High-Throughput Characterization of Electrocatalyst Libraries

Structural and compositional characterization of individual elements of a combinatorial library can be important for the initial validation of a particular combinatorial synthesis method. Many earlier reports on combinatorial synthesis and screening of electrocatalysts fall short of reporting the complete structural and compositional characterization of individual library elements of interest. The workflow described here includes catalyst characterization before and after screening, thereby establishing an activity-composition-structure-stability relationship for electrocatalysts. This can be relevant in light of the extreme conditions present in a conventional fuel cell environment. 

The present workflow includes two complementary high-throughput characterization methods X-ray diffraction (XRD) for structural information and scanning electron microscope/energy dispersive X-ray (SEM/EDX) analysis for quantitative compositional information. Automated rapid-serial XRD and EDX analyses were performed using programmable xy-stages in combination with suitable software. 

To demonstrate the viability of the synthesis and characterization approach in the present workflow, two binary Pt-Fe alloy libraries were designed, synthesized and characterized by XRD (Figs. 11.4 to 11.6) [19]. One library (Fig. 11.5) was characterized as synthesized, while the other (Fig. 11.6) was annealed at 400 °C for 12 h in a hydrogen/argon atmosphere. Pt-Fe is a well-known binary alloy system, exhibiting both substitutional solid solution compositional ranges and intermetal-lic compounds. 

5 and 11.6 show eight XRD patterns as measured along row C. In Fig. 11.5, the dominant (111) diffraction peak for 100% Pt appears at around 20=40°, as expected. The absence of diffraction peaks of pure Pt or Fe in the multi-metal XRD patterns is indicative of complete alloying of the multi-metal thin films by interlayer diffusion. 

As the amount of Fe is increased, the (111) peak shifts to smaller d-spacings, reflecting a contraction of the lattice. The (111) peak positions in Fig. 11.5 show a continuous shift from pure Pt to pure Fe. The Pt-Fe XRD patterns are consistent with a single-phase, substitutional solid solution (disordered alloy) over the entire compositional range. In contrast, Fig. 11.6 clearly displays diffraction from inter-metallic compounds of lower symmetry. Post-deposition annealing has resulted in an ordering of the Pt and Fe atoms, the effect of which is the crystallization of an ordered metal alloy of lower symmetry than 100% Pt. In essence, the applied vacuum deposition method is ideally suited for the preparation of multi-component, 

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