New polymer-based fuel cell

Dr. Neil Canter, Contributing Editor | TLT Tech Beat November 2016

Instead of reducing the cost of the platinum catalyst, researchers focused on increasing the flexibility of fuel cell operation.

 

KEY CONCEPTS
• Currently available fuel cells operate in temperature ranges that are not desirable for use in automobiles.
• A quaternary ammonium biphosphate ion-pair coordinated polymer fuel cell has been developed that operates in a temperature range compatible for use in automobiles.
• The durability of this new fuel cell is at least three times higher than the most successful high-temperature fuel cell. 

DEVELOPMENT OF MORE EFFICIENT FUEL CELLS IS ONGOING. One of the reasons is that fuel cells are seen by the automotive industry as a very promising technology. A number of manufacturers are now offering fuel cell vehicles in specific regional markets around the world, although a lack of hydrogen refueling stations has hindered the widespread use of fuel cell vehicles.

The traditional way for fuel cells to operate is to facilitate the reaction of hydrogen with oxygen from the air using a platinum catalyst in a polymer electrolyte to produce energy and water as a byproduct. A previous TLT article discussed the preparation of a biofuel cell that uses an enzyme cascade to convert a kerosene-based jet fuel into useful energy at room temperature (1). The enzyme system was able to achieve this conversion even though the jet fuel contains a sulfur content ranging between 400 and 1,600 ppm. Sulfur has been known to interfere with metal catalysts previously tried.

The fuel cell membranes most commonly used in automobile fuel cells are based on perfluorosulfonic acid, polytetrafluoroethylene copolymers. Dr. Yu Seung Kim, technical staff at Los Alamos National Laboratory in Los Alamos, N.M., says, “Fuel cells using these copolymers are operated at a relatively low temperature (below 100 C) because water is required for proton conduction. At higher temperatures, the hydration of the fuel cell membranes becomes difficult because of the high partial vapor pressure of water.”

The automotive industry would prefer to use a fuel cell that operates at high temperature without external humidification, according to Kim. He says, “One of the significant technical challenges for current fuel cells is thermal management. When the operating temperature of fuel cells is limited to between 80-100 C, heat rejection is very difficult. With the water management requirement, thermal management can be complex and increase fuel cell cost substantially.”

High-temperature fuel cells have been developed and evaluated. The most successful system is a phosphoric acid (PA)-doped polybenzimidazole (PBI) membrane. Kim says, “This membrane exhibits good performance between 140 C and 180 C. But at temperatures below 140 C, PA will leach out and degrade fuel cell performance.”

Due to their poor water tolerance, PBI-based fuel cells need a high level of PA doping. The high PA doping level of PBI-based fuel cells results in additional problems such as electrode flooding, catalyst poisoning and low mechanical integrity of the membrane. Therefore, the current PBI-based fuel cells are used only for stationary power generation applications.

The problem as noted in Figure 3 is that fuel cells either operate under cool, humid or hot/dry conditions.


Figure 3. Current polymer-based fuel cells run either under hot/dry or cool/humid operating conditions. A different class of fuel cell that bridges the gap between the two operating conditions may reduce fuel cell cost by simplifying thermal and water management. (Figure courtesy of Los Alamos National Laboratory.)

Kim says, “The desirable temperature range for an automobile application is between 80 C and 200 C with good water tolerance. There is need for a fuel cell technology that can bridge the current fuel cell technology gap.”

Such a polymeric fuel cell has now been developed that will be effective in the desired operating conditions.

QUATERNARY AMMONIUM BIPHOSPHATE ION-PAIRS 
Kim and his colleagues have developed a quaternary ammonium biphosphate ion-pair coordinated polymer fuel cell that can operate efficiently between 80 C and 200 C. He says, “While other researchers have focused on reducing the cost of the expensive platinum catalyst, our approach is to develop a simplified fuel cell system by increasing the flexibility of fuel cell operation. Previous efforts to reduce catalyst cost can also be applied to our fuel cell system.”

The researchers felt that the positive charge provided by quaternary ammonium species will work better with phosphoric acid, which has been used for a long period of time without external humidification. A strong ion pair interaction between the quaternary ammonium cation and the phosphate anion leads to greater retention of phosphoric acid as compared to acid-base coordination between benzimidazole and phosphoric acid.

The approach taken is to produce a quaternary ammonium functionalized membrane through use of a polyaromatic membrane followed by bromination to provide sites where trimethylamine can be attached to the membrane to form the quaternary ammonium cationic sites. After soaking in an 85% solution of phosphoric acid, the quaternary ammonium biphosphate ion-pair membrane is prepared. Kim says, “We made significant efforts to prepare quaternary ammonium tethered polymers over the past eight years. Preparation of acid-doped membranes and fuel cell testing took place only in four months.”

Performance and durability testing show that the quaternary ammonium biphosphate ion-pair-based fuel cell displays promising power generation between 80 C and 200 C. Durability is at least three times better than commercial PBI-based fuel cells after thermal cycling experiments between 80 C and 160 C.

Kim says, “We believe that this ion pair approach has produced a fuel cell that bridges the gap between the two currently available types. To further improve performance, we need to design a better electrode for this type of fuel cell. As part of this process, we will be working to produce better electrode materials.

Additional information can be found in a recent article (2) or by contacting Kim at yskim@lanl.gov. 

REFERENCES
1. Canter, N. (2015), “Room temperature biofuel cell,” TLT, 71 (2), pp. 14-15.
2. Lee, K., Spendelow, J., Choe, Y., Fujimoto, C. and Kim, Y. (2016), “An operationally flexible fuel cell based on quaternary ammonium-biphosphate ion pairs,” Nature Energy, 1, Article Number: 16120.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at neilcanter@comcast.net.