Green Energy

The number and scale of projects using and making hydrogen, a gas that releases energy when burned without emitting carbon dioxide, is rapidly growing. If its construction goes to plan, a €2.5 billion (£2.18 billion) undersea pipeline will convey “green hydrogen” from Spain to France from 2030. In the US, some power stations are being upgraded to allow hydrogen to be blended with fossil gas, and the Norwegian oil company Equinor is teaming up with Thermal SSE to build a 1,800 megawatt (MW) “blue hydrogen” power plant in Britain. Meanwhile, China unveiled a plan in March which includes deploying 50,000 hydrogen vehicles by 2025 and early December saw the first hydrogen-fueled tractors and forklifts leave the assembly line at a new plant in Guangdong province. Hydrogen is produced in multiple ways. A color spectrum is used to render it simple. “Gray” and “brown/black” hydrogen come from fossil gas (methane) and coal (brown or black coal) respectively—a process that, for every ton of hydrogen, emits between ten and 12 tons of CO₂ for gray hydrogen and 18 to 20 for brown. “Blue” is the same process except the carbon dioxide is supposed to be captured and stored underground. And “green” hydrogen is conventionally defined as generated from splitting water into hydrogen and oxygen using renewable electricity. But only 0.04% of hydrogen is green, and blue hydrogen is less than 1%. The rest is gray or brown, most of which is used in oil refineries and for manufacturing ammonia and methanol. It’s an enormous industry which emits more CO₂ than all of Britain and France combined. It is widely hoped that a silver lining of today’s high gas prices will be green hydrogen becoming a cost-competitive alternative to dirty fuels in boilers, shipping tankers and steelworks furnaces. Unfortunately, without electricity market reform, this opportunity is likely to be squandered. And while the buzz around the hydrogen economy intensifies, a closer look suggests the fuel is less a spearhead for a green transition and more the subject of an elaborate bait-and-switch operation by oil companies.

Hydrogen’s true colors

Green hydrogen is essential for decarbonization: to replace fossil fuels in steelmaking, ammonia production for fertilizers and possibly shipping and trucking—processes which are difficult to electrify. Some green hydrogen is crosshatched with dirtier hues. So it’s not simply that in its production a lot of energy is wasted in the double transformation from electricity to gas and then fuel. But burning hydrogen also emits nitrogen oxides, air pollutants linked to respiratory illnesses and acid rain. If green hydrogen production is scaled up to play a significant economic role by 2050, its freshwater demand will exceed one-quarter of today’s global annual consumption, risking water scarcity in some regions. Above all, hydrogen is meaningfully green only if the renewable energy that generates it cannot be fed into the grid to replace power from gas or coal plants. Blue hydrogen relies on a similar—but much more harmful—trick of the light. For hydrogen to be true blue, the emissions must be captured and securely stored. In theory, carbon capture and storage works but nearly all plants use the captured carbon to pump more oil and many have been shut down as failures. Only a handful store carbon indefinitely and even these consume lots of energy and capture only some of the CO₂, which can leak. Blue hydrogen’s main source is methane, a powerful greenhouse gas that is notorious for escaping drilling wells and pipelines. Research suggests that these issues make blue hydrogen worse for the climate than fossil gas. In the EU, as in many economies, electricity pricing is based on the principle of marginal costs, which means that the most expensive source (typically fossil gas) sets the wholesale power price. During sunny or windy spells, a glut of renewable energy generation can slash electricity prices, freeing them from the grip of natural gas prices for a few hours at a time. This is often not enough to justify investments in the electrolysers which produce green hydrogen. Green hydrogen won’t gain the necessary price advantage over blue hydrogen and fossil gas until electricity markets are restructured. Meanwhile, the high price of oil and gas has turbocharged the industry’s expansion. The US government is exhorting oil and fracking firms to “drill baby drill.” Britain’s government is to award more than 100 licenses to drill for oil and gas and colossal new fossil fuel investments have been announced across the Middle East and Africa. In a few years when these new sources come onstream, and particularly if economic growth continues to slow and depress energy demand, gas and oil will become cheaper again—until the next price spike prompts new rounds of investment, and the infernal cycle continues. The owners of newly-built wells, pipelines and terminals will fight to defend those assets and stall decarbonisation. Now fossil-fuel firms are rebranding themselves as agents of “carbon management.” The aim is to prevent their assets from getting stranded by repurposing them, presenting a largely fictional substance, blue hydrogen, as a low-carbon “bridge” to an unspecified green future. Other sectors have joined the oil-led coalition. As the engineer Tom Baxter observes, gas network operators and boiler manufacturers see their survival in this ploy. Utilities are similarly keen, as hydrogen’s inefficiencies allow them to sell more power. Tackling this stalling operation requires public policy. Governments will need to regulate or tax carbon out of the market while simultaneously ramping up renewables. The approach to electricity pricing also needs to shift, to decouple the prices of electricity generated from renewables and fossil gas. The marginal pricing system hugely benefits renewable project owners, since they profit from high electricity prices and effectively zero input costs. An alternative market structure would set rewards for generators according to their average costs plus a slight surplus which could be reinvested into deploying more renewables and other green technologies, providing consumers with cheap electricity. This can only be achieved through a robustly regulated market or by nationalizing energy companies and setting prices and production. These interventions would give green hydrogen a competitive advantage over blue or gray variants, one that could be furthered with other subsidies, such as tax credits on the model of the US Inflation Reduction Act. Above all, energy demand must be reduced to ease upward pressure on price. In any future energy system, hydrogen will have a role. But its expansion must be carefully designed, to prevent the promise of green hydrogen disguising the risks of its blue and gray cousins.

Perovskite solar cells (PSCs) can be made with low-cost materials, are highly efficient, can surpass traditional silicon solar cells, and have the potential to revolutionize renewable energy. However, one of the current drawbacks preventing their widespread use has is their lack of operational stability.

Now, scientists at EPFL and Sungkyunkwan University in Korea have found a way to improve the stability of PSCs. The researchers focused on the degradation of perovskite thin films, which can be damaged by exposure to moisture, heat, and light. The study was carried out by the groups of Professors Michael Grätzel (EPFL) and Nam-Gyu Park (Sungkyunkwan University), and published in Science.

The scientists looked at two specific crystal facets, a term that refers to the crystal’s flat surface, characterized by a particular arrangement of atoms. The arrangement of atoms on these facets can affect the properties and behavior of the crystal, such as its stability and its response to external stimuli like moisture or heat.

The researchers looked at the (100) and (111) facets of perovskite crystals. The (100) facet is a plane that is perpendicular to a crystal’s c-axis with its atoms arranged in a repeating pattern in the form of a square grid. In the (111) facet the atoms are arranged in a triangular grid.

The study found that the (100) facet, which is most commonly found in perovskite thin films, is particularly prone to degradation as it can quickly transition to an unstable, inactive phase when exposed to moisture. In contrast, the (111) facet was found to much more stable and resistant to degradation.

The researchers also identified the cause of the degradation and found that it was due to a strong bond between the perovskite and water molecules, which caused the transition from the stable to unstable phase. They used this information to develop a strategy called “facet engineering,” in which they used special ligand molecules to grow the more stable (111) facet. This resulted in perovskite films that were exceptionally stable and resistant to both moisture and heat.

The study represents an important step forward in the development of PSCs, as stability is a major hurdle to their commercialization. The findings provide a better understanding of how the different crystal facets contribute to the stability of the films; by identifying the most stable facets and finding ways to encourage their growth, it may be possible to improve the overall stability of PSCs and accelerate their entry into the market as a reliable and cost-effective source of renewable energy.

Tsunamis, hurricanes, and maritime weather are monitored using sensors and other devices on platforms in the ocean to help keep coastal communities safe—until the batteries on these platforms run out of juice. Without power, ocean sensors can’t collect critical wave and weather data, which results in safety concerns for coastal communities that rely on accurate maritime weather information. Replacing batteries at sea is also expensive. What if this could all be avoided by powering devices indefinitely from the energy in ocean waves? Pacific Northwest National Laboratory (PNNL) researchers are working to make this a reality with the development of a new cylindrical triboelectric nanogenerator (TENG)—a small powerhouse that converts wave energy into electricity to power devices at sea. Larger versions of this generator could be used to power ocean observation and communications systems, including acoustic and satellite telemetry. “TENGs are low cost, lightweight, and can efficiently convert slow, uniform or random waves into power—making them particularly well-suited to powering devices in the open ocean where monitoring and access are challenging and costly,” explained Daniel Deng, a PNNL laboratory fellow and co-developer of the new TENG device. Deng and his team took a novel approach to advance cylindrical TENGs for use on the open ocean. Their patent-pending frequency-multiplied cylindrical triboelectric nanogenerator (FMC-TENG) uses carefully placed magnets to convert energy more efficiently than other cylindrical TENGs and to better transform slow, uniform waves into electricity. So far, the prototype FMC-TENG has been able to produce enough electricity to power an acoustic transmitter—a type of sensor often included on ocean observing platforms that can be used for communications. This is about the same amount of electricity it takes to power an LED lightbulb. “We’re developing the FMC-TENG to power everything from ocean observing platforms with multiple sensors to satellite communications, all using the power of the ocean,” said Deng.

Artificial fur, magnets, and waves for power

If you’ve ever been shocked by static electricity, then you’ve personally experienced the triboelectric effect—the same effect researchers leverage in the FMC-TENG to produce power. A cylindrical TENG is made up of two nested cylinders with the inner cylinder rotating freely. Between the two cylinders are strips of artificial fur, aluminum electrodes, and a material similar to Teflon called fluorinated ethylene propylene (FEP). As the TENG rolls along the surface of an ocean wave, the artificial fur and aluminum electrodes on one cylinder rub against the FEP material on the other cylinder, creating static electricity that can be converted into power. The more a cylindrical TENG moves, the more energy it generates. That’s why fast, frequent waves can generate more energy than the slower, more uniform waves of the open ocean. To come up with a TENG that could power electronics in the open ocean, Deng and his team set out to increase the amount of wave energy converted into electricity in the FMC-TENG. As it turned out, the key was to temporarily stop the FMC-TENG’s inner cylinder from moving. In the FMC-TENG, the team positioned magnets to stop the inner cylinder in the device from rotating until it reached the crest of a wave, allowing it to build up more and more potential energy. Nearing the crest of the wave, the magnets released and the internal cylinder started rolling down the wave very quickly. The faster movement produced electricity more efficiently, generating more energy from a slower wave.

A wave energy converter for the open ocean

Currently, the FMC-TENG prototype can produce enough power to run small electronics, like temperature sensors and acoustic transmitters. As the team iterates on their design for commercial use, the FMC-TENG is expected to produce enough power to run an entire open ocean monitoring platform including multiple sensors and satellite communications. Plus, the FMC-TENG is lightweight and can be used in both free-floating devices and moored platforms. “The FMC-TENG is unique because there are very few wave energy converters that are efficient and able to generate significant power from low-frequency ocean waves,” said Deng. “This type of generator could potentially power integrated buoys with sensor arrays to track open ocean water, wind, and climate data entirely using renewable ocean energy.” The study is published in the journal Nano Energy.

Researchers have developed a system that can transform plastic waste and greenhouse gases into sustainable fuels and other valuable products—using just the energy from the sun.

The researchers, from the University of Cambridge, developed the system, which can convert two waste streams into two chemical products at the same time—the first time this has been achieved in a solar-powered reactor.

The reactor converts the carbon dioxide (CO2) and plastics into different products that are useful in a range of industries. In tests, CO2 was converted into syngas, a key building block for sustainable liquid fuels, and plastic bottles were converted into glycolic acid, which is widely used in the cosmetics industry. The system can easily be tuned to produce different products by changing the type of catalyst used in the reactor.

Converting plastics and greenhouse gases—two of the biggest threats facing the natural world—into useful and valuable products using solar energy is an important step in the transition to a more sustainable, circular economy. The results are reported in the journal Nature Synthesis.

“Converting waste into something useful using solar energy is a major goal of our research,” said Professor Erwin Reisner from the Yusuf Hamied Department of Chemistry, the paper’s senior author. “Plastic pollution is a huge problem worldwide, and often, many of the plastics we throw into recycling bins are incinerated or end up in landfill.”

Reisner also leads the Cambridge Circular Plastics Center (CirPlas), which aims to eliminate plastic waste by combining blue-sky thinking with practical measures.

Other solar-powered “recycling” technologies hold promise for addressing plastic pollution and for reducing the amount of greenhouse gases in the atmosphere, but to date, they have not been combined in a single process.

“A solar-driven technology that could help to address plastic pollution and greenhouse gases at the same time could be a game-changer in the development of a circular economy,” said Subhajit Bhattacharjee, the paper’s co-first author.

“We also need something that’s tunable, so that you can easily make changes depending on the final product you want,” said co-first author Dr. Motiar Rahaman.

The researchers developed an integrated reactor with two separate compartments: one for plastic, and one for greenhouse gases. The reactor uses a light absorber based on perovskite—a promising alternative to silicon for next-generation solar cells.

The team designed different catalysts, which were integrated into the light absorber. By changing the catalyst, the researchers could then change the end product. Tests of the reactor under normal temperature and pressure conditions showed that the reactor could efficiently convert PET plastic bottles and CO₂ into different carbon-based fuels such as CO, syngas or formate, in addition to glycolic acid. The Cambridge-developed reactor produced these products at a rate that is also much higher than conventional photocatalytic CO₂ reduction processes.

“Generally, CO₂ conversion requires a lot of energy, but with our system, basically you just shine a light at it, and it starts converting harmful products into something useful and sustainable,” said Rahaman. “Prior to this system, we didn’t have anything that could make high-value products selectively and efficiently.”

“What’s so special about this system is the versatility and tunability—we’re making fairly simple carbon-based molecules right now, but in future, we could be able to tune the system to make far more complex products, just by changing the catalyst,” said Bhattacharjee.

Over the next five years, the researchers hope to further develop the reactor to produce more complex molecules. The researchers say that similar techniques could someday be used to develop an entirely solar-powered recycling plant.

“Developing a circular economy, where we make useful things from waste instead of throwing it into landfill, is vital if we’re going to meaningfully address the climate crisis and protect the natural world,” said Reisner. “And powering these solutions using the sun means that we’re doing it cleanly and sustainably.”

A device that can harvest water from the air and provide hydrogen fuel—entirely powered by solar energy—has been a dream for researchers for decades. Now, EPFL chemical engineer Kevin Sivula and his team have made a significant step toward bringing this vision closer to reality. They have developed an ingenious yet simple system that combines semiconductor-based technology with novel electrodes that have two key characteristics: they are porous, to maximize contact with water in the air; and transparent, to maximize sunlight exposure of the semiconductor coating. When the device is simply exposed to sunlight, it takes water from the air and produces hydrogen gas. The results have been published on 4 January 2023 in Advanced Materials.

“To realize a sustainable society, we need ways to store renewable energy as chemicals that can be used as fuels and feedstocks in industry. Solar energy is the most abundant form of renewable energy, and we are striving to develop economically competitive ways to produce solar fuels,” says Sivula of EPFL’s Laboratory for Molecular Engineering of Optoelectronic Nanomaterials and principal investigator of the study.

Inspiration from a plant’s leaf

In their research for renewable fossil-free fuels, the EPFL engineers in collaboration with Toyota Motor Europe, took inspiration from the way plants are able to convert sunlight into chemical energy using carbon dioxide from the air. A plant essentially harvests carbon dioxide and water from its environment, and with the extra boost of energy from sunlight, can transform these molecules into sugars and starches, a process known as photosynthesis. The sunlight’s energy is stored in the form of chemical bonds inside of the sugars and starches.

The transparent gas diffusion electrodes developed by Sivula and his team, when coated with a light harvesting semiconductor material, indeed act like an artificial leaf, harvesting water from the air and sunlight to produce hydrogen gas. The sunlight’s energy is stored in the form of hydrogen bonds.

Instead of building electrodes with traditional layers that are opaque to sunlight, their substrate is actually a 3-dimensional mesh of felted glass fibers.

Marina Caretti, lead author of the work, says, “Developing our prototype device was challenging since transparent gas-diffusion electrodes have not been previously demonstrated, and we had to develop new procedures for each step. However, since each step is relatively simple and scalable, I think that our approach will open new horizons for a wide range of applications starting from gas diffusion substrates for solar-driven hydrogen production.”

From liquid water to humidity in the air

Sivula and other research groups have previously shown that it is possible to perform artificial photosynthesis by generating hydrogen fuel from liquid water and sunlight using a device called a photoelectrochemical (PEC) cell. A PEC cell is generally known as a device that uses incident light to stimulate a photosensitive material, like a semiconductor, immersed in liquid solution to cause a chemical reaction. But for practical purposes, this process has its disadvantages; e.g., it is complicated to make large-area PEC devices that use liquid.

Sivula wanted to show that the PEC technology can be adapted for harvesting humidity from the air instead, leading to the development of their new gas diffusion electrode. Electrochemical cells (e.g., fuel cells) have already been shown to work with gases instead of liquids, but the gas diffusion electrodes used previously are opaque and incompatible with the solar-powered PEC technology.

Now, the researchers are focusing their efforts into optimizing the system. What is the ideal fiber size? The ideal pore size? The ideal semiconductors and membrane materials? These are questions that are being pursued in the EU Project Sun-to-X, which is dedicated to advance this technology, and develop new ways to convert hydrogen into liquid fuels.

Making transparent, gas-diffusion electrodes

In order to make transparent gas diffusion electrodes, the researchers start with a type of glass wool, which is essentially quartz (also known as silicon oxide) fibers and process it into felt wafers by fusing the fibers together at high temperature. Next, the wafer is coated with a transparent thin film of fluorine-doped tin oxide, known for its excellent conductivity, robustness and ease to scale-up.

These first steps result in a transparent, porous, and conducting wafer, essential for maximizing contact with the water molecules in the air and letting photons through. The wafer is then coated again, this time with a thin-film of sunlight-absorbing semiconductor materials. This second thin coating still lets light through, but appears opaque due to the large surface area of the porous substrate. As is, this coated wafer can already produce hydrogen fuel once exposed to sunlight.

The scientists went on to build a small chamber containing the coated wafer, as well as a membrane for separating the produced hydrogen gas for measurement. When their chamber is exposed to sunlight under humid conditions, hydrogen gas is produced, achieving what the scientists set out to do, showing that the concept of a transparent gas-diffusion electrode for solar-powered hydrogen gas production can be achieved.

While the scientists did not formally study the solar-to-hydrogen conversion efficiency in their demonstration, they acknowledge that it is modest for this prototype, and currently less than can be achieved in liquid-based PEC cells. Based on the materials used, the maximum theoretical solar-to-hydrogen conversion efficiency of the coated wafer is 12%, whereas liquid cells have been demonstrated to be up to 19% efficient.