The effect of sodium stannate as the electrolyte additive on the electrochemical performance of the Mg–8Li–1Y electrode in NaCl solution

Abstract

The effect of different concentrations of sodium stannate (Na₂SnO₃) as the electrolyte additive in 0.7 mol L⁻¹ NaCl electrolyte solution on the electrochemical performance of the Mg–8Li–1Y alloy electrode prepared by the vacuum induction melting method have been investigated by means of potentiostatic current–time, potentiodynamic polarization, and electrochemical impedance spectroscopy measurements as well as scanning electron microscopy. The performances of the magnesium–hydrogen peroxide (Mg–H₂O₂) semi-fuel cells with the Mg–8Li–1Y alloy as the anode were also determined. From the study, it has been found that the corrosion potential of the Mg–8Li–1Y electrode is slightly shifted to the negative direction and the corrosion current density is markedly decreased when different concentrations of Na₂SnO₃ are added to a 0.7 mol L⁻¹ NaCl electrolyte solution. The electrochemical impedance spectroscopy measurements show that the polarization resistance of the Mg–8Li–1Y electrode reduces in the following order with different concentrations of Na₂SnO₃: 0.20 mmol L⁻¹ > 0.00 mmol L⁻¹ > 0.05 mmol L⁻¹ > 0.10 mmol L⁻¹ > 0.30 mmol L⁻¹. The electrochemical performance, illustrated by potentiostatic current–time curves, of Mg–8Li–1Y electrode in 0.7 mol L⁻¹ NaCl electrolyte solution containing 0.30 mmol L⁻¹ Na₂SnO₃ is better than that in 0.7 mol L⁻¹ NaCl electrolyte solution containing Na₂SnO₃ in other concentrations. The addition of Na₂SnO₃ to the NaCl electrolyte solution loosens the product film and changes the size and thickness of the micro-clumps of the oxidation products. The Mg–H₂O₂ semi-fuel cell with the Mg–8Li–1Y anode in 0.7 mol L⁻¹ NaCl solution containing 0.30 mmol L⁻¹ Na₂SnO₃ presents a maximum power density of 112 mW cm⁻² at room temperature.

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1. Introduction

The Mg–H₂O₂ semi-fuel cell has received considerable attention in recent years, because it has many advantages, such as low cost, high power density, stable discharging current density, high specific energy, ability to work at ambient pressure, environmental friendliness, reliability, and safety.^1–8 These properties make Mg–H₂O₂ semi-fuel cells promising candidates in a wide range of fields such as space flight and automotive design. In addition, they can also be used in portable equipments, automobiles, medical devices and physical appliances for the convenience of movement.

This electrochemical system consists of a magnesium alloy anode, sodium chloride anolyte, cathode catalyst, conductive membrane and catholyte of sodium chloride, sulfuric acid and hydrogen peroxide. Magnesium is an attractive anode material for this type of semi-fuel cell, because it has high faradaic capacity (Mg: 2.2 A h g⁻¹), high specific energy (Mg: 6.8 kW h kg⁻¹), and more negative standard electroreduction potential versus the standard hydrogen electrode (Mg: −2.37 V in neutral solution).⁹ The Mg–H₂O₂ semi-fuel cell has a theoretical voltage of 4.14 V, which is higher than that of an Al–H₂O₂ semi-fuel cell^10,11 or an Al–AgO battery. Magnesium alloys as anodes, such as AZ31 and AZ61, have been widely investigated since the 1960s.

However, in practice, these electrodes operate at significantly less negative potentials because magnesium anodes are normally covered by passive oxide films, which cause a delay in reaching a steady-state and reduce the discharge rate. In addition, the parasitic corrosion reactions or self-discharge result in a reduction in the columbic efficiency of Mg alloys.¹² In general, there are two ways to improve anode performance. Firstly, magnesium is doped with other elements such as lithium, aluminum, cerium, and yttrium.¹³ Lv et al. investigated the effect of the alloy elements (Al, Zn, and Y) on the electrochemical performances of the Mg–8Li alloys in 0.7 mol L⁻¹ NaCl electrolyte solution.^14,15 Secondly, the additives are added into the electrolyte to activate the anode or to prevent the formation of the oxidation products on the anode. For instance, in our previous study, Cao added oxides of gallium into the solution system, and then further improvement in the discharge current densities of Mg–Li alloys were obtained due to the addition of additives.¹² Indeed, similar results were found by some other researchers, indicating that zinc, calcium and indium as well as stannates (aluminium/air batteries) and citrates were discovered to be effective electrolyte additives for inhibiting corrosion and/or boosting the electrode potential.^16–19

In this study, the Mg–8Li–1Y alloy was fabricated and its electrochemical performances were evaluated in 0.7 mol L⁻¹ NaCl electrolyte solution containing different concentrations of Na₂SnO₃. The influence of the concentrations of Na₂SnO₃ in 0.7 mol L⁻¹ NaCl solution on the electrochemical behaviors of the Mg–8Li–1Y electrode was investigated. The performances of Mg–H₂O₂ semi-fuel cells with the Mg–8Li–1Y anodes were also evaluated.

2. Experimental

2.1. Preparation of the Mg–8Li–1Y alloys

The Mg–8Li–1Y alloys were prepared from ingots of pure magnesium (99.99%), pure lithium (99.99%), and Mg–Y alloy with 25.67 wt% Y, which was placed in the vacuum induction melting furnace. The induction furnace with a refractory lined crucible surrounded by an induction coil was located inside a vacuum chamber. The induction furnace was linked to an AC power source at a frequency precisely matched to the furnace size, and then the alloy was melted under the protection of ultrahigh purity argon. The furnace was then evacuated to 1.0 × 10⁻² Pa and charged with ultrahigh purity argon. A preheated tundish-casting mold assembly was inserted through a valve. The refractory tundish was positioned in front of the induction furnace. The molten alloys were then poured through a tundish into a stainless steel cylinder with the inner diameter of 6 cm and the height of 18 cm. Finally, the mold containing hot melts was cooled to environmental temperature under an argon atmosphere in the furnace. The nominal composition of the Mg–8Li–1Y alloy is given in Table 1 and is presented in its cast state.

Table 1 Nominal composition of the alloy (wt%)

Alloys	Mg	Li	Y
Mg–8Li–1Y	91	8	1

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2.2. Electrochemical measurements

The electrochemical measurements were carried out with Princeton Applied Research in a homemade three-electrode electrochemical cell.^12,20 Nickel foam plated with platinum was used as the auxiliary electrode, and the saturated calomel electrode (SCE) was used as the reference electrode. All potentials were quoted with respect to SCE, and the working electrode was prepared as follows. The surface of the Mg–8Li–1Y alloy was successively polished with 600# and 2000# metallographic emery papers in order to make the surface smooth and remove the original oxide films on the metal surface. Then, the electrode was washed with deoxygenated ultrapure water (Milli-Q), soaked in acetone for 15 minutes to eliminate the surface grease, and then rinsed with distilled water. The dimension of the Mg–8Li–1Y electrode was 20 mm × 20 mm × 2 mm, and the geometric surface area was about 0.5024 cm². The electrolyte solutions were 0.7 mol L⁻¹ NaCl solutions with different concentrations of Na₂SnO₃ additive. Before the experiment, the electrolyte solution was purged with N₂ gas for 10 minutes before measurements in order to remove the oxygen dissolved in the solution. During the measurements, N₂ was passed over the solution.

Potentiodynamic polarization curves of the Mg–8Li–1Y electrode were obtained by sweeping the potential from −2.2 V to −0.8 V with a scanning rate of 5 mV s⁻¹. The corrosion current and corrosion potential of the Mg–8Li–1Y electrode were acquired from the potentiodynamic polarization curves. Potentiostatic current–time curves could be surveyed by keeping the Mg–8Li–1Y electrode at a scan rate of 5 mV s⁻¹ under different potentials for 15 minutes. The current density of the anodic oxidation was measured in accordance with the curves and used to analyze the oxidation activity of the electrode. Electrochemical impedance spectra were recorded under open circuit potential with the frequency range from 0.1 to 100 000 Hz and amplitude of 5 mV.

2.3. Mg–H₂O₂ semi-fuel cell tests

The performances of Mg–H₂O₂ semi-fuel cells with Mg–8Li–1Y anode were examined using a home-made flow-through test cell made of stainless steel. The Mg–8Li–1Y alloy was used as the anode, and the Pd-coated nickel foam served as the cathode. The geometric surface area of the cathode and anode was 4.0 cm² (2.0 cm × 2.0 cm). Nafion-115 membrane was used to separate the anode and the cathode compartments. The anolyte (0.7 mol L⁻¹ NaCl with 0.30 mmol L⁻¹ and without Na₂SnO₃) and the catholyte (0.7 mol L⁻¹ NaCl + 0.1 mol L⁻¹ H₂SO₄ + 0.5 mol L⁻¹ H₂O₂) were pumped to the bottom of the anode and the cathode compartments, respectively, and removed from the top of the compartments. The flow rate for the anolyte and the catholyte was 85 mL min⁻¹ and was controlled by individual peristaltic pumps. The performances of the Mg–H₂O₂ were recorded using a computer-controlled E-load system (Arbin) at ambient temperature.

2.4. Characterization of catalysts

The surface morphology of the Mg–8Li–1Y electrode was measured with scanning electron microscopy (SEM, JEOL JSM-6480). The alloy composition was determined using energy dispersive spectrometry (EDS) with a Vantage Digital Acquisition Engine (Thermo Noran, USA).

3. Results and discussion

3.1. The effect of the electrolyte additive on the potentiodynamic polarization curves of the Mg–8Li–1Y electrode

Fig. 1 shows the potentiodynamic polarization curves of the Mg–8Li–1Y electrode in 0.7 mol L⁻¹ NaCl electrolyte solution containing different concentrations of Na₂SnO₃. Table 2 displays the corrosion parameters of the Mg–8Li–1Y electrode measured in 0.7 mol L⁻¹ NaCl electrolyte solution containing different concentrations of Na₂SnO₃. The corrosion current density is significantly decreased when Na₂SnO₃ (0.05 mmol L⁻¹, 0.10 mmol L⁻¹, 0.20 mmol L⁻¹, or 0.30 mmol L⁻¹) was added into the 0.7 mol L⁻¹ NaCl solution. This indicates that the addition of Na₂SnO₃ to the electrolyte solution can improve the corrosion-resistant performance of the Mg–8Li–1Y electrode. The corrosion-resistant order decreases with the concentration of Na₂SnO₃ in the following sequence: 0.20 mmol L⁻¹ (40.784 μA cm⁻²) > 0.10 mmol L⁻¹ (60.750 μA cm⁻²) > 0.05 mmol L⁻¹ (61.554 μA cm⁻²) > 0.30 mmol L⁻¹ (87.497 μA cm⁻²) > 0.00 mmol L⁻¹ (258.794 μA cm⁻²). It is an interesting phenomenon that the corrosion current density is not monotonically dependent on the concentrations investigated. It may be a complicated experimental phenomenon that needs to be studied in future. From our present results, the different concentrations of Na₂SnO₃ in the electrolyte solution possibly have different effects on the electrode surface or on electrode reaction processes. This might be associated with the electrochemical impedance as well as the size and thickness of the micro-clumps of the products on the electrode during the reaction, which may also have a different influence on the corrosion resistance of the electrode.

Fig. 1 The potentiodynamic polarization curves of the Mg–8Li–1Y electrode in 0.7 mol L⁻¹ NaCl solutions containing different concentrations of Na₂SnO₃. (a) 0.00 mmol L⁻¹; (b) 0.05 mmol L⁻¹; (c) 0.10 mmol L⁻¹; (d) 0.20 mmol L⁻¹; (e) 0.30 mmol L⁻¹.

Table 2 The corrosion parameters of the Mg–8Li–1Y electrode measured in 0.7 mol L⁻¹ NaCl solutions containing different concentrations of Na₂SnO₃ (0.00 mmol L⁻¹, 0.05 mmol L⁻¹, 0.10 mmol L⁻¹, 0.20 mmol L⁻¹ and 0.30 mmol L⁻¹)

Na2SnO3/mmol L^−1	Corrosion potential/V (vs. SCE)	Corrosion current density/μA cm^−2	Open circuit potential/V (vs. SCE)
0	−1.522	258.794	−1.649
0.05	−1.546	61.554	−1.694
0.10	−1.534	60.750	−1.614
0.20	−1.530	40.784	−1.668
0.30	−1.552	87.497	−1.623

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3.2. The effect of the electrolyte additive on the potentiostatic current–time curves of the Mg–8Li–1Y electrode

Fig. 2A–D present the potentiostatic oxidation curves of the Mg–8Li–1Y electrode at various constant potentials of −1.4 V, −1.2 V, −1.0 V and −0.8 V, respectively, after different concentrations of Na₂SnO₃ were added to the 0.7 mol L⁻¹ NaCl electrolyte solution. The anodic oxidation current density increases in the initial discharging stage due to the double layer charging, and then decreases slightly at the corresponding discharging potentials of −1.4 V, −1.0 V and −0.8 V. Moreover, it can be observed from Fig. 2B that at the discharging potential of −1.2 V, the oxidation current density gradually increases with an increase in the discharging time. In addition, the larger vibrations in the current–time curves at a lower anodic potential, such as −1.4 V and −1.2 V, can be observed from Fig. 2A and B. This can be attributed to the formation and accumulation of oxidation products that prevent the Mg–8Li–1Y electrode surface from contacting the electrolyte, leading to a decrease in the active electrode surface area on the alloy surfaces. When the oxides fall off from the electrode, the electrode surface area is regenerated, and the discharging currents increase.

Fig. 2 The current–time curves of the Mg–8Li–1Y electrode recorded in 0.7 mol L⁻¹ NaCl solutions containing different concentrations of Na₂SnO₃ at various constant potentials of (A) −1.4 V, (B) −1.2 V, (C) −1.0 V and (D) −0.8 V. (a) 0.00 mmol L⁻¹; (b) 0.05 mmol L⁻¹; (c) 0.10 mmol L⁻¹; (d) 0.20 mmol L⁻¹; (e) 0.30 mmol L⁻¹.

The relatively smooth current–time profiles at anodic potentials (−1.0 V and −0.8 V) can be found in Fig. 2C and D, demonstrating that the formation and shedding of oxidation products are relatively easy.

From the potentiostatic oxidation curves, it can also be observed that the discharge current density is significantly improved when 0.30 mmol L⁻¹ Na₂SnO₃ is added to the 0.7 mol L⁻¹ NaCl electrolyte solution. The stabilized discharge current density of the electrode in the presence of 0.30 mmol L⁻¹ Na₂SnO₃ is approximately 55 mA cm⁻², which is around 2.5 mA cm⁻² higher than that in the absence of Na₂SnO₃ under the same discharge potential (−0.8 V). However, the Mg–8Li–1Y electrode exhibits lower discharge current density in 0.7 mol L⁻¹ NaCl electrolyte solution containing 0.20 mmol L⁻¹ Na₂SnO₃ than that without Na₂SnO₃ under similar experimental conditions. At other concentrations of Na₂SnO₃ (0.05 mmol L⁻¹ and 0.10 mmol L⁻¹), the discharge current densities of the Mg–8Li–1Y electrode exhibit no significant difference without addition of Na₂SnO₃ in the test period at the tested potentials (−1.4 V, −1.2 V, −1.0 V and −0.8 V). Moreover, at a constant potential of −0.8 V, the discharge current density of the Mg–8Li–1Y electrode decreases in the following order: 0.30 mmol L⁻¹ > 0.05 mmol L⁻¹ > 0.00 mmol L⁻¹ > 0.10 mmol L⁻¹ > 0.20 mmol L⁻¹. As stated above, it can be concluded that the optimal concentration of added Na₂SnO₃ is 0.30 mmol L⁻¹ among the tested concentrations of Na₂SnO₃ (0.00 mmol L⁻¹, 0.05 mmol L⁻¹, 0.10 mmol L⁻¹, 0.20 mmol L⁻¹ and 0.30 mmol L⁻¹).

3.3. The effect of the electrolyte additive on the impedance spectra of the Mg–8Li–1Y electrode

The impedance spectra of the Mg–8Li–1Y electrode were measured at the open circuit potential after different concentrations of Na₂SnO₃ were added to the 0.7 mol L⁻¹ NaCl electrolyte solution, and the results obtained are presented in Fig. 3. The impedance spectra recorded for the Mg–8Li–1Y electrode in different solutions are characterized by a single frequency capacitive semicircle. The polarization resistance, R_p, is represented by the diameter of the capacitive loop, as shown in Fig. 3. The polarization resistances for 0.30 mmol L⁻¹ and 0.10 mmol L⁻¹ concentrations of added Na₂SnO₃ are ca. 422 Ω cm⁻² and ca. 541 Ω cm⁻², respectively, which are smaller than that for 0.20 mmol L⁻¹ Na₂SnO₃ (ca. 618 Ω cm⁻²). However, the polarization resistances for 0.00 mmol L⁻¹ and 0.05 mmol L⁻¹ Na₂SnO₃ are similar. The polarization resistance of the Mg–8Li–1Y electrode can be arranged according to the decrease of the capacitive loop in the following order: 0.20 mmol L⁻¹ > 0.00 mmol L⁻¹ > 0.05 mmol L⁻¹ > 0.10 mmol L⁻¹ > 0.30 mmol L⁻¹. According to the literature^21,22 of impedance studies on conventional magnesium alloys, the frequency capacitive loop acquired in the impedance spectra of the Mg–8Li–1Y electrode may result from charge transfer. This indicates that the Mg–8Li–1Y electrode with 0.30 mmol L⁻¹ Na₂SnO₃ in the electrolyte solution presents the most active behavior compared with the Mg–8Li–1Y electrode in the electrolyte solution with other concentrations of Na₂SnO₃. This result is similar to the results obtained from the potentiostatic current–time measurements (Fig. 2A–D).

Fig. 3 The impedance spectra of the Mg–8Li–1Y electrode recorded in 0.7 mol L⁻¹ NaCl solutions containing different concentrations of Na₂SnO₃.

3.4. The effects of the electrolyte additive on the morphology of the Mg–8Li–1Y electrode

Fig. 4a and b present the surface morphologies of the Mg–8Li–1Y alloys after the samples were consecutively discharged at −1.4 V, −1.2 V, −1.0 V and −0.8 V each for 15 minutes in 0.7 mol L⁻¹ NaCl electrolyte solution and in 0.7 mol L⁻¹ NaCl electrolyte solution containing 0.30 mmol L⁻¹ Na₂SnO₃, respectively. Clearly, the morphologies of the oxidized surface of the Mg–8Li–1Y alloy in the absence and presence of Na₂SnO₃ are different. It was found from Fig. 4a that the oxidation products of the Mg–8Li–1Y alloy in 0.7 mol L⁻¹ NaCl electrolyte solution without Na₂SnO₃ are relatively large and dense micro-blocks on the surface, which can prevent the alloy surface from contacting with the electrolyte, leading to the decrease of the active electrode surface area. Moreover, it can also be observed from Fig. 4b that the addition of 0.30 mmol L⁻¹ Na₂SnO₃ to the electrolyte solution can markedly change the morphology of the oxidized surfaces of the Mg–8Li–1Y alloy tested. The product particles become homogeneous, loosely packed and much deeper. Especially, large channels are observed, which allows the electrolyte to penetrate more easily. Consequently, the discharge performance of the Mg–8Li–1Y electrode is enhanced. This result is consistent with the above-mentioned observation of the potentiostatic current–time curves (Fig. 2A–D).

Fig. 4 SEM micrographs of the Mg–8Li–1Y electrodes after the samples were consecutively discharged at −1.4 V, −1.2 V, −1.0 V and −0.8 V each for 15 minutes in (a) 0.7 mol L⁻¹ NaCl solution and (b) 0.7 mol L⁻¹ NaCl solution containing 0.30 mmol L⁻¹ Na₂SnO₃.

3.5. Fuel cell performance

In order to evaluate the effect of Na₂SnO₃ as the electrolyte additive on the performances of the Mg–8Li–1Y alloy as the anode of the metal–hydrogen peroxide semi-fuel cell, the Mg–H₂O₂ semi-fuel cells were assembled and tested in 0.7 mol L⁻¹ NaCl electrolyte solution and 0.7 mol L⁻¹ NaCl electrolyte solution containing 0.30 mmol L⁻¹ Na₂SnO₃ at ambient temperature. The experimental results are shown in Fig. 5 and 6.

Fig. 5 The plots of the cell voltage vs. current density for Mg–H₂O₂ semi-fuel cells with Mg–8Li–1Y anode in (a) 0.7 mol L⁻¹ NaCl solution and (b) 0.7 mol L⁻¹ NaCl + 0.30 mmol L⁻¹ Na₂SnO₃ solution at room temperature. Cathode: Ir/Pd/Ni-foam. Anolyte: 0.7 mol L⁻¹ NaCl. Catholyte: 0.7 mol L⁻¹ NaCl + 0.5 mol L⁻¹ H₂O₂ + 0.1 mol L⁻¹ H₂SO₄. Flow rate: 85 mL min⁻¹.

Fig. 6 The plots of the power density vs. current density for Mg–H₂O₂ semi-fuel cells with Mg–8Li–1Y anode in (a) 0.7 mol L⁻¹ NaCl solution and (b) 0.7 mol L⁻¹ NaCl + 0.30 mmol L⁻¹ Na₂SnO₃ solution at room temperature. Cathode: Ir/Pd/Ni-foam. Anolyte: 0.7 mol L⁻¹ NaCl. Catholyte: 0.7 mol L⁻¹ NaCl + 0.5 mol L⁻¹ H₂O₂ + 0.1 mol L⁻¹ H₂SO₄. Flow rate: 85 mL min⁻¹.

Fig. 5 shows the cell voltage versus current density plots. It can be seen that the gap of voltage of the cell with 0.30 mmol L⁻¹ Na₂SnO₃ as the electrolyte additive or without electrolyte additive is not obvious at current density lower than 30 mA cm⁻². However, with the increase of current density, the presence of 0.30 mmol L⁻¹ Na₂SnO₃ as the additive in the electrolyte solution can distinctly promote the cell performances. Especially, at current density between 130 mA cm⁻² and 140 mA cm⁻², the voltage of the cell with 0.30 mmol L⁻¹ Na₂SnO₃ as the additive in the electrolyte solution is around 300 mV higher than that without the additive in the electrolyte solution.

Fig. 6 shows the curves of the power density versus current density. The peak power density of the semi-fuel cell with the Mg–8Li–1Y anode in 0.7 mol L⁻¹ NaCl anolyte solution containing 0.30 mmol L⁻¹ Na₂SnO₃ reaches 112 mW cm⁻², which is higher than that in the solution without Na₂SnO₃ (83 mW cm⁻²). This indicates that Mg–8Li–1Y electrode as the anode of Mg–H₂O₂ semi-fuel cells with the 0.30 mmol L⁻¹ electrolyte additive displays better performance than that without the electrolyte additive. Moreover, using Na₂SnO₃ as the electrolyte additive can promote the performances of the Mg–H₂O₂ semi-fuel cells with Mg–8Li–1Y as the anode; however, the possible promotion mechanism will need further study.

4. Conclusions

Casting ingots of the Mg–8Li–1Y alloy as the potential anode material were prepared by the induction melting method. The electrochemical performances of the Mg–8Li–1Y electrode in 0.7 mol L⁻¹ NaCl electrolyte solution with different concentrations of Na₂SnO₃ were investigated. The obtained results support the following conclusions.

(1) The corrosion current density of the Mg–8Li–1Y electrode is significantly decreased after Na₂SnO₃ is added to the electrolyte solution and thus, the corrosion resistance of the electrode is also improved. Especially, the corrosion current density of the electrode reduces from 258.794 μA cm⁻² to 40.784 μA cm⁻² after 0.20 mmol L⁻¹ Na₂SnO₃ is added to the 0.7 mol L⁻¹ NaCl electrolyte solution.

(2) The discharge current density of the Mg–8Li–1Y electrode is improved by adding 0.30 mmol L⁻¹ Na₂SnO₃ to the 0.7 mol L⁻¹ NaCl electrolyte solution, which is about 2.5 mA cm⁻² higher than that in 0.7 mol L⁻¹ NaCl electrolyte solution without Na₂SnO₃ under the discharge potential of −0.8 V. Na₂SnO₃ can activate and promote the discharge performance of the Mg–8Li–1Y electrode; therefore, it is an effective electrolyte additive. The effect of the concentration of Na₂SnO₃ in electrolyte solution on the polarization resistance of the Mg–8Li–1Y electrode decreases in the following order: 0.20 mmol L⁻¹ > 0.00 mmol L⁻¹ > 0.05 mmol L⁻¹ > 0.10 mmol L⁻¹ > 0.30 mmol L⁻¹.

(3) The addition of 0.30 mmol L⁻¹ Na₂SnO₃ to the electrolyte solution can markedly alter the size and thickness of the micro-clumps of the oxidation products formed during discharge of the Mg–8Li–1Y electrode. Therefore, the oxidation products become smaller, deeper and much more homogeneous. This facilitates electrolyte penetration, which leads to the high discharge activity of the Mg–8Li–1Y electrode.

(4) The voltage of the cell with 0.7 mol L⁻¹ NaCl + 0.30 mmol L⁻¹ Na₂SnO₃ as the anolyte solution is around 300 mV higher than that with 0.7 mol L⁻¹ NaCl as the anolyte solution at current density between 130 mA cm⁻² and 140 mA cm⁻². Moreover, the peak power density of the semi-fuel cell with the Mg–8Li–1Y alloy as the anode in 0.7 mol L⁻¹ NaCl solution containing 0.30 mmol L⁻¹ Na₂SnO₃ (112 mW cm⁻²) is higher than that in 0.7 mol L⁻¹ NaCl solution (83 mW cm⁻²).

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21203040) and the Natural Science Foundation of Heilongjiang Province of China (B201201).

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