Oxygen Reduction Reaction Activity Research Articles

F‐p‐d Gradient Orbital Coupling Induced Spin State Enhancement of Atomic Fe Sites for Efficient and Stable Oxygen Reduction Reaction

AbstractAdvancing energy conversion technologies requires cost‐efficient electrocatalysts for the oxygen reduction reaction (ORR). Iron phthalocyanine (FePc) emerges as a scalable and economical ORR electrocatalyst. However, the Fe–N4 configuration in FePc still falls short of the satisfied ORR activity and stability under electrocatalytic conditions. Here, an effective f‐p‐d (Eu–O–Fe) gradient orbital coupling strategy is introduced by integrating FePc with Eu2O3 (FePc/Eu2O3) to enhance the spin state and ORR performance of the Fe center through a precisely designed, scalable synthetic approach. The Eu─O bond promotes electron delocalization and shifts the spin state of Fe center from low‐spin to intermediate‐spin, increasing the eg​ orbital occupancy. This modification optimizes the adsorption of oxygen‐containing intermediates and lowers the ORR energy barrier. Notably, the increased spin state of Fe accelerates charge transfer by releasing more unpaired electrons, improving reaction kinetics. Furthermore, the f‐band serves as a buffer layer for electron compensation during ORR, further stabilizing the covalency and electronic configuration of atomic Fe and boosting durability. The one‐batch synthesis produces exceeding 300 g of FePc/Eu2O3, achieving a half‐wave potential of 0.931 V (vs RHE) at a cost less than 1/15 of commercial Pt/C. It demonstrates exceptional ORR performance in aluminum–air batteries, highlighting its significant application potential.

3D Nanowire Pt Catalysts with Enhanced Stability for the Oxygen Reduction Reaction.

We report that self-supporting mesoporous platinum 3D nanowires with a single diamond (SD) morphology and a high specific surface area of 40.4 m2 g-1 demonstrated enhanced stability toward the oxygen reduction reaction (ORR). These were found to be superior to commercially available carbon-supported Pt nanoparticles (Pt/C). After 1000 potential cycles, there was a 21% loss in surface area for SD-Pt, as compared with a 40.3% loss for Pt/C with no reduction in their half-wave potential (measured at J = 3.0 mA cm-2), whereas the Pt/C catalyst showed a 11.9 mV decrease. Our findings revealed that our SD-Pt thin films also exhibited excellent ORR activity, which offers significant potential for their application as high-performance cathode materials in alkaline fuel cells.

From Structure to Power: Monolithic Carbon Air Cathodes with an Optimized Active Distribution for Direct-Formate Fuel Cells.

Non-noble metal cathodes often suffer from limited oxygen reduction reaction (ORR) activity due to poor mass transport and insufficient active site availability, which restricts their application. This study presents a carbon-based air cathode featuring a monolithic, binder-free design aimed at enhancing the mass transfer and distribution of active sites. Characterization results indicate that the carbon-based monolithic air cathodes with a thickness of 1.5 mm (Fe@AC-1.5) possess a well-distributed pore structure that improves the oxygen transport capacity, thereby facilitating catalytic activity. Additionally, evaluation of the active site distribution of the cathode with a thickness of 2.4 mm (Fe@AC-2.4) revealed a comparable exponential distribution pattern. Furthermore, the findings demonstrate that the 1.5 mm thick monolithic air cathode prepared with agar (Fe@AC-1.5) contains numerous active sites for the ORR and an optimal pore distribution, resulting in an improved ORR activity. The direct-formate fuel cell utilizing Fe@AC-1.5 achieves a maximum power density of 22.63 mW cm-2. This study introduces easily prepared carbon-based monolithic air cathodes with remarkable ORR properties and offers insights into active site distribution, thereby guiding future enhancements in the ORR performance.

Oxygen-Coordinated Cr Single-Atom Catalyst for Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells.

Carbon-supported metal single-atom catalysts (M-SACs) are promising oxygen reduction reaction (ORR) catalysts. Their ORR activity and selectivity are significantly affected by the heteroatoms that coordinate the central metal atoms. Previous reports found that oxygen-coordinated M-SACs promoted a 2e- ORR rather than the 4e- ORR that is more desirable for fuel cells. Herein, we report for the first time that oxygen-coordinated M-SACs are capable of promoting the 4e- ORR in acid media. We prepared a Cr(acac)-NC catalyst with the central Cr atom coordinated by two O atoms. The Cr(acac)-NC catalyst not only exhibits excellent ORR activity and stability in acid media, but also delivers high proton exchange membrane fuel cell (PEMFC) performance comparable to N-coordinated M-SACs. Density functional theory (DFT) calculations reveal that Cr-O2 moieties located on the zigzag edge of the carbon support are the ORR-active sites.

Oxygen‐Coordinated Cr Single‐Atom Catalyst for Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells

AbstractCarbon‐supported metal single‐atom catalysts (M‐SACs) are promising oxygen reduction reaction (ORR) catalysts. Their ORR activity and selectivity are significantly affected by the heteroatoms that coordinate the central metal atoms. Previous reports found that oxygen‐coordinated M‐SACs promoted a 2e− ORR rather than the 4e− ORR that is more desirable for fuel cells. Herein, we report for the first time that oxygen‐coordinated M‐SACs are capable of promoting the 4e− ORR in acid media. We prepared a Cr(acac)‐NC catalyst with the central Cr atom coordinated by two O atoms. The Cr(acac)‐NC catalyst not only exhibits excellent ORR activity and stability in acid media, but also delivers high proton exchange membrane fuel cell (PEMFC) performance comparable to N‐coordinated M‐SACs. Density functional theory (DFT) calculations reveal that Cr−O2 moieties located on the zigzag edge of the carbon support are the ORR‐active sites.

Engineering the Lewis Acidity of Fe Single-Atom Sites via Atomic-Level Tuning of Spatial Coordination Configuration for Enhanced Oxygen Reduction.

Nitrogen-doped carbon-supported Fe catalysts (Fe-N-C) with Fe-N4 active sites hold great promise for the oxygen reduction reaction (ORR). However, fine-tuning the structure of Fe-N4 active sites to enhance their performance remains a grand challenge. Herein, we report an innovative design strategy to promote the ORR activity and kinetics of Fe-N4 sites by engineering their Lewis acidity, which is achieved by tuning the spatial Fe coordination geometry. Theoretical calculations indicated that Fe1-N4SO2 sites (with an axial -SO2 group bonded to Fe) offered favorable Lewis acidity for the ORR, leading to optimized adsorption energies for the key ORR intermediates. To implement this strategy, we developed a molecular-cage-encapsulated coordination strategy to synthesize a Fe single-atom site catalyst (SAC) with Fe1-N4SO2 sites. In agreement with theory, the Fe1-N4SO2/NC catalyst demonstrated outstanding ORR performance in both alkaline (E1/2 = 0.910 V in 0.1 M KOH) and acidic media (E1/2 = 0.772 V in 0.1 M HClO4), surpassing commercial Pt/C and traditional Fe SACs with Fe1-N4 sites or planar S-coordinated Fe1-N4-S sites. Moreover, this newly developed catalyst showed great application potential in quasi-solid-state Zn-air batteries, delivering superior performance across a wide temperature range.

Microstructural and Electrochemical Properties of Gd0.3Ba0.7FeO3−δ–xCe0.90Gd0.10O1.95 Composite Cathode for Solid Oxide Fuel Cells

The chemical compatibility, microstructure, and electrochemical performance of Gd0.3Ba0.7FeO3−δ–xCe0.90Gd0.10O1.95 (GBF0.3−xGDC, x = 30–50 wt%) composite cathodes are systematically investigated. Results reveal excellent chemical compatibility between GBF0.3 and GDC. The GBF0.3–30GDC composite cathode exhibits enhanced microstructural properties, including reduced particle size and increased porosity, which improve oxygen reduction reaction activity and decrease polarization resistance. A peak power density of 0.549 W cm−2 is achieved at 800 °C with hydrogen as the fuel gas. These findings underscore the critical role of compositional ratios in optimizing the electrochemical performance of composite cathodes for solid oxide fuel cells.

Utilizing Molybdenum to Tailor the Electronic Structure of Iron Through Electron Complementary Effect for Promoting Oxygen Reduction Activity

AbstractTailoring the electronic structure of later transition metal‐based electrocatalysts by incorporating early transition metal based on the electronic complementary effect is anticipated to enhance the electrocatalytic activity. Herein, the modulation of the electronic structure of Fe3C through the utilization of Mo2C to promote oxygen reduction reaction (ORR) activity is reported. In situ characterizations combined with theoretical calculations reveal that the electron‐donating capability of molybdenum in Mo2C to the active center of iron in Fe3C optimizes the adsorption and activation of oxygen. Concurrently, the d‐band center of Fe is much closer to the Fermi level, which reduces the energy barrier for the rate‐determining step (*OOH → *O), thereby enhancing the ORR activity. In alkaline media, the catalyst delivers a half‐wave potential (E1/2) of 0.89 V and maintains its efficiency with a mere 8 mV decay after 10 000 cycles, surpassing that of Pt/C. Moreover, it can serve as an air cathode in both liquid‐state and all‐solid‐state zinc‐air batteries (ZABs) and shows promising applications in portable devices. This work brings an innovative design concept for highly efficient electrocatalysts suitable for advanced energy devices.

Why Do Weak-Binding M-N-C Single-Atom Catalysts Possess Anomalously High Oxygen Reduction Activity?

Single-atom catalysts (SACs) with metal-nitrogen-carbon (M-N-C) structures are widely recognized as promising candidates in oxygen reduction reactions (ORR). According to the classical Sabatier principle, optimal 3d metal catalysts, such as Fe/Co-N-C, achieve superior catalytic performance due to the moderate binding strength. However, the substantial ORR activity demonstrated by weakly binding M-N-C catalysts such as Ni/Cu-N-C challenges current understandings, emphasizing the need to explore new underlying mechanisms. In this work, we integrated a pH-field coupled microkinetic model with detailed experimental electron state analyses to verify a novel key step in the ORR reaction pathway of weak-binding SACs─the oxygen adsorption at the metal-nitrogen bridge site. This step significantly altered the adsorption scaling relations, electric field responses, and solvation effects, further impacting the key kinetic reaction barrier from HOO* to O* and pH-dependent performance. Synchrotron spectra analysis further provides evidence for the new weak-binding M-N-C model, showing an increase in electron density on the antibonding π orbitals of N atoms in weak-binding M-N-C catalysts and confirming the presence of N-O bonds. These findings redefine the understanding of weak-binding M-N-C catalyst behavior, opening up new perspectives for their application in clean energy.

The Influence of Ionic Liquid Modification on the Restructuring of Trimetallic PtNiMo/C Catalysts During Conditioning

AbstractDuring preconditioning of PtNiMo/C catalyst metal dissolution and redeposition takes place. At slow scan rates this leads to increased steady activity for oxygen reduction reaction (ORR). Furthermore, ionic liquids (IL) influence the activity of Pt‐based catalysts and can alter the leaching of Pt‐alloy catalysts. This study investigates the influence of ILs on the preconditioning of PtNiMo/C catalysts. Therefore, preconditioning parameters as well as the IL employed were varied. ILs with long side chains like [C8fC1][BETI] resulted in a spreading of the activity development during preconditioning over a longer cycle time and with damped amplitude for the activity changes. For shorter side chains like [BMIM][BETI] the development is like the pristine catalyst. Ex‐situ and in‐situ characterization revealed that [BMIM][BETI] leached fast to the electrolyte, which was not observed for the other ILs. Nevertheless, for all ILs a strongly reduced platinum leaching was observed. Even for the leached [BMIM][BETI] a stable wetting layer influencing the leaching seems to remain. Nevertheless, only for the other ILs where an immobilized bulk phase is remaining a difference in catalyst restructuring during the preconditioning is observed and a higher number of small sized Pt clusters and a lower amount of bigger Pt clusters resulted.

Dual exsolution in silver‐functionalized layered and cubic perovskite oxides

AbstractIn this work, exsolution is achieved from both the A‐site and B‐site of (BaGd0.8La0.2)1‐xAgxCo2O6‐δ and (BaLa)1‐xAgxCoFeO6‐δ (x = 0.04, 0.1, and 0.2) perovskites prepared via solid‐state sintering. Through synchrotron radiation powder X‐ray diffraction and X‐ray absorption spectroscopy techniques, the chemical composition of the nanoparticles was determined to constitute both Ag and CoO. Microstructural studies were done using scanning electron microscopy with varying sizes and shapes of nanoparticles present for respective annealing atmospheres and temperatures. By applying a numerical model to experimental data, it was also established that the changes in enthalpy for oxygen vacancy formation decrease with an increase in silver dopant. From thermogravimetric measurements, the single (BaLa)0.95Ag0.10CoFeO6‐δ perovskite had relatively higher water uptake than the layered (BaGd0.8La0.2)0.95Ag0.10Co2O6‐δ perovskite. The total electrical conductivity done in dry and wet conditions decreased with an increase in temperature in the range of 300–800°C for both layered and single perovskites. The results obtained from electrical conductivity relaxation measurements of (BaGd0.8La0.2)0.95Ag0.10Co2O6‐δ with nanoparticles exhibit increased oxygen reduction reaction activity than (BaLa)0.95Ag0.10CoFeO6‐δ with nanoparticles.

High-Density Accessible Iron Single-Atom Catalyst for Durable and Temperature-Adaptive Laminated Zinc-Air Batteries.

Designing single-atom catalysts (SACs) with high density of accessible sites by improving metal loading and sites utilization is a promising strategy to boost the catalytic activity, but remains challenging. Herein, a high site density (SD) iron SAC (D-Fe-N/C) with 11.8 wt.% Fe-loading is reported. The in situ scanning electrochemical microscopy technique attests that the accessible active SD and site utilization of D-Fe-N/C reach as high as 1.01 × 1021 site g-1 and 79.8%, respectively. Therefore, D-Fe-N/C demonstrates superior oxygen reduction reaction (ORR) activity in terms of a half-wave potential of 0.918 V and turnover frequency of 0.41 e site-1 s-1. The excellent ORR property of D-Fe-N/C is also demonstrated in the liquid zinc-air batteries (ZABs), which exhibit a high peak power density of 306.1 mW cm-2 and an ultra-long cycling stability over 1200 h. Moreover, solid-state laminated ZABs prepared by presetting an air flow layer show a high specific capacity of 818.8 mA h g-1, an excellent cycling stability of 520 h, and a wide temperature-adaptive from -40 to 60 °C. This work not only offers possibilities by improving metal-loading and catalytic site utilization for exploring efficient SACs, but also provides strategies for device structure design toward advanced ZABs.

Sulfur Dioxide-Tolerant Core@shell Ru@Pt Catalysts Toward Oxygen Electro-Reduction

Proton exchange membrane fuel cells (PEMFCs) have achieved milestones in performance improvements and commercial launches. In the typical commercialized PEMFCs, the compressed air to cathode is usually supplied from ambient air, assuming that no costly pre-purification system is applied. Therefore, the working PEMFCs may suffer from the negative effects of the air impurities. In this regard, SO2, as the most poisonous species, may be fed along with air at the cathode and strongly adsorbed on the Pt surface, leading to Pt site deactivation. To address this challenge, we published a series of works in terms of poisoning mechanisms, regeneration protocols, and advanced poisoning-tolerant catalysts. Herein, we are aiming at developing a SO2-tolerant electrocatalyst toward a cathodic oxygen reduction reaction (ORR). We reasonably incorporate the Ru, synthesize Ru@Pt core@shell catalysts and investigate the relationships among Ru incorporation, ORR activity and SO2 tolerance. Impressively, the Ru@Pt/C exhibits higher initial ORR activity (0.288 A mg−1Pt), better SO2 poisoning resistance (33% loss in initial activity) than that of commercial Pt/C catalysts (0.252 A mg−1Pt; 62% loss). The engineered affinity between Pt and SO2 in the presence of Ru is uncovered to account for the improvement.

Morphology-Driven Bifunctional Activity of Layered Birnessite-Based Materials toward Oxygen Electrocatalysis.

The chemical, structural, and morphological diversity of birnessite, a 2D layered MnO2, has opened avenues for its application as an electrocatalyst toward both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Among pristine birnessites prepared by different methods, the freestanding flakes (primary structure) obtained from molten salt (MS-KMnO) showed remarkable bifunctional activity as compared to samples with thicker plates or a hierarchical honeycomb-like (type-I secondary structure) morphology. While the ORR onset potential (E onset) and halfwave potential (E 1/2) for MS-KMnO were recorded at 0.89 and 0.81 V vs RHE, respectively, the OER overpotential (η) was found to be 300 mV. We demonstrated heat-induced secondary structure evolution by modification of the molten salt method, which led to a decrease in activity. In contrast to previous studies, the Co-doped birnessite (Co-KMnO) prepared in molten salt showed lower bifunctional activity (ORR, E 1/2 = 0.72 V; OER, η= 460 mV) as compared to MS-KMnO. Co-KMnO showed an interwoven wrinkled sheet-like (type-II secondary structure) morphology, with Co3+ present in both the in-layer and the interlayer. However, in Co-KMnO/360 prepared at a lower reaction temperature, the areal coverage of the type-II structure reduces, leading to an increase in ORR (E 1/2 = 0.76 V) and OER (η = 440 mV) activity. The chronopotentiometry for 100 h at a constant OER current of 50 mA cm-2 showed an increase in potential from 1.62 to 1.89 V and the characterization of the sample post-treatment showed degradation of the layered structure in MS-KMnO. The samples obtained after 1000 CV cycles in both the ORR and the OER regions showed the formation of secondary structures with a substantial decrease in the Mn3+/Mn4+ ratio. This study demonstrates that morphology tuning within the 2D birnessite system has a marked effect on its bifunctional activity.

Localized Negatively Charged Interfaces for Seawater Electrolyte‐Based Zinc‐Air Batteries

AbstractSeawater electrolyte‐based zinc‐air batteries (S‐ZABs) are considered the promising choice for highly efficient marine power supply, owing to their high specific energy, low cost, and eco‐friendliness. However, the air‐cathode suffers from the sluggish oxygen reduction reaction (ORR) kinetics together with the severe adsorption of Cl− in seawater electrolytes, which severely limits the service life and efficiency of S‐ZABs. Herein, precisely decorating axially coordinated Cl ions (Cl−) on Fe atomic sites (Cl‐FeSA/NC) to construct a local negatively charged interface to inhibit the adsorption of Cl− is proposed, which exhibits a desirable ORR activity, and good stability with almost no loss in electrocatalytic performance after long‐term stability test. Moreover, the assembled battery achieves a peak power density of 208.0 mW cm−2, which is 1.45 times higher than commercial Pt/C‐based S‐ZABs. Density functional theory calculations reveal that the axially coordinated Cl− not only constructs a local negatively charged interface to inhibit the corrosion and poisoning of Fe active sites by Cl−, but also regulates the electronic states of Fe active sites to optimize adsorption/desorption energy of intermates, thus improving the electrocatalytic activity and stability of Cl‐FeSA/NC.

Organogel Polymer Electrocatalysts for Two-Electron Oxygen Reduction.

Polymer gels, renowned for unparalleled chemical stability and self-sustaining properties, have garnered significant attention in electrocatalysis. Notably, organic polymer gels that exhibit temperature sensitivity and incorporate suitable polar nonvolatile liquids, enhance electronic conductivity, and impart distinct morphological features, but remain largely unexplored as electrocatalysts for oxygen reduction reaction (ORR). To address this issue, an innovative strategy is proposed for synergistic modulation of the rigidity of mainchain molecular skeleton and length of alkyl sidechains, enabling the development of organogel polymers with a sol-gel temperature-sensitive phase transition that promises high selectivity and enhanced activity in electrocatalytic processes. Notably, the shortening of alkyl sidechain length can significantly affect the gelation behavior and internal microstructure of the catalyst, which modifies the electron state, ultimately impacting the catalytic activity of the gel polymer catalysts. In particular, phenyl-containing Ph-FL1 with short alkyl sidechains demonstrates outstanding 2e- ORR activity in alkaline medium, achieving a remarkable hydrogen peroxide (H2O2) selectivity of 98.6% with an impressive yield of 4.08 mol g-1 h-1. This performance surpasses most metal-free carbon-based electrocatalysts. Through theoretical calculation, the carbon atom (site-3) of C═N group is identified as potential active sites, representing a significant advancement toward designing cost-effective and efficient ORR electrocatalysts.

Edge-substituents and center metal optimization boosting oxygen electrocatalysis in porphyrin-based covalent organic polymers.

Edge-substituents and center metal optimization boosting oxygen electrocatalysis in porphyrin-based covalent organic polymers.

Effect of particle size on the oxygen reduction reaction activity of carbon‐supported niobium-oxide‐based nanoparticle catalysts

In this study, niobium oxynitride nanoparticles were examined to determine the effect of particle size on oxygen reduction reaction (ORR) activity. To this end, catalyst precursors with niobium oxides dispersed on carbon supports were prepared using the irradiation or impregnation method. Polyacrylonitrile was added to each precursor, followed by heat treatment under an ammonia‐containing atmosphere to synthesize niobium oxynitride nanoparticles. The structures of the prepared catalysts were analyzed using transmission electron microscopy, X-ray diffraction, and X-ray absorption spectroscopy. The results indicated that two catalysts with the same crystal phase but different particle sizes were obtained. Comparing their ORR activities revealed that the effect of particle size on ORR activity was limited. Thus, it was inferred that controlling the microelectron conduction paths can help maximize the benefits of particle size reduction. In addition, niobium oxynitride nanoparticles with different structures were obtained by varying the heat-treatment temperatures, and the ORR activity of each prepared catalyst was evaluated. These findings suggest that forming graphitized carbon residues with high electrical conductivity and controlling nitrogen-doping in the oxide nanoparticles are crucial steps for enhancing the ORR activity of oxide-based catalysts. These findings offer valuable insights for developing material design strategies to improve oxide-based catalyst performance.Graphical abstract

Strategic potassium doping in perovskites: A pathway to superior oxygen reduction reaction and hydration activity in reversible proton ceramic electrochemical cells

Strategic potassium doping in perovskites: A pathway to superior oxygen reduction reaction and hydration activity in reversible proton ceramic electrochemical cells

In Situ Electron Tomography Insights into the Curvature Effect of a Concave Surface on Fe Single Atoms for Durable Oxygen Reaction.

Curvature-induced interfacial electric field effects and local strain engineering offer a powerful approach for optimizing the intrinsic catalytic activity of single-atom catalysts (SACs). Investigations into the surface curvature on SACs are still ongoing, and the impact of the concave surface is often overlooked. In this work, theoretical calculations indicate that curved surfaces, particularly those with concavity, can optimize the electronic structures of single Fe sites and facilitate the reductive release of *OH. A carbon sphere featuring uniformly oriented channels and a chiral multi-shelled carbon hollow nanosphere are selected as carbon matrices due to their accessible concave and/or convex surfaces. After loading Fe species, the resulting catalysts with Fe SA in curved surfaces exhibit excellent oxygen reduction reaction activity (E1/2 = ≈0.89V), strong methanol tolerance, and favorable long-term stability. Impressively, a solid-state flexible Zn-air battery based on this catalyst exhibits a remarkable durability over 40h with a high peak power density of 122.1mW cm-2 and excellent charge-discharge performance at different bending angles. This work offers in-depth insights into the rational design of carbon supports with highly curved surfaces, offering new opportunities for the microenvironmental regulation of SACs at the atomic level.