Electrochemical energy conversion and storage processes catalyzed by nanostructured Pd, Au materials [electronic resource]
Keywords: Chemistry.

The rise of CO2 in the atmosphere caused by extensive utilization of fossil fuels has created great demand for renewable energy resources like solar, wind, geothermal, and bio-fuel. However, these renewable energy resources are intermittent and as a consequence there is often a mismatch between supply and demand. Electrochemical energy conversion and storage processes/systems are a promising solution to this problem. Excess energy can be stored in chemicals through electrochemical reactions and the stored energy can be utilized through the reverse reactions. Electrochemical energy conversion and storage systems could also serve as decentralized power supplies and chemical plants. Despite recent progress, the wide application of such systems still faces several challenges, one of which is low overall energy efficiency. Therefore, developing new materials that catalyze the desired electrochemical reactions efficiently has been an important research area both academically and industrially. In this thesis, I will describe catalyst development studies for two important electrochemical energy conversion and storage processes: Pd-catalyzed electrochemical CO2 conversion and Au-catalyzed alkaline O2 reduction. Renewable electricity powered electrochemical CO2 conversion into value-added chemicals is a promising strategy to control and reduce the fast-increasing CO2 concentration in the atmosphere. The pair of CO2 and its reduction product also serves as a medium for the renewable energy conversion and storage. Formate/formic acid is an attractive CO2 conversion product as it is a valuable commodity product with wide applications, a H2 carrier, a promising fuel candidate for transportable fuel cells, and feedstock for chemical productions. Electrochemical CO2 conversion to formate has been studied for decades, in order to replace current fossil fuel-based industrial production process. Though various materials have been proposed as good candidates for such conversion, they still lack good performance with sufficient energy efficiency. In contrast, two previous studies show that Pd catalyzes CO2 conversion to HCO2-- at potential close to its thermodynamic potential, yet with low conversion rates, and the activity diminished when larger overpotential was applied. We chose Pd as a catalyst candidate for the HCO2-- production as it could potentially be a catalyst with high energy efficiency. In this thesis, I will show that well-dispersed Pd nanoparticles (NPs) on carbon support (Pd/C) perform as an excellent catalyst for the CO2 reduction to HCO2-- with high conversion rates and high energy efficiency. In CO2-saturated HCO3-- electrolyte, with a Pd mass loading of 50 μg cm--2, Pd/C electrodes produced HCO2-- with geometric current density up to 10 mA cm--2 with near quantitative selectivity within 200 mV of overpotential. The mass activity reached up to 200 mA HCO2-- production per mg of Pd used. Under similar conditions, other catalyst materials require several hundreds more of overpotential to achieve comparable performance. Systematic electrokinetic studies revealed that CO2 is the actual reduction substrate during electrocatalysis and provided evidence strongly supports an electrohydrogenation mechanism wherein CO2 is chemically reduced on an electrochemically generated Pd hydride surface. Such reduction mechanism avoids the kinetically sluggish initial electron transfer step, which typically as a result leads to the high overpotential required. CO poisoning is identified to be the major cause for the catalyst deactivation observed during electrocatalysis. CO comes as the minor product, binds on Pd surface and therefore inhibits HCO2-- production. Oxidative treatment such as brief air exposure can remove the CO from catalyst surface and restore the activity. In addition, particle aggregation during catalysis is identified to be a second cause for the deactivation. Evaluation of additional Pd materials suggested that high surface area to mass/volume ratio is essential to an efficient Pd catalyst as thus the transformation from Pd to Pd hydride can be finished in a short period of time and the CO2 reduction to HCO2-- becomes the dominant process thereafter. Alkaline electrochemical O2 reduction reaction (ORR) has been studied extensively because it is the primary cathode reaction in many electrochemical energy conversion and storage systems including fuel cells and metal-air batteries. Au is an exceptionally active alkaline ORR catalyst. Many studies have been performed to elucidate structure--activity relationships for Au and the Au (100) facet is believed to be the most active Au surface site. In this thesis, I will describe oxide-derived Au (ODAu) and physically deposited Au on carbon nanotubes (Au/CNT) as catalysts for ORR in alkaline medium. ODAu is a nanocrystalline material wherein the individual crystallites are linked by a continuous network of grain boundaries (GBs). When evaluated in O2-saturated 0.1 M KOH, ODAu demonstrated a high onset potential of ~1 V and an electron transfer number of 3.7~4, indicating a primary 4 e-- reduction pathway. Surface structure characterization revealed that ODAu was mostly exposed with (111) and (110) facets. Despite this faceting, ODAu exhibited comparable specific activity as Au cubes that have a high proportion of (100) facets on their surfaces and ~10-fold higher specific activity than Au octahedrons and dodecahedrons that are bound by (111) and (110) facets. Au/CNT materials are composed of discrete NPs that are readily characterized by TEM. The GB density in these NPs can be modulated by thermal annealing. ORR studies using Au/CNT materials revealed a correlation between specific activity and GB surface linear density. Together, these results suggest that defect sites such as GB surface terminations contain highly active sites for ORR. Though further studies are required to elucidate the exact structure of these active sites, the results open a new avenue for the development of highly active Au ORR catalysts that does not rely on (100) faceting.