Abstract:
An apparatus for providing electrical energy by utilizing energy from absorbed light to dissociate water and thereby provide free electrons is disclosed. In some embodiments, the apparatus comprises a fuel cell having a photolytic front end, a proton-conducting layer, and a catalytic cathode. The photolytic front end uses energy from light to dissociate water molecules into protons and electrons, the proton-conducting layer conducts protons to the catalytic cathode and forces the electrons to travel through an external electrical circuit, and the catalytic cathode recombines the protons and electrons with oxygen to reform water molecules.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The underlying concepts, but not necessarily the language, of the following case is incorporated by reference: 
         [0002]    U.S. Patent Application Ser. No. 60/862,008, filed 18 Oct. 2006. If there are any contradictions or inconsistencies in language between this application and one or more of the cases that have been incorporated by reference that might affect the interpretation of the claims in this case, the claims in this case should be interpreted to be consistent with the language in this case. 
       FIELD OF THE INVENTION 
       [0003]    The present invention relates to energy conversion devices in general, and, more particularly, to fuel cells. 
       BACKGROUND OF THE INVENTION 
       [0004]    Fuel cells and photovoltaic devices are attractive alternatives to conventional means of providing electrical energy. In many cases, these devices can provide electrical energy without substantial generation of pollutants and without the need for the combustion of fossil fuels. 
         [0005]    A photovoltaic device provides electrical energy by means of a conversion of light energy. The solar cell is perhaps the most prominent example of a photovoltaic device. A photovoltaic device commonly comprises a semiconductor that absorbs incident light, such as sunlight. The absorbed light gives its energy to electrons within the semiconductor, which excites them into the conduction band of the semiconductor. When a voltage is applied to the semiconductor, the excited electrons are enabled to flow and give rise to an electric current. This electric current can be used to power an external device or charge an electrical storage device, such as a battery. Unfortunately, photovoltaic devices tend to be quite expensive. In addition, photovoltaic devices are typically quite inefficient. 
         [0006]    A fuel cell provides electrical energy by means of an electrochemical conversion device, similar to a battery. Unlike a battery, however, it is designed for continuous replenishment of the reactants consumed. A fuel cell produces electricity from fuel and oxygen provided externally, as opposed to the limited internal energy storage capacity of a battery. Fuel cells offer the potential for powering electronics and the like without substantial generation of pollutants. In addition, typical fuel cells operate without hydrocarbon-based fuels, such as oil or gasoline. 
         [0007]    Perhaps the most attractive type of fuel cell is the hydrogen/oxygen proton exchange membrane fuel cell. A hydrogen/oxygen fuel cell typically comprises a proton-conducting membrane (e.g., an electrolyte) that separates an anode and a cathode. Typically, hydrogen is delivered to the fuel cell anode, which comprises a catalyst that accelerates a first chemical reaction wherein H 2  molecules dissociate into protons and electrons (i.e., H 2 →2H + +2e − ). The protons are conducted through the membrane to the cathode, while the electrons are forced to travel through an electrical circuit that is connected between the anode and cathode. At the cathode, another catalyst accelerates a second chemical reaction wherein water molecules are formed from oxygen molecules that combine with the conducted protons and electrons upon their return from the external circuit (i.e., O 2 +4H + +4e − →2H 2 O). In an ideal case, the only byproduct of the operation of the fuel cell is water (in either liquid or gas phase). 
         [0008]    There are, however, several problems associated with conventional fuel cells. First, it can be difficult to safely and efficiently deliver hydrogen to the anode. Second, a storage vessel for hydrogen is often necessary in order to ensure a steady supply of H2 molecules to the anode. As a result, the capacity of the fuel cell is limited. Third, the need to store quantities of volatile hydrogen can lead to safety issues and added cost. Fourth, these fuel cells are not well suited to remote operation since the hydrogen must be replenished once depleted. 
         [0009]    A need exists, therefore, for an energy conversion device that avoids at least some of the drawbacks of the prior-art. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention provides a fuel cell for providing electrons to an external circuit that avoids some of the costs and disadvantages of the prior art. 
         [0011]    A fuel cell in accordance with the present invention utilizes energy gained from the absorption of light to dissociate water into byproducts that include protons and electrons. The fuel cell provides electric current for an external circuit that is electrically connected to the fuel cell. Electrons are provided to the circuit via an anode and return back to the fuel cell via a catalytic cathode. At the catalytic cathode, the electrons recombine with the protons and oxygen to reform water molecules. 
         [0012]    The illustrative embodiment comprises a photolytic layer having an anion-vacancy concentration gradient, a proton-conducting layer (e.g., an electrolyte, etc.) that is non-conductive for electrons, and an anode and catalytic cathode for connection with an external circuit. 
         [0013]    The photolytic layer composes the front end of the fuel cell. It utilizes energy from absorbed light to dissociate water molecules, and to thereby provide electrons and protons to the anode and proton-conducting layer. The proton-conducting layer is non-conductive for electrons; therefore, the electrons at the anode are forced through the external circuit. The protons are conducted from the photolytic layer to the catalytic cathode through the proton-conducting layer. At the catalytic cathode, the protons recombine with oxygen and electrons returning from the external circuit to reform water molecules. 
         [0014]    The anion-vacancy concentration gradient in the photolytic layer is formed by heating the photolytic layer to a high temperature, applying an electric field across the photolytic layer, and reducing the temperature while in the presence of the applied electric field. 
         [0015]    In some embodiments of the present invention, the photolytic layer comprises a plurality of sub-layers, each of which is modified to efficiently absorb a different portion of the visible light spectrum. As a result, substantially the entire visible light spectrum can be efficiently absorbed by the collective sub-layers. 
         [0016]    A fuel cell in accordance with the illustrative embodiment comprises: a photolytic layer, wherein the photolytic layer is in a solid state; and a proton-conducting layer, wherein the proton-conducting layer is in a solid state, and wherein the proton-conducting layer is substantially non-conductive for electrons; wherein at least one of water and hydroxide molecules dissociate at a surface of the photolytic layer to provide protons and electrons. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  depicts a schematic diagram of a portion of a fuel cell circuit in accordance with an illustrative embodiment of the present invention. 
           [0018]      FIG. 2  depicts a schematic diagram of details of a fuel cell in accordance with the illustrative embodiment of the present invention. 
           [0019]      FIG. 3  depicts a schematic diagram of a fuel cell in accordance with an alternative embodiment of the present invention. 
       
    
    
       [0020]    Method  400  depicts a method for forming a fuel cell in accordance with the illustrative embodiment of the present invention. 
         [0021]    Method  500  depicts a method for forming a fuel cell in accordance with an alternative embodiment of the present invention. 
       DETAILED DESCRIPTION 
       [0022]      FIG. 1  depicts a schematic diagram of a portion of a fuel cell circuit in accordance with an illustrative embodiment of the present invention. Fuel cell circuit  100  comprises fuel cell  102  and external circuit  104 . 
         [0023]    Fuel cell  102  is a photolytic fuel cell that absorbs light characterized by a wavelength within a range of suitable wavelengths. Fuel cell  102  utilizes the energy gained from absorbed light to dissociate water molecules into oxygen, protons, and electrons. Electrons liberated by fuel cell  102  are provided to external circuit  104  on wire  106 . Electrons return to fuel cell  102  from external circuit  104  on wire  108 . Fuel cell  102  recombines electrons that return on wire  108  with protons and oxygen to reform water molecules. Fuel cell  102  will be described in more detail below and with respect to  FIG. 2 . 
         [0024]    External circuit  104  is an electronics circuit that has well-known functionality. It will be clear to those skilled in the art how to make and use external circuit  104 . 
         [0025]      FIG. 2  depicts a schematic diagram of details of a fuel cell in accordance with the illustrative embodiment of the present invention. Fuel cell  102  comprises photolytic layer  202 , proton-conducting layer  204 , anode  208 , and cathode  210 . 
         [0026]    Method  400  depicts a method for forming a fuel cell in accordance with the illustrative embodiment of the present invention. Method  400  comprises operations suitable for the formation of fuel cell  102 . The fabrication of fuel cell  102  is described below and with respect to  FIGS. 1-2 . 
         [0027]    Method  400  begins with operation  401 , wherein photolytic layer  202  is formed. Photolytic layer  202  is a layer of manganese-oxide that has a thickness suitable absorbing a desired amount of incident visible light. Typically, the thickness of photolytic layer  202  is within the range of approximately 2 nanometers to approximately 1 micron. In some embodiments, photolytic layer is suitable for absorbing solar radiation. Photolytic layer  202  is modified to create an anion-vacancy gradient through the thickness of the layer. The anion-vacancy gradient results in an energy-bandgap gradient through the thickness of the material. In some embodiments, the energy bandgap varies from a high bandgap of approximately five (5) electron-volts at anode  208  to a low bandgap of substantially zero at the interface of photolytic layer  202  and proton-conducting layer  204 . In some embodiments of the present invention the energy bandgap varies from a high bandgap of approximately 2.5 electron-volts at anode  208  to a low bandgap of substantially zero at the interface of photolytic layer  202  and proton-conducting layer  204 . In some embodiments, the energy bandgap varies from its high bandgap to a low bandgap that is higher than substantially zero. 
         [0028]    Photolytic layer  202  is modified to have a high anion-vacancy concentration gradient by heating it to a temperature above 300° C. and applying an electric field of at least 10 6  Volts/cm across the oxide. In order to quench the anion-vacancy concentration gradient in the oxide, photolytic layer  202  is cooled to a temperature below 100° C. while maintaining this electric field. In some embodiments, the anion-vacancy concentration gradient is quenched in the oxide by rapidly cooling photolytic layer  202  to a temperature below 100° C. without maintaining the applied electric field. 
         [0029]    In some embodiments, photolytic layer  202  is not modified to have an anion vacancy gradient. 
         [0030]    Although the illustrative embodiment comprises a photolytic layer that comprises an oxide, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention wherein photolytic layer  202  comprises a semiconductor. Semiconductors suitable for use in photolytic layer  202  include, without limitation, silicon, germanium, III-V compound semiconductors, II-VI compound semiconductors, and sulfides (e.g., lead sulfide, etc.). It will also be clear to one of ordinary skill in the art, after reading this specification, how to make and use embodiments of the present invention wherein photolytic layer  202  comprises solid-oxide film different than manganese oxide. Suitable oxide films for use in photolytic layer  202  include, without limitation, titania, zirconia, tin oxide, tungsten oxide, iron oxide, and strontium titanate. In addition, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention wherein photolytic layer  202  absorbs wavelengths of light other than those in the visible light spectrum. 
         [0031]    Although the illustrative embodiment of the present invention comprises a photolytic layer that is electrically-conductive and conducts protons, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention wherein photolytic layer  202  is either a) conductive for electrons, but not protons, or b) nonconductive for electrons and non-conductive for protons, or c) is electrically conductive but non-conductive for protons. 
         [0032]    In some embodiments, photolytic layer  202  includes nanostructure to a) increase the light absorbing area and/or b) increase the surface area on which the dissociation of water molecules can occur. 
         [0033]    In some embodiments, photolytic layer  202  includes dopants such as sulfur, nitrogen, and the like, to modify the bandgap of the layer or sub-layers. 
         [0034]    In some embodiments wherein photolytic layer  202  is non-conductive for both electrons and protons, an electron and proton conducting layer is provided between photolytic layer  202  and proton-conducting layer  204 . Suitable materials for this electron and proton conducting layer include, without limitation, palladium, Ca—GdNbO 4 , Ca—Tb 2 O 3 , and Ca—LaNbO 4 . 
         [0035]    In some embodiments wherein photolytic layer  202  is electrically conductive but not conductive for protons, proton-conducting channels are included in photolytic layer  202  to conduct protons to proton-conducting layer  204 . 
         [0036]    At operation  402 , proton-conducting layer  204  is formed. Proton-conducting layer  204  is a layer of yttrium-barium zirconate that has a thickness suitable for providing a desired level of electron impermeability. Typically, the thickness of proton-conducting layer  204  is within the range of approximately 5 nanometers to approximately 1 micron. Proton-conducting layer  204  is in intimate contact with photolytic layer  202  and conducts protons from photolytic layer  202  to cathode  210 . In some embodiments, an oxygen ion conducting layer is formed at operation  402 , rather than proton-conducting layer  204 . 
         [0037]    Although in the illustrative embodiment, proton-conducting layer  204  comprises a layer of yttrium-barium zirconate, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention wherein proton-conducting layer  204  comprises any material that conducts protons but is substantially non-conductive for electrons. Suitable materials for use in proton-conducting layer  204  include, without limitation, proton conducting oxides, proton-conducting polymers, porous zeolites, and proton-conducting inorganic particles such as clay. 
         [0038]    Triple-phase boundary  206  exists at the edge of the junction between photolytic layer  202  and proton-conducting layer  204 . Triple-phase boundary  206  is a region of high reactivity for the various reactions associated with the dissociation of water (in either liquid- or gas-phase) into sub-components that include protons, electrons, oxygen, and ionized O-H molecules (i.e., hydroxide). Hydroxide further dissociates into protons and doubly-ionized oxygen atoms, which subsequently give up their electrons and form O 2  molecules. Although triple-phase boundary  206  is a region of particularly high reactivity, each of these reactions can occur anywhere on the exposed surfaces of photolytic layer  202 . 
         [0039]    At operation  403 , anode  208  is formed on photolytic layer  202 . Anode  208  is an electrode comprising indium-tin-oxide, and provides egress to external circuit  104  for electrons that are liberated by the dissociation of water molecules. Other suitable materials for use in anode layer  208  include, without limitation, transparent conductive oxides, and transparent metals. Anode  208  is transparent for the wavelengths of light in the visible light spectrum. In some embodiments, anode  208  comprises a material that is not transparent to visible light. In these embodiments, the total area of anode  208  is kept as small as possible so as to block as little as possible of the light incident upon fuel cell  102 . 
         [0040]    At operation  404 , cathode  210  is formed on proton-conducting layer  204 . Cathode  210  is an electrode comprising metallization and a catalyst for increasing the rate at which protons, oxygen, and electrons react to form water molecules. Region  212  is a region of high reactivity for recombination of protons, electrons, oxygen into water molecules (in either liquid- or gas-phase). This recombination reaction can, however, occur on any exposed surface of proton-conducting layer  204 . In some embodiments, wherein the catalyst comprises a water-permeable material, recombination can occur on any surface of the proton-conducting layer. Cathode  210  provides ingress for electrons returning from external circuit  104 . In order to increase the probability of light being absorbed in photolytic layer  202 , cathode  210  comprises a material having high reflectivity for visible light. Photons that pass completely through photolytic layer  202  are, therefore, reflected. This increases the probability of that the photons will be absorbed. In some embodiments, cathode  210  includes nanostructure to scatter light back into photolytic layer  202 . In some embodiments, cathode  210  does not comprise a catalyst. 
         [0041]      FIG. 3  depicts a schematic diagram of a fuel cell in accordance with an alternative embodiment of the present invention. Fuel cell  300  comprises photolytic layer  302 , anode  308 , proton-conducting layer  204 , and cathode  210 . 
         [0042]    Method  500  depicts a method for forming a fuel cell in accordance with an alternative embodiment of the present invention. Method  500  comprises operations suitable for the formation of fuel cell  300 . Collectively, the operations of method  500  are analogous to operation  401  of method  400 . The fabrication of fuel cell  300  is described below and with continuing reference to  FIG. 3 . 
         [0043]    Photolytic layer  302  comprises a plurality of sub-layers  304 - 1  through  304 - 3  (hereinafter referred to, collectively, as sub-layers  304 ), each of which comprises a solid oxide. Sub-layers  304 - 1  through  304 - 3  are interposed by interface layers  306 - 1  and  306 - 2  (hereinafter referred to, collectively, as interface layers  306 ), each of which comprises a layer of transparent electron-conducting material. Typically, the thickness of each of sub-layers  304  is within the range of approximately 1 nanometer to approximately 1 micron. 
         [0044]    At method  501 , sub-layer  304 - 1  is formed. Sub-layer  304 - 1  is a layer of manganese oxide, and is analogous to photolytic layer  202 . 
         [0045]    At method  502 , interface layer  306 - 1  is disposed on sub-layer  304 - 1 . Interface layer  306 - 1  is a layer of indium-tin-oxide. Sub-layers  304  are provided with electron-conducting interface layers  306  to enable sequential boosting of the electron energy. In some embodiments, interface layers  306 :
       i. reduce electron back transfer; or   ii. increase electron transfer while reducing energy loss; or   iii. provide band matching between sub-layers; or   iv. conduct electrons so as to provide sequential boosting of electron energy; or   v. improve the absorption absorbed light; or   vi. any combination of i, ii, iii, iv, and v.       
 
         [0052]    At operation  503 , oxide layer  304 - 2  is disposed on interface layer  306 - 1 . 
         [0053]    At operation  504 , interface layer  306 - 2  is disposed on oxide layer  304 - 2 . 
         [0054]    At operation  505 , oxide layer  304 - 3  is disposed on interface layer  306 - 2 . 
         [0055]    It will be apparent to one of ordinary skill in the art that any number of oxide layers and interface layers can be provided to collectively form photolytic layer  302 . 
         [0056]    At operation  506 , an anion-vacancy gradient is induced in each of oxide layers  304 . 
         [0057]    It should be noted that operation  506  can be performed multiple times during the fabrication of photolytic layer  302 . For example, operation  506  could be performed after the formation of each oxide layer  304  to induce an individualized anion-vacancy gradient in each layer. It should also be noted that each oxide layer  304  can be deposited in a manner to produce an in-situ anion-vacancy gradient, thereby obviating the need for operation  506 . 
         [0058]    The respective bandgaps of sub-layers  304  collectively provide a bandgap gradient in photolytic layer  302 . Specifically, the bandgaps of sub-layer  304  vary from approximately 3 electron-volts for sub-layer  304 - 1  to substantially zero for sub-layer  304 - 4 . In some embodiments, at least some of sub-layers  304  are modified to form an anion-vacancy gradient (and, therefore, a bandgap gradient) within themselves. In some embodiments, at least some of sub-layers  304  are modified such that these sub-layers absorb different wavelength spectra, thereby increasing the number of wavelengths that sub-layers  304  collectively absorb. In some embodiments, at least some of sub-layers  304  are modified so that sub-layers  304  collectively absorb a substantially continuous portion of a light spectrum, such as the visible light spectrum. 
         [0059]    It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.