Performance Of Flow Batteries Research Articles

All-iron redox flow battery in flow-through and flow-over set-ups: the critical role of cell configuration

All-soluble, all-iron flow battery performance is critically dependent upon cell configuration. Flow-through and flow-over designs exhibit stark differences in efficiency, maximum power density, capacity retention, and self-discharge.

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.

Towards Low Cost and Long Duration Iron-Air Flow Batteries

The electrification of the energy economy necessitates the development of low cost and large scale energy storage technologies that can integrate intermittent renewables (e.g. solar and wind). Among the various technological options (e.g. lithium ion batteries, vanadium redox flow batteries), metal-air batteries are promising alternatives owing to their high energy density, safety, and use of abundant raw materials. Moreover, when combined with the principles of redox flow batteries, mass transfer is enhanced, which results in a more uniform metal deposition and a reduction in the metal electrode passivation [2]. Furthermore, metal-air flow batteries could enable decoupling of power and energy, while retaining the scalability advantages of redox flow batteries. Additionally, they are more compact than conventional redox flow batteries since only one electrolyte tank is needed.Several metal-air flow battery systems have been investigated such as vanadium-air [3], lithium-air [4] and zinc-air batteries [5]. However, they suffer from limitations which challenge their scalability. Specifically, vanadium is costly and only available in certain regions, lithium requires the use of organic solvents which compromises the battery safety, and zinc forms dendrites leading to a lower performance and safety issues. In this context, iron-based batteries are a promising alternative due to their high specific capacity, low cost, large availability, safety, non-toxicity, and recyclability. Compared to zinc, iron is also less prone to dendrite formation during cycling in aqueous electrolytes since iron deposition kinetics is slower[6]. Despite these advantages, the performance of the iron anode is limited by electrode degradation during cycling due to phase transformations, hydrogen evolution, and low utilization of the activate material [6], which motivates fundamental research to advance the performance and durability of the system.In this poster presentation, we will discuss the main goals of our new project FAIR-RFB [7]. Our overall aim is to develop a low cost and durable iron-air flow battery system and, to this goal, we will investigate, (i) the role of porous electrode microstructure in defining the performance of iron-air flow batteries, (ii) the influence of electrode surface properties in determining transport phenomena, kinetics, selectivity and durability, and (iii) new electrochemical reactor architectures for high power iron-air redox flow batteries.

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

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.

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.

Scaling up Electrode Manufacturing with Non-Solvent Induced Phase Separation Technology

Grid-scale energy storage technologies must be deployed at a massive scale to balance the mismatch between energy generation using renewable technologies and demand. Redox flow batteries are an emerging technological option for large-scale and multi-hour electrochemical energy storage thanks to their ability to decouple energy and power1. Despite their intrinsic advantages, the current elevated costs of this technology hinders commercialization2. To improve cost competitiveness, research efforts have focused on developing novel electrolyte chemistries and optimized reactor concepts. To the latter, optimizing the porous electrode microstructure and surface chemistry offers a promising pathway to increase the performance of flow batteries. However, porous electrodes design is challenging as they must fulfil contradictory requirements such as providing high surface area for electrochemical reactions, facilitating mass transport of reactive species, and maintaining low operational pressure drop. Furthermore, state-of-the-art porous electrodes have been repurposed from other mature electrochemical technologies but have not been tailored for the unique requirements of redox flow batteries3.Non-solvent induced phase separation (NIPS) has been recently introduced as a versatile method to synthesize porous structures suitable for RFBs with unique properties, which are unattainable with current fibrous materials4,5. Drawing inspiration from membrane fabrication techniques, we reengineered the method for the synthesis of porous electrodes for use in redox flow batteries. The porous electrodes synthesized with NIPS were tested in a single-electrolyte cell configuration and then in an all-vanadium redox flow battery, showing promising electrochemical performance in comparison with existing commercial fibrous carbon materials. In this presentation, I will discuss our current efforts to scale up the NIPS method to manufacture electrodes at industrial scale. We believe that non-solvent induced phase separation is a cost-effective alternative to traditional carbon fibrous materials thanks to the few simpler steps required for the electrode preparation. Based on technoeconomic modeling, we estimate that the production cost of the NIPS electrodes can be less than half of the production costs of traditional carbon fibrous electrodes6. Moreover, the phase separated electrodes do not require post-manufacturing heat treatment to increase the surface area and wettability, which further translates into savings on energy costs. In this project, we aspire to scale-up the NIPS materials to a final size, after carbonization, of 30·30 cm2 while retaining similar microstructural and electrochemical performance properties as in laboratory scale. We perform a suite of microscopy, spectroscopic and flow cell diagnostics to correlate the electrode microstructure, surface chemistry and device performance. In this presentation, I will discuss the scalability of the method, reproducibility based on statistical analysis of sample properties and performance, and finally electrochemical performance on two flow battery chemistries.References Markard, J. (2018). The next phase of the energy transition and its implications for research and policy. Nature Energy, 3(8), 628-633.Darling, R. M. (2022). Techno-economic analyses of several redox flow batteries using levelized cost of energy storage. Current Opinion in Chemical Engineering, 37, 100855.Forner-Cuenca, A., and Brushett, F. R. (2019). Engineering porous electrodes for next-generation redox flow batteries: recent progress and opportunities. Current Opinion in Electrochemistry, 18, 113-122.Wan, C. T. C., Jacquemond, R. R., Chiang, Y. M., Nijmeijer, K., Brushett, F. R., & Forner‐Cuenca, A. (2021). Non‐Solvent Induced Phase Separation Enables Designer Redox Flow Battery Electrodes. Advanced Materials, 33(16), 2006716.Jacquemond, R. R., Wan, C. T. C., Chiang, Y. M., Borneman, Z., Brushett, F. R., Nijmeijer, K., & Forner-Cuenca, A. (2022). Microstructural engineering of high-power redox flow battery electrodes via non-solvent induced phase separation. Cell Reports Physical Science, 3(7), 100943.Minke, Christine, Ulrich Kunz, and Thomas Turek. "Carbon felt and carbon fiber-A techno-economic assessment of felt electrodes for redox flow battery applications." Journal of Power Sources342 (2017): 116-124.

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

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

Effect of carbon-foam composite-coated electrode on the power storage performance of soluble lead flow batteries

In this study, we meticulously fabricated a carbon-foam composite-coated electrode through a plating electroless copper technique, which was subsequently followed by electrodeposition of lead on a carbon-foam matrix. The investigation focused on exploring the physicochemical properties of the electrodes and the cycling performances of soluble lead flow batteries. In comparison to a conventional graphite electrode, the carbon-foam composite-coated electrode exhibited superior characteristics, including a higher specific surface area characterized (282 m2 g−1) and a higher conductivity (915.37 S cm−1). The utilization of cyclic voltammetry and electrochemical impedance spectroscopy demonstrated a notable enhancement in the reversible activity of the Pb/Pb2+ couple on the novel electrode. The porous structure of the novel electrode facilitated efficient charge dispersion between the poles and effectively suppressed the dendrite growth of the negative lead. As indicated by the result, the battery incorporating the novel electrode exhibited an energy efficiency exceeding 90.0 %, an extended charge duration of 6 h, and an elevated charge/discharge rate of 60 mA cm−2. The cell employing the CFCC-E had been successfully cycled for over 650 cycles with an average Coulombic efficiency of 94.5 % when cycled at 60 mA cm−2 for 1 h.

Improved Vanadium Flow Battery Performance through a Pulsating Electrolyte Flow Regime

AbstractThe interest in flow batteries as energy storage devices is growing due to the rising share of intermittent renewable energy sources. In this work, the performance of a vanadium flow battery is improved, without altering the electrochemical cell, by applying a pulsating flow. This novel flow battery flow regime aims to enhance performance by improving its mass transfer properties. Both the pulse volume and frequency have an influence on the battery performance, and lead to a 38.7 % increase in accessible discharge capacity compared to the conventional steady flow regime at values of 0.40 cm3 and 2.4 Hz in this cell, respectively. Furthermore, at these parameter settings a reduction of the mass transfer resistance by 71.4 % is achieved at 0.2 cm3/min ⋅ cm2. These results show that a pulsating flow can be beneficial in certain conditions, opening new possibilities for boosting performance in flow batteries.

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.

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.

Crystallizing Self-Standing Covalent Organic Framework Membranes for Ultrafast Proton Transport in Flow Batteries.

Covalent organic frameworks (COFs) display great potential to be assembled into proton conductive membranes for their uniform and controllable pore structure, yet constructing self-standing COF membrane with high crystallinity to fully exploit their ordered crystalline channels for efficient ionic conduction remains a great challenge. Here, a macromolecular-mediated crystallization strategy is designed to manipulate the crystallization of self-standing COF membrane, where the -SO3 H groups in introduced sulfonated macromolecule chains function as the sites to interact with the precursors of COF and thus offer long-range ordered template for membrane crystallization. The optimized self-standing COF membrane composed of highly-ordered nanopores exhibits high proton conductivity (75 mS cm-1 at 100 % relative humidity and 20 °C) and excellent flow battery performance, outperforming Nafion 212 and reported membranes. Meanwhile, the long-term run of membrane is achieved with the help of the anchoring effect of flexible macromolecule chains. Our work provides inspiration to design self-standing COF membranes with ordered channels for permselective application.

Crystallizing Self‐Standing Covalent Organic Framework Membranes for Ultrafast Proton Transport in Flow Batteries

AbstractCovalent organic frameworks (COFs) display great potential to be assembled into proton conductive membranes for their uniform and controllable pore structure, yet constructing self‐standing COF membrane with high crystallinity to fully exploit their ordered crystalline channels for efficient ionic conduction remains a great challenge. Here, a macromolecular‐mediated crystallization strategy is designed to manipulate the crystallization of self‐standing COF membrane, where the −SO3H groups in introduced sulfonated macromolecule chains function as the sites to interact with the precursors of COF and thus offer long‐range ordered template for membrane crystallization. The optimized self‐standing COF membrane composed of highly‐ordered nanopores exhibits high proton conductivity (75 mS cm−1 at 100 % relative humidity and 20 °C) and excellent flow battery performance, outperforming Nafion 212 and reported membranes. Meanwhile, the long‐term run of membrane is achieved with the help of the anchoring effect of flexible macromolecule chains. Our work provides inspiration to design self‐standing COF membranes with ordered channels for permselective application.

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.

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.

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.