PHOTOVOLTAIC REDUCTION OF WASTE CATIONS FROM ZINC AIR FUEL CELLS

Abstract

This invention describes an affordable, renewable, sustainable (ARS) system to produce electricity, comprising a zinc air fuel cell (ZAFC) and photovoltaic cell. This ZAFC produces electricity operating in the pH range of 6.5-8.5 such that the primary waste species in the electrolyte are zinc cations of Zn.sup.+2 and ZnOH.sup.+. These cations are reduced by electrons from photovoltaic cells, and this ZAFC is easily rechargeable with a photovoltaic cell, and it can be overcharged. Also the electrolyte can be separated from the cell, and plating with photo reduction of zinc occurs rapidly on graphite, copper, or zinc cathodes. The efficiency of the ZAFC is approximately 58%, plus the added efficiency of reclaimed reduced zinc cations to zinc, either through recharging or plating. The electrolyte can be seawater or saline. The raw materials for this ZAFC are sufficiently available such that this system could provide ARS electricity.

Claims

1. A photovoltaic chemical reduction of a Zn.sup.+2 species and a ZnOH.sup.+ species within an electrolyte of a zinc air fuel cell.

2. The zinc air fuel cell of claim 1 is comprised of zinc, graphite, and seawater or saline.

Description

DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 shows the relative concentrations of zinc species in aqueous solution at differing pH.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The electrolyte for this ZAFC is seawater or 0.1M-1.0 M NaCl. Possible ORR at the cathode include:

TABLE-US-00003 O.sub.2 + 2H.sub.2O + 4e.sup.− .Math. 4OH.sup.− E = 0.40 V O.sub.2 + 4H.sup.+ + 4e.sup.− .Math. 2H.sub.20 E = 1.23 V O.sub.2 + 2H.sup.+ + 2e.sup.− .Math. H.sub.20.sub.2 E = 0.68 V H.sub.2O.sub.2 .fwdarw. H.sub.2O + ½ O.sub.2

[0019] Whether a single ORR occurs at the cathode or mixture, the net result will always be an increase in pH, either by the production OH.sup.− or consumption of H.sup.+. These reactions are not completely equivalent as the consumption of H.sup.+ produces water. This tendency to increase pH will be attenuated by the hydration of atmospheric carbon dioxide in the electrolyte.

[0020] Surrounding the anode plate with graphite increases the surface area, dramatically improving the efficiency of the cell as measured by voltage and pH. The more efficient ORR, the slower the maturation of the fuel cell to precipitation of Zn(OH).sub.2 and slower rising of pH. This may occur from changes in [Zn.sup.+2], which are coupled to changes in the ORR:

Zn.Math.Zn.sup.+2.Math.ZnOH.sup.+.Math.Zn(OH).sub.2

[0021] As the fuel cell matures, there is a rise in pH from approximately 6.5 to 8.5 and later to a pH of approximately 10. When this rise in pH occurs, the species of zinc metal ions in the electrolyte changes and often a milky precipitation appears. The relationship between pH and zinc species is described in FIG. 1. Also, small spacing between anode and cathode is beneficial for maximum efficiency.

Photovoltaic Reduction of Metal Ions

[0022] Zn.sup.+2 and ZnOH.sup.+ are primary metal cations in this ZAFC in the pH range of 6.5-8.5. Unexpectedly, these cations can be reduced with electrons from a photovoltaic cell. The ZAFC is easily rechargeable and can be overcharged. Also, zinc metal can be easily plated on graphite, copper, or zinc cathodes from samples of the electrolyte. Thus, the waste of this ZAFC can be readily reclaimed with energy from the sun, and the overall efficiency of electricity conversion is much high than an isolated ZAFC, which is approximately 58%.

[0023] Experimental and Theoretical Calculations

TABLE-US-00004 Anode: Zn .Math. Zn.sup.+2 + 2e.sup.− E = 0.76 V Cathode: O.sub.2 + 2H.sub.2O + 4e.sup.− .Math. 4OH.sup.− E = 0.40 V Overall: E = 1.16 V Initial E across 1K ohm resistor 0.816 pH = 6.8 Mature E across 1K ohm resistor 0.668 pH = 8.1 Efficiency = E observed/E theoretical 0.668/1.16 = 58%

Nernst Equation for the Anode Under Standard Conditions:

[0024]
aA+mH.sup.++ze.sup.−=bB+H.sub.20

E=E°+0.0591/z*log [A].sup.a/[B].sup.b−m/z*0.0591 pH

a=1; A=[Zn.sup.+2]; m=2; z=2; b=1; B=[Zn]

E=E°+0.0591/2*log [Zn.sup.+2]/[Zn]−0.0591*pH

[0025] As [Zn.sup.+2] decreases and pH increases, E will decrease, which describes the maturity of this ZAFC with ORR unchanged. However, reduction of Zn.sup.+2 and ZnOH.sup.+ with recharging or reduction of Zn.sup.+2 and ZnOH.sup.+ from the separated electrolyte and replacement of the electrolyte with seawater or saline will reestablish the function of the fuel cell to initial E. The reduction reactions are:

Zn.sup.+2+2e.sup.−.Math.Zn

ZnOH.sup.++H.sup.++2e.sup.−.Math.Zn+H.sub.2O

[0026] If plated, zinc can be incorporated into a new anode. Therefore, the overall efficiency of the system will exceed 58%.

Experimental Section

[0027] A ZAFC was constructed with zinc, graphite, and seawater or saline connected to a 1K ohm resistor. The initial pH and voltage of the cell was 6.9 and 0.816 which matured over time to 8.1 and 0.668 The cell was recharged with a 0.5 watt, 5 volt, 100 ma photovoltaic cell in sunlight to original conditions. The ZAFC can be easily overcharged with the photovoltaic cell to 2.35 V as measured across a 1K ohm resistor with a concomitant pH reduction to 6.8.

[0028] A graphite anode and graphite cathode, copper cathode, or zinc cathode was immersed in the electrolyte aspirated from the ZAFC, and the cations were reduced with a 0.5 watt, 5 volt, 100 ma photovoltaic in sunlight. In all instances, the plating of zinc onto the cathode was rapid. The solar reduction of Zn.sup.+2 and ZnOH.sup.+ to reclaimed zinc metal could be incorporated into a subsequent zinc anode of a ZAFC. The plating of the zinc species is presumed to follow Faraday's Law of Electrolysis:

m=(Q/F)(M/z)

m=mass of substance plated on the electrode
Q=total electric charge transferred
F=Faraday constant
M=molecular weight of substance
Z=electrons transferred per ion

Benefits to Society

[0029] Energy consumption per person continues to increase as more countries improve infrastructure and world population increases, leading to energy insecurity which is often a harbinger of conflict among nations. Coupling ZAFC electricity production with photovoltaic reduction of waste zinc species may provide a system of ARS electricity for the future.

REFERENCES

[0030] 1. Goldberg, J. S., Bhatt, A. M., ZINC AIR FUEL CELL FOR RENEWABLE AND STAINABLE ENERGY US 2022/0302485 A1. [0031] 2. Katsaiti, M., Papadogiannis, E., Dracopoul, V., et al, Solar charging of a Zn-air battery. Journal of Power Sources, 2023. 555.