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‘Magic-angle’ graphene is an unconventional superconductor

An ultrathin material consisting of two misaligned sheets of graphene can be easily converted from being a Mott insulator to a superconductor. The surprising discovery, details of which were announced at the March meeting of the American Physical Society (APS), could lead to the development of materials with a range of engineered electronic properties.

Graphene is a sheet of carbon just one atom thick that has a wealth of unique and potentially useful electronic and mechanical properties. Graphene atoms are arranged in a hexagonal lattice and two or more atomic layers can be stacked upon each other to create bilayer and thicker stacks of carbon.

In the new work, which is published in two papers in Nature, Pablo Jarillo-Herrero and colleagues at the Massachusetts Institute of Technology, Harvard University and Japan’s National Institute of Materials Science have shown that a graphene bilayer behaves as a Mott insulator when the two component sheets are oriented at a “magic angle”. In this form of matter, electrical conductivity is supressed by strong interactions between electrons. It can then be transformed into a superconductor by tweaking the electron density of the material

The researchers made their material by taking two sheets of graphene and rotating them out of alignment by a small angle of about 1°. Having the sheets at this magic angle creates a moiré lattice of atoms with a unit cell that is much larger than that of a single sheet of graphene. Electrons are localized at lattice sites and, under certain conditions, electrons can tunnel from one lattice site to another.

Each lattice site can accommodate a maximum of four electrons. If the electron density is high and the sites are all full, electrons cannot tunnel to neighbouring sites because there is no room for them – making the material an electrical insulator. However, if the density is lower and the sites are partially full, tunnelling can occur and the material is an electrical conductor.

By varying an electric field that is applied to the magic-angle bilayer, Jarillo-Herrero and colleagues can adjust the electron density of the material. At high density when the lattice sites contain four electrons each, the material is an insulator as expected. At lower densities and temperatures above about 4 K, the bilayer is a conductor – also as expected.

Unexpected insulator

However, something unexpected occurs at lower temperatures when the electron density is set so that each site contains one, two or three electrons. Instead of being a conductor, the material appears to be a Mott insulator – a state of matter that occurs when there is a strong interaction between electrons that inhibits tunnelling. The situation is even more interesting when the electron density is increased or decreased slightly away from two electrons per site. The material then becomes a superconductor with a transition temperature of about 1.7 K, which is surprisingly high given the relatively low electron density of the material.

The combination of a Mott-insulator phase and superconductivity is something that is also seen in some high-temperature superconductors, which tend to be 2D layered materials. As a result, magic-angle bilayers could provide important insights into the poorly-understood physics of these unconventional superconductors.

“Physicists now have an exciting new platform to probe the unusual properties of high-temperature superconductors, and possibly to design new materials that operate at even higher temperatures,” said Jarillo-Herrero at a news conference at the APS March meeting. “Usually you have to grow different classes of material to explore each different phase. We can explore all of the physics in one device electrically. It couldn’t get any simpler.”

Jarillo-Herrero also pointed out that the structures could be used to create devices with a range of useful electronic properties. “One can also imagine making a superconducting transistor out of graphene, which you can switch on and off, from superconducting to insulating,” he says. “That opens many possibilities for quantum devices.”

Lawrence Krauss banned from Arizona State University campus following misconduct allegations

Arizona State University (ASU) has put the astrophysicist Lawrence Krauss on paid leave following allegations of sexual misconduct that appeared last month in Buzzfeed. Krauss is a prominent physicist and is founder and director of ASU’s Origins Project. He has also written several popular-science books and appeared in TV documentaries. The Origins Project is set to celebrate its 10th anniversary with a series of events between 5-9 April. It is not known whether this will now go ahead.

Following the allegations, ASU stated that the university had not received any complaints from ASU students, faculty or staff about Krauss. However, it added that it had begun a review on 22 February “to discern the facts” and encouraged “anyone who has concerns about faculty, staff or students to report those concerns”.

Yet in a statement released yesterday, and seen by Physics World,  the university has now banned Krauss from the ASU campus. “In an effort to avoid further disruption to the normal course of business as the university continues to gather facts about the allegations, Krauss has been placed on paid leave and is prohibited from being on campus for the duration of the review,” the statement says.

ASU insists that no further details about the review will be released until it is complete. “The university encourages anyone in our community who has concerns about interactions with faculty, staff or students to report those concerns,” it says, adding that the university provides multiple reporting options, including “through the Office of Student Rights and Responsibilities, the Office of Equity and Inclusion or by calling the ASU Hotline”.

A valued member

Krauss has also resigned as chairman of the Bulletin of Atomic Scientists, which is best-known for its “Doomsday Clock”. The closer the clock is to midnight, the more likely it is that nuclear war or climate change will lead to catastrophe. In a press conference in January, attended by Krauss, the clock was moved forward to read two minutes to midnight.

Buzzfeed was provided with abundant counter-evidence that was ignored or distorted in their story

Lawrence Krauss

In a letter dated 6 March to Rachel Bronson, president and chief-executive officer of the Bulletin, Krauss denied the allegations in the Buzzfeed article claiming they were “incorrect”. “Buzzfeed was provided with abundant counter-evidence that was ignored or distorted in their story,” he wrote.  “The board feels that as a result of the various reactions to the article my presence on the Board of Sponsors at this time distracts from the ability of the Bulletin to effectively carry out that work,” he added.

In a short statement, Bronson noted that Krauss “has been a valued member of the Board, and has greatly contributed to the Bulletin’s mission during this perilous moment in global affairs”.

The news comes as several events have cancelled appearances that Krauss was due to make including the American Physical Society, which announced that he would not be speaking at its April meeting that will be held in Columbus, Ohio, on 14-17 April. He was supposed to be speaking at a session on the legacy of Richard Feynman.

  • Update 8 March: Lawrence Krauss has published a nine-page response to the Buzzfeed allegations while ASU has reportedly cancelled the event celebrating the 10th anniversary of the Origins Project.

How does carbon pricing affect revenue and expenditure?

Carbon pricing offers economic incentives to mitigate global greenhouse-gas emissions and encourage investment in cleaner technology. But how do initiatives operating in one country affect prices in another? And how does the picture change if the carbon price is applied at the point where fossil fuels are extracted from the ground, rather than focusing on their use or on the sale of the resulting goods and services?

Jonas Karstensen and Glen Peters from Norway’s CICERO Center for International Climate Research found that using different accounting systems makes a significant difference to revenues and expenditure. In addition, the team highlights that domestic and global trade play a key role in spreading the carbon price between sectors and countries.

Examples of carbon pricing include a tax on the carbon content of fossil fuels, and emissions trading systems that cap the total level of emissions and allow industries with low emissions to sell their extra allowances to larger emitters. So far, at least 40 countries, including seven out of ten of the world’s largest economies, have put in place some form of carbon pricing. The various initiatives translate to around 13% of yearly global greenhouse-gas emissions.

Reporting their results in Environmental Research Letters (ERL), Karstensen and Peters showed that while rising carbon costs bring large relative price increases in the electricity and energy-intensive sectors, the biggest absolute increases in expenditure are in non-energy-intensive and service sectors.

“It’s the volume of expenditure that matters most when considering carbon pricing, not necessarily how much emissions are required to produce a single product,” Peters told environmentalresearchweb. “Ultimately, households bear most of the costs, rather than governments or capital investments.”

Exploring global patterns, the scientists found that emissions income becomes more evenly distributed among nations when the point of accounting is shifted from mining through to production and consumption. The EU’s carbon price revenue jumped by more than €40 billion when the study associated emissions with production rather than mining, reflecting small fossil-fuel reserves and large imports.

“Because of the implementation challenges, climate policies are often a mosaic of different policy levers, but over time we could see carbon pricing initiatives becoming more uniform – in response to industry and consumer demand,” said Peters. “Theoretically, a gradually rising carbon price would be the optimal solution, so long as distortions and exemptions were kept to a minimum, and there was confidence that the carbon price would remain in place for the long-term.”

3D bone-on-a-chip sheds light on bone metastasis

A three-dimensional (3D) bone-on-a-chip with the potential to study cancer metastasis has been developed by researchers at The Pennsylvania State University. The bone-on-a-chip was designed within a microfluidic device – a miniaturized chamber that is continuously fed with nutrient-containing fluids to support the growth of micron-scale bone tissue (Small doi: 10.1002/smll.201702787).

The device demonstrated the ability to support the attachment, growth and spontaneous differentiation of osteoblasts (bone-forming cells) for up to 30 days. The researchers grew the cells on glass inside a cell culture chamber, which received nutrient-containing medium through a dialysis membrane. They observed that the forming bone tissues were able to mature and produce sufficient amounts of alkaline phosphatase (ALP) enzyme and collagen proteins, and accumulate hydroxyapatite minerals – all important for forming bone with structural integrity and functional relevance to real bone.

After maturation of the bone tissue, the team introduced breast cancer cells, mimicking the metastasis of breast cancer to bone. The researchers found that some breast cancer cells were dormant (or quiet) in the 3D bone environment, without any invasive behaviour and showing no noticeable effect on the bone.

On the other hand, they saw that aggressive breast cancer cells were highly invasive in the 3D bone. These cancer cells dug themselves into the bone by eroding the collagen and consuming the bone matrix, resulting in the formation of large holes in the 3D bone. After two weeks, the aggressive breast cancer cells had integrated themselves into the bone.

The 5-year survival rate of patients with breast cancer drops below 30% when the cancer spreads to other parts of the body, with 70% of such metastasis travelling to bone. Once cancer finds its way to bone, it is virtually incurable. For this reason, it is imperative to understand the interactions between cancer cells and bone. The researchers in this study constructed a model that can help reveal the mechanisms behind bone metastasis. With better understanding of the disease using such platforms, therapeutic targets could be developed to treat and prevent metastasis from occurring.

Lighting the way

As you may remember from my column in last month’s issue of Physics World, the editors and I couldn’t initially decide what to call this column and so we put the problem out to readers via Twitter. We offered two options – Transactions or Joseph Swan – and the overwhelming majority chose the former, showing that physicists are clearly more focused on solving a problem than indulging in collective silliness (no Boaty McBoatface for us).

Perhaps if I had put any effort into selling “Joseph Swan” to readers, the name might have got a little further. It isn’t just that a business is like a swan – both look graceful and beautiful from a distance yet face a frantic struggle to take off. More importantly, a column named after Sir Joseph Wilson Swan (1828–1914) would have recognized the efforts of this great British physicist and chemist, who also – you might be surprised to learn – was the inventor of the light bulb.

Swan’s way

Thomas Edison might be the name that people most closely associate with the invention of the light bulb, but it was Swan who was responsible for developing and supplying the electric lights used in the world’s first electrically lit homes and public buildings, including the Savoy Theatre in London, in 1881. Swan had patented his design in 1869 but the trouble with patents is they force you to explain how your invention works. So if you don’t follow through rapidly you can expect competition. And so it proved with the light bulb. Edison’s version, which he patented in 1879, may have merely improved on Swan’s efforts, but ultimately it was more successful.

But why did Swan not do better? After all, the general principle of the incandescent light bulb is simple: if you send an electrical current through a filament, it heats up and glows, producing light. But what’s important is that the interior of the bulb is a vacuum so the filament doesn’t oxidize and lasts a long time. The vacuum in Swan’s bulb was so poor that the filament – made from carbonized paper – disintegrated rapidly and the bulb glowed for barely 15 hours.

Swan’s bulb also had a low resistance, which sounds good until you realize that it meant a high current flowing through the first bulbs. The only practical way to stop the bulb from burning out was therefore to place multiple bulbs in series, but this meant that if one bulb popped, all the lights went out. Swan worked on his bulb product and later patented a better version based on a higher resistance filament at a similar time to Edison – but not before giving his competitors plenty of time to get into the light-bulb business.

Edison had other advantages too. He had a better vacuum pump earlier. He was well funded (from the sale of his telegraph business) and his team tested thousands of materials at his Menlo Park facility in New Jersey, including a higher-resistance filament derived from bamboo. It lasted up to 1200 hours and could be used in parallel circuits, thereby avoiding the all-lights-out-if-one-bulb-pops scenario. And as there was no electrical infrastructure at the time, Edison designed his bulb with the whole system in mind.

Let there be light

To see why Edison’s bulb succeeded, let’s recall the technologies available in the 1880s to deliver light to homes and businesses when the Sun went down. The options were mostly fire based: candles, gas lights and oil lamps. Whale oil was particularly popular as it burned the brightest and didn’t produce much soot – this was a major reason why whales were hunted. The problem was that each source of light had to be individually lit every evening and then put out before you went to bed.

The beauty for businesses was that electric light was a “closed” system, with every manufacturer having their own bulbs, voltages and forms of DC distribution

James McKenzie

Electric light was a revelation. It was clean, relatively safe and multiple lights could be switched on and off simultaneously. In short, everyone wanted it. The beauty for businesses was that it was a “closed” system, with every manufacturer having their own bulbs, voltages and forms of DC distribution. Customers were forced to keep buying from the same firm – Edison’s in Edison’s case.

Edison therefore benefited from four things: a well-thought-through system; a good business model; a working product that was much better than the competition; and patents. As a result, he attracted huge investment from J P Morgan and many others to roll out his electrical DC infrastructure around the world. As the market grew, however, Edison faced growing competition himself, particularly from the alternating-current (AC) system developed by the Westinghouse Electric Corporation. The battle of the currents raged for more than two decades and, ultimately, his DC system itself lost out because AC was better at long-distance transmission.

Lessons from Edison

The physics of both Swan’s and Edison’s bulbs was exactly the same, but it was Edison’s design and product implementation that led him to succeed and spawn a business – General Electric – that is still around today. Yes, Swan patented first and was a successful inventor and entrepreneur. He even sued Edison for patent infringement, with the British courts ruling against Edison, who was forced to make Swan a partner in his UK firm Ediswan. But it is Edison who is recognized as the father of a global revolution.

To succeed in business, you not only need to solve a problem but also work hard on the features and benefits of your product. And that’s why I wanted to call the column Joseph Swan. Still, Transactions is a very good name.

Nanowire arrays help blind mice see again

Photoreceptors made from titanium dioxide nanowires coated with gold nanoparticles could restore vision in blind mice. The devices, which respond to green, blue and near-ultraviolet light when interfaced with the retina of the animals, might help in the development of improved ocular prosthetics that do not require external power sources.

Diseases like retinitis pigmentosa or age-related macular degeneration can irreversibly damage retinal photoreceptors. This leads to vision impairment and eventually blindness, even though the retinal neurons involved in signal processing and the optic nerve remain functional. There is currently no medical treatment for these diseases but researchers have been working on replacing these lost photoreceptors with artificial ones, such as those made from photodiode arrays, for example.

While showing promise, these devices are far from being optimized, with the most important problem being that they require an external power supply and microelectronic processing units. Wiring such hardware into the eye is not only very challenging technically, it is also, needless to say extremely traumatic for a patient.

1D semiconductor nanowire arrays

A team of researchers led by Gengfeng Zheng and Jiayi Zhang of Fudan University in Shanghai, China, have been studying 1D semiconductor nanowire arrays made from titanium dioxide (TiO2) and gold (Au) as possible alternative implants here.

In contrast to previous photoresponsive structures, the 1D nanowire arrays are extremely well aligned in one direction, which means that they are structurally very similar to biological photoreceptors. They can thus very efficiently absorb light and separate charges (electrons and holes) to produce a photocurrent (in the same way as photoconversion devices such as solar cells and photodetectors), so eliminating the need for trans-ocular cables or power supplies. The photocurrent they produce can then be passed to neighbouring retinal neurons to stimulate them so that they fire a signal to the brain.

Restoring the visual response

Zheng and Zhang made their oriented Au-TiO2 arrays by growing them on fluorine-doped tin oxide (FTO) or flexible polymer substrates using a hydrothermal technique. They then decorated the surface of the arrays with gold nanoparticles, which allow the arrays to efficiently photoconvert light in the visible range (as measured by UV-visible absorption spectroscopy). This is because the particles amplify the light electrical field and inject “hot electrons” generated by surface plasmons (collective excitations of conduction electrons at the surface of the gold) into the TiO2 conduction band. These hot electrons then recombine with holes to produce a photocurrent.

In their experiments, the researchers interfaced the nanowire arrays with degenerated retinas in the right eyes of blind mice. The left eyes served as a control.

They found that the arrays are able to restore electrical signalling in the mice retinal ganglion cells when the rodents are exposed to green, blue and near UV light. “More excitingly, we saw that sub-retinally implanted nanowire arrays evoke activity in the primary visual cortex in vivo as well as improved pupil dilation in response to light in the animals. Light-sensitivity, and thus visual function, is restored in about 4–8 weeks in the implanted eyes compared to controls,” says Zhang.

New treatment options for people at risk

Zheng and Zhang believe that their new study could open new treatment options for people at risk of long-term visual degeneration. “Our work could help in the development of a new generation of optoelectronic tool kits for sub-retinal prosthetic devices in human patients one day,” they tell nanotechweb.org.

The researchers, reporting their work in Nature Communications doi:10.1038/s41467-018-03212-0, say that they are now busy improving the nanowire arrays’ sensitivity and their response to the colour red. “We will also be performing more experiments that measure the visual acuity in mice with degenerated retinas,” adds Zheng

Google aims for quantum supremacy

Google was the talk of the APS March Meeting on Monday, as Julian Kelly of the company’s Quantum AI Lab unveiled a 72-qubit quantum processor – the largest so far reported. The team hopes that the chip will allow them to achieve so-called quantum supremacy, the point at which a quantum computer can solve problems that are beyond the power of conventional devices.

The Google engineers are now starting to test the 72-qubit chip, which has been dubbed Bristlecone because its qubits are arranged in pinecone-like pattern. “From what we know so far, we’re very optimistic,” said John Martinis, a physicist at Google and the University of California Santa Barbara. If all goes well, Martinis is confident that quantum supremacy could be demonstrated with the next few months.

The Google announcement follows news in November that IBM Research was starting to test a 50-qubit quantum processor, while Intel revealed a 49-qubit test chip in January. But among these headline-grabbing announcements, James Wootton at the University of Basel – also speaking at the APS meeting – striked a note of caution.

Quantum games

Wootton has been creating simple games and puzzles to benchmark the performance of current prototypes, and his tests so far suggest that they still have a long way to go. “Devices now exist that are larger and cleaner than ever before,” he comments. “They’re on the cloud and they can be accessed using standard programming tools.”

Wootton’s approach stems from a long tradition of using games both to test computer hardware and to teach programming skills. And just as the hardware for quantum computing is only just starting to emerge from university labs and commercial companies, scientists also need to learn a new skill of quantum software engineering – in other words, how to use quantum processes to perform computations.

“When we start programming any sort of system we will first try something really simple,” says Wootton. “A game is a good place to start.”

To begin with Wootton created a version of the game Battleship, and used it to develop a tutorial to help scientists start programming quantum computers. He then moved on to another game, which he called Quantum Awesomeness, that tests the performance of a quantum processor with a series of increasingly difficult puzzles.

His results reveal that noise remains a major limiting factor for the current generation of prototypes, which typically have 16–19 qubits. “Based on tests with devices from IBM Research and Rigetti, even 50-qubit devices won’t be clean enough to achieve quantum supremacy,” he says. “The device size and connectivity determines the complexity of the device, while the noise levels determine how well the device can play the game.”

LIDAR system is faster than a speeding bullet

A new laser system that can make precise measurements of distance at record-breaking speed has been unveiled by researchers at Germany’s Karlsuhe Institute of Technology (KIT) and the Federal Institute of Technology in Lausanne (EPFL) in Switzerland. Made using optical frequency combs, the system can measure the surface profile of a speeding bullet.

Modern technologies such as aerial drones, observation satellites, self-driving cars and manufacturing robots need to make rapid, highly-accurate measurements of distances to surrounding objects. Currently, such measurements are made by laser-based light detection and ranging (LIDAR) systems, but even these are struggling to keep pace.

Optical teeth

To construct a faster ranging system, the KIT and EPFL scientists exploited solitons, which are wave packets consisting of laser light that maintain their shape as they propagate. These solitons can take the form of optical-frequency combs, with “teeth” of light with precisely defined wavelengths, spaced at regular intervals (see figure). The scientists realized that if one comb was fired at and reflected off a moving object, it would interfere with another, newly-produced comb. With knowledge of the comb structure, the phase difference of the superimposed teeth would reveal the distance travelled by the first comb.

To create the combs, the scientists fabricated circular, micron-scale pieces of high-quality silicon nitride, which loses very little light. When fed with continuous-wave lasers, the refractive index of this material responds to changes in electric field. Called the Kerr effect, this causes light waves at certain Kerr frequencies to resonate, producing the optical-frequency combs suitable for the team’s experiment.

“We have developed low-loss optical resonators, in which extremely high optical intensities can be generated – a prerequisite for soliton frequency combs,” explains Tobias Kippenberg of EPFL. “These so-called Kerr frequency combs have rapidly found their way into new applications over the previous years. “

Record breaking

To test their system, the researchers fired a bullet at a speed of 150 m/s and aimed a comb-producing laser perpendicular to its trajectory. By continuously observing comb interference patterns, they generated a 3D image of the bullet. “We managed to sample the surface structure of the projectile on-the-fly, achieving micrometre accuracy,” comments KIT’s Christian Koos. “To this end, we recorded 100 million distance values per second, corresponding to the fastest distance measurement so far demonstrated.”

While the measurement was made at both high precision and high speed, the range of the technique is only around 1 m – a value they wish to increase in the future. Additionally, their system is rather large and generates a huge amount of data. The researchers will now work towards producing a more compact design, which they believe could one day fit inside a matchbox.

The LIDAR system is described in Science.

Physicists cast doubt on ‘twisted’ neutrons

Physicists in the US have cast doubt on a 2015 report that neutrons with orbital angular momentum (OAM) were created and characterized in the lab.

Ronald Cappelletti, Terrence Jach and John Vinson of the National Institute of Standards and Technology (NIST) in Gaithersburg Maryland have calculated that neutron interference effects measured by Dmitry Pushin and colleagues at the University of Waterloo, NIST’s Joint Quantum Institute in Maryland and Boston University are not related to neutron OAM.

Writing in Physical Review Letters, Cappelletti, Jach and Vinson also argue that the technique developed by Pushin’s team is extremely inefficient at creating neutrons with OAM.

Do the twist

Physicists know that particles such as photons and free electrons can have orbital angular momentum. This is a quantum-mechanical effect whereby the wavefront of the electron, for example,  twists around its direction of propagation like a spiral of fusilli pasta.

Three years ago, Pushin and colleagues did an interferometry experiment that involved splitting a beam of slow-moving “cold” neutrons so that they travelled along two different paths. In one path they placed a spiral phase plate (SPP), which is a coin-sized piece of solid aluminium that resembles one twist of a spiral ramp. The other path was free of any objects. Neutrons travelling down the two paths were recombined and the quantum interference was measured. The neutron flux was set so low that the experiment was essentially measuring the interference between individual neutrons in a superposition of having taken the two different paths.

When a neutron travels through aluminium, its speed changes slightly. The spiral nature of the SPP and the fact that the neutron has a wavefront that extends over space mean that the shape of the wavefront changes as the neutron passes though the SPP. Pushin and colleagues argued that this imparted OAM to the neutrons – something that had already been seen for laser light passing through SPPs – and that evidence of neutron OAM was apparent in the resulting interference pattern.

Short on coherence

Cappelletti, Jach and Vinson have now, however, done calculations that they say show that the experiment actually measured phase-contrast interference, which has nothing to do with OAM. They point out that the neutrons used in the experiment had wavefronts that extend coherently about 5 µm in the direction transverse to their motion. This, the trio argues, is much too small relative to the size of the SPP for OAM to be imparted to the vast majority of neutrons passing through the apparatus. They point out that this size mismatch does not occur in SPP-laser experiments, where the coherence of the laser beam extends across the SPP.

The trio then went on to calculate that only about one in a million neutrons would acquire OAM in the 2015 experiment, which is too low to be measured. This number, they calculate, could however be boosted significantly if the neutron coherence length and the SPP were matched in size.

Artificial photosynthesis and bacteria transform CO2 into speciality chemicals

Production of butanol and hexanol may now be environmentally friendly thanks to a process that couples solar-powered electrochemical CO2 reduction and fermentation. Scientists from Siemens, Evonik and Covestro developed the hybrid system, which offers a sustainable, scalable, and efficient way to both reduce CO2 levels and produce value-added chemicals.

Alcohols like butanol and hexanol are widely used in coatings, solvents, cosmetics and fuels. Unfortunately, they are usually produced from depleting fossil sources using a costly and complex process, prompting Guenter Schmid and colleagues to design an alternative.

To drive alcohol production kinetically requires CO. The electrochemical cell in the researcher’s hybrid system uses a porous silver gas-diffusion electrode capable of uniquely high current densities (up to 300 mA cm–2), and an extended lifetime (up to 1200 hours) to produce syngas (CO/H2CO2 from CO2 and H2O, which the bioreactor then converts into valuable alcohols. High current densities are required to drive CO2 reduction at the electrolyser due to the low solubility of CO2 in salt-based electrolytes. The voltage required to drive CO2 reduction is provided by a commercial photovoltaic (PV). This solar cell provided up to 1200 hours of 70% CO-Faradaic efficiency Previously such high efficiencies have only been attained for a few minutes under these conditions or when very high CO2 pressures were used.

Thanks to the high-performance characteristics of the electrochemical cell, the system reached overall photon-to-alcohol efficiencies of 8%, making it one of the most efficient reported for oxygenate production from solar-driven CO2 hydrogenation.

Scaling up: CO2 electroreduction

The researchers also suggest how the process could be scaled up based on the reactions that take place. The cathode reduces CO2 to CO in a reaction that oxidizes water molecules into hydroxide anions (CO2+H2O+2e → CO+2OH–). The water oxidation takes place at the anode, which was composed of an iridium-oxide-coated platinum electrode: H2O-2e → 1/2O2+2H+. The alkaline cathode electrolyte requires an excess of CO2 for its neutralization: full reduction of one CO2 molecule requires three molecules of CO2 (3CO2+H2O+2e → CO+2HCO3-).

Overall, scaling up to produce 10,000 tonnes of hexanol and butanol requires 3.4 × 109 moles of electrons: the CO2 electrolyser provides 16.7% of these electrons, and the H2O electrolyser provides the rest. The plant would consume about 25.5 MW, which 14.6 hectare PV modules would supply. This amount of alcohol would also require 25,000 tonnes of CO2, which amounts to scaling up the CO2 electrolyser by a factor of 270,000, for instance by increasing its surface area of 10 cm2 to 1 m2 (factor 103) and by stacking 270 electrolysis cells.

Scaling up: fermentation

The bioreactor contained two anaerobic cultures, which bioprocessed the alcohols under non-growth conditions: C. autoethanogenum and C. kluyveri. Firstly, C. autoethanogenum transformed CO, H2 and CO2 to acetate and ethanol, which was then further inoculated by C. kluyveri to produce butanol and hexanol.

Scaling the fermentation to produce 10,000 tonnes of alcohols would require an increase in the number of fermenters (bacteria) by a factor of 21.6 × 106. The researchers calculated that increasing the cell concentration by a factor of up to 30 and the volume from 1 litre to 700,000 litres would achieve this. Even though the cultures must be placed in separate containers, these would only require vitamins and minerals for their media.

The hybrid system highlights the importance of research collaboration across diverse scientific fields that can lead to creative and efficient ways to decrease CO2 levels while producing highly desired chemicals. Furthermore, the modular and local alcohol production the research group proposes and the use of renewable energy paves the way to not only eco-friendly but also economically feasible systems for the generation of specialty chemicals.

More information can be found in Nature Catalysis 10.1038/s41929-017-0005-1.

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