Edge-hosted Atomic Co-N4 Sites on Hierarchical Porous Carbon for Highly Selective Two-electron Oxygen Reduction Reaction

Efficient and durable oxygen reduction in alkaline media by doping heteroatomic boron into FeSA-NC catalyst

Despite extensive research has been conducted on atomic dispersion catalysts for various reactions, altering the electronic structure of the central metal to enhance electrochemical reactivity remains a challenging task. Herein, the electrochemical reactivity was considerably enhanced by introducing heteroatomic B to adjust the d-band of single Fe center. In specific, the obtained FeSA-BNC catalyst demonstrated an outstanding ORR performance (E1/2 = 0.87 V) and exhibited greater long-term durability in alkaline media compared to Pt/C. The performance of FeSA-BNC in Zn-air battery was also higher than that of Pt/C. According to theoretical calculations, a downward shift in the d-band center of Fe was induced by introducing B, thereby improving the desorption of intermediates and facilitating the oxygen reduction reaction (ORR).

Coupling Ni Single Atomic Sites with Metallic Aggregates at Adjacent Geometry on Carbon Support for Efficient Hydrogen Peroxide Electrosynthesis

Single atomic catalysts have shown great potential in efficiently electro-converting O2 to H2O2 with high selectivity. However, the impact of coordination environment and introduction of extra metallic aggregates on catalytic performance still remains unclear. Herein, first a series of carbon-based catalysts with embedded coupling Ni single atomic sites and corresponding metallic nanoparticles at adjacent geometry is synthesized. Careful performance evaluation reveals NiSA/NiNP-NSCNT catalyst with precisely controlled active centers of synergetic adjacent Ni-N4S single sites and crystalline Ni nanoparticles exhibits a high H2O2 selectivity over 92.7% within a wide potential range (maximum selectivity can reach 98.4%). Theoretical studies uncover that spatially coupling single atomic NiN4S sites with metallic Ni aggregates in close proximity can optimize the adsorption behavior of key intermediates *OOH to achieve a nearly ideal binding strength, which thus affording a kinetically favorable pathway for H2O2 production. This strategy of manipulating the interaction between single atoms and metallic aggregates offers a promising direction to design new high-performance catalysts for practical H2O2 electrosynthesis.

Understanding Advanced Transition Metal-Based Two Electron Oxygen Reduction Electrocatalysts from the Perspective of Phase Engineering

Non-noble transition metal (TM)-based compounds have recently become a focal point of extensive research interest as electrocatalysts for the two electron oxygen reduction (2e- ORR) process. To efficiently drive this reaction, these TM-based electrocatalysts must bear unique physiochemical properties, which are strongly dependent on their phase structures. Consequently, adopting engineering strategies toward the phase structure has emerged as a cutting-edge scientific pursuit, crucial for achieving high activity, selectivity, and stability in the electrocatalytic process. This comprehensive review addresses the intricate field of phase engineering applied to non-noble TM-based compounds for 2e- ORR. First, the connotation of phase engineering and fundamental concepts related to oxygen reduction kinetics and thermodynamics are succinctly elucidated. Subsequently, the focus shifts to a detailed discussion of various phase engineering approaches, including elemental doping, defect creation, heterostructure construction, coordination tuning, crystalline design, and polymorphic transformation to boost or revive the 2e- ORR performance (selectivity, activity, and stability) of TM-based catalysts, accompanied by an insightful exploration of the phase-performance correlation. Finally, the review proposes fresh perspectives on the current challenges and opportunities in this burgeoning field, together with several critical research directions for the future development of non-noble TM-based electrocatalysts.

General Design Concept of High-Performance Single-Atom-Site Catalysts for H2O2 Electrosynthesis

Hydrogen peroxide (H2O2) as a green oxidizing agent is widely used in various fields. Electrosynthesis of H2O2 has gradually become a hotspot due to its convenient and environment-friendly features. Single-atom-site catalysts (SASCs) with uniform active sites are the ideal catalysts for the in-depth study of the reaction mechanism and structure-performance relationship. In this review, the outstanding achievements of SASCs in the electrosynthesis of H2O2 through 2e- oxygen reduction reaction (ORR) and 2e- water oxygen reaction (WOR) in recent years, are summarized. First, the elementary steps of the two pathways and the roles of key intermediates (*OOH and *OH) in the reactions are systematically discussed. Next, the influence of the size effect, electronic structure regulation, the support/interfacial effect, the optimization of coordination microenvironments, and the SASCs-derived catalysts applied in 2e- ORR are systematically analyzed. Besides, the developments of SASCs in 2e- WOR are also overviewed. Finally, the research progress of H2O2 electrosynthesis on SASCs is concluded, and an outlook on the rational design of SASCs is presented in conjunction with the design strategies and characterization techniques.

Single-Atom Co─O4 Sites Embedded in a Defective-Rich Porous Carbon Layer for Efficient H2O2 Electrosynthesis

The production of hydrogen peroxide (H2O2) 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 H2O2 electrosynthesis via 2e- ORR, providing a high H2O2 selectivity of 98.8% at 0.6 V and a low onset potential of 0.73 V for generating H2O2. 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 H2O2 production. Particularly, a high H2O2 production rate of 0.25 mol g-1 cat h-1 at 0.6 V can be obtained by the flow cell. The in situ-generated H2O2 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 H2O2 by using Co─O4 single atom electrocatalyst and unveil the electrocatalytic mechanism.

Efficient Neutral H2O2 Electrosynthesis from Favorable Reaction Microenvironments via Porous Carbon Carrier Engineering

The efficient electrosynthesis of hydrogen peroxide (H2O2) via two-electron oxygen reduction reaction (2e- ORR) in neutral media is undoubtedly a practical route, but the limited comprehension of electrocatalysts has hindered the system advancement. Herein, we present the design of model catalysts comprising mesoporous carbon spheres-supported Pd nanoparticles for H2O2 electrosynthesis at near-zero overpotential with approximately 95 % selectivity in a neutral electrolyte. Impressively, the optimized Pd/MCS-8 electrocatalyst in a flow cell device achieves an exceptional H2O2 yield of 15.77 mol gcatalyst -1 h-1, generating a neutral H2O2 solution with an accumulated concentration of 6.43 wt %, a level sufficiently high for medical disinfection. Finite element simulation and experimental results suggest that mesoporous carbon carriers promote O2 enrichment and localized pH elevation, establishing a favorable microenvironment for 2e- ORR in neutral media. Density functional theory calculations reveal that the robust interaction between Pd nanoparticles and the carbon carriers optimized the adsorption of OOH* at the carbon edge, ensuring high active 2e- process. These findings offer new insights into carbon-loaded electrocatalysts for efficient 2e- ORR in neutral media, emphasizing the role of carrier engineering in constructing favorable microenvironments and synergizing active sites.

Planar Chlorination Engineering: A Strategy of Completely Breaking the Geometric Symmetry of Fe-N4 Site for Boosting Oxygen Electroreduction

Introducing asymmetric elements and breaking the geometric symmetry of traditional metal-N4 site for boosting oxygen reduction reaction (ORR) are meaningful and challenging. Herein, the planar chlorination engineering of Fe-N4 site is first proposed for remarkably improving the ORR activity. The Fe-N4/CNCl catalyst with broken symmetry exhibits a half-wave potential (E1/2) of 0.917 V versus RHE, 49 and 72 mV higher than those of traditional Fe-N4/CN and commercial 20 wt% Pt/C catalysts. The Fe-N4/CNCl catalyst also has excellent stability for 25 000 cycles and good methanol tolerance ability. For Zn-air battery test, the Fe-N4/CNCl catalyst has the maximum power density of 228 mW cm-2 and outstanding stability during 150 h charge-discharge test, as the promising substitute of Pt-based catalysts in energy storage and conversion devices. The density functional theory calculation demonstrates that the adjacent C─Cl bond effectively breaks the symmetry of Fe-N4 site, downward shifts the d-band center of Fe, facilitates the reduction and release of OH*, and remarkably lowers the energy barrier of rate-determining step. This work reveals the enormous potential of planar chlorination engineering for boosting the ORR activity of traditional metal-N4 site by thoroughly breaking their geometric symmetry.

Ultrasensitive cortisol electrochemical immunosensor amplifying by Au single-atom nanozymes and HRP enzymes

Cortisol, a corticosteroid hormone as a primary stress hormone response to internal and external stress, has been regarded as a gold standard reliable biomarker to evaluate human mental stress. The double enzymes strategy, using nanozyme and enzyme amplifying the electrochemical signal, has been widely used to improve the performance of electrochemical biosensors. An ultra-sensitive electrochemical cortisol sensor based on Au single-atom nanozymes had been fabricated through HRP labeled anti-cortisol antibody binding with Au by Au-S bond. Based on the high catalytic activity of Au single-atom nanozymes and the high selectivity of HRP-labeled anti-cortisol antibodies, the cortisol electrochemical sensor-based Au single-atom nanozymes had an excellent response to cortisol, such as high electrochemical activity, high sensitivity, high selectivity, and wide linear range (0.15-300 ng mL-1) and low detection (0.48 pg mL-1) through the four-parameter logistic model with 95% confidence. The electrochemical cortisol sensor was used to determine the cortisol concentration of human saliva at different times.

Recent advances in electrosynthesis of H2O2via two-electron oxygen reduction reaction

The electrosynthesis of hydrogen peroxide (H2O2) via a selective two-electron oxygen reduction reaction (2e- ORR) presents a green and low-energy-consumption alternative to the traditional, energy-intensive anthraquinone process. This review encapsulates the principles of designing relational electrocatalysts for 2e- ORR and explores remaining setups for large-scale H2O2 production. Initially, the review delineates the fundamental reaction mechanisms of H2O2 production via 2e- ORR and assesses performance. Subsequently, it methodically explores the pivotal influence of microstructures, heteroatom doping, and metal hybridization along with setup configurations in achieving a high-performance catalyst and efficient reactor for H2O2 production. Thereafter, the review introduces a forward-looking methodology that leverages the synergistic integration of catalysts and reactors, aiming to harmonize the complementary characteristics of both components. Finally, it outlines the extant challenges and the promising avenues for the efficient electrochemical production of H2O2, setting the stage for future research endeavors.

Axially Modified Square-Pyramidal CoN4-F1 Sites Enabling High-Performance Zn-Air Batteries

Cobalt-nitrogen-carbon (Co-N-C) catalysts with a CoN4 structure exhibit great potential for oxygen reduction reaction (ORR), but the imperfect adsorption energy toward oxygen species greatly limits their reduction efficiency and practical application potential. Here, F-coordinated Co-N-C catalysts with square-pyramidal CoN4-F1 configuration are successfully synthesized using F atoms to regulate the axial coordination of Co centers via hydrothermal and chemical vapor deposition methods. During the synthesis process, the geometry structure of the Co atom converts from six-coordinated Co-F6 to square-pyramidal CoN4-F1 in the coordinatively unsaturated state, which provides an open binding site for the O2. The introduction of axial F atoms into the CoN4 plane alters the local atomic environment around Co, significantly improving the ORR activity and Zn-air batteries performance. In situ spectroscopy proves that CoN4-F1 sites strongly combine with the OOH* intermediate and facilitate the splitting of O-O bond, making OOH* readily decompose into O* and OH* via a dissociative pathway. Theoretical calculations confirm that the axial F atom effectively reduces the electronic density of the Co centers and facilitates the desorption of the OH* intermediate, efficiently accelerating the overall ORR kinetics. This work advances a feasible synthesis mechanism of axial ligands and provides a route to construct efficient high-coordination catalysts.

Electronic structure modification of metal phthalocyanines by a carbon nanotube support for efficient oxygen reduction to hydrogen peroxide

An active and selective two-electron oxygen reduction reaction (2e- ORR) is required for efficient electrosynthesis of H2O2. This reaction can be promoted by metal phthalocyanines (MPcs), which serve as model catalysts with well-defined structures. MPc molecules have mostly been evaluated on conductive carbon-based substrates, including glassy carbon (GC) and carbon nanotubes (CNTs), yet their influence on the electrocatalytic properties is not well understood. This study demonstrated that the ORR activity per surface area was improved by up to 4-fold with MPc molecules supported on CNTs (MPc/CNTs, M = Co, Mn, and Fe) compared to MPc loaded directly on GC. Ultraviolet photoelectron spectroscopy and density functional theory calculations revealed that the CNTs modified the electronic structure of the MPc molecules to optimize the *OOH binding energy and boost the heterogeneous electron transfer rates. Detailed kinetic analysis enabled multiple reaction pathways to be decoupled to extract the metal-dependent intrinsic 2e-/4e- ORR activities. Finally, MPc/CNT catalysts were employed in an H2O2 electrosynthesis flow cell, which delivered an industrial-scale current density of -200 mA cm-2 and an H2O2 faradaic efficiency of 88.7 ± 0.6% with the CoPc/CNT catalyst in a neutral electrolyte.

Polypyrrole regulates Active Sites in Co-based Catalyst in Direct Borohydride Fuel Cells

Direct borohydride fuel cells (DBFCs) convert borohydride (NaBH4) chemical energy into clean electricity. However, catalytic active site deactivation in NaBH4 solution limits their performance and stability. We propose a strategy to regulate active sites in Co-based catalysts using polypyrrole modification (Co-PX catalyst) to enhance electrochemical borohydride oxidation reaction (eBOR). As an anode catalyst, the synthesized Co-PX catalyst exhibits excellent eBOR performance in DBFCs, with current density of 280 mA ⋅ cm-2 and power density of 151 mW ⋅ cm-2, nearly twice that of the unmodified catalyst. The Co-PX catalyst shows no degradation after 120-hour operation, unlike the rapidly degrading control. In-situ electrochemical attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIRS) and density functional theory (DFT) suggest that polypyrrole-modified carbon support regulate the charge distribution, increasing oxidation state and optimizing adsorption/desorption of intermediates. A possible reaction pathway is proposed. This work presents a promising strategy for efficient polymer-modulated catalysts in advanced DBFCs.

Engineering Co-N-Cr Cross-Interfacial Electron Bridges to Break Activity-Stability Trade-Off for Superdurable Bifunctional Single Atom Oxygen Electrocatalysts

Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts have exhibited encouraging oxygen reduction reaction (ORR) activity. Nevertheless, the insufficient long-term stability remains a widespread concern owing to the inevitable 2-electron byproducts, H2O2. Here, we construct Co-N-Cr cross-interfacial electron bridges (CIEBs) via the interfacial electronic coupling between Cr2O3 and Co-N-C, breaking the activity-stability trade-off. The partially occupied Cr 3d-orbitals of Co-N-Cr CIEBs induce the electron rearrangement of CoN4 sites, lowering the Co-OOH* antibonding orbital occupancy and accelerating the adsorption of intermediates. Consequently, the Co-N-Cr CIEBs suppress the two-electron ORR process and approach the apex of Sabatier volcano plot for four-electron pathway simultaneously. As a proof-of-concept, the Co-N-Cr CIEBs is synthesized by the molten salt template method, exhibiting dominant 4-electron selectively and extremely low H2O2 yield confirmed by Damjanovic kinetic analysis. The Co-N-Cr CIEBs demonstrates impressive bifunctional oxygen catalytic activity (▵E=0.70 V) and breakthrough durability including 100 % current retention after 10 h continuous operation and cycling performance over 1500 h for Zn-air battery. The hybrid interfacial configuration and the understanding of the electronic coupling mechanism reported here could shed new light on the design of superdurable M-N-C catalysts.

Elucidating the impact of oxygen functional groups on the catalytic activity of M-N4-C catalysts for the oxygen reduction reaction: a density functional theory and machine learning approach

Efforts to enhance the efficiency of electrocatalysts for the oxygen reduction reaction (ORR) in energy conversion and storage devices present formidable challenges. In this endeavor, M-N4-C single-atom catalysts (MN4) have emerged as promising candidates due to their precise atomic structure and adaptable electronic properties. However, MN4 catalysts inherently introduce oxygen functional groups (OGs), intricately influencing the catalytic process and complicating the identification of active sites. This study employs advanced density functional theory (DFT) calculations to investigate the profound influence of OGs on ORR catalysis within MN4 catalysts (referred to as OGs@MN4, where M represents Fe or Co). We established the following activity order for the 2eORR: for OGs@CoN4: OH@CoN4 > CoN4 > CHO@CoN4 > C-O-C@CoN4 > COC@CoN4 > COOH@CoN4 > CO@CoN4; for OGs@FeN4: COC@FeN4 > CO@FeN4 > OH@FeN4 > FeN4 > COOH@FeN4 > CHO@FeN4 > C-O-C@FeN4. Multiple oxygen combinations were constructed and found to be the true origin of MN4 activity (for instance, the overpotential of 2OH@CoN4 as low as 0.07 V). Furthermore, we explored the performance of the OGs@MN4 system through charge and d-band center analysis, revealing the limitations of previous electron-withdrawing/donating strategies. Machine learning analysis, including GBR, GPR, and LINER models, effectively guides the prediction of catalyst performance (with an R2 value of 0.93 for predicting ΔG*OOH_vac in the GBR model). The Eg descriptor was identified as the primary factor characterizing ΔG*OOH_vac (accounting for 62.8%; OGs@CoN4: R2 = 0.9077, OGs@FeN4: R2 = 0.7781). This study unveils the significant impact of OGs on MN4 catalysts and pioneers design and synthesis criteria rooted in Eg. These innovative findings provide valuable insights into understanding the origins of catalytic activity and guiding the design of carbon-based single-atom catalysts, appealing to a broad audience interested in energy conversion technologies and materials science.

Ni clusters immobilized on oxygen-rich siloxene nanosheets for efficient electrocatalytic oxygen reduction toward H2O2 synthesis

Hydrogen peroxide (H2O2) electrosynthesis via the two-electron oxygen reduction reaction (2e- ORR) represents a green alternative to the energy-intensive anthraquinone process. However, the practical application of this method is limited by the lack of cost-effective and high-performance electrocatalysts. Reported here is a hybrid catalyst composed of nickel (Ni) clusters immobilized onto the surface of two-dimensional siloxene nanosheets (Ni/siloxene), which exhibits excellent efficiency and selectivity in electrocatalytic oxygen reduction to H2O2 in an alkaline medium, demonstrating a standard 2e- pathway with >95% H2O2 selectivity across a wide potential range. Experimental results disclose that the high performance of Ni/siloxene can be traced to a synergy of the Ni clusters and the oxygen-rich surface of siloxene. Density functional theory (DFT) calculations further reveal a weakened interaction between Ni/siloxene and *OOH and the consequently reduced energy barrier for the *OOH protonation toward H2O2 desorption, thus leading to a high 2e- ORR reactivity and selectivity. This work provides a valuable and practical guidance for designing high-performance 2e- ORR electrocatalysts based on the rational engineering of the metal-support interaction.

Dithiine-linked metalphthalocyanine framework with undulated layers for highly efficient and stable H2O2 electroproduction

Realization of stable and industrial-level H2O2 electroproduction still faces great challenge due large partly to the easy decomposition of H2O2. Herein, a two-dimensional dithiine-linked phthalocyaninato cobalt (CoPc)-based covalent organic framework (COF), CoPc-S-COF, was afforded from the reaction of hexadecafluorophthalocyaninato cobalt (II) with 1,2,4,5-benzenetetrathiol. Introduction of the sulfur atoms with large atomic radius and two lone-pairs of electrons in the C-S-C linking unit leads to an undulated layered structure and an increased electron density of the Co center for CoPc-S-COF according to a series of experiments in combination with theoretical calculations. The former structural effect allows the exposition of more Co sites to enhance the COF catalytic performance, while the latter electronic effect activates the 2e- oxygen reduction reaction (2e- ORR) but deactivates the H2O2 decomposition capability of the same Co center, as a total result enabling CoPc-S-COF to display good electrocatalytic H2O2 production performance with a remarkable H2O2 selectivity of >95% and a stable H2O2 production with a concentration of 0.48 wt% under a high current density of 125 mA cm-2 at an applied potential of ca. 0.67 V versus RHE for 20 h in a flow cell, representing the thus far reported best H2O2 synthesis COFs electrocatalysts.

Lattice Distortion and H-passivation in Pure Carbon Electrocatalysts for Efficient and Stable Two-electron Oxygen Reduction to H2 O2

The exploration of inexpensive and efficient catalysts for oxygen reduction reaction (ORR) is crucial for chemical and energy industries. Carbon materials have been proved promising with different catalysts enabling 2 and 4e- ORR. Nevertheless, their ORR activity and selectivity is still complex and under debate in many cases. Many structures of these active carbon materials are also chemically unstable for practical implementations. Unlike the well-discussed structures, this work presents a strategy to promote efficient and stable 2e- ORR of carbon materials through the synergistic effect of lattice distortion and H-passivation (on the distorted structure). We show how these structures can be formed on carbon cloth, and how the reproducible chemical adsorption can be realized on these structures for efficient and stable H2 O2 production. The work here gives not only new understandings on the 2e- ORR catalysis, but also the robust catalyst which can be directly used in industry.

Dual-Ligand Strategy to Construct Metal Organic Gel Catalyst with the Optimized Electronic Structure for High-Efficiency Overall Water Splitting and Flexible Metal-Air Battery

Non-noble metal catalysts are known for their efficient catalytic performance for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). Metal organic gels (MOGs) can be considered as a promising electrocatalyst owing to the diverse physicochemical properties but usually suffer from its poor electrical conductivity and catalytic stability. Here, a FeCo-MOG is constructed with considerable trifunctional activity. The optimal P-CoFe-H3 prepared by using phytic acid (PA) and 2,4,6-Tris[(p-carboxyphenyl)amino]-1,3,5-triazine benzoic acid (H3 TATAB) as dual ligands), exhibits outstanding ORR, OER, and HER activities as well as stability, exceeding most of state-of-the-art catalysts. As expected, the flexible Zn-air battery applied with P-CoFe-H3 as air cathode displays considerable power density, discharge voltage plateau, and cycling stability. Impressively, it is also capable of driving the overall water-splitting device by applying the P-CoFe-H3 as anode and cathode. Furthermore, theoretical calculations reveal that dual ligands can optimize the coordination environment and charge density of active sites, thereby reducing the absorption energy of intermediate species and boosting the catalytic performance. This work endows the dual-ligands coordination strategy with great potentiality for MOGs-based electrocatalysts in energy conversion devices.

An Ultrastable Bifunctional Electrocatalyst Derived from a Co2+-Anchored Covalent-Organic Framework for High-Efficiency ORR/OER and Rechargeable Zinc-Air Battery

It remains a great challenge to develop alternative electrocatalysts with high stability for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, a bifunctional electrocatalyst composed of hollow CoOx (Co3O4/CoO) nanoparticles embedded in lamellar carbon nanofibers is derived from a Co2+-anchored covalent-organic framework. The as-fabricated electrocatalyst (CoOx@NC-800) exhibits a half-wave potential (E1/2) of 0.89 V with ultrahigh long-term stability (100% current retention after 3000 CV cycles). Together with promising OER performance, the CoOx@NC-800 based reversible Zn-air battery displays a small potential gap (0.70 V), superior to that of the commercial 20% Pt/C + RuO2. The density functional theory (DFT) calculations reveal that the remarkable electrocatalytic performance and stability of CoOx@NC-800 are attributed to the optimized adsorption of the *OOH intermediate and reduced free energy of the potential-limiting step. This study establishes the functionalization of COF structure for fabrication of high-performance carbon-based electrocatalysts.

p-Block Bismuth Nanoclusters Sites Activated by Atomically Dispersed Bismuth for Tandem Boosting Electrocatalytic Hydrogen Peroxide Production

Constructing electrocatalysts with p-block elements is generally considered rather challenging owing to their closed d shells. Here for the first time, we present a p-block-element bismuth-based (Bi-based) catalyst with the co-existence of single-atomic Bi sites coordinated with oxygen (O) and sulfur (S) atoms and Bi nanoclusters (Biclu ) (collectively denoted as BiOSSA /Biclu ) for the highly selective oxygen reduction reaction (ORR) into hydrogen peroxide (H2 O2 ). As a result, BiOSSA /Biclu gives a high H2 O2 selectivity of 95 % in rotating ring-disk electrode, and a large current density of 36 mA cm-2 at 0.15 V vs. RHE, a considerable H2 O2 yield of 11.5 mg cm-2 h-1 with high H2 O2 Faraday efficiency of ∼90 % at 0.3 V vs. RHE and a long-term durability of ∼22 h in H-cell test. Interestingly, the experimental data on site poisoning and theoretical calculations both revealed that, for BiOSSA /Biclu , the catalytic active sites are on the Bi clusters, which are further activated by the atomically dispersed Bi coordinated with O and S atoms. This work demonstrates a new synergistic tandem strategy for advanced p-block-element Bi catalysts featuring atomic-level catalytic sites, and the great potential of rational material design for constructing highly active electrocatalysts based on p-block metals.

Strategies for Sustainable Production of Hydrogen Peroxide via Oxygen Reduction Reaction: From Catalyst Design to Device Setup

An environmentally benign, sustainable, and cost-effective supply of H2O2 as a rapidly expanding consumption raw material is highly desired for chemical industries, medical treatment, and household disinfection. The electrocatalytic production route via electrochemical oxygen reduction reaction (ORR) offers a sustainable avenue for the on-site production of H2O2 from O2 and H2O. The most crucial and innovative part of such technology lies in the availability of suitable electrocatalysts that promote two-electron (2e-) ORR. In recent years, tremendous progress has been achieved in designing efficient, robust, and cost-effective catalyst materials, including noble metals and their alloys, metal-free carbon-based materials, single-atom catalysts, and molecular catalysts. Meanwhile, innovative cell designs have significantly advanced electrochemical applications at the industrial level. This review summarizes fundamental basics and recent advances in H2O2 production via 2e--ORR, including catalyst design, mechanistic explorations, theoretical computations, experimental evaluations, and electrochemical cell designs. Perspectives on addressing remaining challenges are also presented with an emphasis on the large-scale synthesis of H2O2 via the electrochemical route.

Atomically Dispersed Manganese on Carbon Substrate for Aqueous and Aprotic CO2 Electrochemical Reduction

CO₂ utilization and conversion are of great importance in alleviating the rising CO₂ concentration in the atmosphere. Here, a single-atom catalyst (SAC) is reported for electrochemical CO₂ utilization in both aqueous and aprotic electrolytes. Specifically, atomically dispersed Mn-N₄ sites are embedded in bowl-like mesoporous carbon particles with the functionalization of epoxy groups in the second coordination spheres. Theoretical calculations suggest that the epoxy groups near the Mn-N₄ site adjust the electronic structure of the catalyst with reduced reaction energy barriers for the electrocatalytic reduction of CO₂ to CO. The resultant Mn-single-atom carbon with N and O doped catalyst (MCs-(N,O)) exhibits extraordinary electrocatalytic performance with a high CO faradaic efficiency of 94.5%, a high CO current density of 13.7 mA cm^-2 , and a low overpotential of 0.44 V in the aqueous environment. Meanwhile, as a cathode catalyst for aprotic Li-CO₂ batteries, the MCs-(N,O) with well-regulated active sites and unique mesoporous bowl-like morphology optimizes the nucleation behavior of discharge products. MCs-(N,O)-based batteries deliver a low overpotential and excellent cyclic stability of 1000 h. The findings in this work provide a new avenue to design and fabricate SACs for various electrochemical CO₂ utilization systems.

Regulating Spin States in Oxygen Electrocatalysis

Developing efficient and stable transition metal oxides catalysts for energy conversion processes such as oxygen evolution reaction and oxygen reduction reaction is one of the key measures to solve the problem of energy shortage. The spin state of transition metal oxides is strongly correlated with their catalytic activities. In an octahedral structure of transition metal oxides, the spin state of active centers could be regulated by adjusting the splitting energy and the electron pairing energy. Regulating spin state of active centers could directly modulate the d orbitals occupancy, which influence the strength of metal-ligand bonds and the adsorption behavior of the intermediates. In this review, we clarified the significance of regulating spin state of the active centers. Subsequently, we discussed several characterization technologies for spin state and some recent strategies to regulate the spin state of the active centers. Finally, we put forward some views on the future research direction of this vital field.