The Key Role of Water Activity for the Operating Behavior and Dynamics of Oxygen Depolarized Cathodes

Abstract

Advanced chlor‐alkali electrolysis with oxygen depolarized cathodes (ODC) requires 30 % less electrical energy than conventional hydrogen‐evolution‐based technology. Herein, we confirm that the activities of hydroxide and water govern the ODC performance and its dynamics. Experimental characterization of ODC under varying mass transfer conditions on the liquid side reveals large differences in the polarization curves as well as in potential step responses of the electrodes. Under convective transport in the liquid electrolyte, the ODC is not limited by mass transfer in its current density at j>3.9 kA m−2, whereas transport limitations are already reached at j≈1.3 kA m−2 with a stagnant electrolyte. Since gas phase conditions do not differ significantly between the measurements, these results are in contrast the common assumption that oxygen supply determines ODC performance. A dynamic model reveals the strong influence of the electrolyte mass transfer conditions on oxygen availability and thus performance. Dynamic responses of the current density to step‐wise potential changes are dominated by the mass transport of water and hydroxide ions, which is by orders of magnitude faster with convective electrolyte flow. Without convective liquid electrolyte transport, a high accumulation of hydroxide ions significantly lowers the oxygen solubility. Thus, a fast mass transport of water and hydroxide is essential for high ODC performance and needs to be ensured for technical applications. The predicted accumulation of ions is furthermore validated experimentally by means of scanning electrochemical microscopy. We also show how the outlined processes can explain the distinctively different potential step responses with and without electrolyte convection.

Limiting Current Density of Oxygen Reduction under Ultrathin Electrolyte Layers: From the Micrometer Range to Monolayers

“…Although recent simulation work on ORR at catalysts in gas diffusion electrodes provides high current densities, being in good agreement with experimental results, even when assuming continuous thin electrolyte films in the range of 25–300 nm covering the catalyst particles and without considering uptake control, [51] this does not contradict the results reported here. In fact, for the same kind of electrode it was reported that the gas‐liquid interface is by far more complex than a homogenously thin film, rather being characterized by a high density of micro‐droplets, as was revealed by operando X‐Ray imaging [52] .…”

Improvement of Oxygen‐Depolarized Cathodes in Highly Alkaline Media by Electrospinning of Poly(vinylidene fluoride) Barrier Layers

“…Based on these findings, the authors draw the conclusion that a high electrode performance requires a good mass transport of water and hydroxide ions in the liquid phase as well as an extended gas‐liquid interface. The strong influence of water and ion transport on the performance of the electrodes was also confirmed experimentally . As a consequence, it is essential to achieve an optimized electrolyte boundary e. g. by a variation of the hydrophobic part of the GDE.…”

Revisiting Chlor-Alkali Electrolyzers: from Materials to Devices

“…Röhe et al [44] established a dynamic three-phase model of advanced chlor-alkali electrolysis porous ODC, which proved that water transport, especially that of hydroxide ions in the liquid electrolyte, is the overall limiting factor. The oxygen concentration is rapidly consumed near the gas-liquid interface [45]. Considering the non-ideal behavior of highly concentrated water, the water activity coefficient largely depends on ion concentration [46].…”