Optimization Of Ion Transport Behavior At Solid-Liquid Interface And Related Applications Of Supercapacitor

Green and low-carbon energy storage and conversion technology is the key to promote the development and consumption of renewable energy.A typical example is supercapacitor.Supercapacitor is an electrochemical storage device based on electrostatic adsorption and has been intensively studied by researchers,owing to the advantages of high-power density,fast charging and discharging rate and long cycle life.Understanding and regulating ion transport at the solid-liquid interface has been found to be a crucial method to promote the electrochemical performance of supercapacitor.However,the rapid development of two-dimensional nanomaterials makes it difficlult for classical theories to accurately describe the ion transport behavior in the highly confined nano or sub-nano spaces,due to the severe folding of electrostatic double layers.In addition,diversified surface properties are found to have significant influences on ion dynamics,leading to challenges for the design of high-performance electrode materials.To address the above issues,the ion transport behaviors between the highly confined nanochannels（especially near the electrode walls）are investigated through molecular dynamics（MD）simulations and density functional theory（DFT）calculations.The influences of nanochannel spacing and surface properties on ion adsorption/transport are revealed and elaborated.Based on the simulation results,electrode materials with enhanced ion adsorption/transport performance are prepared to realize high-performance capacitive deionization（CDI）and supercapacitor applications.As for the regulation of anion adsorption,the MD simulations are employed to calculate the diffusion coefficients of anions and cations in graphene nanochannels with different interlayer spacing.It is found that the diffusion coefficient of chloride ions is much higher than that of sodium ions in the graphen channels with spacing of 1.2～1.4nm,indicating that chloride ions diffusion dominates the ion transport in these channels.To prepare graphene with the desired pore size,the hydrogen peroxide solution is added to graphene oxide solution during the hydrothermal process to etch the defects on graphene nanosheets.By controlling the volume ratio of hydrogen peroxide,nanoporous graphene film（NGF）electrodes were prepared with pore sizes in the ranges of 0.8～1.0 nm,1.2～1.6 nm and 1.6～3.2 nm.The electrochemical quartz crystal microbalance（EQCM）tests are conducted to track the ion transport behavior of the as-prepared graphene samples with different pore size distributions.Results show that chloride ion adsorption dominated the charging process of graphene samples with pore size of 1.2～1.6 nm,consistent with the MD simulation results.In addition,the result of capacitive deionization measurements also shows that graphene samples with pore size of 1.2～1.6 nm deliver the highest salt adsorption.As for the regulation of cation adsorption,the adsorption energy of sodium atoms on different kinds of functional groups of MXene is analysed by DFT calculations.The sodium atom adsorption energy is found to varies dramatically with the types of functional groups,where the values are in the following order:-O>-F>bare>-Cl>-OH.To verify this conclusion,HCl/Li F etching method is employed to prepare the few layer Ti₃C₂T_x samples,which are then treated by alkali solutions in ice-bath.The alkali treatment leads to increased-OH content and decreased-F content,while the interlayer spacing is almost unchanged.The mass-charge ratio of the samples tested by EQCM is found to decrease dramatically after the ice-bath alkali treatment,indicating less ion adsorption.In addition,freeze-drying method is used to fabricate Ti₃C₂T_x aerogel electrodes,the salt adsorption capacity of which is tested by capacitive deionisation measurements to further validate the obtained conclusion.As for the regulation of ion transport velocity,a novel strategy is proposed to accelerate ion transport within the nanochannel by reducing ion-wall interactions（ionophobic）and enhancing hydrogen bondings（hydrophilic）.According to the above adsorption energy calculations,the-OH functional group has the lowest affinity to cations and thus have the potential to achieve ionophobicity.A similar ice bath alkali treatment is used to increase the ratio of-OH/-O functional groups in Ti₃C₂T_x from 0.69to 1.5,reduce the surface charge from–33.4 m V to–15.0 m V and increase the intermediate water/free water ratio from 0.48 to 0.62.After determining the interlayer spacing using in-situ/ex-situ X-ray diffraction spectroscopy,molecular dynamics simulations are performed to investigate the ion transport mechanisms in two typical models of Ti₃C₂O₂（ionophilic）and Ti₃C₂（OH）₂（ionophobic）nanochannels for a fixed interlayer spacing（1.60 nm）.It is found that the transport velocity of cations near the wall of the Ti₃C₂（OH）₂ electrode is much higher than the Ti₃C₂O₂ electrode.And the number of water molecules and hydrogen bonds within the Ti₃C₂（OH）₂ nanochannel is higher than that of Ti₃C₂O₂.Further MD calculations show that this phenomenon also occurs in nanochannels with the same effective interlayer spacing（0.65 nm）and unconfined space（50 nm）,indicating that the accelerated ion transport enabled by ionophobicity and hydrophilicity can be applied in wide range of pore structures.Electrochemical measurements results show the capacitance retention of Ti₃C₂T_x films at the current density of 50 A g^–1 can be increases from 20.4%to 78.4%(in 1 mol L^–1Na₂SO₄ electrolyte)after the ice-bath alkali treatment,and the ion diffusion resistance decreases from 7.89Ωcm² to 0.66Ωcm².Finally,the specific application of the above ion kinetic regulation strategies in capacitive deionization and supercapacitors is presented.Nanoporous graphene film electrodes with optimised pore size distribution are assembled with MXene areogel electrodes to fabricate a full-cell ion selectivie adsorption system.The salt adsorption capacity（SAC）of 27.2～49.0 mg g^–1 are obtained at salt concentrations of 500～5000mg L^–1,with charge efficiency（CE）of 110%～123%.Besides,no performance degradation is observed during 100 adsorption/desorption cycles.The ionophobic,hydrophilic MXene film electrode is applied in supercapacitors with 1 mol L^–1 H₂SO₄electrolyte and still have a capacity of 289.3 F g^–1(862.1 F cm^–3)even at the current density of 50 A g^–1.The ionophobic,hydrophilic MXene films is further assembled with a redox graphene（r GO）film to fabricate asymmetric supercapacitors,realizing the highest energy density of 36.44 Wh kg^–1(108.59 Wh L^–3)and the highest power density of 11.73 k W kg^–1(34.96 k W L^–3).