# Phenothiazylpropylsulfonate: A High-Potential Posolyte for Redox-Flow Batteries: Study of the Instability in the Charged State

Roman Ishchenko, Matthieu Becuwe, Lionel Dubois, Serge Gambarelli, Christine Cézard, Emmanuel Baudrin

► To cite this version:

Roman Ishchenko, Matthieu Becuwe, Lionel Dubois, Serge Gambarelli, Christine Cézard, et al., Phenothiazylpropylsulfonate: A High-Potential Posolyte for Redox-Flow Batteries: Study of the Instability in the Charged State. ChemistryEurope, 2025, 3 (5), 10.1002/ceur.202500226. [https://doi.org/10.5554/ceur.202500226](https://doi.org/10.5554/ceur.202500226)

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# Phenothiazylpropylsulfonate: A High-Potential Posolyte for Redox-Flow Batteries: Study of the Instability in the Charged State

Roman Ishchenko, Matthieu Becuwe, Lionel Dubois, Serge Gambarelli, Christine Cézard, and Emmanuel Baudrin*

The development of aqueous organic redox-flow batteries (AORFB) requires finding new posolyte alternatives to the ferro-cyanide salts used presently. Among the potential families, phenothiazine has been reported to be of interest if its solubility in aqueous medium is successfully increased. In this article, the phenothiazine propyl sulfonate (PTZPS) is evaluated as a promising high-potential posolyte for neutral-medium aqueous redox-flow battery. It is demonstrated that electrochemical reversibility is highly dependent on pH, with namely a fast capacity loss in

neutral medium. Through the preparation of the oxidized form of this molecule, a kinetic study is performed, confirming the crucial role of the electron transfer step between two molecules at this redox state. Even if density functional theory (DFT) calculations of the electron transfer are not successful due to the significant multireference character of the oxidized form's dyad, using MD simulations, the behavior of the oxidized form in various media is qualitatively predicted, including the effect of addition of chaotropic additives.

## 1. Introduction

As renewable and sustainable energy sources are gaining more and more popularity, we are facing their intrinsic trait, namely the

intermittent nature of electricity generation that complicates their integration into the existing power grid. The generation/ consumption fluctuation is generally a problem for the grid that necessitates a solution to mitigate it. Batteries, and more specifically redox-flow batteries (RFBs), are one of the solutions proposed for this goal due to their adaptability to the power and capacity requirements and good durability in the conditions of regular use.

All-vanadium RFBs were the first to be commercialized. They rely on the rich redox chemistry of vanadium and acceptable solubility of its species offering 20–30 Wh L⁻¹ of capacity using typically a 1.7 M vanadium concentration in sulfuric acid-based electrolytes.[¹,²] Their other advantage is the possibility of rebalancing after the crossover related capacity fading as vanadium is conserved in the system.[³] At the same time, vanadium has limited abundance and subsequently high fluctuating cost that leads to the active search for alternatives.[¹,²]

Organic chemistry can offer a viable alternative to the inorganic materials. This is due to their much higher diversity of the chemical space compared to the one of inorganic compounds. Such diversity allows designing highly tailored molecules in order to maximize the RFB performance. Two classes of organic RFB are being developed: using either aqueous or non-aqueous solvents. We will focus on the former, AORFB (aqueous organic redox-flow batteries), as they offer a range of advantages such as 1) cheap solvent (water) and supporting electrolytes, 2) higher intrinsic conductivity of the electrolyte and 3) lower environmental and fire hazard. However, it comes with pendent limitations such as 1) a narrower potential window available and 2) need of careful structure optimization to ensure high solubility and stability.

Multiple organic molecule families were explored for the purpose of RFB,[⁴] but only a few of them are applicable to AORFB[⁵,⁶]

R. Ishchenko, M. Becuwe, E. Baudrin
Laboratoire de Réactivité et de Chimie des Solides
UMR CNRS 7314
Université de Picardie Jules Verne
Amiens Cedex 1 F-80039, France
E-mail: emmanuel.baudrin@u-picardie.fr

R. Ishchenko, M. Becuwe, E. Baudrin
Réseau sur le Stockage Electrochimique de l'Energie (RS2E)
FR CNRS
3459, France

R. Ishchenko, M. Becuwe, L. Dubois, S. Gambarelli, E. Baudrin
GDR Redox-Flow CNRS
2070, France

L. Dubois, S. Gambarelli
Université Grenoble Alpes

CEA

CNRS

IRIG

SyMMES
Grenoble 38000, France

C. Cézard
Laboratoire de Glycochimie et des Agroressources d'Amiens (LG2A)
UR 7378
Université de Picardie Jules Verne
10 rue Baudelocque, Amiens Cedex 80089, France

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Supporting information for this article is available on the WWW under https://doi.org/10.1002/ceur.202500226

<figure><img src="image_4.png" /></figure>

© 2025 The Author(s). ChemistryEurope published by Chemistry Europe and Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
<figure><img src="image_5.png" /></figure>

due to the limited electrochemical stability of water and its higher reactivity toward the compounds compared to organic solvents. Among the chemistries mostly used for the negolyte are the viologens,[6-9] quinones,[6] and phenazines.[6,10-12] For the posolyte TEMPO-based compounds,[13-15] derivatives of ferrocene[16-18] are being studied along with various azines such as phenothiazine[19-23] and phenoxazine.[24-26] Another viable option consists of repurposing electroactive molecules with appropriate properties from other energy-related application, such as triflilimides[27] or sulfonamides,[28] at the same time, this will require thorough research and structure optimization. However, presently the only long-term viable posolyte in commercial AORFB is still the [Fe(CN)₆]³⁴⁻ redox couple with its limitations in terms of solubility and potential. There is therefore a need to find new alternatives.

Phenothiazines offer rather high redox potentials around 0.7 V versus SHE and fast electron transfer kinetics. However, the unmodified phenothiazine is almost insoluble in water, with thus the need to design specifically highly soluble derivatives. First reports used methylene blue (MB), as it is a cheap phenothiazine derivative possessing a solubility of 0.13 mol L⁻¹ at 25 °C in pure water.[29] It was shown to be highly stable in a 0.5 M H₂SO₄ supporting electrolyte over multiple charge/discharge cycles.[30] The behavior of MB in redox-flow batteries was further studied in detail by Zhang et al.[19] Their main findings include an optimized electrolyte formulation allowing the use of MB at a 1.5 mol L⁻¹ concentration in a mixture of acetic acid and sulfuric acid in water and a crucial role of protonated forms of MB in redox processes, therefore limiting the use of MB to acidic posolytes only. A work to develop similar posolyte materials was carried out resulting in the engineering of an MB analog stable for 18,000 cycles in sulfuric acid and offering high capacity retention and energy density of 47 Ah L⁻¹.[20] It was also outlined that 3,7-substituted phenothiazines are more stable in charged state due to the delocalization effects that prevent attacks at the sites with the lowest/highest atomic charge. The oxidation of the MB-like compounds involves two electrons, generating a closed-shell species as the product. Those products have greater stability when compared to the open-shell (i.e., radical) products of one-electron oxidation.

An alternative approach in the design of novel phenothiazine-based posolytes consists of modifying the phenothiazine by the substitution at nitrogen with a hydrophilic fragment to ensure high solubility. Recently, Chen and coauthors reported the use of phenothiaziyl-propylsulfonate (PTZPS) in RFB.[23] They have shown that PTZPS in mixed H₂SO₄ and acetic acid demonstrates a high solubility (1.95 mol L⁻¹) and rather high cycling stability (62% over 1000 cycles). They however underlined that the compound in the charged/oxidized state suffers from a reaction with water and from crossover problems, which lead to capacity fading.

Herein we present a further detailed study of the behavior of PTZPS and its charged state in aqueous solutions in order to get more insights into the design of phenothiazine-based posolytes for the development of pH-neutral redox-flow batteries.

# 2. Results and Discussion

## 2.1. Synthesis

The first step was the synthesis of the soluble phenothiazine-based molecule through a modification at the nitrogen atom. The PTZPS (PhenoThiaZylPropylSulfonate) was prepared from the phenothiazine (PTZ) under the subsequent action of NaH and 1,3-propanesultone (Scheme 1). The purity of the product was checked using NMR spectrometry (Figure 4b and Supporting Information). The two-step synthesis process led to a yield of 60%.

Then, we determined the solubility which is an important parameter as the capacity of an electrolyte is directly related to the ability to solubilize the electroactive species. A sample of about 500 mg of PTZPS was stirred in 1 mL of water at 25 °C for 24 h, then an aliquot was diluted 10,000 times, and the concentration was calculated using a previously made calibration curve. The product presents rather high solubility of ca. 1 mol L⁻¹ in water due to the presence of highly hydrophilic sulfonate group. While value is lower than the value of 1.95 M obtained by Chen et al. in mixed acid electrolyte (2.7 M H₂SO₄ and 1.8 M acetic acid) and lower than 2.3 M in pure water reported for quaternary ammonium compound reported in the same work,[23] it is still a promising solubility value for the development of posolytes in neutral conditions. Furthermore, it also appears as a potential candidate for redox targeting RFB, where an insoluble electroactive material is inserted into the electrolyte reservoir and acts as main source of capacity while the soluble material acts solely as electron transfer shuttle.

## 2.2. Electrochemical Characterization

For the basic electrochemical characterization, we have opted for cyclic voltammetry (CV) in neutral medium (0.1 M NaCl). PTZPS shows a clear quasi-reversible redox couple with a formal redox potential of 0.69 V versus SHE (Figure 1a). The redox process is limited by diffusion as underlined by Randles-Sevcik plot. This plot allowed the determination of the diffusion coefficients, which are equal to 6.8 × 10⁻⁷ cm² s⁻¹ for both the oxidized and reduced species (Figure 1b).

As expected, the potential of the redox process is virtually independent of pH as seen from Pourbaix diagram (Figure 1d). The only exception here is 3 M H₂SO₄ where redox potential is ca. 50 mV lower. This can be justified by a protonation at nitrogen atom in acidic medium, which is consistent with previous research.[31] At the same time, pH has a major impact on the

<figure><img src="image_6.png" /><figcaption>Scheme 1. Synthesis of PTZPS.</figcaption></figure>
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 1.** a) CV of PTZPS at different scan rates; b) Randles-Sevcik plot; c) CV of PTZPS at different pH; d) oxidation and reduction peaks ratio and Pourbaix diagram (1 mM in buffer solution, 250 mV/s); e) CV of PTZPS with extended potential range (1 mM in 0.1 M NaCl (solid lines) or in 3 M H₂SO₄ (dashed line); 500 mV s⁻¹).</figcaption></figure>

reversibility as exemplified by the evolution of the values of the ratio I<sub>ox</sub>/I<sub>red</sub> (Figure 1d) in agreement with the instability of the oxidized form, as we will detail below. As we can see, the more the pH increases, the more irreversible the redox process becomes.

From the literature, it is generally assumed that phenothiazine compound can react electrochemically through two successive one-electron electrochemical processes. Thus, the potential range was extended toward 1.3 V versus SHE (Figure 1e). A second oxidation process can be identified around 1.1 V versus SHE. However, it is completely irreversible in the studied medium and the intensity highly dependent on the pH of

the electrolyte. As this process is irrelevant for the AORFB application, the following study will be restricted to only the first redox couple at 0.69V vs SHE.

## 2.3. RFB Cycling

Based on this electrochemical activity, the behavior PTZPS in a full RFB in neutral medium was evaluated. For that, an electrochemical test in galvanostatic cycling mode of 0.1 M PTZPS in 0.5 M NaCl using sodium salt of anthraquinone-2,7-disulfonic acid (AQDS) as a nonlimiting negolyte was performed. During the first
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 3.** NMR spectra of the a) posolyte after cycling, b) sulfoxide synthesized separately, and c) pristine posolyte material (DMSO-d₆).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Scheme 2.** Synthesis of phenothiazylpropylsulfonate sulfoxide.</figcaption></figure>

by the bulk electrolysis are identical, as well as to observe the evolution of EPR spectrum during the radical cation degradation (Figure 5b). The evolution of the EPR signal of the radical cation decreases over time after initial electrochemical generation and becomes almost absent after 10 h. The double integral of EPR signal follows a second-order kinetics confirming our assumptions about a bimolecular rate-limiting step of the degradation (Figure 5b).

The same experiments being repeated with more concentrated solutions of PTZPS (50 mM) show a complete loss of the hyperfine structure of the EPR spectrum presumably attributed to a molecular association phenomenon in solution (Figure 5c).

Using the same electrochemical cell coupled to the EPR spectrometer, we followed the evolution of the EPR signal during cyclic voltammetry measurements. The radical cation spectrum

intensity evolves clearly with the imposed potential of the CV experiments—namely the maxima of the EPR signal intensity are well aligned with the oxidation current maxima (Figure 5d). The variations of the intensity between CV cycles can be attributed to the incomplete reduction of the oxidized species due to its diffusion away from the working electrode, as the cell does not allow for a rapid enough scan rate. A progressive decrease in the maximal EPR intensity is also related to the irreversible formation of the sulfoxide that limits the quantity of the available PTZPS.

### 2.4.2. Kinetics Study of the Radical Cation Decomposition

To explore a wider timeframe and to be able to quantify the species concentrations, a kinetic study by means of UV-Vis
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>**Figure 4.** a) potential-capacity curves for the first and last cycle; b) capacity retention graph for PTZPS in 3 M $H_2SO_4$ (25 mL of 0.1 M solution vs. 25 mL of 0.06 M AQDS, 3 M $H_2SO_4$ as supporting electrolyte, 10 mA cm$^{-2}$).</figcaption></figure>

spectroscopy was conducted. To do so, a stock solution of the radical cation was prepared by bulk electrolysis of 0.1 M solution of PTZPS in 60% $HClO_4$. Then, a small aliquot of this stock solution was quickly diluted 1000 times with the appropriate buffer solution and the absorption was recorded at the absorption maximum of the radical cation (514 nm) allowing the determination of its concentration. We tested different reaction orders and it

appears that the best fit was obtained by plotting 1/C over time (Figure 6a) consistent with a second-order degradation mechanism. This confirms our hypothesis about the degradation mechanism. From this, we determined the kinetic constant ($k_{exp}$) and extended the study in other buffers. It is clear that the measured kinetic constant is pH-dependent with order 0.5 in OH- therefore suggesting a more complex mechanism (Figure 6b).

<figure><img src="image_16.png" /><figcaption>**Scheme 3.** Proposed two-step degradation mechanism of N-substituted phenothiazine radical cations ($R = (CH_2)_3SO_3Na$ for PTZPS).</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Scheme 4.** Electrochemical synthesis of PTZPS radical cation from a solution of 0.1 M PTZPS in aqueous acidic solution.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>**Figure 5.** a) Experimental (1 mM in 50% H2SO4), simulated (inset—HFC constants used for simulation), and calculated (PBE0/def2-TZVPD/wB97-D4/def2-SVP) EPR spectra of PTZPS radical cation; b) inverse double integral of EPR spectrum of PTZPS radical cation generated electrochemically time evolution fitted with linear function (50 mM in 0.5 M NaCl, 90 s at 500 mV versus reference electrode); c) EPR spectrum of electrochemically generated PTZPS radical cation in concentrated solution (50 mM in 0.5 M NaCl); d) time evolution of CV current and EPR spectrum double integral of PTZPS (between 0 and 0.7 V at 1 mV s⁻¹, 50 mM in 0.5M NaCl).</figcaption></figure>
<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /><figcaption>**Figure 6.** a) Typical linearized kinetic curve of PTZPS radical cation degradation in buffer solution (phosphate buffer pH 3.15, 514nm; every 500th experimental point is plotted); b) logarithm of measured rate constant plotted versus pH; c) UV-Vis spectra of PTZPS, PTZPS radical cation, and PTZPS sulfoxide; d) typical UV-Vis spectrum of the products mixture and its fit.</figcaption></figure>

Upon analysis of the composition of the solution after the reaction, it was found that both pristine PTZPS and the sulfoxide are present. The quantification was realized by fitting the spectra with the sum of the individual components' spectra (Figure 6c and d). For PTZPS radical cation, a deviation from the expected 1:1 ratio between the nonoxidized molecule and the sulfoxide in favor of starting PTZPS was observed regardless of the pH (Table 1). Note that the radical cation completely disappears after 24 h for pH above 6 with only residual amounts in mildly acidic conditions.

Thus, in coherence between EPR and kinetic data, the degradation of the radical cation follows a second-order kinetics. This suggests a bimolecular rate-limiting step corresponding to the electron transfer between two molecules of PTZPS radical cation. The ratio of the degradation products is not consistent with the mechanism proposed previously for the decomposition of phenothiazine radical cations. This fact combined with pH dependent rate constant may suggest a more complex reaction mechanism.

In order to gain more insights into the observed behavior of PTZPS radical cation, theoretical calculations were carried out.

<table><caption>**Table 1.** Measured rate constants and reaction mixture composition of PTZPS degradation at different pH.</caption><thead><tr><th>pH</th><th>Rate constant<br>[s<sup>-1</sup>M<sup>-1</sup>]</th><th>PTZPS [%]<sup>a)</sup></th><th>PTZPS-SO<br>[%]<sup>a)</sup></th><th>Radical cation<br>[%]<sup>a)</sup></th></tr></thead><tbody><tr><td>3.15</td><td>0.82</td><td>73</td><td>26</td><td>0.8</td></tr><tr><td>4.50</td><td>2.7</td><td>73</td><td>26</td><td>0.5</td></tr><tr><td>5.98</td><td>4.2</td><td>65</td><td>35</td><td>0</td></tr><tr><td>7.15</td><td>846</td><td>75</td><td>25</td><td>0</td></tr><tr><td>8.48</td><td>3043</td><td>65</td><td>35</td><td>0</td></tr><tr><td>9.10</td><td>1554</td><td>57</td><td>43</td><td>0</td></tr></tbody><tfoot><tr><td colspan="5"><sup>a)</sup>Quantified after 24h by UV-Vis spectrum fitting.</td></tr></tfoot></table>

## 2.5. Computational Study

### 2.5.1. DFT Calculations

DFT calculations were performed using ORCA 6.0.1[37] package with default approximations (RIJCCOSX[38,39]), integration grids, convergence criteria, and basis set definitions. Def2-SVP and
<figure><img src="image_24.png" /></figure>

def2-TZVPD were used as basis set[40,41] for geometry optimization and EPR parameters calculations, respectively, in conjunction with wB97-D4[42–44] and PBE0[45,46] density functional. SMD continuous solvation model[47–49] was used throughout all the calculations to account for the aqueous medium. Vibrational frequencies were calculated after the geometry optimization to ensure the true minimum was reached. Molecular graphics was created with UCSF ChimeraX.[50,51]

First, dyad geometry was constructed using GOAT algorithm implemented in ORCA at xTB2[52] theory level so as not to bring any subjective bias about that geometry. This dyad is effectively a biradical; therefore, it is highly likely to have multireference character. To check for the significance of the multireference character of the dyad fractional occupation number weighted electron density (FOD) calculation was performed. The isosurface of such electron density (Figure 7) shows significant delocalization, which is a sign of multireference character. Due to the high intricacy of the multireference calculations, it was decided not to proceed further.

<figure><img src="image_25.png" /><figcaption>**Figure 7.** FOD of PTZPS radical cation dyad (TPSSh/def2-TZVP// wB97-D4/def2-SVP).</figcaption></figure>

## 2.5.2. Molecular Dynamics Simulations

As the systems of multiple PTZPS molecules have significant multireference character, a pivot to MD simulations instead of DFT was made. Lacking any quantum mechanical nature MD should be more robust while simultaneously allowing the inclusion of explicit solvent at appropriate concentration and supporting salt ions to better recreate the experimental conditions. At the same time, it might not be able to reproduce all the effects governing the interaction of two PTZPS molecules, reducing them to electrostatic interactions and standard values of Lennard-Jones interaction parameters defined in force field.

The PTZPS radical cation was parametrized using GAFF2 force field via Antechamber[53] software both in its protonated and deprotonated forms. RESP charges[54] were calculated using Multiwfn[55,56] from the wave function from ORCA (HF/6-31G*). Then, the system containing 50 molecules of PTZPS, 27,778 molecules of water and appropriate quantities of supporting salt's cation and anion in order to ensure electro neutrality was assembled using Packmol.[57] The resulting system was subjected to energy minimization, NVT heating to 300K over 100 ps, NPT equilibration at 1 bar over 100 ps, and the NPT production run over 50 ns using GPU-accelerated pmemd engine[58,59] in Amber22.[60]

According to the mechanism proposed earlier, the crucial step is the electron transfer between two radical cations of PTZPS. Electron transfer rate decreases exponentially with the intermolecular distance, thus requiring two molecules to come into close contact. To evaluate such contacts, the radial distribution function (RDF) between the center of mass of the PTZPS molecules over the MD trajectory was calculated using CPPTRAJ.[61] It is clearly seen that for the case of deprotonated PTZPS radical cation, a sharp peak is present at the distance of 6 Å which is not the case for protonated molecule in the medium of sulfuric acid (Figure 8a). It can be justified by the overall positive charge of the protonated form, which results in electrostatic repulsion.

As one of the strategies to improve capacity retention, the use of chaotropic agents was considered. Such compounds can disrupt the network of the hydrogen bonds in water leading to

<figure><img src="image_26.png" /><figcaption>**Figure 8.** Radial distribution function (0–25 Å, 0.5 Å bin size, between centers of mass) of PTZPS molecules a) in various media and b) with various additives (50 ns NPT, PSA—deprotonated form, PSH—protonated form).</figcaption></figure>
<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /><figcaption>**Figure 9.** Capacity retention with of a PTZPS with phenol or urea as addi- tive compared to the case without any additive (25 mL of 0.1 M PTZPS solution versus 25 mL of 0.06 M AQDS, 0.5M NaCl as supporting electrolyte, 10 mA cm⁻²).</figcaption></figure>

higher solubility ("salt-in effect") and it can be supposed that this
property can be beneficial, as it would hinder close contacts
between molecules.

This assumption was tested in MD simulation using well-
known chaotropic salts such as LiCl and MgCl₂ as well as nonionic
chaotropic additives such as urea and phenol. The simulations
revealed a slight decrease of the peak at 6 Å after the substitution
of NaCl with chaotropic salts, which aligns with their expected
properties but at the same time not enough to improve capacity
retention significantly. With the addition of urea, this peak is as
well reduced only slightly. However, with phenol, an almost com-
plete absence of the RDF peak at 6 Å was observed similarly to
the protonated molecule in 9 M H₂SO₄ where the charged state is
completely stable, which signifies a significant reduction in close
contacts between deprotonated radical cation molecules and
potentially better capacity retention (Figure 8a).

This result was also checked in RFB cell measurements using either urea or phenol as additives. The RFB with phenol added showed a much faster capacity decay probably due to the reaction of phenol with the charged state that cannot be predicted from MD simulation.

Addition of urea showed visually much faster dissolution of
the solid PTZPS and improved the capacity retention (1.8% of
capacity loss per cycle with urea versus 3% for NaCl solution with-
out additives) (Figure 9).

**3. Conclusions**

The origin of phenothiazine instability in aqueous redox-flow bat-
teries originates from the disproportionation reaction of its radi-
cal cation as highlighted in this work. This disproportionation
follows a second-order kinetics, thus affecting the performance
especially when using high concentration as required for its appli-
cation toward high energy density RFB. Furthermore, even if the
kinetic of this phenomenon for the propylsulfonate-substituted
phenothiazine is slower in acidic medium, it is difficult to

completely suppress it in mildly acidic electrolytes. As shown
in the literature, high concentration strong acids or mixtures
of acids are needed to stabilize such systems.

Theoretical calculations were performed. However, extended research by the means of quantum chemistry needed for a better comprehension of the phenothiazine radical cation behavior in solution is severely complicated by the multireference nature of the dyad, thus requiring use of high-end computational methods. We managed to use simple MD simulation for this purpose to show qualitatively correct results that are consistent with the RFB cycling behavior. While being useful, this method can easily fail, as it has no ability to predict chemical reactions between electrolyte components. Nevertheless, it brought some hints for instance that using chaotropic agents could help to reduce the kinetic of the degradation process.

Further efforts are still needed to develop phenothiazine-
based posolytes for neutral medium and should be directed to
the reduction of intermolecular interaction especially in the
charged state either by the means of molecular design (e.g., add-
ing bulky groups) or by electrolyte formulation engineering. We
are presently following such an approach which will be reported
in a forthcoming article.

**Acknowledgements**

This work has benefited from a government grant managed by
the Agence Nationale de la Recherche (ANR) under the France
2030 program, within the DISCOVERY project ("ANR-23-
PEBA-0001"). It is available in full text on HAL.science (CCBY
License, creative commons). Financial support from the IR
INFRANALYTICS FR2054 for conducting the research is gratefully
acknowledged. This work was granted access to HPC resources of
"Plateforme MatriCS" within University of Picardie Jules Verne.
"Plateforme MatriCS" is cofinanced by the European Union with
the European Regional Development Fund (FEDER) and the
Hauts-De-France Regional Council among others.

**Conflict of Interest**

The authors declare no conflict of interest.

**Data Availability Statement**

The data that support the findings of this study are available in
the supplementary material of this article.

Keywords: density functional theory · molecular dynamics ·
phenothiazine · redox-flow batteries

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Manuscript received: July 4, 2025

Revised manuscript received: July 29, 2025

Version of record online: August 16, 2025