The hydrogen fuel cell electrolyte or more commonly the polymer electrolyte membrane (PEM) is a material that looks something like ordinary kitchen plastic wrap. It conducts only positively charged ions and blocks the electrons. The PEM is the key to the fuel cell technology; it must permit only the necessary ions to pass between the anode and cathode. Other substances passing through the electrolyte would disrupt the chemical reaction.
Three major characteristics of membranes are not totally satisfied by today's membranes when applied to a hydrogen automotive fuel cell.
- The demand for continuous peak power in hot ambient environments,
- Cold start capability
- Stable performance at low relative humidity
DOE has set 2010 technical goals of conductivity of 0.1 Ohms-cm2 @ 160 C, oxygen and hydrogen crossover of 2 mA/cm2 or less, cost of $5.00/kW and durability of greater than 5,000 hours for transportation applications and 40,000 hours for stationary applications.
The best-known and most widely used membrane material today is Nafion, a perflorosulfonic acid (PFSA) product, in the same chemical family as DuPont's Teflon, that is cast into film and supplied by DuPont. Since the 1960s, Nafion has been the membrane of choice in specialized fuel-cell applications such as spacecraft. Nafion membrane electrode assemblies (MEAs) set the standard for power density and durability in a variety of fuel cell applications from buses, forklifts, and scooters to laptops, cellular phones, and stationary generators. While effective, membranes made with this material don't last as long as desired, are expensive to make, operate within limited temperatures, and require expensive supporting systems to maintain proper humidity and cooling, i.e.," a water management system". A 20-30 micron thick membrane introduced in October 2005 has seven times the lifetime of previous membranes.
The need to operate at temperatures exceeding 100°C and at low relative humidity presents difficult new challenges for membranes used in fuel cells. This difficulty stems from the decrease in water content of the polymer electrolytes in the desired temperature range. Replacement of water while retaining conductivity is the most difficult problem. However, developing a better membrane remains a challenge to the researchers working on alternative membranes.
3M has developed a PFSA membrane, based on its Kynar® polymers, with characteristics which they claim is superior to the standard Nafion membrane. The membrane has higher strength, higher conductivity at lower water content, and higher temperature operation.
Hydrocarbon membranes are claimed to allow fuel cells to be smaller, lighter, lower cost, more durable and more efficient. SRI spin-off PolyFuel has developed membranes that are made of hydrocarbon molecules converted into polymers, or plastics. By forcing the material to self-assemble into structural blocks and conductive blocks, PolyFuel can make membranes that not only are 16 times stronger, but also allow the hydrogen protons to flow more freely to the cathode side of the cell.
PolyFuel claims to have a hydrocarbon membrane capable of working within a broader range of temperatures and producing more power per square centimeter than fuel cell membranes in use today. Hydrogen fuel cells that use PolyFuel's new hydrocarbon-based material, the company claims, will produce less humidity, create up to 15 percent more power, and cost less to manufacture than traditional membrane materials. Stable operation is possible at 35% relative humidity. The membrane is also able to provide stable performance at temperatures from 2°C to 95°C. In addition, when compared with typical perfluorinated membranes, the PolyFuel membrane is more than twice as strong, more than 16 times as stiff and has 4 times less hydrogen permeability - all of which are important criteria for durability and manufacturability.
Ongoing Membrane Research
Researchers at Georgia Tech have discovered that adding the chemical triazole to membranes increases the conductivity and reduces the moisture dependence of membranes. In addition to improving the conductivity, replacing the water in membranes with triazole increases the operating temperature from the usual 80 C to 120 C, above the boiling point of water, thus permitting the elimination of the water management system. The higher operating temperature permits a slightly less pure hydrogen fuel to be used because it is less sensitive to CO poisoning.
Sandia has demonstrated that Sandia Polymer Electrolyte Alternative (SPEA) (polyphenylenes polymer) could operate as high as 140 degrees C and produce a peak power of 1.1 watts per square centimeter at 2 amps per square centimeter at 80 degrees C. These advances include smaller fuel cell stacks because of better heat rejection, enhanced water management, and significant resistance to carbon monoxide poisoning. These performance properties suggest that the SPEA material may be a potential alternative to Nafion.
Initial performance test results for heteropolyacid (HPA)-based PEMs are very promising for high temperature operation without the need for humidification. The ability of these compounds to retain water to high temperatures and act as super acids makes them excellent proton conductors. The ultimate goal is to develop HPA-based composite materials that can be combined with polymers and other potential supports to manufacture thin films as membrane materials for use in fuel cells.
High temperature polybenzimidizole (PBI) membrane are being developed for Plug Power by RPI. Membranes have been tested for 14,000 hrs but lacked sufficient mechanical stability for further testing. Recent effort has been to adding reinforcing fillers. The fillers did not adversely affect the performance. Durability testing has resumed.
OPM (oxford performance materials) is developing high temperature, 120o C, fuel cell membranes using advanced S-PEKK polyketone polymer. The material has excellent conductivity properties, but durability is a problem. Mechanical reinforcement, blending, and cross-linking show promise with respect to improving membrane durability. A new grade of basis material (polyether ketone ketone) may be required to substantially improve SPEKK membrane durability.
One area of research is the evaluation of inorganic solid state proton conducting systems for high temperature fuel cell membranes. The goal of this research is to acquire an improved fundamental understanding of a class of inorganic proton conductors (heteropoly acids [HPA] and their salts) that exhibit high proton conductivity at elevated temperatures (well above 100°C) and to apply that understanding to fuel cell membrane technology. The HPA exhibit proton conductivity among the highest measured in the solid state, more than an order of magnitude higher than Nafion. The ultimate goal is to develop HPA-based composite materials that can be combined with polymers and other potential supports to manufacture thin films as membrane materials for use in fuel cells.
Resources:
Nafion® Membranes and Dispersions, DuPont Fuel Cells website
New Membranes for PEM Fuel Cells, Steve Hamrock, 3M, April 27,2005
Polyfuel Hydrogen Membrane , website
DOE Hydrogen Program 2005 Annual Progress Report, Section VII.B, Fuel Cell Membranes
Technorati tags: fuel cells, hydrogen, membranes, electrolyte, energy, technology
The Energy Blog: PEM Fuel Cell Membranes
Dear Sir,
I want to develop non-selectriv permionic
membrane for vanadium battery and fuel cell.
What kind of materials I should be interested in.
Anuvat Sirivat
Posted by: Anuvat Sirivat | April 25, 2006 at 07:59 AM
Thanks for sharing this very use full information I will be definatly be back soon. Keep Up the Good Work!
Posted by: HHOGasKitWizad | August 07, 2009 at 08:35 PM
let us know which is the best membrane of the following ,nafion 2030 wx membrane or nafion 982 wx membrane ?
difference in terms of power requirement,efficiencies of membrane ,life,performance for low impurity generation,
Posted by: RAJ PATIL | September 09, 2009 at 01:33 PM