**2. Experimental Section**

#### *2.1. Materials and Methods*

The hexaniobate precursor K7HNb6O19·13H2O, vanadyl phosphate VOPO4·2H2O, TMA10H5[PNb12O40]·30.5H2O, K6[V10O28]·10H2O, and TMA9[PNb12V2O42]·19H2O were prepared according to previous literature methods and identified by IR spectroscopy [10,38–41]. Other starting chemicals and solvents were purchased from commercial source and used without further purification. IR spectra in KBr pellets were collected in the range of 400–4000 cm−<sup>1</sup> using a Bruker FT-IR spectrometer (Leipzig, Germany). Thermogravimetric analyses (TGA) of the compounds were performed using a LABSYS EVO device (Setaram Inc., Lyon, France) from room temperature to 800 ◦C under N2 atmosphere. Elemental analyses (Nb, V, Ni, P) were measured on a Thermo ICP atomic emission spectrometer (Waltham, MA, USA); C, H, N were performed on an ElementarVario EL cube Elmer CHN elemental analyzer (Langenselbold, Germany). X-ray photoelectron spectrum (XPS) analysis were measured on a Thermo ESCALAB 250 spectrometer using Al Kα radiation as the X-ray source (1486.7 eV). Powder X-ray diffraction (PXRD) data were obtained on SHI-MADZU XRD-6000 X-ray diffractometer (Kyoto, Japan) with Cu Kα radiation (λ = 1.54 Å; 2θ = 5–50◦). Gas chromatograph analyses were detected on a Shimadzu GC-2014C instrument with an FID detector equipped with an HP 5 ms capillary column. The hydrogen was detected by a Techcomp GC-9700 gas chromatograph (Shanghai, China) with a 5 Å molecular sieve column (2 m × 2 mm) and a thermal conductivity detector (TCD). Temperatureprogrammed chemisorption of carbon dioxide (CO2-TPD) was performed on a PCA-1200 temperature-programmed chemisorption instrument. The UV-vis spectra were measured on a UV-2600 (Builder, Beijing, China). Electrochemical surface area experiments were measured on a CHI660E electrochemical workstation (CH Instruments, Shanghai, China). Other electrochemical experiments were performed on an Ivium-OctoStat30 multi-channel electrochemical workstation (Eindhoven, The Netherlands).
