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Pilot-scale microbial fuel cells produce electricity from wastewater

In wastewater treatment, aeration is an energy-intensive but necessary process to remove contaminants. Pumps blow air into the wastewater to supply the microbes in the treatment tank with oxygen. In return, these bacteria oxidize organic substances to CO2 and hence remove them from the wastewater. This process is the industrial standard and has proven itself for over a century. If the researchers at Washington State University and the University of Idaho have their way, that is changing now.

In their project, the researchers used a unique microbial fuel cell system they developed to replace aeration. Their novel wastewater treatment system cleans wastewater with the help of microorganisms that produce electricity. These microbes are called electrophiles.

The work should one day lead to less dependence on the energy-intensive treatment processes. Most of the energy in such processes is consumed in the activated sludge and its disposal. The energy consumption in water treatment produces around 4-5% of anthropogenic CO2 worldwide. to put that in perspective, according to the Air Transport Action Group in Geneva, international air transport produced 2.1% CO2 in 2019. The researchers published their work in the journal Bioelectrochemistry. In addition to cutting green house gas emissions, lowering the energy consumption of wastewater treatment would save billions in annual operation and maintenance costs.

Microbial fuel cells allow microbes to convert chemical energy into electricity, much like in a battery. In wastewater treatment, a microbial fuel cell can replace aeration while capturing electrons from wastewater organics. These electrons themselves are in turn a waste product of the microbial metabolism. All living organisms strive to discharge their excess electrons. This process is known as respiration or fermentation. The electricity generated the microbes can be used for useful applications in the wastewater treatment plant itself. The technology kills two birds with one stone. On the one hand, the treatment of the wastewater saves energy. On the other hand, it also generates electricity.

Up until now, microbial fuel cells have been used experimentally in wastewater treatment systems under ideal conditions, but under real and changing conditions they often fail. Microbial fuel cells lack regulation that controls the potential of anodes and cathodes and thus the cell potential. This can easily lead lead to a system failure. The entire cell must then be replaced.

To tackle this problem, the researchers added an additional reference electrode to the system that enables them to control their fuel cell. The system becomes more flexible. It can either work as a microbial fuel cell on its own and consume no energy, or it can be converted so that less energy is used for aeration while it purifies the wastewater more intensively. Frontis Energy uses a similar control system for its electrolysis reactors.

The system was operated for one year without major issues in the laboratory as well as a pilot in a wastewater treatment plant in Idaho. It removed contaminants at rates comparable to those in a classic aeration tanks. In addition, the microbial fuel cell could possibly be used completely independent of grid power. The researchers hope that one day it could be used in small wastewater treatment plants, such as cleaning livestock farms or in remote areas.

Despite the progress, there are still challenges to be overcome. They are complex systems that are difficult to build. At Frontis Energy we specialize in such systems and can help with piloting and commercialization.

(Photo: Wikipedia / National University of Singapore)

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Biochar from waste removes pharmaceuticals from wastewater

Biochar is a coal-like substance that is mainly made from agricultural waste products. It can remove contaminants such as pharmaceuticals from treated wastewater. This is the result of research carried out by scientists of the Pennsylvania State University and the Arid Lands Agricultural Research Center in Arizona. The biochar was made from two agricultural residues common in the US: cotton and guayule.

To test the ability of biochar to adsorb pharmaceuticals from treated wastewater, the scientists compared three common compounds. During adsorption, a material like a pharmaceutical adheres to the surface of solid biochar particles. In the case of absorption, in turn, one material is taken up into another, such as in a sponge.

The shrub guayule grows in the dry southwestern US and its waste was used for the biochar tested. Among bonatics, it is also called Parthenium argentatum. The shrub is cultivated as a source of rubber and latex. The plant is chopped to the ground and its branches crushed to extract the latex. The dry, mushy, fibrous residue that remains after the stalks are chopped up to extract the latex is called bagasse.

The results are important as they demonstrate the potential of biochar made from abundant agricultural waste. If it wasn’t re-used, this waste would have to be disposed at a cost. The production of biochar is an inexpensive additional processing step to reduce contamination in treated wastewater used for irrigation.

At the same time, most wastewater treatment plants are currently not equipped to remove emerging contaminants such as pharmaceuticals. If these toxic compounds were removed by biochar, the wastewater could be reprocessed in irrigation systems. This re-use is crucial in regions where water scarcity is a constraint for agricultural production.

The pharmaceutical compounds used in the study were: sulfapyridine, an antibacterial drug commonly used in veterinary medicine; docusate, a widely used laxative and stool softener, and erythromycin, an antibiotic used to treat infections and acne.

The results, published in the journal Biochar, suggest that biochar can effectively adsorb agricultural waste. The biochar obtained from cotton processing waste was a lot more efficient. It adsorbed 98% of the docusate, 74% of the erythromycin and 70% of the sulfapyridine from aqueous solutions. In comparison, the biochar obtained from guayule residues bagasse adsorbed 50% of the docusate, 50% of the erythromycin and only 5% of the sulfapyridine.

Research found that a temperature rise from about 340°C to about 700°C in the oxygen-free pyrolysis process used to convert agricultural waste materials to biochar resulted in a improved capacity for adsorption.

To date, there have been no studies on the use of guayule bagasse to make biochar and remove contaminants, nor are there any for cotton processing waste. Some research has been carried out into the possible removal of other contaminants. However, this is the first study to use cotton gin waste specifically to remove pharmaceuticals from water.

The research is more than theoretical. At Frontis Energy we hope that the technology will soon be available on industrial scale. With cotton gin waste being widespread even in the poorest regions, we believe this source of biochar holds great promise for decontaminating water. The next step would be to develop a mixture of biochar material to adsorb a wider variety of contaminants from water.

(Photo: Wikipedia)

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Highly efficient desalination using carbon nanotubes

Separating liquid compartments is not only important for generating energy in biological cells, respiration that is, but also for electrochemical cells and desalination through reverse osmosis and other processes. Therefore, scientists and engineers intensively research this field. We have already reported in several posts about promising attempts to make membranes cheaper and more effective. New nanomaterials have also been developed.

As a result of climatic changes caused by global warming, water scarcity is increasingly becoming a problem in many parts of the world. Settlements by the sea can secure their supply by desalinating water from seawater and brackish water sources. The process, however, is very energy intensive.

Now, researchers at California’s Lawrence Livermore National Laboratory (LLNL) have developed artificial pores made of carbon nanotubes that remove salt from water so efficiently that they are comparable to already available commercial desalination membranes. These tiny pores are only 0.8 nanometers in diameter. A human hair with a diameter of 60,000 nm. The researchers published the results in the journal Science Advances.

The predominant technology used to remove salt from water is reverse osmosis. A thin-film composite membrane (TCM) is used to separate water from ions. Hitherto the performance of these membranes has, however, been unsatisfactory. There is, for example, always a tradeoff between permeability and selectivity. In addition, exisiting membranes often show insufficient ion repulsion and are contaminated by traces of impurities. This requires additional cleaning stages, which again increase energy costs.

As is so often the case, the researchers got inspired by nature. Biological water channels, also known as aquaporins, are a great model for the structures that can improve performance. These aquaporins have extremely narrow internal pores that compress the water. This enables extremely high water permeability with transport rates of more than 1 billion water molecules per second per pore. Due to the low friction on the inner surfaces, carbon nanotubes represent one of the most promising approaches for artificial water channels.

The research group developed nanotube porins that insert themselves into artificial biomembranes. These engineered water channels simulate the functionality of aquaporin channels. The researchers measured the water and ion transport through their artificial porins. Computer simulations and experiments using the artificial porins in lipid membranes showed improved flux and strong ion repulsion in the channels of carbon nanotubes.

This measurement method can be used to determine the exact value of the water-salt permselectivity in such narrow carbon nanotubes. Atomic simulations provide a detailed molecular view of the novel channels. At Frontis Energy, we are excited about this promising approach and hope to see a commercial product soon.

(Image: Wikipedia)

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Rechargeable PEM fuel cell with hydrogen storage polymer

Energy-converting devices such as fuel cells are among the most efficient and clean alternative energy-producing sources. They have the potential to replace fossil-fuel-based power generators. More specifically, proton exchange membrane fuel cells (PEMFCs) are promising energy conversion devices for residential, transportation and portable applications owing to their high power density and efficiency at low operating temperatures (ca. 60–80 °C). For the complete approach, with electrolytic hydrogen renewable sources, PEM fuel cells can become one of the cleanest energy carriers. This is because water is the final product of such energy conversion systems. Currently, Nafion™ membranes are regularly used as hydrogen barriers in these fuel cells.

A Proton exchange membrane

Sufficient hydrogen gas supply is crucial for practical application of the PEMFC systems. Currently, expensive high-pressure tanks (70 MPa) are state-of-the-art for hydrogen storage. Besides cost, there are other drawbacks such as portability and safety. In order to address these issues, alternative hydrogen storage materials have been extensively investigated. For example, metal hydrides and organic hydride materials, can fix and release hydrogen via covalent bonding.

Now, Dr. Junpei Miyake and colleagues of the University of Yamanashi, Japan, have proposed an “all-polymer” rechargeable PEMFC system (RCFC). The work has been published in Nature Communications Chemistry. Their strategy was to apply a hydrogen-storage polymer (HSP) sheet (a solid-state organic hydride) as a hydrogen-storage medium inside the fuel cell. With this approach, the issues like toxicity, flammability and volatility as well as concerns related to other components such as the fuel reservoir, feed pump and vaporizer were solved. The HSP structure is based on fluorenol/fluorenone groups that take over hydrogen-storage functionality.

In order to test the performance of their HSP-based rechargeable fuel cell, the scientists attached the HSP sheet of the membrane electrode to the catalyst layer of the anode. At the same time, the cathode side was operated as in a regular PEMFC. The researchers reported that an iridium catalyst has been applied to the inside of the HSP sheet to improve the hydrogen-releasing and fixing processes.

Fuel cell operation, cycle performance and durability were tested using cycles of 6 periodic steps. At first, hydrogen was infused into HSP sheet for 2 h, followed by nitrogen gas flushing to remove hydrogen from the anode. Then, heating of the cell up to 80°C to initiated hydrogen release from the HSP sheet. Together with oxygen gas supplied to the cathode side the fuel cell produced constant electrical current.

The team demonstrated that their HSP sheet released 20%, 33%, 51%, or 96% of the total fixed hydrogen gas in 20, 30, 60, or 360 min, respectively. The temperature was 80°C in the presence of the iridium catalyst. Also, the iridium catalyst could absorb up to 58 mol% hydrogen, which was considerably lower than that stored in the HSP. The maximum operation time was approximately 10.2 s / mgHSP (ca. 509 s for 50 mg of HSP) at a constant current density of 1 mA / cm2. The RCFCs reached cycleability of least 50 cycles. In addition, the utilization of a gas impermeable sulfonated poly-phenylene membrane (SPP-QP, another type of PEM) turned out to be a good strategy to enhance the opration time of the RCFC.

The advantageous features of the reported RCFC system include better safety, easier handling and lower weight. These are perfect for example in mobile application such as fuel cell vehicles. However, for the improvement of the RCFC performance, hydrogen storage capacity and kinetics (H2-releasing/fixing reactions) as well as catalyst stability need further improvements.

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Reverse electrodialysis using Nafion™ membranes to produce renewable energy

In the order to address the global need for renewable and clean energy sources, salinity-gradient energy harvested by reverse electrodialysis (RED) is attracting significant interest in recent years. In addition, brine solution coming from seawater desalination is currently considered as a waste; however thanks to its high salinity it can be exploited as a valuable resource to feed RED. RED is an engineered adaptation of nature’s osmotic energy production where ions flow pass the cell membrane in order to produce the universal biological currency ATP. This energy is also harvested by the RED technology.

Now, more than ever there is need for sustainable and environmentally friendly technological solutions in order to keep up with ever growing demand for clean water and energy. The traditional linear way “produce and throw away” does no longer serve the society anymore and the new approach of circular economy has take a place, where any waste can be considered as a valuable resource for another process. In this respect, reverse electrodialysis is a promising electromembrane-based technology to generate power from concentrated solutions by harvesting the Gibbs free energy of mixing the solutions with different salinity. In particular, brine solutions produced in desalination plants, which is currently considered as a waste, can be used as concentrated streams in RED stack.

Avci et al. of the University of Calabria, Italy, have recently published their solution for brine disposal using RED-stack. They have realized that in order to maximize generated power, the high permselectivity and ion conductivity of membrane components in RED are essential. Although Nafion™ membranes are among the most prominent commercial cation exchange membrane solutions for electrochemical applications, no study has been done in its utilization toward RED processes. This was the first reported RED stack using Nafion™ membranes.

A typical RED unit is similar to an electrodialysis (ED) unit, which is a commercialized technology. ED uses a feed solution and the electrical energy, while producing concentrate and dilute, separately. On the other side, RED uses concentrated and dilute solutions that are mixed together in a controlled manner in order to produce spontaneously electrical energy. In a RED stack, repeating cells comprised of alternating cation and anion exchange membranes that are selective for anions and cations. The salinity gradient over each ion exchange membrane creates a voltage difference which is the driving force for the process. The ion exchange membranes are one of the most important components of a RED stack.

The performance of Nafion™ membranes (Nafion™ 117 and Nafion™ 115) have been evaluated under a high salinity gradient conditions for the possible application in RED. In order to simulate the natural environments of RED operation, NaCl solution as well as multicomponent NaCl + MgCl2 have been tested.

Gross power density under high salinity gradient and the effect of Mg2+ on the efficiency in energy conversion have been evaluated in single cell RED using Nafion™ 117, Nafion™ 115, CMX and Fuji-CEM-80050 as cation exchange membranes. Two commercial cation exchange membranes – CMX and Fuji-CEM 80050, frequently used for RED applications, have served as benchmark.

The results show that under the condition of 0.5 M / 4.0 M NaCl solutions, the highest Pd,max was achieved using Nafion™ membrane. This result is attributed to their outstanding permselectivity compared to other CEMs. In the presence of Mg2+ ions, Pd,max reduction of 17 and 20% for Nafion™ 115 and Nafion™ 117 were recorded, respectively. Both membranes maintained their low resistance; however a loss in permselectivity was measured under this condition. Even though, it was reported that Nafion™ membranes outperformed other commercial membranes such as CMX and Fuji-CEM-80050 for RED application.

(Photo: Wikipedia)

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CO2-neutral traffic

Fossil fuels have made tremendous social and economic advances pssible. This becomes clear, among other things, if you look at the increase in road traffic. Around 90 million vehicles were produced in 2019. In 2000 it was 60 million. It is assumed that the number of vehicles produced by 2030 will grow to 120 million. The increase in road mobility undoubtedly has a positive impact on social mobility and economic growth. However, this also makes the traffic increase a self-accelerating process. Economic growth in the Brics countries (Brazil, Russia, India, China and South Africa) is particularly crucial in this regard. At the same time, it is expected that the proportion of electric vehicles, including hybrids, will also increase sharply. However, whether this is realistic, given the limited lithium reserves, can again be doubted.

In 2010 more than 1 billion cars were registered worldwide. With an annual increase of around 3%, it was already 1.3 billion in 2019. These emit around 6.0 billion tons of CO2 annually (out of a total of 33 billion tons worldwide), making them the largest expanding source of CO2. Energy-related CO2 emissions are generally continuing to rise, although this increase was briefly interrupted by the global health crisis of 2020. In addition, there are 20 to 30% of emissions from the production of fuels and the manufacture and disposal of vehicles.

Life cycle analyzes of vehicles with different drive concepts are the subject of many studies. When it comes to CO2 emissions, the energy source is crucial. Two main developments are discussed today: the electrification of the propulsion system (i.e. fully and partially electrified vehicles) and the electrification of fuels (i.e. hydrogen and synthetic fuels).

In the manufacture of synthetic fuels, water is broken down into oxygen and hydrogen by electrolysis with renewable electricity. Due to the temporary oversupply of renewable electricity, this energy is particularly cheap. The hydrogen can then be used in hydrogen vehicles propelled by fuel cells. Alternatively, CO2 can be converted into hydrocarbons with hydrogen and then used in conventional combustion engines in a climate-neutral manner. The advantage of fuel cell vehicles is their high efficiency and the low cost of electrolysis. The disadvantage is the lack of a hydrogen infrastructure. Converting from hydrocarbons to hydrogen would cost trillions. The cheaper alternative would be synthetic hydrocarbons. However, the development is still in its infancy and the production of synthetic fuels cannot yet be carried out on a large scale.

Hydrogen and synthetic fuels are a necessary addition to electromobility, especially for long-distance and load transport. The widespread view that the low level of efficiency of internal combustion engines makes these fuels uninteresting ignores the possibility of using them to store and transport energy and to enable climate neutrality for air and shipping traffic. If you compare the CO2 emissions from electric motors and electrified fuels, it becomes clear that these mainly depend on the CO2 pollution of the electricity used.

Synthetic fuel sources

The production of synthetic fuel requires renewable electricity, water and CO2. The technical processes are known. However, the first large-scale industrial plants are only in the planning phase. However, pilot projects such as that of the Canadian company Carbon Engineering have shown the technical feasibility of scaling. The generation costs depend mainly on the size of the plant and the electricity price, which results from the local conditions, the structure of the electricity market and the share of renewable electricity.

The decentralized production of these fuels brings not only climate neutrality but also geopolitical gains. Since CO2 and renewable energy – in contrast to lithium – are generally accessible resources, users of this technology become independent of energy imports. At Frontis Energy we think these are strong arguments in favor of synthetic fuels.

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Accelerated deforestation in the EU

Forests are vital to our society. In the EU, forests make up around 38% of the total land area. They are important carbon sinks as they eliminate around 10% of EU greenhouse gases. Efforts to conserve them are a key part of EU climate targets. However, the increasing demand for forest products poses challenges for sustainable forest management.

According to a report recently published in the renowned science magazine Nature, the EU’s deforested area has increased by 49% and with it the loss of biomass (69%). This is due to large-scale deforestation, which reduces the continent’s carbon absorption capacity and accelerates climate change.

The analyzed a series of very detailed satellite data. The authors of the report show that deforestation occurred primarily on the Iberian Peninsula, the Baltic States, and Scandinavia. Deforestation of forest areas increased by 49% between 2016 and 2018. Satellite images also show that the average area of ​​harvested land across Europe has increased by 34 percent, with potential implications for biodiversity, soil erosion and water regulation.

The accelerating deforestation could thwart the EU’s strategy to combat climate change, which aims in particular to protect forests in the coming years, the experts warn in their study. For this reason, the increasing use of forests is challenging to maintain the existing balance between the demand for wood and the need to preserve these key ecosystems for the environment. Typically, industries such as bioenergy or the paper industry are the driving forces behind deforestation.

The greatest acceleration in deforestation was recorded in Sweden and Finland. In these two countries, more than 50% of the increase in deforestation in Europe has been recorded. Next in line are Spain, Poland, France, Latvia, Portugal and Estonia, which together account for six to 30% of the increase, the study said.

Experts suggest linking deforestation and carbon emissions in model calculations before setting new climate targets. The increase in forest harvest is the result of the recent expansion of global wood markets, as evidenced by economic indicators for forestry, timber bioenergy and international trade. If such a high forest harvest continues, the EU’s vision of forest-based mitigation after 2020 could be compromised. The additional carbon losses from forests would require additional emission reductions in other sectors to achieve climate neutrality.

At Frontis Energy, we find the competition between bioenergy and this important carbon sink particularly disturbing, as both are strategies to mitigate global warming.

(Photo: Picography / Pixabay)

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Light-driven process turns greenhouse gases into valuable products

Much research has been done in order to reduce the use of fossil petroleum products as fuels. In that respect syngas (synthetic gas) seems as a great opportunity for sustainable energy developments. Syngas is the mixture composed of hydrogen (H2) and carbon monoxide (CO) as its main components. It represents an important chemical feedstock used widely for industrial processes for generating chemicals and fuels:

Global use of syngas in industrial processes.

Syngas can be produced from methane (CH4) in a reforming reaction with water (H2O), oxygen (O2) or carbon dioxide (CO2). The process called methane dry reforming (MDR) can be combined with carbon dioxide:

CH4 + CO2 → 2 H2 + 2 CO

It is an environmentally friendly path, turning two greenhouse gases into a valuable chemical feedstock.

However, the MDR is process requires chemical catalysts and high temperatures in the range between 700 − 1,000°C. Usually, it suffers from coke deposition and, in consequence, catalyst deactivation.

Some chemists have recently demonstrated that light, and not heat, might be a more effective solution for this energy-hungry reaction.

The photocatalytic solution

A team of researchers at the Rice University in Houston, Texas, together with colleagues from Princeton University and the University of California have developed superior light-stimulated catalysts that can efficiently power MDR reactions without any heat input. This work has been published in the prestigious journal Nature Energy.

They have reported a highly efficient and coke-resistant plasmonic photocatalyst containing precisely one ruthenium (Ru) atom for every 99 copper (Cu) atoms. The isolated single-atom of Ru obtained on Cu antenna nanoparticles provides high catalytic activity for the MDR reaction. On the other side, Cu antennas allow strong light adsorption and under illumination and deliver hot electrons to ruthenium atoms. The researchers suggested that both, hot-carrier generation and single-atom structure are essential for excellent catalytic performance in terms of efficiency and coking resistance.

The optimal Cu-Ru ratio have been investigated in synthesized series of CuxRuy catalysts with varying molar ratios of plasmonic metal (Cu) and catalytic metal (Ru), where x,y are atomic percentage of Cu and Ru. Overall, the Cu19.8Ru0.2 was the most promising composition in terms of selectivity, stability and activity. In comparison to pure Cu nanoparticles, the Cu19.8Ru0.2 mix exhibits increased photocatalytic reaction rates (approx. 5.5 times higher) and improved stability with its performance maintained over 20 h period. Calculations showed that isolated Ru-atoms on Cu lower the activation barrier for the methane dehydrogenation step in comparison to pure Cu without promoting undesired coke formation.

In addition, the research has been supported by different methods (CO-DRIFTS with DFT) in order to unravel and prove single-atom Ru structures on Cu nanoparticles occurring in Cu19.9Ru0.1 and Cu19.8Ru0.2 compositions.

The comparison between thermocatalytic and photocatalytic activity at the same surface for MDR has also been demonstrated. The thermocatalytic reaction rate at 726°C (approx. 60 µmol CH4 / g / s) was less than 25% of photocatalytic reaction rate under white-light illumination with no external heat (approx. 275 µmol CH4 / g / s). This enhancement in the activity is attributed to the hot-carrier generated mechanism which is predominant in the photocatalytic MDR. The role of the hot-carrier is an increase in C−H activation rates on Ru as well as improved H2 desorption.

The scientists also reported the catalyst achieving a turnover frequency of 34 mol H2  / mol Ru / s and photocatalytic stability of 50 h under focused white light illumination (19.2 W / cm2) with no external heat.

As the synthesized photocatalysts is primarily based on Cu which is an abundant element, this approach provides a promising, sustainable catalyst operating at low-temperatures for MDR. This allows cheaper syngas production at higher rates, bringing us closer to a clean burning carbon fuel.

(Photo: Wikipedia)

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Microbial Power-to-Gas in exhausted oilfields as bridge between renewable and fossil energy

An abandoned or unproductive oilfield can be reused for methane production from CO2 using renew­able electrical power. Exhausted oilfields can be reactors for the conversion of renewable energy to natural gas using microbes. To achieve this, an oilfield can be made electrically conductive and catalytically active to produce natural gas from re­newable power sources. The use of natural gas is superior to any battery because of the existing infra­structure, the use in combustion engines, the high energy density and because it can be recycled from CO2. Oil­fields are superior to any on-ground production because of the enormous storage capaci­ties. They are already well explored and these geological formations underwent environmental risk assessments. Lastly, the microbial power-to-gas technology is already available.

Process summary

Whole process (end-to-end via methane)

50% electrical efficiency

Energy density of methane

180 kWh kg1

Storage capacity per oilfield

3 GWh day1

Charge/Discharge cycles

Unlimited

Investment (electrodes, for high densities)

$51,000 MW1

Cost per kWh (>5,000 hours anode lifetime)

<$0.01 kWh1

Electrolyte

Seawater

The Problem

To address the problem of storing renewable energy, batteries have been proposed as a possible so­lution. Lithium ion batteries have a maximum energy storage capacity of 0.3 kWh kg−1. To date, this is consid­ered the best trade-off between cost and efficiency but these batteries are still too inefficient to replace gasoline, which has a capacity of about 13 kWh kg−1. This makes battery driven cars heavier than conventional cars. Lithium air batteries are considered a possible al­ternative because they can reach theoretical capacities of 12 kWh kg−1 but technical difficulties have prevented them from being used for transportation.

In contrast, methane has an energy density of 52 MJ kg−1 corresponding to 180 kWh kg−1 which is sec­ond only to hydrogen with 500 kWh kg−1, not counting in nuclear energy. This high energy den­sity of methane and other hydrocarbons along with their facile usage, is the reason why they are used in combustion and jet engines that drive nearly all transportation to date. While electrical cars seem to be a tempting green alternative, the fact that combustion engines and the fueling infrastructure are so wide-spread makes it difficult to switch.

In addition to the difficulty of changing habits, battery-driven electrical cars need other limited natural resources such as lithium. To equip all 94 million automobiles produced worldwide in 2017, 3 mega tons lithium carbonate would need to be mined annually⁠. This is nearly 10% of the entire recoverable lithium resources of 35 mega tons worldwide. Although lithium and other met­als can be recycled, it is clear that metal based batteries alone will not build the bridge between green en­ergy and tradi­tional ways of transportation due to the low energy densities of metals. And this does not even take into account other energy de­mands such as industrial nitrogen fixation, aviation or heating.

For Germany, with its high proportion of renewable energy, fuel for cars is not the only problem. As re­newable energy is generated in the north, but many energy consumers are in the south, the grid ca­pacity is frequently reached during peak production hours. A steadier energy output can only be ac­complished by decentralizing the production or by en­ergy storage. To decentralize production, home­owners were en­couraged to equip their property with solar panels or wind­mills. As tax incentives phase out, homeowners face the prob­lem of energy storage. The best product for this group of cus­tomers so far are again lithium ion batteries but investment costs of $0.10 kWh−1 are still unattractive espe­cially be­cause these products store the en­ergy as electricity which can only be used for a short time and is less efficient than natural gas when used for heating.

Natural gas is widely used as energy source today and the global energy infrastructure is designed for natural gas and other fossil fuels. Increasing demand and lim­ited resources for these fossil fuels were the main driv­ers of oil and gas prices during the last years, slowed by the recent economic crises and hydraulic fractur­ing (fracking). The high oil price attracted in­vestors to recover oil using techniques that be­come in­creasingly expensive and are environmental risks such as deep-sea drilling or tar sand extraction. Ironically, the high oil price made costly renew­able ener­gies an economically feasible alterna­tive and helped driving down their cost. Since habits are difficult to change and building an entirely new infrastructure only for renewable energies does not seem economically feasible today while CO2 drives global warming, a more realistic solution needs to be found.

Microbial Power-to-Gas could be a bridging technology that integrates renewable energy into the existing fossil fuel in­frastructure. It reaches break even in less than 2 years if certain preconditions are met. This is ac­complished by integrating methane produced from renewable energy into the current oil and gas pro­ducing infrastructure. The principal idea is to use carbon instead of metals as energy carrier because of its high energy density when bound to hydrogen. The benefits are:

  • High energy density of 180 kWh kg−1 methane
  • Low investments due to existing infrastructure (natural gas, oilfield equipment)
  • Carbon is not a limited resource
  • Low CO2 footprint due to CO2 recycling
  • Methane is a transportation fuel
  • Methane is the energy carrier for the Haber-Bosch process
  • Inexpensive catalysts further reduce initial investments
  • Low temperatures due to bio-catalysis
  • No toxic compounds used
  • No additional environmental burden because existing oilfields are reused

The solution

Methane can be synthesized by microbes or chemically. Naturally, methane is produced by anaerobic (oxygen-free) microbial biomass decomposition. The energy for biomass synthesis is provided by sun­light or chemical energy like hydrogen. In the case of methanogens (methane producing microbes), energy is harvested after CO2 and hydrogen were re­leased from biomass de­composition following a 1-to-4 stoichiometry:

CO2 + 4 H2 → CH4 + 2 H2O

Without microbes, methane is produced by the Nobel-prize winning Sabatier reaction and several attempts are currently underway to use it on industrial scale. It is necessary to split water into hydrogen and use this to re­duce CO2 in the gas phase. A major drawback of the Sabatier reaction is the need for high tempera­tures around 385ºC, and a nickel catalyst that becomes quickly spent. Methanogens use iron-nickel enzymes called hydro­genases to harvest energy from hydrogen, but do so at ambient tempera­tures.

To produce abiotic hydrogen, water is split using precious metal catalysts. Microbes split water using hydrogenases in reverse direction and the produced hydro­gen is oxidized by methanogens that grow in the electrolyte or on electrodes to pro­duce methane⁠. This reaction oc­curs at the correct 1-to-4 stoichiometry⁠ at potentials that are near to the theoretical hydro­gen production potential of −410 mV obtained from the Nernst equation in neutral aqueous solu­tions⁠. Methanogenic microorganisms are able to reduce the overpoten­tial.

Power-to-Gas concept for exhausted oilfields. Electrolysis catalyzes water splitting inside the oilfield producing methane gas and O2.

The future challenge will be to accelerate methane production rates as has been reported for a high tem­perature oilfield cul­tures⁠. Besides increasing the temperature, the most obvious solution is to use a higher reactive surface and bringing both electrodes closer together. Using carbon brushes that are poor hydrogen catalysts but provide a higher sur­face for microbial attachment is one possibility. Methane production correlates with microbial cell numbers in the reactors.

The number of methanogens in microbial electrolysis reactors correlates with the electrode surface.

To overcome the problem of expensive carbon (and also steel) brushes for large scale applications,exhausted gas and oilfields could be used. They provide a high surface area and are usually eco­nomic liabili­ties and not assets. Methano­gens inhabit oilfields where they carry out the final step in anaero­bic petroleum degradation⁠. Hence, oilfields can be seen as bioreactors at geological scale. Geological formations provide ideal con­ditions for produc­ing, storing and ex­tracting methane.

Open questions and potential solutions

Oilfield porespace volume

The Californian Summerland oilfield, for instance, has been abandoned and extensively studied in the past. It produced 27 billion barrels of oil and 2.8 billion m3 methane during its lifetime of 90 years. This maximum load of 3.5 billion m3 left the same volume of porespace filled with seawater behind. Only 2% of these pores are larger than 50 µm, which is necessary for microbial growth assuming dimensions of 1 x 2 µm of a methanogen cell. Experiments showed that the resulting 70 million m3 accessible porespace have a storage capacity of 35,000 TW.  That is a lot of methane assuming a solubility of 0.74 kg methane m−3 seawater at 500 m water depth⁠. All German off-shore windfarms together have a capacity of 7,000 MW. Obviously, the limiting factor is not the volumet­ric storage capacity of an oilfield.

Microbial methane production rates

But how fast can microbes produce methane in an hypothetical oilfield? Under optimal conditions, methanogens that grow on electrodes (typically the genus Methanobacterium or Methanobrevibacter) can produce methane at a rate of 100-200 nmol ml−1 day−1 (equals 2.24-4.48 ml l−1 day−1) depending on catalyst and potential. Using a produc­tion rate of 15 J ml−1 day−1 of methane (190 nmol ml−1 day−1), the en­tire microbially accessi­ble oilfield (2%) has a ca­pacity of 3.6 mil­lion MBtu per year. Mi­crobes would theoretical­ly consume 1 TWh per year for 3.6 mil­lion MBtu meth­ane pro­duction if there were no losses and elec­trical power is translate­d into methane 1-to-1. A power genera­tor of 121 MW would be suffi­cient to supply the entire oil­field at these rates. However, all Ger­man off-shore wind­farms produce 7,000 MW mean­ing that only 3% off-peak power can be cap­tured by our ex­ample oilfield. There­fore, the catalytic sur­face and activity must be in­creased to accel­erate methane conversion rates.

Since methanogens produce methane from hydrogen, not only the 2% porespace big enough for cells can be used resulting in an increased catalytic surface to nearly 60%. A hydrogen cata­lyst needs to be found that does not out-pace methanogen growth to keep the reservoir pH within the limits of 6-8 re­quired for methanogen growth. This hydrogen catalyst must be cheap and render an oilfield electrically conductive. A chemical formulation that mimics microbial hydrogen catalysis could be used. It has the potential to turn a non-conductive and non-catalytic oilfield into a con­ductive hydrogen catalyst sufficient to sustain methane produc­tion needed to store all of Germany’s electricity produced by off-shore wind­farms. This catalyst is solu­ble in water when inactive. To become active, it coats mineral surfaces by precipitation that can be triggered by indigenous microbes or by electrical polarization. The investment would be $2.3 mil­lion per MW storage capaci­ty ($16 bil­lion for the entire 7,000 MW). Due to microbial growth, the cat­alytic activity of the system improves dur­ing operation and there is no need for the second component if an immediate production is not crucial. The investments made on the cathode side would then be as low as $600 per MW ($4.2 million for 7,000 MW).

Anodes

As the cathodic side of the reaction can be excluded as limiting factor, the anode needs to be de­signed. Several commercially available anodes such as mixed metal oxides (up to 750 A m−2) with platinum on carbon black or niobium anodes (Pt/C, 5-10 kA m−2) could be used. Anodes based on platinum are the most cost-efficient material available on the market. Invest­ments made for Pt/C (10%, 6 mg cm−2) anodes will amount to $50,000 per MW ($350 million for 7,000 MW). How­ever, the exact amount of Pt needed for the reaction still needs to be evaluated in an experiment be­cause the corrosion rate at 2 V cell voltage is unknown. An often cited value for the life­time of fuel cells is 5,000 hours and is used here to determine the costs per kWh. For 5,000 hours life­time, the costs per kWh will be at the targeted limit of $0.01 but may be well below that because Pt/C anodes can be re­cycled and the Pt load may be reduced to 3 mg cm−2 (5%). Alternatively, steel anodes (SS316, 2.5 kA m−2, $54,000 per MW) can be used but it is unclear when steel anodes fail to elec­trolyze. In conclusion, the anodic side is the cost-driving factor. Hope­fully, better water splitting anodes will lower these costs in future.

Cost estimation summary

Windfarms

Already in place

CO2 injection

Already occurred

Natural gas capturing equipment

Already in place

Microbial seed

Wastewater from oil rig

Cathode costs

$600 MW1

Anode costs

$50,000 MW1

Electrolyte (seawater)

Free

Total (>5,000 hours anode lifespan)

<$0.01 kWh1

Energy and conversion efficiencies

The whole cell voltage for microbial power-to-gas reactions varies from 0.6 to 2.0 V, depending on ca­thodic rates, anodic corrosion and the presence of a membrane. Higher voltages will accelerate an­ode corrosion, again, making anodes the limiting factor. As the voltage decreases, methane production rates become slower but also more efficient. The voltage also depends on the pH of the oil­field. An oil­field that underwent CO2 injection as enhanced recovery method will have a low pH, provid­ing better condi­tions for hydrogen production but not for microbial growth and must be neutralized using seawa­ter. As stated above, the oilfield, being the cathode, is not limiting the the sys­tem. The use of Pt/C an­odes eliminates the overpotential problem on the anode side. Hence, we can assume an ideal system that splits water at 1.23 V. However, the voltage is often 2 V due to anode and cathode overpotentials. Optimized cul­tures and cathodes produce about 190 nmol ml−1 day−1 methane which equals 0.15 J ml−1 day−1 using the energy of combustion of 0.8 MJ mol−1. The same electrolysis cell consumes 0.2 mW at a cell voltage of 2 V which equals 0.17 J ml−1 day−1 and the resulting energy effi­ciency is 91%. The anodes can be simple carbon brushes and the two cham­bers of the cell are separated by a Nafion™ membrane. The system can still be optimized by using Pt/C anodes and by avoiding mem­branes.

The overall electricity-methane-electricity efficiency also depends on the consumption side efficiency where methane is used in com­bustion engines and gas fired power plants. Such power plants fre­quently operate at efficiencies of 40- 60%. Assuming a reasonable power efficiency of 80% (see above), the overall electrical power recov­ery using gas fired power plants will be up to 50%. Besides the high efficiency of gas fired power plants, they are also easy to build and therefore contribute the a better power grid efficiency. Coal fired power plants can be upgraded to gas fired power plants.

Experimental approach

The conversion efficiencies of charge (Coulombs) transported across the circuit are usually between 70-100% in these systems depending on the electrode material⁠. Another efficien­cy limitation could arise from mass transport inhibition. Mass transport can be improved by pump­ing electrolyte adding more costs for pumping which still have to be de­termined. However, since most oil­fields undergo seawater injection for enhanced oil recovery the addi­tional cost may be negligi­ble. The total efficiency has yet to be determined in scale-up experiments and will depend on the fac­tors men­tioned above.

The reactor simulates oilfield conditions using sand as filling material under continuous flow of electrolyte.

Controlling the pH is crucial. Alkaline pHs significantly impede hydrogen pro­duction and therefore methanogenesis. This can be addressed by a software that monitors the pH and adjusts the po­tential accordingly. Addition of acids is not desired as this drives the costs. The software can also act as potentiostat that then fully controls the methane production process. To test the process under more realistic conditions, a drill core from an oilfield must be obtained.

Results show methane production in the simulation reactor. The appearance of methane in the anode compartment was a result of flow from the cathode to the anode, carrying produced methane with it.

Return of investment of the microbial power-to-gas process

The the microbial power-to-gas process in unproductive oilfields is economically superior to all other storage strategies because of the low start-up and operating costs. This is achieved because the ma­jor investments are the installation of oil- and gas production equipment and renewable power plants which are already in place as a precondition. These investments break even in a short amount of time.

But how can the microbial Power-to-Gas process accelerate the return of investment in renewable en­ergy? Only 8 out of 28 active off-shore windfarms reported their investment costs. These 8 produce roughly half the overall power of 1,600 MW corresponding to $7 billion. While the maximum production of an oilfield with unlimited supply of elec­tricity would yield hypothetical 3.6 million MBtu natural gas per year resulting a return of $13 million per year the real production is limited by off-peak power gener­ated by re­newable energy production. Assuming that the maximum annual methane production corresponds to 10% excess electrical power, $15 million per year can by gener­ated by selling 4.3 million MBtu meth­ane per year on the market. These are $15 million that are not lost during off-peak shutdowns. Clearly, this conservative estimate can help to compensate the invest­ment in renewable energy earlier. It also decreases the investment risk because the investment calcu­lations for new wind farms can be made on a more reliable basis.

In the example using all German windfarms (7,000 MW) this compensation roughly doubles. Using the $60 million generated by methane sales per year, the investment of $4 million for the cathodic cata­lyst and the $36 million for the Pt/C anodes are compensated for within less than a year. No other invest­ments are required because the target oilfield already produced oil and gas and all necessary installa­tion are in working condition. The target oilfield is swept using seawater as sec­ondary extraction method. Electrical installations are in place for cathodic protection of production equipment in order to prevent microbial corrosion, which, however, may need to be upgraded to pass the now higher power densities. Moreover, CO2 is used from CO2 injection as tertiary enhanced oil re­covery method. Only the pH may then need to be adjusted to sustain life by sweeping with seawater.

And this is not the end of oilfield storage capacity. In theory, an oilfield can store the entire amount of renewable energy produced in one year globally, allowing more than enough head room for future development and CO2 sequestration.

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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)