Gas Diffusion Resistance Research Articles

Introduction of mesopores effectively enhances the accessibility of volatile organic compounds within the micropores of covalent triazine frameworks

Porous organic polymers (POPs) with abundant pores have great potential for the treatment of volatile organic compounds (VOCs). However, narrow micropores increase gas diffusion resistance and limit the accessibility of VOCs within the micropores. Here, we have synthesized microporous covalent triazine frameworks (mpCTFs) with abundant mesopores using nano-silica. And the effects of the introduced mesopore structure on the sorption performances of CTF were further investigated by 1,2-dichloroethane adsorption. The results indicate that mpCTF12.5 has the most suitable mesopore volume, and its specific surface area and total pore volume are 2.40 and 2.17 times that of CTF, respectively. Meanwhile, the mpCTF12.5 exhibited excellent adsorption performance for methanol, ethanol, acetone, benzene, ethyl acetate, 1,2-dichloroethane and toluene due to the multiple C-H…O, C-H…Cl, O-H…N and C-H…π interactions between VOCs and mpCTFX framework, demonstrated via DFT calculations. Additionally, the two-component gas-competitive adsorption behavior and the impact of water vapor on the adsorption performance of mpCTFX were investigated, with the adsorption mechanism thoroughly analyzed using the GCMC model. The introduction of abundant mesopores undoubtedly shortened the diffusion paths within the micropores of CTFs, significantly enhancing their accessibility for VOC adsorption. This study presents a viable strategy to improve the performance of porous materials in VOC adsorption applications.

Macroporous Structures of Nb-SnO2 Particles as a Catalyst Support Induce High Porosity and Performance in Polymer Electrolyte Fuel Cell Catalyst Layers.

Macroporous niobium-doped tin oxide (NTO) is introduced as a robust alternative to conventional carbon-based catalyst supports to improve the durability and performance of polymer electrolyte fuel cells (PEFCs). Metal oxides like NTO are more stable than carbon under PEFC operational conditions, but they can compromise gas diffusion and water management because of their denser structures. To address this tradeoff, we synthesized macroporous NTO particles using a flame-assisted spray-drying technique employing poly(methyl methacrylate) as a templating agent. X-ray diffraction analysis and scanning electron microscopy confirmed the preservation of crystallinity and revealed a macroporous morphology with larger pore volumes and diameters than those in flame-made NTO nanoparticles, as revealed by mercury porosimetry. The macroporous NTO particles exhibited enhanced maximum current density and reduced gas diffusion resistance relative to commercial carbon supports. Our findings establish a foundation for integrating macroporous NTO structures into PEFCs to optimize durability and performance.

Enhancing multi-channel electrode structures in ceramic fuel cells: Influence of channel geometry on mass transport and electrochemical performance

Solid oxide fuel cell (SOFC) is a promising electrochemical power generation technology that contributes to the decarbonization of the energy field. The electrode-supported design enables the use of thinner electrolytes, but inevitably brings additional gas diffusion resistance, which in turn leads to higher anodic concentration polarization. Therefore, additional structural design of the electrodes is necessary to ensure efficient operation and mass transport resistance minimization. In this study, anode-supported micro-tubular SOFCs with different multi-channel geometries have been attempted, during which samples with 4 channels exhibit unique elliptical structure due to meticulous manipulation. In addition, the 4-channel samples have been compared to the single-channel and 7-channel counterparts, exploring the influential mechanism of the anode micro-structure on gas transfer and electrochemical performances. The study found that the 4-channel cell has the best geometric controllability, with its minimum gas diffusion path reduced to below 50 μm, such thin electrochemical active region (EAR) accounts for >50 % of the entire external surface. Efficient utilization of the geometric surface area considerably facilitates the fuel transport to reach anode/electrolyte interface. By enhancing the mass transport efficiency, the maximum power density of the 4-channel cell reached 1.40 W cm−2 (750 °C), which is 22.8 % and 102.9 % higher than that of the 7-channel and single-channel cells under the same conditions. The highly efficient anode design additionally exhibited exceptional structural integrity and mechanical robustness, thereby ensuring the steady operation of the cell.

Degradation of solid oxide fuel cell anodes by the deposition of potassium compounds

Alkali contents with low melting points in the ash of woody biomass vaporize during the biomass gasification process, damaging various downstream energy conversion devices, such as the solid oxide fuel cells (SOFCs). In this study, the degradation of SOFC anodes by the deposition of potassium compounds (KCl, K2CO3, and KOH) was investigated. An aqueous solution of potassium compounds was dripped onto the anode surface of the SOFC button cell at room temperature. After drying at 343 K, 6.964 × 10-6 mol KCl, 6.964 × 10-6 mol KOH, and 3.482 × 10-6 mol K2CO3 was deposited on the anode. Button cells with the deposition of K compounds were employed for power generation experiments at 1023 K with the supply of artificial syngas from biomass gasification. After the power generation experiments, the surface structures of the anodes were microscopically analyzed using the SEM and EDS. As a result, K compounds hardly affected the OCV of SOFC. With the addition of KCl, no apparent change in the anode structure was observed, and only a slight KCl deposit was detected. However, chloride tends to be chemisorbed on Ni, increasing the ohmic resistance as well as the adsorption/desorption resistance. However, KOH transformed to K2CO3 and then remained massively on the anode, which was clearly observed in the SEM images. K2CO3 significantly decreased the cell voltage under a current density of 100 mA·cm−2. Through impedance analyses, this voltage drop was mainly attributed to the ohmic resistance and gas diffusion resistance. However, there is no evidence that this deposit degrades Ni particles.

The Porous Structure Design of Catalyst Layer by Controlling Particle Size Distribution of PEMFC Catalyst Ink

A general catalyst ink of proton exchange membrane fuel cells(PEMFC) consists of a catalyst, its support, an ionomer, and a solvent. These elements are made in the form of membrane electrode assembly(MEA) through grinding, dispersing, and coating processes. Therefore, the preparation of catalyst ink is an important step that directly affects the structure formation of the catalyst layer and the cell performance. In this study, by controlling the particle size distribution of the catalyst ink, we aim to achieve optimized pore structure in the catalyst layer that leads to improved performance of the fuel cell. Particle size and its distribution of the catalyst ink were confirmed by using particle size analyzer(PSA). Mercury intrusion porosimetry(MIP) was used to determine porous structure, including the pore diameter and total pore volume of MEA. Segmented tomographic evaluation is utilized to evaluate the 3D porous carbon structure in terms of local surface area, pore size distribution, and their 3D networking. MEA were tested to ascertain the influence of the catalyst ink properties on electrochemical character such as H2/air polarization curves, electrochemical impedance spectroscopy(EIS). Also, to analyze the resistance contributed by the molecular and Knudsen diffusion and permeation through the ionomer film, we tested MEA using limiting-current measurements. As a result, We are able to confirm that the particle size has a high correlation with the pore characteristics of the electrode layer that affect the cell performance. The enhanced electrochemical performance is attributed by decreasing fine particles about 0.1um in size. We figured out that the gas diffusion resistance in the catalyst layer increases as the fine particles increase. The results of this study will provide important insights into the design and fabrication of the catalyst layers in PEMFCs and could potentially lead to improvements in the efficiency of fuel cells.

Self-Supporting Microporous Layer for Polymer Electrolyte Fuel Cells

Introduction Improving power density is desired in research and development of polymer electrolyte fuel cells (PEFC) [1-5]. A reduction of gas diffusion resistance by down-thinning the gas diffusion layer (GDL) less than 50 μm is desirable [2]. Whilst both the electrocatalyst layers and electrolyte membrane are around 10 μm, the GDL is still relatively thick at about 150 μm [1]. The previous study investigated the possibility and technical issues in using high-strength and thin, porous metallic GDLs [3]. Based on such previous studies, this study aims to develop a self-supporting thin MPL/GDL with improved power density by creating conductive paths and reducing contact resistance with microporous layer (MPL) deposited on various GDLs. Experimental In this study, MPL was screen-printed on various mesh GDLs as substrates. Cells were fabricated by using these materials, and current-voltage characteristics and microstructural observations were performed. Stainless steel (SUS316 977mesh, 28 μm thick), titanium (200mesh, 152 μm thick), and carbon fiber (45 μm thick) were used as mesh GDL materials. The MPLs were composed of carbon black, carbon nanotubes, polytetrafluoroethylene (PTFE), and solvent (a mixture of polyethylene glycol 600 and pure water), which were mixed, screen printed, and heat treated at 300°C for 30 min to fabricate the MPL/GDL. Conventional materials such as Pt/C (TEC10E50E, Tanaka Kikinzoku, Japan) were used for preparing membrane-electrode-assemblies (MEAs). In the electrochemical characterization of this study, current-voltage characteristics were evaluated using an electrochemical impedance analyzer (SAS SP-240, Bio-Logic Science Instruments, France), and various overvoltages were separated. Results and discussion Figure 1 shows the current-voltage characteristics of cells using various MPL/GDLs. The thickness of MPL/GDLs fabricated was ranging from 62 to 180 μm. For comparison, the characteristics of a cell using a standard material, commercially available MPL/GDL (22BB, SGL Carbon, Germany), are also shown. The results indicate that the use of any kind of these MPLs improves cell performance. The electrochemical performance with metallic GDLs was considerably improved by applying MPLs. In particular, GDLs with metallic meshes made of SUS316 show a significant improvement in current-voltage characteristics by depositing MPLs. The activation overvoltage and ohmic overvoltage were comparable to those of 22BB, the commercial GDL/MPL. This is because the contact resistance between the GDL and the catalyst layer is reduced by the carbon component in the MPL, and the electrical conductivity is improved. The concentration overvoltage was also suppressed by depositing the MPL. The cell performance with MPL3/SUS316 977mesh was comparable to that of 22BB. In the future, we aim to further reduce the concentration overvoltage by optimizing e.g., the content of hydrophilic components in MPLs. Acknowledgment This paper is based on results obtained from a project, JPNP20003, commissioned 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,M. Lyth, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 169, 044523, (2022).New Energy and Industrial Technology Development Organization (NEDO), Roadmap of Fuel Cells for Heavy-Duty Vehicles, 48, (2022), (in Japanese), https://www.nedo.go.jp/content/100944011.pdf Yamamoto, M. Yasutake, Z. Noda, S. M. Lyth, J. Matsuda, M. Nishihara, A. Hayashi, and K. Sasaki, ECS Trans., 109 (9),265 (2022).Larminie and A. Dicks, Fuel Cell Systems Explained, 2nd ed., John Wiley & Sons, England, 2003.Kitahara, T. Konomi, H. Nakajima, and J. Shiraishi, Kikai B, 76 (761), 101, (2010). Figure 1

Concerns on the Effects of Electrode Positions in Electrolyte Container for the Oxygen Evolution Reaction.

Water electrolysis is currently a major technique to produce clean hydrogen, which is regarded as a promising and sustainable energy carrier. The efficiency of water electrolysis is highly dependent on the oxygen evolution reaction (OER) on the anode. The evaluation of an OER electrocatalyst is frequently carried out on a three-electrode system in a container of electrolyte. Herein, we found that the electrode positions in the electrolyte container could significantly affect the data acquisition of OER performance. After a detailed investigation, we reveal that the difference of the OER activity of an electrocatalyst at a different position is originated from their different iRu drop and the gas diffusion resistance. For the first time, this work evokes concerns on the accurate evaluation of electrocatalysts regarding the electrode position. For fair comparisons and reliable results, it is strongly suggested to keep the electrode position unchanged in the electrochemical measurements. In addition, the establishment of a standard electrolyzer setup for electrocatalysis evaluation in the electrochemical community is also called for.

High-performance solid oxide electrolysis cell with the ordered straight pores in the support

The large diffusion resistance of steam in the thick support layer is the main factor limiting the performance of solid oxide electrolysis cell (SOEC). This work demonstrates the importance of the straight pore diffusion channel in enhancing the performance of electrolysis cell. The Cell-A with the straight pore diffusion channel in the support and the Cell-B with randomly distributed pores in the support have been compared and the corresponding reaction mechanisms are elucidated. Under 50% absolute humidity (A.H), Cell-A achieves a very high current density of −4.30 A cm−2@1.5V and −2.37 A cm−2@1.3V at 800 °C. Even if under A.H = 10%, Cell-A also displays a high current density of −2.74A cm−2@1.5V, which is 6 times than that of Cell-B. Electrochemical impedance spectroscopy (EIS) measurements and analysis of distribution of relaxation times (DRT) reveal a significant reduction of the gas diffusion resistance on Cell-A. The rate-determining Cell-A is the diffusion of oxygen ion through the air electrode/electrolyte interface, and the gas diffusion is no longer the main factor limiting steam electrolysis.

Self-Supporting Microporous Layer for Polymer Electrolyte Fuel Cells

The gas diffusion layer (GDL) used in a PEFC is thicker than the electrode catalyst layer and electrolyte membrane. Thinning down the GDL can reduce gas diffusion resistance and volumetric power density of PEFC stacks. In this study, MPL/GDL is prepared by printing microporous layers (MPLs) on carbon meshes of several tens of micrometers thick as substrates for thin-layer GDLs. Through various current-voltage and overvoltage measurements and microstructural analysis of the cells using these thin-layer MPL/GDLs, cell performance has been improved, equivalent to that of the state of the MPL/GDL.

Subfreezing conversion of ALD-derived ZnO layer to ultra-thin ZIF-8 membrane for high-flux C3H6 production

ZIF-8 membrane is well-recognized for its capability in separating C3H6 from C3H6/C3H8 gas mixture. However, the low C3H6 permeance and inferior pressure resistance severely hinder its high-flux C3H6 production. In this study, ultra-thin and well-intergrown ZIF-8 membrane was fabricated through in situ subfreezing conversion of 10-cycle ALD-derived ZnO layer. Benefiting from the deposited 2 nm-thick ZnO layer and the inhibited ZIF-8 grain growth during subfreezing synthesis, the thickness of obtained ZIF-8 membrane was down to 180 nm, indicating low gas diffusion resistance; moreover, owing to well-intergrown grain boundary structure and slightly penetrated membrane layer, obtained ZIF-8 membrane was featured with enhanced pressure resistance. The ultra-thin thickness and strengthened structure endowed our membrane with unprecedented C3H6 flux (5.29 × 10−2 mol m−2 s−1) and decent C3H6/C3H8 separation factor (60.5) at feed pressure up to 7 bar, which was 1–2 orders of magnitude higher than state-of-the-art ZIF-8-based membranes and far beyond their performance limits under comparable conditions, rendering it a superb choice for high-yield C3H6 production.

Study of the Influences of Fuel Utilization on Efficiency and Anode Safety at Different Fuel Flow Rates for Industrial-Size Solid Oxide Fuel Cell

Solid oxide fuel cell (SOFC) is regarded as a promising energy conversion technology that can directly produce electricity from chemical fuels without combustion. Commercialization of SOFC systems has already started with residential and industrial applications, and demonstrations for a broader range of uses have been carried out. However, the widespread commercialization of SOFC systems requires further improvements both in efficiency and durability. The efficiency of a single cell is affected by fuel utilization and voltage. In addition, high fuel utilization may cause an increase in concentration polarization or lead to local Ni oxidation of cell anode. Therefore, it is imperative to study the influence of fuel utilization on the cell's performance, efficiency, and anode safety. In this work, I-V curves at different fuel flow rates were tested. The fuel utilization and voltage corresponding to the peak power density at different flow rates were determined. Based on the gas conversion and diffusion resistance models, the area specific resistance behavior and the fuel utilization corresponding to the beginning of the concentration polarization zone were investigated. Besides, the effects of fuel utilization and cell voltage on local Ni oxidation were also studied. The results indicated that at a low fuel flow rate of 0.1-0.3 L/min, the fuel utilization corresponding to the peak power density condition (which is also the condition of maximum efficiency) was about 90%. Although a significant increase in concentration polarization occurred under such high fuel utilization, no Ni oxidation occurred due to the high voltage. However, at a high fuel flow rate of 0.8-1.0 L/min, the fuel utilization corresponding to the peak power density condition was about 70%. The low voltage led to local Ni oxidation before entering the concentration polarization zone. This finding provides an idea for choosing fuel utilization at different fuel flow rates to realize high efficiency and anode safety.

Transient Nyquist Diagram for Fuel Cells

Transient Nyquist diagrams were calculated by significantly changing the load current of polymer electrolyte fuel cells (PEFCs) while a perturbation current of a constant amplitude of ±2 A (±0.04 A cm−2) was simultaneously applied to the load current. The frequencies used for the calculation ranged from approx. 0.38 Hz to approx. 10 kHz, the number of noise-reduced impedance points was 255, and the measurement time was approx. 2.6 s. Fast Fourier Transform (FFT) electrochemical impedance spectroscopy (EIS) was used to calculate the impedance, and the ergodic hypothesis was used for noise reduction. Instead of a time-averaged impedance, this method uses an ensemble average of impedances with different frequencies (quasi-ensemble-median). The load current was changed in the form of a step or ramp, or in a form equivalent to the US06 drive cycle. The obtained transient Nyquist diagrams were arc-shaped, similar to those obtained by the usual FRA method, and altering RH did not change their shape. For the step response and US06 drive cycle, a local increase in impedance was observed when the load current increased sharply. This is believed to be caused by an increase in gas diffusion resistance or charge transfer resistance.

Roles of lower-side leaf trichomes in diffusion resistance and gas-exchange characteristics across environmental gradients in Metrosideros polymorpha.

Leaf trichomes on the lower leaf surface are common in many plant species, especially those grown under dry and/or low-temperature conditions; however, their adaptive significance remains unclear. Lower-side leaf trichomes can directly decrease gas fluxes through increased gas-diffusion resistance but can indirectly increase gas fluxes through increased leaf temperature owing to increased heat-diffusion resistance. We examined whether the combined direct and indirect effects of trichome resistance increase photosynthetic rates and water-use efficiency (WUE) using Metrosideros polymorpha Gaud., which varies widely in the masses of lower-side non-glandular leaf trichomes across various environments on the Hawaiian Islands. We employed both field surveys, including ecophysiological measurements at five elevation sites, and simulation analyses to predict the gas-exchange rates of leaves with various trichome-layer thicknesses across a wide range of environmental conditions. Field surveys showed that the trichome-layer thickness was the largest at the coldest and driest site and the thinnest at the wettest site. Field surveys, experimental manipulations and simulation analyses demonstrated that leaf trichomes significantly increased leaf temperature owing to the increased heat resistance. Simulation analyses showed that the effect of leaf trichomes on heat resistance was much larger than that on gas-flux resistance. Leaf trichomes can increase daily photosynthesis only in cold dry areas by increasing the leaf temperature. However, the increased leaf temperature with leaf trichomes resulted in a consistent decrease in the daily WUE at all elevation sites. The magnitudes of trichome effects on gas-exchange rates were associated with the temperature difference across the elevational gradient, the strong light intensity in Hawaii, the leaf-size variation and the conservative stomatal behavior of M. polymorpha as well as the trichome-layer thickness. In summary, the lower-side leaf trichomes in M. polymorpha can be beneficial for carbon assimilation in low-temperature environments but not for water conservation in most environments in terms of diffusion resistance.

Effect of granulation on chlorine-release behavior during municipal solid waste incineration.

The preparation of refuse-derived fuel (RDF) is an effective and simple means of rural municipal solid waste utilization. The release of chlorine during RDF combustion is important as it causes high-temperature corrosion and pollutants emission such as HCl, dioxins, etc. In this paper, constant-temperature and increasing-temperature combustion experiments were carried out using an electrically heating furnace to analyse the effects of granulation (pressure and additives) on the release of chlorine in particles. During the constant-temperature combustion below 800 °C, only organic chlorine was released from the RDF. The increase of granulation pressure from 1 MPa to 10 MPa did not affect the total amount of chlorine release, but delayed the organic chlorine release by increasing the gas diffusion resistance. During the constant-temperature combustion above 900 °C, inorganic chlorine was released as well. The increase of granulation pressure enhanced the inorganic chlorine release significantly by promoting the reactants contact. During the increasing-temperature combustion, the increase of granulation pressure delayed the organic chlorine release as well but inhibited the inorganic chlorine release. This was mainly attributed to the slow temperature rise to 900 °C, during which the inherent calcium in the RDF reacted with silicon and aluminium, resulting in less reactants for an inorganic chlorine release reaction. Three calcium-based additives were used to inhibit chlorine release. CaCO3 showed no dechlorination effect, and CaO showed better dechlorination effect than Ca(OH)2. For the constant-temperature combustion at 900 °C, the addition of CaO with a Ca/Cl ratio of 2 achieved a dechlorination efficiency of over 90%, with little influence from the granulation pressure. For the increasing-temperature combustion, the granulation pressure had a significant influence on CaO dechlorination effectiveness. Only at a granulation pressure as high as 10 MPa, did the addition of CaO with the Ca/Cl ratio of 2.5 achieve a dechlorination efficiency of 95%.

Dimeric ionic liquid for improving performance and durability of PEMFCs

A dimeric ionic liquid (1,1'-(butane-1,4-diyl)bis9-methylpyrimido[1,2-a]pyrimidinium) di-nonafluorobutanesulfonate (D-MTBD) is investigated for surface modification to a PtCo/C catalyst for improving performance and durability of the catalyst in a fuel cell membrane electrode assembly (MEA). Compared to the PtCo/C catalyst without modification, the addition of the dimeric ionic liquid (IL) improves the mass activity (MA) of the PtCo/C catalyst in the MEA, as well as fuel cell performance. Moreover, the dimeric IL also improves the durability of the PtCo/C catalyst. Thermogravimetry and differential scanning calorimetry were used to evaluate the IL's thermal stability and melting point. The viscosity of the catalyst inks with and without IL were compared. Electrochemically active surface area, impedance, and gas diffusion resistance were evaluated and compared with the PtCo/C catalyst without IL modification. Owing to its higher molecular weight and melting point, the dimeric IL shows potential as a highly stable catalyst modifier. This work shows that the dimeric IL may serve as a replacement to regular monomeric ILs for catalyst surface modification.

A molecular level based parametric study of transport behavior in different polymer composite membranes for water vapor separation

The membrane dehumidification technology has great energy-saving potential compared to traditional methods. However, the design of composite membrane depends mostly on trial tests. To understand the mechanisms dominating material properties and quantitatively predict the air dehumidification performance of the composite membranes in practical applications, various models combined with different polymeric materials and porous support membranes were developed and investigated by using grand canonical Monte Carlo (GCMC) and Molecular dynamics (MD) simulation methods. The interfacial interactions between the selective layer and the support membrane were analyzed in detail to explore the interface stability and compatibility of various composite membranes. The physical characteristics (density, fractional free volume, solubility parameter and cohesive energy density) and transport properties (solubility, diffusivity, permeability and selectivity) of various composite membranes were parametrically evaluated. The polydimethylsiloxane (PDMS) composite membranes exhibited stronger interfacial interaction in comparison to the PVA composite membranes. The hydrophilicity and polarity of polyvinyl alcohol (PVA) polymer resulted in a stronger interaction between the gas molecules and the PVA membrane. According to the solution–diffusion mechanism, the PVA-PVDF membrane presented the optimal H2O permeability of 3121.38 Barrer among all composite membranes. Generally, polyvinylidene fluoride (PVDF) or polyacrylonitrile (PAN) as the materials of support membrane had good fiber forming characteristics and low gas diffusion resistance, which significantly affects the performance of selective layers. The microscopic mechanisms revealed in this work would lay a solid theoretical foundation for the design of high performance composite membrane for air dehumidification.

Investigation of a Method of Evaluating Proton Transport Resistance in PEFC Catalyst Layers

Since there is no method to directly measure proton conductivity in the catalyst layer in PEFC even though proton conduction in the catalyst layer is one of the critical factors for high performance and durability. A method to directly measure proton transport resistance in the catalyst layers was investigated by sandwiching a sample layer by two Nafion® membranes to block electron transfer, and two gas diffusion electrodes to reduce gas diffusion resistance, in this study. Direct current was applied to the system in the hydrogen atmosphere, and measurements for proton transport resistance were performed in various sample layers. As a result, proton transport resistance in sample layers was successfully evaluated, and such measurements lead to improvement of current-voltage performance of PEFC.

Improve the Activity and Stability of PtCo/C Catalyst by Ionic Liquid in Proton Exchange Membrane Fuel Cell

Ionic liquid ([MTBD][C4F9SO3]) has been explored as a surface modification of PtCo/C catalyst to improve its performance and durability in membrane electrode assembly (MEA). Compared to the pristine catalyst, the incorporation of ionic liquid (IL) increases the oxygen reduction reaction (ORR) activity of PtCo/C and reduces the gas and proton diffusion resistance of the cathode catalyst layer, resulting in considerably improved MEA performance. More importantly, IL largely slows down the ORR activity decay. The PtCo/C-IL outperformed PtCo/C throughout the 30,000 potential cycles test, which is consistent with higher active surface area retention and less Co dissolution of PtCo/C-IL. This work suggests that IL modification can serve as a complementary approach to the development of novel electrocatalysts and electrode structure.

Fractal microstructure effects on effective gas diffusivity of a nanoporous medium based on pore-scale numerical simulations with lattice Boltzmann method

The interaction between gas diffusion and complex microstructures of a nanoporous medium is important to gas diffusion in shale formation, but this interaction is complex and has not been fully understood. Those approaches based on fractal theory can well describe the fractal multiscale microstructure with simple boundaries. The lattice Boltzmann method (LBM) can consider the effects of complex microstructures at one scale. This study proposes a combination approach of fractal theory and LBM to simulate the gas diffusion process in fractal multiscale microstructures of a nanoporous medium. In this study, the gas diffusion in a complex microstructure at base scale is numerically simulated by the LBM and the gas diffusion in finer microstructures is described through a diffusion equation with a local gas diffusion coefficient. First, a microstructure of a nanoporous medium is reconstructed by a random reconstruction algorithm. Then, a local gas diffusion coefficient is proposed based on fractal theory to consider the effects of coupling molecular diffusion and Knudsen diffusion with the fractal structures of a nanoporous medium on the gas diffusion in finer microstructures. This local gas diffusion coefficient is validated by experimental data and introduced into the governing equation of gas diffusion. The LBM simulation is verified with experimental data and two other model results on the effective gas diffusivity of a nanoporous medium. Finally, key parameters for the effective gas diffusivity of a nanoporous medium are identified through sensitivity analysis. It is found that the effective gas diffusivity in a microstructure is higher for a smaller range of Knudsen number. Bigger pore diameter fractal dimension and smaller tortuosity fractal dimension represent lower gas diffusion resistance and have higher effective gas diffusivity. This combination approach provides a powerful tool to estimate the effective gas diffusivity in a complex nanoporous medium.

Optimization of La0.6Sr0.4CoO3-Δ (LSC) Cathode Layer for the High Electrochemical Performance

It is important to optimize the microstructure of the cathode layer to improve the electrocatalytic activity of the solid oxide fuel cells (SOFCs). In this study, the most recent LSC cathodes with various thicknesses in the temperature range of 650-750 oC were considered. This was achieved experimentally with symmetric cells with different cathode layer thicknesses of 6 – 55 µm. Prior to that, optimization of the sintering temperature and minimization of the cathode contact resistance were carried out. The cathode thickness was confirmed by electron microscopy (SEM), and electrochemical impedance spectroscopy was performed to measure the polarization resistance. In addition, the effect of thickness on the oxygen reduction reaction process was also considered. The polarization resistance decreases until the thickness of the cathode increases to 30 μm, and starts to increase at a thickness beyond that. In addition, the main contribution of charge transfer resistance at the electrolyte/cathode interface and the remarkable effect of gas diffusion resistance with cathode thickness were also highlighted.