Effect of the Sulfate Anions on the Oxygen Reduction Reaction Activity on Stabilized Pt Skin-PtCo Cathode Catalysts at Practical Temperatures

Abstract

Polymer electrolyte fuel cells (PEFCs) have been attracting much attention as a clean and efficient power source. However, the degradation of polymer electrolyte membranes (PEMs), such as perfluorosulfonic acid or sulfonated hydrocarbon, made the cell performance decrease via not only a decrease in the proton conductivity but also a decrease in the cathode performance by the decomposition products. In our previous work, we have investigated the effect of the specific adsorption of sulfuric acid (a typical decomposition product from PEMs) in the electrolyte solution on the ORR activity at Nafion-coated commercial Pt/C (c-Pt/C) electrode at 30 to 80 oC by the channel flow double electrode (CFDE) method.1 Very recently, we have developed a new cathode catalyst, by forming two uniform atomic layers of stabilized Pt skin on PtCo nanoparticles supported on graphitized carbon black or high-surface-area carbon black (Pt2AL–PtCo/GCB or Pt2AL–PtCo/C), with high ORR activity and high durability at high temperature.2, 3 In the present research, we have investigated the effect of sulfate anion on the ORR activity at the Pt2AL–PtCo/C electrode in 0.1 M HClO4 supporting electrolyte solution in the practical temperature range from 30 to 80 °C. The Pt2AL–PtCo/C catalyst (30 wt%-metal loading, particle size d = 3 nm) was prepared in the same manner as described previously.2, 3 The kinetically-controlled ORR activities at Nafion-coated Pt2AL–PtCo/C working electrode was evaluated from the hydrodynamic voltammograms in O2-saturated 0.1 M HClO4 + X mM H2SO4 (X = 10-3, 1, 5, and 50) solution by using the CFDE cell.1 At the collecting electrode located downstream of the working electrode, the H2O2 yield, P(H2O2), was quantified. Figure 1 shows the values of P(H2O2) at 0.76 V vs. RHE on the Nafion-coated Pt2AL–PtCo/C and c-Pt/C as a function of log [H2SO4]. The P(H2O2) values on both Pt2AL–PtCo/C and c-Pt/C increased with increasing [H2SO4]. This indicates that the specific adsorption of sulfate anion was the major factor for increasing H2O2 formation on c-Pt/C or Pt2AL–PtCo/C catalysts. However, the values of P(H2O2) at the Pt2AL–PtCo/C were markedly lower than that of c-Pt/C at all temperature. Especially, the P(H2O2) at 80 oC and [H2SO4] ≤ 50 mM on the Pt2AL–PtCo/C was nearly identical value to that in sulfate-free solution. Thus, the use of Pt2AL–PtCo/C in place of c-Pt/C provides a great advantage of suppressing the H2O2generation appreciably, resulting in a mitigation of the decomposition of the PEMs. Figure 2 shows plots of the apparent rate constant k app per active surface area of Nafion-coated Pt2AL–PtCo/C and c-Pt/C electrodes for the ORR at 0.80 V vs RHE as a function of log [H2SO4]. In sulfate-free 0.1 M HClO4 solution, the k app values at Pt2AL–PtCo/C electrode were ca. 2-3 times higher than those of c-Pt/C electrode at all temperatures examined. The k app values at [H2SO4] = 1 μM on both electrodes approximately accord with those obtained in sulfate-free solution, followed by the decrease linearly with log [H2SO4] in the temperature range between 50 and 80 °C. Such a behavior can be explained well with the Frumkin–Temkin adsorption isotherm for sulfate anions.1, 4 In the high-temperature range (≥ 70 °C), the activity loss on the Pt2AL–PtCo/C was greatly suppressed, i.e. the loss of k app value at 80 °C and [H2SO4] = 50 mM was only 7 % in contrast with the case of 50°C (50 %). This is consistently explained by a weakening of the adsorption of sulfate at high temperature on the Pt2AL–PtCo alloy catalysts, probably due to a modified electronic structure. Hence, we have proposed with certainty that a short-term high current density operation is effective to recover the performance loss suffered from the specific adsorption of sulfate anions, because both large amounts of water and heat are produced. This work was supported by the funds for “SPer-FC” projects from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References H. Yano, T. Uematsu, J. Omura, M. Watanabe, and H. Uchida, J. Electroanal. Chem., 747, 91 (2015).M. Watanabe, H. Yano, D. A. Tryk, and H. Uchida, J. Electrochem. Soc., 163, F455 (2016).M. Chiwata, H. Yano, S. Ogawa, M. Watanabe, A. Iiyama, and H. Uchida, Electrochemistry, 84, 133 (2016).J. Omura, H. Yano, D. A. Tryk, M. Watanabe, and H. Uchida, Langmuir, 30, 432 (2014). Figure 1