How Cross-Linking Influences the Electrochemical Properties for Brush-Functionalized Electrodes

Hydrogel brushes comprise cross-linked polymer chains that are attached at one end to a substrate. Their network architecture promotes electrochemical stability, which renders these materials suitable for electrochemical applications. However, the electrochemical properties of such hydrogel brushes have not yet been studied yet. In this work, we investigate the role of the cross-linker in the electrochemical behavior of negatively charged poly(3-sulfopropyl methacrylate) (PSPMA) hydrogel brushes. Using electrochemical impedance spectroscopy, we study the role of layer resistance in a poor solvent (0.1 M KOH) and the role of charge transfer in a good solvent (0.1 M KCl). In 0.1 M KOH, we observe that the polymer layer limits the ion diffusion to the electrode and that this phenomenon depends on the cross-linker content. There is an optimum in cross-linker content, for which the ion diffusion experiences the least resistance. We attribute this optimum to a variance in the flexible and rigid polymer phase. In 0.1 M KCl, we find that the solvated brush thickness is the parameter that determines the extent of the charge transfer resistance. The more cross-linked a hydrogel brush is, the lower the swelling ratio in a good solvent and thus the lower the charge transfer resistance becomes. With this knowledge, we can use the cross-linker content as a tool to minimize the layer resistance or tune the charge transfer resistance, which is useful for corrosion protection, electrochemical sensing, and ion gating applications.