# Is the amount of water the most important parameter in concentrated aqueous electrolytes? The case of aqueous magnesium cells

Malaurie Paillot, Sophie Le Caër, Magali Gauthier

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Malaurie Paillot, Sophie Le Caër, Magali Gauthier. Is the amount of water the most important parameter in concentrated aqueous electrolytes? The case of aqueous magnesium cells. ACS Electro-chemistry, 2025, 10.1021/acselectrochem.5c00082 . hal-05148551

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Distributed under a Creative Commons Attribution 4.0 International License
Is the Amount of Water the Most Important
Parameter in Concentrated Aqueous Electrolytes?
The Case of Aqueous Magnesium Cells

Malaurie Paillot, [a]. [b]. § Sophie Le Caër, */[b] Magali Gauthier *[a]

[a] Université Paris-Saclay, CEA, CNRS, NIMBE, LEEL, 91191 Gif-sur-Yvette Cedex,
France

[b] Université Paris-Saclay, CEA, CNRS, NIMBE, LIONS, 91191 Gif-sur-Yvette Cedex,
France

§ Present address: Laboratoire de Chimie et d'Électrochimie du Solide (LCES), Université de Montréal, Montreal, H2V 0B3, Quebec, Canada

**Corresponding authors**

magali.gauthier@cea.fr and sophie.le-caer@cea.fr
**Abstract**

Aqueous magnesium batteries, based on an abundant ion, are a promising alternative to Li-ion batteries. However, their reactivity in electrochemical full cells remains largely unexplored.

Herein, we focused on the effect of hydrogen evolution reaction in magnesium full cells based on aqueous electrolytes. We screened both the electrolyte structuration and the gas production in aqueous magnesium batteries as a function of the nature of the electrolyte: imide-, acetate-perchlorate-, bi-salt and polymer-based electrolytes. We have particularly focused on H₂ production for assessing solution stability. At low molalities, H₂ production correlates with the kosmotropic-chaotropic properties of anions, consistent with the solution structure and elevated free-water content revealed by infrared spectroscopy. Conversely, at elevated molalities, H₂ production follows the same trend across all electrolytes -even though infrared spectroscopy reveals distinct differences in their structuration. In this regime, H₂ evolution decreases linearly with increasing salt concentration. Thus, while anion identity exerts a major influence on hydrogen-bond network organization, the system's reactivity (as gauged by H₂ production) is predominantly controlled by the available water content.

**KEYWORDS**

aqueous batteries; concentrated electrolytes; hydrogen evolution reaction; gas analysis; infrared spectroscopy
Introduction

One of the most important components of Li-ion batteries is the electrolyte, which consists of a salt dissolved in non-aqueous organic solvents. Although the electrolyte has a wide electrochemical stability window (ESW), the large amount of solvent in solution is a real obstacle because the solvents used are toxic, harmful to the environment and highly flammable.¹ Therefore, one solution is to replace these solvents with water.² However, the use of water is limited by its thermodynamic stability window, which is 1.23 V. To overcome this limit, Suo et al.³ demonstrated that ESW of aqueous lithium batteries could reach values of 3 V by utilizing highly concentrated aqueous electrolytes referred to as water-in-salt electrolytes (WISE). The interest of this approach lies in the modification of the water environment, with the decrease of free water molecules³ and the formation of a protective interphase⁴,⁵ on the electrode, which limit the oxygen evolution reaction (OER), as well as the water reduction reaction (hydrogen evolution reaction, HER). The latter represents one of the greatest challenges for aqueous batteries and has been the subject of extensive investigation and debate,⁶⁻⁹ especially in the case of Zn aqueous batteries. The production of molecular hydrogen from the water reduction at the negative electrode interface irreversibly consumes water from the electrolyte as well as electrons, resulting in irreversible capacity and coulombic efficiency drop, as well as capacity fade.⁸,¹⁰

The sustainability of lithium resources is also a major challenge, particularly in the case of WISEs which require a large amount of salt.¹¹ Switching to divalent ions such as magnesium may thus enable the manufacture of more cost-effective and environmentally-friendly concentrated electrolytes. However, studies on magnesium-based WISE are scarce.¹²⁻¹⁴ The first study reports a full cell based on a Mg(TFSI)₂ electrolyte (magnesium bis(trifluoromethanesulfonyl)imide: Mg((CF₃SO₂)₂N)₂) of molality 4 m (molality: m, moles per kg), which had a stability window of 2 V.¹² The stability window of the Mg(TFSI)₂
electrolyte is lower than aqueous lithium solutions which could be explained by the restricted solubility of magnesium salts in water. To overcome this limitation, various strategies exist to minimize water reactivity. For example, the use of an electrolyte based on molecular crowding such as PEG (polyethylene glycol)-Mg(TFSI)₂-H₂O¹³ or PVA (polyvinyl alcohol)-Mg(NO₃)₂-H₂O¹⁴ extends the stability windows on stainless steel of these electrolytes to 2.3 V and 2.2 V respectively. The molecular crowding agent modifies the hydrogen bond network of water, thus limiting its reactivity. Furthermore, the presence of a crowding agent reduces the amount of salt required in concentrated aqueous electrolytes which considerably reduces the cost of solutions.¹⁵ Other strategies implemented for mono and multivalent technologies include the addition of a co-salt (water-in-bisalt (WIBS) electrolytes)¹¹,¹⁶,¹⁷ to occupy the water molecules, organic solvent (hybrid electrolyte)¹⁸ or ionic liquids.¹⁹

The aim of these strategies is to strongly affect the organization of water molecules in solution. The properties of the anions have a strong influence on the H-bond network.¹⁹⁻²¹ Different classifications of anions exist the literature based on hydrophobic/hydrophobic properties²² or the Hofmeister series.²³,²⁴ This latter classification, which sorts anions according to their kosmotropic or chaotropic character, is the most commonly used and will be taken into account in this study. Kosmotropic anions are small anions with a high surface charge density, favoring the H-bond network of water. As such, they are referred to as “structure making” anions. Conversely, chaotropic anions are generally large anions with a low surface charge density, which limits their hydration and breaks the water hydrogen bond network (“structure breaker”) (Figure 1).²³,²⁴ Chaotropic anions are therefore the most effective at disrupting water molecules in solution.

The kosmotropic or chaotropic character of the anion plays thus a critical role in the electrolyte's structuration, which in turn affects the HER kinetics by changing the reactivity of the water molecules. Yet, although these strategies have a significant impact on the
structuring of water molecules in solution,²² their importance for the long-term reactivity of
aqueous batteries, in particular for limiting in the long-term the molecular hydrogen evolution
reaction (HER) on the cathodic side, has yet to be fully demonstrated.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Diagram of the Hofmeister series classifying kosmotropic and chaotropic anions. Inspired from ref.²³,²⁴ Chemical structures of magnesium acetate, perchlorate and TFSI⁻ salts are shown.</figcaption></figure>

In-depth study of these electrolytes using spectroscopic techniques²⁵⁻²⁸ and electrochemical analysis⁶,²⁹ helps to understand how the molecules are organized in solution, and gives a first idea of the electrolyte's properties. However, these studies do not provide a clear picture of how the electrolyte behaves in a full cell. In fact, cells based on aqueous electrolytes produce gases and suffer from capacity loss due to degradation reactions.³⁰,³¹ Droguet *et al*.³⁰ demonstrated that HER is the main degradation reaction in aqueous lithium full cells based on WISEs, with a large amount of dihydrogen produced during cycling despite the high salt concentration. To better understand what governs reactivity in magnesium cells and particularly the role of the electrolyte's nature, we attempted to correlate both the electrolyte structuration and the gas production by investigating various magnesium electrolytes containing varying amounts of anions with different kosmotropic or chaotropic character. To assess the influence of the nature of the different electrolytes considered, we performed
infrared spectroscopy experiments as well as the physico-chemical properties (conductivity and viscosity). We finally compared these findings with the H₂ production (HER) used here as a critical parameter to describe the reactivity.

# Experimental section

## Electrolyte and electrode preparation

Magnesium bis(trifluoromethanesulfonyl)imide (Mg((CF₃SO₂)₂N)₂.99.5%, Solvionic), magnesium perchlorate (Mg(ClO₄)₂, 99%, Thermo Fischer) and sodium perchlorate (NaClO₄, 98%, Sigma Aldrich) were used as received and stored in a glovebox under argon.

Magnesium acetate (Mg(CH₃COO)₂.4H₂O, 99%, Sigma Aldrich) and potassium acetate (KCH₃COO, 99%, Sigma Aldrich) were used as received and stored in a desiccator. PEG400, polyethylene glycol (H-(O-CH₂-CH₂)ₙ-OH, n = 400, Sigma Aldrich) and acetonitrile (CH₃CN, 99.8%, anhydrous, Sigma Aldrich) were used as received and stored in ambient air.

The various aqueous solutions were prepared by mixing the salt (accurately weighed beforehand in the glove box for the anhydrous salts) with ultrapure water (18.2 MΩ.cm) and stirring for 30 minutes at room temperature to solubilize all the salts.

MgMn₂O₄ was synthesized with a sol-gel method following the protocol of Liu *et al.*³² PTCDA (perylenetetracarboxylic dianhydride, C₂₄H₈O₆, 97%. Sigma Aldrich, see Figure 3) was used as received. Composite electrodes composed of 70 wt% active material, 20 wt% carbon super P (Alfa Aesar) and 10 wt% polytetrafluoroethylene (PTFE, Sigma-Aldrich) were prepared. Active material, carbon and binder were mixed for a few minutes to obtain a homogeneous paste. This mixture was shaped into compact, flat electrodes. These were then pressed onto 12 mm diameter stainless steel grids. The electrodes were stored for at least 24 hours at 60°C in an oven before being cycled.
## Physico-chemical measurements

The viscosity and the density were acquired using a capillary of 1.59 mm diameter in a vibrating-tube densitometer (DMA 4500 M, Anton Parr) coupled to a rolling ball microviscosimeter (Lovis 2000 ME) at 20 °C. The conductivity values were obtained at 20 °C using a Consort C861 conductivity meter and a Consort SK10 electrode. The instrument was calibrated beforehand using two buffer solutions with different concentrations of potassium chloride (KCl), for which the conductivity is known.

## Fourier Transform infrared spectroscopy (FT-IR) measurements

Infrared spectra of the samples were recorded in attenuated total reflection (ATR) mode on a Bruker Tensor 27 spectrometer equipped with a Specac Golden Gate ATR accessory and a diamond crystal. A few microliters of each sample were deposited directly onto the diamond surface, and a background spectrum was first acquired under identical conditions without any sample. Spectra were collected from 500 to 4500 cm⁻¹, accumulating 50 scans at a resolution of 4 cm⁻¹ and a scan speed of 2.2 kHz. Baseline correction was carried out in Origin 2021b using its “Peak Analyzer” tool.

In this study, we focus solely on the O–H stretching region, which is sensitive to hydrogen-bonding environments. Because the ATR signal intensity varies with the penetration depth of the evanescent wave at the diamond–sample interface, we applied a minimal correction by multiplying the raw intensities by their respective wavenumbers.³³ The corrected spectra are hereafter referred to as “corrected ATR spectra.”

## Electrochemistry measurements

Electrochemical stability windows (ESW) were measured using a three-electrode cell. The electrolyte solution was degassed with argon for 20 minutes before each measurement to remove any O₂ or CO₂ molecules. The working electrode used was a stainless steel wire with
a surface estimated at 0.05 cm². Prior to measurement, working and counter-electrodes were washed with ethanol and placed in an ultrasonic bath for 20 minutes, first in ultra-pure water and then in ethanol. The counter-electrode was a platinum grid (1 cm²), and the reference electrode was an AgCl/Ag electrode containing an internal solution of 3 M NaCl (ALS Japan, 1.95 V vs. RHE (reversible hydrogen electrode, 25°C)). To determine the stability limits of each system, cyclic voltammetry was performed at room temperature with a scan rate of 1 mV s⁻¹ using a BioLogic VMP3 potentiostat. The cell was kept closed under argon during the experiment. The 45° tangent method was used to determine the upper and lower ESW limits.

The electrochemical behavior of the negative and positive electrode materials was evaluated with the same configuration as the ESW, replacing the working electrode with a composite electrode on a stainless steel grid. Cyclic voltammetry was performed at room temperature with a scan rate of 0.1 mV·s⁻¹. Moreover, the galvanostatic behavior of each electrode was evaluated in a 3-electrodes setup in a 3 m Mg(TFSI)₂ electrolyte (see Figure S7). The capacity of the MgMn₂O₄ was found to be limited to 60-100 mAh·g⁻¹ (practical capacity around 140 mAh·g⁻¹)³⁴ while the capacity of the PTCDA was around 130 mAh·g⁻¹, consistent with the literature and with the insertion of 1 Mg²⁺ per formula unit.³⁵

The ampoule, specially designed to combine electrochemistry and post-cycling gas analysis, has a metal valve at the top for gas measurement. Two tungsten wires are embedded in the glass and extend to the bottom of the first part of the cell to hold the support grid containing the electrodes at the end. The second part, which contains the electrolyte, is attached to the upper cell by a clip and a Viton ring that provides a seal between the two parts of the cell.

For more details, see the cell description given in reference³⁶ (see also the scheme of these ampoules in Figure 3).
The assembly of the upper and lower parts of the cell and the filling of the ampoule with 3 ml of electrolyte were carried out in a glove box under an inert atmosphere of argon. The ampoule was completely sealed in the glove box and maintained under an inert atmosphere of approximately 1 bar for the duration of cycling. Ten cycles were performed between 0 and 1.7 V at a rate of C/10. The C/10 rate was determined on the basis of the mass of active material in the limiting PTCDA electrode. An N/P (negative/positive) capacity ratio of 0.4-0.5 was used to exacerbate the HER reaction³⁶ (see details in the discussion). The mass loadings of MgMn₂O₄ and PTCDA electrodes ranged between 10.6/13.3 and 3.6/4.9 mg.cm⁻², respectively. All measurements were carried out at least twice to ensure reproducibility of results. H₂ produced after irradiation was quantified by micro-gas chromatography (µGC-R3000 SRA Instrument). Ultrapure argon (99.9999%) was used as a carrier gas to perform the analysis. The detection limit of H₂ was 10 ppm. Each gas measurement was repeated twice to ensure reproducibility. The amount of H₂ was the average of the two measurements, with an error bar indicating the largest and smallest amount of H₂ measured.

## Results and Discussion

*Effect of the electrolyte's nature on the structuration and physico-chemical properties*

The various electrolytes studied were based on acetate, perchlorate and magnesium imide salts at low and high molalities (Figure 1). In addition, mixed cations WIBS electrolytes composed of two acetate or two perchlorate salts dissolved in water were investigated. The acetate electrolyte was prepared with magnesium acetate (Mg(Ac)₂) and potassium acetate (KAc); the perchlorate electrolyte was prepared with magnesium perchlorate (Mg(ClO₄)₂) and sodium perchlorate (NaClO₄). A mixture of magnesium acetate and perchlorate salts (Mg(Ac)₂ and Mg(ClO₄)₂), a mixed anions WIBS, was also studied. Finally, the addition of PEG 400 (HO(CH₂CH₂O)ₙH, M_n = 400) or acetonitrile (ACN, CH₃CN) in a weight ratio of
0.8 (80%wt. of PEG or ACN with 20%wt. of water) in 1 m $Mg(ClO_4)_2$ was examined. The perchlorate salt was preferred in these solutions in place of the $TFSI^-$ anion as it is a more realistic and competitive salt compared to costly and toxic imides. As the perchlorate salt has a chaotropic behavior close to imide (see Figure 1), it appeared as a good choice for molecular crowding electrolytes (MCE) and hybrid electrolytes, as well as to compare them with the WIBS solutions proposed. A high co-solvent content of 80% of PEG or ACN was also selected to remain consistent with prior studies of aqueous electrolytes in the literature.<sup>13,15,37,38</sup> The different characteristics of each solution is given in Table S1. The choice of salt and the shift from dilute to concentrated regimes induce significant alterations in the solution's physicochemical properties, as well as in the structuring and interactions of ions and water molecules. Changes in solvation in the various electrolytes considered were analyzed by FT-IR spectroscopy (Figure 2). The O-H stretching band of water offers valuable insights into electrolyte structuring, particularly concerning the hydrogen-bonding network and water-salt interactions.<sup>33,39</sup> Indeed, an upshift in the O-H stretching band wavenumber signifies weaker hydrogen bonds in water and increased disruption of the hydrogen-bond network by surrounding ions.<sup>39,40</sup> Precisely, the O-H stretching band can be divided into different contributions or environments corresponding to more or less strong H-bonds.<sup>39</sup> Taking the example of the $TFSI^-$ based systems, the first contribution, at the highest wavenumbers around 3600 cm<sup>-1</sup>, corresponds to water molecules that interact fully with the ions, the second one at 3400 cm<sup>-1</sup> illustrates the interaction of water with the anion through one H-bond, and the third at around 3200 cm<sup>-1</sup>, reflects a behavior of bulk water.<sup>25</sup>
<figure><img src="image_3.png" /><figcaption>**Figure 2.** O-H stretching bands of the aqueous electrolytes considered, based on different magnesium salts and with different strategies of formulation. The lines are a guide for the eyes and refer to different H-bond network strengths (from the weaker, at the highest wavenumber, to the stronger, at the lowest wavenumber).</figcaption></figure>

At low molality (1 m, upper part of Figure 2), regardless of the electrolyte considered (Figure 2), the water stretching band is broad and rather similar to the one of pure water, reflecting the presence of bulk water environments, with most probably Mg²⁺ ions coordinated by water molecules and mainly free water molecules. As expected, the O-H stretching band in the acetate solution (a kosmotropic anion) is red-shifted to lower wavenumbers compared with those of perchlorate and TFSI⁻ solutions (chaotropic anions), reflecting a stronger hydrogen-bond network in the acetate system. Upon increasing molality, the acetate-based solutions (pure Mg(Ac)₂ and WIBS acetate, see middle panel in Figure 2) are characterized by a similar
band shape, whatever the molality, reflecting the strong H-bond network formed by the
kosmotropic acetate anion, as water molecules can be tightly bound to acetate anions.⁴¹

In contrast, for chaotropic salts such as Mg(TFSI)₂ and Mg(ClO₄)₂, as the molality increases, the O-H stretching band shifts towards the higher wavenumbers, and, obviously, we can clearly see the change in proportion of the three contributions presented above. The main contribution for the 3 m Mg(TFSI)₂ solution and the WIBS based on perchlorate anions is located around 3600 cm⁻¹ (Figure 2), corresponding to a very weak H-bond network.⁴² Many water-water interactions are therefore replaced by water-anion and water-cation interactions as the molality increases. Thus, the water molecules are occupied by the solvation of the ions, which should limit their reactivity. These trends for chaotropic and kosmotropic anions are consistent with those obtained on sodium-based aqueous solutions with different anions such as acetate, perchlorate and imide.²³

Regarding the WIBS perchlorate solutions, it appears clearly that the electrolyte with the lowest water content has a more structured band and quite logically contains the least free water molecules. Interestingly, we note that the mixed anions WIBS electrolyte combining magnesium acetate and perchlorate salts has a band as structured as the one of the 3 m Mg(TFSI)₂ solution (Figure 2).

When looking at the molecular crowding (with PEG) and hybrid electrolytes (with acetonitrile) strategies, besides the contribution of the alkyl modes observed below 3000 cm⁻¹, one can see that they can also help in tuning the H-bond network and the nature of the water molecules interaction. In the case of the PEG-based solution (Figure 2), even for a low molality of 1 m Mg(ClO₄)₂, the hydrogen bonds network is weakened by the presence of PEG, as already observed in Li aqueous batteries.¹⁵ For a LiTFSI-PEG-based electrolyte, this behavior was attributed to the substitution of water-water interactions by weaker water-PEG
ones.¹⁵ A similar trend is obtained here in the case of Mg-based electrolytes. This process is believed to increase the electrochemical stability of the electrolyte. For the hybrid electrolyte, a similar behavior is observed, with a weakening of the hydrogen network upon replacing water with acetonitrile (Figure 2). This weakening is even more pronounced than in the case of PEG.

In order to study the transport properties of the electrolytes, we determined two essential physico-chemical properties: viscosity and conductivity at 20 °C for the WISE and WIBS solutions. They are presented in Figure S1 and Table S2. As expected, the viscosity increases as the molality of the solutions increases. Conversely, for the conductivity, it decreases with increasing molality, except in the case of Mg perchlorate where conductivity increases when the molality increases from 1 to 4 m. Conductivity of concentrated solutions usually present a Gaussian-like trend with molality, with an increase of conductivity when increasing molality followed by a decrease of the conductivity when reaching higher concentrations.²² The behavior of the perchlorate solutions suggests that even when reaching high concentrations, the conductivity of perchlorate 4 m solution stays high, which can be linked to the low viscosity reached. Overall, the concentrated acetate-based solutions are highly viscous in comparison to the chaotropic perchlorate and imide solutions. This high viscosity is correlated to a low conductivity for the acetate concentrated solutions. Mg(ClO₄)₂ solutions have conductivities 2 to 4 times higher than those of Mg(TFSI)₂ for similar molality values. The WIBS solutions based on perchlorate salts present interestingly quite low viscosity values (around 6-8 mPa.s) with elevated conductivities around 90 mS.cm⁻¹.

## Electrochemical stability windows

The electrochemical stability window (ESW) of each electrolyte solution was determined (Fig. 3a) on a stainless steel working electrode at a scan rate of 1 mV.s⁻¹ (Figures S2-S6).
Firstly, it should be noted that for the dilute electrolytes the 1 m $Mg(Ac)_2$ present the lowest ESW with a value of 2.2V, while the 1 m $Mg(ClO_4)_2$ and 1 m $Mg(TFSI)_2$ show already quite high ESW values of 2.5 V.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** (a) Electrochemical stability windows acquired by cyclic voltammetry at 1 mV.s⁻¹ with a three-electrodes setup with stainless steel as the working electrode. The reference electrode is AgCl/Ag in a 3 M NaCl internal solution (1.95 V vs RHE) and the counter electrode is a platinum wire. Detailed voltamograms and HER and OER potentials are presented in Figures S2-S6 and Table S3, respectively. (b) Schematic diagram of the modified ampoule designed to couple electrochemical measurements and gas quantification. The measurements were carried out with a capacity ratio N/P (negative/positive) <1 with a $MgMn_2O_4$ positive electrode and a PTCDA (perylenetetracarboxylic dianhydride) negative electrode, along with the cyclic voltammograms of the PTCDA and $MgMn_2O_4$ electrodes in 4 m $Mg(ClO_4)_2$ acquired in a three-electrodes setup at 0.1 mV.s⁻¹ and a graphite rod as the counter electrode.</figcaption></figure>

For the more concentrated aqueous electrolytes at 3 or 4 m (i.e. electrolytes meeting WISE conditions, with both a greater mass and volume of salt than water), the electrochemical stability is extended for all the solutions considered. This is due to the reduced amount of
water in these samples (Table S1) that is able to react easily on both the oxidation and
reduction sides with OER and HER reactions respectively.

However, for the acetate ions the ESW is only increased to 2.4 V, from 1 to 4 m. This
difference is due to the kosmotropic nature of the acetate anion;²⁴ the local structure of the
water remains largely unchanged as the molality increases.²²,⁴³ On the other hand, the
chaotropic perchlorate and imide anions weaken and disrupt the hydrogen bond network,
leading to higher ESW values as the molality increases.²³,²⁴,⁴⁴ It should be noted, however,
that acetate solutions contain more water than perchlorate and imide solutions (Table S1) for a
given molality. This could also account for the lower ESW value in the case of acetate
solutions.

Similarly, the ESW of acetate-based WIBS electrolytes does not increase much (maximum of 2.5 V) despite the fact that the amount of water in the electrolyte is much lower than in pure magnesium acetate WISEs (Table S1). However, we note a shift in the ESW reduction limit towards the lower potentials for acetate-concentrated solutions or acetate-containing WIBS (Figure 3a). This is expected from literature data.¹¹,⁴³,⁴⁵,⁴⁶ This behavior was attributed to the higher pH of acetate solutions, which shifts the HER potential to more negative values.⁴⁶ For perchlorate WIBS, increasing the salt content leads to an increase in ESW up to a value of 2.9 V. This demonstrates how the change in the water H-bond network increases the ESW value.

The mixed anions WIBS electrolyte combining magnesium acetate and perchlorate salts
exhibits an intermediate behavior. With the presence of both kosmotropic acetate and
chotropic perchlorate anions, the stability window reaches a value of 2.6 V. In this case, the
chaotropic anion helps to limit water degradation (Figure 3a). Once again, this demonstrates
the effect of the chaotropic or kosmotropic nature of the anions and the resulting difference in
the interactions between water and ions, influencing the reactivity of water through the HER
and OER reactions.²³ Finally, both the organic/aqueous hybrid electrolyte (Figure 3a, 1 m
Mg(ClO₄)₂ in 80%wt. of acetonitrile (ACN) and 20%wt. of water) and molecular crowding (Figure 3a, 1m Mg(ClO₄)₂-0.8PEG-0.2H₂O in weight ratio) strategies were effective in increasing the ESW compared with the dilute electrolyte (1 m Mg(ClO₄)₂). The molecular crowding strategy even presents the widest stability window among all the electrolytes examined in this study, with a value of 3.0 V.

## H₂ production analysis

To go beyond the influence of the electrolyte's nature on the solution structuration, we also analyzed the influence of the electrolyte type on H₂ production resulting from HER. To do so, full cells were assembled using the different solutions as electrolytes. The challenge was to select a positive and a negative electrode that would operate within the potential range of the electrolytes used and, above all, that would use as far as possible the whole electrochemical stability window. The major drawback is the choice of electrode materials, which is limited in the case of Mg-ion batteries. We chose to assemble full cells consisting of a MgMn₂O₄ (MMO) positive electrode and a PTCDA negative electrode (perylenetetracarboxylic dianhydride, an organic molecule, see Figure 3b). As shown in Figure 3b, the potentials of the MMO and PTCDA electrodes fall well within the ESW of the electrolytes studied here, justifying their use. One has however to keep in mind that these electrodes may face some interfacial or structural evolution during cycling. First, concerning PTCDA, it is well known that such organic electrodes are subjected to dissolution. However, this dissolution is limited when using concentrated electrolytes.⁴⁷ For MgMn₂O₄, Cabello et al.³⁴ demonstrated in aqueous electrolytes some possible structural transformation, leading to the formation of the λ-MnO₂ phase. Yet, this do not prevent the cycling of the electrode with satisfactory performance. In both cases, reactivity with the electrolyte at the surface of the electrodes needs to be considered. However, we considered that the reasonable performance obtained justify plainly the use of MMO and PTCDA electrodes. Gas analysis was carried out on full
cells after 10 cycles at C/10 between 0 and 1.7 V for each electrolyte (Figures S8-S11) in
adapted cells for gas measurements (Fig. 3b).<sup>36</sup> The amount of gas was determined by micro-
gas chromatography. To exacerbate H<sub>2</sub> production, we adjusted the balance between the
negative and positive electrodes. With a capacity ratio N/P (negative/positive) < 1, the cell
voltage is mainly driven by the potential of the negative electrode, which favors the HER
reaction.<sup>36</sup>

*Which strategy is most effective in minimizing water reactivity?*

Using this experimental set-up, H₂ was the only gas detected after cycling regardless of the electrolyte considered. Measured H₂ productions are shown in Figure 4a.
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

**Figure 4.** (a) H₂ production after 10 cycles when reduction is favored for cells containing different electrolytes; (b) percentage of irreversible capacity resulting from the direct HER reaction (see the Supporting Information for the calculation of the irreversible capacity) and (c) percentage of “solvent” (including water, PEG or ACN amounts) in the different electrolytes studied.
For the dilute electrolytes, a greater amount of H₂ was produced than in concentrated solutions, particularly for the acetate salt. Increasing salt content systematically limits H₂ release (Figure 4a), which is expected for the perchlorate and imide electrolytes from the structuration data and from the weakening of the H network. Yet, while acetate-based electrolytes consistently maintain a strong hydrogen-bond network across all concentrations (Figure 2), H₂ evolution decreases clearly as salt molality increases (Figure 4a). This is linked to the slight increase of the ESW shown in Figure 3a. Consistently, within each anion family, higher concentration always decreases molecular hydrogen evolution, but this applies for a given anion only. Indeed, even at the lowest molality of 3 m in the case of Mg(TFSI)₂, lower H₂ production is detected than for cells containing purely acetate or perchlorate solutions at 4 m. Interestingly, at identical 4 m molality, Mg(Ac)₂ solution yields comparable -or slightly lower- H₂ production than the Mg(ClO₄)₂ solution, despite the fact the anions differ in their kosmo/chaotropic character (kosmotropic in the first case, chaotropic in the second one) and particularly in their structuring, see Figures 1-2. This counterintuitive result may arise from the higher stability of the 4 m Mg(Ac)₂ solution on the reduction side compared to the 4 m Mg(ClO₄)₂ system (Figure 3a, Table S3), offsetting perchlorate's structuring advantage.⁴⁸ Furthermore, the marked viscosity increase in acetate solutions with rising molality (Figure S1, Table S2) likely limits the HER kinetics.³⁰

For WIBS, H₂ production is very limited, with values equal to or less than 1 µmol after 10 cycles. The mixed anions 4 m Mg(ClO₄)₂ + 4 m Mg(Ac)₂ is the most effective in reducing H₂ production while surprisingly it did not present the highest ESW (Figure 3a). This observation shows that incorporating a second salt to occupy the water is a good strategy for limiting water degradation which is linked to the reduction of free water molecules observed by infrared spectroscopy (Figure 2) as well as the high viscosity, limiting the HER kinetics.³⁰ We note in particular that the very low water content 1 m Mg(ClO₄)₂ with 14 m NaClO₄ (Figure
4c) is characterized by a very low gas production, below 0.1 µmol. This trend is in line with the hypothesis that having the least water in the electrolyte limits its degradation, while a mixed anions strategy seems also useful for decreasing the HER reaction.

Cells made with electrolytes containing a crowding agent or a hybrid electrolyte also show a very low H₂ production (Figure 4a). However, incorporating an organic solvent such as acetonitrile at a high mass percentage runs counter to the strategy of increasing cell safety by using water rather than organic molecules. Conversely, the use of a crowding agent reduces the amount of salt in the electrolyte while minimizing the amount of water. This therefore appears to be an effective strategy for developing green, low-cost electrolytes while limiting water degradation.

Interestingly, the electrolytes that exhibit the lowest HER potentials -such as the WIBS acetate system (1 m Mg(Ac)₂ + 15 m KAc) and PEG-based molecular-crowding formulations- do not necessarily present the lowest H₂ production. This implies that the electrochemical stability window of the bulk electrolyte is not the main parameter governing the HER reaction. Rather, interfacial phenomena -namely, the local structuring of the electrolyte, the solvation environment of water molecules at the electrode surface, and specific electrolyte-electrode interactions- appear to exert a dominant influence. Crucially, all of these interfacial effects might be highly sensitive to the nature and properties of the salt.

To assess the impact of the HER reaction on the cell irreversible capacity and cell stability, the irreversible capacity related to the direct HER reaction³⁰,³⁶ (H₂O+1e⁻ → HO⁻ + 1/2H₂) was calculated from the amount of H₂ produced (see Supporting Information with calculation details) (Figure 4b). For the sake of simplicity, we assumed that the irreversible capacity linked to H₂ release comes mainly from the direct HER reaction. This corresponds to the part of the coulombic efficiency coming from the direct HER reaction. The evolution of the
irreversible capacity from the direct HER reaction is globally the same as that of H₂
production as a function of the nature of the electrolyte. The improvement of the performance
while reducing H₂ production is also clearly observed in the coulombic efficiencies evolution
of the different solutions (Figures S12 to S15). The dilute solutions show very low coulombic
efficiencies, directly linked to H₂ production, while concentrated solutions and WIBS
strategies demonstrate improved coulombic efficiencies. This confirms that limiting
electrolyte degradation by the HER reaction helps to reduce capacity loss.

*Does the nature of the anion have an impact on H₂ production?*

In many works, electrolytes have been compared on the basis of molality.⁴⁹⁻⁵¹ Accordingly, we begin by comparing the electrolytes studied here on the basis of molality. Although molality is the most commonly used concentration unit in the literature, it exhibits several inherent limitations. In particular, when comparing electrolytes composed of cations of different valences, molality does not reflect differences in electrolyte composition.⁵² The mass percentage of water in solution (or also the mass percentage of salt), the water-anion molar ratio or the water-cation molar ratio are thus a more appropriate comparison in this context.⁵² Water-to-anion molar ratio is a convenient parameter for assessing the relative availability of water molecules per anion, especially in systems featuring cations of differing valence. By comparing mole quantities, this ratio can indeed suggest how many water molecules are likely coordinated to ionic species and how many remain “free” in bulk solvent. However, because it does not account for anion size or mass, it can overlook steric and volumetric effects on solution structure—an omission that becomes pronounced with large, bulky anions (e.g., imides). Mass percent composition (wt % water or wt % salt) more accurately reflects the total water inventory available for processes such as the hydrogen evolution reaction (HER). Nonetheless, it provides no direct information on the fraction of water that is bound versus
unbound, nor on the coordination environment around individual ions. Together, these complementary values offer a more complete picture of aqueous-electrolyte solutions.

Figure 5a shows the evolution of H₂ production as a function of the water/anion molar ratio for diluted, concentrated (WISE) and WIBS electrolytes. The hybrid electrolytes containing PEG or ACN were not considered as they act as co-solvents and play a similar role to water. When the water/anion ratio is considered instead of molality, two regions appear for low and high values of this ratio. For a high ratio (i.e., a high water content, Figure 5a), H₂ production follows the kosmo/chaotropic classification of anions. Consequently, at low molality the choice of anion has a significant impact on the reactivity of the cell. This may be linked to differences in the strength of the H-bond network as discussed with Figure 2. In addition, other spectroscopic studies and molecular dynamics simulations have shown that electrolytes with chaotropic anions are composed of fewer water clusters or bulk-like water than electrolytes with kosmotropic anions.<sup>23,53</sup> For a defined molality, the number of water clusters decreases in the following order:<sup>23</sup> Ac⁻ > ClO₄⁻ > TFSI⁻. This corresponds to the order found herein for the hydrogen production. On the other hand, at the lowest water/anion molar ratios (i.e. in the case of concentrated solutions) all the cells show the same H₂ production trend. H₂ production decreased almost linearly with the quantity of water. This observation suggests that the choice of anion does not play a significant role in H₂ production at high molality, while it has a strong effect on the electrolyte structuration. Only the solution composed of Mg(TFSI)₂ behaves slightly differently from the others (Figure 5a, for a water-anion molar ratio <10). This can be explained by the particular behavior of the imide salt which is highly chaotropic (see Figure 1), and by the possible formation of a protective interface on the surface of the negative electrode<sup>4-6</sup> that can limit the HER reaction. The formation of a protective interface in the case of LiTFSI-based WISE has been reported and its nature debated.<sup>4-6,54</sup> Recently, Jommongkol *et al.*<sup>4</sup> have shown the precipitation of the LiTFSI salt
due to a local change in concentration, which leads to the formation of a LiTFSI micro-layer
and also to the formation of a nanometric layer of LiF and LiOH that repels water molecules
and prevents the HER reaction.⁴ On the other hand, perchlorate-based electrolytes could lead
to a less protective layer than that of imide, which may account for the differences in behavior
between imide and perchlorate electrolytes.¹⁵,⁵⁵ A comprehensive study of the interfaces
formed in magnesium-concentrated electrolytes, as a function of salt type, is needed to
determine whether degradation and electrochemical behavior are linked to the nature of the
interface.

<figure><img src="image_8.png" /><figcaption>**Figure 5.** H₂ production after 10 cycles when HER is favored, in aqueous magnesium full cells with different electrolytes, plotted as a function of (a) the water-to-anion molar ratio and (b) the weight percentage of salt. Circles: diluted (1 m) and concentrated electrolytes (3 m or 4 m); stars: mixed cations WIBS electrolytes (Mg(ClO₄)₂ + NaClO₄ or Mg(Ac)₂ + K(Ac)); squares: mixed anions WIBS (4 m Mg(ClO₄)₂ + 4 m Mg(Ac)₂).</figcaption></figure>

While the anion seems to have a minimal impact on H₂ production at high molality, Figure 5b
proves that salt content (or water content) is the predominant parameter. As illustrated in
Figure 5b, H₂ production exhibits an identical dependence on salt weight fraction regardless
of anion identity, indicating that the overall salt (or water) content -rather than merely the
presence of free water molecules that can be observed by IR spectroscopy- predominantly
governs H₂ evolution. A similar trend is observed for the evolution of the irreversible capacity
coming from HER (Figure S16). As the salt content increases, H₂ production decreases,
showing that the amount of water (or salt) in the electrolyte is one of the main parameter
governing H₂ release in Mg WISEs, even when the structuration of the electrolyte changes.

Conclusion

In summary, we compared the reactivity of different anion-based Mg aqueous solutions, water-in-bisalt electrolytes, and polymer-based and hybrid electrolytes, putting in parallel their structuration and propensity to favor or reduce HER. Increasing the molality, mixing cations or anions, or using a polymer increase the stability window while reducing H₂ production from HER. Overall, analysis of H₂ production in magnesium full cells containing concentrated aqueous electrolytes is a good descriptor of the solution reactivity and viability. While we show that the nature of the anion and WIBS and molecular crowding strategies have a strong influence on the water H-bond network and ion-water interaction, H₂ production is mainly determined by the water content for concentrated solutions. Counter-intuitively, this shows that the electrolyte structure has little effect on the HER reaction. Comparative studies on other monovalent or multivalent systems, such as Li, Na or Zn, could verify the universality of this result. This will prove very useful for the design of viable batteries based on concentrated aqueous solutions.

**Supporting Information Available:** Experimental methods, data of viscosity/conductivity and electrochemical measurements.
## AUTHOR INFORMATION

### Corresponding authors

Magali Gauthier - Université Paris-Saclay, CEA, CNRS, NIMBE, LEEL, 91191 Gif-sur-Yvette Cedex, France, magali.gauthier@cea.fr,

Sophie Le Caër - Université Paris-Saclay, CEA, CNRS, NIMBE, LIONS, 91191 Gif-sur-Yvette Cedex, France, sophie.le-caer@cea.fr

### Authors

Malaurie Paillot, Université Paris-Saclay, CEA, CNRS, NIMBE, LEEL, LIONS, 91191 Gif-sur-Yvette Cedex, France

Current address : Laboratoire de Chimie et d'Électrochimie du Solide (LCES), Université de Montréal, Montreal, Quebec, Canada, malaurie.paillot@umontreal.ca

## ACKNOWLEDGMENT

This work was supported by the French National Research Agency (ANR) under the Project ANR-21-CE50-0005-01. We would like to thank CEA for funding the Ph.D. of M.P. The authors thank Bénédicte Montigny (PCM2E, Tours, France), for support in the conductivity and viscosity measurements and Florent Fasquel (CEA, NIMBE, LEEL) for assistance in the CV measurements. We are thankful to Taren Cataldo (University of New South Wales) for the thorough reading of the manuscript.

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