Polymer materials for electrochemical applications: Processing in supercritical fluids

Accepted Manuscript Title: Polymer materials for electrochemical applications: processing in supercritical fluids Authors: Mikhail S. Kondratenko, Igor V. Elmanovich, Marat O. Gallyamov PII: DOI: Reference:

S0896-8446(16)30362-X http://dx.doi.org/doi:10.1016/j.supflu.2017.03.011 SUPFLU 3880

To appear in:

J. of Supercritical Fluids

Received date: Revised date: Accepted date:

11-10-2016 15-3-2017 15-3-2017

Please cite this article as: Mikhail S.Kondratenko, Igor V.Elmanovich, Marat O.Gallyamov, Polymer materials for electrochemical applications: processing in supercritical fluids, The Journal of Supercritical Fluidshttp://dx.doi.org/10.1016/j.supflu.2017.03.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Review

Polymer materials for electrochemical applications: processing in supercritical fluids Mikhail S. Kondratenko a, Igor V. Elmanovich a,b, Marat O. Gallyamov a,b * a

Faculty of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory 1-2, GSP-1, Moscow 119991, Russian Federation b A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova St. 28, GSP-1, Moscow 119991, Russian Federation *

Corresponding author. Tel.: +7 495 939 1430; fax: +7 495 939 2988. E-mail address: [email protected] (M.O. Gallyamov)

Graphical abstract

Highlights  Processing of electrochemically relevant polymers with sc fluids is reviewed.  Electroactive electronically conductive polymers are in primary focus.  Recent literature on ion-conductive polymers is also analyzed.  The achieved advances and demonstrated benefits are highlighted.  Some remaining questions and challenges are mentioned. ABSTRACT In this review we discuss the usage of supercritical fluids for processing (synthesis, modification, forming, etc.) of polymeric materials for electrochemical applications. Our primary focus is on the literature that reveals the benefits of this approach for processing electronically conductive polymers. Recent literature on ion-conductive polymers is also analyzed. The achieved advances as well as remaining questions and challenges still to be faced are generally outlined. Keywords: Supercritical fluid Conducting polymer Membrane Porous matrix Composite Nafion Nomenclature

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Acronyms AN acetonitrile APS ammonium peroxydisulfate BPO benzoyl peroxide CFC-113 1,1,2-trichloro-1,1,2-trifluoroethane DBSA dodecylbenzenesulfonic acid DFHDPA dodecafluoroheptyldipropargyl acetate DFE difluoroethane DMA dimethylacetamide DMFC direct methanol fuel cell DMSO dimethyl sulfoxide DVB divinylbenzene EW equivalent weight GO graphene oxide HFC hydrofluorocarbon MWNT multi-walled carbon nanotubes OLED organic light-emitting diode PANI polyaniline PCA pyrrole-2-carboxylic acid PCFTE polychlorotrifluoroethylene PDMS polydimethylsiloxane PE polyethylene PEDOT poly(3,4-elthylenedioxythiophene) PEMFC polymer electrolyte membrane fuel cell PFA poly(fluoroalkyl acrylate) PFFB poly[(9-ylidene-{2-tetradecyloxy-5-tetrafluorophthalimidephenyl}fluorenyl-2,7diyl)-alt-(1,4-phenyl) PFTE perfluoroalkyl ester substituted polythiophene PTFE polytetrafluoroethylene PLA poly(lactide) PMA poly(methyl acrylate) PMMA poly(methyl methacrylate) PVK poly(N-vinylcarbazole) PPy polypyrrole PS polystyrene PSFTE semifluoroalkyl ester substituted polythiophene PTs polythiophenes PVAc poly(vinyl acetate) PVDF poly(vinylidene fluoride) scCO2 supercritical carbon dioxide scCHF3 supercritical trifluoromethane SCF supercritical fluid SDS sodium dodecyl sulfate TBAHFP tetrabutylammonium hexafluorophosphate TEM transmission electron microscopy TFODPA tridecafluorooctyldipropargyl acetate THF tetrahydrofuran XRD X-ray powder diffraction

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1. Introduction Nowadays, electrochemical applications are strongly oriented towards usage of new and advanced polymer materials. Polymers are used in power sources (primary [1] and rechargeable batteries [2], fuel cells [3], redox flow batteries[4], photovoltaic batteries [5], [6]), supercapacitors [7], sensors of different types [8], [9], electrochromic [10] and other electrochemical devices [11]. Sometimes, even the very word "polymer" is present in the name of modern hi-tech commercial products, such as a well-known Li-polymer battery [12,13]. Additionally, in this regard one can also mention polymer electrolyte membrane fuel cells [14], polymer solar cells [15], smart windows based on polymer dispersed liquid crystal devices [16], which are all on the way to their commercialization. Further, significant part of commercial OLED (organic) displays, polymer light-emitting diodes, is based on high-molecular-weight materials, i.e. polymeric ones [17], [18]. In the majority of modern electrochemical applications mentioned above the polymers play an active role: they can be intrinsically electroactive [19], conductive (with electronic and/or ionic conductivity) or even electrocatalytically active [20]. Yet, somewhat more passive role of polymers is also rather important for certain electrochemical applications. For decades, inert porous polymer materials have been successfully serving as convenient and stable matrices-separators for electrolytes [21]. More uniform gel-like polymer matrices for electrolytes are also required for some types of power sources [22]. Further, for quite a long time polymers are widely used as simple binders [23] for an active phase in different power sources, sensors, smart windows, etc. This is due to their adjustable adhesiveness, wettability (including hydrophobicity), permeability, elasticity as well as generally good mechanical properties and processability. Indeed, from a historical viewpoint it is interesting to mention that the first success of fuel cells was related to the pioneer usage of a hydrophobic polymeric binder (Teflon) [24] in FC electrodes in order to develop an important concept of a partially hydrophobized electrode. This problem is specifically relevant particularly to fuel cell electrodes – in distinct from more simple design of electrodes of other types of power sources – due to the necessity to extend a so-called three-phase-boundary (an interface of electrolyte/electrocatalyst/gas reagent phases) [25] on the whole volume of an active (electrocatalytic) material. Prevention of metal corrosion is another typical electrochemical area, where protective polymeric coatings traditionally play an important role [26]. Yet, it was established that in such applications electroactive polymers (e.g., intrinsically electronically conductive) also behave significantly better as compared to passively protective, i.e., inert ones [27], [28]. Rather advanced applications of polymers may be related to the amazing tendency of block copolymers towards self-assembly [29], [30]. Highly regular nanostructured mesoporous electrodes and membranes may be created in that way, which widens the arsenal and horizons of electrochemical researchers [31], [32], [33]. Block copolymers may help to control spatial organization of electrocatalytic particles in the active phase. Whereas, controlled size, density, regularity, and separation [34] of catalysts particles may affect pathways of electrochemical reactions involved, e.g. intensity of the main reaction (such as, for example, intensity and selectivity of methanol oxidation reaction [35]) and the side reactions (such as, for example, peroxide formation in oxygen reduction reaction [36]). Thus, using block copolymers it is possible to tailor the order in the electrocatalytic phase, which should improve its behavior. Usage of supercritical fluids, including scCO2, definitely offers new benefits for synthesis and processing of polymers in general [37] and for electrochemical applications in particular.

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Indeed, the gas-like absence of any surface-tension-driven effects along with liquid-like density of supercritical fluids make them unique media for processing of different matrices. Besides, it is a common knowledge, that eventual purity of the materials applied is of a paramount importance for electrochemistry, for both research and production. In this regard, supercritical fluids may also offer certain benefits when applied as media for obtaining or processing electrochemistry-related polymers. Indeed, mainly, these fluids are gases or volatile liquids at normal conditions. Therefore, they leave the modified/synthesized product spontaneously and with high degree of completeness. Thus, the typical problem of a residual solvent is automatically solved. Another consequence is that they are ready to be produced cheaply with a high degree of purity or purified easily after usage. Many of them (including scCO2) are environmentally friendly, which is not the least of the advantages. Further, typically they have rather small and simple molecules, which are (electro)chemically and thermally stable, with CO2 again being a typical example here. Therefore, even if their residual traces are still present in the materials, no poisoning of electrocatalytic processes occurs. Also, they typically do not interact with the polymer material itself directly and do not induce its (electro)chemical degradation either. As compared with the usage of typical organic solvents, supercritical fluids usually allow to increase (if necessary) the temperature of the synthesis/treatment procedure, provided that suitable high-pressure setup sustaining both high pressures and temperatures is available. As compared with typical chemical processes performed in plasma or vapor surroundings, the liquid-like density of supercritical fluids mainly allows one to disperse or even dissolve the materials in them rather effectively. Besides, if two or more immiscible fluids are explored, they may form highly tunable (again, by variation of pressure/temperature) biphase systems, which are convenient for wide spectra of heterogeneous processes [38]. Some simplest examples may include the direct deposition of uniform protective films from solutions in scCO2 [39], where uniformity is related to the absence of the disturbing surface-tension-driven effects for this unusual solvent [40]. On the way of general improving of the concept of a partially hydrophobized electrode via minimization of the amount of the polymer hydrophobizer to be introduced, uniform hydrophobic polymeric coatings can be deposited directly from solutions in scCO2 onto gas-diffusion layers [41] and active phase materials [42] of the fuel cell electrodes (or just any gas-breathing electrodes, in general). The possibility to decrease significantly the amount of the polymer hydrophobizer required for optimal performance is also related to the absolute wetting ability of scCO2 together with its non-disturbing manner of leaving porous structure after the deposition/modification process is completed. Usage of self-assembly of amphiphilic compounds in solutions in scCO2 and on a substrate from such solutions allowed to form regular structures on a substrate [43] as well as to load them with electrocatalytically active noble metal nanoparticles [44]. This controllable order in nanoparticles localization, size and separation should allow tailoring electrocatalytic behavior of active phase of power sources [34], [35], [36]. In 2012 Bozbag and Erkey prepared very comprehensive review on the usage of supercritical fluids for fuel cells [45]. All aspects of modern fuel cells research and development problems were carefully described and benefits of transition towards the usage of supercritical fluids as solvents/modifiers were highlighted. Different classes of materials were considered and further researches required for every particular class were outlined. Among the different organic/inorganic materials of the fuel cells components, polymers were also discussed, but mainly from a viewpoint of their usage in a membrane. The membrane of fuel cells should possess high proton (ion) conductivity and reduced permeability with respect to direct crossover of gas reagents (meaning selectiveness), but it must not be electronically conductive.

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Otherwise, it would be unsuitable for fuel cells due to just direct shunt currents appearance. Therefore, electroactive polymers with intrinsic electronic conductivity were mainly out of the scope of the review [45]. Yet, such electroactive polymers [19], [20] are extremely important for many modern electrochemical applications, including some aspects of electrodes of power sources (usage of such polymers instead of more traditional inorganic electron conductors/electrocatalysts), but also for solar cells [5], [6], [15], supercapacitors [7], different sensors [8], [9], smart windows [10], [16], light-emitting diodes for displays [17], [18], actively protective coatings [27], [28], etc. [11]. Therefore, primary focus of our review is processing of electronically conductive polymers with sc fluids. Further, we also describe recent works on the way of improving the balance between ionic conductivity and selectivity of membranes, which were published after the appearance of the review [45]. 2. Supercritical fluids SCF is a substance simultaneously compressed and heated above its critical pressure and temperature. In this state the boundary between liquid and gas phases disappears and SCF has the physical properties intermediate between those of a gas and a liquid. It occupies all the volume available and has high diffusivity as well as low viscosity like a gas, while possessing liquid-like density. Solubility of different compounds in SCFs depends on fluids density and can be easily controlled and tailored by changing the pressure and temperature, making SCFs a powerful instrument for chemical processing. Among all SCFs scCO2 is the most popular one due to its relatively low critical pressure (73 bar) and temperature (31 °C) as well as a number of other advantages [46]: CO2 is a nontoxic and non-flammable compound; it has high vapor pressure (above 60 bar), which allows its complete removal from processed materials; CO2 is inert towards oxidation and has low reactivity; it has low viscosity, high diffusivity and low surface tension, which allows it to wet structured materials more effectively than most other solvents do. Due to the central symmetry of the molecule, CO2 has zero dipole moment, low dielectric constant of 1–2 (depending on the pressure and temperature) and low volume polarizability in the range of 0.012–0.032 at room temperature. However, it cannot be considered absolutely nonpolar one since there is a charge separation between the oxygen atoms and the central carbon atom (the calculated partial charges are –0.36 and +0.72 correspondingly [47]) resulting in a quadrupole moment of 13×10-40 C m2 [48]. CO2 is therefore able to solvate some polar substances through dipole-quadrupole interactions. It can dissolve several classes of polymers with weak polymer-polymer interactions and correspondingly low surface tension. Indeed, perfluoroalkyl acrylates and PDMS are known to have quite high solubility in scCO2. The pressures needed to dissolve the polymers usually rise significantly with increased chain length (Mw) due to decreased entropy contribution (see Fig. 1). An interested reader may refer to the recent review by Girard et al. [47] for more information on polymer/CO2 interactions and phase behavior of polymers in scCO2 However, the low dielectric constant of scCO2 makes it a poor solvent towards polar and ionic species. This becomes an obstacle for conducting electrochemical processes in scCO2 due to low solubility of salts in it and therefore low conductivity of supporting electrolytes based on scCO2. Instead, choosing supercritical HFCs as solvents for electrolytes appears to be a more efficient strategy for such purposes. HFCs have mild critical parameters, wide electrochemical stability window due to their inertness towards reduction and oxidation as well as the

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possibility of dissolving significant amounts of salts composed of bulky ions with low charge/radius ratio due to a non-zero dipole moment. It has been shown [50] that CHF3, a solvent with accessible critical point of 25.9 C and 48 bar (corresponding to critical density of 0.525 g cm-3), having dielectric constant ranging from 1.6 at ~0.2 g cm-3 to 5.7 at ~0.9 g cm-3 at 50 C, seems to be suitable for electrochemical studies under supercritical conditions. The solubility of TBAHPF in CHF3 is substantially higher than in CO2, which provides reasonable conductivities for electrochemical experiments. 3. Processing of conducting polymers in supercritical media 3.1. Introduction to conducting polymers Organic macromolecules with highly π-conjugated polymeric chains exhibiting high electronic conductivity in partially oxidized or reduced state were discovered in 1970-s by Heeger, MacDiarmid and Shirakawa [51–53] and are now considered promising materials for a wide range of applications, including electrochemical energy storage, electrocatalysis, organic electrochemistry, bioelectrochemistry, photoelectrochemistry, electroanalysis, sensors, electrochromic displays, microsystem technologies, electronic devices, microwave screening, corrosion protection, etc. [54]. Typical examples of conducting polymers are polyacetylene, polyaniline, polypyrrole, polythiophene, polyethylenedioxythiophene, polyphenylene vinylene and others (Fig. 2). The conductivity of such polymers may be varied in a wide range from 10 -10 to 104 S cm-1 and they may behave as insulators, semi-conductors or good electronic conductors depending on the oxidation state. Although the π-electrons in conducting polymers are delocalized along the chain, the conductivity measured for pristine polymers is quite low, usually in the range of 10-10–10-5 S cm-1. For example, the conductivity of pure cis-polyacetylene is as low as 1.9 10-9 S cm-1 due to the alteration of single and double bonds [55] resulting in a high band gap. However partial oxidation by halogens such as Cl, Br, I results in significant increase in conductivity of polyacetylene by several orders of magnitude: in the case of partial oxidation by I the conductivity rises up to 38 S cm-1 [51]. Such partial oxidation or reduction of conductive polymer chains through chemical or electrochemical redox reactions is called ‘doping’. Doping is a reversible process resulting in formation of positive or negative charge carriers in the polymer backbone accompanied by entrapping or releasing counter ions from the supporting solution to maintain the overall charge neutrality. Doping can be performed either chemically or electrochemically. The resulting conductivity depends on the type of a dopant as well as on the doping level. Even a very low doping level of about 1% results in an increase of conductivity up to values of around 0.1 S cm-1. Further doping of the conducting polymers leads to a saturation of the conductivity at values around 10 2–104 S cm-1. Various chemical and electrochemical methods of conducting polymer synthesis are known. Polyacetylene, the first prototype of a conducting polymer, was prepared from acetylene using a Ziegler–Natta catalyst. However, polyacetylene is easily oxidized by the oxygen in air and is also sensitive to humidity, which makes it unsuitable for many applications. That is probably why the most widely used approach to conductive polymer synthesis is an oxidative polymerization of cheap, simple, aromatic benzoid (aniline, diphenylamine, phenylenevinylene) or heterocyclic (pyrroles, thiophenes, azines) compounds [54]. General polymerization mechanism cannot be provided due to the chemical diversity of the studied compounds. The first step is usually the oxidation of a monomer and the formation of cation radicals. The further reaction pathways may vary depending on the experimental conditions

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and reactive species. The oxidation may be followed by a dimerization reaction between two cation radicals, and then the stepwise chain growth via the association of radical ions (see the proposed mechanism of pyrrole polymerization in Fig. 3: the polymer chain growth occurs via the repeating sequence of oxidation, coupling and deprotonation acts). The oxidation step can be induced either chemically by the addition of oxidants such as FeCl3, FeClO4, APS etc., or electrochemically by applying anodic polarization over the electrode. Electrochemical polymerization allows one to control the oxidation state by means of electrode potential and produce high-quality polymer films with fine structure on conductive supports, while the chemical way appears to be more suitable for production of large amounts of polymer or creation of composites with non-conducting templates. Red-ox activity of conducting polymers as well as their electronic and ionic conductivity and flexibility makes them promising materials for electrochemical energy storage devices such as supercapacitors and batteries (detailed overview on this topic is given in the recent reviews [57], [58]). They may act both as active electrode materials and as flexible conductive binders. Fast redox transitions in conductive polymers allow one to create pseudocapacitive electrodes for supercapacitors and thus increase the specific capacitance from typical values of 100–200 F g-1 – common for electrical double layer charging of carbon materials [59] – up to 400–600 F g-1 [58], [60]. Redox activity of conducting polymers may also be exploited by using them as active electrode materials in rechargeable batteries such as Li-ion batteries. Using conductive polymers as binders allows one to create flexible devices, optimize ionic and electronic transport as well as to deal with the well-known volume change problem of inorganic materials during the charge-discharge process. Creating an efficient energy storage device with high specific power and energy requires maximizing contact surface area between the phases with ionic and electronic conductivities, providing efficient transport of ions in the electrolyte and inside the electrode, high electronic conductivity in the electrode and current collector as well as facile electron transfer to allow for fast red-ox transformations. An ideal electrode material should probably have a 3D nanostructure similar to the one shown in Fig. 4. Significant efforts are focused on creating nanostructured conducting polymer composites to meet the abovementioned requirements. Using supercritical media for synthesis and processing of conducting polymers opens new possibilities in development of nanostructured composites with complex morphology. Benefits of using SCFs arise from their high penetration ability, the absence of capillary forces, the possibility to control and tailor the solubility of compounds by changing the fluid pressure. Moreover, easy solvent removal upon decompression eliminates the residual solvent problem. Yet, application of supercritical media for conducting polymer synthesis raises several questions regarding the phase behavior of monomers, polymerization products and template materials in supercritical media as well as the influence of supercritical media on the mechanisms of oxidative polymerization, morphology and properties of the resulting polymers. Below we summarize the research experience in this area. 3.2. Synthesis of conducting polymers in supercritical media Both chemical and electrochemical oxidative polymerization of conductive polymer monomers can be performed in SCFs. The monomers are usually mixable with properly selected SCFs. For example, Dandge et al. [61] have shown that pyrrole monomer is miscible with scCO2 in all proportions at different temperatures. A single phase has been observed for aniline/CO2 system at pressures higher than 120 bar and temperature of 35 С by M. Chatterjee et al. [62]. These facts should result in higher diffusivity of the monomers in SCFs in comparison with liquid phase polymerization and therefore different morphology of the resulting products.

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Changing physical properties of SCFs by means of pressure and temperature variation opens intriguing possibilities in controlling the morphology of conducting polymer materials. 3.2.1. Chemical synthesis The first attempt of conducting polymer synthesis in SCFs was made by Kerton et al. [63]. They synthesized polypyrrole via thermal decarboxylation of pyrrole-2-carboxylic acid both in scCO2 and in scCHF3 using ferric salts (FeCl3 or Fe(CF3SO3)3) as oxidants. Physicochemical properties of the obtained materials in general were very similar to those of polypyrrole prepared in conventional solvents, except for conductivity, which was two to three orders of magnitude lower: pressed pellet conductivities of the obtained polymer did not exceed 5×10-2 Scm-1 (see Table 1). Authors explained that conductivity difference by some over-oxidation of conjugated chains at the elevated temperatures required for the chosen synthetic route. Authors also noted the unusual fibrillar morphology of polypyrrole synthesized in supercritical media, which was different from globular morphology usually observed after conventional precipitation polymerization in liquid media (Fig. 5). The authors attributed the difference to the low viscosity of the supercritical fluid. Du et al. [64] proposed polymerization of aniline at scCO2/aqueous solution interface and obtained PANI microtubes with a hollow core. APS, which was used as an oxidant, was dissolved in aqueous phase also containing some amount of SDS surfactant. The aqueous solution was initially separated from aniline monomer by a wall of the glass beaker. The contact between aniline and oxidant was possible only after its dissolution in scCO2 and diffusion to the aqueous interface. Since no microtube formation was observed in the absence of SDS, the authors suggested surfactant-induced formation mechanism: aniline was oxidatively polymerized by APS at the interface of scCO2 and aqueous solution, forming hollow microtubes with narrow diameter distribution around 120 nm. Pham et al. [65] and Akbarinezad et al. [66] synthesized PANI in two phase water/CO2 system without any surfactant using APS as an oxidant. The resulting polymerization products had morphology of highly crystalline (according to XRD data in [65,66]) bulk fibers without hollow cores and with fiber diameter from 30 to 80 nm (see Table 1). The authors of [65] and [66] report high product yields of 63% and 80%, respectively. The difference in the yields may be attributed to the difference in experimental conditions: Pham et al. [65] conducted synthesis in compressed liquid CO2, while Akbarinezad et al. [66] used scCO2, higher mobility of which may be responsible for higher polymer yields. Both teams report high conductivities (Table 1), which are quite typical for PANI synthesized in conventional solvents and washed in hydrochloric acid solutions. Successful two-stage synthesis of PANI in single phase scCO2 in the absence of water phase was reported by Lopatin et al. [67]. CO2-soluble salt of DBSA and aniline was synthesized on the first stage and then polymerized in scCO2 in the presence of APS on the second one. The doping degree of the obtained PANI with DBSA was reported to be about 7%. Wang et al. [68] have shown that using scCO2 for aniline polymerization in the presence of cyclodextrine it is possible to obtain inclusion complex of PANI with cyclodextrine. Zaidi et al. [69] reported PPy, PTs and PPy/PTs copolymer synthesis in scCO2. They found that increased proportion of thiophene in the copolymer results in decreased conductivity. The authors explained that by the decreased number of chloride ions per polymer unit at higher thiophene proportion and by the presence of carbonyl defects acting as interruptions in conjugation and impeding charge transfer along the polymer chain. To clarify the influence of scCO2 on conductivity, Fernando et al. [70] synthesized PANI films using conventional approach and studied the influence of scCO2 treatment on polarons of HCl-doped PANI films. They found out that scCO2 treatment at 300 bar and 40 oC results in

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significant decrease of electrical conductivity of the films and attributed that to decreased amount of polarons due to the removal of HCl dopant by scCO2. Since the solubility of oligomers and polymers in SCFs usually decreases rapidly with increase of chain length (see Fig. 1), the resulting polymers appear to be non-soluble at the reaction conditions and their precipitation occurs during polymerization. However this is not always the case: it has been shown that conjugated polymers soluble in SCFs can be obtained via SCF polymerization of functionalized monomers. Li et al. [71] first proposed to use perfluoroalkyl substituents to prepare CO2-soluble conjugated polymers and showed that regioregular poly(3-pefluorooctyltiophene) synthesized by McCullough’s method [72] in conventional solvent is soluble in scCO2. Ganapathy et al. [73,74] then synthesized CO2-soluble polythiophenes PFTE and PSFTE by functionalizing thiophene rings with fluorinated ester groups with subsequent oxidative polymerization in scCO2. Comparison of PFTE and PSFTE synthesized in scCO2 with those synthesized in CHCl3 did not reveal any noticeable difference in molecular weight, polydispersity, or UV and visible light absorption and emission properties as well as any noticeable difference in electrical conductivity. Keshtov et al. [75,76] used identical approach and obtained a series of CO2-soluble conjugated polythiophenes containing fluoroalkyl ester groups in the side chains using scCО2 as a solvent. The properties of the polymers obtained in scCО2, such as their molecular weight, polydispersity, conjugation, UV absorption, were similar to the properties of PFTE obtained in CHCl3 as well. Naidu et al. [77] synthesized another type of conjugated polymers by conducting cyclopolymerization in scCO2 using MoCl5 as an oxidant for two fluorinated dipropargyl derivatives: 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyldipropargyl acetate and 3,3,4,4,5,5,6,6,7,7-dodecafluoroheptyldipropargyl acetate, respectively. While the first polymerization product was only partially soluble in CFC-113, the second one – with the same chemical structure except for one fluorine atom of the terminal trifluoromethyl group substituted with a hydrogen atom – was reported to be completely soluble in common organic solvents such as acetone, THF, 1,4-dioxane, ethyl acetate, and dimethylformamide. Strong absorption peak at 455 nm, being characteristic for the p-conjugated polyene backbone system, was observed in solution of the later polymerization product in THF. It is worth noticing that a nanofibrillar morphology of conducting polymers is generally observed as a result of SCF polymerization in contrast to a globular morphology, which is common for liquid phase synthesis. The fibrillar morphology may be beneficial for creating electrode materials with high energy and power densities due to enhanced ion and electron transport along the fibers as well as their high surface area. However questions regarding the influence of SCF parameters on the morphology and conductivity of conducting polymers remain open. 3.2.2. Electrochemical synthesis Electrochemical synthesis directly in supercritical fluids is an emerging field due to such advantages as simple separation of reagents and products as well as significantly improved mass transport of dissolved species to the electrode surface in supercritical media [78,79]. However conducting electrochemical processes in supercritical media faces certain complications arising from sophisticated high-pressure reactor design required along with low dissolving power and low dielectric constant of typical supporting electrolytes. Possibly, exploring two-phase systems may also be beneficial [38]. Nevertheless, electropolymerization of conducting polymers in supercritical media is possible and opens up new possibilities to obtain fine-structured well-defined films.

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The first successful electrochemical oxidative polymerization of conducting polymers, PPy and PANI, in scCO2 was reported by Anderson et al. [80]. PPy synthesized in scCO2 containing TBAHFP as electrolyte and AN as a cosolvent exhibited morphology consisting of small granular PPy nodules on a flat PPy surface in contrast to wrinkled texture of PPy typically observed for films synthesized in non-aqueous media (Fig. 6). The conductivity of scCO2synthesized PPy was 4±2 Scm-1, comparable to that of PPy electrochemically synthesized in liquid media. Yan et al. [81] also used AN as a cosolvent and conducted electropolymerization of PPy in CO2/AN system in the presence of TBAHFP. They studied the influence of system parameters (composition and pressure) on the properties of conducting polymer films. It was found out that the films synthesized in a single phase scCO2/AN system were uniform and much smoother than those synthesized in two phase CO2/AN or in pure AN. The authors explained this fact by a higher diffusion rate of the monomer and at the same time slower growth of the film due to sluggish electron transfer in the presence of CO2 (it was shown that the addition of CO2 decreased the polymerization rate in scCO2/AN system). Murata et al. [82] obtained somewhat similar smooth films of PANI electropolymerized in homogeneous scCO2/AN system. The highest conductivity of 130 Scm-1 was observed for the films synthesized at 100 bar and 50 oC. The flatness of the film in the system decreased with increasing CO2 amount. Higher roughness was observed for films obtained in dichloromethane as a solvent. As Yan et al. [81] previously, Murata et al. [82] attributed the formation of the particularly smooth and flat films to the slow growth of the film and the high diffusion rate of the monomer in the supercritical media. Mahito et al. [83] obtained highly regular thin films of PPy and PTs in scCHF3 and on the contrary reported faster polymerization than in conventional liquid media in that case. This may be due to higher conductivity of scCHF3 based electrolyte ensured by an ambivalent nature of this solvent and higher solubility of conducting salts as compared to non-ambivalent CO2. Jikei et al. [84] proposed to conduct electrochemical polymerization of PPy in scCO2-inwater emulsion and studied influence of supporting electrolytes on such polymerization. Among the examined electrolytes, p-toluenesulfonic acid produced a film with the highest conductivity of 13.6 Scm-1. PPy films prepared in the CO2/water emulsion had homogeneous nodular microstructure with diameters of individual nodules of about 500 nm. On the macroscale the films appeared to be much more uniform than those synthesized in water. In their subsequent work [85] the authors showed that such technique allows one to obtain fine texturized films of PANI, similar to that of PPy, as well. However, electrochemical polymerization of EDOT, which is insoluble in water, resulted in irregular and rough PEDOT film. Mahito et al. [86] studied cation transport in PPy membranes electro-oxidatevily polymerized in sub- and supercritical DFE with dissolved TBAHFP as electrolyte. More dense and uniform membranes with higher conductivities (up to 7.4 Scm-1) were formed in scDFE in comparison to subcritical DFE and AN. Such membranes were selective towards potassium cations and demonstrated higher K+/Na+ and K+/Li+ relative permeabilities. As one can see from the results discussed above, the enhanced monomer diffusivity in SCFs promotes formation of more uniform and fine conducting polymer films as compared with those obtained in liquid electrolytes. Electrodeposition of such well-defined films on high surface area porous conducting supports appears to be a promising strategy in creating electrode materials for electrochemical energy storage.

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3.3.Conductive polymer composites Due to high diffusivity of supercritical fluid and high mobility of monomers dissolved in such fluid it can be used for impregnation of high-surface-area-materials with an oxidant and a monomer. This allows to conduct further oxidative polymerization inside even the smallest pores and to obtain different types of composite materials, which can be divided into two major groups: organic polymer-polymer composites and organic/inorganic composites. 3.3.1. Organic composites Various conductive elastomers have been obtained by scCO2 impregnation of porous polymer matrices with monomer or oxidant and subsequent polymerization reaction. Shenoy et al. [87] synthesized conductive elastomeric foams by polymerization of PPy inside porous polyurethane foam. They used scCO2 to introduce Fe(CF3SO3)3 oxidant inside foam pores and then exposed the foam to Py vapor. The addition of small amounts of ethanol allowed to improve the oxidant solubility in sc fluid as well as oxidant and PPy distribution inside the foam which lead to significant improvement in the resulting conductivity of the composites. Conductivity of up to 10-2 Scm-1 was achieved. Abbet et al. [88] polymerized 3-undecylbithiophene inside a polystyrene matrix using two-step batch process: during the first step the host polymer was impregnated with Fe(CF3SO3)3 in scCO2, during the second step the oxidant-impregnated host polymer was placed in contact with a mixture of monomer and scCO2 resulting in polymerization reaction. The highest conductivity of 3×10-4 Scm-1 was obtained in composites prepared at 313 K and 207 bar, which represented conditions that led to significant deposition of the oxidant and formation of interconnected conducting polymer network. In another work [89] this group prepared composites of PPy with films of PCTFE, crosslinked PDMS, PMMA and porous crosslinked PS by impregnating the films with oxidant and bringing them in contact with scCO 2 saturated with Py. Nonporous films of PCFTE and PDMS did not formed conductive composites with Py, while conductivities of PPy composites with swollen PMMA and porous PS were in the range of 3×10-5 to 1×10-4 Scm-1. Kurosawa et al. [90] performed oxidative polymerization of Py inside porous crosslinked polystyrene matrix by impregnating it with iodine and then bringing it in contact with Py. Both steps were carried out with and without scCO2. The use of supercritical CO2 was shown to facilitate the transport and deposition of I2 and Py in the pores leading to composites with higher levels of conductivity. They have shown that the amount of conductive PPy/I complex in the resulting composite was proportional to the amount of impregnated I2. Conductivities of up to 10-3 Scm-1 were achieved for composites with about 1:1 ratio between PPy/I2 and polystyrene. Nikitin et al. [91] synthesized PPy/microporous PE composites by impregnating porous PE matrix with FeCl3 methanol solution with subsequent thermal decarboxylation and polymerization of PCA in a high pressure vessel filled with scCO2. They studied the influence of pressure, temperature, number of synthesis repetitions as well as FeCl3 concentration on PPy content in the resulting composite. The composites with up to 50 % content of conductive polymer were obtained. Tang et al. [92] proposed to conduct blending of host polymer with pyrrole monomer in sc or liquid CO2 prior to soaking it with oxidant and studied influence of the blending conditions on the PPy/PS composites properties. They found out that composites with much higher conductivity may be obtained when the blending process is carried out before oxidant doping. They observed also that conductivity of the polymer composites increased with temperature and pressure during blending process at the same scCO2 density. Conductivities of up to 110-2

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Scm-1 were achieved. In Ref. [93] they further studied influence of doping conditions in water and AN on the resulting composite conductivity and found that conductivity changed nonmonotonically with the concentration of the doping solution and a bell-shape profile was obtained. They also reported that the maximum conductivity of the composites doped with iron (III) compounds decreased in the following order of anion: chloride>sulfate>perchlorate>nitrate. Kayrak-Talay et al. [94,95] used scCO2 to immobilize glucose oxidase on a conductive composite of polyurethane/PPy and studied the influence of impregnation parameters (pressure and temperature) on the enzymatic activity of the resulting composite. Authors concluded that by performing immobilization via the scCO2 route, the activity values were doubled compared to the immobilization at atmospheric conditions. More effective glucose oxidase immobilization in scCO2 was explained by swelling of the polymer matrix in the supercritical fluid as well as by lower surface tension and higher wetting ability of the latter. Authors have also shown that the specific enzymatic activity behaves non-monotonically with variation of pressure and temperature having a maximum at 100 bar and 30 oC. SCF assisted drying can be used to obtain porous conducting polymer composites with improved ion-transport properties. Carlsson et al. [96] prepared conductive aerogel composites of nanofibrillated cellulose and PPy with tunable structural and electrochemical properties. The composites have been obtained by chemically polymerizing Py onto TEMPO-oxidized cellulose nanofibers dispersed in water and applying different drying procedures. ScCO 2 drying resulted in high porosity aerogel composites with the largest surface area (246 m 2 g-1) reported so far for a conducting polymer/paper-based material, while low surface area samples (<1 m2 g-1) were obtained using ambient drying in air. It has been shown that the differences in the porosity result in dramatic changes in the voltammetric behavior of the composites. A specific capacity of 220 C g-1 was obtained for all composites in voltammetric experiments performed at a low scan rate of 1 mV s-1 and this capacity was retained at scan rates up to 50 mV s-1 only for the high porosity composites obtained by scCO2 drying due to improved ion transport (Fig. 7). Therefore, the proposed concept is suitable for creating flexible supercapacitor and battery electrodes. 3.3.2. Organic/inorganic composites Processing of carbon materials in scCO2 is a rapidly emerging approach [97], which can be used to obtain various composites of graphene and nanotubes with conducting polymers for applications in light emitting diodes, solar cells, supercapacitors and batteries. ScCO 2 wets carbon materials efficiently and they can be easily processed and impregnated with various compounds in this media without auxiliary modification of their surface. ScCO 2 may also act as an antisolvent, which facilitates the deposition of modifying compounds from their solutions in ordinary liquid solvents [98]. It has been also shown that CO2 can intercalate between graphene plates in graphite and exfoliate graphene sheets during depressurization [99] which opens wide possibilities in creating conductive polymer/carbon composites. Steinmetz et al. [100] have shown that it is possible to polymerize conducting polymers (photo-conducting PVK and PPy) inside carbon nanotubes by scCO2-assisted impregnation of MWNT with corresponding monomers and subsequent polymerization. Li et al. [101] covered carbon nanotube surface with poly(2-methoxy-5-(3′,7′dimethyloctyloxy)-1,4-phenylenevinylene) conjugated polymer by simple mixing single-walled carbon nanotubes and polymer dispersions in DMSO or DMA in scCO2. The procedure resulted in decoration of nanotubes with polymer agglomerates. Fluorescence spectra of the composites were similar to that of pure polymer.

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Yuvaraj et al. [102] synthesized MWNT/PEDOT composites by conducting oxidative polymerization of EDOT mixed with surface modified MWNT in scCO2. This procedure resulted in formation of tubular layer of PEDOT film coating nanotube surface. By comparing with pure PEDOT, the PEDOT/MWNT composite exhibited enhanced thermal stability. Zheng et al. [103] prepared PFFB/graphene oxide nanohybrids by means of exposition PFFB/graphene oxide dispersions in THF in scCO2 at 160 bar and 40 C. Their results indicated that the fluorescent behavior of the nanohybrids obtained after scCO2 exposition was enhanced significantly in comparison with samples obtained without such exposition. It was also shown that scCO2 induces PFFB crystallization on graphene oxide surface. Several works are dedicated to making conductive polymer/carbon material composites with high specific capacity and charge-discharge cycling stability for supercapacitor applications. Xu et al. [104] prepared graphene oxide/PANI composites as new electrode materials for supercapacitors by oxidative polymerization of aniline mixed with graphene oxide in a biphase scCO2/ethanol system using APS as an oxidant. This procedure resulted in uniform coverage of graphene oxide by PANI nanoparticles. The morphology of nanocomposites was controlled through adjusting the concentration of aniline. The composites synthesized at 0.1 M concentration of aniline exhibited better specific capacitance and cycle stability than pure PANI and graphene oxide (see Fig. 8). High specific capacitance of 425 Fg-1 at a current density of 0.2 Ag-1 was achieved. In their later work [105] this group used scCO2 to obtain free-standing flexible graphene oxide/carbon nanofiber/polypyrrole composite films with enhanced specific capacitance and cycling stability. Graphene oxide/carbon nanofiber films casted from water-based dispersion were exposed to scCO2 followed by rapid decompression prior to pyrrole polymerization procedure in aqueous solution. Such exposition resulted in enlarging the space between graphene oxide sheets and allowed for Py monomers penetration inside the films. Yang et al. developed an approach of filling graphene/Py aerogels obtained via hydrothermal route with PANI-covered nanotubes by simple dipping aerogels in CNT/PANI dispersion to prevent stacking of graphene/Py sheets [106]. The resulting specific capacitance was reported to be 5 times higher than that of graphene/Py aerogel and 2.2 times higher than that of CNT/PANI. In their further work [107] the impregnation of graphene/Py aerogels with PANI-covered nanotubes was performed via scCO2 exposition of ethanol dispersions of this materials. The resulting composites were compared to the ones obtained by the dipping method described above (see preparation scheme in Fig. 9). This allowed to obtain hybrid materials with high specific capacitance of 400 F g-1, which was 1.4 times higher than that of similar materials obtained by dipping method without the assistance of scCO2. Several works are devoted to scCO2-assisted preparation of conducting polymer composites with metal and non-metal oxide based materials. In particular, Yuvaraj et al. [108] used scCO2 to polymerize PEDOT on surface modified silica particles and obtained composites with enhanced thermal stabilities compared to the bare polymer. Authors noticed that the resulting conductivity of the composites increased with increasing silica content. The highest conductivity of 3.45×10-2 Scm-1 was obtained for the sample with 20 wt. % SiO2. Pham et al. [109] prepared PANI/TiO2 composites in scCO2 using two different strategies. In the first method separately synthesized TiO2 particles were mixed with aniline in order to perform polymerization in scCO2. The second method included the preparation of aniline/TiO2 hybrids through a sol–gel reaction of titanium isopropoxide in the presence of aniline. Further polymerization of aniline/TiO2 hybrids in scCO2 produced PANI/TiO2 hybrid particles. In that case the composites showed the intrusion of PANI into the internal structure

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of the TiO2 phase. The second route resulted in significantly enhanced conductivity of 7.7×10 -2 Scm-1 as compared to 3.8×10-2 Scm-1 obtained according to the first one. Atobe et al. [110,111] prepared highly aligned PPy and PTs nanowires by means of templated electropolymerization in scCHF3 inside porous alumina membrane followed by dissolution of the membrane. In contrast, the similar procedure in AN resulted in low density non-oriented nanowires with 550 times lower conductivity (6 ×10-3 Scm-1 in scCHF3 vs. 1.1×10-5 Scm-1 in AN, also see Fig. 10 for morphology comparison). 3.4.Intermediate conclusions The usage of supercritical media opens new opportunities for tuning material morphology on a stage of conducting polymer synthesis: oxidative chemical synthesis in sc media allows one to obtain fine nanofibers with narrow diameter distribution or even hollow microtubes. Electrochemical polymerization in sc solvents results in more dense and uniform membranes than those prepared in conventional solvents. Conducting polymer composites of complex morphology can be easily obtained due to high mobility of supercritical fluids. These possibilities are of great practical importance since conducting polymers have significant issues with processability. However, there is some controversy in the literature data. Indeed, conductivities reported in different works may differ in several orders of magnitude even at similar synthesis conditions. Questions regarding influence of sc media on oxidative chemical and electrochemical polymerization processes remain unsolved. The same stands for the influence on morphology as well as such properties of the resulting polymers as doping level and conjugation, which determine electric conductivity. This is a clear indicator that further research is required in order to solve these questions and to implement the achievements. Making composites of conducting polymers with carbon materials, especially graphene, in SCF based systems appears to be a promising approach. Indeed, it may be possible to perform SCF assisted exfoliation of non-oxidized graphene sheets and simultaneous impregnation of the resulting material with corresponding monomers followed by in situ synthesis of conductive polymers between the sheets. However there are only a few recent studies in this field and much work is yet to be done. 4. Processing of membrane materials in supercritical media Polymeric membranes are widely used as separators in electrochemical power sources. Conductivity, selectivity and thermomechanical stability of a separator must be optimized in order to design an efficient device. The necessity to balance these properties drives the research on polymeric membrane fabrication and modification. Due to the absence of surface tension and gas-like diffusivity supercritical carbon dioxide is a promising medium for membrane modification or grafting. Research on supercritical fluid implementation in membranes development for electrochemical applications focuses mostly on preparing composite Nafion-based membranes. Nafion is a copolymer with a PTFE backbone and perfluorinated ether side chains terminated with sulfonic acid groups (Fig. 11). The backbone of Nafion is organized into hydrophobic domains of polymer bulk and is responsible for chemical and mechanical stability of the membrane, while the side chains provide proton-conductive channels throughout the membrane. Nafion in a fully hydrated stage has high proton conductivity of up to 0.18 Scm-1. Different types of Nafion and its analogues vary in some parameters, e.g. membrane thickness,

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equivalent weight (m index in Fig. 11), side chain length (k index in Fig. 11), chemical nature of initial groups of macromolecules (a fluorination of residual hydrocarbon moieties improves overall macromolecular (electro)chemical stability). Nafion is a commercial standard as a membrane for direct methanol fuel cell and hydrogen-oxygen polymer electrolyte membrane fuel cell. However, in both cases using Nafion is associated with some drawbacks. First, Nafion intolerance towards working at low humidity conditions limits the working temperatures of PEMFC and DMFCs to lower than 80oC. Increasing this temperature would increase kinetic of electrochemical reactions and somewhat eliminate CO-poisoning of Pt catalyst [113]. Thus, developing a composite Nafion-based membrane capable to retain water and having high proton conductivities at elevated temperatures would yield a more efficient membrane both in terms of performance and economics. Another issue is high methanol permeability of Nafion hindering the performance of DMFC. Though, using a thicker Nafion one can decrease methanol permeability [114], it also leads to an increased resistivity of a membrane towards proton transport, which means additional energy losses in a power source. Both of these problems are being addressed in numerous researches on modifying Nafion and testing the modified membranes [115], [116]. In this regard, Nafion-organic and Nafion-inorganic binary and ternary systems are being extensively studied [117]. It is outlined in 2016 review [117] that the main goals of such research is to develop a composite membrane with an increased selectivity than that of a pristine Nafion, ensuring higher power densities and minimizing the usage of Nafion polymer, i.e. working with thinner membranes. Most of the current research is focused on incorporating some filler (e.g. polymer or inorganic nanoparticles such as advanced carbons or metal oxides) into proton-conducting channels of Nafion. Depending on the fillers nature it can enhance mechanical strength of the membrane, hinder methanol permeability (thus increasing the membrane selectivity) or increase water uptake of the membrane at elevated temperatures. Taking into account that many silicon oxide or metal precursors are soluble in scCO2, one may conclude that it would be beneficial to use this solvent to prepare such Nafion-based composites. In 2012 Bozbag & Erkey [45] presented a complete review on material preparation for fuel cell applications using scCO2 with a part focusing on membranes development. Preparation of porous polymeric matrices impregnated with Nafion [118], [119], Pd-impregnated Nafion membranes [120] as well as grafted [121], [122] and structurally modified [123], [124] membranes were also thoroughly reviewed by Bozbag & Erkey. We present here an updated look on the matter, structuring this part of the review according to several basic modifications that can be done using scCO2 for relevant materials. 4.1.Preparation of porous polymeric matrices There are two basic methods of preparing porous polymeric matrices using supercritical fluids. The first one consists of exposing a non-soluble polymeric membrane to scCO2, which causes swelling of the polymer and then formation of pores after supercritical medium leaves the membrane via decompression of a high-pressure vessel. This technique is explored vastly, there are numerous papers and several reviews on the matter [125], [126]. Although there are some evidence that open pore structures can indeed be formed in that way [127], the method generally yields microcellular morphologies, which are not well suited for electrochemical applications. The second method involves preparing a blend of two or more polymers with at least one that is soluble in sc media and at least one that is insoluble. After membrane is casted from such a blend, the soluble polymer can be dissolved in a sc medium and thus removed from

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the matrix leaving the polymeric membrane with well-percolated open-pore morphology. If subsequently filled with continuous ion-conducting phase, either liquid or solid, such a matrix should provide an efficient separator for electrochemical applications [118], [119]. In Refs. [118] and [119] authors filled polymeric matrices formed using scCO2 with Nafion as an ionic conductor. The resulting composite membranes were thinner than pristine Nafion and with higher mechanical strength, although with somewhat lower proton conductivity. 4.2.Synthesis of ionomers in porous polymeric matrices Cho et al. [128] presented a rather peculiar work where pore-filling technique to prepare ion-conductive membrane was implemented without using Nafion as an ion-conductor. Instead, porous polyethylene was used as a backbone and scCO2 was chosen as a reaction medium (Fig. 12). Further, styrene and divinylbenzene comonomers were copolymerized in the pores of the matrix in supercritical carbon dioxide with subsequent sulfonation. Thus, crosslinked resin of polystyrene sulfonate was formed in the porous PE matrix with uniform composite structure. After polystyrene was sulfonated, this grafted membrane was further modified by means of repetitive alternating deposition of poly(vinylimidazole) and poly(2acrylamido-2-methyl-1-propanesulfonic acid) on its surface. Thus, a layer-by-layer assembly was formed using acid-base complex with base polymer. The purpose was to compact porefilling structure as well as to control the hydrophilicity. The authors expected that the composite should combine decreased methanol permeability with high proton conductivity, having more optimal balance of those properties as compared with reference Nafion 115 membrane. Indeed, proton conductivity and methanol permeability were then measured for the resulted ion-conductive membranes. Membranes with 2- to 4-layered structures demonstrated maximum of proton conductivity (~ 0.1 S/cm), which was higher than that of reference Nafion 115. Methanol permeability decreased with the increase of the number of layers, but was lower than that of Nafion 115 for all the composite pore-filled membranes. Thus, improved selectivity was achieved. Moreover, using PE as a porous matrix allowed authors to create mechanically stable membranes with the thickness varied from 34 to 43 µm. In Ref. [122] copolymerization of styrene and divinylbenzene was performed in poly(vinylidene fluoride) matrix in supercritical carbon dioxide. Indeed, PVDF is known to swell significantly in scCO2 medium thus providing increased free volume available for impregnation with the comonomers to be copolymerized. The copolymerization with subsequent sulfonation resulted in formation of interpenetrating polymer network of cross-linked resin of polystyrene sulfonate in the PVDF matrix. The degree of cross-linking was controlled by the relative amount of the DVB comonomer applied. The increase of the DVB amount resulted in decreased proton conductivity of the interpolymer composites but also in reduced methanol permeability. Therefore, optimization of the balance between suppression of reagent cross-over and tending to increase ohmic losses may be performed for operational direct methanol fuel cells by means of proper selection of DVB amount. 4.3.Incorporation of inorganic particles into Nafion There are two strategies currently being explored for decreasing Nafions methanol permeability by impregnating the membrane with inorganic nanoparticles using supercritical medium. Erkey et al. [129] were the first to suggest Pd impregnation into Nafion membrane via supercritical approach. It is known that Pd nanoparticles, while somewhat promoting proton conductivity through the formation of Pd hydrated state, still suppress the methanol

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permeability in the membrane. Indeed, they act as scavengers for fuel and oxidizer molecules in the membrane bulk converting them into water. Therefore, incorporation of precious metal nanoparticles into Nafion leads to self-humidifying membranes with increased operating temperatures [130]. Erkey et al. [129] prepared a Pd-Nafion composite by impregnating of Pd precursor inside Nafion membrane from solution in scCO2 with the subsequent reduction of the precursor using hydrogen. TEM images of the resulted composite revealed a rather high uniformity of the Pd nanoparticles distribution inside the film, which indeed lead to the methanol permeability decrease with the increase of DMFC operation performance. Kim et al. [120] have used a different reduction route with NaBH4 as a reduction agent. They varied NaBH4 concentration and found out that the increase of concentration of the reducing agent leads to formation of larger Pd nanoparticles thus hindering proton conductivity. The amount of 0.5 mM of NaBH4 as being an optimal reducing agent concentration was reported. Iwai et al. [131] further investigated the role of Pd-containing ligand on the properties of Pd/Nafion composites. Varying the chemical nature of the ligands, they showed that increase of precursor concentration does not automatically lead to nanoparticles size growth. However, Iwai et al. [131] confirmed that the excessive particles size increase always deteriorates proton conductivity. They also demonstrated that the smallest size and the most uniform distribution of Pd nanoparticles is to be achieved when using a perfluorinated Pd precursor. Second method to decrease methanol permeability of Nafion consists of filling its hydrophilic pores with pristine or surface-modified SiO2 nanoparticles. However, if a traditional liquid-solvent technique is used to incorporate solid nanoparticles into a membrane, proton conductivity deteriorates along with methanol permeability. It was proposed by Su et al. [132], [133] to use scCO2 as a medium for such modification. They demonstrated that supercritical solution allows to distribute uniformly surface-modified SiO2 nanoparticles, not only in hydrophilic regions, but throughout perfluorinated domains of Nafion. That leads to lower methanol permeability, achieving even a slight increase of proton conductivity, thus improving both conductivity and selectivity of the membrane. 4.4.Restructurization of Nafion In recent years a new and promising approach to decrease methanol permeability of perfluorsulfonic membranes is being developed: it is based on an increase of a Nafion crystallization degree, which is to be achieved by scCO2 treatment of the membrane [123], [124]. The idea is to rearrange Nafion hydrophobic and hydrophilic segments thus making membranes morphology more equilibrium, with increased membrane crystallinity and, at the same time, reduced size of hydrophilic clusters. Since no solid filler is used, the process does not deteriorate proton conductivity. Ayazo et al. [134] have demonstrated that using small polar co-solvents, such as acetonitrile or isopropanol, further decrease of methanol permeability is achieved, while not affecting conductivity. Cai et al. [135] and Li et al. [136] used this technique on commercially available Nafion 212 membranes with subsequent characterization of the modified membranes, including single-cell operation. Using wide range X-ray diffraction data, they demonstrated that the crystallinity of an H-form of Nafion 212 indeed increases from 14.5% for untreated membrane up to 17.3% for the membrane treated with scCO2 at 160 oC. The crystallinity increase after treatment was found to be directly proportional to the temperature, yet Nafion 212 membrane was found to be thermomechanically unstable in scCO2 at temperatures above 160oC. Subsequently to the increase of crystallinity, methanol permeability decreased significantly, i.e. from 2.49 to 1.29×10−6 cm2/s after treatment. Furthermore, while freezable water uptake decreased significantly after treatment, the non-freezable water-uptake was found to be

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increased for both H- and Na-form of Nafion 212. Giving that it is the non-freezable water that partakes in the proton transport, this observation can explain the increased proton conductivity. The increase of non-freezable water uptake can also find its application in increasing operating temperatures of Nafion membranes in both DMFC and PEMFC. Single-cell tests performed in work [135] decisively proved that the resulted modified Nafion 212 has a better efficiency in DMFC than both untreated Nafion 212 and also commercial Nafion 117 as well (Fig. 13). Guerrero-Gutierrez et al. [137] investigated modification of Nafion structure in scCO2 along with counter-ion substitution (Fig. 14). Their data on scCO2-treated pristine Nafion show increase of proton conductivity with decrease in methanol permeability and are in agreement with the previous works on the matter. Ion-exchanged membranes exhibit decrease of both methanol permeability and conductivity. Noting that in some cases (such as for Fe 3+ exchanged membrane) the methanol permeability decrease is an order of magnitude higher than the decrease in conductivity, the authors conclude that combination of ion-exchange and scCO2 treatment of Nafion is a promising method for preparing highly selective membranes. a)Another study on exposing already modified Nafion to scCO2 was performed by Zhu et al. [138]. They propose to incorporate terephthalic acid that acts as a free radical scavenger in Nafion prior to sc-treatment. The authors demonstrate that when small amounts of terephthalic acid are used (0.5–1 wt. %), the resulted membrane, while remains more selective than pristine Nafion, also gains in chemical stability. By means of changing the order of modifications – ion-substitution and sc-treatment – the authors demonstrated that these changes do not affect modified membranes properties. It was demonstrated also by Ai et al. [139] that structural changes of Nafion after scCO2 treatment lead to the reduction of mineralization driven cracking in membrane. Therefore, modified Nafion can be used as a durable membrane in biological environment, for example in implantable bio-sensors. 4.5.Intermediate conclusions Most of the work on supercritical fluids implementation in polymeric membranes for electrochemical power sources is focused on developing Nafion-based composites with lower methanol permeability. Rather promising results were achieved. Among the approaches that have been developed in this area – such as Nafion impregnation with SiO2 nanoparticles or with Pd nanoparticles – the most exiting technique seems to be the modification of Nafion structure by a simple exposure of the membrane to pure supercritical carbon dioxide. This process does not deteriorate proton conductivity, since no filler is used, it is inexpensive and is to be done in a single step. The fact that Nafion re-crystallization can be performed with ion-exchanged Nafion opens a great number of possible modifications. And yet, the scientific data on the dependence of Nafion properties changes on the conditions of CO2 exposure is far from being complete. It seems that a systematical research on determining how the pressure and temperature of the supercritical medium influence the achievable properties of a membrane after an exposure/modification can be useful for further development of this advanced method. Preparation of polymeric matrices with open-pore structure for electrochemical applications using scCO2 is another research area that is being explored. Unlike the case with Pd-impregnated Nafion or Nafion restructurization, no systematical research on this matter has been done yet. However, the results show that it is a promising technique to prepare chemically and mechanically stable separators, suitable for electrochemical devices, where well-percolated structures are generally demanded.

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5. Concluding remarks The analyzed literature clearly indicates that the usage of supercritical fluids in routine laboratory processing of polymer materials indeed enriches capabilities and research potential of the modern electrochemistry. Synthesis of electroactive electronically conductive polymers in the presence of supercritical fluids provides a convenient and powerful tool to tune up desirable morphology of the polymer product. Indeed, well-defined structures of controllable shapes and sizes as required for different types of electrochemical devices were obtained in this manner. The possibility to perform an electrochemical synthesis directly in a high-pressure vessel seems to be even more intriguing and exciting as far as any matrices with rather complex morphologies may be thus applied as a starting seeding backbone for obtaining advanced composites. Concerning processing of ion-conductive polymers, one should mention impressive and simple procedure of Nafion restructuring by means of direct treatment in pure sc fluids. Being further combined with introduction of additional agents promoting ion conductivity or selectivity, this option should result in obtaining promising composites with optimal balance of the main functional properties. The pathways to form well-percolated straight channels for selective transport of a certain type of charge carriers were also revealed. Yet, many challenges are still to be faced on the way towards eventual establishing of sc processes as a reliable tool of an electrochemical laboratory providing predictable and reproducible results. Acknowledgements This work was supported by the Russian Science Foundation (grant number [16-13-10338]). References [1] [2]

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Fig. 1. Cloud-point pressures at 5 wt% polymer concentration and 298 K for binary mixtures of CO2 with poly(methyl acrylate) (PMA), poly(lactide) (PLA), poly(vinyl acetate) (PVAc), poly(dimethyl siloxane) (PDMS), and poly(fluoroalkyl acrylate) (PFA) as a function of a number of repeat units based on Mw. Reprinted with permission from Ref. [49], copyright 2003, Elsevier.



Fig. 2. The chemical structures of typical conducting polymers: polyacetylene, polyaniline, polypyrrole, polythiophene, polyethylenedioxythiophene, polyphenylenevinylene.

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Fig. 3. The scheme of pyrrole polymerization mechanism. Reprinted with permission from Ref. [56], copyright 2014, Royal Society of Chemistry.



Fig. 4. The desired structure of an electrode-electrolyte interphase for an efficient electrochemical energy storage device.

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Fig. 5. Scanning electron micrographs of polypyrroles prepared using FeCl3 in (a) water at 25 °C and (b) scCO2 at 80 °C. Reprinted with permission from Ref. [63], copyright 1997, Royal Society of Chemistry.

Fig. 6. Scanning electron micrograph of PPy grown on ITO in scCO2 at 1400 psi, 50 °C, 0.16 M pyrrole/0.16 M TBAPF6/13.1 vol % acetonitrile (left) and in 0.1 pyrrole/1 M TBAPF6/acetonitrile (right). Reprinted with permission from Ref. [80], copyright 2002, American Chemical Society.

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Fig. 7. Cyclic voltammogramms measured in 2M aqueous NaCl solution at various scan rates for nanofibrillated cellulose and PPy composites dried in air (Comp_Air) and in scCO2 at 100 bar and 36 C (Comp_CO2). Reprinted with permission from Ref. [96], copyright 2012, Royal Society of Chemistry.

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Fig. 8. Electrochemical performance of GO/PANI nanocomposite prepared at 0.1 M concentration of aniline. (a) CV curves of GO, pure PANI, and GO/PANI nanocomposite at 10 mV s -1 in 1 M H2SO4 solution. (b) CV curves of GO/PANI nanocomposite at different scan rates of 5, 10, 20, 50, and 100 mV s-1. (c) Galvanostatic charge-discharge curves of pure PANI and GO/PANI nanocomposite at current density of 200 mA g-1. (d) Specific capacitance of pure PANI and GO/PANI nanocomposite at different current densities. Reprinted with permission from Ref. [104], copyright 2012, American Chemical Society.

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Fig. 9. Scheme of the preparation of graphene/Py foam filled with PANI-covered nanotubes. Reprinted with permission from Ref. [107], copyright 2015, Elsevier.

Fig. 10. SEM images of cylindrical-shaped PPy electrodeposited at 1.4 V vs. Ag wire for 40 min in (a) an acetonitrile solution and (b) scCHF3. Reprinted with permission from Ref. [110], copyright 2012, Elsevier Ltd.



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Fig. 11. Chemical structure of Nafion and its analogues. For Nafion k = 1, m ~ 6.5 (for EW = 1100 g/mol), 100 < n < 1000 [112].

Fig. 12. Schematic representation of the multi-layer proton conducting membrane in the pore of PE/PSS film. Reprinted with permission from Ref. [128], copyright 2006, Elsevier B.V.

Fig. 13. Polarization curves for the MEAs fabricated with different membranes at (a) 25 °C and (b) 60 °C (operation conditions: 1 M methanol solution and pure O2). Reprinted with permission from Ref. [135], copyright 2011, Elsevier B.V.

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b) Fig. 14. a) Methanol permeability for pristine Nafion and Nafion processed with scCO2 and then cation-exchanged, b) proton conductivity for the same membranes. Reprinted with permission from Ref. [137], copyright 2012, Wiley Periodicals, Inc.

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Table 1 Parameters of chemical synthesis of conducting polymers in supercritical media, morphology of the product and electronic conductivity. Polymer

Media

Oxidant

T, oC

P, bar

Yield, %

PPy

scCO2

FeCl3

80

150

63

PPy

scCO2

Fe(CF3SO3)3

80

150

87

PPy

scCHF3 scCO2/1 M HCl + 0.25 M SDS in H2O Liquid CO2/1 M HCl in H2O scCO2/HCl in H2O scCO2 scCO2 scCO2 scCO2 scCO2 scCO2 scCO2 scCO2 scCO2

FeCl3

90

48

57

APS

40

85

–

APS

room temp.

139

63

APS

35

100

80

APS BPO FeCl3 FeCl3 FeCl3 FeCl3 FeCl3 MoCl5 MoCl5

70 35 90 90 40 40 40 40-60 40-60

350 160 83 83 207 207 200 250-345 250-345

– – 39 57 65 60 70-73 50-65 49-61

PANI PANI PANI PANI PANI PPy PTs PFTE PSFTE PFTE poly(TFODPA) poly(DFHDPA)

Product morphology fibers, d ~100200 nm fibers, d ~100200 nm – hollow microtubes, d~120 nm fibers, d ~ 30-70 nm fibers, d ~ 60-80 nm – – – – – – – – –

σ, S cm-1

Ref.

510-2

[63]

210-2

[63]

110-3

[63]

0.36

[64]

4.3

[65]

2.5

[66]

– – 2.510-5 710-9 410-9 310-9 – – –

[67] [68] [69] [69] [73] [73] [75,76] [77] [77]

36