Journal of Electroanalytical Chemistry 460 (1999) 30 – 37 Hydrogen electrosorption in Ni – Pd alloys Michal  Grden ´ a , Andrzej Czerwin ´ ski a, *, Jerzy Golimowski a , Ewa Bulska a , Beata Krasnode ˛bska-Ostre ˛ga a , Roberto Marassi b , Silvia Zamponi b a Department of Chemistry, The Uniersity of Warsaw, Pasteura 1, 02 -093 Warsaw, Poland b Department of Chemistry, Uniersity of Camerino, S. Agostino 1, 62032 Camerino (MC), Italy Received 26 January 1998; received in revised form 17 August 1998 Abstract Hydrogen absorption in Ni – Pd alloys has been investigated. The amount of absorbed hydrogen in alloys containing below 20 at.% of nickel is equal to the amount of hydrogen sorbed in pure palladium. Hydrogen absorption occurs in the range 0–40 at.% of nickel concentration. Cyclic voltammograms recorded at Ni – Pd alloys have characteristic peaks which overlap with the responses due to processes occurring on the surface at Ni and Pd atoms. Also some of the processes characteristic of the pure metals can be distinguished from the recorded voltammograms. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Hydrogen absorption in Ni – Pd electrodes; Nickel; Nickel – palladium alloys; Palladium 1. Introduction Nickel and palladium form a series of alloys with face-centred cubic lattices. The crystal structures of palladium and nickel are the same with similar crystal dimensions a Pd =3.89 and a Ni =3.52 A ˚ (25°C) [1]. In palladium and nickel the number of holes per atom in the d-band is the same at 0.6 [2] and it is expected that the number of holes per atom in the d-band remains constant at 0.6 over the entire range of nickel-palla- dium alloys [3]. The hydrogen is sorbed in both metals as dissolved H ( -phase, H/Me 0.03), and as the metal hydride, ( -phase, H/Me 0.7). The main differ- ence between Pd and Ni is that the solubility of hydro- gen in nickel under normal conditions is much lower [4 – 6]. Under normal conditions, the amount of electro- sorbed hydrogen in palladium generally does not de- pend on solution composition or the way the electrode has been prepared and the maximum H/Pd ratio in basic and acidic solution is about 0.75 0.1 [7–9]. The volume of H electrosorbed in a nickel electrode depends greatly on the process and the kind of prepara- tion of the Ni electrode, i.e. the behaviour of Raney nickel is different from that of solid metal as wire or foil. The presence of surface poisons influenced the hydrogen absorption in nickel. This process was studied by Baranowski and S ´ mial  owski [10–13] in thin layers of nickel in the presence and absence of thiourea and arsenic. Also Soares et al. [14,15] reported the influence of surface poisons on hydrogen electrosorption in Ni cathodes. For Pd–Ni alloys almost a linear decrease of electro- sorption properties with increase of nickel concentra- tion in the alloy was found both for basic [16] and acidic [17] solutions. At an atom ratio Ni/Pd close to 0.3 (30%), the hydrogen concentration value is almost equal to zero. Hoare and Schuldiner [17] found a correlation between hydrogen solubility in nickel – pal- ladium alloys with catalytic activity for the hydrogen (H 2 ) producing reaction as a function of Ni concentra- tion. Barton et al. [18] found that even at 10 at.% of nickel concentration in alloys with palladium, the H/Pd ratio is still above 0.6 and then drastically diminishes to almost zero at nickel concentrations above 17 at.%. * Corresponding author. E-mail: aczerw@chem.uw.edu.pl 0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S0022-0728(98)00330-1