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Research Papers

Solar Thermal Electrolytic Process for the Production of Zn From ZnO: The Electrolysis of ZnO From 1275–1500 K

[+] Author and Article Information
R. Schroeder, L. Matthews, D. Leatzow, J. Kondratko, J. Will, S. Duncan, W. Sheline, N. Lindeke

Mechanical and Electrical Engineering Department,  Valparaiso University, Valparaiso, IN 46383

R. Palumbo1

Mechanical and Electrical Engineering Department,  Valparaiso University, Valparaiso, IN 46383

Pedro Neves

Solar Technology Laboratory,  Paul Scherrer Institute, 5232 Villigen PSI, Switzerland; Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland

1

Corresponding author.

J. Sol. Energy Eng 133(4), 041013 (Oct 18, 2011) (11 pages) doi:10.1115/1.4004706 History: Received January 13, 2010; Revised July 20, 2011; Published October 18, 2011; Online October 18, 2011

The solar thermal electrolytic production of Zn from ZnO was studied in the temperature range of 1275–1500 K in a cavity-solar receiver located at the focal point of a concentrating solar furnace. This study establishes how cathode material, solvent, current levels, and operating temperature influence the electrolytic cell’s performance. For a nominal current density of 0.1 A cm− 2 at temperatures from 1275 to 1425 K, we found that our performance parameters, the back work ratio and substituted-solar fraction, are within 25% and 20% of the ideal values, respectively. This behavior was true whether the cathode was Mo or W and whether the electrolyte was pure cryolite or a 35 mol. % cryolite-CaF2 mixture. When the electrolytes were cryolite-CaF2 mixtures in the temperature range of 1275–1425 K, there was no measurable difference in the performance, but at 1500 K with a MgF2 electrolyte, the performance dropped significantly. We have some evidence that the performance of the cell is better at current densities above 0.1 A cm− 2 when the cathode is Mo as opposed to W. Furthermore, the difference in the performance values can be attributed to higher kinetic over voltages associated with W versus Mo as a cathode. Our data also suggest that kinetic over voltages increase as the operating temperature increases. The experimental evidence suggests the reaction mechanism at the cathode for ZnO in cryolite involves a reaction between Na+  and ZnF2 , and the anode reaction involves a reaction between the anions Al2 OF6 2−  and ZnO2 2−  . Both Mo and W worked as cathode materials, but both the Mo and the W became brittle. Pt worked well as an anode without showing any evidence of degradation. Our SiC crucible may have suffered some carbothermic reaction with ZnO at temperatures exceeding 1275 K, with solvent mixtures of cryolite, CaF2 , and MgF2 .

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

A schematic of an ideal cycle for producing work from the electrothermal decomposition products of ZnO

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Figure 2

A schematic of the solar reactor used to study the electrothermal decomposition of ZnO. Key features are a blackbody cavity made of graphite for absorbing concentrated solar radiation and an electrolytic cell with SiC as the crucible. The gas phase products evolving at the cathode and anode are swept out of the reactor with Ar. They pass through mullite chimneys and the Zn is recovered downstream in a particle filter.

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Figure 3

A typical plot of current versus voltage for the decomposition reaction. The data in this plot correspond to the electrolytic decomposition of 4% by weight ZnO in 3NaF-AlF3 at 1275 K. The cathode was Mo and the anode Pt. Each data point is the average of 21 values.

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Figure 4

Current versus time for the steady state electrolysis of 4% by weight ZnO in 3NaF-AlF3 at 1275 K. Current levels are shown for various cell voltages. The cathode is W and the anode Pt. Each data point is the average of 21 values.

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Figure 5

The percentage of overvoltage versus normalized current for two temperatures that corresponds to irreversible electrode transport and kinetic processes. The cathode was Mo and the anode Pt. The electrolyte was 4 wt. % ZnO in 3NaF-AlF3 . Dividing the I/G ratio by 3.5 cm converts the x-variable to a nominal current density.

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Figure 6

ZnO back work ratios for three I/G ratios in the temperature range of 1250–1500

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Figure 7

A typical plot of back work ratio and substituted-solar fraction versus the I/G ratio. The data in this plot correspond to the electrolytic decomposition of 4% by weight ZnO in 3NaF-AlF3 at 1275 K. The cathode was Mo and the anode Pt.

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Figure 8

Uncertainty flow chart for the back work ratio, r

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Figure 9

Uncertainty flow chart for the substituted-solar fraction, f

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Figure 10

The uncertainty flow chart for the current normalized by the cell constant, j

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