Despite the abundance of nitrogen and hydrogen, industrial ammonia synthesis remains an energy-intensive process. Electrochemical methods have been suggested as potentially lower-energy alternatives to traditional ammonia synthesis, but these newer technologies often struggle with achieving key performance metrics. By integrating a plasma field with an electrochemical reactor, a new technology developed at the State University of New York at Buffalo (SUNY Buffalo; www.buffalo.edu) is able to efficiently produce ammonia from only air and water at room temperature. “Many alternative technologies can perform well in a specific category, such as achieving high ammonia production rates, high Faradaic efficiency, good nitrogen conversion, operation at ambient conditions or eliminating hydrogen inputs or carbon emissions. What sets our technology apart is that we can achieve multiple of these key performance metrics simultaneously,” explains Chris Li, professor of chemistry at SUNY Buffalo.
The dual nature of the technology helps overcome the challenges of other alternative ammonia-synthesis routes. The advantage of plasma is its ability to readily break nitrogen triple bonds, but it lacks the selectivity to direct the reaction toward ammonia. Conversely, electrochemical systems enable conditions for ammonia selectivity, but they experience difficulties with nitrogen activation.
Within the plasma-electrochemical process, air, the primary reactant, is humidified with water and passed through a plasma field, where nitrogen is broken into reactive species. These reactive nitrogen species then recombine with oxygen and hydrogen to form a complex mixture of NOxHy compounds. This mixture is fed to an electrochemical reactor, where the NOxHy compounds are selectively converted into ammonia. Essential to ensuring process efficiency was gaining a clear understanding of the NOxHy mixture. “We’ve detected up to eight different NOxHy species, though there may be more. The catalytic conversion of mixed NOxHy species is quite complex, so we used a computational graph theory approach to map out all possible reaction pathways and identify catalysts that enhance efficiency,” says Li.
Li’s team has stably operated a reactor system producing around 1 g/d of ammonia for over 1,000 h. “In theory, our plasma and electrochemical reactors should have no fundamental barriers scaling to similar levels as fuel cells and electrolyzers. This can typically be achieved through parallel reactor configurations or by increasing the reactor’s geometric volume or surface area. Our top priorities in the lab now are improving intrinsic energy efficiency and demonstrating scale-up feasibility,” adds Li.
Further details about this work were published in the December 2024 issue of the Journal of the American Chemical Society.