Australian researchers have brought the long-awaited dream of an economy running on hydrogen fuel a step closer by developing a device that generates zero-emission fuels from the sun's rays.
The solar-powered fuel cell, which converts chemical energy into electricity, produces hydrogen fuel at 22 per cent energy efficiency, breaking the previous world record of 18 per cent.
"This will provide a cheap, clean and renewable source of hydrogen as the ultimately sustainable fuel," says co-researcher Professor Douglas MacFarlane, of Monash University.
"This breakthrough takes us a step closer towards the hydrogen economy, where vehicles run primarily on hydrogen fuel which produces no greenhouse emissions or other pollutants."
Hydrogen can be used to generate electricity directly in fuel cells. Cars driven by fuel-cell electric engines are already available from a number of car manufacturers.
"As a result of our research, hydrogen could now be used as an inexpensive energy-storage technology in homes to store energy from rooftop solar panels," Professor MacFarlane says.
Nature's example
The scientists achieved artificially what a leaf does in a plant: namely turning water, carbon dioxide and sunlight into compounds that can be stored as energy.
The resemblance to a leaf ends there. "Physically, artificial leaf designs are much bigger; they are closer to black in colour because that helps them absorb a broader fraction of the solar spectrum," Professor MacFarlane says.
The simplest and cheapest practical device resembles a rooftop solar hot-water heater, including a solar panel and a tank where the hydrogen gas is stored.
In a nutshell, the process involves passing an electric current from the solar cell through the water to split it into hydrogen and oxygen.
In itself, electrolysis is nothing new. "Water electrolysis has been known for hundreds of years," Professor MacFarlane says. "What we did was to carefully tune the process to work effectively with high-efficiency commercial solar cells."
The device releases oxygen as a byproduct of the process. "So, that part is the same as a plant's photosynthesis – right down to the catalyst that sets off the oxygen-producing reaction," he says.
Professor MacFarlane says the new fuel cell could form part of a solar farm complex. "This is particularly useful if there is a natural gas pipeline nearby as hydrogen can be mixed with natural gas – up to about 10 per cent – without significantly altering the properties of the natural gas."
Hydrogen
Hydrogen is the universe's lightest and most abundant chemical element, comprising one proton circled by one electron. (As a compound, it prefers to exist as two atoms bonded together.) To some extent, the planet's future depends on it.
The earth's surface has warmed by almost 0.6 degrees over the last century. We are now on course for a rise in temperature of more than two degrees above pre-industrial levels.
Global warming could be significantly reduced by replacing greenhouse emissions from vehicles with water vapour.
Cars running on hydrogen fuel cells are able to do this: the fuel of hydrogen and oxygen is converted into electricity, heat and water.
"It is probably appropriate to think of hydrogen as an important component of the future economy rather than being the hydrogen economy, as such," Professor MacFarlane says. Various energy and energy-storage technologies will offer different advantages, he says.
"In regard to transport, the ability to store relatively large quantities of it for rapid refuelling of a vehicle is a distinct feature for longer-range transport, especially heavy vehicles such as trucks and buses," he notes. At the moment, this seems a more attractive option than the fully "electric truck", which would require an enormous battery and take a long time to recharge.
"The fact that it can be easily generated from water and a solar-cell source, as demonstrated in our research, is a major advantage and the generator cell [the water electrolysis device] can cope very well with variations in power that arise from the intermittent nature of solar and wind power," Professor MacFarlane says.
"Hydrogen production, storage and conversion is challenged by substantial losses, especially if it needs to be compressed or liquefied," says Sydney University engineer Anthony Vassallo. "But there will be cases where it can make sense (energetically and economically).
For his money, Professor Vassallo says he would go for energy storage in batteries, "where the conversion losses are much lower".
Fuel cells explained
In essence, a battery stores chemical energy and then converts this into a flow of electrons, namely electricity.
At either end of the battery is an electrical conductor in the form of a positive and a negative electrode.
The two electrodes – separated by an electrolyte that conducts ions (atoms that have become electrically charged) but not electrons – are usually made of different metals, one of which gives up electrons while the other accepts them.
The battery discharges when it is connected to an external electrical circuit, with chemical reactions occurring at the surface of each electrode.
Electrons, which always carry a negative charge, are liberated by these reactions and gather at the surface of the negative electrode.
Because they cannot travel through the electrolyte, the electrons are unable to take the shortest route to the positive electrode. Instead, they are forced to flow from the negative electrode through a circuit of wires leading outside the battery.
This movement of electrons towards the positive electrode, through the external circuit, is the electrical current. This is what illuminates a light bulb, for example, connected to the circuit.
Like batteries, fuel cells also have negative and positive electrodes that are separated by an electrolyte in the form of a membrane. But, unlike batteries that rely on a fixed supply of internal fuel, fuel cells run for as long as there is an external supply of energy – generally in gaseous form such as hydrogen.
Microbe power
A living fuel cell, using bacteria to generate electricity, was first proposed by British scientists in 1911.
The microbial fuel cell, as it is known, bears some resemblance to a stomach in that it relies on microbes such as colonies of bacteria that are grown in the absence of oxygen. Electrons are released as the micro-organisms digest plant and animal matter, such as biological waste in the form of food scraps, urine or dead flies.
Electricity is produced when the electrons flow through the external circuit. The ions, on the other hand, move across the membrane to the positive electrode where they re-combine with incoming electrons and oxygen to form water.
Not unlike a fuel cell, a flow battery relies on an external supply of energy. But, instead of relying on dead flies, the flow battery runs on two tanks of liquids that are embedded in whatever structure the battery is powering.
The liquids are pumped past a membrane that separates the negative and positive electrodes. This gives rise to chemical reactions producing electrons in the external circuit. The flow battery is recharged by running it backwards.
Batteries like this might power small robots, such as RoboCat, which could recharge by visiting a wireless recharging post that applies an external current across the battery's membrane.
"It might also be possible to develop a hybrid system that combines the benefits of a flow battery and a microbial fuel cell," says Yannis Ieropoulis of the University of West England in Bristol. In principle, Professor Ieropoulis says, the electricity needed to reverse the reactions in the flow battery could be supplied by the waste powering the microbial fuel cell.
Links
Learn more about hydrogen and how it might power tomorrow's world in the textbook Oxford Big Ideas Australian Curriculum Science 8 (OUP)
VCAA link
Please send bright ideas for new topics to pspinks@fairfaxmedia.com.au
