X-Stack w. FF

The X-stack FF can be used for different electrolyser technologies. Redox-flow.com has tested the stack as an alkaline water electrolyser with the following configuration

• 6 cells with 25 cm² active area/electrodes in each cell
• Ni bipolar plates
• 25 mm Ni foam electrodes
• Zirfon UTP 500+ separator
• 6 M KOH – 500 mL (one common electrolyte container for both sides)

1 EPDM Flow field gasket with 1 mm thickness and 2 PTFE Cover gasket with 0.25 mm thickness were used in each half-cell. As Nickel felt electrodes are almost incompressible it is important to combine gaskets that match the electrode thickness. To seal the porous Zirfon separators 0.5 mm PTFE porous separator gaskets were used (to prevent leaking through the separator to the outside of the stack)

Due to the porous nature of the Zirfon separator two separate electrolyte containers cannot be used as even very small pressure differences between the two sides will result in liquid cross-over through the membrane. For this reason, only one common electrolyte container is used.

The hydraulic circuit consisted of ¼’’ PTFE tubes, one bottle and one heating unit placed right after the pump and just before the stack inlet.

First graph below shows initial pressure loss test in the stack with pure water at RT. It was measured using Redox-flow pressure unit, right before stack hydraulic inlet and outlet.

As seen the pressure loss is very small, in particular this is due to the small thickness of the electrode and the interdigitated flow fields on the bipolar plates. In this case the total pressure loss inside the stack, consists of small contributions from the flow channels in the PEEK flow body, stack manifold, shunt channels before the electrodes and the electrodes by them selves,

Although the hydraulic tests were performed with water, the pressure loss should not be significantly different for other electrolytes (and temperatures).

For performance test polarisation curves at both 25° C and 80° C were performed with electrical currents up to 10 A (400 mA/cm2) and 200 mL/min flow rate (33 mL/min per cell). They were recorded by applying constant current for 30 s, and then immediately shifted to the next current. The voltage for the polarisation curve was obtained by averaging the data points of the last 5 s of the constant current measurement. The stack and electrolyte were used ‘as is’ i.e. without preconditioning/pre-electrolysis of the stack.

Graph below shows the results of the test.

As seen from the polarisation curves the performance is significantly better (as expected) for the 80° C experiment compared to the 25° C (i.e. the voltage is almost 2 V lower at 400 mA/cm²). For comparison the X-stack Flat (with 1.8 mm Nickel foam electrode) polarisation curve measured at RT is also included. Interestingly the performance of the Nickel felt is better than that of the foam. In any case these test have not been performed for optimisation, but to test basic functionality of the stack. The 400 mA/cm² corresponds to 10 A, whereby the total electrical power supplied to the stack is more than 100 W. In present case current was limited by power supply, but higher electrical currents can be supplied to the stack.

The heating of the stack was done with Redox-Flow.com heating unit. The temperature of the stack was measured with a thermometer in the thermometer holder inside the stack. The temperature profile when the stack and electrolyte is heated from 25 C to 80 C is shown in the graph below.

It is seen that the stack and 500 mL electrolyte reaches the 80° C  setpoint within less than 45 min and underlines the efficiency of the heating unit.