UNIVERSITA’ DEGLI STUDI DI PAVIA DIPARTIMENTO DI CHIMICA Dottorato di Ricerca in Scienze Chimiche e Farmaceutiche Direttore: Prof. Antonella Profumo Operando spectroscopy for the study of reaction mechanisms in electrochemistry Tutore: Prof. Paolo Ghigna Tesi di dottorato di ricerca di Martina Fracchia TRIENNIO ACCADEMICO 2016-2019 (XXXII CICLO) Coordinatore: Prof. Mauro Freccero Guardavo gonfiare le gemme in primavera, luccicare la mica nel granito, le mie stesse mani, e dicevo dentro di me “Capirò anche questo, capirò tutto, ma non come loro vogliono. Troverò una scorciatoia, mi farò un grimaldello, forzerò le porte”. Era snervante, nauseante, ascoltare discorsi sul problema dell'essere e del conoscere, quando tutto intorno a noi era mistero che premeva per svelarsi: il legno vetusto dei banchi, la sfera del sole di là dai vetri e dai tetti, il volo vano dei pappi nell'aria di giugno. […] Saremmo stati chimici, Enrico ed io. Primo Levi, “Il Sistema Periodico” Abstract The progressive lack of fossil fuels together with the growing awareness of the environmental problems has led to an extensive research on renewable and sustainable energy sources. In this sense, electrochemistry covers a major role, considering that many processes for sustainable energy production are based on electrochemical reactions. While much attention has been devoted to the development of new electrode materials, a comprehensive understanding of the mechanism of electrochemical reactions is still necessary to rationalize the choice of the electrodes and to design new ones, with the final aim of reaching higher efficiencies. Operando X-Ray absorption spectroscopy (XAS) is a powerful and versatile technique which allows information regarding the electronic and structural properties of a given material, while the material is operating under conditions of reaction. In this thesis, operando XAS was carried out on electrodes involved in (photo)electrocatalytic reactions. Attention was especially paid to photosystems for the photocatalytic water splitting, where operando XAS permitted to study the generation and the fate of the photogenerated carriers (recombination/charge transfer) and to clarify the role of the overlayer in composite electrodes. While operando XAS experiments with hard X-rays ( i.e. highly energetic X-rays) are well- established, the same cannot be said for operando XAS with soft X-rays, since their low penetration depth and the severe vacuum limitation have hindered a parallel development. In this work, a pioneering experiment of operando soft-XAS at ambient pressure was performed to study the reactivity of SnO 2 towards reducing gases. Finally, dynamic multi-frequency analysis (DMFA) was employed in the field of electrocatalysis as a new strategy for the study of reaction mechanisms. This technique, which consists in acquiring dynamic impedance spectra while cycling the electrode under investigation, allowed information to be gained regarding the mechanism of the HER reaction on Pt and relevant kinetic parameters, through directly following their trend with the potential. Contents List of abbreviations ........................................................................................................................... 1 Introduction ......................................................................................................................................... 3 1. X-ray Absorption Spectroscopy (XAS) - theory and fundamentals ....................................... 6 1.1 X-ray generation: the synchrotron radiation ................................................................. 6 1.2 X-ray absorption .................................................................................................................. 8 1.3 The EXAFS region ............................................................................................................... 11 1.4 The XANES region .............................................................................................................. 16 2. In situ and operando X-ray absorption spectroscopy in electrochemistry ....................... 19 2.1 Electrochemical reactions for sustainable energy production ................................ 19 2.1.1 The hydrogen evolution reaction .................................................................................. 20 2.1.2 The oxygen evolution reaction ...................................................................................... 23 2.1.3 Photocatalytic water splitting ....................................................................................... 26 2.2 In situ and operando XAS in electrochemistry ............................................................. 27 2.2.1 In situ and operando XAS for the investigation of Pt-based electrodes ........................ 29 2.2.2 In situ and operando XAS for the investigation of catalysts for the oxygen evolution reaction 30 2.2.3 New perspectives .......................................................................................................... 32 3. Investigating reaction mechanisms in electrochemistry: methods and strategies ......... 34 3.1 In situ and operando XAS in (photo)electrocatalysis ................................................. 34 3.1.1 Selection of the sample ................................................................................................. 34 3.1.2 Design of the electrochemical cell ................................................................................ 35 3.1.3 Data acquisition: the operando experiment ................................................................. 37 3.1.4 Data acquisition: ∆μ (Light – Dark) method and Fixed Energy X-ray Absorption Voltammetry (FEXRAV) .................................................................................................................. 39 3.1.5 Data analysis: fit of the EXAFS and of the XANES region .............................................. 41 3.2 Towards new strategies: quasi-operando soft-XAS .................................................... 44 3.3 Towards new strategies: dynamic multi-frequency analysis ................................... 45 3.3.1 Classical electrochemical impedance spectroscopy ...................................................... 46 3.3.2 Describing electrochemical phenomena with equivalent circuits ................................ 48 3.3.3 Dynamic multi-frequency analysis ................................................................................ 50 4. Operando XAS in electrocatalysis: a study of α and γ-FeOOH in HER condition ............... 54 4.1 Theoretical background and motivation for the experiment ................................... 54 4.2 Experimental ...................................................................................................................... 56 4.2.1 Preparation of the sample ............................................................................................. 56 4.2.2 Description of the experiment ...................................................................................... 58 4.3 Results and discussion ..................................................................................................... 58 4.4 Final Remarks .................................................................................................................... 74 5. Operando XAS in photoelectrocatalysis: investigating WO 3 photoanodes ........................ 76 5.1 Theoretical background and motivation for the experiment ................................... 76 5.2 Experimental ...................................................................................................................... 78 5.2.1 Preparation of the sample ............................................................................................. 78 5.2.2 Description of the experiment ...................................................................................... 79 5.3 Results and discussion ..................................................................................................... 79 5.4 Final remarks ..................................................................................................................... 86 6. Operando XAS in photoelectrocatalysis: a study on Cu 2O and Cu xO photocathodes ....... 88 6.1 Theoretical background and motivation for the experiment ................................... 88 6.2 Experimental ...................................................................................................................... 90 6.2.1 Preparation of the samples ........................................................................................... 90 6.2.2 Description of the experiment ...................................................................................... 91 6.3 Results and discussion – Cu 2O electrodes ..................................................................... 92 6.4 Results and discussion – Cu xO electrodes ................................................................... 103 6.5 Final remarks ................................................................................................................... 115 7. Operando XAS in photoelectrocatalysis: a study of α-Fe 2O3/NiO x photoanodes ............ 117 7.1 Theoretical background and motivation for the experiment ................................. 117 7.2 Experimental .................................................................................................................... 119 7.2.1 Preparation of the sample