Relationship between Resistance Due to Bubbles and Electrode Shape in Alkaline Water Electrolysis Cells
Authors: Nishio Yuki , Misumi Ryuta , Mitsushima Shigenori 
Published: December 2024

Introduction

An inherent challenge with alkaline water electrolysis (AWE) is the increased overvoltage due to bubbles covering the electrode surface at high current densities. Therefore, it is necessary to identify and quantify the effect of air bubbles on electrolysis voltage to develop strategies for reducing overpotentials. In this paper, we present a method that uses electrochemical measurements to quantify the bubble-induced overpotential relative to the total overpotential by dividing the voltage applied across the cell by the resistive component.

Experiments

A reversible hydrogen electrode (RHE) was used as the reference electrode and Ni was used as both the cathode and anode. Two electrode shapes, viz. wire (φ = 500 µm) and expanded metal (width: 30 mm; height: 4 mm), were used. The current density i
geo was normalized by dividing the input current value by the electrode area S
geo. In the setup, 2M KOH was used as the electrolyte, and the temperature of the solution was 30 °C. After pretreatment, cyclic voltammetry measurements were performed at a scan rate of 5 mV s −1. Electrochemical impedance spectroscopy measurements were performed with i
geo ranging from 0.03 to 1.2 Acm−2, an amplitude of 8%, and a frequency range of 1.25 × 10 5 to 1 Hz.The solution resistances were determined from Cole–Cole plots and corrected for potential losses.

Results and discussion

The cell voltage E
cell, composed of the theoretical decomposition voltage U
0, voltage iR
m due to the diaphragm and the solution and bubbles near the diaphragm, cathode overvoltage ηc , and anode overvoltage ηa , is expressed by equation (1):

E
cell＝U
0＋iR
m＋η
c＋η
a (1)

Here, ηc and ηa can be expressed as the sum of resistance due to electrochemical reaction and mass transfer resistance as shown in equations (2) and (3):

η
c＝η
el,c＋iR
c (2), η
a＝η
el,a＋iR
a (3)

where ηel represents the reaction overvoltage caused by the performance of the electrode catalysis and is calculated from the Tafel slope ΔT
f of the polarization curve and the exchange current density i
0 using equation (4):

log10
i
geo＝log10
i
0＋η
el /ΔT
f (4)

where R
c and R
a are the mass transfer resistances derived from the hydrogen and oxygen bubbles generated by the electrolytic reactions, respectively.

Fig. 1 shows the changes in the cell voltage and the corresponding resistance components with current density for the wire and mesh electrodes. Irrespective of the electrode shape, in the low current density region (|i
geo| < 0.03 Acm−2), U
0 and ηel mainly account for most of the cell voltage. As i
geo increased, the number of bubbles increased, leading to an increase in the proportion of resistance component derived from the bubbles in the cell voltage. Therefore, the calculated ratios ψ (= (iR
c + iR
a+ iR
m) / (E
cell− U
0 )) of the overvoltage, originating from the bubble and diaphragm (iR
c + iR
a+ iR
m), to the total overpotential (E
cell− U
0 ) at a high current density of | | = 1.2 Acm−2 were ψ
wire = 33.3% for the wire electrode and ψ
mesh = 51.2% for the expanded mesh. The overvoltage caused by the bubbles was greater for the mesh electrodes than for the wire electrodes. This suggests that for mesh electrodes, ψ increases due to entrapment of air bubbles in the electrode openings.

Based on equivalent circuit models [1, 2], the overvoltage (iR
c + iR
a+ iR
m) derived from the bubbles and diaphragm that contribute to the cell voltage E
cell is divided into ohmic resistance and diffusion resistance components. R
c and R
a are expressed as the sums of the ohmic resistance increases R
sb
,
c and R
sb
,
a in the solution between the bubbles near the electrode and the diffusion resistances R
bub (cathode) and R
d (anode) owing to the dense coverage of the bubbles on the electrode surface. Considering that R
m is the sum of the resistance R
mb due to the bubbles remaining near the diaphragm and resistance R
m0 of the diaphragm. We quantified the resistance derived from bubbles via equivalent circuit model analyses. The results indicate that at high current densities, the adverse effect of the resistance derived from bubbles (R
sb
,
c, R
sb
,
a, R
bub, R
d, R
m) is more on mesh electrodes than on wire electrodes.

Acknowledgments

A part of this study was supported by the New Energy and Industrial Technology Development Organization (NEDO, JPNP20003).

References

[1] H. Ikeda, R. Misumi, Y. Nishiki, Y. Kuroda, S. Mitsushima, Electrochimica Acta, 430,141053 2022

[2] D. Kitajima, R. Misumi, S. Mitsushima, 47th Electrolysis Technology Symposium, 11, 2023.

Figure 1