Palladium-Ceria Nanocatalyst for Hydrogen Oxidation in Alkaline Media: Optimization of the Pd-CeO2 Interface
Authors: Yu Haoran , Davydova Elena S. , Ash Uri , Miller Hamish Andrew , Bonville Leonard J. , Dekel Dario R. , Maric Radenka 
Published: February 2020

The development of Pt-free catalyst for anion exchange membrane fuel cells is limited by the sluggish hydrogen oxidation reaction (HOR) at the anode. Previously, the use of CeO2 as a catalyst promoter facilitated drastic ennoblement of Pd for the HOR kinetics in base media. the use of CeO2 in Pd-based electrocatalysts on the anode of H2-AEMFC has led to a 5-fold improvement of power density compared to the undoped Pd/C (1). A maximum power density of 1 W cm-2 was recently demonstrated using Pd-CeO2/C catalyst, with 85% reduction of anode catalyst loading (2). The catalytic promotion of the HOR kinetics of the Pd-CeO2/C composite was ascribed to the OH- donor effect of CeO2 (1). Furthermore, CeO2 could stabilize the surface PdO species (3), which has been shown to promote the HOR kinetics on Pd (4). Although the positive catalytic effect of ceria on the electrocatalytic activity of the metal-ceria composite has clear experimental evidence, the knowledge about the optimal Pd-ceria interface is still lacking.
In the present work, Pd-CeO2/C composite electrocatalysts are synthesized using three different synthetic approaches based on the flame-based reactive spray deposition technology (RSDT) as a flexible technique enabling the regulation of the particle sizes and providing more instruments to optimize the Pd-CeO2 interface. The correlation between the Pd-CeO2 interaction and the HOR activity is established through comparisons of three types of Pd-CeO2/C synthesized catalysts using electrochemical techniques and X-ray photoelectron spectroscopy (XPS).
The distribution of Pd, Ce and carbon species in all the three types of catalysts is illustrated using scanning transmission electron microscopy (STEM) and elemental mapping (Figure 1). Comparing to previous works on Pd-CeO2/C catalysts (1, 5), the RSDT process improves the mixing and interface between Pd and Ce with all three types of catalysts. Particularly, the Type 1 catalyst shows the most homogeneous contact between Pd and Ce. The Pd-CeO2 chemical interaction results in partial charge transfer from metallic Pd atoms to CeO2 particles and thus, higher concentration of Pd (II) suggests stronger interaction of Pd and CeO2 (1). Based on XPS results, the Type 1 catalyst shows the highest Pd (II)/Pd (0) of 82/18 compared to the Type 2 (59/41) and Type 3 (47/53), which is in line with STEM observations (Figure 1). The HOR activity for RSDT-derived catalysts follows the trend of Type 1 > Type 2 > Type 3. This trend corroborates well with the degree of Pd oxidation state (Pd (II)/Pd (0) ratio) and Pd-CeO2 interaction. The specific activity of Type 1 catalyst surpasses the reported activity achieved by wet-chemistry based method. The RSDT technique has shown its feasibility for the development of Pd-CeO2/C composite HOR catalysts. Further implementation of the RSDT process for the optimization of Pd-CeO2/C catalyst design should concentrate on obtaining homogeneous composites with the lowest particle sizes, intimate contact between ceria and Pd, and the optimal Pd-to-Ce ratio. Application of these optimized HOR catalysts in AEMFC testing is the subject of ongoing work in our laboratories.
Figure 1. HAADF images and elemental mapping of three types of Pd-CeO2/C catalysts using XEDS. The elemental maps correspond to the square region in the HAADF images.
Figure 2. Mass-normalized exchange current densities for three types of RSDT-derived Pd-CeO2/C catalysts compared with the state-of-the-art Pd-CeO2/C electrocatalyst. (1)
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Figure 1