Structure Design Of Ti₃C₂T_x-Based Electrodes For High-Rate Pseudocapacitive Energy Storage Investigation

With the rapid depletion of fossil fuels and the increasingly severe trend of global warming,more than 140 countries and regions worldwide have proposed carbon neutrality targets,covering 90%of global carbon emissions.Developing green and low-carbon new energy has become a global consensus.However,the inherent characteristics of randomness,volatility,and intermittency in new energy power generation pose severe challenges to the safety and reliability of the power grid when a large proportion of new energy power is integrated into it.Electrochemical energy storage devices are an effective way to solve the above problems by storing and releasing electricity,thus balancing the supply and demand contradictions of the power system.Compared with lithium-ion batteries,supercapacitors possess advantages such as extremely short charging and discharging time,particularly long cycle life,high power density,high reliability,and no environmental pollution.These characteristics make them well-suited to meet the short-term,high-frequency energy storage needs of electricity.In order to broaden the high-energy application scenarios of supercapacitors,it is necessary to improve their energy density while maintaining their high power density,that is,to develop"double-high"supercapacitors.According to the different energy storage mechanisms,supercapacitors can be divided into electric double-layer capacitors and pseudocapacitors.Compared with electric double-layer capacitors,pseudocapacitors mainly store energy through surface or near-surface Faradaic redox reactions,with higher specific capacitance and energy density at low scan rates or low current densities.However,their electrochemical performance is poor at high rates.Therefore,improving the rate performance of pseudocapacitive electrodes is the main way to achieve"double-high"supercapacitors.Compared to bulk counterparts,two-dimensional nanomaterials have enormous potential for achieving high-rate pseudocapacitance due to their large specific surface area and abundant surface-active sites.However,when assembled into macroscopic electrodes,two-dimensional nanomaterials often suffer from poor rate performance due to low electron and ion transport rates.To address this issue,this thesis utilizes highly conductive Ti₃C₂T_x and their composites as pseudocapacitive active materials,and carries out three aspects of work to improve the ion/electron transport capabilities of the electrodes,thereby enhancing the rate performance of the pseudocapacitive electrodes.To mitigate the problem of ion diffusion obstruction in aerogel electrodes,this thesis proposes a novel approach of reducing pore tortuosity to increase the diffusion coefficient of electrolyte ions,thereby realizing low-temperature high-rate pseudocapacitive energy storage.Compared with traditional disordered Ti₃C₂T_xaerogels,this thesis prepares aligned Ti₃C₂T_x aerogels with vertical pore structures by designing a bidirectional temperature gradient,which significantly reduces the pore tortuosity and thus increases the ion diffusion coefficient.At room temperature,as the scan rate increases from 10 m V s^-1 to 5000 m V s^-1,the capacitance retention of aligned Ti₃C₂T_x aerogels is 52.2%,significantly higher than that of disordered Ti₃C₂T_x aerogels（35.8%）.Within the scan rate range of 10～100 m V s^-1,the areal capacitance and mass loading of aligned Ti₃C₂T_x aerogels exhibit an almost linear relationship,indicating great rate performance even at an industrial-level loading of 10 mg cm^-2.In particular,benefiting from its unique pore structures and ultra-high metal conductivity,aligned Ti₃C₂T_x aerogels can still achieve high-rate pseudocapacitive energy storage in extremely low-temperature environments（i.e.,-30℃）.At scan rates from 10 m V s^-1to 1000 m V s^-1,the rate performance of aligned Ti₃C₂T_x aerogels at-30℃is 63.6%,which is similar to that at room temperature（64.5%）.At a scan rate of 50 m V s^-1,the capacitance retention remained as high as 73%when the testing temperature drops from25℃to-30℃,exceeding that of other reported pseudocapacitive supercapacitors.Such robust low-temperature energy storage performance is mainly due to the low activation energy,fast ion transport and rapid electrochemical reaction in aligned Ti₃C₂T_x aerogels.Regarding the issue of poor conductivity of some pseudocapacitive materials,this thesis proposes an innovative design to enhance the electronic transfer by electrically connecting 1T-Mo S₂ nanosheets with conductive Ti₃C₂T_x skeleton,thus achieving surface-dominant high-rate pseudocapacitive energy storage.Compared with traditional Mo S₂ electrodes synthesized by slurry-coating methods,this thesis prepares1T-Mo S₂/Ti₃C₂T_x hybrid aerogels by the bidirectional freeze-casting method,where Ti₃C₂T_x conductive bridges connect both sides of 1T-Mo S₂ nanosheets to improve the overall electron transfer of the composite electrode.Furthermore,the three-dimensional porous oriented structure of the aerogel ensures rapid ion transport.At a scan rate of 5m V s^-1,the specific capacitance of 1T-Mo S₂/Ti₃C₂T_x aerogel is 392 F g^-1,which is about 2.2 times and 1.5 times that of 1T-Mo S₂ film and 1T-Mo S₂/Ti₃C₂T_x film,respectively.When the scan rate increases from 5 m V s^-1 to 1000 m V s^-1,the capacitance retention of 1T-Mo S₂/Ti₃C₂T_x aerogel is 38%,which is much higher than that of 1T-Mo S₂ film（8%）and 1T-Mo S₂/Ti₃C₂T_x film（13%）.Within the scan rate range of 5～50 m V s^-1,the capacitance contribution of 1T-Mo S₂/Ti₃C₂T_x aerogel ranges from69.6%to 86.9%,higher than that of 1T-Mo S₂/Ti₃C₂T_x film（50.8%～80.8%）,demonstrating the surface-dominant high-rate pseudocapacitive energy storage mechanism.The asymmetric supercapacitor assembled by 1T-Mo S₂/Ti₃C₂T_xaerogel//r GO film exhibits a high specific power of 76.4 k W kg^-1 and a high specific energy of 45.3 Wh kg^-1,while simultaneously possessing the high power of electric double-layer capacitance and the high energy of pseudocapacitance.In response to the problem of ion transport being hindered by the interlayer stacking in film electrodes,this thesis proposes a new approach to improve the ion transport efficiency per unit volume by constructing a bidirectionally porous compact film electrode structure,thereby achieving a synergistic improvement in rate performance and volumetric capacitance.Compared with the traditional Ti₃C₂T_x films with stacked morphology,this thesis prepares bidirectionally porous compact Ti₃C₂T_xfilms by combining the strategy of mild sulfuric acid etching and freeze-casting,significantly shortening the horizontal and vertical ion transport pathways.At a scan rate of 20 m V s^-1,the bidirectionally porous Ti₃C₂T_x film exhibits a high volumetric capacitance of 968 F cm^-3.Even at a high scan rate of 5000 m V s^-1,the volumetric capacitance of the bidirectionally porous Ti₃C₂T_x film is still as high as 391 F cm^-3,much higher than that of stacked Ti₃C₂T_x films(75 F cm^-3).Due to the high packing density and high-rate pseudocapacitive energy storage characteristics,the asymmetric supercapacitor based on bidirectionally porous Ti₃C₂T_x film and r GO film exhibits a high energy density(20.2 Wh L^-1)comparable to that of a lithium-ion battery and a high power density(85.9 k W L^-1)comparable to that of an aluminum electrolytic capacitor.