Application of Ionic Liquids Containing Tricyanomethanide [C(CN) 3 ] - or Tetracyanoborate [B(CN) 4 ] - Anions in Dye-Sensitized Solar Cells Magdalena Marszalek, † Zhaofu Fei, † Dun-Ru Zhu, ‡ Rosario Scopelliti, † Paul J. Dyson,* ,† Shaik Mohammed Zakeeruddin,* ,† and Michael Gra ̈ tzel † † Institut des Sciences et Inge ́ nierie Chimiques, Ecole Polytechnique Fe ́ de ́ rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland ‡ State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, P. R. China * S Supporting Information ABSTRACT: A series of novel ionic liquids composed of imidazolium, pyridinium, pyrrolidinium, and ammonium cations with tricyanomethanide or tetracyanoborate anions were prepared. The ionic liquids were characterized by NMR and IR spectroscopy and ESI-mass spectrometry, and their physical properties were investigated. Solid state structures of the N-propyl-N-methylpyrrolidinium and triethylpropylammo- nium tetracyanoborate salts were obtained by single crystal X- ray diffraction. The salts that are liquid at room temperature were evaluated as electrolyte additives in dye-sensitized solar cells, giving rise to efficiencies 7.35 and 7.85% under 100 and 10% Sun, respectively, in combination with the standard Z907 dye. ■ INTRODUCTION Despite the ongoing interest in ionic liquids (ILs), 1,2 the majority of the research is based on ILs containing a rather limited set of fluorine-containing anions, notably tetrafluor- oborate (BF 4 - ), hexafluorophosphate (PF 6 - ), and bis- (trifluoromethylsulfonyl)imide (Tf 2 N - ). 3 In contrast, ILs based on nonfluorous anions, i.e., tricyanomethanide ([C- (CN) 3 ] - ) 4-6 and tetracyanoborate ([B(CN) 4 ) - ]), 7,8 are some- what underexplored. Indeed, it has been reported that ILs with [C(CN) 3 ] - and [B(CN) 4 ] - anions have low viscosities. 4,8 For example, [EMI][C(CN) 3 ] (where EMI = 1-ethyl-3-methyl- imidazolium cation) has a viscosity of 18 cP at 22 °C and a melting point of 11 °C, 4 and [EMI][B(CN) 4 ] has a viscosity of 19 cP at 21 °C, 8 both being lower than that of [EMI][Tf 2 N], 27 cP at 20 °C. 9 N,N,N′,N′-Tetramethyl-N″,N″-dipentylguani- dinium tricyanomethanide has a viscosity of 88 cP, 6 the value being relatively low given the cation. There are only a limited numbers of ILs that contain [C(CN) 3 ] - and [B(CN) 4 ] - anions and their applications have been scarcely explored, despite the high stability and interesting electrochemical and physicochemical properties of the [B- (CN) 4 ] - anion. 10 Indeed, in nearly all of the applications, including electrolytes, 11-13 as well as others, 14,15 studies are almost exclusively on the [EMI] + -based systems. In this paper, the synthesis and characterization of a series of new ILs with [C(CN) 3 ] - and [B(CN) 4 ] - anions are described. Some of the room temperature ILs were employed as electrolytes in dye-sensitized solar cells giving very good efficiencies exceeding 7%. ■ RESULTS AND DISCUSSION The preparation of ILs with BF 4 - , PF 6 - , and Tf 2 N - anions is well established, 16,17 whereas there are relatively few reports describing the synthesis of ILs with [C(CN) 3 ] - anions. The synthesis of ILs with [B(CN) 4 ] - anions is only covered in a patent, and details of the anion exchange of the alkali metal salts of tetracyanoborate with imidazolium or pyridinium halides have not been reported. 7 In this work, we used a literature protocol to react 1-propyl-3-methylimidazolium chloride [PMI]Cl with Na[C(CN) 3 ] in aqueous solution, and the resulting IL, [PMI][C(CN) 3 ](1a), was extracted with dichloromethane. Similarly, reaction of [PMI]Cl with K[B- (CN) 4 ] gives the IL [PMI][B(CN) 4 ](1b). Using the same approach, ILs containing N-propylpyridinium, N-propyl-N- methylpyrrolidinium, and triethylpropylammonium cations with the [C(CN) 3 ] - anion (2a-4a) and the [B(CN) 4 ] - anion (2b-4b) were prepared (see Figure 1). The [C(CN) 3 ] - containing ILs, 1a-4a, are liquid at room temperature, whereas of the ILs containing the [B(CN) 4 ] - Received: July 15, 2011 Published: October 25, 2011 Article pubs.acs.org/IC © 2011 American Chemical Society 11561 dx.doi.org/10.1021/ic201513m | Inorg. Chem. 2011, 50, 11561-11567