Diffusion Layer Thickness Research Articles

Effect of 3C-SiC Layer Thickness on Lateral Photovoltaic Effect in 3C-SiC/Si Heterojunction

In recent years, the lateral photovoltaic effect (LPE) with its unique working mechanism has been explored as an indispensable method for position detection applications. A promising platform for developing self-powered position-sensitive detectors (PSDs) is the 3C-SiC/Si heterojunction, taking advantage of its large built-in voltage and capabilities to work in harsh environments. In this work, we further demonstrated the superior performance for position sensing of the 3C-SiC/Si heterojunction by optimizing the diffusion layer thickness. We fabricated 3C-SiC/Si heterojunction devices with different thicknesses and evaluated the position-sensing performance under different lighting conditions. A maximum sensitivity of 603.65 mV/mm was achieved in the device with a 90-nm SiC layer at zero bias. We elaborated the working mechanism by examining the generation and diffusion of charge carriers under nonuniform light illumination. The excellent sensing performance can be attributed to the high resistivity of the diffusion layer and the trapping sites at the heterojunction interface. Our study further establishes the potential of 3C-SiC/Si for optoelectronic sensing and provides useful guidelines for developing ultrasensitive PSD.

Reliability of chemical osmosis as a practical method to extract water out of shale and promote shale’s mechanical stability

Wellbore instability in shale formations is one of the most bothersome problems leading to significant changes in shale’s petrophysical, mechanical, and chemical properties, and could cost billions of dollars annually. Drilling fluid’s ionic composition is one of the factors affecting wellbore stability. This paper investigates the impact of chemical osmosis, ionic diffusion, and diffusion osmosis on the stability of shale, identify the critical salt concentration, and examine the impact of ionic type and concentration on shale stability. Experimental methods include utilization of shale I and II through linear expansion and gravimetric measurement tests using different salts. Results show that the critical salt concentration in shale I and II is 8 w/w%. Chemical osmosis is found to be a reliable method for water extraction out of shale if critical concentration value is not surpassed. Beyond this value, the ionic diffusion and diffusion osmosis in shale I and II were adversely affecting shale’s mechanical and chemical stability. The swelling and gravimetric ions and water uptake tests showed that shale is a leaky semi-permeable membrane, and that chemical osmosis could be jeopardized by ionic transport into shale. However, the water transport impact on changes in the thickness of diffuse double layer of clay needs to be investigated. The impact of temperature on water and ions transportation into shale needs to be examined to reflect in-situ conditions. In this study, the cation type that makes up the salt was varied while the anion type was fixed. Since both cations and anions tend to diffuse into shale in the presence of a concentration gradient, it is recommended to vary the anion type to study the combined impact of cation and anion type on shale stability.

A new modelling approach for dissolution of polydisperse powders

A new modelling approach for dissolution of polydisperse powders is developed within the framework of the classical Noyes–Whitney/Nernst–Brunner analysis. Its distinguishing feature is that the underlying continuous particle-size distribution is retained. Two different but related dependencies of the diffusion-layer thickness on particle size are considered. First, a power-law dependence that interpolates between a thickness that is proportional to (or equals) the particle radius (obtained when the exponent equals 1) and a constant thickness (obtained when the exponent is 0). Second, a piecewise linear function such that the thickness equals the particle radius for sufficiently small particles and is constant for larger ones. The modelling approach is exemplified by consideration of a lognormal particle-size distribution. Highly accurate closed-form expressions for the fraction of dissolved drug are obtained for dissolution under sink conditions (which are exact if the diffusion-layer thickness is radius-independent). Moreover, it is demonstrated that any result derived under sink conditions can be reused to determine the fraction of dissolved/absorbed drug under non-sink conditions, using the concept of a retarded time. Comparison with literature data and experiments are used to validate the modelling approach and to demonstrate its usefulness in a practical context.

Fabrication of Cu/Al/Cu Laminated Composites Reinforced with Graphene by Hot Pressing and Evaluation of Their Electrical Conductivity.

Metal laminated composites are widely used in industrial and commercial applications due to their excellent overall performance. In this study, the copper/graphene-aluminum-copper/graphene (Cu/Gr-Al-Cu/Gr) laminated composites were prepared by ingenious hot pressing design. Raman, optical microscope (OM), scanning electron microscope (SEM), van der Pauw (vdP), and X-Ray Diffractometer (XRD) were used to investigate the graphene status, interface bonding, diffusion layer thickness, electrical conductivity, Miller indices and secondary phases, respectively. The results demonstrate that the Cu-Al interfaces in the Cu/Gr-Al-Cu/Gr composites were free of pores, cracks and other defects and bonded well. The number of graphene layers was varied by regulating the thickness of the Cu/Gr layer, with the Cu/Gr foils fabricated by chemical vapor deposition (CVD). The electrical conductivity of the composite was significantly improved by the induced high-quality interfaces Cu/Gr structure. The increased number of graphene layers is beneficial for enhancing the electrical conductivity of the Cu/Gr-Al-Cu/Gr composite, and the highest conductivity improved by 20.5% compared to that of raw Al.

Oxidation behavior of Cr-coated Zr-4 alloy prepared by multi-arc ion plating at 1000-1200°C

The oxidation behavior of Cr-coated Zr-4 alloy is studied in air environment at temperatures ranging from 1000 °C to 1200 °C and for oxidation times ranging from 1 h up to 4 h. The oxidation weight gain, sample appearance, surface and cross-sectional microstructure, Cr-Zr diffusion and phase at different oxidation times for the three temperatures were analyzed in detail. The results reveal that the oxidation weight gain is parabola with oxidation time, and the parabolic rate constant increases with the increase of oxidation temperature. At 1000 °C and 1100 °C, the Cr coating can provide oxidation protection for the internal Zr-4 alloy after 4 h of oxidation. While at 1200 °C, Cr coating can only provide effective oxidation protection for the internal Zr-4 alloy for 1 h. At 1000 °C and 1100 °C, the thickness of the Cr-Zr diffusion layer increases linearly with the oxidation time, while at 1200 °C, the thickness of the Cr-Zr diffusion layer increases linearly with the oxidation time after 1–3 h of oxidation, and after 4 h of oxidation, it suddenly increases sharply. And the XRD results imply that the preferred orientation of Cr2O3 grains is related to the oxidation temperature.

Atomic diffusion behavior in electromagnetic pulse welding

• Establish molecular dynamics model to investigate the interface diffusion behavior. • A narrow diffusion zone of amorphous layer was found at the bonding interface. • Improve the theoretical basis on the bonding mechanism at the atomic level. This paper investigates the atomic diffusion behavior and bonding mechanism in a Cu-Al joint fabricated by electromagnetic pulse welding. The molecular dynamics simulation is proposed to investigate the atomic diffusion behavior. The equation of diffusion layer thickness is deduced to calculate the diffusion layer thickness according to the simulation results. The welding and transmission electron microscope experiments are carried out, and the simulation results are in good agreement with the experiments. There is a diffusion zone of amorphous layer at the bonding interface, but the diffusion thickness is narrow. So this work reveals that the inter-atomic bonding is not a major factor in electromagnetic pulse welding.

PERFORMANCE ANALYSIS ON PROTON EXCHANGE MEMBRANE FUEL CELL BY USING TAGUCHI METHOD OF OPTIMIZATION

The various operational parameters and geometrical characteristics have a significant impact on how well the Proton Exchange Membrane Fuel Cell (PEMFC) performs. Cell temperature, back pressure, anode and cathode inflow velocities, Gas Diffusion Layer (GDL) porosity and thickness, cathode water mass percentage, flow channel dimensions, rib width, and porous electrode thickness are among the operating and design parameters that are optimized in this article. Using the traditional orthogonal array Taguchi approach, the optimization of design and operating parameters in software was done in two steps. From the preliminary analysis, it was concluded that rib width had the least impact on fuel cell performance and that back pressure had the greatest impact.

Prenos mase na rotirajući disk adsorpcionom i elektrohemijskom metodom

One of the ways to intensify the process is the use of rotating elements, which increase the mass transfer coefficients many times over. In this study, the mass transfer coefficient was determined by adsorption and electrochemical methods, using a rotating disk (RD) to study the effect of rotation on the mass transfer intensity. For adsorption method, a dilute solution of methylene blue was used, and silica gel foils on rotation disk were used as adsorbents. The measurements were performed under diffusion-controlled conditions, and the color intensity of the surface was determined using SIGMA Scan Pro. The study of mass transfer by electrochemical method was performed in batch systems with a volume of 20 dm3 and 0.8 dm3 , using a solution of sulfuric acid and copper sulfate, and potassium ferricyanide (III), potassium ferricyanide (II), and potassium chloride. Disks with diameters of 0.8 cm, 1 cm, 2 cm, 5.6 cm, and 20 cm were used in the rotation range up to 800 rpm. After determining the operating voltage, the current intensity was measured at different rpm and the mass transfer coefficients were calculated. It was found that the mass transfer coefficient increases as the rotation speed of the RD increases, and that is does not depend on the diameter of RD. It is shown that the current density increases with the increase of the RD speed, while the diffusion layer thickness decreases. A comparative analysis of the results obtained by the adsorption and electrochemical methods was made and the results were compared with literature correlations.

Hierarchical Porous Electrode Impedance Model Based on Diffusion Dynamics and the Electrode Morphology and Prediction of Electric Double-Layer Structures

The capacitance performance of a hierarchical porous carbon (HPC) electrode mainly depends on the dipole storage capacity and the rate of electric double-layer (EDL) reorganization. However, it is difficult to directly measure the effect of microscopic changes of pores and electrolytic characteristics on the internal ionic mechanism of the EDL. Therefore, a model is proposed to easily and accurately describe both the diffusion and EDL dynamics of the HPC electrode. In general, diffusion in the mesoporous channel can be characterized in the middle-frequency region (5–500 Hz) and in the micropore in the low-frequency region (0.05–5 Hz). What is more, we have proven that the diffusion layer thickness is inversely proportional to the electrolytic concentration and positively proportional to the mesoporous size, microporous depth, and surface roughness. In particular, the thicker diffusion layer makes it easier for ions diffusing into micropores and a better EDL recombination in mesoporous channels. However, the thinner diffusion layer means a better EDL recombination in micropores. The low-frequency region (0.01–0.05 Hz) characterized the compact layer dynamics, which shows the constant phase element behavior obviously. Moreover, the compact layer thickness is inversely proportional to the surface heterogeneity, thus determining the dipole storage capacity. The model offers a general framework for impedance analysis and EDL prediction of HPC.

Effect of Particle Size of MgO on the Sinter Strength before and after Reduction and Its Mechanism Analysis

It is a popular countermeasure to add MgO into sinter to cope with the use of high-Al2O3 iron ore. The effects of particle size with MgO-bearing flux on the strength of a sintered sample (Fe2O3-MgO and Fe2O3-MgO-CaO series) are discussed in the present work for the purpose of improving sinter strength before and after reduction. The experimental results show that (1) with the increase in fine light calcined magnesite (LCM) from 0 to 100%, the compressive strength with the Fe2O3-MgO series increased from 5.66 to 7.42 MPa before reduction and from 2.49 to 6.03 MPa after reduction, and the compressive strength with the Fe2O3-MgO-CaO series increased from 4.62 to 7.01 MPa before reduction and from 4.00 to 6.23 MPa after reduction. The result of the sintering pot experiment conformed with the laboratory result. In actual production, LCM with a 50% fine grind should be added into the sinter based on economic concerns and environmental protection. (2) The ball-to-ball model was introduced to explain the reason that the sample compressive strength increased with the increase in fine LCM addition before reduction. (3) To quantitatively analyze the relationship of mineralization rate and particle size with MgO-bearing flux, the mineralization reaction experiment was carried out in the study and the result was as follows: the diffusion layer thickness and diffusion rates with coarse MgO-bearing flux were 250.8 μm and 12.54 μm·min−1, respectively, and those of fine MgO-bearing flux were 397.1 μm and 19.86 μm·min−1, respectively. The experimental results also explained the reason that more fine LCM increased the compressive strength of the sample before reduction. (4) The more compact and uniform microstructures and less transformation of Fe2O3 to Fe3O4 resulted the increase in compressive strength with finer MgO-bearing flux after reduction. Based on above results, adding fine MgO-bearing flux to the sinter is effective in enhancing the sinter quality.

Warm Rolled Temperature Effect on Microstructure and Mechanical Properties of 18Mn/40Si2CrMo Multilayer Composite Steel

In order to obtain a good strength-plastic/toughness match relationship, 18Mn/40Si2CrMo multilayer composite steels were successfully fabricated by a vacuum hot rolling and warm rolling process in this paper. The effects of different warm rolling temperatures (400–600 °C) on the microstructure and mechanical properties of the multilayer composite steel were systematically investigated. The result shows that the warm rolling process reduces thickness of the interfacial diffusion layer, which improves the interfacial bonding strength of multilayer composite steel. With the increase of warm rolling temperature, the total elongation (TEL) increases but ultimate tensile strength (UTS) decreases. The multilayer composite steel with a warm temperature of 500 °C achieves the balance of strength and plastic of which the UTS and TEL are 1.7 GPa and 12.5%, respectively. This is due to the high work-hardening ability of deformation twins of the 18Mn layer and the precipitates nanoscale carbides of the 40Si2CrMo layer to obscure the dislocation movement.

High temperature oxidation resistance of TNM alloy coated with/without 8YSZ/NiCoCrAlY thermal barrier coatings

The oxidation resistance of the TNM alloy (Ti-44Al-4Nb-1.5Mo (B, Y) (at. %)) coated with/without 8YSZ/NiCoCrAlY thermal barrier coating (TBC) was conducted through cyclic oxidation at 950 ℃. The formation mechanism of oxide scale, interface interdiffusion mechanism, and evolution law of thermal growth oxide (TGO) were systematically investigated. Results show that the weight gains of the TNM alloy coated with/without TBC are 2.45 mg/cm2 and 6.98 mg/cm2, respectively. The Ti-Al-N layer and the (Nb, Mo)-rich layer near the surface play a role in blocking the internal diffusion of air and the external diffusion of metal ions. After a comprehensive analysis of EPMA and SEM results, it is concluded that the diffusion rate of elements in NiCoCrAlY is Ni > Co > Cr. At the same time, the relationship between the interface diffusion layer thickness and oxidation time is built based on Fick’s first law. In addition, as the oxidation time increases, TGO evolves from a dense Al2O3 layer to CS (chromia, spinel oxide) layer, and CSN (chromia, spinel, and nickel oxide) cluster oxides. The mixed oxides induce crack nucleation and growth, thereby reducing TBC lifetime.

Mathematical modeling of a direct urea fuel cell

The performance of an anion exchange membrane-based direct urea/O2 fuel cell (AEM-DUFC) is analyzed via a mathematical model based on mass/charge transport and electrochemical reactions. The present model is verified using published experimental data, featuring a good accuracy with high R2 of 0.989 for polarization curve. The cell performances are evaluated in terms of voltage losses at anode/cathode, ohmic voltage loss in membrane, and urea crossover. The voltage losses in electrodes are dominant at a low current density; however, that in membrane is the highest at a high current density. The mathematical analysis results indicate that the structural design parameters (membrane thickness, porosity, and thickness of diffusion layers) and the operating parameters (urea fuel concentration and flow rate, O2/air, KOH feed concentration, and temperature) are important factors for high performance of AEM-DUFC. Urea and KOH concentrations exhibit optimum levels because of their composite effect. Overall, improve AEM-DUFC performance mainly depends on the structure and operating parameters of anode side.

Benzoylthiourea based polymers as new binding agents for diffusive gradients in thin films technique in labile mercury determination in freshwaters

In this study, the diffusive gradient in thin film (DGT) has been developed using three new materials derived from benzoylthiourea (PBTU, BTP1 and BTP2) for the determination of bioavailable mercury (Hg). The efficiency of these DGT devices was compared with the well-established 3-mercaptopropyl functionalized silica gel (3MFS) in laboratory and field conditions. It was found that PBTU lacked of consistency for handling and assembling in the DGT device, whereas the other two polymers provided mechanically stable binding gels and extracted satisfactorily Hg. However, BTP2 showed no correlation between Hg uptake and Hg present in the aqueous solution, and therefore, BTP1 gels was chosen as the binding layer used for DGT technique. The calculated diffusion coefficient of DGT-BTP1 was 4.0 × 10 −6 cm 2 s −1 at 25 °C in 0.8 mm thick diffusion layer and 2.3% w/v of material in gel, whereas for DGT-3MFS, the D value was 9.1 × 10 −6 cm 2 s −1 . Moreover, it was found that neither the pH (range 4.5 – 9.5) nor the ionic strength (range 0.005–0.1 mol L −1 ) significantly affected the binding behavior of Hg (p > 0.05). Finally, the DGT-BTP1 devices were successfully used for in-situ measurements of Hg in Quito River (Colombia), impacted by artisanal gold mining, and were compared with DGT-3MFS. Results showed a good performance with values of the labile Hg concentration between 5 to 8% of the total Hg in water. This study demonstrates that benzoylthiourea based DGT is a useful tool for in situ monitoring of Hg in freshwaters. • Three new materials derived from benzoylthiourea were used for DGT technique. • BTP1 extracted satisfactorily Hg, with a diffusion coefficient of 4.0 × 10 −6 cm 2 s −1 . • DGT-BTP1 was suitable for aqueous solutions with a wide pH and ionic strength range. • These devices were successfully used in a Colombian river highly impacted by Hg. • Benzoylthiourea based DGT was a useful tool for in situ monitoring of Hg in freshwaters.

Interface characteristics and mechanical properties of Al0.6CoCrFeNi/5052Al matrix composites fabricated via vacuum hot-pressing sintering and annealing

In this study, Al0.6CoCrFeNi high-entropy alloy particle reinforced 5052Al matrix composites prepared via vacuum hot-pressing sintering were annealed. Experimental techniques such as SEM, XRD, EPMA, Vickers hardness, and uniaxial compression were used to characterize the prepared composites' microscopic morphology and mechanical properties. Furthermore, the Boltzmann-Matano diffusion theory and diffusion layer thickness curve fitting methods were used to predict the trend of the thickness of the diffusion layer under various annealing conditions. The results of this study suggest that the thickness of the diffusion layer increased parabolically with an increase in annealing temperature or annealing time. The intermetallic compounds (IMCs) generated in the diffusion layer region were Al13Co4-, Al9Co2-, and Al18Cr2Mg3-type phases. The mechanical properties of the composites were closely related to the change in the thickness of the diffusion layer: when the annealing temperature was 773 K/8 h, the compressive strength and microhardness of the composites reached the maximum values of 345.7 MPa and 82.8 HV0.2, respectively. Furthermore, the linear fitting method with the addition of the incubation period provided the best fit to the experimental values and provided a reference for subsequent modulation of the composite properties by varying the thickness of the diffusion layer.

Evaluation and Application of a Novel Diffusive Gradients in Thin-Films Technique for In Situ Monitoring of Glucocorticoids in Natural Waters.

The potential environmental risks of glucocorticoids (GCs) have attracted attention in the past few decades. In this study, a novel diffusive gradients in thin-films (DGT) device and analytical technique based on the second generation of polar enhanced phase (PEP-2), PEP-2-DGT, were developed for sampling and quantifying natural and synthetic GCs in aquatic systems. The capacity of PEP-2 gels for accumulating all target compounds was >600 μg per disc, sufficient for long-term passive sampling of selected GCs, even in wastewaters. Systematic tests were carried out to verify the application of DGT in natural waters and wastewaters. The performance of PEP-2-DGT devices was independent (CDGT/Csoln was in the acceptable range of 0.9-1.1) of a wide range of environmental conditions: ionic strength (0.001-0.5 mol L-1), dissolved organic matter (0-20 mg L-1), and pH (3.06-9.02). It was tested for various diffusive layer thicknesses (0.565-2.065 mm) and different deployment times (10-168 h). Diffusion coefficients (D) of selected GCs through an agarose-based diffusive gel were determined for the first time (3.80-4.85 × 10-6 cm-2 s -1 at 25 °C). Linear correlations between D and log Kow were established for three groups of target GCs (R2 = 0.96-0.99). This could enable prediction of D values for other GCs with similar structures in the future, which will help for rapid screening and emergency monitoring. Concentrations and distribution patterns of analytes obtained by PEP-2-DGT devices in five rivers after 7- and 14-day deployments were in accordance with those measured from grab samples, with total GC concentrations ranging from 7 to 27 ng L-1 at all sampling sites, confirming the reliability and robustness of the DGT devices for monitoring GCs in natural waters. The development of the new DGT technique will help improve understanding of the behavior and fate of these compounds in the aquatic environments.

Engineering the Electrochemical Reaction Microenvironment to Valorize Nitrate-Polluted Wastewaters

Anthropogenic activities have imbalanced the global nitrogen cycle since the large-scale implementation of Haber-Bosch ammonia manufacturing. The removal of reactive nitrogen species (all inorganic forms besides dinitrogen) has fallen far behind their production, leading to heavy environmental burdens and continuous losses from the nitrogen economy. As the most prevalent waterborne reactive nitrogen pollutant, excessive nitrate jeopardizes both human and ecosystem health. Rather than treat nitrate as a waste, we electrochemically reduce nitrate (NO3RR) to ammonia, a commonly used fertilizer and clean fuel. Electrochemical treatment can enable on-site, modular treatment powered by renewable energy. As an inner-sphere reaction involving multiple hydrogenation and electron-transfer steps, the activity and selectivity of electrochemical NO3RR to NH3 is strongly influenced by properties at the electrode-electrolyte interface. Properties of the electrocatalyst (e.g., chemical composition, crystal structure) are crucial to intrinsic reaction activity and selectivity, while electrolyte properties (e.g., pH, species concentrations) also exert substantial impacts. Together, the electrode and interfacial electrolyte are referred to as the reaction microenvironment. Notably, the reaction environment is dynamically evolving during the reaction process: the electrocatalyst structure can undergo reconstruction, and the interfacial electrolyte properties usually deviate from the initial and bulk electrolyte properties. Titanium (Ti) has been identified as a robust NO3RR electrocatalyst, with appreciable NH3 selectivity and is suitable for real wastewater treatment due to its abundance, moderate cost, and corrosion resistance. We found that NO3RR introduces significant surface reconstruction and forms titanium hydride (TiHx, 0 < x ≤ 2). With a combination of ex situ grazing-incidence X-ray diffraction (GIXRD) and X-ray absorption spectroscopy (XAS), we demonstrated near-surface TiH2 enrichment with increasing NO3RR applied potential and duration. This quantitative relationship facilitated electrochemical treatment of Ti to form TiH2/Ti electrodes for use in NO3RR, thereby decoupling hydride formation from NO3RR performance. A wide range of NO3RR activity and selectivity on TiH2/Ti electrodes between −0.4 and −1.0 VRHE was observed and analyzed with density functional theory calculations. On the electrolyte side, mass transport modifies the interfacial electrolyte properties and thus regulates NO3RR activity and selectivity. In a representative flow-cell configuration with a Ti electrode, we systematically controlled mass transport conditions and demonstrated their impacts on NO3RR performance. With continuum model simulation and in situ infrared absorption spectroscopy, we characterized the interfacial environment under varied mass transport conditions and elucidated the impacts of interfacial electrolyte properties on NO3RR activity and selectivity. We found that diffusion layer thickness and interfacial cation concentration govern NO3RR activity, while interfacial pH steers NO3RR selectivity. In summary, our study underscores the importance of understanding the reaction microenvironment to inform rational catalyst design and electrolyte engineering to advance NO3RR performance.

Unravelling Mass Transport Effects in Electrochemical Nitrate Reduction on Titanium

Nitrate is a prevalent waterborne pollutant that threatens the health of humans and aquatic systems. By selectively producing ammonia, electrochemical nitrate reduction reaction (NO3RR) can directly transform nitrate pollutants into widely used commodity chemicals and fertilizers, thus balancing the nitrogen cycle while reducing energy consumption from the traditional Haber-Bosch process.Although research efforts to date have primarily focused on the development of efficient nitrate reduction catalysts, the electrolyte environment has been found to substantially influence NO3RR activity and selectivity. Specifically, the composition of the first ten nanometers of electrolyte away from the electrode surface dictates the immediate environment within which reactions take place. As has been demonstrated in the electrochemical CO2 reduction reaction, this interfacial reaction microenvironment comprised of near-surface electrolyte and electrocatalyst influences reaction activity and selectivity. In electrochemical NO3RR, where an anionic species reacts at a negatively charged electrode, the reaction rate can often be limited by the mass transport of the reactant nitrate. Additionally, mass transport phenomena define the electrolyte side of the reaction microenvironment by influencing other interfacial properties (i.e., electric potential, pH and ion concentrations). Therefore, we investigated the influence of mass transport phenomena on NO3RR activity and selectivity.In this study, we used a membrane-separated flow cell as a representative and translatable reactor and titanium foil as a common ammonia-selective NO3RR electrode. By varying the electrolyte flow rate, we controlled the mass transport phenomena in the flow cell and empirically determined the diffusion layer thickness. We found that NO3RR activity generally increased with increasing electrolyte flow rate, as predicted from the decreasing diffusion layer thickness, whereas NH3 selectivity slightly decreased, showing an activity-selectivity trade-off.To unravel the origins of the experimentally observed mass transport effects, simulations using the 1D generalized modified Poisson-Nernst-Planck (GMPNP) model were conducted to spatially resolve interfacial properties. Interfacial cation concentration and pH were the two properties that changed most significantly with electrolyte flow rate. Informed by simulation results, we controlled the bulk electrolyte composition to modify the interfacial cation concentration and pH and examine their impacts on NO3RR activity and selectivity. Nitrate removal rate decreased by half when the bulk cation concentration was lowered to 1/100 of the original bulk concentration but the product distribution was similar. Meanwhile, in phosphate buffered electrolytes with different bulk pH values, NH3 selectivity decreased as bulk pH increased, with NO3RR activity remaining largely unchanged. Therefore, we attributed the influence of mass transport on NO3RR selectivity to changes in the interfacial pH during reaction, likely as the result of the varying NO3RR activity under different flow rates. By comparing the nitrogen species product distribution in different electrolytes, we inferred that the interfacial pH increased rapidly to basic in non-buffered systems regardless of their bulk pH.In summary, we systematically investigated the underexplored effects of mass transport on NO3RR and found that it influenced the reaction activity and selectivity by influencing interfacial properties. This study underscores the importance of scrutinizing the interfacial electrolyte environment in electrocatalyst development and provides insight to optimize NH3 production by engineering the bulk electrolyte.

Power Generation Performance of Polymer Electrolyte Fuel Cells with Electrocatalysts Supported on SnO2 in High Current Density Range

1. Introduction Considerable efforts have been made for FCEV commercialization, where both performance and durability have to be achieved1.SnO2-supported electrocatalysts with carbon backbone have a potential to achieve both high catalytic activity and high cycling durability2-3, but it is necessary to further optimize electrochemical performance up to high current densities for such alternative electrocatalysts in the future. The objective of this study is to examine cell performance at high current densities by varying electrocatalyst layer fabrication conditions and selecting suitable gas diffusion layer and sealant thickness for the MEAs. 2. Experimental Membrane-electrode-assemblies (MEAs) were fabricated using a Pt/C standard catalyst (Tanaka Kikinzoku Kogyo, TEC10E50E) for both electrodes, and cell performance up to 2.5 A/cm2 were examined by varying sealant thickness and gas diffusion layers on the cathode side. Using mesoporous carbon (MC) as a support backbone and SnO2 as a support surface layer on the cathode side, cells with Pt/Sn0.92Nb0.08O2/MC electrocatalysts were fabricated. Cell performance and overvoltages up to high current densities were evaluated by varying ionomer ratio and heat treatment temperature in electrocatalyst layer fabrication. Cells prepared under optimized conditions were fabricated and subjected to a 60,000-start-stop cycle tests. 3. Results and discussion The cell with the standard Pt/C catalyst exhibited the lowest concentration overvoltage and the highest performance when the GDL was 22BB (SGL Carbon), the GDL thickness was 225μm, and the sealant thickness was 100 µm on the cathode side, as shown in Fig. 1 (measured at full humidification at 80°C). When the 22BB GDL with microporous layer (MPL) is used, the cell performance was found to be improved by compressing GDL with a thinner sealant.For cells with the alternative SnO2-based catalyst, cell performance reached a maximum when the ionomer was 8 wt.% of the catalyst, when the GDL was 22BB, and when the sealant thickness was 100μm. Furthermore, as shown in Figs. 2 and 3, the activation overvoltage was minimized, and cell performance was maximized when the heat treatment temperature of tin oxide loading was 600°C. Under the optimized conditions, the ECSA retention after 60,000 start-stop voltage cycles was 45% as shown in Fig. 4. This confirms that even when MC is used as the catalyst support backbone, the cycle durability is improved by applying tin oxide.In the future, we aim to further improve the catalytic activity by modifying parameters such as heat treatment temperature and loadings in catalyst preparation to reduce activation overvoltage. In addition, to suppress the increase in concentration overvoltage, we have to improve water management. Based on these efforts, latest results of cell performance will be reported in this presentation. Acknowledgments A part of this study was supported by the New Energy and Industrial Technology Development Organization (NEDO). References Takahashi, T. Ikeda, K. Murata, O. Hotaka, S. Hasegawa, Y. Tachikawa, M. Nishihara, J. Matsuda, T. Kitahara, S. M. Lyth, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 169, 044523 (2022).Nakazato, D. Kawachino, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, K. Sasaki, J. Electrochem. Soc., 165 (14), F1154 (2018).Matsumoto, M. Nagamine, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, K. Sasaki, J. Electrochem. Soc., 165 (14), F1164 (2018). Figure 1

Optimization of Pulsed Polarization Condition for Efficient Electrochemical CO2 Reduction

Introduction Massive emission of CO2 gas from fossil fuel combustion causes the severe global greenhouse effect. Electrochemical reduction of CO2 using renewable energy has been attractive due to its simple methodology, while the conversion efficiency may be lessened due to undesirable side reactions such as water decomposition. In this study, CO2 was covered to CO on Ag electrode in aqueous KHCO3 solution using potentiostatic pulse polarization, and optimal pulse condition to improve the conversion efficiency was explored. Experiment An electrochemical cell was composed of a cathode compartment containing an Ag working electrode (W.E.) and an Ag/AgCl reference electrode and an anode compartment containing a Pt counter electrode. Two compartments were separated by a Nafion membrane. An Ag-W.E. was mechanically polished and potentiostatically pulse-polarized in a CO2-saturated 0.1 M KHCO3 solution to form CO. After some polarization period, the conversion efficiency ( η=[CO]/([CO]+[H2]) ) of CO2 to CO was evaluated by using gas chromatography as a function of various parameters such as cathodic polarization potential (E c) and time (t c), rest potential (E a) and time (t a), KHNO3 concentration, etc. Result and discussion When E c was varied, the highest η was obtained around –1.5 to –1.6 V. When t c was varied at E c = –1.6 V, the highest η was obtained around t c = 3 s, as shown in Fig. 1. The t a did not affect η. These results reveal that the t c influences the conversion efficiency viacontrolling the diffusion layer thickness of reactants such as CO2 molecular. The highest η at the optimal pulse condition was ca. 0.9 and is comparable to the value obtained by the previous work using ionic liquid solution (T. Oguma et al., Electrochemistry, 88 (2020) 451–456). Fig. 1. Relationship between the cathodic polarization time t c and reduction efficiency Figure 1