2.3.2020 Transport Phenomena – MSc thesis project (Physics / Chemical Engineering) Imaging and simulating the local pH of a CO2 electrolyzer Experimental and computational research Starting date: September 2020 Supervisor: David Vermaas ([email protected]) Daily supervisor: Lorenz Baumgartner ([email protected]) Electrochemical CO2 reduction has the potential of reducing greenhouse gas emissions and storing excess electricity from renewable energy sources in the form of useful chemical intermediates such as CO. These can be further converted to hydrocarbon fuels using established refinery processes such as Fischer-Tropsch. During the electrochemical reaction, oxygen is evolved at the anode (2 OH - → ½ O2 + H2O + 2 e - ) and CO2 is reduced to CO at the Ag cathode (2 H2O + CO2 + 2 e - → 2 OH - + CO). The undesired evolution of H2 can also take place at the cathode (2 H2O + 2 e - → H2 + 2 OH - ). [1] The pH value of the electrolyte strongly influences the selectivity for the desired product CO. However, the electrolyte effects on the reaction are difficult to study because the reaction at the cathode surface forms OH - and therefore leads to an increase in local pH. This is further complicated by electrolytes anions (e.g., HCO3 - ) acting as a pH buffer and film diffusion to the bulk depending on hydrodynamic conditions. For this reason, we are going to equip a flow cell for electrochemical CO2 reduction with fluorescence lifetime imaging microscopy (FLIM). This advanced imaging technique allows to determine the local pH environment of a fluorescent dye by measuring the fluorescent lifetime decay. [2] In addition, we are going to use COMSOL simulations to develop a physical model of the electrolyzer. The results of this study can improve the design of reactors and process conditions. Figure 1: Left: FLIM setup with electrolysis cell and Fluorescein dye. Middle: No current applied: No lifetime gradient in electrolyte. Right: Applying - 50 mA cm -2 to the cathode leads to the development of a lifetime gradient corresponding to the increase in local pH. Interdisciplinary project features: • Optimize reactor design and operation using a joint experimental and computational approach • Gain experience in Electrochemistry, FLIM imaging, Computational reactor engineering (COMSOL) • Become member of our diverse Transport Phenomena research group • Be on the cutting of electrochemical research by pioneering online FLIM imaging in flow cells • Optimize the reactor design and process conditions Background reading: [1] Kibria, M. G., et al. (2019). Advanced Materials 31(31): 1807166. van Munster, E. B. and T. W. J. Gadella (2005). Berlin, Heidelberg, Springer: 143-175. [2] de Valença, J., et al. (2018). Langmuir 34(7): 2455-2463.

Transport Phenomena – MSc thesis project (Physics ......2.3.2020 Transport Phenomena – MSc thesis project (Physics / Chemical Engineering) Imaging and simulating the local pH of

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2.3.2020

Transport Phenomena – MSc thesis project (Physics / Chemical Engineering) Imaging and simulating the local pH of a CO2 electrolyzer Experimental and computational research Starting date: September 2020 Supervisor: David Vermaas ([email protected]) Daily supervisor: Lorenz Baumgartner ([email protected])

Electrochemical CO2 reduction has the potential of reducing greenhouse gas emissions and storing excess electricity from renewable energy sources in the form of useful chemical intermediates such as CO. These can be further converted to hydrocarbon fuels using established refinery processes such as Fischer-Tropsch. During the electrochemical reaction, oxygen is evolved at the anode (2 OH- → ½ O2 + H2O + 2 e-) and CO2 is reduced to CO at the Ag cathode (2 H2O + CO2 + 2 e- → 2 OH- + CO). The undesired evolution of H2 can also take place at the cathode (2 H2O + 2 e- → H2 + 2 OH-).[1]

The pH value of the electrolyte strongly influences the selectivity for the desired product CO. However, the electrolyte effects on the reaction are difficult to study because the reaction at the cathode surface forms OH- and therefore leads to an increase in local pH. This is further complicated by electrolytes anions (e.g., HCO3-) acting as a pH buffer and film diffusion to the bulk depending on hydrodynamic conditions.

For this reason, we are going to equip a flow cell for electrochemical CO2 reduction with fluorescence lifetime imaging microscopy (FLIM). This advanced imaging technique allows to determine the local pH environment of a fluorescent dye by measuring the fluorescent lifetime decay.[2] In addition, we are going to use COMSOL simulations to develop a physical model of the electrolyzer. The results of this study can improve the design of reactors and process conditions.

Figure 1: Left: FLIM setup with electrolysis cell and Fluorescein dye. Middle: No current applied: No lifetime gradient in electrolyte. Right: Applying - 50 mA cm-2 to the cathode leads to the development of a lifetime gradient corresponding to the increase in local pH.

Interdisciplinary project features:

• Optimize reactor design and operation using a joint experimental and computational approach • Gain experience in Electrochemistry, FLIM imaging, Computational reactor engineering (COMSOL) • Become member of our diverse Transport Phenomena research group • Be on the cutting of electrochemical research by pioneering online FLIM imaging in flow cells • Optimize the reactor design and process conditions

Background reading: [1] Kibria, M. G., et al. (2019). Advanced Materials 31(31): 1807166. van Munster, E. B. and T. W. J. Gadella (2005). Berlin, Heidelberg, Springer: 143-175. [2] de Valença, J., et al. (2018). Langmuir 34(7): 2455-2463.