Relationship between Resistance Due to Bubbles and Electrode Shape in Alkaline Water Electrolysis Cells

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 η c , and anode overvoltage η a , is expressed by equation (1): E cell ＝ U 0 ＋ iR m ＋ η c ＋ η a (1) Here, η c and η a 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 η el represents the reaction overvoltage caused by the performance of the electrode catalysis and is calculated from the Tafel slope Δ T f of the polarization curve and the exchange current density i 0 using equation (4): log 10 i geo ＝log 10 i 0 ＋ η el /Δ T 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 lo