Neutral magic-angle bilayer graphene: Condon instability and chiral resonances

We discuss the full optical response of twisted bilayer graphene at the neutrality point close to the magic angle within the continuum model. (i) First, we define the full optical response consistent with the underlying $D_3$ symmetry, yielding the total, magnetic, and chiral response that transform according to the irreducible representations $A_1$, $A_2$, and $E$, respectively. Then, we numerically calculate the dissipative and reactive response for twist angles around the magic angle $\theta_m$ and comment on the possibility of a Condon instability. (ii) Second, we numerically calculate the full optical response {\it almost at} $\theta_m$. The total response is characterized by three universal plateaus which can be obtained from an analytical calculation. The magnetic and the chiral response, however, is given by corresponding non-universal plateaus depending on the twist angle $\theta$ via the dimensionless parameter $\alpha\sim\theta_m-\theta$. (iii) Following the discussion on the large magnetic response, we calculate the plasmonic excitations at the neutrality point inside the optical gap of relaxed twisted bilayer graphene. We find that acoustic plasmons extend over almost the whole optical gap and carry the largest oscillator strength. (iv) Finally, we discuss symmetry relations for the response functions as function of the chemical potential and highlight the consequences of the approximate particle-hole symmetry of the continuum model for twisted bilayer graphene. We then discuss a detailed balance relation where the chiral response at charge neutrality can be understood in terms of electron (hole) transitions for which the initial (final) states are energetically closer to charge neutrality than the final (initial) states.Comment: 17 pages, 7 figure

2-Sulfoethylammonium trifluoromethanesulfonate as an Ionic Liquid for High Temperature PEM Fuel Cells

2-Sulfoethylammonium trifluoromethanesulfonate ([2-Sea+][TfO−]) represents a novel class of proton-conducting ionic liquids (PILs) based on aminoalkylsulfonic acids. The fundamental suitability of [2-Sea+][TfO−] for application as a protic electrolyte in high temperature PEM fuel cells (HT-PEFCs) was investigated up to a temperature of 130°C. A comparison was made against a state-of-the-art electrolyte, phosphoric acid. [2-Sea+][TfO−] is electrochemically and thermally stable up to 140°C. The specific conductivity of 95 wt% [2-Sea+][TfO−] aqueous solution at 130°C is ≈20 times lower compared to 95 wt% H3PO4. The strong coupling of ion transport and viscous flow suggests a vehicular ion (proton) transport in [2-Sea+][TfO−]. 95 wt% [2-Sea+][TfO−] shows superior kinetics in terms of oxygen reduction reaction (ORR) on polycrystalline Pt compared to 95 wt% H3PO4 at temperatures greater than 90°C in a fuel cell-applicable potential range. Double layer capacitances suggest a complex double layer structure, including adsorbed [2-Sea+][TfO−] and water, as well as intermediates of oxygen reduction and Pt oxidation. Potential and temperature-dependent ORR kinetics in the presence of 95 wt% [2-Sea+][TfO−] yield different Tafel slopes (b = 82–139 mV) and symmetry factors (β = 0.46–0.96), indicating changes in surface coverages of the adsorbed species and possibly also a change in the reaction mechanism

Untersuchungen zum Einsatz neuer Werkstoffe für SOFC-Anwendungen

Fuel cells based on solid oxides (‘SOFC’) are excellent alternative devices for power generation, when they are operated at high temperature, e.g. above 600°C. Having only fixed parts for the power generating part of the device is only one advantage of the fuel cell. Due to their unique design, these devices offer a maximum of efficiency for energy conversion compared to conventional power generating systems, which are mainly based on turbines. One aim of this thesis is the examination of alternative electrolyte and cathode materials for the SOFC applications at reduced temperatures, which means in the temperature range between 600°C and 750°C. For the first main task, several materials from the oxygen ion conducting electrolytes were selected. Different strontium and magnesium doped lanthanum gallate (LSGM) materials with additional transition metal doping were selected and prepared via two different preparation methods. The optimum calcining conditions were determined using thermal analysis methods. The results of the structural analysis of the sintered electrolyte materials were used to select the most suitable electrolyte materials. As a result, LSGM and iron doped LSGM (LSGMF) were the most promising materials. Further investigations were carried out on LSGMF materials with different strontium content. The influence of chemical cation non-stoichiometry on the perovskite material was investigated. Therefore, measurements to gather information about the crystallographic structure, morphology, electrochemistry and electrical conductivity were carried out. For a selected sample, the correlations between single effects, such as the crystallographic structure, and the electrical properties are shown by combining the different analysis methods. It could be shown that both the electrochemistry and the crystallographic structure have a significant influence on the electrical conductivity of the LSGMF materials. The second aim of the thesis was the selection and preparation of suitable cathode materials for the SOFC operated at reduced temperatures. The focus for this selection was laid on the chemical compatibility with LSGMF and Scandia doped yttria (ScSZ) based electrolyte materials. Most of the cathode materials could be prepared single phased, with the exception of a strontium doped lanthanumcuprate and a strontium doped lanthanumnickelate. The third aim of the thesis was the investigation of the chemical compatibility between the prepared cathode materials and the electrolyte materials ScSZ and LSGM. The combinations of electrolyte-cathode materials were annealed at commonly used co-sintering temperatures but for extended sintering times. X-Ray diffraction patterns (XRD) and scanning electron microscopy (SEM) were used for the investigation. Based on the examination of the appearing primary and secondary phases and the morphology a ranking of the combinations is given. The least chemical reactions with both electrolyte materials were observed for a strontium doped lanthanum manganite. However, a strontium and copper doped lanthanum-ferrite seems to be the most promising cathode material for reduced temperatures

Electrical conductivity and chemical equilibria of the phosphoric acid \u2013 water system at HT-PEM fuel cells relevant condition

High temperature polymer electrolyte membrane (HT-PEM) fuel cells typically work at 120-200\ub0C and are mainly based on phosphoric acid (PA) swollen basic polymer membranes like phosphoric acid doped polybenzimidazole. An overview can be found in [1-7]. Although PA is a widely used material even outside the field of electrochemical transformers, only little is known about the thermodynamical but also physical properties at temperatures above 100\ub0C. Especially the correlation between different parameters provided from different sources in literature is often demanding due to different experimental approaches. In this work, an alternative approach is used simulating directly the conditions inside an operational HT-PEM. The obtained physical and thermodynamic data are critically compared to results from other laboratories

Adsorption process of phosphoric acid on polybenzimidazole membranes: a crucial step inside operational high temperature PEM fuel cells

High temperature polymer electrolyte fuel cells (HT-PEFC) typically work at 120-200\ub0C and are mainly based on phosphoric acid (PA) swollen basic polymer membranes like poly\uacbenzimidazole (PBI). Although PA doped PBI membranes were investigated in several ex\uacperimental studies, the kinetic of the adsorption process, the mo\uaclecular interactions bet\uacween the PA molecules and the polymer chains, poly\uaccon\uacdensation equilibria of the PA molecules as well as the implications on the proton conductivity is not finally illuminated. In this work, we have investigated the adsorption process of PA on m-PBI and a commercial AB-PBI derivative (Fumapem AM-55). A number of membranes (cross and uncross-linked) has been prepared at different doping level and analysed to elucidate the adsorption process of the PA as function of temperature, acid concentration and chemical equilibria between PA derivatives. Impedance measurements, Karl Fischer titration method and RAMAN spectra have been used to characterise the membranes. The thermodynamic aspects related to the adsorption process have been analysed with different kinetic models. Considering own and literature data on non-crosslinked m-PBI, a BET-like adsorption kinetics explains the behaviour of this polymer type satisfactorily. Using the RAMAN data, different regions in the BET-like isotherm can be correlated to the protonation of the polymer chain, formation of H-bonds directly to the chain and indirectly to still adsorbed PA molecules

Liquid \u2013 gas phase equilibria of phosphoric acid at high temperature electrolyte polymer fuel cell condition

Phosphoric acid is used in most high temperature polymer electrolyte membrane fuel cells (HT-PEFC) especially because it exhibits high proton conductivity together with high availability. The nature of the proton conductivity in phosphoric acid and the interaction between the polymer matrix and phosphoric acid were investigated recently by MD simulations [1]. Experimental conductivity data are available extensively for temperatures up to 100\ub0C and different water contents, while it gets scares for higher temperatures [2, 3]. The reason for this is the complex nature of the phosphoric acid, tending to polymerize from its monomeric form at low temperature and surplus of water towards chain or even ring structures at temperatures above 100\ub0C with depletion of water [4]. However, the different species have different chemical but also physical properties. This circumstance is either neglected or not considered for most examinations. In contrast to most published methods starting with P2O5 and defined water content, we start from aqueous orthophosphoric acid (85w%) and impose a fixed water vapour pressure using a climate chamber. The development of the equilibrium state with the gas phase is investigated using time sequenced impedance measurements. Temporal snapshot samples are taken and examined using Karl-Fischer titration method to determine the content of unbound water and Raman spectroscopy to determine changes in molecular interactions. The transitions between the different species and adducts of phosphoric acid can be precisely determined

Uptake of protic electrolytes by polybenzimidazole-type polymers \u2013 Model for the absorption isotherm and electrolyte/polymer interactions

Phosphoric acid doped polybenzimidazol (PBI) membranes are commonly used as proton conducting membrane in high temperature polymer electrolyte fuel cells (HT-PEFC). In this study we want to present a general (thermodynamic) model for the absorption process of protic electrolytes by PBI-type polymers, which is able to describe the whole accessible doping range. We have analysed literature and own data on the uptake of phosphoric, sulphuric and perchloric acid by non-cross-linked m-PBI [1, 2] and AB-PBI [3] and by a commercial cross-linked PBI derivative (Fumapem AM-55\ua9). The uptake of protic electrolytes by PBI-type polymers can be described satisfactorily by a BET-like adsorption isotherm (GAB model), assuming a multilayer-like adsorption process. In addition to the thermodynamic data spectroscopic data from Raman studies are taken into account [4]. It is possible to correlate domains in the adsorption isotherms to the protonation of the polymer chains, the formation of H-bonds directly to the chains and to still absorbed electrolyte molecules