Tailoring the microenvironment in Fe-N-C electrocatalysts for optimal oxygen reduction reaction performance

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

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

Heterogeneous organophotocatalytic HBr oxidation coupled with oxygen reduction for boosting bromination of arenes

Developing mild photocatalytic bromination strategies using sustainable bromo source has been attracting intense interests, but there is still much room for improvement. Full utilization of redox centers of photocatalysts for efficient generation of Br+ species is the key. Herein we report heterogenous organophotocatalytic HBr oxidation coupled with oxygen reduction to furnish Br2 and H2O2 for effective bromination of arenes over Al2O3 supported perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). Mechanism studies suggest that O-vacancy in Al2O3 can provide Lewis-acid-type anchoring sites for O2, enabling unexpected dual-electron transfer from anchored photoexcited PTCDA to chemically bound O2 to produce H2O2. The in-situ generated H2O2 and Br2 over redox centers work together to generate HBrO for bromination of arenes. This work provides new insights that heterogenization of organophotocatalysts can not only help to improve their stability and recyclability, but also endow them with the ability to trigger unusual reaction mode via cooperative catalysis with supports.

Enhanced Activity of Small Pt Nanoparticles Decorated with High-Loading Single Fe─N4 for Methanol Oxidation and Oxygen Reduction via the Assistive Active Sites Strategy

Decorating platinum (Pt) with a single atom offers a promising approach to tailoring their catalytic activity. In this study, for the first time, an innovative assistive active sites (AAS) strategy is proposed to construct high-loading (3.46wt.%) single Fe─N4 as AAS, which are further hybridized with small Pt nanoparticles to enhance both oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) activities. For ORR, the target catalyst (Pt/HFeSA-HCS) exhibits a higher mass activity (MA) of 0.98 A mgPt -1 and specific activity (SA) of 1.39 mA cmPt -2 at 0.90 V versus RHE. As for MOR, Pt/HFeSA-HCS shows exceptional MA (3.21 A mgPt -1) and SA (4.27 mA cmPt -2) at peak values, surpassing commercial Pt/C by 15.3 and 11.5 times, respectively. The underlying mechanism behind this AAS strategy is to find that in MOR, Fe─N4 promotes water dissociation, generating more *OH to accelerate the conversion of *CO to CO2. Meanwhile, in ORR, Fe─N4 acts as a competitor to adsorb *OH, weakening Pt─OH bonding and facilitating desorption of *OH on the Pt surface. Constructing AAS that can enhance dual functionality simultaneously can be seen as a successful "kill two birds with one stone" strategy.

TiN as Radical Scavenger in Fe─N─C Aerogel Oxygen Reduction Catalyst for Durable Fuel Cell

Fe─N─C is the most promising alternative to platinum-based catalysts to lower the cost of proton-exchange-membrane fuel cell (PEMFC). However, the deficient durability of Fe─N─C has hindered their application. Herein, a TiN-doped Fe─N─C (Fe─N─C/TiN) is elaborately synthesized via the sol-gel method for the oxygen-reduction reaction (ORR) in PEMFC. The interpenetrating network composed by Fe─N─C and TiN can simultaneously eliminate the free radical intermediates while maintaining the high ORR activity. As a result, the H2 O2 yields of Fe─N─C/TiN are suppressed below 4%, ≈4 times lower than the Fe─N─C, and the half-wave potential only lost 15 mV after 30 kilo-cycle accelerated durability test (ADT). In a H2 ─O2 fuel cell assembled with Fe─N─C/TiN, it presents 980 mA cm-2 current density at 0.6 V, 880 mW cm-2 peak power density, and only 17 mV voltage loss at 0.80 A cm-2 after 10 kilo-cycle ADT. The experiment and calculation results prove that the TiN has a strong adsorption interaction for the free radical intermediates (such as *OH, *OOH, etc.), and the radicals are scavenged subsequently. The rational integration of Fe single-atom, TiN radical scavenger, and highly porous network adequately utilize the intrinsic advantages of composite structure, enabling a durable and active Pt-metal-free catalyst for PEMFC.

Faraday cage-type self-powered immunosensor based on hybrid enzymatic biofuel cell

Self-powered immunosensors (SPIs) based on enzymatic biofuel cell (EBFC) have low sensitivity and poor stability due to the high impedance of the immune sandwich and the vulnerability of enzymes to environmental factors. Here, we applied the Faraday cage-type sensing mode on a hybrid biofuel cell (HBFC)-based SPI for the first time, which exhibited high sensitivity and stability. Cytokeratin 19 fragment (CYFRA 21-1) was used as a model analyte. Au nanoparticle-reduced graphene oxide (Au-rGO) composite was used as the supporting matrix for immunoprobe immobilized with detection antibody and glucose dehydrogenase (GDH), also the builder for Faraday cage structure on the bioanode in the presence of antigen. After the combination of immunoprobe, antigen, and the antibody on the bioanode, the Faraday cage was constructed in case the AuNP-rGO was applied as a conductive cage for electron transfer from GDH to the bioanode without passing through the poorly conductive protein. With the assistance of the Faraday cage structure, the impedance of the bioanode decreased significantly from 4000 to 300 Ω, representing a decline of over 90%. The sensitivity of the SPI, defined as the changes of open circuit voltage (OCV) per unit concentration of the CYFRA 21-1, was 68 mV [log (ng mL-1)]-1. In addition, Fe-N-C was used as an inorganic cathode material to replace enzyme for oxygen reduction reaction (ORR), which endowed the sensor with 4-week long-term stability. This work demonstrates a novel sensing platform with high sensitivity and stability, bringing the concept of hybrid biofuel cell-based self-powered sensor.

Recent advances in bifunctional dual-sites single-atom catalysts for oxygen electrocatalysis toward rechargeable zinc-air batteries

Rechargeable zinc-air batteries (ZABs) with high energy density and low pollutant emissions are regarded as the promising energy storage and conversion devices. However, the sluggish kinetics and complex four-electron processes of oxygen reduction reaction and oxygen evolution reaction occurring at air electrodes in rechargeable ZABs pose significant challenges for their large-scale application. Carbon-supported single-atom catalysts (SACs) exhibit great potential in oxygen electrocatalysis, but needs to further improve their bifunctional electrocatalytic performance, which is highly related to the coordination environment of the active sites. As an extension of SACs, dual-sites SACs with wide combination of two active sites provide limitless opportunities to tailor coordination environment at the atomic level and improve catalytic performance. The review systematically summarizes recent achievements in the fabrication of dual-site SACs as bifunctional oxygen electrocatalysts, starting by illustrating the design fundament of the electrocatalysts according to their catalytic mechanisms. Subsequently, metal-nonmetal-atom synergies and dual-metal-atom synergies to synthesize dual-sites SACs toward enhancing rechargeable ZABs performance are overviewed. Finally, the perspectives and challenges for the development of dual-sites SACs are proposed, shedding light on the rational design of efficient bifunctional oxygen electrocatalysts for practical rechargeable ZABs.

Immobilization of iron phthalocyanine on MOF-derived N-doped carbon for promoting oxygen reduction in zinc-air battery

Functional carbon nanomaterials play a crucial role in the cathodic oxygen reduction reaction (ORR) for sustainable fuel cells and metal-air batteries. In this study, we propose an effective approach to immobilize iron phthalocyanines (FePc) by employing a porous N-doped carbon material, denoted as NC-1000, derived from a sheet-shaped coordination polymer. The resulting NC-1000 possesses substantial porosity and abundant pore defects. The nitrogen sites within NC-1000 not only facilitate FePc adsorption but also optimize the electron distribution at the Fe-N site. The FePc@NC-1000 composite material exhibits a significant number of active centers in the form of Fe-N4 moieties, showcasing satisfactory ORR activity. Specifically, it demonstrates an onset potential of 0.99 V, a positive half-wave potential of 0.86 V, a large limiting current of 5.96 mA cm-2, and a small Tafel slope of 44.41 mV dec-1. Additionally, theoretical calculations and experimental results confirm the favorable performance and durability of zinc-air batteries assembled using FePc@NC-1000, thereby highlighting their considerable potential for practical applications. Overall, this study provides a comprehensive exploration of the enhanced catalytic performance and increased stability of metal-organic framework-derived functional carbon nanomaterials as cost-effective, efficient, and stable catalysts for the ORR.

Boosting oxygen reduction reaction kinetics through perturbating electronic structure of single-atom Fe-N3S1 catalyst with sub-nano FeS cluster

Single atomic Fe-N4 catalyst exhibits a great prospect for oxygen reduction reaction (ORR) and adjusting the intrinsic coordination structure and the carbon matrix structure effectively improves the catalytic activity. However, controlling the active site coordination structure and its surrounding environment at atomic level remains a challenge. In this paper, Fe-N3S1 and FeS sub-nano cluster were innovatively concatenated on S, N co-doped carbon matrix (SNC), denoted as FeS/FeSA@SNC catalysts, for modulating ORR catalysis performance. Both experimental measurements and theoretical calculations have confirmed that the local electron configuration of Fe center is modulated by this unique structure combination leading to optimized ORR kinetics. Based on this design, the synthesized FeS/FeSA@SNC delivers ORR activity with a half-wave potential of 0.9 V (vs. RHE), excelling that of commercial Pt/C (0.87 V) and the Zn-air battery (ZAB) with this cathode catalyst delivers a peak power density of 126 mW cm-2. This work presents a novel strategy for manipulating the single-atom active sites through control the local coordination structure and provides a reference for the development of novel efficient ORR electrocatalysts.

Developing a class of dual atom materials for multifunctional catalytic reactions

Dual atom catalysts, bridging single atom and metal/alloy nanoparticle catalysts, offer more opportunities to enhance the kinetics and multifunctional performance of oxygen reduction/evolution and hydrogen evolution reactions. However, the rational design of efficient multifunctional dual atom catalysts remains a blind area and is challenging. In this study, we achieved controllable regulation from Co nanoparticles to CoN4 single atoms to Co2N5 dual atoms using an atomization and sintering strategy via an N-stripping and thermal-migrating process. More importantly, this strategy could be extended to the fabrication of 22 distinct dual atom catalysts. In particular, the Co2N5 dual atom with tailored spin states could achieve ideally balanced adsorption/desorption of intermediates, thus realizing superior multifunctional activity. In addition, it endows Zn-air batteries with long-term stability for 800 h, allows water splitting to continuously operate for 1000 h, and can enable solar-powered water splitting systems with uninterrupted large-scale hydrogen production throughout day and night. This universal and scalable strategy provides opportunities for the controlled design of efficient multifunctional dual atom catalysts in energy conversion technologies.

Coordination Chemistry of Large-Sized Yttrium Single-Atom Catalysts for Oxygen Reduction Reaction

Although being transition metals, the Fenton-inactive group 3-4 elements (Sc, Y, La, Ti, Zr, and Hf) can easily lose all the outermost s and d electrons, leaving behind ionic sites with nearly empty outermost orbitals that are stable but inactive for oxygen involved catalysis. Here, it is demonstrated that the dynamic coordination network can turn these commonly inactive ionic sites into platinum-like catalytic centers for the oxygen reduction reaction (ORR). Using density functional theory calculations, a macrocyclic ligand coordinated yttrium single-atom (YN₄ ) moiety is identified, which is originally ORR inactive because of the too strong binding of hydroxyl intermediate, while it can be activated by an axial ligand X through the covalency competition between YX and YOH bonds. Strikingly, it is also found that the binding force of the axially coordinated ligand is an effective descriptor, and the chlorine ligand is screened out with an optimal binding force that behaves self-adaptively to facilitate each ORR intermediate steps by dynamically changing its YCl covalency. These experiments validate that the as-designed YN₄ -Cl moieties embedded within the carbon framework exhibit a high half-wave potential (E_1/2 = 0.85 V) in alkaline media, the same as that of the commercial Pt/C catalyst .

Regulating the N-Coordination Structure of Fe-Fe Dual Sites as the Electrocatalyst for the O2 Reduction Reaction in Metal-Air Batteries

Iron-nitrogen coordinated catalysts are regarded as efficient catalysts for the oxygen (O₂) reduction reaction (ORR), wherein the coordination environment of Fe sites is critical to the catalytic activity. Herein, we explored the effect of the nitrogen-coordination structure of dual-atomic Fe₂ sites (i.e., Fe₂-N₆-C and Fe₂-N₄-C) on the performance of the ORR. The half-wave potential (E_1/2) of Fe₂-N₆-C is 0.880 V vs RHE, outperforming that of the tetracoordinate Fe₂-N₄-C (0.851 V) and commercial Pt/C (0.850 V) in alkaline electrolytes. The Fe₂-N₆-C-based zinc-air battery delivers a maximum power density of (258.6 mW/cm²) and superior durability under 10 mA/cm². Theoretical calculations unveil that the moieties of Fe₂-N₆ profits the d-electron rearrangement of the Fe₂ sites. The electronic and geometrical structure of Fe₂-N₆ promotes the O₂ molecules adsorbed on the Fe₂ site and reduces the dissociation energy barrier of O₂, benefiting fracture of O-O bonds and acceleration of the transformation of O₂ to *OOH (the first step of the ORR process). Such exploration of modulating the local N-coordination environment of Fe₂ dimers paves an in-depth insight to design and optimize dual-atomic catalysts.

Heteroatom Coordination Regulates Iron Single-Atom-Catalyst with Superior Oxygen Reduction Reaction Performance for Aqueous Zn-Air Battery

Platinum group metal (PGM)-free M-N-C catalysts have exhibited dramatic electrocatalytic performance and are considered the most promising candidate of the Pt catalysts in oxygen reduction reaction (ORR). However, the electrocatalytic performance of the M-N-C catalysts is still limited by their inferior intrinsic activity and finite active site density. Regulating the coordination environment and increasing the pore structure of the catalyst is an effective strategy to enhance the electrocatalytic performance of the M-N-C catalysts. In this work, the coordination environment and pore structure exquisitely regulated Fe-N-C catalyst exhibit excellent ORR activity and durability. With the enhanced intrinsic activity and increased active site density, the optimized Fe-N/S-C catalyst shows impressive ORR activity (E_1/2  = 0.904 V vs reversible hydrogen electrode (RHE)) and superior long-term durability in an alkaline medium. As the advanced physical characterization and theoretical chemistry methods illustrate, the S-modified Fe-N_x (Fe-N₃ /S-C) moiety is confirmed as the improved active center for ORR, and the increased active site density further improved ORR efficiency. Based on the Fe-N/S-C cathode, a Zn-air battery is fabricated and shows superior power density (315.4 mW cm^-2 ) and long-term discharge stability at 20 mA cm^-2 . This work would open a new perspective to design atomically dispersed iron-metal site catalysts for advanced electro-catalysis.

Engineering atomic Fe–N–C with adjacent FeP nanoparticles in N,P-doped carbon for synergetic oxygen reduction and zinc–air battery

FeP-900 doped with transition metals (FeP and Fe–N–C) and heteroatoms (N,P) was prepared via pyrolysis of a conjugated microporous polymer constituted by Fe–phthalocyanine and cyclotriphosphazene, in which FeP was wrapped in N,P-rich carbon matrix.