Suppressing H2 evolution by using a hydrogel for reversible Na storage in Na3V2(PO4)3

We report a low-cost hydrogel electrolyte by adding 3 wt% poly(acrylate sodium) (PAAS) into 1 M Na2SO4 aqueous electrolyte, which achieves a widened electrochemical stability window (ESW) of 2.45 V on stainless steel current collector from 2.12 V in 1 M Na2SO4 aqueous electrolytes (AE). Moreover, the H2 evolution potential reaches −1.75 V vs. Ag/AgCl on titanium current collector. The results reveal that the polymer network structure of PAAS has the ability to interact with water molecules and thus the hydrogen evolution reaction can be limited effectively, which broadens the ESW of aqueous electrolyte and allows the reversible Na-ion intercalation/deintercalation of Na3V2(PO4)3 as an anode material in aqueous electrolyte reported for the first time.


Introduction
Sodium-ion batteries have attracted much attention in the eld of energy storage systems due to the rich abundance and low cost of sodium. Nonetheless, conventional sodium-ion batteries are based on toxic and ammable organic electrolytes which may cause security issues in practical applications. [1][2][3][4][5] Aqueous electrolytes for sodium-ion batteries have the advantages of non-ammability, high ionic conductivity, and low cost. However, the thermodynamic ESW of water as an electrolyte solvent is restricted to barely 1.23 V arising from the H 2 and O 2 evolution, and thus the output voltage and energy density of cells are limited. [5][6][7][8][9][10][11][12][13][14] Recently, Wang et al. 8 and Battaglia et al. 9 have made remarkable progress on high concentration (9.26 M) sodium triuoromethanesulfonate (NaCF 3 SO 3 ) and ultrahigh (up to 37 M) concentration sodium bis(uorosulfonyl)imide (NaFSI) in water by forming "water-in-salt" electrolyte (WiSE), which has been the most effective approach to bind "free" water for extending stable electrochemical windows to more than 2.5 V. But the exorbitant cost due to the massive use of organic sodium salts and temperature-dependent solubility of salts hinder the practical application of the WiSE strategy. Therefore, a simple and economical strategy to suppress the electrochemical decomposition of water for aqueous sodium batteries is of great importance.
Besides, it is also meaningful to select an electrode material that is compatible with the electrolytes to achieve Na-ion storage. Nowadays, the oen used anode materials, such as activated carbon (2-2.6 V vs. Na/Na + ), 15 NaTi 2 (PO 4 ) 3 (2.1 V vs. Na/ Na + ), 16 vanadium oxide Na 2 V 6 O 16 $nH 2 O (2.5 V vs. Na/Na + ), 12 or organic anodes polyimide (2.2-2.4 V vs. Na/Na + ), 17,18 possess relatively high electrode potentials in aqueous sodium-ion batteries, which leads to low cell voltage output. Na 3 V 2 (PO 4 ) 3 with a NASICON structure has been intensively investigated for sodium-ion batteries as a cathode. [19][20][21][22][23][24] In fact, Na 3 V 2 (PO 4 ) 3 has the characteristics of dual potential plateaus at 3.4 V vs. Na + /Na and 1.6 V vs. Na + /Na, and the lower one has been utilized for anode use in Na 3 V 2 (PO 4 ) 3 symmetrical full cells with nonaqueous electrolytes that show good cycle stability and high rate capability. 23,24 Provided Na 3 V 2 (PO 4 ) 3 could be also used anode in aqueous electrolytes, a higher cell voltage can be expected. However, since its Na + -intercalation potential (1.6 V vs. Na/Na + ) is much lower than hydrogen evolution potential (2.3 V vs. Na/Na + ), 5 nding a special aqueous electrolyte that ts Na 3 V 2 (PO 4 ) 3 anode is important.
Herein, we introduce a new aqueous electrolyte based on the combination of inexpensive Na 2 SO 4 salt and hydrogel poly(acrylate sodium) (PAAS). The interaction among salt, PAAS and water molecules suppresses hydrogen evolution reaction effectively, which broadens the ESW of aqueous electrolyte and allows the reversible Na-ion intercalation/deintercalation of Na 3 V 2 (PO 4 ) 3 as an anode in aqueous electrolyte.

Results and discussion
Hydrogel electrolytes (HEs) of various concentrations were prepared by mixing 1 M Na 2 SO 4 aqueous electrolytes with an appropriate amount of PAAS (1, 3, 5 wt% respectively, which are noted as HE-1, HE-3, HE-5, M w ¼ 300w-700w, Aladdin), and then by intense stirring at room temperature until dissolved. As shown in Fig. 1a and b, the states of the electrolytes changes from liquid to gel aer the addition of PAAS, and the viscosity of the weakly acidic HEs (pH ¼ 5.5-7) increases with higher PAAS content. The conductivity of HEs at room temperature is about 78-80 mS cm À1 compared to 76 mS cm À1 for AE (Fig. 1c). The higher conductivity with PAAS might be mainly related to the increased Na + content from PAAS. The declined conductivity for 5 wt% PAAS could be explained by the rising viscosity. It is noteworthy that the conductivity of the aqueous gel polymer electrolytes is much higher than those of the organic sodiumion electrolytes (6-12 mS cm À1 ) and most of "water-in-salt" electrolytes ($10 mS cm À1 ). 5,8,9 Fig . 2 shows the ESW of HEs obtained by potential dynamic polarization using a stainless steel (SS) working electrode in a three-electrode cell. The addition of various amounts of PAAS to the AE results in different degrees of ESW broadening and the content of 3 wt% PAAS (HE-3) exerts the strongest effect. Using HE-3, there is a sharp over-potential increase of about 330 mV for H 2 evolution reaction (HER). That is, the initial HER potential shis from À0.98 V for AE to À1.31 V vs. Ag/AgCl for HE-3, and the total ESW is as wide as 2.45 V compared to 2.12 V in AE. It may be ascribed to the strong water absorption effect of PAAS which causes self-ionizing H + of water molecules to interact with -COO À , resulting in a decrease in the mobility of H + in HEs and negative shi of the hydrogen evolution potential.
To better explain the function of PAAS in 1 M Na 2 SO 4 aqueous electrolyte, we have used DMol3 method based on density functional theory (DFT) 25,26 to calculate the binding energy, and determine the possible combined forms of Na + and oxygen atoms with water molecules in the cluster. The results of the binding energy changes among PAAS, Na 2 SO 4 and water molecules are provided in Fig. 3a. The binding energy between the above three is À0.69 eV (<0), which indicates that PAAS and Na 2 SO 4 have some interaction with water molecules in HEs. This may be due to the similar attraction effect between -COO À or SO 4 2À to Na + that makes PAAS and Na 2 SO 4 preferentially form a more structurally stable molecular cluster to restrain the water molecule tightly. Na + ions from both PAAS and the Na 2 SO 4 have solvation with water molecules and the remaining unsolvated water molecules act with the carboxylate in a manner to form hydrogen bonds ( Fig. 3a and b, W1-W7 and Table 1). Moreover, the widened ESW may be also related to the interaction of ion-pair charges between -COOH groups, which are  formed by self-ionizing H + and -COO À groups. The charge interaction between ions expands the polymer network and causes a large osmotic pressure, which enhances the interaction between water molecules and oxygen atoms and also raises salt tolerance ability of PAAS 27 at the same time. Since the part of H + exists in the form of a -COOH group, the reduction resistance of the water molecules is slightly enhanced. These interactions jointly lead to the improvement in cathodic stability observed in HEs as shown in Fig. 2b. Fig. 4a shows the Fourier transform infrared (FTIR) analysis of PAAS, Na 2 SO 4 and water molecules. The strong peak at 1124 cm À1 shown in AE and HEs can be attributed to the antisymmetric stretching vibration of SO 4 2À . For PAAS, the peak at 1454 cm À1 indicates the angle vibration absorption of -CH 2 , while the peaks at 1570 and 1409 cm À1 correspond to the antisymmetric and symmetric stretching vibration absorption peaks of -COO À , respectively. These three characteristic peaks become more pronounced with the increase of PAAS content. The stretching vibration absorption peak of C]O (1710 cm À1 ) disappears when PAAS is added into aqueous solutions, indicating that the combination of the water molecules with carboxylate oxygen may weaken the C]O stretching vibration absorption peak. Besides, the H-O-H bending vibration is red-shied from 1630 cm À1 to 1635 cm À1 when the concentration of PAAS increased from 0% (AE) to 5 wt% (HE-5), which is likely due to an increase of viscosity. It can be observed in Fig. 4b that HE-3 shows a narrower peak compared to the broad Raman band from 3000 cm À1 to 3700 cm À1 of pure water. In addition, the intensity of the peak at 3200 cm À1 corresponding to free water is lower and the peak of coordinated water shis to 3550 cm À1 from 3410 cm À1 . Hence, it can be inferred that HE-3 can achieve better electrochemical stability due to the combination of a fraction of water molecules.   The material type of the anode current collector has an apparent inuence on the HER over-potential. We further measured the ESW of HE-3 by potentiodynamic scanning on different metal electrodes, including stainless steel (SS), Al, Ti and Pt (Fig. 5a). Al is a typical current collector and can suppress the HER potential to À1.55 V vs. Ag/AgCl compared with Pt (À1.0 V) or SS (À1.31 V). Furthermore, the HER potential for Ti electrode can reach À1.75 V (vs. Ag/AgCl) in HE-3. The HER overpotential of the anode current collectors in HE-3 follows the sequence: Ti > Al > SS > Pt.
The electrode materials that have been reported in aqueous electrolyte systems are shown in Fig. 5b. The widened ESW of HE-3 provides more options for electrode materials available for reversible sodium-ion storage. We have selected NASICON structured Na 3 V 2 (PO 4 ) 3 material for a potential Na + intercalation electrode with titanium current collector. The synthesized Na 3 V 2 (PO 4 ) 3 has irregular particle shapes in the sizes mostly below 5 mm ( Fig. 6a and b). Its XRD pattern in Fig. 6c accords with the standard one well. TG result indicates that the Na 3 V 2 (PO 4 ) 3 material contains 4.44% carbon (Fig. 6d), and the weight increases aer 550 C could be attributed to the oxidation of V 3+ in Na 3 V 2 (PO 4 ) 3 . 21,22 Fig. 5c shows cyclic voltammograms of Na 3 V 2 (PO 4 ) 3 electrode in the three-electrode system containing HEs and AE at a scan rate of 5 mV s À1 . As evidenced by the results in Fig. 5c, the intercalation of sodium-ion in Na 3 V 2 (PO 4 ) 3 electrode is accompanied by hydrogen evolution reaction in AE, HE-1 and HE-5. Due to the strong competing reduction, no obvious Na + deintercalation peak can be observed in AE. On the other hand, although the Na + intercalation and Fig. 5 The electrochemical stability windows of HE-3 on different working electrodes, scan rate: 10 mV s À1 (a). The intercalation potential of some electrode materials that could possibly be employed for aqueous sodium-ion storages and comparison of aqueous sodium-ion storages end-of-charge voltages with various salts (b). CV of Na 3 V 2 (PO 4 ) 3 working electrode (coated on titanium) in a three-electrode system containing HEs and AE for the first cycle at scanning rate of 5 mV s À1 (c). CV of Na 3 V 2 (PO 4 ) 3 working electrode (coated on titanium) in a three-electrodes cell containing HE-3 for the first three cycles from À0.7 V to À1.5 V vs. Ag/AgCl at scanning rate of 5 mV s À1 (d). This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 620-625 | 623 hydrogen evolution potentials cannot be distinguished in HE-1 and HE-5, the corresponding Na + deintercalation peaks are still distinct, conrming the PAAS effect in Fig. 2. By contrast, the potential of Na + intercalation in HE-3 is well distinguishable from that of hydrogen evolution despite their small difference. The symmetric redox peaks at À1.36 V and À1.18 V vs. Ag/AgCl (i.e. 1.56 V and 1.74 V vs. Na/Na + ), accorded with the reported results, [21][22][23][24] demonstrate the reversible sodium-ion intercalation/deintercalation of Na 3 V 2 (PO 4 ) 3 in HE-3. This result proves that HE-3 effectively broadens the ESW of aqueous electrolyte and allows the reversible Na-ion intercalation/ deintercalation of Na 3 V 2 (PO 4 ) 3 as an anode in aqueous electrolyte.
The electrochemical cycle reversibility of Na 3 V 2 (PO 4 ) 3 has been further examined by multiple CV test from À0.7 V to À1.5 V (vs. Ag/AgCl). As shown in Fig. 5d, the reduction peaks and oxidation peaks appear at approximately À1.38 V and À1.19 V (vs. Ag/AgCl) respectively for the initial three cycles. These three cycles are highly coincident, reecting the good Na + -intercalation/deintercalation reversibility of Na 3 V 2 (PO 4 ) 3 electrode in HE-3. In addition, the Na + storage capacity of Na 3 V 2 (PO 4 ) 3 can be estimated from the reversible cyclic voltammetry (CV) curves using the following eqn (1). 28 where C NVP and m are the specic capacity and mass of Na 3 V 2 (PO 4 ) 3 material, Q refers to the quantity of electric charge, which is equal to the product of current (I) and time (t), that is, the ratio of the shaded area ( Ð IdV) under the CV curve to the scan rate (v s ).
The specic capacity of Na 3 V 2 (PO 4 ) 3 electrode was calculated from CV curves (Fig. 5d) and eqn (1). The specic capacity of Na 3 V 2 (PO 4 ) 3 electrode is about 37.2 mA h g À1 during the reduction process. Although this capacity is lower than the previously reported capacity of Na 3 V 2 (PO 4 ) 3 in non-aqueous electrolyte (67 mA h g À1 ), 23 it should be mentioned that the dynamic factors, such as the particle size and scanning rate, could inuence the available capacity. The reversible electrode reaction provides the possibility for the research on aqueous Na 3 V 2 (PO 4 ) 3 -based cells and other aqueous cell systems using an anode that has a lower potential than the frequently used NaTi 2 (PO 4 ) 3 .

Conclusions
We have investigated the PAAS hydrogel electrolyte containing 1 M Na 2 SO 4 for potential use in aqueous Na-ion batteries. It is found that the PAAS content has obvious inuence on the hydrogen evolution potential. Adding 3 wt% PAAS to 1 M Na 2 SO 4 aqueous electrolyte effectively inhibits the hydrogen evolution. The ESW of aqueous gel electrolyte is broadened to an extent of 330 mV from 2.12 V (in AE) to 2.45 V (in HE-3) on stainless steel current collector, and particularly the hydrogen evolution potential is shied to À1.75 V vs. Ag/AgCl (i.e. 1.17 V vs. Na/Na + ) on titanium current collector. DFT calculation and spectroscopic analysis conrm the interaction between water molecules and PAAS, which may decrease the water activation. Finally, through CV test and cathodic area calculation we rst prove the feasibility of Na 3 V 2 (PO 4 ) 3 anode for reversible Na storage in this electrolyte with the stable capacity of ca. 37.2 mA h g À1 .

Material synthesis and electrode preparation
The Na 3 V 2 (PO 4 ) 3 powder was synthesized by spray drying method as reported, in which a given amount of NH 4 VO 3 (99.9%, Macklin), NH 4 H 2 PO 4 (99%, Aladdin), Na 2 CO 3 (99%, Energy Chemical) and glucose (as the carbon precursor and the reductive agent) were dissolved in deionized water and stirred at 80 C until blue clear solution formed. The blue solution was spray-dried to obtain the precursor by a spraying dryer (BUCHI Mini Spray Dryer B-290), where the inlet and outlet temperatures were set at 200 C and 105 C, respectively, the feed pump speed was 8% and the gas speed was 5000 mL min À1 . Then, the obtained powder was pre-calcined in a tube furnace at 350 C for 5 h and calcined at 750 C for 12 h in N 2 atmosphere. The Na 3 V 2 (PO 4 ) 3 electrodes were prepared as followed: Na 3 V 2 (PO 4 ) 3 material, Super P and polyvinylidene uoride (PVDF) were mixed with a weight ratio of 8 : 1 : 1 in N-methyl-2-pyrrolidone (NMP), which was spin-coated on a titanium grid, and was dried at 80 C for 8 h under vacuum condition.

Material characterizations
FTIR spectra were measured on a PerkinElmer Spectrum 100. Raman spectra were collected with a DXR Raman Microscope using a 532 nm diode-pumped solid-state laser between 4000 and 2700 cm À1 , with all the samples sealed in a glass test tube. The scanning electron microscopy (SEM) images of Na 3 V 2 (PO 4 ) 3 powder were obtained on Hitachi-X650 microscope (20 kV) scanning electron microscopy. The X-ray diffraction (XRD) patterns were recorded on a scan rate of 2 $min À1 from 10 to 65 by using Rigaku diffractometer (D max -2200) with Cu-K radiation. The carbon content in Na 3 V 2 (PO 4 ) 3 powder was analyzed using a Thermo Gravimetric Analyzer (TGA/Pyris 1 TGA).

Electrochemical measurements
Ionic conductivity was measured with Rex conductivity meter DDS-307 at room temperature. The electrochemical stability windows (ESW) of the electrolytes were examined by cyclic voltammetry (CV) using CHI650C electrochemical workstation and the glassy three-electrode system with Ag/AgCl as reference electrode (2.92 V vs. Na/Na + ), stainless steel (SS 304), aluminum (Al), titanium (Ti), platinum (Pt) and prepared Na 3 V 2 (PO 4 ) 3 as working electrodes, and graphite as counter electrode. The experiments were conducted at scanning rate of 10 or 5 mV s À1 in a given potential range at 25 C.

Conflicts of interest
There are no conicts to declare.