Indian Journal of Chemistry Vol. 4 1 A, February 2002, pp. 245-253

Advances in Contemporary Research

Developments of water electrolysis technology by solid polymer electrolyte

Sang-Do Han, Kee-Bae Park, Ravi Rana*, K C Singh**

Korea Institute of Energy Research, P.O. Box 103, Yusung Taejon, Korea *Dept. of Chemistry, Maharshi Dayanand University, Rohtak, Haryana, India

Received 25 Septelllber 2000; revised 19 October 2001

Water electrolysis by solid polymer electrolyte is the most promising technology for large-scale hydrogen production for future. The concept introduced by General Electri c Co. (U.S.A) in late sixties has been adopted by Brown Boveri Research Center, Baden, Switzerland and more recently by Japan under WE-NET Japanese Hydrogen Programme Project. For the last 25 years, thi s field has witnessed a lot of acti vity, in research and development of energy efficient water electrolyzers . In thi s rev iew, the selec ti on of hi gh ac ti vi ty electrocatalysts, methods to prepare SPE electrocatalyst composites, their stability. the pretreatment of solid polymer electrolyte membrane for the deposition of electrocatalysts, the fab rication of current collectors, the scale up of water electrolyzers and recent advances has been di scussed.

Dr. K.C. Singh did hi s B.Sc. from Govt. College, Hoshiarpur (Punjab University . Chandigarh ). He did hi s M.Sc. in 1973 from Jammu Uni versity, Jammu (J&K) and got Ph .D. on "Solution Thermodynamics" in 1979 fro m M.D. Un iversity, Rohtak. He joined as a lecturer in 1980 and presently he is a Reader in Department of Chemistry in M.D. Un iversity,

Rohtak . Hi s areas of research are (i) Solution Thermodynamics and (ii ) Thin ox ide fi lms on metals.

The present work was carri ed out at the Korea Institu te of Energy Research. Korea, where he was a visiting scientist for six months.

Mr. Rav i Kumar Rana did hi s MSc. degree fro m M.D. Uni versity Rohtak .. At present he is worki ng for Ph .D on the topic "Characterisation of solid polymer electrolyte and its use in water electrolysis" in the Chemistry Department. M.D. University, Rohtak, under the supervi sion of Dr. K.C. Singh and Dr . Han- Sang Do. He has worked

as a Chemi st in R&D in Bhamt Rasayan Ltd for one year.

Dr. Sang-Do Han did BA course from Kyungpook Nati onal Un ivers ity in 1975 and MA course from Chungnam Nati onal Universi ty, Korea, in 1984, and received his Ph.D. degree in So li d State Chemistry from Uni versity of Bordeaux, France in 1994. He worked at LG semi conductor Co. Ltd . from

1978- 1980. and is currently work ing at Korea Institute of Energy Research since 1980. Hi s areas of interest are (i) electronic and electro lyte materials, (ii ) chemical sensors and (iii ) hydrogen production. Curren tly he is the Edi tor of the Journal of Korean Sensors Society.

Introduction

Mr. Kee-Bae Park received hi s BA and MA degree from Daejon Industry University . Korea in 1992 and 1995, respecti vely. He is working at Korea Institute of Energy Research since 1985 His fi eld of interest is electronic cont rol and dev ices.

Water electrolysis IS the simplest method for producing pure hydrogen on a large or small scale. There are three majo r technologies for e lectro lytic hydrogen production depending upon the nature of electrolyte used in e lectro lys is ce ll i. e. alkaline, po lymer membrane and ceramic ox ide elec trolyte. Alkaline electrolyte based e lectrolyzers are the o ldest, least expensive and thus the most widespread of the various electrolysis technologies 1. The al kaline water electrolysers are constructed with a bipo lar configuration in which a series of electrodes are arranged, vertical and parallel to one another. A gas separator is incorporated between two adjacent e lectrodes to form a hydrogen chamber on one side and an oxygen chamber on the other. The electro lyte is ci rculated through each electrode-separator space. By applying a vo ltage across the two end electrodes, each intermediate electrodes becomes bipol ar, evolving hydrogen on one s ide and oxygen on the other. Generally nicke l is used as the electrocata lys t fo r hydrogen and oxygen electrodes . The voltage effic iency of alkaline e lectro lyzer improves at high temperature but the corrosion of electrodes and other related material s also increases. Researchers have

246 INDIAN J CHEM. SEC A, FEBRUARY 2002

attempted to find more rugged substitutes for the b 2-4 d . kid 2.3 . as es tos separator an mc e ano e to lI1crease

the long time stability and energy efficiency. The interest in 0 2- conducting ceramic electrolytes based on yttria stabilized zirconia (YSZ) is due to their high temperature (> 1000DC) operation which not only lowers the activation barriers for various electrode surface chemical reactions but beneficial for engineering designing as one has to work with an exclusively gas/solid system. But the problems of corrosion, thermal expansion and inter mixing of adjacent phases become important in these electrolyzers due to their high temperature operation.

The solid polymer electrolyte technology, now called proton-exchange membrane technology, consists of coating or pressing two electrodes on to a membrane llsed as electrolyte and it is the most promising candidate for low temperature fuel cells (the present review article deals only with this technology). The concept of SPE (solid polymer electrolyte) in water electrolysis was first introduced5.6 by General Electric Company in early 1970. Now, the hydrogen production by this method has been included in WE-NET. Japanese hydrogen program project7. Brown Boveri Company of Switzerland is also very active in this field and has marketed its membrane process8.9. The development of thi s technology has been delayed mainly due to investment costs. SPE or Nafion membrane has excellent chemical and mechanical stability, together with high ionic conductivity and good gas impermeability, but it is an expensive material. Secondly, due to its strong acidity the choice of electrocatalysts is limited only to costly platinum group metals or oxide electrocatalysts. In order to keep capital cost within economic bounds, to decrease the elecLrocatalyst loading and to increase the current densities a lot of work has been done. Typical noble metal loading of a few mg/cm2 and typical current densities of several Alcm2 are currently obtained. During operation of electrolysis cell, the electrocatalysts not only face highly acidic environment but also high mechanical tension due to oas evolution. Therefore, much attention must be b

devoted to the structure of the electrodes, to reduce the noble metal loading and also to strengthen the bonding of the electrodes on to the SPE 10-12.

2. Nafion membrane (solid polymer electrolyte) Perllurosulphonate polymers (Nafion) consist of a

polytetrafluoroethylene back bone with side chains terminating in sulphonate ion exchange groups.

Membranes of various thicknesses and equivalent weights are now commercially available (Dupont de Nemours Nafion products). The equivalent weight of Nafion membrane is defined as the weight of acid polymer that will neutralize one equivalent of base; eq. wt. is equal to the reciprocal of ion exchange capacity.

The water adsorption capacity of Nafion depends upon the pretreatment of the membrane i3

. If it is heated to more than IOODC in air, then put in water, the water uptake is very less (called shrunken form). if it is just immersed in water at room temperature, it absorbs up to 17 weight % water (called Normal form Nafion). Water up take increases to 30% when boiled in water for 30 min (called Expanded [orm I 3

-21). There

is further increase of water absorpt ion if it is heated in water under pressure at higher temperatures. Number of studies 13. 22-28 have been made Lo establish the micro-structure and to explain pecul iar properti es of Nafion. Based on the results obtained by small angle X-ray scattering, mass transfer experiments, including hydraulic permeation and water diffu ion and electron microscopy, Gierke el al. 13 have proposed cluster­network to explain the properties of Nafion membrane. According to him, polymeric ions and absorbed water exist in approximately spherical domains as ionic clusters, separated from the polytetrafluoroethylene (PTFE) matri x. These clusters are connected by short narrow channels which have a diameter of about 10 A. The cluster size grows with increasing amount of absorbed water. However, there is a probability of some intrusion of the fluorocarbon phase into the aqueous phase to form an interfac ial

. hi' f tl t 21.29-31 Th regIOn t at a so contall1s some 0 .le wa er . e permeability coefficients of gases through Nafion depend greatly on the water content, the cation form

. h . 32-35 Th and the Ion exc ange capacIty' e gas permeation rate through the same sample varies with temperature, pressure and membrane thickness. Permeability of H2 gas is about twice as large as that of oxygen. When membrane absorbs water, the narrow channels are filled with water. The gas diffuses through water and it approaches the value of diffusion of H2 and O2 in water and becomes constant.

The Nafion membrane used for water electrolysis has to be pretreated with acids CfhS04 or HN03 or Hel or H20 2) to convert it into H+ form and to remove impurities it has to be boiled with water at high temperature. If the treatment temperature is very hi gh, the water content in the membrane will also be high,

HAN et al.: WATER ELECTROLYSIS BY SOLID POLYMER ELECTROLYTE 247

thus the resistance of the membrane decreases but at the same time the permeation rate of gases through the membrane increases. Generally, for the hydrothermal treatment, temperature range is between 100 to 150°C. This is because at temperatures below 100°C the adhesive strength between the membrane and the deposited metal tends to decrease and at temp­erature above 150°C the mechanical strength of membrane tends to decrease considerabl/6

.

3. Surface roughening treatment An adequate surface roughening of Nafion

membrane before deposition of electrocatalyst is one of the key technologies for developing an SPE water electrolyzer, which can be used at higher current densities. It can be carried out by various methods such as oxygen pl asma, sand blasting and wet abrasive blasting. It gives strong adhesion and larger contact area between membrane and plated electrode for loweri ng cell voltage and rai sing current densities 36-40 G b' I' d ' h h - . as permea I Ity stu les t roug membranes and their composites4 1

-43

, and cun'ent efficiencies and gas . 44 . .

punty 111 an operatIonal SPE electrolyzer have revealed that the porous structure of the electrode formed by surface roughening facilitates the release of the evolving gas, leading to a decrease in the resistance due to gas bubbles and a decrease in the gas permeation rate through the membrane (i.e. an increase in current efficiency). A homogeneous roughness on membrane surface with an average depth of etching of 3-4 mi crometer produces SPE-Pt composites45, which have the lowest values of cell vo ltage, I R drop and highest current densities. Further, the non-treated membranes are found to suffer damage due to void formation during water electrolysis, which led to a significant decrease in its mechanical strength.

Takenaka et 01. ,36.46 have etched the samples with oxygen plasma treatment at Radio frequency power vary ing from 100 w to 500 w. The etching depends upon the treatment time and Radio frequency power. The surface roughening treatment is also described as

. 147 essentla to prepare good SPE composite by hot press method. There the surface roughening is done by sand blast or by silicon carbide sheet to produce an etching of an average depth of ±6 microns.

However, the observations of Millet et 0/.48 are

contrary to the above. They determined the active areas of Pt electrode of etched and unetched membranes, and found the difference to be low. They

concluded that membrane surface etching results in the reduction of membrane thickness and as a consequence in lower mechanical strength, hi gher gas diffusion and lower faradic yield. They have prepared SPE composi tes49

.5o without etching the SPE

membrane and electrolysis cells are found to be quite stable at large-current densities and at high temperature for a long time. According to Liu el 01.51

and Takenaka et at. 52, the adhesive strength of electrocatalyst with membrane and its porosity depends upon factors like concentration, temperature and time of deposition of noble metals. In the experiments, all composites were prepared without surface etching treatment.

4. Basic design of cell In SPE cells, Nafion membranes of varying

thicknesses are used as the electrolytes . Charge carriers in the membrane are hydrated hydrogen ions (H+.xH20) which move through the solid electrolyte by passing from one fixed suI phonic acid group to the adjacent one. On to the two faces of afion membrane generally platinum (for the cathode) and Ir or Ir02/alloys of Ir02 (for anode) are bonded (chemically or by hot press technique) using proprietary procedure. A stable conductive materi al used as a separator between the anode chamber of one cell and the cathode chamber of the adjacent cell al so serves as a current collector in a bipolar configurat ion. During electrolysis deionized pure water is circulated, at a sufficiently high flow rate to remove the waste heat (Fig.I) . Water decomposes electrochemically in the anode chamber producing O2 gas, hydrogen ions and electrons. The hydrogen ions move through the SPE and recombine electrochemically with electrons,

2 3 4 5 6 7

Fig I-Cross sectional schematic of cell.

248 INDIAN J CHEM , SEC A, FEBRUARY 2002

which pass via external circuit, to form hydrogen gas in the cathode chamber (Fig.2).

In order to achieve highly efficient SPE electrolysis operating at low voltage and at high current densities an ideal SPE electrode must have the following characteristics.

(i) Metal film deposition predominantly within the afi on in a thin (sub micron) layer adjacent to the

membrane surface; (ii) large electrode - SPE contact area to provide electrochemically large active surface area;(iii) good inter particle contact for low electronic res istance; and (iv) porous structure for free mass transfer of water and gases .

In order to achieve the above goals following techniques are employed to prepare SPE electrodes: (a) Chemical deposition ; (b) electro-deposition; (c) hot press method; and (d) deposition of lr02 or alloys or platinum on porous titanium sheet/mesh followed by impregnation with Nafion and hot press.

5. Selection of electrocatalyst For several years , studies on SPE water electrolysis

technology had been mainly concerned with the survey of electrocatalysts and the methods of preparation of SPE-electrocatalyst composite. Since electrocatalyst has to be placed in highly acidic environment, the choice of metals as electrocatalysts is almost totally restricted to noble metals and their

53-57 d h alloys. Many workers have reporte t e characteri stics of electrocatalysts of noble metals and alloys in acids solutions. Now it is confirmed that they behave in the same way as in acids when plated on SPE.

It has been found that platinum or other noble metal alloys have the same catalytic activity for the hydrogen evolution reaction but electrodes consisting of Ru , Ru-Ir, Ru02fTi021Ir02, or lr are the best noble metal electrocatalysts for oxygen evolution. The ox ides of Ir or Ru (lr02 and RU02) are unusual with respect to their high electronic conductivity. Pure ruthenium has the high initial catalytic actively, but, it corrodes during oxygen evolution. Therefore, it is not recommended as anode electro-catalyst. The anodic over voltage for different noble metals and alloys follow the sequence: Ir<Rh<Rh-Pt<Pt-Ru<Pt<Pd.

Low catalytic activity of Pt and Pd at the anodic side may be due to the formation of high resistance oxide films on their surfaces .

Now, Pt or lr-Pt as cathode and Ir-Pt. Jr, Ir02 as anode are generally used as electrocatalysts. The

+ - i + , -2H + 2e --..- H2 Ca thod e Sipolar e.lcct r ode AnOd~- H20 ----+ 2 H + 1/ 2° 1 r..L2e.

SPE '

Cu rre nt coUcclor s

Curre.nl collec tors

p ow er Su p p ly

Fig.2 - Schematic representation of two SPE sing le ce lls corrected in series.

electrocatalytic activity for hydrogen evolution of RU02, Ir02 and mixed RU02 and Ir02 have been studied by many researchers58

-63 and their use as

cathode electrocatalysts have been proposed due to their insensitivity towards catalytic poisoning by metal ions.

6. Chemical deposition Chemical deposition of electrocatalysts on SPE

faces can be carried out either by impregnating Nafion membrane with cations of noble metal s, then reducing, these cations by suitable reducing agent or by circulating the anionic solution of metal salts to one side of membrane and reducing agent to the other side of it. The catalytic activity and the adhesive strength of the composite depend on factors such as the kind of metal salt and reducing agent used, concentrations and temperature of their respective solutions, the time of reduction and the conditions needed for the pretreatment of membrane(e.g. hydro­thermal treatment and surface roughening treatment).

6.1. Chemical deposition of platinum Generally, dilute solution of Pt(NH3)4ChH20 is

used to saturate the SPE with Pt ions then reduction of the same is carried out with NaBH4 solution.

According to Her et al. 5 1.64-66 the best fabrication conditions found, are the impregnation of SPE for 40 minutes in 0.6 mM Pt(NH3)4Cb solution followed by reduction with 1 mM NaBH4 solution for 2 h. The temperature for impregnation and reduction is 50°C and Pt-Ioading obtained varied from 0.4 to 0.6 mg/cm2

• The platinum film is found to be dense but porous and is concentrated with in 0.5 urn (micrometer) from the surface. Sakai et 01.45 have

HAN et a/.: WATER ELECTROLYSIS BY SOLID POLYMER ELECTROLYTE 249

obtained thin continuous layer ofPt (0.8 mg/cm2) by

saturating SPE with Pt(NH3)4CIz solution, then reduction with NaBH4 at SO-60°C. The second cycle of deposition was carried out to prepare an efficient composite with I.S mg to 2.S mg/cm2 loading. The two cycle preparation of SPE/Pt composite is also achieved by Millet el aL. 49 by loading membrane with 0.0 I molar Pt(NH3)4CIz solution for IS min. and then reduction with 0.3% NaBH4 solution at 2SoC for 2 hours. Platinum loading of 1.13 mg/cm2 was obtained.

Fedkiw and Raleigh67 have described the formation of Pt and Pt alloys (Pt-Pd, Pt-Ru) to form thin continuous, porous films at and within the Nafion membrane surface using 0.026% of Pt(NH3)4':12 salt dissolved in I :2: :CH30H:H20 solvent. The membrane was dipped for 24 h at SO°c. Then reduction was carried out with 0.1 M NaBH4 for 2 h to obtain a 10-micron thick Pt film of loading 4 mg/cm2. Effect on the morphology and other properties of the film formation by other reducing agents like SnCIz, N2H4 and methanol have also been discussed. The reductant NaBH4 is found best to prepare a good SPEIPt composite suitable for water electrolysis. With the use of 3% H2PtCI6 solution on one-side of membrane and 1% NaBH4 or N2H4 solution on the other side of the membrane, the deposition of platinum was carried out by Takenaka et al.46

.

Electrolytic deposition of platinum has been achieved by Nagel et al. 68 first dipping SPE in O.S% solution of Pt(NH3MN02)2 complex at 90°C for 30 min and then pressing it between platinum anode and graphite cathode. The electrolysis was carried out for I h at O.SAlcm2 current density to obtain 0.7 mg/cm2

deposition of Pt on cathode side. Banziger et al.69 have described a method for continuously coating the SPE membrane electrolytically. Fir:;t O.S% Pt(NH3MN02)2 solution at 90°C was ion-exchanged with Nafion for 30 min . Then electrolysis was carried out to result in the formation of O.Smg Icm2 of Pt layer on the cathode side.

Chemical plating of platinum anode is achieved by number of workers37.38.52.70 by impregnating membrane in Pt(NH3)4CIz solution and reducing in NaBH4. Further in the second step, an additionally required amount of the same metal or a different metal is plated on the thin metal layers in a chemical plating bath containing a mixed solution of metal complexes, a reducing agent, and buffer agent to maintain pH. The electrodes with lr plated on platinum are used because of high cell performances in cell voltage and gas purity and good durability. A one-side catalyst loading obtained is 1-2mg/cm2.

~al hod e

discharge J:'or t

Sf fvic€: por i

Ca lh ode en pi al.

Packing sheet Pac king

Membrane electrode assem bly

Fig.3 - Basic structure with five 2500 cm2 cell stacked e lectrolyzer.

6.2. Deposition of iridium Millet et al.49 have deposited iridium

electrochemically inside the platinum surface coated on SPE membrane by impregnating the membrane with the cationic solution of [lr(OH)z(H20)4]+[lr(OH)(H20)sJ"+. An ilidium deposition of 0.2 mg/cm2 on the cathodic side was found to be sufficient for its catalytic activity as anode. Takenaka et al.36 have reduced the mi xed solution of lrCh and RuCI} on the SPE surface by NaBH4 solution that penetrated through the membrane. The plating of iridium on the earlier Pt deposited SPE surface has been obtained37-39 in a chemical plating bath having amine complex of iridium cations, reductant and a buffer agent. The amount of plated catalyst was controlled by the concentration of metal ions. Yasuyoshi et al.71 have described a plating bath solution consisting of H2PtCI6, IrC14, and NH2NH2 compounds. The pH of the solution was adjusted with NaOH and maintained between S-6 by a buffer solution of citric acid and sodium citrate. The NaOH solution is allowed to penetrate from one side of membrane and the reducing reaction of Pt and Ir with N2H4 in the presence of penetrating alkali is brought out on the other surface of membrane. An electro less iridium plating bath containing iridium halide and hydrazi ne or hydroxylamine salt having pH(3-10) has been described in ref.(72). The deposition of iridium is carried out electrochemically68, first impregnating the Ir cations from the mixed solution of IrChH20 , NH40H and N2~H20 at 80°C for 30min. The condition for electrolytic deposition were 140°C at 17 atmosphere pressure in a closed vessel at a current density of 0.03SAlcm2 for 30 min . A shining metallic iridium layer resulted on the cathode side.

Hiroaki et aL. 73 have described number of electroless plating baths for the deposition of iridium on the earlier deposited platinum surface on SPE membrane. The details of three baths are as follows:

250 INDIAN J CHEM, SEC A, FEBRUARY 2002

K[lr(N2H5)C15J = 1.0 gm, water = 750 ml, pH = 2.8, temp.= 70°C, pH adjuster = 0.1 N N2H4H20, deposition = 4.04 mg/cm2 in 4 h, and plating yield = 94.2%.

K21rCI6 = 1.1 g, N2H4 HCl = 0.3 Ig, water = 750 ml , p H = 2.8, temp. = 70°C, pH adjuster = 0.1 N. N2H5Cl , deposition = 4.35 mg/cm2 in 4 h and plating yield = 99.9%.

Na21rCI c, = 1.0 g, N2H4HCl = 0.28 g, water = 750 ml , pH = 2.8, pH adjuster = 0 .1 N2H4.H20, temp = 70°C, deposition = 3.83 mg/cm2 in 4 h, and plating yield = 89.1 %.

7. Hot press method In the pioneering work of the U.S . firm General

Electric Company for the development of large scale hydrogen generators. the preparation of electrodes is based on the coating by heat and/or pressure of a mi xture of a catalyst powder and a binder (such as teflon) on each side of the SPE74

-77

. In number of U.S . patents7S

•7lJ

, a thin layer of metallic particles was prepared in the hydrophilic, thermo-plastic ion exchange material solution and it was then mechanically hot pressed to Nafion to form a SPE electrocatalyst composite. Lawrance et al.47 have also prepared SPE. electrocatalyst composite by hot pressing II' and Pt black powder on the abraded surface of Nafion membrane at 182-188°C at 980 Ibs/cm2 pressure for 8 min . With 1.4 mglinch2 and 2 mg/c m2 loading of Ir and Pt respectively an efficient SPE metal composite suitable for water electrolysis has been obtained_

Nakanore et al. 8o have prepared a cell of electrode area 200 cm2 by hot press method with 11'02 (4mg/cm2) as anode and Pt (3 mg/cm2) as cathode while, Nagai et al. 14 have prepared a cell of 50 cm2

area with Ir02 loading (3mg/cm2) for efficient water e lectrolysis. The SPE electrode composite obtained by Petrov et al.15 by hot press technique consists of platinum plated titanium mesh as cathode and Ru, Ir and Ta oxide coated Ti mesh as anode. In order to create a three dimensional reaction zone, Ti meshes were loaded with 0.8 g/cm2 with Nafion with the help of Nafiq.n solution . These were hot pressed to Nafion at 130°C at 200 atm/cm2 pressure. In place of titanium mesh , titanium porous sheets of Imm thickness were used to prepare Ir02 electrodes by Andolfalta et al. 5')

These were again impregnated with Nafion solution before hot press ing with Nafion membrane.

8. Current collectors The role of current collectors in SPE water

electrolysis system is as important as electrocatalysts . Through these, the transfer of electrons from anode to cathode takes place in the outer circuit. They not only act as separators of hydrogen and oxygen evolved in cathode and anode chambers ut also give a mechanical support to SPE membrane under operational differential pressures.

These materials should have high electric conductivity, low contact resistance with the electrocatalysts, sufficient corrosion resistance, impervious to hydrogen and oxyoen and stable for

. 77 b

long tllne . Expanded titanium meshes or perforated titanium screens plated with plati nu m were used as cathode and anodic support collectors at the earl y stage. However, the hydrogen embrittlernent takes place on the cathodic current collec tor in lon o time b

operation. According to Laconti et al.16 the hydrogen embrittlement of material s follow the sequence.

Ti = Ta > Nb > Zr > graphite

Therefore the use of moulded carbon cun-ent collectors was proposed. Takenaka et al. 17 have developed a new type of porous carbon sheets and made special graphite filters as cathodic current collectors. It was further observed that Ti mesh anode collectors caused pin holes on the membrane­electrocatalyst composites due to the large current­concentration locally at the contact poi nts. A new anode current collector with a double layer structure of thin and finely porous layer and a coarsely porous substrate was developed. The new collectors are prepared by sintering or pl as ma spraying titanium powders on a substrate of porous sheet made of titanium fibers. Nakanori et al. 80 have used sintered titanium fiber plate electroplated with platinum as anode and sintered stainless fiber plate electroplated with gold as cathode, because stainless fiber is more resistant to hydrogen embrittlement than titanium fiber. Arai et al. 18 have proposed a method for preparing electrode and current collectors ill situ for SPE water electrolysis. Porous metal sheets of Ti were used as current collectors on which catalyst was deposited and catalyst layer was then coated with solid polymer electrolyte.

Two pl::!tinum current collectors made of Pt gauze (196 mesh cm-2) welded on to a perforated Pt fo il (0.2 mm) thick are used by Millet et al. 4

<) .

HAN et at.: WATER ELECTROLYSIS BY SOLID POLYMER ELECTROLYTE 251

' ·9

'" 01 '·8 .8 0 > c 0 17 '" E

'" u 1-6

',5 2 3 4

Current density(A/cm2 )

Fig 4-Current vo ltage curve for 2500cm2-cell Electrolyzer at 80°C

9. Sensitivity to poisoning In SPE water electrolysi s generally, metallic

platinum is used as cathodic electrocatalyst due to its high catalytic activity. However, its activity is not selective and it is also highly sensitive to poisoning by under potential deposition of monolayers of other metals81.82 such as copper83, lead84 , nickel48

.59 and iron59

. Once the platinum surface is covered by such monolayers, hydrogen di scharge takes place on the new surface and the cathodic over voltage increases drastically . In water electrolysis, the deioni sed water circulated in the cell stack becomes poi soned by metallic cations because of the s low but steady corrosion of the steel piPing . Therefore the incorporation of ion exchangers in the anodic and cathodic water circuits have been proposed by Andolfatto et al. 59

. Another alternative proposed by many researchers59

.58.48, is to use the IrO? or RuO? based alloys as cathodic materials in - place of platinum, because of their insensitivity to poisoning.

The use of Nafion also increases the problems of electrode poi soning. The ec;uilibrium between the membrane and the soluti on leads to an exchange of protons of the membrane with the cations of the solutions. The amount of particular ion exchanged can be determined quantitatively from the selectivity co­efficient of various ions85

. Therefore, a large increase in poly cations concentrations inside the membrane can take place with time. The electrode in membrane faces more impurities than in solution and the electrodes kinetics can, therefore, be modified. Thi s ion exchange of protons with metallic cations also leads to an increase o f the resistivity of the membrane. The resistivity is at leas t, five times hi gher for a Nafion-membrane in Ni2+ form than in H+ form and a dramatic increase of the ohmic drop is observed, when it is ion exchanged with Ni+2 ions59

•

' ·9

'" 01 ' ·8 .8 0 > c 0 '·7 '" E

'" 1-6 u

' ,5 2 3 4

Current density(A/cm2 )

Fig 4-Current voltage curve for 2500cm2-cell Electrolyzer at 80°C

10. Recent advances and scale up of SPE water electrolyzer

SPE electrolyzers of small size with (50 cm2

electrode area) were made available as early as in mid seventies. But these units were not practical and economical for large scale production of hydrogen 78 .

Now a lot of progress has been made in this area. Hot press method and chemical plating methods have been adopted for the preparation of SPE electrocatalysts composites by various research groups.

In early eighties an electrolysis module consisting of two cells with electrode area 1,600 cm2 (40 cmx40 cm) was tested successfullyl ? which operated at 80°C

2 ' I Ncm current density and 5 atmospheric pressure with 1.7 volts . H2 and O2 purities obtained were 99.999% and 99.88% respectively .

Recently a novel method 15.59 to increase reaction area in three dimensional zone has been proposed and tested at laboratory scale with 5 cm2 electrode area. RU2Ir, Ta oxide or Ir02, coated Ti gauze or perforated Ti sheets are used as anode and Ti or stainless steel gauze coated with Pt or Ir02 used as cathode. The electrodes are impregnated with Nafion with the help of Nafion solution to create a three dimensional reaction zone before hot press to Nafion membrane. The cell has been tested for continuous operation up to 5,000 hours at 25°C and 80°C at I A/cm2 cUlTent density with cell voltage 2.23 volts and 1.85 volts respectively .

Mc El roy8? has designed a SPE water electrolyser for space applications based on nuclear submari ne SPE electrolyzer, with incorporated capability to operate at high differential pressures of O2 and H2 up to 3,000 psi.

At present , an electrolyzer with 2,500 C l11 ~ electrode area with four stacks prepared by hot press and chemical plating method has been tes ted

252 INDIAN J CHEM , SEC A, FEBRUARY 2002

successfully at 80De and at current densities ranging from I A to 4A/cm2

. The ener~y efficiency obtained is more than 94%(88.89) (Figs 3-5). ]n WE-NET project of Japan Hydrogen Program (1998-2003), phase II, the projected scale up of electrode area for the water electrolyser is greater than 10,000 cm2 which should operate at I to 3 A Icm2 current density, with energy efficiency >90%.

]n order to reduce initial investment cost, the development of new solid polymer electrolyte material is the need of the hour. Partially fluorinated membrane (with low cost) has been suggested as a substitute for Nafion90

. Linkowu et 01.90.91 have

studied several polymeric materials like aromatic polyesters, polybenzimidazoles, polyphenylene sulphides, polysulphones, polyethersulphones, polyketones and polyimides to develop a new solid polymer electrolyte. A new ionomer with the same capabilities as that of Nafion , has been reported. But a cheap and better alternative of Nafion is still awaited

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processes, edited by 0 H SmIth & A T Kuhn, (Elsevier Amsterdam).( 1971). Chapter 4.

2 Murray] N, Advanced alkaline eleClrolysis cell development: Final report. (Brookhaven National Laboratory Report No. 5169695. Teledyne energy systems. Timonium. M. D), 1983.

3 Inlemarional Energy Agency Task IV: Annllal progress report (U S Department of Energy, Washington DC), (1984).

4 Arribart H, Piffard Y & Doremieux Morin C, Solid state ionies , 7 ( 1982) 91.

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