(4fl) Carbon-Free Connected Pt–Co Nanoparticle Catalysts with Chemically Ordered Structures for Enhancing Oxygen Reduction Reaction Activity in Polymer Electrolyte Fuel Cells

The necessity of renewable energy sources is driven by the urgent need to mitigate climate change and reduce our dependence on fossil fuels. Transitioning to a hydrogen society is critical for expanding the use of renewable energy sources and achieving a sustainable energy future. In this context, fuel cells play a vital role due to their ability to convert chemical energy directly into electrical energy with high efficiency and minimal emissions. Among various types of fuel cells, polymer electrolyte fuel cells (PEFCs) are particularly promising as a next-generation energy-conversion technology owing to their high energy efficiency and wider operating temperature range. However, a sluggish oxygen reduction reaction (ORR) rate and the low durability of conventional catalysts in PEFCs limit the wider commercialization. A significant challenge faced by conventional catalysts of carbon-supported Pt-based nanoparticles is carbon corrosion, which occurs during the start-stop cycles and under high potential conditions, leading to the degradation of the catalyst and a reduction in overall fuel cell performance.

To address these issues, our group has developed a carbon-free, connected Pt₅₀–Fe₅₀ nanoparticle catalyst with a porous hollow capsule structure. This catalyst comprised the nano-sized network formed by the connection of Pt₁–Fe₁ nanoparticles. The connected Pt₅₀–Fe₅₀ catalyst exhibits an enhanced ORR specific activity (9 times higher than a commercial Pt nanoparticle on carbon black (Pt/C)) as well as excellent durability against start-stop operations, due to the carbon-free structure.^[1,2] On the other hand, load cycle operation refers to the repeated cycling of potential, simulating real fuel cell conditions and accelerating the catalyst's dissolution. During the load cycle operation, iron is dissolved from the connected Pt₅₀–Fe₅₀ catalyst, leading to the loss of ORR activity and the degradation of polymer electrolytes causing by the Fenton reaction.

In this work, a carbon-free and iron-free, connected Pt_x-Co_100-x nanoparticle catalysts with chemically ordered structures has been developed, as shown in Fig. 1(a). Chemically ordered (superlattice) structures are crystal structures with an ordered arrangement of atoms and are thermodynamically stable. The metal compositions and chemically ordered degrees in the catalysts are controlled to discuss the structural effects on ORR activity and durability of the catalysts.

Herein, we employed our silica-coating method for the catalyst synthesis,^[2] which involves the combination of silica coating and high temperature annealing, as shown in Fig. 1(b). High temperature annealing requires the formation of beaded network by connected Pt–Co nanoparticles as well as chemically ordered structures. Before annealing, the surface of the Pt_x-Co_100-x/SiO₂ samples was coated with thin silica layers to prevent the detachment of Pt_x–Co_100-x nanoparticles from the SiO₂ template as well as the large catalyst agglomeration and coalescence during the annealing process.

The XRD patterns of the obtained catalysts showed the peaks corresponding to L1₀ type and L1₂ type chemically ordered structures for the connected Pt₅₀–Co₅₀ and Pt₇₀–Co₃₀ catalysts annealed at 600 °C, respectively. The ordering parameter (S) of the catalysts as the indicator of chemically ordered degree was calculated from the peak intensity ratio of the ordered plane to the ordered and disordered plane. Both catalysts had S = ca. 55%. The SEM and TEM images of the connected Pt_x–Co_100-x catalysts showed a hollow capsule structure, indicating the formation of a stable connected Pt_x–Co_100-x network.

The electrochemical measurements of ORR activity and durability for the prepared catalysts were conducted in a 0.1 M HClO₄ aq.. The connected Pt₅₀–Co₅₀ and Pt₇₀–Co₃₀ catalysts achieved 6 and 10 times higher ORR specific activities than that of Pt/C, respectively. These high activities are mainly contributed to the lattice strain induced by alloying and the formation of a Pt-bulk-like structure of the carbon-free connected network. Subsequently, the ORR activity of the connected Pt₇₀–Co₃₀ catalyst with different ordered degree (S = 0%, 28%, and 58%) were evaluated. As the ordering parameter in the connected Pt₇₀–Co₃₀ catalysts was higher, the ORR specific activity was increased. Based on the STEM-EDX line mapping results, most of Co in the disordered catalyst (S = 0%) was leached out during the electrochemical pretreatment (50 cycles of cyclic voltammetry), leading to a significant reduction in the catalytic enhancement provided by the alloy structure. In contrast, the highly ordered catalyst (S = 58%) exhibited minimal Co leaching during the electrochemical pretreatment. Thus, the alloy structure and the carbon-free connected structure enhanced ORR specific activity of the highly ordered Pt₇₀–Co₃₀ catalyst.

The load cycle durability of the connected catalysts was tested using the following potential cycling: 0.6 V for 3 s Û 1.0 V for 3 s, 60 °C in a 0.1 M HClO₄ aq.. After 10,000 load cycles, the unique connected network structure successfully maintained and the specific activity of the ordered catalyst (S = 58%) was higher than the disordered catalysts (S = 0%) and a commercial Pt/C. According to the STEM-EDX results, 83% of the alloyed Co retained in the catalyst with a highly ordered structure even after 10000 load cycles, indicating the suppression effect of ordered structure on metal dissolution. In addition, the L1₂ ordered Pt₇₅–Co₂₅ structure remained in the catalyst after load cycle test, confirmed by the TEM diffraction results.

In summary, we successfully developed the connected Pt_x-Co_100-x catalysts with chemically ordered structures for the first time. Our findings demonstrate the significant potential of chemically ordered, carbon-free connected Pt_x-Co_100-x network catalysts in enhancing ORR activity and durability for polymer electrolyte fuel cells, thus contributing to the broader adoption of hydrogen energy.

Acknowledgement

The part of this presentation is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).