In situ X-ray absorption spectroscopy (XAS) along with electrochemical measurements were used to explore the redox activity of Prussian blue (PB) and its contribution to pseudocapacitance in an asymmetrical solid-state supercapacitor (SC). The SC, with a configuration of (activated carbon)/polymer electrolyte membrane/(activated carbon + PB), was fabricated with an embedded KCl saturated Ag/AgCl reference electrode. The potentials of the two electrodes were generally displaced proportionally with increase in the cell voltage. Upon negative polarization from the open circuit voltage (OCV) condition to −3 V, the charge capacity was greater than that upon positive polarization from the OCV condition to +3 V, in agreement with the higher fraction of Fe(III) than that of Fe(II) and the low concentration of K ions in PB. Detailed analysis of cyclic voltammetry data in the range between 0 and −3 V indicates that the battery-like contribution to the charge current was 96% that of the capacitor-like contribution at a scanning rate of 2.5 mV s ⁻¹ but only 15% at 100 mV s ⁻¹ . The in situ XAS characterization results indicate that the majority of Fe was reversibly charged and discharged between Fe(III) and Fe(II) during positive and negative polarizations with minimal changes in local atomic structure.

We examined the redox activity of commercially obtained K ₄ Fe(CN) ₆ ·xH ₂ O and K ₃ Fe(CN) ₆ incorporated in an asymmetric solid-state supercapacitors (SCs) using combined cyclic voltammetry and X-ray absorption spectroscopy (XAS) under in situ conditions. The cyclic voltammetry measurements were done at 10 mV s ⁻¹ between −2 and + 2 V while the XAS measurements were conducted in transmission mode at the Advanced Photon Source. The asymmetric SCs were constructed with one electrode containing activated carbon (AC) and another electrode containing a mixture of AC and K ₄ Fe(CN) ₆ ·xH ₂ O or K ₃ Fe(CN) ₆ sandwiched between a PVDF/LiTFS membrane. We show that K ₄ Fe(CN) ₆ ·xH ₂ O is stable while Fe(III) in K ₃ Fe(CN) ₆ is reduced to Fe(II) as a result of the electrode synthesis procedures. The complete reduction of Fe(III) to Fe(II) in K ₃ Fe(CN) ₆ during the synthesis procedures is believed to be due to the interaction with lithium from the LiTFS salt and the formation of LiK ₃ Fe(CN) ₆ . Based on the energy shifts in the XANES data, about 50% of the Fe in both K ₄ Fe(CN) ₆ ·xH ₂ O and the reduced form of K ₃ Fe(CN) ₆ is oxidized and reduced between Fe(III) and Fe(II) during electrochemical cycling between −2 and +2 V. The associated changes in the coordination distances and disorders of the first (Fe-C) and second (Fe-N) shells were also quantified and discussed.

Co-curing holds great promise to minimize assembly weight, time, and cost for stiffened aerospace structures, which are conventionally fabricated separately and then integrated either through mechanical fastening or adhesive bonding—also known as secondary bonding. This study presented a low-cost co-curing process using VARTM to fabricate stiffened shells, particularly composite box beams. The experimental investigation was performed and the co-curing process was improved by scrutinizing the critical process parameters, such as foam strength and coating, and curing cycle. This work was also intended to present the demonstration of the proposed co-curing method and its comparison with the conventional secondary bonding technique for three-cell carbon fiber-reinforced polymer (CFRP) composite box beams. Fiber volume fraction measurements were carried out to the specimens extracted from the various section of the co-cured part, namely top skin, web, and bottom skin and as a result, around 60% of fiber volume fraction was measured, which was in good agreement with the results obtained from optical microscopy-based image analysis. Structural-level four-point bending test results showed that the weight normalized maximum and the ultimate load of the part increased by 44% and 45% with the use of the co-curing process, respectively. The improved mechanical properties indicated that stronger structural integration can be achieved by integrally curing structures. SEM micrographs revealed a favorable fiber-matrix interface, bolstering the superior integration of the co-cured part. These findings suggest that the low-cost co-curing process can be a potential candidate for the fabrication of stiffened aerospace structures, such as composite box beams.