The power and energy densities of electrochemical storage cells depend on the surface area responsible for facilitating the electron transfer. Using suspensions containing conducting particles such as slurries has been recently identified as promising for extending the electrode area in some electrochemical cells. This can be attributed to their ability to offer a dynamic electrode area by improving the associated electronic conduction. Besides offering a large surface area, slurry electrodes have also been used to perform heterogeneous reactions that other methods cannot achieve.

Among the available conducting slurries, graphene, graphite and carbon nanotubes exhibit the greatest potential owing to their low weight and widespread use in supercapacitors. However, below the percolation limit, the suspended particles are generally not expected to improve conductivity except for a few cases characterized by the ordering of particles because of external factors like external fields. The appearance of a dipole in the presence of electric fields when conducting materials are immersed in electrolytes has been associated with a decrease in the impedance even when the concentrations are lower than the theoretical percolation limits.

When the dipole has a sufficient voltage difference, polarization occurs, giving rise to what is commonly known as bipolar electrochemistry, a phenomenon that holds potential applications in different fields. This phenomenon could also provide a better understanding of the underlying reasons responsible for the significant impedance decrease. Various studies have been carried out to describe the drop in cell resistance due to the bipolar electrochemistry effects. However, more studies are still needed to clarify the existing controversies.

To this note, a team of researchers in Spain from the Institut de Ciència de Materials de Barcelona-CSIC and other CSIC centers: Dr. Laura Fuentes-Rodríguez, Dr. Llibertat Abad, Dr. Eulalia Pujades, Dr. Pedro Gómez-Romero, Dr. Dino Tonti and Professor Nieves Casañ-Pastor studied the significant drop in the resistance of electrochemical cells through dipole and bipolar electrochemistry induction. In their approach, macroscopic conducting pieces were immersed in the electrolyte without electrical contact. Additionally, the mediation of soluble redox species and its effects on additional mechanisms like charge transfer as well as the effects of various parameters like geometry and size, were examined. The work is currently published in the Journal of the Electrochemical Society.

The authors demonstrated the modification of the physical and chemical properties of the electrochemical system when the electrolyte contained immersed conducting pieces that were not in contact and below the percolation limits. Through dipole interaction, the induced dipoles created electrophysical mediation that favored the electrochemical mediation and shuttling among adjacent poles, thereby contributing to further impedance decrease. A significant decrease in the cell impedance was reported, with a 30% decrease observed for the first few immersed conducting pieces. Additional charge-transfer mechanism and polarizations effects were also observed, showing lower overpotential for the respective redox processes without percolation or ordering. The dramatic decrease in the resistance of the electrochemical cell was also attributed to the effects of various parameters like the number of pieces, their shape, size and location.

In summary, a significant drop in cell resistance was experimentally shown by Professor Nieves Casañ-Pastor and colleagues to elucidate the various responsible factors and processes. The findings were comparable to the effects observed in the carbon suspensions, both in the presence and absence of redox species, an evidence showing that direct electrical percolation, including order and particle contact, is not needed to explain the remarkable performance of low impedance. The changes in resistance and shuttling among adjacent dipoles suggested the possibility of nee cell engineering with larger power and current outputs. In a statement to Advances in Engineering, Professor Nieves Casañ-Pastor, the lead and corresponding author stated that the new study contributes to the design of new electrochemical cells with improved currents for various practical applications like energy storage.

About the author

DR. Nieves Casañ-Pastor, Research Profesor at Institut of Materials Science of Barcelona-CSIC

Prof. Nieves Casañ-Pastor initiated her Chemistry studies at the University of Valencia, (Bs 1981, Ms 1982). She obtained her PhD degree in Chemistry from Georgetown University (1988), on reversible redox processes in polyoxometalates and their structural and magnetic changes. After a brief postdoc at the University of Valencia, she obtained a permanent job as Staff Researcher at the Institute of Materials Science of Barcelona, one of the institutes of the State run research agency, CSIC, in Spain. She has received Volkswagen-CSIC and Foundation Domingo Martinez and Foundation Marato TV3 prices for her research on electrochemical systems related to energy storage, catalysis and bioelectrodes. She has been member of the ACS, Sigma Xi, MRS and ECS societies.

Her work has focused on room temperature electrochemical techniques for doping and deposition of materials, as well as for the preparation of new phases including mixed oxides and pristine graphene, and hybrid nanostructured phases, and the design of electrodes for electrostimulation and for energy storage. More recently she has developed a significant effort on the electrochemistry available by inducing dipoles in conducting materials immersed in electrolytes.

Neural cell response has been observed wirelessly, involving the possibility of designing transparent unwired electrodes for implantation in the nervous system. Deep characterization with local resolution (synchrotron x-ray absorption, SEM-EDX, XPS, etc) of the processes involved within the material are helping to define the differences among the effects on cells. On the other hand, significant differences in the global cell impedance have been observed. The study continues and has direct implications in electrochemical cell design, energy storage systems, catalysis and bioelectrodes protocols.

Reference

Dramatic Drop in Cell Resistance through Induced Dipoles and Bipolar Electrochemistry. 
Fuentes-Rodríguez, L., Abad, L., Pujades, E., Gómez-Romero, P., Tonti, D., & Casañ-Pastor, N. (2022). 
Journal of the Electrochemical Society, 169(1), 016508.