Effect of Thermal Treatment on Structure and Properties of Niobium Oxide Aerogels as Electrolyzer Catalyst Supports
Date
2020-08
Authors
Albiter, Luis A.
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Abstract
Oxygen evolution reaction (OER) catalyst limitations including cost, activity, and stability remain bottlenecks to the wide scale adoption of proton exchange membrane (PEM) water electrolyzers used to split water into hydrogen and oxygen. Distributing the catalyst on a high surface area support can reduce the loading of the active noble-metal OER catalyst (e.g., ruthenium and iridium) and lower the cost of PEM electrolyzers. Carbon, which is typically used as a support material, is highly unstable under the conditions required for OER which makes it unfeasible for long-term use within electrolyzers. Therefore, alternative catalyst supports are needed. Niobium oxide, Nb2O5, is stable under the oxidative potentials and highly corrosive acidic conditions of PEM electrolyzers; however, the low electronic conductivity of Nb2O5 significantly limits its use as a catalyst support material. In this work, approaches to obtain high surface area and conductive niobium oxides, NbOx, for stable supports for OER catalysts were investigated. The effects of synthesis and processing conditions, metal substituents and temperature/atmosphere treatments on the structure, morphology, surface area, and electronic conductivity were evaluated. Niobium oxide was synthesized using a metal alkoxide sol-gel method and dried under either ambient or supercritical drying conditions. To increase the electronic conductivity of NbOx, different metal (e.g. W5+, W6+, Ti4+ and Ru4+) substituents were incorporated during the synthesis process. Thermal treatment under air or hydrogen was investigated to determine the effects of temperature and atmosphere on structure and physical properties. The structure, morphology, composition and surface area were determined by X-ray diffraction (XRD), scanning electron microscopy, energy dispersive spectroscopy (EDS), thermogravimetric analysis, and nitrogen physisorption measurements. Electronic conductivities were determined using two-point probe measurements. Results show that a high Brunauer-Emmett-Teller (BET) surface area of 454 m2 g-1 and a mesoporous NbOx structure was obtained by supercritical drying (aerogel) as compared to 389 m2 g-1 and microporous NbOx structure obtained by ambient evaporative drying (xerogel). The incorporation of W5+ and Ti4+ within Nb2O5 was supported based on XRD and EDS analysis, but no measurable increase in conductivity was observed. The mixed oxide of RuOx-NbOx heated to 600 ⁰C in ambient air had a significantly higher conductivity of 1x10-2 S cm-1 which was a substantial improvement from NbOx which had a conductivity below our detection limit (~10-9 S cm-1). Thermal treatment under hydrogen at various temperatures was investigated, and results showed that the NbOx aerogel heated in 600 ⁰C had an adequate BET surface area of 45 m2 g-1 and an increased conductivity of 1x10-5 S cm-1. The development of a high surface area and electrically conductive catalyst support that can withstand the harsh environments of PEM electrolyzers brings the field one step closer to a grid-scale chemical energy storage solution.
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Keywords
Catalyst, Electrolyzer
Citation
Albiter, L. A. (2020). <i>Effect of thermal treatment on structure and properties of niobium oxide aerogels as electrolyzer catalyst supports</i> (Unpublished thesis). Texas State University, San Marcos, Texas.