Abstract:
Lightweight photoelectrochemical system for real-time hydrogen production from water and sunlight, using lightweight multi-junction photo electrodes made from the highly reliable and efficient copper indium selenide thin films, preferably made by low-cost electrodeposition on flexible foil.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of photoelectrolysis of water, with sunlight as the sole power source. More specifically, this invention relates to the design of photoelectrochemical (PEC) systems comprising hybrid photoelectrodes based on copper indium selenide material for generation of hydrogen (H 2 ) and oxygen (O 2 ) gases. 
     BACKGROUND OF THE INVENTION 
     The development of low cost, renewable energy capabilities is critical for future air, terrestrial and space transportation, as well as for distributed electric power generation. H 2  can replace fossil fuels for the production and storage of energy. H 2  can be produced from various resources: renewables, nuclear energy, and coal. High efficiency and low emissions are achieved through use of fuel cells. H 2  fuel cells can power cars, boats and aircraft. Its generation in large quantities at low cost can lead to a new energy resource and provide energy self-sufficiency. A renewable method for generating H 2  uses only sunlight and water, considerably reducing the costs and environmental impacts of fossil and nuclear fuels. 
     PEC technology affords real-time H 2  production with water and solar energy. A PEC H 2  production system integrates a semiconductor photoelectrode with an electrolyzer into a single, monolithic device, to produce H 2  directly from water, using only sunlight. High performance solar PEC systems could offer the most efficient option for low cost, safe, lightweight H 2  production to fuel the emerging fuel cell systems. Harvesting energy from the environment makes it possible to power micro fuel cells in real-time on board transportation vehicles or remote locations. The fuel cell becomes regenerative when the system is integrated with in-situ H 2  production. 
     An integrated PEC cell offers the potential for cost effective, renewable hydrogen generation. Both n- and p-type semiconductors can be used for PEC splitting of water into H 2  and O 2 . A PEC cell can provide about 30% efficiency advantage over a separate p/n photovoltaic (PV) system that is coupled to an electrolysis cell; it avoids the energy losses in the ohmic contact due to the mismatch of the Fermi levels and the band edges. So far, no single semiconductor has been identified, that can provide: (1) Correct energetics: bandgap, band edge overlap to drive the electrolysis reactions; (2) Fast charge transfer, and (3) Stability in an aqueous environment. Obstacles to direct photoelectrolysis of water are the lack of efficient light absorption when bandgap &lt;2.0 eV, corrosion of the semiconductor, and unmatched energetics. The bandgaps of photochemically stable semiconductors are too large for efficient light absorption. Semiconductors with bandgaps in the optimal solar absorption range are typically thermo-dynamically unstable with respect to oxidation. The theoretical limit for water-splitting voltage is 1.23V. Practically, however, due to the existence of overpotentials at the electrolyte/electrode interfaces, the voltage needed is approximately 1.6V or greater. Thus a PV structure generates a voltage of approximately 1.6V or greater when operating under solar radiation. 
     A number of approaches have been tried, to overcome some of the obstacles to the direct splitting of water with a single electrode, including using: (a) simultaneously illuminated bi-photoelectrodes, (b) hybrid or bi-polar photoelectrode comprising a PEC cell and PV cell, and (c) Multiple-junction PEC cells. Several prior inventions and publications have disclosed designs for a variety of PEC cells. Most prior art PEC designs suffer from several shortcomings, including insufficient voltage to split water; the need for an external electrical bias for the electrolysis; corrosion in the electrolyte during operation for extended periods; expensive fabrication methods for the photoelectrodes; and low conversion efficiency. Therefore, there is a need to devise an efficient, stable, cost effective PEC cell with sufficient voltage to produce hydrogen from water, with a solar-to-hydrogen efficiency &gt;10%. 
     SUMMARY OF THE INVENTION 
     Accordingly the main objective of this invention is to present an efficient and stable, lightweight PEC system with sufficient voltage to drive the photoelectrolysis of water using renewable solar energy. The invention further seeks to provide two alternate PEC cell configurations to address the problem of voltage matching. These configurations derive special benefits by basing both photoelectrodes on the proven high efficiency and stability CIS and its alloys. The primary PEC cell configuration increases the cell voltage with a new CIS based bi-hybrid photoelectrode PEC system. Each photoelectrode combines a solid-state device with a transparent conducting material (TCM) that forms a liquid junction with the electrolyte. The alternate cell configuration is also based on the CIS photoelectrodes; it reduces the voltage requirements for water splitting by using an intermediary step for splitting a hydrogen halide. Several variants of these main configurations are designed to provide (i) PEC systems with inexpensively made efficient and stable photoelectrodes; (2) Optimum photoelectrode configuration and low weight for efficient, in-situ hydrogen generation on site; and (3) PEC cell design that maximizes the efficiency of hydrogen and oxygen production for different applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  Shows a cross sectional schematic of an exemplary PEC cell for water splitting into H 2  at the photocathode and into O 2  at the photoanode with the two compartments separated by an H +  and OH −  ion conducting membrane. 
         FIG. 2  Shows a cross sectional representative schematic of a photoanode comprising substrate/contact/n-absorber/p-buffer/TCM making electrolytic contact with the solution. 
         FIG. 3  Shows a cross sectional representative schematic of a photocathode comprising substrate/contact/p-absorber/n-buffer/TCM (catalyst) making electrolytic contact with the solution. 
         FIG. 4  Shows a PEC cell with the photoanode and the photocathode, mounted in a side-by-side configuration to generate H 2  and O 2  separately. 
         FIG. 5  Shows a PEC cell with the two photoelectrodes, combined on a single central conducting substrate for bifacial illumination. 
         FIG. 6  Shows a PEC cell with the two photoelectrodes constructed on (a) opposite walls of the electrolytic cell for bifacial illumination, and adjacent (b) angular and (c) circular walled cells with a membrane separating the two compartments in each cell. 
         FIG. 7  Shows a PEC cell with the two photoelectrodes combined into a monolithically stacked device on either side of a single TCM coated transparent substrate to maximize photon absorption with one-sided illumination. The device is illuminated through a wider gap p-type photocathode. 
         FIG. 8  Shows a PEC cell with an inert metal anode and photocathode which combines n-CIS and p-CIS based devices in tandem through a tunnel junction. 
         FIG. 9  Shows the PEC cell of  FIG. 1  based on lightweight, low-density transparent polymer materials. 
         FIG. 10  Shows one side illuminated PEC cell comprising a hybrid photoanode based on a conducting substrate that makes electrical contact to an inert metal cathode. 
         FIG. 11  Shows PEC cell designs with electrically attached electrodes similar to that in  FIG. 10 , using (a) multijunction photocathode comprising two or more PV cells with an inert metal anode, (b) multijunction photoanode comprising two or more PV cells and metal cathode, and (c) multijunction photoanode and photocathode, each comprising two or more PV cells that absorb light at different wavelengths. 
         FIG. 12  Shows hydrogen generation through successive reactions involving the photogeneration of I 2  at the n-CIS/p-CuISe 3  photoanode, chemical generation of HI and O 2 , and the reduction of HI at the cathode. 
         FIG. 13  Shows hydrogen generation at a cathode during photooxidation of I −  at the n-CIS/p-CuISe 3  photoanode; a third compartment controls the pH in the cathode compartment. 
       
         
           
                 
               
                 
                 
                 
               
             
                 
                     
                 
                 
                   Reference Numerals 
                 
                 
                     
                 
               
               
                 
                     
                 
               
            
             
                 
                     
                   20 
                   PEC system 
                 
                 
                     
                   21 
                   Illumination 
                 
                 
                     
                   22 
                   Electrolysis cell 
                 
                 
                     
                   23 
                   Electrolyte 
                 
                 
                     
                   24 
                   Separator membrane 
                 
                 
                     
                   25 
                   Anode compartment 
                 
                 
                     
                   26 
                   Cathode compartment 
                 
                 
                     
                   27 
                   Electrolytic junction  
                 
                 
                     
                   28 
                   Anode, photoanode 
                 
                 
                     
                   29 
                   Cathode, photocathode 
                 
                 
                     
                   30 
                   Substrate 
                 
                 
                     
                   31 
                   Contact 
                 
                 
                     
                   32 
                   np solid-state PV junction 
                 
                 
                     
                   33 
                   TCM 
                 
                 
                     
                   34 
                   n-CIS absorber 
                 
                 
                     
                   35 
                   p-buffer 
                 
                 
                     
                   36 
                   Encapsulant 
                 
                 
                     
                   37 
                   pn junction 
                 
                 
                     
                   38 
                   p-CIS absorber 
                 
                 
                     
                   39 
                   n-buffer 
                 
                 
                     
                   40 
                   Metal catalyst 
                 
                 
                     
                   41 
                   Third compartment 
                 
                 
                     
                   42 
                   Fourth compartment 
                 
                 
                     
                   43 
                   Fifth compartment 
                 
                 
                     
                   44 
                   Hydrophobic membrane 
                 
                 
                     
                 
               
            
           
         
       
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This invention provides a new hybrid bi-photoelectrode PEC system for hydrogen generation. A preferred embodiment combines the concepts of bi-photoelectrode and hybrid configurations to produce a hybrid bi-photoelectrode PEC system. The schematic of the PEC system  20  in  FIG. 1  illustrates the basic concept for splitting H 2 O into H 2  and O 2  under illumination  21 . The system  20  comprises an electrolysis cell  22 , containing an electrolyte  23 . Electrolysis of water can proceed in either acidic electrolyte, such as H 2 SO 4  or an alkaline electrolyte such as KOH. Half reactions in acidic electrolyte typically include:
 
2H + +2 e   − →H 2   (reduction at cathode)
 
2H 2 O→4H + +O 2 +4 e   −   (oxidation at anode)
 
Half reactions in alkaline electrolyte include:
 
2H 2 O+2 e   − →H 2 +2OH −   (reduction at cathode)
 
4OH − →O 2 +2H 2 O+4 e   −   (oxidation at anode)
 
For both types of electrolyte the half reactions lead to an overall reaction of: 2H 2 O→H 2 +O 2 .
 
     The electrolyte parameters such as pH, ionic strength, solution composition, etc. can be adjusted to be compatible with the energetics and stability of the photoanode and the photocathode. 
     A separator membrane or frit  24  divides a PEC cell  22  into two compartments  25  and  26  in which the oxidation and reduction half reactions take place, respectively. The separator  24  allows exchange of H +  and OH −  ions for the electrolysis, but separates and confines the H 2  and O 2  gases into two different compartments of the cell. The membrane  24  can be applied onto a porous substrate. The porous substrate allows the electrolyte to flow through the substrate, yet provides mechanical strength to support the membrane  24 . The membrane  24  can be made to be extremely thin to reduce cost. An example of the supporting material is micro-porous polypropylene. The membranes are generally installed between the two electrodes in a way that allows for maximum radiation to reach the photoelectrodes. 
     The anode compartment  25  contains an anode or photoanode  28  for O 2  evolution and the cathode compartment  26  contains a cathode or photocathode  29  for H 2  evolution. Each photoanode  28  and photocathode  29  is a composite of a thin film semiconductor heterojunction PV device and a PEC junction  27  between the TCM layer and the electrolyte. A typical photoanode  28  will have a configuration of substrate/n/p PV cell/TCM-electrolyte junction and a typical photocathode  29  will have a configuration of substrate/p/n PV cell/TCM (catalyst)/electrolyte junction. In combination they can generate sufficient driving force for H 2  and O 2  generation from water in the PEC cell. High efficiency for photoelectrolysis of water can be obtained by matching the voltage generated between the photoanode  28  and photocathode  29  to the operating voltage of the electrolyzer. Illumination of each electrode will produce photovoltage (V hv )
 
 V   hV   A  for photoanode= V   hV   A ( PV )+ V   hV   A ( PEC )
 
 V   hV   C  for photocathode= V   hV   C ( PV )+ V   hv   C ( PEC )
 
     The resultant photovoltage V hV   A +V hV   C  is expected to be greater than 1.6V to induce spontaneous photoelectrolysis. It can drive the water reduction reaction at the photocathode  29  and water oxidation at the photoanode  28 . 
     The composite electrodes avoid some of the individual problems of bi-photoelectrode and hybrid systems. For example, the more efficient, low bandgap semiconductor in the solid state PV cell is paired with a wide bandgap stable, but less efficient TCM such as a metal oxide. The semiconductor with lowest bandgap determines the efficiency. In the preferred embodiment, both photoelectrodes  28 ,  29  are based on the proven high efficiency and stability CIS semiconductor material and its alloys. The CIS material may be a single crystal, co-evaporated film or electrodeposited film, on a conducting foil substrate or glass with a conducting contact. Electrochemical fabrication of CIS on a flexible lightweight foil offers substantial cost and weight reduction potential, while still retaining high efficiency. The glass substrate provides rigidity and transparency for specific PEC cell designs. 
       FIG. 2  shows a cross sectional view of the photoanode  28 . It comprises a double-junction device fabricated on a substrate  30  which may be provided with a conducting contact layer  31 . The substrate  30  may be a metal foil, or glass coated with a conducting contact  31  such as Mo or ITO. The device includes an n/p solid-state PV junction  32  in tandem with a TCM  33  that forms an electrolytic junction with the electrolyte  23 . The photoanode  28  is illuminated through the top TCM/electrolyte junction  27 . However most of the absorption takes place in the n/p solid-state PV junction  32  below the electrolytic junction. 
     In the preferred embodiment, the n/p PV junction  32  is made with an n-CIS absorber  34  and a p-type buffer  35  to form n/p PV junction  32 , as described in our US patent application No. 20030230338. A thin film of n-CIS  34  may be electrodeposited directly on a conducting foil substrate  30  without the need for a contact  31  layer. CIS films with n-type conductivity and ordered defect chalcopyrite stoichiometries with Cu:In:Se ratios of 1:1:2, 1:2:3.5, 1:3:5, . . . etc. may electrodeposited in one-step as described in our US patent application No. 20030230338. The n-CIS films include CIS alloys with other elements from groups I-III, V and VI that may increase their efficiency or stability. The n-CIS films are recrystallized using rapid thermal processing. In the preferred embodiment the p-buffer is CuISe 3 , created by PEC surface conversion in acidic Cu-polyiodide electrolyte as described in U.S. Pat. No. 5,286,306. The resulting n-CIS/p-CuISe 3  heterostructure  32  minimizes defects in the crucial space charge region and reduces recombination for enhanced device efficiency. 
     The top surface of the n/p heterostructure  32  is coated with a TCM layer  33 . In addition to being transparent and conducting, this TCM layer  33  should be corrosion resistant in the electrolyte  23  and have catalytic properties for O 2  evolution reaction. The TCM layer  33  can be selected from a number of materials that are resistant to electrochemical corrosion and have catalytic properties for O 2  evolution reaction, such as TiO 2 , Fe 2 O 3 , WO 3 , RuO 2  SnO 2  Sn(F)O 2 , etc. and modifications from these basic materials. These materials may be alloyed with other elements such as Ca or Mg to adjust the bandgap or to remove the rectifying junction between these oxides and electrodes as suggested in US patent application No. 20050211290. Other materials such as mixed oxides like SrTiO 3 , p-CuInO 2 , i-ZnSnO 2  offer the possibility of tailoring their band gap by altering their composition to be compatible with an n-CIS/p-CuISe 3  type device  32 , as well as the electrolyte  23 . 
     Various approaches including doping, alloying, and surface modification can be used to achieve an ohmic contact between the TCM  33  and electrolyte  23 . Nitrides and carbides of Group IB, III and IV can also be used as TCM  33  coatings. Transparent conducting carbon nanotubes and corrosion resistant polymer nanocomposites can serve as TCMs  33 . Organic nanocomposites can be made sufficiently conductive for use as the TCM  33  coating by adding a small amount of metal. Various methods, such as spin coating or spray pyrolysis, sputtering, evaporation, chemical vapor deposition, chemical bath deposition or electrodeposition may be used for these coatings. The edges of the photoanode  28  and the photocathode  29  are insulated from exposure to the solution  23  by an encapsulating film  36 . 
       FIG. 3  shows a cross sectional view of the photocathode  29  comprising a substrate  30 , a back contact  31 , p/n junction  37  PV cell in tandem with a TCM  33  and electrolyte  23  junction. Typical substrate  30  may be a metal foil or glass/Mo. Commercial Mo/p-CIGS/n-CdS PV cells can be adapted to produce a stable and efficient photocathode for H 2  generation. The preferred embodiment uses an absorber  38  made from p-CIS. The p-CIS absorber  38  includes CIS alloys with gallium, sulfur or other Group I, III and VI elements. The p-CIS absorber  38  is preferably electrochemically fabricated in order to reduce cost. Alternately, the p-CIS films  38  can be made by conventional vapor phase methods. An n-type buffer  39 , such as n-CdS, n-ZnS and other commonly used buffer materials for p-CIS PV cells, will form a p/n PV junction  37  with the p-CIS absorber  38 . 
     The p/n junction  37  is coated with TCM  33  containing a H 2  evolution catalyst  40  such as Pt. The catalyst may be applied directly on the n-buffer  39  layer to reduce the overvoltage losses associated with the noncatalytic buffer/electrolyte interface  27 . For example, a thin layer of platinum catalyst  40  may be electrodeposited on the CdS surface from a 20 mM H 2 PtCl 6  solution. Photoassisted galvanostatic deposition can be performed at a cathodic current density of 1 mA/cm 2 , with a Pt deposit quantity corresponding to a charge of 10 mC/cm 2 . J. K. Nørskov et al indicate that Pt is a better electrocatalyst than other metals for H 2  evolution, primarily because the evolution reaction is thermo-neutral on Pt at the equilibrium potential. The platinum catalyst also may provide corrosion-protection for the semiconductor [Bogdanoff et al]. A discontinuous layer of Pt particles can be electrodeposited from 1 mM K 2 PtCl 6 +0.1M HClO 4  using a constant current of 100 μA (500 μA/cm 2 ) and a variable deposition time from 100 to 700s. 
     Various other methods are available for incorporating Pt catalyst into the photocathode, such as using the ceramic catalyst of Pt dispersed alumina [Kinumoto et al], a two-stage spray pyrolysis of 1% by weight solution of H 2 PtCl 6  [Ho et al], freeze-drying H 2 PtCl 6  on an oxide layer followed by reduction in hydrogen [Abbaro et al], etc. Alternate catalytic metals such as Co, Ru, Pd, Rh and Ni can also be used. The catalyst may be supported on a carrier layer comprising at least one member selected from the group consisting of alumina, zirconia, titania, ceria, magnesia, lanthania, niobia, yttria and iron oxide and mixtures thereof. 
     Alternately a wide gap TCM  33  selected from the oxides, nitrides, carbides or polymers can be used for the photocathode provided that the TCM  33  forms an efficient junction with the electrolyte to generate sufficient current for water reduction. The TCM  33  is also required to be compatible with the n-type buffer component of the photocathode  29 . The edges of the photocathode are insulated from exposure to the solution  23  by an encapsulant  36 . 
     The photocathode  29  and the photoanode  28  are combined in a PEC cell containing the electrolyte  23  as shown in  FIG. 1 . The combination can provide a total photovoltage of V (cell)=V hV   A +V hV   C  for the device to allow spontaneous water splitting. Coupling the 2 photoelectrodes is anticipated to exceed the minimum (1.6V) that is required to spontaneously drive the water splitting reaction. 
     The photoanode  28  and photocathode  29  used for the PEC cell of  FIG. 4  are mounted in a side-by-side stacked configuration to generate H 2  and O 2  separately. They may be constructed on flexible foil substrate  30 . The PEC cell design dictates the placement of the membrane  24  in the various embodiments. For example the membrane  24  can be installed in the same direction as the radiation or behind one substrate  30 . A cation-exchange membrane  24  can be used for the conduction of H +  for the acidic electrolyte and anion-exchange membrane can be used for the conduction of OH −  in the alkaline electrolyte. When the membranes  24  are installed perpendicular to the photoelectrode (for example, vertically), the H 2  and O 2  gases can be separated by gravity, away from the membrane  24 . In this way, the required thickness of membrane  24  is small, leading to significant reduction in material cost and increased conduction of ions. 
     In  FIG. 5 , the photoanode  28  and photocathode  29  are combined on a single central substrate. By using a common substrate  30  made from a conducting material such as steel, the external electrical circuit can be eliminated. The electrodes are illuminated from opposite sides of the PEC cell with the light radiation impinging on both electrode faces in direction perpendicular to the plane of the substrate. Such devices may be used in applications where bifacial illumination is available, such as on certain aircraft or spacecraft parts. 
     In  FIG. 6   a , the photoanode  28  and photocathode  29  are constructed directly on the wall of the electrolysis cell  22  in superstrate configuration, i.e. the cell  22  wall serves as the superstrate for each of the electrodes and the light comes in through the cell  22  wall and back contact  31  into each absorber  34 ,  38  layer. The surface of the photoanode  28  and photocathode  29 , in contact with the electrolyte should be catalytic for the O 2  and H 2  evolution reactions, respectively. A TCM is also required to serve as the contact  31  material between the cell  22  wall and the absorber  34 . 
     The photoelectrodes  28 ,  29  may also be constructed on adjacent walls of the cell  22 .  FIG. 6   b  shows a 2-dimensional view of a PEC cell  20  with the photoelectrodes  28 ,  29  on either side of the cell  22 . The separator membrane is impervious to the gases and is only provided near the lower end of the device to maintain separation between the gases. The illumination for the photoelectrodes  28 ,  29  is provided through the transparent walls of the cell  22 . Such superstrate-based photoelectrodes can also be constructed on curved walls of an oval or circular shaped cell  22  in  FIG. 6   c . Further such cell  22  wall based photoelectrodes  28 ,  29  may also be configured with appropriate components in a substrate configuration. The main consideration for the substrate version is that the back contact of each photoelectrode is a TCM and the respective front component contacting the electrolyte has catalytic properties for H 2  and O 2  evolution reactions. 
     In  FIG. 7 , the n-CIS based  28  and a p-CIS based  29  photoelectrodes are combined into a monolithically stacked device on a single TCM coated transparent substrate  30  to maximize photon absorption with one-sided illumination. In this case the system  20  can be illuminated through a wider gap p-type photocathode  29  based on p-CuGaSe 2 , p-CuO, p-CuI or p-CuSCN absorber  38 . Efficiency gains are possible if the n-CIS device is used in a tandem structure with a p-type PV cell perpendicular to incoming light. The n-CIS photoanode can be constructed on TCM coated substrate  30  either in superstrate or substrate configuration. 
     In  FIG. 8 , the n-CIS and p-CIS based photoelectrodes separated by a tunnel junction are combined into tandem photocathode  29 . The p-CIS may be constructed from a wider band material such as copper gallium sulfide. The photocathode is combined with an inert metal anode  28  in electrical contact with the conducting substrate of the n-CIS PV device in the PEC cell. 
     The glass or metal substrates  30  used for the photoanode  28  or the photocathode  29  may be replaced by TCM coated low-density transparent polymer foil to decrease weight of the system  20 . The PEC cell container  22  can also be constructed from a transparent plastic.  FIG. 9  shows a polymer-based version of  FIG. 1 . The polymer electrodes can be used in any of the embodiments shown in  FIGS. 4-8 . 
       FIG. 10  shows a simplified design of system  20  in which an inert metal cathode  29  such as platinum, titanium, graphite, steel, platinized metal, doped polymer or TCM replaces the photocathode. This cathode  29  contacts the back of the conducting substrate  30  of an n-CIS photoanode. 
       FIG. 11   a  shows a PEC cell such as that shown in  FIG. 8 , but using multijunction photoanode  28  and including additional PV junctions to absorb light of different wavelengths. This photoanode  28  can generate a cell voltage that is high enough to split water. The individual solid state junctions  32 ,  33  constituting the multijunction photoelectrode may be constructed from CIS based absorbers in the form of thin film or nanoparticles combined with conducting organic polymers. Subcells made from variants of the basic n-CIS PV cell  28  are described in our U.S. patent application Ser. No. 11/420,674. Such a multijunction PV device may be also constructed from p-CIS based PV subcells to serve as photocathode  29 ,  FIG. 11   b . Alternately multijunction photoanode  28  and photocathode  29  can be combined in a single PEC cell to realize a very high efficiency water splitting device,  FIG. 11   c.    
     An alternate approach to circumvent the problem of matching the cell voltage with the water splitting voltage is by PEC decomposition of a hydrogen halide. Hydrogen can be produced from a hydrogen halide at a much lower voltage than by direct electrolysis of water. This approach provides the advantage of reducing the amount of electrical energy required. Water can be chemically reacted with a halogen gas to form a hydrogen halide, and then the resulting hydrogen halide can be electrolyzed to form hydrogen as described in U.S. Pat. No. 5,709,791. 
     To achieve such a path with CIS based photoelectrodes, the basic PEC system of  FIG. 1  is modified to include intermediate chemical reactions. The preferred embodiment uses HI formation and decomposition reactions for the immediate steps.  FIG. 12  shows water splitting via iodide (I − ) photooxidation in a PEC system  20  containing CuI—I − /I 2 —HI solution and an n-CIS/p-CuISe 3  n/p junction  32  preferably without a TCM  33  layer. The anode compartment  25  is connected to a cathode compartment  26  through an anion selective membrane  24  that is impermeable to Cu +  diffusion. The membrane  24  may contain a solid polymer and a ceramic electrolyte with Ittria. The anode compartment  25  is connected to 2 additional compartments  41  and  42 . Compartment  41  provides for heating the electrolyte to 160° C. Compartment  41  is connected to an additional compartment  43  through a hydrophobic membrane  44  having a pore diameter of preferably 1-3 μm. The cathode compartment  26  contains a Pt metallized inert metal cathode or photocathode  29  similar to that shown in  FIG. 3 . 
     Upon illumination, iodine is generated at the photoanode  28  via reaction 2I − →I 2− +2e − . The solution gets pumped through the compartments  41  and  43  through various exits and inlet valves. The 160° C. temperature in compartment  41  sublimes the I 2 , which then passes though the hydrophobic membrane  44  into compartment  43  as described in U.S. Pat. No. 6,071,327. The I 2  gas is separated by passing the I 2 -containing phase through a porous hydrophobic separation membrane  44 , which retains the I 2  gas on the upstream side of the phase flow and removes the I 2  gas-free phase during the downstream side of the separation membrane. In addition, a fluororesin membrane such as polyethylene tetrafluoride, which itself is hydrophobic, i.e. impermeable to water but permeable to steam is used in combination with this separation membrane. 
     Alternatively, a water-repellent membrane based on SiO 2  may be formed by coating a hydrophobic resin such as a fluororesin onto a porous material such as a metal (hastelloy or stainless steel, etc.), a ceramic, a glass filter or a foam material, or by a sol-gel method. The separation membrane  44  is secured by metal or ceramic porous plates (60 μm pore size). The valves are opened when the pressure reaches approximately 0.2 MPa. gas is collected with the valves open until the pressure falls to 0.1 MPa. Furthermore, the temperature of the pipe is adjusted with heater, to prevent the I 2  from clogging the pipe at low temperatures. The I 2  reacts with water in chamber  43  through a catalytic reaction with activated carbon or Pt/γ-alumina catalyst via reaction: I 2 −+H 2 O→HI+O 2 , to produce O 2  gas. The O 2  gas is collected and HI solution is pumped into the cathode compartment  26  where it is reduced at the cathode via reaction: HI+e−→½H 2 +I −  to generate H 2  gas. The H 2  gas is collected and I— diffuses into the anode chamber  25 . This chamber  25  is also fed by the I 2  free solution from chamber  41  that is pre-cooled to room temperature in chamber  42 . 
     The PEC system of  FIG. 12  also generates H 2  and O 2  from water and sunlight only. The other chemicals used are regenerated in the total cycle. This system offers the advantage of not requiring the complex electrode structures need to generate large voltages for spontaneous photoelectrolysis in the multijunction photoelectrode based PEC cells of  FIG. 11 . This system trades the photoelectrode complexity with a more elaborate chamber design. It is to be understood that the invention system of  FIG. 12  contemplates many alternate designs, configuration and several variations of methods to make the photoelectrodes and other components in different chambers. In fact most of the photoelectrode and PEC cell designs shown in  FIGS. 4-8  may be adapted for the iodide based H 2  generation system. 
     Alternate chemical reactions and chamber designs are also possible, such as the variant shown in  FIG. 13 , depicting H 2  generation at a cathode  29  driven by the photooxidation of I −  at an n-CIS/p-CuISe 3  photoanode  28 . The anode  25  and cathode  26  sections are separated by a membrane or frit  24 . The anode compartment  25  contains a HI—I − —CuI solution saturated with sufficient CuI to reduce the photogenerated I 2 . The anode  28  is n-CIS/p-CuISe 3  junction  32 . The cathode compartment  26  contains acidified water with no Cu or I 2 . The same HI acid may be used in both compartments. Illumination of the n-CIS electrode  28  leads to I −  oxidation. The CuI will reduce the I 2  formed to I −  and Cu 2+ . The cathodic reaction will be H 2  evolution. The changes in pH in the cathode compartment  26  require adjustment by acidifying, to neutralize the OH −  ions produced. 
     The above PEC cell designs require a small amount of electrolyte and low cost materials, making the system lightweight, portable and low cost. The above PEC systems can lead to a self-sustained, stand-alone system for H 2  generation by combining it with gas collecting containers, electrolyte circulating pumps, supporting structures and auxiliary components. Various alternative embodiments can be devised to reduce costs and weight. For example the PEC cell can be designed to make the best use of gravity to separate the gases generated in different compartments. In this way, the use of membrane can be minimized or totally avoided. 
     The above disclosed PEC cell and system has been outlined in broad terms to state the more important features of the invention. The invention is capable of other embodiments and of being practiced and carried out in various other ways not specifically described here. The invention offers significant advantages such as reliable energy conversion efficiency, efficient electrolysis, low cost, and high durability. For multijunction photoelectrodes, the voltage can be &gt;1.6 V, which is sufficient for water electrolysis. The energy can be stored in form of H 2 . When used in combination with portable fuel cells, this PEC system can provide portable power, transportation, residential, industrial and distributed power generation systems in remote locations, backup power stations, battery replacement, portable power generation, electric vehicles, and marine power stations. 
     The above detailed description of the present invention is given for explanatory purposes. All references disclosed herein are expressly incorporated herein by reference. It will be apparent to those skilled in the art that numerous changes and modifications can be made without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be construed in an illustrative and not a limitative sense, the scope of the invention being defined solely by the appended claims.