US20040211679A1

US20040211679A1 - Electrochemical hydrogen compressor

Info

Publication number: US20040211679A1
Application number: US10/478,852
Authority: US
Inventors: Terrance Wong; Francois Girard; Thomas Vanderhoek
Original assignee: National Research Council of Canada
Current assignee: National Research Council of Canada
Priority date: 2002-03-07
Filing date: 2003-03-06
Publication date: 2004-10-28
Also published as: WO2003075379A3; AU2003208221A8; WO2003075379A2; AU2003208221A1

Abstract

The invention disclosed relates to an apparatus and process for electrochemical compression of hydrogen. The apparatus comprises a membrane electrolyte cell assembly (MEA), including planar gas distribution plates sandwiching the MEA, the assembly being held together by end-plates, the end-plates having complementary peripheral grooves for seating an intervening seal between the end-plates and the MEA, the end-plate on the anode side further including a hydrogen supply inlet and the end-plate on the cathode side further including a compressed hydrogen outlet. Both single cell and multi-cell assemblies are disclosed. The multi-cell assemblies comprise a plurality of such single cells connected in series, such that the compressed hydrogen from the outlet of a first cell is connected to the hydrogen outlet of the next cell in series, where each cell is electrically isolated from the adjacent cell in the series. The process involves the electrochemical compression of hydrogen in such cells, whereby pressures of up to 12,000 psi are achieved by multi-cell assemblies.

Description

BACKGROUND OF THE INVENTION

This invention relates to an apparatus and process for electrochemical compression of hydrogen.

Fuel cells offer an environmentally friendly method of efficient energy generation, and the use of hydrogen as the fuel of choice is attractive as the conversion to electrical energy is emissions-free, with water and heat being the only by-products. The delivery of hydrogen in gaseous or liquid form or as an absorbed species (e.g. metal hydride, on activated carbon, or in carbon nanotubes) depends on the fuel cell application, and re-fueling frequency and related autonomy are important factors to consider in the selection of the appropriate mode of fuel storage. As liquid hydrogen involves energy-intensive and sophisticated cryogenic technologies and has a high boil-off rate and absorption technology is still in its infancy, gaseous hydrogen is a convenient and common form for storage, usually by pressurized containment for increased energy density. To this end, mechanical compression is the most common means by which to achieve pressurization; however, it suffers from limitations due to 1) intensive energy use, 2) wear-and-tear of moving parts, 3) hydrogen embrittlement, 4) excessive noise, 5) bulky equipment, and 6) contamination of the gas usually by compressor lubricants. Non-mechanical pressurization by thermal cycling is possible, but this is also energy intensive and not commercially practical yet.

Electrochemical transfer of hydrogen through proton-conductive materials is known, and fundamental studies on single-stage transfer applications can be found reported in the literature. For example, the use of thin perovskite-type oxide proton-conducting ceramics is well documented for single-stage separation of hydrogen from gas mixtures [1-3]. In these applications, the single cell operates at elevated temperatures (500-1000° C.) in order to maintain sufficiently high protonic conductivity through the separator. Reports on electrochemical hydrogen compression are scarce and most describe the use of single cells with a polymer electrolyte membrane (PEM), i.e. Nafion®, as the proton-conductive separator and Pt as the electrocatalyst on carbon electrodes (both anode and cathode) [4-7]. Operation of these cells to pressure differentials of ˜43 atm (with anodic compartment pressure=1 atm) occurs without excessive energy demands; at greater differentials, however, rapid loss of H ₂due to leakage around the cell seals causes an exponential increase in power consumption. Use of Nafion PEM in electrochemical transfer of H₂has also been reported [8]; in this case, H₂was directed to the cathodic compartment filled with water in order to de-oxygenate the water by reaction of the H₂with the dissolved O₂. It is important to note that in all these applications, electrochemical transport is selective to H₂only due to the proton conductive nature of the separator and, in a gas mixture, hydrogen is not only concentrated (pressurized) but also purified by such means.

In U.S. Pat. No. 6,361,896 of Eberle et al. [10], the disclosure indicates that for single cell devices, differential pressures of up to about 10 bar can be achieved. This compares with earlier prior art devices that can only achieve a 5 bar differential pressure. Also disclosed is the use of a second cell to increase the differential pressure theoretically to “more than about 15 bar”. The higher pressure differential is said to be achieved by means of a planar porous gas distribution support layer on the anode side (see col. 2). However, it is significant that there is no experimental proof provided that this was achieved. Moreover, no specific structure is described

Also described in Ströbel et al. [5] is a multi-cell stack. It is noted that the cells in the stack are connected in-parallel, so that there is no H ₂transport from one cell to the next. Accordingly, the H₂output pressure from each cell is the same. The maximum pressure differential achieved was about 54 bar.

SUMMARY OF THE INVENTION

According to the invention, an apparatus and process are provided for pressurizing hydrogen electrochemically.

As will be discussed later, this technology targets the hydrogen supply and gas storage industries as well as the emerging fuel cell industry. With potential application of the technology in the fuel cell industry, high-pressure compression is desired and, more specifically, pressurization up to 12,000 psi is targeted, as this level is deemed necessary by the transportation industry for practical implementation of fuel cell vehicles.

According to one aspect of the invention, an apparatus is provided for compression of hydrogen, comprising a membrane electrolyte cell assembly (MEA), including a proton-conducting electrolyte membrane, an anode on one side of the membrane and a cathode on the other side of the membrane, the anode having an electrochemically active material for oxidizing hydrogen to protons, the cathode having an electrochemically active material for reducing protons to hydrogen, and further comprising next to the anode and cathode, planar gas distribution and support plates sandwiching the MEA, the assembly being held together by end-plates, the end-plates having complementary peripheral grooves for seating an intervening seal between the end-plates and the MEA, the end-plate on the anode side further including a hydrogen supply inlet and the end-plate on the cathode side further including a compressed hydrogen outlet.

According to another aspect of the invention, a process is provided for the compression of hydrogen by means of the apparatus described in the preceding paragraph, wherein hydrogen is compressed electrochemically by the MEA cell by oxidation of the hydrogen to protons at the anode, which having passed through the membrane to the cathode side are reduced back to hydrogen and discharged under pressure.

According to yet another aspect of the invention, in order to achieve even higher total or system pressure, we provide a plurality of such cells connected in series. The more cells connected in series, the higher the final outlet (overall) pressure that is achieved. By connection in series, we mean connected such that there is a progressive increase in pressure from cell to cell in the series. It would be expected by those skilled in the art that compression to higher pressures than those achievable by a single cell design according to our invention are achievable by including additional cells connected in series. For example, by setting a pressure differential of 1000 psi per stage (at each cell), compression to 10,000 psi overall would require ten cells.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram showing the concept of electrochemical hydrogen compression. [0011]
FIG. 2 is a diagram showing the concept of multi-stage electrochemical hydrogen compression. [0012]
FIG. 3 is a concept diagram showing a cross-sectional view of a multi-stage electrochemical hydrogen compressor with an overall cylindrical configuration. [0013]
FIG. 4 is a diagram showing the design of planar gas distribution and support plates according to the invention with complementary grooves for intervening seals to provide a leak-free seal between the MEA and the plates. [0014]
FIG. 5 is a diagram showing the unassembled view of a single-stage electrochemical hydrogen compressor unit according to the invention. [0015]
FIG. 6 is a schematic circuit design for a two-stage electrochemical hydrogen compressor system according to the invention. [0016]
FIG. 7 is a graph showing the results of electrochemical hydrogen compression to 45 psia (T=22° C.; i=0.6 A). [0017]
FIG. 8 is a graph showing the results of electrochemical hydrogen compression to 75 psia (T=22° C.; i=0.6 A). [0018]
FIG. 9 is a graph showing the voltage applied during electrochemical hydrogen compression to 45 psia (T=22° C.; i=0.6 A) (---ΔE derived using equation 4). [0019]
FIG. 10 is a graph showing the voltage applied during electrochemical hydrogen compression to 75 psia (T=22° C.; i=0.6 A) (---ΔE derived using equation 4). [0020]
FIG. 11 is a graph showing the voltage applied during electrochemical hydrogen compression to 45 psia (T=65° C.; i=0.6 A) (---ΔE derived using equation 4). [0021]
FIG. 12 is a graph showing the voltage applied during electrochemical hydrogen compression to 45 psia (T=80° C.; i=0.6 A) (---ΔE derived using equation 4). [0022]
FIG. 13 is a graph showing the voltage applied during electrochemical hydrogen compression to 45 psia (T=65° C.; i=0.1 A) (---ΔE derived using equation 4). [0023]
FIG. 14 is a graph showing the results of dual-stage, electrochemical hydrogen compression to 75 psia (T=22° C.). [0024]
FIG. 15 is a graph showing the voltage applied during dual-stage, electrochemical hydrogen compression to 75 psia (T=22° C.) (---ΔE derived using equation 4). [0025]
FIG. 16 is a graph showing the results of electrochemical hydrogen compression to 535 psia (T=22° C.; i=0.6 A). [0026]
FIG. 17 is a graph showing the voltage applied during electrochemical hydrogen compression to 535 psia (T=22° C.; i=0.6 A) (---ΔE derived using equation 4). [0027]
FIG. 18 is a graph showing the results of dual-stage, electrochemical hydrogen compression to 2000 psia (T=22° C.). [0028]
FIG. 19 is a graph showing the voltage applied during dual-stage, electrochemical hydrogen compression to 2000 psia (T=22° C.) (---ΔE derived using equation 4).[0029]

DETAILED DESCRIPTION OF THE INVENTION

Electrochemical compression of hydrogen is accomplished by the application of an electric potential across a proton-conductive polymer electrolyte material separating anode and cathode compartments to effect the transport of hydrogen from one side to the other. The process is based on the following anodic and cathodic reactions: [0030]
H₂→2H⁺+2e⁻ anodic (oxidation) (1)
2H⁺+2e⁻→H₂cathodic (reduction) (2)
The use of electrocatalysts facilitates these reactions, and the principle of operation can be illustrated as shown in FIG. 1. Oxidation of H[0031] ₂at the anode 10, located in anodic compartment 12 generates hydrogen ions (protons) and electrons; the hydrogen ions migrate across the proton-conductive polymer electrolyte separator 14 while the electrons travel via an external circuit to the cathodic compartment 16 where reduction back to H₂takes place at the cathode 18. From thermodynamic considerations using the Nemst equation, the theoretical applied potential to effect a desired final pressure of H₂exiting the cathodic compartment can be determined. For example, the thermodynamic cell potential is represented by the following equation: $\begin{matrix} E_{cell} = E_{c} - E_{a} = E_{cell}^{\circ} - \frac{RT}{2 F} \ln \frac{a_{H_{2}, c}}{a_{H_{2}, a}} & (3) \end{matrix}$
E[0032] _cell=thermodynamic cell potential, V
E[0033] _c=cathode half-cell potential, V
E[0034] _a=anode half-cell potential, V
E[0035] _cell°=thermodynamic cell reference potential, 0.00 V
a[0036] _H ₂ _,c=activity of H₂at the cathode
a[0037] _H ₂ _,a=activity of H₂at the anode
R=gas constant, 8.3144 mol[0038] ⁻¹K⁻¹L kPa
T=temperature, K [0039]
F=Faraday constant, 96487 C/mol e[0040] ⁻
With hydrogen as a gas, a[0041] _H ₂equates to pressure, P_H ₂, and the applied potential, ΔE, is determined from the following equation: $\begin{matrix} applied potential = Δ E = \langle E_{cell} \rangle = \frac{RT}{2 F} \ln \frac{P_{H_{2}, c}}{P_{H}} & (4) \end{matrix}$
For a more rigorous mathematical treatment, the thermodynamic property, fugacity (f), is used and is the effective pressure when the non-ideality of gases is taken into consideration. Fugacity relates to P by the following equation: [0042]
fugacity, f=φP (5)
where φ is the fugacity coefficient, akin to the activity coefficient (γ) in the thermodynamic treatment of non-ideal solutions. Fugacity coefficients have been tabulated for a number of gases and, for hydrogen, φ is essentially 1.0 for pressures up to 1000 psia (68 atm) [9]; at higher pressures, f becomes significant. The applied potential is then determined more accurately from the following equation: [0043] $\begin{matrix} applied potential = Δ E = \langle E_{cell} \rangle = \frac{RT}{2 F} \ln \frac{f_{H_{2}, c}}{f_{H}} = \frac{RT}{2 F} \ln \frac{φ_{H_{2}, c}}{φ_{H}} + \frac{RT}{2 F} \ln \frac{P_{H_{2}, c}}{P_{H}} & (6) \end{matrix}$
φ[0044] _c=cathodic compartment fugacity coefficient
φ[0045] _a=anodic compartment fugacity coefficient
For example, for a ten-fold increase in pressure with P[0046] _H ₂ _,a=1 atm and P_H ₂ _,c=10 atm at room temperature (25° C.), the applied potential is ΔE=29.7 mV (with (φ_H ₂ _,a=1.000 and φ_H ₂ _,c=1.006). For a further ten-fold increase with P_H ₂ _,a=10 atm and P_H ₂ _,c=100 atm, ΔE=30.3 mV (φ_H ₂ _,a=1.006 and φ_H ₂ _,c=1.063) and, with P_H ₂ _,a=1 atm and P_H ₂ _,c=100 atm, ΔE=59.9 mV (φ_H ₂ _,a=1.000 and φ_H ₂ _,c=1.063). As evident, a relatively small applied potential results in significant pressurization, and the device functions essentially as a concentration cell. In actuality, the required applied voltage, E_working, would be higher due to electrode overpotentials and resistance (circuit and ohmic drop across the separator) and due to application of an electric current to effect a timely increase in pressure:
E _working =ΔE+E _polarzation
E _polarization=|η_a|+|η_c |+iR _separator +iR _circuit (7)
η[0047] _a=overpotential of the anode, V
η[0048] _c=overpotential of the cathode, V
i=applied current, A [0049]
R[0050] _separator=resistance across the proton-conductive separator, ohm
R[0051] _circuit=resistance of the electrical circuit, ohm
The overpotentials of the anode and cathode represent chemical kinetic barriers, i.e. the energy required for electron transfer during the anodic and cathodic electrochemical reactions, and the use of electrocatalysts (e.g. Pt) and/or higher temperatures can reduce these values. For resistance, the ohmic drop across the separator can be minimized, for instance, by the use of thinner materials and, across the circuit, with appropriate electrical materials. [0052]
With the initial condition, P[0053] _H ₂ _,a=P_H ₂ _,c=P_i, the energy consumption for single-stage compression, from P_H ₂ _,c=P_ito P_f, can be determined from the following equation: $\begin{matrix} energy = w = Δ nRT \ln \frac{P_{f}}{P_{i}} + E_{polarization} (i) (Δ t) & (8) \end{matrix}$
Δn=no. of moles of H[0054] ₂transferred, mol
Δt pressure increase time period, sec [0055]
ΔnRTln(P[0056] _f/P_i)=w_t, thermodynamic work of compression,
The efficiency (%) of electrochemical hydrogen compression is referenced to the applied voltage and is a measure of the deviation from thermodynamic work: [0057] $\begin{matrix} efficiency = \frac{w_{t}}{w} \times 100 & (9) \end{matrix}$
In practice, multi-stage compression is preferred for high-pressure applications whereby pressure differentials between stages can be set to reasonable values in order to accommodate material limitations (e.g. structural integrity, effective sealing, and H[0058] ₂back-diffusion phenomenon [5]). For an electrochemical compressor with N stages with P₁, P₂. . . P_Ninitially established and kept constant (FIG. 2), the total energy consumption can be determined simply from summation of the compression energies at each stage: $\begin{matrix} total energy = \sum_{1}^{N} w_{N} = \sum_{1}^{N} E_{polarization}^{N} (i) (Δ t) & (10) \end{matrix}$
Methodology [0059]
The design of an electrochemical hydrogen compressor is similar to that of a fuel cell, and it is proposed that a multi-stage unit be modeled after a PEM fuel cell stack. Nafion® is employed as the proton-conductive polymer membrane separator with Pt as the electrocatalyst dispersed on carbon to function as the anode and cathode electrodes in the overall membrane-electrode-assembly (MEA). [0060]
As shown in FIG. 3, an overall cylindrical multi-cell stack configuration, having, for example, hemispherical end-[0061] plates 26 provides good mechanical stability. Hydrogen supply inlet 33 is provided in the end-plate on the anode side of the first cell and compressed hydrogen outlet 35 in the other end-plate on the cathode side of the last cell. The plates are connected by tie-bolts 28. The design of a multi-stage unit is as illustrated where electrically non-conductive separators 20 ensure electrical separation of compression stages. It will be appreciated by those skilled in the art that other configurations will also work effectively. As with fuel cells, graphite support plates 22 could be used sandwiching the MEA's 24, but these require separate charge collectors for good electrical conductivity (cf. copper endplates in a fuel cell stack).
As best seen in FIG. 4, porous stainless [0062] steel support plates 22 are used, which are positioned adjacent to the MEA 24 with seals (e.g. in the form of an elastomeric o-ring) disposed in grooves 30 to ensure a leak-free seal between the plate and the membrane of the MEA (i.e. the peripheral area outside of the active area). Unlike a fuel cell stack, complex serpentine flow fields are not necessary, and access of H₂to the MEA's is simply achieved by perforating the plates 22 e.g. in a central area 23 of the plate, or by use of sintered frit plates. The sintered metal frit plates are made of a powdered metal material such as stainless steel, which is compressed into the form of a plate. Such material provides a structurally strong, yet porous material to provide for passage of gases to and from the active area of the MEA.
It is believed that the high differential pressures are achieved by means of the porous supporting [0063] plate 22 on the anode side and its seating in socket 25 a. On the cathode side, the porous plate maintains contact during pressurization with the active area of the MEA via use of a spring means, including a spring 29 and spring support 31, (i.e. see FIG. 5) for ensuring adequate electrical contact. High-pressure stability is provided because the plates 22 immobilize the MEA during pressurization, such that the membrane does not rupture due to a ballooning effect.
Commercially available materials (MEA, stainless steel plating, and seals) are used in the construction of single- and two-stage compressors (see later). Examples of other proton-conductive membranes include sulfonated-polystyrene and the partially fluorinated ionomeric membranes, IonClad R-1010 and R-4010 (Pall Co.), as these represent more economical alternatives to Nafion. Also, as H[0064] ₂is the only species of interest, complications of slow membrane deterioration, as reported in fuel cells and attributed to the formation of hydrogen peroxide (from reaction of H₂with O₂) within the membrane, is not expected to be a problem, and the use of non-fluorinated materials such as sulfonated-polystyrene will suffice in electrochemical compression applications.
The design of supporting [0065] plates 22 incorporates porosity or perforation characteristics in order to allow sufficient exposure of H₂to catalytic active sites on the surface of the MEA and, at the same time, permit the plates to give sufficient structural support to the membrane, thus minimizing its deformation under conditions of high-pressure differentials.
In the embodiment shown in FIGS. 3 and 4, the design of the supporting [0066] plates 22 also incorporates complementary peripheral grooves 30 for disposition of seals, e.g. an elastomeric o-ring to insure a leak-free seal between the MEA and the plates.
FIG. 5 shows the unassembled view of a single stage of the working system responsible for establishing proof-of-concept, multi-stage electrochemical compression. This electrochemical compressor unit comprises a membrane-electrode-assembly (MEA) [0067] 24 supported by stainless steel sintered frit plates 22 a and contained within cylindrical stainless steel housing 26 that make up the anodic and cathodic compartments. The stainless steel housing 26 is a high-pressure filter holder (Fisher Scientific, cat. no. 09-753-13M) adapted for its present use. The membrane-electrode-assembly 24 (Palcan Fuel Cell Co. Ltd., Vancouver, Canada) is circular in design (FIG. 4) with an active area of 11.34 cm²and comprises of gas-diffusion electrodes (anode and cathode), comprising Pt (1 mg/cm²) as the electrocatalyst supported on carbon (40 wt. % Pt/C), and Nafion® 115 as the electrolyte. Use of this unit is documented in examples described below for single- and dual-stage hydrogen compression.
As shown in FIG. 5, complementary pairs of [0068] grooves 25 and pocket 25 a are machined into the inside face of both end-plates 26. Upon assembly, the seal between the end-plates 26 and the MEA 24, is provided by an o-ring 27 of an elastomeric material, disposed in the grooves 25, and the frit plate 22 a is seated in pocket 25 a.
A [0069] spring 29 and spring support 31 are provided on the cathode side. Both the spring and spring support are conveniently made of stainless steel. This spring and spring support arrangement provides for equalization of the force exerted on the MEA by the frit plate on the cathode side of the MEA 24, regardless of the pressure differential across the MEA, such that the MEA can move together, i.e. without separating as a result of the high pressure.
As H[0070] ₂is a small molecule able to permeate through many types of materials, the selection and design of appropriate sealing material is important. Examples include Viton®, Santoprene®, and PTFE.
The multi-stage compressor embodiment includes a plurality of PEM cells connected in series, such that the compressed hydrogen from the outlet of a first cell in the series is fed to the hydrogen inlet of the next cell in series, wherein each cell is electrically isolated from the adjacent cell in the series. Note that while it is apparent that in the Strobel et al. publication, the cells are clearly shown to be connected in parallel, U.S. Pat. No. 6,361,896 states that the cells are connected “in series”. However, it is apparent that by “in series” the authors mean that the cells are arranged or placed adjacent to each other, i.e. as illustrated in the publication, for increased hydrogen flux. However, the hydrogen outputs and inputs are not connected in series and, therefore, progressive increases in pressure from cell to cell are not possible. [0071]
The circuit diagram for a two-stage unit connected in series showing the balance-of-plant is illustrated in FIG. 6, wherein PG refers to pressure gauges; PCV refers to pressure check valves; CV refers to check valves; FM refers to flow meters; PT refers to pressure transducers; HUM refers to the gas humidifier; HTR refers to the heater; RH refers to the relative humidity ports; and T/C refers to the thermocouple ports. Separate power supplies are used for each electrochemical compressor unit. The system is purged with nitrogen prior to hydrogen compression. Hydrogen is humidified by HUM101 and initially introduced to the entire system at atmospheric pressure. Thereafter, power is applied to the electrochemical compressor unit(s), and the pressure is monitored via PT101, PT102, and PT103. The system temperature is monitored via thermocouples at all T/C ports. In the circuit diagram, the stages are electrically isolated by use of electrically insulating (e.g. Teflon®) tubing, or by Swagelok dielectric fittings. This provides electrical isolation of [0072] stage 1 from stage 2.
In single-stage compression, one electrochemical compressor unit is employed and, as examples of its performance, FIGS. 7 and 8 show temporal plots for compression from atmospheric pressure (15.9 psia) to approx. 45 and 75 psia hydrogen, respectively. FIGS. 9 and 10 show corresponding temporal plots of the applied voltages along with the thermodynamic applied potential (ΔE) as determined from equation 4. For compression to 45 psia, 0.6 A was applied galvanostatically, and a linearly increase in pressure in the cathodic compartment (volume=18.2 mL) was effected. At t=610 sec, the pressure reached 45.7 psia (FIG. 7), and the voltage applied during compression increased from 58.2 to 70.3 mV (FIG. 9). The current source was then discontinued (i=0 A), and the equilibrium potential across the cell was measured with ★E[0073] _cell|=14.1 mV. For compression to 75 psia, a linear pressure increase in the cathodic compartment also took place and, at t=1270 sec, P_H ₂ _,c=76.4 psia. The applied voltage during compression increased from 57.1 to 78.4 mV (FIG. 11). The current source was then discontinued, and the equilibrium cell potential measured was |E_cell|=22.0 mV. In both studies, the system temperature recorded was 22.0° C. There is excellent agreement in the profile of the plots of the applied voltages and ΔE (E_polarization=58 mV), and between ΔE and the measured |E_cell|. For compression to 45 psia, the energy consumption, determined using equation 8, was 25.1 J (with w_t=3.9 J). The efficiency, as determined from equation 9, was 16%. For compression to 75 psia, w=56.1 J (with w_t=11.9 J), and the efficiency was 21%. It is noted that efficiency improves with increasing temperature and with decreasing applied current (due to lower i²R losses), and upwards of 80% has been reported for single-stage electrochemical compressors [4,5]. For the present system, compression to 45 psia at 65 and 80° C. with i=0.6 A yields the applied voltage profiles as illustrated in FIGS. 11 and 12 (E_polarization≈43 mV at 65° C.; E_polarization=33 mV at 80° C.), and the respective efficiencies were 22 and 28%. Likewise, with i=0.1 A and T=65° C., the profile of the applied voltage for compression to 45 psia is shown in FIG. 13 (E_polarization=7.8 mV), and the efficiency here was 60%.
In dual-stage compression, two electrochemical compressor units connected in series are employed, and FIG. 14 shows an example of the pressure change at each stage (unit) from application of electrical power. Here, 45 and 75 psia were chosen as final pressures for the first and second stages, respectively, both stages initially at atmospheric pressure (15.9 psia). A current of 2.4 A was applied galvanostatically to stage 1 and 0.6 A to [0074] stage 2. A linear increase in pressure was effected at both stages; 46.7 psia was reached at t=315 sec for stage 1 (volume=27.2 mL), and 75.7 psia was attained after 1230 sec for stage 2 (volume=18.2 mL). At stage 1, the current was reduced to 0.6 A to maintain the pressure until the final pressure at stage 2 was reached. Afterwards, the current source was discontinued at both stages (i=0 A), and the equilibrium cell potential was measured at both stages, with |E_cell|=15.0 mV for stage 1 and |E_cell|=6.9 mV for stage 2.
FIG. 15 shows the temporal plots of the applied voltages with comparison to those for ΔE (as determined from equation 4), and there is good agreement of the profile of the plots (E[0075] _polarization=40 mV (0.6 A) and 158 mV (2.4 A) for stage 1, and E_polarization=57 mV (0.6 A) for stage 2). The system temperature was 22.0° C. The energy consumption for priming the dual-stage compressor to the chosen stage pressures was 400.7 J, as determined using equation 8.
Single-stage compression to higher pressures was also performed. For example, compression to 535 psia (FIG. 16) was carried out at T=22° C. and using i=0.6 A. The profile of the applied voltage is shown in FIG. 17, and there is good agreement with that of ΔE (E[0076] _polarization≈52 mV). The energy consumption, as determined from eq. 8, was 362.4 J (with W_t=231.4 J), and the efficiency, as determined from eq. 9, was 39%.
Dual-stage compression to higher to pressures has also been carried out. For example, at T=22° C., compression to 2000 psia, with [0077] stage 1 at 1000 psia, is illustrated in FIG. 18. The profile of the applied voltages is shown in FIG. 19, and there is good agreement with ΔE derived from eq. 4. The applied current was 4.0 A for stage 1 and 2.0 A for stage 2. The equilibrium cell potentials were |E_cell|=53.0 mV for stage 1 and |E_cell|=8.7 mV for stage 2.
For the fuel cell industry, the electrochemical hydrogen compressor can be applied interfacing: 1) a hydrogen production device (i.e. fuel processor, electrolyzer, etc.) and a fuel cell; 2) a hydrogen production device and a hydrogen storage device; and 3) a hydrogen storage device and a fuel cell. For industries concerned with hydrogen supply and storage, the compressor can be applied interfacing a hydrogen production device and a hydrogen storage device. [0078]

REFERENCES

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Claims

1. An apparatus for compression of hydrogen, comprising a membrane electrolyte cell assembly (MEA), including a proton-conducting electrolyte membrane, an anode on one side of the membrane and a cathode on the other side of the membrane, the anode having an electrochemically active material for oxidizing hydrogen to protons, the cathode having an electrochemically active material for reducing protons to hydrogen, and further comprising next to the anode and cathode, planar gas distribution and support plates sandwiching the MEA, the assembly being held together by end-plates, the end-plates having complementary peripheral grooves for seating an intervening seal between the end-plates and the MEA, the end-plate on the anode side further including a hydrogen supply inlet and the end-plate on the cathode side further including a compressed hydrogen outlet.

2. The apparatus according to claim 1, wherein the gas distribution and support plates include a central gas distribution area.

3. The apparatus according to claim 2, wherein the gas distribution area is in the form of pores.

4. The apparatus according to claim 1, the gas distribution and support plates are made of a porous sintered metal frit material.

5. The apparatus according to claim 4, wherein the metal frit is stainless steel frit.

6. The apparatus according to claim 2, wherein the end-plates each include an additional complementary pocket for seating the metal frit plates.

7. The apparatus according to claim 6, additionally comprising on the cathode side between the gas distribution and support plate and the end plate, a spring means for ensuring adequate electrical contact.

8. The apparatus according to claim 7, wherein both the spring and spring support are made of stainless steel.

9. The apparatus according to claim 7, wherein the proton conducting membrane is of a material selected from the group consisting of Nafion®, sulfonated-polystyrene and the partially fluorinated ionomeric membranes, lonClad® R-1010 and R-4010.

10. The apparatus according to claim 1, additionally comprising means for applying an electric potential to the cell, wherein the applied potential to effect a final pressure of hydrogen exiting the cathode side of the cell is determined by the Nernst equation.

11. The apparatus according to claim 1, comprising a plurality of MEA cells connected in series, such that the compressed hydrogen from the hydrogen outlet of a first cell in the series is fed to the hydrogen inlet of the next cell in series, wherein each cell is electrically isolated from the next cell in the series.

12. A process for the compression of hydrogen by means of the apparatus according to claim 1, wherein hydrogen is compressed electrochemically by the MEA by oxidation of the hydrogen to protons at the anode, which having passed through the membrane to the cathode side are reduced back to hydrogen and discharged under pressure.

13. The process according to claim 12, wherein hydrogen is pressurized to 12,000 psi or greater.

14. The process according to claim 12, wherein a plurality of MEA cells are connected in series, each cell being electrically isolated from the next cell in the series, such that hydrogen discharged under pressure from the hydrogen outlet of a first cell in the series is fed to the hydrogen inlet of the next cell in series, and hydrogen is discharged from the hydrogen outlet of the next cell at a higher pressure.

15. An apparatus for compression of hydrogen, comprising a membrane electrolyte cell assembly (MEA), including a proton-conducting electrolyte membrane, an anode on one side of the membrane and a cathode on the other side of the membrane, the anode having an electrochemically active material for oxidizing hydrogen to protons, the cathode having an electrochemically active material for reducing protons to hydrogen, and further a hydrogen supply inlet and a compressed hydrogen outlet.