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Disciplines : Chimie Physique
Laboratoire : CHIMIE, BIOLOGIE, INNOVATION
Institution d’accueil : Paris Sciences et Lettres - PSL
Ecole doctorale : Chimie physique et chimie analytique de Paris Centre - ED 388
Description :
Energy can be generated from the reversible mixing of salt solutions with different concentrations and is called salinity gradient power (SGP).
In classical systems, the salted solution diffuses toward the non salted solutions through selective membranes. The ions flux is converted into electrons flux at the electrodes.
In this work, we plan to use new membranes to increase the efficiency of the process. Our approach relies on recent finding of one of us. We shown recently that the ionic flux is larger at least by a factor of five in a boron nitride nanotube compared than in a carbon nanotube or in a pore inside a polymer membrane. These experiments were performed at the level of a single nanotube. This result originates in the anomalously high surface charge carried by the boron nitride nanotube’s internal surface in water at large pH, which we independently quantify in conductance measurements. Such increase of the ionic flux will allow to increase the efficiency of the classical process by at least a factor of 10. We target a power of 100 mW/ cm2 i.e 10 W / m2.
Context and Motivation :
Energy can be generated from the reversible mixing of salt solutions with different concentrations and is called salinity gradient power (SGP). The energy that theoretically can be generated per m3 river water is 1.7 MJ when mixed with the same volume sea water or even 2.5 MJ when mixed with a large surplus of sea water.
A Reverse electrodialysis stack, a RED stack consists of a large number of cells. A cell consists of an anion exchange membrane (AEM), a sea water compartment, a cation exchange membrane (CEM) and a river water compartment. The ions in the sea water diffuse through the membranes to the river water : the Na+ ions through the CEM and the Cl- ions through the AEM. The positive Na+ movement to the right and the negative Cl-movement to the left add together to give a positive ionic current to the right. At the electrodes, the ionic current has to be converted into an electron current. The value of the ionic flux depends upon the nature of the membrane [Harmsen]. We shown recently [Bocquet] that the ionic flux is larger at least by a factor of five in a boron nitride nanotube compared than in a carbon nanotube or in a pore inside a polymer membrane. These experiments were performed at the level of a single nanotube. This result originates in the anomalously high surface charge carried by the boron nitride nanotube’s internal surface in water at large pH, which we independently quantify in conductance measurements. In those experiments, we use Ag/AgCl electrodes to measure the electric current passing through the nanotube.
Scientific Objectives :
In this project, our aim is to build a cell using a membrane of 10 cm2 and characterize the amount of harvestable energy. An important lock is to be able to convert important ionic flux into electric current. We propose in this project to test various kind of electrodes and to select the one allowing the conversion of large ionic fluxes into electronic current. We target a power of 100 mW/ cm2 i.e 10 W / m2. The ionic flux predicted by the previous experiments is at least one order of magnitude greater than this power.
At the academic level our aim is to better understand the flow of ions and the conversion between ions into electrons at the level of the electrodes. A comprehensive modeling of the process will be done. We plan to optimize the salt concentrations in each stack in order to get the most efficient process.
If the expected impacts are reached, we will be able to build a prototype set-up allowing us to enter a transfer of technology process and convince industry partners to enter in this market. Note that we already have good relationships with industrial partners in this domain (Plastic Omnium, Solvay)
Methodology and Planning :
For this purpose, we will take advantage of millifluidic devices.
Two channels will be separated by a boron nitride nanotube’s membrane. In these channels we will inject respectively fresh water and salted water. Due to the salt concentration difference a ionic flux exacerbated by the properties of the membrane will be created. The membrane of the boron nitride nanotube will be prepared by our collaborators in Montpellier and Rennes. The channels will be connected to a system of electrodes. We will also use membrane of TiO2 nanopores prepared in the laboratory. We are able to prepare single pore (50-100 nm), multipores (1000 nanopores) or membrane with a pore density of 109 pores/cm2 with this material.
We plan to use three kinds of electrodes :
– Systems where electrodes play an active role in the redox process :one is growing and the other one is dissolving. (Ag/AgCl, Zn,ZnSO4, MnO2 electrodes as a Na+ intercalation material).
– Supercapacitor electrodes such as carbon aerogels.
– Flowing electrodes such as the one developed in [Colin].
– Systems where the electrodes are not involved in the redox process.
– Systems of conductive polymers such as PSS-PDOT
Note that, in the two first kind of systems (such as Ag/AgCl or carbone aerogels) the feed waters have to be interchanged periodically to invert the direction of the electrical current and with that the electrode processes. This imposes limitations on the stack design : the stack should be equipped with identical sea and river water compartments. A less attractive solution is to mechanically perform the periodical interchange of anode and cathode.
In the last situation, the system will be equipped with inert electrodes such as graphite electrodes in contact with a reversible redox couple (Fe2+/Fe3+) or Fe(CN)6 4-/Fe(CN)6
3-. The potential difference needed for reduction of Fe3+ to Fe2+ on the cathode will be counterbalanced by the oxidation of Fe2+ to Fe3+ at the anode. The Fe3+/ Fe2+ ratio will be kept constant by recirculation the combined anolyte and catholyte through the electrode compartments.
We will also vary the concentration of salt in both channel and the amplitude of the salt gradient. Due to ohmic losses, it is required to have a given amount of salt in the low salt channel to be able to have an ion flux.
A particular care will be brought to the studies of the ion concentration into the cell. We will follow quantitatively the concentration in Na+ and Cl- by interferometry in each part of the cell. Our aim is to measure the concentration field inside the system, its dynamic and to model it. This will allow us to understand the limiting step of the process. Back and fourth, with our collaborators in Rennes are envisaged to enhance the properties of the membrane. The influence of flow rate, values of salt gradients will be studied in details as well as the ageing of the electrodes.
Compétences requises :
The candidate must have a strong knowledge electrochemistry and fluid mechanics. The candidate must be able to understand theory and have some skills for experiments.