Study On Cement-based Structural Supercapacitor

In response to scientific policy of“carbon neutrality”and“carbon peaking”,it is urgent to optimize power energy structure and solve carbon emission problem in the construction field.The combination of large-scale structural electrochemical energy storage technology and renewable clean energy is an effective way to realize electric energy generation and storage in building structures.Cement-based materials with many pore structure can be regarded as natural solid ionic conductors,which can offer effective channels for ions storage and movement.Therefore,cement electrolytes can be employed to prepare structural supercapacitors to realize novel integration of electrochemical energy storage technology and building materials.However,due to limited movement of ions in the cement electrolytes,mechanical-ionic conductivity contradiction as well as narrow voltage window of cement electrolytes,cement-based structural supercapacitors exhibit low mechanical strength and electrochemical energy storage capacity.This paper focuses on electrode and electrolyte material systems to systematically investigate reduced graphene oxide（rGO）/metal oxide composite electrodes,polymer cement electrolytes and assembled cement-based structural supercapacitors.For the first time,ionic conductive phases and polymers are doped into cement paste to prepare polymer cement electrolyts and an organic salt lithium trifluoromethanesulfonate（LiOTf）is employed as the ionic conductive phase of polymer cement electrolyte.Molecular dynamics simulation and mechanical-ionic conductivity data fitting are combined to clarify influence mechanism of pore structure on polymer cement electrolyte performance.By investigating micromorphology,capacitive nature and cycle stablility of rGO/metal oxide composite electrodes,it is confirmed that composite electrodes have combined excellent electrochemical stability of graphene and high theoretical specific capacitance of metal oxides.As content of metal oxide in the composite electrode increases,the capacitance rate of composite electrode increases to peak value.Compared with rGO/single metal oxide composite electrodes rGO@Mn O₂ and rGO/CuO,rGO/bimetallic oxide composite electrodes rGO/Ni Co₂O₄ and rGO@Cu Co₂O₄ present higher capacitance and cycle stability.In this paper,four composite electrodes prepared exhibit high capacitance,capacitance rate and cycle stability,which are ideal electrode candidates for cement-based structural supercapacitors.The influence of dosage and type of ionic conductive phase on ionic conductivity and compressive strength of cement electrolyte is studied.It is found that the ionic conductivity and compressive strength of cement electrolyte increases and decreases with the content increase of ionic conductive phase.It is concluded that there is a contradiction between compressive strength and ionic conductivity of cement electrolyte,that is,increase of compressive strength will inevitably lead to decrease of ionic conductivity.Compared with inorganic salt KOH,organic salt LiOTf as ionic conductive phase can broaden voltage window of cement electrolyte.The influence of dosage and type of polymer on ionic conductivity and compressive strength of polymer cement electrolyte is studied.It is found that molecular chains of polyacrylic acid（PAA）or polyethylene oxide（PEO）permeate with cement hydration products,which can regulate pore structure of cement electrolyte and improve contradictory relationship between compressive strength and ionic conductivity of cement electrolyte.In their respective systems,5wt%KOH-6wt%PAA cement electrolyte,5wt%LiOTf-6wt%PAA cement electrolyte,5wt%KOH-2wt%PEO cement electrolyte and 5wt%LiOTf-2wt%PEO cement electrolyte display the best mechanical-ionic conductivity performance,which are ideal electrolyte candidates for cement-based structural supercapacitors.Migration behavior of KOH solution in the pore structure of PAA cement composite is analyzed.It is observed that migration depth of ions in the pore structure of PAA cement composite gradually increase to equilibrium with the increase of pore size.It is predicted that when ionic conductive phase is sufficient,ionic conductivity of PAA cement electrolyte will gradually increase to a peak value with the increase of pore size.Relationships between ionic conductivity and compressive strength of PAA cement electrolyte and total porosity and（<50 nm）pore content of pore structure are fitted.The results demonstrate that the ionic conductivity and compressive strength of PAA cement electrolyte slightly decrease with the increase of total porosity;the ionic conductivity of PAA cement electrolyte basically increases with the increase of（<50nm）pore content;the compressive strength of PAA cement electrolyte first decreases and then increases with the increase of（<50 nm）pore content.Compressive strength,mechanical efficiency η_m,voltage window,capacitance,energy density and electrochemical energy storage efficiencyη_eof 16 cement-based structural supercapacitors assembled with four kinds of rGO/metal oxide composite electrodes and four kinds of polymer cement electrolytes are studied.It is shown that the compressive strength andη_m are determined by compressive strength of polymer cement electrolyte.When using same electrode,capacitance andη_e of cement-based structural supercapacitor are mainly determined by ionic conductivity of polymer cement electrolyte,while its energy density is mainly determined by voltage window and ionic conductivity of polymer cement electrolyte.When using same polymer cement electrolyte,the capacitance and energy density of cement-based structural supercapacitor are not only related to capacitance of composite electrode,but also related to compatibility between composite electrode and polymer cement electrolyte.Cement-based structural supercapacitors assembled with rGO/metal oxide composite electrodes and polymer cement electrolytes show capacitance of 114～318m F/cm²,energy density of 0.01～0.5 m Wh/cm² and electrochemical energy storage efficiencyη_e of 0.12～0.20,which are much better than reported structural supercapacitors.These demonstrate that material systems of rGO/metal oxide composite electrodes and polymer cement electrolytes have promising potential application value in the field of structural energy storage.