Studies on Direct Methanol Fuel Cell: An electro-chemical energy conversion device

Jay Pandey

Research Scholar

Department of Chemical Engineering

Indian Institute of Technology Delhi, New Delhi

Outline

Introduction

Objectives

Experimental details

Membrane characterization

DMFC performance

Conclusions

Fuel Cell

Electrochemical device which converts chemical

energy into electrical energy

Invented by W.R.Groove, 1839 Introduced the IEMs in FCs (1963, J.W.Niedrach)

Fuel cell type Op. Temp. (oC)

Transported ion

Membrane used Power density mW/cm2

Fuel cell efficiency

Polymer electrolyte membrane fuel cell (PEMFC)

50-80 H+ Polymeric membrane 350 45-60

Alkaline fuel cell (AFC) 60-90 OH- Aqueous alkaline solution

100-200 40-60

Phosphoric acid fuel cell (AFC)

150-200 H+ Molten phosphoric acid 200 55

Molten carbonate fuel cell (MCFC)

600-700 CO32- Molten alkaline

carbonate

100 60-65

Solid oxide fuel cell (SOFC) 800-1000 O2- Ceramics 240 55-65

Int. J. Hydrogen Energy, 35, 2010, 9349-9384

Direct Methanol Fuel Cell (DMFC)

Sub-category of PEMFC

Fuel at anode: Methanol ; Oxidant at cathode: Oxygen

Membrane used: Proton exchange membrane (PEM)

Operating temperature: 50-1200C

Power density: 240 mW/cm2

Fuel cell efficiency: ~60%

Power output: 0.1 – 15W

Contd.....

Why methanol is preferred over hydrogen fuel ?

Energy density: Methanol: 4.8 Wh/cm3

Hydrogen: 2.7 Wh/cm3

Easy transportation and handling Readily available, relatively lesser cost Stable at all atmospheric conditions

(Silva et al, 2005)

Electrochemical reactions involved in DMFC

Anodic reaction(Oxidation): 0.03 V

CH3OH + H2O CO2 + 6H + + 6e-

Cathodic reaction (Reduction): 1.22 V3/2 O2 + 6H+ + 6e- 3H2O

Overall reaction: 1.19 VCH3OH + 3/2 O2 CO2 + 2H2O

(Silva at al. 2005)

Applications of DMFC

All kinds of portable, automotive and mobile applications like,

• Powering laptop, computers, cellular phones, digital cameras

• Fuel cell vehicles (FCVs)

• Spacecraft applications

• Any consumables which require long lasting power compare to Li-ion batteries

(Dyre et al., 2002)

Objectives

Synthesis of proton conductive PWA membrane for potential application in

DMFC

Physico-chemical characterization of membrane in order to characterize the

surface morphology, phase identification, intermolecular bonding, thermal

stability of the membrane

Electrochemical characterization of the membrane to analyze the

electrochemical behavior of membrane such as specific conductivity,

transport number, areal resistance of the membrane

Study of the DMFC performance using synthesized PWA membrane

Synthesis protocol of PWA membrane

PWA membrane

Physico-chemical characterization

FT-IR spectra of PWA membrane

XRD patterns of PWA membrane

FT-IR spectra confirms the stable intermolecular interaction between silica and tungustate ions.

Silanol ion peak ~1532 cm-1

Tungstate ion peak~1079, 984, 828, 815 cm-1

XRD patterns show the presence of silica and phosphotungustic acid in the membrane even after the heat treatment up to 150oC for 2 h.

PWA peak

Silica Peak

SEM analysis of membrane

SEM images of PWA membrane

SEM images of graphite support

The SEM images show the surface uniformity as well as proper dispersion of active sol (PWA and TEOS) on graphite support.

Electrochemical characterization

Membrane potential and transport number measurements

Experimental specifications

Volume of each compartment

27 cm3

Concentration of NaCl 0.1 M/0.01 M

Maximum cell voltage 0.118 V

Photographic image of diffusion cell

EIS specifications

Frequency range 1Hz- 1 MHz

AC voltage 5 mV

Area of membrane 12.56 cm2

Concentration of NaCl in both the compartments

0.5 M

Specific conductivity (S/cm) measurements

Nyquist Plot for resistance measurement

Nyquist plot

Membrane potential and transport number

*As the PWA/TEOS ratio is increased the transport as well as the membrane potential is increased significantly due to increase in the surface charge density of the synthesized membrane

Specific conductivity and water uptake

As the wt% of PWA was increased specific conductivity was also found to be increased i.e. more ionic conduction occurred through the PWA membrane.

Maximum value of water uptake was found around 30% for 1 molar ratio of PWA and TEOS. It indicates that membranes has high hydration content at higher wt% of PWA that will result into high proton conduction.

Fig. 1: Variation of specific conductivity with molar ratio of PWA and TEOS

Fig. 2: Variation of water uptake with molar ratio of PWA and TEOS

Experimental Setup for DMFC

DMFC performance

0.5 PWA/TEOSPower density= 29 mW/cm2

OCV= 0.65 V

1.5 PWA/TEOSPower density= 35 mW/cm2

OCV= 0.75 V

Experimental specifications:Cell temperature= 25oCMeOH flow rate= 5 ml/minOxygen flow rate= 100 ml/min

*It can be inferred that 1.5 PWA/TEOS has better DMFC performance than 0.5 PWA/TEOS membrane, mainly due to high proton conductivity of membrane for 1.5 PWA/TEOS

Conclusions

• The PWA membrane was synthesized using sol-gel method followed by solution casting on graphite support

• The highest obtained value of transport number was 0.90 for the synthesized PWA membrane

• Higher value of transport number indicates that maximum current is being carried across the membrane

• The maximum value of specific conductivity was found 5 mScm-1 at room temperature (32oC)

• Proton conductivity for inorganic membranes being used in DMFC is in the range of 5-14 mScm-1

• Maximum obtained power density was 35 mW/cm2 for 1.5 PWA/TEOS, and OCV was 0.75 V

• Synthesized PWA membrane has the potential for wide applications in DMFC

• The membrane properties can be further improved by changing the synthesis protocol or final treatment methods

References

S.K., Kamarudin, F., Achmad, W.R.W., Daud. Overview on application of direct methanol fuel

cell (DMFC) for portable electronic devices. Int. J. Hydrogen Energy, 34, 6902-6916. 2009.

U.S.D., Energy. Fuel cell handbook. Science Applications International Corporation E&G

Services, 5th ed., Parson Inc., 2000.

R., O’Hayre, S.W., Cha. Fuel cell fundamentals. Wiley, 113, 267-268, 2007.

S.Q., Song, W.J., Jhou, W.J., Li. Direct methanol fuel cells: Methanol crossover and its

influence on single DMFC performance. Solid State Ionic, 10, 458-462. 2004.

Z.G., Shao, P., Joghee, I.M., Hsing. Preparation and characterization of hybrid Nafion-silica

membrane doped with phosphotungustic acid for high temperature operation of PEMFC. J.

Membr. Sci. 229, 43–51, 2004.