2.2.3. The Assembly Structure of Cations in Compounds **1** and **2**

Both compounds **1** and **2** crystallize with Na<sup>+</sup> as counter cations, and four crystally independent Na<sup>+</sup> ions exist in every asymmetric unit. Taking compound **1** as the example, Na1, Na2, and Na3 are located on the symmetric plane and therefore possess the site occupancy of 0.5. For Na4, though it is not located at any symmetry element, and has an apparent site occupancy of 1, it is treated as a disorder due to the following properties: (1) Na4 and its symmetric atom Na4 have a distance of 2.128 Å, which is much shorter than that between two normal sodium cations with a distance over 3.0 Å, and is even shorter than the Na–O bond length with a general value over 2.3 Å. The abnormally short distance between Na4 and Na4 indicates that the two Na<sup>+</sup> ions should exist alternately, showing a disorder in space. (2) Na4 has a thermal displacement parameter over 0.1, which is much higher than those of Na1, Na2, and Na3 with an average value of about 0.04. After the disorder treatment, the thermal displacement parameter of Na4 decreases to about 0.04 and remains stable in the anisotropic state after several refinement cycles. (3) The whole polyanion has a total charge of −4, which needs two positive charges in the asymmetric unit for charge neutralization. Additionally, in the presence of three Na<sup>+</sup> ions with half site occupancies, the fourth one should also occupy the 0.5 site, which is also in accordance with the above analysis conditions. Finally, the elemental analysis result of Na also supports this treatment.

Four Na+ ions express different coordination environments as well as coordination numbers (Figure 3a). Na1 is coordinated by six water molecules and one terminal O atom of polyanion, showing a seven-coordination type. Na2, Na3, and Na4 are all in a six-coordination type, in which all the coordination sites of Na2 and Na3 are occupied by water molecules. As a comparison, the coordination environment of Na4 is completed by five water molecules and one O atom from polyanion. One important feature of these Na+ ions is that the coordinated water molecules are generally shared by two or three cations, resulting in their possessing wide edge- or face-connection with each other. This feature also facilitates the formation of the extended structure. As shown in Figure 3b, a regular honeycomb structure formed by Na+ ions and its coordinating O atoms can be obtained, showing one-dimensional (1D) channels along the c axis. Here, to better exhibit the architecture of the packing of Na<sup>+</sup> ions, polyanions are omitted for clarity except for two O atom links, with Na+ being kept. The passageways constructed here are helpful for the fast migration of ions, which can be used as a proton conductor.

**Figure 3.** (**a**) The coordination environments of four independent Na+, and (**b**) the packing model of counter cation Na+ in compound **1**. The polyanions are omitted for clarity. Blue ball: Na, red ball O, blue polyhedron: {NaO6} or {NaO7}.

#### 2.2.4. FT-IR, XPS and TGA Curves of Compounds **1** and **2**

Except for single crystal X-ray diffraction analysis, the structures of compounds **1** and **2** are also confirmed by FT-IR spectra. As shown in Figure S4a, the FT-IR spectrum of compound **1** in the low wavenumber region expresses characteristic vibration of the inorganic cluster sourced from V–O, Mo–O, and Mo=O. The existence of wavenumbers over 1000 cm−<sup>1</sup> can be ascribed to the vibrations of C–O, C–H, and H–O, showing the occurrence of organic components in the compound. Compound **2** has a similar FI-IR pattern as that of compound **1**, which is shown in Figure S4b.

XPS was used to identify the bond valence of Mo and V in the clusters. For compound **1**, as shown in Figure S5a, the binding energy peaks located at 235 and 232 eV are sourced from MoVI3d3/2 and MoVI3d5/2, indicating that the bond valence of Mo is +6. The bond valence of V is determined in Figure S5b, in which two binding energy peaks at 517 and 524 eV are VV2p1/2 and VV2p3/2, respectively, showing a +5 bond valence for V. As shown in Figure S6, the XPS spectra of compound **2** are similar to those of compound **1**, which also indicate the existence of MoVI and VV in the cluster.

To verify the purity of the prepared compounds, the powder X-ray diffraction (PXRD) patterns of compounds **1** and **2** was checked. As shown in Figure S7, the as-synthesized and simulated PXRD patterns are similar for each compound, indicating that the powder products maintained the same architectures as those in the single crystal state.

The thermal stability of compounds **1** and **2** were evaluated through TGA analysis. As shown in Figure S8, both compounds **1** and **2** quickly lose their lattice water molecules in the range of room temperature to 100 ◦C. Additionally, the organic species leave the cluster, followed by the decomposition of polyanion, showing the supporting role of triol ligand in the cluster.

#### 2.2.5. Stability of Compounds **1** and **2** in Aqueous Solution

The existing states of compounds **1** and **2** in an aqueous solution were examined using 1H NMR spectra. As shown in Figure S9, compound **1** is not very stable in the aqueous solution and decomposition occurred in the process of dissolving. The signals belonging to the free triol ligand appeared at the beginning and increased over time. Calculated from the integration value of the peaks, the initial decomposition ratio is about 6.6%, and reaches 7.9% after 3 days. The different chemical environments of methene groups in compound **1** generate the splitting of the signal, showing double peaks at around 4.9 ppm. Compound **2** shows a similar behavior to that of compound **1** in an aqueous solution (Figure S10). The difference is that compound **2** seems to have a relatively higher stability, and only

2.1% decomposes at the beginning, reaching 6.2% after 3 days. It should be noted that the decomposition process can be accelerated by the addition of acid or base.

#### *2.3. Proton Conductivity of Compounds* **1** *and* **2**

Proton-conducting materials play essential roles in fuel cells, which have seen rapid progress due to the demand for clean energy. For building high-proton-conductivity materials, an extensive hydrogen bonding network is needed, which accelerates the transport of the proton and induces energy loss. POMs are good candidates for proton-conducting materials due to their abundant O atoms on the surface of the cluster, which can serve as hydrogen bonding acceptors. Here, the proton conductivity of compounds **1** and **2** under different temperatures are investigated in the condition of a controlled 75% relative humidity (RH). As shown in Figure 4a, with the temperature changing from 25 to 60 ◦C, the proton conductivity of compound **1** increased from 2.25 × 10−<sup>5</sup> to 7.74 × 10−<sup>5</sup> S cm<sup>−</sup>1. Based on the proton conductivities at various temperatures, the activation energy (Ea) of the proton conductivity of compound **1** was evaluated. As shown in Figure S11a, a linear fitness can be obtained with the ln(σT) as longitudinal ordinate and 1/T as horizontal ordinate, from which the activity energy Ea is calculated as 0.32 eV. The relatively low Ea value (smaller than 0.4 eV) indicates that the proton conduction process in compound **1** is ascribed to a Grotthuss mechanism. This result is also in accordance with the crystal structure described above, in which a 1D channel exists and facilitates the transfer of protons. In addition, the water molecules on the wall of the channel also promote proton transport. The proton conductivity behavior of compound **2** was also investigated to evaluate the influence of the terminal group of the triol ligand. As shown in Figure 4b, in the range of 25 to 60 ◦C, the proton conductivity increases from 1.97 × 10−<sup>5</sup> to 7.48 × 10−<sup>5</sup> S cm<sup>−</sup>1. It can be seen that compounds **1** and **2** have similar proton conductivity, which sources their similar crystal structures as well as packing models. The slight decrease of proton conductivity of compound **2** compared to that of compound **1** is ascribed to originate from the slightly higher hydrophobicity of ethyl to methyl. In addition, the Ea of proton conduction for compound **2** was calculated as 0.36 eV (Figure S11b), which also indicates a Grotthuss mechanism.

**Figure 4.** Nyquist plots of compounds (**a**) **1**, and (**b**) **2** at various temperatures and 75% RH; (**c**) Nyquist plots of compounds **1** and **2** under the conditions of 75% and 98% RH at 30 ◦C; (**d**) proton conductivity of compounds **1** and **2** under different conditions.

We further investigated the influence of RH on proton conductivity. As shown in Figure 4c, when the RH increases to 98%, the proton conductivity of compound **1** increases to 1.24 × 10−<sup>4</sup> S cm−<sup>1</sup> at 30 ◦C, showing a 4.4 times larger elevation compared with that under 75% RH. In a higher RH condition, the number of dissociated water molecules increases and promotes the transport of protons in the 1D channel through the stable hydrogen bonding network. Compound **2** also shows similar proton conductivity behaviors and has a value of 1.05 × 10−<sup>4</sup> S cm−<sup>1</sup> at 30 ◦C and 98% RH, which also shows a 4.3 times improvement. By collecting proton conductivity under different conditions, as shown in Figure 4d, it can be concluded that RH has a more obvious influence than that of temperature. In addition, as listed in Table S5, we have also collected the recent results of the proton conductivity of POMs, showing that compounds **1** and **2** possess a relatively high performance.
