Performance Of Redox Flow Battery Research Articles

A poly(2-ethylaniline) blend membrane for vanadium redox flow batteries

Processable E-PANI polymer blend SPES membrane for enhancement of vanadium redox flow battery performance.

Electrolyte Flow through Porous Carbon Electrodes Under Compression in the Hydrogen-Bromine Redox Flow Battery

The conditions of electrolyte flow affect the performance of redox flow batteries through the combined effect of pumping energy and electrode overpotential. In the hybrid hydrogen-bromine flow battery the conditions of electrolyte flow depend on the trans-membrane pressure exerted by hydrogen gas in the membrane electrode assembly, which has a compressive effect on the liquid side electrode.This study aims at characterizing the effect of hydrogen pressure on flow characteristics and investigate its effect on transport behavior. We selected two types of porous carbon electrodes, carbon paper and carbon cloth, with and without surface conditioning, and study liquid flow at the compression ratios expected in a hydrogen-bromine cell under increasing operation pressure.Combination of electrode characterization with electrolyte velocity measurements obtained by particle image velocimetry in flow-through configuration allow characterization of liquid flow through porous electrodes, and attempt to relate it to electrode morphology. Electrochemical testing is then is used to reveal the effect of electrode compression on electrochemical active surface area, estimated by the capacitance of the double layer, and cell polarization.The velocity field is revealed to be highly dependent on the fiber structure of the electrode and surface conditioning. The effect of compression manifests early and impacts the uniformity of flow through the porous structure. Additionally, the changes in active area and cell resistance result in a complex combination of factors that determine cell polarization. Figure 1

Electrode Reactions on Conducting Polymers

The electrocatalysis behind the direct chemical-to-electrical energy interconversion is one of key facets in the development of sustainable economy. This results in a technological demand for a variety of applications such as electrical transportation, electrified synthesis and the grid balancing. Here are three examples of problematized electrocatalytic processes. (A) The sluggish kinetics and multistep character of oxygen reduction reaction (ORR) are causing losses in electrical energy conversion and poor selectivity, which limit the wide implementation of oxygen-associated energy conversion technologies. (B) The water electrolysis yielding the hydrogen evolution reaction (HER), the key process in technologies of green hydrogen, provides only 4% of worldwide hydrogen production due to the high catalyst cost. (C) The control of the proton-coupled electron transfers on organic redox molecules such as benzenediols. These three illustrate the stimulus of the intensive research on noble-metal-free electrocatalysts. Conducting polymers, the organic materials synthesised from abundant element, constitute a distinct class of molecular electrocatalysts [1, 2] attributed with behaviour of mixed ion-electron conductors (MIEC). Firstly, the landscape of ORR phenomena happening on p- and n-type conducting polymers at the mechanistic and device levels are discussed [1-3]. Secondly, the effect of proton supply is rationalized at both mechanistic and device levels for HER on PEDOT-triflate [4]. Thirdly, the significant effect of ionic transport on the rate of the proton-coupled electron transfers was observed and conceptualized as a ion-selective electrocatalysis (ISEC) [5].[1] E. Mitraka, M. Gryszel, M. Vagin, M.J. Jafar, A. Singh, M. Warczak, M. Mitrakas, M. Berggren, T. Ederth, I. Zozoulenko, X. Crispin, E.D. Głowacki, Electrocatalytic Production of Hydrogen Peroxide with Poly(3,4‐ethylenedioxythiophene) Electrodes, Advanced Sustainable Systems, 3 (2019) 1800110.[2] Z.X. Wu, P.H. Ding, V. Gueskine, R. Boyd, E.D. Glowacki, M. Oden, X. Crispin, M. Berggren, E.M. Bjork, M. Vagin, Conducting Polymer-Based e-Refinery for Sustainable Hydrogen Peroxide Production, Energy & Environmental Materials.[3] M. Vagin, V. Gueskine, E. Mitraka, S.H. Wang, A. Singh, I. Zozoulenko, M. Berggren, S. Fabiano, X. Crispin, Negatively-Doped Conducting Polymers for Oxygen Reduction Reaction, Advanced Energy Materials, 11 (2021) 2002664.[4] R. Valiollahi, M. Vagin, V. Gueskine, A. Singh, S.A. Grigoriev, A.S. Pushkarev, I.V. Pushkareva, M. Fahlman, X.J. Liu, Z. Khan, M. Berggren, I. Zozoulenko, X. Crispin, Electrochemical hydrogen production on a metal-free polymer, Sustainable Energy & Fuels, 3 (2019) 3387-3398.[5] M. Vagin, C.Y. Che, V. Gueskine, M. Berggren, X. Crispin, Ion-Selective Electrocatalysis on Conducting Polymer Electrodes - Improving the Performance of Redox Flow Batteries, Advanced Functional Materials, 30 (2020) 2007009.

Electrochemical Evaluation of Manganese Electrolytes for Redox Flow Batteries

Redox flow batteries (RFBs) are promising candidates for long-duration energy storage mainly due to the lower cost of improving the energy density compared to lithium-ion batteries. However, the full sustainability of RFBs has not been achieved yet, and there is a lot of room for further improvement. One of the main parameters showing a strong influence on the sustainability of RFBs is redox active materials. In this work, manganese was selected for electrochemical evaluation as it fulfills the sustainability requirements. Manganese is a non-toxic, low-cost and abundant material with sufficient aqueous solubility[1]. Moreover, the standard potential of manganese (1.51 V) is higher than that of vanadium (1.26 V).For electrochemical parameters, we have, through cyclic voltammetry (CVs) and polarization curves, estimated the diffusion coefficient (D0 ) and heterogeneous standard electron transfer rate (k 0) of manganese in an acidic electrolyte in the presence of titanium as an additive. In order to investigate the performance of an RFB system using a manganese electrolyte, a hybrid system was designed that uses hydrogen gas on the negative side and an acidic manganese electrolyte the positive side. We have studied the resistance behavior of the hybrid RFB through electrochemical impedance spectroscopy at different states of charge. From EIS data, cell ohmic resistance, and a general idea of the different processes taking place at different state of charge were obtained. Moreover, we studied the performance of the hybrid RFB during 70 cycles, obtaining an average coulombic efficiency of 98.1%, voltage efficiency of 77.8% and capacity retention of 97.7%. Carney, J.R., B.R. Dillon, and S.P. Thomas, Recent Advances of Manganese Catalysis for Organic Synthesis. European Journal of Organic Chemistry, 2016. 2016(23): p. 3912-3929.

Improved Vanadium Redox Flow Battery Performance through a Pulsating Electrolyte Flow Regime

The increase in emissions of greenhouse gases and their environmental impact lead to a transition from primarily fossil fuel-based energy sources to renewable and eco-friendly ones, such as solar panels and wind turbines. However, these sources are intermittent, resulting in fluctuating energy production levels that require buffering, which poses significant challenges in matching production-consumption profiles. [1,2] In order to stabilise the grid and reduce dependency on fossil fuels, integrating electrical energy carriers into a smart grid to store excess energy produced during peak periods and employ it during periods of low production is a potential solution. Redox flow batteries hold promise in this regard, as the storage capacity can be decoupled from the battery size, enabling capacity to be scaled independently of power output by increasing the volume of the electrolyte. [3] In contrast to conventional secondary battery types, such as lead-acid or lithium-ion batteries, the electrolyte in a redox flow battery is non-stationary, inducing convective mass transport effects that are critical for achieving a high power output.In this work we introduced pulsations to the electrolyte flow of an all-vanadium redox flow battery, leading to a reduced stagnant boundary layer attributed to shear forces. Previously, Pérez-Gallent et al. demonstrated that the conversion can be increased by a factor of two while selectivity gains of 15-20% can be made in a electrolyser by introducing a pulsating flow. [4] Furthermore, Vranckaert et al. studied a pulsating flow combined with pillar field electrodes, using the well-known ferri-ferrocyanide redox couple, and concluded that the average limiting current density can be increased fourfold compared to working with a conventional steady state flow regime. [5] Overall, a pulsating flow behaviour yields an enhanced mass transfer of electroactive species by increasing the local velocity through pulsations. The net residence time of the reactants remains unchanged since the pulsating flow has no influence on the overall flow velocity. However, the resulting improvement of mass transfer leads to an increase in conversion. As it is clear that mass transfer enhancement strategies open the door for more efficient systems, our work will for the first time focus on the influence of a pulsating electrolyte flow on the redox flow battery performance. The results show a significant improvement: the discharge capacity at 100 mA/cm2 increased by 38.7% when applying a pulse with pulse amplitude (PA) 0.4ml and pulse frequency (PF) 2.4 Hz at a flow rate of 5 ml/min (Figure 1 A). Furthermore, this 38.7% increase in discharge capacity surpassed the performance achieved at a constant steady-state flow of 25 ml/min. Therefore, a fivefold reduction of the net flow rate is possible without performance losses, decreasing necessary pumping costs. When regarding the stability of an all-vanadium redox flow battery operated with a pulsating electrolyte flow initial fluctuations of the current efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) were recorded. However, the efficiencies became stable at 98.4%, 70.5% and 69.4% respectively, showing no decline over time (Figure 1 B). This novel and improved redox flow battery system allows to take the next step towards integrating electrical energy carriers into a smart grid suitable for renewable and green energy sources.[1] P. D. Lund, J. Lindgren, J. Mikkola, J. Salpakari, Renew. Sustain. Energy Rev. 2015, 45, 785–807.[2] H. Kondziella, T. Bruckner, Renew. Sustain. Energy Rev. 2016, 53, 10–22.[3] E. Sánchez-Díez, E. Ventosa, M. Guarnieri, A. Trovò, C. Flox, R. Marcilla, F. Soavi, P. Mazur, E. Aranzabe, R. Ferret, J. Power Sources 2021, 481, 228804.[4] E. Pérez-Gallent, C. Sánchez-Martínez, L. F. G. Geers, S. Turk, R. Latsuzbaia, E. L. V. Goetheer, Ind. Eng. Chem. Res. 2020, 59, 5648–5656.[5] M. Vranckaert, H. P. L. Gemoets, R. Dangreau, K. Van Aken, T. Breugelmans, J. Hereijgers, Electrochim. Acta 2022, 436, 141435. Figure 1

Manufacturing Strategies for Engineering Carbon Electrodes Via Non-Solvent Induced Phase Separation

Porous electrodes are at the core of emerging electrochemical systems and tuning the electrode properties is critical to overcome system-specific limitations. To meet the stringent performance and cost requirements, there is a need for tailored microstructures generated through inexpensive and scalable synthesis methods with precursors originating from reliably-sourced, globally-abundant feedstocks. To this end, we recently introduced a manufacturing route using non-solvent induced phase separation (NIPS), yielding porous carbon electrodes with multimodal and interconnected porosity. The synthesized NIPS electrodes demonstrated a beneficial trade-off between transport properties (permeability, mass transfer) and electrochemically active surface area (ECSA) resulting in high performance in iron and all-vanadium redox flow batteries (RFBs)[1,2]. Moreover, NIPS manufacturing offered control over electrode characteristics (pore size, surface area) which are critical parameters for broader electrochemical devices. Besides its use for RFB application, we hypothesize that the electrode morphologies resulting from NIPS could be applied to a broader range of electrochemical systems employing porous electrodes. Consequently, here we focus on extending the library of electrode microstructures achievable via NIPS through the alteration of simple process parameters.In this presentation, I will discuss our latest advances on the bottom-up manufacturing of porous electrodes using NIPS targeting porosity gradients and high ECSA materials. For the former, we introduce the use of a control layer, casted on top of the NIPS polymer solution at the interface with the coagulation bath, thereby altering the rate of the phase separation process. We investigate various process parameters such as the composition, viscosity, and thickness of the control layer and assess their effect onto the resulting microstructures. The NIPS method assisted by a control layer generates porosity gradients, resulting in a slight increase of the electrode ECSA at the cost of a higher pressure drop. To further increase the ECSA of the prepared electrodes, we also propose nano-patterning approaches to significantly enhance ECSA through addition of nano-scale porosity without compromising the pressure drop through the porous structure. Finally, both porosity gradient and nano-patterned NIPS electrodes are tested in a all-vanadium redox flow cell to extract microstructure-performance relationships. Despite being in its early technological stage, this work illustrates the versatility of NIPS to manufacture porous electrodes through a very broad microstructural design space.

Effect of Parasitic Gas Evolution Reactions on the All-Vanadium Redox Flow Battery Performance

All-vanadium redox flow batteries (VRFB) as the most promising candidates for large-scale energy storage can overcome the inherent defect of natural energy, such as intermittent and fluctuating, et al. During the operation of VRFB, the parasitic gas evolution reactions (GERs) are unavoidable due to thermodynamic limitation, which yet will cause a series of issues, such as imbalance of state of charge (SOC), graphite electrode degradation, security risk, etc. To have a deeper understanding of the parasitic GERs in VRFB, the present work developed a transient, multidimensional, multiphysics VRFB model coupling the multiphase species transport and electrochemical reactions. The model was validated with in-house experimental data and published literature, which showed great consistency. Following, the temporal-spatial characteristics of GERs and their competition with main cell reactions were analyzed in detail under different charging current densities and electrolyte flowrate. The results showed that the rate of hydrogen evolution reaction (HER) at the negative electrode is several magnitudes higher than that of the oxygen evolution reaction (OER) at the positive electrode. Besides, the HER happens during the whole charge-discharge process since the activation overpotential remains negative while the OER occurs only at the end of the charging process under high charging current density. It is notable that the HER mainly occurs in the region near the bipolar plate and increases with increasing charging current density. Since HER is favored thermodynamically over the reduction reaction of trivalent vanadium ions, the activation overpotential of the HER remains much higher than that of the main cell reaction during the whole charge-discharge process, albeit the volumetric current density for hydrogen evolution accounts for only part of the overall current due to the limitation of the reaction kinetics. The HER increases with the SOC and causes a SOC imbalance of 0.54% within 10 charge-discharge cycles, thereby accounting for an 11.0% degradation in the VRFB’s capacity. A high current density decreases the amount of hydrogen generated during a charge-discharge cycle yet also causes hydrogen accumulation in the porous electrode due to faster hydrogen generation. The present work takes a comprehensive and detailed discussion about the characteristics of parasitic gas evolution reactions and their effect on the VRFB performance, which gives a deeper understanding of the energy loss mechanism of VRFB for its development and optimization. Figure 1

Lignin-Based Electrolytes for Redox Flow Batteries

Redox flow batteries (RFBs) are promising candidates for long duration energy storage applications thanks to their unique feature of independent scaling of energy and power. Organic electrolytes and an alternative to the present metal-based have been proposed as potentially low-cost and environmentally friendly electrolytes to alleviate the capital costs of RFBs. Lignin, the second most abundant bio-based polymer in nature, demonstrates the advantages of high carbon content, rich in aromatic groups and is a potential electrolyte basefor RFBs [1] [2] [3] [4]. In this work, soda lignin was considered as redox species modified in optimum oxidative conditions. The cyclic voltammetry analysis confirmed its electrochemical reversibility, so that for RFB tests, modified soda lignin was used on the negative side versus ferrocyanide on the positive at highly alkaline media.Following RFB cycling tests, a substantial improvement was achieved in the charge/discharge capacities and coulombic efficiency of RFB using modified soda lignin compared to pristine. For further performance optimization, it was found that the flow ratehas a substantial influence. This is more likely due to the thixotropic behaviour of the lignin electrolyte. Further explorations are under way of studying different membranes/electrolyte additives and cycling with higher concentration of lignin towards enhancing the RFB performance by improving the conductivity thus voltage efficiencies. Thecurrent approach of introducing bio-based electrolytes could enlighten a new path in the future research direction of organic RFBs and add a valuable point towards the carbon neutralization. Keywords: Lignin, Bio-based electrolyte, Organic redox flow battery, Sustainability, Energy storageAcknowledgement:The authors acknowledge Miscancell, Hondiusstraat 28, 6827 DE Arnhem, Netherlands for providing the lignin material for this work.

Designing Metal Complexes for Redox Flow Batteries By High-Throughput Synthesis

Redox flow batteries (RFBs) are an attractive device for high-capacity energy storage systems. Due to the disadvantages of vanadium RFBs such as high cost and limited temperature operability [1], researchers have been developing new redox couples to replace vanadium. The use of relatively inexpensive transition metals (Fe, Cr, Co, etc.) in combination with ligands to form metal-ligand complexes has received considerable attention [2]. The redox potential of a metal ion of a metal-ligand complex can be tuned by forming a complex with a ligand, and the metal-ligand complex form helps alleviate the crossover problem [3].A major challenge in the discovery and development of functional materials is to find and select the most optimized candidates from millions of possible materials [4]. High-throughput Synthesis (HTSy) enables simultaneous screening, synthesis and characterization of a large number of different material classes, providing unprecedented throughput and reproducibility. Therefore, HTSy can save energy, time, and cost for more efficient discovery of advanced functional materials [5]. In this study, based on HTSy methods, we used an automatic synthesis device (OT-2 liquid pipetting robot) to mix and heat metals (Ni, Cr, Cu, etc.) and ligand liquids to automatically design metal complexes. At the same time, a robot platform is developed to perform high-throughput electrochemical tests to quickly select suitable metal complexes for RFBs, and then conduct further related RFBs performance tests.

Electrochemical Residence Time Distribution as a Diagnostic Tool for Redox Flow Batteries

The fluid dynamic and electrochemical performance of redox flow batteries (RFBs) stems from the relationship between the flow field and the porous electrode, whose interplay determines how active species move and react during device operation. While characterization techniques, such as residence time distribution, offer insights into species mobility within a reactive volume for a traditional chemical reactor, electrochemical reactors also enable simultaneous measurement of the redox reactions, unlocking another dimension of analysis. Herein, we demonstrate how potentiodynamic measurements, using injections of electrolyte examined through moment analysis, can provide electrode-specific performance scaling relationships across a matrix of carbon paper and cloth electrodes with flow through and interdigitated flow fields. We further combine experimental campaigns with multiphysics simulations to demonstrate how electrode surface area can be estimated with this technique, which we then validate with activated and unactivated commercial carbon cloth electrodes. These studies reveal the multiscale observations that potentiodynamic measurements afford, augmenting existing electrochemical techniques for holistic electrochemical reactor diagnostics.

Proton conducting zeolite composite membrane boosts the performance of vanadium redox flow battery

In a vanadium redox flow battery (VRFB), the selectivity of protons over vanadium ions for an ion-conducting separator is crucial to achieving capacity retention in charge/discharge cycles. Conventionally used perfluorinated or hydrocarbon-based membranes are not able to resolve cross-contamination of vanadium during VRFB operation. Engineered inorganic materials like zeolites have inherent selectivity towards specific ions based on the size and charge of the ions. In this work, ZSM-5 zeolites with different Si/Al ratios were modified by a facile ion-exchange process to obtain proton rich zeolites, followed by successful infiltration into the SPEEK matrix. The composite membranes demonstrated excellent oxidative stability in 2 M H2SO4 containing 1.5 M VO2+ ions as well as favourable electrochemical properties. In a single-cell VRFB, the membrane with the highest proton conductivity i.e., SPEEK-ZSM 5 (25) 2H+ was able to achieve an average Coulombic efficiency of 97.0% and average energy efficiency of 78.0% over 300 charge/discharge cycles at 80 mA cm−2 current density. The polarization curve and self-discharge experiment results illustrated 21.7% higher peak power density and 2.5 times slower self-discharge compared to state-of-the-art Nafion-117 in identical operating conditions indicating zeolite-composite membranes as a potential candidate to enhance the durability and performance of VRFBs.

Low-Index Facet Polyhedron-Shaped Binary Cerium Titanium Oxide for High-Voltage Aqueous Zinc-Vanadium Redox Flow Batteries.

Aqueous zinc-vanadium hybrid redox flow battery systems are an efficient strategy to address the problems of low voltage and high cost of conventional all-vanadium redox flow batteries. However, the low electrochemical activity of carbon-based electrodes toward a vanadium redox reaction limits the performance of redox flow batteries. In this study, polyhedral binary cerium titanium oxide (Ce2/3TiO3, CTO) is synthesized using molten salt synthesis. CTO is fabricated by adjusting the temperature and composition. Notably, the prepared CTO obtained at 1000 °C shows the highest catalytic activity for a VO2+/VO2+ redox reaction. Further, CTO is prepared as a composite electrocatalyst and applied to a high-voltage aqueous zinc-vanadium redox flow battery. The cell adopts an alkali zinc electrolyte containing a Zn/[Zn(OH)4]2- redox pair and exhibits a high operating voltage of 2.26 V. Remarkably, a zinc-vanadium redox flow battery using the composite electrocatalyst exhibits a high energy density of 42.68 Wh L-1 at 20 mA cm-2 and an initial voltage efficiency of 90.3%. The excellent cell performance is attributed to structural defects caused by A-site deficiency in the perovskite oxide structure as well as oxygen vacancies resulting from the low valence state of the metal ion, which enhance the catalytic activity of the vanadium ions.

Impact of Pendent Ammonium Groups on Solubility and Cycling Charge Carrier Performance in Nonaqueous Redox Flow Batteries.

The synthesis, characterization, electrochemical performance, and theoretical modeling of two base-metal charge carrier complexes incorporating a pendent quaternary ammonium group, [Ni(bppn-Me3)][BF4], 3', and [Fe(PyTRENMe)][OTf]3, 4', are described. Both complexes were produced in high yield and fully characterized using NMR, IR, and UV-vis spectroscopies as well as elemental analysis and single-crystal X-ray crystallography. The solubility of 3' in acetonitrile showed a 283% improvement over its neutral precursor, whereas the solubility of complex 4' was effectively unchanged. Cyclic voltammetry indicates an ∼0.1 V positive shift for all waves, with some changes in reversibility depending on the wave. Bulk electrochemical cycling demonstrates that both 3' and 4' can utilize the second more negative wave to a degree, whereas 4' ceases to have a reversible positive wave. Flow cell testing of 3' and 4' with Fc as the posolyte reveals little improvement to the cycling performance of 3' compared with its parent complex, whereas 4' exhibits reductions in capacity decay when cycling either negative wave. Postcycling CVs indicate that crossover is the likely source of capacity loss in complexes 3, 3', and 4' because there is little change in the CV trace. Density functional theory calculations indicate that the ammonium group lowers the HOMO energy in 3' and 4', which may impart stability to cycling negative waves while making positive waves less accessible. Overall, the incorporation of a positively charged species can improve solubility, stored electron density, and capacity decay depending on the complex, features critical to high energy density redox flow battery performance.

Asymmetric batteries based on customized positive and negative electrodes-an effective strategy to further improve the performance of vanadium redox flow batteries

There are significant differences in the cathodic and anodic kinetics of vanadium redox flow batteries (VRFBs), thus the use of a single composition and structure of electrode materials is unbefitting according to the barrel principle. Herein, W and Sb based electrocatalysts are evenly embedded into the electrospun carbon nanofibers (ECNFs), and the active functional groups and surface defects of the W-ECNFs and Sb-ECNFs electrodes are increased effectively, which resulted in a significant improvement in their hydrophilicity, specific surface area and electrocatalytic activity towards the VO2+/VO2+ and V3+/V2+redox reactions. The electrode reaction rate constant k and diffusion coefficient D of the positive and negative reactions on W-ECNFs and Sb-ECNFs are more similar and closely matched, which is more conducive to the effective improvement of battery performance. As a results, the asymmetric battery with W-ECNFs and Sb-ECNFs as positive and negative electrode shows the highest energy efficiency of 81.33% at 200 mA/cm2, higher than that of the VRFBs with pristine ECNFs as both two electrodes (75.66%) or that only optimizing one electrode (79.76% and 78.15%). The excellent battery performance further verifies the feasibility of constructing asymmetric batteries according to the difference of the positive and negative reaction kinetics in VRFBs.

A DFT-based FD-KMC Simulation for Electrodeposition of Copper Nanoparticles on Carbon Electrode Surface

Electrodeposition is often used to load catalysts onto electrode surfaces to enhance their electrochemical activity, thereby improving the performance of redox flow batteries. The kinetic Monte Carlo (KMC) method was used to successfully simulate the nucleation and growth of nanoparticles during the electrodeposition process. However, the reliability of KMC simulation results is closely related to the atomic kinetic parameters derived from quantum-scale calculations. Meanwhile, the electrochemical reaction behaviors during electrodeposition rely on the mass transport of electroactive ions near the electrode surface. To address these issues, density functional theory (DFT) was introduced to obtain the energy barriers required in the calculation of KMC. Simultaneously, the finite difference (FD) method was integrated into the KMC algorithm to provide the transient concentration distribution of the diffusion layer near the electrode surface. This DFT-based FD-KMC method was used to simulate the early stage of electrodeposition of copper (Cu) nanoparticles on carbon electrode surfaces and investigate the effects of bulk concentration and applied potential on the characteristics of deposition morphology of Cu nanoparticles. Additionally, carbon electrode surfaces with different defect site numbers were generated to reveal the influence of surface defect sites on the morphology of the deposited Cu nanoparticles during electrodeposition process.

First-principles study of V3+ adsorption on MXene toward vanadium redox flow battery

The sluggish kinetic of V2+/V3+ redox reaction limits the performance of vanadium redox flow battery (VRFB). As one of the critical steps of negative reaction, the adsorption process of V3+ on catalyst plays a significant role. Owing to the high conductivity and catalytic activity of MXene materials, the adsorption behavior of V3+ on MXenes (Ti3C2O2, Nb3C2O2, Sc3C2O2) and MXenes with oxygen vacancy (Vo-Ti3C2O2, Vo-Nb3C2O2, Vo-Sc3C2O2) were revealed through first-principles based on density functional theory (DFT). Band structure, adsorption energy, Mulliken charge population, density of state, partial density of states and charge density difference were calculated. The results show that the adsorption mechanism of V3+ on MXenes is chemisorption and oxygen vacancy efficiently promotes the adsorption energy for V3+. The strong interaction between V3+ and MXenes is ascribed to orbital hybridization and overlapping. The adsorption energy of V3+ on Sc3C2O2 with oxygen vacancy (Vo-Sc3C2O2) reaches −98.255 kcal/mol. Meanwhile, the most obvious electron transfer phenomenon exists on the surface of Vo-Sc3C2O2. Vo-Sc3C2O2 shows the strongest metallicity and the largest adsorption energy for V3+, which presents potential candidate as catalyst for V2+/V3+ redox reaction.

Investigating the coupled influence of flow fields and porous electrodes on redox flow battery performance

At the core of redox flow reactors, the design of the flow field geometry –which distributes the liquid electrolyte through the porous electrodes– and the porous electrode microstructure –which provides surfaces for electrochemical reactions– determines the performance of the system. To date, these two components have been engineered in isolation and their interdependence, although critical, is largely overlooked. Here, we systematically investigate the interaction between state-of-the-art electrode microstructures (a paper and a cloth) and prevailing flow field geometries (flow through, serpentine and four variations of interdigitated). We employ a suite of microscopic, fluid dynamics, and electrochemical diagnostics to elucidate structure-property-performance relationships. We find that interdigitated flow fields in combination with paper electrodes –which features a uniform microstructure with unimodal pore size distribution– and flow-through configurations combined with cloth electrodes –which have a hierarchical microstructure with bimodal pore size distribution– provide the most favorable trade-off between hydraulic and electrochemical performance. Our analysis evidences the importance of carrying out the co-design of flow fields and electrode microstructures in tandem.

A Comparative Study of Nafion 212 and Sulfonated Poly(Ether Ether Ketone) Membranes with Different Degrees of Sulfonation on the Performance of Iron-Chromium Redox Flow Battery.

For an iron-chromium redox flow battery (ICRFB), sulfonated poly(ether ether ketone) (SPEEK) membranes with five various degrees of sulfonation (DSs) are studied. To select the SPEEK membrane with the ideal DS for ICRFB applications, the physicochemical characteristics and single-cell performance are taken into consideration. Following all the investigations, it has been determined that the SPEEK membrane, which has a DS of 57% and a thin thickness of 25 μm, is the best option for replacing commercial Nafion 212 in ICRFB. Firstly, it exhibits a better cell performance according to energy efficiency (EE) and coulombic efficiency (CE) at the current density range between 40 mA cm-2 and 80 mA cm-2. Additionally, it has a more stable EE (79.25-81.64%) and lower discharge capacity decay rate (50%) than the Nafion 212 (EE: 76.74-81.45%, discharge capacity decay: 76%) after 50 charge-discharge cycles, which proves its better oxidation stability as well. In addition, the longer self-discharge time during the open-circuit voltage test further demonstrates that this SPEEK membrane could be employed for large-scale ICRFB applications.

Leveraging sulfonated poly(ether ether ketone) for superior performance in zinc iodine redox flow batteries

Sulfonated poly (ether ether ketone) SPEEK membranes with a degree of sulfonation of approximately 73 % were synthesized and investigated as the cation exchange membrane (CEM) for the zinc iodine (ZI) redox flow battery (RFB). Specifically, SPEEK was used in ZI RFB with 1 mol L−1 electrolyte and its performance was compared to the current benchmark CEM, Nafion™. Notably, columbic and energy efficiencies of 92.9 % and 75.9 %, respectively, were measured for the ZI battery with SPEEK at 17 mA cm−2, which was a 370 % increase in energy density compared to those with Nafion 212 membranes. RFBs with SPEEK exbibit a peak power density of 122 mW cm−2 at a current density of 166 mA cm−2 (98 mW/cm2 at 133 mA/cm2 with Nafion 212) because of their lower overpotential and higher discharge voltage. These results demonstrate the high cation selectivity of the SPEEK membranes in neutral, salt-based electrolytes, and provide the groundwork to develop a suitable replacement to Nafion for low-cost RFBs.

Enhancing Vanadium redox flow batteries performance through local compression ratio adjustment using stiffness gradient carbon felt electrodes

Vanadium redox flow batteries (VRFBs) are assembled by compressing the electrodes to reduce the contact resistance between electrodes and bipolar plates and prevent liquid electrolyte leakage. However, an excessive compression ratio can impede the mass transfer and cause high-pressure drops in the electrolyte, ultimately reducing energy and system efficiencies. Accordingly, conventional stacks require a compromised compression ratio of 20 to 30%, resulting in substantial contact resistance and lower cell performance. To address this issue, we proposed a new approach to adjust the local compression ratio of carbon felt electrodes by creating a gradient in compressive stiffness along the through-thickness direction. We achieved this by selectively treating the carbon felt electrode to increase the interaction of carbon fibers in a treated region. This approach deforms the electrode non-uniformly, increasing the electrical contact point between the electrode and bipolar plate in regions with higher deformation and improving the convection and diffusion of ions at the interface between the electrode and membrane in regions with lower deformation, where porosity is relatively high. In a VRFB single-cell test, the electrode with the stiffness gradient demonstrated the highest energy efficiency (86.5%) under a current density of 100 mA∙cm−2, 10% higher than that of neat CF.