Evaluation of Factors for Promoting Bubble Detachment from Anodes for Alkaline Water Electrolysis

Abstract

Introduction Toward the realization of a hydrogen society without any carbon dioxide emission, water electrolysis to produce hydrogen using renewable electricity is attracting attention. Alkaline water electrolysis (AWE) is one of the water electrolysis technology, which uses low-cost materials. To reduce energy consumption of the AWE, the electrolysis bubble around the electrodes, which inhibits mass transfer of reactant and limits the effective electrochemical surface area [1], should be removed effectively. In this study, as a basic evaluation of the effect of bubbles on the polarization of porous electrodes, we have investigated the effect of operation pressure and pore size on apparent activity of oxygen evolution reaction (OER) for Ni porous electrodes. Experimental Working electrodes were the Ni Celmet® (Sumitomo Electric Industries, Ltd., pore size of 1.95, 0.85, and 0.51mm, thickness of 3 mm). A counter and a reference were a Ni coil and a reversible hydrogen electrode (RHE), respectively. All measurements were performed with a three-electrode electrochemical cell at 303±1K with 7.0 M of KOH in a pressurized vessel.After pretreatment, the effect of pressure on the activity was evaluated with chronopotentiometry for pore size of 0.51 mm electrode in the current range from 50 to 1000 mA cm-2-geo under pressure range from ambient to 0.3 MPa. Also, the effect of pore size on the activity was evaluated with chronoamperometry for mean pore diameter of 1.95, 0.85, 0.51 mm electrodes in the potential range from 1.5 to 2.0 V vs. RHE under ambient pressure. In addition, we observed bubble behavior during electrolysis with the high-speed camera (Photron, FASTCAM Nova S12) in visualized three-electrode electrolyzer under ambient pressure. Results and discussion Figure 1 shows the chronopotentiogram as a function of logarithm of time under various pressure at 400 mA cm-2-geo. The visualization of bubble behavior under 0.1 MPa pressure showed that the bubble formed for 15 ms of electrolysis, because of charging of the electric double layer and dissolving of generated oxygen. After 500 ms of electrolysis, initial bubble detachment began, and the bubble detachment became steady after 1000 ms of electrolysis with increase of potential vibration. Under higher pressure, the trend of the chronopotentiograms was same, but the slopes decreased with the increase of pressure after 15 ms, and the potential increase was also smaller than that of 0.1 MPa.Figure 2 shows the steady state polarization curves and the potential increase from 15 ms to 20000 ms as a function of current of the CPs. The potential decreased with the increase of pressure, simultaneously the potential increase decreased with the increase of pressure. This suggests that pressurization reduced the flow resistance of the bubbles, accelerated their detachment, reduced the bubble volume fraction in the porous medium, and increased the effective surface area of an electrode.Figure 3 shows the decrease of current density during the chronoamperometry at 1.8 V vs. RHE as a function of charge passed. The visualization of bubble behavior showed that the timing of bubble generation was about 1, 3, and 7 mC cm-2-geo on the mean pore diameter of 1.95, 0.85, and 0.51 mm electrodes, respectively. Also, the timing of bubble detachment was 50, 90, and 110 mC cm-2-geo. The smaller the pore size electrodes were larger current decrease with later bubble formation. Bubble formation occurs after gas dissolution into the electrolyte on an electrode surface reaches local supersaturation [2]. Supersaturation of a smaller pore size electrode was higher than that of larger pore size one because of larger charge passed before bubble generation. If steady state supersaturation is irrelevant to the pore size, bubble occupancy of smaller pore electrode must become larger than that of larger pore electrode during relaxation period. Figure 4 shows the polarization curves and the current decrease during bubble generation to detachment as a function of potential. The current decrease increased with the decrease of the pore diameter. This suggests that the flow resistance of the bubbles is larger with smaller pore size electrodes, because the pore occupancy of bubble increases with decrease of the pore diameter. Acknowledgements A part of this study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Reference [1] S. Marini, P. Salvi, P. Nelli, R. Pesenti, M. Villa, Electrochim. Acta, 82, 384-391 (2012)[2] M. E. Tawfik, F. j. Diez, Electrochim. Acta, 146, 792-797 (2014) Figure 1