Using microscale flow cells to study the electrochemical reduction of carbon dioxide

An electrochemical process to reduce CO2 has potential to store electrical energy from renewable sources such as wind and solar in chemical form via the production of chemical fuels and feedstocks. When combined with a “green” renewable source, this technology provides a means of (a) reducing dependence on foreign oil via enhancing the penetration of renewable technologies into the transportation sector and (b) reducing CO2 emissions by moving away from fossil fuels (Chapter 1). The goal of this research is to use a microfluidic platform to study the influence of electrolytes, reactor conditions and catalysts to address the three primary challenges for the electrochemical reduction of CO2 to CO: (1) low reaction energetic efficiency; (2) low reaction rate; and (3) poor reaction selectivity. This dissertation reports an investigation of roles electrolytes, operating conditions, and catalysts can play in the reduction of CO2. The anion and cation size, along with pH, were found to drastically influence both the current density and selectivity of the product distribution (Chapter 2). Specifically, small anions and large cations were found to favor the production of CO and hinder the evolution of H2. The microfluidic reactor was used to look at the performance of a larger cation, EMIM BF4, an ionic liquid. With the ionic liquid-based electrolyte, an early cathode onset potential was observed at the expense of poor anode performance. The poor anode performance was overcome via the addition of a secondary electrolyte stream. Using the modified reactor, in the presence of an EMIM BF4 electrolyte solution, CO production was achieved at a reactor potential of only 1.5 V, which constitutes a maximum cathode overpotential of 0.17 V (Chapter 3). This low onset potential constitutes a drastic reduction in the onset potential for CO production in an arrangement that achieves excellent selectivity for CO. While the energetic efficiency in the dual electrolyte setup was drastically improved, the achieved current density was quite low because the EMIM+ cation poisoned the cathode. Because the poor anode performance was from the BF4 anion, the membrane could be eliminated by substituting a BF4 anion in the ionic liquid electrolyte solution for either a OH or Cl anion. The substitution of the BF4 anion for either a OH or Cl anion enabled drastic improvements in the partial current density for CO when using either an EMIM OH or an EMIM Cl electrolyte solution (Chapter 4). Further improvements were achieved via bolstering the conductivity with the addition of a secondary salt, KOH. Amine-based novel organometallic complexes have been developed for the electrochemical reduction of CO2 to CO (Chapter 5). Specifically, several silver-based organometallic catalysts, AgDAT, AgPc, and AgPz, have comparable or better current densities with increased selectivity for CO as compared to Ag nanoparticle-based catalysts. Furthermore, when comparing the performance relative to the silver loading, the amine-based organometallic catalysts outperform the silver catalyst by more than 20 fold. Furthermore, when using copper-based organometallic catalysts (i.e., CuDAT), the product selectivity changed. This resultant change in selectivity and current density demonstrates that organometallic complexes have potential to “tune” catalysts to produce a wide range of desired products. The electrochemical CO2 conversion reactor used in the work described here (Chapters 2 through 5) is based on a microfluidic fuel cell developed earlier in the Kenis group. Chapter 6 reports on the influence of boundary layer depletion on performance in an alkaline, air-breathing laminar flow fuel cell as a function of electrode length, electrode arrangement, and flow rates. Higher current densities were achieved when using shorter electrodes. Furthermore, alternative electrode arrangements (aspect ratios and multiple electrodes in series) enable higher fuel utilization and power output per catalyst area. Looking forward, these studies will shed insight in regards to optimal electrolyte composition and catalyst development with the aim of further improving the current density and energetic efficiency of the reaction