Hydrogen and Nitrogen Evolution Reactions in Acid Liquid Ammonia

Abstract

The hydrogen and nitrogen evolution reactions were investigated on various electrode surfaces in acid (NH4NO~) liquid ammonia solutions at temperatures ranging from 20 ~ to --60~ Tafel slopes, transfer coefficients, and exchange currents were determined from the potential-current measurements. The relative rates of the hydrogen evolution reaction on various surfaces in liquid ammonia is in the order Pt, Ir >> Ni2B, Au > Mo, Cr, W >> Ta, similar to the relative reactivities found in aqueous solutions. The Tafel slopes for the cathodic reaction are also similar to Tafel slopes determined in aqueous solutions at comparable temperatures. However, the apparent cathodic transfer coefficients tend to decrease significantly at low temperatures. On platinum surfaces, the slow step appears to change from the chemical reccmbination of adsorbed hydrogen atoms to an electrochemical process as the temperature decreases. Studies of the nitrogen evolution reaction are complicated by possible interfering anodic reactions. However, the results indicate that there is an extremely high overvoltage associated with the nitrogen evolution reaction, and that electrochemical process rather than chemical recombination steps control the reaction rate. * Electrochemical Society Active Member.