Electrochemical breakdown in hydrogel ionotronic devices.

In recent years, hydrogel, as a stretchable, transparent, ionic conductor, has attracted considerable attention and its integration with various materials has enabled new functions: hydrogel ionotronics. These hybrid systems rely on both mobile ions and mobile electrons. However, coupling of ions and electrons brings a new challenge: electrochemical breakdown. Here, we study the breakdown behaviors of a typical ionotronic system-a hydrogel-elastomer device at high DC voltage, which consists of three elements: hydrogel, dielectric elastomer, and metal. We develop a phase diagram of the possible failure modes through theory and experiment, and find a new failure mode, electrochemical breakdown, caused by ion-electron exchange at the metal-hydrogel interface. Our experiments show that the breakdown voltage of the dielectric elastomer decreases when the capacitance of the electrical double layer formed at the metal-hydrogel interface is below a certain value. It is found that the failure mode and its transition are determined by three material properties: the electrical breakdown strength of the dielectric elastomer, the capacitance of the metal-hydrogel interface per unit area, and the electrochemical window of the hydrogel electrolyte. These findings will guide the characterization and improvement of the reliability of hydrogel ionotronic devices.