The first iron-based catalyst to convert hydrogen directly to electricity has moved cheap fuel cells one step closer.
Researchers at the Center for Molecular Electrocatalysis at the US Department of Energy's Pacific Northwest National Laboratory have outlined their discovery in the latest issue of Nature Chemistry.
"A drawback with today's fuel cells is that the platinum they use is more than a thousand times more expensive than iron," said chemist R. Morris Bullock, who leads the research.
His team has been developing catalysts that use cheaper metals such as nickel and iron and now they have found one that can split hydrogen as fast as two molecules per second, with an efficiency approaching those of commercial catalysts.
Hydrogen fuel cells work by breaking the bond within a hydrogen molecule, where two electrons connect two hydrogen atoms like a barbell.
Most fuel cells use a platinum catalyst – essentially a chunk of metal – to crack a hydrogen molecule open like an egg so the electron whites run out and form a current an electric current.
Because platinum's chemical nature gives it the ability to do this chemists can't simply replace the expensive metal with the cheaper iron or nickel, but instead Bullock and his PNNL colleagues, chemists Tianbiao "Leo" Liu and Dan DuBois have taken inspiration from an enzyme called hydrogenase which uses iron to split hydrogen.
One of the properties they needed the catalyst to have was the ability to split hydrogen atoms into all of their parts by moving both the protons and electrons around in a controlled series of steps, sending the protons in one direction and the electrons to an electrode, where the electricity can be used to power things.
To do this, they need to split hydrogen molecules unevenly in an early step of the process. One hydrogen molecule is made up of two protons and two electrons, but the team needed the catalyst to tug away one proton first and send it away, where it is caught by a molecule called a proton acceptor. In a real fuel cell, the acceptor would be oxygen.
Once the first proton with its electron attracting force is gone, the electrode easily plucks off the first electron before another proton and electron are similarly removed, with both of the electrons being shuttled off to the electrode.
The speed of the team’s new catalyst peaked at about two molecules per second, thousands of times faster than the closest, non-electricity making iron-based competitor.
In addition, they determined its overpotential, which is a measure of how efficient the catalyst is. Coming in at 160 to 220 millivolts, the catalyst revealed itself to be similar in efficiency to most commercially available catalysts.
Now the team is figuring out the slow steps so they can make them faster, as well as determining the best conditions under which this catalyst performs.