Improved Vanadium Redox Flow Battery Performance through a Pulsating Electrolyte Flow Regime

Abstract

The increase in emissions of greenhouse gases and their environmental impact lead to a transition from primarily fossil fuel-based energy sources to renewable and eco-friendly ones, such as solar panels and wind turbines. However, these sources are intermittent, resulting in fluctuating energy production levels that require buffering, which poses significant challenges in matching production-consumption profiles. [1,2] In order to stabilise the grid and reduce dependency on fossil fuels, integrating electrical energy carriers into a smart grid to store excess energy produced during peak periods and employ it during periods of low production is a potential solution. Redox flow batteries hold promise in this regard, as the storage capacity can be decoupled from the battery size, enabling capacity to be scaled independently of power output by increasing the volume of the electrolyte. [3] In contrast to conventional secondary battery types, such as lead-acid or lithium-ion batteries, the electrolyte in a redox flow battery is non-stationary, inducing convective mass transport effects that are critical for achieving a high power output.In this work we introduced pulsations to the electrolyte flow of an all-vanadium redox flow battery, leading to a reduced stagnant boundary layer attributed to shear forces. Previously, Pérez-Gallent et al. demonstrated that the conversion can be increased by a factor of two while selectivity gains of 15-20% can be made in a electrolyser by introducing a pulsating flow. [4] Furthermore, Vranckaert et al. studied a pulsating flow combined with pillar field electrodes, using the well-known ferri-ferrocyanide redox couple, and concluded that the average limiting current density can be increased fourfold compared to working with a conventional steady state flow regime. [5] Overall, a pulsating flow behaviour yields an enhanced mass transfer of electroactive species by increasing the local velocity through pulsations. The net residence time of the reactants remains unchanged since the pulsating flow has no influence on the overall flow velocity. However, the resulting improvement of mass transfer leads to an increase in conversion. As it is clear that mass transfer enhancement strategies open the door for more efficient systems, our work will for the first time focus on the influence of a pulsating electrolyte flow on the redox flow battery performance. The results show a significant improvement: the discharge capacity at 100 mA/cm2 increased by 38.7% when applying a pulse with pulse amplitude (PA) 0.4ml and pulse frequency (PF) 2.4 Hz at a flow rate of 5 ml/min (Figure 1 A). Furthermore, this 38.7% increase in discharge capacity surpassed the performance achieved at a constant steady-state flow of 25 ml/min. Therefore, a fivefold reduction of the net flow rate is possible without performance losses, decreasing necessary pumping costs. When regarding the stability of an all-vanadium redox flow battery operated with a pulsating electrolyte flow initial fluctuations of the current efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) were recorded. However, the efficiencies became stable at 98.4%, 70.5% and 69.4% respectively, showing no decline over time (Figure 1 B). This novel and improved redox flow battery system allows to take the next step towards integrating electrical energy carriers into a smart grid suitable for renewable and green energy sources.[1] P. D. Lund, J. Lindgren, J. Mikkola, J. Salpakari, Renew. Sustain. Energy Rev. 2015, 45, 785–807.[2] H. Kondziella, T. Bruckner, Renew. Sustain. Energy Rev. 2016, 53, 10–22.[3] E. Sánchez-Díez, E. Ventosa, M. Guarnieri, A. Trovò, C. Flox, R. Marcilla, F. Soavi, P. Mazur, E. Aranzabe, R. Ferret, J. Power Sources 2021, 481, 228804.[4] E. Pérez-Gallent, C. Sánchez-Martínez, L. F. G. Geers, S. Turk, R. Latsuzbaia, E. L. V. Goetheer, Ind. Eng. Chem. Res. 2020, 59, 5648–5656.[5] M. Vranckaert, H. P. L. Gemoets, R. Dangreau, K. Van Aken, T. Breugelmans, J. Hereijgers, Electrochim. Acta 2022, 436, 141435. Figure 1