# High Potential Isoindoline-Based Nitroxides Posolytes for Aqueous Organic Redox Flow Batteries

Karim Boutamine, Gilles Casano, Patricia Bassil, Sébastien Gauden, Cecilia Poderi, Emilie O Pepe, Frédéric Favier, Steven Le Vot, Olivier Ouari

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Karim Boutamine, Gilles Casano, Patricia Bassil, Sébastien Gauden, Cecilia Poderi, et al. High Potential Isoindoline-Based Nitroxides Posolytes for Aqueous Organic Redox Flow Batteries. Chem-SusChem, 2026, 19 (3), 10.1002/cssc.202502461. [https://doi.org/10.1002/cssc.202502461](https://doi.org/10.1002/cssc.202502461)

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# Isoindoline-based Nitroxides as Posolytes for Aqueous Organic Redox Flow Batteries

Karim Boutamine,[a,b,c], Gilles Casano,[a] Patricia Bassil[b,c], Sébastien Gauden,[a] Cecilia Poderi,[a] Emilie Pepe,[a] Frédéric Favier[b,c], Steven Le Vot*[b,c] and Olivier Ouari*[a,c]

[a] Dr. K. Boutamine, Dr. G. Casano, S. Gauden, Dr. C. Poderi, E. Pepe, Prof. O. Ouari
Aix-Marseille Univ, CNRS, ICR UMR 7273, Marseille, France
E-mail: steven.le-vot@umontpellier.fr; olivier.ouari@univ-amu.fr

[b] Dr. K. Boutamine, Dr. P. Bassi, Dr. F. Favier, Dr. S. LeVot,
ICGM, University Montpellier, CNRS, ENSCM, Montpellier 34095, France

[c] Dr. K. Boutamine, Dr. P. Bassil, Dr. F. Favier, Dr. S. Le Vot, Prof. O. Ouari
Réseau sur le Stockage 'Electrochimique de l'Energie (RS2E), CNRS, Amiens 80000, France

Supporting information for this article is given via a link at the end of the document.

## Abstract:

The growing transition from fossil fuels to renewable energy sources such as wind and solar requires efficient stationary energy storage systems to ensure grid stability. Among emerging technologies, redox flow batteries (RFBs) offer a promising solution due to their unique decoupling of energy and power capacities, allowing flexible system design. Recent advances in organic redox flow batteries (ORFBs) have highlighted redox-active organic molecules (ROMs) as sustainable alternatives to conventional vanadium-based systems, addressing issues of cost and corrosivit. In particular, nitroxide radicals such as TEMPO derivatives have demonstrated high reversibility and fast kinetics in aqueous systems, though the stability of their oxidized N-oxoammonium form remains a challenge for long term storage. Isoindoline-based nitroxides offer potential for enhanced stability but have been limited by complex and low-yield synthetic routes. Herein, we present a convenient metal-catalyzed [2+2+2] intermolecular cycloaddition strategy for the synthesis of isoindoline-based nitroxides and their aza analogues, including two new candidates, TC-TMIO and PPO. Electrochemical characterization reveals that PPO, a cationic 2,3-dihydropyrrolo[3,4-c]pyridinium nitroxide, exhibits an oxidation potential 220 mV higher than the benchmark 4-TMA-TEMPO and achieves solubility exceeding 3 M in 1 M NaCl aqueous solution. Preliminary stability assessments of the PPO and redox flow battery testing using a methylviologen/PPO system demonstrate its potential as a high-performance, sustainable posolyte for aqueous organic redox flow batteries.

## Introduction

Renewable power plants such as wind farms and photovoltaics are critical today to the shift from fossil fuels to renewable and sustainable energy sources. Stationary energy storage technologies are key partners to the widespread integration of these inherently intermittent, non-dispatchable energy sources into the grid while ensuring grid stability. Redox flow batteries (RFB) technology is emerging as a suitable and promising option, notably due to the decoupling of power output and energy. The amount of energy stored is determined by the size of the electrolyte tanks, while the power output is dictated by the size of the electrochemical stack. This decoupling enables RFBs to be scaled independently to meet specific energy and power requirements. [1] [2] [3] Thereby, RFB are easily tailored to match the needs of a broad range of applications (off grid systems, backup power, demand response batteries), in comparison with other technologies such as lithium ion batteries on the one hand and pumped storage hydropower on the other. Recent developments have shown that redox active organic molecules (ROM) can be valuable alternatives to counterbalance the limitations of commercially implemented vanadium RFB, notably their high and volatile cost, the use of corrosive acid-based electrolytes and their limited energy densities. [4] [5] [6] [7] Organic RFBs are seen as a promising alternative due to their use of abundant and sustainable materials, but their performance and
durability still require further improvement. The rational design of ROMs relies on chemical tailoring to improve key properties such as water-solubility, redox potentials and kinetics, and stability to propose efficient low-cost, sustainable and safe aqueous organic RFBs. In the quest for efficient organic posolytes for RFB, nitroxide radicals such as tetramethylpiperidinyloxyl (Tempo) derivatives have recently attracted a lot of attention for aqueous organic redox flow batteries (AORFB). [8] [9] [10] [11] [12] [13] [14] [15] The fast and reversible one-electron reaction between the nitroxide and N-oxoammonium couple (Figure 1), the suitable redox potentials and the rich chemistry to tailor the nitroxide molecules have opened promising perspectives. Although nitroxides are stable in aqueous solution in a wide range of pH values, their respective N-oxoammonium cation appeared to undergo slow decomposition reactions in aqueous solutions. Studies of the N-oxoammonium fade in aqueous systems have proposed the involvement of the acidic methylene protons in β-position in an irreversible ring opening reaction. [16] [17]

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Structure of TEMPO-based nitroxide radical and its one-electron oxidation product (N-oxoammonium ion)</figcaption></figure>

In 2016, Janoscka et al. introduced **4-TMA-Tempo**, 4-trimethylammonium-Tempo salt, as promising posolyte for AORFB. [18] The higher and reversible oxidation potential of **4-TMA-Tempo** respective to **4-OH-Tempo** (0.99 V and 0.84 V vs SHE, respectively), its fast redox kinetics, and its high solubility (2.3 M) in 1.5 M NaCl aqueous solution provided it as one of the most studied and promising posolytes for AORFB applications. Subsequently, it has been suggested by several studies that the trimethylammonium group plays a favorable role on the stability of the N-oxoammonium cation but the origin of this effect has not been identified. To our knowledge, limited examples of non Tempo-based nitroxides have been reported in the field of AORFBs, with only one examples of isoindoline nitroxide in non-aqueous RFB. [19] The fused ring scaffold in isoindoline nitroxides have been reported to be less prone to ring-opening degradation [20] and could provide opportunities to improve stability of oxoammonium salts in aqueous systems. Despite their numerous advantages, widespread applications of isoindoline nitroxides may have been restricted by accessibility limitations (chemically and commercially). Three main synthetic approaches have been reported but they all present moderate to low yield schemes and usually restricted scope (scheme 1). The tetraalkylation of N-benzylphthalimide by excess of MeMgBr has been the most commonly used synthetic procedure [21] [22] to access to 1,1,3,3-tetramethyldibenzo[e,g]isoindolin-2-ylloxyl nitroxide and further functionalization approaches have been described in the last years. [23] [24] Hideg et al. have reported a strategy based on a Diels-Alder reaction involving a pyrroldinediene and diethyl acytelenedicarboxylate to generate the isoindoline nitroxide frame after oxidative aromatization. [25] Nevertheless, the approach requires the six-step synthesis of the diene precursor and sometimes the long heating period for the Diels-Alder reaction which causes degradation or polymerization of the starting materials. Later, the same group reported the electrocyclic reaction of 3-bromo-4-formyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-1-ylloxyl nitroxide to access isoindoline based nitroxide [26] [27] [28] but again, the synthetic pathway is long, with an overall yield inferior to 15% and exhibits limited structural functionalities.

<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Reported synthetic routes for accessing isoindoline-based nitroxides</figcaption></figure>

Herein, we report a convenient and straightforward route for the preparation of isoindoline-based nitroxides and their aza counterparts such as 2,3-dihydro-1H-pyrrolo[3,4-c]pyridine derivatives, using a metal-catalyzed [2+2+2] intermolecular cycloaddition approach. Two new nitroxides, **TC-TMIO** and **PPO**, as candidate posolytes for
aqueous organic redox flow batteries have been synthesized and their electrochemical properties including the redox potential and kinetics are reported. **PPO**, a cationic 2,3-dihydropyrrolo[3,4-c]pyridinium nitroxide, provides an redox potential surpassing state of the art **4-TMA-Tempo** nitroxide posolyte by 220 mV and exhibits a high solubility (> 3 M) in 1M NaCl aqueous solutions. Preliminary data on the stability of the **PPO** oxoammonium cation is also reported. The performance of a methylviologen/PPO RFB is discussed and compared to the state-of the art compound **4-TMA-Tempo**.

# Results and Discussion

## Synthesis

The construction of functional fused rings using intermolecular cyclotrimerization reactions has attracted a growing of interest in the last decade. This approach usually provides very efficient methods for the synthesis of complex products in a single step from simple building blocks, providing a high atom economy and broad reaction scope. Among them, the [2+2+2] cyclotrimerization of triple bounds are well-established reactions for the synthesis of polysubstituted benzenes. [29] [30] [31] [32] [33] [34] In the recent years, the use of transition metal such as Co, Fe, Ni, Rh, and Ru complexes has been reported to proceed cycloadditions under mild conditions using a broad range of unactivated nitriles, and alkyne substituted at the α and β positions substrates. [35] [36] [37] Therefore, the metal-catalyzed intermolecular cycloaddition of alkynes and nitriles with α,ω-dialkynylamines appears as an appealing method to access isoindoline and 2,3-dihydropyrrolo[3,4-c]pyridine nitroxides, respectively. As starting dialkynyl amines are readily accessible, we decided to further elaborate on the isoindoline and dihydropyrrolo[3,4-c]pyridine synthesis using dialkyl acetylenedicarboxylate and acetonitrile substrates, respectively.

For **TC-TMIO**, we selected Rh-catalyzed cyclotrimerization route, using Wilkinson's catalyst (5 mol %) and bis 4-(methyl 4-methylpent-2-ynoate)amine (2) with diethylacetylene carboxylate as model reaction (Scheme 2).

<figure><img src="image_4.png" /><figcaption>**Scheme 2.** Synthesis of TC-TMIO</figcaption></figure>

Isoindoline 3 was formed in 62% yield at 85°C in toluene with 5 mol% of catalyst and under inert atmosphere, and these conditions enable a good conversion with limited side product formation. Oxidation of amine 3 by peracetic acid in a water/dichloromethane mixture at room temperature afford in 90% yield diethyl, dimethyl 1,1,3,3-tetramethylisoindoline-N-oxyl-4,5,6,7-tetracarboxylate (TC-TMIO), that has not been reported yet to the best of our knowledge. The X-ray crystal structure analysis confirmed the structure of nitroxide TC-TMIO (Figure 2) and features a nearly planar isoindoline core with the tetracarboxylate groups pointing out of the plane.
<figure><img src="image_5.png" /><figcaption>**Figure 2:** A view of the molecular structure of the nitroxide TC-TMIO.</figcaption></figure>

To document the applicability of the approach for the preparation of fused heterocycles, 2,3-dihydropyrrolo[3,4-c]pyridine was prepared using a Co-catalyzed [2 + 2 + 2] cycloaddition, where the active catalyst was generated *in situ* from inexpensive and commercially available materials. Reaction of the bis(2-methylbut-3-yn-2-yl)amine α,ω-diynes (1) with acetonitrile in the presence of 5 CoCl₂·6 H₂O (10%), dppe (12%), Zn powder (20%) at 60°C yields the amine 5 in 40% yield (Scheme 3).

<figure><img src="image_6.png" /><figcaption>**Scheme 3.** Synthesis of nitroxide PPO</figcaption></figure>

Selective oxidation of the secondary amine of 5 using peracetic acid at room temperature yields the corresponding nitroxide free radical in good yield (63%). Finally, the pyridine ring was further alkylated with methyl iodide to generate 1,1,3,3,5,6-hexamethyl-2,3-dihydro-1H-pyrrolo[3,4-c]pyridin-5-ium -N-oxyl nitroxide (PPO) in 92% yield.

Electrochemical characterization of PPO and TC-TMIO

The electrochemical properties of the newly reported PPO and TC-TMIO nitroxides were characterized using cyclic voltammetry (CV) and rotating disk electrode (RDE) experiments in 1 M NaCl aqueous electrolytes. CV is a convenient method for preliminary assessing the reversibility and redox potential of the system.

<table><thead><tr><th></th><th>Redox potential (E°) vs. SHE</th><th>Electron transfer rate (k<sub>0</sub>) (10<sup>-3</sup> cm.s<sup>-1</sup>)</th><th>Diffusion constant (D) (10<sup>-6</sup> cm<sup>2</sup>.s<sup>-1</sup>)</th><th>Solubility (M, in 1 M NaCl) solution)</th></tr></thead><tbody><tr><td>PPO</td><td>1.16 <sup>[a]</sup></td><td>6.4 <sup>[a]</sup><br>4.3 <sup>[b]</sup></td><td>2.3 <sup>[a]</sup><br>2.9 <sup>[b]</sup></td><td>3</td></tr><tr><td>4-TMA-Tempo</td><td>0.94 <sup>[a]</sup></td><td>11.9 <sup>[a]</sup><br>4.4 <sup>[b]</sup></td><td>3.3 <sup>[a]</sup><br>2.8 <sup>[b]</sup></td><td>2.5</td></tr><tr><td>TC-TMIO</td><td>0.91 <sup>[a]</sup></td><td>0.1 <sup>[a]</sup><br>nd</td><td>1.6 <sup>[a]</sup><br>nd</td><td>nd</td></tr><tr><td>sulfateTempo [38]</td><td>0.81</td><td>1.91</td><td>2.98</td><td>nd</td></tr></tbody></table>
<caption>**Table 1.** Electrochemical Properties and diffusion coefficients for nitroxide posolytes ([a] measured using CV experiments, [b] measured using rotating disk electrode experiments). Values for sulfate Tempo taken from reference [38].</caption>

Figure (S18) shows a quasi-reversible redox potential (E°') at 1.16 V (vs SHE) of PPO revealing a peak separation potential (ΔEp) of 70 mV that is almost independent of the scan rate, and with a cathodic to anodic peak current ratio (|Ipa|/|pcl|) close to 1.

It is worth mentioning that **4-TMA Tempo**, one of the cationic posolytes of choice for today's AORFBs, shows a potential of 0.94 V (vs SHE) (Fig S19), revealing an improved oxidation potential (+220 mV) for **PPO** (Figure 3) still suitable for aqueous voltage window. Solubility of **PPO** reaches at least 3 M in 1 M NaCl aqueous electrolyte for both its reduced (uncharged) and oxidized (charged) states. These two properties are favorable criteria for AORFB full cell relative to reported nitroxide posolytes.

<figure><img src="image_7.png" /><figcaption>**Figure 3:** Cyclic voltammograms of 0.1 M PPO (red), 4-TMA Tempo (black) and Methyl viologen (blue) in 1 M NaCl (aq) solution. In parentheses the half-wave potentials (E°').</figcaption></figure>

To determine the redox kinetic characteristics of PPO, further investigations were conducted using CV at various scan rates and rotating disk electrode (RDE) experiments at different rotation speeds. The apparent electron transfer rate constant (K₀,app) and diffusion coefficient (D) were determined for PPO, TC-TMIO and 4-TMA-Tempo, and the results are reported in Table 1 (additional details are provided in SI). For PPO, the K₀,app values were estimated to be 6.4 × 10⁻³ cm s⁻¹ and 4.3 × 10⁻³ cm s⁻¹ by CV (Fig S17) or RDE (Fig S20), respectively. The diffusion coefficients (D) of PPO has been determined as 2.3 × 10⁻⁶ cm² s⁻¹ and 2.9 × 10⁻⁶ cm² s⁻¹ from CV and RDE, respectively. Both kinetic characteristics are satisfying the targeted performances and are in line with values obtained for small size nitroxides. Beyond the quantitative analysis, the values of K₀,app and D for PPO are very close to those of the state-of-the-art **4-TMA-Tempo** posolyte (Fig. S19 and S22), indicating that, based on these preliminary electrochemical assessments, PPO is suitable for use in a redox flow battery.

**TC-TMIO** shows a redox potential of 0.91 V (vs SHE) (Fig S18). The electron transfer rate constant (K₀,app) and diffusion coefficient (D) were found to be 0.1 × 10⁻³ cm s⁻¹ and 1.6 × 10⁻⁶ cm² s⁻¹. Associated values for an anionic nitroxide posolyte (Tempo-sulfate) are indicated in Table 1 for comparison. Preliminary investigation of the cycling performances of **TC-TMIO** for long time experiment has been poor, mainly due to acid-induced partial precipitation during cycling. For the rest of the study (battery cycling) only **PPO** and **4-TMA-Tempo** are discussed.

**Charge-discharge performance of PPO in AORBs**

Redox flow batteries were assembled using PPO (0.1 M) and methyl viologen (1,1'-bis(methyl)-4,4'-bipyridinium dichloride, 0.1 M) dissolved in 1.0 M NaCl as the posolyte and negolyte, respectively. A Fumasep FAA-3-50 (thickness 45 - 55 µm, surface area 25 cm²) membrane was employed as ion exchange membrane.

Methyl viologen (MV) was selected as negolyte because of numerous studies using MV in AORFB reported in the literature and its commercially-availability.[ref5] As shown in Figure 3, PPO resulted in a 16% improvement in cell voltage compared to the reference **4-TMA-Tempo**. Moreover, the larger PPO solubility provides further improvements in energy density thanks to enhanced posolyte capacity. Considering a solubility of 3 M (1 e⁻), the theoretical capacity of PPO is 80 Ah L⁻¹, and the energy density of a battery combining PPO with MV (2.4 M, 1e⁻) as negolyte is 57 Wh L⁻¹, placing it as a promising organic posolyte when compared to data reported for nitroxides and highlighted in Figure 4.
<figure><img src="image_8.png" /><figcaption>**Figure 4:** Redox potential and solubility of some nitroxide-based posolytes reported for AORFB.</figcaption></figure>

4-COONa TEMPO [43], PSS-TEMPO [44], MiAcNH-TEMPO [45], N₂-TEMPO [13], CT [10], g⁺-TEMPO [46], Y-Type TEMPO [15], N+TEMPOD [11], 4-OH-TEMPO [47] [48] [49], TMAP-TEMPO [8] [12] [50], TMAAcNH-TEMPO [8] [50] [51], 4-NH₂-TEMPO [48] [49] [52], 4-TMA-TEMPO [8] [16] [50] [18] [53] [54], TMA-Proxyl [14], TPP-TEMPO [9], 4-SO₄Na-TEMPO [43], CPD [10], CPL [10].

One of the key properties for long-duration energy storage in AORBs is the stability of the system, notably the long-term cycling. The charge-discharge experiment of **PPO** based redox flow battery was conducted in the voltage range of 0.900 – 1.725 V at a current density of 10 mA cm⁻² (Fig. S6) and data were compared to **4-TMA-Tempo** based AORFB (Fig S7). Detailed cycling protocols are provided in the SI. Since the posolyte is the focus of this study, the volume of the negolyte reservoir was selected three times larger than for the posolyte, maintaining the concentration identical, resulting in a configuration with a capacity-limiting side (CLS) and a non-capacity-limiting side (NCLS). By using this unbalanced configuration, performances of the battery, including fade rate, can be solely attributed to the compartment of interest (CLS, in this case the posolyte). [39] The cell voltage is 1.54 V and the battery exhibits a low overpotential for charge and discharge leading to good voltage efficiency. Using galvanostatic cycling at constant current followed by a potentiostatic regime at constant voltage [7] [40] [41], a discharge capacity of 60 mAh was obtained, recovering 90% of the theoretical capacity. As shown in Figure 5, the capacity then slightly decreases with time while maintaining excellent coulombic efficiency (> 99%). For **4-TMA-Tempo**, the first discharge led to a capacity of 62 mAh (Figure S23), with a subsequent decrease in capacity but to a lower extend compared to **PPO**.

<figure><img src="image_9.png" /><figcaption>**Figure 5:** Charge/discharge capacity and coulombic efficiency as a function of the cycles number for **MV** / **PPO** AORFB. Conditions: anolyte: 75 ml of 0.1 M **MV** in 1 M NaClq; catholyte: 25 ml of 0.1 M **PPO** in 1 M NaCl; Fumasep FAA-3-50 anion-exchange membrane; current density 10 mA cm⁻²; temperature 23°C, time of cycling 6.8 days</figcaption></figure>
From Figure 5 and S23, temporal capacity fade rates of 3.22% and 1.01% per day were found for **PPO** and **4-TMA-Tempo**, respectively. The fade rate of **PPO** is in the range usually observed for other nitroxides, with **4-TMA-Tempo** as one of the best standards. It is worth noting that most of the capacity loss occurs during the first cycles. Table S1 shows that from cycle 150 to cycle 200, the capacity fade rate is 1.9% and 0.70% per day for **PPO** and **4-TMA-Tempo**, respectively.

In lab-scale redox flow batteries, the capacity fade rate has two main origins: the crossover of redox-active molecules from one tank to the other, and/or the chemical degradation of the redox species, which leads to capacity loss. In AORFBs, the latter phenomenon is usually the most critical, even if the degradation process is not understood. To identify the origin of observed capacity loss in both systems (**MV** – **PPO** and **MV** – **4-TMA-Tempo**), cyclic voltammetry (CV), UV-visible spectroscopy, and ¹H NMR spectroscopy analysis of electrolyte solutions (either posolytes and negolytes) have been performed before and after cycling (330 cycles i.e. 6.8 days).

CVs of the posolyte solution containing **PPO** (Figure 6), recorded over an extended potential range, show the response of the nitroxide / N-oxoammonium redox couple at 1.16 V. Before battery cycling, during the negative scan, the reduction of the nitroxide moiety into hydroxylamine can be observed at -0.54 V. It can be noted also that a small redox system is also observed at around 0.7 V. This peak disappears quickly after a few cycles, likely due to the presence of residual iodine impurities, originating from non-fully quantitative anion exchange metathesis. After cycling experiments, two comments are clear from the CV results. First, the current of the nitroxide / oxoammonium system has decreased, with an intensity loss (22.4%) matching closely the battery's capacity loss (21.9%). Second, oxidized **MV** is detected in the posolyte tank, indicating that **MV** passes through the membrane, confirming the non-negligible crossover effect in a practical cell.

<figure><img src="image_10.png" /><figcaption>**Figure 6:** Cyclic voltammograms of PPO posolyte before (line) and after RFB 330 cycles (dash line).</figcaption></figure>

Figure S25 shows that, unlike **MV**, **PPO** nitroxide does not cross the membrane from the posolyte to the negolyte compartments. For **4-TMA-Tempo**, a similar trend, for both electrolytes, is noted from CV, before and after cycling (Figure S10). However, the decrease in the current intensity corresponding to the nitroxide/oxoammonium couple is less pronounced (6.5%) for **4-TMA-Tempo**, which is in good agreement with the battery's capacity fade (6.9%).

In the recent years, it has been observed that the degradation of the Tempo-based posolyte stems mainly from the decay of the oxoammonium species (charged state) rather than the nitroxide species (discharged state). Numerous electrochemical studies have hypothesized possible chemical degradation mechanisms of the N-oxoammonium species, mostly by a Hofmann-like elimination or / and a ring-opening reaction. [16] [17] Recently, Barbon et al. shown that the N-oxoammonium cation at pH 9 react quickly with hydroxide ions to generate back the corresponding nitroxide alongside H₂O and O₂ in a 4 / 2 / 1 ratio, respectively. [42] This process can be seen as less detrimental than chemical alteration such as ring opening. The posolyte can thus be recovered in the discharged state and the reaction corresponds to the self-discharge of the battery.

Post-mortem UV-visible and ¹H NMR investigation of the **PPO** electrolyte solution revealed degradation products (Figure 7b). The appearance of a new chromophore at 324 nm and the decrease in the intensity of the two main bands at 220 and 268 nm indicate the transformation of the N-oxoammonium (Figure 7a). Moreover, new peaks at 1.58, 1.83, 2.37, 7.99, and 8.81 ppm were observed by ¹H NMR (Figure 7b).
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 7:** a) UV-visible spectrum and b) ¹H NMR spectra of PPO electrolytes (before and after cycling. Ascorbic acid has been added to the solution for the NMR experiment. Stars indicate the additional peaks observed after cycling.</figcaption></figure>

To better understand **PPO** and **4-TMA-Tempo** electrolytes degradation and the possible influence of cycling conditions and/or the presence of methylvilogene therein, two electrolytes prepared from the same solutions were charged to 100% SOC in two different batteries using methyl viologen as the negolyte. These 100% SOC electrolytes were then isolated from their respective cells and stored under the same conditions as the previously discussed batteries, i.e., an inert atmosphere, same temperature, same humidity level, and a strictly identical storage time.

Analyses of the 100% SOC electrolytes after 7 days of storage showed a current intensity decrease of 19% and 13% for **PPO** and **4-TMA-Tempo**, respectively, as measured by cyclic voltammetry (Figure S11). These experiments report a loss of electrochemically active specie concentrations, clearly indicating that the degradation of nitroxides is independent of battery cycling. It is confirmed by ¹H NMR analyses of aged posolyte showing peaks at 1.58, 1.83, 2.37, 7.99, and 8.81 ppm for **PPO**, with signal intensities nearly the same as for the cycled electrolyte (Figure 7).

**Conclusion**

An efficient and general method for the catalytic preparation of substituted isoindoline and 2,3-dihydro-1H-pyrrolo[3,4-c]pyridine based nitroxides is described with high functional compatibility and using diverse unactivated triple bonds and α,ω-diynes. The scope of the reaction is broader than methods usually reported to access this family of nitroxides.

The electrochemical properties of two nitroxide posolytes, a cationic (**PPO**) and an anionic (**TC-TMIO**), were evaluated, revealing an enhanced oxidation potential (+220 mV) for **PPO** compared to the benchmark **4-TMA-Tempo**.

AORFB has been constructed using **PPO** as posolyte and **MV** as negolyte at neutral pH and displaying a significantly higher cell voltage of 1.56 V regarding other reported nitroxide posolytes, and representing a 16% improvement regarding the **MV/4-TMA-Tempo** system. The higher solubility of **PPO** offers further improvements in energy density, potentially reaching 57 Wh L⁻¹ due to a theoretical capacity of 80 Ah L⁻¹. Cycling results for the **MV/PPO** battery showed a capacity loss of 3.22%, compared to 1.01% for the **MV/4-TMA-Tempo** system. Post-mortem studies of cycled electrolytes and electrolytes stored outside the battery at 100% SOC revealed that the degradation of **PPO** is primarily due to the oxidized form of the molecules. Despite the less-than-optimal stability of **PPO**, our results constitute a crucial step in the development of new redox-active molecules that are easily tailored for high energy and power densities. The proposed synthetic method and the interesting structural and redox properties of **PPO** opens new perspectives and will enable further progress in the energy storage field.
Acknowledgements

This work was supported by a French government grant from the Agence Nationale de la Recherche (ANR) under the France 2030 program, reference ANR-23-PEBA-0005. ANR is also gratefully acknowledged for funding through the Labex STORE-EX (Grant ANR-10-LABX-76-01)

**Keywords:** Aqueous Organic Redox Flow Battery • nitroxide • posolyte • isoindoline • [2+2+2] cyclotrimerization

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A convenient metal-catalyzed [2+2+2] cycloaddition enables the synthesis of isoindoline-based nitroxides as new posolytes for aqueous organic redox flow batteries. The cationic PPO exhibits a 220 mV higher oxidation potential than 4-TMA-TEMPO, 3 M solubility in 1 M NaCl and cell voltage of 1.56 V. Flow battery tests reveal PPO's promising performance for sustainable energy storage.

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