The two polymorphs of lithium cobalt oxide, LiCoO 2 , present an opportunity to contrast the structural requirements for reversible charge storage (battery function) vs. catalysis of water oxidation/oxygen evolution (OER; 2H 2 O → O 2 + 4H + + 4e − ). Previously, we reported high OER electrocatalytic activity from nanocrystals of the cubic phase vs. poor activity from the layered phase – the archetypal lithium-ion battery cathode. Here we apply transmission electron microscopy, electron diffraction, voltammetry and elemental analysis under OER electrolysis conditions to show that labile Li + ions partially deintercalate from layered LiCoO 2 , initiating structural reorganization to the cubic spinel LiCo 2 O 4 , in parallel with formation of a more active catalytic phase. Comparison of cubic LiCoO 2 (50 nm) to iridium (5 nm) nanoparticles for OER catalysis (commercial benchmark for membrane-based systems) in basic and neutral electrolyte reveals excellent performance in terms of Tafel slope (48 mV dec −1 ), overpotential ( η = ∼420 mV@10 mA cm −2 at pH = 14), faradaic yield (100%) and OER stability (no loss in 14 hours). The inherent OER activity of cubic LiCoO 2 and spinel LiCo 2 O 4 is attributed to the presence of [Co 4 O 4 ] n+ cubane structural units, which provide lower oxidation potential to Co 4+ and lower inter-cubane hole mobility. By contrast, the layered phase, which lacks cubane units, exhibits extensive intra-planar hole delocalization which entropically hinders the four electron/hole concerted OER reaction. An essential distinguishing trait of a truly relevant catalyst is efficient continuous operation in a real electrolyzer stack. Initial trials of cubic LiCoO 2 in a solid electrolyte alkaline membrane electrolyzer indicate continuous operation for 1000 hours (without failure) at current densities up to 400 mA cm −2 and overpotential lower than proven PGM (platinum group metal) catalysts.
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Shocked silica aerogel radiance transition
Silica (SiO2) aerogel is widely used in high-energy-density shock experiments due to its low and adjustable density. Reported here are measurements of the shock velocity, optical radiance, and reflectivity of shocked SiO2 aerogel with initial densities of 0.1, 0.2, and 0.3 g/cm3. These results are compared with similar data from three solid polymorphs of SiO2, silica, quartz, and stishovite with initial densities 2.2, 2.65, and 4.3 g/cm3, respectively. Interestingly, below a brightness temperature of Tbright≈35,000 K, the slope of the radiance vs shock velocity is the same for each of the SiO2 aerogels and solid polymorphs. At Tbright≈35000 K, there is an abrupt change in the radiance vs shock velocity slope for aerogels, but not seen in the solid polymorphs over the pressures and temperatures explored here. An empirical model of shock front radiance as a function of SiO2 density and laser drive parameters is reported to aid in the design of experiments requiring maximum shock front radiance.
more » « less- Award ID(s):
- 2020249
- NSF-PAR ID:
- 10439908
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- Journal of Applied Physics
- Volume:
- 132
- Issue:
- 9
- ISSN:
- 0021-8979
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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