Optimization of Performance of Fe-N-C Catalysts on Tantalum-Doped Titanium Dioxide Supports for ORR

The combination of hydrogen and oxygen into water by way of fuel cells suggests a promising avenue for the conversion of stored chemical energy into usable electrical energy. Due to membrane stabilities in electrolyte solutions, proton-exchange membrane fuel cells (PEMFCs) have exhibited the most potential for viability in current applications. The performance of these fuel cells is dictated mainly by the sluggish oxygen reduction reaction (ORR) at the cathode. Traditional ORR catalysts are composed of platinum group metals (PGMs), which are highly active but expensive noble metals. However, reaching comparable activity with less expensive materials serves as an alternative to PGM catalysts. An example of these novel, non-PGM catalysts is the Fe-N-C class of catalysts.

Fe-N-C catalysts are frequently supported by conductive carbon. Due to the highly oxidizing environment of the cathode, carbon corrosion, in which the support is used as a carbon source in the formation of carbon dioxide, can occur. This corrosion decreases activity by inducing agglomeration and detachment of the active sites. For this reason, conductive non-carbon supports investigated in conjunction with Fe-N-C catalysts could potentially serve to increase the stability and longevity of non-Pt catalysts for cathodic ORR. Over the course of this project, synthesis and mass-loading parameters were varied to find optimal activity on oxide supports, creating a novel Fe-N-C @ Ta_0.05Ti_0.95O₂ electrocatalyst.

To evaluate the potential for this TiO₂-based support, synthesis by pyrolysis and performance testing by cyclic voltammetry were completed. Each material was produced by dry mixing and heat treating known precursors for Fe-N-C catalysts, aminoantipyrine (AAP) and iron(III) nitrate (Fe(NO₃)₃), in the presence of the Ta-doped TiO₂ support. Certain synthesis parameters were varied, such as the mass ratio of AAP to Ta_0.05Ti_0.95O₂ and the mass ratio of Fe(NO₃)₃ to AAP. Both in situ and ex situ synthesis techniques were studied, as was a sacrificial support method (SSM). Materials underwent a first heat treatment in a H₂/N₂ mixture and a second heat treatment in ammonia (NH₃). Once heat treatments were completed, the products were ground into fine powders and made into inks using a Nafion solution, DI water, and isopropyl alcohol. The inks were sonicated and then applied to a glassy carbon and platinum rotating ring-disk electrode (RRDE) via drop casting. Each catalyst was tested using dual electrode cyclic voltammetry (DECV) with a Pine WaveNow potentiostat in both acid and alkaline electrolyte solutions using a graphite counter and a reference hydrogen electrode while holding the ring at potentials for peroxide oxidation. After performance testing, characterization techniques such as BET, XRD, and XPS were used to further analyze the structural composition of the synthesized catalysts.

A novel set of Fe-N-C catalysts supported by conductive, non-carbon, Ta-doped TiO₂ was synthesized and tested to show considerable ORR activity. Ample opportunity for future work within this project exists, with respect to durability testing and physical characterization to elucidate a structure-property-performance relationship. Overall, this research demonstrated the viability of this new class of electrocatalyst supports for ORR.