Porous Electrode Model Research Articles

(Invited) Low Temperature Electrolysis for Hydrogen at Scale

This presentation will focus on durable, high-performance materials and interfaces for advanced water splitting, enabling a clear pathway for achieving <$2/KgH2 (on scale) with efficiency of 43 KWh/Kg using anion exchange membrane interface. Advances via fundamental understanding of both hydrogen and oxygen evolution reactions (HER/OER) leading to novel materials will be in conjunction with critical improvements in membrane and ionomers and gas evolution electrodes with corresponding characterization and testing. Progress towards these goals under a three-year multifaceted and comprehensive effort will be described wherein Northeastern University (NEU) will present catalyst development and characterization (both in situ and ex situ). University of Delaware (UD) will showcase improvements in ionomer and membrane materials. In addition, close collaboration with National Laboratory partners with Lawrence Berkeley National Lab (LBNL) participating in multiscale modeling and computation in close concert with Sandia National Laboratory (SNL) providing MD simulations of the membrane catalyst interface and National Renewable Energy Laboratory (NREL) providing advanced ionomers, durability protocols and validation will be described. Anion exchange membrane electrolyzers (AEMELs) are ideally suited with a low-cost profile enabled by platinum group metal (PGM)-free catalysts, low fluorine content membranes, and a less corrosive environment for cell separators. This presentation will showcase state of the art stable, high-conductivity, and high-strength AEMs, stable and active PGM-free catalysts for hydrogen and oxygen evolution reaction (HER/OER), and high performance electrode architectures that together can unlock the cost advantages of AEMELs. If successful, the developed technology can meet FCTO efficiency targets, delivering carbon-neutral hydrogen at $2/kg while simultaneously enabling higher penetrations of wind and PV electricity on the grid. The overall goal is cell level performance of 1.62 V at 1 A/cm2, which meets the FCTO efficiency target of 43 kWh/kg. Component performance targets have been established using a porous electrode model to support the overall cell performance target. This is at the modeled scale of 50,000 kg/day and operating at 1 A/cm2 resulting in hydrogen cost at $2.15, $1.82, or $1.76/kg, respectively (2, 20, 200 plants). In the low-volume manufacturing case, it is still possible to meet the cost target by operating near 2 A/cm2, sacrificing some efficiency.

Electrochemical Modeling and Simulation of a Three-Electrode Lead Acid Cell

The low initial cost, ease of manufacture and relatively mature state of technology make lead-acid batteries the most popular energy storage choice for a variety of applications. Lead-acid batteries exist in different designs and configurations, each suitable for different use cases and performance targets such as cycle life, rate performance, and cost. Further performance and design improvements remain of significant salience to battery manufacturers.1–3 Electrochemical modeling can greatly complement the product development process by enabling the quick evaluation new lead-acid designs, reducing the amount of testing and validation required, and their attendant temporal and economic demands. Several continuum-level models with varying predictive capabilities have been reported in the literature. These have been used to examine the effect of various material and cell design parameters on performance.4–7 This talk describes an electrochemical modeling study for a lead-acid test cell comprising two positive electrodes engaging a single negative electrode. The one-dimensional electrochemical model employed herein is adapted from classical porous electrode models for flooded lead-acid cells. Along with the porous electrode, separator and reservoir domains of a single cell model, the test cell includes additional cathode and reservoir regions. This configuration may be observed in the terminal plates in a commercial battery layout. Classical electrochemical models,4,6 which analyze a single repeat unit in a full battery, are extended to this non-standard configuration by modeling additional porous electrode and reservoir regions. The non-standard model necessitates appropriate boundary conditions to define the coupling between electrochemical variables in different regions, since the symmetry boundary conditions typically used are no longer applicable. The modified model equations and new boundary conditions are then simulated using a finite-difference scheme for discharge and compared against experimental data for standard performance tests, namely C20, Reserve Capacity (RC) and Cold Cranking Amperage (CCA). The salient modeling and simulation differences with a standard lead-acid model are also discussed, as are the trends in predicted electrochemical variables. We expect this model to be of substantial utility in guiding the evaluation of new battery designs through the improved prediction of performance in the test cell configuration. Acknowledgements This research was supported by EXIDE Technologies. The authors also acknowledge financial support from the Department of Chemical Engineering and the Clean Energy Institute at the University of Washington. References G. J. May, A. Davidson, and B. Monahov, J. Energy Storage, 15, 145–157 (2018)J. Lannelongue, M. Cugnet, N. Guillet, and A. Kirchev, J. Power Sources, 352, 194–207 (2017).A. Jaiswal and S. C. Chalasani, J. Energy Storage, 1, 15–21 (2015).H. Gu, T. V. Nguyen, and R. E. White, J. Electrochem. Soc., 134, 2953–2960 (1987).W. B. Gu, C. Y. Wang, and B. Y. Liaw, J. Electrochem. Soc., 144, 2053–2061 (1997).M. Cugnet, S. Laruelle, S. Grugeon, B. Sahut, J. Sabatier, J.-M. Tarascon, and A. Oustaloup, J. Electrochem. Soc., 156, A974–A985 (2009).Nguyen, T. V., R. E. White, and H. Gu, J. Electrochem. Soc., 137, 2998–3004 (1990).

Global Sensitivity Analysis To Characterize Operational Limits and Prioritize Performance Goals of Capacitive Deionization Technologies.

Capacitive deionization (CDI) technologies couple electronic and ionic charge storage, enabling improved thermodynamic efficiency of brackish desalination by recovering energy released during discharge. However, insight into CDI has been limited by discrete experimental observations at low desalination depths (Δ c, typically reducing influent salinity by 10 mM or less). In this study, the performance and sensitivity of three common CDI configurations [standard CDI, membrane CDI (MCDI), and flowable electrode CDI (FCDI)] were evaluated across the operational and material design landscape by varying eight common input parameters (electrode thickness, influent concentration, current density, electrode flow rate, specific capacitance, contact resistance, porosity, and fixed charge). All combinations of designs were evaluated for two influent concentrations with a calibrated and validated one-dimensional (1-D) porous electrode model. Sensitivity analyses were carried out via Monte Carlo and Morris methods, focusing on six performance metrics. Across all performance metrics, high sensitivity was observed to input parameters which impact cycle length (current, resistance, and capacitance). Simulations demonstrated the importance of maintaining both charge and round-trip efficiencies, which limit the performance of CDI and FCDI, respectively. Accounting for energy recovery, only MCDI was capable of operating at thermodynamic efficiencies similar to reverse osmosis.

Wettability in electrodes and its impact on the performance of lithium-ion batteries

Wettability by the electrolyte is claimed to be one of the challenges in the development of high-performance lithium-ion batteries. Non-uniform wetting leads to inhomogeneous distribution of current density and unstable formation of solid electrolyte interface film. Incomplete wetting influences the cell performance and causes the formation of lithium plating in the anode, which leads to safety issue. Research has pointed out that insufficient wetting could be found in the electrode, and the wetting characteristics would be different in each electrode. Here we use lattice Boltzmann simulation to show the electrolyte distribution and understand the wetting characteristics in the cathode and anode. We develop a multiphase lattice Boltzmann model with the reconstruction of electrode microstructure using a stochastic generation method. We use a porous electrode model to identify the effect of wettability on the cell performance and to elucidate the dependence of capacity on the wettability. Our results would lead to more reliable lithium-ion battery designs, and establish a framework to inspect the wettability inside electrodes.

Porous Electrode Model with Particle Stress Effects for Li(Ni1/3Co1/3Mn1/3)O2 Electrode

A porous electrode model, incorporating particle stress effects, is developed for the electrode kinetic processes in the positive Li(Ni1/3Mn1/3Co1/3)O2 or NMC111 electrode. The model is used to analyze experimental data from galvanostatic intermittent titration technique (GITT) during charging at the beginning of life. The equilibrium potential accounts for the influence of mechanical stress in the electrode particles. While the standard Newman-based model proves unable to capture the dynamic performance of NMC111, the extended model with stress allows good fits of the GITT responses for NMC half cells for a voltage range from 3.7–4.1 V vs Li/Li+ at 10°C, 25°C and 40°C. Four physical parameters are extracted to analyze the underlying diffusive, kinetic, thermodynamic and stress phenomena from polarization to relaxation during a GITT transient. Strong dependencies of the kinetic rate constant k, slope of the open-circuit potential curve dEconc/dxpos and stress proportionality factor ϒstress with lithium concentration are found. The effective diffusion coefficients Ds,eff are ∼10−14 – 10−13 cm2/s across voltages and temperatures. Diffusion limitation and particle surface stress are more profound at higher voltages and at higher temperatures. This leads to large lithium concentration gradient near particle surface, requiring longer relaxation time during GITT.

Proton Transport in Catalyst Layers of a Polymer Electrolyte Water Electrolyzer: Effect of the Anode Catalyst Loading

The understanding of loss terms in polymer electrolyte water electrolysis (PEWE) cells is essential to maximize efficiency and minimize cost and enable the technology for energy applications, such as “power-to-X”. We adapt the use of the transmission-line model for porous electrodes in conjunction with electrochemical impedance spectroscopy measurements of the cell in the H2/N2 mode known for fuel cells to PEWE cells to quantify proton transport resistance and double layer capacitance of the anode catalyst layer. Anode catalyst loading was varied between 0.05 and 3.2 mgIrO2 cm−2. A non-linear relationship was found between the anode IrO2 loading and the proton transport resistance. Catalyst layers with very low IrO2 loading (0.05–0.16 mgIrO2 cm−2) had ∼4 times higher mass-specific activity and ∼2 times higher mass-specific capacitance, revealing an inhomogeneous utilization of the catalytic material. The kinetic and transport limitations associated with the anode catalyst layer have been correlated with its morphological features.

Asymptotic Reduction of a Porous Electrode Model for Lithium-Ion Batteries

We present a porous electrode model for lithium-ion batteries using Butler--Volmer reaction kinetics. We model lithium concentration in both the solid and fluid phase, along with solid and liquid e...

Multi-Species, Multi-Reaction Model for Porous Intercalation Electrodes: Part II. Model-Experiment Comparisons for Linear-Sweep Voltammetry of Spinel Lithium Manganese Oxide Electrodes

In Part 1 of this series, we formulated a porous electrode model including multiple lithium-insertion species and associated electrochemical and homogenous reactions. In this paper, the multi-species multi-reaction (MSMR) model is used to simulate linear-sweep voltammetry (LSV) data of a porous electrode of spinel lithium manganese oxide (LMO) at different scan rates, and values of transport, kinetic, and thermodynamic parameters are fit to the measured data. Twenty model parameters are optimized to simultaneously fit the slowest and fastest sweep rates. The optimization is computationally intensive, but the calculation times were reduced by almost one half using a perturbation formula valid at the lowest sweep rate, developed in Part 1, together with numerical solutions at higher sweep rates. A sensitivity analysis determines the importance of different model parameters on the results. The fitting is most sensitive to the values of the Open Circuit Voltage (OCV) and it was necessary include a small amount of hysteresis, so that the OCV is slightly different on forward and backward scans. The next most important fitting parameter was the solid phase diffusion coefficient. Of smaller importance are the kinetic parameters and ohmic losses in the solid and liquid phases of the electrode.

Faster Lead-Acid Battery Simulations from Porous-Electrode Theory: Part II. Asymptotic Analysis

Electrochemical and equivalent-circuit modeling are the two most popular approaches to battery simulation, but the former is computationally expensive and the latter provides limited physical insight. A theoretical middle ground would be useful to support battery management, on-line diagnostics, and cell design. We analyze a thermodynamically consistent, isothermal porous-electrode model of a discharging lead-acid battery. Asymptotic analysis of this full model produces three reduced-order models, which relate the electrical behavior to microscopic material properties, but simulate discharge at speeds approaching an equivalent circuit. A lumped-parameter model, which neglects spatial property variations, proves accurate for C-rates below 0.1C, while a spatially resolved higher-order solution retains accuracy up to 5C. The problem of parameter estimation is addressed by fitting experimental data with the reduced-order models.

Faster Lead-Acid Battery Simulations from Porous-Electrode Theory: Part I. Physical Model

An isothermal porous-electrode model of a discharging lead-acid battery is presented, which includes an extension of concentrated-solution theory that accounts for excluded-volume effects, local pressure variation, and a detailed microscopic water balance. The approach accounts for three typically neglected physical phenomena: convection, pressure diffusion, and variation of liquid volume with state of charge. Rescaling of the governing equations uncovers a set of fundamental dimensionless parameters that control the battery’s response. For the discharge situations considered here, total volume change and nonuniform pressure effects prove to be negligible because variations occur in just one spatial dimension. A numerical solution of a simplified model is developed and exploited to predict transient cell voltages and internal concentration profiles in response to a range of C-rates. The dependence of discharge capacity on C-rate deviates substantially from Peukert’s simple power law: charge capacity is concentration-limited at low C-rates, and voltage-limited at high C-rates. The model is fit to experimental data, showing good agreement.

A Physics-Based Model Capacity Fade Analysis of LiMn2O4/Graphite Cell at Different Temperatures

The capacity fading behavior of a LiMn2O4/graphite lithium ion cells at different temperatures is analyzed using a physics-based porous composite electrode model and a parameter estimation technique. The parameter estimation technique is used to extract capacity fade dependent model parameters from experimental cycling data. Although the capacity fading mechanism of the LiMn2O4/graphite lithium ion cells are greatly influenced by temperature, major capacity fading mechanism is closely related to the trapping of Li ions into solid electrolyte interphase on the graphite negative electrode and the reduction in the volume fraction of the active material in the LiMn2O4 positive electrode. At 25°C, the dominant capacity fading mechanisms is the formation of the solid electrolyte interphase while at 60°C the dominant capacity fading mechanism is the reduction in the volume fraction of the positive active material. The efficacy of the physics-based composite electrode model is validated with experimental discharge profiles obtained from cells cycled at 25 and 60°C.

Multi-scale impedance model for supercapacitor porous electrodes: Theoretical prediction and experimental validation

In this paper, a multi-scale impedance model is developed for evaluating the charge storage capacity and ion-transfer rate of supercapacitor porous electrodes. This model can be used to theoretically understand the contributions of the intra-particle pore at nano-scale, the inter-particle pore at micron-scale, and the porous electrode at millimeter-scale to the performance of supercapacitor. Then, impedance spectrum characteristics of porous electrodes are screened via numerical simulation based on the developed multi-scale impedance model, especially for different electrode thicknesses and various faradaic processes with different time constants. Subsequently, seven supercapacitor samples, four of them with electrode thicknesses of 60, 100, 180, and 370 μm respectively, and three of them with different types of current collectors (nickel, carbon-coated stainless-steel, and stainless-steel), are fabricated and characterized to validate the developed model. The fitting results and residuals of the measured impedance data validate the linear relationship of the scaled equivalent resistance of electrolyte v. s. electrode thickness and the scaled characteristic time constant v. s. the square of electrode thickness. The validated multi-scale impedance model might offer an effective method to rationally balance or optimize charge storage and charge transfer via morphology design for porous electrodes.

Critical Role of Supporting Electrolyte Selection on Flow Battery Cost: Combined Effects of Cell Materials and Operation

With commercial development of redox flow batteries (RFBs) underway and given the importance of capital cost, we analyze RFB supporting electrolyte charge carrier, membrane, open-circuit voltage (OCV), cell performance, and active species size as possible major cost drivers. Physics-based and techno-economic modeling includes a porous electrode model and accounts for chemical and material selection and amounts, stack and tank sizing, and flow-dependent pumping power.1 The 1D porous electrode model, validated in a prior study,2 couples mass transfer, charge transfer, and ohmic transport to estimate the electrode contribution to full cell area specific resistance (ASR). Primary findings for a 5-h discharge of an aqueous RFB are that present costs are in general $10 ~ 100/kWh lower with 1.4 V versus 1.0 V OCV, size selective separators (SSS) versus ion exchange membranes (IEM), as shown in Figure 1a. Large active species size alone would increase costs slightly due to higher viscosity and thus higher pumping cost, reduced mass transfer, and higher mass-based electrolyte cost versus lower molecular weight actives. However, large actives enable SSS use, and the slight incremental cost is more than offset by cost reductions from the high conductivity and inherent low cost of SSS membranes. In the future, the costs of different scenarios converge assuming significant cost reductions in the stack, membranes, and unit cost less materials. In Figure 1b, multiple options, led by H+/IEM with small active species and OCV > 1.3 V, can meet a Joint Center for Energy Storage Research target adjusted to $100/kWh for 5-h storage. Implications of the work here are: 1) recommendations on the path to enable low cost aqueous RFBs, and 2) a modeling tool and opportunity to quantify cost implications of additional factors held constant in the present study. One recommendation is the further exploration of IEMs using H+ or Na+ and SSS, as these membranes have relatively favorable likelihoods to enable system cost targets. We also suggest development work towards redox couples and conditions for high OCV, which enhances electrolyte energy content and stack power. Subsequently, engineering effort and adoption of RFBs is needed to enable significant reduction in cost through high volume manufacturing of stacks, membranes, and overall RFB systems. Lastly, one can use the present model to explore the effect of various additional parameters on cost. Such parameters include: the cell components (e.g., electrode properties, membrane thickness) affecting ASR, the operation (e.g., flow rate, V-I operating point) affecting ASR, and the application (e.g., discharge duration) affecting power-related cost (e.g., stack) versus energy-related cost (e.g., electrolyte volume, tank size). Acknowledgments This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy. JDM acknowledges additional support from the NSF Graduate Research Fellowship.

(Invited) On the EIS Response of Composite Insertion Electrodes – Toward a Better Understanding of Porous Electrodes

Electrochemical Impedance Spectroscopy (EIS) is a powerful tool to assess and understand the degradation of Li-ion battery (LIB) performances during operation (upon charge and discharge cycles). The full interpretation of EIS spectra of LIB can be however tedious because these systems are composed of porous electrodes and complex electrolyte. Moreover, side reactions, such as the formation of an insulating layer around the particles of the porous electrodes, can occur in battery operation modifying the impedance response. A way to improve the interpretation of impedance data of LIB is the use of modeling. Most of the time, data interpretation is limited to the use of Electrical Equivalent Circuits (EEC) but the link between EEC and the physical processes involved in the system is ambiguous and difficult to establish. However, in the 60’s, Newman et al. [1] devised a model for porous battery electrodes. Their model describes the whole battery system thanks to equations that are directly related to the physical phenomena occurring in the composite electrodes. This model is still widely used in the battery field to simulate galvanostatic charge and discharge. Based on their work, and inspiring by works already done on modeling of EIS [2], it is possible to devise a model by linearizing all the equations [3]. The model obtained can then be directly connected to the physical phenomena occurring inside the porous electrodes which will greatly help to interpret EIS spectra. In this work, we developed an analytical solution for the electrochemical impedance of a porous electrode composed of spherical intercalation particles in contact with concentrated electrolyte. The proposed model which is based on the concentrated solution theory but which also considers kinetics limitations and both solution-phase and solid-phase diffusion limitations, can predict the EIS response of a porous electrode at any state of charge using a limited set of parameters. Moreover, the developed model can predict the optimal porosity of the electrode to be targeted during the manufacturing process for the best cycling performances depending on the composition of the electrode (volume fractions of active material, binder, and conductive agent). (1) Newman, J.S.; Tobias, C.W. Journal of the Electrochemical Society 1962 , 109(12), page 1183-1191. (2) Meyers, J.P. et al. ; Journal of the Electrochemical Society 2000 , 147(8), page 2930-2940. (3) Huang, J.; Zhang, J.B. Journal of the Electrochemical Society 2016 , 163(9), page A1983-A2000.

(Norman Hackerman Young Author Award) Multiphase Porous Electrode Theory

As batteries become increasingly common and important in products we use in our daily lives, it becomes similarly important to be able to model their behavior at full cell length and time scales. This capability facilitates both their design and use. The diverse applications in which batteries are applied each require careful design optimization in order to provide the most cost-effective solution satisfying the product needs, and experimentally exploring the full design space in order to find the optimum is impractical. Cell models help drastically narrow the space, enabling cost-effective product development. With existing cells, the models can help provide estimates of difficult-to-measurable internal variables, which may inform usage constraints. In order for the models to provide the most value and ability to extrapolate beyond their training data, their underlying physics must be as representative as possible of the real electrochemical system. Porous electrode theory has been developed, validated, and extended for many battery chemistries for decades and serves as the basis for most physical models at full cell length scales. In this work, we continue in that tradition by exploring the incorporation of new models for both the active particles and the interfacial electron transfer reaction kinetics. In the active particles, we employ non-equilibrium thermodynamics, enabling us to consistently describe particles with internal phase separation within a Cahn-Hilliard-type phase field framework. Many practically relevant electrode materials exhibit phase separation during lithium insertion, and various approaches have been used to model these systems, many of which either neglect the details of the resulting concentration profiles or describe the phase interfaces using numerically inconvenient phase boundaries with no interfacial tension. In the phase field approach, phase boundaries appear and disappear spontaneously according to the governing free energy functional without specialized numerical methods for interface tracking, and they naturally structure themselves in energy-minimizing configurations according to their interfacial tension, system stresses, etc. Rather than being a fit input, the equilibrium voltage emerges as a result of the free energy functional and the highly non-uniform concentration profiles as the average system concentration varies. The lithium in the active materials is consistently coupled to the electrolyte phase by variationally defined reaction kinetics, represented either by a generalized Butler-Volmer expression or quantum-mechanical electron transfer theories such as Marcus-Hush-Chidsey kinetics extended for concentrated solutions. This generalization of porous electrode models reduces directly to the more familiar “Newman” model with concentrated solution theory in the electrolyte in the case of solid solution active materials. Here, we will explore the theoretical developments in the model and present relevant examples, with a focus on comparisons with previous models and some experimental evidence in support of the approach.

A two-dimensional porous electrode model for designing pore structure in a quinone-based flow cell

Abstract A two-dimensional porous electrode model for designing a pore structure in a quinone-based flow battery was developed and validated by using experimental data from a metal-free quinone-based flow battery. The effects of porosity and pore size distribution on electrode performance and concentration fields were analyzed, the appropriate porosity and pore size distribution were obtained. In addition, the comprehensive effect of pore volume, specific surface area and pore size were considered, the dimensionless number shape factor SF was introduced to characterize the morphology, and the effect of morphology on the electrode performance was also investigated. This model was expected to understand the effect of morphology on the electrode performance and guide the preparation of the electrode. Extensions to the model in actual tests and future work were suggested.

Effect of Formation Cycling Schemes on Li-Ion Cell Capacity

Li-ion batteries (LIBs) have the highest gravimetric and volumetric energy density than other rechargeable batteries and are a potential candidate which can replace internal combustion engines (ICEs) in the present and future automobiles to reduce the transport sector’s problems of oil supplies and CO2 emission climate change. The limitations associated with adopting this green technology to completely replace the traditional propulsion technology from automobiles are higher cost, and capacity fade of batteries with cycling and time. The capital cost of the battery includes the material cost and production cost, including initial testing and formation cycle cost. Therefore, reducing the cost of electrode materials and battery fabrication together with developing high energy density materials and novel electrode processing techniques are promising challenges for the academic research community as well as battery manufacturers. Initial formation cycles of cells are required for growth and stabilisation of the solid electrolyte interphase (SEI) film over the surface of the electrode which protects the sites from further electrolyte reduction and thereby controls the irreversible loss of lithium and electrolyte. So far, the formation process is done at very low current, i.e., C/30~C/20, for three to five cycles followed by resting to achieve a uniform growth of the SEI film over the reactive surface of the electrodes1. Therefore it typically requires 1-3 weeks of industrial space, environmental chambers and battery cycler2, and leads to high costs. The cost of the formation period is the second most expensive process in cell manufacture after electrode processing3. A recent study by An et al. 2 proposed a fast formation cycling method and shortened the formation time six fold with improved cycling performance of the cell. However, a comprehensive study of the effect of operational current and voltage on formation cycling of Li-ion cells needs to be conducted to optimise the formation strategy. This optimisation can minimise the formation time without compromising the quality of SEI layer and cell performance that will finally contribute to overall cost reduction of the battery pack. The objectives of the present study are the quantitative and qualitative analysis of SEI film growth for various formation cycling approaches of the Graphite-Nickel Manganese Cobalt (NMC) oxide cells. The SEI film resistance/thickness will be gauged by electrochemical impedance spectroscopy (EIS), scanning electron spectroscopy (SEM) and X-ray photoelectron spectroscopy (XPS). This testing will be followed by the charge-discharge cycling of the cells to elucidate the effect of formation schemes on the subsequent capacity degradation. Furthermore, this experimental framework will also be simulated and comprehensively analysed using an electrolyte reduction based SEI growth model coupled with the porous electrode model. The results obtained through this study will be valuable to the cell manufacturers in the selection of the formation approach for the reduced cell and battery pack cost.

A Mechanistic Investigation of Crossover-Induced Capacity Fade in Redox Flow Batteries with Non-Selective Separators

The independent control of power capability, which is affected by reactor design, and energy capacity, which is affected by stored electrolyte volume, enables redox flow batteries (RFBs) to be potential candidates for large-scale energy storage systems. Typical RFBs consist of two large tanks with positive and negative electrolyte pumped to an electrochemical reactor through a network of pipes, while the reactor consists of high surface area porous electrodes (such as carbon felt) separated by an ion exchange membrane (IEM) or a non-selective separator (NSS). Previous research has established inorganic (such as vanadium) and organic/semi-organic (such as TEMPO) molecules for use in RFBs with aqueous or non-aqueous electrolytes. IEMs are used to electronically insulate the two electrodes and reduce crossover of the redox species by charge selectivity. IEMs are expensive and can contribute as much as 20% to the total battery cost depending on the size of the reactor [1]. Reduction of RFB capital costs could incentivize the adoption of RFBs for grid-energy arbitrage [2], and, hence, the development of RFBs with inexpensive NSSs with minimal crossover could enhance RFB use. In this regard, redox active polymers (RAPs) have been developed in both aqueous and non-aqueous systems to replace IEMs with NSSs. Here, NSSs mitigate crossover by size exclusion of RAPs [3,4]. However, the viscosity of such polymer based redox molecules increases with concentration leading to higher pumping pressures required to maintain the flow rate, thus adding to the pumping costs. In addition, higher pumping pressures increase electrolyte crossover which induces RFB capacity fade from cycle-to-cycle. The objectives of the present work are (1) to model multi-component transport during simultaneous electrochemical reactions within RFBs using NSSs, (2) to assess the tradeoffs in rate capability and cycle life incurred when replacing IEMs with NSSs, and (3) to predict performance as a function NSS design (including pore size and thickness). In this context, we are developing a two-dimensional model using porous electrode theory that explicitly captures porous-media flow, electronic current conservation, and conservation of molecular species (typically four redox-active species and two supporting ionic species that are inert) with simultaneous electrochemical reactions. These processes produce coordinated molecular fluxes can result in crossover and shuttling of redox species. In this model, molecular fluxes arise due to (1) pressure differences across the membrane (advection) (2) concentration gradient between the two electrolytes in the reactor (diffusion) and (3) migration due to the gradient in the electrolyte potential. In addition, we observe electrolyte volume losses which arises due to bulk electrolyte flow through the porous membrane because of viscosity (and therefore pressure) difference between the two electrolytes. Each electrode experiences primary reactions due to the major redox couple in the electrolyte and secondary reactions arising from the species that crossover from the counter electrode. We find that the secondary reactions experience a much higher overpotential than the primary reactions. The shuttling process occurs when the products of the secondary reactions diffuse back into the parent electrode. This shuttling process is used in bio sensing devices to amplify and detect the amount of catechol in human nervous system [5] and for overcharge protection in Li-ion batteries [6]. In the context of RFBs, this must be controlled to minimize capacity fade. The present porous-electrode model and associated reduced-order models will enable mechanistic interpretation of experimental data from which performance will be correlated as a function of several non-dimensional parameters that will aid in the design of NSSs. Further, we will validate our porous electrode model with experiments using aqueous RFBs having interdigitated flow fields using various NSSs and IEMs. We gratefully acknowledge the financial support of the Joint Center for Energy Storage Research (JCESR). References Arora, P., Zhang, Z. M., Chem. Rev., 2004, 104, 4419−4462.Darling et al., Energy Environ. Sci., 7 ,2014.Gavvalapalli et al., Am. Chem. Soc., 136 (46),2014Janoschka et al., Nature 527, 78–81, 2015Bernhard Wolfrum et al., Analytical Chemistry, Vol. 80, No. 4, 2008Jun Chen et al., Solid-State Lett., volume 8, issue 1, A59-A62, 2005

Experimental Validation of the Transmission Line Model Via Impedance Spectroscopy of an Ordered Array on Porous Carbon Electrode

In this paper, the interface of a porous substrate with an arranged and well-defined geometry is investigated for the impedance response with different electrolytes by using the well known electrochemical technique of Electrochemical Impedance Spectroscopy (EIS). Based on De Levie's Transmission Line Model (TLM) for porous electrodes a modification has been done in the model by incorporating the CPE behavior to reflect the non-ideal behaviour for real electrodes at low frequencies. The geometry used to validate the proposed model was fabricated from a smooth Glassy carbon electrode (GCE) surface to correlate the EIS response to that obtained by the model. The desired array was obtained by using laser micromachining tool on a glassy carbon substrate with a goal to study the influence of different electrolytes, varying electrolyte concentrations, the morphological features of pore- depth and radii of pore as well as different pore shapes. The outcomes of the experiments carried out on the in-house setup are compared with the MATLAB simulated results of the impedance. The transition from resistive to capacitive behavior is found to be shifted to lower frequency side and TLM behavior at high frequencies is more apparent as the concentration is increased. For the same concentration, different electrolytes are found to behave differently due to varying conductivity and ion sizes. In all the experiments carried out the impedance response showed CPE (constant phase element) behavior at the low frequency end while for high frequencies the nature/appearance of TLM behavior is found to be dependent on the pore depth to radius ratio. In addition the concept of time constant is also dealt with which exhibits differences for different geometries of the pores on glassy carbon. The results suggest that the GC electrode engaged here could provide a tractable model for studying electrochemical impedance behavior of porous electrodes without using any surface area analysis or pore volume analysis technique. Keywords: Transmission Line Model, Glassy carbon, EIS, MATLAB

Water Balance in Polymer Electrolyte Fuel Cells with Ultra-Low Pt Loading: From Modeling to Design

The commercial deployment of polymer electrolyte fuel cells (PEFCs) hinges on breakthroughs in design and integration of highly performing and durable catalyst layers with drastically reduced platinum loading. In this context, experimental studies have shown an unexpectedly large increase of voltage losses upon a marked reduction of the Pt content. In an effort to unravel this behavior, physical modeling of porous electrodes is employed to analyze a wide range of experimental literature data. Detailed evaluation of the data reveals correlated trends in essential fuel cell parameters. These findings are interpreted in view of the tipping water balance that affects the interplay of transport and reaction in catalyst layer and gas diffusion media. As will be demonstrated, the performed model-based analysis warrants the definition of a correlation exponent that should be used to assess the merit of different approaches to catalyst layer fabrication.