Harnessing the power of hydrogen gas presents one of the most promising options available for obtaining a large-scale sustainable energy solution. However, there are numerous and significant challenges present in the production of pure hydrogen, one of the most prominent of which is the high costs associated with the use of rare and expensive chemical elements such as platinum. Accordingly, the team at the Brookhaven National Laboratory set out to create a catalyst with high activity and low costs, that could facilitate the production of hydrogen as a high-density, clean energy source.
The key component in the production of pure hydrogen is one of the most abundant elements on the planet. Water provides a cheap and plentiful source and is free of harmful greenhouse gas by-products. Electrolysis, or the splitting of water (H2O) into oxygen (O2) and hydrogen (H2), requires an external source of electricity and an efficient catalyst. It's the latter of these that causes the difficulties.
An effective catalyst has to combine high catalytic activity, high durability and high surface area. The most effective material for use as the catalyst for electrolysis is platinum: a highly expensive commodity. The rising cost of platinum (currently around US$50,000 per kilogram) has gone a long way to discouraging widespread investment in the production of hydrogen gas.
However, the high cost isn't the only issue associated with use of the rare element. James Muckerman, senior chemist on the project, said “People love platinum, but the limited global supply not only drives up price, but casts doubt on its long-term viability ... There may not be enough of it to support a global hydrogen economy”.
Creating an alternative to platinum presents a significant challenge. The strength of an element's bond to hydrogen determines its level of reaction – too strong and the initial activity will poison the catalyst, too weak and there is little or no activity. “We needed to create high, stable activity by combining one non-noble element that binds hydrogen too weakly with another that binds too strongly," said Muckerman.
The new catalyst is initially made up of nickel and metallic molybdenum, but the resulting compound is still unable to match the performance levels of platinum. To solve this issue, nitrogen is introduced through a complicated procedure in which the compound is subjected to a high-temperature ammonia environment. This process infuses the nickel-molybdenum with the nitrogen, but also produces an unexpected result.
Nitrogen has been used for similar applications in the past, but only for bulk materials, or objects larger than one micrometer. When applying the element for use on nanoscale materials, with dimensions measuring billionths of a meter, the reaction transformed the resulting compound into unexpected two-dimensional nanosheets. These structures provide highly accessible reactive sites, and therefore more reacting potential. The paper's lead author, Wei-Fu Chen, comments on the resulting compound, stating that “Nitrogen has 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."
The combined cost of the components required for the compound is about one one-thousandth that of platinum, but with a comparable (though slightly lower) performance level. The production process is also simple and scalable, making it viable for industrial applications.
Although this new compound falls short of providing a complete solution to the multi-faceted challenge of creating affordable hydrogen gas, it does provide a significant reduction in the cost of the required materials. “We needed to figure out fundamental approaches that could potentially be game-changing, and that's the spirit in which we're doing this work," said Muckerman. "It's about coming up with a new paradigm that will guide future research”.
It's all about fundamental exploration, without which the surprising discovery of the nanosheet structure would never have been made.