Experimental Study of the Gravity-Fed Laminar Electrolyser

Alkaline water electrolysis is praised as one of the most promising technologies for sustainable hydrogen production. Still, gas crossovers at low current densities limit its operating range, especially when paired with intermittent renewable energy sources. This study investigates the Gravity-Fed Laminar Electrolyser concept, designed to separate gas products and electrolyte flow at the production site and reduce gas crossovers below the industrial safety limit of 2% at low current densities, addressing the limitations of traditional zero-gap electrolysers.

In-situ experiments were conducted using a custom-built prototype, focusing on performance and crossover concentration at low current densities (0.13 A/cm², 0.09 A/cm², 0.06 A/cm² and 0.03 A/cm²). The study tested various electrode-membrane combinations, including nickel meshes and felts paired with Zirfon 220, Zirfon 500 and ePTFE, and three channel widths (1 mm, 0.7 mm, and 0.5 mm). Hydrogen in oxygen (HTO) concentrations were measured using a gas chromatograph. The volumetric flow rate of the electrolyte (30 w% KOH) was measured using a flow metre at the cell outlet. The electrolyte influx temperature was simultaneously controlled using a magnetic hot plate stirrer and a thermocouple.

The experiments revealed that low Reynolds numbers (< 1300) that arise from decreasing the channel widths create a laminar flow regime that reduces hydrogen advection to the electrolyte bulk. However, stagnant flow regions may develop in the vicinity of the inert channel boundaries, where diffusion dominates and leads to the increase of crossover rates at low current densities. In terms of anodic hydrogen concentration (HTO), Zirfon assemblies exhibited similar trends: an increasing anodic hydrogen concentration with declining current densities results in a peculiar drop at the ultimate measurement point of 0.03 A/cm². Concerning ePTFE configurations, the crossover rate showed an unmistakable correlation with the current density.

During the iterative process of optimising performance, numerous findings were reported. Among the tested electrode setups, nickel mesh exhibited superior performance compared to nickel felt, attributed to its open structure and ability to conform to the Zirfon layer, thereby enhancing contact and facilitating hydrogen release. Substituting the anode with a stainless steel mesh improved performance in the ePTFE configuration, while, for their Zirfon counterpart, an adverse effect was observable. Furthermore, hot pressing showed a positive impact, allowing effective integration of meshes with Zirfon 500 and ePTFE materials. Increasing the circulated electrolyte temperature improved catalytic activity, reduced ohmic resistances, and increased the current density starting at a lower onset voltage.

Based on the insights gained in this study, the Gravity-Fed Laminar Electrolyser has a vast potential to improve its performance further and proves to be a viable concept for operating within safety limits at low current densities.