Stabilization of Metal Alloy Nanocatalysts by Encapsulation with a Metal Oxide

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The threat of climate change motivates us to reduce our reliance on fossil fuels and transition to renewable energy infrastructure. Many clean energy technologies including hydrogen fuel cells, direct methanol fuel cells, and water electrolyzers utilize electrocatalysts to generate electricity from chemical energy or vice versa. One direction for increasing the catalytic performance and tolerance to contaminants of the catalyst layers is the development of metal alloy-based catalysts. For instance, ruthenium (Ru) can be alloyed with platinum (Pt) to increase the tolerance of Pt to carbon monoxide, a contaminant that can be found in hydrogen fuel. However, metal alloy catalysts have an additional route for degradation beyond the well-established Ostwald ripening, coalescence, and dissolution mechanisms that are an issue for nanoparticle electrocatalysts. Harsh fuel cell conditions can lead to a separation of the two metals that were originally alloyed in the nanocatalyst and, as a result, this leads to a loss in catalytic performance and tolerance to contaminants.1 This project focuses on the durability of alloyed PtRu electrocatalysts, which facilitate the methanol oxidation reaction in a direct methanol fuel cell and the hydrogen oxidation reaction in a hydrogen fuel cell.This project explored an encapsulation of electrodeposited PtRu nanoparticle catalysts with a porous niobium oxide as a method to stabilize the PtRu alloy. Niobium oxide was chosen due to its high corrosion resistance and previous studies showing its stabilization of Ru.2 PtRu alloyed nanoparticles were synthesized via electrodeposition on a glassy carbon electrode substrate and subsequently encapsulated by an electrophoretic deposition method adapted from Jha et al.3 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, linear sweep voltammetry, chronoamperometry, and electrochemical impedance spectroscopy. Carbon monoxide stripping and the double layer capacitance methods were 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 through tuning the potentials applied during the deposition of the niobium oxide as a means 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. Yang, Z.; Yu, X.; Zhang, Q. RSC Adv., 2016, 6, 114014-114018.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. Mater. 2018, 30, 6549-6558.