Low-temperature PEM fuel cells that use Nafion are at present being commercialized in fuel cell vehicles, but these cells can operate only at relatively low temperatures and high hydration levels; therefore, they require humidified inlet streams and large radiators to dissipate waste heat. High-temperature PEM fuel cells that use phosphoric acid (PA)-doped polybenzimidazole (PBI) could address these issues, but these PBI-based cells are difficult to operate below 140 ˚C without suffering loss of PA. The limited operating temperature range makes them unsuitable for automotive applications, where water condensation from frequent cold start-ups and oxygen reduction reactions at the fuel cell cathode occur during normal vehicle drive cycles.

In this work, we demonstrate that fuel cells based on quaternary ammonium (QA)-biphosphate ion-pair-coordinated polyphenylene (PA-doped QAPOH) PEMs can avoid the limitations of Nafion and PBI-based fuel cells, enabling operation under a wide range of conditions that are off-limits for existing fuel cell technology. These PEMs can conduct protons through stable ionic pair complexes and enable fuel cell operation at temperatures from 80 to 180 ˚C.

—Lee et al.

Currently, two main classes of polymer-based fuel cells exist. One is the class of low-temperature fuel cells that require water for proton conduction and cannot operate above 100 °C. The other type is high-temperature fuel cells that can operate up to 180 °C without water; however, the performance degrades under water-absorbing conditions below 140 °C.

There’s a huge benefit to running fuel cells at the widest possible operating temperature with water tolerance. But current fuel-cell vehicles need humidified inlet streams and large radiators to dissipate waste heat, which can increase the fuel-cell system cost substantially, so people have looked for materials that can conduct protons under flexible operating conditions. It is very exciting that we have now found such materials.

—Yu Seung Kim, the project leader at Los Alamos

The operating temperature window of a PEM fuel cell is determined by the interactions between the acid (for example, tethered sulfonic acid or free phosphoric acid) and the base moieties (for example, free water or tethered QA) in the membrane, the researchers explained.

For low-temperature fuel cells, the hydrogen bonding interactions between the sulfonic acid group and water molecules in Nafion is only 15.4 kcal mol⁻¹; this does not provide enough stability above the boiling temperature of water, leading to membrane dehydration.

For high-temperature fuel cells, The intermolecular interaction energy between benzimidazole and PA is 17.4 kcal mol⁻¹—only 4.8 kcal mol⁻¹ greater than that between PA and one water molecule, about 12.6 kcal mol⁻¹. Due to the relatively weak interaction, benzimidazole tends to lose PA easily with water absorption. PA-doped PBI requires a high concentration of base moieties and a high acid content to impart sufficient anhydrous conductivity.

The research team found that a phosphate-quaternary ammonium ion-pair that has much stronger interaction, which allows the transport of protons effectively even under water-condensing conditions.

The discovery happened when we were investigating alkaline hydroxide conducting membranes, which have quaternary ammonium groups. While the alkaline membranes work only under high pH conditions, the idea came across that alkaline membranes can be used under low pH conditions by combining with phosphoric acid. This was a breathtaking moment, when Choe brought the calculation data that showed the interaction between quaternary ammonium and biphosphate is 8.7 times stronger than conventional acid-base interaction.

—Yu Seung Kim

The Los Alamos team collaborated with Fujimoto at Sandia to prepare quaternary ammonium functionalized polymers. The prototype fuel cells made from the ion-pair-coordinated membrane demonstrated excellent fuel-cell performance and durability at 80-200 °C, which is unattainable with existing fuel cell technology.

The performance and durability of this new class of fuel cells could even be further improved by high-performing electrode materials, said Kim, citing an advance expected within five to ten years that is another critical step to replace current low-temperature fuel cells used in vehicle and stationary applications.

Researchers on this project include Kwan-Soo Lee (Los Alamos National Laboratory, Chemistry Division), Jacob Spendelow, Yu Seung Kim (Los Alamos National Laboratory, Materials Physics and Applications Division), Yoong-Kee Choe (National Institute of Advanced Industrial Science & Technology, Japan), and Cy Fujimoto (Sandia National Laboratories).

Los Alamos has been a leader in fuel-cell research since the 1970s. Fuel cell technologies can significantly benefit the nation’s energy security, the environment and economy through reduced oil consumption, greenhouse gas emissions, and air pollution. The current research work supports the Laboratory’s missions related to energy security and materials for the future.

Resources

• Kwan-Soo Lee, Jacob S. Spendelow, Yoong-Kee Choe, Cy Fujimoto & Yu Seung Kim (2016) “An operationally flexible fuel cell based on quaternary ammonium-biphosphate ion pairs” Nature Energy 1, Article number: 16120 doi: 10.1038/nenergy.2016.120