Ionic liquids in electrocatalysis

Ionic liquids in electrocatalysis

Accepted Manuscript Ionic liquids in electrocatalysis Gui-Rong Zhang , Bastian J.M. Etzold PII: DOI: Reference: S2095-4956(16)00008-5 10.1016/j.jech...

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Accepted Manuscript

Ionic liquids in electrocatalysis Gui-Rong Zhang , Bastian J.M. Etzold PII: DOI: Reference:

S2095-4956(16)00008-5 10.1016/j.jechem.2016.01.007 JECHEM 93

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

18 November 2015 17 December 2015 21 December 2015

http://www.journals.elsevier.com/ journal-of-energy-chemistry/

Please cite this article as: Gui-Rong Zhang , Bastian J.M. Etzold , Ionic liquids in electrocatalysis, Journal of Energy Chemistry (2016), doi: 10.1016/j.jechem.2016.01.007

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Ionic liquids in electrocatalysis Gui-Rong Zhanga, Bastian J. M. Etzolda,b,* a

Lehrstuhl für Chemische Reaktionstechnik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen 91058, Germany

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Ernst-Berl-Institut für Technische und Makromolekulare Chemie, Technische Universität Darmstadt,

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Darmstadt 64287, Germany Article history: Received 18 November 2015 Revised 17 December 2015

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Accepted 21 December 2015 Available online

Abstract

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The performance of an electrocatalyst, which is needed e.g. for key energy conversion reactions such as hydrogen evolution, oxygen reduction or CO2 reduction, is determined not only by the inherent structure

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of active sites but also by the properties of the interfacial structures at catalytic surfaces. Ionic liquids (ILs), as a unique class of metal salts with melting point below 100 oC, present themselves as ideal

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modulators for manipulations of the interfacial structures. Due to their excellent properties such as good chemical stability, high ionic conductivity, wide electrochemical windows and tunable solvent properties

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the performance of electrocatalysts can be substantially improved through ILs. In the current minireview, we highlight the critical role of the IL phase at the microenvironments created by the IL, the liquid

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electrolyte, catalytic nanoparticles and/or support materials, by detailing the promotional effect of IL in electrocatalysis as reaction media, binders, and surface modifiers. Updated exemplary applications of IL in electrocatalysis are given and moreover, the latest developments of IL modified electrocatalysts following the “Solid Catalyst with Ionic Liquid Layer (SCILL)” concept are presented. Key words: Electrocatalysis; Ionic liquid; Solid catalyst with ionic liquid layer; Ligand effect; Mass transfer; Fuel cell; Water electrolysis *

Corresponding author.

Tel:

+49 6151 1629984;

Fax:

+49 6151 1629982;

E-mail:

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[email protected] This work was supported by the funding of the German Research Council (DFG), which, within the framework of its Excellence Initiative, supports the Cluster of Excellence “Engineering of Advanced Materials” (www.eam.uni-erlangen.de) at the University of Erlangen-Nürnberg. 1. Introduction

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Ionic liquids (ILs), which are usually defined as salts with melting points below 100 oC [1], represent a unique class of fluids combining many highly interesting properties such as good thermal stability, wide range of fluidity, high ionic conductivity, wide electrochemical potential window and low/negligible volatility. Origin of these unique properties stems mainly from the interplay of various

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interactions between ions (e.g., Coulombic force, hydrogen bond, intermolecular force) [2,3]. Thus, ILs can be strategically designed for specific applications, when the ionic interactions are understood in detail and on a molecular level. Thanks to numerous experimental data concerning the specific properties and applications of ILs being published in the past decade [4–7], and the significant development in the theoretical predication methods [8–12], the application oriented design of IL

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structures has become more and more feasible. Among their numerous applications highly interesting is

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the use of ILs in electrocatalysis. In the past decade application of ILs either as reaction media (solvent), binders, or catalyst surface modifiers for some electrocatalytic key energy conversion reactions (Figure

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1) has attracted tremendous research interest. The introduction of the IL phase to electrocatalytic systems would give birth to a unique local microenvironment created by the solid electrode, liquid electrolyte and

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other adjacent species. The sensitivity of the electrocatalytic reaction on this local microenvironment can be employed to alter selectivity, activity and stability [13]. The IL phase is supposed to affect the

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electrocatalytic behavior through changing thermodynamic properties, reaction pathway, mass transportations, surface accessibility and local concentration of reactants at electrode surfaces, while detailed understandings of electrocatalysis involving IL and electrocatalytic systems specifically designed for use in IL are still needed.

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Figure 1. Scheme illustrating typical properties and applications of ILs in electrocatalysis.

Considering that there have been many excellent reviews concerning ILs in surface electrochemistry

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[3,14], electrified interfaces [2], bioelectrochemistry [15], we will not detail all sides of IL applied in electrochemistry. In the current minireview, we will focus on applications of ILs in electrocatalysis. The purpose is to summarize first how ILs can be employed to affect electrocatalysis and how electrocatalytic properties of ILs can be basis for rationally using IL to achieve performance

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involves any form of electrocatalysis.

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enhancement and second to highlight some exemplary applications of ILs in the literature which

2. How ILs would affect electrocatalysis

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2.1. As reaction media

ILs have long been realized as alternative reaction media and solvent free electrolyte for

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electrochemical process [3,16]. In particular, facile structural manipulation and excellent stability over wide potential/temperature range further make IL a promising electrolyte for electrocatalysis. For

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instance, IL ([C4mim][BF4]) has been used as electrolyte for hydrogen oxidation reaction (HOR) on transition metals such as molybdenum, which were highly active and fairly stable for HOR in IL [17]. The exchange current density is three times higher than that of HOR on platinum [17]. The measurements of activation energies indicate that water could be more easily activated on Mo than on Pt in IL. These results present a prospect of searching low-cost and highly efficient electrocatalysts for HOR in ILs. Electrocatalytic oxidation of hydrogen sulfide at platinum in IL was also studied [18,19]. It is found that the reactants tended to build up at the double layer, reflecting high solubility of the reactant

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in IL and possible attractive interactions with the IL anions at the electrode [19]. Al Nashef et al. reported that the ILs would facilitate the generation of superoxide ions (O2-) which were highly active species for electrooxidation reaction [20]. It is reported that using [C2mim][NTf2] and [C4mpyrr][NTf2] as the reaction media, phenol and 4-tert-butyl-phenol could be efficiently converted to corresponding phenyl triflate molecule [21], while using [C4min][BF4] and [C4mim][PF6], benzyl alcohol could be selectively electrooxidized to benzaldehyde on platinum electrode [22]. Besides, IL has also been widely

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investigated as electrolyte for fuel cell applications, and a number of these studies suggested that fuel cells using protic ILs as electrolyte possessed better stability and performance than conventional inorganic acid electrolyte [23–27]. The promotional effect of IL as electrolyte could originate from the high solubility of reactant gases, weak moisture absorbing properties, and good mass transport properties

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which are desirable to improve the kinetics of the electrode reactions [27]. 2.2. As binders

Due to their good conductivity and suitably viscous, ILs have also been employed as a new kind of binders for replacing conventional non-conductive organic ones in fabricating carbon paste electrode

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(CPE) for electrocatalytic applications [28]. It is found that IL modified CPE (IL-CPE) gave a very large current response from electroactive substrates. e.g., Maleki et al. prepared an IL-CPE composite

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electrode based on the use of pyridinium-based ionic liquid. They found that the composite electrode showed surprisingly high electrochemical performance, reflecting by its ability to lower the

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overpotential of electroactive compounds, and to increase the rate of electron-transfer processes, which

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is believed to be due to the modification of the microstructure of the electrode surface using ionic liquid as the binder [29]. A metal containing IL of [(C4H9)2-bim]3[La(NO3)6] was used for the fabrication of

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IL-CPE, which showed good electrocatalytic activities towards the reduction of hydrogen peroxide, nitrite, bromate, and trichloroacetic acid [30]. The presence of IL ([C4mpyr][NTf2]) in a modified CPE was also reported to improve the electrooxidation rate of ascorbic acid (AA), dopamine (DA) and uric acid (UA), enabling simultaneous determination of these organic molecules with high efficiency and wide linear responses (Figure 2) [31]. Moreover, it is also well documented that incorporation of ionic liquid into the structure of CPE could improve the electrocatalytic activity of CPE-supported active species such as phosphomolybdic acid [32,33], haemoglobin [34], Pd [35,36], Ag [37], PdAg alloy nanoparticles [38], and Cu(OH)2 [39]. A survey has been undertaken to clarify the possible reasons for

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the electrocatalytic activity enhancement by the presence of IL in CPE [40]. It was suggested that different factors, such as the increase in the ionic conduction of the binder, decrease in the resistance of the modified electrode, increase in ion exchange properties of the electrode and the inherent catalytic

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activity of ILs would be responsible for the considerably improved electrochemical response [40].

Figure 2. The CPE was modified with multiwalled carbon nanotubes and an IL. After optimization the

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electrode was further modified with palladium nanoparticles. The resulting electrode gives three sharp and well separated oxidation peaks for ascorbic acid, dopamine and uric acid. Reprinted with permission

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2.3. As surface modifiers

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from ref. [31]. Copyright 2014 Springer.

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Furthermore, ILs show potential for surface modification of carbon materials to improve their compatibility and stability, and introduce more abundant binding sites to anchor metal nanoparticles for

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electrocatalytic applications. It is reported that IL-modified carbon materials exhibit switchable solubility [41], high charge-transfer activity [42] and high electronic conductivity [43]. In addition, low interfacial tensions of IL resulted in high nucleation rates, allowing formation of very small particles [44]. Au/CNT-IL nanohybrides were synthesized by immobilizing Au nanoparticles onto IL functionalized CNT [45]. Based on the excellent physicochemical properties of ILs, Au/CNT-IL nanohybrids showed good electrocatalytic performance toward oxygen reduction. Similarly, Chen et al. developed a strategy for the synthesis of Pt or PtRu/CNT nanohybrids based on IL-functionalized CNTs (Figure 3). As a result of the uniform distribution of the surface functional groups provided by IL, Pt and

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PtRu nanoparticles supported on the CNTs-IL have a smaller particle size, better dispersion, and higher active surface area than those on CNTs without IL modification [46]. The PtRu/CNTs-IL (or Pt/CNTs-IL) electrocatalyst shows better performance in the direct electrooxidation of methanol than the

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PtRu/CNTs (or Pt/CNTs) electrocatalyst.

Figure 3. Schematic diagram of the modification of CNTs with polymer IL and the preparation of Pt/CNTs-PIL nanohybrids. EG: ethylene glycol; AIBN: 2,2’-azobisisobutyronitrile. Reprinted with

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permission from ref. [46]. Copyright 2009 Wiley.

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2.4. Solid catalysts with ionic liquid layer (SCILL) concept in electrocatalysis In 2007, a concept called ‘solid catalysts with ionic liquid layers’ (SCILL) has been developed. In

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this system, a solid heterogeneous catalyst is coated with a thin film of IL (see Figure 4) [47]. It has been

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demonstrated experimentally that such systems may exhibit better selectivity and even higher activities than their uncoated analogs in various hydrogenation reactions [5,47–50]. The promotional effect of the

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IL layer may stem from a twofold manner: (1) ILs may directly interact with the active center as a ligand; (2) ILs would modify the effective concentration of the reactant at the catalytically active sites [50,51]. Especially the importance of the promotional effect as a ligand could be deduced in a unique manner due to the possibility of molecular variation of the IL and possibilities to carry out ultra-high vacuum (UHV)-based characterization due to the extremely low vapor pressure of most ILs [52–54].

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Figure 4. Schematic illustration of the SCILL concept for cathodic oxygen reduction on a Pt/C material. On the carbon support surface a hydrophobic IL-layer is deposited and in direct contact to the aqueous

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electrolyte, supplying oxygen for the reaction. Depending on the IL-layer thickness the platinum nanoparticle is partially or fully covered. Reprinted with permission from ref. [55]. Copyright 2015 American Chemical Society.

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The mechanisms known to alter catalytic properties in thermal catalysis can in principle also be effective to enhance the performance of electrocatalysts. The pioneer work transferring this innovative

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SCILL concept to electrocatalysis was achieved by Erlebacher et al., who constructed a series of nanoporous PtNi-ionic liquid composite electrocatalysts by incorporating a hydrophobic protic ionic

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liquid, [MTBD][beti] into the pores of either nanoposrous PtNi bulk materials [56] or nanoparitlces [57], as shown in Figure 5. The intrinsic activity of these composite electrocatalysts for oxygen reduction was

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found to be 2–3 times higher than those of PtNi catalsts without IL, and dramatically 10 times higher than that of the commercial Pt/C catalyst. The promotional effect of IL was ascribed to the high O2

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solubility in IL and associated biased reactant O2 to the catalytic surface, which improved the reaction kinetics on PtNi catalysts. Similarly, a Pt-ionic liquid composite catalyst was fabricated by impregnating graphene supported Pt nanoparticles with an oxygen-philic IL, [MTBD][NTf2] [58]. The composite electrocatalysts exhibited enhanced intrinsic activity and methanol tolerance for oxygen reduction relative to its counterpart without IL, which was believed to arise from the high O2 solubility and less methanol-philic nature of the involved IL. While some hypotheses on these principles leading to the enhancement effect were formulated in these preliminary works, the reason behind the activity and

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stability increase when transferring the SCILL from thermal to electrocatalysis is not fully understood. Thus, the ILs employed for the modification up to now were not designed task specific for the SCILL modification. Recently, we have shown that engineering the IL modification through the amount of IL added the pore system of Pt/C is highly important to leverage the boost in intrinsic activity but avoid inducing mass transfer limitations [55]. Through this and a smart IL design the world record in activity

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of pure Pt in oxygen reduction could be achieved [59].

Figure 5. Scheme of encapulsated nanoporous nanoparticle catalysts for high-performance PEMFC

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oxygen reduction. (a) Cartoon illustrating the key components: the nucleus of the catalyst is a high surface area nanoporous nanoparticle, which allows the particle to be encapsulated by an IL. The

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composite catalyst is attached to a conductive substrate to aid integration into a PEMFC cathode catalyst layer. (b) High resolution transmission electron microscope image (HRTEM) of nano-porous

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nickel-platinum (np-NiPt) nanoparticles encapsulated with [MTBD][beti] IL, supported on carbon.

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Reprinted with permission from ref. [57]. Copyright 2013 Wiley.

3. Exemplary applications of ILs in electrocatalysis 3.1. HOR and Hydrogen evolution reaction (HER) HOR and HER are of practical and fundamental significance, and regarded as model system where it is know that the hydrogen-metal bond strength dictates reaction efficiency [60]. In acidic electrolyte, the reaction process can be described as:

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H + + e- ↔ H 2 Hads is considered to be an adsorbed reaction intermediate, which is sensitive to the nature of electrode materials and the microenvironment at electrode surfaces. The dependence of the the HER/HOR activity on the nature of electrode materials has been widely investigated, and employed as a prototype in describing the electrocatalyst activity trend. A volcano plot highlighting activity trends in HER on varied

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metal catalysts versus metal hydride bond strength can be observed in aqueous electrolyte, while the nature behind is still the topic of debate. The beneficial effect of IL towards HER/HOR was mainly demonstrated by directly using ILs as reaction media. For instance, an impressive overall cell efficiency of 67% can be obtained in a H2-O2 fuel cell device using the [C4mim][BF4] as electrolyte [61]. Electrochemical properties of HER/HOR in ILs have also been widely investigated at varied electrode

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surfaces (e.g., Pt, Pd, Au), highlighting the importance of the nature of electrode materials and IL characteristics in determining the electrocatalytic behavior. ILs usually feature significant volume expansion/contraction during phase change, which would influence the dissolution of H2 molecules. Meanwhile, the high viscosity of ILs would slow down the diffusion properties of electroactive species

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and consequently affect the overall rate of electrochemical processes [62]. In aqueous electrolyte, the proton transport follows the well-established Grotthuss mechanism. While this is not the case for proton

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transport in ILs, where the solvated protons in IL have a very large solvodynamic radii and are not dissociated [63], which will make the diffusion coefficient of protons much smaller than that of H2

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molecules [62]. This will result in rapid accumulation of one species over the other at the electrode surface [64], which can be properly utilized to rationally tune the selectivity of electrochemical

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reactions. Compton et al. found that HER process can easily proceed in [NTf2]- based ILs, but can hardly occur in [C6mim]Cl or [C4mim][NO3] [65,66]. These results emphasize the important role of anion

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protonation in determining the HER/HOR properties in ILs. In recent years, development of non-precious metal electrocatalysts for HER applications has

attracted tremendous attention because of their great variety, low cost and flexible properties. Maschmeyer et al. demonstrated that, using non-coordinating ILs with planar heterocyclic cation as synthesis media, MoS2 crystals with unique de-layered structures can be selectively prepared, which exposes high number of active edge sites for HER [67]. It is also documented that the HER can proceed much more efficiently on Mo catalyst in IL electrolyte than that in aqueous solution, due to the easier

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proton-donating properties on the IL pre-adsorbed surface with the decreased activation energy [68]. 3.2. Oxygen reduction reaction Polymer electrolyte membrane fuel cells (PEMFCs) are supposed to play a major role in the future clean energy scenario due to the growing concerns on the depletion of petroleum resources and environmental issues. The cathodic oxygen reduction reaction (ORR) is a relatively sluggish process

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with reaction rate constant being orders of magnitude lower than the anodic HOR, which is considered as the primary bottleneck in broad based applications of PEMFC technology. The ORR process can proceed through two possible reaction pathways: the direct 4-electron pathway and the two stage pathway with 2 electrons transferred in each stages and H2O2 intermediate. It is always highly desirable

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to develop innovative electrocatalytic systems which is inexpensive, stable and can catalyze ORR exclusively proceeding through the 4-electron pathway.

Over the past decade, the electrochemical reduction of O2 involving ILs has been widely studied, mainly using ILs directly as the electrolyte material. Watanabe et al. successfully fabricated a

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membrane-type fuel cell using a protic IL ([dema][TfO]) as electrolyte, and found that both anodic hydrogen oxidation and cathodic oxygen reductions proceed efficiently in IL with an impressive cell (Figure 6) [23]. Similarly, ethylammonium nitrate has also been used

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open circuit potential of 1.03 V

as electrolyte for fuel cell applications, which enable the authors to patent IL as an alternative electrolyte

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for fuel cell applications [69]. However, the anhydrous and high operating temperature permitted by using IL electrolyte raised the possibility of working under conditions that would remove poisonous

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species (e.g., CO) and avoid catalyst corrosion. Furthermore, IL electrolyte based fuel cells could even exhibit superior cell performance at low current densities to those conventional ones, and there is no

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need for humidification or pressure to sustain performance [70].

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Figure 6. Application of ILs in a non-humidifying fuel cell. Reprinted with permission from ref. [23]. Copyright 2010 American Chemical Society.

ORR properties in ILs have also been investigated using the half-cell configuration. Munakata et al. studied the ORR performance on Pt catalysts in protic ILs with anions of [N(SO2F)2]-, [N(SO2CF3)2]-

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and [N(SO2CF3)2]-, and found that the reduction current increased with the degree of fluorination [71]. In situ FTIR measurements evidenced that anion adsorption on Pt surfaces can be significantly reduced as fluorination increased, leading to more available Pt sites for ORR and consequent improved reaction kinetics in ILs with higher degree of fluorination [71]. In recent years, it is disclosed that ILs can be

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more successfully applied as electrocatalyst modifiers than as pure electrolyte. For instance, it has been demonstrated that introduction of a small amount of ILs into the conventional pure Pt or bimetallic PtNi catalysts, which will lead to the formation of IL based composite catalyst or SCILL samples, could significantly enhance the ORR kinetics [56,57]. The superior performance of these SCILL/IL composite catalysts is considered to arise from the higher O2 solubility in IL and hydrophobic microenvironment

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created by the additional IL phase which would help reduce the nonreactive species on Pt surfaces [55]. We also disclosed that the additional IL phase could also improve the electrochemical stability of Pt

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nanoparticles (Figure 7).

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catalysts through suppressing the carbon corrosion and protecting against agglomeration of Pt

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Figure 7. Durability measurements of reference Pt/C and SCILL systems. ORR polarization curves of

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Pt/C-fresh (a) and Pt/C-SCILL-50 (b) before and after 2000 potential cycles between 0.4 to 1.4 V in

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O2-saturated 0.1 M HClO4. The insets show the corresponding CV curves of each sample before and after the potential cycling. Summary of the electrochemically active surface area (c) and half-wave potential (d) of each sample before and after the durability test. Reprinted with permission from ref. [55].

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Copyright 2015 American Chemical Society.

Most recently, we have made effort to investigate the origin of the boosting effect of IL on Pt for

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ORR, and found that the hydrophobic IL would selectively locate at the surface Pt sites with low coordination number [59]. The hydrophobicity conveyed by IL help then to protect these active sites from being poisoned non-reactive oxygenated species (e.g., OHad), as illustrated in Figure 8, further resulting in more available active sites and thus improved reaction kinetics for ORR. Moreover, the enhanced activity of the IL-modified sample can be stabilized up to 30000 electrochemical potential cycles. The degraded SCILL sample is still far more active than the fresh Pt/C. These results verify that the activity boosting effect from IL-modification can be well maintained and the possible IL leaching

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into aqueous electrolyte is actually negligible. It is suggested that the hydrophobicity is a key property required from an IL to improve the performance of a Pt catalyst in an aqueous reaction media [59]. It is well documented that, even for hydrophobic ILs, they can dissolve water with pronounced concentrations [72]. The presence of small amount of water in IL would facilitate the formation of interconnected network/water channel within the IL [73,74], which could aid proton transportation

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through direct hydronium diffusion, or proton hopping between hydronium and water molecules [57].

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Figure 8. Schemes for the interfacial microenvironments at the surfaces of (a) Pt/C, and (b) Pt/C-SCILL, showing that the additional IL phase would selectively locate at the defect sites (atoms coloured in

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yellow) and help protect Pt sites from being occupied by hydroxyl species. Reprinted with permission

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from ref. [59]. Copyright 2015 Wiley.

We have also found that ILs could also impose some passivation effect to the electrocatalytic system.

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In our recent work, we reported a volcano dependent behavior of the ORR activity on the IL pore filling degree of the SCILL (Pt/C-[MTBD][NTf2]) catalysts with the maximum activity obtained at pore filling

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degree of 50% [55]. It is revealed that the reactant molecules (O2) have a slower diffusion rate in the IL phase than that in an aqueous solution, and too high pore filling degree will cause complete flooding of the pore system and consequently lead to a restricted mass transportation and reduced catalytic activity. Most recently, we also disclosed that the length of the alkyl chain of imidazolium based ILs could also play a key role in affecting the ORR activity of the IL-modified Pt/C catalysts, and the IL with too long alkyl chain will cause a significant passivation effect to the Pt catalysts, as evidence by the ca. 50% decrease in electrochemically active surface area, and reduced intrinsic activity for ORR on the

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IL-modified Pt/C relative to the fresh Pt/C. Moreover, the combination of IL with non-precious metal catalysts also leads to the creation of high performance electrocatalysts for ORR applications. Mao et al. prepared a highly efficient ORR catalyst by immobilizing cobalt porphyrin and Prussian blue nanoparticles onto IL-carbon nanotube composites, yielding an overall apparent 4-electron reduction of O2 molecules (Figure 9). The presence of IL was

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supposed to stabilize the active nanoparticles and considered to be crucial for obtaining the active ORR catalyst [75]. Ding et al. compared ORR performance on multiwalled carbon nanotubes (MWCNTs) and graphite electrode in [BF4]- based ILs, and found MWCNTs exhibited 2 to 3 times higher rate constants for ORR than the graphite electrode, and the reaction rate on both catalysts were sensitive to the size of the IL cation [76]. In summary, these aforementioned works imply that the ORR properties could be

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boosted or manipulated by introducing IL species in the electrocatalytic system, as pure electrolyte or catalyst modifier. Considering the great varieties of ILs, it is highly promising to develop more active

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ORR electrocatalysts by rationally tuning the cationic or anionic structures of the ILs.

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Figure 9. Scheme showing the apparent four electron reduction of O2, using CoP (structure shown) and PB-NPs (Prussian blue nanoparticles); (bottom) schematic illustration of the two electrocatalysts immobilized in a multiwalled carbon nanotube–IL gel. Reprinted with permission from ref. [75]. Copyright 2008 American Chemical Society.

3.3. Water electrolysis

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Electrolysis of water, which is usually referred to as water splitting, is the decomposition of water into hydrogen and oxygen molecules by applying a proper potential difference on water. This process can be used to produce breathable oxygen and more importantly high purity hydrogen, which can be used as fuels for hydrogen-oxygen fuel cell applications. The main research target in the field is to reduce the overpotential, or more straightforwardly the energy lost during the process by developing robust electrocatalytic materials. Besides, electrolytes are also considered to play a major role in

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determining the efficiency of electrolysis systems. Introducing IL phase into water electrolysis systems is supposed to be one of the most promising approaches to enhancing the electrolysis efficiency, since both replacement of aqueous electrolyte with IL or mixing IL with conventional aqueous electrolyte are capable to modify the intrinsic structure of electrode-electrolyte interfaces mainly by influencing the

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intermolecular interactions (molecular forces, H bonds etc.)

A representative work was conducted by de Souza et al., who reported that imidazolium ILs such as [BMI][BF4] and [BMI][PF6] can be used as electrolyte for hydrogen production through water electrolysis, and the system exhibited efficiency of more than 94.5% and current density of 20 mA/cm2

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which is not affected during the electrolysis due to the chemical stability of ILs [77]. They also compared the water electrolysis properties in mixtures of IL and water on a range of electrode materials,

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including Pt, Ni, stainless steel (SS) and low carbon steel (LCS) [78]. They found that the precise ratio of IL to water was crucial to obtaining higher current densities (Figure 10) on all the electrode materials.

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The best combination of IL with water was achieved at 10 vol% IL in water, giving an impressive current density as high as 42 mA/cm2 on LCS, which is even higher than that on Pt electrode. They

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suggested that too low IL concentration would limit the ionic conductivity, while too high IL concentration would lead to the formation of aggregates of ionic pairs. This work also demonstrated the

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possibility to replace precious Pt with inexpensive materials as electrocatalysts for water electrolysis by simply introducing a proper amount of IL phase, which makes this system extremely attractive to industrial scale hydrogen generation via water electrolysis.

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Figure 10. Current density dependency on the IL concentrations in water obtained at −1.7 V comparatively for all electrocatalysts. Cathodic current densities of platinum (■); 304 stainless steel (●);

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low carbon steel (▾); nickel (▴) working electrodes at different concentrations of BMI·BF4 in water taken at −1.7 V. Reprinted with permission from ref. [78]. Copyright 2006 Elsevier.

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3.4. Electrochemical reduction of CO2

The electrochemical reduction of CO2 is considered an efficient approach to producing fuels and

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other commodity chemicals under mild conditions, and also has tremendous positive impacts on reducing CO2 emission into the atmosphere [79]. However, CO2 reduction reaction usually proceeds at

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very negative potentials, which corresponds to very high energy consumption and poor efficiency, mainly due to the fact that CO2 molecule is thermodynamically stable and kinetically inert [62]. The

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major limiting step for electroreduction of CO2 lies in the transfer of the first electron of CO2 to form an anion radical, which needs to be overcome by applying unusually high overpotential. The most

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commonly explored electrocatalysts for CO2 reduction are (transition) metals (Fe, Co, Ni, Cu, Ru, Sn, Pb etc.) and their associated compounds (e.g., complexes, oxides) [80], while improvements in energy efficiency and selectivity for desired products are still needed. ILs have long been investigated as environment friendly materials for CO2 capture due to their high CO2 solubility [81,82]. Most recently, ILs have also shown promise in modulating the CO2 electroreduction process as co-catalyst and/or reaction medium, by decreasing activation energy, altering reaction pathway, and selectively stabilizing reaction intermediates. For instance, Grills et al. proposed that an interaction between [C2mim]+ cation and partially reduced Re complex catalyst would result in lowering of activation energy for CO2

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reduction based on their time-resolved IR measurements [83]. Most recently, they have made attempts to investigate the role of imidazolium based IL in the electroreduction of CO2 by calculating the vertical electron affinities of intermediate species using DFT methods, and proposed that the imidazolium cation could facilitate attack on proton by reducing the impact of the electrolyte cation on the energy levels of pπ orbitals of the metallocarboxylic acid, and at the same time, enhances the kinetic and/or thermodynamic electron affinity via π-π interactions between the imidazolium and bipyridine

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heteroaromatic rings and subsequently results in higher reduction potential of CO2 in IL than that in conventional electrolyte [84]. Zhou et al. found that Ag electrocatalyst in chloride containing IL ([C4mim][Cl]) displayed unprecedentedly high activity for reduction of CO2 to CO [85]. While Sun et al. found the presence of ILs would shift the reaction course during CO2 reduction on Pb, from widely

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reported oxalate to CO and an imidazolium carboylate complex [86]. Most recently, it is also reported that the anion of a superbase IL ([P66614][124Triz]) could chemically bind with neutral CO2 molecule, resulting in significantly reduced the activation energy required for reduction of CO2 and lowest overpotential for CO2 reduction to formate with high efficiencies [87].

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The presence of IL is also reported to improve the catalytic performance of CO2 reduction on non-precious metal electrocatalysts. A benchmark work is conducted by Rosen et al., who disclosed that

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CO2 molecules can be selectively converted to CO in presence of IL by using an N-doped carbon nanofiber catalyst at overpotential of 0.17 V [88]. In contrast, CO production cannot be detected on the

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same catalyst without presence of IL phases, implying that IL could act as a possible co-catalyst. They suggested that formation of [CO2]- could be promoted in presence of IL, due to the complexation

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between IL cation and [CO2]-, which would help reduce the initial reduction barrier [88].

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3.5. Electrooxidation of formic acid, methanol The electrooxidation of small organic molecules such as methanol and formic acid has attracted great

interest in the past two decades, due to their fundamental importance from science aspects and their potential applications as alternative energy carriers to hydrogen in fuel cell applications [89–93]. Pt and Pd are generally considered as most efficient electrocatalysts for electrooxidation of methanol and formic acid, however, accumulation of poisonous species (identified mainly as adsorbed CO) which is more resistant to oxidation, would usually cause rapid degradation of Pt and Pd electrocatalysts. Higher reaction temperature usually favors the oxidative desorption of poisonous species, but is restricted by the

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limited operating temperature in aqueous electrolytes. Replacement of conventional aqueous electrolyte with ILs is supposed to increase the possible operating temperature and consequently improve the anti-poison ability of these electrocatalysts. Feng et al. demonstrated that n-butylammonium nitrate IL could be employed as an electrolyte for formic acid oxidation reaction (FAOR) on Pd catalyst [94]. They found that the Pd activity for FAOR

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could be promoted in presence of IL and was highly sensitive to the concentration of IL in the aqueous solution, with the highest activity obtained at IL fraction of 0.12. The presence of IL is supposed to accelerate the reaction rate through promoting the formation of surface oxides on Pd which is prerequisite for oxidative desorption of poisonous species. Similarly, the oxidation of methanol and CO has also been investigated in IL electrolytes, and the formation of surface oxides on Pt catalysts with

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assistance of trace amount of dissolved water in IL seems play a key role in determining the electrooxidation properties [95]. So far, it seems that pure IL cannot fully replace aqueous solution as electrolyte for the electrooxidation reactions, since the surface oxidation of these Pt and/or Pd electrocatalysts, which is a major limiting factor in electrooxidation of these small molecules, can hardly

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proceed in absolute non-aqueous IL electrolyte. Anyway, the superior performance of electrocatalysts in the mixed IL/water electrolyte shows some promises to take the fully advantage of the unique properties

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of ILs in developing more efficient electrocatalytic systems.

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4. Conclusions

In the current minireview, we highlighted the effect of ILs in electrocatalysis by summarizing the

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latest research, including directly using ILs as electrolyte or catalyst promotors. IL phase could play a key role in determining the structure of the interfaces between solid electrode and liquid electrolyte, and

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affect the adsorption, mass transportation, and electron transfer behavior at catalytic surfaces. Rational design the structures of ILs shows great promise to more accurately manipulate catalytic behavior of electrocatalysts, given that ILs have great variety and can be task-specifically designed. Although great successes have already been achieved by introducing IL into electrocatalytic systems, and intensive spectroscopy characterizations have been applied to characterize the structures of ILs, the detailed understandings of the inherent structures, configurations, distribution and orientation of IL at electrode interfaces are still lacking, and might differ significantly from one IL to another. It is still highly desirable to conduct systematic investigations over a wide range of ILs in a hope to gain more insights

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into these innovative systems. The promising promotional effect of IL in electrocatalysis inspires us to further fully take advantage of the unique properties of ILs, and we ascertain that it’s going to be an exciting research area and a wide scope of improvements, discoveries and applications are waiting.

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Graphical abstract

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Ionic liquids, as versatile molecules with numerous unique properties, have shown great promise to boost the performance of electrocatalytic systems.

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