Mechanism Of Ion Transport In Charged Two-dimensional Nanochannels And Applications Of Low-temperature Supercapacitors

Energy storage technology is the key technique for energy utilization.Recently,supercapacitors are attracting enormous attentions around the world for their high power density,fast charging/discharging rate and long cycle life.Typically,supercapacitors can be classified as electric double-layer capacitors（EDLCs）and pseudocapacitors based on the charge storage mechanism.The progress of nanotechnology promotes the development of supercapacitor electrode materials.Two-dimensional（2D）nanomaterials（e.g.,graphene and MXenes）are emerging as ideal electrode materials owing to their large surface area and superb electron conductivity.Supercapacitors are urgently required to maintain high performance in a wide temperature range for practical applications.The energy storage performance of supercapacitors is sensitive to their operational temperatures.Generally,the capacitance attenuates as the temperature decreases.This is because ion transport slows down at low temperatures,leading to reduced ions diffusing on the electrode surface and thus limited charge storage.Therefore,the low-temperature performance of supercapacitors is closely related to the ion transport in electrodes.However,the ion transport confined in nanospcace of 2D nanomaterials,which stimulates‘size effects’in supercapacitors,is still unclear.Herein,molecular dynamics（MD）simulations are employed to investigate the temperature dependence of ion diffusion coefficients in graphene nano-/subnano-channels.Based on the simulation results,effective solutions are proposed to enhance the ion transport and low-temperature performance of graphene-based EDLCs and pseudocapacitors,i.e.,constructing ion transport paths and assembling three-dimensional（3D）hierarchical architectures.High-performance low-temperature supercapacitors are thus obtained.Moreover,the concept is applied to the new-class 2D material,MXenes,leading ultrahigh-rate low-temperature pseudocapacitors.Experimental measurements are technically obstructed to directly reveal the ion transport process within nanoconfined spaces of<2 nm.In this work,MD simulations are employed to study the ion diffusion coefficients confined in graphene nanochannels at different temperatures,providing fundamental data for ion dynamics in nanoconfined spaces.Results show that ions surrounded by hydration shells exhibit distinct layer structure in nanochannels.The flat graphene surfaces with solvophobic nature can facilitate the motion of water molecules,which further accelerates the diffusion of ions in hydration shells.Consequently,confined ions diffuse significantly faster（along channel surfaces）than those in bulk solution.In charged nanochannels where the electrostatic interaction between counter-ions and charged channel surfaces suppress the motion of ions,the diffusion coefficients are lower than those in neutral counterparts.Besides,the increase of temperature leads to enhanced vibrant thermal motion of ions,as well as the increased diffusion coefficients.Due to the significant role of electrolyte-surface interactions,ion diffusion coefficients in nanoconfined spaces are more stable with varied temperatures than those in bulk solution.The vibration of electrolyte conductivity with temperature also decreases with the decreasing channel width.To enhance the low-temperature performance of EDLCs,ion transport pathways in electrode materials are constructed.Holey graphene electrodes and mixed-solvent organic electrolyte are matched to assemble an ultralow-temperature EDLC working at-60°C.The abundant mesopores and macropores on the surface of reduced holey graphene oxide（r HGO）increase the accessible surface area,leading to an improved capacitance compared to reduced graphene oxide（r GO）.As the temperature decreases to-60°C,the r HGO EDLC delivers a high capacitance of 106.2 F g^-1 with a retention of 70.6%,superior to the r GO counterpart（52.3%）.Owing to the ion transport pathways and the reduced ion diffusion length offered by the holey morphology,the ion diffusion resistance in r HGO is significantly smaller than that in r GO and less influenced by temperature with a lower activation energy.Specifically,at-60°C,the energy density of EDLC reaches up to 26.9 Wh kg^-1 with a maximum power density of 18.7 k W kg^-1.Pseudocapacitors possess higher specific capacitance than EDLCs albeit with poorer low-temperature performance retention.Herein,a rational design is proposed to enhance the low-temperature stability of pseudocapacitors.Specifically,well-aligned hierarchical pseudocapacitive electrodes are fabricated featuring run-through ion-buffering reservoirs in a graphene network（GN）,open intersheet channels between vertical graphene nanosheets（VGNSs）for fast ion transport,conductive scaffold as electron transport path and Mn O₂ nanopetals on VGNSs for efficient interfacial redox reactions.With reduced ion diffusion length and charge-transfer resistance and improved ion transport rate,the electrode capacitance decreases from 541 to 490 F g^-1at 1 A g^-1 as the temperature drops from 25 to 0°C,revealing a capacitance retention of 90.7%.At-30°C,the symmetric device based on the hierarchical electrodes maintains 80.8%of the room-temperature capacitance,which is comparable to EDLCs.Moreover,86.0%of capacitance is retained after 5000 cycles under repeated heating and cooling at temperatures ranging from-30 to 60°C.Asymmetric supercapacitors with the hierarchical architecture as the positive electrode exhibit stable performance over a wide temperature range from-30 to 60°C.The maximum energy and power density at-30°C are 21.6 Wh kg^-1 and 16.6 k W kg^-1,respectively.The above design concespt is generalized to the emerging 2D material,MXenes.3D porous Ti₃C₂T_x films are assembled wih nanosheets of enlarged interlayer space.MD simulations are conducted to investigate the ion diffusion and concentration in MXene nanochannels.Results show that ion diffusion coefficients and concentrations increase with the increasing channel width at different temperatures,suggesting higher electrolyte conductiviy and larger accessible active surface area in wider channels.Based on the simulation data,a KOH-modified freeze-casting method is used to synthesize porous Ti₃C₂T_x films with large interlayer distance（1.42 nm）.At room temperature,3D Ti₃C₂T_x films deliver a capacitance of 207.9 F g^-1 at 10 V s^-1,which demonstrates 58.6%capacitance retention with a 1000-fold scan rate increase.The capacitive performance is almost independent of film mass loading up to a practical level of 16.18 mg cm^-2.Electrochemical impedance spectroscopy measurements show that the ion diffusion resistance is significantly lower than that in stacked structures.As the temperature decreases from 25 to-30°C,the ion diffusion resistance in symmetric device is less influenced by the decreased temperature than charge transfer resistance.The specific capacitance of the symmetric pseudocapacitor exhibits a retention of 79.6%from 278.9 to 222.1 F g^-1,which is at the same level of EDLCs.