Progress and prospects of pH-neutral aqueous organic redox flow batteries:Electrolytes and membranes

Aqueous organic redox flow batteries(AORFBs),which exploit the reversible electrochemical reactions of water-soluble organic electrolytes to store electricity,have emerged as an efficient electrochemical energy storage technology for the grid-scale integration of renewable electricity.pH-neutral AORFBs that feature high safety,low corrosivity,and environmental benignity are particularly promising,and their battery performance is significantly impacted by redox-active molecules and ion-exchange membranes(IEMs).Here,representative anolytes and catholytes engineered for use in pH-neutral AORFBs are outlined and summarized,as well as their side reactions that cause irreversible battery capacity fading.In addition,the recent achievements of IEMs for pH-neutral AORFBs are discussed,with a focus on the construction and tuning of ion transport channels.Finally,the critical challenges and potential research opportunities for developing practically relevant pH-neutral AORFBs are presented.

Towards an all-vanadium redox flow battery with higher theoretical volumetric capacities by utilizing the VO^2＋/V^3＋ couple

An all-vanadium redox flow battery with V（IV） as the sole parent active species is developed by accessing the VO2＋/V3＋ redox couple. These batteries, referred to as V4RBs, possess a higher theoretical volumetric capacity than traditional VRBs. Copper ions were identified as an effective additive to boost the battery performance.

Steric hindrance shielding viologen against alkali attack in realizing ultrastable aqueous flow batteries

Viologens known as a kind of promising negolyte materials for aqueous organic redox flow batteries,face a critical stability challenge due to the S_N2 nucleophilic attack by hydroxide ions(OH-)during the battery cycling.In this work,a N-cyclic quaternary ammonium-grafted viologen molecule,viz.1,1'-bis(4,4'-dime thylpiperidiniumyl)-4,4'-bipyridinium tetrachloride((DBPPy)Cl_(4)),is developed by the molecular engineering strategy.The obtained(DBPPy)Cl_(4) molecule shows a decent solubility of 1.84 M and a redox potential of-0.52 V vs.Ag/AgCl,Experimental and theoretical results reveal that the grafted N-cyclic quaternary ammonium groups act as the steric hindrance to prevent nucleophilic attack by OH~-,increasing the alkali resistance of the electroactive molecule.The symmetrical battery with 0.50 M(DBPPy)Cl4shows negligible decay during the 13-day cycling test.As demonstration,the flow battery utilizing 1.0 M(DBPPy)Cl_(4) as the negolyte and 1-(1-oxyl-2,2',6,6'-tetramethylpiperidin-4-yl)-1'-(3-(trimethylammonio)propyl)-4,4'-bipyridinium trichloride as the posolyte exhibits a high capacity retention rate of 99.99%per cycle at 60 mA cm^(-2).

Tetrathiafulvalene esters with high redox potentials and improved solubilities for non-aqueous redox flow battery applications

The exploitation of high performance redox-active substances is critically important for the development of non-aqueous redoxflow batteries.Herein,three tetrathiofulvalene(TTF)derivatives with different substitution groups,namely TTF diethyl ester(TTFDE),TTF tetramethyl ester(TTFTM),and TTF tetraethyl ester(TTFTE),are prepared and their energy storage properties are evaluated.It has been found that the redox potential and solubility of these TTF derivatives in conventional carbonate electrolytes increases with the number of ester groups.The battery with a catholyte of 0.2 mol L^(-1) of TTFTE delivers a specific capacity of more than 10 Ah L^(-1) at the current density of 0.5 C with two discharge voltage platforms locating at as high as 3.85 and 3.60 V vs.Li/Liþ.Its capacity retention can be improved from 2.34 Ah L^(-1) to 3.60 Ah L^(-1) after 100 cycles by the use of an anion exchange membrane to block the crossover of TTF species.The excellent cycling stability of the TIF esters is supported by their well-delocalized electrons,as revealed by the density function theory calculations.Therefore,the introduction of more and larger electron-withdrawing groups is a promising strategy to simultaneously increase the redox-potential and solubility of redox-active ma-terials for non-aqueous redoxflow batteries.

Multiple-dimensioned defect engineering for graphite felt electrode of vanadium redox flow battery

The scarcity of wettability,insufficient active sites,and low surface area of graphite felt(GF)have long been suppressing the performance of vanadium redox flow batteries(VRFBs).Herein,an ultra-homogeneous multipledimensioned defect,including nano-scale etching and atomic-scale N,O codoping,was used to modify GF by the molten salt system.NH_(4)Cl and KClO_(3) were added simultaneously to the system to obtain porous N/O co-doped electrode(GF/ON),where KClO_(3) was used to ultra-homogeneously etch,and O-functionalize electrode,and NH4Cl was used as N dopant,respectively.GF/ON presents better electrochemical catalysis for VO_(2)+/VO_(2)+ and V3+/V2+ reactions than only O-functionalized electrodes(GF/O)and GF.The enhanced electrochemical properties are attributed to an increase in active sites,surface area,and wettability,as well as the synergistic effect of N and O,which is also supported by the density functional theory calculations.Further,the cell using GF/ON shows higher discharge capacity,energy efficiency,and stability for cycling performance than the pristine cell at 140 mA cm^(−2) for 200 cycles.Moreover,the energy efficiency of the modified cell is increased by 9.7% from 55.2% for the pristine cell at 260 mA cm^(−2).Such an ultra-homogeneous etching with N and O co-doping through“boiling”molten salt medium provides an effective and practical application potential way to prepare superior electrodes for VRFB.

Implications of electrode modifications in aqueous organic redox flow batteries

Aqueous organic redox flow batteries(RFBs)exhibit favorable characteristics,such as tunability,multielectron transfer capability,and stability of the redox active molecules utilized as anolytes and catholytes,making them very viable contenders for large-scale grid storage applications.Considerable attention has been paid on the development of efficient redox-active molecules and their performance optimization through chemical substitutions at various places on the backbone as part of the pursuit for high-performance RFBs.Despite the fact that electrodes are vital to optimal performance,they have not garnered significant attention.Limited research has been conducted on the effects of electrode modifications to improve the performance of RFBs.The primary emphasis has been given on the impact of electrode engineering to augment the efficiency of aqueous organic RFBs.An overview of electron transfer at the electrode-electrolyte interface is provided.The implications of electrode modification on the performance of redox flow batteries,with a particular focus on the anodic and cathodic half-cells separately,are then discussed.In each section,significant discrepancies surrounding the effects of electrode engineering are thoroughly examined and discussed.Finally,we have presented a comprehensive assessment along with our perspectives on the future trajectory.

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.

Unraveling the coordination behavior and transformation mechanism of Cr^(3+) in Fe–Cr redox flow battery electrolytes

Currently,the iron chromium redox flow battery(ICRFB)has become a research hotspot in the energy storage field owing to its low cost and easily-scaled-up.However,the activity of electrolyte is still ambiguous due to its complicated solution environment.Herein,we performed a pioneering investigation on the coordination behavior and transformation mechanism of Cr^(3+)in electrolyte and prediction of impurity ions impact through quantum chemistry computations.Based on the structure and symmetry of electrostatic potential distribution,the activity of different Cr^(3+)complex ions is confirmed as[Cr(H2O)5Cl]^(2+)>[Cr(H2O)4Cl2]+>[Cr(H2O)6]^(3+).The transformation mechanism between[Cr(H2O)6]^(3+)and[Cr(H2O)5Cl]^(2+)is revealed.We find the metal impurity ions(especially Mg^(2+))can exacerbate the electrolyte deactivation by reducing the transformation energy barrier from[Cr(H2O)5Cl]^(2+)(24.38 kcal mol^(−1))to[Cr(H2O)6]^(3+)(16.23 kcal mol^(−1)).The solvent radial distribution and mean square displacement in different solvent environments are discussed and we conclude that the coordination configuration limits the diffusivity of Cr^(3+).This work provides new insights into the activity of electrolyte,laying a fundamental sense for the electrolyte in ICRFB.

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.

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.

KHCO_3 activated carbon microsphere as excellent electrocatalyst for VO^(2+)/VO_2^+ redox couple for vanadium redox flow battery

In this paper,carbon microsphere prepared by hydrothermal treatment was activated by KHCO_3 at high temperature,and employed as the catalyst for VO^(2+)/VO_2^+redox reaction for vanadium redox flow battery(VRFB).Carbon microsphere can be etched by KHCO_3 due to the reaction between the pyrolysis products of KHCO_3 and carbon atoms.Moreover,KHCO_3 activation can bring many oxygen functional groups on carbon microsphere,further improving the wettability of catalyst and increasing the active sites.The electrocatalytic properties of carbon microsphere from hydrothermal treatment are improved by high temperature carbonization,and can further be enhanced by KHCO_3 activation.Among carbon microsphere samples,the VO^(2+)/VO_2^+redox reaction exhibits the highest electrochemical kinetics on KHCO_3 activated sample.The cell using KHCO_3 activated carbon microsphere as the positive catalyst demonstrates higher energy efficiency and larger discharge capacity,especially at high current density.The results reveal that KHCO_3 activated carbon microsphere is an efficient,low-cost carbon-based catalyst for VO^(2+)/VO_2^+redox reaction for VRFB system.

From aqueous Zn-ion battery to Zn-MnO_(2) flow battery:A brief story

Aqueous Zn-ion battery(AZIB)has become an attractive technology because of its unique features of low cost,high safety and the eco-friendliness.MnO_(2) is the model cathode material for AZIB since the first report on reversible Zn-MnO_(2) battery,but recent studies have unveiled different charge storage mechanisms.Due to revamping of the electrochemistry and redesigning of the electrolyte and interface,there is tremendous performance enhancement in AZIB.This mini Review will first give a brief introduction of ZIB,including fundamentals of materials and components,and the progress in recent years.Then,a general classification of working mechanisms related to MnO_(2) in neutral and mildly acidic electrolyte is elaborated.Our focus is put on the recent blossoming Zn-MnO_(2) electrolytic mechanism,which has given birth to the Zn-MnO_(2) redox flow batteries that are highly promising for large-scale static energy storage.

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.

A phosphotungstic acid coupled silica-Nafion composite membrane with significantly enhanced ion selectivity for vanadium redox flow battery

An ultra-high ion-selective Nafion composite membrane modified by phosphotungstic acid(PWA)coupled silica for vanadium redox flow battery(VRB)was constructed and prepared through solution casting.The composite membrane exhibits excellent proton conductivity and vanadium ions blocking property by incorporating the nanohybrid composed of silica and PWA into the Nafion ionomer.Simple tuning for the filling amount of the nanohybrid endows the obtained membranes preeminent vanadium barrier property including a minimum vanadium permeability of 3.13×10-7cm2min-1and a maximum proton conductivity of 0.081 S cm-1at 25°C.These indicate an ion selectivity of 2.59×105S min cm-3,which is 6.8times higher than that of recast Nafion(0.33×105S min cm-3).As a result,the VRB with the composite membrane shows superior battery performance containing a lower self-discharge rate,higher capacity retention and more robust cyclic stability compared with recast Nafion over a range of current densities from 40 to 100 m A cm-2.

Facile fabrication of amphoteric semi-interpenetrating network membranes for vanadium flow battery applications

For improvement of vanadium redox flow battery （VRB） performance, novel amphoteric semi- interpenetrating membranes （ASIPN） were prepared using poly（ether ether ketone） （PEEK） and polysul- fone （PSf）, the former bearing sulfonic groups and the latter imidazolium. These two groups form ionic crosslinks between PEEK and PSf; meanwhile, covalent cross links were built between PSf chains with ad- dition of N-（3-aminopropyl）-imidazole. The amphoteric nature of the membrane allows facile proton and anion transport; the IPN structure and the presence of imidazolium cation effectively suppress vanadium ion crossover through the membrane. Therefore, the ASIPN based VRBs show higher Coulombic efficiency and energy efficiency than that assembled with pristine SPEEK and Nation 212 membranes. Our work demonstrates that the ASIPN membranes are promising for VRB applications.

High performance of zinc-ferrum redox flow battery with Ac^-/HAc buffer solution

A green low-cost redox flow battery using Zn/Znredox couple in HAc/NaAc medium and Fe/Feredox couple in HSOmedium was first proposed and investigated for potential stationary energy storage applications. The presence of HAc/NaAc in the negative electrolyte can keep the pH between 2.0 and 6.0even when a large amount of Hions move into negative electrolyte from positive electrolyte through ion exchange membrane. In the pH range of 2.0–6.0, the chemical reaction of Zn species with Hspecies is very insignificant; furthermore, the electroreduction of Hion on the negative electrode is significantly suppressed at this pH range. The zinc-ferrum redox flow battery（Zn/Fe RFB） operated within a voltage window of 0.5–2.0 V with a nearly 90% utilization ratio, and its energy efficiency is around 71.1% at room temperature. These results show that Zn/Fe RFB is a promising option as a stationary energy storage equipment.

A highly concentrated vanadium protic ionic liquid electrolyte for the vanadium redox flow battery

A protic ionic liquid is designed and implemented for the first time as a solvent for a high energy density vanadium redox flow battery.Despite being less conductive than standa rd aqueous electrolytes,it is thermally stable on a 100 ℃ temperature window,chemically stable for at least 60 days,equally viscous and dense with typical aqueous solvents and most importantly able to solubilize to 6 mol L^(-1) vanadium sulfate,thus increasing the VRFB energy density by a factor of 2.5.Electrochemical measurements revealed quasi-reversible redox transitions for both catholyte and anolyte at 25 ℃ while a proof-of-concept redox flow cell with the proposed electrolyte was tested for a total of 150 cycles at 25 ℃,showing an open circuit potential of 1.39 V and energy and coulombic efficiencies of 65% and 93%,respectively.What’s more,the battery can be equally cycled at 45℃ showing good thermal stability.This study underlines a new route to improve the energy-to-volume ratio of energy storage system.

Broad temperature adaptability of vanadium redox flow battery-part4:Unraveling wide temperature promotion mechanism of bismuth for V^(2+)/V^(3+) couple

Vanadium flow battery （VFB） is a fast going and promising system for large-scale stationary energy stor- age. However, drawbacks such as low power density and narrow temperature window caused by poor catalytic activity of graphite felt （GF） electrodes limit its worldwide application. In this paper, bismuth, as a low-cost, no-toxic and high-activity electrocatalyst, is used to modify the thermal activated GF （TGF） via a facile hydrothermal method. Bismuth can effectively inhibit the side reaction of hydrogen evolution in wide temperature range, while promoting the V2＋/V3＋ redox reaction. As a result, the VFB assembled with Bi/TGF as negative electrode demonstrates outstanding rate performance under the current density up to 400 mAcm-2, as well as a long-term stability over 600 charging/discharging cycles at a high cur- rent density of 150mA cm-2. Moreover, it also shows excellent temperature adaptability from -10 ℃ to 50 ℃ and high durability for life test at the temperature of 50 ℃.

Sucrose pyrolysis assembling carbon nanotubes on graphite felt using for vanadium redox flow battery positive electrode

In the present paper, multi-walled carbon nanotubes（MWCNTs） are successfully assembled on graphite felt（GF） using sucrose pyrolysis method for the first time. The in situ formed pyrolytic carbon is chosen as the binder because it is essentially carbon materials as well as CNTs and GF which has a natural tendency to achieve high bonding strength and low contact resistance. The MWCNTs/GF electrode is demonstrated to increase surface area, reduce polarization, lower charge transfer resistance and improve energy conversion efficiency comparing with GF. This excellent electrochemical performance is mainly ascribed to the high electro-catalytic activity of MWCNTs and increasing surface area.