Patent Publication Number: US-11377747-B2

Title: Solar fuels generator with pH separation

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application is a continuation of U.S. application Ser. No. 15/909,764, filed Mar. 1, 2018, which application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/465,556, filed on Mar. 1, 2017, and incorporated herein in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with government support under Grant No. DE-5C0004993-T-112188 awarded by the Department of Energy. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to solar generators, and more particularly, to solar fuels generators. 
     BACKGROUND 
     Solar fuels generators create fuels through the use of paired half reactions. Examples half reactions are the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and the CO 2 R reduction reaction (CO 2 R). One of the half reactions occurs in an anolyte and the other half reaction occurs in a catholyte. The anolyte and the catholyte are separated by a separator. The half reaction in the anolyte is often more efficient at a different pH than the half reaction in the catholyte. For instance, the CO 2 R reduction reaction (CO 2 R) can be efficiently carried out in a neutral catholyte, the hydrogen evolution reaction (HER) can be efficiently carried out in an acidic catholyte, and the oxygen evolution reaction (OER) can be efficiently carried out in an alkaline anolyte. However, solar fuels generators generally operate with the anolyte and the catholyte at substantially the same pH. Accordingly, at least one or both the half reactions occur under inefficient conditions. As a result, there is a need for solar fuels generators that allow the half reactions to occur under efficient conditions. 
     SUMMARY 
     A solar fuels generator includes an anolyte in contact with a separator and a catholyte in contact with the separator. The pH of the anolyte and the pH of the catholyte are each held at a different steady state pH level during operation of the solar fuels generator. In some instances, the separator is constructed such that water dissociates in the separator during the operation of the solar fuels generator. In some instances, hydroxide anions enter the anolyte from the separator during operation of the solar fuels generator and protons enter catholyte from the separator during operation of the solar fuels generator. 
     Another embodiment of a solar fuels generator includes a first reactor configured to contain an anolyte in contact with a separator and a second reactor configured to contain a catholyte in contact with the separator. The separator is configured to keep the pH of the anolyte and the pH of the catholyte at a steady state pH level during operation of the solar fuels generator. The steady state pH level of the anolyte is different from the steady state pH level of the catholyte. 
     In some instances, a solar fuels generator includes an anolyte in contact with a separator and a catholyte in contact with the separator. The separator includes an anion exchange membrane and a cation exchange membrane arranged such that a component of the anolyte and/or the catholyte cannot travel across through the separator without traveling through both the anion exchange membrane and the cation exchange membrane. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a diagram illustrating the active portion of a solar fuels generator. 
         FIG. 2A  is a cross-section of a solar fuels generator. 
         FIG. 2B  is a cross-section of another embodiment of a solar fuels generator. 
         FIG. 2C  is a perspective drawing of an exploded view of an interface between a separator, a catholyte reservoir and a anolyte reservoir that is suitable for use in the solar fuels generator of  FIG. 2A  and/or  FIG. 2B . 
         FIG. 3A  is a schematic diagram of a solar fuels generator. 
         FIG. 3B  is a perspective drawing of an exploded view of an interface between a separator, cathode and anode that is suitable for use in the solar fuels generator of  FIG. 3A . 
         FIG. 4A  shows current density versus time for a solar fuels generator constructed according to  FIG. 2B . 
         FIG. 4B  shows collected oxygen volume versus time for a solar fuels generator constructed according to  FIG. 2B . 
         FIG. 5  shows current density versus time for a solar fuels generator constructed according to  FIG. 2A . 
     
    
    
     DESCRIPTION 
     As used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a solar cell” includes a plurality of solar cells and reference to “the material” includes reference to one or more materials and equivalents thereof known to those skilled in the art, and so forth. 
     Also, the use of “or” means “and/or” unless stated otherwise. Similarly, “comprise,” “comprises,” “comprising” “include,” “includes,” and “including” are interchangeable and not intended to be limiting. 
     It is to be further understood that where descriptions of various embodiments use the term “comprising,” those skilled in the art would understand that in some specific instances, an embodiment can be alternatively described using language “consisting essentially of” or “consisting of.” 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although any methods and reagents similar or equivalent to those described herein can be used in the practice of the disclosed methods and compositions, the exemplary methods and materials are now described. 
     A solar fuels generator includes an anolyte and a catholyte in contact with a separator. The separator is configured such that the pH of the anolyte and the pH of the catholyte are each held at different steady state pH levels during operation of the solar fuels generator. The ability to operate the solar fuels generator with the anolyte and the catholyte at different pH levels allows the pH level of the anolyte to be selected so the half reaction occurring in that anolyte occurs under efficient conditions. Additionally, the pH level of the catholyte can be selected so the half reaction occurring in that catholyte occurs under efficient conditions even if the pH level selected for the anolyte is different from the pH level selected for the catholyte. For instance, in a solar fuels generator pairing an oxygen evolution reaction (OER) with a hydrogen evolution reaction (HER), the pH level of the anolyte where the OER occurs can be alkaline while the pH level of the catholyte where the HER occurs can be acidic. 
     Selecting different pH levels for the anolyte and the catholyte can improve the overall efficiency of the solar fuels generator. For instance, the inventors have achieved solar-to-fuel (STF) conversion efficiencies of around 10% when pairing an oxygen evolution half reaction (OER) with a hydrogen evolution half reaction (HER). The inventors have also been able to achieve solar-to-fuel (STF) conversion efficiencies of around 10% when pairing an oxygen evolution half reaction (OER) with a CO 2 R reduction half reaction that generates formate. Further, these solar-to-fuel (STF) conversion efficiencies have been achieved using only light as the bias source and without the use of an electrical bias source such as a battery. In contrast, prior solar fuels generators have achieved solar-to-fuel (STF) conversion efficiencies in the range of 4.6-6.5% when pairing the oxygen evolution half reaction (OER) and a CO 2 R reduction half reaction. 
       FIG. 1  illustrates the active portion of a solar fuels generator. The illustrated portion of the solar fuels generator includes an anolyte  10  and a catholyte  12  separated by a separator  14 . The anolyte  10  can be a solid, liquid, gas, vapor, polymeric electrolytes or a combination thereof. The catholyte  12  can be a solid, liquid, gas, vapor, polymeric electrolytes, or a combination thereof. The anolyte  10  and catholyte  12  each contacts the separator  14 . The separator  14  includes an anion exchange membrane  16  and a cation exchange membrane  18  arranged such that a component cannot travel between the anolyte  10  and the catholyte  12  through the separator  14  without traveling through both the anion exchange membrane  16  and the cation exchange membrane  18 . A cation exchange membrane  18  is cationically conductive with limited conductivity for nonionic atoms or nonionic compounds, and close to zero conductivity for anions. An anion exchange membrane  16  is anionically conductive with limited conductivity for nonionic atoms or nonionic compounds, and close to zero conductivity fro cations. As a result, ions, nonionic atoms, and nonionic compounds from the anolyte  10  do not substantially travel across the separator  14  to the catholyte  12  and ions, nonionic atoms, and nonionic compounds from the catholyte  12  do not substantially travel across the separator  14  to the anolyte  10 . Accordingly, protons do not readily travel between the anolyte  10  and the catholyte  12  allowing the anolyte  10  and catholyte  12  to be at substantially different pH levels. 
       FIG. 1  illustrates a catalyst layer  20  located between the anion exchange membrane  16  and the cation exchange membrane  18 . The catalyst layer  20  is optional and the anion exchange membrane  16  can be in direct physical contact with the cation exchange membrane  18 . For instance, the anion exchange membrane  16  can be laminated directly to the cation exchange membrane  18 . When the separator  14  includes a catalyst layer  20 , the catalyst layer  20  can include, consist of, or consist essentially of a catalyst such as a water dissociation catalyst or water self-ionization catalyst. Suitable water dissociation catalyst or water self-ionization catalysts include, but are not limited to, metal oxides and/or metal hydroxides such as TiOH, ZrOH, SiOH, polymeric materials such as poly(ferrocenyldimethysilane), poly(acrylamide), graphene and graphene oxides. Although  FIG. 1  shows the catalyst included in a layer that is distinct from the anion exchange membrane  16  and the cation exchange membrane  18 , the anion exchange membrane  16  and/or the cation exchange membrane  18  can include the catalyst. For instance, the catalyst can be embedded in the anion exchange membrane  16  and/or the cation exchange membrane  18 . Alternately, the anion exchange membrane  16  and/or the cation exchange membrane  18  can impregnated or implanted with the catalyst. 
     Suitable anion exchange membranes  16  for use in the separator  14  include, but are not limited to, polyaromatic polymers, fluorinated polymers functionalized with sulfonic acid groups. An example of a suitable fluorinated polymer functionalized with sulfonic acid groups is sold under the trademark NAFION®. Suitable cation exchange membranes  18  for use in the separator  14  include, but are not limited to, polymeric materials functionalized with quaternary ammonium groups. An example of a suitable polymer functionalized with quaternary ammonium groups is sold under the trademark SELEMION®. 
     A suitable thickness for the anion exchange membrane  16  includes, but is not limited to, a thickness greater than 1 μm, 5 μm, or 10 μm and/or less than 500 μm, 1000 μm. A suitable thickness for the cation exchange membrane  18  includes, but is not limited to, a thickness greater than 1 μm, 5 μm, or 10 μm and/or less than 500 μm, 1000 μm. A suitable thickness for the catalyst layer  20  includes, but is not limited to, a thickness greater than 1 nm, or 2 nm and/or less than 10 μm, or 100 μm. 
     The illustrated portion of the solar fuels generator includes a cathode  22  that contacts the catholyte  12 . The cathode  22  can include an optional cathode catalyst layer  24  on a cathode conductor  26 . The cathode catalyst layer  24  can include one or more cathode catalysts selected to catalyze the half reaction that occurs at the cathode  22 . Although the one or more cathode catalysts are shown as being included in a cathode catalyst layer  24 , the one or more cathode catalysts can be included in the cathode conductor  26 . Suitable cathode catalysts include, but are not limited to, reduction catalysts. When the half reaction at the cathode  22  is the hydrogen evolution reaction (HER), a suitable cathode catalyst includes, but is not limited to, Pt, Ni, NiPx, CoPx, NiMo, and combinations thereof. When the half reaction at the cathode  22  is a CO 2 R reaction, a suitable cathode catalyst includes, but is not limited to, Pd, Cu, Cu/Au, Ag, and combinations thereof. 
     The illustrated portion of the solar fuels generator includes an anode  28  that contacts the anolyte  10 . The anode  28  can include an optional anode catalyst layer  30  on an anode conductor  32 . The anode catalyst layer  30  can include one or more anode catalysts selected to catalyze the half reaction that occurs at the anode  28 . Although the one or more anode catalysts are shown as being included in an anode catalyst layer  30 , the one or more anode catalysts can be included in the anode conductor  32 . Suitable anode catalysts include, but are not limited to, oxidation catalysts. When the half reaction at the anode  28  is the oxygen evolution reaction (OER), a suitable anode catalyst includes, but is not limited to, FeNiOx, IrOx, RuOx, CoOx and combinations thereof. 
     One or more electrical conductors  34  provide electrical communication between the anode  28  and the cathode  22 . Suitable electrical conductors include, but are not limited to, metal wires, conductive polymers, conductive pastes and combinations thereof. An electrical pathway includes the anode  28 , the one or more electrical conductors  34  and the cathode  22 .  FIG. 1  illustrates an external bias source  36  positioned along the electrical pathway so as to apply a bias between the cathode  22  and the anode  28 . Suitable external bias sources  36  include, but are not limited to, batteries, fuel cells, and grid electricity. In some instances, the external bias source  36  is a photoelectrode that converts incident light into excited electron-hole pairs that drive a chemical reaction. The external bias source  36  is optional. For instance, in addition to the external bias source  36  or as an alternative to the external bias, the anode  28  and/or cathode  22  can be configured to act as a bias source. For instance, the anode  28  can be a photoanode  28  that converts incident light into excited electron-hole pairs that drive a chemical reaction and/or the cathode  22  can be a photocathode  22  that converts incident light into excited electron-hole pairs that drive a chemical reaction. The solar fuels generator is constructed to include one or more bias sources selected from the group consisting of the anode  28 , the cathode  22 , and the external bias source  36 . 
     When the anode  28  is not photoactive, suitable anode conductors  32  include, but are not limited to, metals, metal alloys, metal phosphide and metal oxides such as Ni, Cu, Cu/Au, NiPx, CoPx, CoOx, and NiFeOx. When the anode  28  is or includes a photoanode  28 , the anode conductor  32  can include or consist of a photoanode light absorber selected to absorb light at a wavelength to which the photoanodes  28  will be exposed during operation of the solar fuels generator. When the cathode  22  is not photoactive, suitable cathode conductors  26  include, but are not limited to, metals and metal oxides such as metals, metal alloys, metal phosphide and metal oxides such as Ni, Cu, Cu/Au, NiPx, CoPx, CoOx, NiFeOx. When the cathode  22  is or includes a photocathode  22 , the cathode conductor  26  can include a photocathode light absorber selected to absorb light at a wavelength to which the photocathode  22  will be exposed during operation of the solar fuels generator. When the external bias source  36  is or includes a photoelectrode, the photoelectrode includes an external light absorber selected to absorb light at a wavelength to which the photocathode  22  will be exposed during operation of the solar fuels generator. 
     Suitable materials for the photoanode light absorbers, photocathode light absorbers, and external light absorbers include, but are not limited to, semiconductors. In some instances, the photoanode light absorbers include or consist of one or more semiconductors, the photocathode light absorbers include or consist of one or more semiconductors, and/or the external light absorbers include or consist of one or more semiconductors. Suitable semiconductors for the photoanode light absorbers include, but are not limited to, metal oxides, oxynitrides, sulfides, and phosphides that are stable in an oxidizing environment such as WO 3 , TiO 2 , and TaON. Suitable semiconductors for the photocathode light absorbers include, but are not limited to, p-type silicon, InP, Cu 2 O, GaP, and WSe 2 . Suitable semiconductors for the external light absorbers include, but are not limited to, Si, GaAs, CdTe, dopped indium gallium (di)selenide (CICS), and combinations thereof. 
     In some instances, the external light absorbers, the photoanode light absorbers and/or the photocathode light absorbers are doped. For instance, a photoanode light absorber can be an n-type semiconductor while the photocathode light absorber can be a p-type semiconductor. One or more pn junctions can also be present within one or more light absorbers selected from the group consisting of external light absorbers, photocathode light absorbers, and photoanode light absorber, and can be arranged so that electrons flow from the cathode  22  to a cathode catalyst and holes flow from the anode  28  to an anode catalyst. 
     The following discussion describes operation of a solar fuels generator constructed as shown in  FIG. 1 . For the purposes of this discussion, there is no external bias source  36  on the electrical pathway, the cathode  22  is not photoactive, and the anode  28  is a photoanode  28 . During operation of the solar fuels generator, the anode  28  is illuminated as shown by the arrow labeled L in  FIG. 1 . The photoanode light absorber included in the anode  28  absorbs at least a portion of the incident light. The absorption of light within the photoanode light absorber excites hole-electron pairs within the photoanode light absorber. The position of an n-type photoanode light absorber in the anolyte  10  produces an electrical field that causes the holes to move to the surface of the photoanode light absorber and then the surface of the anode catalyst where the oxidation of the water in the first phase is catalyzed as shown in  FIG. 1 . 
     The electrical field at the anode  28  also causes the electrons that were excited in the anode  28  to move along the electrical pathway to the cathode catalyst where the electrons react with protons in the catholyte  12  to form hydrogen gas. The resulting hydrogen gas can be stored for use as hydrogen fuel. 
     The oxidation of the water generates gaseous oxygen and hydrogen cations (H + , called protons below). Since the separator  14  includes an anion exchange membrane  16  and a cation exchange membrane  18 , the anolyte  10  and catholyte  12  do not exchange protons across the separator  14 . However, the anion exchange membrane  16  and/or cation exchange membrane  18  can be constructed with sufficient permeability for water to be present between or at an interface of the anion exchange membrane  16  and cation exchange membrane  18 . An electrical potential between or at the interface of the anion exchange membrane  16  and cation exchange membrane  18  causes water to dissociate between or at the interface of the anion exchange membrane  16  and the cation exchange membrane  18 . For instance, the water can dissociate where the anion exchange membrane  16  contacts the cation exchange membrane  18  or in the catalyst layer between the anion exchange membrane  16  and cation exchange membrane  18 . 
     The water dissociation within the separator  14  generates hydroxide anions and protons in the interface of the anion exchange membrane  16  and the cation exchange membrane  18  and/or between the interface of the anion exchange membrane  16  and cation exchange membrane  18 . The anion exchange membrane  16  is on the anolyte  10  side of the separator  14 . As a result, the hydroxide anions can travel through the anion exchange membrane  16  to the anolyte  10  without traveling through the cation exchange membrane  18  as shown in  FIG. 1 . The hydroxide anions enter the anolyte  10  and combine with the protons from the anode  28  to form water. As a result, the protons generated from the anode  28  do not change or do not substantially change the pH of the anolyte  10  during operation of the solar fuels generator. Accordingly, the pH of the anolyte  10  remains stable during operation of the solar fuels generator. 
     The cation exchange membrane  18  is on the catholyte  12  side of the separator  14 . As a result, the protons generated by the water dissociation travel through the cation exchange membrane  18  to the catholyte  12  without traveling through the anion exchange membrane  16  as shown in  FIG. 1 . The protons enter the catholyte  12  and replace the protons that are consumed in the generation of the hydrogen gas. Since the protons consumed at the cathode  22  are replaced by protons from the separator  14 , the pH of the catholyte  12  does not change or does not substantially change during operation of the solar fuels generator. Accordingly, the pH of the catholyte  12  remains stable during operation of the solar fuels generator. 
     Since the pH of the anolyte  10  and the catholyte  12  remain stable during operation of the solar fuels generator, the pH level of the anolyte  10  and the catholyte  12  are each maintained at a steady state pH level during operation of the solar fuels generator. For instance, the pH of the anolyte  10  and the catholyte  12  can remain constant or substantially constant for at least 60 minutes, or 600 minutes during operation of the solar fuels generator. The steady state pH level of the anolyte  10  and catholyte  12  can be substantially different. For instance, the absolute value of the difference between the steady state pH level of the anolyte  10  and the steady state pH level of the catholyte  12  can be more than 0, or 2, and/or less than 14, or 15. In some instances, the steady state pH level of the catholyte  12  is less than the steady state pH level of the anolyte  10 . In some instances, the steady state pH level of the catholyte  12  is less than 3, 7, or 15 and greater than or equal to zero while the steady state pH level of the anolyte  10  is greater than 0, 3, or 7 and less than 15. Selecting different steady state pH levels for the anolyte  10  and catholyte  12  so they are efficient for the half reaction occurring in the anolyte  10  and catholyte  12  can increase the efficiency of the solar fuels generator. For instance, the solar fuels generator can have a solar-to-fuel (STF) conversion efficiency greater than 7% or 9%, and/or less than 20%, or 100% for at least 1 hour, 10 hour, 100 hour, or 1000 hour during operation of the solar fuels generator. In some instances, the pH at which a half reaction occurs efficiently is a function of the catalyst for that half reaction. As a result, the steady state pH selected for an anolyte  10  and/or catholyte  12  can be function of the half reaction catalyst in addition or as an alternative to being a function of the half reaction. 
     The solar fuels generator of  FIG. 1  is disclosed using the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) as paired half reactions, however, the disclosed solar fuels generator can be used with other paired half reactions. For instance, the solar fuels generator can use the oxygen evolution reaction (OER) paired with the CO 2 R reduction reaction (CO 2 R). Using the CO 2 R reduction reaction (CO 2 R) can be used to generate other fuels that include hydrocarbons such as methane. Hydrocarbon fuels include or consists of carbon and hydrogen and may include or consist of carbon, hydrogen, and oxygen. The following generalized reaction can represent the overall reaction used when pairing the oxygen evolution reaction (OER) paired and the CO 2 R reduction reaction (CO 2 R):
 
M CO 2 +N H 2 O→C M H 2 NO (2M+N−2P) +P O 2   (Formula I)
 
where M, N, and P are non-negative numbers and, in some instances, are integers. C M H 2 NO (2M+N−2P)  represents the fuel produced in this reaction and CO 2  serves as the reactant included in the catholyte  12 . Examples of the fuels that can be produced using this reaction in combination with the disclosed solar fuels generator include carbon monoxide, methanol, methane, ethanol, and formic acid. The following table 1 presents values for M, N and P that can be used to generate a particular fuel.
 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                 Chemical 
               
               
                 M 
                 N 
                 P 
                 Fuel 
                 Name 
               
               
                   
               
             
            
               
                 1 
                 2 
                 2 
                 CH4 
                 Methane 
               
               
                 2 
                 4 
                 3 
                 2 molecules of 
                 Methanol 
               
               
                   
                   
                   
                 CH 3 OH produced 
               
               
                 2 
                 2 
                 1 
                 2 molecules of 
                 Formic acid 
               
               
                   
                   
                   
                 HCOOH produced 
               
               
                 2 
                 2 
                 2 
                 CH 3 COOH 
                 Acetic Acid 
               
               
                 2 
                 3 
                 3 
                 C 2 H 6 O 
                 Ethanol 
               
               
                 3 
                 3 
                 4 
                 CH 3 CH 2 COH 
                 Propanol 
               
               
                 3 
                 4 
                 4 
                 HOCH 2 CH 2 CH 2 OH 
                 1,3- 
               
               
                   
                   
                   
                   
                 Propanediol 
               
               
                 4 
                 3 
                 4 
                 CH 3 CH 2 COCOOH 
                 2- 
               
               
                   
                   
                   
                   
                 Oxybutyric 
               
               
                   
                   
                   
                   
                 acid 
               
               
                 4 
                 5 
                 6 
                 CH 3 CH 2 CH 2 COH 
                 Butanol 
               
               
                 6 
                 6 
                 6 
                 C 6 H 12 O 6   
                 Glucose 
               
               
                   
               
            
           
         
       
     
     The half reactions for each of the above fuels illustrate how the solar fuels cell generates a particular one of the hydrocarbon fuels in the above Table 1. For instance, when using the solar fuels cell to generate methanol, the half reaction at the anode, the half reaction at the cathode and the overall reaction are as follows: 
                     3   ⁢           ⁢     (         H   2     ⁢     O   ⁡     (   g   )         ⁢           →         O   2     ⁡     (   g   )       ⁢           +           ⁢     4   ⁢     H   +       +           ⁢     4   ⁢     e   -           )     ⁢           ⁢     (     reaction   ⁢           ⁢   at   ⁢           ⁢   the   ⁢           ⁢   anode   ⁢           ⁢   28     )                   2   ⁢           ⁢     (         CO   2     ⁢           +           ⁢     6   ⁢     H   +       +           ⁢     6   ⁢     e   -         →         CH   3     ⁢   OH     +           ⁢       H   2     ⁢   O         )       ⁢                       (     reaction   ⁢           ⁢   at   ⁢           ⁢   the   ⁢           ⁢   cathode   ⁢           ⁢   22     )                 4   ⁢     H   2     ⁢   O     +     2   ⁢     CO   2         →       2   ⁢     CH   3     ⁢   OH     +     3   ⁢     O   2     ⁢           ⁢       (     overall   ⁢           ⁢   reaction     )     .                 
Since the overall reaction is Formula I with M=N=P=6, these half reactions show the relationship between the overall reaction and the half reactions at the anodes and the cathodes. Additionally, the reaction at the anode is the same as the reaction disclosed in the context of  FIG. 1 . As a result, the primary chemical change needed to generate methanol instead of hydrogen is the delivery of CO 2  to the cathode as a reactant.
 
     As is evident from Formula I, each of the hydrocarbon fuels generated through the use of Formula I is generated by including CO 2  in the catholyte as a reactant. It is believed that a particular one of the hydrocarbon fuels can be generated by controlling variables such as the proportions (or partial pressures) of the reactant, the temperature of the reaction, the voltages applied to the catalysts, and the chemical composition of the catalysts. 
     When the catholyte  12  includes a reactant, the cathode catalyst can catalyze the reaction at the cathode  22 . For instance, when the catholyte  12  includes CO 2  as a reactant, a suitable cathode catalyst can include one or more components selected from the group consisting of copper (Cu), zinc (Zn), tin (Sn), nickel (Ni), platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), metal porphyrins and phthalocyanines. Other metals can also serve as a cathode catalyst when the catholyte  12  includes CO 2  as a reactant. 
       FIG. 2A  illustrates a solar fuels generator having an active portion constructed according to a version  FIG. 1  where the anode  28  is a photoanode  28  and the cathode  22  is not photoactive. The solar fuels generator includes an anolyte reservoir  40  that acts as a reactor for the half reaction at the anode  28 . The anolyte reservoir  40  contains the anode  28 , an anolyte  10 , and an agitation mechanism  42  such as a stir bar. During operation of the solar fuels generator, the agitation mechanism  42  can be used to agitate and/or mix the components of the anolyte  10 . The anolyte reservoir  40  includes an illumination port  44  through which light can enter the anolyte reservoir  40 . The anode  28  is positioned such that light entering the anolyte reservoir  40  through the illumination port  44  can be incident on the anode  28 . The anolyte reservoir  40  includes three other utility ports  46  that provide a pathway into the interior of the anolyte reservoir  40 . Each of the utility ports  46  is sealed such that the contents of the anolyte reservoir  40  are not exposed to the ambient atmosphere. Suitable methods of sealing the utility ports  46  include, but are not limited to, conventional sealing devices  56  such as rubber stoppers. One or more conduits  48  extend through one of the sealing devices  56  into the interior of the anolyte reservoir  40 . The one or more conduits  48  can be used to collect and/or analyze the anolyte  10  and/or the gas above the anolyte  10 . One of the utility ports  46  is sealed. The electrical conductor  34  extends through another one of the sealing devices  56  and is electrically connected to the anode  28 . 
       FIG. 2A  shows the electrical conductor  34  connected to an external bias source  36 . However, an external bias source  36  is optional as disclosed above. 
     The solar fuels generator includes a catholyte reservoir  49  that acts as a reactor for the half reaction at the cathode  22 . The catholyte reservoir  49  contains a cathode  22  that is not photoactive, a catholyte  12 , and an agitation mechanism  42  such as a stir bar. The catholyte reservoir  49  includes three utility ports  46  that each provides a pathway into the interior of the anolyte reservoir  40 . Each of the utility ports  46  is sealed such that the contents of the anolyte reservoir  40  are exposed to the ambient atmosphere. Suitable methods of sealing the utility ports  46  include, but are not limited to, conventional sealing devices  56  such as rubber stoppers. One or more conduits  48  extend through one of the sealing devices  56  into the interior of the anolyte reservoir  40 . The conduits  48  can be used to collect and/or analyze the catholyte  12  and/or gas above the catholyte  12 . In some instances, one or more of the conduits  48  can be used to deliver a reactant into the catholyte  12 . For instance, when the half reaction in the catholyte  12  is the CO 2 R reduction reaction (CO 2 R), one or more of the conduits  48  can be used to deliver CO 2  to the catholyte  12 . The electrical conductor  34  extends through another one of the sealing devices  56  and is electrically connected to the cathode  22 . 
     The catholyte reservoir  49  includes a re-circulation system. The re-circulation system includes two or more re-circulation conduits  50  arranged such that at least two of the re-circulation conduits  50  extend through one or more of the utility ports  46  on the catholyte reservoir  49 . For instance,  FIG. 2A  shows two of the re-circulation conduits  50  that each extends through a different one of the utility ports  46 . The re-circulation conduits  50  are in liquid communication with a pump  52 . The pump  52  can be used to re-circulate the catholyte  12  from the catholyte reservoir  49  back into the catholyte reservoir  49 . An end of one or more fluid conduits  48  can be placed near a surface of an electrode and/or a surface of a separator  14 . For instance,  FIG. 2A  illustrates an end of a fluid conduit  48  placed near the surface of the separator  14 . The location of the end of the fluid conduit  48  near a surface can reduce formation and/or retention of gas bubbles on the surface. For instance, a flow of the catholyte  12  onto and/or across the surface of the separator  14  can remove gas bubbles from the surface of the separator  14  through mechanisms such as forced convective flux. 
     The catholyte reservoir  49  and the anolyte reservoir  40  each includes a separator port  54 . The separator  14  is held between the separator port  54  of the catholyte reservoir  49  and the separator port  54  of the anolyte reservoir  40  such that the anolyte  10  and the catholyte  12  are each in contact with the separator  14 . 
     Although  FIG. 2A  illustrates the re-circulation system operating with the catholyte reservoir  49 , a re-circulation system can be used with the anolyte reservoir. For instance,  FIG. 2B  illustrates the solar fuels generator of  FIG. 2A  modified to include a re-circulation system that re-circulates the anolyte  10 . As noted above, an end of one or more fluid conduits  48  can be placed near a surface of an electrode and/or a surface of a separator  14 . Accordingly,  FIG. 2B  illustrates an end of a fluid conduit  48  placed near the surface of the anode  28 . As noted, the locating the end of the fluid conduit  48  near the surface of the anode  28  can reduce formation and/or retention of gas bubbles on the surface of the anode  28 . A suitable distance for an end of a fluid conduit  48  near a surface of an electrode and/or a separator  14  includes a distance less than 10 cm, 5 cm, or 2 cm and/or greater than 1 mm, 5 mm, or 10 mm. 
     The solar fuels generator can include more than one re-circulation system. For instance, a first re-circulation system can be used with the catholyte reservoir  49  and a second re-circulation system can be used with the anolyte reservoir. Alternately, both the catholyte reservoir  49  and anolyte reservoir can exclude a re-circulation system. 
     In  FIG. 2A  and  FIG. 2B , the interface between the separator  14 , the catholyte reservoir  49  and the anolyte reservoir  40  is constructed such that the separator  14 , the anolyte  10 , and the catholyte  12  are not exposed to the ambient atmosphere. For instance,  FIG. 2C  illustrates one possible construction of the interface between the separator  14 , the catholyte reservoir  49  and the anolyte reservoir  40 . The interface includes multiple sealing devices  56  such as o-rings located between different interface components. For instance, a sealing device  56  is located between a flange of the catholyte reservoir  49  and a first separator support  58 . A sealing member is also between the first separator support  58  and the separator  14 . Another sealing member is between the separator  14  and a second separator support  60 . Another sealing member is also between the second separator support  60  and a flange of the anolyte reservoir  40 . Suitable second separator supports  60  and first separator supports  58  include but are not limited to, metal substrates, rubber, Teflon, and glass. 
     The interface is formed by clamping the flange of the anolyte reservoir  40  to the flange of the catholyte reservoir  49  with the components of the interface between the flange of the anolyte reservoir  40  and the flange of the catholyte reservoir  49 . When the interface is assembled, the separator  14  is located between the first separator support  58  and the second separator support  60 . An opening  62  extends through the first separator support  58  and the anolyte  10  can contact the separator  14  through the opening  62  in the first separator support  58 . An opening  62  extends through the second separator support  60  and the catholyte  12  can contact the separator  14  through the opening  62  in the second separator support  60 . 
       FIG. 3A  illustrates another embodiment of a solar fuels generator having an active portion constructed according to  FIG. 1 . In this embodiment, the interface between the separator  14 , the catholyte reservoir  49  and the anolyte reservoir  40  are moved outside of the reservoirs. For instance, rather than the interface being clamped between the reservoirs, the electrodes are moved outside of the interface and over the openings  62  in the separator supports  58 . More particularly, the anode  28  is positioned over the opening  62  through the first separator support  58  and the cathode  22  is positioned over the opening  62  through the second separator support  60 . 
     Lumens  64  extend through the first separator support  58  to the opening  62  through the first separator support  58 . A cathode  22  re-circulation system includes a catholyte pump  66  and fluid conduits  48  that are configured to re-circulate the catholyte  12  from a catholyte reservoir  49 , through one of the lumens  64  into the opening  62  in the first separator support  58 , out another of the lumens  64  and back to the catholyte reservoir  49 . During operation of the solar fuels generator, the catholyte  12  contacts the separator  14  and the cathode  22  when passing through the opening  62  in the first separator support  58 . Accordingly, the opening  62  in the first separator support  58  effectively serves as a reactor for the half reaction at the cathode  22 . 
     Lumens  64  extend through the second separator support  60  to the opening  62  through the second separator support  60 . An anode  28  re-circulation system includes an anolyte pump  68  and fluid conduits  48  that are configured to re-circulate the anolyte  10  from an anolyte reservoir, through one of the lumens  64  into the opening  62  in the second separator support  60 , out another of the lumens  64  and back to the anolyte reservoir. During operation of the solar fuels generator, the anolyte  10  contacts the separator  14  and the anode  28  when passing through the opening  62  in the second separator support  60 . Accordingly, the opening  62  in the first separator support  58  effectively serves as a reactor for the half reaction at the anode  28 . 
     One or more conduits  48  extend through the catholyte reservoir  49  into the interior of the catholyte reservoir  49 . The conduits  48  can be used to collect and/or analyze the catholyte  12  and/or gas above the catholyte  12 . In some instances, one or more of the conduits  48  can be used to deliver a reactant into the catholyte  12 . For instance, when the half reaction in the catholyte  12  is the CO 2 R reduction reaction (CO 2 R), one or more of the conduits  48  can be used to deliver CO 2  to the catholyte  12 . 
       FIG. 3B  illustrates a possible construction of the interface between the separator  14 , cathode  22  and the anode  28  as shown in  FIG. 3A . The interface includes multiple sealing devices  56  such as o-rings located between different interface components. For instance, a sealing device  56  is located between the cathode  22  and a first separator support  58 . A sealing member is also between the first separator support  58  and the separator  14 . Another sealing member is between the separator  14  and a second separator support  60 . Another sealing member is also between the second separator support  60  and the anode  28 . The interface is formed by clamping the components of the interface together. When the interface is assembled, the separator  14  is located between the first separator support  58  and the second separator support  60 . 
     Although  FIG. 1  through  FIG. 3B  are disclosed in the context of the anode  28  being a photoanode  28 , the cathode  22  can be a photocathode  22  in addition or as an alternative to the anode  28  being a photoanode  28 . When the cathode  22  is a photocathode  22 , the absorption of light by the photocathode light absorber generates hole-electron pairs within the photocathode light absorber. The presence of a p-type photocathode light absorber in the catholyte  12  produces an electrical field that causes the electrons within the photocathode light absorber to move to the surface of the cathode  22  light absorber and then the surface of the cathode catalyst where they react with the protons to form hydrogen gas. The holes generated in the photocathode light absorber move from the photocathode light absorber, along the electrical pathway to the photoanode light absorber as a result of the electrical field. 
     Although  FIG. 1  through  FIG. 3B  are disclosed in the context of the anode  28  being a photoanode  28 , the external bias source  36  can be a photoelectrode in addition or as an alternative to the anode  28  being a photoanode  28 . When the external bias source  36  is a photoelectrode, the absorption of light by the photocathode light absorber excites hole-electron pairs within the external light absorber. The photo-excited electrons and holes in the external light abosrber produce an electrical field that causes the electrons within the external light absorber to move along the electrical pathway toward the cathode  22  and the holes within the external light absorber to move along the electrical pathway toward the anode  28 . 
     The separator disclosed above can be used in conjunction with other solar fuels generator constructions in order to improve pH conditions in the anolyte and catholyte. For instance, the separator can replace the separator in solar fuels generators having electrodes that are attached directly to a separator and/or are immobilized relative to the separator as disclosed in U.S. patent application Ser. No. 12/176,065, filed on Jul. 18, 2008, now U.S. Pat. No. 8,110,898; and U.S. patent application Ser. No. 12/956,422, filed on Nov. 30, 2010, now U.S. Pat. No. 9,530,912; and U.S. patent application Ser. No. 13/855,515, filed on Apr. 2, 2013, now U.S. Pat. No. 9,476,129; each of which is incorporated herein in its entirety. 
     EXAMPLES 
     Example 1 
     A solar fuels generator was constructed according to  FIG. 2B  and  FIG. 2C . The solar fuels generator was configured to generate hydrogen gas at the cathode by the hydrogen evolution reaction (HER) and oxygen gas at the anode by the oxygen (OER). 
     The catholyte was aqueous H 2 SO 4  with pH=0 (J.T. Baker, ACS 88%). The anolyte was a 0.5M potassium borate (KBi) solution with pH=9.3 prepared using a 0.5 M KOH (aq.) solution made from potassium hydroxide pellets (KOH, Macron Chemicals, ACS 88%) and a 1 M boric acid (H 3 BO 3 , Sigma Aldrich, BioReagent&gt;99.5%) aqueous solution. 
     The cathode was not photoactive and included a Pt mesh or a Ti mesh as the cathode conductor. The cathode had a cathode catalyst layer coated on the cathode conductor. The cathode catalyst layer included CoP as a cathode catalyst selected to catalyze the hydrogen evolution reaction (HER). 
     The anode was photoactive with an anode conductor that included a tandem-junction photoabsorber. The photoabsorber included planar GaAs layers contacting planar InGaP layer. The planar layers were grown epitaxially by metal organic chemical-vapor deposition (MOCVD) on an n + -GaAs wafer that had a (100)-oriented polished surface. The anode conductor also included an amorphous hole-conductive protection layer. The protection layer was a 62.5 nm thick layer of TiO 2  grown on the exposed InGaP by atomic-layer deposition at 150° C. with tetrakis (dimethylamido) titanium and water as precursors. A optically transparent layer of Ni metal (˜2 nm thick) was RF sputter deposited (AJA International) onto the exposed TiO 2  surface at 130 W with a constant deposition rate of ˜0.1 A/s at a constant working pressure of 5 mTorr maintained by an Ar flow rate of 10 sccm. The layer of Ni metal provided an ohmic contact to the anolyte. Additionally, the layer of Ni metal served as the anode catalyst layer with the Ni serving as an anode catalyst selected to catalyze the oxygen evolution reaction (OER). The anode had an area of 1.06 cm 2  that was illuminated during operation of the solar fuels generator. The anode was electrically connected directly to the cathode without a bias source being present along the electrical pathway between the anode and cathode. 
     The separator included 100 micrometers of Nafion as the cation exchange membrane in contact with 100 micrometer of Selemion as an anion exchange membrane. The bipolar separator did not include a catalyst layer. The separator was cut into 3×3 cm pieces and thoroughly rinsed with deionized water before use. 
     The anolyte reservoir and the catholyte reservoir  49  both included stir bars for agitating the anolyte and catholyte during operation of the solar fuels generator. The anolyte reservoir included a re-circulation system. The re-circulation system included a peristaltic pump system (Simply Pumps PM300F) with a minimal flow rate of ˜500 mL/min controlled by a tunable power supply. The re-circulation system included curved glass tubing as the fluid conduits. The curved glass tubing was connected to the pump by polyimide tubing. An end of the curved glass tubing was placed close to the surface of the anode to facilitate removal of the bubbles from the surface of the anode and to reduce dissolution of the Ni anode catalyst at near-neutral pH conditions. 
     The solar fuels generator was operated by re-circulating the anolyte while illuminating the anode with a halogen lamp at 1 sun for over 100 hours. Additionally, the catholyte and anolyte were stirred using stir bars. During operation of the solar fuels generator, the pH of the bulk anolyte was monitored and was maintained at a steady state of pH=0. During operation of the solar fuels generator, the pH of the bulk catholyte was monitored and was maintained at a steady state of pH=9.3. 
     The current density was monitored during operation of the solar fuels generator and the results are presented in  FIG. 4A . The halogen lamp failed at about 70 hours of operation and illumination was resumed at about 5 hours later. The right side of  FIG. 4A  shows the solar-to-fuel (STF) conversion efficiency (solar-to-hydrogen (STH) conversion efficiency). The solar fuels generator maintained a STH conversion efficiency above 8% and even above 9% for more than 100 hours. 
     The volume of oxygen produced as a function of time was determined by gas collection measurements. The results are presented in  FIG. 4B . Additionally,  FIG. 4B  shows the volume of oxygen that would be produced based on the current passed as a function of time assuming 100% Faradic efficiency for oxygen evolution. A near Faradic efficiency of oxygen production at the anode was observed over the course of ˜20 hours of continuous operation suggesting minimal corrosion of the anode under these conditions. 
     Example 2 
     A solar fuels generator was constructed according to  FIG. 2A  and  FIG. 2C . The solar fuels generator was configured to generate formate gas at the cathode by the CO 2 R reduction reaction (CO 2 R) and oxygen gas at the anode by the oxygen (OER). 
     The catholyte was aqueous 2.8M KHCO 3  with pH=8.0 saturated with a stream of CO 2  at 1 atm. The catholyte was prepared by vigorously bubbling CO 2  (g) through K 2 CO 3  solutions until the pH did not change. The anolyte was a 1.0M KOH solution with pH=13.7 prepared from potassium hydroxide pellets. 
     The cathode was not photoactive and included a Ti mesh as the cathode conductor coated with a Pd/C nanoparticle coating (Pd mass loading of 250 μg/cm 2 ) as the cathode catalyst layer. The cathodes were fabricated by drop-casting a solution containing 2 mg/mL Pd/C nanoparticles and ˜0.15 wt % Nafion in isopropanol on the Ti mesh. The cathodes had a geometric area of about 0.040 cm 2 . 
     The anode was the photoactive anode of Example 1. The anode had an area of 0.03 cm 2  that was illuminated during operation of the solar fuels generator. The anode was electrically connected directly to the cathode without a bias source being present along the electrical pathway between the anode and cathode. 
     The separator included 100 micrometers of Nafion as the cation exchange membrane in contact with 100 micrometer of Selemion as an anion exchange membrane. The bipolar separator did not include a catalyst layer. The separator was cut into 1.5×1.5 cm pieces and thoroughly rinsed with deionized water before use. 
     The anolyte reservoir and the catholyte reservoir  49  both included stir bars for agitating the anolyte and catholyte during operation of the solar fuels generator. The catholyte reservoir  49  included a re-circulation system. The re-circulation system included a peristaltic pump system (Simply Pumps PM300F) with a minimal flow rate of ˜500 mL/min controlled by a tunable power supply. The re-circulation system included curved glass tubing as the fluid conduits. The curved glass tubing was connected to the pump by polyimide tubing. An end of the curved glass tubing was placed close to the surface of the separator to facilitate removal of CO 2  bubbles from the surface of the separator and accordingly to reduce voltage loss caused by the bubbles and/or fluctuations in the current and voltage. 
     The solar fuels generator was operated by re-circulating the anolyte while illuminating the anode at 100 mW/cm 2  of simulated AM1.5 illumination. Additionally, the catholyte and anolyte were stirred using stir bars. During operation of the solar fuels generator, the pH of the bulk anolyte was monitored and was maintained at a steady state of pH=13.7. During operation of the solar fuels generator, the pH of the bulk catholyte was monitored and was maintained at a steady state of pH=8.0. 
     The current density was monitored during operation of the solar fuels generator and the results are presented in  FIG. 5 . The photocurrent density was 8.7 +/−0.5 mA/cm 2 . Accordingly, the solar fuels generator maintained an average current density above 7 mA/cm 2  or even 8 7 mA/cm 2  for more than 150 minutes of operation time. Additionally, stability tests were performed showing that the Faradic efficiency of CO 2  reduction to formate was about ˜100%, 98%, 95%, and 94% after 30 minutes, 1 hour, 2 hours, and 3 hours respectively. The corresponding solar to the solar-to-fuel (STF) conversion efficiency (solar-to-formate conversion efficiency) at these times was about 10.5%, 10.3%, 10.0%, and 9.9% respectively. 
     Other embodiments, combinations and modifications of this invention will occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.