Low-Cost Nanoscale Catalyst Splits Hydrogen from Water

(Energy.gov)

Researchers at Brookhaven National Laboratory in New York, part of the U.S. Department of Energy, have developed a new electrocatalyst that generates hydrogen gas from water cleanly and with much less expensive materials than current catalysts. Their findings are described online this week in the journal Angewandte Chemie International Edition (paid subscription required).

Traditional methods of producing pure hydrogen for fuel cells can be problematic, either in their release of harmful carbon dioxide into the atmosphere or the rare and expensive metals such as platinum required for production. The new catalyst, say the researchers — a form of nickel-molybdenum-nitride — turns out to be a high-performing catalyst with a nanosheet structure.

Water provides an abundant source of hydrogen, free of harmful greenhouse gas byproducts. The process of splitting water (H2O) into oxygen (O2) and hydrogen (H2), called electrolysis, requires external electricity and an efficient catalyst to break chemical bonds while shifting around protons and electrons. To be a feasible energy source, the amount of energy put into the reaction must be as small as possible while still exceeding the minimum required for the thermodynamics of electrolysis.

That catalyst needs to encourage an efficient reaction, combining high durability, high catalytic activity, and high surface area. It must also hit a “sweet spot,” where the bond with hydrogen is not so weak that it does not stimulate activity, nor too strong where it poisons the catalyst.

An element that meets those chemical requirements is platinum, but it’s price — more than $55,000 a kilogram, as of 30 April — makes it uneconomical for large-scale use. Nor is there enough platinum for it to be used in catalysts on any large scale.

To find an alternative, the team led by Brookhaven chemist James Muckerman devised a compound of nickel, which reacts much the same way as platinum, spiked with metallic molybdenum to enhance its reactivity. While that compound had some success and a lower cost — $20 per kilogram for nickel and $32 per kilogram for molybdenum — it still did not have the performance of platinum.

To boost its power further, the Brookhaven team applied nitrogen to modify the structure of the nickel-molybdenum, producing as expected discrete, sphere-like nanoparticles. What the researchers did not expect, however, was the resulting formation of the nanoparticles into sheets, which offered large surface areas, one of the key requirements of the catalyst.

“Nitrogen made a huge difference,” says Wei-Fu Chen, first author of the paper and chemistry research associate. “It expanded the lattice of nickel-molybdenum, increased its electron density, made an electronic structure approaching that of noble metals, and prevented corrosion.”

The researchers say the new catalyst performs nearly as well as platinum, achieving electrocatalytic activity and stability unmatched by any other non-noble metal compounds. While the catalyst is not a complete solution to the challenge of creating affordable hydrogen gas, it does offer a major reduction in the cost of essential equipment.

Read more:

• Robot Jellyfish Runs on Hydrogen, Oxygen in Water
• Nanoparticle Size Affects Hydrogen Release in Fuel Cells

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