Electroreduction of Oxygen on Carbide-Derived Carbon Supported Pd Catalysts

Abstract

In recent years there have been multiple studies trying to improve the activity and stability of Pd-based catalysts for oxygen reduction reaction (ORR).1 The primary goal is to substitute Pt used for the fuel cell applications. In this work the standard borohydride reduction method was used to deposit Pd on different carbon materials, which were prepared from various metal carbides. These carbide-derived carbon (CDC) materials had varying amounts of micro and mesopores and were further ball-milled to obtain smaller carbon particle sizes. The influence of the support material on the prepared catalysts was studied. From the obtained catalysts it was observed that low micro-porosity lead to a higher order of agglomeration of the deposited Pd particles with electrochemical surface area being 3 times smaller in alkaline solution as compared to highly microporous Pd/CDC2. Obtained Pd/C catalysts were studied using thermogravimetry, scanning electron microscopy, cyclic voltammetry, and CO stripping. The ORR was explored using the rotating disk electrode (RDE) method. From Figure 1 it is evident that the Pd particles have a higher order of agglomeration on Pd/CDC3 and Pd/CDC4, which were the two catalysts with the lowest amount of micropores in the original CDC material. The results of thermogravimetric analysis indicated around 20% of Pd deposition for all studied catalysts except for Pd/CDC3 characterized with smaller mass loss compared with the others, which could be due to fewer sites where Pd can deposit. The results of the thermogravimetric analysis also indicated that the Pd deposition shifted the thermooxidation temperature of the materials to lower values compared to the Pd-free CDC materials. RDE polarization curves were measured in both 0.1 M KOH and 0.1 M HClO4 solution. According to the Koutecký–Levich analysis the number of electrons transferred per O2 molecule for all the catalyst was around 4. There are notable differences in the diffusion-limited current plateaus, which could be due to different coverage of the Pd/C catalysts due to the larger carbon particles present in some of the CDC materials, even though the CDC materials were ball-milled, still rather large (2-6 μm) carbon particles could be observed depending on the CDC material. In alkaline media, the specific activity (SA) values increased with the number of mesopores and decreased with the increase of the number of micropores, where highest SA could be observed for Pd nanoparticles supported on CDC3 and CDC4. Figure 1 . SEM images of a) Pd/CDC1, b) Pd/CDC2, c) Pd/CDC3, d) Pd/CDC4, e) Pd/CDC5, and f) Pd/Vulcan carbon. Reference H. Erikson, A. Sarapuu, J. Solla-Gullon, and K. Tammeveski, J. Electroanal. Chem., 780, 327-336 (2016). Figure 1