Transition metal dichalcogenides: nanostructuring strategies and engineering for water electrolysis

Transition metal dichalcogenides (TMDs) are a family of ubiquitous and inexpensive inorganic layered materials, with an ever-growing set of physicochemical properties. Namely, the edge site confined hydrogen evolution reaction (HER) electrocatalysis found in pristine TMDs is of high interest to replace the scarce precious metals currently employed in proton exchange membrane electrolysers. The core of this thesis is devoted to the maximization of the electrocatalytic activity of TMDs, here being MoS2 and WS2, towards the HER in acidic electrolytes by use of physical and electrochemical techniques. Activation of the electrochemically inert sulfur edge sites in MoS2 was undertaken by preparation of Ni-MoS2 hybrid nanoclusters using a dual-target magnetron sputtering and gas condensation technique. The HER enhancement observed is limited by the sulfur-deficient inherent nature of the size-selected MoS2 nanoclusters, which hampers their crystallinity and electrochemical stability, amended here by a post-sulfidation treatment consisting of sulfur evaporation and annealing. Edge site exposure is alternatively explored for crystalline TMD flakes by fabrication of nanopillar/nanocone array structures, investigating their morphology-dependent mass transport properties and chalcogen-dependent HER catalysis. Insight on the pH-dependent HER activity and stability of electrodeposited amorphous molybdenum sulfide is thoroughly investigated, proposing the moieties responsible for the observed HER catalysis. Lastly, the potential of tungsten sulfide decoration to mitigate iridium corrosion under acidic oxygen evolution reaction conditions is evaluated

Phase- and Surface Composition-Dependent Electrochemical Stability of Ir-Ru Nanoparticles during Oxygen Evolution Reaction

The increasing scarcity of iridium (Ir) and its rutile-type oxide (IrO$_{2}$), the current state-of-the-art oxygen evolution reaction (OER) catalysts, is driving the transition toward the use of mixed Ir oxides with a highly active yet inexpensive metal (Ir$_{x}$M$_{1-x}$O$_{2}$). Ruthenium (Ru) has been commonly employed due to its high OER activity although its electrochemical stability in Ir-Ru mixed oxide nanoparticles (Ir$_{x}$Ru$_{1-x}$O$_{2}$ NPs), especially at high relative contents, is rarely evaluated for long-term application as water electrolyzers. In this work, we bridge the knowledge gap by performing a thorough study on the composition- and phase-dependent stability of well-defined Ir$_{x}$Ru$_{1-x}$O$_{2}$ NPs prepared by flame spray pyrolysis under dynamic operating conditions. As-prepared NPs (Ir$_{x}$Ru$_{1-x}$O$_{y}$) present an amorphous coral-like structure with a hydrous Ir-Ru oxide phase, which upon post-synthetic thermal treatment fully converts to a rutile-type structure followed by a selective Ir enrichment at the NP topmost surface. It was demonstrated that Ir incorporation into a RuO$_{2}$ matrix drastically reduced Ru dissolution by ca. 10-fold at the expense of worsening Ir inherent stability, regardless of the oxide phase present. Hydrous Ir$_{x}$Ru$_{1-x}$O$_{y}$ NPs, however, were shown to be 1000-fold less stable than rutile-type Ir$_{x}$Ru$_{1-x}$O$_{2}$, where the severe Ru leaching yielded a fast convergence toward the activity of monometallic hydrous IrO$_{y}$. For rutile-type Ir$_{x}$Ru$_{1-x}$O$_{2}$, the sequential start-up/shut-down OER protocol employed revealed a steady-state dissolution for both Ir and Ru, as well as the key role of surface Ru species in OER activity: minimal Ru surface losses (<1 at. %) yielded OER activities for tested Ir$_{0.2}$Ru$_{0.8}$O2 equivalent to those of untested Ir$_{0.8}$Ru$_{0.2}$O2. Ir enrichment at the NP topmost surface, which mitigates selective subsurface Ru dissolution, is identified as the origin of the NP stabilization. These results suggest Ru-rich Ir$_{x}$Ru$_{1-x}$O$_{2}$ NPs to be viable electrocatalysts for long-term water electrolysis, with significant repercussions in cost reduction

Microkinetic Analysis of the Oxygen Evolution Performance at Different Stages of Iridium Oxide Degradation

The microkinetics of the electrocatalytic oxygen evolution reaction substantially determines the performance in proton-exchange membrane water electrolysis. State-of-the-art nanoparticulated rutile IrO$_{2}$ electrocatalysts present an excellent trade-off between activity and stability due to the efficient formation of intermediate surface species. To reveal and analyze the interaction of individual surface processes, a detailed dynamic microkinetic model approach is established and validated using cyclic voltammetry. We show that the interaction of three different processes, which are the adsorption of water, one potential-driven deprotonation step, and the detachment of oxygen, limits the overall reaction turnover. During the reaction, the active IrO$_{2}$ surface is covered mainly by *O, *OOH, and *OO adsorbed species with a share dependent on the applied potential and of 44, 28, and 20% at an overpotential of 350 mV, respectively. In contrast to state-of-the-art calculations of ideal catalyst surfaces, this novel model-based methodology allows for experimental identification of the microkinetics as well as thermodynamic energy values of real pristine and degraded nanoparticles. We show that the loss in electrocatalytic activity during degradation is correlated to an increase in the activation energy of deprotonation processes, whereas reaction energies were marginally affected. As the effect of electrolyte-related parameters does not cause such a decrease, the model-based analysis demonstrates that material changes trigger the performance loss. These insights into the degradation of IrO$_{2}$ and its effect on the surface processes provide the basis for a deeper understanding of degrading active sites for the optimization of the oxygen evolution performance

Electrochemical sulfidation of WS2 nanoarrays:strong dependence of hydrogen evolution activity on transition metal sulfide surface composition

The activity of transition metal sulfides for the hydrogen evolution reaction (HER) can be increased by sulfur-enrichment of active metal-sulfide sites. In this report, we investigate the electrochemical sulfidation of atmospherically aged WS2 nanoarrays with respect to enhancing HER activity. In contrast to MoS2, it is found that sulfidation diminishes HER activity. Electrochemical and XPS experiments suggest the involvement of insoluble tungsten oxides in the altered HER and electron transfer properties. This demonstrates the strong dependence of the transition metal dichalcogenide (TMD) composition with the successful sulfur incorporation and subsequent HER activity

Heating up the OER: Investigation of IrO 2 OER Catalysts as Function of Potential and Temperature**

Abstract Despite intensive investigations for unravelling the water splitting reaction, the catalyst behavior during the oxygen evolution reaction (OER) is still not fully understood. This is especially true under more demanding conditions like high potentials and high temperatures. Rotating disk electrode measurements show a gradual increase of OER current when increasing the temperature up to 80 °C. However, strong bubble formation at elevated temperatures makes in‐situ characterization of the catalyst challenging. Here we utilize an in‐situ electrochemical and heated flow cell, which aims at an efficient removal of bubbles from the catalyst surface and enables structural studies by X‐ray absorption spectroscopy (XAS) at temperatures up to 80 °C. Changes in the Ir L3‐edge X‐ray absorption near edge spectra (XANES) were observed with respect to the white line position and principal components related to structural changes were extracted. At temperatures of 60 °C and above, the white line position of XANES spectra reaches a steady state, which is possibly caused by an equilibrium of different Ir oxidation states. These findings provide first spectroscopic insights in the behavior of OER catalysts at elevated temperatures which are typical for industrial applications and rarely addressed until now

Increased Ir–Ir Interaction in Iridium Oxide during the Oxygen Evolution Reaction at High Potentials Probed by Operando Spectroscopy

The structure of IrO$_{2}$ during the oxygen evolution reaction (OER) was studied by operando X-ray absorption spectroscopy (XAS) at the Ir L$_{3}$-edge to gain insight into the processes that occur during the electrocatalytic reaction at the anode during water electrolysis. For this purpose, calcined and uncalcined IrO$_{2}$ nanoparticles were tested in an operando spectroelectrochemical cell. In situ XAS under different applied potentials uncovered strong structural changes when changing the potential. Modulation excitation spectroscopy combined with XAS enhanced the information on the dynamic changes significantly. Principal component analysis (PCA) of the resulting spectra as well as FEFF9 calculations uncovered that both the Ir L$_{3}$-edge energy and the white line intensity changed due to the formation of oxygen vacancies and lower oxidation state of iridium at higher potentials, respectively. The deconvoluted spectra and their components lead to two different OER modes. It was observed that at higher OER potentials, the well-known OER mechanisms need to be modified, which is also associated with the stabilization of the catalyst, as confirmed by in situ inductively coupled plasma mass spectrometry (ICP-MS). At these elevated OER potentials above 1.5 V, stronger Ir–Ir interactions were observed. They were more dominant in the calcined IrO$_{2}$ samples than in the uncalcined ones. The stronger Ir–Ir interaction upon vacancy formation is also supported by theoretical studies. We propose that this may be a crucial factor in the increased dissolution stability of the IrO$_{2}$ catalyst after calcination. The results presented here provide additional insights into the OER in acid media and demonstrate a powerful technique for quantifying the differences in mechanisms on different OER electrocatalysts. Furthermore, insights into the OER at a fundamental level are provided, which will contribute to further understanding of the reaction mechanisms in water electrolysis

On the limitations in assessing stability of oxygen evolution catalysts using aqueous model electrochemical cells

Recent research indicates a severe discrepancy between oxygen evolution reaction catalysts dissolution in aqueous model systems and membrane electrode assemblies. This questions the relevance of the widespread aqueous testing for real world application. In this study, we aim to determine the processes responsible for the dissolution discrepancy. Experimental parameters known to diverge in both systems are individually tested for their influence on dissolution of an Ir-based catalyst. Ir dissolution is studied in an aqueous model system, a scanning flow cell coupled to an inductively coupled plasma mass spectrometer. Real dissolution rates of the Ir OER catalyst in membrane electrode assemblies are measured with a specifically developed, dedicated setup. Overestimated acidity in the anode catalyst layer and stabilization over time in real devices are proposed as main contributors to the dissolution discrepancy. The results shown here lead to clear guidelines for anode electrocatalyst testing parameters to resemble realistic electrolyzer operating conditions

Monitorización del balance redox durante el último tercio de la gestación en vacas de aptitud cárnica suplementadas con hidroxitirosol

Proyecto financiado por MCIN/ΑΕΙ/10.13039/501100011033 (FETALNUT). Contrato predoctoral de N. Escalera-Moreno de la Universitat de Lleida