The Effect and Prevention of Trace Metal Contaminants on CO2 Reduction by Heterogenous Copper Catalyst

Abstract

Transforming carbon dioxide into a chemical feedstock or usable fuel has been one of the most widely studied topics in electrochemistry. Solid-state heterogeneous Cu surfaces and Cu-containing catalysts are among the most extensively studied electrochemical systems for the CO2 reduction reaction (CO2RR). However, reported electrocatalytic studies of the CO2RR by Cu catalysts under putatively identical conditions show large variances in measured product distributions. In this dissertation, I share my efforts to quantify the effects of trace metal contamination on product distributions for the CO2RR by Cu catalysts. Specifically, I show that Cu is extremely sensitive to trace metal impurities, and even sub-ppm levels of some impurities can measurably influence the product distributions. In this work, I identify the sources of trace metal contamination in typical electrochemical electrolysis experiments, quantify the amount of trace metal present under different electrolysis conditions, and demonstrate that seemingly trivial differences in experimental set up, purity of material, and material pre-treatment can have significant impacts on product distribution. In Chapter 1, the background and motivation of my research is outlined. Although my initial dissertation research focused on studying the effect of controlled mass transport to Cu surfaces to control CO2RR activity and product distribution, many of my initial attempts at reproducing experiments reported in the literature show very different product distributions than those reported. The discrepancies between my data and the literature reports became the motivation that led to my exploration of each step in the experimental procedure and material preparation of Cu CO2RR, and the associated changes in trace metal contamination as well as the effect in product distribution. In Chapter 2, I present my work showing that the reference electrode can be a source of trace metal contamination that influences CO2RR product distributions. One of the most common choices of reference electrodes, Ag/AgCl/KCl(sat.), was found to be a source of trace Ag+ contamination., to which Cu surfaces were found to be extremely sensitive. Even ppb levels of Ag+ in solution were shown to have dramatic effects on CO2RR product distribution—for instance, decreasing Faradaic efficiency for CH4 production and increasing Faradaic efficiency for CO production. We demonstrated that this problem could be mitigated by employing a different configuration of the reference electrode. In Chapter 3, other materials used in CO2RR experiments were explored as possible sources of trace metal contamination. It was observed that the ion exchange membrane and auxiliary electrode introduce trace metals into the electrochemical system. In addition, the starting purity of the foil and electrolyte and the pre-treatments of the electrolyte all influence the product distribution of CO2RR. My results suggest that the Cu system is highly complex. The change in product distribution based on differences in experimental procedure cannot be easily predicted. In Chapter 4, the major conclusions and future directions of Cu CO2RR research are addressed. The use of a protective overlayer, such as a polymer coating, is critical for future research on the Cu system. Preliminary data as well as recommendations on future experiments are also included. This dissertation work demonstrates the importance of understanding the true nature of the electrochemical system when performing CO2RR, developing carefully designed control experiments, and incorporating standardized experimental procedure across research groups for meaningful comparisons between CO2RR studies.