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Photoelectrochemical Generation of HydrogenUsing Heterostructural Titania Nanotube Arrays

Mano MisraPrincipal Investigator

Metallurgical and Materials Engineering,University of Nevada, Reno,

Reno, Nevada, 89557Phone: 775-784-1603Email: misra@unr.edu

June 9th-13th, 2008

DOE Hydrogen Program Review 2008

Project ID # PDP15

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Overview

• Barriers addressed:- AP. Materials efficiency- AQ. Materials durability- AR. Bulk material synthesis- AS. Device configuration and

scale up

• Total project funding: $ 3,650 K– DOE share: $ 2,970 K– Contractor share: $ 680 K

• Funding for FY06: $ 3,650 K

Timeline

Budget

Barriers

Partners

• John Turner, National Renewable Energy Laboratory

• M.K. MazumderUniversity of Arkansas at Little Rock

• Project start date: October, 2006• Project end date: September, 2009• Percent complete: 60

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Objectives

Overall Develop high efficiency hybrid-semiconductor materials for hydrogen generation by water splitting.

2006-2007

• Develop new anodization technique to synthesize high quality and robust TiO2 nanotubes with wide range of nanotube architecture.

• Develop single step, low band gap TiO2 nanotubes.• Develop kinetics and formation mechanism of the titanium dioxide nanotubes under different synthesis conditions.

2007-2008

2008-2009

• Develop organic-inorganic hybrid photoanodes.• Develop combinatorial approach to synthesize hybrid photo-anodes having multiple semiconductors in a single photo-anode.

• Develop cost-effective cathode materials.

• Develop mixed metal oxide nanotubular photoanodes.• Develop multi-junction photoanodes.• Design PEC systems for on-field testing under real solar irradiation.

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Approach

Task A. Synthesis and fabrication of metal and mixed metal oxidenanotubular arrays by electrochemical anodization method.

Task B. Band-gap modification and engineering.

Task C. Application of the nanotubular materials for photo-electrochemical generation of H2 from H2O.

Task D. Materials stability of hybrid TiO2 nanotubular photo-anodes.

Task E. Scale-up and process evaluation.

• Ultrasonic mediated metal (Ti, Fe, W and Ta) oxide nanotube arrays • Synthesis in organic as well as inorganic medium• Annealing and characterization of TiO2 nanotubes• Coupling of nanotubes with low band gap semiconductors

• Photo-anode (doping with hetero-elements and design composite photoanodes) • Photo-cathode (Pt and Ni nanoparticles/TiO2)

• Electrochemical methods• Spectroscopic analysis

• Scale-up (photoanodes and cathodes)• Photoelectrochemical hydrogen generation under real solar irradiation

• Test hybrid photoanodes• Test hybrid cathodes• Reducing e-h recombination with organics by solution chemistry

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Novel methods for the formation of titania nanotubes

• 20-150 nm diameter • 0.5 -15 μm length• Smooth, compact and robust

Bi-facial photoanode :

Organo-fluoride solution in the presence of ultrasonic waves.

Ultrasonicator

Water

Anodizing solutionPt mesh Pt mesh

Ti foil for anodization

Fig. Experimental set-up for bifacial photoanodeCharacteristics:

(B)Fig. (A) SEM and (B) TEM images of titania nanotubes

Ti + 2H2O → TiO2 + 4H+ --- (1)

TiO2 + 6 F– + 4H+ → [TiF6]2– + 2 H2O -- (2)

Formation mechanism:

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1

2 2

3

45

7

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1. Quartz cell, 2. light source, 3. 1MKOH solution, 4. Ag/AgCl electrode, 5. cathode, 6. teflon lid, 7. photoanode, and 8. cathode.

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-1.4 -0.9 -0.4 0.1

Light illumination on one sideLight illumination on both the sides

Pho

tocu

rrent

(mA

)

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0 20 40 60 80 100

Pho

tocu

rrent

(mA

)

Time (sec)

1st light source off

2nd light source off

2nd light source on

1st light source on

V (Ag/Cl)

High surface area in small geometrical areaDouble efficiency per footprintSolar concentrating technology can be exploitedReduced recombination losses due to intermediate metallic contacts

Advantages:

Photoelectrolysis using bi-facial photoanode and Pt/TiO2 cathode

(A)

(B)

Fig. Schematic of PEC test arrangement using bifacial electrodes

Fig. Potentiodynamic and potentiostatic hydrogen generationusing bifacial electrodes (geometrical area=16 cm2)

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Stability test of titania nanotubes under PEC conditions: Evaluation of the materials for long term operation)

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0 2 4 6 8Time (hr)

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ocur

rent

(mA

)

,

Light off

Dark current

Light on

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Inte

nsity

(Cou

nts)

Before test

After testAnatase (101)

0123456

0 10 20 30Time (days)

Phot

ocur

rent

(mA

)

0 80 160 240Time (hr)

Fig. H2 generated continuously for (a) 8 h and (b) for 30 days (8h/day). Photoanode consisting of nitrogen annealed titania nanotubes are used (geometrical area = 3.5 cm2) in these experiments

(A)

(B)

Fig. XRD of the photoanode before and after thelong term test. It shows the photoanode is

stable after one month operation. This is also confirmed by DRUV-Vis and SEM measurements

8Fig. (A) The hybrid photoanode showed better photoactivity than titania nanotubes alone. (B) Potentiostatic measurements showed that the material is stable for long operation

Organic-inorganic hybrid photoanode: Surface functionalization of titania nanotubes with 2,6-dihydroxyanthraquinone (DHA)

Fig. The hybrid photoanode absorbs visible photons efficiently than titania nanotubes alone

(A) (B)

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Ti Metal

Sonoelectrochemical Anodization

TiCl4

500 0C, O2

Ti

TiO2

Ti

TiO2

TiO2

(A)

Fabrication of titania nanotubes with nanoparticles (Synthesis of nanoparticle/nanotube heterostructure photoanode)

Fig. Schematic of TiO2 nanoparticle/nanotube heterostructural photoanode preparation with (A) SEM image and

(B) TEM image of the particles

(B)

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Nanoparticle/nanotube heterostructure photoanode

Fig. H2 generation using the TiO2 nanotube/nanoparticlehybrid material. A three fold

enhancement in the photocurrent density was observed using the hybrid photoanode

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1.1

1.3

1.5

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200 400 600 800

Wavelength (nm)

Abs

orpt

ion

(a)

(b)

(a) TiO2 nanotube(b) TiO2 nanotube/nanoparticle hybrid

Fig. Increase in the absorption spectrum wasobserved in the TiO2 nanotube/nanoparticle

hybrid material

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Applied Potential (V vs Ag/AgCl)

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t Den

sity

(mA

/cm

2 )

TiO2 nanotube

TiO2 nanoparticle

TiO2 nanotube/nanoparticle hybrid

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TiO2

TiO2

CdS

Fig. SEM image showing the CdS/TiO2 core-

shell particles on the top of the TiO2 nanotube

arrays

Fabrication of titania nanotubes using low band gap semiconductors: CdS/TiO2 core-shell photoanode

Fig. CdS/TiO2 core shell photoanode absorb

better visible light compared to TiO2 nanotubes

alone

(B)

-0.2-0.1

00.10.20.30.40.50.60.70.8

200 300 400 500 600 700 800

Wavelength (nm)

Abs

orba

nce

(a.u

.)

TiO2

CdS

CdS/TiO2

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t den

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(mA

/cm

2)

(a)

(b)

(c)

(d)

Effect of CdS sensitization

Fig. Potentiodynamic plot of (a) CdS/TiO2 nanotubes

under dark conditions, and (b) ING-TiO2, (c) ORG-TiO2,

(d) CdS-TiO2 (ORG) nanotubes under illumination

(87 mW/cm2). The measurements are carried out in

sulfide-sulfite electrolyte

0

102030405060708090

100VisibleUV

% C

ontri

butio

n

TiO2 C-TiO2CdS/C-TiO2

Photoelectrolysis of water using CdS/TiO2 core/shell photoanode

Fig. A comparison with past results. CdS/TiO2 nanotube

photoanode showed more than 60% visible light contribution.

The results are calculated using AM 1.5 filter (solar spectrum)

and band pass filter (λ ≥ 420 nm; visible spectrum)

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Photoelectrolysis of water using CdS/TiO2 core/shell photoanode (Contd.)

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Phot

ocur

rent

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sity

(mA

/cm

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Need to be improved

Fig. Contribution of various wavelength regions to the photoactivity of CdS/TiO2 (ORG) nanotube composite material. It shows that most of the activity is contributed from the solar spectrum of wavelengths in the range of 400-550 nm. Interferencefilters with CWL of ±15 nm and FWHM of 50 nm are

used for the potentiodynamic measurements

Fig. Potentiostatic (I-t) graph obtained using CdS/TiO2 (ORG) nanotubes as photoanode in sulfide-sulfite electrolyte at –0.8 V Ag/AgCl (under the illumination of 87 mW/cm2 solar light). The photocurrent becomes zero when the light is off and the original current comes back as soon as the light is illuminated

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Ti

e1

e2

e3

e

Light illumination

TiO2 nanotubes

CdS/TiO2particles

Pote

ntia

l vs

SHE

(V)

-1

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-2

n -TiO2

n -CdS

ConductionBand

2.25 eV3.2 eV

ValenceBand

Electron transfer from CdS to TiO2is thermodynamically favorable

ee

ee

ee

Fig. A schematic showing the thermodynamic favorable energy bands for CdS and TiO2. The photogenerated electrons transfer from the CdS CB to the TiO2 and the holes transfer from the CB of the CdSTo the solution.

e1 = photoelectrons generated from the thin film coated on the CdS particles

e2 = photoelectrons generated from the CdS particlese3 = photoelectrons generated from the TiO2 nanotubese (e1 + e2 + e3) = overall photoelectrons generated from the system

Fig. A schematic view to show the effectiveness of core-shell-nanotube approach to harvest sunlight

Photoelectrolysis of water using CdS/TiO2 core/shell photoanode: Tentative mechanism

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Development of UNR easy-H2☺ PEC cell to be used under

solar light irradiation (on-field H2 generation)

Preliminary results indicate that H2 generation from on-field experiments is comparable to the experiments under simulated solar light conditions (AM 1.5)

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Future Work

• Synthesis of heterostructural photoanodes:

Synthesis of mixed metal oxide nanotubes (sputtering-anodization, pulsed electrodeposition-oxidation, electrochemical deposition-anodization).

Synthesis of metal oxide and compound semiconductors (electrodeposition, incipient wetness method and spin rotor coatings)

• Synthesis of low-cost cathodes:

• Preparation of inexpensive and robust cathode by fabricating TiO2 with Ni.

• Investigation of the photoanode and cathode by microstructural and electrochemical techniques.

• Kinetics studies of the titania nanotubes formation by the H2O2 titration and ICP analysis.

• Stability studies of photoanodes by various characterization techniques and Kelvin-Probe measurements.

• Incident photon to current conversion efficiency (IPCE) measurements

• Scale-up the system

• Design PEC system for on-field testing under real solar irradiation.

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Summary

• Relevance: Develop a stable and efficient photoelectrochemical cell for solar hydrogen generation by water splitting.

• Approach: Synthesize hybrid nanotubular TiO2 composite arrays as photoanode and nanoparticles decorated cathodes for improved photo conversion process.

• Technical accomplishments and process: Developed a hybrid composite photoanode comprising of TiO2 nanotubes and CdS core-shell configuration having more than 6% solar-to-hydrogen conversion efficiency under AM 1.5 conditions.

• Technology transfer/collaboration: Active partnership with NREL and University of Arkansas at Little Rock.

• Proposed future research: (a) Mixed oxides (oxides of Fe, Ta and W) and composite photoanodes to harvest full spectrum of sunlight, (b) develop inexpensive cathodes using Ni nanoparticles (c) scale-up the PEC system and (d) on-field testing under real solar irradiation.