SciBar: Splitting Water

Designing Alternative Electrode Materials

Words by Nicola Simcock, edited by Calum Kirk.

For many, the face of Greta Thunberg now symbolises society’s battle with climate change, and for Januarys SciBar Audience, the face of PhD student Steve Ward nowsymbolises an exciting branch of materials chemistry being used to address that verysame topic! Steve joined us from the School of Engineering, Newcastle University to talk about his work creating novel materials for renewable energy applications.

To kick off the new decade Steve opened with a familiar message:

CLIMATE CHANGE IS REAL

Global temperatures have been steadily increasing since the industrial revolution and show few signs of slowing. Steve showed a graph (below) displaying a dramatic increase in temperature since the 1980’s independently recorded by five separate organisations (Picture credit: NASA), demonstrating a level of data reproducibility that many scientists can only dream of (trust me…I’m a scientist!).

This temperature increase is linked to burning fossil fuels and the subsequent enhanced ‘greenhouse effect’

The greenhouse effect

The greenhouse effect is a natural process that ensures life on earth. Energy from the sun travels through space and hits the earth. Some of this energy is reflected back into space and the rest is absorbed by land and sea (heating it up) or becoming trapped and ‘reradiated’ in our atmosphere by greenhouse gases such as water vapour (H2O), methane (CH4), carbon dioxide (CO2) and ozone (O3), amongst others. For life as we know it, this heating is necessary; however, it is enhanced – with wide ranging consequences – by an increase in greenhouse gases. Burning fossil fuels such as coal and oil, releases the components trapped within them as greenhouse gases, such as CO2.  An increase in greenhouse gas quantity increases ‘trapped’ energy, and consequently the global temperature.

As the global temperature rises towards ‘extreme’, so do the impacts (Just ask Greta! Or more specifically, the evidence driven research she wants everyone to know about). To slow this temperature rise we need to slow, or stop, the release of greenhouses gases. One major way to do this is to stop burning fossil fuels, but due to our dependence on these resources, we need a suitable energy alternative.

Such an alternative comes with some stipulations:

  • Be readily available
  • High energy constant (comparable to fossil fuels)
  • Have manageable environmental effects (e.g. ‘green energy’)

There are a variety of alternative fuels on the market, but for Steve, Hydrogen is Number 1.0 (that’s right, a Periodic table pun…).

Firstly, Hydrogen is the most abundant element in the known universe, making it readily available󠅇. Secondly, Hydrogen has a higher specific energy value than any fossil fuel, meaning you get more bang for your buck! (we’ll talk more about Hydrogen going ‘BANG’ later). Finally, Hydrogen burns with a clean flame –the only waste product is water – making it green. Note: while water vapour is a greenhouse gas, it can be considered less problematic than others as it remains in the atmosphere for a period of days rather than years – or in some cases centuries!

2H2 (g) + O2 (g) ­–> 2H2O (g) + energy

Hydrogen sounds pretty great right? While this apparent ‘wonder fuel’ ticks most of the necessary boxes it does come with one major drawback: safety. As you can see from the equation above, burning Hydrogen is most efficient when it’s gaseous and being highly flammable, this adds danger. There are a number of historical events that demonstrate this, such as The Hindenburg disaster.

The Hindenburg Disaster

In 1987 a German passenger airship, LZ 129Hindenburg, exploded in the New Jersey sky as it tried to dock with its mooring mast. Being lighter than air, Hydrogen was chosen as the gas to keep the airship in the sky. After a fire broke out on board it is thought that this escalated to an explosion when the highly flammable Hydrogen ignited. Sadly, this disaster resulted in 36 fatalities and understandably, stunted the airship industry. 

Due to its flammability, storing large quantities of Hydrogen gas can carry a significant risk. Whereas storing hydrogen as a liquid, particularly as water (~66% hydrogen), is much safer.

From liquid water, we just put the previous equation in reverse and extract the hydrogen:

2H2O (l) + energy –> 2H2 (g) + O2 (g) ­

However, the keen-eyed will notice one glaring problem. Splitting water apart (termed electrolysis) has been around since the 19th century, but it is a non-spontaneous’ reaction, meaning it requires energy. To carry out electrolysis we must use an electrochemical cell.

Electrochemical cell

An electrochemical cell for water electrolysis contains a DC power source connected to two electrodes. One is positively charged (the cathode) and the other is negatively charged (the anode). Both electrodes are submerged in an electrolyte – a substance that conducts electricity when dissolved in water. When an electric charge is applied, the ions in the electrolyte move to the electrode with the opposite charge. Charge is then transferred at the electrodes resulting in a new chemical product. Hydrogen gas is produced at the cathode (the chemical reaction is known as reduction, or the gain of an electron), and oxygen is produced on the anode (oxidation, or the loss of an electron).

Obtaining both the Hydrogen and the Oxygen from water consists of two separate reactions within the electrochemical cell. Gaining the Hydrogen is relatively simple; however getting the O2 is very slow. In order for the electrochemical cell to work successfully, both reactions must take place. As the production of Oxygen takes the longest, it represents the ‘Rate Limiting Reaction’ – a single step in the overall process that slows everything down. There is such focus on gaining O2 that this part of the process is given its own title: the Oxygen Evolution Reaction (OER) and it is the OER that needs improving to make Hydrogen production from water feasible.

To speed up the OER a catalyst can be used.

What is a catalyst?

A catalyst is any substance that increases the rate of the reaction without undergoing any permanent chemical change itself. Meaning, whatever is added as a catalyst at the start of the reaction, will be returned unchanged at the end. For example, modern cars use a ‘catalytic converter’ in the exhaust. The catalytic converter uses Platinum as its catalyst to convert toxic carbon monoxide (CO) into relatively safe carbon dioxide (CO2). At the end of the chemical reaction, the platinum is unchanged and can be re-used.

Steve’s research at Newcastle focusses on designing materials to increase the rate of the Oxygen Evolution Reaction. OER catalysts are used as anode materials within the electrochemical cell. Lucky for Steve, some materials already work well such as Nobel metal oxides like Rubidium Oxide (RuO2) and Iridium oxide (IrO2) and currently provide the ‘gold-standard’ in terms of performance. Less lucky for Steve, is the fact that these materials are very rare and expensive.

To design a good oxygen evolution catalyst, three important things must be considered:

  • Cost
  • Chemical suitability
  • Availability

Transition metals such as Iron (Fe), Nickle (Ni) and Cobalt (Co) have shown promise as OER catalysts. However, like much scientific endeavor, this is not as simple as it appears. The transition metals must be housed in a crystal structure known as perovskite oxides or perovskite crystals.

We can think of a perovskite oxide as a 6-sided dice:

On the corners there are large atoms that house the general structure e.g. Calcium. In the middle of the ‘dice’ is the transition metal e.g. Iron (Fe). On the faces of the ‘dice’ are oxygen atoms each connected to the central transition metal.

The Oxygen atoms are a problem (pesky Oxygen strikes again!) as they block access to the surface of the transition metal. If the Oxygen atoms were removed this would improve access and thus improve the rate of the Oxygen Evolution Reaction.

So why can’t we just take those onerous Oxygens away and free up the transition metal atom in the middle? Stability. Turns out those Oxygen atoms are quite important for holding everything together. However, removal of Oxygen is possible with various techniques, which research like Steve’s will work to improve.

The performance of perovskite oxides as OER anode materials isn’t great compared to noble metals. Noble metal catalysis can produce more hydrogen gas using lower energy requirements – it is more expensive to buy the rare Nobel metals, but with higher performance, they are cheaper to run overall. Also, the perovskite transition metals appear to have some ‘operation stability’ issues, so for many applications they may degrade over time.

However, it wasn’t all doom and gloom as Steve reminded us why this research is important. Transition metals are cheaper, more abundant, and have greater ionic conductivity than Nobel metals, so getting these materials to work efficiently will come with a huge money-saving bonus. Perovskites are highly tuneable, meaning the perovskite crystal structure (6-sided dice) can be made with lots of elemental combinations, which can be manipulated to improve catalyst efficiency. Steve and his colleagues in the field just have to figure out which are the most useful combinations!

Thankfully, there are hundreds of research groups all working towards this goal, with recent technological developments showing promise. As Steve so clearly stated, climate change is a real issue, becoming increasingly important with time. Replacing fossil fuel reliance with alternatives such as water-derived Hydrogen is one such method in a range of possibilities. Additionally, Steve added that changes in government policy are vital and we can all work towards achieving those.

During the break the audience mulled over this new energy driven knowledge and a few interesting questions were raised. Particularly ethical considerations; whether we should be using a precious resource like water to pursue clean energy when there are places in the world that lack it? As Steve reasoned, every ethical aspect of such research must be considered. Water shortages are an important issue and one that may become increasingly prevalent with continued climate change. Is short-term sacrifice for long-term gain a suitable compromise? Not an easy question, but one that certainly needs thinking about!  

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