Approaches and technical challenges to high temperature operation of proton exchange membrane fuel cells

Question

1. What are the contributions in "Approaches and technical challenges to high temperature operation of proton exchange membrane fuel cells" ?

2. What is the way to keep the membranes hydrated?

3. What is the chemical potential of water in a polymer membrane?

4. Why is the hydrogen adsorption required to be higher than CO?

Accepted Answer

The authors report here approaches to the development of high temperature membranes for proton exchange membrane fuel cells ; composite perfluorinated sulfonic acid membranes were prepared to improve water retention, and non-aqueous proton conducting membranes were prepared to circumvent the loss of water.

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For optimal performance, these membranes must be well hydrated, which swells the membranes and allows for bridging between ionic inclusions facilitating proton conductivity.

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The modifications to the membrane structure by the addition of the inorganic phase that interacts strongly with water would lead to lower chemical potential of water in the membrane and could account for the improved fuel cell performance at reduced relative humidity.

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Because hydrogen adsorption is less exothermic than CO and hydrogen adsorption requires two adsorption sites, increased temperature leads to a beneficial shift towards lower CO coverage and higher H coverage.

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For water-dependent membranes, the fuel cell conditions that were most often used were a total pressure of 3 atm and several temperatures between 80 and 1408C.

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Composite membranes could be expected to improve the performance of fuel cells operating at elevated temperatures by allowing operation at reduced relative humidity.

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CO tolerance is defined as operation of a fuel cell in the presence of CO where the significant polarization at the hydrogen electrode does not occur (i.e. the voltage loss is less than 10– 20 mV).

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It is clear that operation at higher temperatures will increase the abilityof the fuel cell anode to perform in the presence of small amounts of CO by decreasing the coverage of CO on the catalyst surface, thereby increasing hydrogen coverage.

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The reactant gas partial pressure at both the anode and cathode and relative humidity in the cell were higher at lower temperatures, which would improve cell performance.

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Solid oxide conductors have been employed for many years in high temperature fuel cells but work is beginning on low temperature solid state proton conductors for fuel cells [11].

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At high temperatures (160–1808C),its conductivity ( 0.1 S/cm) was comparable to hydrated Nafion at low temperatures and over an order of magnitude higher than pure imidazole (5 10 3 S/cm at 1308C) [9].

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The form of zirconium phosphate in the membrane is believed to be one of the crystalline forms of a-zirconium phosphate (Zr(HPO4)2) [13].

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The third approach the authors explored was the use of a solid state protonic conductor whose conduction mechanism occurs in the absence of water.

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The typical membranes for these low temperature proton exchange membrane fuel cells (PEMFCs) are DuPont NafionTM or other perfluorinated sulfonic acid polymers (e.g. Aciplex from Ashai Chemicals).

Accepted Answer

The ultimate limits of vapor pressure reduction with improved composite membranes and optimized chemical structure have not been reached, as further increases in composite materials can improve membrane hydration characteristics.

Approaches and technical challenges to high temperature operation of proton exchange membrane fuel cells

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Most frequently asked questions

1. What are the contributions in "Approaches and technical challenges to high temperature operation of proton exchange membrane fuel cells" ?

2. What is the way to keep the membranes hydrated?

3. What is the chemical potential of water in a polymer membrane?

4. Why is the hydrogen adsorption required to be higher than CO?

5. What are the common conditions used for fuel cell membranes?

6. What is the effect of a composite membrane on the performance of fuel cells?

7. What is the definition of a tolerance for CO?

8. What is the effect of higher temperatures on the performance of the fuel cell?

9. What is the effect of the reactant gas on the cell performance?

10. What is the problem with the use of solid oxide conductors in fuel cells?

11. What temperature was the conductivity of the recast Nafion membrane?

12. What is the form of zirconium phosphate in the membrane?

13. What is the third approach to the use of a solid state protonic conductor?

14. What are the typical membranes for low temperature proton exchange membrane fuel cells?

15. What is the ultimate limit of vapor pressure reduction with improved composite membranes?

Figures

Citations

Mechanical Stability of H3PO4-Doped PBI/Hydrophilic-Pretreated PTFE Membranes for High Temperature PEMFCs

Proton exchange membranes for fuel cells operated at medium temperatures: Materials and experimental techniques

Anhydrous proton conductivities of squaric acid derivatives.

Side chain flexibility in perfluorosulfonic acid ionomers: an ab initio study.

Polymer Electrolyte Application in Electrochemical Devices

References

On the development of proton conducting materials for technological applications

Solid acids as fuel cell electrolytes

Imidazole and pyrazole-based proton conducting polymers and liquids

Polymer electrolyte fuel cells.

High‐Temperature Proton Conducting Membranes Based on Perfluorinated Ionomer Membrane‐Ionic Liquid Composites

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