High-Entropy Oxide of (BiZrMoWCeLa)O2 as a Novel Catalyst for Vanadium Redox Flow Batteries

In this study, new fluorite high-entropy oxide (HEO), (BiZrMoWCeLa)O2, nanoparticles were produced using a surfactant-assisted hydrothermal technique followed by calcination and were used as novel catalytic materials for vanadium redox flow batteries (VRFBs). The HEO calcined at 750 °C (HEO-750) demonstrates superior electrocatalytic activity toward V3+/V2+ and VO2+/VO2+ redox couples compared to those of cells assembled with other samples. The charge–discharge tests further confirm that VRFBs using the HEO-750 catalyst demonstrate excellent Coulombic efficiency, voltage efficiency, and energy efficiency of 97.22, 87.47, and 85.04% at a current density of 80 mA cm–2 and 98.10, 74.76, and 73.34% at a higher current density of 160 mA cm–2, respectively. Moreover, with 500 charge–discharge cycles, there is no discernible degradation. These results are attributed to the calcination heat treatment, which induces the formation of a new single-phase fluorite structure, which facilitates the redox reactions of the vanadium redox couples. Furthermore, a high surface area, wettability, and plenty of oxygen vacancies can give more surface electroactive sites, improving the electrochemical performance, the charge transfer of the redox processes, and the stability of the VRFBs’ electrode. This is the first report on the development of fluorite structure HEO nanoparticles in VRFBs, and it opens the door to further research into other HEOs.

Table S3 The cyclic voltammetry results obtained from Figure S6.The CV curves of the HEO-750 electrode for the VO 2+ /VO 2 + were conducted at different scan rates, as shown in Figure S7a.The peak current (i pa and i pc ) as a function of the square root of the scanning rate for the HEO-750 electrode (Figure S7b).The cathodic and anodic peak current densities of the VO 2+ /VO 2 + redox pair are proportional to the square root of the scan rate, indicating that the electrochemical behavior of the redox couple at the electrode was diffusion controlled. 1 Because peak current versus square root of scan rate is nonlinear, CV is not the best way to determine kinetic parameters.Therefore, the kinetic parameters were determined using the rotating disc electrode (RDE) method, i.e., LSV curves.The electrode reaction rates of HEO-750 were investigated using linear sweep voltammetry (LSV), as shown in Figure S8.The rotation speed was varied from 200 rpm to 2000 rpm (Figure S8a).LSV was scanned from 0 V to 2 V on the positive side versus Ag/AgCl at the scan rate of 2 mV s -1 .The linear relationship between limiting current, i L , and the square root of rotation speed (ω 1/2 ) is shown in Figure S8b, which reveals the good electrochemical activity of HEO-750. Figure S8b plots the Levich behavior of the HEO-750 assembled electrodes governed by the equation below. 2,3  = 0.62nFAD 2/3 ω 1/2 υ -1/6 C 0 (1) From the slopes of the curves given in Figure S8b, the calculated diffusion coefficients for VOSO 4 using Equation 1 are 2.1246×10 -5 cm 2 s -1 .Figure S8c is obtained by taking the logarithm of the reciprocal of the y-intercepts of Figure S8d.The values of k 0 were obtained from i 0 using Equation 2. The exchange current and estimated standard rates constant are 7.75×10 -3 A and 2.5×10 -4 cm s -1 , respectively.Linearly fitted Koutecky-Levich plots of i −1 versus ω −1/2 .2400 2800 3200 3600 Intensity (a.u.) Raman Shift (Cm -1 ) Intensity (a.u.) Raman Shift (Cm -1 ) Raman Shift (Cm -1 ) Raman Shift (Cm -1 )

Figure
Figure S7 (a) CV curves of HEO-750 electrode in 1.6 M VOSO 4 in 4.6 M H 2 SO 4 electrolyte at scan rates ranging from 10 to 100 mV s -1 .(b) Peak current (i pa and i pc ) as a function of the square root of the scanning rate for the HEO-750 electrode.

Figure S9 Figure S10
Figure S9 Potential-dependent in situ Raman spectra measured at (a, b) low wavenumber and (c, d) high wavenumber regimes of HEO-750 in 0.05 M VOSO 4 + 2 M H 2 SO 4 .

Figure S14 .Figure S15 .
Figure S14.(a, c) the CV curves of TGF and TGF-HEO-750 and the corresponding values of I pa /I pc and ΔE p with a scan rate of 5 mV s −1 ; (b, c) the Nyquist plots of the TGF and TGF-HEO-750, and the corresponding R s and R ct values at open-circuit potential 5 mV, towards VO 2+ /VO 2 + in 0.05 M VOSO 4 + 2 M H 2 SO 4 solutions after charge-discharge cycles.

Table S2
The fitting results of XPS O 1s spectra obtained from Figure S5.

Table S4
EIS results obtained from FiguresS10a and S10b.

Table S5
Comparison of the CE, VE, and EE of the TGF-HEO-750 material with those of previously reported metal and metal oxide-based materials.