Today's electrochemical reactor design is a less de veloped discipline as compared to the electrocataly tic synthesis. While catalysts show increasing conversi on rates, they are often operated without measures for the reduction of concentration polarization effects . As a result, a stagnant boundary layer forms at t he electrode-electrolyte interface. This stagnant boun dary layer presents an additional voltage drop and reduces the energy efficiency. It is generally acce pted that this phenomenon is caused by a combinatio n of fast electrode reactions and slow diffusion of t he reacting species. Our earlier work demonstrated the potential of non-conducting static mixers to reduce concentration polarization effects. However, there are few studies on conductive static mixers applied as electrodes. In this study, we present a new concept of additive manufactured flow through electrode mixers . Our electrode geometry combines a high surface area with mixing properties, diminishing concentrat ion polarization effects of transport-limited react ions. Mass transport properties of these conductive stati c mixers are evaluated in an additive manufactured electrochemical reactor under controlled conditions by applying the limiting-current method. Keywords: printed electrodes, additive manufacturing, kinetic s, interfaces, concentration polarization 1. Introduction Commonly, electrochemical research focuses on catal yst materials for new applications. Often standard electrochemical methods are used to quantify the ch aracteristic features of the electrocatalytic react ion with ever growing depth in understanding molecular details. [1-2] Significantly less effort is spent on understanding details of electrodes in real reactor configurations. [3-6] Concentration polarization at the electrode-electrolyte interface is systematically e liminated in ideal electrochemical characterization methods such as the rotating disk electrode, while they are omnipresent in real reactors. [7-13] Fluid control at electrodes is hence a pre-requisite to obtain el ectrochemical conversions as characterized and tail ored for new electro-catalysts on a microscale. [14-20] Additive manufacturing serves as an example to cre ate new electrodes and electrochemical reactors with functi onal geometries. [21-25] Below, we present additive manufactured flow through electrode mixers with yet unprecedented geometry and enhanced hydrodynamics. These electrodes serve as an example how to diminish concentration polarization as evidenced by increasing limiting current densities. The electrode geometry is based on our earlier published work applied as turbulence promoter in de vices for membrane separation processes. [26] The geometry has been proven to influence concentration polarization effects while operated with complex fluids during filtration. [27] Here we prove that such geometries when made elect rically conductive can act as electrodes with improved mass transport properti es. In our manuscript we use the term electrode mix er rather than the frequently used term turbulence pro moter to avoid technical misconception. 2. Materials and methods Printing electrodes The electrode mixer was designed following the detai ls of Fritzmann et al. [26] According to our specifications the electrode of helical architectur e was fabricated at the Fraunhofer Institute for La ser Technology ILT applying selective laser melting of stainless steel (X2CrNiMo 17-12-2). [28-30] As shown in Figure 1 and 3(c) the structure is composed of repe ating unit cells with two oppositely twisted filame nts which have a single point of contact. The center of the upper filament is positioned on the outer diam eter of the lower filament. This allows defining the ent ire geometry by two dimensionless parameters Lm/Dm and dm/Dm as well as the number of twisted filaments nm.[26-27] Geometrical characteristics of the additive manufactured electrode as well as the reactor housi ng are reported in Table 1. Figure 1. (a) Electrode mixer of helical architectu re, (b) cross-section of the reactor housing includ ing the electrode mixer and the upper flat electrode. Reactor design To evaluate mass transport properties for different electrode configurations, an electrochemical react or housing was designed and fabricated using polyjet 3 D printing (Stratasys, Objet Eden 260V).[31] The reactor housing shown in Figure 1(b) and Figure 2 was print ed layer by layer with a transparent photopolymer (Stratasys, RGD810). During printing the internal f luid channels need supporting structures (Stratasys , (a) (b) SUP705) which were later cleaned using a high-press ure washer (Krumm-tec, RK Top 5). Any remaining support was subsequently removed in a stirred bath of 1 mol/L sodium hydroxide for approximately 12 hours. Figure 2. Scheme of the electrochemical reactor con sisting of the following parts: (1) flow inlet, (2 & 4) profile gaskets, (3) reactor housing with build-in electrod e mixer and two flat electrodes, (5) flow outlet, ( 6) electrode connector. The reactor consists of the following functional parts: (1) inlet with an integrated mesh flow distributor with 200 µm pores which generates a uniform flow prof ile over the entire channel width, (2 & 4) additive manufactured flexible profile gaskets (Stratasys, FLX9 50), (3) reactor housing with the electrode mixer guided by a rail to maintain the interelectrode gap and two flat stainless steel electrodes, (5) outlet with an embedded gold spring probe pin (Ingun Prüfmittelb au, GKS-003-0040) to connect the electrode mixer, (6) electrical connector for the flat electrodes. Table 1. Nominal geometrical characteristics of the electrode mixer and the reactor housing. The elect rode mixer surface area was obtained from CAD models. Filament diameter Dm 5.2 mm Filament thickness dm 0.8 mm Number of filaments nm ~12 - Filament length (single twist) Lm 10.4 mm Electrode mixer height hm 6.8 mm Electrode mixer surface area Am 14195.0 mm² Electrode mixer void fraction φm 77.9 % Reactor channel height hch 7.6 mm Reactor channel width wch 30.0 mm Reactor channel length lch 100.0 mm Flat electrode surface area Ae 3000.0 mm² Fluid dynamics To perform the experiments under well defined flow co nditions, fluid dynamics simulations with Comsol Multiphysics were carried out during the design proces s of the electrochemical reactor. In particular the inflow conditions were optimized with an integrated m esh flow distributor as shown in Figure 2. In Figure 3 we demonstrate the three dimensional flow distribution at (a) the inlet without flow distributor and (b) the inlet with flow distributor. The three dimensional flow field inside the electrode mixer is visualized by (d) six layers along the flow path of (c) the repeati ng unit cell. Here, the velocity magnitude is shown b y a color-coded background and arrows denote the flow dir ection. The two different inlet configurations were simulated with laminar inflow conditions and zero pre ssure at the outlet. Symmetry and periodic boundary conditions were applied to the simulations to reduce calculation effort. Characterization of the mass transport Mass transport properties were evaluated for two diffe rent electrode configurations using the limiting- current method. [32] An electrolyte was prepared of 5 mM potassium ferrocy anide and a twentyfold excess of potassium ferricyanide in deionised water. Additi onally 1 mol/L potassium hydroxide was added as supporting electrolyte. This composition ensures that o nly the anode reaction regenerates potassium ferricyanide. [4] All prepared chemicals were of analytical grade an d used without further purification. The aqueous electrolyte was circulated from a stirred res ervoir (Heidolph, MR Hei-Standard) to the reactor compartment via a gear pump (Ismatec, MCP-Z Process IP65) at a constant non-fluctuant flow rate of 40 mL/min. The discharged electrolyte was recycled to the stirred reservoir. Prior to the measurement the electrodes were cleaned, mounted in the reactor housing and connected to a potentiostat (Metrohm, Autolab PGSTAT302N). Once a potential was applied the reversible redox-reaction of potassium ferrocyanide and potassium ferricyanide started to evo lve in the reactor compartment. [Fe(CN) 6]-4 ⇄ [Fe(CN) 6]-3 + e - Current-potential curves were recorded with chronoamp erometry by stepwise increasing the potential from +0.0 V to a maximum of +1.2 V while plotting the current over time. With each change in potential a new steady state was allowed to develop. Steady state cu rrent-potential data were obtained after a maximum of 240 seconds. As recorded by the potentios tat we report space averaged data normalized for the effective electrode area. These data were also repl otted as the reciprocal resistance against the reciprocal current density. The reciprocal resistanc e and the reciprocal current density were calculated of the applied potential, the measured current in steady state and the anode surface area. Here, diffusion resistance related to the arising boundary layer can b e identified. [33-34] Typically s-shaped curves are observed with under-limiting and over-limiting region s. The point of inflection is designated to the limiting current density Ilim /Aeff . According to the limiting-current condition a maximu m concentration gradient is obtained and potassium ferrocyanide is locally deplete d at the anode surface. In steady state, the concentration of potassium ferrocyanide at the anode s urface Ci,0 is considered virtually zero. Since the anolyte and the catholyte are not separated, a depletio n of potassium ferrocyanide in the bulk is fully compensated by the cathodic reaction. Hence, the bulk concentration Ci,bulk can be considered constant. With these assumptions the mass transfer coefficient kL can be estimated according to Equation 1.[33-34]  ∙ ∙  ∙, (1) Here ne equals the number of transferred electrons, F is known as the Faraday constant, Aeff accounts for the effective anode surface area and Ci,bulk is the concentration of the component potassium ferroc yanide in the bulk. [35] The effective anode surface area Aeff was determined of the designed models and is shown in Table 1. 3. Results and discussions To perform the experiments under well defined flow co nditions, fluid dynamics simulations were carried out during the design process of the additive manufactu red electrochemical reactor. At the inlet, flow conditions were optimized towards a homogeneous distr ibution by embedding a mesh as visualized in Figure 2, 3(a) and 3(b). Hence, the formation of dea d zones and channeling effects inside the reactor housing and at the electrode mixer were prevented. Figure 3. Simulations of the 3D flow field inside ( a) the inlet without flow distributor, (b) the inle t with flow distributor and the electrode mixer visualized by (d) six layer s along the flow path of (c) the repeating unit cel l. The velocity magnitude is shown by a color-coded background and arrows denote the flow direction. In this article, the effect of the inflow conditions on the electrochemical conversion were not studied. However, former studies showed the positive effect of homogeneous flow conditions on the current (a) (b) (c) (d) density. [36] The flow field inside the electrode mixer is visuali zed in Figure 3(d) by six layers along the flow path of the repeating unit cell shown in Figure 3(c). The helical electrode mixer leads to a rotational flo w profile reducing the concentration polarization layer at the electrode-electrolyte interface. The designed electrochemical reactor shown in Figure 2 was operated as an empty channel with two flat electrodes as cathode and anode. By adding the electro de mixer to the reference system the second configuration was set up. Here the electrode mixer wa s connected as anode and the two flat electrodes were operated as cathodes. The measured raw data are summarized in Figure 4(a). For both electrode configurations the measurements show a similar trend : With increasing applied potentials a limitation of diffusion develops as observed by changes in curren t curvature. These changes are more pronounced for the configuration without mixer as observed by the form ation of a distinct plateau. The formation of a plateau indicates that the entire electrode operates und er mass transfer limitation resulting in an evenly formed boundary layer across the entire electrode-el ectrolyte interface. In contrast a non-uniform laminar boundary layer forms at the surface of the electrode mi xer as indicated by a less distinctive plateau. In t his case the electrode mixer is disrupting the laminar bo undary layer at the electrode-electrolyte interface shifting mass transfer limitation towards a higher pote ntial. Figure 4. Evaluation of mass transport properties f or the reactor configured as empty channel and with electrode mixer. (a) measured raw data, (b) resistance plot. All measurements were repeated three times in a ran dom order. Error bars indicate the measured uncertainty of the reciprocal resistance. To quantify both electrode configurations in terms of mass transport properties, the current is first normalized by the electrode surface area as indicate d by dashed lines in Figure 4(a). When data are visualized in Figure 4(b) as a plot of the reciproc al resistance against the reciprocal current density, mass transport limitations and the formation of a concentr ation polarization boundary layer appear as rapid changes in resistance. [33] The inflection points representing the limiting curre nt densities are located at 2.28 A/m² (0.17 V) and 4.02 A/m² (0.36 V) for the m easurements without and with electrode mixer, respectively. Comparing the mass transport properties reveals an increase of 76 % in mass transport coefficient as the electrode mixer is applied. The inc reasing mass transport properties are caused by hydrodynamic effects imposed by the electrode mixer : Increased shear forces at the electrode-electrolyte interface lower the thickness of the laminar boundary layer and thereby shorten the distance of diffusion. [37-38] Our measurements show the feasibility to make use of additive fabrication methods to generate new electrode geometries. The approach, in conjunction with systematic CFD multiphysics modeling, presents a relatively easy way to quickly t est and optimize various electrode geometries to suit different electrochemical reactions. Whether the presente d electrode geometry also helps to improve the physico-chemical hydrodynamics in gas evolution rea ctions remains to be investigated in the future. We are confident that the presented evidence stimulates the field of electrochemistry beyond the search of electrocatalysts only. 4. Conclusion The article describes new electrode architectures pri nted of stainless steel by an additive manufacturing technique. The geometry has improved mass transport p roperties as revealed by increased limiting current densities. This work opens a plethora of questions as indicated in the discussion part. In particular, it motivates further research on new electrode geometries in relation to electrochemical phenomena such as gas bubble evolution and polarization in complex 3D architectures. Acknowledgements This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No 694946). It was supported by the Alexander-von-Humboldt Foundation and the Cluster of E xcellence “Tailor-made Fuels from Biomass” (TMFB) funded by the Excellence Initiative of the German federal and state governments. J. Hereijgers greatly acknowledges the Research Foundati on – Flanders (FWO) for support through a Post- Doctoral grant (28761).