Novel Wide Area Hydrogen Sensing Technology

W. Hoagland

D. K. Benson

R. D. Smith

Outline

New developmental hydrogen gas indicators

Technologies Response kinetics Possible applications

What are Chemochromic H2 Detectors?

Chemochromic hydrogen detectors change color when exposed to gaseous hydrogen. This color change is caused by a chemical reaction between hydrogen and the active material. It may be reversible or non-reversible.

Reversible partial reductionof tungsten trioxide

Coloration

1. H2 + 2(Pd) 2(Pd.H)

2. 2(Pd.H) + 2(WO3) 2(W+5O2.OH) + 2(Pd)

Bleaching

3. 2(W+5O2.OH) + 1/2O2 2WO3 + H2O

ChemochromicChemical Reactions

Optical absorption by electrons localized at oxide vacancies

Electrons are localized in the vicinity of W5+ ions as small polarons

hv + W5+(A) + W6+(B) W6+(A) + W5+(B)

Technology addresses a recognized need

“Although safety-by-design and passive mitigation systems are preferred, it will still be necessary to develop technologies to detect hydrogen releases or other system failures.For example, coatings that change color upon exposure to hydrogen can provide immediate visual evidence of a leak.”

U.S. DOE Multi-year Plan 2003-2010

Critical Research Topic

“The overall goal is to develop low-cost sensor technologies that are not based on conventional practices, that can be directly integrated with hydrogen systems, and that are resistant to contamination.”

…Goal #3: Develop a sensing technology for a wide-area determination of hydrogen presence prior to any combustion or local temperature rise.”

U.S. DOE - February 2004

Advantages of Chemochromic Hydrogen Indicators

Reversible color change Inherently safe – no ignition source Convenient indication from a distance No instrumentation needed Applicable in various forms

Paints, inks, coatings Tape/Stretch Films Conformable Wrap Decals

Very low cost

Typical structures of chemochromic indicator material

Substrate polymer sheet

Tungsten oxide 500 nm

Palladium 3 nm

PTFE 100 nm Vacuum depositedmulti-layerthin film

Nano-powderTungsten oxide (~50 nm)

PlatinizedChemically (~5%)

Chemically synthesizednano-particles

Thin film indicator responds quickly and reversibly

Thin Film Indicator

Could be used for lettering on safety decals

No hydrogen present

Hydrogen present

Flexible Plastic Indicating Tape

Could be used to wrap around pipe fittings

Shrink wrap indicator appliedto a low pressure cylinder with an intentional 50 micron hole

Nano-particles could be used in “smart paints” or printing inks

• Epoxy (100% solids)

• Polyurethane

• Acrylic (Solvent based)

• Acrylic (Water Based)

Paint formulations shown to work include:

Indicators turn dark blue when exposed to hydrogen

Optical Absorption

0

20

40

60

80

500 600 700 800 900 1000

Wavelength (nm)

Ab

so

rpti

on

(%

)

Transmittance at 830 nm

0

50

100

0 100 200 300

Time (s)

T (

%)

Response of Chemochromicindicator is exponential

– thin film

Switch to air

H2

Response mechanism

Double exponential decay in transmittance is characteristic of two parallel first-order chemical reactions.

Speculation: The reactions occur at different kinds of catalytic sites -

At the catalyst metal/tungsten oxide interface At catalyst metal/tungsten oxide/gas interface

Response of Chemochromic– nano-particles

Nano-Particles

0

20

40

60

80

100

0 50 100

Time (s)

Re

lati

ve

Tra

ns

mit

tan

ce

(%

)

Response of a Chemochromic– PVA water-base

“Smart Paint”

40

50

60

70

80

90

100

0 200 400 600 800

Time (s)

Rel

ativ

e T

ran

smit

tan

ce (

%)

tau1= 97 s; 100%

Thin film indication threshold in air is less than 400 ppm H2

0

0.005

0.01

0 500 1000 1500

Hydrogen Conc. (ppm)

Init

ial

Slo

pe

of

res

po

ns

e,

(-%

/s)

Chemochromic indicators respond more quickly at higher temperatures

1

10

100

1000

10000

-50 -30 -10 10 30

Temperature (C)

Tim

e c

on

sta

nt

tau

1 (

s)

A protective coating reduces the effect of humidity

02468

0 20 40 60 80 100

Relative Humidity (%)

Tim

e c

on

sta

nt

(s)

PTFE coating

No coating

Indicator response slows over time

05

10152025303540

0 20 40 60

Exposure (days)

tau

1 (

s)

Different film structures reduce the rate of response slowing

0102030

0 20 40 60 80

Exposure (days)

tau

1 (

s)

NASA/KSC Project

H2 Dispensing Station

H2 Dispensing Station

H2 Dispensing Station

RFID H2 Sensor Network

RFID Hydrogen Sensors

RFID Response

On-going R&D Activities

Optimize nano-particle pigment Catalyst choice Pd, Pt, or mixed Catalyst concentration Possible protective coatings

Select suitable paint vehicles Optimize paint formulations for various applications Field test prototype indicators in realistic

environments Optimize thin film coating for polymer substrates

Optimize film component proportions Improve protective coatings to stabilize response

over longer time

Range of Products

Conclusions

Present indicators are promising for leak detection in many applications

New designs are being developed with slower rates of degradation

Performance parameters can be modified for specific applications and requirements

Partnerships

Actively seeking industry partnersStrategicTestingManufacturing

Thank you!

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