Electrocatalysis for Energy Conversion and Electrosynthesis - In-situ Studies on Model Systems

This thesis explores the energy conversion and electrosynthesis using in-situ analytical methods. To achieve this, we used techniques such as called electrochemical infrared reflection absorption spectroscopy (EC-IRRAS) and cyclic voltammetry (CV). An ultraclean environment was ensured during all measurements. Starting with the simplest surfaces – single crystal electrodes – we first studied the influence of the IL 1-ethyl-3-methylimidazolium trifluromethanesulfonate ([C2C1Im][OTf]) on the selectivity control and suppression of undesired side reactions for the electrooxidation of 2,3-butanediol on Pt(hkl) single crystal electrodes. This reaction is a surface structure sensitive reaction. On Pt(111) and Pt(110), the main product is diacetyl, whereas on Pt(100), acetoin formation dominates. Besides, C-C bond cleavage takes places only on Pt(100) due to the ensemble effect, producing poisoning species COads. In the presence of the IL (1 mM [C2C1Im][OTf]), although the overall conversion of 2,3-butanediol is decreased, the selectivity for the intermediate acetoin is increased from ~ 50% to 80% on Pt(100). More importantly, the C-C bond cleavage – and consequently CO formation – is completely suppressed at higher IL concentrations (10 mM), avoiding CO poisoning. We attribute these effects mainly to the potential dependent and reversible adsorption of the anion [OTf]-, along with additional interionic interactions. The active surface sites are blocked by the adsorbed [OTf]- anions, resulting in a decrease in activity. Besides, surface active sites responsible for the formation of OH are also blocked in the potential range of the 2nd oxidation step forming diacetyl. This results in an increase in the selectivity towards acetoin. Finally, the ensembles of active Pt(100) surface sites needed for the C-C bond cleavage are also blocked by the anion, which results in the suppression of CO formation. In the next step, we studied the influence of the IL 1-ethyl-3-methylimidazolium dicyanamide ([C2C1Im][DCA]) on the stability of the Au(111) single crystal electrode, using various in-situ methods, including CV, EC-IRRAS, online inductively coupled plasma mass spectrometry (ICP-MS), and electrochemical scanning tunneling microscopy (EC-STM). The IL [C2C1Im][DCA] adsorbs specifically and reversibly on the Au(111) surface. We identified two adsorbed species: the weakly and strongly adsorbed anions. The adsorption of the former takes place within seconds, while the adsorption of the latter takes place in the range of minutes. We tentatively attribute the slow adsorption/desorption kinetics to a reorientation process. The IL [C2C1Im][DCA] has drastic effects on the stability of Au. Already, in the presence of low concentrations of IL (10 mM), Au dissolution is enhanced by more than 40 times compared to the situation in the absence of the IL. While the cathodic transient dissolution of Au is dominant in the absence of the IL, the anodic transient dissolution becomes the main channel in the presence of the IL. In the absence of the IL, small pits and islands with monoatomic heights are formed during potential cycles. However, we observe a higher number of larger and deeper holes with more cycles in the presence of the IL. We attribute these observations to the complexation of Au ions with the IL, which suppresses the redeposition of dissolved Au. In the next step, we increased the surface complexity by alloying Pt with Ru under ultra-high vacuum (UHV) conditions using a surface science approach. We studied the electrooxidation of isopropanol on Pt-Ru electrocatalysts. The structure and composition of this model surface were investigated by synchrotron radiation photoelectron spectroscopy (SRPES) (Alexander Simanenko from our group) and STM (Jan Skvara from the group of Dr. Josef Myslivecek). The activity and selectivity were investigated by CV and EC-IRRAS. The original alloy surface shows a high overpotential (CV peak at 0.75 VRHE) for the reaction. However, we created Pt aggregates on the Ru(0001) surface by a dealloying procedure (i.e., cycling potential between 0.0 and 1.5 VRHE), which are active sites with low overpotential (CV peak at 0.15 VRHE) for the oxidation, drastically improving the final activity of the catalyst. We then increased the system complexity even further to complex powder electrocatalysts, i.e., trimetallic PtNiMo/C for ORR. Bilal Danisman from the group of Prof. Dr. Bastian Etzold synthesized the catalyst and determined its activity. The activity of the PtNiMo/C catalyst increases initially within ten preconditioning cycles (between 0.05 and 1.2 VRHE) before decreasing. We first studied the activity changes during the initial pretreatment (23 cycles). We used CO as the probe molecule to investigate the adsorption sites by CV and EC-IRRAS. In CVs, we identify adsorbed CO on different adsorption sites: Pt sites near NiOx (ca. 0.47 VRHE), Pt facet sites (ca. 0.73 VRHE), and small Pt clusters (ca. 0.80 VRHE). In EC-IRRA spectra, we identify CO adsorbed on Pt in two positions: on-top bonded CO (COt) and bridge bonded CO (COb). The former includes bands at 2062, 2027 and 2007 cm-1, which are assigned to CO adsorbed on Pt facets, small Pt clusters, and Pt atoms near alloying metals, respectively. The COb band is only visible at 1843 cm-1, indicating the formation of larger Pt facets. Together with the dissolution studies, we attribute the activity increase to the formation of small size Pt clusters. However, to reach a steady state, more preconditioning cycles are required for PtNiMo/C catalyst. In addition, different conditioning atmospheres (inert gas and O2) and scan rates (20 and 500 mV s-1) are also important factors. Therefore, in a follow-up work, we studied these effects. For the samples pre-treated in Ar and O2 using a slow scan rate (20 mV s-1) after 150 cycles, i.e., C2-150-Ar and C2-150-O2, we identify adsorbed CO on different adsorption sites from CVs: Pt sites near NiOx (ca. 0.48 VRHE), Pt facet sites (ca. 0.72 VRHE), and small Pt clusters (0.78 VRHE). The highest number of small clusters are formed for the sample treated in Ar with slow scan rate and 150 cycles. For the sample pre-treated in Ar using a fast scan rate (500 mV s-1) and extended cycles (800), i.e., R-800-Ar, we identify only two adsorbed CO species: on Pt sites near NiOx (ca. 0.48 VRHE) and on Pt facet sites (ca. 0.72 VRHE). No small Pt clusters were detected. EC-IRRA spectra of these samples show distinct differences. For R-800-Ar, we identify adsorbed COt on Pt from only two contributions: Pt facets (2055 cm-1) and Pt atoms near alloying metals (2007 cm-1). For both C2-150-Ar and C2-150-O2, we observe adsorbed COt on Pt, including at least three contributions at 2055, 2039 and 2007 cm-1, which are assigned to CO adsorbed on Pt facets, a combination of low-coordinated Pt sites and small Pt clusters, and Pt atoms near alloying metals, respectively. However, we identified adsorbed COb on Pt at 1843 cm-1 only in the case of C2-150-Ar. This indicates that larger Pt particles form during this conditioning, which we attribute to Ostwald ripening and/or coalescence. In the last step, we studied the influence of the ILs, especially 1-butyl-3-methylimidazolium bis(perfluoroethylsulfonyl)imide [BMIM][BETI] and 1-(6-methylheptane)-3-methylimidazolium bis(perfluoroethylsulfonyl)imide [C8fC1][BETI], on the activity dynamic of the trimetallic PtNiMo/C for ORR during preconditioning. PtNiMo/C-[BMIM][BETI] shows similar activity development to the pristine sample. We observe by EC-IRRAS that the less hydrophobic IL [BMIM][BETI] is leaching out and dissolving into the electrolyte solution during conditioning. We then performed CO stripping by CV and EC-IRRAS after 150 cycles in Ar. In the case of PtNiMo/C-[C8fC1][BETI], a higher number of small Pt clusters with relatively smaller sizes are formed, being responsible for the higher activity observed. On the other hand, larger Pt particles are formed in the case of PtNiMo/C-[BMIM][BETI], allowing CO to adsorb on Pt in the bridge-bonded position. We demonstrate that in-situ EC-IRRAS play an important role in monitoring activity dynamic of complex powder electrocatalysts and in identification of adsorption sites.