Ionic liquids, solvent-free electrolytes

While impressive progress has been made in the development of stable, non-volatile electrolyte formulations, the conversion yields obtained with these systems are presently in the 7-10% range, i.e., below the 11.1% reached with volatile solvents. Future research efforts will be dedicated to bridge the performance gap between these systems. The focus will be on hole conductors and solvent-free electrolytes such as ionic liquids. The latter are a particularly attractive choice for the first commercial modules, due to their high stability, negligible vapor pressure and excellent compatibility with the environment. 

The use of solvent-free electrolytes, i.e., high temperature ionic liquids (molten salt) electrolytes, which only become ionically conductive upon melting when heated to high temperatures. Another variant is the use of low rate, room temperature solid electrolytes. 

Anodic limit, potential referred to Li+/Li, cutoff current density in parentheses. Scan rate 5 mV s. Activated carbon as working surface. Scan rate 10 mV s. Supporting electrolyte 0.1 M BU4NBF4. Scan rate 100 mV s . The solvent-free condition was realized by using an ionic liquid based on Imldazolium cation, at 80 °C. Scan rate 20 mV s . ... 

Two devices are prepared. In the case of the device A, the incident photon-to-collected electron conversion efficiency (IPCE) exceeds 80% from 410 to 590 nm, reaching the maximum of 93% at 530 nm. The short-circuit photocurrent density (/sc), open-circuit photovoltage (Voc), and fill factor (FF) of device A with an acetonitrile-based electrolyte under an irradiance of AM 1.5 G full sunlight are 14.33 mA cm-12, 734 mV, and 0.76, respectively, yielding an overall conversion efficiency (jf) of 8.0%. The photovoltaic parameters of device B with a solvent-free ionic liquid electrolyte are 14.06 mA cm 12, 676 mV, 0.74, and 7.0%, respectively. 

The sieving effect of the carbon host was also demonstrated by measuring the capacitance values of an AC in a series of solvent-free ionic liquids (ILs) of increasing cation size [17], Since ions are not solvated in pure ILs, it was easy to interpret the electrochemical properties by comparing the nanoporous characteristics of carbon and the size of cations calculated by molecular modeling. It was found that the overall porosity of the carbon is noticeably underused, due to pores smaller than the effective size of the cations. The results with ILs confirm that the optimal pore size depends on the kind of electrolyte, i.e., the dimensions of pores and ions must match each other. 

Balducci A, Soavi F, Mastragostino M. The use of ionic liquids as solvent-free green electrolytes for hybrid supercapacitors. Applied Physics A 2006 82 627-632. 

The plasma ionic liquid interface is interesting from both the fundamental and the practical point of view. From the more fundamental point of view, this interface allows direct reactions between free electrons from the gas phase without side reactions - once inert gases are used for the plasma generation. From the practical point of view, ionic liquids are vacuum-stable electrolytes that can favorably be used as solvents for compounds to be reduced or oxidised by plasmas. Plasma cathodic reduction may be used as a novel method for the generation of metal or semiconductor particles, if degradation reactions of the ionic liquid can be suppressed sufficiently. Plasma anodic oxidation with ionic liquids has yet to be explored. In this case the ionic liquid is cathodically polarized causing an enhanced plasma ion bombardment, that leads to secondary electron emission and fast decomposition of the ionic liquid. 

Arbizzani, C., M. Biso, D. Cericola, M. Lazzari, F. Soavi, and M. Mastragostino. 2008. Safe, high-energy supercapacitors based on solvent-free ionic liquid electrolytes. Journal of Power Sources 185 1575—1579. 

Wang, R, Zakeeruddin, S. M., Moser, J. E., Humphry-Baker, R, and Gratzel, M. (2004). A solvent-free, SeCN-/(SeCN)3-based ionic liquid electrolyte for high-efficiency dye-sensitized nanocrystalline solar cells./ Am. Chem. Soc., 126, pp. 7164-7165. 

Prediction of salt electrochemical stability in the context of Li-ion batteries has mainly involved predicting the Eox of novel lithium salt anions, frequently without any focus on the subsequent decomposition reaction products and mechanisms. However, with recent results on oxidation promoted solvent-anion reactions [57] and the rapid development of solvent-free ionic liquid (IL) electrolytes, investigations of both anion and cation decomposition products are foreseen by us to become more frequent and important - particularly in connection with the passivation phenomena at the negative electrode. As for solvents, we will here follow the historical development of studies and methods, followed by some more recent works that together with our remarks outline our perspective on the future. 

Electrolytes are distinguished from pure electronic conductors by the fact that the passage of an electric current is only insured by displacement of charged species called ions and hence accompanied by a transfer of matter. Therefore, electrolytes are entirely ionic electrical conductors without exhibiting any electronic conductivity (i.e., no free electrons). They can be found in the solid state (e.g., fluorite, beta-aluminas, yttria-stabilized zirconia, and silver iodide), liquid state (e.g., aqueous solutions, organic solvents, molten salts and ionic liquids), and gaseous state (e.g., ionized gases and plasmas). The ions (i.e., anions or cations)... 

Lazzari M, Mastragostino M, Soavi F (2007) Capacitance response of carbons in solvent-free ionic liquid electrolytes. Electrochem Commun 9 1567-1572... 

In recent developments (Bai et al., 2008), the somewhat problematic liquid electrolyte has been replaced by an ionic liquid consisting of a solvent-free ternary eutectic melt as the redox electrolyte. Further information on the history of the Gratzel cell, and details of the latest breakthroughs, can be found at http //en. Wikipedia, org/wiki/ Dye - sensitized soIar ceU (2008). 

Appetecchi, G. B. Kim, G.-T. Montanino, M. Alessandrini, F. Passerini, S. (2008). Solvent-free, PYR14TFSI ionic liquids-based ternary polymer electrolyte systems. II. Battery tests. ECS Transactions, 11, 29, (2008, Rechargeable Lithium and Lithium Ion Batteries), 119-129... 

Quasi-solid-state electrolytes include gel polymer electrolytes, ionic liquids, and plastic crystal systems. It is important to distinguish polymer electrolytes and gel polymer electrolytes. In polymer electrolytes, charged cationic or anionic groups are chemically bonded to a polymer chain, while gel polymer electrolytes are solvated by a high dielectric constant solvent and are free to move. In a classical gel electrolyte, polymer and salts are mixed with a solvent, usually having a concentration above 50 wt%, and the role of the polymer is to act as a stiffener for the solvent, creating a three-dimensional network, where cations and anions move freely in the liquid phase [88]. The solid polymer electrolyte includes poly(ethylene oxide) (PEO)-based lithium ion conductors that typically show conductivities of 10 S cm while the gel polymer electrolytes have semisolid character with much higher ionic conductivities of the order 10 —10 S cm . ... 

Zhao J, Shen X, Yan F, Qiu L, Lee S, Sun B (2011) Solvent-free ionic liquid/poly(ionic liquid) electrolytes for quasi-soUd-state dye-sensitized solar cells. J Mater Chem 21(20) 7326-7330. doi 10.1039/ClJM10346F... 

Most of the solid polymer electrolytes used can be classified as follows (1) polymer or gel matrixes swollen with liquid electrolyte solutions (e.g. ethylene carbonate (EC)/PAN/sodium perchlorate (NaC104)) (2) singleion systems in which only one ionic species is mobile within a polymer matrix (e.g. perfiuorosulphonate ionomer Nafion ) (3) solvent-free ion-coupled systems consisting of ion-solvating polymers mixed with salts, so that cations and ions become mobile within the polymer network, e.g. PEO mixed with salts. 

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