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Converting atmospheric carbon dioxide into carbon nanotubes for use in batteries

The Solar Thermal Electrochemical Process (STEP) converts atmospheric carbon dioxide into carbon nanotubes that can be used in advanced batteries. (credit: Julie Turner, Vanderbilt University)

The electric vehicle of the future will be carbon negative (reducing the amount of atmospheric carbon dioxide) not just carbon neutral (not adding CO2 to the atmosphere), say researchers at Vanderbilt University and George Washington University (GWU).

The trick: replace graphite electrodes in lithium-ion batteries (used in electric vehicles) with carbon nanotubes and carbon nanofibers recovered from carbon dioxide in the atmosphere. The new technology could also be used in sodium-ion batteries, currently under development for large-scale applications, such as the electric grid.

How to convert CO2 to carbon nanotubes

As described in an open-access paper in the Mar. 2 issue of the journal ACS Central Science, the project builds on a solar thermal electrochemical process (STEP) that can create carbon nanofibers from ambient carbon dioxide (see “‘Diamonds from the sky’ approach to turn CO2 into valuable carbon nanofibers“).

STEP uses solar energy to provide both the electrical and thermal energy needed to break down carbon dioxide into carbon and oxygen and to produce carbon nanotubes, which are stable, flexible, conductive and stronger than steel.

In lithium-ion batteries, the nanotubes replace the carbon anode used in commercial batteries. The team demonstrated that the carbon nanotubes gave a small boost to the performance, which was amplified when the battery was charged quickly.

In sodium-ion batteries, the researchers found that small defects in the carbon, which can be tuned using STEP, can unlock stable storage performance more than 3.5 times above that of sodium-ion batteries with graphite electrodes.

Both carbon-nanotube batteries were exposed to about 2.5 months of continuous charging and discharging and showed no sign of fatigue.

Depending on the specifications, making one of the two electrodes out of carbon nanotubes means that up to 40 percent of a battery could be made out of recycled CO2, according to Vanderbilt Assistant Professor of Mechanical Engineering Cary Pint, not including packaging (which could also be replaced in the future).

Cost benefits

This approach also reduces end-user battery cost, unlike most efforts to reuse CO2 aimed at low-valued fuels, like methanol, which “cannot justify the cost required to produce them,” Pint said.

“Other applications for the carbon nanotubes include carbon composites for strong, lightweight construction materials, sports equipment and car, truck and airplane bodies,” said GWU Professor of Chemistry Stuart Licht.

The researchers estimate that with a battery cost of $325 per kWh (the average cost of lithium-ion batteries reported by the Department of Energy in 2013), a kilogram of carbon dioxide has a value of about $18 as a battery material — six times more than when it is first converted to methanol — a number that increases when moving from large batteries used in electric vehicles to the smaller batteries used in electronics.

And unlike methanol, combining batteries with solar cells provides renewable power with zero greenhouse emissions.

Comparison of conventional natural-gas plant (A), which has CO2 as an exhaust, with carbon nanofiber/carbon nanotube-based natural-gas plant (B) (steam turbine cooling/electricity-generating process (pink), common to both, omitted) (credit: Stuart Licht et al./ACS Central Science)

Licht also proposed that the STEP process could be coupled to a natural gas-powered electrical generator. The generator would provide electricity, heat, and a concentrated source of carbon dioxide that would boost the performance of the STEP process.

At the same time, the oxygen released in the process could be piped back to the generator, where it would boost the generator’s combustion efficiency to compensate for the amount of electricity that the STEP process consumes. The end result could be a fossil fuel electrical power plant with net-zero CO2 emissions.

The research was partially supported by National Science Foundation and NSF Graduate Research Fellowship grants.

Abstract of Carbon Nanotubes Produced from Ambient Carbon Dioxide for Environmentally Sustainable Lithium-Ion and Sodium-Ion Battery Anodes

The cost and practicality of greenhouse gas removal processes, which are critical for environmental sustainability, pivot on high-value secondary applications derived from carbon capture and conversion techniques. Using the solar thermal electrochemical process (STEP), ambient CO2 captured in molten lithiated carbonates leads to the production of carbon nanofibers (CNFs) and carbon nanotubes (CNTs) at high yield through electrolysis using inexpensive steel electrodes. These low-cost CO2-derived CNTs and CNFs are demonstrated as high performance energy storage materials in both lithium-ion and sodium-ion batteries. Owing to synthetic control of sp3 content in the synthesized nanostructures, optimized storage capacities are measured over 370 mAh g–1 (lithium) and 130 mAh g–1 (sodium) with no capacity fade under durability tests up to 200 and 600 cycles, respectively. This work demonstrates that ambient CO2, considered as an environmental pollutant, can be attributed economic value in grid-scale and portable energy storage systems with STEP scale-up practicality in the context of combined cycle natural gas electric power generation.

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How to efficiently convert carbon dioxide from air to methanol fuel

The carbon dioxide-to-methanol process (credit: Surya Prakash)

Researchers at the University of Southern California (USC) Loker Hydrocarbon Research Institute have created fuel out of thin air — directly converting carbon dioxide from air into methanol at relatively low temperatures for the first time. While methanol can’t currently compete with oil, it will be there when we run out of oil, the researchers note.

The researchers bubbled air through an aqueous solution of pentaethylenehexamine (PEHA), adding a Ru-Macho-BH ruthenium catalyst to encourage hydrogen to latch onto the CO2 under pressure. They then heated the solution, converting 79 percent of the CO2 into methanol.

Though mixed with water, the resulting methanol can be easily distilled, said G.K. Surya Prakash, professor of chemistry and director of the Loker Hydrocarbon Research Institute.

Proposed reaction sequence for CO2 capture and in situ hydrogenation to methanol (credit: J. Kothandaraman et al./Journal of the American Chemical Society)

Scaling up

Methanol (aka “wood alcohol” or “rubbing alcohol”) is attractive because it can be directly used as a clean-burning liquid fuel for internal combustion engines and for fuel cells. It’s also a hydrogen storage medium and a chemical feedstock for producing a myriad of chemicals and products, including ethylene and propylene. It’s one of the most important building blocks in the chemical industry, with an annual production of more than 70 million tons.

The research is part of a broader effort to use renewable energy to transform greenhouse gas into its combustible form —  attacking global warming from two angles simultaneously.

Prakash and Olah hope to refine the process to the point that it could be scaled up for industrial use within five to 10 years. “Of course it won’t compete with oil today, at around $30 per barrel,” Prakash said. “But right now we burn fossilized sunshine. We will run out of oil and gas, but the sun will be there for another five billion years. So we need to be better at taking advantage of it as a resource.”

Lower temperatures

Previous efforts have required a slower multistage process with the use of high temperatures and high concentrations of CO2, meaning that renewable energy sources would not be able to efficiently power the process.

The new system operates at around 125 to 165 degrees Celsius (257 to 359 degrees Fahrenheit), minimizing the decomposition of the catalyst — which occurs at 155 degrees Celsius (311 degrees Fahrenheit). The system uses a homogeneous catalyst, making it a quicker “one-pot” process. In a lab, the researchers demonstrated that they were able to run the process five times with only minimal loss of the effectiveness of the catalyst.

The new process was published in the Journal of the American Chemical Society on Dec. 29. The research was supported by the USC Loker Hydrocarbon Research Institute.

Abstract of Conversion of CO2 from Air into Methanol Using a Polyamine and a Homogeneous Ruthenium Catalyst

A highly efficient homogeneous catalyst system for the production of CH3OH from CO2 using pentaethylenehexamine and Ru-Macho-BH (1) at 125–165 °C in an ethereal solvent has been developed (initial turnover frequency = 70 h–1 at 145 °C). Ease of separation of CH3OH is demonstrated by simple distillation from the reaction mixture. The robustness of the catalytic system was shown by recycling the catalyst over five runs without significant loss of activity (turnover number > 2000). Various sources of CO2 can be used for this reaction including air, despite its low CO2 concentration (400 ppm). For the first time, we have demonstrated that CO2 captured from air can be directly converted to CH3OH in 79% yield using a homogeneous catalytic system.

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Shaking out the nanomaterials: a new method to purify water

Extracting one- and two-dimensional nanomaterials from contaminated water (credit: Michigan Tech)

A new study published in the American Chemical Society’s journal Applied Materials and Interfaces has found a novel—and very simple—way to remove nearly 100 percent of nanomaterials from water.

Water and oil don’t mix, of course, but shaking them together is what makes salad dressing so great. Only instead of emulsifying and capturing bits of shitake or basil in tiny olive oil bubbles, this mixture grabs nanomaterials.

Dongyan Zhang, a research professor of physics at Michigan Tech, led the experiments, which covered tests on carbon nanotubes, graphene, boron nitride nanotubes, boron nitride nanosheets and zinc oxide nanowires. Those are used in everything from carbon fiber golf clubs to sunscreen.

“These materials are very, very tiny, and that means if you try to remove them and clean them out of contaminated water, that it’s quite difficult,” Zhang says, adding that techniques like filter paper or meshes often don’t work.

What makes shaking work is the shape of one- and two-dimensional nanomaterials. As the oil and water separate after some rigorous shaking, the wires, tubes and sheets settle at the bottom of the oil, just above the water. The oils trap them. However, zero-dimensional nanomaterials, such as nanospheres do not get trapped.

Green Nanotechnology

We don’t have to wait until the final vote is in on whether nanomaterials have a positive or negative impact on people’s health and environmental health. With the simplicity of this technique, and how prolific nanomaterials are becoming, removing nanomaterials makes sense. Also, finding ways to effectively remove nanomaterials sooner rather than later could improve the technology’s market potential.

“Ideally for a new technology to be successfully implemented, it needs to be shown that the technology does not cause adverse effects to the environment,” the authors write. “Therefore, unless the potential risks of introducing nanomaterials into the environment are properly addressed, it will hinder the industrialization of products incorporating nanotechnology.”

Purifying water and greening nanotechnology could be as simple as shaking a vial of water and oil.


Michigan Technological University | Shaking the Nanomaterials Out: New Method to Purify Water

Abstract of A Simple and Universal Technique To Extract One- and Two-Dimensional Nanomaterials from Contaminated Water

We demonstrate a universal approach to extract one- and two-dimensional nanomaterials from contaminated water, which is based on a microscopic oil–water interface trapping mechanism. Results indicate that carbon nanotubes, graphene, boron nitride nanotubes, boron nitride nanosheets, and zinc oxide nanowires can be successfully extracted from contaminated water at a successful rate of nearly 100%. The effects of surfactants, particle shape, and type of organic extraction fluids are evaluated. The proposed extraction mechanism is also supported by in situ monitoring of the extraction process. We believe that this extraction approach will prove important for the purification of water contaminated by nanoparticles and will support the widespread adoption of nanomaterial applications.

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Chemicals that make plants defend themselves could replace pesticides

Researchers used the relative induction of GUS activity as a screening tool for identifying new chemical elicitors that induce resistance in rice to the white-backed planthopper Sogatella furcifera (credit: Xingrui He et al./Bioorganic & Medicinal Chemistry Letters)

Chemical triggers that make plants defend themselves against insects could replace pesticides, causing less damage to the environment. New research published in an open-access paper in Bioorganic & Medicinal Chemistry Letters identifies five chemicals that trigger rice plants to fend off a common pest — the white-backed planthopper, Sogatella furcifera.

Pesticides have a detrimental effect on ecosystems, ravaging food chains and damaging the environment. One of the problems with many pesticides is that they kill indiscriminately.

Sogatella furcifera (credit: BIO Photography Group/CNC, Biodiversity Institute of Ontario)

For rice plants, this means pesticides kill the natural enemies of one of their biggest pests, the white-backed planthopper Sogatella furcifera. This pest attacks rice, leading to yellowing or “hopper burn,” which causes the plants to wilt and can damage the grains. It also transmits a virus disease called, southern rice black-streaked dwarf virus, which stunts the plants’ growth and stops them from “heading,” which is when pollination occurs.

Left untreated, many of the insects’ eggs would be eaten, but when pesticides are used, these hatch, leading to even more insects on the plants. What’s more, in some areas as many as a third of the planthoppers are resistant to pesticides.

“The extensive application of chemical insecticides not only causes severe environmental and farm produce pollution but also damages the ecosystem,” explained Dr. Jun Wu, one of the authors of the study and professor at Zhejiang University 
in China. “Therefore, developing safe and effective methods to control insect pests is highly desired; this is why we decided to investigate these chemicals.”

Enhancing plants’ natural defense mechanisms

Plants have natural self-defense mechanisms that kick in when they are infested with pests like the planthopper. This defense mechanism can be switched on using chemicals that do not harm the environment and are not toxic to the insects or their natural enemies.

In the new study, researchers from Zhejiang University 
in China developed a new way of identifying these chemicals. Using a specially designed screening system, they determined to what extent different chemicals switched on the plants’ defense mechanism. The team designed and synthesized 29 phenoxyalkanoic acid derivatives. Of these, they identified five that could be effective at triggering the rice plants to defend themselves.

The researchers used bioassays to show that these chemicals could trigger the plant defense mechanism and repel the white-backed planthopper, which suggests potential use in insect pest management.

“We demonstrate for the first time that some phenoxyalkanoic acid derivatives have the potential to become such plant protection agents against the rice white-backed planthopper,” said Dr. Yonggen Lou, one of the authors of the study and professor at Zhejiang University 
in China. “This new approach to pest management could help protect the ecosystem while defending important crops against attack.”

The next step for the research will be to explore how effective the chemicals are at boosting the plants’ defenses and controlling planthoppers in the field.

Abstract of Finding new elicitors that induce resistance in rice to the white-backed planthopper Sogatella furcifera

Herein we report a new way to identify chemical elicitors that induce resistance in rice to herbivores. Using this method, by quantifying the induction of chemicals for GUS activity in a specific screening system that we established previously, 5 candidate elicitors were selected from the 29 designed and synthesized phenoxyalkanoic acid derivatives. Bioassays confirmed that these candidate elicitors could induce plant defense and then repel feeding of white-backed planthopper Sogatella furcifera.

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New inventions track greenhouse gas, remediate oil spills

Camera test at Foljesjon, a lake in a research area west of Vanersborg, Sweden (credit: Linkoping University)

A new camera that can image methane in the air, allowing for precision monitoring of a greenhouse gas, has been developed by a team of researchers from Linköping and Stockholm Universities.

The new camera should help us better understand the rapid but irregular increase of methane in the atmosphere (which has puzzled researchers) and identify the sources and sinks of methane in the landscape. It may also suggest ways to reduce emissions.

”The camera is very sensitive, which means that the methane is both visible and measureable close to ground level, with much higher resolution [less than a square meter and at ambient levels (~1.8 ppmv, or parts per million volume)] than previously. Being able to measure on a small scale is crucial,” says Magnus Gålfalk, Assistant Professor at Tema Environmental Change, Linköping University who led the study.

An image of methane gas from the hyperspectral infrared camera, visualized in purple (credit: Linköping University)

The advanced hyperspectral (across the spectrum) thermal infrared camera weighs 30 kilos and measures 50 x 45 x 25 centimeters. It is optimized to measure the same portion of the solar radiation spectrum that methane absorbs and which makes methane such a powerful greenhouse gas.

The camera can be used to measure emissions from many environments including sewage sludge deposits, combustion processes, animal husbandry, and lakes.

For each pixel in the image (320 x 256 pixels), the camera records a precise spectrum range (in the 7.7 microns thermal IR region), which makes it possible to quantify the methane separately from the other gases.

The camera was developed by a team with expertise in astronomy, biogeochemistry, engineering. and environmental sciences. “We’re working to make it airborne for more large-scale methane mapping,” says principal investigator David Bastviken, professor at Tema Environmental Change, Linköping University.

The research was recently published in Nature Climate Change.

Super-absorbent material to soak up oil spills

Boron nitride material supported by a plant spike, demonstrating its light weight (credit: Weiwei Lei et al./Nature Communications)

In hopes of limiting the disastrous environmental effects of massive oil spills, materials scientists from Drexel University and Deakin University (Australia) have teamed up to manufacture and test a new “boron nitride nanosheet” material that can absorb oils and organic solvents up to 33 times its weight. That could make it possible to quickly mitigate these costly, environmentally damaging accidents.

The material, which literally absorbs oil like a sponge, is now ready to be tested by industry after two years of refinement in the laboratory at Deakin’s Institute for Frontier Materials (IFM).

Deakin Professor Ying (Ian) Chen, PhD, the lead author of the project’s research paper, recently published in Nature Communications, said the material is the most exciting advancement in oil spill remediation technology in decades.

“Oil spills are a global problem and wreak havoc on our aquatic ecosystems, not to mention they cost billions of dollars in damage,” Chen said. “Everyone remembers the Gulf Coast disaster, but here in Australia they are a regular problem, and not just in our waters. Oil spills from trucks and other vehicles can close freeways for an entire day, again amounting to large economic losses,” Chen said.

The nanosheet is made up of flakes just several nanometers (one billionth of a meter) in thickness with tiny holes. This strecture enables the nanosheet to increase its effective surface area to 273 square meters (3000 square feet) per gram.

Researchers from Drexel’s College of Engineering helped to study and functionalize the material, which started as boron nitride powder, commonly called “white graphite.” By forming the powder into atomically thin sheets, the material could be made into a sponge.

“The mechanochemical technique developed meant it was possible to produce high-concentration stable aqueous colloidal solutions of boron nitride sheets, which could then be transformed into the ultralight porous aerogels and membranes for oil clean-up,” said Vadym Mochalin, PhD, a co-author of the paper, who was a research associate professor at Drexel while working on the project, and is now an associate professor at Missouri University of Science and Technology.

The Drexel team used computational modeling to help understand the intimate details of how the material was formed. In the process, the team learned that the boron nitride nanosheets are flame resistant — which means they could also find applications in electrical and heat insulation.

The nanotechnology team at Deakin’s Institute for Frontier Materials has been working on boron nitride nanomaterials for two decades and has been internationally recognized for its work in the development of boron nitride nanotubes and nanosheets. This project is the next step in the IFM’s continued research to discover new uses for the material.

Abstract of Making methane visible

Methane (CH4) is one of the most important greenhouse gases, and an important energy carrier in biogas and natural gas. Its large-scale emission patterns have been unpredictable and the source and sink distributions are poorly constrained. Remote assessment of CH4 with high sensitivity at a m2 spatial resolution would allow detailed mapping of the near-ground distribution and anthropogenic sources in landscapes but has hitherto not been possible. Here we show that CH4 gradients can be imaged on the <m2scale at ambient levels (~1.8 ppm) and filmed using optimized infrared (IR) hyperspectral imaging. Our approach allows both spectroscopic confirmation and quantification for all pixels in an imaged scene simultaneously. It also has the ability to map fluxes for dynamic scenes. This approach to mapping boundary layer CH4 offers a unique potential way to improve knowledge about greenhouse gases in landscapes and a step towards resolving source–sink attribution and scaling issues.

Abstract of Boron nitride colloidal solutions, ultralight aerogels and freestanding membranes through one-step exfoliation and functionalization

Manufacturing of aerogels and membranes from hexagonal boron nitride (h-BN) is much more difficult than from graphene or graphene oxides because of the poor dispersibility of h-BN in water, which limits its exfoliation and preparation of colloidal solutions. Here, a simple, one-step mechano-chemical process to exfoliate and functionalize h-BN into highly water-dispersible, few-layer h-BN containing amino groups is presented. The colloidal solutions of few-layer h-BN can have unprecedentedly high concentrations, up to 30 mg ml−1, and are stable for up to several months. They can be used to produce ultralight aerogels with a density of 1.4 mg cm−3, which is ~1,500 times less than bulk h-BN, and freestanding membranes simply by cryodrying and filtration, respectively. The material shows strong blue light emission under ultraviolet excitation, in both dispersed and dry state.

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These self-propelled microscopic carbon-capturing motors may reduce carbon-dioxide levels in oceans

Nanoengineers have invented tiny tube-shaped micromotors that zoom around in water and efficiently remove carbon dioxide. The surfaces of the micromotors are functionalized with the enzyme carbonic anhydrase, which enables the motors to help rapidly convert carbon dioxide to calcium carbonate. (credit: Laboratory for Nanobioelectronics, UC San Diego Jacobs School of Engineering)

Nanoengineers at the University of California, San Diego have designed enzyme-functionalized micromotors the size of red blood cells that rapidly zoom around in water, remove carbon dioxide, and convert it into a usable solid form.

The proof-of-concept study represents a promising route to mitigate the buildup of carbon dioxide, a major greenhouse gas in the environment, said the researchers.

The team, led by distinguished nanoengineering professor and chair Joseph Wang, published the work this month in the journal Angewandte Chemie.

In their experiments, the nanoengineers demonstrated that the micromotors rapidly decarbonated water solutions that were saturated with carbon dioxide. Within five minutes, the micromotors removed 90 percent of the carbon dioxide from a solution of deionized water.

The micromotors were just as effective in a sea-water solution and removed 88 percent of the carbon dioxide in the same time frame.

“In the future, we could potentially use these micromotors as part of a water treatment system, like a water decarbonation plant,” said Kevin Kaufmann, an undergraduate researcher in Wang’s lab and a co-author of the study.

The micromotors are essentially six-micrometer-long tubes that help rapidly convert carbon dioxide into calcium carbonate, a solid mineral found in eggshells, the shells of various marine organisms, calcium supplements and cement. The micromotors have an outer polymer surface that holds the enzyme carbonic anhydrase, which speeds up the reaction between carbon dioxide and water to form bicarbonate. Calcium chloride, which is added to the water solutions, helps convert bicarbonate to calcium carbonate.

The fast, continuous motion of the micromotors in solution makes the micromotors extremely efficient at removing carbon dioxide from water, according to the researchers. The team explained that the micromotors’ autonomous movement induces efficient solution mixing, leading to faster carbon dioxide conversion.

Self-propulsion from oxygen gas bubbles

To fuel the micromotors in water, researchers added hydrogen peroxide, which reacts with the inner platinum surface of the micromotors to generate a stream of oxygen gas bubbles that propel the micromotors around. When released in water solutions containing as little as two to four percent hydrogen peroxide, the micromotors reached speeds of more than 100 micrometers per second.

Vdeo frames showing the movement of a micromotor in sea water (credit: Laboratory for Nanobioelectronics, UC San Diego Jacobs School of Engineering)

However, the use of hydrogen peroxide as the micromotor fuel is a drawback because it is an extra additive and requires the use of expensive platinum materials to build the micromotors. As a next step, researchers are planning to make carbon-capturing micromotors that can be propelled by water.

“If the micromotors can use the environment as fuel, they will be more scalable, environmentally friendly and less expensive,” said Kaufmann.

Abstract of Micromotor-Based Biomimetic Carbon Dioxide Sequestration: Towards Mobile Microscrubbers

We describe a mobile CO2 scrubbing platform that offers a greatly accelerated biomimetic sequestration based on a self-propelled carbonic anhydrase (CA) functionalized micromotor. The CO2 hydration capability of CA is coupled with the rapid movement of catalytic micromotors, and along with the corresponding fluid dynamics, results in a highly efficient mobile CO2 scrubbing microsystem. The continuous movement of CA and enhanced mass transport of the CO2 substrate lead to significant improvements in the sequestration efficiency and speed over stationary immobilized or free CA platforms. This system is a promising approach to rapid and enhanced CO2 sequestration platforms for addressing growing concerns over the buildup of greenhouse gas.

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Carbon dioxide capture by a novel material that mimics a plant enzyme

Atomic structure of the adsorbed* carbon dioxide (gray sphere bonded to two red spheres) inserted between the manganese (green sphere) and amine (blue sphere) groups within the novel metal-organic framework, forming a linear chain of ammonium carbamate (top). Some hydrogen atoms (white sphere) are omitted for clarity. (credit: Image courtesy of Thomas McDonald, Jarad Mason, and Jeffrey Long)

A novel porous material that achieves carbon dioxide (CO2) capture-and-release with only small shifts in temperature has been developed by a team of researchers at the Center for Gas Separations Relevant to Clean Energy Technologies, led by the University of California, Berkeley (a DOE Energy Frontier Research Center), and associates.

This metal-organic framework (MOF) structure, which adsorbs* CO2, closely resembles an enzyme found in plants known as RuBisCO, which captures CO2 from the atmosphere for conversion into nutrients.

The discovery* paves the way for designing more efficient materials that dramatically reduce overall energy cost of carbon capture. Such materials could be used for carbon capture from fossil-fuel-based power plants as well as from the atmosphere, mitigating the greenhouse effect.

The enhanced carbon capture efficiency of the new class of materials could allow for dramatic reductions in the overall energy cost of carbon capture in power plants or even from the atmosphere, according to the researchers.

* Adsorbed CO2 is captured on the surface of a material; absorbed CO2 is captured inside the material.

** The cooperative mechanism for carbon dioxide (CO2) adsorption in porous MOF materials:

First, a CO2 molecule gets inserted between a metal ion and an amine group within the cylindrical pore of the MOF. Interestingly, the chemical environment of the MOF with the adsorbed CO2  is very similar to that of plant enzyme RuBisCO with a bound CO2.

RuBisCO plays an essential role in biological carbon fixation by plants and conversion into nutrients. In the case of the newly synthesized diamine-appended MOFs, however, the inserted CO2 reorganizes the chemical environment at the adjacent metal ion site to be just right for the insertion of the next CO2.

As more CO2 enters the pore, a cooperative domino effect ensues that leads to the formation of linear chains of ammonium carbamate along the cylindrical pore surfaces of the MOF.

Gas adsorption measurements show the high selectivity of the material for CO2 from the typical composition of flue gas from fossil-fuel-based power plants that contains nitrogen, water, and CO2.

Furthermore, the material has large working capacities — the amount of CO2 adsorbed and desorbed for a given amount of material — that are enabled by only moderate temperature shifts for the adsorption and desorption processes.

Finally, the research points out that changing the strength of the metal-diamine bond through metal substitution allows for rational tuning of the adsorption and desorption properties.

Abstract of Cooperative insertion of CO2 in diamine-appended metal-organic frameworks

The process of carbon capture and sequestration has been proposed as a method of mitigating the build-up of greenhouse gases in the atmosphere. If implemented, the cost of electricity generated by a fossil fuel-burning power plant would rise substantially, owing to the expense of removing CO2 from the effluent stream. There is therefore an urgent need for more efficient gas separation technologies, such as those potentially offered by advanced solid adsorbents. Here we show that diamine-appended metal-organic frameworks can behave as ‘phase-change’ adsorbents, with unusual step-shaped CO2 adsorption isotherms that shift markedly with temperature. Results from spectroscopic, diffraction and computational studies show that the origin of the sharp adsorption step is an unprecedented cooperative process in which, above a metal-dependent threshold pressure, CO2 molecules insert into metal-amine bonds, inducing a reorganization of the amines into well-ordered chains of ammonium carbamate. As a consequence, large CO2 separation capacities can be achieved with small temperature swings, and regeneration energies appreciably lower than achievable with state-of-the-art aqueous amine solutions become feasible. The results provide a mechanistic framework for designing highly efficient adsorbents for removing CO2 from various gas mixtures, and yield insights into the conservation of Mg2+ within the ribulose-1,5-bisphosphate carboxylase/oxygenase family of enzymes.

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A high-efficiency, sustainable process using solar and carbon dioxide to produce methane for natural gas

Artificial photosynthesis provides electrical current to produce hydrogen gas from water; the hydrogen then synthesizes carbon dioxide (via microbes) into methane (CH4) (credit: Berkeley Lab)

A team of researchers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has developed a hybrid system that produces hydrogen and uses it (via microbes) to synthesize carbon dioxide into methane, the primary constituent of natural gas.

“We can expect an electrical-to-chemical efficiency of better than 50 percent and a solar-to-chemical energy conversion efficiency of 10 percent if our system is coupled with state-of-art solar panel and electrolyzer,” says Peidong Yang, a chemist with Berkeley Lab’s Materials Sciences Division and one of the leaders of this study.

“Natural photosynthesis, a solar-to-chemical energy conversion process that combines light, water, and CO2 to make biomass, operates at less than 1% efficiency,” UC Berkeley prof. Chris Chang explained to KurzweilAI. “We have now done a order of magnitude better than nature in this artificial photosynthesis system, albeit in one prototype system where we make methane,” he said. “The advance is that most artificial photosynthesis systems only use light and water, and operate at lower efficiencies to boot. The ability to incorporate CO2 fixation is also a big advance.”

Yang, who also holds appointments with UC Berkeley and the Kavli Energy NanoScience Institute (Kavli-ENSI) at Berkeley, is one of three corresponding authors of a paper describing this research in the Proceedings of the National Academy of Sciences (PNAS). 

Sustainable, efficient

Solar energy, a sustainable source of energy, is used to generate hydrogen from water via the hydrogen evolution reaction (HER). The HER is catalyzed by earth-abundant nickel sulfide nanoparticles that operate effectively under biologically compatible conditions.

“Water is the most sustainable starting feedstock for hydrogen,” Chang said. In comparison, “most hydrogen now comes from hydrocarbons, which gives off CO2.”

“We selected methane as an initial target owing to the ease of product separation, the potential for integration into existing infrastructures for the delivery and use of natural gas, and the fact that direct conversion of carbon dioxide to methane with synthetic catalysts has proven to be a formidable challenge,” said Chang.

“Since we still get the majority of our methane from natural gas, a fossil fuel, often from fracking, the ability to generate methane from a renewable hydrogen source (solar) is another important advance.”

Abstract of Hybrid bioinorganic approach to solar-to-chemical conversion

Natural photosynthesis harnesses solar energy to convert CO2 and water to value-added chemical products for sustaining life. We present a hybrid bioinorganic approach to solar-to-chemical conversion in which sustainable electrical and/or solar input drives production of hydrogen from water splitting using biocompatible inorganic catalysts. The hydrogen is then used by living cells as a source of reducing equivalents for conversion of CO2 to the value-added chemical product methane. Using platinum or an earth-abundant substitute, α-NiS, as biocompatible hydrogen evolution reaction (HER) electrocatalysts and Methanosarcina barkeri as a biocatalyst for CO2 fixation, we demonstrate robust and efficient electrochemical CO2 to CH4 conversion at up to 86% overall Faradaic efficiency for ≥7 d. Introduction of indium phosphide photocathodes and titanium dioxide photoanodes affords a fully solar-driven system for methane generation from water and CO2, establishing that compatible inorganic and biological components can synergistically couple light-harvesting and catalytic functions for solar-to-chemical conversion.

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Soaking up carbon dioxide and turning it into valuable products

Conceptual model showing how porphyrin COFs could be used to split CO2 into CO and oxygen (credit: Omar Yaghi, Berkeley Lab/UC Berkeley)

Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a system that absorbs carbon dioxide and also selectively reduces it to carbon monoxide (which serves as a primary building block for a wide range of chemical products including fuels, pharmaceuticals and plastics).

The trick: they’ve incorporated molecules of carbon dioxide reduction catalysts into the sponge-like crystals of covalent organic frameworks (COFs).

How to soak up carbon dioxide in the area of a football field

With the reduction of atmospheric carbon dioxide emissions in mind, Yaghi and his research group at the University of Michigan in 2005 designed and developed the first COFs as a means of separating carbon dioxide from flue gases. A COF is a porous three-dimensional crystal consisting of a tightly folded, compact framework that features an extraordinarily large internal surface area — a COF the size of a sugar cube were it to be opened and unfolded would blanket a football field. The sponge-like quality of a COF’s vast internal surface area enables the system to absorb and store enormous quantities of targeted molecules, such as carbon dioxide.

Structural model showing a covalent organic framework (COF) embedded with a cobalt porphyrin (credit: UC Berkeley)

“There have been many attempts to develop homogeneous or heterogeneous catalysts for carbon dioxide, but the beauty of using COFs is that we can mix-and-match the best of both worlds, meaning we have molecular control by choice of catalysts plus the robust crystalline nature of the COF,” says Christopher Chang, a chemist with Berkeley Lab’s Chemical Sciences Division, and a co-leader of this study.

“To date, such porous materials have mainly been used for carbon capture and separation, but in showing they can also be used for carbon dioxide catalysis, our results open up a huge range of potential applications in catalysis and energy.”

Chang and Omar Yaghi, a chemist with Berkeley Lab’s Materials Sciences Division who invented COFs, are the corresponding authors of a paper in Science that describes this research.

Chang and Yaghi both hold appointments with the University of California (UC) Berkeley. Chang is also a Howard Hughes Medical Institute (HHMI) investigator. Yaghi is co-director of the Kavli Energy NanoScience Institute (Kavli-ENSI) at UC Berkeley.

The notoriety of carbon dioxide for its impact on the atmosphere and global climate change has overshadowed its value as an abundant, renewable, nontoxic and nonflammable source of carbon for the manufacturing of widely used chemical products, the researchers point out.

Now, through another technique developed by Yaghi, called “reticular chemistry,” which enables molecular systems to be “stitched” into netlike structures that are held together by strong chemical bonds, the Berkeley Lab researchers were able to embed the molecular backbone of COFs with a porphyrin catalyst, a ring-shaped organic molecule with a cobalt atom at its core. Porphyrins are electrical conductors that are especially proficient at transporting electrons to carbon dioxide.

Among most efficient CO2 reduction agents

“A key feature of COFs is the ability to modify chemically active sites at will with molecular-level control by tuning the building blocks constituting a COF’s framework,” Yaghi says. “This affords a significant advantage over other solid-state catalysts where tuning the catalytic properties with that level of rational design remains a major challenge. Because the porphyrin COFs are stable in water, they can operate in aqueous electrolyte with high selectivity over competing water reduction reactions, an essential requirement for working with flue gas emissions.”

In performance tests, the porphyrin COFs displayed exceptionally high catalytic activity — a turnover number up to 290,000, meaning one porphyrin COF can reduce 290,000 molecules of carbon dioxide to carbon monoxide every second. This represents a 26-fold increase over the catalytic activity of molecular cobalt porphyrin catalyst and places porphyrin COFs among the fastest and most efficient catalysts of all known carbon dioxide reduction agents. Furthermore, the research team believes there’s plenty of room for further improving porphyrin COF performances.

“We’re now seeking to increase the number of electroactive cobalt centers and achieve lower over-potentials while maintaining high activity and selectivity for carbon dioxide reduction over proton reduction,” Chang says. “In addition we are working towards expanding the types of value-added carbon products that can be made using COFs and related frameworks.”

This research was supported by the DOE Office of Science in part through its Energy Frontier Research Center (EFRC) program. The porphyrin COFs were characterized through X-ray absorption measurements performed at Berkeley Lab’s Advanced Light Source, a DOE Office of Science User Facility.

Abstract of Covalent organic frameworks comprising cobalt porphyrins for catalytic CO2 reduction in water

Conversion of carbon dioxide to carbon monoxide and other value-added carbon products is an important challenge for clean energy research. Here, we report modular optimization of covalent organic frameworks (COFs), in which the building units are cobalt porphyrin catalysts linked by organic struts through imine bonds, to prepare a catalytic material for aqueous electrochemical reduction of CO2 to CO. The catalysts exhibit high Faradaic efficiency (90%) and turnover numbers (up to 290,000 with initial turnover frequency 9400 hours−1) at pH 7 with an overpotential of –0.55 V, equivalent to a 60-fold improvement in activity compared to the molecular cobalt complex, with no degradation over 24 hours. X-ray absorption data reveal the influence of the COF environment on the electronic structure of the catalytic cobalt centers.

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How to capture and convert CO2 from a smokestack in a single step

Using a novel catalyst, a single chemical assembly (UiO-66-P-BF2) could capture CO2 and also transform it and hydrogen into formic acid (HCOOH) via a two-step (yellow arrows) reaction (credit: Ye and Johnson/ACS Catalysis)

University of Pittsburgh researchers have invented (in computations) a cheap, efficient catalyst that would capture carbon dioxide (CO2) from coal-burning power plants before it reaches the atmosphere and converts the CO2  into formic acid — a valuable chemical that would create a revenue return. 

One current method for capturing CO2 uses Metal–organic frameworks (MOFs), which have a porous, cage-like structure that can absorb CO2, but require expensive catalysts, like platinum.

Instead, the researchers looked for lower-cost non-metallic catalysts, and found that a compound known as UiO-66-PBF2 would do the job.

The method is similar to one developed by Rice University using a combination of amine-rich compounds and carbon-60 molecules.

Those methods both differ from the “diamonds from the sky” approach, which turns CO2 from the air into carbon nanofibers, by capturing the CO2 before it leaves a smokestack. It will be interesting to compare these approaches in terms of CO2 capture efficiency, energy cost, and revenue return.

All three of these methods avoid the costs and energy expenditures from transporting and depositing CO2 at a storage site, required in carbon sequestration.

Abstract of Design of Lewis Pair-Functionalized Metal Organic Frameworks for CO2 Hydrogenation

Efficient catalytic reduction of CO2 is critical for the large-scale utilization of this greenhouse gas. We have used density functional electronic structure methods to design a catalyst for producing formic acid from CO2 and H2 via a two-step pathway having low reaction barriers. The catalyst consists of a microporous metal organic framework that is functionalized with Lewis pair moieties. These functional groups are capable of chemically binding CO2 and heterolytically dissociating H2. Our calculations indicate that the porous framework remains stable after functionalization and chemisorption of CO2 and H2. We have identified a low barrier pathway for simultaneous addition of hydridic and protic hydrogens to carbon and oxygen of CO2, respectively, producing a physisorbed HCOOH product in the pore. We find that activating H2 by dissociative adsorption leads to a much lower energy pathway for hydrogenating CO2 than reacting H2 with chemisorbed CO2. Our calculations provide design strategies for efficient catalysts for CO2 reduction.