Über die Stabilität von Anoden(photo-)elektrokatalysatoren in der Solar zu Wasserstoff Verbindung

The sustainable production of hydrogen for energy storage and utilization is an important cornerstone to overcome global warming, one of the biggest challenges of our time. One method to produce hydrogen is (photo-)electrochemical water splitting. It facilitates the sustainable splitting of water into hydrogen and oxygen from electricity or directly from the sunlight. While tremendous progress has been made in the last decades towards utilizing (photo-)electrochemical water splitting, both technologies are still not at the needed technology readiness level for widespread application. One of the issues that both technologies share is limited operational stability. Proton exchange membrane water electrolysis (PEMWE) the most promising technology for electrochemical water splitting, suffers from high demand of scarce iridium on the anode side, where the oxygen evolution (OER) reaction takes place. Testing in aqueous model systems (AMS) shows high degradation rates of iridium under OER conditions. However, recent reports question the applicability of these results for PEMWE. In the field of photoelectrochemistry, on the other hand, stability is still widely neglected. Stability is mostly estimated with thermodynamical data or measured with simple photoelectrochemical methods without gaining profound knowledge of the underlying degradation mechanisms. This work aims at deepening the understanding of the degradation mechanisms of anode catalysts (photo-)electrochemical water splitting. The discrepancy between iridium degradation in AMS and membrane electrode assemblies (MEA), as used in PEMWE is confirmed by comparing iridium dissolution in a scanning flow cell coupled to an inductively coupled plasma mass spectrometer (SFC-ICP-MS), an AMS, to iridium in a specifically designed (MEA). The reasons for the measured discrepancy are determined by a thorough parameter study, identifying overestimated acidity and stabilization with time in MEA as the main contributors to the discrepancy. In order to determine the stability of photoelectrocatalysts in real-time, the SFCICP- MS system is modified by introducing a light source. This modified setup is used to measure the dissolution of WO3, a seemingly stable photoanode, as proof of concept. In further studies, the dissolution stability of WO3 was determined in different electrolytes, identifying varying reaction kinetics at the photoanode surface as the origin for diverging stability between electrolytes. The application of iridium as co-catalyst to the WO3 surface, as the final study, gives insights into photoanode stability under varied catalytic conditions and the best utilization of scarce iridium.Die nachhaltige Erzeugung von Wasserstoff zur Energiespeicherung und Nutzung ist ein wichtiger Grundpfeiler um die globale Erwärmung, eine der größten Herausforderungen unserer Zeit, aufzuhalten. Eine Methode zur Erzeugung von Wasserstoff ist (photo-)elektrochemische Wasserspaltung. Diese ermöglicht die Spaltung von Wasser in Wasserstoff und Sauerstoff aus nachhaltigem elektrischem Strom, oder direkt aus dem Sonnenlicht. Obwohl in den letzten Jahrzehnten bereits große Fortschritte bezüglich der Nutzung von (photo-)elektrochemischer Wasserspaltung gemacht wurden, sind beide Technologien noch nicht auf dem nötigen Technologiereifegrad für eine flächendeckende Anwendung. Eine Schwäche beider Technologien ist ihre Betriebsstabilität. Die vielversprechendste Technologie für elektrochemische Wasserspaltung, Protonaustauschmembranwasserelektrolyse (PEMWE), bedarf großen Mengen Iridiums, eines raren Edelmetalls, auf der Anodenseite, an der die Sauerstoffentwicklungsreaktion (OER) stattfindet. Untersuchungen in wässrigen Modellsystemen (AMS) zeigen hierbei hohe Iridiumdegradationsraten unter OER-Bedingungen. Allerdings stellen kürzlich veröffentlichte Resultate die Anwendbarkeit auf PEMWE Systeme in Frage. In der photoelektrochemischen Forschung wird die Betriebsstabilität hingegen immer noch weitgehend ignoriert. Stabilität wird hier meistens thermodynamisch abgeschätzt oder mit einfachen photoelektrochemischen Methoden, die keinen tieferen Einblick in die zugrundeliegenden Degradationsmechanismen gewähren, ermittelt. Diese Arbeit zielt darauf ab, das Verständnis über die Degradationsmechanismen von Anodenkatalysatoren in der (photo-)elektrochemischen Wasserspaltung zu erweitern. Die Diskrepanz in der Degradation von iridiumbasierten Katalysatoren in AMS sowie Membranelektrodeneinheiten (MEA), wie in der PEMWE verwendet, wird durch den Vergleich der Iridiumauflösung in einer Rasterflusszelle gekoppelt an ein induktiv gekoppeltes Plasmamassenspektrometer (SFC-ICP-MS) und einer speziell entwickelten MEA bestätigt. In einer detaillierten Parameterstudie werden eine überschätzte Azidität sowie eine Stabilisierung über die Betriebsdauer in der MEA als Hauptverursacher für die Diskrepanz ermittelt. Um auch die Auflösungsraten von Photoelektrokatalysatoren in Echtzeit bestimmen zu können, wird das SFC-ICP-MS System durch Einbauen einer Lichtquelle modifiziert. Der erweiterte Teststand ermöglicht es die Auflösungsraten vonWO3, einem mutmaßlich stabilen Photoanodenmaterial, alsMachbarkeitsnachweis zu bestimmen. In weiteren Studien wird die Auflösungsstabilität von WO3 in verschiedenen Elektrolyten bestimmt. Hierbei zeigt sich, dass verschiedene Reaktionskinetiken auf der Oberfläche der Photoelektrode ursächlich für divergierene Stablität zwischen den Elektrolyten sind. Die finale Studie, in der Iridium als Co-Katalysator auf der WO3 Oberfläche aufgetragen wird gibt Einblicke in die Stabilität von Photoanoden unter veränderten katalytischen Bedingungen und die bestmögliche Nutzung von Iridium

Calcium binding to a disordered domain of a type III-secreted protein from a coral pathogen promotes secondary structure formation and catalytic activity

Strains of the Gram-negative bacterium Vibrio coralliilyticus cause the bleaching of corals due to decomposition of symbiotic microalgae. The V. coralliilyticus strain ATCC BAA-450 (Vc450) encodes a type III secretion system (T3SS). The gene cluster also encodes a protein (locus tag VIC_001052) with sequence homology to the T3SS-secreted nodulation proteins NopE1 and NopE2 of Bradyrhizobium japonicum (USDA110). VIC_001052 has been shown to undergo auto-cleavage in the presence of Ca2+ similar to the NopE proteins. We have studied the hitherto unknown secondary structure, Ca2+-binding affinity and stoichiometry of the "metal ion-inducible autocleavage" (MIIA) domain of VIC_001052 which does not possess a classical Ca2+-binding motif. CD and fluorescence spectroscopy revealed that the MIIA domain is largely intrinsically disordered. Binding of Ca2+ and other di- and trivalent cations induced secondary structure and hydrophobic packing after partial neutralization of the highly negatively charged MIIA domain. Mass spectrometry and isothermal titration calorimetry showed two Ca2+-binding sites which promote structure formation with a total binding enthalpy of -110 kJ mol(-1) at a low micromolar K-d. Putative binding motifs were identified by sequence similarity to EF-hand domains and their structure analyzed by molecular dynamics simulations. The stoichiometric Ca2+-dependent induction of structure correlated with catalytic activity and may provide a "host-sensing" mechanism that is shared among pathogens that use a T3SS for efficient secretion of disordered proteins

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

Accessing In Situ Photocorrosion under Realistic Light Conditions: Photoelectrochemical Scanning Flow Cell Coupled to Online ICP-MS

High-impact photoelectrode materials for photoelectrochemical (PEC) water splitting are distinguished by synergistically attaining high photoactivity and stability at the same time. With numerous efforts toward optimizing the activity, the bigger challenge of tailoring the durability of photoelectrodes to meet industrially relevant levels remains. In situ photostability measurements hold great promise in understanding stability-related properties. Although different flow systems coupled to light-emitting diodes were introduced recently to measure time-resolved photocorrosion, none of the measurements were performed under realistic light conditions. In this paper, a photoelectrochemical scanning flow cell connected to an inductively coupled plasma mass spectrometer (PEC-ICP-MS) and equipped with a solar simulator, Air Mass 1.5 G filter, and monochromator was developed. The established system is capable of independently assessing basic PEC metrics, such as photopotential, photocurrent, incident photon to current efficiency (IPCE), and band gap in a high-throughput manner as well as the in situ photocorrosion behavior of photoelectrodes under standardized and realistic light conditions by coupling it to an ICP-MS. Polycrystalline platinum and tungsten trioxide (WO3) were used as model systems to demonstrate the operation under dark and light conditions, respectively. Photocorrosion measurements conducted with the present PEC-ICP-MS setup revealed that WO3 starts dissolving at 0.8 VRHE with the dissolution rate rapidly increasing past 1.2 VRHE, coinciding with the onset of the saturation photocurrent. The most detrimental damage to the photoelectrode is caused when subjecting it to a prolonged high potential hold, e.g., at 1.5 VRHE. By using standardized illumination conditions such as Air Mass 1.5 Global under 1 Sun, the obtained dissolution characteristics are translatable to actual devices under realistic light conditions. The gained insights can then be utilized to advance synthesis and design approaches of novel PEC materials with improved photostability

Time-resolved analysis of dissolution phenomena in photoelectrochemistry – A case study of WO3 photocorrosion

Photocorrosion is one of the main challenges in photoelectrochemical water splitting. Traditional methods for degradation assessment, such as chronoamperometry or electrolyte/electrode post-analysis, provide limited information on the degradation mechanisms and kinetics. To address this issue, a new setup, which is based on a light source, an electrochemical cell, and an on-line inductively coupled plasma mass spectrometer (ICP-MS), has been developed. The first results on the photocorrosion of a commercial WO3 powder on Au are demonstrated in this work. It is shown that, in the absence of light, WO3 is stable in a wide potential range. On the other hand, in the presence of light, it dissolves proportionally to the anodic photocurrent. The latter is explained by the formation of aqueous tungsten complexes with the electrolyte that are thermodynamically more stable than WO3. As can be anticipated from these initial results, the novel method will enable the characterization of a wide range of photoelectrochemical materials, and thus eventually lead to the development of long-term stable devices

Photocorrosion of WO 3 Photoanodes in Different Electrolytes

Photocorrosion of an n-type semiconductor is anticipated to be unfavorable if its decomposition potential is situated below its valence band-edge position. Tungsten trioxide (WO3) is generally considered as a stable photoanode for different photoelectrochemical (PEC) applications. Such oversimplified considerations ignore reactions with electrolytes added to the solvent. Moreover, kinetic effects are neglected. The fallacy of such approaches has been demonstrated in our previous study dealing with WO3 instability in H2SO4. In this work, in order to understand parameters influencing WO3 photocorrosion and to identify more suitable reaction environments, H2SO4, HClO4, HNO3, CH3O3SH, as electrolytes are considered. Model WO3 thin films are fabricated with a spray-coating process. Photoactivity of the samples is determined with a photoelectrochemical scanning flow cell. Photostability is measured in real time by coupling an inductively coupled plasma mass spectrometer to the scanning flow cell to determine the photoanode dissolution products. It is found that the photoactivity of the WO3 films increases as HNO3 < HClO4 ≈ H2SO4 < CH3O3SH, whereas the photostability exhibits the opposite trend. The differences observed in photocorrosion are explained considering stability of the electrolytes toward decomposition. This work demonstrates that electrolytes and their reactive intermediates clearly influence the photostability of photoelectrodes. Thus, the careful selection of the photoelectrode/electrolyte combination is of crucial importance in the design of a stable photoelectrochemical water-splitting device

Photocorrosion of n‐ and p‐Type Semiconducting Oxide‐Covered Metals: Case Studies of Anodized Titanium and Copper

Illumination aects the corrosion of oxide-covered metals via photoinitiated dissolution processes. Anodized titanium with n-typesemiconducting anatase TiO2, and anodized copper with p-type semiconducting cuprite Cu2O of thicknesses up to ca. 90 nm areprepared, and investigated under controlled convection in photoelectrochemical experiments illuminated at grazing incidence. Il-lumination with photon energies above the band gap triggers anodic photocurrents for titanium above the at band potential, andcathodic photocurrents for copper below the at band potential. On both systems, increased corrosion rates are evidenced via mea-surements of polarization curves, electrochemical impedance spectra (EIS) and concentration determination via inductively coupledplasma mass spectrometry (ICP-MS). The increase in corrosion current with illumination is slightly lower than photocurrents, andscales linearly with intensity as expected if triggered by linear absorption. Following the Gerischer model, for titanium, electron-holepairs cause oxide dissolution by hole annihilation through cation dissolution. For copper, increased corrosion rates are caused by in-creased cathodic reaction rate through photoexcitation. The maximum non-thermal increase in corrosion rate is ca. 1 μA cm2 forcopper, and ca. 7 μA cm2 for titanium, thus few μm per year and negligible for structural materials. Photocorrosion may aect lo-calized corrosion, and nanostructures, e.g. if a morphology is crucial for a functional surface, such as in catalysts

CrO x -Mediated Performance Enhancement of Ni/NiO-Mg:SrTiO 3 in Photocatalytic Water Splitting

By photodeposition of CrOx on SrTiO3-based semiconductors doped with aliovalent Mg(II) and functionalized with Ni/NiOx catalytic nanoparticles (economically significantly more viable than commonly used Rh catalysts), an increase in apparent quantum yield (AQYs) from ∼10 to 26% in overall water splitting was obtained. More importantly, deposition of CrOx also significantly enhances the stability of Ni/NiO nanoparticles in the production of hydrogen, allowing sustained operation, even in intermittent cycles of illumination. In situ elemental analysis of the water constituents during or after photocatalysis by inductively coupled plasma mass spectrometry/optical emission spectrometry shows that after CrOx deposition, dissolution of Ni ions from Ni/NiOx-Mg:SrTiO3 is significantly suppressed, in agreement with the stabilizing effect observed, when both Mg dopant and CrOx are present. State-of-the-art electron microscopy and energy-dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) analyses demonstrate that upon preparation, CrOx is photodeposited in the vicinity of several, but not all, Ni/NiOx particles. This implies the formation of a Ni–Cr mixed metal oxide, which is highly effective in water reduction. Inhomogeneities in the interfacial contact, evident from differences in contact angles between Ni/NiOx particles and the Mg:SrTiO3 semiconductor, likely affect the probability of reduction of Cr(VI) species during synthesis by photodeposition, explaining the observed inhomogeneity in the spatial CrOx distribution. Furthermore, by comparison with undoped SrTiO3, Mg-doping appears essential to provide such favorable interfacial contact and to establish the beneficial effect of CrOx. This study suggests that the performance of semiconductors can be significantly improved if inhomogeneities in interfacial contact between semiconductors and highly effective catalytic nanoparticles can be resolved by (surface) doping and improved synthesis protocols

Platinum Dissolution in Realistic Fuel Cell Catalyst Layers

Pt dissolution has already been intensively studied in aqueous model systems and many mechanistic insights were gained. Nevertheless, transfer of new knowledge to real‐world fuel cell systems is still a significant challenge. To close this gap, we present a novel in‐situ method combining a gas diffusion electrode (GDE) half‐cell with inductively coupled plasma mass spectrometry (ICP‐MS). With this setup, Pt dissolution in realistic catalyst layers and the transport of dissolved Pt species through Nafion membranes are evaluated directly. We observe that (i) specific Pt dissolution is increasing significantly with decreasing Pt loading, (ii) in comparison to experiments on aqueous model systems with flow cells, the measured dissolution in GDE experiments is considerably lower and, (iii) by adding a membrane onto the catalyst layer, Pt dissolution is reduced even further. All these phenomena are attributed to the varying mass transport conditions of dissolved Pt species, influencing re‐deposition and equilibrium potential