Relationship between Bubble Generation Behavior and Hydrogen Evolution Performance at High Current Densities during Alkaline Water Electrolysis

Abstract

Introduction Alkaline water electrolysis (AWE) has attracted considerable attention as a method to produce hydrogen. However, one of the problems with AWE is the increase in overpotential, which is attributed to bubbles covering the electrode surface at high current densities. Therefore, it is important to understand the bubble generation behavior to develop strategies for reducing the overpotential. In this study, we observed bubble generation behavior on Platinum (Pt) electrodes using a high-speed camera [1] and conducted electrochemical measurements to investigate the relationship between the current density/electrode potential and bubble generation behavior. Experiments Pt wires with diameters of 200 μm were used as the working electrodes. The reference and counter electrodes were a reversible hydrogen electrode (RHE) and Ni rods with a diameter of 3 mm, respectively. All measurements were conducted using a three-electrode electrochemical cell with a 2 M KOH aqueous solution as the electrolyte at 303±1K. To minimize the solution resistance, the working and reference electrodes were positioned as close as possible to each other, with a separation distance of approximately 2 mm. To ensure a uniform current-density distribution, a Ni-rod counter electrode was inserted on both sides of the cell. Observation windows were installed on the sides of the cells. Only a 3-mm parallel section of the working-electrode wire was exposed to the electrolyte to observe the electrolytic reaction, while the rest of the wire was covered with PTFE heat-shrink tube and epoxy resin. After appropriate pretreatment, cyclic voltammetry (CV) measurements were performed at a scan rate of 5 mV s-1 to obtain the polarization curves. Electrochemical impedance spectroscopy (EIS) measurements were conducted at −0.2 to −0.8 V vs. RHE with a potential amplitude of 10 mV, over a frequency range of 3×105 to 1 Hz, where 60 repeated measurements were performed for each frequency. The solution resistance was obtained from Cole–Cole plots and corrected for potential losses. The current density (i geo / A cm-2) was normalized by the geometric surface area of the cylindrically shaped working electrode. Constant current electrolysis was performed under twelve conditions of i geo = −0.05 to −1.5 A cm-2. Under each condition, the formation behavior of hydrogen bubbles was photographed using a high-speed video camera. Results and discussion Figures 1 and 2 show the polarization curves at the cathode and anode, respectively [2]. The overvoltage increased with i geo, as evidenced by the shift of the polarization curves from Tafel slope, in the range of −i geo ＜ 0.9 A cm−2 at the cathode and in the range of i geo > 0.6 A cm−2 at the anode. Figure 3 shows Cole–Cole plots obtained at i geo = −1.0 A cm−2 at the cathode and Figure 4 shows Cole–Cole plots obtained at i geo = 1.0 A cm−2 at the anode [2]. Figure 5 shows the hydrogen bubble generation behavior obtained at i geo = −1.0 A cm−2 at the cathode, and Figure 6 shows oxygen bubble generation behavior obtained at i geo = 1.0 A cm−2 at the anode [2]. The Cole–Cole plots indicate the resistance component on the low-frequency side at high current densities in addition to the charge transfer resistance. Moreover, the arc shapes of the resistance components differ between the anode and cathode, suggesting that different mechanisms are responsible for the increase in the overvoltage at high current densities. We believe that these resistances arise from reactant transfer limitations due to the hydrogen and oxygen bubble layers covering the electrode surfaces. Moreover, it was also confirmed that the charge-transfer resistance is inversely proportional to the current density, indicating that the effective electrode surface area does not change over a wide range of current densities. By comparing Figures 5 and 6, it was observed that oxygen bubbles at the anode where a larger overvoltage increase than cathode was measured had larger diameters and were less likely to detach from the electrode. Furthermore, the bubble layer formed on the electrode surface was thicker on the anode than on the cathode. Acknowledgements Part of this study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in the Advancement of Hydrogen Technologies and Utilization Project (JPNP14021), commissioned by the New Energy and Industrial Technology Development Organization (NEDO).