Electroquímica inalámbrica bipolarnuevos hitos y aplicaciones

Abstract

This thesis focusses on new aspects of bipolar wireless electrochemistry not previously developed. Bipolar electrochemistry occurs when an induced potential is generated among poles on an immersed conducting material (bipolar electrode) using driving external electrodes. This without a cable connection with the bipolar electrode, thanks to the induction of a dipole opposite to the external field imposed between the edges of the material. This makes it possible to generate electrochemical reactions in the induced anode and cathode of the immersed material, and opens the door to a wide variety of wireless electrochemical applications. The creation of opposing poles in the immersed material and the associated electrochemistry cause changes both in the global cell and in the material, which in turn may change the final phases and properties, and induce phenomena like previously observed wireless neural electrostimulation. In this work, several essential highlights have been observed and studied. The presence of one or several conducting pieces immersed in the electrolyte decreases substantially the resistance of the electrochemical cell, even without electron percolation. This is due to the dipole formation and ionic redistribution in the cell. In addition, a decrease in charge transfer resistance is observed, and additional pathways found for redox mediation, thanks to the possible conversion of reduced species on the adjacent induced anode, or vice versa, in what may be described as a cascade effect. The electrochemical reactions at the bipolar electrode depend on the external applied voltage, geometrical configuration (including the position within the field forces, shape and volume of occupation of the material), the electrolyte and the materials chemical reactivity. Thus, a noble metal is inert and at its induced poles reactions occurring correspond to the solvent and redox species present in it. A metal like copper, in alkaline media, undergoes an induced anodization that, as we have observed, yields a number of oxide and hydroxide stripes perpendicular to the field, with oscillating oxidation states due to the evolution of resistance of the metal surface, which in turn modify the induced dipoles. If the material allows redox intercalation processes (being a mixed ionic electronic conductor), the local resolution studies ex situ or in operando modes show intercalation of Na+ present in the media, at the induced cathode, and its propagation towards the induced anode, due to changes in resistance. That renders eventually a redox gradient material. This gradient maybe responsible of the great effect that IrOx PEDOT:PSS, bipolar electrode, have on neuronal wireless stimulation. In both cases, a relaxation of the gradient exists at different time scales. The anionic mobility in CoN allows also the reduction reaction to metallic Co at the induced cathode and, therefore, the generation of ferromagnetism with a significant magnetization, at low potentials in Wireless conditions. Depending on the geometry position of the material vs the electric field, a gradient material or a homogeneous one is formed and, therefore, such ferromagnetism may be volatile or permanent. Each of the results found offer a new paradigm related Wireless applications, from electrostimulation already described to energy storage systems, electronics and magneto-ionic devices and others. And above all, a new comprehension of the phenomena involved has evolved, even in very complex systems like copper oxidation, and will continue opening possibilities.