Modeling a Flow-Cell for Electrochemical Reduction of Carbon Dioxide to Formate

Modeling a Flow-Cell for Electrochemical Reduction of Carbon Dioxide to Formate

Gadgil, Nitish (TU Delft Mechanical, Maritime and Materials Engineering)

Haverkort, Willem (mentor)

Delft University of Technology

2018-10-29

Increasing CO2 levels in the atmosphere due to over-dependence on fossil fuels is a growing concern. Its alarming impact on global climate has prompted the United Nations (UN) to set up its sustainable goal of carbon-neutrality. Electrochemical reduction of CO2 (ERC) is one of the solutions of carbon neutral cycle, which can be driven by renewable energy sources for utilizing CO2 to produce high energy density fuels/feedstocks. In this way, CO2 becomes a carbon source for the production of renewable fuels/feedstocks. However, ERC in aqueous solutions is a severely mass-transfer limited process, mainly due to low CO2 solubility in such solutions. Flow-cells have been found to be promising in mitigating this problem as compared to H-cells, making them more suitable for the commercial realization of ERC. Moreover, high pressures and continuous flow of electrolyte have been shown to lower the mass-transfer limitations. However, their simultaneous influence on ERC in flow-cells has not been explicitly analyzed.

This work primarily focuses on ERC using tin (Sn) electrodes, in which formate is the main product of interest. During ERC in aqueous electrolytes, CO2 is known to undergo an electrode reaction as well as a homogeneous reaction. This dual action increases the modeling complexity. In this study, a simplified analytical 1D model is developed to find the conditions under which the previously mentioned CO2 homogeneous reaction can be safely neglected. It is found that CO2 can be assumed to undergo only the electrode reaction at low current densities (lower than 7 mA/cm2). However, the current densities for commercial realization are about 100- 200 mA/cm2, which therefore requires incorporating the CO2 homogeneous reactions in the model.

Therefore, a multiphysics model of a flow-cell is developed. The model demonstrates how different pressures, velocities and cell potentials influence the current densities and the faradaic efficiencies of different products as well as pH near the cathode (surface pH). At atmospheric pressure, for a constant cell potential, increasing the velocity by an order of magnitude is observed not only to increase the faradaic efficiency of formate by about 20% but it is also found to lower the surface pH. In addition, at a fixed velocity, to maintain a constant faradaic efficiency of formate at high total current densities (for instance, 100 mA/cm2) requires using higher pressures. The results from this work will help future modeling studies to simplify their models when investigating low current densities, and it will guide the experimental studies on flow cells to select appropriate operating conditions.

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Student theses

master thesis

© 2018 Nitish Gadgil