In Situ XAS Investigation of K 4 Fe(CN) 6 ·xH 2 O and K 3 Fe(CN) 6 Redox Activity in Solid-State Supercapacitors

Abstract

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.