Relationship between Evaluation of Oer Performance and Oxygen Bubble Generation Behavior in Alkaline Water Electrolysis

Abstract

Introduction Alkaline Water Electrolysis (AWE) is anticipated as a mean of producing hydrogen as an energy carrier for sustainable development goals. One of the problems in alkaline water electrolysis is the rise in anode overvoltage at the high current density range. To further improve the efficiency of hydrogen production, grasping the bubble generation behavior and development of electrode materials based on it are important. In this study, using nickel electrodes with different diameters, the formation behavior of oxygen bubbles on the electrodes was observed with a high-speed video camera, and the relationship between the current density / electrode potential and the bubble formation behavior was investigated. Experimental Working electrode were Ni wire (NiW) (two types with diameters of 100 and 200 μm). A reference and a counter electrode were a reversible hydrogen electrode (RHE) and Ni rods (diameter 3 mm), respectively. All measurements were performed with a three-electrode electrochemical cell [1] at 303±1K with KOH aqueous solution. To reduce the solution resistance as much as possible, the working electrode and the reference electrode were placed as close as possible (about 1 mm). To equalize the current density distribution, Ni rods counter electrode were inserted from both sides of the cell. Observation windows were fitted on the sides of the cell. The working electrode was exposed only the horizontal part about 3 mm where we want to observe the electrolytic reaction, and the other parts were covered by PTFE heat-shrink tube and epoxy resin. After appropriate pretreatments, cyclic voltammetry (CV) measurement was performed at a scan rate of 5 mV s-1 to obtain polarization curves. Electrochemical impedance spectroscopy (EIS) measurement was performed at 1.5-1.8 V vs. RHE with a potential amplitude 10 mV, a frequency range of 1-106 Hz and 20 repeated measurements for each frequency. The solution resistance was obtained from the Nyquist plots, and the potential loss was corrected. The current density i geo / A cm-2 was normalized by the geometric surface area of the cylindrical shaped working electrode. Constant current electrolysis was performed under five conditions of i geo = 0.05-1.0 A cm-2. Under each condition, the formation behavior of oxygen bubbles was photographed with a high-speed video camera. From the obtained photographed images, the diameters of hundreds of oxygen bubbles detached from the electrodes were analyzed to calculate the Sauter mean diameter d 32. Results and discussion Figure 1 shows the polarization curves of the Ni wire electrodes with different diameters. In the range of i geo < 0.3 A cm-2 where the logarithmic value of the current density i geo changed linearly with respect to the potential, there was no significant difference in an OER performance among the two types of electrodes. In the range of i geo > 0.3 A cm-2, the overvoltage increased with i geo regardless of the electrode diameter, and it was confirmed that the smaller the diameter of the electrode was, the smaller the overvoltage at the same current density was, especially in the high current density range.Figure 2 shows the relationship between current density and overvoltage. It was confirmed that the overvoltage increased as the current density increased, and the smaller the surface curvature of the electrode was, the larger the overvoltage was.Figure 3 shows the relationship between the current density and oxygen bubble diameter. The larger the current density was, the larger the oxygen bubble diameter was, and the oxygen bubble diameter was in proportion to the 1/3 power of i geo. From the in-situ observation of the oxygen bubble generation behavior on the electrode with a high-speed video camera [2], the oxygen bubbles generated on the electrode coalesced and detached from the upper end of the Ni wire electrode placed horizontally. From the above, it was considered that the larger the surface curvature of the electrode was, i.e. the smaller the electrode diameter was, the smoother the detachment of oxygen bubbles from the electrode was. It is implied that large curvature of electrode surface contributed to the smaller overvoltage. 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] Y. Kojima, R. Misumi, M. Kaminoyama, S. Mitsushima, 43rd Electrolysis Technology Debate, 14 (2019)[2] H. Ikeda, Y. Kojima, R. Misumi, M. Kaminoyama, S. Mitsushima, 22nd Society of Chemical Engineers Student Presentation, D08 (2020) Figure 1