Research On The Activation And Modification Of Carbon Paper

Supercapacitors are considered to be a type of novel electrochemical energy storage device because of quick charge/discharge rate,long cycle life.Graphite carbon electrode materials are widely used in electrode materials of supercapacitors because they have the unique advantages of good electrochemical stability,high conductivity and rich source.Graphite carbon electrode materials,which are usually based on the electrochemical double layer energy storage mechanism,have lower specific capacitance and limit the high energy density energy storage applications of carbon-based supercapacitors.In this paper,the carbon material itself is modified to improve the electrochemical performance,including the following three parts:?1??1?expanded activated carbon paper?ACP?,the graphite carbon paper was prepared by a two-step electrochemical exfoliated process,including nitrate?NO₃^-?and Sulfate(SO₄^2-)ion assisted stripping,which expands the graphite layer spacing and increase the effective specific surface area of the carbon paper.The specific capacitance of carbon paper electrode material is significantly increased.?2?nitrogen-doped activated carbon paper?N-ACP?,the effective improvement of specific capacitance of graphite carbon materials was achieved by the introduction of the pseudocapacitance from the nitrogen containing functional groups?3?sulfur,nitrogen co-doped activated carbon paper?S,N-ACP?,the further improvement of specific capacitance of graphite carbon materials was achieved by the introduction of the pseudocapacitance from the sulfur and nitrogen containing functional groups.The specific research content of this thesis mainly includes the following three aspects:?1?Preparation and electrochemical properties of expanded activated carbon paper electrode materialsThe ACP electrode material was prepared by a two-step electrochemical exfoliated method and a hydrothermal reduction of ethylene glycol.Graphite carbon paper was used as precursor,and then expanded graphite carbon paper was prepared by NO₃^-and SO₄^2-ion assisted two-step electrochemical stripping method.Then,the unstable oxygen-containing functional group was removed by hydrothermal reduction of ethylene glycol.The graphite layer has an enlarged spacing and a graphite sheet layer structure on the surface of the ACP,which is beneficial to increase the effective specific surface area and promote ion diffusion and transfer.The ACP has a specific capacitance of 380 mF cm^-2 at 1 mA cm^-2,which is significantly higher than the pure carbon paper specific capacitance of 21 mF cm^-2.The current density increased from 1mA cm^-2 to 10 mA cm^-2,and the ACP specific capacitance was attenuated from 380 mF cm^-2to 277 m F cm^-2,and the specific capacitance retention rate was 73%,showing better rate performance.It is noteworthy that nearly 98%capacitance retention was obtained after 5000charge/discharge cyclesa nd good cycle stability is maintained.Therefore,the ACP electrode has a significantly improved specific capacitance performance.All-solid-state supercapacitor was constructed using ACP as electrode and using H₂SO₄-PVA gel as electrolyte without any current collectors and binders.ACP supercapacitor were conducted at a voltage window of 1.6 V.The specific capacitance of ACP supercapacitor reaches 97 mF cm^-22 at 1 mA cm^-2 and still achieves 68 mFcm^-2 even at 5 mA cm^-2,presenting the rate capacity retention of 70%.Moreover,the energy density declines from 0.343 to 0.242mWh cm^-3 when the power density increases from 8 to 40 mWcm^-3.The capacitance retention of 97%after 3000 cycles keeps good cycling life.?2?Preparation and electrochemical properties of nitrogen doped activated carbon paperThe N-ACP electrode material was prepared by ammonia gas nitriding using ACP as the precursor and ammonia as the nitrogen source at high temperature.The doping amount of nitrogen in N-ACP reaches 10.8%.The introduction of the nitrogen atom produces pseudocapacitance effect.The specific capacitance of N-ACP at 1 mA cm^-2 is 525 mF cm^-2,which is significantly higher than the ACP specific capacitance(380 mF cm^-2).When the current density is increased to 10 mA cm^-2,the specific capacitance is attenuated to 394 mF cm^-2,and the specific capacity retention rate is 75%.After 5000 cycles at the current density of 10mA cm^-2,the specific capacity retention can reach 98%,indicating excellent cycle performance.Therefore,the N-ACP electrode material exhibited an effectively improved specific capacitance performance.All-solid-state supercapacitor was constructed using N-ACP as electrode and using H₂SO₄-PVA gel as electrolyte without any current collectors and binders.N-ACP supercapacitor were conducted at a voltage window of 1.5 V.The N-ACP symmetrical supercapacitor has a specific capacitance of 150 mF cm^-2,an energy density of 0.47mWh cm-3,and a power density of 7.5 mW cm^-3 at a current density of 1 mA cm^-2.The capacitance retention of 96%after 3000 cycles keeps good cycling life.?3?Preparation and electrochemical properties of sulfur,nitrogen co-doped activated carbon paperBased on N-ACP,sublimed sulfur is used as a sulfur source to prepare S,N-ACP electrode materials.The doping amount of sulfur and nitrogen in S,N-ACP reached 18.3%and 6.3%,respectively.Sulfu,nitrogen co-doping modification can increase the content of heteroatoms,provide various active sites and exert various hetero atom advantages,and introduce nitrogen-containing functional groups?pyridyl nitrogen,pyrrole nitrogen,etc.?and sulfur-containing functional groups?thiophene sulfur,etc.?.Pseudocapacitance effect is produced to improve the S,N-ACP specific capacitance performance.The specific capacitance of S,N-ACP at 1 mA cm^-2 is 616 mF cm^-2,which is significantly higher than the N-ACP specific capacitance(525mF cm^-2).When the current density is increased to 10 mA cm^-2,the specific capacitance is attenuated to 514 mF cm^-2,and the specific capacity retention rate is 83%.After 5000 cycles at the current density of 10 mA cm^-2,the specific capacity retention of S,N-ACP-650 can reach99%,keeping excellent specific capacitance cycle performance.Therefore,the S,N-ACP electrode material exhibits a further increased specific capacitance.All-solid-state supercapacitor was constructed using S,N-ACP as electrode and using H₂SO₄-PVA gel as electrolyte without any current collectors and binders.S,N-ACP-650symmetric supercapacitor has a specific capacitance of 211 mF cm^-2 and an energy density of0.66mWh cm^-3 at the current density of 1 mA cm^-2 at a voltage window of 1.5 V.The power density is 7.5 mW cm^-3.Under the current density of 5 mA cm^-2,the specific capacitance retention can reach to 98%after 3000 cycles,keeping good cycle stability.