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

doi:10.21203/rs.3.rs-2611870/v1

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

https://doi.org/10.21203/rs.3.rs-2611870/v1

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published 16 May, 2023

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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 chloride, 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. Consideration of the improvements for the application in a real cell is given.

AZ63 alloy

Voltage

Specific capacity

Energy

Power

Over the last decades, we are witnesses of the astonishing development of different electrode materials for alkali-ion and metal-air batteries [1–7] supercapacitors [8–12^], and supercapattery [13–15]. Even so, some rather old electrochemical systems have still space for further improvement. For example, lead acid battery discovered in 1859 by Gaston Planté are still challenging and has a high market share among other electrochemical energy storage systems. Adding porous, high surface area, carbonaceous electrode parallel to negative, lead electrode, the pronounced enhancement in specific power and the number of cycles, in the so-called UltraBattery is achieved [16–18]. Also, the addition of different additives in the electrolyte improved the characteristics of lead-acid batteries [19].

Seawater-activated primary batteries, as 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], are used for a wide range of applications, like sonobuoys, radio buoys, lifebuoy, life raft, torpedo, and different seawater detecting devices due to the needs for continuous detecting of pollutions and climate changes [23]. At the 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 [22–24]. Among many different cathode materials, like CuCl, K₂S₂O₇, CuSO₄, PbO_2, [22, 23], silver chloride (AgCl), and lead chloride (PbCl₂) are most widely applied [23]. As an 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, is used [25]. Silver chloride cathode is usually made by melting into ingots that can be simply rolled into thin 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 PbCl₂ powder with some conductive materials like activated carbon or graphite, and a binder are formed into a sheet, in which a conductive net is inserted. For Mg-PbCl₂ based cells, the open-circuit voltage is around 1.2 V, and the typical operating voltage is nominally between 0.9 V to 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 effectively applied to form carbon felt-silver chloride cathode materials. In the cell with magnesium AZ63 anode in the current range from ~ 170 A g^− 1 to ~ 480 A g^− 1, it is shown that the specific capacity of the cell decrease from ~ 130 Ah kg^− 1 to ~ 100 Ah kg^− 1, specific energy from ~ 145 Wh kg^− 1 to ~ 70 Wh kg^− 1, and specific power increase from ~ 170 W kg^− 1 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 Pb-PbCl₂ cathode using cyclic voltammetry on pure lead in 1 M HCl and investigated discharge behavior with different Mg-alloy 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 including also 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 PbCl₂ and AgCl on pure metals [35–39]. For example, Barradas et al. [40] examined the formation of lead chloride on lead in 1M HCl during anodic polarization by electrochemical methods and SEM. They showed the existence of two different surface layers of PbCl₂ 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 NaC1O₄. They observed two cathodic peaks and suggested that these two peaks may characterize the reduction of two types of AgCl nuclei, formed instantaneously and formed by progressive nucleation. Pargar et al. [43] investigated the galvanostatic formation of AgCl on the pure silver electrode in the current range of 0.5 to 4 mA cm^− 2. They showed that at a high current density, 4 mA cm^− 2, a thicker and more complex, bi-layer structure of AgCl is formed.

Therefore, the aim of this work is to investigate the electrochemical synthesis of AgCl and PbCl₂ on pure metals, and their characteristics as rechargeable cathode materials in 3.5% sodium chloride. 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.

Working electrodes, pure silver, and lead (Koch-Light Laboratories, LDT, England), with a width of 3 mm and exposed surface area of 1 cm² are used. After polishing, before experiments, possible oxides are removed by dipping in 8 M HNO₃ for 30 s. In all experiments, three compartment glass cell with a volume of 100 cm³ and 3.5% NaCl (p.a. Merck) as the electrolyte is used. As the reference electrode served saturated calomel electrode (SCE), while the lead counter electrode 1 cm ⋅ 5 cm placed in a microporous polypropylene bag is used to avoid hypochlorite formation, for example on platinum or graphite that can change pH, and disturb experiments.

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 V to 2 V is performed (see text for the explanations), followed by further formation during 50 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 PbCl₂ to convert all chlorides to the metallic state. Simultaneously with charge-discharge measurements, the magnesium alloy AZ63 plate, 2 mm ⋅ 5 cm is used as a second quasi-reference electrode (E = − 1.54 V vs. SCE) to avoid the formation of the Mg(OH)₂ and change of the pH, and the voltage is simultaneously 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 PbCl₂ of 4.4 mg cm^− 2 is estimated from charge capacity using Faraday law.

All the experiments are performed using Gamry 1010E potentiaostat/galvanostat. Optical micrographs are obtained with an optical microscope Olympus CX41 connected to a PC.

3.1. Polarization measurements and formation and characterization of the electrodes

Figures 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(H₂O|H₂) = − 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 PbCl₂|Pb corresponding potentials is at − 0.485 V vs. SCE. The reversible electrode potentials for AgCl|Ag and PbCl₂|Pb electrodes are given as:

$${E_{\text{r}}}({\text{AgCl|Ag}})=E_{{\text{r}}}^{{\text{\varvec{\theta}}}}({\text{A}}{{\text{g}}^{\text{+}}}{\text{|Ag}})+\frac{{2.3RT}}{F}\log {L_{{\text{sp}}}} - \frac{{2.3RT}}{F}\log a({\text{C}}{{\text{l}}^ - })$$

$${E_{\text{r}}}({\text{PbC}}{{\text{l}}_2}{\text{|Pb}})=E_{{\text{r}}}^{{\text{\varvec{\theta}}}}({\text{P}}{{\text{b}}^{{\text{2+}}}}{\text{|Pb}})+\frac{{2.3RT}}{{2F}}\log {L_{{\text{sp}}}} - \frac{{2.3RT}}{F}\log a({\text{C}}{{\text{l}}^ - })$$

where E_r^θ(Ag⁺|Ag) = 0.799 V vs. SHE and E_r^θ(Pb²⁺|Pb)= − 0.268 V vs. SHE are the standard electrode potential, L_sp is a solubility product of AgCl and PbCl₂ (L_sp(AgCl) = 1.6⋅10^− 10 M² and L_sp(PbCl₂) = 1.7⋅10^− 5 M³). The activity of chloride ions a(Cl^–) = c(Cl^–)γ(Cl^–) = 0.39, where the activity coefficient, γ(Cl^–) = 0.65, for the concentration of chloride ions of c(Cl^–) = 0.6 M is estimated using the empirical Davies equation suitable for seawater [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, Figs. 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 V to 0.6 V are performed, Fig. 2a), and the corresponding capacity is followed, shown in the inset. 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 V to − 0.2 V even after one hundred cycles charge was very small less than 0.1 mAh cm^− 2, indicating that a compact PbCl₂ layer did not form. For that reason, the Pb electrode is five times charged to 2 V where more compact insoluble PbO₂ 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 V to − 0.2 V, the discharge capacity is stabilized to ~ 0.55 mAh cm^− 2, inset b) in Fig. 2b). Even the AgCl and PbCl₂ 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 PbCl₂ is associated with the presence of metallic silver or lead in the active mass. The potentials of the electrodes are determined by the ratio of a(MCl_n) = x and a(M) = 1 - x. Once the whole Ag or Pb is converted to AgCl or PbCl₂, as a nonconductive phase, a sharp increase of the potential is observed, and vice versa.

For considered cases, Scheme 1a) and b) shows possible formation mechanism of AgCl and PbCl₂ in the potentials range of − 0.6 V to 0.6 V and − 0.8 V to − 0.2 V, respectively. Initially, for silver, dissolved Ag⁺ reacts with Cl^– near the electrode surface and due to low solubility product of L_sp(AgCl) = 1.6⋅10^− 10 M² precipitate as dense, microporous thin inner layer film. Through micropores, Ag⁺ diffuses and reacts with Cl^– forming the outer AgCl layer. When the outer layer fills 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, Scheme 1b, due to the much higher solubility product of L_sp(PbCl₂) = 1.7⋅10^− 5 M³ dissolved Pb²⁺ have a less tendency to form thin microporous film, and during the initial period only small fraction of metallic lead surface is cowered with macroporous PbCl₂. After a prolonged time, most of the dissolved lead, probably diffuse into the solution and produce particles of PbCl₂ that precipitates at the bottom of the cell (which is visually observed), and the very low mass of PbCl₂ 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 PbO₂ is formed, which is further reduced to PbCl₂ and metallic lead. After the initial formation of compact PbCl₂, the situation becomes similar to the process at Ag, and the formation of PbCl₂ practically stops after ~ 50 cycles of charge-discharge, inset b) in Fig. 2b).

Figure 3a shows typical stable cyclic voltammograms of AgCl|Ag, and PbCl₂|Pb in 3.5% NaCl. Formation of AgCl starts at ~ 0 V and occurred via solid-state reaction through one broad peak 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 around 0 V small current could be seen in a magnified CV. 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 PbCl₂|Pb is similar to silver counterpart, but in the potential range from − 1 V to 0 V. Reduction of PbCl₂ to metallic lead, also proceeds via two packs, (B) and (C) that probably corresponds to reduction of the outer and inner layer.

Optical micrographs of AgCl and PbCl₂ are shown in Figs. 3b) and c). AgCl has a practically nonporous low crystalline structure. PbCl₂ is with irregular structure, and some whiskers can be observed, probably developed from the deposition of Pb²⁺ from the solution produced by the small dissolution of PbCl₂.

3.2. Charge-discharge characteristics

The charge-discharge curves for AgCl|Ag and PbCl₂|Pb in the current density range from 2 to 15 mA cm^–2 are shown in Figs. 4a) 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 establishing 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. 4a), 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 PbCl₂|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 PbCl₂ layer, a small amount of metallic Pb necessary for establishing the reversible potentials is present, Fig. 4b). Obtained discharge capacity decreased with increases of applied current density, from 0.55 mAh cm^− 2 to 0.4 mAh cm^− 2, inset in Fig. 4b), with Coulombic efficiency of the charge-discharge ranging from 75–85%. The increase of Coulombic efficiency with the increase of applied current is controversial and could be suggested that during charge with smaller current density, some of the initially formed Pb²⁺ has enough time to diffuse into the solution and could not precipitate onto the electrode surface as PbCl₂. At higher current density, the saturation concentration of PbCl₂ 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 connections of Faraday law with obtained charge capacities, insets in Figs. 4a) and b):

$$m({\text{MC}}{\operatorname{l} _n})={Q_{\text{c}}}\frac{{M({\text{MC}}{\operatorname{l} _n})}}{{nF}}$$

the mass for every corresponding charge is calculated, and shown in the inset in Fig. 5 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 PbCl₂ 4.4 mg cm^− 2 is estimated. With the obtained mass the specific currents are calculated by dividing current densities by mass. As can be seen in Fig. 5 for AgCl specific current ranges from 0.4 to 2.9 A g^− 1, while for PbCl₂ ranges from 0.45 to 3.44 A g^− 1. The theoretical specific capacity per gram of corresponding metal chlorides is determined using the following equation:

$${q_{{\text{s,T}}}}=\frac{{M({\text{MC}}{\operatorname{l} _n})}}{{nF}}$$

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 PbCl₂ is 193 mAh g^− 1. From Fig. 5 can be seen that obtained specific capacity for AgCl tends to the theoretical values for low current densities, while for PbCl₂ obtained discharge capacity of ~ 125 mAh g^− 1 for smaller current density is significantly reduced compared to 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 PbCl₂, so it could be suggested that solid-state conversion of PbCl₂ to Pb is not so fast 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. 6. For AgCl, after initial practically constant capacity, some small decay is observed followed by ~ 20% increase of the initial capacity, inset in Fig. 6. PbCl₂ 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 PbCl₂|Pb, due to the extensive mechanical straining some cracks in the relatively compact chloride deposits are formed, allowing further dissolution of underlying metals and formation of more metal chlorides onto the electrode surface.

3.3. Potential characteristics of magnesium quasi-rechargeable cells

To investigate the potential characteristic of quasi-rechargeable 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 alloy [27], 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 AZ63|AgCl system very flat potentials around 1.5 V are obtained, while for AZ63|PbCl₂ around 1 V, Figs. 7a) and b). Reactions that occurred during the charge are, at the positive electrode:

Ag + Cl^– → AgCl + e^– (5)

Pb + 2Cl^– → PbCl₂ + 2e^– (5a)

and at a negative AZ63 electrode, because Mg²⁺ cannot be deposited from an aqueous solution, will be hydrogen evolution:

2H₂O + 2e^– → H₂ + 2OH^– (6)

Discharge can be associated with the following reactions, at the positive electrode:

AgCl + e^– → Ag + Cl^– (7)

PbCl₂ + 2e^– → Pb + 2Cl^– (7a)

and at the negative electrode:

Mg → Mg²⁺ + 2e^– (8)

To roughly calculate the specific capacity, energy, and power of the potential cell, first, the specific current, I_s is calculated based on the following equation:

$${I_{\text{s}}}=\frac{I}{{m[{\text{MC}}{\operatorname{l} _n}]+m({\text{Mg}})]}}$$

The mass of magnesium in grams is calculated using the Faraday law and obtained discharge capacities, Q_d in Ah, of the AgCl and PbCl₂, shown in the insets of Figs. 4a) and b):

$$m({\text{Mg}})={Q_{\text{d}}}\frac{{M({\text{Mg}})}}{{2F}}$$

where M(Mg) = 24.3 g mol^− 1, and F = 26.8 Ah mol^− 1. The estimated mass of the Mg for AgCl system (1 mAh cm^− 2 for 2 mA cm^− 2 to 0.7 mAh cm^− 2 for 15 mA cm^− 2), is in the range of 0.45 mg to 0.32 mg. The corresponding dissolved Mg mass for the PbCl₂, for discharge capacities from 0.55 mAh cm^− 2 to 0.4 mAh cm^− 2 are in the range of 0.25 mg to 0.18 mg. It is obvious that dissolved Mg mass is much below 10% of the mass of the positive electrodes, so using the few centimeters thick Mg alloy and taking into account Mg corrosion, ~ 50 µA cm^− 2 [27], the cell that will be charged-discharged many times could be easily produced. For example, for the corrosion current density of 50 µA cm^− 2 the loss of AZ63 due to corrosion will be 0.2 g cm^− 2 or ~ 0.1 mm per year. Therefore, depending on the applied current and time the thickness of the negative electrode could be calculated for the desired number of charge-discharge cycles. Using calculated specific currents, that for the AZ63|AgCl ranges from 0.35 A g^–1 to 2.7 A g^–1, and for AZ63|PbCl₂ from 0.45 A g^–1 to 3.3 A g^–1, the specific capacity of the possible cells is calculated and shown in the inset of Fig. 7c. For AZ63|AgCl is in the range of 180 mAh g^− 1 to 130 mAh g^− 1, while for AZ63|PbCl₂ are in the range of 118 mAh g^− 1 to 80 mAh g^− 1.

The theoretical capacity of the cell can be estimated as:

$${Q_{{\text{cell}}}}=\frac{{{m_+}{q_{{\text{s,}}+}}+{m_ - }{q_{{\text{s,}} - }}}}{2}$$

and specific capacity as:

$${q_{{\text{s,cell}}}}=\frac{{{Q_{{\text{cell}}}}}}{{{m_+}+{m_ - }}}$$

The mass of the positive electrode m₊ could be taken as 1 g, and the corresponding mass of, the negative, magnesium, the electrode can be calculated from:

$${m_ - }=\frac{{{m_+}{q_{{\text{s,+}}}}}}{{{q_{{\text{s,-}}}}}}$$

The theoretical specific capacity for AgCl is 188 mAh g^− 1, for PbCl₂ is 193 mAh g^− 1, and for Mg is 2.2 Ah g^− 1. Hence, the Mg dissolved mass for the cell with AgCl will be 0.085 g, and for the cell with PbCl₂ 0.088 g. The theoretical capacity of the AZ63|AgCl cell will be 187.5 mAh, or the theoretical specific capacity 172.8 mAh g^− 1, while for the AZ63|PbCl₂ will be 190 mAh and 174.6 mAh g^− 1. For AgCl based system theoretical value is in good agreement with obtained value, but for PbCl₂ is smaller, probably due to low active mass utilization that can be connected with slow solid-state transformation.

Specific discharge energy, w_s,d, of the cell is estimated by the integration of discharge curves and using the following equation:

$${w_{{\text{s,d}}}}=\frac{{{I_{\text{s}}}\int\limits_{0}^{{{t_{\text{d}}}}} {{U_{\text{d}}}} {\text{d}}t}}{{3600}}$$

while specific discharge power, P_s,d, of the cell is calculated by:

$${P_{{\text{s,d}}}}=\frac{{{w_{{\text{s,d}}}}}}{{{t_{\text{d}}}}}$$

The calculated values are shown in Fig. 7c). Specific energy for AZ63|AgCl is in the range of 260 to 190 Wh kg^− 1, and for the AZ63|PbCl₂ in the range from 125 to 80 Wh kg^− 1. The estimated specific power, for AZ63|AgCl, ranges from 260 to 4000 W kg^− 1, and for AZ63|PbCl₂ from 450 to 3300 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 µm 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. [44]. It should be also mentioned that all silver and lead could be regenerated by dissolving in HNO₃ which is not the case with classical casted or pressed electrodes.

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 PbCl₂ 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 PbCl₂. 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 PbCl₂, with specific capacity for AZ63|AgCl cell, are in the range of 180 mAh g^− 1 to 130 mAh g^− 1, while for AZ63|PbCl₂ are in the range of 118 mAh g^− 1 to 80 mAh g^− 1. Specific energy for AZ63|AgCl is in the range of 260 to 190 Wh kg^− 1, and for the AZ63|PbCl₂ in the range from 125 to 80 Wh kg^− 1, and estimated specific power, for AZ63|AgCl ranges from 260 to 4000 W kg^− 1, and for AZ63|PbCl₂ from 450 to 3300 W kg^− 1.

Acknowledgment

This work was supported by the Ministry of Science, Technological Development and Innovation of the Republic of Serbia (Contract No. 451-03-47/2023-01/200135)

CRediT authorship contribution statement

Aleksandra Popović: Investigation, Data Curation, Formal analysis, Writing. Branimir Grgur: Fundraising, Conceptualization, Methodology, Supervising, Writing an original draft, Writing review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

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Scheme 1 is available in the Supplemental Files section.

Scheme1a.tif
Scheme 1. a) Formation of the low porous AgCl film, and b) formation of the porous PbCl₂ film.
Scheme1b.tif
Scheme 1. a) Formation of the low porous AgCl film, and b) formation of the porous PbCl₂ film.

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

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Abstract

Figures

1. Introduction

2. Experimental

3. Results And Discussion

3.1. Polarization measurements and formation and characterization of the electrodes

3.2. Charge-discharge characteristics

3.3. Potential characteristics of magnesium quasi-rechargeable cells

4. Conclusions

Declarations

References

Scheme

Supplementary Files

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