Electrocapillary maximum and potential of zero charge of carbon aerogel.

We present an electrochemical study of carbon aerogel (CA) in aqueous sodium fluoride solutions, focusing on the comparison of two quantities that are related to the potential of zero charge (pzc): the capacitance minimum and the 'electrocapillary maximum' of the surface forces. Capacitance minima are well resolved in our samples. Their potential emerges reproducibly as around 90 mV (vs. Ag/AgCl in KCl), similar to the value, 70 mV, of bulk glassy carbon which we use for comparison, and similar to previous reported pzc values for carbon materials. Significantly, no electrocapillary maximum is found in this potential range. This demonstrates that the pzc does not necessarily coincide with the potential of the maximum of surface stress. We also determined the area-specific capacitances, c(a) = 2.8 microF cm(-2), which agrees well with reports for the basal-plane of graphite single crystals. Our experiments yield large reversible strain amplitudes, up to 0.45%.

Exploring the Reversibility of Phase Transformations in Aluminum Anodes through Operando Light Microscopy and Stress Analysis.

Aluminum is well-known to possess attractive properties for possible use as an anode material in Li-ion batteries (LIBs), but effort is still needed to understand how and why it degrades. Herein, investigations of the delithiation and the re-lithiation processes in Al thin films using an established operando light microscopic platform are pursued. Operando videos highlight that the extraction of Li from the ß phase (LiAl) is accompanied by fracture and crack formation leading to the detachment of the α phase (Al) from the rest of the electrode. The evolution of mechanical stress in Al thin film electrodes is tracked and shows severe stress asymmetry as phase transformations progress. Combining with the observations from light and electron microscopy, the mechanical stress during dealloying can be explained by Li solubility with the ß phase, formation of cracks and of a highly porous Al nanostructure. Although the results pave a difficult path for utilization of the Al/LiAl/Al (α/ß/α) phase transformations in future LIBs, they also suggest excellent opportunities when structural changes can be prevented, which otherwise impact the stability of Al-based electrodes.

Improvement of the Cycling Performance of Aluminum Anodes through Operando Light Microscopy and Kinetic Analysis.

Aluminum is an attractive anode material for lithium-ion batteries (LIBs) owing to its low cost, light weight, and high specific capacity. However, utilization of Al-based anodes is significantly limited by drastic capacity fading during cycling. Herein, a systematic study is performed to investigate the kinetics of electrochemical lithiation of Al thin films to understand the mechanisms governing the phase transformation, by using an operando light microscopy platform. Operando videos reveal that nuclei appear at random positions and expand to form quasi-circular patches that grow and merge until the phase transformation is complete. Based on this direct evidence, models of the lithiation processes in Al anodes are discussed and reaction-controlled kinetics are suggested. The growth rate of LiAl depends on the potential and increases considerably as higher overpotentials are approached. Lastly, improved cycling performance of Al-based anodes can be realized by two approaches: 1) by controlling the lithiation extent, the cycling life of Al thin film is extended from 5 cycles to 25 cycles; 2) the performance can be optimized by adjusting the kinetics. Together, this work offers a renewed promise for the commercialization of Al-based anodes in LIBs where the performance requirements are compatible with the proposed cycle life-extending strategies.

Rapid process synthesis supported by a unified modular software framework.

Although known to be very powerful, the widespread application of model-based techniques is still significantly hampered in the area of bio-processes. Reasons for this situation can be found along the whole chain to set up and implement such approaches. In a time-consuming step, models are typically hand-crafted. Whether alternatives of better models exist to actually fulfill the final goals is undocumented, most often even unknown. In a next step, model-based process control methods are hand-coded in an error-prone procedure. For many of these methods given in the literature, only simulation studies are shown, leaving the interested reader with the unanswered question whether the implementation of a specific method in a real process is viable. As the potentially time-consuming implementation of such a method presents a risk for a rapid process development, promising candidates may be overlooked. To remediate this unsatisfactory situation, a combination of theoretical methods and information technology is proposed here. By an exemplarily realized software tool, it is shown how such an environment helps to promote model-based optimization, supervision, and control of bio-processes and allows for an inexpensive test of new ideas as well in real-life experiments. The contribution concentrates on an overview of a possible software architecture with respect to necessary methods and a meaningful information strategy, highlighting some of the more crucial building blocks. Experimental results exploiting parts of the proposed methods are given for a yeast strain synthesizing a product of industrial interest.

Dependence of surface stress, surface energy and surface tension on potential and charge.

A toy model and simple model functions are used to exemplify the relation between surface tension, surface energy and surface stress given by Shuttleworth's equation. Variations of the surface tension of charged interfaces must obey Lippmann's equation. Variations Deltaf of the surface stress of electrodes would be either equal to or proportional to and smaller than those variations Deltagamma of the surface tension, if the potential of zero charge (pzc) did not depend on the surface strain epsilon. However, since the pzc E(0) is a function of strain epsilon, the basic dependence of the surface stress on the charge, f(q), is described by a sum of three terms: the first one is the surface stress of the uncharged surface. The second one varies linearly with the surface tension, gamma(q), as long as the amount of specific adsorption remains constant, and is quadratic in E and q for a potential-independent double layer capacitance. The third summand that contributes to f(q) is linear in q and is a direct consequence of the potential dependence E(0)(epsilon) of the pzc. This result should help to resolve the seeming discrepancy between previous work on the surface stress changes of electrodes: Most experimental and theoretical results supported either the view that the variations of surface stress are identical or similar to that of surface tension, i.e. have essentially a quadratic dependence on the electrode potential, or that the basic dependence is a linear function of the charge. The more comprehensive model description presented here allows an explanation of the different results and their seeming discrepancies without assuming experimental errors or fundamental thermodynamic problems. Furthermore, an estimate of the potential dependence of the surface modulus can be obtained.