In Situ XAS Investigation of K₄Fe(CN)₆·xH₂O and K₃Fe(CN)₆Redox Activity in Solid-State Supercapacitors

We examined the redox activity of commercially obtained K4Fe(CN)6 xH2O and K3Fe(CN)6 incorporated in an asymmetric solidstate 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 K4Fe(CN)6 xH2O or K3Fe(CN)6 sandwiched between a PVDF/LiTFS membrane. We show that K4Fe(CN)6 xH2O is stable while Fe(III) in K3Fe(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 K3Fe(CN)6 during the synthesis procedures is believed to be due to the interaction with lithium from the LiTFS salt and the formation of LiK3Fe(CN)6. Based on the energy shifts in the XANES data, about 50% of the Fe in both K4Fe(CN)6 xH2O and the reduced form of K3Fe(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.