Comparative Study of Platinum Nanoparticle Deposition on TiO2/BP and SnO2/BP Nanocomposites for the Oxygen Reduction Reaction

Abstract

Proton exchange membrane fuel cells (PEMFCs), in their low temperature (LT, 60-80 °C) and high temperature (HT, 120-200 °C) variants, can be used for broad applications in the automotive and stationary sector. However, the most commonly used PEMFC catalyst, consisting of platinum nanoparticles supported on a carbon black, suffers from degradation under the relevant working conditions e.g. low pH environment and potentials in the range of 0.6-1.5 V vs. RHE. These processes include carbon corrosion, Pt dissolution and detachment as well as agglomeration.[1-3] Especially, carbon corrosion leads to an increase of the support hydrophilicity, decreased conductivity and loss of Pt particles resulting in an overall performance loss of PEMFCs.[1] Different studies already reported a positive effect of metal oxide-carbon composite supports using TiO2-Vulcan®XC-72[1,2], SnO2-Vulcan®XC-72[2,3], or fluorine-doped SnO2 on reduced graphene oxide[4] on the carbon and Pt stability. Moreover, Ruiz Camacho et al. showed higher ORR activity for Pt/TiO2-Vulcan and Pt/SnO2-Vulcan compared to Pt/C and a lower potential for CO oxidation during stripping experiments.[2] This can be beneficial for PEMFC operation with reformate.To further push the activity and stability of Pt/metal oxide-carbon catalysts homogeneous distribution of metal oxide and Pt nanoparticles is necessary. In contrast to previous studies, that uses low surface area Vulcan®XC-72[1-3] the implementation of high surface area Black Pearls (BPs) can enable more homogenous distribution of metal oxides and Pt nanoparticles which can positively impact the activity. In this comparative study, Pt/metal oxide-carbon catalysts using SnO2 and TiO2 nanoparticles on BPs are analyzed towards their physical properties and electrochemical ORR activity and stability. Metal oxide/carbon composites were fabricated by deposition of 50 wt.% commercial SnO2 or TiO2 nanoparticles on Black Pearls® 2000. Thermogravimetric analysis (TGA) reveals the successful deposition of metal oxides TiO2 (41 wt.%) and SnO2 (47 wt.%) on BP. Next, deposition of 40 wt.% Pt nanoparticles with diameters between 1-2 nm on the metal oxide-BP composites is done. Transmission electron microscope (TEM) images display successful deposition of Pt with uniform distribution of Pt for both composite catalysts in Figure 1 a) and c). The elemental mapping of Pt and Sn or Ti, using scanning TEM with energy dispersive spectroscopy (EDS) for analysis of the interaction between metal oxide displays homogenous distribution of Pt over the metal oxide-BP supports (Fig 1, b), d)). In the case of Pt/SnO2-BP also uniform Sn distribution is observed whereas for Pt/TiO2-BP partial agglomeration of TiO2 is found.Further analysis of Pt and metal oxides will be given using high resolution-TEM for analysis of lattice distance and ICP-MS for determination of Pt content. Moreover, electrochemical characterization using rotating ring disc electrode will be carried out for comparison of ORR activity and selectivity. Furthermore, an accelerated stress test including 5000 cycles in the range of 0.6-1.5 V vs. RHE in N2-saturated 0.1 mol L-1 HClO4 is applied to analyze the overall catalyst stability. The results will reveal the most promising candidate in terms of activity and stability for future application in HT-PEM half- and single-cell setups.