Ongoing innovation: electrolysis, hydrogen and oxygen creation from seawater and sun

At Smithsonian.com 

Could Giant “Solar Rigs” Floating On the Ocean Convert Seawater To Hydrogen Fuel?  Scientists at Columbia University have designed a device that could make the process economically viable

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(Courtesy Flickr user Austin Valley)

Usually, when we think about energy production at sea, we imagine giant oil rigs, or perhaps rows of towering wind turbines.  Recently, though, floating solar panels have been added to the mix, including a solar farm the size of 160 football fields that went into operation in China last year.

Now, a team of researchers at Columbia University wants to go a step farther. They say it’s possible to use solar panels on the ocean surface to power devices that can produce hydrogen fuel from seawater.

Hydrogen is a clean form of energy, but it’s most commonly produced from natural gas in a process that also releases carbon dioxide, a key driver of climate change. The Columbia scientists say their device, called a floating photovoltaic electrolyzer, eliminates that consequence by instead utilizing electrolysis to separate oxygen and hydrogen in water molecules, and then storing the latter for use as fuel.

Team leader Daniel Esposito, an assistant professor of chemical engineering, points out that using existing commercial electrolyzers to generate hydrogen is pretty costly. “If you take off-the-shelf solar panels and commercially available electrolyzers, and you use sunlight to split water into hydrogen and oxygen, it’s going to be three to six times more expensive than if you were to produce hydrogen from natural gas,” he says.

He also notes that those electrolyzers require membranes to keep the oxygen and hydrogen molecules separated once they’re split apart. That not only adds to the cost, but those parts would tend to degrade quickly when exposed to the contaminants and microbes in saltwater.

Being able to safely demonstrate a device that can perform electrolysis without a membrane brings us another step closer to making seawater electrolysis possible,” Jack Davis, a researcher and lead author of the proof-of-concept study, said in a statement. “These solar fuel generators are essentially artificial photosynthesis systems, doing the same thing that plants do with photosynthesis, so our device may open up all kinds of opportunities to generate clean, renewable energy.”

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Two mesh electrodes are held at a narrow separation distance (L), and generate H2 and O2 gases concurrently. The key innovation is the asymmetric placement of the catalyst on the outward facing surfaces of the mesh, such that the generation of bubbles is constrained to this region. When the gas bubbles detach, their buoyancy causes them to float upward into separate collection chambers. (Daniel Esposito/Columbia Engineering)

Bubbling up

So, what makes their electrolyzer distinctive?

The device is built around electrodes of titanium mesh suspended in water and separated by a small distance. When an electrical current is applied, the oxygen and hydrogen molecules split apart, with the former developing gas bubbles on the electrode that’s positively charged, and the latter doing the same on the one with a negative charge.

It’s critical to keep these different gas bubbles separated, and the Columbia electrolyzer does this through the application of a catalyst to only one side of each mesh component—the surface farthest away from the other electrode. When the bubbles get larger and detach from the mesh, they float up along the outside edges of each electrode instead of mixing together in the space between them.

Not only have the scientists avoided using expensive membranes, but they also didn’t have to incorporate the mechanical pumps that some models use to move liquids. Instead, their device relies on buoyancy to float the hydrogen bubbles up into a storage chamber. In the lab, the process was able to produce hydrogen gas with a 99 percent purity.

Alexander Orlov, an associate professor of materials science and chemical engineering at Stony Brook University in New York, agrees that the elimination of membranes is a “substantial” development. “The membranes are weak points in the technology,” he says. “There are some more sophisticated solutions, but Esposito’s approach is extremely simple and quite practical. It has been published and peer-reviewed in very high-impact publications, so despite its simplicity, the science and novelty are solid.”

Thinking big

Esposito and Davis readily acknowledge that it’s a big leap from the small model tested in their lab to the massive kind of structure that could make the concept economically viable. It might need to comprise hundreds of thousands of connected electrolyzer units to generate a sufficient amount of hydrogen fuel from the sea.

In fact, says Esposito, it might be necessary to make some design changes as the project scales up and becomes more modular, so many pieces can fit together to cover a large area. Also, they face the challenge of finding materials that can survive for a long time in saltwater.

That said, both believe their approach has potential to affect the country’s energy supply in a meaningful way. Hydrogen already is heavily used in the chemical industry, for instance, to make ammonia and methanol. And, demand is expected to keep rising as more auto manufacturers commit to cars that run on hydrogen fuel cells.

image: https://public-media.smithsonianmag.com/filer/db/92/db929a99-8f4a-4a11-bcd3-e358c3d1e9fa/schematic_of_floating_pv-electroylzer.jpgschematic of floating pv-electroylzer.jpg
(Left) Photo of the stand-alone PV-electrolyzer prototype floating in a liquid reservoir of sulfuric acid. Photovoltaic cells positioned on top of the “mini rig” convert light into electricity that is used to power the membraneless electrolyzer submerged below. (Right) A rendering of a hypothetical large-scale “solar fuels rig” operating on the open sea. ((Left) Jack Davis and (right) Justin Bui/Columbia Engineering)

Their long-term vision is of giant “solar fuel rigs” floating in the ocean, and Esposito has gone so far as to estimate how much cumulative area they would need to cover to generate enough hydrogen fuel to replace all the oil used on the planet. His calculation: 63,000 square miles, or an area slightly less than the state of Florida. That sounds like a lot of ocean, but he points out that the total area would cover about .045 percent of the Earth’s water surface.

It’s a bit of a pie-in-the-sky projection, but Esposito has also thought about the real-world challenges that would face a floating energy production operation not tethered to the sea bed. For starters, there are big waves.

“Certainly, we’d need to design the infrastructure for this rig so that it can withstand stormy seas,” he says. “It’s something you would take into account when you’re thinking where a rig is located.”

And maybe, he adds, these rigs could be able to move out of harm’s way.

“There’s the possibility of a rig like this being mobile. Something that could perhaps expand, and then contract. It probably wouldn’t be able to move fast, but it could move out of the way of a storm.

“That would be really valuable,” he says.

Columbia Engineers Develop Floating Solar Fuels Rig for Seawater Electrolysis  

Design is the first practical floating solar hydrogen-generating device to perform water electrolysis without pumps or membranes; could lead to low-cost, sustainable hydrogen production

Schematic side-view of passive membraneless electrolysis. Two mesh electrodes are held at a narrow separation distance (L), and generate H2 and O2 gases concurrently. The key innovation is the asymmetric placement of the catalyst on the outward facing surfaces of the mesh, such that the generation of bubbles is constrained to this region. When the gas bubbles detach, their buoyancy causes them to float upward into separate collection chambers. —Image credit: Daniel Esposito/Columbia Engineering

In a single hour, more energy from the sun hits the Earth than all the energy used by humankind in an entire year. Imagine if the sun’s energy could be harnessed to power energy needs on Earth, and done in a way that is economical, scalable, and environmentally responsible. Researchers have long seen this as one of the grand challenges of the 21st century.

Daniel Esposito, assistant professor of chemical engineering at Columbia Engineering, has been studying water electrolysis—the splitting of water into oxygen (O2) and hydrogen (H2) fuel—as a way to convert electricity from solar photovoltaics (PVs) into storable hydrogen fuel. Hydrogen is a clean fuel that is currently used to propel rockets in NASA’s space program and is widely expected to play an important role in a sustainable energy future. The vast majority of today’s hydrogen is produced from natural gas through a process called steam methane reforming that simultaneously releases CO2, but water electrolysis using electricity from solar PV offers a promising route to produce H2 without any associated CO2 emissions.

Esposito’s team has now developed a novel photovoltaic-powered electrolysis device that can operate as a stand-alone platform that floats on open water. His floating PV-electrolyzer can be thought of as a “solar fuels rig” that bears some resemblance to deep-sea oil rigs, except that it would produce hydrogen fuel from sunlight and water instead of extracting petroleum from beneath the sea floor. The study (DOI: 10.1016/j.ijhydene.2017.11.086), “Floating Membraneless PV-Electrolyzer Based on Buoyancy-Driven Product Separation,” was published today by International Journal of Hydrogen Energy.

High speed video comparison of asymmetric and symmetric electrodes during operation at a current density of 150 mA cm-2. For asymmetric electrodes, the product bubbles are generated only on the outward facing surfaces and do not mix with each other as they float upwards. On the symmetric electrodes, bubbles are also generated on the inside surface, and thus they can more easily cross-over into the opposite collection chamber, thereby reducing the product purity.
—Video credit: Jack Davis/Columbia Engineering

The researchers’ key innovation is the method by which they separate the H2 and O2 gases produced by water electrolysis. State-of-the-art electrolyzers use expensive membranes to maintain separation of these two gases. The Columbia Engineering device relies instead on a novel electrode configuration that allows the gases to be separated and collected using the buoyancy of bubbles in water. The design enables efficient operation with high product purity and without actively pumping the electrolyte. Based on the concept of buoyancy-induced separation, the simple electrolyzer architecture produces H2 with purity as high as 99 percent.

Side by side schematic of the floating PV-electrolyzer: (Left) Photo of the stand-alone PV-electrolyzer prototype floating in a liquid reservoir of sulfuric acid. Photovoltaic cells positioned on top of the “mini rig” convert light into electricity that is used to power the membraneless electrolyzer submerged below. The generated H2 bubbles are harvested within the interior of the device as they float upwards, while O2 bubbles are allowed to vent to the atmosphere. (Right) Rendering of a hypothetical large-scale “solar fuels rig” operating on the open sea where it uses abundant sunlight to split seawater into H2, which is temporarily stored on the rig before being piped or shipped back to shore. —Image credit: (left) Jack Davis (right) Justin Bui / Columbia Engineering

“The simplicity of our PV-electrolyzer architecture—without a membrane or pumps—makes our design particularly attractive for its application to seawater electrolysis, thanks to its potential for low cost and higher durability compared to current devices that contain membranes,” says Esposito, whose Solar Fuels Engineering Laboratory develops solar and electrochemical technologies that convert renewable and abundant solar energy into storable chemical fuels. “We believe that our prototype is the first demonstration of a practical membraneless floating PV-electrolyzer system, and could inspire large-scale ‘solar fuels rigs’ that could generate large quantities of H2 fuel from abundant sunlight and seawater without taking up any space on land or competing with fresh water for agricultural uses.”

Commercial electrolyzer devices rely on a membrane, or divider, to separate the electrodes within the device from which H2 and O2 gas are produced. Most of the research for electrolysis devices has been focused on devices that incorporate a membrane. These membranes and dividers are prone to degradation and failure and require a high purity water source. Seawater contains impurities and microorganisms that can easily destroy these membranes.

Being able to safely demonstrate a device that can perform electrolysis without a membrane brings us another step closer to making seawater electrolysis possible,” says Jack Davis, the paper’s first author and a PhD student working with Esposito. “These solar fuels generators are essentially artificial photosynthesis systems, doing the same thing that plants do with photosynthesis, so our device may open up all kinds of opportunities to generate clean, renewable energy.”

Crucial to the operation of Esposito’s PV-electrolyzer is a novel electrode configuration comprising mesh flow-through electrodes that are coated with a catalyst only on one side. These asymmetric electrodes promote the evolution of gaseous H2 and O2 products on only the outer surfaces of the electrodes where the catalysts have been deposited. When the growing H2 and O2 bubbles become large enough, their buoyancy causes them to detach from the electrode surfaces and float upwards into separate overhead collection chambers.

The team used the Columbia Clean Room to deposit platinum electrocatalyst onto the mesh electrodes and the 3D-printers in the Columbia Makerspace to make many of the reactor components. They also used a high-speed video camera to monitor transport of H2 and O2 bubbles between electrodes, a process known as “crossover.” Crossover between electrodes is undesirable because it decreases product purity, leading to safety concerns and the need for downstream separation units that make the process more expensive.

In order to monitor H2 and O2 crossover events, the researchers incorporated windows in all of their electrolysis devices so that they could take high-speed videos of gas bubble evolution from the electrodes while the device was operating. These videos were typically taken at a rate of 500 frames per second (a typical iPhone captures video at a rate of 30 frames per second).

The team is refining their design for more efficient operation in real seawater, which poses additional challenges compared to the more ideal aqueous electrolytes used in their laboratory studies. They also plan to develop modular designs that they can use to build larger, scaled-up systems.

Esposito adds: “There are many possible technological solutions to achieve a sustainable energy future, but nobody knows exactly what specific technology or combination of technologies will be the best to pursue. We are especially excited about the potential of solar fuels technologies because of the tremendous amount of solar energy that is available. Our challenge is to find scalable and economical technologies that convert sunlight into a useful form of energy that can also be stored for times when the sun is not shining.”

About the Study

The study is titled “Floating Membraneless PV-Electrolyzer Based on Buoyancy-Driven Product Separation.” Authors include: Jonathan Davis, Ji Qi, Xinran Fan, Justin Bui, and Daniel V. Esposito, all in the department of chemical engineering, Columbia Engineering. The study was funded by Columbia University start-up funds.  The authors declare no financial or other conflicts of interest.

http://engineering.columbia.edu/news/daniel-esposito-solar-fuels-seawater-electrolysis

Jonathan T. Davis, Ji Qi, Xinran Fan, Justin C. Bui, Daniel V. Esposito. Floating membraneless PV-electrolyzer based on buoyancy-driven product separationInternational Journal of Hydrogen Energy, 2017; DOI: 10.1016/j.ijhydene.2017.11.086

Columbia University School of Engineering and Applied Science. “Floating solar fuels rig created for seawater electrolysis: Design is the first practical floating solar hydrogen-generating device to perform water electrolysis without pumps or membranes; could lead to low-cost, sustainable hydrogen production.” ScienceDaily. ScienceDaily, 15 December 2017. <www.sciencedaily.com/releases/2017/12/171215094443.htm>.

In the 20th century, scientists developed electrolysis devices for seawater that could separate hydrogen from water using fine membranes. But these contraptions were prone to clogging and biofouling.  “Magnesium and calcium ions precipitate and clog the pores of the membranes, and the microbes and other life forms attach to them,” Esposito explains. “You can purify them, but it adds cost.”  To solve this problem, Esposito and his team have created a first of its kind membrane-free hydrogen rig. In their design, the reactive sides of the electrodes, which do the actual work of making hydrogen and oxygen, face away from each other. As the electrical charge is applied and the water breakdown reaction occurs, hydrogen forms at the negative electrode, while oxygen congregates at the positive one, with both gasses naturally separated from each other.

This arrangement keeps the hydrogen and oxygen gasses far enough apart to prevent them from recombining into water molecules. Instead, the molecules float up along the vertical electrodes, forming bigger bubbles as they rise. At the top, the hydrogen is collected into a container while the oxygen is vented into the atmosphere—possibly adding a whiff of fresh air to Esposito’s basement lab.  Esposito says commercial seawater hydrogen production is still far on the horizon, but once the technology matures, the floating rigs would function somewhat similarly to oil rigs, without the fear of polluting spills.  If operating close to shore, the hydrogen could be brought to land in pipes. If the rig is in the deep sea, the gas would have to be pumped into tankers. Another possibility, Esposito says, is to use blimps or air balloons to deliver the gas, capitalizing on hydrogen’s natural buoyancy.  If we wanted to use offshore hydrogen production to replace all the oil used in the world today, the floating rigs would have to cover only a tiny fraction of the world’s ocean—about 162,000 square kilometers, Esposito says. That’s slightly smaller than Florida, and bigger than New York State—or about 0.032 percent of our globe’s surface. “That’s all you need to power the planet with seawater and sun,” Esposito says.

Turning Water Into Fuel