Water molecules on the surface of an electrode flip just before they give up electrons to form oxygen – a feat of nanoscale gymnastics that explains why the reaction takes more energy than it theoretically should. After observing this flipping in individual water molecules for the first time, scientists at Northwestern University in the US say that the next step is to find ways of controlling it. Doing so could improve the efficiency of the reaction, making it easier to produce both oxygen and hydrogen fuel from water.

The water splitting process takes place in an electrochemical cell containing water and a metallic electrode. When a voltage is applied to the electrode, the water splits into oxygen and hydrogen via two separate half-reactions.

The problem is that the half-reaction that produces oxygen, known as the oxygen evolution reaction (OER), is difficult and inefficient and takes more energy than predicted by theory. “It should require 1.23 V,” says Franz Geiger, the Northwestern physical chemist who led the new study, “but in reality, it requires more like 1.5 or 1.8 V.” This extra energy cost is one of the reasons why water splitting has not been implemented on a large scale, he explains.

Determining how water molecules arrange themselves

In the new work, Geiger and colleagues wanted to test whether the orientation of the water’s oxygen atoms affects the kinetics of the OER. To do this, they directed an 80-femtosecond pulse of infrared (1034 nm) laser light onto the surface of the electrode, which was in this case made of nickel. They then measured the intensity of the reflected light at half the incident wavelength.

This method, which is known as second harmonic and vibrational sum-frequency generation spectroscopy, revealed that the water molecules’ alignment on the surface of the electrode depended on the applied voltage. By analysing the amplitude and phase of the signal photons as this voltage was cycled, the researchers were able to pin down how the water molecules arranged themselves.

They found that before the voltage was applied, the water molecules were randomly oriented. At a specific applied voltage, however, they began to reorient. “We also detected water dipole flipping just before cleavage and electron transfer,” Geiger adds. “This allowed us to distinguish flipping from subsequent reaction steps.”

An unexplored idea

The researchers’ explanation for this flipping is that at high pH levels, the surface of the electrode is negatively charged due to the presence of nickel hydroxide groups that have lost their protons. The water molecules therefore align with their most positively charged ends facing the electrode. However, this means that the ends containing the electrons needed for the OER (which reside in the oxygen atoms) are pointing away from the electrode. “We hypothesized that water molecules must flip to align their oxygen atoms with electrochemically active nickel oxo species at high applied potential,” Geiger says.

This idea had not been explored until now, he says, because water absorbs strongly in the infrared range, making it appear opaque at the relevant frequencies. The electrodes typically employed are also too thick for infrared light to pass through. “We overcame these challenges by making the electrode thin enough for near-infrared transmission and by using wavelengths where water’s absorbance is low (the so-called ‘water window’),” he says.

Other challenges for the team included designing a spectrometer that could measure the second harmonic generation amplitude and phase and developing an optical model to extract the number of net-aligned water molecules and their flipping energy. “The full process – from concept to publication – took three years,” Geiger tells Physics World.

The team’s findings, which are detailed in Science Advances, suggest that controlling the orientation of water at the interface with the electrode could improve OER catalyst performance. For example, surfaces engineered to pre-align water molecules might lower the kinetic barriers to water splitting. “The results could also refine electrochemical models by incorporating structural water energetics,” Geiger says. “And beyond the OER, water alignment may also influence other reactions such as the hydrogen evolution reaction and CO₂ reduction to liquid fuels, potentially impacting multiple energy-related technologies.”

The researchers are now exploring alternative electrode materials, including NiFe and multi-element catalysts. Some of the latter can outperform iridium, which has traditionally been the best-performing electrocatalyst, but is very rare (it comes from meteorites) and therefore expensive. “We have also shown in a related publication (in press) that water flipping occurs on an earth-abundant semiconductor, suggesting broader applicability beyond metals,” Geiger reveals.

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If a water droplet flowing over a surface gets stuck, and then unsticks itself, it generates an electric charge. The discoverers of this so-called depinning phenomenon are researchers at RMIT University and the University of Melbourne, both in Australia, and they say that boosting it could make energy-harvesting devices more efficient.

The newly observed charging mechanism is conceptually similar to slide electrification, which occurs when a liquid leaves a surface – that is, when the surface goes from wet to dry. However, the idea that the opposite process can also generate a charge is new, says Peter Sherrell, who co-led the study. “We have found that going from dry to wet matters as well and may even be (in some cases) more important,” says Sherrell, an interdisciplinary research fellow at RMIT. “Our results show how something as simple as water moving on a surface still shows basic phenomena that have not been understood yet.”

Co-team leader Joe Berry, a fluid dynamics expert at Melbourne, notes that the charging mechanism only occurs when the water droplet gets temporarily stuck on the surface. “This suggests that we could design surfaces with specific structure and/or chemistry to control this charging,” he says. “We could reduce this charge for applications where it is a problem – for example in fuel handling – or, conversely, enhance it for applications where it is a benefit. These include increasing the speed of chemical reactions on catalyst surfaces to make next-generation batteries more efficient.”

More than 500 experiments

To observe depinning, the researchers built an experimental apparatus that enabled them to control the sticking and slipping motion of a water droplet on a Teflon surface while measuring the corresponding change in electrical charge. They also controlled the size of the droplet, making it big enough to wet the surface all at once, or smaller to de-wet it. This allowed them to distinguish between multiple mechanisms at play as they sequentially wetted and dried the same region of the surface.

Their study, which is published in Physical Review Letters, is based on more than 500 wetting and de-wetting experiments performed by PhD student Shuaijia Chen, Sherrell says. These experiments showed that the largest change in charge – from 0 to 4.1 nanocoulombs (nC) – occurred the first time the water contacted the surface. The amount of charge then oscillated between about 3.2 and 4.1 nC as the system alternated between wet and dry phases. “Importantly, this charge does not disappear,” Sherrell says. “It is likely generated at the interface and probably retained in the droplet as it moves over the surface.”

The motivation for the experiment came when Berry asked Sherrell a deceptively simple question: was it possible to harvest electricity from raindrops? To find out, they decided to supervise a semester-long research project for a master’s student in the chemical engineering degree programme at Melbourne. “The project grew from there, first with two more research project students [before] Chen then took over to build the final experimental platform and take the measurements,” Berry recalls.

The main challenge, he adds, was that they did not initially understand the phenomenon they were measuring. “Another obstacle was to design the exact protocol required to repeatedly produce the charging effect we observed,” he says.

Potential applications

Understanding how and why electric charge is generated as liquids flow during over surfaces is important, Berry says, especially with new, flammable types of renewable fuels such as hydrogen and ammonia seen as part of the transition to net zero. “At present, with existing fuels, charge build-up is reduced by restricting flow using additives or other measures, which may not be effective in newer fuels,” he explains. “This knowledge may help us to engineer coatings that could mitigate charge in new fuels.”

The RMIT/Melbourne researchers now plan to investigate the stick-slip phenomenon with other types of liquids and surfaces and are keen to partner with industries to target applications that can make a real-world impact. “At this stage, we have simply reported that this phenomenon occurs,” Sherrell says. “We now want to show that we can control when and where these charging events happen – either to maximize them or eliminate them. We are still a long way off from using our discovery for chemical and energy applications – but it’s a big step in the right direction.”

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Scientists in the US have developed a new type of photovoltaic battery that runs on the energy given off by nuclear waste. The battery uses a scintillator crystal to transform the intense gamma rays from radioisotopes into electricity and can produce more than a microwatt of power. According to its developers at Ohio State University and the University of Toledo, it could be used to power microelectronic devices such as microchips.

The idea of a nuclear waste battery is not new. Indeed, Raymond Cao, the Ohio State nuclear engineer who led the new research effort, points out that the first experiments in this field date back to the early 1950s. These studies, he explains, used a 50 milli-Curie ⁹⁰Sr-⁹⁰Y source to produce electricity via the electron-voltaic effect in p-n junction devices.

However, the maximum power output of these devices was just 0.8 μW, and their power conversion efficiency (PCE) was an abysmal 0.4 %. Since then, the PCE of nuclear voltaic batteries has remained low, typically in the 1–3% range, and even the most promising devices have produced, at best, a few hundred nanowatts of power.

Exploiting the nuclear photovoltaic effect

Cao is confident that his team’s work will change this. “Our yet-to-be-optimized battery has already produced 1.5 μW,” he says, “and there is much room for improvement.”

To achieve this benchmark, Cao and colleagues focused on a different physical process called the nuclear photovoltaic effect. This effect captures the energy from highly-penetrating gamma rays indirectly, by coupling a photovoltaic solar cell to a scintillator crystal that emits visible light when it absorbs radiation. This radiation can come from several possible sources, including nuclear power plants, storage facilities for spent nuclear fuel, space- and submarine-based nuclear reactors or, really, anyplace that happens to have large amounts of gamma ray-producing radioisotopes on hand.

The scintillator crystal Cao and colleagues used is gadolinium aluminium garnet (GAGG), and they attached it to a solar cell made from polycrystalline CdTe. The resulting device measures around 2 x 2 x 1 cm, and they tested it using intense gamma rays emitted by two different radioactive sources, ¹³⁷Cs and ⁶⁰Co, that produced 1.5 kRad/h and 10 kRad/h, respectively. ¹³⁷Cs is the most common fission product found in spent nuclear fuel, while ⁶⁰Co is an activation product.

Enough power for a microsensor

The Ohio-Toledo team found that the maximum power output of their battery was around 288 nW with the ¹³⁷Cs source. Using the ⁶⁰Co irradiator boosted this to 1.5 μW. “The greater the radiation intensity, the more light is produced, resulting in increased electricity generation,” Cao explains.

The higher figure is already enough to power a microsensor, he says, and he and his colleagues aim to scale the system up to milliwatts in future efforts. However, they acknowledge that doing so presents several challenges. Scaling up the technology will be expensive, and gamma radiation gradually damages both the scintillator and the solar cell. To overcome the latter problem, Cao says they will need to replace the materials in their battery with new ones. “We are interested in finding alternative scintillator and solar cell materials that are more radiation-hard,” he tells Physics World.

The researchers are optimistic, though, arguing that optimized nuclear photovoltaic batteries could be a viable option for harvesting ambient radiation that would otherwise be wasted. They report their work in Optical Materials X.

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A research team headed up at Linköping University in Sweden and Cornell University in the US has succeeded in recycling almost all of the components of perovskite solar cells using simple, non-toxic, water-based solvents. What’s more, the researchers were able to use the recycled components to make new perovskite solar cells with almost the same power conversion efficiency as those created from new materials. This work could pave the way to a sustainable perovskite solar economy, they say.

While solar energy is considered an environmentally friendly source of energy, most of the solar panels available today are based on silicon, which is difficult to recycle. This has led to the first generation of silicon solar panels, which are reaching the end of their life cycles, ending up in landfills, says Xun Xiao, one of the team members at Linköping University.

When developing emerging solar cell technologies, we therefore need to take recycling into consideration, adds one of the leaders of the new study, Feng Gao, also at Linköping. “If we don’t know how to recycle them, maybe we shouldn’t put them on the market at all.”

To this end, many countries around the world are imposing legal requirements on photovoltaic manufacturers, to ensure that they collect and recycle any solar cell waste they produce. These initiatives include the WEEE directive 2012/19/EU in the European Union and equivalent legislation in Asia and the US.

Perovskites are one of the most promising materials for making next-generation solar cells. Not only are they relatively inexpensive, they are also easy to fabricate, lightweight, flexible and transparent. This allows them to be placed on top of a variety of surfaces, unlike their silicon counterparts. And since they boast a power conversion efficiency (PCE) of more than 25%, this makes them comparable to existing photovoltaics on the market.

A shorter lifespan

One of their downsides, however, is that perovskite solar cells have a shorter lifespan than silicon solar cells. This means that recycling is even more critical for these materials. Today, perovskite solar cells are disassembled using dangerous solvents such as dimethylformamide, but Gao and colleagues have now developed a technique in which water can be used as the solvent.

Perovskites are crystalline materials with an ABX₃ structure, where A is caesium, methylammonium (MA) or formamidinium (FA); B is lead or tin; and X is chlorine, bromine or iodine. Solar cells made of these materials are composed of different layers: the hole/electron transport layers; the perovskite layer; indium tin oxide substrates; and cover glasses.

In their work, which they detail in Nature, the researchers succeeded in delaminating end-of-life devices layer by layer, using water containing three low-cost additives: sodium acetate, sodium iodide and hypophosphorous acid. Despite being able to dissolve organic iodide salts such as methylammonium iodide and formamidinium iodide, water only marginally dissolves lead iodide (about 0.044 g per 100 ml at 20 °C). The researchers therefore developed a way to increase the amount of lead iodide that dissolves in water by introducing acetate ions into the mix. These ions readily coordinate with lead ions, forming highly soluble lead acetate (about 44.31 g per 100 ml at 20 °C).

Once the degraded perovskites had dissolved in the aqueous solution, the researchers set about recovering pure and high-quality perovskite crystals from the solution. They did this by providing extra iodide ions to coordinate with lead. This resulted in [PbI]⁺ transitioning to [PbI₂]⁰ and eventually to [PbI₃]⁻ and the formation of the perovskite framework.

To remove the indium tin oxide substrates, the researchers sonicated these layers in a solution of water/ethanol (50%/50% volume ratio) for 15 min. Finally, they delaminated the cover glasses by placing the degraded solar cells on a hotplate preheated to 150 °C for 3 min.

They were able to apply their technology to recycle both MAPbI₃ and FAPbI₃ perovskites.

New devices made from the recycled perovskites had an average power conversion efficiency of 21.9 ± 1.1%, with the best samples clocking in at 23.4%. This represents an efficiency recovery of more than 99% compared with those prepared using fresh materials (which have a PCE of 22.1 ± 0.9%).

Looking forward, Gao and colleagues say they would now like to demonstrate that their technique works on a larger scale. “Our life-cycle assessment and techno-economic analysis has already confirmed that our strategy not only preserves raw materials, but also appreciably lowers overall manufacturing costs of solar cells made from perovskites,” says co-team leader Fengqi You, who works at Cornell University. “In particular, reclaiming the valuable layers in these devices drives down expenses and helps reduce the ‘levelized cost’ of electricity they produce, making the technology potentially more competitive and sustainable at scale,” he tells Physics World.

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