Hybrid 3D pseudocapacitive nanocrystals and porous rGO electrodes in capacitive deionization

Significance 

The surge in population worldwide has caused serious strain on existing fresh water resources. To counteract this effect, the capacitive deionization (CDI) technology has been employed to eliminate salt ions from water with carbon electrodes by the electrical double layer formation. So far, good results have been achieved, nonetheless, the search for new and improved materials that could be used for electrode manufacturing for CDI still continues. Until now, carbon nanomaterials have shown incremental adsorptive capacity.

Previous reports revealed redox and faradic active intercalation materials based on redox or faradaic reactions as new fronts for investigation of better electrode materials. These classes of materials offer equally competitive electro-sorption capacity than EDL with fast electro-sorption/desorption properties as well. However, mass loading limitation in these materials prevents the practical utilization of such electrochemical active materials in capacitive deionization.

Recently Khalifa University of Science and Technology researchers led by Professor Linda Zou proposed a novel mechanism to resolve the inherent drawback that limits the pseudocapacitive behavior to thin film electrodes with nanometer thickness, consequently inhibiting their practical application.

The mixed oxide spinel crystals that contains two cations (A, B) are less studied and reported in literature.  MnFe2O4 offers not only offers redox reaction of the ferrite phase that involves charge transfer at both the Mn and Fe ions over different potential ranges but also is balanced by insertion/extraction of salt ions into/from the lattice. The redox reaction of MnFe2O4 involves a very small lattice expansion, so it has both cycling stability and fast electrosorption/desorption of ions in salt solution. The authors believe such pseudocapacitive metal oxide materials present a valuable possibility of different electrode materials other than common carbon materials. Their work co-authored by Dr. Hammad Younes, Dr. Florent Ravaux, Dr. Nabil El Hadri and Prof Linda Zou is currently published in the research journal, Electrochimica Acta.

The research team achieved the new approach in a two-times of hydrothermal synthesis approach by not only strictly controlling the size and crystallinity of the MnFe2O4 nanocrystals but also by incorporating graphene sheets that were in close contacting with the mixed spinel oxides to provide more conductivity as well as many openings via nanopores that are randomly distributed across the material structure. The two-times of hydrothermal synthesis strategy encompassed: first, synthesis of the dense structure of mixed spinel oxides and reduced graphene oxide framework; second, production of the chemically etched porous GO dispersion followed by it utilization together with the dense structured composite to synthesis more macro-porous MnFe2O4/PrGO composite.

The authors reported that the carefully designed morphology showed the potential to improve the redox charge transfer of metal center surface, consequently resulting in better performance in removal of ions from aqueous solution than MFO only electrode. Of greater significance, the designed MFO/PrGO nanocomposite was considered as an environmentally-friendly nanomaterial.

In summary, Professor Linda Zou and her colleagues at Khalifa University of Science and Technology demonstrated for the first time that MFO/PrGO nanocomposite electrode can be designed and synthesized by a two-times of hydrothermal synthesis approach and applied as pseudocapacitive electrode in capacitive deionization experiments. The carefully designed architecture; the dense structure of MFO/rGO prepared by first-time hydrothermal process provided good connections of MFO nanoparticle with rGO framework for efficient redox charge transfer; the chemically-etched porous rGO used in the second time hydrothermal process offered hierarchical pores for easier access of the electrode materials, both contributed in enhanced pseudocapacitive electro-sorption. These results opened up the possiblity for using hybrid 3D electrochemical-active nanocrystals and graphene nanocomposite as electrodes for efficient electrosorption and desalination.

Hybrid 3D pseudocapacitive nanocrystals and porous rGO electrodes in capacitive deionization - Advances in Engineering
Characterization of crystalline structure of the nanocomposite electrode materials (a) TEM image of agglomerated MnFe2O4 nanoparticles; (b) Diffraction SAED pattern with identified MnFe2O4 rings; (c) Filtered HRTEM image of the crystal structure in [111] zone axis. (Credit: Electrochimica Acta with permission).
Hybrid 3D pseudocapacitive nanocrystals and porous rGO electrodes in capacitive deionization - Advances in Engineering
Figure. Schematic illustration of the synergistic electrosorption mechanism of pseudocapacitive MnFe2O4 and nanoporous rGO (MFO/PrGO) composite electrodes. (Source: Electrochimica Acta, Volume 306, 20 May 2019, Pages 1-8.)

About the author

Prof. Linda Zou joined Khalifa University of Science and Technology as a full professor in October 2014. Her research interests include employing nanotechnology to develop novel desalination and water purification solutions, such as membranes and electrodes. Dr. Zou’s research outcomes have been published in more than 150 journal articles and conference presentations with frequent citations. During 2000-2013, she was the chief investigator of many frontier research projects sponsored by the Australian Research Council and the Center of Excellence in Desalination Australia. At Khalifa University since 2014, Dr. Zou leads a ground-breaking research project using nanotechnology to develop cloud seeding materials, which was awarded by the inaugural UAE Research Program on Rain Enhancement Science for 2016-2018.

Reference

Hammad Younes, Florent Ravaux, Nabil El Hadri, Linda Zou. Nano-structuring of pseudocapacitive MnFe2O4/Porous rGO electrodes in capacitive deionization. Electrochimica Acta, volume 306 (2019) page 1-8.

Go To Electrochimica Acta

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