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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, including supercapacitors, 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, solvation state and concentration. Traditionally, the capacitance of electrochemical interfaces can be divided into two types:

  1. Double-layer capacitance: ions are adsorbed based on their charge. Ion adsorption is non-specific.
  2. Faradaic pseudocapacitance: specific ions are adsorbed, 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 – charge 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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Energy storage market in Germany

Germany’s electricity portfolio

In our last posts we introduced electrical energy storage (EES) and the EU market for EES. Now, we focus on some important EU members, beginning with Germany. The country’s electrical energy portfolio reflects its status among the most progressive countries in the world in terms of climate action. As of November 2016, Germany had produced ~35% of its 2016 electricity needs from renewable sources as outlined in the Figure below.

Electricity Production in Germany (Source: Fraunhofer ISE)

The growth of renewable energy has been driven by Germany’s strong energy transition policy – the “Energiewende” – a long-term plan to decarbonize the energy sector. The policy was enacted in late 2010 with ambitious GHG reduction and renewable energy targets for 2050 (80-95% reduction on 1990 GHG levels and 80% renewable-based electricity).
A major part of the 2010 Energiewende policy was the reliance on Germany’s 17 nuclear power plants as a “shoulder fuel” to help facilitate the transition from fossil fuels to renewables. In light of the Fukushima disaster just six months after the enactment of the Energiewende, the German government amended the policy to include an aggressive phase-out of nuclear by 2022 while maintaining the 2050 targets. This has only magnified the importance of clean, reliable electricity from alternative sources like wind and solar.

Existing Energy Storage Facilities

As of late 2016, there is 1,050 MW of installed (non-PHS) energy storage capacity in Germany. The majority of this capacity is made up of electro-mechanical technologies such as flywheels and compressed air energy storage (CAES; see figure below).

Capacities of EES Types in Germany (Source: Sandia National Laboratories)

However, these numbers are somewhat skewed based on the fact that the electro-mechanical category is essentially two large capacity CAES plants. In reality, electro-chemical projects (mainly batteries) are much more prevalent and represent the vast majority of growth in the German storage market. There are currently 11 electro-chemical type energy storage projects under development in Germany and no electro-mechanical projects under development (see figure below).

Number of EES Projects by Type (Sandia National Laboratories)

Services Uses of Energy Storage

As outlined earlier, there are a multitude of service uses for EES technologies. Currently the existing EES fleet in Germany serves grid operations and stability applications (black start, electric supply capacity), and on-site power for critical transmission infrastructure. A breakdown of service uses in the German market is shown below.

Service Uses of Energy Storage Facilities in Germany (Sandia National Laboratories)

Most notable in is the fact that renewables capacity firming only represents 0.3% of EES currently operating in Germany, excluding pumped hydro storage. In order to understand this, it must be noted that Germany is a net exporter of electricity (next figure below). Having one of the most reliable electrical grids in the world and an ideal geographical location give Germany excellent interconnection to a variety of neighboring power markets; making it easy to export any excess electricity.

This “export balancing” is a primary reason why the EES market has not seen similar growth as renewable energy in Germany − it is easy for Germany to export power to balance the system load during periods of peak renewable production. However, there are negative aspects of this energy exporting such as severe overloading of transmission infrastructure in neighboring countries.

Net Exports of Electricity with Average Day-Ahead Market Pricing for Germany in 2015 (Source: Fraunhofer ISE)

Energy Storage Market Outlook

Logic seems to indicate that with aggressive renewable energy targets, a nuclear phase-out, and increased emphasis on energy independence Germany will need to develop more EES capacity. However, many have conjectured that the lagging expansion of EES in the short and medium term will not pose a barrier to the Energiewende. In fact, some claim that EES will not be a necessity in the next 10-20 years. For example, even when Germany reaches its 2020 wind and solar targets (46 GW and 52 GW, respectively), these would generally not exceed 55 GW of supply and nearly all of this power will be consumed domestically in real-time. Thus, no significant support from EES would be required.

The German Institute for Economy Research echos these sentiments and argue that the grid flexibility needed with significant renewable energy capacity could be provided by more cost-effective options like flexible base-load power plants and better demand side management. Additionally, innovations in power-to-heat technologies which would use surplus wind and solar electricity to feed district heating systems present significant opportunity, while creating a new market of energy service companies.

Power-to-Gas

Germany’s Federal Ministry of Transport and Digital Infrastructure found that P2G is ideally suited for turning excess renewable energy into a diverse product that can be stored for long periods of time and Germany has been the central point for P2G technology development in recent years. There are currently seven P2G projects either operating or under construction in Germany.

While there is work being done, economically feasible production of P2G is currently not achievable due to limited excess electricity and low guaranteed capacity. This limited excess electricity, is an example of the effect of power exports discussed earlier. While there may not be a significant commercial market in the short-term, introduction of P2G for transport could act as an additional driver behind continued renewable energy development in Germany.

In our next post, we cover the energy storage market of the United Kingdom.

(Jon Martin, 2019)

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Energy storage in the European Union

Grid integration of renewables

In our previous post of this blog series on Electrical Energy Storage in the EU we briefly introduced you to different technologies and their use cases. Here, we give you a short overview over the EU energy grid.  Supplying approximately 2,500 TWh annually to 450 million customers across 24 countries, the synchronous interconnected system of Continental Europe (“the Grid”) is the largest interconnected power network in the world. The Grid is made up of transmission system operators (TSOs) from 24 countries stretching from Greece to the Iberic Peninsula in the south, Denmark and Poland in the north, and up to the black sea in the east. The European Network of Transmission System Operators (ENTSO-E) serves as the central agency tasked with promoting cooperation between the TSOs from the member countries in the Grid. The ENTSO-E, in essence, acts as the central TSO for Europe. With over 140 GW of installed wind and solar PV capacity, the EU trails behind only China in installed capacity. A breakdown of the individual contributions of EU member states is shown below in the figure above.

Energy Storage in the EU

For this study a number of European countries were selected for more detailed investigation into energy storage needs. These countries were selected based on a combination of existing market size, intentions for growth in non-dispatchable renewable energy and/or energy storage, and markets with a track record of innovation in the energy sector.

On a total capacity basis (installed and planned MW) the top three energy storage markets within the EU are: Italy, the UK, and Germany. These countries were selected on the basis of these existing market sizes.

Spain and Denmark were selected based on their large amounts of existing renewable energy capacity and − in the case of Denmark − the forecasted growth in renewable energy and energy storage capacity.

While still lagging behind the rest of the EU in terms of decarbonization efforts and having a small portion of their energy from renewable sources, the Netherlands were also selected for further investigation.

Each of the selected countries (Germany, UK, Italy, Spain, Denmark, Netherlands) are discussed in the proceeding sections, providing a more detailed overview outlining their current electricity portfolios and decarbonization efforts, current energy storage statistics, and a brief discussion on market outlook.

Pumped Hydro Storage

With over 183 GW of installed capacity worldwide, pumped hydro storage is the most widely implemented and most established form of energy storage in the world. Due its extensive market penetration, technology maturity, and the fact that this blog is aimed at emerging new storage technologies, the data presented in the following posts excludes this technology.

Find more details about the energy storage market of selected European countries in our next postings.

(Jon Martin, 2019)

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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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EU market summary for energy storage

Electrical energy storage (EES) is not only a vital component in the reliable operation of modern electrical grids, but also a focal point of the global renewable energy transition. It has been often suggested that EES technologies could be the missing piece to eliminating the technical hurdles facing the implementation of intermittent renewable energy sources. In the following blog posts, selected EES markets within the European Union will be evaluated in detail.

With over 80 MW of installed wind and solar capacity, Germany is by far the leading EU nation in the renewable energy transition. However, experts have argued that Germany’s need for widespread industrial scale energy storage is unlikely to materialize in any significant quantity for up to 20-years. This is due to a number of factors. Germany’s geographic location and abundance of connections to neighbouring power grids makes exporting any electricity fluctuations relatively easy. Additionally, when Germany reaches its 2020 targets for wind and solar capacity (46 GW and 52 GW, respectively) the supply at a given time would generally not exceed 55 GW. Nearly all of this would be consumed domestically, with no/little need for storage.

When evaluating energy storage in the UK, a different story emerges. Being an isolated island nation there is considerably more focus on energy independence to go along with their low-carbon energy goals. However, the existing regulatory environment is cumbersome, and poses barriers significant enough to substantially inhibit the transition to a low-carbon energy sector – including EES. The UK government has acknowledged the existence of regulatory barriers and pledged to address them. As part of this effort, a restructuring of their power market to a capacity-based market is already underway. The outlook for EES in the UK is promising, there is considerable pressure from not only industry, but also the public and the government to continue developing EES facilities at industrial scale.

Italy, once heavily hydro-powered, has grown to rely on natural gas, coal, and oil for 50% of it’s electricity (gas representing 34% alone). The introduction of a solar FIT in 2005 lead to significant growth in the solar industry (Italy now ranks 2nd in per capita solar capacity globally) before the program ended in July 2014. In recent years there has been notable growth in electro-chemical EES capacity (~84 MW installed), primarily driven by a single large-scale project by TERNA, Italy’s transmission system operator (TSO). This capacity has made Italy the leader in EES capacity in the EU, however the market is to-date dominated by the large TSOs.

However, the combination of a reliance on imported natural gas, over 500,000 PV systems no longer collecting FIT premiums, and increasing electricity rates presents a unique market opportunity for residential power-to-gas in Italy.
Denmark is aggressively pursing a 100-percent renewable target for all sectors by 2050. While there is still no official roadmap policy on how they will get there, they have essentially narrowed it down to one of two scenario: a biomass-based scenario, or a wind + hydrogen based scenario. Under the hydrogen-based scenario there would be widespread investment to expand wind capacity and couple this capacity with hydrogen power-to-gas systems for bulk energy storage. With the Danish expertise and embodied investment in wind energy, one would expect that the future Danish energy system would be build around this strength, and hence require significant power-to-gas investment.

The renewable energy industry in Spain has completed stagnated due to retroactive policy changes and taxes on consumption of solar generated electricity introduced in 2015. The implementation of the Royal Decree 900/2015 on self-consumption has rendered PV systems unprofitable, and added additional fees and taxes for the use of EES devices. No evidence was found to suggest a market for energy storage will materialize in Spain in the near future.

The final country investigated was the Netherlands, which has been criticized by the EU for its lack of progress on renewable energy targets. With only 10% of Dutch electricity coming from renewable sources, there is currently little demand for large-scale EES. While the Netherlands may be lagging behind on renewable electricity targets, they have been a leader in EV penetration; a trend that will continue and see 1-million EVs on Dutch roads by 2025. In parallel with the EV growth, there has been a large surge in sub-100kW Li-ion installations for storing energy at electric vehicle (EV) charging stations. It is expected that these applications will continue to be the primary focus of EES in the Netherlands.

Similar to Italy, the Dutch rely heavily on natural gas for energy within their homes. This fact, coupled with an ever-increasing focus on energy independent and efficient houses could make the Netherlands a prime market for residential power-to-gas technologies.

Read more about electrical energy storage here.

Jon Martin, 2019

(Photo: NASA)

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Machine learning makes smarter batteries

Renewable energies, such as wind and solar energy are naturally intermittent. To balance their demand and supply, batteries of, for example, electric vehicles can be charged and act as an energy buffer for the power grid. Cars spend most of their time idle and could, at the same time, feed their electricity back into the grid. While this is still a dream of the future, commercialization of electric and hybrid vehicles is already creating a growing demand for long-lasting batteries, both for driving as well as grid buffering. Consequently, methods for evaluating the state of the battery will become increasingly important.

The long duration of battery health tests is a problem, hindering the rapid development of new batteries. Better battery life forcasting methods are therefore urgently needed but are extremely difficult to develop. Now, Severson and her colleagues report in the journal Nature Energy that machine learning can help to predict computer battery life by creating computer models. The published algorithms use data from early-stage charge and discharge cycles.

Normally, a figure of merit describes the health of a battery. It quantifies the ability of the battery to store energy relative to its original state. The health status is 100% when the battery is new and decreases with time. This is similar to the state of charge of a battery. Estimating the state of charge of a battery is, in turn, important to ensure safe and correct use. However, there is no consensus in the industry and science as to what exactly a battery’s health status is or how it should be determined.

The state of health of a battery reflects two signs of aging: progressive capacity decline and impedance increase (another measure of electrical resistance). Estimates of the state of charge of a battery must therefore take into account both the drop in capacity and the increase in impedance.

Lithium ion batteries, however, are complex systems in which both capacity fade and impedance increase are caused by multiple interacting processes. Most of these processes cannot be studied independently since they often occur in simultaneously. The state of health can therefore not be determined from a single direct measurement. Conventional health assessment methods include examining the interactions between the electrodes of a battery. Since such methods often intervene directly in the system “battery”, they make the battery useless, which is hardly desired.

A battery’s health status can also be determined in less invasive ways, for example using adaptive models and experimental techniques. Adaptive models learn from recorded battery performance data and adjust themselves. They are useful if system-specific battery information are not available. Such models are suitable for the diagnosis of aging processes. The main problem, however, is that they must be trained with experimental data before they can be used to determine the current capacity of a battery.

Experimental techniques are used to evaluate certain physical processes and failure mechanisms. This allows the rate of future capacity loss to be estimated. Unfortunately, these methods can not detect any intermittent errors. Alternative techniques use the rate of voltage or capacitance change (rather than raw voltage and current data). In order to accelerate the development of battery technology, further methods need to be found which can accurately predict the life of the batteries.

Severson and her colleagues have created a comprehensive data set that includes the performance data of 124 commercial lithium-ion batteries during their charge and discharge cycles. The authors used a variety of rapid charging conditions with identical discharge conditions. This method caused a change of the battery lives. The data covered a wide range of 150 to 2,300 cycles.

The researchers then used machine learning algorithms to analyze the data, creating models that can reliably predict battery life. After the first 100 cycles of each experimentally characterized battery their model already showed clear signs of a capacity fade. The best model could predict the lifetime of about 91% data sets studied in the study. Using the first five cycles, batteries could be classified into categories with short (<550 cycles) or long lifetimes.

The researchers’ work shows that data-driven modeling using machine learning allows forecasting the state of health of lithium-ion batteries. The models can identify aging processes that do not otherwise apparent in capacity data during early cycles. Accordingly, the new approach complements the previous predictive models. But at Frontis Energy, we also see the ability to combine generated data with models that predict the behavior of other complex dynamic systems.

(Photo: Wikipedia)

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Faster photoelectrical hydrogen

Achieving high current densities while maintaining high energy efficiency is one of the biggest challenges in improving photoelectrochemical devices. Higher current densities accelerate the production of hydrogen and other electrochemical fuels.

Now a compact, solar-powered, hydrogen-producing device has been developed that provides the fuel at record speed. In the journal Nature Energy, the researchers around Saurabh Tembhurne describe a concept that allows capturing concentrated solar radiation (up to 474 kW/m²) by thermal integration, mass transport optimization and better electronics between the photoabsorber and the electrocatalyst.

The research group of the Swiss Federal Institute of Technology in Lausanne (EPFL) calculated the maximum increase in theoretical efficiency. Then, they experimentally verified the calculated values ​​using a photoabsorber and an iridium-ruthenium oxide-platinum based electrocatalyst. The electrocatalyst reached a current density greater than 0.88 A/cm². The calculated conversion efficiency of solar energy into hydrogen was more than 15%. The system was stable under various conditions for more than two hours. Next, the researchers want to scale their system.

The produced hydrogen can be used in fuel cells for power generation, which is why the developed system is suitable for energy storage. The hydrogen-powered generation of electricity emits only pure water. However, the clean and fast production of hydrogen is still a challenge. In the photoelectric method, materials similar to those of solar modules were used. The electrolytes were based on water in the new system, although ammonia would also be conceivable. Sunlight reaching these materials triggers a reaction in which water is split into oxygen and hydrogen. So far, however, all photoelectric methods could not be used on an industrial scale.

2 H2O → 2 H2 + O2; ∆G°’ = +237 kJ/mol (H2)

The newly developed system absorbed more than 400 times the amount of solar energy that normally shines on a given area. The researchers used high-power lamps to provide the necessary “solar energy”. Existing solar systems concentrate solar energy to a similar degree with the help of mirrors or lenses. The waste heat is used to accelerate the reaction.

The team predicts that the test equipment, with a footprint of approximately 5 cm, can produce an estimated 47 liters of hydrogen gas in six hours of sunshine. This is the highest rate per area for such solar powered electrochemical systems. At Frontis Energy we hope to be able to test and offer this system soon.

(Photo: Wikipedia)

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Ammonia energy storage #3

As a loyal reader or loyal reader of our blog, you will certainly remember our previous publications on ammonia energy storage. There, we describe possible ways to extract ammonia from the air, as well as the recovery of its energy in the form of methane (patent pending WO2019/079908A1). Since global food production requires large amounts of ammonia fertilizers, technologies for extraction from air is already very mature. These technologies are essentially all based on the Haber-Bosch process, which was industrialized at the beginning of the last century. During this process, atmospheric nitrogen (N2) is reduced to ammonia (NH3). Despite the simplicity of the molecules involved, the cleavage of the strong nitrogen−nitrogen bonds in N2 and the resulting nitrogen−hydrogen bonds pose a major challenge for catalytic chemists. The reaction usually takes place under harsh conditions and requires a lot of energy, i.e. high reaction temperatures, high pressures and complicated combinations of reagents, which are also often expensive and energy-intensive to manufacture.

Now, a research group led by Yuya Ashida has published an article in the renowned journal Nature, in which they show that a samarium compound in aqueous solution combined with a molybdenum catalyst can form ammonia from atmospheric nitrogen. The work opens up new possibilities in the search for ways to ammonia synthesis under ambient conditions. Under such conditions, less energy is required to produce ammonia, resulting in higher energy efficiency for energy storage. In today’s Haber-Bosch process, air and hydrogen gas are combined via an iron catalyst. The resulting global ammonia production of this process ranges from 250 to 300 tonnes per minute, delivering fertilizers that provide nearly 60% of the world’s population (The Alchemy of Air, available at Amazon).

Comparison of different approaches to produce ammonia. Top: In the industrial Haber-Bosch synthesis of ammonia (NH3), nitrogen gas (N2) reacts with hydrogen molecules (H2), typically in the presence of an iron catalyst. The process requires high temperatures and pressures, but is thermodynamically ideal because only little energy is wasted on side reactions. Center: Nitrogenase enzymes catalyze the reaction of six-electron (e) nitrogen and six protons (H+) under ambient conditions to form ammonia. However, two additional electrons and protons form one molecule of H2. The conversion of ATP (the biological energy “currency”) into ADP drives the reaction. This reaction has a high chemical overpotential. It consumes much more energy than is needed for the actual ammonia forming reaction. Bottom: In the new reaction proposed by Ashida and colleagues, a mixture of water and samarium diiodide (SmI2) is converted to ammonia using nitrogen under ambient conditions and in the presence of a molybdenum catalyst. SmI2 weakens the O−H bonds of the water and generates the hydrogen atoms, which then react with atmospheric nitrogen.

On industrial scale, ammonia is synthesized at temperatures that exceed 400°C and pressures of approximately 400 atmospheres. These conditions are often referred to as “harsh”. During the early days, these harsh conditions were difficult to control. Fatal accidents were not uncommon in the early years of the Haber-Bosch development. This has motivated many chemists to find “milder” alternatives. After all, this always meant searching for new catalysts to lower operating temperatures and pressures. The search for new catalysts would ultimately reduce capital investment in the construction of new fertilizer plants. Since ammonia synthesis is one of the largest producers of carbon dioxide, this would also reduce the associated emissions.

Like many other chemists before them, the authors have been inspired by nature. Nitrogenase enzymes carry out the biological conversion of atmospheric nitrogen into ammonia, a process called nitrogen fixation. On recent Earth, this process is the source of nitrogen atoms in amino acids and nucleotides, the elemental building blocks of life. In contrast to the Haber-Bosch process, nitrogenases do not use hydrogen gas as a source of hydrogen atoms. Instead, they transfer protons (hydrogen ions, H+) and electrons (e) to each nitrogen atom to form N−H bonds. Although nitrogenases fix nitrogen at ambient temperature, they use eight protons and electrons per molecule N2. This is remarkable because the stoichiometry of the reaction requires only six each. This way, nitrogenases provide the necessary thermodynamic drive for nitrogen fixation. The excess of hydrogen equivalents means that nitrogenases have a high chemical overpotential. That is, they consume much more energy than would actually be needed for nitrogen fixation.

The now published reaction is not the first attempt to mimic the nitrogenase reaction. In the past, metal complexes were used with proton and electron sources to convert atmospheric nitrogen into ammonia. The same researchers have previously developed 8 molybdenum complexes that catalyze nitrogen fixation in this way. This produced 230 ammonia molecules per molybdenum complex. The associated overpotentials were significant at almost 1,300 kJ per mole nitrogen. In reality, however, the Haber-Bosch process is not so energy-intensive given the right catalyst is used.

The challenge for catalysis researchers is to combine the best biological and industrial approaches to nitrogen fixation so that the process proceeds at ambient temperatures and pressures. At the same time, the catalyst must reduce the chemical overpotential to such an extent that the construction of new fertilizer plants no longer requires such high capital investments. This is a major challenge as there is no combination of acids (which serve as a proton source) and reducing agents (the electron sources) available for the fixation at the thermodynamic level of hydrogen gas. This means that the mixture must be reactive enough to form N−H bonds at room temperature. In the now described pathway with molybdenum and samarium, the researchers have adopted a strategy in which the proton and electron sources are no longer used separately. This is a fundamentally new approach to catalytic ammonia synthesis. It makes use of a phenomenon known as coordination-induced bond weakening. In the proposed path, the phenomenon is based on the interaction of samarium diiodide (SmI2) and water.

Water is stable because of its strong oxygen-hydrogen bonds (O−H). However, when the oxygen atom in the water is coordinated with SmI2, it exposes its single electron pair and its O−H bonds are weakened. As a result, the resulting mixture becomes a readily available source of hydrogen atoms, protons and electrons, that is. The researchers around Yuya Ashida use this mixture with a molybdenum catalyst to fix nitrogen. SmI2-water mixtures are therefore particularly suitable for this type of catalysis. In them, a considerable coordination-induced bond weakening was previously measured, which was used inter alia for the production of carbon-hydrogen bonds.

The extension of this idea to catalytic ammonia synthesis is remarkable for two reasons. First, the molybdenum catalyst facilitates ammonia synthesis in aqueous solution. This is amazing because molybdenum complexes in water are usually degraded. Second, the use of coordination-induced bond weakening provides a new method for nitrogen fixation at ambient conditions. This also avoids the use of potentially hazardous combinations of proton and electron sources which are a fire hazard. The authors’ approach also works when ethylene glycol (HOCH2CH2OH) is used instead of water. Thus, the candidates for proton and electron sources are extended by an additional precursor.

Ashida and colleagues propose a catalytic cycle for their process in which the molybdenum catalyst initially coordinates to nitrogen and cleaves the N−N bond to form a molybdenum nitrido complex. This molybdenum nitrido complex contains the molybdenum-nitrogen triple bond. The SmI2-water mixture then delivers hydrogen atoms to this complex, eventually producing ammonia. The formation of N−H bonds with molybdenum nitrido complexes represents a significant thermodynamic challenge since the N−H bonds are also weakened by the molybdenum. Nevertheless, the disadvantages are offset by the reduction of the chemical overpotential. The SmI2 not only facilitates the transfer of hydrogen atoms, but also keeps the metal in a reduced form. This prevents undesired molybdenum oxide formation in aqueous solution.

The new process still has significant operational hurdles to overcome before it can be used on an industrial scale. For example, SmI2 is used in large quantities, which generates a lot of waste. The separation of ammonia from aqueous solutions is difficult in terms of energy consumption. However, if the process were used for energy storage in combination with our recovery method, the separation would be eliminated from the aqueous solution. Finally, there is still a chemical overpotential of about 600 kJ/mol. Future research should focus on finding alternatives to SmI2. These could be based, for example, on metals that occur more frequently than samarium and promote coordination-induced bond weakening as well. As Fritz Haber and Carl Bosch have experienced, the newly developed method will probably take some time for development before it becomes available on industrial scale.

(Photo: Wikipedia)

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A Durable Aluminum-Air-Battery

Non-rechargeable batteries, which depend on a reaction between aluminum and oxygen, can store significantly more energy than conventional lithium-ion batteries. The biggest limitation of such aluminum-air batteries is their short shelf life. An improved battery design could help eliminate this limitation. Aluminum and air batteries are based on the property of aluminum to corrode, which is also their weak spot:

4 Al + 3 O2 + 6H2O → 4 Al (OH)3

While an aluminum-air battery is not used, its electrodes corrode causing unwanted discharge. This self-discharge drastically shortens the shelf life of the battery. Brandon Hopkins, of the Massachusetts Institute of Technology in Cambridge, and his colleagues developed an aluminum-air battery that uses a conventional electrolyte during operation. When stored, however, the electrolyte is replaced by oil. Their article was recently published in the journal Science.

The new battery reaches a storage capacity of almost 900 Wh / kg. This makes the prototype comparable to other aluminum-air batteries. In contrast, the new corrosion protection extends the storage time 10,000-fold. The authors suggest that such a battery could be used in long-range drones and grid-independent power generation. At Frontis Energy, we believe that batteries with high storage capacity and durability can be used almost anywhere, for example for sensors and other applications.

(Photo: George Hodan)

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Ammonia energy storage #1

The ancient, arid landscapes of Australia are not only fertile soil for huge forests and arable land. The sun shines more than in any other country. Strong winds hit the south and west coast. All in all, Australia has a renewable energy capacity of 25 terawatts, one of the highest in the world and about four times higher than the world’s installed power generation capacity. The low population density allows only little energy storage and electricity export is difficult due to the isolated location.

So far, we thought the cheapest way to store large amounts of energy was power-to-gas. But there is another way to produce carbon-free fuel: ammonia. Nitrogen gas and water are enough to make the gas. The conversion of renewable electricity into the high-energy gas, which can also be easily cooled and converted into a liquid fuel, produces a formidable carrier for hydrogen. Either ammonia or hydrogen can be used in fuel cells.

The volumetric energy density of ammonia is almost twice as high than that of liquid hydrogen. At the same time ammonia can be transported and stored easier and faster. Researchers around the world are pursuing the same vision of an “ammonia economy.” In Australia, which has long been exporting coal and natural gas, this is particularly important. This year, Australia’s Renewable Energy Agency is providing 20 million Australian dollars in funding.

Last year, an international consortium announced plans to build a $10 billion combined wind and solar plant. Although most of the 9 terawatts in the project would go through a submarine cable, part of this energy could be used to produce ammonia for long-haul transport. The process could replace the Haber-Bosch process.

Such an ammonia factories are cities of pipes and tanks and are usually situated where natural gas is available. In the Western Australian Pilbara Desert, where ferruginous rocks and the ocean meet, there is such an ammonia city. It is one of the largest and most modern ammonia plants in the world. But at the core, it’s still the same steel reactors that work after the 100 years-old ammonia recipe.

By 1909, nitrogen-fixing bacteria produced most of the ammonia on Earth. In the same year, the German scientist Fritz Haber discovered a reaction that could split the strong chemical bond of the nitrogen, (N2) with the aid of iron catalysts (magnetite) and subsequently bond the atoms with hydrogen to form ammonia. In the large, narrow steel reactors, the reaction produces 250 times the atmospheric pressure. The process was first industrialized by the German chemist Carl Bosch at BASF. It has become more efficient over time. About 60% of the introduced energy is stored in the ammonia bonds. Today, a single plant produces and delivers up to 1 million tons of ammonia per year.

Most of it is used as fertilizer. Plants use nitrogen, which is used to build up proteins and DNA, and ammonia delivers it in a bioavailable form. It is estimated that at least half of the nitrogen in the human body is synthetic ammonia.

Haber-Bosch led to a green revolution, but the process is anything but green. It requires hydrogen gas (H2), which is obtained from pressurized, heated steam from natural gas or coal. Carbon dioxide (CO2) remains behind and accounts for about half of the emissions. The second source material, N2, is recovered from the air. But the pressure needed to fuse hydrogen and nitrogen in the reactors is energy intensive, which in turn means more CO2. The emissions add up: global ammonia production consumes about 2% of energy and produces 1% of our CO2 emissions.

Our microbial electrolysis reactors convert the ammonia directly into methane gas − without the detour via hydrogen. The patent pending process is particularly suitable for removing ammonia from wastewater. Microbes living in wastewater directly oxidize the ammonia dissolved in ammonia and feed the released electrons into an electric circuit. The electricity can be collected directly, but it is more economical to produce methane gas from CO2. Using our technology, part of the CO2 is returned to the carbon cycle and contaminated wastewater is purified:

NH3 + CO2 → N2 + CH4