Engineering Active Sites in Metal-Organic Frameworks for Efficient Electrochemical Carbon Dioxide Reduction

Abstract

Electrochemical carbon dioxide (CO2) reduction (CO2RR) is a form of carbon recycling aimed to create a net-carbon-neutral loop through the production of valuable chemical feedstocks and fuels through electrochemical means. Due to the global urgency to reduce rising global carbon emissions, CO2RR research has gained significantly more traction in the last two decades accelerating the advances and discoveries of efficient catalyst materials. With significant advances made in catalyst discovery, surface-science mechanistic understanding, and electrochemical cell development, there still remains the challenge of long term stability and single selectivity towards products. Within this research, a multi faceted approach is used to improve overall CO2RR stability and selectivity.Mono- and bimetallic based metal-organic frameworks (MOFs) were developed to improve overall catalytic activity while decreasing metal usage. Bimetallic bismuth based MOFs were developed to tune selectivity towards the two-electron electrochemical reduction of CO2 towards formate (HCOO– ). Monometallic copper based MOFs were made for the efficient reduction of CO2 to carbon monoxide ( CO ) and subsequently methane ( CH4). To improve the selectivity and long term longevity of the MOFs, a custom-designed flow cell electrolyzer was employed to decrease mechanical stress. In addition, a bipolar membrane (BPM) was used in the flow cell to modulate local pH to mitigate chemical degradation on the catalyst. Finally, pulsed electrolysis was tested and used to improve long term longevity of both the working and counter electrode ultimately determined to be a crucial aspect of prolonged electrolysis at industrial relevant current densities and voltages. To minimize total cell voltages, an efficient bi-functional nickel-iron ( NiFe) nano catalyst was developed and demonstrated lower overpotentials for the oxygen evolution reaction (OER) and significantly lower input costs compared to traditionally used platinum group metals. In addition to improving overall cell architecture, significant mechanistic studies were done to understand catalyst activity at relevant voltages. In situ Raman spectroscopy was performed to determine active sites, reaction intermediates, and material oxidation states during electrolysis. Further, Raman mapping was done on the catalyst surface to determine before and after effects of reduction and oxidation voltage pulses. In-line nuclear magnetic resonance (NMR) was used to determine real time production of liquid products and degradation mechanisms of the catalyst. Finally, Mott-Schottky measurements were done on the MOFs to determine the tuning of the flat-band potential (Vf b) with the addition of secondary metal nodes. To determine market viability of CO2RR, thorough techno-economic analyses (TEA) were done. Theoretical studies to direct industrial products (such as CH4) were done in addition to real analyses of experimental results. CO2RR to HCOO– , in particular, experimental results indicate a particular good market fit when scaled up. To more common reduction products such as CH4, more niche applications such as aerospace incorporation, may be necessary.