Light-driven hydrogen evolution assisted by covalent organic frameworks

Altres ajuts: RSCCovalent organic frameworks (COFs) are crystalline porous organic polymers built from covalent organic blocks that can be photochemically active when incorporating organic semiconducting units, such as triazine rings or diacetylene bridges. The bandgap, charge separation capacity, porosity, wettability, and chemical stability of COFs can be tuned by properly choosing their constitutive building blocks, by extension of conjugation, by adjustment of the size and crystallinity of the pores, and by synthetic post-functionalization. This review focuses on the recent uses of COFs as photoactive platforms for the hydrogen evolution reaction (HER), in which usually metal nanoparticles (NPs) or metallic compounds (generally Pt-based) act as co-catalysts. The most promising COF-based photocatalytic HER systems will be discussed, and special emphasis will be placed on rationalizing their structure and light-harvesting properties in relation to their catalytic activity and stability under turnover conditions. Finally, the aspects that need to be improved in the coming years will be discussed, such as the degree of dispersibility in water, the global photocatalytic efficiency, and the robustness and stability of the hybrid systems, putting emphasis on both the COF and the metal co-catalyst

Unraveling Charge Transfer in CoFe Prussian Blue Modified BiVO4 Photoanodes

Catalyst modification of metal oxide photoanodes can result in markedly improved water oxidation efficiency. However, the reasons for improvement are often subtle and controversial. Upon depositing a CoFe Prussian blue (CoFe-PB) water oxidation catalyst on BiVO4, a large photocurrent increase and onset potential shift (up to 0.8 V) are observed, resulting in a substantially more efficient system with high stability. To elucidate the origin of this enhancement, we used time-resolved spectroscopies to compare the dynamics of photogenerated holes in modified and unmodified BiVO4 films. Even in the absence of strong positive bias, a fast (pre-ms), largely irreversible hole transfer from BiVO4 to CoFe-PB is observed. This process retards recombination, enabling holes to accumulate in the catalyst. Holes in CoFe-PB remain reactive, oxidizing water at a similar rate to holes in pristine BiVO4. CoFe-PB therefore enhances performance by presenting a favorable interface for efficient hole transfer, combined with the catalytic function necessary to drive water oxidation

Impact of Oxygen Vacancy Occupancy on Charge Carrier Dynamics in BiVO4 Photoanodes

Oxygen vacancies are ubiquitous in metal oxides and critical to performance, yet the impact of these states upon charge carrier dynamics important for photoelectrochemical and photocatalytic applications remains contentious and poorly understood. A key challenge is the unambiguous identification of spectroscopic fingerprints which can be used to track their function. Herein, we employ five complementary techniques to modulate the electronic occupancy of states associated with oxygen vacancies in situ in BiVO4 photoanodes, allowing us to identify a spectral signature for the ionization of these states. We obtain an activation energy of ∼0.2 eV for this ionization process, with thermally activated electron detrapping from these states determining the kinetics of electron extraction, consistent with improved photoelectrochemical performance at higher temperatures. Bulk, un-ionized states, however, function as deep hole traps, with such trapped holes energetically unable to drive water oxidation. These observations help address recent controversies in the literature regarding oxygen vacancy function, providing new insights into their impact upon photoelectrochemical performance

UV-Vis operando spectroelectrochemistry for (photo)electrocatalysis: Principles and guidelines

Electrocatalytic and photoelectrocatalytic technologies are key players in the transition to a circular economy. In order to rationally design the new generation of efficient (photo)electrocatalysts it is critical to establish which performance bottlenecks must be overcome during the complex catalytic transformations. Obtaining such detailed mechanistic and kinetic knowledge requires the use of operando spectroscopic techniques that are capable of probing the system under operating conditions. Of all the methods available UV–Vis spectroelectrochemistry stands out for its versatility and experimental simplicity which enables the systematic study of samples under a wide range of reaction conditions. From a mechanistic viewpoint, this technique allows us to quantify the accumulation of reactive species at the catalyst–electrolyte interface and, through a kinetic population model, opens the door to characterising the kinetics of the rate determining step of catalysis. Here, we review the different information that can be extracted from UV–Vis spectroelectrochemistry and how it can be used to understand catalytic mechanisms in solid (photo)electrodes

Ruthenium complexes with polynucleating ligands and their capacity to catalytically oxidize water to dioxyen

Descripció del recurs: el 01 setembre 2012La tesi es presenta com a compilació de manuscrits (tots ells situats a l'Annex), tres d'ells ja publicats en revistes internacionals d'elevada qualitat científica dins els camps de la química de coordinació i la catàlisi i dos més a punt per a ser enviats a revistes de similar impacte. La redacció en anglès de tots els manuscrits presents en aquesta tesi permetrà el ràpid enviament dels corresponents treballs per a publicació. L'apartat de "Resultats i discussió" d'aquesta tesi (dividit en dos parts i situat a l'inici d'aquesta) constitueixen una reflexió general sobre els resultats assolits i àmpliament detallats en cadascún dels artícles científics annexats. Després d'una introducció a les propietats bàsiques dels compostos de ruteni i les propietats redox que els fan ideals per a catalitzar processos d'oxidació (fent especial èmfasi a l'oxidació d'aigua a oxigen molecular i les seves potencials aplicacions per a la generació d'energies netes i renovables), el primer capítol de resultats d'aquesta tesi presenta una nova família de complexos de ruteni polipiridílics. Aquesta nova família consisteix en la modificació del ligand Hbpp o de la terpiridina, per aconseguir uns complexos més robustos i per poder ser suportats sobre TiO2. S'ha evaluat la capacitat d'oxidar aigua d'aquests nous complexos, tant en fase homogènia com heterogènia, fent servir Ce(IV) com a oxidant. En tots els casos s'ha detectat una cogeneració de O2 i CO2 deguda a la degradació de la part orgànica del catalitzador. L'estudi d'aquest procés de desactivació ha permès saber que l'origen d'aquest CO2 és una reacció bimolecular entre dos catalitzadors. A més a més, en aquest mateix capítol, s'ha dut a terme l'oxidació de l'aigua per activació electroquímica, demostrant que en els casos descrits no es genera oxigen. Aquesta inactivitat, demostra que hi ha molts factors a tenir en compte alhora de heterogeneïtzar un catalitzador homogeni, com ara: El procés d'ancoratge, l'orientació dels centres actius, la naturalesa de la nova espècie formada i la posició i tipus de grup d'ancoratge. El segon capítol d'aquest manuscrit, es descriu la síntesi i la complexació d'un nou lligand, que inspirat en l'Hbpp, és capaç de dur a terme processos d'isomerització, basats en la diferent afinitat del Ru(II) i Ru(III) pel N i l'O. L'objectiu d'aquest caràcter hemilàbil és el de l'estabilització dels estats d'oxidació alts mitjançant la coordinació a través de l'oxigen. Aquest procés s'ha estudiant mitjançant diferent tècniques, posant de manifest que tenen lloc dos processos d'isomerització diferents. El tercer capítol de la memòria preparada és el conjunt de resultats i conclusions obtingudes al llarg de tot el treball. En conjunt, la tesi presentada per Laia Francàs Frocada és d'alta qualitat tant científica com formal. El text és coherent i ben organitzat i la bibliografia proposada extensa i actualitzada. Les conclusions extretes d'aquesta memòria permetran continuar el camp de treball cap a la cerca de catalitzadors actius, tant en fase homogènia com heterogènia de potencial aplicació per a la generació de cel·les de combustible per a la fotoproducció d'hidrogen.This PhD thesis in title "Ruthenium Complexes with Polynucleating Ligands and their Capacity to Catalytically Oxidize Water to Dioxgyen" is presented as a compendium of publications, three of them have already publicated and two of them are still in preparation. The thesis is divided in four chapters and the publications are placed in the annexes. The first chapter is a general introduction in the field. It is about: Photosynthesis which is the biological process by which photons from the sun are captured and the energy is stored into the energy rich carbon molecules needed to power life. Inspired by this process, artificial photosynthesis seeks to use water and sunlight to obtain H2 as a carbon free fuel. In this context the aqua polypyridilics ruthenium complexes are interesting in order to carry out the water oxidation reaction. All these concepts are developed in this chapter. The second chapter contains the objectives of the work, and the main chapter of results and discussion is divided into two parts. The first one containing the information of: The synthesis of new tetra- and octadentate Hbpp modified ligands have been described. In addition, their correspondent di- and tetranuclear complexes have been synthesized and thoroughly characterized. Their catalytic activity towards water oxidation has been tested, and the deactivation pathway has been studied. With the aim to anchor the dinuclear complex onto TiO2 the Hbpp-Bz ligand has been modificated by the addition of a carboxylic moiety. Their complexes have been synthesized, characterisized and supported onto TiO2 rutile surface. The new heterogeneized catalyst has been used to oxidize water using Ce(IV) as chemical oxidant. Complexes [RuII2(L-L)(bpp)(trpy-Pe)2]n+ (L-L= μ-Cl and n = 2; L-L= μ-Ac and n = 2, L-L = (H2O)2 and n = 3) have been prepared in order to anchor them onto FTO-TiO2 films. The properties of the homogeneous and the supported catalysts have been studied. Together with the modified electrode FTO-TiO2-[RuII2(bpp-Ra)(H2O)2(trpy)]2+, they have been tested as water oxidation catalysts by electrochemical activation. The second part of the results and discussion is about: The synthesis and complexing abilities in front of Ru of a new hemilabile bridging ligand is described. The aim of the work is to develop a new family of ruthenium complexes capable to perform linkage isomerism in order to stabilize the Ru high oxidation states. This isomerism linkage is based in the fact that Ru (II) has more affinity to the nitrogen than to the oxygen and Ru(III) the affinity change and the oxygen coordination becomes more favoured than the nitrogen one. The fourth chapter contains the summary and a general conclusions of the work

Water oxidation kinetics of nanoporous BiVO4 photoanodes functionalised with nickel/iron oxyhydroxide electrocatalysts

In this work, spectroelectrochemical techniques are employed to analyse the catalytic water oxidation performance of a series of three nickel/iron oxyhydroxide electrocatalysts deposited on FTO and BiVO4, at neutral pH. Similar electrochemical water oxidation performance is observed for each of the FeOOH, Ni(Fe)OOH and FeOOHNiOOH electrocatalysts studied, which is found to result from a balance between degree of charge accumulation and rate of water oxidation. Once added onto BiVO4 photoanodes, a large enhancement in the water oxidation photoelectrochemical performance is observed in comparison to the un-modified BiVO4. To understand the origin of this enhancement, the films were evaluated through time-resolved optical spectroscopic techniques, allowing comparisons between electrochemical and photoelectrochemical water oxidation. For all three catalysts, fast hole transfer from BiVO4 to the catalyst is observed in the transient absorption data. Using operando photoinduced absorption measurements, we find that water oxidation is driven by oxidised states within the catalyst layer, following hole transfer from BiVO4. This charge transfer is correlated with a suppression of recombination losses which result in remarkably enhanced water oxidation performance relative to un-modified BiVO4. Moreover, despite similar electrocatalytic behaviour of all three electrocatalysts, we show that variations in water oxidation performance observed among the BiVO4/MOOH photoanodes stem from differences in photoelectrochemical and electrochemical charge accumulation in the catalyst layers. Under illumination, the amount of accumulated charge in the catalyst is driven by the injection of photogenerated holes from BiVO4, which is further affected by the recombination loss at the BiVO4/MOOH interface, and thus leads to deviations from their behaviour as standalone electrocatalysts

Separating bulk and surface processes in NiOx electrocatalysts for water oxidation

Nickel oxide-based catalysts currently represent the state of the art in electrochemical water oxidation in alkaline pH. However, much of their functionality remains poorly understood, particularly regarding catalytically active sites and mechanism. Herein, we conduct a thickness dependent study of sputter deposited NiOx films by electrochemical impedance spectroscopy and spectroelectrochemistry in order to differentiate bulk oxidation from catalytic activation. We find that while catalytic activation occurs throughout the film bulk, only the upper ≤5 nm of these films are able to participate in the water oxidation reaction, a result that may be critical in the design of next generation co-catalysts to maximise performance and minimise light absorption losses