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

Abstract

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 coupledmore » 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.« less

Authors:

Hemmatifar, Ali  ^[1];

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

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+ Show Author Affiliations

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

Publication Date:

Fri Oct 20 00:00:00 EDT 2017

Research Org.:

Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)

Sponsoring Org.:

USDOE National Nuclear Security Administration (NNSA)

OSTI Identifier:

1513135

Alternate Identifier(s):

OSTI ID: 1550073

Report Number(s):

LLNL-JRNL-770141
Journal ID: ISSN 0043-1354; 961101

Grant/Contract Number:  

AC52-07NA27344; 15-ERD-068

Resource Type:

Accepted Manuscript

Journal Name:

Water Research

Additional Journal Information:

Journal Volume: 122; Journal Issue: C; Journal ID: ISSN 0043-1354

Publisher:

Elsevier

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY