Fe-Ni-Mo Electrodeposits to Cathodes for Hydrogen Evolution
Authors: Mascaro Lucia Helena 
Published: February 2020

Several methods have been used to prepare metal films, but the electrodeposition techniques is probably the most economic and was the first method used for alloy films. Alloys of group iron combined to Mo or W have shown significant promise for application in the contexts of catalysts, batteries and electrochromics devices. The mechanism of codeposition of these alloys is not yet completely elucidated and some papers has mentioned the possible multi-step reduction of some Mo chemical species. In this present work Fe-Ni-Mo electrodeposition process was studied at different experimental conditions by cyclic voltammetry for obtain information about the induced codeposition mechanism, The coatings were characterized by the X-ray photoelectron spectroscopy (XPS) and scanning electronic microscopy (SEM). The electrocatalysis properties to hydrogen evolution rection (HER) was evaluated.
Electrochemical experiments were performed in a conventional three-electrode cell controlled by potentiostat/galvanostat, PAR 273 under microcomputer command. Platinum disc (0.196 cm2 ) was used as working electrodes (WE), the reference electrode was always a saturated calomel electrode (SCE) and counter electrode (CE) was a large surface platinum spiral wire. Chemicals reagents: FeSO4·6H2O, NiSO4·7H2 O and Na3C6H5O7·2H2O, were all of analytical grade purity. The Fe:Ni:Mo electrolyte composition was 1:10:1 and pH was adjusted to 5.0 with citric acid solution. The Ni-Fe-Mo cathodes were prepared potentiostatically (at –0.7 V, -1.2V and –1.5 V) or galvanostatically (at 30 mA cm-2) during 30 min or 60 min, using steel as substrate. The surface morphology was analyzed by scanning electron microscope (SEM) and quantitative chemical analyses were performed by X-ray photoelectron spectroscopy (XPS) experiments.
From cyclic voltammograms results it was possible to observe the presence of two cathodic peaks in -0,7 and -1,2 V. In the anodic region, from -1,5 V is only observed two peaks of dissolution, confirming that the formation of the alloys can occurs in more negative potentials. By the XPS analysis it was possible to detect the presence of Ni (0), Fe (0) and Fe (II) in all evaluated deposition conditions. However, the presence of Mo (V) in -0.7 V and Mo (IV) were observed in -1,2 and -1,5 V. From these results it was possible to propose that deposition of different Mo specieswas induced by the presence of Fe and Ni. The film composition determined by XPS was about 9% Fe, 8% Ni, 20% Mo and 63% Of O. That is, almost 50% of the coating is formed by oxides. The morphological analysis presented a globular structure for this material, independent of the deposition techiniques used. In order to determine the electrocatalytic efficiency of Fe-Ni-Mo alloys in HER, the Tafel parameters in 6.0 mol L-1 KOH solution were determined. The deposits presented a good performance with Tafel inclination values of 90.1 mV and 58.9 for potentiostatic deposition at -1.5 V and galvanostatic at 30 mA cm-2, respectively. The overpotential values at 135 mA cm-2 was -84.5 mV and in electrolysis the overpotential remains stable for more than 200 hours. These results indicate that Ni-Fe-Mo alloys obtained by electrodeposition may perform well for RDH.