Direct Observation of Two Types of Proton Conduction Tunnels

Apr 7, 2014 - Feng-Ming Zhang , Long-Zhang Dong , Jun-Sheng Qin , Wei Guan , Jiang Liu , Shun-Li Li , Meng Lu , Ya-Qian Lan , Zhong-Min Su , and Hong-...
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Direct Observation of Two Types of Proton Conduction Tunnels Coexisting in a New Porous Indium−Organic Framework Xiang Zhao,† Chengyu Mao,† Xianhui Bu,*,‡ and Pingyun Feng*,† †

Department of Chemistry, University of California, Riverside, California 92521, United States Department of Chemistry and Biochemistry, California State University, Long Beach, 1250 Bellflower Boulevard, Long Beach, California 90840, United States

‡

S Supporting Information *

F

ligand approach is able to create proton conduction tunnels with varied chemical and structural environments, which represents a new direction for designing proton conducting materials. One of the recent studies involving mixed trisulfonate and triphosphonate anions has resulted in conductivity over 10−2 S cm−1.31 Here, we focus on a unique combination between oxalate and 4,5-imidazoledicarboxylate (imdc). The use of imdc is because it is more versatile than oxalate. H3imdc has three deprotonatable protons and can provide an extra proton to promote conduction (compared to H2ox), without compromising its ability to form the chelating mode similar to that of oxalate (Supporting Information Figure S1). Even though imdc has been studied in MOFs,34 this is the first time that it is introduced for the purpose of enhancing proton conduction. Instead of commonly used divalent metal ions,24−26 In3+ is chosen in this work because In-MOFs are well-known for their tendency to form a charged framework with mobile chargebalancing species such as NH4+ and H3O+, and such features may enhance the proton conductivity. Another consideration is that the new material should be moisture stable both for watermediated proton conduction enhancement and for tolerance toward moisture under operating conditions. Taking these factors into consideration, our syntheses were performed in a highly basic aqueous solution with the addition of ammonia, in which indium nitrate hydrate, oxalic acid, and 4,5-imidazoledicarboxylic acid were dissolved. Square shaped colorless crystals were obtained. The structure of the compound (denoted CPM-102) was determined by single crystal crystallography under low temperature (∼150 K), which gives a formula of [In(imdcH)(ox)]·(NH4)(H2O)1.5. The crystal structure can be understood as negatively charged [In(imdcH)(ox)]− layers with charge balancing cation NH4+ and neutral H2O guests located at the interlayer space. Each indium ion is eight-coordinated in a chelating manner with two 4,5-imidazoledicarboxylate and two oxalate ligands (Supporting Information Figure S9). Due to the coexistence of two kinds of ligands, three types of 4-membered rings are found in each layer alternatively, with the ring compositions of In4(ox)4 (I), In4(imdc)4 (II), and In4(ox)2(imdc)2 (III), respectively, in a 1:1:2 ratio (Figure 1a). These layers adopt the ABCDABCD stacking sequence

ast ion conductors lie at the foundation of many energyrelated applications such as batteries and fuel cells.1−3 Proton conduction is a particular class of ionic conduction and is especially relevant to fuel cell applications. The state-of-theart proton conduction materials are mainly based on sulfonated polymers such as Nafion.4 There has been an increasing interest in other types of materials that may offer advanced applications at lower cost, higher efficiency, or different operating conditions.5 Metal−organic frameworks (MOFs) have been widely studied for its porosity and the capability to adsorb and separate gas molecules.6−10 Recently, the interest in MOFs has been broadened to many other areas such as ionic conduction.11−23 The well-defined structure of MOFs makes it possible to pinpoint the proton conduction pathway, which can lead to a better understanding of the proton conduction mechanism. Furthermore, the designable architecture, the ease with which functional ligands can be introduced into framework design, and the capability to incorporate different guest species provide numerous opportunities to improve the proton conduction property by materials design. Metal oxalates are known to exhibit high proton conduction, especially under high humidity environment.24−29 One example is Humboldtine, which is a 1-D coordination polymer with the formula of Fe(ox)·2H2O. It is believed that the strong hydrogen bonds between water molecules and oxygen atoms from oxalate ligands provide a pathway for proton conduction.24 Another example is a honeycomb layered structure of zinc oxalate with the formula of (NH4)2(adp)[Zn2(ox)3]· 3H2O.25 In this structure, an extensive hydrogen bonding network was formed between oxalate oxygens and guest species including NH4+ and adipic acid. A recent advance was observed for a closely related compound, which also contains similar [Zn2(ox)3]2− honeycomb structure, but interpenetrated with another cationic net with the formula of [(Me2NH2)3SO4]+. This compound has shown the highest water assisted conductivity so far with a value comparable to that of Nafion.26 Oxalate, phosphonates, and sulfonates are also among the fast proton conducting materials.30−33 The common feature of these ligands is a high density of oxygen atoms. These oxygen sites contribute to the formation of extended H-bonding network, and also exposed Lewis base sites can function as relays in proton transport. However, the above-mentioned ligands only represent a small portion of the large pool of potential useful ligands. Many opportunities can be realized through the combination of two or more ligands. The mixed © 2014 American Chemical Society

Received: February 8, 2014 Revised: February 28, 2014 Published: April 7, 2014 2492

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conduction (Figure 1c). The type I proton tunnel goes through the interlayer space between type I and type III rings, in a direction where two oxalate ligands from adjacent layers are stacked to each other. In contrast, the type II proton tunnel goes through the interlayer space between type II and type III rings, in a direction where two adjacent imdc ligands are stacked. The two types of proton tunnels are arranged alternatively in the same layer, but adopt the perpendicular orientation from those in the adjacent layer. As illustrated in Figure 2a, the type I channel is highly hydrophilic, with 16 oxygen atoms surrounding four guest molecules. The guest species are so well-defined that even the hydrogen atoms can be precisely located from X-ray data. This allows us to probe the detailed mechanism for the proton conduction. Two NH4+ cations and two H2O molecules are found within the interlayer space between a type I ring and a type III ring. These four guest molecules are connected with each other through H-bonds to form a square. The hydrogen atoms pointing away from the square form 12 additional hydrogen bonds with the oxygen atoms from surrounding oxalate and imdc ligands (Figure 2b). In fact, the distance between the closest N and O atoms (N3···O9c ∼ 3.37 Å) from adjacent squares also allows the formation of a hydrogen bond, although the hydrogen atoms observed at 150 K are not at appropriate positions to form hydrogen bonds between adjacent squares (N3···O9c) because they tends to be located at their most stable position in the “frozen” condition. However, at elevated temperatures, the higher thermal energy may activate these guest species and allow them to vibrate and rotate. Thus, all the NH4+ and H2O guest molecules in the channel form railway-shaped double chains through hydrogen bonding (Figure 2c), which provide a straight pathway for proton conduction.

Figure 1. Illustration of (a) the periodical pattern of each layer constructed from three types of 4-membered rings; (b) the ··· ABCDABCD··· layer stacking sequence and (c) two types of proton tunnels embedded in the interlayer space and their spatial distribution.

(Figure 1b). It is found that the type III ring is sandwiched between a type I ring and a type II ring from adjacent layers. In contrast, both type I and type II rings are sandwiched between two type III rings. An analysis of the structure revealed an extensive H-bonding network which includes both guest NH4+ and H2O species and oxygen atoms from the host layer (Supporting Information Table S1 and Figure S4). In fact, these H-bonds help connect adjacent layers together to create a 3-D network. Interestingly, there are two distinctly different groups of hydrogen bonds, both of which forms tunnels that contribute to proton

Figure 2. (a) Perspective view along [110] direction of type I proton tunnel; (b) the interlayer space between type I and type III rings and related hydrogen bonds; (c) hydrogen bonds along type I channel; (d) perspective view along [110] direction of type II proton tunnel; (e) the interlayer space between type II and type III rings; and (f) hydrogen bonds along type II channel. (Gold: In; red: O; blue: N; gray: C; yellow: H.) 2493

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crystal data, we are able to get a close look for the possible proton conduction pathway, which is quite valuable for us to understand its proton conduction mechanism. Therefore a detailed analysis of the H-bonding networks in both channels has been performed. Interestingly, this compound has integrated two types of proton carriers in its structure, the guest counterion NH4+ and a nondeprotonated carboxylic acid group from imdc ligand in the host. In the type I tunnel, the major proton carrier is NH4+. Under applied electrical field, a proton from the ammonium ion can hop to an adjacent water molecule and form a hydronium ion (Supporting Information Figure S11). Then a proton can be dissociated from the hydronium ion and further hop to another ammonia molecule, either on the side or in front. Alternatively, the proton from the ammonium ion can also hop to the coordinated oxygen atoms on the host and then the proton can either migrate along the surface of the host or hop back to the guest chains through hydrogen bonding. Compared to the guest−host pathway, the guest−guest direct pathway is straighter but the longer distance between adjacent nonhydrogen atoms requires higher energy to activate the proton to cross over the barrier. Thus, it is expected that both pathways will contribute to the proton conduction, and they will achieve a balance with each other. With increased temperature, the guest−guest direct pathway will ultimately dominate the conduction. In the type II tunnel, the major proton carrier becomes the carboxylic acid group. When electrical field is applied, the proton can be activated and can hop between carboxyl groups on adjacent imdc ligands and achieve conduction. Alternatively it can also hop from a carboxyl group to a guest water molecule to form a hydronium ion, and then the proton from the hydronium ion can either hop back to another carboxyl group or hop to an oxalate ligand and migrate on the surface of the layer. In Supporting Information Figure S3, the hydrogen bond networks are illustrated under different hydrogen bond lengths, which can give a simple map for the proton conduction pathway (the shorter, the easier). It is necessary to point out that the proton conduction is a complicated phenomenon and that the distance between donor and acceptor atoms is only one factor. Other important factors such as the electronegativity of the element and the coordination geometry also need to be considered. In summary, we have created a unique layered compound in which two types of proton tunnels coexist. This phenomenon has been never reported before and results from our use of the mixed oxygen-rich ligand strategy that helps to create two distinct tunnels. Impedance spectroscopy studies of both bulk sample and single crystals have shown good conductivity. The proton conduction pathway through H-bonding has been mapped out, and the possible conduction pathway is proposed. The introduction of imdc ligand for building proton conducting MOFs has been demonstrated as successful for its dual role: it helps to create an extensive H-bond network as well as to serve as a proton carrier in its dinegative form. We believe that this research will open up new opportunities for developing advanced MOFs with superb proton conducting properties by taking advantage of properly designed or chosen oxygen-rich ligands.

Type II channel is based on the dangling carboxylate oxygens from imdc ligands and the guest water molecule and is narrower than the type I channel. Still, it has a high density of oxygen atoms and is hydrophilic (Figure 2d). Unlike in type I channel, the positions of hydrogen atoms in the type II channel are not well-defined, likely due to the orientational disorder of H2O, and are thus modeled into statistic distribution, which correlates well with its thermal ellipsoids (Supporting Information Figure S10). Still, the location of the water molecule is always between a type II ring and type III ring, near the type III ring end (Figure 2e). The distance between adjacent carboxylate oxygens and water oxygen allow the formation of hydrogen bonding, which is responsible for the second type of proton conduction pathway (Figure 2f). Since the imdc ligand only lost two protons in coordination with indium ion, it contributes an extra hydrogen for proton conduction, by hopping from one oxygen site to another through the H-bonding network. AC impedance spectroscopy was performed on a compressed pellet of the crystalline powder sample coated with Pt/Pd electrodes. The Nyquist plot is shown below. The semicircle in the high frequency region is likely to be attributed to bulk and grain boundary resistance, and the tail at low frequency is related to the limited diffusion of mobile ions at the electrode− electrolyte interface. Under 98.6% relative humidity (RH) and 23.5 °C, the proton conductivity reaches 0.82 × 10−3 S cm−1 (Figure 3), which is comparable to the best known proton

Figure 3. Nyquist plots of (a) the pellet sample at 23.5 °C and 98.6% relative humidity and (b) the single crystal along the ab plane at 22.5 °C and 98.5% relative humidity.

conducting MOF materials developed so far (Supporting Information Table S2). The Nyquist plots obtained under different humidity conditions (from 70% to 98.6% RH) at room temperature show that the proton conductivity is strongly humidity dependent (Supporting Information Figure S7). To further investigate the conductivity along specific directions in the crystalline solid, single crystal conductivity measurement was performed on a square plate shaped crystal with dimensions of about 0.35 mm × 0.35 mm × 0.075 mm. The electrodes are made by applying a melting-resolidification cycle of gallium metal (Supporting Information Figure S8). The inplane conductivity was measured as 1.11 × 10−2 S cm−1 under 98.5% RH and 22.5 °C. The strong humidity dependence was also observed for the single crystal sample. Due to the small channel, it is unlikely for the guest ammonium and water species to transport for a long distance within the channel. Instead, local molecular motions such as vibration and rotation are possible. Thus the vehicle mechanism for proton conduction can be excluded, and it is expected that the proton conduction within this compound is dominated by the Grotthuss mechanism.35,36 Thanks to the precisely refined 2494

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, powder X-ray diffraction, thermal analysis, IR spectrum, additional figures and tables, crystallographic table, and CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(X.B.) E-mail: [email protected]. *(P.F.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DEFG02-13ER46972.

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REFERENCES

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