We studied the half cell performance of a slurry-based vanadium redox ow battery via the polarization and electrochemical impedance spectroscopy methods. First, the conductive static mixers are examined and lower ohmic and di usion resistances are shown. Further analyses of the slurry electrodes for the catholyte (VO2+- VO 2+) and anolyte (V3+- V2+) are presented for the graphite powder slurry containing up to 15.0 wt.% particle content. Overall, the anolyte persists as the more resistive half cell, while ohmic and di usion- related limitations are the dominating resistances for both electrolytes. The battery is further improved by the addition of Ketjen black nanoparticles, which results in lower cell resistances. The best results are achieved when 0.5 wt.% Ketjen black nanoparticles are dispersed with graphite powder since the addition of nanoparticles reduces ohmic, charge transfer and mass di u- sion resistances by improving particle-particle dynamics. The results prove Preprint submitted to ChemElectroChem February 13, 2020 the importance of understanding resistances in a slurry electrode system. Keywords: vanadium, redox flow battery, slurry electrode, conductive static mixer, conducting materials 1. Introduction The interest in electrical energy storage systems for the grid is growing steadily due to supply uctuations caused by the integration of renewable energy sources [1]. Large-scale energy storage systems are necessary as a bu er between energy production and grid consumption for applications like peak shaving and load leveling [2]. One of the most promising technologies is redox ow batteries (RFBs), which enable a broad range of energy stor- age capacity with variable power outputs [3, 4]. RFBs are advantageous as they decouple power and energy capacities. Most RFB technologies have electrolyte-based redox reactions, meaning that the electrodes are not con- sumed during battery operation. This enables the power to be controlled by the volume of the cell and the amount of stored energy to be controlled by the electrolyte volume stored in the external tanks. One of the most researched RFBs is the vanadium redox ow battery (VRB) [5{9]. VRBs use vanadium as redox-active species, where redox re- actions of VO2+- VO 2+as catholyte and V3+- V2+as anolyte happen si- multaneously. Solid porous electrodes, such as carbon felts or papers, are widely used in these batteries [10{12]. The electrodes do not contribute to the redox reactions, except for the fact that they provide sucient surface area for electrolyte reactions, leading to higher current densities. Recently, a novel approach has been introduced that renders the traditional concept of 2 static solid porous electrodes to a more dynamic level [13]. In this approach, conductive particle networks, called slurry electrodes, are used to transport the electrons to the electrolyte. Slurry electrodes are conductive particles dispersed in an electrolyte. The dispersion of the particles is recirculated with the electrolyte in an electro- chemical ow cell. The particles form a continuous charge transfer mech- anism via inherently induced particle-particle and particle-current collector contacts. When the conductive particle bridge is maintained, either the stor- age of the charge in the particle or the direct transfer of the charge to the redox-active species can be achieved. Several applications are already avail- able in the literature: ow capacitive deionization [14], electrochemical ow capacitor [15], and redox ow battery [16]. A system with slurry electrodes bene ts from: (1) high surface area of the particles; (2) ease of production; (3) scalability of the electrode surface area independently from the cell vol- ume; (4) maintenance through particle recycling. Carbon particles are usually the material of choice for the slurry elec- trodes in di erent applications [16{18]. Particle properties such as size, shape, electrical conductivity, and porosity play a critical role in the charge transfer for each application. A speci c volume ratio of particle-to-electrolyte is required to enable the highest charge transfer. This phenomenon has al- ready been described with the particle percolation theory that predicts better charge transfer with higher particle content [15, 19]. However, more pump- ing power is required as viscosity increases with higher particle-to-electrolyte ratio; therefore, detailed analyses of the particle properties and the rheology of the suspension are necessary to nd the best performing suspension [20]. 3 Figure 1: A slurry vanadium redox ow battery (left) and particle ow pro les for laminar and turbulent ow (right) One of the main challenges is that the slurry electrodes are shear-thinning uids. Therefore, the ow pro le under laminar conditions will su er from stagnant ow rates in proximity to the electrode surface (Fig. 1). On the contrary, slurry particles require more turbulent ow regimes, which force particles to mix and increase the contact between particles and the current collector. A simple approach to increasing the turbulent ow is using static mixers in membrane separation and electrochemical processes [21{23]. Static mixers are ow promoters that induce additional secondary ows to the ow eld; therefore, they help to mitigate the mass transport related limitations. As recently shown by our group, static mixers improve the particle move- ments in the ow cell and enable the slurry electrodes to reach higher polar- ization current densities [13]. To have a better understanding of resistances that limit the performance, slurry electrodes should be studied thoroughly in a VRB. 4 In this work, we further investigate the utilization of slurry electrodes and thereby elucidate the half cell resistances in a VRB. Polarization and elec- trochemical impedance spectroscopy (EIS) measurements are used to evalu- ate overpotentials and resistances resulting from ohmic, charge transfer and di usion processes during charge and discharge. Subsequently, a particle ad- ditive is introduced to increase the performance of the slurry electrodes by improving the electrical conductivity, thus reducing the overpotentials and their resistances. 2. Materials and Methods 2.1. Electrolyte Preparation Commercial vanadium electrolyte was purchased as a mixture of total 1:6 M VOSO 4/V2(SO 4)3in 4 M sulfuric and phosphoric acid media (GfE Metalle und Materialien GmbH, Germany). The experiments were all done at 50% state of charge (SOC). For this, the electrolyte was charged in a VRB using standard graphite felts (GFD4.6 EA, SGL Group, Germany). Slurry electrodes were prepared by manually adding conductive particle powders to the solutions on a mass fraction base. Synthetic graphite powder (GP) (Sigma-Aldrich Co., Germany) with an average size below 20 µm was used as the main slurry particle and Ketjen black®(KB) EC600-JD (Fitz Chem LLC, USA), a type of carbon black, with an average size of 34 nm was used as the additive particle. The weight fractions of the slurries were set to 0.0, 5.0 , 7.5, 10.0, and 15.0 wt. %. The KB percentage was established as 0.5 and 1.0 wt. %, while the total weight fraction of the particles was kept constant at 15.0 wt. %. Slurry electrodes containing KB were then 5 mixed for at least half an hour with magnetic stirrers. The electrolyte ow was maintained at 20 mL ©min with a peristaltic pump (Master ex L/S, Cole Parmer). 2.2. Cell A laboratory sized ow cell was used as described in our previous paper [13]. The static mixers (SM) were produced via additive manufacturing with a single layer of twisted laments (Stratasys Objet Eden 260V, RGD525). The SMs were etched in 75 % sulfuric acid for 10 minutes and then spray coated with conductive graphite lacquer (Graphit, Cramolin). 2.3. Electrochemical Measurements A potentiostat/galvanostat with an integrated impedance module (PG- STAT302N, Metrohm GmbH) was used for all measurements and overpoten- tials were measured with multimeters (PeakTech 2025). The experimental setup for overpotential measurements is illustrated in Fig. 2a. Half cell over- potentials were resolved via external voltage measurements. Therefore, PVC connectors were placed as measurement cells for both electrolyte. A glassy carbon electrode (PalmSens BV, The Netherlands) and a Hg/HgSO 4ref- erence electrode lled with 0 :5 M sulfuric acid (Xylem Analytics GmbH., Germany) were inserted in the measurement cells and used for overpotential and half cell potential evaluations. The surface area for the current density was de ned from the active surface area of the graphite plates. Constant current steps of 5 mA cm2were applied alternately between charging and discharging. The currents were applied for 30 s and the resulting cell poten- tial values were recorded. The open circuit potential (OCP) was kept in a 6 constant range of 20 mV before every new measurement. The polarization experiments were done until the charge potential of 1 :85 V, and discharge potential of 0 :8 V were reached. Figure 2: Schematic showing the experimental setup used for (a) polarization measure- ments, (b) electrochemical impedance spectroscopy with symmetric electrolyte ow (single electrolyte owing through both half cells) 7 EIS experiments were performed with a symmetrical cell setup as shown in Fig. 2b. Only one electrolyte was circulated through both sides of the cell, by connecting the outlet of one chamber to the inlet of the other. The electrolyte is fed to a ow dampener right after the peristaltic pump to reduce uid pulsations, which may have an e ect on the impedance results. Settling of the particles in the dampener was prevented with a magnetic stirrer. An alternating potential signal with an RMS amplitude of 5 mV was applied. The frequency of the signal is measured from 30 kHz to 0 :1 Hz. Direct potential during the EIS experiments was set to 0 :1 V over the OCP values. No negative potentials were used, because the symmetric electrolyte ow provides similar results. Between each measurements, a waiting time of 30 s was applied to let the particles reach a steady state of OCP. The EIS data was handled and further modelled in ZView®(Scribner, USA) software. 3. Results and Discussions All the EIS data are modelled using the equivalent circuit shown in Fig. 3a. The equivalent circuit of the model is adapted from Rommerskirchen et al., which describes an EIS methodology for a ow capacitive deionization (FCDI) process using porous activated carbon particles as slurry electrodes [24]. The resistances that accumulate in a FCDI module resemble highly those of a VRB that utilizes slurry electrodes. The di erence between the two processes is that the FCDI process does not engage faradaic reactions, while the VRB exclusively focuses on the faradaic reactions of redox species. On the other hand, due to extremely high porosity of activated carbon in the FCDI process, the convective properties become critical. Since we could 8 not determine a di erence between modelling with or without convective resistances, they are neglected in this study. The ohmic resistances (R ohm) are de ned at the high frequency region (1000 Hz$!) modelled with a resistor. R ohmincludes contact resistances between electrolyte and electrodes, electrolyte resistances, and membrane re- sistance in the cell. The middle range frequencies (10 Hz $!$1000 Hz) characterize mostly the charge transfer resistances (R ct). A resistor, R ct, depicts both faradaic reactions near the current collector and the charge transfer resistance of the particles that make contact with the surface. Here, the traditional parallel capacitor is exchanged with a constant phase element (CPE ct), as the capacitance of the current collector surface and the parti- cles itself are constantly disturbed by the redox reactions, causing imperfect capacitor behavior. Towards the low frequency region ( !$10 Hz) mass dif- fusion related resistances (R md) are slightly prominent giving the Nyquist plot a slightly at zone [18]. Very low frequencies are highly sensitive to pulsating electrolyte ow caused by pumps. A ow dampener is used to eliminate some of the oscillations. Yet, some disturbance can be observed at very low frequencies before the second semi circle of the Nyquist plots is complete. Therefore, we decided to stop the measurements right before the oscillations ( !0:1 Hz). 3.1. E ects of static mixers on cell resistances The bene t of using SMs has been shown previously through only full cell polarization experiments [13]. For that reason, EIS experiments are conducted to evaluate the improvements on the cell resistances, and analyzed for each anolyte (V3+- V2+) and catholyte (VO2+- VO 2+). The Nyquist plots 9 /s48/s46/s49/s48 /s48/s46/s49/s53 /s48/s46/s50/s48 /s48/s46/s50/s53 /s48/s46/s51/s48/s48/s46/s50/s48 /s48/s46/s49/s53 /s48/s46/s49/s48 /s48/s46/s48/s53 /s48/s46/s48/s48 /s90/s39 /s32/s47/s32/s45/s90/s39/s39 /s32/s47/s32 /s32 /s119/s47/s32/s83/s77/s32/s67/s97/s116/s46/s119/s47/s111/s32/s83/s77/s32/s67/s97/s116/s46 /s119/s47/s32/s83/s77/s32/s65/s110/s46/s119/s47/s111/s32/s83/s77/s32/s65/s110/s46/s98Figure 3: (a) EIS frequency spectrum (top) with the modi ed [24] equivalent circuit model (bottom), (b) Comparison of static mixer usage (with SM ( v) and without SM ( u)) for catholyte (blue) and anolyte (black) with 15.0 wt.% slurry electrodes via EIS are presented in Fig. 3b. The anolyte shows overall higher resistances as the catholyte, which is in correlation with previous published research [13]. A direct observation can be made through checking R ohmwhere the Nyquist plots intersects with y-axis (y=0). Using SMs drastically reduce the ohmic resistances for each catholyte and anolyte. With the help of better mixing in the ow eld, the concentration polarization is reduced, resulting in higher ionic conductivity [22, 25]. Similar behavior can be calculated for R mdfrom the modelling results, where using SMs lowers the mass di usion resistances from 0.29 to 0.10 at the catholyte and 0.27 to 0.14 at the anolyte. The improvement of the ion transport can also be explained by better mixing in the ow eld. A clear increase of R ctcan be seen from the length of the rst semicircles. R ctis four times higher when SMs are used for catholyte and two times higher for anolyte. We can explain the increase of charge resistances by the high surface roughness of the SMs. The conductive graphite coating 10 on the SMs creates surface roughness, on which micron-sized particles may easily be attached. This prolongs the charge transfer pathway and thus, higher charge transfer resistances occur. Nevertheless, as the results of our previous polarization experiments with 15.0 wt.% graphite powder and SM show improved polarization behavior, we can conclude that R ohmand R md have a higher impact than R ctfor the battery application [13]. 3.2. Half cell overpotential and resistances The primary goal in this study is to resolve the overpotentials in each half cell, and identify the main contributing resistances. Firstly, full cell ex- periments are done at 50 % SOC and the performance of the overall battery among di erent slurry concentrations are evaluated. Approximately 1.4 V OCP is set as the constant before each measurement point. Fig. 4a depicts the resulting polarization curves. The cell potential is corrected for ohmic resistances, which is measured by full cell EIS measurement. The highest particle concentration is set to 15.0 wt.% to avoid clogging issues. Both charge and discharge polarization curves show positive correlation to the in- creased particle concentration by having higher current densities for higher content of slurry electrodes. The charge side of the polarization (positive current densities) reaches 70 mA cm2current density with 15.0 wt.% slurry electrodes, as does the discharge side (negative current densities). Compared to our previous study, we can distinguish a slight di erence between polar- ization at 0 %-100 % SOC for charge and discharge and polarization at 50 % SOC [13]. The half-charged state of the electrolytes shows more realistic resistance values, as the losses caused by activation and mass transport are equally e ective. Therefore, maximum current densities that can be achieved 11 by this study show slightly lower results. /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/s32/s99/s109/s45/s50/s48/s32/s119/s116/s37/s49/s53/s32/s119/s116/s37/s48/s32/s119/s116/s37/s97 /s45/s56/s48 /s45/s52/s48 /s48 /s52/s48 /s56/s48/s45/s50/s48/s48/s45/s49/s48/s48/s48/s49/s48/s48/s50/s48/s48/s79/s118/s101/s114/s112/s111/s116/s101/s110/s116/s105/s97/s108/s32/s47/s32/s109/s86 /s67/s117/s114/s114/s101/s110/s116/s32/s68/s101/s110/s115/s105/s116/s121 /s32/s47/s32/s109/s65/s32/s99/s109/s45/s50/s99/s97 /s111/s104/s109 /s98 /s97/s110/s111/s104/s109 /s99/s97/s97/s110 Figure 4: (a) Full cell polarization of slurry electrodes from 0 to 15 wt.%, (b) Catholyte (ca), anolyte ( an), and ohmic ( ohm) overpotentials of 15 wt.% slurry electrodes Half cell contributions on the 15.0 wt.% slurry electrodes are shown in Fig. 4b. The catholyte and anolyte overpotentials are relatively similar being approximately 30 mV for both charge and discharge. The highest contribu- tion to the total overpotential is the ohmic overpotential ( ohm) with up to 180 mV at 70 mA cm2. It is therefore obvious that the ohmic resistances that persist when using slurry electrodes are one of the limiting factors in the system. The breakdown of the half cell resistances with EIS is presented in Fig. 5. The single electrolyte system evaluates catholyte and anolyte separately, therefore more distinctive analysis of the resistances can be made. The Nyquist plots of the catholyte with increasing particle content are presented in Fig. 5a and the resolved resistances in Fig. 5b. Ohmic resistances show a slight increase with higher particle content. R ohmincreases from 0 :09 to 0:11 for the catholyte. A decrease in the charge transfer resistances can 12 be seen, with it having the lowest resistance values among all. R ctdecreases from 0:010 to 0:007 with higher particle content. Meaning that the more particles the slurry electrodes have, the better the charge transfer is. The nal and the highest resistance is the mass di usion resistance. Surprisingly, Rmddecreases from 0 :25 to 0:20 . A swift decrease of R mdoccurs between 5 and 7.5 wt.% slurry, where presumably particle-particle interactions become observable. Particle percolation theory describes a critical ratio of particle content for the dispersion to form highly conductive particle percolations [19, 26, 27]. Although it has been suggested that better charge transfer may be obtained with particle percolations, however, the low values of R ctdo not reveal an apparent proof for better charge transfer phenomena. The improve- ment of R mdis more distinct in this case and further improvements should be considered for further reduction. We investigate further on the anolyte resistances in Fig. 5c-d. Similar to the catholyte, anolyte ohmic resistance shows an increase with the higher particle content. Only the slurry electrodes with 15.0 wt.% showed a de- crease at the R ohmright after the 10.0 wt.% slurry electrodes. Total increase of R ohmis from 0:14 to 0:19 . It is worth noticing that anolyte ohmic re- sistances are almost twice as high as the catholyte ohmic resistances. The charge transfer resistances in anolyte demonstrates a slight increase from 0:04 to 0:05 , which con icts with the behavior of R ctfor the catholyte. Nevertheless, the charge transfer resistances of the anolyte are higher com- pared to the catholyte. Finally, R mdis again the highest resistance for the anolyte, beginning at 0 :46 and decreasing to 0 :31 with higher particle content. The decrease starts after the 10.0 wt.% slurry content, which veri es 13 /s90/s39 /s32/s47/s32 /s32/s45/s90/s39/s39 /s32/s47/s32 /s32/s97 /s48/s32/s119/s116/s46/s37 /s49/s53/s32/s119/s116/s46/s37 /s48 /s53 /s55/s46/s53 /s49/s48 /s49/s53/s48/s46/s48/s48/s46/s49/s48/s46/s50/s48/s46/s51/s82/s101/s115/s105/s115/s116/s97/s110/s99/s101/s32/s47/s32 /s83/s108/s117/s114/s114/s121 /s32/s67/s111/s110/s116/s101/s110/s116/s32/s47/s32/s119/s116/s46/s37/s82 /s99/s116/s82 /s111/s104/s109/s82 /s109/s100/s98 /s90/s39 /s32/s47/s32/s45/s90/s39/s39 /s32/s47/s32/s99 /s48/s32/s119/s116/s46/s37 /s49/s48/s32/s119/s116/s46/s37/s49/s53/s32/s119/s116/s46/s37 /s48 /s53 /s55/s46/s53 /s49/s48 /s49/s53/s48/s46/s48/s48/s46/s50/s48/s46/s52/s48/s46/s54/s82/s101/s115/s105/s115/s116/s97/s110/s99/s101/s32/s47/s32 /s83/s108/s117/s114/s114/s121 /s32/s67/s111/s110/s116/s101/s110/s116/s32/s47/s32/s119/s116/s46/s37/s82 /s109/s100 /s82 /s111/s104/s109 /s82 /s99/s116/s100Figure 5: Half cell resistances; (a) the Nyquist plot for the catholyte, (b) resolved resis- tances for the catholyte, (c) the Nyquist plot for the anolyte, (d) resolved resistances for the anolyte, mass di usion presented with R md, ohmic with R ohm, and charge transfer with R ct the previous ndings on the catholyte, namely that the particle percolation threshold has a distinct impact on mass transport resistances. Overall all the anolyte resistances that are presented, show higher values than the catholyte, which is the same relation as is shown in the literature [28]. 14 3.3. Improving resistances by Ketjen black According to our results obtained from full cell and half cell polarization and EIS studies, the main limitations of the slurry electrodes are due to the ohmic and the di usion related resistances in the VRB. Therefore, reducing these resistances would be bene cial for the applicability of slurry based systems. GP used in the slurry electrodes are micro sized ( $20µm) and sharply shaped. Electrical conductivity depends on the contact area between these particles, which is mainly limited by the shape. One possible approach is to use a conductive additive within the slurry electrodes to increase the conductivity [29, 30]. In this work, we used KB nanoparticles as conductive additive for the GP slurries. The conductivity of the particle network is improved by the nanoparticles lling the GP-GP voids, thus creating higher surface area for the charge transfer. /s45/s56/s48 /s45/s52/s48 /s48 /s52/s48 /s56/s48/s45/s52/s48/s48/s45/s50/s48/s48/s48/s50/s48/s48/s52/s48/s48/s79/s118/s101/s114/s112/s111/s116/s101/s110/s116/s105/s97/s108/s32/s47/s32/s109/s86 /s67/s117/s114/s114/s101/s110/s116/s32/s68/s101/s110/s115/s105/s116/s121 /s32/s47/s32/s109/s65/s32/s99/s109/s45/s50/s98 /s49/s53/s32/s119/s116/s46/s37/s32/s71/s80 /s49/s52/s46/s53/s32/s119/s116/s46/s37/s32/s71/s80 /s32/s32/s32/s32/s32/s32/s32/s32/s32/s43 /s32/s32/s48/s46/s53/s32/s119/s116/s46/s37/s32/s75/s66 /s49/s52/s32/s119/s116/s46/s37/s32/s71/s80 /s32/s32/s32/s32/s32/s32/s32/s32/s32/s43 /s32/s32/s49/s32/s119/s116/s46/s37/s32/s75/s66/s49/s52/s32/s119/s116/s46/s37/s32/s71/s80 /s32/s32/s32/s32/s32/s32/s32/s32/s32/s43 /s32/s32/s49/s32/s119/s116/s46/s37/s32/s75/s66 Figure 6: (a) Illustration of KB addition to slurry electrodes, (b) full cell polarization of the KB added slurry electrodes with total particle content of 15 wt.% The use of KB with GP slurry electrodes is illustrated in Fig. 6a. KB 15 addition to the GP slurry electrode is done by 0.5 and 1.0 wt.%, respectively. The total weight percentage is kept constant at 15.0 wt.% slurry electrode. The addition of KB could not be increased further, as the viscosity of the slurry electrodes become inapplicable. Fig. 6b shows the full cell polarization of the 15.0 wt.% GP, 14.5 wt.% GP + 0.5 wt.% KB, and 14.0 wt.% GP + 1.0 wt.% KB. Both KB added slurry electrodes perform better on the charge and discharge polarization by having higher current densities. Interestingly, 0.5 wt.% KB addition shows the best performance increasing the discharge current density from 65 to 80 mA cm2at a constant overpotential of approx. 340 mV. The same behavior can be seen on the charge side where the current density increase from 60 to 75 mA cm2at a constant overpotential of approx. 250 mV. At the same time, the 1.0 wt.% addition of KB results in slightly lower current densities than 0.5 wt.% on the discharge side, while the charge side is very similar to the results of 0.5 wt.% KB addition. This suggests that using small amounts of conductive additive can improve the slurry battery performance, but higher nanoparticle content may not have the best e ect. Further EIS analysis is done to resolve the resistances that change with the addition of KB in each half cell. Fig. 7a-b present the resistances for the catholyte and Fig. 7c-d for the anolyte. R ohmdecreases from 0 :11 to 0:09 with 0.5 wt.% KB addition in the catholyte. A greater decrease in R ohmcan be observed at the anolyte side, R ohmdecreasing from 0 :19 to 0:14 with 0.5 wt.% KB addition. For both electrolytes, R ohmwith 1.0 wt.% KB addition shows lower resistances than only 15.0 wt.% GP but higher than 0.5 wt.% KB. The decrease in the ohmic resistances proves the hypothesis of the possible increase in the electrical conductivity by better particle-particle contact when 16 /s97 /s48/s46/s48 /s48/s46/s53 /s49/s46/s48/s48/s46/s48/s48/s46/s49/s48/s46/s50/s48/s46/s51/s82/s101/s115/s105/s115/s116/s97/s110/s99/s101/s32/s47/s32 /s75/s101/s116/s106/s101/s110/s32/s66/s108/s97/s99/s107/s32/s80/s101/s114/s99/s101/s110/s116/s97/s103/s101/s32/s47/s32/s119/s116/s46/s37/s98 /s82 /s109/s100 /s82 /s111/s104/s109 /s82 /s99/s116 /s49/s53/s32/s119/s116/s37/s32/s71/s80/s99 /s90/s39 /s32/s47/s32 /s48/s46/s48 /s48/s46/s53 /s49/s46/s48/s48/s46/s48/s48/s46/s49/s48/s46/s50/s48/s46/s51/s82/s101/s115/s105/s115/s116/s97/s110/s99/s101/s32/s47/s32 /s75/s101/s116/s106/s101/s110/s32/s66/s108/s97/s99/s107/s32/s80/s101/s114/s99/s101/s110/s116/s97/s103/s101/s32/s47/s32/s119/s116/s46/s37/s100 /s82 /s109/s100 /s82 /s111/s104/s109 /s82 /s99/s116Figure 7: Half cell resistances for the KB addition to the GP slurry electrodes; (a) the Nyquist plot for the catholyte, (b) resolved resistances for the catholyte, (c) the Nyquist plot for the anolyte, (d) resolved resistances for the anolyte the nanoparticles are used. The 1.0 wt.% KB addition however, increases the ohmic resistances again, which is due to presumably the dispersion quality that gets visibly worsened. The charge transfer resistances for catholyte and anolyte show similar trend. R ctshows minimum resistance for 0.5 wt.% KB addition in both catholyte and anolyte with relatively low values. A noticeable di erence is observed in the anolyte side for R ct, by which the 17 resistance increase is even higher than slurry electrodes without KB addition. Therefore, it is clear that a higher content of nanoparticles in the slurry electrodes has a negative e ect on the charge transfer mechanism. The highest improvement for the addition of the nanoparticles can be observed in the di usion related resistances. R mdfor the catholyte decreases from 0:22 to 0:13 for 1.0 wt.% KB addition, surprisingly even lower than 0.5 wt.%. Nevertheless, the overall observation recurs for the anolyte, since the 0.5 wt.% KB addition shows the lowest R md, which decreases from 0 :27 to 0:23 . We can explain the improved di usion resistances by the disturbed particle percolations of the GP. Smaller KB particles constantly break the percolated GP particle network locally. Thus, local di usion of the ions is highly improved. However, the overall recurrence of particle percolations becomes more frequent, leading to an enhancement of the ohmic and charge transfer resistances. 4. Conclusions In this work, we further investigated the polarization behavior and limit- ing resistances for each half cell of a slurry electrode-based vanadium redox ow battery. Firstly, the e ect of using static mixers was analyzed with the half cell impedance measurements that indicate improved ohmic and mass transfer limitations when static mixers are utilized. Further analysis of each half cell showed a relatively high contribution of the ohmic and mass transfer resistances compared to charge transfer resistances. Higher particle content in the slurry electrodes improved the charge transfer and more explicitly the mass transfer resistances, whereas the ohmic resistances increased. Overall 18 the catholyte half cell of the battery showed less resistive behavior than the anolyte side in the slurry VRB. After resolving and identifying the resistances for the slurry electrodes containing only graphite powder, nanosized Ketjen black particles were added to increase the conductivity and reduce the resistances in the battery. An addition of 0.5 and 1.0 wt.% Ketjen black was investigated in the graphite powder slurry with a total particle content of 15.0 wt.%. The best result was achieved with 0.5 wt.% Ketjen black addition which lowers the values of ohmic, charge transfer, and mass di usion resistances. The e ect of KB addition was demonstrated by the polarization study that shows the high- est current densities with the 0.5 wt.% KB content, reaching 80 mA cm2 when discharging. Ultimately, as a result of EIS and polarization studies, we can evaluate more on the nature of the slurry electrodes via analyzing and de ning the resistances that exist and further improvement strategies can be established. Acknowledgements M. Wessling acknowledges the support of the Gottfried Wilhelm Leib- niz award; This project has received funding from the European Research Council (ERC) under the grant agreement No. 694946. 19