New Method for Energy Harvesting from Salinity Gradient  

Introduction
Blue energy or salinity difference energy (SGE) deals with harvesting energy released when two different solutions are mixed.   Several SGE technologies were developed to convert the entropic energy to electricity. Among them, pressure retarded osmosis (PRO) or reverse electrodialysis (RED) have been prototyped. In the case of PRO, osmotic membrane is placed between two solutions and osmotic flow feeds a turbine. In the case of RED, the pairs of ion exchange membranes are used and ion transport constitutes electricity.  To these two methods, one has to add capacitive mixing (CAPMIX) that was developed lately1. The process is based on mixing of two solutions and allows to obtain an electrical current by switching periodically high- and low-saline solutions. When ion-exchange membranes are used, the energy is generated by Donnan potential. Hence, electrodes accumulate cations and anions without any need for external voltage. When the external solution is changed, ions move back.

Two approaches can be used for assembling the electrode with ion-exchange barriers:  i) wrapping electrodes with ‘polymeric’ ion-exchange membranes and ii) depositing of polyelectrolytes on the electrodes. The first assembly forms ‘hard’ electrode, while the second – ‘soft’ electrode. 

 

Material and Methods
All electrodes for CAPMIX system were prepared from YP-50F (Kuraray Chem. Com.) activated carbon with poly(vinyl chloride) as a binder. 

Cation and anion exchange membranes were obtained from interpolymer polyethylene/styrene-co-divinylbenzene thick films. They were chlorosufonated and hydrolyzed to obtain cation exchange membranes or aminated to obtain anion exchange membranes.  

Poly(acrylic acid), PAA MW 50 kDa, was adsorbed on the electrode surface to obtain a cation-exchange layer. That layer was modified with excess of poly(ethylene imine), PEI MW 25 kDa   to get an anion-exchange layer.