Bubble and Dissolved Gas Distribution in Flow through Membraneless Water Electrolysis

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Membraneless water electrolysis devices offer an alternative method for hydrogen production without the use of a membrane separator used in traditional alkaline (porous diaphragm) or PEM water electrolysis (polymer electrolyte membrane). This could offer lower capital cost devices (only requiring two electrodes and a cell body) with simple and scalable manufacturing. It could also result in lower operating costs if highly conductive liquid electrolytes are used. Membraneless devices come in a variety of forms including parallel plate, diverging flow through and buoyancy driven devices to name a few. However, the key concept between them is using fluid forces in the bulk electrolyte (controlled by device or porous electrode structure) to remove bubbles from the electrode surfaces and attempt to reduce cross-over of dissolved hydrogen and oxygen, as well as their two-phase flow [1]. Since there is no separator or membrane to restrict transport between the anode and cathode the fluid convection with laminar flows must be used to guide the products to the separate outlets with minimal cross-over and to minimise bubble accumulation on the electrode surfaces. Bubbles on the electrodes increase the activation overpotential by reducing the active electrochemical surface area and decrease the effective conductivity of the flowing electrolyte by inducing tortuous pathways in the electrode gap. In this study, we look at the single-phase and two-phase flow response of flow through membraneless electrolyser by coupling secondary current distribution (which includes the effects of ionic and activation overpotentials) with the volume of fluid method in OpenFOAM. The effects of increasing the length of the device and the electrolyte flow rate on the product cross-over and the dynamic potential of the devices are evaluated to give guidelines for process scale up from 1 to 10 cm. Furthermore, the effect of device porous electrode microstructure on the current density distribution and bubble accumulation is evaluated using custom microstructure reconstruction algorithms built into PMG 1.6 [2]. It was found that there is a balance in flow rate required for uniform flow over the electrodes which depends on the device length and the porous electrode properties. Ordered, graded or unstructured electrodes have different electrochemical performance, and this study highlights some electrode designs that could improve if future manufacturing efforts are increased. Depending on the current density distribution, dissolved gas accumulation and the wettability conditions of the porous electrodes, bubbles may emerge in the electrode gap.Figure 1 - Effect of the hydrogen and oxygen bubble accumulation (yellow) on the ionic current density distribution in a flow through membraneless water electrolyser