Thin Diffusion Layer Research Articles

Three-dimensional porous SiC/BDD electrode with long-term robustness and enhanced electrochemical degradation performance for refractory organic pollutants

Boron-doped diamond (BDD) electrodes with a three-dimensional (3D) porous network structure can overcome the mass transfer limitations and increase substantially the utilization rate of active oxygen species. Nonetheless, the 3D porous network structure was invariably built using metallic substrates, which inherently have inferior stability because of their non-corrosion resistance and the severe thermal mismatch between BDD films and substrates. Herein, we reported, for the first time, the use of a commercially porous silicon carbide (SiC) as the substrate to construct a 3D SiC/BDD electrode to simultaneously alleviate the problematics of substrate corrosion and thermal mismatch. This novel electrode exhibited a long-term service lifetime of 800 h in 3 M H2SO4 under the current density of 50 mA cm−2, over 500 times longer than the conventional metallic substrate-based BDD electrodes. The optimum p-SiC/BDD electrode exhibited a high oxygen evolution potential of 2.04 V vs. SHE, a low charge transfer resistance of 18.4 Ω, a high mass transfer coefficient of 2.78 × 10−5 m s−1, and a thin diffusion layer thickness of 25.2 μm. The proposed electrode had a threefold larger pseudo-first-order reaction rate constant than a flat BDD electrode, although it consumed just a fourth of the latter’s electric energy. The superior performance was attributable to an increased mass transfer rate confirmed by computational fluid dynamics (CFD) simulations as well as a high yield of reactive oxygen species verified by electron spin resonance (ESR). Furthermore, the proposed SiC/BDD electrode could work effectively under different water matrices in the presence of diverse salt electrolytes, thus paving a new avenue for the structure design of robust non-active anode materials.

Bubble rising in the presence of a surfactant at very low concentrations

This paper analyzes experimentally and numerically the steady bubble rising in water with a surfactant dissolved at very low concentrations. We explain how traces of surfactant can significantly change the bubble dynamics. The tiny surface tension variation produced by the surfactant monolayer has a negligible effect on the capillary pressure. However, this variation occurs within an extremely thin diffusive boundary layer, which produces a Marangoni stress three orders of magnitude larger than the tangential viscous stress in a surfactant-free bubble. Although the Marangoni stress is confined within the surface boundary layer, it manages to immobilize most of the bubble's south hemisphere. The increase in skin friction and the reduction of the terminal velocity cannot be attributed to the viscous stress exerted on the immobilized interface but to the stress in the diffusive surface boundary layer. The stagnant-cap approximation applies despite the small surfactant concentration considered.

Oxidation resistance of Mo/Cr bilayer coating on Zr alloy in a 1200 °C steam environment

The Cr-coated Zr alloys demonstrate excellent resistance to high-temperature steam oxidation. However, the rapid diffusion pathways for oxygen formed by the inter-diffusion between the coating and alloy at high temperatures significantly affect the steam oxidation resistance of the coated alloys. To address this issue, we developed a Mo/Cr bilayer coating on Zr alloy by a combination of dc-MS and HiPIMS surface treatments. The coating exhibits outstanding steam oxidation resistance at high temperatures, resulting in a mass gain approximately 86.6% and 44.1% lower than that of the bare Zr alloy and Cr coating, respectively, after 30 min of steam oxidation at 1200 °C. This is mainly because, during the oxidation process, the Mo interface layer undergoes a transformation into a thin and high-quality double diffusion layer structure, effectively avoiding high-temperature inter-diffusion between the Cr coating and Zr alloy, thereby inhibiting the formation of oxygen diffusion pathways.

Interfacial Design Contributing to High Conversion Efficiency in Mg3(Sb, Bi)2/Bi2Te3 Thermoelectric Module with Superior Stability

AbstractMg3(Sb, Bi)2‐based materials possess excellent room‐temperature thermoelectric performance, while poor interfacial behaviors occur when connected with metal electrodes due to the strong chemical activity and volatility of Mg element. In this study, a high efficiency of 7.1% under a temperature difference of 230 K is achieved in n‐Mg3(Sb, Bi)2/p‐Bi2Te3 thermoelectric module. When changing the interfacial layer from Fe powder to Fe foil, it effectively prevents a significant diffusion of both Mg and Bi elements from the material matrix to the interfacial layer, resulting in an extremely low contact resistivity ≈3.4 µΩ cm2 that is almost one order lower than of that of Fe powder/Mg3(Sb, Bi)2 junction ≈30 µΩ cm2. Particularly, a thin diffusion layer with a width of ≈2 µm is initially observed in the unannealed Fe foil/Mg3(Sb, Bi)2 junction. Even after thermal aging at 573 K for 28 days, the diffusion‐layer width is basically unchanged and its corresponding contact resistivity maintained as low as ≈5.8 µΩ cm2. Overall, this work provides deep insights into interfacial design and paves the way for high‐performance and sustainable low‐grade waste heat recovery.

30 μm Thin Anode Gas Diffusion Layers for Optimized PEM Fuel Cell Operation at 120 °C and Low Relative Humidity

Achieving higher temperature operation above 100 °C, while increasing the volumetric power density are key targets in current fuel cell development. In this work, a novel type of polyvinylidene fluoride and graphite based microporous film of only 30 μm thickness, acting as thin alternative to typical gas diffusion layers (GDLs) on the anode side of a hydrogen fuel cell is reported. Ex situ measurements (contact angle, electric conductivity, gas permeability measurements, and scanning electron microscopy (SEM)) reveal that the thin, porous layers achieve comparable chemical, electrical, and transport properties to commonly employed and significantly thicker gas diffusion layers. In‐situ fuel cell tests reveal considerable improvements, particularly in the target operation range beyond 100 °C: At 120 °C and 30% RH and 0.65 V cell voltage, the best thin anode layer enables a 51% higher current density (1342 vs 885 mA cm−2) and a 9% lower high frequency resistance (76 vs 84 mΩ cm−2) compared to a commercial state‐of‐the‐art GDL.

Design and Fabrication of Segmented GeTe/(Bi,Sb)2Te3 Thermoelectric Module with Enhanced Conversion Efficiency

AbstractGeTe and (Bi,Sb)2Te3 are two representative thermoelectric (TE) materials showing maximum performance at middle and low temperature, respectively. In order to achieve higher performance over the whole temperature range, their segmented one‐leg TE modules are designed and fabricated by one‐step spark plasma sintering (SPS). To search for contact and connect layers, the diffusion behavior of Fe, Ni, Cu, and Ti metal layers in GeTe is studied systematically. The results show that Ti with a similar linear expansivity (10.80 × 10−6 K−1) to GeTe, has low contact resistance (3 µΩ cm2) and thin diffusion layer (0.4 µm), and thus is an effective metallization layer for GeTe. The geometric structure of the GeTe/(Bi,Sb)2Te3 segmented one‐leg TE module and the ratio of GeTe to (Bi,Sb)2Te3 are determined by finite element simulation method. When the GeTe height ratio is 0.66, its theoretical maximum conversion efficiency (ηmax) can reach 15.9% without considering the thermal radiation and thermal/electrical contact resistance. The fabricated GeTe/(Bi,Sb)2Te3 segmented one‐leg TE module showed a ηmax up to 9.5% with a power density ≈ 7.45 mW mm−2, which are relatively high but lower than theoretical predictions, indicating that developing segmented TE modules is an effective approach to enhance TE conversion efficiency.

Correlation of hierarchical porosity in nanoporous gold with the mass transport of electron transfer-coupled-chemical reactions

Optimization of mass transfer within a porous material is a highly promising strategy to improve the efficiency of electrode reactions. Herein, nanoporous gold (NPG) modified gold electrode is investigated to study the mass transport of ascorbic acid (AA), which oxidation process is characterized by an electrochemical coupled 1st order chemical reaction, commonly termed as EC1 reaction. The template-assisted synthesis of NPG results into the formation of a highly pure and porous film of gold. However, the surface porosity of NPG depends on the choice of electrodeposition parameters, such as deposition time (td) and potential (Ed) and the size of the substrate. Such porosity variation of NPG strongly influences the voltammetric profile of AA anodic reaction, displaying sigmoidal, non-symmetric and symmetric peak features. The analysis of mass transport behaviour of AA reveals a combination of diffusion and thin layer EC1 mechanism, predominance of which is determined by the Ed and td selected for NPG synthesis, as well as the size of the Au substrate. The mass transport of AA on NPG prepared on Au microelectrodes experienced a significant diffusion from bulk solution, owing to the larger pores, which permits the easier exchange of redox species between the NPG volume and the bulk solution. On the contrary, mass transport of AA on NPG deposited on big Au electrode has a significant contribution of thin layer diffusion, attributed to the smaller surface pores of NPG, which limits the exchange of AA and its oxidized form from the bulk solution.

Effects of hydrophobic slips in non-uniform electrokinetic transport of charged viscous fluid in nozzle-diffuser

The mixing efficiency and electro-osmotic flow enhancement over a hydrophobic structured microchannel with nozzle-diffuser under an external electric field is studied. The interfacial surface zeta potential is varied to generate a strong convection effect between two injecting fluids from the reservoirs for a wide range of Reynolds numbers. The Poisson–Nernst–Plank model is incorporated to deal with thick and thin diffuse layers formed by a non-Newtonian electrolyte solution for the numerical simulation of the mobility of ions. To avoid the high-pressure drop complications in the computation, we have scaled the mixing efficiency with the average pressure drop. The analytical validation of the velocity and potential for thin and thick electric double layer with the existing experimental results corroborated and bridged the performance of the present model to achieve faster mixing by reducing pressure gradient. It is demonstrated that hydrodynamic slip increases the flow velocity producing larger mobility; however, the heterogeneous zeta potential generates a backflow that prevents the driving fluids leading to higher mixing efficiency, discussed in the first phase of the work. It is found that the mixing performance of Newtonian fluid is maximum for a nozzle-diffuser-shaped microchannel when all other physical parameters are constant. In the next phase, the mixing performance of shear thickening, shear thinning, and Newtonian fluid has been discussed for various physio-chemical parameters, i.e., slip length, Debye parameter, channel conical angle/slope, and heterogeneous zeta potential strength. The mixing efficiency and the pressure gradient decrease with the increase in the Debye parameter and the slip length. It is observed that the mixing efficiency can further be enhanced by increasing the heterogeneity of zeta potential strength and channel conical angle. This study can be used as a benchmark model for fabrication of chaotic mixers in hydrophobic slips with wall-mounted heterogeneous zeta potential and can be suitable for handling the power-law fluids.

A thin-layer diffusion model-based intelligent cyclic voltammetry analysis method to capture electrochemical parameters in flow batteries

Contrary to the conventional approach that utilizes the planar electrode model to achieve the electrochemical parameters of the electrode in flow batteries, this work proposes an intelligent cyclic voltammetry (CV) analysis method to accurately capture the electrochemical parameters of the carbon-based porous electrode: (i) the transient diffusion process is elaborated by the Fick's second law; (ii) the thin-layer diffusion boundary condition is determined based on the structural characteristics of porous electrodes; (iii) CV response on the porous electrode is obtained by the Bulter-Volmer equation in assocition with the Fick's first law; and (iv) the electrochemical parameters of the porous electrode are automatically extracted from the experimental data via the genetic algorithm. The feasibility of the intelligent CV analysis method is proven by studying the electrochemical parameters including the diffusion coefficient D and the standard rate constant k0 of [Fe(CN)6]4−/[Fe(CN)6]3−. Additionally, this analysis method is applied to investigate the vanadium redox flow battery by providing accurate electrochemical parameters for the 3D multi-physics model to predict the charge/discharge performance at different current densities. The simulation results demonstrate a maximum relative error of 2% in comparison to the experimental data, suggesting the good reliability in developing the advanced model. The proposed intelligent CV analysis method provides a potential application in accurately capturing the electrochemical parameters in other electrochemical energy conversion and storage devices.

Investigating Comproportionation in Multielectron Transfers via UV-Visible Spectroelectrochemistry: The Electroreduction of Anthraquinone-2-sulfonate in Aqueous Media.

UV-vis spectroelectrochemistry is assessed as a tool for the diagnosis and quantitative in situ investigation of the incidence of comproportionation in multielectron transfer processes. Thus, the sensitivity of the limiting current chronoabsorptometric signals related to the different redox states to the comproportionation kinetics is studied theoretically for different working modes (normal and parallel light beam arrangements) and mass transport regimes (from semi-infinite to thin layer diffusion). The theoretical results are applied to the spectroelectrochemical study of the two-electron reduction of the anthraquinone-2-sulfonate in alkaline aqueous solution, tuning the thermodynamic favorability of the comproportionation reaction through the electrolyte cation. The quantitative analysis of the experimental results reveals the occurrence of comproportionation in the three media examined, showing different kinetics depending on the cationic species in solution.

Understanding Carbon Nanotube Voltammetry: Distinguishing Adsorptive and Thin Layer Effects via "Single-Entity" Electrochemistry.

Cyclic voltammetry of ensembles of nanotube-modified electrodes fails to distinguish between signals from electroactive material adsorbed on the tubes from those due to a thin-layer response of analyte material occluded in the pores of the ensemble. We demonstrate that the distinction can be clearly made by combining cyclic voltammetry with single-entity measurements and provide proof of concept for the case of b-MWCNTs and the oxidation of 4-hexylresorcinol (HR), where the increased signals seen at the modified electrode are concluded to arise from thin-layer diffusion and not adsorptive effects. The physical insights are generic to porous, conductive composites.

Impact of Separator Thickness on Temperature Distribution in Single Cell of Polymer Electrolyte Fuel Cell Operated at Higher Temperature of 90 °C and 100 °C

The New Energy and Industry Technology Development Organization (NEDO) road map (Japan, 2017) has proposed that a polymer electrolyte fuel cell (PEFC) system, which operates at a temperature of 90 °C and 100 °C, be applied for stationary and mobility usage, respectively. This study suggests using a thin polymer electrolyte membrane (PEM) and a thin gas diffusion layer (GDL), at the same time, to achieve better power-generation performance, at a higher temperature than usual. The focus of this paper is to clarify the effect of separator thickness on the distribution of temperature at the reaction surface (Treact), with the relative humidity (RH) of the supply gasses and initial operation temperature (Tini), quantitatively. In this study, separator thickness is investigated in a system using a thin PEM and a thin GDL. Moreover, this study investigates the difference between the maximum temperature and the minimum temperature obtained from the distribution of Treact as well as the relation between the standard deviation of Treact − Tini and total voltage, to clarify the effect of separator thickness. The impact of the flow rates of the supply gases on the distribution of Treact is not large, among the investigated conditions. It is noticed that the temperature distribution is wider when a separator thickness of 2.0 mm is selected. On the other hand, it is observed that the temperature increases along with the gas flow through the gas channel, by approximately 2 °C, when using a separator thickness between 1.5 mm and 1.0 mm. The impact of the RH on the distributions of Treact − Tini is larger at Tini = 100 °C, when a separator thickness of 1.0 mm is selected. It is revealed that the wider temperature distribution provides a reduction in power-generation performance. This study proposes that the thin separators, i.e., with a thickness of 1.5 mm and 1.0 mm, are not suitable for higher temperature operation than usual.

Detachable all-carbon-linked 3D covalent organic framework films for semiconductor/COF heterojunctions by continuous flow synthesis

Detachable all-carbon-linked 3D covalent organic framework films for semiconductor/COF heterojunctions by continuous flow synthesis

Accurate Solutions to Non-Linear PDEs Underlying a Propulsion of Catalytic Microswimmers

Catalytic swimmers self-propel in electrolyte solutions thanks to an inhomogeneous ion release from their surface. Here, we consider the experimentally relevant limit of thin electrostatic diffuse layers, where the method of matched asymptotic expansions can be employed. While the analytical solution for ion concentration and electric potential in the inner region is known, the electrostatic problem in the outer region was previously solved but only for a linear case. Additionally, only main geometries such as a sphere or cylinder have been favoured. Here, we derive a non-linear outer solution for the electric field and concentrations for swimmers of any shape with given ion surface fluxes that then allow us to find the velocity of particle self-propulsion. The power of our formalism is to include the complicated effects of the anisotropy and inhomogeneity of surface ion fluxes under relevant boundary conditions. This is demonstrated by exact solutions for electric potential profiles in some particular cases with the consequent calculations of self-propulsion velocities.

Peridynamic modeling of freeze-thaw damage in concrete structures

The peridynamic model introduced in this paper is capable of simulating the entire process of concrete freeze-thaw failure. The model considers the heterogeneity of concrete and a new loading method implementing the freezing force during freeze-thaw cycles is applied. The model is verified by comparing with the damage pattern of a cement paste sample captured in the experiment. Considering concrete’s porosity, randomly distributed pre-damage is introduced to simulate freeze-thaw damaging in actual macroscopic concrete. The unique contribution to the field in this work, is the way to couple the damage with loading condition, through diffuse layers, not on mathematical interfaces. Highlights Damage initiation and crack propagation in freeze-thaw cycling are simulated. The peridynamic simulation adopts a stochastically homogenized material model. Ice crystallization pressure is applied via an equivalent displacement condition. Freezing forces exert on material nodes near crack surfaces to mimic a thin water diffusion layer. Stochastic model retains heterogeneity while cost the same as a homogenized one.

Influence of conductive porous electrodes on the apparent electrode kinetics of fenitrothion

• FT’s cyclic voltammetry response at planar and porous modified electrode is simulated. • Simulations reveal the occurrence of thin-layer effects at porous modified electrode. • Cyclic voltammetry experiment validates the simulation results. Planar bare electrodes fabricated with surface-modified multiwalled carbon nanotubes (MWCNT) are used to detect organophosphate (OP) compounds, which are used as herbicides and fungicides but are harmful to human health. Deposition of carbon nanotubes on the surfaces of bare electrodes enhances electrocatalysis, increasing the electrode to analyte current response and narrowing peak-to-peak potential separation during cyclic voltammetry (CV). We hypothesize that, as the thickness of the deposited MWCNT layer decreases, Fenitrothion (FT, an OP) mass transport between the layers of porous multiwalled carbon nanotubes changes from semi-infinite to thin-layer diffusion. This influences the electrode electrochemical response to the analyte. Using simulations and experiments, we show that when porous MWCNT are deposited on a conductive glassy carbon electrode the mass transport of FT changes from semi-infinite to thin-layer diffusion during CV. This alters the electrochemical response of the electrode and reduces peak-to-peak potential separation. To simulate CV response to multi-step and multi-electron electrochemical reactions of FT, both the semi-infinite and thin-layer diffusion models are employed for the planar bare and modified porous surface electrodes. The transition from thin layer to semi-infinite diffusion is clear when the nanotube layer thickness on the bare electrode increases. The model is applicable to other toxic chemicals, such as 4-nitrophenol, parathion, or methyl parathion that have similar electrode kinetics.

Local diffusion indicators: A new tool for analysis of electrochemical mass transport

• Numerical simulation of chronoamperometry on a plane-truncated sphere is performed. • A local diffusion indicator is introduced. • Linear, convergent and thin layer diffusion contributions to flux are identified. • Otherwise indistinguishable diffusional processes are resolved. The recently introduced diffusion indicator (Haonan et al, J. Electroanal. Chem., 2019, 855, 113602) is generalised to characterise local mass transport fluxes to a non-uniformly accessible electrode to probe both qualitatively and quantitatively the nature of the diffusion to different parts of an electrode surface at different times following electrochemical perturbation. The approach is illustrated in respect of diffusion to a truncated sphere on a plane surface. The relative contributions of thin layer, linear and convergent diffusion following potential step chronoamperometry are evaluated. Comparison is made with analogous measurements on a micro-disc electrode. The results have implications for the transients accompanying the impact of nanoparticles on planar electrodes and for hanging mercury drop electrodes.

Self-diffusiophoresis of Janus particles that release ions

Catalytic Janus swimmers demonstrate a diffusiophoretic motion by self-generating the gradients of concentrations and electric potential. Recent work has focused on simplified cases, such as a release of solely one type of ions or low surface fluxes of ions, with limited theoretical guidance. Here, we consider the experimentally relevant case of particles that release both types of ions, and obtain a simple expression for a particle velocity in the limit of the thin electrostatic diffuse layer. Our approximate expression is very accurate even when ion fluxes and surface potentials are large and allows one to interpret a number of intriguing phenomena, such as the reverse in the direction of the particle motion in response to variations of the salt concentration or self-diffusiophoresis of uncharged particles.

First-Order or Second-Order? Disproportionation of Lithium Superoxide in Li-O2 Batteries.

The disproportionation of LiO2 to Li2O2 is a key step in Li-O2 batteries, and it is regarded as a second-order reaction. However, its mechanism is not well addressed, and its kinetics is rarely studied due to the difficulties of quantifying the rate constants, particularly for high concentrations of superoxide (>10 mM). Here, we quantified the kinetic rate constant by a microkinetic model using a microelectrode tip with a thin diffusion layer and fast response. We report that the reaction order of LiO2 transitions from 1 at high concentrations of superoxide (∼20 mM) to 2 at low concentrations of superoxide (∼1 mM). LiO2 is chemically reduced by free superoxides to form Li2O2 and O2, instead of reacting with another LiO2 via a disproportionation step. This chemical-reduction mechanism explained the change of reaction order and the kinetics profile. As a rate-determining step, this step restricts the overall kinetics of the discharging process and should be the focus of future catalyst design.

Impacts of Separator Thickness on Temperature Distribution and Power Generation Characteristics of a Single PEMFC Operated at Higher Temperature of 363 and 373 K

The aim of this study is to investigate the effects of the separator thickness on not only the heat and mass transfer characteristics, but also the power generation characteristics of a polymer electrolyte membrane fuel cell (PEMFC) with a thin polymer electrolyte membrane (PEM) and thin gas diffusion layer (GDL) operated at higher temperatures of 363 and 373 K. The in-plane temperature distributions on the back of the separator at the anode and cathode, which are the opposite sides to the GDL, are measured using a thermograph at various initial cell temperatures (Tinit), relative humidity (RH) levels, and supply gas flow rates. The total voltage corresponding to the load current is measured in order to evaluate the performance of the PEMFC. As a result, it is revealed that the effect of the RH on the power generation characteristics is more significant when the separator thickness decreases. It is revealed that the power generation performance obtained at high current densities decreases with the increase in Tinit with thinner separator thicknesses. According to the investigation of the in-plane temperature distribution, it is clarified that the temperature decreases at corner positions in the separator with the separator thickness of 2.0 mm, while the temperature gradually increases along with the gas flow with separator thicknesses of 1.5 mm and 1.0 mm.