Abstract:
A fuel cell is used to create pressure by reverse biasing the fuel cell. A voltage is applied across the fuel cell to change the liquid near the fuel cell into gas, and expand its volume. The volume expansion is used for work function, either to expand a housing or move a piston or the like. By removing the voltage, the fuel cell can regenerate by absorbing the gas to again create energy, thereby retracting the volume expansion. In an embodiment, water may be the electrolyte which is electrolyzed to form hydrogen and oxygen gas, and then recombined into water.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims benefit of U.S. Provisional Application No. 60/282,951, filed Apr. 13, 2001. 
     
    
     BACKGROUND  
       [0002]     Many pressure producing elements use a moving part to create a pressure increase. The use of a moving part may produce drawbacks, especially in difficult operating conditions. Recently there has been considerable interest in the development of materials to convert electrical energy directly to mechanical energy and a number of new actuating materials are being developed to this end. These include electrochemically responsive conducting polymers, capacitance-driven carbon nanotube actuators, pH responsive hydrogels, ionic polymer metal composites, electric field responsive elastomers, and field electrostrictive polymers. An impetus behind this development is the desire to create more efficient transduction which can be scaled to size or weight demands that cannot be fulfilled by conventional electric motors, pumps, and switches. These constraints are particulary relevant to the emerging fields of microfluidics, microelectromechanical systems (MEMS), and robotics. While many of the new materials under investigation exhibit useful specific properties, e.g., large stresses, sizable strains, or fast cycling time, they commonly suffer from inherent limitations that severely restrict their general applicability.  
       SUMMARY  
       [0003]     According to the present system, an electrically operable cell, one without moving parts, is disclosed. This device uses electrolytic phase transformation. This system may use electrochemically generated gas for the reversible and controllable application of pressure and/or motion as used for actuation. In an embodiment, the device uses high surface area electrodes for rapid electrochemical response, and the separation of the electrochemical half reactions, as in, for example, a fuel cell, for full control of the volume and pressure change processes. In an embodiment, the cell is constructed in a flexible housing, as in a membrane, for direct application of pressure/volume. Another embodiment uses a rigid housing for the external direction of the pressure/volume changes by fluid flow in or out of the cell via tubing.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]     These and other aspects will now be described in detail with reference to the accompanying drawings, wherein:  
         [0005]      FIGS. 1 and 2  show a flexible walled embodiment the pressure generating device, respectively in its deenergized and energized states; and  
         [0006]      FIG. 3  shows a rigid walled device being used to drive an external device, e.g. a piston; and  
         [0007]      FIGS. 4 and 5  show representative results. 
     
    
     DETAILED DESCRIPTION  
       [0008]     The present system describes a fuel cell configured for electrolytic generation of gas from a liquid. The generation of gas from the liquid may produce volume/pressure changes. According to the present system, the volume may change-theoretically by large factors. In addition, the process is reversible, that is the gas can be recombined into a liquid, and occurs at a controlled rate.  
         [0009]     Rapid recombination of gases is facilitated by electrodes having large effective catalyst surface area. Control of the recombination is achieved by physically separating the half reactions. Both conditions are present in fuel cells, that are well-known in the art. A common fuel cell configuration seperates the two half-reactions by an ionically conducting membrane.  
         [0010]     The present application operates by repeatedly running a cell, e.g. a electrochemically-reacting cell such as a fuel cell, “in reverse, that is to generate the necessary gases electrolytically, for the forward or generating portion of the cycle. This occurs in a 3:2 stoichiometric ratio of gas to liquid.  
         [0011]     Specifically, for a hydrogen-oxygen electrochemical reaction: 
 
2H 2 O( l )         O 2 ( g )+4H + ( aq )+ 4   e   −   (1) 
 
4H( aq )+ 4   e   −           2H 2 ( g )  (2) 
 
Net: 2H 2 O( l )         O 2 ( g )+2H 2 ( g )  (3) 
 
         [0012]     The fuel cell may then be operated in the conventional way to consume the gas, and return the system to its initial state. In this part of the operation, some energy may be recouped.  
         [0013]     A reversible actuator, according to an embodiment, is shown in  FIGS. 1, 2 , and  3 . Platinum impregnated carbon cloth fuel cell electrodes  102 ,  104  (e.g. as from Etech, Inc.) are held against a Nafion  117  proton exchange membrane  106  by perforated steel mesh elements  110 ,  112 , which also provide electrical contact to the electrodes  102 ,  104 . Wires  114 ,  116  connect the the mesh elements to contact the electrodes. Additional steel or platinum wire electrodes may be fitted and selected via an external control mechanism, for example, electrical relays. The cell is fitted within a flexible membrane housing  120  are shown in  FIGS. 1 and 2 . In an alternative embodiment, the housing is a rigid housing with fluid connectors running in parallel to prevent differential pressure buildup between the two compartments.  
         [0014]     The compartments  130 ,  131  are filled with an aqueous electrolyte solution that may be comprised of 1 molar sulphuric acid or a phosphate buffer. The cell assembly is sealed either by an epoxy seal  122  or by a rubber gasket.  
         [0015]      FIG. 2  shows how the result of the electrochemical reaction causes generated gas within the compartments  131 ,  130 . This causes the flexible wall  120  to expand in the area  133 . This expansion may itself be used for work, or may be used for sealing an orifice such as  134 .  
         [0016]     The device embodiment with a rigid housing  299  is shown with its control system in  FIG. 3 . The operation of the actuator may be computer controlled by controller  300 , which may include relays  302  or switches therein. The fuel cell  310  is operated in electrolysis mode or in recombination mode by configuring relays  302  to apply a current across electrodes  312 ,  314  or to draw a current from electrodes  312 ,  314 , respectively. In electrolysis mode, the applied current causes water in the cell to be converted to hydrogen at the cathode, and to oxygen at the anode.  
         [0017]     The generation of gas increases the pressure and/or volume in the cell. In the  FIGS. 1 and 2  embodiment, this causes the expansion of the flexible membrane  120  to the shape shown in  FIG. 2 . In the  FIG. 3  embodiment, this causes the application of fluid force and motion through fluid conduits  322  to, for example, a piston  320 . An applied potential of 3V may be sufficient under moderate conditions. In recombination mode, a lower applied voltage, or short circuit, between the fuel cell electrodes leads to the oxidation of hydrogen and reduction of oxygen to water. Current flows through the cell as the gases are consumed, and the rate of the process is controlled by the external electrical circuit.  
         [0018]     The consumption of gas continues until the device has returned to its initial condition.  
         [0019]     Since an actuator device based on these reactions is powered by the gas it generates, its response will be governed approximately by the ideal gas law: 
 
 PV=nRT   (4) 
 
         [0020]     The relationship provides two limiting scenarios:  
         [0021]     (A) Expansion under isobaric conditions: This represents the maximum fractional change in length ΔL/L, or strain, that can be achieved in the form of linear displacement of a piston as the volume of the system grows. The charge passed during electrolysis and the reaction stoichiometry determine the volume of gas produced. The maximum strain that can be achieved with a piston driven by this process is a function of the volume of gas produced (equation 5) and the volume of water consumed (equation 6).  
         [0022]     At constant pressure, 
 
 V   gas =(3/2) n   H   2   O RT/ρ  (5) 
 
 V   liquid   =n   H   2   O   ×M   H   2   O/ρH   2   O   (6) 
 
 Where M H   2   O  and ρ H   2   O  are the molecular weight and density of water respectively, and n H   2   O  moles of water are transformed. The maximum relative strain under ambient conditions can be calculated:  
             strain   =         (     actuated   ⁢           ⁢   length     )     -     (     unactuated   ⁢           ⁢   length     )         (     unactuated   ⁢           ⁢   length     )               (   7   )                     V   _     gas     ⁢         -   V     _     liquid       =     V   liquid             (   8   )                       ⁢     ≅       V   gas       V   liquid                 (   9   )                       ⁢     =           (     3   /   2     )     ⁢     RT   /     ρ   atm           M   ⁢           ⁢     H   2     ⁢     O   /     ρH   2       ⁢   O       ⁢     
     ⁢           ≅     136   ⁢     ,     ⁢   000   ⁢           ⁢   %                 (   10   )             
 
         [0023]     (B) Pressurization at constant volume: The buildup of pressure within the system by electrolysis represents the maximum force per cross-sectional area, or stress, that can be generated and applied through a piston. In the absence of piston motion (and flex in the system components), the maximum stress is reached when the gas is confined to the small volume made available by the water consumed (see equation 6):  
             P   =         (     3   /   2     )     ⁢   n   ⁢           ⁢     H   2     ⁢   O   ⁢           ⁢   RT       V   liquid               (   11   )                       ⁢     =           (     3   /   2     )     ⁢   RT       M   ⁢           ⁢     H   2     ⁢     O   /     ρH   2       ⁢   O       ⁢     
     ⁢           ≅     200   ⁢           ⁢   MPa                 (   12   )             
 
         [0024]     The performance of the device has been demonstrated under these two limiting conditions. In both cases, it was shown that the device behaves in accordance with the predicted response indicated above within experimental error. Representative results are presented in  FIG. 4  for stress and in  FIG. 5  for strain.  
         [0025]     Although only a few embodiments have been disclosed in detail above, other modifications are possible. All such modifications are intended to be encompassed within the following claims.