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Capacitive storage for “blue energy” production and desalinization

Capacitive storage for “blue energy” production and desalinization

Researchers from PHENIX (Paris) and CIRIMAT (Toulouse) published in Physical Review X their works on the use of capacitive technologies to create osmotic energy.

“Blue energy”, or osmotic energy, is all the energy we could be able to obtain – or to store – using the salinity difference between sea salty water and river pure water in estuaries where the two kinds of water mix. Nowadays, the chosen method to obtain this energy is based on the use of a semi-permeable membrane: water molecules circulate through it and the flow produced by the difference of pressure is used to produce energy. 

RS2E researchers published a paper in which they proposed an alternative technology using capacitors. They have tried to explain the mechanisms behind the charge storage and the salt adsorption phenomena in capacitors comprised of carbon nanoporous electrodes and aqueous electrolytes. Theirs works can be applied to desalinization processes. The study combining experimental and molecular simulations was published in Physical Review X.

CapMix and CDI: a sustainable alternative to the current semi-permeable membranes

Blue energy seems to be a promising renewable energy. Contrary to wind or solar energy, it can be produced continuously. However, it is the least exploited “marines” energies.

A team of researchers from PHENIX lab (modelling expertise) in Paris and from CIRIMAT lab (experimental expertise) in Toulouse focused on their supercapacitors knowledge to go beyond the current state of the art. Their previous works on the subject showed two important factors to blue energy production: the complex structure of the electrode material and the electrolyte polarization.

In their new study, researchers focus on two technologies: the capacitive mix (CapMix) and the capacitive deionization (CDI). Both of them consist in charge/discharge cycles in more or less salty solution. Supercapacitors are already using nanoporous electrodes to store charges. These components could be used for blue energy to improve the efficiency of CapMix and CDI processes.

A unique modelling/experimental way

Researchers benefited from the support of the RS2E, the city of Paris with the Emergence(s) project and the EoCoE and ERC IONACES European projects. With this help, they combined experimental studies and molecular simulations to work on the subject. The scientists could evaluate for two capacitors the electrodes charge and the water and ions quantities in the electrodes pores. The electrodes materials are the same but the salt concentration differs.

They could also predict the capacity and the adsorbed salt quantity – two essential parameters for the device conception. Then, modelling provides a lot of microscopic information about the electrolyte structure, in particular about the solvation of confined ions. 

Eventually, researchers conducted analysis on the uses of these results for CapMix and CDI processes. They highlighted the benefits of nanoporous carbons (from carbides, or CDCs, also used in supercapacitors), the limits of the usual modelling models and the relevance of their approach. This approach can do realistic predictions and say more about microscopic mechanisms. Nonetheless, it cannot replace simpler models for systematic predictions.

The next step for this collaboration is to find more about the electrodes structure effect (studying other CDCs) and the ions nature (studying other salt). Researchers will find more about transport properties (diffusion and electrolyte resistance in the electrodes), that determine the charge/discharge dynamism in capacitors.

To know more, listen the interview of Benjamin Rotenberg (PHENIX) in the French radio show La Une de la Science on France Inter.

References :

Blue energy and desalination with nanoporous carbon electrodes: capacitance from molecular simulations to continuous models

Michele Simoncelli, Nidhal Ganfoud, Assane Sene, Matthieu Haefele, Barbara Daffos, Pierre-Louis Taberna, Mathieu Salanne, Patrice Simon, et Benjamin Rotenberg. Physical Review X, DOI : 10.1103/PhysRevX.8.021024

Researcher contact: Benjamin Rotenberg (benjamin.rotenberg@sorbonne-universite.fr)

[1] On the molecular origin of supercapacitance in nanoporous carbon electrodes , C. Merlet et al, Nature Mater., 11, 306 (2012)

Highly confined ions store charge more efficiently in supercapacitors , C. Merlet et al, Nature Comm., 4, 2701 (2013)

Simulating Supercapacitors : Can We Model Electrodes As Constant Charge Surfaces ? , C. Merlet et al, J. Phys. Chem. Lett., 4, 264 (2013)