Redox flow batteries
As renewable energies are intermittent it is necessary to develop new methods for large-scale storage.
For this reason, redox flow batteries constitute a very promising solution for the best storage of large amounts of energy from renewable sources. They present different advantages such as low cost and long life. However, current systems are not satisfactory because the electrolytes used are very corrosive and toxic.
Our team has been a pioneer in the development of systems based on organic molecules or metal compounds solubilized in aqueous media which are less toxic and synthesized at low cost. Among the molecules studied, the Fe-triethanolamine complex was found to be a good candidate for redox flow batteries due to its very negative potential, high solubility and low cost.
The high solubility of this complex, synthesized according to a method developed in the laboratory, makes it possible to achieve large volumetric capacities (greater than 10 Ah.L-1) for high powers (80-120 mW.cm-2) (Electrochim. Acta 2019).
In order to increase the energy capacity of the redox flow batteries, we also sought to increase the solubility of redox active molecules in water. Ionic liquids have thus been shown to be effective as additives for dissolving quinonics derivatives (PCT Int. Appl. WO 2017212179).
All of the team's work on redox batteries is made in symbiosis with the start-up Kemiwatt created by two researchers of the team.
This work has in particular contributed to the production in 2016 of the world's first prototype of organic redox flow batteries delivering a power of 10 to 20 kW and more recently of a demonstrator set up on the University campus from Rennes.
Microbial biofuel cells
Since 2007, we have been developing electroactive microbial biofilms, which are interesting as catalysts electrode since they are ubiquitous and inexpensive (from inocula of the compost type or wastewater by example), self-renewing through cell division and durable (at least several years).
Bacteria electroactive devices also have the particularity, under adequate selective pressure, to electrically connect their cellular metabolism to solids such as anodes and cathodes (Nat. Rev. Chem.2017). We had shown that a relevant and controlled modification of surfaces allowed to improve the development and performance of anode biofilms (Bioelectrochem. 2018; Bioresour. Technol. 2015). These microbial anodes can cleanse the wastewater from their organic matter while producing electrical energy in a configuration of Microbial biofuel.
The challenges and limitations of installing batteries microbials in wastewater treatment plants have recently published. This reflection led us to reflect on a passive short-circuited system for the decontamination of water nitrates lagoon areas (the electricity produced is not recovered but upgraded directly to the electrodes) in particular for nitrate reduction.
Since 2015, our efforts have also focused on the development of photoelectrochemical cells. These systems, also called artificial leaves, are based on the use of semiconductor surfaces immersed in a electrolyte. Under solar illumination, photo-generated charge carriers perform electrochemical reactions at the solid / liquid interface. We are using this concept to convert solar energy into chemical vectors, storable and transportable, which can be used when this energy is no longer available.
This theme has several obstacles, the most important being the electrode materials stability and their cost. For this, developments have been worn on the photocathode part to efficiently perform the hydrogen evolution reaction. In this context, our research has focused on the use of p-type silicon (p-Si) with which we have set up methods of simple modification by inexpensive catalysts (Nanoscale, 2017).
In addition, the study of clusters polyoxometallate-Mo3S4 catalytic agents on these surfaces allowed us to highlight remarkable synergies phenomena (J. Am. Chem. Soc. 2019).
Finally, the developments carried out on the photocathode material also led us to prepare hybrid p-Si / molecular catalyst assemblies effective for the photoelectrochemical reduction of CO2 to CO (ACS Catal. 2015).
The photoanode part which, due to the instability of semiconductors in anode regime, is a well-identified technological barrier has also been strongly developed on n-type Si (n-Si) (Curr. Opin. Colloid Interface Sci. 2019) as well as on semiconductor oxides (ChemElectroChem 2019).
In this context, we have highlighted unexpected phenomena in terms of surface protection (Energy Environ. Sci. 2018) which enabled us to develop a series of inexpensive electrochemical methods for the preparation of stable and particularly effective photoanodes for the oxidation of water (Adv. Energy Mater. 2019) and also for urea oxidation (Nat. Commun. 2019).
Finally, our progress in this area has made it possible to prepare a complete photoelectrochemical cell entirely based on Si and allowing, without energy input other than solar illumination, to electrolyze water with photo-currents of several mA for 7 hours, being the first at the national level (Adv. Sustainable. Syst. 2018).