H2SO4 under N2. Figure 4: Medium platinum particle diameter as a function of pH after 5000 potential cycles. Figure 5 shows the logarithm of normalized electrochemical surface area as a function of the cycling number with the triangular potential pattern in H2SO4 under nitrogen atmosphere. The decreases of the surface area were significantly larger for higher concentration of sulfuric acid. Up to 2000 cycles, the surface area decreased with linear relation in this figure, and after 2000 cycles, the surface area decrease rate decreased, especially lower concentration. This behaviour was the same to all other conditions in this study. Here, we analyzed up to 2000th cycle. Proceedings WHEC2010 210.11.00 2000 4000 6000Normalized ECA /  ]Cycle number / -0.50.1 M0.5 M1 M0.2 M Figure 5: Electrochemical surface area as a function of potential cycling number with triangular potential cycles in various concentration of H2SO4 at 25°C -4-30 0.5 1.0 1.5pH / -RectangularTriangularStep to sweepSweep to stepLog[ k / -]0 1.5Log[ k / -]H2SO4 in O2H2SO4 in N2HClO4 in O2 HClO4 in N2pH / -0.5 1.0-5-4-3-2a) Triangular wave in H2SO4 and HClO4 under O2 or N2. b) Various potential pattern in H2SO4 under N2. Figure 6: Rate constant of the electrochemical surface area decrease as a function of pH. If the electrochemical surface area decrease rate is proportional to the electrochemical surface area: S, the following equation is obtained. δS/δn = - k S (1) The n and k were the cycling number and the rate constant of the surface area decrease, respectively. The rate constants were determined by the slope of Figure 5. Figure 6 shows the rate constant as a function of pH. The rate constants decreased with the increase of pH, which were expected from the particle diameter. Figure 6 a) is the rate for triangular pattern in H2SO4 and HClO4 under N2 or O2. The difference between N2 and O2 was not significant, 22 Proceedings WHEC2010and the rate constant in H2SO4 was larger than that in HClO4, which would correspond to the particle growth. Figure 6 b) is the rate for various potential cycling pattern in H2SO4 under N2. In the 4 potential cycling patterns, the combination with the sweep oxidation and the step reduction showed the smallest rate constant in all pH. The combination with the step oxidation and the sweep reduction showed the largest rate constant in lower pH region. The dependence of the rate of the triangular pattern on pH was larger than others. Therefore, the kinetics of the simultaneously reactions of oxidation and reduction would affect to these behaviours. The dissolution reaction follows the acidic dissolution in acid solution under oxidizing atmosphere [5]. PtO2 + H2O + H+ = Pt(OH)3+ (2) However, the rate determining reaction under potential cycling would not same to the equilibrium. At least, there are 2 kinds of dissolution reactions which are anodic and cathodic processes [4]. Here, we consider the simplest simultaneously reactions of dissolution and protection during anodic and cathodic processes. The reactions and the reaction rate equation might be as follows: (Anodic) Pt → Pt4+ + 4e- vad = kad[Pt4+]-1exp [{4F(Ε−Εο3)}/(RT)] (3) Pt + 2H2O→ PtO2 + 4H+ + 4e- vo = kod[H+]-4exp[{4F(Ε−Εο4)}/(RT)] (4) (Cathodic) PtO2 + 4H+ + 2e- → Pt2+ + 2H2O vcd = kcd[H+]4[Pt2+]-1exp[{4F(Ε−Εο5)}/(RT)] (5) PtO2 + 4H+ + 4e- → Pt + 2H2O vr = kr[H+]4exp[{4F(Ε−Εο6)}/(RT)] (6) Here, v, E, and Eoi were the reaction rate, the potential, and the equilibrium potential for each reaction, respectively.  The dissolution reactions are equations (3) and (5), and the protection reactions are equations (4) and (6). The protection reaction rates would be lower than that of dissolution reaction, because the protection reactions were more complex than the dissolution reactions. At that time, the step oxidation would enhance the dissolution, because the dissolution amount before the oxidation layer formation would be larger than that of sweep oxidation, and the sweep reduction would enhance the dissolution, because the oxide which was the reactant of the dissolution would be maintained longer time than step reduction during reduction process. Proceedings WHEC2010 234 Conclusion In order to clarify degradation mechanism of Pt/C, the electrochemical surface area under potential cycling has been investigated in various conditions. The electrochemical surface area decrease seems to be proportional to the surface area up to 2000 potential cycles, and the rate constant of electrochemical surface area decrease increased with the decrease of pH. The rate constant in H2SO4 was larger than that in HClO4. These behaviours were same to the solubility of platinum bulk metal in acidic medium. The rate constant was affected by potential cycling pattern. The rate constant of the combination of sweep oxidation and step reduction was smaller than that of rectangular, triangular, and the combination of step oxidation and sweep reduction. These behaviours could be explained by the kinetics of simultaneously reactions of anodic dissolution and oxide formation, and cathodic dissolution and surface reduction. Acknowledgement This study was supported by the project for “Research and Development of Polymer Electrolyte Fuel Cell” from the New Energy and Industrial Technology Development Organization (NEDO). References [1] P. J. Ferreira, G. J. La O', Y. Shao-Horn, D. Morgan, R. Makharia, S. Kocha and H. A. Gasteiger, J. Electrochem. Soc., 152, A2256 (2005). [2] S. Chen, H. A. Gasteiger, K. Hayakawa, T.  Tada and Y. Shao-Horn, J. Electrochem. Soc., 157, A82 (2010). [3] S. Koh, N. Hahn, C. Yu and P. Strasser, J. Electrochem. Soc., 155, B1281 (2008). [4] S. Mitsushima, S. Kawahara, K. Ota and N. Kamiya, J. Electrochem. Soc., 154, B153 (2007). [5] S. Mitsushima, Y. Koizumi, S. Uzuka and K. Ota, Electrochim. Acta, 54, 455 (2008). [6] S. Kawahara, S. Mitsushima, K. Ota and N. Kamiya, ECS Trans., 3(1), 625 (2006). 24 Proceedings WHEC2010

Relationship between Degradation of Pt/C and Property of Electrolyte for PEFCs

http://juser.fz-juelich.de/record/135309/files/FC1_5_Mitsushima_rev0423.pdf

Relationship between Degradation of Pt/C and Property of Electrolyte for PEFCs

Abstract

Similar works

Full text

Available Versions

Juelich Shared Electronic Resources