Proton Exchange Membrane Fuel Cells (PEMFCs) Cell Technology Proton Exchange Membrane Fuel Cells (PEMFCs) Docent Jinliang Yuan November, 2008 Department of Energy Sciences Lund Institute

Fuel Cell Technology

Proton Exchange Membrane Fuel Cells (PEMFCs)

Docent

Jinliang YuanNovember, 2008

Department of Energy Sciences Lund Institute of Technology (LTH), Sweden


Four PEMFC stacks illustrating developments through the 1990s. The 1989model on the left has a power density of 0.1kWL−1. The 1996 model on the

right has a power density of 1.1kWL−1.


•The PEMFC developments have reached the current densities up to around1Acm−2

or more,

•While at the same time reducing the use of platinum by a factor of over 100 (from 28

mg/cm2

to 0.2

mg/cm2

or less).

•These improvements have led to huge reduction in cost per kilowatt of power, and much improved power density (0.1

to

over 1 kW/L).

•For various applications, two aspects are very similar: the electrolyte used, and the electrode structure and catalyst (both not discussed extensively during the lecturing sessions)

Current Statues


Water management – a vital topic for PEMFCs.

The method of cooling the cell and stack.

The method of connecting cells in series. The bipolar plate designs vary greatly.

At what pressure to operate the PEMFCs?

PEMFC Special Issues


• they are chemically resistant,

•

they are strong (mechanically), and so can be made into very thin films, down to 50μm,

•

they are acidic, they can absorb large quantities of water,

•

if they are well hydrated, the H+

ions can move quite freely within the material –

they are good proton conductors, 0.1

Scm−1, but the water content falls, the conductivity falls in a more or less linear fashion.

Main features of Nafion and other fluorosulphonate

ionomers

The structure of Nafion-type membrane materials.

Fuel Cell TechnologyThe structure of carbon-

supported catalyst

Electrodes

Catalyst layer

Diffusion layer (not a good name)

•gas diffusion (transport?),

•current

collecting,

•water removing,

•physical

support.

The structure of a PEMFC electrode (Carbon paper or carbon cloth material)


PEMFC Water Management

Water content should

be well

balanced, Why

so critical?•too

dry, less proton

conductivity,

•too

wet, water flood and blocking

gas.


Several Complications

•Electro-osmotic drag

-the H+

ions moving from the anode to the cathode pull water molecules with them. Typically between one and five

water molecules are

dragged fro each proton. It becomes more critical at high current densities…

•Another major problem is the drying effect of air at high temperatures.

Solution

to solve these problems is to humidify the air, the hydrogen, or both, before they enter the fuel cell, i.e., add a by-product to the inputs…


Air Flows

•Air flows

supplied

at a rate

faster than

that needed and removes

the generated

water in the cathods.

•Air flows

supplied

at a rate

faster than

that needed (stoichiometric

rate) and removes

the generated

water in the cathods.

•Non-linear

relationship

between

the dry

effect

of air and temperature,which

is based

on the relative

humidity, water content

and saturated

vapour pressure, etc.


Important Variables

•Humidity ratio

between mw

the mass of water present in the mixture and ma

the mass of dry air. The total mass of the air is mw

+ ma

.

•Relative humidity between Pw

the partial pressure of the water and Psat

the saturated vapour

pressure of the water. It gives a correct feeling of drying effect

or not, e.g., fully

humidified air means that air is unable to hold any more water…


•The saturated vapour

pressure varies with temperature in a highly non-linear way –

Psat

increases more rapidly at higher temperatures.

For air at 20◦C, relative humidity 70%, to be heated to 60◦C at constant pressure without adding water, so the new relative humidity is only 8%


Humidity of PEMFC Air

The humidity of the exit air of a fuel cell

Pressure ratio or molar fraction based on the number of moles of species leaving the cell per second

λ, the air stoichiometry, is 2 when T=70oC. E.g., 67% is too dry..

Fuel Cell TechnologyRelative humidity versus temperature for the exit air with air stoichiometry

of 2 and 4. The entry air is dry, and the total pressure is1bar.


•At temperatures above about 60◦C, the relative humidity of the exit air is below or well below 100% at all reasonable values of stoichiometry

•Extra humidification of the reactant gases is essential in PEM fuel cells operating at above about 60◦C

Very critical to optimize operating temperatures, i.e., a higher temperature gives a better performance, mainly because the cathode overvoltage reduces. However, once over 60◦C the humidification problems increase, and the extra weight and cost of the humidification equipment can exceed thesavings coming from a smaller and lighter fuel cell.


Contra flow of reactant gases to spread

humidification.

•Running PEMFCs

without

external humidification is to set the air stoichiometry

so that the relative humidity of the exit air is

about 100% and to ensure that the cell design enables water balanced within the cell.

•Air and hydrogen flow in opposite directions across the MEA.•The water flow from anode to cathode is the same in all parts, (electro-osmotic drag). •The back diffusion from cathode to anode varies, but is compensated for by the gas circulation.


External HumidificationExample: PEMFC operating at 90◦C, inlet pressure of 220 kPa

and exit pressure of 200 kPa, typical air stoichiometry

is 2.

Case 1. inlet ‘normal air’

at 20◦C and 70% relative humidity.

Case 2. inlet air at 80◦C and 90% relative humidity.


Fuel Cell Thermal Management

• The electrochemical reactions are exothermic, i.e., the heat is generated together with the contribution of the ohmic resistance;

• For instance, excessive heat generation may result in dehydration of membrane, and in such case, decreased conductivities and thermal stresses are expected, even mechanical failure of fuel cell components.

• Heat removal and proper thermal management are critical design and operating issues in fuel cells;

• Local temperature distribution has significant effects on the PEMFC saturation pressure/phase change.


PEMFC Heating Generated•How much heat generated?

In converting the hydrogen energy into electricity, efficiencies are normally about 50%, this means that a fuel cell of power X watts will also have to dispose of about X watts of heat.

•Small Temperaturer difference?

The driving

force

of the heat rejection

from the radiator is naturally

small because

of the low

operating

temperature

(80oC in PEMFC vs. 120 oC in ICE).


PEMFC Cooling-Overall

Power Range

Cooling Method

Remarks

Below 100W

Combined reactant

and

cooling

gas

No extra component, too

dry

if

λ

is too

big.100W-1kW Extra

channelsSeperate

reactant

and cooling

airAbove

1kW Extra

channelsWater cooling


Best Case: assume power Pe

Watts, operating at 50◦C, 40% of the heat removed by air, specific heat capacity cp

flowing at a rate of m

kg/s, and subject to a temperature change (50-20)oC.


Three cells stack, with the bipolar

plate modified for air cooling

using separate

reactant and cooling air.


•Air cooling is simpler, but it becomes harder to cool it to a similar temperature, as it gets larger.

•The air channels make the fuel cell stack larger than it needs to be

•The need of PEMFC water cool is greater than with a petrol engine, as the fuel cell performance is more affected by variation in temperature.

Why/When Water Cooling Needed?

PEMFC stack with several

kWe

needs

water cooling…

Documents

Proton Exchange Membrane Fuel Cells (PEMFCs) Cell Technology Proton Exchange Membrane Fuel Cells (PEMFCs) Docent Jinliang Yuan November, 2008 Department of Energy Sciences Lund Institute