Local Current Density Distribution Research Articles

A 1 + 1-D Multiphase Proton Exchange Membrane Fuel Cell Model for Real-Time Simulation

A 1 + 1-dimensional (1 + 1-D) multiphase model of proton exchange membrane fuel cell (PEMFC) is developed to simulate the dynamic behaviors of the fuel cell under various operating conditions. Electrochemical, fluidic, and thermal physical phenomena are considered, and the model can accurately describe the multiphysical processes inside the PEMFC. Meanwhile, the water phase change and nitrogen crossover are also considered in this model. The developed model can simulate the fuel cell behaviors in both normal temperature and cold start conditions. Selection of the time step size is discussed, and the solution method of the real-time model is proposed. Computational efficiency is greatly improved by simplifying the multicomponent diffusion inside the membrane electrode assembly (MEA), which results in the shorter calculation time than the simulated physical time. The proposed model is suitable for embedded applications, such as real-time simulation or online diagnostic control. The comprehensive model validation is carried out by comparing the simulation results, including polarization curves, ohmic resistance, local current density distribution, and voltage evolution, under various working conditions of steady state, load change, and cold start, and a good agreement is achieved. Finally, some simulation cases are studied to further demonstrate the simulation ability of the model.

J c and Composition Distribution of YBCO Coated Conductors Fabricated by the TFA-MOD Process

The Trifluoroacetate-Metal Organic Deposition (TFA-MOD) process is one of the most promising fabrication processes for low-cost superconducting coated conductors (CCs) because of its high yield and non-vacuum system. However, in the doping and multi-coating process for the enhancement of critical current (<i>I</i>_c), degradation of <i>I</i>_c often occurs. In this study, to clarify the origin of <i>I</i>_c degradation, we have investigated the local critical current density (<i>J</i>_c) distribution, in-field <i>J</i>_c properties and the variation of composition for TFA-MOD processed YBa₂Cu₃O_y (YBCO) CCs. The local <i>J</i>_c distributions were measured by Scanning Hall-Probe Microscopy for confirming the location of the high&#x002F;low-<i>J</i>_c regions. The CCs were then cut into 3 mm&#x00D7;3 mm or 3 mm&#x00D7;4 mm for dc magnetization measurement. The <i>T</i>_c of the all samples were about 87 K to 90 K. In-field <i>J</i>_c at 77 K was obtained from <i>M</i>-<i>H</i> measurements and all the samples were compared with each other. The in-field <i>J</i>_c of high-<i>J</i>_c sample showed about 100 times higher value than that of the low-<i>J</i>_c sample. Then we have observed the microchemistry and the variation of composition by SEM-EDS. In the high-<i>J</i>_c sample, the grains of the superconducting phase were large in size and were connected to each other. This means that superconducting current could form through the whole sample area and generate a large magnetic moment. On the other hand, in the low-<i>J</i>_c sample, though the superconducting phase could be observed, the grains were small in size and sparse. Therefore, the magnetic moment measured was small. However, it was also suggested that the intra-grain <i>J</i>_c would be almost the same as that of high-<i>J</i>_c sample.

Snowflake Bionic Flow Channel Design to Optimize the Pressure Drop and Flow Uniform of Proton Exchange Membrane Fuel Cells.

The flow channel design of bipolar plates plays a significant role in the proton exchange membrane fuel cells operation, particularly in thermal and water management. The pursuit of low-pressure drop supply and flow field uniformity in PEM fuel cells has not stopped, resulting in numerous new bipolar plate flow channel designs. The biomimetic leaf vein shape-based flow channel and lung flow channel designs can significantly improve gas supply uniformity and reduce pressure drop. Therefore, we propose a snowflake-shaped bionic channel design by integrating the advantages of the leaf vein shape and lung shape channel. A 3D multi-physics fuel cell model is used to verify the feasibility and superiority of the bionic snowflake design in improving fuel cell performance, especially in reducing the pumping work. The local pressure distribution, oxygen distribution, water distribution, and current density distribution are used to reveal the enhancement mechanism of the new snowflake flow channel. The flow uniformity is further enhanced by using multi-objective (13 target parameters) and multi-parameter (18 independent variables) genetic algorithm optimization. The general goal of this work is to provide a new strategy for the thermal and water management of PEM fuel cells.

Influence of Local Inhomogeneities in the REBCO Layer on the Mechanism of Quench Onset in 2G HTS Tapes

The resistive-type of superconducting fault current limiters (R-SFCL) using the second generation of high-temperature superconductors (2G HTS) are well adapted to power grid protection. Almost electrically invisible in normal operation, it quenches and becomes highly resistive in the event of a fault. The tape operation at the transition from the superconducting state to the resistive state is the basis of a R-SFCL. In this scope, we studied the onset of a quench occurring in 2G HTS tapes using high-speed imaging to record the bubble generation in liquid nitrogen. In the first milliseconds of operation, the bubble generation appears highly inhomogeneous. Dissipation is initiated on multiple spots over the tape surface, which first expands in the direction of the width of the tape and then along its length. A detailed analysis of the REBCO layer is performed by means of scanning Hall probe microscopy (SHPM) after the optical study of the tapes is completed. Analysis of the local current density distribution demonstrates that the bubble generation occurs at positions of local inhomogeneities in the REBCO layer. These inhomogeneities, well recognized through SHPM, are not reliably described by the critical current as a function of the position derived magnetically (e.g., by traditional TapeStar measurements). Similar results were obtained on samples from Superpower and SuperOx.

Two-Dimensional simulation of purge processes for dead-ended H2/O2 proton exchange membrane fuel cell

ABSTRACT Proton exchange membrane fuel cells (PEMFCs) with the dead-ended anode (DEA) and dead-ended cathode (DEC) can achieve high utilization of hydrogen and oxygen but also cause the accumulation of liquid water in the cell and performance degradation. In this study, a two-dimensional multiphase transient PEMFC model is developed to investigate the purge strategy of the PEMFC with DEA and DEC. This model is well validated with the experimental data. The simulation results show that the voltage increases to a certain extent at the beginning of the dead-ended operation and then decreases gradually. During the purging period, the voltage reaches a peak value and then decreases. Therefore, the optimal purge duration is defined as the purge stops when the voltage reaches the maximum value, and the optimal purge duration decreases with the increase of scavenging velocity and increases with the increase of voltage drop rate, however, the energy efficiency under the optimal purge duration is not the highest. Besides, compared with the purging anode and cathode operation, the peak value of the purging cathode operation decreases gradually. Compared with the counter-flow operation, the co-flow operation leads to earlier purging moment, larger ohmic resistance, and voltage decay rate. The excessive accumulation of liquid water leads to a more poor uniformity of the local current density distribution. The purging cathode and counter-flow operation is the most ideal purge strategy during short-time operation, due to the highest energy efficiency about 52.3%, the second-highest output performance, and the second-longest purge interval about 180.01 s.

Unraveling the mechanism of non-uniform zinc deposition in rechargeable zinc-based batteries with vertical orientation

• The non-uniform zinc deposition in the vertical direction is investigated. • Optical observations and stripping/platting model are combined in this work. • The non-uniform deposited zinc is started by nucleation site fracture. • The inhomogeneity is exacerbated by the non-uniform current and hydrogen evolution. Secondary zinc-based batteries demonstrate a bright prospect as a candidate for next-generation electrochemical energy storage devices. However, the inhomogeneous zinc deposition with vertical orientation during electrochemical stripping/platting cycles leads to battery failure quickly and thereby limits commercial applications of zinc-based batteries. In this work, the deposited electrochemical reaction is deeply investigated and the mechanism of inhomogeneous electrochemical deposition is proposed via in-situ optical observation and numerical analysis. During the deposited reaction, zinc atoms prefer to deposit on the nucleation sites on the electrode surface, changing the morphology of nucleation sites and leading to possible fractures. The fractural zinc fragments then accumulate at the bottom and connect to the bulk electrode, which initiate the inhomogeneous distribution in the vertical direction where the protrusion appears at the bottom, and the middle and top parts have rare deposited zinc. The inhomogeneity is exacerbated during stripping/platting reaction cycles due to the non-uniform local current density distribution on the emerging protrusion. Besides, the generated bubbles caused by hydrogen evolution exacerbate the inhomogeneous distribution via changing the ion distribution. Based on the conclusions, a uniform deposition behavior is achieved via modulating the ion distribution by the flowing electrolyte as a demonstration. The unraveled mechanisms can serve as a reference for the design of advanced zinc-based batteries with stable electrodes.

Experimental study of key operating parameters effects on the characteristics of proton exchange membrane fuel cell with anode recirculation

Water management is essential to ensure high performance and stable operation of the proton exchange membrane fuel cell (PEMFC). However, little is known about water management in the hydrogen circulation anode (HCA) mode which can achieve high hydrogen utilization. In this work, the effects of hydrogen circulation rate, a key parameter of HCA mode, on water management and cell performance are experimentally investigated. In particular, transparent fuel cell and printed circuit board segmented fuel cell technologies are used to study the water distribution and the local current density distribution, respectively. Furthermore, the electrochemical properties and PEMFC stability at different hydrogen circulation rates are discussed in detail. The experimental results indicate that increasing the hydrogen circulation rate reduces liquid water in the cathode and anode flow channels, which can effectively alleviate the adverse effects of flooding on overall performance and cell stability. However, subsequent drying of the membrane and catalyst layer results in an increase in ohmic resistance and a decline in cell performance, especially at low current densities. The current density distribution shows that severe local flooding is likely to occur in the hydrogen and air outlet regions at a low hydrogen circulation rate. For a high hydrogen circulation rate, membrane drying will occur in some areas, especially near the hydrogen inlet. To achieve uniform current density distribution and high fuel cell performance, the hydrogen circulation rate should be increased with the load current.

Operando X-ray absorption tomography for the characterization of lithium metal electrode morphology and heterogeneity in a liquid Li/S cell

Lithium/sulfur batteries have been under intense study for the last two decades, due to the high specific capacity promised by elemental sulfur as an active material. Sulfur is also cheap, abundant and non-toxic compared to conventional transition metal oxides. The Li/S electrochemical cell is traditionally composed of a sulfur-based electrode, an organic liquid electrolyte and a lithium metal electrode. Lithium metal technologies have received a renewed interest, due to the light weight and low operating voltage of the lithium electrode, which makes it the ultimate choice to be combined with high-energy positive electrode materials. In this paper, operando X-ray absorption and phase contrast tomography were employed to characterize the lithium metal electrode morphology and heterogeneity in a liquid Li/S cell. It was observed that oxidation of the lithium foil occurs in a heterogeneous fashion with pit formation from the beginning of discharge, with the pits growing in diameter, depth and quantity during the discharge. Using Faraday's law, the local current density distribution was calculated to confirm the strongly heterogeneous behavior of the lithium metal electrode. We demonstrate that decreasing the current density leads to lithium stripping/plating occurring more homogenously, with the formation of a smaller number of pits. • X-ray absorption tomography is a powerful tool to characterize the lithium. • Presence of large number of pits during lithium oxidation at high current density. • Determination of the local current density distribution. • Lithium heterogeneity starts at the beginning of the cell life.

3D Printing Architecting Reservoir‐Integrated Anode for Dendrite‐Free, Safe, and Durable Zn Batteries

AbstractThe growing demand for safe and renewable energy storage systems has driven the recent renaissance of Zn‐ion batteries (ZIBs). Nevertheless, the intrinsic drawbacks of inhomogeneous electric distribution and sluggish ion replenishment worsen the Zn dendrite issues that seriously impede their practical application. Herein, for the first time, a functional 3D printed reservoir‐integrated N‐doped carbon host (3DP‐NC) is designed to remodel the electric/ionic fields. The customized 3D printed structure equipped with regular micron‐sized holes induces reduced local current density and homogeneous electric distribution. The micron‐sized holes function as reservoirs to ensure unobstructed ion diffusion and quasi‐steady‐state ionic supplements. A N‐doping interfacial modification strategy is further employed to encourage a highly zincophilic surface, hence reducing the nucleation energy barrier and motivating uniform Zn nucleation. As a result, the Zn‐deposited 3DP‐NC electrode (3DP‐NC@Zn) affords dendrite‐free morphology and highly reversible Zn plating/stripping with an ultra‐small overpotential of 15.3 mV even at 10 mA cm−2. Additionally, these appealing features also endow the 3DP‐NC@Zn electrode with an outstanding lifespan over 380 h at 1 mA cm−2 and 1 mAh cm−2. The thrilling performance establishes a new roadmap that advances the development of dendrite‐free and durable metal batteries by exploiting this unique 3D printing technique.

Modeling and characterization of the biochar electrodes for vanadium redox flow battery

A three-dimensional transient model of a vanadium redox flow battery (VRFB) with biochar electrode was developed in this study. The model was built to simulate the microscopic particle accumulation that is applicable to the biochar electrode. The electrode assembly was constructed using the classical mass transfer theory of porous media. The charge/discharge curves, cell efficiency, local current density distribution and species concentration distribution at various moments of time were investigated using this model. The results showed that the particle accumulation model of biochar electrode is both a realistic and reasonable approximation of events in the electrode. Analysis of the charg/discharg process and electrochemical reaction rates in the cell found that when the average particle radius of 1 × 10−6 m, an electrode porosity of 0.3, an electrode thickness of 0.003 m and the average current density of 40 A/m2, the charging voltage of the battery was lower and the discharging voltage was higher.

(Invited) Current Density Distribution In a Pilot-Scale Electrodialysis Unit - 3D Mathematical Modeling and Experimental Investigation

Electrodialysis (ED) represents a matured and well-established desalination and membrane separation technology. In order to satisfy application demands, there is a trend for increasing dimensions of not only the industrial ED, but also of the other electrochemical membrane units, like electrolyzers, fuel cells etc.), capable of high process intensity and efficiency. These activities are driven mainly by cost reduction. This is also the case of reverse electrodialysis as an allied process, currently meeting rapidly growing popularity as a promising green energy source. However, experiences in a development of larger scale membrane electrochemical devices document scale-up from laboratory or pilot system to industrial ones not to be straightforward. The scale-up based on an direct increase of smaller devises dimensions, like e.g. increasing membrane active area or number of membranes without appropriate modifications of entire unit design, often leads to pronounced nonuniformity of local current density, mass and heat distribution inside the system. Consequently, the performance, reliability and lifetime of industrial scale apparatuses is significantly exacerbated. A uniformity of the above-mentioned fluxes distribution strongly depends on numerous aspects, particularly on the geometry of electrodes, manifolds and fluid distributors. Due to these facts, there is an urgent need of fundamental understanding of the relations between the system geometry and uniformity of the fluxes, especially on an industrial scale.The presented work focuses at investigation on local current density distribution in the pilot-scale ED unit consisting of 200 vertically orientated membrane pairs with working area of 0.64 × 0.32 m2 per membrane. Planar-plate electrodes are horizontally segmented into the 6 identical and electrically insulated segments allowing determination of current distribution along main direction of the solutions flow. The current distribution is investigated at different operating conditions (salt concentration, applied current, solution flow rate, electrode compartment solutions properties etc.). In parallel, a macrohomogeneous (volume-averaged) 3D mathematical model is validated and employed to calculate and visualize the local fields of electric potential, current density and salt concentrations. A nonuniformity in the current distribution mainly due to the by-pass current through the manifold is observed. The qualitative relationships between the nonuniformity of current density distribution and process performance and reliability observed for pilot- or industrial-scale can be generalized to be valid also for other similar electromembrane systems with plate-and-frame design.

Performance of PEFC Under Different Gas-Mixing Conditions

Introduction For PEFCs’ grand-scale commercialization, their performance should be further improved. The performance of PEFC is the results of the distribution of the local current density and materials concentration, especially oxygen and water. In addition, the in-plane transport of gas in the gas diffusion layer (GDL), which is difficult to measure, affects the distribution of oxygen partial pressure and relative humidity (RH) by mixing the gas with different residence time. In this study, the distribution of local current density as well as oxygen partial pressure and RH was estimated by varying active area and gas velocity. Furthermore, two kinds of different gas channels, parallel and serpentine, were utilized to estimate the effect of gas mixing in the GDL. The concept of space time was used to discuss the effects of gas mixing in the perspective of chemical engineering. The results may provide a theory basis for cell design procedure. Theory The ORR kinetics can be expressed as Eq.1.-r SO = k gc p O (1)where -r SO is the oxygen consumption rate per active area [mol/(m2s)], p O is the local oxygen partial pressure [Pa] which was experimentally proved to be 1st-order to the reaction rate[1]. k gc represents the partial-pressure-based kinetics constant [mol/(Pa.m2s)] which is the function of cathode electromotive force and RH at a fixed temperature[2].PEFC has the gas-mixing behavior between plug flow reactor (PFR) and perfectly mixed flow reactor (CSTR), whose design equations can be written as Eqs.2 and 3.-r SO d(A/v 0) = C O0 Al dx O (2)-r SO (A/v 0) = C O0 Al x O (3)where A is the active area from the gas inlet [m2], v 0 is the inlet gas velocity [m/s], C O0 is the oxygen concentration in the inlet gas [mol/m3], Al is the cross-section area of a single gas channel [m2], and x O is the oxygen conversion. Experimental The membrane electrode assembly (MEA) with 10 mm thick catalyst layer and Nafion® ionomer and membrane (DuPont NR-212) was set up in a JARI-type cell. The polarization curves were measured with parallel and serpentine channels under different gas flow rates. The different areas of gaskets opening and GDLs were utilized to obtain different active areas, as shown in Fig. 1. The reactant gas was humidified in bubblers. The cell was operated at 80 °C and the pressure at the cell outlet was 1 atm. Results and Discussion Fig. 2 shows the experimental results of polarization curve. The integrated current per active area is higher in 4 cm2 active area comparing to 2 cm2 at high cell voltage in case of parallel channels, and vice versa at low cell voltage. Since the ORR-produced water humidifies the cell, which increases the activity of the catalyst, the current density near the outlet is higher than that near the inlet. When the current density is high enough, liquid water floods the cell, which causes the performance decrease. On the other hand, the polarization characteristics with different active areas are nearly the same in case of serpentine channels. Higher velocity in the gas channel promote the liquid water removal so that the flooding is not remarkable.Fig. 3 shows the relationship between oxygen conversion and area/velocity, which is proportional to the space time. The convex downward curve in case of the parallel channels indicates that the ORR in the cell behaves like an autocatalytic reaction in PFR. Whereas, the curve of the cell with serpentine channels is slightly convex upward, which infers the gas mixing in the cell is more remarkable, approaching to CSTR.As the oxygen conversion increasing, the water vapor partial pressure increases and the oxygen partial pressure decreases, so that a maximal reaction rate exists, as shown in Fig. 4. Therefore, a turning point of oxygen conversion exists, below which a gas channel working like CSTR e.g. serpentine channels should be chosen, and above which a gas channel working like PFR e.g. parallels channel should be chosen, as shown in Fig. 5. Conclusions The distribution of local current density and gas composition as well as gas mixing can be estimated by varying active area and gas velocity. The ORR in the cell with parallel channels can be regarded as an autocatalytic reaction in PFR. The gas mixing in the cell with serpentine channels is more remarkable than parallel channels. When the active area and gas flow rate are certain, an optimal gas channel which the gas-mixing character of is known can be selected in cell design. Acknowledgment This work was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan. Figure 1

Electrochemical and Flow Fields Simulation in Ordered and Disordered Fibrous Electrodes By High Schmidt Number Lattice Boltzmann Method

Redox flow batteries (RFBs) are promising candidates for a large-scale electrical energy storage device that can be utilized to compensate for intermittent renewable energy sources [1]. For the massive implementation of the RFBs in the grid, further reduction of irreversible losses corresponding to charge/discharge processes is highly required. Fibrous electrodes have been gathering much attention to improving cell performance in this decade [2,3]. The electrode properties considerably affect irreversible losses in RFBs and optimal electrode architecture was presented according to continuum-scale modeling and simulation [4]. In numerical simulations, a correlation equation has been applied to determine the bulk mass transfer properties of the electrodes. In the correlation equation, the normalized mass transfer coefficient, i.e. Sherwood (Sh) number, is modeled by Reynolds (Re) number and Schmidt (Sc) number and fitting parameters determined by experiments are applied.Pore-scale modeling and simulations enable us to resolve inhomogeneous flow fields in the electrodes with associated chemicals and reaction distributions and have versatile potentials to explore the electrode properties with the correlation equation. As one of the pore-scale simulations, the Lattice Boltzmann method has been applied to fibrous electrodes of RFBs [5-7]. In typical RFBs, metal ions as reactive species have slow mass diffusion in comparison to momentum diffusion. Thus the flow fields have a high Schmidt number, Sc=v/D, where v is the kinematic viscosity of an electrolyte and D is the diffusivity of a reactive species. We developed a Lattice Boltzmann method based on the model by Sullivan [8], that was applicable for high-Sc electrochemical and flow fields simulation and investigated the inhomogeneous distribution of flows, chemicals and local reactions with a pore-scale resolution [9].In this study, we applied the high-Sc LBM to ordered and disordered fibrous electrodes to examine irreversible losses and a series of in silico tests had been carried out to obtain the bulk mass transfer properties of the electrodes. In the simulation, the mass and the momentum conservation equation for the electrolyte solution were solved, and the chemical species conservation equation for the vanadium ion (V3+) in the electrolyte solution and the charge conservation equation for the electrode and the electrolyte solution were solved. Numerical analysis of electrochemical and flow fields in the negative fibrous electrode under the charge was performed with the fixed pressure drop.Figure 1 shows local current density distribution in an ordered electrode (Fig.1(a)) and a disordered electrode (Fig.1(b)) in the vanadium redox flow battery under the charge. Although the porosity was the same at 0.6 in each case, inflow velocity was different. The disordered fibrous electrode showed a higher electrolyte velocity in comparison to the ordered electrode. The higher electrolyte velocity can enhance the mass transfer around the fiber. However, the calculated overpotential in the disordered electrode was almost the same as the ordered electrode. This was attributed to the utilization of the fibrous electrode in each case. The electrode surface of the disordered electrode was not fully utilized due to inhomogeneity of the electrolyte flow and therefore an advantage of the higher electrolyte velocity was mitigated for reduction of irreversible losses in the electrode, suggesting both the electrolyte velocity and the effective utilization of the electrode surface to be balanced for engineered electrodes to reduce the irreversible losses. Acknowledgements This research was supported by JSPS KAKENHI Grant Number 21H04540.

Performance of PEFC Under Different Gas-Mixing Conditions

For PEFCs’ grand-scale commercialization, their performance should be further improved. The performance of PEFC is the results of the distribution of local current density and materials concentration, especially oxygen and water. In addition, the in-plane transport of reactant gas in the gas diffusion layer (GDL), which is difficult to measure, affects the distribution of oxygen partial pressure and relative humidity (RH) by mixing the gas with different residence time. In this study, the distribution of local current density as well as oxygen partial pressure and RH was estimated by varying active area and gas flow rate. Furthermore, two kinds of different gas channels, parallel and serpentine, were utilized to estimate the effect of gas mixing in the GDL. The concept of space time was used to discuss the effects of gas mixing in the perspective of chemical engineering. The results may provide a theory basis for a cell design procedure.

Current density and temperature distribution measurement and homogeneity analysis for a large-area proton exchange membrane fuel cell

Homogeneous distribution of electro-chemical reaction rates among the activation surface is critical for improving the performance and durability of automotive proton exchange membrane fuel cells (PEMFCs). Segmented measurement technology is commonly used to characterize the local physical-parameter distribution in the PEMFC. In this study, the local current density (LCD) and temperature distributions of a PEMFC with the activation area of 108 cm2 and various cathode flow fields are experimentally investigated. A homogeneity parameter is introduced to evaluate the homogeneity of LCD distribution. The results show that the performance and LCD distribution uniformity of the cell with dot-parallel flow field are much better than that with parallel and parallel-serpentine flow fields. The temperature distribution is generally positively correlated with LCD distribution. For the LCD distribution, the high LCD region firstly appears in the cathode downstream region, and gradually transfers upstream with increasing the current load. With the increase of inlet humidity, the LCD near the cathode inlet is improved due to the improvement of membrane hydration. Increasing the cathode stoichiometry can effectively improve the uniformity of LCD distribution, and mitigate the local oxygen starvation, especially at high loads.

A 3D electrochemical-thermal coupled model for electrochemical and thermal analysis of pouch-type lithium-ion batteries

A three-dimensional electrochemical-thermal coupled model is developed to investigate the interactive electrochemical and thermal characteristics of pouch-type lithium-ion batteries under natural convection conditions. The heat generation rate calculated by the electrochemical model is applied to the thermal model as the heat source, while the temperature derived from the thermal model is regarded as the initial condition for the electrochemical model. The simulations are verified by the experimental data under different discharge rates (1, 3, and 5 C). Numerical results reveal that the average particle size of electrodes directly affects the heat generation rate of the battery during the discharge process. More importantly, it is found that in the in-plane direction, the maximum local current density appears near the tabs initially and moves to the bottom side with the progress of the discharge as the regions away from tabs becomes more favorable for electrochemical reactions. The uneven distribution of local current density results in a non-uniform distribution of the heat generation rate and thus the uneven temperature distribution. In addition, the temperature gradient in the through-plane direction is relatively small under natural convection conditions. This work offers more insights into heat generation mechanisms in lithium-ion batteries, which will assist the design of efficient battery thermal management systems.

Complex Growth Behavior of Li Dendrites in Al2O3 Nanoparticles‐Driven Viscoelastic Electrolytes for Lithium Metal Batteries: Dynamic versus Quasistatic Rheology

AbstractViscoelastic electrolytes have been focused as innovative electrolytes for Li metal batteries because of their unique mechanical and electrochemical properties compared to conventional Newtonian liquid electrolytes. However, the role of the mechanochemical properties of viscoelastic electrolytes in the electrochemical performance of Li metal has not been fully understood yet. In particular, dynamic rheology of viscoelastic electrolytes has not been considered one of significant factors accelerating Li dendrites growth. Herein, Al2O3 nanoparticles‐driven viscoelastic electrolytes are introduced to improve the electrochemical performance of Li metal. In contrast to conventional Newtonian liquid electrolytes, viscoelastic electrolytes significantly suppress Li dendrites growth during cycling, resulting in excellent electrochemical performance, such as stable capacity retention over 400 cycles. Moreover, the complex growth mechanism of Li dendrites in viscoelastic electrolytes is demonstrated in terms of dynamic versus quasistatic rheology. Dynamic rheology prevails over quasistatic rheology as Al2O3 weight fraction and current density is increased in viscoelastic electrolytes. Dynamic rheology gives rise to spatial non‐uniformity in the mechanical and rheological properties of viscoelastic electrolytes, leading to promoting Li dendrites growth due to the uneven distribution of local current density on the Li metal surface. These findings provide fundamental insights into strategies to design advanced electrolytes for Li metal batteries.

Characterization of Spatial Distribution of Local Critical Current Density in a Co-Doped BaFe2As2 Film Based on Magnetic Microscopy

We have investigated spatial distribution of local critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) in a Co-doped BaFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (Ba122:Co) film based on scanning Hall-probe microscopy (SHPM). In-plane J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> distribution was derived from the magnetic field distribution. Thanks to the high-resolution characterization, local obstacle can be detected clearly, as well as a local spot with extremely high J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> over 7 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which is among the highest value at remanent state reported for this series of films so far. The SHPM allows us to separate the influence of current blocking obstacles and intrinsic flux pinning properties on J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . Validity of the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> analysis was checked by a magnetization measurement based on a SQUID magnetometer. Moreover, we investigated the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> distributions at different temperatures by SHPM and studied their probability distribution functions (PDFs). PDFs are found to be scaled by a Weibull function with the same shape parameter, while J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> s are decreasing with higher temperature. These results indicate that the intrinsic in-plane pinning is controlled by the same mechanism.

Oxide Formation during Transpassive Material Removal of Martensitic 42CrMo4 Steel by Electrochemical Machining.

The efficiency of material removal by electrochemical machining (ECM) and rim zone modifications is highly dependent on material composition, the chemical surface condition at the break through potential, the electrolyte, the machining parameters and the resulting current densities and local current density distribution at the surfaces. The ECM process is mechanistically determined by transpassive anodic metal dissolution and layer formation at high voltages and specific electrolytic compositions. The mechanisms of transpassive anodic metal dissolution and oxide formation are not fully understood yet for steels such as 42CrMo4. Therefore, martensitic 42CrMo4 was subjected to ECM in sodium nitrate solution with two different current densities and compared to the native oxide of ground 42CrMo4. The material removal rate as well as anodic dissolution and transpassive oxide formation were investigated by mass spectroscopic analysis (ICP-MS) and (angle-resolved) X-ray photoelectron spectroscopy ((AR)XPS) after ECM. The results revealed the formation of a Fe3−xO4 mixed oxide and a change of the oxidation state for iron, chromium and molybdenum, e.g., 25% Fe (II) was present in the oxide at 20.6 A/cm2 and was substituted by Fe (III) at 34.0 A/cm2 to an amount of 10% Fe (II). Furthermore, ECM processing of 42CrMo4 in sodium nitrate solution was strongly determined by a stationary process with two parallel running steps: 1. Transpassive Fe3−xO4 mixed oxide formation/repassivation; as well as 2. dissolution of the transpassive oxide at the metal surface.

Role of Chromate in the Cathodic Current Availability in an Al Alloy-316 Stainless Steel Galvanic Couple Under Simulated Atmospheric Conditions

Numerous studies have demonstrated that the Cu-rich intermetallic particles (IMPs) are the major facilitators of localized corrosion of high strength Al alloys in chloride-containing environments.[1-4] In the context of cathodic activity, these Cu-rich IMPs catalyze the fast oxygen reduction reaction (ORR) rates required to sustain anodic dissolution of the peripheral Al matrix and/or preferential dealloying of active elemental constituent(s) of the Cu-rich IMPs. The latter process results in Cu replating on the surface which increases the cathodic surface area. The situation is exacerbated when these Al alloys are coupled with more noble fasteners such as stainless steel (SS) as found in aircraft structures; macro-galvanic interactions between the Al plates and SS fasteners lead to substantial damage, including within the fastener holes.[5-8] The SS fastener is speculated to increase the driving force for Cu replating, and the replated Cu in turn sustains fast cathodic reaction kinetics.[4] Traditionally, soluble chromates have been used in conversion coatings and as pigments in protective coatings to mitigate localized corrosion of Al alloys.[9-10] Chromate cathodically reduces to form an insoluble Cr3+-rich film on the alloy surface that blocks cathodic sites and hinders the ORR as well as the dealloying of the S-phase (Al2CuMg).[10-13] While chromate is well known to be carcinogenic, and efforts are underway to replace it with an environmentally-friendly inhibitor, efforts to date on potential alternatives have proven generally unsuccessful.[14] Consequently, chromate remains the choice inhibitor for Al alloy applications particularly in the aerospace industry. As such, it remains of utmost academic and industrial interest to continue to gain more in-depth understanding of chromate inhibition mechanism(s), especially as it pertains to the corrosion behavior of complex structures with micro- and macro-galvanic couples with the intent to enhance the knowledge required to facilitate the establishment of non-toxic chromate replacement systems.In the present work, the rotating disk electrode (RDE) technique is used for a comparative study of the inhibitive effect of sodium chromate on the ORR on polished and pre-corroded AA7050 electrodes. The results will serve as boundary conditions for the computational modeling of galvanic corrosion inhibition of AA7050 coupled to 316SS in a plate-fastener arrangement. The RDE is used to simulate thin electrolyte films with varying diffusion boundary layer thicknesses as rotation rate is increased. In the inhibitor-free chloride solution, a Pt working electrode is used to determine boundary layer thickness as a function of rotation rate, eliminating the effects of a surface oxide film. Initial results on polished electrodes show that at optimal chromate concentrations where the cathodic current density is significantly reduced on AA7050 and 316SS, the ORR kinetics on pure Cu is still substantial. This implies that higher inhibitor concentrations may be required to suppress the ORR on corroded AA7050 replated with Cu. AA7050 pre-corrosion is simulated by employing two approaches: 1) free corrosion at the open circuit potential in inhibitor-free solution and 2) electroplating Cu onto AA7050. The results are compared to the ORR kinetics on pure Cu. Furthermore, the scanning vibrating electrode technique (SVET) will be carried out on a typical AA7050/316SS galvanic couple to map local current density distributions as a function of chromate concentration. Surface characterization will be performed using optical microscopy and scanning electron microscopy (SEM) and surface film analyses will be conducted utilizing x-ray photoelectron spectroscopy (XPS).