Importance of Hydroxide Ion Conductivity Measurement for Alkaline Water Electrolysis Membranes

Abstract

Alkaline water electrolysis (AWE) refers to a representative water electrolysis technology that applies electricity to synthesize hydrogen gas without the production of carbon dioxide. The ideal polymer electrolyte membranes for AWE should be capable of transporting hydroxide ions (OH^-) quickly in harsh alkaline environments at increased temperatures. However, there has not yet been any desirable impedance measurement method for estimating hydroxide ions' conduction behavior across the membranes, since their impedance spectra are significantly affected by connection modes between electrodes and membranes in the test cells and the impedance evaluation environments. Accordingly, the measurement method suitable for obtaining precise hydroxide ion conductivity values through the membranes should be determined. For this purpose, Zirfon^®, a state-of-the-art AWE membrane, was adopted as the standard membrane sample to perform the impedance measurement. The impedance spectra were acquired using homemade test cells with different electrode configurations in alkaline environments, and the corresponding hydroxide ion conductivity values were determined based on the electrochemical spectra. Furthermore, a modified four-probe method was found as an optimal measurement method by comparing the conductivity obtained under alkaline conditions.

Keywords: Zirfon; alkaline water electrolysis; electrode configuration; hydrogen generation; hydroxide ion transport; impedance measurement; ion conductivity; polymer electrolyte membrane.

Figure 1

(a) H-type cell and (b) electric circuit: membrane immersed in a KOH condition. (V_H: voltage high; V_L: voltage low; I_H: current high; IL: current low; R_a: ohmic resistance of an alkaline solution; R_m: ohmic resistance of a membrane coupon; Ed: electrode to electrode distance.)

Figure 2

Zero gap cell employing a membrane immersed in a KOH condition (Ed: electrode to electrode distance).

Figure 3

(a) Four-probe cell and (b) electric circuit: the cell immersed in a KOH solution. (V_H: voltage high; V_L: voltage low; I_H: current high; IL: current low; R_a: ohmic resistance of an alkaline solution; R_m: ohmic resistance of a membrane coupon; Ed: electrode to electrode distance.)

Figure 4

(a) Modified four-probe cell and (b) electric circuit: the cell without immersing in a KOH solution only installed a wetting membrane. (V_H: voltage high; V_L: voltage low; I_H: current high; IL: current low; R_a: ohmic resistance of an alkaline solution; R_m: ohmic resistance of a membrane coupon; Ed: electrode to electrode distance.)

Figure 5

Hydroxide ion conductivity of aqueous KOH solutions as a function of temperature and concentrations.

Figure 6

Nyquist plot via the H-type cell in 30 wt.% KOH solution at 30 °C condition.

Figure 7

Nyquist plot via the zero gap cell in 30 wt.% KOH solution at 30 °C (a) with membrane, (b) without membrane.

Figure 8

Hydroxide ion conductivity measured using the 2-probe method in 30 wt.% KOH solution.

Figure 9

Hydroxide ion conductivity measured using the 4-probe method in 30 wt.% KOH solution immersing under 30 wt.% KOH condition.

Figure 10

Hydroxide ion conductivity measured using the 4-probe method in 30 wt.% KOH solution (a) immersing in 30 wt.% KOH condition (Figure 3 mode), (b) without immersing in 30 wt.% KOH solution (Figure 4 mode).

Figure 11

Hydroxide ion conductivity measured using the 4-probe method (Figure 4 mode) without immersing in a KOH condition only installed wetting membranes immersed in 1–8 M KOH before the measurement.