Bubble Management through Electrolyte Engineering to Reduce Bubble Induced Resistance in Zero-Gap Water Electrolyzers
Authors: Obata Keisuke , Qiu Huihang , Nagato Keisuke , Takanabe Kazuhiro 
Published: December 2024

Hydrogen production through water electrolysis will play a central role to obtain a sustainable energy carrier. Although the community targets to operate at high current density (>1 A cm−
2) to reduce the production cost, the intense operation forms massive gas bubbles leading to detrimental effects on the resultant cell voltage. Gas bubbles may reduce the electrochemical active surface area leading to an increase in kinetic overpotential. The voids in the electrolyte inhibit the ion conduction and the mass transfer. Surface modifications and structuring are often performed on electrodes to control the wettability.[1] In this study, we demonstrate bubble management through electrolyte engineering. Bubble detachment diameters measured in various electrolytes were analyzed through an analytical equation and multiphase fluid dynamics simulations to clarify the major contributions from the electrolytes.[2] The knowledge was further implemented in a homemade zero-gap water electrolyzer to reduce bubble induced ohmic losses.

Ni plate was employed as a model electrode to produce oxygen gas bubbles at 50 mA cm−
2. Electrolyte solutions were various concentrations of alkaline KOH and potassium-based buffer solutions (phosphate, borate, and carbonate) at non-extreme pH levels and room temperature. Bubble detachment diameters measured by a high-speed camera were found to continuously decrease in these electrolyte solutions by increasing the molality. After quantifying the electrolyte properties (density, surface tension and viscosity), the measured bubble diameters were compared with an analytical equation, which considers the force balance between the buoyancy force and the surface tension force on a textured surface,[3] to clarify the contributions from the electrolytes and the electrode surface to determine the diameters. The density of buffer solutions significantly increases in the concentrated solutions, which helps to reduce the diameter due to the buoyancy force. The deconvoluted surface contribution was also found to help reducing the diameter in the dense solutions. The electrode surface may be reconstructed during the oxygen evolution in the concentrated conditions, which was also inferred from contact angle measurements after the reactions. The analytical bubble detachment diameter is just derived from the force balance on a bubble at static condition. To cover the viscous force, which appears during the deformation of fluids, numerical simulations using volume of fluid method were performed. In viscous solutions and at high gas inlet assuming high current density, the diameters were found to increase indicating that the viscous force is working on the bubbles towards the electrode surface.

Having clarified the major roles from electrolytes in the bubble detachment diameter, ohmic resistance in a homemade zero-gap water electrolyzer was investigated (schematics in Figure 1). NiMoOx on Ni felt (0.2 mm thick), NiFeOx on Ni felt and hydrophilic polyethersulfone (PES, 0.14 mm thick, porosity of 80%) were used as a cathode, an anode, and a porous separator, respectively, in various molality of potassium carbonate solutions at pH 10.5 and 80 °C. Uncompensated resistances were obtained from high-frequency intersects in Nyquist plots obtained from electrochemical impedance spectroscopies at an open circuit voltage (OCV) and a potential at 800 mA cm−
2. A volcano trend of the resistance at OCV (blue bars in Figure 1) reflects the bulk electrolyte conductivity.[4] Notably, additional resistance (red bars in Figure 1) was observed at 800 mA cm−
2 indicating the inhibition of ion conduction due to the presence of massive gas bubbles in the stacked structure. The increment of resistance at 800 mA cm−
2 also exhibited a volcano trend. The ratio of resistances between OCV and 800 mA cm−
2 reflects how the effective ion conduction path was inhibited by the void fraction due to the gas bubbles.[5] The estimated void fractions also demonstrated the volcano trend as a function of the solute molality. Its decreasing trend agreed with the detachment diameters from the Ni plate model electrode discussed above indicating that the quantitative understanding of the bubble dynamics is also effective to reduce the ohmic losses.[2] Accumulation of gas bubbles were indeed observed by the high-speed camera using a transparent dialyzer separator which was closely stacked with NiMoOx/Ni cathode. Overall, our study demonstrates the significance of the electrolyte engineering to control the gas bubbles and the resultant losses.

Reference

[1] R. Iwata, et al., Joule, 2021, 5, 887.

[2] H. Qiu, et al., Langmuir, 2023, 39, 4993.

[3] J. Li, et al., Langmuir, 2022, 38, 3180.

[4] T. Nishimoto, et al., ACS Catal., 2023, 13, 14725.

[5] D. A. G. Bruggeman, Ann. Phys. 1935, 39, 4993.

Figure 1