Enhancing Ionic Conductivity in Metal-Organic Frameworks
Ricardo F. Mendes, Filipe A. Almeida Paz
Department of Chemistry, CICECO – Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal
rfmendes@ua.pt
Research on Metal-Organic Frameworks is currently driven towards the need to employ such materials in technological areas. Our research group has focused on the design of networks based on polyphosphonic acid ligands and rare-earth cations now envisaging energy applications. This work presents our recent efforts on designing better performing MOFs that take advantage of proton/ionic mobility and/or exchange. In one approach the conductivity of a positively-charged MOF based on a triphosphonic acid with Cl⁻ ions in the channels ([Gd(H₄nmp)(H₂O)₂]Cl∙2H₂O, H₆nmp = nitrilo(trimethylphosphonic) acid) was increased during the humidity-induced single-crystal to single-crystal transformation, reaching a value of 0.51 S cm⁻¹ at 98% relative humidity (RH) and 94 ºC.⁽¹⁾ This increase was observed during the transformation process, attributed to the high mobility of the Cl⁻ anions. On a second approach, we were able to improve the conductivity of a 3D material by a simple incorporation of a cation in the channels. The incorporation of K⁺ into the pores of [Ln(H₃pptd)]·xSolvent (H₆pptd = (5’-(4-Phenylphosphonic acid)-[1,1’:3’,1’’-terphenyl]-4,4’’-diyl)diphosphonic acid) led to a considerable increase in conductivity up two orders of magnitude, reaching 2.1 x 10⁻² S/cm at 40 ºC and 20% (RH) and 0.19 S cm⁻¹ at 94 °C and 98% RH.⁽²⁾ These two examples are, up to this moment, ranked among the highest conductivity values reported for these type of materials.
Fig.1 – (a) Schematic representation of the Cl⁻ mobility in [Gd(H₄nmp)(H₂O)₂]Cl∙2H₂O at high temperature and humidity. (b) Conductivity improvement of [Ln(H₃pptd)]·xSolvent by incorporation of K⁺ cations.
This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, FCT Ref. UIDB/50011/2020 & UIDP/50011/2020, financed by national funds through the FCT/MCTES and when appropriate cofinanced by FEDER under the PT2020 Partnership Agreement. RFM also gratefully acknowledge FCT for the Junior Research Position (CEECIND/00553/2017). The authors also acknowledge the contribution of Sérgio M. F. Vilela, Jorge A. R. Navarro, Paula Barbosa, Germán Pérez-Sánchez, Harriott Nowell, Duarte Ananias, Filipe Figueiredo, José R. B. Gomes, João P. C. Tomé, Eddy M. Domingues and Patrícia Silva for work developed.
References
- “Enhanced proton conductivity in a layered coordination polymer”, R. F. Mendes, P. Barbosa, P. Silva, E. M. Domingues, F. Figueiredo, F. A. Almeida Paz, Chem. Science (2020), Volume 11, 6305-6311
- “Multifunctionality in an Ion-Exchanged Porous Metal-Organic Framework”, S. M. F. Vilela, J. A. R. Navarro, P. Barbosa, R. F. Mendes, G. Pérez-Sánchez, H. Nowell, D. Ananias, F. Figueiredo, J. R. B. Gomes, J. P. C. Tomé, F. A. Almeida Paz, Journal American Chemical Society (2021), Volume 143, 1365−1376