Rational Synthesis of Iron/Nitrogen-Doped Carbon Catalyst through a Spatial Isolation Strategy for Efficient Oxygen Reduction in Acidic and Alkaline Media

It remains challenging to rationally synthesize iron/nitrogen-doped carbon (Fe/N-C) catalysts with rich Fe−N_x atomic active sites for improved oxygen reduction reaction (ORR) electrocatalysis. A highly efficient Fe/N-C catalyst, which has been synthesized through a spatial isolation strategy, is reported. Derived from bioinspired polydopamine (PDA)-based hybrid microsphere precursors, it is a multifunctional carrier that loads atomically dispersed Fe³⁺/Zn²⁺ ions through coordination interactions and N-rich melamine through electrostatic attraction and covalent bonding. The Zn²⁺ ions and melamine in the precursor efficiently isolate Fe³⁺ atoms upon pyrolysis to form rich Fe−N_x atomic active sites, and generate abundant micropores during high-temperature treatment; as a consequence, the resultant Fe-N/C catalyst contains rich catalytically active Fe−N_x sites and a hierarchical porous structure. The catalyst exhibits improved ORR activity that is superior to and close to that of Pt/C in alkaline and acidic solutions, respectively.     

Inducing Covalent Atomic Interaction in Intermetallic Pt Alloy Nanocatalysts for High-Performance Fuel Cells

The harsh working environments of proton exchange membrane fuel cells (PEMFCs) pose huge challenges to the stability of Pt-based alloy catalysts. The widespread presence of metallic bonds with significantly delocalized electron distribution often lead to component segregation and rapid performance decay. Here we report L1₀−Pt₂CuGa intermetallic nanoparticles with a unique covalent atomic interaction between Pt−Ga as high-performance PEMFC cathode catalysts. The L1₀−Pt₂CuGa/C catalyst shows superb oxygen reduction reaction (ORR) activity and stability in fuel cell cathode (mass activity=0.57 A mg_Pt⁻¹ at 0.9 V, peak power density=2.60/1.24 W cm⁻² in H₂-O₂/air, 28 mV voltage loss at 0.8 A cm⁻² after 30 000 cycles). Theoretical calculations reveal the optimized adsorption of oxygen intermediates via the formed biaxial strain on L1₀−Pt₂CuGa surface, and the durability enhancement stems from the stronger Pt−M bonds than those in L1₁−PtCu resulted from Pt−Ga covalent interactions.     

改性炭黑-LaMnO3复合材料的制备及其共价复合效应对氧还原性能的影响

以VulcanXC-72炭黑为载体,通过对炭载体石墨化处理和表面化学修饰,将其与化学沉淀法制备的纳米级LaMnO₃颗粒共混,再经特定温度下煅烧,制备出改性炭黑-LaMnO₃复合材料.X射线光电子能谱和热重分析表明,当煅烧温度在300℃时,炭载体与LaMnO₃纳米颗粒之间形成了大量C-O-M（M=La,Mn）化学键.扫描电子显微镜和高分辨透射电子显微镜分析发现,纯相LaMnO₃纳米颗粒主要呈现短棒、三支棒或竹节棒的形貌特征,炭载体则为具有完整石墨层的空心球结构,LaMnO₃均匀分散在炭载体上.在25℃,1mol/LNaOH溶液中的电化学测试结果表明,成分比（LaMnO₃：C）为2：3的复合材料具有很高的氧还原电催化活性,氧还原反应电子数为3.81,中间产物H₂O₂产率为9.5%,其活性接近商业Pt/C催化剂（E-TEK）.高的氧还原电催化活性主要归因于LaMnO₃纳米颗粒与炭载体之间形成了大量共价键.     

Pt-Like Oxygen Reduction Activity Induced by Cost-Effective MnFeO2/N-Carbon

The development of effective and affordable electrocatalysts for the oxygen reduction reaction (ORR) is critical for the renewable-energy technologies. Here, we present a new manganese iron oxide (MnFeO₂) as a cost-effective material for the ORR with Pt-like electrochemical properties. Pyrolysis of hybrid agar hydrogel on NaCl nanocrystals furnishes a unique structure in which the active MnFeO₂ particles are uniformly immobilized in the nitrogen-doped porous carbon aerogels (MnFeO₂/NPC). Nitrogen-doped carbon is introduced to construct porous mass-transfer channels and reduce self-aggregation of the MnFeO₂ particles. It is found that the formation of the MnFeO₂ phase greatly depends on the pyrolysis temperature. Benefiting from the synergy of MnFeO₂ and NPC, the MnFeO₂/NPC can actually be as good as the Pt/C catalyst for the ORR, with an onset-potential of 0.98 V and a half-wave potential of 0.86 V, combined with demonstrating a superior stability and tolerance to methanol.     

Construction of Co4 Atomic Clusters to Enable Fe−N4 Motifs with Highly Active and Durable Oxygen Reduction Performance

Fe−N−C catalysts with single-atom Fe−N₄ configurations are highly needed owing to the high activity for oxygen reduction reaction (ORR). However, the limited intrinsic activity and dissatisfactory durability have significantly restrained the practical application of proton-exchange membrane fuel cells (PEMFCs). Here, we demonstrate that constructing adjacent metal atomic clusters (ACs) is effective in boosting the ORR performance and stability of Fe−N₄ catalysts. The integration of Fe−N₄ configurations with highly uniform Co₄ ACs on the N-doped carbon substrate (Co₄@/Fe₁@NC) is realized through a “pre-constrained” strategy using Co₄ molecular clusters and Fe(acac)₃ implanted carbon precursors. The as-developed Co₄@/Fe₁@NC catalyst exhibits excellent ORR activity with a half-wave potential (E_1/2) of 0.835 V vs. RHE in acidic media and a high peak power density of 840 mW cm⁻² in a H₂−O₂ fuel cell test. First-principles calculations further clarify the ORR catalytic mechanism on the identified Fe−N₄ that modified with Co₄ ACs. This work provides a viable strategy for precisely establishing atomically dispersed polymetallic centers catalysts for efficient energy-related catalysis.     

CO2 Laser-Induced Graphene with an Appropriate Oxygen Species as an Efficient Electrocatalyst for Hydrogen Peroxide Synthesis

Oxygen species functionalized graphene (O−G) is an effective electrocatalyst for electrochemically synthesizing hydrogen peroxide (H₂O₂) by a 2 e⁻ oxygen reduction reaction (ORR). The type of oxygen species and degree of carbon crystallinity in O−G are two key factors for the high catalytic performance of the 2 e⁻ ORR. However, the general preparing method of O−G by the precursor of graphite has the disadvantages of consuming massive strong oxidant and washing water. Herein, the biomass-based graphene with tunable oxygen species is rapidly fabricated by a CO₂ laser. In a flow cell setup, the laser-induced graphene (LIG) with abundant active oxygen species and graphene structure shows high catalytic performance including high Faraday efficiency (over 78 %) and high mass activity (814 mmolg_catalyst⁻¹ h⁻¹), superior to most of the reported carbon-based electrocatalysts. Density function theory demonstrates the meta-C atoms at nearby C−O, O−C=O species are the key catalytic sites. Therefore, we develop one facile method to rapidly convert biomass to graphene electrocatalyst used for H₂O₂ synthesis.     

FeNi Nanoparticles Coated on N-doped Ultrathin Graphene-like Nanosheets as Stable Bifunctional Catalyst for Zn-Air Batteries

High-performance and low-cost bifunctional catalysts are crucial to energy conversion and storage devices. Herein, a novel oxygen electrode catalyst with high oxygen evolution reaction and oxygen reduction reaction (OER/ORR) performance is reported based on bimetal FeNi nanoparticles anchored on N-doped graphene-like carbon (FeNi/N−C). The complete 2D ultrathin carbon nanosheet is induced by etching and stripping of molten sodium chloride and its ions in the carbonization process at suitable temperature. The obtained FeNi/N−C catalyst exhibits rapid reaction kinetics for OER, efficient four electron transfer for ORR, and outstanding bifunctional performance with reversible oxygen electrode index of 0.87 V for OER/ORR. Zn-air batteries with a high open-circuit voltage of 1.46 V and a stable discharge voltage of 1.23 V are assembled using liquid electrolytes, zinc sheet as Zn-electrode and FeNi/N−C coating on carbon cloth as air-electrode. The specific capacity is as high as 816 mAh g⁻¹ and there is extremely little decay after charge-discharge cycle time of 275 h for the FeNi/N−C as oxygen electrode catalyst in Zn-air battery, which are much better than that assembled with Pt/C−RuO₂ catalyst.     

Insight into Iron Leaching from an Ascorbate-Oxidase-like Fe−N−C Nanozyme and Oxygen Reduction Selectivity**

Ascorbate (H₂A) is a well-known antioxidant to protect cellular components from free radical damage and has also emerged as a pro-oxidant in cancer therapies. However, such “contradictory” mechanisms underlying H₂A oxidation are not well understood. Herein, we report Fe leaching during catalytic H₂A oxidation using an Fe−N−C nanozyme as a ferritin mimic and its influence on the selectivity of the oxygen reduction reaction (ORR). Owing to the heterogeneity, the Fe-N_x sites in Fe−N−C primarily catalyzed H₂A oxidation and 4 e⁻ ORR via an iron-oxo intermediate. Nonetheless, trace O₂⋅⁻ produced by marginal N−C sites through 2 e⁻ ORR accumulated and attacked Fe-N_x sites, leading to the linear leakage of unstable Fe ions up to 420 ppb when the H₂A concentration increased to 2 mM. As a result, a substantial fraction (ca. 40 %) of the N−C sites on Fe−N−C were activated, and a new 2+2 e⁻ ORR path was finally enabled, along with Fenton-type H₂A oxidation. Consequently, after Fe ions diffused into the bulk solution, the ORR at the N−C sites stopped at H₂O₂ production, which was the origin of the pro-oxidant effect of H₂A.     

Fe3C coupled with Fe-Nx supported on N-doped carbon as oxygen reduction catalyst for assembling Zn-air battery to drive water splitting

Fe-N-C structures have been considered as a candidate to replace noble metal catalysts towards oxygen reduction reaction (ORR) due to their excellent electrocatalytic activity and durability. Herein, a zinc-mediated synthesis strategy is proposed for N-doped graphitic porous carbon encapsulated uniform dispersed Fe₃C nanoparticles coupled with atomically dispersed Fe-N_x moieties (NPC/Fe-N-C) derived from biomass coconut shell. The introduction of zinc species could be conductive to the dispersion of iron species and formation of porous structures. Density functional theory calculations demonstrate that the N-doped carbon coating structures can weaken the oxygen intermediates adsorption energy barrier of Fe₃C. Beside, the graphitic carbon could promote the electron transfer during the electrochemical reaction. These special structures enable NPC/Fe-N-C to have excellent ORR activity with an E_onset of 1.0 V, which is much better than Pt/C. Furthermore, the zinc-air battery assembled by pairing NPC/Fe-N-C with a high-efficiency oxygen evolution reaction (OER) catalyst can continuously and stably operate a charge-discharge potential gap of 0.8 V at 10 mA/cm² for more than 600 h. More importantly, the assembled batteries could drive overall water splitting device, realizing the effective energy conversion.     

Singlet Oxygen Induced Site-Specific Etching Boosts Nitrogen-Carbon Sites for High-Efficiency Oxygen Reduction

Targeted construction of carbon defects near the N dopants is an intriguing but challenging way to boost the electrocatalytic activity of N-doped carbon toward oxygen reduction reaction (ORR). Here, we report a novel site-specific etching strategy that features targeted anchoring of singlet oxygen (¹O₂) on the N-adjacent atoms to directionally construct topological carbon defects neighboring the N dopants in N-doped carbon (¹O₂−N/C). This ¹O₂−N/C exhibits the highest ORR half-wave potential of 0.915 V_RHE among all the reported metal-free carbon catalysts. Pyridinic-N bonded with a carbon pentagon of the neighboring topological carbon defects is identified as the primary active configuration, rendering enhanced adsorption of O₂, optimized adsorption energy of the ORR intermediates, and a significantly decreased total energy barrier for ORR. This ¹O₂-induced site-specific etching strategy is also applicable to different precursors, showing a tremendous potential for targeted construction of high-efficiency active sites in carbon-based materials.     

Integrated Ultrafine Co0.85Se in Carbon Nanofibers: An Efficient and Robust Bifunctional Catalyst for Oxygen Electrocatalysis

Transition-metal selenides are emerging as alternative bifunctional catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR); however, their activity and stability are still less than desirable. Herein, ultrafine Co_0.85Se nanoparticles encapsulated into carbon nanofibers (CNFs), Co_0.85Se@CNFs, is reported as an integrated bifunctional catalyst for OER and ORR. This catalyst exhibits a low OER potential of 1.58 V vs. reversible hydrogen electrode (RHE) (E_{J=10, OER}) to achieve a current density (J) of 10 mA cm⁻² and a high ORR potential of 0.84 V vs. RHE (E_{J=−1, ORR}) to reach −1 mA cm⁻². Thus, the potential between E_{J=10, OER} and E_{J=−1, ORR} is only 0.74 V, indicating considerable bifunctional activity. The excellent bifunctionality can be attributed to high electronic conduction, abundant electrochemically active sites, and the synergistic effect of Co_0.85Se and CNFs. Furthermore, this Co_0.85Se@CNFs catalyst displays good cycling stability for both OER and ORR. This study paves a new way for the rational design of hybrid catalysts composed of transition-metal selenides and carbon materials for efficiently catalyzing OER and ORR.     

Direct Observation of Oxygen Dissociation on Non‐Stoichiometric Metal Oxide Catalysts

Oxygen dissociation on metal oxides is a key reaction step, limiting the efficiency of numerous technologies. The complexity of the multi‐step oxygen reduction reaction (ORR) makes it difficult to investigate the oxygen dissociation step independently. Direct observation of the oxygen dissociation process is described, quantitatively, on perovskites La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ and (La_0.8Sr_0.2)_0.95MnO_3±δ, using gas‐phase isotope‐exchange with a 1:1 ¹⁶O₂:¹⁸O₂ ratio. Oxygen transport mechanisms between gas–surface reactions and surface–bulk exchange are deconvoluted. Our findings show that regardless of participation of lattice oxygen, La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ is better at oxygen dissociation than (La_0.8Sr_0.2)_0.95MnO_3±δ. Heteroexchange, involving lattice oxygen, dominates on La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ. In contrast, (La_0.8Sr_0.2)_0.95MnO_3±δ shows both homoexchange and heteroexchange, with the latter only happening above 600 °C. Using a 1:1 isotope mixture, a simple method is presented for separation of the oxygen dissociation step from the overall ORR.     

A Bifunctional Hybrid Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions: Nano-Co3O4-Deposited La0.5Sr0.5MnO3 via Infiltration

For rechargeable metal–air batteries, which are a promising energy storage device for renewable and sustainable energy technologies, the development of cost-effective electrocatalysts with effective bifunctional activity for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) has been a challenging task. To realize highly effective ORR and OER electrocatalysts, we present a hybrid catalyst, Co₃O₄-infiltrated La_0.5Sr_0.5MnO_3-δ (LSM@Co₃O₄), synthesized using an electrospray and infiltration technique. This study expands the scope of the infiltration technique by depositing ~18 nm nanoparticles on unprecedented ~70 nm nano-scaffolds. The hybrid LSM@Co₃O₄ catalyst exhibits high catalytic activities for both ORR and OER (~7 times, ~1.5 times, and ~1.6 times higher than LSM, Co₃O₄, and IrO₂, respectively) in terms of onset potential and limiting current density. Moreover, with the LSM@Co₃O₄, the number of electrons transferred reaches four, indicating that the catalyst is effective in the reduction reaction of O₂ via a direct four-electron pathway. The study demonstrates that hybrid catalysts are a promising approach for oxygen electrocatalysts for renewable and sustainable energy devices.     

Gelatin-Derived 1D Carbon Nanofiber Architecture with Simultaneous Decoration of Single Fe−Nx Sites and Fe/Fe3C Nanoparticles for Efficient Oxygen Reduction

Exploring high-performance non-precious-metal electrocatalysts for the oxygen reduction reaction (ORR) is critical. Herein, a scalable and cost-effective strategy is reported for the construction of one-dimensional carbon nanofiber architectures with simultaneous decoration of single Fe−N_x sites and highly dispersed Fe/Fe₃C nanoparticles for efficient ORR, through the Fe^III-complex-assisted electrospinning of gelatin nanofibers with subsequent pre-oxidation and carbonization. Results show that the presence of a Fe^III complex enables the 1D gelatin nanofibers to be well retained during the pre-oxidation process. Owing to the distinct 1D nanofiber structure and the synergistic effect of Fe/Fe₃C and Fe−N_x sites, the resulting electrocatalyst is highly active for ORR with a half-wave potential of 0.885 V (outperforming commercial Pt/C) and a superior electrochemical stability in alkaline electrolytes. Similarly, it also shows a high power density (144.7 mW cm⁻²) and a superior stability in Zn-air batteries. This work opens a path for the design and synthesis of 1D carbon electrocatalyst for efficient ORR catalysis.     

Importing Antibonding-Orbital Occupancy through Pd−O−Gd Bridge Promotes Electrocatalytic Oxygen Reduction

The active-site density, intrinsic activity, and durability of Pd−based materials for oxygen reduction reaction (ORR) are critical to their application in industrial energy devices. This work constructs a series of carbon-based rare-earth (RE) oxides (Gd₂O₃, Sm₂O₃, Eu₂O₃, and CeO₂) by using RE metal–organic frameworks to tune the ORR performance of the Pd sites through the Pd−RE_xO_y interface interaction. Taking Pd−Gd₂O₃/C as a representative, it is identified that the strong coupling between Pd and Gd₂O₃ induces the formation of the Pd−O−Gd bridge, which triggers charge redistribution of Pd and Gd₂O₃. The screened Pd−Gd₂O₃/C exhibits impressive ORR performance with high onset potential (0.986 V_RHE), half-wave potential (0.877 V_RHE), and excellent stability. Similar ORR results are also found for Pd−Sm₂O₃/C, Pd−Eu₂O₃/C, and Pd−CeO₂/C catalysts. Theoretical analyses reveal that the coupling between Pd and Gd₂O₃ promotes electron transfer through the Pd−O−Gd bridge, which induces the antibonding-orbital occupancy of Pd−*OH for the optimization of *OH adsorption in the rate-determining step of ORR. The pH-dependent microkinetic modeling shows that Pd−Gd₂O₃ is close to the theoretical optimal activity for ORR, outperforming Pt under the same conditions. By its ascendancy in ORR, the Pd−Gd₂O₃/C exhibits superior performance in Zn-air battery as an air cathode, implying its excellent practicability.     

Co3+-Rich Na1.95CoP2O7 Phosphates as Efficient Bifunctional Catalysts for Oxygen Evolution and Reduction Reactions in Alkaline Solution

Implementing sustainable energy conversion and storage technologies is highly reliant on crucial oxygen electrocatalysis, such as the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). However, the pursuit of low cost, energetic efficient and robust bifunctional catalysts for OER and ORR remains a great challenge. Herein, the novel Na-ion-deficient Na_2−xCoP₂O₇ catalysts are proposed to efficiently electrocatalyze OER and ORR in alkaline solution. The engineering of Na-ion deficiency can tune the electronic structure of Co, and thus tailor the intrinsically electrocatalytic performance. Among the sodium cobalt phosphate catalysts, the Na_1.95CoP₂O₇ (NCPO5) catalyst exhibits the lowest ΔE (E_J10,OER−E_J−1,ORR) of only 0.86 V, which favorably outperforms most of the reported non-noble metal catalysts. Moreover, the Na-ion deficiency can stabilize the phase structure and morphology of NCPO5 during the OER and ORR processes. This study highlights the Na-ion deficient Na_2−xCoP₂O₇ as a promising class of low-cost, highly active and robust bifunctional catalysts for OER and ORR.     

One-Step Preparation of Fe/N/C Single-Atom Catalysts Containing Fe−N4 Sites from an Iron Complex Precursor with 5,6,7,8-Tetraphenyl-1,12-Diazatriphenylene Ligands

Fe/N/C single-atom catalysts containing Fe−N_x sites prepared by pyrolysis are promising cathode materials for fuel cells and metal-air batteries due to their high oxygen reduction reaction (ORR) activities. We have developed iron complexes containing N2- or N3-chelating coordination structures with preorganized aromatic rings in a 1,12-diazatriphenylene framework tethering bromo substituents as precursors to precisely construct Fe−N₄ sites in an Fe/N/C catalyst. One-step pyrolysis of the iron complex with carbon black forms atomically dispersed Fe−N₄ sites without iron aggregates. X-ray absorption spectroscopy (XAS) and electrochemical measurements revealed that the iron complex with N3-coordination is more effectively converted to Fe−N₄ sites catalyzing ORR with a TOF value of 0.21 e site⁻¹ s⁻¹ at 0.8 V vs. RHE. This indicates that the formation of Fe−N₄ sites is controlled by precise tuning of the chemical structure of the iron complex precursor.     

A Bifunctional Hybrid Electrocatalyst for Oxygen Reduction and Evolution: Cobalt Oxide Nanoparticles Strongly Coupled to B,N-Decorated Graphene

The electrocatalyzed oxygen reduction and evolution reactions (ORR and OER, respectively) are the core components of many energy conversion systems, including water splitting, fuel cells, and metal–air batteries. Rational design of highly efficient non-noble materials as bifunctional ORR/OER electrocatalysts is of great importance for large-scale practical applications. A new strongly coupled hybrid material is presented, which comprises CoO_x nanoparticles rich in oxygen vacancies grown on B,N-decorated graphene (CoO_x NPs/BNG) and operates as an efficient bifunctional OER/ORR electrocatalyst. Advanced spectroscopic techniques were used to confirm formation of abundant oxygen vacancies and strong Co−N−C bridging bonds within the CoO_x NPs/BNG hybrid. Surprisingly, the CoO_x NPs/BNG hybrid electrocatalyst is highly efficient for the OER with a low overpotential and Tafel slope, and is active in the ORR with a positive half-wave potential and high limiting current density in alkaline medium.     

Co3O4@Cu-Based Conductive Metal–Organic Framework Core–Shell Nanowire Electrocatalysts Enable Efficient Low-Overall-Potential Water Splitting

In the work reported herein, the electrocatalytic properties of Co₃O₄ in hydrogen and oxygen evolution reactions have been significantly enhanced by coating a shell layer of a copper-based metal–organic framework on Co₃O₄ porous nanowire arrays and using the products as high-performance bifunctional electrocatalysts for overall water splitting. The coating of the copper-based metal–organic framework resulted in the hybridization of the copper-embedded protective carbon shell layer with Co₃O₄ to create a strong Cu−O−Co bonding interaction for efficient hydrogen adsorption. The hybridization also led to electronically induced oxygen defects and nitrogen doping to effectively enhance the electrical conductivity of Co₃O₄. The optimal as-prepared core–shell hybrid material displayed excellent overall-water-splitting catalytic activity that required overall voltages of 1.45 and 1.57 V to reach onset and a current density of 10 mA cm⁻², respectively. This is the first report to highlight the relevance of hybridizing MOF-based co-catalysts to boost the electrocatalytic performance of nonprecious transition-metal oxides.     

Synthesis of Non-precious N-doped Mesoporous Fe3O4/CoO@NC Materials towards Efficient Oxygen Reduction Reaction for Microbial Fuel Cells

N-doped transition metal oxides are strategic materials towards the efficient oxygen reduction reaction (ORR) of microbial fuel cells (MFCs). Non-precious N-doped Fe₃O₄/CoO@NC−T (T represents carbonization temperature) catalysts are prepared by an efficient two-step strategy for ORR. Fe₃O₄/CoO@NC-750 exhibits the best performance with an efficient four-electron transfer pathway. The optimal power density of MFCs by using Fe₃O₄/CoO@NC-750 as the cathode catalyst (1243.4 mW ⋅ m⁻²) is superior to that of the MFCs with commercial Pt/C catalyst (1080 mW ⋅ m⁻²), which shows an outstanding activity towards ORR. No significant decrease in output voltage results over 70 days, which shows an excellent electrochemical stability.