Niklas Lindahl

Determination of the Bending Rigidity of Graphene via Electrostatic Actuation of Buckled Membranes

Classical continuum mechanics is used extensively to predict the properties of nanoscale materials such as graphene. The bending rigidity, κ, is an important parameter that is used, for example, to predict the performance of graphene nanoelectromechanical devices and also ripple formation. Despite its importance, there is a large spread in the theoretical predictions of κ for few-layer graphene. We have used the snap-through behavior of convex buckled graphene membranes under the application of electrostatic pressure to determine experimentally values of κ for double-layer graphene membranes. We demonstrate how to prepare convex-buckled suspended graphene ribbons and fully clamped suspended membranes and show how the determination of the curvature of the membranes and the critical snap-through voltage, using AFM, allows us to extract κ. The bending rigidity of bilayer graphene membranes under ambient conditions was determined to be 35.5−15.0 +20.0 eV. Monolayers are shown to have significantly lower κ than bilayers.

Templated Growth of Covalently Bonded Three‐Dimensional Carbon Nanotube Networks Originated from Graphene

A template-assisted method that enables the growth of covalently bonded three-dimensional carbon nanotubes (CNTs) originating from graphene at a large scale is demonstrated. Atomic force microscopy-based mechanical tests show that the covalently bonded CNT structure can effectively distribute external loading throughout the network to improve the mechanical strength of the material.

Carbon Nanotube Field Effect Transistors with Suspended Graphene Gates

Novel field effect transistors with suspended graphene gates are demonstrated. By incorporating mechanical motion of the gate electrode, it is possible to improve the switching characteristics compared to a static gate, as shown by a combination of experimental measurements and numerical simulations. The mechanical motion of the graphene gate is confirmed by using atomic force microscopy to directly measure the electrostatic deflection. The device geometry investigated here can also provide a sensitive measurement technique for detecting high-frequency motion of suspended membranes as required, e.g., for mass sensing.

Depth probing of the hydride formation process in thin Pd films by combined electrochemistry and fiber optics-based in situ UV/vis spectroscopy

We demonstrate a flexible combined electrochemistry and fiber optics-based in situ UV/vis spectroscopy setup to gain insight into the depth evolution of electrochemical hydride and oxide formation in Pd films with thicknesses of 20 and 100 nm. The thicknesses of our model systems are chosen such that the films are thinner or significantly thicker than the optical skin depth of Pd to create two distinctly different situations. Low power white light is irradiated on the sample and analyzed in three different configurations; transmittance through, and, reflectance from the front and the back side of the film. The obtained optical sensitivities correspond to fractions of a monolayer of adsorbed or absorbed hydrogen (H) and oxygen (O) on Pd. Moreover, a combined simultaneous readout obtained from the different optical measurement configurations provides mechanistic insights into the depth-evolution of the studied hydrogenation and oxidation processes.

Chemically Soft Solid Electrolyte Interphase Forming Additives for Lithium-ion Batteries

The solid electrolyte interphase (SEI) layer is a key element of lithium-ion batteries (LIBs) enabling stable operation and significantly affecting the cycling performance including life-length. Here we present the concept of chemically soft SEI-forming additives, created by introducing aromatic ring based derivatives of already well-known SEI-formers to render them chemically soft, resulting in 1,3,2-benzodioxathiole 2,2-dioxide (DTDPh), 3H-1,2-benzoxathiole 2,2-dioxide (PSPh), and 1,4,2-benzodioxathiine 2,2-dioxide (PSOPh). A computational DFT based comparison predicts promise with respect to both early and controlled reduction processes. These predictions are verified by basic electrochemical studies targeting appropriate additive reduction potentials i.e. prior to any electrolyte solvent or salt decomposition. In addition, the decomposition paths of the SEI-formers are projected and the end products compared with spectroscopic data for the SEI-layers formed in LIB cells. The SEI-layers formed finally show very good properties in terms of improved capacity retention, improved coulombic efficiency, and reduced resistance for the graphite/electrolyte/LFP full cells made, especially observed for PSOPh. That is due to the preferred C–O bond breaking mechanism, observed also for DTDPh, and supported by the S–C bond breaking mechanism, together resulting in well conductive and good adhesion properties of the SEI-layers. This is expedited by higher softness, eventuating in a formation process stabilizing some of the radicals and/or lowering the kinetic barriers. These positive effects are confirmed both when applying a commercial style electrolyte and for a new generation electrolyte based on the LiTDI salt, where suppression of the TDI anion reduction is truly crucial.

Fuel Cell Measurements with Cathode Catalysts of Sputtered Pt3Y Thin Films

Fuel cells are foreseen to have an important role in sustainable energy systems, provided that catalysts with higher activity and stability are developed. In this study, highly active sputtered thin films of platinum alloyed with yttrium (Pt3 Y) are deposited on commercial gas diffusion layers and their performance in a proton exchange membrane fuel cell is measured. After acid pretreatment, the alloy is found to have up to 2.5 times higher specific activity than pure platinum. The performance of Pt3 Y is much higher than that of pure Pt, even if all of the alloying element was leached out from parts of the thin metal film on the porous support. This indicates that an even higher performance is expected if the structure of the Pt3 Y catalyst or the support could be further improved. The results show that platinum alloyed with rare earth metals can be used as highly active cathode catalyst materials, and significantly reduce the amount of platinum needed, in real fuel cells.

Free-Standing 3D-Sponged Nanofiber Electrodes for Ultrahigh-Rate Energy-Storage Devices

We have designed a self-standing anode built-up from highly conductive 3D-sponged nanofibers, that is, with no current collectors, binders, or additional conductive agents. The small diameter of the fibers combined with an internal spongelike porosity results in short distances for lithium-ion diffusion and 3D pathways that facilitate the electronic conduction. Moreover, functional groups at the fiber surfaces lead to the formation of a stable solid-electrolyte interphase. We demonstrate that this anode enables the operation of Li-cells at specific currents as high as 20 A g-1 (approx. 50C) with excellent cycling stability and an energy density which is >50% higher than what is obtained with a commercial graphite anode.