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...

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...

No abstract

We have developed a simplified model for the steady-state operation of a parallel-plate electrochemical reactor for hydrocarbon fluorination. Model equations accommodate reduced conductivity of the initially liquid reaction mixture, two-phase hydrodynamics, and phase equilibrium due to the generation of gas in the process. Using continuum-model computer simulations, we investigate different reactant feed temperatures and exit pressures and calculate steady-state, one-dimensional profiles of current density, temperature, liquid volume fraction, and conversion for different applied potentials. A qualitative description of the gas-liquid flow is also provided. Different flow regimes have a significant effect on the liquid-volume-fraction profile and hence the current distribution to some extent. The local current density decreases along the reactor due to the increasing presence of gas. However, the majority of resistance exists in an anodic film. In general, trends of increased conversions with increased applied potential, increased exit pressure, and decreased feed temperature are verified. Conversions of less than 1% are calculated for all cases considered. * Electrochemical Society Student Member. * * Electrochemical Society Fellow.tially constant values of U and Rium can accurately represent current-potential behavior with kinetic resistance.Mass-transfer resistance at the anode due to the evolution of gas is negligible. Experimental data for H2 gasa The properties of KF are chosen for the species CE. KF is one of several additives used. Although KF poses some practical problems,9 its well-established conductivity aids in modeling the process.Various hydrocarbons are fluorinated by the Simons process including alkanes, ethers, sulfonates, and amines.l5 By molecular weight, hexane falls within the wide range of hydrocarbon sizes fluorinated electrochemically.

Independent transport measurements of carbon dioxide permeation were undertaken to evaluate a standard assumption of attributing cathodic CO 2 flux in direct methanol fuel cells ͑DMFCs͒ entirely to methanol crossover. With a humid air cathode at ambient conditions, a DMFC cathode emanated 3.5 to 4 ϫ 10 Ϫ8 mol/(cm 2 s) of CO 2 ͑20 to 25 mA/cm 2 ''leak current density''͒. From a methanol-free anode feed of either carbonated liquid water or humid CO 2 /H 2 gas ͑90%:10%͒, carbon dioxide permeated to the cathode at 0.7 ϫ 10 Ϫ8 mol/(cm 2 s) or 20% of the total DMFC flux. Under current ͑enabled with the presence of H 2 in CO 2 ), the permeation rate rose to 1 ϫ 10 Ϫ8 mol/(cm 2 s). The rise with cell current density increase from 24 to 80 mA/cm 2 was quantitatively consistent with convection of CO 2 with electro-osmosis of water. The indication that anodic CO 2 contributes to the total cathodic flux has a positive implication for fuel efficiency in DMFCs.Direct methanol fuel cells ͑DMFCs͒ offer tremendous advantages in energy density, in the ease of liquid fuel storage and delivery, and in negligible refuel time. Methanol has a theoretical energy density of 6.09 Wh/g. Even a 20% efficient cell far exceeds the 0.4-Wh/g density of advanced lithium batteries. However, DMFCs face fuel loss via crossover as a key obstacle to practical application. Methanol crossover translates to fuel inefficiency of the cell, decreases cell potential, and reduces performance of the cathode. Thus, methanol permeation to the cathode is important to quantify. Figure 1 shows the typical arrangement for measuring methanol crossover. In the standard model for methanol crossover, carbon dioxide flux from the cathode of a DMFC is wholly attributed to methanol crossover.