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Promising hydrophilic membranes with fast and selective ion transport for energy devices

In addition to well-established Nafion™ membranes which are currently the best trade-off between high-performance and cost in proton exchange fuel cells (PEM), methanol fuel cells, electrolysis cells etc. As our energy resources are diversifying, there is a growing demand for efficient and selective ion-transport membranes for energy storage devices such as flow batteries.

A Sumitomo Electric flow battery for energy storage of a solar PV plant. (Photo: Sumitomo Electric Co.)

Redox flow batteries – the energy storage breakthrough

The high demand for a reliable and cost-effective energy storage systems is reflected in the increased diversity of technologies for energy storage. Among different electrochemical storage systems, one of the most promising candidates are redox-flow batteries (RFBs). They could meet large-scale energy storage requirements scoring in high efficiency, low scale-up cost, long charge/discharge cycle life, and independent energy storage and power generation capacity.

Since this technology is still young, the development of commercially and economically viable systems demands:

  • improvement of the core components e.g. membranes with special properties,
  • improvement of energy efficiency
  • reduction in overall cost system.

Meeting demands for redox flow batteries

Two research teams in the United Kingdom, one from Imperial College and the other from the University of Cambridge, pursued a new approach to design the next generation of microporous membrane materials for the redox-flow batteries. They recently published their data in the well renown journal Nature Materials. Well-defined narrow microporous channels together with hydrophilic functionality of the membranes enable fast transport of salt ions and high selectivity towards small organic molecules. The new membrane architecture is particularly valuable for aqueous organic flow batteries enabling high energy efficiency and high capacity retention. Importantly, the membranes have been prepared using roll-to-roll technology and mesoporous polyacrylonitrile low-cost support. Hence, these innovative membranes could be cost effective.

As the authors reported, the challenge for the new generation RFBs is development of low-cost hydrocarbon-based polymer membranes that features precise selectivity between ions and organic redox-active molecules. In addition, ion transport in these membranes depends on a formation of the interconnected water channels via microphase separation, which is considered a complex and difficult-to-control process on molecular level.

The new synthesis concept of ion-selective membranes is based on hydrophilic polymers of intrinsic microporosity (PIMs) that enable fast ion transport and high molecular selectivity. The structural diversity of PIMs can be controlled by monomer choice, polymerization reaction and post-synthetic modification, which further optimize these membranes for RFBs.

Two types of hydrophilic PIM have been developed and tested: PIMs derived from Tröger’s base and dibenzodioxin-based PIMs with hydrophilic and ionizable amidoxime groups.

The authors consider their approach innovative because of

  1. The application of PIMs to obtain rigid and contorted polymer chains resulting in sub-nanometre-sized cavities in microporous membranes;
  2. The introduction of hydrophilic functional groups forming interconnected water channels to optimize hydrophilicity and ion conductivity;
  3. The processing of the solution to produce a membrane of submicrometre thickness. This further reduces ion transport resistance and membrane production costs.

Ionic conductivity has been evaluated by the real-time experimental observations of water and ion uptake. The results suggest that water adsorption in the confined three-dimensional interconnected micropores leads to the formation of water-facilitated ionic channels, enabling fast transport of water and ions.

The selective ionic and molecular transport in PIM membranes was analyzed using concentration-driven dialysis diffusion tests. It was confirmed that new design of membranes effectively block large redox active molecules while enabling fast ion transport, which is crucial for the operation of organic RFBs.

In addition, long-term chemical stability, good electrochemical, thermal stability and good mechanical strength of the hydrophilic PIM membranes have been demonstrated.

Finally, it has been reported that the performance and stability tests of RFBs based on the new membranes, as well as of ion permeation rate and selectivity, are comparable to the performances based on a Nafion™ membranes as benchmark.

(Mima Varničić, 2020, photo: Wikipedia)

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Electrical energy storage

Electrical Energy Storage (EES) is the process of converting electrical energy from a power network into a form that can be stored for converting back to electricity when needed. EES enables electricity to be produced during times of either low demand, low generation cost, or during periods of peak renewable energy generation. This allows producers and transmission system operators (TSOs) the ability to leverage and balance the variance in supply/demand and generation costs by using stored electricity at times of high demand, high generation cost, and/or low generation capacity.
EES has many applications including renewables integration, ancillary services, and electrical grid support. This blog series aims to provide the reader with four aspects of EES:

  1. An overview of the function and applications of EES technologies,
  2. State-of-the-art breakdown of key EES markets in the European Union,
  3. A discussion on the future of these EES markets, and
  4. Applications (Service Uses) of EES.

Table: Some common service uses of EES technologies

Storage Category

Storage Technology

Pumped Hydro

Open Loop

Closed Loop

Electro-chemical

Batteries

Flow Batteries

Capacitors

Thermal Storage

 

Molten Salts

Heat

Ice

Chilled Water

Electro-mechanical

Compressed Air Energy Storage (CAES)

Flywheel

Gravitational Storage

Hydrogen Storage

 

Fuel Cells

H2 Storage

Power-to-Gas

Unlike any other commodities market, electricity-generating industries typically have little or no storage capabilities. Electricity must be used precisely when it is produced, with grid operators constantly balancing electrical supply and demand. With an ever-increasing market share of intermittent renewable energy sources the balancing act is becoming increasingly complex.

While EES is most often touted for its ability to help minimize supply fluctuations by storing electricity produced during periods of peak renewable energy generation, there are many other applications. EES is vital to the safe, reliable operation of the electricity grid by supporting key ancillary services and electrical grid reliability functions. This is often overlooked for the ability to help facilitate renewable energy integration. EES is applicable in all of the major areas of the electricity grid (generation, transmission & distribution, and end user services). A few of the most prevalent service uses are outlined in the Table above. Further explanation on service use/cases will be provide later in this blog, including comprehensive list of EES applications.

Area

Service Use / Case

Discharge Duration in h

Capacity in MW

Examples

Generation

Bulk Storage

4 – 6

1 – 500

Pumped hydro, CAES, Batteries

Contingency

1 – 2

1 – 500

Pumped hydro, CAES, Batteries

Black Start

NA

NA

Batteries

Renewables Firming

2 – 4

1 – 500

Pumped hydro, CAES, Batteries

Transmission & Distribution

Frequency & Voltage Support

0.25 – 1

1 – 10

Flywheels, Capacitors

Transmission Support

2 – 5 sec

10 – 100

Flywheels, Capacitors

On-site Power

8 – 16

1.5 kW – 5 kW

Batteries

Asset Deferral

3 – 6

0.25– 5

Batteries

End User Services

Energy Management

4 – 6

1 kW – 1 MW

Residential storage

Learn more about EES in the EU in the next post.

(Jon Martin, 2019)

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Fuel Cells

Fuel cells are a special type of galvanic cells. They can be fueled by solid, liquid, or gaseous fuel. The electrochemical oxidation of the fuel is coupled to energy gain, which is captured in form of electricity – as opposed to heat during chemical oxidation. Hence, fuel cells are direct energy converters with high efficiency. Most fuel cells achieve an energy conversion efficiency of 70-90%. If the conversion is 100%, no waste heat is produced. This ideal case of energy conversion is called ‘cold combustion’ which has been demonstrated for the first time in 1955 by Justi & Winsel. The fuel for this process is hydrogen gas, H2. It enters a porous nickel tube (gas diffusion electrode) where it is dissociated into protons and electrons according to:

H2 → 2 H+ + 2 e

Hydrogen fuel (H2) and oxygen (O2) are pumped into a fuel cell where two electrodes and the electrolyte fuse them to water.

During desorption, each H atom releases a proton (H+) and an electron (e). The electron is discharged onto the electrode, called anode, and the proton into the electrolyte. As a result of the dissociation process, the anode becomes negatively charged. On the second electrode, called cathode, oxygen gas, O2, is then charged with the electron and converted into O2− ions. The cathode becomes positively charged. Both electrodes are submerged in electrolytes which is in most cases a potassium hydroxide, KOH, solution of water. In the electrolyte, cations (H+) and anions (O2−) form water by chemical fusion. Theoretically, the efficiency is 92% accompanied by little waste heat – as opposed to normal combustion where heat of ~3,000ºC is produced.

2 H2 + O2 → H2O

Unlike heat power generators, fuel cells achieve high direct energy conversion efficiency because they avoid the additional step of heat generation. Besides shortcutting heat generation, fuel cells operate without mechanical parts and emit no noise, flue gas, or radioactivity, which puts them in focus of future developments. Due to their high energy efficiency and the high energy density of hydrogen, fuel cells are ideal for electric vehicles. In space flight, fuel cells were first used during Apollo Program between 1968 and 1972, in the Skylab Project 1973, the Apollo-Soyus Program, the Space Shuttle Program, and on board the International Space Station. There, they provide the electrical power for tools and water treatment. One benefit is that the final product of cold combustion in fuel cells is that water is the final product which is used by astronauts on their missions.

There are various types of fuel cells but all have in common that they consist of electrodes for fuel and O2 activation, and electrolytic conductors between these electrodes. Recent variations of fuel cells include methane fuel cells and microbial fuel cells. Due to the high activation energy of methane, methane fuel cells usually operate at high temperature using solid electrolytes. Microbial fuel cells, use microbes as anodic catalyst and organic matter in water as fuel. This makes them ideal for wastewater treatment.

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A Graphene Membrane Becomes a Nano-Scale Water Gate

Biological systems can control water flow using channels in their membranes. This has many advantages, for example when cells need to regulate their osmotic pressure. Also artificial systems, e.g. in water treatment or in electrochemical cells, could benefit from it. Now, a group of materials researchers behind Dr. Zhou at the University of Manchester in the United Kingdom have developed a membrane that can electrically switch the flow of water.

As the researchers reported in the journal Nature, a sandwiched membrane of silver, graphene, and gold was fabricated. At a voltage of more than 2 V channels it opens its pores and water is immediately channeled through the membrane. The effect is reversible. To do this, the researchers used the property of graphene to form a tunable filter or even a perfect barrier to liquids and gases. New ‘smart’ membranes, developed using a low-cost form of graphene called graphene oxide, allow precise control of water flow by using an electrical current. The membranes can even be used to completely block water when needed.

To produce the membrane, the research group has embedded conductive filaments in the electrically insulating graphene oxide membrane. An electric current passed through these nanofilaments created a large electric field that ionizes the water molecules and thus controls the water transport through the graphene capillaries in the membrane.

At Frontis Energy we are excited about this new technology and can imagine numerous applications. This research makes it possible to precisely control water permeation from ultrafast flow-through to complete shut-off. The development of such smart membranes controlled by external stimuli would be of great interest to many areas of business and research alike. These membranes could, for instance, find application in electrolysis cells or in medicine. For medical applications, artificial biological systems, such as tissue grafts, enable a plenty of medical applications.

However, the delicate material consisting of graphene, gold, and silver nano-layers is still too expensive and not as resistant as our Nafion™ membranes. But unlike Nafion™ you can tune them. We stay tuned to see what is coming next.

(Illustration: University of Manchester)