Diffusion Through PTFE

Experimental Study of Diffusion of Carbon Dioxide, Methanol and Formic acid through PTFE Membrane.

Salman Zafar

Content

• Objectives

• Literature Survey

• Experimental Setup

• Results and Comparisons

• Summary

Indian Institute of Technology Roorkee -- 2

Objectives

• Better understanding of the transport of selected species

through the PTFE membrane using DEMS

• Studying the Effect of change in Pore size of the membrane

• Studying the Effect of movement of solution in the cell

• Studying the Effect of temperature change

• Studying the Effect of presence of Catalyst


Content

• Objectives




• Summary


Literature Survey

• Ashton, S., J. (2011)

The Design, Construction and Research Application of a

Differential Electrochemical Mass Spectrometer (DEMS),

PhD Dissertation TU Munich.

The work deals with the design, construction and characterization of DEMS

which has emerged as an important tool for the analysis of the half-cell

studies in the fuel cells and other electro-chemical areas of application. The

study uses the DEMS to elucidate the chemical reactions that occur while

the oxidation of methanol on carbon based Pt-Ru catalyst. And the

corrosion of the high surface area carbon based catalysts. The study

presents a detailed discussion about the various operational features of

DEMS


Literature Survey

• Lindermeir, A., Rosenthal, G., Kunz, U. (2004)

On the question of MEA preparation for DMFCs. J. Power

Sourc., Vol. 129(2), pp. 180-187

The study presents two methods of preparation of the membrane electrode

assembly. In the first method the preparation started with the carbon

backing, then the substrate along with some PTFE is added to it, followed

by spraying of the catalyst mixture. This is then combined with the

membrane to give the MEA. In the second method the catalyst mix is

applied directly to the membrane. The assemblies from the second method

were found to be better due to a greater availability of active sites of the

catalyst.


Literature Survey

• Cappiello, A., Famiglini, G., Palma, P. (2003)

Peer reviewed: electron ionization for LC/MS. Anal. Chem.,

Vol. 75, pp. 496A–503A

The study highlights the use of Teflon membranes in DEMS studies. The

Teflon membrane being hydrophobic in nature is able the separate out the

aqueous medium from going into vacuum, resulting in the ionic species

being the majority of what goes into the DEMS. The output data from the

DEMS may be converted to quantitative data by calibrating the ion current

to actual mass or actual quantity of a specific ion or species.


Literature Survey

• Sundmacher K. (1999)

Cyclone flow cell for the investigation of gas-diffusion

electrodes, App. Eletroche., Vol. 3 (29), pp. 919-926

The study throws light on a new design for the electrochemical half cell

studies that is well suited for the Gas diffusion Electrodes. The design

earlier used for the study of surface electrochemical reactions was the

rotating disc electrode, but this is not suited well with GDE’s. The cylcone

flow cell suggested by Sundmacher, makes use of cyclone vortex flow over

a stationary electrode surface that is diffusive in nature to the gas phase.


Literature Survey

• Wang, H., Rus, E., Abruna, H.D. (2010)

Letters to Analytical Chemistry New Double-Band-

Electrode Channel Flow Differential Electrochemical Mass

Spectrometry Cell: Application for Detecting Product

Formation during Methanol Electro-oxidation. Anal. Chem.,

Vol. 82, pp. 4319–4324

The study shows the behaviour of mass spectrometer ion current with change in flow

rate of the solution over the electrode surface. The mass current is plotted as a

function of the flow rate for carbon dioxide, methanol and other species. The trends

given in this study were observed to be similar to one found in experiments.


Content

• Objectives




• Summary


System Description


Apparatus

Cyclone Flow Cell

Membrane

PTFE Membranes

0.2μm and 0.45μm

Mass Spectrometer

Vacuum Pumps

Vacuum Pump 1 80 l.s-1

Vacuum Pump 2 200 l.s-1

Solution Pump

Peristaltic Pump

Operation Range- 24RPM(27.3 ml/min) to 600RPM(579ml/min)


Experiments

Carbon Dioxide 1. De-ionized water bubbled with Nitrogen for 15min

2. Then bubbled with CO2 for 30min

3. Cell and connections set up, vacuum pumps started (with vacuum valve

closed).

4. Solution bottle bubbled and stirred continuously

5. Cell filled with solution

6. Vacuum valve is opened to evacuate the space between membrane and the

valve, valve closed again.

7. Pump is started and cell fills with ‘fresh’ solution

8. Pump is closed, and the valve is opened (1min Delay)

9. Record the diffusion curve and close the valve

10. Repeat from step 7 again


CO2 Diffusion Measurement


Experiments

Methanol and Formic Acid

1. De-ionized water bubbled with Nitrogen for 15min

2. 0.5M Solution Made

3. Cell and connections set up, vacuum pumps started (with vacuum valve

closed).

4. Cell filled with solution

5. Vacuum valve is opened to evacuate the space between membrane and the

valve, valve closed again.

6. Pump is started and cell fills with ‘fresh’ solution

7. Pump is closed, and the valve is opened (1min Delay)

8. Record the diffusion curve and close the valve

9. Repeat from step 6 again


Content

• Objectives




• Summary


Results

Carbon Dioxide – Diffusion Through 0.2μm Membrane

Almost similar transport – no significant effect of catalyst presence.


Results

Carbon Dioxide – Diffusion Through 0.45μm Membrane

Presence of catalyst slows down the transport


Results

Carbon Dioxide – Diffusion Membrane Comparison

Transport through 0.2μm membrane is slower


Results

Carbon Dioxide – Diffusion at different temperatures

No significant effect on the diffusion curve


Results

Carbon Dioxide – Diffusion with varying degree of solution convection

No significant effect on the diffusion curve


Results

Methanol – Diffusion Through 0.2μm Membrane



Results

Methanol – Diffusion Through 0.45μm Membrane



Results

Methanol – Membrane Comparison

Greater and faster diffusion for bigger pore size


Results

Methanol – Effect of convection on Mass signal

The shape of the Diffusion Curve remains the same


Results

Formic Acid – Diffusion Through 0.2m Membrane

Presence of catalyst slows down the transport significantly


Results

Formic Acid – Diffusion Through 0.45μm Membrane

Presence of catalyst slows down the transport significantly


Results

Formic Acid – Membrane Comparison



Results




Results




Results – Conformation with Earlier Study

New Double-Band-Electrode

Channel Flow Differential

Electrochemical Mass

Spectrometry Cell: Application for

Detecting Product Formation

during Methanol Electro-oxidation

Wang, H., Rus, E., Abruna, H.,D.

(Analytical Chemistry, Vol. 82, No.

11, June 1, 2010)



Carbon Dioxide - Hydrodynamic Behaviour of MS signal



Methanol - Hydrodynamic Behaviour of MS signal



Formic Acid - Hydrodynamic Behaviour of MS signal


Content

• Objectives




• Summary and Further Developments


Summary

• The flux for non polar species (CO2) is higher compared to polar species

(Methanol and formic acid).

• The flux and rate of diffusion for 0.45μm membrane is greater than 0.2 μm

membrane.

• The presence a catalyst has a little effect on transport for 0.2 μm membrane

in case of CO2 as compared to methanol and formic acid .

• The speed of pump has no significant effect on the diffusion curve for all the

species.

• The temperature dependence is not profound over range of 10 °C

• Hydrodynamic behaviour of the species in the system was in conformation

with an earlier study.


Further Developments

• The data of the MS may be used

quantitatively and be compared

with a model if the system is

calibrated for the three species

giving a calibration constant that

would relate the mass currents to

actual mass flow rates.

• The diffusion curve can be

studied in an exact manner when

the dead space between the

membrane and the vacuum pump

can be removed some how.


References

• Sundmacher K. (1999)

Cyclone flow cell for the investigation of gas-diffusion electrodes, App.

Eletroche., Vol. 3 (29), pp. 919-926

• Lindermeir, A., Rosenthal, G., Kunz, U. (2004)

On the question of MEA preparation for DMFCs. J. Power Sourc., Vol.

129(2), pp. 180-187

• Cappiello, A., Famiglini, G., Palma, P. (2003)

Peer reviewed: electron ionization for LC/MS. Anal. Chem., Vol. 75, pp.

496A–503A

• Ashton, S., J. (2011)

The Design, Construction and Research Application of a Differential

Electrochemical Mass Spectrometer (DEMS), PhD Dissertation TU

Munich.


References

• Wang, H., Abruña, H.D. (2011)

Electrocatalysis of direct alcohol fuel cells: quantitative DEMS studies.

Struct. Bond., Vol. 141, pp. 33–83

• Planes, G.A., García, G., Pastor, E. (2007)

High performance mesoporous Pt electrode for methanol

electrooxidation. A DEMS study. Electrochem. Commun. Vol. 9, pp. 839–

844

• Wolter, O., Heitbaum, J. (1984)

Differential Electrochemical Mass Spectroscopy (DEMS) — a New

Method for the Study of Electrode Processes. Ber. Bunsenges. Phys.

Chem., Vol. 88, pp. 2– 6


References

• Munk, J., Christensen, P.A., Hamnett, A., Skou, E. (1996)

The electrochemical oxidation of methanol on platinum and platinum +

ruthenium particulate electrodes studied by in-situ FTIR spectroscopy

and electrochemical mass spectrometry. J. Electroanal. Chem., Vol. 401,

pp. 215-222

• Wang, H., Rus, E., Abruna, H.D. (2010)

Letters to Analytical Chemistry New Double-Band-Electrode Channel

Flow Differential Electrochemical Mass Spectrometry Cell: Application

for Detecting Product Formation during Methanol Electro-oxidation. Anal.

Chem., Vol. 82, pp. 4319–4324

---xxx---


Thank You!


Extra - Apparatus


Extra

• Dimensions of the Cell (mm)


Extra

• The Methanol Run curve


Extra - Run Comparisons


Extra - Catalyst Coating


• The nanoparticles are mixed with distilled water and isopropanol and then with nafion solution which acts as a binder.

• The mixture is then sonicated and it ensures evenly dispersed particles.

• The suspension is then directly sprayed on the PTFE membrane.

• The catalyst loading is then directly calculated by the difference in the weights of the naked membrane and the coated membrane.

MATLAB Model for Carbon Dioxide

Assumptions

• Diffusion is purely Diffusive in nature.

• Concentration gradient across the

membrane is linear

• Tortuosity of the membrane is ≈ 1

• Initially the concentration of CO2 is

uniform (At saturation)

• The Diffusion is one dimensional


MATLAB Model

Development of the Model

Mass Balance Equation (Unsteady State)

Finite Difference method was used to define the derivates, and equation were written for 1st, i th and last

element and the membrane and solved simultaneously


MATLAB Results


Documents

Diffusion Through PTFE