Significant reduced use of platinum catalyst is essential for full-scale implementation of polymer electrolyte fuel cells (PEFCs). Toshiba has developed an alternating catalyst layer structure (ACLS) catalytic electrode fabricated by a sputtering process and is studying its application in PEFCs.1 The ACLS cathode has been modified and previous studies have reported good power generation characteristics and high durability at a platinum loading of 0.15 mg cm-2 using a modified ACLS (M-ACLS).2 The power generation characteristics of PEFCs strongly depends on oxygen transport in the catalyst layer in addition to the catalytic activity. Because the catalyst layer structure of the M-ACLS electrode is characterized by nanosheet platinum, which differs from commonly used nanoparticulate Pt/C, it is necessary to understand the oxygen transport capability in order to reduce the platinum loading. In recent years, measurement of oxygen transport resistance by the limiting current method using a PEFC single cell is almost established.3,4 In this presentation, we report the result of investigating the oxygen transport resistance of the M-ACLS electrode using the limiting current method
A catalytic electrode precursor containing platinum and pore-forming material was deposited on a commercially available gas diffusion layer substrate by using a sputtering device, and then the pore-forming material was removed to prepare the M-ACLS anode and the M-ACLS cathode. The electrode structure was observed by SEM. Various properties such as the specific surface area of the platinum catalyst, the roughness factor (the ratio of the platinum surface area to the electrode area), and power generation characteristics were evaluated with reference to the Fuel Cell Commercialization Conference of Japan protocol. Furthermore, the limiting current was obtained in temperature range of 50 °C to 80 °C using two different kinds of diluted oxygen gas (oxygen dry mole fraction 0.01, dilution gas N2 or He). Transport resistance was separated by utilizing the difference in oxygen diffusibility between the two kinds of dilution gasses or pressure dependency.3,4
３．Results and discussion
Figure 1 shows an example of a cross-sectional SEM photograph of a M-ACLS catalytic electrode laminated with a porous platinum sheet, which is found to have a high porosity without carrier. Figure 2 shows a typical I-V curve using the limiting current method. It can be seen that the value of the limiting current density changes, reflecting the difference in oxygen diffusibility in the two kinds of dilution gases. In this presentation, we report the oxygen transport resistance analysis results of M-ACLS cathode, oxygen transport resistance per unit platinum surface area, and superiority of M-ACLS catalytic electrode to over nanoparticulate Pt/C.
(1) W. Mei, T. Fukazawa, Y. Nakano, Y. Akasaka, K. Naito, ECS Trans., 50(2), 1377 (2012)
(2) W. Mei, T. Fukazawa, T. Yang, N. Yoshinaga, Y. Kanai, ECS Trans., 69(17), 755 (2015)
(3) T. Mashio, A. Ohma, S. Yamamoto and K. Shinohara, ECS Trans., 11(1), 529 (2007)