Understanding the Effects of Power Ultrasound on the Hydrogen Evolution Reaction (HER) and the Oxygen Evolution Reaction (OER) on Polycrystalline Pt and Ni in Alkaline and Acidic Solutions

Abstract

The aim of this Ph.D. project was to investigate the effect of power ultrasound on the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) on polycrystalline nickel (Ni) and platinum (Pt) in alkaline and acidic electrolytes in order to understand the kinetics and mechanisms of HER and OER at these electrodes under ultrasonic conditions. The secondary objective was to elucidate the mechanism(s) for the observed decrease in cell voltages and the anodic and cathodic overpotentials in the presence of ultrasonication.

In the present dissertation, firstly, we investigated the effects of power ultrasound (26 kHz, up to 100% acoustic amplitude) on the HER on polycrystalline Pt disc electrode in 0.5 M H2SO4 by cyclic and linear sweep voltammetry at 25 oC. We also studied the formation of molecular hydrogen bubbles on a Pt wire in the absence and presence of power ultrasound using ultra-fast camera imaging. It was found that ultrasound significantly increases currents towards the HER i.e., a ~250% increase in current density was achieved at maximum ultrasonic power. The potential at a current density of ₋10 mA cm₋2 under silent conditions was found to be ₋46 mV and decreased to ₋27 mV at 100% acoustic amplitude i.e., a E shift of +20 mV, indicating the influence of ultrasound on improving the HER activity. A nearly 100% increase in the exchange current density (jo) and a 30% decrease in the Tafel slope (b) at maximum ultrasonic power, was observed in the low overpotential region, although in the high overpotential region, the Tafel slopes (b) were not significantly affected when compared to silent conditions.

Motivated by the fact that Ni and Ni-based materials have attracted great interest in the development of renewable energy technologies, including fuel cells and water electrolysers, and due to their good stability, catalytic activity, abundance, and lower cost when compared to the currently used Platinum Group Metals (PGM)-based materials, we designed a facile and scalable approach for activating the surface of metallic Ni for the HER in aqueous alkaline media through continuous and pulsed ultrasound (24 kHz, 44 W, 60% acoustic amplitude). Sonoactivated Ni showed a remarkably enhanced HER activity with a much lower overpotential at −10 mA cm−2 of −277 mV vs. RHE when compared to non-sonoactivated Ni. The outcome of our research offers a novel route for activating Ni-based materials by ultrasonic treatment to tune the chemical composition and electrocatalytic activity of the Ni surface for the electrochemical water splitting reaction.

Understanding the activity dependence of the KOH concentration (pH) of alkaline electrolytes is essential for designing durable and active HER catalysts. Motivated by this fact, the HER activity and kinetics of polycrystalline and nanostructured nickel-based catalysts were evaluated in various pH’s and KOH concentrations. The results for nanostructured NiMo catalyst indicate that both electrochemical active surface area and reaction order have a promoting region under various pH and KOH concentrations (0.01 M-1.0 M, pH=12-14) accompanied by better HER activity (a lower overpotential for achieving ₋10 mA cm-2) and Tafel slope decreases from around 180 mV dec-1 to 60 mV dec-1 in the same pH and KOH concentration range. The polycrystalline Ni displays different behaviour where a promoting (0.01 to 0.10 M, pH = 12-13), stabilizing (0.1 to 1.0 M, pH 13-14), and an inhibiting region (2.0 M, pH > 14) are present. However, Tafel slopes of around 120 mV dec-1 are obtained for polycrystalline Ni at all KOH concentrations. The HER characteristics are inhibited at 2.0 M KOH for both catalysts due to slower OH- transport kinetics. The results confirmed the importance of tuning catalyst-pH/KOH concentration for better HER activity and kinetics.

The development of cost-effective and active water-splitting electrocatalysts is an essential step towards the realisation of sustainable energy. Its success requires an intensive improvement in the kinetics of the anodic half-reaction of the OER, which determines the overall system efficiency to a large extent. Motivated by this, we developed a facile and one-route strategy to activate the surface of metallic nickel (Ni) for the OER in alkaline media by ultrasound (24 kHz, 44 W, 60% acoustic amplitude). Sonoactivated Ni showed enhanced OER activity with a much lower potential at +10 mA cm-2 of +1.594 V vs. RHE after 30 min ultrasonic treatment compared to +1.617 V vs. RHE before ultrasonication. In addition, lower charge transfer resistance of 11.1 Ω was observed for sonoactivated Ni as compared to 98.5 Ω for non-sonoactivated Ni.

Finally and for completeness of the Ph.D. project, the electrochemical kinetics and mechanism of Raney-Ni towards the HER and the OER under silent (no ultrasound) and ultrasonic (408 kHz) conditions have been investigated in 30 wt.- % aqueous KOH solution at different temperatures (T = 25, 40 and 60 °C). It was observed that there is a significant difference between the effect of ultrasonication on the HER and the OER. Ultrasonication significantly shifts the overpotential at ₋300 mA cm-2 (ƞ300) of HER by +34 mV at 25 °C due chiefly to the effective bubble removal while it does not influence the OER overpotential. It was also shown that the ultrasonic effect on the HER depends upon temperature, and ultrasonication does not play a remarkable role at high temperatures. Moreover, ultrasonication cannot overcompensate for the decreasing HER activity by lowering the temperature.