Dramatic Drop in Cell Resistance through Induced Dipoles and Bipolar Electrochemistry

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 save 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 and due to the bipolar electrochemistry effects. However, more studies rea 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.