DMFC Fuel Cells

Question 1

What are direct/ liquid fuel cells?

Answer 1

If the fuel does not need to be processed before entering the fuel cell since the electrocatalyst of the fuel cell disintegrates that fuel, it is called a direct fuel cell, provided it is any other fuel than hydrogen. Mostly, but not necessarily these fuels are liquid.

Question 2

Advantages of Direct/ Liquid FCs.

Answer 2

• High theoretical energy density.
• Convenient handling, particularly in smaller amounts, such as needed for portable fuel cells.
• Much less BoP (Balance of plant) needed compared to reforming routes → simpler systems.
• Lower temperatures possible than via reforming routes.

Question 3

Disadvantages of Direct/ Liquid FCs.

Answer 3

• The electrocatalyst needs to break down the molecule. For low temperature fuel cells this
occurs some hundred degrees below the temperatures for reforming. These processes are not
fully comparable, yet it gives an idea of the challenge.
• The larger the molecule is, which is to be broken down the harder it is. It is much harder to
crack a C-C binding compared to a C-H Binding or even the H-H binding in H2.
• This leads to substantial voltage drops in direct FCs compared to H2 cells.
• It works better the higher the temperature and the simpler the molecule in LT-FCs.
• The more intermediates can be formed the more challenging it gets.
- Hence, the theoretical advantages cannot be fully harnessed.
- Direct FCs are useful yet limited to smaller power.

Question 4

Advantages of DMFC.

Answer 4

• No fuel processing necessary
• Low systems weight and small system size
• Methanol/water mixture as fuel and cooling
• No complex water and temperature management

Question 5

Applications of DMFC.

Answer 5

Systems for on-board / back-up / off-grid power; some 10W – lower kW range

Question 6

DMFC Electrodes.

Answer 6

Porous current conductors (Carbon)

Question 7

DMFC Electrolyte.

Answer 7

Nafion (Proton conducting membrane)

Question 8

DMFC Catalyst.

Answer 8

Pt/PtRu

Question 9

DMFC Anode reaction.

Answer 9

CH3OH +H2O → CO2 +6H+ +6e-

Question 10

DMFC Cathode reaction.

Answer 10

3/2O2 +6H+ +6e- → 3H2O

Question 11

DMFC total reaction.

Answer 11

3/2O2 + CH3OH → CO2 + 2H2O

Question 12

What are active DMFC Systems?

Answer 12

• Fans and pumps being suitable for up to a couple of kW of FC power
• operates on 100% methanol in the tank
• Active systems generally operate above 50 up to 70 °C
• Needs no separate cooling cycle; cooling occurs via the methanol/water flow

Question 13

What are DMFC Passive systems?

Answer 13

• Passive systems rely on convection for mass transport
• Passive systems are suitable in the 10 W range up to some 100W

Question 14

What is an issue in DMFC due to Methanol being the fuel?

Answer 14

Methanol crosses over the electrode via diffusion and electro-osmotic drag with subsequent oxidation. This leads to fuel loss and creates a „mixed potential“, i.e. reducing the potential of the cathode by partially
inducing the same reaction as it occurs at the anode. This fuel crossover to the cathode is a difference in flows between DMFC and PEFC.

Question 15

Why is Pt/Ru Catalyst used instead of Pt Catalyst?

Answer 15

Because removing CO takes another step with pure Pt, whereas Ru removes absorbed CO on Pt directly. The loadings are high with approx. 2 mg/cm^2 at either side.