**5. New types of electrodes for electrochemical cells**

The potential applications of plasma polymers as materials for electrochemistry are primarily associated with the possibility of designing their electronic structure and thereby with the designing of their electrical properties. Plasma deposition from diverse precursors and their mixtures, performed under various conditions of the process, leads to a huge variety of thin films characterized by a broad spectrum of electrical properties. In Fig. 9 a diagram of the conductivity typical for different plasma polymer types is presented. As one can see, the whole range from 10−18 S/m to 106 S/m is covered by plasma deposited films. Insulating, semiconducting (of different types of conductivity) and metallic films can be obtained in this way (Tyczkowski, 2004). Some of these films can reveal a significant activity in electrochemical processes. Photocatalytic activity of such films was already discussed in the previous section (*Sec. 4.2.*).

Taking into account the electrical properties mentioned above as well as another important feature of plasma polymers, namely the membrane nature, which is very often revealed by these films, a new fascinating electrocatalyst structure has been proposed. An effective electrocatalyst must satisfy many requirements, such as high activity, high electrical conductivity, and long-term stability, which may be in conflict with each other. One possible way to solve these conflicts is the use of composite materials, where the matrix and the dispersed phase are independently selected. A successful approach is that of associating a highly conducting (though catalytically inert) matrix with an active (though less conducting) dispersed phase. When the matrix is permeable, one has a three-dimensional (3D) catalyst: all catalytic particles are active, irrespective of their position in the composite. Fig. 10 schematically represents the operation of such a system.

Cold Plasma – A Promising Tool for the Development of Electrochemical Cells 121

of acrylonitrile, and cobalt atoms playing the role of active centers, which were introduced to the matrix during the deposition process from bis(acetylacetonate)cobalt(II) precursor (Doblhofer & Dürr, 1980). Just this one example shows how the great possibilities for the construction of electrochemical electrodes are inherent in the plasma deposition technique.

Fuel cells are electrochemical membrane reactors that are able to convert chemical energy from a fuel directly to electrical energy through a chemical reaction with oxygen. Hydrogen is the most common fuel, but alcohols like methanol and hydrocarbons such as CH4 are also used. Although there are many types of fuel cells, all of them consist of two electrodes (negative anode and positive cathode) and an electrolyte (liquid or solid) that allows ionic charges to move between the electrodes. Recently, a lot of effort has gone into improving the quality, reliability, and efficiency of these components by their modification and introduction of new materials (Carrette et al., 2000; Sundmacher, 2010). The cold plasma

For more than a decade, the polymer electrolyte (membrane) fuel cells (PEFC), which can be fed with hydrogen (when proton-exchange membranes are used, such fuel cells are called PEMFC) or, for example, fed directly with methanol (direct methanol fuel cells (DMFC)), are one of the most extensively investigated types of fuel cell worldwide. This results from their high energy-conversion efficiency, relatively low operating temperature, and high power density. The basic catalytic reactions taking place at the electrodes of PEMFC are as follows

The anode reaction consists of hydrogen oxidation to protons (hydrogen oxidation reaction – HOR). The protons migrate through the membrane to the cathode. At the cathode, oxygen is reduced (oxygen reduction reaction – ORR) and then recombines with the protons to form water. The electrodes have to be porous to gas diffusion to ensure the supply of the reactant gases to the active zones, where a catalytic material is in contact with the ionic (membrane) and electronic (catalyst substrate) conductors. Similar reactions occur at the electrodes of

Platinum-based materials (Pt or Pt alloys) are by far the best catalysts for the hydrogen and methanol oxidation (Eqs. (5) and (7)) as well as oxygen reduction reactions (Eqs. (6) and (8)). Unfortunately, Pt is a precious, very expensive metal, which limits the widespread commercialization of Pt-based fuel cells. Besides, the stability of Pt and Pt alloys becomes a serious problem for long-term operation of the cells. Hence, extensive research is underway to overcome these difficulties. The works are going in two directions. Firstly, the new methods to ensure consumption of smaller amounts of platinum and at the same time providing a more stable and effective catalyst are developed. And secondly, the new

anode H2 → 2H+ + 2*e*− (5)

anode CH3OH + H2O → CO2 + 6H+ + 6*e*− (7)

cathode ³/2 O2 + 6H+ + 6*e*<sup>−</sup> → 3H2O (8)

/2 O2 + 2H+ + 2*e*<sup>−</sup> → H2O (6)

**5.1 Catalytic electrodes for fuel cells** 

(Carrette et al., 2000):

technology is also widely involved in this process.

cathode 1

DMFC (for a proton-exchange membrane) (Carrette et al., 2000):

Fig. 9. Room-temperature conductivity *σ* of the main groups of plasma polymers (Tyczkowski, 2004).

Fig. 10. Model of a three-dimensional (3D) electrocatalytic membrane (Tyczkowski, 2010).

One of the first 3D-electrocatalyst electrodes was tested for the reduction of molecular oxygen. The electrode was made of a conducting matrix, formed by plasma polymerization of acrylonitrile, and cobalt atoms playing the role of active centers, which were introduced to the matrix during the deposition process from bis(acetylacetonate)cobalt(II) precursor (Doblhofer & Dürr, 1980). Just this one example shows how the great possibilities for the construction of electrochemical electrodes are inherent in the plasma deposition technique.

#### **5.1 Catalytic electrodes for fuel cells**

120 Electrochemical Cells – New Advances in Fundamental Researches and Applications

Fig. 9. Room-temperature conductivity *σ* of the main groups of plasma polymers

Fig. 10. Model of a three-dimensional (3D) electrocatalytic membrane (Tyczkowski, 2010).

One of the first 3D-electrocatalyst electrodes was tested for the reduction of molecular oxygen. The electrode was made of a conducting matrix, formed by plasma polymerization

(Tyczkowski, 2004).

Fuel cells are electrochemical membrane reactors that are able to convert chemical energy from a fuel directly to electrical energy through a chemical reaction with oxygen. Hydrogen is the most common fuel, but alcohols like methanol and hydrocarbons such as CH4 are also used. Although there are many types of fuel cells, all of them consist of two electrodes (negative anode and positive cathode) and an electrolyte (liquid or solid) that allows ionic charges to move between the electrodes. Recently, a lot of effort has gone into improving the quality, reliability, and efficiency of these components by their modification and introduction of new materials (Carrette et al., 2000; Sundmacher, 2010). The cold plasma technology is also widely involved in this process.

For more than a decade, the polymer electrolyte (membrane) fuel cells (PEFC), which can be fed with hydrogen (when proton-exchange membranes are used, such fuel cells are called PEMFC) or, for example, fed directly with methanol (direct methanol fuel cells (DMFC)), are one of the most extensively investigated types of fuel cell worldwide. This results from their high energy-conversion efficiency, relatively low operating temperature, and high power density. The basic catalytic reactions taking place at the electrodes of PEMFC are as follows (Carrette et al., 2000):

$$\text{anode}\qquad\text{H}\_2 \to 2\text{H}^\* + 2e^-\tag{5}$$

$$\text{cathode} \qquad \text{l/} \_2\text{O}\_2 + 2\text{H}^\* + 2e^- \to \text{H}\_2\text{O} \tag{6}$$

The anode reaction consists of hydrogen oxidation to protons (hydrogen oxidation reaction – HOR). The protons migrate through the membrane to the cathode. At the cathode, oxygen is reduced (oxygen reduction reaction – ORR) and then recombines with the protons to form water. The electrodes have to be porous to gas diffusion to ensure the supply of the reactant gases to the active zones, where a catalytic material is in contact with the ionic (membrane) and electronic (catalyst substrate) conductors. Similar reactions occur at the electrodes of DMFC (for a proton-exchange membrane) (Carrette et al., 2000):

$$\text{anode}\qquad \text{CH}\_3\text{OH} + \text{H}\_2\text{O} \to \text{CO}\_2 + 6\text{H}^\* + 6e^-\tag{7}$$

$$\text{cathode} \quad \text{ }^\text{\S}\text{\!\!\/\_2\text{O}} + \text{\!\!\/\_2\text{H}} + \text{\!\!\/}\text{\!\text{\"}} + \text{\!\!\text{\"}} \text{\!\text{\"}} \text{\!\text{\"}} \text{\!\text{\"}} \text{\!\text{\"}} \text{\!\text{\"}} \text{\!\text{\"}} \text{\!\text{\"}} \text{\!\text{\"}} \text{\!\text{\"}} \text{\!\text{\"}} \text{\!\text{\"}} \text{\!\text{\text\"}} \text{\!\text{\text\"}} \text{\!\text{\text\"}} \text{\!\text{\text\"}} \text{\!\text{\text\"}} \text{\!\text{\text\"}} \text{\!\text{\text\"}} \text{\!\text{\text\"}} \text{\!\text{\text\"}} \text{\!\text{\text\"}} \text{\!\text{\text\"}} \text{\!\text\text\"} \text{\!\text\text\"} \text{\!\text\text\"} \text{\!\text\text\"} \text{\!\text\text\"} \text{\!\text\text\"} \text{\!\text\text\"} \text{\!\text\text\text\"} \text{\!\text\text\text\"} \text{\!\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\text\} \text \text\text\text\text\text\text\text\text\text\$$

Platinum-based materials (Pt or Pt alloys) are by far the best catalysts for the hydrogen and methanol oxidation (Eqs. (5) and (7)) as well as oxygen reduction reactions (Eqs. (6) and (8)). Unfortunately, Pt is a precious, very expensive metal, which limits the widespread commercialization of Pt-based fuel cells. Besides, the stability of Pt and Pt alloys becomes a serious problem for long-term operation of the cells. Hence, extensive research is underway to overcome these difficulties. The works are going in two directions. Firstly, the new methods to ensure consumption of smaller amounts of platinum and at the same time providing a more stable and effective catalyst are developed. And secondly, the new

Cold Plasma – A Promising Tool for the Development of Electrochemical Cells 123

nanometers (13–80 nm) and a length of micrometers (2–20 μm) decorated with 2–5 nm Pt nanoclusters were already the object of research. It was found that such systems prepared on the electrode substrate (carbon cloth or carbon paper) significantly improve the performance of both PEMFC and DMFC compared with the conventional electrodes (Soin et al., 2010). By the way, it should be added that vertically aligned carbon nanotubes as well as graphene layers fabricated by PECVD have recently attracted research interest as

In addition to extensive research into the production of platinum catalysts by the sputtering method, it was also trying to get them through the plasma polymerization method (PECVD). A platinum-containing organic complex, bis(acetylacetonate)platinum(II) (Pt(acac)2), which is characterized by a relatively low sublimation temperature (160–170°C), was used as a precursor of PECVD carried out in an RF discharge. The plasma-polymerized film was then calcined to drive off organic material, leaving behind a catalyst-loaded substrate (Dhar et al., 2005a). The same procedure was used to prepare a composite consisting of ZrO2 support and Pt catalyst. The support and the catalyst were deposited on a metallic substrate by PECVD as alternate layers from Zr(acac)4 and Pt(acac)2, respectively. It was found that Pt agglomerates were embedded in the zirconia support (Dhar et al., 2005b). Other potential path of development of fuel cells, in addition to improving the properties of platinum electrodes, is searching for new catalytic materials. This research is mainly focused on the cathode materials, at which the oxygen reduction reactions (ORR) (Eqs. (6) and (8)) constituting the bottleneck in the fuel cell operation proceed (Wang, 2005). The cold plasma technology creates potential and real opportunities in this regard (Brault, 2011). Like the platinum catalyst, also in this case the plasma sputtering technique was used. For example, CoS2-based thin films were prepared by this method. Electrochemical assessment indicated that the films had significant ORR catalytic activity (Zhu, L. et al., 2008). Similarly, a high ORR catalytic activity showed niobium oxinitride (Nb-O-N) films prepared by plasma (RF) reactive sputtering from a Nb metal plate under various partial pressures of N2 and O2

Taking into account the promising electrocatalytic activity for ORR demonstrated by nanoparticles of cobalt oxides (Manzoli & Boccuzzi, 2005), we have undertaken in our laboratory an attempt to produce such a material for the PEMFC electrodes by the plasma polymerization method. The films containing CoOX were deposited in a parallel plate RF (13.56 MHz) reactor shown in Fig. 2. Cyclopentadienyl(dicarbonyl)cobalt(I) (CpCo(CO)2) was used as a precursor. As a result of the plasma deposition process, very thin films (25– 750 nm) composed of a hydrocarbon matrix and amorphous CoOX were obtained. The amorphousity was determined by the electron diffraction pattern. However, only a moderate thermal treatment was enough to transform the amorphous films into films with nanocrystalline structure of cobalt spinel (Co3O4) (Fig. 11). The creation of cobalt spinel nanocrystals was supported by Raman spectroscopy measurements. The electron diffraction and Raman spectroscopy measurements also allowed us to determine the nanocrystals size. It was found that this size can be controlled by parameters of the plasma deposition process. As an example, Fig. 12 shows a dependence of the average size of Co3O4 nanocrystals on the flow rate of CpCo(CO)2 vapor through the plasma reactor. This simple example is just enough to show that the cold plasma is a very useful technique, not only for the fabrication of new materials, but also for the precise control of their structure. In the same way as the

supercapacitor electrode materials (Amade et al., 2011; Zhao et al., 2009).

(Ohnishi et al., 2010).

alternative catalysts that are cheaper than platinum and exhibit at least a comparable catalytic activity are sought (Sundmacher, 2010; Wang, 2005).

As far as the methods of reducing the amount of platinum catalyst are concerned, the most promising appears to be the plasma sputtering technique. By this method, the catalytic electrodes can be prepared with a platinum loading down to 0.005 mgPt/cm2, that is drastically lower than that for conventional Pt electrodes (0.5–1.0 mgPt/cm2), with no detrimental effect on fuel cell performance. The Pt catalyst is dispersed as nano-clusters of controlled size (sometimes, until less than 2 nm) and controlled crystalline structure that determines the concentration of catalytically active centers. It should be emphasized that such a possibility, in principle, is given only by the application of cold plasma (Caillard et al., 2005; Caillard et al., 2009; Saha et al., 2006; Xinyao et al., 2010). As it was already mentioned, this technology also allows to put the deposited material on virtually any substrate. Thus, the Pt catalyst can be sputtered on both a porous carbon substrate forming the electrode and the surface of a polymer electrolyte (e.g. Nafion), to which the carbon electrode is then pressed.

The main requirement of a good electrode is a three-phase boundary between the fuel supply, the catalyst particle and the ionic (polymer) electrolyte. The catalyst particles also must be in direct contact with the electron conducting electrode (Carrette et al., 2000). To ensure such a contact and at the same time maximize the interphase boundary, cosputtering or co-deposition of carbon-based material and platinum was used. Materials classified as 3D-electrocatalysts can be obtained in this way. For example, a simultaneous co-sputtering of carbon and platinum on a conventional carbon porous substrate led to high electrodes efficiency for both the hydrogen oxidation and oxygen reduction reactions. The PEMFC tested in this case achieved a specific power of 20 kW per 1 g of platinum, which is one of the best results reported so far (Cavarroc et al., 2009). Carbon and platinum can be also deposited by subsequent sputtering processes. First a porous columnar carbon film (column diameter of 20 nm) is deposited, and then these nanocolumns are decorated by Pt nanoclusters (Rabat & Brault, 2008).

Recently, interesting results have been obtained in this field by combining the plasma polymerization and sputtering methods. For instance, the synthesis of composite thin films made of platinum nanoclusters (3–7 nm) embedded in a porous hydrocarbon matrix was carried out by simultaneous PECVD of pp-ethylene and sputtering of a platinum target. The metal content in the films could be controlled over a wide range of atomic percentages (5– 80%) (Dilonardo et al., 2011). Aniline mixed with functionalized platinum nanoparticles as a precursor of PECVD was, in turn, used to prepare a typical 3D-catalyst. The plasma deposition was performed under atmospheric pressure conditions. Plasma polymerized aniline (pp-aniline), which is characterized by both electronic and ionic conductivity, associated with the Pt catalyst in a 3D porous network, without doubt lead to the development of the three-phase boundary (Michel et al., 2010).

Another idea is to deposit on the electrode surface carbon nanofibers (nanotubes) by PECVD and then decorating them by sputtered platinum nanoclusters. Generally, three consecutive plasma deposition steps are carried out to this end: the sputtering of a catalyst used to initiate the growth of the carbon nanofibers (e.g. Fe, Ni, Co), the creation of the nanofibers by PECVD from a mixture of precursors (e.g. CH4/H2, CH4/N2) and the sputtering of platinum. The plasma produced systems consisting of carbon nanofibers with a diameter of

alternative catalysts that are cheaper than platinum and exhibit at least a comparable

As far as the methods of reducing the amount of platinum catalyst are concerned, the most promising appears to be the plasma sputtering technique. By this method, the catalytic electrodes can be prepared with a platinum loading down to 0.005 mgPt/cm2, that is drastically lower than that for conventional Pt electrodes (0.5–1.0 mgPt/cm2), with no detrimental effect on fuel cell performance. The Pt catalyst is dispersed as nano-clusters of controlled size (sometimes, until less than 2 nm) and controlled crystalline structure that determines the concentration of catalytically active centers. It should be emphasized that such a possibility, in principle, is given only by the application of cold plasma (Caillard et al., 2005; Caillard et al., 2009; Saha et al., 2006; Xinyao et al., 2010). As it was already mentioned, this technology also allows to put the deposited material on virtually any substrate. Thus, the Pt catalyst can be sputtered on both a porous carbon substrate forming the electrode and the surface of a polymer electrolyte (e.g. Nafion), to which the carbon

The main requirement of a good electrode is a three-phase boundary between the fuel supply, the catalyst particle and the ionic (polymer) electrolyte. The catalyst particles also must be in direct contact with the electron conducting electrode (Carrette et al., 2000). To ensure such a contact and at the same time maximize the interphase boundary, cosputtering or co-deposition of carbon-based material and platinum was used. Materials classified as 3D-electrocatalysts can be obtained in this way. For example, a simultaneous co-sputtering of carbon and platinum on a conventional carbon porous substrate led to high electrodes efficiency for both the hydrogen oxidation and oxygen reduction reactions. The PEMFC tested in this case achieved a specific power of 20 kW per 1 g of platinum, which is one of the best results reported so far (Cavarroc et al., 2009). Carbon and platinum can be also deposited by subsequent sputtering processes. First a porous columnar carbon film (column diameter of 20 nm) is deposited, and then these nanocolumns are decorated by Pt

Recently, interesting results have been obtained in this field by combining the plasma polymerization and sputtering methods. For instance, the synthesis of composite thin films made of platinum nanoclusters (3–7 nm) embedded in a porous hydrocarbon matrix was carried out by simultaneous PECVD of pp-ethylene and sputtering of a platinum target. The metal content in the films could be controlled over a wide range of atomic percentages (5– 80%) (Dilonardo et al., 2011). Aniline mixed with functionalized platinum nanoparticles as a precursor of PECVD was, in turn, used to prepare a typical 3D-catalyst. The plasma deposition was performed under atmospheric pressure conditions. Plasma polymerized aniline (pp-aniline), which is characterized by both electronic and ionic conductivity, associated with the Pt catalyst in a 3D porous network, without doubt lead to the

Another idea is to deposit on the electrode surface carbon nanofibers (nanotubes) by PECVD and then decorating them by sputtered platinum nanoclusters. Generally, three consecutive plasma deposition steps are carried out to this end: the sputtering of a catalyst used to initiate the growth of the carbon nanofibers (e.g. Fe, Ni, Co), the creation of the nanofibers by PECVD from a mixture of precursors (e.g. CH4/H2, CH4/N2) and the sputtering of platinum. The plasma produced systems consisting of carbon nanofibers with a diameter of

catalytic activity are sought (Sundmacher, 2010; Wang, 2005).

electrode is then pressed.

nanoclusters (Rabat & Brault, 2008).

development of the three-phase boundary (Michel et al., 2010).

nanometers (13–80 nm) and a length of micrometers (2–20 μm) decorated with 2–5 nm Pt nanoclusters were already the object of research. It was found that such systems prepared on the electrode substrate (carbon cloth or carbon paper) significantly improve the performance of both PEMFC and DMFC compared with the conventional electrodes (Soin et al., 2010). By the way, it should be added that vertically aligned carbon nanotubes as well as graphene layers fabricated by PECVD have recently attracted research interest as supercapacitor electrode materials (Amade et al., 2011; Zhao et al., 2009).

In addition to extensive research into the production of platinum catalysts by the sputtering method, it was also trying to get them through the plasma polymerization method (PECVD). A platinum-containing organic complex, bis(acetylacetonate)platinum(II) (Pt(acac)2), which is characterized by a relatively low sublimation temperature (160–170°C), was used as a precursor of PECVD carried out in an RF discharge. The plasma-polymerized film was then calcined to drive off organic material, leaving behind a catalyst-loaded substrate (Dhar et al., 2005a). The same procedure was used to prepare a composite consisting of ZrO2 support and Pt catalyst. The support and the catalyst were deposited on a metallic substrate by PECVD as alternate layers from Zr(acac)4 and Pt(acac)2, respectively. It was found that Pt agglomerates were embedded in the zirconia support (Dhar et al., 2005b).

Other potential path of development of fuel cells, in addition to improving the properties of platinum electrodes, is searching for new catalytic materials. This research is mainly focused on the cathode materials, at which the oxygen reduction reactions (ORR) (Eqs. (6) and (8)) constituting the bottleneck in the fuel cell operation proceed (Wang, 2005). The cold plasma technology creates potential and real opportunities in this regard (Brault, 2011). Like the platinum catalyst, also in this case the plasma sputtering technique was used. For example, CoS2-based thin films were prepared by this method. Electrochemical assessment indicated that the films had significant ORR catalytic activity (Zhu, L. et al., 2008). Similarly, a high ORR catalytic activity showed niobium oxinitride (Nb-O-N) films prepared by plasma (RF) reactive sputtering from a Nb metal plate under various partial pressures of N2 and O2 (Ohnishi et al., 2010).

Taking into account the promising electrocatalytic activity for ORR demonstrated by nanoparticles of cobalt oxides (Manzoli & Boccuzzi, 2005), we have undertaken in our laboratory an attempt to produce such a material for the PEMFC electrodes by the plasma polymerization method. The films containing CoOX were deposited in a parallel plate RF (13.56 MHz) reactor shown in Fig. 2. Cyclopentadienyl(dicarbonyl)cobalt(I) (CpCo(CO)2) was used as a precursor. As a result of the plasma deposition process, very thin films (25– 750 nm) composed of a hydrocarbon matrix and amorphous CoOX were obtained. The amorphousity was determined by the electron diffraction pattern. However, only a moderate thermal treatment was enough to transform the amorphous films into films with nanocrystalline structure of cobalt spinel (Co3O4) (Fig. 11). The creation of cobalt spinel nanocrystals was supported by Raman spectroscopy measurements. The electron diffraction and Raman spectroscopy measurements also allowed us to determine the nanocrystals size. It was found that this size can be controlled by parameters of the plasma deposition process. As an example, Fig. 12 shows a dependence of the average size of Co3O4 nanocrystals on the flow rate of CpCo(CO)2 vapor through the plasma reactor. This simple example is just enough to show that the cold plasma is a very useful technique, not only for the fabrication of new materials, but also for the precise control of their structure. In the same way as the

Cold Plasma – A Promising Tool for the Development of Electrochemical Cells 125

Fig. 12. Dependence of the average size of Co3O4 nanocrystals on the flow rate of CpCo(CO)2

Fig. 13. Fuel cell characteristic for various cathode catalytic materials: (a) – CuOX (*t* = 12 min); (b) – Co3O4 (*t* = 12 min); (c) – Co3O4+CuOX (*t* = 12 min); (d) – Co3O4 (*t* = 60 min); (e) –

There has been a significant research work done in the recent past in the development of lithium-ion batteries, which are extensively applied in various electric and portable electronic devices. Although a lot of attractive cathode and anode materials for these batteries are already known, it is still lay much stress on finding new solutions in this area. Increasingly, the cold plasma technology is also used to this end. The first attempts were made in the eighties of the 20th century in Sanyo Electric Co. in Japan, where AF plasma (6.5

vapor through the plasma reactor (Tyczkowski, 2011).

Co3O4 (*t* = 80 min); (f) – Pt (Tyczkowski, 2011).

**5.2 Electrodes for lithium-ion batteries** 

films with CoOX, films containing CuOX were deposited using bis(acetylacetonate)copper(II) (Cu(acac)2). After feeding the reactor by a mixture of CpCo(CO)2 and Cu(acac)2 vapors, thin films of Co–Cu mixed oxides were also fabricated (Tyczkowski, 2011; Tyczkowski et al., 2007).

Fig. 11. Outline of the preparation process of nanocrystalline Co3O4 films by plasma deposition.

To test the electrocatalytic activity of the above-mentioned films, electrodes for PEMFC were prepared by the deposition of these films on a carbon paper substrate. Then the samples were thermally treated forming anode materials. The opposite electrode (cathode) was prepared from the same carbon paper covered with 10% Platinum on Vulcan XC-72 catalyst (1.56 mg/cm2) (Kazimierski et al., 2010; Kazimierski et al., 2011). In Fig. 13, preliminary results concerning the current–voltage dependence for the tested fuel cell with various types of anode catalytic materials are shown. Although the characteristics obtained for plasma deposited materials are still far from the model system, in which both electrodes are prepared from Pt (curve f), nevertheless these results are very promising. It is enough to notice that the concentration of catalytic active centers increases with the increase of the deposition time, which is reflected in the improvement of the fuel cell characteristic (curves b, d and e). A simple calculation showed that after 80 min deposition of CoOX, the whole deposited material loading is only 0.1 mg/cm2 (moreover, this value is drastically reduced by the annealing) (Kazimierski et al., 2011). However, it seems to be possible to significantly increase the concentration of the centers by optimizing the plasma deposition parameters. Equally important as the centers concentration is the structure of the oxide material. One can see in Fig. 13 that the film composed of Co3O4 and CuOX reveals much higher activity (curve c) than each of these oxides separately (curves a and b). Thus, it is no wonder that further intensive works in this field are planned.

films with CoOX, films containing CuOX were deposited using bis(acetylacetonate)copper(II) (Cu(acac)2). After feeding the reactor by a mixture of CpCo(CO)2 and Cu(acac)2 vapors, thin films of Co–Cu mixed oxides were also fabricated (Tyczkowski, 2011; Tyczkowski et al.,

Fig. 11. Outline of the preparation process of nanocrystalline Co3O4 films by plasma

To test the electrocatalytic activity of the above-mentioned films, electrodes for PEMFC were prepared by the deposition of these films on a carbon paper substrate. Then the samples were thermally treated forming anode materials. The opposite electrode (cathode) was prepared from the same carbon paper covered with 10% Platinum on Vulcan XC-72 catalyst (1.56 mg/cm2) (Kazimierski et al., 2010; Kazimierski et al., 2011). In Fig. 13, preliminary results concerning the current–voltage dependence for the tested fuel cell with various types of anode catalytic materials are shown. Although the characteristics obtained for plasma deposited materials are still far from the model system, in which both electrodes are prepared from Pt (curve f), nevertheless these results are very promising. It is enough to notice that the concentration of catalytic active centers increases with the increase of the deposition time, which is reflected in the improvement of the fuel cell characteristic (curves b, d and e). A simple calculation showed that after 80 min deposition of CoOX, the whole deposited material loading is only 0.1 mg/cm2 (moreover, this value is drastically reduced by the annealing) (Kazimierski et al., 2011). However, it seems to be possible to significantly increase the concentration of the centers by optimizing the plasma deposition parameters. Equally important as the centers concentration is the structure of the oxide material. One can see in Fig. 13 that the film composed of Co3O4 and CuOX reveals much higher activity (curve c) than each of these oxides separately (curves a and b). Thus, it is no wonder that further

2007).

deposition.

intensive works in this field are planned.

Fig. 12. Dependence of the average size of Co3O4 nanocrystals on the flow rate of CpCo(CO)2 vapor through the plasma reactor (Tyczkowski, 2011).

Fig. 13. Fuel cell characteristic for various cathode catalytic materials: (a) – CuOX (*t* = 12 min); (b) – Co3O4 (*t* = 12 min); (c) – Co3O4+CuOX (*t* = 12 min); (d) – Co3O4 (*t* = 60 min); (e) – Co3O4 (*t* = 80 min); (f) – Pt (Tyczkowski, 2011).

#### **5.2 Electrodes for lithium-ion batteries**

There has been a significant research work done in the recent past in the development of lithium-ion batteries, which are extensively applied in various electric and portable electronic devices. Although a lot of attractive cathode and anode materials for these batteries are already known, it is still lay much stress on finding new solutions in this area. Increasingly, the cold plasma technology is also used to this end. The first attempts were made in the eighties of the 20th century in Sanyo Electric Co. in Japan, where AF plasma (6.5

Cold Plasma – A Promising Tool for the Development of Electrochemical Cells 127

(optoelectronically active materials, electrode materials), but also there, where this conductivity should be as low as possible. Plasma polymers with very low electrical conductivity (see: Fig. 9) are often utilized as a variety of thin-film insulators. In turn, the semi-permeable properties of some plasma polymers allow them to be used as selective membranes. By combining the low electrical conductivity and selective permeability, one can get great barrier materials. Thus, for example, very thin barrier layers for direct methanol fuel cells (DMFC) were produced. For the technical realization of the DMFC, a highly proton conducting polymer electrolyte is necessary. Perfluorosulfonic acid membranes, such as Nafion®, are widely used to this end. Indeed, these membranes have high proton conductivity, but their great disadvantage is too high permeability of methanol molecules that migrate from anode to cathode lowering the cell performance. Deposition of a thin plasma polymer film, produced by PECVD from perfluoroheptane (C7F16), on the membrane surface decreases the methanol permeability by two-orders of magnitudes (Lue

The possibility of plasma copolymerization and the production of composite materials allows to design at the molecular level thin films of very low electronic conductivity, but with very high ionic conductivity. Such films can be obtained in the polymer-like form (polymer electrolytes) and as ionic glasses (solid oxide electrolytes). These new solid electrolyte systems enable us to replace the conventional solid electrolytes by the much thinner elements, which in addition have all the other advantages of plasma fabricated materials, for example, selective permeability. Thin films of solid electrolytes produced by

The most common polymer electrolytes (often called the ion-exchange membranes) for fuel cells are composed of crosslinked macromolecular chains making up a three-dimensional structure on which are distributed some ionizable functional groups giving the membrane its specificity. To maintain the electroneutrality of the material, ionized sites are compensated for by an equivalent number of mobile ions of opposite charge. By jumping between ionized sites, these mobile ions give the membrane its ionic conduction ability. To prepare ion-exchange membranes by plasma polymerization, it is necessary first to choose a precursor with long and flexible chains or, at best, containing spacers in its structure (phenyl groups, for example) and then to initiate the deposition process with a "soft" plasma discharge in order to safely preserve those elements of the precursor likely to constitute the skeleton of the final material. The next criterion, namely, a large quantity of ionizable functional groups favorably distributed in the polymer matrix, requires the selection of a second precursor containing the appropriate ionizable functional group in its structure, which will be embedded in the polymer matrix without defects. A schematic representation of basic processes that occur in the "soft" plasma polymerization of a proton-exchange membrane from styrene and trifluoromethane sulfonic acid (CF3SO3H) is shown in Fig. 14. The first works devoted to the development of plasma-polymerized ion-exchange membranes for fuel cells were carried out in the late 1980's by the Inagaki's group at the Shizuoka University in Japan (Inagaki, 1996). Plasma polymerization of a mixture of fluorinated benzene (C6F6, C6F5H or C6F4H2) and SO2 gave a Nafion®-like plasma polymer that contains sulfonic acid and sulfonate groups. Such a membrane has the cation-exchange

cold plasma deposition techniques have been of particular interest recently.

et al., 2007).

**6.1 Solid electrolytes for fuel cells** 

kHz) polymerized pyrrole as a conducting cathode layer was use for a Li-ion battery. The pp-pyrrole was deposited on one side of a porous polypropylene separator sheet, a Li layer was vapor deposited on the other site of the sheet, and stainless steel collector layers were formed on both sides by spattering. This battery showed very good properties. Research on the use of polypyrrole (also produced by plasma polymerization) for cathodes of Li-ion batteries is currently being pursued (Cho, S.H. et al., 2007). Another interesting conductive polymer, which was used as the cathode material, is plasma polymerized carbon disulfide (pp-CS2). These films (approx. 0.5–1 μm) deposited on Pt foil showed satisfactory electrochemical activity in cells vs. Li/Li+. Compared to poly(carbon disulfide) prepared by conventional chemical means, cells having the pp-CS2 improved cycle life because the plasma polymerized material is more crosslinked and does not depolymerize as readily (Sadhir & Schoch, 1996).

A lot of attention is also paid to entirely new anode materials. Although carbon in nowadays is used as the commercial anode material, it has several shortcomings such as, for instance, low reversible capacity that is usually ranged in 250–300 mAh/g. In order to increase this parameter, silicon with the highest theoretical capacity (e.g., 4000 mAh/g) has been proposed as a new negative electrode material. Unfortunately, this material has also serious drawbacks, which are related to the poor electrical conductivity and drastic volume changes during electrochemical reactions. To solve these problems, silicon and silicide powders with conducting materials such as metals, oxides, and nitrides are used as the composite anodes. The potential of plasma technology for the fabrication of both powders and composite systems are particularly useful in this case. For example, a complex procedure of plasma deposition was used to produce an anode material in the form of copper silicide-coated graphite particles. The graphite particles with mean diameter of 6.0 μm were covered with a very thin film of a-Si:H (30–50 nm) in the PECVD process from SiH4. Then, copper layer was deposited on the surface of silicon-coated graphite using the next plasma technique, namely RF sputtering. After annealing at a temperature of 300°C, copper silicide was formed. This material used as the anode in Li-ion batteries revealed high capacity properties and good electrical performance (Kim, I.C. et al., 2006).

An example of particles formed by plasma methods for use in the anodes can be the synthesis of monodisperse and non-agglomerated SiOX nanoparticles by the PECVD method from a mixture of SiH4 and O2 in a plasma reactor specially designed for this purpose. It should be emphasized that in this case the nanoparticle size can be controlled by the flow rate of the reactant gases through the reactor. The SiOX nanoparticles mixed with graphite particles constitutes the anode material (Kim, K. et al., 2010).

More sophisticated materials for Li-ion electrodes were also fabricated by the cold plasma technology. Reactive co-sputtering of Sn and Ru in oxygen plasma has allowed to obtain SnO2–RuO2 composite thin films, which reveal unique electrochemical properties (Choi et al., 2004). As the anode, cobalt oxide thin films deposited by reactive sputtering of Co in O2 plasma were also tested with success. It was found that these films contained Co3O4 grains with the size of 4–25 nm (Liao et al., 2006).

#### **6. Other components of electrochemical cells**

Plasma deposited thin films are not only very useful for creating electrochemical cell elements where they must be characterized by high electrical conductivity (optoelectronically active materials, electrode materials), but also there, where this conductivity should be as low as possible. Plasma polymers with very low electrical conductivity (see: Fig. 9) are often utilized as a variety of thin-film insulators. In turn, the semi-permeable properties of some plasma polymers allow them to be used as selective membranes. By combining the low electrical conductivity and selective permeability, one can get great barrier materials. Thus, for example, very thin barrier layers for direct methanol fuel cells (DMFC) were produced. For the technical realization of the DMFC, a highly proton conducting polymer electrolyte is necessary. Perfluorosulfonic acid membranes, such as Nafion®, are widely used to this end. Indeed, these membranes have high proton conductivity, but their great disadvantage is too high permeability of methanol molecules that migrate from anode to cathode lowering the cell performance. Deposition of a thin plasma polymer film, produced by PECVD from perfluoroheptane (C7F16), on the membrane surface decreases the methanol permeability by two-orders of magnitudes (Lue et al., 2007).

The possibility of plasma copolymerization and the production of composite materials allows to design at the molecular level thin films of very low electronic conductivity, but with very high ionic conductivity. Such films can be obtained in the polymer-like form (polymer electrolytes) and as ionic glasses (solid oxide electrolytes). These new solid electrolyte systems enable us to replace the conventional solid electrolytes by the much thinner elements, which in addition have all the other advantages of plasma fabricated materials, for example, selective permeability. Thin films of solid electrolytes produced by cold plasma deposition techniques have been of particular interest recently.

### **6.1 Solid electrolytes for fuel cells**

126 Electrochemical Cells – New Advances in Fundamental Researches and Applications

kHz) polymerized pyrrole as a conducting cathode layer was use for a Li-ion battery. The pp-pyrrole was deposited on one side of a porous polypropylene separator sheet, a Li layer was vapor deposited on the other site of the sheet, and stainless steel collector layers were formed on both sides by spattering. This battery showed very good properties. Research on the use of polypyrrole (also produced by plasma polymerization) for cathodes of Li-ion batteries is currently being pursued (Cho, S.H. et al., 2007). Another interesting conductive polymer, which was used as the cathode material, is plasma polymerized carbon disulfide (pp-CS2). These films (approx. 0.5–1 μm) deposited on Pt foil showed satisfactory electrochemical activity in cells vs. Li/Li+. Compared to poly(carbon disulfide) prepared by conventional chemical means, cells having the pp-CS2 improved cycle life because the plasma polymerized material is more crosslinked and does not depolymerize as readily

A lot of attention is also paid to entirely new anode materials. Although carbon in nowadays is used as the commercial anode material, it has several shortcomings such as, for instance, low reversible capacity that is usually ranged in 250–300 mAh/g. In order to increase this parameter, silicon with the highest theoretical capacity (e.g., 4000 mAh/g) has been proposed as a new negative electrode material. Unfortunately, this material has also serious drawbacks, which are related to the poor electrical conductivity and drastic volume changes during electrochemical reactions. To solve these problems, silicon and silicide powders with conducting materials such as metals, oxides, and nitrides are used as the composite anodes. The potential of plasma technology for the fabrication of both powders and composite systems are particularly useful in this case. For example, a complex procedure of plasma deposition was used to produce an anode material in the form of copper silicide-coated graphite particles. The graphite particles with mean diameter of 6.0 μm were covered with a very thin film of a-Si:H (30–50 nm) in the PECVD process from SiH4. Then, copper layer was deposited on the surface of silicon-coated graphite using the next plasma technique, namely RF sputtering. After annealing at a temperature of 300°C, copper silicide was formed. This material used as the anode in Li-ion batteries revealed high capacity properties and good

An example of particles formed by plasma methods for use in the anodes can be the synthesis of monodisperse and non-agglomerated SiOX nanoparticles by the PECVD method from a mixture of SiH4 and O2 in a plasma reactor specially designed for this purpose. It should be emphasized that in this case the nanoparticle size can be controlled by the flow rate of the reactant gases through the reactor. The SiOX nanoparticles mixed with graphite

More sophisticated materials for Li-ion electrodes were also fabricated by the cold plasma technology. Reactive co-sputtering of Sn and Ru in oxygen plasma has allowed to obtain SnO2–RuO2 composite thin films, which reveal unique electrochemical properties (Choi et al., 2004). As the anode, cobalt oxide thin films deposited by reactive sputtering of Co in O2 plasma were also tested with success. It was found that these films contained Co3O4 grains

Plasma deposited thin films are not only very useful for creating electrochemical cell elements where they must be characterized by high electrical conductivity

(Sadhir & Schoch, 1996).

electrical performance (Kim, I.C. et al., 2006).

with the size of 4–25 nm (Liao et al., 2006).

**6. Other components of electrochemical cells** 

particles constitutes the anode material (Kim, K. et al., 2010).

The most common polymer electrolytes (often called the ion-exchange membranes) for fuel cells are composed of crosslinked macromolecular chains making up a three-dimensional structure on which are distributed some ionizable functional groups giving the membrane its specificity. To maintain the electroneutrality of the material, ionized sites are compensated for by an equivalent number of mobile ions of opposite charge. By jumping between ionized sites, these mobile ions give the membrane its ionic conduction ability. To prepare ion-exchange membranes by plasma polymerization, it is necessary first to choose a precursor with long and flexible chains or, at best, containing spacers in its structure (phenyl groups, for example) and then to initiate the deposition process with a "soft" plasma discharge in order to safely preserve those elements of the precursor likely to constitute the skeleton of the final material. The next criterion, namely, a large quantity of ionizable functional groups favorably distributed in the polymer matrix, requires the selection of a second precursor containing the appropriate ionizable functional group in its structure, which will be embedded in the polymer matrix without defects. A schematic representation of basic processes that occur in the "soft" plasma polymerization of a proton-exchange membrane from styrene and trifluoromethane sulfonic acid (CF3SO3H) is shown in Fig. 14.

The first works devoted to the development of plasma-polymerized ion-exchange membranes for fuel cells were carried out in the late 1980's by the Inagaki's group at the Shizuoka University in Japan (Inagaki, 1996). Plasma polymerization of a mixture of fluorinated benzene (C6F6, C6F5H or C6F4H2) and SO2 gave a Nafion®-like plasma polymer that contains sulfonic acid and sulfonate groups. Such a membrane has the cation-exchange

Cold Plasma – A Promising Tool for the Development of Electrochemical Cells 129

fluorinated carboxylic membranes from H2O and C4F8 (Thery et al., 2010), phosphorousdoped silicon dioxide membranes from SiH4, PH3 and N2O (Prakash et al., 2008) as well as membranes produced by plasma polymerization from heptylamine (C7H15NH2) or 1,7-

The cold plasma technology is also tested for the preparation of anion-exchange membranes, which are presently becoming significant materials for application in alkaline fuel cells, where hydroxyl ions OH− are the ion charge carriers. Similarly, as in the case of the protonexchange membranes, the plasma deposition provides formation of very thin crosslinked films with high ion (OH−) conductivity, high chemical stability and low fuel permeability. The first attempt to obtain such membranes was undertaken only in 2006, when precursors containing tertiary amine groups were plasma polymerized and then the deposited films were quaternized by methyl iodide (Schieda et al., 2006). Very recently, an opposite procedure was applied, namely, the films were plasma polymerized from vinylbenzyl chloride, then the benzyl chloride groups (−CH2Cl) present in the films were quaternized by trimethylamine into −CH2N+(CH3)3Cl− groups, and finally these groups were alkalized by KOH into −CH2N+(CH3)3OH− groups. This material proves to be an excellent hydroxide ion conductor with great potential for application in alkaline direct alcohol fuel cells (Zhang et

Lastly, we should also mention the solid oxide fuel cells (SOFC), which are currently of great interest. As far as the plasma technology is concerned, the thermal plasma (see: Fig. 1) is particularly relevant in this case (Henne, 2007). However, the cold plasma is trying to use as well. In addition to the reactive sputtering method that is quite justified when we want to obtain thin films of inorganic oxides (e.g. La-Si-O, which is a potential candidate as electrolyte material for intermediate-temperature solid oxide fuel cells (Briois et al., 2007)), attempts to employ the PECVD method have been also made. Popular solid oxide electrolyte material including yttria-stabilized zirconia (YSZ) was prepared by microwave plasma polymerization from (acetylacetonate)zirconium(I) and tris(dipivaloylmethanato) yttrium(III) as precursors (Itoh & Matsumoto, 1999). Recently performed physicochemical investigations of such deposited solid electrolyte films have shown characteristic nanostructures that are strongly affected by the variation of plasma parameters and the precursor mixture composition. Thus, we can obtain the films with exactly the desired

The polymer electrolytes for Li-ion batteries are fundamentally the same as those used in fuel cells, with the only difference that ions transported in this case are Li+ ions. These membranes should be very good electronic insulators that separate the anode from the cathode, but very good lithium-ion conductors. They should also have adequate chemical resistance as well as adequate mechanical strength to withstand the pressure changes and stresses of the electrodes during discharge/charge cycling of the battery. All these requirements can be satisfied by plasma polymerized thin films. The first reports on the preparation of such films appeared at the end of the 1980's. The films were deposited from precursors containing alkoxy, siloxane and vinyl groups in one molecule. Then, the films were sprayed with a solution of LiOCl4 to introduce Li+ into the plasma polymer structure (Ogumi et al., 1989). In subsequent years, more complex systems were prepared. For

structure and properties, for example, with appropriate ionic conductivity.

**6.2 Solid electrolytes for lithium-ion batteries** 

octadiene (C8H14) and then their treatment by SO2 plasma (Siow et al., 2009).

al., 2011).

Fig. 14. Schematic representation of the synthesis procedure of proton-exchange plasma polymers (Roualdès et al., 2007).

ability and its main role is to provide the transport of protons from the anode where they are produced by the oxidation of fuel (Eqs. (5) and (7)), to the cathode where they are consumed by the reduction of oxygen into water (Eqs. (6) and (8)). Until recently, the results obtained in this respect were not as good as those for conventional membranes (Roualdès et al., 2007). However, the latest reports provide much more promising results. For example, the mentioned already proton-exchange membranes, which are plasma deposited from styrene and CF3SO3H (Fig. 14), can have a higher percentage of proton exchange groups, a higher proton conductivity and a lower fuel permeability, compared with commercially available Nafion® membranes, when appropriate parameters of the plasma process are chosen. Moreover, various plasma procedures are examined in search of these membranes with the best properties. Apart from the typical PECVD method (Ennajdaoui et al., 2010; Roualdès et al., 2007), the remote (after glow) plasma technique (Jiang et al., 2011) and the plasma deposition under high pressure conditions (Merche et al., 2010) have been employed. Other types of proton-exchange membranes prepared by cold plasma deposition, different from those from styrene and CF3SO3H, have also been investigated, for example,

Fig. 14. Schematic representation of the synthesis procedure of proton-exchange plasma

ability and its main role is to provide the transport of protons from the anode where they are produced by the oxidation of fuel (Eqs. (5) and (7)), to the cathode where they are consumed by the reduction of oxygen into water (Eqs. (6) and (8)). Until recently, the results obtained in this respect were not as good as those for conventional membranes (Roualdès et al., 2007). However, the latest reports provide much more promising results. For example, the mentioned already proton-exchange membranes, which are plasma deposited from styrene and CF3SO3H (Fig. 14), can have a higher percentage of proton exchange groups, a higher proton conductivity and a lower fuel permeability, compared with commercially available Nafion® membranes, when appropriate parameters of the plasma process are chosen. Moreover, various plasma procedures are examined in search of these membranes with the best properties. Apart from the typical PECVD method (Ennajdaoui et al., 2010; Roualdès et al., 2007), the remote (after glow) plasma technique (Jiang et al., 2011) and the plasma deposition under high pressure conditions (Merche et al., 2010) have been employed. Other types of proton-exchange membranes prepared by cold plasma deposition, different from those from styrene and CF3SO3H, have also been investigated, for example,

polymers (Roualdès et al., 2007).

fluorinated carboxylic membranes from H2O and C4F8 (Thery et al., 2010), phosphorousdoped silicon dioxide membranes from SiH4, PH3 and N2O (Prakash et al., 2008) as well as membranes produced by plasma polymerization from heptylamine (C7H15NH2) or 1,7 octadiene (C8H14) and then their treatment by SO2 plasma (Siow et al., 2009).

The cold plasma technology is also tested for the preparation of anion-exchange membranes, which are presently becoming significant materials for application in alkaline fuel cells, where hydroxyl ions OH− are the ion charge carriers. Similarly, as in the case of the protonexchange membranes, the plasma deposition provides formation of very thin crosslinked films with high ion (OH−) conductivity, high chemical stability and low fuel permeability. The first attempt to obtain such membranes was undertaken only in 2006, when precursors containing tertiary amine groups were plasma polymerized and then the deposited films were quaternized by methyl iodide (Schieda et al., 2006). Very recently, an opposite procedure was applied, namely, the films were plasma polymerized from vinylbenzyl chloride, then the benzyl chloride groups (−CH2Cl) present in the films were quaternized by trimethylamine into −CH2N+(CH3)3Cl− groups, and finally these groups were alkalized by KOH into −CH2N+(CH3)3OH− groups. This material proves to be an excellent hydroxide ion conductor with great potential for application in alkaline direct alcohol fuel cells (Zhang et al., 2011).

Lastly, we should also mention the solid oxide fuel cells (SOFC), which are currently of great interest. As far as the plasma technology is concerned, the thermal plasma (see: Fig. 1) is particularly relevant in this case (Henne, 2007). However, the cold plasma is trying to use as well. In addition to the reactive sputtering method that is quite justified when we want to obtain thin films of inorganic oxides (e.g. La-Si-O, which is a potential candidate as electrolyte material for intermediate-temperature solid oxide fuel cells (Briois et al., 2007)), attempts to employ the PECVD method have been also made. Popular solid oxide electrolyte material including yttria-stabilized zirconia (YSZ) was prepared by microwave plasma polymerization from (acetylacetonate)zirconium(I) and tris(dipivaloylmethanato) yttrium(III) as precursors (Itoh & Matsumoto, 1999). Recently performed physicochemical investigations of such deposited solid electrolyte films have shown characteristic nanostructures that are strongly affected by the variation of plasma parameters and the precursor mixture composition. Thus, we can obtain the films with exactly the desired structure and properties, for example, with appropriate ionic conductivity.

#### **6.2 Solid electrolytes for lithium-ion batteries**

The polymer electrolytes for Li-ion batteries are fundamentally the same as those used in fuel cells, with the only difference that ions transported in this case are Li+ ions. These membranes should be very good electronic insulators that separate the anode from the cathode, but very good lithium-ion conductors. They should also have adequate chemical resistance as well as adequate mechanical strength to withstand the pressure changes and stresses of the electrodes during discharge/charge cycling of the battery. All these requirements can be satisfied by plasma polymerized thin films. The first reports on the preparation of such films appeared at the end of the 1980's. The films were deposited from precursors containing alkoxy, siloxane and vinyl groups in one molecule. Then, the films were sprayed with a solution of LiOCl4 to introduce Li+ into the plasma polymer structure (Ogumi et al., 1989). In subsequent years, more complex systems were prepared. For

Cold Plasma – A Promising Tool for the Development of Electrochemical Cells 131

electrochemical cell engineering is a problem, however, so vast that a separate chapter

I would like to thank all the members of my team: prof. P. Kazimierski, dr. S. Kuberski, dr. J. Sielski, R. Kapica, as well as my doctor students: P. Makowski, W. Redzynia, A. Twardowski, and I. Ludwiczak, for their excellent cooperation. I also thank Ms. K.M.

Amade, R.; Jover, E.; Caglar, B.; Mutlu, T. & Bertran, E. (2011). Optimization of

Artero, V.; Chavarot-Kerlidou, M. & Fontecave, M. (2011). Splitting Water with Cobalt.

Ayllón, J.A.; Figueras, A.; Garelik, S.; Spirkova, L.; Durand, J. & Cot, L. (1999). Preparation of

Barreca, D.; Devi, A.; Fischer, R.A.; Bekermann, D.; Gasparotto, A.; Gavagnin, M.; Maccato,

Battiston, G.A.; Gerbasi, R.; Gregori, A.; Porchia, M.; Cattarin, S. & Rizzi, G.A. (2000).

Belmonte, T.; Henrion, G. & Gries, T. (2011). Nonequilibrium Atmospheric Plasma

Biederman, H. (Ed.). (2004). *Plasma Polymer Films*, Imperial College Press, ISBN 1-86094-

Borrás, A.; Sánches-Valencia, J.R.; Garrido-Molinero, J.; Barranco, A. & González-Elipe, A.R.

Brault, P. (2011). Plasma Deposition of Catalytic Thin Films: Experiments, Applications,

Brenner, J.R.; Harkness, J.B.L.; Knickelbein, M.B.; Krumdick, G.K. & Marshall, C.L. (1997).

Briois, P.; Lapostolle, F. & Billard, A. (2007). Investigations of Apatite-Structure Coatings

Molecular Modeling. *Surf. Coat. Technol.*, Vol. 205, pp. S15-S23

MnO2/Vertically Aligned Carbon Nanotube Composite for Supercapacitor

TiO2 Powder Using Titanium Tetraisopropoxide Decomposition in a Plasma Enhanced Chemical Vapor Deposition (PECVD) Reactor. *J. Mater. Sci. Lett.*, Vol. 18,

C.; Tondello, E.; Bontempi, E.; Depero, L.E. & Sada, C. (2011). Strongly Oriented Co3O4 Thin Films on MgO(100) and MgAl2O4(100) Substrates by PE-CVD. *Cryst.* 

PECVD of Amorphous TiO2 Thin Films: Effect of Growth Temperature and Plasma

(2009). Porosity and Microstructure of Plasma Deposited TiO2 Thin Films. *Micropor.* 

Microwave Plasma Synthesis of Carbon-Supported Ultrafine Metal Particles.

Deposited by Reactive Magnetron Sputtering Dedicated to IT-SOFC. *Plasma Process.* 

Palinska for her technical assistant in the preparation of this Chapter.

Application. *J. Power Sources*, Vol. 196, pp. 5779-5783

Gas Composition. *Thin Solid Films*, Vol. 371, pp. 126-131

Deposition. *J. Therm. Spray Techn.*, Vol. 20, pp. 744-759

*Angew. Chem. Int. Ed.*, Vol. 50, pp. 7238-7266

*Eng. Comm.*, Vol 13, pp. 3670-3673

*Mesopor. Mat.*, Vol. 118, pp. 314-324

*NanoStructured Mater.*, Vol. 8, pp. 1-17

*Polym*., Vol. 4, pp. S99-S103

should be devoted to it.

**8. Acknowledgment** 

**9. References** 

pp. 1319-1321

467-1, London

example, to suppress a reaction between electrodes and the electrolyte, especially to suppress the dendritic growth of lithium during battery charging, a concept of functional gradient solid polymer electrolyte was developed. This electrolyte system was obtained by changing the composition of the mixture of precursors (dimethyl-2-[(2-ethoxyethoxy) ethoxy]vinylsilane and 1,1-didifluoroethylene) during the plasma polymerization process (Ogumi et al., 1997).

The PECVD technique appears to be a unique method that allows for the implementation of Li-ion batteries with particularly sophisticated architecture. A new type of 3D microbatteries with anode or cathode post-arrays has been recently developed in the Tolbert Lab at University of California (Los Angeles). However, such systems require a solid electrolyte in the form of conformal coatings that will evenly cover the high aspect ratio electrodes. Plasma deposited polyethyleneoxide-like electrolyte films, which are electronic insulating and can be intercalated with lithium ions, have been chosen to this end. Currently, these films are intensively investigated (Dudek, 2011).
