ORR Properties for Model Pt-Shell Layers Prepared on Nitrogen-Beam Irradiated Pt25Ni75(111) Substrate

Abstract

Introduction Pt–M (M = Fe, Co, Ni, Cu, Pd, etc.) alloy and Pt/M core–shell nanoparticles have been intensively studied as oxygen reduction reaction (ORR) catalysts for polymer electrolyte fuel cells (PEFCs). As for the core–shell catalysts, the core elements easily dissolve in the electrolyte under the PEFCs operating conditions, leading to degradation of the catalytic activity. To control dissolution behavior of the core elements, Kuttiyiel et al. proposed the structure of Pt–shell/nitride–core nanostructures and demonstrated that nitride core is effective not only for enhancing ORR activity but also for improving durability [1]. For developing the practical Pt–M catalysts, it is significant to clarify a role of the nitrogen atoms located at the core–shell interface. In this study, we fabricate model catalyst structures that composed of Pt(111) epitaxial layers on low–energy N2–beam irradiated Pt25Ni75(111) substrate surfaces by using molecular beam epitaxy (MBE) and discuss the ORR activity and the durability. Experimental Model catalyst sample surfaces are fabricated as follows. First, Pt25Ni75(111) single crystal substrate was cleaned by Ar+ sputtering and annealing at 1173 K in ultra–high vacuum (UHV). Next, neutralized N2–beam (≤ 100eV) was irradiated onto the cleaned Pt25Ni75(111) at room temperature by using low–energy neutralized nitrogen ion beam gun. Then, 3ML–thick–Pt was deposited on the N2–beam irradiated Pt25Ni75(111) substrate by an electron–beam evaporation method at room temperature, followed by annealing at 673 K for 10 minutes in UHV to flatten the topmost surfaces. The resulting surface structure was verified by scanning tunneling microscopy (STM). The sample thus fabricated was transferred to electrochemical evaluation systems set in a N2–purged glove box without air exposure [2]. CV and LSV measurements were conducted in N2–purged and O2–saturated 0.1 M HClO4, respectively, and ORR activity was evaluated from j k values at 0.9 V vs. RHE by using Koutecky–Levich equation. The electrochemical stability of the sample was investigated by applying potential cycles between 0.6 V (3s) and 1.0 V (3s) in O2–saturated 0.1 M HClO4. Results and Discussion UHV–STM images of the as–prepared 3ML–Pt/Pt25Ni75(111) and 3ML–Pt/100eV–N2–beam irradiated Pt25Ni75(111) (3ML–Pt/N2–beam–Pt25Ni75(111)) surfaces are shown in Fig. 1(a). The line profiles of white arrows are depicted below the corresponding STM images. The STM images clearly show that atomically flat surfaces can be obtained through the UHV–process, irrespective of the 100eV–N2–beam irradiation. Furthermore, the both surfaces exhibited moiré–like height modulations (line profiles) that derived from lattice mismatches between the topmost Pt–shell layers and corresponding substrates. The results suggest that compressive surface strain should be induced for both the shell layers. Corresponding CV curves of the 3ML–Pt/Pt25Ni75(111) (blue), 3ML–Pt/N2–beam–Pt25Ni75(111) (red) and Pt(111) (black–dashed) are summarized in Fig. 1(b). Compared with the clean Pt(111), the total Hupd charges for the 3ML–thick–Pt surfaces considerably decreased and the onset of Oads positively shifted. Such CV features are common for highly–active Pt skin–type surfaces [2, 3]. Furthermore, the oxidation of the Pt shell layers for the latter N2–beam irradiated sample is suppressed in comparison to the former. ORR activities evaluated before and after 1000 potential cycles loading are summarized in Fig. 1(c). As for the initial activity enhancement factor, the 3ML–Pt/Pt25Ni75(111) andthe N2–beam irradiated one reveals ×15 and ×8 vs. Pt(111), respectively. The results suggest that the nitrogen atoms located at the Pt–shell/Pt25Ni75(111) interface should release the surface strain of the Pt–shell layers, leading to decrease in pristine ORR activity. In contrast, N2–beam irradiated sample is relatively stable against the potential cycle loading, indicating that the interface nitrogen atoms dominate not only activity but also durability of the Pt– shell/nitride–core type catalysts. At the PRiME 2016, I will discuss ORR activity and durability for various–thick Pt shells prepared on N2–beam–Pt25Ni75(111). Acknowledgement This study was supported by new energy and industrial technology development organization (NEDO) of Japan.