NANOCOMPOSITE CATION EXCHANGE MEMBRANE WITH FOULING RESISTANCE AND ENHANCED SALINITY GRADIENT POWER GENERATION FOR REVERSE ELECTRODIALYSIS

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1 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 1

2 2 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 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 Osmotic power plant, Norway

3 3 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 rinse solution (Fe 2+ and Fe 3+ ). Simplified schematic view of an RED stack representing the fluid transport through the ion-exchange membranes. Hong, J. G. et al 2014.

4 4 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 (Ca 2+ and Mg 2+ ) for CEMs); RED optimized ion-exchange membranes are NOT available.

5 Membrane Properties 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 (%); ΔV measured : measured membrane potential difference (V) between 0.1 M and 0.5 M NaCl solutions; ΔV theoretical : theoretical membrane potential difference (V) (estimated to be 37.9V from the Nernst equation). Hong, J. G. et al 2014; Scheme of permselective ion transport property of ion exchange membranes 5

6 Membrane Properties Ion exchange capacity (IEC): number of fixed charges per unit weight of dry membrane, was measured using a titration method. C NaOH : the concentration of NaOH (M) used; V NaOH : the volume of NaOH (ml); Swelling degree (SD): the amount of water content in the membrane per unit weight of dry membrane. W wet : mass (g) of wet membrane samples; W dry : mass (g) of dried membrane samples, Fixed Charge Density (CD): ratio of ion exchange capacity and swelling degree. Hong, J. G. et al 2014; 6

7 7 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: Fe 2 O 3, SiO 2, 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. Hong, J. G. et al 2014; Xu, T

8 8 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. [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.

9 9 Fabrication of Membranes O-MWCNTs SPPO sonication mix cast DMSO Dispersed O-MWCNTs Homogeneous Polymer solution Ion Exchange Membrane

10 10 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). Published in Journal of Membrane Science, 2016

11 11 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 %. Published in Journal of Membrane Science, 2016

12 12 Membrane Electrochemical Properties Optimal amounts of O-MWCNTs ( wt%) enhanced the electrochemical properties.

13 13 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 V was maintained, current changes were monitored during two hours time range. Composition and concentration of model solutions used in anti-fouling tests. Test Concentrated water Diluted Water Test 1 NaCl (0.5 M) CaCl 2 (0.01 M) NaHCO 3 ( M) NaCl (0.017 M) CaCl 2 ( M) NaHCO 3 ( M) Test 2 NaCl (0.5 M) NaCl (0.017 M)

14 14 Membrane Anti-fouling Tests Current change with time for Test 1 and Test 2 Ratio of permselectivity and ionic resistance of CEMs after anti-fouling Test 2

15 15 Membrane Anti-fouling Tests Performance potentials (α 2 / 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) Membranes Potential Before Test Potential After Test Percentage (used/ unused) SPPO % SPPO-0.1 O-MWCNT % SPPO-0.2 O-MWCNT % SPPO-0.3 O-MWCNT % SPPO-0.5 O-MWCNT % SPPO-0.8 O-MWCNT % CSO % FKS Water contact angle, surface mean roughness and surface charge density of nanocomposite CEMs. Membranes Contact angle [ ] S a [nm] Surface charge density [meq /m 2 ] SPPO SPPO-0.1 O-MWCNT SPPO-0.2 O-MWCNT SPPO-0.3 O-MWCNT SPPO-0.5 O-MWCNT SPPO-0.8 O-MWCNT

16 16 Membrane RED Performance 0.5 wt% O-MWCNT membrane achieved maximum power density (30% higher than pristine SPPO membrane, and 14% higher than commercial FKS membrane).

17 17 Conclusion Nanocomposite membranes were found to be attractive candidates for application in electrochemical systems like RED. Membranes with 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). Published in Journal of Membrane Science, 2016

18 18 Acknowledgement This research was partially supported by the U.S. National Science Foundation (NSF Grant No. CBET ).

19 19 Thank You for Your Attention