The combustion of petrochemicals for energy continues to be a dominant contributor to anthropogenic carbon dioxide (CO2) emissions. The resultant increase of atmospheric CO2 levels has been linked to global climate change, ocean acidification, and health issues. The severity of these problems continues to generate interest in how energy is produced and consumed. Besides the detrimental effects of its byproducts, combustion is a relatively inefficient way of recovering energy from chemical bonds. Conversely, renewable energy sources like wind and solar are becoming cheaper and more abundant, but suffer from intermittent supply. An alternative is to use electrochemical means to store renewable energy in chemical bonds in a recoverable manner, to create a more efficient and environmentally benign cycle. Our research seeks to understand: (1) how to store renewable energy in chemical bonds as solar fuels via artificial photosynthesis and (2) how to recover this energy with catalysts in fuel cells. Fuel cells can operate at higher efficiency and competitive energy density to combustion engines using fuels like formic acid and methanol, which can be produced from CO2. Using Ni with a tetrapyridyl-containing hexadentate ligand framework, we have observed efficient and selective CO2 reduction to CO and H2O in the presence of a proton source. Such transformations are relevant to fuel production and the cathodic half reaction of a fuel cell, respectively. Initial mechanistic studies suggest that similar to Ni(cyclam)2+, CO loss from Ni(I) becomes the limiting step at applied potential. With sufficient phenol present as a proton source, icat/ip, which has a relationship to catalytic rate kcat, reaches ~7. Spectroelectrochemical studies show the persistence of the Ni-CO species under these conditions, consistent with this hypothesis. Kinetic experiments point to proton-coupled electron transfer to cleave the C-OH bond from hydroxycarbonyl is the rate-limiting step in the catalytic cycle.

Polypyridyl electrocatalysts for the reduction of CO2

255th National Meeting (2018)

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