A 3D open-framework iron hydrogenophosphate showing high proton conductance under water and aqua-ammonia vapor

Abstract

Herein we report the first example of the proton conductivity of an open-framework metal phosphate (NH₃(CH₂)₃NH₃)₂–[Fe₄(OH)₃(HPO₄)₂(PO₄)₃]·4H₂O under aqua-ammonia vapor. Its optimized proton conductivity is 5 × 10⁻² S cm⁻¹ at 313 K and aqua-ammonium vapor from 1 M NH₃·H₂O solution. That is approximately two orders of magnitude greater than the maximum value under water vapor (8.0 × 10⁻⁴ S cm⁻¹ at 317 K and 99% RH). The proton transfer mechanism has been proposed in terms of the structural analyses, activation energy calculations, and PXRD determinations.

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Open-framework metal phosphates, as a popular kind of porous material, have been widely explored for their potential applications in catalysis, gas separation and magnetism.^1–4 Recently, proton-conductive of open-framework metal phosphates have been becoming an emerging area in the field of fuel cells, in virtue of their porous structure, good proton carriers and hydrogen-bond supported nets, etc.^5–11 Similar to metal–organic frameworks (MOFs),^12–18 open-framework metal phosphates have a designable porous structure, in which the channel can provide an efficient path for proton transmission; then usually inorganic metal skeletons are negatively charged, the counter ions in the channels such as NH₄⁺, enH₂⁺ are good proton carriers and also help to establish efficient hydrogen-bonding networks for proton hopping or moving. Furthermore, the rigid framework of metal phosphates, which consist of metals and phosphates, endow them with better water and chemical stability than MOFs. Indeed, open-framework metal phosphates have been considered to have important application prospects as proton conductors.

High proton conductivity is one of the important properties of proton conductors. Two methods, which can promote proton conductivity of porous solid proton conductors, like MOFs, porous coordination polymers (PCPs)^19,20 and covalent organic frameworks (COFs),^21–24 have been well documented, including (1) through modifying the pores of the framework with functional groups (–SO₃H, –COOH, etc.);^25–27 (2) through introducing proton conducting chemicals or proton carriers into the pores or layers of these materials¹⁶ and (3) through varying the proton conduction environment (such as temperature, relative humidity (RH) and the vapor environment). For example, Luo²⁸ and coauthors doped imidazole into the pores of the MOF-808, raising the proton conductivity of MOF-808 to 3.45 × 10⁻² S cm⁻¹ at 338 K and 99% RH. We introduced Na⁺ ions into the inter-layer of a two-dimensional layered inorganic–organic hybrid metal hydrogenophosphate (C₂H₁₀N₂)[Mn₂(PO₄)₂]·2H₂O via ion exchange method. The Na⁺ ions sharply improve the amount of water sorption of (C₂H₁₀N₂)[Mn₂(PO₄)₂]·2H₂O, and promote the formation of a dense and extensive hydrogen-bonding network with water molecules, thus the proton conductivity of the ion exchange product rises to 10⁻² S cm⁻¹ at 283 K and 99% RH.²⁹ That was more than 900 times higher than (C₂H₁₀N₂)[Mn₂(PO₄)₂]·2H₂O under the same conditions. In addition, the role of water molecule is the key to achieve high proton conductivity for water-mediated proton-conducting materials; therefore, adjusting RH is a common method to vary the proton conductivity. In general, it is found that proton conductivity increases with the rising of RH. In particular, ammonia has a molecular system most similar to that of water, specifically three proton donor sites and one acceptor site with a tetrahedral geometry, a small molecular size (kinetic diameter 2.60 Å) and a degenerate conjugate acid–base system (NH₄⁺ + NH₃ ↔ NH₃ + NH₄⁺).^30,31 Therefrom, we can speculate that ammonia may be a nest promising proton conducting medium. Very recently, Li^32,33 group reported the proton conductivity of two isostructural 3D Co(II) MOFs under aqua-ammonia vapor, both displays that the proton conductivities under aqua-ammonia vapor are significantly enhanced than those under water vapor. This exciting result inspires us to study the open-framework metal phosphates in similar situations to explore the methods to improve the conductivity.

Herein we synthesized a three-dimension metal phosphate (NH₃(CH₂)₃NH₃)₂–[Fe₄(OH)₃(HPO₄)₂(PO₄)₃]·4H₂O (1),³⁴ and investigated its proton conductivity under water and aqua-ammonia vapor. Our investigation shows that 1 exhibit high proton conductivity in water vapor; then in the aqua-ammonium vapor, proton conductivity sharply increased and reached 5 × 10⁻² S cm⁻¹ (at 313 K and under aqua-ammonium vapor from 1 M NH₃·H₂O solution).

Compound 1 crystallizes in the tetragonal space group I4₁/a, its asymmetric unit contains two crystallographically independent Fe ions (labeled as Fe(1) and Fe(2)), one HPO₄²⁻ ion (labeled as P(1)), two different PO₄³⁻ ions (labeled as P(2) and P(3)), four coordination water molecules (labeled as O(13), O(14), O(15) and O(16)) and together with two protonated diction of propylenediammonium.

As shown in Fig. 1a, two Fe atoms (Fe(1) and Fe(2)) are both coordinated with six oxygen atoms to form FeO₆ octahedra, respectively. Fe(1) is hexacoordinated by three oxygen atoms from PO₄³⁻ anions, one oxygen atoms from HPO₄²⁻ anions and another two bridging oxygen atoms from OH⁻. Different with that, Fe(2) is ligated by two oxygen atoms from PO₄³⁻ anions, two oxygen atoms from HPO₄²⁻ anions and the rest two bridging oxygen atoms from OH⁻. The connectivity of the three PO₄³⁻ tetrahedral are shown in Fig. 1b. For HPO₄²⁻ (P1) and PO₄³⁻ (P2), three oxygen atoms connected with three Fe²⁺, respectively, and the rest oxygen atoms naked; while for PO₄³⁻ (P3), all oxygen atoms are connected with Fe²⁺.

Fig. 1 (a) The Fe₄O₂₀ cluster unit with the symmetric codes: H* = 1 − x, 2 − y, z; M* = 1 − x, 1.5 − y, 0.25 − z; (b) the connectivity of three HPO₄²⁻ anions; (c) polyhedral representation for a single chain structure constructed by PO₄³⁻ and Fe³⁺; (d) polyhedral view of the structure of 1 along the a axis.

Two crystallographically different Fe(2)O₆ edge-sharing octahedral into a dimmer and it further linked with two Fe(1)O₆ octahedral into a Fe₄O₂₀ cluster unit (Fig. 1a). Along the a-axis, the Fe₄O₂₀ cluster unit form an infinite chain via –Fe(1)–O(12)–Fe(1)– bonds with the neighboring two tetramers (Fig. 1c); the chains are interlinked with each other via phosphate tetrahedral to generate a three-dimensional framework structure with large tunnels parallel to (1 0 0) and the diprotonated 1,3-diaminopropane and water molecules order arranged in the channel along the a-axis (Fig. 1d).

As indicated in Fig. 2, 1,3-diaminopropane and water molecules located at the channel via hydrogen bonding interaction. Water molecules are assembled into an infinite chain parallel to a-axis in the sequence O15W–O16W –O16W–O15W–O14W–O13W–O13W–O14W and with O⋯O distances vary from 2.448 to 2.580 Å, which are much shorter than 2.850 Å in liquid water. Then, the short O⋯N distances with dO(14)⋯N(2) = 2.558 Å exist in the 1,3-diaminopropane and water molecules, and dN(1)⋯O(1) = 2.840 Å, dN(1)⋯O(2) = 2.964 Å, dN(1)⋯O(3) = 2.918 Å, dN(1)⋯O(6) = 2.931 Å, dN(1)⋯O(8) = 3.044 Å and dN(2)⋯O(10) = 2.763 Å between 1,3-diaminopropane and the inorganic framework. Such highly ordered infinite water chains and strong hydrogen bonding interaction between 1,3-diaminopropane and inorganic framework are the potential proton conducting pathway.

Fig. 2 Hydrogen-bonding interactions in water molecules and between water and ammonia molecules.

Fig. S1† shows the PXRD profiles of 1 and its sample after exposure in water solution for a month, together with the simulated PXRD pattern of 1. All of the profiles show high similarity, which indicates that 1 has ultrastrong storage stability. TG and DTA curves of 1 are displayed in Fig. S2.† Three-step decomposition process appeared in the TG curves. The first decomposition step in the range of 298–400 K exhibited 8.3% weight loss, which is close to the calculated value (7.96%) that corresponds to the loss of crystal water in 1. This weight-losing process is demonstrated in the DTA plot of 1, in which a peak temperature at 380 K is associated with this process. The second decomposition process starting at 400 K can be attributed to the loss of –OH groups in the inorganic framework. The rest decomposition step that started at 503 K corresponds to the decomposition of charge-compensating cations. The TG and DTA results demonstrated that 1 is thermally stable below ca. 500 K.

The temperature dependence of PXRD profiles are shown in Fig. S3.† The PXRD patterns below 423 K are almost the same as 303 K, indicating that the structure of 1 is stable although the water molecules in 1 had lost in the temperature range of 303–400 K. As the temperature further rises to 453 K, the PXRD profiles come to slight change, the diffraction peak of (2 0 0) planes slightly shift toward to higher 2θ degree indicating that the loss of the water molecules in the framework reduces the layer distance. When the temperature reached 513 K, the framework of 1 began to collapse with the strongest diffraction peak of (1 0 1) disappeared. The PXRD datas are consistence with TG that the structure is stable before 500 K.

The ac impedance of powdered sample of 1 under different RH and temperature was carried out. Notably, as shown in Fig. S4,† the proton conductivity of compound 1 is so low (7.3 × 10⁻¹² S cm⁻¹ at 403 K and 3.1 × 10⁻¹⁰ S cm⁻¹ at 503 K) under anhydrous condition that it can be ignored. However, after the compound 1 was equilibrated at various relative humidity for 72 h, they exhibit stable conductivity values. To clarify the effect of RH on proton conductivity, the humidity-dependent Nyquist plots of the proton conductivity at 303 K and in the RH range of 60% to 99% were measured. The resultant Nyquist plots are displayed in Fig. S5.† It is to be pointed out that 1 shows unregular semi-circle in the high frequency at low humidity. Nevertheless, at high humidity, the unregular arc in the high frequency and a spike in the low frequency range were observed corresponding to the boundary resistance and a electrode contribution, respectively. Proton conductivity was calculated by fitting the Nyquist plots with the Z-View program. The calculated conductivities, shown in Fig. 3, vary from 2.1 × 10⁻⁹ S cm⁻¹ (60% RH) to 1.62 × 10⁻⁸ S cm⁻¹ (75% RH), and reach a maximum value of 4 × 10⁻⁴ S cm⁻¹ (99% RH). The results indicate that the proton conductivity of 1 is humidity-independent and the conductivity value increases with increasing RH, which may be attribute to the increasing amount of water molecules at high humidity can form more H-bonds with 1, and promote the transport of protons, increasing the conductivity.

Fig. 3 Plots of σ vs. RH at 303 K for compound 1.

We further measured the Nyquist curves at fixed humidity (99% RH) and varied temperature range of 287 K and 317 K (Fig. 4). The resultant resistance of 1 decreased with elevating temperature, indicating that proton conductivity increased as the rising of temperature. For example, the proton conductivity of 1 is 2.0 × 10⁻⁴ S cm⁻¹ at 287 K, while with the temperature was raised to 317 K, the proton conductivity increased to 8.0 × 10⁻⁴ S cm⁻¹, which is comparable to some recently reported MOFs, such as Zn-(m-H₆L) 1.39 × 10⁻⁴ S cm⁻¹ (at 314 K and 98% RH),³⁵ PA@Tp-Azo 9.9 × 10⁻⁴ S cm⁻¹ (at 332 K and 98% RH).³⁶

Fig. 4 Nyquist plots of compound 1 at 99% RH.

Fig. 5 gives the plot of ln(σT) against 1000/T at 99% RH. The activation energies (E_a) for proton conduction of 1 were calculated by fitting the curve with Arrhenius equation.

Fig. 5 The corresponding conductivity in the form of ln(σT) vs. 1000/T for 1.

In equation, the symbol σ represents the proton conductivity, E_a is the proton transport activation energy, k_B is the Boltzmann constant, and A is the pre-exponential factor. The calculated E_a value is 0.32 eV indicating that 1 follow the Grotthuss mechanism, which may that the formation of hydrogen bonding for the immersion of water molecules into the pores are favorable to proton hopping. Besides that, samples used for proton conductivity testing under water vapor of 1 were determined by PXRD. As shown in Fig. S1,† the PXRD peaks are similar with as-synthesized and simulated data, which demonstrates that the structure of 1 keeps robust after ac impedance determination under water vapor condition.

Based on the above descriptions, we should notice that the optimization proton conductivity of 1 (8.0 × 10⁻⁴ S cm⁻¹ at 99% RH and 317 K) is apparently smaller than the Nafion-based proton-conducting material (σ = 10⁻¹ to 10⁻² S cm⁻¹), which is the unique proton conductive material for industrial application at present, and its proton conductivity is recognized as a benchmark for transcendence. Inspired by some recent research results and the summarized methods to improve the proton conductivity of porous solids, we hope to vary the vapor environment for obtaining high proton conductivity. Specially, we attempted to improve the proton conductivity of 1 by changing the pure water vapor into aqua-ammonia vapor.

The proton conductivity of 1 under aqua-ammonia vapor, obtained from NH₃·H₂O solutions, was performed by the ac impedance method. Fig. 6 gives the impedance spectra for 1 at 303 K and different aqua-ammonia vapors from 0.01 to 1 M of NH₃·H₂O. The conductivity, calculated by fitting the Nyquist plots with Z-View program, increased from 3.6 × 10⁻⁴ S cm⁻¹ (0.01 M) to 4.5 × 10⁻² S cm⁻¹ (1 M), indicating two orders of magnitude increase with the increasing concentration of NH₃·H₂O solution. Not only that, the proton conductivity of 1 also showed temperature depended, and the resultant proton conductivity increased with elevating temperature.

Fig. 6 Nyquist plots of compound 1 at different concentration of NH₃·H₂O solution.

For example, the proton conductivity is 2.2 × 10⁻³ S cm⁻¹ at 283 K and it increases to 6.5 × 10⁻³ S cm⁻¹ at 313 K with 0.1 M NH₃·H₂O solution; the proton conductivity increased from 2.3 × 10⁻² S cm⁻¹ (283 K) to 5.0 × 10⁻² S cm⁻¹ (313 K) with 1 M NH₃·H₂O solution (Fig. S6, Table S1†). These results demonstrated that both high temperature and high concentration aqua-ammonia vapor can promote the improvement of proton conductivity. The maximum proton conductivity value of 1 (5.0 × 10⁻² S cm⁻¹) is comparable to that of Nafion and has exceed some reported MOFs under aqua-ammonia vapor and several MOFs containing NH₃ or NH₄⁺ components, such as {[Co₃(DMPhIDC)₂(H₂O)₆]·2H₂O}_n (4.11 × 10⁻³ S cm⁻¹ at 373 K with 1.5 M NH₃·H₂O solution), {[Co₃(m-BrPhIDC)₂(H₂O)₆]·2H₂O} (5.07 × 10⁻⁴ S cm⁻¹ at 373 K with 1.5 M NH₃·H₂O solution),³³ Ca-PiPhtA-NH₃ (6.6 × 10⁻³ S cm⁻¹ at 297 K and 98% RH).³⁷

It also should be noted that, even with relatively low concentration NH₃·H₂O solution (0.1 M to 0.5 M), the proton conductivity values can be promoted. For instance, the proton conductivity is 6.5 × 10⁻³ S cm⁻¹ at 313 K with 0.1 M NH₃·H₂O solution and it is 1.3 × 10⁻² S cm⁻¹ at 313 K with 0.5 M NH₃·H₂O solution. That is approximately one order and two orders of magnitude greater than the proton conductivity of 1 at 317 K and 99% RH.

The activation energy of 1 under aqua-ammonia vapor was estimated by using the Arrhenius equation to fit its Nyquist plots. The E_a values are listed in Table S1,† and indicating that E_a > 0.4 eV while the NH₃·H₂O solution is less than 0.1 M versus E_a < 0.4 eV as the NH₃·H₂O solution in the range of 0.5 M and 1 M. These results imply that the proton transfer in 1 follows an efficient Grotthuss mechanism with higher concentration of NH₃·H₂O solution, while the vehicle mechanism is under lower concentration of NH₃·H₂O solution. This may that higher concentration of NH₃·H₂O solution form more dense hydrogen bonding network and favor to proton hopping; while lower concentration of NH₃·H₂O solution form a relatively sparse hydrogen bonding network and favor to proton transport.

In addition, the PXRD patterns of samples of 1 underwent proton conductivity testing under aqua-ammonia vapor were shown in Fig. S1.† The peaks are similar with as-synthesized and simulated data of 1, demonstrating that the structure of 1 has strong stability. This is one of the important properties as a good proton conductor material in the practical application.

In summary, we successfully prepared a 3D open-framework iron hydrogenophosphate 1 with excellent thermal and chemical stability and explored its proton conduction under water and aqua-ammonia vapor. We found that compound 1 possessed high proton conductivity at room temperature, especially under aqua-ammonia vapor. The corresponding proton-conducting mechanisms under water and aqua-ammonia vapors were discussed according to experimental results. In addition, the excellent structural stability of 1 underwent electrochemical experiment under water and aqua-ammonia vapor was confirmed by PXRD measure, respectively.

Our work provides a new easy-to-execute method to drastically improve the proton conductivity of open-framework metal phosphates.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the National Natural Science Foundation of China (Grant No. 51604155), the Foundation of the Jiangsu Education Committee (17KJB150028), the School Project of Nanjing Xiaozhuang University (2019NXY19) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra00270d

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