PROBLEM OF INSITU REAL-AREA DETERMINATION IN EVALUATION OF PERFORMANCE OF ROUGH OR POROUS, GAS-EVOLVING ELECTROCATALYSTS .2. UNFOLDING OF THE ELECTROCHEMICALLY ACCESSIBLE SURFACE OF ROUGH OR POROUS-ELECTRODES - A CASE-STUDY WITH AN ELECTRODEPOSITED POROUS PT ELECTRODE

The problem of evaluation of the electrochemically effective area of porous electrocatalyst materials is treated by examination of the kinetics of the hydrogen evolution reaction (HER) from aqueous KOH at a plated porous Pt electrode in relation to in situ real-area determination by means of scanning electron microscopy (SEM), cyclic voltammetry, potential relaxation and impedance measurements. With regard to its electrochemical accessibility, the rough or shallow-pore electrode surface can be 'unfolded' experimentally by increasing the electrolyte conductivity, i.e. simplifying the transmission-line model by diminishing the distributed electrolyte resistance. It was found in the impedance studies that, for the same porous plated Pt electrode, the constant phase element exponent, alpha, associated with double-layer capacitance varies from 0.56 to 1.0 for the HER in 0.5 and 2.0 mol dm-3 KOH solutions, respectively. That is, alpha is not an intrinsic property of a given porous electrode, but depends strongly on the conductivity of electrolyte solution in the pores.