Skip to main content

Making H2 while the sun shines

sunrise

While the process of splitting water into hydrogen and oxygen by electrolysis has been well known for years, recent breakthroughs have placed the technology firmly at the cutting edge of scientific research, due to its huge potential for large-scale and renewable hydrogen production.

To discover more about this exciting field, cH2ange spoke to Prof. Kevin Sivula, Head of the Laboratory for Molecular Engineering of Optoelectronic Nanomaterials at the École Polytechnique Fédérale De Lausanne (EPFL).

Kevin Sivula, electrolysis expert and Head of the Laboratory for Molecular Engineering of Optoelectronic Nanomaterials at the EPFL

What projects are you currently involved in?

We work on the development of semiconductor technology, ranging from organic semiconductors to nano-particulate semiconductors, which have many applications including flexible electronics, transistors and solar cells. A large focus of our group is on finding ways to convert solar energy into fuels using inexpensive and stable semiconductor materials.

For example, we are working with Françoise Barbier from Air Liquide on the development of economically viable solar hydrogen production. For this we use photoelectrochemical devices that both absorb sunlight and then directly convert it into chemical energy via an electrochemical reaction. In this case, water is reduced by the photogenerated electrons, producing hydrogen molecules.

Right now, our society uses hydrogen produced from natural gas. In a hundred or so years, when we run out of natural gas, we’ll still need a way to make fertilizer to grow crops.

 

Photoelectrochemical water splitting was first demonstrated in the 1970s, albeit at a low performance level. Today, we are working on a totally new approach, based on new materials that our lab has recently discovered, namely oxide semiconductors. The project involves a particular family of oxides called delafossites — which have a specific chemical formula based on Copper (I) and a 3 valence transition metal, or other metal such as iron, chromium, gallium or aluminum. These oxides form specific crystal structures that have very good optoelectronic properties to suit the applications of this device.

Many oxides have the advantage of being very stable, since they cannot be oxidized further under ambient conditions. We’ve also shown our materials to be stable under device operation conditions.

This is how it works:

Electrolysis is a simple way to produce hydrogen. When coupled to a renewable energy source, it is also eco-friendly.

What are the main applications?

Energy storage is a main application of hydrogen production from electrolysis. There are big advantages in terms of scalability and timescale. For example, let’s think about alternative storage methods such as lithium ion batteries. The timescale over which energy can be stored in these electrochemical devices is relatively short. You’ve probably picked up a camera or phone that hasn’t been used for a while, and turned it on to find the batteries are dead.

That’s because these devices naturally discharge relatively quickly. Hydrogen, however, can sit in a tank for months, years and even decades.

Moreover, hydrogen produced from electrolysis can also serve as a renewable feedstock for the chemical industry, at a scale that is commensurate with the world population. This is a really important point because, eventually, in an energy economy that is completely sustainable without fossil fuels, we will still need a way to have chemical energy for the production of fertilizers, pharmaceuticals, plastics and many other materials.

Right now, our society uses hydrogen produced from natural gas. In a hundred or so years, when we run out of natural gas, we’ll still need a way to make fertilizer to grow crops.

How efficient can electrolysis be in future?

Firstly, efficiency is not really the important issue here. The most important factor is the price of the hydrogen produced. You can make a very complicated and very efficient system, but nobody will use it if it’s prohibitively expensive.

Nevertheless, to consider efficiency, let’s first look at dark electrolysis. If you want to store electricity in the form of bonds through water splitting, then today’s commercially available electrolyzers have LHV [The most commonly used metric for the comparison of efficiencies is the lower heating value (LHV) of hydrogen] efficiencies of about 65% to 70%.

With hydrogen I can store as much energy as I want, it only depends on the size of the tank. And it’s easy to make a larger one to store more hydrogen gas. In a battery, this storage potential is very limited by the amounts of lithium and raw materials present in the battery.

Regarding the direct conversion of solar energy into hydrogen, two processes are involved — the absorption of light and the conversion of this light into chemical energy — both of which have associated efficiencies. So, if we consider the overall solar energy to hydrogen production, right now we can convert about 16% to 20% of the sun’s light into hydrogen.

Potentially I think this could be increased to as much as 40% one day, if the right combination of materials is found. However, it may never be practical to reach this level of efficiency. Economically speaking a device that converts only 10% of incident solar energy in Hydrogen could be sufficient, if the device were cheap enough.

How does hydrogen technology compare with batteries?

Some people ask why we can’t just use batteries for electricity storage. And there are two strong arguments against this. The most compelling is that we won’t only use hydrogen for the chemical storage of energy, to be converted back to electricity, like we use batteries. Instead we can take advantage of the chemistry for industry. Hydrogen produced from natural gas is already used on a massive industrial scale for extremely important chemical transformations.

The second argument concerns scalability. With hydrogen I can store as much energy as I want, it only depends on the size of the tank. And it’s easy to make a larger one to store more hydrogen gas. In a battery, this storage potential is very limited by the amounts of lithium and raw materials present in the battery.

If the world’s governments decide to support renewably produced hydrogen, by implementing a carbon tax on natural gas or giving subsidies to hydrogen produced from renewable energy, then I’m convinced hydrogen would play an immediate and important role, because we have access to the technology right now.

This is the price we pay for the stability of the chemical hydrogen bond. We have to get over an activation energy to first make a bond, and then break it to get the energy back. At each of these steps we lose a little bit of the energy input. This means the “round trip efficiency”, from the initial power to the final power, is about 50%. However, this price is worth paying if we want to store energy for a long time scale — which will be essential in future. This is one of the main benefits of renewable hydrogen, along with its sustainability and the fact it is carbon neutral and can be stored at a large scale.

And let’s be clear, there is always a round trip efficiency loss with any technology. Even batteries don’t give back 100% of the energy that you put in. It is thermodynamically impossible because we always have to increase the entropy of the system.

Will electrolysis be useful for addressing intermittency in renewable power generation?

This depends on the timescale of the intermittency. We’ve already mentioned that hydrogen storage is ideal for the long term. In order to cope with the supply and demand flux over the course of a year, from summer to winter for example, hydrogen can be extremely important.

When it comes to shorter-term intermittency, I think the round trip efficiency of the power to gas to power cycle, being only about 50% at best, will limit hydrogen as a solution for daily or hourly intermittency. Other technologies such as batteries or mechanical energy storage like pumping water uphill can tackle this short-term intermittency.

Roughly 90% of hydrogen produced today comes from natural gas reforming. How can we shift towards renewably generated hydrogen from electrolysis?

There are two factors. The first is economic: industry and consumers want the cheapest option when it comes to fuel. So there has to be an economic advantage for producing hydrogen by electrolysis. This might occur because the cost of natural gas increases, either due to political issues or because eventually we are going to run out. On the other hand, if we can lower the cost of the electrolysis, then we can bring down the cost of hydrogen produced in this way, making it more economically competitive.

The other factor is political. If the world’s governments decide to support renewably produced hydrogen, by implementing a carbon tax on natural gas or giving subsidies to hydrogen produced from renewable energy, then I’m convinced hydrogen would play an immediate and important role, because we have access to the technology right now.

The good thing is that we will find a solution. There’s no other option except the complete destruction of humanity! The question is, how long will it take?

The tipping point depends on the competitor, in this case natural gas hydrogen production. If there were to be a political crisis next week involving one of the major players in the natural gas industry, then overnight it could become economical to produce hydrogen by electrolysis. Then the technology’s implementation would accelerate.

Without political intervention, with only economic forces to influence the adoption of the technology, then we’re in for the long road. In this case I think fifty years is a conservative estimate before we see the widespread adoption of this type of energy storage and transformation.

The good thing is that we will find a solution. There’s no other option except the complete destruction of humanity! The question is, how long will it take?

How can we make electrolysis more economically viable?

There is a lot of ongoing research dedicated to developing better catalysts for electrolyzers. There is a kinetic barrier that has to be overcome to make the chemical hydrogen and oxygen bonds. Catalysts can lower this kinetic barrier, making the reaction easier, meaning we can put in less overpotential and thus make these chemical bonds with less energy input.

We really want to do this with inexpensive and scalable materials. We need to find new materials, that are less expensive and just as effective and stable as platinum, which is used today. This is a major challenge for all researchers in the field and the future looks promising.