Technoeconomic and mechanistic insights into the electroreduction of carbon dioxide to value added chemicals
Addressing societal needs of improving the standard of living for the rising human population has placed a tremendous stress on the energy supply driving global economic growth. Historically, such increased energy demands have been satisfied by the combustion (burning) of fossil fuels such as coal, oil, and natural gas. However, the increased utilization of fossil fuels has come with a penalty: a rapid rise in the atmospheric carbon dioxide (CO2) levels, especially in the past few decades, with the daily average value crossing and staying above the 400 ppm mark in 2016 for the first time in recorded human history. These increased CO2 levels (along with other greenhouse gases) have been shown to negatively affect the earth’s surface energy balance, leading to an increase in the global mean temperature anomaly (commonly known as global warming) and deleterious climate change effects. Owing to the large scale and growing nature of excess CO2 emissions (currently 4GtC yr–1), a variety of mitigation, adaptation, and utilization approaches need to be implemented together (the stabilization wedges approach), to enable the transition of modern society towards a carbon neutral future. This dissertation focuses on one such CO2 utilization approach i.e., the renewable electricity driven electroreduction of CO2 to value-added carbon chemicals such as carbon monoxide (CO), formic acid (HCOOH), methane (CH4), methanol (CH3OH), ethylene (C2H4), and ethanol (C2H5OH). These chemicals are currently manufactured on the industrial scale using fossil fuel based methods. The renewable electricity driven electroreduction of CO2 could be a sustainable alternative to such methods. This dissertation employs a multiscale approach to investigate both the system level technoeconomic and molecular level mechanistic aspects of CO2 electroreduction. Chapter 2 of this dissertation introduces a comprehensive, yet easy to use, gross-margin model to evaluate the technoeconomic prospects of CO2 electroreduction. The model helps answer key questions such as: (i) what products are the best to produce? and (ii) what performance parameters are required to develop an economically viable process? The model shows the commercialization of CO and HCOOH to be viable in the near future. Interestingly, the model also shows that co-producing an economically less viable product (CH3OH, C2H5OH, C2H4) with a more viable product (CO, HCOOH) could be a strategy for offsetting the economic limitations on individual products. Chapter 3 of this dissertation utilizes some of the technoeconomic insights gained in Chapter 2 to develop an alternative CO2 electroreduction approach i.e., the co-electrolysis of CO2 and glycerol. Thermodynamic analysis of the conventional CO2 electroreduction approach (i.e., CO2 reduction at the cathode coupled to the oxygen evolution reaction (OER) at the anode) indicates the OER (and not the CO2 reduction) to be the energetically intense step, consuming nearly 90% of the electricity input. Hence, identifying and utilizing anode reactions with lower energy requirements than the OER could result in a radical lowering (i.e., a step change) in the electricity consumption. The results in Chapter 3 show that several alternate anode reactions can be utilized. In particular, the anodic oxidation of glycerol (waste byproduct of biodiesel production) in combination with the cathodic reduction of CO2 (co-electrolysis of CO2 and glycerol) seems promising, with the resulting system requiring 37-53% less electricity than the conventional CO2 electroreduction process with the OER at the anode, thus drastically improving the techno-economic prospects of CO2 electroreduction. Chapters 4 and 5 of this dissertation focuses on analyzing the effect of electrolytes and developing better electrocatalytic systems for the electroreduction of CO2 to CO. In Chapter 4, the effect of electrolyte concentration and the role of anions on the electroreduction of CO2 on a silver coated gas diffusion layer (GDL) electrode is studied using aqueous solutions of KOH, KCl, and KHCO3. Multiple fold improvement in the activity for CO was obtained on increasing the electrolyte concentration from 0.5 to 3.0 M with a maximum current density of 440 mA cm–2 (one of the highest values reported to date) being obtained at an energy efficiency of 42% when using 3.0 M KOH as the electrolyte. The electrolyte anions were found to play an important role in the process as well, with the onset potential of CO changing in the order OH– (–0.13 V vs. RHE) < HCO3– (–0.46 V vs. RHE) < Cl– (–0.60 V vs. RHE). In Chapter 5, sub 5-nm gold nanoparticles supported on polybenzimidazole wrapped carbon nanotubes are reported as catalysts for the electroreduction of CO2 in a GDL electrode based alkaline flow electrolyzer. An onset cell potential of just –1.50 V and an onset cathode potential of just –0.02 V vs. RHE was observed for CO production. Additionally, activity levels as high as 99 and 158 mA cm–2 were obtained at cell overpotentials of just –0.7 and –0.94 V, respectively, corresponding to energetic efficiencies of 63.8 and 49.3%. These results represent the lowest onset cell and cathode potential as well as the highest activity for CO production at high energetic efficiency reported in the literature. This electrochemical system was further used to interrogate the mechanism of CO2 electroreduction under alkaline conditions. Combinations of the onset cathode potential data, Tafel slopes, and kinetic isotope effect demonstrated the rate determining step for CO production to be the pH independent single electron transfer step instead of the commonly assumed concerted proton electron transfer step, resulting in an intrinsic lowering of the overpotentials at high pH. Overall, the studies reported in this dissertation provide both system and molecular level insights into the design of electrochemical processes, electrolytes, and catalysts for the electroreduction of CO2 at high levels of activity while minimizing the energy requirements. Such insights will help guide the design of even better CO2 electroreduction systems in the future.
CO2 Electroreduction, CO2 Utilization, Electrocatalysis, Technoeconomic analysis
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign