Periodically inserting energetic relaxations into Reverse Monte Carlo fits improves the accuracy of model structures with minimal additional computational cost.

To inform the development of advanced electrodes for energy storage, water treatment, and catalysis, among other applications, we need to improve our understanding of how material structure evolves during electrochemical operation. Insight into the evolution of local atomic structure during electrochemical operation is accessible through a range of sophisticated in operando probes, but techniques for in operando observation of macroscale electrode phenomena (e.g., swelling, dissolution, and chemical degradation) are limited. This macroscale understanding is critical to establish a full picture of electrochemical material behavior. Here, we report a multimodal cell for simultaneous electrochemical quartz crystal microbalance (EQCM) and in operando spectroscopic ellipsometry (SE). This SE-EQCM cell allows for the measurement of mass, thickness, optical properties, and electrochemical properties together in one device. Using polyaniline (PANI) as a test case, we demonstrate the use of this SE-EQCM cell to rapidly measure known phenomena and reproduce a range of prior results during the electrodeposition, electrochemical cycling, and electrochemical degradation of PANI. In particular, the simultaneous mass and thickness measurement afforded by this cell allows us to distinguish known qualitative differences in the degradation of PANI under oxidative and reductive potentials. The SE-EQCM cell we report promises to reveal new insights into the electrochemical behavior of thin film materials for a range of applications.

Nanoscale films of redox-active amine polymers such as polypyrrole (Ppy) are of interest for aqueous energy storage, water treatment, and chemical sensors. Unfortunately, the electrochemical properties of Ppy are constrained by the local material structures that form during typical synthesis. In this study, we examine how crosslinking Ppy postsynthesis with short-chain bifunctional alkyl-halide crosslinkers influences the charge storage properties of Ppy. Specifically, we employ dibromoethane (EtBr 2 ), dibromopropane (PrBr 2 ), and dibromobutane (BuBr 2 ) crosslinkers to link amines from adjacent Ppy polymer chains in nanoscale Ppy films formed by electrodeposition. We study the electrochemical performance of the resulting structures using an electrochemical quartz crystal microbalance complemented by density-functional theory studies. We identify that the shortest (ethyl) crosslinker sterically traps free anions from the electrolyte within the Ppy structure. These trapped anions lead to a qualitative shift in the electrochemical mechanism from anion-insertion to cation-insertion behavior. We identify that the propyl crosslinker, with just one carbon more than ethyl, allows for more rapid anion motion than intrinsic Ppy, accessing electrochemical capacities up to 60% higher than that with no crosslinker. These results reveal the strong impact of the local molecular structure on the electrochemical properties of redox-active polymers and demonstrate the use of short-chain bifunctional crosslinkers to control their qualitative electrochemical response.

Understanding the atomic structure of ultrathin (<20 nm) atomic layer deposition (ALD) coatings is critical to establish structure−property relationships and accelerate the application of ALD films to stabilize battery interfaces. Previous studies have measured the atomic structure of nanoscale ALD films using cryogenic electron diffraction with a large (∼200 nm) beam diameter. However, for ultrathin ALD coatings, these measurements provide only ensemble average structural information and cannot be used to directly measure differences in atomic structure through the depth of the ALD film. In this study, we localize the electron beam to a small (∼5 nm) spot size using cryogenic scanning transmission electron microscope (STEM), and we collect electron diffraction data at multiple points along the depth of a 12 nm thick ALD AlO x film deposited onto a carbon nanotube (CNT) substrate without a contribution from the substrate. We couple these diffraction measurements with pair distribution function (PDF) analysis and iterative reverse Monte Carlo-molecular statics (RMC-MS) modeling to compare atomic structure metrics at different positions in the film depth. We interpret the modeling results considering the three-dimensional (3D) concentric cylindrical sample geometry of a CNT with uniform AlO x coating. These measurements confirm a two-phase bulk/interface structural model proposed previously for ALD AlO x and indicate that the interfacial layer at the CNT−AlO x interface is 2.5 nm thick�5 times larger than previously reported. This report demonstrates direct measurement of atomic structural variations across nanoscale material interfaces that is of broad interest for electrochemical applications and will help inform the use of ALD coatings to stabilize lithium-ion battery interfaces.

Conjugated amine polymers (CAPs) such as polyaniline and polypyrrole have high electrical conductivities (>500 S/cm), high specific capacities (>500 F/g), and are one of few known material classes containing redox-active sites for binding negatively-charged anions. These properties make CAPs appealing for a range of electrochemical applications including semiconductor devices, energy storage devices, sensors, and electrochemical separation processes. Unfortunately, existing synthesis routes for CAPs rely on rapid chemical or electrochemical oxidation reactions that provide limited control over the molecular structure and thickness of CAPs. This restricts the ability to use CAPs to address technological needs. In this report, we describe ongoing research in our group to deposit conformal CAP films with molecular-scale control of thickness and composition using sequential and self-limiting gas-phase surface reactions. Our approach is in line with established molecular layer deposition (MLD) techniques, and uses an oxidant as one of the MLD precursors in so-called oxidative molecular layer deposition (oMLD). Using select examples, we reveal new insights into oMLD growth behavior, characterize the molecular structures of the oMLD CAP films, and connect structural differences under varying deposition conditions with electrical and electrochemical properties of the films. The insights we report enable the deposition of electrically conductive and redox-active CAP films conformally onto high-surface area samples, opening new avenues to enable advanced electrochemical technologies based on ultrathin and conformal CAP films.