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Abstract
Alkaline water electrolysis (AWE) is a mature and technically viable technology for large-scale green hydrogen production and therefore plays a central role in global decarbonization strategies. Its ability to support net-zero transition, depends on continued reductions in capital cost, improvements in overall energy efficiency, and wider operable current-density range, which today remains limited to 0.8 A cm−2. Achieving these targets requires the development of advanced membrane materials with long-term stability, enhanced ionic conductivity, and suppressed gas crossover.
This thesis focuses on the development of next-generation membranes incorporating intrinsically porous fillers to strengthen polymer matrices and to introduce additional ion-transport pathways. Several classes of porous fillers, including ceramics and metal organic frameworks (MOFs) offer potential for such enhancements. Particular attention was given to MOFs, which have gained increasing prominence due to their modular tunability, extremely high surface area, and unlimited structural diversity enabled by distinct metal/linker combinations. Their micropores are expected to facilitate hydroxide-ion diffusion, while their chemical versatility allows post-synthetic modification to improve ion mobility within the membrane.
Two overarching questions guide this work: (i) whether membranes composed almost entirely of porous filler, with minimal polymer binder, can serve as alternatives to current separators, and (ii) whether MOFs can act as viable fillers for future alkaline membranes, provided they exhibit sufficient alkaline stability under harsh operating conditions. Considerations of scalability and cost were also addressed, particularly in view of ongoing efforts in green MOF synthesis, including solvent-free and energy-efficient routes.
This thesis investigates the alkaline stability, ionic conductivity, and membrane-integration potential of MOFs containing nitrogen-based linkers, focusing on Ni3(BTP)2 as the primary candidate. Initial screening revealed that carboxylate-based MOFs are unstable in strongly alkaline media: UiO-66 and UiO-66-NH2, despite their high connectivity, undergo complete degradation in 6 M KOH. In contrast, N-donor frameworks such as PCN-601 and Ni3(BTP)2 retain their crystallinity far longer, highlighting the superior basicity and strong M–N coordination of these systems. Ni3(BTP)2 was therefore studied in depth. A new low-temperature sol–gel synthesis route is developed, offering a cost-effective and scalable alternative to solvothermal synthesis and enabling the preparation of nanosized particles ideally suited for membrane integration. Optimisation of the modulator-to-linker ratio identifies an optimum value of 2.0, yielding Ni3(BTP)2 nanoparticles. When pelletized and assembled in a coin-cell configuration, these particles exhibit an ionic conductivity of 56 mS cm−1 in 6 M KOH at 80 °C. Long-term stability studies revealed that even the most robust Ndonor MOFs undergo gradual transformation into β-Ni(OH)2 under extremely harsh conditions. Nevertheless, the parent MOF structure still remains detectable for more than five months at 80°C and over seven months at 110°C, making Ni3(BTP)2 one of the most alkaline-resistant MOFs reported to date.
The second major component of this thesis involves the fabrication and evaluation of Ni3(BTP)2-based mixed matrix membranes (MMMs). Initial studies using PSU/PVP blends provided insight into casting behaviour and filler dispersion but showed insufficient ionic conductivity for electrolysis. PBI-based MMMs demonstrated improved performance: incorporation of Ni3(BTP)2 or UiO-66 significantly enhanced hydroxide-ion transport at lower KOH concentrations. At 5 wt.% KOH, the Ni3(BTP)2 MMM reached 9.5 mS cm−1 conductivity and 47.2 % KOH uptake, compared to 6.0 mS cm−1 and 22.7% for neat PBI.
The main focus then shifted to polyisatin-based MMMs, where nanosized sol–gel Ni3(BTP)2 enabled uniform dispersion and mechanically robust films. These MMMs outperformed both neat polyisatin and commercial Zirfon® 500. At 1 A cm−2, in 6 M KOH and at 80 °C, using a nickel catalyst coated expanded mesh electrode, the Ni3(BTP)2 MMM achieved a cell voltage of 1.814 V, corresponding to a voltage reduction of 131–186 mV relative to neat polyisatin, and a reduction of 48 mV relative to Zirfon® 500. When noble-metal electrocatalysts were used, the Ni3(BTP)2 MMM cell voltage further decreased to 1.647 V under the same operating conditions. Durability tests demonstrated stable operation until pinhole formation after 700 h, with performance decline correlating with increased gas crossover.
Beyond electrolysis, the membranes also demonstrate good performance in alkaline redox flow batteries, where incorporation of Ni3(BTP)2 increases the power density from 671 mW cm−2 to 1076 mW cm−2, and in electrodialysis, where the selective ion-sieving properties of the MOF effectively suppress Fe(CN)3− 6 crossover. These results highlight the multi-domain applicability of MOF MMMs across multiple energy-relevant technologies.
Overall, this work demonstrates that MOF MMMs, in particular Ni3(BTP)2, represent a promising and potentially transformative approach to improving the efficiency and sustainability of hydrogen production through alkaline electrolysis. Their combination of intrinsic porosity, tunability, and functional versatility offers unique opportunities for designing tailored ion-transport pathways. Although long term degradation occurs under harsh alkaline environments, high ionic conductivity, and processability of Ni3(BTP)2 highlight its strong potential for next-generation membrane technologies. Nickel-nitrogen MOFs in general appear to define a promising direction for future research in alkaline electrolysis. Their robust M–N coordination suggests that stability may be further enhanced by exploring higher framework connectivity. Isoreticular analogues with larger pore apertures should also be explored as they might provide more efficient hydroxide-ion transport. Overall, this thesis provides foundational insights into the design of MOF-based separators and their prospective role in enabling advanced membrane technologies, not least for the clean energy transition.
This thesis focuses on the development of next-generation membranes incorporating intrinsically porous fillers to strengthen polymer matrices and to introduce additional ion-transport pathways. Several classes of porous fillers, including ceramics and metal organic frameworks (MOFs) offer potential for such enhancements. Particular attention was given to MOFs, which have gained increasing prominence due to their modular tunability, extremely high surface area, and unlimited structural diversity enabled by distinct metal/linker combinations. Their micropores are expected to facilitate hydroxide-ion diffusion, while their chemical versatility allows post-synthetic modification to improve ion mobility within the membrane.
Two overarching questions guide this work: (i) whether membranes composed almost entirely of porous filler, with minimal polymer binder, can serve as alternatives to current separators, and (ii) whether MOFs can act as viable fillers for future alkaline membranes, provided they exhibit sufficient alkaline stability under harsh operating conditions. Considerations of scalability and cost were also addressed, particularly in view of ongoing efforts in green MOF synthesis, including solvent-free and energy-efficient routes.
This thesis investigates the alkaline stability, ionic conductivity, and membrane-integration potential of MOFs containing nitrogen-based linkers, focusing on Ni3(BTP)2 as the primary candidate. Initial screening revealed that carboxylate-based MOFs are unstable in strongly alkaline media: UiO-66 and UiO-66-NH2, despite their high connectivity, undergo complete degradation in 6 M KOH. In contrast, N-donor frameworks such as PCN-601 and Ni3(BTP)2 retain their crystallinity far longer, highlighting the superior basicity and strong M–N coordination of these systems. Ni3(BTP)2 was therefore studied in depth. A new low-temperature sol–gel synthesis route is developed, offering a cost-effective and scalable alternative to solvothermal synthesis and enabling the preparation of nanosized particles ideally suited for membrane integration. Optimisation of the modulator-to-linker ratio identifies an optimum value of 2.0, yielding Ni3(BTP)2 nanoparticles. When pelletized and assembled in a coin-cell configuration, these particles exhibit an ionic conductivity of 56 mS cm−1 in 6 M KOH at 80 °C. Long-term stability studies revealed that even the most robust Ndonor MOFs undergo gradual transformation into β-Ni(OH)2 under extremely harsh conditions. Nevertheless, the parent MOF structure still remains detectable for more than five months at 80°C and over seven months at 110°C, making Ni3(BTP)2 one of the most alkaline-resistant MOFs reported to date.
The second major component of this thesis involves the fabrication and evaluation of Ni3(BTP)2-based mixed matrix membranes (MMMs). Initial studies using PSU/PVP blends provided insight into casting behaviour and filler dispersion but showed insufficient ionic conductivity for electrolysis. PBI-based MMMs demonstrated improved performance: incorporation of Ni3(BTP)2 or UiO-66 significantly enhanced hydroxide-ion transport at lower KOH concentrations. At 5 wt.% KOH, the Ni3(BTP)2 MMM reached 9.5 mS cm−1 conductivity and 47.2 % KOH uptake, compared to 6.0 mS cm−1 and 22.7% for neat PBI.
The main focus then shifted to polyisatin-based MMMs, where nanosized sol–gel Ni3(BTP)2 enabled uniform dispersion and mechanically robust films. These MMMs outperformed both neat polyisatin and commercial Zirfon® 500. At 1 A cm−2, in 6 M KOH and at 80 °C, using a nickel catalyst coated expanded mesh electrode, the Ni3(BTP)2 MMM achieved a cell voltage of 1.814 V, corresponding to a voltage reduction of 131–186 mV relative to neat polyisatin, and a reduction of 48 mV relative to Zirfon® 500. When noble-metal electrocatalysts were used, the Ni3(BTP)2 MMM cell voltage further decreased to 1.647 V under the same operating conditions. Durability tests demonstrated stable operation until pinhole formation after 700 h, with performance decline correlating with increased gas crossover.
Beyond electrolysis, the membranes also demonstrate good performance in alkaline redox flow batteries, where incorporation of Ni3(BTP)2 increases the power density from 671 mW cm−2 to 1076 mW cm−2, and in electrodialysis, where the selective ion-sieving properties of the MOF effectively suppress Fe(CN)3− 6 crossover. These results highlight the multi-domain applicability of MOF MMMs across multiple energy-relevant technologies.
Overall, this work demonstrates that MOF MMMs, in particular Ni3(BTP)2, represent a promising and potentially transformative approach to improving the efficiency and sustainability of hydrogen production through alkaline electrolysis. Their combination of intrinsic porosity, tunability, and functional versatility offers unique opportunities for designing tailored ion-transport pathways. Although long term degradation occurs under harsh alkaline environments, high ionic conductivity, and processability of Ni3(BTP)2 highlight its strong potential for next-generation membrane technologies. Nickel-nitrogen MOFs in general appear to define a promising direction for future research in alkaline electrolysis. Their robust M–N coordination suggests that stability may be further enhanced by exploring higher framework connectivity. Isoreticular analogues with larger pore apertures should also be explored as they might provide more efficient hydroxide-ion transport. Overall, this thesis provides foundational insights into the design of MOF-based separators and their prospective role in enabling advanced membrane technologies, not least for the clean energy transition.
| Original language | English |
|---|
| Place of Publication | Kgs. Lyngby |
|---|---|
| Publisher | Technical University of Denmark |
| Number of pages | 258 |
| Publication status | Published - 2025 |
UN SDGs
This output contributes to the following UN Sustainable Development Goals (SDGs)
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SDG 7 Affordable and Clean Energy
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Dive into the research topics of 'Novel Separators for Alkaline Water Electrolysis'. Together they form a unique fingerprint.Projects
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Novel Separators for Alkaline Electrolysis
Ribeiro, R. (PhD Student), Jensen, J. O. (Main Supervisor), Chatzichristodoulou, C. (Supervisor), Kraglund, M. R. (Supervisor), Lohmann-Richters, F. P. (Examiner) & Singh, J. (Examiner)
01/12/2022 → 02/03/2026
Project: PhD
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