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.