Science & Tech

Green Energy Breakthrough: Hydrogen Harvesting with Seawater Electrolysis

An interview with Daniel Esposito, Assistant Professor of Chemical Engineering at Columbia University.

A view of what the membraneless electrolysis technology looks like.

Hydrogen is one of the many promising clean fuel sources of the future. Unfortunately, the dominant approaches used to date to produce it in a sufficient volume involve the use of methane, itself a greenhouse gas, in a reaction that also releases carbon dioxide (CO2).

Daniel Esposito, an assistant professor at Columbia University’s Chemical Engineering department, is working with a team to invent a very different way of producing clean energy: efficiently and with no harmful emissions.

The technology they are creating is what appears to be a solution that allows the production of hydrogen using membrane-free electrolysis, itself a first-of-its-kind innovation, envisioned to be used on a rig that would float on seawater and itself be powered solely by solar energy. If and when the technology becomes a mass-manufacturable reality, it could change the world of clean energy for billions around the globe.

Trillions spoke to Dr. Esposito in his offices at Columbia University.

Trillions: Could you describe your role at Columbia and what you do there?

Daniel Esposito: I am in my fourth year as an assistant professor in the Chemical Engineering department here at Columbia. As an assistant professor, my responsibilities include not only teaching classes but also running a research lab.

The particular lab that I run here is called the Solar Fuels Engineering Lab. The overarching goal and objective of our research in this lab is to develop materials and devices that will help more efficiently convert solar energy, or electricity from solar photovoltaics, into storable chemical fuels.

 

Trillions: It is certainly something that will make a big difference. One of the things that a lot of readers out there probably aren’t even aware of is how much hydrogen is being pushed globally [as an alternate fuel]. Toyota is putting an entire ecosystem into place, over in Japan, where even delivery trucks and [the means of] grabbing your hydrogen “on the go” are all being set up as infrastructure. With innovations like what you’re talking about, it should make it possible to make use of this sooner than a lot of people realize.

What exactly is this innovation that you’ve been working on, on the nature of water electrolysis using solar energy?

Daniel Esposito: The innovation in what we’ve done, in this most recent study, is at two different levels. At one level, the innovation is a novel electrode architecture that allows us to split water into oxygen and hydrogen without the presence of a membrane or divider separating those two electrodes or a pump that’s used to flow the liquid electrolyte through the device. With this architecture that we’ve developed for these electrodes, it’s very simple.

With that simplicity comes potentially low-cost manufacturing and assembly of these devices. This is particularly important in a renewable energy future, because the price of electricity from renewable energy generators like solar and wind has dropped substantially in recent years and will continue to do so going forward. So, when you look at the economics of these water electrolysis systems for producing hydrogen, that puts a big emphasis on decreasing capital costs in order to make hydrogen production competitive with conventional hydrogen production technology, which is steam methane reforming.

Steam methane reforming is a cheap process right now because natural gas is very cheap. Of course, a downside of steam methane reforming is that it also emits CO2 in the process. So, water electrolysis driven by renewable resources is a carbon-free method of producing a carbon-free fuel.

That was innovation number one – the simplicity of the electrode architecture that’s used within these electrolysis devices.

The second innovation is more at the device or system level, where we have implemented this membrane-free architecture into a bigger system or device that floats on the surface of the water. Hence the name we came up with, which is “solar fuels rig.”

There are examples of other people in literature who have thought of doing something along the lines of this. But, to our knowledge, this is the first demonstration of a practical device that in principle could be scaled up and operated to generate large quantities of CO2- free hydrogen.

Trillions: I have this image of seeing … this … outside. You see wind energy now in areas where that might have been a surprise. Or you see solar panel installations … [including] a massive one that went into West Virginia recently over an old mine, which is a wonderful way of making use of it. They’re also doing a similar thing in China with that. Here we have the idea that we might have the floating rigs [you’ve described] out there, so you’re basically mining the water in a very different way.

Let’s go into a little more detail into the technology, the nature of electrolysis, how it works and how your approach with membrane-less electrolysis works in this particular situation.

Daniel Esposito: At a high level, let’s talk about what water electrolysis is. Another name for it is “water splitting.” As is implied by that name, you need to put energy into an electrolyzer system to split water into its two components: oxygen and hydrogen. Based on the stoichiometry of water, which is H2O, you get two times the amount of hydrogen as you get oxygen.

This is a thermodynamic uphill reaction, meaning you need to put energy into it to go from water to oxygen and hydrogen. There are a number of ways you can do that, but electrolysis is one that we and many other people believe is really attractive for facilitating this reaction. In electrolysis, the input source of energy to drive water splitting is electricity, and this is done through an electrochemical process. So, as is implied by that word “electrochemical,” you are converting electrical energy into chemical energy in the form of the chemical bond within the hydrogen fuel that’s produced.

In electrochemical water splitting, one often views the overall electrochemical reaction as two separate half reactions. The two half reactions that are relevant for water splitting are the oxygen and hydrogen evolution reactions. The hydrogen evolution reaction occurs at the cathode, which is one of two electrodes within the electrolyzer. The oxygen evolution reaction takes place at the anode, which is the other electrode within the electrolyzer.

Artist\'s conception of the the seaborne version of the hydrogen harvester.

During electrolyzer operation, a first important task is to produce the oxygen and the hydrogen by electrolysis, and the second important task is to ensure that these two product species remain separate from each other. Hydrogen is really, for the vast majority of applications, the valuable fuel that we’re most interested in. If you get this mixture of oxygen and hydrogen, that’s going to decrease the value of the hydrogen because it will be less pure. Secondly, it can create a safety hazard, because a mixture of hydrogen and oxygen can be highly flammable. So it’s really important during electrolysis to make sure the oxygen and the hydrogen that are being produced remain separate from each other.

In conventional electrolyzers, membranes or nanoporous or microporous dividers called diaphragms are used to separate the two electrodes. They still allow ionic current to transfer between the two electrodes, but they serve as physical barriers to block the oxygen and hydrogen from crossing between the gap that separates the two electrodes.

In our research, a key innovation that allowed us to avoid the use of these dividers was related to the structure of the electrodes used within the device. These electrodes are what are called “porous flow-through electrodes.” They look just like the screen that you would find in a window or a screen door that allows air through but keeps the bugs out. What we did was to take two pieces of mesh electrodes and deposit a special metallic material, called the catalyst, on only the outer surfaces of these two mesh electrodes. So, if you are looking at a side view of our configuration, the two mesh screens are placed in a face-to-face configuration and the metal catalyst, where the reaction takes place, is deposited only on the outer surfaces. The catalyst is where the reaction occurs, as I just mentioned. So, because it’s placed only on the outer surface, that’s where the gaseous oxygen and hydrogen products form.

As a gaseous product, these products want to float upwards within the aqueous electrolyte from which they form. That buoyancy force causes those bubbles to float upwards into separate collection chambers before the bubbles have a chance to cross over between the gap that separates those two electrodes. We refer to this phenomenon as “buoyancy-induced separation” of the oxygen and hydrogen products.

Trillions: That’s very clever, in terms of how you’re separating the two gases in there. [You made] a very good point, that the hydrogen, of course, is the fuel and the O2 is a wonderful place to burn things in, so you want to be careful about keeping the two of them separated.

How far along is this, and what kind of experimental results have you seen in efficiency of operation or scalability?

Daniel Esposito: Everything that we’ve done so far has involved lab-scale demonstrations. The miniature solar fuels rig that we demonstrated in our lab is about eight inches or so across. So, the quantity of hydrogen that’s produced by this is very small. [It’s] maybe enough to power the electronics for your phone, but to have a meaningful impact on global energy use, this is something that would have to be scaled up over acres, if you wanted to service the fuel needs of, say, a town. Significantly larger areas would be needed if you wanted to expand to the level of impacting state or nationwide energy use.

In principle, though, what we’ve done is really promising, because the basic design should be scalable without impacting the efficiency or the product purity. There are certainly some adjustments and some optimization that we and others will need to do to get there, but I think they are largely solvable engineering problems.

The other question behind this is the economics. How much is it going to cost to construct these things, and will the price of the hydrogen produced with this technology be able to compete with hydrogen produced by the conventional steam methane reforming process? At this point, we really don’t know, but we are very optimistic that the simplicity of our devices and likelihood of improving their performance further will create opportunities to compete not only with conventional electrolyzer designs but also fossil fuels.

Trillions: That’s a challenge that, until you actually have begun to try scaling it up, as well as think about the logistics of how you move what you’ve gathered to where it needs to get to, [you can’t be sure].

Daniel Esposito: So there needs to be a larger infrastructure in place. Which is something that some countries – Japan is one that you mentioned – have made a lot of progress in these regards in recent years. It’s also probably going to be location-dependent. It’s also going to be helpful if there’s already an industry that’s consuming large amounts of hydrogen. When hydrogen is discussed as a fuel, people generally think of it as a carbon-free “fuel of the future.” But the truth of the matter is that, globally, we consume massive amounts of hydrogen already today, using large amounts of energy to produce it. To be exact, we use about eight quads worth of energy every year to produce hydrogen. A quad is [equal to] a quadrillion BTUs. Just as a reference, the United States’ total annual energy use is 100 quads. So, we’re talking about a big amount of energy that’s already used to produce hydrogen, and there’s a large associated amount of hydrogen with that. A lot of that goes to the chemical industry, for example, for making chemicals like ammonia, which is really important for fertilizers and feeds billions of people on this planet. Another important chemical is methanol, which uses hydrogen as a feedstock.

So, if you can co-locate these water electrolysis facilities close to where you have some of these large chemical plants that are using hydrogen from steam methane reforming, then that can have a big impact. And, in principle, the infrastructure [to make use of the hydrogen] is already there today.

Hydrogen can [also] be a universal CO2-free fuel. Hydrogen can be used not only as transportation fuel but also as the fuel for space heating and cooking. You can, using fuel cells, convert hydrogen back to electricity. So, it’s helpful for the electrical utility [industry too]. When you have that infrastructure in place, it benefits all these different sectors.

Trillions: What’s next for the project? What are your plans and what do you hope to do?

Daniel Esposito: I think [there are] a lot of really neat things to work on here, both at the level of the material that goes into the electrolyzers and then the engineering that involves the design and optimization of the devices and the platforms themselves. One particularly important area moving forward is to optimize and develop better catalysts that are coated on the outer surfaces of those mesh electrodes, particularly for seawater electrolysis. A lot of the experiments that we’ve done up to this point have actually been done in acidic electrolyte, whereas, in contrast, seawater is relatively pH-neutral and contains a lot of chloride ions. Chloride ions are a concern for water electrolysis because (a) they can be fairly corrosive, and (b) chloride ions can recombine to form chlorine gas at the anode of electrolysis cells. So that represents a reaction, at the anode, that’s competing with the oxygen evolution reaction. Moving forward, we would need to put more efforts into developing catalytic materials that will selectively promote oxygen evolution as opposed to chlorine evolution. Also, just in general, [it is important to develop] catalysts for both electrodes that are robust and stable for operation in seawater for long periods of time.