Flow-Engineered 3-D Printed Electrodes for Alkaline Water Electrolysis

Abstract

Initially, industrial alkaline electrolyzers had 2-D bipolar plates as electrodes in a configuration in which there is an important physical gap between the electrodes (e.g., 2 mm). This setup presented a high ohmic resistance and is commonly limited to a current density in the order of 0.5 A·cm-2 due to gas screening, i.e., the produced gases form a non-conductive layer over the entire electrode surface, limiting current density. The more advanced designs of today include the use of a zero-gap cell where the interelectrode distance is limited to the thickness of the separator (hundreds of micrometers for diaphragms and tens of micrometers for membranes). Commonly, 2-D electrodes such as perforated plates or an expanded metal mesh are used. As the electrode is pressed against the separator, the gases are forced to be evacuated at the backside of the electrode, avoiding the screening effect. As a consequence, much higher current densities can be achieved. More recently, there has been an increasing interest in the integration of 3-D electrodes in the zero-gap configuration, as they present higher surface areas than their counterpart 2-D electrodes. Nevertheless, bubbles can get trapped inside the intricate 3-D electrode structures. Several strategies are used to mitigate bubble entrapment, including the use of a forced electrolyte flow.The current study aims to investigate the possible electrochemical performance improvement of a single alkaline water electrolysis cell by using 3-D printed electrodes with graded structures, engineered to enhance bubble removal under the effect of a forced electrolyte flow. As the majority of the current is concentrated near the separator, the tailored 3-D printed electrodes present low strut distances and, consequently, a high surface area in this region. Further away from the separator, the distance between the struts gradually increases, decreasing flow-related energy dissipation and avoiding bubble entrapment. This tailored laterally graded structure promotes a difference in pressure along the direction perpendicular to the separator, allowing to extract bubbles from the region near the separator and to evacuate them towards the region with higher strut distances more efficiently. Furthermore, our flow cell was designed to promote a homogeneous upstream flow at the entrance of the electrode. Based on previous findings [1], a Schwarz triply periodic minimum surface was chosen as electrode geometry, as this geometry can channel the flow and reduce bubble-induced ohmic resistance. This geometry is defined by the following mathematical equation: t = sin(2·π·z/L)·sin(2·π·x/L) - 0.4·sin(2.4·π·y/L)·cos(2·π·z/L)·cos(2·π·x/L)with L the periodicity (e.g., an L = 1 mm means that the structure repeats itself in the space at each 1 mm), and t relates to void fraction.Different electrodes with a Schwarz geometry were analyzed in a combined experimental and simulation approach. Electrodes were 3-D printed using selective laser melting, and a strut in the order of 500 μm was achieved. Explicit computational fluid dynamics simulations were performed to estimate bubble residence time and path. Experimental analyses were conducted in a flow-through setup operating under industrial standard conditions of 30 wt% KOH electrolyte at 70°C to elucidate the performance disparities between these structures. Our findings indicate that such tailored 3-D printed electrodes are capable of significantly outperforming previously reported performances in the literature using more stochastic 3-D electrodes like foams. Furthermore, results show that the printed structures are more sensitive to electrolyte flow, presenting an important overvoltage reduction with increase in electrolyte flow rate. We also identified the most critical structural parameters controlling bubble evacuation, thereby significantly reducing the cell's ohmic resistance. Ultimately, this study also emphasizes the critical role of efficient gas evacuation for high-rate alkaline electrolysers using a zero-gap design.[1] Rocha F, Delmelle R, Georgiadis C, Proost J. Electrochemical Performance Enhancement of 3D Printed Electrodes Tailored for Enhanced Gas Evacuation during Alkaline Water Electrolysis. Advanced Energy Materials 2022;13:2203087.