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Energy & Fuels 2007, 21, 1092-1097
Feasibility Study of an Aqueous Zinc-Persulfate Battery Zhang Shimin* College of Chemistry and Chemical Engineering, Central South UniVersity, Changsha, Hunan, China ReceiVed June 8, 2006. ReVised Manuscript ReceiVed December 15, 2006
In a zinc-persulfate battery, active metal-zinc pieces were used as an anode and ammonium chloride as an electrolyte in an anode zone, with soluble oxidants-persulfate as an active cathode substance and electrolyte in the cathode zone. Carbon felt was used as an inert cathode, with a PE-01 homogeneous membrane between the anode and cathode zones as well as 100 mL of solution in both the anode and cathode zones. The discharge characteristics of the battery at 5 Ω was investigated for various temperatures and concentrations of ammonium persulfate, sodium persulfate, and ammonium chloride, and a flat discharge characteristic was obtained. The output energy of the zinc-persulfate battery increases remarkably on increasing the concentrations of persulfate and ammonium chloride, as well as temperature. The output energy of the zinc-sodium persulfate battery is greater than that of a zinc-ammonium persulfate battery. If the concentration of ammonium persulfate is less than or equal to 1.25 M, crystals did not form in the cathode zone after discharge when the temperature was above 0 °C. Crystals formed are a mixture of (NH4)2Zn(SO4)2‚6H2O, (NH4)2Zn(SO4)2, (NH4)2SO4, and (NH4)3H(SO4)2. (NH4)2Zn(SO4)2 is the major constituent, and the others are the minor ones. The maximal and least actual gravimetric energy densities of the Zn-(NH4)2S2O8 battery are 48.15 and 20.78 W h kg-1, respectively.
1. Introduction In the table of standard electrode potentials of substances, those very common and cheap substances with very negative electrode potentials and without environmental contamination include manganese, aluminum, and magnesium, etc. Batteries using magnesium as the negative electrode have been investigated widely. Batteries using manganese or aluminum as the negative electrode have also been investigated.1-4 Other very common and cheap substances with very positive electrode potentials and without environmental contamination include potassium permanganate, hydrogen peroxide, persulfate, and so on. Marsh and Licht5 investigated an aqueous aluminumhydrogen peroxide battery using hydrogen peroxide as the active positive substance; Licht6 investigated an aqueous aluminumpermanganate fuel cell using potassium permanganate as the active positive substance. However, investigations with persulfate as the active positive substance of a battery have not been carried out. Thus, in the present investigation, persulfate was used as the active positive substances of the battery and the electrolyte in the cathode zone, zinc, as the active negative substance, ammonium chloride, as the electrolyte in the anode zone, and carbon felt, as an inert cathode, with a PE-01 homogeneous membrane between the anode and cathode zones. The discharge characteristic of the battery was measured for
various temperatures and concentrations of ammonium persulfate, sodium persulfate, and ammonium chloride. 2. Experimental The analytical grade reagents and electrode materials used were ammonium chloride, ammonium persulfate, sodium persulfate, 99.99% zinc pieces (thickness of 0.15-0.25 mm), and carbon felt (thickness of 3 mm). The discharge characteristics were investigated with NEWARE charge/discharge equipment; Tafel and cyclic voltammetry plots were measured with a CHI potentiostat. The crystals formed in the cathode zone were analyzed by a D/MAX2500 X-ray diffraction apparatus. The electrolytic cell was made of organic glass. The total volume of the cell was 5 × 5 × 10 cm3, which was divided into two equal parts of 5 × 5 × 5 cm3 by a membrane. The solution volume in both the anode and cathode zones was 100 mL. The width of the zinc pieces and the carbon-felt electrode was 4.5 cm. The electrodes were immersed in solution approximately 4 cm. The zinc electrode was formed by the superposition of more than one zinc piece, each of which was polished and washed with deionized water before experiments. The solutions were prepared using deionized water. The cell was discharged at a constant resistance of 5 Ω.
3. Results and Discussion * Tel.: 86-731-8837703. Fax.: 8710006. E-mail:
[email protected]. (1) Tajima, S. Aluminium and manganese as anodes for dry and reserve batteries. J. Power Sources 1984, 11 (1-2), 155-161. (2) Despic, A. R. Design characteristics of an aluminium-air battery with consumable wedge anodes. J. Appl. Electrochem. 1985, 15 (2), 191-200. (3) Licht, S. Peramunage, Dharmasena. Novel aqueous aluminium/sulfur batteries. J. Electrochem. Soc. 1993, 140 (1), 4-6. (4) Verma, L. K. Studies on an aluminium-carbon cell. J. Power Sources 1994, 50 (1-2), 187-192. (5) Marsh, C.; Licht, S. Novel aqueous dual-channel aluminium-hydrogen peroxide battery. J. Electrochem. Soc. 1994, 141 (6), L61-L63. (6) Licht, S. A novel aqueous aluminium/permanganate fuel cell. Electrochem. Commun. 1999, 33-66.
3.1. Tafel and Cyclic Voltammetry Plots. Tafel and cyclic voltammetry plots for S2O82-/SO42- on the carbon-felt electrode in 1.5 M ammonium persulfate solution are shown in Figures 1 and 2, respectively. A platinum sheet with a large surface area was used as the counter electrode and a 1-cm2 carbon felt was used as the working electrode, with the back of the carbon-felt electrode exposed to the solution for both Figures 1 and 2. Figure 1 shows that the difference value of the two electrode potentials obtained by scanning in both positive and negative
10.1021/ef0602628 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/15/2007
Aqueous Zinc-Persulfate Battery
Figure 1. Tafel plot for S2O82-/SO42- on a carbon-felt electrode in 1.5 M ammonium persulfate solution: initial E, 0 V; final E, 2 V; segment, 2; scan rate, 0.01 V s-1.
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Figure 3. Discharge characteristics of Zn-(NH4)2S2O8 batteries with various ammonium persulfate concentrations in the cathode zone and 5 M ammonium chloride in the anode zone. The concentration of ammonium persulfate is as follows: (a) 1; (b) 1.5; (c) 2 M.
of the batteries were much smaller than their standard electromotive force. The difference values between E and E° were caused by the lower electrode potentials of ammonium persulfate because the electrode potential of ammonium persulfate was slightly larger than 1.0 V vs saturated calomel electrode (SCE) by measure. But, the electrode potential was not consistent with that obtained from the Tafel plot (see Figure 1) and was approximately the mean of the two electrode potentials obtained from Figure 1. Figure 3 shows the discharge characteristic of Zn-(NH4)2S2O8 batteries with various ammonium persulfate concentrations in the cathode zone and 5 M ammonium chloride in the anode zone. The discharge current can be calculated from the following formula: Figure 2. Cyclic voltammetry plot for S2O82-/SO42- on a carbon-felt electrode in 1.5 M ammonium persulfate solution: initial E, 2 V; high E, 2 V; low E, 0 V; segment, 2; scan rate, 0.1 V s-1.
directions was very large. This means that the reversibility of the ammonium persulfate electrode was very poor. Figure 2 shows that the ammonium persulfate electrode could be polarized very easily. So when ammonium persulfate is discharged on the carbon-felt electrode, there will be a very high overpotential. 3.2. Discharge Characteristics of Zn-(NH4)2S2O8 Batteries with Various Ammonium Persulfate Concentrations. For Zn-(NH4)2S2O8 batteries with 5 M ammonium chloride in the anode zone and 1, 1.5, and 2 M ammonium persulfate in the cathode zone, their electromotive forces or open circuit voltage E (measured by voltmeter) were 2.042, 2.092, and 2.040 V, respectively. The cathode reaction of the batteries was
S2O82- + 2e ) 2SO42- φ° ) 2.01 V The anode reaction of the batteries was
Zn ) Zn2+ + 2e φ° ) -0.763 V The standard electromotive force E° of the batteries is E° ) 2.01 V - (-0.763 V) ) 2.773 V. So the electromotive forces
I(A) )
V(V) - 0.006(V) 5(Ω)
where V is an ordinate in Figure 3, (V - 0.006) is the output voltage of the batteries, and 0.006 V is an instrumental correction factor. Figure 3 indicates that the output voltages of the batteries are 0.7-0.8 V smaller than their electromotive forces and are approximately 1.2-1.3 V. Voltage of more than 200 mV of this 700-800 mV was lost through the membrane and solution, with the remainder lost at the carbon-felt cathode; no voltage loss occurred on the zinc electrode (the potential losses in different parts of the cell were estimated by two saturated calomel electrodes inserted in two salt bridges which were placed in two side of membrane, respectively). So, the polarization potential of ammonium persulfate is approximately 0.5 V (vs normal hydrogen electrode (NHE)) at the present discharge current. When the carbon-felt electrode was replaced with a platinum one with the same surface area, the overpotential of ammonium persulfate increases instead of decreasing. Thus, such high overpotential perhaps relates to the larger molecular weight and nonmetal character of (NH4)2S2O8. Figure 3 also shows that when the concentrations of ammonium persulfate increase, the discharge time of the battery increases remarkably. When the concentrations of ammonium persulfate are larger than 1.5 M, its discharge time is longer than 20 h and the discharge current becomes very flat. When
1094 Energy & Fuels, Vol. 21, No. 2, 2007
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Figure 4. X-ray powder diffraction pattern of crystals formed (the sample was dried at 60 °C).
the concentration of ammonium persulfate reaches 1.5 M, a few crystals form on the bottom edge of the carbon-felt cathode and membrane on the side of the cathode zone after discharge. When the concentration of ammonium persulfate reaches 2.0 M, more crystals form on the membrane, especially on the bottom edge of the membrane on the side of the cathode zone, but a few crystals form on the bottom edge of the carbon-felt cathode. The X-ray powder diffraction pattern shows that these crystals are a mixture of (NH4)2Zn(SO4)2‚6H2O, (NH4)2Zn(SO4)2, (NH4)2SO4, and (NH4)3H(SO4)2 (see Figure 4). (NH4)2Zn(SO4)2 is the major constituent, and the others are minor ones. The larger concentration of ammonium persulfate and more Zn2+ (after longer time discharge) are the cause of the formation of (NH4)2Zn(SO4)2 because the formation of (NH4)2Zn(SO4)2 needs more Zn2+ and (NH4)2S2O8. The discharge current decreases sharply at the end of the discharge for curve c in Figure 3. The reason for this is that not all of the ammonium persulfate has been exhausted, but membrane holes have been blocked by the crystals formed. This blocking of membrane holes will affect the use of the membrane once again. Thus, preventing crystals from forming is a problem we must investigate further. 3.3. Discharge Characteristics of Zn-Na2S2O8 Batteries with Varying Na2S2O8 Concentrations. Figure 5 shows the discharge characteristics of Zn-Na2S2O8 batteries with 5 M ammonium chloride in the anode zone and 1-2 M sodium persulfate in the cathode zone. The electromotive forces of these batteries are 1.930, 1.958, and 1.871 V, respectively. It is obvious that the electromotive forces of zinc-sodium persulfate batteries are smaller than those of zinc-ammonium persulfate batteries. Figure 5 indicates that the output energy of batteries increases remarkably on increasing the concentrations of sodium persulfate. The discharge current decreases sharply at the end of discharge for curve c in Figure 5. The reason for this is not that the sodium persulfate has been exhausted but that membrane holes have been blocked by crystals formed because when the concentration of sodium persulfate is 1 or 1.5 M, there are no crystals on the membrane and carbon-felt cathode after discharge; when the concentrations of sodium persulfate is 2 M, there are more crystals on the membrane on the side of the cathode zone. 3.4. Comparison of the Discharge Characteristics of Zn(NH4)2S2O8 and Zn-Na2S2O8 Batteries. Figure 6 shows the
Figure 5. Discharge characteristics of Zn-Na2S2O8 batteries with various sodium persulfate concentrations in the cathode zone and 5 M ammonium chloride in the anode zone. The concentrations of sodium persulfate were as follows: (a) 1; (b) 1.5; (c) 2 M.
discharge characteristics of zinc-sodium persulfate and zincammonium persulfate batteries with 5 M ammonium chloride in the anode zone and 2 M persulfate in the cathode zone. The output energy of the zinc-sodium persulfate battery is slightly greater than that of the zinc-ammonium persulfate battery. The reason for this is that crystals form slightly more difficultly in the zinc-sodium persulfate battery. Sodium persulfate is much more expensive than ammonium persulfate. Thus, it is more economical to use ammonium persulfate. 3.5. Discharge Characteristics of Zn-(NH4)2S2O8 Batteries with Various Ammonium Chloride Concentrations. Figure 7 shows the discharge characteristics of Zn-(NH4)2S2O8 batteries with 2 M ammonium persulfate in the cathode zone and 4-6 M ammonium chloride in the anode zone; 6 M exceeds the solubility of ammonium chloride. Hence, ammonium chloride was not completely dissolved. But, it was completely dissolved a moment after discharge. It is obvious that ammonium chloride was consumed by Zn2+ and zinc-ammonium complex ions were formed after discharge. It is interesting to note that the discharge time of Zn(NH4)2S2O8 batteries increases with increasing ammonium
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Figure 6. Discharge characteristics of (a) Zn-(NH4)2S2O8 and (b) ZnNa2S2O8 batteries with 2 M persulfate in the cathode zone and 5 M ammonium chloride in the anode zone.
Figure 8. Discharge characteristics of Zn-(NH4)2S2O8 batteries with 2 M ammonium persulfate in the cathode zone and 5 M ammonium chloride in the anode zone at (a) 24 and (b) 40 °C.
Figure 7. Discharge characteristics of Zn-(NH4)2S2O8 batteries with 2 M ammonium persulfate in the cathode zone and (a) 4, (b) 5, and (c) 6 M ammonium chloride in the anode zone.
Figure 9. Discharge characteristics of Zn-(NH4)2S2O8 batteries with 1.5 M ammonium persulfate in the cathode zone and 5 M ammonium chloride in the anode zone at (a) 14 and (b) 24 °C.
chloride concentration in the anode zone. This confirms that the formations of crystals in the cathode zone relate to Zn2+. The larger the concentrations of ammonium chloride, the more zinc-ammonium complex ions form, and the fewer dissociative Zn2+ ions are present. The lower the level of dissociative Zn2 ions, the fewer crystals form. Thus, it is more difficult to stop membrane holes. In fact, when the concentration of ammonium chloride was 4 M, there were fewer crystals on the bottom edge of the carbon-felt cathode and there were many crystals on the membrane on the side of the cathode zone. When the concentration of ammonium chloride was 5 M, there were fewer crystals on the bottom edge of the carbon-felt cathode and there were few crystals on the membrane, especially, on the bottom edge of the membrane. When the concentration of ammonium chloride was 6 M, there were fewer crystals on the bottom edge of the membrane.
zone and 5 M ammonium chloride in the anode zone at 24 and 40 °C (Figure 8), (2) 1.5 M ammonium persulfate in the cathode zone and 5 M ammonium chloride in the anode zone at 14 and 24 °C (Figure 9), and (3) 1, 1.25, and 1.4 M ammonium persulfate in the cathode zone and 5 M ammonium chloride in the anode zone at 2-3 °C (Figure 10). After discharge, there were fewer crystals on the bottom edge of the carbon-felt cathode and there were many crystals on the membrane on the side of the cathode zone for curve a in Figure 8; there were no crystals on the carbon-felt cathode and there were many crystals on the membrane on the side of the cathode zone for curve b in Figure 8. Therefore, the increase of temperature from 24-40 °C did not prevent crystals from forming. But from Figure 8, we know that the increase of temperature made the discharge voltage and time increase, i.e., it made the output energy of the batteries increase. The reason for this is that the higher the temperature, the fewer crystals are formed. The situation for Figure 9 is similar to that for Figure 8. But, the curves in Figure 9 are not smooth. There were fewer crystals on the bottom edge of the carbon-felt cathode, and there were
3.6. Discharge Characteristics of Zn-(NH4)2S2O8 Batteries at Various Temperatures. The discharge characteristics of Zn(NH4)2S2O8 batteries at various temperatures are shown in Figures 8-10: (1) 2 M ammonium persulfate in the cathode
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Figure 10. Discharge characteristics of Zn-(NH4)2S2O8 batteries with (a) 1, (b) 1.25, and (c) 1.4 M ammonium persulfate in the cathode zone and 5 M ammonium chloride in the anode zone at 2-3 °C.
Figure 11. Calculation of energy density for a Zn-(NH4)2S2O8 battery with 2 M ammonium persulfate in the cathode zone and 5 M ammonium chloride in the anode zone at 40 °C.
more crystals on the membrane on the side of the cathode zone for curves a and b in Figure 9. After discharge, there were no crystals on the carbon-felt cathode and membrane for curves a and b in Figure 10, but there was a thin layer of crystals on the membrane on the side of the cathode zone for curve c in Figure 10. 3.7. Calculation of the Energy Density for the Zn(NH4)2S2O8 Battery. We calculate the energy density of the Zn-(NH4)2S2O8 battery according to the following reaction
Supposing that the density of the solvent is 1 and, thus, the mass of the solvent is 1.57624 kg, the mass of 2Zn + 2(NH4)2S2O8 + 5NH4Cl + solvent is
0.85461 + 1.57624 ) 2.43085(kg) Ignoring the mass of the electrolytic cell, the total mass of the battery is 2.43085 kg. Supposing that the output voltage of the battery changed linearly with time (see Figure 11),
V(V) ) -
Zn + (NH4)2S2O8 f resultant The mass of Zn+(NH4)2S2O8 is
65.38 g + 2 × 18.04 g + 2 × 32.06 g + 8 × 16 g ) 293.58 g ) 0.29358 kg Because E° ) 2.773 V, the theoretical gravimetric energy density of the Zn-(NH4)2S2O8 battery is
2.773 × 2 × 9.6485 × 10 ) 506.30(W h kg-1) 3600 × 0.29358 4
while the theoretical gravimetric energy density of a Pb-PbO2 battery is 170 W h kg-1. The actual gravimetric energy density of a Zn-(NH4)2S2O8 battery with 2 M ammonium persulfate in the cathode zone and 5 M ammonium chloride in the anode zone at 40 °C is calculated as follows: the mass of 2Zn + 2(NH4)2S2O8 + 5NH4Cl is
2 × 65.38 g + 2 × 228.2 g + 5 × (35.45 + 18.04) g ) 854.61 g ) 0.85461 kg The densities of Zn, (NH4)2S2O8, and NH4Cl are 7.13, 1.982, and 1.527, respectively. The volume of 2Zn + 2(NH4)2S2O8 + 5NH4Cl is
2 × 65.38/7.13 + 2 × 228.2/1.982 + 5 × (35.45 + 18.04)/1.527 ) 423.76(mL) 2Zn + 2(NH4)2S2O8 + 5NH4Cl is the matter of 10 batteries. So, the volume of solvent for 2Zn + 2(NH4)2S2O8 + 5NH4Cl is
2000 - 423.76 ) 1576.24(mL)
1.51 - 1.29 t(h) + 1.51 29.8
The actual gravimetric energy density of the Zn-(NH4)2S2O8 battery is
10 2.43085 10 2.43085
∫0
29.8
(V - 0.006)2 10 dt ≈ R 2.43085
- 1.29 t + 1.51) ∫029.8 51(- 1.5129.8
2
2
∫029.8 VR dt )
dt ) 48.15 (W h kg-1)
This is the maximal gravimetric energy density of a Zn(NH4)2S2O8 battery, while the actual gravimetric energy density of a Pb-PbO2 battery is 17 W h kg-1. The actual volumetric energy density of a Zn-(NH4)2S2O8 battery is
10 2
2
∫029.8 VR dt ) 58.52(W h dm-3)
Using the following formula (see Figure 12)
V(V) ) -
1.29 - 1.08 t(h) + 1.29 17.0
we can calculate the actual gravimetric energy density of a Zn(NH4)2S2O8 battery with 1.25 M ammonium persulfate in the cathode zone and 5 M ammonium chloride in the anode zone at 2-3 °C and the result obtained is 20.78 W h kg-1. This is the least gravimetric energy density of a Zn-(NH4)2S2O8 battery. The actual volumetric energy density of the battery is 23.93 W h dm-3. In summary, the most remarkable advantages of a Zn(NH4)2S2O8 battery are that both zinc and ammonium persulfate
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battery is very simple. The battery can be applied to outlying mountainous areas and pastures, as well as to other areas short of electricity. Some features of the battery make it superior in deep-sea applications. 4. Conclusions
Figure 12. Calculation of energy density for a Zn-(NH4)2S2O8 battery with 1.25 M ammonium persulfate in the cathode zone and 5 M ammonium chloride in the anode zone at 2-3 °C.
are cheap and friendly to the environment; the PE-01 homogeneous membrane is also cheap (its cost is only one-tenth of a Nafion membrane) and durable in ammonium persulfate solution; the discharge current of the battery is very smooth, and its energy density is not too low; and the structure of the
(1) The output energy of a zinc-persulfate battery increases remarkably on increasing the concentration of persulfate and ammonium chloride, as well as temperature. (2) The output energy of a zinc-sodium persulfate battery is greater than that of a zinc-ammonium persulfate battery, but ammonium persulfate is much cheaper. (3) If the concentration of ammonium persulfate is less than 1.25 M, crystals will not form in the cathode zone after discharge when the temperature is greater than 0 °C. (4) The crystals formed in the cathode zone after discharge are a mixture of (NH4)2Zn(SO4)2‚6H2O, (NH4)2Zn(SO4)2, (NH4)2SO4, and (NH4)3H(SO4)2. (NH4)2Zn(SO4)2 is the major constituent, and the others are minor ones. (5) The maximal and least actual gravimetric energy density of a Zn-(NH4)2S2O8 battery are 48.15 and 20.78 W h kg-1, respectively, while the actual gravimetric energy density of a Pb-PbO2 battery is 17 W h kg-1. EF0602628
