Oxygen Reduction Reaction Properties of Dry-Process Synthesized Pt-Non PGM Transition Metal Multi-Component Alloys: Influence of Transition Metal Elements

Abstract

Introduction Multi-component alloys (MCAs) have been attracted much attention in various engineering application fields. [1] By changing numbers of constituent elements (n) as well as their composition ratios (𝑥) of MCAs, the mixing-entropy (ΔSmix = −𝑅∑(𝑖=1→n) 𝑥𝑖 ln 𝑥𝑖) increases with increasing n, resulting in reduced Gibbs free-energy (ΔG = ΔH - TΔS) of the MCAs, which correlates with interesting bulk material’s properties. Furthermore, the MCAs surfaces possess specific chemical properties, for example, high thermodynamic and chemical stabilities, probably caused by the reduced ΔG, that are much different from those of binary alloys, e.g., Pt-Co or Pt-Ni. Therefore, the MCAs surfaces can be applied for novel nano-materials developments, including electrocatalytic materials. [2] Indeed, our previous study for single-crystal surfaces of the MCAs system of Pt and non-PGM Cr-Mn-Fe-Co-Ni (known as Cantor alloy [3]) demonstrates that enhanced oxygen reduction reaction (ORR) activity at a pristine state and, furthermore, electrochemical structural stability even through the potential cycle (PC)-loading. [4] Thermodynamical and chemical properties of the MCAs surfaces are, in general, a superposition of specific properties of the constituent elements and newly emerging properties that stemming from multi-component alloying. Therefore, catalytic influences of the multiple alloying-elements on ORR properties (activity and structural stability) of Pt-containing MCAs (referred to as Pt-MCAs) surfaces are much complicated, due mainly to the sluggish diffusion and segregation behaviors of the constituent elements. Thus, optimizing the constituent elements (kinds and n) and their composition ratios (𝑥) at the surface vicinity is surely essential for considering an application of the MCAs as practical ORR catalysts. In this study, we synthesized Pt-MCAs on Pt(111) substrate surface with various n and kinds of the constituent elements by controlling vacuum-deposition amounts of the elements and conducted fundamental investigations aimed for high performance cathode alloy catalyst’s developments of polymer electrolyte membrane fuel cells. Experimental In this study, we focused on equi-composition-ratio of the MCAs by picking up 2~5 elements of Cr, Mn, Fe, Co, and Ni as alloying elements of Pt. 10 monolayer (ML)-thick (1 ML = ca. 0.3 nm) MCAs layer was first deposited on the surface-cleaned Pt(111) substrate by using an arc-plasma deposition method at 300 K, and subsequently annealed in vacuum at 773 K for 30 min. Then, 4 ML-thick Pt layer was deposited on the MCAs layer at 300 K and post-annealed at 623 K in vacuum. The samples thus prepared are designated as Pt/MCAs/Pt(111). The atomic-level micro-structures and composition ratios of the surface-vicinity of resulting Pt/MCAs/Pt(111) were characterized by cross-sectional STEM-EDS and XPS. CVs and LSVs were conducted in N2-purged and O2-saturated 0.1 M HClO4 by using an RDE apparatus. ORR activity was evaluated from j k values estimated at 0.9 V vs. RHE by using Koutecky-Levich equation and structural stability (ORR durability) was discussed based upon the activity transitions during applying 5,000 potential cycles (PCs) of 0.6(3s)‐1.0(3s) V vs. RHE in O2 saturated 0.1 M HClO4 at room temperature. Results and Discussion The ORR activity trends under applying the PC loading (a), initial CVs (b), and XPS-estimated surface compositions of Pt/MCAs/Pt(111) are summarized in Fig. 1(a-c), where Cr-Mn-Fe-Co-Ni, Mn-Fe-Co-Ni, Cr-Fe-Co-Ni, and Fe-Co-Ni are adopted for the MCAs. Notably, the ORR activity of Cr containing Pt/Cr-Fe-Co-Ni/Pt(111) (green) decreased remarkably up to 2,000 PCs, compared to Cr-less Pt/Mn-Fe-Co-Ni/Pt(111) (red), Pt/Fe-Co-Ni/Pt(111) (yellow), and both Cr and Mn containing Pt/Cr-Mn-Fe-Co-Ni(Cantor alloy)/Pt(111) (blue). Furthermore, Pt/Cr-Fe-Co-Ni/Pt(111) exhibited increased electric double layer charge in the potential region of 0.4 to 0.6 V at the pristine state (b). Additionally, XPS-estimated relatively smaller and larger composition ratios of the Pt and Cr, respectively, among the tested Pt/MCAs/Pt(111) (c), suggesting that Cr of the MCAs preferentially diffuses and segregates at the surface-vicinity through the heat treatment process of the sample fabrication, which might cause severe decrease in ORR activity at the early stage of the PCs loading (up to 2,000 PCs). [5] At the meeting, correlations between the ORR properties and atomic-level microstructures of Pt/MCAs/Pt(111) surfaces will be discussed in detail. Acknowledgement This study was supported by new energy and industrial technology development organization (NEDO) of Japan, JST SPRING, Grant Number JPMJJSP2114 (Y.C.) and Grant-in-Aid for JSPS Fellows (Y.C.).