Equilibria model for pH variations and ion adsorption in capacitive deionization electrodes

Hemmatifar, Ali; Oyarzun, Diego I.; Palko, James W.; Hawks, Steven A.; Stadermann, Michael; Santiago, Juan G.

doi:10.1016/j.watres.2017.05.036

Equilibria model for pH variations and ion adsorption in capacitive deionization electrodes

Journal Article · Fri Oct 20 00:00:00 EDT 2017 · Water Research

DOI:https://doi.org/10.1016/j.watres.2017.05.036· OSTI ID:1513135

^[1]; Oyarzun, Diego I. ^[1]; Palko, James W. ^[1]; Hawks, Steven A. ^[2]; Stadermann, Michael ^[2]; Santiago, Juan G. ^[1]

Stanford Univ., CA (United States). Dept. of Mechanical Engineering
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

Ion adsorption and equilibrium between electrolyte and microstructure of porous electrodes are at the heart of capacitive deionization (CDI) research. Surface functional groups are among the factors which fundamentally affect adsorption characteristics of the material and hence CDI system performance in general. Current CDI-based models for surface charge are mainly based on a fixed (constant) charge density, and do not treat acid-base equilibria of electrode microstructure including so-called micropores. Here, we expand current models by coupling the modified Donnan (mD) model with weak electrolyte acid-base equilibria theory. In our model, surface charge density can vary based on equilibrium constants (pK's) of individual surface groups as well as micropore and electrolyte pH environments. In this initial paper, we consider this equilibrium in the absence of Faradaic reactions. The model shows the preferential adsorption of cations versus anions to surfaces with respectively acidic or basic surface functional groups. We introduce a new parameter we term “chemical charge efficiency” to quantify efficiency of salt removal due to surface functional groups. We validate our model using well controlled titration experiments for an activated carbon cloth (ACC), and quantify initial and final pH of solution after adding the ACC sample. We also leverage inductively coupled plasma mass spectrometry (ICP-MS) and ion chromatography (IC) to quantify the final background concentrations of individual ionic species. Our results show a very good agreement between experiments and model. The model is extendable to a wide variety of porous electrode systems and CDI systems with applied potential.

View Accepted Manuscript (DOE)

Research Organization:: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)

Sponsoring Organization:: USDOE; USDOE National Nuclear Security Administration (NNSA)

Grant/Contract Number:: AC52-07NA27344

OSTI ID:: 1513135

Alternate ID(s):: OSTI ID: 1550073

Report Number(s):: LLNL-JRNL--770141; 961101

Journal Information:: Water Research, Journal Name: Water Research Journal Issue: C Vol. 122; ISSN 0043-1354

Publisher:: ElsevierCopyright Statement

Country of Publication:: United States

Language:: English

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