Structure Regulation Of Pbo_x(1<x?2)and Its Effects On Charge-Discharge Perfomance Of Soluble Lead Flow Battery

It is an effective way to achieve the complementary development of new energy and public grid by developing a new energy off-grid system,which integrates "power generation,power storage and power consumption".Reliable,cheap and safe energy storage system is the key to develop the new energy off-grid system.The flow battery has attracted much attention from off-grid energy storage due to its kW-MW energy storage power,safe and reliable operating conditions and flexible configuration available everywhere.The soluble lead single flow battery has a simple structure without any membrane,and employed soluble lead salts,which has the lower cost than other flow batteries.However,the structural stability and reversibility of PbO₂ deposition on the positive electrode is one of factors limiting the commercial application of soluble lead single flow battery.In order to solve the problem,the influence of current density,H+concentration of electrolyte and operating temperature on the crystal structure of PbO₂ are systematically studied in the solution of lead methanesulfonate/methanesulfonic acid.Different PbO₂ crystals are prepared by controlling electrodeposition conditions.The effects of different crystal structure of PbO₂ on the charge-discharge efficiency and cycle stability of soluble lead flow battery are investigated.The anodic deposited materials with different contents of O_lattice,i.e.PbO_x(1<x?2)are obtained by changing deposition potentials and electrolytes.The influence of O_lattice content on the redox activity of PbO_x is analyzed.The preparation conditions of PbO_x with the high content of O_lattice are proposed.It would provide a theoretical basis and experimental reference for efficient and stable operation of soluble lead single flow battery.According to the different physichemical properties of different PbO₂ crystals,the effects of current density,composition of electrolyte and operating temperature on PbO₂ crystal in the lead methanesulfonate/methanesulfonic acid system are studied.It has been found that the change of deposition conditions affects the deposition potential of PbO₂.The high deposition potential is beneficial to the formation of ?-PbO₂,and the low potential is favor for the formation of ?-PbO₂.?-PbO₂ is formed when the electrodeposition potential is below 1.50 V vs SCE.When the deposition potential is higher than 1.55 V vs.SCE,?-PbO₂ appears in the deposition layer.It has been also found the characteristics of overlying of two oxidation peaks,and two apparent reduction peaks in CV Curve.This further confirms that the deposition potentials of different PbO₂ crystal are different.The mixed crystal of ?-PbO₂+?-PbO₂ and the single crystal of ?-PbO₂ are prepared,and the effect of crystal form on discharge capacity is revealed.In view of the decrease of battery efficiency and the falling off of positive electrode deposits during the constant current charge/discharge cycle,it has been found that the charge voltage decreases with the charge discharge cycle.It could be inferred that the accumulation of PbO₂ on the cathode surface reduced the nucleation driving force of subsequent PbO₂,and the growth of PbO₂ on the electrode surface during cycling enlarged the effective electrode area,leading to the falling down of charge voltage.The decrease of voltage will cause the low redox efficiency and the formation of ?-PbO₂ which is easy to fall off.The constant voltage charge could avoid the problem of voltage dropping down during cycling.Its cycling performance is more stable,and its efficiency is also better than that in the constant current charge and discharge cycle.Since ?-PbO₂+?-PbO₂ mixed crystals formed at different potentials have different discharge capacities,the effect of anoxic structure of PbO_x on discharge capacity is studied.The ratio of O:Pb in PbO_x deposited at different potentials is characterized by XPS.The influence of O:Pb ratio on electrochemical impedance and discharge capacity of PbO_x is revealed.The increase of O_lattice content in PbO_x by deposition potential could be attributed to the promotion of OH· formation.Under the conditions to avoid oxygen evolution from the positive electrode,the O_lattice content of PbO_x generated by the high constant voltage is higher.The flow battery exhibits stable during cycle.The average coulomb efficiency of the battery with 2.15 V constant voltage charging -40 mA cm-2 constant current discharge is 95.3±1.5%during the first 10 cycles.The electrodeposition of PbO₂ is a step-by-step oxidation process of OH' to Pb²⁺,PbO_x with different O_lattice content can also be prepared by controlling the diffusion rate of Pb²⁺in different electrolyte systems at the same potential.In three electrolyte systems of Pb(CH₃SO₃)₂/CH₃SO₃H,Pb(CF₃SO₃)₂/CF₃SO₃H and Pb(ClO₄)₂/HClO₄,the O_lattice content of PbOx deposited in Pb(CF₃SO₃)₂/CF₃SO₃ H is the highest,and its discharge capacity is also better than that of PbOx deposited in other two electrolytes.The charge and discharge performance of the battery in Pb(CF₃SO₃)₂/CF₃SO₃H and Pb(CH₃SO₃)2/CH₃SO₃H are compared.The cycling stability of the battery in Pb(CF₃SO₃)2/CF₃SO₃H is better than that of Pb(CH₃SO₃)₂/CH₃SO₃H.The reason is that the adsorption of Pb(CF₃SO₃)₂/CF₃SO₃H is beneficial to obtain high quality PbO₂ deposition layer and its inhibition effect on free H+ ions,and thus the pH of electrolyte is kept stable in long-term cycle.In Pb(CF₃SO₃)2/CF₃SO₃H electrolyte,the coulomb efficiency is more than 80 % and the energy efficiency is more than 60 % after 244 cycles at a current density of 40 mA cm-2.Finally,the preparation conditions of high O_lattice content and the synergistic effect of PbO₂ crystal form and O_lattice content on the efficiency and cycle life of lead flow battery are comprehensively analyzed.