The new catalyst performs nearly as well as platinum, achieving electrocatalytic activity and stability unmatched by any other non-noble metal compounds, and the production process is simple and scalable, according to Dr. Muckerman.

Splitting water (H₂O) into oxygen (O₂) and hydrogen (H₂), requires external electricity and an efficient catalyst to break chemical bonds while shifting around protons and electrons. For a catalyst to facilitate an efficient reaction, it must combine high durability, high catalytic activity, and high surface area. The strength of an element’s bond to hydrogen determines its reaction level—too weak, and there’s no activity; too strong, and the initial activity poisons the catalyst.

We needed to create high, stable activity by combining one non-noble element that binds hydrogen too weakly with another that binds too strongly. The result becomes this well-balanced Goldilocks compound—just right.

—James Muckerman

Platinum is the gold standard for electrocatalysis, combining low overpotential with high activity for the chemical reactions in water-splitting. However, rapidly rising costs—already hovering around $50,000 per kilogram—are discouraging widespread use of platinum and other noble metals. The limited global supply of platinum not only drives up price, but casts doubts on its long-term viability, Muckerman said, adding that “There may not be enough of it to support a global hydrogen economy.”

In contrast, the principal metals in the new compound developed by the Brookhaven team are both abundant and cheap: $20 per kilogram for nickel and $32 per kilogram for molybdenum.

In this new catalyst, nickel takes the reactive place of platinum, but it lacks a comparable electron density. The scientists needed to identify complementary elements to make nickel a viable substitute, and they introduced metallic molybdenum to enhance its reactivity. While effective, it still couldn’t match the performance levels of platinum.

We needed to introduce another element to alter the electronic states of the nickel-molybdenum, and we knew that nitrogen had been used for bulk materials, or objects larger than one micrometer. But this was difficult for nanoscale materials, with dimensions measuring billionths of a meter.

—Wei-Fu Chen, the paper’s lead author

The scientists expected the applied nitrogen to modify the structure of the nickel-molybdenum, producing discrete, sphere-like nanoparticles. Instead, however, subjecting the compound to a high-temperature ammonia environment infused the nickel-molybdenum with nitrogen, but also transformed the particles into unexpected two-dimensional nanosheets. The nanosheet structures offer highly accessible reactive sites, and therefore more reaction potential.

Using a high-resolution transmission microscope in Brookhaven Lab’s Condensed Matter Physics and Materials Science Department, as well as X-ray probes at the National Synchrotron Light Source, the scientists determined the material’s 2D structure and probed its local electronic configurations.

Despite the fact that metal nitrides have been extensively used, this is the first example of one forming a nanosheet. Nitrogen made a huge difference—it expanded the lattice of nickel-molybdenum, increased its electron density, made an electronic structure approaching that of noble metals, and prevented corrosion.

—Wei-Fu Chen

Additional collaborators on this research were: Anatoly Frenkel of Yeshiva University; Nebojsa Marinkovic of the University of Delaware; and Chao Ma, Yimei Zhu and Radoslav Adzic of Brookhaven Lab.

The research was funded by Brookhaven’s Laboratory Directed Research and Development (LDRD) Program. The National Sychrotron Light Source and other Brookhaven user facilities are supported by the DOE Office of Science.

Resources

• Chen, W.-F., Sasaki, K., Ma, C., Frenkel, A. I., Marinkovic, N., Muckerman, J. T., Zhu, Y. and Adzic, R. R. (2012), Hydrogen-Evolution Catalysts Based on Non-Nobel Metal Nickel–Molybdenum Nitride Nanosheets. Angew. Chem. Int. Ed.. doi: 10.1002/anie.201200699