Oxygen Reduction Reaction Catalysis Research Articles

Bismuth Nanoparticles and Single Iron Atoms on Carbon Derived from a Covalent Organic Framework Synergistically Catalyze the Oxygen Reduction Reaction.

The utilization of catalysts comprising metal nanoparticle has been beneficial for enhancing the performance of oxygen reduction reaction (ORR). However, the inadequate intrinsic activity of these catalysts still presents a significant challenge, limiting their overall effectiveness. This issue can be addressed by introducing single atoms, which can create a synergistic effect with the nanoparticles to catalyse and thereby improve performance. Nevertheless, the synergistic catalysis of nanoparticles and single atoms is still under investigation. In this study, we fabricated a core-shell structured carbon framework incorporating Fe single atoms and Bi nanoparticles through the pyrolysis of COF and MOF core-shell structures. Introducing Fe single atoms into ZIF-8, with Fe-ZIF-8 as the core and Bi-containing COF as the shell, resulted in higher ORR activity. The catalyst exhibited a half-wave potential of 0.867 V and a high current density of 6.68 mA cm-2 in 0.1 M KOH, which were comparable to those of Pt/C equivalent. This study provides new research concepts for exploring the application of single atoms and nanoparticles in catalytic oxygen reduction reactions through synergistic effects.

Tuning the structure and properties of carbon cloth by FeCl3intercalation for efficient two-electron oxygen reduction catalysis.

The advancement of various energy conversion and storage technologies hinges on the development of efficient and stable electrocatalysts for the oxygen reduction reaction (ORR). In this study, we report the enhancement of carbon cloth (CC) for robust ORR through an FeCl3intercalation reaction. Utilizing a thermal annealing method, FeCl3was intercalated into the graphite structure on the surface of CC, resulting in the creation of numerous defects and the incorporation of Fe species. These newly introduced defects play a pivotal role in activating the ORR via a two-electron pathway. The presence of Fe species further stabilizes the catalytic activity, leading to efficient and stable ORR performance. Our findings highlight the significance of defect engineering and Fe species incorporation in carbon-based materials for improved ORR catalysis and pave the way for the development of advanced electrocatalysts for energy-related applications.

Machine Learning‐Aided Discovery of Low‐Pt High Entropy Intermetallic Compounds for Electrochemical Oxygen Reduction Reaction

Advancing the design of cathode catalysts to significantly maximize platinum utilization and augment the longevity has emerged as a formidable challenge in the field of fuel cells. Herein, we rationally design a high entropy intermetallic compound (HEIC, Pt(FeCoNiCu)3) for catalyzing oxygen reduction reaction (ORR) by an efficient machine learning stategy, where crystal graph convolutional neural networks are employed to expedite the multicomponent design. Based on a dataset generated from first‐principles calculations, the model can achieve a high prediction accuracy with mean absolute errors of 0.003 for surface strain and 0.011 eV atom‐1 for formation energy. In addition, we identify two chemical features (atomic size difference and mixing enthalpy) as new descriptors to explore advanced ORR catalysts. The carbon supported Pt(FeCoNiCu)3 catalyst with small particle size is successfully synthesized by a freeze‐drying‐annealing technology, and exhibits ultrahigh mass activity (4.09 A mgPt‐1) and specific activity (7.92 mA cm‐2). Meanwhile, The catalyst also shows significantly enhanced electrochemical stability which can be ascribed to the sluggish difussion effect in the HEIC structure. Beyond offering a promising low‐Pt electrocatalysts for fuel cell cathode, this work offers a new paradigm to rationally design advanced catalysts for energy storage and conversion devices.

Machine Learning-Aided Discovery of Low-Pt High Entropy Intermetallic Compounds for Electrochemical Oxygen Reduction Reaction.

Advancing the design of cathode catalysts to significantly maximize platinum utilization and augment the longevity has emerged as a formidable challenge in the field of fuel cells. Herein, we rationally design a high entropy intermetallic compound (HEIC, Pt(FeCoNiCu)3) for catalyzing oxygen reduction reaction (ORR) by an efficient machine learning stategy, where crystal graph convolutional neural networks are employed to expedite the multicomponent design. Based on a dataset generated from first-principles calculations, the model can achieve a high prediction accuracy with mean absolute errors of 0.003 for surface strain and 0.011 eV atom-1 for formation energy. In addition, we identify two chemical features (atomic size difference and mixing enthalpy) as new descriptors to explore advanced ORR catalysts. The carbon supported Pt(FeCoNiCu)3 catalyst with small particle size is successfully synthesized by a freeze-drying-annealing technology, and exhibits ultrahigh mass activity (4.09 A mgPt-1) and specific activity (7.92 mA cm-2). Meanwhile, The catalyst also shows significantly enhanced electrochemical stability which can be ascribed to the sluggish difussion effect in the HEIC structure. Beyond offering a promising low-Pt electrocatalysts for fuel cell cathode, this work offers a new paradigm to rationally design advanced catalysts for energy storage and conversion devices.

Highly asymmetrically configured single atoms anchored on flame-roasting deposited carbon black as cathode catalysts for ultrahigh power density Zn-air batteries

Iron group element-based single atom (SA) catalysts are highly regarded as promising alternatives to commercial Pt/C for catalysis of oxygen reduction reaction (ORR). For applications in rechargeable zinc-air batteries (ZABs), achieving the necessary high catalytic efficiency of the SAs toward oxygen evolution reaction (OER) however remains a significant challenge. Here, highly asymmetrically configured Fe SAs created with N,S co-coordination and anchored on flame-roasting deposited carbon black (CB), Fe-N3S1/CB, are developed, achieving outstanding bifunctional oxygen catalytic efficiency, with an ultra-small potential gap of 0.661 V at 10 mA cm-2 (ΔE10), outperforming the (Pt/C+RuO2) composite catalyst (0.697 V). With a newly proposed binder-free composite air cathode design, the Fe-N3S1/CB based ZAB achieves an ultrahigh power density of 365.7 mW cm-2 at a current density of 511.3 mA cm-2, largely outperforming the (Pt/C+RuO2) based ZAB (225.9 mW cm-2 at 344.7 mA cm-2). Furthermore, the Fe-N3S1/CB based ZAB demonstrates excellent long-term stability, with only 8.2% decay in round-trip efficiency over 1000 (333.3 h) charge-discharge cycles at 10 mA cm-2. Density functional theory calculations elucidate that incorporation of sulfur into the coordination sphere of Fe facilitates the electrochemical dehydroxylation step for ORR and accelerates the electrochemical O2 desorption step for OER, thereby reducing the corresponding free energy differences on Fe SAs for largely enhanced catalytic efficiency.1. A large size hetero-atom element, sulfur, is introduced to create highly asymmetrically configured Fe single atoms for enhancements in catalytic efficiency toward both oxygen reduction reaction and oxygen evolution reaction, and a binder-free composite air cathode design is proposed to improve electrochemical performances of zinc-air batteries.2. An ultra-small potential gap of 0.661 V at 10 mA cm-2 (ΔE10) is achieved for the air cathode, and an ultrahigh discharge power density of 365.7 mW cm-2 at a current density of 511.3 mA cm-2 is acquired for the zinc-air battery.

Increasing catalytic efficiency of cobalt centers in oxygen reduction reaction - Evidence from cobalt with salen ligands polymer complex

Substantial research is performed for cobalt-based catalysts for various applications in catalysis, including oxygen reduction reaction (ORR) catalysis in nonaqueous electrolytes. Cobalt exhibits high catalytic activity due to partially filled d-orbitals, which promote the formation of chemisorption bonds with various molecules. The catalytic activity of metal centers can be influenced both by changing their immediate environment and by varying the reaction conditions. This paper investigates the influence of solvent type, electrolyte concentration, and organic framework conductivity on the efficiency of ORR catalysis in non-aqueous electrolytes as illustrated by poly-CoSal polymer complex. The findings show that the increase in the solvent donor number of and conductivity of the organic backbone leads to highly efficient cobalt catalysts, which can be potentially be applied in oxygen cathodes of Li-O2 batteries. CoSal-based catalysts increase the discharge voltage of Li-O2 cells and can operate in dry atmospheric air.

Pt Ternary Alloy with Rare Earth Metal - Pt5Ce-Co Intermetallic Nanoparticles as Highly Active Catalyst for Oxygen Reduction Reaction in PEMFCs

Alloying with rare earth elements is an important direction of exploring in Pt-based alloy design in oxygen reduction reaction (ORR), due to its special electronic configurations. Here we reported a ternary intermetallic alloy Pt5Ce-Co/C. The electronic valence state of Pt alloys was modified by the unique 4f orbital and the contraction effect of Ce, plus the additional active sites provided by the Co incorporation, optimizing the adsorption energy of the reaction intermediates, and effectively improving the electrocatalytic performance. The Pt5Ce-Co/C achieved 2.8 A∙mgPt -1 of mass activity in ORR catalysis, which can maintain 1.4 A∙mgPt -1 after 30k of accelerated stress test (AST). For membrane electrode assembly test, Pt5Ce-Co/C shows an initial mass activity of 724 mA∙mgPt -1, and a current density of 1876 mA∙cm-2 at 0.7 V under heavy-duty vehicle testing. This work provides an avenue to design advanced PGM ternary alloys by incorporating rare-earth metals to promote the catalyst performance.

Bifunctional electrode materials: Enhancing microbial fuel cell efficiency with 3D hierarchical porous Fe3O4/Fe-N-C structures

The rational development of high-performance anode and cathode electrodes for microbial fuel cells (MFCs) is crucial for enhancing MFC performance. However, complex synthesis methods and single-performance electrode materials hinder their large-scale implementation. Here, three-dimensional hierarchical porous (3DHP) Fe3O4/Fe-N-C composites were prepared via the hard template method. Notably, Fe3O4/Fe-N-C-0.04-600 demonstrated uniformly dispersed Fe3O4 nanoparticles and abundant Fe-Nx and pyridinic nitrogen, showing excellent catalytic performance for oxygen reduction reaction (ORR) with a half-wave potential (E1/2) of 0.74 V (vs. RHE), surpassing Pt/C (0.66 V vs. RHE). Moreover, Fe3O4/Fe-N-C-0.04-600 demonstrated favorable biocompatibility as an anode material, enhancing anode biomass and extracellular electron transfer efficiency. Sequencing results confirmed its promotion of electrophilic microorganisms in the anode biofilm. MFCs employing Fe3O4/Fe-N-C-0.04-600 as both anode and cathode materials achieved a maximum power density of 831.8 ± 27.7 mW m−2, enduring operation for 38 days. This study presents a novel approach for rational MFC design, emphasizing bifunctional materials capable of serving as anode materials for microorganism growth and as cathode catalysts for ORR catalysis.

Catalysis of oxygen reduction reaction by iron porphyrin and its sensing application

Hemin, or iron porphyrin, serves as a non-noble metal catalyst for the oxygen reduction reaction (ORR). The stability of hemin is maintained in high salt concentrations, functioning as an ORR redox catalyst in the aqueous phase. Hemin facilitates the development of an oxygen sensor that transitions the ORR mechanism from a 2-electron to a more efficient 4-electron process. This sensor demonstrates excellent reproducibility (RSD<5%), with a wide linear range of 38.9–613 μM, a sensitivity of 0.136 ± 0.003 μA μM−1, and a limit of detection (3SB/m) of 11.8 μM.

Ternary ordered L10-Pt-Co-Fe intermetallics for efficient ORR catalysis through dissociation pathway

Developing efficient and durable Pt-based electrocatalysts for oxygen reduction reaction (ORR) is critical for the practical application of fuel cells but still remains challenge at present. Here we successfully synthesized a series of ternary L10-PtCoxFe1-x (x=0.33, 0.50 and 0.67) intermetallic nanoparticles (NPs) supported on reduced graphene oxide for ORR catalysis. L10-PtCo0.50Fe0.50 exhibits the highest mass activity (MA) of 0.93 A mg−1Pt at 0.9 V (1.82 times the corresponding binary L10-PtCo intermetallics) and minimal activity loss (24.73 % loss in MA) after 30,000 potential cycles. By Density Functional Theory calculations, the excellent performance of ternary L10-PtCo0.50Fe0.50 can be ascribed to: (1) more efficient electronic structure regulation caused by dual-element driven electron transfer, which leads to more electron accumulation on Pt and weakens the over-binding of oxygen-containing species, (2) the unique two-center bridge pattern of O2 adsorption over Pt-Fe site leads to ORR proceeding via. the dissociative mechanism, avoiding the formation of OOH*.

Revealing Significant Electronic Effects on the Oxygen Reduction Reaction with Iron Porphyrins.

Understanding electronic effects on catalysis from a mechanism point of view is of fundamental significance but is also challenging. We herein report on electronic effects on the oxygen reduction reaction (ORR) with Fe porphyrins. By using FeIII tetraphenylporphyrin (TPP-Fe) and FeIII tetra(pentafluorophenyl)porphyrin (TPFP-Fe), we showed their different electrochemical and chemical behaviors for ORR. Mechanism studies revealed that the FeIII-superoxo species of TPP-Fe can undergo smooth protonation with trifluoroacetic acid (TFA) but the electron-deficient FeIII-superoxo species of TPFP-Fe cannot be protonated with TFA. The FeIII-superoxo reactivity difference between TPP-Fe and TPFP-Fe is the origin of their different catalytic ORR behaviors.

Nature of the Active Center for the Oxygen Reduction on Ag-Based Single-Atom Alloy Clusters.

The development of alternative alloy catalysts with high activity, surpassing platinum group metals, for the oxygen reduction reaction (ORR) is urgently needed in the field of electrocatalysis. The Ag-based single-atom alloy (AgSAA) cluster has been proposed as a promising catalyst for the ORR; however, enhancing its activity under operational conditions remains challenging due to limited insights into its actual active site. Here, we demonstrate that the operando formation of the MO x (OH) y complex serves as the key active site for catalyzing the ORR over AgSAA cluster catalysts, as revealed through comprehensive neural network potential molecular dynamics simulations combined with first-principles calculations. The volcano plot of the ORR over the MO x (OH) y complex addresses the gaps inherent in traditional metallic alloy models for pure AgSAA cluster catalysts in ORR catalysis. The appropriate orbital hybridization between OH and the dopant metal in the MO x (OH) y complexes indicated that the Ag54Co1, Ag54Pd1, and Ag54Au1 clusters are optimal AgSAA catalysts for the ORR. Our work underscores the significance of theoretical modeling considering the reaction atmosphere in uncovering the true active site for the ORR, which can be extended to other reaction systems for rational catalyst design.

Co/CoSe heterojunction as bifunctional oxygen electrocatalysts for rechargeable Zinc–Air batteries

The construction of suitable heterostructures in materials can tune the Fermi energy levels and electron transport rates of materials to achieve attractive oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) properties. In this study, a catalyst anchored in nitrogen-doped carbon nanowires (NCNW) with a heterogeneous interface of Co and CoSe is constructed to realize high-performance ORR and OER bifunctional catalysis. These Co and CoSe heterostructure interfaces can optimize the adsorption of intermediates. The Co/CoSe@NCNW catalyst performs excellent ORR catalytic activity (E1/2 = 0.89 V), and good OER activity (potential at 10 mA/cm2, Ej = 10 = 1.55 V). More importantly, the material shows excellent performance in rechargeable zinc-air batteries (ZABs). The open-circuit voltages of 1.47 and 1.50 V and the peak power densities of 122.56 and 74.49 mW/cm2 in liquid and solid electrolytes, respectively, are superior to those of the ZABs prepared from noble metals. Consequently, the construction of heterogeneous structures can convincingly boost the commercialization of rechargeable ZABs, and be extended to other fields involving ORR and OER.

Atomically Engineered Defect-Rich Palladium Metallene for High-Performance Alkaline Oxygen Reduction Electrocatalysis.

Defect engineering is a key chemical tool to modulate the electronic structure and reactivity of nanostructured catalysts. Here, it is reported how targeted introduction of defect sites in a 2D palladium metallene nanostructure results in a highly active catalyst for the alkaline oxygen reduction reaction (ORR). A defect-rich WOx and MoOx modified Pd metallene (denoted: D-Pd M) is synthesized by a facile and scalable approach. Detailed structural analyses reveal the presence of three distinct atomic-level defects, that are pores, concave surfaces, and surface-anchored individual WOx and MoOx sites. Mechanistic studies reveal that these defects result in excellent catalytic ORR activity (half-wave potential 0.93V vs. RHE, mass activity 1.3AmgPd-1 at 0.9 V vs. RHE), outperforming the commercial references Pt/C and Pd/C by factors of ≈7 and ≈4, respectively. The practical usage of the compound is demonstrated by integration into a custom-built Zn-air battery. At low D-Pd M loading (26µgPdcm-2), the system achieves high specific capacity (809mAhgZn -1) and shows excellent discharge potential stability. This study therefore provides a blueprint for the molecular design of defect sites in 2D metallene nanostructures for advanced energy technology applications.

Investigation of the Chemical Changes from N-Doped Graphene and MOF to N-G/MOF Composites for the Enhancement of Electrocatalytic Properties

N-doped graphene (N-G) materials showed high potential as electrocatalysts for oxygen reduction reaction (ORR) in electrochemical energy systems and increasingly gaining ground as alternatives to the expensive and degradation-prone Platinum Group Metal (PGM)-based electrocatalysts. Numerous studies demonstrated that as N-G materials are integrated with Metal-Organic Frameworks (MOFs), the N-G/MOF composites exhibited higher electrocatalytic activities than the individual precursors; on several occasions, even higher than the PGM-based ones. In this essence, it is crucial to analyze and understand the chemical changes that occur through the integration of N-G and MOF, as these changes play key roles in enhancing electrocatalytic properties of the materials by introducing different electrochemically active sites. To investigate such chemical changes, we have synthesized an N-G/MOF composite by integrating an N-G with ZIF-8 (which is the most studied member of the MOF family) following a facile mechanochemical wet ball milling process. In the electrochemical characterization, the N-G/MOF composite performed better than N-G and ZIF-8 for ORR catalysis by generating a higher cathodic current density.Multiple samples of N-G, ZIF-8, and N-G/MOF composite were analyzed by X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy to detect the changes in elemental ratios, chemical bonds, and chemical functional groups from the precursors (N-G and ZIF-8) to the N-G/MOF composite. An increase in the elemental Nitrogen (N) ratio was observed in N-G/MOF compared to its precursors; which signifies an increased N-doping ratio in the material which is advantageous for ORR catalysis. The XPS C 1s spectra of N-G/MOF shifted to a higher binding energy (~1eV) compared to the precursors indicating an overall increase in the oxidation states of Carbon (C) atoms in the material which could facilitate oxygen molecules’ adsorption better on these sites at the beginning of ORR process. Comparing the analyzed XPS N 1s spectra revealed an increased relative ratio of the Pyridinic-N functional group in N-G/MOF, which also partly justifies the composite’s enhanced ORR electrocatalytic performance. FTIR analysis revealed interactions between methyl groups (-CH3) of ZIF-8 with oxygen functional groups (i.e. -OH) of N-G while forming the N-G/MOF composite. Besides, a transition from methyl groups (-CH3) to methylene group ( >CH2) was evident during the synthesis, increasing charge density near those C atoms of the N-G/MOF which can as well be advantageous for ORR electrocatalysis. The combined impact of these chemical alterations from N-G and ZIF-8 to N-G/MOF yielded a synergistic effect, elucidating its superior electrocatalytic properties. The findings of this experimental investigation provide insights into the chemical reasons for the enhancement of electrocatalytic properties of N-G/MOF composites; and hereby offer guidance to the design and optimization endeavors of novel catalysts with N-G and MOF materials for diverse electrochemical applications.

The Impact of Carbon Corrosion Towards Oxygen Reduction Reaction Activity on Atomically Dispersed M-N-C Catalysts

Atomically dispersed Pt group metal free (PGM free) M-N-C catalysts offer a promising pathway towards a clean energy future.1 This class of materials has proven to be a viable substitute for commercial Pt/C to catalyze oxygen reduction reaction (ORR) with similar activity at only a fraction of the raw material cost.2 To gain a wider market adoption, however, M-N-C catalysts need to significantly improve its durability compared to Pt and its alloys in a real-world operating condition. Towards this end, substantial efforts have focused on making more durable M-N-C catalysts while simultaneously maintaining and improving its ORR activity.3,4 As part of the effort to understand the fundamental aspects that control M-N-C catalysts’ durability during ORR condition, we employ a combined approach using first principles density functional theory (DFT) and density functional tight binding theory (DFTB) to identify the impact of carbon corrosion occurring on M-N-C catalyst towards ORR activity. Our starting atomic representation of the ORR active site is the “C10” type local structure composed of FeN4C10 local defect5 embedded in a monolayer graphene support of varying cell size. As carbon corrosion occurs and consequently creating local carbon vacancies surrounding the active site, we then obtain a family of FeN4C11 and FeN4C12 type of local structures. Our DFT calculations of OH binding energy on these corrosion derived structures suggest that the presence of C vacancies and the subsequent H passivation on dangling C introduces elastic structural deformation to various degree that decreases the theoretical ORR limiting potential. This decrease in DFT calculated ORR limiting potential across a family of FeN4C10, FeN4C11, and FeN4C12 local structures can be attributed to a mechanical energy arising from the surface reconstructions. Reference D. A. Cullen et al., Nat Energy, 6, 462–474 (2021).G. Wu, K. L. More, C. M. Johnston, and P. Zelenay, Science, 332, 443–447 (2011).E. F. Holby, G. Wang, and P. Zelenay, ACS Catal., 10, 14527–14539 (2020).T. Patniboon and H. A. Hansen, ACS Catal., 11, 13102–13118 (2021).H. T. Chung et al., Science, 357, 479–484 (2017).

New Insights into the ORR Catalysis on Pt Alloy Nanoparticles from an Element Specific d-Band Analysis.

Bolstered by their unique atomic structures and tailored compositions, nanoalloys exhibit extraordinary properties making them ideal materials to solve challenges in energy storage and conversion catalysis. However, a quantitative description of the structure-property relationships using an accurate descriptor-based model for nanoalloys, ranging from bimetallic to multimetallic compositions, is needed to drive efficient material design toward high-performance catalysis. In this work, we highlight the electronic property and catalytic activity relationship from an element specific d-band analysis of Pt-based alloy catalysts using X-ray absorption near-edge spectroscopy (XANES). Using a series of L10-MPt/Pt (M = Fe, Co, Ni) core/shell alloy catalysts with well-defined atomic structures, we quantified subtle differences in the Pt d-electron states and correlated the Pt d-band structure to their superior catalytic activity toward the oxygen reduction reaction (ORR). Our analysis used the upper d-band edge position as a predictive descriptor for the mass activity toward the ORR instead of the commonly used d-band center position. Together with density functional theory calculations and Nørskov d-band theory, the upper d-band edge position for the Pt states, derived from experimental measurements, elucidates new physical insights into the ORR performance of the L10-MPt/Pt core/shell catalysts. An element specific Pt d-band analysis using XANES overcomes challenges in traditional X-ray photoelectron spectroscopy-based valence d-band analysis, which cannot distinguish signals from independent elements in nanoalloys. Thus, the insights from the element specific d-band analysis presented in this work are a promising approach to determine structure-property relationships in a variety of transition metal nanoalloys and will be useful in the design of future high-performance catalysts.

Fe2N5P dual-atom catalysts: A pathway to efficient hydrogen evolution in renewable energy applications

Dual-atom catalysts (DACs) are expected to improve the catalytic activity compared to single-atom catalysts (SACs). This improvement is attributed to the synergistic interaction between adjacent bimetallic atoms in DACs. Recently, the highly active Fe2N5P sites embedded in covalent organic framework-derived carbon have been demonstrated to catalyze oxygen reduction reaction (doi:10.1021/acscatal.3c02186). Motivated by this fascinating finding of Fe2N5P-based DACs, their potential for catalyzing hydrogen evolution reaction (HER) has been investigated. Thus, electrocatalytic activities of the Fe2N6 and Fe2N5P-based DACs embedded in graphene layer were systematically investigated toward the HER. Our results show that the Fe2N6 site embedded in graphene shows high catalytic activity and selectivity toward the HER. However, our results reveal that the P atom in the Fe2N5P dual-site facilitates the × H formation and thus improves its catalytic activity toward HER due to a greater degree of synergistic effect. The computed Gibbs free energies confirm the higher feasibility of HER on the Fe center of Fe2N5P compared to the Fe2N6 DAC.

Role of Vacancy Defects and Nitrogen Dopants for the Reduction of Oxygen on Graphene.

Disentangling the roles of nitrogen dopants and vacancy defects (VG) in metal-free carbon catalysts for the oxygen reduction reaction (ORR) ideally requires studying both the dopants and defects separately. Here, we systematically introduced nitrogen dopants and VGs via plasma treatment into the basal plane of monolayer graphene as a model carbon catalyst to investigate their specific roles in ORR catalysis. An increased defect density including dopants is positively associated with boosted ORR activity. Nitrogen dopants are responsible for an improved current via a 2e- pathway generating hydroperoxide, while VGs result in enhanced kinetics and water production. We therefore infer that VGs in graphene are responsible for the improved ORR kinetics, while nitrogen dopants majorly influence the selectivity of ORR reaction products. The nitrogen dopants without VGs lead to a higher overpotential compared with the pristine graphene. Instead of the attribution of the ORR active site to only nitrogen species in carbon materials, the improved ORR activity in nitrogen-doped carbon materials should be attributed to the active sites constituted of VGs, oxygen dopants, and nitrogen dopants. Through this work, we provide important insights into the intertwined roles of nitrogen and VGs as well as oxygen dopants in nitrogen-doped metal-free catalysts for a more efficient ORR.

A twin S-scheme CoFe2O4/VCNDA/BiVO4 nanozyme for photo-auto-enzyme-coupled catalytic degradation of carbamazepine

The photo-enzyme-coupled catalytic (PECC) system combines the high selectivity of enzymatic reaction with the unique activity of photocatalytic reaction, which shows great application potential in the field of organic pollutant degradation. However, the traditional PECC system usually requires additional to enzyme substrates to achieve the enzymatic reaction, which increases the risk of environmental pollution. Herein, the twin S-scheme CoFe2O4/vesicular g-C3N4-FeCoDA/BiVO4 (CVBDA) nanozyme with good horseradish peroxidase (HRP) activity was prepared and used to construct the unique photo-auto-enzyme-coupled catalytic (PAECC) system. The twin S-scheme mechanism was confirmed by the XPS and UPS. In this heterojunction, the strong redox ability of CVBDA nanozyme can be effectively retained for the efficient degradation of pollutants and catalysis of oxygen reduction reactions. And the abundant H2O2 provided by the oxygen reduction reaction lays the foundation for excellent HRP activity and effectively increases the amount of active group to improve the oxidation performance of CVBDA nanozyme. With 1 mg/L carbamazepine as the target contaminant, the degradation efficiency of CVBDA nanozyme can reach 90.9 % in the PAECC system. This study provides a new idea for the design of twin S-scheme nanozyme to improve the catalytic performance of photo-enzyme-coupled catalytic system.