Activity Descriptor Identification for Oxygen Reduction on Nonprecious Electrocatalysts: Linking Surface Science to Coordination Chemistry

Developing nonprecious group metal based electrocatalysts for oxygen reduction is crucial for the commercial success of environmentally friendly energy conversion devices such as fuel cells and metal–air batteries. Despite recent progress, elegant bottom-up synthesis of nonprecious electrocatalysts (typically Fe–N_<i>x</i>/C) is unavailable due to lack of fundamental understanding of molecular governing factors. Here, we elucidate the mechanistic origin of oxygen reduction on pyrolyzed nonprecious catalysts and identify an activity descriptor based on principles of surface science and coordination chemistry. A linear relationship, depicting the ascending portion of a volcano curve, is established between oxygen-reduction turnover number and the Lewis basicity of graphitic carbon support (accessed via C 1s photoemission spectroscopy). Tuning electron donating/withdrawing capability of the carbon basal plane, conferred upon it by the delocalized π-electrons, (i) causes a downshift of e_g-orbitals (d_<i>z</i>²) thereby anodically shifting the metal ion’s redox potential and (ii) optimizes the bond strength between the metal ion and adsorbed reaction intermediates thereby maximizing oxygen-reduction activity

Highly active oxygen reduction non-platinum group metal electrocatalyst without direct metal–nitrogen coordination

International audienceReplacement of noble metals in catalysts for cathodic oxygen reduction reaction with transition metals mostly create active sites based on a composite of nitrogen-coordinated transition metal in close concert with non-nitrogen-coordinated carbon-embedded metal atom clusters. Here we report a non-platinum group metal electrocatalyst with an active site devoid of any direct nitrogen coordination to iron that outperforms the benchmark platinum-based catalyst in alkaline media and is comparable to its best contemporaries in acidic media. In situ X-ray absorption spectroscopy in conjunction with ex situ microscopy clearly shows nitrided carbon fibres with embedded iron particles that are not directly involved in the oxygen reduction pathway. Instead, the reaction occurs primarily on the carbon–nitrogen structure in the outer skin of the nitrided carbon fibres. Implications include the potential of creating greater active site density and the potential elimination of any Fenton-type process involving exposed iron ions culminating in peroxide initiated free-radical formation

Elucidating Oxygen Reduction Active Sites in Pyrolyzed Metal–Nitrogen Coordinated Non-Precious-Metal Electrocatalyst Systems

Detailed understanding of the nature of the active centers in non-precious-metal-based electrocatalyst, and their role in oxygen reduction reaction (ORR) mechanistic pathways will have a profound effect on successful commercialization of emission-free energy devices such as fuel cells. Recently, using pyrolyzed model structures of iron porphyrins, we have demonstrated that a covalent integration of the Fe–N_<i>x</i> sites into π-conjugated carbon basal plane modifies electron donating/withdrawing capability of the carbonaceous ligand, consequently improving ORR activity. Here, we employ a combination of <i>in situ</i> X-ray spectroscopy and electrochemical methods to identify the various structural and functional forms of the active centers in non-heme Fe/N/C catalysts. Both methods corroboratively confirm the single site 2e^– × 2e^– mechanism in alkaline media on the primary Fe²⁺–N₄ centers and the dual-site 2e^– × 2e^– mechanism in acid media with the significant role of the surface bound coexisting Fe/Fe_<i>x</i>O_<i>y</i> nanoparticles (NPs) as the secondary active sites

Spectroscopic insights into the nature of active sites in iron–nitrogen–carbon electrocatalysts for oxygen reduction in acid

International audienceDeveloping efficient and inexpensive catalysts for the sluggish oxygen reduction reaction (ORR) constitutes one of the grand challenges in the fabrication of commercially viable fuel cell devices and metal–air batteries for future energy applications. Despite recent achievements in designing advanced Pt-based and Pt-free catalysts, current progress primarily involves an empirical approach of trial-and-error combination of precursors and synthesis conditions, which limits further progress. Rational design of catalyst materials requires proper understanding of the mechanistic origin of the ORR and the underlying surface properties under operating conditions that govern catalytic activity. Herein, several different groups of iron-based catalysts synthesized via different methods and/or precursors were systematically studied by combining multiple spectroscopic techniques under ex situ and in situ conditions in an effort to obtain a comprehensive understanding of the synthesis-products correlations, nature of active sites, and the reaction mechanisms. These catalysts include original macrocycles, macrocycle-pyrolyzed catalysts, and Fe−N–C catalysts synthesized from individual Fe, N, and C precursors including polymer-based catalysts, metal organic framework (MOF)-based catalysts, and sacrificial support method (SSM)-based catalysts. The latter group of catalysts is most promising as not only they exhibit exceptional ORR activity and/or durability, but also the final products are controllable. We show that the high activity observed for most pyrolyzed Fe-based catalysts can mainly be attributed to a single active site: non-planar Fe–N4 moiety embedded in distorted carbon matrix characterized by a high potential for the Fe2+/3+ redox transition in acidic electrolyte/environment. The high intrinsic ORR activity, or turnover frequency (TOF), of this site is shown to be accounted for by redox catalysis mechanism that highlights the dominant role of the site-blocking effect. Moreover, a highly active MOF-based catalyst without Fe–N moieties was developed, and the active sites were identified as nitrogen-doped carbon fibers with embedded iron particles that are not directly involved in the oxygen reduction pathway. The high ORR activity and durability of catalysts involving this second site, as demonstrated in fuel cell, are attributed to the high density of active sites and the elimination or reduction of Fenton-type processes. The latter are initiated by hydrogen peroxide but are known to be accelerated by iron ions exposed to the surface, resulting in the formation of damaging free-radicals