Electrocatalytic studies on imidazolium based ionic liquids: defining experimental conditions

Abstract

The number of publications devoted to studying electrochemical reactions in room temperature ionic liquids (RTILs) is constantly growing, but very few of them have been devoted to defining proper experimental conditions to obtain reproducible electrochemical results. In this work, we demonstrate that the combination of a proper RTIL purification treatment and a filtered Ar gas stream allow us to obtain featureless voltammograms in [C4mim][BF4], [C4mim][NTf2], and [C4m2im][NTf2], which otherwise present signals associated with different types of impurities such as water and some minor electroactive impurities acquired during the RTIL synthesis process. Moreover, we demonstrate that bubbling Ar, or another inert gas, through the electrolyte in order to purge O2 dissolved in RTILs is one of the major sources of water and O2 impurities incorporated in RTILs within the electrochemical cell. To overcome this source of water uptake, we have incorporated a gas stream purification filter before the gas reaches the RTIL in the electrochemical cell. To illustrate the effect of these impurities in relevant electrocatalytic studies, we study the electrocatalytic reduction of CO2 on Pt nanoparticles and the key role of an appropiate filter when the CO2 gas stream is bubbled within imidazolium based RTILs. Our cyclic voltammetric studies point out that CO2 electroreduction on Pt nanoparticles only presents activity in [C4mim][NTf2] and [C4m2im][NTf2], thus suggesting that the C-2 position on the imidazolium ring is not the key position in CO2 electrochemical reduction. In contrast, the same Pt nanoparticles are inactive towards CO2 electroreduction in [C4mim][BF4], which is a more hydrophilic RTIL.

Role of H2O for CO2 Reduction Reactions at Platinum/Electrolyte Interfaces in Imidazolium Room‐Temperature Ionic Liquids

“…Prior of usage the as received [EMIM][DCA] electrolyte (>98 %, Iolitec, Germany) was cleaned by adding molecular sieve (3 Å, 400 mg/mL) and heated to 80 °C under vacuum (10 −3 mbar), as described previously . [EMIM][BF 4 ] (>98 %, Iolitec, Germany), [BMIM][BF 4 ] and [OMIM][BF 4 ] (99 %, Iolitec, Germany) were cleaned by stirring under vacuum . The resulting residual water concentrations were 31 mM for [EMIM][DCA], 50 mM for [EMIM][BF 4 ] and [BMIM][BF 4 ] and were measured by Karl‐Fischer titration.…”

Electrocatalytic reduction of CO2 in neat and water-containing imidazolium-based ionic liquids

“…Several other studies have confirmed that the presence of water enhances the CO2RR in RTILs [8,10,21,[26][27][28]. This is not surprising because most pathways of the CO2RR require transfer of protons as well as electrons, and water is a good proton donor.…”

“…(iii) Experimental work with single-crystal electrodes in RTILs aimed at understanding the role of surface structure and surface composition. This is a formidable challenge, due to the need of ultraclean conditions when working with this kind of electrodes and the well-known presence of impurities in RTILs [21,45,46], the levels of which can vary from one batch of product to the next. An additional problem in the case of neat RTILs is the absorption of water (from the atmosphere or from a gas being bubbled through the liquid), which acts as an impurity and can obscure relevant voltammetric features [21].…”

“…Moreover, they showed that the imidazoliumbased enhancement of the CO2RR is greatest for silver cathodes and suggest a mechanism in which a purely chemical step is followed by an electron transfer rate-determining step (usually denoted chemical-electrochemical, CE), involving desorption of the RTIL cation before the electron transfer to CO2. Recent results by Montiel et al [21] also suggest that the C2 position on the imidazolium ring does not play a key role in the CO2RR. These authors noted that the CO2 reduction activity of Pt nanoparticles was unaffected when using an imidazolium RTIL with a methyl group at the C2-position.…”

Acid Catalysis with Alkane/Water Microdroplets in Ionic Liquids

“… 72 , 73 These results, together, strongly support the formation of very strong catalytic acid sites in the water microdroplets of the more hydrophilic [bmim][BF 4 ] IL. 74 It must be noted here that the high density of water microdroplets in the IL significantly increases the surface area of the IL in contact with water, since a simple calculation leads to a value of 480 m 2 ·mL –1 of IL ( Figure S32 ). This result suggests that the increase in surface could also be behind the enhanced catalytic activity.…”