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Photogeneration of hydrogen by Halobacterium halobium MMT22 coupled with silicon PN junction semiconductor without external bias potential — II
0360-3199/92 $5.00 + 0.00 Pergamon Press Ltd. ~) 1992 International Association for Hydrogen Energy.
hit. J. 14wlrogen Energy, VoL 17, No. 2, pp. 89-91. 1992. Printed in Great Britain.
P H O T O G E N E R A T I O N OF H Y D R O G E N BY HALOBACTERIUM HALOBIUM MMT22 C O U P L E D WITH SILICON PN J U N C T I O N S E M I C O N D U C T O R W I T H O U T E X T E R N A L BIAS POTENTIAL - - II M. M. TAQUI KHAN, M. R. ADIGA and J, P. BHATT Discipline of Coordination Chemistry and Homogeneous Catalysis, Central Salt & Marine Chemicals Research Institute, Bhavnagar 364 002, India (Received for publication 9 October 1991)
Abstract--Hydrogen generation by the biophotoelectrochemical cell using Halobacterium halobium MMT22 for photoproton pumping and platinum coated silicon for photoelectron generation requires 1 - 2 h of illumination or 15-20 min of external potential of 0.6-0.9 V for the development of required hydrogen reduction potential. The evolution of H2 continues as long as illumination is provided to the surfaces. The rate of generation of H2 is about 5.45 I h - i from 1 m 2 of H. halobium surface.
between H. halobium and silicon to drive the photogenerated protons. The H2 generation is possible only at reduction potential of - 0 . 4 1 V between silicon and electrolyte which takes sufficient time under illumination. The open circuit voltage development under illumination in the Bio-PEC cell with a 25% NaC1 electrolyte solution is shown in Fig. 1. It shows that the potential increases with time and reaches a saturation value at about 0.25 V. Instead of illuminating the system for 1 - 2 h after initiation of a measurable amount of H2 production, an external potential of 0.9 V between platinum electrodes kept in the H. halobium compartment and silicon compartment was applied for 1 5 - 2 0 rain with simultaneous illumination of the Bio-PEC system. The system produced H2 continuously under illumination even after cutting off the power supply. This is due to the development of reduction potential of H2 at the platinum coated silicon surface. The current between the platinum electrode in the H. halobium compartment and silicon show that after cutting off the external potential, the current decreases at a faster rate in the initial period and then reaches saturation (as shown in Fig. 2) for 25 % NaC1 electrolyte concentration. The pH measured in the H. halobium compartment after completion of experiment shows that it is almost neutral around 6.9, while in the silicon compartment the pH was found to be 8.9. However, the rate of H2 production is not affected and the pH remained the same in the H. halobium compartment which is an ideal condition for photoinduced proton pumping, pH stability is very important for H2 production by H. halobium [ 2 - 7 ]. Figure 3 explains the mechanism of H2 evolution. The absorption of a photon results in the deprotonation of the Schiff base linkage, and the apoprotein undergoes a conformational change which allows the unidirectional migration
INTRODUCTION Experiments on Halobacterium halobium MMT22 coupled with a PN junction silicon semiconductor in a biophotoelectrochemical(Bio-PEC) cell demonstrated the evolution of hydrogen. The rate of evolution of H2 on the platinum coated silicon surface depends on the applied external potentials between the platinum electrode kept in the Halobacterium halobium chamber and the silicon electrode [ 1 ]. The experiments under illumination by a xenon lamp without application of an external potential, however, did not show an appreciable amount of H2. When an external potential of 0 . 6 - 0 . 8 V was applied for a few minutes, the H2 generation was initiated and continued even after cutting off of the external power source. The H2 generation was also initiated when the apparatus was illuminated continuously for a sufficiently long time in order to develop an open circuit voltage of = 0 . 2 4 - 0 . 3 0 V between H. halobium and silicon. As a continuation of the work reported earlier [ 1 ] the present paper reports the variations in the H2 generation at different NaC1 electrolyte concentrations. EXPERIMENTAL The xenon light source, Bio-PEC cell and other experimental conditions were the same as reported earlier [ 1 ]. OBSERVATIONS AND DISCUSSION An appreciable amount of hydrogen evolution was not observed immediately after illumination of the Bio-PEC cell. This may be due to insufficient voltage development 89
90
M.M. TAQUI KHAN et al. 2.8
240
2.4
220 200
< E
180
¢-.
2.0 1.6
== 1.2 -t
160
> E 140 -g 120
--O
0.8
O
0.4
t--
O 13_
0
100
4
1 0 160 200 240 280 320 360 4 0
Time, s
80
Fig. 2. Variation of current with time in Bio-PEC cell having 25% NaCI.
60
40 20 0
/ 1
20
1
1
40
60
I
I
|
I
I
80 100 120 140 160 Time, rain
Fig. 1. Open circuit voltage developmentunder illuminationin Bio-PEC cell having 25% NaCI solution.
of a proton from the opposite side of the membrane by water splitting. This is supported by an increase in the basicity of the medium from pH 7.0, in the beginning of the experiment, to pH 8.9 during the reaction. The silicon PN junction semiconductor with the N-type surface coated with platinum black, when illuminated with photons with energy greater than 1.15 eV, causes the generation of electrons
purple membrane
and holes which are separated by the existing semiconductor solid PN junction. The electrons are collected at the N-type surface. The platinum black coating over the N-type surface conducts the electrons and also serves as catalyst for reduction of protons. Thus protons from H. h a l o b i u m and electrons from silicon combine to produce H2. The H2 generation at different concentrations of NaCI electrolyte starting from sea water, i.e. 3.5%, to near saturation level of 25 % under illuminationis shown in Fig. 4. For a lower concentration of NaC1, namely 3.5% and 10.0%, though a few H2 bubbles were formed at the silicon surface it was not, however, possible to measure the amount of H2 bubbles due to small exposed areas of H. h a l o b i u m - - 6.6 cmZs and 6.0 cm 2 of silicon; but the amounts measured for 15.0% NaCI, 20% NaCI and 25.0% NaC1 concentrations are 0.0047, 0.025 and 0.06 cm 3 min-~ respectively. This indicates the higher amounts of H2 generated when the salt concentration is at near saturation levels. The graph in Fig. 5 drawn for the rate of H2
H
LJ
I N Si--+-
hv I %
~-N--CH2_ ...........
OH H H O + ~ ~
tl
~ NH+ CH
H+"etl~
:
~
-holes
thick black Pt layer
e /J---,nsu,ation
Fig. 3. Mechanismof hydrogen evolutionby H. halobium MMT22coupled with PN junction silicon semiconductor.
H2 PHOTOGENERATION BY HALOBACTERIUM HALOBIUM MMT22
2.4//
3.0
91
(25% NaCI)
2.8
0.06
I t-
2.6
004
2.2
2.0 x~ 1.8
.2
(20% NaCI)
"o £ 1.6 o. 1.4 £ 1.2 1.0i~
f
0
0.8
I
5%
]
,
10% 15% 20% 25% NaCI concentration
Fig. 5. Rate of hydrogen production vs NaCI concentration.
0.6
0.4 0.2
(15% NaCI) q3.....~ ~''g"
0 10 2? 3'0 4'0
7'0
Time, min Fig. 4. Hydrogenproductionat different concentrationsof NaC1
external potential of 0 . 6 - 0 . 9 V between the H. halobium compartment and silicon for 1 5 - 2 0 min. The hydrogen is continuously produced under illumination at a rate of about 5.45 1 h -~ for one square of H. halobium at = 1265 W m 2 light intensity in a nearly saturated NaCI solution of 25% concentration.
electrolyte. REFERENCES production vs NaC1 concentration shows that there is a steep rise in H2 generation rates with a concentration of NaCI which acts as a supporting electrolyte. This suggests that in photoinduced H2 generation the saturated salt solution gives the best results, giving about 5.45 1 h-L for 1 m 2 of exposed 1t. halobium. CONCLUSIONS Direct illumination of H. halobium, though releasing protons, requires sufficient time to develop the potentials to drive these protons towards the platinum coated silicon electrode. The time duration can be reduced by applying an
1. M. M. Taqui Khan, M. R. Adiga Hydrogen Energy 17, 93 (1992). 2. M. M. Taqui Khan and J. P. Bhatt, 14, 643-645 (1989) 3. M. M. Taqui Khan and J. P. Bhatt, 15, 473-476 (1990). 4. M. M. Taqui Khan and J. P. Bhatt, 15, 477-480 (1990). 5. M. M. Taqui Khan and J. P. Bhatt, 16. 9 - 1 1 (1991). 6. M. M. Taqui Khan and J, P. Bhatt, 16, 83-85 (1991). 7. M. M. Taqui Khan and J. P. Bhatt, 16, 683-685 (1991).
and J. P. Bhatt, Int. J,
hu. J. Hydrogen Energy Int. J. 14ydrogen Energy Int. J. Hydrogen Energy Int. J. Hydrogen Energy Int. J. Hydrogen Energy Int. J. Hydrogen Energy
hit. J. 14wlrogen Energy, VoL 17, No. 2, pp. 89-91. 1992. Printed in Great Britain.
P H O T O G E N E R A T I O N OF H Y D R O G E N BY HALOBACTERIUM HALOBIUM MMT22 C O U P L E D WITH SILICON PN J U N C T I O N S E M I C O N D U C T O R W I T H O U T E X T E R N A L BIAS POTENTIAL - - II M. M. TAQUI KHAN, M. R. ADIGA and J, P. BHATT Discipline of Coordination Chemistry and Homogeneous Catalysis, Central Salt & Marine Chemicals Research Institute, Bhavnagar 364 002, India (Received for publication 9 October 1991)
Abstract--Hydrogen generation by the biophotoelectrochemical cell using Halobacterium halobium MMT22 for photoproton pumping and platinum coated silicon for photoelectron generation requires 1 - 2 h of illumination or 15-20 min of external potential of 0.6-0.9 V for the development of required hydrogen reduction potential. The evolution of H2 continues as long as illumination is provided to the surfaces. The rate of generation of H2 is about 5.45 I h - i from 1 m 2 of H. halobium surface.
between H. halobium and silicon to drive the photogenerated protons. The H2 generation is possible only at reduction potential of - 0 . 4 1 V between silicon and electrolyte which takes sufficient time under illumination. The open circuit voltage development under illumination in the Bio-PEC cell with a 25% NaC1 electrolyte solution is shown in Fig. 1. It shows that the potential increases with time and reaches a saturation value at about 0.25 V. Instead of illuminating the system for 1 - 2 h after initiation of a measurable amount of H2 production, an external potential of 0.9 V between platinum electrodes kept in the H. halobium compartment and silicon compartment was applied for 1 5 - 2 0 rain with simultaneous illumination of the Bio-PEC system. The system produced H2 continuously under illumination even after cutting off the power supply. This is due to the development of reduction potential of H2 at the platinum coated silicon surface. The current between the platinum electrode in the H. halobium compartment and silicon show that after cutting off the external potential, the current decreases at a faster rate in the initial period and then reaches saturation (as shown in Fig. 2) for 25 % NaC1 electrolyte concentration. The pH measured in the H. halobium compartment after completion of experiment shows that it is almost neutral around 6.9, while in the silicon compartment the pH was found to be 8.9. However, the rate of H2 production is not affected and the pH remained the same in the H. halobium compartment which is an ideal condition for photoinduced proton pumping, pH stability is very important for H2 production by H. halobium [ 2 - 7 ]. Figure 3 explains the mechanism of H2 evolution. The absorption of a photon results in the deprotonation of the Schiff base linkage, and the apoprotein undergoes a conformational change which allows the unidirectional migration
INTRODUCTION Experiments on Halobacterium halobium MMT22 coupled with a PN junction silicon semiconductor in a biophotoelectrochemical(Bio-PEC) cell demonstrated the evolution of hydrogen. The rate of evolution of H2 on the platinum coated silicon surface depends on the applied external potentials between the platinum electrode kept in the Halobacterium halobium chamber and the silicon electrode [ 1 ]. The experiments under illumination by a xenon lamp without application of an external potential, however, did not show an appreciable amount of H2. When an external potential of 0 . 6 - 0 . 8 V was applied for a few minutes, the H2 generation was initiated and continued even after cutting off of the external power source. The H2 generation was also initiated when the apparatus was illuminated continuously for a sufficiently long time in order to develop an open circuit voltage of = 0 . 2 4 - 0 . 3 0 V between H. halobium and silicon. As a continuation of the work reported earlier [ 1 ] the present paper reports the variations in the H2 generation at different NaC1 electrolyte concentrations. EXPERIMENTAL The xenon light source, Bio-PEC cell and other experimental conditions were the same as reported earlier [ 1 ]. OBSERVATIONS AND DISCUSSION An appreciable amount of hydrogen evolution was not observed immediately after illumination of the Bio-PEC cell. This may be due to insufficient voltage development 89
90
M.M. TAQUI KHAN et al. 2.8
240
2.4
220 200
< E
180
¢-.
2.0 1.6
== 1.2 -t
160
> E 140 -g 120
--O
0.8
O
0.4
t--
O 13_
0
100
4
1 0 160 200 240 280 320 360 4 0
Time, s
80
Fig. 2. Variation of current with time in Bio-PEC cell having 25% NaCI.
60
40 20 0
/ 1
20
1
1
40
60
I
I
|
I
I
80 100 120 140 160 Time, rain
Fig. 1. Open circuit voltage developmentunder illuminationin Bio-PEC cell having 25% NaCI solution.
of a proton from the opposite side of the membrane by water splitting. This is supported by an increase in the basicity of the medium from pH 7.0, in the beginning of the experiment, to pH 8.9 during the reaction. The silicon PN junction semiconductor with the N-type surface coated with platinum black, when illuminated with photons with energy greater than 1.15 eV, causes the generation of electrons
purple membrane
and holes which are separated by the existing semiconductor solid PN junction. The electrons are collected at the N-type surface. The platinum black coating over the N-type surface conducts the electrons and also serves as catalyst for reduction of protons. Thus protons from H. h a l o b i u m and electrons from silicon combine to produce H2. The H2 generation at different concentrations of NaCI electrolyte starting from sea water, i.e. 3.5%, to near saturation level of 25 % under illuminationis shown in Fig. 4. For a lower concentration of NaC1, namely 3.5% and 10.0%, though a few H2 bubbles were formed at the silicon surface it was not, however, possible to measure the amount of H2 bubbles due to small exposed areas of H. h a l o b i u m - - 6.6 cmZs and 6.0 cm 2 of silicon; but the amounts measured for 15.0% NaCI, 20% NaCI and 25.0% NaC1 concentrations are 0.0047, 0.025 and 0.06 cm 3 min-~ respectively. This indicates the higher amounts of H2 generated when the salt concentration is at near saturation levels. The graph in Fig. 5 drawn for the rate of H2
H
LJ
I N Si--+-
hv I %
~-N--CH2_ ...........
OH H H O + ~ ~
tl
~ NH+ CH
H+"etl~
:
~
-holes
thick black Pt layer
e /J---,nsu,ation
Fig. 3. Mechanismof hydrogen evolutionby H. halobium MMT22coupled with PN junction silicon semiconductor.
H2 PHOTOGENERATION BY HALOBACTERIUM HALOBIUM MMT22
2.4//
3.0
91
(25% NaCI)
2.8
0.06
I t-
2.6
004
2.2
2.0 x~ 1.8
.2
(20% NaCI)
"o £ 1.6 o. 1.4 £ 1.2 1.0i~
f
0
0.8
I
5%
]
,
10% 15% 20% 25% NaCI concentration
Fig. 5. Rate of hydrogen production vs NaCI concentration.
0.6
0.4 0.2
(15% NaCI) q3.....~ ~''g"
0 10 2? 3'0 4'0
7'0
Time, min Fig. 4. Hydrogenproductionat different concentrationsof NaC1
external potential of 0 . 6 - 0 . 9 V between the H. halobium compartment and silicon for 1 5 - 2 0 min. The hydrogen is continuously produced under illumination at a rate of about 5.45 1 h -~ for one square of H. halobium at = 1265 W m 2 light intensity in a nearly saturated NaCI solution of 25% concentration.
electrolyte. REFERENCES production vs NaC1 concentration shows that there is a steep rise in H2 generation rates with a concentration of NaCI which acts as a supporting electrolyte. This suggests that in photoinduced H2 generation the saturated salt solution gives the best results, giving about 5.45 1 h-L for 1 m 2 of exposed 1t. halobium. CONCLUSIONS Direct illumination of H. halobium, though releasing protons, requires sufficient time to develop the potentials to drive these protons towards the platinum coated silicon electrode. The time duration can be reduced by applying an
1. M. M. Taqui Khan, M. R. Adiga Hydrogen Energy 17, 93 (1992). 2. M. M. Taqui Khan and J. P. Bhatt, 14, 643-645 (1989) 3. M. M. Taqui Khan and J. P. Bhatt, 15, 473-476 (1990). 4. M. M. Taqui Khan and J. P. Bhatt, 15, 477-480 (1990). 5. M. M. Taqui Khan and J. P. Bhatt, 16. 9 - 1 1 (1991). 6. M. M. Taqui Khan and J, P. Bhatt, 16, 83-85 (1991). 7. M. M. Taqui Khan and J. P. Bhatt, 16, 683-685 (1991).
and J. P. Bhatt, Int. J,
hu. J. Hydrogen Energy Int. J. 14ydrogen Energy Int. J. Hydrogen Energy Int. J. Hydrogen Energy Int. J. Hydrogen Energy Int. J. Hydrogen Energy