Separators and Membranes for Advanced Alkaline Water Electrolysis

Abstract

Traditionally, alkaline water electrolysis (AWE) uses diaphragms to separate anode and cathode and is operated with 5-7 M KOH feed solutions. The ban of asbestos diaphragms led to the development of polymeric diaphragms, which are now the state of the art material. A promising alternative is the ion solvating membrane. Recent developments show that high conductivities can also be obtained in 1 M KOH. A third technology is based on anion exchange membranes (AEM); because these systems use 0-1 M KOH feed solutions to balance the trade-off between conductivity and the AEM's lifetime in alkaline environment, it makes sense to treat them separately as AEM WE. However, the lifetime of AEM increased strongly over the last 10 years, and some electrode-related issues like oxidation of the ionomer binder at the anode can be mitigated by using KOH feed solutions. Therefore, AWE and AEM WE may get more similar in the future, and this review focuses on the developments in polymeric diaphragms, ion solvating membranes, and AEM.

Figure 1

Literature on the topic “water electrolysis”, and percentage of the subgroups “alkaline”, “alkaline & catalyst”, and “alkaline & membrane or diaphragm or separator” (web of knowledge, 2024-01).

Figure 2

Structural formula of polyphenylene sulfide (PPS).

Figure 3

(A) Viscosity of the casting suspension (18 wt % PSU/NMP + 10 vol % ZrO₂) as a function of the shear rate (stress-ramp experiment) and the sintering temperature of the zirconia particles. (B) Time dependent rheological behavior of 80 wt % ZrO₂+20 wt % PSU casting suspensions with sintered zirconia. Reproduced with permission from reference (65). Copyright 2006 American Chemical Society.

Figure 4

Published performances of AWE cells using diaphragms and uncoated electrodes. Details and references are listed in Table 1.

Figure 5

Published performances of AWE cells using diaphragm and coated Ni-electrodes. Details and references are listed in Table 2.

Figure 6

(A) (a) Cross sectional overview on membrane M21 indicating two distinct regions of the membrane, top layer and bulk; (b) schematic representation of membrane cross-section with distinct two microstructures (c) detailed cross sectional view on top layer; (d) bottom surface of the membrane M21; (e) top surface of membrane M21. (B) Variation of the total resistivity as a function of membrane thickness. (a) Schematic representation of top layer (Lt) and bulk microstructure (Lb) ratios for membranes of various thicknesses. (b) Influence of membrane thickness on total resistivity (M21). Reproduced with permission from reference (54). Copyright 2015, Elsevier.

Figure 7

(A) Cross-sectional SEM images of (a) Zirfon PERL and (b) fabricated separator Z85 from Seoultech. (B) Amount of (a) Zr 3d and (b) S 2p atomic percentage of the prepared separators from Seoultech and Zirfon PERL separator as a function of the etching time from 0 to 1220 s through XPS depth profile. (c) Contact angle of the prepared separators measured at room temperature in KOH 30 wt % solution. Reproduced with permission from reference (67). Copyright 2020, Elsevier.

Figure 8

Acid–base equilibrium of m-PBI and structure analogues thereof in alkaline environment.

Figure 9

Summary of non-PBI polymer systems that have been explored for alkaline ion-solvating membranes.

Figure 10

Degradation of AEM in alkaline conditions, reproduced with permission from reference (21). Copyright 2021, American Society of Mechanical Engineers.

Figure 11

(a) Dependence of conductivity on KOH concentration, reproduced from. (b) Conductivity, proton mobility, and proton concentration of Nafion 117 in sulfuric acid, (c) Conductivity of KOH solutions; (a) reproduced with permission from reference (121). Copyright 2021, Elsevier.

Figure 12

Development of polyimidazolium membranes and their main degradation pathway under alkaline conditions.

Figure 13

Half-lives of cationic model compounds in 3 M NaOD/D₂O/CD₃OD.

Figure 14

Preparation of poly(biphenyl alkylene)s by polyhydroxyalkylation and quaternization carried out by Bae et al.

Figure 15

PiperION by Versogen (a) and Orion CMX by Orion polymer (b) are two commercial AEMs prepared by polyhydroxyalkylations.

Figure 16

Examples of arene monomers with different functionalities used in polyhydroxyalkylations to prepare AEM materials.

Figure 17

Examples of ketone monomers with different functionalities used in polyhydroxyalkylations to prepare AEM materials.

Figure 18

Principal mechanism of the Friedel–Crafts type hydroxyalkylation reaction exemplified with a trifluoromethyl ketone and an arene monomer. If R₂ = Ph, the corresponding polyhydroxyalkylation results in a poly(biphenyl alkylene) in which R₁ may potentially contain a cation or a group that can be transformed into a cation. (H⁺A^– = TFSA = triflic acid, DCM = dichloromethane).

Figure 19

Examples of poly(arylene piperidinium)s with different arylene groups: (a) p-terphenyl, (b) 9,9-dimethylfluorene, (c) α,ω-diphenyl alkylene, (d) p,p-quaterphenyl, (e) m,p,m-quinquephenyl, (f) 1,6-diphenylpyrene, (g) 1,1′-binaphthyl, (h) dibenzo-18-crown-6, and (i) 9-ethylcarbazole.

Figure 20

Examples of poly(arylene piperidinium)s with modified piperidinium cations: (a–e) N-spirocyclic cations, (f) N-tethered hydrophilic side chain, and (g–l) various N-tethered cationic side chains.

Figure 21

Poly(arylene piperidinium)s with alkyl side chains (a and b) and branching sites (c).

Figure 22

Poly(arylene piperidinium) AEMs cross-linked by (a) multicationic cross-links via Menshutkin reaction, (b) thermally activated reaction of styrenic groups during membrane casting, and (c) cross-linking by a blending approach using a bromoalkylated SEBS.

Figure 23

Poly(arylene alkylene)s with backbones based on (a) bi- and quaterphenyl, (b–f) fluorene, and (g) carbazole, all tethered with QA cations via flexible alkyl chains.

Figure 24

Poly(arylene oxindole)s tethered with QA cations via flexible alkyl chains.

Figure 25

Poly(arylene alkylene)s with (a) xanthene backbones, (b) benzylic QA cations, (c) cationic side chains prepared by ATRP, (d) different backbone configuration and QA placement, and (e) protected imidazolium cations in the backbone (ionene), and (f) bis-piperidinum cations in the backbone (ionene).

Figure 26

Styrene-based polymers and synthesis of AEMs. QA: Quaternary ammonium.

Figure 27

Gas impurities in dependence of current density at solar operation electrolysis within 1 day (24 June 1993). Ten kilowatt alkaline electrolyzer: 20 cells, membranes on the basis of polysulfone are used as separators. Reproduced with permission from reference (31). Copyright 1996 Elsevier, Inc.

Figure 28

Volumetric permeation flux density of electrolyte through Zirfon sample as a function of the absolute pressure difference at a cell temperature of 80 °C. Reproduced with permission from reference (348). Copyright 2016, The Electrochemical Society.

Figure 29

Length swelling of different AEM (chloride form) and Nafion from dry to wet state at 30 °C, then to wet state at 60 °C, and after cooling to room temperature after 1 h, 1 week, 2 weeks and 20 weeks. PI-15 and PI-20 are PiperION membranes from Versogen, and FAA3-PK-75, PI-15 and PBI/mTPN membranes are reinforced.

Figure 30

(A) Sketch of the CCM fabrication. 1: Preparation of the catalyst slurry. 2: Cutting the PVC template and assembly on the Zirfon to define the electrode geometry. 3: Blade-coating of catalyst slurry on the diaphragm (Zirfon). 4: Drying the CCM under ambient conditions, removing the PVC template from the Zirfon substrate and hot drying at 80 °C in a furnace under air conditions. 5: Cutting the electrodes with a punch machine into seven separate electrodes. 6: Leaching the aluminum in 32 wt % KOH solution and sodium tartrate at 80 °C for 24 h. 7: Mounting in an alkaline single cell for electrochemical characterization on the cathode side. (B) Polarization curves of zero-gap and nonzero-gap electrodes based on Raney nickel compared to the benchmark and CCM with optimum loading. Reproduced with permission from reference (74). Copyright 2022, The Electrochemical Society.