Strong Interaction Between Redox Mediators and Defect-Rich Carbons Enabling Simultaneously Boosted Voltage Windows and Capacitance for Aqueous Supercapacitors

Energy density,the Achilles’heel of aqueous supercapacitors,is simultaneously determined by the voltage window and specific capacitance of the carbon materials,but the strategy of synchronously boosting them has rarely been reported.Herein,we demonstrate that the rational utilization of the interaction between redox mediators(RMs)and carbon electrode materials,especially those with rich intrinsic defects,contributes to extended potential windows and more stored charges concurrently.Using 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxyl(4OH-TEMPO)and intrinsic defect-rich carbons as the RMs and electrode materials,respectively,the potential window and capacitance are increased by 67%and sixfold in a neutral electrolyte.Moreover,this strategy could also be applied to alkaline and acid electrolytes.The first-principle calculation and experimental results demonstrate that the strong interaction between 4OH-TEMPO and defectrich carbons plays a key role as preferential adsorbed RMs may largely prohibit the contact of free water molecules with the electrode materials to terminate the water splitting at elevated potentials.For the RMs offering weaker interaction with the electrode materials,the water splitting still proceeds with a thus sole increase of the stored charges.The results discovered in this work could provide an alternative solution to address the low energy density of aqueous supercapacitors.

Strategies to suppress the shuttle effect of redox mediators in lithium-oxygen batteries

Rechargeable lithium-oxygen(Li-O_(2))batteries are the next generation energy storage devices due to their ultrahigh theoretical capacity.Redox mediators(RMs)are widely used as a homogenous electrocatalyst in non-aqueous Li-O_(2)batteries to enhance their discharge capacity and reduce charge overpotential.However,the shuttle effect of RMs in the electrolyte solution usually leads to corrosion of the Li metal anode and uneven Li deposition on the anode surface,resulting in unwanted consumption of electrocatalysts and deterioration of the cells.It is therefore necessary to take some measures to prevent the shuttle effect of RMs and fully utilize the soluble electrocatalysts.Herein,we summarize the strategies to suppress the RM shuttle effect reported in recent years,including electrolyte additives,protective separators and electrode modification.The mechanisms of these strategies are analyzed and their corresponding requirements are discussed.The electrochemical properties of Li-O_(2)batteries with different strategies are summarized and compared.The challenges and perspectives on preventing the shuttle effect of RMs are described for future study.This review provides guidance for achieving shuttle-free redox mediation and for designing Li-O_(2)cells with a long cycle life,high energy efficiency and highly reversible electrochemical reactions.

Atomic/nano-scale in-situ probing the shuttling effect of redox mediator in Na-O_(2) batteries

Sodium-oxygen batteries(Na-O_(2))have attracted extensive attention as promising energy storage systems due to their high energy density and low cost.Redox mediators are often employed to improve Na-O_(2) battery performance,however,their effect on the formation mechanism of the oxygen reduction product(NaO_(2))is still unclear.Here,we have investigated the formation mechanism of NaO_(2) during the discharge process in the presence of a redox mediator with the help of atomic/nano-scale in-situ characterization tools used in concert(e.g.atomic force microscope,electrochemical quartz crystal microbalance(EQCM)and laser nano-particle analyzer).As a result,real-time observations on different time scales show that by shuttling electrons to the electrolyte,the redox mediator enables formation of NaO_(2) in the solution-phase instead of within a finite region near the electrode surface.These findings provide new fundamental insights on the understanding of Na-O_(2) batteries and new consequently perspectives on designing high performance metal-O_(2) batteries and other related functions.

Improving the True Cycling of Redox Mediators-assisted Li-O_(2) Batteries

The application of redox mediators has been considered as a promising strategy to boost the performance of aprotic Li-O_(2)batteries.However,the issues brought with redox mediators,especially on the Li anode side have been overlooked.Here,we propose a facile approach of preparing a gel polymer membrane that not only allow uniform Li plating/stripping withlarge current densities over extended cycling but also inhibit the diffusion of redox mediators and avoid redox shuttling,self-discharge,and internal shortcircuiting.More importantly,the gel polymer membrane prevents the penetration of O_(2)and superoxide intermediates from the Li anode.Therefore,it ensures the successful application of both lithium anode and redox mediators in Li-O_(2)batteries to achieve the desired high capacity and rate performance.Meanwhile,it helps understand the benefit and problems of added redox mediators and reactive oxygen species so that theperformance of such Li-O_(2)batteries can be truly evaluated.

Enhanced cycle stability of aprotic Li-O_(2) batteries based on a self-defensed redox mediator

LiBr as a promising redox mediator(RM)has been applied in Li-O_(2)batteries to improve oxygen evolution reaction kinetics and reduce overpotentials.However,the redox shuttle of Br_(3)^-can induce the unexpected reactions and thus cause the degradation of LiBr and the corrosion of Li anode,resulting in the poor cyclability and the low round-trip efficiency.Herein,MgBr_(2)is firstly employed with dual functions for Li-O_(2)batteries,which can serve as a RM and a SEI film-forming agent.The Br^(–)is beneficial to facilitating the decomposition of Li_(2)O_(2)and thus decreasing the overpotential.Additionally,a uniform SEI film containing Mg and MgO generates on Li anode surface by the in-situ spontaneous reactions of Mg^(2+)and Li anode in an O_(2)environment,which can suppress the redox shuttle of Br_(3)^-and improve the interface stability of Li anode and electrolyte.Benefiting from these advantages,the cycle life of Li-O_(2)battery with MgBr_(2)electrolyte is significantly extended.

Chemical Activation of S/Li_(2)S in Li-S Batteries by a Bidirectional Organic Redox Mediator

The energy density and lifespan of prototype Li-S batteries under high sulfur loading and lean electrolyte have been mainly restricted by the incomplete interconversion between insulating S8 and Li2S.The introduction of an electrocatalyst has been preserved as an effective way to breakthrough the bottleneck of the interconversion rate.Herein,we demonstrate a novel bidirectional redox mediator,insoluble dithiobisphthalimide(DTPI),as the electrocatalyst for both S8 reduction and Li2S oxidation.Due to the dual-functional role of both electron/Li^(+)donor and acceptor,DTPI can efficiently accelerate the redox reactions during charge/discharge and significantly alleviate the incomplete conversion of sulfur species.Consequently,the Li-S batteries with DTPI deliver superior specific capacity and cycling stability in comparison with those without DTPI.Especially,the redox mediator is scalable for synthesis and the DTPI-based 5 A·h pouch cell delivers a specific discharge capacity of around 870 mA·h·g^(−1) at 0.1 C(1 C=1675 mA/g)without capacity fading over 80 cycles.The bidirectional catalysis mechanism has been studied through theoretical calculation and ex-situ characterization of the cathode materials.This work approves the effectiveness of bidirectional organic redox mediator in the construction of practical Li-S batteries.

Remarkable improvement of cyclic stability in Li-O2 batteries using ruthenocene as a redox mediator

Nonaqueous Li-O2 batteries attract attention for their theoretical specific energy density.However,due to the difficulty of decomposition of Li2 O2,Li-O2 batteries have high charge overpotential and poor cycling life.So all kinds of catalysts have been studied on the cathode.Compared to heterogeneous solid catalysts,soluble catalysts achieve faster and more effective transport of electrons by reversible redox pairs.Here,we first report ruthenocene(Ruc) as a mobile redox mediator in a Li-O2 battery.0.01 mol/L Ruc in the electrolyte effectively reduces the charging voltage by 610 mV.Additionally,Ruc greatly increases the cycling life by four-fold(up to 83 cycles) with a simple ketjen black(KB) cathode.The results of SEM,XPS and XRD confirm that less discharge product residue accumulated after recharge.To verify the reaction mechanisms of the mediato r,free energy profiles of the possible reaction pathways based on DFT are provided.

Viologens-based redox mediators with tunable HOMO–LUMO energy gap for highly effective hydrogen peroxide electrosynthesis

In comparison with the developing nano-carbon catalysts,some small organic molecules are also emerging as catalysts with typical features,however,their working mechanism is still unclear.Here,we synthesized a series of viologen-based heterogeneous catalysts with the same molecular skeleton but different substituent groups through anion exchange engineering.These viologen-based molecules were used as a model catalyst to investigate the underlying structure–function relationship for small molecules-based H_(2)O_(2) electrosynthesis.Differing from the commonly reported carbon-based electrocatalysts,viologens can produce H_(2)O_(2) in a synergistic manner,which means that viologens can not only directly catalyze oxygen reduction but also serve as a redox mediator.We found that the ring current and H_(2)O_(2) selectivity of viologens deliver an increasing trend with the increase of the alkyl chain length of alkyl-substituted viologens and further increase when using benzyl as the substituent group.As a result,a benzyl-substituted viologen(BV)delivers the best electrocatalytic performance among the samples,including the highest H_(2)O_(2) selectivity of 96.9%at 0.6 V and the largest ring current density of about 13.6 mA·mmol-1.Furthermore,density functional theory(DFT)calculations disclose that the carbon atoms bonded with positively charged N are the active sites and the small highest occupied molecular orbital(HOMO)–lowest unoccupied molecular orbital(LUMO)energy gap of BV is beneficial to the synergistic mechanism for H_(2)O_(2) production.This work sheds new insight into the efficient H_(2)O_(2) production in a synergistic manner for small molecules-based electrocatalysts.

Redox Mediator Chemistry Regulated Aqueous Batteries:Insights into Mechanisms and Prospects

Redox mediators(RMs),serving as intermediate electron carriers or reservoirs,play vital roles in developing new charge transfer energy storage systems with high voltage or capacity in aqueous batteries.However,the underlying mechanism and selection criteria of RMs remain unclear in aqueous batteries,which hinders the further exploitation of new RMs and aqueous battery chemistries.

A molecular sieve-containing protective separator to suppress the shuttle effect of redox mediators in lithium-oxygen batteries

Lithium-oxygen(Li-O_(2))batteries have a great potential in energy storage and conversion due to their ultra-high theoretical specific energy,but their applications are hindered by sluggish redox reaction kinetics in the charge/discharge processes.Redox mediators(RMs),as soluble catalysts,are widely used to facilitate the electrochemical processes in the Li-O_(2)batteries.A drawback of RMs is the shuttle effect due to their solubility and mobility,which leads to the corrosion of a Li metal anode and the degradation of the electrochemical performance of the batteries.Herein,we synthesize a polymer-based composite protective separator containing molecular sieves.The nanopores with a diameter of 4Åin the zeolite powder(4A zeolite)are able to physically block the migration of 2,2,6,6-tetramethylpiperidinyloxy(TEMPO)molecules with a larger size;therefore,the shuttle effect of TEMPO is restrained.With the assistance of the zeolite molecular sieves,the cycle life of the Li-O_(2)batteries is significantly extended from~20 to 170 cycles at a current density of 250 mA·g^(-1)and a limited capacity of 500 mAh·g^(-1).Our work provides a highly effective approach to suppress the shuttle effects of RMs and boost the electrochemical performance of Li-O_(2)batteries.

Supercapacitors as redox mediators for decoupled water splitting

With the help of the redox mediator, decoupled water-splitting allows O_(2)and H_(2)to be produced at different times, at different rates, and even in different cells, which promotes both the operation safety and the utilization of renewable power sources. However, the current densities and stabilities of these redox mediators are commonly low, which require further improvements for practical applications. Here, we propose to use supercapacitors as solid state redox mediators for decoupled water splitting. For demonstration, Na_(0.5)MnO_(2)(pseudocapacitor) and active carbon(double layer capacitor), are both used as the redox mediator. These supercapacitors show superior current density(1 A/cm^(2)) and ultralong cycle-life(8000 cycles) compared with commonly investigated battery-based mediators(NiOOH/Ni(OH)_(2)). Our research proves supercapacitors can be used as redox relay with high current density and stability, which may bring new insights in the design of decoupled water splitting systems.

Dual-function redox mediator enhanced lithium-oxygen battery based on polymer electrolyte

The polymer electrolyte based lithium-oxygen battery has showed higher safety than that of organic liquid electrolyte.However,the energy efficiency and cycling stability are still the challenges for the practical application of lithium-oxygen battery.Herein,the 1,4 para benzoquinone has been demonstrated as dual-function redox mediator for promoting both oxygen reduction and oxygen evolution reactions of lithium-oxygen battery with polymer electrolyte,which have been confirmed by the Cyclic Voltammetry and discharge/charge test of battery under O_(2) gas,as well as the theoretical calculations.Furthermore,the composite cathode that in-situ constructed by polymerizing electrolyte precursors with redox me-diator can be beneficial for the electrochemical reactions.Combing composite cathode and lithium ions source,the polymer electrolyte based lithium-oxygen batteries can operate for long lifetime with low charge potentials and good rate performances.Thus,this work has highlighted the promising implementation of lithium-oxygen battery based on polymer electrolyte,in which the dual-function redox mediator are employed for both discharge and recharge processes.

A chiral salen-Co(Ⅱ)complex as soluble redox mediator for promoting the electrochemical performance of Li-O_(2) batteries

Low discharge capacity and poor cycle stability are the major obstacles hindering the operation of Li-O_(2)batteries with highenergy-density.These obstacles are mainly caused by the cathode passivation behaviours and the accumulation of by-products.Promoting the discharge process in solution and accelerating the decomposition of discharge products and by-products are able to alleviate above problems to some extent.Herein,chiral salen-Co(Ⅱ)complex,(1R,2R)-(-)-N,N-bis(3,5-di-t-butylsalicylidene)-1,2-cyclohexanediaminocobalt(Ⅱ)(Co(Ⅱ))as a multi-functional redox mediator was introduced into electrolyte to induce solution phase formation of Li_(2)O_(2)and catalyze the oxidation of Li_(2)O_(2)and main by-products Li_(2)CO3.Due to the Co(Ⅱ)has the solvation effect towards Li+,it can drive solution phase formation of Li_(2)O_(2),to prevent electrode from passivation and then increase the discharge capacity with a high Li_(2)O_(2)yield of 96.09%.Furthermore,the Co(Ⅱ)possesses suitable redox couple potentials,and it does so while simultaneously boosting the oxidization of Li_(2)O_(2)and the decomposition of Li_(2)CO3,reducing charge overpotential,and promoting cycle lifespan.Thereby,a cell with Co(Ⅱ)achieved a long cycling stability at low charge plateau(3.66 V)over 252 cycles with a specific capacity of 500 mAh·gcarbon^(−1).

Nonmacrocyclic Iron(II)Soluble Redox Mediators Leading to High-Rate Li-O_(2)Battery

The lithium-oxygen(Li-O_(2))battery is highly promising but suffers from poor cycling life,especially at high rates;hence,the need for high-efficient accelerating agents is crucial.Recently macrocyclic Fe-based redox mediators,such as iron(II)phthalocyanine(FePc)and heme,have been developed and anticipated to be ideal due to their bifunctional charge and superoxide shuttling capabilities.However,they still operate far below expectations,which could result from the low concentrations in electrolyte due to the strongπ-πinteraction at carbon cathode.Herein,the authors report a new type of nonmacrocyclic Fe-based redox mediators,iron(II)acetylacetonate[Fe(acac)2]and iron(II)glycinate[Fe(gly)2],which have weakπ-πinteraction with the carbon cathode,thus,remain at high concentrations in the electrolyte.The Fe(gly)2@Li-O_(2)battery reaches a long life of 321 cycles at 0.5 A g^(−1),which is much superior to the counterpart with the typical macrocyclic FePc,and particularly exhibits a long life of 167 cycles at 2.0 A g^(−1)and 136 cycles at ultrahigh 5.0 A g^(−1).This study demonstrates an efficient strategy to achieve a high-rate performance of Li-O_(2)batteries by developing nonmacrocyclic Fe-based redox mediators with high-efficient electron and superoxide shuttling.

Boosting sulfur redox kinetics by a pentacenetetrone redox mediator for high-energy-density lithium-sulfur batteries

Lithium-sulfur(Li-S)battery is considered as a promising energy storage system due to its ultrahigh theoretical energy density of 2,600 Wh·kg^(−1).Redox mediation strategies have been proposed to promote the sluggish sulfur redox kinetics.Nevertheless,the applicability of redox mediators in practical high-energy-density Li-S batteries has seldomly been manifested.In this work,5,7,12,14-pentacenetetrone(PT)is proposed as an effective redox mediator to promote the sulfur redox kinetics under practical working conditions.A high initial specific discharge capacity of 993 mAh·g^(−1) is achieved at 0.1 C with high-sulfur-loading cathodes of 4.0 mgS·cm^(−2)and low electrolyte/sulfur(E/S)ratio of 5μL·mg_(S)^(−1).More importantly,practical Li-S pouch cells with the PT mediator realize an actual initial energy density of 344 Wh·kg^(−1)and cycle stably for 20 cycles wih a high capacity retention of 88%.This work proposes an effective redox mediator and further verifies the redox mediation strategy for practical high-energydensity Li-S batteries.

Linear paired electrolysis of furfural to furoic acid at both anode and cathode in a multiple redox mediated system

Implementing a new energy-saving electrochemical synthesis system with high commercial value is a strategy of the sustainable development for upgrading the bulk chemicals preparation technology in the future.Here,we report a multiple redox-mediated linear paired electrolysis system,combining the hydrogen peroxide mediated cathode process with the I2 mediated anode process,and realize the conversion of furfural to furoic acid in both side of the dividedflow cell simultaneously.By reasonably controlling the cathode potential,the undesired water splitting reaction and furfural reduction side reactions are avoided.Under the galvanostatic electrolysis,the two-mediated electrode processes have good compatibility,which reduce the energy consumption by about 22%while improving the electronic efficiency by about 125%.This system provides a green electrochemical synthesis route with commercial prospects.

Development of High Areal Capacity Electrolytic MnO_(2)-Zn Battery via an Iodine Mediator

The commercialization of electrolytic MnO_(2)-Zn batteries is highly applauded owing to the advantages of cost-effectiveness,high safety,high energy density,and durable working performance.However,due to the low reversibility of the cathode MnO_(2)/Mn^(2+)chemistry at high areal capacities and the severe Zn anode corrosion,the practical application of MnO_(2)-Zn batteries is hampered by inadequate lifespan.Leveraging the full advantage of an iodine redox mediator,here we design a highly rechargeable electrolytic MnO_(2)-Zn battery with a high areal capacity.The MnO_(2)-Zn battery coupled with an iodine mediator in a mild electrolyte shows a high discharge voltage of 1.85 V and a robust areal capacity of 10 mAh cm^(-2)under a substantial discharge current density of 160 mA cm^(-2).The MnO_(2)/I_(2)-Zn battery with an areal capacity of 10 mAh cm^(-2)exhibits prolonged stability of over 950 cycles under a high-capacity retention of~94%.The scaled-up MnO_(2)/I_(2)-Zn battery reveals a stable cycle life under a cell capacity of~600 mAh.Moreover,our constructed MnO_(2)/I_(2)-Zn battery demonstrates a practical energy density of~37 Wh kg^(-1)and a competitive energy cost of<18 US$kWh^(-1)by taking into account the cathode,anode,and electrolyte.The MnO_(2)/I_(2)-Zn battery,with its remarkable reversibility and reasonable energy density,enlightens a new arena of large-scale energy storage devices.

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.

Decoupled water electrolysis:Flexible strategy for pure hydrogen production with small voltage inputs

Hydrogen gas is widely regarded as an ideal green energy carrier and a potential alternative to fossil fuels for coping with the aggravating energy crisis and environmental pollution.Currently,the vast majority of the world's hydrogen is produced by reforming fossil fuels;however,this hydrogen-making technology is not sustainable or environmentally friendly because ofits high energy consumption and large carbon emissions.Renewables-driven water splitting(2H_(2)0-2H_(2)+0_(2))becomes an extensively studied scheme for sustain-able hydrogen production.Conventional water electrolysis requires an input voltage higher than 1.23 V and forms a gas mixture of H_(2)/O_(2),which results in high electricity consumption,potential safety hazards,and harmful reactive oxygen species.By virtue of the auxiliary redox mediators(RMs)as the robust H^(+)/e^(-)reservoir,decoupled electrolysis splits water at a much lower potential and evolves O_(2)(H_(2)O+RMS_(ox)-O_(2)+H-RMS_(red))and H_(2)(H-RMS_(red)-H_(2)+RMS_(ox))at separate times,rates,and spaces,thus pro-ducing the puretarget hydrogen gas safely.Decoupled electrolysis has accelerated the development ofwater electrolysis technology for H_(2) production.However,itis still lack of a comprehensive and in-depth review in this field based on different types of RMs.This review highlights the basic principles and critical progress of this emerging water electrolysis mode over the past decade.Several representative examples are then dis-played in detail according to the differences in the RMs.The rational choice and design of RMs have also been emphasized.Subsequently,novel applications of decoupled water splitting are briefly discussed,including the manufacture of valuable chemicals,Cl_(2) production,pollutant degradation,and other half-reactions in artificial photosynthesis.Finally,thekey characteristics and disadvantages of each type of mediator are sum-marized in depth.In addition,we present an outlook for future directions in decoupled water splitting.Thus,the flexibility in the design of mediators provides huge space for improving this electrochemical technology.@2024 Science Press and Dalian Institute of Chemical Physics,Chinese Academy of Sciences.Published by ELSEVIER B.V.and Science Press.All rights reserved.

Zinc-doped g-C_3N_4/BiVO_4 as a Z-scheme photocatalyst system for water splitting under visible light

A two‐step photocatalytic water splitting system,termed a“Z‐scheme system”,was achieved using Zn‐doped g‐C3N4for H2evolution and BiVO4for O2evolution with Fe2+/Fe3+as a shuttle redox mediator.H2and O2were evaluated simultaneously when the doping amount of zinc was10%.Moreover,Zn‐doped(10%)g‐C3N4synthesized by an impregnation method showed superior active ability to form the Z‐scheme with BiVO4than by in‐situ synthesis.X‐ray diffraction,UV‐Vis spectroscopy,scanning electron microscopy,and X‐ray photoelectron spectroscopy were used to characterize the samples.It was determined that more Zn?N bonds could be formed on the surface of g‐C3N4by impregnation,which could facilitate charge transfer.