Performance metrics for the objective assessment of capacitive deionization systems

Abstract

In the growing field of capacitive deionization (CDI), a number of performance metrics have emerged to describe the desalination process. Unfortunately, the separation conditions under which these metrics are measured are often not specified, resulting in optimal performance at minimal removal. Here we outline a system of performance metrics and reporting conditions that resolves this issue. Our proposed system is based on volumetric energy consumption (Wh/m 3 ) and throughput productivity (L/h/m 2 ) reported for a specific average concentration reduction, water recovery, and feed salinity. To facilitate and rationalize comparisons between devices, materials, and operation modes, we propose a nominal standard testing condition of removing 5 mM from a 20 mM NaCl feed solution at 50% water recovery for CDI research. Using this separation, we compare the desalination performance of a flow-through electrode (fte-CDI) cell and a flow between membrane (fb-MCDI) device, showing how significantly different systems can be compared in terms of generally desirable desalination characteristics. In general, we find that performance analysis must be considered carefully so to not allow for ambiguous separation conditions or the maximization of one metric at the expense of another. Additionally, for context we discuss a number of important underlying performance indicators and cell characteristics that are not performance measures in and of themselves but can be examined to better understand differences in performance.

Rapid Inversion of Surface Charges in Heteroatom‐Doped Porous Carbon: A Route to Robust Electrochemical Desalination

“…After the complete inversion of CDAC, there was no observable decrease in desalination performance, and the SAC value after the long‐term operation (6.67 mg g −1 ) was substantially higher than those obtained with the commercial ACs, which were 0.11, 0.66, and 1.47 mg g −1 for MSP20X, S51HF, and YS2, respectively. The current profiles of initial and final cycle are displayed in Figure S9 (Supporting Information), and the average salt adsorption rates (ASARs), energy consumption, and charge efficiencies (CEs) are also shown in Table S1 (Supporting Information) . It was noteworthy that commercial ACs manifested higher CEs than that of CDAC in early stage of CDI operations; however, due to the drop and rise of CEs in commercial ACs and CDAC, respectively, CDAC's CE was more than four times larger than those of commercial ACs.…”

Theory of Water Desalination with Intercalation Materials

“…4 shows, after reversal of the current direction, there is also a small "area" which corresponds to energy "output" (e.g. from 1 to 1.15 hr), i.e., where energy recovery is in theory possible [41], after which a much longer period follows where energy must be invested to run the cell (from 1.15 to 3 hr for Mode 2).…”

Frequency analysis and resonant operation for efficient capacitive deionization

“…where, , v res E is the volumetric (resistive) energy consumption at resonant operation, P is the throughput productivity (volume of freshwater produced per unit electrode area, per unit time), and the water recovery is implicitly 50% (see Hawks et al, 2018b for a discussion of these metrics).…”

“…Moreover, CDI is inherently periodic 3 because electrical charging and discharging forcing functions result in periodic salt removal and regeneration phases. CDI performance can be evaluated using a recently proposed set of metrics (Hawks et al, 2018b).…”