Spotlights pubs.acs.org/JPCL
Spotlights: Volume 7, Issue 24
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DETERMINATION OF PEUKERT’S CONSTANT USING IMPEDANCE SPECTROSCOPY: APPLICATION TO SUPERCAPACITORS Benjamin Franklin may have borrowed the military term “battery” during his early experiments with electricity, and Alessandro Volta’s famous voltaic pile may have been the first true electric battery, but neither of these pioneers could have imagined the enormous capacity of 21st century energy storage devices. Batteries have become essential in modern life, and researchers continue to work toward higher, more efficient performance from these crucial tools. One key factor in the performance of energy storage devices is the relationship between energy and power density. Higher power densities are generally correlated with lower available energy. In batteries, this relationship is empirically described using Peukert’s equation, or Peukert’s law, which relates the capacity of a battery to the discharge rate or current. Peukert’s equation has recently attracted attention for application to supercapacitors as a way of modeling and evaluating their performance. In their Letter, Mills and Kim (DOI: 10.1021/acs.jpclett.6b02441) present a method for evaluating Peukert-type behavior of energy storage devices using impedance spectroscopy. They compared this method to galvanostatic charge/discharge measurements through application to three commercial supercapacitors. Both approaches were found to be consistent, but the impedance spectroscopy method allowed the determination of Peukert’s constant more precisely than conventional directcurrent methods. The authors made this finding by measuring the device’s electrical properties in the frequency domain rather than in the time domain. The method could facilitate simple and accurate characterization of energy storage devices and may be particularly useful for characterizing supercapacitors.
the individual cells, and the evolution of water-induced device degradation. The results show the importance of detecting and ameliorating these effects and could lead to better efficiency through the management of the local optical and electronic properties of the multilayer structures.
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IONIZATION OF CELLOBIOSE IN AQUEOUS ALKALI AND THE MECHANISM OF CELLULOSE DISSOLUTION At first glance, newspapers, grated cheese, home insulation, and your favorite sweater do not appear to have that much in common, aside from the happiness they may add to your life. But all of these things can be linked by a common polymer: cellulose. Cellulose is present in most plants, making it one of the world’s most abundant renewable resources. Its uses are seemingly endless, and research continues to find new ways to exploit its strengths. One challenge is the hierarchically ordered structure of cellulose, which makes it difficult to shape. Cellulose is insoluble in most common solvents but will dissolve in aqueous alkali under a narrow range of conditions. Although a large number of cellulose solvents are known, only a few are suitable for industrial applications, and the process can produce significant pollutants. In their Letter (DOI: 10.1021/ acs.jpclett.6b02346), Bialik et al. provide a mechanistic view of cellulose dissolution in aqueous alkali using cellobiose as a model system in combination with two-dimensional nuclear magnetic resonance spectroscopy, density functional theory, and molecular dynamics simulations. They found that approximately one OH group per cellulose anhydroglucose unit is deprotonated, which turns cellulose into a soluble polyelectrolyte in strongly alkaline solutions. This insight could contribute to the discovery of new cellulose solvents that optimize its dissolution, in turn aiding scientists in the creation of additional sustainable cellulose-based materials.
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PROBING PHOTOCURRENT NONUNIFORMITIES IN THE SUBCELLS OF MONOLITHIC PEROVSKITE/SILICON TANDEM SOLAR CELLS When U.S. President Jimmy Carter had solar panels installed at the White House in 1979, many people viewed them as a curiosity, and the use of solar power did not take the nation by storm. Progress may have been slow, but today photovoltaic panels can be found on homes, businesses, and telephone poles across the United Stated and all over the world. Research is ongoing to make better and more efficient solar cells, and scientific interest has recently focused on hybrid perovskite solar cells. Optimization of the technology is challenging because conventional characterization methods do not give clear feedback on the localized chemical and physical factors that limit performance within individual subcells. In their Letter, Song et al. (DOI: 10.1021/acs.jpclett.6b02415) examine perovskite/silicon tandem solar cells with a light beam-induced current mapping technique to probe photocurrent collection nonuniformities in the individual subcells that comprise the tandem. The choice of laser wavelengths and light biasing conditions allowed the investigation of efficiency-limiting effects related to processing defects, optical interference within © 2016 American Chemical Society
Published: December 15, 2016 5307
DOI: 10.1021/acs.jpclett.6b02856 J. Phys. Chem. Lett. 2016, 7, 5307−5307
