Investigations on Pt-Pd Aerogels for Oxygen Reduction Reaction

Carbon supported Pt nanoparticles are widely used as electrocatalysts for polymer electrolyte fuel cells (PEFCs). However, the carbon corrosion associated with particle detachment leads to a strong loss of electrochemically active surface area (ECSA) under operating fuel cell conditions. In order to increase the long-term performance, a promising strategy is the utilization of unsupported high surface area metallic electrocatalysts. For instance, nanostructured thin films (NSTF) showed significantly improved activity and durability for the sluggish kinetics of oxygen reduction reaction (ORR). [1] The challenge, therefore, is the development of unsupported high surface area metallic electrocatalysts for the ORR. Recently, we reported that multimetallic aerogels show considerable catalytic performance and durability over the time. They consist of a three-dimensional network of well-connected nanoparticles, having high porosity, high conductivity and high surface area of up to 180 m² g^-1. [2-3] The benefits for ORR activity and durability have recently been published. [4 - 5]

In this work, we show two approaches for the preparation of Pt-Pd aerogel: the spontaneous one-step gelation route and the two-step gelation route. Both methods lead to the formation of the aerogel structure. However, the spatial distribution of the dissimilar metals and the homogeneity within the aerogels are expected to be strongly different. Ex-situ Extended X-ray Adsorption Fine Structure (EXAFS) spectroscopy was performed to provide chemical and structural information about homogeneity and distribution for the differently synthesized Pt-Pd aerogel materials. Afterwards, electrochemical investigations on the differently synthesized Pt-Pd aerogels were performed by using RDE technique. An electrochemical protocol was developed to establish the ECSA for the highly complex Pt-Pd aerogels. Based on the previous work, the durability of the Pt-Pd aerogels were traced out by monitoring of the ECSA and ORR activity over time combined with the changes of particle size, network structure and chemical composition.

Our data show that the chemical and structural homogeneity results from the two differently used synthetic approaches and have strong influences on the catalytic performance and long-term stability. The one-step gelation process forms almost alloy particles, while non-alloyed connected particles are generated by using two-step gelation process. The homogeneously alloyed Pt-Pd aerogels exhibit higher activity and durability compared to non-alloyed systems. In-situ electrochemical XAS studies were performed to point out the changes of structure and composition during the electrochemical cycling.

Our work provides new insights into the relationship between structure, composition, reactivity and robustness for unsupported high surface area bimetallic aerogels.

[1]Debe, M..K., Schmoeckel, A. K., Vernstrom, G. D., Atanasoski R. J. Power Sources. 2006, 161(2), 1002-11.

[2] Liu, W. et al., Angew. Chem. Int. Ed. 2012, 51, 5743-5747.

[3] Herrmann, A.-K. et al., Chem. Mat., 2014, 26(2), 1074–1083.

[4] Liu W. et al., Angew. Chem. Int. Ed. 2013, 52, 9849–985.

[5] Schmidt, T. J. et al.; European Patent Application EP 12177908, 2012