Int. J. Hydrogen Energy, Vol. 15, No. 11, pp. 783 788, 1990. Printed in Great Britain. 0360-3199/90 $3.00 + 0.00 Pergamon Press plc. :~, 1990 International Association for Hydrogen Energy. ELECTRODE KINETIC PARAMETERS FOR THE HYDROGEN EVOLUTION REACTION ON Pd IN ACID MEDIUM: INFLUENCE OF THE ELECTRODE ACTIVATION 0 . SAVADOGO,* K. AMUZGAR and D. L. PIRON D6partement de g6nie m6tallurgique, l~cole Polytechnique de Montr6al, C.P. 6079, succ. A. Montrbal, Quebec H3C 3A7, Canada (Received for publication 1 May 1990) Abstract--The hydrogen evolution reaction (h.e.r.) on the activated Pd electrode was studied. It was observed that its electrocatalytic performance changed with the derivatization time. The lowest overvoltage for the h.e.r, is obtained on Pd electromodified during 90 min. The sensitivity of the electrodes to impurity effects during the h.e.r, decreases on modified electrodes. The exchange current density of Pd decreases remarkably when Hg 2+, Cd 2+ or Pb 2+ are present in the electrolysis electrolyte. Accordingly, Pd is very sensitive to these impurities. In contrast, modified Pd surfaces with SiW~20~ 0 (Pd90SiW~2) leads to better resistance to the impurities effect. The h.e.r, mechanism on Pd and Pd90SiWI2 was studied between 298 and 358 K. The influence of the electrode surface activation on the kinetic parameters such as exchange current density and heat of activation of the constituent steps of the h.e.r, was deduced. The effect of the electrode surface activation on the variation of the overvoltage t/ and the Tafel slope with the temperature was also studied. It was shown that the temperature-dependence of r/and the temperature-independence of b are confirmed experimentally if the electrochemical barrier symmetry factor (~) is taken as the temperature- independence. The activation of the electrode surface has no effect on this behaviour. INTRODUCTION The fuel cell electrodes for aqueous electrolytes contain finely dispersed platinum or palladium on a carbon support [1-3]. Until the present time, these electrodes used as an electrocatalyst are the most expensive materials in the fabrication of components for aqueous electrolyte fuel cells. It has been shown [4] that, apart from cost considerations, but also from the availability standpoint of these noble metals resource, it is necessary to minimize their use in fuel cell power plants for automobiles [5] and other terrestrial applications [6]. On the other hand, it is necessary to increase the power density (doubling it) if we want to lower the capital cost of the fuel cell power plant [4]. Furthermore, research on active materials can allow a significant increase in the performance of solid polymer electrolyte fuel cells in the activation, ohmic and mass transport-controlled regions. In order to improve the electrocatalytic properties of the h.e.r, on electrodes in acid medium, it has been demon- strated that metallic and semiconductor materials can be persistently modified and activated for this reaction using heteropolyanions (HPA) [7-14]. It was found that the surface modifications of different electrode materials resulted in an increase of the exchange current density and a decrease of the overvoltage for the h.e.r. Some *To whom correspondence should be addressed. detailed studies [15-19] have shown that the slope, b (b = dq/dlni) in the Tafel or Butler-Volmer equations for the h.e.r, process from various proton donors and in various solvents, which is conventionally represented by the classical relationship b = 2.3RT/otF ( 1) is not confirmed experimentally if ~ is taken as tem- perature-independent electrochemical barrier symmetry factor in electrode kinetics. The value e characterizes the electrochemical linear energy relation represented by the Butler-Volmer or Tafel equations [20, 21]. The accumulated experimental results [15-19] show that the observed experimental behaviour is not usually rep- resented by equation (1) and that ~ contains apparently a temperature dependent term. Thus, the lack of tem- perature-dependence of b can be attributed to a poten- tial-dependence of the entropy of activation and the energy of activation which finally results in an interesting variation of e with temperature [21]. The purpose of this communication is to present the results of a study on the optimization of the electrocatalytic properties of Pd electrodes with the derivatization time. The influence of the impurities on the electrocatalytic properties of the electrodes was analysed. The influence of the electrode activation on the temperature-dependence of b and 7 and on the heat of activation of the steps of the h.e.r, was studied. 783