Carbon-Based Electrocatalysts for Acidic Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) is vital for clean and renewable energy technologies, which require no fossil fuel but catalysts. Platinum (Pt) is the best-known catalyst for ORR. However, its high cost and scarcity have severely hindered renewable energy devices (e.g., fuel cells) for large-scale applications. Recent breakthroughs in carbon-based metal-free electrochemical catalysts (C-MFECs) show great potential for earth-abundant carbon materials as low-cost metal-free electrocatalysts towards ORR in acidic media. This article provides a focused, but critical review on C-MFECs for ORR in acidic media with an emphasis on advances in the structure design and synthesis, fundamental understanding of the structure-property relationship and electrocatalytic mechanisms, and their applications in proton exchange membrane fuel cells. Current challenges and future perspectives in this emerging field are also discussed.     

Bioinspired Single-Atom Sites Enable Efficient Oxygen Activation for Switching Anodic/Cathodic Electrochemiluminescence

Exploring advanced co-reaction accelerators with superior oxygen reduction activity that generate rich reactive oxygen species (ROS) has attracted great attention in boosting luminol-O₂ electrochemiluminescence (ECL). However, tuning accelerators for efficient and selective catalytic O₂ activation to switch anodic/cathodic ECL is very challenging. Herein, we report that enzyme-inspired Fe-based single-atom catalysts with axial N/C coordination structures (FeN₅, FeN₄© SACs) can generate specific ROS for cathodic/anodic ECL conversion. Mechanistic studies reveal that FeN₅ sites prefer to produce highly active hydroxyl radicals and afford direct cathodic luminescence by promoting the cleavage of O−O bonds through N-induced electron redistribution. In contrast, FeN₄© sites tend to produce superoxide radicals, resulting in inefficient anodic ECL. Benefiting from the enhanced cathodic ECL, FeN₅ SAC-based immunosensor was constructed for the sensitive detection of cancer biomarkers.     

Continuous Modulation of Electrocatalytic Oxygen Reduction Activities of Single-Atom Catalysts through p-n Junction Rectification

Fine-tuning single-atom catalysts (SACs) to surpass their activity limit remains challenging at their atomic scale. Herein, we exploit p-type semiconducting character of SACs having a metal center coordinated to nitrogen donors (MeN_x) and rectify their local charge density by an n-type semiconductor support. With iron phthalocyanine (FePc) as a model SAC, introducing an n-type gallium monosulfide that features a low work function generates a space-charged region across the junction interface, and causes distortion of the FeN₄ moiety and spin-state transition in the Fe^II center. This catalyst shows an over two-fold higher specific oxygen-reduction activity than that of pristine FePc. We further employ three other n-type metal chalcogenides of varying work function as supports, and discover a linear correlation between the activities of the supported FeN₄ and the rectification degrees, which clearly indicates that SACs can be continuously tuned by this rectification strategy.     

MnO_2电极上氧还原的电催化机理

研究了MnO2 催化氧还原的电催化性质 ,通过比较MnO2 的自还原和催化氧还原的相关性 ,结合极化曲线分析和中间产物检测 ,提出了氧在MnO2 上电催化机理 .根据这一机理 ,MnO2 首先还原为MnOOH ,随之氧的还原通过化学氧化MnOOH ,两者协同进行 .依此导出的极化曲线形式能够较好地解决实验中观测到的动力学特征     

Altering Oxygen Binding by Redox-Inactive Metal Substitution to Control Catalytic Activity: Oxygen Reduction on Manganese Oxide Nanoparticles as a Model System**

Establishing generic catalyst design principles by identifying structural features of materials that influence their performance will advance the rational engineering of new catalytic materials. In this study, by investigating metal-substituted manganese oxide (spinel) nanoparticles, Mn₃O₄:M (M=Sr, Ca, Mg, Zn, Cu), we rationalize the dependence of the activity of Mn₃O₄:M for the electrocatalytic oxygen reduction reaction (ORR) on the enthalpy of formation of the binary MO oxide, Δ_fH°(MO), and the Lewis acidity of the M²⁺ substituent. Incorporation of elements M with low Δ_fH°(MO) enhances the oxygen binding strength in Mn₃O₄:M, which affects its activity in ORR due to the established correlation between ORR activity and the binding energy of *O/*OH/*OOH species. Our work provides a perspective on the design of new compositions for oxygen electrocatalysis relying on the rational substitution/doping by redox-inactive elements.     

Selective Four-Electron Reduction of Oxygen by a Nonheme Heterobimetallic CuFe Complex

We report herein the first nonheme CuFe oxygen reduction catalyst ([Cu^II(bpbp)(μ-OAc)₂Fe^III]²⁺, CuFe−OAc ), which serves as a functional model of cytochrome c oxidase and can catalyze oxygen reduction to water with a turnover frequency of 2.4×10³ s⁻¹ and selectivity of 96.0 % in the presence of Et₃NH⁺. This performance significantly outcompetes its homobimetallic analogues (2.7 s⁻¹ of CuCu−OAc with %H₂O₂ selectivity of 98.9 %, and inactive of FeFe−OAc ) under the same conditions. Structure-activity relationship studies, in combination with density functional theory calculation, show that the CuFe center efficiently mediates O−O bond cleavage via a Cu^II(μ-η¹ : η²-O₂)Fe^III peroxo intermediate in which the peroxo ligand possesses distinctive coordinating and electronic character. Our work sheds light on the nature of Cu/Fe heterobimetallic cooperation in oxygen reduction catalysis and demonstrates the potential of this synergistic effect in the design of nonheme oxygen reduction catalysts.     

Epoxy-rich Fe Single Atom Sites Boost Oxygen Reduction Electrocatalysis

Electrocatalysts for highly efficient oxygen reduction reaction (ORR) are crucial for energy conversion and storage devices. Single-atom catalysts with maximized metal utilization and altered electronic structure are the most promising alternatives to replace current benchmark precious metals. However, the atomic level understanding of the functional role for each species at the anchoring sites is still unclear and poorly elucidated. Herein, we report Fe single atom catalysts with the sulfur and oxygen functional groups near the atomically dispersed metal centers (Fe1/NSOC) for highly efficient ORR. The Fe1/NSOC delivers a half-wave potential of 0.92 V vs. RHE, which is much better than those of commercial Pt/C (0.88 V), Fe single atoms on N-doped carbon (Fe1/NC, 0.89 V) and most reported nonprecious metal catalysts. The spectroscopic measurements reveal that the presence of sulfur group induces the formation of epoxy groups near the FeN4S2 centers, which not only modulate the electronic structure of Fe single atoms but also participate the catalytic process to improve the kinetics. The density functional theory calculations demonstrate the existence of sulfur and epoxy group engineer the charges of Fe reactive center and facilitate the reductive release of OH* (rate-limiting step), thus boosting the overall oxygen reduction efficiency.     

Understanding Synergistic Catalysis on Cu-Se Dual Atom Sites via Operando X-ray Absorption Spectroscopy in Oxygen Reduction Reaction

The construction and understanding of synergy in well-defined dual-atom active sites is an available avenue to promote multistep tandem catalytic reactions. Herein, we construct a dual-hetero-atom catalyst that comprises adjacent Cu-N₄ and Se-C₃ active sites for efficient oxygen reduction reaction (ORR) activity. Operando X-ray absorption spectroscopy coupled with theoretical calculations provide in-depth insights into this dual-atom synergy mechanism for ORR under realistic device operation conditions. The heteroatom Se modulator can efficiently polarize the charge distribution around symmetrical Cu-N₄ moieties, and serve as synergistic site to facilitate the second oxygen reduction step simultaneously, in which the key OOH*-(Cu₁-N₄) transforms to O*-(Se₁-C₂) intermediate on the dual-atom sites. Therefore, this designed catalyst achieves satisfied alkaline ORR activity with a half-wave potential of 0.905 V vs. RHE and a maximum power density of 206.5 mW cm⁻² in Zn-air battery.     

Approaching Theoretical Performances of Electrocatalytic Hydrogen Peroxide Generation by Cobalt-Nitrogen Moieties

Electrocatalytic oxygen reduction reaction (ORR) has been intensively studied for environmentally benign applications. However, insufficient understanding of ORR 2 e⁻-pathway mechanism at the atomic level inhibits rational design of catalysts with both high activity and selectivity, causing concerns including catalyst degradation due to Fenton reaction or poor efficiency of H₂O₂ electrosynthesis. Herein we show that the generally accepted ORR electrocatalyst design based on a Sabatier volcano plot argument optimises activity but is unable to account for the 2 e⁻-pathway selectivity. Through electrochemical and operando spectroscopic studies on a series of CoN_x/carbon nanotube hybrids, a construction-driven approach based on an extended “dynamic active site saturation” model that aims to create the maximum number of 2 e⁻ ORR sites by directing the secondary ORR electron transfer towards the 2 e⁻ intermediate is proven to be attainable by manipulating O₂ hydrogenation kinetics.     

Decorating Single-Atomic Mn Sites with FeMn Clusters to Boost Oxygen Reduction Reaction

The regulation of electron distribution of single-atomic metal sites by atomic clusters is an effective strategy to boost their intrinsic activity of oxygen reduction reaction (ORR). Herein we report the construction of single-atomic Mn sites decorated with atomic clusters by an innovative combination of post-adsorption and secondary pyrolysis. The X-ray absorption spectroscopy confirms the formation of Mn sites via Mn-N₄ coordination bonding to FeMn atomic clusters (FeMn_ac/Mn-N₄C), which has been demonstrated theoretically to be conducive to the adsorption of molecular O₂ and the break of O−O bond during the ORR process. Benefiting from the structural features above, the FeMn_ac/Mn-N₄C catalyst exhibits excellent ORR activity with half-wave potential of 0.79 V in 0.5 M H₂SO₄ and 0.90 V in 0.1 M KOH as well as preeminent Zn-air battery performance. Such synthetic strategy may open up a route to construct highly active catalysts with tunable atomic structures for diverse applications.     

Enhanced Four-Electron Oxygen Reduction Selectivity of Clamp-Shaped Cobalt(II) Porphyrin(2.1.2.1) Complexes

The molecular structure, electrochemistry, spectroelectrochemistry and electrocatalytic oxygen reduction reaction (ORR) features of two Co^II porphyrin(2.1.2.1) complexes bearing Ph or F₅Ph groups at the two meso-positions of the macrocycle are examined. Single crystal X-ray analysis reveal a highly bent, nonplanar macrocyclic conformation of the complex resulting in clamp-shaped molecular structures. Cyclic voltammetry paired with UV/Vis spectroelectrochemistry in PhCN/0.1 M TBAP suggest that the first electron addition corresponds to a macrocyclic-centered reduction while spectral changes observed during the first oxidation are consistent with a metal-centered Co^II/Co^III process. The activity of the clamp-shaped complexes towards heterogeneous ORR in 0.1 M KOH show selectivity towards the 4e⁻ ORR pathway giving H₂O. DFT first-principle calculations on the porphyrin catalyst indicates a lower overpotential for 4e⁻ ORR as compared to the 2e⁻ pathway, consistent with experimental data.     

Adapting Synthetic Models of Heme/Cu Sites to Energy-Efficient Electrocatalytic Oxygen Reduction Reaction

In nature, cytochrome c oxidases catalyze the 4e⁻ oxygen reduction reaction (ORR) at the heme/Cu site, in which Cu^I is used to assist O₂ activation. Because of the thermodynamic barrier to generate Cu^I, synthetic Fe-porphyrin/Cu complexes usually show moderate electrocatalytic ORR activity. We herein report on a Co-corrole/Co complex 1-Co for energy-efficient electrocatalytic ORR. By hanging a Co^II ion over Co corrole, 1-Co realizes electrocatalytic 4e⁻ ORR with a half-wave potential of 0.89 V versus RHE, which is outstanding among corrole-based electrocatalysts. Notably, 1-Co outperforms Co corrole hanged with Cu^II or Zn^II. We revealed that the hanging Co^II ion can provide an electron to improve O₂ binding thermodynamically and dynamically, a function represented by the biological Cu^I ion of the heme/Cu site. This work is significant to present a remarkable ORR electrocatalyst and to show the vital role of a second-sphere redox-active metal ion in promoting O₂ binding and activation.     

An In Situ Investigation of the Protein Corona Formation Kinetics of Single Nanomedicine Carriers by Self-Regulated Electrochemiluminescence Microscopy

Protein coronas are present extensively at the bio-nano interface due to the natural adsorption of proteins onto nanomaterials in biological fluids. Aside from the robust property of nanoparticles, the dynamics of the protein corona shell largely define their chemical identity by altering interface properties. However, the soft coronas are normally complex and rapidly changing. To real-time monitor the entire formation, we report here a self-regulated electrochemiluminescence (ECL) microscopy based on the interaction of the Ru(bpy)₃³⁺ with the nanoparticle surface. Thus, the heterogeneity of the protein corona is in situ observed in single nanoparticle “cores” before and after loading drugs in nanomedicine carriers. The label-free, optical stable and dynamic ECL microscopy minimize misinterpretations caused by the variation of nanoparticle size and polydispersity. Accordingly, the synergetic actions of proteins and nanoparticles properties are uncovered by chemically engineered protein corona. After comparing the protein corona formation kinetics in different complex systems and different nanomedicine carriers, the universality and accuracy of this technique were well demonstrated via the protein corona formation kinetics curves regulated by competitive adsorption of Ru(bpy)₃³⁺ and multiple proteins on surface of various carriers. The work is of great significance for studying bio-nano interface in drug delivery and targeted cancer treatment.     

Non-planar Nest-like [Fe2S2] Cluster Sites for Efficient Oxygen Reduction Catalysis

Metal-nitrogen-carbon catalysts, as promising alternative to platinum-based catalysts for oxygen reduction reaction (ORR), are still highly expected to achieve better performance by modulating the composition and spatial structure of active site. Herein, we constructed the non-planar nest-like [Fe₂S₂] cluster sites in N-doped carbon plane. Adjacent double Fe atoms effectively weaken the O−O bond by forming a peroxide bridge-like adsorption configuration, and the introduction of S atoms breaks the planar coordination of Fe resulting in greater structural deformation tension, lower spin state, and downward shifted Fe d-band center, which together facilitate the release of OH* intermediate. Hence, the non-planar [Fe₂S₂] cluster catalyst, with a half-wave potential of 0.92 V, displays superior ORR activity than that of planar [FeN₄] or [Fe₂N₆]. This work provides insights into the co-regulation of atomic composition and spatial configuration for efficient oxygen reduction catalysis.     

Modulating Electronic Structures of Iron Clusters through Orbital Rehybridization by Adjacent Single Copper Sites for Efficient Oxygen Reduction

The atom-cluster interaction has recently been exploited as an effective way to increase the performance of metal-nitrogen-carbon catalysts for oxygen reduction reaction (ORR). However, the rational design of such catalysts and understanding their structure-property correlations remain a great challenge. Herein, we demonstrate that the introduction of adjacent metal (M)−N₄ single atoms (SAs) could significantly improve the ORR performance of a well-screened Fe atomic cluster (AC) catalyst by combining density functional theory (DFT) calculations and experimental analysis. The DFT studies suggest that the Cu−N₄ SAs act as a modulator to assist the O₂ adsorption and cleavage of O−O bond on the Fe AC active center, as well as optimize the release of OH* intermediates to accelerate the whole ORR kinetic. The depositing of Fe AC with Cu−N₄ SAs on nitrogen doped mesoporous carbon nanosheet are then constructed through a universal interfacial monomicelles assembly strategy. Consistent with theoretical predictions, the resultant catalyst exhibits an outstanding ORR performance with a half-wave potential of 0.92 eV in alkali and 0.80 eV in acid, as well as a high power density of 214.8 mW cm⁻² in zinc air battery. This work provides a novel strategy for precisely tuning the atomically dispersed poly-metallic centers for electrocatalysis.     

Dual-Atom Catalysts Derived from a Preorganized Covalent Organic Framework for Enhanced Electrochemical Oxygen Reduction

Dual-atom catalysts (DAC) are deemed as promising electrocatalysts due to the abundant active sites and adjustable electronic structure, but the fabrication of well-defined DAC is still full of challenges. Herein, bonded Fe dual-atom catalysts (Fe₂DAC) with Fe₂N₆C₈O₂ configuration were developed through one-step carbonization of a preorganized covalent organic framework with bimetallic Fe chelation sites (Fe₂COF). The transition from Fe₂COF to Fe₂DAC involved the dissociation of the nanoparticles and the capture of atoms by carbon defects. Benefitting from the optimized d-band center and enhanced adsorption of OOH* intermediates, Fe₂DAC exhibited outstanding oxygen reduction activity with a half-wave potential of 0.898 V vs. RHE. This work will guide more fabrication of dual-atom and even cluster catalysts from preorganized COF in the future.     

Constructing Fe-N4 Sites through Anion Exchange-mediated Transformation of Fe Coordination Environments in Hierarchical Carbon Support for Efficient Oxygen Reduction

Metal single atoms (SAs) anchored in carbon support via coordinating with N atoms are efficient active sites to oxygen reduction reaction (ORR). However, rational design of single atom catalysts with highly exposed active sites is challenging and urgently desirable. Herein, an anion exchange strategy is presented to fabricate Fe-N₄ moieties anchored in hierarchical carbon nanoplates composed of hollow carbon spheres (Fe-SA/N-HCS). With the coordinating O atoms are substituted by N atoms, Fe SAs with Fe-O₄ configuration are transformed into the ones with Fe-N₄ configuration during the thermal activation process. Insights into the evolution of central atoms demonstrate that the SAs with specific coordination environment can be obtained by modulating in situ anion exchange process. The strategy produces a large quantity of electrochemical accessible site and high utilization rate of Fe-N₄. Fe-SA/N-HCS shows excellent ORR electrocatalytic performance with half-wave potential of 0.91 V (vs. RHE) in 0.1 M KOH, and outstanding performance when used in rechargeable aqueous and flexible Zn-air batteries. The evolution pathway for SAs demonstrated in this work offers a novel strategy to design SACs with various coordination environment and enhanced electrocatalytic activity.     

Boosting Electrocatalytic Carbon Dioxide Reduction via Self-Relaxation of Asymmetric Coordination in Fe-Based Single Atom Catalyst

Addressing the limitations arising from the consistent catalytic behavior observed for various intermediates during the electrochemical carbon dioxide reduction reaction (CO₂RR) poses a significant challenge in the optimization of catalytic activity. In this study, we aimed to address this challenge by constructing an asymmetric coordination Fe single atom catalyst (SCA) with a dynamically evolved structure. Our catalyst, consisting of a Fe atom coordinated with one S atom and three N atoms (Fe−S₁N₃), exhibited exceptional selectivity (CO Faradaic efficiency of 99.02 %) and demonstrated a high intrinsic activity (TOF of 7804.34 h⁻¹), and remarkable stability. Using operando XAFS spectra and Density Functional Theory (DFT) calculations, we elucidated the self-relaxation of geometric distortion and dynamic evolution of bond lengths within the catalyst. These structure changes enabled independent regulation of the *COOH and *CO intermediate adsorption energies, effectively breaking the linear scale relationship and enhancing the intrinsic activity of CO₂RR. This study provides valuable insights into the dynamic evolution of SACs and paves the way for targeted catalyst designs aimed to disrupt the linear scaling relationships.     

Catalytic Linkage Engineering of Covalent Organic Frameworks for the Oxygen Reduction Reaction

Metal-free covalent organic frameworks (COFs) have been employed to catalyze the oxygen reduction reaction (ORR). To achieve high activity and selectivity, various building blocks containing heteroatoms and groups linked by imine bonds were used to create catalytic COFs. However, the roles of linkages of COFs in ORR have not been investigated. In this work, the catalytic linkage engineering has been employed to modulate the catalytic behaviors. To create single catalytic sites while avoiding other possible catalytic sites, we synthesized COFs from benzene units linked by various bonds, such as imine, amide, azine, and oxazole bonds. Among these COFs, the oxazole-linkage in COFs enables to catalyze the ORR with the highest activity, which achieved a half-wave potential of 0.75 V and a limited current density of 5.5 mA cm⁻². Moreover, the oxazole-linked COF achieved a conversion frequency (TOF) value of 0.0133 S⁻¹, which were 1.9, 1.3, and 7.4-times that of azine-, amide- and imine-COFs, respectively. The theoretical calculation showed that the carbon atoms in oxazole linkages facilitated the formation of OOH* and promoted protonation of O* to form the OH*, thus advancing the catalytic activity. This work guides us on which linkages in COFs are suitable for ORR.