Fe-N-Doped Carbon Capsules with Outstanding Electrochemical Performance and Stability for the Oxygen Reduction Reaction in Both Acid and Alkaline Conditions

Modulating the Iron Microenvironment for a Cooperative Interplay Between Fe-N-C Single Atoms and Fe3C Nanoclusters on the Oxygen Reduction Reaction

The coexistence of single atoms and nanoparticles is shown to increase the oxygen reduction performance in Fe-N-C electrocatalysts, but the mechanisms underlying this synergistic effect remain elusive. In this study, model Fe-N-C electrocatalysts with controlled ratios of FeN4 sites and Fe3C nanoclusters is systematically designed and synthesized. Experiments and density functional theory (DFT) computations reveal that Fe3C nanoclusters near FeN4 sites modulate the electron density of the Fe single-atom microenvironment through an electron withdrawing effect. This substantially alters the oxygen reduction reaction (ORR) mechanisms and boosts the catalytic performance of FeN4 sites. This study provides fundamental insights into the dynamic catalytic impact of single atoms and nanoparticle coexistence in advanced Fe-N-C electrocatalysts for the ORR, paving the way for further refinement through various combinations.

Fe-N catalyst derived from and supported on Lycopodium clavatum sporopollenin exine capsules for the oxygen reduction reaction

A carbon-supported electrocatalyst, featuring carbon nanotubes anchored on 3D porous graphitic carbon, was developed with the aim to perform in the operating conditions of alkaline fuel cells and metal air batteries. The catalyst was developed via two steps: first powders of Sporopollenin exine capsules used as a bio-based carbon support were activated via CO2 gasification to obtain a high specific area and porosity, second the derived porous carbons were impregnated by an iron salt and a nitrogen source, to be carbonised in Nitrogen at high temperature. The prepared catalyst demonstrated an efficient oxygen reduction reaction activity showing a half-wave potential of ~ 0.775 V vs. Reversible hydrogen electrode, comparable with that of commercial 20 wt% Pt/C in alkaline conditions, a good stability after accelerated degradation testing, retaining ~ 86% of the initial limiting current density, and a higher diffusion limited current density (6.3 vs. 5.1 mA cm- 2) than the commercial counterpart. Overall, we show the suitability of Sporopollenin exine capsule as support for electrocatalysis and a promising methodology to develop sustainable catalysts for the oxygen reduction reaction.

Polymetal-Chelated Fabrication of Bimetallic Nanophosphides as Electrocatalysts for Zinc-Air Batteries

Bimetallic phosphides are considered as promising electrocatalysts for zinc-air batteries toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). To address the semi-conductor inherent low electronic conductivity and catalytic activity, a polymetal-chelated strategy is employed to in situ fabricate bimetallic nanophosphides within carbon matrix anchoring by chemical bonding. The employment of biomolecule polydopamine (PDA) efficiently anchors various transition metal ions due to its strong chelating capability via inherent functional groups. Furthermore, the chelation of multi-metal ion is proved to promote the formation of graphitic nitrogen. The bimetallic FexCoyP phosphides nanoparticles are intimately encapsulated in carbon matrix through in situ carbonization and phosphatization processes. When utilized in Zinc-air batteries, Fe0.20Co0.80P anchored within N, P co-doped sub-microsphere (Fe0.20Co0.80P /PNC) exhibit a maximum power density of 167 mW cm-2 and cycle life up to 270 cycles, with a round-trip voltage of 0.955 V. The mechanisms for catalytic activity passivation are ascribed to the etching of nitrogen and oxidation of phosphorus in carbon matrix, as well as the oxidation of the surface phosphide on the sub-microspheres. This study presents a promising candidate for advancing the further development of energy conversation catalysis.

A novel 3D nitrogen-doped porous carbon supported Fe-Cu bimetallic nanoparticles composite derived from lignin: an efficient peroxymonosulfate activator for naphthalene degradation

A novel 3D nitrogen-doped porous carbon supported Fe-Cu bimetallic nanoparticles composite (Fe-Cu-N-PC) was prepared via direct pyrolysis by employing black liquor lignin as a main precursor, and it was utilized as a novel catalyst for PMS activation in degrading naphthalene. Under the optimum experimental conditions, the naphthalene degradation rate was up to 93.2% within 60 min in the Fe-Cu-N-PC/PMS system. The porous carbon framework of Fe-Cu-N-PC could facilitate the quick molecule diffusion of reactants towards the inner bimetallic nanoparticles and enriched naphthalene molecules from the solution by a specific adsorption, which increased the odds of contact between naphthalene and reactive oxygen species and improved the reaction efficiency. The quenching reaction proved that the non-free radical pathway dominated by 1O2 was the main way in naphthalene degradation, while the free radical pathway involving SO4·- and ·OH only played a secondary role. Moreover, owing to its high magnetization performance, Fe-Cu-N-PC could be magnetically recovered and maintained excellent naphthalene degradation rate after four degradation cycles. This research will offer a theoretical basis for the construction of facile, efficient, and green technologies to remediate persistent organic pollutants in the environment.

Elucidating the performance of hexamethylene tetra-amine interlinked bimetallic NiCo-MOF for efficient electrochemical hydrogen and oxygen evolution

Bimetallic metal-organic frameworks (MOFs) play a significant role in the electrocatalysis of water due to their large surface area and availability of increased numbers of pores. For the inaugural time, we examine the effectiveness of a hexamethylene tetra-amine (HMT)-induced 3D NiCo-MOF-based nanostructure as a potent bifunctional electrocatalyst with superior performance for overall water splitting in alkaline environments. The structural, morphological, and electrochemical properties of the as-synthesized bifunctional catalyst were examined thoroughly before analyzing its behavior towards electrochemical water splitting. The HMT-based NiCo-MOF demonstrated small overpotential values of 274 mV and 330 mV in reaching a maximum current density of 30 mA cm-2 for hydrogen and oxygen evolution mechanisms, respectively. The Tafel parameter also showed favorable HER/OER reaction kinetics, with slopes of 78 mV dec-1 and 86 mV dec-1 determined during the electrochemical evaluation. Remarkably, the NiCo-HMT electrode exhibited a double-layer capacitance of 4 mF cm-2 for hydrogen evolution and 23 mF cm-2 for oxygen evolution, while maintaining remarkable stability even after continuous operation for 20 hours. This research offers a valuable blueprint for implementing a cost-effective and durable MOF-based bifunctional catalytic system that has proven to be effective for complete water splitting. Decomposition of water under higher current densities is crucial for effective long-term generation and commercial consumption of hydrogen.

The application of plasma technology for the preparation of supercapacitor electrode materials

With the rapidly growing demand for clean energy and energy interconnection, there is an urgent need for rapid and high-capacity energy storage technologies to realize large-scale energy storage, transfer energy, and establish the energy internet. Supercapacitors, which have advantages such as high specific capacitance, fast charging and discharging rates, and long cycle lifetimes, are being widely used in electric vehicles, information technology, aerospace, and other fields. The performance of supercapacitors is crucially dependent on electrode materials. These can be categorized into electric double-layer capacitors and pseudocapacitors, primarily made from carbon and transition metal oxides, respectively. However, effectively monitoring the physicochemical properties of electrode materials during preparation and processing is challenging, which limits the improvement of supercapacitors' performance. Plasma materials preparation technology can effectively affect the materials preparation processing by energetic electrons, ions, free radicals, and multiple effects in plasma, which are easily manipulated by operation parameters. Therefore, plasma material preparation technology is considered a promising method to precisely monitor the physicochemical and electrochemical properties of energy storage materials and has been widely studied. This paper provides an overview of plasma materials preparation mechanisms, and details of the plasma technology application in the preparation of transition metal hybrids, carbon, and composite electrode materials, as well as a comparison with traditional methods. In conclusion, the advantages, challenges, and research directions of plasma materials preparation technology in the field of electrode materials preparation are summarized.

Atomically Dispersed Zn/Co-N-C as ORR Electrocatalysts for Alkaline Fuel Cells

Hydrogen fuel cells have drawn increasing attention as one of the most promising next-generation power sources for future automotive transportation. Developing efficient, durable, and low-cost electrocatalysts, to accelerate the sluggish oxygen reduction reaction (ORR) kinetics, is urgently needed to advance fuel cell technologies. Herein, we report on metal-organic frameworks-derived nonprecious dual metal single-atom catalysts (SACs) (Zn/Co-N-C), consisting of Co-N4 and Zn-N4 local structures. These catalysts exhibited superior ORR activity with a half-wave potential (E1/2) of 0.938 V versus RHE (reversible hydrogen electrode) and robust stability (ΔE1/2 = -8.5 mV) after 50k electrochemical cycles. Moreover, this remarkable performance was validated under realistic fuel cell working conditions, achieving a record-high peak power density of ∼1 W cm-2 among the reported SACs for alkaline fuel cells. Operando X-ray absorption spectroscopy was conducted to identify the active sites and reveal catalytic mechanistic insights. The results indicated that the Co atom in the Co-N4 structure was the main catalytically active center, where one axial oxygenated species binds to form an Oads-Co-N4 moiety during the ORR. In addition, theoretical studies, based on a potential-dependent microkinetic model and core-level shift calculations, showed good agreement with the experimental results and provided insights into the bonding of oxygen species on Co-N4 centers during the ORR. This work provides a comprehensive mechanistic understanding of the active sites in the Zn/Co-N-C catalysts and will pave the way for the future design and advancement of high-performance single-site electrocatalysts for fuel cells and other energy applications.

Pyridine Functionalized Carbon Nanotubes: Unveiling the Role of External Pyridinic Nitrogen Sites for Oxygen Reduction Reaction

Pyridinic nitrogen has been recognized as the primary active site in nitrogen-doped carbon electrocatalysts for the oxygen reduction reaction (ORR), which is a critical process in many renewable energy devices. However, the preparation of nitrogen-doped carbon catalysts comprised of exclusively pyridinic nitrogen remains challenging, as well as understanding the precise ORR mechanisms on the catalyst. Herein, a novel process is developed using pyridyne reactive intermediates to functionalize carbon nanotubes (CNTs) exclusively with pyridine rings for ORR electrocatalysis. The relationship between the structure and ORR performance of the prepared materials is studied in combination with density functional theory calculations to probe the ORR mechanism on the catalyst. Pyridinic nitrogen can contribute to a more efficient 4-electron reaction pathway, while high level of pyridyne functionalization result in negative structural effects, such as poor electrical conductivity, reduced surface area, and small pore diameters, that suppressed the ORR performance. This study provides insights into pyridine-doped CNTs-functionalized for the first time via pyridyne intermediates-as applied in the ORR and is expected to serve as valuable inspiration in designing high-performance electrocatalysts for energy applications.

Review on the Degradation Mechanisms of Metal-N-C Catalysts for the Oxygen Reduction Reaction in Acid Electrolyte: Current Understanding and Mitigation Approaches

One bottleneck hampering the widespread use of fuel cell vehicles, in particular of proton exchange membrane fuel cells (PEMFCs), is the high cost of the cathode where the oxygen reduction reaction (ORR) occurs, due to the current need of precious metals to catalyze this reaction. Electrochemists tackle this issue in the short/medium term by developing catalysts with improved utilization or efficiency of platinum, and in the longer term, by developing catalysts based on Earth-abundant elements. Considerable progress has been achieved in the initial performance of Metal-nitrogen-carbon (Metal-N-C) catalysts for the ORR, especially with Fe-N-C materials. However, until now, this high performance cannot be maintained for a sufficiently long time in an operating PEMFC. The identification and mitigation of the degradation mechanisms of Metal-N-C electrocatalysts in the acidic environment of PEMFCs has therefore become an important research topic. Here, we review recent advances in the understanding of the degradation mechanisms of Metal-N-C electrocatalysts, including the recently identified importance of combined oxygen and electrochemical potential. Results obtained in a liquid electrolyte and a PEMFC device are discussed, as well as insights gained from in situ and operando techniques. We also review the mitigation approaches that the scientific community has hitherto investigated to overcome the durability issues of Metal-N-C electrocatalysts.

Microwave-Assisted Synthesis of Highly Active Single-Atom Fe/N/C Catalysts for High-Performance Aqueous and Flexible All-Solid-State Zn-Air Batteries

The development of low-cost single-atom electrocatalysts for oxygen reduction reaction (ORR) is highly desired but remains a grand challenge. Superior to the conventional techniques, a microwave-assisted strategy is reported for rapid production of high-quality Fe/N/C single-atom catalysts (SACs) with profoundly enhanced reaction rate and remarkably reduced energy consumption. The as-synthesized catalysts exhibit an excellent ORR performance with a positive half-wave potential up to 0.90 V, a high turnover frequency of 0.76 s-1 , as well as a satisfied stability with a lost half-wave potential of just 27 mV over 9000 cycles (much better than that of Pt/C with 107 mV lost) and good methanol resistance. The open-circuit voltages of as-constructed aqueous and flexible all-solid-state Zn-air batteries (ZABs) are 1.56 and 1.52 V, respectively, higher than those of 20% Pt/C-based ones (i.e., 1.43 and 1.38 V, respectively). Impressively, they afford a peak power density of 235 mW cm-2 , which exceeds that of Pt/C (i.e., 186 mW cm-2 ), and is comparable to the best ones of Fe/N/C-based ZABs ever reported.

Microstructure Design Strategy for Molecularly Dispersed Cobalt Phthalocyanine and Efficient Mass Transport in CO2 Electroreduction

Cobalt phthalocyanine (CoPc) has attracted particular interest owing to its excellent activity during the electrochemical CO₂ conversion to CO. However, the efficient utilization of CoPc at industrially relevant current densities is still a challenge owing to its nonconductive property, agglomeration, and unfavorable conductive substrate design. Here, a microstructure design strategy for dispersing CoPc molecules on a carbon substrate for efficient CO₂ transport during CO₂ electrolysis is proposed and demonstrated. The highly dispersed CoPc is loaded on a macroporous hollow nanocarbon sheet to act as the catalyst (CoPc/CS). The unique interconnected and macroporous structure of the carbon sheet forms a large specific surface area to anchor CoPc with high dispersion and simultaneously boosts the mass transport of reactants in the catalyst layer, significantly improving the electrochemical performance. By employing a zero-gap flow cell, the designed catalyst can mediate CO₂ to CO with a high full-cell energy efficiency of 57% at 200 mA cm^-2 .

Fe-N co-doped coral-like hollow carbon shell toward boosting peroxymonosulfate activation for efficient degradation of tetracycline: Singlet oxygen-dominated non-radical pathway

Fe-N co-doped coral-like hollow carbon shell (Fe-N-CS) was synthesized via a simply impregnation-pyrolysis method. The Fe-N-CS showed an excellent ability for activating peroxymonosulfate (PMS), which could degrade about 93.74% tetracycline (20 mg/L) in 12 min. The Fe-N-CS/PMS system exhibited a good anti-interference capacity of various pH, inorganic anions, HA and different water qualities. More importantly, the Fe nanoparticles were anchored uniformly in the carbon layer, effectively limiting the metal leaching. The quenching tests and electron spin resonance (ESR) manifested that non-radical singlet oxygen (¹O₂) was the main reactive oxygen species (ROS) for TC degradation. The mechanism study showed that Fe nanoparticles, defect and graphite N played a key role in activating PMS to produce ROS. Moreover, three probable degradation pathways were proposed by using LC-MS measurements. Generally, this work had a new insight for the synthesis of heterogeneous Fe-N-C catalysts in the advanced oxidation process based on PMS.

A disposable enzymatic biofuel cell for glucose sensing via short-circuit current

It is essential to construct a biofuel cell-based sensor and develop an effective strategy to detect glucose without any potentiostat circuitry in order to create a simple and miniaturized device. In this report, an enzymatic biofuel cell (EBFC) is fabricated by the facile design of an anode and cathode on a screen-printed carbon electrode (SPCE). To construct the anode, thionine and flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) are covalently immobilized via a crosslinker to make a cross-linked redox network. As a cathode, the Pt-free oxygen reduction carbon catalyst is employed alternative to the commonly used bilirubin oxidase. We proposed the importance of EBFC-based sensors through the connection of anode and cathode; they can identify a short-circuit current by means of applied zero external voltage, thereby capable of glucose detection without under the operation of the potentiostat. The result shows that the EBFC-based sensor could be able to detect based on a short-circuit current with a wide range of glucose concentrations from 0.28 to 30 mM. Further, an EBFC is employed as a one-compartment model energy harvester with a maximum power density of (36 ± 3) μW cm^{- 2} in sample volume 5 μL. In addition, the constructed EBFC-based sensor demonstrates that the physiological range of ascorbic acid and uric acid shows no significant effect on the short-circuit current generation. Moreover, this EBFC can be used as a sensor in artificial plasma without losing its performance and thereby used as a disposable test strip in real blood sample analysis.

Cu/CuOx@C Composite as a High-Efficiency Electrocatalyst for Oxygen Reduction Reactions

Among clean energy transformation devices, fuel cells have gained special attention over the past years; however, advancing appropriate non-valuable metal impetuses to halfway supplant the customary Pt/C impetus is still in progress. In this paper, we propose a specific electrocatalyst in the formula of highly-active Cu species, associated with coated carbon (Cu@C-800), for oxygen reduction reaction (ORR) through post-treatment of a self-assembled precursor. The optimized catalyst Cu@C-800 showed excellent ORR performance (i.e., the onset potential was 1.00 V vs. RHE, and half-wave potential of 0.81 V vs. RHE), high stability, resistance to methanol, and high four-electron selectivity. The enhancement is attributed to the synergy between the carbon matrix and a high explicit surface region and rich Cu nano-species.

Controllable Solid-Phase Fabrication of an Fe2O3/Fe5C2/Fe-N-C Electrocatalyst toward Optimizing the Oxygen Reduction Reaction in Zinc-Air Batteries

Preparing advanced electrocatalysts via solid-phase reactions encounters the challenge of low controllability for multiconstituent hybridization and microstructure modulation. Herein, a hydrothermal-mimicking solid-phase system is established to fabricate novel Fe₂O₃/Fe₅C₂/Fe-N-C composites consisting of Fe₂O₃/Fe₅C₂ nanoparticles and Fe,N-doped carbon species with varying morphologies. The evolution mechanism featuring a competitive growth of different carbon sources in a closed hypoxic space is elucidated for a series of Fe₂O₃/Fe₅C₂/Fe-N-C composites. The size and dispersity of Fe₂O₃/Fe₅C₂ nanoparticles, the graphitization degree of the carbonaceous matrix, and their diverse hybridization states lead to disparate electrocatalytic behaviors for the oxygen reduction reaction (ORR). Among them, microspherical Fe₂O₃/Fe₅C₂/Fe-N-C-3 exhibits an optimal ORR performance and the as-assembled zinc-air battery shows all-round superiority to the Pt/C counterpart. This work presents a mild solid-phase fabrication technique for obtaining a variety of nanocomposites with effective control over composition hybridization and microstructural modulation, which is significantly important for the design and optimization of advanced electrocatalysts.

Nanofiber-Based Oxygen Reduction Electrocatalysts with Improved Mass Transfer Kinetics in a Meso-Porous Structure and Enhanced Reaction Kinetics by Confined Fe and Fe3C Particles for Anion-Exchange Membrane Fuel Cells

The development of high-performance nonprecious metal catalysts for oxygen reduction reactions is critical for the commercialization of fuel cells. In this paper, we report a non-precious catalyst with high-performance, in which Fe and Fe3C is embedded in nitrogen-doped carbon nanofibers (MIL-N-CNFs) by co-electrospinning Fe-MIL and polyacrylonitrile (PAN) and pyrolyzing. The mass ratio of Fe-MIL to PAN in the precursors and the pyrolysis temperature were optimized to be 1.5 and treated at 800 °C, respectively. The optimized catalyst exhibited an onset potential of 0.950 V and a half-wave potential of 0.830 V in alkaline electrolytes, thanks to the improved mass transfer kinetics in a meso-porous structure and enhanced reaction kinetics by confined Fe and Fe3C particles. Additionally, the optimized catalyst showed a better methanol tolerance than the commercial 20 wt.% Pt/C, indicating a potential application in direct methanol fuel cells. Serving as the cathode in CCM, the anion-exchange membrane fuel cell reaches a power density of 192 mW cm−2 at 428 mA cm−2 and 80 °C.

In-Situ Silica Xerogel Assisted Facile Synthesis of Fe-N-C Catalysts with Dense Fe-Nx Active Sites for Efficient Oxygen Reduction

In the past decade, atomically dispersed Fe active sites (coordinated with nitrogen) on carbon materials (FeNC) have emerged rapidly as promising single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) to substitute precious group metal (PGM) catalysts, owing to their earth abundance and low cost. Nonetheless, the production of highly active FeNC SACs is largely restricted by material cost, low product yield and difficulty of microstructure design. Herein, the authors demonstrate a facile in-situ xerogel (ISG) assisted synthetic strategy, using cheap materials, to construct FeNC SACs (ISG FeNC). The porous silica xerogel, formed in-situ with the FeNC precursors, encourages the emergence of enormous micropores/mesopores and homogeneous confinement/protection to the precursors during pyrolysis, benefiting to the formation of abundant accessible active sites (27.6 × 10¹⁹ sites g^-1 ). Correspondingly, the ISG FeNC exhibits excellent ORR activity with a half-wave potential (E_1/2  = 0.91 V) in alkaline medium. The Zn-air battery assembled using the ISG FeNC SACs as the bifunctional catalyst of air cathode, demonstrates commendable performance with high peak power density of 249.1 mW cm^-2 and superior long-term stability (660 cycles with 220 h). This work offers an economic and efficient way to fabricate PGM-free SACs for diverse applications.

Directional modification of oxygen functional groups by N heteroatoms on soft/hard carbons for sodium storage

Different oxygen functionalization processes occur for N-doped soft carbon (N-SC) and hard carbon (N-HC), and there may be a competitive relationship between the formation of pyridinic-N and C=O groups. For...

Enhancing ORR Activity of Fullerene-Derived Carbons by Implanting Fe in Assembled Diamine-C60 Spheres

Fullerene-derived carbons have been demonstrated as effective electrode materials for electrocatalytic reactions. The rational arrangement of heteroatoms in the carbon structure is essential to yield high catalytic activity. Herein, assembled...

Substrate strain tunes operando geometric distortion and oxygen reduction activity of CuN2C2 single-atom sites

Single-atom catalysts are becoming increasingly significant to numerous energy conversion reactions. However, their rational design and construction remain quite challenging due to the poorly understood structure–function relationship. Here we demonstrate the dynamic behavior of CuN₂C₂ site during operando oxygen reduction reaction, revealing a substrate-strain tuned geometry distortion of active sites and its correlation with the activity. Our best CuN₂C₂ site, on carbon nanotube with 8 nm diameter, delivers a sixfold activity promotion relative to graphene. Density functional theory and X-ray absorption spectroscopy reveal that reasonable substrate strain allows the optimized distortion, where Cu bonds strongly with the oxygen species while maintaining intimate coordination with C/N atoms. The optimized distortion facilitates the electron transfer from Cu to the adsorbed O, greatly boosting the oxygen reduction activity. This work uncovers the structure–function relationship of single-atom catalysts in terms of carbon substrate, and provides guidance to their future design and activity promotion.

The rational design of single-atom catalysts is challenging. This work reveals a substrate-strain tuned geometry distortion of CuN₂C₂ single-atom site, which greatly boosts oxygen reduction activity by facilitating electron transfer to adsorbed O.

Electrospun ZIF-derived cavity porous carbon nanofibers as a freestanding cathode for lithium-oxygen batteries with ultralow overpotential

Construction of an efficient electrocatalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with low overpotential and cycling stability for lithium-oxygen batteries still remains a puzzling challenge. Herein, we propose a scalable approach to integrate ZIF derivatives into cavity porous carbon nanofibers (CPCNFs) via an electrospinning technique and thermal treatment (Zn/CoNC@CPCNFs). The ultralong interconnected nanofiber matrix is beneficial, and the developed Zn/CoNC@CPCNFs catalyst with excellent flexibility can be utilized as a free-standing electrode based on an air-cathode. Moreover, this confinement strategy ensures the dispersion of Co-based species and abundant porosity structure, which contributes to the transport and adsorption of oxygen and exposes more Co-N coordination catalytic centers, as a result of a drastically ultralow voltage gap. Consequently, a cell based on a Zn/CoNC@CPCNF electrode presents remarkably decreased charge-discharge polarization (0.36 V), a high initial discharge capacity with an ultra-low overpotential of 0.59 V, and long-term cyclability with a cut-off capacity of 0.2 mA h cm^-2 at 0.02 mA cm^-2. We hope that our protocol will offer instruction for the design and application of oxygen electrocatalysts for energy conversion and storage.

Hollow nitrogen-doped carbon nanospheres as cathode catalysts to enhance oxygen reduction reaction in microbial fuel cells treating wastewater

Hollow nanospheres play a pivotal role in the electro-catalytic oxygen reduction reaction (ORR), which is a crucial step in microbial fuel cell (MFC) device. Herein, the hollow nitrogen-doped carbon nanospheres (HNCNS) were synthesized with the sacrifice of silica coated carbon nanospheres (CNS@SiO₂) as template. HNCNS remarkably enhanced the ORR activity compared to the solid carbon and solid silica spheres. By tuning calcination temperature (800-1100 °C), the surface chemistry properties of HNCNS were effectively regulated. The optimal HNCNS-1000 catalyst which was calcined at 1000 °C exhibited the highest ORR activity in neutral media with the onset potential of 0.255 V and half-wave potential of -0.006 V (vs. Ag/AgCl). Single chamber MFC (SCMFC) assembled with HNCNS-1000 cathode unveiled comparable activity to a conventional Pt/C reference. It showed the highest maximum power density of 1307 ± 26 mW/m², excellent output stability of 5.8% decline within 680 h, chemical oxygen demand (COD) removal of 94.0 ± 0.3% and coulombic efficiency (CE) of 7.9 ± 0.9%. These excellent results were attributed to a cooperative effect of the optimized surface properties (e.g., structural defects, relative content of pyrrolic nitrogen and specific surface area) and the formation of hollow nanosphere structure. Furthermore, the positive linear relationship of the structural defects and pyrrolic nitrogen species with the maximum power generation in SCMFC were clearly elucidated. This study demonstrated that the cost effective HNCNS-1000 was a promising alternative to commercial Pt/C catalyst for practical application in MFCs treating wastewater. Our result revealed the effectiveness of MFC fabricated with HNCNS-1000 cathode catalyst in terms of power generation and wastewater treatment.

Improving the Stability of Non-Noble-Metal M-N-C Catalysts for Proton-Exchange-Membrane Fuel Cells through M-N Bond Length and Coordination Regulation

An effective and universal strategy is developed to enhance the stability of the non-noble-metal M-N_x /C catalyst in proton exchange membrane fuel cells (PEMFCs) by improving the bonding strength between metal ions and chelating polymers, i.e., poly(acrylic acid) (PAA) homopolymer and poly(acrylic acid-maleic acid) (P(AA-MA)) copolymer with different AA/MA ratios. Mössbauer spectroscopy and X-ray absorption spectroscopy (XAS) reveal that the optimal P(AA-MA)-Fe-N catalyst with a higher Fe³⁺ -polymer binding constant possesses longer FeN bonds and exclusive Fe-N₄ /C moiety compared to PAA-Fe-N, which consists of ≈15% low-coordinated Fe-N₂ /N₃ structures. The optimized P(AA-MA)-Fe-N catalyst exhibits outstanding ORR activity and stability in both half-cell and PEMFC cathodes, with the retention rate of current density approaching 100% for the first 37 h at 0.55 V in an H₂ -air fuel cell. Density functional theory (DFT) calculations suggest that the Fe-N₄ /C site could optimize the difference between the adsorption energy of the Fe atoms on the support (E_ad ) and the bulk cohesive energy (E_coh ) relative to Fe-N₂ /N₃ moieties, thereby strongly stabilizing Fe centers against demetalation.

Recent Insights on Catalyst Layers for Anion Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) performance have significantly improved in the last decade (>1 W cm-2 ), and is now comparable with that of proton exchange membrane fuel cells (PEMFCs). At high current densities, issues in the catalyst layer (CL, composed of catalyst and ionomer), like oxygen transfer, water balance, and microstructural evolution, play important roles in the performance. In addition, CLs for AEMFCs have different requirements than for PEMFCs, such as chemical/physical stability, reaction mechanism, and mass transfer, because of different conductive media and pH environment. The anion exchange ionomer (AEI), which is the soluble or dispersed analogue of the anion exchange membrane (AEM), is required for hydroxide transport in the CL and is normally handled separately with the electrocatalyst during the electrode fabrication process. The importance of the AEI-catalyst interface in maximizing the utilization of electrocatalyst and fuel/oxygen transfer process must be carefully investigated. This review briefly covers new concepts in the complex AEMFC catalyst layer, before a detailed discussion on advances in CLs based on the design of AEIs and electrocatalysts. The importance of the structure-function relationship is highlighted with the aim of directing the further development of CLs for high-performance AEMFC.

Robust Protection of III-V Nanowires in Water Splitting by a Thin Compact TiO2 Layer

Narrow-band-gap III-V semiconductor nanowires (NWs) with a suitable band structure and strong light-trapping ability are ideal for high-efficiency low-cost solar water-splitting systems. However, due to their nanoscale dimension, they suffer more severe corrosion by the electrolyte solution than the thin-film counterparts. Thus, short-term durability is the major obstacle for using these NWs for practical water-splitting applications. Here, we demonstrated for the first time that a thin layer (∼7 nm thick) of compact TiO₂ deposited by atomic layer deposition can provide robust protection to III-V NWs. The protected GaAs NWs maintain 91.4% of its photoluminescence intensity after 14 months of storage in ambient atmosphere, which suggests the TiO₂ layer is pinhole-free. Working as a photocathode for water splitting, they exhibited a 45% larger photocurrent density compared with unprotected counterparts and a high Faraday efficiency of 91% and can also maintain a record-long highly stable performance among narrow-band-gap III-V NW photoelectrodes; after 67 h photoelectrochemical stability test reaction in a strong acid electrolyte solution (pH = 1), they show no apparent indication of corrosion, which is in stark contrast to the unprotected NWs that fully failed after 35 h. These findings provide an effective way to enhance both stability and performance of III-V NW-based photoelectrodes, which are highly important for practical applications in solar-energy-based water-splitting systems.

Si doped Fe-N/C catalyst for oxygen reduction reaction directed by ordered mesoporous silica nanospheres template strategy

Template-assisted synthesis strategy is an effective approach to prepare high performance oxygen reduction catalyst. The Fe-N/C catalysts were prepared via high temperature pyrolysis of the composites containing Fe-loaded mesoporous silica nanospheres and polypyrrole wrapped on it (Fe/mSiO₂@PPY). Fe loading ways combined with polymerization means of pyrrole greatly influence the structure and morphology of the final catalysts. By controlling the type of templates (mesoporous, microporous and nonporous templates) and synthesis conditions, Si doped Fe-N/C (Si-Fe-N/C) catalyst with hollow shell structures was obtained. The multiple heteroatom co-doping of Si, Fe and N in carbon framework are confirmed by EDS, XPS and Raman. The co-doping of Fe and N increases the oxygen reduction reaction (ORR) catalytic activities, while the doping of Si facilitates graphitization degree of carbon framework. The electrochemical performance of the Si-Fe-N/C catalyst was evaluated by the linear sweep voltammograms (LSV). It exhibits higher current density (5.4 mA cm^-2) and more positive half-wave potential (0.83 V vs. RHE), which is comparable to commercial Pt/C catalyst. Stability tests show that the Si-Fe-N/C catalyst possesses excellent durability and more than 90% of its original activity can be retained after 50,000 s running at 0.68 V (vs. RHE).

Chitosan-derived hydrothermally carbonized materials and its applications: A review of recent literature

Chitosan (CS) is a linear polysaccharide biopolymer, one of the most abundant biowastes in the environment. This makes chitosan a potential material for a wide range of applications. To improve CS's properties, chitosan has to be chemically modified. Hydrothermal carbonization (HTC) is a sustainable process for converting chitosan to solid carbonized material. This article presents a review on the applications of hydrothermally treated chitosan in different fields such as water treatment, heavy metals adsorption, carbon dioxide capturing, solar cells, energy storage, biosensing, supercapacitors, and catalysis. Moreover, this review covers the impact of HTC process parameters on the properties of the produced carbon material. The diversity of applications indicates the great possibilities and multifunctionality of hydrothermally carbonized chitosan and its derivatives. The utilization of HTC-CS is expected to further expand as a result of the movement toward sustainable, environmentally-friendly resources. Thus, this review also recommends a few suggestions to improve the properties of HTC chitosan and its comprehensive applications.

Enhanced electrocatalytic oxygen reduction reaction for Fe-N4-C by the incorporation of Co nanoparticles

Oxygen reduction reaction (ORR) catalytic activity can be improved by means of enhancing the synergy between transition metals. In this work, a novel porous Fe-N₄-C nanostructure containing uniformly dispersed Co nanoparticles (CoNPs) is prepared by an assisted thermal loading method. The as-prepared Co@Fe-N-C catalyst shows enhanced ORR activity with a half-wave potential (E_1/2) of 0.92 V vs. RHE, which is much higher than those of the direct pyrolysis CoNP-free sample Fe-N-C (E_1/2 = 0.85 V) and Pt/C (E_1/2 = 0.90 V) in alkaline media. It exhibits remarkable stability with only a 10 mV decrease in E_1/2 after 10 000 cycles and an outstanding long-term durability with 85% current remaining after 60 000 s. In acidic media, this catalyst exhibits catalytic activity with an E_1/2 of 0.79 V, comparable to Pt/C (E_1/2 = 0.82 V). X-ray absorption fine spectroscopy analysis revealed the presence of active centres of Fe-N₄. Density functional theory calculations confirmed the strong synergy between CoNPs and Fe-N₄ sites, providing a lower overpotential and beneficial electronic structure and a local coordination environment for the ORR. The incorporation of CoNPs on the surface of Fe-N₄-C nanomaterials plays a key role in enhancing the ORR catalytic activity and stability, providing a new route to prepare efficient Pt-free ORR catalysts.

Development of a highly active FeNC catalyst with the preferential formation of atomic iron sites for oxygen reduction in alkaline and acidic electrolytes

Nitrogen-doped porous carbons containing atomically dispersed iron are prime candidates for substituting platinum-based catalysts for oxygen reduction reaction (ORR) in fuel cells. These carbon catalysts are classically synthesizedviacomplicated routes involving multiple heat-treatment steps to form the desired Fe-N_x sites. We herein developed a highly active FeNC catalyst comprising of exclusive Fe-N_x sites by a simplified solid-state synthesis protocol involving only a single heat-treatment. Imidazole is pyrolyzed in the presence of an inorganic salt-melt resulting in highly porous carbon sheets decorated with abundant Fe-N_x centers, which yielded a high density of electrochemically accessible active sites (1.36 × 10¹⁹ sites g^-1) as determined by the in situ nitrite stripping technique. The optimized catalyst delivered a remarkable ORR activity with a half-wave potential (E_1/2) of 0.905 V_RHE in alkaline electrolyte surpassing the benchmark Pt catalyst by 55 mV. In acidic electrolyte, an E_1/2 of 0.760 V_RHE is achieved at a low loading level (0.29 mg cm^-2). In PEMFC tests, a current density of 2.3 mA cm^-2 is achieved at 0.90 V_iR-free under H₂-O₂ conditions, reflecting high kinetic activity of the optimized catalyst.

In situ synthesis of Co–B-doped porous carbon through laser thermal reduction for an efficient oxygen reduction reaction

High dispersibility Co/B–C catalysts are prepared by laser thermal reduction on boron phenolic resin, exhibiting an efficient ORR electrocatalytic activity.

Active site engineering of atomically dispersed transition metal-heteroatom-carbon catalysts for oxygen reduction

Owing to the advantage of atomic utilization, the single-atom catalyst has attracted much attention and been employed in multifarious catalytic reactions. Its definite site configuration is favorable for exploring the actual active centers and corresponding reaction mechanism. At the atomic scale, the tunable site configuration, from central metal atoms, coordinated heteroatoms, peripheral dopants, and feasible polymetallic centers to the synergetic intrinsic carbon defects, can effectively augment the intrinsic activity for oxygen reduction reaction (ORR). From a practical viewpoint, the propagation strategies of single-atom sites, the loading-activity relation and the structural retention during practical tests are crucial for the industrial applications. Furthermore, the activity contribution of multiple additional active centers including the active carbon sites and the pony-size well-wrapped metal species should be acknowledged. From the perspective mentioned above, this paper thoroughly analyses the consensuses, controversies, challenges and possible solutions based on the current research progress, thereby providing inspiration and guidance for the active center engineering of single-atom catalysts.

Novel joint catalytic properties of Fe and N co-doped graphene for CO oxidation

Using density functional theory, we have performed detailed calculations of the joint catalytic activity of graphene co-doped with Fe and N atoms. The Fe atom can be located on single vacancy graphene and acts as the active site. Due to the strong attraction of the Fe ion, the O-O bond length of the O2 molecule is elongated, which decreases the bonding energy between the O atoms. The energy barrier of CO oxidization is 0.84 eV. When N atoms are doped into the graphene, the interactions between the Fe ions and O2 molecules are stronger, and the O-O bond lengths are elongated further, which makes the desorption of the quasi-CO2 molecule easier. The energy barriers are reduced to 0.62 eV, 0.49 eV, and 0.33 eV for graphene doped with one, two and three N atoms, respectively. The O atom remaining on the Fe ion can form a CO2 molecule with an additional CO molecule. The produced CO2 molecule can be released with a small or even zero energy barrier by adsorbing an O2 molecule. The adsorbed O2 molecule is then involved in the next reaction process, and the material can be used as a recycled catalyst.

Optimization Strategies of Preparation of Biomass-Derived Carbon Electrocatalyst for Boosting Oxygen Reduction Reaction: A Minireview

Oxygen reduction reaction (ORR) has attracted considerable attention for clean energy conversion technologies to reduce traditional fossil fuel consumption and greenhouse gas emissions. Although platinum (Pt) metal is currently used as an electrocatalyst to accelerate sluggish ORR kinetics, the scarce resource and high cost still restrict its further scale-up applications. In this regard, biomass-derived carbon electrocatalysts have been widely adopted for ORR electrocatalysis in recent years owing to their tunable physical/chemical properties and cost-effective precursors. In this minireview, recent advances of the optimization strategies in biomass-derived carbon electrocatalysts towards ORR have been summarized, mainly focusing on the optimization of pore structure and active site. Besides, some current challenges and future perspectives of biomass-derived carbon as high-performance electrocatalysts for ORR have been also discussed in detail. Hopefully, this minireview will afford a guideline for better design of biomass-derived carbon electrocatalysts for ORR-related applications.

Activation of Humic Acid in Lignite Using Molybdate-Phosphorus Hierarchical Hollow Nanosphere Catalyst Oxidation: Molecular Characterization and Rice Seed Germination-Promoting Performances

Although solid-phase activation of lignite using a nanocatalyst has great potential in producing low-cost and sustainable humic acid, the large-scale application of this technology still faces challenges because of the high price and toxicity of the nanocatalyst. Additionally, the specific molecular components of humic acid in activated lignite remain unknown. In this work, a multifunctional molybdate-phosphorus hierarchical hollow nanosphere (Mo-P-HH) catalyst was successfully manufactured by a simple way followed by phosphorization. In comparison with a commercial Pd/C catalyst, the multifunctional Mo-P-HH catalyst was more effective in producing water-soluble humic acid with small molecular functional groups from lignite via solid-phase activation. Moreover, Fourier transform ion cyclotron resonance mass spectrometry revealed the molecular compositions of humic acid in activated lignite. Compared with that from raw lignite, the humic acid after Mo-P-HH activation had less aromatic structure but higher content of lipids, proteins, amino sugar, and carbohydrates. In addition, the activated humic acid simulated seed germination and seedling growth. Therefore, this study provided a high-performance hierarchical hollow nanocatalyst for activation of humic acid and also offered the theoretical basis for the application of humic acid in agriculture.

Spinach-Derived Porous Carbon Nanosheets as High-Performance Catalysts for Oxygen Reduction Reaction

Biomass-derived porous carbon materials are effective electrocatalysts for oxygen reduction reaction (ORR), with promising applications in low-temperature fuel cells and metal-air batteries. Herein, we developed a synthesis procedure that used spinach as a source of carbon, iron, and nitrogen for preparing porous carbon nanosheets and studied their ORR catalytic performance. These carbon sheets showed a very high ORR activity with a half-wave potential of +0.88 V in 0.1 M KOH, which is 20 mV more positive than that of commercial Pt/C catalysts. In addition, they showed a much better long-term stability than Pt/C and were insensitive to methanol. The remarkable ORR performance was attributed to the accessible high-density active sites that are primarily from Fe-N _x moieties. This work paves the way toward the use of metal-enriching plants as a source for preparing porous carbon materials for electrochemical energy conversion and storage applications.

Reconstruction of pH-universal atomic FeNC catalysts towards oxygen reduction reaction

Constructing of single atom catalysts that can stably exist in various energy conversion and storage devices is still in its infancy. Herein, a geometrically optimized three-dimensional hierarchically architectural single atomic FeNC catalyst with fast mass transport and electron transfer is rationally developed by post-molecule pyrolysis assisted with silicon template and reconstructs by ammonia treating. The ammonia-assisted secondary pyrolysis not only compensates for the volatilization of nitrogen species contained in organic precursors but also optimizes the surface structure of FeNC catalyst, thus increasing the content of pyridinic nitrogen and boosting the density of active sites (FeN_x) in FeNC samples. In addition, the pyridinic nitrogen adjusts the electronic distribution in Fe 3d active center and promotes the catalytic performances. Therefore, this hollow spherical atomically dispersed FeNC catalyst delivers outstanding oxygen reduction reaction (ORR) activity in pH-universal electrolyte and surpasses the most reported values.

Recent Advances in Single-Atom Electrocatalysts for Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) plays significant roles in electrochemical energy storage and conversion systems as well as clean synthesis of fine chemicals. However, the ORR process shows sluggish kinetics and requires platinum-group noble metal catalysts to accelerate the reaction. The high cost, rare reservation, and unsatisfied durability significantly impede large-scale commercialization of platinum-based catalysts. Single-atom electrocatalysts (SAECs) featuring with well-defined structure, high intrinsic activity, and maximum atom efficiency have emerged as a novel field in electrocatalytic science since it is promising to substitute expensive platinum-group noble metal catalysts. However, finely fabricating SAECs with uniform and highly dense active sites, fully maximizing the utilization efficiency of active sites, and maintaining the atomically isolated sites as single-atom centers under harsh electrocatalytic conditions remain urgent challenges. In this review, we summarized recent advances of SAECs in synthesis, characterization, oxygen reduction reaction (ORR) performance, and applications in ORR-related H₂O₂ production, metal-air batteries, and low-temperature fuel cells. Relevant progress on tailoring the coordination structure of isolated metal centers by doping other metals or ligands, enriching the concentration of single-atom sites by increasing metal loadings, and engineering the porosity and electronic structure of the support by optimizing the mass and electron transport are also reviewed. Moreover, general strategies to synthesize SAECs with high metal loadings on practical scale are highlighted, the deep learning algorithm for rational design of SAECs is introduced, and theoretical understanding of active-site structures of SAECs is discussed as well. Perspectives on future directions and remaining challenges of SAECs are presented.

Highly active Fe-N-doped porous hollow carbon nanospheres as oxygen reduction electrocatalysts in both acidic and alkaline media

Hierarchical iron-nitrogen-codoped porous hollow carbon spheres have been synthesized by using melamine-formaldehyde (MF) resin spheres as templates, nitrogen sources and pore-forming agents. FeCl3, 1,10-phenanthroline and carbon black were used as iron, nitrogen and carbon sources. The as-obtained porous hollow carbon spheres possess a high specific surface area of 807 m2 g-1, as well as exhibited excellent electrocatalytic activity for the oxygen reduction reaction (ORR) in both acidic and alkaline media. In 0.1 M HClO4 solution, the onset potential was 0.857 V (vs. RHE) and the half-wave potential was 0.715 V, which are only 78 and 80 mV less than those of the 20% Pt/C catalyst, respectively. In addition, in 0.1 M KOH solution, the onset potential was 1.017 V and the half-wave potential was 0.871 V for the ORR, which are 22 and 28 mV more positive than those of the Pt/C catalyst, respectively. Meanwhile, the catalyst also exhibited excellent methanol tolerance and long-term durability with a more effective four-electron pathway compared to the 20% Pt/C catalyst in both acidic and alkaline media. When used as an air-cathode catalyst for a Zn-air battery, the maximum power density of a Zn-air battery with the MF-C-Fe-Phen-800 cathode was 235 mW cm-2 at a high current density of 371 mA cm-2, and a high open-circuit potential of 1.654 V, superior to that of Pt/C (199 mW cm-2, 300 mA cm-2, 1.457 V). A series of designed experiments suggested that the remarkable performance was attributed to the high specific area, hollow carbon spheres, unique hierarchical micro-mesoporous structures, high contents of pyridinic-N and graphitic-N. The superiority of the as-prepared catalyst makes it promising for use in practical applications.