Quantifying Mass Transfer Rates in Redox Flow Batteries

Abstract

Engineering the electrochemical reactor of a redox flow battery (RFB) is critical to delivering sufficiently high power densities, as to achieve cost-effective, grid-scale energy storage. Cell-level resistive losses reduce RFB power density and originate from ohmic, kinetic, or mass transfer limitations. Mass transfer losses affect all RFBs and are controlled by the active species concentration, state-of-charge, electrode morphology, flow rate, electrolyte properties, and flow field design. The relationship among flow rate, flow field, and cell performance has been qualitatively investigated in prior experimental studies, but mass transfer coefficients are rarely systematically quantified. To this end, we develop a model describing one-dimensional porous electrode polarization, reducing the mathematical form to just two dimensionless parameters. We then engage a single electrolyte flow cell study, with a model iron chloride electrolyte, to experimentally measure cell polarization as a function of flow field and flow rate. The polarization model is then fit to the experimental data, extracting mass transfer coefficients for four flow fields, three active species concentrations, and five flow rates. The relationships among mass transfer coefficient, flow field, and electrolyte velocity inform engineering design choices for minimizing mass transfer resistance and offer mechanistic insight into transport phenomena in fibrous electrodes. Grid-scale energy storage has been identified as a key technology for improving sustainability in the electricity generation sector 1 by increasing the efficiency of the existing fossil fuel infrastructure, 2 alleviating intermittency of renewables (e.g., wind, solar), 3 and providing regulatory services.2 In particular, redox flow batteries (RFBs) have emerged as attractive devices for grid storage.1 In these rechargeable electrochemical cells, energy is stored and released by reducing or oxidizing electroactive species that are dissolved in liquid-phase electrolyte solutions. [4][5][6][7] The electrolytes are housed in large, inexpensive tanks and pumped through a power-converting electrochemical reactor. Within the reactor, a selective membrane, which permits transport of charge-balancing ions but blocks active species, separates two porous electrodes where the respective reduction and oxidation reactions take place. The decoupling of the electrochemical reactor and the energy storage tanks enables independently scalable power (reactor size) and energy capacity (tank size), as well as simplified manufacturing, easy maintenance, and improved safety.4-7 Designing the electrochemical reactor to deliver sufficiently high power is a critical consideration toward achieving the low battery costs required for economic viability and enabling the efficient delivery of various grid services. [8][9][10] Cell-level resistive losses in RFBs can originate from one of three areas: ohmic, charge transfer, or mass transport losses. Ohmic losses arise from the current collector, the porous electrode...

Towards Low Resistance Nonaqueous Redox Flow Batteries

“…7,8,27 AqRFB development has recently benefited from a series of studies implementing a single electrolyte diagnostic flow cell technique 31 to systematically evaluate cell designs 32 and to better elucidate cell-level performance limitations relating to ohmic, charge transfer, and mass transfer losses. 27,[33][34][35] A schematic of this single electrolyte technique is provided in Figure 1a, where a flow cell is connected to a single electrolyte reservoir at 50% state-of-charge (SOC). The electrolyte passes through the positive side of the cell, where the active species are oxidized, and then loops back through the negative side of the cell, where the charged species are reduced.…”

The Critical Role of Supporting Electrolyte Selection on Flow Battery Cost

“…Importantly, this steady-state model assumes that variations in active species concentration in an electrode are negligible in the directions parallel to the separator, which holds true when the electrolyte flow rate is very high relative the rate of change in battery SOC (i.e., low reactant conversion per pass). 82 This assumption aligns with high power RFB cell design, where typically the mass transfer increases afforded by pumping the electrolyte faster lead to improvements in power density that vastly outweigh the pumping losses associated with the faster flowing electrolyte. 46 To briefly summarize the full derivation and analysis available in Ref.…”

“…80 R DC = 2(R contact + R electrode ) + R mem [7] Electrode resistance model.-The electrode resistance contribution to R DC is computed using a one-dimensional, steady-state porous electrode polarization model from a recent publication, and this model was validated with experimental flow cell polarization data employing various flow fields and electrolyte flow rates. 82 The model calculates individual electrode polarization, accounting for overpotential losses due to the convective mass transfer, Butler-Volmer reaction kinetics, and the electrolyte resistivity in the pore-phase of the porous electrode. Importantly, this steady-state model assumes that variations in active species concentration in an electrode are negligible in the directions parallel to the separator, which holds true when the electrolyte flow rate is very high relative the rate of change in battery SOC (i.e., low reactant conversion per pass).…”

“…46 To briefly summarize the full derivation and analysis available in Ref. 82, Equations 8 through 16 describe how the overpotential distribution and, subsequently, the current distribution in the porous electrode are computed, using all dimensionless parameters.…”

Dithiolene Complexes of First‐Row Transition Metals for Symmetric Nonaqueous Redox Flow Batteries

“…Galvanostatic battery experiments were performed by using ac onventional zero-gap flow-cell manufactured in house;t he "Gen 2 flow-cell" was reproduced from ar eported method [47,48] (see the Supporting Information for further details). Experiments were conducted by using af low-through flow field (FTFF), 1mmc arbon paper electrodes (Technical Fibre Products Ltd.,p olyvinyl alcohol binder,2 .08 cm 2 active area) and either aC elgard membrane (Celgard 2500 Microporous Membrane, 25 mmt hickness) or F-930 cation exchange membrane (fumapem F-930, FuMA-Tech GmbH, 30 mmt hickness).…”