| dc.description.abstract | The Ni-Mo electrocatalyst has been recognised as one of the most promising earth-abundant materials for the alkaline hydrogen evolution reaction. In recent research it was found that during electrolysis of water, molybdenum migrates to the surface of the cathode and leaches out into the electrolyte solution as molybdate. Furthermore, the cation used in the electrolyte influenced the strength of the leaching.
In this thesis, the influence of different cations and the hydroxide concentration on the leaching of molybdenum was researched in depth. By varying the pH between 8 and 14.8 and using lithium, sodium and potassium as cation, the effects on Ni-Mo surface change were studied systematically. The Ni-Mo electrocatalysts were synthesized using electrodeposition and were employed for 3-day electro-catalysis at a current density of -10 mA/cm2. The surface displayed changes in roughness and composition, when studied before and after catalysis. This surface change was mainly studied with SEM-EDX, capacitance measurements, ICP-AES and liquid AFM. Transient chrono-potentiometry, linear sweep voltammetry and GC were used to study the efficiency changes when influenced by a change in electrolyte solution.
The hydroxide concentration is the main incentive that pushes the molybdenum equilibrium towards molybdate, overcoming the reductive potential around the electrode. It was noticed with EDX and capacitance measurements that when using potassium as cation the amount of molybdenum that leached from the electrode doubled, most likely due to a solubility effect.
The activation time observed for the catalyst is caused by the increase in surface area due to leaching of molybdenum as was seen with liquid AFM. Further proof was obtained when the faradaic efficiency was calculated, where efficiency of up to 115% was obtained. This efficiency over 100% was explained to be due to extra hydrogen production from the molybdenum oxidation.
The high surface area due to the leaching of molybdenum appears to be the most important facet for its low overpotentials. Molybdenum is necessary in certain amounts to circumvent the formation of nickel hydrides.
Increasing the current density to -500 mA/cm2 showed far lower molybdenum loss in 1M KOH. SEM displayed a restorative effect on the Ni-Mo alloy, restoring cracks that were there before catalysis. | |
