High-efficiency Oxygen Reduction Reaction Research Articles

Preface

The 2022 8th International Forum on Manufacturing Technology and Engineering Materials (IFEMMT 2022) was successfully held on October 14th-16th, 2022 in Chongqing, China (virtual conference). IFEMMT 2022 brought together academics and experts in the field of Manufacturing Technology and Engineering Materials to a common forum, making it a great platform for people to share views and experiences in related areas.We were honored to have Prof. Xiansheng Ning from Shenyang Ligong University, China to serve as our General Conference Chair. The conference covered keynote speeches, oral presentations, and online Q&A discussion, attracting 150 individuals. Firstly, keynote speakers were each allocated 30-45 minutes to hold their speeches. Then in the oral presentations, the excellent papers selected were presented by their authors in sequence.During the conference, five sophisticated professors were invited to make keynote speeches. Among them, Prof. Vijay Kumar Thakur from Scotland’s Rural College delivered a keynote speech on Polymer Nanocomposites as Promising Materials for the Future. Polymer nanocomposites are rapidly emerging as novel materials for a number of advanced engineering applications. The combination of both the polymers and fillers provides enhanced properties depending on the type and nature of polymer matrices as well as fillers. Therefore, this presentation mainly discussed about the work on the synthesis and processing of polymer-based materials for advanced application. Additionally, Assoc. Prof. Xiao Lu from Shenyang Ligong University made a report on Structure Design of Defective Electrocatalysts and Effect of High Magnetic Field on Electrocatalysis Process. The report proposed a method to achieve high efficiency oxygen reduction reaction by regulating the defect structure of carbon materials and metal catalyst materials and put forward the preparation of catalyst materials under strong magnetic field and the influence of strong magnetic field on oxygen evolution reaction and oxygen reduction reaction process. Their excellent presentations had sparked heated discussion in the conference. Moreover, every participant praised this conference for disseminating insightful knowledge.After months of well preparation and hard work, the proceedings of IFEMMT 2022 covering a bunch of papers are smoothly published. These papers feature but are not limited to the following topics: Advanced Design Technology, Manufacturing Systems and Automation, New Materials and Advanced Materials, etc. All the papers have been checked through rigorous review and processes to meet the requirements of publication.We would like to express our heartfelt appreciation to all the keynote speakers, peer reviewers, and all the participants. Particularly, we would like to acknowledge the Journal of Physics: Conference Series, for the endeavor of all its colleagues in publishing this paper volume. We are sure that IFEMMT 2022 will contribute to not only scientific and engineering progress but also other new products and processes.The Committee of IFEMMT 2022List of Committee member is available in this Pdf.

High-Efficiency Oxygen Reduction Reaction Revived from Walnut Shell.

The development of inexpensive and efficient electrocatalysts for oxygen reduction reactions (ORR) remains a challenge with respect to renewable energy technologies. In this research, a porous, nitrogen-doped ORR catalyst is prepared using the hydrothermal method and pyrolysis with walnut shell as a biomass precursor and urea as a nitrogen source. Unlike past research, in this study, urea is not directly doped; instead, a new type of doping is carried out after annealing at 550 °C. In addition, the sample's morphology and structure are analyzed and characterized by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). A CHI 760E electrochemical workstation is used to test NSCL-900's performance in terms of oxygen reduction electrocatalysis (ORR). It has been found that the catalytic performance of NSCL-900 is significantly improved compared with that of NS-900 without urea doping. In a 0.1 mol/L KOH electrolyte, the half-wave potential can reach 0.86 V (vs. RHE) and the initial potential is 1.00 V (vs. RHE). The catalytic process is close to four-electron transfer and there are large quantities of pyridine nitrogen and pyrrole nitrogen.

A microstructure tuning strategy on hollow carbon nanoshells for high-efficient oxygen reduction reaction in direct formate fuel cells

The microstructure of carbon supports for electrocatalysts plays a crucial role in dispersing active sites, establishing oxygen/ion transport channels, and thus determining the oxygen reduction reaction (ORR) activity. A facile and simple microstructure tuning strategy is adopted to synthesize hollow carbon nanoshells (CNSs) with tuneable microstructure in this study. The CNSs with expected macroporous (300–966 nm) and mesoporous (2–5 nm) structures were achieved by adjusting heating ramp rates during pyrolysis. The CNS prepared at 1 °C min−1 (CNS-1) possesses a well-developed mesoporous/macroporous structure, which is conducive to dispersing iron phthalocyanine during the synthesis and establishing triple-phase interfaces in the catalyst layer during the ORR. Therefore, the electrocatalyst (Fe–N/CNS-1), derived from the CNSs-1, exhibits a 44 mV higher half-wave potential (0.879 V vs. RHE) in a rotating disk electrode and a 14% higher maximum power density (16.1 mW cm−2) in a membrane-less direct formate fuel cell than that of Pt/C. This study delivers a promising microstructure tuning strategy for carbon supports to construct high-efficient ORR electrocatalysts.

Rational Design of Atomically Dispersed Metal Site Electrocatalysts for Oxygen Reduction Reaction.

Future renewable energy supply and a cleaner Earth greatly depend on various crucial catalytic reactions for the society. Atomically dispersed metal site electrocatalysts (ADMSEs) have attracted tremendous research interest and are considered as the next-generation promising oxygen reduction reaction (ORR) electrocatalysts due to the maximum atom utilization efficiency, tailorable catalytic sites, and tunable electronic structures. Despite great efforts have been devoted to the development of ADMSEs, the systematic summary for design principles of high-efficiency ADMSEs is not sufficiently highlighted for ORR. In this review, the authors first summarize the fundamental ORR mechanisms for ADMSEs, and further discuss the intrinsic catalytic mechanism from the perspective of theoretical calculation. Then, the advanced characterization techniques to identify the active sites and effective synthesis methods to prepare catalysts for ADMSEs are also showcased. Subsequently, a special emphasis is placed on effective strategies for the rational design of the advanced ADMSEs. Finally, the present challenges to be addressed in practical application and future research directions are also proposed to overcome the relevant obstacles for developing high-efficiency ORR electrocatalysts. This review aims to provide a deeper understanding for catalytic mechanisms and valuable design principles to obtain the advanced ADMSEs for sustainable energy conversion and storage techniques.

Molten-salt confined synthesis of nitrogen-doped carbon nanosheets supported Co3O4 nanoparticles as a superior oxygen electrocatalyst for rechargeable Zn-air battery

Rational design and preparation of high-efficiency oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts is crucial for the large-scale practical application of rechargeable Zn-air batteries. In this work, employing a combined strategy of self-sacrificial template and molten-salt confinement effect, a facile one-step pyrolysis route has been developed to synthesize defect-rich N-doped carbon nanosheets supported Co3O4 nanoparticles (Co3O4@NCNs). The pyrolytic precursor is built up of NaCl encapsulated ZnO@zeolitic imidazolate framework-67 core-shell particles. Thanks to the pore-forming and oxygen source functions of ZnO and the confinement effect of NaCl, the holed NCNs in Co3O4@NCNs are interconnected into a 3D porous carbon structure with a high N-dopant level. The as-prepared Co3O4@NCNs catalyst exhibits excellent bifunctional ORR/OER electrocatalytic activities with a half-wave ORR potential (E1/2) of 0.84 V, an ultralow gap of 0.63 V between OER Ej = 10 mA/cm2 and ORR E1/2 potentials, and prominent long-term durability. Its rechargeable Zn-air battery displays a high power density up to 173.8 mW cm−2 and superior cycling stability. This work highlights a novel strategy for the component and architecture design of high-performance carbon-based electrocatalysts in energy conversion and storage systems.

Heterostructure-induced enhanced oxygen catalysis behavior based on metal cobalt coupled with compound anchored on N-doped carbon nanofiber for microbial fuel cell

High-efficiency oxygen reduction reaction (ORR) electrocatalyst in microbial fuel cells (MFCs) is important to boost the power production efficiency and reduce overall cost. Herein, we demonstrate a novel nitrogen (N)-doped carbon nanofiber (N-CNF) supported metal and metal compound heterostructure derived from metal-organic frameworks (MOFs), which endows superior electrocatalytic activity by optimizing the coupling modulation effect. The resulting cobalt/cobalt phosphide and cobalt/cobalt sulfide nanoparticles embedded in N-doped carbon nanofiber (Co/CoP/Co2P@N-CNF, Co/CoS2@N-CNF) present superior ORR activity and methanol tolerance. Moreover, the assembled MFCs modified with Co/CoP/Co2P@N-CNF and Co/CoS2@N-CNF composite also achieve higher power density (375.16 and 400.06 mW m−2) as well as coulombic efficiency (11.2 %, 12.4 %), superior than that of Pt/C electrode (333.70 mW m−2, 10.4 %). Impressively, the Co/CoS2@N-CNF electrode exhibits long-term stability and durability in dual-chamber MFCs. A high-performance heterostructure cathode with an effective strategy for bridging nanocatalysis and practical MFCs is reported and presented.

Construction of MWNT supported ultra-small Ag@MnO2 nanoparticles for the ORR and Al–air batteries

The development of non-platinum catalysts for high efficiency oxygen reduction reaction (ORR) is one of the research hotspots of fuel cells and metal air cells.

Carbon-based single-atom catalysts: impacts of atomic coordination on the oxygen reduction reaction.

Slow oxygen reduction reaction (ORR) kinetics is the main factor restricting the development of fuel cells and metal-air batteries. Carbon-based single-atom catalysts (SACs) have the advantages of high electrical conductivity, maximal atom utilization, and high mass activity, thus showing great potential in exploring low-cost and high-efficiency ORR catalysts. For carbon-based SACs, the defects in the carbon support, the coordination of non-metallic heteroatoms, and the coordination number have a great influence on the adsorption of the reaction intermediates, thus significantly affecting the catalytic performance. Consequently, it is of vital importance to summarize the impacts of atomic coordination on the ORR. In this review, we focus on the regulation of the central atoms and coordination atoms of carbon-based SACs for the ORR. The survey involves various SACs, from noble metals (Pt) to transition metals (Fe, Co, Ni, Cu, etc.) and major group metals (Mg, Bi, etc.). At the same time, the influence of defects in the carbon support, the coordination of non-metallic heteroatoms (such as B, N, P, S, O, Cl, etc.), and the coordination number of the well-defined SACs on the ORR were put forward. Then, the impact of the neighboring metal monomers for SACs on the ORR performance is discussed. Finally, the current challenges and prospects for the future development of carbon-based SACs in coordination chemistry are presented.

Unveiling the inter-molecular charge transfer mechanism of N-doped graphene/carbon nanotubes heterostructure toward oxygen reduction process for Zn-air battery

Heteroatom-doped carbon-based materials have been deemed as highly efficient electrocatalysts for cathodic oxygen reduction reaction (ORR) in the sustainable energy conversion technologies. However, the limited active sites caused by the aggregation of carbon layers ineluctably hinder the mass transfer, thereby generating insufficient ORR performance. Herein, we offer a rational and facile method to manufacture a hybrid heterostructure of N-doped graphene and carbon nanotubes at the pyrolysis temperature of 900 °C (N-G/CNTs-900), which exhibits a better ORR activity than most of reported carbon-based ORR electrocatalysts, and comparable to that of Pt/C. Thus-fabricated N-G/CNTs-900 features hierarchically porous architecture, high specific surface area, abundant defects, and high active N content, and what’s more, it also affords active sites for oxygen activation and subsequent 4e− reduction processes due to the hetero-interlayer charge transfer and the electron accumulations of the carbon atoms on the surface layer by theoretical calculations. Compared with commercial Pt/C, N-G/CNTs-900-based Zn-air battery presents a higher peak power density (133.60 mW/cm2) and a larger specific capacity (707 mAh/gZn). This work provides a novel idea to design the high-efficiency and long-term durability ORR catalyst, which is benefit for boosting the commercial applications of metal-free carbon-based materials in the renewable energy conversion devices.

Iron/cobalt-decorated nitrogen-rich 3D layer-stacked porous biochar as high-performance oxygen reduction air-cathode catalyst in microbial fuel cell

Developing low-cost and high-efficiency oxygen reduction reaction (ORR) catalysts is crucial to the commercial application of microbial fuel cell (MFC). Herein, Fe/Co-decorated nitrogen-rich three-dimensional (3D) layer-stacked porous biochar (Fe/Co-NC-x) have been synthesized from silk gel through secondary carbonization of activated carbons which firstly adsorbed metal ions. The multilayer porous structure of Fe/Co-NC-3 contributes to construction of high specific surface area (576 m2 g-1), large pore volume (1.27 cm3 g-1) and many defect structure (ID/IG = 1.004). As expected, with Fe/Co synergistic effect, Fe/Co-NC-3 exhibits excellent ORR performance through 4e- pathway with good methanol resistance. In addition, the performance of MFC using Fe/Co-NC-3 as air-cathode catalyst is more prominent with higher maximum power density (1059.62 ± 30.00 mW m-2) compared to that using NC (668.19 ± 9.84 mW m-2) and commercial Pt/C catalyst (957.33 ± 10.50 mW m-2). Therefore, Fe/Co-NC-3 should be a prospective catalyst in the practical application of fuel cells and other energy devices.

Metal-organic frameworks derived Co/N-doped carbon nanonecklaces as high-efficient oxygen reduction reaction electrocatalysts

Metal/carbon-based materials derived from metal-organic frameworks have recently attracted great attention for oxygen reduction reaction (ORR) electrocatalysts, but poor electrical conductivity and insufficient stability owing to granular structures, as well as low production hinder their practical applications. Herein, we design and synthesize Co nanoparticles decorated on N-doped carbon nanonecklaces (Co/N-CNNs) by way of electrospinning and following carbonization. The Co/N-CNNs hybrid exhibits excellent ORR performance with a positive half-wave potential (0.865 V) and a large limiting current density (5.35 mA cm−2), even comparable to those of Pt/C. Such superior electrocatalytic properties are attributed to the distinctive necklace-like structure, which supplies high specific surface area, hierarchical porous structure, well-dispersed and covered Co nanoparticles, and hierarchical architecture of N-doped carbon. This work provides an effective strategy to fabricate ORR electrocatalysts with well-designed structure and mass production via electrospinning.

Multi-sites synergistic modulation in oxygen reduction electrocatalysis

Revealing the types of and interplays among multiple active-sites in iron-nitrogen-carbon (FeNC) materials is of great significance for developing high-performance, Fe-based non-precious metal catalysts for oxygen reduction reaction (ORR). In this paper, a single-atom FeNC catalyst is prepared through high-temperature pyrolyzing of melamine foam (MF), iron phthalocyanine (FePc), phthalocyanine (Pc), and zinc (Zn)-salts composite. The catalyst is found to contain a variety of active-sites, including carbon atom next to pyridinic-N (pyridinicNC), Fe-N4 and pore defect. It is shown that MF with high N-content is responsible for the formation of the main pyridinicNC sites and in the meantime acts as the self-sacrificed template for framework of the catalyst. The presence of Pc can facilitate the formation of the predominant Fe-N4 sites, since the interplay between Pc and FePc results in a confinement of Fe-N4. Zn-salts serve as the pore-forming additives to create sufficient pore defects which can also anchor pyridinicNC and Fe-N4 structures. The results of density functional theory (DFT) calculations suggest that the multiple active-sites function synergistically to enable high-efficiency ORR electrocatalysis. The optimal FeNC catalyst shows superior ORR activity with a half-wave potential of ∼0.88 V (vs. RHE), as well as high methanol tolerance and electrochemical stability compared to the commercial carbon-supported platinum (Pt/C) catalyst.

Nitrogen-doped carbon nanotubes filled with Fe3C nanowires for efficient electrocatalytic oxygen reduction

It is of great significance to develop the highly active, durable and inexpensive oxygen reduction reaction (ORR) electrocatalysts for fuel cells and metal-air batteries. Nitrogen-doped carbon nanotube (NCNTs) was considered to be one of the most significant non-noble metal ORR electrocatalysts, but its catalytic activity has some shortcomings due to its unfavorable ORR intermediates adsorption characteristics. Herein, we report a new kind of nitrogen-doped carbon nanotubes, which have filled with Fe3C nanowires (NCNTs@Fe3C NWs), working as a stable and effective ORR catalyst. The half-wave potential of the obtained catalyst in a 0.1 M KOH solution is 0.88 V, which is significantly higher than the pristine NCNTs (0.79 V) and the commercial Pt/C (0.85 V). Compared with NCNTs, the improvement in ORR activity of NCNTs@Fe3C NWs can be attributed to the strain effect on the outermost carbon layer due to the high filling of Fe3C NWs. Most importantly, NCNTs@Fe3C NWs shows higher durability and better methanol tolerance than the commercial Pt/C catalyst. Density functional theory (DFT) calculation shows that the tensile strain can optimize the binding energy of ORR intermediates to a certain extent. This work provides new research ideas for the rational design of high-efficiency carbon-based ORR electrocatalysts through the direction of strain engineering.

N-Doped CrS2 Monolayer as a Highly-Efficient Catalyst for Oxygen Reduction Reaction: A Computational Study.

Searching for low-cost and highly-efficient oxygen reduction reaction (ORR) catalysts is crucial to the large-scale application of fuel cells. Herein, by means of density functional theory (DFT) computations, we proposed a new class of ORR catalysts by doping the CrS2 monolayer with non-metal atoms (X@CrS2, X = B, C, N, O, Si, P, Cl, As, Se, and Br). Our results revealed that most of the X@CrS2 candidates exhibit negative formation energy and large binding energy, thus ensuring their high stability and offering great promise for experimental synthesis. Moreover, based on the computed free energy profiles, we predicted that N@CrS2 exhibits the best ORR catalytic activity among all considered candidates due to its lowest overpotential (0.41 V), which is even lower than that of the state-of-the-art Pt catalyst (0.45 V). Remarkably, the excellent catalytic performance of N@CrS2 for ORR can be ascribed to its optimal binding strength with the oxygenated intermediates, according to the computed linear scaling relationships and volcano plot, which can be well verified by the analysis of the p-band center as well as the charge transfer between oxygenated species and catalysts. Therefore, by carefully modulating the incorporated non-metal dopants, the CrS2 monolayer can be utilized as a promising ORR catalyst, which may offer a new strategy to further develop eligible electrocatalysts in fuel cells.

Mesoporous sulfur-modified metal oxide cathodes for efficient electro-Fenton systems

Development of cathode materials with high activity, stability, and wide operating pH range is crucial for electro-Fenton reactions. Herein, mesoporous sulfur-modified metal oxides were coated onto stainless steel meshes and applied as a cathode to promote in-situ generation of H2O2 for electro-Fenton processes. Different mesoporous metal oxides with and without sulfur-modification were synthesized using Fe, Ni, and Zn precursors to investigate the effect of sulfur doping. Among them, sulfur doped Fe2O3 showed the best electrocatalytic performance, and it was attributed to several factors such as large specific surface area, high-efficiency oxygen reduction reaction activity, and enhanced electron/charge transfer. The effect of operational parameters such as applied voltage, catalyst dosage, initial pH, initial concentration of phenol, and electrolyte concentration on the catalytic performance was investigated in terms of degradation efficiencies and rate constants. The results indicate that for high concentration 100 ppm phenol solution, 78% removal efficiency with a rate constant of 0.186 h−1 was achieved with sulfur-doped Fe2O3 on stainless steel mesh electrode under optimal conditions in 8 h. This cathode worked effectively in a wide pH range (3–10) due to its pH self-control ability and exhibited superior stability and reusability with insignificant deterioration in the catalytic activity. This study offers an alternative approach to fabricate electrocatalysts with low-cost sulfur-doped metal oxides for electro-Fenton systems.

Cobalt phosphide nanoparticles encapsulated in manganese, nitrogen co-doped porous carbon nanosheets with rich nanoholes for high-efficiency oxygen reduction reaction

It is a challenging task to research oxygen reduction electrocatalysts with cost-effectiveness, high-performance and ultra-stability to replace traditional noble metal catalysts in renewable energy conversion/storage devices. Herein, cobalt phosphide (Co2P) nanoparticles encapsulated in Mn, N co-doped porous carbon nanosheets with abundant nanoholes (Co2P/Mn,N-PCNS) were prepared by a alizarin complexone coordination regulated pyrolysis at 800 °C. In the controlled experiments, the pyrolysis temperature and metal types were investigated in details. The resultant catalyst exhibited rapid mass/charge transfer and superior oxygen reduction reaction (ORR) performance (Eonset = 0.96 V; E1/2 = 0.86 V vs RHE), surpassing commercial Pt/C. This work presents some constructive guidelines for synthesis of appealing ORR electrocatalysts in renewable energy technology.

Construction of N, P Co-Doped Carbon Frames Anchored with Fe Single Atoms and Fe2 P Nanoparticles as a Robust Coupling Catalyst for Electrocatalytic Oxygen Reduction.

A coupling catalyst of highly dispersed N, P co-doped carbon frames (NPCFs) anchored with Fe single atoms (SAs) and Fe2 P nanoparticles (NPs) is synthesized by a novel in situ doping-adsorption-phosphatization strategy for the electrocatalytic oxygen reduction reaction (ORR). The optimized Fe SAs-Fe2 P NPs/NPCFs-2.5 catalyst shows a superior ORR activity and stability in 0.5 m H2 SO4 and 0.1 m KOH, respectively. Theoretical calculations reveal a synergistic effect, in that the existence of Fe2 P weakens the adsorption of ORR intermediates on active sites and lowers the reaction free energy. The doped P atoms with a strong electron-donating ability elevate the energy level of Fe-3d orbitals and facilitate the adsorption of O2 . The active Fe atoms exist in a low oxidation state and are less positively charged, and they serve as an electron reservoir capable of donating and releasing electrons, thus improving the ORR activity. Operando and in situ characterization results indicate that the atomically dispersed FeN4 /FeP coupled active centers in the Fe SAs-Fe2 P NPs/NPCFs-2.5 catalyst are characteristic of the different catalytic mechanisms in acidic and alkaline media. This work proposes a novel idea for constructing coupling catalysts with atomic-level precision and provides a strong reference for the development of high-efficiency ORR electrocatalysts for practical application.

Conductive tungsten oxynitride supported highly dispersed cobalt nanoclusters for enhanced oxygen reduction

Herein, a high-performance electrocatalyst for oxygen reduction reaction (ORR), which is composed of highly dispersed Co nanoclusters loaded on conductive oxygen-deficient tungsten oxynitride framework structured with three-dimensionally ordered macropores (3DOM Co@WNO) was proposed. The tungsten oxynitride carrier decorated with oxygen vacancies can improve the conductivity and electrochemical stability of the catalyst. At the same time, it offers strong anchoring interaction and ensures good dispersion for the loaded Co metals, thus achieving high catalytic activity and durability. Theoretical calculations show that electron transfer occurs at the interface between WNO and Co4 leading to a higher accumulation of electrons around the Co site, promoting the ORR reaction kinetics. Due to these advantages, 3DOM Co@WNO was found to possess ORR activity comparable to Pt/C and superior primary Zn-air battery performance over Pt/C and excellent stability as well. This study advances the evolvement of high-efficiency and durable ORR electrocatalysts using intrinsically robust and conductive oxynitride support strategy with strong metal-support interactions.

Coupling nano-Fe3O4 with oxygen vacancies on a hypercrosslinked iron porphyrin-coated ZIF-8 as a high-efficiency oxygen reduction reaction electrocatalyst

In this study, a novel and highly efficient ORR catalyst is designed via the pyrolysis of zeolitic imidazole framework-8 (ZIF-8) under the protection of hypercrosslinked iron porphyrins. Fe3O4 nanoparticles with oxygen vacancies are formed in situ and promoting catalytic efficiency. Benefiting from the synergistic effect among the Fe3O4 and ZIF-8 skeleton, the hybrid catalyst exhibits excellent ORR activity even surpassing commercial Pt/C. The pyrolyzation temperature is studied in detail and determined to be a crucial parameter for ORR performance. Nano-Fe3O4, which has high catalytic efficiency, is formed in situ at 800 ℃. Amorphous iron is formed at 600 ℃ and 700 ℃, and zero-valent iron is formed at 900 ℃. The content of oxygen vacancies, Fe–N, and graphite–N, which are beneficial to the ORR reaction, are all reached to a maximum at 800 ℃. This study provides a new approach to design highly active Fe3O4-based electrocatalytic materials.

Local Coordination Regulation through Tuning Atomic-Scale Cavities of Pd Metallene toward Efficient Oxygen Reduction Electrocatalysis.

Moderate adsorption of oxygenated intermediates takes a significant role in rational design of high-efficiency oxygen reduction reaction (ORR) electrocatalysts. Long-serving as a reliable strategy to tune geometric structure of nanomaterials, defect engineering enjoys the great ability of adjusting the coordination environment of catalytic active sites, which enables dominant regulation of adsorption energy and kinetics of ORR catalysis. However, limited to controllable nanocrystals fabrication, inducing uniformly dispersed high-coordinated defects into ultrathin 2D nanosheets remains challenging. Herein, atomic-scale cavities (ASCs) are proposed as a new kind of high-coordinated active site and successfully introduced into suprathin Pd (111)-exposed metallene. Due to its atomic concave architecture, leading to elevated CN and moderately downshifted d-band center, the as-made Pd metallene with ASCs (c-Pd M) exhibits excellent ORR performance with mass activity of 2.76 A mgPd -1 at 0.9V versus reversible hydrogen electrode (RHE) and half-wave potential as high as 0.947V, which is 18.9 (2.7) times higher and 104 (46) mV larger than that of commercial Pt/C (Pd metallene without ASCs). Besides, the durability of c-Pd M exceeds its commercial counterpart with ≈30% loss after 5000 cycles. This work highlights a new-style mentality of designing fancy active sites toward efficient ORR electrocatalysis.