Abstract:
An electrochemical energy conversion system comprises an electrochemical energy conversion device, in fluid communication with a source of liquid carrier of hydrogen and an oxidant, for receiving, catalyzing and electrochemically oxidizing at least a portion of the hydrogen to generate electricity, a hydrogen depleted liquid, and water. A method of electrochemical energy conversion includes the steps of directing a liquid carrier of hydrogen to an electrochemical conversion device and electrochemically dehydrogenating the liquid carrier of hydrogen in the presence of a catalyst to produce electricity.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application relates to, and claims priority from, provisionally filed US patent application having docket number 205589-1 and Ser. No. 60/910,092, entitled “HYDROGEN CARRIERS BASED ON AROMATIC NITROGEN CONTAINING HETEROCYCLIC COMPOUNDS”, filed on Apr. 4, 2007, which application is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The invention relates generally to a method and apparatus for electrochemical energy conversion and more specifically to methods and apparatus of electrochemical energy conversion using a liquid carrier of hydrogen. 
         [0003]    Proton exchange membrane (PEM) based fuel cells are considered to be effective electricity generators for both stationary and mobile applications. PEM fuel cells electrochemically react air with an external supply of fuel to produce electricity and typically have an energy density that is greater than conventional electrochemical batteries. Typical fuel for a PEM fuel cell is hydrogen. Effective hydrogen storage remains a challenge, especially for mobile applications. High pressure or liquid hydrogen storage options are too expensive and typically have a low volumetric energy density. Current solid materials for hydrogen storage operating at temperatures below the typical operating temperatures of PEM fuel cells (100 C) are currently capable of storing only about 4 wt. % and require a sophisticated heat management system that reduces total system capacity by about 50%. In addition such materials require total redesign of cars and refueling infrastructure. Liquid fuels like methanol also can be used in PEM fuel cells. However, these fuels generate CO 2  and CO that poisons the fuel cell catalyst. The most effective type of fuel for a PEM fuel cell is methanol that is a very toxic and highly flammable liquid. The use of a diluted methanol fuel reduces these risks but also substantially reduces the system energy density. 
         [0004]    To improve the energy density of the PEM fuel cell system, many efforts are focused on improvement of the hydrogen storage subsystem. Some high capacity metal hydride options currently exist but they are either irreversible or work reversibly at much higher temperatures than the fuel cell operates. The recharge of these hydrides involves a high rate of heat dissipation and therefore additional components such as a heat exchanger. 
         [0005]    Accordingly, there is a need in the art for an improved electrochemical energy conversion system that overcomes some of the limitations of the current PEM fuel cell and hydrogen storage limitations. 
       BRIEF DESCRIPTION 
       [0006]    An electrochemical energy conversion system comprises an electrochemical energy conversion device, in fluid communication with a source of liquid carrier of hydrogen and an oxidant, for receiving, catalyzing and electrochemically oxidizing at least a portion of the hydrogen to generate electricity, a hydrogen depleted liquid, and water. A method of electrochemical energy conversion includes the steps of directing a liquid carrier of hydrogen to an electrochemical conversion device and electrochemically dehydrogenating the liquid carrier of hydrogen in the presence of a catalyst to produce electricity. 
     
    
     
       DRAWINGS 
         [0007]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0008]      FIG. 1  is a schematic illustration of one embodiment of the instant invention. 
           [0009]      FIG. 2  is a schematic illustration of another embodiment of the instant invention. 
           [0010]      FIG. 3  is picture of an experimental setup in accordance with another embodiment of the instant invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    An electrochemical energy conversion system  10  comprises an electrochemical energy conversion device  12  in fluid communication with a source of a liquid carrier of hydrogen  14  (LQH n ) and an oxidant  15 , typically air, purified oxygen or their mixture, as shown in  FIG. 1 . The electrochemical energy conversion device  12  receives, catalyzes and electrochemically oxidizes at least a portion of the hydrogen  16 , contained in the liquid carrier of hydrogen LQH n    14  to generate electricity  18 , a hydrogen depleted liquid LQ  20 , and water  22 . Hydrogen depleted liquid LQ  20  may include both fully hydrogen depleted liquids and partially hydrogen depleted liquids. While the appropriate liquid carrier of hydrogen  14  will vary from system to system, the selection process will typically be based on criteria such as the hydrogen storage capacity of the carrier, the rate and the heat of dehydrogenation of the carrier, the boiling point of the carrier and the overall cost of the carrier. 
         [0012]    In one embodiment, electrochemical energy conversion device  12  comprises a Proton Exchange Membrane (PEM) fuel cell that includes a solid electrolyte  24  that separates an anode portion  26  and a cathode portion  28 . PEM fuel cell  12  further comprises a catalyst  30 , typically disposed on the anodic side of the solid electrolyte  24 , for accelerating the disassociating and oxidation of hydrogen  16  from the liquid carrier of hydrogen  14 . In one embodiment, catalyst  30  comprises palladium, platinum, rhodium, ruthenium, nickel and combinations thereof. In another embodiment, the catalyst  30  is a group VIII metal, such as finely dispersed metal alloys and transition metal complexes with multidentate P- or N-containing ligands (e.g. “pincer” type) on high-surface-area conductive supports like carbon or conductive polymers. In one embodiment, the catalyst  30  will be anchored to the anode portion  26  via formation of chemical bonds between the catalyst  30  and the anode portion  26 , for example, by using functionalized silanes or by adsorption on a ligand-modified surface. In one embodiment, the system  10  may further comprise a catalyst material (not shown) on the cathode portion  28  to increase the electrochemical cell potential and improve the oxygen reduction reaction. 
         [0013]    System  10  further comprises a storage tank  32  for storing the hydrogen depleted liquid  20 . The solid electrolyte  24  typically comprises a membrane, for example Nafion®, which membrane is compatible with the liquid carrier of hydrogen  14  and the catalyst  30 . In another embodiment, the solid electrolyte is a high-temperature membrane based on composites of proton-conductive ceramics and high-heat polymers (for example, polysulfones or polybenzimidazoles). In addition to the oxidant  15 , a quantity of water vapor may be directed into the system to keep the solid electrolyte  24  hydrated for better proton conductivity, 
         [0014]    In one embodiment, the liquid carrier of hydrogen  14  is an organic liquid carrier of hydrogen. In another embodiment, the liquid carrier of hydrogen  14  is a cyclic hydrocarbon. In another embodiment, the liquid carrier of hydrogen is a partially or fully hydrogenated nitrogen-containing aromatic heterocycle, for example, 2-aminopyridine, 4-methylpyrimidine, dipyrimidinemethane, dimethyltetrazine, dipirimidine, diazacarbazole, alkylcarbazole, 4-aminopyridine, dipyrazinemethane, tripyrazinemethane, tripyrazineamine, dipyrazine, tetrazacarbazole, isoquinoline, di(2-pyridyl)amine, quinazoline, and combinations thereof. In yet another embodiment, the liquid carrier of hydrogen  14  is a partially or fully hydrogenated aromatic hydrocarbon, for example naphthalene, benzene, anthracene, and combinations thereof. In yet another embodiment, the liquid carrier of hydrogen  14  is one of perhydro-N-ethylcarbazole, cyclohexane, tetrahydroisoquinoline, tetraline, decaline, and combinations thereof. 
         [0015]    In some embodiments, the liquid carrier of hydrogen  14  may include certain additives to improve its flow characteristics or enhance the electrochemical reaction that occurs at the electrochemical energy conversion device  12 . 
         [0016]    In operation, the liquid carrier of hydrogen  14 , for example an organic liquid carrier, is directed (typically from a tank  34  or the like) to the electrochemical energy conversion device  12 , where the liquid carrier is electrochemically dehydrogenated in the presence of a catalyst to produce electricity  18 . As discussed above, several limitations exist in the current PEM fuel cell and hydrogen storage systems including the lack of a high-capacity hydrogen storage medium and the incompatibility of such systems with the existing fueling and transportation infrastructure. The current invention, however, provides a high-capacity energy storage solution, as several liquid carriers of hydrogen exceed 7-wt % hydrogen storage capacity. At a capacity of 7 wt. % hydrogen, a 20-gallon tank of an organic liquid carrier will provide about 5 kg equivalent of hydrogen enabling about a 300-mile drive. In addition, because the energy storage solution is based on a liquid carrier, the existing re-fueling and transportation infrastructure can be utilized without substantial modification. 
         [0017]    Other benefits of the instant invention are that the electricity  18  is produced from the electrochemical energy conversion device  12  without the production of a hydrogen gas, making utilization and storage concerns, safety and size much easier to deal with. Furthermore, the hydrogen-depleted organic liquid  20  can be re-hydrogenated via on-board electrolysis (when used in a plug-in mode) or off-board (when used in a fuel cell vehicle mode). Thus, system  10  is both an attractive hydrogen storage solution and a high-capacity energy storage solution. Accordingly, this system provides a single hydrogen/energy storage solution for a combined plug-in electric and pure hydrogen fuel cell vehicles. As a hydrogen storage solution, the system  10  has the advantage of being able to use the existing re-fueling infrastructure. As a plug-in solution, the system  10  can be recharged at night, thus regenerating fuel cost effectively and easing the distribution of at least part of the overall energy for transportation via existing electrical grids instead of through fuel transportation and distribution networks. 
         [0018]    Another embodiment of an electrochemical energy conversion system  100  is shown in  FIG. 2 . System  100  combines the two storage tanks required in system  10  and utilizes a single storage tank or vessel  102  comprising a separator  104 , for example a membrane separator, that divides the storage tank or vessel  102  into multiple portions to store both the liquid carrier of hydrogen  14  and the hydrogen depleted liquid  20 . In another embodiment, the membrane separator  104  is a flexible membrane. Such an arrangement makes the system  100  much more compact and efficient, especially in the re-fueling process. While the system  100  shows the liquid carrier of hydrogen  14  in a bottom portion of the tank  102  and the hydrogen depleted liquid  20  in a top portion of the tank  102 , it is contemplated that those positions could be altered and potentially many other segmentation configurations would be within the spirit of this invention. 
         [0019]    The chemistry involved within the instant invention can be summarized as follows. Partial electro-oxidation of the liquid carrier of hydrogen  14 , for example an organic carrier, in the presence of an electrocatalyst  24  generates protons (Equation 1), where LQ stands for a hydrogen depleted organic carrier molecule. 
         [0000]      LQ*H n →LQ+ n H +   +ne   −   (1) 
         [0020]    Generated protons travel through the solid electrolyte  24  and combine with reduced oxygen at the cathode  28  to generate water  22  (Equation 2). 
         [0000]        n/ 2O 2   +n H +   →n/ 2H 2   O−ne   −   (2) 
         [0021]    The total reaction is described by Equation 3. 
         [0000]      LQ*H n   +n/ 2O 2 →LQ+ n/ 2H 2 O  (3) 
         [0022]    In these equation, all reactions are reversible, which allows the fuel cell to be used as an electrolyzer for recharging of the organic carrier. In the electrolyzer, the cell is a flow battery in which high energy hydrogenated fuel is stored separately from the electrochemical cell thus increasing the system energy density. Known flow batteries (vanadium, zinc-bromine) with liquid electrolyte have flexible layouts, and high power and capacity but cannot be used for most applications, including mobile applications, due to the low energy density of the electrolyte (75-140 Wh/kg). The calculated energy densities of certain liquid carriers of hydrogen  14 , such as organic liquid carriers are in the range between about 1550 to about 2000 Wh/kg. The use of a direct rechargeable fuel cell as a flow battery will make the energy density of the total hydrogen storage and utilization system close to the theoretical limit, and suitable for mobile applications. 
         [0023]    The off-board hydrogenation of organic cyclic and heterocyclic molecules can be accomplished in relatively mild conditions (e.g. 100° C., 7 bar hydrogen) with the appropriate catalyst (high surface area Ni or supported Pt) to yield the saturated molecule with good conversion and turnover rates. However, dehydrogenation, the reverse reaction, is highly endothermic and strongly limited by thermodynamic equlibria. Catalytic thermal dehydrogenation of cyclic hydrocarbons usually requires high temperature (&gt;200° C.) and has slow kinetics. Electrochemical dehydrogenation, as discussed in the current invention, however, can be conducted at lower temperatures and at higher rates. 
         [0024]    To calculate the theoretical open circuit voltage (OCV) of an exemplary electrochemical energy conversion device  12  for different carriers, the ΔG (Gibbs energy) of reaction was used (3), as shown above. ΔG can be calculated from ΔG of two reactions (4 and 5). The parameters of hydrogen oxidation reaction are well known, and ΔG of reaction 4 is known for some molecules and can be estimated based on theoretical calculations for others. This approach gives OCV values for various organic carriers in the range between about 950 to about 1020 mV. The higher the heat of dehydrogenation, the lower the fuel cell OCV. 
         [0000]      LQ*H n →LQ+ n/ 2H 2   (4) 
         [0000]      H 2 +½O 2 →H 2 O  (5) 
       EXAMPLE 
       [0025]    Experiments with the use of a liquid hydrogenated carbazole demonstrated the use of liquid organic compounds as a fuel in an electrochemical energy conversion device. A hydrogen fuel cell with platinum catalyst, as shown in  FIG. 3 , was filled with a diluted solution of dodecahydrocarbazole in acetonitrile and demonstrated an OCV of 340 mV with oxygen as an oxidant. Using a carbon black/Ni/Pt—C electrode with dodecahydrocarbazole as the carrier increased the OCV to 650 mV. 
         [0026]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.