On the effect of the uncompensated solution resistance on the applicability of the galvanostatic relaxation techniques and comparison to steady-state techniques for electrode kinetic measurements

Abstract

The effect of the uncompensated resistance between the reference and working electrodes was investigated on the applicability fields of the single and double pulse galvanostatic relaxation techniques for the determination of exchange current density. The applicability field is defined in terms of three characteristic times of the electrode system: τ c, the time constant of the double layer capacitance—reaction resistance system; τ d, the time constant of the diffusion impedence—reaction resistance system; and τ r, the time constant of the double layer capacitance—uncompensated solution resistance system. Within the boundaries of this field, the exchange current density can be determined with a maximum error of ±20%. It was found that the techniques are limited by the uncompensated solution resistance to τ r ⩽ 0.2τ c. An applicability diagram was also constructed for steady-state measurement techniques. In this case, the applicability field is defined in terms of k 0 (the apparent standard rate constant of the reaction), k m (rate constant of diffusion), and τ r and τ c. A comparison reveals that, at small τ c values, the relaxation techniques can be used for systems with larger k o values than can the steady-state techniques, but at large τ c the situation is reversed. The crossover point depends on the specific techniques compared; for example, for the double pulse galvanostatic technique with computer curve-fitting data evaluation compared to the rotating disc electrode steady-state technique, the crossover is τ c τ~ 10 −2 s.