MOF-Derived Isolated Fe Atoms Implanted in N-Doped 3D Hierarchical Carbon as an Efficient ORR Electrocatalyst in Both Alkaline and Acidic Media

ZIFs-Derived Hollow Nanostructures via a Strong/Weak Coetching Strategy for Long-Life Rechargeable Zn-Air Batteries

Recently, zeolitic imidazolate frameworks (ZIFs) composites have emerged as promising precursors for synthesizing hollow-structured N-doped carbon-based noble-metal materials with diverse structures and compositions. Here, a strong/weak competitive coordination strategy is presented for synthesizing high-performance electrocatalysts with hollow features. During the competitive coordination process, the cubic zeolitic-imidazole framework-8 (Cube-8)@ZIF-67 with core-shell structures are transformed into Cube-8@ZIF-67@PF/POM with yolk-shell nanostructures employing phosphomolybdic acid (POM) and potassium ferricyanide (PF) as the strong chelator and the weak chelator, respectively. After calcination, the hollow Mo/Fe/Co@NC catalyst exhibits superior performance in both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Interestingly, the Mo/Fe/Co@NC catalyst exhibits efficient electrocatalytic performance for Zn-air batteries (ZABs), with a high power density (≈150 mW cm-2) and superior cycling life (≈500 h) compared to commercial platinum/carbon (Pt/C) and ruthenium dioxide (RuO2) mixture benchmarks catalysts. In addition, the density functional theory further proves that after the introduction of Mo and Fe atoms, the adsorption energy with the adsorption intermediates is weakened by adjusting the d-band center, thus weakening the reaction barrier and promoting the reaction kinetics of OER. Undoubtedly, this study presents novel insights into the fabrication of ZIFs-derived hollow structure bifunctional oxygen electrocatalysts for clean-energy diverse applications.

Enhancing activity and stability of FeNC catalysts through co incorporation for oxygen reduction reaction

FeNC single atom catalysts (SACs) have attracted great interest due to their highly active FeN4 sites. However, the pyrolysis treatment often leads to inevitable metal migration and aggregation, which reduces the catalytic activity. Moreover, due to the Fenton reaction caused by FeNC in alkaline and acidic solutions, the presence of Fe and peroxide in electrodes may generate free radicals, resulting in serious degradation of the organic ionomer and the membrane. Herein, we report an original strategy of introducing Co single atoms into FeNC catalysts, forming atomically dispersed bimetallic active sites (FeCoNC) and improving the activity and stability of the catalyst. Benefiting from this strategy, FeCoNC catalyst exhibits excellent oxygen reduction reaction (ORR) activity in alkaline media (E1/2 = 0.88 V) and in acidic media (E1/2 = 0.77 V). As the cathode of Zn-air battery (ZAB), FeCoNC shows an excellent peak power density of 142.8 mW cm-2 and a specific capacity of 806.6 mAh/gZn. This work provides a novel avenue to optimize and enhance the ORR performance of atomic dispersed FeNC catalysts.

Hierarchical Porous N-Doped Carbon Particles Derived from ZIF-8 as Highly Efficient H2S Selective Oxidation Catalysts

In the selective oxidation of H2S, the catalytic activity over N-doped carbon-based catalysts is significantly influenced by the accessibility of active sites and the mass transfer rates of reactant molecules (e.g., H2S and O2) as well as generated sulfur monomers. Therefore, it is crucial for enhancing the initial performance via the controlled synthesis of carbon-based catalysts with highly exposed active sites and unique porous structures. Herein, we reported on an efficient strategy to synthesize nanosized N-doped carbon particles with hierarchical porous structures by directly pyrolyzing an oversaturated NaCl-encapsulated ZIF-8 precursor mixture. The introduction of NaCl not only serves as a pollution-free template to promote the formation of graphitic carbon layers but also acts as an intercalating agent to guide the derivation of hierarchical porous structures, as well as enhances the amount of active nitrogen species in the catalysts. As a result, the as-prepared H-NC800 catalyst shows excellent H2S selective oxidation performance (sulfur formation rate is 794 gsulfur·kgcat-1·h-1), good stability (>80 h), and antiwater vapor properties. The characterization results and DFT calculations indicate the crucial role of pyridinic N in the adsorbing and activating reactant molecules (H2S, O2). Furthermore, nanoscale N-doped carbon particles accelerated the rapid transport of generated sulfur monomers under a hierarchical porous structure. This investigation introduces a distinctive strategy for synthesizing ZIF-8-derived N-doped carbon nanosized with a hierarchical porous structure, while its efficient and stable H2S selective oxidation performance highlights significant potential for practical implementation in the industrial desulfurization process.

Regulation of Electronic Structures of the Urchin-Like NiCoP/CoP Nanocatalysts for Fast Hydrogen Evolution

The exploration of stable, efficient, and low-cost catalysts toward ammonia borane hydrolysis is of vital significance for the practical implementation of this hydrogen production technology. Integrating interface engineering and nano-architecture engineering is a favorable strategy to elevate catalytic performance, as it can modify the electronic structure and provide sufficient active sites simultaneously. In this work, urchin-like NiCoP/CoP heterostructures are prepared via a three-step hydrothermal-oxidation-phosphorization synthesis route. It is demonstrated that the original Ni/Co molar ratio and the amount of phosphorus are crucial for adjusting the morphology, enhancing the exposed surface area, facilitating charge transfer, and modulating the adsorption and activation of H2O molecules. Consequently, the optimal Ni1Co2P heterostructure displays remarkable catalytic properties in the hydrolysis of ammonia borane with a turnover frequency (TOF) value of 30.3 molH2 ⋅ min-1 ⋅ molmetal -1, a low apparent activation energy of 25.89 kJ ⋅ mol-1, and good stability. Furthermore, by combining infrared spectroscopy and isotope kinetics experiments, a possible mechanism for the hydrolysis of ammonia borane was proposed.

Copper oxides supported sulfur-doped porous carbon material as a remarkable catalyst for reduction of aromatic nitro compounds

Synthesis and manufacturing of metal-organic framework derived carbon/metal oxide nanomaterials with an advisable porous structure and composition are essential as catalysts in various organic transformation processes for the preparation of environmentally friendly catalysts. In this work, we report a scalable synthesis of sulfur-doped porous carbon-containing copper oxide nanoparticles (marked CuxO@CS-400) via direct pyrolysis of a mixture of metal-organic framework precursor called HKUST-1 and diphenyl disulfide for aromatic nitro compounds reduction. X-ray diffraction, surface area analysis (BET), X-ray energy diffraction (EDX) spectroscopy, thermal gravimetric analysis, elemental mapping, infrared spectroscopy (FT-IR), transmission electron microscope, and scanning electron microscope (FE-SEM) analysis were accomplished to acknowledge and investigate the effect of S and CuxO as active sites in heterogeneous catalyst to perform the reduction-nitro aromatic compounds reaction in the presence of CuxO@CS-400 as an effective heterogeneous catalyst. The studies showed that doping sulfur in the resulting carbon/metal oxide substrate increased the catalytic activity compared to the material without sulfur doping.

Graphene-edge-supported iron dual-atom for oxygen reduction electrocatalysts

Pyrolyzed Fe-N-C-based catalysts, particularly FeN4, are reported to show enhanced catalytic activity for some chemical reactions, particularly for the oxygen reduction reaction (ORR). Here, we present a computational study to investigate another pyrolyzed Fe-N-C-based catalyst, i.e. Fe2N6, adsorbed on graphene with special emphasis on the edges of graphene nanoribbons (both zig-zag and armchair configurations) as a candidate for Fe dual-atom catalysts (Fe-DACs). Utilizing density functional theory calculations along with microkinetic simulations, we investigate the influence of graphitic edges on the stability and ORR activity of Fe-DAC active sites. Our findings indicate that the presence of graphitic edges, particularly the zig-zag configuration, significantly lowers the formation energy of Fe-DAC active sites, making them more likely to form at the edges. Furthermore, several Fe-DAC active sites at graphitic edges exhibit exceptional ORR performance, surpassing the commonly employed FeN4 active site in SAC systems and even exceeding the benchmark Pt(111) surface. Notably, the (Fe2N6)o@z1 active site demonstrates outstanding performance in both associative and dissociative mechanisms. These results highlight the role of graphitic nanopores in enhancing the catalytic behavior of Fe-DAC active sites, providing valuable insights for designing efficient non-precious metal catalysts for ORR applications.

Co(II)-Chelated Polyimines as Oxygen Reduction Reaction Catalysts in Anion Exchange Membrane Fuel Cells

In this paper, a cobalt (Co)-chelated polynaphthalene imine (Co-PNIM) was calcined to become an oxygen reduction reaction (ORR) electrocatalyst (Co-N-C) as the cathode catalyst (CC) of an anion exchange membrane fuel cell (AEMFC). The X-ray diffraction pattern of CoNC-1000A900 illustrated that the carbon matrix develops clear C(002) and Co(111) planes after calcination, which was confirmed using high-resolution TEM pictures. Co-N-Cs also demonstrated a significant ORR peak at 0.8 V in a C-V (current vs. voltage) curve and produced an extremely limited reduction current density (5.46 mA cm-2) comparable to commercial Pt/C catalysts (5.26 mA cm-2). The measured halfway potential of Co-N-C (0.82 V) was even higher than that of Pt/C (0.81 V). The maximum power density (Pmax) of the AEM single cell upon applying Co-N-C as the CC was 243 mW cm-2, only slightly lower than that of Pt/C (280 mW cm-2). The Tafel slope of CoNC-1000A900 (33.3 mV dec-1) was lower than that of Pt/C (43.3 mV dec-1). The limited reduction current density only decayed by 7.9% for CoNC-1000A900, compared to 22.7% for Pt/C, after 10,000 redox cycles.

Designing a Hierarchical Porous Carbon with Optimized Nitrogen Doping for Efficient Oxygen Reduction Reaction

Nitrogen-doped carbon is considered one of the most promising oxygen reduction catalysts due to its low cost and high activity, however, it still falls short of Pt/C. In this study, we report a strategy for the preparation of highly reactive N-doped hierarchical porous carbon by primary pyrolysis using zinc acetate as a stand-alone zinc source and amino-rich reactants as carbon and nitrogen sources to introduce Zn-Nx structures into mesoporous structures generated by the hard template method using the strong coordination of zinc and amino groups. Benefited from the simultaneous optimization of the hierarchical porous structure and nitrogen-doping, the half-wave potential of Zn(OAc)2 -DCD/HPC is as high as 0.909 V vs. RHE, much better than that of commercial Pt/C catalysts (0.872 V vs. RHE). In addition, zinc-air batteries assembled with Zn(OAc)2 -DCD/HPC (Pmax =198 mW cm-2 ) as the cathode exhibit higher peak power density compared to Pt/C (Pmax =168 mW cm-2 ). This strategy might open up new opportunities for designing and developing highly active metal-free catalysts.

Solvent environment engineering to synthesize FeNC nanocubes with densely Fe-Nx sites as oxygen reduction catalysts for Zn-air battery

Zeolitic imidazole framework (ZIF)-derived iron-nitrogen-carbon (FeNC) materials are expected to be high-efficiency catalysts for oxygen reduction reaction (ORR). However, increasing the density of active sites while avoiding metal accumulation still faces significant challenges. Herein, solvent environment engineering is used to synthesize the FeNC containing dense Fe-N_x moieties by adjusting the solvent during the ZIF precursor synthesis process. Compared with methanol and water/methanol, the aqueous media can provide a more moderate Fe content for the ZIF precursor, which facilitates the construction of high-density Fe-N_x sites and prevent the appearance of iron-based nanoparticles during pyrolysis. Therefore, the FeNC(C) nanocubes synthesized in an aqueous media have the highest single atom Fe loading (0.6 at%) among the prepared samples, which presents excellent oxygen reduction properties and durability under alkaline and acidic conditions. The advantage of FeNC(C) is proven in Zn-air batteries, with outstanding performance and long-term stability.

1,2,4-triazole-assisted metal-organic framework-derived nitrogen-doped carbon nanotubes with encapsulated Co4N particles as bifunctional oxygen electrocatalysts for rechargeable zinc-air batteries

The design of high-performance oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) dual-functional catalysts is not only important for the further applications of zinc-air batteries (ZABs) but also a major challenge in the field of energy conversion. The cheap 1,2,4-triazole (1,2,4-TZ) can be decomposed easily by heat, making it a high research value in carbon catalysts derived from metal-organic frameworks (MOFs). Here, Co₄N particles encapsulated at the top of N-doped carbon nanotubes (Co₄N@NCNTs) were conveniently prepared by 1,2,4-TZ-assisted pyrolysis of Co-MOF-74 for the first time. Owing to the excellent activity of Co₄N particles and the highly graphitized N-doped carbon nanotubes (NCNTs), Co₄N@NCNTs obtained at 900 °C (Co₄N@NCNT-900) exhibited astonishing catalytic performance in both ORR and OER, and high reversible oxygen bifunctional activity (ΔE = 0.685 V). Moreover, Co₄N@NCNT-900 displayed a larger discharge power density (122 mW cm^-2), a better specific capacity (811.8 mAh g^-1), and more excellent durability during the ZAB test, implying that Co₄N@NCNT-900 can act as a bifunctional high active catalyst in ZABs.

Competitive Coordination-Oriented Monodispersed Cobalt Sites on a N-Rich Porous Carbon Microsphere Catalyst for High-Performance Zn-Air Batteries

Metal/nitrogen-doped carbon single-atom catalysts (M-N-C SACs) show excellent catalytic performance with a maximum atom utilization and customizable tunable electronic structure. However, precisely modulating the M-N_x coordination in M-N-C SACs remains a grand challenge. Here, we used a N-rich nucleobase coordination self-assembly strategy to precisely regulate the dispersion of metal atoms by controlling the metal ratio. Meanwhile, the elimination of Zn during pyrolysis produced porous carbon microspheres with a specific surface area of up to 1151 m² g^-1, allowing maximum exposure of Co-N₄ sites and facilitating charge transport in the oxygen reduction reaction (ORR) process. Thereby, the monodispersed cobalt sites (Co-N₄) in N-rich (18.49 at%) porous carbon microspheres (CoSA/N-PCMS) displayed excellent ORR activity under alkaline conditions. Simultaneously, the Zn-air battery (ZAB) assembled with CoSA/N-PCMS outperformed Pt/C+RuO₂-based ZABs in terms of power density and capacity, proving that they have good prospects for practical application.

Fe3C nanoclusters integrated with Fe single-atom planted in nitrogen doped carbon derived from truncated hexahedron zeolitic imidazolate framework for the efficient transfer hydrogenation of halogenated nitrobenzenes

The control of morphology, structure and composition of metal-organic frameworks derived metal-nitrogen doped porous carbon (M-N-C) with high precision and accuracy is essential for the catalytic performance. While single-atom or small-sized nanometer catalysts show notable effects in catalysis, one catalyst combining the advantages of single-atom and nanometer catalysts may cultivate more benefits. Herein, we designed and successfully fabricated a series of Fe-doped ZIF-x with different morphologies (cube→truncated hexahedron→truncated octahedron) in one pot by simply adjusting the adding amount of vitamin C. After high-temperature calcination, Fe₃C integrated with Fe single-atom planted in N-doped carbon (Fe_SA/Fe_NC-N-C-x) with various morphology, structure and composition could be acquired. Among them, Fe_SA/Fe_NC-N-C-0.75 exhibited the best catalytic performance for the transfer hydrogenation of halogenated nitrobenzenes with N₂H₄·H₂O under room temperature. Acid-leaching tests, poisoning experiments, and the density functional theory calculations showed that Fe₃C integrated with Fe single-atom had a better catalytic effect than the separated Fe₃C or Fe single-atom.

Single-Atom Fe-N4 Sites for Catalytic Ozonation to Selectively Induce a Nonradical Pathway toward Wastewater Purification

Nonradical oxidation has been determined to be a promising pathway for the degradation of organic pollutants in heterogeneous catalytic ozonation (HCO). However, the bottlenecks are the rational design of catalysts to selectively induce nonradicals and the interpretation of detailed nonradical generation mechanisms. Herein, we propose a new HCO process based on single-atom iron catalysts, in which Fe-N₄ sites anchored on the carbon skeleton exhibited outstanding catalytic ozonation activity and stability for the degradation of oxalic acid (OA) and p-hydroxybenzoic acid (pHBA) as well as the advanced treatment of a landfill leachate secondary effluent. Unlike traditional radical oxidation, nonradical pathways based on surface-adsorbed atomic oxygen (*O_ad) and singlet oxygen (¹O₂) were identified. A substrate-dependent behavior was also observed. OA was adsorbed on the catalyst surface and mainly degraded by *O_ad, while pHBA was mostly removed by O₃ and ¹O₂ in the bulk solution. Density functional theory calculations and molecular dynamics simulations revealed that one terminal oxygen atom of ozone preferred bonding with the central iron atom of Fe-N₄, subsequently inducing the cleavage of the O-O bond near the catalyst surface to produce *O_ad and ¹O₂. These findings highlight the structural design of an ozone catalyst and an atomic-level understanding of the nonradical HCO process.

Impact of Different Lignin Sources on Nitrogen-Doped Porous Carbon toward the Electrocatalytic Oxygen Reduction Reaction

Lignin is an ideal carbon source material, and lignin-based carbon materials have been widely used in electrochemical energy storage, catalysis, and other fields. To investigate the effects of different lignin sources on the performance of electrocatalytic oxygen reduction, different lignin-based nitrogen-doped porous carbon catalysts were prepared using enzymolytic lignin (EL), alkaline lignin (AL) and dealkaline lignin (DL) as carbon sources and melamine as a nitrogen source. The surface functional groups and thermal degradation properties of the three lignin samples were characterized, and the specific surface area, pore distribution, crystal structure, defect degree, N content, and configuration of the prepared carbon-based catalysts were also analyzed. The electrocatalytic results showed that the electrocatalytic oxygen reduction performance of the three lignin-based carbon catalysts was different, and the catalytic performance of N-DLC was poor, while the electrocatalytic performance of N-ELC was similar to that of N-ALC, both of which were excellent. The half-wave potential (E_1/2) of N-ELC was 0.82 V, reaching more than 95% of the catalytic performance of commercial Pt/C (E_1/2 = 0.86 V) and proving that EL can be used as an excellent carbon-based electrocatalyst material, similar to AL.

Catalytic advancements in carbonaceous materials for bio-energy generation in microbial fuel cells: a review

Microbial fuel cells (MFCs) are a sustainable alternative for wastewater treatment and clean energy generation. The efficiency of the technology is dependent on the cathodic oxygen reduction reaction, where the sluggish reaction kinetics hampers its propensity. Carbonaceous materials with high electrical conductivity have been widely explored for oxygen reduction reaction (ORR) catalysts. Here, incorporating transition metal (TM) and heteroatom into carbon could further enhance the ORR activity and power generation in MFCs. Nitrogen (N)-doped carbons have also been a popular research hotspot due to abundant active sites formed, resulting in superior conductivity, stability, and catalytic activity over carbons. This review summarizes the progress in the carbon-based materials (primary focus on the cathode) for ORR and their utilization in MFCs. Furthermore, we discussed the conceptualization of MFCs and carbonaceous materials to instigate the ORR kinetics and power generation in MFC. Furthermore, prospects of carbon-based materials for actual application in bio-energy generation have been discussed. Carbonaceous catalysts and biomass-derived carbons exhibit good potential to replace precious Pt catalysts for ORR. M-N-C catalysts were found to be the most suitable catalysts. Electrocatalysts with MN_x sites are able to achieve excellent activity and high-power output by taking advantage of the active site exposure and rapid mass transfer rate. Moreover, the use of biomass-derived carbons/self-doped carbons could further reduce the overall cost of catalysts. It is anticipated that the research gaps and future perspectives discussed will show new avenues to develop excellent electrocatalysts for better performance and transformation of technology to industrial applications.

Synthesis of hierarchically structured Fe3C/CNTs composites in a FeNC matrix for use as efficient ORR electrocatalysts

Fe-N-C has a high number of FeN _x active sites and has thus been regarded as a high-performance oxygen reduction reaction (ORR) catalyst, and combining Fe₃C with Fe-N-C typically boosts ORR activity. However, the catalytic mechanism remains unknown, limiting further research and development. In this study, a precipitation-solvothermal process was used in conjunction with pyrolysis to produce a series of Fe-N-C catalysts derived from a zeolitic imidazolate framework (ZIF) that was composited with Fe₃C. The prepared catalysts had a multiscale structure of ZIF-like carbon particles and rod-like structures, as well as bamboo-like carbon nanotubes (CNTs) and carbon layers wrapped with Fe₃C particles while a series of studies revealed the origin of the rod-like structures and Fe₃C phase. The hierarchical structure was beneficial to the enhanced electrocatalytic performance of catalysts for ORR. The optimal sample had the highest half-wave potential of 0.878 V vs. RHE, which was higher than that of commercial Pt/C (0.861 V vs. RHE). The ECSA of the optimal sample was 1.08 cm² μg^-1, with an electron transfer number close to 4, and functioning kinetics. The optimal sample exhibited high durability and methanol tolerance for the ORR. Finally, blocking different Fe active sites with coordination ions demonstrated that Fe(ii) was the main active site, indicating that Fe₃C primarily served as a cocatalyst to optimize the electron structure of Fe-N-C, thereby synergistically improving the ORR activity.

Emerging single-atom iron catalysts for advanced catalytic systems

Due to the elusive structure-function relationship, traditional nanocatalysts always yield limited catalytic activity and selectivity, making them practically difficult to replace natural enzymes in wide industrial and biomedical applications. Accordingly, single-atom catalysts (SACs), defined as catalysts containing atomically dispersed active sites on a support material, strikingly show the highest atomic utilization and drastically boosted catalytic performances to functionally mimic or even outperform natural enzymes. The molecular characteristics of SACs (e.g., unique metal-support interactions and precisely located metal sites), especially single-atom iron catalysts (Fe-SACs) that have a similar catalytic structure to the catalytically active center of metalloprotease, enable the accurate identification of active centers in catalytic reactions, which afford ample opportunity for unraveling the structure-function relationship of Fe-SACs. In this review, we present an overview of the recent advances of support materials for anchoring an atomic dispersion of Fe. Subsequently, we highlight the structural designability of support materials as two sides of the same coin. Moreover, the applications described herein illustrate the utility of Fe-SACs in a broad scope of industrially and biologically important reactions. Finally, we present an outlook of the major challenges and opportunities remaining for the successful combination of single Fe atoms and catalysts.

MOF-Derived Bimetallic Pd-Co Alkaline ORR Electrocatalysts

The development of highly active, durable, and low-cost electrocatalysts for the oxygen reduction reaction (ORR) has been of paramount importance for advancing and commercializing fuel cell technologies. Here, we report on a novel family of Pd-Co binary alloys (Pd_xCo, x = 1-6) embedded in bimetallic organic framework (BMOF)-derived polyhedral carbon supports. BMOF-derived Pd₃Co, annealed at 300-400 °C, exhibited the most promising ORR activity among the family of materials studied, with a half-wave potential (E_1/2) of 0.977 V vs RHE and a mass activity of 0.86 mA/μg_Pd in 1 M KOH, both values being superior to those of commercial Pd/C electrocatalysts. Moreover, it maintained robust durability after 20,000 potential cycles with a minimal degradation in E_1/2 of 10 mV. The enhanced performance and stability are ascribed to the uniform elemental distribution of Pd and Co and the Co-containing N-doped carbon (Co-N-C) structures. In anion exchange membrane fuel cell (AEMFC) tests, the peak power density of the cell employing a BMOF-derived Pd₃Co cathode reached 1.1 W/cm² at an ultralow Pd loading of 0.04 mg_Pd/cm². Strategies developed herein provide promising insights into the rational design and synthesis of highly active and durable ORR electrocatalysts for alkaline fuel cells.

Exploring the structural dependence of metal-free carbon electrocatalysts on zinc-based metal-organic framework types

Metal-organic frameworks (MOFs) have been widely used as precursors to derive carbon-based electrocatalysts for the oxygen reduction reaction (ORR) due to their high porosity and tunable chemical composition/structure. However, the influence of MOF type on the structure and further ORR activity of derived metal-free carbon catalysts is still elusive. In the present work, a series of different Zn-based MOFs were employed as precursors to explore this issue. Meanwhile, prepare N-doped metal-free carbon catalysts were prepared for the ORR under the activation of sacrificial urea (which is effective to enhance the ORR activity of carbon-based catalysts). By analyzing the intermediates during pyrolysis, it is found that the decisive role of MOF types on the doped N and the morphology of derived carbon catalysts was played by the Zn coordination environment of MOFs and its reactivity with the decomposition intermediate of urea. Although the structure and porosity of derived carbon catalysts from different MOFs are very different, they all showed superior ORR activity and Zn-air battery performance up to 20 wt% Pt/C benchmark catalysts. From the above analyses, the combination of urea and compounded Zn is also a promising activation method for the preparation of highly-efficient metal-free carbon electrocatalysts.

CO2 bubble-assisted in-situ construction of mesoporous Co-doped Cu2(OH)2CO3 nanosheets as advanced electrodes towards fast and highly efficient electrochemical reduction of nitrate to N2 in wastewater

The development of high-efficient and cost-effective electrocatalysts is crucial to remove nitrate pollutant in wastewater. Herein, we design and prepare mesoporous Co-doped Cu₂(OH)₂CO₃ malachite nanosheets as an electrocatalyst toward highly efficient nitrate reduction using a facile CO₂ bubble-assisted coprecipitation synthesis. The electrocatalytic performance is subject to the Co/Cu ratio of this malachite. Remarkably, compared with the pristine monometal Cu or Co-based electrocatalyst, the optimal electrocatalyst, 0.3Co@Cu₂(OH)₂CO₃, displays fast and highly efficient removal capacity of nitrate with an impressive high total nitrogen (TN) removal of 8628.99 mg N g^-1_CoCu (398.79 mg N g_cat^-1 h^-1), N₂ selectivity of 97.11% as well as negligible nitrite product at 100 mg L^-1 NO₃^--N and 2000 mg L^-1 Cl^- neutral electrolyte. Above all, high total nitrogen removal efficiency (81.92%) and chemical oxygen demand (73.74%) in actual wastewater. Its excellent electrocatalytic performance is achieved by regulating the electronic structure and the adsorption/desorption of the intermediate. This study discovers a new type of electrode materials for nitrate removal in wastewater.

Efficient iron single-atom catalysts for selective ammoxidation of alcohols to nitriles

Zeolitic imidazolate frameworks derived Fe₁-N-C catalysts with isolated single iron atoms have been synthesized and applied for selective ammoxidation reactions. For the preparation of the different Fe-based materials, benzylamine as an additive proved to be essential to tune the morphology and size of ZIFs resulting in uniform and smaller particles, which allow stable atomically dispersed Fe-N₄ active sites. The optimal catalyst Fe₁-N-C achieves an efficient synthesis of various aryl, heterocyclic, allylic, and aliphatic nitriles from alcohols in water under very mild conditions. With its chemoselectivity, recyclability, high efficiency under mild conditions this new system complements the toolbox of catalysts for nitrile synthesis, which are important intermediates with many applications in life sciences and industry.

Fe-N-C catalyst with Fe-NX sites anchored nano carboncubes derived from Fe-Zn-MOFs activate peroxymonosulfate for high-effective degradation of ciprofloxacin: Thermal activation and catalytic mechanism

Developing high-efficient catalysts is crucial for activating peroxymonosulfate (PMS). Fe-N-C catalysts exhibit excellent performance for PMS activation because of the contribution of doped N, Fe-Nx and Fe₃C sites. In our work, a series of Fe-N-C catalysts with high-performance was obtained by pyrolyzing Fe-Zn-MOFs precursors. During pyrolysis process, the change of chemical bonds and formation of active sites in the precursor were elucidated by characterization analysis and related catalytic experiments. Graphitic N, Fe-Nx and Fe₃C were confirmed to activate PMS synergistically for ciprofloxacin (CIP) degradation. Besides, the catalytic performance was proportional to the amount of doped iron and calcination temperature. Moreover, the Fe-N-C-3-800/PMS system not only displayed good recycling performance, but also had high anti-interference ability. Integrated with quenching and electron paramagnetic resonance (EPR) experiments, a non-radical pathway dominated by ¹O₂ was proposed. Furthermore, PMS could bond to Fe-N-C-3-800 to form intermediate for charge transfer, thus accelerate electron transfer between CIP and PMS to realize degradation of CIP. Six main pathways of CIP degradation were proposed, which include bond fission of N-C on piperazine ring and direct oxidation of CIP. This study provided a new idea for the design of heterogeneous carbon catalysts in advanced oxidation field.

Tuning Active Species in N-Doped Carbon with Fe/Fe3C Nanoparticles for Efficient Oxygen Reduction Reaction

Transition metal-nitrogen-carbon (M-N-C) catalysts (M = Fe, Co, etc.) are the most promising substituents of Pt-based catalysts for oxygen reduction reaction (ORR). However, the insufficient active species in catalysts inevitably hamper their widespread applications. Herein, we report the regulation of the active species in the catalysts of multicomponent N-doped carbon with Fe/Fe₃C nanoparticles by polydopamine (PDA) coating. It is found that the PDA is conducive to increasing the pyridinic, graphitic, and total N content in the carbon matrix. Benefiting from the chelating effects, the PDA further profits the formation of Fe-Nx structures and the implantation of Fe/Fe₃C nanoparticles in the matrix during the pyrolysis. As expected, the resultant catalysts exhibit over 15 times mass activity toward ORR than nitrogen-doped carbon. Moreover, our developed catalysts show long-term stability as well as high methanol tolerance, which is superior to that of the commercial Pt/C electrode. This work provides a new avenue to explore a wider range of high-performance ORR electrocatalysts by regulating the active species.

Deep-Breathing Fe-Doped Superstructure Modified by Polyethyleneimine as Oxygen Reduction Electrocatalysts for Zn-Air Batteries

The development of economical, robust and high active non-precious metal oxygen reduction reaction (ORR) electrocatalysts to replace the precious metal is extremely crucial for the widespread applications of metal-air batteries....

V. S, Menon RS, Panda SK, Sahu AK. In-situ fabrication of cobalt sulfide decorated N, S co-doped mesoporous carbon and its application as electrocatalyst for efficient oxygen reduction reaction

Designing an efficient electrocatalyst for facile oxygen reduction reaction (ORR) is essential to achieve higher fuel cell performance. Herein, we demonstrate the simple in-situ process to synthesize cobalt sulfide decorated...

Recent Advancements in the Synthesis and Application of Carbon-Based Catalysts in the ORR

Fuel cells are a promising alternative to non-renewable energy production industries such as petroleum and natural gas. The cathodic oxygen reduction reaction (ORR), which makes fuel cell technology possible, is sluggish under normal conditions. Thus, catalysts must be used to allow fuel cells to operate efficiently. Traditionally, platinum (Pt) catalysts are often utilized as they exhibit a highly efficient ORR with low overpotential values. However, Pt is an expensive and precious metal, posing economic problems for commercialization. Herein, advances in carbon-based catalysts are reviewed for their application in ORRs due to their abundance and low-cost syntheses. Various synthetic methods from different renewable sources are presented, and their catalytic properties are compared. Likewise, the effects of heteroatom and non-precious metal doping, surface area, and porosity on their performance are investigated. Carbon-based support materials are discussed in relation to their physical properties and the subsequent effect on Pt ORR performance. Lastly, advances in fuel cell electrolytes for various fuel cell types are presented. This review aims to provide valuable insight into current challenges in fuel cell performance and how they can be overcome using carbon-based materials and next generation electrolytes.

High piezocatalytic capability in CuS/MoS2 nanocomposites using mechanical energy for degrading pollutants

Piezocatalysis, driven by mechanical energy and piezoelectric effect, is of great potential in addressing the environmental issues. In this work, a piezoelectric catalyst was fabricated by growing few-layer MoS₂ nanosheets onto CuS, for the piezocatalytic degradation of Rhodamine B (RhB), methylene blue (MB) and hexavalent chromium (Cr (VI)). The excellent removal efficiency of Cr (VI) and RhB can be reached 100% within 180 s, through the piezocatalysis of CuS/MoS₂-0.6 driven by mechanical stirring in the dark. Impressively, the piezoelectric current of CuS/MoS₂-0.6 is 48 and 35.7 times higher than that of pure CuS and MoS₂, respectively. The significantly enhanced piezocatalytic performance can be ascribed to the formation of CuS/MoS₂ heterojunction and the piezoelectric field generated by MoS₂ nanosheets, which promotes the efficient separation of electrons and holes. This study provides insights into strategies to improve catalytic performance through utilizing mechanical energy and opens a new horizon for environmental remediation.

A Ferrocene Metal-Organic Framework Solid for Fe-Loaded Carbon Matrices and Nanotubes: High-Yield Synthesis and Oxygen Reduction Electrocatalysis

Using a carbon-rich designer metal-organic framework (MOF), we open a high-yield synthetic strategy for iron-nitrogen-doped carbon (Fe-N-C) nanotube materials that emulate the electrocatalysis performance of commercial Pt/C. The Zr(IV)-based MOF solid boasts multiple key functions: (1) a dense array of alkyne units over the backbone and the side arms, which are primed for extensive graphitization; (2) the open, branched structure helps maintain porosity for absorbing nitrogen dopants; and (3) ferrocene units on the side arms as atomically dispersed precursor catalyst for targeting micropores and for effective iron encapsulation in the carbonized product. As a result, upon pyrolysis, over 89% of the carbon component in the MOF scaffold is successfully converted into carbonized products, thereby contrasting the easily volatilized carbon of most MOFs. Moreover, over 97% of the iron ends up being encased as acid-resistant Fe/Fe₃C nanoparticles in carbon nanotubes/carbon matrices.

N- and O-doped hollow carbons constructed by self- and extrinsic activation for the oxygen reduction reaction and flexible zinc-air Batteries

Zinc-air batteries (ZAB), especially those assembled on flexible substrates, have attracted great research attention in electronics and wearable electronics. However, the air-cathode reaction-oxygen reduction reaction (ORR) has limited the development of ZAB technology. In this study, a hollow carbon catalyst, NOC-1000-1, was prepared by pyrolysis of a mixture of a N-enriched Zn/bispyrozolate-based metal-organic framework and urea to replace the labile Pt-based catalysts for ORR. The employment of sacrifical urea eliminated the requirement for complicated post-treatment compared to the template method. Combined with self-activation (Zn evaporation), the obtained carbon showed a micro- and mesopore-dominant hierarchical structure coexisting with some macropores. Moreover, the doped N and O species were also tailored in a preferable configuration for ORR by simply screening the pyrolysis conditions. Under the synergistic effect of the preferable N and O configurations and pore structure, the derived carbon catalyst displayed superior ORR activity of 0.977 V onset potential and 0.867 V half-wave potential; these values are slightly better than those of the 20% Pt/C benchmark catalyst (0.985 and 0.861 V, respectively). Flexible solid-state ZABs were further assembled by employing the derived carbon catalyst as an air-cathode, and they exhibited a higher peak power density of 100.92 mW cm^-2 than a 20% Pt/C-RuO₂ battery as well as previously reported similar batteries and very high stability for up to 30 h. The flexible solid-state ZABs could drive a red light-emitting diode and run a 130-type motor for hours, which indicates their promising applications in real-world technologies.

Simultaneously Engineering the Coordination Environment and Pore Architecture of Metal-Organic Framework-Derived Single-Atomic Iron Catalysts for Ultraefficient Oxygen Reduction

Designing highly efficient and durable electrocatalysts that accelerate sluggish oxygen reduction reaction kinetics for fuel cells and metal-air batteries are highly desirable but challenging. Herein, a facile yet robust strategy is reported to rationally design single iron active centers synergized with local S atoms in metal-organic frameworks derived from hierarchically porous carbon nanorods (Fe/N,S-HC). The cooperative trithiocyanuric acid-based coating not only introduces S atoms that regulate the coordination environment of the active centers, but also facilitates the formation of a hierarchically porous structure. Benefiting from electronic modulation and architectural functionality, Fe/N,S-HC catalyst shows markedly enhanced ORR performance with a half-wave potential (E_1/2 ) of 0.912 V and satisfactory long-term durability in alkaline medium, outperforming those of commercial Pt/C. Impressively, Fe/N,S-HC-based Zn-air battery also presents outstanding battery performance and long-term stability. Both electrochemical experimental and density functional theoretical (DFT) calculated results suggest that the FeN₄ sites tailored with local S atoms are favorable for the adsorption/desorption of oxygen intermediate, resulting in lower activation energy barrier and ultraefficient oxygen reduction catalytic activity. This work provides an atomic-level combined with porous morphological-level insights into oxygen reduction catalytic property, promoting rational design and development of novel highly efficient single-atom catalysts for the renewable energy applications.

Facile Synthesis of Boron and Nitrogen Dual-Doped Hollow Mesoporous Carbons for Efficient Reduction of 4-Nitrophenol

The development of heteroatom-doped carbons with fascinating hierarchical porosity is of great significance for the improvement of catalytic properties of carbon catalysts. In this work, we report a boron and nitrogen codoped hollow mesoporous carbon (denoted as BN/HMC) via a simple synthesis route by direct pyrolysis of phenylboronic acid/melamine/ZIF-8 precursors. Thanks to their high specific surface area, unique hollow mesoporous nanoarchitecture, rich defects, and boron and nitrogen codoping, the obtained BN/HMC-0.05 can be employed as a high-efficiency carbon-based catalyst for the reduction of 4-nitrophenol. Theoretical calculations reveal that the B and N codoping in a carbon matrix are essential for the adsorption and activation of 4-nitrophenol. The present work might pave a new way in construction of metal-free carbon catalysts with both heteroatom doping and hierarchical porosity for various applications.

Hierarchically porous Fe,N-doped carbon nanorods derived from 1D Fe-doped MOFs as highly efficient oxygen reduction electrocatalysts in both alkaline and acidic media

Rationally designing low-cost yet highly efficient electrocatalysts for the oxygen reduction reaction (ORR) in both alkaline and acidic media remains highly challenging. Herein, we report the facile synthesis of Fe,N-doped carbon nanorods (denoted as Fe-N/C-NR) with abundant hierarchical pores and highly active sites by the pyrolysis of a one-dimensional (1D) Fe-doped zeolitic imidazolate framework (Fe-ZIF-8) as a self-sacrificing template. The unique 1D nanoarchitecture of the resultant Fe-N/C-NR can provide fast electron and electrolyte transport towards exposed active sites, and their hierarchically porous structures with large surface areas can efficiently facilitate mass diffusion and increase the density of exposed active sites. Furthermore, it is demonstrated that the coexistence of highly dispersed Fe-Nx sites and Fe3C/Fe nanoparticles (NPs) in these electrocatalysts can provide a large number of desired catalytic centers with highly intrinsic activity and structural stability. As a result, the optimized 5Fe-N/C-NR exhibits excellent catalytic activity for the ORR, with a high half-wave potential (E1/2) of 0.90 V vs. RHE in alkaline medium, superior to that of commercial Pt/C (0.86 V vs. RHE), and also a high E1/2 of 0.81 V vs. RHE in acidic medium, comparable to that of commercial Pt/C (0.81 V vs. RHE). Moreover, its robust ORR durability can far surpass that of commercial Pt/C in both acidic and alkaline media, further highlighting the merit of this MOF-templated strategy. Our findings might shed light on the rational design of cost-effective and highly efficient ORR electrocatalysts for practical applications.

Recent advances in single-atom catalysts for advanced oxidation processes in water purification

Single-atom catalysts (SACs) have attracted considerable attention from researchers because of their distinct structures and characteristics, especially in maximizing atomic utilization and elevating the intrinsic catalytic activity. More recently, SACs have been becoming a burgeoning area of the environmental field and are extensively applied to remove various refractory organic pollutants. This review summarizes the emerging synthetic and characterization strategies of SACs and analyzes their development tendency. Besides, the application of SACs in advanced oxidation processes (AOPs, e.g., catalysis of H₂O₂, activation of persulfates and photocatalysis) is discussed. The excellent removal of pollutants depends on the fast generation of reactive oxygen species (SO₄^•-, ^•OH, ¹O₂, and O₂^•-). The advantages of SACs in AOPs are summarized, and constructive opinions are put forward for the stability and activity of the catalyst. Finally, the opportunities and challenges faced by SACs and its future development direction in the AOPs catalytic field are proposed.

Recent Advances on MOF Derivatives for Non-Noble Metal Oxygen Electrocatalysts in Zinc-Air Batteries

Oxygen electrocatalysts are of great importance for the air electrode in zinc-air batteries (ZABs). Owing to the high specific surface area, controllable pore size and unsaturated metal active sites, metal-organic frameworks (MOFs) derivatives have been widely studied as oxygen electrocatalysts in ZABs. To date, many strategies have been developed to generate efficient oxygen electrocatalysts from MOFs for improving the performance of ZABs. In this review, the latest progress of the MOF-derived non-noble metal-oxygen electrocatalysts in ZABs is reviewed. The performance of these MOF-derived catalysts toward oxygen reduction, and oxygen evolution reactions is discussed based on the categories of metal-free carbon materials, single-atom catalysts, metal cluster/carbon composites and metal compound/carbon composites. Moreover, we provide a comprehensive overview on the design strategies of various MOF-derived non-noble metal-oxygen electrocatalysts and their structure-performance relationship. Finally, the challenges and perspectives are provided for further advancing the MOF-derived oxygen electrocatalysts in ZABs.