Reverse Electrodialysis for Energy Recovery: Material Development and Performance Evaluation

Salinity Gradient Power- Reverse Electrodialysis (SGP-RED), so-called blue energy, is a promising untapped membrane based renewable and sustainable energy generation technology. Salinity gradient energy can be defined as the energy reveals during the mixing of two solution having different concentration. Creating a controlled mixing in a RED stack gives the opportunity to transfer the mixing energy directly to electricity by redox reactions. Alternate arrangement of cation exchange membranes (CEM) and anion exchange membranes (AEM) form the required compartment design for controlled mixing. When high and low concentration solutions are fed from neighboring compartments, electrochemical potential difference of the solutions drive the ions from high to low concentrations. However, only charges opposite to membrane fixed charge can diffuse through, i.e. for an ideal membrane only cations can transport through CEM. Therefore, an ionic flux can be generated inside of the stack.

Understanding the fundamentals of the technology and the present challenges of SGP-RED is very important for the evaluation of the experimental study. Therefore, Chapter 1 deals with the theory behind SGP-RED, potential of current state of art and challenges on performance and commercialization.

Most of the RED literature investigate RED performance by using artificial solutions that only contains NaCl. In Chapter 2, the effect of real river and seawater solutions (collected from river of Amantea, Italy) is experimentally investigated on lab-scale RED stack prototype. Different flow rates and temperature are studied to find an optimized condition. RED effluents are characterized to have a better understanding on transport mechanisms of monovalent and multivalent ions. Ion characterization results indicate multivalent ions tends to transport against their concentration gradient. Moreover, investigations on electrochemical properties concludes Mg2+ has the most severe effect on RED performance by causing an order of magnitude reduction on CEM conductivity.

After concluding drastic negative effect of Mg2+ on power generation in the second chapter, Chapter 3 is dedicated to investigate broad range of magnesium content in mixing brine and seawater. Magnesium is known as second most abundant cation in the natural seawater solution and concentration varies from region to region. 0.5 and 4 molal solutions from 0 to 100 % Mg2+ content are tested in RED setup. Ionic characterization of outlet solution is completed to see effect of concentration on transport of ions. It is observed that uphill transport is limited to 0 – 30% of MgCl2. Ohmic and non-ohmic resistance of the CEM and AEM characterized in the test solutions. Resistance characterization reveals that cation exchange membrane resistance is critically affected by Mg2+ concentration while resistance of AEM remains unaffected.

Due to RED is a non-commercialized technology, there is no commercial ion exchange membranes designed for RED. Therefore, most of the RED studies investigates electrodialysis (ED) membranes because of the similarity. In Chapter 4, cation exchange membranes are prepared considering the needs of RED. A well-known polymer, polysulfone, is sulfonated by chlorosulfonic acid to obtain negatively charged polymer. After the characterization of the polymer, CEMs are prepared with an asymmetric porous morphology by wet phase inversion method. Phase inversion parameters, e.g. solvent type, co-solvent ratio, are studied to optimize the membrane resistance and permselectivity. Among the prepared membranes, most promising one is further characterized for different NaCl concentration to estimate the power density. The results encourage to consider wet phase inversion method as a fabrication method for CEM.

Commercial cation exchange membranes are produced as dense homogeneous membranes by functionalized polymeric materials as standalone or into a support to have a mechanical stability. In Chapter 5, sulfonated polyethersulfone membranes are prepared by wet phase inversion and solvent evaporation method. In solvent evaporation method, polyethersulfone/sulfonated polyethersulfone blend ratio is optimized considering electrochemical and mechanical properties. In wet phase inversion, effect of co-solvent, evaporation time, coagulation bath composition and concentration are studied to optimize the membrane electrochemical properties. Best performing wet phase inversion membrane, solvent evaporation membrane with corresponding ion exchange capacity and a benchmark commercial membrane CMX (Neosepta, Japan) are characterized to estimate RED performance for different solution concentration. Competitive results point out the possibility of CEM production by wet phase inversion.

Chapter 6 is dedicated to conclude and discuss the achievements of the conducted work. In addition, some outlook for the future works was mentioned based on the deductions of the experimental work