Conductive 2D MOF Coupled with Superprotonic Conduction and Interfacial Pseudo-capacitance

It remains challenging to integrate protonic conductive metal-organic frameworks (MOFs) with efficient electrical conductivity. Recently in Matter, Su et al. reported a novel two-dimensional (2D) MOF with carboxylic acid groups showing high electrical conductivity, originating from the superprotonic conduction and protonic/interfacial pseudo-capacitance coupling. It remains challenging to integrate protonic conductive metal-organic frameworks (MOFs) with efficient electrical conductivity. Recently in Matter, Su et al. reported a novel two-dimensional (2D) MOF with carboxylic acid groups showing high electrical conductivity, originating from the superprotonic conduction and protonic/interfacial pseudo-capacitance coupling. Proton-conducting materials have attracted considerable attentions for their role as electrolytes in sensors, batteries, fuel cells, and so on.1Hossain H. Abdalla A.M. Jamain S.N.B. Zaini J.H. Azad A. A review on proton conducting electrolytes for clean energy and intermediate temperature-solid oxide fuel cells.Renew. Sustain. Energy Rev. 2017; 79: 750-764Crossref Scopus (260) Google Scholar Metal-organic frameworks (MOFs) have recently been investigated as possible candidates for proton-conducting applications,2Ramaswamy P. Wong N.E. Shimizu G.K. MOFs as proton conductors--challenges and opportunities.Chem. Soc. Rev. 2014; 43: 5913-5932Crossref PubMed Google Scholar by the virtue of high crystallinity, rich pore structure, easy tailorable chemistry, and systematic structural variation. It is noted that previous works are mainly focused on investigation of three-dimensional (3D) proton-conductive MOFs,3Kim S. Joarder B. Hurd J.A. Zhang J. Dawson K.W. Gelfand B.S. Wong N.E. Shimizu G.K.H. Achieving superprotonic conduction in metal-organic frameworks through iterative design advances.J. Am. Chem. Soc. 2018; 140: 1077-1082Crossref Scopus (201) Google Scholar and the 2D proton-conductive MOFs that are promising for development of new nano/quantum devices are rarely touched. More significantly, single 2D MOF coupled with protonic (ion) and electrical conductance has never been explored. Very impressively, Su et al.4Su J. He W. Li X.M. Sun L. Wang H.Y. Lan Y.Q. Ding M.N. Zuo J.L. High electrical conductivity in a 2D MOF with intrinsic superprotonic conduction and interfacial pseudo-capacitance.Matter. 2020; 2: 711-722Abstract Full Text Full Text PDF Scopus (77) Google Scholar recently reported that the enhanced conductivity and a mixed protonic/electrical conduction could be realized in a novel 2D MOF containing redox-active tetrathiafulvalene (TTF)-based linkers. Owing to the designability of MOFs, three rational strategies might be adopted to incorporate proton carries into MOFs: (i) selecting high-valence metal ions as nodes; (ii) introducing protonated counterions or guest molecules in pores; and (iii) functionalizing the frameworks with acidic groups. In this issue of Matter, Su and colleagues prepared two 2D proton-conductive MOFs, [(CH3)2NH2][In(m-TTFTB)] (MOF-1) and [(CH3)2NH2][In(TTFOC)] (MOF-2), assembled from high-valence In3+ and TTF-based linkers (tetrathiafulvalene tetrabenzoic acid (m-H4TTFTB) and tetrathiafulvalene octacarboxylates (H8TTFOC)) following the above design principles (Figure 1A). Compared with 2D MOF-1, 2D MOF-2 with additional uncoordinated carboxylic acid groups exhibits higher solvent-accessible volume, superior protonic conduction, favorable proton conduction pathway, and unique interfacial pseudo-capacitance. The electrochemical impedance measurement is known to be a powerful method to disclose the bulk proton conductivity behavior.5Müller F. Ferreira C.A. Azambuja D.S. Alemán C. Armelin E. Measuring the proton conductivity of ion-exchange membranes using electrochemical impedance spectroscopy and through-plane cell.J. Phys. Chem. B. 2014; 118: 1102-1112Crossref Scopus (75) Google Scholar In this study, Su and colleagues determined the alternating current (AC) impedance of 2D MOF-1 and MOF-2 in pressed powder pellets under variable humidity and temperature. Notably, the highest proton conductivity of MOF-1 and MOF-2 (343 K and 98% RH) reaches 7.39 × 10−3 and 1.69 × 10−2 S cm−1, respectively. In addition, the temperature and proton conductivity extracted from AC impedance measurement is fitted with the Einstein-Nernst equation, further extrapolating the activation energy of these materials (Figure 1B). The summarized results for MOF-1, MOF-2, and other reported representative materials are listed in Figure 1C at around 303 K and 98% RH. Very impressively, MOF-2 indicates the comparative proton conductivity and much lower activation energy (0.09 eV) even compared with commercial proton conductor Nafion. This low active energy suggests that MOF-2 follows the proton hopping (Grotthuss) mechanism6Grancha T. Soria J.F. Cano J. Amorós P. Seoane B. Gascon J. Garcĺa M.B. Losilla E.R. Cabeza A. Armentano D. et al.Insights into the dynamics of grotthuss mechanism in a pronton-conducting chiral bioMOF.Chem. Mater. 2016; 28: 4608-4615Crossref Scopus (93) Google Scholar (Ea < 0.4 eV), where the conduction of protons migrates among absorbed coordinated water molecule, balanced dimethylammonium cations, and strong hydrogen bond networks formed with uncoordinated carboxylic groups. As for state-of-the-art proton (ion) conductive MOFs, one major challenge lies in how to integrate MOF conductor into real devices because the proton conductive pathway usually blocks at the MOF and electrode interface, thus leading to an overall capacitive charging behavior. Despite numerous progresses that have been made on the mixed electronic and ionic conductivity for the conjugated polymers,7Berggren M. Crispin X. Fabiano S. Jonsson M.P. Simon D.T. Stavrinidou E. Tybrandt K. Zozoulenko I. Ion electron coupled functionality in materials and devices based on conjugated polymers.Adv. Mater. 2019; 31: e1805813Crossref Scopus (83) Google Scholar there is no suitable solution for conductive MOFs until now. In Su’s work, what’s exciting is that 2D superprotonic conductive MOF-2 demonstrates excellent overall direct current electrical conductivity (4.05 × 10−3 S cm-1 at 303 K and 90% RH) as it is placed in contact with metal electrode (Figure 1D), which is comparable with the intrinsic electron conductive MOFs.8Sun L. Campbell M.G. Dincă M. Electrically conductive porous metal-organic frameworks.Angew. Chem. Int. Ed. Engl. 2016; 55: 3566-3579Crossref PubMed Scopus (1169) Google Scholar It deserves to be stressed that the typical current-voltage (I-V) curve of MOF-2 in vacuum only displays a linear behavior and much lower conductivity (1.69 × 10−8 S cm-1), revealing the negligible intrinsic electron conduction of MOF-2. Why does the ionic conductive MOF-2 show high overall electrical conductivity without intrinsic electron conduction? Su et al. innovatively propose a pseudo-capacitance process to explain interfacial conductivity between MOF and electrode. The authors comprehensively elucidate the interfacial pseudo-capacitance (electrical) mechanism with cyclic voltammetry (CV) tests for pure m-H4TTFTB, H8TTFOC, MOF-1, and MOF-2. Upon anodic scanning, both m-H4TTFTB and H8TTFOC in N,N-dimethylformamide (DMF) solution containing 0.1 M LiBF4 display reversible one-electron processes at 0.18 V/0.50 V and 0.17 V/0.42 V (versus Fc/Fc+), which are attributed to the TTF/TTF⋅+ and TTF⋅+/TTF2+ redox couples, respectively. The redox activity of the ligands could inherit to 2D MOF-1 and MOF-2, based on the fact that solid-state direct current CV study on MOF-1 and MOF-2 also shows similar two quasi-reversible one-electron processes (Figure 1E) at 0.17 V/0.50 V and 0.27 V/0.59 V, respectively (versus Fc/Fc+). Importantly, the typical I-V curve of MOF-2 placed in contact with metal electrode at 303 K and 90% RH also exhibits a non-linear I-V behavior and two couples of peaks in the range of −2 V to 2 V (inset of Figure 1D). These two couples of peaks are quite similar to TTF/TTF⋅+ and TTF⋅+/TTF2+ redox couples. Evidently, owing to the redox activity of TTF-based ligands, the conductivity of MOF-electrode interface is considerably enhanced. Hence, the authors deduce that the mixed proton/interfacial pseudo-capacitance conductive mechanism is responsible for overall electrical conduction in the device. Still, some key issues need to be addressed in future work. For instance, there is a long way to integrate proton (ion) conductive MOFs into functional devices, such as electrochemical transistor, electrochromic devices, and surface switches. And furthermore, it keeps unclear whether the interfacial pseudo-capacitance conductive mechanism is general and applicable for the MOFs constructed with other redox ligands. In conclusion, a mixed proton/interfacial pseudo-capacitance conductive mechanism is first demonstrated in 2D MOFs. This work opens the door toward rational design and synthesis of new ionically and electrically conductive MOFs for emerging energy and information devices. High Electrical Conductivity in a 2D MOF with Intrinsic Superprotonic Conduction and Interfacial Pseudo-capacitanceSu et al.MatterJanuary 22, 2020In BriefIntegrating efficient ionic and electrical conduction in metal-organic frameworks (MOFs) is desired for their applications in clean energy technologies. We present the rational design and synthesis of an MOF with unbound carboxyl groups that facilitate high proton conductivity and redox-active ligands that mediate efficient electrical conduction at the MOF-metal interface through a coupled ionic/pseudo-capacitive conduction mechanism. The design strategy presented here offers guidance to the future development of ionically and electrically conductive MOFs for energy-storage devices. Full-Text PDF Open Archive