Spectroscoptic characterization of extended surface catalysts

Abstract

To reduce greenhouse gasses and improve air quality it is imperative that the US and other leading countries increasingly add renewable energy to their energy portfolio. Replacing fossil fuel combustion for transportation applications and hydrogen generation with polymer electrolyte membrane (PEM) fuel cells and electrolyzers would have a substantial environmental impact. Extended surface catalysts are a promising alternative to state- of the art catalysts used in these devices. Extended surface PtNi catalysts derived by spontaneous galvanic displacement (SGD) using Ni nanowire templates have demonstrated high activity towards the oxygen reduction reaction at the cathode of a PEM fuel cell and showed improved durability relative to the state-of-the-art Pt-based nanoparticles. Similarly, extended surface IrNi and IrCo catalysts derived from Ni and Co nanowires showed great promise as effective catalysts for oxygen evolution reaction in PEM electrolyzers. The goal of this work is to understand the effect of extended surface PtNi catalyst composition, structure and morphology on catalytic activity and durability. To this end, as received and post- modified catalysts were studied using an extensive set of spectroscopy and microscopy methods, including X-ray photon spectroscopy (XPS), energy dispersive x-ray spectroscopy (EDS) via transmission electron spectroscopy (TEM), x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopies, and transmission x-ray microscopy (TXM). For the PtNi catalyst, it was found that at low Pt loadings the SGD synthesis does not result in an extended Pt surface, instead Pt nanoparticles are formed. Post-synthesis annealing in a hydrogen environment resulted in the formation of a homogeneous extended surface and a boost in specific activity due to PtNi alloying. Further, the ability of various acid treatments to effectively remove bulk Ni, and equally important, their effect on the Pt and Ni surface speciation, wire integrity, and PtNi alloy were analyzed. It was determined that weaker acids did not remove enough Ni and thus had negligible effect on durability while harsher treatments were effective at removing bulk Ni, but resulted in dealloying, disintegration of the nanowires, and loss of activity. Post-treatment with 0.1M nitric acid resulted in removal of Ni from the bulk of the nanowires while leaving the PtNi alloy more in-tact, resulting in optimum balance between activity and durability. Annealing in oxygen was found to generate a protective surface layer therefore mitigating Ni leaching and further improving durability. Based on the success of PtNi nanowires as effective ORR catalysts, IrNi and IrCo nanowires were studied as catalysts for OER. The IrNi and IrCo nanowires with extended surfaces demonstrated high activity and durability relative to the state-of-the-art IrO2 nanoparticles. An extensive XPS characterization, in conjunction with microscopy analysis was used to determine surface composition of as-received and acid-treated materials, both as a function of Ir displacement. In addition to valuable information regarding potential active species, this work reports detailed deconvolution of XPS spectra, which will be valuable for applications of Ir-based materials beyond electrolysis. When incorporated into an electrode the overall catalyst layer structure affects performance of the catalyst, and in particular, the extent of wire to wire contact, wire distribution, and void space are thought to be important. Transmission X-ray microscopy was used for 2D and 3D visualization of nanowire-based extended surface catalysts within the catalyst layer, providing unprecedented structural details complementary to those obtained with electron microscopy techniques. Methodology developed in this work is applicable to future studies of PEM fuel cells and electrolyzers.