(541c) Permethylcobaltocenium Based Hydroxide Exchange Membranes With High Stability

Permethylcobaltocenium Based Hydroxide Exchange Membranes with High Stability

Shuang Gu, Junhua Wang, Qianrong Fang, Robert B. Kaspar, Bingzi Zhang and Yushan Yan*

Department of Chemical & Biomolecular Engineering, University of Delaware

5 Innovation Way, Newark, DE 19716, USA

*yanys@udel.edu

Hydroxide (OH⁻) exchange membranes (HEMs) have become the research focus in energy conversion electrochemical devices (fuel cells, electrolyzers, and solar hydrogen generators) during the past decade owing to their intrinsic ability to work with non-precious-metal catalysts as well as their desirable tolerance to CO₂ contamination^[1]. However, unlike their counterpart—proton (H⁺) exchange membranes (PEMs) that are based on the highly stable sulfonic-acid functional group (−SO₃H) and thus have excellent chemical and thermal stability, HEMs suffer from limited chemical stability due to their currently used ammonium hydroxide cationic functional group (−NR₃⁺OH⁻). The sensitive degradation pathways (such as nucleophilic substitution, Hoffman elimination, and ylide formation) of ammonium hydroxide not only limit the working temperatures for HEM applications but also raise the strong concern for long-term HEM stability. Alternative cationic functional groups based on the same nitrogen element (such as pyridinium, guadinium, and imidazolium) and other elements (phosphorus: phosphonium, sulfur: sulfonium, and ruthenium: bis(terpyridine)ruthenium complex^[2]) have also been introduced, bringing new features (improved solubility, improved thermal stability, and/or increased charge number) for HEMs but the practice suggests their chemical stability still remain limited. The limited chemical stability of ammonium and other cations may be rationalized as the result of their flexible valence electron structures, allowing for many degradation pathways possible.

Alkali metal cations (typically, Li⁺, Na⁺, and K⁺) have excellent chemical stability, because of their non-flexible closed-shell valence electron configurations ([1s²], [2s²2p⁶], [3s²3p⁶], respectively) that are the same as those of noble gases, but they are very challenging to immobilize for HEM applications. Bearing a unit of positive charge, biscyclopentadienyl cobalt sandwich cation (e.g., (C₅H₅)₂Co(III)⁺, or cobaltocenium) satisfies the 18-electron rule for transition metal complexes, in which Co(III) also has closed-shell valence electron configuration. The cobaltocenium hydroxide is considered as a strong base, and in fact the cobaltocenium is the most stable metallocenium in alkaline media^[3]. Equally importantly, cobaltocenium can also be immobilized to polymers in both main-chain and side-chain.

Permethylcobaltocenium, i.e., decamethylated cobaltocenium, is expected to have even higher structure stability than the normal cobaltocenium, due to the charge delocalization caused by strong electron donation from the ten methyl groups^[4]. This is evidenced by over 600 mV negative shift of formal potential observed [−1.24 of (C₅Me₅)₂Co(III)⁺/(C₅Me₅)₂Co(II) vs. −0.63 V of (C₅H₅)₂Co(III)⁺/(C₅H₅)₂Co(II) referring the SHE, the same solvent of CH₂Cl₂^[5]]. Here we present, for the first time, the permethylcobaltocenium-functionalized polymer HEMs that show very high thermal and chemical stability, promising to applications in durable and robust electrochemical devices.

References

[1]          J. R. Varcoe, R. C. T. Slade, Fuel Cells 2005, 5, 187-200.

[2]          Y. P. Zha, M. L. Disabb-Miller, Z. D. Johnson, M. A. Hickner, G. N. Tew, J Am Chem Soc 2012, 134, 4493-4496.

[3]          G. Wilkinson, F. A. Cotton, Prog Inorg Chem 1959, 1, 1-124.

[4]          N. G. Connelly, W. E. Geiger, Chem Rev 1996, 96, 877-910.

[5]          U. Koelle, F. Khouzami, Angewandte Chemie-International Edition in English 1980, 19, 640-641.

Key words: Cobaltocenium, Hydroxide exchange membrane, Stability, Fuel cells, Electrochemical devices.