CO2 electroreduction on model catalyst surfaces

Abstract

The ongoing rise in global temperature and resulting change in climate strongly motivates discontinuing the combustion of fossil fuels as a source of energy. In place of fossil fuels, we need to transition to renewable energy sources such as solar, wind and hydro. Renewable energy sources are, however, intermittent in nature, necessitating the development and use of energy storage technologies. Electrochemical conversion of carbon dioxide into fuels and chemicals is one such technology and is the focus of this thesis. Chapter 1 is a general motivation for the use of electrochemical CO2 reduction. Chapter 2 is an overview of the CO2 and CO electroreduction field with a focus on Cu and ends with listing areas in need of further investigation, motivating the work presented in the remainder of the thesis; i) a benchmark for CO reduction on polycrystalline Cu, so as to better understand the activity of nanostructured Cu catalysts, ii) the presence (if any) of oxidised phases on polycrystalline Cu during CO reduction conditions and surface reconstruction during reaction conditions, iii) the ability to track surface bound intermediates during CO2 and CO reduction. Chapter 3 deals with the experimental tools, methods and related theory used for the experiments described in the following chapters. Chapter 4 deals with measuring CO reduction on planar polycrystalline Cu in 0.1 M KOH. Selectivity and activity were measured at -0.4, -0.50 and -0.59 V vs RHEusingacombinationofGC,HS-GCand NMRforanalysis. PolycrystallineCu was found to yield more than 50 % Faradaic efficiency towards CO reduction products, including high selectivity for C2 and C3 aldehydes and alcohols and ethene. Polycrystalline Cu was found to have an intrinsic activity comparable to nanostructured Cu catalysts (Cu nanoparticles and oxide-derived Cu), when normalised to electrochemically active surface area, despite the large difference in geometric current densities. Polycrystalline and nanostructured catalysts were seen to have a different selectivity which was attributed to the different potential windows made available by the different roughness factors of the catalyst materials, increased readsorption and reduction of aldehydes in a porous structure, and the presence of undercoordinated sites. Issues with stability of activity and selectivity were also observed and were linked to Si from the glassware in alkaline conditions. Chapter 5 describes experiments using grazing incidence X-ray diffraction (GIXRD) with synchrotron radiation to observe the reduction of the native oxide on a polycrystalline Cu thin film. The reduction was performed in Ar and CO saturated 0.1 M KOH in a spectroelectrochemical flow cell whilst monitoring the Cu(111) and Cu2O(111) diffraction peaks on a 2-dimensional detector. Analysis indicated that all oxidised phases were reduced around +0.3 V vs RHE in a reductive scan. Additionally, there were indications of surface restructuring towards (100)-like facets when going to negative potentials in CO saturated electrolyte. Chapter 6 uses attenuated total reflection surface enhanced IR absorption spectroscopy (ATR-SEIRAS) to track surface bound CO and other adsorbates at conditions relevant for CO2 and CO reduction. CO reduction on Cu in 0.1 M KHCO3 from 0.2 to -1.1 V vs RHE revealed record breaking intensities for the CO peak (0.056 a.u.). The CO peak was deconvoluted into two bands which were tracked as a function of potential from the initial adsorption of CO and to the band disappears again at the most cathodic potentials. The possible origins of the two band are discussed along with variations from otherreported literature. CO was used as a probe molecule on Au to investigate the interaction of different electrolytes with the catalyst surface. Stronger competition for the surface in bicarbonate resulted in a vastly smaller potential window for adsorbed CO compared to perchlorates (NaClO4, HClO4). The combined use of ATR-SEIRAS and Pb UPD on Au revealed that the CO signal on Au is related to (211) and (110) type sites rather than the (111) facets. Chapter 7 is a general conclusion. Areas in need of continued research and procedures for improved investigation are suggested. The main results of chapters 4 and 5 have already been published as papers and these are attached in the appendix. Papers based on the content of chapter 6 are planned.