Chapter 1

Introduction to Redox Polymers: Classification, Characterization Methods and Main Applications

Nerea Casado*a and David Mecerreyes*ab
a University of the Basque Country UPV/EHU, Joxe Mari Korta Center, 20018 Donostia-San Sebastian, Spain. E-mail: Nerea.casado@ehu.eus, david.mecerreyes@ehu.eus
b Ikerbasque, Basque Foundation for Science, 48011 Bilbao, Spain.


In this chapter the field of redox polymers and the main contributions of the following chapters are introduced. First, we present the definitions and classification of redox polymers. The type of redox polymer will depend on the location of the redox center, whether it is located in the polymer backbone or as a pendant group, on the conjugation of the polymer backbone and the chemical nature of the redox center. Next, the main characterization methods used for assessing the properties of redox polymers such as cyclic voltammetry or electrochemical impedance spectroscopy will be discussed. The main applications of redox polymers in energy will be discussed focusing mostly on the use of redox polymers in different battery technologies as well as emerging technologies such as biofuel cells or thermoelectric cells. Finally, the emerging applications of redox polymers in medicine, in new technologies such as tissue engineering, drug delivery, actuators or bioelectronics will be presented.


1.1 Introduction

Redox polymers are those polymers that can undergo reversible oxidation (loss of electrons) and reduction (gain of electrons) processes, as defined by the International Union of Pure and Applied Chemistry (IUPAC). They contain electroactive sites or groups that sustain these redox processes, which can be located in the main polymer backbone, as in the case of electrically conducting polymers such as polypyrrole,1 or in the polymer's side-chain, as in the case of a polymer bearing a ferrocene group.2

The redox reaction implies the variation of the oxidation state of the polymer, which provokes changes in the properties of the polymeric material. Therefore, depending on the oxidation state (oxidized or reduced), the polymer may exhibit different chemical, optical, electronic or mechanical properties. The reversibility and easy external control of redox processes have made redox polymers interesting for different applications and for the development of new electrochemical devices such as organic batteries, electrochromic devices, optoelectronic devices, biosensors or biofuel cells.3,4

Moreover, the applications of redox polymers in medicine have increased in the last 20 years, for example, on the development of new types of actuators and drug-delivery systems.5,6 As already mentioned, redox polymers are also interesting for electrochromic and electroluminescence devices and organic solar cells, due to their optoelectronic properties.7,8 However, the developments of optoelectronic polymers are not reviewed in this book, as they are commonly known as conducting, conjugated, electroluminescent or electrochromic polymers, rather than as redox polymers.

Thus, this book highlights current trends in the chemistry, characterization and application of redox polymers. In this chapter, we will introduce the topic by showing an overview of the different types of redox polymers, state-of-the-art characterization techniques and their applications. In this last part, the role of redox polymers in energy devices, such as batteries, supercapacitors, solar cells, biofuel cells, as well as in medical applications such as tissue engineering, drug delivery, actuators and bioelectronic devices, will be highlighted.

1.2 Classification of Redox Polymers

Redox polymers became popular in the early 1980s as a new class of electroactive polymer. The first examples were based on conducting polymer backbones such as polyacetylene1 and polymers containing ferrocene groups. Nowadays, the redox polymer portfolio is formed by a wide variety of chemical structures, including various conjugated polymer backbones and/or electroactive moieties which can be organic or inorganic/organometallic species. Historically, redox polymers have been classified mainly using the following criteria: Location of the redox center: in the polymer backbone or as a pendant group, Nature of polymer backbone: conjugated/semiconducting or nonconjugated, Nature of redox center: organic or inorganic.

These classifications always include exceptions and hybrid forms due to the large variety of redox polymers that have been synthesized over the years.

In this chapter, we will show first examples of organic redox polymers (formed by C, H, N, O and S), where we distinguish between semiconducting polymers and nonconjugated redox polymers. Next, we describe redox polymers having organometallic or alternative atoms in their redox moiety. Then, we will explain state-of-the-art characterization techniques of redox polymers and their most important applications in energy and medicine (Figure 1.1).

Fig. 1.1 Oxidation and reduction processes of representative organic and inorganic moieties included in redox polymers.

1.2.1 Organic Redox Polymers

1.2.1.1 Conjugated/Semiconducting Polymers

Conjugated polymers (CPs) are composed of fully conjugated sequences of double bonds along the polymeric backbone. The extended conjugation and the additional doping provoke that conjugated polymers may exhibit bulk electric conductivity. Since the discovery of conductive polyacetylene by Shirakawa and co-workers in 1977,9 conducting polymers have been widely investigated due to their interesting and tunable properties. In general, conducting polymers possess high electrical conductivity, easy processability, flexibility, low weight, low cost and the possibility of large-scale production.10–12 Therefore, CPs have been employed in a wide range of applications including actuators, organic light-emitting diodes, batteries, supercapacitors, biosensors and drug-delivery systems.10–15

The most studied conducting polymers due to their high conductivity and easy synthesis are polypyrrole (Ppy), polyaniline (PANI), polythiophene (PT) and poly(3,4-ethylenedioxythiophene) (PEDOT). Among them, PEDOT is nowadays the most popular one due to its thermal and chemical stability, electro(chromic) properties and transparency, which are some of the reasons behind the success of its commercialization. Conducting polymers require partial oxidation or reduction processes to give rise to charged species, and thus, they can be considered as redox polymers. The generated charges are compensated by the appropriate counter ions, named dopants, to maintain the electroneutrality of the polymer.16 The partial oxidation and reduction are known as p-doping and n-doping, respectively.17 As a result of the doping process, charged defects such as polarons and bipolarons are created in the polymer chains, which are responsible for the electron conduction.

1.2.1.2 Nonconjugated Polymers

Organic redox-active groups, such as nitroxides, quinones, phenothiazines or viologens, have been incorporated in polymer structures either as pendant groups or in the main polymeric backbone. The synthesis of new redox polymers and copolymers has been actively pursued in recent years for their application in batteries, biofuel cells, sensors and drug-delivery systems, among others.

Nitroxide radicals are one family of redox-active organic groups. Nishide and co-workers pioneered the synthesis of polymers containing nitroxide radicals such as 2,2,6,6,-tetramethyl-1-piperidinyloxy (TEMPO) (see Figure 1.2, polymer 1).18,19 This type of radical is very stable and possesses fast electron transfer kinetics, thus they have been of great interest for organic batteries and supercapacitors.20–22 Block copolymers, containing nitroxide radicals in one of the blocks, have been developed to tune the properties and obtain responsive polymers,23 organic cathodes24 and polymers with resistive memory properties.25 The TEMPO group has also been incorporated as a pendant group in conjugated polymers, such as polythiophene26 and PEDOT (polymer 4).27 These polymers showed a synergetic effect between the redox properties of the conjugated backbone and the stable nitroxide radical, which are of great interest for battery applications.

Fig. 1.2 Chemical structures of redox polymers containing electroactive organic moieties: as a pendant group (A, B) or in the polymer backbone (C, D), which can have at the same time conjugated (B, D) or aliphatic (nonconjugated) (A, C) backbones.

Other types of stable radicals such as phenoxyl and verdazyl have been also incorporated in redox polymers with tuneable properties.28 Phenoxyl radical polymers have found applications in energy storage.29 For example, Schubert and co-workers synthesized phenoxyl-containing polymers with norbornene and methacrylate backbones30 (polymer 2), which were applied as anode-active material in organic batteries due to the low redox potential. In a similar way, Price et al. reported the synthesis of 6-oxoverdazyl polymers with poly(phenyl methacrylate) backbone.31

Carbonyl-containing polymers are another relevant redox polymer family.32 Carbonyls represent a common organic structural group, which can be found in a variety of redox moieties, such as anthraquinone, quinone, imide or anhydrides. Some examples in this area include polymers with anthraquinone derivatives33 (polymer 5) or polyimides with naphthalene (polymer 6) or pyromellitic compounds.34 Redox-active polyimides have been widely investigated as electrode materials for lithium,34,35 sodium,36 lithium-sulfur37 and all-organic batteries,38 as well as for applications in aerospace and electronics.39 Quinone-containing polymers are very attractive for energy storage applications due to their high charge-storage capacity. Moreover, quinones are the principal redox center in natural organic materials, which makes them interesting in terms of sustainability.40 Some examples include hybrid materials between conducting polymers, such as polypyrrole or PEDOT, and biopolymers like ligninsulfonates as electrode materials for batteries and supercapacitors.41–43 Another interesting example is catechol-containing polymers,44 which were applied as organic cathodes for lithium batteries,45 while their ability to host various cations (H+, Li+, K+, Zn2+, Mg2+, Ca2+ and Al3+) in aqueous batteries has been recently demonstrated.46 This kind of bioinspired catechol polymer has also been synthesized as nanoparticles, which could be used in a variety of battery technologies.47

Other organic functional groups such as viologen, triphenylamine and phenothiazine are known to possess redox properties. Viologen and triphenylamine moieties have been incorporated into different polymer backbones to obtain linear, cross-linked or porous polymers.48,49 Polymers with viologen moieties (polymer 7) combine redox and electrochromic properties, which make them interesting for several fields, including electrochromism, energy storage, gas storage and separation, and biochemistry.50 Polymers with carbazole and phenazine units (polymer 9) and its derivatives (phenothiazine, thianthrene) have been used for energy storage,51 photovoltaic52 and optoelectronic devices.53 The application of phenothiazine-containing redox polymers (polymer 10) in energy storage devices has increased in the last decade due to their fast redox process and high redox potential (3.5 V vs. Li/Li+). Thus, they have been employed not only as electrode materials in Li/organic,54,55 redox-flow56 and all-organic57,58 batteries but also as redox mediators in Li/O2 batteries.59

The last family of organic redox polymers consists of organosulfur compounds. Polymers with disulfide bonds and polysulfide moieties (polymer 8) have been investigated as electrode materials in batteries.60–62 Specially, copolymers with high sulfur content synthesized by inverse vulcanization are interesting cathode materials to replace elemental sulfur in lithium/sulfur (Li/S) batteries with good cycling stability.63 Disulfide bonds are also very interesting for drug-delivery applications, as they degrade under physiological reducing conditions.6 Tetrathiafulvalene (TTF) is an important redox moiety with disulfide linkages, which can undergo two reversible redox processes to radical cation (TTF+) and dication (TTF2+). TTF-containing polymers (polymer 3) have been widely studied due to their interesting charge-transfer properties for organic electronics.64

1.2.2 Inorganic Redox Polymers

Inorganic redox moieties have also been incorporated into polymer materials either in the backbone or as pendant groups. Ferrocene is the gold-standard organometallic moiety due to its redox stability and reversibility. The ferrocene group has been integrated into several polymer backbones, from the simplest poly(vinylferrocene)60 (Figure 1.3, polymer 11) to block copolymers with ferrocene in one of the blocks. In this manner, functional polymers with ferrocene units have been developed with interesting mechanical robustness, photo-physical, optoelectronic properties and stimuli-responsive properties.61,65 Poly(vinylferrocene), for example, has been copolymerized with poly(methyl methacrylate) to form nanocapsules,63 or with poly(ethylene oxide) to obtain water-soluble star polymers62 for biomedical applications. Poly(ferrocenylsilane) polymers (polymer 15) are the most important polymers where the ferrocene group is located in the polymer backbone.66 As an example, poly(ferrocenylsilane)-based gels and hydrogels were synthesized as redox-responsive materials.64

Fig. 1.3 Chemical structures of redox polymers containing electroactive inorganic moieties.

In contrast to ferrocene polymers, cobaltocene-containing polymers have been much less developed, probably due to the difficulty of preparing substituted derivatives. Cobaltocenium-containing polymers (polymer 12) have been used in electrochemistry, catalysis and biosensing.67,68

Conjugated polymers comprising inorganic elements in their structure such as selenium, tellurium or phosphorous have been investigated to modify their redox and optoelectronic properties. In the last decade, polyselenophene conjugated polymers and copolymers (polymer 13) has been the most studied ones owing to their optical properties and their potential use in low-cost electronic devices.69,70 As an example, Hollinger et al. synthesized selenophene–thiophene block copolymers, which phase-separate and exhibit interesting absorbance features for optoelectronic uses.70 Moreover, the development of solution-processable poly(3-alkyltellurophene) polymers (polymer 14) proved their potential application for optoelectronic applications.71 On the other hand, phosphole-containing polymers allow post-functionalization in the phosphorous center, as they are able to tune the electronic properties. Phosphole-containing polymers have been applied in optoelectronic and sensing devices.72,73

Selenium is a nonmetal located in the chalcogen group of the periodic table between sulfur and tellurium elements. It possesses many similarities to the elements of its group, however, the bond energy of diselenide bond is smaller than the disulfide (Se–Se 172 kJ mol−1vs. S–S 226 kJ mol−1). Therefore, selenium-containing polymers are promising materials as redox-responsive drug carriers.74 As an example, a polyurethane triblock copolymer with diselenide bonds was reported by Xu and Zhang's group (polymer 16). This block copolymer forms multiresponsive micellar aggregates, which are responsive to both oxidant and reductant conditions and suitable for controlled drug-delivery systems.75

Polymers with transition metals, such as zinc, copper, platinum and iridium, have been widely developed due to their intrinsic properties including redox, catalytic, magnetic, light absorption and emission properties (polymer 17).75–77 Depending on the interaction of the metal with the polymeric chain and the nature of the chain, whether conjugated or not, the electronic and electrochemical properties may vary.78,79 In this family, zinc-containing polymers have been the most studied ones due to the coordination ability of Zn(ii) to develop a wide variety of structures.74 Moreover, platinum-, osmium- and palladium-containing polymers have been also investigated in biomedical applications such as biosensors and drug delivery (polymer 18).80–82

1.3 Characterization of Redox Polymers

Apart from the conventional polymer characterization techniques for molecular weight distribution, glass transition, crystallization, physicochemical, thermal and mechanical properties, redox-active polymers are commonly analyzed by electrochemical techniques. Cyclic voltammetry (CV) is the most widespread characterization method, together with electrochemical impedance spectroscopy (EIS). Moreover, CV can be combined simultaneously with additional techniques such as UV–Vis–NIR, infrared spectroscopy, photoemission, electrochemical quartz microbalance (EQCM) and atomic force microscopy to study the changes taking place in the polymer during the redox processes. As electrochemical characterizations are very sensitive, besides the intrinsic factors defined by the polymer nature, external factors related to the sample preparation and electrochemical technique have to be taken into account during the characterization of redox polymers. The most important external factors affecting the electrochemical characterization are: sample preparation, dimensions of the sample (thickness, weight and area), kinetics of the redox process, nature of the electrolyte, type of set-up, temperature and formulation of the electrode when binders and conductive additive are included.

1.3.1 Cyclic Voltammetry (CV)

Cyclic voltammetry is a powerful and popular electrochemical method to study the oxidation and reduction processes of redox polymers. It provides information about the redox behavior of the material at different potentials and is a fast and reliable characterization technique.83 In this experiment, a potential is applied to the working electrode (E), which is swept at a certain rate (scan rate) and the resulting current (i) is measured. The obtained trace (i vs. E) is called a voltammogram or cyclic voltammogram, which is dependent on the type of process occurring at the electrode.

Redox polymers can present different voltammogram shapes depending on the redox behavior (see Figure 1.4). Conducting polymers present typically two types of currents: the faradaic current caused by the redox reaction and the capacitive current resulting from the electrical double layer generated on the surface of the electrode.3 These two current responses can be observed in the voltammogram trace of Figure 1.4a. Redox polymers with localized redox moieties, either in the backbone or as a pendant group, show only the faradaic current. The maximum current peak corresponds to the oxidation potential (Eox), while the minimum current peak to the reduction potential (Ered) is shown in Figure 1.4b. The redox potential is usually determined by the average potential (E1/2) between the oxidation and reduction potentials, and is specific to the experimental conditions [E1/2E=(Eox+Ered)/2], as the two peaks are separated due to the diffusion of the analyte to and from the electrode surface. The separation of oxidation and reduction peaks reveals the reversibility of the redox reaction. Reversible redox reactions present small ΔE values (ΔE<57 mV),84 while in quasi-reversible processes ΔE increases showing two separated and less sharp peaks in the cyclic voltammogram (Figure 1.4c). Moreover, if the process is completely irreversible, only one peak, oxidation or reduction, will be present in the cyclic voltammogram. Figure 1.4d shows a typical trace for a polymer with irreversible oxidation process.

Fig. 1.4 Cyclic voltammograms of (a) conducting polymer, (b) diffusion-controlled reversible redox process, (c) quasi-reversible process and (d) irreversible process.

The current response of redox polymers can be limited by two different processes. The first limiting process is related to the electron transfer kinetics between the electrode and the redox species, while the second one is due to the diffusion of the redox species to and from the electrode. Thus, diffusion is negligible for thin-layer electrodes, as the electroactive polymer deposited on the electrode surface is reacted very rapidly. On the contrary, electron transfer kinetics are not important for diluted solutions as the redox process will be affected only by the motion of the redox species. Most of redox polymers present an intermediate behavior between these two cases, which is called “finite diffusion.”3

The determination of the limiting process is usually carried out by cyclic voltammetry experiments at different scan rates. The scan rate determines how fast the applied potential is changed. Faster scan rates result in the decrease of the diffusion layer size, generating higher current responses. When the redox polymer is deposited on the electrode, a linear variation of the peak current with the scan rate shows that the process is not limited by diffusion. The current response is described by the following equation: where ip is the peak current, n is the number of electrons, F is the Faraday constant, v is the scan rate, A is the electrode surface area, Γ is the amount of redox-active material deposited in the electrode surface, R is the gas constant and T is the temperature in Kelvin.

When the redox polymer is dissolved in the electrolyte and involves freely diffusing redox species, the Randles–Sevcik equation describes how the peak current increases linearly with the square root of the scan rate. where C is the bulk concentration of the redox-active species and D is the diffusion coefficient of the redox-active species.83

Moreover, the cyclic voltammetry can be used to obtain additional information. For example, if the relationship between the oxidation and reduction current peaks is equal to one, the process is reversible, meaning that all the oxidized species are reduced back. The charge related to the oxidation and reduction processes can be quantified by measuring the area under the curve and dividing it by the scan rate. Additionally, by dividing this charge by the mass of the redox-active species, the specific capacity of the polymer is obtained, while the capacitance is the relationship between the current and the scan rate.85

Cyclic voltammetry characterization can be done using a rotating disc electrode (RDE) hydrodynamic working electrode to analyze the mechanism and kinetics of the redox processes. When the RDE reaches a steady state, a constant laminar flow of the electrolyte solution is created at the electrode surface. In this way, the mass transport of the redox species is not a limiting factor and the kinetics of the redox reaction can be calculated. This technique has been employed for the determination of electrocatalytic behavior of redox polymers.86

In conclusion, cyclic voltammetry is a versatile and powerful electrochemical technique to analyze the redox potential of the electroactive polymers. This redox potential value is specific and crucial for each application of the redox polymers. Figure 1.5 shows the redox potential of most common electroactive moieties included in redox polymers, however, it is important to note that these values are dependent on the experimental conditions, such as the type of electrolyte, cell configuration or sample preparation. In energy storage applications like batteries, the redox potential of the polymer is important as it will determine its application, whether it is used as an anode or cathode material, together with the voltage of the device, which is given by the potential difference between the two electrodes. In supercapacitors, polymers with fast and wide redox processes are required to obtain high specific capacitances. On the other hand, for applications such as biosensors and biofuel cells, as the polymers are used as redox mediators, their redox potential should be similar to the enzyme or the reaction where they will be involved.

Fig. 1.5 Redox potentials of common electroactive groups included in redox polymers. *Potential values are taken from Li/Li+ reference electrode and changed to Ag/AgCl (Li/Li+ is −3.26 V vs. Ag/AgCl). Reproduced from ref. 3 with permission from Elsevier, Copyright 2016.

1.3.2 Electrochemical Impedance Spectroscopy (EIS)

Together with cyclic voltammetry, electrochemical impedance spectroscopy (EIS) is a powerful and complex technique to analyze the electrical characteristics of the polymer films. The electrochemical impedance is the response of a system (cell) to an applied potential. In this technique, the current is measured when a small-amplitude alternating current (AC) potential is applied to a cell in equilibrium. In order to have a pseudo-linear response, the excitation signal should be small (∼10 mV). In this way, when the sinusoidal potential is applied, the current response at the same frequency is a phase-shifted sinusoid. The frequency dependence of this impedance can reveal complex chemical processes. EIS also provides information about the rate of charge transfer and transport processes.3

The response of the system is usually represented in Nyquist plots, where the imaginary impedance is plotted against the real impedance. The variation of the impedance with the frequency is characteristic of an electrical circuit. Therefore, an equivalent electrical circuit model, which describes the system, is generally used to interpret the obtained EIS data. Parameters such as the charge transfer resistance, Warburg diffusion coefficient or the capacitance can be obtained from the interpretation of the equivalent circuit. It is worth mentioning that each electrochemical system should be modeled with the best appropriate equivalent circuit. For example, a simple electrode reaction can be described by the Randle equivalent circuit, containing the solution resistance (Rs), charge transfer resistance (Rct), Warburg impedance (Zw) and double-layer capacitance (Cdl).87 The main advantage of EIS is that both the diffusion coefficient and the redox capacitance can be obtained from the same experiment, while multiple experiments are required to analyze these two values with cyclic voltammetry.

Another electrochemical technique used in the characterization of redox polymers is the chronocoulometry. This involves the measurement of charge as a function of time when a potential step waveform is applied. Chronocoulometry is used to determine the electrode surface area, diffusion coefficients, adsorption of electroactive species, and the kinetics of electron transfer and mechanisms of chemical reactions.3

1.3.3 Coupling of Electrochemical Methods with Additional Characterization Techniques

The coupling of electrochemical techniques with other characterization methods, such as UV–Vis spectroscopy, is a very interesting approach to get a better under understanding of the redox processes. These in situ analyses may also give information about the mechanism of the redox process. Next, the most widespread techniques that have been coupled with cyclic voltammetry will be explained. Optical absorption characterization. UV–Vis–NIR spectroscopy provides information about absorption bands and the optical band gap of redox polymers. The coupling of this technique with electrochemistry is named spectroelectrochemistry.88,89 It is also used to determine the stability of radical species,90 polaron and bipolaron formation,91 as well as color change during the redox reaction.92 Therefore, spectroelectrochemistry is mostly used for the analysis of electrochromic materials and the development of photovoltaic and electrochromic devices. As an example, the UV–Vis–NIR spectrum of a PEDOT:PSS sample in Figure 1.6a at different potentials denotes the absorbance changes, showing a polaron band at −0.2 V vs. Ag/AgCl and increasing π–π* transition band at higher potentials. Other spectroscopy techniques such as Raman and FTIR have also been coupled to electrochemistry to analyze the structural change in the polymer during the redox reaction. The appearance of new peaks, or the shifting or intensity changes in the peaks during the oxidation and reduction processes can provide information on the alteration in the bonding nature of the polymers (Figure 1.6b).89,93,94 Surface characterization techniques can be also coupled with electrochemistry. Thus, surface morphology of redox polymers can be analyzed by scanning electron microscopy (SEM),95 scanning tunneling microscopy (STM) or atomic force microscopy (AFM).96 As an example, Figure 1.6c shows AFM images for PEDOT at reduced and oxidized states. Electrochemical quartz crystal microbalance (EQCM) is a powerful in situ technique to complement electrochemical experiments. It is able to detect mass changes in the deposited film during electrochemical experiments. Thus, it is used to analyze processes involving mass changes such as adsorption, electrodeposition, polymerization or doping.97 For example, the in situ CV-EQCM spectrum of a poly(amine-imide) polymer is shown in Figure 1.6d. During its oxidation, the ions of the electrolyte are absorbed into the polymer film to compensate the ions formed and thus, increasing the total mass, while in the reduction the contrary process is observed, desorption of ions together with the decrease of the total mass. Electron spin resonance (ESR) spectroscopy is another interesting method to analyze the presence of polaron charge carriers, as they have spin, opposite to bipolaron charge carriers. This technique provides the concentration of polaronic species and charged species. Therefore, it is used to study the polymerization processes that occur via radical intermediates, the redox processes in radical polymers and doping levels in conducting polymers. For example, polyaniline and PEDOT conducting polymers have been intensely characterized by ESR spectroscopy.90

Fig. 1.6 Examples of characterization techniques coupled with electrochemical methods. (a) UV–Vis spectrum of PEDOT:PSS film; (b) Raman spectrum of polyaniline oxidation; (c) AFM images of reduced and oxidized states of PEDOT; and (d) EQCM graph of a poly(amine-imide). Reproduced from ref. 3 with permission from Elsevier, Copyright 2016.

1.4 Applications of Redox Polymers

The growing interest in redox polymers is due to their versatility and wide range of applications. The most important fields where redox polymers have been employed are batteries and biosensors. However, the search for more sustainable and environmentally friendly materials in the last decade has increased the application of redox polymers in other energy devices such as supercapacitors, solar cells, biofuel cells or thermoelectric devices. Moreover, redox polymers are finding new opportunities in the development of medical systems such as actuators, drug-delivery systems, tissue engineering and bioelectronic devices. In the following sections, we will review the most important advances in the application of redox polymers in energy and medicine technologies.

1.4.1 Applications in Energy Conversion and Storage Devices

Energy has become one of the key issues of this century due to the growing population and the technological development. For this reason, the development of sustainable and green energy technologies has become of great importance in our society. New clean technologies which are able to create electricity from the natural abundant energy from the sun, waves or wind as well as to store that electricity are actively being searched. Redox polymers are playing an important role in the development of some of these technologies as will be highlighted in this section.

The most popular use of redox polymers nowadays is in electrochemical energy storage devices such as batteries, supercapacitors or flow cells. This will be extensively discussed in several chapters throughout this book, and due to its importance, we will dedicate a special subsection to this next. However, there are other technologies which are still in their infancy that are expected to have rapid development in the coming years. This is the case of the biofuel cells, where naturally occurring sugars are converted into electricity through an enzymatic process similar to the electrochemical biosensors using a redox polymer as the key ingredient.87

Two important types of devices that make use of similar redox polymers are thermoelectric cells and solar cells.98,99 However, those devices that convert heat or light into electricity, respectively, are not discussed in this book. The reason behind this is that the polymers used in these cases, although they present redox type activity, are mostly known as (semi)conducting, semiconjugated or optoelectronic polymers (Figure 1.7).

Fig. 1.7 Examples of main applications of redox polymers in the energy field. (a) Batteries and supercapacitors. Adapted from ref. 22 with permission from the Royal Society of Chemistry. (b) Biofuel cells. Adapted from ref. 100https://doi.org/10.1038/s41467-018-07137-6, under the terms of the CC BY 4.0 licence, http://creativecommons.org/licenses/by/4.0/. (c) Thermoelectric cells. Adapted from ref. 98 with permission from the Royal Society of Chemistry. (d) Solar cells. Adapted from ref. 99 with permission from John Wiley and Sons, © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

1.4.1.1 Batteries and Supercapacitors

Redox polymers are actively being investigated in several electrochemical energy storage or battery technologies. Back in the 1980s there was a great deal of hype about the use of conducting polymers in batteries. However, the expectations did not became reality due to the stability issues of the batteries and supercapacitors based on most conducting polymers at that time. Nowadays there is renewed interest in the use of redox polymers in several emerging battery technologies, such as flexible thin batteries, fast-charging batteries, organic batteries, metal-air, redox-flow batteries or aqueous batteries. In these technologies, redox polymers can play a different role, for example, as active materials in the cathode or anode, as a redox-active binder or mediator, or as soluble catholyte or anolyte in aqueous redox flow batteries.

Through the chapters of this book, the authors will discuss the challenges and characteristics of the most important redox polymers in battery technologies. In other words, how the different polymers families such as radical polymers, phenothiazine-type polymers, carbonyl polymers or catechol polymers are used in emerging battery technologies.44,101–104 Chapter 8 is devoted to discussing the new battery configurations led by the use of redox polymers (Figure 1.8).

Fig. 1.8 Examples of the main types of redox polymers actually investigated in batteries and discussed in the chapters of this book: (a) Radical polymers. Adapted from ref. 101 with permission from John Wiley and Sons, © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Phenothiazine polymers. Adapted from ref. 58 with permission John Wiley and Sons, © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Carbonyl polymers. Reproduced from ref. 103, with permission from Elsevier, Copyright 2017. (d) Catechol polymers.

1.4.2 Applications in Medicine

Although energy is the most popular field of use of redox polymers, medicine is expected to be the most important one in the coming decades. Historically, the most important application of redox polymers has been in the area of electrochemical biosensors such as glucose or other types of bioactive compounds. However, as will be discussed in this book, there is a high demand for new materials such as redox polymers in emerging healthcare technologies which are specially important nowadays.

For instance, in tissue engineering redox polymers are very popular in the development of conducting scaffolds for the growth or sensing of tissues based on electrically sensitive cells such as neurons or cardiomyocytes.105 In Chapter 10, the use of redox-sensitive polymers presenting mostly disulfide cleavable bonds in drug delivery will be discussed. These new redox-sensitive drug-delivery vectors are becoming very important nowadays due to their sensitiveness to reactive oxygen species (ROS). ROS are key intermediates in a number of biological mechanisms and their role and high concentration in different diseases are important factors. In Chapter 13, it will be discussed how through the use of redox polymers electric signals can be transformed in movement through devices such as electromechanical actuators.106 Those devices are part of the global development of bioinspired devices known as artificial muscles of importance in a number of new technologies in the robotics or microelectronics areas. Last but not least, the use of redox polymers such as PEDOT in different bioelectronic devices such as biosensors or electrodes to interface with the biological signals will be discussed (Figure 1.9).

Fig. 1.9 Examples of the main applications of redox polymers in medicine and discussed in the chapters of this book. (a) Tissue engineering. Reproduced from ref. 105 with permission from the American Chemical Society, Copyright 2019. (b) Drug delivery. Reproduced from ref. 107 with permission from the Royal Society of Chemistry. (c) Actuators. Adapted from ref. 108 with permission from the Royal Society of Chemistry. (d) Bioelectronics. Adapted from ref. 109, https://doi.org/10.3390/polym10090989, under the terms of the CC BY 4.0 licence, https://creativecommons.org/licenses/by/4.0/.

1.5 Conclusions

Through this chapter, the field of redox polymers and the topics that will be more deeply discussed in the different chapters of this book are introduced. First, the different types of redox polymers and their classification are presented based on the chemical nature and how the redox-active site is distributed within the polymer. Next, the main characterization methods used for assessing the properties of redox polymers are introduced. Finally, the main applications of redox polymers in energy and medicine are indicated as they will be discussed in depth through the different chapters of the book.

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