Abstract:
In general, in one aspect, a graphene film is used as a protective layer for current collectors in electrochemical energy conversion and storage devices. The graphene film inhibits passivation or corrosion of the underlying metals of the current collectors without adding additional weight or volume to the devices. The graphene film is highly conductive so the coated current collectors maintain conductivity as high as that of underlying metals. The protective nature of the graphene film enables less corrosion resistant, less costly and/or lighter weight metals to be utilized as current collectors. The graphene film may be formed directly on Cu or Ni current collectors using chemical vapor deposition (CVD) or may be transferred to other types of current collectors after formation. The graphene film coated current collectors may be utilized in batteries, super capacitors, dye-sensitized solar cells, and fuel and electrolytic cells.

Description:
BACKGROUND 
       [0001]    Conventional electrochemical energy storage devices (e.g., batteries, super capacitors) and energy conversion devices (e.g., dye-sensitized solar cells, fuel and electrolytic cells) consist of a pair of electrodes (positive and negative) separated by an electrolyte (e.g., polymer gel electrolyte, perforated or microporous polymeric membrane soaked in a liquid electrolyte). The electrode materials are usually coated on metallic foils that are used to collect the charge generated during discharge, and to permit connection to an external power source during recharge. The charge transfer reactions and electrolyte decomposition in the proximity of the current collectors usually result in corrosion behavior during cycling. The corrosion behavior may include one or more of: oxidization of current collectors at the positive electrode side (e.g., formation of thick surface oxide layers); ion intercalation at the negative electrode side (e.g., plating of metallic alloys and subsequent pulverization of current collectors); and etch and dissolution of exposed current collector surface. The corrosion behavior may result in passivation of the current collectors resulting in increased internal resistance and voltage drop at high current loading, or deterioration in device lifetime, performance and ultimate collapse during successive charge/discharge cycling. 
         [0002]    Current energy and environmental concerns are driving the development of energy storage devices towards the fields demanding high power output, such as electrical automotives, integration of renewable energy and smart electric grids. To meet the operation requirements, these energy storage devices need to have fast charge/discharge capability at high load current, and possess low internal resistance to suppress voltage degradation and energy dissipation in the form of waste heat. Accordingly, high-quality metals that are less susceptible to corrosion are required to be used as current collectors. Current collectors in conventional energy conversion and storage devices are usually limited to copper (Cu) for the negative side and aluminum (Al) for the positive side in non-aqueous electrolytes, or platinum (Pt), stainless steel and iron-nickel (Fe—Ni) alloy in aqueous electrolytes. 
         [0003]    To further achieve high power density and long lifetime, additional treatments are necessary to diminish corrosion at the current collectors. For example, introduction of non-corrodible conducting metal powders into electrode materials, or plating non-corrodible metal coatings onto current collectors facing the electrode sides. However, substantial quantities of noble metals such as silver, gold or platinum are needed to ensure long-term robustness. Another strategy is to induce electrically conducting organic protective layers onto current collectors or organic additives into the electrolytes. All these attempts led to significant increases in the cost and manufacture complexity of the final devices. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The features and advantages of the various embodiments will become apparent from the following detailed description in which: 
           [0005]      FIGS. 1A-1C  illustrate an example process of forming a corrosion resistant current collector/electrode for use in energy conversion and storage devices, according to one embodiment; 
           [0006]      FIGS. 2A-2F  illustrate an example process for transferring the graphene film from the current collector it was grown on to a different current collector, according to one embodiment; 
           [0007]      FIG. 3  illustrates a high level representation of an example energy conversion and/or storage device, according to one embodiment; and 
           [0008]      FIG. 4  illustrates a high level representation of an example energy conversion and/or storage device, according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Graphene is an allotrope of carbon. Its structure is one-atom-thick planar sheets of sp 2 -bonded carbon atoms that are densely packed in a honeycomb crystal lattice. A graphene film may be made of a single graphene sheet or several layers of graphene sheets. The graphene film may be impermeable to gas and ion diffusion and have excellent chemical and mechanical stability. The graphene film may therefore be used as anti-corrosion protective layers for metallic current collectors in electrochemical energy conversion and storage devices. The graphene film may be a continuous coating inserted between electrode materials (anode and cathode) and a corresponding face of the metallic current collector. Alternatively, the graphene film may cover the entire current collector. The use of graphene film provides protective layers that are efficient and reliable in inhibiting passivation or corrosion of the underlying metals without adding additional weight or volume to the system. 
         [0010]    Furthermore, the graphene film is highly conductive. Thus, the coated current collectors maintain conductivity as high as that of fresh metals. The mobility of charge carriers (electrons) between the current collectors and electrode materials can readily pass through the conducting graphene intermediate. This represents an attractive pathway to enhance the power delivery and cycling life of energy conversion and storage devices. Moreover, it may enable additional choices in the metals utilized for the current collectors. For example, less costly and/or lighter weight metals may be utilized. 
         [0011]    The graphene film may be grown on metal films, such as copper (Cu) or nickel (Ni), by a chemical vapor deposition (CVD) process. CVD processes are known to those skilled in the art. The CVD process may be conducted between approximately 500 and 1200 degrees Celsius (° C.). 
         [0012]      FIGS. 1A-1C  illustrates an example process of forming a corrosion resistant current collector/electrode for use in energy conversion and storage devices.  FIG. 1A  illustrates the process beginning with a metal layer  100  (e.g., Cu, Ni).  FIG. 1B  illustrates the metal layer  100  after a graphene film  110  is grown thereon using CVD. The graphene film  110  acts as a protective layer and may be a single graphene sheet or several layers of graphene sheets. As illustrated, the graphene film  110  was grown on both sides of the metal layer  100  and covers the entire surface of each side. According to one embodiment, the graphene film  110  may be removed from one side and utilized elsewhere. Alternatively, the graphene film  110  may be grown on a single side. The graphene film  110  on bottom side is illustrated in dotted lines to indicate it is optional. The resultant coated metallic substrate  100 ,  110  may serve as passivation/corrosion inhibitive current collectors in energy conversion and/or storage devices. The coated metallic substrates  100 ,  110  may be applied to the energy conversion and/or storage devices directly. 
         [0013]      FIG. 1C  illustrates the coated metallic substrate  100 ,  110  after electrode materials  120  are coated onto the graphene film  110 . The electrode materials  120  may be coated on a side that will face electrolyte in an energy conversion and storage devices. According to one embodiment, the electrode materials  120  may be coated on both sides of the substrate. The electrode materials  120  on bottom side are illustrated in dotted lines to indicate it is optional. The metallic substrate  100  having electrode materials  120  on both sides may, for example, be utilized between multi-stacked electrodes or cells where it acts as a cathode on one side and an anode on the other side. 
         [0014]    The electrode materials may include, but are not limited to, graphite, lithium iron phosphate, nickel oxide, manganese oxide, titanium oxide and alkaline metal hydride. The electrode materials may be coated thereon by tape casting, hot pressing, sputtering or thermal deposition. The processes for coating the electrode materials may be known to those skilled in the art. The type of electrode materials  120  used may be based on amongst other things the type of energy conversion and storage device the resultant current collector/electrode  100 ,  110 ,  120  are to be used in and whether the electrode is an anode or cathode. For the embodiment where the electrode materials  120  are on both sides, the electrode materials  120  on the two sides may be the same or may be different depending on the use thereof. 
         [0015]    The use of the graphene film  110  between the current collector  100  and the electrode material  120  may inhibit passivation or corrosion of the current collector  100  that may typically occur without affecting the conductivity thereof or adding any noticeable weight or volume thereto. 
         [0016]    The current collectors  100  (e.g., Cu, Ni) may be utilized in energy conversion and storage devices when appropriate. However, some devices may be better served with a different metal layer, such as an aluminum (Al) or iron (Fe). Furthermore, the use of the graphene film  110  may enable arbitrary metals to be utilized as current collectors. The arbitrary metals may be more susceptible to corrosion, may be lighter weight, and/or may be less expensive. The graphene film  110  grown via CVD on the metal layer  100  (e.g., Cu, Ni) may be mechanically transferred to other metal layers. 
         [0017]      FIGS. 2A-F  illustrate an example process for transferring the graphene film  110  from the current collector (e.g., Cu, Ni) it was grown on to a different current collector.  FIG. 2A  illustrates the metallic substrate  100  (e.g., Cu, Ni) coated with the graphene film  110  on one side as a starting point (e.g.,  FIG. 1B ).  FIG. 2B  illustrates the substrate after a photoresist film  200  is casted onto the graphene film  110  by spray coating, dip coating, spin coating, casting or lamination. The photoresist film  200  may be a polymethyl methacrylate (PMMA) film but is not limited thereto. These processes are known to those skilled in the art. Following the application of the photoresist film  120  the substrate is dried or baked to enhance adhesion between the graphene  110  and the photoresist film  200 . 
         [0018]      FIG. 2C  illustrates the substrate after the metal layer  100  (e.g., Cu, Ni) is removed leaving the graphene film  110  coated with the photoresist film  200  on one side and nothing on the other side. The metal layer  100  may be etched off using any number of etching methods, including dry etching or wet etching, known to those skilled in the art. If the metal layer  100  is etched it cannot be reused which may increase the overall cost of the resulting current collector/electrode. Alternatively, the graphene film  110  may be detached from the metal layer  100  by electrochemical peeling which is known to those skilled in the art. If the graphene film  110  is peeled off of the metal layer  100 , it can be reused to grow additional graphene films  110 . 
         [0019]      FIG. 2D  illustrates the substrate after the released side of graphene film  110  is attached to a target metal substrate  210 . The target metal substrate  210  may be selected based on various parameters, including but not limited to, the type of energy conversion and/or storage device the resultant current collector/electrode are to be used in, whether the electrode is an anode or cathode, the price point for the device, the weight requirements of the device. For example, the target metal substrate  210  may be Al, Fe, or any number of other metals that are not as high quality as the standard metals used for current collectors and may be cheaper and lighter weight metals. The graphene film  110  may be attached to the target metal substrate  210  directly upon drying or using known methods including the use of extrusion equipment to strengthen the adhesion. 
         [0020]      FIG. 2E  illustrates the substrate after removal of the photoresist film  200 . The photoresist film  200  may be removed using known methods, including but not limited to, rinsing the substrate in a solvent, such as acetone or annealing in air. The substrate may be dried before incorporation into electrodes. The graphene transfer procedure can be repeated upon needs to create coatings on both sides of the metal substrate  210 . The graphene film  110  on bottom side is illustrated in dotted lines to indicate it is optional. 
         [0021]      FIG. 2F  illustrates the substrate after electrode materials  220  are coated onto the graphene film  110 . As noted above, the electrode materials  220  may be coated either on one side or on both sides. The electrode materials  220  on bottom side are illustrated in dotted lines to indicate it is optional. The electrode materials may include, but are not limited to, graphite, lithium iron phosphate, nickel oxide, manganese oxide, titanium oxide and alkaline metal hydride. The electrode materials may be coated thereon by tape casting, hot pressing, sputtering or thermal deposition. The processes for coating the electrode materials may be known to those skilled in the art. The type of electrode materials  220  used may be based on various different parameters. The electrode materials  220  may be the same as the electrode materials  120  utilized for the current collectors  100  (e.g., Cu, Ni) or may be different based on the different material used for the current collector  210 . For the embodiment where the electrode materials  220  are on both sides, the electrode materials  220  on the two sides may be the same or may be different. 
         [0022]      FIG. 3  illustrates a high level representation of an example energy conversion and/or storage device  300 . The device  300  includes a pair of current collectors  310 ,  315  each having a surface covered with a graphene film  320 . The current collectors  310 ,  315  are metallic conductor layers. The current collectors  310 ,  315  may be made of the same metal or may be made of different metals (current collector  310  may be made of a first material while the current collector  315  is made of a second material). The current collectors  310 ,  315  may be Cu or Ni, where the graphene film  320  was grown thereon (e.g.,  FIGS. 1A-1B ). Alternatively, the current collectors  310 ,  315  may be an arbitrary metal, where the graphene film  320  is transferred thereto (e.g.,  FIGS. 2A-2E ). The graphene film  320  may be a single sheet or multiple sheets and provide corrosion protection to the current collectors  310 ,  315  while not affecting their conductivity. 
         [0023]    A cathode material  330  forms an electrode on one side of the device (on current collector  310 ) and an anode material  140  forms an electrode on an opposite side (on current collector  315 ). The cathode/anode materials  330 ,  340  may include, but are not limited to, graphite, lithium iron phosphate, nickel oxide, manganese oxide, titanium oxide and alkaline metal hydride. An electrolyte  350  is provided between the electrodes  330 ,  340 . The electrolyte  350  may be, for example, a polymer gel, or a perforated or microporous polymeric membrane soaked in a liquid. 
         [0024]    A load  360  is connected to the current collectors  310 ,  315 . The device  300  may be, for example, a battery, a supercapacitor, or a fuel cell. As one skilled in the art would know, the fuel cell generates oxygen (not illustrated) between the current collector  310  and the cathode material  330  and hydrogen (not illustrated) between the current collector  315  and the anode material  340 . 
         [0025]      FIG. 4  illustrates a high level representation of an example energy conversion and/or storage device  400 . The device  400  includes a pair of current collectors  420 ,  425  each mounted to a glass substrate  410  and having a surface covered with a graphene film  430 . The current collectors  420 ,  425  are metallic conductor layers. The graphene film  430  may have been grown on the current collectors  420 ,  425  (e.g.,  FIGS. 1A-1B ) or may have grown on other metallic layers and transferred thereto (e.g.,  FIGS. 2A-2E ). The graphene film  430  may be a single sheet or multiple sheets and provide corrosion protection to the current collectors  420 ,  425  while not affecting their conductivity. 
         [0026]    A dye absorbed photo catalyst  440  is formed on the current collector  420 . An electrolyte  450  is provided between the current collectors  420 ,  425 . The electrolyte  450  may be, for example, a polymer gel, or a perforated or microporous polymeric membrane soaked in a liquid. A load  460  is connected to the current collectors  420 ,  425 . The device  400  may be, for example, a dye-sensitized solar cell. 
         [0027]    Although the disclosure has been illustrated by reference to specific embodiments, it will be apparent that the disclosure is not limited thereto as various changes and modifications may be made thereto without departing from the scope. Reference to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described therein is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment” appearing in various places throughout the specification are not necessarily all referring to the same embodiment. 
         [0028]    The various embodiments are intended to be protected broadly within the spirit and scope of the appended claims.