Telling researchers how to design a fuel-cell structure to overcome one of the most recalcitrant engineering challenges of hydrogen power, Héctor D. Abruña said that, with an alkaline medium, it was possible to use nonprecious metals such as nickel, iron, cobalt, manganese, and so on, which are 500 to 1,000 times cheaper than precious metals such as platinum and palladium.
Fuel cells have always been an embarrassment to clean-energy systems: they are theoretically efficient, are attractive due to their transport and back-up capabilities, but they are loaded with catalysts composed of metals that are scarce, expensive, and non-recyclable. Platinum is still predominant in conventional acidic fuel cells, due to the scarcity of materials that can tolerate the chemistry and still catalyzed the hydrogen oxidation reaction sufficiently quickly to be useful. That reliance has influenced the economy of the technology as much as its functionality.
The more recent Cornell and collaborator work changes the chemistry to alkaline conditions, allowing less exotic metals to be employed. The core development is the nickel catalyst to be encased in an ultra-thin shell of nitrogen-doped carbon, which is about 2 nanometers thick. The surface is coated sufficiently to allow passage of electrons, but is strong enough to prevent oxidation of the nickel to an inactive form. That is important since bare nickel had long ago proven to be promising in alkaline systems and is quickly passivated under operating conditions, thereby becoming inactive as a catalyst.
The outcome does not simply consist of a cheaper ingredient list. The highest current density, when cobalt-magnesium was used as the cathode, was over 200 milliwatts per square centimeter, which was reported by the team when paired with an anode, and a cell with a power of over 1 watt per square centimeter was described by a later Cornell account. That degree is noteworthy since it puts platinum-free alkaline fuel cells into the performance arena previously limited to precious-metal systems. Another benefit of operation proposed in the same work is better carbon monoxide resistance of hydrogen fuel, making optional cleanup equipment less necessary and facilitating the design of the systems. The more difficult test is the durability.
Even researchers who introduce the next big hurdle, lifetime, are defining it as the next big challenge. The existing chemistry has come to approximately 2,000 hours of operation, which is still short of the U.S. Department of Energy stability goal of 15,000 hours in fuel cells. That is one of the reasons that platinum alternatives can create a buzz in the laboratory, but require a long time to become relevant in vehicles, stationary generators, or distributed energy systems. Catalysts have to endure not only optimal test conditions, but also thousands of start-stop cycles, pollution incidents and temperature variations.
Nonetheless, the nickel finding fits into a greater trend of hydrogen studies. Other camps have recently reduced the platinum content drastically instead of eliminating it altogether, such as catalysts with only 4% platinum by weight to produce hydrogen. Simultaneously, iron-nitrogen-carbon materials are under pressure to be developed as platinum-group-metal-free cathodes, and the results are enhanced by the meticulous regulation of thermal treatment and carbon structure.
The combination of these efforts is an indication of a larger change than any one headline discovery: hydrogen devices are being re-architectured to use abundant materials, nanoscale protective structures, and catalytic points that have been optimized to work in a particular operating environment. Making the most recent nickel-carbon design significant not because it resolves the debate on the fuel-cell, but because it forms an assault on the cost bottleneck at the reaction surface itself. In the case of hydrogen power, that is where cheapness has been lurking all along.