To gain initial mechanistic insights into the fundamental electrochemical performance of {MV13} relevant for NRFB operation, we analyzed charge transfer behaviour using Randles-Sevcik analyses. These data indicate that the observed redoxprocesses are diffusion-limited, suggesting that charge-transfer at the electrode/electrolyte interface is fast compared with diffusion. Thus, mass-transport to/from the electrode surface is considered a future limitation to be optimized for these systems. Redox-kinetics were further studied using Nicholson and Koutecký-Levich methods, to determine diffusion coefficients (D0) and electron transfer rate constants (k0) for {MV13}. D0 and k0 obtained from the two methods are generally consistent, in the order of 10 -6~-5 cm 2 s -1 (D0) and 10 -4~-3 cm s -1 (k0), respectively (see SI, Section 2.2 and Table ). This data is in line with previous reports on redox-active POMs in organic electrolytes. The data also reveal that, despite their size (diameter ~1 nm), {MgV13} and {V14} show suitable mass transport and fast electron transfer in acetonitrile. Both features are important prerequisites for optimum NRFB operation. Next, we investigated the membrane crossover rate of {MV13} through a standard commercial Celgard 2400 membrane. Membrane permeability was analyzed in an electrochemical Hcell consisting of two compartments separated by the membrane. Our analyses gave a permeability of 2.35×10 -11 cm 2 s -1 and only ~1.5% of the active species crossed the membrane over a period of 7 days in a static diffusion test. This is significantly lower crossover compared to previous literature reports on similar species (details see SI, Section 2.3). Given the promising features of {MV13}, we designed a symmetric NRFB operated using an acetonitrile solution containing 1 mM {MV13} and 0.1 M (nBu4N)PF6. Identical solutions were used as anolyte and catholyte, and the half-cells were separated by a Celgard 2400 membrane, see Figure . All NRFB tests reported in this work were performed at room temperature under N2 atmosphere to avoid undesirable reactions with oxygen or water. The as-constructed symmetric NRFBs were tested under galvanostatic conditions at current densities ranging from 0.4 mA cm -2 to 1.0 mA cm -2 . To avoid overoxidation of the graphite electrode and possible degradation of the electrolyte components, the cutoff voltages for charging and discharging were set to 1.8 V and 0.2 V. Note that the NRFB was operated in 2-electrode configuration, so voltages given for the NRFB are absolute potential differences between the cathode and anode. According to the polarization curve shown in Figure , the NRFB reaches the highest power density (3.3 mW cm -2 ) at a current density of 0.6 mA cm -2 . Notably, the polarization curve is almost linear with no obvious plateau, indicating there is no observable redox kinetic limitations for the electrode reactions of the battery. This is in line with the high kinetic rate constant k0 for {MV13} described above. As shown in Figure , distinct charge and discharge plateaus were observed between 0.5 V and 1.0 V at a constant current density of 0.6 mA cm -2 . These values closely match the average potential difference of the redox processes in anolyte and catholyte. Theoretically, the average potential difference between the anolyte and catholyte based on the CV analysis in Figure shows that the thermodynamic potential of the NRFB is calculated to be 1.7 V. However, for the actual fullcell, an average potential of 0.75 V at a discharge current density of 0.6 mA cm -2 is observed. The difference between theoretical and experimental value is likely due to the overlapping faradic response for the forward electrode reaction processes and is often observed in RFBs. Under the given conditions, the system features a discharge specific capacity of 22.6 mAh and a coulombic efficiency of 93% at 0.6 mA cm -2 . Also, stable galvanic charge (23.9 mAh) and discharge capacities (22.2 mAh) are observed during galvanostatic cycling, resulting in an excellent capacity retention of 97% after 50 cycles (reduction rate = 0.074% per cycle, see Figure ). The cycling behaviour of the NRFB in constant current mode was then studied in detail: over 50 cycles (≈ 52 h), the coulombic efficiency was maintained at ~93%, confirming the excellent redox chemical and stability of {MV13} (Figure ). The NRFB also shows good voltage efficiency of 72% and energy efficiency of 67% with negligible efficiency fading after 50 cycles. The average number of stored and released electrons per cluster unit during the charge/discharge test was calculated by integration of the dQ/dV curves (SI, Figure ), suggesting that ~4.1 electrons are transferred for the anolyte and ~3.8 electrons are transferred for the catholyte. The alternating current area-specific resistance (AC-ASR) of the battery was determined by high-frequency electrochemical impedance spectroscopy (EIS) at the open circuit potential (OCP). We note that the high frequency resistance curve shows no obvious polarization plateaus, indicating that the main resistances inside the NRFB are caused by electrode and membrane resistances (SI, Figure ). Furthermore, the rate capability of the as-assembled {MV13}based NRFB was evaluated at current densities ranging from 0.4 mA cm -2 to 1.0 mA cm -2 . Figure shows that the highest charge capacity (25.3 mAh) and discharge capacity (23.5 mAh) are observed at a current density of 0.4 mA cm -2 , while the drop with increasing current density (Figure ). This is most likely due to incomplete electrolyte reaction under higher current density. Coulombic efficiency and energy efficiency of the NRFBs were also investigated at varying current densities (Figure ). The coulombic efficiency of the NRFB remains above 93%, while the energy efficiency gradually decreases from 72% to 50% with increasing current density (SI, Figure ). The voltage profiles of the NRFB remain nearly identical throughout the 50 cycles test at a constant current density, indicating stable electrode reactions (Figure ). Minor differences on the voltage profile for the first cycle are most likely be due to an electrode/electrolyte activation process. The ability to regenerate spent electrolytes is a key target for sustainable RFB research. To this end, the anolyte and catholyte solutions were recovered after cycling, mixed, and re-set to the original oxidation state (i.e., regenerated), by bulk electrolysis at the OCP (-0.5 V). This regenerated electrolyte was then re-deployed in the NRFB and cycled at a current density of 0.6 mA cm -2 under our standard conditions. As shown in Figure , 87.5% of the original capacity was recovered, which is a promising first demonstration of the recyclability and regeneration of the {MV13} electrolyte. Redox stability of the active electrolyte component is crucial for the durability of the RFB. Thus, comparative CV analyses for both anolyte and catholyte were performed before and after the cycling test (see SI, Figure ). The CVs show that all general redox features of the original anolyte and catholyte are still present. Minor shifts and a general broadening of the redox signals are observed. This indicates that overall, the system operates robustly, while the minor changes observed led us to perform further spectroscopic analyses of the system, see below. The capacity fading observed has two likely causes: first, after prolonged cycling, we noted formation of some precipitate in the tubing. This is most likely due to crystallization of {MV13}, which effectively removes some active component from the electrolyte. Second, trace amounts of atmospheric O2 or H2O might have diffused into the system, e.g. through the tubing, resulting in undesired side reactions leading to capacity fading. To gain initial mechanistic insights into the redox cycling and charge-transfer of {MV13} during charge/discharge, we performed operando ultraviolet-visible (UV-vis) spectroscopy for both, anolyte and catholyte over the cycling test. UV-vis spectroscopy of {MV13} (Figure and Figure , SI) shows that the cluster features two prominent absorption bands, the broad V(IV/V) intervalence charge transfer (IVCT) absorption band centered at λ~900 nm (Figure ), and the ligand to metal charge transfer (LMCT) absorption band centered at λ~380 nm (Figure ). For the anolyte, we observed during charging (0 min -30 min), electro-oxidation occurs, as verified by operando UV-vis spectroscopy (Figure and SI, Figure ), where cluster oxidation was indicated by a loss of the characteristic IVCT transition. This indicates the oxidation of {MV13} to an all V(V) state. This is in line with the dQ/dV analyses discussed above, which indicate the removal of approximately four electrons per {MV13} unit. During the discharging process (30 min -60 min), the anolyte was electro-reduced, and the characteristic IVCT transition absorption was recovered. The IVCT spectral features before and after redox cycling are virtually identical, indicating structural integrity and identical charge-state of {MV13}. For the catholyte, operando UV-Vis spectroscopy was also used to monitor the electro-reduction during charging (Figure and SI, Figure ). Here, we observed that the characteristic absorption peak at λ~380 nm decreased during charging, which is indicative of the generation of additional V(IV) centers in {MV13}. During discharge (i.e., electro-oxidation) of the catholyte, excellent redox reversibility was observed, and the characteristic absorption at ~380 nm was fully recovered, and the LMCT spectral features of {MV13} before and after redox cycling are virtually identical (Figure ). To further verify the structural integrity of {MV13}, we performed ex-situ Raman spectroscopy. Thus, electrolyte samples were extracted from the NRFB at different oxidization states (fresh, oxidized, reduced and regenerated) and Raman spectra were measured under identical conditions. As shown in SI, Figure , the samples feature the characteristic {MV13} signals, and no significant changes of the signals in terms of shifts or changes of intensity are observed at different oxidation states. This indicates that the {MV13} metal oxo framework is retained during the electrochemical cycling, further emphasizing that redoxequilibrated mix-valent polyoxovanadates are promising active components for NRFBs.