Oscillatory patterns in redox gradient materials through wireless bipolar electrochemistry. The dynamic wave-like case of copper bipolar oxidation

Bipolar electrochemistry allows the development of processes in a wireless manner, with reactions occurring at the induced anodes and cathodes of an immersed conducting material in the electrolyte. As a result, a gradient oxidation state may appear along the main axis field on the surface or bulk of the material depending on the type of reaction available at each induced potential. Redox intercalation gradients have been observed, metal anodization, or deposition, and also reactions at the electrolyte in the nearby environment of the poles induced. The complex oxidation of copper and interconversion between phases formed yields in this work an oscillating redox gradient, thanks to the great resistance change when the oxidized phases are formed. Parallel stripes containing mainly Cu2O, CuO, and Cu(OH)2 with large resistance are formed perpendicular to the electric field, forming a sequence of secondary dipoles in intermediate Cu stripes, that depends on the external voltage applied, and that oscillates in time at the same spatial coordinates. With longer times, copper solubilizes at the larger induced potential zones, probably as Cu(OH)42−. A simple finite element electrostatic model defines the complex potential waves induced in the piece. The resulting dynamics offer an example of the complexity of order in unwired conducting materials in wet media, either in catalysis, bioelectrodes, electronics, photovoltaics, or energy storage.The authors thank financing from the Ministry of Science of Spain (MAT2015-65192-R, and RTI2018-097753-B-I00), and Severo Ochoa Program (SEV-2017-0706 and CEX2019-000917-S). ICN2 is funded by the CERCA program/Generalitat de Catalunya. The authors would like to acknowledge data acquisition by Anna Esther Carrillo (SEM) and Guillaume Sauthier (XPS)