The cell-impedance-controlled lithium transport through LiMn2O4 film electrode with fractal surface by analyses of ac-impedance spectra, potentiostatic current transient and linear sweep voltammogram

Lithium transport through the fractal LiMn₂O₄ film electrode under the cell-impedance-controlled constraint was investigated by employing ac-impedance spectroscopy, potentiostatic current transient technique and linear sweep voltammetry. For this purpose, the flat and fractal LiMn₂O₄ film electrodes were prepared on the as-deposited Pt/polished Al₂O₃ substrate and the surface modified Pt/unpolished Al₂O₃ substrate, respectively. From the analysis of the ac-impedance spectra obtained from the flat and fractal electrodes, it is found that the apparent self-similar fractal dimension reduces the charge-transfer resistance, and the phase angle of the diffusion impedance under the semi-infinite diffusion condition positively deviates in absolute value from 45° with increasing fractal dimension. All the potentiostatic current transients experimentally measured from the flat and fractal LiMn₂O₄ electrodes showed non-generalised Cottrell behaviour which resulted from the cell-impedance-controlled constraint during lithium transport. In the case of linear sweep voltammogram theoretically calculated and experimentally measured from the flat and fractal LiMn₂O₄ electrodes, the power dependence of the peak current on the scan rate hardly exhibited the generalised Randles-Sev?ik behaviour. From the analyses of the potentiostatic current transients and the linear sweep voltammograms, furthermore, it is experimentally confirmed that the internal cell resistance mainly determining the cell-impedance-controlled lithium transport strongly depends upon the fractal dimension as well as such external parameters as the applied potential step and the amount of lithium transferred during lithium transport.     

Analysis of anodic current transient measured on Pd electrode with fractal surface: Hydrogen diffusion coupled with interfacial charge transfer

Hydrogen transport through Pd electrode with fractally rough surface was investigated in 0.1 M NaOH solution by analysis of the anodic current transient. The Pd electrode surfaces were electrochemically modified to be rough by redox cycling in 1 M H₂SO₄ solution. From the triangulation analysis of the atomic force microscopy images, it was found that the modified electrode surfaces exhibited self-similar scaling properties with different fractal dimensions, depending upon the number of redox cycles. The anodic current transient measured on the surface-modified fractal electrode subjected to the hydrogen discharging potentials of 0.3-0.7 V reversible hydrogen electrode, (RHE) showed the non-generalised Cottrell behaviour, which resulted from the constraint of hydrogen diffusion mixed with interfacial charge transfer during hydrogen transport. Especially, it displayed an inflexion point at the time that corresponds to the temporal outer cut-off of fractality, i.e. the crossover time required for the fractal to flat transition. In addition, the temporal outer cut-off under the constraint of mixed control was observed to be shortened with increasing hydrogen discharging potential, which could be accounted for by the increased growth rate of diffusion layer in the electrode accompanying the facilitated charge transfer kinetics at the electrode/electrolyte interface.     

An experimental study on cell-impedance-controlled lithium transport through Li1−δCoO2 film electrode with fractal surface by analyses of potentiostatic current transient and linear sweep voltammogram

The effect of the surface roughness on the cell-impedance-controlled lithium transport through the Li_1−δCoO₂ film electrode was experimentally investigated in a 1 M LiClO₄-PC solution by the analyses of the potentiostatic current transient (PCT) and the linear sweep voltammogram (LSV). The flat and fractal Li_1−δCoO₂ film electrodes were prepared on the Pt/polished Al₂O₃ substrate and the surface-modified Pt/unpolished Al₂O₃ substrate, respectively. From the ac-impedance spectra obtained from the flat and fractal electrodes, it is found that the apparent self-similar fractal dimension reduces the charge-transfer resistance. All the PCTs did not exhibit the generalised Cottrell behaviour until the characteristic time t_ch and all the power dependence of the peak current on the potential scan rate positively deviated from the generalised Randles-Sevcik behaviour above the characteristic scan rate ν_ch in the LSVs. From the analyses of the PCTs and the LSVs in terms of t_ch and ν_ch, furthermore, it is experimentally confirmed that the surface roughness plays a significant role in the kinetic facilitation of the interfacial charge-transfer reaction during the whole lithium intercalation and deintercalation processes.     

Lithium transport through a sol-gel derived LiMn2O4 film electrode: analyses of potentiostatic current transient and linear sweep voltammogram by Monte Carlo simulation

Lithium transport through a sol-gel derived LiMn₂O₄ film electrode was theoretically investigated by analyses of the potentiostatic current transient and the linear sweep voltammogram in consideration of the interactions between lithium ions by using Monte Carlo simulation. The anodic current transients experimentally measured on the film electrode ran with the slope of logarithmic current with logarithmic time flatter than −0.5 in the early stage, and then did in an upward concave shape in the time interval between t_T1 and t_T2. The linear sweep voltammograms experimentally measured on the film electrode showed two anodic peak currents I_p1 and I_p2 which increased linearly with scan rate v to the power of 0.66 and 0.70, respectively, (i.e. I_p1∝v^0.66 and I_p2∝v^0.70) at the scan rates higher than 0.5 mV s⁻¹. Moreover, the higher v was, the larger appeared the positive deviations of the first and second peak potentials E_p1 and E_p2 from the first and the second transition potentials E°_p1 and E°_p2, respectively, in the inverse derivative of the electrode potential curve. The current transients and the linear sweep voltammograms were analyzed in consideration of the interactions between lithium ions in the electrode by using the Monte Carlo simulation under two different constraints of the diffusion-controlled lithium transport and the cell-impedance-controlled lithium transport. The current transients and the linear sweep voltammograms, theoretically calculated under the cell-impedance-controlled constraint in consideration of the interactions between lithium ions, were in good agreement with the experimental results in shape. The disorder to order phase transition in the LiMn₂O₄ film electrode during the cell-impedance-controlled lithium transport at the potential jump and scan was discussed with the aid of the concentration profiles and the local cross-sectional snapshots of the configuration of lithium ions simulated by the Monte Carlo method.     

Theoretical approach to cell-impedance-controlled lithium transport through Li1 − δMn2O4 film electrode with partially inactive fractal surface by analyses of potentiostatic current transient and linear sweep voltammogram

Lithium transport through the partially inactive fractal Li_1 − δMn₂O₄ film electrode under the cell-impedance-controlled constraint was theoretically investigated by using the kinetic Monte Carlo method based upon random walk approach. Under the cell-impedance-controlled constraint, all the potentiostatic current transients calculated from the totally active and partially inactive fractal electrodes hardly exhibited the generalised Cottrell behaviour and they were significantly affected in shape by the interfacial charge-transfer kinetics. In the case of the linear sweep voltammogram determined from the totally active and partially inactive fractal electrodes, all the power dependence of the peak current on the scan rate above the characteristic scan rate deviated from the generalised Randles-Sev?ik behaviour. From the analyses of the current transients and the linear sweep voltammograms simulated with various values of the simulation parameters, it was further recognised that the cell-impedance-controlled lithium transport through the partially inactive fractal Li_1 − δMn₂O₄ film electrode strongly deviates from the generalised diffusion-controlled transport behaviour of the electrode with the totally active surface, which is attributed to the impeded interfacial charge-transfer kinetics governed by the surface inhomogeneities including the fractal dimension of the surface and the surface coverage by active sites and by the kinetic parameters including the internal cell resistance.