Capillary-driven pumping for passive degassing and fuel supply in direct methanol fuel cells

  • Nils Paust
  • Christian Litterst
  • Tobias Metz
  • Michael Eck
  • Christoph Ziegler
  • Roland Zengerle
  • Peter Koltay
Research Paper

Abstract

In this paper we present a new concept of creating and using capillary pressure gradients for passive degassing and passive methanol supply in direct methanol fuel cells (DMFCs). An anode flow field consisting of parallel tapered channels structures is applied to achieve the passive supply mechanism. The flow is propelled by the surface forces of deformed CO2 bubbles, generated as a reaction product during DMFC operation. This work focuses on studying the influence of channel geometry and surface properties on the capillary-induced liquid flow rates at various bubbly gas flow rates. Besides the aspect ratios and opening angles of the tapered channels, the static contact angle as well as the effect of contact angle hysteresis has been identified to significantly influence the liquid flow rates induced by capillary forces at the bubble menisci. Applying the novel concept, we show that the liquid flow rates are up to thirteen times higher than the methanol oxidation reaction on the anode requires. Experimental results are presented that demonstrate the continuous passive operation of a DMFC for more than 15 h.

Keywords

Capillary pump Tapered channels Contact angle hysteresis Passive direct methanol fuel cells 

List of symbols

AMEA

Chemical active area of the DMFC (cm2)

Cf

Concentration of the aqueous methanol water solution (m l−1)

F

Faraday constant (F = 96485°C mol−1)

g

Ground acceleration (9.81 m s−1)

h

Channel height at the channel inlet (mm)

H

Channel height at the inlet of the double tapered structure (mm)

I

Electric current (mA)

i

Electric current density (mA cm−2)

l

Length of the channel (mm)

Lbub

Length of a gas bubble (mm)

\({\dot{m}}\)

Mass flow (kg s−1)

\( M_{{{\text{CO}}_{2} }} \)

Molar weight of CO2 (\( M_{{{\text{CO}}_{2} }} \) = 44 g mol−1)

\( M_{{{\text{CH}}_{ 3} {\text{OH}}}} \)

Molar weight of CH3OH (\( M_{{{\text{CH}}_{ 3} {\text{OH}}}} \) = 32 g mol−1)

p

Pressure (Pa)

Δpbub

Pressure difference over a deformed gas bubble in a tapered channel (Pa)

peff

Pump efficiency defined as the ratio between the liquid flow rate induced by the moving bubbles to the bubbly gas flow rate (−)

P

Electric power (mW)

r

Radius of a liquid gas interface (mm)

V

Volume of the reservoir filling (ml)

w

Channel width (mm)

W

Channel width of the centre channel of the double-tapered channel (mm)

x

Horizontal coordinate

xf

x position of bubble front meniscus (mm)

xb

x position of bubble back meniscus (mm)

y

Vertical coordinate

α

Opening angle of the tapered channel (°)

β

Opening angle of the side channels of the double-tapered channel (°)

θ

Contact angle (°)

θhys

Contact angle hysteresis (°)

θad

Advancing contact angle (°)

θrec

Receding contact angle (°)

θpininlet

Contact angle caused by pinning at the gas inlet (°)

Φ

Flow rate (μl min−1)

κ

Curvature of the liquid/gas interface (mm−1)

ρ

Density (at ambient conditions: methanol solution 4 M: ρl = 968 kg m−3; gas: \( \rho_{{{\text{CO}}_{2} }} \) = 1.78 kg m−3)

σ

Surface tension (methanol solution 4 M: 0.053 N m−1)

Notes

Acknowledgments

This work was supported by the German Federal Ministry of Education and Research within Project 03SF0311B, the German Federal Ministry of Economics and Labour (BMWA) within the VDI/VDE InnoNet-program PlanarFC and the German Research Council within Project 527/3.

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Copyright information

© Springer-Verlag 2009

Authors and Affiliations

  • Nils Paust
    • 1
  • Christian Litterst
    • 1
  • Tobias Metz
    • 1
  • Michael Eck
    • 1
  • Christoph Ziegler
    • 1
  • Roland Zengerle
    • 1
  • Peter Koltay
    • 1
  1. 1.Laboratory for MEMS Applications, Department of Microsystems Engineering (IMTEK)University of FreiburgFreiburgGermany

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