Mechanistic insights into electrocatalysts for green technologies
Date
2019
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Publisher
University of Delaware
Abstract
Addressing climate change requires new technologies to reduce today's CO2 emissions. Electrochemical devices, coupled renewable energy generation from solar and wind, are capable of producing energy and/or chemicals with zero carbon emissions. The economic sectors that produce over a third of the carbon emissions in the United States are the transportation, industrial, and agricultural sectors and all of these sectors could reduce if not eliminate carbon emissions using electrochemical devices. Because there is no combustion in the electrochemical processes, the efficiency of these devices can be almost twice that of traditional combustion cycles. There are a variety of different types of electrochemical devices, with the distinguishing feature being the electrolyte, however, this thesis will focus on polymer exchange membrane-based devices which have a solid polymer electrolyte. Polymer electrolyte membranes can transport protons or hydroxide between the electrodes while operating at near ambient conditions and maintaining high efficiency. Commercialization of these electrochemical devices are limited due to the energy losses associated with the catalysis as well as the expensive construction materials. In order to develop inexpensive and more efficient electrocatalysts, a mechanistic understanding of the reaction is critical. The aim of this thesis is to gain a mechanistic understanding of the hydrogen oxidation and nitrogen reduction reactions to help aid in rational catalyst design for these devices which could be crucial for green technologies capable of reducing carbon emissions and mitigating climate change. ☐ Renewable production of hydrogen, and use of hydrogen in fuel cell vehicles can reduce the carbon emissions from the transportation sector. Widespread adoption of fuel cells is still lacking though due to the high cost of fuel cell vehicles. The current technology, which is implemented in cars like the Toyota Mirai, uses an acidic environment which requires expensive materials for catalysts as well as the ancillary components for durability. Moving to an alkaline environment allows for cheaper materials, and could help reduce the cost. However, the catalytic activity of the Platinum group metals (PGMs) decreases by two orders of magnitude for the hydrogen oxidation and evolution reactions. By studying the most active catalyst in base, PtRu, we demonstrate that the apparent hydrogen binding energy (HBEapp) is the key descriptor for the reaction, and the reaction does not proceed through the bifunctional mechanism. The HBEapp change from acid to base was determined by the water adsorption and structure which is pH dependent. Cations, which have been proposed to cause the changes between acid and base, can have a modest erect on the HBEapp, however, the solution pH dominates the HBEapp. ☐ Ammonia production requires a significant amount of fossil fuels and energy, both of which contribute to the large carbon emissions. Electrochemical nitrogen reduction (ENR) could reduce the overall energy required for producing ammonia, a building block for fertilizers. However, the selectivity of ENR actually favors hydrogen production for metal catalysts. By comparing the activity of PGMs in acid and base, we demonstrate that the increase in pH increases the selectivity toward ENR, however it is still below the required selectivity to compete with the current Haber-Bosch process. Alternatively, metal nitrides had computationally been estimated to be more selective catalysts for ENR, but had very little experimental proof. By testing a variety of metal nitrides, we determined reaction mechanism and deactivation mechanisms common between metal nitrides, although the active sites were not universal. The mechanisms for the reaction and deactivation will be discussed for vanadium and chromium nitrides.