A soft processible polyoxometalate-zwitterion eutectic electrolyte for superprotonic conduction

Abstract

Proton conductors are important for propelling the development of energy and electronic technologies. Polyoxometalates (POMs), as a class of crystalline solid superacids with nanocluster form and high proton conductivity, are intriguing candidates for this. However, the use of POMs as practical proton conductors is hindered by their low processability coming from inherent brittleness, and their humidity-dependent conductivity. Here, we report a deep eutectic strategy to convert brittle POM powders to soft processible electrolytes through supramolecular complexation with hydroxyl‑containing zwitterions. The resulting POM-zwitterion eutectics exhibit good malleability and high modulus over 1 MPa, promoting them easily processed into self-supporting eutectics. Due to the soft state and percolated hydrogen bonding networks, these eutectics can transport protons by both diffusing and hopping processes, that is, a Vehicle-Grotthuss synergistic mechanism, which enables them to show superprotonic conductivities up to 1.0 × 10⁻³ S cm⁻¹ at 30 °C (25% RH) and 2.4 × 10⁻² S cm⁻¹ at 150 °C (anhydrous condition). This work paves a new route to designing advanced electrolyte materials based on the supramolecular eutectic concept.

Graphical abstract

A supramolecular eutectic strategy is developed to convert brittle polyoxometalates into soft superprotonic conducting electrolytes through simply solid-state mixing with hydroxylated zwitterions. These electrolytes are well processible and show superprotonic conducting behaviors based on a hopping-diffusing synergistic mechanism.

1. Introduction

Proton conductors have widespread applications in energy and electronic technologies, such as fuel cells, batteries, and sensors [[1], [2], [3], [4]–5]. Achieving high proton conductivity and excellent processibility is highly required to develop such materials. To this end, various candidates including ceramic oxides, solid acids, and acid-containing polymers have been explored [[6], [7], [8], [9], [10]–11]. Among them, polyoxometalates (POMs), as a kind of nanoscale metal-oxide clusters with super strong Brønsted acidity, have attracted considerable attention [[12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]–23]. However, the proton conductivity of POMs is tightly dependent on their hydration level, which can decrease several orders of magnitude after POMs lose water [24,25]. This drawback seriously limited the use of POMs to transport protons at low humidity or anhydrous conditions. On the other hand, POMs are generally prepared as brittle crystals or powders [[26], [27], [28], [29], [30], [31], [32]–33], which makes it difficult to convert the complexes with high content of POMs into flexible materials for device requirements. In this context, a strategy to modify POMs into well-processible electrolytes with high proton conductivity at low humidity is of great significance.

Recently, the supramolecular eutectic strategy has been widely used to modulate the melting point and mechanical properties of multicomponent systems, creating soft materials with good fluidity and malleability, like deep eutectic solvents [[34], [35], [36], [37], [38]–39]. Moreover, the weak bonding nature in a eutectic system can facilitate ion migration [40,41]. Particularly, the abundant hydrogen bonding networks in eutectic systems can act as continuous pathways for proton hopping, which endows these eutectics with special advantages as highly proton-conductive electrolytes [[42], [43]–44]. Therefore, the mechanically softening and conductively enhancing effect afforded by the eutectic strategy is well-suitable to develop processible POM-based proton conductors.

Here, we report the conversion of brittle H₄SiW₁₂O₄₀ (HSiW) powders into soft superprotonic conductors by a supramolecular eutectic strategy. Solid zwitterions containing hydroxyl groups are used to dissociate the proton from HSiW and meanwhile form eutectic hydrogen bonding networks with HSiW, which leads to the transformation of HSiW to a flexible self-supporting material with good malleability. Moreover, due to the percolated hydrogen bonding networks serving as proton transport highways, the HSiW-zwitterion eutectics exhibit high proton conductivities up to 1.0 × 10⁻³ S cm⁻¹ at 30 °C (25% RH) and 2.4 × 10⁻² S cm⁻¹ at 150 °C (anhydrous condition). The eutectic mechanism, mechanical behaviors, and proton conductive properties of this system are studied.

2. Experimental section

2.1. Materials and methods

Detailed synthetic procedures and computational methods are given in SI Appendix and Figs. S1-S6.

2.2. Characterization

MALDI-TOF spectra were measured on a Bruker Autoflex™ MALDI-TOF mass spectrometer using α-Cyano-4-hydroxycinnamic acid (CHCA) as the matrix. Samples were typically dissolved in H₂O at 1.0 mg/mL. ¹H NMR spectra were recorded on a Bruker AVANCEIII 500 MHz spectrometer at room temperature. FT-IR spectra (KBr pellet) were collected on a Bruker Vertex 80 V spectrometer equipped with DTGS detector (32 scans) in a resolution of 1 cm⁻¹. TGA was performed on a TA Q500 Thermal Analyzer in flowing N₂ with a heating rate of 10 °C min⁻¹ from 35 °C to 800 °C. DSC measurements were carried out at temperature ramp rates of 5 K min⁻¹ using a TA Instruments DSC Q20. Samples were equilibrated at 25% relative humidity and placed in a sealed crucible for cyclic testing. All data reported are selected from the third cycle. X-ray diffraction (XRD) data were recorded on a Rigaku SmartLab (Japan) X-ray diffractometer using Cu Kα1 radiation at a wavelength of 1.542 Å. Rheological properties were measured by a TA DHR-2 rheometer, using 8/16 mm parallel plates with a gap size of ∼1 mm. 0.1–1% strain was chosen to ensure all measurements were taken within the linear viscoelastic. Temperature sweeps measurements were taken at a rate of 3 °C/min and 1 rad/s fixed frequency. Oscillatory shear rheology measurements were performed at angular frequencies (ω) in the range of 10⁻¹ rad/s ≤ ω ≤ 10² rad/s. Samples were tested at least twice to ensure repeatability. Mechanical performances of SiW-nHPS were measured by an electronic universal material testing machine (Shimadzu AG-I) at room temperature. The strain rate is 100 mm min⁻¹. Their length, width, and thickness were about 15 mm, 8 mm, and 2 mm.

2.3. AC electrochemical impedance measurements

The proton conductivity was measured with a Solartron 1260A impedance/gain-phase analyzer by a two-probe AC impedance method from 0.1 Hz to 100 kHz with the amplitude voltage of the signal at 20 mV. Experiments were performed in a heating/drying oven with program control and forced convection. Each test temperature was stabilized for 5 min. The samples were placed in the groove of Teflon mold and sandwiched between two flat copper electrodes. Samples under low humidity conditions were tested after equilibration in about 25% relative humidity chamber. The conduction of samples in the anhydrous state is pre-heated for removing water. The proton conductivity (σ) was calculated by Eq. 1:

$$\sigma =L/RS$$ (1)

where L (cm) is the distance between two electrodes (the thickness of sample), R (Ω) is the sample resistance and S (cm²) is the cross-sectional area of the electrodes.

The relationship between conductivity and activation energy is consistent with the Arrhenius equation. From the slope, the activation energy (E_a) of conductivity can be evaluated using Eq. 2:

$$\begin{array}{c}\hfill \sigma T={\sigma }_0\exp (-{\mathit{E}}_{\mathit{a}}/\kappa T)\end{array}$$ (2)

where E_a denotes the activation energy of conductivity, ${\sigma }_0$ is the pre-exponential factor, T is the temperature and κ is the Boltzmann constant.

3. Results and discussion

3.1. Mechanism for formation of HSiW-nHPS eutectics

Considering that hydrogen bonds can favor the charge delocalization in ionic compounds and cause a low eutectic temperature, a hydrogen-bond-active zwitterion HPS was synthesized to interact with HSiW. HPS is an ammonium-sulfonate zwitterion with three −OH groups on the ammonium part (Fig. 1). The −OH groups can serve as hydrogen bond donors and the sulfonate groups can accept protons from POMs. The solid-phase mixing of the powders of HSiW and HPS at low relative humidity (RH) of 25% with a proper molar ratio can create soft eutectics. The whole process does not need the assistance of additional solvents. The photos of HSiW-nHPS mixtures with varied molar ratios from 1:1 to 1:8 are shown in Fig. 1. In the range from 1:2 to 1:6, transparent and shapeable eutectics are formed. However, opaque powders are obtained at other molar ratios. To clarify the contribution of −OH groups to the eutectic effect, PPS and MPS were synthesized by changing the hydroxyethyl groups to propyl and methyl groups (Fig. S7). PPS and MPS cannot form eutectics with HSiW (Fig. S8), which demonstrates the crucial role of −OH groups.

Fig. 1.

(a) Schematic illustration and photos of the preparation of HSiW-nHPS mixtures. (b) Binary phase diagram of HSiW-nHPS, HSiW-nPPS, and HSiW-nMPS mixtures.

X-ray diffraction (XRD) and differential scanning calorimetry (DSC) measurements are used to probe the eutectic state of HSiW-HPS mixtures. As shown in Fig. 2a, the intrinsic crystalline peaks of HSiW and HPS are weakened in their mixtures and completely vanished in the XRD patterns of HSiW-2HPS, HSiW-4HPS, and HSiW-6HPS. Accordingly, a glass transition temperature (T_g) is observed in the DSC curves of HSiW-2HPS, HSiW-4HPS, and HSiW-6HPS, appearing at −15 °C, −30 °C, and −40 °C, confirming the amorphous eutectic state of these samples. The T_g lowers with the increasing content of HPS. In comparison, the mixtures of HSiW with PPS or MPS show typical crystalline peaks in the XRD patterns (Fig. S9), and no glass transitions appear in the DSC curves (Fig. 2b), which reflects that the lack of −OH groups make the zwitterions lose the ability to suppress the crystallization of HSiW. In addition, scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) were employed to further characterize the eutectic state of HSiW-4HPS (Fig. S10). EDS indicated that HSiW and HPS were uniformly distributed without phase separation. This result confirms that HSiW and HPS are homogeneously mixed at the microscopic scale.

Fig. 2.

(a) XRD patterns of HPS, HSiW, and HSiW-nHPS. (b) DSC curves of different POM-zwitterion mixtures. (c) Characteristic IR vibrations of S=O bonds in HSiW-nHPS and KSiW-nHPS.

Except for the influence of zwitterion, we also study the role of HSiW, particularly its protons, in forming the eutectics. A potassium salt K₄SiW₁₂O₄₀ (KSiW) is used as the control. KSiW has the same anionic cluster as HSiW but the countercations are K⁺. KSiW-nHPS (n = 1,4,8) is obtained as brittle powders, and its crystalline state is revealed by XRD results (Fig. S11). The DSC curve of KSiW-4HPS does not exhibit glass transition. These results indicate that protons are necessary for the eutectic process. In previous works, we found that the strong Brønsted acidity of HSiW enables it to donate protons to sulfonate groups through hydrogen bonds, forming −SO₃H [45]. This protonation process can be identified by IR spectra. The characteristic S=O vibration peak of pure HPS appears at 1177 cm⁻¹. With the incorporation of HSiW, this peak displays a red shift for HSiW-nHPS (n decreases from 6, 4, to 2), and appears at 1158 cm⁻¹ for HSiW-2HPS (Fig. 2c). The redshift means an increased length of the S=O bond due to the averaging of electron cloud density induced by hydrogen bonds. For KSiW-nHPS (n = 2, 4, 6), their S=O vibrations all occur at 1185 cm⁻¹ despite the content of KSiW, showing a blue shift to HPS. This tendency is opposite to HSiW-nHPS, which means the absence of hydrogen bonding but an electrostatic interaction between −SO₃⁻and K⁺ is a key driving force for the crystallization.

Based on the above analysis, the co-existence of −OH and −SO₃H is a prerequisite for eutectic formation. We presume that the hydrogen bonds on the surface oxygen of HSiW contributed by −OH and −SO₃H can effectively disturb the ionic lattices of HSiW-nHPS mixtures. Generally, the strong electrostatic force between oppositely charged ions creates rigid ionic lattices and causes the brittleness of ionic crystals. In our HSiW-nHPS system, the −OH and −SO₃H groups favor the formation of abundant hydrogen bonding networks, which can shield the electrostatic attraction between the anionic HSiW and the cationic ammonium of HPS. So, the crystalline stacking is broken, instead of an amorphous eutectic state.

3.2. Soft processing properties of the eutectics

The mechanical properties of three eutectic mixtures HSiW- nHPS (n = 2, 4, 6) were studied by rheological and tensile measurements. Two conditions, 30−80 °C (25% RH) and 100−150 °C (anhydrous) are used to implement the rheological tests. Overall, these samples gradually become soft with the increasing content of HPS. From HSiW-2HPS, HSiW-4HPS to HSiW-6HPS, their storage modulus at 30 °C shows a decrease from 10⁸, 10⁶ to 10⁵ Pa (Fig. 3a-3c). Moreover, the storage modulus gradually becomes lower than the loss modulus, displaying a transition from a solid-like to a liquid-like state. Notably, HSiW-4HPS possesses a comparable storage modulus and loss modulus of 10⁶ Pa at the temperature range of 30−60 °C, which makes it a semi-solid material that is readily shaped at room temperature. This polymer-like soft processing feature also indicates the existence of network structures in the system, thus allowing for maintaining the association of HSiW and HPS under external strain. The possible supramolecular networks in HSiW-4HPS are proposed in Fig. 3d. The multiple hydrogen bonding sites provided by -OH and -SO₃H groups make HPS act as supramolecular crosslinkers to connect with adjacent HSiW. Due to the presence of adsorbed water in HSiW, the HSiW-nHPS contain ∼ 10 wt% water at 25% RH (Fig. S12). TGA results indicate that the hydrogen bonding network formed by HPS promotes a higher proportion of structural water in the overall water content compared to PPS. These water molecules should also participate in forming hydrogen bonds and softening the system. When testing temperatures are at 100−150 °C (anhydrous condition), the rheological curves of HSiW-2HPS, HSiW-4HPS, and HSiW-6HPS show a similar tendency to 25% RH, corresponding to solid-like, semi-solid, and liquid-like behaviors, respectively (Fig. S13). The semi-solid temperature window of HSiW-4HPS is 100−125 °C, wherein it displays the comparable storage modulus and loss modulus of 10⁷ Pa. The increase in modulus of HSiW-4HPS after drying further demonstrates the softening effect of water in the system.

Fig. 3.

(a-c) Storage modulus G′ and loss modulus G″ of HSiW-nHPS (n = 2, 4, 6) at 25% RH. The inset is the proposed interaction model of HSiW-nHPS. (d) Scheme of the supramolecular networks in HSiW-4HPS. (e) Tensile stress-strain curves of HSiW-nHPS (n = 2, 4, 6) at 25 °C. (f) Viscosity η versus frequency ω of HSiW-4HPS at 25% RH. (g) The malleability illustration of HSiW-4HPS.

The tensile tests of HSiW-2HPS show a brittle fracture curve with a yielding strength over 3 MPa and a short breaking elongation of 1% (Fig. 3e). In comparison, HSiW-4HPS and HSiW-6HPS display viscoelastic tensile behavior, in which a continuous strain-softening occurs until the stress decays to zero but the samples do not break. The yielding strength of HSiW-4HPS and HSiW-6HPS are 0.35 and 0.12 MPa, and their elongation at zero stress reaches 600% and 420%. Such excellent malleability reflects the existence of supramolecular network structures. To confirm this, frequency-dependent viscosities of HSiW-4HPS and HSiW-6HPS are measured (Fig. 3f and Fig. S14). Typical shear thinning phenomena are observed, which indicates the formation of associated networks. When increasing the temperature, the shear thinning behavior gradually disappears, implying that these networks are mainly based on hydrogen bonds and are thermally unstable. Due to the good malleability and high modulus, HSiW-4HPS is easily processed into self-supporting eutectics, as shown in Fig. 3g and a video in the supporting information. Considering that HSiW-4HPS exhibits reversible transitions between semi-solid and liquid states, we utilized this transition to prepare a mechanosensing material by solidifying HSiW-4HPS in an elastic porous polymer matrix. The resulting polymer composite displays a good tensile response signal and exhibits favorable stability (Fig. S15)

3.3. Superprotonic conduction properties of the eutectics

The proton conductivities of HSiW-nHPS (n = 2, 4, 6) were measured by impedance spectroscopy at 30−60 °C (25% RH) and 100−150 °C (anhydrous condition), as shown in Fig. 4a, 4b and Tables S1, S2. The original Nyquist plots are given in Fig. S16. At 25% RH, the elevated HSiW content in HSiW-nHPS leads to a higher proton conductivity and lower activation energy (E_a) for conduction. In this case, HSiW-2HPS exhibits the highest proton conductivity of 1.0 × 10⁻³ S cm⁻¹ at 30 °C (25% RH). However, at the anhydrous condition, HSiW-4HPS show the highest proton conductivities from 100 to 150 °C, reflecting that an equal molar ratio of HPS to the protons of HSiW is superior for proton conduction when there are no adequate watermolecules to assist the proton conduction [46]. In addition, as the hydroxyl groups of HPS can facilitate the formation of hydrogen bonding networks, the E_a is decreased with the increased HPS content. Notably, HSiW-4HPS shows an excellent anhydrous proton conductivity up to 2.4 × 10⁻² S cm⁻¹ at 150 °C (Fig. S17). To the best of our knowledge, this anhydrous proton conductivity is the highest among the POM-based proton conductors reported so far. In general, electrolytes with proton conductivity higher than 10⁻⁴ S cm⁻¹ are regarded as superprotonic conductors. HSiW-nHPS (n = 4, 6) well fits this standard. Furthermore, we also use Walden rule to evaluate the decoupling state of mechanical and conductive properties in HSiW-nHPS (n = 4, 6). The decoupling parameters α of these samples are all below 1 and their ionicity curves are all in the superionic regime above the ideal line in the Walden plots, under both 25% RH and anhydrous conditions (Fig. 4c), demonstrating that they are typical superprotonic electrolytes. To evaluate the influence of zwitterion type on the proton conduction, HSiW-4PPS is used as a control to compare with HSiW-4HPS (Table 1). The proton conductivities of HSiW-4PPS are 7.2 × 10⁻⁵ S cm⁻¹ at 30 °C (25% RH) and 7.8 × 10⁻⁵ S cm⁻¹ at 150 °C (anhydrous condition), which are 14 times and 3 orders of magnitude lower than HSiW-4HPS at the same condition. This result confirms the importance of hydroxyl groups of zwitterion in realizing highly efficient proton conduction. Besides, the E_a values of HSiW-4PPS are 0.59 eV and 0.86 eV at 25% RH and anhydrous conditions, and those of HSiW-4HPS are 0.44 eV and 0.50 eV. The magnitude of E_a can reflect the proton conducting mechanism. An E_a lower than 0.4 eV means that the Grotthuss mechanism is dominated, in which protons hop via hydrogen bonding networks. When E_a is larger than 0.5 eV, the vehicle mechanism is dominated, and protons are transported by diffusion. Following this principle, the proton conduction facilitated by HPS corresponds to a vehicle-Grotthuss synergistic mechanism (Fig. 4d and Fig. S18). The soft state and the dense hydrogen bonding networks of HSiW-4HPS enable it to transport protons by both diffusing and hopping modes. In contrast, HSiW-4PPS shows a typical Vehicle conducting mechanism, indicative of an insufficient hydrogen bonding network in it.

Fig. 4.

Temperature-dependent proton conductivity of HSiW-nHPS (n = 2, 4, 6) and HSiW-4PPS at (a) 25% RH and (b) anhydrous condition. (c) Walden plot of HSiW-nHPS (n = 4, 6). The ideal line is established using 1 mol L⁻¹ aqueous KCl solutions. α is decoupling parameter. (d) Scheme of the vehicle-Grotthuss synergistic mechanism in HSiW-nHPS. DFT results of the key energy state during proton transport from HSiW to (e) HPS and (f) PPS.

Table 1.

Conductive parameters of HSiW-4HPS and HSiW-4PPS.

Samples	σa (mS cm−1)	Eab (eV)	σc (mS cm−1)	Ea (eV)
HSiW-4HPS	1.0 × 10−3	0.44	2.4 × 10−2	0.50
HSiW-4PPS	7.2 × 10−5	0.59	7.8 × 10−5	0.86

^aThe proton conductivity at 30 °C under 25% RH.

^bActivation energy of conduction.

^cThe proton conductivity at 150 °C under anhydrous conditions.

To clarify how the molecular structure of zwitterions affects the proton conduction process of HSiW, we performed the DFT calculation to study the energy change of proton transport from HSiW to HPS and PPS, respectively, as shown in Fig. 4c, d. The energy differences between two key intermediates are calculated: one is protons on HSiW and forms a hydrogen bond adducts with HPS, SiW(H)⋅⋅⋅HPS; the other is that protons are transferred to the sulfonated group, SiW⋅⋅⋅(H)HPS. The proton transport energy is relative to the difference (ΔE). The HPS system shows a smaller ΔE of 4.90 kcal mol⁻¹ than the PPS system whose ΔE is 7.05 kcal mol⁻¹. This suggests that the proton transport from HSiW to HPS is easier than to PPS, which is consistent with the experimental result that HSiW-4HPS has a higher proton conductivity and a lower E_a than HSiW-4PPS. Moreover, the hydrogen bonds between the −OH groups of HPS and the surface oxygen of HSiW lead to a stable molecular conformation. With such an interaction, the absolute energy of SiW⋅⋅⋅(H)HPS is lowered. Thus, the −OH groups of HPS can stabilize HSiW and facilitate it to donate protons.

4. Conclusion

In summary, we present a deep eutectic strategy to prepare processible POM-based proton conductors. The supramolecular hydrogen bonding network of POMs with hydroxyl‑containing zwitterions can effectively shield the electrostatic interaction between POMs and the zwitterions, leading to a conversion of brittle crystalline POM powders to soft amorphous eutectics. The dynamic crosslinking effect dominated by hydrogen bonds endows these eutectics with good malleability and high modulus. Moreover, superprotonic transport is realized in these materials owing to the synergy of eutectic-state-promoted diffusion and hydrogen-bond-based hopping. This work demonstrates that the supramolecular eutectic concept has a promising perspective in developing advanced ionic conductors.

Declaration of competing interest

The authors declare that they have no conflicts of interest in this work.

Biographies

Shengchao Chai is a doctoral student at State Key Laboratory of Supramolecular Structure and Materials, Jilin University. His main research interests are focused on supramolecular hybrid electrolytes based on polyoxometalates.

Haolong Li (BRID: 09950.00.97827) is currently a professor in Jilin University of China. He received his B.S. (2003) and Ph.D. (2008) from Jilin University. From 2009 to 2011, he conducted postdoctoral research at the Max Planck Institute for Polymer Research in Germany supported by Alexander von Humboldt Scholarship. In 2011, he joined Jilin University as an associate professor and became a full professor in 2016. He was granted by the Excellent Young Scientists Fund of NSFC in 2016. His research interests focus on the development of advanced electrolyte materials, particularly high-performance proton conductors, for new energy technologies.