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(x)/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(z(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.

Experimental Observation of Redox-Induced Fe-N Switching Behavior as a Determinant Role for Oxygen Reduction Activity.

The commercialization of electrochemical energy conversion and storage devices relies largely upon the development of highly active catalysts based on abundant and inexpensive materials. Despite recent achievements in this respect, further progress is hindered by the poor understanding of the nature of active sites and reaction mechanisms. Herein, by characterizing representative iron-based catalysts under reactive conditions, we identify three Fe-N4-like catalytic centers with distinctly different Fe-N switching behaviors (Fe moving toward or away from the N4-plane) during the oxygen reduction reaction (ORR), and show that their ORR activities are essentially governed by the dynamic structure associated with the Fe(2+/3+) redox transition, rather than the static structure of the bare sites. Our findings reveal the structural origin of the enhanced catalytic activity of pyrolyzed Fe-based catalysts compared to nonpyrolyzed Fe-macrocycle compounds. More generally, the fundamental insights into the dynamic nature of transition-metal compounds during electron-transfer reactions will potentially guide rational design of these materials for broad applications.

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

Replacement 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 x 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 in situ 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 x O y nanoparticles (NPs) as the secondary active sites.