Abstract:
Disclosed are methods and systems for generating hydrogen gas at pressures high enough to fill a hydrogen storage cylinder for stationary and transportation applications. The hydrogen output of an electrochemical hydrogen gas generating device, a hydrogen-producing reactor, or a diluted hydrogen stream is integrated with an electrochemical hydrogen compressor operating in a high-differential-pressure mode. The compressor brings the hydrogen produced by the hydrogen generating device to the high pressure required to fill the storage cylinder.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/942,239, filed on Aug. 29, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to systems and methods for generating hydrogen, and more particularly to systems and methods for generating hydrogen gas at pressures high enough to fill gas storage cylinders. 
     BACKGROUND OF THE INVENTION 
     Hydrogen gas must be generated at high pressures to fill hydrogen storage cylinders for stationary and transportation applications, including on board a vehicle and at refueling stations. To produce hydrogen for use or storage at high pressure, water electrolysis may be performed at the required high pressure, generating both hydrogen and oxygen at high pressure. Alternatively, differential-pressure electrolysis may be employed to generate hydrogen at high pressure and oxygen at substantially atmospheric pressure. To date, high pressure water electrolyzers have been fabricated that either generate both hydrogen and oxygen at 3000 psia, where psia is the pressure in pounds per square inch, absolute, or generate hydrogen at 2500 psia and oxygen at atmospheric pressure. For example, Giner Electrochemical Systems, LLC has fabricated a water electrolyzer that operates at a differential pressure (H 2 &gt;O 2 ) of 2500 psia using plastic materials as frames and proton-exchange membranes (PEMs) as solid-polymer electrolytes. A low-pressure pump provides liquid water at near-ambient pressure to the anode side of the electrolyzer. When DC current is applied, the water is decomposed at the anode to oxygen, protons and electrons. The oxygen is separated from the excess circulating water, which acts as a reactant and coolant, with a low-pressure gas/water separator. All functions on the anode side are conducted at near-ambient pressure. The protons, along with some water, are electrochemically transported across the membrane to the cathode, where they react with the externally transported electrons to produce hydrogen at the required higher operating pressure. The hydrogen is separated from the transported water in a high-pressure gas/water separator. 
     Electrolyzers operating totally or partially at high pressure may be expensive, involve complex construction, and present safety hazards. Therefore, a need exists in the art for simple, safe, and inexpensive systems and methods for generating hydrogen gas at high pressures. 
     SUMMARY OF THE INVENTION 
     The systems of the present invention can generate hydrogen gas at pressures high enough to fill a gas storage cylinder for stationary and transportation applications, including on board a vehicle and at refueling stations. The electrochemical process for generating hydrogen at pressures that may be greater than 3000 psia features feeding the hydrogen output of a water electrolyzer or other hydrogen gas generating device operated at atmospheric or moderate pressure to an electrochemical hydrogen compressor operating in a high-differential-pressure mode. “Atmospheric or moderate pressure,” as used herein, means from about 0 psia to about 3000 psia. The electrochemical hydrogen compressor has an anode operating at the same pressure as the cathode of the electrochemical hydrogen generator or other hydrogen gas generating device, and a cathode operating at the higher pressure required to fill the gas storage cylinder. The compressor, which may be operated at a 3000 psia or greater pressure differential, elevates hydrogen produced by the electrochemical hydrogen generator or other hydrogen gas generating device to the desired high pressure, for example, 6000 psia. 
     The electrochemical hydrogen generator and compressor of the invention are stacks comprising one or more cells connected electrically in series or in parallel. In some preferred embodiments, each cell contains a membrane and electrode assembly (MEA) comprising an anode and a cathode in intimate contact with and separated by an ionic conductive membrane such as a proton-exchange membrane (PEM) or solid alkaline membrane. When power is applied to each cell in the electrochemical hydrogen generator stack, protons and electrons are generated at the anode. The protons are electrochemically transported across the membrane to the cathode, where they combine with the externally transported electrons to form hydrogen gas. This hydrogen gas is fed to the hydrogen compressor, where it is oxidized at the anode of each cell to form protons and electrons. The protons are transported across the membrane to the cathode, where they are reduced by the externally transported electrons to form hydrogen at the desired higher pressure. 
     The anticipated benefits of the invention include safety of operation and relative simplicity of constructing a differential-pressure hydrogen compressor cell compared to an electrolyzer with the same pressure difference, which translates into cost savings. The two-cell system of the invention is safer to operate than a high-pressure electrolyzer. Membrane failure in the compressor cell presents little hazard as long as there is a pressure shut off valve, and membrane failure in the low-pressure electrolyzer is less dangerous than it would be in a high-pressure electrolyzer. Thus, the two-cell system allows for the use of thinner membranes, resulting in lower voltage. This compensates for the somewhat higher overall voltage and power inefficiency anticipated when using two cells instead of one. In addition, less risk of explosion exists in recirculating water accumulated at the anode or cathode side of the compressor to the low-pressure electrolyzer of the two-cell system than in feeding a pressurized reactor with cathode water, even if the water contains some hydrogen. 
     A water electrolyzer alone could be used to generate hydrogen at high pressure; however, above 2500 psia differential pressure, difficulties arise in supporting the MEA as mechanical properties of the membrane, metallic support structures and compression pads rapidly deteriorate. A high-pressure PEM electrolyzer is also more expensive than an integrated low-pressure electrolyzer and electrochemical hydrogen compressor. Low-cost materials that may be used in the compressor, but not in the high-pressure electrolyzer, include carbon-supported electrode structures, stainless steels, inconels, hastelloys, low-cost hydrocarbon PEMs, and anion exchange (hydroxide transport) solid alkaline membranes. In the compressor, carbon, graphite, hastelloys, stainless, and inconels may replace the costly valve metals (Ti, Zr, Nb) used in electrolyzers. In addition, small noble metal loadings are required due to the high reversibility of the hydrogen electrode in the absence of carbon monoxide and other inhibiting gas traces. 
     Further, there are sizable voltage efficiency losses associated with operating a high-pressure (or high-pressure-differential) water electrolyzer, because the oxygen electrode in the water electrolyzer is quite irreversible. Operating the electrolyzer at near atmospheric pressure also allows for the use of lower current densities at the electrolyzer stack without a substantial decrease in faradaic efficiency (efficiency losses due to gas crossover), which is close to 100%. Decreased current density may be achieved by distributing approximately the same amount of electro-catalyst over a larger membrane surface, resulting in higher voltage efficiency of the electrolyzer. These advantages more than compensate for the additional voltage required by the hydrogen compressor cell (which may contribute 5 to 10% to the overall system voltage compared to single cell voltage) and the existence of two stacks versus one. 
     These and other benefits and features of the present invention will be more fully understood from the following detailed description, which should be read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an integrated electrolyzer stack and electrochemical hydrogen compressor stack. 
         FIG. 2  is a perspective view of an electrolyzer stack. 
         FIG. 3  is a cross-sectional view of a single water electrolysis cell or electrochemical compressor cell. 
         FIG. 4  is a cross-sectional view of a water electrolysis cell. 
         FIG. 5  is a schematic diagram of an integrated and unitized electrolyzer stack and hydrogen compressor stack. 
         FIG. 6  is a graph showing the performance of the electrochemical compressor described in Example 1 at various temperatures. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a system  10  of the present invention is shown, comprising an integrated electrochemical hydrogen generating stack  12  and electrochemical hydrogen compressor stack  14 . In the embodiment of  FIG. 1 , the electrochemical hydrogen generating stack  12  is a water electrolysis stack comprising an anode  16 , a membrane and electrode assembly (MEA)  18 , and a cathode  20 . At the anode  16 , water is used as a reactant and oxygen is generated at a near-ambient pressure P 0 . The operable range for P 0  is from about 0 psia to about 2,500 psia, with a preferred range of from about 0 psia to about 200 psia, where a low pressure pump could be used. Protons formed at the anode  16  are electrochemically driven across the MEA  18  to the cathode  20  where they combine with externally driven electrons to form hydrogen at an elevated pressure P 1 , which is greater than P 0 . The operable range for P 1  is from about 30 psia to about 3,000 psia, with a preferred range of from about 300 psia to about 2,500 psia. The generated moist hydrogen from the electrolyzer is fed to an anode  22  of the electrochemical hydrogen compressor stack  14 , where it is oxidized to form protons. The protons are electrochemically driven across an MEA  24  to a cathode  26 , where they combine with externally driven electrons to form hydrogen at a high pressure P 2 , which is greater than P 1 . The operable range for P 2  is from about 500 psia to about 10,000 psia, with a preferred range of from about 2,500 psia to about 5,000 psia. The electrode reactions are as follows, where P 0 &lt;P 1 &lt;P 2 : 
                                         H 2 O → ½O 2 (P 0 ) + 2H +  + 2e −     @ electrolyzer anode   [1]       2H +  + 2e −  → H 2 (P 1 )   @ electrolyzer cathode   [2]       H 2 O → ½O 2 (P 0 ) + H 2 (P 1 )   Electrolyzer net reaction   [3]       H 2 (P 1 ) → 2H +  + 2e −     @ compressor anode   [4]       2H +  + 2e −  → H 2 (P 2 )   @ compressor cathode   [5]       H 2 (P 1 ) → H 2 (P 2 )   Compressor net reaction   [6]                    
A power conditioner (DC power generator)  28  provides the power required for electrochemical hydrogen generation and compression. The power supplied to the compressor  14  is equal to the difference between the total power of the power conditioner  28  and the power supplied to the electrolyzer  12 .
 
     The electrochemical hydrogen generating stack  12  and hydrogen compressor stack  14  comprise one or more cells  30  connected electrically in series, as shown in  FIG. 5 . In some alternative embodiments, the cells  30  may be connected in parallel.  FIG. 2  shows a perspective view of a typical water electrolyzer stack  12 . The cells  30  are held between a set of end plates  32 . Water enters the stack  12  through an opening  35 . Product humidified hydrogen leaves the stack  12  through an opening  37 . Product oxygen and water leave the stack  12  through an opening  39 . A water purge opening  41  may be used to remove hydrogen from the cells  30  and manifolds during extended periods (days) of shutdown. A typical hydrogen compressor stack  14  has a similar configuration to the electrolyzer stack  12 . 
       FIG. 3  shows a cross-sectional view of a typical single cell  30 , with substantially the same configuration for the electrolyzer cell  30  as for the compressor cell  30 . The cell  30  may have an active cell area of from about 5 cm 2  to about 1 M 2  per cell, with a preferred active cell area of from about 50 cm 2  to about 1000 cm 2 . The cell  30  includes an MEA  100  comprising an ionic conductive membrane such as a thin proton-exchange membrane (PEM)  102  to which an anode  104  and a cathode  106  are attached. The ionic conductive membrane of the MEA  100  may be, for example, perfluorocarbon sulfonic acid sold under the trade name Nafion (DuPont, Wilmington, Del.) for PEM electrolyzers or compressors; sulfonated grafted polystyrene-TFE based materials such as R-4010 sold by RAI (Hauppauge, N.Y.) for PEM hydrogen compressors; and grafted, quaternary ammonium hydroxide polystyrene-TFE based materials such as R-4030 sold by RAI (Hauppauge, N.Y.) for alkaline hydrogen compressors. The ionic conductive membrane of the MEA  100  may have a thickness of from about 18 micrometers to about 200 micrometers, with a preferred thickness of from about 25 micrometers to about 100 micrometers. The electrodes of the MEA  100  may be, for example, of the types disclosed in U.S. Pat. Nos. 3,992,271; 4,039,409; and 4,311,569, the teachings of which are incorporated herein by reference. The electrodes of the MEA  100  may have noble metal (N.M.) catalyst loadings of from about 0.05 mg N.M./cm 2  to about 8 mg N.M./cm 2 , with preferred N.M. catalyst loadings ranging from about 0.1 mg N.M./cm 2  to about 0.8 mg N.M./cm 2 . Suitable noble metal catalysts include, for example, Pt, Ir, Ru, Pd, Rh, Re, Os, and their oxides. Such catalysts are especially preferred for PEM embodiments. In some alternative embodiments, the electrodes of the MEA  100  may include non-noble metal catalysts and their oxides, for example, Ti, Nb, Ta, Zr, W, Mo, Ni, Ag, Co, Fe, and La, used in combination with one another and also supported on high surface area catalyst supports such as carbons, graphites, carbides, nitrides, valve metal oxides and transition metal oxides. Such catalysts are especially preferred for alkaline embodiments of the invention. 
     Expanded metal distributor screens  108  on each side of the MEA  100  conduct current and improve the flow distribution of gas and liquid products and reactants through the cell  30 . Suitable materials for current collectors and fluid, electric current distributors of the invention include, for example, valve metals, transition metals, carbons, graphites, carbides, and composites thereof with polymers such as Kynar (Elf Atochem, Philadelphia, Pa.) and polysulfone. Preferred materials include Ti for PEM electrolyzer anodes; graphite, Zr, and carbon for PEM electrolyzer cathodes; and graphite, carbon, stainless, hastalloys, and inconels for PEM hydrogen compressors and alkaline systems applications. The metal screens  108  mechanically support the MEA  100 , which can easily withstand differential pressures (H 2 &gt;O 2 ) in excess of 2500 psia. Separators  114 ,  116  contain fluids on the face of the screens  108  or active area of the cell assembly. 
     The reactants, water in a water electrolyzer cell  30  and hydrogen in a hydrogen compressor cell  30 , enter the applicable cell  30  through an opening  107  on the anode side. The reactants flowing along the screen  108  on the anode side contact the anode  104 , where protons and electrons are produced. In a water electrolyzer cell  30 , oxygen is also produced at the anode  104 . The oxygen flows along the screen  108  and leaves the cell  30 , along with excess water, through an opening  109  on the anode side, at the opposite end of the cell  30  from the opening  107  through which water enters the cell  30 . Protons produced at the anode  104  are electrochemically transported across the PEM  102  to the cathode  106 , where they combine with the externally transported electrons, which flow from anode  104  (loss of electrons) to cathode  106  (gain of electrons), to form hydrogen gas at an elevated pressure relative to the anode-side gas. Hydrogen gas produced at the cathode  106  flows along the screen  108  and leaves the cell  30  through an opening  111  on the cathode side. 
       FIG. 4  depicts a typical single water electrolysis cell  30  (see also  FIG. 2 ). The MEA  100  is contacted on each side by metal screen  108  comprising a multi-layer screen package. The screen package is preferably about 0.03 inches thick and may be, for example, of the type disclosed in U.S. Pat. No. 6,179,986, the teachings of which are incorporated herein by reference. For example, the screen package may be an integral lightly platinized (0.2 mg/cm 2 ) Ti current collector/water distribution mesh package fabricated by spot welding two or more expanded meshes, sold by Exmet Corp. of Naugatuck, Conn. under the trade name Exmet, until the total mesh thickness is equivalent to the thickness of cell frame  112 . Alternatively, the screen package may be a grooved carbon current collector/water distribution structure made of materials such as porous C, solid C, or molded C or TiC composites with polymers such as Kynar (Elf Atochem, Philadelphia, Pa.), polysulfone, polyesters, terephthalates, or liquid crystal polymers (e.g., Kevlar (DuPont, Wilmington, Del.), Vectra (Ticona, Summit, N.J.), and polybenzimidazole). 
     The screens  108  form fluid cavities for the water, hydrogen, and oxygen. Behind each screen  108  on the cathode side is an insert  110 , which, along with oxygen separator  114  and hydrogen separator  116 , contains the fluids on the face of the screens  108  or active area of the cell assembly. The insert  110  may be a solid foil, a porous foil, a mesh, or a combination thereof, with a total thickness ranging from about 0.01 cm to about 0.15 cm, preferably from about 0.025 cm to about 0.075 cm. Suitable materials for the insert  110  include, for example, metals such as Ti and Nb, stainless, and carbon. The separators  114 ,  116  may, for example, comprise thin composite, conductive plates made of materials such as Nb or Ti foil on the anode side and Zr foil or molded carbon on the cathode side. In some embodiments, the separators  114 ,  116  may be of the types described in U.S. Pat. Nos. 6,179,986, 4,214,969 and 4,339,322, the teachings of which are incorporated herein by reference. 
     Each screen  108  is surrounded by a cell frame  112 , made of polysulfone or another material with similar properties, that externally contains the fluids and manifold or ports that direct fluids in and out of the screen cavities. Suitable materials for the cell frame  112  include, for example, organic and inorganic polymers or plastics, carbons, graphites, composites of carbons or graphites with polymers, ceramics and electrically inerted or coated metals. The cell frame may have a total thickness of from about 0.05 cm to about 0.5 cm, with a preferred thickness of from about 0.15 cm to about 0.3 cm. Liquid water, preferably distilled or deionized, enters the cell  30  through an opening  107  in the frame  112  on the anode side and flows along the screen  108 . The water is uniformly distributed along the screen  108  and is oxidized along the anode  104  of the MEA  100  to form oxygen, protons, and electrons. The oxygen and some water are released from the cell  30  through an opening  109  in the frame  112  on the anode side, at the opposite end of the cell  30  from the opening  107  through which water enters the cell  30 . The protons and some water are transported across the PEM  102  to the cathode side of the cell  30 . The protons are reduced along the cathode  106  by externally transported electrons to form humidified hydrogen, which is released from the cell  30  through an opening  111  in the cathode side of the frame  112 . This process is repeated for any additional cells  30  in the electrolyzer stack  12 , and the humidified product hydrogen is fed to the hydrogen compressor stack  14 . A water purge opening  113  in the cathode side of the frame  112  may be used to remove hydrogen from the cells  30  and manifolds during extended periods (days) of shutdown. 
     The screens  108  also serve as current collectors, conducting electrons from the cell anode  104  to the oxygen separator sheet  114 , from which they pass through the adjacent hydrogen separator  116  and screen package  108  to the cathode  106  of the next cell in a bipolar configuration. Gaskets  118 , preferably 0.005-inch-thick plastic, seal the cell frame  112  to the metal separators  114 ,  116 , while the membrane  102  seals the frame  112  on the opposite side. A pressure pad assembly  120 , for example made of silicone and woven metal strips, and breather screen  122  between two adjacent hydrogen and oxygen metal separators  114 ,  116  provide the contact pressure against the cell active area through the separators  114 ,  116 . A plastic manifold gasket  124  surrounds the pressure pad assembly  120  between the separators  114 ,  116  to seal the fluid manifold parts between cells  30  in the stack  12 . 
     The mechanical design configuration for a typical hydrogen compressor cell  30  of the invention is similar to that of the water electrolysis cell  30  shown in  FIG. 4 , except that humidified hydrogen gas is introduced to the anode side of the cell  30  instead of liquid water, so hydrogen is oxidized to protons and electrons but no oxygen is produced. The hydrogen-generating reaction at the cathode side is the same for the compressor as for the electrolyzer, the reduction of protons with electrons to form hydrogen, and hydrogen may be generated at 2500 psia above the anode side. The concept of a PEM hydrogen concentrator/compressor is described by Sedlak et al., Int. J. Hydrogen Energy 6:45-51 (1981), the teachings of which are incorporated herein by reference. 
       FIG. 5  shows an example of a preferred system  10  of the invention, comprising an integrated electrochemical hydrogen generating stack  12  and electrochemical hydrogen compressor stack  14 , and a power conditioner  28 . The electrochemical hydrogen generating stack  12  is a 2-inch high water electrolyzer stack composed of 14 cells  30  in electrical series, each with 0.05 ft 2  (3-inch diameter) of active area. The stack  12  generates oxygen at between about 40 psia and about 200 psia and hydrogen at about 1000 psia. The stack  12  runs at 15.5 amps (310 mA/cm 2 ) at 1.58 volts per cell. The cell voltage and diffusional losses of 0.5 amps result in a stack efficiency of slightly above 90% on the basis of the higher heating value of hydrogen (1.48 volts), with 37 Watts of waste heat. The electrochemical hydrogen compressor stack  14  comprises cells  30  of similar number and size to those of the electrolyzer stack  12 , in electrical series with one another, and in parallel with the electrolyzer stack  12 . The compressor elevates hydrogen from about 1,000 psia to about 5,000 psia. The electrolyzer and electrochemical compressor stacks  12 ,  14  are unitized and held between a single set of 6-inch diameter end plates  32  to minimize weight, volume and cost. Polysulfone cell frames  112  of 4.5-inch diameter with ridges and containment rings surround the cells  30  in each stack  12 ,  14 . An electrical bus  34  separates the stacks  12 ,  14  and allows them to be operated at different currents as required by their different production rates. A control circuit  36  controls the voltage across the electrochemical hydrogen compressor  14  at the required 0.05 to 0.1 volt/cell. If the hydrogen feed stream to the electrochemical hydrogen compressor  14  is interrupted, causing the voltage to rise, the current of 15.5 amps is shunted around the compressor  14  to prevent the voltage from rising to the point where oxygen would be generated on the anode side of the compressor  14 . 
     The equation for the thermodynamic voltage of the electrochemical hydrogen compressor cell of the invention is as follows, if the anode is fed at 30 psia (˜2 Atm, a, where Atm, a is the pressure in atmospheres, absolute): 
                     E   =     29.5   ⁢     T   298     ⁢   log   ⁢     P     P     a   ⁢           ⁢   tm             ,     m   ⁢           ⁢   V             [   7   ]               
At 100° C. (373° K) and 3000 psia (˜200 Atm, a) pressure differential, E=74 mV. The thermodynamic voltage required by the hydrogen compressor cell is equal to the additional thermodynamic voltage of a differential-pressure electrolyzer with a cathode operating at the same high pressure over the potential of an all-atmospheric-pressure electrolyzer, i.e., there is no difference in thermodynamic voltage attributable to the high-pressure cathode in the two approaches. However, the hydrogen compressor cell contributes to overall voltage through an additional polarization due to current multiplied by ionic resistance and very slight activation overvoltages. Operable voltages for electrolyzers of the invention range from about 1.4 V to about 3.0 V, with a preferred range of from about 1.5 V to about 2.0 V. Operable voltages for hydrogen compressors of the invention range from about 0 V to about 0.5 V, with a preferred range of from about 0.05 V to about 0.30 V. Operable current densities for electrolyzers and electrochemical compressors of the invention range from about 0 mA/cm 2  to about 10,000 mA/cm 2 , with a preferred range of from about 300 mA/cm 2  to about 3,000 mA/cm 2 .
 
     A consequence of high-pressure hydrogen cathode operation is permeation of hydrogen through the PEM, which significantly reduces faradaic efficiency. This applies for both a high-pressure electrolyzer and a high-differential-pressure hydrogen compressor, but an electrolyzer with pressurized anode and cathode has significant additional inefficiency due to oxygen permeation. A much smaller oxygen permeation inefficiency exists when the anode operates at low pressure. The faradaic inefficiency effects a power efficiency loss, which is about 10 times higher for a high-pressure electrolyzer than for a high-differential-pressure hydrogen compressor because the cell voltage of the electrolyzer is about ten times higher. 
     In a system with electrolyzer and compressor stacks connected electrically in series and having the same number of cells, the hydrogen migration from high-pressure cathode to low-pressure anode in the compressor stack would create a continuous pressure buildup in the low-pressure hydrogen space, the gas space of the compressor anode, the electrolyzer cathode, and the associated plumbing. If the number of cells in each stack is sufficiently large, pressure balance in the low-pressure hydrogen space may be achieved by using one or more additional cells in the electrochemical compressor stack than in the electrolyzer stack. The number of additional cells in the compressor stack will depend on total cell number, cell characteristics, and operating pressures. As demonstrated in Example 3 below, a typical system of the invention may comprise 15 cells per stack, requiring one additional cell in the electrochemical compressor stack. This solution does not lend it self to a perfect match due to the stepwise introduction of additional cells. 
     Alternatively, or to fine-tune a design with a slightly larger electrochemical compressor stack than electrolyzer stack, an additional power source may be connected between the terminals of the electrochemical compressor (low voltage) stack. Other alternative or additional measures include shunting the electrolyzer (high voltage) stack by a resistor or periodically venting hydrogen pressure. The latter two approaches are recommended only if the unbalance is small, such that the lost power or hydrogen represents a small fraction of the supplied power or pressurized hydrogen. Low pressure in the hydrogen space, for example due to over-correction in selecting the number of cells by which the electrochemical compressor stack exceeds the electrolyzer stack, may be remedied by shunting the hydrogen compressor (low voltage) stack by a resistor or by connecting an additional power source between the terminals of the electrolyzer (high voltage) stack. 
     Those of skill in the art will appreciate that adding water to a low-pressure electrolyzer is relatively straightforward and routinely practiced. Net water addition is not required in the hydrogen compressor cells, but humidification of anode hydrogen is usually necessary in PEM cells since the transported proton is hydrated. Hydrogen produced by the electrolyzer is humidified, and the humidity can be regulated by controlling the temperature of the electrolyzer or the hydrogen gas. The moderate pressure and the temperature of the electrolyzer is selected to produce optimal electrochemical PEM hydrogen compressor anode operation, especially with respect to humidity. In addition to requiring two stacks, the systems of the present invention require a gas-diffusion electrode, which is the anode of the hydrogen compressor cells. However, this electrode is straightforward to operate, especially since the hydrogen gas from the electrolyzer is humidified at saturation and the electrolyzer temperature may be controlled to create ideal humidification for the hydrogen compressor cell. The operational temperature ranges for the system are from about 25° C. to about 130° C., with a preferred temperature range of from about 25° C. to about 80° C. The operating pressure of the system ranges from about 0 psia to about 10,000 psia, with a preferred pressure range of from about 30 psia to about 5,000 psia. Net removal of water from the cells at the high- or low-pressure end is reasonably straightforward. 
     As shown in the figures and demonstrated in the examples below, the electrochemical hydrogen generating stack of the invention may be a PEM water electrolyzer stack. However, the electrochemical hydrogen generating stack may be any ionic conductive membrane electrochemical stack operating at atmospheric or moderate pressure to produce protons and electrons at the anode by oxidation and generate hydrogen gas at an elevated pressure at the cathode by electrochemical reduction of the protons with the electrons. Alternative electrochemical hydrogen generating stacks of the invention include cells that generate hydrogen gas at the cathode and also produce protons at the anode and/or hydroxide ions at the cathode. For example, an alkaline liquid electrolyzer that generates hydroxide ions as well as hydrogen at the cathode may be used, as long as precautions are taken to eliminate any NaOH entrapped in the low-pressure hydrogen stream from the integral electrochemical compressor. In some embodiments, sodium sulfate electrolyzers may be used, wherein sulfuric acid (from protons) and oxygen gas are generated at the anode, and sodium hydroxide (from hydroxide ions) and hydrogen are produced at the cathode. In other alternative embodiments, electrolyzers of the invention include brine, chloralkali electrolyzers, which generate chlorine at the anode and hydrogen gas and hydroxide ions at the cathode, and HCl or HBr electrolyzers, which generate chlorine or bromine, respectively, and protons at the anode and hydrogen gas at the cathode. In still other alternative embodiments of the invention, a methanol/hydrogen stack may be used, in which methanol is oxidized at the anode to produce protons, electrons and carbon dioxide, and hydrogen gas is evolved at an elevated pressure at the cathode by reduction of the protons with the electrons. In still other embodiments, the electrochemical hydrogen generating stack is a reformate/hydrogen stack, in which the reformate gas contains hydrogen gas diluted with carbon dioxide, nitrogen, and water vapor. The diluted hydrogen gas is oxidized at the anode to form protons and electrons, and pure hydrogen gas at an elevated pressure is produced at the cathode from the reduction of the protons and electrons. 
     As shown in the figures and examples herein, the electrochemical hydrogen compressor stack of the invention may be a PEM hydrogen compressor. In some alternative embodiments of the invention, the electrochemical hydrogen compressor may be a solid alkaline membrane electrochemical hydrogen compressor. An example of a solid alkaline membrane is RAI 4030, supplied by RAI, Hauppauge, N.Y., a fluorocarbon-grafted quaternary ammonium hydroxide anion exchange membrane. A solid alkaline membrane generally is not sufficiently stable for use in electrolyzers or fuel cells where hydrogen and oxygen are present, but stability is significantly enhanced in an electrochemical hydrogen compressor where only a hydrogen atmosphere is present. 
     In an alternative embodiment to an electrochemical H 2  generator, a reactor that converts a hydrogen-containing compound to a hydrogen-containing gas is used as a hydrogen source for the electrochemical hydrogen compressor. Non-limiting examples of such reactors include hydrocarbon processors, such as, e.g., hydrocarbon gas reformers, and ammonia crackers. For example, certain embodiments employ a gas reformer such as described by Rostrop-Nielson et al., “Steam Reforming, ATR, Partial Oxidation: Catalysts and Reaction Engineering” in  Handbook of Fuel Cells, Vol.  3,  Fuel Cell Technology and Applications , p. 159 (Vielstich et al. eds., John Wiley &amp; Sons, 2003) or an ammonia cracker such as described by Hacker et al., “Ammonia Crackers” in  Handbook of Fuel Cells, Vol.  3,  Fuel Cell Technology and Applications , p. 121 (Vielstich et al. eds., John Wiley &amp; Sons, 2003), the teachings of the foregoing references being incorporated herein by reference. In yet another alternative embodiment to an electrochemical H 2  generator, a PEM fuel cell that exhausts an effluent of a not completely reacted, dilute hydrogen stream is used as the H 2  source. In some of these alternative embodiments, the hydrogen-containing gas produced by the reactor is treated using a gas conditioner (e.g., a humidifier) before it enters the hydrogen compressor. 
     Thus, in a first system, a reactor (e.g., an electrolyzer, hydrocarbon processor, or ammonia processor) converts a hydrogen-containing compound to a hydrogen-containing gas at a first pressure P 1 . Alternatively, the first system is a PEM fuel cell that supplies a diluted H 2  stream at pressure P 1 . Integrated with the first system is a second system, an electrochemical hydrogen compressor that produces hydrogen at a second pressure P 2 . The electrochemical hydrogen compressor includes at least one cell containing an anode, which is, in at least some instances, a gas diffusion electrode, a cathode, and an ionic conductive membrane in intimate contact with and separating the anode and the cathode. The electrochemical hydrogen compressor anode oxidizes hydrogen at pressure P 1  supplied by the first system, and the cathode evolves hydrogen at pressure P 2 , which is greater than P 1 . 
     As an alternative to a conventional hydrogen-evolving cathode (e.g., Pt black particles with binder alone, or Pt dispersed on carbon with binder), in some embodiments, the electrochemical hydrogen compressor cathode includes a solid metallic film such as, for example, palladium (Pd) or a Pd alloy. Hydrogen atoms are formed at the inner surface of the metallic film, which is in contact with the ionic conductive membrane, and transported to the outer surface of the metallic film, where they combine to form hydrogen gas at pressure P 2 , which is greater than the pressure P 1  at the anode. 
     In particular embodiments, to accelerate atomic hydrogen transport, one or both surfaces of the metallic film, e.g., a palladium (Pd) or a Pd alloy film, are treated. The treatment includes, for example, roughening the metallic surface and/or depositing noble metals or particulate materials containing noble metals (e.g., Ir or Rh) or transition metals (e.g., Co) on one or both sides of the Pd foil. 
     In some embodiments, the ionic conductive membrane in the electrochemical hydrogen compressor is a solid alkaline membrane. In this case, the following electrochemical reactions take place:
 
Anode: H 2 (P 1 )+2OH − →2H 2 O+2 e   −   [8]
 
Cathode: Pd foil surface facing the solid alkaline membrane
 
2H 2 O+2 e   − →2OH − +2H.  [9]
 
     Outer surface of Pd foil
 
2H.→H 2 (P 2 )  [10]
 
Atomic hydrogen (2H.) is transported through the Pd foil and combines at the outer surface to form hydrogen at pressure P 2 .
 
     In some alternative embodiments, the ionic conductive membrane in the electrochemical hydrogen compressor is a proton exchange membrane. In this case the following electrochemical reactions take place:
 
Anode: H 2 (P 1 )→2H + +2 e   −   [11]
 
Cathode: Pd foil surface facing the PEM
 
2H + +2 e   − →2H.  [12]
 
     Outer surface of Pd foil
 
2H.→H 2 (P 2 )  [13]
 
Atomic hydrogen (2H.) is transported through the Pd foil and combines at the outer surface to form hydrogen at pressure P 2 .
 
     The following nonlimiting examples further illustrate certain preferred embodiments of the present invention: 
     EXAMPLE 1 
     A hydrogen compressor cell  30  with active cell area of 0.05 ft 2  was assembled with a 5-mil (dry) membrane  102  (Dow Chemical, Midland, Mich., Dow XUS 13204), having an equivalent weight of approximately 800 and an ion-exchange capacity (meq of H +  ion/g of dry polymer) of 1.25. Pt black electrode structures  104 ,  106  were integrally bonded to each surface of the membrane  102 . Ambient-pressure hydrogen was fed to the anode  104  of the PEM compressor cell  30 , where it was continually oxidized, by the application of a direct electric current from a DC power generator, to form protons, which were electrochemically transported across the membrane  102  to the cathode  106 . At the cathode  106 , the protons were reduced back to hydrogen, which was allowed to rise to a higher pressure. Because the hydrogen oxidation and reduction reactions are highly reversible, essentially all of the voltage losses were due to current multiplied by ionic resistance. The hydrogen gas was compressed from approximately 15 psia to approximately 30 psia. The performance of the electrochemical compressor  14  at 30° C., 60° C., and 80° C. is shown in  FIG. 6 . 
     EXAMPLE 2 
     A hydrogen compressor cell  30  with an active cell area of 0.23 ft 2  was assembled with 10-mil (dry) Nafion 120 membrane  102  (E.I. DuPont, Wilmington, Del.) and successfully operated by the application of a direct electric current from a DC power generator at 1000 mA/cm 2 , compressing hydrogen gas from approximately 15 psia to approximately 1000 psia. Cell voltage was approximately 0.32 V at 40° C. 
     EXAMPLE 3 
     A compressor stack  14  with a 15-mil Nafion membrane  102  (E.I. DuPont, Wilmington, Del.), having a water content of 0.37 g of water per 1 g of dry membrane operates at a differential pressure of 3000 psia (˜200 Atm, a). The compressor stack  14  is integrated with an electrolyzer stack  12  with the same number of cells  30  operating at 80° C. and near-atmospheric pressure, with a current density of 1000 mA/cm 2  by the application of a direct electric current from a DC power generator. The hydrogen lost from the high-pressure to the low-pressure side of the compressor stack  14  is 6.7% of the total hydrogen generated; the electrolyzer  12  has a negligible effect on the amount of hydrogen transferred to the low-pressure hydrogen space. The 6.7% hydrogen loss is offset by adding 1 cell per every 15 cells of a theoretical (no hydrogen permeability) compressor stack  14  while leaving the electrolyzer stack  12  untouched. 
     While the foregoing invention has been described with reference to its preferred embodiments, various alterations and modifications will occur to those skilled in the art. All such alterations and modifications are intended to fall within the scope of the invention and the appended claims.