Belinda Smith
Belinda Smith is a science and technology journalist in Melbourne, Australia.
Future fuel cells
Imagine driving a car, but instead of exhaust fumes and carbon dioxide, it emits only pure water. Cars powered by hydrogen fuel cells do just that, converting energy stored in molecular bonds into electrical energy. The Toyota Mirai (Japanese for ‘future’) is one of the first commercially sold vehicles powered by fuel cells and rated the most fuel-efficient hydrogen fuel cell vehicle by the US Environmental Protection Agency. It made its debut at the November 2014 Los Angeles Auto Show and, last year, landed in Australia for a three-year trial.
So what’s under the hood? Instead of an internal combustion engine, where hot gases push a piston to turn a crankshaft, it has a stack of hydrogen fuel cells.
There are a few different types of hydrogen fuel cells but their basic principles are the same. Cars like the Mirai use what are known as proton exchange membrane fuel cells. They comprise two electrodes – an anode and a cathode – separated by an electrolyte membrane that lets specific types of charged particles pass through.
Pressurised hydrogen gas (H2) is pumped from a storage tank to the anode, where it’s forced through a catalyst – a thin layer of platinum. The catalyst tears the H2 molecule apart into two positively charged (or ionised) hydrogen atoms and two electrons.
The ionised hydrogen atoms cruise through the electrolyte to the cathode – but the electrons are blocked. So they travel around the circuit to the cathode, creating the current.
“Why aren’t we all driving around in electric cars powered by proton exchange membrane fuel cells?”
At the cathode, air is pumped in. Oxygen gas molecules (O2) in air are also broken apart to create two negatively charged oxygen atoms. These react with the hydrogen ions that passed through the membrane and electrons that traversed the circuit to form water molecules (H2O).
German-Swiss chemist Christian Friedrich Schoenbein first published fuel cells’ underlying principles in a magazine in 1838. He knew that running an electric current through water could split it into oxygen and hydrogen. Could that process be reversed to produce electricity? A year later, Welsh physicist William Robert Grove built a working prototype: hydrogen and oxygen mixed in a sulfuric acid bath – a liquid electrolyte – with porous platinum electrodes produced a small electrical current.
In the late 1800s, chemists Ludwig Mond and Charles Langer tried to build a more practical model. For the next 50 years or so, chemists added a few modifications and tweaks here and there.
Then Willard Thomas Grubb at General Electric Research Laboratory in 1955 made a huge advance in fuel-cell technology. Instead of a liquid electrolyte, he used a more reliable membrane of sulfonated polystyrene. The proton exchange membrane fuel cell was born.
Since then, chemists have invented better polymer electrolytes. They’ve also boosted the fuel cells’ efficiency by, for instance, roughening the platinum catalyst layer to increase surface area and the rate of reaction. To propel a car, for instance, fuel cells are stacked to produce more power.
So why aren’t we all driving around in electric cars powered by proton exchange membrane fuel cells? The simple answer is cost. Building fuel cells is a pricey endeavour: platinum catalysts aren’t cheap, nor are proton exchange membranes. To compete with petrol-powered or even hybrid cars, fuel cell systems need to halve in price. A new Mirai will set you back around $70,000 – twice as much as the Toyota Prius, a hybrid.
Materials science labs are working to find ways to drop fuel cell production costs, as well as find better ways to produce and store hydrogen.
Hydrogen production
Hydrogen is the most abundant element in the universe, but on Earth, most is locked up in molecules such as water. Fuel cells need H2 to work. So how is it produced?
More than 90% of hydrogen fuel is made when natural gas is reacted with super-hot steam, producing synthesis gas, or syngas – a mix of hydrogen, carbon monoxide and carbon dioxide. Running an electric current through water to split it into hydrogen and oxygen – a technique called electrolysis – is a promising green method if it uses low-emission electricity such as nuclear power or solar concentrators.
Algae, too, have been harnessed to make hydrogen from water and sunlight. Researchers are working on developing specialised semiconductors that do the same.
Better batteries
When it comes to batteries, lithium-ion is king. They’re lightweight and very energy dense, meaning they pack more power per weight than other batteries.
But they’re also prone to catching fire if damaged. And if every car went electric or hybrid, there wouldn’t be enough lithium to go around. So researchers are looking at ways to make them safer and replace lithium with more plentiful, cheaper metals.
Different lithium-ion batteries have different compounds at the anode and cathode. Briefly, the lithium-ion battery you might find in your smartphone comprises a positive electrode (anode) made of lithium cobalt oxide and a negative electrode (cathode) of carbon bathed in electrolyte. The electrodes are separated by a thin plastic membrane that selectively lets lithium ions through.
As the battery charges, lithium ions move from the anode through the membrane to the cathode. As it discharges, the ions move back to the anode. Electrons, blocked by the membrane, travel to the anode through an external wire to produce electricity. Sodium is a contender to replace lithium, but because sodium ions are larger and heavier, the batteries aren’t as energy-dense. And while that might rule them out for smartphones, they’re ideal for jobs such as storing solar energy. Calcium, magnesium, aluminium and potassium are also in the running, with each at varying degrees of research, prototyping and commercialisation.
What about safety? Samsung’s notoriously faulty Note 7 batteries caught fire when the separator membrane wore thin or tore and the electrodes touched. This short-circuited the battery, igniting the flammable electrolyte. To this end, materials scientists are exploring non-flammable, solid electrolytes.
Stable storage
One kilogram of hydrogen yields 120 megajoules – that’s nearly three times as much energy as the same mass of petrol. By volume, though, petrol wins: a litre of petrol can provide 32 megajoules while a litre of liquid hydrogen yields just eight megajoules. Storing huge volumes of hydrogen for fuel cells isn’t so much a problem for stationary power generators or even large vehicles. But for standard cars, where space is at a premium, carting around a giant tank of hydrogen is simply not an option.
How hydrogen is stored, too, is important from a safety perspective. Compressed hydrogen gas can be explosive in a car accident. So researchers are finding ways to store hydrogen at high density – to yield the most power per tank – but not high pressure, so it’s safer to cart around.
Promising candidates come in the form of metallic-organic frameworks, or MOFs. MOFs are materials that look a bit like molecular scaffolds, with metal atoms joined up with organic molecules.
Gaps in the crystal pattern can snare other molecules, such as carbon dioxide, oil or hydrogen.
With limitless number of combinations of metals and organic molecules, MOFs can be customised to do just about anything – from capturing carbon dioxide from coal-fired power plant emissions to cleaning oil from water – and, of course, storing hydrogen.
When hydrogen molecules are trapped by some MOFs, they seem to calm down. An ideal MOF packs loads of hydrogen molecules in – boosting density – while stopping them from jiggling around too much, which eases pressure.
Super solar cells
While the first working photovoltaic (solar) cell was made of gold and selenium by American inventor Charles Fritts in 1883, silicon underpins most solar cell technology today and has dominated since the mid-20th century.
Silicon solar cells produce electricity when photons knock electrons from one layer of silicon semiconductor to another. These layers are doped with different atoms to give one layer an excess of electrons and the other a deficit. If the circuit is complete, the electrons can be captured to produce electricity. Many cells bundled together form a solar panel.
The first silicon solar cells’ efficiency hovered around 5%. Refining silicon’s purity and treating a solar cell’s surface to better absorb light have helped boost efficiency to around 25%. Commercially produced solar cells currently sit around 20%. {%recommended 5823%}
Meanwhile, other materials have been explored for solar cells, such as organic semiconductors. They’re made mostly made of carbon and hydrogen atoms.
There are benefits to organic solar cells. Organic semiconductors can be printed on flexible materials. Plus, they’re cheap.
What they’re not, though, is terribly efficient. Current organic solar cell technology has efficiencies around the 10% mark (although this is tipped to rise). So they’re not necessarily seen as a replacement for silicon solar cells but can be used alongside them.
For instance, organic solar cells were installed on rooftops in rural southern African villages that couldn’t access the electricity grid, giving those populations a safer and cheaper alternative to burning kerosene.
