Development of alternative energy sources is crucial to tackle challenges encountered by the growing global energy demand. Hydrogen fuel, a promising way to store energy produced from renewable power ...sources, can be converted into electrical energy at high efficiency via direct electrochemical conversion in fuel cells, releasing water as the sole byproduct. One important drawback to current fuel‐cell technology is the high content of platinum‐group‐metal (PGM) electrocatalysts required to perform the sluggish oxygen reduction reaction (ORR). Addressing this challenge, remarkable progress has been made in the development of low‐cost PGM‐free electrocatalysts synthesized from inexpensive, earth‐abundant, and easily sourced materials such as iron, nitrogen, and carbon (Fe–N–C). PGM‐free Fe–N–C electrocatalysts now exhibit ORR activities approaching that of PGM electrocatalysts but at a fraction of the cost, promising to significantly reduce overall fuel‐cell technology costs. Herein, recent developments in PGM‐free electrocatalysis, demonstrating increased fuel‐cell performance, as well as efforts aimed at understanding the key limiting factor, i.e., the nature of the PGM‐free active site, are summarized. Further improvements will be accomplished through the controlled and/or rationally designed synthesis of materials with higher active‐site densities, while at the same time establishing methods to mitigate catalyst degradation.
Remarkable progress has been made in the development of Fe‐based precious‐group‐metal‐free (PGM‐free) catalysts for the oxygen reduction reaction, enabling much improved fuel cell performance. Since active site and support are so closely interlinked, efforts to obtain the desired catalyst structure, as well as to better understand the key limiting factors for activity and durability, are reviewed.
Platinum group metal-free (PGM-free) materials based on pyrolyzed M–N–C precursors offer a promising approach to replacing rare and expensive platinum group metal-based oxygen reduction reaction ...(ORR) electrocatalysts in proton exchange fuel cells (PEFCs). A major issue, however, is the stability of these materials in acidic environments and at potentials experienced in situ in PEFC cathodes and rotating disk electrode (RDE) experiments. Density functional theory (DFT)-based approaches have been valuable to understand how atomic scale structures couple to ORR activity. Little has been reported, however, on quantification of active site structure stability. This work proposes a set of DFT-accessible descriptors for M dissolution (demetalation) that directly address this need. Through the application of this approach to a specific Fe–N4 bilayer graphene-hosted active site structure, the roles of the environment (pH and potential), ORR intermediates, and graphene underlayers are explored. Ranges of stability are reported and hypotheses explaining previously reported experimental behavior based on these findings are proposed. In particular, proposed are model implications for experimental trends in stability with respect to alkaline and acidic conditions; experimental trends for dissolution to occur below a given potential; and observed discrepancies in stability for materials in O2-bearing vs O2-purged environments. Based on these findings, suggestions for improving active site resistance to metal dissolution are provided.
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 Fe2+/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.
Atomic-scale structures of oxygen reduction reaction (ORR) active sites in non-platinum group metal (non-PGM) catalysts, made from pyrolysis of carbon, nitrogen, and transition-metal (TM) precursors ...have been the subject of continuing discussion in the fuel cell electrocatalysis research community. Quantum chemical modeling is one path forward for understanding of these materials and how they catalyze the ORR. We here demonstrate through literature examples of how such modeling can be used to better understand non-PGM ORR active site relative stability and activity and how such efforts can also aid in the interpretation of experimental signatures produced by these materials.
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Platinum group metal–free (PGM-free) metal-nitrogen-carbon catalysts have emerged as a promising alternative to their costly platinum (Pt)–based counterparts in polymer electrolyte fuel cells (PEFCs) ...but still face some major challenges, including (i) the identification of the most relevant catalytic site for the oxygen reduction reaction (ORR) and (ii) demonstration of competitive PEFC performance under automotive-application conditions in the hydrogen (H₂)–air fuel cell. Herein, we demonstrate H₂-air performance gains achieved with an iron-nitrogen-carbon catalyst synthesized with two nitrogen precursors that developed hierarchical porosity. Current densities recorded in the kinetic region of cathode operation, at fuel cell voltages greater than ∼0.75 V, were the same as those obtained with a Pt cathode at a loading of 0.1 milligram of Pt per centimeter squared. The proposed catalytic active site, carbon-embedded nitrogen-coordinated iron (FeN₄), was directly visualized with aberration-corrected scanning transmission electron microscopy, and the contributions of these active sites associated with specific lattice-level carbon structures were explored computationally.
The structure of active sites in Fe-based nonprecious metal oxygen reduction reaction catalysts remains unknown, limiting the ability to follow a rational design paradigm for catalyst improvement. ...Previous studies indicate that N-coordinated Fe defects at graphene edges are the most stable such sites. Density functional theory is used for determination of stable potential oxygen reduction reaction active sites. Clusters of Fe–N x defects are found to have N-coordination-dependent stability. Previously reported interedge structures are found to be significantly less stable than in-edge defect structures under relevant synthesis conditions. Clusters that include Fe–N3 defects are found to spontaneously cleave the O–O bond.
Metal and nitrogen doped carbon materials (denoted as M-N-C) synthesized through high-temperature pyrolysis have been found to exhibit activity for oxygen reduction reaction (ORR) approaching that of ...Pt and electrochemical stability higher than previous MN
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-containing macrocyclic molecular catalysts. Tremendous efforts have thus been devoted to the advancement of M-N-C catalysts as an economical alternative to Pt-based catalysts for proton exchange membrane fuel cell cathodes with a focus on simultaneously improving activity and stability. To this end, novel computational modeling techniques have been developed and applied to acquire knowledge crucial for accelerating the pace of M-N-C catalyst development. In this review, recent progress in computational method development, as well as the predictions of chemical structure of active sites, reaction pathways, ORR kinetics, and catalyst stability in electrochemical environments, are critically surveyed. Moreover, the crucial role of computational modeling to elucidate the functional mechanism of M-N-C catalysts for ORR in acid media and enable rational design of M-N-C catalysts is discussed with a visionary outlook for the field.
Computational modeling has been used to acquire knowledge of the active site structure, reaction kinetics, and stability of metal, nitrogen co-doped carbon electrocatalysts, which exhibit encouraging activity for oxygen reduction reaction.
This work demonstrates the essential role of particle size and crossover hydrogen on the degradation of platinum polymer electrolyte membrane fuel cell (PEMFC) cathodes. One of the major barriers to ...implementation of practical PEMFCs is the degradation of the cathode catalyst under operating conditions. This work combines both experimental and theoretical techniques to develop a validated and thermodynamically consistent kinetic model for the coupling of degradation and the catalyst particle size distribution. Our model demonstrates that, due to rapid changes in the Gibbs-Thomson energy, particle size effects dominate degradation for similar2 nm particles but play almost no role for similar5 nm particles. This result can help guide synthesis of more stable distributions. We also identify the effect of hydrogen molecules that cross over from the anode, demonstrating that in the presence of this crossover hydrogen surface area loss is greatly enhanced. We demonstrate that crossover hydrogen changes the surface area loss mechanism from coarsening to platinum loss through dissolution and precipitation off of the carbon support.
Experimental values of hydrogen diffusion coefficients in bulk α-uranium obtained at elevated temperatures have significant scatter, leading to dramatic differences in extrapolated diffusion ...coefficients at room temperature. Previous density functional theory (DFT) calculations predicted higher diffusion barriers than experiments, suggesting hydrogen diffusion through undefected bulk is not possible. In this manuscript DFT calculations of hydrogen diffusion coefficients as a function of crystallographic orientation in α-uranium are reported. The findings explain previous discrepancies, show that diffusion of hydrogen through bulk uranium is consistent with experiments, and suggest a reasonable value for room temperature diffusion coefficient based on proposed pathway.
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