Electrochemical formation and behavior of silver and lead chlorides as potential cathodes for quasi-rechargeable magnesium seawater cell

Primary seawater magnesium-based cell with AgCl or PbCl2 cathodes is widely used as power sources. In this paper, we consider the cyclic galvanostatic formation of silver and lead chlorides and their electrochemical behavior for potential applications in the new concept of the seawater quasi-rechargeable magnesium cell. For potential cells, the voltage for Mg alloy AZ63 and AgCl is ~ 1.5 V, and for the PbCl2, ~ 1 V. High discharge specific capacity, energy, and power are obtained under the very high discharge rate. It is also presented that systems could be potentially used in emergency situations for a few days up to a few weeks as a power source in the life-saving boat for sporadic emitting GPS-SOS tacking signals and night signal lights.


Introduction
Over the last decades, we are witnesses to the astonishing development of different electrode materials for alkali-ion and metal-air batteries [1][2][3][4][5][6][7] supercapacitors [8][9][10][11][12], and supercapattery [13][14][15]. Even so, some rather old electrochemical systems have still space for further improvement. For example, the lead acid battery discovered in 1859 by Gaston Planté is still challenging and has a high market share among other electrochemical energy storage systems. Adding a porous, high surface area, carbonaceous electrode parallel to the negative, lead electrode, the noticeable enhancement in specific power and the number of cycles, in the so-called UltraBattery is achieved [16][17][18]. Also, the addition of different additives in the electrolyte improved the characteristics of lead-acid batteries [19].
Seawater-activated primary batteries based on magnesium alloys and silver chloride are another example of the old systems. Discovered in 1878 [20] and developed by Bell Telephone Laboratories in 1945 as the power source for electric torpedoes [21,22], today are used as power sources for a wide range of applications like: life-saving boats, life belts, life raft, electric torpedos, and different seawater detecting devices and sensors due to the needs for continuous detecting of pollutions and climate changes [23]. At present, the applications of the seawater-activated primary cell, with high power and energy content, and discharge time depending on the size of a few minutes to one hundred hours, are emphasized in many countries [22][23][24]. Among many different cathode materials, like CuCl, K 2 S 2 O 7 , CuSO 4 , PbO 2, [22,23], silver chloride (AgCl), and lead chloride (PbCl 2 ) are most widely used [23]. As anode due to the very low standard electrode potentials of -2.37 V, magnesium is usually considered. Owing to the fast corrosion of the pure magnesium in the seawater, typically magnesium alloys with different alloying elements (Al, Mn, Zn, Pb, Ce, Y, Ga, Ca, Li, etc.), that lower the rate of the hydrogen evolution reaction and alloy corrosion rate, are used [25]. Silver chloride cathode is usually made by melting into ingots that can be rolled into sheets of desired thickness. The operating voltage of the Mg-AgCl cell is maintained at a relatively high constant value of 1.1-1.5 V [22]. Lead chloride active material is in a powdered form and cannot be melted and rolled like AgCl. Consequently, a mixture of PbCl 2 powder with some conductive materials like active carbon or graphite, and a binder are formed into a sheet, in which a conductive net is inserted. For Mg-PbCl 2 based cells, the open-circuit voltage is around 1.2 V, and the typical operating voltage between 0.9 and 1.05 V, and as a curiosity, it can operate in river water [23]. For both systems, it is characteristic that during discharge, cathodes are disintegrated with the utilization of the active mass lower than 1. Also, all the silver is lost, as a relatively expensive metal (* 21 US $ per ounce, 31.1 g [26]), and lead can be considered an environmental pollution metal. An improvement of AgCl synthesis is reported in our previous work [27] in which the fast and low-cost modified process of successive ion layer adsorption and reaction (SILAR) is applied to form carbon felt-silver chloride cathode materials. In the cell with magnesium AZ63 anode in the current range from * 170 to * 480 mA g -1 , it is shown that the specific capacity of the cell decrease from * 13 to * 100 Ah kg -1 , specific energy from * 145 to * 70 Wh kg -1 , and specific power increase from * 170 to 340 W kg -1 , with the voltage plateau in the range of 1.2 V to -0.8 V depending on applied current.
Rechargeable seawater-activated cells could have a lot of advantages, but to the best of our knowledge, only a few papers are published on this topic. For example, El-Halim et al. [28] prepared a Pb-PbCl 2 cathode using cyclic voltammetry on pure lead in 1 M HCl and investigated discharge behavior with different Mg alloys and aluminum. Also, polypyrrole-zinc self-rechargeable cells [29,30], the cell with BiOCl anode and silver-silver chloride as the cathode in 1 M NaCl [31], and Ag-Zn battery with alkaline and a mild electrolyte containing chlorides that form AgCl [32] are investigated. As mentioned by Senthilkumar et al. [33] ''new concepts or chemistry is an urgent requirement for rechargeable seawater batteries to achieve a low-cost, user-friendly nature with adequate energy densities and high levels of safety''. These new concepts consider metal-air rechargeable cells with Na, K, and Mg as anode materials in organic electrolytes separated by a suitable membrane from the oxygen cathode in seawater and are still in the R&D phase [33,34].
Many authors using different techniques investigated the formation of PbCl 2 and AgCl on pure metals [35][36][37][38][39]. For example, Barradas et al. [40] studied by electrochemical methods and SEM, the formation of lead chloride onto the lead in 1 M HCl during anodic polarization. They showed the existence of two dissimilar surface layers of PbCl 2 , a basal layer consisting of relatively small crystals, and an upper layer of larger crystals. Similar investigations are performed by El Rehim et al. [41]. Birss and Smith [42] investigated the initial stages of growth and reduction of thin silver chloride films in the solutions of 0.1 M NaCl and 1 M NaC1O 4 . They observed two cathodic peaks and suggested that these two peaks may characterize the reduction of two types of AgCl nuclei, first formed instantaneously, and second formed by progressive nucleation. Pargar et al. [43] investigated the galvanostatic formation of AgCl on the pure silver electrode in the current range from 0.5 to 4 mA cm -2 . They showed that at a higher current density, 4 mA cm -2 , a thicker and more complex, bi-layer structure of AgCl is formed.
Therefore, this work aims to investigate the electrochemical synthesis of AgCl and PbCl 2 on pure metals, and their characteristics as rechargeable cathode materials in 3.5% sodium chloride, as simulated seawater. Also, such cathode will be initially tested in a new concept of quasi-rechargeable magnesium seawater cell. Because during the charge on the magnesium alloy electrode, hydrogen will be evolved, such a system cannot be considered as a classical rechargeable cell, so we attributed this new concept to a quasi-rechargeable cell. It will be also presented that systems could be potentially used in emergency situations for a few days up to a few weeks as a power source in life-saving boats for sporadic emitting GPS-SOS tacking signals and night signal lights.

Experimental
Working electrodes, pure silver, and lead (Koch-Light Laboratories, LDT, England), with a width of 3 mm and exposed surface area of 1 cm 2 are used. After polishing, before experiments, possible oxides are removed by dipping in 8 M HNO 3 for 30 s. In all experiments, three compartment glass cell with a volume of 100 cm 3 and 3.5% NaCl (p.a. Merck) as the electrolyte, simulating seawater, is used.
Silver chloride is formed by 20 charge-discharge cycles with the current density of 10 mA cm -2 , in the potential range from -0.6 V to 0.6 V until capacity is stabilized. For the formation of lead chloride initially, five cycles, with the current density of 10 mA cm -2 , of charge-discharge from -0.8 to 2 V is performed (see text for the explanations), followed by further formation during 50 charge-discharge cycles with the same current density, in the potential range from -0.8 V to -0.2 V, until capacity is stabilized. Charge-discharge behavior is investigated in the current density range from 2 mA cm -2 to 15 mA cm -2 . Before each charge-discharge, the electrode is conditioned for 300 s, at -0.6 V for AgCl, and -0.85 V for PbCl 2 to convert all chlorides to the metallic state. As the reference electrode served saturated calomel electrode (SCE), and to avoid the formation of the Mg(OH) 2 and change of the pH in the limited cell volume, instead of an Mg-alloy negative electrode, the lead counter electrode 1 cm Á 5 cm placed in a microporous polypropylene bag is used. Simultaneously with charge-discharge potential measurements, versus SCE, the magnesium alloy AZ63 plate, 2 mm Á 5 cm is used as a second quasireference electrode (E = -1.54 V vs. SCE) and the cell voltage, between positive and quasi-reference AZ63 electrode, is measured using the digital voltmeter ISO-Tech IDM 73, RS-232 interfaced to a PC. The active mass of the AgCl of 5.2 mg cm -2 and for PbCl 2 of 4.3 mg cm -2 is estimated from the discharge capacity of the electrodes using Faraday law. The dependence of the real part of the impedance and materials capacitance is evaluated in the entire investigated potential ranges from the Mott-Schottky experiments at 10 Hz.
All the electrochemical experiments are performed using Gamry 1010E potentiaostat/galvanostat. In order to examine the surface morphology of the deposits, a field emission scanning electron microscope (FESEM) Tescan, model Mira 3 XMU, at 20 kV is used. X-ray diffraction is done on a Philips PW 1050 diffractometer with Cu-Ka 1,2 radiation (Ni filter) at room temperature. The diffraction data are collected with a scanning step width of 0.05°and 6 s time per step in a 2h range of 10-70°. EVA 9.0 software is used for phase identification and semiquantification of the samples. The X-ray Line Profile Fitting Program (XFIT) with a Fundamental Parameters convolution approach to generating line profiles [44] is used to estimate crystallite size. XFIT uses a convolution approach to X-ray line-profile fitting in which the line shape is synthesized from the Cu-Ka emission profile, the diffractometer's dimensions, and the specimen's physical variables.
3 Results and discussion 3.1 Polarization measurements and formation and characterization of the electrodes Figure 1a and b, show polarization curves of pure metals in 3.5 wt% NaCl, starting from the cathodic potentials of -0.8 V for silver and -1.3 V for lead. During the forward scan of the silver electrode, Fig. 1a, hydrogen evolution could proceed, thermodynamic potentials is E r (H 2 O|H 2 ) = -0.655 V vs. SCE, but taking into account high overpotentials, the cathodic curve is probably controlled with mixed activation-diffusion oxygen reduction reaction. Corrosion potentials are established at -0.164 V vs. SCE, followed by pseudo-passive like behavior. At * 2 mV, sharp increases in the current density can be connected with the formation of AgCl onto the silver surface, characterized by a small peak (A) at the current density of * 10 mA cm -2 . After point (A) pseudo-passive behavior connected with the growth of AgCl is observed in the forward and backward scans. Below * 2 mV solid-state reaction of AgCl reduction to metallic silver occurred, through two peaks, (B) and (C). As pointed out by Pargar et al. [43], those two peaks could be connected with the reduction of the bi-layer structure. The first peak (B), could be connected with an outer layer, while peak (C) with the inner layer. Similar behavior is obtained for the polarization curve of the lead electrode, Fig. 1b. The corrosion potential of -0.617 V vs. SCE is determined, while for the PbCl 2 |Pb corresponding potential is at -0.485 V vs. SCE. The reversible electrode potentials for AgCl|Ag and PbCl 2 |Pb electrodes are given as:.  [27]. Hereafter, the reversible potential of the silver-silver chloride electrode for the given conditions, Eq. (1) is * 0.247 V vs. SHE or * 2 mV vs. SCE, and for the lead-lead chloride electrode, Eq. (2), is -0.243 V vs. SHE or -0.484 V vs. SCE that is identical to the observed values, Fig. 1a, and b.
To form the AgCl layer at the silver electrode, twenty successive charge-discharge cycles with a current density of 10 mA cm -2 , from -0.6 to 0.6 V are performed, Fig. 2a, and the corresponding capacity is followed, shown in the inset of Fig. 2a. As can be seen, the capacity is stabilized after * 15 cycles to 0.8 mAh cm -2 . For the Pb electrode, performing a similar procedure, but in the potential range from -0.8 to -0.2 V even after one hundred cycles charge was very small, less than 0.1 mAh cm -2 , indicating that a compact PbCl 2 layer did not form. For that reason, the Pb electrode is five times charged to 2 V where more compact insoluble PbO 2 is formed, inset (a) in Fig. 2b, and then reduced to Pb during discharge. After that, during fifty charge-discharge cycles in the potential range from -0.8 to -0.2 V, the discharge capacity is stabilized to * 0.55 mAh cm -2 , inset (b) in Fig. 2b. Even the AgCl and PbCl 2 are nonconductive, during charge and discharge, potential plateaus are connected with the solid-state conversion of metallic silver or lead to silver or lead chlorides. Therefore, the conductivity of AgCl and PbCl 2 is associated with the presence of metallic silver or lead in the active mass. Once when the whole Ag or Pb surface is converted to AgCl or PbCl 2 , as a nonconductive phase, a sharp increase of the potential is observed, and vice versa.
SEM images of AgCl and PbCl 2 are shown in Fig. 3a-d. AgCl has a practically compact, low porous nanometric structure. PbCl 2 is with irregular porous structure, Figs, 3c and d, and some rod-like and needle-like structures can be observed, Fig. 3c. At the higher magnification it is obvious that the lead  Fig. 3d, is larger and much more porous than the silver deposit, Fig. 3b.
X-ray diffraction patterns of the cycled Ag and Pb electrodes, Fig. 4, confirmed the presence of metal chloride phases AgCl (PDF# 31-1238) and PbCl 2 (PDF# 26-1150), respectively. In the case of the lead electrode, a minor phase Pb(OH)Cl (PDF# 89-7363), mineral laurionite, is also present in the amount of 30 wt%. The most probable occurrence of Pb(OH)Cl is five initial cycles to the 2 V during the initial formation of PbCl 2 . During the anodic oxidation of lead, this phase is also detected by XRD in 3% NaCl but not in HCl by Barradas et al. [45]. The mean crystallite sizes of formed AgCl and PbCl 2 are estimated to be 70 nm and 30 nm, respectively. Silver appears to be more prone to chloride formation than lead. SEM images of the electrodes support the XRD findings. Namely, the silver electrode has a uniform particle morphology, while the lead electrode has two different particle shapes (rod-like and needle-like) that can be attributed to two different crystalline phases. According to literature data, rod-like particles can be ascribed to the PbCl 2 phase [40], while needle-like particles are characteristic of the Pb(OH)Cl phase [46].
For considered cases, Fig. 5a and b shows possible formation mechanism of AgCl and PbCl 2 . Initially, for silver, dissolved Ag ? reacts with Clnear the electrode surface and due to the low solubility product of L sp (AgCl) = 1.6Á10 -10 M 2 precipitate as dense, microporous thin inner layer film. Through micropores, Ag ? diffuses and reacts with Clforming the outer AgCl layer. When the outer layer filled all the micro-pores of the inner film, the AgCl growth stops, and capacity is stabilized, inset in Fig. 2a. On the contrary, for the lead electrode, Fig. 5b, due to the much higher solubility product of L sp (PbCl 2 )-= 1.7Á10 -5 M 3 dissolved Pb 2? have a less tendency to form thin microporous film, and during the initial period only a small fraction of metallic lead surface is covered with the macro-porous PbCl 2 . After a prolonged time, most of the dissolved lead, probably diffuse into the solution and produce particles of PbCl 2 that precipitate at the bottom of the cell (which is visually observed), and a very low mass of PbCl 2 precipitate to the lead surfaces (corresponding to * 0.1 mAh cm -2 ). For that reason, we charged the electrode in five successive cycles to * 2 V, where compact PbO 2 is formed, which is further reduced to PbCl 2 and metallic lead. After the initial formation of compact PbCl 2 , the situation becomes similar to the process on Ag, and the formation of PbCl 2 practically stops after * 50 cycles of charge-discharge cycles, inset b in Fig. 2b. Formation of Pb(OH)Cl, probably occurred at higher anodic potentials via chemical reaction Pb 2? ? OH -? Cl -= Pb(OH)Cl. Figure 6a shows typical stable cyclic voltammograms of AgCl|Ag, and PbCl 2 |Pb in 3.5% NaCl. Formation of AgCl starts at * 0 V and occurred via the solid-state reaction through one broad peak (A) and shoulder (A 0 ) above * 0.2 V, up to the potentials of * 0.5 V. Reduction of AgCl to metallic silver starts at * -0.1 V (probably due to the overpotentials necessary to produce metallic Ag, even though around 0 V small cathodic current could be seen in a magnified CV (not shown). Reduction of AgCl, proceeds through one large peak (B) and a smaller one (C), as observed in a polarization curve, and could be associated with the reduction of the outer and inner layer. The cyclic voltammogram of PbCl 2 |Pb is similar to silver counterpart, but in the potential range from -1 V to 0 V. Oxidation of metallic lead occurred in the potential range from -0.5 to * 0 V, via one well-defined peak (A) and one shoulder (A 0 ). Reduction of PbCl 2 to metallic lead also proceeds via two packs, a larger one (B) and a smaller one (C) that probably corresponds to the reduction of the outer and inner layer. One more assumption should be considered. As investigated by Meyer [47] slow solid-state diffusion controlled electrochemical reduction of Pb(OH)Cl occurred in 1 M KCl in the potential range of -0.8 V to -1.15 V vs. SCE, with the peak at the potential of * -1.05 V vs. SCE.
The electrochemical impedance spectroscopy is an effective tool to characterize battery materials, as for example shown by Gong et al. [48,49] but in our case due to constant changes in materials composition and structure, we used the qualitative method of determining the dependence of the real part of the electrochemical impedance and capacitance of the investigated materials, which are shown in Fig. 6b and c. For the AgCl|Ag electrode, Fig. 6a, starting from the negative potentials, Z 0 has a practically constant value of * 1 X cm 2 up to the reversible potentials that could be attributed to the rough pure metal surface. Above reversible potentials, Z 0 starts to increase that corresponds to the formation of AgCl Approaching the reversible potentials, capacitance rapidly decreases with increasing potentials that could be connected with the formation of semiconducting AgCl [50]. In The case of Pb, Fig. 6c, the situation is more complicated. Even the electrode is discharged and conditioned before experiments, it is possible that some small amounts of PbCl 2 still exist in the bulk of the Pb. So the variation of the capacitance and impedance below reversible potentials could be connected with reductions and phase transformations of PbCl 2 remains. Above reversible potentials, Z 0 behave similarly as in the case of Ag, but capacitance first increases up to the potential of *-0.25 V and then decreases, which could be connected with the first formation of surface PbCl 2 and then with solid-state controlled growth of PbCl 2 and phase transformations within Pb.

Charge-discharge characteristics
The charge-discharge curves for AgCl|Ag and PbCl 2 |Pb in the current density ranges from 2 to 15 mA cm -2 are shown in Fig. 7a and b. The charge of the AgCl|Ag electrode occurred under the very flat potential plateau near the reversible potentials. Discharge proceeds initially with a very fast potential drop to the potential of * -0.6 V that could be connected with the formation of metallic silver necessary for the establishment of the reversible AgCl|Ag potential, and discharge continues at the potentials of * -0.1 V. Obtained capacity depends on applied current density, inset in Fig. 7a, and ranges from 1 mAh cm -2 for 2 mA cm -2 to 0.7 mAh cm -2  for 15 mA cm -2 , with practically 100% Coulombic efficiency. The charge-discharge of PbCl 2 |Pb also occurred over a very flat plateau near reversible potentials. Contrary to AgCl, a huge potential drop for discharge is not observed, suggesting that in the PbCl 2 layer, a small amount of metallic Pb necessary for establishing the reversible potentials is present, Fig. 7b. Obtained discharge capacity decreased with increases of applied current density, from 0.55 to 0.4 mAh cm -2 , inset in Fig. 7b, with Coulombic efficiency (C.I.) of the charge-discharge ranging from 75 to 85%. The increase of Coulombic efficiency with the increase of applied current is controversial and could be suggested that during charge with smaller current densities, some of the initially formed Pb 2? has enough time to diffuse into the solution and could not precipitate onto the electrode surface as PbCl 2 or more Pb(OH)Cl is formed that cannot be reduced at potentials of [ -0.6 V vs. SCE. At higher current density, the saturation concentration of PbCl 2 is much more easily formed and precipitates onto the electrode surface.
To determine the specific capacity of the investigated materials, the following procedure is performed. Using the connections of the Faraday law with obtained charge capacities, insets in Fig. 7a and b: the mass for every corresponding charge is calculated and shown in the inset in Fig. 8 as the dependence of the mass on applied current and extrapolated to zero current density. In that way, the maximum available AgCl mass of 5.2 mg cm -2 and for PbCl 2 4.3 mg cm -2 is estimated. With the obtained mass the specific currents, I s , are calculated by dividing current densities by the mass. As can be seen in Fig. 8 for AgCl specific current ranges from 0.37 to 2.9 A g -1 , while for PbCl 2 ranges from 0.48 to 3.5 A g -1 . The theoretical specific capacity per gram of corresponding metal chlorides is determined using the following equation: where M, g mol -1 , is the molar mass of metal chlorides, n is the number of exchanged electrons, and F, 26.8 Ah mol -1 , Faraday constant. The theoretical specific capacity for AgCl is 188 mAh g -1 , and for PbCl 2 is 194 mAh g -1 . From Fig. 8 can be seen that obtained specific capacity for AgCl tends to the theoretical values for low current densities, while for PbCl 2 obtained discharge capacity of * 125 mAh g -1 for smaller current density is significantly reduced Fig. 6 a Typical, stable cyclic voltammograms of AgCl|Ag and PbCl 2 |Pb in 3.5% NaCl. The dependence of the real part of the impedance and capacitance over potentials for b AgCl and c PbCl 2 compared with the theoretical values. Considering theoretical specific capacity and specific currents it can be calculated that electrodes are discharged with a very high Q-rate, from 2Q to 15.4Q for AgCl, and from 2.3Q to 17.8Q for PbCl 2 , consequently, it could be suggested that solid-state conversion of PbCl 2 to Pb is very slow and utilization of the active mass is limited.
The cyclic stability of the materials is investigated over 200 cycles, at the current density of 15 mA cm -2 . Some unusual behavior is observed, as can be seen in Fig. 9. For AgCl, after initial practically constant capacity, some small decay is observed followed by * 20% increase of the initial capacity, inset in Fig. 9. PbCl 2 electrodes show a practically constant increase of charge-discharge capacity and after 200 cycles are higher for * 50% than the initial value. The increase of the capacity could be explained that under the very high charge-discharge rate, 15.4Q for AgCl|Ag and 17.5Q for PbCl 2 |Pb, due to the extensive mechanical straining some cracks in the relatively compact chloride deposits are formed, allowing further dissolution of the underlying metals and formation of more metal chlorides onto the electrode surface.

Potential characteristics of magnesium quasi-rechargeable cells
To investigate the potential characteristic of quasirechargeable Mg-based cells, during the charge-discharge experiments, potentials are also recorded versus the quasi-reversible Mg alloy AZ63 reference electrode. Due to the very small polarization of Mg AZ63 alloy [27,51], the potential could be considered, in the first approximation, as cell voltage. As mentioned in the experimental, this is done to avoid the formation of the Mg(OH) 2 and pH change in limited cell volume, which is not the case for the real systems potentially applied in seawater. For the AZ63|AgCl system very flat potentials around 1.5 V are obtained, while for AZ63|PbCl 2 around 1 V, Fig. 10a and b.
Reactions that occurred during the charge are, at the positive electrode: or Pb þ 2Cl À ! PbCl 2 þ 2e À ð6Þ and at a negative AZ63 electrode, because Mg 2? cannot be deposited from an aqueous solution, will be hydrogen evolution: Discharge can be associated with the following reactions, at the positive electrode: AgCl þ e À ! Ag þ Cl À ð8Þ or and at the negative electrode: The reaction for Mg dissolution given by Eq. (8), is only theoretical, due to the existence of a negative difference effect (NDE) [52], when under anodic electrochemical dissolution reaction (discharge of the cell) occurred simultaneously with chemical dissolution: and the utilization of Mg AZ63 anode mass, u -, is around 0.55 [53].
The theoretical capacities and the Coulombic efficiency of the hypothetical mass-balanced cell can be estimated by knowing the specific capacity of charge and discharge. Because, during the charge on the negative electrode hydrogen is evolved, the capacity corresponds only to the positive electrode. On the contrary, the discharge capacity of the cell is: The corresponding theoretical masses of, the negative, magnesium, electrode can be calculated knowing the discharge capacity of the positive electrode, from: where M(Mg) = 24.3 g mol -1 , uis taken to be 0.55, and F = 26.8 Ah mol -1 . The theoretical specific capacity for AgCl is 188 mAh g -1 , for PbCl 2 is 193 mAh g -1 , and for Mg is 2.2 Ah g -1 . Therefore, the total mass of the dissolved Mg can be also calculated by: By the calculation of the total dissolved mass of AZ63 and taking into account masses of AgCl = 5.2 mg, and of PbCl 2 = 4.3 mg, the specific capacities of the AZ63|AgCl cell for the charge will vary from 183 to 135 mAh g -1 , and for the discharge from 165 to 125 mAh g -1 . For the AZ63|PbCl 2 cell charge will vary from 170 to 100 mAh g -1 , and for discharge from 115 to 80 mAh g -1 , Fig. 11. Finally by dividing q d,cell with q c,cell the Coulombic efficiency (C.E.) of the chargedischarge ranging from 65 to 80% for AZ63|PbCl 2 cell, and * 93% for AZ63|AgCl is estimated and shown in the inset of Fig. 11. Even the above shown calculations is only theoretically based, it can serve to calculate mass of the negative electrode for desired numbers, N, of charge-discharge cycles: For example for a cell with AgCl cathode and nominal capacity of 1 Ah, the mass of AgCl will be m(AgCl) = Q/q s = 5.3 g. Therefore the mas of the negative electrode, for example for 100 charge-discharge cycles will be approximately 83 g.
For the real cell, because the mass of the positive electrode is much smaller than magnesium mass, due to the irreversibility of Mg charge-discharge and the unimportance of the total cell mass for the seawater applications, we will estimate specific capacity, energy, and power focused only on the positive electrode.
Using calculated discharge specific currents for the positive electrodes, that for the AgCl ranges from 0.37 to 2.9 A g -1 , and for PbCl 2 from 0.48 to 3.5 A g -1 , the specific capacities of the possible cells is calculated and shown in the inset of Fig. 12. For AZ63|AgCl is in the range of 194 mAh g -1 to 135 mAh g -1 , while for AZ63|PbCl 2 is in the range of 125 mAh g -1 to 84 mAh g -1 .
Specific discharge energy, w s,d , of the cell, is estimated by the integration of discharge curves and using the following equation: while specific discharge power, P s,d , of the cell is calculated by:  The calculated values are shown in Fig. 12. Specific energy for the AZ63|AgCl cell is in the range of 260 to 200 Wh kg -1 , and for the AZ63|PbCl 2 cell in the range from 132 to 86 Wh kg -1 . The estimated specific power, for AZ63|AgCl, ranges from 560 to 4100 W kg -1 , and for AZ63|PbCl 2 from 490 to 3540 W kg -1 .
Even though the obtained specific capacity, energy, and power are promising for the real applications of such systems, obtained areal capacities are relatively small, * 0.5 to 1 mAh cm -2 . So for some hypothetical real cells, the positive electrode surface will be very high. For that reason, it could be suggested that some other method for preparing metal chlorides should be considered. Pargar et al. [43] using the galvanostatic anodization of the silver in 0.1 M HCl, obtained much ticker AgCl, and for example, with 4 mA cm -2 during one hour 40 lm of AgCl is obtained, corresponds to AgCl mass of 0.022 g cm -2 (4 mAh cm -2 ). Similar results are obtained by Lin et al. [54]. It should be also mentioned that all the silver and lead could be regenerated by dissolving in HNO 3 which is not the case with classical AgCl cast or PbCl 2 pressed electrodes.

Potential applications of the cells for life-saving boats
Based on the obtainable results, the potential uses of Mg-sweater batteries with AgCl that shows much better characteristics than PbCl 2 could be suggested.
With the astounding development of microelectronic components with low energy consumption, the possible application of the investigated system could be for life-saving boats. Namely, presented systems could be used in emergency situations where the cell can be used for limited times, a few days up to a few weeks as a power source for sporadic emitting GPS-SOS tacking signals via low energy consumption radio transmitters, Fig. 13. During the night, due to the recent development of the low-consuming currents LED's, 1-2 mA, few LED's can provide a constant or flashing signal light. A cell can be charged during the day with a small photovoltaic cell. One advantage of such a cell is its low price and negligible self-discharge rate during storage, which is not the case with classical battery systems. Relatively low voltage of the cell could be increased by serial connection of two cells with proper elimination of the current leaking, or by simple electronic miniature micro-power synchronous step-up DC/DC converters like LT3525-3 V or LTC3525-5 V. The system management could be accomplished with small programmable micro-controllers.

Conclusions
Using the cyclic charge-discharge at a constant current in 3.5% NaCl silver and lead chlorides are successfully obtained on the pure metal surfaces. AgCl and PbCl 2 are investigated as positive electrode materials and good specific capacities are obtained with a very high Q-rate, from 2Q to 15.4Q for AgCl, and from 2.3Q to 17.8Q for PbCl 2 . Such electrode materials are used to demonstrate the new concept of Mg-based quasi-rechargeable seawater cells. Namely, during charge silver or lead will be oxidized to corresponding chlorides, and on Mg alloy hydrogen evolution reaction will occur. During discharge, chlorides will be reduced to metals and Mg alloy will be dissolved. The estimated discharge voltage of the cells will be 1.5 V for AgCl, and * 1 V for PbCl 2 , with specific capacity for AZ63|AgCl cell, in the range of 194 mAh g -1 to 135 mAh g -1 , while for AZ63|PbCl 2 are in the range of 125 mAh g -1 to 84 mAh g -1 . Specific energy for AZ63|AgCl is in the range of 260 to 200 Wh kg -1 , and for the AZ63|PbCl 2 in the range from 132 to 86 Wh kg -1 , and estimated specific power, for AZ63|AgCl ranges from 560 to 4100 W kg -1 , and for AZ63|PbCl 2 from 490 to 3540 W kg -1 . It is also proposed that systems could be potentially used in emergency situations for a few