(404i) Demystifying Mass Transfer in Electrolyzers through 3D Printed Parallel Plate Cells: The Importance of Inlet Effects

As a result of the global energy transition, a
large percentage of the world’s power supply will become electric. For
electrochemical engineers this transition generates a sea of opportunities.
After all, it is electrolyzers that use electricity to produce some of life’s
essential chemicals. However, fully embracing these opportunities in a
sustainable way is challenging: Future electrolyzers must be safer, cleaner,
and far more flexible, while simultaneously increasing productivity and
cost-effectiveness. Essentially, they will have to achieve more with less.

One key parameter in the performance of
electrolyzers is the rate of mass transfer. Most often, electrochemical
reactions are mass transfer limited due to the fast kinetics that depend
exponentially on the applied overpotential. Numerous correlations in literature
exist to describe the rate of mass transfer in a specific reactor.^[1]
Though, their predictions can vary up to almost an order of magnitude (see figure
1). The reason for this is the large geometric variety between the reactors
that were measured. The design of the inlets in particular is a crucial
contributor to the rate of mass transfer. ^[2] Moreover, only a few
lab-scale electrolyzers are commercially available, often with sub-optimal
designs, which limits further research. As a consequence, it is difficult to establish
a generalized description of the mass transfer performance of parallel plate electrolyzers.
Furthermore, most existing correlations only describe the average rates of mass
transfer and therefore do not consider any local variation despite the
importance thereof.^[3]

Figure 1: Mass transfer performance in terms of
Sherwood vs. Reynolds. Up to an order of magnitude of difference in performance
is visible between reactors. The red triangle, square and circle markers denote
a selection of inlet manifolds tested with the 3D printed prototype. Small insert:
sketches of the inlet manifolds, from left to right: tube inlet, conic inlet,
divider-type inlet.

The goal of our study has been to demystify the
fundamentals of mass transfer in parallel plate electrolyzers. As this would have
required testing a large geometric variety of cells, traditional manufacturing
techniques were impractical due to their lead time and cost. Therefore, we have
used 3D printing to rapidly construct many different reactor prototypes.
Moreover, each prototype could be finetuned to the exact specifications each measurement
required. This way, each geometric parameter could be varied and investigated
individually.

Figure 2: 3D printed electrolyzer prototype for
mass transfer studies. The electrode area is 40 cm².

Nearly the entire reactor can be 3D printed in less
than a workweek’s time (see figure 2). Inlet manifolds, the channel and the
rubber sealings could all be printed using commercially available equipment. Furthermore,
by using 32 independently operated electrode segments we were able to measure
the mass transfer performance on a local scale. Sherwood – Reynolds
correlations were established for designs with different shapes of turbulence
promoters, 3D electrodes and inlet manifolds. The effect of an increased or
decreased hydrodynamic entrance length was also investigated.

In the end these prototypes lead to enhanced
understanding of electrochemical reactors, allowing us to intensify processes
such as alkaline water electrolysis. As a proof of concept, we are developing a
3D printed lab-scale electrolyzer for alkaline water splitting which can be
used to test the effect of geometry on the production of hydrogen directly.

References

[1] T.R.
Ralph, M.L. Hitchman, J.P. Millington, F.C. Walsh, Mass Transport in an
electrochemical laboratory filterpress reactor and its enhancement by
turbulence promoters, Electrochem. Acta, 1994, pp. 591-603.

[2] A.
Frías-Ferrer, J. Gonzálex-García, V.Sáez, C. Ponce de León, F.C. Walsh, The
effects of manifold flow on mass transport in electrochemical filter-press reactors,
AIChE J.,  2008, pp. 811-823.

[3] A.A.
Wragg, A.A. Leontaritis, Local mass transfer and current distribution in
baffled and unbaffled parallel plate electrochemical reactors, Chem. Eng.
J., 1997, pp. 1-10.