Hydrogen fuel cells (FCs) are key energy devices that can accelerate our transition away from non-renewable, carbon-intensive fuels especially in the grid-scale storage and transportation sectors. FC cathodes perform the oxygen reduction reaction (ORR), and despite decades of progress towards higher performance electrocatalysts, the ORR remains kinetically sluggish and limits the efficiency of assembled FC devices. Modern, commercially available FC cathodes typically contain platinum (Pt)-based catalysts that can cause the overall FC to become prohibitively expensive for some applications so identifying alternate non-platinum group ORR catalyst formulations could increase deployment of FCs. Physics-based calculations with density functional theory (DFT) have predicted many first-row transition metal oxides (MOx, M = Mn, Fe, Cr, Ni, Co) as having favorable ORR performance trends. Additionally, there is experimental validation of nanoscale ternary and quaternary transition metal oxide systems with a mixture of transition metals used for catalyzing the ORR where relatively modest activity enhancements are reported. Despite this progress, an important limitation is present in the low material stability of non-precious transition metal oxides especially in acidic environments such as those encountered in proton exchange membrane FCs. Using synthetic material strategies to overcome such stability limitations in a variety of FC operating conditions will provide a deeper understanding of the relationship between structure, activity, and degradation.In this work, we employ two simultaneous strategies to work towards enhancing the activity and stability of transition metal oxides: (1) addition of antimony (Sb) as a ligand and (2) functionalization with nitrogen or sulfur. We utilized a previously reported colloidal synthesis for uniform ~ 50 nm oxide nanoparticles for manganese, nickel, cobalt, and iron to produce four sets of nanoparticles based on each of these metals. By using a colloidal synthesis that concurrently introduces Sb with the secondary metal and drying the particles afterwards, we incorporate Sb evenly throughout the polycrystalline structure of the of these nanoparticle catalysts. A high temperature surface modification reaction with reactive gas such as air, ammonia, or hydrogen sulfide was used to convert any remaining transition metal antimonate into an oxide, nitride, or sulfide, respectively. In total, twelve materials are used in this study: the oxide, nitride, and sulfide (collectively “X-ides”) of each of the four transition metal containing, antimony incorporated nanoparticles. We optimize this material synthesis and evaluate the nanoparticle catalyst’s ability to enhance electrochemical ORR activity in both acidic (pH 1) and alkaline (pH 13) conditions which are simulating operating conditions of proton-exchange membrane and anion-exchange membrane FCs, respectively. In addition to activity modulation, we measure the ORR selectivity change towards hydrogen peroxide formation for each material and note the role of nitrogen and sulfur modification in suppressing hydrogen peroxide formation in alkaline environments. In addition to electrochemical characterization, we employed a vast array of bulk and surface characterization techniques to understand catalyst dynamics and contextualize observed activity trends. Transmission electron microscopy (TEM) enabled determination of particle sizes and morphologies alongside electron diffraction patterns to validate the catalyst synthesis. X-ray diffraction (XRD) then provided further information about crystallinity that connected bulk-level insights to the local information from TEM. X-ray photoelectron spectroscopy (XPS) enabled determination of oxidation state changes in the transition metal and in Sb that serve as important descriptors for estimating the activity of these catalysts. We also employed in situ/operando techniques to probe catalyst dynamics and stability under various conditions. Using manganese K-edge synchrotron x-ray absorption spectroscopy (XAS) on the manganese antimony X-ides, we find significant changes to oxidation state and coordination of manganese in the nitride and sulfide as a function of potential indicating processes that displace nitrogen and sulfur in the lattice under certain conditions. Lastly, we use an in situ electrochemical flow cell coupled to inductively coupled plasma-mass spectrometry (ICP-MS) to measure corrosion as a function of ORR current density, potential sweeping, and electrolyte gas saturation to determine degradation patterns. Insights from all these techniques provide a fascinating perspective on what factors in a material and its environment are responsible for enhanced activity, selectivity, and stability. We perform DFT modeling of these systems to further elucidate material properties that are principally responsible for achieving key metrics in activity and stability especially as non-precious catalysts are evaluated for their performance in assembled device conditions. Combining electrochemical experiments, physics-based modeling, and material characterization to better understand structure-performance relationships is a promising path towards accelerating the process of materials discovery for eventual deployment in critical energy applications for the future.
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Open Access
Jan 1, 2024
Yee Wei Foong + 1
