On Energy Cycle Of Aluminum As Carrier And The Enhancement Of Energy Density And Power Density Of Aluminum-air Battery

In order to resolve the problems of energy crisis and environmental pollution and promote the low-carbon and sustainable economy,it is important to develop renewable energy resources and enhance the percentage of electricity in terminal energy application in China.The concept of aluminum as energy carrier is proposed for energy storage.First the power generated from solar and wind is stored in Al fuel in the form of chemical energy produced with carbon-free electrolysis process,and then the chemical energy is converted to electricity by aluminum-air battery in the terminal application.Different from hydrogen energy storage and Li-ion battery energy storage,there are several advantages of aluminum energy storage,including abundance in resources,low cost in infrastructure,easy to store and transport,and low cost in terminal application.Therefore,the aluminum energy storage is a major means to improve the utilization of solar and wind energy resources in three northern regions of China(North-western,North-central,and North-eastern).Enhancement of the energy density of aluminum-air battery is favorable to increasing the energy efficiency,and decreasing the cost of power in terminal application.And the availability of aluminum-air battery is promoted with higher power density and lower corrosion resistance.In order to improve the power density and energy density of aluminum-air battery in alkaline electrolyte,the aluminum-air battery with micro anode-cathode distance(ACD)and flow electrolyte is constructed and investigated.The lower ACD resulted in reduction of both polarization and ohimc drop.With increase of operation temperature,the electrode reaction rate is accelerated due to highly reactive reactants and high diffusion rate of oxygen,but the self-corrosion of Al anode is increased and the dissolved oxygen level in electrolyte is lower.And the concentration polarization is decreased with the increase of electrolyte flow rate.Thus,the optimum operation conditions of aluminum-air battery is of ACD of 0.5 mm,electrolyte flow rate of 15m L min^-1 at 60?,in which the peak energy density is 3880 m Wh g_Al^-1.Thus the enhancement of total energy conversation efficiency of aluminum energy storage is about 0.5-1%,and the cost of electricity in terminal application is about 2.22 Yuan k Wh^-1.Pure O₂ atmosphere can help keeping high energy efficiency at high power density for Al-air flow battery due to the increased oxygen solubility,but slightly unbenifiting anode efficiency.Under pure O₂ atmosphere,the peak power density is up to 545 m W cm^-2,the anode efficiency reaches 96.2%.The optimum energy efficiency is 42.6%with output power density of 430.5 m W cm^-2.Because of the low working voltage(?1.6 V)of conventional Al-air battery with alkaline electrolyte,the energy density is below 4000 m Wh g_Al^-1,less than half of the theoretical value.So it is important to increase the operation voltage to enhance the energy density for Al-air battery.In this paper,the battery operates with membrane separating alkaline anolyte and acid catholyte.As a result,by simply tuning the environment for the cathode from alkaline to acid,the thermodynamic potential of battery will significantly increase from 2.71 to 3.54 V.And the Al-air battery based on fluid alkaline-acid hybrid electrolyte with anion exchange membrane(AEM)displays an open circuit voltage of 2.57 V,and approaches a high energy density of4591 m Wh g_Al^-1 at discharge voltage of around 2.08 V at 10 m A cm^-2.Hence,the total energy conversation efficiency of aluminum energy storage is up to 3.84-14.76%,and the cost of electricity in final application reduces to 1.88 Yuan k Wh^-1,lower than the cost of Li-ion battery energy storage.In this study,the concept of an ultrahigh voltage Al-air battery based on aqueous alkaline-acid hybrid electrolyte is demonstrated and the working mechanism is investigated.And the fluid electrolyte might solve the discharge voltage decline caused by p H changing of both anolyte and catholyte during discharge when AEM is using as separator.Moreover,these results provide a feasible approach to improve the performance of Al-air battery.Due to the competitive reaction between reduction of oxygen and hydrogen in alkaline electrolyte,the conventional Al-air battery cannot maintain stability during a long-time operation at low working current density or open circuit.To suppress the open circuit corrosion,the alkaline electrolyte has to be drained from electrolyte chamber or displaced by oil,which causes increase of the energy consumption and the device complicated.Herein,a proof-of-concept of novel Al-air battery is proposed by using room temperature ionic liquid(RTIL)as anolyte and aqueous alkaline solution as catholyte.The dual electrolyte battery configuration successfully combines the unique advantage of stability and activity of Al in ionic liquid,and avoids the blockage of cathode in alkaline catholyte.The working mechanism of dual electrolyte Al-air battery based on ionic liquid anolyte is investigated,and the results show the increasing of discharge voltage has a good linear response with the reduction of Al³⁺dissociation activation energy,and the control step is dissociation of Al³⁺.Furthermore,the porous Al foil can increase the reaction area and enhance discharge current density and power density.Because of the reversible process of Al deposition/dissolution in the ionic liquid,the rechargeable performance of dual electrolyte Al-air battery is studied.Though the poor reversibility of air cathode for oxygen reduction and evolution reaction result in poor cycle performance,the novel design of Al-air battery with dual electrolyte based on ionic liquid anolyte provides certain theory and carries out a reference for rechargeable Al-air battery.In summary,it is investigated from various perspectives to comprehensively analyze the viability of aluminum energy storage as a promising way to promote the application of renewable energy resource in China,including technology,economy and environment.Furthermore,study on the enhancement of power density,energy density and stability is carried out to solve the problems of Al-air battery for terminal application.Therefore,the results provide theoretical guidance for designing and using Al-air battery,and lay a foundation for its application.