A Combined Modeling and Experimental Study Assessing the Impact of Fluid Pulsation on Charge and Energy Efficiency in Capacitive Deionization

Abstract

Cell-cycling performance in capacitive deionization (CDI) can suffer from various charge-efficiency loss mechanisms. In conventional CDI, we show that salt residue within electrodes introduces a temporal lag between charge and desalination stages of a CDI cycle. Without accounting for this effect in the collection of effluent, significant performance degradation occurs as current density increases. To overcome this we use pulse-flow operation to control fresh-and brine-water concentrations. The charge and energy efficiency performance between the two flow-modes is compared using a porous electrode model that is calibrated and validated with experimental data. To quantify specific contributions to charge efficiency losses, the model captures local salt variations resulting from a combination of electrosorption, leakage current, and immobile surface charge. Compared to traditional continuous-flow operation, simulation results show that charge efficiency increases up to 23% in the pulse-flow operation at a current density up to 20 A/m 2 , which leads to a 73% decrease of specific energy consumption (SEC). In addition, the SEC predicted by the pulse-flow operation model closely aligns with the predictions of the continuous-flow model after accounting for the temporal lag in effluent salinity. Both simulations and experimental results suggest that pulse-flow operation closely approximates the performance in continuous-flow operation.

Reducing impedance to ionic flux in capacitive deionization with Bi-tortuous activated carbon electrodes coated with asymmetrically charged polyelectrolytes

“…2B). At higher voltage windows, the transition of current from capacitive to faradaic leads to an increase in leakage current at low current densities due to longer charging times (Shang et al., 2017). At low current densities, faradaic reactions (e.g.…”

“…Observed charge efficiency (ηCE), which relates the effective charge adsorbed to the total charge passed in the circuit during charging, incorporates losses due to faradaic reactions, co-ion repulsion during electrosorption, desalination of brine residue in the flow-channel and electrode from the previous cycle (Shang et al., 2017). Charge efficiency was calculated as a ratio of charge equivalent of salt removed to the total charge passed during charging.ηCE=ΔNeff.Fqin…”

“…A typical cell architecture involves the flow of feed solution in the channel created by the separation of porous carbon electrodes, called flow-by or flow-between CDI (fb-CDI) (Bouhadana et al., 2010; E. Suss et al., 2012; Porada et al., 2013). The simplicity of this architecture could translate to low system costs and fouling potential, however, the charge efficiency (ratio of charge equivalent of salt adsorbed to the charge input (Zhao et al., 2010)) and adsorption capacity remain low due to co-ion transport and repulsion (cation repulsion at the positive electrodes and vice-versa), as well as residual brine in the flow channel and electrode macropores preceding the discharge stroke (Suss et al., 2015; Kim and Choi, 2010; Li and Zou, 2011; Shang et al., 2017).…”

“…Alternatively, electrodes can be coated with charged polyelectrolytes or fabricated with charged binders to cover macropores ion-selective layer of low resistance to retain co-ions (Ahualli et al., 2017; Kim et al., 2019b). While polyelectrolyte coating could serve as a cost-effective means promoting counter ion flux into and out of electric double layers, it remains unclear if selectivity enhancements observed under constant voltage charging would hold under constant current operation, in which charge efficiency is limited by salt residue from preceding discharge strokes (Hawks et al., 2018; Shang et al., 2017).…”

Thermodynamics of Ion Separation by Electrosorption

“…In its simplest form, the major contributor to volumetric resistive consumption , can be estimated as / where , , and are respectively the series resistance, current magnitude, and flow rate. To put the energetic performance of current CDI technology into perspective, we show in [43][44][45][46][47][48][49] In addition to published data, we include measurements from a cell which we constructed specifically for the current study. Our CDI cell here features a low-resistance (for each electrode pair) design which we realized by pressing into and embedding titanium mesh current collectors into 270 μm thick commercially available porous carbon electrodes.…”