Zinc Air Flow Battery

Zinc Air Flow Battery

The objectives of this task were to develop an electrolyte, with high conductivity and energy density, that allows efficient zinc deposition and stripping. Additional requirements for the electrolyte were that it minimised self discharge on open circuit, be tolerant to carbon dioxide and compatable with the air electrode.

The major problem with the negative electrode in the zinc air flow battery is the dendritic zinc growth which can cause internal short circuits and the non-uniform zinc distribution during repeated charge/discharge cycles (shape change). First we investigated the physical properties like viscosity, conductivity, zinc oxide solubility, oxygen solubility and so on of different kind of electrolytes. On the basis of these finding a certain concentration of a base like KOH or NaOH was selected to dissolve the zinc oxide. The zinc deposition in different electrolytes was investigated in Rota Hull-cell experiments and Flow cell experiment. First the experiments were done without the use of additives, which can suppress dendrite formation then additive systems were evaluated. Therefore numerous deposition experiments in additive free KOH based electrolytes were performed Fig 1

Figure 1: Brass cylinder electrode: 30 minutes deposition 8M KOH/0,5M ZnO at 60C and90 rpm (2,8 cms-1)

At low current densities, filamentous mossy zinc was deposited and at higher current densities the formation of dendrites was observed. The mossy zinc has a high surface area that could be beneficial for the use in a battery, but unfortunately the adherence of such a layer is insufficient, thus material loss during discharging is inevitable. This results in a lower coulombic efficiency. The formed dendrites could lead to cell shorting and must therefore inhibited.

The addition of bismuth (Figure 2a) to the alkaline zincate electrolyte improves the zinc morphology. Only at current densities above 100 mAcm- dendrites are formed. Below 100 mAcm-, the layer was compact and dendrite-free. It was found that the best and the most effective dendrite suppressor is an organic inhibitor. The zinc deposit obtained from an electrolyte containing the organic inhibitor (Figure 2b) was compact and dendrite-free over the whole range of current densities.

Figure 2.a) 30 minutes deposition from 8M KOH/0.5M ZnO electrolyte at 60C and 90 rpm (2,8cms-1) with the addition of bismuth, b) 30 minutes deposition from 8M KOH/0,5M ZnO electrolyte at 60C and 90 rpm (2,8cms-1) with addition of an organic additive.

After the successful dendrite-free deposition with Rota-Hull cell, further investigations with the test flow cell were performed. Idea was to prolong zinc deposition time and to increase the coulombic efficiency through deposition morphology control at minimum required flow rate. A higher operating temperature is advantageous for the oxygen electrode in a zinc-air battery, but higher temperature also enhances the self- discharge. Therefore 60C was chosen for the operating temperature for the flow cell experiments, which should provide a compromise in working conditions for the negative zinc electrode and the positive air electrode in the battery.

In conclusion a zincate electrolyte was developed with

  • a very low rate of self discharge
  • low cost (10 $/kWh)
  • reasonable energy density (65 Wh/l)
  • dendrite free Zn deposition up to 100 mA cm- using stable additive system
  • high current efficiency during plating and stripping
 

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