Electrochemical Oxygen Reduction Reaction Research Articles

Site-specific reactivity of stepped Pt surfaces driven by stress release.

Heterogeneous catalysts are widely used to promote chemical reactions. Although it is known that chemical reactions usually happen on catalyst surfaces, only specific surface sites have high catalytic activity. Thus, identifying active sites and maximizing their presence lies at the heart of catalysis research1-4, in which the classic model is to categorize active sites in terms of distinct surface motifs, such as terraces and steps1,5-10. However, such a simple categorization often leads to orders of magnitude errors in catalyst activity predictions and qualitative uncertainties of active sites7,8,11,12, thus limiting opportunities for catalyst design. Here, using stepped Pt(111) surfaces and the electrochemical oxygen reduction reaction (ORR) as examples, we demonstrate that the root cause of larger errors and uncertainties is a simplified categorization thatoverlooks atomic site-specific reactivity driven by surface stress release. Specifically, surface stress release at steps introduces inhomogeneous strain fields, with up to 5.5% compression, leading to distinct electronic structures and reactivity for terrace atoms with identical local coordination, and resulting in atomic site-specific enhancement of ORR activity. For the terrace atoms flanking both sides of the step edge, the enhancement is up to 50 times higher than that of the atoms in the middle of the terrace, which permits control of ORR reactivity by either varying terrace widths or controlling external stress. Thus, the discovery of the above synergy provides a new perspective for both fundamental understanding of catalytically active atomic sites and design principles of heterogeneous catalysts.

Co(O)4(N)‐type single‐atom‐based catalysts and ligand‐driven modulation of electrocatalytic properties for reducing oxygen molecules

AbstractSingle‐atom‐based catalysts are intriguing electrocatalytic platforms that combine the advantages of molecular catalysts and conductive carbon‐based materials. In this work, hybrids (Co‐NrGO‐1 and Co‐NrGO‐2) were generated by wet‐reactions between organometallic complexes (Co(CH3COO)2 and Co[CH3(CH2)3CH(C2H5)COO]2, respectively) and N‐doped reduced graphene oxide at 25°C. Various characterizations revealed the formation of atomically dispersed Co(O)4(N) species in Co‐NrGO‐2. Density functional theory (DFT) calculations explained the effect of the aliphatic C7 group in Co2 on the formation processes. The Co‐NrGO‐2 hybrid showed excellent catalytic performance, such as onset (0.94 V) and half‐wave (0.83 V) potentials, for electrochemical oxygen reduction reaction (ORR). Co‐NrGO‐2 outperformed Co‐NrGO‐1, which was explained by more back donation to the antibonding orbitals of O2 from electron‐rich aliphatic groups. DFT calculations support this feature, with mechanistic investigations showing favored ORR reactions and facile breakage of double bonds in O2.

Enriched Ti3+ in TiONC electrocatalyst for efficient H2O2 generation via 2-electron oxygen reduction reaction

The 2-electron electrochemical oxygen reduction reaction (ORR) is considered a viable method for synthesizing H2O2 because of its efficiency and environmental friendliness. However, conventional catalysts for this reaction predominantly rely on high-cost metals and require complex preparation procedures. Herein, we synthesized an electrocatalyst containing TiN and N-doped carbon via the pyrolysis of melamine and MIL125(Ti)–NH2. The effects of melamine and pyrolysis temperature on the chemical composition and defect site formation were investigated. Importantly, the obtained results suggest that the generated Ti3+ sites are more important in enhancing the performance of the 2-electron ORR, surpassing defective carbon and oxygen functional groups. In particular, the resulting TiONC-MA-1000 catalyst prepared through pyrolysis at 1,000 °C contained abundant Ti3+ sites, resulting in high 2-electron ORR activity with a molar selectivity exceeding 91 % and high stability over a 12 h testing period. The electrochemical H2O2 generation experiments conducted in an H-cell configuration employing TiONC-MA-1000 as the electrocatalyst yielded 6.99 mol⋅gcatalyst−1⋅h−1 H2O2. Therefore, the present study proposes a facile approach for fabricating highly active electrocatalysts suitable for large-scale H2O2 production.

Single-Atom Co─O4 Sites Embedded in a Defective-Rich Porous Carbon Layer for Efficient H2 O2 Electrosynthesis.

The production of hydrogen peroxide (H2 O2 ) via the two-electron electrochemical oxygen reduction reaction (2e- ORR) is an essential alteration in the current anthraquinone-based method. Herein, a single-atom Co─O4 electrocatalyst is embedded in a defective and porous graphene-like carbon layer (Co─O4 @PC). The Co─O4 @PC electrocatalyst shows promising potential in H2 O2 electrosynthesis via 2e- ORR, providing a high H2 O2 selectivity of 98.8% at 0.6V and a low onset potential of 0.73V for generating H2 O2 . In situ surface-sensitive attenuated total reflection Fourier transform infrared spectra and density functional theory calculations reveal that the electronic and geometric modification of Co─O4 induced by defective carbon sites result in decreased d-band center of Co atoms, providing the optimum adsorption energies of OOH* intermediate. The H-cell and flow cell assembled using Co─O4 @PC as the cathode present long-term stability and high efficiency for H2 O2 production. Particularly, a high H2 O2 production rate of 0.25molg-1 cat h-1 at 0.6V can be obtained by the flow cell. The in situ-generated H2 O2 can promote the degradation of rhodamine B and sterilize Staphylococcus aureus via the Fenton process. This work can pave the way for the efficient production of H2 O2 by using Co─O4 single atom electrocatalyst and unveil the electrocatalytic mechanism.

ZnMn2O4 spinel nanocrystals-decorated multi-walled carbon nanotubes for oxygen reduction: Experimental and theoretical studies on the strong coupling facilitated four-electron selectivity

In this work, we have successfully formulated a single-step hydrothermal synthesis of mixed transition metal oxide nanocrystals (ZnMn2O4) strongly coupled on carbon support (MWCNTs) that acts as an efficient electrocatalyst for the electrochemical oxygen reduction reaction (ORR). The results depict that the composite, MWCNTs@ZnMn2O4 shows better electrocatalytic activity as compared to the activity exhibited by its individual components MWCNTs and ZnMn2O4 combined, attributed to the strong coupling between the two components. Transmission electron images of the composite depict that ZnMn2O4 nanocrystals have been successfully integrated on MWCNTs resulting in high conductivity and activity. The composite, MWCNTs@ZnMn2O4 exhibits an onset potential of 0.9 V vs. RHE, 99 % four-electron ORR selectivity, and high stability. The high selectivity and stability are assigned due to the strong coupling of ZnMn2O4 to MWCNTs through Zn/Mn–O–C bonds which are proved from DFT studies. The presence of Zn/Mn–O–C bonds is verified from X-ray photoelectron spectroscopy.

Enhanced Kinetics of Oxygen Reduction Reactions in Alkaline Solutions inside Mesoporous Electrodes

Oxygen reduction reactions (ORR) in alkaline solutions attract increasing attention for their use as the cathodic reaction in metal-air batteries and alkaline fuel cells. However, the commercialization of these systems is impeded by high activation and concentration overpotentials of ORR, which originate from the sluggish kinetics on the catalyst surface and oxygen transport in the electrolyte, respectively. Although the practical electrode enables a decrease of the overpotentials with the porous structure, the effect of inner pores has not been understood. Among various pore sizes, it is crucial to investigate an electrochemical behavior in mesopores with a size of a few nm since such confined structures possibly change the intrinsic catalytic kinetics.1 In this study, we evaluated the ORR activity in the mesopore using Pt model electrodes with arrays of cylindrical pores with uniform pore sizes (Fig. a). We investigated the activity dependence on the electrode thickness to separate activation and concentration overpotentials, and found that the activation overpotential inside the mesopore with a size of 1.8 nm is smaller than that measured with the planer electrode. Pt model electrodes were electrodeposited from platinum salt in a liquid crystalline phase of a surfactant.2 The liquid crystalline phase was obtained by mixing 29 wt% of H2O, 29 wt% of hexachloroplatinic acid, and 42 wt% of octaethylene glycol monododecyl ether. The three-electrode cell consisted of an Au disk (φ 4 mm) working electrode, a Pt wire counter electrode, and an Ag|AgCl|sat’d KClaq reference electrode. The deposition was conducted at –0.055 V and 25 °C, and the charge densities were adjusted to obtain the electrode thickness of ca. 45, 100, 380, and 900 nm. Transmission electron microscopy (TEM) was performed to observe the pore structure. Electrochemical surface area (ECSA) and ORR activities were evaluated by cyclic voltammetry in N2/O2-saturated 0.1 mol dm–3 KOH solutions with a rotating disk electrode system, consisting of the porous model or a polished planar Pt working electrode, a Pt wire counter electrode, and a Hg|HgO reference electrode. The scan rate and rotation speed were 20 mV s–1 and 2000 rpm, respectively. TEM image exhibits the honeycomb arrays of mesopores with a pore size of 1.8 nm and a pore distance of 4.9 nm (Fig. a). The model electrodes possessed polycrystalline facets, and ECSA was calculated from the hydrogen adsorption peak area in blank voltammetry (Fig. b). The ECSA matched closely with the geometrically calculated surface area, suggesting that the cylindrical pores are uniformly distributed on the electrodes with defined pore lengths (Fig. c). ORR activities were evaluated with the Tafel plots (Fig. d), with current densities corrected for ECSA and concentration overpotential outside the mesopores. Since the oxygen transport resistance or the concentration overpotential is higher in longer pores, the electrodes with longer pore lengths caused smaller ORR current densities. In contrast, the concentration overpotential was suppressed enough to extract the activation overpotential when the pore length was 45 nm and 100 nm, and these electrodes led to higher ORR current density than the planar electrode. The result indicates that the activation overpotential inside the mesopore with a size of 1.8 nm is smaller than that measured with the planer electrode. We will report the origin of the enhanced kinetics inside the mesopore from the viewpoint of the electronic structure change.

Elucidation of the Interplay between the Features of the Precursor and the Physicochemical Properties of “Core-Shell” Hierarchical Carbon Nitride Electrocatalysts for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is a major bottleneck in the operation of proton-exchange membrane fuel cells (PEMFCs) since it introduces large overpotentials (no less than 250-300 mV), which degrade significantly the energy conversion efficiency of the device. In addition, the electrocatalysts (ECs) yielding the lowest ORR overpotentials in the acidic environment found at the PEMFC cathode are based on Pt, a very scarce critical raw material (CRM). Pt raises concerns as it might trigger serious supply bottlenecks in the event of a practical widespread implementation of PEMFCs in the framework of today’s energy transition. Hence, the development of high-performing and durable ECs for the ORR is a major stepping stone towards the large-scale rollout of PEMFCs.In our research group a new breakthrough approach has been devised for the synthesis of ORR ECs, that is completely different form the preparation routes that are commonly adopted in the state of the art [1, 2]. Briefly, the approach consists in the pyrolysis and subsequent treatment of a precursor obtained by coupling suitable carbon species/sacrificial supports with a zeolitic inorganic-organic polymer electrolyte (Z-IOPE). The latter comprises anionic complexes including the Pt “catalyst” and other “co-catalysts”, which are bridged by organic binders [1,2].On one hand, the proposed approach has already shown its potential to yield ECs exhibiting a performance and durability improved with respect to Pt/C benchmark ECs. On the other hand, the proposed approach is extremely flexible, especially with respect to the synthesis of the initial EC precursor. In principle, it is possible to modulate a number of crucial features of such EC precursor, with a particular reference to: (i) the chemical composition (e.g., in terms of Pt, other “co-catalysts” and N; the latter plays a crucial role to stabilize the active sites into “coordination nests” on the EC surface, raising durability); and (ii) the morphology, that can be “templated” on suitable species (e.g., carbon species, sacrificial supports). The very flexibility of the proposed synthetic process needs to be leveraged appropriately to maximize the performance of the final ORR ECs, at the same time minimizing the efforts wasted on approaches with a low development potential.Herein it is elucidated how the modulation in the parameters involved in the synthesis of the EC precursor (e.g., the binder and the approaches to couple the Z-IOPE with the support) impact the physicochemical and electrochemical properties of the final ECs, with a particular reference to the chemical composition, morphology and distribution of the ORR active sites. An extensive characterization of the final ECs is carried out. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and microanalysis are adopted to determine the bulk chemical composition. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is used to elucidate the surface chemical composition and the chemical states of the surface elements. Ultra-high resolution scanning electron microscopy (UHR-SEM) and transmission electron microscopy (TEM) allow for the elucidation of EC morphology. The porosimetric features are probed by automatic gas adsorption instrumentations. Wide-angle X-ray diffraction (WAXD) is crucial to resolve the crystal structure. Cyclic voltammetry with the rotating ring-disk electrode method (CV-TF-RRDE) is used to clarify the “ex-situ” electrochemical performance and ORR reaction mechanism. Finally, the most promising ECs are used in the fabrication of single PEMFCs, that are tested as a function of the operating conditions.It is revealed that, in comparison with the Pt/C benchamark, some ECs are characterized by a much higher intrinsic kinetic performance due to the electronic and bifunctional effects bestowed by the carbon nitride support and the «co-catalysts». Finally, the binder has a marked effect on the stoichiometry, size, and distribution of the PtMx aggregates bearing the active sites.

From Synthesis to Application: Ni3 Fe N As Oxygen Reduction Electrocatalyst in Anion Exchange Membrane Fuel Cells

Catalysis has played a crucial role in advancing and commercializing energy conversion technologies. It is essential to identify abundant, active, and stable materials to enable the reliable and cost-efficient use of catalysts in renewable technologies such as fuel cells (FCs) and electrolyzers. Suitable candidates, like nonprecious metals, can be found in the transition metals' first row, where materials like bimetallics, metal oxides, and metal nitrides can be obtained. Recently, these materials have shown high activity toward the Oxygen Reduction Reaction (ORR) and the Oxygen Evolution Reaction (OER) in an alkaline medium, which can be related to good performance in FCs and electrolyzers. However, most of the studies do not go further than half-cell reactions. In this study, we explored the synthesis of a metal nitride, Ni3FeN, and its application as a cathode material in hydrogen fuel Anion Exchange Membrane Fuel Cells (AEMFC). A one-step ammonolysis synthesis was done to obtain Ni3FeN supported on carbon. Various metal loadings were synthesized with nanoparticles sizes of 7 ± 2 nm and 10 ± 4 nm for 40% and 60%, respectively, revealed with Transmission Electron Microscopy (TEM). High-resolution Scanning Transmission Electron Microscopy (STEM) images and Energy Dispersive X-ray (EDX) mapping analysis revealed a metal nitride core and a metal oxide shell of ca. 3 nm. The bulk electronic structure was further evaluated using X-ray Absorption Spectroscopy (XAS), where the EXAFS modeling suggests a bulk nitride electronic structure. Consequently, cyclic voltammetry (CV) was used to investigate the electrochemical properties and ORR performance of the Ni3FeN catalyst in 1 M KOH. The catalyst exhibited a stable electrochemical structure with a slight decrease in E1/2 of 10 mV (from 0.85 V to 0.84 V) during a 100 K ORR accelerated durability test (ADT). In addition, Levich and Rotating Ring Disk Electrode (RRDE) analyses showed that the number of electrons transferred during the ORR was consistent with a 4-electron pathway. Furthermore, the Ni3FeN catalyst was tested as the cathode in an AEMFC, obtaining a power density output of 400 mW/cm2, highlighting its potential for alkaline medium fuel cell applications.

(Invited) Effects of Nano-Structure on the Kinetics of the Oxygen Reduction over N-Doped Carbon

Nitrogen-doped carbon catalysts for the electrochemical oxygen reduction reaction (ORR) have great potential to substitute precious metal alloy catalysts for the electrochemical synthesis of H2O2 based on a fuel cell setup. Consequently, obtaining a kinetic understanding of nitrogen-doped carbon catalysts in acidic media is essential. Our research group has established a mathematically and experimentally modified RRDE approach to calculate the corresponding kinetic rate constants, which significantly reduced the calculation noise by avoiding a rotation speed study. Furthermore, the overestimation of the 4-e reduction process, which is related to the (2+2)-e pathway (k 2 + k 3) in the catalyst layer matrix, was corrected by studying the effect of the catalyst loading density. The established method was successfully applied to the ORR over Fe/N/C and N/C catalysts in acidic and alkaline media (Table 1). The results indicate that Fe/N/C catalysts follow mixed reduction pathways of 4-e and 2-e reductions in acidic media, and a pure 4-e reduction pathway in alkaline media, whereas the N/C catalyst exhibits a pure 2-e reduction pathway in acidic media, and mixed reduction pathways of 4-e and 2-e reductions in basic media. Among the catalysts mentioned above, the N/C catalyst is the most promising for the electrochemical synthesis. To minimizing the effect of k 3, introducing mesoporous structure would be beneficial, because the H2O2 produced by k 2 can be smoothly eliminated without the further reduction to H2O by k 3 (Figure 1). In this context, we have synthesized a N-doped mesoporous carbon by pyrolyzing a N and C source in a mesoporous silica, KIT-6, as a hard-template. This synthesis resulted in a mesoporous carbon with a gyroid structure. The electrochemical study has revealed that this catalyst exhibits a quite high selectivity over 90 % to the 2-e reduction in acidic media. Figure 1

(Invited) Enhanced Material Durability of Transition Metal-Antimony X-Ide Nanoparticles in Oxygen Reduction Electrocatalysis

Hydrogen fuel cells (FCs) are key energy devices that can accelerate our transition away from non-renewable, carbon-intensive fuels especially in the grid-scale storage and transportation sectors. FC cathodes perform the oxygen reduction reaction (ORR), and despite decades of progress towards higher performance electrocatalysts, the ORR remains kinetically sluggish and limits the efficiency of assembled FC devices. Modern, commercially available FC cathodes typically contain platinum (Pt)-based catalysts that can cause the overall FC to become prohibitively expensive for some applications so identifying alternate non-platinum group ORR catalyst formulations could increase deployment of FCs. Physics-based calculations with density functional theory (DFT) have predicted many first-row transition metal oxides (MOx, M = Mn, Fe, Cr, Ni, Co) as having favorable ORR performance trends. Additionally, there is experimental validation of nanoscale ternary and quaternary transition metal oxide systems with a mixture of transition metals used for catalyzing the ORR where relatively modest activity enhancements are reported. Despite this progress, an important limitation is present in the low material stability of non-precious transition metal oxides especially in acidic environments such as those encountered in proton exchange membrane FCs. Using synthetic material strategies to overcome such stability limitations in a variety of FC operating conditions will provide a deeper understanding of the relationship between structure, activity, and degradation.In this work, we employ two simultaneous strategies to work towards enhancing the activity and stability of transition metal oxides: (1) addition of antimony (Sb) as a ligand and (2) functionalization with nitrogen or sulfur. We utilized a previously reported colloidal synthesis for uniform ~ 50 nm oxide nanoparticles for manganese, nickel, cobalt, and iron to produce four sets of nanoparticles based on each of these metals. By using a colloidal synthesis that concurrently introduces Sb with the secondary metal and drying the particles afterwards, we incorporate Sb evenly throughout the polycrystalline structure of the of these nanoparticle catalysts. A high temperature surface modification reaction with reactive gas such as air, ammonia, or hydrogen sulfide was used to convert any remaining transition metal antimonate into an oxide, nitride, or sulfide, respectively. In total, twelve materials are used in this study: the oxide, nitride, and sulfide (collectively “X-ides”) of each of the four transition metal containing, antimony incorporated nanoparticles. We optimize this material synthesis and evaluate the nanoparticle catalyst’s ability to enhance electrochemical ORR activity in both acidic (pH 1) and alkaline (pH 13) conditions which are simulating operating conditions of proton-exchange membrane and anion-exchange membrane FCs, respectively. In addition to activity modulation, we measure the ORR selectivity change towards hydrogen peroxide formation for each material and note the role of nitrogen and sulfur modification in suppressing hydrogen peroxide formation in alkaline environments. In addition to electrochemical characterization, we employed a vast array of bulk and surface characterization techniques to understand catalyst dynamics and contextualize observed activity trends. Transmission electron microscopy (TEM) enabled determination of particle sizes and morphologies alongside electron diffraction patterns to validate the catalyst synthesis. X-ray diffraction (XRD) then provided further information about crystallinity that connected bulk-level insights to the local information from TEM. X-ray photoelectron spectroscopy (XPS) enabled determination of oxidation state changes in the transition metal and in Sb that serve as important descriptors for estimating the activity of these catalysts. We also employed in situ/operando techniques to probe catalyst dynamics and stability under various conditions. Using manganese K-edge synchrotron x-ray absorption spectroscopy (XAS) on the manganese antimony X-ides, we find significant changes to oxidation state and coordination of manganese in the nitride and sulfide as a function of potential indicating processes that displace nitrogen and sulfur in the lattice under certain conditions. Lastly, we use an in situ electrochemical flow cell coupled to inductively coupled plasma-mass spectrometry (ICP-MS) to measure corrosion as a function of ORR current density, potential sweeping, and electrolyte gas saturation to determine degradation patterns. Insights from all these techniques provide a fascinating perspective on what factors in a material and its environment are responsible for enhanced activity, selectivity, and stability. We perform DFT modeling of these systems to further elucidate material properties that are principally responsible for achieving key metrics in activity and stability especially as non-precious catalysts are evaluated for their performance in assembled device conditions. Combining electrochemical experiments, physics-based modeling, and material characterization to better understand structure-performance relationships is a promising path towards accelerating the process of materials discovery for eventual deployment in critical energy applications for the future.

Engineering FeCo Dual Sites on Tube-on-Plate Hollow Structure for Efficient Oxygen Electroreduction.

Atomically dispersed single-atom catalysts are intriguing catalysts in the field of electrocatalysis for nearly 100% exploitation of metal atoms. However, they are still far from practical usage due to the scaling relationship limit and metal loading limit. Generation of a diatomic complex would offer superior catalytic performance through the cooperation of two neighboring atoms as active sites. Herein, Fe/Co dual atomic sites embedded in a tube-on-plate hollow structure are designed and fabricated for an efficient electrochemical oxygen reduction reaction (ORR). The unique structure composed of ultrathin nanotube building blocks dramatically maximizes the surface area for copious active site exposure. Thanks to the synergetic interaction between Fe/Co pairs, the obtained FeCo/NC exhibits outstanding ORR activity and stability in alkaline media. Furthermore, density functional theory calculations have revealed that the remarkable activity is attributed to the electron-deficient Fe sites in FeCoN6. This work may pave the way for the innovative design of highly dispersed dual-site catalysts for broader applications in the realm of electrochemical catalysis.

Design strategies towards transition metal single atom catalysts for the oxygen reduction reaction – A review

The electrochemical oxygen reduction reaction (ORR) is pivotal in energy conversion <i>via</i> a 4e⁻ ORR pathway and green hydrogen peroxide production <i>via</i> 2e⁻ ORR pathway. Transition metal single atom catalysts (TM SACs) have attracted intense attention in recent years for ORR due to their high activity and near maximum metal atom utilization. The future development of TM SACs for ORR requires improved understanding of reaction pathways, since currently the true origin of activity remains contentious owing to the lack of qualitative/quantitative information about active sites. Knowledge-guided design is imperative for the optimization of TM SACs performance in terms of activity and selectivity. This review focuses on the latest progress in the design of TM SACs for ORR, placing particular attention on efforts to elucidate reaction mechanisms. Experimental evidence based on <i>in-situ/operando</i> characterization measurements, along with theoretical predictions, are summarized to deepen understanding of the structure-performance relationships at both atomic and molecular level. Finally, some perspectives are offered relating to the fundamental science needed for TM SACs to find practical application in energy storage and conversion devices. We hope this review will inspire the development of new synthetic routes towards high-performance ORR electrocatalysts for the energy sector.

Simultaneous ozone and hydrogen peroxide electrosynthesis via defect modulation in Ni, Sb‐doped SnO2 electrocatalysts

AbstractElectrochemical synthesis of green oxidants O3 and H2O2 is valuable for applications, but challenges persist in enhancing the O3 and H2O2 generation activity and combined application. Herein, we modulate the surface Ni active sites and oxygen vacancy defects content in Ni‐Sb‐SnO2 electrocatalysts to enhance selectivity for electrochemical ozone generation (EOP) and two‐electron electrochemical oxygen reduction reactions (2e− ORR). The Ni active sites and oxygen vacancy defects enriched electrocatalysts resulting in an ozone faradaic efficiency of 48.1%, while non‐enriched electrocatalyst obtained 90% selectivity for H2O2. Theoretical calculations revealed that Ni‐Sb‐SnO2 efficiently captures O2 with defective Ovac2 stabilize intermediates, facilitating O3 and H2O2 synthesis. Moreover, concerted EOP and 2e− ORR enable concurrent generation of O3 and H2O2 for efficient synergistic degradation of organic pollutants, while attenuating the energy demands of the electrolyzer. This study provides an appealing strategy for simultaneous production of O3 and H2O2 with applications in wastewater treatment.

4-6 carbophene: A two-dimensional carbon material with Dirac cone and superior activity for electrochemical oxygen reduction reactions

The positive exploration of efficient and cost-effective electrocatalysts for the oxygen reduction reaction (ORR) is crucial for the sustainable development of energy conversion and storage technologies. Recently, a new type of two-dimensional (2D) carbon crystals, called 4-6 carbophene, has been successfully synthesized. It consists of a four-carbon ring and a six-carbon ring. In this work, we have discovered that 4-6 carbophene is a two-dimensional Dirac cone material through density-functional theory (DFT) calculations. Therefore, we further investigated the oxygen reduction reaction on 4-6 carbophene to understand its catalytic properties. The results demonstrate that on 4-6 carbophene, the ORR prefers a direct four-electron pathway. Under acidic conditions, the ORR overpotential of 4-6 carbophene is as low as 0.59 V, which is comparable to some metal benchmark catalysts, demonstrating that 4-6 carbophene is a metal-free ORR candidate catalyst. In addition, the richly charged carbon atoms on 4-6 carbophene exhibit remarkable charge transfer when binding to the adsorbed intermediates. This results in a strong binding strength. Thus, 4-6 carbophene has favorable ORR catalytic activity. This work not only provides promising low-cost catalysts in pure carbon materials for ORR, promoting the development of metal-free ORR catalysts, but also enhances the understanding of Dirac cone two-dimensional materials for applications in catalysis.

Highly efficient degradation of antibiotic residues with reagent-free photoelectro-fenton via hollow fiber gas diffusion electrode

Fenton advanced oxidation technology is a potential treatment option for emerging organic pollutants in wastewaters. The challenges in the effective activation and generation of H2O2 limit its practical applications. We designed a chemical reagent-free Photoelectric-Fenton (PEF) system. Using an electro-chemical oxygen reduction reaction (ORR) employing Cu hollow fiber gas diffusion electrodes (Cu-hf-GDE) H2O2 was generated and was subsequently converted to OH using Fe/CN thin film on batch wall reactors. This PEF system demonstrated efficient elimination of florfenicol at a concentration of 10 ppm. The study revealed that O2− were the key species for the electrical generation of H2O2, while OH were the dominant reactive oxygen species (ROSs) for florfenicol degradation. A mechanism for the degradation of florfenicol was proposed, and the ecotoxicity of florfenicol and its degradation intermediates was investigated. The reusability, stability, and application potential of the PEF system in real water bodies were also assessed. Overall, this work demonstrates an example of pollution control resulting from philosophy of green chemistry, which offers a promising pathway to reduce the chemical demand for advanced oxidation processes.

Oxygen-vacancy-type Mars–van Krevelen mechanism drives ultrafast dioxygen electroreduction to hydrogen peroxide

The electrochemical oxygen reduction reaction (ORR) along a two-electron transfer pathway has been considered as an eco-friendly route for producing hydrogen peroxide (H2O2). However, large-scale industrial application of this ORR technology calls for ultrafast and effective generation of H2O2 under operating conditions (current densities >1 A/cm2 and Faradaic efficiency ≈ 100%). This imposes strict criteria for exploring innovative strategies for enhancing the adsorption and activation of O2 under vigorous reaction condition, which represents a significant challenge thus far. Here, we report an ‘oxygen-vacancy-type’ Mars–van Krevelen mechanism for promoting ORR. Our theoretical calculations show that the structural oxygen vacancies of zinc oxide catalysts effectively alter the electron densities of nearby metal active sites, producing a more electron-deficient Zn center, which, in turn, assists the adsorption and activation of O2. A catalyst electrode designed as that exhibits superior ORR activities with a Faradaic efficiency of 98.1% at a current density of 1 A/cm2 (H2O2 yield rate of 621.88 mg/h/cm2). Further mechanism study has been performed through in situ Raman spectroscopy to monitor the adsorption and activation of oxygen intermediate (∗O2) of ORR, providing additional experimental evidence for the Mars–van Krevelen mechanism.

Dual Single-Atomic Co-Mn Sites in Metal-Organic-Framework-Derived N-Doped Nanoporous Carbon for Electrochemical Oxygen Reduction.

Synthesizing dual single-atom catalysts (DSACs) with atomically isolated metal pairs is a challenging task but can be an effective way to enhance the performance for electrochemical oxygen reduction reaction (ORR). Herein, well-defined DSACs of Co-Mn, stabilized in N-doped porous carbon polyhedra (named CoMn/NC), are synthesized using high-temperature pyrolysis of a Co/Mn-doped zeolitic imidazolate framework. The atomically isolated Co-Mn site in CoMn/NC is recognized by combining microscopic as well as spectroscopic techniques. CoMn/NC exhibited excellent ORR activities in alkaline (E1/2 = 0.89 V) as well as in acidic (E1/2 = 0.82 V) electrolytes with long-term durability and enhanced methanol tolerance. Density functional theory (DFT) suggests that the Co-Mn site is efficiently activating the O-O bond via bridging adsorption, decisive for the 4e- oxygen reduction process. Though the Co-Mn sites favor O2 activation via the dissociative ORR mechanism, stronger adsorption of the intermediates in the dissociative path degrades the overall ORR activity. Our DFT studies conclude that the ORR on an Co-Mn site mainly occurs via bridging side-on O2 adsorption following thermodynamically and kinetically favorable associative mechanistic pathways with a lower overpotential and activation barrier. CoMn/NC performed excellently as a cathode in a proton exchange membrane (PEM) fuel cell and rechargeable Zn-air battery with high peak power densities of 970 and 176 mW cm-2, respectively. This work provides the guidelines for the rational design and synthesis of nonprecious DSACs for enhancing the ORR activity as well as the robustness of DSACs and suggests a design of multifunctional robust electrocatalysts for energy storage and conversion devices.

Regulating N Species in N-Doped Carbon Electro-Catalysts for High-Efficiency Synthesis of Hydrogen Peroxide in Simulated Seawater.

Electrochemical oxygen reduction reaction (ORR) is an attractive and alternative route for the on-site production of hydrogen peroxide (H2 O2 ). The electrochemical synthesis of H2 O2 in neutral electrolyte is in early studying stage and promising in ocean-energy application. Herein, N-doped carbon materials (N-Cx ) with different N types are prepared through the pyrolysis of zeolitic imidazolate frameworks. The N-Cx catalysts, especially N-C800 , exhibit an attracting 2e- ORR catalytic activity, corresponding to a high H2 O2 selectivity (≈95%) and preferable stability in 0.5m NaCl solution. Additionally, the N-C800 possesses an attractive H2 O2 production amount up to 631.2mmolg-1 h-1 and high Faraday efficiency (79.8%) in H-type cell. The remarkable 2e- ORR electrocatalytic performance of N-Cx catalysts is associated with the N species and N content in the materials. Density functional theory calculations suggest carbon atoms adjacent to graphitic N are the main catalytic sites and exhibit a smaller activation energy, which are more responsible than those in pyridinic N and pyrrolic N doped carbon materials. Furthermore, the N-C800 catalyst demonstrates an effective antibacterial performance for marine bacteria in simulated seawater. This work provides a new insight for electro-generation of H2 O2 in neutral electrolyte and triggers a great promise in ocean-energy application.

A defective iron-based perovskite cathode for high-performance IT-SOFCs: Tailoring the oxygen vacancies using Nb/Ta co-doping

The sluggish kinetics of the electrochemical oxygen reduction reaction (ORR) in intermediate-temperature solid oxide fuel cells (IT-SOFCs) greatly limits the overall cell performance. In this study, an efficient and durable cathode material for IT-SOFCs is designed based on density functional theory (DFT) calculations by co-doping with Nb and Ta the B-site of the SrFeO3−δ perovskite oxide. The DFT calculations suggest that Nb/Ta co-doping can regulate the energy band of the parent SrFeO3−δ and help electron transfer. In symmetrical cells, such cathode with a SrFe0.8Nb0.1Ta0.1O3−δ (SFNT) detailed formula achieves a low cathode polarization resistance of 0.147 Ω cm2 at 650 °C. Electron spin resonance (ESR) and X-ray photoelectron spectroscopy (XPS) analysis confirm that the co-doping of Nb/Ta in SrFeO3−δ B-site increases the balanced concentration of oxygen vacancies, enhancing the electrochemical performance when compared to 20 mol% Nb single-doped perovskite oxide. The cathode button cell with Ni-SDC|SDC|SFNT configuration achieves an outstanding peak power density of 1.3 W cm−2 at 650 °C. Moreover, the button cell shows durability for 110 h under 0.65 V at 600 °C using wet H2 as fuel.

Morphological and microstructural engineering of Mn-N-C with strengthened Mn-N bond for efficient electrochemical oxygen reduction reaction

Revealing the correlation between the micro–macro structures and active sites of transition metal-nitrogen-carbon (M−N−C) for electrochemical oxygen reduction reaction (ORR) is essential to develop non-precious metal catalyzed fuel cells and metal-air batteries. Herein, Mn-N-C catalysts with various morphologies and microstructures were prepared using Mn2+ coordinated bis(imino)-pyridine-based polymer with N-rich content as a new platform of precursor, which was synthesized via a condensation reaction of 1H-1, 2, 4-Triazole-3, 5-Diamine and 2, 6-Diacetylpyridine. Results demonstrate that morphologies and fine structures (pore structure, N dopants, surface area, and atomic Mn-N bond length, etc.) of Mn-N-C catalysts are strongly dependent on the growth of the specific precursor with and without NaCl as a template and the secondary calcination temperature can further optimize the bond length of Mn-Nx moieties, resulting in significant activity differences between correspondingly derived Mn-N-C catalysts for ORR. Compared to Mn-N-C derived from the synthesized precursor without NaCl, Mn-N-C obtained by the precursor grown directly on NaCl features an ultra-thin nanosheet-like structure with a hierarchical pore distribution and a shorter Mn-N bond, presenting high ORR performance with a half-wave potential of 0.88 V in 0.1 M KOH and a tiny potential loss of 11 mV after 30 K cycles, which is also proved by high-performance practical single Zn-Air battery (139 mW·cm−2) and proton exchange membrane fuel cell (400 mW·cm−2). This work provides a new understanding of the critical role of morphology and Mn-N bonding length of M−N−C for enhancing ORR and a new route of developing non-Fe/Co-based M−N−C catalysts for electrocatalysis.