Metal-Nitrogen-Carbon Catalysts for Oxygen Reduction in PEM Fuel Cells: Self-Template Synthesis Approach to Enhancing Catalytic Activity and Stability

Advances in the determination of trace amounts of iron cations through electrochemical methods: A comprehensive review of principles and their limits of detection

Quantification of trace amounts of iron is of great importance in various fields. In the industrial sector, it is crucial to monitor the release of iron out of corrosion, pickling treatment, and steel manufacturing to address potential environmental and economic challenges. In biological systems, despite its indispensability, it is essential to maintain iron concentration below a specific threshold. Electrochemical (EC) methods provide significant analytical capabilities due to their simplicity, ease of use, and cost-effectiveness. This review focuses on the fundamental principles of EC methods for iron detection, including potentiometry, amperometry, coulometry, voltammetry, and electrochemical impedance spectroscopy (EIS). It further explains the process of obtaining calibration curves, and subsequently, determining the concentration of unknown ions. Additionally, technical notes are presented on selecting the initial signal value, reducing the duration of tests, excluding non-faradaic signals, and extending the linear region with the lowest detection limit. These notes are supported by key findings from relevant case studies.

Non-Noble-Metal-Based Electrocatalysts for Acidic Oxygen Evolution Reaction: Recent Progress, Challenges, and Perspectives

The oxygen evolution reaction (OER) plays a pivotal role in diverse renewable energy storage and conversion technologies, including water electrolysis, electrochemical CO2 reduction, nitrogen fixation, and metal-air batteries. Among various water electrolysis techniques, proton exchange membrane (PEM)-based water electrolysis devices offer numerous advantages, including high current densities, exceptional chemical stability, excellent proton conductivity, and high-purity H2. Nevertheless, the prohibitive cost associated with Ir/Ru-based OER electrocatalysts poses a significant barrier to the broad-scale application of PEM-based water splitting. Consequently, it is crucial to advance the development of non-noble metal OER catalysis substance with high acid-activity and stability, thereby fostering their widespread integration into PEM water electrolyzers (PEMWEs). In this review, a comprehensive analysis of the acidic OER mechanism, encompassing the adsorbate evolution mechanism (AEM), lattice oxygen mechanism (LOM) and oxide path mechanism (OPM) is offered. Subsequently, a systematic summary of recently reported noble-metal-free catalysts including transition metal-based, carbon-based and other types of catalysts is provided. Additionally, a comprehensive compilation of in situ/operando characterization techniques is provided, serving as invaluable tools for furnishing experimental evidence to comprehend the catalytic mechanism. Finally, the present challenges and future research directions concerning precious-metal-free acidic OER are comprehensively summarized and discussed in this review.

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

Fe─N─C is the most promising alternative to platinum-based catalysts to lower the cost of proton-exchange-membrane fuel cell (PEMFC). However, the deficient durability of Fe─N─C has hindered their application. Herein, a TiN-doped Fe─N─C (Fe─N─C/TiN) is elaborately synthesized via the sol-gel method for the oxygen-reduction reaction (ORR) in PEMFC. The interpenetrating network composed by Fe─N─C and TiN can simultaneously eliminate the free radical intermediates while maintaining the high ORR activity. As a result, the H2O2 yields of Fe─N─C/TiN are suppressed below 4%, ≈4 times lower than the Fe─N─C, and the half-wave potential only lost 15 mV after 30 kilo-cycle accelerated durability test (ADT). In a H2─O2 fuel cell assembled with Fe─N─C/TiN, it presents 980 mA cm-2 current density at 0.6 V, 880 mW cm-2 peak power density, and only 17 mV voltage loss at 0.80 A cm-2 after 10 kilo-cycle ADT. The experiment and calculation results prove that the TiN has a strong adsorption interaction for the free radical intermediates (such as *OH, *OOH, etc.), and the radicals are scavenged subsequently. The rational integration of Fe single-atom, TiN radical scavenger, and highly porous network adequately utilize the intrinsic advantages of composite structure, enabling a durable and active Pt-metal-free catalyst for PEMFC.

Single-Atom Mn Catalysts via Integration with Mn Sub Nano-Clusters Synergistically Enhance Oxygen Reduction Reaction

Integrating single atoms and clusters into one system represents a novel strategy for achieving the desired catalytic performance. In comparison to single-atom catalysts, catalysts combining single atoms and clusters harness the advantages of both, thus displaying greater potential. Nevertheless, constructing single-atom-cluster systems remains challenging, and the fundamental mechanism for enhancing catalytic activity remains elusive. In this study, a directly confined preparation of a 3D hollow sea urchin-like carbon structure (MnSA/MnAC-SSCNR) is developed. Mn single atoms synergistically interact with Mn clusters, optimizing and reducing energy barriers in the reaction pathway, thus enhancing reaction kinetics. Consequently, in contrast to Mn single-atom catalysts (MnSA-SSCNR), MnSA/MnAC-SSCNR exhibits significantly improved oxygen reduction activity, with a half-wave potential (E1/2) of 0.90 V in 0.1 m KOH, surpassing that of MnSA-SSCNR and Pt/C. This work demonstrates a strategy of remote synergy between heterogeneous single atoms and clusters, which not only contributes to electrocatalytic reactions but also holds potential for reactions involving more complex products.

Regulation Strategies for Fe-N-C and Co-N-C Catalysts for the Oxygen Reduction Reaction

Proton exchange membrane fuel cells (PEMFCs) and alkaline membrane fuel cells (AEMFCs) have received great attention as energy devices of the next generation. Accelerating oxygen reduction reaction (ORR) kinetics is the key to improve PEMFC and AEMFC performance. Platinum-based catalysts are the most widely used catalysts for the ORR, but their high price and low abundance limit the commercialization of fuel cells. Non-noble metal-nitrogen-carbon (M-N-C) is considered to be the most likely material class to replace Pt-based catalysts, among which Fe-N-C and Co-N-C have been widely studied due to their excellent intrinsic ORR performance and have made great progress in the past decades. With the improvement of synthesis technology and a deeper understanding of the ORR mechanism, some reported Fe-N-C and Co-N-C catalysts have shown excellent ORR activity close to that of commercial Pt/C catalysts. Inspired by the progress, regulation strategies for Fe-N-C and Co-N-C catalysts are summarized in this Review from 5 perspectives: (1) coordinated atoms, (2) environmental heteroatoms and defects, (3) dual-metal active sites, (4) metal-based particle promoters, and (5) curved carbon layers. We also make suggestions on some challenges facing Fe-N-C and Co-N-C research.

The Effect of Sulfur and Nitrogen Doping on the Oxygen Reduction Performance of Graphene/Iron Oxide Electrocatalysts Prepared by Using Microwave-Assisted Synthesis

The synthesis of novel catalysts for the oxygen reduction reaction, by means of a fast one-pot microwave-assisted procedure, is reported herein and deeply explained. In particular, the important role of doping atoms, like sulfur and nitrogen, in Fe2O3-reduced graphene oxide nanocomposites is described to address the modification of catalytic performance. The presence of dopants is confirmed by X-ray Photoelectron Spectroscopy analysis, while the integration of iron oxide nanoparticles, by means of decoration of the graphene structure, is corroborated by electron microscopy, which also confirms that there is no damage to the graphene sheets induced by the synthesis procedure. The electrochemical characterizations put in evidence the synergistic catalysis effects of dopant atoms with Fe2O3 and, in particular, the importance of sulfur introduction into the graphene lattice. Catalytic performance of as-prepared materials toward oxygen reduction shows values close to the Pt/C reference material, commonly used for fuel cell and metal-air battery applications.

Microporous Polymer-Modified Glassy Carbon Electrodes for the Electrochemical Detection of Metronidazole: Experimental and Theoretical Insights

The persistence and potential toxicity of emergent pollutants pose significant threats to biodiversity and human health, emphasizing the need for sensors capable of detecting these pollutants at extremely low concentrations before treatment. This study focuses on the development of glassy carbon electrodes (GCEs) modified by films of poly-tris(4-(4-(carbazol-9-yl)phenyl)silanol (PTPTCzSiOH), poly-4,4'-Di(carbazol-9-yl)-1,1'-biphenyl (PCBP), and poly-1,3,5-tri(carbazol-9-yl)benzene (PTCB) for the detection of metronidazole (MNZ) in aqueous media. The films were characterized using electrochemical, microscopy, and spectroscopy techniques, including scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Monomers were electropolymerized through cyclic voltammetry and chronoamperometry techniques. Computational methods at the B3LYP/def2-TZVP level were employed to investigate the structural and electrochemical properties of the monomers. The electrochemical detection of MNZ utilized the linear sweep voltammetry technique. Surface characterization through SEM and XPS confirmed the proper electrodeposition of polymer films. Notably, MPN-GCEs exhibited higher detection signals compared to bare GCEs up to 3.6 times in the case of PTPTCzSiOH-GCEs. This theoretical study provides insights into the structural, chemical, and electronic properties of the polymers. The findings suggest that polymer-modified GCEs hold promise as candidates for the development of electrochemical sensors.

Rechargeable Zinc-Air Batteries: Advances, Challenges, and Prospects

Rechargeable zinc-air batteries (Re-ZABs) are one of the most promising next-generation batteries that can hold more energy while being cost-effective and safer than existing devices. Nevertheless, zinc dendrites, non-portability, and limited charge-discharge cycles have long been obstacles to the commercialization of Re-ZABs. Over the past 30 years, milestone breakthroughs have been made in technical indicators (safety, high energy density, and long battery life), battery components (air cathode, zinc anode, and gas diffusion layer), and battery configurations (flexibility and portability), however, a comprehensive review on advanced design strategies for Re-ZABs system from multiple angles is still lacking. This review underscores the progress and strategies proposed so far to pursuit the high-efficiency Re-ZABs system, including the aspects of rechargeability (from primary to rechargeable), air cathode (from unifunctional to bifunctional), zinc anode (from dendritic to stable), electrolytes (from aqueous to non-aqueous), battery configurations (from non-portable to portable), and industrialization progress (from laboratorial to practical). Critical appraisals of the advanced modification approaches (such as surface/interface modulation, nanoconfinement catalysis, defect electrochemistry, synergistic electrocatalysis, etc.) are highlighted for cost-effective flexible Re-ZABs with good sustainability and high energy density. Finally, insights are further rendered properly for the future research directions of advanced zinc-air batteries.

Construction of highly active FeN4@Fex(OH)y cluster composite sites for the oxygen reduction reaction and the oxygen evolution reaction

Seeking cost-effective and earth-abundant electrocatalysts with excellent activity for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in zinc-air batteries (ZABs) is critically important. In this work, the ORR and OER performance of the Fex cluster supported on FeN4 composite sites (FeN4@Fex) is investigated based on density functional theory. Based on the charge density difference between the Fex cluster and the FeN4 substrate, the conclusion that the decreased charge density of the chemical bond between the metal and the adsorbate can weaken the adsorption of the adsorbate can be drawn. The results of the d-band center also confirm this. Furthermore, the ORR and OER free energy change profiles show that FeN4@Fe8 exhibits the best ORR and OER activity. This is because the electronic environment regulated by the Fex cluster can improve the adsorption of intermediates, which is conducive to enhancing catalytic activity. Further considering the solution environment, the activity of FeN4@Fex with preadsorbed OH (FeN4@Fex(OH)y) was studied. It is found that FeN4@Fe8(OH)6 is still the best catalyst. This work introduces new highly active composite sites for catalyzing the ORR in an acid medium.

Facilitating interface charge transfer via constructing NiO/NiCo2O4 heterostructure for oxygen evolution reaction under alkaline conditions

Designing high-activity electrocatalysts to enhance the slow multielectron-transfer process of the oxygen evolution reaction (OER) is of great importance for hydrogen generation. Here, we employ hydrothermal and subsequent heat-treatment strategies to acquire nanoarrays-structured NiO/NiCo₂O₄ heterojunction anchored Ni foam (NiO/NiCo₂O₄/NF) as efficient materials for catalyzing the OER in an alkaline electrolyte. Density functional theory (DFT) results demonstrate that NiO/NiCo₂O₄/NF exhibits a smaller overpotential than those of single NiO/NF and NiCo₂O₄/NF owing to interface-triggered numerous interface charge transfer. Moreover, the superior metallic characteristics of NiO/NiCo₂O₄/NF further enhance its electrochemical activity toward OER. Specifically, NiO/NiCo₂O₄/NF delivered a current density of 50 mA cm^-2 at an overpotential of 336 mV with a Tafel slope of 93.2 mV dec^-1 for the OER, which are comparable with those of commercial RuO₂ (310 mV and 68.8 mV dec^-1). Further, an overall water splitting system is preliminarily constructed via using a Pt net as cathode and NiO/NiCo₂O₄/NF as anode. The water electrolysis cell performs an operating voltage of 1.670 V at 20 mA cm^-2, which outperform the Pt net||IrO₂ couple assembled two-electrode electrolyzer (1.725 V at 20 mA cm^-2). This study proposes an efficient route to acquire multicomponent catalysts with rich interfaces for water electrolysis.

Optimizing d-Orbital Electronic Configuration via Metal-Metal Oxide Core-Shell Charge Donation for Boosting Reversible Oxygen Electrocatalysis

Tuning the d-orbital electronic configuration of active sites to achieve well-optimized adsorption strength of oxygen-containing intermediates toward reversible oxygen electrocatalysis is desirable for efficient rechargeable Zn-Air batteries but extremely challenging. Herein, this work proposes to construct a Co@Co₃ O₄ core-shell structure to regulate the d-orbital electronic configuration of Co₃ O₄ for the enhanced bifunctional oxygen electrocatalysis. Theoretical calculations first evidence that electron donation from Co core to Co₃ O₄ shell could downshift the d-band center and simultaneously weak spin state of Co₃ O₄ , result in the well-optimized adsorption strength of oxygen-containing intermediates on Co₃ O₄ , thus contributing a favor way for oxygen reduction/evolution reaction (ORR/OER) bifunctional catalysis. As a proof-of-concept, the Co@Co₃ O₄ embedded in Co, N co-doped porous carbon derived from thickness controlled 2D metal-organic-framework is designed to realize the structure of computational prediction and further improve the performance. The optimized 15Co@Co₃ O₄ /PNC catalyst exhibits the superior bifunctional oxygen electrocatalytic activity with a small potential gap of 0.69 V and a peak power density of 158.5 mW cm^-2 in ZABs. Moreover, DFT calculations shows that the more oxygen vacancies on Co₃ O₄ contribute too strong adsorption of oxygen intermediates which limit the bifunctional electrocatalysis, while electron donation in the core-shell structure can alleviate the negative effect and maintain superior bifunctional overpotential.

First-principles-driven catalyst design protocol of 2D/2D heterostructures for electro- and photocatalytic nitrogen reduction reaction

Ammonia synthesis from nitrogen is a vital process and a necessity in a variety of applications including energy, pharmaceutical, agricultural, and chemical applications. The electro- and photocatalytic nitrogen reduction reactions (NRRs) are promising sustainable processes operated under milder conditions than the conventional Haber-Bosch process. However, the main pain points of these catalytic processes are their low selectivity and low efficiency. This perspective presents the recent status and the design protocols for developing promising 2D/2D heterojunction catalysts for the NRR, using the first-principles approach. The current theoretical studies are briefly discussed, and available methods are suggested for the development and design of new potential 2D/2D heterojunctions as efficient electro- and photo-NRR catalysts.

Mn-N-C catalysts derived from metal triazole framework with hierarchical porosity for efficient oxygen reduction

Manganese and nitrogen co-doped porous carbon (Mn-N-C) are proposed as one of the most up-and-coming non-precious metal electrocatalysts to substitute Pt-based in the oxygen reduction reaction (ORR). Herein, we chose metal triazole frameworks as carbon substrate with hierarchical porosity for trapping and anchoring Mn-containing gaseous species by a mild one-step pyrolysis method. The optimized Mn-N-C electrocatalyst with a large metal content of 1.71 wt% and a volume ratio of 0.86 mesopores pore delivers a superior ORR activity with a half-wave potential (E_1/2) of 0.92 V in 0.1 M KOH and 0.78 V in 0.1 M HClO₄. Moreover, the modified Mn-N-C catalyst showed superior potential cyclic stability. TheE_1/2remained unchanged in 0.1 M KOH and only lost 6 mV in 0.1 M HClO₄after 5000 cycles. When applied as the cathode catalyst in Zn-air battery, it exhibited a maximum peak power density of 176 mW cm^-2, demonstrating great potential as a usable ORR catalyst in practical devices.

A Review of In-Situ Techniques for Probing Active Sites and Mechanisms of Electrocatalytic Oxygen Reduction Reactions

Electrocatalytic oxygen reduction reaction (ORR) is one of the most important reactions in electrochemical energy technologies such as fuel cells and metal-O2/air batteries, etc. However, the essential catalysts to overcome its slow reaction kinetic always undergo a complex dynamic evolution in the actual catalytic process, and the concomitant intermediates and catalytic products also occur continuous conversion and reconstruction. This makes them difficult to be accurately captured, making the identification of ORR active sites and the elucidation of ORR mechanisms difficult. Thus, it is necessary to use extensive in-situ characterization techniques to proceed the real-time monitoring of the catalyst structure and the evolution state of intermediates and products during ORR. This work reviews the major advances in the use of various in-situ techniques to characterize the catalytic processes of various catalysts. Specifically, the catalyst structure evolutions revealed directly by in-situ techniques are systematically summarized, such as phase, valence, electronic transfer, coordination, and spin states varies. In-situ revelation of intermediate adsorption/desorption behavior, and the real-time monitoring of the product nucleation, growth, and reconstruction evolution are equally emphasized in the discussion. Other interference factors, as well as in-situ signal assignment with the aid of theoretical calculations, are also covered. Finally, some major challenges and prospects of in-situ techniques for future catalysts research in the ORR process are proposed.

Simply prepared electrocatalyst of CoFe alloy and nitrogen-doped carbon with multi-dimensional structure and high performance for rechargeable zinc-air battery

Simple and green preparation of highly-performed electrocatalysts for reaction both at cathode (oxygen reduction reaction (ORR)) and anode (oxygen evolution reaction (OER)) is crucial for boosting the application of meta-air battery. CoFe alloy and nitrogen doped carbon (CoFe-NC) material was prepared by a one-step carbonization procedure to construct a highly efficient electrocatalysis in this work. CoFe-NC displays a three-dimensional (3D) flower-like morphology composed of ordered stacked 2D nanosheets, which is entangled by 1D carbon nanotubes (CNTs). Its structure and electrocatalytic performance are compared with that of nitrogen doped carbon materials obtained from 2D zeolitic-imidazolate frameworks (ZIF) with no metal or single metal, as well as 3D ZIF with bimetal. Benefiting from the multi-dimensional structure of bimetal nanoparticles, 1D CNTs, 2D nanosheets, and 3D flowers, as well as the abundant active sites of Co/Fe-N_xand pyridine nitrogen, CoFe-NC displays a high half-wave potential of 0.896 V for ORR and low overpotential of 370 mV at 10 mA cm^-2for OER. Furthermore, compared with the primary and rechargeable Zn-air batteries fabricated with commercial Pt/C-RuO₂catalysts, the CoFe-NC catalysts assembled Zn-air batteries show a higher specific capacity (812.2 mAh g^-1), open circuit potential (1.59 V), power density (183.4 mW cm^-2), and stability. Hence, a facile and environmental-friendly strategy is provided for rational design and synthesis of bifunctional electrocatalysts for zinc-air batteries.

One-pot production of a sea urchin-like alloy electrocatalyst for the oxygen electro-reduction reaction

Designing a cost-effective catalyst with high performance towards the oxygen electro-oxidation reaction (ORR) is of great interest for the development of green energy storage and conversion technologies. We report herein a facile self-assembly strategy in a mild reducing environment to realize an urchin-like NiPt bimetallic alloy with the domination of the (111) facets as an efficient ORR electrocatalyst. In the rotating-disk electrode test, the as-obtained NiPt nanourchins (NUCs)/C catalyst demonstrates an increase in both onset potential (0.96 V_RHE) and half-wave potential (0.92 V_RHE) and a direct four-electron ORR pathway with enhanced reaction kinetics. Additionally, the as-made NiPt NUCs/C electrocatalyst also shows impressive ORR catalytic stability compared to a commercial Pt NPs/C catalyst after an accelerated durability test with 15.29% degradation in mass activity, which is 3.04-times lower than 46.48% of the Pt NPs/C catalyst. The great ORR performance of the as-made catalyst is due to its unique urchin-like morphology with the dominant (111) facets and the synergistic and electronic effects of alloying Ni and Pt. This study not only provides a robust ORR electrocatalyst, but also opens a facile but effective route for fabricating 3D Pt-based binary and ternary alloy catalysts.

Evidence of carbon-supported porphyrins pyrolyzed for the oxygen reduction reaction keeping integrity

Fe(III) 5,10,15,20-(tetraphenyl)porphyrin chloride (FeTPP) and Co(III) 5,10,15,20-(tetraphenyl)porphyrin chloride (CoTPP) were adsorbed on carbon Vulcan and studied as electrocatalysts for the oxygen reduction reaction (ORR) before and after pyrolysis. The pyrolysis process was also simulated through ab initio molecular dynamic simulations and the minimum energy path for the O₂ dissociation after the interaction with the metal center of the FeTPP and CoTPP were calculated. After the pyrolysis the FeTPP showed the best performances reducing O₂ completely to H₂O with increased limiting current and lower overpotential. Tafel slops for the various catalysts did not change after the pyrolytic process suggesting that the mechanism for the ORR is not affected by the heat treatment. TEM images, X-ray diffraction, XPS spectroscopy, ⁵⁷Fe Mössbauer, and DFT simulations, suggest that there is no breakdown of the macrocyclic complex at elevated temperatures, and that the macro cyclic geometry is preserved. Small variations in the Metal-O₂ (M-O₂) binding energies and the M–N bond length were observed which is attributed to the dispersive interaction between the macrocycles and the irregular surface of the Vulcan substrate induced by the heat treatment and causing better interaction with the O₂ molecule. The theoretical strategy herein applied well simulate and explain the nature of the M–N–C active sites and the performances towards the ORR.

Prussian blue analogues derived electrocatalyst with multicatalytic centers for boosting oxygen reduction reaction in the wide pH range

Due to the complex of oxygen reduction reaction (ORR), designing catalysts with multicatalytic centers is considered as a promising way for boosting the ORR. Herein, a multicatalytic centers electrocatalyst Fe₃C/Mn₃O₄ encased by N-doped graphitic layers (FeMn PDA-900) is synthesized using iron manganese Prussian blue analogues and dopamine as the precursor. It exhibits a half-wave potential (E_1/2) of 0.86 V for ORR and yields of H₂O₂ lower than 5% in 0.1 M KOH. Moreover, the prepared catalyst has also shown high catalytic ORR performance in both acidic and neutral electrolyte solutions, which exhibits the potential application in both the proton exchange membrane fuel cell and the microbial electrolysis cell. It is found that the good performance can be well explained by proton-coupled electron transfer mechanism due to the multicatalytic centers from Fe-N_x, Fe₃C and Mn₃O₄ for providing enough active sites at the same time and the N-doped graphitic layers as a bridge for facilitating the electron transfer between the interfaces of Fe₃C/Mn₃O₄ nanoparticles, which paves the way for protons and electrons transfer simultaneously and rapidly, and thus lowing the energy barrier and facilitating the ORR process. Therefore, FeMn PDA-900 is a promising candidate to replace precious metal-based ORR electrocatalysts at the whole pH range.

Boosting Oxygen Reduction for High-Efficiency H2 O2 Electrosynthesis on Oxygen-Coordinated CoNC Catalysts

Atomically dispersed CoNC is a promising material for H₂ O₂ selective electrosynthesis via a two-electron oxygen reduction reaction. However, the performance of typical CoNC materials with routine CoN₄ active center is insufficient and needs to be improved further. This can be done by fine-tuning its atomic coordination configuration. Here, a single-atom electrocatalyst (Co/NC) is reported that comprises a specifically penta-coordinated CoNC configuration (OCoN₂ C₂ ) with Co center coordinated by two nitrogen atoms, two carbon atoms, and one oxygen atom. Using a combination of theoretical predictions and experiments, it is confirmed that the unique atomic structure slightly increases the charge state of the cobalt center. This optimizes the adsorption energy towards *OOH intermediate, and therefore favors the two-electron ORR relevant for H₂ O₂ electrosynthesis. In neutral solution, the as-synthesized Co/NC exhibits a selectivity of over 90% over a potential ranging from 0.36 to 0.8 V, with a turnover frequency value of 11.48 s^-1 ; thus outperforming the state-of-the-art carbon-based catalysts.

Engineering Co and Ru dual-metal atoms on nitrogen-doped carbon as highly efficient bifunctional oxygen electrocatalysts

Designing dual-metal atoms efficient bifunctional oxygen electrocatalyst by a one-step adsorption and a pyrolysis steps.

Deep learning for the automation of particle analysis in catalyst layers for polymer electrolyte fuel cells

The rapidly growing use of imaging infrastructure in the energy materials domain drives significant data accumulation in terms of their amount and complexity. The applications of routine techniques for image processing in materials research are often ad hoc, indiscriminate, and empirical, which renders the crucial task of obtaining reliable metrics for quantifications obscure. Moreover, these techniques are expensive, slow, and often involve several preprocessing steps. This paper presents a novel deep learning-based approach for the high-throughput analysis of the particle size distributions from transmission electron microscopy (TEM) images of carbon-supported catalysts for polymer electrolyte fuel cells. A dataset of 40 high-resolution TEM images at different magnification levels, from 10 to 100 nm scales, was annotated manually. This dataset was used to train the U-Net model, with the StarDist formulation for the loss function, for the nanoparticle segmentation task. StarDist reached a precision of 86%, recall of 85%, and an F1-score of 85% by training on datasets as small as thirty images. The segmentation maps outperform models reported in the literature for a similar problem, and the results on particle size analyses agree well with manual particle size measurements, albeit at a significantly lower cost.

A Rhenium Single-Atom Catalyst for the Electrocatalytic Oxygen Reduction Reaction

Single atom catalysts (SACs) have received a great deal of attention due to their extremely high active site utilization and superior activities. The exploration of metal SACs has been carried out by screening the elemental periodic table from first-row to second-row, and even third-row transition metals. However, Re SACs have not been reported, even if Re metal sites also play essential roles in catalyzing many important reactions. The construction of Re SACs may maximize Re catalytic sites and provide new Re active sites for higher activity. Herein, we used 1,10-phenanthroline to complex Re cations on carbon black, followed by heat treatment to obtain Re SAC. The Re SAC exhibited an oxygen reduction reaction (ORR) half-wave potential of 0.72 V versus reversible hydrogen electrode (RHE) in 0.1 M KOH, superior to Re nanoparticles catalyst (0.67 V vs. RHE). Re SAC exhibited better stability at 0.5 V vs. RHE than Pt/C, showing potential as a new electrocatalyst for ORR.

Active site engineering of single-atom carbonaceous electrocatalysts for the oxygen reduction reaction

The electrocatalytic oxygen reduction reaction (ORR) is the vital process at the cathode of next-generation electrochemical storage and conversion technologies, such as metal-air batteries and fuel cells. Single-metal-atom and nitrogen co-doped carbonaceous electrocatalysts (M-N-C) have emerged as attractive alternatives to noble-metal platinum for catalyzing the kinetically sluggish ORR due to their high electrical conductivity, large surface area, and structural tunability at the atomic level, however, their application is limited by the low intrinsic activity of the metal-nitrogen coordination sites (M-N x ) and inferior site density. In this Perspective, we summarize the recent progress and milestones relating to the active site engineering of single atom carbonous electrocatalysts for enhancing the ORR activity. Particular emphasis is placed on the emerging strategies for regulating the electronic structure of the single metal site and populating the site density. In addition, challenges and perspectives are provided regarding the future development of single atom carbonous electrocatalysts for the ORR and their utilization in practical use.

Fe-N4 Doped Carbon Nanotube Cathode Catalyst for PEM Fuel Cells

The earth-abundant iron and nitrogen doped carbon (Fe-N-C) catalyst has great potential to substitute noble metal catalysts for oxygen reduction reaction (ORR) in H₂-O₂ proton exchange membrane fuel cells (PEMFCs). Herein, we report the preparation of Fe-N₄ moiety doped carbon nanotubes (CNTs) by ball milling and two-step pyrolysis with dual metal-organic frameworks (MOFs) as the precursor. This catalyst shows high ORR catalytic performance and stability. Different from traditional inorganic iron sources, the MOF structure can effectively prevent the iron metal from aggregating during pyrolysis. In PEMFC, the catalyst shows high current density (0.39 A/cm² at 0.7 V) and power density (850 mW/cm²). Such a method brings inspiration for the reasonable design of FeNC catalysts with high catalytic activity for H₂-O₂ PEMFCs.

Emerging carbon shell-encapsulated metal nanocatalysts for fuel cells and water electrolysis

The development of low-cost, high-efficiency electrocatalysts is of primary importance for hydrogen energy technology. Noble metal-based catalysts have been extensively studied for decades; however, activity and durability issues still remain a challenge. In recent years, carbon shell-encapsulated metal (M@C) catalysts have drawn great attention as novel materials for water electrolysis and fuel cell applications. These electrochemical reactions are governed mainly by interfacial charge transfer between the core metal and the outer carbon shell, which alters the electronic structure of the catalyst surface. Furthermore, the rationally designed and fine-tuned carbon shell plays a very interesting role as a protective layer or molecular sieve layer to improve the performance and durability of energy conversion systems. Herein, we review recent advances in the use of M@C type nanocatalysts for extensive applications in fuel cells and water electrolysis with a focus on the structural design and electronic structure modulation of carbon shell-encapsulated metal/alloys. Finally, we highlight the current challenges and future perspectives of these catalytic materials and related technologies in this field.

Proton Exchange Membrane Fuel Cells (PEMFCs): Advances and Challenges

The study of the electrochemical catalyst conversion of renewable electricity and carbon oxides into chemical fuels attracts a great deal of attention by different researchers. The main role of this process is in mitigating the worldwide energy crisis through a closed technological carbon cycle, where chemical fuels, such as hydrogen, are stored and reconverted to electricity via electrochemical reaction processes in fuel cells. The scientific community focuses its efforts on the development of high-performance polymeric membranes together with nanomaterials with high catalytic activity and stability in order to reduce the platinum group metal applied as a cathode to build stacks of proton exchange membrane fuel cells (PEMFCs) to work at low and moderate temperatures. The design of new conductive membranes and nanoparticles (NPs) whose morphology directly affects their catalytic properties is of utmost importance. Nanoparticle morphologies, like cubes, octahedrons, icosahedrons, bipyramids, plates, and polyhedrons, among others, are widely studied for catalysis applications. The recent progress around the high catalytic activity has focused on the stabilizing agents and their potential impact on nanomaterial synthesis to induce changes in the morphology of NPs.

Metal-Nitrogen-Carbon Catalysts of Specifically Coordinated Configurations toward Typical Electrochemical Redox Reactions

Metal-nitrogen-carbon (M-N-C) material with specifically coordinated configurations is a promising alternative to costly Pt-based catalysts. In the past few years, great progress is made in the studies of M-N-C materials, including the structure modulation and local coordination environment identification via advanced synthetic strategies and characterization techniques, which boost the electrocatalytic performances and deepen the understanding of the underlying fundamentals. In this review, the most recent advances of M-N-C catalysts with specifically coordinated configurations of M-N_x (x = 1-6) are summarized as comprehensively as possible, with an emphasis on the synthetic strategy, characterization techniques, and applications in typical electrocatalytic reactions of the oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, CO₂ reduction reaction, etc., along with mechanistic exploration by experiments and theoretical calculations. Furthermore, the challenges and potential perspectives for the future development of M-N-C catalysts are discussed.

Ionic liquid-derived Fe, N, S, F multiple heteroatom-doped carbon materials for enhanced oxygen reduction reaction

Heteroatoms doped carbon catalysts have been intensively studied to take the place of Platinum based catalysts for oxygen reduction reaction (ORR) because of their ideal catalytic activity. Herein, the microporous-mesoporous carbon material catalysts doped with Fe, N, S and F were synthesized through a plain one-pot pyrolysis method with ionic liquid 1-butyl-3-methyli-midazolium bis((trifluoromethyl)sulfonyl)imide ([Bmim][TF₂N]) and melamine as precursors. Electrochemical analysis shows that the ORR activity and stability of the obtained catalysts are obviously better than Pt/C under alkaline condition. Meanwhile, the catalysts show similar ORR activity and much better durability in 0.1 M HClO₄comparing to Pt/C. Moreover, the tolerance of methanol in both basic and acid solutions is greatly better than Pt/C. The high activity is ascribed to the large specific surface area, porous structure and the synergistic effect between S, F, pyridine N, graphite N and Fe-N_x. The high stability possibly comes from the appropriate graphitization and the carbon-coating effect. The strategy proposed here has the advantages of facile, low cost, high efficiency and easy large-scale production, which provides new ideas for the preparation of high-performance Fe-N-C electrocatalysts.

Template-Free Formation of Regular Macroporosity in Carbon Materials Made from a Folded Polymer Precursor

Porous carbon materials attract great interest in a wide range of applications such as batteries, fuel cells, and membranes, due to their large surface area, structural and compositional tunability, and chemical stability. While micropores are typically obtained when preparing carbon materials by pyrolysis, the fabrication of mesoporous, and especially macroporous carbons is more challenging, yet important for enhancing mass transport. Herein, template-free regular macroporous carbons are prepared from a mixture of unfolded (linear) and folded (single-chain nanoparticles, SCNP) polyvinylpyrrolidone chains. While having the same chemical composition, the different molecular architectures lead to phase separation even before pyrolysis, creating a dense cell architecture, which is retained upon carbonization. Upon increasing the SCNP content, the homogeneity of the pore network increases and the specific surface area is enlarged 3-5-fold, until ideal properties are obtained at 75% SCNP, as observed by high-resolution scanning electron microscopy and N₂ physisorption porosimetry. The materials are further investigated as hydrazine oxidation electrocatalysts, demonstrating the link between the evolving morphology and current density. Importantly, this study demonstrates the role of polymer architecture in macroporosity templating in carbon materials, providing a new approach to develop complex carbon architectures without the need for external templating.

Proton Exchange Membrane (PEM) Fuel Cells with Platinum Group Metal (PGM)-Free Cathode

Proton exchange membrane (PEM) fuel cells have gained increasing interest from academia and industry, due to its remarkable advantages including high efficiency, high energy density, high power density, and fast refueling, also because of the urgent demand for clean and renewable energy. One of the biggest challenges for PEM fuel cell technology is the high cost, attributed to the use of precious platinum group metals (PGM), e.g., Pt, particularly at cathodes where sluggish oxygen reduction reaction takes place. Two primary ways have been paved to address this cost challenge: one named low-loading PGM-based catalysts and another one is non-precious metal-based or PGM-free catalysts. Particularly for the PGM-free catalysts, tremendous efforts have been made to improve the performance and durability-milestones have been achieved in the corresponding PEM fuel cells. Even though the current status is still far from meeting the expectations. More efforts are thus required to further research and develop the desired PGM-free catalysts for cathodes in PEM fuel cells. Herein, this paper discusses the most recent progress of PGM-free catalysts and their applications in the practical membrane electrolyte assembly and PEM fuel cells. The most promising directions for future research and development are pointed out in terms of enhancing the intrinsic activity, reducing the degradation, as well as the study at the level of fuel cell stacks.

Advanced Electrocatalysis for Energy and Environmental Sustainability via Water and Nitrogen Reactions

Clean and efficient energy storage and conversion via sustainable water and nitrogen reactions have attracted substantial attention to address the energy and environmental issues due to the overwhelming use of fossil fuels. These electrochemical reactions are crucial for desirable clean energy technologies, including advanced water electrolyzers, hydrogen fuel cells, and ammonia electrosynthesis and utilization. Their sluggish reaction kinetics lead to inefficient energy conversion. Innovative electrocatalysis, i.e., catalysis at the interface between the electrode and electrolyte to facilitate charge transfer and mass transport, plays a vital role in boosting energy conversion efficiency and providing sufficient performance and durability for these energy technologies. Herein, a comprehensive review on recent progress, achievements, and remaining challenges for these electrocatalysis processes related to water (i.e., oxygen evolution reaction, OER, and oxygen reduction reaction, ORR) and nitrogen (i.e., nitrogen reduction reaction, NRR, for ammonia synthesis and ammonia oxidation reaction, AOR, for energy utilization) is provided. Catalysts, electrolytes, and interfaces between the two within electrodes for these electrocatalysis processes are discussed. The primary emphasis is device performance of OER-related proton exchange membrane (PEM) electrolyzers, ORR-related PEM fuel cells, NRR-driven ammonia electrosynthesis from water and nitrogen, and AOR-related direct ammonia fuel cells.