Hydrogen Crossover in PEM and Alkaline Water Electrolysis: Mechanisms, Direct Comparison and Mitigation Strategies

Abstract

This study provides a direct comparison of hydrogen crossover in PEM (Nafion 117) and alkaline water electrolysis (Zirfon) at a temperature of 60 • C applying state-of-the-art separating unit materials. To this end, occurring crossover mechanisms are described first, before experimental data of the anodic hydrogen content are shown in dependence of current density, system pressure and process management strategy. The results suggest that permeation in PEM electrolyzers is mainly governed by diffusion due to a supersaturated concentration of dissolved hydrogen within the catalyst layer, showing a share of 98% of the total permeation flux at 1 A cm −2 and atmospheric pressure. Permeation in alkaline electrolyzers also exhibits a significant influence of supersaturation, but the overall crossover is mainly influenced by mixing the electrolyte cycles, which makes up a share of 90% at 0.7 A cm −2 and 1 bar. Generally it becomes evident that hydrogen permeation across the separating unit is more than one order of magnitude smaller in alkaline electrolysis, which is mainly a consequence of the significantly lower hydrogen solubility in concentrated KOH electrolyte. Finally, this study concludes with an assessment of the impact of separating unit thickness and provides mitigation strategies to reduce hydrogen crossover.

The synthesis of a Zirfon‐type porous separator with reduced gas crossover for alkaline electrolyzer

“… where C is a geometric factor (m 5 ), ρ is the density of the liquid (kg m −3 ), γ is the surface tension (N m −1 ) between the liquid and vapor phases, η is the viscosity of the liquid (Pa·s), and t is the time (s). Generally, the value of C is determined by using a complete wetting (cos θ = 1) liquid (eg, C 6 ‐C 10 alkanes) for porous powders (Al 2 O 3 , SiO 2 , ZrO 2 , etc) . The prepared separators were composed of more than 75 wt.% of ZrO 2 nanoparticles.…”

“…Generally, the value of C is determined by using a complete wetting (cosθ = 1) liquid (eg, C 6 -C 10 alkanes) for porous powders (Al 2 O 3 , SiO 2 , ZrO 2 , etc). 28 The prepared separators were composed of more than 75 wt.% of ZrO 2 nanoparticles. Thus, octane was utilized as the complete wetting liquid in this study.…”

Numerical Investigations of Additively Manufactured Structures as Promising New Catalyst Supports with Adjustable Flow Characteristics

“…That way, transport and reaction processes in multiphase systems can be precisely controlled and significantly enhanced. In the field of heterogeneous catalysis, additively manufactured so-called periodic open cellular structures (POCS) as catalyst supports have shown their advantages over conventional randomly packed beds in terms of heat management [1] and pressure drop [2]. By inserting one POCS based on a diamond unit cell in the cavities of a second diamond cell structure, a completely new type of so-called interpenetrating POCS (interPOCS) is formed which enables in operando flow field and mass transport adjustments by shifting the (relative) position of the structures [3].…”

interPOCS – A sophisticated structured system for in operando flow control

“…This represents a challenge for conventional clarification and capture methods and leads to a bottleneck within the downstream process, which must be considered especially for large-scale production. A promising concept is the application of aqueous twophase extraction (ATPE) as a combined cell harvest and capture step [2].The integration of ATPE in a downstream process for monoclonal antibodies is demonstrated. A model is used that allows early prediction of settler dimensions with high prediction accuracy.…”

“…This represents a challenge for conventional clarification and capture methods and leads to a bottleneck within the downstream process, which must be considered especially for large-scale production. A promising concept is the application of aqueous twophase extraction (ATPE) as a combined cell harvest and capture step [2].…”