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Harvesting energy from salinity differences using battery electrodes in a concentration flow cell Taeyoung Kim, Mohammad Rahimi, Bruce E. Logan, and Christopher A. Gorski Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02554 • Publication Date (Web): 12 Aug 2016 Downloaded from http://pubs.acs.org on August 22, 2016
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Submitted to: Environmental Science & Technology
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Harvesting energy from salinity differences using battery
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electrodes in a concentration flow cell
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Taeyoung Kim,a Mohammad Rahimi,b Bruce E. Logan,a and Christopher A. Gorski*a
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Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, PA 16802, USA
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Department of Chemical Engineering, The Pennsylvania State University,
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University Park, PA 16802, USA
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*Corresponding Author:
[email protected];
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+1-814-865-5673 (phone), +1-814-863-7304 (Fax)
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Abstract
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Salinity-gradient energy (SGE) technologies produce carbon-neutral and renewable
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electricity from salinity differences between seawater and freshwater. Capacitive mixing
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(CapMix) is a promising class of SGE technologies that captures energy using capacitive or
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battery electrodes, but CapMix devices have produced relatively low power densities and
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often require expensive materials. Here, we combined existing CapMix approaches to
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develop a concentration flow cell that can overcome these limitations. In this system, two
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identical battery (i.e., faradaic) electrodes composed of copper hexacyanoferrate (CuHCF)
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were simultaneously exposed to either high (0.513 M) or low (0.017 M) concentration NaCl
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solutions in channels separated by a filtration membrane. The average power density 1
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produced was 411±14 mW m–2 (normalized to membrane area), which was twice as high as
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previously reported values for CapMix devices. Power production was continuous (i.e., it did
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not require a charging period and did not vary during each step of a cycle) and was stable for
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20 cycles of switching the solutions in each channel. The concentration flow cell only used
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inexpensive materials and did not require ion-selective membranes or precious metals. The
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results demonstrate that the concentration flow cell is a promising approach for efficiently
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harvesting energy from salinity differences.
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Introduction
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Salinity differences between seawater and freshwater contains an enormous amount of
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entropic energy, which – if harvested – could produce approximately 1 TW of renewable
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electricity from coastal sites worldwide.1 Several salinity-gradient energy (SGE) technologies
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have been developed to convert this energy into electrical power, including pressure-retarded
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osmosis (PRO),1-4 reverse electrodialysis (RED),1, 4-6 and capacitive mixing (CapMix).7-18 In
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PRO, electrical power is produced by creating pressure differences between two waters with
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different salinities using semipermeable membranes. In RED, potential differences developed
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across an array of ion-exchange membranes produce electrical power using electrodes and
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soluble redox-active compounds. CapMix is a relatively new approach that encompasses
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several technologies inspired by energy storage devices, such as supercapacitors and batteries.
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CapMix devises produce electrical power from salt concentration-dependent electrode
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potentials, which can be created from capacitive (i.e., non-faradaic) or battery (i.e., faradaic)
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electrodes or Donnan potentials developed across ion-exchange membranes. Of the SGE
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technologies, the largest reported electrical power densities have been generated for PRO
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(~10 W m–2-membrane area) and RED (~1 W m–2-membrane area). However, these two
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technologies require the use of ion-exchange or semipermeable membranes (i.e., “selective
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membranes”), which are currently cost-prohibitive or susceptible to fouling.1, 4 A significant
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advantage of some CapMix technologies over PRO and RED is that they can produce
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electrical power without the need for selective membranes.
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Several CapMix approaches have been developed with different reactor configurations
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and electrode materials. Capacitive energy extraction based on double layer expansion
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(CDLE), uses two porous carbon electrodes that undergo capacitive charging/discharging
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reactions, which produce power by exploiting the concentration-dependent double layer
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thickness and subsequent voltage rise and fall at each electrode.7-10 Generating electrical
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power using CDLE has major challenges, including charge leakage, low power densities
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(0.05 W m–2), and intermittent power production.9 Producing electrical power using
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capacitive energy extraction based on Donnan Potentials (CDPs) is similar to CDLE, except
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that potentials are developed across ion-exchange polymers or membranes placed on the
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surface of carbon electrodes.11-16 CDP has multiple advantages over CDLE, including higher
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power densities (~0.2 W m–2) through external electrode charging13 and continuous power
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production using flow electrodes.15, 16 The major disadvantage of CDP is that, like RED, it
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requires expensive ion-exchange membranes. Unlike CDLE and CDP, electrical power
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production in a mixing entropy battery (MEB) is based on battery electrodes that develop
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concentration-dependent electrode potentials through faradaic reactions.8, 17, 18 MEB has
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shown a reasonable power density of ~0.1 W m–2 without ion-exchange membranes,17 but
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electrical power production is intermittent due to the need for periods of charging, and
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current systems use precious metals, such as silver, as an electrode material. A recently
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suggested approach inspired by a concentration cell,19 called the same-electrode-kind
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CapMix (SEK-CapMix), partly addressed these challenges by using two identical chemically
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modified carbon electrodes,10 but the voltage rise (110 mV) and power density (
