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A structure-mechanism-performance relationship of metal-free carbon catalysts for outstanding H2O2 production activity and selectivity in alkaline media.

A H2/Br2 flow battery operating with redox-mediated bromate anions ( BrO 3 − ) has shown several beneficial features induced by the bromate-bromide chemical reaction, particularly in terms of cell performance and safety issues. In this study, a kinetic model of the chemical reaction is developed by thoroughly accounting for detailed reaction steps and by incorporating a three-dimensional, transient H2/Br2 flow battery model. First, the H2/Br2 flow battery model with BrO 3 − is validated against experimental data where cell voltage recovery is observed until the BrO 3 − is completely consumed. The model successfully reproduces the measured voltage evolution data during discharge, thus accurately capturing the transient behavior of an H2/Br2 cell influenced by the bromate-bromide chemical reaction. In addition, H2/Br2 cells are simulated with and without BrO 3 − and model predictions reveal that the reversible voltage, ohmic overpotential in the membrane, and activation overpotential of the Br2 reduction reaction are all improved due to the use of BrO 3 − in the catholyte. The present model for the H 2 - Br 2 - BrO 3 − system enables the quantitative analysis of the beneficial effects for using redox-mediated bromate anion and thus becomes a useful tool for designing and optimization of H2/Br2 cell with BrO 3 − .

A three-dimensional (3-D) transient numerical model of an alkaline water electrolysis (AWE) cell with potassium hydroxide solution is developed by rigorously accounting for the hydrogen and oxygen evolution reactions and resulting species and charge transport through various AWE components. First, the AWE model is experimentally validated against a polarization curve corresponding to a wide range of currents as high as 2.0 A·cm−2. In general, the simulation results compare well with the measured data and further reveal the operating characteristics of AWE cells, showing key distributions of solid/electrolyte potentials and multidimensional contours of reactant and product concentrations at various current densities. In particular, the contribution of hydroxide ion (OH−) diffusion to the ohmic losses through porous electrodes and a porous separator are quantitatively examined at low and high electrolyte flow rates. The present full 3-D AWE model provides a basic understanding of the electrochemical and transport phenomena and can be further applied to practical large-scale AWE cell and stack geometries for grid-scale hydrogen production.

In actual PEM fuel cell systems, the coolant flow rate is generally controlled to maintain a preset temperature at the coolant outlet. This implies that a change in coolant supply flow rate is a good early indicator of a malfunctioning PEM fuel cell stack and system components. In this study, various fuel cell malfunctions are simulated based on the practical coolant flow control strategy by using a three-dimensional, two-phase, multiscale PEM fuel cell model developed in our previous studies. The focus is on analysis of the characteristics of coolant flow rate change along with voltage degradation in various fuel cell malfunction cases. The model predictions show that in general, the coolant flow rate tends to increase proportionally with the degree of voltage degradation, but the increase in temperature inside the membrane electrode assembly (MEA) is not always related to the voltage drop and is influenced more directly by local current density distribution. Although the present numerical comparison between the normal and malfunctioning cases is conducted at the low current density of 0.3 A cm−2, the general cell behavior will not be altered at higher current densities due to inverse relationship between cell performance and waste heat generation. The present work elucidates the complex interplay among increase in coolant flow rate, increase in MEA temperature, voltage drop, and change in local current density distribution when a PEM fuel cell malfunctions.

CeO2 has been established as an effective scavenger for destructive oxygen radicals in fuel cell membranes. The effect of ceria (CeO2) surface defect on •OH and •OOH radical scavenging efficacy is ...

In this study, a three-dimensional transient metal hydride model is applied to two different depleted uranium (DU) bed designs. One bed is designed to contain 1.86 kg DU for a hydrogen isotope stor...

The oxygen transport resistance in the cathode catalyst layer (CL) of polymer electrolyte fuel cells (PEFCs) has been reported to be significantly higher than expected, especially when the platinum (Pt) loading is low and/or the degree of CL degradation is severe. In this paper, the oxygen transport resistance behavior in the cathode CL is numerically analyzed under various CL design and operating conditions. Particular emphasis is placed on the aged CL wherein Pt particle growth and active Pt surface area loss are observed. For this study, a previously developed micro-scale catalyst model is improved upon to account for Pt particle size. The new model includes calculations of catalyst activity and electrochemically active surface areas, as well as various transport resistances through the ionomer and liquid films. After coupling the micro-scale CL model with a three-dimensional PEFC model, multi-scale simulations are carried out under various PEFC catalyst designs (varying Pt loading, ionomer fraction, oxygen permeation rate through the ionomer film) and operating conditions (drying or flooding of the electrode, high or lower current density). The simulation results agree well with experimental oxygen transport resistance data and further indicate that CL design with low Pt loading is more susceptible to degradation. Providing extensive multi-dimensional contours of species concentration, temperature, and current density inside the PEFC, this study provides a comprehensive understanding of oxygen transport resistance in the cathode CL in different PEFC situations.

A multi-dimensional transient numerical alkaline water electrolyzer (AWE) model is applied to zero-gap circular shaped cells in which either a porous separator (Zirfon) or a dense polybenzimidazole (PBI) membrane is sandwiched between the negative and positive electrodes of nickel foam. In general, not only do the model predictions compare well with the experimental data collected under various wt% KOH electrolyte conditions, but they also reveal key features that favor the use of the PBI membrane over the porous separator. Moreover, a lower ohmic polarization and resulting higher cell performance are achieved by using the PBI membrane owing to its higher ionic conductivity and thinner thickness compared to those of the separator. Although a thin PBI membrane of around 50 μ m is employed, simulation results demonstrate that the crossover of hydrogen and oxygen gases is effectively suppressed with a 10-fold thicker separator (500 μ m ).

A novel cathode flow-field design suitable for a passive air-cooled polymer electrolyte membrane (PEM) fuel cell stack is proposed to enhance the water-retaining capability under excess dry air supply conditions. The innovative cathode flow-field is designed to supply more air to the cooling channels and further enables deceleration of the reactant air in the gas channels and acceleration of the coolant air in the cooling channels simultaneously along the air flow path. Therefore, the design facilitates the waste heat removal through the cooling channels while the water removal by the reactant air is minimized. The conceptual cathode flow-field design is validated using a three-dimensional PEM fuel cell model. The detailed simulation results clearly demonstrate that the new cathode flow-field design exhibits superior water-retaining capability compared with a conventional cathode flow-field design (parallel flow channel configuration) under typical air-cooled fuel cell operating conditions. This study provides a new strategy to design cathode flow-fields to alleviate notorious membrane dehydration and unstable performance issues in a passive air-cooled PEM fuel cell stack.

The proton exchange membrane fuel cell (PEMFC) is promising technology for efficient power generation and has wide applications. In PEMFC development, it is important to diagnose malfunctions in a system with defective components and a PEMFC stack can act as an effective sensor to detect the various malfunctioning modes. Hence, the focus of this study is to analyze the response of a PEMFC under various malfunction conditions including humidifier, air blower, and coolant pump, catalyst layer degradation, and membrane aging based on 3D PEMFC simulations. Except for the coolant supply malfunction, other malfunctions exhibit similar behavior in terms of voltage drop and temperature rise, requiring more detailed measurement techniques such as Electrochemical Impedance Spectroscopy to identify the cause of malfunctions. In addition, measuring the relative humidity of the outlet gas may not provide sufficient information to distinguish the malfunction of the anode or cathode humidifier. The results of the study suggest fault detection and isolation methods under these malfunction conditions to prevent more severe failure of the PEMFC stack and system. An extensive multi-dimensional contour comprising temperature, relative humidity, liquid saturation, water content, and current density is also provided for the better analyzation of system malfunctioning behaviors.

We present a three-dimensional (3-D) steam-methane-reforming (SMR) model consisting of a steam-reforming (SR) reactor, water gas shift reactor, preferential oxidation reactor, catalytic burner, hea...

The purpose of this study is to investigate the influence of design parameters for the scale-up of the depleted uranium (DU) bed. The actual DU bed chosen for this study has a DU loading of 1.86 kg...

In general, a passive air cooled proton exchange membrane fuel cell (PEMFC) operates under an excessive amount of air flow and low humidification conditions, indicating effective control of stack temperature and membrane hydration is of paramount importance for achieving high and stable performance. In this study, we numerically investigate the effects of cathode bipolar plate flowfield design on temperature and water distributions inside a passive air cooled PEMFC. The three-dimensional (3D) non-isothermal PEMFC model is applied to various PEMFC geometries where three different flowfield designs with different ratio and dimensions of air supply channel and cooling channels are taken into account. Simulation results show that the flowfield design with narrow air supply channel and/or wide cooling channel exhibits superior water management performance whereas the effect of oyxgen depletion is minimal due to sufficiently a large amount of air supply conditions. This study clearly demonstrates that the membrane dehydration issue in a passive type air cooled PEMFC can be mitigated by proper design of cathode flowfield.

The effect of side-chain length on the nanophase-segregated structure and transport in perfluorinated sulfonic acid (PFSA)-based and perfluorinated phosphoric acid (PFPA)-based membranes is investigated at 20 and 5 wt % water content conditions using a molecular dynamics simulation method. It is found using the pair correlation analysis that the longer side chain leads to more developed local water structures in the water phase at 20 wt % water content, observable in both membrane chemistries albeit more distinct in PFPA-based membranes. It is also confirmed from the structure factor analysis that large-scale nanophase segregation is enhanced with increasing side-chain length for PFPA membranes, whereas no significant change is observed at these scales for PFSA membranes. Next, it is revealed that the proton transport is increased by 0.004 S/cm in PFSA-based membranes with increasing side-chain length due to the enhanced vehicular and hopping mechanisms, whereas the proton transport in PFPA-based membranes is decreased by 0.002 S/cm despite improved nanophase segregation. As confirmed by the pair correlation function analysis, the diminished proton transport in PFPA-based membranes is attributed to the molecular association of phosphate groups with hydronium ions via hydrogen bond in the longer side-chain case, which is namely a hydronium-mediated bridge configuration. Such bridge configurations and correspondingly similar trends in proton transport are also observed at 5 wt % water content condition to a lesser extent. Our simulation study demonstrates that the proton transport is affected by the hydrogen-bonding network as well as by the nanophase segregation.

This paper reports novel cathode flow-field designs for passive typed air-cooled polymer electrolyte membrane fuel cells (PEMFCs) to help alleviate electrolyte dehydration and performance degradation issues under excess dry air supply conditions. The proposed flow-field designs include 7 three-dimensional (3D) patterned designs in addition to a parallel channel configuration equipped with rectangular baffles to control the airflow for more efficient heat removal. The designs were evaluated numerically using 3D, two-phase PEMFC simulations. Compared to a typical parallel flow channel configuration, the proposed flow-field designs show better heat removal and water retention capability. The improvement in single cell voltage was around 13–75 mV at an operating current density of 0.5 A cm−2, whereas the larger pressure drops around ∆ P = 6.9–317.6 Pa cm−1 were required because of the more complex flow-field configurations compared to the simple straight parallel channel geometry ( ∆ P = 7.4 Pa cm−1). This work presents a comprehensive understanding of air-cooled PEMFC operating characteristics under excessive dry air supply conditions and a new design strategy for cathode flow-fields.

In this study, we propose a novel method of pKa prediction in a diverse set of acids, which combines density functional theory (DFT) method with machine learning (ML) methods. First, the DFT method with B3LYP/6-31++G**/SM8 is used to predict pKa, yielding a mean absolute error of 1.85 pKa units. Subsequently, such pKa values predicted from the DFT method are employed as one of 10 molecular descriptors for developing ML models trained on experimental data. Kernel Ridge Regression (KRR), Gaussian Process Regression, and Artificial Neural Network are optimized using three Pipelines: Pipeline 1 involving only hyperparameter optimization (HPO), Pipeline 2 involving HPO followed by a relative contribution analysis (RCA) and recursive feature elimination (RFE), and Pipeline 3 involving HPO followed by RCA and RFE on an expanded set of composite features. Finally, it is demonstrated that KRR with Pipeline 3 yields optimal pKa prediction at an MAE of 0.60 log units. This algorithm was then utilized to predict the pKa of 37 novel acids. The two most important features were determined to be the number of hydrogen atoms in the molecule and the degree of oxidation of the acid. The predicted pKa values were documented for future reference.

Zoster-related pain (ZRP) has many negative effects on a patient's quality of life. The transforaminal steroid injection (TFESI), which reduces neural inflammation and pain, has been advocated by pain physicians. Many reports demonstrated that early administration of TFESI showed better efficacy; however, the golden period during which TFESI is most effective remains unclear.This multicentre retrospective cohort study aimed to identify the golden period by which TFESI yields the best outcome in patients with ZRP.Multicenter, retrospective cohort study.University-affiliated hospitals.After performing the TFESI in patients with ZRP, the patients were classified into two groups: the effective group (E) and the not effective group (N) based on the changes in the pain intensity 3 months after the TFESI. The receiver operating characteristic (ROC) curve analysis was used to assess the cut-off time point for predicting TFESI effectiveness. Furthermore, a logistic regression analysis was performed to identify patients' factors associated with a successful treatment outcome.Of the 302 patients, 186 and 116 patients were classified into the E and N group, respectively. ROC curve analysis showed that the best cut-off time point for TFESI was 12 weeks (95% confidence interval [CI]; 10-14 weeks) after the onset of HZ. The only variable associated with a favorable outcome was a symptom duration of ? 12 weeks compared with12 weeks (Odd ratio, 0.107; 95% CI, 0.055-0.205; P0.001). Other patient variables were not significantly associated with the effectiveness of TFESI. TFESI was most effective when administered within 12 weeks of the onset of herpes zoster.This study was not a prospective randomized controlled trial (RCT) and the follow-up period was only 3 months after TFESI.TFESI is more effective when administered within 12 weeks of onset of herpes zoster.

3D customized Pt nanoarchitecture realizes large surface area and porosity for excellent performance and durability in fuel cells., Unsupported Pt electrocatalysts demonstrate excellent electrochemical stability when used in polymer electrolyte membrane fuel cells; however, their extreme thinness and low porosity result in insufficient surface area and high mass transfer resistance. Here, we introduce three-dimensionally (3D) customized, multiscale Pt nanoarchitectures (PtNAs) composed of dense and narrow (for sufficient active sites) and sparse (for improved mass transfer) nanoscale building blocks. The 3D-multiscale PtNA fabricated by ultrahigh-resolution nanotransfer printing exhibited excellent performance (45% enhanced maximum power density) and high durability (only 5% loss of surface area for 5000 cycles) compared to commercial Pt/C. We also theoretically elucidate the relationship between the 3D structures and cell performance using computational fluid dynamics. We expect that the structure-controlled 3D electrocatalysts will introduce a new pathway to design and fabricate high-performance electrocatalysts for fuel cells, as well as various electrochemical devices that require the precision engineering of reaction surfaces and mass transfer.

Water crossover through the membrane of a vanadium redox flow battery system is not desirable because it floods one half-cell, diluting the vanadium solution on one side and consequently increasing the concentration of vanadium in the other half-cell. To analyze the effect of water crossover and the resultant electrolyte imbalance issue in the vanadium redox flow battery, herein we newly develop a water transport model and incorporate it into our previously developed 3D vanadium redox flow battery model. The model rigorously accounts for water production/consumption by the redox reaction of VO2+/VO2 and side reactions as well as various mechanisms of water crossover through the membrane arising from diffusion, electro-osmotic drag (EOD), and vanadium crossover. The numerical model is successfully validated against in situ data collected during experiments in which the electrolyte volumes and cell voltages are measured during charge–discharge cycles carried out under various current densities. The detailed simulation results clearly elucidate water crossover behaviors at different stages of charging and discharging, and further reveal the individual contributions of water crossover mechanisms to the overall electrolyte imbalance between the negative and positive sides of the VRFB system.

A new direction for developing electrocatalysts for hydrogen fuel cell systems has emerged, based on the fabrication of 3D architectures. These new architectures include extended Pt surface building blocks, the strategic use of void spaces, and deliberate network connectivity along with tortuosity, as design components. Various strategies for synthesis now enable the functional and structural engineering of these electrocatalysts with appropriate electronic, ionic, and electrochemical features. The new architectures provide efficient mass transport and large electrochemically active areas. To date, although there are few examples of fully functioning hydrogen fuel cell devices, these 3D electrocatalysts have the potential to achieve optimal cell performance and durability, exceeding conventional Pt powder (i.e., Pt/C) electrocatalysts. This progress report highlights the various 3D architectures proposed for Pt electrocatalysts, advances made in the fabrication of these structures, and the remaining technical challenges. Attempts to develop design rules for 3D architectures and modeling, provide insights into their achievable and potential performance. Perspectives on future developments of new multiscale designs are also discussed along with future study directions.

Proton exchange membrane fuel cell (PEMFC) are actively replacing fossil fuel-based energy systems in commercial applications. Evaluation of cell performance and degradation is critical and usually performed by analyzing the polarization curve and/or Nyquist plot. The polarization test provides an insight into the entire cell, whereas electrochemical impedance spectroscopy (EIS) is used to obtain the Nyquist plot that facilitates the assessment of individual voltage losses occurring in the inner components of a PEMFC. This paper highlights the degradation assessment of PEMFCs using a coupled one-dimensional (1-D) two-phase PEMFC model and Randles-TLM equivalent circuit model. The 1-D model includes a micro-scale catalyst layer (CL) model to more accurately assess electrochemical catalyst activity and mass transport inside the agglomerated porous structure of CL. This model is utilized to estimate key input parameters for various PEMFC operating conditions and degradation scenarios, which are then applied to the equivalent circuit model. The coupled model simulations successfully reproduce both experimental polarization curves and Nyquist plots for various PEMFC conditions. This study enhances the understanding of underlying physical phenomena occurring during long-term PEMFC operations.

This study applies, tests, and compares comprehensive surrogate-based optimization techniques to optimize the performance of polymer electrolyte membrane fuel cells (PEMFCs). Moreover, parametric cases considering four important design variables, i.e., gas diffusion layer thickness ( δ g d l ), channel depth ( d c h ), channel width ( w c h ), and land width ( w l a n d ), are defined using the latin hypercube sampling technique under reasonable constraint conditions. Multidimensional, two-phase PEMFC model simulations are performed to generate the training and test data under these design conditions. Three famous surrogate models, i.e., response surface approximation (RSA), radial basis neural network (RBNN), and kriging (KRG), are employed to construct objective functions for the PEMFC cell voltages operating in the galvanostatic mode, and their accuracies are tested and compared using root mean square error and adjusted R-square. The surrogates are then linked to stochastic optimization algorithms, i.e., genetic algorithm and particle swarm optimization, to determine the optimal design points. A comparative study of these surrogates reveals that the KRG model outperforms the RBNN and RSA models in terms of prediction capability. Furthermore, the PEMFC model simulations at the optimal design points clearly demonstrate that performance improvements of around 56–69 mV at 2.0 A cm−2 are achieved with the optimum design values compared to typical PEMFC design conditions.

The high temperature-direct methanol fuel cell (HT-DMFC) based on phosphoric acid (PA)-doped polybenzimidazole (PBI) membranes shows promise as a passive-type DMFC system because it can operate using highly concentrated methanol fuel. In this paper, the methanol crossover and cell performance behaviors of the HT-DMFCs were investigated using a one-dimensional (1-D) HT-DMFC system model that fully accounts for the electrochemical reactions, key species transport and heat generation inside a cell, and the evaporation processes of liquid methanol/water fuel in the evaporator. The model was first validated against experimental HT-DMFC data measured over a wide range of methanol feed concentrations and operating current densities, and operating characteristics of HT-DMFCs were then explored in detail. Particular emphasis was placed on conducting a comparative study of HT-DMFCs with traditional liquid feed low temperature-DMFCs based on perfluorosulfonic acid membranes. The simulation results showed that a HT-DMFC can operate well under highly concentrated methanol fuel above 12 M due to the minimal degree of methanol crossover through the PA-doped PBI membrane.

In this paper, the effects of using porous metal foam based bipolar plates (BPs) are investigated under practical automotive fuel cell operations with low humidification reaction gases. Particular emphasis is placed on evaluating water management capabilities of metal foam based BP designs, compared to the traditional serpentine flow field BP designs. A three-dimensional, two-phase fuel cell model developed in a previous study is applied to 25cm2 real-scale fuel cell geometries with metal foam and serpentine flow modes, and then successfully validated against the experimental data measured under different operating pressures and current densities. The detailed simulation results clearly elucidate advantages of using metal foam as flow distributor through extensive multidimensional contours of flow velocity, species, and current density.

Using a two-phase polymer electrolyte fuel cell (PEFC) model, we numerically investigated the influence of metal foam porous properties and wettability on key species and current distributions within a PEFC. Three-dimensional simulations were conducted under practical low humidity inlet hydrogen and air gases, and numerical comparisons were made for different metal foam design variables. These simulations were conducted to elucidate the detailed operating characteristics of PEFCs using metal foams as a flow distributor, and the simulation results showed that two-phase transport and the resulting flooding behavior in a PEFC are influenced by both the metal foam porous properties and the porous properties of an adjacent layer (e.g., the gas diffusion layer). This paper offers basic directions to design metal foams for optimal water management of PEFCs.

In this paper, a three-dimensional (3-D) hydrogen absorption/desorption model is applied to two different real-scale metal hydride vessels (MHVs). The model is then validated against experimentally measured metal hydride temperature and H/M ratio evolution data from a hydrogen charge/discharge cycle. The two experimental vessels were designed to have identical dimensions and contain the same mass of ZrCo (125 g), but one is loaded with copper fins and the other with copper foams. Since the real-scale vessel geometries involve several million computational grid points, a parallel computational methodology has been employed to reduce the computational turnaround time. This comparison highlights this substantial improvement in agreement between simulation and experiment obtained by conducting full-scale simulations of the whole MHVs. In addition to model validation, detailed key contours are analyzed to elucidate hydrogen charge/discharge characteristics in whole-scale MHV geometries. This study demonstrates the validity of our 3-D hydrogen absorption/desorption model for optimization of practical MHV design and operating conditions.

In all-vanadium redox flow battery (VRFB) systems, the electrolyte imbalance between the negative and positive electrodes inevitably occurs and subsequently necessitates costly electrolyte rebalancing procedures for long-term VRFB operation. We propose a new operating strategy for VRFBs that alleviates the electrolyte imbalance issue, which is to employ water and sulfuric acid at different concentrations as initial supporting electrolytes between the negative and positive electrodes. Using a three-dimensional VRFB model that was developed and validated in our previous study, we numerically demonstrate that the degree of electrolyte imbalance can be reduced by using appropriate compositions of the initial anolyte and catholyte. Detailed simulation results reveal that the diffusive water crossover flux between the negative and positive electrodes, which is a main contributor to electrolyte imbalance during charge-discharge cyclic operations, can be effectively suppressed by controlling the initial anolyte and catholyte compositions.

Natural gas (NG) is delivered to homes and businesses through a complex pipeline network that mainly consists of high-pressure main feed lines and many low-pressure local distribution lines. To date, throttling valves have been employed to reduce the NG pressure between the two types of pipeline, leading to the loss of the work potential generated by the expansion of pressurized NG in the NG supply chain. As a result, research is currently underway to replace throttling valves with turbo expander generators (TEGs) to obtain additional work power. In this study, a multi-energy hub system consisting of phosphoric acid fuel cell (PAFC) and TEG modules is proposed and analyzed using ASPEN-HYSYS® modeling and simulations. The key concept underlying the system configuration is to maximize overall system efficiency through the integration of exothermic PAFC and endothermic TEG operations. When the TEG system is combined with four 460kWe PAFC stacks, the simulation results show that roughly 1.73 MWe of power can be produced in the TEG system via the expansion of a 28,000 kg/h NG stream. However, when the PAFC stack runs under hydrogen production mode (which requires high voltage operation), an additional heating source is required to fully deliver the expansion work generated by the NG flow within the TEG system. This study clearly illustrates the beneficial features of the proposed multi-energy hub system in terms of thermal integration and overall efficiency but also reveals that the system configuration and operation need to be carefully optimized in order to advance the technology.

Precise monitoring of electrolyte leaching in high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) devices during lifetime tests is helpful in making a diagnosis of their quality changes and analyzing their electrochemical performance degradation. Here, we investigate electrolyte leaching in the performance degradation of phosphoric acid (PA)-doped polybenzimidazole (PBI) membrane-based HT-PEMFCs. We first perform quantitative analyses to measure PA leakage during cell operation by spectrophotometric means, and a higher PA leakage rate is detected when the current density is elevated in the cell. Second, long-term degradation tests under various current densities of the cells and electrochemical impedance spectroscopy (EIS) analysis are performed to examine the influence of PA loss on the membrane and electrodes during cell performance degradation. The combined results indicate that PA leakage affect cell performance durability, mostly due to an increase in charge transfer resistance and a decrease in the electrochemical surface area (ECSA) of the electrodes. Additionally, a three-dimensional (3-D) HT-PEMFC model is applied to a real-scale experimental cell, and is successfully validated against the polarization curves measured during various long-term experiments. The simulation results highlight that the PA loss from the cathode catalyst layer (CL) is a significant contributor to overall performance degradation.

© 2017 Elsevier Ltd Designing and optimizing the flow-field structure for the liquid phase Br2/HBr electrolyte solution of H2/Br2 redox flow batteries (RFBs) is important for improving cell performance. In this study, two electrolyte flow modes, i.e. the flow-by and flow-through modes, are simulated by using a three-dimensional H2/Br2 RFB model. The model is first applied to real-scale H2/Br2 cell geometries and then validated against the experimental polarization curves acquired using the two different flow modes. The model predictions compare well with the experimental data and further highlight the advantages of using the flow-through mode relative to the flow-by mode. Detailed multi-dimensional contours of the electrolyte flow velocity and key species distributions reveal that more uniform diffusion and stronger convective transport are achieved by using the flow-through mode, which alleviates the ohmic loss associated with charge transport in the Br2 electrode.

A three-dimensional (3-D) model for H2/Br2 redox flow batteries (RFBs) is developed by rigorously accounting for the redox reactions of hydrogen and bromine species, and the resulting species and charge transport through various cell components. First, the H2/Br2 RFB model is experimentally validated against the discharge and charge voltage curves measured over a wide range of current densities up to 1.4 A cm−2. In general, the model predictions compare well with the experimental data, and they further reveal key electrochemical and transport phenomena inside the cell. Particular emphasis is placed on analyzing the crossover of bromine species through the membrane at detailed levels where the model calculates electro-osmotic and diffusive fluxes of bromine species across the membrane; the relative magnitudes are compared under various charging and discharging stages. In addition, a parametric study is carried out to examine the effects of two key design variables on cell performance and crossover behavior, i.e., the thicknesses of the membrane and bromine porous media. This full 3-D H2/Br2 RFB model can be applied to realistic large-scale cell geometry for grid-scale energy storage applications and directly utilized to determine the optimal design and operating conditions for H2/Br2 RFBs.

The effects of the volumetric expansion of uranium due to hydride formation during hydrogen absorption are examined numerically. To assess the effects of the volume changes, a three-dimensional transient metal hydride model is applied to multiple computational domains, i.e., generated as a function of the H/M atomic ratio during hydrogen absorption. The numerical simulations are carried out with or without considering the volume change effect. The simulation results clearly show that a higher temperature rise and slower hydrogen absorption reaction are predicted when the volumetric expansion of uranium hydride is taken into consideration in the model simulations. The comparison results highlight that neglecting the volumetric expansion of metal hydride during hydrogen absorption can lead to an overestimation of the hydrogen charging performance.

Substantial efforts have been made to advance automotive fuel cell technologies with high performance, low cost, and long durability. The dry or low-humidified reactant inlet conditions are typical of automotive fuel cell operations in order to simplify the fuel cell system and reduce parasitic power consumption for external humidification. The low-humidity conditions can cause severe membrane dehydration and thus lead to lower overall cell performance. On the other hand, even under low-humidity operations, oxygen access to the reaction sites can be significantly limited by the liquid film formation in the catalyst layer (CL) due to the hydrophilic nature of electrolyte inside the CL. The purpose of this work is to numerically investigate the characteristics of low-humidity fuel cell operations using a comprehensive two-phase fuel cell model. The main focus is placed on analyzing the water content distribution in the membrane and film formation in the cathode CL under different inlet pressures and operating current densities. The CL is assumed to exhibit cylindrical and spherical pore structures and thus two liquid film growth modes are analyzed and discussed under different pressures and liquid saturation levels. The simulation results clearly elucidate detailed behaviors of membrane dehydration and liquid film formation/growth phenomena and their influences on overall cell performance.

We present a high-temperature proton exchange membrane fuel cell (HT-PEMFC) system model that accounts for fuel reforming, HT-PEMFC stack, and heat-recovery modules along with heat exchangers and balance of plant (BOP) components. In the model developed for analysis, the reaction kinetics for the fuel reforming processes are considered to accurately capture exhaust gas compositions and reactor temperatures under various operating conditions. The HT-PEMFC stack model is simplified from the three-dimensional HT-PEMFC CFD models developed in our previous studies. In addition, the parasitic power consumption and waste heat release from the various BOP components are calculated based on their heat-capacity curves. An experimental fuel reforming reactor for a 5.0 kWe HT-PEMFC system was tested to experimentally validate the fuel reforming sub model. The model predictions were found to be in good agreement with the experimental data in terms of exhaust gas compositions and bed temperatures. Additionally, the simulation revealed the impacts of the burner air-fuel ratio (AFR) and the steam reforming reactor steam-carbon ratio on the system performance and efficiency. In particular, the combined heat and power efficiency of the system increased up to 78% when the burner AFR was properly adjusted. This study clearly illustrates that an HT-PEMFC system requires a high degree of thermal integration and optimization of the system configuration and operating conditions.