Active Sites For Oxygen Reduction Reaction Research Articles

Self-assembled flower-like MnO2 grown on Fe-containing urea-formaldehyde resins based carbon as catalyst for oxygen reduction reaction in alkaline direct methanol fuel cells

The urea-formaldehyde resins carbon (UFC) and Fe-containing UFC (Fe-UFC) have been prepared by direct carbonization of urea-formaldehyde resins (UF resins) without any surfactants. On this basis, N-doped materials of UFC/MnO2 and Fe-UFC/MnO2 composites were successfully synthesized by the situ redox method. UF resins played both roles of carbon source and nitrogen source. The specific surface areas are 265.7 m2·g−1 and 229.4 m2·g−1 for UFC/MnO2 and Fe-UFC/MnO2, respectively. Direct methanol fuel cells (DMFCs) were assembled with the prepared catalyst as cathode catalyst, polymer fiber membrane (PFM) as electrolyte film and PtRu/C as anode catalyst. DMFCs and cyclic voltammetry (CV) performance tests indicate that Fe-UFC/MnO2-based DMFC displays superior oxygen reduction reaction (ORR) catalytic activity than UFC/MnO2 owing to Fe-Nx existing and a large number of pyridine-N in Fe-UFC/MnO2. The lone pair electrons of pyridine-N can coordinate with transition metal elements and form Fe-Nx groups on Fe-UFC/MnO2, which can be used as the electrocatalytic active sites for ORR. On the other hand, the presence of iron on the prepared catalyst can reduce the band gap, facilitate the transfer from MnIII to MnIV, and provide free electrons for adsorbed O2 or active oxygen species, thus accelerating the conductivity and ORR catalytic efficiency.

Biowaste derived activated carbon electrocatalyst for oxygen reduction reaction: Effect of chemical activation

Conversion of bio-wastes to useful doped carbon materials for various energy applications is emerging as a cost-effective strategy. A novel activated carbon electrocatalyst derived from chicken feather rachis (RCF), a poultry industry bio-waste is explored for oxygen reduction reaction (ORR) catalysis. The rachis is the central stem from which the fibrous ramus is completely removed and it is more crystalline compared to feather ramus. Nitrogen doped activated carbon (CNAx) electrocatalyst is prepared by chemical activation coupled pyrolysis. The chemical activators used include potassium hydroxide (KOH), phosphoric acid (H3PO4) and zinc chloride (ZnCl2) followed by pyrolysis at 500, 700 and 900 °C. Electrochemical performance has been evaluated using cyclic voltammetry (CV) and linear sweep voltammetry (LSV) using a rotating disk electrode (RDE). KOH activated electrocatalyst exhibits remarkable improvement in surface area favouring improved onset potential (−0.02 V vs Ag/AgCl). This increased activity is due to increase in number of well-exposed ORR active sites on activation. The effect of chemical activators on the structure and morphology of the activated carbons are discussed using Raman spectroscopy, adsorption-desorption isotherm study, electron microscopic techniques, atomic force microscopy (AFM), and XPS studies. KOH activated CNAx-900 exhibits best combination of properties and confirms its feasibility to be a suitable electrocatalyst for PEMFC. Hence, RCF derived electrocatalysts are propitious alternates for ORR catalysis.

Unique Half Embedded/Exposed PdFeCu/C Interfacial Nanoalloy as High‐Performance Electrocatalyst for Oxygen Reduction Reaction

AbstractDesign of high‐performance non‐Pt electrocatalyst for fuel cell applications is greatly anticipated. Herein, we have developed a unique half‐embedded and half‐exposed interfacial PdFeCu nanoalloy anchored onto carbon matrix. The stable electronic coupling between the carbon matrix and PdFeCu nanoalloy possess very fast interfacial electron transfer which in turn enhances the electron conductivity. This makes the trimetallic nanoalloy high performing oxygen reduction reaction (ORR) electrocatalyst in both basic and acidic media. The PdFeCu/C nanoalloy exhibits enhanced electrochemically active surface area than various PdFe/C bimetallics as well as benchmark 20 wt% Pt/C and Pd/C. As a result, it offers larger active sites for ORR and eased the electron transport during the electrocatalysis. It exhibits 1.5‐ and 2.4‐fold higher mass activity in comparison to the Pt/C and Pd/C. Furthermore, it exhibits long‐term stability and low onset potential compared to those of the other catalysts. Thus, the present investigation shows potential strategy for the design and synthesis of Pt‐free electrocatalyst with remarkable catalytic activity and stability.

Facile synthesis of nitrogen, sulfur dual-doped porous carbon via carbonization of coal tar pitch and MgCl2·6H2O for oxygen reduction reaction

Developing heteroatom-doped carbon catalysts toward oxygen reduction reaction (ORR) to replace platinum (Pt)-based catalysts is a promising strategy. Herein, nitrogen (N), sulfur (S) dual-doped porous carbons (NS@C-x-y) were prepared via a one-step carbonization of the mixture of acid treatment coal tar pitch (CTP) and MgCl2·6H2O, through in situ self-generating MgO template route. N and S were successfully introduced into the carbon framework of CTP by mixed acid treatment, leading to N, S co-doping especially pyridinic-N, graphitic-N, and C-S-C as ORR active sites. Benefiting from the fluffy lump graphitic structure, large surface areas, and synergistic effect between N and S electrocatalytic sites, the resulting samples demonstrate superior ORR performances with the highest diffusion-limited current density of 5.97 mA cm−2 and half-wave potential of 0.82 V (vs. RHE), long-term stability, and excellent methanol tolerance in alkaline medium, which are competitive with Pt/C catalyst.

Catalytic Activity of Titanium and Ruthenium Oxide Nanosheets in the Oxygen Reduction Reaction

Abstract:Monolayer molecular electrodes composed of titanium oxide nanosheets (TiO2ns) or ruthenium oxide nanosheets (RuO2ns) were prepared and their activities in the oxygen reduction reaction (ORR) were evaluated to investigate the ORR active sites in oxide catalysts. In TiO2ns, the influence of physical distortion sites in the crystal structure formed by introducing oxygen vacancies was determined. The ORR activity of TiO2ns was improved by introducing physical distortion sites. In RuO2ns, the effects of both the type of crystal structure and electrochemical distortion sites arising from redox reactions on ORR performance were studied. The type of crystal structure had almost no effect on ORR activity. In contrast, electrochemical distortion sites were expected to behave as the ORR active sites because the on-set potential of the ORR was similar to the redox peak position for the RuO2ns. Thus, the distortion sites in oxide crystal structures may behave as the active sites in the ORR independent of the metal species.

A Phthalocyanine‐Based Layered Two‐Dimensional Conjugated Metal–Organic Framework as a Highly Efficient Electrocatalyst for the Oxygen Reduction Reaction

Layered two-dimensional (2D) conjugated metal-organic frameworks (MOFs) represent a family of rising electrocatalysts for the oxygen reduction reaction (ORR), due to the controllable architectures, excellent electrical conductivity, and highly exposed well-defined molecular active sites. Herein, we report a copper phthalocyanine based 2D conjugated MOF with square-planar cobalt bis(dihydroxy) complexes (Co-O4 ) as linkages (PcCu-O8 -Co) and layer-stacked structures prepared via solvothermal synthesis. PcCu-O8 -Co 2D MOF mixed with carbon nanotubes exhibits excellent electrocatalytic ORR activity (E1/2 =0.83 V vs. RHE, n=3.93, and jL =5.3 mA cm-2 ) in alkaline media, which is the record value among the reported intrinsic MOF electrocatalysts. Supported by in situ Raman spectro-electrochemistry and theoretical modeling as well as contrast catalytic tests, we identified the cobalt nodes as ORR active sites. Furthermore, when employed as a cathode electrocatalyst for zinc-air batteries, PcCu-O8 -Co delivers a maximum power density of 94 mW cm-2 , outperforming the state-of-the-art Pt/C electrocatalysts (78.3 mW cm-2 ).

A Phthalocyanine‐Based Layered Two‐Dimensional Conjugated Metal–Organic Framework as a Highly Efficient Electrocatalyst for the Oxygen Reduction Reaction

AbstractLayered two‐dimensional (2D) conjugated metal–organic frameworks (MOFs) represent a family of rising electrocatalysts for the oxygen reduction reaction (ORR), due to the controllable architectures, excellent electrical conductivity, and highly exposed well‐defined molecular active sites. Herein, we report a copper phthalocyanine based 2D conjugated MOF with square‐planar cobalt bis(dihydroxy) complexes (Co‐O4) as linkages (PcCu‐O8‐Co) and layer‐stacked structures prepared via solvothermal synthesis. PcCu‐O8‐Co 2D MOF mixed with carbon nanotubes exhibits excellent electrocatalytic ORR activity (E1/2=0.83 V vs. RHE, n=3.93, and jL=5.3 mA cm−2) in alkaline media, which is the record value among the reported intrinsic MOF electrocatalysts. Supported by in situ Raman spectro‐electrochemistry and theoretical modeling as well as contrast catalytic tests, we identified the cobalt nodes as ORR active sites. Furthermore, when employed as a cathode electrocatalyst for zinc–air batteries, PcCu‐O8‐Co delivers a maximum power density of 94 mW cm−2, outperforming the state‐of‐the‐art Pt/C electrocatalysts (78.3 mW cm−2).

In-situ enriching active sites on co-doped Fe-Co4N@N-C nanosheet array as air cathode for flexible rechargeable Zn-air batteries

The sluggish oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) kinetics in air cathode severely refrain the development of the reversible Zn-air battery. Herein, we report the design and synthesis of co-doped Fe-Co4N@N-C nanosheet array derived from MOF precursor on carbon cloth as the dual-functional electrocatalyst to boost the reaction kinetics. The Fe, N co-doping significantly promotes the generation of abundant Pyridinic-N-M active sites for ORR due to the strong coordination effect between metal center and pyridinic nitrogen. The enriched Co3+ sites concurrently motivate the formation of targeted *OOH intermediates during OER with decreased charge transfer resistance. Such electrocatalyst therefore delivers high catalytic activity for both ORR and OER. When applied as air cathode for liquid Zn-air battery, the device exhibits a high specific energy density of 934 Wh kg−1 with excellent cycling stability, which is superior to the referenced Pt and Ru-based Zn-air batteries. Flexible solid-state Zn-air battery can also achieve a high volumetric power density of 72 mW cm-3 and good cycling durability under different bending states. This work gives a facile and cost-efficient strategy to construct bifunctional flexible electrode with enriched active sites for Zn-air batteries.

Boosting ORR/OER Activity of Graphdiyne by Simple Heteroatom Doping

AbstractNitrogen‐doped graphdiynes recently emerged as promising metal‐free oxygen reduction reaction (ORR) electrocatalysts. However, which type of N‐dopants contributing to the enhanced catalytic performance and the catalytic performances of other heteroatom‐doped graphdiynes has not been explored systematically. Herein, ORR and oxygen evolution reaction (OER) catalytic performances of X‐doped graphdiynes are examined by means of DFT computations (X = B, N, P, and S). It is revealed that the graphitic S‐doped graphdiyne (Model S1), the sp‐N‐doped graphdiyne (Model N3) and the graphitic P‐doped graphdiyne (Model P1) exhibit comparable or even better ORR/OER activities than Pt/C or RuO2, with ORR activity trend as Model S1 > Pt/C > Model N3 and OER trend as Model P1 > RuO2 > Model N3. The carbon atoms near N‐ and S‐dopants and featuring large positive charge are the ORR active sites in Models N3 and S1, whereas the carbon atoms near N‐ and P‐dopants and possessing high spin are the OER active sites in Models N3 and P1. Overall, this study not only gains deep insights into the catalytic activity of N‐doped graphdiyne for ORR, but also guides developing of graphdiyne‐based ORR/OER catalysts beyond N‐doping.

Structure-Function Relationships of PGM-Free ORR Electrocatalysts from Density Functional Theory

Pyrolyzed platinum group metal free (PGM-free) oxygen reduction reaction (ORR) electrocatalysts are a promising class of earth-abundant materials for low-temperature polymer electrolyte fuel cell (PEFC) cathodes. Understanding of the atomic scale structure of PGM-free ORR active sites remains a key focus of research efforts with the aim of improving performance of these materials. The pyrolysis process leads to highly heterogeneous catalyst systems which complicates direct active site study. In particular, (i) understanding how these active sites give rise to ORR activity, (ii) how they interact with the environment during material degradation, (iii) their interactions with probe molecules for the purposes of active site quantification, and (iv) their spectroscopic signatures are all important aspects governed by atomic scale structure. Through the use of density functional theory (DFT), we investigate these four structure-function relationships. ORR activity, one of the greatest challenges faced by PEFCs, is explored via several binding energy parameterized descriptor models. These models include the computational hydrogen electrode (CHE) model which yields thermodynamic limiting potentials, and the linearized Gibbs energy relationship (LGER) model which yields reversible potentials for reaction steps. Additionally, kinetics of OOH bond dissociation via nudged elastic band (NEB) DFT calculations are also considered as this has been proposed by some groups to be rate determining for pathways that include binding to local C sites. Combined, these models give insight into how local arrangement of atoms affects reaction pathways and enables exploration of varied reaction pathways on and local to the active site. Durability of PGM-free electrocatalysts remains a key challenge in these materials. While a variety of degradation mechanisms have been proposed, the relation between environment and degradation mechanism has not been firmly established, complicating any mitigation strategies that might be applied. Through the use of an automated ab initio molecular dynamics (AIMD)-based model, the kinetics of bond breaking local to the active site is explored. Resulting degraded structures can then be studied via the activity models previously mentioned to show how such degradation affects calculated ORR activity descriptors. Additionally, the use of reaction rate models applied to activity loss curves can also enable some discrimination of degradation mechanism indicating either a single autocatalytic process or two mechanisms with different temporal behavior. Another issue faced by PGM-free electrocatalysts is the lack of a method for quantifying the number of ORR active sites. Unlike Pt-based systems where integration of charge transferred in the H adsorption/desorption region can give information about density of active sites, no such method has been fully established for determining the number of PGM-free active sites, a value required to deconvolute turn over frequency from ORR current densities. Promising approaches include use of probe molecules that, if bound to ORR active sites (and only ORR active sites) could provide insight into how many active sites are in a given sample. This is highly dependent on the specificity of binding for these molecules which can be explored via binding energy calculations with DFT. Finally, DFT can also be a valuable tool for understanding spectroscopic signatures of highly ORR active materials. The main issue faced in such studies is focusing on just the active sites which generally requires considering changes in the Fe states, particularly for so called atomically dispersed catalysts that exhibit the highest ORR activities to date. X-ray adsorption spectroscopy (XAS, in particular XANES and EXAFS) can give information about local structure and the state of Fe with and without probe molecules. DFT and related theoretical approaches can simulate these spectra as a function of local environment. Vibrational calculations with and without probe molecules give insight into Fe-specific nuclear resonance vibrational spectroscopy (NRVS). Calculation of energy density at the Fe nucleus and electric field gradient gives input regarding isomer shift and quadrupole splitting, giving much needed interpretation of Mössbauer spectroscopy, especially in instances where no standard exists or changes in measured parameters with ligands/probes are required. Combined, these coupled experimental/theoretical approaches give insight into the nature of active sites in PGM-free systems. In particular, in this contribution we will focus on the presence of spontaneously evolved ligands that, using DFT, are shown to be likely contributors to electrocatalyst activity in-situ and experimental signatures thereof.

Simultaneous SECM-AFM-Ters Mapping of Electrocatalytic Reactions on a Nanometric Scale

We have developed and used Scanning Electrochemical Microscopy-Atomic Force Microscopy (SECM-AFM) imaging technique that can provide high resolution detailed electrochemical and spectral information of oxygen reduction reaction (ORR) reaction at a lateral resolution of less than 50 nm. The ORR activity of an individual unsupported Pt nanoparticle, carbon-supported Pt aggregates and Fe, N surface modified graphitic carbon was mapped under selected reduction conditions. This newly developed method utilizes a gold coated SiO2 tip imbedding a 50 nm diameter Pt electrode. positioned at a constant working distance of ~4-8 nm above the catalyst surface using force feedback control. A 532 nm Raman laser directed to the apex of the tip was applied to collect tip enhanced Raman spectral (TERS) signal of the reaction byproduct (H2O2). Thus, we simultaneously mapped the electrochemical oxygen reduction by the Pt tip electrode and measured the H2O2 oxidation currents on the gold coating secondary electrode and at the same time reordered the peroxide tip enhanced Raman signal under the same scanned area. A direct evidence was seen, showing for the first time that the same active ORR sites are also responsible for peroxide formation (figure 1). Moreover, on a nanometric level ORR proceed mostly via 2 electron reaction to hydrogen peroxide rather than the literature reported 4 electrons reduction of O2 to water, as widely accepted from other electrochemical techniques. Fig. 1. Nanoscale Mapping of Catalytic Hotspots on Fe, N -modified HOPG by Scanning Electrochemical Microscopy-Atomic Force Microscopy [1]. [1] Srikanth Kolagatla, Palaniappan Subramanian and Alex Schechter*, Nanoscale, 2018, 10, 6962 Figure 1

Influence of the Dopant Atom and the Oxygen Vacancy Site for the Oxygen Reduction Reaction on the Titanium Oxide

Polymer electrolyte membrane fuel cells (PEMFCs) are expected as one of the next generation power sources because its exhaust gas is, theoretically, only water. On the other hand, the selling price of the PEMFC mounted devices expensive, and this price inhibits the wide-spread commercialization. It is one reason for this thing that the platinum-based catalyst, which is high cost and low durability, is used as the electrocatalyst and is required large quantity to obtain the cell performance. As one of the ultimate solutions for this problem, usage of the non-platinum oxide catalyst is proffered. Recently, the group 4 and 5 transition-metal oxides are attracted attention as the non-platinum oxide catalyst since these show the activity of oxygen reduction reaction (ORR) and the high durability. In these oxides, introducing of the oxygen vacancy site in oxide surface to get high ORR activity is the common design guide for the new catalyst development. Addition, it is well known that doping niobium to the titanium oxide with the oxygen vacancy site is also effective for obtaining the high ORR activity. In other words, introducing the oxygen vacancy site and doping the other metal atom is necessary to prepare for the high ORR catalyst based on group 4 and 5 transition-metal oxide in this time. Although the active site for ORR on the oxide surface is not clear, it is speculated that the oxygen vacancy site would play the important role for ORR. However, the relationship between the dopant atom and the oxygen vacancy site has not been discussed enough in the viewpoint from ORR. In fact, the dopant atom in the previous works of literature in acidic environmental is almost the niobium, and other atoms are very few. The other hand, it is reported the calculated formation energy of oxygen vacancy site on anatase Ti M O2 (101) depends on the dopant atom. Therefore, this study describes the influence of the dopant atom and the oxygen vacancy site on the ORR activity of titanium oxide. In this study, niobium, manganese, and ruthenium were chosen as the dopant atom because their formation energy of oxygen vacancy site decreased step by step in literature. All samples were prepared by the typical sol-gel method. The oxygen vacancy site was introduced by the chemical reduction method using the sodium borohydride. Electrochemical studies were conducted in Ar- or O2-saturated 0.1 M HClO4 (60ºC) with carbon plate counter-electrode and Ag/AgCl reference-electrode. The ORR activity was evaluated by the difference current of current in Ar-saturated condition from the current in O2-saturated condition. The on-set potential of ORR current was notated as E on-set in bellow. Crystal structures before chemical reduction treatment were a mixture of anatase and rutile in all dopant species. The crystallinity of samples was decreased by chemical reduction treatment, but the clear phase transition did not observe. In the case of niobium doping, the E on-set was increased to ~0.6 V from ~0.4 V by the chemical reduction treatment. This phenome agreed with previous works, and it is thought that the enhancement effect was provided by the formation of the oxygen vacancy site. The ruthenium doped titanium oxide was also started ORR at ~0.4V. Thus, the difference of enhancement effect between niobium and ruthenium for ORR activity of titanium oxide was little before introducing the oxygen vacancy site. After introducing the oxygen vacancy site, E on-set of ruthenium doped titanium oxide was ~0.75 V. In addition, its ORR current was the highest in this study. If the ruthenium dopant was removed from titanium oxide lattice and formed the ruthenium oxide during the chemical reduction treatment, its quantity of ruthenium oxide is very low. Because the doping ratio of ruthenium was 5 mol%. Moreover, the E on-set of ruthenium oxide was ~0.6 V which was a lower value than that of reduced ruthenium doped titanium oxide. After the introducing oxygen vacancy site, the enhancement effect for ORR ability depended on the dopant species. Herein, the formation energy of oxygen vacancy site of niobium doped titanium oxide was higher than that of ruthenium doped titanium oxide. In ruthenium doped titanium oxide with oxygen vacancy site, it is speculated that the oxygen molecule is easier adsorption/desorption in comparison with niobium doped titanium oxide. At this moment, we think that the ORR activity of titanium oxide depends by the dopant atom but needs not only dopant species but also the oxygen vacancy site.

Nuclear Resonance Vibrational Spectroscopy and Mössbauer Spectroscopy Studies of Atomically Dispersed (AD)57fe-N-C Oxygen Reduction Reaction Catalysts for Polymer Electrolyte Fuel Cells

Polymer electrolyte fuel cells (PEFCs) are a key technology to realize a hydrogen-based economy in the transportation sector, expected to significantly reduce greenhouse gas emission. Platinum (Pt) continues to be the electrocatalyst of choice for oxygen reduction reaction (ORR) in PEFCs, despite the fact that the scarcity, high cost, and monopolized global distribution of this metal severely impede its large-scale commercialization. As an alternative approach, substantial efforts have been made to move towards PGM-free catalysts, the best performing of which are heat-treated iron, nitrogen, carbon (Fe-N-C) materials. While promising, the Fe-N-C catalysts face significant challenges on the way to becoming viable. One of the challenges is the need for identifying the ORR active site as a prerequisite for further development of Fe-N-C catalysts. The nature of the active site has puzzled researchers for decades, with the very presence of Fe being often questioned in the highly acidic environment of the PEFCs. Demonstrating the presence of surface Fe and its coordination environment would thus represent a major step towards a better understanding of the origins of catalytic activity in PGM-free catalysts. In this work, we applied Fe-specific characterization techniques, nuclear resonance vibrational spectroscopy (NRVS) and Mössbauer spectroscopy (MS) for the purpose of detecting Fe on the surface of Fe-N-C catalysts. While NRVS and MS are bulk methods, the use of a differential technique, in which the spectrum for catalyst recorded without an Fe-specific surface probe is subtracted from the probe-treated material, allows for obtaining spectrum representative of the surface Fe only. In the case described in this presentation, we applied this differential technique using NO (an O2 analog) as a probe of the surface Fe. Then, by combining the NRVS and MS signatures with DFT modeling we were able to obtain electronic and structural information for the surface Fe sites. For the described approach to be successful it is critical for Fe in the catalyst to remain in a single chemical form, relevant to the ORR. A majority of PGM-free synthesis methods based on heat-treatment of a mixture of iron, nitrogen, and carbon yield highly heterogeneous materials, containing Fe in various chemical forms, including in particular Fe-rich nanoparticles. These nanoparticles contribute to the NRVS and MS signals, making detection of the ORR-relevant atomically dispersed surface Fe species difficult or altogether impossible. To address this challenge, LANL recently developed a nanoparticle-free, metal organic framework (MOF)-derived PGM-free ORR catalyst, (AD)Fe-N-C. For the purpose of NRVS and MS experiments, this catalyst was also fully enriched with the 57Fe isotope of iron. The (AD)57Fe-N-C catalyst was then treated with NO as a probe of surface Fe sites and characterized using differential NRVS and MS. In this presentation, we will summarize the results of NRVS and MS experiments that attest to the presence of Fe sites on the surface of the atomically dispersed Fe-N-C catalysts for ORR. Acknowledgements This research is supported by DOE Fuel Cell Technologies Office, through the Electrocatalysis Consortium (ElectroCat). This research used resources of the Advanced Photon Source (APS) at sector 3, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357.

Well-dispersed iron oxide stabilized Fe[sbnd]N4 active sites in porous N-doped carbon spheres as alternative superior catalyst for oxygen reduction

The fabrication of highly active oxygen reduction reaction (ORR) catalysts with strong anti-poisoning and durability at low cost is extremely desirable but still remains a great challenge. Herein, a novel hierarchically structured material of N-doped porous carbon spheres embedded with monodispersed Fe2O3 clusters (Fe2O3/NPCS) is constructed. The composition and microstructure of Fe2O3/NPCS are carefully characterized, and the existed Fe2O3-stabilized FeN4 active sites for ORR are verified. The Fe2O3/NPCS catalyst exhibits a superior ORR activity with a positive half-wave potential of 0.95 V, showing an obvious around 90 mV positive-shift over commercial Pt/C (20 wt%). Additionally, the high ORR catalytic activity is also accompanied by a smaller Tafel slope of 45.3 mV decade−1 than Pt/C, implying a fast kinetics at high potentials. Furthermore, the high-active catalyst also demonstrates an outstanding anti-poisoning capacity and long-term durability. The excellent ORR activites are attributed to the synergistic effect between the rich FeN4 active sites protected by Fe2O3 clusters, the porous structures facilitating electron transport as well as the electrolyte and oxygen diffusion.

Effect of particle size of carbon catalyst on oxygen reduction activity for fuel cell applications

Graphite powders with particle sizes of 40, 100, 400, and 1000 nm were used as cathode catalysts. The influence of graphite particle size on the electrocatalytic behavior towards the oxygen reduction reaction (ORR) was investigated. The catalytic activity and kinetics of the ORR, and methanol tolerance were tested in alkaline solutions with saturated oxygen via cyclic voltammetry (CV), rotating disk electrode (RDE) and amperometric i-t curve techniques. Experimental results show that the ORR process of the graphite catalyst with a particle size of 40 nm was close to the four-electron pathway at -0.6 V, exhibiting the best ORR activity. This is ascribed to a greater number of edge defects as ORR active sites in the carbonmaterial. The number of sites can be characterized with X-ray diffraction (XRD) and Raman spectroscopy. Additionally, these graphite catalysts have been demonstrated a superior tolerance to methanol than the commercial 20 wt.% Pt/C electrode in alkaline solutions.

Atomically dispersed Fe-N-P-C complex electrocatalysts for superior oxygen reduction

Development of cost-effective electrocatalysts as an alternative to platinum for oxygen reduction reaction (ORR) is of great significance for boosting the applications of green energy devices such as fuel cells and metal-air batteries. Here we report a nitrogen and phosphorus tri-doped hierarchically porous carbon supported highly cost-effective, efficient and durable Fe single-site electrocatalyst derived from biomass. Combined aberration-corrected HAADF-STEM, XPS and XAFS measurements and theoretical calculations reveal the atomically dispersed Fe-N-P-C-O complex as the dominant active sites for ORR. This work also shows the design principle for enhancing the ORR activity of single Fe site catalysts with higher Fe charge, which can be manipulated by the coordinated structure in the active centre. Theoretical calculations reveal that the main effective sites are singleN-P-O-Fe-O centers, where the associated P-O-Fe bond can significantly lower the stability of strongly adsorbed O* and OH* on the catalytically active sites and thus give rise to enhanced ORR performance. The insights reported here open a new avenue for constructing highly efficient molecule-like heterogeneous catalysts in electrochemical energy technologies.

Bottom-Up Construction of Active Sites in a Cu-N4-C Catalyst for Highly Efficient Oxygen Reduction Reaction.

Bottom-up construction of efficient active sites in transition metal-nitrogen-carbon (M-N-C) catalysts for oxygen reduction reaction (ORR) from single molecular building blocks remains one of the most difficult challenges. Herein, we report a bottom-up approach to produce a highly active Cu-N4-C catalyst with well-defined Cu-N4 coordination sites derived from a small molecular copper complex containing Cu-N4 moieties. The Cu-N4 moieties were found to be covalently integrated into graphene sheets to create the Cu-N4 active sites for ORR. Furthermore, the activity was boosted by tuning the structure of active sites. We find that the high ORR activity of the Cu-N4-C catalyst is related to the Cu-N4 center linked to edges of the graphene sheets, where the electronic structure of the Cu center has the right symmetry for the degenerate π* orbital of the O2 molecule. These findings point out the direction for the synthesis of the M-N-C catalysts at the molecular level.

Freestanding 1D Hierarchical Porous Fe‐N‐Doped Carbon Nanofibers as Efficient Oxygen Reduction Catalysts for Zn–Air Batteries

High‐performance nonprecious metal‐doped 1D carbon catalysts for oxygen reduction reactions (ORR) are viable candidates in lieu of platinum‐based catalysts. Hierarchical porous Fe‐N‐doped carbon nanofibers (Fe‐NHCFs) are fabricated via carbonization of MOF nanofibers with a specific surface area of 294 m2 g−1 and inherent hierarchical porosity. Benefiting from the Fe‐N doping‐induced active sites, unique hierarchical porosity, and 1D honeycomb conductive networks to facilitate electron transfer, the freestanding Fe‐NHCFs confer a current density (5.2 mA cm−2) at 0.70 V (vs reversible hydrogen electrode), comparable with the commercial 20 wt% Pt/C (5 mA cm−2) in alkaline medium. Especially, the parallel characteristics with Pt/C is acquired in an assembled Zn–air battery, which deliver a discharge current density of 90 mA cm−2 and an output peak power density of 61 mW cm−2. This simple synthesis strategy would leverage a new geometry for tailored utility of active sites for ORR in a 1D carbon framework.

Probing the Active Sites of Carbon‐Encapsulated Cobalt Nanoparticles for Oxygen Reduction

AbstractGreat effort has been contributed to exploring efficient and cost‐effective oxygen reduction reaction (ORR) catalysts for fuel cell applications in the past decades. Now various electrocatalysts can be synthesized for high‐performance ORR catalysis. However, the identification of the ORR active sites in many nonprecious metal‐based catalysts is still difficult. This is due to the heterogeneity and complexity of the catalyst structures. For example, the active site of core–shell ORR electrocatalysts has been a continuously debatable issue, hampering the exploration of new ORR catalysts. Herein, a carbonized Co metal organic framework (Co@C) is used to uncover the ORR active sites in core–shell electrocatalysts. The surface Co particles in the Co@C sample are removed by HCl wash, and the Co cores are removed using an electrochemical activation method. The characterizations reveal that both the samples before and after the electrochemical activation show the existence of single Co species. The corresponding electrocatalysis test results indicate that neither the surface Co particles nor the encapsulated Co cores influence the ORR performance of the samples. It is deduced that the single Co species coordinated with the nitrogen in the carbon layers of the core–shell catalysts are the actual ORR active sites.

Why and how to tailor the vertical coordinate of pore size distribution to construct ORR-active carbon materials?

Pore size distribution and specific surface area of carbon-based materials are extensively reported to play a key role in catalyzing oxygen reduction reaction (ORR). As far as pore size distribution is concerned, carbon materials with the same pore size ranges always show different ORR performances. Why? It is well accepted that there are two coordinates for pore size distribution, horizontal and vertical ones. The former is related to the pore size range and studied maturely, and the latter is closely related to the quantity of any kind of specific pores (including meso-/micro-/macro-pore) contributed to the pore volume or area, i.e., the variation of the vertical coordinate of pore size distribution is closely linked to the variation of the meso-/micro-/macro-pore volumes, which should make great contribution to the mass transfer and density of ORR active sites of carbon materials and never be reported. Herein, this work synthesizes a series of N, P co-doped micro/mesoporous carbon materials with tunable/unalterable/negligible meso-/micro-/macro-pore volumes to pioneer the relationship of the mesopore volume on behalf of the vertical coordinate of pore size distribution with the - mass transfer and ORR active site density of such metal-free heteroatom-doped porous carbon materials and investigate why and how to tailor the coordinates of pore size distribution to construct ORR-active carbon materials i.e., the mass transfer and density of ORR active site are improved via tailoring the vertical coordinate of pore size distribution. We also develop a universal pore-forming strategy for extensively synthesizing carbon materials with expected pore size range (horizontal coordinate) and mesopore volume (vertical coordinate) from widely available biomass in nature to guide the synthesis and selection of carbon-based ORR-active materials to meet the demands of high-performance ORR electrocatalysts.