Photon Factory Activity Report 2016 #34 (2017)
AR-NE5C/2013G620 , BL-18C/2015G083

Polyiodides in Room-Temperature Ionic LiquidHiroshi Abe1,*, Hiroaki Kishimura1, Mayumi Takaku2, Mai Watanabe2 and Nozomu Hamaya2
1Department of Materials Science and Engineering, National Defense Academy, Yokosuka
239-8686, Japan
2
Graduate School of Humanities and Sciences, Ochanomizu University, 2-1-1 Otsuka, Bunkyo-ku,
Tokyo 112-8610, Japan
1 Introduction
Polyiodides (Im-) have been studied in various scientificfields since a long time.[1] Dye-sensitized solar cells(DSSCs) have been examined for high conversion
efficiency. Recently, room-temperature ionic liquids(RTILs) were assembled into DSSCs on the basis of theirelectrochemical stability.[2] In redox couples, polyiodides
combined with the RTILs were fabricated in the DSSCs.
Very recently, 127I-NMR experiments in the [Cnmim][Im]system have supported the Grotthuss exchangemechanism by the doublet and triplet peak splitting at a
relatively large m.[3] Here, the Cnmim+ cations were 1alkyl-3-methylimidazolium, and n denoted the alkyl-side
chain length of Cnmim+.
2 ExperimentHigh pressure (HP) behavior of transparent pure[C3mim][I] was examined using diamond anvil cell(DAC) in BL-18C. In the mixed system, we performed
HP experiments in NE5C with a multi-anvil assembly(MAX80), since ruby inside the DAC is useless as apressure marker in the dark colored [Cnmim][Im].
3 Results and Discussion
Low-temperature phase (LT) behaviors
wereinvestigated using simultaneous X-ray diffraction anddifferential scanning calorimetry.[4] Crystallization ofpure [C3mim][I] was suppressed both at LT and HP (Fig.
1). In contrast, stoichiometric [C3mim][I3]havingpolyiodide easily crystalized at LT and HP. Hence, I3(one of polyiodides) is regarded as a crystal forming
factor, although I- contributes to glass forming. This is asignificant finding of an essential property of polyiodides.

Non-stoichiometric [C3mim][I3.66] demonstrated an
entirely different phase behavior at LT. In the coolingprocess, the crystallization of [C3mim][I3.66] was notobserved. While, upon heating, metastable frozen[C3mim][I3.66] exhibited cold crystallization. In order tointerpret the complicated behaviors, we introduce thefollowing assumption. Since [C3mim][I3] crystallized
simply, [C3mim][I3] is defined newly to be pure RTIL
system considering I3- anion as a crystal forming factor.
Then, [C3mim][I3.66] is rewritten by [C3mim][I3] – 7.1
mol% I2. Thus, 7.1 mol% I2 is regarded as an additive to
new defined pure system of [C3mim][I3]. In some binary
system, 7 mol% additive changes the phase behaviors
drastically. It is pointed out that fluctuations of excessiodine occurs between C3mim+ and I3-. Thus, we deducethat complicated phase behaviors of non-stoichiometric[C3mim][I3] – 7.1 mol% I2 are originated from thedynamic fluctuations of excess iodide/iodine.
Excessiodide/iodineofnon-stoichiometric[C3mim][I3.66] (or [C3mim][I3] – 7.1 mol% I2) caused
complicated behavior in the HP phases; (i) HP crystal
polymorph, (ii) spatial heterogeneous solid phases (edgeor
centralparts),and(iii)
decompression
crystallization.[4]In the liquid state, the excess iodide/iodinedestabilizes by propagating between cation and anion.
The unbalanced fluctuation contributes to the nonstoichiometric anomalies at LT and HP. A significant
finding of effect of the non-stoichiometric anomalies
provides a new insight to polyiodide migration inside
RTILs assembled in the DSSCs devices.
Acknowledgement
We thank Dr. T. Takekiyo and Prof. Y. Yoshimura ofNational Defense Academy for the helpful discussions.
We also thank the Photon Factory Program Advisory
Committee (Proposal Numbers: 2013G620, 2015G083).
References[1] P. H. Svensson and L. Kloo, Chem. Rev. 103, 1649
(2003).
[2] R. Kawano and M. Watanabe, Chem. Commun., 330(2003).
[3] H. Abe et al., Phys. Chem. Chem. Phys. 18, 32337(2016) .
[4] H. Abe et al., Faraday Discuss. (2007), in press.
*ab@nda.ac.jp

Fig. 1: Phase behaviors of [C3mim][Im] at LT and HP.