The electrochemical and electrocatalytic behaviour of glassy alloys.
Crosby, Christine Mary.
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The aim of this study was to investigate the electrochemical and electrocatalytic properties of a selection of glassy alloys for the hydrogen evolution reaction in base. The glassy alloy compositions tested included the known alloys Fe67Co18B14Si1, Co66Fe4Si16B12Mo2, Fe40Ni40B20 and Fe40Ni40P14B6 and an entirely new alloy Zr74Ti19Cu2Fe5. The electrochemical techniques employed were cyclic voltammetry and slow sweep polarisation. Electrochemical techniques were used in conjunction with the surface analysis techniques of scanning electron microscopy (SEM) and energy dispersive x-ray spectrometry (EDS) to gain insight into the morphology and chemical compositions of the electrode surfaces after various treatments. The aims included: 1) To obtain an understanding of the field of electrochemistry of glassy alloys. 2) To develop systems, techniques and procedures to enable the testing of a new alloy to be performed with confidence. 3) To this end, techniques were firstly developed and then compared with published data on the known alloys. Once the handling techniques were satisfactory the new and previously untested Zr74Ti19Cu2Fe5 glassy alloy was characterised, in particular its catalytic properties and its corrosion resistant properties were investigated. The physical properties of the Zr74Ti19Cu2Fe5 alloy are under investigation by another group in the School of Physics. My findings are presented here. The corrosion resistance of the alloys was determined in their as-polished state and after surface pretreatment from slow sweep anodic polarisation studies and cyclic voltammetry. Glassy Fe67Co18Bl4Si1 and Co66Fe4Si16B12Mo2 displayed the poorest corrosion resistance of the alloy compositions tested. The anodic polarisation curve of the Zr74Ti19Cu2Fe5 alloy produced no active region and displayed potentially excellent anticorrosive properties in the basic media which was attributed to highly passivating Zr oxide and Ti oxide surface films. The electrocatalytic activity of the glassy alloys for hydrogen evolution was evaluated in 1MKOH. Cathodic polarisation curves were used to construct Tafel plots from which the kinetic Tafel parameters, i0 and b, were calculated. The least corrosion resistant glassy alloy compositions, Fe67Co18B14Sil and Co66Fe4Si16B12Mo2, displayed the highest catalytic activity for hydrogen evolution in the as-polished state. The most corrosion resistant alloy, Zr74Ti19Cu2Fe5, showed the poorest catalytic activity for the reaction in the as-polished state and only a slight improvement was obtained by increasing the electrolyte temperature in comparison to the other alloys tested. This was again attributed to passivating Zr oxide and Ti oxide surface layers that inhibited the HER. It was found that the Zr-based alloy displayed no substantial advantages over the other glassy alloys or more expensive noble metal surfaces in basic media, unless pre-treated as described in this thesis. The influence of ex situ chemical pretreatment on the electrocataytic activity of the glassy alloys for the HER was determined using pure HF and HF/HNo3 mixtures. Acid pretreatment of glassy C066Fe4SiI6B12M02 and Fe40Ni40Pl4B6 with IM HF/lM HN03 (10 minutes) and Zr74Til9Cu2Fe5 with 1M HF (10 seconds) resulted in a significant improvement in the activity of the alloys in comparison to their as-polished state. SEM/EDS analysis indicated that preferential dissolution of a P-enriched surface region on the Fe40Ni40P14B6 electrode created a porous structure with a greatly enlarged surface area at which the HER could occur. In comparison, the P-free, Fe40Ni40B20, composition displayed a much lower improvement in activity after acid pretreatment with only slight surface roughening observed. The Zr component of glassy Zr74Ti19Cu2Fes was selectively leached by acid pretreatment to produce a porous surface, however, the corrosion resistance of the alloy was also reduced, as indicated from anodic polarisation curve that showed an active and passive region of greater current density than the as-polished electrode. Hence the beneficial effect of acid pretreatment in activating the alloy surface for the HER was countered by a reduction in the general corrosion resistance of the alloy. In view of the dramatic effect on the HER shown by prior ex situ (acidic) oxidation of the glassy alloy surface, the influence of in situ (anodic) oxidation in the basic medium was investigated for comparison. For all the glassy alloy compositions tested, anodic activation was found to be less effective than acidic activation. Anodic pretreatment of glassy Zr74Ti19Cu2Fe5 (3000µA.cm-2) resulted in the greatest improvement in activity in comparison to the as-polished state out of the alloy compositions tested. In addition, the corrosion resistance of the alloy was not reduced by anodic pretreatment and consequently formed a less destructive activation procedure than acidic pretreatment. In this regard, anodic pretreatment would produce a more durable electrocatalyst and is the preferred technique for activating the glassy alloy surfaces for the HER. Initial characterisation of the surface deposits formed by anodic oxidation, using SEM and EDS techniques, indicates that the composition of these deposits and the mechanism by which anodic activation activates the glassy alloy surfaces requires further investigation.