Negative Half-cell Research Articles

Active Material Transport in Carbon Porous Electrode for Vanadium Redox Flow Batteries

Introduction The Vanadium Redox Flow Battery (VRFB) is an energy storage device that supplies vanadium electrolyte solutions and is expected to be used as a storage system for renewable energy due to its advantages such as being able to design output and capacity independently. To charge and discharge at high currents, VRFBs need to enhance not only the electrode activity but also the active material transport within the porous electrodes due to low diffusion coefficients and high viscosity of electrolyte solutions. However, there are only a few previous works focusing on material transport [1,2], and the mass transport phenomena in boundary film is still unclear. We are proposing Seamless Carbon (SC) materials as electrode materials for VRFBs, which have a uniform interconnected pore structure in all directions [3]. This pore structure should be advantage for the analysis of material transport. The objective of this work is to evaluate the limitation of material transport rate in positive and negative half-cells. By comparing the current-voltage curves of symmetric cells and conventional full cells, mass transport limiting half-cell was clarified. Experimental SC materials were prepared as follows; a precursor was carbonized at 800 °C under a N2 gas atmosphere, heat treated at 1500 °C under an Ar gas atmosphere, and oxidized at 420 °C under an air atmosphere. The interconnected pore diameter of the SC material was set to 13 μm. A single cell was prepared by sandwiching electrodes and a membrane (Nafion 117) between carbon blocks with interdigitated flow channels. SC material with a thickness of ca. 0.4 mm was set into both positive and negative half-cells. Current-overpotential curves were obtained under flowing electrolyte solution at rates of 5 to 20 mL min−1. Results and Discussion Figure 1 shows the current-overvoltage curves of the negative half-cell. While the limiting current was observed in the negative half-cell, it was not observed in the positive half-cell. Comparing with the current-voltage curves in the full cell, the insufficient active material transport at the negative half-cell dominates the limiting current of the full cell. The one-pass conversion was ca. 0.3 at the limiting current, which indicates the impact of bulk concentration change was small, while active material transport within the boundary film should impacted for the limiting current. The equilibrium of ion exchange between vanadium complex and surface oxygen groups on the negative electrode does not favor the disassociation of proton due to the acidic condition. Consequently, the vanadium concentration on the electrode surface becomes higher than zero, suggesting that the concentration gradient within the boundary layer is relatively small which resulted insufficient active material transport in the negative half-cell. Acknowledgement This research was supported by JSPS KAKENHI C JP22K04814.

Exploring Iron-Containing Electrodes for Use as Negative Electrodes of All-Iron Redox Flow Batteries

The transition towards a renewables-based energy economy is motivated by pressing environmental concerns and economic growth rates. This new energy paradigm necessitates efficient, robust and cost-effective, large-scale energy storage systems. Aqueous, all-iron redox flow batteries (AIRFBs) are a promising contender for large-scale energy storage, motivated by their reliance on environmentally friendly and abundant raw materials, their intrinsic safety, and projected high energy density (ca. 135 Ah L-1) [1]. However, the performance and lifetime of state-of-the-art AIRFBs are greatly limited the poor kinetics and uneven plating taking place in the negative half-cell. Traditionally, the electrochemical stack leverages carbon fiber-based electrodes (e.g. felts), which provide moderate surface area for reaction but suffer from sluggish kinetics and plated layers of poor quality [2].Hereby, we explore the use of iron-based (e.g. pure iron, steel types 302, 316, 420) electrode alternatives to overcome the challenges of prevalent carbon fiber electrodes or pure iron. Steels have been investigated for numerous electrochemical systems, such as (microbial) fuel cells, electrolyzers and various types of batteries, displaying high performance and excellent chemical stability, robust mechanical properties and low production costs [3]. More precisely, in this work we investigate iron-based electrodes as a substitute for conventional carbon felts due to their enhanced kinetic activity in the negative half cell [Fe2+(aq)|Fe(s)]. Firstly, different metal electrodes were tested in flow-by configuration to obtain their polarization performance. Employing a flat electrode of pure alpha-iron as a baseline, we find that it displays balanced behavior between charging and discharging mode, although it exhibits very limited utilization of its projected gravimetric capacity[4]. Following this, we test a set of steel sheets of different nature, that is, ferritic, martensitic and austenitic, which contain various alloying elements (e.g., Cr, Ni, Mn, Mo, etc.) in different ratios, resulting in different electrochemical performance. We find that materials containing low to moderate Ni:Cr ratios result in enhanced iron plating and stripping kinetics, with increases ranging from ~20 to ~60% with respect to the pure alpha-iron electrode. Furthermore, we observe that the crystallographic structure of austenitic materials tend to favor reactions involving iron ions, as opposed to martensitic structures of which feature higher solid-to-gas surface energy, which favor the competing hydrogen evolution reaction [5]. We additionally investigate advanced three-dimensional structures in a symmetric, flow-through configuration, by selecting the best performing steel composition. By screening various fiber arrangements, we aim to elucidate the structural effects on mass transfer and overall cell performance.Overall, we studied the interplay between the composition and architecture of different metal-based electrodes. We hope that these findings will help shed light on their potential to improve the performance of the challenging iron plating-stripping reaction and ultimately introduce them in a full cell system to enable higher capacity and rate performance of the promising all-iron flow battery Acknowledgments IGG gratefully acknowledges financial support by “la Caixa Foundation” (ID 100010434) under the fellowship number LCF/BQ/EU20/11810076. AFC gratefully acknowledges funding by the European Union (ERC, FAIR-RFB, ERC-2021-STG 101042844). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them.

Effect of Electrolyte pH during Pretreatment of Carbon Electrodes on the Subsequent Kinetics of the VII/VIII and VIV/VV Reactions in Vanadium Flow Batteries

Growing environmental concerns and the depletion of fossil fuel reserves have prompted increased focus on renewable energy sources like wind and solar power.1-3 However, integrating these renewables into power grids is challenging due to their intermittent nature. Large-scale energy storage systems are seen as a solution to improve the reliability and efficiency of renewable energy. Vanadium flow batteries (VFBs) are gaining attention for their potential in large-scale energy storage due to the advantage of utilizing the same active element on both sides of the battery, which eliminates cross-contamination issues and extends the lifetime of the battery.4-6 A typical VFB consists of a cell stack and two external tanks containing electrolytes with redox active species, circulated through the cell stack via pumps. The negative half-cell contains V2+/V3+ (VII/VIII) species and the positive half-cell contains VO2+/VO2 + (VIV/VV) species, separated by an ion exchange membrane. Carbon or graphite felts are commonly used as electrodes in VFBs due to their high electrical conductivity, chemical stability, and availability.2,9 However, voltage inefficiencies arising from sluggish electrode kinetics can lead to increased overpotentials and reduced coulombic efficiency, impacting the overall energy efficiency of the battery.5,6 Therefore, it is important to investigate the electrode kinetics to better understand and improve the performance of VFBs.Various electrode treatments, including chemical, electrochemical, and thermal treatments, have been reported to have significant impacts on the carbon electrode kinetics.7-9 We have previously reported that cathodic treatment enhances the kinetics of the positive (VIV/VV) electrode but inhibits the kinetics of the negative (VII/VIII) electrode, while anodic treatment inhibits the kinetics of the positive electrode but enhances the kinetics of the negative electrode. 9-18 Other factors affecting the electrode activity include ageing of the electrode, the final treatment potential, the redox rest potential, and the treatment potential window.14,15 Recently we showed that the electrochemical activity of the VIV/VV and VII/VIII redox reactions in highly acidic vanadium electrolytes at electrochemically treated glassy carbon electrodes is also affected by the pH of the treatment electrolyte.19,20 The effect of pH of the treatment electrolyte was investigated for electrolytes from pH 0 to pH 5. The activities of carbon electrodes towards both the VIV/VV and VII/VIII redox reactions increase with increasing pH of the treatment electrolytes.In this presentation, we report detailed results on treating glassy carbon electrodes electrochemically at different pH and the subsequent effects on kinetics. The activity of variously pretreated electrodes towards both the VIV/VV and VII/VIII redox couples will be compared and contrasted, and the important differences discussed. Acknowledgements Varsha Sasikumar S P would like to thank the Irish Research Council (IRC) for a postgraduate scholarship to perform this research and acknowledges the facilitation of this research by the University of Limerick (UL) and the South East Technological University (SETU) in Ireland. References M. J. Leahy et al., Wind Power Generation and Wind Turbine Design, ed. W. Tong (WIT Press, Southampton) 661 (2010).C. Roth et al., (ed.), Flow Batteries: From Fundamentals to Applications (New York, Wiley) (2022).Z. Zhang et al., Renew. Sustain. Energy Rev., 148, 111263 (2021).M. Zarei-Jelyani et al., J. Appl. Electrochem., 54, 719 (2024).H. Agarwal et al., Adv. Sci., 11, 2307209 (2024).Z. Huang et al., ACS Sustainable Chem. Eng., 10, 7786 (2022).R. K. Sankaralingam et al., J. Energy Storage, 41, 102857 (2021).K. J. Kim et al., J. Mater. Chem. A, 3, 16913 (2015).M. A. Miller et al., J. Electrochem. Soc., 163, A2095 (2016).A. Bourke, et al., ECS Trans., 64, 1 (2014).A. Bourke et al., ECS Trans., 61, 15 (2014).A. Bourke et al., ECS Trans., 53, 59 (2013).A. Bourke et al., ECS Trans., 66, 181 (2015).M. Al-Hajji-Safi et al., ECS Trans., 109, 67 (2022).M. Al-Hajji-Safi et al., ECS Trans., 77, 117 (2017).A. Bourke et al., J. Electrochem. Soc., 163, A5097 (2016).A. Bourke et al., J. Electrochem. Soc., 162, A1547 (2015).A. Bourke et al., J. Electrochem. Soc., 170, 030504 (2023).V. Sasikumar et al., ECS Trans., 109, 107 (2022).V. Sasikumar et al., Electrochim. Acta., Under revision (2024).

Crossover analysis in a commercial 6 kW/43kAh vanadium redox flow battery utilizing anion exchange membrane

The volumetric and species transport is a significant source of capacity decay in vanadium redox flow batteries (VRFBs). However, even with the prevalent use of anion exchange membranes (AEMs) in commercial systems, the understanding of the crossover mechanisms in this context remains limited. The primary objective of this study was to gain a deeper comprehension of these mechanisms through experimental investigations conducted on a 6 kW/43kAh VRFB system using AEMs. It is concluded that protons serve as the primary charge carriers, owing to the high ionic concentrations of the electrolyte. During normal operation, there was a consistent pattern of volume transfer from the positive to the negative half-cell (∼0.5 % of the total electrolyte per cycle/day). Experimental assessments performed at different states-of-charge (SoCs) revealed that the volumetric transport towards the anolyte increases with SoC. The osmotic effects are concluded to be the main contributors to the volumetric transport. The osmotic pressure difference is hypothesized to arise from the asymmetric diffusion coefficients of the vanadium ions, changes in pH affecting sulfate and bisulfate equilibrium alongside their different diffusion coefficients, and a likely predominance of the bisulfate concentration gradient in the water transport through the membrane.

Electrochemical Deposition of Bismuth on Graphite Felt Electrodes: Influence on Negative Half-Cell Reactions in Vanadium Redox Flow Batteries

In this paper, bismuth (Bi) was successfully deposited on graphite felts to improve the electrochemical performances of vanadium redox flow batteries. Modified graphite felts with different Bi particle loadings were obtained through electrochemical deposition at voltages of 0.8 V, 1.2 V and 1.6 V in 0.1 M BiCl3 solution for 10 min. The optimal Bi particle loading was confirmed by scanning electron microscopy (SEM), single cells and electrochemical tests. The SEM images revealed the deposition of granular Bi particles on the fiber surface. The Bi-modified felts which were electro-chemically deposited at 1.2 V (Bi/TGF-1.2V) showed excellent electrochemical performances in cyclic voltammetry curves and impedance spectroscopy. Meanwhile, the single cells assembled with Bi/TGF-1.2V as negative electrodes exhibited higher voltage efficiencies than the others. The optimized Bi particle loading induced better catalysis of the V3+/V2+ reaction and hence significantly improved the cell performances. In addition, the prepared Bi-modified felts showed stable cell performances and slower charge–discharge capacity declines than the other electrodes at current densities between 20 mA/cm2 and 80 mA/cm2. Compared with the pristine felt, the voltage efficiency of the vanadium redox flow battery assembled with Bi/TGF-1.2V graphite felt was 9.47% higher at the current density of 80 mA/cm2. The proposed method has considerable potential and guiding significance for the future modification of graphite felt for redox flow batteries.

Investigating the V(II)/V(III) electrode reaction in a vanadium redox flow battery – A distribution of relaxation times analysis

Vanadium Redox Flow Batteries (VRFBs) are already commercially available and promise to provide excellent prerequisites to face the challenge of large-scale energy storage. Nevertheless, the VRFB itself has to overcome challenges regarding lifetime and efficiency. Polarization and pumping losses due to a high electrolyte flow-through resistance in the electrode contribute to much of the efficiency losses. We investigated the reaction and processes in the negative VRFB half-cell using electrochemical impedance spectroscopy combined with the distribution of relaxation times analysis. We identify the individual processes in the negative half-cell by varying several parameters. One peak is observed in the low-frequency range below 2 mHz, attributed to the ion transport. In the range from 2 mHz to 1 Hz, several peaks are identified and assigned to the redox-active species' transport processes through the electrode's porous structure. In the high-frequency range above 1 Hz, the single peak was assigned to the electrochemical reaction in the negative half-cell. The processes in this half-cell are slower than in the positive half-cell. The technique is beneficial to gain a fundamental understanding of the catalytic process of the V(II)/V(III) reaction to optimize novel electrode materials or monitor electrode degradation processes.

Insights into the hydrogen evolution reaction in vanadium redox flow batteries: A synchrotron radiation based X-ray imaging study

The parasitic hydrogen evolution reaction (HER) in the negative half-cell of vanadium redox flow batteries (VRFBs) causes severe efficiency losses. Thus, a deeper understanding of this process and the accompanying bubble formation is crucial. This benchmarking study locally analyzes the bubble distribution in thick, porous electrodes for the first time using deep learning-based image segmentation of synchrotron X-ray micro-tomograms. Each large three-dimensional data set was processed precisely in less than one minute while minimizing human errors and pointing out areas of increased HER activity in VRFBs. The study systematically varies the electrode potential and material, concluding that more negative electrode potentials of −200 mV vs. reversible hydrogen electrode (RHE) and lower cause more substantial bubble formation, resulting in bubble fractions of around 15%–20% in carbon felt electrodes. Contrarily, the bubble fractions stay only around 2% in an electrode combining carbon felt and carbon paper. The detected areas with high HER activity, such as the border subregion with more than 30% bubble fraction in carbon felt electrodes, the cutting edges, and preferential spots in the electrode bulk, are potential-independent and suggest that larger electrodes with a higher bulk-to-border ratio might reduce HER-related performance losses. The described combination of electrochemical measurements, local X-ray micro-tomography, AI-based segmentation, and 3D morphometric analysis is a powerful and novel approach for local bubble analysis in three-dimensional porous electrodes, providing an essential toolkit for a broad community working on bubble-generating electrochemical systems.

(Invited) Investigating the Influence of Electrolyte Additives and Anode Structures on Electroplating Performance Towards Use in Iron-Based Batteries

Environmental concerns and economic development motivate a shift towards an energy economy based on renewable energy sources. In this new paradigm, large scale energy storage plays a pivotal role in the integration of intermittent and fluctuating renewable energy technologies, such as wind or solar. Among the available solutions, redox flow batteries (RFBs) are promising contenders as they allow independent scaling of power and energy, have a projected long lifetime and lower costs than other battery technologies[1]. However, the most developed RFB systems to date, i.e. all vanadium-RFBs, are still not extensively deployed, due to the elevated and fluctuating costs of vanadium, and the geographical dependency on resource extraction. Iron-based flow batteries, on the contrary, rely on earth-abundant active materials and benign water as solvent, granting potential for cost-effective energy storage systems[2]. However, the efficiency of these electrochemical reactors is limited by challenges such as unoptimized electrolyte compositions, which cause parasitic generation of hydrogen gas or irregular coating distributions[3], and electrode materials that have not been optimized for this specific application. These can severely affect current density distribution, therefore conditioning the plating and stripping or detachment of plated regions (irreversible capacity loss), which limit battery efficiency and operational lifetime. Current cycle life of iron-based batteries has been reported around 2,000 while the US DOE establishes targets comprised between 1,000 to 10,000 cycles[4] Therefore, in this research we aim to understand the underlying mechanisms of iron electroplating and the key parameters which influence the resulting structures, to enhance overall iron-based battery performance.In this talk, we present our advances to improve iron-RFB efficiency and durability, focusing on the iron plating electrode. In this research, we first leverage planar, model iron surfaces in a Swagelok-type cell and employ a suite of electrochemical techniques (e.g., chronoamperometry, electrochemical impedance spectroscopy) to gain insight into the poorly understood process of iron electroplating. The experimental data is coupled with parameter fitting to pre-existing physical models[5]. Additionally, we study the influence of chelating agents, such as buffering agents, surfactants and levelers (e.g., ascorbate, tetrabutylammonium ion or thiourea, respectively), on plating phenomena and elucidate their effects on the kinetics and structure of the resulting iron coatings, as well as their cycling behavior in symmetric cell configuration. Overall, we find that buffering agents achieved the greatest improvement in terms of faradaic efficiency, resulting in moderate cell currents, while other additives, such as levelers, allow larger plating currents at the expense of limited faradaic efficiencies.In addition to electrolyte-induced effects, another bottleneck of iron-RFBs are the electrodes employed in the negative half-cell. Common electrodes rely on iron-containing films or sintered iron oxide particles to form 3D structures[6]. However, comprehensive analysis relating anode structure and composition to flow cell performance have not been addressed to date. Accordingly, we hereby study the effects of diverse types of negative electrodes. To minimize complexities emerging in porous electrodes, we first analyze the effects of electrode composition in 2D films (e.g., pure iron, doped iron or iron-carbon hybrid systems) on the cell performance via polarization. Building on these findings, and to further improve iron-RFB performance, we study the impact of electrode architecture, by introducing complex, three-dimensional porous electrodes that enable higher surface areas and mass transport rates. We find that these three-dimensional electrode structures with large pore sizes produce an enhancement in exploitable active species and increased capacity, although there arises a trade-off with the structural integrity of these 3D electrodes. With this research we hope to shed light onto the control of ion-ion, solid-liquid and solid-solid interplay for iron-based batteries to aid the design of tailored electrolytes and advanced electrodes to realize low-cost iron-based battery systems.

Investigating the Impedance of an Iron/Iron Redox Flow Battery at Different State of Charge Conditions – a Distribution of Relaxation Times Analysis

New energy storage technologies are needed to offset the erratic power supply of renewable energy sources due to the rising global energy demand and the urgency of climate change action. The Iron/Iron redox flow battery is a viable contender for large-scale energy storage, even if there are still issues with operational parameters and efficiency. Due to the positive electrode's moderate pH and low voltage, which preserves minimum toxicity and aggressivity, iron is a very affordable active material, making it feasible to develop inexpensive energy storage systems and batteries. The Electrochemical impedance spectroscopy technique was utilized to understand the degradation of the iron/iron redox flow battery during charging and discharging operation modes. However, discriminating between processes occurring at similar time scales is difficult since they overlap in the generally used Nyquist or Bode representation. Distribution of relaxation times (DRT) analysis tackles this issue by converting the EIS data with respect to the time constants of the individual processes. Individual processes such as electrochemical reactions inside the iron/iron redox flow cell, transport processes through porous structures, and iron ion transport were investigated.The impedance measurements were carried out to evaluate the ohmic resistance during the charging and discharging phases of the iron/iron redox flow battery with varied states of charge ranging from 0-100% while charging and 100-0% while discharging. In this series, a DC offset load of 100 mA in the charging and discharging directions was used to test the resistance differential associated with the battery's state of charge. In the charging phase of the battery of three DC offset load parameters, the iron ions were transported faster in the process while the state of charge of a battery increased at lower frequencies. The impedance increased when the state of charge decreased at a higher frequency range, involving the iron ions transported through the negative half-cell in an iron/iron redox flow battery. The overall ohmic resistance in the charging phase of the battery decreased up to 6 % until the battery was fully charged, but in discharging phase, the overall ohmic resistance increased to 2 %. This indicated that the DC offset load in the charging and discharging direction were dependent on the battery's state of charge. Figure 1

A zinc–iodine hybrid flow battery with enhanced energy storage capacity

Zinc–Iodine hybrid flow batteries are promising candidates for grid scale energy storage based on their near neutral electrolyte pH, relatively benign reactants, and an exceptional energy density based on the solubility of zinc iodide (up to 5 M or 167 Wh L−1). However, the formation of zinc dendrites generally leads to relatively low values for the zinc plating capacity, limiting the cycle duration to <2 h even at relatively low current densities. In this study we investigate the effects of various cell configurations as well as complexing Zn2+ with gluconate with the aim of increasing the cycle duration and increasing the current density during charge and discharge. The addition of a non-conductive felt to the negative half-cell provided a significant increase in the plating capacity, while the addition of a non-conductive felt to the positive half-cell was shown to be effective in preventing electrical shorts resulting from dendrite growth at the cost of additional resistive losses. Ultimately, cycles with up to 4.8-h charge/discharge were demonstrated at a current density of 100 mA cm−2.

Revealing the Multifaceted Impacts of Electrode Modifications for Vanadium Redox Flow Battery Electrodes.

Carbon electrodes are one of the key components of vanadium redox flow batteries (VRFBs), and their wetting behavior, electrochemical performance, and tendency to side reactions are crucial for cell efficiency. Herein, we demonstrate three different types of electrode modifications: poly(o-toluidine) (POT), Vulcan XC 72R, and an iron-doped carbon-nitrogen base material (Fe-N-C + carbon nanotube (CNT)). By combining synchrotron X-ray imaging with traditional characterization approaches, we give thorough insights into changes caused by each modification in terms of the electrochemical performance in both half-cell reactions, wettability and permeability, and tendency toward the hydrogen evolution side reaction. The limiting performance of POT and Vulcan XC 72R could mainly be ascribed to hindered electrolyte transport through the electrode. Fe-N-C + CNT displayed promising potential in the positive half-cell with improved electrochemical performance and wetting behavior but catalyzed the hydrogen evolution side reaction in the negative half-cell.

Online and noninvasive monitoring of battery health at negative-half cell in all-vanadium redox flow batteries using ultrasound

Hydrogen evolution is one of the major side reactions that is detrimental to the health of all-vanadium redox flow batteries, especially for long-term cycling. Effective, low-cost, and accurate online prediction and detection methods for hydrogen generation are not yet available. In this work, we designed an online, noninvasive ultrasonic probing approach for monitoring the state of charge (SoC), predicting the hydrogen generation, and detecting hydrogen gas bubbles in anolyte solutions. The technique employs a pulse-echo method to measure the sound speed and the acoustic attenuation coefficient of the anolyte solution. Through static offline experiments and online in operando experiments, we have demonstrated that when hydrogen gas is generated in anolyte solutions, large variations are observed in both sound speed and acoustic attenuation coefficient measurements. We found that the variations of acoustic attenuation coefficient are highly correlated (correlation coefficients >0.9) with the gas flow rate. The designed acoustic method can monitor the SoC of anolyte, predict the hydrogen generation, and detect the presence of gas bubbles in an anolyte solution and, thus, provide information about the state of health for operation and management of flow battery systems.

The impact of in-situ hydrogen evolution on the flow resistance of electrolyte flowing through the carbon felt electrode in a redox flow battery

The parasitic hydrogen evolution reaction (HER) leads to capacity fade of aqueous redox flow batteries. In addition, the evolved hydrogen gas bubbles stagnating inside the porous electrode may block the flow of electrolyte, increase the flow resistance, and reduce the battery performance. By precisely controlling the HER and electrolyte flow rates, we explore the impact of the mA cm−2 scale HER on the relative permeability of HCl-based aqueous electrolyte flowing through a carbon felt electrode. Experimental results show that the HER with a current density of 2 mA cm−2 reduces the quasi-steady two-phase flow relative permeability in the negative half-cell from 1.0 to 0.84, even at a high electrolyte flow velocity of 16 mm s−1. With an ultrasonic gas sensor, the delay of gas release out of the cell from the beginning of HER has been measured. After the halt of HER, the recovery of the liquid permeability lasts for more than an hour if the electrolyte velocity is maintained at 8 mm s−1. Moreover, significant instability of pressure drop is observed at a low electrolyte velocity of 2 mm s−1.

Low Concentration Slurry Electrodes for Redox Flow Batteries

Iron redox flow batteries are a promising option for utility scale energy storage. In redox flow batteries (RFB), the power and energy storage capacities are decoupled, making them highly scalable 1,2. Due to its high abundance, low cost, and low toxicity, iron is very attractive as a reactive species for both the positive and negative half cells in large scale redox flow batteries 3. The Fe(II)/Fe(III) reaction is utilized at the positive electrode while the Fe(0)/Fe(II) reaction is used at the negative electrode. Unfortunately, the Fe(II) reduction reaction used in the negative cell involves plating solid iron onto the electrode during charge. This plating reaction limits the battery’s capacity based on the spatial constraints of the flow cell, coupling the power and storage capacities of the flow battery and limiting its scalability 2.Slurry electrodes, consisting of a dispersion of conductive particles in the electrode, have been proposed as solution for this issue4. By having the metal deposit onto the mobile dispersion of particles, as in Figure 1B, instead of the stationary electrode as in Figure 1A, the power and storage capacities of a hybrid flow battery can be decoupled. Slurry electrodes have also been proposed in a number of other applications such as water deionization and supercapacitors5. Their use has also been studied for use in fully soluble RFB chemistries, such as all-vanadium6,7. However, nearly all of the previous work in slurry electrodes has been in highly concentrated slurries in order to take advantage of the conductivity of the percolated particle network. Unfortunately, these highly loaded slurries can be viscous and can cause clogs and failures in a flowing system such as an RFB4,7.In this work, the electrochemical behavior of slurries below the percolation threshold are investigated via voltammetry in a custom flow cell. The percolation threshold of a slurry is identified and the modified behavior of the Fe (II)/Fe (III) reaction is measured as a function of slurry concentration and flow rate. The results suggest that significant enhancement of the electrochemically active surface area can be achieved below the percolation threshold.(1) Dinesh, A.; Olivera, S.; Venkatesh, K.; Santosh, M. S.; Priya, M. G.; Inamuddin; Asiri, A. M.; Muralidhara, H. B. Iron-Based Flow Batteries to Store Renewable Energies. Environ. Chem. Lett. 2018, 16 (3), 683–694. https://doi.org/10.1007/s10311-018-0709-8.(2) Weber, A. Z.; Mench, M. M.; Meyers, J. P.; Ross, P. N.; Jeffrey, T.; Liu, Q. Redox Flow Batteries , a Review Environmental Energy Technologies Division , Lawrence Berkeley National Laboratory , Department of Mechanical , Aerospace and Biomedical Engineering , University of Tennessee , Department of Chemical Engineering , McGill Un. 1–72.(3) Petek, T. J. Enhancing the Capacity of All-Iron Flow Batteries: Understanding Crossover and Slurry Electrodes. Ph.D. Thesis 2015, No. May.(4) Petek, T. J.; Hoyt, N. C.; Savinell, R. F.; Wainright, J. S. Slurry Electrodes for Iron Plating in an All-Iron Flow Battery. J. Power Sources 2015, 294, 620–626. https://doi.org/10.1016/j.jpowsour.2015.06.050.(5) Mourshed, M.; Niya, S. M. R.; Ojha, R.; Rosengarten, G.; Andrews, J.; Shabani, B. Carbon-Based Slurry Electrodes for Energy Storage and Power Supply Systems. Energy Storage Mater. 2021, 40 (April), 461–489. https://doi.org/10.1016/j.ensm.2021.05.032.(6) Percin, K.; van der Zee, B.; Wessling, M. On the Resistances of a Slurry Electrode Vanadium Redox Flow Battery. ChemElectroChem 2020, 7 (9), 2165–2172. https://doi.org/10.1002/celc.202000242.(7) Lohaus, J.; Rall, D.; Kruse, M.; Steinberger, V.; Wessling, M. On Charge Percolation in Slurry Electrodes Used in Vanadium Redox Flow Batteries. Electrochem. commun. 2019, 101 (March), 104–108. https://doi.org/10.1016/j.elecom.2019.02.013. Figure 1

Exploring the Effectiveness of Carbon Cloth Electrodes for All-Vanadium Redox Flow Batteries

Redox flow batteries (RFBs) have proven to be a promising technology for the integration of intermittent renewable energy sources into the existing electrical grid due to their ability to decouple energy and power ratings, and the associated cost-effectiveness in long-duration energy storage [1]. Among various flow battery chemistries, vanadium redox flow battery (VRFB) is one of the most prominently studied flow battery system [2]. In comparison to other chemistries, four soluble oxidation states of vanadium are available and accessible. This characteristic of VRFBs eliminates cross-contamination between the positive and negative half-cells that would otherwise deteriorate the life and durability of the battery [3]. In a flow battery cell, porous electrode is the key component that determines the performance of the system by delivering liquid electrolytes, facilitating ion/charge transfer, and providing sites for electrochemical reactions [4]. Recently, it has been found that significant performance improvements can be achieved with fabric (carbon cloth) electrodes when coupled with a wetting electrolyte [5].In this talk, we will report our experimental approach to evaluate the short- and long-term effectiveness of various carbon cloth electrode samples for use in VRFBs. Results generated from symmetric flow cell cycling coupled with electrochemical impedance spectroscopy and polarization curve measurements will be discussed. The key physico-chemical properties and their evolution during cycling will be elucidated.

Identification of Overpotentials in Vanadium Redox Flow Battery with Reference Electrodes and Determination of Apparent Electrochemical Rate Constants

Vanadium redox flow battery (VRFB) is an amateur technology that passed pilot stages in fulfilling the stationary applications at the energy storage market with a high share of renewable power generation. The commercial applications of VRFBs require the optimization of their internal losses and developing the optimal control strategies for in-grid operation. This calls for a deep understanding of the internal VRFB processes, which greatly benefit from both the mathematical modeling and experimental testing for the model validation and refinement.However, the available experimental data for VRFBs is usually limited by open-circuit voltage (OCV) and potential (OCP), charge/discharge voltage profiles at constant load current, full cell polarization curves at the fixed state of charge (SOC), and internal stack ohmic resistances. This also complicates the decoupling and accessing the voltage loss sources, with the prevailing ones usually classified as ohmic, concentration, and activation overpotentials. As such, Aaron et al. (1) showed that decoupling of overpotentials at positive and negative half-cell requires the half-cell potentials measurements with reference electrodes (RE) in addition to full cell data. Different configurations of RE incorporation (2–4) were suggested with a limited agreement with full cell experimental data. The alternative approach with extrapolation of parameters presented in the literature is also challenging due to their strong dependence on an experimental protocol (e.g., the electrode material and its pre-treatment procedure, a membrane, and a setup geometry). As an illustration, the comprehensive review Roznyatsovskaya et al. (5) highlights the four orders of magnitude difference in reported surface area-normalized rate constants. Therefore, the development of a self-consistent experimental setup is crucial for understanding and decoupling internal losses for flow batteries. Combined with VRFB modeling, this may extend accurate predictions of its operation at extreme SOCs and load currents, where voltage losses hamper the efficiency of the battery with the dominating ones usually attributed to concentration (mass-transport in solutions) and activation (sluggish kinetics of reactions on electrodes) overpotentials.Herein, we developed the experimental setup adopting a 5cm2 single flow cell equipped with reference electrodes at positive and negative sides. The experiments were performed in symmetric and full cell designs. The polarization curves were measured at different SOCs [10 – 90 %], electrolyte flow rates [5-30 ml min-1], and load currents. The additional effort was made to compare the results of the experiments in symmetric and full cell designs and extract the apparent kinetic rate constants for positive and negative half-cells. The latter was performed within the 0D modeling approach, which is based on general mass and charge conservation principles. The key feature of the model is the approach for the determination of the integral mass-transfer coefficient on the electrode-electrolyte interface that allows describing non-linear polarization behaviour at high current densities. The effective thermodynamically consistent single-step reaction mechanisms were considered at both half cells, thus decreasing the number of free kinetic parameters. Overall, the proposed approach demonstrated the growing impact of the membrane losses at high current densities. The extracted apparent rate constants under conditions of a real flow battery provide important insights for developing reliable and efficient VRFBs applicable for energy storage systems.1 D. Aaron, Z. Tang, A. B. Papandrew and T. A. Zawodzinski, J. Appl. Electrochem., 2011, 41, 1175–1182.2 H. Lim, J. S. Yi and D. Lee, J. Power Sources, 2019, 422, 65–72.3 S. Ressel, A. Laube, S. Fischer, A. Chica, T. Flower and T. Struckmann, J. Power Sources, 2017, 355, 199–205.4 M. Cecchetti, A. Casalegno and M. Zago, J. Power Sources, 2018, 400, 218–224.5 N. Roznyatovskaya, J. Noack, K. Pinkwart and J. Tübke, Curr. Opin. Electrochem., 2020, 19, 42–48.

Computational design of microarchitected porous electrodes for redox flow batteries

Porous flow-through electrodes are used as the core reactive component across electrochemical technologies. Controlling the fluid flow, species transport, and reactive environment is critical to attaining high performance. However, conventional electrode materials like felts and papers provide few opportunities for precise engineering of the electrode and its microstructure. To address these limitations, architected electrodes composed of unit cells with spatially varying geometry determined via computational optimization are proposed. Resolved simulation is employed to develop a homogenized description of the constituent unit cells. These effective properties serve as inputs to a continuum model for the electrode when used in the negative half cell of a vanadium redox flow battery. Porosity distributions minimizing power loss are then determined via computational design optimization to generate architected porosity electrodes. The architected electrodes are compared to bulk, uniform porosity electrodes and found to lead to increased power efficiency across operating flow rates and currents. The design methodology is further used to generate a scaled-up electrode with comparable power efficiency to the bench-scale systems. The variable porosity architecture and computational design methodology presented here thus offers a novel pathway for automatically generating spatially engineered electrode structures with improved power performance.

In-Situ Monitoring of Electrode Degradation in Vanadium Redox-Flow Batteries By Distribution of Relaxation Times Analysis

Recent studies[1,2] suggest that ion crossover through the membrane of a vanadium redox-flow battery (VRFB) can not be considered the only source of irreversible capacity loss. Instead, degradation of the macroporous carbon felt electrodes has to be taken into account as well. Especially in the negative half-cell, any degradation-induced alteration of the fibres’ surface composition will significantly impede the kinetics of the V2+/V3+ redox-system. Since the resulting increase in kinetic overvoltage will limit the accessible capacity, a reasonable lifetime prediction of a VRFB is only possible if all aspects of capacity fade mechanisms, including electrode degradation are well understood. This raises the demand for a measurement technique, which allows for in-operando state-of-health-assessment of the electrodes in a non-invasive fashion. Employing electrochemical impedance spectroscopy (EIS), the characteristics of any electrochemical system can be easily probed by applying non-harmful, small-amplitude sinusoidal perturbations. Interpretation of EIS usually relies on fitting suitable equivalent circuit models to experimental data. However, validation of such models can be a cumbersome task due to the inherent ambiguity of EIS, meaning that different circuits may approximate the same data set equally well. Furthermore, a full cell spectrum will always yield a superposition of the respective half-cell impedances. Discriminating the contributions of the individual half-cell electrodes to the impedance of a VRFB has therefore been attempted by implementing reference electrodes. However, these are known to be a source of artefacts and distortion in the high frequency range.In the present study we apply the distribution of relaxation times (DRT) analysis[3] to EIS data of a single cell VRFB. This innovative approach allows us to resolve up to three different time constants per frequency decade. It therefore enables processes that overlap in the classical Bode- and Nyquist representation to be displayed as distinct peaks. Consequently, the individual half-cells can be studied from full cell spectra, circumventing the necessity of finding appropriate equivalent circuit models and rendering the implementation of reference electrodes obsolete.We outline our experimental strategy for unravelling the physical nature of the processes related to the individual signals in the DRT spectrum and finally demonstrate how these findings enable an online monitoring of electrode degradation in VRFB. Keywords: electrochemical impedance spectroscopy, distribution of relaxation times, electrode degradation, vanadium redox-flow battery

Voltage Loss Analysis of Zinc-Cerium Redox Flow Batteries

Redox flow batteries (RFBs) are a relatively new generation of electrochemical devices suitable for large-scale energy storage applications. The separation between the electrolyte storage tanks and the electrochemical cell in RFBs simplifies the battery scale-up and facilitates the energy/power ratio tuning. In comparison to current energy storage devices, RFBs are beneficial due to their portability, flexibility and low maintenance cost [1].Among the different types of RFBs investigated, those based on zinc and cerium are very attractive due to the large negative and positive electrode potentials in an aqueous media. Thus, zinc-cerium RFBs are capable of providing one of the highest cell voltages (~ 2.4 V) among flow batteries and a large theoretical energy density [2]. To date, Zn-Ce RFBs have primarily been investigated galvanostatically to determine their charge, voltage and energy efficiencies and attempts have been made to suppress the rate of the hydrogen and oxygen evolution side reactions [3-6]. In order to further optimize the performance of these batteries and to elucidate the future pathways to enhance their efficiency, the sources of voltage loss in the battery during discharge must be identified and the role of the positive and negative half-cells in the voltage loss determined. Toward this goal, we have conducted in situ polarization and EIS experiments on a full-cell Zn-Ce RFB with reference electrodes inserted in the system [7]. The insertion of the reference electrodes in the RFB enables us to decouple the contribution of negative and positive electrodes to the total performance loss. At low and intermediate current densities, the main contributor to the voltage loss during discharge is the kinetic overpotential of the negative Zn/Zn2+ half-cell. On the other hand, at high current densities, mass transfer limitations at the positive Ce3+/Ce4+ half-cell cause a large potential drop in the system. From in situ kinetic studies, we have measured an exchange current density of ∼ 7.4×10−3 A cm−2 for Zn oxidation and ∼ 24.2×10−3 A cm−2 for Ce4+ reduction, which is consistent with our previous findings from battery operation that the kinetics of the negative electrode reaction is slow compared to that of the positive electrode at low-to-intermediate current densities. The use of an alternative mixed methanesulfonate-chloride negative electrolyte to reduce the kinetic overpotential of the negative half-cell reaction and the influence of the flow rate on the mass-transfer rate of the positive half-cell reaction have also been investigated and will be discussed in this presentation.[1] De Leon, C. P., Frías-Ferrer, A., González-García, J., Szánto, D. A., & Walsh, F. C. (2006). Redox flow cells for energy conversion. Journal of power sources, 160(1), 716-732.[2] Walsh, F. C., Ponce de Léon, C., Berlouis, L., Nikiforidis, G., Arenas‐Martínez, L. F., Hodgson, D., & Hall, D. (2015). The development of Zn–Ce hybrid redox flow batteries for energy storage and their continuing challenges. ChemPlusChem, 80(2), 288-311.[3] Leung, P. K., Ponce-de-León, C., Low, C. T. J., & Walsh, F. C. (2011). Zinc deposition and dissolution in methanesulfonic acid onto a carbon composite electrode as the negative electrode reactions in a hybrid redox flow battery. Electrochimica Acta, 56(18), 6536-6546.[4] Nikiforidis, G., Berlouis, L., Hall, D., & Hodgson, D. (2014). An electrochemical study on the positive electrode side of the zinc–cerium hybrid redox flow battery. Electrochimica Acta, 115, 621-629.[5] Amini, K., & Pritzker, M. D. (2018). Electrodeposition and electrodissolution of zinc in mixed methanesulfonate-based electrolytes. Electrochimica Acta, 268, 448-461.[6] Amini, K., & Pritzker, M. D. (2019). Improvement of zinc-cerium redox flow batteries using mixed methanesulfonate-chloride negative electrolyte. Applied Energy, 255, 113894.[7] Amini, K., & Pritzker, M. D. (2020). In situ polarization study of zinc–cerium redox flow batteries, Journal of Power Sources, 471, 228463

A combined in situ monitoring approach for half cell state of charge and state of health of vanadium redox flow batteries

Efficient vanadium redox flow battery operation requires reliable half cell specific state of charge and capacity information at each point in time. This study uses long term correlations of electrolyte densities with negative half cell capacities and half cell potentials in Nernst equations for a joint in situ measurement approach. In principle, no ex situ reference data are needed for the calibration since a combination of Coulomb counting and half cell electrolyte potentials enables a self consistent calibration during battery operation cycles. By this way state of charge results of a negative half cell feasibility study are confirmed for long term data and extended to the positive half cell. The explicit consideration of H+ concentration and acid dissociation for the positive half cell leads to consistent half cell measurements with lower errors. Experiments have been carried out with two different cation exchange membranes achieving comparable measurement errors in the few per cent range.