Scientific Discovery

Candle soot can power the batteries of electric cars, researchers find

The carbon in candle soot could be a better option for lithium batteries

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Left image: candle soot being deposited on stainless steel. Right image: high resolution microscopy image of carbon nanoparticles from candle soot; the inset shows the candle soot deposited on a substrate. (The images were featured in the original research article by Kakunuri and Sharma in <em>Electrochimica Acta,</em> October 2015).

For centuries, they have been used to light the way, tell the time and banish bad smells. Now researchers say candles could be the key to cheaper, more efficient electric car batteries. But how would this work, and why has no one thought of it before?

Chandra Shekhar Sharma, PhD“Generally we overlook the simpler things; candle soot is not new, but we’re only now looking at it as a potential source of carbon,” said Dr. Chandra Shekhar Sharma, Assistant Professor in the Department of Chemical Engineering at the Indian Institute of Technology in Hyderabad, India. In fact, as a carbon source, candle soot may be more effective than what’s currently available for making powerful batteries.

Dr. Sharma and his colleague Dr. Manohar Kakunuri came across a study published a few years ago in Science that showed candle soot has superhydrophobic surface properties. “If you put a water droplet on candle soot, it rolls off. However, from the material’s perspective, candle soot carbon has electric potential. So why not use it as an electrode?” asked Dr. Sharma. “We looked into it and saw it also shows some exceptional electrochemical properties, so we decided to test it further.”

Manohar Kakunuri, PhDIn a new study published in Electrochimica Acta, Dr. Sharma and Dr Kakunuri found that because of the shape and configuration of the tiny carbon nanoparticles, the carbon in candle soot is suitable for use in powerful lithium ion batteries. What’s more, because candle soot can be produced quickly and easily, it is a scalable approach to making batteries.

The magic of candle soot

When a candle burns, it gives off clouds of black soot made of carbon. Masking bad odors isn’t the only thing this soot can do, according to Dr. Sharma. He wanted to find out what was so special about the structure of the particles in candle soot and why they had electrical properties, so the team analyzed soot using x-ray crystallography and microscopy.

The researchers looked at the soot collected from the tip of a candle flame and from the middle of the flame and compared the size, shape and structure of the carbon. The results showed that the burning process forms nanoparticles of carbon that are 30-40 nanometers across and are joined together in an interconnected network. They also found that the soot recovered from the tip of a candle flame, which burns at 1400˚C, has fewer impurities, such as wax, making it perform better as an electrical conductor.

“The structure of the carbon nanoparticles in the candle soot were joined together in a really particular pattern, like a fractal,” said Dr. Sharma. “We think this explains why soot behaves in this way – why it repels water and has an electrical charge unlike other sources of carbon.”

The next step was to find out whether the soot is suitable to use in lithium ion batteries.

Using carbon to charge batteries

Lithium ion batteries power many devices, from smartphones and digital cameras all the way up to cars and even aircraft. The batteries work by having two electrically charged materials – electrodes – suspended in a liquid to produce a current. Carbon is used as one of those materials in smaller batteries, like the one in your smartphone. But for bigger, more powerful batteries, such as those used in electric cars, carbon is not usually suitable. In graphite form, carbon has a stable crystalline structure that is not sufficient to produce the required current density.

In small lithium batteries, the positive electrode (called the anode) is made of a material containing the reactive metal lithium, or oxides of other metals like magnesium or nickel, and the negative electrode (called the cathode) is made form a carbon material. Traditional lithium batteries use graphite as the carbon source, similar to what pencils are made of. However, graphite does not work well in more powerful batteries, like those used in plug-in hybrid electric cars, for which higher current rates are required.

The researchers then analyzed the effectiveness of the soot as a conducting material to use in a battery. The effectiveness of batteries and materials used in batteries can be tested through a technique called cyclic charge-discharge (CCD). The rate of charge/discharge reflects how powerful the battery is; the higher the rate, the more powerful the battery can be used for electric vehicle to speed up the car; the results showed that the candle soot carbon performed best at higher rates.

The shape and size of the carbon nanoparticles, and the way they are joined together, means candle soot is a suitable material to use in electric car batteries. Not only is the technology efficient and cost-effective, it’s also scalable. Dr. Sharma estimates that one hybrid car would need 10 kilograms of carbon soot, which would be deposited in about an hour using candles.

“We’re very excited about the results. This new approach is very easy, and the costs involved are minimal – it would make battery production cheaper,” said Dr. Sharma. “Carbon materials are generally not used in electric vehicle batteries. We strongly believe this material can have a positive impact on the currently available technological solutions for lithium batteries. This is the first report, and people may now look into it much more carefully. Our research opens up new possibilities for the use of candle soot as an electrode material.”

The researchers now plan to develop a candle soot battery to test the technology further. They are also planning to test hybrid materials that contain candle soot to see if they can make it an even better material for batteries.

The weird world of batteries

This is not the first time a strange new material has been used to generate power. Cola, for example, works well as the electrolyte in a homemade battery – all you need is a strip of copper, from a pipe, for example, and a piece of aluminum – the soda can works well for this. Similarly, you may have made a potato battery or a lemon battery to power a clock at school: sticking metallic electrodes into the potato or lemon, which act as the electrolyte, generates a current.

However, according to, no battery breakthroughs have been made in the last five years. “This is not surprising when considering that few other products have requirements as stringent as the battery. A battery must have high energy storage capability, provide a long service life, be safe to use, and require little maintenance. In addition, the battery must work at hot and cold temperatures, deliver high power on demand, charge quickly, and cost little.”

Some battery innovations have produced useful characteristics, and working batteries:

  • Nickel-zinc batteries are recyclable and contain no toxic materials. They don’t work well if they’re trickle-charged, though.
  • Zinc-air batteries work well for small devices like hearing aids but are not very powerful.
  • Sodium-sulfur batteries, used in missiles, work at extremely high temperatures (400˚-700˚C) and can be stored for up to 50 years in a cooler atmosphere.

Read the study

Elsevier has made the following article free access until 16 January 2016:

Electrochimica Acta is the official journal of the International Society of Electrochemistry (ISE). The journal, published by Elsevier, covers disciplines within electrochemistry, including analytical, molecular and physical electrochemistry, bioelectrochemistry and electrochemical energy conversion and storage. The ISE was founded in 1949 by leading European and American electrochemists to serve the growing needs of electrochemistry. Since then the ISE has evolved to comprise more than 2,000 individual members from more than 60 countries.

The authors

Dr. Chandra Shekhar Sharma is an assistant professor in the department of chemical engineering at the Indian Institute of Technology, Hyderabad, India. His research interests are carbon based hierarchical materials, bioinspired polymer functional surfaces, electrospun nanofibers and carbon-MEMS. He has received many awards, including the DST Inspire Faculty Award (2015), INAE Innovative Project Award (2011) and Gandhian Young Technological Innovation Award (2014 & 2015). Dr. Sharma has 22 peer-reviewed publications in reputed international journals, including Carbon, Langmuir, ACS Applied Materials and Interfaces, Small and Electrochimica Acta. He has four patent applications filed and two book chapters to his credit.

Dr. Manohar Kakunuri is a visiting scholar at University of Exeter, UK. He obtained his doctoral degree under the supervision of Dr. Chandra Shekhar Sharma in 2015 at the Indian Institute of Technology, Hyderabad, India.

Elsevier Connect Contributor

Lucy Goodchild-van
  HiltenAfter a few accidents, Lucy Goodchild van Hilten discovered that she’s a much better writer than a scientist. Following an MSc in the History of   Science, Medicine and Technology at Imperial College London, she became Assistant Editor of Microbiology Today. A stint in the press office at Imperial saw her stories on the front pages, and she moved to Amsterdam to work at Elsevier as Senior Marketing Communications Manager for Life Sciences. She’s now a freelance writer at Tell Lucy. Tweet her @LucyGoodchild.

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