Research Of The Redox Mediators In Metal-Air Batteries

The aprotic metal-air battery has attracted ever-increasing attention because it represents a promising technology with high specific energy density which overhelmingly exceeds commercial lithium-ion batteries.Despite of the high energy density,the metal-air battery is confronted with several daunting challenges,especially the high overpotential,low energy efficiency,poor reversibility,and short cycle life.These challenges have a common origin centering around the discharge product,which is kinetically difficult to decompose and thermodynamically inert in the contact with the electrode.Thus,more power energy or higher charge overpotetnial should be applied to trigger the charging process.Many heterogeneous catalysts have been used to reduce the high overpotential on charging.Nevertheless,a common problem of these solid catalysts is the limited number of contact reactive sites and poor electric conduction among solid catalysts,the discharge products,and electrode.As an alternative to solid catalysts and functioning via a solution-based homogeneous catalysis mechanism,soluble redox mediators(RMs)can circumvent the deactivation by the discharge products,provide a larger number of active sites via solid-electrolyte contact,and have better activity tunability to avoid catalyzing unwanted parasitic reactions.In this paper,we focused on the study of the redox mediators in energy storage and conversion system.The effect the redox mediators toward their applications in metal-air batteries were carefully studied.Excellent performances were obtained in this work.The main points are outlined as follows:(1)A highly-efficient redox mediator of N,N'-Diphenyl-p-phenylenediamine(DPPD)is introduced for the first time to catalyze the oxygen evolution reaction(OER)in Li-O2 batteries.During charging process,DPPD is electrochemically oxidized to DPPD+,which can chemically decompose Li2O2 to O2 on the cathode at a much-reduced overpotential.The in-situ differential electrochemical mass spectrometry(DEMS)confirms that the charging process is dominated by Li2O2 decomposition.Multifaceted mechanistic investigation unravels that the discharge product,Li2O2,could be completely decomposed with the assisence of DPPD.With the assistance of DPPD,the Li-O2 cells can achieve lower charging potential,improved cycling stability,and prolonged lifespan which is three times longer than that of tetrathiafulvalene(TTF)counterpart.This work is another example of regulating interfacial reactions in Li-O2 batteries by using RM.(2)4,4'-Dipyridyl disulfide(DPDS)is proposed as soluble catalyst to reduce the overpotential and facilitate better reversible formation and decomposition of Li2CO3 by a solution mediated mechanism for Li-CO2 battery.It is demonstrated that CO2 is successfully captured and released on the basis of disulfide.Moreover,the discharge and charge processes are dramatically tailored with the remarkable synergistic effect of pyridine nitrogen.Multifaceted mechanistic investigation unravels that the Li-CO2 battery is discharged with Li2CO3 and carbon as the main discharge product,and that disulfide and pyridinic "N" in DPDS facilitate Li2CO3 formation and decomposition via a solution-phase homogeneous catalysis mechanism.The results elucidated that stable cycling of Li2CO3 relying on the assistance of effective catalysts and DPDS presents a credible and viable opportunity for long lifespan Li-CO2 battery.(3)A new concept of CO2-assisted sodium-phenanthrenequinone(Na-PQ)battery is reported that can capture CO2 to heighten its load voltage and specific energy upon discharge and reversibly release CO2 on recharge.A mechanistic study,combining cyclic voltammetry(CV),in situ surface-enhanced Raman spectroscopy(SERS),differential electrochemical mass spectrometry(DEMS)and density functional theory calculation(DFT),reveals that CO2 is involved in the discharge reaction by bonding to the carbonyl moieties(C=O)of the reduced PQ species(PQ2-in particular),which lowers the energy of the final discharge product PQ2-CO2(Na+)2 and therefore increases the formal potential of the redox couple PQ-Na+/PQ2-CO2(Na+)2.Specifically,the CO2-assisted Na-PQ battery has achieved a higher discharge voltage and an improved specific energy.The CO2-assisted Na-PQ battery reported here exemplifies that electrochemical energy storage would have great potential to address the grand challenges of CO2 mitigation,utilization,and storage.