The research on electrochemical reactors is mostly limited to planarly designed modules. In this study, we compare a tubular and a planar electrochemical reactor for the utilization of the slurry electrodes. Cylindrical formed geome- tries demonstrate a higher surface-to-volume ratio, which may be favorable in terms of current density and volumetric power density. A tubular shaped elec- trochemical reactor is designed with conductive static mixers to promote the slurry particle mixing, and the vanadium redox ow battery is selected as a showcase application. The new tubular design presents similar cell resistances to the previously designed planar battery and shows increased discharge polar- ization behavior up to 100 mA cm2. The volumetric power density reaches up to 30 mW cm3, which is two times higher than that of the planar one. The ∗DWI-Leibniz Institute for Interactive Materials, Forckenbeckstr. 50, 52074 Aachen, Ger- many †RWTH Aachen University, Aachener Verfahrenstechnik-Chemical Process Engineering, Forckenbeckstr. 51, 52074 Aachen, Germany, Email: manuscripts.cvt@avt.rwth-aachen.de, Tel: +49 24180-95470; Fax: +49 24180-92252 ‡Corresponding author: Prof. Dr.-Ing. Matthias Wessling 1 battery performance is further investigated and 85% coulombic, 70% voltage and 60% energy eciency is found at 15 mA cm2with 15 wt.% slurry content. Keywords: tubular, redox flow battery, vanadium, slurry electrode, conductive static mixer 2 Introduction Recent concerns about the environmental impact and sustainability of fossil-fuel based energy sources have accelerated the research on renewable forms of energy. Fluctuations caused by these sources remain problematic and need to be handled before renewables can be eciently utilized in the current electricity grid [1, 2]. A promising way of overcoming this challenge is to use electrochemical storage and conversion processes, which are energy ecient, e ective, and clean [3{5]. However, further studies on electrochemical reactors are needed to improve their selectivity and the eciency. All electrochemical reactors require of two electrodes and an electrolyte for redox reactions to take place. Electrode materials and properties are one of the most researched components of electrochemical reactors. Usually, solid porous electrodes are used as high-surface-area electrode materials [6{8]. Nonetheless, these electrodes still need improvement regarding their surface area as well as exibility and kinetic behaviors. Slurry electrodes, also known as ow electrodes, have been recently consid- ered to be a dynamic replacement for solid porous electrodes in electrochemi- cal systems [9{13]. The scalability of conventional electrochemical reactors is mainly limited by the electrodes being not scaled up energy eciently and cost e ectively.[14] By using conductive particle networks, a owable electrolyte- electrode dispersion can be maintained, which can be scaled up or down in- 2 dependently of reactor size. In this context, slurry electrodes enable a suit- able environment for electrochemical processes, where the reactor design can be altered according to the electrode, gasket or spacer requirements. Moreover, slurry electrodes are easy to produce and recyclable via a simple post-process [9]. Some applications have already proven to be suitable for slurry electrodes, such as ow-electrode capacitive deionization systems [15], electrochemical ow capacitors [16], redox ow batteries [17], and semi-solid lithium-ion batteries [18]. Commonly, solid porous electrodes are employed in these systems, but these electrodes perform best when pressed tightly to the current collectors to achieve the highest possible conductivity [19]. In contrast to porous electrodes, no compression is necessary for the usage of the slurry electrodes in the cell. Instead, conductive static mixers can simply enable a sucient activity for the slurry electrodes [20]. This facilitates an easier realization of cell geometries di- verging from the usual planar designs. Yet, there are no attempts to investigate di erent cell geometries for the slurry electrode applications. One of the particular geometry that has been investigated for traditional elec- trochemical reactors is the cylindrical geometry. Some tubular PEMFC studies suggest better polarization performance at higher overpotential regions than classical planar designs [21, 22]. Solid oxide fuel cells are also another choice of application for tubular structured modules, mostly because of their superior thermal stability [23{25]. There are several studies on tubular shaped direct methanol fuel cells, where a better weight-to-volume ratio was hypothesized [26{28]. Finally, Ressel et al. introduced a tubular module design for a tradi- tional vanadium redox ow battery (VRB), which showed up to 70 mA/cm-2 polarization behavior. The study mentions 12 times higher area speci c cell resistance (ASR), which is proposed to be caused by the lack of compression on the graphite felt electrodes [29]. 3 Figure 1: Cuboid and cylinder active surface area (left) and volume increase by active surface area graph (right) The e ect of compression on the graphite felts have also been studied widely [19, 30, 31]. These studies indicate that higher degree of compression yields in lower ASR values, which is favorable for energy eciency of the battery. However, porosity of the felt is hindered by this compression, which causes higher pressure drops for the electrolyte ow and accordingly results in lower round trip eciency for the system. This trade-o has been concluded to be most ecient in the range of 20-40 % of felt compression in the VRB systems. Realizing sucient compression in a tubular system is more challenging than in a planar system due to geometrical limitations. Therefore, higher cell resistances in tubular systems are expected when felt electrodes are used. On the contrary, slurry electrodes are promising for using in the tubular module design, since compression is not necessary. An advantage of the tubular electrochemical systems is that cylindrical ge- ometry enables a higher surface-to-volume ratio than planar systems. This can be proven if similar active electrode surface area is chosen and the electrolyte volumes are compared. For instance, Fig. 1 presents a cuboid with a plane active electrode surface area ( hL) and cuboidal electrolyte volume ow ( hLW). Secondly, a cylinder with a circular active electrode surface area (2 hr0) 4 and cylindrical electrolyte volume ow ( hr2r2 0). Surface area increase in the cylindrical geometry has lesser gain on the volume compared to the cuboid geometry (Fig. 1). In another saying, if the surface area of both geometries are scaled up, the volume increase of the cylindrical shape would be smaller. Fur- ther, smaller volume per surface area of tubular shape would be advantageous in terms of power density of the electrochemical reactor. Therefore, having a cylindrical reactor for an electrochemical process may be more versatile in terms of scaling up the modules without increasing the volume as exceedingly as it is for the planar systems. Thus, volumetric power density (power per volume) of the systems can be improved. In this study, we discuss if a tubular geometry may be bene cial in terms of electrochemical activity for electrochemical reactors. The activity increase is expected because of the higher packing density per volume that can be achieved by tubular geometries. Therefore, a design and comparison of a lab-scale tubular electrochemical reactor to a planar one is presented in this research. Previously, we established a slurry VRB combined with the static mixers which enables the usage of slurry electrodes [20]. Slurry electrodes are suitable candidates for transforming the planar cell designs to tubular ones, due to the limitations in the standard solid electrodes. Herein, the tubular reactor is designed for the slurry vanadium system including conductive static mixers, and the battery performance is evaluated in terms of battery characteristics. Furthermore, vol- umetric power density evaluation is discussed by comparing the results to the previous planar slurry VRB. 5 3 Materials and Methods 3.1 Materials The current collectors of the module are made of impregnated graphite (M uller & R ossner GmbH, Germany), machined as a hollow cylinder (28 mm inner di- ameter, 3 mm wall thickness) and a rod (9 mm diameter). A Na onTMTub- ing from PermaPure LLC is used as cation exchange membrane (17 mm inner diameter, 100 µm thickness). All supporting structures and static mixers are created by rapid polymer prototyping (Stratasys, Objet Eden 260V). RGD525 is chosen for 3D-print material as it provides sucient mechanical and chemical durability. Static mixers were made conductive by rstly etching in 75% sul- furic acid for 10 min and followed by spray coating with a conductive lacquer (Cramolin) for about ten layers. A conductive adhesive, Leit-C (Sigma-Aldrich Chemie GmbH), is used to decrease the contact resistances between static mix- ers and current collectors. Several di erent O-ring sealings (EPDM, Landefeld Druckluft und Hydraulik GmbH) were placed in the assembly to prevent liquid leakages. VRB experiments are conducted with commercially available vanadium elec- trolyte (GFE-AMG Titanium Alloys & Coatings) consisting of 0.8 M V(III), 0.8 M V(IV) in total 4.5 M sulphate concentration. Slurry electrodes are prepared by dispersing graphite powder (synthetic, 20 µm, 20 m2g1, Sigma- Aldrich Chemie GmbH) into the electrolyte. 3.2 Cell design Main layers of the tubular redox ow battery can be seen from Fig. 2. The tubular design consists of an inner and an outer half-cell separated by a tubular cation exchange membrane. Both half-cells includes conductive static mixers 6 Figure 2: Design layers of the tubular assembly with spot in the inner half- cell structure; (a) single-twisted static mixers, (b) global mixers, (c) support columns as ow distributors for slurry electrodes. A rod shaped current collector (1) contacts the inner half-cell and placed at the very center of the design. The active geometrical surface area of the rod is de ned as 17 :5 cm2. The inner half-cell (2) consists of static mixers in the core that forms the active inner volume. It also comprises inlet and outlet for the inner electrolyte ow. The at tubular surfaces on both edges serve for membrane fastening with the help of O-rings. The central part of the static mixer is consisting of (a) single-twisted static mixers adapted from Fritzmann et al. [32], (b) global mixers, (c) support columns for mechanical stability. The whole part is pushed over the current collector rod (1) and the electrical conductivity is improved by applying the conductive adhesive. A cylindrical current collector (5) is positioned at the outer shell of the module and contacts the outer half-cell (4). The outer half- cell static mixers are made of only the single-twisted static mixers (a) and the global mixers (b). It is also linked with the conductive adhesive to the outer current collector that radially encloses the outer half-cell. The overall illustration and the completed module are presented in Fig. 3. The design is nalized with the end pieces on both sides of the module. The 7 end pieces distribute the outer and inner electrolyte to the static mixers while preventing leakage from the half-cells. Two anges on the outer current collector connect both end pieces to the module and tightens the module by means of screws. Thread seal tape (PTFE) is used throughout the assembly to prevent against the leakages. Finally, conductive copper tapes are rounded on the outer and the inner current collector for the connection to the power supplies. Figure 3: Complete tubular module illustration (left) and realization (right) 3.3 Electrochemical measurements All experiments are carried out with the novel designed tubular module. Glass vessels are used for each electrolyte with outlets at the bottom and inlets on the upper sides. Vessels are purged with constant N 2feed to avoid air oxidation of vanadium species. An electrolyte ow rate of 20 mL min1is maintained by a peristaltic pump (Master ex L/S, Cole Palmer) and each electrolyte vessel is lled with 50 mL electrolyte. Slurry electrodes are prepared by dispersing graphite powder with weight ratios of 5, 10, and 15% into vanadium electrolytes and stirring until well dispersed mixture is maintained. 8 A potentiostat/galvanostat is used with an impedance module (PGSTAT302N, Metrohm GmbH) for the electrochemical measurements. Three di erent types of measurements were conducted, namely electrochemical impedance spectroscopy (EIS), polarization analysis, and charge-discharge experiments. Firstly, the elec- trolytes are charged to 50% state of charge (SOC) by using a previously built standard VRB. Then, EIS and polarization experiments are conducted, respec- tively for each slurry content with 0 (only electrolyte) to 15 wt.-%. The EIS measurements are done in the frequency region of 100 kHz to 100 mHz at a sine wave signal amplitude of 10 mV. The area speci c resistance (ASR) is calculated from high frequency EIS data to calculate ohmic drop corrected potential values for the polarization study. EIS measurements are followed by the polarization study, where series of galvanostatic charge and discharge steps are performed. Constant current (CC) is applied for 30 s and corresponding cell potential is recorded with cuto voltage of 1 :9 V for charge and 0 :8 V for discharge. After each CC step, the open circuit potential (OCP) of the battery is brought to the initial OCP value, where the SOC is set to 50%. Finally, charge-discharge experiments are performed in the tubular module utilizing discharged 15 wt.- % slurry electrodes by applying CC of 15 mA cm2. Consequently, energy and coulombic eciencies are evaluated for each charge-discharge cycle. 4 Results and Discussion The tubular electrochemical module is designed to showcase a possible surface- to-volume ratio advantage against traditional planar-designed electrochemical reactors. The slurry electrode VRB is chosen to demonstrate the capabilities of the tubular design, and furthermore, to incorporate the advantages of the dynamic feature of the slurry electrodes. The limiting active surface area of tubular battery is built to be similar to our previously published static-mixer 9 installed planar VRB to moderate the surface area dependent inaccuracies [20]. The limiting active surface area of the tubular battery is 17 :5cm2, while the planar battery's active surface area is 18 cm2. Thereby, all the results in the following section are presented in comparison to our previous planar VRB. The tubular vanadium redox ow battery is rstly evaluated by the EIS measurements. The evaluation of the Nyquist plot is done for three basic re- sistances (1) ohmic resistances (R ) where the imaginary resistances are zero at high frequency region, (2) charge transfer resistances (R ct) are characterized at the mid-range frequencies with a semi-circle, followed by the (3) mass trans- port resistances (R m) at low frequencies with an inclining tail at the end of the nyquist plot. Fig. 4a shows the Nyquist plots of the tubular slurry reactor with particle content from 0 wt.% to 15 wt.%. Ohmic resistance of the system increases grad- ually with the increase of the particle concentration. R of the 0 wt.% slurry is 4:4 cm2and increases up to 4 :9 cm2for the 15 wt.%. The increase of ohmic resistance with increasing particle content has been also shown by Rom- /s45/s90/s34 /s90/s39/s82/s82 /s99/s116/s82 /s109/s45/s90/s34/s32/s47/s32/s79/s104/s109/s46/s99/s109/s50 /s90/s39/s32/s47/s32/s79/s104/s109/s46/s99/s109/s50/s48/s32/s119/s116/s46/s32/s37 /s49/s53/s32/s119/s116/s46/s32/s37/s97 /s53 /s54 /s55/s48/s49/s50/s45/s90/s34/s32/s47/s32/s79/s104/s109/s46/s99/s109/s50 /s90/s39/s32/s47/s32/s79/s104/s109/s46/s99/s109/s50/s80/s108/s97/s110/s97/s114 /s84/s117/s98/s117/s108/s97/s114/s98 /s55/s32/s107/s72/s122 /s48/s46/s53/s32/s107/s72/s122 Figure 4: Nyquist plots of a) slurry concentration e ect on the tubular module, 0], 5[, 10 c, 15 vwt.% b) comparison of planar to tubular battery with each 15 wt.% slurry 10 merskirchen et al. for the ow capacitive deionization systems where activated carbon particles used as active material [33]. This phenomena can be explained by the particles blocking the ionic transport in the cell. Higher particle content yield in a longer ionic pathway, thus higher ohmic resistance. Following the ohmic resistance, we observe a drastic decrease for the charge transfer resis- tances being signi cantly smaller with increasing slurry content. Overall charge transfer resistances decreased from 2 :8 cm2(0 wt.%) to 1 :1 cm2(15 wt.%). Lohaus et al. simulated a VRB slurry system, where a distinct correlation be- tween particle concentration and particle percolation is found [34]. Due to the particle percolation that develops in the slurry ow eld, a greater charge trans- fer is expected from the current collectors to the particles. Therefore, we observe less R ctwhile increasing the amount of the particle content in the electrolyte. The interpretation of increasing ohmic resistance and decreasing charge trans- fer resistance can be better understood, when the di erence between the charge transfer through ions and particles are de ned. The increasing amount of par- ticles cause a low ionic conductivity due to blocked ionic pathways. In contrast, higher particle content improve the charge transfer from the current collectors to the particles, because more particles enables more electronic conductivity [35]. Low frequency region directly after the half circles shows relatively low an- gled tail meaning less capacitive di usion limitations. However, all the slurry concentrations show nearly identical low frequency behavior with only a shift by virtue of the changes in R and R ct. Therefore, we conclude that the di u- sion limitations are not highly dependent on the particle content in the tubular reactor. We choose 15 wt.% particle content as comparison experiment to the pre- viously designed planar slurry VRB and present EIS results for both 15 wt.% 11 tubular and planar VRB on the Fig. 4b. The planar module presents slightly lower R with a di erence of approx. 0 :1 cm2, which may be caused by the di erence at the electrolyte gap between both modules. The tubular design has larger distance between the current collectors and membrane leading to higher electrolyte gap, thus higher R . However, the di erence is negligible when we consider the total ohmic resistances. Surprisingly, R ctof the planar module shows nearly three times lower resistance, while mass transport related resis- tances are comparable. It can be reasonably assumed according to these results that the planar module has less resistances, which would be a rst indication for higher energy eciencies. Furthermore, frequency regions for both tubular and planar setups show similar values being roughly 7 kHz for R and 0:5 kHz for the beginning of R m. /s67/s101/s108/s108/s32/s80/s111/s116/s101/s110/s116/s105/s97/s108/s32/s47/s32/s86 /s67/s117/s114/s114/s101/s110/s116/s32/s68/s101/s110/s115/s105/s116/s121 /s32/s47/s32/s109/s65 /s46/s99/s109/s45/s50/s48/s32/s119/s116/s46/s32/s37/s49/s53/s32/s119/s116/s46/s32/s37/s97 /s67/s101/s108/s108/s32/s80/s111/s116/s101/s110/s116/s105/s97/s108/s32/s47/s32/s86 /s67/s117/s114/s114/s101/s110/s116/s32/s68/s101/s110/s115/s105/s116/s121 /s32/s47/s32/s109/s65 /s46/s99/s109/s45/s50/s84/s117/s98/s117/s108/s97/s114 /s80/s108/s97/s110/s97/s114/s98 Figure 5: IR corrected cell polarization graphs a) 0 ], 5 [, 10 c, 15vwt.% slurry content in the tubular cell, b) comparison of planar to tubular battery with each 15 wt.% slurry content Fig. 5 shows the results of the polarization study on the tubular slurry VRB with IR corrected potential values. The measurements are all maintained at 50% SOC and IR correction is done according to the R values from Fig. 5a. Polarization study is done by performing constant current charge and discharge 12 on the battery and the voltage response of the system is recorded after it reaches to a stable state. In such battery applications, it is desired to have the highest possible potentials during the discharge and lowest possible potential during the charge process. The potential cut-o range is usually kept between 0.9 and 1.7 V for charge and discharge, respectively. The results from Fig.5a indicate a pos- itive correlation between particle concentration and the polarization behavior. 15 wt.% slurry shows the lowest overpotentials on both charge and discharge sides while reaching almost 100 mA cm2for discharging and 60 mA cm2for charging in the potential range of 0.8 to 1.8 V. Furthermore, the polarization curves become attered with increasing content of the slurries, which is desired for the VRB application. In Fig. 5b, we compare the polarization of the tubular and the planar module con gurations consisting of 15 wt.% slurry. A di erence can be detected on the discharge side, while the charge side is relatively similar. On the discharge side, the planar battery can only reach up to 75 mA cm2. This di erence can be explained with the better cell and static mixer design in the tubular module. The tubular module has a high packing density for the conductive static mixers. The conductive static mixers behave also as current collectors. Yet, the surface area of the both modules is still calculated by the geometrical surface area of the graphite plate (planar) and graphite rod (tubu- lar). By using more conductive static mixers, there is a larger contact surface available for the particles while the geometrical surface area is not changed. Con- sequently, higher current densities can be reached while the geometrical surface areas are similar. Secondly, as a result of having better mixing in tubular mod- ule, the polarization at high current region shows less steep voltage decrease. This region, where the high current density causes steep overpotential changes, is usually described by the mass transport related limitations [36]. One of the main reasons to strive for a tubular module design is the possible 13 higher volumetric power density due to the improved surface-to-volume ratio of tubular structures. Therefore, the polarization data should be transformed to volumetric power density. However, both planar and tubular slurry VRBs are not fully optimized in terms of sealing and housing, which highly a ects the total volume. Hence, we consider a real and equal basis for an objective comparison by reducing the volume to the core structure. The core of the planar cell is de ned as outer bounds of graphite plates from each side, while the core of the tubular cell is de ned as the outer diameter of the outer graphite tube. The comparison of the volumetric power density of tubular and planar modules for the 15 wt.% slurry electrodes can be seen from Fig. 6. Higher discharge power density can be reached with the tubular design up to 30 mW cm3. Additionally, for the planar design the power density reaches its plateau region at about 15 mW cm3, while the tubular module does not indicate any plateau region yet for the discharge process. An insigni cant di erence on the charge side can be detected for the planar module showing about 5 mW cm3higher power density. Overall, we conclude a better performance for the tubular module because of the greater volumetric discharge power density, since we aimed to prove the advantage of the tubular structures due to their surface-to-volume ratio. Besides, the tubular module can be eciently designed longer than current tubular modules resulting in a larger surface area without a ecting the total volume as it would be for the planar module. In addition, even higher increase in volumetric power density may be achieved. Further, the battery performance of the tubular VRB is brie y studied by cyclic charge-discharge testing with 15 wt.% slurry electrodes. An interme- diate current density has been chosen concerning the polarization studies as 15 mA cm2for both charging and discharging. Fig. 7 presents the cell poten- tial among eight cycles. An ohmic overpotential drop approx. 0 :25Vcan be 14 /s45/s49/s48/s48 /s45/s53/s48 /s48 /s53/s48 /s49/s48/s48/s45/s51/s48/s48/s51/s48/s86/s111/s108/s117/s109/s101/s116/s114/s105/s99/s32/s80/s111/s119/s101/s114/s32/s68/s101/s110/s115/s105/s116/s121/s32/s47/s32/s109/s87/s46/s99/s109/s45/s51 /s67/s117/s114/s114/s101/s110/s116/s32/s68/s101/s110/s115/s105/s116/s121 /s32/s47/s32/s109/s65 /s46/s99/s109/s45/s50/s80/s108/s97/s110/s97/s114 /s84/s117/s98/s117/s108/s97/s114Figure 6: Volumetric power density comparison of planar and tubular battery with each 15 wt.% slurry content detected after each charge cuto . We reported similar results in our previous work, where we introduced the slurry electrodes for the VRB applications by us- ing conductive static mixers [20]. Furthermore, the ohmic resistances from EIS results at Fig. 4b suggests a similar ASR for both planar and tubular modules. The ohmic overpotential is caused by the cell resistances that persists with the utilization of the slurry electrodes. We suggest that this can be improved by in- creasing the electrical conductivity of the static mixers. It is important to note that static mixers are produced by additive manufacturing to be able to create their complex geometry. The material for the static mixers is not conductive. Therefore, a graphite coating is applied on top of the polymer material, creating a conductive layer. Same procedure is applied for the tubular designed static mixers. However, the graphite coating is still relatively resistive ( $100 cm) resulting higher contact resistances. Hence, improving the resistivity of these static mixers may reduce the overall ohmic losses. The performance of the battery is further evaluated by means of coulombic 15 /s48 /s50/s53 /s53/s48 /s55/s53 /s49/s48/s48 /s49/s50/s53/s48/s46/s56/s49/s46/s48/s49/s46/s50/s49/s46/s52/s49/s46/s54/s67/s101/s108/s108/s32/s80/s111/s116/s101/s110/s116/s105/s97/s108/s32/s47/s32/s86 /s84/s105/s109/s101/s32/s47/s32/s104Figure 7: Cyclic charge and discharge for 15 wt.% slurry electrodes at 15 mA cm2constant current (CE), energy (EE), and voltage (VE) eciencies. Fig. 8 shows the eciencies over the cycles of charge and discharge. The CE represents the ratio of the charge amount during the charge and discharge. An average CE of 85% is observed for the tubular VRB. Slightly low level of CE can be explained by usage of the slurry electrodes. Recently, we proved that better mixing and conductivity of the static mixers provides better charge transfer to the particles, thus higher CE can be achieved [20]. We believe that improving the contact resistances between the static mixers and the graphite current collectors may yield in desired results. Moreover, we introduced a novel way of static mixing by means of global mixers around the whole tubular structure. The mixing properties of these global mixers should be studied and validated in terms of ow pro le of the electrolyte and the particles. Undesired ow patterns might have been produced, due to introduction of global mixing, causing low particle- current collector interactions. The EE is the ratio of the consumption of the total energy for the charge process and the total energy released during the discharge process. The EE of the battery is found to be approx. 60%, which is also relatively low compared to the literature values (75-90%) for standard 16 /s49 /s50 /s51 /s52 /s53 /s54 /s55 /s56/s53/s48/s54/s48/s55/s48/s56/s48/s57/s48/s49/s48/s48/s69/s102/s102/s105/s99/s105/s101/s110/s121/s32/s47/s32/s37 /s67/s121 /s99/s108/s101/s32/s78/s117/s109/s98/s101/s114/s67/s111/s117/s108/s111/s109/s98/s105/s99 /s86/s111/s108/s116/s97/s103/s101 /s69/s110/s101/s114/s103/s121 Figure 8: Tubular battery coulombic ( [), voltage (c), and energy ( ]) ecien- cies VRBs [37]. On the other hand, it is similar to the ndings of the tubular VRB made from standard graphite felt electrodes [29]. Finally voltage eciency is found to be around 70% being slightly lower than literature values (80-95%), which is expected as a result of both lower CE and EE [37{39]. 5 Conclusion In this research, a comparison of tubular and planar electrochemical reactors is studied using a slurry VRB as a showcase application. According to our previous ndings, slurry electrodes can be utilized in a VRB by using conduc- tive static mixers. Therefore, a tubular electrochemical reactor with 3D-printed static mixers is designed matching the active current collector surface area with our previous planar design. Similar cell resistances are found through electro- chemical impedance spectroscopy showing no extra ohmic or mass transport related resistances, making comparison between tubular and planar modules easier. The polarization study of the tubular cell shows up to 100 mA cm2dis- 17 charge current density while the planar battery can reach only up to 75 mA cm2. Furthermore, when comparing the volumetric power densities of the two bat- tery modules, an almost twice as high volumetric power density of the two can be reached by using the tubular module, which results from higher surface-to- volume ratio. We state that, due to the surface-to-volume ratio of cylindrical forms, increasing the active surface area would not result in explicitly higher vol- umes as would be the case for a cuboid form. Therefore, scaling up the tubular electrochemical reactor would lead to an even higher volumetric power density. Even though this study focuses only on the electrochemical performance of such reactor designs and proves higher activity, the challenges manufacturing such systems persist. Further electrochemical processes should be considered that may bene t from the higher power densities likely to be delivered by a tubular formed electrochemical reactors. Acknowledgements M. Wessling acknowledges the support of the Gottfried Wilhelm Leibniz award; This project has received funding from the European Research Council (ERC) under the grant agreement No. 694946. 18