lnt. J. Hydrogen Energy, Vol. 6, pp. 45-51. Pergamon Press Ltd. 1981. Printed in Great Britain ~) International Association for Hydrogen Energy

0360-3199/81/0101~045 $02.00/0

HYDROGEN RECOVERY AND PURIFICATION USING THE SOLID POLYMER ELECTROLYTE ELECTROLYSIS CELL

J. M. SEDLAK, J. F. AUSTIN and A. B. LACONT!

General Electric Company, Direct Energy Conversion Programs, Wilmington, MA 01887, U.S.A.

(Received for publication 13 May 1980)

Abstraet--A process is described of obtaining high-purity hydrogen characterized by bringing a hydrogen containing gas in contact with a hydrated solid polymer electrolyte cell comprised of a catalytic anode, perfluorocarbon sulfonate ion exchange membrane and a catalytic cathode. Direct current energy is supplied between the electrodes to overcome the internal resistance of the cell, dissociate the hydrogen in the gas-to- be-treated to protons, and drive the protons through the cation membrane for recovery at the opposite electrode. Experimental data on the device indicate very high efficiency and low voltages over the current

2 density range 0-1100 mA cm- . Applications of the device are demonstrated for electrochemically pumping hydrogen from a low to a high pressure and separation of hydrogen from an inert gas to provide high-purity hydrogen.

INTRODUCTION

IN SEVERAL experimentally demonstrated cycles for thermochemical production of hydrogen from water the hydrogen is generated in conjunction with other gaseous substances. Cox has pointed out the need for an efficient means of separating pure hydrogen from other constituents [1]. One functional method involves electrolytic transport of hydrogen in the form of protons across a solid polymer electrolyte electrolosis cell [2, 3]. Apart from brief descriptions of electrochemical hydrogen recovery devices [1-5], there is little experimental information on the efficiency of hydrogen recovery as a function of operating current density.

EXPERIMENTAL

Hydrogen purification in a solid polymer electrolyte cell is shown schematically in Fig, 1. A gas mixture containing hydrogen at partial pressure Pl and inert gases such as nitrogen is fed to the anode compartment of the cell. Hydrogen contacts a thin layer of platinum deposited directly onto the membrane (< 4mgPt cm-2). Protons formed on passage of direct current migrate through the membrane according to a proton conduction mechanism. The ion exchange capacity of the Nation ®* membrane typically is 0.82--0.86 m-equiv H + g-I dry polymer. On reaching the platinum cathode, protons are reduced back to hydrogen at partial pressure P0.

A series of eight separate experiments were carried out at 25°C in a solid polymer electrolyte cell with an electrode area of 46.6cm 2. Anode feed streams of either pure H2 or 30% H2/70% N2 were saturated with water vapor and flowed to the cell at slightly above atmospheric pressure. The latter step was necessary to preclude membrane dryout. Flow rates to the cell were maintained high enough in these experiments to restrict H2 depletion to < 5%.

T H E O R E T I C A L

The current density, J, at any point in the electrochemical hydrogen purifier/compressor is given as follows

j = _ _ V b ln (po /p ) , (1) P P

* Registered trademark of E. I. Dupont de Nemours

45

46 J. M. SEDLAK, J. F. AUSTIN AND A. B. LACONTI

H2-2H + +2e

DILUTED H 2 FEED

t P2

SOLID

- I ~ 2 H + (H20)

J \ , , / " Pl

.H2• 'PURE'' PRODUCT

H 2

Po

FIG. 1. Schematic of hydrogen purification in the solid polymer electrolyte cell.

where V is the uniform cell potential drop across the cell, p is cell resistivity in ohm cm 2 and b is R T / 2 F . Since the hydrogen partial pressure decreases across the anode compartment the average current density, J, is obtained by integrating equation (1) from Pl to P2

J= (/)2

After integration and the substitution 0: =

: = v + b[ l n /9 PL

The voltage efficiency ev of the hydrogen energy contained in the recovered hydrogen

1 (p2j dp. (2)

p2/p~

+ 0:ln 0:_ 1] (t,,/t,o) (3)

0:--1 J

transfer can be computed relative to the thermal

V E~ = 1 -- 1. 48-----4" (4)

In equation (4) 1.484 V is the voltage equivalent of the heat of combustion of hydrogen. Note also that equation (3) contains terms pertaining to the reversible work of separating and then compressing hydrogen to pressure P0. This work cannot be avoided however efficiently the electrolyzer is operated. A current inefficiency also arises because of back diffusion of hydrogen from the high-pressure cathode to the anode. This reverse transport of hydrogen is ordinarily expressed in terms of an equivalent current density, Jo. JD increases with temperature according to Arrhenius law and is inversely proportional to membrane thickness. Jo depends linearly on the hydrogen partial pressure difference across the membrane, but is independent of J. The current efficiency is computed as follows

ei = 1 - J o/J . (5)

The total cell efficiency is e =ev" ei. Another important factor is the purity of the recovered hydrogen. Since the solid polymer/

electrode system is hydrated, the product hydrogen will contain water vapor. At 25°C and 2 000 kPa hydrogen collection pressure, for example, the water content will be 0.16%. If necessary, moisture can easily be removed by conventional gas driers. The other source of gaseous impurity is the inert component of the anode feed. The diffusion rate m (mol/sec -~ cm -2) of the inert gas to the cathode is proportional to its partial pressure at the anode and is inversely proportional to

HYDROGEN RECOVERY AND PURIFICATION 47

membrane thickness. Also, m follows an Arrhenius temperature dependence but is independent of electrolysis current density. The % "inert" is given by equation (6)

100m % "inert" (J - J D ) / z F + m (6)

Fror~ the viewpoint of hydrogen product ~l~urity, low-temperature and high-current-density operation is preferred.

DISCUSSION

The data for representative experiments presented in Fig. 2 indicate linear voltage vs current density relationships for anodic and cathodic reactions of H2 in the solid polymer electrolyte. This finding, first of all, has importance in the interpretation of polarization curves for water electrolysis to H2 and 02 in this type of cell. In theory, the bottom line in Fig. 2 should extrapolate back to 0.000 V at zero current since anode and cathode H2 partial pressures were equal. We observed, however, a small intercept amounting to 0.016 V (0.008 V/electrode). This intercept is equivalent to a pH difference across an acid cell of 0.27 units and probably arises as a consequence of a slight excess of protons near the anode. In the interpretation of water electrolysis data we have always assumed that essentially all of the cell polarization can be assigned to the oxygen anode. The present results confirm the validity of our assumptions. That is, the H2 cathode polarizes only to the extent of a few millivolts.

0.3 i i i i ' i

0.2

0.1

0 ,

0 21111 4OO 6OO 8OO 1111111 121111

CURRENT DENSITY, mA/crn 2

FIG. 2. Polarization characteristics of solid polymer electrolyte cell at 25°C: ~p~ = 29.6 kPa, Po = 792 kPa, 0.0254 cm membrane; ©p~ = 100 kPa, P0 = 792 kPa, 0.0254 cm membrane; []Pl = 29.6 kPa, Po = 100 kPa, 0.0127 cm membrane; Ap~ = 100 kPa, P0 = I00 kPa, 0.0127 cm membrane.

48 J. M. SEDLAK, J. F. AUSTIN AND A. B. LACONTI

The first sets of hydrogen recovery measurements were taken on a 0.025 cm (10 mil) membrane at H2 partial pressures of 30 and 100 kPa in the anode feed. The H2 product was in both cases collected at 792 kPa. Cell resistances were 9.7 and 11.7 mr2, respectively. Total pressure drop across the cell (cathode vs anode) was 689 kPa. With a proper membrane support structure, pressure differentials up to 2000 kPa can be maintained. This permits abstraction of hydrogen from dilute low-pressure feed streams with concurrent generation of a moderately high-pressure pure H2 stream. In effect, the solid polymer electrolyte cell functions as a single-stage purifier/compressor. The uppermost line in Fig. 2 corresponds to a hydrogen compression ratio of Po/Pl = 26.4. At 300 mA cm -2 the voltage efficiency was 87%. Approximately 86% of the energy expended in the transfer of H2 was associated with resistive (iR) losses. The remainder is attributed to reversible work of separation/compression and electrode polarization. Reversible work (i.e. minimum) is unavoidable and must be charged against any H2 separation process. These results were encouraging, but they pointed out the need to minimize iR losses in the electrochemical cell.

TABLE 1. Parameters selected for calculation of projected hydrogen recovery efficiency

P0 Anode feed H2 partial pressure Pl Anode effluent H2 partial pressure p2 H2 collection pressure Ap Cross cell pressure difference (at inlet) -- Total anode side pressure (at inlet) JD H2 diffusion loss

H2 stripping factor (P2/Pl) p Total cell resistivity T Temperature R Gas constant F Faraday constant

20.7 kPa 1.03 kPa 1378 kPa 1171 kPa 207 kPa 3.6 mA cm -2 0.05 0.214 ohm.cm 2 25°C

The electrolyte/electrode assembly was reconstructed using a 0.0125 cm (5 mil) membrane. Experiments in this case were designed to determine current limitations (if any) and the resistive component of total cell voltage. In separate tests the thinner membrane was capable of sustaining cross-cell pressure differentials of at least 689 kPa. The slopes of the two lower lines in Fig. 2 correspond to total cell resistances of only 4.6 mg2. No current limitations were observed even at 1100 mA cm -2. As with the thicker membrane, the iR contribution is the most significant source of power consumption. However, the magnitude of the iR loss has been reduced by > 50%.

Knowledge of system resistivity (0.0046 ohm x 46.6 cm 2 = 0.214 ohm.cm2), the voltage--current density linearity, and the magnitude of the membrane polarization at zero current (0.016 V) permitted the calculation of projected efficiency of hydrogen recovery and hydrogen purity as functions of current density. The conditions chosen for these calculations are listed in Table 1 together with the measured cell parameters. We have assumed as a plausible practical situation an input stream to the anode composed of 10% H2 in N2 at a total pressure 207 kPa. Ninety-five per cent of the hydrogen is to be abstracted and recovered on the cathode side at 1378 kPa. Voltage was first computed as a function of current density using equation (3). Then 0.016 V was added to all voltages to account for zero current polarization. Voltage efficiencies were derived from equation (4). In order to compute current efficiencies, numerical values of H2 diffusion loss, Jo, were required. Jn values shown in Fig. 3 as a function of temperature (at the pressure conditions of Table 1) had previously been determined for this membrane in a Linde diffusion cell. At 25°C, Jo = 3.6 mA cm -2. The current efficiency thus followed directly from equation (5). Total efficiency e = eie~ is plotted vs current density in Fig. 4. The lower curve represents efficiency when reversible work of H2 separation and compression is charged against the process. The upper curve was derived by exclusion of reversible work from the efficiency calculations.

HYDROGEN RECOVERY AND PURIFICATION 49

I I I I I

1.2 9.3

~ 0.8 9.1 ~

~ o . 6 9.o r,

0.4 s.9 ~

I 0.2 8.8

0 .0 I I I i i 8 .7 2.8 2.8 3.0 3.2 3.4 3.6

103/T. OK

FiG. 3. Arrhenius plots for H 2 a n d N 2 diffusion across the 0.0125 cm solid polymer electrolyte membrane under H E partial pressure gradient 1370 kPa or N 2 partial pressure

gradient 186 kPa.

95

90

%

85

/ a 0 ~ i i I i i

0 200 400 600 800 1000

J, mA/cm 2

FiG. 4, Calculated overall energy efficiency vs current density of hydrogen purification for conditions of Table 1: [] including reversible work; 0 excluding reversible work.

50 J. M. SEDLAK, J. F. AUSTIN AND A. B. LACONTI

Efficiency vs current density curves exhibit a maximum because ei increases and ev decreases with increasing current density. Even at the high current density of 1000 mA cm -2 the overall efficiency is still 80%. Selection of operating current density (and thus electrolyzer size) for a particular feed stream involves an economic trade-off: capital cost vs increased energy cost per cubic metre of H2 recovered.

A final consideration if the expected purity of the H2 product as estimated from equation (6). Included in Fig. 3 are diffusion data as a function of temperature for migration of N2 across the solid polymer electrolyte. Since the N2 content at the cathode is small the partial pressure at the

r I I I I

0.2

:z~0.1

0 I I I I I 0 200 400 600 800 1000

CURRENT DENSITY, m~ulcm 2

FIG. 5. Nitrogen content of purified hydrogen vs current density for conditions of Table 1.

anode (186 kPa) is the diffusional driving force. At this N2 pressure and at 25°C, m = 5.45 x 10 -1° tool sec -1 cm -z. As indicated in Fig. 5, the N2 content of the product gas drops sharply from 0.2% at 50 mA cm -2 to 0.01% at 1000 mA cm -2. In view of the 90% N2 content of the original feed stream this signifies a very high degree of removal of N2 from the product H2 gas in a single- stage process. Passage of the product H2 through a second electrolyzer stage could, if desired, reduce the N2 content to negligible levels.

It is envisioned that the solid polymer electrolyte system described in this work could be applied economically to a wide variety of H2 recovery applications. The information presented in this paper provides sufficient theoretical development and experimental detail to enable projection of energy efficiency of hydrogen abstraction from dilute gaseous streams.

Detailed consideration of design features of a solid polymer electrolyte hydrogen purification plant is beyond the scope of this paper. It is appropriate, however, to comment briefly on plant investment costs and on H2 stream gaseous contaminants.

Investment costs for a large solid polymer electrolyte water electrolysis plant have been published recently [6]. At 400 ma cm -2, for example, the total investment (in 1976 dollars) was estimated to be $350 per kilowatt-equivalent of 1-12 produced. The investment cost of the electrochemical H2 purifier should be less than that of the water electrolyzer. A key factor is the smaller d.c. power supply cost for the purifier. Operating costs will also be lower for the purifier because of smaller energy consumption.

The output of a Luigi gasifier is a representive "contaminated" H2 stream. CH4, N2 and CO2 have little effect on operation of the electrochemical H2 purifier; they act simply as diluents. H2S presents a more difficult problem. Removal of H2S from the gas stream is, of course, a solution.

HYDROGEN RECOVERY AND PURIFICATION 51

Alternatively, sulfur-tolerant anode catalysts might be feasible. In particular, RuO2, WO2.5, MoS2, WS2 and PtSx have been investigated in this laboratory in connection with fuel cell oxidation of HE contaminated with H2 S.

REFERENCES

1. K. E. Cox, in Proc. 2nd World Hydr. Eng. Conf., Zurich, Vol. 2, p. 471 (21-24 August 1978). 2. H. J. R. MAGET, U. S. Patent 3,489,670 (1970). 3. L. J. NtrrrALL and J. H. RUSSELL, in Proc. 2nd World Hydr. Eng. Conf., Zurich, Vol. 1, p. 391 (21-24

August 1978). 4. S. H. LANOER and R. G. HALDEMAN, U. S. Patent 3,475,302 (1969). 5. H. J. R. MAOET, U. S. Patent 4,118,299 (1978). 6. J. M. SEDLAK, J. H. RUSSELL, A. B. LACONTI, D. K. GUPTA, J. F. AUSTIN and J. S. NUGENT, Hydrogen

Production Using Solid-Polymer-Electrolyte Technology for Water Electrolysis and Hybrid Sulfur Cycle, EM-1185, Research Project 1086-3, Final Report, September 1979 (Electrical Power Research Institute sponsored study).