A strand is nearly 10 000 times thinner than your hair, conducts electricity better than copper and is 1 000 times stronger than steel, but lighter than a single strand of a spider’s web. This is why researchers all over the world are looking for new applications for carbon nanofibres.
On a microscopic scale, a carbon nanofibre is like a tube made up of a rolled honey comb: carbon atoms organised as a network of hexagonal rings. At the University of Witwatersrand we are making these nanotubes out of fly ash, a toxic by-product of coal-fired power stations, and using them to generate electricity.
Carbon nanofibres can also be used in clothing to turn friction on clothes into electricity to power mobile devices, as additives to polymers to increase their mechanical strength (such as in improving bullet proof suits), and in gas storage to store hydrogen for energy. In batteries, these little fibres can store four times more energy than normal lithium battery electrodes.
For some context on the problem of fly ash, the World Energy Council 2013 report estimated that the average coal consumption was seven billion tonnes a year and was projected to increase to 10 billion tonnes a year by 2020. At this rate of consumption, close to a billion tonnes of fly ash will be produced every year. This is the same weight as about 10 million blue whales stacked on top of each other. Finding ways to safely recycle this waste material is critical. Fly ash contains toxic elements such as lead, mercury, chromium and arsenic that are easily air-borne if left exposed to the elements.
They damage animal and human brains, cause cancer and asthma and adversely affect the functioning of the heart. During rainy seasons these toxic elements drain into underground water systems and could potentially poison people and animals who depend on these affected water systems. Our group is turning this fly ash into carbon nanofibres.
There are three commonly used ways of making these remarkable nanofibres: laser ablation, arc discharge and chemical vapour deposition. During laser ablation, a pellet made up of graphite (the same substance that is found in your pencil) infused with metal nanoparticles is heated up using powerful lasers.
These lasers break down the graphite into hot gaseous carbon atoms (which reach more than 1200?C – nearly a quarter of the temperature on the surface of the sun), and these atoms are blown into a cooler region of the reactor using a nonreactive gas like argon. When the hot gas cools, the atoms naturally arrange themselves into carbon nanofibres.
In arc discharge, high electrical currents are applied to two graphite rods, which are made to touch to form hot gaseous carbon in the arcing chamber. The inner surface of the chamber is cooled using water and as soon as the hot gaseous carbon touches the cool surface of the chamber it also arranges the carbon atoms into carbon nanofibres. But neither of these methods is easily scaled up because they use a lot of energy and require extremely specialised equipment.
Our group has investigated chemical vapour deposition as a way to make nanofibres. This process uses metal nanoparticles to break down a gas comprising carbon and hydrogen atoms. The hydrogen escapes the reaction chamber as hydrogen gas and the carbon sticks to the surface of the metal particle.
As the process continues, more carbon atoms collect on the metal particle and form nanofibres which are anchored on the metal particles. The metal particles used in this process are called catalysts, a substance that speeds up a chemical reaction without chemically changing itself. Commonly used catalysts are iron, nickel and cobalt.
Fly ash is made up of spherical particles, like the surface of a basketball decorated with small spots of iron metal particles.
Because these metal particles are present in the fly ash, carbon nanofibres can grow on the surface of the fly ash particles through chemical vapour deposition.
Rings of carbon nanofibre appear to grow out of the surface of the iron particles like very long tentacles. Half a teaspoon of fly ash can yield up to eight grams (about one and a half teaspoons) of carbon nanofibres. Practically, the structure continues to grow as long as the temperature is suitable and there is enough carbon.
Of all the research groups worldwide that have made carbon nanofibres from fly ash, our team at Wits is the first to use them to generate electricity. We do this through photocatalysis, which is a chemical process that starts with sunlight. We bind the metal oxide particles (a compound made from a metal and one or more oxygen atoms) on the surface of the carbon nanofibres.
We use titanium dioxide as the metal oxide, and use it to break water its constituent atoms, namely oxygen and hydrogen, releasing energy and electrons in the process. The hydrogen can be used to generate electricity or as an alternative energy source.
This research is being developed for state power utility Eskom to possibly introduce better fuel and alternative energy resources into the grid with minimal expense to the company.
The doors opened by making carbon nanofibres from fly ash are endless, and our photocatalysis is only one of their many applications. This is only the beginning of a possibly brighter future.
Arthur Moya attends Wits University.