Zinc Air Battery Unit Cell

Zinc Air Battery Unit Cell

In order to produce a battery the two half-cell reactions (positive air and negative zinc) must be combined into a functional unit flow cell. One of the more significant challenges in the project was the ability to satisfactorily scale up the bi-functional air electrode to 600 cm2 (A4) while maintaining the desired in-cell performance. Much of the initial focus of was on establishing scalable air electrodes and then testing them in the unit cell before feeding into the stack development. The objectives for the initial battery integration were:

  • Design, construct and characterise a variety of bespoke unit cells to demonstrate the operation of the POWAIR system culminating in a significant scale up (electrode area ca. 600 cm2).
  • Select and test materials of construction.
  • Characterise the unit cell performance.
  • Validate the unit cell design for use in multi-cell stacks

The size of the unit cell was scaled progressively from a small (electrodes of 2 cm x 4 cm) cell with simplified inlet/outlet manifolds up to a stackable cell capable of incorporating electrodes with area of 600 cm2.

Given that the operating temperature is 60 oC and the electrolyte is 8 mol dm-3 NaOH or KOH, then the material selection of the cell components was a critical task. The selections at unit cell and stack scale were:

  • Cell frames A cheap and readily available thermoplastic, which is suitable for detailed injection moulding and welding with good chemical resistance to alkali: PP, HDPE
  • Bipolar electrodes carbon polymer Seals: EPDM or HypalonR.
  • Current collector: nickel plated brass or copper).
  • End plates: powder coated carbon steel or stainless steel.
  • Air electrode: nickel foam.

During scale-up of the air electrode, a number of architectures and materials were considered. Substrates for the catalyst layer included wove metal cloth, carbon paper, ion exchange membrane and metal foam. Nickel foam impregnated with catalyst paste was the final selection. This electrode architecture offered the best combination of electrochemical performance and chemical stability. An example of a 600 cm2 electrode is provided in figure 7.

Figure 7: 600 cm2 air electrode based on nickel foam impregnated with catalyst paste

The unit cell fitted with the Ni-foam air electrode showed stable charge/discharge performance with repeated cycling. Figure 8 shows the voltage vs. time plot for 14 x 1-hour charge periods along with associated discharge periods. The electrolyte initially comprised 0.5 mol dm-3 ZnO dissolved in 8 mol dm-3 KOH maintained at 60 oC and pumped through the cell at 7 cm3 s-1. A constant current density of 10 mA cm-2 was used for charge and discharge. This corresponds to over 24 hours continuous operation with a negligible drop in cell efficiency.

Figure 8: Charge/discharge cycling of the unit cell, demonstrating good efficiency with stable voltage and charge efficiencies.

The unit cell work demonstrated:

  • Confirmed operation of a 600 cm2 air electrode (using a spinel catalyst with better bi-functional performance than Pt-based electrode) within an operational Zn-air flow battery.
  • The unit flow cells have been cycled with several electrode architectures. Voltage and charge efficiencies greater than 60 % and 80 % respectively have been achieved on long-term cycling. Power densities up to 86 mW cm-2 have been achieved on discharge.
  • Ni foam based positive electrodes have shown best-to-date cycling behaviour within the flow cell. The larger-format version returning >100 hours in-cell operation without significant drop-off in performance.
  • The negative electrode performs with a charge efficiency over 80 %.
  • Cell cycling experiments have demonstrated that the combined negative (Zn deposition and dissolution) and positive (oxygen evolution and reduction) half-cells operate successfully as a zinc-air battery.
 

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