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

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.

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

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

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.

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.

Fabrication of Membranes

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SPPO

DMSO

sonication cast

Ion Exchange MembraneDispersed O-MWCNTs HomogeneousPolymer solution

O-MWCNTs

mix

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

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

Membrane Electrochemical Properties

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Optimal amounts of O-MWCNTs (0.3 – 0.5 wt%) enhanced the electrochemical properties.

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

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.

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).

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

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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