Electrolyte Distribution Research Articles

Optimization of electrolyte flow and vanadium ions conversion by utilizing variable porosity electrodes in vanadium redox flow batteries

The proper flow and uniform distribution of vanadium electrolyte inside the electrode are crucial factors to improve battery performance. In this study, the novel gradient and double-layered porous electrode (GPE and DLPE) with optimized properties were investigated. More vanadium ions conversion occurs due to more uniform distribution of electrolyte flow with promptly supplementing of reactants and timely removing of products. Thus, the larger cell capacity and output power as well as higher energy efficiency are obtained in the electrodes with a linear or stepwise increasing in porosity at the same inlet flow rate. In the other case with the same pressure difference between the inlet and outlet, the larger cell capacity and efficiencies occur in the electrodes with a constant maximum porosity or a linear decreasing porosity due to the larger average inlet flow velocity. The better battery performance can be obtained by optimizing porosity arrangement in the electrodes.

Visualization of electrolyte flow in vanadium redox flow batteries using synchrotron X-ray radiography and tomography – Impact of electrolyte species and electrode compression

The electrolyte distribution inside the porous electrodes of vanadium redox flow batteries is critical to the performance, as it determines the electrochemically active surface area. Herein, the influence of thermal activation, compression, and the injected electrolyte species on the pressure drop and the wetting is investigated by means of synchrotron X-ray radiation. The saturation versus the through-plane position is quantitatively displayed as a function of time to resolve the wetting process. The initial state after the imbibition is then quantitatively compared to the saturation after flow-through conditions. It was concluded that thermal activation plays a major role in the wetting, resulting in an up to six times higher saturation. Only a minor increase in saturation between the initial wetting state and after flow-through was observed. Additionally, there are only minor differences in the wetting behavior between the vanadium species in the electrolyte, the V(III) electrolyte shows the highest saturation. Increasing compression leads to a higher pressure drop and the saturation decreases only at compression ratios higher than 50%. Air pocket formation inside the liquid column was observed and the displacement and re-emergence of air pockets after flow-through is displayed.

Towards Bottom-up Engineered Electrode Microstructures for Redox Flow Batteries

Redox flow batteries (RFBs) are promising rechargeable electrochemical devices for grid-scale energy storage, but further cost reductions are needed for widespread implementation. While research efforts have primarily focused on molecular discovery, there has been significantly less attention paid to the engineering aspects (i.e. transport, reactor design) of the battery. Of particular importance are the porous electrodes used in the electrochemical stack. Today’s electrodes largely draw from the fuel cell material set, but, within a RFB, the porous electrode must perform a number of different roles including providing active surfaces for electrochemical reactions, facilitating uniform liquid electrolyte distribution, and supporting low pressure drops. Accordingly, before purpose-built electrodes can be developed for RFB applications, performance-limiting factors for the present materials set must be quantified. Unambiguous analysis is challenging in an operating RFB due to the complex coupling of transport and reactions, which vary as a function of state of charge during cell cycling. Deconvoluting the role of electrode properties on flow battery performance requires the development of diagnostic techniques that enable electrode characterization under well-controlled but application-relevant conditions. To this end, we systematically evaluate the performance of several different carbon paper, felt, and cloth electrodes using the single-electrolyte cell configuration [1] and a model organic redox couple (TEMPO/TEMPO+) [2]. Using polarization and electrochemical impedance spectroscopy, we quantify the impact of electrode microstructure on battery performance. To further understand the role of electrode microstructure on the performance, we use tomography, and numerical simulation in tandem with the aforementioned electrochemical diagnostics. We find that woven electrodes, which feature a bimodal pore size distribution, provide an outstanding electrochemical performance and low pressure drop [3]. Inspired on these learnings, our most recent work focuses on developing bottom-up synthetic methods to fabricate porous electrodes with engineered microstructures, with a particular interest in bimodal pore size distributions and porosity gradients across the electrode thickness. In this talk, I will discuss our most recent experimental strategies to engineer electrodes with highly-controlled microstructures tailored for specific RFB chemistries. Darling et al., J. Electrochem. Soc., 161, A1381 (2014).D. Milshtein et al., J. Power Sources, 327, 151 (2016).Forner-Cuenca et al., Under Review at Electrochem. Soc. Acknowledgments We gratefully acknowledge the financial support of the Swiss National Science Foundation (P2EZP2_172183) and the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy.

Structural modification of vanadium redox flow battery with high electrochemical corrosion resistance

In the conventional vanadium redox flow battery, the bipolar plates are usually designed with flow fields to improve the battery performance by facilitating the homogeneous distribution of electrolytes. The introduction of flow field results in severer oxidation corrosion, owing to the sharp edges and corners formed on the flow fields. In this study, a modified battery structure for the vanadium redox flow battery is proposed to alleviate the oxidation corrosion of the bipolar plates and flow fields. The flow fields are segmented from the bipolar plates, and inserted between the porous electrodes and membrane as independent components. To improve the service life and processability, the flow fields are prepared with photosensitive resin by means of the three-dimensional printing technique. A battery with the modified structure is assembled to examine the battery performance and oxidation resistance. Morphology characterization and elemental composition analysis are employed to investigate the oxidization on the bipolar plate surface. The modified battery structure contributes to decreasing the contact resistance. The pressure drop and charging/discharging tests indicate that the battery with the modified structure exhibits maintained flow behavior and energy efficiency, and provides a higher Coulombic efficiency compared to the conventional vanadium redox flow battery. Moreover, the bipolar plates in the modified battery structure demonstrate a higher capacity to restrain oxidation corrosion during the charging process. The modified battery structure can enhance the service life of bipolar plates and flow fields, and is significant in the application of vanadium redox flow battery.

(Energy Technology Division Supramaniam Srinivasan Young Investigator Award Address) Towards Deterministic Electrode Design: Elucidating the Role of Surface Chemistry and Microstructure on Flow Battery Performance

Redox flow batteries (RFBs) are promising for energy-intensive grid storage applications, but further improvements are needed for universal adoption.1,2 While research efforts have primarily focused on molecular engineering of redox couples as a means of reducing system cost, advances in other critical systems components also hold significant cost reduction potential. Of particular importance are the porous electrodes which are responsible for multiple critical functions in the flow cell related to thermodynamics, kinetics, and transport including providing surfaces for electrochemical reactions, enabling uniform liquid electrolyte distribution with low hydraulic resistance, as well as conducting electrons and heat. However, there is limited knowledge on how to systematically design and implement these materials in emerging RFB applications, forcing the repurposing of available materials that are not tailored for this electrochemical system. Moreover, current generation materials, which are typically developed via empirical approaches, lack control of surface chemistry (e.g., compositional heterogeneity) and morphology (e.g., broad pore size distribution). This fundamentally limits the performance, durability and, consequently, the cost of resultant systems. In this talk, I will discuss methods for disaggregating and quantifying resistive losses in various porous electrodes using model redox couples, diagnostic flow cells, and electrochemical modeling.3,4 When applied in combination with suitable spectroscopy and microscopy techniques, structure-performance relations can be elucidated5,6 which may eventually lead to design rules that enable the fabrication of chemistry-specific electrodes based solely on the knowledge of the physical and electrochemical properties of the redox active electrolyte.

Calcium ion effects on the water/oil interface in the presence of anionic surfactants

The influence of ion type at constant ionic strength on the surfactant layer present between a polar and a non-polar liquid has been investigated. The investigated systems are composed of sodium bis(2-ethylhexyl) sulfosuccinate (AOT), dodecane and brines with different electrolyte compositions. Parallel and coordinated sampling have been performed via molecular dynamics simulation and experiments. By varying the molar ratio between calcium and sodium ions at constant ionic strength, we determined the relative influence of the bivalent ion. In the present work, firstly, we provide an atomistic description via molecular dynamics (MD) simulations of the surfactant and electrolyte distributions in respect to the dodecane-water interface. Secondly, we report the interfacial tension (IFT) and the estimated Debye length based on the theory of the electric double layer, comparing the simulation results with a classical interface description.

The effect of short-range interaction and correlations on the charge and electric field distribution in a model solid electrolyte

A simple lattice model of a solid electrolyte presented as a xy-slab geometry system of mobile cations on a background of energetic landscape of the host system and a compensating field of uniformly distributed anions is studied. The system is confined in the z-direction between two oppositely charged walls, which are in parallel to xy-plane. Besides the long-range Coulomb interactions appearing in the system, the short-range attractive potential between cations is considered in our study. We propose the mean field description of this model and extend it by taking into account correlation effects at short distances. Using the free energy minimization at each of z-coordinates, the corresponding set of non-linear equations for the chemical potential is derived. The set of equations was solved numerically with respect to the charge density distribution in order to calculate the cations distribution profile and the electrostatic potential in the system along z-direction under different conditions. An asymmetry of charge distribution profile with respect to the midplane of the system is observed. The effects of the short-range interactions and pair correlations on the charge and electric field distributions are demonstrated.

Reduced graphene oxide aerogel as stable host for dendrite-free sodium metal anode

Sodium (Na) metal has attracted great attention as a promising anode for next-generation energy storage systems because of its abundant resources, potentially low cost, and high theoretical capacity. However, severe dendrite growth, large volume expansion during plating/stripping lead to poor cycle performance and limit the practical application of sodium metal anodes. Here, oriented freeze-drying and molten infusion are used to synthesize a Na-infused reduced graphene oxide aerogel (Na@rGa) composite anode using a reduced graphene oxide aerogel (rGa) as a stable host. This unique host shows ultra-light weight which guarantees high capacity (1064 mAh g−1), and the large specific surface area and uniform pores effectively lower the local current density and uniform electrolyte distribution from the inner to the outside of electrode. The Na@rGa anode exhibits low overpotential (50 mV) and stable cycle performance for 1000 cycles at 5 mA cm−2 in carbonate electrolyte system. The electrochemical performance of a full cell using the Na@rGa composite anode is clearly superior to that of a reference cell. This work provides a new horizon for construction of three-dimensional stable hosts and safe sodium metal anodes.

A two-dimensional mathematical model for vanadium redox flow battery stacks incorporating nonuniform electrolyte distribution in the flow frame

Lumped models have been widely adopted to predict the performance of the vanadium redox flow battery (VFRB) stack, which mainly due to its simplicity in modeling transient behaviors during operation cycles. However, average transport and electrochemical properties in previous lumped models make it impossible to obtain the information of electrolyte distributions in stacks. To address this issue, in this work, we report a two-dimensional mathematical model for a VRFB stack considering the effect of nonuniform electrolyte distributions in the flow frame via a flow network equivalence method. With this new model, battery performance and its temperature at different operating conditions are determined accurately. It is demonstrated that (i) temperature fluctuations of the stack reach up to 10 K at different current densities and flow rates; (ii) 25% blockage in the middle cell can lead to the capacity reduction by up to 80%.

Studies of Lithium Heterogeneities in Li-Ion Batteries

Energy storage media based on different technologies have gained in importance for a wide field of applications ranging from supplying portable devices to large electric vehicles. In recent decades the energy storage technology, based on lithium ions, dominates to the market due to the best compromise between energy and power densities (both gravimetric and volumetric), low self-discharge when not in use, tiny memory effect etc. Despite the overall success of Li-ion technology, a further progress permanently demands for lower-cost, longer-life, higher energy/power density batteries resulting in active development and research to the field. Nowadays modern Li-ion batteries are sophisticated electrochemical devices, possessing numerous degrees of freedom (chemical, morphological, transport) along with complicated geometries of the electrode integration. This along with the need to minimize the risks for possible materials oxidation, electrolyte evaporation, cell charge changes etc. requires new dedicated experimental techniques capable to reveal “live” information about processes occurring inside the cell. In such instance neutron scattering is already a well-established experimental technique for the characterization of Li-ion batteries1. Neutron scattering methods when combined with electrochemical characterization undergo an increasing relevance for studies of lithium-ion based electrochemical energy storage systems on different length scales, e.g. neutron imaging, reflectometry, small-angle neutron scattering, quasielastic neutron scattering and powder diffraction. Nowadays in situ experiments with neutrons are performed on different self-developed/special test cells and commercial Li-ion cells of diverse designs depending on the research targets and needs. Simple in principle, but complicated in practice, designs of modern Li-ion batteries may result in spatial inhomogeneity of current, lithium or electrolyte distribution, which are often difficult to quantify, but they will surely affect performance, cycling stability and/or safety. Despite the increasing popularity of neutron scattering studies of batteries at their operating conditions the problem of cell non-uniformity (indirectly pointed by electrochemical measurements) and its effect is often not properly accounted in literature. Here a combination of three neutron-based experimental techniques, namely computed neutron tomography, high-resolution neutron powder diffraction and spatially-resolved neutron powder diffraction, applied in situ for studies on commercial Li-ion cells of the 18650-type will be presented along with results of electrochemical studies. The details of the cell organization on different length scales and its evolution on various factors like state-of-charge, temperature and fatigue will be presented in light of 3D lithium distribution in cylinder-type2-3 and prismatic4 Li-ion batteries. Reference s : [1] H. Ehrenberg, A. Senyshyn, M. Hinterstein, H. Fuess, in: E.J. Mittermeijer & U. Welzel (Eds.), Modern Diffraction Methods, Weinheim: Wiley-VCH, 2012, pp. 491-518. [2] A. Senyshyn, M. J. Mühlbauer, O. Dolotko, M. Hofmann, H. Ehrenberg, Sci. Rep. 5 (2015) 18380 [3] M.J. Mühlbauer, O. Dolotko, M.Hofmann, H.Ehrenberg, A.Senyshyn, J. Power Sources 348 (2017) 145-149 [4] M.J. Mühlbauer, A. Schökel, M. Etter, V. Baran, A. Senyshyn, J. Power Sources 403 (2018) 49-55

Optimization of local porosity in the electrode as an advanced channel for all-vanadium redox flow battery

Experimental and numerical studies have been carried out to improve the flow distribution by optimizing the local porosity of the electrodes of the all-vanadium redox flow batteries (VFBs) to increase the energy efficiency at high current density. We control the local porosity by inserting extra layer of electrode at inlet and outlet, and the flow field of electrolyte is analyzed numerically. First, the flow field of electrolyte is analyzed numerically to understand distribution of electrolyte in the electrode. Then, the charge and discharge curve is analyzed to understand the effect of local porosity of electrode on the energy efficiency. At 50 mA/cm2, the energy efficiency is the highest when using electrode with uniform porosity. At 150 mA/cm2, however, the energy efficiency of the cell using the electrode with low porosity at inlet is similar to that using the uniform electrode which is 66.6%. Lastly, we suggest an empirical equation for optimal local porosity distribution of the electrode according to current density. Using the empirical equation, we can increase the energy efficiency of the cell to 67.7% at 150 mA/cm2. This study shows the possibility of increase of the energy efficiency of VFBs by controlling local porosity of the electrode.

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.

Study on architecture design of electroactive sites on Vanadium Redox Flow Battery (V-RFB)

Numerous researches have been conducted to look for better design of cell architecture of redox flow battery. This effort is to improve the performance of the battery with respect to further improves of mass transport and flow distribution of electroactive electrolytes within the cell. This paper evaluates pressure drop and flow distribution of the electroactive electrolyte in three different electrode configurations of vanadium redox flow battery (V-RFB) cell, namely square-, rhombus- and circular-cell designs. The fluid flow of the above-mentioned three electrode design configurations are evaluated under three different cases i.e. no flow (plain) field, parallel flow field and serpentine flow field using numerically designed three-dimensional model in Computational Fluid Dynamics (CFD) software. The cell exhibits different characteristics under different cases, which the circular cell design shows promising results for test-rig development with low pressure drop and better flow distribution of electroactive electrolytes within the cell. Suggestion for further work is highlighted.

Comparison between 1-D and grey-box models of a SOFC

Solid Oxide Fuel Cells (SOFCs) have shown unique performance in terms of greater electrical efficiency and thermochemical integrity with the power systems compared to gas turbines and internal combustion engines. Nonetheless, simple and reliable models still must be defined. In this paper, a comparisonbetween a grey-box model and a 1-D model of a SOFC is performed to understand the impact of the heat transfer inside the cell on the internal temperature distribution of the solid electrolyte. Hence, a significant internal temperature peak of the solid electrolyte is observed for a known difference between anode and cathode inlet temperatures. Indeed, it highlights the difference between the 1-D model andthe grey-box model regarding the thermal conditioning of the SOFC. Therefore, the results of this study can be used to investigate the reliability of the thermal results of box models in system-level simulations.

Flow field research on electrochemical machining with gas film insulation

Electrochemical machining (ECM) is an efficient method for fabricating crucial components of aircraft engines, such as integral blisks and diffusers. This paper proposes a new method for decreasing the stray corrosion in trepanning ECM, in which air is supplied around the machined workpiece to remove stray electrolyte. Specifically, compressed air was blown into the cathode and through the workpiece in a forward flow mode, thereby forming a gas film on the machined surface. Simulations were performed using the ANSYS software to determine the distributions of compressed air and electrolyte in the machined area. The simulation results demonstrated a uniform air–electrolyte distribution and a stable flow field under the optimal parameters. Experiments were conducted to verify these results, and the gas film was found to exert an obvious influence on reducing the taper angle and improving the surface quality of the workpiece. Severe corrosion of the workpiece occurred in the absence of air flow, whereas the stray corrosion was significantly reduced when air was supplied through the cathode. Upon increasing the feed rate from 0.5 to 1.5 mm/min, the taper angle decreased from −0.012° to −0.003° and the surface roughness decreased from Ra 0.841 μm to Ra 0.266 μm, whereas the optimal values for the samples obtained without the gas film were 0.096° and Ra 0.993 μm at the feed rate limit of 1.0 mm/min.

X-ray Based Visualization of the Electrolyte Filling Process of Lithium Ion Batteries

The electrolyte filling process constitutes the interface between cell assembly and formation of lithium ion batteries. Electrolyte filling is known as a quality critical and also time consuming process step. To avoid limitations in battery quality a homogeneous electrolyte distribution is necessary. Therefore, especially large sized cells are stored for hours. To accelerate filling and wetting processes the effect of materials- and process parameters on electrolyte distribution needs to be investigated. Unfortunately, in situ methods to characterize the filling and wetting state are still rare, limited in availability or even time-consuming in preparation. To overcome these drawbacks this paper introduces X-ray as an innovative method to visualize the electrolyte filling process in large scaled lithium ion batteries. Therefore, an experimental setup was developed to enable in situ X-ray measurements during the filling process of large scaled cells. Additionally, an evaluation process for the optical data was proposed. Based on these images the suitability of X-ray as visualization method is shown considering three exemplary filling parameters.

Electrochemical performance of bulk-type all-solid-state batteries using small-sized Li7P3S11 solid electrolyte prepared by liquid phase as the ionic conductor in the composite cathode

A high lithium-ion conductive solid electrolyte with Li7P3S11 phase and small particle size was prepared by a liquid phase process, and the solid electrolyte was used as the ionic conductor in the composite cathode for bulk-type all-solid-state batteries. The electrochemical performance of the all-solid-state cell using Li7P3S11 solid electrolyte prepared by liquid phase was studied and compared to that using Li7P3S11 solid electrolyte prepared by ball milling.Solid electrolytes prepared by the liquid phase and ball milling processes consisted mainly of the Li7P3S11 crystal phase and the local structure was composed by PS43−, P2S74− and P2S64− units. Both solid electrolytes exhibited a comparable high ionic conductivity over 10−3 Scm−1. A particle size around 500 nm was obtained by the liquid phase process, while a particle size larger than 10 μm was obtained by the ball milling process. The composite cathode using the solid electrolyte obtained by the liquid phase process displayed a better distribution of the solid electrolyte and the active material, verified by scanning electron microscopy and energy-dispersive X-ray spectroscopy.The all-solid-state cell using LiNi1/3Co1/3Mn1/3O2 and the solid electrolyte prepared by the liquid phase exhibited a better electrochemical performance than that using the solid electrolyte prepared by ball milling. The all-solid-state cells exhibited a first discharge capacity of 154 mAh g−1 and 46 mAh g−1, respectively. Furthermore, the structure of the solid electrolyte prepared by the liquid phase process, after charge-discharge measurements with charge-end voltages of 4.6 V vs Li, was investigated by using ex-situ Raman spectroscopy. Significant structural changes were not observed, indicating that Li7P3S11 is stable against charge-discharge processes.

Electrostatic Adsorption of Uniformly Charged Electrolytes within Like-charged Electrodes

The classical-fluids density functional theory has been developed for studying the structural and the electrical properties of electrolyte solutions containing uniformly charged hard-spherical ions. The modified fundamental-measure theory has been used to evaluate the hard-sphere contribution. The mean-field approximation has been employed to calculate the cross correlation between the hard sphere contribution and the Coulomb interaction. The Poisson equation for ions carrying charges that are spatially separated has been solved. The present theory shows reasonably good agreement with the corresponding Monte Carlo simulation results. The calculated results show that the attraction between like-charged planar surfaces is the result of the intra-ionic correlation and depends strongly on the ion size, valence, mole fraction, and charge distribution of electrolytes.

Asymmetric electrolytes near structured dielectric interfaces.

The ion distribution of electrolytes near interfaces with dielectric contrast has important consequences for electrochemical processes and many other applications. To date, most studies of such systems have focused on geometrically simple interfaces, for which dielectric effects are analytically solvable or computationally tractable. However, all real surfaces display nontrivial structure at the nanoscale and have, in particular, a nonuniform local curvature. Using a recently developed, highly efficient computational method, we investigate the effect of surface geometry on ion distribution and interface polarization. We consider an asymmetric 2:1 electrolyte bounded by a sinusoidally deformed solid surface. We demonstrate that even when the surface is neutral, the electrolyte acquires a nonuniform ion density profile near the surface. This profile is asymmetric and leads to an effective charging of the surface. We furthermore show that the induced charge is modulated by the local curvature. The effective charge is opposite in sign to the multivalent ions and is larger in concave regions of the surface.

Effect of flow field geometry on operating current density, capacity and performance of vanadium redox flow battery

Addition of flow fields to carbon paper electrodes in a vanadium redox flow battery (VRFB) can improve the peak power density through uniform distribution of electrolyte in the electrodes. However, it is unclear whether flow fields have a similar effect with graphite felt electrodes, as VRFBs with felt electrodes reported in literature show a large anomaly in obtained power density. In this work, we evaluate three flow fields; viz. serpentine, interdigitated and conventional (without flow pattern) type with felt electrodes and compare their performance with a serpentine flow field using carbon paper electrodes under identical experimental conditions. The conventional flow field provides highest energy efficiency (75%) followed by serpentine (64%) and interdigitated (55%) at 0.2 A cm−2 attributable to the deteriorating electrolyte distribution in the electrodes. Computation fluid dynamic simulations confirm the experimental finding of worsening electrolyte distribution (conventional < serpentine < interdigitated). A power density of 0.51 W cm−2 at 60 mL min−1 flow rate is obtained for serpentine and conventional flow fields with felt electrodes; comparable to the highest power density reported in literature for high performing zero-gap flow field architecture. This paper gives comprehensive insights on flow fields for VRFBs that can be extended to other flow batteries.