Deeply virtual Compton scattering has recently been studied by three HERA experiments, H1, ZEUS and HERMES, covering a broad range of kinematic regimes. We present cross section measurements of the two collider experiments in the kinematic region 2<Q^2<100 GeV^2 and 30<W<140 GeV, and compare them to QCD-based calculations. HERMES measurements of azimuthal asymmetries and their kinematical dependences are presented for Q^2>1 GeV^2 and 2<W<7 GeV.

Jochen Volmer

Separated longitudinal and transverse structure functions for the reaction 1H(e,eprime pi+)n were measured in the momentum transfer region Q2=0.6-1.6 (GeV/c)**2 at a value of the invariant mass W=1.95 GeV. New values for the pion charge form factor were extracted from the longitudinal cross section by using a recently developed Regge model. The results indicate that the pion form factor in this region is larger than previously assumed and is consistent with a monopole parameterization fitted to very low Q2 elastic data.

J. Volmer

We investigate the formation of lattice matched single-crystalline Fe$_3$Si/GaAs(001) ferromagnet/semiconductor hybrid structures by Volmer-Weber island growth, starting from the epitaxial growth of isolated Fe$_3$Si islands up to the formation of continuous films as a result of island coalescence. We find coherent defect-free layers exhibiting compositional disorder near the Fe$_3$Si/GaAs-interface for higher growth rates, whereas they are fully ordered for lower growth rates.

B Jenichen Z Cheng M Hanke J Herfort A Trampert

H$_2$ evolution reaction (HER) requires an electrocatalyst to reduce the reaction barriers for the efficient production of H$_2$. Platinum-group metal (PGM) elements such as Pt, Pd, etc. and their derivatives show excellent electrocatalytic activity for HER. The high cost and lack of availability of PGM elements bring constraints over their wide commercial applications, so discovering noble metal-free electrocatalysts with lower possible reaction barriers is paramount important. Two-Dimensional Transition Metal Dichalcogenides (2D TMDs) have emerged as a pinnacle group of materials for many potential applications, including HER. In this work, we have computationally designed a pristine 2D monolayer tungsten diselenide (WSe$_2$) TMD using the first principle-based hybrid Density Functional Theory (DFT) to investigate its structural, electronic properties and the electrocatalytic performance for HER. The possible Volmer-Heyrovsky and Volmer-Tafel reaction mechanisms for HER at the W-edge of the active site of WSe$_2$ were studied by using a non-periodic finite molecular cluster model W$_{10}$Se$_{21}$. Our study shows that the pristine 2D monolayer WSe$_2$ follows either the Volmer-Heyrovsky or the Volmer-Tafel reaction mechanisms with a single-digit low reaction barrier about 6.11, 8.41 and 6.61 kcal/mol during the solvent phase calculations of H-migration, Heyrovsky and Tafel transition (TS) states, respectively. The lower reaction barriers, high turnover frequency (TOF) ~ 4.24 x $10^6$ sec$^{-1}$ and 8.86 x $10^7$ sec$^{-1}$ during the Heyrovsky and Tafel reaction steps and the low Tafel slope 29.58 mV.dec$^{-1}$ confirm that the pristine 2D monolayer WSe$_2$ might be a promising alternative to PGM based electrocatalyst.

Srimanta Pakhira Vikash Kumar

An electrocatalyst is needed to efficiently lower the reaction barriers to produce hydrogen through the H2 evolution reaction (HER). Recently, two-dimensional transition metal dichalcogenides (2D TMDs), such as the pure 2D monolayer MoTe2 TMD, have become attractive materials for HER. Using the first principle-based hybrid DFT-D method, we have computationally designed a pure 2D monolayer MoTe2 TMD and examined its structural and electronic properties and electrocatalytic efficacy towards HER. A non-periodic finite molecular cluster model Mo10Te21 system was employed to explore the feasibility of both the Volmer-Heyrovsky and Volmer-Tafel reaction mechanisms for the HER. The solvent-phase calculations of the HER on the 2D monolayer MoTe2 TMD demonstrate that this material can effectively undergo either Volmer-Heyrovsky or Volmer-Tafel reaction pathways. This conclusion is supported by our determination of low reaction barriers for the H*-migration, Heyrovsky, and Tafel transition states (TSs), which were found to be approximately 9.80, 12.55, and 5.29 kcal.mol-1, respectively. These results highlight the potential utility of MoTe2 TMD as a promising electrocatalyst for HER. The unusual electrocatalytic activity of the pure 2D monolayer MoTe2 TMD is evidenced by its ability to significantly reduce reaction barriers, achieving impressive turnover frequency during the Heyrovsky and Tafel reaction steps, respectively. Additionally, it demonstrates a remarkably low Tafel slope of 29.58 mV.dec-1. Further exploration of its potential applications in electrocatalysis is warranted. The present work provides valuable insights into the atomic modulation of active sites for enhanced electrocatalytic performance towards HER, paving a way for designing advanced non-noble metal free electrocatalysts.

Vikash Kumar Srimanta Pakhira

In this article the Butler-Volmer equation used in describing Lithium-ion (Li-ion) batteries is discussed. First, a complete mathematical model based on a macro-homogeneous approach developed by Neuman is presented. Two common mistakes found in the literature regarding a sign in a boundary conditions and the use of the transfer coefficient are mentioned. The paper focuses on the form of the Butler-Volmer equation in the model. It is shown how practical problems can be avoided by taking care in the form used, particularly to avoid difficulties when the solid particle in the electrodes approaches a fully charged or discharged state or the electrolyte gets depleted. This shows that the open circuit voltage and the exchange current density must depend on the lithium concentration in both the solid and the electrolyte in a particular way at the extremes of the concentration ranges.

A. M. Ramos C. P. Please

Two different metrics are used to assess Forster resonance energy transfer (FRET) between fluorophores in the steady state: (1) acceptor-quenching of donor fluorescence, E (a.k.a. transfer efficiency); and (ii) donor-excited acceptor fluorescence, F-A-Dex. While E is still more widely used, F-A-Dex has been gaining in popularity for practical reasons among experimentalists who study biomembranes. Here, for the special case of membrane-bound fluorophores, we present a substantial body of experimental evidence that justifies the use of simple Stern-Volmer expressions when modeling either FRET metric under dilute-probe conditions. We have also discovered a dilute-regime correspondence between our Stern-Volmer expression for E and Wolber and Hudson's series approximation for steady-state Forster quenching in 2D. This novel correspondence allows us to interpret each of our 2D quenching constants in terms of both (i) an effective Forster distance, and (ii) two maximum acceptor-concentration limits, each of which defines its own useful experimental regime. Taken together, our results suggest a three-step strategy toward designing more effective steady-state FRET experiments for the study of biomembranes.

Jeffrey T. Buboltz Charles Bwalya Santiago Reyes Dobromir Kamburov

We use a unique combination of electrochemical techniques to elucidate the dependency of hydrogen evolution reaction (HER) and absorption on pH and overpotential for iron and nickel. Impedance spectroscopy shows the dominance of the Volmer-Heyrovsky reaction pathway, challenging the common consideration of Volmer-Tafel dominance. Polarization slopes agree with the Volmer or Heyrovsky rate-determining step, with limitations at high overpotential. The evolution of steady-state permeation current density with overpotential is rationalised through newly-developed theory. Surface activity and absorption trends are captured. Combined with modelling, this work provides a path for quantifying hydrogen uptake and establishing an equivalent fugacity for aqueous electrolytes.

L. Cupertino-Malheiros M. Duportal T. Hageman E. Martínez-Pañeda

Most mathematical models of the transport of charged species in battery electrodes require a constitutive relation describing intercalation of Lithium, which is a reversible process taking place on the interface between the electrolyte and active particle. The most commonly used model is the Butler-Volmer relation, which gives the current density as a product of two expressions: one, the exchange current, depends on Lithium concentration only whereas the other expression depends on both Lithium concentration and on the overpotential. We consider an inverse problem where an optimal form of the exchange current density is inferred, subject to minimum assumptions, from experimental voltage curves. This inverse problem is recast as an optimization problem in which the least-squares error functional is minimized with a suitable Sobolev gradient approach. The proposed method is thoroughly validated and we also quantify the reconstruction uncertainty. Finally, we identify the universal features in the constitutive relations inferred from data obtained during charging and discharging at different C-rates and discuss how these features differ from the behaviour predicted by the standard Butler-Volmer relation. We also identify possible limitations of the proposed approach, mostly related to uncertainties inherent in the material properties assumed known in the inverse problem. Our approach can be used to systematically improve the accuracy of mathematical models employed to describe Li-ion batteries as well as other systems relying on the Butler-Volmer relation.

Lindsey Daniels Smita Sahu Kevin J. Sanders Gillian R. Goward Jamie M. Foster Bartosz Protas

Now-a-days, the development of clean and green energy sources is the prior interest of research due to increasing global energy demand and extensive usage of fossil fuels that create pollutants. Hydrogen has the highest energy density by weight among all chemical fuels. For the commercial-scale production of hydrogen, water electrolysis is the best method which in turn requires an efficient, cost-effective and earth-abundant electrocatalyst. Recent studies have shown that the 2D Janus TMDs are highly effective in the electrocatalytic activity for HER. Herein we report a 2D monolayer WSeTe Janus TMD electrocatalyst for HER. We studied the electronic properties of 2D monolayer WSeTe Janus TMD using periodic DFT calculations, and the direct electronic band gap was obtained to be 2.39 eV. After the calculations of electronic properties, we explored the HER intermediates including various transition state structures (Volmer TS, Heyrovsky TS, and Tafel TS) using a molecular cluster model of WSeTe noted as W10Se9Te12. The present calculations revealed that the 2D monolayer WSeTe Janus TMD is a potential electrocatalyst for HER. It has the lowest energy barriers for all the TSs among other TMDs, such as MoS2, Mn-MoS2, MoSSe, etc. The calculated Heyrovsky energy barrier (= 8.72 kcal.mol-1) for the Volmer-Heyrovsky mechanism is larger than the Tafel energy barrier (=3.27 kcal.mol-1) in the Volmer-Tafel mechanism. Hence our present study suggests that the formation of H2 is energetically more favorable via the Vomer-Tafel mechanism. This work helps shed light on the rational design of effective HER catalysts.

Vikash Kumar Shrish Nath Upadhyay Dikeshwar Halba Srimanta Pakhira