Redox flow batteries (RFB) are a kind of hybrid between a fuel cell and a battery. They can be described as a fuel cell in which H₂ and O₂ gases are replaced by electrolytes capable of storing electrical charges. Their architecture makes it possible to dissociate the quantity of chemical energy stored and the electrical power produced, as in a fuel cell, with the high efficiency and storage capacity of a battery. For this reason, they are particularly well suited to the stationary storage of solar or wind energy.

Redox flow batteries on the market today use vanadium chemistry, with the resulting environmental problems. To overcome these problems, researchers at the CEA are attempting to replace vanadium with organic molecules capable of storing charge and, if possible, biosourced. Among these compounds are quinone derivatives, used on a large scale as dyes and present in a large number of living organisms (animals, plants, lichens). 

High-resolution NMR, the leading technique for analyzing organic molecules.

However, to date, this approach suffers from several limitations: low energy density and performance that degrades much more rapidly than in the case of vanadium-based RFBs. Improving the performance of organic RFBs therefore requires a better understanding of the chemical phenomena associated with charge storage in electrolytes.
High-resolution NMR in solution is the leading technique for analyzing organic molecules, but how can electrodes and a circulation system be added in the very confined space of an NMR, without disturbing the high (over 10 teslas) and perfectly homogeneous magnetic field required for high resolution? 

Based on the NMR expertise of IRAMIS and the electrochemistry of our laboratory, the researchers rose to the challenge!

They have developed a miniaturized, operational RFB device, whose architecture and individual components have been optimized to preserve magnetic field homogeneity as far as possible. They tested the effectiveness of their device with a model electrolyte (2,7-anthraquinone disulfonate or AQDS), using several modalities (imaging, spectroscopy, velocimetry).
- Using additive manufacturing and capitalizing on their experience combining micro-sensing and fluidics, they succeeded in building a mini-accumulator with redox flux, to be inserted into the cavity of a high-resolution NMR spectrometer magnet (with a diameter of less than 18 mm).
- By installing this device in an MRI micro-imaging probe base, they were able to carry out velocimetry measurements to characterize electrolyte solution flows in the battery during operation.
- They were able to carry out localized NMR spectroscopic analyses (¹H and ¹³C), to avoid regions of high magnetic disturbance (at the electrode, for example).

By repeating cycles of reduction and oxidation of the model electrolyte, they demonstrate the dimerization of the oxidized and reduced species of the electrolyte, as well as the formation of a hetero-dimer combining the two species. This dimerization can only reduce the mobility of the active molecules, increase the viscosity of the electrolyte and, ultimately, limit battery performance.

The original device, designed in the laboratory, can also be used for in situ analysis of other electrochemical processes using liquid-state NMR. Several avenues for improvement will also be explored (addition of temperature micro-sensors, etc.).

Contacts:
IRAMIS: Patric​k Berthault (NIMBE/LSDRM), Saclay, France
SyMMES: Lionel Dubois (SyMMES/CAMPE), Grenoble, France