Experimental

In this work, a 3D-printed electrolyzer was used. The structural parts of the 3D-printed electrolyzer were printed both in-house and externally. In the former case, a Makergear M3-ID or a Prusa MK3S was used. In both printers, PETG was extruded during the fused filament fabrication process. In the latter case parts were ordered from the 3D printing company ZiggZagg, who used the multi jet fusion of Nylon. The gaskets were printed in-house using TPU filament Ninjatek-Ninjaflex and the Makergear M3-ID. The electrolyzer is shown in figure 3 and consisted of exchangeable inlets and electrode assemblies. In table 2 the characteristic dimensions of the electrolyzer are shown. The electrode assembly was built by affixing a 100x50mm nickel plate to a printed substrate using epoxy resin. The electrical connection was provided by two wires soldered to the backside of the nickel plate on one end, and 2 banana plugs on the other end.
[FIGURE 3]
[TABLE 2]
[FIGURE 4]
For the 3D-printed electrolyzer, three different inlets were used in this work: a tube inlet, a conic inlet, and a divider inlet (see figure 4). Additionally, it was possible to add a calming section with printed extender pieces that were either 100 mm, 150 mm or 200 mm long, thereby increasing the hydrodynamic entrance length. These extender pieces contained an empty rectangular channel with the same cross-section as the part containing the electrodes. Measurements with longer calming sections of 300 mm and 550 mm were carried out by using multiple extender pieces.
Turbulence promoters could be inserted into the channel of the electrolyzers. Four different designs were tested: tube grid, large gyroid, medium gyroid and small gyroid (figure 5). The designs were printed in a small form (100x40 mm) built to fit the non-extended electrolyzer and a longer 250x40 mm form meant to fit a channel extended by 150 mm (figure 6).
[FIGURE 5]
[FIGURE 6]
The setup consisted of an electrolyte vessel connected via a gear pump to the printed electrolyzer and is schematically shown in figure 7. A variable area flow meter (Swagelok) was used to measure the flow rate. The pressure drop was measured by means of an 800 mm tall U-tube filled with water. The electrolyte vessel was continuously flushed with nitrogen. A water lock was used to prevent backmixing of atmospheric oxygen. The concentration of the oxidant (hexachloroiridate(IV)) was tracked throughout the experiment using an inline UV-VIS flow cell (Avantes).
[FIGURE 7]
The hexachloroiridate(IV)/hexachloroiridate(III) redox couple was used for the limiting current density measurements. The electrolyte consisted of 0.5 mM potassium hexachloroiridate(IV) (>99.99% from Merck/Sigma Aldrich) and 1.0 mM hexachloroiridate(III) (>99.9% from Merck/Sigma Aldrich) in a solution of 0.5 M KNO3 (>99% from VWR Chemicals), containing 0.1 M of potassium acetate pH 4 buffer (made from >99% potassium acetate and >99% acetic acid from VWR Chemicals). More background on the performance of the hexachloroiridate redox couple can be found in previous work [16]. A list of the electrolyte properties is available in table 3.
[TABLE 3]
Before the experiments, the electrodes were pretreated following the recommendations of Szanto et al. [9] The procedure consisted of polishing the electrodes using felt paper and a descending series of alumina particles (namely 1.0 μm, 0.3 μm and 0.05 μm), followed by two times 15 minutes sonication and 15 minutes of hydrogen evolution in 0.5 M KOH solution. The setup was then thoroughly rinsed with a solution containing 0.1 M pH 4 acetate buffer and 0.5 M KNO3. During this time, a baseline measurement of the UV-VIS probe was taken. After this, the setup was drained, flushed with nitrogen and the hexachloroiridate electrolyte was added. For another 15 minutes, nitrogen was bubbled through the electrolyte vessel to remove dissolved oxygen from the solution.
Chronoamperometry experiments were performed in order to determine the limiting current density. The procedure was as follows: First, the gear pump was set in motion to produce the desired flow rate. Then, a cell potential of -0.8 V was applied and a waiting time of at least 7 seconds was implemented in order to reach a steady state situation. The limiting current was determined from the average of 30 data points measured over a period of 3 seconds after the current had stabilized.
In our work we record all current data after 7 seconds. Since for low flow rates (see figure 2) the current has not always completely stabilized after 7 seconds, the Sherwood numbers obtained at lower flow rates (and hence lower Reynolds numbers) may be overestimated slightly in our work.