Transition Metal-Based Catalysts on Stable and Corrosion-Resistant Supports for Oxygen Reduction Reaction
Authors: Cho Sung Beom , He Cheng , Parrondo Javier , Sankarasubramanian Shrihari , Ramani Vijay K , Mishra Rohan 
Published: February 2020

The current proton exchange fuel cells (PEFC) employ platinum group metal (PGM) catalysts in their electrode for oxygen reduction reaction (ORR). However, the excessive cost of PGM catalysts is a major hurdle for realizing the potential of PEFCs, thus necessitating the development of inexpensive non-PGM catalysts. Amongst the different PGM-free catalysts, a complex mixture of carbon (C), nitrogen (N), and transition metals (M) forming M-N-C clusters (M=Co, Fe, Ni, Mn, etc.) are cuurently the leading candidates due to their comparable activity to PGM catalysts and cost-effectiveness [1]. This complex is generally synthesized by the pyrolysis of carbon-based support with metal and nitrogen precursors [2]; however, the carbon-based support suffers from catastrophic failure during repeated start-up/shut-down events wherein the high cathode interfacial potential results in the irreversible corrosion of carbon and hence to the loss of active centers [3]. More distressingly, the reactions typically yield a complex structure, where the active site and reaction mechanism is still under the debate.

To overcome this issue, we have investigated the stability and catalytic activity of M-N-C clusters on corrosion-resistive supports, using a combination of theoretical and experimental methods. We have identified conductive and corrosion-resistive binary metal carbides by surveying the Materials Project [4] and Open Quantum Materials Databases [5]. For selected support materials, we have identified the atomic configuration of the M-N-C active sites and investigated the reaction mechanism using density-functional theory (DFT) calculations. We find M-N4 clusters to be energetically preferable on the rocksalt-type carbide supports. The bond distance and electronic structure of the active site are highly dependent on the choice of transition metals of the complex and supports. This dependency affects the adsorption energetics of the reaction intermediates and also plays a determining role on the reaction pathway and overall catalytic activity. Based on this understanding, we have synthesized M-N-C clusters on the most promising carbide supports and measured their activity and corrosion resistivity. Overall, our results demonstrate the rapid discovery of catalysts using a synergistic combination of predictive theory and experiments.

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