Superior Oxygen Reduction Reaction Research Articles

Tunable Catalytic Vertex Wall Chemistry in Metal-free Covalent Organic Frameworks for Enhanced Oxygen Reduction.

Metal-free covalent organic frameworks (COFs) have emerged as promising catalysts for the oxygen reduction reaction (ORR) because of their unique structural properties and notable stability. To enhance both catalytic activity and selectivity, a variety of linkers and linkages have been investigated in efforts to precisely engineer COFs. However, the impact of vertex structures within COFs on ORR catalysis remains largely underexplored. Here, to modulate COF catalytic performance, we introduce tunable catalytic vertex wall chemistry by introducing diverse triazine and thiophene units. The catalytic vertex wall approach allows the fine-tuning of electronic surface states, leading to improved intermediate adsorption characteristics and accelerated ORR activity. Remarkably, the engineered COF achieved a half-wave potential of 0.76 V, surpassing COFs modified by linker or linkage strategies. Theoretical calculations suggest that this enhanced activity arises from the strong binding affinity of OOH* intermediates to carbon atoms adjacent to the thiophene vertex, facilitating OOH* reduction to a O2 molecule, which is the rate-limiting step of the ORR. These findings reveal the pivotal role of vertex wall engineering in conjugated COF frameworks, and offer critical insights to advance COFs design toward superior ORR performance.

Synergistic Effects of Fe3O4/N-Doped Porous Carbon Nanospheres for Enhanced Oxygen Reduction Reaction.

Designing transition metal oxide (TMO)/porous carbon composite materials for the oxygen reduction reaction (ORR) is a promising strategy in high-performance fuel cell technology. In this study, we used the isolation effect and pore-creating properties of Zn2+ to fabricate a composite material comprising ultrasmall Fe3O4 particles anchored on hierarchically N-doped porous carbon nanospheres. This material, referred to as CPZ1.0-Fe0.1, serves as an efficient ORR electrocatalyst in alkaline fuel cells. CPZ1.0-Fe0.1 exhibits outstanding ORR catalytic activity with a half-wave potential of 0.86 V in a 0.1 M KOH solution, surpassing that of the Pt/C catalyst (0.82 V). Furthermore, CPZ1.0-Fe0.1 exhibited excellent stability with minimal current degradation after 18 h of continuous testing. The superior ORR catalytic performance can be attributed to the synergistic effect between the catalytic sites and the high conductivity of porous carbon. The porous carbon nanospheres effectively address the low conductivity of Fe3O4 nanoparticles, while the ultra-small Fe3O4 nanoparticles anchored on the carbon surface provide efficient catalytic sites for the ORR.

Assembly-foaming synthesis of hierarchically porous nitrogen-doped carbon supported single-atom iron catalysts for efficient oxygen reduction.

High-performance electrocatalysts are highly concerned in oxygen reduction reaction (ORR) related energy applications. However, facile synthesis of hierarchically porous structures with highly exposed active sites and improved mass transfer is challenging. Herein, we develop a novel assembly-foaming strategy for synthesizing hierarchically porous nitrogen-doped carbon supported single-atom iron catalysts. Incorporation of a Fe3+/histidine complex into the block copolymer F127/resol assembly system not only enables an assembly-foaming process forming hierarchical pores, but also promotes the creation of abundant nitrogen-coordinated single-atom Fe (FeNX) sites on well-graphitized carbon skeletons. The obtained materials possess interconnected macropores (1.5-11.5µm), large mesopores (5-30nm) and rich micropores, high surface areas (534-970m2 g-1), large pore volumes (0.68-1.04cm3 g-1) and rich FeNX sites. The optimized sample exhibits a superior ORR activity (onset potential 1.03V and half-wave potential 0.89V) to the commercial 20wt% Pt/C catalyst, a high kinetic current density and excellent stability and methanol tolerance.The prominent performance stems from the coeffects of the hierarchical pore structure and the rich accessible FeNX sites. The significance of the pore structure is revealed by the positive linear relationship between the double-layer capacitances of the obtained materials and their ORR activities.

Pt2Gd Alloy Nanoparticles from Organometallic Pt and Gd Complexes and Hollow Mesoporous Carbon Spheres: Enhanced Oxygen Reduction Reaction Activity and Durability.

Pt2Gd alloy nanoparticles supported in hollow mesoporous carbon spheres (HMCS; Pt2Gd/HMCS) were successfully prepared by the thermal reduction of organometallic Pt and Gd complexes without oxygen atoms supported in the pores of HMCS. The structures of Pt2Gd alloy nanoparticles were fully characterized by TEM, HAADF-STEM-EDS, XRD, XAFS, and XPS, suggesting the formation of uniform Pt2Gd alloy nanoparticles with an average particle size of 5.9 nm. Pt2Gd/HMCS showed superior oxygen reduction reaction activity (2.4 times higher mass-specific activity to Pt nanoparticles on HMCS (Pt/HMCS)) and remarkable durability even after the 100,000 cycles of accelerated degradation tests compared to Pt/HMCS and a commercial Pt/C catalyst. The structure of the Pt2Gd alloy nanoparticles after initial aging and the durability tests suggested that the Pt2Gd core-Pt shell structure with a particle size similar to the pore size of HMCS was stably formed inside the porous structure of HMCS and maintained under the oxygen-reduction working conditions.

Rational Design of Atomically Immobilized Fe‐N4 Sites via Acid‐Amine Chemistry: Leveraging Metal Ionic Assistance for Precise Morphology Control

ABSTRACTTailoring atomically dispersed single‐atom catalyst (Fe‐SAC) holding well‐defined coordination structure (Fe‐N4) along with precise control over morphology is a critical challenge. Herein, we propose a novel acid‐amine coupling reaction between metal‐chelated ionic liquid ([1‐(3‐aminopropyl) 3‐methylimidazolium tetrachloroferrate(III)] [APIM]+[FeCl4]−) and carboxylic groups of carbon allotropes (C = GO, CNT, CNF, and vX‐72) to precisely immobilize Fe‐Nx sites. Out of designed single‐atom catalyst (IL‐Fe‐SAC‐C), Fe‐N4 on graphene (IL‐Fe‐SAC‐Gr) delivered superior oxygen reduction reaction (ORR) activity by holding higher halfwave potential of 0.882 V versus RHE in 1.0 M KOH akin to Pt/C (0.878 V vs. RHE) and surpassing recently reported M–N–C catalysts with superior ethanol tolerance. Thanks to higher graphitization degree, enhanced surface characteristics, and richness in high‐density Fe‐N4 sites of IL‐Fe‐SAC‐Gr confirmed by XPS, X‐ray absorption spectroscopy (XAS), and HAADF analysis. The IL‐Fe‐SAC‐Gr catalyst‐coated cathode on testing in flexible direct ethanol fuel cells (f‐DEFC) delivered higher peak power density of 18 mW cm−2 by outperforming Pt/C‐based cathode by 3.5 times as a result of excellent ethanol tolerance. Further, the developed f‐DEFC successfully powered the Internet of Things (IoT)‐based health monitoring system. This method demonstrates novel strategy to tailor high‐performance single‐atom (Fe‐SAC‐C) sites on desired morphologies to meet specific application requirements with feasibility and versatility.

Enhanced H2O2 Production by Two-Electron Oxygen Reduction on Co3O4/MXene Catalyst

The two-electron oxygen reduction reaction (ORR) using an electrochemical method is considered a viable green technology for generating H2O2. Platinum group metals demonstrate excellent H2O2 generation performance due to their superior ORR activity and stability. However, the high cost of these electrocatalysts is a significant barrier to the widespread adoption of this technology, prompting research into non-precious metal catalysts as alternatives. In this work, Co3O4 nanoparticles with sized from 5 to 10 nm were synthesized on MXene sheets. Compared with Co3O4/C and MXene, the results indicated that Co3O4/MXene exhibited the best electrochemical performances, with an electron transfer number of ∼3.1 during oxygen reduction and an H2O2 selectivity of ∼47%. Co3O4/MXene also demonstrated great stability. After 5000 cycles, the H2O2 selectivity and electron transfer number remained at 50% and 3.0, respectively. After 12 h of continuous testing, Co3O4/MXene exhibited a high Faraday efficiency of 50% and H2O2 yield of 0.14 mol h−1 g−1, with H2O2 selectivity increasing to 68.7% and an electron transfer number of 2.6. The excellent electrochemistry is attributed to the synergistic effect between MXene and Co3O4. This study provides valuable insights into non-precious metal electrocatalysts for H2O2 generation.

Biomass-derived single atom catalysts with phosphorus-coordinated Fe-N3P configuration for efficient oxygen reduction reaction

Exploiting non-precious metal catalysts with excellent oxygen reduction reaction (ORR) performance for energy devices is paramount essential for the green and sustainable society development. Herein, low-cost, high-performance biomass-derived ORR catalysts with an asymmetric Fe-N3P configuration was prepared by a simple pyrolysis-etching technique, where carboxymethyl cellulose (CMC) was used as the carbon source, urea and 1,10-phenanthroline iron complex (FePhen) as additives, and Na3PO4 as the phosphorus dopant and a pore-forming agent. The CMC-derived FeNPC catalyst displayed a large specific area (BET:1235 m2 g−1) with atomically dispersed Fe-N3P active sites, which exhibited superior ORR activity and stability in alkaline solution (E1/2 = 0.90 V vs. RHE) and Zn-air batteries (Pmax = 149 mW cm−2) to commercial Pt/C catalyst (E1/2 = 0.87 V, Pmax = 118 mW cm−2) under similar experimental conditions. This work provides a feasible and cost-effective route toward highly efficient ORR catalysts and their application to Zn-air batteries for energy conversion.

Ordered Pt3Ni intermetallic compound: A high-performance oxygen reduction reaction catalyst for Zn-air batteries

The development of advanced catalysts for oxygen reduction reaction (ORR) is crucial for Zinc-air batteries (ZABs). In this study, we focus on the controlled synthesis of ordered Pt3Ni intermetallic compounds through impregnation and high-temperature annealing, aiming to enhance the ORR performance for ZABs. The ordered Pt3Ni (denoted as Pt3Ni) and conventional alloy Pt3Ni (denoted as A-Pt3Ni) are synthesized at 600 ℃ and 200 ℃, respectively. Detailed structural characterizations confirm the formation of both ordered and disordered structures. Pt3Ni demonstrates superior ORR catalytic activity and stability compared to A-Pt3Ni and commercial Pt/C catalyst, as evidenced by higher half-wave potentials, lower Tafel slopes, and enhanced electrochemical stability. Furthermore, Pt3Ni exhibits exceptional performance in ZABs, with higher power density, specific capacity, and longer cycle life than its counterparts. These findings highlight the potential of ordered Pt3Ni intermetallic compounds as efficient ORR catalysts in ZABs, offering a promising route for the development of advanced energy conversion and storage systems.

Revealing the Critical Role of the Lone Pair Electrons of Bi3+ in Electrocatalytic Oxygen Reduction Reaction on Mn‐Based BiMn2O5 Mullite Surface

AbstractThere is a research gap regarding the role of lone pair electrons of post‐transition metal cations in catalysis field, yet such studies hold significant implications for expanding their applications. Herein, the role of Bi3+ ion lone pair electrons in electrocatalytic oxygen reduction reaction (ORR) is elucidated using ternary mullite oxides as prototype catalysts. Among the three synthesized mullites ((Sm/Y/Bi)Mn2O5), BiMn2O5 exhibits notably superior ORR catalytic activity. Through advanced characterization techniques, including in situ Raman, in situ infrared, and X‐ray absorption spectroscopy, combined with theoretical calculations, it is determined that the superior electrocatalytic performance of BiMn2O5 originates from the lone pair electrons that activate catalytic sites through stereochemical effects. Specifically, the Bi3+ lone pair electrons in the 6s orbital near Fermi level interact with O 2p orbitals, causing a shift of the oxygen ligands, and significant variance in A─O bond lengths. This structural distortion of BiO8 coordination unit directly results in a large Mn─O bond angle at the active center of BiMn2O5 and strong covalent interactions, facilitating charge transfer and refining the adsorption behavior of oxygen intermediates to achieve the overpotential of 0.25 V vs. RHE. This study provides insights into developing next‐generation catalysts by harnessing the lone pair electrons of post‐transition metal cations.

NiMn-Layered Double Hydroxides@Metal Anchored Nitrogen-Doped Carbon Framework As a Bifunctional Catalyst for Rechargeable Zinc-Air Batteries

The rechargeable zinc-air battery (ZAB) holds promise as an energy storage system for the next generation due to its high theoretical energy density, safe aqueous electrolyte, environmental friendliness, and cost-effectiveness of zinc. However, the efficiency of ZABs is hindered by the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Noble metals such as platinum (Pt), iridium dioxide (IrO2), and ruthenium oxide (RuO2) are commonly used as advanced electrocatalyst materials. Nonetheless, their scarcity, high cost, unstable catalytic properties, and susceptibility to carbon monoxide poisoning limit the large-scale applications of ZABs.Therefore, exploring non-noble bifunctional electrocatalysts for ZABs is crucial, with transition metals being viable alternatives. Recently, bimetallic transition metal layered double hydroxide (LDH) has emerged as a promising candidate for OER catalysts due to its high specific surface area, abundant active sites, and synergistic effects between two transition metals. However, LDH still faces challenges such as poor electrical conductivity and self-aggregation during fabrication.In this work, we developed a three-dimensional core-shell structure, enabling the growth of LDH on nitrogen-doped porous carbon framework (N-C) materials to address these challenges. The N-C material, derived from zeolitic-imidazole framework (ZIF) material after pyrolysis, shows promise as an ORR catalyst. ZIF, a subclass of metal-organic framework (MOF), boasts high nitrogen content and inherits MOF advantages such as a high surface area, abundant active sites, and chemical stability. Furthermore, utilizing mesoporous silica as a hard template allows us to maintain the dodecahedral structure of ZIF and create mesoporous structures after annealing.The core-shell structure developed in this study demonstrates superior OER and ORR catalytic activity, offering a promising direction for the design of bifunctional catalysts for high-performance ZABs.

Impact of Local Structure and Spin on the ORR Performance of Single Atom M-N-C Catalysts

Transition metal single-atom catalysts supported on N-doped graphene (M-N-Cs) offer significant advantages in metal utilization and uniformity of active sites compared to traditional heterogeneous catalysts. They are particularly notable for their superior electrocatalytic Oxygen Reduction Reaction (ORR) performance. Various modifications have been explored, including different metal combinations, different first shell elements (N, C, S, B), and coordination numbers, to enhance electrocatalytic ORR activities. Another emerging focus is the link between their local structure and spin state. The Zhenan Bao and Jaramillo groups recently found that the catalytic activity of the NiNx moiety stems from its tetrahedrally distorted structure because it stabilizes the high-spin Ni state more than the low-spin state.[1] The local structure and its distortion can be modified by adjusting the carbon nanotube diameter,[2] pore size,[3] layering,[4] or by adding ligands at the first or second shell.[5]In this study, we have conducted a computational screening of M-N-Cs with 3d, 4d, and 5d metals and local structures including square planar, tetragonal pyramidal, and tetrahedral symmetries. Metals in their divalent states within the MN4 units exhibited a V-shaped trend in formation energy correlated with increasing atomic numbers across each series. This pattern appears to be linked to the interaction between the two electrons of the N4 ligand and the d electron count of the metals, and it also emerged in the *O/*OH adsorption energy plots. Additionally, the preferred spin state varied with the metal type and structural symmetry, influencing electron distribution and crystal field splitting. These factors collectively impacted the binding energies with adsorbents, subsequently altering oxygen reduction reaction (ORR) activity. Further, we explored the adsorption energies of various intermediates (*CO, *H, *Cl, and *N2H) relevant to carbon dioxide reduction reaction (CO2RR), hydrogen evolution reaction (HER), chlorine evolution reaction (ClER), and nitrogen reduction reaction (NRR), respectively. Our primary objective was to identify optimal metal-structure combinations for enhanced catalytic performance. To achieve this, we considered several descriptors including partial Integrated Crystal Orbital Hamilton Population (ICOHP), metal d-band center, Madelung energy, and metal ionization potential. Moving forward, we aim to extend these findings by incorporating distorted coordination environments and integrating experimental synthesis strategies to induce such distortions, potentially unlocking new pathways for catalytic efficiency enhancement.[1] Koshy, David M., et al. "Investigation of the Structure of Atomically Dispersed NiNx Sites in Ni and N-Doped Carbon Electrocatalysts by 61Ni Mössbauer Spectroscopy and Simulations." Journal of the American Chemical Society 144.47 (2022): 21741-21750.[2] Cepitis, Ritums, et al. "Surface curvature effect on dual-atom site oxygen electrocatalysis." ACS Energy Letters 8.3 (2023): 1330-1335.[3] Glibin, Vassili P., et al. "Non-PGM electrocatalysts for PEM fuel cells: thermodynamic stability and DFT evaluation of fluorinated FeN4-based ORR catalysts." Journal of The Electrochemical Society 166.7 (2019): F3277.[4] Wu, Yahui, et al. "Boosting CO2 electroreduction over a cadmium single‐atom catalyst by tuning of the axial coordination structure." Angewandte Chemie International Edition 60.38 (2021): 20803-20810.[5] Choi, Daeeun, et al. "Bridging the Catalytic Turnover Gap Between Single‐Atom Iron Nanozymes and Natural Enzymes by Engineering the First and Second Shell Coordination." Advanced Materials (2023): 2306602.

Investigation on High Stable Ordered Ptｍ (M=Fe, Co, Ni, Cu) Catalysts Via Operando X-Ray Absorption Spectroscopy Analysis

In the development of society, energy stands as the cornerstone for human survival and progress. Historically, traditional fossil fuels served as the primary energy source; however, the widespread utilization of fossil fuels exacerbates issues like climate change and environmental pollution and necessitates the development of new, clean, and efficient energy alternatives to supplant conventional fossil fuels. Hydrogen, with its high energy density and emission-free properties, emerges as a promising alternative energy carrier, particularly in fuel cell technology. 1 However, the widespread adoption of fuel cells is hindered by challenges, notably the high cost associated with platinum catalysts used in oxygen reduction reactions (ORR). Addressing this challenge requires innovative approaches to enhance catalytic performance while reducing cost concern. 2 This has led to the exploration of alloying Pt with non-noble metals (Fe, Co, Ni, Cu), which improves the ORR activity than benchmark Pt/C and simultaneously solves the high catalyst cost issues. 3 Previous analyses of these alloys show that the significant improvement in ORR activity is due to optimized Pt 5d orbital vacancy, which shortens the Pt-Pt bond distance and weakens the binding of intermediates. The Pt-Pt bond distance has been widely regarded as a crucial factor in analyzing the structure-activity relationship.This research aimed to explore the relationship between catalytic performance and Pt-Pt bond distance, combining electrochemical performance with the operando X-ray absorption spectroscopy analysis during ORR conditions. In this study, various ordered PtM (M=Fe, Co, Ni, Cu) alloys were synthesized by thermal treatment under NH3 atmosphere using mesoporous carbon supports, which restrict particle growth during thermal treatment. The morphological attributes analyzed by TEM images clearly show that Pt-M alloys is located inside the pores of the mesoporous carbon layer. The detailed electrochemical investigation by Cyclic voltammetry (CV) and Linear sweep voltammetry (LSV), shows three times superior activity than benchmark Pt/C. The Operando HERFD-XAS analysis, conventional EXAFS combined with electrochemical investigations shows a positive correlation between specific activity and the Pt-Pt bond, which indicated that contracted lattice structure would contribute to less formation of the Pt oxide species and ultimately results in superior ORR activity and durability. Therefore, our results demonstrate the importance of the Pt-Pt bond length in assessing ORR activity and validate that structural changes during operation can be a key enabling factor in predicting catalytic performance and durability. Acknowledgment This work is supported by the PEFC and the NEDO FC-Platform commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Reference: A. Risco-Bravo, C. Varela, J. Bartels and E. Zondervan, Renewable Sustainable Energy Rev., 2024, 189, 113930. J. Wang, W. Ding and Z. Wei, Acta. Phys. Sin., 2021, 37, 2009094. C. Kim, F. Dionigi, V. Beermann, X. Wang, T. Moeller and P. Strasser, Adv. Mater., 2019, 31, 1805617. Figure 1

High Activity pH-Universal Oxygen Reduction Reaction Catalyst: Tri-Doped Porous Carbon Material Derived from Agricultural and Mining Wastes

Introduction Fuel cells still command great attention even as the need for an alternative source of energy increases annually. Following the unison conclusion by various researchers on the use of platinum and precious group metals (PGM) catalysts, as an expensive venture, many researchers are still committed to alternative and effective catalysts for fuel cell application. First-row transitional metals (TM) have been proposed as effective alternatives for oxygen reduction reaction (ORR) applications due to their promising high ORR activity and durability besides their abundance and cheaper unit cost compared to PGM [1]. In the last decade in particular, many researchers have reported catalysts derived from carbon doped with iron (Fe) and nitrogen (N). Such catalysts have shown high ORR activity, especially under alkaline conditions. The formation of Fe-N4 moieties in the carbon lattice through the interaction between the doped heteroatoms has been reported as an effective active site during the ORR process [2,3]. However, most of these catalysts have only shown high activity under alkaline conditions while their activity in neutral and acidic mediums still lags hindering wide applications in fuel cells [4]. This negative trend has been attributed to the limited number of active sites and the ease of catalyst degradation in acidic or neutral medium hence low durability in these media. To address these challenges, an increase in the number and concentration of dopants has been considered and shown positive results towards increasing the activity. However, such ventures have a net effect on the sustainability of this technology. Herein an eco-friendly and sustainable approach is presented. In this approach, we harness waste rice husk biomass rich in silica as a source of carbon (C) and self-doped silica (Si) together with pyrite, an abundant byproduct of chalcopyrite smelting following the increased demand of copper for the electric vehicle (EV) industry, as a source of Fe. These materials are then synthesized through a chain of hydrothermal and template carbonization synthesis processes to yield a tri-doped catalyst which is referred to as Fe-N-Si-C in this work. The synthesized catalyst was characterized for both physiochemical properties using TEM, SEM, XAFS and DFT while the electrochemical properties were analyzed using linear sweep voltammetry, cyclic voltammetry, and durability in both alkaline, neutral, and acidic mediums. Results and discussion The template carbonization method adopted in the synthesis in this work, resulted in a porous carbon material with successfully doped heteroatoms as confirmed by the TEM and SEM analysis. A homogenous and evenly distributed of C, Fe, N and Si elements was further confirmed by the EDS mapping. The significance of this heteroatom doping that was achieved from wastes was exhibited in the Fourier transform EXAFS where the coexistence of Fe-N4 moieties and Fe clusters with surface oxides was confirmed by the presence of Fe-N and Fe-O peaks were detected on the catalyst spectrum. Similarly, a Si-K-edge X-ray absorption revealed the formation of an amorphous SiO2 peak on the Fe-N-Si-C catalyst. These Fe-N4 porphyrin-like moieties and the SiO2 provided effective active sites for ORR as demonstrated in the LSV analysis in all universal pH media, where Fe-N-Si-C showed an onset potential of 1.02V, 0.96V and 0.86V which was close to those of commercial PtC ;1.04V, 0.99V and 0.91V in ORR analysis conducted in 0.1M KOH (alkaline),0.1M PB (neutral) and 0.1M HClO4 (acidic) respectively as highlighted in Table 1. Motivated by superior ORR performance in alkaline conditions, a Zn-air battery (ZAB) was assembled with 6M KOH and 0.2M Zinc acetate. Fe-N-Si-C cell achieved a 103mW/cm2 power density, nearly 50% higher than what the PtC (78mW/cm2) cell achieved. This work not only showcases value addition to unused environmental waste resources considered pollutants but also introduces a cost-effective and environmentally friendly catalyst, serving as a substitute for precious and expensive metal-derived electrocatalysts.

Bimetallic Cofe Nanoparticles Embedded in Hierarchically Meso-Macroporous N-Doped Carbon Nanospheres As Electrocatalyst for Excellent Oxygen Reduction Reaction

Proton exchange membrane fuel cells (PEMFCs) are regarded as a highly promising technology for clean and sustainable energy conversion, but face challenges due to their reliance on costly noble metals and sluggish oxygen reduction reaction (ORR) kinetics[1]. Therefore, development of effective catalysts to serve as the alternatives of platinum group metals is required. So far, metal-nitrogen-carbon (M-N-C) materials are considered as the most promising electrocatalysts for ORR[2]. Their performance, however, are still impeded by the low density and accessibility of active metal sites. In this work, we developed a remarkable ORR electrocatalysts by assembling bimetallic CoFe nanoparticles on hierarchically meso-macroporous N-doped carbon nanospheres. This hierarchically structured meso-macroporous N-doped carbon serves as a conductive carrier and provides abundant exposed sites for the active metal species. In turn, the bimetallic CoFe nanoparticles significantly increase the total density of active catalytic sites for ORR. As a result, the optimized CoFe nanoparticles embedded in meso-macroporous N-doped carbon (CoFe@MesoMacroNC) delivered superior ORR activities in both alkali (half-wave potential = 0.943 V vs. RHE) and acid media (half-wave potential = 0.811 V vs. RHE). In addition, when the CoFe@MesoMacroNC was assembled in a Zn-air battery, it showed a higher power density of 155 mW cm-2 than commercial Pt/C (137 mW cm-2) as well as a better stability.

Controlled Synthesis of Ultrahigh-Loading Pt–Co Intermetallic Compounds for Superior Oxygen Reduction Reaction

Controlled Synthesis of Ultrahigh-Loading Pt–Co Intermetallic Compounds for Superior Oxygen Reduction Reaction

Synergistic effects of metal single atoms and clusters on graphene-supported electrocatalysts: Insights into the mechanism of oxygen reduction reaction

In recent years, non-precious-metal-doped single-atom catalysts have emerged as promising alternatives to Pt-based catalysts due to their high active site density and excellent oxygen reduction reaction (ORR) activity. However, challenges arise in precisely controlling the doping precision of metal atoms during experimental preparation, often resulting in clustering of metal atoms. The impact of these metal clusters on catalytic activity remains unclear. Here employing density functional theory, we investigated a series of N-doped graphene based catalysts, including MN4, MN4@M4˗Py, MN4@M4˗Qu (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Ir, Pt), to elucidate changes in catalyst properties upon metal cluster loading and assess how different configurations of metal clusters influence ORR activity. Our findings reveal synergistic interactions between metal clusters and single atoms, which can either enhance or inhibit catalytic activity depending on elemental types and cluster configurations. Remarkably, the CoN4@Co4˗Py catalyst exhibits superior ORR catalytic performance, with an overpotential of 0.277 V. This study not only sheds light on the synergistic effects between metal clusters and catalysts, but also offers theoretical insights for the experimental preparation of catalysts.

Hydrothermal-mediated in-situ nitrogen doping to prepare biochar for enhancing oxygen reduction reactions in microbial fuel cells

Nitrogen-doped carbon materials are deemed promising cathode catalysts for microbial fuel cells (MFCs). The challenge lies in reducing costs and enhancing the proportion of electrocatalytically active nitrogenous functional groups. This study proposes a hydrothermal-mediated in-situ doping method to produce nitrogen-doped biochar from aquatic plants. The nitrogen atoms are anchored in the carbon structure during hydrothermal treatment. Subsequent pyrolysis converts the hydrochar into a catalyst with highly catalytically active aromatic ring structure (HC-N+PY). The as-prepared HC-N+PY electrocatalyst demonstrates superior oxygen reduction reaction activity with half-wave potentials of 0.82 V. The MFC with HC-N+PY exhibits excellent performance, with a peak power density of 1444 mW/m2. Theoretical calculations demonstrate that the synergistic effect of graphitic nitrogen and C–O groups at defect sites enhances O2 adsorption and protonation. This work highlights the potential of utilizing nitrogen-doped biochar derived from aquatic plants as an effective catalyst for enhancing the performance of microbial fuel cells.

Electrodeposition of Ni/Cu Bimetallic Conductive Metal-Organic Frameworks Electrocatalysts with Boosted Oxygen Reduction Activity for Zinc-Air Batteries.

Zinc-air batteries employing non-Pt cathodes hold significant promise for advancing cathodic oxygen reduction reaction (ORR). However, poor intrinsic electrical conductivity and aggregation tendency hinder the application of metal-organic frameworks (MOFs) as active ORR cathodes. Conductive MOFs possess various atomically dispersed metal centers and well-aligned inherent topologies, eliminating the additional carbonization processes for achieving high conductivity. Here, a novel room-temperature electrochemical cathodic electrodeposition method is introduced for fabricating uniform and continuous layered 2D bimetallic conductive MOF films cathodes without polymeric binders, employing the organic ligand 2,3,6,7,10,11-hexaiminotriphenylene (HITP) and varying the Ni/Cu ratio. The influence of metal centers on modulating the ORR performance is investigated by density functional theory (DFT), demonstrating the performance of bimetallic conductive MOFs can be effectively tuned by the unpaired 3d electrons and the Jahn-Teller effect in the doped Cu. The resulting bimetallic Ni2.1Cu0.9(HITP)2 exhibits superior ORR performance, boasting a high onset potential of 0.93V. Moreover, the assembled aqueous zinc-air battery demonstrates high specific capacity of 706.2mA h g-1, and exceptional long-term charge/discharge stability exceeding 1250 cycles.

Heteroatoms doping for modulation of sp3-hybridized carbon sites for highly efficient bio-electroreduction of oxygen in microbial fuel cells

Due to the fact that single chamber microbial fuel cells (SC-MFCs) generally require efficient, stable, and inexpensive cathode catalysts to promote bioelectricity generation performance, metal-free catalysts have attracted increasing attention in SC-MFCs. However, enhancing the electrocatalytic activity of the oxygen reduction reaction (ORR) using metal-free catalysts remains a challenge. This study introduces a metal-free porous catalyst by synchronously doping phosphorus (P) and nitrogen (N) atoms into the biomass-derived carbon skeleton (AB-N-P). The AB-N-P catalyst, displaying half-wave potentials (E1/2) of 0.89 (alkaline) and 0.832 V (neutral), exhibits superior ORR performances compared to most metal-free catalysts and is comparable to some non-precious metal catalysts. Remarkably, it (E1/2: 0.890 to 0.880 V) matches or even exceeds the stability of commercial Pt/C catalyst (E1/2: 0.864 to 0.849 V) after 3000 cyclic voltammetry cycles. Density functional theory (DFT) calculations demonstrate that P and N co-doping synergistically increases the sp3-hybridized carbon content and redistributes charge density, thereby enhancing the catalytic properties and altering the electronic properties of the active sites. In practical applications in SC-MFCs, the AB-N-P electrocatalyst shows exceptional performance, achieving a higher power density (896.4 mW m−2) than that of commercial Pt/C (538.2 mW m−2). This study offers insightful revelations into the modulation of carbon active-sites by doping heteroatoms to carbon skeleton as metal-free catalyst in bioenergy-generation systems.

Axial Coordination Engineering on Fe-N-C Materials for Oxygen Reduction: Insights from Theory.

Axial coordination engineering has emerged as an effective strategy to regulate the catalytic performance of metal-N-C materials for oxygen reduction reaction (ORR). However, the ORR mechanism and activity changes of their active centers modified by axial ligands are still unclear. Here, a comprehensive investigation of the ORR on a series of FeN4-L moieties (L stands for an axial ligand) is performed using advanced density functional theory (DFT) calculations. The axial ligand has a substantial effect on the electronic structure and catalytic activity of the FeN4 center. Specially, FeN4-C6H5 is screened as a promising active moiety with superior ORR activity, as further revealed by constant-potential calculations and kinetic analysis. The enhanced activity is attributed to the weakened *OH adsorption caused by the altered electronic structure. Moreover, microkinetic modeling shows that at pH=1, FeN4-C6H5 possesses an impressive theoretical half-wave potential of ~1.01 V, superior to the pristine Fe-N-C catalysts (~0.88 V) calculated at the same level. These findings advance the understanding of the ORR mechanism of FeN4-L and provide guidance for optimizing the ORR performance of single-metal-atom catalysts.