Minimizing bubble losses in membraneless flow-through water electrolysis

Abstract

In an effort to make water electrolysis more efficient and simple, flow-through electrolyzers eliminate the need for a gas-separating membrane between the electrodes. However, replacing the membrane with a forced flow field brings new challenges and considerations. In this research, the design of a flow-through electrolyzer is optimized for minimum voltage losses due to bubble formation, pressure drop, and electrolytic resistance. These parameters are all influenced by the potassium hydroxide concentration, the inter-electrode gap and the feed flow rate. The effect on the power dissipation of a flow-through electrolyzer is analytically modelled and validated using experiments, where the design variables are varied.

Additionally, the effect on the resistance in the electrolyte of keeping the gasses dissolved in the electrolyte is studied using IV curves of degassed electrolyte. The gasses in the electrolyte were purged with a vacuum chamber. This degassed electrolyte showed a reduction in overpotential of 150 mV at max. However, this reduction in overpotential was outweighed by the energy requirements to degas the electrolyte. Besides degassing, the effect of suppressing bubble formation by different flow rates was investigated. It was found that there was no noticeable reduction in overpotential between the state where bubbles are suppressed and bubbles were formed.

In addition to the effect of keeping the gasses dissolved, an analytical model was constructed to describe the necessary flow rates to mitigate the electrical resistance due to bubbles in the electrolyte. In the analytical model, solubility plays a big role in determining the necessary flow rate, as solubility is related to the emergence of bubbles. Contrarily, experiments showed the effect of solubility was found to be rather low. By varying the gap width, it was shown that the shear rate at the wall is a better indicator for bubble removal and therefore reduction in resistance due to bubbles.

Furthermore, the shape of the discharge channel was changed to promote uniform flow across the electrodes. The uniform flow should make the product removal at every part of the electrolyzer equal, such that there are no stagnant zones where gas accumulation could build up. This new discharge channel shape was analysed using COMSOL Multiphysics by comparing it with a conventional straight discharge channel. The variable discharge channel outperformed the straight discharge channel in creating a uniform flow across the electrodes for Euler numbers bigger than 10, meaning that the inertial forces are negligible compared to the pressure drop. During experiments, the electrolyzer with a variable discharge channel was tested. This electrolyzer configuration did not perform as well as expected from the theory and simulations, having a higher pressure drop and electric resistance than the conventional electrolyzer. The reason why the variable discharge channel performed poorly was inconclusive.

Lastly, the performance of the various variables was evaluated using the total power dissipation as a function of the current density. This took both the pressure drop across the system and the electrical power consumption of the electrolyzer into account. It was found that using a high electrolyte of 6M potassium hydroxide (KOH) together with a small inter-electrode gap gave the lowest energy dissipation per kg produced hydrogen gas. However, increasing the KOH concentration increases the viscosity and thereby the pressure drop. The theoretical optimum for KOH concentration was calculated to be 5M for this flow-through electrolyzer.