Increase in Interfacial Adhesion and Electrochemical Charge Storage Capacity of Polypyrrole on Au Electrodes Using Polyethyleneimine
High-performance devices based on conducting polymers (Cps) require the fabrication of a thick Cp-coated electrode with high stability. Herein, we propose a method for enhancing the interfacial adhesion strength between a gold electrode and an electropolymerized polypyrrole (ppy) layer by introducing a polyethyleneimine (peI) layer. Although this insulating layer hinders the initial growth of the pPy layer on the Au surface, it improves the adhesion by up to 250%. Therefore, a thick layer of
... a thick layer of ppy can be fabricated without delamination during drying. X-ray photoelectron spectroscopy shows that the peI layer interacts with the Au surface via polar/ionic groups and van der Waals interactions. scanning electron microscopy reveals that the cohesion of the ppy layer is stronger than the interfacial adhesion between the peI layer and the ppy layer. Importantly, the electroactivities of ppy and its dopant are unaffected by the PEI layer, and the electrochemical storage capacity of the pPy layers on the PEI-coated Au electrodes increases with thickness, reaching ~530 mC/cm 2 . Negative potential sweep is detrimental to pPy layer adhesion: pPy layers on a bare Au electrode peel off instantly as the potential is swept from 0.2 to −0.7 V, and most of the charge stored in the layer becomes inaccessible. In contrast, ppy layers deposited on peI coated Au electrode are mechanically stable and majority of the charge can be accessed, demonstrating that this method is also effective for enhancing electrochemical stability. Our simple approach can find utility in various applications involving CP-coated electrodes, where thickness-dependent performance must be improved without loss of stability. Conducting polymers (CPs) such as polypyrrole (pPy) are attracting considerable interest because their redox activity, conductivity, and biocompatibility are useful for various applications, including electrochemical energy storage devices, actuators, and neural interfaces. To fabricate high-performance CP electrodes, it is often desirable to deposit a thick CP layer on the electrode because its performance is highly dependent on this layer's thickness; for instance, the areal energy/power densities (Wh/cm 2 , W/cm 2 ) or actuating force of a CP electrode are proportional to its thickness 1,2 . However, it is challenging to develop a thick CP layer because of the low interfacial adhesion strength between the CP layer and the underlying electrode. Accordingly, the CP layer can easily peel off from the electrode during fabrication and operation. One strategy to overcome this delamination issue is to roughen the electrode surface by either etching or electroplating its surface 3 . Although mechanical interlocking between the overlaying CP layer and the rough metal surface has been shown to improve interfacial adhesion 3,4 , the CP layers were only a few microns thick. Additionally, the etching process could expose an electrochemically reactive underlying layer (such as Ti), making the CP-coated electrode unstable during long-term application. An alternative strategy involves the formation of covalent bonds between the electrode surface and the pPy layer using modified pyrrole monomers 5,6 . However, this method requires additional effort to synthesize the monomers, and some covalent bonds such as Au surface and thiolated pyrrole cannot be used for electrochemical cycling of the CP layer due to their instability 6 .