27 September 2023

It gets a bad rap but carbon helps make solar panels safer

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Dr Jacob Martin* explains how carbon materials can improve solar power, green hydrogen and battery technology

Carbon has a (justifiably) bad reputation for its role in climate change. You’ve heard we need to reduce our carbon emissions, our carbon footprint, our carbon miles.

That’s certainly the case, but this is typically carbon that is emitted into the atmosphere as carbon dioxide, methane and soot.

Other carbon materials also need a bit of respect.

They will be critical to our transition from burning fossil fuels.

Lithium-ion batteries, hydrogen fuel cells and solar panels will all help us reduce our reliance on these old sources of energy — and they all use carbon materials.

Let’s explore a few of the carbon materials that will improve green energy technology.

Without carbon, lithium-ion batteries tend to catch fire

Lithium-ion batteries are a cornerstone technology for decarbonisation.

They will power our electric cars and help buffer renewable energy fluctuations in the grid.

But lithium metal reacts violently with air and water, leading to early lithium-cobalt batteries catching fire.

Japanese chemist Akira Yoshino solved this problem in the 1980s by adding carbon to the lithium-cobalt battery.

Specifically, Professor Yoshino added graphite — a form of crystalline carbon.

Graphite soaks up lithium metal, forming a stable compound where, for every six carbon atoms, one lithium atom is stored in ordered arrays between graphite’s layers.

The “lithiated graphite” also changes colour from a dull black to a beautiful gold.

If the battery is exposed to air, the oxygen and water cannot as easily get to the lithium, making it safe enough to store in your pocket.

Professor Yoshino shared the Nobel prize in Chemistry in 2019 for developing lithium-ion batteries as we know them today.

It is only really in the past decade that these batteries have started to decarbonise our lives through electric cars and in our electricity grids.

And while lithium-ion batteries appear set to take over the world, there are still some issues that could lead to manufacturing bottlenecks.

Most of the graphite used in lithium-ion batteries is mined and is not pure enough to be used directly in batteries.

It also requires washing in acid to remove contaminating metals, leading to an environmentally damaging waste stream.

To create an alternative, researchers in the Carbon Group at Curtin University have been working on producing graphite for batteries from, for instance, construction waste.

“We are able to convert polyvinyl chloride, a common waste material used in plastic piping, into high-quality graphite,” said Jason Fogg, a PhD student who studies carbon materials science.

But this waste-derived graphite is still more energy-expensive compared with mined graphite.

“The high temperatures required are the current challenge,” Mr Fogg said.

“You must heat the plastic pipe to almost 3,000 degrees Celsius, which is half the surface temperature of the Sun, before it converts into graphite.”

Irene Suarez-Martinez, who co-leads the Carbon Group, has been turning to supercomputers to find ways to lower this temperature and, therefore, make waste-derived graphite cheaper.

How does carbon help green hydrogen?

Countries are investing billions in “green hydrogen” — hydrogen gas produced with renewable energy — which can be used as an energy store.

But there are vanishingly small amounts of naturally occurring hydrogen gas here on Earth.

This is because hydrogen likes to bond with oxygen to form water.

To make green hydrogen, electrical energy from renewables splits the water molecule into hydrogen and oxygen in a device called an electrolyser.

A fuel cell device can then later recombine the hydrogen gas and oxygen, providing electrical energy on demand — in, say, a hydrogen fuel-cell electric car.

One of the main challenges holding hydrogen back as an energy source is the cost of the platinum metal needed in fuel cells and electrolysers to let the reactions occur.

But due to the scarcity of platinum in the earth’s crust, it is horrendously expensive, and makes up around 77 per cent of the cost of a fuel cell, as estimated by the National Renewable Energy Laboratory in the US.

Yuan Chen from the University of Sydney is an expert in using carbon materials to reduce the cost of hydrogen fuel cells.

“We are replacing the platinum with single-atom catalysts, where the atom is iron, nickel and cobalt, embedded into carbon,” he explained.

Further work is underway to improve the stability of these platinum replacements, but they have so far reached the milestone of performing at a similar efficiency as the more expensive platinum catalyst.

Carbon can improve solar efficiency too

Timothy Schmidt is leading a team at the University of New South Wales to improve the efficiency of solar cells using carbon-based coatings.

“The best silicon solar cells max out at 26 per cent efficiency and engineers can scramble to make them more efficient, but they are running up against a ceiling,” he said.

The reason silicon solar cells are not more efficient is that they can only transform specific colours of light into electricity.

Redder light is converted most efficiently, but as the light gets bluer, it produces more heat.

Professor Schmidt’s team is working on a coating to put atop a silicon solar cell to convert blue light into redder light that silicon can efficiently absorb.

This has the sci-fi name of a “singlet-fission solar cell”.

If the carbon-based coating can increase the amount of energy produced by the solar cell, it could improve panel efficiency by up to 35 per cent in the next five to 10 years.

While a hike from 26 to 35 per cent doesn’t sound like a lot, it makes a huge difference when it comes to solar payback time.

For example, increasing the efficiency of a solar cell from 12 per cent to 14 per cent halved the payback time for a solar system from four to two years, according to researchers from Utrecht University in the Netherlands (although solar efficiency isn’t the only factor that affects payback time).

These are just a handful of many projects in Australia and abroad using carbon materials to build and refine green energy technology.

And despite its bad reputation, carbon can help us reach our climate goals.

*Dr Jacob Martin is a materials and combustion scientist at Curtin University.

This article first appeared at abc.net.au

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