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NANOCOMPOSITE CATION EXCHANGE MEMBRANE WITH FOULING RESISTANCE AND ENHANCED
SALINITY GRADIENT POWER GENERATION FOR REVERSE ELECTRODIALYSIS
X I N T O N G , B O P E N G Z H A N G , A N D Y O N G S H E N G C H E N
G E O R G I A I N S T I T U T E O F T E C H N O L O G Y
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Salinity Gradient Power (SGP)
• Global warming and energy shortage helped creating interest in development of renewable energy. Salinity gradient power (SGP) is a new type of clean energy.
• Electrical energy generated from inevitable entropy increase of mixing of two solutions of different salt concentrations [1,2].
• Estimated to have a total global potential for power
production placed at 2.4-2.6 terawatts (TW) (more
than 80% of the current global electricity demand) [3,4].
• Different technologies available: reverse electrodialysis
(RED) and pressure retarded osmosis (PRO), etc.
[1] Norman 1974, [2] Weinstein et al 1976, [3] Guler et al 2012, [4] Ramon et al 2011.
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Osmotic power plant, Norway
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Reverse Electrodialysis (RED)
• Reverse process of electrodialysis.
• Alternating cation exchange membranes
(CEMs) and anion exchange membranes
(AEMs) between electrodes.
• Alternating river water and seawater channels.
• Salinity gradient results in a potential difference
over each membrane.
• Chemical potential difference causing ions to
transport from concentrated to diluted solution.
• Conversion of ionic current to electron current at
the electrodes via redox reactions.
• Redox reaction is facilitated by electrode rinsesolution (Fe2+ and Fe3+).
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Hong, J. G. et al 2014.
Simplified schematic view of an RED stack representing the fluid transport through the ion-exchange membranes.
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Advantages and Challenges
Advantages of RED system
• Limitless supply (if river and sea water is used);
• No production of green house gas (GHG), thermal pollution, or radioactive waste;
• No daily fluctuation in the productions due to variations in wind speed or sunshine.
Technical Challenges for RED system
• Low energy efficiency and low power density;
• Membrane fouling (organic fouling for AEMs, and inorganic fouling/ scaling (Ca2+ and Mg2+) for CEMs);
• RED optimized ion-exchange membranes are NOT available.
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5Hong, J. G. et al 2014;
Ionic resistance: the ability of the membrane to oppose
the passage of ionic current.
Permselectivity: the ability of the membrane to select
counter-ions and repulse co-ions.
α: measured apparent permselectivity (%);
ΔVmeasured : measured membrane potential difference (V) between 0.1 M and 0.5 M NaCl solutions;
ΔVtheoretical: theoretical membrane potential difference (V) (estimated to be 37.9V from the Nernst equation).
Scheme of permselective ion transport property of ion exchange membranes
Membrane Properties
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6Hong, J. G. et al 2014;
Ion exchange capacity (IEC): number of fixed charges
per unit weight of dry membrane, was measured using a
titration method.
Swelling degree (SD): the amount of water content
in the membrane per unit weight of dry membrane.
Fixed Charge Density (CD): ratio of ion exchange capacity
and swelling degree.
CNaOH : the concentration of NaOH (M) used;
VNaOH : the volume of NaOH(mL);
Wwet : mass (g) of wet membrane samples;
Wdry : mass (g) of dried membrane samples,
Membrane Properties
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RED- Specific Nanocomposite Membranes
Optimal membrane characteristics for RED power generation• Low ionic resistance;
• High selectivity of ions (e.g., Na+ and Cl-);
• High ion exchange capacity (IEC);
• Low swelling degree (SD).
Nanocomposite ion exchange membranes for RED • Incorporation of inorganic materials into organic polymer matrix (e.g., inorganic materials: Fe2O3, SiO2,
carbon nanotubes, graphene oxide; organic materials: PPO, PES, PVA).
• Deriving optimal synergized properties by combining unique features of inorganic with those of organic material.
• Enhancing chemical, thermal and mechanical stability.
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Hong, J. G. et al 2014; Xu, T. 2005.
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Synthesis of Nanocomposite RED Membranes
Organic material: SPPO (sulfonated poly (2,6-dimethyl-1,4-phenylene oxide))
Good chemical and thermal stability, as well as good mechanical properties [1].
Inorganic material: Oxidized mutli-walled carbon nanotubes (O-MWCNTs)
Enhanced dispersion property and better chemical compatibility with polymer compared to pristine CNTs;
long-distance ionic pathways could be formed when elongated nanomaterials (nanotubes or nanofibers) are used, which facilitate ion transport in membrane;
Effectively improve the anti-fouling properties of pressure-driven membranes due to their ability to change membrane surface morphology, surface charge density and hydrophilicity.
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[1] Hong, J. G. et al 2014; [2] Spitalsky, Z. et al 2010; [3] Yao, Y. et al 2011; [4] Vatanpour, V. et al 2011; Celik, E. et al 2011.
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Fabrication of Membranes
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SPPO
DMSO
sonication cast
Ion Exchange MembraneDispersed O-MWCNTs HomogeneousPolymer solution
O-MWCNTs
mix
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Morphologies of Nanocomposite Membranes
• SEM images of O-MWCNTs and nanocomposite cation exchange membranes
(a) oxidized multi-walled carbon nanotubes; (b) pristine SPPO; (c) 0.5 wt % O-MWCNT membrane; (d) pristine SPPO membrane (higher magnification); (e) 0.5 wt % O-MWCNT membrane (higher magnification); and (f) 1.5 wt % O-MWCNT membrane (higher magnification).
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Published in Journal of Membrane Science, 2016
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Morphologies of Nanocomposite Membranes
• Cross section of nanocomposite membranes
(a) SPPO; (b) 0.1 wt % O-MWCNT; (c) 0.2 wt % O-MWCNT; (d) 0.3 wt % O-MWCNT; (e) 0.5 wt % O-MWCNT; (f) 0.8 wt %.
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Published in Journal of Membrane Science, 2016
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Membrane Electrochemical Properties
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Optimal amounts of O-MWCNTs (0.3 – 0.5 wt%) enhanced the electrochemical properties.
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Membrane Anti-fouling Tests
Chosen synthesized CEMs were tested at the same time; commercial CSO was tested for comparison.
Two different groups of model solutions were used for two test runs.
A constant applied voltage of 10.52 V was maintained, current changes were monitored during two hours time range.
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Composition and concentration of model solutions used in anti-fouling tests. 1
Test Concentrated water Diluted Water
Test 1
NaCl (0.5 M)
CaCl2 (0.01 M)
NaHCO3 (2.5×10-3 M)
NaCl (0.017 M)
CaCl2 (3.8×10-4 M)
NaHCO3 (9.6×10-4 M)
Test 2 NaCl (0.5 M) NaCl (0.017 M)
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Membrane Anti-fouling Tests
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Ratio of permselectivity and ionic resistance of CEMs after anti-fouling Test 2
Current change with time for Test 1 and Test 2
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Membrane Anti-fouling Tests
MembranesPotential
Before Test
PotentialAfter Test
Percentage (used/
unused)
SPPO 11367 6125 53.9%
SPPO-0.1 O-MWCNT 13425 8167 60.8%
SPPO-0.2 O-MWCNT 14227 8658 60.9%
SPPO-0.3 O-MWCNT 16416 10274 62.6%
SPPO-0.5 O-MWCNT 20034 11107 55.4%
SPPO-0.8 O-MWCNT 13415 7217 53.8%
CSO 3968 2319 58.4%
FKS 5280 -- --
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Performance potentials (α2/ R) of CEMs before and after
anti-fouling test (Test 1) (The FKS membrane was not
included in the anti-fouling test, and only the original
potential is listed).
(α ---- apparent permselectivity; R ---- ionic resistance)
MembranesContact
angle [°]Sa [nm]
Surface charge density [meq /m2]
SPPO 81.5 3.5 2.6
SPPO-0.1 O-MWCNT75.9 7.0 2.9
SPPO-0.2 O-MWCNT67.1 10.0 3.0
SPPO-0.3 O-MWCNT64.1 14.6 3.0
SPPO-0.5 O-MWCNT50.8 26.5 3.1
SPPO-0.8 O-MWCNT73.9 36.7 2.8
Water contact angle, surface mean roughness
and surface charge density of nanocomposite CEMs.
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Membrane RED Performance
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0.5 wt% O-MWCNT membrane achieved maximum power density (30% higher than pristine SPPO membrane, and 14% higher than commercial FKS membrane).
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Conclusion
• Nanocomposite membranes were found to be attractive candidates for application in electrochemical systems like RED.
• Membranes with 0.3-0.5 wt% O-MWCNT showed best anti-fouling performance.
• There is a correlation between CEM anti-fouling property and membrane surface hydrophilicity and surface charge density.
• Membrane with 0.5 wt% O-MWCNT showed best RED power generation performance (about 33% higher than pristine SPPO membrane).
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Published in Journal of Membrane Science, 2016
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Acknowledgement
This research was partially supported by the U.S. National Science Foundation (NSF Grant No. CBET-1235166).
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Thank You for Your Attention
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