Alloyed and chemically modified metal catalysts for the electrochemical reduction of CO2 [electronic resource]
Keywords: Chemical Engineering.

While fossil fuels have helped our society grow and technology advance rapidly in the last century, the mounting evidence of their detrimental eﬀects on air quality, public health, and climate change, in addition to the desire for energy independence from geopolitically instable suppliers, are strong motivations for shifting to cleaner, independent energy sources. Wind and solar are promising alternatives, but their intermittency leads to either curtailing or the need for energy storage. One approach is the carbon dioxide reduction reaction (CO2RR), which combines CO2 and H2O using renewable energy to produce high energy density fuels and commodity chemicals, providing an attractive, lower-carbon alternative process to fossil fuels and petrochemicals. However, a wide variety of products can be produced from this reaction, and there is still much work to be done in developing catalysts to steer the selectivity, increase activity, and maximize energy eﬃciency for the CO2RR in order for the process to be cost-competitive with other solutions. Transition metal surfaces have been explored as electrocatalysts for the CO2RR, with the activity of several metals reported in seminal work by Yoshio Hori. The Jaramillo laboratory designed a new electrolysis cell with excellent product detection sensitivity that not only was able to reproduce Hori's results, but also was able to detect additional products. Additionally, it was used to explore a wider set of potentials to obtain a more complete view of the catalytic activity of several transition metal catalysts. The results of several metals - Pt, Fe, Ni, Cu, Au, Ag, and Zn - were collectively analyzed to provide new insights into the CO2RR on metals, particular with the selectivity of minor products such as methanol and methane. Volcano plots based on theoretically predicted CO binding energy were constructed, and our experimental results correlate well with theoretical predictions. After establishing a reliable testing protocol for the CO2RR, we have since moved to exploring novel materials. Amines are commonly used as CO2 capture sorbents and adsorbents for ﬂue gases of coal-ﬁred power plants, as amines can interact with the CO2 molecules to form carbamates through N-C bond formation, and the CO2 can be released through a temperature or pressure change to produce pure CO2 streams. Additionally, pyridine has been added to solution to modify the activity of metal and semiconductor catalyst surfaces to produce methanol at fairly high Faradaic eﬃciencies, and the formation of a weak N-C bond has been detected and proposed as a mechanism for the enhanced CO2RR activity. Ionic liquids containing amine groups have also shown interesting activity for the CO2RR, and these works inspired our work to use a thin ﬁlm amine- containing polymer as a surface modiﬁer to aﬀect the activity of metal catalysts. Speciﬁcally, thin ﬁlms of 10-20nm of polyaniline (PAni) were electrodeposited on a polycrystalline Pt foil and explored for the CO2RR. Up to a 5x enhancement in the production of formate as well as an increase in the production of CO at high overpotentials were observed for the PAni-Pt ﬁlms relative to the Pt foils, and these enhancements were conﬁrmed to not be due to PAni degradation. These results show a promising method for modifying CO2RR activity that could be used and optimized for other similar systems in the future. Alloys have been interesting systems to explore for a wide variety of electrocatalytic reactions, and high-throughput testing as well as theoretical predictions are two methods to try and reduce the time needed to ﬁnd active catalysts. Due to the need for product detection, it is diﬃcult to design a high-throughput CO2RR system, and the complexity of the reaction also makes it diﬃcult to ﬁnd theoretical studies that have accurately predicted active catalysts to this point. Recent theoretical studies have provided some intriguing candidates by simplifying the reaction to just a few rate- determining steps, and some progress has been made in correlating theoretical studies with experimental results. We have developed a "medium throughput" testing protocol to be able to synthesize, physically characterize, and electrochemically test about 4 alloy compositions each day and have worked with Jens Norskov's group to explore potential alloy candidates for the CO2RR. A dual e-beam system was designed by my colleague, Christopher Hahn, that can coevaporate a variety of alloy thin ﬁlms of desired compositions. Code has been written to quickly analyze individual materials and compare results with a growing database of materials to improve the speed of analysis and assessment for trying to narrow down candidates and ﬁnd interesting CO2RR alloy catalysts. One of the most promising results from the alloy screening to this point has been the PtInx system of alloys. While Pt is known to bind CO strongly and produce primarily H2 and In is known to bind CO weakly and produce primarily formate, several synthesized PtInx alloys predominantly produced CO. These ﬁlms were then further characterized to gain a more complete understanding of the alloy compositions, crystallinity, and morphology of the alloys, and the results were then analyzed based on the knowledge gained of the catalytic surfaces. The most active alloys had bulk compositions of PtIn4 and PtIn12 , but the surfaces were mostly a crystalline Pt3In7 phase, with phase-separated In particles on the surface as well. It is postulated that the increased activity for CO was due to the Pt3In7 alloy phase, which has a binding energy for CO that is in between that for Pt or In. This was a promising initial results that shows that the activity of alloys can be very diﬀerent than that for either of the individual elements, and our system will hopefully continue to ﬁnd additional active alloys and provide insight into the mechanisms for the CO2RR.