Impedance analysis of hydrogen adsorption on palladium in 0.1 M NaOH solution

The behavior of hydrogen adsorption on palladium has been investigated by a.c. impedance measurements in 0.1 M NaOH solution of pH 13 at 298 K. The electrode impedance has been measured by superimposing an a.c. voltage of 5 mV amplitude ranging between 10-1 and 104 Hz on a d.c. potential range from -0.76 to 0.44 V (standard hydrogen electrode (SHE)). The equivalent circuit representing the hydrogen adsorption reaction is proposed from an analysis of the frequency dependence of the electrode admittance. The capacitance, arising from the adsorbed hydrogen, has been determined as a function of the applied potential by a complex nonlinear least-squares curve fitting method based on the proposed equivalent circuit. As the cathodic applied potential increased from -0.36 to -0.76 V (SHE), the adsorption capacitance considerably increased owing to the enhanced coverage of hydrogen adsorbed on the palladium surface. In the applied potential range from -0.36 to 0.24 V (SHE), the adsorption capacitance exhibited a minimum value, characterizing a double-layer region.