Diffusion Of Reactants Research Articles

Characterization of bipolar plate-free polymer electrolyte membrane fuel cells operating in low and high relative humidity

The bipolar plate (BP), a crucial component of the polymer electrolyte membrane fuel cell (PEMFC), has been a significant concern due to its substantial influence on the volume, weight, and cost of the fuel cell electric vehicle (FCEV) stack. Consequently, extensive efforts have been dedicated to resolving this issue. In this study, the performance analysis was conducted by removing either the anode or cathode bipolar plate under varying relative humidity (RH) conditions. Under the An-Ca RH 100% - 100% environment, the peak power density was 0.653 W cm−2 for the conventional cell, 0.715 W cm−2 for the anode BP-free cell, and 1.08 W cm−2 for the cathode BP-free cell. This is due to the improvement in reactant diffusion and the influence of pressure gradients resulting from the removal of the BP. Moreover, the cell without the BP exhibited improved water retention, accentuating its advantages in harsh conditions. In the An-Ca RH 0% - 100% case, the anode BP-free cell demonstrated the highest performance at 0.565 W cm−2, while in the An-Ca RH 100% - 0% case, the cathode BP-free cell showed a performance of 0.830 W cm−2, representing over 300% higher performance compared to the conventional cell.

Partial-Interpenetration-Controlled UiO-Type Metal-Organic Framework and its Catalytic Activity.

An unprecedented correlation between the catalytic activity of a Zr-based UiO-type metal-organic framework (MOF) and its degree of interpenetration (DOI) is reported. The DOI of an MOF is hard to control owing to the high-energy penalty required to construct a partially interpenetrated structure. Surprisingly, strong interactions between building blocks (inter-ligand hydrogen bonding) facilitate the formation of partially interpenetrated structures under carefully regulated synthesis conditions. Moreover, catalytic conversion rates for cyanosilylation and Knoevenagel condensation reactions are found to be proportional to the DOI of the MOF. Among MOFs with DOIs in the 0-100% range, that with a DOI of 87% is the most catalytically active. Framework interpenetration is known to lower catalytic performance by impeding reactant diffusion. A higher effective reactant concentration due to tight inclusion in the interpenetrated region is possibly responsible for this inverted result.

Flash-back, blow-off, and symmetry breaking of premixed conical flames

Hydrodynamic and thermal-diffusive effects subject premixed flames to intrinsic instabilities that strongly influence their shape and propagation/stabilization characteristics. However, the interaction and coupling of intrinsic flame dynamics with background flow gradients and boundary conditions remain poorly understood. This paper presents a global nonlinear bifurcation analysis of burner-stabilized laminar premixed conical flames with varying reaction rates and reactant diffusivities, respectively parameterized by the Damköhler number Da and the Lewis number Le. Using a dimensionless formulation of the reacting, weakly-compressible Navier–Stokes equations, the dynamics of flash-back, blow-off, and cellular instability are explored in a fully-coupled framework. Our analysis identifies steady conical flame states over a finite range of Da and Le, limited by saddle–node bifurcations corresponding to spontaneous blow-off of the axisymmetric flame below a Le-dependent lower critical Da value and spontaneous boundary-layer flash-back beyond a higher critical Da value. Furthermore, the analysis reveals that the conical flame loses its axisymmetry via circle–pitchfork bifurcations as Le or Da decrease or increase beyond respective critical values. These bifurcations are shown to correspond to stationary three-dimensional global modes describing steady polyhedral or tilted flame structures, each associated with distinct azimuthal periodicities. Statement of SignificanceThis work presents a bifurcation analysis of laminar premixed conical flames within a fully-coupled flame/flow model. By considering variations in reaction rate and reactant diffusivity, the analysis identifies saddle–node bifurcations corresponding to spontaneous flash-back and blow-off of the axisymmetric flame. It also identifies axisymmetry breaking bifurcations associated with transitions to steady three-dimensional polyhedral and tilted flame states. These global instabilities indicate that hydrodynamic and thermal-diffusive effects strongly influence a flame’s steady structure in the azimuthal dimension even when no instabilities appear in the plane of symmetry. As such, future analyses of flame dynamics should rule out symmetry-breaking behaviors before adopting a two-dimensional computational framework.

Design and synthesis of ZnGlu MOF/SBA-16 nanocomposite, and its performance as an environmentally friendly nanocomposite for solvent-free chemical fixation of CO2 to epoxides for high-yield synthesis of cyclic carbonates

In this study, ZnGlu MOF has been immobilized into hexagonal mesoporous silica SBA-16 to fabricate ZnGlu MOF/SBA-16 hybrid material. The prepared hybrid material was introduced as a sustainable green nanocomposite for CO2 epoxidation to cyclic carbonates under solvent-free conditions at low pressure of CO2 (5 bar). The structure of the obtained nanocomposites is authenticated using various analytical techniques like FT-IR, XRD, SAXRD, CO2-TPD, NH3-TPD, BET, TEM, FE-SEM, EDX, EDX-mapping, and ICP-OES. For making a greener process, ZnGlu MOF/SBA-16 nanocomposite showed remarkable catalytic activity for converting CO2 with diverse epoxides toward five-membered cyclic carbonates prepared without using any toxic metals and reagents. The presence of the mesoporous silica SBA-16 has improved reactants diffusion and molecular accessibility for suitable interactions in Lewis acid centers, which led to MOFs outperforming in the efficient chemical fixation of CO2. Also, the essential role of free amino groups in ZnGlu MOF was shown as base sites and H-bonding groups when suggesting a possible reaction mechanism. This efficient, and convenient catalytic system can be separated by filtration, and its original activity was maintained for five successive cycles under the same reaction conditions. The above nanocomposite, with great potential, can be used to catalyze different chemical reactions for industrial purposes.

Enhanced water retention in carbon nanotube sheets-sandwiched gas diffusion layer in polymer electrolyte membrane fuel cells operated under low humidity conditions

This study introduces the polymer electrolyte membrane fuel cell (PEMFC) using sandwiched-carbon nanotube (CNT) sheets into the gas diffusion layer (GDL). The CNT sheegts are synthesized via floating catalyst chemical vapor deposition and formed into sheets using the direct spinning technique. The sandwiched-CNT sheets PEMFC indicates high electrochemical performance across a range of relative humidity conditions, remarkably in non-humidified conditions of the cathode. This notable achievement can be attributed to the distinctive physical properties of the CNT sheets. In contrast to the microporous layers, the CNT sheets possess a nanoporous structure that enhances reactant diffusion, and appropriate hydrophobicity enhances the water retention capacity in GDL. These properties are confirmed by polarization curves, electrochemical impedance spectroscopy (EIS), quantitative capillary pressure, and water removal rate test in constant current mode at 0.4 A/cm2. In the non-humidified cathode condition, the peak power density of the sandwiched-CNT sheets PEMFC is 45% higher than conventional PEMFC. Moreover, it demonstrates significantly reduced ohmic resistance in EIS. The water retention capability of the PEMFC is assessed through quantitative capillary pressure and water removal rate test. Based on these assessments, we validate that the sandwiched-CNT sheets PEMFC exhibits high performance even under non-humidified cathode conditions.

Investigation of Harmonic Response in Non-Premixed Swirling Combustion to Low-Frequency Acoustic Excitations

The propagation mechanism of flow disturbance under acoustic excitations plays a crucial role in thermoacoustic instability, especially when considering the effect of non-premixed combustion on heat release due to reactant mixing and diffusion. This relationship leads to a complex coupling between the spatial distribution of the equivalence ratio and the propagation mechanism of flow disturbance. In the present study, the response of a methane-air non-premixed swirling flame to low-frequency acoustic excitations was investigated experimentally. By applying Proper Orthogonal Decomposition (POD) analysis to CH* chemiluminescence images, the harmonic flame response was revealed. Large Eddy Simulation (LES) was utilized to analyze the correlation between the vortex motion within the shear layers and the harmonic response under non-reacting conditions at excitation frequencies of 20 Hz, 50 Hz, and 150 Hz. The results showed that the harmonic flame response was mainly due to the harmonic velocity pulsations within the shear layers. The acoustically induced vortices within the shear layer exhibited motion patterns susceptible to harmonic interference, with spatial distribution characteristics closely related to the oscillation modes of the non-premixed combustion.

Oxidation of NiCr and NiCrMo Alloys at Low Temperatures

Oxidation of Ni-Cr and Ni-Cr-Mo was studied in operando with near ambient pressure x-ray photoelectron spectroscopy in the Cabrera-Mott regime. The oxidation temperature was 200°C—a severely diffusion-limited regime. The near-surface alloy is Cr-enriched after the reduction of native oxide in vacuum, and especially so for Ni-15Cr-6Mo. Mo-cations are integrated into the oxide and Mo(VI) dominates at the surface. The surface chemistry-driven promotion of chromia by Mo predicted by theory is negated by the limited surface diffusion of reactants. Preoxidation processing is proposed to control the oxide properties for the use of Ni-Cr superalloys at low temperatures.

Nitrogen-doped hollow mesoporous carbon spheres for efficient oxygen reduction

Extensive investigations have been devoted to nitrogen-doped carbon materials as catalysts for the oxygen reduction reaction (ORR) in various conversion technologies. In this study, we introduce nitrogen-doped carbon materials with hollow spherical structures. These materials demonstrate significant potential in ORR activity within alkaline media, showing a half-wave potential of 0.87 V versus the reversible hydrogen electrode (RHE). Nitrogen-doped hollow carbon spheres (N-CHS) exhibit unique characteristics such as a thin carbon shell layer, hollow structure, large surface area, and distinct pore features. These features collectively create an optimal environment for facilitating the diffusion of reactants, thereby enhancing the exposure of active sites and improving catalytic performance. Building upon the promising qualities of N-CHS as a catalyst support, we employ heme chloride (1 wt%) as the source of iron for Fe doping. Through the carbonization process, Fe-N active sites are effectively formed, displaying a half-wave potential of 0.9 V versus RHE. Notably, when implemented as a cathode catalyst in zinc-air batteries, this catalyst exhibits an impressive power density of 162.6 mW cm−2.

Catalytic activity of two-dimensional cobalt-ZIF catalyst for NaBH4 hydrolysis

In this work, cobalt-ZIF catalysts were synthesized by water-based synthesis methods. Various hydrogen bond acceptor (tetramethylammonium bromide (TMAB), tetraethylammonium bromide (TEAB), tetrapropylammonium bromide (TPAB), tetrabutylammonium bromide (TBAB) and trimethylphenylammonium bromide (TMPAB)) were co-added during cobalt-ZIF catalyst synthesis procedures to promote the deprotonation of 2-methyl imidazole and the deprotonated 2-methyl imidazole was then further coordinated with cobalt ion to form a ZIF framework. Co-addition of TMPAB during synthesis resulted in 3D imperfect rhombic dodecahedrals with high surface area (432.3 m2/g), while, the rest of used hydrogen bond acceptors (TMAB, TEAB, TPAB and TBAB) provided 2D ZIF-L structure with relatively small surface area and pore volume. All synthesized cobalt-ZIF with different structures and morphologies were used as a catalyst for H2 generation by NaBH4 hydrolysis reaction. It was found that all 2D cobalt-ZIF structures exhibited higher hydrogen production rate than that of the 3D imperfect rhombic dodecahedral structure. This can be explained by the cushion-shaped cavity of a two-dimensional framework exhibiting higher flexibility which can promote more diffusion of reactants than the tetrahedral structure of a 3D-framework. X-ray absorption spectroscopy was used to investigate the electronic state of cobalt and to monitor the changing of cobalt probe atom during hydrolysis reaction. The electronic state of cobalt for all catalysts was Co2+ with the same tetrahedral symmetry. The lowering of pre-edge peaks of cobalt K edge XANES spectra during hydrolysis reaction indicated to distortion of the symmetry which is related to the reduction of the catalytic performance.

The reactive Graetz problem for a first order homogeneous decomposition reaction, and the limit of moderately large reaction constants

The long term goal of the present study is to facilitate the inversion of experimental data on cluster evaporation in laminar flow inside tubes. This requires the determination of reactant and product fluxes as functions of three dimensionless governing parameters: the axial position x along the tube, the reaction rate coefficient K, and the ratio γ of product and reactant diffusivities. The reactant flux is first calculated by separation of variables, including the accurate determination of the first 35 eigenvalues for 13 discrete values of K in the range 0<K<500. This approach is far less efficient for the product flux because the eigenfunctions are not standard functions. A numerical solution is also implemented for the reactant and its product, though it can hardly cover the full range of the three relevant parameters. The most common experimental situation involves relatively large values of K, for which a three term asymptotic solution is developed in negative powers of K1/2, with errors of order K−2. These approximate results represent well the numerically calculated reactant fluxes for K > 10 in terms of two functions, H0(s) and H1(s), reported in tables. The product fluxes require two functions, I0(s,γ), I1(s,γ). These we compute for six values of γ, and report as simple analytical fits covering continuously the full physically relevant range 1<γ<1.5. The desired ability to analyze experimental data to yield kinetic information is therefore achieved without the need for additional numerical solutions to partial differential equations.

Three-Dimensional Porous ZnO-Supported Carbon Fiber Aerogel with Synergistic Effects of Adsorption and Photocatalysis for Organics Removal

A three-dimensional (3D) ZnO-supported carbon fiber aerogel (ZnO/CFA) was successfully prepared by using natural cotton with hydrophilicity as the precursor. The facile synthetic strategy includes two steps: Zn2+ exchange on the surface of cotton and thermal treatment at high temperatures. Particularly, the calcination temperature was found to greatly affect the content, dispersity, and size of supported ZnO nanoparticles, and the product obtained at 600 °C (ZnO/CFA-600) exhibited both high ZnO loading and well-dispersed ZnO nanoparticles. Therefore, ZnO/CFA-600 has superior photocatalytic activity for tetracycline (TC) degradation under UV light irradiation compared with others. Additionally, the unique 3D crosslinking network inside the ZnO/CFA generates an open channel for the rapid migration and diffusion of reactants and products. In a dynamical water-treated system, the 3D porous ZnO/CFA-600 continuously works for TC removal without any separation operation and maintains high synergistic performance of adsorption and photocatalysis for at least 8 h. Consequently, the 3D porous ZnO/CFA product, with its large adsorbability and high photoactivity, shows a lot of industrial potential in wastewater treatments.

(Invited) How to Identify, Parameterise and Use Complex Electrode Kinetic Models

Electrochemical engineering transfers knowledge from surface/material level to higher levels to build well performing porous electrodes, cells and systems. Modeling accompanies this process at all levels, as it allows for deeper insights into limitations and for subsequent knowledge-driven design and optimization. Modeling tools at higher levels are already well established and predictive, when it comes to heat and mass transfer, as these phenomena are widespread in engineering. They struggle, however, to reproduce or predict the electrochemical behavior of electrodes.The situation at the electrode level becomes tricky, as there frequently are complex reaction networks at the surface. These lead to a relationship of current, voltage and reactants, which cannot be accurately described by a Tafel- or Butler-Volmer-type rate equation. Identifying suitable reaction network models including reaction kinetic equations and parameterizing them is a challenge; this talk will present promising solutions and their application to selected examples from the electrolysis and battery field.Besides the electrode surface itself, changes in electrolyte composition close to the electrode surface may occur, which alter electrode performance dramatically. Models may need to account not only for diffusion and migration of the main reactants, but also for transport of intermediates and chemical reactions in the electrolyte as well as phase changes. How can one be sure to have implemented proper processes and parameters? Also here, the talk will give an insight into promising solutions and the application to selected examples including oxygen reduction and CO2 reduction.The thus identified and parameterized models reveal process limitations and sensitivities which allow for better design and operation of the electrodes. They are further suitable for integration into porous electrode or cell models for design and diagnosis purposes on higher levels.

Construction of imidazole@defective hierarchical porous UiO-66 and fibrous composites for rapid and nonbuffered catalytic hydrolysis of organophosphorus nerve agents

Hydrolytic destruction of toxic organophosphorus nerve agents by metal-organic framework (MOF) catalysts is commonly reliant on bulk water and volatile liquid base, preventing real-world implementation. Poor accessibility to MOF-based active sites in heterogeneous catalysis is also a crucial factor since reactants diffusion is limited by inherently small micropores. To overcome these practical limitations, a ligand-selective pyrolysis strategy was used to construct unsaturated Zr defects and additional mesopores in UiO-66(Zr). Owing to synergistic effect of Zr defects and hierarchical pores, hydrolysis rate constant (k) of nerve agent simulant DMNP (dimethyl 4-nitrophenyl phosphate) on optimal DHP-UiO-30% (defective hierarchical porous UiO-66) is 3.2 times higher than counterpart UiO-30% in N-ethylmorpholine buffer. Encapsulating imidazole (Im) into DHP-UiO-30% affords Im@DHP-UiO, mimicking phosphotriesterase. Im-72@DHP-UiO exhibits rapid DMNP detoxification with 99% conversion in 12 min and initial half-life (t1/2) of 1.8 min in nonbuffered water. As the first example of ‘three-in-one’ detoxifier, Im@DHP-UiO is further integrated onto nonwoven fabric to construct Im@DHP/Fiber, achieving solid-phase detoxification at ambient humidity with t1/2 of 19.6 min and final conversion of 91%. This is comparable to many powdered catalysts in aqueous solution buffered by volatile bases. This unified strategy is critical and viable to efficiently hydrolyze nerve agents in practical settings.

Preparation and optimization of eggshell-type Cu/Zn/Al2O3 pellet catalysts for methanol reforming

The pellet-type catalyst demonstrates a usable region for the chemical reaction based on the reaction rate and reactant diffusivity. Therefore, achieving selective metal infiltration on the outer region of the support enhances the catalyst's efficiency. To address this issue, we prepared the eggshell-type Cu/ZnO/Al2O3 catalysts using the repulsion between a hydrophobic solution and a hydrophilic solution. For the methanol steam reforming reaction, the eggshell-type catalysts outperformed the homogeneously metal distributed catalysts. By analyzing the elemental distribution at both the pellet-scale and nanoscale of the catalyst, we revealed that the improved performance of the eggshell-type catalyst resulted from the selective distribution of copper particles on the outer region of the catalysts.

Excellent Performance and Feasible Mechanism of ErOx-Boosted MnOx-Modified Biochars Derived from Sewage Sludge and Rice Straw for Formaldehyde Elimination: In Situ DRIFTS and DFT

To avoid resource waste and environmental pollution, a chain of ErOx-boosted MnOx-modified biochars derived from rice straw and sewage sludge (EryMn1-y/BACs, where biochars derived from rice straw and sewage sludge were defined as BACs) were manufactured for formaldehyde (HCHO) elimination. The optimal 15%Er0.5Mn0.5/BAC achieved a 97.2% HCHO removal efficiency at 220 °C and exhibited favorable EHCHO and thermal stability in a wide temperature window between 180 and 380 °C. The curbed influences of H2O and SO2 offset the boosting effect of O2 in a certain range. Er–Mn bimetallic-modified BACs offered a superior HCHO removal performance compared with that of BACs boosted using Er or Mn separately, owing to the synergistic effect of ErOx and MnOx conducive to improving the samples’ total pore volume and surface area, surface active oxygen species, promoting redox ability, and inhibiting the crystallization of MnOx. Moreover, the support’s hierarchical porous structure not only expedited the diffusion and mass transfer of reactants and their products but also elevated the approachability of adsorption and catalytic sites. Notably, these prominent features were partly responsible for the outstanding performance and excellent tolerance to H2O and SO2. Using in situ DRIFTS characterization analysis, it could be inferred that the removal process of HCHO was HCHOad → dioxymethylene (DOM) → formate species → CO2 + H2O, further enhanced with reactive oxygen species. The DFT calculation once again proved the removal process of HCHO and the strengthening effect of Er doping. Furthermore, the optimal catalytic performance of 15%Er0.5Mn0.5/BAC demonstrated its vast potential for practical applications.

CuO–ZnO catalyst decorated on porous CeO2-based nanosheets in-situ grown on cordierite for methanol steam reforming

Development of efficient and durable structured catalysts for hydrogen production by methanol steam reforming (MSR) is becoming extremely appealing owing to their low pressure drop and large open area. In this work, porous CeO2-based nanosheet array is in-situ grown on the surface of cordierite honeycomb ceramic by a simple hydrothermal approach. The generated porous nanosheet structure not only surpasses the obstacle of low specific surface area of cordierite (increasing from 4.4 to 252.6 m2 g-1), but also the peak area ratio of oxygen vacancies on the support surface rises from 19.7% to 29.7%, which are favorable to promote the catalytic activity and reduce by-product CO selectivity of structured catalysts in MSRreaction. Importantly, the structured catalysts formed by decorating CuO–ZnO catalysts on CeO2-based nanosheets via sol-gel combustion method remain porous structure, offering an ideal space for reactant entry and mass transfer diffusion, which facilitates reaction triggering and product discharge. The optimal structured catalyst achieves 100% methanol conversion at 260 °C without CO and the methanol conversion remains at 94.7% after 100 h of the reaction, exhibiting the excellent catalytic activity and durability. Thus, the structured catalyst has potential for industrial-scale preparation and application, especially for on-site hydrogen production in vehicles.

A novel fitted numerical scheme for singularly perturbed delay parabolic problems with two small parameters

In this paper, fitted operator finite difference methods are presented for two-parameter singularly perturbed one-dimensional parabolic partial differential equations with a delay in the time variable. Such boundary value problems are frequently encountered in the spatial diffusion of reactants and in control systems. To approximate the solution of the problem, we consider the Crank–Nicolson method for time discretization and the non-standard scheme for space discretization on uniform meshes. We prove that the proposed numerical method is uniformly convergent, having convergence of order two in time and one in space. Further, we present numerical results for two test problems in support of our theoretical findings and to demonstrate that the non-standard scheme is more effective than the hybrid scheme comprising central difference, upwind, and mid-point.

CALCULATION OF THERMODYNAMIC AND KINETIC PARAMETERS OF CATALYTIC CRACKING REACTIONS ON LEWIS AND BRØNSTED ACID SITES

Link for citation: Nazarova G.Y., Ivashkina E.N., Maltsev V.V. Calculation of thermodynamic and kinetic parameters of catalytic cracking reactions on Lewis and Brønsted acid sites. Bulletin of the Tomsk Polytechnic University. Geo Аssets Engineering, 2023, vol. 334, no. 7, рр. 214-225. In Rus. The relevance of the research is caused by the emerging necessity of developing a mathematical model to optimize the heterogeneous process of catalytic cracking. This tools should take into account both the chemical transformations of a wide range of hydrocarbon groups (different feedstock types), as well as the stages of adsorption, reactants diffusion, conversion of hydrocarbons on the catalyst surface, acid characteristics and pore size of the catalysts. The study of the hydrocarbon conversion patterns on Lewis or Brønsted acid sites using quantum-chemical modeling methods allow us to quantify the thermodynamic parameters of reactants adsorption, the kinetic parameters of carbocations formation and cracking on acid sites. These results are necessary to develop a mathematical model based on the of heterogeneous catalytic reaction mechanism. The aim of this work is to identify the level of quantum chemical theory and to determine the thermodynamic and kinetic parameters of reactants adsorption, carbenium ions formationand hydrocarbons cracking on Lewis and Brønsted acid sites. Methods: quantum-chemical modeling methods to optimize the molecular geometry of reactants and products of catalytic cracking reactions, calculate vibrational frequencies, thermodynamic parameters of adsorption and catalytic cracking of hydrocarbons and heteroatomic compounds with the participation of Bronsted and Lewis acid sites. Results. The chosen level of quantum-chemical theory allowed obtaining the results that are consistent with the laws of the process and the experimental reactivity of hydrocarbons in cracking reactions on acid catalysts. The thermodynamic parameters of the adsorption of C6–C16 hydrocarbons and thiophenes on Lewis and Brønsted acid sites were identified. We found that during the cracking of n-hexane on the Lewis acid site, the reaction is limited by the carbenium ion formation stage.The activation energy of this stage was 281,3 kJ/mol whereas the value for the cracking stage was 277,2 kJ/mol. Further study shows that the activation energy of carbenium ion formation from izohexane and C8–C10 alkanes with normal structure on the Lewis acid site was 257,6 and 279,2…277,9 kJ/mol. The most energetically favorable is the formation of carbocation from hexene at Brønsted acid sites (76,59 kJ/mol). The results of the work will be used to create a mathematical model of a heterogeneous process based on the Langmuir–Hinshelwood equations.

Job-synergistic engineering from Fe3N/Fe sites and sharp-edge effect of hollow star-shaped nitrogen-doped carbon structure for high-performance zinc-air batteries

Due to the limited sources, high cost, and poor long-term stability of noble metal catalysts, the large-scale commercial application of zinc-air batteries has been restricted. Therefore, designing non-noble metal catalysts with excellent activity and stability for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) blossoms in the ascendant. Herein, we report a hollow star-shaped dual-functional electrocatalyst with uniform and dispersed Fe3N/Fe nanoparticles derived from zeolitic imidazolate framework (ZIFs) by a simple sacrificial template strategy. This hollow structure has the characteristics of low density, thin shell, and high permeability, and its internal cavity not only provides additional three-phase interfaces to accelerate O2 reduction and evolution but also promotes the diffusion of reactants onto the catalyst. By controlling the input amount of iron metal salt, the dispersion degree of FeNP can be effectively adjusted, and the generated Fe3N introduces abundant Fe-NX active sites. Density functional theory calculations show that the strong affinity between Fe3N and FeNP is favorable for promoting the oxygen intermediate reaction, achieving a job-synergistic catalytic effect and high reaction kinetics. The synthesized Fe3N/FeNP-N-C-3.3% exhibits excellent dual-functional activity for OER and ORR, with a high half-wave potential of 0.867 V for ORR (better than the commercial Pt/C of 0.82 V) and an overpotential of only 294 mV for OER at 10 mA cm−2. The rechargeable Zn-air battery prepared with Fe3N/FeNP-N-C-3.3% as an air cathode shows a long-term cycling performance of over 750 h and a maximum power density of 205 mW cm−2. The flexible Zn-air battery has a high open-circuit voltage of 1.48 V and a maximum power density of 52.6 mW cm−2, showing glamorous commercial prospects.

Methods for Preparing Hierarchical Zeolites by Chemical Etching and Their Catalytic Applications: A Review

AbstractZeolites are widely used in petrochemical processes and refineries due to their well‐ordered microporous network and large surface area. However, the diffusion of reactants and products is hampered by the narrow microporous channels, causing limitations. To overcome this challenge, modifying the pore structure is crucial, and the chemical etching technique is a powerful tool that introduces mesopores and macropores, consequently enhancing mass transfer and accessibility. Diverse chemical etching methods have been invented, including exposure to both acids (organic/inorganic acids), alkali (organic/inorganic alkali), and neutral etchants (e. g., ammonium fluoride). This review summarizes and assesses the chemical etching methods and their relevance to catalytic cracking reactions, methanol to hydrocarbons (MTH), and biomass conversion. The potential of zeolites with modified pore structures has motivated researchers to develop novel methods to tackle the practical challenges associated with their applications.