Ion-conducting polymers are important materials for a variety of electrochemical applications. Perfluorinated ionomers, such as Nafion, are the benchmark materials for proton conduction and are widely used in fuel cells and other electrochemical devices including solar-fuel generators, chloralkali cells, and redox flow batteries. While the behavior of Nafion in bulk membranes (10 to 100s μm thick) has been studied extensively, understanding its properties under thin-film confinement is limited. Elucidating the behavior of thin Nafion films is particularly important for the optimization of fuel-cell catalyst layers or vapor-operated solar-fuel generators, where a thin film of ionomer is responsible for the transport of ions to and from the active electrocatalytic centers. Using a combination of transport-property measurements and structural characterization, this work demonstrates that confinement of Nafion in thin films induced thickness-dependent proton conductivity and ionic-domain structure. Confining Nafion films to thicknesses below 50 nm on a silicon substrate results in a loss of microphase separation of the hydrophilic and hydrophobic domains, which drastically increases the material's water uptake while in turn decreasing its ionic conductivity.

Bubbles are known to influence energy and mass transfer in gas evolving electrodes. However, we lack a detailed understanding on the intricate dependencies between bubble evolution processes and electrochemical phenomena.This review discusses our current knowledge on the effects of bubbles on electrochemical systems with the aim to identify opportunities and motivate future research in this area. We first provide a base background on the physics of bubble evolution as it relates to electrochemical processes. Then we outline how bubbles affect energy efficiency of electrode processes, detailing the bubble-induced impacts on activation, ohmic and concentration overpotentials.Lastly, we describe different strategies to mitigate losses and how to exploit bubbles to enhance electrochemical reactions. Context & ScaleElectrochemical reactors will play a key role in the electrification of the chemical industry and can enable the integration of renewable electricity sources with chemical manufacturing. Most large-scale industrial electrochemical processes, including chloro-alkali and aluminum production, involve gas evolving electrodes. The evolution of bubbles at the surface of redox reaction sites often lead to the reduction of the active electrode area, the increase of ohmic resistance in the electrolyte and the formation of undesirable concentration gradients. All of these effects result in energy losses which reduce the efficiency of electrochemical systems. This review synthesizes our current understanding on the relationship between bubble evolution and energy losses in electrochemical reactors. By presenting a thorough account on the state of the research in this area, we aim to provide a common ground for the research community to improve our understanding on the complex processes involved in multiphase electrochemical systems. Increasing our knowledge on the relationship between bubbles and electrochemistry will lead to new strategies to mitigate and exploit bubble-induced phenomena leading to design guidelines for high-performing electrochemical reactors.

Nafion is an ion-containing random copolymer used as a solid electrolyte in many electrochemical applications thanks to its remarkable ionic conductivity and mechanical stability. Understanding the mechanism of ion transport in Nafion, which depends strongly on hydration, therefore requires a complete picture of its morphology in dry and hydrated form. Here we report on a nanoscale study of dry versus hydrated as-cast 100 nm Nafion membranes using analytical transmission electron microscopy (TEM) and cryogenic TEM tomography, respectively. For the dry membrane, spherical clusters ∼3.5 nm in diameter corresponding to the hydrophilic sulfonic-acid-containing phase are identified. In contrast, cryo TEM tomography of the hydrated membrane reveals an interconnected channel-type network, with a domain spacing of ∼5 nm, and presents the first nanoscale 3D views of the internal structure of hydrated Nafion obtained by a direct-imaging approach.