Functionalized and fully characterized graphene-based lubricant additives are potential 2D materials for energy-efficient tribological applications in machine elements, especially at macroscopic ...contacts. Two different reduced graphene oxide (rGO) derivatives, terminated by hydroxyl and epoxy-hydroxyl groups, were prepared and blended with two different molecular weights of polyethylene glycol (PEG) for tribological investigation. Epoxy-hydroxyl-terminated rGO dispersed in PEG showed significantly smaller values of the friction coefficient. In this condition, PEG chains intercalate between the functionalized graphene sheets, and shear can take place between the PEG and rGO sheets. However, the friction coefficient was unaffected when hydroxyl-terminated rGO was coupled with PEG. This can be explained by the strong coupling between graphene sheets through hydroxyl units, causing the interaction of PEG with the rGO to be non- effective for lubrication. On the other hand, antiwear properties of hydroxyl-terminated rGO were significantly enhanced compared to epoxy-hydroxyl functionalized rGO due to the integrity of graphene sheet clusters.
Herein, the use of highly concentrated sunlight for materials science research is reviewed. Specific research directions include: (1) the generation of inorganic nanostructures, some of which had ...eluded experimental realization with conventional synthetic processes, and (2) elucidating the processes governing the degradation of organic and perovskite‐based photovoltaic materials and devices, along with accelerated assessment of their stability. Both approaches employ solar concentrators capable of producing flux densities exceeding those of terrestrial solar radiation by up to three orders of magnitude, and are geared toward either creating extensive ultrahot reactor conditions conducive to the rapid, safe synthesis of unusual nanomaterials or judiciously interrogating photovoltaic devices.
The use of highly concentrated sunlight for generating nanomaterials, and for elucidating how material properties of organic and perovskite photovoltaics change over time, is reviewed. Solar concentrators developed for these projects span three orders of magnitude in attainable target flux density for experiments performed indoors under controlled conditions.
The photochemical stability of encapsulated films of mixed halide perovskites with a range of MAPb(I1−xBrx)3 (MA=methylammonium) compositions (solid solutions) was investigated under accelerated ...stressing using concentrated sunlight. The relevance of accelerated testing to standard operational conditions of solar cells was confirmed by comparison to degradation experiments under outdoor sunlight exposure. We found that MAPbBr3 films exhibited no degradation, while MAPbI3 and mixed halide MAPb(I1−xBrx)3 films decomposed yielding crystallization of inorganic PbI2 accompanied by degradation of the perovskite solar light absorption, with faster absorption degradation in mixed halide films. The crystal coherence length was found to correlate with the stability of the films. We postulate that the introduction of Br into the mixed halide solid solution stressed its structure and induced more structural defects and/or grain boundaries compared to pure halide perovskites, which might be responsible for the accelerated degradation. Hence, the cause for accelerated degradation may be the increased defect density rather than the chemical composition of the perovskite materials.
To be or not to be (efficient and stable): Photochemical stability studies of MAPb(I1−xBrx)3 (MA=methylammonium) photovoltaic materials show that mixed halide compositions are less stable than pure halide ones. We postulate that these solid solutions contain internal stresses and structural defects, which might be responsible for their accelerated degradation.
Perovskite photovoltaics offer a highly efficient and low-cost solar energy harvesting technology. However, the presence of lead (Pb) cations in photovoltaic halide perovskite (HaPs) materials is ...concerning, and quantifying the environmental hazard of accidental Pb2+ leaching into the soil is crucial for assessing the sustainability of this technology. Pb2+ from inorganic salts was previously found to remain in the upper soil layers due to adsorption. However, Pb-HaPs contain additional organic and inorganic cations, and competitive cation adsorption may affect Pb2+ retention in soils. Therefore, we measured, analyzed by simulations and report the depths to which Pb2+ from HaPs penetrates into 3 types of agricultural soil. Most of the HaP-leached Pb2+ is found to be retained already in the first cm of the soil columns, and subsequent rain events do not induce Pb2+ penetration below the first few cm of soil surface. Surprisingly, organic co-cations from the dissolved HaP are found to enhance the Pb2+ adsorption capacity in clay-rich soil, compared to non-HaP-based Pb2+ sources. Our results imply that installation over soil types with improved Pb2+ adsorption, and removal of only the contaminated topsoil, are sufficient means to prevent ground water contamination by HaP-leached Pb2+.
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•We characterized soil depth profiles of lead leached from photovoltaic perovskites.•Lead from damaged perovskite solar cells is unlikely to contaminate groundwater.•Organic co-cations in perovskites enhance the lead adsorption capacity of soil.•Perovskite-lead retained at topsoil is x100 less than the soil adsorption capacity.•Solar cell installation over soils with increased organic/clay content is preferred.
Hematite (α-Fe2O3) is one of the most studied photoanode materials due to its stability in alkaline electrolytes and visible light absorption. However, its reported performance lags significantly ...behind its theoretical limit. Toward determining routes for efficient photo-oxidation on hematite, we investigated the role of hole-trapping surface states as reaction intermediates using sacrificial reductant reagents as hole scavengers, H2O2 and FeCN. Photoelectrochemical characterization at low scavenger concentrations and intermediate potentials, as opposed to previous studies at high scavenger concentrations, has shown the reaction mechanism to include competitive photo-oxidation between water molecules and the hole scavengers, similar for both H2O2 and FeCN. Using operando Raman spectroelectrochemistry, we show similar transient features for both scavengers, interpreted as scavenger adsorption to a two-site reaction intermediate participating in the photogenerated hole transfer of water photo-oxidation, hence the competition. These findings strengthen the significance of hole-trapping surface states for water photo-oxidation on hematite and the previously suggested two-site reaction pathway for efficient hole transfer in this reaction. A better understanding of the mechanisms of photoelectrochemical water splitting can assist in improving the efficiency of solar hydrogen production.
Lead halide perovskites attract much attention in recent years as a realistic solution for efficient and low‐cost solar cells. One of the interesting solar cell structures is the fully ...mesoporous‐carbon‐based perovskite solar cells. The mesoporous layers can be fabricated entirely by screen printing with the potential for upscaling. Herein, the two‐step deposition of perovskite in mesoporous‐carbon‐based perovskite solar cells is studied. The influence of the dipping time on the photovoltaic parameters is investigated using charge extraction and intensity‐modulated photovoltage spectroscopy (IMVS) measurements. A power conversion efficiency of 15% is observed for cells fabricated using two‐step deposition which is one of the highest reported for this solar cell structure. Stability characterizations at maximum power point (MPP) tracking show degradation with time, however a complete recovery of the devices in the dark is revealed. Analyzing the mechanism for this shows that the perovskite's unit cell shrinks during the recovery process due to internal stress relief. This interesting phenomenon opens the possibility to optimize the stability of these solar cells for commercial applications.
Herein, the two‐step deposition of perovskite in mesoporous‐carbon‐based perovskite solar cells with repeatable power conversion efficiency over 12% is analyzed. Stability characterizations show degradation with time, however, a complete recovery of the devices in the dark was revealed. Analyzing the mechanism for this shows that the perovskite's unit cell shrinks during the recovery process due to internal stress relief.
The efficiency of charge transfer in electrochemical devices is largely determined by the ion concentration profile near the electrode surface, i.e., the electrical double layer (EDL). Room ...temperature ionic liquids (RTILs) are attractive for electrochemical applications due to their high charge density as well as for their tunable anion/cation design, low vapor pressure, and wide electrochemical window. The EDL structure in RTILs is profoundly different from that in traditional (dilute) electrolytes in that opposite charges tend to layer in a spatially alternating, segregated structure that decays toward the bulk region. Such charge layering becomes crucial for applications that require confinement of RTILs into narrow spaces, where RTILs are interfaced with nanostructured electrodes. Layering in the EDL is frequently explained by electrostatic interactions of the ions with the electrode, assuming that RTILs are homogeneous liquids made of ions only. However, a growing evidence points to the presence of neutral and charged multi-ion species in RTIL bulk and that the EDL structure is related to this bulk heterogeneity. These relations imply rich spatiotemporal competitions between bulk phases and structures, the electrode morphology, and the EDL layering. However, a mechanistic understanding of how the bulk properties of RTILs affect the mesoscale order in EDLs, as well as charge transport/transfer in the EDL, is currently lacking. To advance this gap, a mean-field approach that extends the standard Poisson–Nernst–Planck model to account for ion association/dissociation and bulk phases is being suggested. An inclusive theoretical modeling can help with an understanding of the factors and mechanisms governing EDL structure formation and predict the effect of the EDL structure on the interfacial electrochemical properties.
The development of hybrid organic–inorganic halide perovskite solar cells (PSCs) that combine high performance and operational stability is vital for implementing this technology. Recently, ...reversible improvement and degradation of PSC efficiency have been reported under illumination–darkness cycling. Quantifying the performance and stability of cells exhibiting significant diurnal performance variations is challenging. We report the outdoor stability measurements of two types of devices showing either reversible photo-degradation or reversible efficiency improvement under sunlight. Instead of the initial (or stabilized) efficiency and T 80 as the figures of merit for the performance and stability of such devices, we propose using the value of the energy output generated during the first day of exposure and the time needed to reach its 20% drop, respectively. The latter accounts for both the long-term irreversible degradation and the reversible diurnal efficiency variation and does not depend on the type of process prevailing in a given perovskite cell.
Photosynthesis is used by plants, algae and bacteria to convert solar energy into stable chemical energy. The initial stages of this process--where light is absorbed and energy and electrons are ...transferred--are mediated by reaction centres composed of chlorophyll and carotenoid complexes. It has been previously shown that single small molecules can be used as functional components in electric and optoelectronic circuits, but it has proved difficult to control and probe individual molecules for photovoltaic and photoelectrochemical applications. Here, we show that the photocurrent generated by a single photosynthetic protein-photosystem I-can be measured using a scanning near-field optical microscope set-up. One side of the protein is anchored to a gold surface that acts as an electrode, and the other is contacted by a gold-covered glass tip. The tip functions as both counter electrode and light source. A photocurrent of ∼10 pA is recorded from the covalently bound single-protein junctions, which is in agreement with the internal electron transfer times of photosystem I.
Interfacial energy band alignment is crucial for applications involving charge transfer and transport. Specifically, efficient photovoltaic (PV) devices require fine-tuning of the energy levels at ...interfaces between the absorber and the electrodes for extraction of photogenerated charges. Infrared (IR) absorbers have a small bandgap, hence such tuning is difficult using common electrode materials. Semiconductor sensitized PV devices typically utilize transparent metal oxide photoanodes, since many semiconductor absorbers can inject photoexcited electrons into their conduction bands. Transparent photocathode electrodes, to which photoexcited holes are transferred from semiconductor absorbers, are less common. Transparent nanostructured photocathodes with beneficial energy level alignment with small bandgap semiconductors can widen the material choices for IR photoelectrodes. Herein we show that CuSCN nanowire (NW) arrays can serve as such photocathodes, with efficient transfer of photogenerated holes from bulk-like PbS absorbing in the IR and near-IR range. Pulsed electrodeposition was used to fabricate high-quality CuSCN NW arrays. Significant NIR and IR photocurrents showed that the CuSCN NW array is an efficient photocathode for PbS sensitized IR photovoltaic devices. Comparison of various electrode materials verified the photogenerated hole injection vs competing processes such as electron injection and recombination and suggested that this material system can be used for studying photosensitized hole injection from other small bandgap semiconductors.
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