In the search for clean and sustainable alternative energy sources, pure hydrogen is king. Currently, hydrogen is produced by reforming natural gas, but that process relies on a limited supply of fossil fuels and generates carbon dioxide as a byproduct. The hydrogen created from natural gas also contains impurities, which degrades fuel cell performance.
The ‘nitridization’ of a nickel-molybdenum composite results in a unique nanostructure, in which thin, flat nanosheets are stacked with an average stacking number of 6. The nickel atoms segregate and spread across the surface of the nanosheets, allowing for higher surface area for interaction with hydrogen.
Another process for extracting pure hydrogen is electrolysis – or the splitting of water – which can be done with proton exchange membrane (PEM) electrolyzers or artificial photosynthetic systems that use sunlight. The result is the creation of clean energy in the form of hydrogen stripped from water. The best catalyst for hydrogen production from water is known to be platinum. But platinum is expensive ($50,000 per kilogram) and not sufficiently abundant to meet the demand of worldwide hydrogen production.
Now, chemists at the Department of Energy’s (DOE) Brookhaven National Laboratory have created the best performing non-noble metal catalyst for hydrogen production, and at $52 per kilogram, their nickel-molybdenum composite brings the possibility of cost-efficient hydrogen fuel cells closer to reality.
“Hydrogen is clean and sustainable energy with high density and it is a promising alternative to fossil fuels,” said Brookhaven chemist Kotaro Sasaki, who led the research with his colleagues, Wei-Fu Chen and Jim Muckerman. “By electrolysis of water, we can generate pure hydrogen without carbon dioxide emission.”
It’s the unique combination of nickel and molybdenum that makes the catalyst so high-performing. Nickel binds hydrogen weakly, while molybdenum binds it strongly. The mixture of the two non-noble metals allows for an optimum bond strength, which increases the production of hydrogen to a level that nears that of platinum.
The key to increasing the performance of this alloy is creating composite NiMo nitrides by introducing nitrogen to the metals. This suppresses corrosion of the metals in acidic solutions, which allow for more efficient HER than alkaline solutions. ‘Nitriding’ also results in a unique nanostructure, in which thin and flat nanosheets are stacked with an average stacking number of 6, creating a higher surface area for interaction with hydrogen.
“To make a viable alternative energy, durability is very important. Non-noble catalysts have been made for hydrogen extraction, but corrosion has always been a problem,” said Sasaki. “Our catalyst combines the best of two worlds – nickel and molybdenum – and the combination nears the efficiency of platinum.”
Using in-situ x-ray absorption spectroscopy (XAS) at the National Synchrotron Light Source, the team observed that the high performance of their composite happens because nickel and molybdenum have low valence states, meaning they are not oxidized, and their electronic states have been modified to give a moderate metal-hydrogen binding strength. XAS also revealed that the nickel atoms had segregated and spread across the surface of the nanosheet structures.
“XAS is a very powerful tool for catalyst development because we can know the details of controlling factors, such as electronic and oxidation states, and the atomic structures of our catalyst,” said Sasaki.
The nickel-molybdenum nitride catalyst is more economical and more efficient than other non-noble metal catalysts, and the process to scale up to commercial production is a simple one. Thanks to Brookhaven’s Laboratory Directed Research and Development (LDRD) Program, which funded the work, this finding could significantly advance sustainable fuel production. The team will continue to search for other combinations of elements that can catalyze hydrogen production for future fuel cells.
This research was published in May 2012 in Angewandte Chemie International Edition.