Intermediate Temperature Proton Conducting Membrane Fuel Cells Biology Essay

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A PEMFC unit is made up of a proton conducting polymer membrane that is sandwiched between two electrodes. These electrodes are in contact with current collectors that deliver electrons to the external load e.g a bulb, as seen in figure 2. The electricity is generated by the oxidation of the fuel at the anode, which produced electrons and the reduction of oxygen takes place at the cathode, which consumes electrons.

Currently PEMFCs use a Nafion® membrane that operate at a temperature of around 70 to 80oC and is limited by the inability to operate at higher temperatures. This is due to the boiling point of water, which would result in dehydration.

The Proton Exchange Membrane Fuel Cells (PEMFC) also called "Solid polymer fuel cell" (SPFC). And the typical PEMFCs work at low temperatures, bringing the advantage that a PEMFC can start quickly. And the thinness of the MEAs (Membrane electrode assemblies) means that compact fuel cells can be made reducing the size of the unit considerably. Another appealing advantage of PEMFC is that there are no corrosive fluid hazards, and it can work in any orientation making it ideal for a portable or residential application.

Early versions of PEMFC that was used in the NASA Gemini spacecraft had a lifetime of only about 500 hours (Warshay, 1990) [1] .

Figure 1: PEM fuel cell in the Gemini Space Programme

(Courtesy of Smithsonian Institute, Fuel Cell History website)

The developments over the recent years has increased the current densities up to around 1 or more, in the meantime reducing the use of platinum by a factor of over 100. These had let to a huge reduction in cost per kW of power with improved power densities making then applicable of residential use.

Figure 2: Simple PEM Schematic (Courtesy of Johnson Matthey Fuel Cells)

Figure 2 shows a simple PEM schematic, and a more detail schematic can be seen in figure 3 below showing the basic components of a PEMFC.

Figure 3: Basic components of a PEMFC

As the electrolyte is a solid rather than a liquid, the sealing of the anode and cathode gases is far easier and this in turn makes the unit cheaper to manufacture than some other types of fuel cell. Furthermore, the solid electrolyte can lead to a longer cell and stack life as it is less prone to corrosion than some other electrolyte materials.

Achieving Intermediate Temperature PEMFC

Operating a PEMFC at a higher temperature is greatly desirable. But if the fuel cell it to operate above temperature of 120oC, any efficiency gains reached can be outweighed due to the pressurization.

A possible solution to this would be the development of membranes capable of operating at elevated temperatures. The currently existing commercial membranes such as Nafion are unsuitable for the use at high temperatures. This is because the propensity of these membranes to undergo dehydration, loss of mechanical strength, and higher levels of gas permeation.

A number of different particles for composite PEMs have been used; including silica (SiO2), titania (TiO2), zirconia (ZrO2), clays, alumina (Al2O3), zeolites, zirconium phosphate, and heteropoly acids (HPAs) and SPEEK have also been used.

The issues can be overcome by modifying the Nafion by incorporating proton conductive hydrophobic oxides or framework hydrates into Nafion or using alternative membranes such as PBI which is discussed later in this essay.

This will improve the proton conductivity, good water retention properties and suitably low water solubility.

By incorporating inorganic particles we can provide hydrogen bonding sites for water molecules through its surfaces and therefore prevent water from fast evaporation. The following research was carried out at the Penn State University and we can see how the powders of the inorganic additive were synthesized.

A large number of prospective hydrophilic inorganic compounds were analysed. Namely, ZrO2, TiO2, Al2O3, SiO2, α-Zr(HPO4)2, and H3OZr2(PO4)3 with respect to their acid-base properties.

And the Nafion/inorganic additive composite membranes were made-up using different techniques and tested in a H2/O2 PEM fuel cell over a range of temperatures of 80 and 120 oC. It was found that the incorporation of selected inorganic additives led to noteworthy improvement of the membrane water retention properties and the PEMFC performance, especially.

Figure 4: Performance of H2O2 PEMFC based on Nafion composite membranes with different inorganic additives at 120oC [2] 

The observed improvement in the performance of the composite membranes as seen in figure 4 above is mainly due to two factors:

Enhanced water retention of the new membranes due to hydrophilicity of inorganic particles, which in turn maintains high Nafion conductivity.

Enhanced proton conductivity of the membranes due to the contribution of the highly protonated surface of inorganic additives.

Proton conductors capable of operating at intermediate temperatures and low relative humidities currently have a large interest because of their advantages over Nafion-type fluoropolymers that are conventionally used in proton exchange membrane fuel cells (PEMFCs).

At low temperatures, carbon monoxide (CO) poisoning of the platinum (Pt) catalyst is also a problem. Traces of CO, present in the hydrogen feed, are preferentially adsorbed on catalyst sites resulting in reduced efficiency and possible permanent poisoning of the catalyst.

One solution to these problems is operating the PEMFC at intermediate temperatures. At such temperatures, CO poisoning becomes less prominent, electrode kinetics are faster, and water would exist primarily in the vapour phase precluding problems associated with water management and mass transport limitations. However, since Nafion® requires a high-water content to maintain proton conductivity, it is very ineffective at temperatures above the boiling point of water. Therefore, alternative membranes, which maintain mechanical strength and chemical stability at elevated temperatures, are needed.

One such membrane is polybenzimidazole (PBI). PBI has the advantage of being lightweight as well as stable at intermediate temperatures, and it achieves proton conductivity when doped with phosphoric or sulphuric acid [3] . Protons are transported through the solid matrix so, its conductivity is less dependent on water content than Nafion® [4] . Proton conduction in PBI may be based on the Grotthuss mechanism [5] .

PBI membrane thicknesses are lower than Nafion® because of its higher mechanical strength. A reduction in membrane thickness leads to lower ionic resistance, since resistance is proportional to thickness. In addition, PBI has a lower permeability to hydrogen and methanol than Nafion®, resulting in less crossover of fuel at elevated temperatures. PBI's conductivity can be further enhanced by alloying with inorganic composites. Several studies have been conducted on the feasibility of using PBI and its composites as membranes in PEMFCs [6] .

Composites of PBI have been developed with sulfonated polysulfones (SPSF) [7] ; zirconium phosphate (ZrP), phosphotungstic acid (PWA), silicotungstic acid (SiWA) [8] ; inorganic phosphomolybdic acid (PMo12) [9] ; imidazole and 1-methyl imidazole (Me-Im) [10] ; silicotungstic acid and silica [11] . The conductivity of PBI and its composites reportedly depend on the temperature, humidity, doping level of acid treatment, and method of preparation of the membrane.

PBI-based membranes have relatively lower conductivities than Nafion®. However, since PBI's permissible membrane thickness is less than Nafion®, its ionic resistance is of the same order of magnitude.

σ (Sm−1)

lm (µm)

Total resistance (Ω cm2)









Table 1 [12] 

Table 1 above compares typical values of conductivity and the membrane thickness for PBI-based membranes and Nafion®. It can be seen that for a given cross-sectional area, the ionic resistances of Nafion® and PBI are in the same order of magnitude.

There has been a considerable amount of work done in PEMFC modelling over the past 15 years, on PEMFCs using Nafion® membranes, which is low-temperature PEMFCs [13] with temperature of 70-100oC. Although through research article we can see that many intermediate temperature membranes have been investigated for use in PEMFCs, until now there are no mathematical models that have been published for intermediate temperature PEMFC.

Advantages and disadvantages of organic and inorganic polymers

Inorganic polymers such as Polphosphazenes which is composed of phosphorus and nitrogen repeat units are also looked at for use as PEMs. This has a relatively high thermal, chemical, and oxidative stability and the polymer backbone in polyphosphazenes is also very easy to functionalize with a wide variety of different groups (from the precursor polymerpoly(dichlorophosphazene), -[PCl2N-]-).

One of the first proton exchange membranes was based on sulfonated Polystyrene. Where divinylbenzene was used as a cross-linking unit for extra stability. This was developed by General Electric [14] and it was used for fuel cells in the Gemini space program. The advantage of it being that it was cheap and easy to manufacture.

The disadvantage was that due to the sensitivity of the benzylic hydrogen to radical attack, the lifetimes for these membranes under general fuel cells operating conditions were quite low.

Sulfonamide-substituted polyphosphazenes are of keen interested as they have exhibited very high power densities that are comparable with Nafion and may be suitable for use in PEMFC applications.

Another inorganic polymer system, sulfonated polysiloxanes, has also been investigated for potential use as PEMs due to the potential high thermal stability of the Si-O backbone.

Benefits of Intermediate Temperature PEMFCs

Due to the high operating temperature, heat can easily be removed from the system.

There is no need for a liquid cooling loop. Therefore, complicated, bulky and expensive liquid cooling systems can be avoided [15] .

Greater system efficiency can be obtained. And it also provides a high tolerance to CO. Due to the higher temperatures, hydrogen with a high CO concentration can be used.

Improved fuel oxidation kinetics.

Simpler yet compact and robust design running independent of humidification.

High fuel flexibility.

And lower cost.

The waste-heat from the fuel cell can be employed to evaporate liquid-energy-carriers which boil at a temperature lower than the operating temperature of the fuel cell. Ensuring tight integration between endothermic and exothermal processes greatly increases the overall system efficiency.