PROBLEMS IN THE DETERMINATION OF ADSORPTION BEHAVIOR OF INTERMEDIATES IN FARADAIC REACTIONS - DISTINCTION BETWEEN DOUBLE-LAYER AND ADSORPTION CAPACITANCE OF ELECTROCATALYSTS DETERMINED FROM FAST POTENTIAL RELAXATION TRANSIENTS

In recent papers, we have employed open-circuit potential relaxation transients for evaluation of the adsorption behaviour of chemisorbed intermediates in electrochemical ps evolution reactions, through analysis of the interfacial capacitance. In the present paper, we examine theoretically and by means of computer simulation the significance of such measurements, in particular conditions under which capacitance of the double layer can be distinguished from that due to pseudocapacitance associated with the intermediates. The initial potential relaxation slopes, (dV/dt)t=0, required for double layer capacitance evaluation, can be determined accurately by a procedure involving fitting of digitally acquired potential relaxation transient [V(t)] curves, recorded over a time scale of microseconds to milliseconds, by a polynomial or simpler function. Coupled with the current density prior to current interruption, i(t = 0), the procedure enables both the double layer capacitance and "iR" drops to be determined. The accuracy of this method is examined. As examples, the electrochemical H-2, Cl2 and F2 evolution reactions are examined. In the case of the H-2 evolution reaction, both Ni and electrodeposited Ni(80%)-Mo(19%)-Cd(1%) were studied. The curve-fitting results show C(dl) to be ca 40-mu-F cm-2 at Ni wire and ca 90 mF cm-2 at the electrodeposited electrode. For the anodic Cl2 evolution at preoxidized Pt surfaces, the C(dl) values obtained by means of the fitting method are in good agreement with those from ac impedance and the conventional "whole decay curve" analysis. In the case of F2 evolution at carbon, problems of a different kind arise in evaluation of the interfacial capacitance due to its unusually low value of ca 1-mu-F cm-2, or less, which arises on account of the small dielectric capacitance associated with the barrier-layer film of "CF" species generated on F2-evolving C electrodes.