Ionic liquids as tailored media for the synthesis and processing of energy conversion materials

Abstract

Though in its infancy stage, ionic liquid (IL)-assisted synthesis and processing of energy conversion materials is triggering a wide interest to eco-friendly production of already existing and novel materials. ILs possess the potential of overcoming the limitations of conventional synthesis approaches. Due to their unique characteristics such as high chemical and thermal stabilities, nearly negligible vapour pressure, wide electrochemical window, broad liquidus range, tunable polarity, hydrophobicity/hydrophilicity, ionic liquids are opening new frontiers in the ionothermal synthesis of tailored materials, enabling products that are difficult or impossible to achieve by using other, more conventional preparation routes. Trying to offer an exhaustive review on the burgeoning role of ionic liquids for the synthesis and processing of energy conversion materials, this perspective article focuses on IL-assisted production of electrode materials and catalysts for fuel cells, photo-induced water splitting and dye-sensitized solar cells. A brief excursion on the use of ionic liquids for the processing of natural fuels for biofuel cells is also made. The state-of-the-art research endeavours are firstly evaluated, followed by the exploration of future research directions with some thoughts on emerging challenges and opportunities.

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Gebrekidan Gebresilassie Eshetu

Gebrekidan G. Eshetu earned his Multiple MSc degrees in Materials Science and Chemical Engineering from the Picardie Jules Verne University (France), Paul Sabatier University (France), Warsaw University of technology (Poland), University of Cordoba (Spain) and University of Marseille (France) under the Erasmus Mundus EU programme. He earned his PhD in Materials Science in 2013, working with Prof. Stephane Laruelle, Prof. Michel Armand, Dr Sylvie Grugeon and Dr Guy Marlair at the LRCS laboratory and INERIS research institute (France). Since February 2014, he is a postdoctoral researcher at the Helmholtz Institute of Ulm (Germany). His research focuses on electrochemical energy storage technologies.

Michel Armand

Michel Armand was educated in France and USA; PhD in Physics (1978); emeritus Directeur de Recherche at CNRS; invited scholar, Lawrence Berkeley Laboratory (1982–1983); Senior scientist at CIC Energigune in Spain (2010…); Honorary professor, Deakin University. Michel Armand has brought several concepts for energy-related electrochemistry: intercalation compounds (1972); polymer electrolytes (1978); highly delocalized salts (perfluorosulfonimides and Hückel-stabilized anions) for liquid, polymer electrolytes and as base for ionic liquids; organic electrodes for positive (polyquinones) or negative (dicarboxylates) electrodes. The carbon “nanopainting” process has made LiFePO4@C a safe, widely used cathode. He has authored or co-authored 360 publications and 168 patents.

Hiroyuki Ohno

Hiroyuki Ohno received his PhD degree in 1981 from Waseda University, Tokyo, Japan. After working at Waseda University and Case Western Reserve University (USA), he was invited to Tokyo University of Agriculture and Technology as an associate professor in 1988. He was promoted to a full professor in 1997. His recent research activities are concentrated on the science of ionic liquids, especially design of functional ionic liquids. He has published 420 papers and 180 reviews and books. He is now a dean of his university since 2013, and FRSC since 2008.

Bruno Scrosati

Bruno Scrosati, formerly Full Professor at the University of Rome, is presently affiliated with the Italian Institute of Technology (Italy). He received the Doctor in Science “honoris causa” from the University of St. Andrews (UK), Chalmers University (Sweden) and University of Ulm (Germany). He received the Research Award from the Battery Division of the Electrochemical Society, the XVI Edition of the Italgas Prize and the Vittorio de Nora award of the Electrochemical Society. He has been President of the Italian Chemical Society and the Electrochemical Society. He was European Editor of the “Journal of Power Sources” and is presently a member of Editorial Boards of various international journals. Professor Scrosati is the author of more than 550 scientific publications (H-index of 68) various books and chapters in books and 18 patents.

Stefano Passerini

Stefano Passerini is Professor at the Karlsruhe Institute of Technology, Helmholtz Institute Ulm (Ulm, Germany) since January 1, 2014. Formerly Professor at the University of Muenster (Germany), he co-founded the MEET battery research centre at the University of Muenster (Germany). His research activities are focused on electrochemical energy storage in batteries and supercapacitors. Co-author of more than 300 scientific papers (H-index of 52), a few book chapters and several international patents, he has been awarded in 2012 the Research Award of the Electrochemical Society Battery Division. Since 2015 he has been appointed as Editor-in-Chief of the Journal of Power Sources.

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Broader context

The advent of ionic liquids (ILs) as eco-friendly and designer reaction media has opened new avenues in the field of energy storage and conversion devices. ILs offer many distinct advantages, such as negligible vapour pressure, good thermal stability, wide liquidus range, broad electrochemical window, and high synthetic flexibility. These peculiar properties endow ILs as designer reaction media in production and processing of highly engineered and tailored products via acting as advanced solvents, precursors, charge-compensating groups, and structure-directing/inducing agents. IL-assisted synthesis help to obtain task-oriented specific materials with non-typical features, suitable for a variety of specific applications. The synthesis and processing of energy conversion materials is one of the most rapidly growing applications of ILs. Herein, a critical assessment of the current status quo of the application of ILs in the fabrication and processing of materials for use in fuel cells, photoelectrochemical water splitting, dye sensitized solar cells (DSSC), and biofuels is made. This review also critically highlights crucial issues that should be addressed and provides perspectives for the future research directions in the field.

1. Introduction

Global environmental issues associated with climatic changes induced by greenhouse gases are dramatic concerns of today's energy-conscious society.^1–4 The ever-increasing world population and its increasing affluence with associated higher energy consumption put huge pressure for expanding our current energy infrastructure, which has been, and still is, heavily relying on limited and environmentally unfriendly sources, such as coal, oil and gas. The increasing atmospheric pollution, global warming, depletion of fossil-fuels and lack of energy security, represent the most important imperatives to be urgently addressed for a sustainable future. These challenges can only be met through the development of new materials, systems and technologies, enabling the transformation of environmentally friendly and renewable energy sources, such as sunlight, wind and biomass. However, all these sources rely on the availability of novel materials and systems capable of exploiting their full possibilities to the grand energy challenge. For instance, electrical energy is converted from chemical energy in fuel cells and from light in photovoltaics. Chemical energy, on the other hand, can be generated from water splitting by using appropriate catalysts. Key to the effective performance of all the aforementioned conversion systems are the materials that rule the primary process, such as catalysts and electrode materials in fuels cells, semiconductors in photovoltaics, processing strategies in biofuels, and so on. In this respect, the procedure for the materials fabrication plays a key role in tuning their properties and performance. Amid the synthetic routes reported in literature, the ionothermal method is expected to be economically competitive due to the low processing temperature coupled with its eco-friendliness. Indeed, this method has gained great attention in the materials science community since it allows obtaining and processing a wide range of attractive high performance energy conversion materials, otherwise not accessible via conventional synthesis approaches. As a result, ionothermal synthesis routes, utilizing ionic liquids (ILs) as tailored and green reaction media, have opened new frontiers in the field of energy storage and conversion devices. In a recent review,⁵ we have shown the impact of IL-based synthesis and processing on electrochemical energy storage devices, such as batteries and supercapacitors. Here we complete the information by critically discussing the role of ionic liquids on the preparation of materials for the development of other classes of timely important technologies, including fuel cells (comprising PEMFCs, DMFCs and SOFCs), dye sensitized solar cells (DSSC), water splitting and biofuel systems and processing.

ILs can be broadly defined as molten salts formed by the combination of weakly coordinating cations and anions, with a melting point well below 100 °C.^6–10 The small lattice enthalpies and large entropy changes, resulting from the large size and conformational flexibility of the weakly coordinating ions, favor the occurrence of the liquid state.^9,11 Accordingly, some ILs possess unique physical properties compared to conventional electrolyte media, e.g., organic solvents, including, among others, negligible vapor pressure (thus, low flammability), high thermal stability, broad liquidus range, tunable electrochemical window, and ability to dissolve and process a wide range of materials (e.g., synthesis precursors, enzymes) as well as to stabilize metastable compounds.^10–13 Such a wide range of unique properties endows ILs as suitable alternatives to molecular solvents for the synthesis of engineered materials, by acting as advanced solvents, reactants, dopants, as well as capping, templating agents and catalysts. For instance, owing to a series of beneficial aspects, electrochemical deposition is usually rated as the most convenient method to fabricate semiconducting materials for energy conversion (e.g., water splitting). However, electrodeposition is strongly temperature-dependent since throwing power and film quality are enhanced by operating at high temperatures in order to prevent yields of discrete materials being affected by poor oxygen evolution efficiency, as instead is the case for low temperature operations. Conventional aqueous, and even, non-aqueous electrolytes (e.g., used in the SIGAL process¹⁴), do not allow high deposition temperatures, and, consequently, the search for thermally stable media has been one of the top priorities in the field. In this regard, IL-based electrolytes have been identified as perfect candidates due to their high thermal stability, as well as to their ability to allow atom diffusion on the surface during growth, a factor which positively influences the quality of the final film.^15–19 Another bonus of ILs is that theoretically they can be formed by unlimited cation/anion combinations. While the number of conventional solvents used in industry is of the order of about 6 × 10², that of ILs that can be simply prepared in laboratory exceed 10⁶, this resulting in more than 10¹² binary and about 10¹⁸ ternary combinations. This in turn provides the opportunity to generate the required system of ILs of improved reactivity, viscosity, solubility, purification and recyclability, and thereby open their way towards industrial applications. However, the life-cycle cost, sensitivity to contaminants, more efficient and scalable commercial synthesis, precise recyclability data, green and optimized distillation and recyclable strategies are still crucial in evaluating ILs viability in commercial applications. In other words, there is a long way to go before large-scale implementation of ILs for the fabrication of energy conversion materials and their eco-friendly processing. However, despite the excitement behind the use of ILs-mediated syntheses, the methodology is yet at its early stage being still affected by a series of issues, such as high cost and lack of well documented toxicity data. Nevertheless, these apparent drawbacks appear to be compensated by recyclability and re-usability, actions that can be performed without affecting the quality of the resulting products, as in fact demonstrated in practice by the well-known BASIL process, see also later discussion in Section 3.

Here we will critically evaluate these aspects by discussing the use of ionic liquids for the synthesis of various timely and relevant materials outlining the technological importance, as well as the issues which may possibly affect their application for large scale production and at the same time, providing suggestions on how to successfully address them.

For the sake of convenience for readers, the acronyms/abbreviations along with the descriptions of ionic liquids utilized in this review manuscript are provided in Table 1.

Table 1 Inclusive list of acronyms/abbreviations and descriptions of ionic liquids used in this review

Acronyms/abbreviation	Ionic liquid
[EMI][TFSI]	1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide
[BMI][TFSI]	1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide
[MTBD][BETI]	7-Methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene-bis(pentafluoroethanesulfonyl)imide
[BMP][DCA]	N-Butyl-3-methylpyrrolidinium cyanamide
[C4mim][Br]	1-Butyl-3-methylimidazolium bromide
[3-MBP][DCA]	1-Butyl-3-methylpyridinium dicyanamide
[C16mim][Br]	1-Hexadecyl-3-methylimidazolium bromide
[BMP][TF2N]	1-Butylpyrrolidinium bis(trifluoromethanesufonyl)imide
[C4mim][PF6]	1-Butyl-3-methylimidazolium hexafluorophosphate
[C4mim][CH3CO2]	1-Butyl-3-methylimidazolium acetate
[C2mim][Cl]	1-Ethyl-3-methylimidazolium chloride
[C4mim][BF4]	1-Butyl-3-methylimidazolium tetrafluoroborate
[Cmim][HSO4]	3-Carboxymethyl-1-methylimidazolium bisulfate
[C4C1Py][TF2N]	1-Butyl-3-methylpyridinium bis(trifluoromethanesulfonyl)imide
[C8mim][PF6]	1-Octyl-3-methylimidazolium hexafluorophosphate
[VEI][NO3]	1-Vinyl-3-ethylimidazolium nitrate
[C2mim][DCA]	1-Ethyl-3-methylimidazolium dicyanide
[Ch Sac]	Cholinium saccharinate
[C4mpyr][DCA]	N-Butyl-N-methylpyrrolidinium dicyanide
[Ch][dhp]	Cholinium dihydrogen phosphate
[C4mim][dhp]	1-Butyl-3-methylimidazolium dihydrogen phosphate
[Ch][dbp]	Cholinium dibutyl phosphate
[C4mim][CH3CO2]	1-Butyl-3-methylimidazolium acetate
[C4mim][MeSO4]	1-Butyl-3-methyl imidazolium methanesulfonate
[Ch][Lac]	1-Butyl-3-methylimidazolium lactate
(Hy[Ch][dhp])	Hydrated choline dihydrogen phosphate
[Dimim][MeO][H]PO2	1,3-Dimethylimidazolium methylphosphonate

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2. Synthesis of energy conversion materials

2.1. Catalysts for fuel cell technologies (FCs)

2.1.1. General aspects. Among the various sustainable energy conversion systems, fuel cells (FCs) are gaining increasing popularity due to their higher environmental compatibility compared to conventional combustion-based power generation systems.^20–22 FCs are in fact not restricted by the Carnot cycle, thus, intrinsically capable of providing higher efficiency combined with the advantage of heat co-generation or hybrid applications. As well known, a fuel cell operates by oxidizing the fuel (usually hydrogen, but also methanol or glucose and other carbon and hydrogen-containing compounds) at the anode (+) and reducing oxygen at the cathode (−) to produce water (sometimes CO₂) as the only waste. Fuel cells (FCs) can be classified into various categories according to the choice of electrolyte and operating temperature, the most common being Proton Exchange Membrane Fuel Cells (PEMFCs), Direct Methanol Fuel Cells (DMFCs) and Solid-Oxide Fuel Cells (SOFCs).²² In general, FCs are very appealing as energy conversion devices and this accounts for the extensive research and development activities devoted to their practical development. Although fuel-cell powered cars are already in the market, there still various issues to be solved for assuring a wide commercial impact for these devices. A major one is in the availability of effective catalysts, both for the oxygen reduction (ORR) and oxidation (OER) reactions, i.e. characterized by low price/performance ratio at moderate temperatures. This condition does not apply for PEMFCs where the cost of the most commonly used catalysts (e.g., Pt or Pt-based compounds) contributes for more than 55% of the total cost.^21,23 In addition, these precious metal catalysts are also affected by low durability, with poor resistance to carbon monoxide poisoning. While for the description of the basic electrochemistry which rules the fuel cell operation we refer the reader to other, more specialized reviews,^21,22 here we may only reiterate that a wide commercial output of fuel cells is obviously hampered by the lack of suitable catalysts and, consequently, current research trends are targeted at developing electrocatalysts of acceptable performance with minimal Pt-loading or, ideally, precious metal-free, with particular attention on the ORR, which is six or more orders of magnitude slower than OER.

From the synthesis methodology perspective, ILs have emerged as tailorable media for the fabrication of highly active, molecularly engineered electrocatalysts, since they benefit by a series of relevant specific advantages, including: (i) the production of materials with very low particle size, due to low surface tension, resulting in a slow coarsening during the growth phase; (ii) electro-deposition at high temperature, due to the high thermal stability; (iii) flexibility in the solubility of precursors, due to the tunable polarity, and (iv) rapid crystal growth in microwave (MW) synthesis with a high susceptibility. In the following paragraphs, the role of IL-based synthesis is discussed for the various types of fuel cells.

2.1.2. Proton exchange membrane fuel cells (PEMFCs). In general, the aforementioned peculiar features of ILs enable the synthesis of tailored materials of improved catalytic activities, such as carbon-supported Pt nanoparticles with low Pt loading. In this respect, Zhao et al.²⁴ reported the use of novel ILs for the synthesis of carbon-nanotube-supported platinum (IL–Pt/CNT) catalysts for PEMFCs. This synthetic procedure implied first the activation via acids of the chemically inert CNT with the goal of generating surface functional groups for optimizing metal dispersion on the CNTs surface. However, the key aspect of this particular synthesis, i.e., decorating the CNTs with metal particles, keeping homogeneous size distribution and dispersion while preventing agglomeration without additional pre-treatment, can only be achieved with the use of ionic liquids as reaction media, see Fig. 1. The Pt-decorated CNTs obtained by IL-assisted microwave ionothermal synthesis provide materials with improved catalytic activity due to a series of beneficial features associated with: (i) high degree of disorder of the CNTs and (ii) large number of surface defects and the mono-dispersion of Pt particles, as demonstrated by the fact that IL-based Pt/CNTs catalyst showed considerable improvement in the specific current resulting from their electrochemical surface area, being 21% higher than that of commercial Pt/C catalysts.

Fig. 1 Schematic drawing of the synthesis of Pt/MWCNT electrocatalyst with and without ionic liquids. The use of IL promotes the homogeneous dispersion of Pt nanoparticles, by preventing agglomeration as, e.g., when only ethylene glycol is used. This is due to the specific interactions between the IL and the carbon material, resulting in the surface modification of the MWCNTS.

Carbon nanomaterials (e.g., graphene) doped with heteroatoms, due to a series of specific features, which include excellent electrocatalytic activity, high conductivity and large surface area,^25–27 are among the strongest contenders for use as metal-free catalysts in PEMFCs. However, the available synthetic methods such as chemical vapor deposition (CVD), arc discharge, segregation growth, Hummer's oxidation followed by hydrazine (N₂H₄) reduction or N-doping by nitrogen plasma treatment, require rigorous reaction conditions such as high temperature and vacuum, as well as the use of highly toxic and explosive reagents, and possible metal contaminants. Thus, the fabrication of trace-metal-free N-doped-graphene via low energy and benign synthesis conditions is of paramount importance. In this regard, Lu et al.²⁸ showed a clever way of fabricating N-doped graphene via electrochemical exfoliation, followed by in situ nitrogen doping of graphite, using nitrate (ethyl ammonium nitrate – EAN)-based ILs, serving both as exfoliating and doping agent. The electrochemical exfoliation of graphite in the presence of EAN, containing 10% (v/v) water, resulted in a metal-free, N-doped graphene having an excellent catalytic activity towards ORR. The presence of water facilitates the degree of exfoliation via oxidation, generating oxygen species. While the nitrate ions of the ionic liquid are responsible for the fast and low stage intercalation, the ethylammonium cations serve for the in situ N-doping of the electrogenerated intermediate material.

The sluggish ORR kinetics can be further improved by engineering the coating of the nanoparticle catalysts via ionic liquids, as demonstrated in the case of IL-impregnated nanoporous Ni–Pt, where ILs have been efficaciously applied to engineer metal–Pt alloy coatings.²⁹ It is important in this respect to point out that the incorporation of the hydrophobic, but protic [MTBD][BETI] into the pores of the high surface-area Ni–Pt alloy nanoparticles leads to consistent improvements in the kinetics of ORR.^29,30 The high O₂ solubility in [MTBD][BETI], combined with the confined environment (due to encapsulation in Ni/Pt nanoparticles), boosts the attempt frequency, namely the parameter that characterizes the rate of surface diffusion through a jumping process of adatoms residing at adsorption sites to nearer-neighbour ones, thus finally enhancing the ORR rate. Indeed, the IL-impregnated Ni–Pt carbon-supported nanoporous nanoparticle catalyst (IL–Ni–Pt/C) shows a catalytic activity one order of magnitude higher than that of commercial Pt/C. Another evidence of the key role of ILs for the synthesis of PEMFC catalysts is provided by the work of Hasché et al.,³¹ who evaluated mesoporous N-doped, carbon-supported Pt catalysts, synthesized in n-butyl-3-methylpyridinium dicyanamide, [BMP][DCA], which acts as N and C precursor. Compared to the benchmark commercial Pt ones, the new catalyst fabricated via the IL-assisted method presented a comparable activity, however, with a consistent enhancement in the electrochemical active surface area (ECSA) and long-term stability, thus, finally confirming its suitability for promoting ORR. Finally, improvements in the ORR kinetics in PEMFCs, have been also reported for other electrocatalysts such as Pt-loaded carbon aerogels,³² and IL-functionalized MWCNTs decorated with Au nanoparticles.³³

2.1.3. Direct methanol fuel cells (DMFCs). DMFC, due to its high energy density, ease of refuelling, and low environmental impact, is among the most promising fuel cell technologies. However, the sluggish ORR kinetics and the methanol crossover are the crucial issues that still have to be successfully addressed to allow a full commercialization of this important power generating system. Moreover, in DMFCs, CO species are formed and adsorbed during the electro-oxidation of methanol, inferring that the catalyst for the methanol electrode also requires to be CO-tolerant. The most commonly used electrocatalysts in DMFCs are again Pt-based compounds; however, the high cost and CO-poisoning have also in this case hindered their large-scale application, calling for low-cost and high-performance alternative materials. In this respect, bimetallic particles are very attractive since the shape, size and composition of their structures significantly influence the catalytic activity. In this scenario, Li et al.³⁴ have recently demonstrated that core–shell-structured Au–Pt bimetallic particles may be successfully obtained by an ionothermal synthesis using [C₄mim][Br] IL and a mixture of HAuCl₄·4H₂O and H₂PtCl₆·5H₂O (1 : 3 ratio) at room temperature. When tested as catalyst in a DMFC, these particles showed a much superior peak current density than that obtained by using a single element, i.e., 9.56 mA cm⁻²versus 0.17 mA cm⁻² (Au only) and 1.93 mA cm⁻² (Pt only). The corresponding mass activity of the Au–Pt catalyst is 3.2 times higher than that reported for the Pt/C, i.e., 133.5 mA mg⁻¹versus 41.1 mA mg⁻¹. The key role of the IL in promoting the synthesis of the Au–Pt bimetallic particles was finally demonstrated by comparing their catalytic activity when prepared by a similar, but [C₄mim][Br]-free procedure.

As part of the research efforts towards the development of metal-free catalysts, it worth to stress that heteroatom (N, S or B)-doped carbon-based catalysts have shown promising features, such as high immunity towards methanol crossover (i.e., they do not catalyze the oxidation of CH₃OH directly by oxygen), long-term stability, low cost and high electrocatalytic activity for ORR. Han et al.³⁵ successfully demonstrated the synthesis of N-doped hollow carbon spheres using ILs with a nitrile group anion/cation as carbon precursor and silica as the templating agent. The obtained material, having high nitrogen content (∼10.9%) and degree of graphitization, combined with other features, demonstrated an excellent electrocatalytic activity with ∼4e⁻ transfer, super-immunity towards methanol crossover, and a stability superior to commercial Pt/C. In addition, high surface area S and N-co-doped hierarchical porous carbon, obtained using [BMP][TF₂N] as N, S and C precursors, showed improved catalytic activity, high methanol crossover tolerance, and good thermal stability.³⁶ The ORR potential peaks were found to be directly related to the heteroatom loading, demonstrating the need to have a N/S optimal ratio for obtaining the best catalytic activity when both dopants were used.

It has been reported that graphene, due to its high surface area, can be exploited as an active support to load noble metals, such as Pt, Au, Ag or bimetallic alloys (e.g., PtAg), for use as catalysts in DMFCs. For example, PtAg nanostructures decorated on reduced graphene oxide (PtAg/RGO) have been synthesized again using an IL, [C₁₆mim][Br], which serves both as surface-capping and structure-directing agent.³⁷ The IL-prepared PtAg/RGO nanocomposite has a large electrochemically active surface area, namely, 92% and 16%, higher than that of Pt/C and Pt-black, respectively, as well as higher catalytic activity, methanol tolerance and long-term durability for ORR in alkaline solution. Fig. 2 illustrates the IL-based synthesis of PtAg-decorated graphene.

Fig. 2 Illustration of the fabrication of PtAg/RGO composite electrocatalyst via an IL-mediated approach. The IL plays the role of structure directing agent and surface capping, leading to a high electrochemically active surface area and, thus, high catalytic activity.

Another promising approach towards the fabrication of highly efficient hybrid nanocatalysts for DMFC sees the direct mixing of IL-functionalized carbon nanotubes (CNTs) with metal (e.g., Pt) precursors.³⁸ The imidazolium group, attached through N,N′-dicyclohexyl carbodiimide (DCC) coupling with the counter anion (PtCl₄²⁻), plays a crucial role in the formation of IL–CNTs/Pt hybrid materials. More recently, Do et al.³⁹ reported the synthesis of GnP-supported Pt catalysts (Pt/GnP) by using a microwave process based on [C₄mim][PF₆] and [C₄mim][CH₃CO₂] and using H₂PtCl₆·6H₂O as Pt precursor and ethylene glycol (EG) as reducing agent. The IL addition to EG–GnP after that of the Pt precursor assured a uniform distribution of Pt on GnP, as promoted by the direct interaction of Pt ions with the π sites on the GnP basal plane and residual oxygen groups at the GnP edge surface. The size of Pt nanoparticles was found to be controlled by the polarity of the ILs, as well as by the IL-/GnP molar ratio. For instance, deposition in [C₄mim][CH₃CO₂] produced small Pt nanoparticles, due to the higher basicity of the CH₃CO₂⁻ anion with respect to PF₆⁻, this being associated with its unsymmetrical arrangement of oxygen atoms. Electrochemical comparisons of IL-assisted Pt/GnP and commercial Pt/XC-72R catalysts towards methanol oxidation revealed that the former exhibited much higher (an 80% increase) catalytic activity, in turn ascribed to the higher dispersion of Pt particles and more favorable Pt-support interaction with GnP. Finally, it is here worth noting that a large number of electrocatalysts for use in DMFCs have been obtained via the ionothermal synthesis route. The most important of them are PtRu,³² Pt/C,⁴⁰ C–PtAu-nanoparticle hybrids,⁴¹ flower-like Pd nanoparticles, Pt–WC/TiO₂,⁴² polymeric ionic liquids (PIL)/Pt nanoparticle hybrids,⁴³ CuPt–/IL–graphene,⁴⁴ Pt/XC-72 coated N-doped carbon (Pt/XC-72@C–N),⁴⁵ mixed ILs/graphene-supported Pt nanoparticles,⁴⁶ Pt–Ru/C,⁴⁷ and Pd/C.⁴⁸

2.1.4. Solid oxide fuel cells (SOFCs). The SOFC, a viable high temperature (500–1000 °C) technology, has recently received large attention as a major source of electrical energy conversion system, mainly owing to its efficiency which overpass that of Carnot cycle power sources.^49,50 Similar to the aforementioned FCs, the efficiency of SOFCs is also highly dependent on the quality of the employed electrocatalyst,⁵¹ usually of the rare earth-type. Among them, dysprosium oxide (Dy₂O₃) is the one so far reported having the best catalytic behaviour. However, due to its negative potential and high reactivity with atmospheric components, dysprosium cannot be electrodeposited from aqueous solutions. Once again, ionic liquids have been successfully employed for addressing the issue. Indeed, by a single electrodeposition step from an IL based on eutectic choline chloride/urea in a 1 : 2 molar ratio, followed by controlled oxidation, a highly performing nanostructured Au–Dy₂O₃ composite electrocatalyst could be easily obtained.⁵¹ In general, though fuel cells are considered as relatively exotic technologies, they are not yet a major commercial success. This is mainly attributed to their high cost, durability and related issues. IL-mediated synthesis has demonstrated to enable the synthesis of designable catalysts, one of the critical issues that needs to be solved with FCs. Moreover, the question of how well the synthesis methodology can be scaled up from the laboratory to large scale needs to be critically addressed.

2.1.5. Photoelectrochemical water splitting. The effective capture of sunlight and its subsequent conversion into chemical fuels, such as hydrogen, represents a significant step towards the exploitation of a truly sustainable and environmentally benign energy source.⁵² The production of H₂ by photo-splitting of water, based on the following reaction series (1)–(3), is an important alternative to the current industrial process, primarily relying on reforming fossil fuels.

2H₂O → O₂ + 4H⁺ + 4e⁻ (1)

4H⁺ + 4e⁻ → 2H₂ (2)

2H₂O → 2H₂ + O₂ (3)

Photoelectrochemical water splitting can be regarded as a cost-effective, convenient and simple technique, which involves the simultaneous oxidation/reduction of oxygen (eqn (1)) and hydrogen (eqn (2)) with the production of the two gases (eqn (3)). Compared to that associated with hydrogen production, the oxidation reaction, also denoted the oxygen evolution reaction (OER), is the most sluggish and rate-determinant step, indeed requiring an overpotential exceeding 0.45 V to drive the reaction at useful rates, which results in a substantial energy loss, usually as heat. This clearly accounts for the tremendous efforts recently devoted to the search of effective and tailor-made electrocatalysts based on earth-abundant elements (e.g., Mn, Co), aimed to allow the replacement of the precious metals (e.g., RuO₂, IrO₂,…)-based catalysts currently used in commercial electrolysers.

In this scenario, particular attention has been addressed to manganese oxides, MnO_x (x = 1.5–2), which have been synthesized using a number of approaches, including precipitation, hydrothermally, sol–gel route, classical aqueous electrode deposition, etc. Once again, ILs have shown fascinating outcomes as solvents/electrolytes for the preparation of MnO_x and related catalysts. A relevant example is provided by Zhou et al.⁵³ who recently reported the use of ethylammonium nitrate as electrolyte for the electrosynthesis of MnO_x layers at moderate temperatures (100–150 °C). This process was made possible only by exploiting an ionic liquid (ethylammonium nitrate, EAN), which in fact, allows the use of water (reactant) at high temperatures without need of a high-pressure vessel. As it can be seen from Fig. 3, the IL-synthesized catalyst presented a very good electrochemical performance, allowing significantly high currents at low overpotentials with an energy loss limited to 5.8%, i.e. a value much lower than the 23.5% associated with the production of MnO_x-based catalyst via conventional synthesis routes. Unfortunately, the practical application of this catalyst is still hindered by its degradation over a relatively short operation time due to the fact that MnO_x, as like the majority of the transition metal-based oxides, is affected by the oxo-anion solubility under prolonged exposure at high (oxidizing) potentials in aqueous solutions. This issue has been addressed attempting the stabilization of the metal oxide-based catalyst by developing water-insoluble multi-layer structures, again with the assistance of ionic liquids, as shown by Izgorodin et al.⁵⁴ who demonstrated the incorporation of phosphates into the MnO_x surface at high concentration of phosphate ions, enabling a suitable stabilization of the oxide surface, hence assuring its prolonged catalytic activity.

Fig. 3 Performance of various MnO_x-based electrocatalysts synthesised in 1 M NaOH (right), 0.4 M di(butylammonium) sulfate (BAS, pH = 10) (middle), and 2 M di(butylammonium) sulfate–ethylammonium nitrate (BAS–IL, pH = 10) (left). Preparation of MnO_x films from IL electrolytes enables obtaining films with less water, higher conductivity, and improved water oxidation performance.

Still with the support of ethylammonium nitrate, EAN, Zhou et al.⁵⁵ succeeded in electrodepositing a variety of MnO_x phases having different catalytic performance, depending on the acidity and/or alkalinity of the IL used as the electrolyte. In this respect, while the film deposited from a highly basic medium mainly resulted in the hausmannite, spinel-structured (Mn₃O₄), those from slightly alkaline, acidic and neutral electrolytes, yielded Mn₂O₃ and birnessite-like MnO_x films, having the highest catalytic activity for water oxidation among all those deposited from a neutral electrolyte. Recently, a rational design of highly performing boron (B)- and fluorine (F)-doped 2D-graphitic carbon nitrides (g-CN) via self-polymerization of urea in [C₄mim][BF₄] IL has also been reported.⁵⁶ In this synthesis, the urea is used as delamination agent, while the IL serves as a multifunctional modifier to control the texture and surface chemistry, as well as to tune the semiconductor properties of the g-CN imparted by B and F doping. Finally, IL-assisted of efficient electrocatalysts, such as self-organized TiO₂ nanotubes,⁵⁷ TiO₂ nanotube arrays,⁵⁸ Ag/Pd nanoalloys,⁵⁹etc. have also been reported to be effective novel catalysts in photoinduced water splitting.

The standard Gibbs free energy change (ΔG) for water splitting into H₂ and O₂ is +237.2 kJ mol⁻¹, which corresponds to 1.23 eV per electron transferred, indicating that to run the overall process, a semiconductor photoelectrode with a band gap wider than 1.23 eV, along with suitable band edge positions, is needed. Owing to its chemical and photostability, environmental-friendly nature and low cost, titanium oxide, TiO₂, is in principle a very appealing candidate. However, its wide band gap (over 3 eV) and the rapid recombination rate of photogenerated electron–hole pairs, is reflected in a very low efficiency of the overall H₂ production. The recombination issue has been addressed by developing carbon–TiO₂ composites, which provide enhanced interfacial contact with adsorbates. A recent example is provided by a TiO₂/reduced graphene oxide (RGO) nanocomposite, prepared via the hydrothermal method using [C₄mim][PF₆] IL as a solvent. While the graphene sheets act as electron transfer channels, reducing the recombination of the photogenerated electron–hole pairs, ILs serve as tailorable solvents for yielding a material with engineered morphology and boosted properties. The obtained TiO₂/RGO showed in fact, a much higher efficiency of hydrogen production than that achievable by composites synthesized making use of traditional solvents.⁶⁰

Surface modification of TiO₂ by doping with heteroatoms is another proposed way to improve its photocatalytic activity. This strategy involves the preparation of nitrogen (N) and carbon (C) co-doped mesoporous TiO₂via an evaporation-induced self-assembly method using [C₂mim][Cl] ionic liquid, as schematically shown in Fig. 4. C and N co-doping are expected to tune the band gap, making it decrease to a value of 2.92 eV, compared to 3.2 eV for commercial TiO₂, reflected by a significant increase in the absorbance edge and intensity of visible light.⁶¹ All these material improvements lead to a photocatalytic hydrogen generation of 81.8 μmol g⁻¹ h⁻¹, which is ∼37 times higher than that achievable by commercial TiO₂. Other important and highly performing electrocatalysts that have been obtained by IL-assisted synthesis include: mesoporous TiO₂ (using [C₄mim][PF₆]^62,63 and [C₄mim][BF₄]⁶⁴), mesoporous CeO₂–TiO₂ ([C₁₆mim][Br]⁶⁵) and TiO₂/RGO ([C₄mim][PF₆]⁶⁶).

Fig. 4 Schematic representation of the synthesis of carbon (C) and nitrogen (N) co-doped TiO₂ from ionic liquids. The IL serves both as C and N source and mesopore creator via interaction with the titania surface.

2.2. Dye sensitized solar cells (DSSCs)

The dye-sensitized solar cell (DSSC), also occasionally referred to as the “2^nd generation PV” or “Grätzel” cell,⁶⁷ has the prospective to be technically and economically more viable than traditional semiconductor-based photovoltaics (PVs). A typical DSSC device consists of three components: the dye-sensitized, nanoparticle TiO₂ photoanode, the redox electrolyte (most commonly iodide/triiodide), and the conducting, electrocatalytic layer, acting as counter electrode. Even if DSSCs use a number of the above described advanced dye and electrode materials, they are still less costly compared to the silicon-based solar cells, especially in terms of manufacturing steps.

Due to these expected favorable properties, DSSCs have attracted considerable attention both in academic and industrial laboratories. However, despite the large efforts spent in developing the related technology, DSSCs are only commercialized on a small scale, since the required improvements in efficiency (presently limited to 13%⁶⁸) and the stability requirement (<10% drop in performance after 1000 hours aging at 60 °C) are still key targets to be met. One promising strategy to address these issues is to optimize the porous-structured oxide film photoanode with the aim of enhancing its light absorption. In this scenario, a number of binary (e.g., TiO₂, ZnO, SnO₂) and ternary (BiOX, X = Cl, Br, I) oxide photoanodes have been developed, using IL-assisted fabrication, either alone or in synergy with microwave, ultrasound and other similar synthesis techniques.⁶⁹

Optimized anatase TiO₂, i.e., the photoanode of choice for DSSCs, may be prepared by sol–gel using [C₄mim][BF₄] IL as templating agent, so as to obtain a material characterized by high surface area, stable crystal structure, controlled porosity, tailored-designed pore size distribution, and good thermal stability.⁷⁰ Due to these unique structural and morphological features, this modified TiO₂ photoanode could benefit by larger absorption of dye molecules and longer electron lifetime, compared to commercial, non-porous TiO₂, this finally allowing its operation with higher photocurrent, photovoltage and efficiency. In addition, the photocatalytic activity of this IL-prepared nanocrystalline TiO₂ was found be at 100 °C comparable with that of standard TiO₂, however obtained at the much higher temperature of 400 °C.

The best DSSC performance, in terms of short-circuit photocurrent (13.15 mA cm⁻²) and power conversion efficiency, was in fact achieved using TiO₂ films having a nanoflower morphology, as that obtained with a [C_xmim][HSO₄] IL-assisted synthesis.⁷¹ Another electrode choice for application in DSSCs is zinc oxide, ZnO, a material that also greatly benefits by IL-based synthesis. In this respect, it was found that the morphology of the IL-assisted prepared ZnO material varies from rod-like to star-like and flower-like depending on the nature of the ILs utilized.⁷² The control of the surface, which is assumed to play an important role in favouring the charge transfer mechanism at the ZnO-based heterojunction, is another aspect recently receiving attention. Amongst all the suggested strategies to enhance the surface properties, the fabrication of ZnO–organic hybrid materials has been reported to be a very promising approach because the complementary benefits of ZnO and the organic moieties can judiciously be combined to tune the formed material.⁷³ In this regard, ILs can again be the electrolytes of choice, not only because of their self-assembling and templating properties, but also as convenient sources of ions which may interact electrostatically with the inorganic solid oxide surface, so as to promote its tuneable functional properties. An impressive proof of concept on the use of ILs as all-in-one (i.e., reaction media, morphology templating agent, and surface modifier) media was recently reported by Azaceta et al.,⁷³ who, by exploiting this route, succeeded in preparing a nanostructured ZnO/[C₄C₁py][TF₂N] having very promising properties as DSSC anode in terms of power conversion efficiency, electrochemical performance, extended absorption edge, and open circuit voltage. For instance, the power conversion efficiency recorded for the ZnO–IL hybrid film increased by a factor of about 2, compared to the IL-free ZnO one.

2.3. Ionic liquids for biofuel processing

Biofuel cells, see scheme of Fig. 5, are attracting a continuously growing interest. There are a few different strategies to prepare these cells whose function and performance depend on the materials used as catalysts, among which, enzymes are the most efficient. There are many studies on the fabrication and performance analysis of enzyme-based biofuel cells, but all of them are carried out in aqueous (buffer) solutions. In spite of being the best medium for biosystems, water is not a suitable solvent from the viewpoint of the required performance. Aqueous solutions, in fact, easily evaporate and require pasteurization to retain cleanliness, especially when used under open air. Accordingly, the challenge in the field is to replace these solutions with more efficient solvents. Ionic liquids, due to their properties of non-volatility and high ionic conductivity, are potentially excellent candidates. However, there is a critical requirement for the use of IL-based biofuel cells, namely whether they can allow the operation of enzymes. At present, there is no ionic liquid that completely satisfies this tough condition; however, several studies on enzyme-based biofuel cells are under way to address this issue. There are several challenges to dissolve proteins in ionic liquids. For instance, while lipase, a rather strong enzyme, can be stably dispersed in some polar ionic liquids still keeping its enzymatic activity, there are only few reports on the dissolution of active proteins, such as cytochrome C, in pure ionic liquids. Though a few chloride-based ILs have a low melting point, chloride generally forms strong H-bonds with proteins amide linkages which favour dissolution. Along this line, 1-allyl-3-methylimidazolium cation was coupled with a chloride anion to prepare a low melting point polar ionic liquid in which cytochrome C was reported to be soluble at 80 °C with a redox activity detected up to 140 °C.⁷⁴ It was also reported that both the π* and β values, i.e., the Kamlet–Taft parameters representing polarizability and hydrogen bond basicity, respectively, of ionic liquids are in fact sufficiently high to dissolve cytochrome C. On the other hand, the overall stability of proteins is not sufficiently long in pure ionic liquids.

Fig. 5 Schematic representation of the operating mechanism of a biofuel cell. The biofuel requires preliminary processing which can be proficiently performed in IL media.

Therefore, various strategies have been attempted to address this issue. Fujita et al. have reported that cholinium dihydrogen phosphate mixed with small amount of water (Hy-IL) is an excellent solvent for proteins.⁷⁵ Such a Hy-IL contains 3–5 water molecules per ion pair, all of them strongly hydrated to ions. Consequently, Hy-IL has no unbound water and thus, it can be regarded as an ionic liquid derivatives. It has been reported that the electron transfer activity of cytochrome C dissolved in cholinium dihydrogen phosphate, containing a small amount of water, maintained for more than one year. This exceptional long shelf life is suitable for application in enzyme-based biofuel cells. Indeed, Fujita et al. succeeded, for the first time, to obtain the enzymatic oxidation of cellobiose with cellobiose dehydrogenase (CDH) in the Hy-IL, with two electrons taken from cellobiose to the CDH, and further transferred to cytochrome C, to finally promote its oxidation, as spectroscopically demonstrated.⁷⁶ It was also shown that some metalloproteins, such as horseradish peroxidase, ascorbate oxidase, azurin II and fructose dehydrogenase, all dissolve in the Hy-IL with enzymatic activity.⁷⁷ Although some property changes depending on their species cannot be excluded, most of these enzymes in the Hy-IL showed an improvement in thermal stability with respect to that obtained in buffered aqueous solutions. Harvesting electrons from saccharides by corresponding dehydrogenases in the Hy-IL, was also reported to be a successful strategy to obtain prototypes of enzyme-based biofuel cells.⁷⁸ Ohno et al. reported that ionic liquids may dissolve cellulose,⁷⁹ as well as promoting its extraction not only from biomass,^80,81 but also even from wood,⁸² finally demonstrating that exploiting ionic liquids is a powerful method to make biofuel cells become a practical reality. On this line, it is expected that in the near future grass, wood chips, used papers and even cotton T-shirts may be thrown into ionic liquids to generate electrons with the aid of enzyme-based biofuel cells.

3. Challenges, prospects and opportunities

In this review, we have shown that tailorable ionic liquids show some potential for assisting the production and processing of energy conversion materials. Table 2 summarises all materials presented in this review.

Table 2 Summary of materials for use in electrochemical energy conversion devices produced in IL media. The used IL in terms of the material functional role are also listed

Material obtained	IL(s) used	Role of the IL
IL–Pt/CNT	[EMI][TFSI], [BMI][TFSI]	Reaction media
N-Doped graphene	EAN-based IL + 10% (v/v) H2O	Exfoliating agent and in situ dopant
(IL–Ni–Pt)/C	[MTBD][BETI]	Advanced solvent, surface modifier
Pt-loaded carbon aerogels	[C4mim][BF4]	Stabilizer
IL-functionalized MWCNTS–Au	1-(3-Aminopropyl)-3-methylimidazolium bromide	Surface modifier
N-doped Pt/C	[BMP][DCA]	N- and/or C-precursor
Core–shell Au–Pt bimetallic	[C4mim][Br]	Solvent and morphology directing
N-doped hollow C spheres	[3-MBP][DCA]	C precursor
S- and N-co-doped porous C	[BMP][TF2N]	N, S and C precursor
PtAg/RGO	[C16mim][Br]	Surface capping, structure-directing
IL–CNTS/Pt	1-(3-Aminopropyl)-3-methylimidazolium bromide (NH2-IL)	Surface functionalizing
Pt/GnP	[C4mim][PF6], [C4mim][CH3CO2]	Structure-directing
PtRu	[C4mim][BF4]	Stabilizer
Pt/C	[C4mim][BF4]	Advanced solvent
Pd nanoparticles	[C4mim][Br], [C4mim][Cl]	Morphology/structure directing
Pt–WC/TiO2	[C4mim][BF4]	Structure and morphology directing
CuPt–/IL–graphene	[C8mim][PF6]	Increase conductivity
PIL/Pt hybrids	[BviM][Br]	Monomer
Pt/XC-72@C–N	[VEIM][NO3], [C2mim][DCA]	Dopant
ILs/graphene supported Pt	[C4mim][BF4], [C4mim][PF6]	Surface functionalizing
Pt–Ru/C	[C4mim][BF4], [C4mim][PF6]	Advanced solvent
Pd/C	[C4mim][BF4]	Advanced solvent
Dy2O3	Eutectic choline chlorine/urea (1 : 2, molar ratio)	Electrolyte for electrodeposition
MnOx (x = 1.5–2)	EAN (ethyl ammonium nitrate)-based IL	Electrolyte for electrodeposition
Phosphate surface-modified MnOx	Phosphate buffer ionic liquid	Surface modification
B- and F-doped graphitic carbon carbides	[C4mim][BF4]	Texture and chemistry modifier
TiO2/RGO	[C4mim][PF6]	Advanced solvent
N- and C-co-doped TiO2	[C2mim][Cl]	N- and C-precursor
Mesoporous TiO2	[C4mim][PF6], [C4mim][BF4]	Templating
CeO2–TiO2	[C16mim][Br]	As template
TiO2/RGO	[C4mim][PF6]	Reducing agent
Anatase TiO2	[C4mim][BF4]	Templating agent
TiO2 films	[Cmim][HSO4]	Structure-directing agent
ZnO	[C4C1Py][TF2N]	Reaction media, functional surface modifier
Cytochrome C protein	[C4mim][MeSO4], [C4mim][CH3CO2], [Ch][dbp], [C4mim][dhp], [Ch][dhp], [C4mpyr][DCA], [Ch][Sac], [Ch][Lac]	Advanced solvent and extracting agent
Oxidized cellobiose	[Ch][dbp]	Advanced solvent and extracting agent
Metalloprotein	(Hy[Ch][dhp])	Advanced solvent and extracting agent
Fructose dehydrogenase mobilized on Au or C	(Hy[Ch][dhp])	Advanced solvent and extracting agent
Cellobiose dehydrogenase mobilized on Au or C	(Hy[Ch][dhp])	Advanced solvent and extracting agent
Dissolved cellulose	[C2mim][MeO][R][PO2], R = H, Me, MeO	Advanced solvent and extracting agent
Extracted cellulose and xylan from wheat bran	[C1mim][MeO][PO2]	Advanced solvent and extracting agent

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Indeed, ILs can play multiple roles, such as functional solvents, electrolytes for electrodeposition, structure-directing agents, stabilizers and precursors, opening a low-temperature and eco-friendly synthesis approach, and new avenues towards the synthesis of highly effective materials for fuel cells, photocatalytic water splitting and dye sensitized solar cells. In addition, ILs have capability to process biofuels, a newly emerging area of potential practical importance. However, the research accounts that have been so far observed represent the tip of an iceberg and intensified research work is still needed to fully understand the effect of the IL structure in promoting advancements of the above listed materials. For instance, the interactions between ILs and precursors/intermediate products, as well as the detailed mechanism of enzyme dissolution in ILs are matters to be methodically investigated. Understanding of the governing mechanisms of the ILs towards tuning the reaction conditions via theoretical modeling is also another desirable, but not yet achieved goal. As announced by BASF in the large-scale and multi-ton BASIL (biphasic acid scavenging using ILs) process, ILs are starting to leave academic labs and find their way into industrial-scale applications.⁸³ However, for the not yet fully commercialized technologies (e.g., FCs, DSSCs, water splitting) a suitable approach of scaling up of the IL-assisted ionothermal synthesis and processing need to be well thought and developed. As the concepts and paradigms of IL-assisted synthesis in the aforesaid applications is new, a very close cooperation between academia and industry is necessary towards their deployment in large-scale applications. Considering the research work in this field is still at its early stage, there is hope that these aspects will be successfully clarified in the near future.

Acknowledgements

Dr G. G. Eshetu and Prof. Dr S. Passerini would like to acknowledge the financial support of the Helmholtz Institute of Ulm (HIU). Prof. Dr B. Scrosati is grateful to HIU and to the Tokyo University of Agriculture and Technology (TUAT) for the visiting professor positions.

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