| The large-scale consumption of renewable energy is the driving force to support the world’s sustainable development.However,the working mode of renewable energy is intermittent and random,resulting in low conversion efficiency and unstable power output,which greatly limit its application.The synergistic model of "power generation and energy storage" is the main way to achieve the safe and stable supply of renewable energy.supercapacitor have both the high-power characteristics of physical capacitors and the high energy density of batteries,making up for the shortcomings of two energy storage devices,which can be used as an advanced energy storage device in the future.The material used for electrode determines the comprehensive performance of supercapacitor device.For realizing the large-scale application,scientists are committed to developing various advanced and low-cost carbon electrode materials.Porous carbon presents great advantages,such as well-developed pore structure,stable physicochemical characteristics,low-cost and easy industrialization,which is the most pursued goal of human science.Our country has large coal reserves,and the preparation of functional porous carbon electrode materials is expected to solve the problem of low-price and large-scale production.However,coal as a solid carbon source,which is usually etched with corrosive and polluting strong alkali or acid to prepare porous carbon,resulting in uneven pore structure,low graphitization degree and poor yield.Focusing on large-scale application,how to prepare coal-based porous carbon with multi-dimensional regulation of pore structure,conductive properties,and surface physicochemical properties by a simple,eco-efficient and inexpensive modification strategy is the key issue to be solved.In view of this,aiming at the energy storage of coal-based porous carbon for supercapacitor electrode,using plasma-assisted activation,ultrasound-assisted activation,mechanochemistry activation methods on strengthening porous carbon.On this basis,the energy storage mechanism of coal-based porous carbon in supercapacitor is revealed.The specific work is as follows:(1)Porous carbon materials prepared by physical activation have poor specific surface area and pore structure,the strong alkali/acid used in chemical activation is easy to corrode the equipment and bring environmental pollution,and both of them are difficult to meet the large-scale preparation requirements of energy storage electrode materials.Therefore,water vapour plasma-assisted Mg-based activation strategy for porous carbon regulation is proposed.Non-corrosive MgCO3 was used as activator,combined with the bombardment pore-forming and surface chemical modification functions of water vapour plasma,coal-based porous carbon with specific surface area of 1989 m2·g-1,mesopores ratio of 39%and O/C ratio of 13.93%was obtained.The assembled supercapacitor has high specific energy of 10.35 Wh·kg-1 at 125 W·kg-1.(2)Apart from regulating pore structure and improving surface functionalization,high graphitization degree is critical for fast charge transport of supercapacitor.Hencein,on the basis of pore structure and surface chemical modification,ultrasound-assisted Fe/Mg-based activation strategy for porous carbon is proposed.Fe-based component can guide the formation and decomposition of Fe3C during ultrasound-assisted activation process,catalyze the conversion of amorphous carbon into graphite-like crystallite.Mg-based component can be converted into special nanoparticles during the pyrolysis,and the pore structure can be formed by subsequent washing.Furthermore,ultrasound induced cavitation promotes the breaking of chemical bonds and the generation of free radicals,which changes the surface chemical properties of the materials.As a bridge connecting pore structure,surface chemical properties and conductive properties,ultrasound technology has prepared coal-based porous carbon with rich micro-mesopores(Vmic 0.92 m3·g-1,Vmes 0.40 m3·g-1),excellent oxygen doping level(13.65 at.%)and high graphitization degree(ID/IG 0.94).The assembled supercapacitor exhibits excellent energy/power performance(9.6 Wh·kg-1/125 W·kg-1),rate performance(205 F·g-1/20 A·g-1)and cycle stability(capacitance is decayed by 9.7%after 10000 cycles at 1 A·g-1).(3)Based on the synergistic regulation of pore structure,surface chemical properties and conductive properties,it is a key issue to seek a simple,eco-efficient and inexpensive preparation process of porous carbon.Hence,taking advantage of remarkable advantages of Mg-based activator in the preparation of porous carbons,dry mechanochemistry coupled Mg-based activation strategy for porous carbon is proposed.Chemical reactions are induced by extrusion,impact,shear and collision between grinding medium and materials(MgCO3 and carbon precursor),so as to change the internal structure,crystal phase structure and surface chemical properties of the materials.The obtained coal-based porous carbon with micropores ratio of 64.03%,O/N doping content of 10.38 at.%and graphitization degree(ID/IG)of 0.88.The assembled supercapacitor illustrates superior energy/power performance(10.17 Wh·kg-1/125 W·kg-1),rate performance(219 F·g-1/20 A·g-1)and cycle stability(capacitance is decayed by 7.1%after 10000 cycles at 1 A·g-1).(4)Mechanochemistry technology manifests the advantages of short modification time,high reactivity,environmental friendly and easy large-scale.However,dry mechanochemistry still has problems such as particle easy agglomeration and uneven surface chemical modification.Therefore,wet mechanochemistry coupled Fe-based activation strategy for porous carbon is further proposed.Through the mechanical force effect of grinding medium and materials(K2FeO4 and precursor)in liquid environment.the secondary agglomeration of particle is reduced,the surface functional group distribution of materials is uniformly changed,and the graphitization degree is improved.The strategy produces unique physicochemical effects during the synthesis process,producing highly heteroatom doped(O 8.53 at.%,N 9.17 at.%,S 0.67 at.%),abundant micro-mesopores(Vmic 47.32%,Vmes 52.68%),large specific surface area(1817 m2·g-1)and high graphitization degree(ID/IG 1.07)of coal-based porous carbon.The assembled supercapacitor has excellent rate performance(specific capacitance retention is 72.63%at A·g-1),cycle performance(capacitance is decayed by 3.8%after 10000 cycles at 1 A·g-1)and energy/power performance(9.9 Wh·kg-1/125 W·kg-1).The comparison of different control strategies shows that the plasma-assisted Mg-based activation has the advantages of green reagent,short modification time and high energy density,but the procedure is complicated,the economy and large-scale application potential are low.The cycle stability and economy of ultrasound-assisted Fe/Mg-based activation are slightly improved,but the activation reagent has trace pollution.The potential for large-scale application is low.Dry mechanochemistry coupled Mg-based activation has the advantages of green reagent,simple procedure,high economy and cycle stability,which has the potential for large-scale application.Compared with the dry process,the wet mechanochemistry coupled Fe-based activation time is relatively reduced,the modification process is simple and the cycle stability is significantly improved,showing high economy and large-scale application potential.The above research is expected to provide a scientific theoretical basis for the transformation of low-rank coal to clean and efficient energy storage electrode materials. |
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