Nonlinear dynamics of ion concentration polarization in capacitive deionization

Hyungsub Lee, Dongho Kim, Ji Yoon Kang, Ki Wan Bong, Soo Hyun Lee, Rhokyun Kwak

Research output: Contribution to journalArticle

2 Citations (Scopus)

Abstract

Capacitive deionization (CDI) is an emerging desalination technique by utilizing the electrosorption of porous electrodes. Thus far, only linear concentration profiles in bulk electrolyte between the electrodes have been observed, during ion charging (ion depletion) or ion discharging (ion enrichment). This so-called linear ion concentration polarization (ICP) can be predicted by the simple convective-diffusion model. In this study, we demonstrate that capacitive charging can induce nonlinear concentration profiles in the electrolyte. To visualize nonlinear ICP dynamics, a miniaturized CDI platform is developed to observe in situ ion concentration profiles. Determined by the ion flux mismatching of electrosorption and bulk ion transport, we discovered four ICP regimes in CDI – steady, linear ICP, delayed-nonlinear ICP, and nonlinear ICP. We directly observed the signatures of nonlinear ICP: i) flat depletion zone with extremely low ion concentration, and ii) corresponding exponential current drop. Based on this observation, two scaling models are formulated to predict i) when the nonlinear ICP occurs, and ii) the propagation speed of the flat depletion zone. To the best of our knowledge, this is the first microscopic characterization of ICP in CDI systems, and it firmly establishes the existence of nonlinear ion transport dynamics in CDI's bulk electrolyte.

Original languageEnglish
Pages (from-to)14-24
Number of pages11
JournalDesalination
DOIs
Publication statusPublished - 2019 May 15

Keywords

  • Capacitive deionization
  • Desalination
  • Ion concentration polarization

ASJC Scopus subject areas

  • Chemistry(all)
  • Chemical Engineering(all)
  • Materials Science(all)
  • Water Science and Technology
  • Mechanical Engineering

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