Hydrogen evolution on porous Ni cathodes modified by spontaneous deposition of Ru or Ir

Porous Ni deposits, prepared by cathodic deposition, were modified by immersing them in acid deaerated solutions containing Ru(III) or Ir(IV) chloride complexes with which they readily reacted, without any activation procedure, giving rise to spontaneous deposition of either Ru or Ir. The obtained electrodes were investigated by cyclic voltammetry, impedance spectroscopy and scanning electron microscopy. All data showed that the initial large area of the Ni deposits further increased upon immersion in solutions of noble metal complexes. EDX analyses proved that the deposition of Ru reached a limiting situation in some hours, while that of Ir was slower and continued for a longer time. The persistence of intense peaks due to the Ni(II)/Ni(III) redox system showed that Ru and Ir did not form a continuous layer able to prevent the contact between Ni and electrolyte. Hydrogen evolution was studied in 1 M NaOH solutions. Spontaneous deposition of both noble metals markedly improved the performance of porous Ni. The best results were achieved with Ir-modified electrodes, after immersion in Ir(IV) solution for 6 h. Tafel slopes and overpotentials of Ru-modified electrodes were not as low as those of Ir-modified electrodes.