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Hydrophobic membrane with nanopores developed for efficient energy storage
FEATURE
Hydrophobic membrane with nanopores developed for efficient energy storage Scientists in Germany and South Korea say they have improved a key component of energy storage systems that has the potential to aid their development. Their research shows that a membrane produced from a hydrophobic polymer of intrinsic microporosity can be used as a proton conducting separator in an all-vanadium redox flow battery with unprecedented infinite proton/vanadium selectivity. Storing energy from fluctuating sources and delivering a stable supply of electrical power are central issues when using renewable energy obtained from solar plants or wind-power stations. Here, efficient and flexible energy storage systems need to accommodate fluctuations in energy gain. Scientists from Germany’s Leibniz Institute for Interactive Materials (DWI), RWTH Aachen University, and Hanyang University, in Seoul South Korea have significantly improved a key component for the development of new, energy storage systems.
Efficient energy storage Redox flow batteries are considered a viable next-generation technology for highly efficient energy storage. They use electrolytes, chemical components in solution, to store energy. A vanadium redox flow battery, for example, uses vanadium ions dissolved in sulphuric acid. Being separated by a membrane, two energystoring electrolytes circulate in the system. The storage capacity depends on the amount of electrolyte and can be easily increased or decreased depending on the application. To charge or discharge the battery, the vanadium ions are chemically oxidised or reduced whilst protons pass through the separating membrane.
The current benchmark is a Nafion membrane. This membrane is chemically stable and permeable for protons, and is well known for its use in H2 fuel-cell applications. However, Nafion and similar polymers swell when exposed to water and loose their barrier function for vanadium ions. Polymer chemists try to prevent “vanadium leakage” by changing the molecular structure of such membranes.
Different approach The researchers from Aachen and Seoul came up with a completely different approach, as Prof. Dr.Ing. Matthias Wessling, Vice Scientific Director, Leibniz Institute for Interactive Materials and chair of Chemical Process Engineering at RWTH Aachen University explained. He said: ‘We use a hydrophobic membrane instead. This membrane keeps its barrier functions since it does not swell in water.’ ‘We were pleasantly surprised when we discovered tiny pores and channels in the hydrophobic material and they appear to be filled with water. These water channels enable protons to travel through the membrane with high speed. The vanadium ions, however, are too large to pass through the membrane.’ According to the researchers, the diameter of the channels is less than 2 nm and the barrier
Central role The membrane plays a central role in this system: on the one hand it has to separate the electrolytes to prevent energy loss by shortcircuiting, whilst on the other hand, protons need to pass through the membrane when the battery is charged or discharged. To enable efficient, commercial use of a redox flow battery, the membrane needs to combine both these functions which, so far, still remains a significant challenge for membrane developers. 8
Membrane Technology
function seems to be stable over time – even after one week or 100 charging and discharging cycles vanadium ions were not able to pass through the membrane. ‘We reached an energy efficiency of up to 99%, depending on the current. This shows that our membrane is a true barrier for the vanadium ions,’ added Professor Wessling. At all current densities tested, between 1 and 40 mA per square centimetre, the scientists reached 85% energy efficiency or more, whereas conventional systems do not exceed 76%.
Transport model The researchers say that the results suggest a new transport model. Instead of swelling, the polymer with intrinsic microporosity, named PIM, condensed significantly. Water molecules that accumulate in the pores, but not in the polymer itself, might be the reason for this phenomenon. The researchers hope to initiate further studies to analyse this effect in detail. The scientists are planning to perform additional application tests to find out whether or not they can improve the hydrophobic membrane to enable it to be used in a redox flow battery, and to investigate its long-term stability. Ultimately, the hydrophobic membrane might advance the practical use of redox flow batteries and similar energy storage systems. The researchers are highly motivated by the idea of a stable energy supply when using sustainable energy sources, by making a contribution to power system and frequency stability. Contact: Prof. Dr.-Ing. Matthias Wessling, DWI – Leibniz-Institut für Interaktive Materialien Ev, Forckenbeckstr. 50, D-52056 Aachen, Germany. Tel: +49 241 80 23179, Email: [email protected]
PhD student Tao Luo and postdoc Il Seok Chae are part of the research team that developed the new hydrophobic membrane with nanopores (photograph courtesy of DWI – LeibnizInstitut für Interaktive Materialien Ev).
(Further details of this research are presented in a paper entitled ‘Ultra-high proton/vanadium selectivity for hydrophobic polymer membranes with intrinsic nanopores for redox flow battery’, which appears in Advanced Energy Materials Vol. 6, Issue 16).
January 2017
Hydrophobic membrane with nanopores developed for efficient energy storage Scientists in Germany and South Korea say they have improved a key component of energy storage systems that has the potential to aid their development. Their research shows that a membrane produced from a hydrophobic polymer of intrinsic microporosity can be used as a proton conducting separator in an all-vanadium redox flow battery with unprecedented infinite proton/vanadium selectivity. Storing energy from fluctuating sources and delivering a stable supply of electrical power are central issues when using renewable energy obtained from solar plants or wind-power stations. Here, efficient and flexible energy storage systems need to accommodate fluctuations in energy gain. Scientists from Germany’s Leibniz Institute for Interactive Materials (DWI), RWTH Aachen University, and Hanyang University, in Seoul South Korea have significantly improved a key component for the development of new, energy storage systems.
Efficient energy storage Redox flow batteries are considered a viable next-generation technology for highly efficient energy storage. They use electrolytes, chemical components in solution, to store energy. A vanadium redox flow battery, for example, uses vanadium ions dissolved in sulphuric acid. Being separated by a membrane, two energystoring electrolytes circulate in the system. The storage capacity depends on the amount of electrolyte and can be easily increased or decreased depending on the application. To charge or discharge the battery, the vanadium ions are chemically oxidised or reduced whilst protons pass through the separating membrane.
The current benchmark is a Nafion membrane. This membrane is chemically stable and permeable for protons, and is well known for its use in H2 fuel-cell applications. However, Nafion and similar polymers swell when exposed to water and loose their barrier function for vanadium ions. Polymer chemists try to prevent “vanadium leakage” by changing the molecular structure of such membranes.
Different approach The researchers from Aachen and Seoul came up with a completely different approach, as Prof. Dr.Ing. Matthias Wessling, Vice Scientific Director, Leibniz Institute for Interactive Materials and chair of Chemical Process Engineering at RWTH Aachen University explained. He said: ‘We use a hydrophobic membrane instead. This membrane keeps its barrier functions since it does not swell in water.’ ‘We were pleasantly surprised when we discovered tiny pores and channels in the hydrophobic material and they appear to be filled with water. These water channels enable protons to travel through the membrane with high speed. The vanadium ions, however, are too large to pass through the membrane.’ According to the researchers, the diameter of the channels is less than 2 nm and the barrier
Central role The membrane plays a central role in this system: on the one hand it has to separate the electrolytes to prevent energy loss by shortcircuiting, whilst on the other hand, protons need to pass through the membrane when the battery is charged or discharged. To enable efficient, commercial use of a redox flow battery, the membrane needs to combine both these functions which, so far, still remains a significant challenge for membrane developers. 8
Membrane Technology
function seems to be stable over time – even after one week or 100 charging and discharging cycles vanadium ions were not able to pass through the membrane. ‘We reached an energy efficiency of up to 99%, depending on the current. This shows that our membrane is a true barrier for the vanadium ions,’ added Professor Wessling. At all current densities tested, between 1 and 40 mA per square centimetre, the scientists reached 85% energy efficiency or more, whereas conventional systems do not exceed 76%.
Transport model The researchers say that the results suggest a new transport model. Instead of swelling, the polymer with intrinsic microporosity, named PIM, condensed significantly. Water molecules that accumulate in the pores, but not in the polymer itself, might be the reason for this phenomenon. The researchers hope to initiate further studies to analyse this effect in detail. The scientists are planning to perform additional application tests to find out whether or not they can improve the hydrophobic membrane to enable it to be used in a redox flow battery, and to investigate its long-term stability. Ultimately, the hydrophobic membrane might advance the practical use of redox flow batteries and similar energy storage systems. The researchers are highly motivated by the idea of a stable energy supply when using sustainable energy sources, by making a contribution to power system and frequency stability. Contact: Prof. Dr.-Ing. Matthias Wessling, DWI – Leibniz-Institut für Interaktive Materialien Ev, Forckenbeckstr. 50, D-52056 Aachen, Germany. Tel: +49 241 80 23179, Email: [email protected]
PhD student Tao Luo and postdoc Il Seok Chae are part of the research team that developed the new hydrophobic membrane with nanopores (photograph courtesy of DWI – LeibnizInstitut für Interaktive Materialien Ev).
(Further details of this research are presented in a paper entitled ‘Ultra-high proton/vanadium selectivity for hydrophobic polymer membranes with intrinsic nanopores for redox flow battery’, which appears in Advanced Energy Materials Vol. 6, Issue 16).
January 2017