The hydrogen peroxide (H2O2) generation via the electrochemical oxygen reduction reaction (ORR) under ambient conditions is emerging as an alternative and green strategy to the traditional energy‐intensive anthraquinone process and unsafe direct synthesis using H2and O2. It enables on‐site and decentralized H2O2production using air and renewable electricity for various applications. Currently, atomically dispersed single metal site catalysts have emerged as the most promising platinum group metal (PGM)‐free electrocatalysts for the ORR. Further tuning their central metal sites, coordination environments, and local structures can be highly active and selective for H2O2production via the 2e−ORR. Herein, recent methodologies and achievements on developing single metal site catalysts for selective O2to H2O2reduction are summarized. Combined with theoretical computation and advanced characterization, a structure–property correlation to guide rational catalyst design with a favorable 2e−ORR process is aimed to provide. Due to the oxidative nature of H2O2and the derived free radicals, catalyst stability and effective solutions to improve catalyst tolerance to H2O2are emphasized. Transferring intrinsic catalyst properties to electrode performance for viable applications always remains a grand challenge. The key performance metrics and knowledge during the electrolyzer development are, therefore, highlighted.
Homogeneous catalysis of dioxygen reduction by molecular Mn complexes
Continually increasing global energy demand perpetuates the need for effective alternative energy sources and ‘green’ industrial processes. The oxygen reduction reaction (ORR) is crucial to the development of hydrogen fuel cells, a key device in the development of alternative energy sources. Further, the ORR to hydrogen peroxide by electrochemical means can provide an environmentally friendly alternative to its industrial production, which is capital and energy intensive. While Pt has traditionally been the best electrocatalyst for the ORR, inspiration from active sites in nature that bind and transport O 2 has led to the development of earth-abundant transition metal catalysts. However, despite the prevalence of Mn-based active sites that bind and activate O 2 in biological systems, there remains a lack of developed Mn-centered catalysts for ORR in comparison to Fe and Co. Here, we summarize known Mn-based molecular electrocatalysts for the ORR and describe their activity as well as future directions of the field.
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- Award ID(s):
- 2102156
- NSF-PAR ID:
- 10399080
- Date Published:
- Journal Name:
- Chemical Communications
- Volume:
- 58
- Issue:
- 84
- ISSN:
- 1359-7345
- Page Range / eLocation ID:
- 11746 to 11761
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
The catalytic reduction of dioxygen (O2) is important in biological energy conversion and alternative energy applications. In comparison to Fe- and Co-based systems, examples of catalytic O2 reduction by homogeneous Mn-based systems is relatively sparse. Motivated by this lack of knowledge, two Mn-based catalysts for the oxygen reduction reaction (ORR) containing a bipyridine-based non-porphyrinic ligand framework have been developed to evaluate how pendent proton donor relays alter activity and selectivity for the ORR, where Mn(p-tbudhbpy)Cl (1) was used as a control complex and Mn(nPrdhbpy)Cl (2) contains a pendent –OMe group in the secondary coordination sphere. Using an ammonium-based proton source, N,N′-diisopropylethylammonium hexafluorophosphate, we analyzed catalytic activity for the ORR: 1 was found to be 64% selective for H2O2 and 2 is quantitative for H2O2, with O2 binding to the reduced Mn(II) center being the rate-determining step. Upon addition of the conjugate base, N,N′-diisopropylethylamine, the observed catalytic selectivity of both 1 and 2 shifted to H2O as the primary product. Interestingly, while the shift in selectivity suggests a change in mechanism for both 1 and 2, the catalytic activity of 2 is substantially enhanced in the presence of base and the rate-determining step becomes the bimetallic cleavage of the O–O bond in a Mn-hydroperoxo species. These data suggest that the introduction of pendent relay moieties can improve selectivity for H2O2 at the expense of diminished reaction rates from strong hydrogen bonding interactions. Further, although catalytic rate enhancements are observed with a change in product selectivity when base is added to buffer proton activity, the pendent relays stabilize dimer intermediates, limiting the maximum rate.more » « less
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The urgent need to address the high-cost issue of proton-exchange membrane fuel cell (PEMFC) technologies, particularly for transportation applications, drives the development of simultaneously highly active and durable platinum group metal-free (PGM-free) catalysts and electrodes. The past decade has witnessed remarkable progress in exploring PGM-free cathode catalysts for the oxygen reduction reaction (ORR) to overcome sluggish kinetics and catalyst instability in acids. Among others, scientists have identified the newly emerging atomically dispersed transition metal (M: Fe, Co, or/and Mn) and nitrogen co-doped carbon (M–N–C) catalysts as the most promising alternative to PGM catalysts. Here, we provide a comprehensive review of significant breakthroughs, remaining challenges, and perspectives regarding the M–N–C catalysts in terms of catalyst activity, stability, and membrane electrode assembly (MEA) performance. A variety of novel synthetic strategies demonstrated effectiveness in improving intrinsic activity, increasing active site density, and attaining optimal porous structures of catalysts. Rationally designing and engineering the coordination environment of single metal MN x sites and their local structures are crucial for enhancing intrinsic activity. Increasing the site density relies on the innovative strategies of restricting the migration and agglomeration of single metal sites into metallic clusters. Relevant understandings provide the correlations among the nature of active sites, nanostructures, and catalytic activity of M–N–C catalysts at the atomic scale through a combination of experimentation and theory. Current knowledge of the transferring catalytic properties of M–N–C catalysts to MEA performance is limited. Rationally designing morphologic features of M–N–C catalysts play a vital role in boosting electrode performance through exposing more accessible active sites, realizing uniform ionomer distribution, and facilitating mass/proton transports. We outline future research directions concerning the comprehensive evaluation of M–N–C catalysts in MEAs. The most considerable challenge of current M–N–C catalysts is the unsatisfied stability and rapid performance degradation in MEAs. Therefore, we further discuss practical methods and strategies to mitigate catalyst and electrode degradation, which is fundamentally essential to make M–N–C catalysts viable in PEMFC technologies.more » « less
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Platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) with atomically dispersed FeN 4 sites have emerged as a potential replacement for low-PGM catalysts in acidic polymer electrolyte fuel cells (PEFCs). In this work, we carefully tuned the doped Fe content in zeolitic imidazolate framework (ZIF)-8 precursors and achieved complete atomic dispersion of FeN 4 sites, the sole Fe species in the catalyst based on Mößbauer spectroscopy data. The Fe–N–C catalyst with the highest density of active sites achieved respectable ORR activity in rotating disk electrode (RDE) testing with a half-wave potential ( E 1/2 ) of 0.88 ± 0.01 V vs. the reversible hydrogen electrode (RHE) in 0.5 M H 2 SO 4 electrolyte. The activity degradation was found to be more significant when holding the potential at 0.85 V relative to standard potential cycling (0.6–1.0 V) in O 2 saturated acid electrolyte. The post-mortem electron microscopy analysis provides insights into possible catalyst degradation mechanisms associated with Fe–N coordination cleavage and carbon corrosion. High ORR activity was confirmed in fuel cell testing, which also divulged the promising performance of the catalysts at practical PEFC voltages. We conclude that the key factor behind the high ORR activity of the Fe–N–C catalyst is the optimum Fe content in the ZIF-8 precursor. While too little Fe in the precursors results in an insufficient density of FeN 4 sites, too much Fe leads to the formation of clusters and an ensuing significant loss in catalytic activity due to the loss of atomically dispersed Fe to inactive clusters or even nanoparticles. Advanced electron microscopy was used to obtain insights into the clustering of Fe atoms as a function of the doped Fe content. The Fe content in the precursor also affects other key catalyst properties such as the particle size, porosity, nitrogen-doping level, and carbon microstructure. Thanks to using model catalysts exclusively containing FeN 4 sites, it was possible to directly correlate the ORR activity with the density of FeN 4 species in the catalyst.more » « less
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Proton-exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) are promising power sources from portable electronic devices to vehicles. The high-cost issue of these low-temperature fuel cells can be primarily addressed by using platinum-group metal (PGM)-free oxygen reduction reaction (ORR) catalysts, in particular atomically dispersed metal–nitrogen–carbon (M–N–C, M = Fe, Co, Mn). Furthermore, a significant advantage of M–N–C catalysts is their superior methanol tolerance over Pt, which can mitigate the methanol cross-over effect and offer great potential of using a higher concentration of methanol in DMFCs. Here, we investigated the ORR catalytic properties of M–N–C catalysts in methanol-containing acidic electrolytes via experiments and density functional theory (DFT) calculations. FeN 4 sites demonstrated the highest methanol tolerance ability when compared to metal-free pyridinic N, CoN 4 , and MnN 4 active sites. The methanol adsorption on MN 4 sites is even strengthened when electrode potentials are applied during the ORR. The negative influence of methanol adsorption becomes significant for methanol concentrations higher than 2.0 M. However, the methanol adsorption does not affect the 4e − ORR pathway or chemically destroy the FeN 4 sites. The understanding of the methanol-induced ORR activity loss guides the design of promising M–N–C cathode catalyst in DMFCs. Accordingly, we developed a dual-metal site Fe/Co–N–C catalyst through a combined chemical-doping and adsorption strategy. Instead of generating a possible synergistic effect, the introduced Co atoms in the first doping step act as “scissors” for Zn removal in metal–organic frameworks (MOFs), which is crucial for modifying the porosity of the catalyst and providing more defects for stabilizing the active FeN 4 sites generated in the second adsorption step. The Fe/Co–N–C catalyst significantly improved the ORR catalytic activity and delivered remarkably enhanced peak power densities ( i.e. , 502 and 135 mW cm −2 ) under H 2 –air and methanol–air conditions, respectively, representing the best performance for both types of fuel cells. Notably, the fundamental understanding of methanol tolerance, along with the encouraging DMFC performance, will open an avenue for the potential application of atomically dispersed M–N–C catalysts in other direct alcohol or ammonia fuel cells.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2CC04628H
