Single Metal Site Research Articles

17O-EPR determination of the structure and dynamics of copper single-metal sites in zeolites

The bonding of copper ions to lattice oxygens dictates the activity and selectivity of copper exchanged zeolites. By 17O isotopic labelling of the zeolite framework, in conjunction with advanced EPR methodologies and DFT modelling, we determine the local structure of single site CuII species, we quantify the covalency of the metal-framework bond and we assess how this scenario is modified by the presence of solvating H216O or H217O molecules. This enables to follow the migration of CuII species as a function of hydration conditions, providing evidence for a reversible transfer pathway within the zeolite cage as a function of the water pressure. The results presented in this paper establish 17O EPR as a versatile tool for characterizing metal-oxide interactions in open-shell systems.

Chemical Vapor Deposition for N/S-Doped Single Fe Site Catalysts for the Oxygen Reduction in Direct Methanol Fuel Cells

Single metal site catalysts are the most promising candidates to replace platinum-group-metal (PGM) catalysts for the oxygen reduction reaction (ORR), yet insufficient performance and scalable preparation approaches remain great challenges. Here, we report a nitrogen (N)/sulfur (S) codoped single Fe site catalyst (Fe–N/S–C) through a chemical vapor deposition (CVD) strategy. Using the cyclopentadiene-shielded Fe atom ferrocene (Fc) as the precursor, atomically dispersed single Fe sites were successfully embedded into the N, S codoped 2D carbon nanosheets. The superior catalytic activity for the ORR in alkaline media is stemmed from the N, S codoping, tuning the optimal charge distribution of Fe sites. In addition, the CVD approach could surpress the formation of iron-carbide-containing iron clusters (“FexC/Fe”), thereby leading to high surface areas and porosity. Furthermore, the Fe–N/S–C catalyst was further studied as a cathode catalyst in direct methanol fuel cells showing encouraging performance.

Tailoring the redox capabilities of organic ligands for metal-ligand coordination with vanadium single-sites

The creation of single-site metal centers (SSMCs) through the formation of metal-organic coordination networks is an area of interest due to the proven ability of SSMCs to improve selectivity for heterogeneous catalysts. In order to better understand the reactivity potential for the SSMCs it is necessary to study the ligand-metal interaction in the metal-organic coordination networks. In the work reported here, we demonstrate the ability to tune the oxidation state of vanadium from II to IV through the tailoring of redox-active ligands. Using the N-heterocyclic ligands of bipyrimidine (BP), bispyrimidinyltetrazine (BMTZ), and biimidazole (BIM) complexed with metallic V, we have shown that the oxidation state of the V metal centers can be tuned to V(II) for BP, V(III) for BMTZ, and V(IV) for BIM. These redox-active ligands provide similar coordination environments when complexed into one dimensional chains but result in different oxidation states for the single-site metal center.

Microenvironment and Nanoreactor Engineering of Single-Site Metal Catalysts for Electrochemical CO2 Reduction

Electrochemical CO2 reduction simultaneously achieves conversion and storage of electrical energy as chemical energy through carbon recycling. Electrocatalysts with single-site and high efficiency for CO2 reduction have recently attracted much attention. In this review, the rational design strategies of single-site metal catalysts (SSMCs) for CO2 reduction are summarized. Particularly, we present the microenvironment engineering of SSMCs which entails atomic-scale design and nanoreactor construction. We further discuss the catalytic mechanisms, effects of the types, and coordination environments among metal species on tailoring catalytic activity and those nanoreactor features toward high-performance electrochemical CO2 reduction. Finally, we overview the current challenges and prospects of single-site metal catalysts for electrochemical CO2 reduction.

Noble-Metal-Free Nanostructured and Graphene Supported Electrocatalysts for Water Splitting

Clean hydrogen production through water splitting is a direct and promising sustainable energy pathway to access and store non-fossil chemical fuels. Synthetic design strategies to access low cost, robust and high performance noble metal-free electrocatalysts are vital for forthcoming water splitting applications. To accomplish this goal, the nanoscale design of multifunctional catalysts based on cost-effective graphene supports, hollow nano-architectures and well-coordinated hybrid organic-inorganic structures has been attracting increasing research interest because of the flexible tuning options of their electronic states, chemical composition and local coordination environments.Over the last years, we have been implementing systematic investigations based on synthetic design, spectroscopy approaches and computational modeling toward the understanding of the mechanism of action in a series of novel, robust and efficient bi-functional electrocatalysts.Recently, we explored synergistic effects of NiFe alloys encapsulated into N-doped carbon nanotubes (N-CNTs) supported on reduced graphene (rGO) nanosheets for overall water splitting.[1] This so-called NiFe-N-CNT-rGO electrocatalyst was synthesized through a direct and flexible strategy using g-C3N4 as a precursor, where thermally reduced NiFe nanoparticles act as surface nucleation sites for C species to assist N-CNT growth through an unconventional reduction-nucleation-growth mechanism. The catalyst has a low overpotential of 270 mV for oxygen evolution reaction (OER), being superior in performance to a wide range of reported noble-metal-free catalysts. As revealed from density functional theory (DFT) calculations, N-doping and charge transfer at the CNT walls tune the free energy in the electronic structure toward adsorption of intermediates, which greatly enhanced catalytic performance. This synthetic strategy not only leads to excellent performance and opens up new avenues for other carbon-supported alloy catalysts, but its scalability and easy applicability also render it quite promising for large-scale water splitting applications.In parallel, we explored molecular strategies to conjugate single active sites of metal-phthalocyanine molecules MPc (M=Ni, Co, Fe) on GO.[2] We aim for a thorough understanding of the role of the local coordination environment around the single metal site in the synergistic mechanisms and the catalytic performance. With an advanced combination of ADF (annular dark field)-STEM, operando X-ray absorption spectroscopy and DFT calculations, we observed changes in the electronic and local structure environment of MPc-GO under operando electrochemical OER conditions. Single active metal Ni, Co, Fe sites attain high valence states due to chemisorbed OH− species upon catalytic activation, while the local orders suffer from structural distortion. These changes in the electronic and structural properties lead to the formation of HO-M-N4 moieties with high OER reactivity under operando conditions. These results provide new insight into the fundamental understanding of structure-performance relationships for single active site catalysts.Likewise, we studied hollow transition metal sulfide nanoboxes synthesized through anion exchange processes[3] between S2- and CN- species that lead to the formation of Prussian blue (PB), M-S@PB (M=Co, Fe, Ni) heterostructures. We found that the high performance of those nanostructures arises from in situ formation of active Co-Fe oxide/hydroxide species during OER, attaining a superior performance compared to noble-metal-catalysts such as RuO2, with a low overpotential of 286 mV and high durability over longer operational periods. The flexible tuning options of the electronic, chemical and local-atomic coordination of those nanostructures render them promising hybrid catalysts for dual OER and HER over an extended pH range.We have further explored new soft templating strategies for the first 1D cobalt coordination polymer electrocatalyst with bio-inspired features, and we applied state-of-the-art analytical techniques and computational modeling to monitor its formation in situ and to unravel its structure.[4] The highly durable 1D-cobalt coordination polymer catalyst implements strategies to transfer key motifs of inorganic materials into a hydrogen-bonded ligand environment, namely the embedment of the essential {H2O-Co2(OH)2-OH2} edge-site motif of cobalt oxide catalysts into a flexible and robust organic matrix. While this unconventional catalyst shows exceptional performance in commercial alkaline electrolyzers and organic transformations, its flexible structure offers new pathways toward the rational design of mixed molecular-solid disordered catalysts.[1] W. Wan, S. Wei, J. Li, C. A. Triana, Y. Zhou, G. R. Patzke, J. Mater. Chem. A 2019 (7) 15145.[2] W. Wan, C. A. Triana, J. Lan, J. Li, C. S. Allen, Y. Zhao, M. Iannuzzi, G. R. Patzke, ACS Nano 2020 (14) 13279.[3] Y. Zhao, C. K. Mavrokefalos, P. Zhang, R. Erni, J. Li, C. A. Triana, G. R. Patzke, Chem. Mater. 2020 (32) 1371.[4] C.A. Triana, R. More, A. J. Bloomfield, P. V. Petrovic, S. G. Ferroon, G. Stanley, S. D. Zaric, T. Fox, E. N. Brothers, S. W. Sheehan, P. T. Anastas, G. R. Patzke, Matter 2019 (1) 1354.

Atomically Dispersed Single Metal Site Catalysts for Oxygen Reduction in Acidic Media

Proton exchange membrane fuel cells (PEMFCs) are leading candidates in utilizing clean energy resources for application in transportation, stationary, and portable devices. In PEMFCs, cathode catalysts are crucial for overall performance and durability due to kinetically slow oxygen reduction reactions (ORR). Because platinum (Pt), a state-of-the-art ORR catalyst, is rare and expensive, the development of high-performance platinum metal group (PGM)-free catalysts is highly desirable for future fuel cell technologies. Among various PGM-free catalyst formulations, metal and nitrogen co-doped carbon (M-N-C, M: Fe, Co, or Mn) catalysts have exhibited encouraging activity and stability in acidic media for the ORR, which possess great potential to replace Pt in the future. Therefore, based on our extensive experience in the field of ORR catalysis, the talk will comprehensively summarize the basic principles in the design and synthesis of M-N-C catalysts for durable, inexpensive, and high-performance PEMFCs with an emphasis on Co- and Mn-N-C catalysts to avoid Fenton reactions between Fe2+ and H2O2, which can generate free radicals and lead to the degradation of catalysts, ionomers, and membranes in PEMFCs. Furthermore, template-free 3D hydrocarbon frameworks as attractive precursors to advanced M-N-C catalysts will be discussed to enhance intrinsic ORR activities in acidic media significantly. Also, the long-term performance durability of M-N-C cathodes will be discussed extensively to provide potential solutions to enhance catalyst stability in PEMFCs. Finally, this talk will provide an overall perspective on the progress, challenges, and solutions of PGM-free catalysts for future PEMFC technologies.

Synergy of Pt-Free Single Metal Sites for Promoting Pt and Pt3co Ordered Intermetallic Catalysts for Fuel Cells: Performance and Durability Improvements

The reduction of platinum use and improvement of its catalytic performance has become the most critical steps to accelerate the development of proton-exchange membrane fuel cells (PEMFCs). Here we report a novel and facile method to boost the performance of the Pt-cased catalyst by integrating with an atomic platinum-group metal (PGM)–free active sites. Using iron-doped zeolitic imidazolate framework-8 (ZIF-8) derived carbon as support, we achieved a uniform deposition of Pt and intermetallic Pt-Co nanoparticles (NPs) through one-step synthesis, with average particle size 2.4 nm for Pt, 3.4 nm for L10 PtCo, and 4.2 nm for L12 Pt3Co. Besides, easy phase transfer during synthesis between ordered intermetallic structures L10 PtCo and L12 Pt3Co was achieved, and a comprehensive comparison between their catalytic performance was established. Synergistic catalysis between Pt or Pt-Co NPs over a PGM–free catalytic substrate derived from iron-doped ZIF-8 led to excellent oxygen reduction reaction (ORR) performance under both rotating disk electrode (RDE) and membrane electrode assembly (MEA) testing. Resulting pure Pt catalysts achieved ORR mass activities of 0.451 A/mgPt and retained 80% initial values after 30,000 voltage cycles in a fuel cell, superior to the DOE 2020 targets without using Pt alloy. Both PtCo and Pt3Co catalysts achieved better performance and exhibited all-sided excellence among mass activity, stability, and power density. Among them, Pt3Co reached the power density at 0.67 V of 923 mW/cm2 and retained 86% initial mass activity after 30,000 voltage cycles in a fuel cell. Their performance dependences on support material particle size and ionomer content were discussed and optimized in MEAs.

Bioinspired Catalytic Reduction of Aqueous Perchlorate by One Single-Metal Site with High Stability against Oxidative Deactivation

Reduction of perchlorate (ClO4–) with an active and stable catalyst is of great importance for environmental, energy, and space technologies. However, after the rate-limiting oxygen atom transfer (OAT) from inert ClO4–, the much more reactive ClOx– (x ≤ 3) intermediates can cause catalyst deactivation. The previous Re–Pd/C catalyst contained a [ReV(O)(hoz)2]+ site (Hhoz = 2-(2′-hydroxyphenyl)-2-oxazoline) and readily reduced ClO4–, but ClOx– intermediates led to rapid formation and hydrolysis of [ReVII(O)2(hoz)2]+. While microbes use delicate enzymatic machinery to survive the oxidative stress during ClO4– reduction, a synthetic catalyst needs a straightforward self-protective design. In this work, we introduced a methyl group on the ligand oxazoline moiety and achieved a substantial enhancement of catalyst stability without sacrificing the performance of ClO4– reduction. A suite of kinetics measurement, X-ray photoelectron spectroscopy characterization, reaction modeling, stopped-flow photospectrometry, and 1H NMR monitoring revealed the underlying mechanism. The most critical and unexpected effect of the methyl group is the deceleration (for 2 orders of magnitude) of OAT from ClO3– to [ReV(O)(Mehoz)2]+. However, the rate of OAT with ClO4– was not affected. The methyl group also slowed down the hydrolysis of [ReVII(O)2(Mehoz)2]+ and allowed the introduction of methoxy onto the phenolate moiety to further accelerate ClO4– reduction. With 1 atm H2 at 20 °C, the Re–Pd/C catalyst used [ReV(O)(MehozOMe)2]+ as the only reaction site to reduce multiple spikes of 10 mM ClO4– into Cl– without decomposition. This work showcases the significant effect of simple ligand modification in improving catalyst stability for high-performance ClO4– reduction.

Supported Single Atom Catalysts for C−H Activation: Selective C−H Oxidations, Dehydrogenations and Oxidative C−H/C−H Couplings

AbstractThe activation of C−H bonds in hydrocarbons (saturated, unsaturated and aromatics) by single atom catalysts immobilized in a solid support is an active research front in catalysis. On the one hand, different examples of transition metal catalysts atomically dispersed on inorganic, organic and hybrid supports are constantly designed, synthesized and characterized using advanced microscopy and spectroscopic techniques. On the other hand, active, selective and stable single metal sites are recently developed as high performant heterogeneous catalysts for the C−H activation of organic molecules. This review covers recent examples of selective alkane and arene C−H oxidations, dehydrogenations of alkanes, as well as C−H/C−H oxidative couplings involving arenes, alkenes and alkynes, based on the use of supported single atom catalysts.

Carbon Dioxide Electroreduction: Efficient Electron Transfer from Electron‐Sponge Polyoxometalate to Single‐Metal Site Metal–Organic Frameworks for Highly Selective Electroreduction of Carbon Dioxide (Small 20/2021)

By combining the superiority of polyoxometalate (POM) and single-metal site Co in porphyrin, a POM-based metal–organic framework composite has been constructed by Wen-Wen He, Ya-Qian Lan, Shengqian Ma, and co-workers (article number 2100762). Directional electron transfer from POM to the single-metal site Co in PCN-222(Co) accelerates electron-transfer efficiency thereby affording high electrocatalytic CO2 reduction reaction activity.

The modulating effect of N coordination on single-atom catalysts researched by Pt-Nx-C model through both experimental study and DFT simulation

N-doped carbon-based single-atom catalysts (NC-SACs) are widely researched in various electrochemical reactions due to high metal atom utilization and catalytic activity. The catalytic activity of NC-SACs originates from the coordinating structure between single metal site (M) and the doped nitrogen (N) in carbon matrix by forming M-Nx-C structure (1≤ x≤ 4). The M-N4-C structure is widely considered to be the most stable and effective catalytic site. However, there is no in-depth research for the “x” modulation in Pt-Nx-C structure and the corresponding catalytic properties. Herein, atomically dispersed Pt on N-doped carbon (Pt-NC) with Pt-Nx-C structure (1≤ x≤ 4), as a research model, is fabricated by a ZIF-8 template and applied to catalytic oxygen reduction. Different carbonization temperatures are used to control N loss, and then modulate the N coordination of Pt-Nx-C structure. The Pt-NC has the predictable low half-wave potential (E1/2) of 0.72 V vs RHE compared to the Pt/C 20% of 0.81V due to low Pt content. Remarkably, the Pt-NC shows a high onset potential (1.10 V vs RHE, determined for j = -0.1 mA cm2) and a high current density of 5.2 mA cm−2, more positive and higher than that of Pt/C 20% (0.96 V) and 4.9 mA cm−2, respectively. As the structural characterization and DFT simulation confirmed, the reducing Pt-N coordination number induces low valence of Pt atoms and low free energy of oxygen reduction, which is responsible for the improved catalytic activity. Furthermore, the Pt-NC shows high mass activity (172 times higher than that of Pt/C 20%), better stability and methanol crossover resistance.

Water oxidation with transition metal catalysts with non-innocent ligands and its mechanisms

• Transition metal catalysts derived from non-innocent ligands in water oxidation. • Role of redox active non-innocent ligands are discussed. • Various types of mechanisms are illustrated for such an oxidation process. • Cyclic voltametric, kinetic, electrochemical and theoretical studies are considered. Transition metal complexes acting as catalysts towards water oxidation concern a challenging research topic with high relevance in solar and fuel conversion technologies. Non-innocent ligands with redox behavior can contribute with an important role on this field. We review herein the use as catalysts of complexes (containing single or double metal sites) with such a type of ligands on water oxidation and the corresponding reaction mechanisms as established by kinetic, electrochemical and theoretical studies.

A Visible-Light-Harvesting Covalent Organic Framework Bearing Single Nickel Sites as a Highly Efficient Sulfur-Carbon Cross-Coupling Dual Catalyst.

Covalent Organic Frameworks (COFs) have recently emerged as light-harvesting devices, as well as elegant heterogeneous catalysts. The combination of these two properties into a dual catalyst has not yet been explored. We report a new photosensitive triazine-based COF, decorated with single Ni sites to form a dual catalyst. This crystalline and highly porous catalyst shows excellent catalytic performance in the visible-light-driven catalytic sulfur-carbon cross-coupling reaction. Incorporation of single transition metal sites in a photosensitive COF scaffold with two-component synergistic catalyst in organic transformation is demonstrated for the first time.

A Visible‐Light‐Harvesting Covalent Organic Framework Bearing Single Nickel Sites as a Highly Efficient Sulfur–Carbon Cross‐Coupling Dual Catalyst

AbstractCovalent Organic Frameworks (COFs) have recently emerged as light‐harvesting devices, as well as elegant heterogeneous catalysts. The combination of these two properties into a dual catalyst has not yet been explored. We report a new photosensitive triazine‐based COF, decorated with single Ni sites to form a dual catalyst. This crystalline and highly porous catalyst shows excellent catalytic performance in the visible‐light‐driven catalytic sulfur–carbon cross‐coupling reaction. Incorporation of single transition metal sites in a photosensitive COF scaffold with two‐component synergistic catalyst in organic transformation is demonstrated for the first time.

Efficient Electron Transfer from Electron-Sponge Polyoxometalate to Single-Metal Site Metal-Organic Frameworks for Highly Selective Electroreduction of Carbon Dioxide.

In this work, by combining the superiority of polyoxometalates (POMs) and catalytic single-metal site Co of metalloporphyrin, a series of mixed-valence POM-based metal-organic frameworks (MOFs) composites is synthesized by a post-modification method. The electron-transfer property of POM@PCN-222(Co) composite is significantly enhanced owing to the directional electron-transfer from POM to single-metal site Co in PCN-222(Co). In particular, H-POM@PCN-222(Co) gives a high Faradaic efficiency of 96.2% for electroreduction of CO2 into CO and good stability over 10h. DFT calculations confirm that the directional electron transfer, which accelerates the multi-electron transfer from the electrode to active single-metal site Co, enriches the electron density of the Co center, and ultimately reduces the energy of the rate-determining step, thus increasing the catalytic activity of CO2 reduction reaction (CO2 RR). This work therefore suggests some new insight for the design of efficient electrocatalysts for CO2 RR.

Carbon‐Supported Single‐Atom Catalysts: Carbon‐Supported Single Metal Site Catalysts for Electrochemical CO2 Reduction to CO and Beyond (Small 16/2021)

The carbon-supported single metal site catalysts have great potential for electrochemical CO2 reduction due to their flexibility to regulate metal centers, coordination environments, and local structures for enhancing catalytic activity and selectivity. Yi Mei, Gang Wu, and co-workers report that besides the high efficiency of CO2 to CO conversion, single metal sites could provide a synergy to design composite catalysts for producing other C2 products (article number 2005148).

Simultaneous oxidative and reductive reactions in one system by atomic design

Single-atom catalysts often exhibit unexpected catalytic activity for many important chemical and biological reactions with respect to their bulk counterparts, and have been recognized as potential substitutes for natural enzymes. Here we report a biomimetic composite, yolk–shell Pd1@Fe1, that features two compatible single-atom systems with atomically dispersed Fe1 sites in a N-doped carbon shell and Pd1 sites in a yolk derived from a metal–organic framework. Directly utilizing the O2 and H2 sources generated on-site from the electrocatalytic overall water splitting, the as-synthesized yolk–shell Pd1@Fe1 could simultaneously catalyse nitroaromatic hydrogenation and alkene epoxidation reactions and lead to a cascade synthesis of amino alcohols. Our findings provide a versatile strategy to integrate different single metal sites within one system to allow the continuous and easy synthesis of complex compounds for various challenging reactions. Single-atom catalysts can exhibit improved catalytic performance with respect to their bulk counterparts. Now, the authors introduce a yolk@shell catalyst with spatially separated Pd and Fe single sites that simultaneously catalyse nitroaromatic hydrogenation and alkene epoxidation reactions, leading to a cascade synthesis of amino alcohols.

Carbon-Supported Single Metal Site Catalysts for Electrochemical CO2 Reduction to CO and Beyond.

The electrochemical CO2 reduction reaction (CO2 RR) is a promising strategy to achieve electrical-to-chemical energy storage while closing the global carbon cycle. The carbon-supported single-atom catalysts (SACs) have great potential for electrochemical CO2 RR due to their high efficiency and low cost. The metal centers' performance is related to the local coordination environment and the long-range electronic intercalation from the carbon substrates. This review summarizes the recent progress on the synthesis of carbon-supported SACs and their application toward electrocatalytic CO2 reduction to CO and other C1 and C2 products. Several SACs are involved, including MNx catalysts, heterogeneous molecular catalysts, and the covalent organic framework (COF) based SACs. The controllable synthesis methods for anchoring single-atom sites on different carbon supports are introduced, focusing on the influence that precursors and synthetic conditions have on the final structure of SACs. For the CO2 RR performance, the intrinsic activity difference of various metal centers and the corresponding activity enhancement strategies via the modulation of the metal centers' electronic structure are systematically summarized, which may help promote the rational design of active and selective SACs for CO2 reduction to CO and beyond.

Synthesis Strategies, Catalytic Applications, and Performance Regulation of Single‐Atom Catalysts

AbstractThe recent dramatic increase in research on isolated metal atoms has received extensive scientific interest in the new frontier of single‐atom catalysis. As newly advanced materials in catalysis, single‐atom catalysts (SACs) have received enormous interest from the perspectives of both scientific research and industrial applications due to their remarkable activity. In addition, other catalytic properties of single metal atoms, including stability and selectivity, can be further improved by tuning their electronic/geometric structures and modulating the metal–support interactions. SACs usually consist of dispersed atoms and appropriate support materials, which are employed to anchor, confine, and/or coordinate with isolated metal atoms. Therefore, the nature of single metal sites allows acquiring a maximum atom utilization approaching 100%, which is of significance, particularly for the development of noble‐metal‐based catalysts. In order to systematically understand the structure–property relationships and the underlying catalytic mechanisms relationship of SACs, the representative scientific research efforts in their synthesis strategies, catalytic applications, and performance regulation are discussed here. Typical single‐atom catalysis processes and the corresponding mechanisms in electrochemistry, photochemistry, organic synthesis, and biomedicine are also summarized. Finally, the challenges and prospects for the development of single‐atom catalysis and SACs are highlighted.

Active site engineering of single-atom carbonaceous electrocatalysts for the oxygen reduction reaction.

The electrocatalytic oxygen reduction reaction (ORR) is the vital process at the cathode of next-generation electrochemical storage and conversion technologies, such as metal–air batteries and fuel cells. Single-metal-atom and nitrogen co-doped carbonaceous electrocatalysts (M–N–C) have emerged as attractive alternatives to noble-metal platinum for catalyzing the kinetically sluggish ORR due to their high electrical conductivity, large surface area, and structural tunability at the atomic level, however, their application is limited by the low intrinsic activity of the metal–nitrogen coordination sites (M–Nx) and inferior site density. In this Perspective, we summarize the recent progress and milestones relating to the active site engineering of single atom carbonous electrocatalysts for enhancing the ORR activity. Particular emphasis is placed on the emerging strategies for regulating the electronic structure of the single metal site and populating the site density. In addition, challenges and perspectives are provided regarding the future development of single atom carbonous electrocatalysts for the ORR and their utilization in practical use.