Porous Electrode Model Research Articles

Mechanical Characterization and Modeling of Large-Format Lithium-Ion Battery Cell Electrodes and Separators for Real Operating Scenarios

This study presents a novel application-oriented approach to the mechanical characterization and subsequent modeling of porous electrodes and separators in lithium-ion cells to gain a better understanding of their real mechanical operating behavior. An experimental study was conducted on the non-linear stiffness of LiNi0.8Co0.15Al0.05O2 and graphite electrodes as well as PE separators, harvested from large-format lithium-ion cells, using compression tests. The mechanical response of the components was determined for different operating conditions, including nominal stress levels, mechanical loading rates, and mechanical cycles. The presented work describes the test procedure, the experimental setup, and an objective evaluation method, allowing for a detailed summary of the observed mechanical behavior. A distinct nominal stress level and mechanical cycle dependency of the non-linear stiffnesses of the porous materials were found. However, no clear dependency on compression rate was observed. Based on the experimental data, a poroelastic mechanical model was utilized to predict the non-linear behavior of these porous materials under real mechanical operating scenarios with a normalized root-mean-squared error less than 5.5%. The results provide essential new insights into the mechanical behavior of porous electrodes and separators in lithium-ion cells under real operating conditions, enabling the accelerated development of high-performing and safe batteries for various applications.

Finite Element Analysis of Local pH Variations in Electrolysis with Porous Electrodes, Considering Water Self-Ionization.

A finite element model was developed to simulate ion fluxes and local pH changes within and around porous electrodes during the H2 evolution reaction (HER) in acidic electrolytes. This model is particularly characterized by its ability to simulate scenarios in which the local pH inside and near the cathode exceeds 7, even under bulk acidic conditions (e.g., pH = 1), by considering the self-ionization of water. Steady-state calculations using meshes with an appropriate spatial distribution inside and near the cathode revealed that the relationship between the local pH and the double-layer potential at the interface between the porous catalyst layer and the electrolyte domains changed notably when the local pH exceeded the threshold of 7. By comparing the fluxes of H+ and OH- ions at the interface and using the thickness of the catalyst layer as a variable, we determined that the presence of H+ ions within the pores or the supply of OH- ions from the pores to the interface was responsible for the characteristic change in the local pH observed for the porous electrode. The porous electrode model constructed in this study can potentially serve as a basis that can be extended to a wide range of electrolysis systems, including not only the HER, but also the reduction of CO2, H2O2, and O2, and even oxidation reactions such as the O2 evolution reaction.

Enhanced Porous Electrode Theory Based Electrochemical Model for Higher Fidelity Modelling and Deciphering of the EIS Spectra

Electrochemical impedance spectroscopy (EIS) is essential for non-invasive battery characterization. This paper addresses the challenge of adequate interpretation of EIS spectra, which are often complicated by overlapping internal phenomena occurring on similar time scales. We present, for the first time, a high-fidelity numerical time-domain electrochemical model that can virtually replicate experimental EIS spectra with three superimposed high-frequency semicircles, a transition to the diffusion tail at elevated imaginary values, and a tilted diffusion tail at low frequencies. These advanced features were made possible by extending state-of-the-art porous electrode model with innovative sub-models for the double layer phenomenon at the carbon black/electrolyte and metal Li-anode/electrolyte interfaces, and transport phenomena of charged species through the solid electrolyte interphase at the Li-anode interface. Additionally, we modelled the diffusion tail inclination by introducing representative active particles of varying sizes. Results from custom-made half-cells confirm the model’s ability to decipher EIS spectra more accurately compared to existing models. Moreover, innovative physics-based battery model that is capable of accurately modelling intra-cell phenomena can reveal internal states and physical parameters of batteries using measured EIS spectra. The model, therefore, also enables functionality of an advanced virtual sensor, which is an important diagnostics feature in next-generation battery management systems.

Modeling of pulse and relaxation of high-rate Li/CFx-SVO batteries in implantable medical devices

We present a Hybrid Multiphase Porous Electrode Theory (Hybrid-MPET) model that accurately predicts the performance of Medtronic’s implantable medical device battery lithium/carbon monofluoride (CFx) - silver vanadium oxide (SVO) under both low-rate background monitoring and high-rate pulsing currents. The distinct properties of multiple active materials are reflected by parameterizing their thermodynamics, kinetics, and mass transport properties separately. Diffusion limitations of Li+ in SVO are used to explain cell voltage transient behavior during pulse and post-pulse relaxation. We also introduce change in cathode electronic conductivity, Li metal anode surface morphology, and film resistance buildup to capture evolution of cell internal resistance throughout multi-year electrical tests. We share our insights on how the Li+ redistribution process between active materials can restore pulse capability of the hybrid electrode, allow CFx to indirectly contribute to capacity release during pulsing, and affect the operation protocols and design principles of batteries with other hybrid electrodes. We also discuss additional complexities in porous electrode model parameterization and electrochemical characterization techniques due to parallel reactions and solid diffusion pathways across active materials. We hope our models can complement future experimental research and accelerate development of multi-active material electrodes with targeted performance.

Advancing the prediction of bath penetration and electrochemical degradation in Hall-Héroult cell cathodes: Insights into ionic species transport in a porous electrode model

This study presents a groundbreaking approach to modeling the Hall-Héroult cathode used in aluminum production. Our innovative model is grounded in a sophisticated porous electrode methodology coupled with state-of-the-art numerical simulations. This enables us to capture the intricate physicochemical processes within the system precisely, encompassing the migration, diffusion, and convection of ionic species. A key feature of our model is the integration of detailed electrochemical reaction kinetics at the microscale, providing a nuanced understanding of the internal dynamics of the cathode. Furthermore, we have incorporated a unique penalization method that rigorously enforces the principles of electroneutrality and ionic thermodynamic equilibrium, ensuring the model's fidelity to real-world phenomena. Computational simulation using the finite element method (FEM) serves as the backbone of our model, offering unparalleled accuracy and robustness. This has been confirmed through validation against empirical data, underlining the model's potential to significantly enhance both the efficiency and sustainability of aluminum production processes.•Development of an advanced porous electrode model for the Hall-Héroult process, utilizing numerical simulations42 to unravel complex physicochemical dynamics.•Incorporation of a novel penalization method for ensuring electroneutrality and thermodynamic equilibrium, enhancing 44 model accuracy.•Validation of the model against empirical data using FEM, demonstrating potential improvements in aluminum 46 production efficiency and sustainability.

Solar fuels design: Porous cathodes modeling for electrochemical carbon dioxide reduction in aqueous electrolytes

The reduction of carbon dioxide emissions is crucial to reduce the atmospheric greenhouse effect, fighting climate change and global warming. Electrochemical CO2 reduction is one of the most promising carbon capture and utilization technologies, that can be powered by solar energy and used to make added-value chemicals and green fuels, providing grid-stability, energy security, and environmental benefits. A two-dimensional finite-elements model for porous electrodes was developed and validated against experimental data, allowing the design and performance improvement of a porous zinc cathode morphology and its operational conditions for an electrolyzer producing syngas via the co-electrolysis of CO2 and water. Porosity, pore length, fiber geometric shape, inlet pressure, system temperature, and catholyte flow rate were explored, and these parameters were thoroughly tuned by using the smart-search Nelder-Mead's multi-parameter optimization algorithm to achieve pronouncedly higher, industrial-relevant current density values than those previously reported, up to 263.6 mA/cm2 at an applied potential of −1.1 V vs. RHE.

Upscaling of Reactive Mass Transport through Porous Electrodes in Aqueous Flow Batteries

Porous electrodes (PEs) are an important component of modern energy storage devices, such as lithium-ion batteries, flow batteries or fuel cells. Their complicated multiphase structure presents a considerable challenge to modeling and simulation. In this paper, we apply the volume-averaging method (VAM) as an efficient approach for the evaluation of effective macroscopic transport parameters in PEs. We consider the transport of electro-active species coupled to heterogeneous Butler-Volmer type reactions at the electrode surface. We identify the characteristic scales and dimensionless groups for the application to aqueous flow batteries. We validate the VAM-based model with direct numerical simulation results and literature data showing excellent agreement. Subsequently, we characterize several simplified periodic PE structures in 2D and 3D in terms of hydraulic permeability, effective dispersion and the effective kinetic number. We apply the up-scaled transport parameters to a simple macroscopic porous electrode to compare the overall efficiency of different pore-scale structures and material porosity values over a wide range of energy dissipation values. This study also reveals that the Bruggeman correction, commonly used in macroscopic porous electrode models, becomes inaccurate for realistic kinetic numbers in flow battery applications and should be used with care.

Environment-Adaptive Online Learning for Portable Energy Storage Based on Porous Electrode Model

Environment-Adaptive Online Learning for Portable Energy Storage Based on Porous Electrode Model

Modeling, Model Calibration, and Characterization of Graphite Anodes from Coal Derived Carbon for Battery Applications

Most current production electric vehicles (EVs) contain cells with a battery pack or module in order to maintain electrical conductivity, prevent fatigue due to vibrations, and provide efficient cooling. Various module hardware has to be designed in order to facilitate the efficient rejection of heat and contain volume change due to operation and battery cell aging. As we see a shift towards EVs both globally and domestically, the time from concept development to vehicle production needs to rapidly decrease. The use of model-based engineering is critical to driving rapid design and virtual prototyping of electric vehicles to ensure that battery module components meet EV requirements.Currently, empirical data is used to characterize and parameterize battery models that are used to drive design decisions at the battery cell and battery module level. Typical vehicle level use profiles such as the Hybrid Pulse Power Characterization (HPPC) profile, constant rate discharge, constant rate charging, and US06 drive cycles are traditionally used to parameterize battery models. In this work, we discuss select cell level testing profiles, evaluate the electrochemical transport and reaction parameters that are estimated using a porous electrode model, and provide insight and recommendations based on data from coal derived graphite for battery applications.References Garrick, T. R., Gao, J., Yang, X., & Koch, B. J. (2021). “Modeling Electrochemical Transport within a Three-Electrode System.” Journal of The Electrochemical Society, 168(1), 010530. Lopata, J. S., Garrick, T. R., Wang, F., Zhang, H., Zeng, Y., & Shimpalee, S. (2023). “Dynamic Multi-Dimensional Numerical Transport Study of Lithium-Ion Battery Active Material Microstructures for Automotive Applications.” Journal of The Electrochemical Society, 170(2), 020530.

Theoretical and Experimental Performance Characterization of Battery Anodes Comprised of Coal Derived Graphite

As demand for energy storage expands, securing reliable supply-chains for raw battery materials is critical in mitigating the threats of delayed manufacturing and inflated manufacturing costs. These issues have pushed researchers and industry to consider alternative sources for traditional lithium-ion battery materials, such as graphite. Commonly available sources of graphite for Li-ion anodes include natural graphite and petroleum coke derived synthetic graphite. While many carbon sources have been studied, most require complicated processing and result in sub-par graphite anode materials. This talk highlights raw coal and coal derived materials as promising graphite sources thanks to their abundance of sp2 hybridized carbon, resulting in simpler processing, and low material costs.We show that coal derived graphite can be implemented as a lithium-ion anode, however, many chemical, microstructural, and morphological considerations are necessary to compare with commercial graphite performance. We will experimentally compare the morphology at the microstructure for graphite from various carbon sources, and then apply the virtual microstructure analysis method as detailed by Lopata et al to quantify potential differences between the anode microstructures with respect to effective active material particle size (diffusion length calculation), the electrochemical surface area, and the tortuosity of the liquid phase domain. These calculations from representative 3D microstructures can then be extended to a porous electrode model as detailed in Garrick et al to probe the impact in ionic, kinetic, and ohmic losses in the system.References Joseph S. Lopata et al 2023 J. Electrochem. Soc. 170 020530Taylor R. Garrick et al 2021 J. Electrochem. Soc. 168 010530

(Invited) The Effect of Surfactants on Vanadium Electrochemistry in Porous Carbon Electrodes

We present a study of vanadium electrochemistry in porous electrodes with and without sodium dodecyl sulfate (SDS) additives present. Carbon paper and felt electrodes are compared. Carbon felt electrodes are the de facto standard in flow batteries. There are significant differences in the electronic conductivity of these media. The carbon paper has a much higher conductivity than the felt, which has an electronic conductivity similar to the ionic conductivity of the electrolytes used. The latter fact motivates an extension of available theory for polarization curve fitting to explicitly include both electronic and ionic contributions in the porous electrode. We demonstrate agreement between our model and an existing model evaluated at the limit where electrode conductivity is significantly greater than electrolyte conductivity. Our model uses the product of the electrode surface area per volume ratio and the heterogeneous electron transfer rate constant, and the Damköhler number as fitting parameters for carbon paper and felt electrodes. We discuss the effect of SDS additives on kinetics in the context of the cation hydration shell for the vanadium ions. To this point, we apply Marcus-Hush-Chidsey formalism to our porous electrode model to quantify reorganization energies obtained through polarization curve fitting. This work was supported as part of the Breakthrough Electrolytes for Energy Storage (BEES2), an Energy Frontier Research Center funded by the United States. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0019409.

(Invited) Mixed Conduction in Ceramic Electrolytes For Intermediate-Temperature Fuel Cells and Electrolyzers

High-temperature fuel cells and electrolyzers (e.g., T > 700 ˚C) rely on oxide electrolytes such as stabilized cubic zirconia that conduct a single defect, oxygen vacancies. Intermediate-temperature electrochemical cells (e.g., T < 650 ˚C) utilize mixed conducting ceramic electrolytes, that conduct multiple defects. Operating at T < 600 ˚C facilitates lower-cost interconnect materials and balance-of-plant components, but the mixed conductor behavior can reduce fuel cell voltages and lower electrolyzer faradaic efficiencies. Predicting behavior of these mixed conductors, even at open-circuit voltage, requires modeling the coupled transport of the multiple conducting defects in the electrolyte. Detailed models of mixed conductors coupled to porous electrode models can simulate cell performance over a broad range of operating conditions. This presentation highlights models of two types of cells with mixed conducting oxide electrolytes. Firstly, gadolinium-doped ceria (GDC) primarily conducts oxygen vacancies but also some electrons via a reduced-ceria small polaron, but it performs well in intermediate temperature solid-oxide fuel cells [1]. Secondly, yttrium-doped barium zirconates (BZY) primarily conducts protons but also oxygen vacancies and small polarons, which contribute to electronic leakage. Variants of BZY electrolytes perform well in fuel cells and electrolyzers [2-4]. This paper focuses on cell-level models of these mixed-conductors and how to identify favorable regions for high performance in fuel cells and electrolyzers. Zhu, A. Ashar, R.J. Kee, R.J. Braun, G.S. Jackson, “Physics-based model to represent the membrane-electrode assemblies of solid-oxide fuel cells based on gadolinium-doped ceria,” J. Electrochem. Soc., Under revision, 2023.J. Kee, S. Ricote, H. Zhu, R.J. Braun, G. Carins, J.E. Persky, “Perspectives on technical challenges and scaling considerations for tubular protonic-ceramic electrolysis cells and stacks ,” J. Electrochem. Soc. 169:054525 (2022).Zhu, Y. Shin, S. Ricote, R.J. Kee, “Defect incorporation and transport in dense BaZr0.8Y0.2O3-d membranes and their impact on hydrogen separation and compression,” J. Electrochem. Soc., Under revision, 2023.Zhu, S. Ricote, R.J. Kee, “Thermodynamics, transport, and electrochemistry in proton-conducting ceramic electrolysis cells,” in High Temperature Electrolysis, W. Sitte and R. Merkle, Editors, IOP Publishing, 2023.

Three-Dimensional Microstructure-Based Deformation Predictions in Battery Electrodes

To give insight on the deformation behavior of the porous electrodes for a lithium-ion battery induced by mechanical stresses, a three-dimensional microstructure-based modeling method (3DMS) is developed [1,2]. The 3DMS simulation applies a Finite Element Analysis (FEA) based solid-stress model to track the composite (active material (AM) and binder) porous electrodes response to compression (external forces), reversible expansion (intercalation dependent), and irreversible expansion (aging dependent).The microstructures used for the electrodes were generated using a modified version of the opensource program MATBOX, developed by NREL, in conjunction with in-house code to create a cutout section of a representative lithium-ion cell. Both anode and cathode structures are stochastically generated and applied with an advanced volume element generation technique to resolve small interface and boundary domains created by binder generation. The applied solid-stress model couples different stresses from local mechanical behavior for the different materials present in the system, considering the individual materials respective mechanical properties. The coupled stresses are used to predict deformation in the microstructure domain. Further analysis on the behavior of the deformation is enacted to gain insights on the design of the porous electrodes.Predictions of the electrodes’ deformation will be validated using experimental work at various length scales and compared to continuum porous electrode models from previous works. Furthermore, development of techniques using the FEA solid stress-model platform will be implemented to predict full cell-level deformation for various macro cell designs including jell-roll, pouch cells, and full module assemblies [3-8]. Ultimately, insights gained from the simulations will be used to understand irreversible deformation in electrodes after cycling and inform microstructure design- particularly for automotive-relevant applications [9]. J. S. Lopata, T. R. Garrick, F. Wang, H. Zhang, Y. Zeng and S. Shimpalee, ECSarXiv (2022).J. S. Lopata, T. R. Garrick, F. Wang, H. Zhang, Y. Zeng and S. Shimpalee, J Electrochem Soc, 170, 020530 (2023).D. J. Pereira, J. W. Weidner and T. R. Garrick, J Electrochem Soc, 166, A1251 (2019).D. J. Pereira, M. A. Fernandez, K. C. Streng, X. X. Hou, X. Gao, J. W. Weidner and T. R. Garrick, J Electrochem Soc, 167, 080515 (2020).D. J. Pereira, A. M. Aleman, J. W. Weidner and T. R. Garrick, J Electrochem Soc, 169, 020577 (2022).T. R. Garrick, K. Kanneganti, X. Y. Huang and J. W. Weidner, J Electrochem Soc, 161, E3297 (2014).T. R. Garrick, K. Higa, S.-L. Wu, Y. Dai, X. Huang, V. Srinivasan and J. W. Weidner, J Electrochem Soc, 164, E3592 (2017).T. R. Garrick, X. Huang, V. Srinivasan and J. W. Weidner, J Electrochem Soc, 164, E3552 (2017).T. R. Garrick and J. W. Weidner, in Electrochemical Society Meeting s 233, p. 1345 (2018).

Quantifying Volume Change in Battery Electrode Microstructures

A three-dimensional microstructure-based modeling method (3DMS) is developed [1, 2] to predict the mechanical behaviors of porous electrode that includes both active material (AM) and binders inside lithium-ion battery during compression, reversible expansion (intercalation dependent), and irreversible expansion (aging dependent). The geometry of both AM and binder is generated stochastically to create microstructures by using a modified MATBOX opensource program with our in-house code. The advanced technique of volume element generation is also established to overcome the interfacial error due to complexity of binder distribution. The Finite Element Analysis (FEA) based solid-stress model is applied to predict local mechanical activities including stress-strain relationship and microstructure evolution during compression, intercalation, and aging.The predictions of behavior in the porous electrode under compressive loads will be presented and the insights into the microstructure deformation will be discussed. Further, the analysis results will be validated using experimental testing results acquired at various length scales. This work can be used for the model-based FEA platform to predict the impact of volume change under charge/discharge cycles, volume change of a jelly-roll during formation, and the response of the active material in typical pouch cells during battery module assembly.The predictions of reversible expansion in the porous electrode during cycling will be presented and the insights into the microstructure expansion as compared to continuum porous electrode models from previous work [3-8] will be discussed.Finally, insights into the linkage between irreversible volume expansion in the porous electrode during aging cycling will be discussed and compared to testing acquired for automotive-relevant large format [9] pouch cells.References J. S. Lopata, T. R. Garrick, F. Wang, H. Zhang, Y. Zeng and S. Shimpalee, ECSarXiv (2022).J. S. Lopata, T. R. Garrick, F. Wang, H. Zhang, Y. Zeng and S. Shimpalee, J Electrochem Soc, 170, 020530 (2023).D. J. Pereira, J. W. Weidner and T. R. Garrick, J Electrochem Soc, 166, A1251 (2019).D. J. Pereira, M. A. Fernandez, K. C. Streng, X. X. Hou, X. Gao, J. W. Weidner and T. R. Garrick, J Electrochem Soc, 167, 080515 (2020).D. J. Pereira, A. M. Aleman, J. W. Weidner and T. R. Garrick, J Electrochem Soc, 169, 020577 (2022).T. R. Garrick, K. Kanneganti, X. Y. Huang and J. W. Weidner, J Electrochem Soc, 161, E3297 (2014).T. R. Garrick, K. Higa, S.-L. Wu, Y. Dai, X. Huang, V. Srinivasan and J. W. Weidner, J Electrochem Soc, 164, E3592 (2017).T. R. Garrick, X. Huang, V. Srinivasan and J. W. Weidner, J Electrochem Soc, 164, E3552 (2017).T. R. Garrick and J. W. Weidner, in Electrochemical Society Meeting s 233, p. 1345 (2018).

Electrochemical impedance response of a thick and porous calcium carbonate layer deposited by thermal growth on a carbon steel electrode

Thermal calcareous scaling has been studied on a simulated heat-exchange carbon steel cylinder set-up in a CaCO3 -rich CO2-saturated solution (SIcalcite=2) using in situ electrochemical methods and ex situ surface characterization. Open circuit potentiometry (OCP) and electrochemical impedance spectroscopy (EIS) indicate dramatic increases to the impedance of the electrode over the first two days. The response quickly changes from a single capacitive loop in the early hours to an impedance characterized by a 45° slope at low frequency in Nyquist format, suggesting fast precipitation kinetics towards a protective state. At the end of 14-day tests, SEM and µ-Raman spectroscopy detected a distinctly layered aragonite-calcite scale. The bi-layered precipitate film is proposed to limit the diffusion of cathodic reactants across the porous, permeable scale, resulting in the 45° slope linear tail characteristic of the so-called transmission line response. EIS responses are experimentally re-evaluated away from open circuit, prior to proposing a bespoke equivalent circuit based on De Levie’s porous electrode model to link the electrochemical data to surface observations.

A multiscale electrochemo-mechanical model of porous electrode in lithium-ion batteries: The coupling of reaction and finite deformation

In this study, we present a novel multiscale electrochemo-mechanical model for porous electrodes in lithium-ion batteries (LIBs). Building upon the theory of finite deformation and the pseudo-two-dimensional framework introduced by Doyle et al. (Journal of the Electrochemical Society, 140, 6(1993)), our model incorporates the interaction between particle deformation and multiscale electrochemical processes, which has been lacking in previous research. The model is validated by the rate performance tests on SiO-based electrodes. By utilizing the model, we analyze the coupling effects among reaction distribution, porosity variation, and local geometrical distortion of the electrode at high current rates. We also investigate the relative importance of different electrochemo-mechanical coupling paths for materials with large volume expansion ratios. Comparing the comprehensive model with simplified versions, we find that diffusion stress is a priority due to its significant impact on ionic diffusion within particles. Porosity variation becomes equally significant at high current rates, as ionic transport in the electrolyte becomes the rate-limiting step. This work contributes to enhance understanding of the interplay between electrochemical and mechanical phenomena in LIB electrodes, especially for silicon-based materials.

Modeling, Model Calibration, and Characterization of Graphite Anodes from Coal Derived Carbon

Most current production electric vehicles (EVs) contain cells with a battery pack or module in order to maintain electrical conductivity, prevent fatigue due to vibrations, and provide efficient cooling. Various module hardware has to be designed in order to facilitate the efficient rejection of heat and contain volume change due to operation and battery cell aging. As we see a shift towards EVs both globally and domestically, the time from concept development to vehicle production needs to rapidly increase. The use of model-based engineering is critical to driving rapid design and virtual prototyping of electric vehicles to ensure that battery module components meet EV requirements.Currently, empirical data is used to characterize and parameterize battery models that are used to drive design decisions at the battery cell and battery module level. Typical vehicle level use profiles such as the Hybrid Pulse Power Characterization (HPPC) profile, constant rate discharging, constant rate charging, and US06 drive cycles are traditionally used to parameterize battery models. In this work, we discuss select cell level testing profiles, evaluate the electrochemical transport and reaction parameters that are estimated using a porous electrode model, and provide insight and recommendations based on data from coal derived graphite for battery applications.

(Invited) How to Identify, Parameterise and Use Complex Electrode Kinetic Models

Electrochemical engineering transfers knowledge from surface/material level to higher levels to build well performing porous electrodes, cells and systems. Modeling accompanies this process at all levels, as it allows for deeper insights into limitations and for subsequent knowledge-driven design and optimization. Modeling tools at higher levels are already well established and predictive, when it comes to heat and mass transfer, as these phenomena are widespread in engineering. They struggle, however, to reproduce or predict the electrochemical behavior of electrodes.The situation at the electrode level becomes tricky, as there frequently are complex reaction networks at the surface. These lead to a relationship of current, voltage and reactants, which cannot be accurately described by a Tafel- or Butler-Volmer-type rate equation. Identifying suitable reaction network models including reaction kinetic equations and parameterizing them is a challenge; this talk will present promising solutions and their application to selected examples from the electrolysis and battery field.Besides the electrode surface itself, changes in electrolyte composition close to the electrode surface may occur, which alter electrode performance dramatically. Models may need to account not only for diffusion and migration of the main reactants, but also for transport of intermediates and chemical reactions in the electrolyte as well as phase changes. How can one be sure to have implemented proper processes and parameters? Also here, the talk will give an insight into promising solutions and the application to selected examples including oxygen reduction and CO2 reduction.The thus identified and parameterized models reveal process limitations and sensitivities which allow for better design and operation of the electrodes. They are further suitable for integration into porous electrode or cell models for design and diagnosis purposes on higher levels.

Exploring Surfactant Electrolyte-Porous Electrode Interactions

Microemulsions are an alternative to aqueous organic electrolytes for redox flow batteries (RFB) that can be modified without synthesis. Water, oil, and emulsifiers, when combined using optimized chemistries and compositions, produce stable mixtures with aggregate structures (microemulsions). Design limitations imposed by the interdependency of redox solubility and electrolyte conductivity are circumvented by solubilizing redox active molecules in the oil phase and supporting ions in the aqueous phase. While macroscopically homogeneous, microemulsions are biphasic at the nanometer scale, effectively decoupling solubility from conductivity. Recently, the first microemulsion RFB was demonstrated using ferrocene solubilized in an anionic surfactant system. Polarization curve analysis revealed performance limitations seemingly arising from electrolyte interactions with the cell components including porous electrodes. Further optimization requires a deeper understanding of microemulsion-porous electrode interactions.Due to the complexity of microemulsion electrochemistry, a first step towards understanding these interactions is made by investigating the relationship between surfactants and porous electrodes. Vanadium redox flow battery (VRFB) electrolytes with surfactant additives are used as a model system to explore fundamental interactions, in situ, between a surfactant-containing electrolyte and a porous electrode. Transport and kinetics are evaluated as a function of electrode, electrolyte, and surfactant properties. Redox species transport is quantified through mass transfer coefficients, using a generalization of an existing porous electrode model fit to experimental polarization curve data. Electrochemical impedance spectroscopy, interpreted using existing kinetic models, is also employed to gain insight into the effects of surfactant on electron transfer rates.

Modeling the Coulombic Efficiency of the Aqueous Iron Electrode

The aqueous iron electrode is attractive for large-scale energy storage because of its long life and low materials cost. The redox potential of the Fe ↔ Fe2+ reaction is approximately 50 mV negative of that of the hydrogen evolution reaction. Hydrogen evolution therefore causes self discharge of the iron electrode during rest and competes with the iron reaction during charge. Here we model the effect of electrode design and cell operation on the coulombic efficiency of charging the iron electrode at modest charging rates (<C/20). Kinetic parameters (exchange current density and transfer coefficients) for the iron dissolution-precipitation reaction and for the hydrogen evolution reaction are estimated from experimental measurements of electrode overpotential and hydrogen generation rate. The volume change from Fe(OH)2 to Fe metal results in a significant increase in porosity during charge and results in change in electrochemically active surface area. The porous-electrode model includes transport in the electrolyte, Butler-Volmer kinetics, and change in volume and surface area.The model provides a theoretical explanation for experimental observations that coulombic efficiency decreases with decreasing charge rate, and that coulombic efficiency is lower in thicker electrodes. We then use the model to explore the impact for large-format cells of electrode size and design on coulombic efficiency.