ELECTROCHEMICAL СHARACTERISTICS OF THE PHOTOELECTRODES ON BASED OF HIGHLY POROUS SILICON WITH THE SILICIDE COATINGS

K. B. Tynyshtykbaev, T. Aytmukan, V. B. Glazman, A. T. Issova, V. Klimenov, V.A.Mamonov, D. Muratov, B. Rakymetov, M. Yeleuov, N.S.Tokmoldin, S. J. Tokmoldin

Physical-Technical Institute of the RK, Almaty, Kazakhstan, e'mail: kt011@sci.kz

HYDROGEN ENERGY

• Hydrogen energy – Energy Future, in which hydrogen chemical energy is use as electricity and heat.

• The transformation of chemical hydrogen energy in Fuel Cells (electrochemical cells) occurs in result redox reaction 2H + O2 = 2H2O

at continuous supply of fuel from the special reservoir.• The undeniable advantage of fuel cells is that they do

not pollute the environment, operate silently and have not the mechanical damage . Efficiency FC reaches 45 - 60% and higher, than efficiency of the internal combustion engine (15%).

The main problems of Hydrogen energy 1. Reducing the cost of the currently existing methods for the

production of hydrogen - electrolysis of water, steam reforming of natural gas and methane, coal gasification and pyrolysis, biotechnology, processes of partial oxidation, thermolysis and radiolysis. Cost of hydrogen 2$/kg (USA).

2. Increased work service and reduce the cost of existing types of fuel cells. High temperature of hydrogen energy conversion and use of expensive high-temperature materials. The high cost of platinum catalyst material with help which occur the chemical conversion processes . Without the catalytic effect of platinum is not possible to achieve the desired conversion rate.

3. Storage, transportation, use of hydrogen, ecology. Contact of clean gaseous or liquid hydrogen with the environment leads to the destruction of Earth ozone layer.

The most perspective method the production of pure hydrogen is electrolysis of water

• The main problem of water electrolysis - decrease specific energy consumption of the production of hydrogen from water (Eel=4kW/h > Ech=3kW/h), can be solved by using a photosensitive for solar spectrum of the porous electrode with a catalytically active surface.

• Perspectively the application of porous silicon electrodes with high photosensitivity and by catalytically active coating

Por-Si PhotoelectrodesThe main problem - energy malnutrition of pure

photoelectrolysis of water ( Ech.b.(н2о) > 1.23 eV at Eg (Si)=1.1 eV) is decided by the use of porous Si.

• Wide gap and the developed surface of the por-Si

• Catalytically active silicides-FeSi, NiSi, PdSi,PtSi.

• Schottky barrier Me/Si

• External power supply on based silicon solar cells

• Modern silicon technology

Energetic diagram of por-Si/p-Si and a metal electrodes for photoelectrolysis water (pH7)

Technique of Experimentp-Si (B) samples with the orientation (100), resistivity of 0,01 Ω∙cm and 10 Om.sm,

350 μm thickness were used. Indium ohmic contacts for etching processes of p-Si were made on the backside wafer by

Thermal annealing at 300 °C for 30 min on air. Por-Si formed by etching of in electrolytes: HF:C2H5OH, HF:(СН3)2СНОН, HF:H2O2. The anode was p-Si sample, the counter electrode – Ni. Density of anode current was about ja = 1.7 mA/cm2 at U=0,8V. In Ni/HF-H2O2/p-Si electrochemical system take place the internal difference of electrical

potential in the process etching and arises electromotive force (emf), the internal current source.Etching time varied from 1 h to 4h. Samples after etching and washing in distilled water were dried and stored in a desiccators

with silica gel and pre-heated at 200°C. Ni electrochemical deposition on Si samples occurs in 0,1M electrolyte NiCl2(6H2O) : H2O.Physical evaporation of Ni-target occurred at P= 4x10(E-4) Torr by Ar+ ion, E = 2,5 keV, j (Ni-target) = 0,2mA/sm2 on equipment ARC 2000. Time evaporation tevap.=1h, dNi= 600 nm.

Annealing at T = 300oC and 400oC in vacuum 8x10-4 Torr at different time tann.=1h, 2h,3h, 4h in equipment of Ion Beam Evaporation.

Ohmic Al-contact to p-Si annealed at the same conditions, that Ni-contact.Current- voltage characteristics of por-Si sample measured in aqueous electrolytes of 0,1M

NiCl2(6H2O) and 0,1M HCl in glass electrochemical cell.

2-chamber glass electrochemical cell

Cross-sectional SEM image of por-Si sample. Initial sample p-Si (100), 0,01 Om.sm. Electrolyte HF: (CH3)2CHOH, tetch. =2h

Cross-sectional SEM image of porous layer deep p-Si (100), 0,01 Om.sm, HF: (CH3)2CHOH, tetch. =1h.

SEM (1), AFM (2, 3) and Optical images (4) of por-Si/Ni

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3

4

Current- voltage characteristics of por-Si sample in an aqueous solution of 0,1M NiCl2.

p-Si (100), 0,01 Om.sm. Porosity in HF: (CH3)2CHOH tetch. =2h.

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-8

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0

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-2 -1,5 -1 -0,5 0 0,5 1 1,5 2 2,5

mI

V

Ряд1

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-4

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0

2

4

6

-3 -2 -1 0 1 2 3

mA

V

I-V characteristics of por-Si samples in an aqueous solution of 0.1M NiCl2 at different porosity of Si substrate

p-Si (100), 0,01 Om.sm

Comparative I-V of por-Si with NiSi and FeSi silicides coatings. Right CVC - increased low-voltage plot for water electrolysis.

I, mA

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0

10

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-5 -4 -3 -2 -1 0 1 2 3 4 5

FeSi

NiSi

I,mA

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-0,8

-0,6

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0

0,2

0,4

0,6

0,8

1

-1 -0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1

FeSi

NiSi

Solar light and dark I-V Characteristics of samples por-Si with FeSi coatings

FeSi

I, mA

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0

10

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-4 -3 -2 -1 0 1 2 3 4

El/Ch-Light

El/Ch dark

Ph light

Ph dark

I-V characteristics of the por-Si with FeSi coatings, obtained by various methods, depending on etching time por-Si

FeSi (Phys)

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0

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-5 -3 -1 1 3 5

I.mA

V 1h

2h

4h

FeSi(El/Chem)

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-5 -4 -3 -2 -1 0 1 2 3 4 5

I,mA

V 1h

2h

4h

I, mA

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0

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-5 -4 -3 -2 -1 0 1 2 3 4 5

V El/Ch

Ph

I, mA

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-1,5

-1

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0

0,5

1

1,5

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-2 -1,5 -1 -0,5 0 0,5 1 1,5 2

V El/Ch

Ph

Comparative I-V of por-Si with NiSi silicide coatings, produced by various methods - electrochemical deposition in the aqueous solution 0.1M NiCl2 (El/Ch) and ion-beam sputtering of Ni by Ar ions, E = 2,5 keV (Phys) . Right CVC - increased low-voltage plot CVC .

I-V characteristics of the por-Si with NiSi coatings, obtained by various methods: - electrochemical (green), physical (red) and

combined (yellow) NiSi

I,mA

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0

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20

30

-5 -3 -1 1 3 5

El/Ch

Ph

Initial

Comparative I-V of por-Si with NiSi silicides with different coatings. Current- voltage characteristics of por-Si sample in an aqueous solution of 0,1M HCl

I,mA

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PS-Ni

PS-Pt

I-V Dependence of the yield hydrogen for different electrode pairs. 0.1Mol electrolyte aqueous solution of hydrochloric acid

Conclusion

• 1. Heterostructure por-Si/p-Si demonstrated the high cathode properties at the electrolysis of aqueous solutions of electrolytes.

• 2. NiSi coating of the surface of por-Si electrodes allows to improve the I-V characteristics of the cathode process of hydrogen release.

• 3. Modification of the original surface of of monocrystalline silicon sample and the combination of electrochemical processes the pore-formation and nickel deposition on the surface pores form the high stability nickel silicides for the process of water electrolysis.

Acknowledgment

The authors are grateful to K.Mit’ for atomic force microscopic images. This work was supported financially by the Ministry of Science and Education of Republic of Kazakhstan. No grants 169/2013 and 217/2013.

Springtime in foothills Ala Tau, region of Almaty, Kazakhstan

Pik Talgar, 5600 m, Ala Tau, Zailiyskii Tien Shan

Pik Talgar, 5600 m, Ala Tau, Zailiyskii Tien Shan

Khan Tengri, 6995 m, Terskey Ala Tau, Tien Shan

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