Addressing the worldwide problems of escalating energy demand and rising CO₂ emissions will require an increase in the use of carbon-neutral, sustainable energy sources. Electrocatalysts are needed for the conversion between chemical energy (bonds such as the H–H bond of hydrogen) and electricity in future systems for the storage and use of energy. Hydrogen is attractive as an energy carrier, but a major barrier to its more wide-spread use is the requirement for efficient and inexpensive catalysts.

Electricity is produced from the oxidation of hydrogen in low-temperature fuel cells, but the best catalyst is platinum, a precious metal of low abundance. A burgeoning effort by chemists studying many areas of catalysis has focused on ‘cheap metals for noble tasks’. Iron is particularly attractive because of its very high earth-abundance, as well as its low cost and toxicity, leading Bolm to suggest the advent of a ‘new iron age’. Here we show that a rationally designed iron catalyst offers substantial promise as an alternative to precious metal catalysts.

—Liu et al.

R. Morris Bullock and his PNNL colleagues, chemists Tianbiao Liu and Dan DuBois, took inspiration for their iron-based catalyst from a hydrogenase. First Liu created several potential molecules for the team to test. Then, with the best-working molecule up to that point, they determined and tweaked the shape and the internal electronic forces to make additional improvements.

The catalyst needs 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 kind of molecule called a proton acceptor. In a real fuel cell, the acceptor would be oxygen.

Once the first proton with its electron-wooing force is gone, the electrode easily plucks off the first electron. Then another proton and electron are similarly removed, with both of the electrons being shuttled off to the electrode.

The team determined the shape and size of the catalyst and also tested different proton acceptors. The final complex has pendent amines in the diphosphine ligand that function as proton relays.

The catalyst split molecular hydrogen at a peak rate of about two molecules per second, thousands of times faster than the closest, non-electricity making iron-based competitor. With an overpotential of 160 to 220 millivolts, the catalyst is 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.

This work was supported by the Department of Energy, Office of Science.

Resources

• Tianbiao Liu, Daniel L. DuBois and R. Morris Bullock (2013) An iron complex with pendent amines as a molecular electrocatalyst for oxidation of hydrogen, Nature Chemistry doi: 10.1038/NCHEM.1571