*3.1. Instruments and Materials*

All chemicals used here were purchased from Aladdin and used directly. All reactions were conducted with double-distilled water as solvent. C and H elemental analysis was made on a vario MICRO cube from the Elementar Company of Germany. Elemental analysis of V, Mo, and Na was conducted on a PLASMA-SPEC (I) inductively coupled plasma atomic emission spectrometer. FT-IR spectra were obtained from a Bruker Vertex 80v spectrometer in KBr pellets, in which the detector was DTGS and the resolution was 4 cm<sup>−</sup>1. Thermal stability of compounds was evaluated by a Q500 Thermal Analyzer (New Castle TA Instruments, New Castle, DE, USA) with a flowing N2 atmosphere and a heating rate of 10 ◦C·min<sup>−</sup>1. XPS data were acquired on an ESCALAB-250 spectrometer with a monochromic X-ray source (Al Kα line, 1486.6 eV). The charging shift of XPS was corrected at a binding energy of C1s at 284.6 eV. Electrochemical impedance spectroscopy was recorded on a Solartron 1260 A impedance analyzer. Powder X-ray diffraction data were recorded on a Rigaku SmartLab X-ray diffractometer with a Cu Kα radiation at a wavelength of 1.54 Å. 1H NMR spectra were obtained from a Bruker AVANCE 500 MHz spectrometer. The proton conductivities of samples were calculated according to the following equation:

σ = l/SR

where σ is the proton conductivity (S/cm), and l, and S, and R are the thickness, area, and resistance of the sample, respectively.

Single-crystal X-ray diffraction data of compounds **1** and **2** were collected on a Bruker D8 VENTURE diffractometer with graphite-monochromated Mo Kα (λ = 0.71073 Å) at 293 K. Both crystals were solved by *SHELXT* and refined by full-matrix-least-squares fitting for *F*<sup>2</sup> using the Olex2 software. All non-H atoms were refined with anisotropic thermal parameters. A summary of the crystallographic data and structural refinements for compounds **1** and **2** is listed in Table 2. The files with CCDC numbers 2206198 and 2206199 contain the supplementary crystallographic data for this paper. These data are available free of charge from www.ccdc.cam.ac.uk/data\_request/cif (accessed on 9 September 2022), or by emailing data\_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK.
