Indian Journal of Chemi stry Vol. 4 1A, May 2002. pp. 955-959

Hydrogen production by water electrolysis using solid polymer electrolyte

Sang-Do Han, K C Singh*t, R K Rana & Kee-Bae Park

Korea Institute of Energy Research, P.O. Box 103, Yusung Taejon, Korea

t Department of Chemistry, M.D. Uni versity, Rohtak, India

Received 20 August 2000; revised 24 January 2002

Electro- catalysts platinum and iridium are deposited on solid polymer electro lyte [(SPE) i.e. Nafi on-117] membrane by chemi cal deposition method. The minimum loading of Pt and Ir required on SPE fo r effic ient working of electro lysis cell are found to be between 1. 15mg/cm2 and 2.8mg/cm2 respecti vely. The best hydrothermal treatment temperature is fo und to be between 11 0°C and 120°e. The effect of the operati ng temperature on the ce ll performance is also studied. The cell voltage has been found to decrease with increase in temperature .

In recent years, considerable amount of work on the development of highly efficient solid polymer electrolyte composites for water electrolysis has been carried out t

-6

. Generally Nafion-117 (cationic exchange membrane) is used as solid polymer electrolyte (SPE) and platinum and iridium/iridium oxide are found to be suitable materials as cathode and anode respectively for water electrolysis due to their low over voltages. Different techniques like chemical deposition6

.7

, Electro depositionS, hot press method9 and deposition of iridium or platinum on porous titanium have been tried by various

k 10.11 ffi . Ch . I wor ers to prepare an e lClent system. emlca deposition can be carried out either by saturating Nafion membrane by cationic solution of Ir or Pt and then reducing the noble metal cations by a suitable reducing agent, or by circulating the anionic solution of metal salts to one side of membrane and reducing agent to other s ide of it. In this note, the chemical

__ deposition of pl atinum is carried out by former method and iridium is deposited by electroless plating method4

.

Hydrothermal treatment at high temperature to increase the water absorption capacity of the membrane and ion plasma treatment for roughening the surface of membrane, to enhance the adhesi ve strength of catalyst-membrane are generally carri ed out before chemical depos ition. But we have prepared all SPE composites withou t ion plasma treatment at

different hydrothermal treatment temperatures to observe systematically the effect on the performance of electrolytic cell.

Experimental Perfluorosulphonic acid polymer membrane

(Dupont de Nemours Nafion products) Nafion-117 (eq .wt.lIOO and dry thickness 0.178mm) was chosen as SPE. JrCl )H20, Pt (NH))4ChH20 , NaBH4 and hydrogen dichloride (GR grade) were suppl ied by Aldrich chemicals. Before chemical deposition, Nafion membrane (5x5 cm2) sample was cleaned by boiling in HNO): H20 (1 : I vol) for 30 min and then boiled in deionized water(resistivity =18M ohm cm at 20°C) under pressure at 100 to 150°C for 1 h for hydrothermal treatment.

Deposition of platinum The chemical deposition of platinum was carried

out by the same procedure as described by Millet et al. 2 Nafion was dipped in O.OIM Pt(NH))4Ch H20 solution for 15 min at room temperature and then the reduction of cationic platinum on membrane was carried out by dipping it in 0.3% NaBH4 solution for two hours. Generally, two cycles for the deposition are sufficient to prepare a good PtlNafion composite with 1.07 to 1.15 mg/cm2 loading.

But if the efficiency of electrolytic cell is lacking, It may be increased by a third cycle of deposition, the film is dipped in platinum solution only for 5 min . If higher loading of platinum is carried out by repeated cycles, the deposition of platinum takes place too far in the membrane and this causes a decrease in the hydrogen production efficiency of the cell (low faradic yield) .

Deposition of iridium Iridium was deposited on the platinum deposited

surface of Nafion in a electrolessplating bath4 at 70°C. In 250 ml of MI25 solution of IrCI)'2H20 , 100 ml of MIlO solution of hydrogen dichloride and I ml of conc. HCI were added by stirring. The solution was concentrated to 25 ml at 90°C. Thi s solution (5 ml ) was diluted to 350 ml with water and the pH of the solution was adjusted to 2.8 by NIiO NaOH solution . In thi s elec trolessplating bath the deposition automatica ll y takes place on pl atinum surface at

956 INDIAN J CH EM, SEC A, MAY 2002

70°C. The deposition was carried out for I to 4 h. by keeping pH of solution between 2.7 and 2.8 using NIIO NaOH solution. The deposits from I mg/cm 2 to 4 mg/c m2 were obtained depending upon the time of deposition. Generally, a period of 3 to 4 h is suffici ent to give good composite of Pt-Ir.

M/SPE/M (M=Pt,lr or Pt, SPE = Solid polymer electrolyte or afion membrane) compos ite was sandwiched between two platinum plated titanium current collectors made of titanium gauze (120x I I 0 mesh diameter 0.1 mm) . Further titanium gauzes were pressed to M/SPE/M composite by two platinum plated titanium plates ( lOx lOx I cm\ Titanium plates were in sulated from each other by a tefl on sheet of suitable thickness as shown in Fig. I . On the pressed side titanium plates were engrooved to I mm depth and with same breadth. Each groove is 2mm apart from the adj acent groove. These grooves ensure sufficient and uniform supply of water during electrolysis and also a escape route of O2 and H2 gases during electrolysis. This whole set-up is represented in Fig. 2 as elecytrolysis cell. The titanium plates were connected to the D.C. power supply, which can work either at constant vol tage or at constant CUITent. There are two thermocouples (TC in Fig. 2) inserted in the titanium plates to measure the temperature of electrolysis cell. Tap water was filtered (PF & R.O.) and stored in a tank. It was deionised (01) (registi vity IBM ohm cm at 20°C) and pumped to anodi c chamber (A) and cathodic chamber (B). Each chamber was

~=>

02+H20 +-

8

H20 --+ +- 1120

2 3 4 5 6 7

Fi g I- Cross sectional schemati c diagram of cell. [I , Titanium plate (anode); 2. Pt-plated T i gauze (anode support collector); 3, Tenon insul ator; 4, Nafion mem bra ne; 5, Tenon in sulator; 6, Pt­plated Ti guaze (cathode support co llector; 7, Titanium plate (ca thode); 8, Ir-Pt elcc trocatalyst layer).

made up of copper and teflon coated from inside having capacity of two liters and 3/4 of each chamber was filled with deionized water, which was found to be sufficient to can'y out the electrolysis fo r many hours.

Water from chamber A and chamber B was pumped to the electrolys is cell through heat exchanger (HE) to mainta in a constant temperature After pass ing through electrolys is ce ll (EC), excess water, H2 and O2 gases were pumped to chamber B and A respecti vely . O2 and H2 gases escaped from the top of chambers A and B respectively and their fl ow and purity were checked by a fl owmeter and a gas analyser.

The whole system was connected to DA (data aqui siti on and control system) which automatically controls the flow and temperature of water and pressure of the gases . The current and voltage were also monitored by this unit.

Result and discussion The bonding of electrodes (Pt and Ir) on the SPE is

critical. In order to reali ze the efficient composi tes for SPE water electrolys is, two conditions must be met:

(i) Metallic electrode must be located simultaneously inside the membrane (act as electrocatalyst) and outside on the surface of membrane (act as current collector); and

(ii ) metallic electrode must be located not too far inside the membrane to avoid low farad ic yield due to gas diffusion through the membrane. Furthermore these two regions must be in contact to ensure electron tran port and good ad herence of the electrode under high mechanical tens ion during gas .... vo lution .

As the electrocatalysts (Pt and Ir) are costl y metal s their loadings on the SPE should be as low as possible (w ithout decrease of electrolysis cell effici ency) so as to make these cells economically viable.

The Ptl SPEI Pt composites prepared with different loadings by depositing Pt as described in the experi mental section : Current voltage (CY) curves were ploued as shown in Fig. 3 and the electrolysis was carri ed out for each composite at 800 mA/cm" (c urrent density) at 80°C for many hours. The req uired vol tage for electrolys is fo r each composi te is summarized in Table I.

It is clear from Table I that two cycles deposition of Pt is sufficient for the efficient working of electrolysis cell.

NOTES 957

B

TAP WATER

PF · RO

11 1'11

LC

cv

PUMP

········ 00 DO DO

DA

HE: Heat exchanger, DI : Deion izer, PF: Prefilter, RO:RO cartridge, CV : Control valve, P: Pressure gauge, TC: Thermocouple, LC: Leve l controller, DC: DC Power supply,DA : Data acquisition and control system, EC : Electrolysis cell,A: Anodic chamber, B: Cathodic chamher

Fig. 2- Flow di agram of water electro lysis system.

Similarly composite with different Ir. loading but with same Pt loading ( 1.07- 1.15 mg/cm2

) were prepared and electrolys is was carried out for each sample at 800mA/cm2 at 80°C and results are summarized in Table 2. CY curves at 25°C and 80°C are plotted in Fi g. 3.

The CY curves for all composites hav ing Pt loading 1.15mg/cm2 and Ir loading 2.8 mg/cm2 coincide with each other, but as the Pt loading is increased, the current effi ciency of the ce ll decreases as in Table 1. On the other hand with the increase of Ir loading the

current efficiency remains more or less same but the minimum amount of Ir required fo r loading is 2.8 mg/cm2as in Table 2.

In these experiments the nafion membranes were pretreated at 110°C for hydro thermal treatment for one hour in water before chemical deposition of Ir or Pt.

The Pt/SPE IPt and Pt Ir/SPE IPtIr composites were found to be light golden yellow and light gray in colour respecti ve ly with smooth spotless shining surface to the naked eye .

958 INDIAN J CHEM, SEC A, MAY 2002

2·6,....----- -·----------------,

2·2

-» -; 2·0

'" o

:i 1. 8

1.6

/

1.40!-----:2~OO,----~.0-:-O ---:60~O,-----8~0-=-O ---:::10';;-;00:----'1200

Fig. 3--Cell voltage and current density relation with temp. [(i) : Platinum as anode and cathode; (ii): Platinum- Iridium as anode and cathodel

If some flaws like dark spots, non-uniformity of colour appeared on the deposited film, It was observed that it does not form a good SPE composite.

SEM photograph of the surface and cross section of IrPt/SPE composite for optimum loading of Pt (1.15 mg/cm2

) and Ir (2.8 mg/cm2) were recorded.

The porous structure of the layer was seen which is necessary for evolution of gases and absorption of water to membrane. Layer of approximate 5 /lm thickness of IrPt was found to be directly bound inside and above the nafion membrane. The deposition of Ir was not only on the outer surface of Pt but also crept deeply inside of the Pt layer as has been revealed by the RDX analysis.

Hydrothermal treatment

The process of boiling nafion membrane in water at high temperature is called hydro thermal treatment. This treatment increases the water absorption capacity of the membrane. Generally 100°-150°C are found to be the desirable temperature range. This is because at temperature below 100°C, the adhesive strength between membrane and the deposited metals tended to decrease, and at temperature above 150°C the mechanical strength of the membrane tends to decrease considerably I, It has also been observed that the effective resistance of the membrane also decreases slightly with increase in hydrothermal temperatures. In order to find out the effect of hydrothermal treatment on the overall efficiency of the cell, six Pt/SPE/Pt (1.15 mg/cm2 loading) composites were prepared, these were pre-treated at tempreatures 100-ISO°C at an interval of 100C in nickel autoclave at high pressure.

Table I-Dependence of number of cycles of Pt deposition on Nafion and the effect of Pt loading on electrolysis voltage and

current efficiency.

No of cycle Pt loading Electrolysis voltage of Pt (mg/cm2) at 800 mA

deposition

I 0.53-0.58 2.1-2.4 (not stable)

2 1.07-1.15 2.02 ±0.01

3 1.72-1.80 2.02 ±0.01

4 2.30-2.38 2.02 ±0.01

Current efficiency*

(%) ,

98.7

97.2

90.1

*current effic:ency %=[Volume of gene-rated H2 in mllsx 96500x2xI OOl/[Current xl sx 22400 mil

Table 2-Time dependence of iridium loading on Nafion and effect of iridium loading on electrolysis vo ltage and current

efficiency

Time of electroless

plating (h)

2

3

4

5

6

Loading (mg/cm2)

0.9-1.0

1.9-1.96

2.8-2.85

3.6-3.7

4.5-4.55

5.40-5.50

Electrolysis voltage at 800 rnA

2.02

1.65-2.02 (not stable)

1.62±.01

1.62±.01

1.62± .01

1.62±.01

Current efficiency

(%)

99_2

99 .1

99 .2

99.15

Each composite was tested for several hours fo r water electrolysis at 80°C. Current voltage curves for all the samples were found to be same as shown in Fig. 3, But the amount of gas evolution or current efficiency decreases as the hydrothermal treatment temperature increases.

It was observed that the most suitable hydrothermal treatment temperature ranges between 110° and 120°C. The behaviour for Ir Pt/SPE/lr Pt composite has not been tested but it is assumed that it will be similar to Pt'SPE/Pt composite.

Surface roughening treatment Various techniques iike sand paper, sandblast and

ion plasma sputtering are used for roughening the surface of membrane. It is considered that this treatment tends to increase the adhesive strength of the catalyst with membrane.

However, it was also observed by Millet et ai. 6 that surface roughening treatment has practically no effect on the efficiency of the cell. However, it may decrease the effective thickness of the membrane, thus causing more permeation of gases through the

NOTES 959

membrane. The present investi gation is carried out without this treatment. The current effici ency and energy effi ciency (two parameters to judge the effici ency of elec trolys is ce ll ) are found to be 99 .2% and 90.3% respectively for Pt and Ir loading of l.15

mg/cm2 and 2.8 mg/cm2 respecti ve ly at 110°C hydrothermal treatment temperature.

E ff' . { 1.48V (thermoneutra l vOltage) )

nergy e IClency= Elec trolysis vo ltage (v)

x current efficiency

1t can be conc luded that the procedure described for preparation of M/SPE/M (M= Pt or Ir) composite is simple and reproducible. A good composite can be prepared within seven hours. The composite thus obtained presents good electrochemical properties, low catalyst's loading, long time stability and high energy effi ciency.

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