Abstract
The electrochemical and corrosion behaviour of Co50Ni23Ga27-xAlx (x = 0 and 1.0 wt%) magnetic shape memory alloys (MSMAs) was studied in 0.5 M NaCl solutions using various electrochemical techniques. Results showed remarkable activation of the tested MSMA toward pitting corrosion upon alloying it with Al. XPS examination confirmed the activation influence of alloyed Al. It proved that the presence of Al in the alloy's matrix weakens its passivity, as manifested by a lower amount of gallium oxides and Cl- adsorption in the aluminium containing MSMA sample. Alloyed Al also activated significantly the tested MSMA for the hydrogen evolution reaction (HER), as indicated by cathodic polarization, electrochemical impedance spectroscopy (EIS), and faradaic efficiency (FE) measurements. Such measurements were performed in 0.1 M KOH solutions and showed that the Co50Ni23Ga26Al1.0 alloy is much more active for the HER than Co, Ni, Co50Ni50, and Co50Ni23Ga27 electrodes. The catalytic impact of pitting corrosion, proved to be catalyzed by Al, on the HER activity of the CoNiGaAl alloy was also studied. The pitted Co50Ni23Ga26Al1.0 MSMA, the best catalyst here, exhibited high HER catalytic performance with an exchange current density (j(o)) of 0.2 mA cm(-2) and FE 96%, and thus approached Pt/C (j(o) = 0.6 mA cm(-2) and FE similar to 100%). Our best catalyst also showed good stability and durability after 3000 cycles of cathodic polarization between the corrosion potential (E-corr) and -1.0 V vs. RHE, and 24 h of electrolysis at a high cathodic current density of 100 mA cm(-2). Microstructure changes made by Al, together with findings obtained from SEM/EDX mapping and XPS studies, were used to interpret its activation influence towards the HER.