Increase In Diffusion Resistance Research Articles

Evaluation of Electrochemical Techniques for Characterizing the Kinetics of Porous Insertion-Type Electrodes

The composite-type porous electrode remains the most commonly used electrode for a wide variety of electrochemical energy storage devices, including lithium-ion batteries, electrochemical capacitors, Li-S/Li-air batteries, and even all-solid-state batteries. This is due to its ease of manufacturability and ability to maintain a high electrochemical surface area with adequate ion and electron transport. Understanding porous electrode kinetics is essential to maximize the active material utilization at high rates. A variety of electrochemical techniques exist to characterize the kinetic behavior of porous electrodes. The most popular ones include electrochemical impedance spectroscopy, galvanostatic charge/discharge, galvanostatic intermittent titration technique, and cyclic voltammetry. The purpose of this work was to evaluate which of these four techniques provides the most relevant kinetic information for understanding porous electrode kinetics. To do this, we utilized porous electrodes composed of LiCoO2 as a model insertion-type material, and with varying thicknesses (10 - 40 µm). We find that GITT is an accessible and reliable method for obtaining the effective diffusion coefficient of the porous electrode. It yields consistent results, regardless of electrode thickness and component ratio. On the other hand, the cyclic voltammetry peak current behavior can be easily compromised, even at slow scan rates of 0.01 - 0.1 mV/s, and when using a high conducting domain ratio. When comparing the dQ/dV plot and cyclic voltammetry for observing peaks indicating phase transitions in the insertion material, the dQ/dV plot is less kinetically limited. We utilized the differential relaxation time (DRT) method to analyze electrochemical impedance spectroscopy data. Our findings demonstrate that as thickness increases, charge transfer resistance decreases, while diffusion resistance increases. This study provides a comprehensive evaluation on the differences between electrochemical methods to characterize the kinetics of composite porous electrodes.

A comprehensive study on effect of carbon nanomaterials as conductive additives in EDLCs

This study performs a comprehensive investigation to objectively analyze the effects of various carbon nanomaterials using as conductive additives (5 wt%) in activated carbon (AC) based electrodes for electrical double-layer capacitors (EDLCs). A variety of carbon nanomaterials including carbon black (CB), single-walled carbon nanotubes (SWCNT), graphene nanoplatelets (GNP), fullerenes (C60), and activated carbon nanofibers (ACNF) are considered in this work as they provide a diverse combination of intrinsic properties such as specific surface area, conductivity, micro-mesoporous structures, and geometry of nanomaterials. These additives are incorporated into freestanding carbon electrodes, and their EDLC performance in a neutral aqueous electrolyte is investigated by means of electrochemical measurements. The findings from this work provide useful insight into how the properties of various carbon nanomaterials influence EDLC performance in different aspects such as capacitive behavior, resistivity, performance at different current densities, charge propagation properties, and stability over long-term cycling. The results show that each carbon additive has its own strengths and limitations for different performance parameters considered. The unfavorable material properties of C60 cause a significant decrease in capacitance at high current density, leading to the lowest device performance incorporated with C60 as additives. While GNP and SWCNT show high crystallinity, low ID/IG ratios, the EDLCs still exhibit decreased specific capacitance and moderate energy densities due to their relatively limited specific surface area and high resistivity of the resultant composite electrodes compared to the pristine AC. The CB additives-based samples demonstrate the highest conductivity of dry electrodes and low equivalent series resistance (ESR) in EDLCs. On the other hand, a reduction in capacitance at high current densities and material degradation during long-term cycling are observed for CB samples, which could be attributed to the high dispersibility of CB causing limited interparticle voids and thus increasing diffusion resistance. In contrast, the use of ACNFs significantly reduces the diffusion resistance and improves the cyclic stability compared with CB based samples due to the advantages of larger specific surface area and superior micro-mesoporous structures of ACNFs. In particular, EDLCs with ACNFs demonstrate an increase in specific capacitance and energy density by 99 % and 76 %, respectively compared to CB based EDLCs at a current density of 20 A/g. This work elucidates the complex interplay between carbon nanomaterials using as additives and corresponding EDLC performance. These results provide important information to the community for future development of new energy conversion and storage devices.

CO2 capture using steam ejector condenser under electro hydrodynamic actuator with non-condensable gas and cyclone separator: A numerical study

The concept for condensation of steam and CO2 separation in a negative CO2 emission gas power plant involves the utilization of a steam ejector condenser (SEC) for direct-contact condensation of vapor with inert gas (CO2) on a spray of subcooled liquid, integrated with a separator to produce pure CO2. Due to the increasing diffusion resistance and reduced convective heat transfer between the steam and subcooled water phases in the presence of non-condensable gas (CO2), the study utilized an electrohydrodynamic (EHD) actuator to enhance heat transfer rate in the SEC. To optimize CO2 purification, the effect of single, dual and quadruple inlets on separation efficiency was analysed. In the SEC, the Eulerian-Eulerian multiphase model is employed, treating water as the continuous phase and the compressible gas mixture (steam and CO2) as the dispersed phase. The standard k-ε model is chosen to depict the turbulence in the ejector. The separator is transient, turbulent, and three-dimensional, using the control volume method. The RSM turbulent model and mixture model are utilized to simulate the turbulent two-phase flow in the gas–liquid separator. The findings indicated that when the mass flux of steam and voltage are increased, the condensation heat transfer coefficient also increases. For a mass flux of steam of 51 (kgm2.s), the condensation heat transfer coefficients were measured to be 0.98, 1.029, 1.08, and 1.134 (MWm2.K) at electrode voltages of 0, 20, 25, and 30 kV, respectively. In addition, a single-inlet cyclone attains a separation efficiency of 95.1 %, while incorporating two inlets improves the performance to 97.9 %. However, the most remarkable outcome is witnessed in cyclones with four inlets, where an impressive separation efficiency of 99.9 % is achieved.

Study of high burnup effect on steam oxidation of Zircaloy and its regulatory implications via the developement of a pre-transient oxide model of TRANOX

A mechanisitc model that gives oxygen distribution of Zircaloy cladding with pre-transient oxide and absorbed hydrogen under steam oxidation environments has been developed. The model has been incorporated into TRANOX. The model has been validated against steam oxidation experiments with Zircaloy-4 specimens that were pre-oxidized in simulated PWR environments and past experimental data available in open literature. The high burnup effects on cladding oxidation owing to pre-transient oxide and hydrogen can be categorized into: thinning of initial Zr-matrix by the formation of pre-transient ZrO2, increasing diffusion resistance of oxygen with pre-transient ZrO2, and increasing oxygen diffusion rate with a thicker β-phase by β-stabilizing hydrogen. Relative magnitudes of these effects with temperature, pre-transient oxide thickness, and hydrogen content determine the dynamics of high burnup fuel oxidation. Mechanistically modeling the high burnup effect, the model comprehensively simulates the integrated effects of pre-transient oxide and absorbed hydrogen. This study confirms that the corrected Cathcart Pawel correlation-based Equivalent Cladding Reacted evaluation method employed by the U.S Nuclear Regulatory Commission is an effective, yet accurate, simplification of the high burnup cladding oxidation modeling for safety analysis of representative LBLOCA of current PWR systems. The application of the developed model to safety analysis revealed allowable discharge burnup limits of 59 and 76 MWd/kgU for Zircaloy-4 and Zr-Nb alloy, respectively from the view point of fuel accident safety.

High-capacity amino-functionalized walnut shell for efficient removal of toxic hexavalent chromium ions in batch and column mode

This research investigated the feasibility of removing hexavalent chromium (Cr(VI) (e.g., HCrO4−, CrO42− and Cr2O72− ions) from water using walnut shells functionalized with amino groups (ACWNS) in a batch and continuous adsorption mode. The characterization of ACWNS was carried out utilizing FTIR, XPS, SEM, and BET methods. Adsorption was investigated in terms of pH, dose, concentration, and contact duration. Furthermore, at different temperatures, the adsorption isotherms, kinetics, and thermodynamics were studied. The Langmuir model described the experimental batch and column modes, while the Thomas and pseudo-second-order kinetics models predicted continuous and batch adsorption, respectively. The maximum uptake quantity toward Cr(VI) was 236.8 mg g–1 in batch mode and 308.4 mg g–1 in column mode at 303 K. There is some selective adsorption for Cr (VI) with some dyes coexisting in solution. Electrostatic interactions and the reduction process mediated Cr(VI) adsorption by ACWNS. The mass transfer potential (B) and the affinity between ACWNS and Cr(VI) (β) were computed using the modified mass transfer factor model. Since the attraction of Cr (VI) to ACWNS is more robust, the B value increases as the concentration increases. Again, MTZ assessment reveals that as MTZ length rises, diffusion resistance increases, resulting in increased adsorption. The adsorbent demonstrated improved regeneration capacity and therefore showed potential for the uptake of Cr(VI) ions from wastewater. Availability of data and materialThe dataset produced and processed during this research is available upon reasonable request from the corresponding author.

Alleviating the adverse effects of deficit irrigation in Flame seedless grapevine via Paulsen interstock

Abstract Using interstock with a potential genetic base is considered more recent and sustainable strategy for mitigating the water deficit. This investigation was carried out on transplant of Flame seedless (Vitis vinifera) grapevine grafted onto two rootstocks namely; Freedom (Vitis champinii x 1613C) and 1103Paulsen (vitis berlandieri x Vitis rupestris) with or without 1103Paulsen as interstock to determine its performance under deficit irrigation condition (50% of field capacity). The results indicated that Paulsen as rootstock or as interstock significantly increased the growth vigor of Flame seedless scion as well as the leaf content of total proline, phenols and sugars. Paulsen rootstock has decreased stomatal conductance, leaf transpiration rate and increased diffusion resistance under 50% deficit irrigation compared with grafting on Freedom rootstock. Moreover, Paulsen as interstock for Flame seedless grafted onto Freedom rootstock significantly increased relative water content accompanied by an increase in thickness of leaf anatomical characters such as midvein, lamina, palisade, xylem and phloem tissue under deficit irrigation compared with grafts without Paulsen interstock. This study suggests that using Paulsen as interstock, can be an adaptation strategy for water stress through controlling in some morphological, chemical physiological and anatomical responses of scion.

External resistance acclimation regulates bio-anode: new perspective from biofilm structure and its correlation with anode performance.

External resistance is important for the anode and cell performance. However, little attentions were paid on the effect of external resistance on the variation of biofilm structure. Here, we used external resistance ranged from 4000 to 500 Ω for anodic acclimation to investigate the correlation between anode performance and biofilm structure. With the reduce of external resistance, the maximum current density of anode increased from 1.0 to 3.4 A/m2, which was resulted from a comprehensive effect of reduced charge transfer resistance and increased diffusion resistance. Biological analysis showed that with the reduce of external resistance, biomass and extracellular polymeric substances content increased by 109 and 286%, cell viability increased by 22.7%, which contributed to the reduced charge transfer resistance. But the porosity of anodic biofilm decreased by 27.8%, which led to an increased diffusion resistance of H+. This work provided a clear correlation between the electrochemical performance and biofilm structure.

Features of isothermal crystallization of ammonium sulphate

Soil fertility is rapidly decreasing due to irrational land use, erosion and soil pollution and adverse weather conditions. The use of composite granular organic-mineral fertilizers that contain mineral and organic components allows the soil fertility to be increased in the most effective and safe manner for the environment. In the production of granular composite fertilizers in a fluidized bed granulator, the process of isothermal mass crystallization is the main parameter that affects the quality.
 The effect of impurities on the morphological structure of ammonium sulfate microcrystals during mass crystallization on the surface was investigated by means of microscopic analysis. The influence of temperature and pH of the solution on the structure and shape of the microcrystal layer was studied.
 The process of mass crystallization depends on the presence of impurities, pH and temperature. Given the fact that there is no analytical or theoretical dependence describing the influence of impurities on the morphological structure, the influence is determined experimentally. The presence of even a small content of surfactants and other impurities in the ammonium sulfate solution has a significant effect on the morphological structure of ammonium sulfate microcrystals, which is revealed as reduction in the concentration of impurities in ammonium sulfate obtained by coke-chemical method. The pH level of the medium affects the structure and shape of microcrystals: smaller crystals with a layered structure are formed in the case of weakly alkaline medium with pH = 8 and smaller microcrystals with a granular structure are formed in the case of pH = 4. Therefore, it is important to maintain the required pH level of the medium during the mass crystallization of ammonium sulfate.
 In isothermal mass crystallization of saturated aqueous solutions of ammonium sulfate with humic compounds and bone meal proceeding on a surface with temperature 90 °C, small elongated crystals with microcrystal sizes from 10 to 100 μm are formed. Impurities of bone meal and humic compounds in the form of inclusions are placed between the crystals of ammonium sulfate: namely, the endosegregation of impurities in the microcrystalline framework is observed. As the concentration of organic matter in the solution increases, which crystallizes as droplets on the surface, rubble and dendrites are formed outside the initial droplet placement. This phenomenon is explained by the increase of diffusion resistance at the center of the drop; as a result, ammonium sulfate crystallizes in the zone with lower resistance.
 The process of isothermal mass crystallization of saturated aqueous solutions of ammonium sulfate with admixtures of humic compounds, bone meal and other target components in the specified ratios will create new highly effective fertilizers. The obtained results allow formulating theoretical bases for the formation of solid structures based on ammonium sulfate with admixtures of organic and mineral components.

Fundamental understanding on the effect of Cr on corrosion resistance of weathering steel in simulated tropical marine atmosphere

The role of Cr on the corrosion resistance of weathering steel in simulated tropical marine atmosphere was investigated by the dry/wet cyclic corrosion test. The addition of Cr in steel greatly promotes the transformation of α-FeOOH. Cr-doped α-(Fe,Cr)OOH and FeCr2O4 present spherical particles of about ten nanometers, improving the compactness of the rust layer. Cr forms Cr2O3 and Cr(OH)3 with good corrosion resistance, and promotes the enrichment of Cu to form CuO in the inner rust layer. These products increase the nucleation position and effectively refine the rust grains, leading to the increase of diffusion resistance for corrosive species.

Revealing the structural degradation mechanism of the Ni-rich cathode surface: How thick is the surface?

Improving the cycling performance of Ni-rich LiNixCoyMnzO2 (NCM, 0 ≤ x,y,z < 1) is critical for commercializing rechargeable batteries based on Ni-rich NCM cathodes. Herein, we studied the structural degradation of Ni-rich NCM/graphite cylindrical 18650-type cells as a function of the cutoff voltage in the 4.2–4.4 V range by electrochemical impedance spectroscopy (EIS), scanning transmission electron microscopy–electron energy loss spectroscopy (STEM–EELS), and high-angle annular dark-field (HAADF) STEM, and modeled the Ni-rich NCM surface using density functional theory (DFT). We verified that the phase changes continuously rather than discretely from the surface into the bulk through cation mixing. Our results suggest that the thickness of the phase-change region at the surface causes the battery performance to suddenly degrade at a certain value. We found that the deterioration in cell performance is mainly due to increasing diffusion resistance in the positive electrode. A 10–25 nm cation mixing layer was observed at the cathode surface after 300 cycles, and this surface layer thickened with increasing charging voltage. Further, simulations revealed that the cathode surface spontaneously evolves oxygen at higher electrochemical potentials.

Structural and functional relationships of activated char briquettes from pyrolysis of sewage sludge for methylene blue removal

Activated char briquettes from pyrolysis residue of sewage sludge were prepared and characterized for methylene blue removal. The briquettes were prepared with different binder (phenolic resin) content (15–25 wt%), compression pressure (2.5–7.5 MPa) and thickness (3.5–14 mm), followed by CO2 or H2O activation under different temperatures. The effects of these parameters on the compositions, surface properties, apparent densities, mechanical properties, methylene blue adsorption kinetics and isotherms were systematically investigated. The results indicated that all of the prepared briquettes showed a characteristic of hierarchical porous structures, as well as high mechanical performance with impact resistant index much higher than 50. The increase of binder content from 15 to 25 wt% can result in marked increase of surface area (SBET), total pore volume (VT), apparent density and mechanical performance of the briquettes. According to the adsorption kinetics of methylene blue, the highest equilibrium adsorption capacity (qe,exp) for CO2-activated and H2O-activated briquettes were 194.6 and 203.6 mg/g. Both the increase of binder content and activation temperature markedly enhanced qe,exp, due to the increase of SBET and VT. qe,exp was much lowered by increasing the compression pressure or the thickness of briquettes, owing to the increased diffusion resistance and blockage of diffusion pathways inside briquettes. The adsorption kinetics data of the briquettes fit well into a pseudo-first order model system, suggesting physical adsorption of methylene blue onto briquettes surface. The adsorption isotherms of methylene blue indicated that both the CO2-activated and the H2O-activated briquettes followed a Langmuir isotherm model. The leaching tests of briquettes revealed that the leaching concentrations of Cd, Cr, Cu, Pb, Ni and Zn were much lower than the Chinese standard limits.

A numerical study on gasification of a single-pore char particle in supercritical water

Gasification models of a char particle based on the true porous structure are essential for the accurate simulation of gasifiers, and pore-scale study might provide important information for the development of the porous char particle gasification models. In this paper, a numerical study was conducted on the gasification of a single-pore char particle in supercritical water, and the emphasis was put on the gasification process inside the pore with the effects of surrounding fluid, pore structure and pore position considered. The results showed that the gasification in a pore was quite affected by pore diffusion. The increase in temperature and particle Reynolds number promoted the gasification in the pore, and convection mainly enhanced the heat transfer but had limited promotion on mass transfer in kinetically controlled regime. Increasing pore length and decreasing pore diameter caused the increase in diffusion resistance and the former had more obvious effects. However, the decreased pore diameter increased the specific surface area and benefited the whole char conversion. The pore position affected the species distribution inside the pore for non-diffusive gasification, and the impact was limited in kinetically controlled regime. Finally, study in this work will be further extended to the gasification of the porous char particle.

Dependence of deposition method on the molecular structure and stability of organosilanes revealed from degrafting by tetrabutylammonium fluoride.

We probe the structure of self-assembled monolayers (SAMs) comprising organosilanes deposited on flat silica-based surfaces prepared by liquid and vapor deposition by removing the organosilane molecules gradually from the underlying substrate via tetrabutylammonium fluoride (TBAF). Removal of organosilanes from the surface involves the cleavage of all pertinent Si-O bonds that anchor the organosilane molecules to the SAM, i.e., direct organosilane-surface linkages and in-plane crosslinks between neighboring organosilanes. We gain insight into the organosilane structure and stability by monitoring the organosilane density as a function of exposure time to TBAF. Degrafting of trifunctional chloro- and methoxy-alkylsilanes deposited from solution yields similar degrafting kinetics. We observe fast degrafting for organosilane SAMs deposited from the vapor phase, indicating that SAMs prepared in this manner form more loosely packed arrays, with less in-plane connectivity, compared to their solution-deposited counterparts. Bulkier, fluorinated silanes form more stable SAMs due to their ability to readily align and form a network with few aggregates and a relatively high fraction of surface bonds. The addition of a polymer brush to an anchored organosilane molecule demonstrates that increased bond tension accelerates the degrafting process despite the increased diffusion resistance.

Shell-Side Condensation Characteristics of R410A on Horizontal Enhanced Tubes

Abstract An experimental investigation into heat transfer characteristics during condensation on two horizontal enhanced tubes (EHTs) was conducted. All the tested EHTs s have similar geometries with an outer diameter of 12.7 mm, and a plain tube was also tested for comparison. Investigated enhanced surfaces consist of dimples, protrusions, and grooves, which may produce more flow turbulence and enhanced the liquid drainage effect. The effects of mass fluxes and vapor quality were compared and analyzed. Test conditions were as follows: saturation temperature fixed at 45 °C, mass flux varying from 100 to 200 kg m−2 s−1, and vapor quality ranging from 0.3 to 0.8. The heat transfer coefficient was presented, and the results show that the proposed enhanced surfaces seem to have worse performance than the conventional tubes when the mass flux is less than 150 kg m−2 s−1, while one of the enhanced tubes (2EHT-1) produce an enhanced ratio of 1.03–1.14 when G = 200 kg m−2 s−1. Besides, it was found that the heat transfer coefficient increases with increasing vapor quality, which can be attributed to the increasing diffusion resistance. Mass flux seems to have little effect on the heat transfer performance of smooth tubes, while that of 1EHT increases obviously with increasing mass flux, especially for high vapor qualities.

Biomass ash stabilized MgO adsorbents for CO2 capture application

Supported MgO adsorbents were prepared from the calcination of MgCl2·6H2O preloaded on several biomass wastes including sugarcane bagasse, coffee grounds, rice husk, and saw dust. CO2 adsorption behaviors of the adsorbents with different supports and MgO loadings were investigated using a fixed-bed reactor. The modified Avrami fractional kinetic model was adopted to correlate their CO2 uptakes to evaluate the CO2 adsorption kinetic performance. Amongst the prepared MgO adsorbents, the rice husk ash supported sample (MgO-RHA) featured high CO2 adsorption capacity, due to the good textural properties, nano-crystallization of MgO particles, the uniform dispersion of active components and the enriched surface basicity. CO2 uptakes of MgO-RHA increased first and then decreased with the increasing MgO loading. CO2 adsorption kinetic would be hampered by higher MgO contents because of the increased diffusion resistance and decreased MgO utilization. The adsorbent with 20 wt% MgO loading exhibited the highest CO2 uptake of 4.56 mmol CO2/g. Besides, the desired adsorbent presented good working stability with 7.68% loss-in-capacity in 10 repeated cycles. Recycling waste MgCl2·6H2O and biomass residues to synthesize CO2 adsorbents provides remarkable economic and environmental implications, from the prospective of CO2 emission mitigation and waste management.

Evaluation of Salt Tolerance and Its Relationship with Carbon Isotope Discrimination and Physiological Parameters of Barley Genotypes

ABSTRACTSoil management through the cultivation of salt-tolerant plants is a practical approach to combat soil salinization. In this study, salt tolerance of 35 barley (Hordeum vulgare L.) genotypes was tested at four salinity levels (0, 100, 200, and 300 mM NaCl in Hoagland nutrient solution) at two growth stages (germination and vegetative). The relationship between salinity tolerance and carbon isotope discrimination (CID) was also accessed. Results of the study carried out under laboratory conditions showed that a negative linear relationship was observed between salt concentration and germination as well as other growth parameters. Some genotypes showed good salt tolerance at germination but failed to survive at seedling stage. However, five genotypes, namely, Jau-83, Pk-30109, Pk-30118, 57/2D, and Akermanns Bavaria showed better tolerance to salinity (200 mM) both at germination and at vegetative growth stage. The salt tolerance of these barley genotypes was significantly correlated with minimum decrease in K+:Na+ ratio in plant tissue with increase in the root zone salinity. However, the case was reversed in sensitive genotypes. CID was decreased linearly with increase in root zone salinity. However, salt-tolerant genotypes maintained their turgor by osmotic adjustment and by minimum increase in diffusive resistance and showed minimum reduction in CID (Δ) with gradual increase in rooting medium salt concentration. Results suggested that the tolerant genotypes make osmotic adjustments by selective uptake of K+ and by maintaining a higher K+:Na+ ratio in leaves. Moreover, CID technique can also be good criteria for screening of salt-tolerant germplasm.

Experimental and Modeling Study on the Effect of Shale Composition and Pressure on Methane Diffusivity

In this study, we present a comprehensive experimental and modeling study on six shale samples to investigate the effect of shale composition and pressure on methane diffusivity. By correlating shale composition, pressure, and methane diffusivity, it was found that both macro- and micropore diffusivity decreased with increasing pressure, while total organic carbon content had little effect on macropore diffusivity and negatively affected micropore diffusivity. This phenomenon may be a result of nonporous organic matter (OM) in transitional shale acting as solid material instead of porous media, occupying micropore volume, prolonging gas diffusion length, and increasing diffusion resistance. Clay minerals with connected microstructures positively affect micropore diffusivity while negatively affecting macropore diffusivity, which may be a result of the filling of macropores with clay particles, blocking gas diffusion pathways, and increasing gas diffusion resistance. Brittle minerals have a positive effect...

Theoretical modeling and simulation of AGMD and LGMD desalination processes using a composite membrane

Most previous studies of air- and liquid-gap membrane distillation (AGMD and LGMD) processes using a composite membrane have been focused on an experimental approach. In this paper, rigorous theoretical investigations of the AGMD and LGMD processes were performed with a flat sheet type module using a composite membrane comprised of a polytetrafluoroethylene (PTFE) active layer and a polypropylene (PP) support layer. The model predictions were verified by comparing with measured data, where good agreement between the prediction results and experimental data was obtained. It was observed that as the gap size increased the AGMD permeate flux decreased exponentially with increased diffusion resistance. On the other hand, the LGMD permeate flux decreased exponentially and then increased asymptotically after attaining a minimum at a certain liquid-gap size (5–7 mm). This phenomenon was due to the onset and enhancement of a natural convection, resulting in an improvement in heat and mass transfer in the liquid gap.

Reaction and Mass Transport Simulation of Polymer Electrolyte Fuel Cell for the Analysis of the Key Factors Affecting the Output Performance in the Catalyst Layer

Polymer electrolyte fuel cell (PEFC) has been developed as a power source for fuel cell vehicle (FCV). To have FCV used more commonly, it is required to reduce PEFC system cost[1]. Increasing output density is effective measures for cost reduction because it is necessary to reduce total cost of PEFC in addition to quantity of platinum catalyst. In recent years, high performance cathode catalyst layers (CLs) are proposed, which made contrivances to its catalyst structure[2-3]. However, the method for developing catalyst is trial and error and there is no clear design guideline for further improvement. The purpose of this study is to propose the optimum design of cathode catalyst (Pt/CB) by simulating the electrochemical reaction and PEFC transport phenomena. In this study, based on the our previous research[4], diffusion phenomenon in the catalyst is newly reflected on the 1D model to compare catalyst structures. Fig.1 shows the image of transport phenomena in the catalyst pore. As shown in the structure (A), if Pt is covered with ionomer, oxygen reduction reaction (ORR) rate per unit Pt surface area is reduced by sulfonic group in ionomer. Thus it is assumed that if Pt exists in the pore as shown in the structure (B), ORR rate increases compared with structure (A) because there is no sulfonic group in the pore. However, oxygen concentration on the surface of Pt decreases compared with structure (A) because oxygen diffusion resistance increases. In this research, reflecting this trade-off between ORR rate and diffusion resistance, we constructed the model to compare catalyst structures. Based on the porous electrode theory[4], ORR and mass transport at the fuel cell evaluation test were simulated. In this research, generated water was considered as vapor, carbon corrosion reaction and suppress elution of platinum were neglected. The temperature profile in the cell was presumed to be uniform (80 ℃). Fig.2 shows some catalyst structures for simulation. Spherical diameter of carbon black (CB) and Pt catalyst carrying rate of CB are same in all structures. Assuming that pores of the porous CB is filled with the generated water. In addition, diffusion coefficient of GDL and MPL are determined by the experimental correlation equation for porous structures of PEFC. Each structure was simulated under various exchange current density. And oxygen distribution and relative reaction ratio were investigated in the catalyst. Fig. 3 shows the current-voltage characteristic curves obtained from the simulation (Exchange current density in the pore is five times as large as covered with ionomer.) Str.2-6 has larger output density at low current density region. In this case, this result shows porous carbon black has larger output density than Str.1 for PEFC because practical voltage region is 0.6 V - 0.8 V. Fig.4 shows the radial relative reaction rate in the pore.Reaction rate decreasing for the center direction, we were able to reflect oxygen diffusion resistance and compare catalyst structures. Furthermore, we attempt to separate factors of performance degradation of fuel cell and evaluate each factors quantitatively to confirm the appropriateness of the model.

Thermal Stability Study of Dye-Sensitized Solar Cells with Cobalt Bipyridyl–based Electrolytes

Dye-sensitized solar cells (DSSCs) with cobalt bipyridyl–based electrolytes can display higher solar cell performance than their iodide/triiodide counterpart. There is, however, little knowledge on their long term stability, which is a crucial aspect for potential commercial application. Herein, we studied the thermal stability of DSSCs using Co(bpy)32+/3+ redox electrolyte at 70°C in the dark for 50 days, combining 3 different additives, 4-tert-butylpyridine (TBP), 1-methylimidazole (MBI) and 2,2′-bipyridyl (BPY), in a nonvolatile solvent 3-methoxypropionitrile (MPN). Significant voltage decreases were found for all the studied solar cells, with a mechanism involving both a positive shift of the conduction band edge potential of TiO2 and a decreased electron lifetime, characterized by time resolved transient modulation techniques. Furthermore electrochemical impedance spectroscopy and differential pulse voltammetry studies indicate that the stability of Co(bpy)33+ is limited, causing an increased diffusion resistance in the electrolyte, but, surprisingly, no substantial change of the short-circuit current density (Jsc) in the devices. Overall, the DSSCs fabricated with the addition of both MBI and BPY in the electrolyte show the highest stability, maintaining 96% of its initial efficiency after 50 days, resulting from the overall compensation effects between the open circuit voltage decrease and the Jsc increase. These results provide insights about the degradation mechanism and emphasize the importance of the stability of TiO2/dye/electrolyte interface for the device stability under thermal stress.