Evaluation of Leak and Reverse Current in a Bipolar Electrolyzer

Introduction In order to solve depletion of fossil fuels and global warming by CO 2 emission, the introduction of renewable energies such as solar and wind power has been promoted. Here, these renewable energies are fluctuated with uneven distribution. So the technologies of the energy conversion from renewable electric power to hydrogen by water electrolysis and the energy storage and transportation with hydrogen have been paid attention. With a perspective of practice use, alkaline and polymer electrolyte water electrolysis have potential to these applications. Especially, alkaline water electrolysis has advantage in large-scale energy system due to lower plant costs with common materials. In a bipolar type alkaline water electrolysis, electrolyte is fed via manifolds to anode or cathode chambers. Then leak circuits through ionic conduction are formed and it leads to several problems. During electrolysis, leak current flows through electrolyte in the manifolds, and decreases current efficiency. After electrolysis, reverse current, which flows through the bipolar plates in the opposite direction to that during electrolysis, leads to the degradation of electrodes 1) - 3) . However its mechanism has not been understood enough. Therefore, the mechanism should be understood to reduce these currents. In this study, we have investigated the relationship among the operating conditions of alkaline water electrolyzer, the cell voltage and leak or reverse current to clarify the mechanism. Experimental The electrolyzer consisted of two cells connected in a series with external manifold. The anodes and the cathodes were Ni mesh, and Nafion membranes (NRE212CS) were used as the separator. Projected area of the electrodes was 27.8 cm 2 . The electrolyte of 7.0 M (=mol・dm -3 ) NaOH solution was fed to electrode chambers at 25 ml・min -1 . The temperature of the electrolyte was controlled at 25 ℃. Leak current was measured during electrolysis in the current density region from 100 to 600 mA・cm -2 for 60 min. Electrolysis was stopped by opening the external circuit, and reverse current was measured. The electronic current through external circuit, the ionic currents through manifolds, and the cell voltages were measured to determine leak and reverse currents. Here, U 1 and U 2 were the cell voltages of the anode terminal side cell and the cathode terminal side cell, respectively. The ionic current was measured by DC milliampere clamp meter (KEW 2500). Results and discussion Figure 1 shows ionic currents through manifold (a) and the cell voltages (b) during and after electrolysis as a function of time. The leak current, during electrolysis over the first 60 min, increased with loading current density. Since the potential difference between the ends of the manifold increased with the terminal voltage of the electrolyzer, the leak current increased following Ohm’s law. At this time, the ratio of leak current to the loading current increased with the decrease of the loading current. Even if each cell voltage is lower than theoretical decomposition voltage, the sum of the cell voltages of the electrolyzer will be able to be larger than theoretical decomposition voltage. This moment, most of current flows through manifold, and the ratio of leak current is very large. After electrolysis, the reverse current flowed around 80 min for all loading current, and the current increased with loading current. The U 2 was about 1.3 V for 80 min while reverse current was observed, and then decreased to about 0.3 V regardless of loading current density while on the other hand U 1 was almost constant around 1.3 V. Since the state of the terminal electrodes could not change after electrolysis with open circuit, the decrease of the reverse current and the U 2 should result from the anode on the bipolar plate. Considering the electromotive force for reverse current, the possible redox should be reduction of the oxidized anode surface of NiOOH to Ni(OH) 2 or dissolved oxygen to OH - and oxidation of the reduced surface of Ni to Ni(OH) 2 or dissolved hydrogen to OH - . In these couples, only the combination of [NiOOH/Ni(OH) 2 ] and [H 2 /OH - ] should show 1.3 V. Therefore these reactions should be the electromotive force for the reverse current. Figure 2 shows electric charge of the reverse current as a function of the loading current density during electrolysis. The electric charge increased with loading current density. Since both U 1 and U 2 were independent from loading current density, the amount of oxide on the anode of bipolar plate increased with loading current density. References 1) WO 2013/141211 A1. 2) J. Divisek, J. Appl . Electrochem ., 20 , 186, (1990). 3) F. Hine, Handbook of Chlor-Alkari Technology, vol. 2 , p.394 (2005). Figure 1