Recent progresses in the development of tubular segmented-in-series solid oxide fuel cells:Experimental and numerical study

Solid oxide fuel cells(SOFCs)have attracted a great deal of interest because they have the highest efficiency without using any noble metal as catalysts among all the fuel cell technologies.However,traditional SOFCs suffer from having a higher volume,current leakage,complex connections,and difficulty in gas sealing.To solve these problems,Rolls-Royce has fabricated a simple design by stacking cells in series on an insulating porous support,resulting in the tubular segmented-in-series solid oxide fuel cells(SIS-SOFCs),which achieved higher output voltage.This work systematically reviews recent advances in the structures,preparation methods,perform-ances,and stability of tubular SIS-SOFCs in experimental and numerical studies.Finally,the challenges and future development of tubular SIS-SOFCs are also discussed.The findings of this work can help guide the direction and inspire innovation of future development in this field.

Advancements,strategies,and prospects of solid oxide electrolysis cells(SOECs):Towards enhanced performance and large-scale sustainable hydrogen production

Solid oxide electrolysis cells(SOECs)represent a crucial stride toward sustainable hydrogen generation,and this review explores their current scientific challenges,significant advancements,and potential for large-scale hydrogen production.In SOEC technology,the application of innovative fabrication tech-niques,doping strategies,and advanced materials has enhanced the performance and durability of these systems,although degradation challenges persist,implicating the prime focus for future advancements.Here we provide in-depth analysis of the recent developments in SOEC technology,including Oxygen-SOECs,Proton-SOECs,and Hybrid-SOECs.Specifically,Hybrid-SOECs,with their mixed ionic conducting electrolytes,demonstrate superior efficiency and the concurrent production of hydrogen and oxygen.Coupled with the capacity to harness waste heat,these advancements in SOEC technology present signif-icant promise for pilot-scale applications in industries.The review also highlights remarkable achieve-ments and potential reductions in capital expenditure for future SOEC systems,while elaborating on the micro and macro aspects of sOECs with an emphasis on ongoing research for optimization and scal-ability.It concludes with the potential of SOEC technology to meet various industrial energy needs and its significant contribution considering the key research priorities to tackle the global energy demands,ful-fillment,and decarbonization efforts.

From concept to commercialization:A review of tubular solid oxide fuel cell technology

The reduced sealing difficulty of tubular solid oxide fuel cells(SOFCs)makes the stacking of tubular cell groups relatively easy,and the thermal stress constraints during stack operation are smaller,which helps the stack to operate stably for a long time.The special design of tubular SOFC structures can completely solve the problem of high-temperature sealing,especially in the design of multiple single-cell series integrated into one tube,where each cell tube is equivalent to a small electric stack,with unique characteristics of high voltage and low current output,which can significantly reduce the ohmic polarization loss of tubular cells.This paper provides an overview of typical tubular SOFC structural designs both domestically and internationally.Based on the geometric structure of tubular SOFCs,they can be divided into bamboo tubes,bamboo flat tubes,single-section tubes,and single-section flat tube structures.Meanwhile,this article provides an overview of commonly used materials and preparation methods for tubular SOFCs,including commonly used materials and preparation methods for support and functional layers,as well as a comparison of commonly used preparation methods for microtubule SOFCs,It introduced the three most important parts of building a fuel cell stack:manifold,current collector,and ceramic adhesive,and also provided a detailed introduction to the power generation systems of different tubular SOFCs,Finally,the development prospects of tubular SOFCs were discussed.

Boosting oxygen reduction activity and CO_(2) resistance on bismuth ferrite-based perovskite cathode for low-temperature solid oxide fuel cells below 600℃

Developing efficient and stable cathodes for low-temperature solid oxide fuel cells(LT-SOFCs) is of great importance for the practical commercialization.Herein,we propose a series of Sm-modified Bi_(0.7-x)Sm_xSr_(0.3)FeO_(3-δ) perovskites as highly-active catalysts for LT-SOFCs.Sm doping can significantly enhance the electrocata lytic activity and chemical stability of cathode.At 600℃,Bi_(0.675)Sm_(0.025)Sr_(0.3)FeO_(3-δ)(BSSF25) cathode has been found to be the optimum composition with a polarization resistance of 0.098 Ω cm^2,which is only around 22.8% of Bi_(0.7)Sr_(0.3)FeO_(3-δ)(BSF).A full cell utilizing BSSF25 displays an exceptional output density of 790 mW cm^(-2),which can operate continuously over100 h without obvious degradation.The remarkable electrochemical performance observed can be attributed to the improved O_(2) transport kinetics,superior surface oxygen adsorption capacity,as well as O_(2)p band centers in close proximity to the Fermi level.Moreover,larger average bonding energy(ABE) and the presence of highly acidic Bi,Sm,and Fe ions restrict the adsorption of CO_(2) on the cathode surface,resulting in excellent CO_(2) resistivity.This work provides valuable guidance for systematic design of efficient and durable catalysts for LT-SOFCs.

An innovative and facile synthesis route of(La,Sr)_(2)FeO_(4+δ)–La_(0.4)Sr_(0.6)FeO_(3−δ)composite as a highly stable air electrode for reversible solid oxide cell applications

Achieving thermal cycle stability is an imperative challenge for the successful commercialization of solid oxide cell(SOC)technology.Ruddlesden‒Popper(R‒P)oxides,whose thermal expansion coefficient(TEC)is compatible with common electrolytes,are promising candidates for SOC applications.However,the two-dimensional conduction characteristic of R‒P oxides leads to insufficient catalytic activity,which hinders their performance.Here,we propose a win‒win strategy for self-assembly decoration by employing a one-pot method to address this issue.By using a single perovskite oxide(La_(0.4)Sr_(0.6)FeO_(3))to modify R‒P oxide(La_(0.8)Sr_(1.2)FeO_(4+δ)),we enhanced the electrochemical performance without compromising the stability of the composite electrode.The strategic incorporation of a 10 mol%perovskite phase at 800℃ resulted in a significant 49%reduction in the polarization resistance(R_(p)),an impressive 86%increase in the maximum power density under power generation mode,and a notable 33%increase in the electrolysis current density under electrolysis mode.Furthermore,the perovskite-decorated R‒P oxide composite also exhibited high thermal and chemical stability,with negligible performance degradation observed under both thermal cycling and charge/discharge cycling conditions.Our results demonstrate that such dual-phase composites,which are simultaneously produced by a onestep process with outstanding catalytic activity and stability,can be considered an effective strategy for the advancement of SOCs.

Lattice Boltzmann simulation study of anode degradation in solid oxide fuel cells during the initial aging process

For present solid oxide fuel cells(SOFCs),rapid performance degradation is observed in the initial aging process,and the dis-cussion of the degradation mechanism necessitates quantitative analysis.Herein,focused ion beam-scanning electron microscopy was em-ployed to characterize and reconstruct the ceramic microstructures of SOFC anodes.The lattice Boltzmann method(LBM)simulation of multiphysical and electrochemical processes in the reconstructed models was performed.Two samples collected from industrial-size cells were characterized,including a reduced reference cell and a cell with an initial aging process.Statistical parameters of the reconstructed microstructures revealed a significant decrease in the active triple-phase boundary and Ni connectivity in the aged cell compared with the reference cell.The LBM simulation revealed that activity degradation is dominant compared with microstructural degradation during the initial aging process,and the electrochemical reactions spread to the support layer in the aged cell.The microstructural and activity de-gradations are attributed to Ni migration and coarsening.

Active Cu and Fe Nanoparticles Codecorated Ruddlesden-Popper-Type Perovskite as Solid Oxide Electrolysis Cells Cathode for CO_(2)Splitting

Solid oxide electrolysis cells(SOECs),displaying high current density and energy efficiency,have been proven to be an effective technique to electrochemically reduce CO_(2)into CO.However,the insufficiency of cathode activity and stability is a tricky problem to be addressed for SOECs.Hence,it is urgent to develop suitable cathode materials with excellent catalytic activity and stability for further practical application of SOECs.Herein,a reduced perovskite oxide,Pr_(0.35)Sr_(0.6)Fe_(0.7)Cu_(0.2)Mo_(0.1)O_(3-δ)(PSFCM0.35),is developed as SOECs cathode to electrolyze CO_(2).After reduction in 10%H_(2)/Ar,Cu and Fe nanoparticles are exsolved from the PSFCM0.35 lattice,resulting in a phase transformation from cubic perovskite to Ruddlesden-Popper(RP)perovskite with more oxygen vacancies.The exsolved metal nanoparticles are tightly attached to the perovskite substrate and afford more active sites to accelerate CO_(2)adsorption and dissociation on the cathode surface.The significantly strengthened CO_(2)adsorption capacity obtained after reduction is demonstrated by in situ Fourier transform-infrared(FT-IR)spectra.Symmetric cells with the reduced PSFCM0.35(R-PSFCM0.35)electrode exhibit a low polarization resistance of 0.43Ωcm^(2)at 850℃.Single electrolysis cells with the R-PSFCM0.35 cathode display an outstanding current density of 2947 mA cm^(-2)at 850℃and 1.6 V.In addition,the catalytic stability of the R-PSFCM0.35 cathode is also proved by operating at 800℃with an applied constant current density of 600 mA cm^(-2)for 100 h.

Investigation and optimization of high-valent Ta-doped SrFeO_(3-δ)as air electrode for intermediate-temperature solid oxide fuel cells

To explore highly active and thermomechanical stable air electrodes for intermediate-temperature solid oxide fuel cells(ITSOFCs),10mol%Ta5+doped in the B site of strontium ferrite perovskite oxide(SrTa_(0.1)Fe_(0.9)O_(3-δ),STF)is investigated and optimized.The effects of Ta^(5+)doping on structure,transition metal reduction,oxygen nonstoichiometry,thermal expansion,and electrical performance are evaluated systematically.Via 10mol%Ta^(5+)doping,the thermal expansion coefficient(TEC)decreased from 34.1×10^(-6)(SrFeO_(3-δ))to 14.6×10^(-6) K^(-1)(STF),which is near the TEC of electrolyte(13.3×10^(-6) K^(-1) for Sm_(0.2)Ce_(0.8)O_(1.9),SDC),indicates excellent thermomechanical compatibility.At 550-750℃,STF shows superior oxygen vacancy concentrations(0.262 to 0.331),which is critical in the oxygen-reduction reaction(ORR).Oxygen temperature-programmed desorption(O_(2)-TPD)indicated the thermal reduction onset temperature of iron ion is around 420℃,which matched well with the inflection points on the thermos-gravimetric analysis and electrical conductivity curves.At 600℃,the STF electrode shows area-specific resistance(ASR)of 0.152Ω·cm^(2) and peak power density(PPD)of 749 mW·cm^(-2).ORR activity of STF was further improved by introducing 30wt%Sm_(0.2)Ce_(0.8)O_(1.9)(SDC)powder,STF+SDC composite cathode achieving outstanding ASR value of 0.115Ω·cm2 at 600℃,even comparable with benchmark cobalt-containing cathode,Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-δ)(BSCF).Distribution of relaxation time(DRT)analysis revealed that the oxygen surface exchange and bulk diffusion were improved by forming a composite cathode.At 650℃,STF+SDC composite cathode achieving an outstanding PPD of 1117 mW·cm^(-2).The excellent results suggest that STF and STF+SDC are promising air electrodes for IT-SOFCs.

Rapid Fabrication of Electrodes for Symmetrical Solid Oxide Cells by Extreme Heat Treatment

Symmetrical solid oxide cells(SSOCs)are very useful for energy generation and conversion.To fabricate the electrode of SSOC,it is very time-consuming to use the conventional approach.In this work,we design and develop a novel method,extreme heat treatment(EHT),to rapidly fabricate electrodes for SSOC.We show that by using the EHT method,the electrode can be fabricated in seconds(the fastest method to date),benefiting from enhanced reaction kinetics.The EHT-fabricated electrode presents a porous structure and good adhesion with the electrolyte.In contrast,tens of hours are needed to prepare the electrode by the conventional approach,and the prepared electrode exhibits a dense structure with a larger particle size due to the lengthy treatment.The EHT-fabricated electrode shows desirable electrochemical performance.Moreover,we show that the electrocatalytic activity of the perovskite electrode can be tuned by the vigorous approach of fast exsolution,deriving from the increased active sites for enhancing the electrochemical reactions.At 900℃,a promising peak power density of 966 mW cm^(-2)is reached.Our work exploits a new territory to fabricate and develop advanced electrodes for SSOCs in a rapid and high-throughput manner.

Recovery of Solid Oxide Fuel CellWaste Heat by Thermoelectric Generators and AlkaliMetal Thermoelectric Converters

A Solid Oxide Fuel Cell(SOFC)is an electrochemical device that converts the chemical energy of a substance into electrical energy through an oxidation-reduction mechanism.The electrochemical reaction of a solid oxide fuel cell(SOFC)generates heat,and this heat can be recovered and put to use in a waste heat recovery system.In addition to preheating the fuel and oxidant,producing steam for industrial use,and heating and cooling enclosed rooms,this waste heat can be used for many more productive uses.The large waste heat produced by SOFCs is a worry that must be managed if they are to be adopted as a viable option in the power generation business.In light of these findings,a novel approach to SOFC waste heat recovery is proposed.The SOFC is combined with a“Thermoelectric Generator and an Alkali Metal Thermoelectric Converter(TG-AMTC)”to transform the excess heat generated by both the SOFC and the TG-AMTC.The proposed TG-AMTC is evaluated using a number of performance indicators including power density,operating temperature,heat recovery rate,exergetic efficiency,energy efficiency,and recovery time.The experimental results state that TG-AMTC has provided an exergetic efficiency,energetic efficiency,and recovery time of 97%,98%,and 23%,respectively.The study proves that the proposed TG-AMTC for SOFC is an efficient method of recovering waste heat.

Effects of operating conditions on the performance degradation and anode microstructure evolution of anode-supported solid oxide fuel cells

Performance degradation shortens the life of solid oxide fuel cells in practical applications.Revealing the degradation mechanism is crucial for the continuous improvement of cell durability.In this work,the effects of cell operating conditions on the terminal voltage and anode microstructure of a Ni-yttria-stabilized zirconia anode-supported single cell were investigated.The microstructure of the anode active area near the electrolyte was characterized by laser optical microscopy and focused ion beam-scanning electron microscopy.Ni depletion at the anode/electrolyte interface region was observed after 100 h discharge tests.In addition,the long-term stability of the single cell was evaluated at 700℃for 3000 h.After an initial decline,the anode-supported single cell exhibits good durability with a voltage decay rate of 0.72%/kh and an electrode polarization resistance decay rate of 0.17%/kh.The main performance loss of the cell originates from the initial degradation.

Fabrication of Gd_(2)O_(3)-doped CeO_(2)thin films through DC reactive sputtering and their application in solid oxide fuel cells

Physical vapor deposition(PVD)can be used to produce high-quality Gd_(2)O_(3)-doped CeO2(GDC)films.Among various PVD methods,reactive sputtering provides unique benefits,such as high deposition rates and easy upscaling for industrial applications.GDC thin films were successfully fabricated through reactive sputtering using a Gd_(0.2)Ce_(0.8)(at%)metallic target,and their application in solid oxide fuel cells,such as buffer layers between yttria-stabilized zirconia(YSZ)/La0.6Sr0.4Co0.2Fe0.8O_(3−δ)and as sublayers in the steel/coating system,was evaluated.First,the direct current(DC)reactive-sputtering behavior of the GdCe metallic target was determined.Then,the GDC films were deposited on NiO-YSZ/YSZ half-cells to investigate the influence of oxygen flow rate on the quality of annealed GDC films.The results demonstrated that reactive sputtering can be used to prepare thin and dense GDC buffer layers without high-temperature sintering.Furthermore,the cells with a sputtered GDC buffer layer showed better electrochemical performance than those with a screen-printed GDC buffer layer.In addition,the insertion of a GDC sublayer between the SUS441 interconnects and the Mn-Co spinel coatings contributed to the reduction of the oxidation rate for SUS441 at operating temperatures,according to the area-specific resistance tests.

Recent advances and influencing parameters in developing electrode materials for symmetrical solid oxide fuel cells

This article delivers a robust overview of potential electrode materials for use in symmetrical solid oxide fuel cells(S-SOFCs),a relatively new SOFC technology.To this end,this article provides a comprehensive review of recent advances and progress in electrode materials for S-SOFC,discussing both the selection of materials and the challenges that come with making that choice.This article discussed the relevant factors involved in developing electrodes with nano/microstructure.Nanocomposites,e.g.,non-cobalt and lithiated materials,are only a few of the electrode types now being researched.Furthermore,the phase structure and microstructure of the produced materials are heavily influenced by the synthesis procedure.Insights into the possibilities and difficulties of the material are discussed.To achieve the desired microstructural features,this article focuses on a synthesis technique that is either the most recent or a better iteration of an existing process.The portion of this analysis that addresses the risks associated with manufacturing and the challenges posed by materials when fabricating S-SOFCs is the most critical.This article also provides important and useful recommendations for the strategic design of electrode materials researchers.

Pulsed electrolysis of carbon dioxide by large-scale solid oxide electrolytic cells for intermittent renewable energy storage

CO_(2) electrolysis with solid oxide electrolytic cells(SOECs)using intermittently available renewable energy has potential applications for carbon neutrality and energy storage.In this study,a pulsed current strategy is used to replicate intermittent energy availability,and the stability and conversion rate of the cyclic operation by a large-scale flat-tube SOEC are studied.One hundred cycles under pulsed current ranging from -100 to -300 mA/cm^(2) with a total operating time of about 800 h were carried out.The results show that after 100 cycles,the cell voltage attenuates by 0.041%/cycle in the high current stage of−300 mA/cm^(2),indicating that the lifetime of the cell can reach up to about 500 cycles.The total CO_(2) conversion rate reached 52%,which is close to the theoretical value of 54.3% at -300 mA/cm^(2),and the calculated efficiency approached 98.2%,assuming heat recycling.This study illustrates the significant advantages of SOEC in efficient electrochemical energy conversion,carbon emission mitigation,and seasonal energy storage.

A robust fluorine-containing ceramic cathode for direct CO_(2) electrolysis in solid oxide electrolysis cells

Stro ntium-doped lanthanum ferrite(LSF)is a potential ceramic cathode for direct CO_(2) electrolysis in solid oxide electrolysis cells(SOECs),but its application is limited by insufficient catalytic activity and stability in CO_(2)-containing atmospheres.Herein,a novel strategy is proposed to enhance the electrolytic performance as well as chemical stability,achieved by doping F into the O-site of the perovskite LSF.Doping F does not change the phase structure but reduces the cell volume and improves the chemical stability in a CO_(2)-rich atmosphere.Importantly,F doping favors oxygen vacancy formation,increases oxygen vacancy concentration,and enhances the CO_(2) adsorption capability.Meanwhile,doping with F greatly improves the kinetics of the CO_(2) reduction reaction.For example,kchem increases by 78%from3.49×10^(-4) cm s^(-1) to 6.24×10^(-4) cm s^(-1),and Dchem doubles from 4.68×10^(-5) cm^(2) s^(-1) to 9.45×10^(-5)cm^(2) s^(-1).Consequently,doping F significantly increases the electrochemical performance,such as reducing R_(p) by 52.2%from 0.226Ωcm^(2) to 0.108Ωcm^(2) at 800℃.As a result,the single cell with the Fcontaining cathode exhibits an extremely high current density of 2.58 A cm^(-2) at 800℃and 1.5 V,as well as excellent durability over 200 h for direct CO_(2) electrolysis in SOECs.

Interface engineering of an electrospun nanofiber-based composite cathode for intermediate-temperature solid oxide fuel cells

Sluggish oxygen reduction reaction(ORR)kinetics are a major obstacle to developing intermediate-temperature solid-oxide fuel cells(IT-SOFCs).In particular,engineering the anion defect concentration at an interface between the cathode and electrolyte is important for facilitating ORR kinetics and hence improving the electrochemical performance.We developed the yttria-stabilized zirconia(YSZ)nanofiber(NF)-based composite cathode,where the oxygen vacancy concentration is controlled by varying the dopant cation(Y2O3)ratio in the YSZ NFs.The composite cathode with the optimized oxygen vacancy concentration exhibits maximum power densities of 2.66 and 1.51 W cm^(−2)at 700 and 600℃,respectively,with excellent thermal stability at 700℃ over 500 h under 1.0 A cm^(−2).Electrochemical impedance spectroscopy and distribution of relaxation time analysis revealed that the high oxygen vacancy concentration in the NF-based scaffold facilitates the charge transfer and incorporation reaction occurred at the interfaces between the cathode and electrolyte.Our results demonstrate the high feasibility and potential of interface engineering for achieving IT-SOFCs with higher performance and stability.

Temperature Gradient Analyses of a Tubular Solid Oxide Fuel Cell Fueled by Methanol

Thermal management in solid oxide fuel cells(SOFC)is a critical issue due to non-uniform electrochemical reactions and convective fl ows within the cells.Therefore,a 2D mathematical model is established herein to investigate the thermal responses of a tubular methanol-fueled SOFC.Results show that unlike the low-temperature condition of 873 K,where the peak temperature gradient occurs at the cell center,it appears near the fuel inlet at 1073 K because of the rapid temperature rise induced by the elevated current density.Despite the large heat convection capacity,excessive air could not eff ectively eliminate the harmful temperature gradient caused by the large current density.Thus,optimal control of the current density by properly selecting the operating potential could generate a local thermal neutral state.Interestingly,the maximum axial temperature gradient could be reduced by about 18%at 973 K and 20%at 1073 K when the air with a 5 K higher temperature is supplied.Additionally,despite the higher electrochemical performance observed,the cell with a counter-fl ow arrange-ment featured by a larger hot area and higher maximum temperature gradients is not preferable for a ceramic SOFC system considering thermal durability.Overall,this study could provide insightful thermal information for the operating condition selection,structure design,and stability assessment of realistic SOFCs combined with their internal reforming process.

Low molecular weight alkane-fed solid oxide fuel cells for power and chemicals cogeneration

This paper presents a review of low molecular weight alkane-fed solid oxide fuel cells(SOFCs),which,unlikely the conventional use of SOFCs for only power production,are utilized to cogenerate produce useful chemicals at the same time.The cogeneration processes in SOFC have been classified according to the different types of fuel.C_(2)and C_(3)alkenes and synthesis gas are the main cogenerated chemicals together with electricity.The chemicals and energy cogeneration in a fuel cell reactor seems to be an effective alternative to conventional reactors for only chemicals production and conventional fuel cells for only power production.Although,the use of SOFCs for chemicals and energy cogeneration has proved successful in the industrial setting,the development of new catalysts aimed at obtaining the desired chemicals together with the production of a high amount of energy,and optimizing SOFC operation conditions is still a challenge to enhance system performance and make commercial applications workable.

A defective iron-based perovskite cathode for high-performance IT-SOFCs:Tailoring the oxygen vacancies using Nb/Ta co-doping

The sluggish kinetics of the electrochemical oxygen reduction reaction(ORR)in intermediatetemperature solid oxide fuel cells(IT-SOFCs)greatly limits the overall cell performance.In this study,an efficient and durable cathode material for IT-SOFCs is designed based on density functional theory(DFT)calculations by co-doping with Nb and Ta the B-site of the SrFeO_(3-δ)perovskite oxide.The DFT calculations suggest that Nb/Ta co-doping can regulate the energy band of the parent SrFeO_(3-δ)and help electron transfer.In symmetrical cells,such cathode with a SrFe_(0.8)Nb_(0.1)Ta_(0.1)O_(3-δ)(SFNT)detailed formula achieves a low cathode polarization resistance of 0.147Ωcm^(2) at 650℃.Electron spin resonance(ESR)and X-ray photoelectron spectroscopy(XPS)analysis confirm that the co-doping of Nb/Ta in SrFeO_(3-δ)B-site increases the balanced concentration of oxygen vacancies,enhancing the electrochemical performance when compared to 20 mol%Nb single-doped perovskite oxide.The cathode button cell with NiSDC|SDC|SFNT configuration achieves an outstanding peak power density of 1.3 W cm^(-2)at 650℃.Moreover,the button cell shows durability for 110 h under 0.65 V at 600℃ using wet H_(2) as fuel.

Electricity Storage With High Roundtrip Efficiency in a Reversible Solid Oxide Cell Stack

We theoretically investigate the electricity storage/generation in a reversible solid oxide cell stack. The system heat is for the first time tentatively stored in a phase-change metal when the stack is operated to generate electricity in a fuel cell mode and then reused to store electricity in an electrolysis mode. The state of charge （H2 frication in cathode） effectively enhances the open circuit voltages （OCVs） while the system gas pressure in electrodes also increases the OCVs. On the other hand, a higher system pressure facilitates the species diffusion in electrodes that therefore accordingly improve electrode polarizations. With the aid of recycled system heat, the roundtrip efficiency reaches as high as 92% for the repeated electricity storage and generation.