Carbon based flow-electrodes are an increasing research field and find potential application in water treatment processes as well as energy conversion and storage. Flow-electrodes usually consist of a pumpable carbon slurry made of carbon particles suspended in a liquid electrolyte solution. One application for flowelectrodes is flow-electrode capacitive deionization (FCDI), which is a membrane-based, electrically-driven desalination method using mostly activated carbon as active material. In contrast to capacitive deionization (CDI) systems based on static electrodes, the use of flow-electrodes enables a continuous operation and the treatment of high salinity solutions. However, it was observed that the performance of FCDI processes heavily relies on the activated carbon quality. The process performance results from a wide range of parameters, including the activated carbon sample characteristics, which are usually not sufficiently covered and predicted by standard carbon analyses. With this article, we establish a foundation for applying electrochemical impedance spectroscopy (EIS) as predictive characterization method for flow-electrode materials. This includes the investigation of influencing system parameters and carbon characteristics, and the development of an equivalent circuit model. Finally, we demonstrate the possibility to predict and match the desalination performance of flow-electrodes based on different activated carbon types using EIS.

Utilizing carbon dioxide (CO2) as a resource for carbon monoxide (CO) production using renewable energy requires electrochemical reactors with gas diffusion electrodes that maintain a stable and highly reactive gas/liquid/solid interface. Very little is known about the reasons why gas diffusion electrodes suffer from unstable long‐term operation. Often, this is associated with flooding of the gas diffusion electrode (GDE) within a few hours of operation. A better understanding of parameters influencing the phase behavior at the electrolyte/electrode/gas interface is necessary to increase the durability of GDEs. In this work, a microfluidic structure with multi‐scale porosity featuring heterogeneous surface wettability to realistically represent the behavior of conventional GDEs is presented. A gas/liquid/solid phase boundary was established within a conductive, highly porous structure comprising a silver catalyst and Nafion binder. Inoperando visualization of wetting phenomena was performed using confocal laser scanning microscopy (CLSM). Non‐reversible wetting, wetting of hierarchically porous structures and electrowetting were observed and analyzed. Fluorescence lifetime imaging microscopy (FLIM) enabled the observation of reactions on the model electrode surface. The presented methodology enables the systematic evaluation of spatio‐temporally evolving wetting phenomena as well as species characterization for novel catalyst materials under realistic GDE configurations and process parameters.

Wetting of the membrane to displace air or conditioning liquids is important to exploit the complex porosity of a filtration membrane. This study reveals the details of wetting in multibore membrane based filtration modules. Using magnetic resonance imaging (MRI), we quantify the fluid distribution patterns during initial membrane wetting in dead-end permeation mode. The spatio-temporal evolution of aqueous copper sulfate solution wetting the membrane fibers was investigated as a function of the applied flux, packing density, and position along the membrane module length. Three initial wetting conditions were examined: delivery-state membranes, ethanol-washed and dried (air-filled) membranes, and ethanol-filled membranes. Significant changes in wetting patterns were observed due to interfacial and polymer swelling effects. This in-situ investigation reveals a slow wetting progression over six hours and more to obtain complete wetting, even at high fluxes of 200 LMH. However, an increased flux leads to faster wetting kinetics as the evolving wetting patterns are flux dependent. Packing density of the multibore fibers additionally impacts the wetting kinetics by shifting the prevalent pressure conditions. Although in dead-end mode, the wetting progression is non-uniform along the membrane module length. In addition to this parameter study, different pre-wetting agents' effect on the displacement behavior was investigated in depth. This study helps to understand (a) complex wetting phenomena inside multibore membranes in dead-end filtration, (b) the membranes' interaction with their surroundings due to neighboring membranes, and (c) the effect of the used fluid system for displacement on the resulting wetting patterns.