Carbon-Free Connected Platinum–Iron Catalysts with Enhanced Chemically Ordered Structures As Durable Oxygen Reduction Electrocatalysts for PEFCs

Abstract

To realize the widespread uses of polymer electrolyte fuel cells (PEFCs), the improvement of the activity and durability of Pt-based electrocatalysts for oxygen reduction reaction (ORR) is essential. In our group, a carbon-free connected Pt–Fe catalyst with a porous hollow capsule structure has been developed. [1–3] This catalyst consists of a nanosized beaded network by the connection of Pt–Fe nanoparticles. The beaded metal network is electrically conductive, enabling the removal of carbon supports form a catalyst layer. The carbon-free cathode catalyst layer using a connected Pt–Fe catalyst exhibits high durability against start/stop cycles, because of the elimination of carbon corrosion problems. In addition, the ORR specific activity of a connected Pt–Fe catalyst is about 9 times higher than that of a commercial Pt nanoparticle catalyst supported on carbon black (Pt/C). Thus, a connected Pt–Fe catalyst provides a high ORR activity and excellent start/stop durability. On the other hand, the dissolution of catalyst metals during a load cycle operation is one of critical issues for Pt-based ORR catalysts. Our previous studies using Pt-alloy nanoparticle catalysts on carbon black demonstrate that chemical ordered (superlattice) structures improve load cycle durability due to the suppression of metal dissolution. [4–8] Therefore, as shown in Figure 1A, this study developed a carbon-free connected Pt–Fe catalyst with a high chemical ordering structure to achieve high durability against both start/stop and load cycle operations.In this study, we proposed a new synthesis method using the combination of a SiO2 coating and an annealing treatment (Figure 1B) to control chemically-ordered (face centered tetragonal: fct) structures in a connected Pt–Fe catalyst. The SiO2 layer coated on catalyst surfaces prevents the detachment of Pt–Fe nanoparticles and the large catalyst agglomeration and coalescence during the high-temperature annealing. Using this method, the connected Pt–Fe catalyst having a nano-sized network and a high fct degree (70–80%) was successfully prepared. This catalyst exhibited ca. ten times higher ORR specific activity, compared with that of the Pt/C. Furthermore, as shown in Fig. 1C, the connected Pt–Fe catalyst with the high fct degree (76%) showed the higher retention of the ORR specific activity after 10000 load cycles (0.6 V for 3 s and 1.0 V for 3 s in N2-saturated 0.1 M HClO4 solution at 60 °C), compared to the catalyst with the low fct degree (47%). The results of the STEM-EDX line mapping indicated that a highly ordered fct structure greatly contributes to the suppression of metal dissolution from the catalyst.This study demonstrated, for the first time, the synthesis method as shown in Figure 1B to prepare a connected Pt–Fe catalyst with a nanosized network and a high chemical ordering structure. The carbon-free connected Pt–Fe catalyst with an enhanced chemically ordered structure exhibited a high ORR activity as well as improved durability against both start/stop and load cycle operations. The results obtained in this study provide useful guidelines for the design of high-performance ORR catalysts for PEFCs. Acknowledgements This work was financially supported by Kanagawa Institute of Industrial Science and Technology (KISTEC) and Core Research for Evolutionary Science and Technology at the Japan Science and Technology Agency (JST-CREST, JPMJCR1543).