Selective Catalysis for Electrocatalytic Hydrogen Oxidation and Evolution Reactions Via Carbon Nanotube Encapsulation and Oxide Nano-Layer Deposition

Abstract

Catalysts undergo poisoning and degradation during their utilisation in many different fields. Common examples are the CO poisoning of Pt,1 or the degradation of Pt/C catalysts during start-stop procedures,2 both occurring in polymer electrolyte fuel cells (PEMFCs). Another energy technology that features prominent catalyst degradation is the hydrogen bromine redox flow battery (H2-Br2 RFB). H2-Br2 RFBs are a cheap and efficient solution to large scale energy storage.3 The main hinderance of the technology is the poisoning of the hydrogen evolution reaction/hydrogen oxidation reaction (HER/HOR) catalyst by bromine species which have crossed over the proton exchange membrane.4 Without a revolution in membrane development, this crossover appears unpreventable.5,6 Therefore, we sought to protect the catalyst locally. Two possible solutions were found to impart catalyst selectivity: Pt encapsulation in single-walled carbon nanotubes (SWCNTs) and metal oxide deposition on catalysts via atomic layer deposition (ALD). Platinum nanoparticles were synthesised within the internal cavities of small diameter SWCNTs, through a simple impregnation and drying procedure. High resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and scanning tunnelling electron microscopy (STEM) were used to characterise the particles and demonstrate that they were confined within the SWCNTs. The typical hydrogen under potential deposition peaks were observed on a cyclic voltammogram of the sample, whereas the oxide region was heavily suppressed compared to Pt/C. Some diffusion limitation was observed during the HOR, indicating that the electrolyte diffusion pathway is through the SWCNTs, but the same mass transport limited current was attained. The oxygen reduction reaction (ORR) mass transport limited current was much lower than expected for Pt (2 mA cm-2), indicating a selectivity for hydrogen over oxygen. The stability of these platinum nanoparticles in the presence of bromide/tribromide solution was vastly increased compared to the standard 50% Pt/C catalyst, shown by x-ray photoelectron spectroscopy (XPS) and electrochemistry. It is proposed that this effect is caused by steric and electrostatic repulsion of the large tribromide ion by the SWCNT cavity (internal diameter of 2 nm). The encapsulated platinum also features a vastly higher mass activity when cycled in a cell, indicating better Pt utilization due to the small particle size. This opens a new route for imparting selective access to active sites of a catalyst, hence increasing the stability of the catalyst, a potential solution to many problems faced by technologies that rely on catalysts. Another possible solution to prevent catalyst poisoning is through a protective oxide coating on its surface. ALD was chosen due to its highly controllable, conformal deposition, but also as it can be applied to a wide range of commercial catalysts, making it highly applicable for real world applications. Vanadium oxide was deposited on a commercial 50% Pt/C catalyst via ALD, chosen for its precise control in both layer thickness and conformity on the surface of the catalyst. XPS and HRTEM confirmed the ALD process had deposited vanadium oxide species on the catalyst. The HOR was unaffected by this deposition, indicating diffusion of hydrogen could occur through the oxide layer. Stability of the material in the presence of bromide/tribromide solution was superior to the uncoated commercial catalyst, showing the oxide coating successfully protected the catalyst. 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