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Paper Journal of Materials Chemistry A the anode side, sodium ions are transported from the catholyte through NASICON and reduced to Na metal during charging, whereas discharging oxidises Na metal and returns Na+ to the catholyte. Since saltwater (1 M) has a neutral pH ($7), the following two types of overall reactions can be expected, during charging 4NaCl (aq) + 2H2O (l) / 4Na(s)+O2 (g)+4HCl(aq);E 1⁄43.53V(pH1⁄47) rate of 0.025 mA cm2 (Fig. 2a, purple curve). The charge and discharge voltage plateaus were found to be 3.84 and 2.72 V, from which the overpotentials were estimated to be 0.31 and 0.81 V, respectively. The discharge overpotential was relatively large compared to the charge overpotential. The pH of saltwater was maintained $7 throughout the charge/discharge processes. Since the measured charge voltage was lower than the theoret- ical potential of the CER (4.07 V), the main electrochemical reaction during charging would be the OER rather than the CER. Our previous studies on seawater batteries showed the evidence of the CER during charging, as the concentration of Cl decreased with the charging time.20 In this study we also observed that the concentration of Cl decreased monotonically with increasing charging time for the battery with 0.4 M salt- water, which is comparable to the NaCl concentration in seawater (Fig. S3, ESI†). To further investigate the occurrence of the CER and OER in saltwater during charging, we measured the cyclic voltammo- gram (CV) of 1 M NaCl (aq) using a three-electrode half-cell at a scan rate of 10 mV s1 and compared it to that obtained using 1 M Na2SO4 (aq). The latter has no Cl ions, therefore no CER would occur.31 As shown in Fig. 2b, within a potential window of 0–1.1 V vs. SHE (2.7–3.8 V vs. Na+/Na), 1 M NaCl displayed a couple of redox current signals with a voltage difference, which could be assigned to the OER and ORR in the anodic and (1) 2NaCl(aq)/Cl2 (g)+2Na(s);E 1⁄44.07V (2) and during discharging 4Na(s)+2H2O(l)+O2 (g)/ 4NaOH (aq); E 1⁄4 3.53 V (pH 1⁄4 7) (3) According to the reaction potentials, the OER route (reaction (1), 3.53 V) is thermodynamically favourable than the CER route (reaction (2), 4.07 V) during charging. However, the CER has been reported to occur together with the OER in NaCl (aq) due to the latter's high overpotential.29,30 This implies the possibility of the CER during the charging process in our saltwater batteries. First, we investigated the rst galvanostatic charge and discharge curves of the battery with 1 M saltwater at a current Fig. 2 Reaction mechanism and salt concentration effect. (a) Galvanostatic charge and discharge voltage profiles of (Na|saltwater) half-cells with 1 M and 5 M saltwater at a current rate of 0.025 mA cm2. Inset: the ion conductivities of saltwater at various concentrations. (b) Cyclic vol- tammograms of 1 M saltwater and 1 M Na2SO4 aqueous solution at a scan rate of 10 mV s1. An enlarged portion (inset) shows no notable difference between two sets of CV curves. (c) Photographs of the anode side before and after charging over 200 h at a current rate of 0.1 mA cm2, showing dark areas of Na metal deposition. (d) Polarisation plots of the half-cells with different salt concentrations as a function of the current rate (0.01–0.5 mA cm2). The inset indicates the theoretical redox potentials with respect to the salt concentration. This journal is © The Royal Society of Chemistry 2016 J. Mater. Chem. A, 2016, 4, 7207–7213 | 7209PDF Image | Saltwater as the energy source for low-cost batteries
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