Phosphoric acid-doped polybenzimidazole-based HT-PEMFCs are a mature technology, as several companies have commercialized the system for stationary applications. However, these HT-PEMFCs have not been seriously considered for transportation applications because they work only well at a temperature range of 140-180 °C under steady-state operating conditions. Under dynamic automotive operating conditions, phosphoric acid loss from the membrane electrode assembly is significant and the fuel cell performance rapidly decreases.
The breakthrough to resolve the acid loss issue came from Los Alamos National Laboratory (LANL) about seven years ago by accident. At that time, a LANL team led by Yu Seung Kim was developing cation functionalized anion exchange membranes for alkaline membrane fuel cells. While they were struggling with the stability of alkaline membrane fuel cells, they doped one of the AEMs with phosphoric acid and tested it under hot and dry conditions. Surprisingly, the fuel cell performance was more stable than a commercial benzimidazole-based HT-PEMFCs at 120-200 °C. They found the exceptional stability came from a much stronger interaction between biphosphate-ammonium ion pairs than that between conventional phosphoric acid-benzimidazole acid-base. They called the new system ion-pair HT-PEMFCs and the cell performance result was published in Nature Energy in 20161.
When I came to work at LANL as Director’s Postdoc Fellowship in 2017, the performance of ion-pair HT-PEMFCs was relatively poor (peak power density = 300 mW cm-2) in spite of good acid retention stability. What we found then was that the ion-pair HT-PEMFCs contained only 10-20% phosphoric acid molecules compared to the conventional polybenzimidazole-based HT-PEMFCs. With such a small amount of phosphoric acid within the membrane electrode assembly of ion-pair HT-PEMFCs, industry standard PTFE electrode binders did not produce high performance. So I had to find better electrode binders for the ion-pair HT-PEMFCs. After one and half years of research, we found that phosphonated polymers synthesized from the University of Stuttgart (Professor Vladimir Atanasov) worked very well with ion-pair HT-PEMFCs. It was a fascinating thing that all phosphonated polymers developed so far did not perform well under HT-PEMFC operating conditions. However, the HT-PEMFCs using ion-pair membrane and phosphonated polymer binder showed higher performance (peak power density = 480 mW cm-2) than conventional polybenzimidazole-based HT-PEMFCs. This exciting result was published under the title “Synergistically integrated phosphonated poly(pentafluorostyrene)for fuel cells” in Nature Materials last year2.
After we achieved good performance with the phosphonated polymers, we were looking for ionomers that performed well even under low-temperature fuel cell conditions (< 100°C). Under such conditions, water can condense, requiring more hydrophobic electrodes. We believed that perfluorosulfonic acid ionomers (Nafion) can help as Nafion is more hydrophobic and the conductivity can be further increased with the sulfonic acid group. So we tested several fuel cells with having composite ionomers consisting of Nafion and phosphonic acid. To our surprise, we obtained higher fuel cell performance with this composite ionomer not only under low-temperature (< 120°C) but also under hot and dry conditions. This was an unexpected result because Nafion is a non-proton conductor without water. The reason for the unexpected high fuel cell performance with the composite ionomer was not elucidated until we visited Klaus-Dieter Kreuer at Max Planck Institute, Germany in August 2019. During our discussion, Kreuer indicated that protonation of phosphonic acid by Nafion may increase the degree of frustration of the hydrogen-bond network and increase proton conduction.
After coming back from Germany, we tried to detect the protonation of phosphonic acid by phosphorous NMR. We found that one peak out of four peaks in the phosphorus NMR spectrum was related to the protonation of phosphonic acid (Figure 1). The other three peaks were characterized to polar interactions. Compared to the non-protonated phosphonic acid ionomer, the protonated phosphonic acid ionomer exhibited high anhydrous proton conductivity across a wide range of temperatures from 80-240 °C. After this finding, we designed high-performance HT-PEMFCs based on this composite ionomer. Multiple institutions including LANL (Katie Lim), Sandia National Labs (Cy Fujimoto), Korea Institute of Science and Technology (Jiyoon Jung), University of New Mexico (Ivana Gonzales), University of Connecticut (Jasna Jankovic), and Toyota Research Institute of North America (Zhendong Hu and Hongfei Jia) were involved in this project. In our most recent Nature Energy paper, we reported the HT-PEMFC performance and durability improvements based on this ionomer technology3, achieving a peak power density of 800 mW/cm2 at 160 °C and stable performance under steady-state and dynamic operating conditions.
Advent Technologies (Emory De Castro) and other US national labs are working together to develop 1 kW stack based on the ion-pair HT-PEMFC technology through US DOE’s L’Innovator program as shown in Figure 2. Combined with the advantages of HT-PEMFCs, this benchmark performance can offer a solution for clean large-scale mobility applications in which LT-PEMFCs are limited by heat rejection in stacks exceeding 80 kW. Such applications include heavy-duty trucks, large-scale maritime ships and submarines, subways, trains, and possibility even airplanes. Finally, as HT-PEMFCs are tolerant to carbon monoxide, the potential use of direct or reformed bio-renewable liquid fuel such as methanol, dimethyl ether, or liquid organic hydrogen carriers can drastically simplify fuel transportation for faster realization of a hydrogen economy and net-zero carbon future4.
1 Lee, K. S., Spendelow, J. S., Choe, Y. K., Fujimoto, C. & Kim, Y. S. An operationally flexible fuel cell based on quaternary ammonium-biphosphate ion pairs. Nature Energy 1, 16120 (2016).
2 Atanasov, V. et al. Synergistically integrated phosphonated poly(pentafluorostyrene)s for fuel cells. Nat Mater 20, 370-377 (2020).
3 Lim, K. H. et al. Protonated phosphonic acid electrodes for high power heavy-duty vehicle fuel cells. Nature Energy DOI https://www.nature.com/articles/s41560-021-00971-x (2022).
4 Gittleman, C. S., Jia, H. F., De Castro, E. S., Chisholm, C. R. I. & Kim, Y. S. Proton conductors for heavy-duty vehicle fuel cells. Joule 5, 1660-1677.
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