Report

Controlled current microelectrode techniques • The experiment is carried out by applying the controlled current between the working and auxiliary electrodes with a current source and determining the potential between the working electrode and a reference electrode. • Chronopotentiometry: E is determined as a function of time. Simplified block diagram of apparatus for chronopotentiometric measurements Current Step • When the RsCd circuit is charged by a constant current i, for a current step, assuming a constant Cd, the potential increases linearly with time. E i(Rs t / Cd ) Different types of controlled current techniques: (a) (b) (c) (d) Constant current chronopotentiometry Chronopotentiometry with linearly increasing current Current reversal chronopotentiometry Cyclic chronopotentiometry Current step E I 0 t Signal 0 t Resulting E-t curve Transition time, τ • After the concentration of electroactive species drops to zero at the electrode surface, the flux of electroactive species to the surface is insufficient to accept all of the electrons being forced across the electro-solution interface. The potential of the electrode will rapidly change toward more negative values until a new, second reduction process can start. • The time after application of the constant current for this potential transition to occur is called τ, the transition time. Sand equation • The measured value of τat known i can be used to determine C* or D. A lack of constancy of the transition time constant, iτ1/2/C*, with i or C* indicates complications to the electrode reaction from coupled homogeneous chemical reactions, adsorption, double-layer charging or the onset of convection. i 1/ 2 nFAD1/ 2 1/ 2 * C 2 Reversible waves • For rapid electron transfer, the Nernst applies. The following equation is obtained. RT 1/ 2 t1/ 2 E E / 4 ln nF t1/ 2 • Where Eτ/4, the quarter-wave potential, is DO RT ` E / 4 E ln 2nF DR • So that Eτ/4 is the chronopotentiometric equivalent of the voltammetric E1/2 value. Totally irreversible waves • For a totally irreversible cathodic reaction, I is related to E by the following equations: 0` n ( E E ) i 0 k C (0, t ) exp nFA RT • The E-t relation is depicted by the following equation: 0 RT RT 2 k 0` 1/ 2 1/ 2 E E ln ln( t ) 1/ 2 n F ( D) nF • The whole E-t wave shifts toward more negative potentials with increasing current. Quasi-reversible waves • Usually the study of the kinetics of quansi – reversible electrode reaction by constant current techniques involves the use of such small current perturbations that the potential change is small. RT 2t1/ 2 1 1 1 i * 1/ 2 1/ 2 * 1/ 2 nF nFA CO DO CR DR i0 • Thus a plot of η vs. t1/2, for small values of η, will be linear, and i0 can be obtained from the intercept. Muticomponent systems and multistep reactions • According the following equation and the transition time ratio, the electrons per reactive species: FA (n1D C n2 D C ) 2 1/ 2 1 * 1 1/ 2 2 * 2 2 2n2 n2 1 n1 n1 2 1/ 2 1/ 2 i(1 2 ) Charging profiles, voltage, current and battery temperature (a )Two-step charging with 0.5 + 0.05 C (b) Two-step charging with 0.7 + 0.05 C (c) Three-step charging with 0.5 + 0.2 + 0.05 C (d) Three-step charging with 1.0 + 0.2 + 0.05 C [1] T. Ikeya, N. Sawada, S. Takagi, J. Murakami, K. Kobayashi, T. Sakabe, E. Kousaka, H. Yoshioka, S. Kato, M. Yamashita, H. Narisoko, Y. Mita, K. Nishiyama, K. Adachil, K. Ishihar. Multi-step constant-current charging method for electric vehicle, valve-regulated, leadracid batteries during night time for load-levelling. Journal of Power Sources 75 1998 101–107. Constant-current discharge capacity of battery system with two-step or three-step charging Constant-current discharge capacity of battery system with multistep charging