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Solar collectors have the potential for significant climate change mitigation by substituting heat produced with fossil fuels. To achieve this, collectors with highly efficient solar absorbers are essential. Carbon nanotubes are highly absorbing, sustainable, cheap, and thermally stable, making them a promising material for solar absorbers. However, achieving a high solar absorptance and low thermal emittance (solar selectivity), while maintaining good thermal stability and scalability is challenging. Here, we present a selective coating based on multi-walled carbon nanotubes and silica (SiO₂). A water-based dispersion enabled by carboxyl functionalization of the carbon nanotubes (CNT_F) is spray coated on a stainless steel (SS) substrate and fixated with sol-gel dip coated silica. The SS/CNT_F/SiO₂ surface exhibits an optical selectivity dependent on CNTF area load and with 0.83 gCNT m⁻² a solar absorptance and thermal emittance of 0.94 and 0.40, respectively, is achieved. The coating also demonstrates excellent thermal stability, with an estimated lifetime of >25 years at working temperatures ≤222°C. All together, we show that by using scalable and cheap technology, concurrent with sustainable materials and a simple structural design, we can manufacture a coating that exhibits properties suitable for low-to-mid-temperature applications. Our study highlights the potential of carbon-based solar absorbers.

MoS₂ is widely praised as a promising replacement for Pt as an electrocatalyst for the hydrogen evolution reaction (HER), but even today, it still suffers from low performance. This issue is tackled by using Mn³⁺ as a surface modifier to trigger sulfur vacancy formation and enhance electron transport in few-layered 2H MoS₂. Only 10% of Mn is sufficient to transform the semiconductive MoS₂ into an active HER electrocatalyst. The insertion of Mn reduces both HER onset potential and Tafel slope which allows reaching 100 mA/cm² at an overpotential of 206 mV, ten times larger of what undoped MoS₂ can achieve. The enhanced activity arises because Mn³⁺ introduces electronic states near the conduction band, promotes sulfur vacancies, and increases the hydrogen adsorption. In addition to its facile production and extended shelf-life, Mn–MoS₂ exhibits an efficiency of 73% at 800 mA/cm² and 2.0 V when used in proton exchange membrane water electrolyzers.

Proton exchange membrane water electrolysis (PEMWE) is a promising technology to produce high-purity renewable hydrogen gas. However, its operation efficiency is highly dependent on the usage of expensive noble metals as electrocatalysts. Replacing, decreasing, or simply extending the operational lifetime of these precious metals have a positive impact on the hydrogen economy. Mo-based electrocatalysts are often praised as potential materials to replace the Pt used at the cathode to catalyse the hydrogen evolution reaction (HER). Most electrocatalytic studies are performed in traditional three-electrode cells with different operational conditions than those seen in PEM systems, making it difficult to predict the expected material’s performance under industrially relevant conditions. Therefore, we investigated the viability of using three selected Mo-based nanomaterials (1T′-MoS2, Co-MoS2, and β-Mo2C) as HER electrocatalysts in PEMWE systems. We investigated the effects of replacing Pt on the catalyst loading, charge transfer resistance, kinetics, operational stability, and hydrogen production efficiency during the PEMWE operation. In addition, we developed a methodology to identify the individual contribution of the anode and cathode kinetics in a PEMWE system, allowing to detect the cause behind the performance drop when using Mo-based electrocatalysts. Our results indicate that the electrochemical performance in three-electrode cells might not strictly predict the performance that could be achieved in PEMWE cells due to differences in interfaces and porosity of the macroscopic catalyst layers. Among the catalysts studied, 1T′-MoS2 is truly an excellent candidate to replace Pt as an HER electrocatalyst due to its low overpotential, low charge transfer resistance, and excellent durability, reaching a high efficiency of ∼75% at 1 A cm-2 and 1.94 V. Our study highlights the importance of a continuous development of efficient noble-metal free HER electrocatalysts suitable for PEMWE systems.

The creation of effective Pd-based architectures with numerous electrocatalytic active sites and efficient charge transfer is of key importance for improving the electrocatalytic performance in water electrolyzer and fuel cell applications. On the other hand, MoS2, possessing multiple electrocatalytic active sites, can act both as support and booster to Pd-based electrocatalytic structures. Herein, MoSx@Pd hybrids were successfully synthesized by using a one-pot liquid phase solvothermal strategy with stoichiometric excess of Pd. The optimized MoSx@Pd proves to be an excellent bifunctional electrocatalyst for both hydrogen evolution reaction and oxygen reduction reaction (ORR). Optimized MoSx@Pd operates the process for hydrogen evolution at the same potential as Pt/C and achieves a low overpotential of 76 mV at −10 mA cm−2 due to improved reaction kinetics and charge transfer processes between Pd and MoS2. On top of that, MoSx@Pd exhibits excellent performance and stability as cathode electrocatalyst in a polymer electrolyte membrane water electrolyzer. Simultaneously, the bifunctional electrocatalyst shows enhanced electrocatalytic ORR activity and stability by maintaining 93% of its initial activity outperforming commercial Pt/C. Finally, rotating ring disk electrode analysis reveals that ORR proceeds through the energy efficient 4e− pathway, with water being the main product, rendering MoSx@Pd a promising component for fuel cells.

The development of photo/electroactive catalysts sustainably producing hydrogen from water splitting and selectively hydrogen peroxide is of paramount importance to alleviate climate change effects. Herein, an anionic cobalt porphyrin (CoP) derivative is electrostatically interfaced with a positively charged modified molybdenum disulfide (MoS₂), forming CoP/MoS₂, which is accordingly employed as nonprecious photo/electrocatalyst for water oxidation reaction (WOR) and selective H₂O₂ production. According to the results, CoP/MoS₂ shows remarkable bifunctional photo/electrocatalytic performance for WOR and 2e⁻ pathway O₂ reduction reaction (ORR) in alkaline electrolyte. Upon visible light irradiation, electrochemical measurements on a fluorine-doped tin oxide (FTO) coated glass electrode reveal an onset potential of 0.595 mV (ORR) and 1.575 mV (WOR) vs. reversible hydrogen electrode, being improved by approximately 80 mV, in both cases, compared to the dark conditions. Notably, the use of the FTO set-up not only enabled us to evaluate the photo/electrocatalytic activity of the CoP/MoS₂ nanoensemble but also mimics the practical conditions in photo/electrochemical devices. The outstanding bifunctional photo/electrocatalytic performance of CoP/MoS₂ is attributed to (a) the use of CoP as versatile single-atom molecular catalyst and photosensitizer (b) the strong ion-pair interactions between cationic modified MoS₂ and the anionic CoP derivative, which prevent aggregation, ensuring better accessibility of the reactants to cobalt active sites, and (c) the co-existence of 1T and 2H phase at modified MoS₂, offering improved electrical conductivity and intrinsic electrocatalytic activity along with enhanced intraensemble electronic interactions upon illumination. This work is expected to inspire the design of advanced and low-cost materials for the sustainable production of renewable fuels.

To meet the demanding requirements for clean energy production, the need to develop advanced electrocatalysts for efficiently catalysing the water splitting reactions attracts a continuously increased attention. However, to meet the anticipated expansion in green hydrogen production from renewable energy sources, the catalysts used for the water splitting reaction not only need to satisfy the required figures of merit but should concurrently be based mainly on abundant, non-critical materials with low environmental impact. In last decades, non-noble metal catalysts, based on transition metals, rare-earth metals, dichalcogenides, and light elements such as phosphorus, nitrogen, and sulphur have shown improved performance. Moreover, in recent years increased interest has been focused on variations of such materials, more specifically on the introduction of defects to further boost their catalytic performance. Through the many studies performed over the last years, it is now possible to summarize, understand and describe the role of these defects for the water splitting reactions, namely the hydrogen and oxygen evolution reactions, and thereby to suggest strategies in the development of next generation electrocatalysts. This is the goal of the current review; we critically summarize the latest progress on the role of introduced defects for catalytic electrolysis applications by scrutinizing the structure–performance correlation as well as the specific catalytic activity. A broad class of nanomaterials is covered, comprising transition metal dichalcogenides, transition metal oxides and carbides, carbon-based materials as well as metal–organic frameworks (MOFs). Finally, the main challenges and future strategies and perspectives in this rapidly evolving field are provided at the end of the review.

The oxygen reduction reaction (ORR) 2e⁻ pathway provides an alternative and green route for industrial hydrogen peroxide (H₂O₂) production. Herein, the ORR photo/electrocatalytic activity in the alkaline electrolyte of manganese and iron porphyrin (MnP and FeP, respectively) electrostatically associated with modified 1T/2H MoS₂ nanosheets is reported. The best performing catalyst, MnP/MoS₂, exhibits excellent electrocatalytic performance towards selective H₂O₂ formation, with a low overpotential of 20 mV for the 2e⁻ ORR pathway (E_ons = 680 mV vs RHE) and an H₂O₂ yield up to 99%. Upon visible light irradiation, MnP/MoS₂ catalyst shows significant activity enhancement along with good stability. Electrochemical impedance spectroscopy assays suggest a reduced charge transfer resistance value at the interface with the electrolyte, indicating an efficient intra-ensemble transfer process of the photo-excited electrons through the formation of a type II heterojunction or Schottky contact, and therefore justifies the boosted electrochemical activities in the presence of light. Overall, this work is expected to inspire the design of novel advanced photo/electrocatalysts, paving the way for sustainable industrial H₂O₂ production.

The hydrogen evolution reaction (HER) is a key process in electrochemical water splitting. To lower the cost and environmental impact of this process, it is highly motivated to develop electrocatalysts with low or no content of noble metals. Here, we report on an ingenious synthesis of hybrid Pt_xNi1_-x electrocatalysts in the form of a nanoparticle-nanonetwork structure with very low noble metal content. The structure possesses important features such as good electrical conductivity, high surface area, strong interlinking, and substrate adhesion, which render an excellent HER activity. Specifically, the best performing Pt_0.05Ni_0.95 sample demonstrates a Tafel slope of 30 mV dec^-1 in 0.5 M H₂SO₄ and an overpotential of 20 mV at a current density of 10 mA cm^-2 with high stability. The impressive catalytic performance is further rationalized in a theoretical study, which provides insight into the mechanism on how such small platinum content can allow for close-to-optimal adsorption energies for hydrogen.