Enhancing Mass Transport in Redox Flow Batteries by Tailoring Flow Field and Electrode Design

In this study, we investigate the mass transport effects of various flow field designs paired with raw and laser perforated carbon paper electrodes in redox flow batteries (RFBs). Previously, we observed significant increases in peak power density and limiting current density when perforated electrodes were used in conjunction with the serpentine flow field. In this work, we expand on our earlier findings by investigating various flow field designs (e.g., serpentine, parallel, interdigitated, and spiral), and continuously measuring pressure drop in each configuration. In all cases, these perforated electrodes are found to be associated with a reduction in pressure drop from 4% to 18%. Flow field designs with a continuous path from inlet to outlet (i.e., serpentine, parallel, spiral) are observed to exhibit improved performance (up to 31%) when paired with perforated electrodes, as a result of more facile reactant delivery and resulting greater utilization of the available surface area. Conversely, flow fields with discontinuous paths which force electrolyte to travel through the electrode (e.g. interdigitated), are adversely affected by the creation of perforations due to the high permeability 'channels' in the electrode. These results demonstrate that mass transport can significantly limit the performance of RFBs with carbon paper electrodes. (C) 2015 The Electrochemical Society. All rights reserved

Modeling of Ion Crossover in Vanadium Redox Flow Batteries: A Computationally-Efficient Lumped Parameter Approach for Extended Cycling

In this work, we have developed a zero-dimensional vanadium redox flow battery (VRFB) model which accounts for all modes of vanadium crossover and enables prediction of long-term performance of the system in a computationally-efficient manner. Using this model, the effects of membrane thickness on a 1000-cycle operation of a VRFB system have been investigated. It was observed that utilizing a thicker membrane significantly reduces the rate of capacity fade over time (up to similar to 15%) at the expense of reducing the energy efficiency (up to similar to 2%) due to increased ohmic losses. During extended cycling, the capacity of each simulated case was observed to approach an asymptote of similar to 60% relative capacity, as the concentrations in each half-cell reach a quasi-equilibrium state. Simulations also indicated that peak power density and limiting current density exhibit a similar asymptotic trend during extended cycling (i.e., an similar to 10-15% decrease in the peak power density and an similar to 20-25% decrease in the limiting current density is observed as quasi-equilibrium state is reached). (C) 2015 The Electrochemical Society. All rights reserved

Effect of Oxidation of Carbon Material on Suspension Electrodes for Flow Electrode Capacitive Deionization

Flow electrode deionization (FCDI) is an emerging area for continuous and scalable deionization, but the electrochemical and flow properties of the flow electrode need to be improved to minimize energy consumption. Chemical oxidation of granular activated carbon (AC) was examined here to study the role of surface heteroatoms on rheology and electrochemical performance of a flow electrode (carbon slurry) for deionization processes. Moreover, it was demonstrated that higher mass densities could be used without increasing energy for pumping when using oxidized active material. High mass-loaded flow electrodes (28% carbon content) based on oxidized AC displayed similar viscosities (∼21 Pa s) to lower mass-loaded flow electrodes (20% carbon content) based on nonoxidized AC. The 40% increased mass loading (from 20% to 28%) resulted in a 25% increase in flow electrode gravimetric capacitance (from 65 to 83 F g^–1) without sacrificing flowability (viscosity). The electrical energy required to remove ∼18% of the ions (desalt) from of the feed solution was observed to be significantly dependent on the mass loading and decreased (∼60%) from 92 ± 7 to 28 ± 2.7 J with increased mass densities from 5 to 23 wt %. It is shown that the surface chemistry of the active material in a flow electrode effects the electrical and pumping energy requirements of a FCDI system

Angewandte Chemie

Renewable energy sources (solar, wind etc.) can provide a substantial amount of energy, their intermittent nature requires low cost, safe and highly efficient electrochemical energy storage systems (EESs). Tremendous efforts to improve EESs like batteries and supercapacitors have been reported but they mainly address small scale storage (e. g. portable electronics) and grid-scale energy storage still remains a challenging To address this challenge, recently, a novel concept to store grid-scale electrical energy called the electrochemical flow capacitor (EFC) has been reported by our group The fundamental goal of this work is to optimize the electrochemical performance of the flowable electrode. Here, we addressed two different approaches to optimize the electrode; activation of active material (carbon spheres) and the nature of electrolyte (aqueous, nonaqueous etc.). We performed series of physical activations on carbon spheres having diameters between 250-350 µm using CO 2 as an activation agent at four different temperatures 800, 900, 950 and 1000 ˚C for duration of 1hour. The resultant activated carbons were characterized using N 2 adsorption, scanning electron microscope (SEM), cyclic voltammetry, galvanostatic charge/discharge cycling and impedance spectroscopy. Changes in the surface area and pore distribution were observed at different activation temperatures. Between 950 and 1000 ˚C the pore distribution appeared to become more uniform In general, the higher the activation temperature the greater was the capacitance across all rates studie