ReviewPolymer materials for electrochemical applications: Processing in supercritical fluids
Graphical abstract
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. 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. 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].
Section snippets
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
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], [52], [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
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
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
Acknowledgement
This work was supported by the Russian Science Foundation (grant number [16-13-10338]).
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