Patent Application
20140030636
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.
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.
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.
The features and advantages of the various embodiments will become apparent from the following detailed description in which:
Graphene is an allotrope of carbon. Its structure is one-atom-thick planar sheets of sp2-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.
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.
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.).
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.
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.
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.
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.
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.
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.
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.
The various embodiments are intended to be protected broadly within the spirit and scope of the appended claims.
