Cumulative effect of transition metals on nitrogen and fluorine co-doped graphite nanofibers: an efficient and highly durable non-precious metal catalyst for the oxygen reduction reaction

Deactivation Mechanism and Mitigation Strategies of Single-Atom Site Electrocatalysts

Single-atom site electrocatalysts (SACs), with maximum atom efficiency, fine-tuned coordination structure, and exceptional reactivity toward catalysis, energy, and environmental purification, have become the emerging frontier in recent decade. Along with significant breakthroughs in activity and selectivity, the limited stability and durability of SACs are often underemphasized, posing a grand challenge in meeting the practical requirements. One pivotal obstacle to the construction of highly stable SACs is the heavy reliance on empirical rather than rational design methods. A comprehensive review is urgently needed to offer a concise overview of the recent progress in SACs stability/durability, encompassing both deactivation mechanism and mitigation strategies. Herein, this review first critically summarizes the SACs degradation mechanism and induction factors at the atomic-, meso- and nanoscale, mainly based on but not limited to oxygen reduction reaction. Subsequently, potential stability/durability improvement strategies by tuning catalyst composition, structure, morphology and surface are delineated, including construction of robust substrate and metal-support interaction, optimization of active site stability, fabrication of porosity and surface modification. Finally, the challenges and prospects for robust SACs are discussed. This review facilitates the fundamental understanding of catalyst degradation mechanism and provides efficient design principles aimed at overcoming deactivation difficulties for SACs and beyond.

N, F Co-Doped Carbon Derived from Spent Bleaching Earth Waste as Oxygen Electrocatalyst Support

Affordable nitrogen and fluorine co-doped carbon nanostructure was prepared from the hazardous industrial waste of edible oil refinery, spent bleaching earth (SBE), and used as raw material for obtaining high-performance non-noble metal bifunctional oxygen electrocatalysts. Waste SBE contains 35 % residue non-saturated oil as a carbon source and the assistance of montmorillonite (MMT) as the template. This study converts waste SBE into a fluorine-doped carbon nanostructure through a pyrolysis process followed by removing the aluminosilicate layers of the MMT by HF etching. Furthermore, the impregnation of the support with Co and Fe nitrates readily gives rise to N, F co-doped carbon (NFC) electrocatalysts, as confirmed by XPS analysis. Electrochemical results evidenced that the Co-NFC catalyst proved to be a valuable bifunctional competitor for oxygen reduction reaction and oxygen evolution reaction in alkaline media, showing activity in both reactions and superior stability compared with the Fe-NFC catalyst in accelerated tests. This work offers a straightforward, economical, and eco-friendly strategy for designing N, F co-doped carbon-based electrocatalysts for oxygen reactions in electrochemical devices.

A comprehensive review of microbial fuel cells considering materials, methods, structures, and microorganisms

Microbial fuel cells (MFCs) are promising for generating renewable energy from organic matter and efficient wastewater treatment. Ensuring their practical viability requires meticulous optimization and precise design. Among the critical components of MFCs, the membrane separator plays a pivotal role in segregating the anode and cathode chambers. Recent investigations have shed light on the potential benefits of membrane-less MFCs in enhancing power generation. However, it is crucial to recognize that such configurations can adversely impact the electrocatalytic activity of anode microorganisms due to increased substrate and oxygen penetration, leading to decreased coulombic efficiency. Therefore, when selecting a membrane for MFCs, it is essential to consider key factors such as internal resistance, substrate loss, biofouling, and oxygen diffusion. Addressing these considerations carefully allows researchers to advance the performance and efficiency of MFCs, facilitating their practical application in sustainable energy production and wastewater treatment. Accelerated substrate penetration could also lead to cathode clogging and bacterial inactivation, reducing the MFC's efficiency. Overall, the design and optimization of MFCs, including the selection and use of membranes, are vital for their practical application in renewable energy generation and wastewater treatment. Further research is necessary to overcome the challenges of MFCs without a membrane and to develop improved membrane materials for MFCs. This review article aims to compile comprehensive information about all constituents of the microbial fuel cell, providing practical insights for researchers examining various variables in microbial fuel cell research.

2D MXene Nanomaterials as Electrocatalysts for Hydrogen Evolution Reaction (HER): A Review

MXenes, a novel family of 2D transition metal carbide, nitride and carbonitride materials, have been gaining tremendous interest in recent days as potential electrocatalysts for various electrochemical reactions, including hydrogen evolution reaction (HER). MXenes are characterized by their etchable metal layers, excellent structural stability, versatility for heteroatoms doping, excellent electronic conductivity, unique surface functional groups and admirable surface area, suitable for the role of electrocatalyst/support in electrochemical reactions, such as HER. In this review article, we summarized recent developments in MXene-based electrocatalysts synthesis and HER performance in terms of the theoretical and experimental point of view. We systematically evaluated the superiority of the MXene-based catalysts over traditional Pt/C catalysts in terms of HER kinetics, Tafel slope, overpotential and stability, both in acidic and alkaline electrolytic environments. We also pointed out the motives behind the electro catalytic enhancements, the effect of synthesis conditions, heteroatom doping, the effect of surface terminations on the electrocatalytic active sites of various MXenes families. At the end, various possible approaches were recommended for a deeper understanding of the active sites and catalytic improvement of MXenes catalysts for HER.

Facilitating the acidic oxygen reduction of Fe-N-C catalysts by fluorine-doping

As the alternatives to expensive Pt-based materials for the oxygen reduction reaction (ORR), iron/nitrogen co-doped carbon catalysts (FeNC) with dense FeN_x active sites are promising candidates to promote the commercialization of proton exchange membrane fuel cells. Herein, we report a synthetic approach using perfluorotetradecanoic acid (PFTA)-modified metal-organic frameworks as precursors for the synthesis of fluorine-doped FeNC (F-FeNC) with improved ORR performance. The utilization of PFTA surfactants causes profound changes of the catalyst structure including F-doping into graphitic carbon, increased micropore surface area and Brunauer-Emmett-Teller (BET) surface area (up to 1085 m² g^-1), as well as dense FeN_x sites. The F-FeNC catalyst exhibits an improved ORR activity with a high E_1/2 of 0.83 V (VS. RHE) compared to the pristine FeNC material (E_1/2 = 0.80 V). A fast decay occurs in the first 10 000 potential cycles for the F-FeNC catalyst, but high durability is still maintained up to another 50 000 cycles. Density functional theory calculations reveal that the strongly withdrawing fluorine atoms doped on the graphitic carbon can optimize the electronic structure of the FeN_x active center and decrease the adsorption energy of ORR intermediates.

Structural regulation of Co-based coordination polymers by adjusting solvent polarity toward electrocatalytic hydrogen evolution performance

A slight change of the solvent plays an important role in the synthesis process, and a small change in the crystal structure can also lead to a large difference in performance.

Progress of carbon-based electrocatalysts for flexible zinc-air batteries in the past 5 years: recent strategies for design, synthesis and performance optimization

The increasing popularity of wearable electronic devices has led to the rapid development of flexible energy conversion systems. Flexible rechargeable zinc-air batteries (ZABs) with high theoretical energy densities demonstrate significant potential as next-generation flexible energy devices that can be applied in wearable electronic products. The design of highly efficient and air-stable cathodes that can electrochemically catalyze both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly desirable but challenging. Flexible carbon-based catalysts for ORR/OER catalysis can be broadly categorized into two types: (i) self-supporting catalysts based on the in situ modification of flexible substrates; (ii) non-self-supporting catalysts based on surface coatings of flexible substrates. Methods used to optimize the catalytic performance include doping with atoms and regulation of the electronic structure and coordination environment. This review summarizes the most recently proposed strategies for the synthesis of designer carbon-based electrocatalysts and the optimization of their electrocatalytic performances in air electrodes. And we significantly focus on the analysis of the inherent active sites and their electrocatalytic mechanisms when applied as flexible ZABs catalysts. The findings of this review can assist in the design of more valuable carbon-based air electrodes and their corresponding flexible ZABs for application in wearable electronic devices.

Three-dimensional MOF-derived Co and N co-doped porous carbon bifunctional catalyst for the Zn–air battery

Considering their actual commercial large-scale applications, research on non-precious metal catalysts has garnered much interest.

Incorporation of Pt-Cr nanoparticles into highly porous MOF-5 as efficient oxygen reduction electrocatalysts

Developing new materials that can enhance the efficiency of energy conversion and storage systems is critical to meeting the rising energy demand of low-carbon economies. Mesoporous materials have the advantages of large specific surface area and multiple channels, which can increase efficiency and flexibility in terms of energy and power density. An active catalyst for oxygen reduction reaction (ORR) based on Pt-Cr nanoparticles with ultralow Pt content (0.90 wt%) has been studied in this paper. In contrast, electrocatalyst Pt/Cr/NPC-900 exhibited an ORR activity with onset potential (E _o) of 1.01 V vs. RHE in an alkaline solution that was superior to commercial Pt/C (20 wt%) (0.96 V vs. RHE). The presence of metal oxides and optimal Pt content enhanced the ORR activity. Therefore, the synergistic effect of the high surface area increased charge transfer, and excellent structural stability can achieve significant ORR efficiency, which is conducive to excellent activity. These findings provide a new perspective for economical and practical ORR electrocatalysts to be designed and synthesized rationally.

Structurally Modulated Graphitic Carbon Nanofiber and Heteroatom (N,F) Engineering toward Metal-Free ORR Electrocatalysts for Polymer Electrolyte Membrane Fuel Cells

The present study designates the heteroatom (N,F)-doped various graphitic carbon nanofibers (GNFs) viz. GNF-linear segmented platelets, antlers, herringbone type, and their structural deformations from pristine fiber with many open-edge active centers as metal-free, cost-effective electrocatalysts for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs). Introduction of heteroatoms to GNF frameworks enlarges the lattice spacing between graphene platelets and leads to structural modulation. The developed GNF/N-F catalysts show excellent ORR activity with insensitivity to CH₃OH and demonstrated outstanding electrochemical potential cycling stability of 10,000 cycles with well-retained ORR kinetics without much loss in the activity. X-ray photoelectron spectroscopy investigation of GNF/N-F catalysts explicitly shows the highly active forms of N (pyridinic, pyrrolic, and graphitic-N) and semi-ionic, ionic C-F of F in the catalysts. The deep-rooted synergistic effect among N and F atoms creates more active centers entrenched with extensive C-C bond polarization and larger charge delocalization with larger spin density differences accomplished in GNF/N-F catalysts. Wide open-edge cavities, opened tips, and many extensively accessible facets collectively enhance the ORR activity of the GNF-H/N-F catalyst. The present study provides a deep insight into the understanding of advanced metal-free electrocatalysts for efficient ORR in PEMFCs and metal-air batteries.

A review of recent progress on electrocatalysts toward efficient glycerol electrooxidation

Glycerol electrooxidation has attracted immense attention due to the economic advantage it could add to biodiesel production. One of the significant challenges for the industrial development of glycerol electrooxidation process is the search for a suitable electrocatalyst that is sustainable, cost effective, and tolerant to carbonaceous species, results in high performance, and is capable of replacing the conventional Pt/C catalyst. We review suitable, sustainable, and inexpensive alternative electrocatalysts with enhanced activity, selectivity, and durability, ensuring the economic viability of the glycerol electrooxidation process. The alternatives discussed here include Pd-based, Au-based, Ni-based, and Ag-based catalysts, as well as the combination of two or three of these metals. Also discussed here are the prospective materials that are yet to be explored for glycerol oxidation but are reported to be bifunctional (being capable of both anodic and cathodic reaction). These include heteroatom-doped metal-free electrocatalysts, which are carbon materials doped with one or two heteroatoms (N, B, S, P, F, I, Br, Cl), and heteroatom-doped nonprecious transition metals. Rational design of these materials can produce electrocatalysts with activity comparable to that of Pt/C catalysts. The takeaway from this review is that it provides an insight into further study and engineering applications on the efficient and cost-effective conversion of glycerol to value-added chemicals.

Fluorine-enriched mesoporous carbon as efficient oxygen reduction catalyst: understanding the defects in porous matrix and fuel cell applications

Herein, fluorine enrichment in mesoporous carbon (F-MC) was explored to introduce maximum charge polarization in the porous matrix, which is beneficial for the preferential orientation of O2 molecules and their subsequent reduction. Ex situ doping of F to porous carbon derived from phloroglucinol-formaldehyde resin using Pluronic F-127 as a structure-directing agent is standardized. The optimized F-MC catalyst exhibited excellent electrocatalytic activity towards the oxygen reduction reaction (ORR) in alkaline media (0.1 M KOH) with an onset potential of -0.10 V vs. SCE and diffusion-limiting current of 4.87 mA cm-2, while displaying only about 50 mV overpotential in the half-wave region compared to Pt-C (40 wt%). In the stability test, the catalyst showed only 10 mV negative shift in its half-wave potential after 10 000 potential cycles. The rotating ring disk electrode (RRDE) experiments revealed that F-MC follows the most preferable 4e - pathway (n = 3.61) with a moderate peroxide (HO2 -) yield. This was further supported by density functional theory calculations and also deeply explains the existence of defects being beneficial for the ORR. The F-MC catalyst owing to its promising ORR activity and long-term electrochemical stability can be viewed as a potential alternative ORR catalyst for anion exchange membrane fuel cell applications.

A room-temperature interfacial approach towards iron/nitrogen co-doped fibrous porous carbons as electrocatalysts for the oxygen reduction reaction and Zn-Air batteries

The development of nonprecious and efficient catalysts to boost the oxygen reduction reaction (ORR) is imperative. However, the majority of previously reported approaches suffered from a complicated fabrication procedure, both time consuming and difficult to scale up. Herein, large-scale iron ion embedded polyaniline fibers were successfully fabricated as precursors for preparing iron/nitrogen co-doped fibrous porous carbons (Fe/NPCFs) through an interfacial engineering strategy at room temperature. As ORR electrocatalysts in an alkaline medium (0.1 M KOH), Fe/NPCFs display a positive half-wave potential of 0.827 V (vs. RHE), and high limited current density (up to 5.76 mA cm-2), which are better than those of commercial Pt/C (E1/2 = 0.815 V, JL = 5.47 mA cm-2). Also, Fe/NPCFs exhibit a high ORR catalysis activity (E1/2 = 0.632 V, JL = 5.07 mA cm-2) in acidic medium (0.5 M H2SO4). When used as an air cathode in a primary Zn-air battery, high power density (158.5 mW cm-2) and specific capacity (717.8 mA h g-1) can be easily achieved, outperforming the commercial Pt/C.

Stable and Efficient Nitrogen-Containing Carbon-Based Electrocatalysts for Reactions in Energy-Conversion Systems

High activity and stability are crucial for the practical use of electrocatalysts in fuel cells, metal-air batteries, and water electrolysis, including the oxygen reduction reaction, hydrogen evolution reaction, oxygen evolution reaction, and oxidation reactions of formic acid and alcohols. Electrocatalysts based on nitrogen-containing carbon (N-C) materials show promise in catalyzing these reactions; however, there is no systematic review of strategies for the engineering of active and stable N-C-based electrocatalysts. Herein, a comprehensive comparison of recently reported N-C-based electrocatalysts regarding both electrocatalytic activity and long-term stability is presented. In the first part of this review, the relationships between the electrocatalytic reactions and selection of the element to modify the N-C-based materials are discussed. Afterwards, synthesis methods for N-C-based electrocatalysts are summarized, and strategies for the synthesis of highly stable N-C-based electrocatalysts are presented. Multiple tables containing data on crucial parameters for both electrocatalytic activity and stability are displayed in this review. Finally, constructing M-N_x moieties is proposed as the most promising engineering strategy for stable N-C-based electrocatalysts.

Nitrogen and Fluorine-Codoped Porous Carbons as Efficient Metal-Free Electrocatalysts for Oxygen Reduction Reaction in Fuel Cells

The severe dependence of oxygen reduction reaction (ORR) in fuel cells on platinum (Pt)-based catalysts greatly limits the process of their commercialization. Therefore, developing cost-reasonable non-precious-metal catalysts to replace Pt-based catalysts for ORR is an urgent task. Here, we use the composite of inexpensive polyaniline and superfine polytetrafluoroethylene powder as precursor to synthesize a metal-free N,F-codoped porous carbon catalyst (N,F-Carbon). Results indicate that the N,F-Carbon catalyst obtained at the optimized temperature 1000 °C exhibits almost the same onset (0.97 V vs RHE) and half-wave potential (0.84 V vs RHE) and better durability and higher crossover resistance in alkaline medium compared to commercial 20% Pt/C, which is attributed to the good dispersion of fluorine and nitrogen atoms in the carbon matrix, high specific surface area, and the synergistic effects of fluorine and nitrogen on the polarization of adjacent carbon atoms. This work provides a new strategy for in situ synthesis of N,F-codoped porous carbon as highly efficient metal-free electrocatalyst for ORR in fuel cells.

Gas-Flow Tailoring Fabrication of Graphene-like Co-Nx-C Nanosheet Supported Sub-10 nm PtCo Nanoalloys as Synergistic Catalyst for Air-Cathode Microbial Fuel Cells

In this work, we presented a novel, facile, and template-free strategy for fabricating graphene-like N-doped carbon as oxygen reduction catalyst in sustainable microbial fuel cells (MFCs) by using an ion-inducing and spontaneous gas-flow tailoring effect from a unique nitrogen-rich polymer gel precursor which has not been reported in materials science. Remarkably, by introduction of trace platinum- and cobalt- precursor in polymer gel, highly dispersed sub-10 nm PtCo nanoalloys can be in situ grown and anchored on graphene-like carbon. The as-prepared catalysts were investigated by a series of physical characterizations, electrochemical measurements, and microbial fuel cell tests. Interestingly, even with a low Pt content (5.13 wt %), the most active Co/N codoped carbon supported PtCo nanoalloys (Co-N-C/Pt) exhibited dramatically improved catalytic activity toward oxygen reduction reaction coupled with superior output power density (1008 ± 43 mW m^-2) in MFCs, which was 29.40% higher than the state of the art Pt/C (20 wt %). Notability, the distinct catalytic activity of Co-N-C/Pt was attributed to the highly efficient synergistic catalytic effect of Co-Nx-C and PtCo nanoalloys. Therefore, Co-N-C/Pt should be a promising oxygen reduction catalyst for application in MFCs. Further, the novel strategy for graphene-like carbon also can be widely used in many other energy conversion and storage devices.

CNTs-Modified Nb3O7F Hybrid Nanocrystal towards Faster Carrier Migration, Lower Bandgap and Higher Photocatalytic Activity

Novel semiconductor photocatalysts have been the research focus and received much attention in recent years. The key issues for novel semiconductor photocatalysts are to effectively harvest solar energy and enhance the separation efficiency of the electron-hole pairs. In this work, novel Nb₃O₇F/CNTs hybrid nanocomposites with enhanced photocatalytic activity have been successfully synthesized by a facile hydrothermal plus etching technique. The important finding is that appropriate pH values lead to the formation of Nb₃O₇F nanocrystal directly. A general strategy to introdue interaction between Nb₃O₇F and CNTs markedly enhances the photocatalytic activity of Nb₃O₇F. Comparatively, Nb₃O₇F/CNTs nanocomposites exhibit higher photodegradation efficiency and faster photodegradation rate in the solution of methylene blue (MB) under visible-light irradiation. The higher photocatalytic activity may be attributed to more exposed active sites, higher carrier migration and narrower bandgap because of good synergistic effect. The results here may inspire more engineering, new design and facile fabrication of novel photocatalysts with highly photocatalytic activity.