Public Abstract

DE-SC0016537: Low temperature, ambient pressure electrochemical ammonia synthesis in alkaline media Mechanistic studies and catalyst design

Award Status: Inactive

Institution: University Of Delaware, Newark, DE
UEI: T72NHKM259N3
DUNS: 059007500

Most Recent Award Date: 07/06/2018
Number of Support Periods: 3
PM: Fitzsimmons, Timothy

Current Budget Period: 09/15/2018 - 09/14/2019
Current Project Period: 09/15/2016 - 09/14/2019
PI: Xu, Bingjun

Supplement Budget Period: N/A

Public Abstract

Ammonia synthesis via the Haber-Bosch process is a pillar of modern agriculture, which converts the abundant but inert dinitrogen in the atmosphere to nitrogen-based fertilizers. Despite more than a century of optimization, the Haber-Bosch process remains energy intensive and reliant on fossil fuels, and produces large amounts of CO2 (>9,400,000 tons in 2014). Distributed and modular ammonia synthesis via the electrochemical nitrogen reduction reaction (ENRR) at or close to ambient conditions, is an attractive alternative. Studies on low temperature and ambient pressure ENRR processes have so far focused on the acidic environment with proton exchange membrane-based devices, which typically exhibit <1% selectivity for ENRR and are incompatible with the alkaline nature of ammonia. The main challenge in making ENRR a viable process is the lack of selective ENRR catalysts and catalyst design principles. Computational studies suggest that conventional monometallic and alloy catalysts cannot achieve optimal ENRR activity because the binding energies of many nitrogen containing intermediates are linearly correlated, i.e., scaling relations. Metal/nitride bifunctional catalysts could defy the scaling relations because they have two distinctive types of sites and a discontinuous d-band. The central hypothesis of the proposed research is that ENRR in alkaline media with hydroxide exchange membranes and metal/nitride bifunctional electrocatalysts could simultaneously increase the ENRR activity and suppress the competing hydrogen evolution reaction. The investigators plan to quantify the ammonia production rate and columbic efficiency on representative monometallic surfaces at well-defined cathode potentials. In addition, they also plan to elucidate reaction pathways on these monometallic surfaces via surface sensitive spectroscopic investigations by identifying surface intermediates and understanding the interplay among them at the molecular level. Finally, this will be used to develop scaling relation-defying ENRR metal/nitride bifunctional catalysts and establish structure-activity relations. The proposed concept of ENRR in alkaline media represents a paradigm shift from the existing PEM-based devices in that it not only eliminates the possibility of the produced ammonia reacting with the acidic PEM, but also retards the competing hydrogen evolution reaction. The systematic mapping of ENRR activities on monometallic catalysts will provide experimental verification for computational predictions. Moreover, quantification of ammonia production rates and selectivities at well-defined potentials, along with identification of adsorbed reaction intermediates with in-situ surface enhanced spectroscopies, will provide mechanistic information that could enable rational catalyst design. Further, the proposed research will provide foundational knowledge to support potential development of modular and distributed ENRR devices.

Ammonia synthesis via the Haber-Bosch process is a pillar of modern agriculture, which converts the abundant but inert dinitrogen in the atmosphere to nitrogen-based fertilizers. Despite more than a century of optimization, the Haber-Bosch process remains energy intensive and reliant on fossil fuels, and produces large amounts of CO2 (>9,400,000 tons in 2014). Distributed and modular ammonia synthesis via the electrochemical nitrogen reduction reaction (ENRR) at or close to ambient conditions, is an attractive alternative. Studies on low temperature and ambient pressure ENRR processes have so far focused on the acidic environment with proton exchange membrane-based devices, which typically exhibit <1% selectivity for ENRR and are incompatible with the alkaline nature of ammonia. The main challenge in making ENRR a viable process is the lack of selective ENRR catalysts and catalyst design principles. Computational studies suggest that conventional monometallic and alloy catalysts cannot achieve optimal ENRR activity because the binding energies of many nitrogen containing intermediates are linearly correlated, i.e., scaling relations. Metal/nitride bifunctional catalysts could defy the scaling relations because they have two distinctive types of sites and a discontinuous d-band. The central hypothesis of the proposed research is that ENRR in alkaline media with hydroxide exchange membranes and metal/nitride bifunctional electrocatalysts could simultaneously increase the ENRR activity and suppress the competing hydrogen evolution reaction. The investigators plan to quantify the ammonia production rate and columbic efficiency on representative monometallic surfaces at well-defined cathode potentials. In addition, they also plan to elucidate reaction pathways on these monometallic surfaces via surface sensitive spectroscopic investigations by identifying surface intermediates and understanding the interplay among them at the molecular level. Finally, this will be used to develop scaling relation-defying ENRR metal/nitride bifunctional catalysts and establish structure-activity relations.

The proposed concept of ENRR in alkaline media represents a paradigm shift from the existing PEM-based devices in that it not only eliminates the possibility of the produced ammonia reacting with the acidic PEM, but also retards the competing hydrogen evolution reaction. The systematic mapping of ENRR activities on monometallic catalysts will provide experimental verification for computational predictions. Moreover, quantification of ammonia production rates and selectivities at well-defined potentials, along with identification of adsorbed reaction intermediates with in-situ surface enhanced spectroscopies, will provide mechanistic information that could enable rational catalyst design. Further, the proposed research will provide foundational knowledge to support potential development of modular and distributed ENRR devices.