A P r o m i s i n g n e w Te c h n o l o g y f o r R e d u c i n g E n e r g y C o n s u m p t i o n

Reversible Electrochemical Mirror (REM) Systems

Andrew Licini, under the guidance of Zachary Detweiler, Matt Vallon and Dr. Steven Bernasek

The Bernasek Research Group(Department of Chemistry)

Specialization: Surface Chemistry and Structure, Heterogeneous Reactions

Introduction to Reversible Electrochemical Mirrors (REMs) and Ionic Liquids

• Windows are important sources of natural light but also greatly compromise the integrity of heating and cooling systems.

• Reversible electrochemical mirrors (REMs) offer a promising alternative to these systems by reversibly depositing metal onto clear substrates to reflect radiant thermal energy

• Most REM systems currently under research use high-boiling organic solvents1 that preclude space applications and present possible environmental hazards.

bis(trifluoromethylsulfonyl)imide (TFSI)

N

N

CH3

NS

O

O

CF3

S OO

CF3

1-butyl-3-methylimidazolium (BMIM)

1.) Source: Araki, S., Nakamura, K., Kobayashi, K. et. al. Electrochemical Optical-Modulation Device with Reversible Transformation between Transparent, Mirror, and Black. Advanced Optical Materials 24 (2012), OP122-OP126.

•The Bernasek Group aims to address this problem by replacing such conventional electrolytes with ionic liquids: organic salts that are liquid at room temperature and yet possess exceedingly low vapor pressures.

Layout of a Reversible Electrochemical Mirror (REM)

-+Apply Potential0

-+

M+

M+

M+

M+

M+

M+

M

M

M

M

M

M

M

M

Light Light

Apply Potential

+-

Indium Tin Oxide (ITO)-Coated Glass

Previous Results: Optical Reflectivity

• Originally planned to study the optical reflectivity of our REM system, but study was halted when preliminary study created poor results.

• Maximum reflectivity only 50% vs. industrial-grade mirror.

• Black residue on surface, can’t be removed with potential.

Electrochemical Study: Silver Deposition Kinetics

-1.5 -1 -0.5 0 0.5 1 1.5

-6.00E-04

-4.00E-04

-2.00E-04

0.00E+00

2.00E-04

4.00E-04

6.00E-04

8.00E-04

1.00E-03

1.20E-03

10-Cycle Cyclic Voltammetry of BMIM TFSI Cell - Silver Deposition onto ITO pane

12345678910

Voltage (V)

Cu

rre

nt

(A)

Cycle #

Overpotential—silver has to overcome barrier to deposit

Stripping current (positive potential) drops rapidly—less silver is removed each time, resulting in an accumulation on the surface.

CONCLUSION:The first time the silver deposits, it creates a seed layer of silver, which more silver deposits onto preferentially over the ITO surface. Surface modification with stable metal atoms like platinum might allow surface to apply more uniformly and remove more reversibly.

SEM Imaging Study: Silver Deposition Kinetics

To right: a secondary electron microscope (SEM) image of silver-deposited ITO slide

• Dendritic growth rather than smooth surface.

• Confirmation of voltammetry results.

• Good news: surface can be altered to alter deposition pattern.

Improvement of Ionic Liquid Optical Purity

BMIM TFSI prepared withoutdichloromethane (DCM) treatments

BMIM TFSI prepared with DCM wash

“Ultra-Pure”-grade BMIM TFSI from manufacturer used as “blank”

Quantitative Purity Analysis: UV/Vis Spectroscopy

200 300 400 500 600 700 800 900 1000 1100

-0.5

0

0.5

1

1.5

2

2.5

3

Absorptivity of BMIM TFSI Samples in the UV/Visible/Near IR Range

Without DCM Wash With DCM Wash

Wavelength (nm)

Ab

so

rba

nce

(A

U)

Earlier ionic liquid without DCM wash has high absorbance in the UV/blue range, making it appear yellow.

Ionic liquid prepared with new DCM wash method has substantially reduced absorbance.

Future Research

• Surface modification of ITO for more even deposition and stripping

• Coupled reaction with copper ions to aid in reversibility of stripping.

• Identification of yellow contaminant removed with dichloromethane with nuclear magnetic resonance (NMR).

• Replication with IR-transparent substrates, like aluminum oxide.

The Princeton Summer Research Experience!

Special Thanks

Many thanks are required for:

• Dr. Steven Bernasek, (the newly) Dr. (!!!) Zachary Detweiler, and Matthew Vallon for their advice, supervision, and support throughout my research (as well as the entire Bernasek and Bocarsly groups in general).

• Our collaborators on the Reversible Electrochemical Mirror project at Edwards Air Force Base and Kirkenland, who provided excellent input, advice and suggestions.

• The United States Air Force, for funding the project at large, and, most importantly…

• The Princeton Environmental Institute, for giving me the funds and opportunity to participate in this amazing research experience!