(330g) Polymer Threaded Metal Organic Frameworks with Ionic and Electrical Conductivity for Electrochemical Applications

We discuss the recent progress made in modifying metal organic frameworks (MOF) to render ionic and electrical conductivity and enable their use in electrodes and membranes. When threaded with polyelectrolytes, MOF can be ion-exchangeable with high capacity [1, 2] as well as ionic conducting with high selectivity [3].

Poly(sodium vinyl sulfonated-co-acrylic acid)~MIL-53(Al) is prepared on anodic aluminum oxide substrate via steps of MOF MIL-53(Al) growth followed by in-situ polymerization [3]. The poly(VS-co-AA)~MIL-53(Al) membrane demonstrates highly specific selectivity in transport of alkali metal cations. Rate of ion transport correlates inversely with the hydrated diameter of the ion reaching a low limiting rate near 0.7 nm hydrated diameter. Charge exclusion is demonstrated with blockage of anion transport under a concentration gradient. The highly uniform porous nanostructure of MOF and ionic function of polyelectrolyte offers the MOF membrane with synergistic selectivity based on exclusion forces of the framework and Coulomb forces from fixed charges of polyelectrolytes in nanochannels.

The latest example is the electrical conducting polyaniline threaded MIL-101 and its application as a electrode material[4]. The combination of a 0.01 S cm^–1 conductivity with ultrahigh surface area of 2065 m² g^–1 offers unique opportunity for application in sensing and energy conversion. The Nernstian-like current response of the Fe²⁺/Fe³⁺ redox couple indicates very rapid transport in the polymer thread MOF electrode, while the high surface area provides orders of magnitude higher current compared to ultramicroelectrode. The potential use of these electrodes in high current applications such as redox flow batteries is discussed.

Acknowledgement

The work described was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R).

[1] L. Gao, C.Y. V. Li, K.Y. Chan, and Z.N. Chen, J. Am. Chem. Soc. 136 (2014) 7209-7212.

[2] L. Gao, C.Y. V. Li, and K.Y. Chan, Chem. Mater. 27(10) (2015) 3601-3608.

[3] L. Gao, K.-Y. Chan, C.-Y. V. Li, L. Xie, and J. F. Olorunyomi, Nano Lett. 2019, doi.org/10.1021/acs.nanolett.9b01211.

[4] C.K. Ho, C.V.Y. Li, L. Gao, K.Y. Chan, J. Chen, J. Tang, J.F. Oloruyomi, C. Liao, and T.S. Zhao, ACS Energy Lett. 2021, 6, 11, 3769–3779.

Figure 1 Cyclic voltammetry curves of PANI∼MIL-101 showing stable Fe²⁺/Fe³⁺ ion selective activity for the first 50 cycles using 0.01 M aqueous FeCl₂/FeCl₃ in 1 M HCl, at a scan rate of 50 mV/s.