Electrochemical Reduction of Carbon Dioxide

Abstract

Due to rising concerns about climate change, a lot of research is currently underway with respect to the development of new technologies which can contribute to the decline of atmospheric CO2, and will allow further penetration of renewables into the energy mix. A promising technology which is currently actively researched is the electrochemical reduction of CO2 (ERC). This technology utilizes otherwise polluting and unwanted CO2 and converts
it into value-added products under the influence of an electrical current. The process can therefore be designed as an energy storage mechanism since electrical energy is stored as chemical bonds. In this research, ERC towards formic acid has been investigated from two perspectives.

First, the feasibility of the commercial production of formic acid compared to other products of ERC was investigated. The electrochemical production of the most common reduction products have been compared based on production costs, energy storage capabilities, toxicity and manageability. Due to the relatively low energy consumption for 2-electron products, namely formic acid, carbon monoxide and oxalic acid, it is found that these products have the most promising business case. Additionally, ERC to formic acid is best studied compared to other products, and high selectivities are commonly reported. Formic acid and methanol are liquid at atmospheric conditions, which is beneficial as relatively large amounts of energy per unit volume can be stored without the need of additional compression or cooling. This will also allow for easy transportation. As hydrogen carrier, formic acid has the advantage that it can be decomposed in H2 and CO2 near room temperature. In the second part of this research, the use of numerical modeling to study the reduction of CO2 in an electrochemical cell towards formic acid/formate at elevated CO2 pressures is presented. The model investigates to impact on the cathodic half-cell of a cell designed for the reduction of CO2 in aqueous electrolyte solutions at a constant temperature of 25℃, simultaneously assuming non-limiting conditions with respect to the anodic half-cell. The modeled part of the cell has been divided in three main regions, namely the bulk, cathode surface region and the electrode surface, which are discussed separately. The bulk is assumed to be the region of equilibrated concentrations which are constant in time, as they are not dynamically influenced by any mass transport phenomena. Reactants are supplied from the bulk to the electrode surface and products are removed vice versa via the cathode surface region, which is a thin region in the vicinity of the electrode. The transfer of species within this region and the chemical reactions between the species, form a system of diffusion-reaction equations. This system is solved numerically using appropriate boundary conditions. The actual reduction of CO2 occurs on the electrode surface, and the kinetics of the electrochemical reactions towards HCOO-, CO and H2 are described using Tafel-type kinetics.

The electrochemical model has been verified and compared with experimental data, and despite various simplifications has proven to be predictive of the electrochemical reduction of CO2. It is found that the potentially beneficial effects of an elevated CO2 pressure on both the production rate and selectivity, as experimentally observed, can be reproduced with reasonable accuracy. The CO2 concentration at the electrode surface is identified as the main limiting factor for achieving both a high selectivity towards formate and a higher production rate on formate producing metals. The model shows that with an increased CO2 pressure the amount of CO2 dissolved into the solution is increased significantly, resulting in a higher concentration of CO2 at the electrode and less mass transfer limitations.