Matthias Hilder

Direct electro-deposition of graphene from aqueous suspensions

We describe the direct electro-chemical reduction of graphene oxide to graphene from aqueous suspension by applying reduction voltages exceeding -1.0 to -1.2 V. The conductivity of the deposition medium is of crucial importance and only values between 4-25 mS cm(-1) result in deposition. Above 25 mS cm(-1) the suspension de-stabilises while conductivities below 4 mS cm(-1) do not show a measurable deposition rate. Furthermore, we show that deposition can be carried out over a wide pH region ranging from 1.5 to 12.5. The electro-deposition process is characterised in terms of electro-chemical methods including cyclic voltammetry, quartz crystal microbalance, impedance spectroscopy, constant amperometry and potentiometric titrations, while the deposits are analysed via Raman spectroscopy, infra-red spectroscopy, X-ray photoelectron spectroscopy and X-ray diffractometry. The determined oxygen contents are similar to those of chemically reduced graphene oxide, and the conductivity of the deposits was found to be ∼20 S cm(-1).

One dimensional energy transfer in lanthanoid picolinates. Correlation of structure and spectroscopy

Optical and structural properties of rare earth complexes with 2-pyridine carboxylic acid (‘Hpic’) are evaluated by luminescence spectroscopy, decay measurements, X-ray crystal structure determination, FTIR, DTA and metal content analysis. Corresponding Tb3+and Eu3+ complexes of this ligand are extraordinarily efficient with respect to their luminescence. In the crystalline state the series is isostructural and composed of M[Ln(pic)4]·nH2O (M = Na, NH4; Ln = Eu, Gd, Tb, Ho) with pic-linked [Ln(pic)4]− units forming a chain-like structure, which gives rise to a one-dimensional exchange communication between the rare earth ions; this energy transfer being confined to the chains. Energy transfer of the Coulomb type between the ligands appears to be of significance only, if suitable rare earth acceptor states are not accessible, as is shown for a series of [La(pic)4]− series, in which La3+ is gradually substituted by Tb3+ or Eu3+.

Graphene/zinc nano-composites by electrochemical co-deposition

We describe for the first time the electrochemical co-deposition of composites based on a reactive base metal and graphene directly from a one-pot aqueous mixture containing graphene oxide and Zn(2+). In order to overcome stability issues the Zn(2+) concentration was kept below a critical threshold concentration, ensuring stable graphene oxide suspensions in the presence of cationic base metal precursors. This approach ensures the compatibility between the cationic base metal precursor and graphene oxide, which is more challenging compared to previously reported anionic noble metal complexes. Spectroscopic evidence suggests that the reason for destabilisation is zinc complexation involving the carboxylate groups of graphene oxide. The composition of the electrodeposited co-composites can be tuned by adjusting the concentration of the precursors in the starting mixture. The nano-composites show zinc particles (<3 nm) being uniformly dispersed amongst the graphene sheets. It is also demonstrated that the composites are electrochemically active and suitable for energy storage and energy conversion applications. However, a factor limiting the discharge efficiency is the reactivity of the base metal (low reduction potential and small particle size) which undergoes rapid oxidation when exposed to aqueous electrolytes.

In-Situ-Activated N-Doped Mesoporous Carbon from a Protic Salt and Its Performance in Supercapacitors

Protic salts have been recently recognized to be an excellent carbon source to obtain highly ordered N-doped carbon without the need of tedious and time-consuming preparation steps that are usually involved in traditional polymer-based precursors. Herein, we report a direct co-pyrolysis of an easily synthesized protic salt (benzimidazolium triflate) with calcium and sodium citrate at 850 °C to obtain N-doped mesoporous carbons from a single calcination procedure. It was found that sodium citrate plays a role in the final carbon porosity and acts as an in situ activator. This results in a large surface area as high as 1738 m2/g with a homogeneous pore size distribution and a moderate nitrogen doping level of 3.1%. X-ray photoelectron spectroscopy (XPS) measurements revealed that graphitic and pyridinic groups are the main nitrogen species present in the material, and their content depends on the amount of sodium citrate used during pyrolysis. Transmission electron microscopy (TEM) investigation showed that sodium citrate assists the formation of graphitic domains and many carbon nanosheets were observed. When applied as supercapacitor electrodes, a specific capacitance of 111 F/g in organic electrolyte was obtained and an excellent capacitance retention of 85.9% was observed at a current density of 10 A/g. At an operating voltage of 3.0 V, the device provided a maximum energy density of 35 W h/kg and a maximum power density of 12 kW/kg.

Phosphonium plastic crystal salt alloyed with a sodium salt as a solid-state electrolyte for sodium devices: phase behaviour and electrochemical performance

Mixtures of triisobutylmethylphosphonium bis(fluorosulfonyl)imide (P1i444FSI) with different concentrations of NaFSI display composition-dependent phase behaviour with depression of the melting point upon NaFSI addition and suppression of crystallisation in the middle of the phase diagram and a higher-melting-point, NaFSI-rich mixed-phase region at compositions beyond 60 mol% NaFSI. Thermal treatment of the intermediate compositions results in complete crystallisation of the materials. All compositions showed high ionic conductivity (>10−5 S cm−1) at 20 °C, and Na symmetric cells containing high concentration of NaFSI (Na|[Na0.9(P1i444)0.1]FSI|Na) were cycled efficiently at 50 and 90 °C at 0.05 and 0.1 mA cm−2, respectively. Intermediate composition (Na|[Na0.6(P1i444)0.4]FSI|Na) cells were cycled at room temperature and 50 °C at 0.25 mA cm−2 with very low and stable cell polarisation (200 mV).

Electrochemical Behavior of PEDOT/Lignin in Ionic Liquid Electrolytes: Suitable Cathode/Electrolyte System for Sodium Batteries

Biomass-derived polymers, such as lignin, contain quinone/ hydroquinone redox moieties that can be used to store charge. Composites based on the biopolymer lignin and several conjugated polymers have shown good charge-storage properties. However, their performance has been only studied in acidic aqueous media limiting their applications mainly to supercapacitors. Here, we show that PEDOT/lignin (PEDOT: poly(3,4-ethylenedioxythiophene)) biopolymers are electroactive in aprotic ionic liquids (ILs) and we move a step further by assembling sodium full cell batteries using PEDOT/lignin as electrode material and IL electrolytes. Thus, the electrochemical activity and cycling of PEDOT/lignin electrodes was investigated in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPyrTFSI), 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (BMPyrFSI), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMImFSI) IL electrolytes. The effects of water and sodium salt addition to the ILs were investigated to obtain optimum electrolyte systems for sodium batteries. Finally, sodium batteries based on PEDOT/lignin cathode with imidazolium-based IL electrolyte showed higher capacity values than pyrrolidinium ones, reaching 70 mAhg-1 . Our results demonstrate that PEDOT/lignin composites can serve as low cost and sustainable cathode materials for sodium batteries.

Spectroscopic Characterization of the SEI Layer Formed on Lithium Metal Electrodes in Phosphonium Bis(fluorosulfonyl)imide Ionic Liquid Electrolytes

The chemical composition of the solid electrolyte interphase (SEI) layer formed on the surface of lithium metal electrodes cycled in phosphonium bis(fluorosulfonyl)imide ionic liquid (IL) electrolytes are characterized by magic angle spinning nuclear magnetic resonance (MAS NMR), X-ray photoelectron spectroscopy (XPS), fourier transformed infrared spectroscopy, and electrochemical impedance spectroscopy. A multiphase layered structure is revealed, which is shown to remain relatively unchanged during extended cycling (up to 250 cycles at 1.5 mA·cm-2, 3 mA h·cm-2, 50 °C). The main components detected by MAS NMR and XPS after several hundreds of cycles are LiF and breakdown products from the bis(fluorosulfonyl)imide anion including Li2S. Similarities in chemical composition are observed in the case of the dilute (0.5 mol·kg-1 of Li salt in IL) and the highly concentrated (3.8 mol·kg-1 of Li salt in IL) electrolyte during cycling. The concentrated system is found to promote the formation of a thicker and more uniform SEI with larger amounts of reduced species from the anion. These SEI features are thought to facilitate more stable and efficient Li cycling and a reduced tendency for dendrite formation.