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 m V 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 microtomography,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.

Organized macro-scale membrane size reduction in vanadium redox flow batteries:Part 1.General concept

The high costs of the currently used membranes in vanadium redox flow batteries(VRFBs)contribute to the price of the vanadium redox flow battery systems and therefore limit the market share of the VRFBs.Here we report a detailed simulation and experimental studies on the effect of membrane reduction of single-cell VRFB.Different simulated designs demonstrate that a proposed centred and double-strip membrane coverage showed a promising performance.Experimental charge-discharge profile of different membrane size reduction,which showed good agreement with simulated data,suggests that the membrane size can comfortably be reduced by up to 20%without severe efficiency or discharge capacity loss.Long-term cycling of 80%centred membrane coverage showed improved capacity retention during the latter cycles with almost 1%difference in capacity and only 2%in energy efficiency when compared to the fully covered-membrane cell.The results hold great promise for the development of cheap RFB stacks and facilitate the way to develop new cell designs with non-overlapping electrodes geometry.Therefore,giving more flexibility to improve the overall performance of the system.

Soft Template-Induced Porous Polyvinylidene Fluoride Membrane for Vanadium Flow Batteries

Vanadium flow batteries(VFBs)are considered ideal for grid-sc ale,long-duration energy storage applications owing to their decoupled output power and storage capacity,high safety,efficiency,and long cycle life.However,the widespread adoption of VFB s is hindered by the use of expensive Nafion membranes.Herein,we report a soft template-induced method to develop a porous polyvinylidene fluoride(PVDF)membrane for VFB applications.By incorporating water-soluble and flexible polyethylene glycol(PEG 400)as a soft template,we induced the aggregation of hydrophilic sulfonated poly(ether ether ketone),resulting in phase separation from the hydrophobic PVDF polymer during membrane formation.This process led to the creation of a porous PVDF membrane with controllable morphologies determined by the polyethylene glycol content in the cast solution.The optimized porous PVDF membrane enabled a stable VFB performance for 200 cycles at a current density of 80 mA/cm^(2),and the VFB exhibited a Coulombic efficiency of 95.2%and a voltage efficiency of 87.8%.These findings provide valuable insights for the development of highly stable membranes for VFB applications.

Lattice Boltzmann modeling of transport phenomena in fuel cells and flow batteries

Fuel cells and flow batteries are promising technologies to address climate change and air pollution problems. An understanding of the complex multiscale and multiphysics transport phenomena occurring in these electrochemical systems requires powerful numerical tools. Over the past decades, the lattice Boltzmann (LB) method has attracted broad interest in the computational fluid dynamics and the numerical heat transfer communities, primarily due to its kinetic nature making it appropriate for modeling complex multiphase transport phenomena. More importantly, the LB method fits well with parallel computing due to its locality feature, which is required for large-scale engineering applications. In this article, we review the LB method for gas-liquid two-phase flows, coupled fluid flow and mass transport in porous media, and particulate flows. Examples of applications are provided in fuel cells and flow batteries. Further developments of the LB method are also outlined.

Modified carbon cloth as positive electrode with high electrochemical performance for vanadium redox flow batteries

Carbon cloth modified by hydrothermal treatment in ammonia water is developed as the positive electrode with high electrochemical performance for vanadium redox flow batteries. The SEM shows that the treatment has no obvious influence on the morphology of carbon cloth. XPS measurements indicate that the nitrogenous functional groups can be introduced on the surface of carbon cloth successfully. The electrochemical performance of V(IV)/V(V) redox couple on the prepared electrode is evaluated with cyclic voltammetry and linear sweep voltammetry measurements. The N-doped carbon cloth exhibits outstanding electrochemical activity and reversibility toward V(IV)/V(V) redox couple. The rate constant of V(IV)/V(V) redox reaction on carbon cloth can increase to 2.27 x 10(-4) cm/s from 1.47 x 10(-4) cm/s after nitrogen doping. The cell using N-doped carbon cloth as positive electrode has larger discharge capacity and higher energy efficiency compared with the cell using pristine carbon cloth. The average energy efficiency of the cell using N-doped carbon cloth for 50 cycles at 30 mA/cm(2) is 87.8%, 4.3% larger than that of the cell using pristine carbon cloth. It indicates that the N-doped carbon cloth has a promise application prospect in vanadium redox flow batteries. (C) 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

Progress on the electrode materials towards vanadium flow batteries (VFBs) with improved power density

The vanadium flow battery （VFB） has been considered as one of the most promising large-scale energy storage technologies in terms of its design flexibility, long cycle life, high efficiency and high safety. How- ever, the high cost prevents the VFB technology from broader market penetration. Improving the power density of the VFB is an effective solution to reduce its cost due to the reduced material consumption and stack size. Electrode, as one of the main components in the VFB, providing the reactions sites for redox couples, has an important effect on the voltage loss of the VFB associated with electrochemical polariza- tion, ohmic polarization and concentration polarization. Extensive research has been carried out on the electrode modification to reduce polarizations and hence improve the power density of the VFB. In this review, state-of-the-art of various modification methods on the VFB electrode materials is overviewed and summarized, and the future research directions helpful to reduce polarization loss are presented.

Influence of solvent on ion conductivity of polybenzimidazole proton exchange membranes for vanadium redox flow batteries

Polybenzimidazole(PBI)is a kind of proton transport membrane material,and its ion conductivity is a key factor affecting its application in vanadium redox flow batteries(VRFBs).The casting solvent of PBI has a significant influence on the acid doping level of PBI membranes which is closely related to ionic conductivity.In this paper,3,3′-diaminobenzidine(DABz)and 4,4′-Dicarboxydiphenylether(DCDPE)were used as raw materials by solution condensation to prepare the PBI with ether bond groups.The chemical structure of PBI was determined by1H NMR and FT-IR,and the prepared PBI had good solubility which can be dissolved in a variety of solvents.The PBI proton exchange membranes were prepared by solution coating with 5 different solvents of N,N-dimethylformamide(DMF),N,N-dimethylacetamide(DMAc),dimethyl sulfoxide(DMSO),1-methyl-2-pyrrolidone(NMP),methane sulfonic acid(MSA).The effects of different solvents on the ion conductivity and physicochemical properties were discussed in detail.The results showed that the PBI membrane prepared by using MSA as solvent(the PBI+MSA membrane)exhibits high water uptake,acid doping level and low vanadium ion permeability.The VRFB assembled with the PBI+MSA membrane exhibited higher coulombic efficiency(CE)99.87%and voltage efficiency(VE)84.50%than that of the commercial Nafion115 membrane at100 m A·cm-2,and after 480 cycles,the EE value can still be maintained at 83.73%.The self-discharge time of a single battery was recorded to be as long as 1000 h.All experimental data indicated that MSA is the best solvent for casting PBI membrane.

Eu-based anolytes for high-voltage and long-lifetime aqueous flow batteries

Aqueous flow batteries(AFBs) are among the most promising electrochemical energy storage solutions for the massive-scale adoption of renewable electricity because of decoupled energy and power, design flexibility, improved safety and low cost. The development of high-voltage AFB is, however, limited by the lack of stable anolytes that have low redox potential. Here we report Eu-based anolytes for high-voltage p H-neutral AFB applications. Eu^(3+) has a reduction potential of -0.39 V vs. SHE, which can be dramatically lowered when forming stable complex with inexpensive organic chelates. A typical complex, Eu DTPA,features a low redox potential of -1.09 V vs. SHE, fast redox kinetics, and a high water solubility of 1.5 M. When paired with ferrocyanide, the battery had an open-circuit voltage of 1.56 V and demonstrated stable cell cycling performance, including a capacity retention rate of 99.997% per cycle over500 cycles at 40 m A cm^(-2), a current efficiency of >99.9%, and an energy efficiency of >83.3%. A high concentration anolyte at 1.5 M exhibited a volumetric capacity of 40.2 Ah L^(-1), which is one of the highest known for p H-neutral AFBs, promising a potent solution for the grid-scale storage of renewable electricity.

A highly stable membrane with hierarchical structure for wide pH range flow batteries

A membrane with high stability and ion conductivity in wide pH range is essential for energy storage devices.Here,we report a novel membrane with hierarchical core-shell structure,which demonstrates high stability and ion conductivity,simultaneously under a wide pH range applications.Spectral characterizations and theoretical calculation indicate that the non-solvent induces the chain segment configuration and eventually leads to polymer-polymer phase separation,thus forming hierarchical porous core-shell structure.Benefiting from this structure,an acidic vanadium flow battery(VFB)with such a membrane shows excellent performance over 400 cycles with an energy efficiency(EE)of above 81%at current density of 120 mA cm^(-2) and an alkaline zinc-iron flow battery(AZIFB)delivers a cycling stability for more than 200 cycles at 160 mA cm^(-2),along with an EE of above 82%.This paper provides a cost-effective and simple way to fabricate membranes with high performance for variety of energyrelated devices.

Theoretical insights on the hydration of quinones as catholytes in aqueous redox flow batteries

Quinones have been widely studied as a potential catholyte in water-based redox flow batteries(RFBs)due to their ability to carry both electrons and protons in aqueous solutions.The wide variety of quinones and derivatives offers exciting opportunities to optimize the device performance while poses theoretical challenges to quantify their electrochemical behavior as required for molecular design.Computational screening of target quinones with high performance is far from satisfactory.While solvation of quinones affects their potential application in RFBs in terms of both electrochemical windows,stability,and charge transport,experimental data for the solvation structure and solvation free energies are rarely available if not incomplete.Besides,conventional thermodynamic models are mostly unreliable to estimate the properties of direct interest for electrochemical applications.Here,we analyze the hydration free energies of more than 1,400 quinones by combining the first-principles calculations and the classical density functional theory.In order to attain chemical insights and possible trends,special attention is placed on the effects of"backbones"and functional groups on the solvation behavior.The theoretical results provide a thermodynamic basis for the design,synthesis,and screening of high-performance catholytes for electrical energy storage.

A review of electrolyte additives and impurities in vanadium redox flow batteries

As one of the most important components of the vanadium redox flow battery （VRFB）, the electrolyte can impose a significant impact on cell properties, performance and capital cost. In particular, the electrolyte composition will influence energy density, operating temperature range and the practical applications of the VRFB. Various approaches to increase the energy density and operating temperature range have been proposed. The presence of electrolyte impurities, or the addition of a small amount of other chemical species into the vanadium solution can alter the stability of the electrolyte and influence cell perfor- mance, operating temperature range, energy density, electrochemical kinetics and cost effectiveness. This review provides a detailed overview of research on electrolyte additives including stabilizing agents, im- mobilizing agents, kinetic enhancers, as well as electrolyte impurities and chemical reductants that can be used for different purposes in the VRFBs.

The catalytic effect of bismuth for VO2+/VO2+and V3+/V2+redox couples in vanadium flow batteries

The effect of bismuth (Bi) for both VO2+/VO2+ and V3+/V2+ redox couples in vanadium flow batteries (VFBs) has been investigated by directly introducing Bi on the surface of carbon felt (CF). The results show that Bi has no catalytic effect for VO2+/VO2(+) redox couple. During the first charge process, Bi is oxidized to Bi3+ (never return back to Bi metal in the subsequent cycles) due to the low standard redox potential of 0.308 V (vs. SHE) for Bi3+/Bi redox couple compared with VO2+/VO2+ redox couple and Bi3+ exhibit no (or neglectable) electro-catalytic activity. Additionally, the relationship between Bi loading and electrochemical activity for V3+/V2+ redox couple was studied in detail. 2 wt% Bi-modified carbon felt (2%-BiCF) exhibits the highest electrochemical activity. Using it as negative electrode, a high energy efficiency (EE) of 79.0% can be achieved at a high current density of 160 mA/cm(2), which is 5.5% higher than the pristine one. Moreover, the electrolyte utilization ratio is also increased by more than 30%. Even the cell operated at 140 mA/cm(2) for over 300 cycles, the EE can reach 80.9% without obvious fluctuation and attenuation, suggesting excellent catalytic activity and electrochemical stability in VFBs. (C) 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

Systematic approaches to improving the performance of polyoxometalates in non-aqueous redox flow batteries

Polyoxometalates have been explored as multi-electron active species in both aqueous and non-aqueous redox flow batteries. Although non-aqueous systems in principle offer a wider voltage window for redox flow battery operation, realization of this potential requires a judicious choice of solvent as well as polyoxometalate properties. We demonstrate here the superior performance of N,N-dimethylformamide(DMF)compared to acetonitrile as a solvent for redox flow batteries based on Li3PMo12O40. This compound displays two 1-electron transfers in acetonitrile but can access an extra quasi-reversible 2-electron redox process in DMF. A cell containing 10 mM solution of Li3PMo12O40 in DMF produced a cell voltage of 0.7 V with 2-electron transfers(State of Charge = 60%) and showed a good cyclability. As a means to boost energy density, operation of the redox flow battery at a higher concentration of 0.1 M Li3PMo12O40 produced cells with cell voltage of 0.6 V in acetonitrile and a cell voltage of 1.0 V in DMF;both showed excellent coulombic efficiencies of more than 90% over the course of 30 cycles. Energy density was also increased by employing an asymmetric cell with different polyoxometalates on each side to extend cell voltage.Li6P2W18O62 exhibited 3 quasi-reversible 2-electron transfers in the potential range between-2.05 V and-0.5 V vs. Ag/Ag+. 10 mM Li6P2W18O62/Li3PMo12O40 in DMF produced a cell with cell voltage of 1.3 V involving 4-electron transfers(State of Charge = 50%) with coulombic efficiency of nearly 100% and energy efficiency of nearly 70% throughout the test with more than 20 cycles. These promising results demonstrate proof-of-concept approaches to improving the performance of polyoxometalates in non-aqueous redox flow batteries.

O/N/S trifunctional doping on graphite felts:A novel strategy toward performance boosting of cerium-based redox flow batteries

The cerium-based redox flow battery(RFB)is regarded as a compelling gridscale energy storage technology to revolutionize the utilization of renewable energy by storing the energy in liquid electrolytes.However,its widespread implementation is impeded by the cerium redox reactions that exhibit slow kinetics on commercial graphite felt(GF)electrodes.Surface functionalization may be an available activation strategy to achieve a significant boost in the electrochemical performance of GFs.However,conventional chemical and/or electrochemical routes for the surface functionalization of GFs suffer from the issues of complication,and the deterioration of the resulting modified electrode surface over long-term cycle processes leads to catalytic activity decline.Here,we develop a facile and general strategy for introducing the functional groups to the electrode through the addition of L-cysteine into electrolytes.The-COOH,-NH_(2),and-SH groups in L-cysteine can induce oxygen/nitrogen/sulfur trifunctional doping on GF surfaces with lower deterioration rates,which enables the activated GFs to demonstrate a promising electrocatalytic activity toward cerium redox reactions and excellent durability when used as a cerium-based RFB electrode.This study proposes a rational strategy to overcome the intrinsic limitations of existing modification techniques for GFs and provides a potential pathway toward high-performance RFBs.

Redox flow batteries based on insoluble redox-active materials.A review

The ever-increasing demand for energy has stimulated the development of economical non-fossil fuels.As representative of clean energy,solar and wind have been identified as the most promising energy sources due to their abundance,cost efficiency,and environmental friendliness.The intrinsic intermittent of the clean energy leads to the urgent requirements large-scale energy storage technique.Redox flow batteries(RFBs)are attractive technology due to their independent control over energy and power.Insoluble redox-active flow battery is a new type of electrochemical energy storage technology that disperses redox-active particles in the electrolyte.Compared with traditional flow batteries,insoluble flow batteries have advantages of large energy density and are very promising in the development of large-scale energy storage systems.At present,three types of insoluble flow batteries have been explored:slurry-based flow batteries,metal/slurry hybrid,and redox-mediator-assisted flow batteries.This Review summarizes the research progress of insoluble flow batteries,and analyzes the key challenges from the fundamental research and practical application perspectives.

Polyoxometalate-based electrolyte materials in redox flow batteries:Current trends and emerging opportunities

Redox flow batteries have received wide attention for electrochemical energy conversion and storage devices due to their specific advantage of uncoupled power and energy devices,and therefore potentially to reduce the capital costs of energy storage.Terrific structural features of polyoxometalates exhibit unique advantages in redox flow batteries,such as,stable chemical properties,multi-electron reaction,good redox reversibility,low permeability,etc,which furnishes a novel perspective for settling various problems of redox flow batteries.This was a comprehensive and critical review of this type of batteries,focusing mainly on the chemistry of polyoxometalate electrolyte materials and introducing a systematic classification.Finally,challenges and perspectives of polyoxometalate electrolyte materials and polyoxometalate redox flow batteries are discussed.

ZIF-derived holey electrode with enhanced mass transfer and N-rich catalytic sites for high-power and long-life vanadium flow batteries

Electrode materials with good redox kinetics,excellent mass transfer characteristics and ultra-high stability play a crucial role in reducing the life-cycle cost and prolonging the maintenance-free time of the vanadium flow batteries(VFB).Herein,a nitrogen-doped porous graphite felt electrode(N-PGF)is proposed by growing ZIF-67 nanoparticles on carbon fibers and then calcinating and acid etching.The multi-scale structure of“carbon fiber gap(electrolyte flow),micro/nano pore(active species diffusion)and Nitrogen active center(reaction site)”in N-PGF electrode effectively increases the catalytic sites and promotes mass transfer characteristics.Reasonable electrode design makes the battery show excellent rate performance and ultra-high cycling stability.The peak power density of the battery reaches 1006 mW cm^(-2).During 1000 cycles at 150 mA cm^(-2),the average discharge capacity and average discharge energy of N-PGF increase substantially by 11.6%and 23.4%compared with the benchmark thermal activated graphite felt,respectively.More excitingly,after ultra-long term(5000 cycles)operation at an ultra-high current density(300 mA cm^(-2)),N-PGF exhibits an unprecedented energy efficiency retention(99.79%)and electrochemical performance stability.

Ultrathin free-standing electrospun carbon nanofibers web as the electrode of the vanadium flow batteries

Ultrathin free-standing electrospun carbon nanofiber web（ECNFW） used for the electrodes of the vanadium flow battery（VFB） has been fabricated by the electrospinning technique followed by the carbonization process in this study to reduce the ohmic polarization of the VFB. The microstructure, surface chemistry and electrochemical performance of ECNFW carbonized at various temperatures from 800 to 1400 °C have been investigated. The results show that ECNFW carbonized at 1100 °C exhibits the highest electrocatalytic activity toward the V;/V;redox reaction, and its electrocatalytic activity decreases along with the increase of carbonization temperature due to the drooping of the surface functional groups.While for the VO;/VO;redox couple, the electrocatalytic activity of ECNFW carbonized above 1100 °C barely changes as the carbonization temperature rises. It indicates that the surface functional groups could function as the reaction sites for the V;/V;redox couple, but have not any catalytic effect for the VO;/VO;redox couple. And the single cell test result suggests that ECNFW carbonized at 1100 °C is a promising material as the VFB electrode and the VFB with ECNFW electrodes obtains a super low internal resistance of 250 mΩ cm;.

Perspective of alkaline zinc-based flow batteries

Energy storage technologies have been identified as the key in constructing new electric power systems and achieving carbon neutrality,as they can absorb and smooth the renewables-generated electricity.Alkaline zinc-based flow batteries are well suitable for stationary energy storage applications,since they feature the advantages of high safety,high cell voltage and low cost.Currently,many alkaline zinc-based flow batteries have been proposed and developed,e.g.,the alkaline zinc–iron flow battery and alkaline zinc–nickel flow battery.Their development and application are closely related to advanced materials and battery configurations.In this perspective,we will first provide a brief introduction and discussion of alkaline zinc-based flow batteries.Then we focus on these batteries from the perspective of their current status,challenges and prospects.The bottlenecks for these batteries are briefly analyzed.Combined with the practical requirements and development trends of alkaline zinc-based flow battery technologies,their future development and research direction will be summarized.

Towards high power density aqueous redox flow batteries

With the increasing penetration of renewable energy sources in the past decades,stationary energy storage technologies are critically desired for storing electricity generated by non-dispatchable energy sources to mitigate its impact on power grids.Redox flow batteries(RFBs)stand out among these technologies due to their salient features for large-scale energy storage.The primary obstacle to the successful industrialization and broad deployment of RFBs is now their high capital costs.A feasible route to cost reduction is to develop high-power RFBs,since the increase in power performance has a pronounced impact on the cost of RFB systems.In this review,an in-depth inspection of the power performance of RFBs is presented.Perspectives for the future development of high-power RFBs along with implementable strategies addressing both the intrinsic and extrinsic limiting factors are summarized,which are expected to provide useful references steering the further improvement in the power density of RFBs.