Conclusion

The different design choices made in the construction of an electrolyzer greatly affect the mass transfer performance. Between the electrolyzers found in literature, up to a factor 10 difference is observed (see figure 8). This variation is the result of several different geometric design choices in the electrolyzer. Using a 3D printed electrolyzer, we were able to investigate the effect of some of these choices.
Depending on the type of inlet that was used, up to a factor 2.2 difference in Sherwood numbers was seen. Furthermore, an earlier transition to the turbulent regime was found. The tube inlet already produced turbulent flow at Re = 65, the conic and divider inlet transitioned around Re = 300. The higher than expected mass transfer is due to the sudden expansion of the inlet to the channel. In turbulent flow, the correlation by Djati et al. predicts mass transfer fairly well. [19] This correlation uses the ratio of cross-sectional area of the inlet and channel as parameter to predict the magnitude of expansion turbulence. In the conic inlet, the cross-sectional area varies throughout the inlet and this was accounted for by using the geometric mean cross-sectional area. The correlation deviated from the experimental results by <18% for the tube inlet, <8% for the divider inlet and <6% for the conic inlet.
The addition of a calming section minimized these effects. With a calming section of 550 mm, the type of inlet no longer seemed to affect the rate of mass transfer. This length is over twice the predicted hydrodynamic entrance length of 240 mm at Re = 1200 (based on eq. 5). Therefore, it is reasonable to assume that the flow is fully developed and that the inlets no longer matter. Despite this, our results did not completely match the correlations for hydrodynamically developed flow established by Ong. [12-13] This is likely the result of the limitations of 3D printing, as this process can result in imperfections in the printed parts that may disturb the flow.
Turbulence promotors generally lead to an enhancement of mass transfer. The presence of a calming section significantly changed the enhancement effect of the promoters. Without calming section, most of our promoters resulted in comparable Sherwood numbers, whereas with an entrance length a larger variance in performance was found. Inlet turbulence therefore greatly influences the effect of a turbulence promoter.
The pressure drop was measured for the different configurations of inlet length, inlet type and turbulence promoters. Overall, only very small differences between each configuration were observed (in the order of 100 pascal). Though the difference is marginal, it appears that higher pressure drops result in higher mass transfer rates.
Mass transfer in electrolyzers can be significantly enhanced by turbulence promoters or turbulence causing inlets. The added pressure drop for these is minimal, which implies that large performance increases can be achieved for little extra pumping costs. However, due diligence must be taken in extrapolating results from the lab-scale to the industrial scale. Since the importance of the inlet effect diminishes as the electrolyzer scales up, mass transfer may be slower than expected from the lab-scale. As we have shown, a good inlet design or a calming section can reduce inlet turbulence in smaller electrolyzers, so that they are more representative of their larger counterparts.