Abstract:
This specification discloses a thin-film gas diffusion electrode (GDE) and the method for making the same. The thin-film GDE is formed in a unitary way. A dual-nature porous thin film is used as the substrate. A surface processing is performed to make one surface of the thing film hydrophlic while the other surface hydrophobic. The hydrophlic area serves as the active layer for electrochemical reactions after chemical processing. The hydrophobic area is kept dry to form a smooth gas channel, functioning as a gas diffusion layer. In this method, the thin-film GDE is free from the use of binders and high-temperature high-pressure manufacturing processes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to a thin-film gas diffusion electrode (GDE) and the method for making the same. In particular, the invention relates to a thin-film GDE formed in a unitary way and the method there for making it. 
     2. Related Art 
     The gas diffusion electrode (GDE) plays an important role in chemical power sources. The GDE uses gas, such as hydrogen and oxygen, as the active material. Through the dissolving and diffusion steps, the gas active material has electrochemical reactions with the ions in the electrolyte to produce an electric current. The GDE using oxygen as the active material is called the air electrode. The combination of the oxygen GDE and the hydrogen GDE forms the main part of fuel cells. 
     In observation of the impacts on the environment due to the rapid population growth and industrial over-development, how to provide an environment-friendly business has become an important subject for all companies. The low-pollution fuel cells are seen as the most important energy source in the 21st century to provide a clean electric power. Such fuel cells can be widely used in electric cars, generators, and even the 3C products. The fuel cells produce electric power through electrochemical reactions but not burning. The hydrogen and oxygen react to produce electric currents, water and heat. There is almost no pollution at all. 
     The conventional GDE is mainly composed of three parts: a current collector, a gas diffusion layer, and an active layer. The current collector is responsible for electron transmissions. It is made of a good conductive metal. The gas diffusion layer provides a channel for gas to diffuse. It has to be kept completely dry, preventing the electrolyte from entering and blocking the diffusion channel. The active layer is the place to undergo a inhomogeneous electrochemical reactions. To accelerate the reaction speed, the active layer is often added with electrocatalysts. 
     The current GDE manufacturing technology is to make the above-mentioned three parts separately and combine them together afterwards. For the active layer, one first has to adsorb electrocatalysts directly onto a porous carbon substance and then add 5% to 15% of binder, which is usually a hydrophobic polytetrafluoroethylene (PTFE). They are homogeneously mixed and formed by rolling. For the gas diffusion layer, one mixes carbon powders and a high-concentration (15% to 25%) hydrophobic binder, usually hydrophobic PTFE, and performs formation by rolling. After finishing the active layer and the gas diffusion layer, they along with the current collect are combined to form a single GDE by hot-pressing. In this method, one needs to use a binder that may cause the following problems. First, during hot-pressing the binder experiences a temperature higher than its glass temperature. Therefore, it becomes mobile so that under pressure it often clogs the gaps and holes on the electrode. Therefore, larger gas channels in the gas diffusion layer are blocked, increasing the resistance of gas transportation. At the same time, the binder is likely cover the electrocatalysts disposed on the carbon powder surface on the active layer. The usage rate of the electrocatalysts and therefore the reaction rate are lowered. All the above effects significantly reduce the output power of the ensemble. 
     In addition, the conventional manufacturing method of the GDE inevitably requires the use of high-temperature and high-pressure processes. Each step of the temperature rising, lowering, and mold withdrawing results in limits on the production speed. The high-temperature and high-pressure processes also worry people with safety concerns. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an objective of the invention to provide a thin-film gas diffusion electrode (GDE) and the method for making the same. The thin-film GDE is formed in a unitary way to avoid the use of a binder and hot-pressing. A porous thin film is used as the substrate. Both surfaces of the thin film are processed in different ways. One surface is a hydrophilic area, whereas the other is a hydrophobic area. The hydrophilic area is chemically processed to become an active layer for chemical reactions. The hydrophobic area is kept dry to form a smooth gas channel as the gas diffusion layer. 
     To achieve the above objective, the main body of the disclosed thin-film GDE has a hydrophobic area on one surface and a porous thin-film hydrophilic area on the other. The hydrophobic area is the gas diffusion layer for gas to pass through. The hydrophilic area is the active layer. In order for the active layer to be electrochemically active, the surface of the hydrophilic area has to be covered with an active substrate. Electrocatalysts are then deposited on the active substrate by a chemical or electrochemical method. This enables electrochemical reactions to be performed on the active layer with the hydrophilic property. Therefore, any place that the electrolyte can reach can undergo electrochemical reactions. 
     For such an innovative thin-film GDE formed in a unitary fashion, one has to employ an appropriate manufacturing method. First, both surfaces of a porous thin film are processed in different ways, so that one surface is a hydrophilic area and the other a hydrophobic area. This results in a dual-nature porous thin film. The hydrophilic area undergoes a wet chemical processing and is covered by an active substrate. Electrocatalysts that facilitate reactions are deposited on the active substrate so that the hydrophilic area of the porous thin film is electrochemically active. The porous thin film can be a polymer thin film. The technology of modifying the surface nature is pretty mature nowadays. One can perform different surface processes on a hydrophobic or hydrophilic porous thin film to obtain the same dual-nature porous thin film. 
     On the other hand, the method of covering the hydrophilic area with an active substrate by a wet chemical process can be achieved by using chemical plating to form a thin metal layer. An alternative method is to deposit a conductive polymer layer as the active substrate using a chemical method. The step of depositing electrocatalysts on the active substrate is achieved by self-assembly, electrophoretic deposition, or electrochemical deposition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a flowchart of making the disclosed thin-film GDE; 
         FIGS. 2 and 3  compare the binding energy of the sulfur element before and after electrocatalysts are bonded to the active substrate using the self-assembly method; and 
         FIG. 4  is a CV analysis diagram of the active layer in the disclosed GDE before and after the electrocatalysts bonding. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     To describe the invention in further detail, please refer to  FIG. 1 . First, a porous thin film is provided (step  110 ). The surfaces of the porous thin film are processed to form a dual-nature porous thin film (step  120 ). One surface of this dual-nature thin film is a hydrophilic area and the other a hydrophobic area. The hydrophilic area goes through a wet chemical process to form an active substrate (step  130 ). Finally, the active substrate is covered with electrocatalysts so that it becomes electrochemically active (step  140 ). 
     The porous thin film material can be selected from porous polymer thin films such as polyurethane (PU), polyethylene (PE), and polytetrofluoroethylene (PTFE) or carbon cloths. In the step of processing the surfaces of the porous thin film to form a dual-nature thin film, the surfaces of different materials have to be processed in different ways. For a hydrophobic porous polymer thin film (such as a PE film), one surface is protected by a special holding tool, whereas the other is exposed to a alkaline solution to perform a hydrophilic processing. One may also choose a hydrophilic porous polymer thin film. In this case, one surface is protected by a special holding tool, whereas the other is covered with a hydrophobic porous polymer thin film with a thickness over 0.05 mm. Either way can provide a dual-nature porous thin film. 
     In the step of performing wet chemical processing on the hydrophilic area to form an active substrate, the wet chemical processing is chemical plating. The hydrophilic area is metal-activated and then coated with an appropriate metal layer. Alternatively, the hydrophilic area is covered with a conductive polymer layer, such as the polypyrrole, in a chemical way as the active substrate. 
     Finally, the step of covering electrocatalysts on the active substrate so that it becomes electrochemically active can be achieved by the self-assembly distribution technology or the electrochemical deposition method. The self-assembly method is to add specific functional groups, such as the —SH and —COOH groups, to the surrounding substitute groups of macrocyclic compounds that can speed up reactions, such as CoTMPP and CoTPP, during the synthesis process. These functional groups are able to bond with specific metals (such as gold and aluminum). Afterwards, such macrocyclic compounds (the electrocatalysts) are distributed in an organic solution, such as acetone, dimethylformamide (DMF) or tetrahydrofuran (THF). The electrode substrate that uses a metal layer as its active substrate is also immersed into the organic solution, so that the electrocatalysts can automatically bond with the active substrate. 
     To prove the self-assembly behavior of electrocatalysts, refer to  FIGS. 2 and 3  to compare the binding energy distribution of the sulfur element (S) before and after the electrocatalysts are bonded with the active substrate using the self-assembly method. This ensures that the self-assembly method is indeed able to bond the electrocatalysts with the metal active substrate. We employ the X-ray photoelectron spectroscopy (XPS) to detect the spectrum. The experimental details are as follows. An electrocatalysts with the S functional groups is dissolved into the THF, forming a 0.1 mM solution. A “wafer gold” is then used as the electrode substrate and immersed in the solution for 24 hours. The wafer gold is formed by first coating a silicon chip surface with a layer of 1500 Å thick TiW (to increase the adhesion of gold) and then a layer of 1000 Å thick gold. Afterwards, the immersed electrode is taken out and immersed in clean THF for 20 minutes. Finally, the electrode is washed to remove non-bonding electrocatalysts from its surface. This step guarantees that the electricatalyst with the S functional groups are combined to the electrode by bonding. The S element is bonded to the functional groups of the electrocatalysts molecules, as shown in  FIG. 2 . The two vertical lines at 165 eV and 163.6 eV represent the atomic binding energy of the S(2p1) and S(2p3) states, respectively. One sees from there that the binding energy of the S element after the electrocatalysts is bonded with the active substrate are shifted to the right, as shown in  FIG. 3 . The dashed lines in  FIG. 3  indicate where the solid lines are in  FIG. 2 . This means that the electrocatalysts molecules automatically bond to the active substrate after the self-assembly. 
     If one uses the electrochemical deposition method to cover the electrocatalysts on the active substrate, a metal oxide or metal (such as ruthenium oxide, iridium oxide, MnO 2 , NiO, Pt, Ag, etc) is directly grown on a metal substrate. One may also deposit nanometer powders of metal or metal oxide that can speed up chemical reactions on the metal substrate by electrophoretic deposition as the electrocatalysts. To prevent the deposited electrocatalysts from coming off the metal substrate, one may impose an appropriate thermal process to increase their adhesiveness. 
     In the following, we use an embodiment to explain the actual procedure of the invention. First, a 3 mm thick PU polymer film is placed in a special holding tool that only allows one surface to have reactions with the solution. The PU polymer film along with the holding tool is placed into a KOH solution to perform the hydrophilic processing. Afterwards, they are washed by de-ion water. The polymer film then goes through a electroless plating activating step. The activated polymer film is first coated with a thin layer of nickel in a electroless plating fashion and then put into immersion gold, allowing gold to replace nickel on the polymer film surface. The polymer is then left quietly for a period. A homogeneous gold-plated layer is thus formed as the active substrate. This method renders a dual-nature polymer film with an active substrate. Finally, the dual-nature polymer film is disposed into an organic solution distributed with electrocatalysts until the electrocatalysts are completely bonded with the gold-plated layer. One then obtains a thin-film GDE formed in a unitary method. 
     For further explanation of the effects of the disclosed thin-film GDE, please refer to  FIG. 4  that analyzes the voltage and current before and after the electrocatalysts bonding. The scan speed is 10 mV/s. The vertical axis indicates the current density in units of A/cm 2 . The horizontal axis indicates the voltage in units of V. We employ the cyclic voltammetry (CV) to measure the oxidation and reduction reactions. The voltage is continuously changed to measure the corresponding current. As shown in  FIG. 4 , the blue contour represents the CV contour before the electrode and the electrocatalysts are bonded. The red contour represents the CV contour after the electrode and the electrocatalysts are bonded. Comparing these two contours, one sees that the disclosed thin-film GDE has more stable oxidation and reduction properties. 
     The invention discloses a new thin-film gas diffusion electrode structure and the method for preparing the same. It is completely free from the troubles of using binders and high-temperature high-pressure processes in the prior art. Aside from no worries about the safety concerns, most important of all, the invention does not need to use binders. This can increase the electrocatalysts usage rate and keep the gas transportation channel from being clogged or blocked. Moreover, due to its unitary formation, the electrode is very thin. This can reduce the weight and volume of the fuel cells. Because of its flexibility, the disclosed thin-film GDE can be widely used in cell designs of various kinds of shapes. 
     Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments, will be apparent to persons skilled in the art. It is, therefore, contemplated that the appended claims will cover all modifications that fall within the true scope of the invention.