Mihail Krumov

Addressing global challenges

The Center for Alkaline-Based Energy Solutions (CABES), a Cornell University-led  Energy Frontier Research Center (EFRC) established in 2018, is advancing the scientific understanding of electrochemical energy conversion in alkaline media at the fundamental level. Using this new knowledge, CABES aims to solve the technological challenges that have hindered the widespread integration of fuel cells into our energy infrastructure.

Fuel cells are well poised to address growing clean energy demands faced at the national and global levels. A fuel cell is an energy conversion device analogous to the internal combustion engine (ICE). However, unlike an ICE, a fuel cell converts a chemical fuel directly into electricity. This electrochemical route has a higher theoretical (100%) and practical (>60%) energy efficiency as compared to the thermal route for energy conversion used by ICEs (50% theoretical, 25% practical efficiency). Hydrogen fuel cells have the advantage of using a carbon-free fuel, which means they exhaust pure water and no CO2. Sourcing hydrogen can also be done in an environmentally benign process by using renewably generated electricity to split water into hydrogen and oxygen, commonly called water electrolysis.

Alkaline anion exchange membrane requirements

Hydrogen fuel cells can be divided into two classes: acidic and alkaline. Acidic fuel cells are a more mature technology, enabled by DuPont's chance discovery of Nafion (a proton conducting membrane) in the 1960s. However, these acidic-based fuel cells require expensive noble metal catalysts, which limits widescale deployment. Alkaline fuel cells enable the use of inexpensive transition metal catalysts, and significant advancements have recently been made.1 A remaining challenge is to develop an anion exchange membrane (AEM) that matches—or preferably exceeds—the performance and durability of Nafion.

Tackling cation stability

Figure 1. The anion exchange membrane (AEM) facilitates hydroxide ion transport in a fuel cell. Degradation of the polymer backbone or the cation functional groups leads to a decrease in cell performance.

Incorporating mobile anions, specifically hydroxide, is necessary to minimize cell resistance and achieve high current/power. This incorporation is achieved by attaching cationic groups (small molecules with a positive charge) onto a polymeric backbone with which the anions associate electrostatically. The degradation of these cationic groups under harsh oxidative conditions inside a fuel cell leads to a loss of ionic conductivity, as shown in Figure 1, and is one of the primary factors that limits the operating lifetime of alkaline fuel cells. CABES researchers have designed a systematic protocol to assess the stability of cationic small molecules with high throughput and quick turnaround.2 In this study, the cation was dissolved in 1 M KOH, and the solution was kept at 80°C for 30 days. The chemical composition of the solution was tracked by nuclear magnetic resonance (NMR) to measure cation degradation kinetics and byproducts. From these results, researchers can infer degradation mechanisms.

A systematic study of 26 model cations was performed as follow-up work to identify trends between cation chemical structure and its stability in alkaline conditions.3 The 26 model cations were classified into six categories based on their structure. These categories cover the majority of structures reported in the literature. Benzyl nucleophilic substitution was found to be the dominant degradation mechanism for benzyl ammonium cations; SN2 and Hofmann elimination are degradation pathways for alkyl ammonium cations. Such fundamental insights are vital to strategically designing small molecules with improved stability. For example, it was hypothesized and confirmed that steric hindrance plays a crucial role in enhancing alkaline stability for N-conjugated systems. Most importantly, the study identified several cations that show negligible degradation after one month at 80 °C in 1 M KOH.

Recently, many of these cations were successfully incorporated into polyethylene-based polymers.4 Seventeen different AEMs were synthesized, and their performance was systematically compared regarding ionic conductivity and chemical stability. Not all cations that performed well in small molecule studies retained the same degree of chemical stability when incorporated into an AEM. The reason is that the covalent linkage to the polyethylene backbone may introduce additional degradation pathways. The solvation shell is very different when in polymer form, which affects degradation kinetics. Four of the AEMs exhibited satisfactory ionic conductivity and chemical stability. The best of these was tested in a fuel cell and achieved an impressive 1 W/cm2 peak power density, which is competitive with Nafion-based acidic fuel cells.

Dynamic characterization and optimization

Water management is essential to consider when optimizing fuel cells. While membranes need to be well hydrated to enable high ionic conductivity, excess water may compromise the mechanical toughness of the membranes if not supported by the polymer backbone. The membrane hydration and ion exchange processes have been monitored in situ during electrode reactions using advanced mass-sensitive electrochemical techniques.5

Water management properties of the membrane are affected by the AEM's ion-exchange capacity (IEC). The IEC is a measure of the quantity of cationic functional groups (which are responsible for ionic conduction) in a given amount of polymer. In a follow-up study, it was found that the polymer's solubility increased with increasing IEC. In contrast, the rigidity of the polymer films was reduced due to the higher content of the hydrophilic cations6; there is a trade-off between solubility and mechanical toughness. This evidence suggests IEC can be used to tune polymer properties so that optimized materials can be rationally designed for specific applications. Specifically, polymers with higher IEC are predicted to function better as ionomers because they have higher conductivity and can be homogeneously distributed on the catalysts. On the other hand, those with a lower IEC could be used as membranes to take advantage of increased rigidity and mechanical strength.

Facile synthesis: putting it all together

To keep fuel cells cost competitive with ICEs, the AEM synthetic strategies must be cost-effective and scalable. To this end, several studies have been undertaken to simplify the synthesis of these materials. A one-pot synthesis of piperidinium-functionalized monomer precursor was developed, which improved on the previous complicated multi-step synthesis.7 Monomer polymerization was accomplished via the usual ring-opening metathesis polymerization strategy. However, this route still requires post-polymerization hydrogenation, which restricts the types of functional groups that can be incorporated into the polymer.

Recently, a novel coordination-insertion polymerization direct synthesis strategy was developed, allowing access to a greater diversity of materials.8 Besides the facile synthesis, these polymers are solution processible and have tunable IEC. Of the several AEMS studied, it was found that those with piperidinium cations exhibited the highest fuel performance. A preliminary peak power density of 730 mW/cm2 was achieved.

Conclusion

Recent advancements in the development of transition metal catalysts have allowed alkaline fuel cells to match the power output of their acidic counterparts while reducing costs. Alkaline membranes have been developed with the necessary ionic conductivity to sustain high performance requirements. The primary research challenge that remains is improving the durability of AEMs. Recent advances in synthesis methodology will allow the exploration of more diverse materials, and advanced electrochemical techniques will elucidate the complex relationships between chemical structure and material properties. Further details on alkaline membranes and ionomers can be found in section 6 of the recent article published by CABES researchers in Chemical Reviews.9

More Information

(1) Krumov, M. The Energy Crisis, Episode VI: Return of the Fuel Cells. Frontiers in Energy Research Newsletter. 2023. https://www.energyfrontier.us/The-Energy-Crisis-Episode-VI-Return-of-the-Fuel-Cells.

(2) Hugar, K. M.; You, W.; Coates, G. W. Protocol for the Quantitative Assessment of Organic Cation Stability for Polymer Electrolytes. ACS Energy Lett. 2019, 4 (7), 1681–1686. https://doi.org/10.1021/acsenergylett.9b00908.

We gratefully acknowledge financial support from the Center for Alkaline-based Energy Solutions (CABES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0019445. This study made use of the NMR facility supported by NSF Grant CHE-1531632.

(3) You, W.; Hugar, K. M.; Selhorst, R. C.; Treichel, M.; Peltier, C. R.; Noonan, K. J. T.; Coates, G. W. Degradation of Organic Cations under Alkaline Conditions. J. Org. Chem. 2021, 86 (1), 254–263. https://doi.org/10.1021/acs.joc.0c02051.

This work was primarily supported as part of the Center for Alkaline-based Energy Solutions (CABES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DESC0019445. M.T. and K.J.T.N. also acknowledge the support from National Science Foundation (NSF, CHE-1809658) for the synthesis of cations 10 and 13. This study made use of the NMR facility supported by NSF Grant CHE-1531632.

(4) Peltier, C. R.; You, W.; Fackovic Volcanjk, D.; Li, Q.; Macbeth, A. J.; Abruña, H. D.; Coates, G. W. Quaternary Ammonium-Functionalized Polyethylene-Based Anion Exchange Membranes: Balancing Performance and Stability. ACS Energy Lett. 2023, 8 (5), 2365–2372. https://doi.org/10.1021/acsenergylett.3c00319.

This work was supported as part of the Center for Alkaline based Energy Solutions (CABES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DESC0019445.

(5) Lu, X.; Abruña, H. D. Anion Exchange and Water Dynamics in a Phosphonium-Based Alkaline Anion Exchange Membrane Material for Fuel Cells: An Electrochemical Quartz Crystal Microbalance Study. ACS Appl. Mater. Interfaces 2021, 13 (9), 10979–10986. https://doi.org/10.1021/acsami.0c22738.

This work was supported as part of the Center for Alkaline Based Energy Solutions (CABES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award DESC0019445, and the Multidisciplinary Research program of the University Research Initiative (MURI) funded by the U.S. Air Force, Office of Scientific Research, under Award No. N00014-17-S-F006.

(6) Lu, X.; You, W.; Peltier, C. R.; Coates, G. W.; Abruña, H. D. Influence of Ion-Exchange Capacity on the Solubility, Mechanical Properties, and Mass Transport of Anion-Exchange Ionomers for Alkaline Fuel Cells. ACS Appl. Energy Mater. 2023, 6 (2), 876–884. https://doi.org/10.1021/acsaem.2c03210.

This work was supported as part of the Center for Alkaline Based Energy Solutions (CABES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under award DESC0019445.

(7) You, W.; Ganley, J. M.; Ernst, B. G.; Peltier, C. R.; Ko, H.-Y.; DiStasio, R. A.; Knowles, R. R.; Coates, G. W. Expeditious Synthesis of Aromatic-Free Piperidinium-Functionalized Polyethylene as Alkaline Anion Exchange Membranes. Chem. Sci. 2021, 12 (11), 3898–3910. https://doi.org/10.1039/D0SC05789D.

This work was primarily supported as part of the Center for Alkaline-based Energy Solutions (CABES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award #DESC0019445. R. R. K. provided monomer materials to the work with financial support from the National Institutes of Health (R35 GM134893). This study made use of the NMR facility supported by the National Science Foundation (NSF, CHE1531632) and the Cornell Center for Materials Research Shared Facilities supported by NSF MRSEC (DMR-1719875). Computational resources were provided by the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility operated under Contract No. DE-AC02-05CH11231.

(8) Hsu, J. H.; Peltier, C. R.; Treichel, M.; Gaitor, J. C.; Li, Q.; Girbau, R.; Macbeth, A. J.; Abruña, H. D.; Noonan, K. J. T.; Coates, G. W.; Fors, B. P. Direct Insertion Polymerization of Ionic Monomers: Rapid Production of Anion Exchange Membranes. Angew Chem Int Ed 2023, 62 (30), e202304778. https://doi.org/10.1002/anie.202304778.

This work was supported by the Center for Alkaline-Based Energy Solutions (CABES), an Energy Frontier Research Center program supported by the U.S. Department of Energy, under Grant DE-SC0019445. This work made use of the Cornell Center for Materials Research Shared Facilities that are supported by the NSF MRSEC program (DMR1719875). This work made use of the NMR facility, which is supported, in part, by the NSF through MRI award CHE-1531632.

(9) Yang, Y.; Peltier, C. R.; Zeng, R.; Schimmenti, R.; Li, Q.; Huang, X.; Yan, Z.; Potsi, G.; Selhorst, R.; Lu, X.; Xu, W.; Tader, M.; Soudackov, A. V.; Zhang, H.; Krumov, M.; Murray, E.; Xu, P.; Hitt, J.; Xu, L.; Ko, H.-Y.; Ernst, B. G.; Bundschu, C.; Luo, A.; Markovich, D.; Hu, M.; He, C.; Wang, H.; Fang, J.; DiStasio, R. A.; Kourkoutis, L. F.; Singer, A.; Noonan, K. J. T.; Xiao, L.; Zhuang, L.; Pivovar, B. S.; Zelenay, P.; Herrero, E.; Feliu, J. M.; Suntivich, J.; Giannelis, E. P.; Hammes-Schiffer, S.; Arias, T.; Mavrikakis, M.; Mallouk, T. E.; Brock, J. D.; Muller, D. A.; DiSalvo, F. J.; Coates, G. W.; Abruña, H. D. Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies. Chem. Rev. 2022, 122 (6), 6117–6321. https://doi.org/10.1021/acs.chemrev.1c00331.

This work was supported by the Center for Alkaline-Based Energy Solutions, an Energy Frontier Research Center program supported by the U.S. Department of Energy, under Grant DE-SC0019445. This work acknowledges the long-term support of TEM facilities at the Cornell Center for Materials Research (CCMR) which are supported through the National Science Foundation Materials Research Science and Engineering Center (NSF MRSEC) program (DMR1719875), and Cornell high-energy synchrotron sources (CHESS), which is supported by the National Science Foundation under Award DMR-1332208.

About the author(s):

Mihail Krumov is a Ph.D. candidate at Cornell University, working in the laboratory of Prof. Abruña. ​Mihail’s research is in utilization of spatially and temporally resolved electroanalytical techniques such as SECM in the investigation of reaction pathways for fundamental and energy storage applications. In his free time Mihail enjoys hiking, traveling, and reading. ORCID ID #0000-0003-2954-3642.