Significant enhancement of proton conductivity in solid acid at the monolayer limit

Zhang, Zhangcai; Liang, Lixin; Feng, Jianze; Hou, Guangjin; Ren, Wencai

doi:10.1038/s41467-024-46911-7

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Published: 27 March 2024

Significant enhancement of proton conductivity in solid acid at the monolayer limit

Nature Communications volume 15, Article number: 2706 (2024) Cite this article

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Abstract

Proton transport in nanofluidic channels is not only fundamentally important but also essential for energy applications. Although various strategies have been developed to improve the concentration of active protons in the nanochannels, it remains challenging to achieve a proton conductivity higher than that of Nafion, the benchmark for proton conductors. Here, taking H₃Sb₃P₂O₁₄ and HSbP₂O₈ as examples, we show that the interactions between protons and the layer frameworks in layered solid acid H_nM_nZ₂O_3n+5 are substantially reduced at the monolayer limit, which significantly increases the number of active protons and consequently improves the proton conductivities by ∼8 ‒ 66 times depending on the humidity. The membranes assembled by monolayer H₃Sb₃P₂O₁₄ and HSbP₂O₈ nanosheets exhibit in-plane proton conductivities of ~ 1.02 and 1.18 S cm⁻¹ at 100% relative humidity and 90 °C, respectively, which are over 5 times higher than the conductivity of Nafion. This work provides a general strategy for facilitating proton transport, which will have broad implications in advancing both nanofluidic research and device applications from energy storage and conversion to neuromorphic computing.

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Introduction

Constructing nanofluidic channels for proton transport is not only fundamentally important for exploring the unusual nanofluidic phenomena^1,2,3,4,5 but also essential for applications in energy storage and conversion, such as fuel cells, batteries, and supercapacitors^6,7,8,9. The fast proton transfer in aqueous systems is commonly governed by the Grotthuss mechanism, which takes place on a hydrogen-bonded network and entrails a collective proton motion similar to Newton’s cradle^10,11,12,13. The efficiency of Grotthuss-type proton conduction mainly relies on the proton bridges interconnected by the hydrogen bonds, and short hydrogen bonds would generate a superharmonic behavior of proton motion, triggering proton transfer in a nearly barrierless manner^13,14. Thus, improving the concentration of active protons in the nanochannels to shorten the hydrogen bonds is vital to achieve a high proton conductivity.

Nafion (DuPont Company), the benchmark for proton conductor, is composed of a perfluorinated polymer skeleton and side chains containing sulfonic acid groups (-SO₃H) as proton donors^15,16. The low content of -SO₃H in the nanochannels is a hurdle to get a high proton concentration, resulting in a conductivity up to ∼0.2 S cm⁻¹ at high relative humidity (RH) and temperature^16,17. Metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) are two emerging porous materials for proton conductors thanks to their regular nanochannels^18,19,20,21. Although various strategies, such as frameworks’ functionalization and guest molecule inclusion, have been developed to improve the concentration of protons in the nanochannels, it is usually difficult to achieve a proton conductivity higher than that of Nafion^20,21. Recently, negatively charged vacancies have been found to be an efficient proton donor to improve the proton conductivity of 2D materials assembled membranes²². However, such membranes are unstable in an acidic environment, and thus it remains a challenge to obtain pure proton conductors.

Solid acids H_nM_nZ₂O_3n+5 (M = Sb, Nb, Ta; Z = P, As; n = 1, 3) are a large family of layer-structured proton conductors with abundant protons lying in M_nZ₂O^n-_3n+5 interlayers^23,24,25,26. Taking H_nSb_nP₂O_3n+5 as an example, there are two kinds of protons trapped on the bridging oxygen atoms of P-O-Sb and Sb-O-Sb²⁴, respectively, for H₃Sb₃P₂O₁₄ (n = 3) (Fig. 1a–c), while only one kind of protons are trapped on the bridging oxygen atoms of P-O-Sb for HSbP₂O₈ (n = 1)²⁵. Bulk H_nSb_nP₂O_3n+5 has been used previously as proton conductors by pressing H_nSb_nP₂O_3n+5 particles together into a pellet because big enough crystals were not generally possible to obtain²⁶. However, the weak binding between H_nSb_nP₂O_3n+5 particles limits their use to RH lower than 95% and temperature below 20 °C, beyond which the pellet suffers severe expansion and cracking. The reported proton conductivities of bulk H_nSb_nP₂O_3n+5 are typically in the range of 10⁻⁵ to 10⁻² S cm⁻¹ from 20 to 95% RH at 20 °C²⁶.

**Fig. 1: Synthesis and characterizations of m-H₃Sb₃P₂O₁₄ nanosheets.**

Here, we show that the interactions between protons and the layer frameworks in H_nSb_nP₂O_3n+5 are substantially reduced at the monolayer limit, which significantly increases the number of active protons and consequently improves the proton conductivities by ∼8‒66 times depending on the humidity. The membranes assembled by monolayer H₃Sb₃P₂O₁₄ (m-H₃Sb₃P₂O₁₄) and HSbP₂O₈ (m-HSbP₂O₈) nanosheets exhibit in-plane proton conductivities of ~1.02 and 1.18 S cm⁻¹ at 100% relative humidity and 90 °C, respectively, which are over five times higher than the conductivity of Nafion, the benchmark for proton conductors.

Results

Synthesis and characterizations of m-H₃Sb₃P₂O₁₄ membranes

We synthesized m-H₃Sb₃P₂O₁₄ nanosheets by exfoliating H₃Sb₃P₂O₁₄ crystals in deionized water by stirring followed by centrifugation (Fig. 1d–i and Supplementary Fig. 1). The products are negatively charged and have excellent dispersibility in deionized water due to electrostatic repulsion (Fig. 1e and Supplementary Fig. 2). Transmission electron microscopy (TEM) image and the corresponding selective-area electron diffraction (SAED) pattern confirm that they are highly crystalline atomically thin nanosheets (Fig. 1f). Atomic force microscopy (AFM) measurements show that the nanosheets are predominantly monolayers with a thickness of ~1 nm and lateral size of ~0.5‒1.5 μm (Fig. 1g and Supplementary Fig. 3). X-ray photoelectron spectroscopy (XPS) measurements indicate that the chemical compositions and valent states of Sb, P and O remain unchanged before and after exfoliation (Supplementary Fig. 4). Raman spectra and Fourier transform infrared (FT-IR) spectra suggest that m-H₃Sb₃P₂O₁₄ nanosheets have almost the same crystal and bonding structure with their bulk crystals (Fig. 1h, i). The slight broadening of Raman peaks and small shift of FT-IR peaks for m-H₃Sb₃P₂O₁₄ nanosheets might be caused by the greatly reduced interlayer coupling.

Free-standing m-H₃Sb₃P₂O₁₄ membranes were fabricated from their aqueous dispersion by vacuum filtration. The high optical transparency indicates that they are an electronic insulating material (Fig. 2a). The cross-sectional scanning electron microscopy (SEM) image and X-ray diffraction (XRD) pattern show that the membranes have well-ordered layered structure (Fig. 2b and Supplementary Fig. 5a). Moreover, they are highly hydrophilic (inset of Fig. 2c), with a contact angle of ~26°, enabling good water uptake under humidity atmosphere. According to the XRD pattern, the interlayer distance (d) is increased by ~0.3 nm compared to that of bulk crystals (0.62 nm)²⁴ even at 0% RH (Supplementary Fig. 5a), indicating that the nanochannels are pre-inserted with one-layer water molecules (~0.28 nm). Based on the water adsorption-desorption isotherms (Supplementary Fig. 6), additional water uptake as high as ~2.5 and 10.2 mol mol⁻¹ was obtained after achieving equilibrium at 30 and 100% RH, respectively. As a result, the d of m-H₃Sb₃P₂O₁₄ membranes reaches 1.51 nm at 100% RH (Fig. 2c and Supplementary Fig. 5b), which is increased by ~0.9 nm compared to that of bulk crystals. This yields up to a trilayer water molecule network in the nanochannels.

Proton transport behaviors of m-H₃Sb₃P₂O₁₄ membranes

Chrono amperometry and linear sweep voltammetry (LSV) were used to evaluate the electronic conductivity of m-H₃Sb₃P₂O₁₄ membranes. They show typical characteristics of electronic insulators with no detectable current at 0% RH, consistent with the high optical transparency (Fig. 2a), while a noticeable electrical current was obtained as RH increased to 100%, suggesting that the membranes are ionic conductors (Supplementary Fig. 7a–c). Such electrical behaviors are similar to those of Nafion 117 film (Supplementary Fig. 7d–f). We further measured the proton transference number (t(H⁺)) of m-H₃Sb₃P₂O₁₄ membranes by H⁺/NO^-₃ transference experiments. The obtained t(H⁺) is 0.954, larger than that of Nafion 117 films (0.897) (Fig. 2d), suggesting that m-H₃Sb₃P₂O₁₄ membranes are pure proton conductors.

We then measured the proton conductivities of m-H₃Sb₃P₂O₁₄ membranes at 100% RH and different temperatures with two-electrode alternating current impedance. Before measurements, the membranes were stored at 100% RH for more than 24 hours to achieve equilibrium (Supplementary Figs. 6a,8). In sharp contrast to the pellets made by H₃Sb₃P₂O₁₄ particles, m-H₃Sb₃P₂O₁₄ membranes show good operation stability even at 100% RH and 90 °C (Supplementary Fig. 9). They exhibit typical impedance spectra for proton conductors²⁷, with a semicircle and an incline spur in the high-frequency and low-frequency regions, respectively (Fig. 2e and Supplementary Fig. 10). The proton conductivities were calculated using the equation:

$$\sigma=\frac{L}{{RS}}$$

(1)

where σ (S cm⁻¹) is ion conductivity, L (cm) is the length of the membrane between the blocked electrodes, R (Ω) is the resistance calculated from Nyquist plots, and S (cm²) is the cross-sectional area of the membrane. Notably, the membranes show superior in-plane proton conductivity of ∼0.49 S cm⁻¹ even at a low temperature of 30 °C, and it increases monotonically to ∼1.02 S cm⁻¹ as the temperature increases to 90 °C (Fig. 2f). This is different from Nafion membrane, which shows decreased proton conductivity when the temperature is higher than 80 °C¹⁷. These in-plane proton conductivities are over 5 times higher than the conductivity of Nafion under the same conditions¹⁷.

We calculated the activation energy (E_a) for proton transport in m-H₃Sb₃P₂O₁₄ membranes by using the equation

$${{{{\mathrm{ln}}}}}(\sigma {\rm T})={{{{\mathrm{ln}}}}}{\sigma }_{0}-\frac{{{{\rm{E}}}}_{{{\rm{a}}}}}{{{\rm{RT}}}}$$

(2)

where σ₀ is a preexponential factor, R is the gas constant, and T is temperature. This yields a value of ~0.141 eV (Fig. 2g), which is almost the same as the theoretical upper limit (0.09‒0.13 eV)²⁸, indicating the very small obstacles of proton transport in m-H₃Sb₃P₂O₁₄ membranes. We further studied the kinetic isotope effect (KIE) of proton transport in the membranes, which yields a similar value of ~1.70 – 2.00 over the temperature 30‒90 °C (Fig. 2h, i). Both E_a and KIE values suggest that the proton transport in m-H₃Sb₃P₂O₁₄ membranes is governed by the Grotthuss mechanism (E_a ≤ 0.4 eV, KIE ≥1.4)²⁸, in which the protons hop among the hydrogen-bonded water networks in the 2D nanochannels.

We further studied the proton transport of m-H₃Sb₃P₂O₁₄ membranes at low temperatures, the influence of membrane thickness and nanosheet size on the proton conductivity, and the stability of the membranes. Notably, the m-H₃Sb₃P₂O₁₄ membranes still show high in-plane conductivity from 0.41 S cm⁻¹ to 0.01 S cm⁻¹ at low temperature from 20 to −30 °C (Supplementary Fig. 11). In contrast, Nafion membrane shows conductivity of 0.063‒0.007 S cm⁻¹ at the same temperature (Supplementary Fig. 11). Moreover, both the membrane thickness and nanosheet size have negligible influence on the proton conductivity (Supplementary Figs 3b, 12–14). For instance, the proton conductivity of the m-H₃Sb₃P₂O₁₄ membranes is ~0.52, 0.53, and 0.56 S cm⁻¹ for a membrane thickness of ~7, 11, and 15 μm, respectively, at 30 °C and 100% RH (Supplementary Fig 12). It also remains similar value of ~0.49, 0.52, 0.52, and 0.53 S cm⁻¹ for the m-H₃Sb₃P₂O₁₄ membranes assembled by the nanosheets with sizes of 0.92, 0.38, 0.22, and 0.15 μm, respectively (Supplementary Figs. 3b, 13,14). In addition, m-H₃Sb₃P₂O₁₄ membranes are very stable in a strong acid environment. They show almost the same Raman, FT-IR spectra, and proton conductivity before and after immersing in 10 M H₂SO₄ for 12 days (Supplementary Fig. 15).

Origin of the superior proton conductivity in m-H₃Sb₃P₂O₁₄ membranes

We studied the influence of the thickness of H₃Sb₃P₂O₁₄ nanosheets on the proton transport in their assembled membranes, and found that the reduced nanosheet thickness plays a key role in boosting the proton transport (Fig. 3, Supplementary Figs. 3, 16–21, and Supplementary Discussion 1). The pellets made by H₃Sb₃P₂O₁₄ particles have engineering issues and may not have highly oriented nanochannels as m-H₃Sb₃P₂O₁₄ membranes do. To rule out the influence of channel orientation on the proton transport, we synthesized H₃Sb₃P₂O₁₄ membranes using nanosheets with different average thicknesses (~1.0, 1.4, 3.1, and 8.6 nm) (Supplementary Figs. 3a, 16), named as 1.0 nm- (i.e., monolayer), 1.4 nm-, 3.1 nm- and 8.6 nm-H₃Sb₃P₂O₁₄ membranes. The XPS, Raman, and FT-IR spectra indicate that the membranes have almost the same chemical composition, bonding, and crystal structure (Supplementary Figs. 17, 18). Moreover, they have similar average sizes from ~0.92 to ~1.27 μm (Supplementary Figs. 3b, 19). Importantly, all these membranes have no engineering issue. The cross-sectional SEM images and XRD patterns indicate that all these membranes have well-ordered layered structures (Fig. 3a–d and Supplementary Fig. 20). We quantitatively characterized the orientation degree of the nanosheets in these membranes using wide-angle X-ray scattering (WAXS), where the orientation degree is expressed by Herman’s orientation factor (f). As shown in Fig. 3e–h, all the membranes show very similar WAXS patterns and f values from 0.97 to 0.99 for the (003) peak, confirming that these membranes have highly oriented structures along (00 l) crystal plane with almost the same orientation degree.

**Fig. 3: Origin of the superior proton conductivity of m-H₃Sb₃P₂O₁₄ membranes.**

We then investigated the proton transport properties of the four kinds of membranes. Figure 3i show that the proton conductivity of H₃Sb₃P₂O₁₄ membranes increases greatly with reducing the average thickness of H₃Sb₃P₂O₁₄ nanosheets. The m-H₃Sb₃P₂O₁₄ membranes show the highest proton conductivity over the investigated temperature range at 100% RH, which is about two, three, and four times larger than that of the membranes assembled from H₃Sb₃P₂O₁₄ nanosheets with an average thickness of ~1.4, 3.1, and 8.6 nm, respectively. Considering almost the same structure of the four membranes, these results give strong evidence that the significantly improved proton conductivity in m-H₃Sb₃P₂O₁₄ membranes is mainly attributed to the reduced thickness of the nanosheets. However, all the membranes show a similar low E_a of 0.13‒0.15 eV (Supplementary Fig. 21a) and KIE of 1.4–2.1 over the temperature from 30 to 90 °C at 100% RH (Supplementary Fig. 21b), indicating that the proton transport in all these membranes is governed by the Grotthuss mechanism with similar hindrance. Thus, the dependence of proton conductivity on the average thickness of H₃Sb₃P₂O₁₄ nanosheets should be dominantly attributed to the difference in the number of active protons with high mobility in different membranes.

To further reveal the origin of the significantly improved proton transport in the m-H₃Sb₃P₂O₁₄ membranes, we studied the proton transport behaviors in m-H₃Sb₃P₂O₁₄ nanosheets, membranes, and bulk H₃Sb₃P₂O₁₄ at 100% RH and 30 °C by solid-state nuclear magnetic resonance (NMR) spectroscopy (Fig. 3j, k, Supplementary Figs. 22, 23, and Supplementary Discussion 1). Notably, m-H₃Sb₃P₂O₁₄ membranes and nanosheets show almost identical ¹H and ³¹P NMR spectra, both of which substantially differ from those of bulk H₃Sb₃P₂O₁₄. These results suggest that the chemical environment and dynamics of protons change significantly as the thickness of H₃Sb₃P₂O₁₄ is reduced to monolayer, but they remain nearly unchanged when the m-H₃Sb₃P₂O₁₄ nanosheets are assembled into membranes with d of 1.51 nm.

Figure 3j shows that for bulk H₃Sb₃P₂O₁₄, m-H₃Sb₃P₂O₁₄ membranes, and nanosheets, their ¹H spin-echo magic angle spinning (MAS) NMR spectra are dominated by a strong peak at 8.1, 7.6, and 7.6 ppm, respectively. Such dominant and sharp ¹H signals are attributred to those active protons with high mobility in the water between the Sb₃P₂O^3-₁₄ layer frameworks. The downshifts of ¹H signals for m-H₃Sb₃P₂O₁₄ nanosheets and membranes are due to the higher water content (Supplementary Fig. 6)²⁹. It is worth noting that several weak ¹H signals at 4.2, 3.8, and 1.2 ppm are also observed for bulk H₃Sb₃P₂O₁₄. Suggested by the observed ¹H-³¹P dipolar coupling in ¹H{³¹P} symmetry-based resonance-echo double-resonance (S-REDOR) experiments (Supplementary Fig. 23), the protons at 4.2 and 1.2 ppm are less mobile and attached to the Sb₃P₂O^3-₁₄ layers. The ¹H-³¹P dipolar coupling exists between the fixed and spatially close ¹H and ³¹P atoms. In contrast, no immobile protons attaching to the Sb₃P₂O^3-₁₄ layers are observed for the m-H₃Sb₃P₂O₁₄ membrane and nanosheets, as evidenced by the absence of ¹H-³¹P dipolar coupling (Supplementary Fig. 23).

The ³¹P NMR spectra were also acquired using both single-pulse (SP) and ³¹P{¹H} cross-polarization (CP) experiments (Fig. 3k). The ³¹P SP spectra show the signals of all the phosphates groups in samples, while the ³¹P{¹H} CP spectra reveal the connectivity of phosphate groups to protons. Specifically, signal in ³¹P{¹H} CP NMR spectra appears only if the phosphate group and proton possess low mobility, and are in proximity to each other. According to the ³¹P SP NMR spectra, the ³¹P signals of three samples are mainly from the Q³ sites³⁰ (phosphate group with three bridging oxygen) forming hydrogen bonds with H₂O. Notably, for bulk H₃Sb₃P₂O₁₄, ³¹P{¹H} CP signals show both the hydrogen-bonded (from −6.9 to −14.7 ppm) and protonated (−25.6 ppm) Q³ sites³⁰, indicating extensive bindings between protons and the Sb₃P₂O^3-₁₄ layers. In contrast, for m-H₃Sb₃P₂O₁₄ nanosheets, all the ³¹P NMR signals that appeared in the SP NMR spectrum are not observed in the ³¹P{¹H} CP NMR spectrum. Similarly, only a weak ³¹P{¹H} CP signal at −14.6 ppm is presented in m-H₃Sb₃P₂O₁₄ membranes. This weak signal indicates the existence of a small amount of rigid hydrogen bonds at the layer surface, which arise from the nanosheets stacked with a small d in the membranes (Supplementary Fig. 5b). The differences in ³¹P{¹H} CP spectra of the three materials suggest that the interactions between protons and Sb₃P₂O^3-₁₄ layer frameworks in m-H₃Sb₃P₂O₁₄ membranes and nanosheets are significantly reduced compared to bulk H₃Sb₃P₂O₁₄, which results in substantial increase in the number of active protons and consequently the superhigh proton conductivities of m-H₃Sb₃P₂O₁₄ membranes.

In addition to providing hydrogen-bonded networks for proton hopping, inserting water molecules into the nanochannels can decouple the adjacent layers by increasing d and weaken the interactions between protons and Sb₃P₂O^3-₁₄ layer frameworks. Therefore, the proton transport of m-H₃Sb₃P₂O₁₄ membranes strongly depends on the RH. XRD results show that d gradually increases from 0.94 to 1.13 nm as the RH increases from 0 to 93% RH (Supplementary Fig. 24a–c), indicating the inserting of about one-layer water molecules (bilayer in total). Importantly, d rapidly increases to 1.51 nm when further increasing RH to 100%, attributed to the inserting of an additional water layer, totally up to trilayer water in the nanochannels. As a result, the proton conductivity of m-H₃Sb₃P₂O₁₄ membranes gradually increases with RH below 95% RH and then shows a sharp increase until 100% RH (Supplementary Fig. 25). Different from m-H₃Sb₃P₂O₁₄ membranes, only bilayer water molecules are inserted in bulk H₃Sb₃P₂O₁₄ at 100% RH (Supplementary Fig. 24d–f), which is not sufficient for decoupling the adjacent layers as confirmed by the NMR measurement results.

Surprisingly, the reduced proton/Sb₃P₂O^3-₁₄ layer interaction in m-H₃Sb₃P₂O₁₄ enables its membranes more pronounced advantages over bulk H₃Sb₃P₂O₁₄ pellets at low RHs. Notably, under 40% RH, the m-H₃Sb₃P₂O₁₄ membrane shows a proton conductivity ~66 times larger than that of the bulk pellets. In contrast, under 90% RH, the proton conductivity is increased approximately eight times (Fig. 3l). Moreover, under low RHs, the proton conductivities of m-H₃Sb₃P₂O₁₄ membranes are about one order of magnitude higher than those of Nafion¹⁷, which is larger than the improvement under high RHs.

Discussion

Our work illustrates a strategy for improving the concentration of proton carriers in the nanochannels to boost proton transport by using monolayer solid acids. To demonstrate the versatility of this strategy, m-HSbP₂O₈ (with only one kind of protons) nanosheets assembled membranes were also fabricated (Fig. 4a–g and Supplementary Figs. 26, 27), which exhibit a highly ordered lamellar structure with excellent hydrophilicity and d of 1.54 nm (trilayer water intercalation) at 100% RH. Similar to m-H₃Sb₃P₂O₁₄ membranes, their proton conductivities are substantially improved by ~30 times compared with the bulk HSbP₂O₈ pellets (Fig. 4h). Significantly, they show a high in-plane proton conductivity of ~1.18 S cm⁻¹ with a low E_a of 0.127 eV at 100% RH and 90 °C (Fig. 4i and Supplementary Fig. 28), which is better than that of m-H₃Sb₃P₂O₁₄ membranes. As an example, using m-HSbP₂O₈ membranes and MXene membranes as electrolytes and electrodes, respectively, we fabricated a high-performance all-2D flexible solid-state micro-supercapacitor with rate capability up to 300 mV s⁻¹ and volumetric energy density of ~18.5 mWh cm⁻³ (Supplementary Methods, Supplementary Figs. 29–32, Supplementary Movie 1, and Supplementary Discussion 2), showing the great potential of m-HSbP₂O₈ membranes for energy applications.

**Fig. 4: Structure and proton transport behaviors of m-HSbP₂O₈ membranes.**

Considering the availability of a vast number of solid acids, we envision that the proton transport performance could be further improved by selecting suitable layered solid acids. Such membranes not only have great potentials in the applications of supercapacitors, batteries, sensors, and neuromorphic computing, but also provide possibilities for discovering unusual nanofluidic phenomena in the nano-confined capillaries, which will expand the unique properties of 2D materials at the monolayer limit in addition to the fascinating electronic, optical, thermal and mechanical properties.

Methods

Materials

Nitric acid (Analytical reagent (AR), HNO₃) were purchased from Sinopharm Chemical Reagents (Shanghai) Co., Ltd. Ammonium dihydrogen phosphate (AR, 99%, NH₄H₂PO₄), antimony trioxide (Sb₂O₃), strontium chloride (AR, 99.5%, SrCl₂), magnesium chloride (AR, MgCl₂), lithium chloride (AR, >99%, LiCl) and phosphorus pentoxide (AR, >98%, P₂O₅) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Magnesium nitrate (AR, Mg(NO₃)₂) was purchased from Damao Chemical Reagent Factory. Potassium nitrate (ACS, >98%, KNO₃) was purchased from Alfa Aesar. Deuterium oxide (99.9 at% D, D₂O) was purchased from Beijing Innochem Science & Technology Co., Ltd.

Synthesis of H₃Sb₃P₂O₁₄ nanosheets

We synthesized bulk H₃Sb₃P₂O₁₄ by the solid-state reaction and the ion exchange processes²³. A mixture of NH₄H₂PO₄ (4.6 mmol), Sb₂O₃ (3.4 mmol), and KNO₃ (6.8 mmol) was well-mixed with agate mortar. It was heated in air to 300 °C for 10 h to decompose NH₄H₂PO₄, and then annealed at 1000 °C for 24 h to yield K₃Sb₃P₂O₁₄. The K₃Sb₃P₂O₁₄ powders (~1.60 g) were dispersed in a ~200 mL HNO₃ solution (8 M) at 50 °C for >24 h to exchange the potassium ions by protons. In the ion exchange process, the white sediments were filtrated on polyethersulfone (pore size, 0.22 μm) membrane filters and the HNO₃ solution was refreshed at least three times to thoroughly exchange potassium ions by protons, producing high-purity H₃Sb₃P₂O₁₄ phase. The obtained H₃Sb₃P₂O₁₄ powders were rinsed in deionized water to remove residual HNO₃. After rinsing, the H₃Sb₃P₂O₁₄ powders were exfoliated in deionized water by stirring at 30 °C for at least 12 h. Differential centrifugation was used to obtain H₃Sb₃P₂O₁₄ nanosheets with different average thicknesses. The dispersion was first centrifuged at 13,000 × g (g = 9.80 m s⁻²) for 10 min to obtain m-H₃Sb₃P₂O₁₄ nanosheet dispersion (supernatant). Then, the sediment was again dispersed in water, followed by centrifugation at 7312 × g for 30 minutes, which yielded H₃Sb₃P₂O₁₄ with an average thickness of ~1.4 nm in the supernatant. Similar to the above differential centrifugation steps, 3250 × g and 1106 × g relative centrifugal forces were used to yield aqueous dispersions of H₃Sb₃P₂O₁₄ nanosheets with an average thickness of ~3.1 nm and 8.6 nm, respectively. The m-H₃Sb₃P₂O₁₄ nanosheets with average lateral sizes of ~0.38, 0.22, and 0.15 μm were synthesized at ultrasonic power and time of 160 W and 5 min, 160 W and 20 min, and 320 W and 20 min, respectively.

Synthesis of m-HSbP₂O₈ nanosheets

We synthesized bulk HSbP₂O₈ as follows^25,31. A mixture of NH₄H₂PO₄ (9.24 mmol), Sb₂O₃ (2.310 mmol), and KNO₃ (4.6 mmol) was well-mixed with an agate mortar. It was first annealed at 200 °C for 4 h and then at 850 °C for 15 h. The product was ground and then acidified by 8 M HNO₃ at 50 °C for >24 h. In the acidification process, the HNO₃ solution was refreshed three times to completely exchange potassium ions with protons. The obtained bulk HSbP₂O₈ was rinsed in deionized water to remove residual HNO₃. After rinsing, the bulk HSbP₂O₈ was exfoliated in deionized water by stirring at 30 °C for at least 12 h. Finally, the resulting dispersion was centrifuged at 13,000×g for 10 min to obtain m-HSbP₂O₈ nanosheet aqueous dispersion (supernatant).

Fabrication of H₃Sb₃P₂O₁₄ and HSbP₂O₈ nanosheet membranes

The vacuum filtration method was used to fabricate H₃Sb₃P₂O₁₄ and HSbP₂O₈ nanosheet membranes. The H₃Sb₃P₂O₁₄ and HSbP₂O₈ nanosheets aqueous dispersions were filtrated under vacuum with polycarbonate (PC, Whatman^TM, pore size 0.2 μm) membranes as a filter. After drying in a vacuum at 50 °C for 24 h, the products were peeled off from the filters to obtain free-standing membranes.

Water vapor adsorption-desorption measurements

The water vapor adsorption-desorption of bulk H₃Sb₃P₂O₁₄ and m-H₃Sb₃P₂O₁₄ membranes was measured in a Dynamic Vapor Sorption (DVS Intrinsic, Surface Measurement Systems Ltd., UK) equipped with an ultrasensitive microbalance of mass resolution (±1 μg) and a high precision RH control system (±1%) at 303 K. Prior to measurements, both the membranes and bulk samples were vacuumed at 393 K for more than 48 h. The equilibration time for adsorption-desorption at each RH was determined by the value of dm/dt (<0.0002% min⁻¹).

Electronic conductivity measurements

Chrono amperometry and LSV measurements were performed to evaluate the electronic conductivities of m-H₃Sb₃P₂O₁₄ membranes and Nafion 117 film (DuPont Company) on an electrochemical workstation (Autolab M204 (Metrohm), potentiostat–galvanostat). To void the ionic current, before testing, both samples were dried at 80 °C in vacuum with P₂O₅ as a desiccant for 5 days.

Proton transference number measurements

The proton transference numbers of m-H₃Sb₃P₂O₁₄ membranes and Nafion 117 films were measured using a two-cell apparatus³². Both samples were first soaked in 2.0 M HNO₃ solution for 72 h at 25 °C, and then mounted between the cells. The two chambers of the cells were filled with HNO₃ solutions with different concentrations (1.0‒3.0 M). The open circuit potential (E_ocp) equals the value of the membrane/film potential due to the Donnan exclusion, and it was measured by connecting both cells and Ag/AgCl electrode through saturated chloride potassium bridges. The proton transference number was calculated by the following equation:

$${E}_{{{{{{\rm{ocp}}}}}}}=-({t}_{+}-{t}_{-})\frac{RT}{F}\,{{{{\mathrm{ln}}}}}\,\frac{{{{\rm{a}}}}_{2}}{{{{\rm{a}}}}_{1}}$$

(3)

where ${t}_{+}$, ${t}_{-}$, R, T, F, and $\frac{{a}_{2}}{{a}_{1}}$ are proton transference number, NO^-₃ transference number, gas constant, temperature, faraday constant, and activity gradient, respectively.

Proton conductivity measurements

Electrochemical impedance spectroscopy measurements were performed by using an Autolab M204 (Metrohm, PGSTAT204) electrochemical workstation with a tunable frequency range from 1 MHz to 10 Hz at a potential of 0 with 150 mV amplitude. The proton conductivities of H₃Sb₃P₂O₁₄ and HSbP₂O₈ based materials at different RHs and temperatures were tested in a climatic test chamber (LabEvent, Weiss-Voetsch Environmental Testing Instruments (Taicang) Co., Ltd.). The 100% RH atmosphere was created by water sealed in a dish. For testing, the free-standing H₃Sb₃P₂O₁₄ and HSbP₂O₈ nanosheet membranes were cut into strips (40.0 × 5.0 mm²) and put on the quartz holder with a square hole (20.0 × 20.0 mm²). For bulk H₃Sb₃P₂O₁₄ and HSbP₂O₈, they were ground to powders by agate mortar and then pressed under a pressure of ~0.76 GPa to obtain pellets, and finally, the pellets were cut into strips (25.0 × 5.0 mm²). The platinum strips (purity: 99.99%, ZhongNuo Advanced Material (Beijing) Technology Co., Ltd.) were attached to both ends of the membranes/bulk pellets and used as blocked electrodes. The thicknesses of the tested membranes and pellets at certain RHs were measured by micrometer (MDC-25PX, accuracy ±1 μm, Mitutoyo Corporation (Japan)). At least three different samples were measured for each kind of membranes/pellets.

KIE measurements

Before measurements, the m-H₃Sb₃P₂O₁₄ membranes were dried at 80 °C in a vacuum with P₂O₅ as a desiccant for 5 days to exclude the influence of water vapor. Then, the membranes were stored in the vapor of D₂O for more than 24 h. EIS measurements were performed at different temperatures ranging from 30 to 90 °C.

NMR measurements

The ¹H spin-echo and ³¹P SP/CP NMR spectra were acquired in a 14.1 T Bruker AVIII 600 NMR spectrometer, using a 4 mm triple-resonance MAS probe at 12 kHz spinning frequency. Excitation of the ¹H and ³¹P signals used a 90° pulse with a pulse length of 9.1 and 6.2 μs, respectively. For ¹H spin-echo NMR experiments, the echo duration was 83.3 μs. For ³¹P{¹H} CP NMR experiments, the CP contact used 24.8 and 28.6 kHz radiofrequency (RF) amplitude for ¹H and ³¹P nuclei, respectively. For ¹H{³¹P} S-REDOR experiments, the RF amplitude of $SR{4}_{1}^{2}$ dipolar recoupling on the ¹H channel was 24 kHz (twice the spinning frequency), and the recouping time was 5.0 ms. The reference spectra (S₀) were obtained by omitting the refocusing pulse at the ³¹P channel, while the spectra containing ¹H-³¹P dipolar dephase (decreasing signal intensity) were obtained by applying the ³¹P refocusing pulse. In the ¹H-¹H 2D exchange experiments of the H₃Sb₃P₂O₁₄ bulk sample, the excitation 90° pulse was 9.8 μs, with 10.0 ms mixing time. The 2D ¹H-¹H exchange NMR spectra (EXSY) were acquired in a 9.4 T Bruker AVANCE III HD 400 NMR spectrometer, operating at a 4 mm HX MAS probe at 12 kHz spinning frequency. The ¹H pulse field gradient (PFG) NMR spectra were acquired in a wide-bore Bruker AVIII 600 spectrometer with a 14.1 T magnet, using a 5 mm PFG probe. In ¹H PFG experiments, the RF amplitude for ¹H excitation was 18 kHz. In each experiment, the diffusion time was 8.0 ms, the duration of gradient pulses was 1.0 ms, with the gradient strength increase from 36 to 999.61 in 16 steps.

XRD measurements at different RHs

To measure the XRD patterns of m-H₃Sb₃P₂O₁₄ membranes and bulk H₃Sb₃P₂O₁₄ pellets at different RHs, they were first dried at 80 °C in vacuum (P₂O₅ as a desiccant) for 5 days and then stored in a sealed dish with different saturated salt slurries including LiCl (11%), MgCl₂ (33%), Mg(NO₃)₂ (53%), SrCl₂ (70%), KNO₃ (93%) at 25 °C for at least 72 h. To achieve 0% RH, we put excess P₂O₅ as a desiccant inside the sealed dish. The 100% RH atmosphere was created by water sealed in the dish.

Structure characterizations

The morphology and structure of H₃Sb₃P₂O₁₄ and HSbP₂O₈ nanosheets, membranes and their counterpart bulks were characterized by XRD (Rigaku diffractometer with Cu-Kα radiation between 5°‒45° and an incident wavelength of 0.154056 nm), SEM (Verios G4 UC), TEM (FEI Tecnai T20, 120 kV), Helium ion microscope (HIM, Zeiss Orion NanoFab), and AFM (Bruker Multimode 8). XPS measurements were performed in an ESCALAB 250 spectrometer using monochromatic Al Kα radiation (1486.6 eV). The hydrophilicity of the membranes was analyzed at 298 K using a contact angle analyzer (Dataphysics, OCA20). The Raman spectra were recorded with JY Labram HR 800 using a 532 nm laser. The FT-IR spectra were acquired on a Smart OMNI-Transmission spectrometer (Thermo Scientific). Zeta-potential was measured with Malvern Zetasizer Nano-ZS90. WAXS measurements were performed on Xeuss 3.0 HR (Xenocs) SAXS/WAXS system using an incident Cu-Kα X-ray beam parallel to the membrane plane. The distance between the sample and the detector was 20.0 cm. For measurements, the membranes were cut into 5.0-mm-wide and 15.0-mm-long strips. The scattering patterns were collected by a Pilatus 3 R 300 K detector. The orientation degree of H₃Sb₃P₂O₁₄ nanosheets in the membranes was quantified by using Herman’s orientations factor (f), which was defined as follows^33,34,

$$f=\frac{3 < {\cos }^{2}\phi > -1}{2}$$

(4)

where <cos²ϕ> is the average value of the square of the cosine of the azimuthal angle for the (003) peak of H₃Sb₃P₂O₁₄ membranes, which was calculated as follows,

$$ < {\cos }^{2}\phi > =\frac{{\int }_{0}^{{{{{{\rm{\pi }}}}}}/2}I(\phi )\sin \phi {\cos }^{2}\phi d\phi }{{\int }_{0}^{\pi /2}I(\phi )\sin \phi d\phi }$$

(5)

where I(ϕ) is the peak intensity at an azimuthal angle of ϕ.

Data availability

The authors declare that the experimental data supporting the results of this study can be found in the paper and its Supplementary Information file. The detailed data for the study is available from the corresponding author upon request.

References

Daiguji, H. Ion transport in nanofluidic channels. Chem. Soc. Rev. 39, 901–911 (2010).
Article CAS PubMed Google Scholar
Koltonow, A. R. & Huang, J. Two-dimensional nanofluidics. Science 351, 1395–1396 (2016).
Article ADS CAS PubMed Google Scholar
Gopinadhan, K. et al. Complete steric exclusion of ions and proton transport through confined monolayer water. Science 363, 145–148 (2019).
Article ADS CAS PubMed Google Scholar
Radha, B. et al. Molecular transport through capillaries made with atomic-scale precision. Nature 538, 222–225 (2016).
Article ADS CAS PubMed Google Scholar
Joshi, R. K. et al. Precise and ultrafast molecular sieving through graphene oxide membranes. Science 343, 752–754 (2014).
Article ADS CAS PubMed Google Scholar
Jiao, K. et al. Designing the next generation of proton-exchange membrane fuel cells. Nature 595, 361–369 (2021).
Article ADS CAS PubMed Google Scholar
Park, M., Ryu, J., Wang, W. & Cho, J. Material design and engineering of next-generation flow-battery technologies. Nat. Rev. Mater. 2, 16080 (2016).
Article ADS Google Scholar
Yang, X. et al. Liquid-mediated dense integration of graphene materials for compact capacitive energy storage. Science 341, 534–537 (2013).
Article ADS CAS PubMed Google Scholar
Lukatskaya, M. R. et al. Ultra-high-rate pseudocapacitive energy storage in two-dimensional transition metal carbides. Nat. Energy 2, 17105 (2017).
Article ADS CAS Google Scholar
Marx, D., Tuckerman, M. E., Hutter, J. & Parrinello, M. The nature of the hydrated excess proton in water. Nature 397, 601–604 (1999).
Article ADS CAS Google Scholar
Garczarek, F. & Gerwert, K. Functional waters in intraprotein proton transfer monitored by FIIR difference spectroscopy. Nature 439, 109–112 (2006).
Article ADS CAS PubMed Google Scholar
Wolke, C. T. et al. Spectroscopic snapshots of the proton-transfer mechanism in water. Science 354, 1131–1135 (2016).
Article ADS CAS PubMed Google Scholar
Shi, B. et al. Short hydrogen-bond network confined on COF surfaces enables ultrahigh proton conductivity. Nat. Commun. 13, 6666 (2022).
Article ADS CAS PubMed PubMed Central Google Scholar
Dereka, B. et al. Crossover from hydrogen to chemical bonding. Science 371, 160–164 (2021).
Article ADS CAS PubMed Google Scholar
Mauritz, K. A. & Moore, R. B. State of understanding of nafion. Chem. Rev. 104, 4535–4585 (2004).
Article CAS PubMed Google Scholar
Kusoglu, A. & Weber, A. Z. New insights into perfluorinated sulfonic-acid ionomers. Chem. Rev. 117, 987–1104 (2017).
Article CAS PubMed Google Scholar
Liu, L., Chen, W. & Li, Y. An overview of the proton conductivity of nafion membranes through a statistical analysis. J. Membr. Sci. 504, 1–9 (2016).
Article CAS Google Scholar
Ramaswamy, P., Wong, N. E. & Shimizu, G. K. H. MOFs as proton conductors – challenges and opportunities. Chem. Soc. Rev. 43, 5913 (2014).
Article CAS PubMed Google Scholar
Lim, D.-W. & Kitagawa, H. Proton transport in metal-organic frameworks. Chem. Rev. 120, 8416–8467 (2020).
Article CAS PubMed Google Scholar
Lim, D.-W. & Kitagawa, H. Rational strategies for proton-conductive metal-organic frameworks. Chem. Soc. Rev. 50, 6349 (2021).
Article CAS PubMed Google Scholar
Sahoo, R., Mondal, S., Pal, S. C., Mukherjee, D. & Das, M. C. Covalent-organic frameworks (COFs) as proton conductors. Adv. Energy Mater. 11, 2102300 (2021).
Article CAS Google Scholar
Qian, X. et al. CdPS₃ nanosheets-based membrane with high proton conductivity enabled by Cd vacancies. Science 370, 596–600 (2020).
Article CAS PubMed Google Scholar
Gabriel, J.-C. P. et al. Swollen liquid-crystalline lamellar phase based on extended solid-like sheets. Nature 413, 504–508 (2001).
Article ADS CAS PubMed Google Scholar
Piffard, Y., Verbaere, A., Lachgar, A., Deniard-courant, S. & Tournoux, M. The layerd phosphatoantimonic acid H₃Sb₃P₂O₁₄, xH₂O. Rev. Chim. Miner. 23, 766–775 (1986).
CAS Google Scholar
Piffard, Y., Verbaere, A., Oyetola, S., Deniard-courant, S. & Tournoux, M. The layerd phosphatoantimonic acid HSb(PO₄)₂, xH₂O: synthesis, structure, thermal behavior and ion exchange properties. Eur. J. Solid State Inorg. Chem. 26, 113–127 (1989).
CAS Google Scholar
Deniard-courant, S., Piffard, Y., Barboux, P. & Livage, J. Relative humidity influence on the water content and on the protonic conductivity of the phosphatoantimonic acids H_nSb_nP₂O_3n+5, xH₂O (n = 1,3,5). Solid State Ion. 27, 189–194 (1988).
Article CAS Google Scholar
Yang, F. et al. A flexible metal–organic framework with a high density of sulfonic acid sites for proton conduction. Nat. Energy 2, 877–883 (2017).
Article ADS CAS Google Scholar
Agmon, N. The Grotthuss mechanism. Chem. Phys. Lett. 244, 456–462 (1995).
Article ADS CAS Google Scholar
Mäemets, V. & Koppel, I. ¹⁷O and ¹H NMR chemical shifts of hydroxide and hydronium ion in aqueous solutions of strong electrolytes. J. Chem. Soc. Faraday Trans. 93, 1539–1542 (1997).
Article Google Scholar
Griffith, C. S., Luca, V., Cochrane, J. & Hanna, J. V. Lanthanide/actinide ion-exchange and structural investigations of the layered phosphatoantimonic acid, H₃Sb₃P₂O₁₄·ZH₂O. Microporous Mesoporous Mater. 111, 387–403 (2008).
Article CAS Google Scholar
Oyetola, S., Verbaere, A., Piffard, Y. & Tournoux, M. The layered compounds AM^v(PO₄)₂ (A=K, Rb, Cs and M=Sb, Nb, Ta). Eur. J. Solid State Inorg. Chem. 25, 259–278 (1988).
CAS Google Scholar
Tanaka, Y. Ion Exchange Membranes Fundamentals and Applications 2nd edn (Elsevier, 2015).
Wan, S. et al. High-strength scalable MXene films through bridging-induced densification. Science 374, 96–99 (2021).
Article ADS CAS PubMed Google Scholar
Wan, S. et al. High-strength scalable graphene sheets by freezing stretch-induced alignment. Nat. Mater. 20, 624–631 (2021).
Article ADS CAS PubMed Google Scholar

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Acknowledgements

We thank Z. B. Liu and Q. Wang for the help on TEM measurements and X. Yu for the discussions on MSCs fabrication. This work was supported by the National Natural Science Foundation of China (Nos. 52188101 and 21773230), the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (No. ZDBS-LY-JSC027), and the LiaoNing Revitalization Talents Program (No. XLYC2201003).

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These authors contributed equally: Zhangcai Zhang, Lixin Liang.

Authors and Affiliations

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110016, China
Zhangcai Zhang, Jianze Feng & Wencai Ren
School of Materials Science and Engineering, University of Science and Technology of China, 72 Wenhua Road, Shenyang, 110016, China
Zhangcai Zhang & Wencai Ren
State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China
Lixin Liang & Guangjin Hou

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Zhangcai Zhang
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Lixin Liang
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Jianze Feng
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Wencai Ren
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Contributions

W.R. conceived and supervised the project. Z.Z. carried out the experiments and measurements. L.L. carried out the NMR measurements and analyzed the NMR data under the supervision of G.H. J.F. synthesized MXene, fabricated and measured MSCs. W.R. and Z.Z. designed the experiments, analyzed the data, and wrote the manuscript with input from other authors.

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Correspondence to Wencai Ren.

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Zhang, Z., Liang, L., Feng, J. et al. Significant enhancement of proton conductivity in solid acid at the monolayer limit. Nat Commun 15, 2706 (2024). https://doi.org/10.1038/s41467-024-46911-7

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Received: 05 August 2023
Accepted: 11 March 2024
Published: 27 March 2024
DOI: https://doi.org/10.1038/s41467-024-46911-7

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