Determination of the mass-transport properties of vanadium ions through the porous electrodes of vanadium redox flow batteries

Tenny

Barton

et al. 2017

151

176

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...

mentioning

confidence: 99%

Quantifying Mass Transfer Rates in Redox Flow Batteries

Tenny

Barton

et al. 2017

151

176

“…Thus, electrolyte design for NAqRFBs must consider viscosity as a key materials optimization parameter, and, in the case of this work, MeCN vastly outperforms PC as a base solvent for a low viscosity and high conductivity electrolyte. Figure 4d hint that mass transfer rates are critical in determining the total ASR for RFBs, but only a handful of prior reports have systematically studied mass transfer effects in AqRFBs, 32,34,[60][61][62] with no reports directly relating to NAqRFBs. To begin addressing this knowledge gap, Figure 5 highlights nonaqueous flow cell performance for the PC-and MeCN-based electrolytes, with a Daramic separator, at 4 flow rates spanning greater than an order of magnitude.…”

Section: Nyquist Plots Inmentioning

confidence: 99%

“…We hypothesize that, as the flow rate increases, the cell approaches the limit of infinitely fast mass transfer; typically, mass transfer coefficients in porous media increase with flow velocity to a power ≤1. 60,63 Thus, in the limit of high electrolyte velocity, the ohmic and charge transfer losses dominate the cell's resistive characteristics. As such, selecting an optimal flow rate will require balancing the cell ASR with pumping losses; beyond a certain critical flow rate, the ASR reduction will be smaller than the magnitude of pumping loss required to boost the flow rate.…”

Section: Nyquist Plots Inmentioning

confidence: 99%

Towards Low Resistance Nonaqueous Redox Flow Batteries

Barton

Carney

et al. 2017

Nonaqueous redox flow batteries (NAqRFBs) are a promising, but nascent, concept for cost-effective grid-scale energy storage. While most studies report new active molecules and proof-of-concept prototypes, few discuss cell design. The direct translation of aqueous RFB design principles to nonaqueous systems is hampered by a lack of materials-specific knowledge, especially concerning the increased viscosities and decreased conductivities associated with nonaqueous electrolytes. To guide NAqRFB reactor design, recent techno-economic analyses have established an area specific resistance (ASR) target of <5 cm 2 . Here, we employ a state-of-the-art vanadium flow cell architecture, modified for compatibility with nonaqueous electrolytes, and a model ferrocene-based redox couple to investigate the feasibility of achieving this target ASR. We identify and minimize sources of resistive loss for various active species concentrations, electrolyte compositions, flow rates, separators, and electrode thicknesses via polarization and impedance spectroscopy, culminating in the demonstration of a cell ASR of ca. 1.7 cm 2 . Further, we validate performance scalability using dynamically similar cells with a ten-fold difference in active areas. This work demonstrates that, with appropriate cell engineering, low resistance nonaqueous reactors can be realized, providing promise for the cost-competitiveness of future NAqRFBs. Grid-scale energy storage has emerged as a critical technology for alleviating the intermittency of renewables, 1 improving the efficiency of the existing grid infrastructure, and offering frequency or voltage regulation.2 Redox flow batteries (RFBs) are particularly attractive storage devices for energy-intensive grid applications.2,3 In a typical RFB, charge-storing active species are dissolved in liquid electrolytes, which are housed in large and inexpensive tanks. The electrolytes are pumped through a power-generating electrochemical reactor, where the active species undergo reduction or oxidation on the surfaces of porous electrodes to charge or discharge the battery. [3][4][5][6] This unique architecture offers several advantages over conventional battery systems, including decoupled power and energy scaling, long operational lifetimes, easy maintenance, simplified manufacturing, and high active-to-inactive materials ratio (particularly at long storage durations). Despite many attractive features, state-of-the-art RFBs are currently too expensive for widespread adoption.7 While the majority of RFB electrolytes are aqueous, 6 transitioning to nonaqueous chemistries could enable higher cell potentials via wider electrolyte stability windows, subsequently increasing the electrolyte energy density and reducing chemical costs. 6,[8][9][10][11] Furthermore, the use of nonaqueous solvents enables a broader palette of potentially inexpensive active materials, which could significantly lower RFB costs. 7,12 In contrast to their aqueous counterparts, nonaqueous redox flow batteries (NAqRFBs) are a nascent technology,...

“…A typically overlooked characteristic, electrolyte viscosity is of critical importance as it directly impacts mass transfer rates in the porous electrodes of a RFB, as well as required pumping power through the entire battery. [55][56][57] As mentioned before, cell potential is a key parameter that affects the power and energy density of the RFB, both of which define RFB cost. 23,26 Second, to link these materials properties to RFB cost, fullcell ASR is calculated by implementing a one-dimensional porous electrode model that solves for electrode polarization as a function of electrolyte resistivity, charge-transfer kinetics, and convective mass transfer rate.…”

Section: A3884mentioning

confidence: 99%

“…We employ an empirical relationship (Equation 5) for the porescale mass transfer rate (Figure 1c), measured for the case of vanadium transport in a porous carbon-felt electrode, 55 where d f is the electrode fiber diameter (m) and Re is the Reynolds number (-) for the electrolyte within the porous electrode. For this work, the electrode fiber diameter is assumed to be that of SGL 25AA (d f ≈ 7 μm).…”

Section: Estimating Materials Parametersmentioning

confidence: 99%

The Critical Role of Supporting Electrolyte Selection on Flow Battery Cost

Darling

Drake

et al. 2017

Redox flow batteries (RFBs) are promising devices for grid energy storage, but additional cost reductions are needed to meet the U.S. Department of Energy recommended capital cost of $150 kWh −1 for an installed system. The development of new active species designed to lower cost or improve performance is a promising approach, but these new materials often require compatible electrolytes that optimize stability, solubility, and reaction kinetics. This work quantifies changes in RFB cost performance for different aqueous supporting electrolytes paired with different types of membranes. A techno-economic model is also used to estimate RFB-system costs for the different membrane and supporting salt options considered herein. Beyond the conventional RFB design incorporating small active species and an ion-exchange membrane (IEM), this work also considers size-selective separators as a cost-effective alternative to IEMs. The size selective separator (SSS) concept utilizes nanoporous separators with no functionalization for ion selectivity, and the active species are large enough that they cannot pass through the separator pores. Our analysis finds that SSS or H + -IEM are most promising to achieve cost targets for aqueous RFBs, and supporting electrolyte selection yields cost differences in the $100's kWh Energy storage has emerged as a key technology for improving the sustainability of electricity generation 1 by improving the efficiency of existing fossil-fuel infrastructure through load-leveling or price arbitrage, 2 alleviating the intermittency of renewables (i.e., solar, wind) to promote their broad implementation, 3 and providing high-value services such as frequency regulation, voltage support, or back-up power.2 Redox flow batteries (RFBs) are promising devices for low-cost grid energy storage due to decoupled capacity and power scaling, long operational lifetime, easy thermal management, and good safety features.2,4-9 Unlike enclosed batteries (i.e., lithium-ion, nickel-metal hydride), RFBs implement soluble redox active species dissolved in liquid electrolytes, which are stored in large, inexpensive tanks. Specifically, the electrolyte is comprised of a supporting electrolyte, which contains solvent (e.g., water) and a supporting salt (e.g., sulfuric acid, sodium chloride), and the redox active species (e.g., bromine). The electrolyte is pumped through an electrochemical stack where the active species are oxidized or reduced to charge or discharge the battery. The size of the electrochemical stack determines the power rating, while the tank volume determines the energy capacity, enabling scalability unique to the RFB architecture. The introduction of new redox chemistries is a strategy for substantially lowering the electrolyte (energy) cost contribution to the total battery cost via decreased chemical costs or increased electrolyte energy density. 23,26 Key active species characteristics in determining the RFB electrolyte cost are the solubility (M), molar mass (kg mol −1 ), number of electrons stored pe...