Stabilization of PtRu Alloy Nanocatalysts by Encapsulation with Niobium Oxide

The threat of climate change motivates us to reduce our reliance on fossil fuels and transition to renewable energy infrastructure. Direct methanol fuel cells (DMFCs), in which electrocatalysts harness electricity from the reaction of methanol and oxygen, are an attractive clean energy technology because of the high energy density and ease of transport and storage of methanol compared to hydrogen. However, there remains much to improve upon in terms of the durability, and as a consequence the economic viability, of DMFCs. This project focuses on the durability of the anodic electrocatalysts which facilitate the methanol oxidation reaction. A platinum-ruthenium alloy is the most commonly used electrocatalyst towards the methanol oxidation reaction due to its high performance. The high activity and selectivity of the platinum is additionally imparted high tolerance for poisoning byproducts of methanol oxidation [carbon monoxide (CO)] by alloying with ruthenium. Unfortunately, the ruthenium is known to dissolve from the alloy and reduce the CO tolerance of the catalysts.<br/>This project explored encapsulating the PtRu nanoparticle catalysts with porous niobium oxide to stabilize the PtRu alloy. Niobium oxide was chosen due to its high corrosion resistance and previous studies showing its stabilization of ruthenium. PtRu alloyed nanoparticles were synthesized <i>via</i> electrodeposition on a glassy carbon electrode substrate and subsequently encapsulated by an electrophoretic deposition method adapted from Penner <i>et al</i>.¹ The morphology and composition of the catalyst and encapsulating layer were characterized by scanning and transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and optical microscopy. The activity and durability of pristine and encapsulated PtRu catalysts were evaluated by cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy. Carbon monoxide stripping and the double layer capacitance method was used to study changes in the electrochemically active surface area before and after deposition of the niobium oxide. Access to the electrocatalyst surfaces through the encapsulating layer was maximized by the addition of surfactants and the application of different potentials during the deposition of the niobium oxide to improve its porosity. The niobium oxide was shown to improve the stability of the PtRu alloy and maintain the CO tolerance of the alloy well beyond that of the un-encapsulated catalyst. The stabilization of PtRu imparted by the encapsulation of niobium oxide herein is promising for the stabilization of alloyed nanoparticle catalysts towards a wide range of electrocatalytic applications.<br/>1. Jha, G.; Tran, T.; Qiao, S.; Ziegler, J. M.; Ogata, A. F.; Dai, S.; Xu, M.; Thai, M. L.; Chandran, G. T.; Pan, X.; Penner, R. M. Electrophoretic Deposition of Mesoporous Niobium(V)Oxide Nanoscopic Films. <i>Chem. Mater. </i><b>2018</b>, <i>30</i>, 6549-6558.