Scale up of ow-electrode capacitive deionization is hindered due to the reliance on thick brittle graphite current collectors. Inspired by developments of electrochemical technologies we present the use of felxible membrane electrode assemblies (MEA) to solve these limitations. We tested di erent carbon- ber fabrics as current collectors and laminated them successfully with ion-exchange membranes. The use of thinner ion-exchange membranes is now possible due to the reinforcement with the carbon ber fabric. Desalination experiments reveal that a MEA setup can achieve salt transfer rates equal to standard setups. Hence we deduce that charge percolation also acts outside the electric eld. In a single point of contact, ionic and electric charges are exchanged at the carbon surface of the MEA. The use of thinner membranes leads to a reduced potential drop. Together with a more homogenous electric eld across the feed water section, this can compensate for the reduction of contact surface between ow electrode and current collector. Keywords: Flow-electrode Capacitive Deionization, Capacitive Deionization, Membrane Electrode Assemblies, Charge Transfer, Desalination 1. Introduction The development of environmentally friendly and energetically ecient technolo- gies for the desalination of water poses a major challenge on the way towards a su- cient and safe water supply. Additionally, many industrial processes produce salt-rich wastewater streams. [1] Recovery and concentration of valuable ionic components from aqueous streams may improve the sustainability and economic feasibility of Phone: +49 241 80-95470, Fax: +49 241 80-92252, manuscripts.cvt@avt.rwth-aachen.de 1 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 2 various large scale industrial processes. The implementation of new technologies for recycling of valuable ionic compounds from salt solutions reduces the water de- mand to prepare these solutions and minimize the impact to the environment by not disposing them to the surface water body. [2, 3] The approaches to solve these prob- lems can be divided by the applied driving force. Besides pressure (reverse osmosis, nano ltration) and heat driven processes (distillation, vapor compression, membrane distillation) [4, 5, 6], electrochemical processes attain more and more attention since they allow the direct use of electricity from renewable resources and, thus, reduce carbon emissions. [7]. In this eld a vast variety of electrochemical processes evolved over the past years, such as electrodialysis (ED), which uses an electric eld to move ions between di erent ows separated by ion-exchange membranes (IEMs). ED is used to set salinities in a large variety of applications; drinking water production is a major eld, but it is also widely applied in the food industry. The technology is readily adaptable to the demands and can be scaled-up by stacking 50 to 100 cells, or layers of IEMs. However, the technology is not well applied yet at high salt concentrations and relies on faradaic reactions to couple ionic and electric charge transport. A more recent development is capacitive deionization (CDI), which is considered most suitable for the desalination of low concentrated salt waters. CDI uses also an electric eld as driving force; the ions are removed from the solution and are stored in the electric double layer at the interface between solution and electrode via electrosorption. With the implementation of ion-exchange membranes, the elec- trical eciency improved, the process was commercialised and applied for o -grid desalination.[8, 9, 10, 11] A continuous version of CDI is ow-electrode capacitive deionization (FCDI), which is also applicable to high salt concentrations.[12, 13] Di erent strategies of 2 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 3 charging and discharging the ow-electrodes were studied [14, 15, 16] as well as a possible application enabeled by the unique separated transport of cations and an- ions in di erent ow electrodes[17]. Since the FCDI technology has many possible applications like salt metathesis and outperforms other technologies with its reduc- tion of the energy demand [18, 17], scale up strategies are studied and developed. Yang et. al. showed an FCDI Stack using graphite current collectors to charge the ow-electrodes [19]. Cho et. al. continued with a segmented approach increasing the packing density of an FCDI cell [20] using a ceramic support structure. Yang et. al. continued on our early cell designs by introducing an electrically conductive titanium mesh next to the ion-exchange membranes showing the reduction of the charge transfer distance. [21] This work also leads to hybrids between FCDI and ED con gurations. [22] In this paper, we are taking an important step forward by introducing a membrane electrode assembly (MEA) to FCDI which uses low-cost, exible and chemically stable carbon ber fabric to conduct the electric charge and reinforce the ion-exchange membranes at the same time. Earlier work always looked at charge transport from the conductive surface via the slurry electrode to the membrane as displayed in Figure 1a. [23, 24, 25] Contrary, when using the proposed MEAs, only the surface facing the membrane is conductive. To describe this e ect, we need a new approach to explain the charge transport in ow electrodes using MEAs. As shown in Figure 1b, our hypothesis is that the slurry particles exchange both, electrical and ionic charges, starting from a single point of contact. The electric charge is supplied by the carbon ber fabric and ions which passed the ion-exchange membranes can adsorb to a ow-electrode particle at the same location. This is comparable to descriptions in literature, where particles are reported to take charge from a surface where particles take charge from a surface. [26] In addition, charge percolation can still emerge forming percolation networks 3 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 4 Conductive fiber fabric electrode + +++ e-e- + +e-Single point ofcontact : aligned transport e-e-e-e- e- e- Cation exchange membraneMembrane electrode assembly +++ e-e- e-Non-conductive housing Flow++ +++e- + +e-e-e- Cation exchange membrane+++ e-e- e- e- e- e- e-e- + ++ FlowGraphite current collector(a) (b) Figure 1. Di erent explanation of charge transport between a surface and particles of a ow elec- trode. (a) Charge transport between graphite current collector and membrane. [23, 24] Electrical charge transport by percolation of particles and ionic charge transport in the liquid phase. (b) Charge transport at the surface of a MEA. Direct exchange of electrical and ionic charges in a single point of contact. from the carbon ber fabric towards the non-conductive ow- eld. Apart from desalination, ion-exchange membranes and membrane electrode as- semblies have a larger application in the eld of electrolysis processes, fuel cells and batteries. [27, 28] In PEFCs, the front- and backside of the membranes are covered with a catalyst layer, where the electrochemical half cell reactions take place. On top of this cata- lyst layer, the gas di usion layer (GDL) is placed, which ensures fast supply of the 4 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 5 reactants to the catalyst layer and simultaneously acts as a current collector. Re- actions in the gas phase are prominent since di usion coecients are of magnitudes larger compared to a possible liquid phase. The materials applied for the GDL are commonly carbon based papers or woven fabrics, but also nickel and metal based alloy foams. [29, 30, 31, 32, 33] For proton exchange membrane fuel cells (PEMFC), the membrane electrode assemblies are made by hot pressing. Suitable conditions have been identi ed to lie in the temperature range between 100-160C and a pressing time of 2-2 :5 min. Yang et al. used a temperature of 100C for SPEEK membranes and a temperature of 130C for Na on®membranes. The applied pressures vary between 275- 650 N =cm2 and depend on the gas di usion layer material and catalyst coating strategy. Koziara et al. determined a temperature of 160C from which desulfonation of SPEEK membranes starts. [34, 35]. For a high performance alkaline AEM, Zhao et al. used a temperature of 60C and a pressure of 100 N =cm2. [31, 32, 36, 37] Process parameters for MEA fabrication with Na on®membranes were also found to be successful at temperatures from room temperature to 127-137C with pressure range 2700-16 000 N =cm2for 1-2:5 min, depending on the membrane type and gas di usion layer material. [36, 38, 39] The extreme forces on the membrane can cause unseen damage and change the membrane properties, e.g. deterioration of functional groups. Therefore, the tem- perature and pressure should be chosen carefully. The ion-exchange capacity of the membranes before and after the hot-pressing procedure has been investigated, show- ing that their properties stay the same. [31, 40] The hypothesis of this work is that by integrating the developments in designing a membrane electrode assembly (MEA) for fuel cell applications and the developments in FCDI, one can improve the overall process of FCDI. The aim is to achieve a 5 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 6 reduction of resistances in the cell as well as a thinner and more exible stack design. Therefore, we report a speci c new MEA design by adaption of the production procedure from literature and demonstrate the implementation in an FCDI cell. 2. Experimental 2.1. Manufacture of Membrane Electrode Assemblies The membrane electrode assemblies (MEAs) we fabricated and tested in this study consist of sandwiched from ion-exchange membranes (FAS 30 and FKS 30, FUMATECH BWT GmbH) and di erent carbon- ber fabrics (CF). For clarity, each carbon ber fabric was abbreviated with CF and their speci c surface weight. CF46 weight surface: 46 g/ m2, thickness: 60 µm, thread diameter: 7 µm thread count: 7000, weave: plain, non-coated provided by R & G Faserverbundwerksto e GmbH, SAMURAI Carbon Fabric CF92 weight surface: 92 g/ m2, thickness: 150 µm, thread diameter: 6 µm thread count: 1000, weave: plain, non-coated provided by Goodfellow GmbH, C003510 CF200 weight surface: 200 g/ m2, thickness: 300 µm, thread diameter: 7 µm thread count: 3000, weave: plain, non-coated provided by Goodfellow GmbH, C003531 Table 1. Di erent carbon ber fabrics applied in this study. We used a hot pressing procedure to join the single layers. Membrane and carbon ber fabric were stacked and covered by a layer of aluminum foil for surface protec- tion and placed in a hot press (Agila PE30, Belgium) ensuring an even distribution of heat and pressure. The structures were pressed for 2 :5 min at 120C and 8:5 bar, 6 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 7 immediately followed by a second 2 min pass at the same pressure but at a temper- ature of 25C. To ensure the MEA has good sealing properties an FCDI module, the side of the MEA with exposed carbon ber fabric was sealed on the rim with polydimethylsiloxane (Sylgard®184 by Dowsil). The active area was spared out, so ions can pass through the membrane and ber fabric and and are stored in the ow electrode. The electrical contact was made by a copper pop rivet clenched into the bers at an extension (see Figure 6). 2.2. FCDI modules In the present study, two sizes of FCDI modules were used. They are termed as 'standard' and 'small' modules. The construction of the 'standard' module was almost identical to the module used in our previous works [14, 17, 41], but no tita- nium eeces were placed between the ion-exchange membranes and current collectors [18, 23]. Those modules were made from polymer end-plates, rubber gaskets, epoxy- impregnated graphite electrodes (M uller & R ossner GmbH & Co. KG, 180 180 10 mm3) with engraved ow-channels for the ow electrodes (3 2 mm2and 200 cm total length), cation- and anion-exchange membranes (Fumasep FKB-PK-130/ED- 100 and Fumasep FAB-PK-130/ED-100, FUMATECH BWT GmbH) with an e ec- tive area of 121 cm2, and a 0:5 mm mesh spacer (ED-100 Spacer, FUMATECH BWT GmbH). The 'small' modules were made from the same materials, but in smaller dimensions of polymer end-plates and rubber gaskets. We used epoxy-impregnated graphite electrodes (M uller & R ossner GmbH Co. KG, 62 12810 mm3) with engraved ow-channels for the ow electrodes (3 2mm2and 470 mm total length), cation- and anion-exchange membranes (Fumasep FKB-PK-130/ED-40 and Fumasep FAB-PK-130/ED-40, FUMATECH BWT GmbH) with an e ective area of 22 :4 cm2, and a 0:5 mm mesh spacer (ED-40 Spacer, FUMATECH BWT GmbH) for the ref- 7 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 8 erence experiments. For the MEA experiments the same ow-channel geometry as in the graphite electrodes was engraved to polycarbonate plates, and the MEAs were used instead of the aforementioned ion-exchange membranes. The MEAs were installed such that the carbon ber fabric always faced the polycarbonate plates. EndplateIEMsEndplate EndplateIEMsEndplate Non-conductive Flow-fieldIsolated GraphiteStandard Graphite EndplateIEMsEndplateCation Flow - electrodeAnion Flow - electrode Figure 2. Di erent con gurations of desalination modules. Left: Setup with MEAs and a non- conductive ow- eld. Center: Setup with graphite current collectors and isolated ow electrode bore holes. Right: Setup with standard graphite current collectors. 2.3. Ion-Exchange Capacity The in uence of the proposed heat pressing procedure on the membrane func- tionality was evaluated by the ion-exchange capacity (IEC). For determination of the IEC, the procedure proposed by Sata and Dlugolecki was applied. In Equation 1 the 8 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 9 ion-exchange capacity is de ned as total amount of charged groups c total[mequiv] over dry weight m dry[g] of the membrane. [42, 43] IEC =ctotal mdry(1) As it is not possible to measure these groups directly, the amount of corresponding ions that are replaced by other ions is determined. Therefore, the membranes are rst brought into a speci c form, meaning functional groups are occupied by one species of ion. Prior to this, the exact dry weight of the investigated piece of membrane has to be measured. Cation-exchange membranes were brought into H+-form by immersing them into 1 mol=L hydrogen chloride solution (Carl Roth GmbH & Co. KG) for at least 24h, replacing the solution three times. Afterwards, the membranes were thoroughly rinsed with ultra-pure water to remove any remaining protons on the membrane surface. The rinsing water was tested for remaining chloride ions by addition of silver nitrate. Subsequently, the membranes were brought into Na+-form by laying them into 3 mol=L sodium chloride solution (Carl Roth GmbH & Co. KG, 99.8 %) under continuous stirring. To ensure complete exchange of protons with sodium ions, the sodium chloride solution was replaced three times within 24 h. The replaced solution was collected and its proton concentration was determined by acid-base titration. Anion-exchange membranes were brought into Cl--form by soaking them into 3 mol=L sodium chloride solution for at least 24 h during which the solution is re- placed three times. Further, the membrane was rinsed with ultrapure water to remove adsorbed chloride ions. Again, the rinsing water was tested on remaining chloride ions by addition of silver nitrate. The membrane, in balance with ultrapure water was then leached in 0 :2 mol=L sodium nitrate solution (Carl Roth GmbH & Co. KG, purity 99 %) under continuous stirring. The solution was replaced three times to 9 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 10 ensure complete exchange of chloride ions through nitrate ions. Again, the replaced solution was collected and its chloride content was determined by high performance liquid chromatography (Agilent 1200, HPLC). 2.4. Electrochemical Impedance Spectroscopy Electrochemical impedance spectroscopy was performed on the set of membranes and membrane electrode assemblies applied in this study to get an insight into the resistance and double layer formation on the membrane surfaces. A four electrode setup was used with a two compartment tubular test cell. The surface under exami- nation had a size of 1 :77 cm2. Working and counter electrodes were made of graphite with a diameter of 5 mm. Working sense and reference electrode were made from a 50m platinum wire. As electrolyte a 1 mol =L sodium chloride solution was used. We used a Gamry Instruments Reference 3000 potentiostat and applied a sinusoidal alternating current at a potential of 10 mV with frequencies ranging from 300 kHz to 0.1 Hz. Before each experimental cycle a speci c amount of electrolyte was provided, to be used for all following measurements. Its inherent resistance and behavior in the test cell was determined by running a rst measurement without the application of a membrane to the test cell. Each test run was repeated three times to verify consistency of the results. 2.5. Operation of FCDI Experiments All experiments were performed with 5 g =L sodium chloride water feed solutions. The solutions were prepared using ultrapure water and sodium chloride (Carl Roth GmbH & Co. KG, 99.8 %). The water feed streams were pumped by peristaltic pumps (Ismatec Reglo ICC peristaltic pump with up to three independent channels) setting the feed ow rates to 2 mL =min each. Conductivity analysis of both, diluate 10 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 11 and concentrate streams, were preformed with conductivity probes (Knick SE 615/1- MS, Knick Elektronische Messger ate GmbH & Co. KG). For the ow electrodes activated carbon powder (Carbopal SC11PG, Donau carbon GmbH) was suspended in high-purity water with a mass fraction of 20 wt% and left to stir for at least 24 hours. The ow electrodes were pumped by peristaltic pumps (Cole Parmer Master ex Easyload II, Tube No.16) and the ow rate was set to 200 mL =min for all experiments. For each ow electrode 100 mL activated carbon suspension was given into a glass vessel, where it was stirred with agitators to prevent settling of the activated carbon particles and pumped circularly through the modules back to the glass vessels. Te on and polyethylene tubes with an inner diameter of 4 mm connected the vessel with the used modules and the modules among each other. A potential of 1:2 V was provided to all modules by power supplies which also measured the re- sponding current (HM8143, Rohde & Schwarz GmbH & Co. KG, E3640A, Keysight Technologies, Inc.). All experiments were operated continuously with two modules as presented in Figure 3 until a steady state was established. The key intention was to examine the behavior of the di erent membrane electrode assemblies in contrast to a graphite cur- rent collector. The right module represents the 'small' desalination module equipped with either graphite current collectors or MEAs, whereas the left module displays the 'standard' module used for regeneration of the ow electrodes. A larger module was chosen to avoid a possible bottleneck during regeneration of the ow electrodes, since the desalination performance was subject of examination. The ow electrodes were circulated between both modules, allowing a continuous operation. As feed, a sodium chloride solution was given into both modules. In order to counter the pressure loss in the module and fouling of the tubing, the ow rate was adjusted be- 11 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 12 Flow electrodesFeed 5 g/L NaClVaried desalination moduleConcentration moduleConcentrate Diluate Figure 3. Module setup of two FCDI Modules for comparing Experiments. The left module for concentration has a active membrane area of 100 cm2. The right desalination module was mounted with MEAs or graphite electrodes. fore every procedure and checked during operation. The conductivities of the water streams were recorded and used for further calculations to determine salt concen- trations. A calibration curve was used to calculate the salt concentration from the measured conductivity. The diagrams in the results section give the concentrations normalized for the feed water concentration and the area-related salt transport rate calculated by Equation 2. 12 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 13 _ m00 salt=(cdesalcfeed)_Vfeed Aactive(2) 3. Results and Discussion In electrochemical applications, membrane electrode assemblies (MEAs) are well established. The present study is the rst on introducing this technology to ow- electrode capacitive deionization. We produced specialized membrane assemblies and tested di erent componends for the MEAs. These MEAs were evaluated with the key parameters used for electrochemical membrane characterisation. Desalina- tion experiments were conducted to compare the new product with currently used graphite current collectors. 3.1. MEA production The membrane electrode assemblies were composed from ion-exchange mem- branes and di erent carbon ber fabrics, as described in Section 2.1. During the hot-pressing procedure a homogeneous distribution of heat and surface load is nec- essary to achieve uniform MEAs. Pre-studies showed that the best results were achieved with the setup and hot-pressing procedure described in this section. We used three di erent carbon ber fabrics. All fabrics have no surface coating to prevent undesired electrical resistances. The three fabrics di er in thickness and width of the roving. Small samples were produced for each combination of carbon ber fabric and anion- or cation-exchange membrane, respectively. 3.2. MEA swelling behavior Swelling is a major issue in at-sheet membrane modules. The size change of the polymer material is usually limited by the sealing of such modules, which can lead to 13 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 14 the formation of wrinkles in the membranes. Especially unreinforced membranes have a high tendency to wrinkle. These undulations in the membrane may cause blockage of the ow-channels next to the membrane. Hence, we used reinforced membranes in our previous studies and compare their behavior with the newly manufactured MEAs. Figure 4 displays the MEAs made from anion-exchange membranes in combina- tion with carbon ber fabrics with di erent surface weights. The pictures for MEAs made from cation-exchange membranes are displayed in the supplementary Figure S4. As previously described in Section 2.1, di erent carbon ber fabrics were used to cre- ate anion and cation-exchange membrane electrode assemblies (AMEAs/CMEAs) via hot pressing. Each carbon ber fabric (CF) was successfully combined with an anion- (FAS 30) and cation- (FKS 30) exchange membrane. Sub- gures 4a, 4c and 4e display the AMEAs after hot-pressing. Both layers of the AMEAs are joined and cannot be detached. AMEA-CF46 shows no signi cant sign that it is pressed together with an IEM. The other two carbon ber fabrics, however have distinct round patches where the IEM is combined with the carbon ber fabric. In areas of roving crossing points the surface load has a maximum, allowing the IEM to bond with the carbon ber fabric and forming the patches. The dimensions of the roving cross sections di er, also resulting in unequal patch sizes. CF46 has a cross section area of 0 :5 cm2, CF92 has 0 :1 cm, whereas, CF200 has 0:2 cm. Between the patches, the membranes fold themselves into the free space between the rovings creating grooves. This e ect is di erent for each membrane material and is displayed in Figure S3 of the supplementary material. In Figures 4b, 4d and 4f, the AMEAs are shown after 30 days of storage in deionized water. Di erences between the fresh MEAs and the the swollen MEAs become apparent. The membranes have many wrinkles all around the surface of 14 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 15 1cm 1cm 1cm 1cm 1cm 1cm(a) (b) (c) (d) (e) (f) Figure 4. Photographs of anion-exchange membrane-electrode assemblies manufactured from three di erent carbon ber fabrics; (a) AMEA with CF92, (c) AMEA with CF200, (e) AMEA with CF46 and the same samples after swelling in water for 30 days; (b) swollen AMEA with CF92, (d) swollen AMEA with CF200, (f) swollen AMEA with CF46 the carbon ber fabric. For AMEA-CF46 these wrinkles become so large that they cause detachment of the membrane from the carbon ber fabric. The other two AMEAs also have many wrinkles, but only in the area where the rovings do not cross. The round patches are still visible. The interconnections leave space to let the membrane swell at these points. Similar occurrences can also be seen with CMEAs in Figure S4 of the supplementary material. Attempts to improve the connection for 15 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 16 AMEA-CF46 by tuning the hot-pressing parameters were not successful, no stable connection using CF46 could be achieved. In contrast, the connections in AMEA- CF92 and AMEA-CF200 are very durable and support the membrane. In these cases the carbon bre fabric also acts as reinforcement of the membranes. 3.3. MEA IEC Temperature control during the hot-pressing of the MEAs is essential, since the active groups of ion-exchange membranes could be destroyed. Thus, we set the tem- perature to 120C to stay always below the critical range of 160C and above. The critical value is a threshold from which desulfonation of sulfonated active groups inside of sulfonated polyether ether ketone (SPEEK) cation-exchange membranes (CEMs), such as the Fumatech FKS 30 applied in this study, starts. [44] For the anion-exchange membranes, no critical temperatures could be identi ed in the man- ufacturers data sheet or literature, so empirical analysis was necessary. For this pur- pose, the ion-exchange capacity (IEC) of the membranes used for MEA fabrication was determined before and after the hot pressing procedure at a chosen temperature. The following Table 2 shows the determined values in [mequiv/g]. Table 2. Results of ion-exchange capacity analysis in [mequiv/g]. Type of membraneManufacturer dataUntreated sampleHotpressed at120 C FKS 30 1.3-1.4 1.36 1.37 FAS 30 1.6-1.8 0.85 0.87 No changes in IEC were observed for the unsupported thin lm IEMs (FKS 30, FAS 30), which were used for the MEA fabrication. However, the IEC values of the 16 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 17 FAS 30 membranes determined in this study di er signi cantly from the manufactur- ers data. This mismatch can be explained by the analytical procedure applied here, which is only capable of determining the amount of strong basic groups of the mem- brane material. It is likely that the FAS 30 anion-exchange membrane by Fumatech consists of strong basic as well as weak basic groups. Dlugolecki et al. discovered this for the Fumatech FAD anion-exchange membrane. To verify if weak basic groups are altered or destroyed during hot pressing, the application of other analytical methods would be required, which would go beyond the scope of this work. [45] 3.4. MEA EIS 121 51 82 10-3-6-9-12Z real [/s937]Zimg [/s937]C EMEA 200CEMEA 46C EMEA 92 Figure 5. Nyquist plot of the hot-pressed cation-exchange MEAs. Figure 5 shows electrical impedance spectroscopy (EIS) measurement results of the fabricated MEAs in a Nyquist plot. The left boundary of all three semi-circles refers to a solution resistance of around 12 . This value was also measured as back- ground resistance in the same measuring cell without a MEA installed. The width 17 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 18 of the semi circles represents the double layer resistance and capacity referring to a simple Randles equivalent circuit. In this description, the measured double layer e ects are a sum of the membrane and the carbon ber fabric. Figure 5 displays that the MEA double layer resistance changes signi cantly depending on the type of carbon ber fabric. The membrane resistance is equal, since the same membrane type (FKS 30) was used for the MEAs and thus, di erences are caused by the dif- ferent carbon ber fabrics. It appears in this comparison that the MEA fabricated with 92 g=m2has the smallest resistance. A reason can be the tight weave of the carbon bers and the overall thin fabric, which could leave paths for the ions to pass through easier. In addition, the CMEA fabricated with 200 g =m2could show the combarable results, however, the thickness of this carbon ber fabric is doubled compared to the carbon ber fabric with 92 g =m2, revealing a greater overall resis- tance. In case of the carbon ber fabric with 46 g =m2, the weave is atter with a smaller thickness of 60 m. This fabric has much more threads than the other two carbon ber fabrics, which could lead to a higher resistance with both, anion- exchange and cation-exchange membranes. In summary, the total resistance depends on the interplay between density of the threads of the weaving and the thickness of the fabric. The carbon ber fabric with 92 g =m2shows the overall best performance. Hence, we applied this carbon ber fabric for the following desalination tests. The equivalent circuit used for the EIS analysis and the results for the AMEAs can be found in the supplementary Figure S1. 3.5. MEA FCDI Module Implementation After testing di erent MEA con gurations, we fabricated MEAs from the CF92 carbon ber fabric and Fumatech FAS 30 and FKS 30 membranes in Fumatech ED- 40 module size. The di erent MEAs were made as described in Section 2.1. Figure 6a 18 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 19 shows the side facing the ow electrode with the open carbon ber fabric in the center. The rim is sealed with PDMS. Three smaller holes are punched through the MEA for distribution of both, ow electrodes and the salt water within the module. The holes along the sides are intended for the screws which bolt the module. The extension with the copper pop rivet can be seen on the right hand side of the image. The membrane side of the MEA is shown in 6b, where the holes have the same purposes as mentioned before. These MEAs were successfully implemented in Fumatech ED- 40 size FCDI modules. 3.6. Desalination experiments Desalination experiments were performed to examine the performance of the MEAs in an FCDI setup. The performance of the MEA experiments was com- pared to the performance of experiments with the isolated and non-isolated graphite current collectors shown in Figure 2. Figure 7a displays the performance over time for the isolated graphite electrode as example. A concentrate and a diluate are pro- duced during the experiments, which can be seen by an increase or decrease in the normalized concentration, respectively. Both curves reach a plateau, indicating that the overall system reached its steady state. Due to the fact that the module used for concentration o ers 5.4 times more membrane surface area than the desalination module, the steady state is limited by the desalination speed only. Water samples were taken at the end of each experiment and the concentration of both product streams were validated via HPLC. With the ow rate of the streams and the active surface area of the desalination module of 22 :4 cm2the salt transport balances were calculated for desalination and concentration module of each experiment. Both bal- ances, calculated via concentrate and diluate, showed equal results and the arithmetic mean was used for comparison. Figure 7b displays the comparison of the calculated 19 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 20 (a) (b) Figure 6. Cation-exchange membrane electrode assembly for implementation in Fumatech ED-40 sized modules. (a) Fiber-side view with electrical contact and PDMS sealant. (b) Membrane-side view. salt transport and current eciencies by Faraday's law. We tested three di erent setups, (a) the standard graphite current collector, (b) the MEAs, and (c) the same standard graphite current collector with isolated bore holes. Thus, in setup (c) the slurry can only exchange charge within the ow eld of the current collector. All setups were designed such that they can be supplied with the ow electrodes and 20 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 21 feed water from the same side. This was done to scale up the system more easily. Each con guration of graphite collectors or MEA were tested at least three times. It is clearly visible that the MEA setup o ers around 175% times more salt trans- port compared to the standard graphite, but also around 30% less than the isolated graphite. This result is remarkable because (1) the active surface area for charge exchange of the MEA is only 1274 mm2compared to the graphite setups 3078 mm2, and (2) because of the signi cant e ect of isolating the bore holes in graphite cur- rent collectors. Additionally, both MEA setups and the isolated graphite setup show current eciencies of almost 100% compared to less than 70% achieved with the standard setup. 0 20 40 60 800.60.81.01.21.41.6NormalizedconcentrationC/C0[-] Time[min]46810 Salttransportrate [mg/h cm²] IsolatedGrap hite MEA Stand ardGrap hite 60708090100 Curre ntEfficie ncy[%](a) (b) Figure 7. Comparing FCDI experiments of a standard graphite FCDI module for concentration and di erent module setups for desalination. Liquid ow rates: 2mL/min; Slurry ow rates 200 mL/min; Potential at both modules: 1.2 V. (a) Course of normalized concentration over time showing the good comparability of three desalination experiments with isolated graphite current collectors. (b) Comparison of salt removal rate in steady state. 21 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 22 As shown in Figure 2, the anion ow electrode has to pass through the cathode current collector. Apparently the small contact area in the bore hole is enough to interfere with the charging at the anode. This leads to the reduced salt adsorption within that module. This result should be discussed in future work in more detail and needs to be taken into account when designing an FCDI module. Compared to the standard setup, the MEA setup achieves higher salt transfer rates and also adds bene cial properties of being exible and thin. Regarding (1), the applied ow- eld has a rectangular cross-section. Thus, slurry which is owing through this ow- eld carved into graphite is in contact with three conductive surfaces. Only the side facing the membrane surface is not conductive as displayed in Figure 1a. Contrary, when using the MEAs, only the surface facing the membrane is conductive but the experiments show a comparable desalination capacity. To describe this e ect, the most natural explanation is that the slurry particles exchange both, electrical and ionic charges, starting from a single point of contact, as shown in Figure 1b. Charge percolation can still emerge forming percolation networks from the carbon ber fabric towards the non-conductive ow- eld. In total, this better and shorter charge transport paths leads to higher salt transport rates as standard graphite setups, even with less electrical charge exchange surface. This interpretation can be further proven through simulations as done by Lohaus et al. [46] 4. Conclusion In electrochemical applications, membrane electrode assemblies (MEAs) are well established. In the present study, the technology is introduced to ow-electrode ca- pacitive deionization. We tested di erent membranes and carbon ber fabrics to produce the MEAs. These MEAs were evaluated with the key parameters used in 22 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 23 the characterisation of membranes for electrochemical applications. Desalination ex- periments were conducted to compare the new product to the currently used graphite current collectors. MEAs were successfully fabricated using a hot pressing procedure. The ion- exchange membranes swell and show many wrinkles in aqueous environment, but stay in good contact with the carbon ber fabric. Thus, in addition to distributing the applied currents, the carbon ber fabrics act as functional reinforcement. Depending on the thread density and thickness of the carbon ber fabric, the swelling behavior di ers. The same parameters of the carbon ber fabric also have an in uence on the MEA resistances obtained by electrical impedance spectroscopy. A thin and relatively open weaving is more favorable than thick and dense structures. In desalination experiments, the performance of the most suitable MEA type was compared with our standard FCDI setup with graphite current collectors. When using graphite current collectors, the results di er signi cantly if the ow-electrode passing through the graphite current collector is electrically isolated from it. Since the MEAs are very thin, both ow-electrodes can enter the module unilaterally, without considerable losses in desalination or concentration performance. The MEA setup achieves salt transport rates comparable to the ones obtained with the two graphite current collector setups. The MEA setup performs better than the setup using non-insulated graphite plates, but not as good as the setup with insulated graphite current collectors. The current collector surface available for charge transfer is almost three times smaller in case of MEAs compared to the available charge transfer surface of the applied graphite current collectors. Thus, the transport mechanisms di er. We propose a new concept of an aligned charge transfer from a single point of contact, which combines shorter charge transfer paths and, hence reduces overpotentials. 23 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 24 When using the MEAs in FCDI, there is no need for a conductive ow eld anymore. Combined with the inherent exibility of the MEAs, this creates a solid base for the upscaling FCDI systems in the future. 24 C. J. Linnartz et al. / Journal of Membrane Science 00 (2019) 1{32 25 Acknowledgement This work was supported by the German Federal Ministry of Education and Re- search (BMBF) under the project ElektroWirbel (FKZ 13XP5008) and by the Eu- ropean Research Council (ERC) under the European Unions Horizon 2020 research and innovation program (694946). The authors thank Fumatech BWT GmbH and Donau Carbon GmbH for the provision of material samples. The authors thank Karin Faensen for scanning electron microscope and microtomography images. The authors thank Tao Luo for his valuable input on ion-exchange membrane charac- teristics. The authors thank Zhaowei Zou and Georg Gert for their support and constructive e ort.