Imidazoles-Derived PGM-Free Cathode Catalysts for Oxygen Reduction Reaction

Abstract

Conventionally platinum based catalysts are most commonly used cathodic catalysts in fuel cells. The high cost of platinum is a serious limiting factor for the widespread deployment of fuel cell technology. Fe-N-C materials have been explored to be some of the best performing substitutes. However the cost of synthesis, activity and durability are issues that still need to be addressed. In this work we explore the use of novel synthesis techniques with small organic molecules to synthesize Fe-N-C catalysts. Hydrothermal and pyrolysis synthesis strategies are used on templated and template free precursors to obtain the desired catalytic materials. Benzimidazole carboxylic acid is treated with a dual templating protocol that involves sodium salt formation and sacrificial silica hard template. The process then involves high temperature pyrolysis followed by template removal to yield the final product. A highly active and stable Fe-N-C electrocatalyst possessing a unique 3D porous graphene “nano-stack” morphology is thus synthesized. Control experiments with single template were performed to prove the advantage of the dual template. The oxygen reduction catalytic activity of these benzimidazole derived catalysts in alkaline electrolyte is studied rotating disk electrode methods. The dual templated catalyst shows an improvement over the commercial Pt/C catalyst along with excellent stability (as performed under the Department of Energy- Durability Working Group protocols). Furthermore, template-free synthesis of an Fe-N-C ORR catalyst is achieved by the direct pyrolysis of ethylenediaminetetraacetic acid ferric sodium salt. Detailed physical characterization of the catalyst is performed by surface area measurement, X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy. Rotating Ring Disk Electrode measurements. A detailed analysis of the effects of synthesis temperature on graphitization, surface area and their concurrent effects on the catalytic performance is brought forth in this work. Finally, we also establish a novel hydrothermal-pyrolysis approach to generate an active Fe-N-C catalyst from molecules like glucose and methyl-imidazole. The process simultaneously involves formation of metal-nitrogen bonds and partial carbonization resulting in in-situ active site formation. The glucose-imidazole catalysts termed as GLU-IMID-C exhibit good ORR activity in acidic and alkaline electrolyte. We perform detailed structural analysis of these catalysts using such techniques as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), BET surface area analysis and Transmission Electron Microscopy (TEM). The electrocatalytic behavior of the catalysts towards oxygen reduction is studied separately in acidic and alkaline electrolytes by using rotating ring disk electrode measurements. A general precursor-structure-performance relationship of these catalysts and their individual performance trends in both electrolytes has been established in this work.