Spectroelectrochemical studies of multi-redox catalysts for water splitting

Time-resolved absorption spectroscopy and electrochemistry have been applied to the investigation of catalyst materials that facilitate water splitting for the conversion of water and renewable energy into hydrogen and oxygen. The goal of this thesis has been to better understand structure-property relationships in different water splitting catalysts by identifying multi-redox states and establishing their role in the catalytic mechanism and the catalytic activity. Chapters 1-2 investigate photocatalytic electrodes based on molecular organometallic catalysts for water reduction and carbon dioxide reduction immobilised on mesoporous TiO2. This research seeks to parametrise the interplay between charge accumulation, recombination, and catalytic reaction pathways, and their impact on the catalytic efficiency of photocatalytic systems. Chapter 1 focuses on investigating the charge transfer and accumulation processes from the TiO2 to the catalyst in acetonitrile. Chapter 2 studies the activity of H2-production systems in the presence of water to monitor protonated multi-reduced catalytic intermediates. It has been possible to minimise recombination and to optimise the catalytic production by regulating the applied potential, the excitation light intensity, and the photoelectrode surface coverage. Chapters 4-5 focus on iridium-based water oxidation electrocatalysts and aim at measuring their activity per iridium atom or state. In Chapter 4, a mathematical method is developed to deconvolve the absorption coefficient, the potential-dependent concentration and the water-oxidation kinetics of different redox states in electrodeposited hydrous iridium oxide IrOx under different conditions. Chapter 5 compares the results of IrOx with different iridium content to those of a molecular dimeric iridium complex immobilised on mesoporous ITO. Active redox states are identified under different applied potentials, and their kinetics are measured. Their mechanism and the role of the coordination sphere and chemical surrounding is discussed. Finally, Chapter 7 explores the photoreducing properties of an iridium-based molecular photocatalyst by measuring the reaction kinetics of the photoexcited reduced state.Open Acces

Designing Materials Acceleration Platforms for Heterogeneous CO2 Photo(thermal)catalysis

Materials acceleration platforms (MAPs) combine automation and artificial intelligence to accelerate the discovery of molecules and materials. They have potential to play a role in addressing complex societal problems such as climate change. Solar chemicals and fuels generation via heterogeneous CO2 photo(thermal)catalysis is a relatively unexplored process that holds potential for contributing towards an environmentally and economically sustainable future, and therefore a very promising application for MAP science and engineering. Here, we present a brief overview of how design and innovation in heterogeneous CO2 photo(thermal)catalysis, from materials discovery to engineering and scale-up, could benefit from MAPs. We discuss relevant design and performance descriptors and the level of automation of state-of-the-art experimental techniques, and we review examples of artificial intelligence in data analysis. Based on these precedents, we finally propose a MAP outline for autonomous and accelerated discoveries in the emerging field of solar chemicals and fuels sourced from CO2 photo(thermal)catalysis

Insights from transient absorption spectroscopy into electron dynamics along the Ga-gradient in Cu(In,Ga)Se2 solar cells

Cu(In,Ga)Se2 solar cells have markedly increased their efficiency over the last decades currently reaching a record power conversion efficiency of 23.3%. Key aspects to this efficiency progress are the engineered bandgap gradient profile across the absorber depth, along with controlled incorporation of alkali atoms via post-deposition treatments. Whereas the impact of these treatments on the carrier lifetime has been extensively studied in ungraded Cu(In,Ga)Se2 films, the role of the Ga-gradient on carrier mobility has been less explored. Here, transient absorption spectroscopy (TAS) is utilized to investigate the impact of the Ga-gradient profile on charge carrier dynamics. Minority carriers excited in large Cu(In,Ga)Se2 grains with a [Ga]/([Ga]+[In]) ratio between 0.2–0.5 are found to drift-diffuse across ≈1/3 of the absorber layer to the engineered bandgap minimum within 2 ns, which corresponds to a mobility range of 8.7–58.9 cm2 V−1 s−1. In addition, the recombination times strongly depend on the Ga-content, ranging from 19.1 ns in the energy minimum to 85 ps in the high Ga-content region near the Mo-back contact. An analytical model, as well as drift-diffusion numerical simulations, fully decouple carrier transport and recombination behaviour in this complex composition-graded absorber structure, demonstrating the potential of TAS.Y.-H.C. Chang thanks the Ministry of Education of Taiwan for her Ph.D. scholarship, and Dr. Michael Sachs for fruitful discussions on TA data. J.R.D. would like to thank the UKRI Global Challenge Research Fund project SUNRISE (EP/P032591/1). L.S. acknowledges funding from the European Research Council (H2020-MSCA-IF-2016, Grant No. 749231). This work received financial support from the Swiss State Secretary for Education, Research and Innovation (SERI) under contract number 17.00105 (EMPIR project HyMet). The EMPIR programme is co-financed by the Participating States and by the European Union’s Horizon 2020 research and innovation programme

Operando spectroelectrochemistry of redox state kinetics in water-oxidation IrOx electrocatalysts

Hydrous iridium oxides (IrOx) are the best oxygen evolution electrocatalysts available for operation in acidic environments. In this study, we employ time-resolved operando spectroelectrochemistry to investigate the redox states kinetics of IrOx electrocatalyst films for both water and hydrogen peroxide oxidation. Three different redox species involving Ir3+, Ir4+ and Ir4.x are identified spectroscopically and their concentrations are quantified as a function of applied potential. The generation of Ir4.x+ states is found to be the potential determining step for catalytic water oxidation, whilst H2O2 oxidation is observed to be driven by the generation of Ir4+ states. The reaction kinetics for water oxidation, determined from the optical signal decays at open circuit, accelerate from ~ 20 s to < 0.5 s with increasing applied potential above 1.3V vs. RHE (i.e. TOFs per active Ir state increasing from 0.05 to 2 s-1). In contrast, the reaction kinetics for H2O2 are found to be almost independent of the applied potential (increasing from 0.1-0.3 s-1 over a wider potential window), indicative of a first order reaction mechanism. These spectroelectrochemical data quantify the increase of both the density of active Ir4.x+ states and the TOFs of these states with applied positive potential, resulting in the observed sharp turn on of catalytic water oxidation current. We reconcile these data with the broader literature while providing a new kinetic insight into IrOx electrocatalytic reaction mechanisms, indicating a first order reaction mechanism for H2O2 oxidation driven by Ir4+ states, and a higher order reaction mechanism involving the co-operative interaction of multiple Ir4.x+ states for water oxidation

Charge accumulation kinetics in multi-redox molecular catalysts immobilised on TiO2.

Multi-redox catalysis requires the accumulation of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. Herein, we investigate charge accumulation in two model multi-redox molecular catalysts for proton and CO2 reduction attached onto mesoporous TiO2 electrodes. Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO2 to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. At high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO2 to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO2 is observed to be faster than microseconds, while electron transfer from multi-reduced species to the conduction band or the electrolyte occurs in the millisecond timescale. Finally, under light irradiation, we show how charge accumulation on the catalyst is regulated as a function of the applied bias and the excitation light intensity

Charge Accumulation Kinetics in Multi-redox Molecular Catalysts Immobilised on TiO2

Multi-redox catalysis requires the transfer of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. Herein, we investigate charge accumulation in two model multi-redox molecular catalysts for proton and CO2 reduction attached onto mesoporous TiO2 electrodes. Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO2 to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. At high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO2 to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO2 is observed to be faster than microseconds, while electron transfer from multi-reduced species to the conduction band or the electrolyte occurs in the millisecond timescale. Finally, under light irradiation, we show how charge accumulation on the catalyst is regulated as a function of the applied bias and the excitation light intensity

Spectroelectrochemistry of water oxidation kinetics in molecular versus heterogeneous oxide iridium electrocatalysts

Water oxidation is the step limiting the efficiency of electrocatalytic hydrogen production from water. Spectroelectro-chemical analyzes are employed to make a direct comparison of water oxidation reaction kinetics between a molecu-lar catalyst, the dimeric iridium catalyst [Ir2(pyalc)2(H2O)4-(µ-O)]2+ (IrMolecular, pyalc = 2-(2’pyridinyl)-2-propanolate) immobilized on a mesoporous indium tin oxide (ITO) substrate, with that of an heterogenous electrocatalyst, an amorphous hydrous iridium (IrOx) film. For both systems, four analogous redox states were detected, with the for-mation of Ir(4+)-Ir(5+) being the potential-determining step in both cases. However, the two systems exhibit distinct water oxidation reaction kinetics, with potential-independent first-order kinetics for IrMolecular contrasting with poten-tial-dependent kinetics for IrOx. This is attributed to water oxidation on the heterogenous catalyst requiring co-operative effects between neighboring oxidized Ir centers. The ability of IrMolecular to drive water oxidation without such co-operative effects is explained by the specific coordination environment around its Ir centers. These distinc-tions between molecular and heterogenous reaction kinetics are shown to explain the differences observed in their water oxidation electrocatalytic performance under different potential conditions

Spectroelectrochemistry of Water Oxidation Kinetics in Molecular versus Heterogeneous Oxide Iridium Electrocatalysts.

Funder: bp International Centre for Advanced MaterialsWater oxidation is the step limiting the efficiency of electrocatalytic hydrogen production from water. Spectroelectrochemical analyses are employed to make a direct comparison of water oxidation reaction kinetics between a molecular catalyst, the dimeric iridium catalyst [Ir2(pyalc)2(H2O)4-(μ-O)]2+ (IrMolecular, pyalc = 2-(2'pyridinyl)-2-propanolate) immobilized on a mesoporous indium tin oxide (ITO) substrate, with that of an heterogeneous electrocatalyst, an amorphous hydrous iridium (IrOx) film. For both systems, four analogous redox states were detected, with the formation of Ir(4+)-Ir(5+) being the potential-determining step in both cases. However, the two systems exhibit distinct water oxidation reaction kinetics, with potential-independent first-order kinetics for IrMolecular contrasting with potential-dependent kinetics for IrOx. This is attributed to water oxidation on the heterogeneous catalyst requiring co-operative effects between neighboring oxidized Ir centers. The ability of IrMolecular to drive water oxidation without such co-operative effects is explained by the specific coordination environment around its Ir centers. These distinctions between molecular and heterogeneous reaction kinetics are shown to explain the differences observed in their water oxidation electrocatalytic performance under different potential conditions

Delocalized, Asynchronous, Closed-Loop Discovery of Organic Laser Emitters

Contemporary materials discovery requires intricate sequences of synthesis, formulation and characterization that often span multiple locations with specialized expertise or instrumentation. To accelerate these workflows, we present a cloud-based strategy that enables delocalized and asynchronous design–make–test–analyze cycles. We showcase this approach through the exploration of molecular gain materials for organic solid-state lasers as a frontier application in molecular optoelectronics. Distributed robotic synthesis and in-line property characterization, orchestrated by a cloud-based AI experiment planner, resulted in the discovery of 21 new state-of-the-art materials. Automated gram-scale synthesis ultimately allowed for the verification of best-in-class stimulated emission in a thin-film device. Demonstrating the asynchronous integration of five laboratories across the globe, this workflow provides a blueprint for delocalizing – and democratizing – scientific discovery