Abstract
Carbon materials doped with transition metal and nitrogen are highly active, non-precious metal catalysts for the electrochemical conversion of molecular oxygen in fuel cells, metal air batteries, and electrolytic processes. However, accurate measurement of their intrinsic turn-over frequency and active-site density based on metal centres in bulk and surface has remained difficult to date, which has hampered a more rational catalyst design. Here we report a successful quantification of bulk and surface-based active-site density and associated turn-over frequency values of mono- and bimetallic Fe/N-doped carbons using a combination of chemisorption, desorption and 57Fe Mössbauer spectroscopy techniques. Our general approach yields an experimental descriptor for the intrinsic activity and the active-site utilization, aiding in the catalyst development process and enabling a previously unachieved level of understanding of reactivity trends owing to a deconvolution of site density and intrinsic activity.
Highlights
Carbon materials doped with transition metal and nitrogen are highly active, non-precious metal catalysts for the electrochemical conversion of molecular oxygen in fuel cells, metal air batteries, and electrolytic processes
non-precious metal catalysts (NPMC) still suffer from poor durability[2,4,11,21,22], which is in part attributed to corrosion of the carbon support and the active sites by by-products such as H2O2
Our analysis correlates the active-site density with the apparent catalytic reactivity in acid and alkaline media, yielding surface site-based metal-specific catalytic turnover frequencies (TOFs)
Summary
Carbon materials doped with transition metal and nitrogen are highly active, non-precious metal catalysts for the electrochemical conversion of molecular oxygen in fuel cells, metal air batteries, and electrolytic processes. Current key goals in non-precious metal ORR catalysis comprise molecular insight in the catalytically active site[15,23], optimized synthesis and interrogation tools for the active-site density, as well as strategies to stabilize the active catalytic sites during oxygen reduction[5,9,24,25] In this respect, particular attention was placed on a specific family of Me-N-C catalysts derived from high-nitrogen content molecules, such as cyanamide and from heterocyclic polymers such as polyaniline (PANI), or combinations thereof[5,9,25]. The direct comparison of TOFs of mono- and bimetallic catalysts uncovers previously unexplored synergistic effects between two dissimilar metal ions in the enhancement of the ORR activity in alkaline
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