Ta-Based Nanostructured Materials for Proton Exchange Membrane Fuel Cells

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The focus of this work has been to assess the suitability of nanostructured TaOxNy, primarily as nanotubes (NTs), as a catalyst and/or catalyst support material for proton exchange membrane fuel cell (PEMFC) applications. It was found that this n-type material could be switched between an insulating state at > 0.6 V vs RHE to a conducting state at < 0.6 V in both aqueous and organic media. In the conducting state, the redox activity was proposed to be due to the Ta4+/5+ reaction, along with insertion/de-insertion of solution cations. The conductivity switching behavior was correlated with electrochromism, with the NTs being yellow-orange in the oxidized state and blue-black when reduced. TaOxNy was then tested for its activity in catalyzing hydrogen oxidation (HOR) and oxygen reduction (ORR) in acidic medium. Although TaOxNy alone is inactive towards these reactions, the presence of Pt nanoparticles (NPs) changed its behaviour. When deposited on a thin compact TaOxNy film, normal Pt electrochemistry was seen with a ~60 mV⋅decade-1 ORR Tafel slope. When Pt NPs were deposited on TaOxNy NTs, a 105 mV⋅decade-1 ORR Tafel slope was seen attributed to porosity or resistance of the TaOxNy. Both the TaOxNy NTs alone and the Pt NPs/TaOxNy NTs material were found to be very resistant to corrosion under PEMFC operating conditions, evaluated through an in-house accelerated durability test. Further, due to its conductivity switching characteristics, the HOR showed better performance than the ORR, but not as good as for state-of the art Pt/Carbon. Interestingly, some hydrogen oxidation was seen even when TaOxNy is non-conducting, attributed to stabilization of the Ta4+ state in H2. TaOxNy was also anchored on colloid imprinted carbon (CIC) to improve its conductivity and durability. Although the CIC improved the conductivity, all CIC-based materials failed the corrosion accelerated durability tests (ADT). The presence of TaOxNy or N-doping of CIC (N is doped during synthesis) also did not improve the corrosion resistance. Overall, TaOxNy was shown to be a very durable material, resistant to corrosion even at very high anodic potentials. It is also a promising support material for Pt NPs for the catalysis of the HOR, but its conductivity likely needs to be improved further in order to catalyze the ORR.
Abhayawardhana, A. D. (2018). Ta-Based Nanostructured Materials for Proton Exchange Membrane Fuel Cells (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/33130