PEM-like alkaline water electrolysis using flow-engineered 3-D electrodes

As shown in the Figure below, the alkaline water electrolysis performance obtained in this work significantly outperforms the ones reported in the recent literature for Ni-based electrodes separated by a Zirfon diaphragm at typical operating conditions (70-80°C, 30wt% KOH). Reasons for the performance enhancement include plasma spray coating, forced upstream electrolyte flow and the use of 3-D zerogap electrodes in bi-layer configuration flow-engineered for enhanced gas evacuation. Firstly, plasma spraying caused a reduction up to 18% in cell voltage at a current density of 2 A·cm-2. This is partly related to a significant increase in surface area. The plasma coating is done with a powder containing Ni and Al. The Al is then leached out in order to obtain a Raney Ni alloy. What remains is a highly porous and rough surface with a higher surface area: 205 cm2·cm-3 compared to 119 cm2·cm-3 before coating, according to X-ray tomography analysis. The second factor is related to the presence of Mo in the powder used to prepare the cathode. Like Al, Mo is leached out, causing an increase in surface area. Moreover, a reduction in Tafel slope was observed when Mo was present, from 168-190 mV·dec-1 to 44-55 mV·dec-1, indicating a significant electro-catalytic effect. Secondly, subtracting out the Zirfon separator resistance, an increase in electrolyte superficial velocity from 0.05 m·s-1 to 0.22 m·s-1 caused a decrease in ohmic resistance of 12-14%. This is a direct result of an improved bubble removal efficiency. The presence of gas bubbles at the electrode surface is indeed known to decrease the active surface area, while their presence within the electrolyte also increases the electrolyte resistance. Finally, with the help of computational fluid dynamics simulations, a bi-layer electrode configuration was designed to enhance gas removal. It consists of two layers. The first one, a thin layer (1.6 mm) of a Ni foam with a relatively low pore size (450 µm), acts as the gas production layer. The second one is a thick (4 mm) plasma coated Ni foam with a relatively high characteristic pore size (3000 µm), acting as a gas extraction layer. As a result of enhanced gas evacuation under forced upstream electrolyte flow, the ohmic resistance when using this bi-layer configuration was as low as 0.13-0.15 Ω·cm2.