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Transition between double-layer and Faradaic charge storage in porous carbon nano-material

In electrochemical cells, such as fuel cells or electrolyzers, electric double-layer (EDL) formation occurs on their electrode surfaces. These EDL act as both, capacitors and resistors and impact therefore the performance of electrochemical cells. Understanding the structure and dynamics of EDL formation could significantly improve the performance of electrochemical systems, for example in energy storage and conversion, water desalination, sensors and so forth.

On a planar electrode, electrolyte ions and the solvent are adsorbed at the electrode surface. The resulting capacitance depends on charge, salvation state and concentration. Traditionally, the capacitance of electrochemical interfaces can be divided into two types:

  1. Non-Faradaic capacitance: ions are adsorbed based on their charge. Ion adsorption is non-specific.
  2. Faradaic pseudocapacitance: specific ions are absorbed, for example through chemical interactions the electrode surface. This may involve charge transfer.

The electrode interface in the most energy application-based technology is, however, not planar but porous. Layer materials in such situations have various degrees of electrolyte confinement and thus different capacitive adsorption mechanisms. Understanding electrosorption in such materials requires a refined view of electrochemical capacitance and charge storage.

A team of researchers from the North Carolina State University, the Paul Sabatier University in Toulouse and the Karlsruhe Institute of Technology reported new insights in electrolyte confinement at the non-planar interfaces in the journal Nature Energy.

Electric double-layer at planar electrodes

The degree of ion solvation (the process of reorganizing solvent and solute molecules) at ideal (planar) electrochemical interfaces determines the ions interaction with the electrodes. There are two distinct cases:

  1. Ions are non-specifically adsorbed: this is the case with strong ion solvation. The electrode’s interactions are primarily electrostatic. This type of interactions can be considered as the induction – change is induced but not transferred.
  2. Ions are specifically adsorbed: in this case, ions are not solvated and can undergo specific adsorption and chemical bonding to the electrode. This process can be described as charge transfer reaction between the electrode and the adsorbed ion. However, the charge transfer reaction depends on the bonding between the ion and the electrode. This correlates with the state of ion solvation.  Thus, it can be expected that the ion solvation is crucial for understanding the ion-electrode interactions in a nano-confined environment such as porous materials.

Carbon based EDL capacitor – the confinement effect

There is a great interest for understanding the relationship between the porosity of carbon nano-materials and their specific capacitance.

When electric double-layer formation occurs in a nano-confined micro-environment, the EDL capacitor in porous carbon materials deviates from the classic EDL model on flat interfaces. The degree of the ion solvation under confinement is determined by the pore size in nano-porous materials and by the inter-layer distance in layered materials that is, 2D-layer materials.

Confinement of ions in sub-nanometer pores results in their desolvation, leading to the capacitance increase and deviation from the typical linear behavior on the surface area. During negative polarization of porous carbon materials with the pore sizes <1 nm, a decrease of capacitance  is observed. This is due to the ion selection limiting ion transport.

These insights are important for effectively tailoring carbon pore structures and for increasing their specific capacitance. Since carbon material is not an ideal conductor, it is important to consider its specific electric structure. For graphite materials, the availability of the charge carriers increases during the polarization which leads to increased conductivity.

Unified model of electrochemical charge storage under confinement

Since the electrochemical interface in the most technological application is non-planar, the researchers proposed a detailed evaluation and different concept of electrochemical capacitance on such non-ideal interfaces. The team evaluated electrosorption on 2D surfaces and 3D porous carbon surfaces with a continuous reduction in pore size in a step-by-step approach of increasing complexity.

The example provided relates to the charge storage characteristics of lithium ions (Li+) in the graphene sheets of organic lithium-containing electrolytes depending on the number of graphene layers. In a single graphene layer, the capacitive response is potential independent due to the absence of specific adsorption. However, with an increase of graphene sheets, redox peaks emerged that are associated with the intercalation of desolvated lithium ions. Lithium intercalation is responsible for battery wear. The team’s hypothesis was that the transition of solvated lithium ion adsorption on a single graphene sheet into subsequent intercalation of desolvated lithium ions occurs with a continuous charge storage behavior. There can be a seamless transition based on the increased charge transfer between an electrolyte ion and host associated with the extent of desolvation and confinement.

In the presented research, a unified approach was proposed that involves the continuous transition between double-layer capacitance and Faradaic intercalation under confinement. This approach excludes the traditional “single” view of electrochemical charge storage in nano-materials regarded as purely electrostatic or purely Faradaic phenomenon.

The increasing degree of ion confinement is followed by decreasing degree of ion solvation thus the increase ion-host intercalation. This results in a continuum from EDL formation through transitioning state to Faradaic intercalation, typical for EDLC nanomaterial.

Image: Pixabay

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Humidity-resistant composite membranes for gas separation

Hydrogen (H2) is a lightweight alternative fuel with a high energy density. However, its environmental impact and life cycle efficiency are determined by how it is produced. Today, the main processes of hydrogen production is either by coal gasification or steam reforming of natural gas where in the last step the produced carbon dioxide (CO2) is produced. Usually, this CO2 is released to the environment. The hydrogen produced by these processes lead is called black/brown or grey hydrogen. To improve its carbon footprint, CO2 capture is necessary. This hydrogen is then call blue hydrogen. However, to obtain zero-emission green hydrogen, electrolysis of water using renewable energy is necessary. During the electrolysis process, hydrogen and oxygen are produced on two electrodes (download our more about hydrogen production and utilization as fuel can be found in our latest DIY FC manual).

Climate-related economic pressure for more efficient gas separation processes

The produced hydrogen is not pure in any of the mentioned instances. For example, using steam methane reforming reaction there are many byproduct gases like carbon monoxide, CO2, water, nitrogen and methane gas.

Typically, the CO2 of hydrogen gas is up to 50% contributing to the greenhouse effect caused by burning fossil fuels. Currently, around 80% of CO2 emissions come from fossil fuels. It has been predicted that the concentration of CO2 in the atmosphere will increase up to 570 ppm in 2,100 which increases the global temperature of about 1.9°C.

The traditional processes of gas separation such as cryogenic distillation and pressure swing adsorption have certain disadvantages, for example high energy consumption. Therefore, developing high-quality and low-cost technologies for gas separation is an important intermediate step to produce cheap hydrogen while reducing CO2 emissions.

Application of 2D material towards gas separation

Finding low cost alternatives like membrane-based separation methods for hydrogen-CO2 separation is a potentially lucrative research and it is therefor not surprising that numerous publications have investigated the matter. The various membrane materials for gas separation range from polymeric membranes, nano-porous materials, metal–organic frameworks and zeolite membranes. The goal is to reach a good balance between selectivity and permeance of gas separation. Both are key parameters for hydrogen purification and CO2 capture processes.

A study published the journal Nature Energy by researchers of the National Institutes of Japan, offered a material platform as advanced solution for the separation of hydrogen  from humid gas mixtures, such as those generated by fossil fuel sources or water electrolysis. The authors showed that the incorporation of positively charged nanodiamonds into graphene oxide (GO/ND+) results in humidity repelling and high performance membranes. The performance of the GO/ND+ laminates excels particularly in hydrogen separation compared with traditional membrane materials.

Strategy and performance of new membrane materials

Graphene oxide laminates are considered as step-change materials for hydrogen-CO2 separation as ultra permeable (triple-digit permeance) and ultra-selective membranes. Still, graphene oxide films lose their attractive separation properties and stability in humid conditions.

After lamination, graphene oxide sheets have an overall negative charge and can be disintegrated due to the electrostatic repulsion if exposed to water. The strategy to overcome this obstacle was based on the charge compensation principle. That is, the authors incorporated positively and negatively charged fillers as stabilizing agents, and tested different loadings as well as graphene oxide flake sizes. So-prepared membranes were tested for stability in dry and humid conditions while separating either hydrogen from CO2 or oxygen.

The GO/ND+ composite membranes retained up to 90% of their hydrogen selectivity against CO2 exposure to several cycles and under aggressive humidity test. A GO30ND+ membrane with 30% positively charged nano-diamond particles exhibited exceptional hydrogen permeance with more than 3,700  gas permeatin units (GPU) and high hydrogen-CO2 selectivity. Interestingly, incorporation of negatively charged nano-diamond particles had no stabilizing effect. The researcher attributed this mostly to the generation of macro scale voids in ND systems resulting in the loss of selectivity. This phenomenon is commonly observed in polymer-based nano-composite membranes with poor interfacial interactions

The gas separation properties of the composite membranes were also investigated using an equimolar hydrogen-CO2 feed mixture. The hydrogen permeance decreased by 6% and hydrogen-CO2 selectivity of the GO30ND+ membrane by 13%.

The stability test of the membranes exposure to wet and dry feeds of the equimolar hydrogen-CO2 mixture  and hydrogen-oxygen mixture showed that GO/ND+ membranes were reversible membrane properties. On the other hand, graphene oxide-only membranes could not survive a single complete cycle exposure, becoming fully permeable to both gases. The researchers explained that the advantages of GO/ND+ membranes over graphene oxide-only membranes were caused by changes of the pore architecture such as dimensions and tortuosity, which could be improved by optimizing the nano-diamond loading. This results in better permeability without any notable loss of selectivity.

X-ray diffraction analysis showed that the incorporation of nanodiamonds has two major effects on the membrane microstructure: increasing the overall pore volume and reducing the average lateral size. Both make the membrane structure more accessible for molecular transport.

Nevertheless, this relatively new class of humid-resistant membranes still needs more optimization to compete with current industrial separation processes.

Image: Pixabay / seagul

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Nanomaterials in bio-electrical systems could improve performance

Since Professor Potter’s discovery of the ability of microbes to turn organic molecules into electricity using microbial fuel cells (MFC) more than a century ago (Potter MC, 1911, Proc Roy Soc Lond Ser B 84:260–276), much research was done to improve the performance. Unfortunately, this did not not produce an economically viable technology. MFCs never made it out of the professors’ class rooms. This may change now that we have advanced nanomaterials available.

The testing of nanomaterials in bio-electrical systems has experienced a Cambrian explosion. The focus usually was on electrodes, membranes, and in the electrolyte with infinite possibilities to find high performing composites. The benefits of such materials include a large surface area, cost savings, and scalability. All are required to successfully commercialize bio-electrical systems. The large-scale commercial application could be wastewater treatment. In our recently published literature survey we discovered that there is no common benchmark for performance, as it is usual in photovoltaics or for batteries. To normalize our findings, we used dollar per peak power capacity as ($/Wp) as it is standard in photovoltaics. The median cost for air cathodes of MFCs is $4,700 /Wp ($2,800 /m²). Platinum on carbon (Pt/C) and carbon nanofibers are the best performing materials with $500 /Wp (Pt/C $2,800 /m²; nanofibers $2,000 /m²).

We found that carbon-based nanomaterials often deliver performance comparable to Pt/C. While MFCs are still far away from being profitable, microbial electrolysis cells already are. With these new carbon-based nanomaterials, MFCs however, are moving closer to become an economic reality. Graphene and carbon nanotubes are promising materials when they are combined with minerals such as manganese or iron oxides. However, the price of graphene is still too expensive to let MFCs become an economic reality in wastewater treatment. The costs of microbial electrolysis, however, are already so low that it is a serious alternative to traditional wastewater treatment as we show in the featured image above. For high strength wastewater, a treatment plant can in fact turn into a power plant with excess power being offered to surrounding neighborhoods. Reducing the costs of microbial electrolysis is accomplished by using a combination of cheap steel and graphite.

Relationship between MEC reactor capacity and total electrode cost including anode and cathode. Errors are standard deviations of four different tubular reactor designs. Anodes are graphite granules and cathodes are steel pipes

 

Graphite, in turn, is the precursor of graphene, a promising material for MFC electrodes. When graphite flakes are reduced to a few graphene layers, some of the most technologically important properties of the material are greatly improved. These include the overall surface and the elasticity. Graphene is therefore a very thin graphite. Many manufacturers of graphene use this to sell a material that is really just cheap graphite. In the journal Advanced Materials Kauling and colleagues published a systematic study of graphene from sixty manufacturers and find that many high-priced graphene products consist mainly of graphite powder. The study found that less than 10% of the material in most products was graphene. None of the tested products contained more than 50% graphene. Many were heavily contaminated, most likely with chemicals used in the production process. This can often lead to a material having catalytic properties which would not have been observed without contamination, as reported by Wang and Pumera.

There are many methods of producing graphene. One of the simplest is the deposition on a metallic surface, as we describe it in our latest publication:

Single-layer graphene (SLG) and multilayer graphene (MLG) are synthesized by chemical vapor deposition (CVD) from a carbon precursor on catalytic metal surfaces. In a surface-mediated CVD process, the carbon precursor, e.g. Isopropyl alcohol (IPA) is decomposed on the metal surface, e.g. Cu or Ni. In order to control the number of graphene layers formed, the solubility of the carbon precursor on the metal catalyst surface must be taken into account. Due to the low solubility of the precursor in Cu, SLG can be formed. It is difficult to grow SLG on the surface of a metal with a high affinity for the precursor.

Protocol:
The protocol is a cheap, safe and simple method for the synthesis of MLG films by CVD in 30-45 minutes in a chemistry lab. A nickel foil is submersed in acetic acid for etching and then transferred to an airtight quartz tube. The same protects the system from ambient oxygen and water vapor. Nitrogen gas is bubbled through the IPA and the resulting IPA saturated gas is passed through the closed system. The exhaust gases are washed in a beaker with a water or a gas wash bottle. The stream is purged for 5 minutes at a rate of about 50 cc/min. As soon as the flame reaches a Meker burner 575-625 °C, it is positioned under the nickel foil, so that sufficient energy for the formation of graphene is available. The flame is extinguished after 5-10 minutes to stop the reaction and to cool the system for 5 minutes. The graphene-coated Ni foil is obtained.

But how thin must graphite flakes be to behave as graphene? A common idea supported by the International Organization for Standardization (ISO) is that flakes with more than ten graphene layers consist essentially of graphite. Thermodynamics say that each atomic layer in a flake with ten or fewer layers at room temperature behaves as a single graphene crystal. In addition, the stiffness of the graphite flakes increases with the layer thickness, which means that thin graphene flakes are orders of magnitude more elastic than thicker graphite flakes.

Unfortunately, to actually use graphene in bioelectric reactors, you still have to make it yourself. The ingredients can be found in our DIY Shop.

 
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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)