A. Ivanovskaya, V. Beck, S. Chandrasekaran, B. Moran, T. Weisgraber, E. Draves, M. Worsley, S. Baker, E. Duoss*Lawrence Livermore National Laboratory,United States*

Keywords: flow battery, mass transport coefficient, flow-through electrode, 3D-printed electrode, porous electrode, graphene electrode, ferrocyanide

Summary:

Since electrochemical reactions occur at electrode surfaces and rely on fast mass transport, liquid type reactors gain efficiency from forcing solutions through 3-dimentional porous electrodes with high surface area and enhanced mass transport properties. Uneven velocity distribution through disordered pore channel networks of conventional foam, felt or fiber paper electrodes is known to cause loss in efficiency due to channeling. Recent technological advances in additive manufacturing offer resources for fabrication of electrodes with controllable periodic pore structures that are free from the channeling and, therefore, potentially more efficient. Manufacturing controllable porous networks opens the door to the engineering of flow inside flow-through electrodes and finding optimal electrode designs that through enhanced mass transport maximize power generation. In this report, by means of numerical simulation and experiment, we study mass transport coefficients of periodic porous lattice flow-through electrodes. Graphene electrodes with simple cubic and face centered cubic structures were printed using the direct ink write method. Limiting currents were measured in ferrocyanide solutions as a function of flow rates and volumetric mass transport coefficients serving as a performance indicator were derived. Measured volumetric mass transport coefficients were found to be in a good agreement with that predicted by numerical simulations performed for the same electrode structures. Results show that while staying at laminar regime (Re < 100), the scaling factor of mass transfer coefficient as a function of flow velocity increases from 0.3 to 0.6 after threshold velocity is reached due to a transition in the mass transport regime from viscous to inertial. Previously sharp increases in the scaling factor were observed only due to changing from laminar to turbulent regimes at much higher Re numbers (Re > 2000). Observation of scaling factor increase at much lower Re numbers in this work was possible due to controlled geometry of printed electrode. Electrodes that yield high mass transport coefficients at low flow velocity are advantageous due to the pumping power inefficiencies present at high velocities. As a result, volumetric mass transport coefficients of FCC electrodes with 0.8 mm lattice parameter are comparable to Ni foam in the viscous regime and approach the better performing carbon felts in the inertial regime. Further improvements in the mass transport properties will be achieved by 3D printing lattices with modified design parameters (e.g. pore size, filament diameter, etc.). The results will be discussed in the context of theory and previous literature.