In-situ activated hydrogen evolution from pH-neutral electrolytes
Sammanfattning: The goal of this work was to better understand how molybdate and trivalent cations can be used as additives to pH neutral electrolytes to activate the Hydrogen Evolution Reaction (HER). Special emphasis was laid on the chlorate process and therefore also to some of the other effects that the additives may have in that particular process.Cathode films formed from the molybdate and trivalent cations have been investigated with electrochemical and surface analytical methods such as polarization curves, potential sweep, Electrochemical Impedance Spectroscopy (EIS), current efficiency measurements, Scanning Electron Microscope (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), X-Ray Fluorescence (XRF) and Inductively Coupled Plasma (ICP) analysis.Trivalent cations and molybdate both activate the HER, although in different ways. Ligand water bound to the trivalent cations replaces water as reactant in the HER. Since the ligand water has a lower pKa than free water, it is more easily electrochemically deprotonated than free water and thus catalyzes the HER. Sodium molybdate, on the other hand, is electrochemically reduced on the cathode and form films which catalyze the HER (on cathode materials with poor activity for HER). Molybdate forms films of molybdenum oxides on the electrode surface, while trivalent cation additions form hydroxide films. There is a risk for both types of films that their ohmic resistance increases and the activity of the HER decreases during their growth. Lab-scale experiments show that for films formed from molybdate, these negative effects become less pronounced when the molybdate concentration is reduced.Both types of films can also increase the selectivity of the HER by hindering unwanted side reactions, but none of them as efficiently as the toxic additive Cr(VI) used today in the chlorate process. Trivalent cations are not soluble in chlorate electrolyte and thus not suitable for the chlorate process, whereas molybdate, over a wide pH range, can activate the HER on catalytically poor cathode materials such as titanium.
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