Aqueous Electrolytes in Nanoporous Media: Structure, Dynamics and Electrochemo-Mechanical Actuation

≪p≫An electrolyte solution near a charged solid surface forms an electric double layer (EDL). While the theory of EDLs has been known since the 1850s, their structure and dynamics are much less understood in the confined geometry of nanoporous media. There, large interfacial curvatures and overlapping EDLs of opposing interfaces can lead to significant property changes compared to planar geometries. Furthermore, EDL formation can lead to changes in mechanical interfacial stresses at the single pore scale and thus to macroscopic material deformation. In this project, this electrochemo-mechanical actuation will be related to the structure and dynamics of the EDL for aqueous electrolytes in nanoporous carbons. For this purpose, experiments and molecular simulation will be combined from the nanoscopic single-pore scale to the macroscopic scale of the porous medium.We will synthesize electrically conducting nanoporous carbons with high specific surface area, defined pore sizes (1-10 nm) and pore geometries. This will allow fine-tuning of the effect of the interfacial region compared to the rest of the pore volume. By adding heteroatoms to the carbon structure and controlling the defect concentrations, different surface chemistries (hydrophobic vs. hydrophilic) are introduced. Depending on the applied electrical voltage between electrolyte and solid, the surface chemistry and the pore diameter, we will investigate EDL formation and mechanical actuation in the material. We will focus on aqueous solutions of simple salts and investigate the role of salt concentration among other parameters. We will use synchrotron-based X-ray scattering to study the EDL in-operando, with special attention to the charge/discharge and thus ion transport kinetics. We will use molecular dynamics simulations to represent the structural properties of the surface of the nanoporous medium and the electrolyte. This should allow us to derive the structural, thermodynamic and transport properties of the geometrically confined electrolyte and relate them to the experimental results, both in terms of the electrochemo-mechanical couplings and the charge capacities at the scale of the single pore and the porous medium.In addition to these fundamental insights into aqueous electrolytes in geometric confinement, these studies provide the basis for supercapacitors with green, water-based electrolytes. They will also contribute to the development of materials for electromechanical actuators based on novel actuation principles, avoiding the usual piezoceramic systems that contain predominantly environmentally hazardous materials, such as lead.≪/p≫