Photocorrosion of WO3 Photoanodes in Different Electrolytes

Harnessing potassium peroxymonosulfate activation of WO3/diatomite composites for efficient photocatalytic degradation of tetracycline

The increasing prevalence of pharmaceutical contaminants in aquatic ecosystems poses profound challenges for both environmental sustainability and public health. Addressing this pressing issue requires the development of innovative, cost-effective, and efficient remediation approaches. Here we report the synthesis of WO3/diatomite composites and their photocatalytic degradation in conjunction with potassium peroxymonosulfate (PMS) activation. By leveraging the synergistic effects, we observe a remarkable degradation of tetracycline, a significant pharmaceutical contaminant, under visible light. Analytically, we have elucidated the driving factors for the enhanced performance, emphasizing the optimal amount of WO3 (10%) in the composite and PMS concentration (3 mM). Specifically, the WO3/diatomite catalyst presents a degradation rate of 80.75% tetracycline (40 mg L-1) after 180 min of visible light irradiation. Also, we elucidate the primary roles of ˙SO4 - radicals in driving the photocatalytic reaction using free radical trapping studies. Our approach not only offers a direct solution to controlling pharmaceutical contamination but also opens new possibilities for advancing the design of composite-based photocatalysts by taking advantage of nature-derived materials.

Stability and degradation of (oxy)nitride photocatalysts for solar water splitting

Advancing towards alternative technologies for the sustainable production of hydrogen is a necessity for the successful integration of this potentially green fuel in the future. Photocatalytic and photoelectrochemical water splitting are promising concepts in this context. Over the past decades, researchers have successfully explored several materials classes, such as oxides, nitrides, and oxynitrides, in their quest for suitable photocatalysts with a focus on reaching higher efficiencies. However, to pave the way towards practicability, understanding degradation processes and reaching stability is essential, a domain where research has been scarcer. This perspective aims at providing an overview on recent progress concerning stability and degradation with a focus on (oxy)nitride photocatalysts and at providing insights into the opportunities and challenges coming along with the investigation of degradation processes and the attempts to improve the stability of photocatalysts.

Allotrope-dependent activity-stability relationships of molybdenum sulfide hydrogen evolution electrocatalysts

Molybdenum disulfide (MoS2) is widely regarded as a competitive hydrogen evolution reaction (HER) catalyst to replace platinum in proton exchange membrane water electrolysers (PEMWEs). Despite the extensive knowledge of its HER activity, stability insights under HER operation are scarce. This is paramount to ensure long-term operation of Pt-free PEMWEs, and gain full understanding on the electrocatalytically-induced processes responsible for HER active site generation. The latter are highly dependent on the MoS2 allotropic phase, and still under debate. We rigorously assess these by simultaneously monitoring Mo and S dissolution products using a dedicated scanning flow cell coupled with downstream analytics (ICP-MS), besides an electrochemical mass spectrometry setup for volatile species analysis. We observe that MoS2 stability is allotrope-dependent: lamellar-like MoS2 is highly unstable under open circuit conditions, whereas cluster-like amorphous MoS3-x instability is induced by a severe S loss during the HER and undercoordinated Mo site generation. Guidelines to operate non-noble PEMWEs are therefore provided based on the stability number metrics, and an HER mechanism which accounts for Mo and S dissolution pathways is proposed.

Investigation of BiVO4-based advanced oxidation system for decomposition of organic compounds and production of reactive sulfate species

Growth of population and expansion of industries lead to increasing contamination of environment with various organic pollutants. If not properly cleaned, wastewater contaminates freshwater resources, aquatic environment and has huge negative impact on ecosystems, quality of drinking water and human health, therefore new and effective purification systems are in demand. In this work bismuth vanadate-based advanced oxidation system (AOS) for the decomposition of organic compounds and production of reactive sulfate species (RSS) was investigated. Pure and Mo-doped BiVO₄ coatings were synthesized using sol-gel process. Composition and morphology of coatings were characterized using X-ray diffraction and scanning electron microscopy techniques. Optical properties were analyzed using UV-vis spectrometry. Photoelectrochemical performance was studied using linear sweep voltammetry, chronoamperometry and electrochemical impedance spectroscopy. It was shown that increase in Mo content affects the morphology of BiVO₄ films, reduces charge transfer resistance and enhances the photocurrent in the solutions of sodium borate buffer (with and without glucose) and Na₂SO₄. Mo-doping of 5-10 at.% leads to 2- to 3-fold increase in photocurrents. Faradaic efficiencies of RSS formation ranged between 70 and 90 % for all samples irrespective of Mo content. All studied coatings demonstrated high stability in long-lasting photoelectrolysis. In addition, effective light-assisted bactericidal performance of the films in deactivation of Gram positive Bacillus sp. bacteria was demonstrated. Advanced oxidation system designed in this work can be applied in sustainable and environmentally friendly water purification systems.

Electrolyte Engineering Stabilizes Photoanodes Decorated with Molecular Catalysts

Molecular catalysts are promising oxygen evolution promoters in conjunction with photoanodes for solar water splitting. Maintaining the stability of both photoabsorber and cocatalyst is still a prime challenge, with many efforts tackling this issue through sophisticated material designs. Such approaches often mask the importance of the electrode-electrolyte interface and overlook easily tunable system parameters, such as the electrolyte environment, to improve efficiency. We provide a systematic study on the activity-stability relationship of a prominent Fe₂ O₃ photoanode modified with Ir molecular catalysts using in situ mass spectroscopy. After gaining detailed insights into the dissolution behavior of the Ir cocatalyst, a comprehensive pH study is conducted to probe the impact of the electrolyte on the performance. An inverse trend in Fe and Ir stability is found, with the best activity-stability synergy obtained at pH 9.7. The results bring awareness to the overall photostability and electrolyte engineering when advancing catalysts for solar water splitting.

Boosting Reactive Oxygen Species Generation Using Inter-Facet Edge Rich WO3 Arrays for Photoelectrochemical Conversion

Water oxidation reaction leaves room to be improved in the development of various solar fuel productions, because of the kinetically sluggish 4-electron transfer process of oxygen evolution reaction. In this work, we realize reactive oxygen species (ROS), H₂ O₂ and OH⋅, formations by water oxidation with total Faraday efficiencies of more than 90 % by using inter-facet edge (IFE) rich WO₃ arrays in an electrolyte containing CO₃ ^2- . Our results demonstrate that the IFE favors the adsorption of CO₃ ^2- while reducing the adsorption energy of OH⋅, as well as suppresses surface hole accumulation by direct 1-electron and indirect 2-electron transfer pathways. Finally, we present selective oxidation of benzyl alcohol by in situ using the formed OH⋅, which delivers a benzaldehyde production rate of ≈768 μmol h^-1 with near 100 % selectivity. This work offers a promising approach to tune or control the oxidation reaction in an aqueous solar fuel system towards high efficiency and value-added product.

High-throughput exploration of activity and stability for identifying photoelectrochemical water splitting materials

The experimental high-throughput (HT) exploration for a suitable solar water splitting photoanode has greatly relied on photoactivity as the sole descriptor to identify a promising region within the searched composition space. Although activity is essential, it is not sufficient for describing the overall performance and excludes other pertinent criteria for photoelectrochemical (PEC) water splitting. Photostability in the form of (photo)electrocatalyst dissolution must be tracked to illustrate the intricate relation between activity and stability for multinary photoelectrocatalysts. To access these two important metrics simultaneously, an automated PEC scanning flow cell coupled to an inductively coupled plasma mass spectrometer (PEC-ICP-MS) was used to study an Fe-Ti-W-O thin film materials library. The results reveal an interrelation between composition, photocurrent density, and element-specific dissolution. These structure-activity-stability correlations can be represented using data science tools like principal component analysis (PCA) in addition to common data visualization approaches. This study demonstrates the importance of addressing two of the most important catalyst metrics (activity and stability) in a rapid and parallel fashion during HT experiments to adequately discover high-performing compositions in the multidimensional search space.

Modeling the Photostability of Solar Water-Splitting Devices and Stabilization Strategies

The practical implementation of photoelectrochemical devices for hydrogen generation is limited by their short lifetimes. Understanding the factors affecting the stability of the heterogeneous photoelectrodes is required to formulate degradation mitigation strategies. We developed a multiscale and multiphysics model to investigate and quantify the photostability of photoelectrodes. The model considers the photophysical processes in an electrocatalyst-coated semiconductor immersed in electrolyte, and the kinetics of the competing water-splitting and photocorrosion reactions. When applied to 12 promising compound semiconductors for use as photoanodes (GaAs, GaP, InP, GaN, SiC, AlP, AlAs, CdTe, CdS, CdSe, ZnS, and ZnSe), the semiconductor-electrocatalyst interfacial charge transfer rate constant was found to be the most significant parameter for stabilizing the photoelectrodes. Its increase induced a sigmoid-like increase in photostability, and its optimization made it possible to stabilize nine semiconductors. The semiconductor surface back-bond energy also increased the photostability in a sigmoid-like response, and its optimization allowed to stabilize five semiconductors. The increase of irradiance induced a logarithmic drop in photostability, and limiting it to 100 W/m² could stabilize three semiconductors. We further observed that the photostability in a device of centimeter-scale can vary by more than 50%, showing stable zones and photocorrosion hotspots. The photoelectrode's length as well as the electrocatalyst and electrolyte conductivities were found to be relevant parameters to impact this heterogeneity in the photostability. Thus, the photostability is not only defined by material properties but also by the specific combination of materials and by the device architecture. The model presented in this study can also be applied to photocathodes and other PEC designs, and can serve as a design tool for understanding, quantifying, and improving the stability.

Photoelectrochemical Behavior of WO3 in an Aqueous Methanesulfonic Acid Electrolyte

N-type semiconducting WO3 is widely investigated as a photoanode operating in water and seawater splitting devices. Because of the propensity of WO3 to favor photo-oxidation of acidic electrolyte anions and, in parallel, the formation on the electrode surface of the peroxo species, the choice of the appropriate electrolyte to allow stable operation of the photoanode is of critical importance. Our results from structural and photoelectrochemical tests performed using mesoporous WO3 photoanodes exposed to 80 h long photoelectrolysis in a 1 M aq. methanesulfonic acid supporting electrolyte demonstrate the photostability of both the WO3 photomaterial and the CH3SO3H electrolyte. The reasons for the stability of aqueous solutions of CH3SO3H are discussed on the basis of earlier literature reports.

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.