Introduction

In recent years, a renewed interest in electrolyzers has emerged. They are expected to play a pivotal role in enabling the ongoing global energy transition, as they are able to directly convert electricity into chemical energy. Moreover, the process can be done cleanly if green energy is used, thereby opening up the way to a sustainable chemical industry.
A key property of an electrolyzer is its mass transfer performance. A high mass transfer performance indicates that the reactants can quickly reach the electrode, in this way enabling faster reactions and higher currents. Typically, mass transfer is expressed as a Sherwood-Reynolds correlation. In the past decades, many such correlations have been established [1-5], but there is a large variance in the reported mass transfer performance. For empty parallel plate electrolyzers, the difference can be up to an order of magnitude depending on the configuration and specific design choices that were used, such as e.g. the design of the inlet [1]. This complicates the comparison of results between them. Moreover, the importance of certain mass transfer enhancing effects depends on the scale. Small-scale electrolyzers for instance will be affected more by inlet effects than large scale cells. Therefore, when results from a small cell are extrapolated to a larger electrolyzer, significantly over- or underestimations can occur if these scaling effects are not considered.
In order to increase our understanding of mass transfer in electrolyzers more configurations need to be tested. Therein lies another complication since most of the cells described in the literature were built in-house or produced decades ago, resulting in them no longer being available for present-day research. For new research, it can therefore be difficult to find a suitable cell with known mass transfer behavior. Building new ones is not straightforward either, as it requires design work and complex machining. A solution to this problem is to 3D print electrolyzers, as it allows the quick construction of numerous prototypes.
The purpose of this work is to carry out a systematic investigation into electrolyzer mass transfer performance. 3D printed parallel plate electrolyzers are used to investigate the effects of different inlet designs, inlet lengths, and turbulence promoters and results are compared to previous work.