Analysis of droplet dynamics on the GDL surface of a PEM fuel cell cathode
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hdl:2099.1/26169
Document typeMaster thesis
Date2013-06-28
Rights accessOpen Access
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Attribution-NonCommercial-NoDerivs 3.0 Spain
Abstract
A fuel cell is an electrochemical device that has the ability to turn the chemical
energy in a fuel directly into electricity with high e ciency. Inside the fuel cell,
oxidation and reduction electrochemical reactions take place producing low-voltage
current (DC) and heat. The former is used to do useful work while the latter is
wasted or can be used in cogeneration applications. Fuel cells are usually compared
with other energy convertors, like reciprocating engines or batteries. Batteries and
fuel cells have the same operating principle, based on the electrochemical reactions
at the anode and the cathode. The main di erence being that batteries store the
reactant inside the cell instead of in a separate storage tank.
There are di erent types of fuel cells depending on the materials used in the
electrolytes, the substances that react in the anode and the cathode or the working
temperature. The current work focuses on Polymer Electrolyte Membrane (PEM)
fuel cells, which can work between -40 and 100 C and use hydrogen as fuel, oxygen
as cathode reactant and Na on® as the electrolyte (Figure 1.1).
The working principle of the PEM fuel cell is based on two electrochemical
reactions. The process starts at the anode, where the hydrogen
ows in the anode
Gas Flow Channel (GFC) and di uses through the pores in the Gas Di usion Layer
(GDL). Attached to the GDL is the Catalyst Layer (CL). The CL is made using
a platinum-based ink which is painted on either the PEM or the GDL. The ink
contains carbon, Pt and electrolyte. The resulting coating is a thin (about 10 m)
porous layer. The Pt catalyses the rst reaction: the hydrogen oxidation reaction.H2 ! 2H+ + 2e- (1.1)
After this reaction, the next layer is the membrane which is made of Na on®.
The membrane allows the protons to travel across its section but it is impermeable
for the electrons and gases. The electrons have to go back through the GDL and
the current collector (that act as the walls of the anode gas
ow channel) in order
to meet the protons at the other side of the membrane, thus generating the desired
electric current. In the cathode, the oxygen
ows in the cathode gas channels,
di uses through the GDL and in the catalyst layer reacts with the protons from the
membrane, performing the second reaction:
2H+ + 2e- +
1
2
O2 ! 2H2OThe union of the anode GDL and CL, membrane and cathode CL and GDL is also
known as Membrane Electrode Assembly (MEA). When fuelled with hydrogen, the
fuel cell has zero emissions, since the only product of the electrochemical reaction
is water and heat. The water generated in the reaction is one of the key factors
in
uencing the fuel cell performance. The membrane needs water in order to conduct
the protons; if there is not enough water, the membrane dries out and the fuel cell
cannot work any longer. Alternatively, if there is too much water, the pores in the
CL and GDL
ood preventing the reactant gases from di using through it. The
exceeding water has therefore to be evacuated through the cathode gas channels.
This is the starting point of the present work.
There are three types of two-phase
ow in the gas channels depending on di erent
factors. Their names change depending on the author, however, these
ows are
usually known as droplet, lm and slug
ow [2], as shown on Figure 1.2.
The present work focuses on droplet
ow and the conditions that lead to lm
and slug
ow. Since it is very di cult to develop an analytical equation for the
shape of a water lm, the best way to proceed is the analysis of a single static and
deformed droplet and then identify the conditions that lead to lm and slug
ow
formation. In addition, the area coverage of the formed droplets is another variable
that needs to be considered, and this variable takes values from 0 to 1 only when
the
ow is identi ed as droplet
ow.
DegreeMÀSTER UNIVERSITARI EN MÈTODES NUMÈRICS EN ENGINYERIA (Pla 2012)
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