Добірка наукової літератури з теми "Polysulfide adsorption"

The combination of the physical adsorption of lithium polysulfides onto porous graphene and the chemical binding of polysulfides to N and S sites promotes reversible Li₂S/polysulfide/S conversion, realizing high performance Li–S batteries with long cycle life and high-energy density.

The Li-S battery is exceptionally appealing as an alternative candidate beyond Li-ion battery technology due to its promising high specific energy capacity. However, several obstacles (e.g., polysulfides’ dissolution, shuttle effect, high volume expansion of cathode, etc.) remain and thus hinder the commercialization of the Li-S battery. To overcome these challenges, a fundamental study based on atomistic simulation could be very useful. In this work, a comprehensive investigation of the adsorption of electrolyte (solvent and salt) molecules, lithium sulfide, and polysulfide (Li2Sx with 2 ≤x≤ 8) molecules on the amorphous Al2O3 atomic layer deposition (ALD) surface was performed using first-principles density functional theory (DFT) calculations. The DFT results indicate that the amorphous Al2O3 ALD surface is selective in chemical adsorption towards lithium sulfide and polysulfide molecules compared to electrolytes. Based on this work, it suggests that the Al2O3 ALD is a promising coating material for Li-S battery electrodes to mitigate the shuttling problem of soluble polysulfides.

Lithium sulfur batteries (LSBs) are a promising candidate to be used in modern commodities like electric vehicles, grid energy storage, electric aviation, and many others because of the exceptionally high theoretical capacity of sulfur (1675 mAh g-1), almost 5 times higher than the conventional lithium-ion batteries. However, the problem of polysulfide shuttling effect originating from the dissolved lithium polysulfides in the electrolyte results in poor cycling stability, hindering the commercialization of LSB. Significant advancement has been made over the years due to a great deal of research on novel materials development and structural design for lithium polysulfide (Li2Sn: 4≤ n ≤8) adsorption synergy to counter the polysulfide shuttling effect 1. However, rather than focusing only on the adsorption synergy (Physical confinement and chemical binding), novel catalysts that can accelerate the polysulfide conversion reaction kinetics are needed to design the next generation LSB. Previously, our group investigated shape-controlled cerium oxide (CeO2) to accelerate the polysulfide conversion reactions by generating the intermediate steps of thiosulfate and polythionate 2, 3. Copper oxide (CuO), being a p-type semiconducting material, is another promising material that can activate thiosulfate formation as its redox potential is 2.53 V vs Li/Li+, which lies in the potential window of 2.4 V < E° ≤ 3.05 V that selectively triggers the formation of thiosulfate. Herein, we investigated 10 wt% of CuO impregnated on the CeO2 nanorods (10 wt%CuO/CeO2) as a cathode host for LSB. The CuO impregnation on the surface of CeO2 nanorods attributed strong interaction between the surface defect rich CeO2 nanorods and the copper oxides (CuOx: Cu2O and CuO) promoting excellent electrocatalytic activity. The 10 wt%CuO/CeO2 sample provides adsorption-catalysis dual synergy to chemically bind and further catalyze the polysulfide conversion by polythionate and thiosulfate generation. As a result, the derived LSB exhibited excellent electrochemical performance with high capacity of 1141 mAh g-1 at 0.2 C with a sulfur loading of 1.33 mg cm-2 and a capacity loss of only 0.04% per cycle after 60 cycles. Key words: lithium sulfur batteries, lithium polysulfides, shuttle effect, cerium oxide, catalysis. Xiong, D. G.; Zhang, Z.; Huang, X. Y.; Huang, Y.; Yu, J.; Cai, J. X.; Yang, Z. Y., Boosting the polysulfide confinement in B/N–codoped hierarchically porous carbon nanosheets via Lewis acid–base interaction for stable Li–S batteries. Journal of Energy Chemistry 2020, 51, 90-100. Azam, S.; Wei, Z.; Wang, R., Cerium oxide nanorods anchored on carbon nanofibers derived from cellulose paper as effective interlayer for lithium sulfur battery. J Colloid Interface Sci 2022, 615, 417-431. Wei, Z.; Li, J.; Wang, R., Surface engineered polar CeO2-based cathode host materials for immobilizing lithium polysulfides in High-performance Li-S batteries. Applied Surface Science 2022, 580.

Abstract Some vital challenges are main obstacles for further development of lithium–sulfur (Li–S) batteries such as low capacity and poor cycle stability resulted from polysulfide shuttling behavior, the physical/chemical entrapment is regarded as an effective method to inhibit and catalyze polysulfides. Herein we design a cross-linked framework of reduced graphene oxide anchored with Cu2−x Se nanoparticles (Cu2−x Se@rGO) by building an electrolyte/Cu2−x Se/graphene triple-phase interface to be a high-efficiency electrocatalyst for Li–S batteries. Importantly, this three-dimensional conductive network possesses a large specific surface area with high ion transport capability, meanwhile providing strong physical constraint for efficient adsorption of soluble polysulfides. Further, this triple-phase catalytic interface provides strong chemical adsorption and abundant Cu2−x Se nanoparticle sulfiphilic active sites, effectively inhibiting the dissolution of polysulfides and guaranteeing the efficient polysulfide adsorption catalysis as well as rapidly uniform Li2S nucleation. Consequently, with the Cu2−x Se@rGO separator, a lower capacity decay rate about 0.059% per cycle after 500 cycles at 2 C is obtained. What’s more, with a higher areal sulfur loading of 3.0 mg cm−2, the capacity is still maintained at 805 mAh g−1 over 100 cycles. Therefore, this work will open new avenue to construct 2D transition metal selenide for superior performance Li–S batteries.

Lithium-sulfur batteries have attracted attention because of their high energy density. However, the “shuttle effect” caused by the dissolving of polysulfide in the electrolyte has greatly hindered the widespread commercial use of lithium-sulfur batteries. In this paper, a novel two-dimensional TiS2/graphene heterostructure is theoretically designed as the anchoring material for lithium-sulfur batteries to suppress the shuttle effect. This heterostructure formed by the stacking of graphene and TiS2 monolayer is the van der Waals type, which retains the intrinsic metallic electronic structure of graphene and TiS2 monolayer. Graphene improves the electronic conductivity of the sulfur cathode, and the transferred electrons from graphene enhance the polarity of the TiS2 monolayer. Simulations of the polysulfide adsorption show that the TiS2/graphene heterostructure can maintain good metallic properties and the appropriate adsorption energies of 0.98–3.72 eV, which can effectively anchor polysulfides. Charge transfer analysis suggests that further enhancement of polarity is beneficial to reduce the high proportion of van der Waals (vdW) force in the adsorption energy, thereby further enhancing the anchoring ability. Low Li2S decomposition barrier and Li-ion migration barrier imply that the heterostructure has the ability to catalyze fast electrochemical kinetic processes. Therefore, TiS2/graphene heterostructure could be an important candidate for ideal anchoring materials of lithium-sulfur batteries.

The poor cycle stability caused by the shuttle effect of polysulfides which have been key scientific issue in the development of high-efficiency lithium–sulfur (Li–S) batteries. In this work, the authors report a Fe-doped Co3O4 (named FCO) that was used as a sulfur-loaded host material for Li–S batteries. We demonstrate the important roles of well-designed Co3O4 particles and Fe atoms in regulating polysulfide conversion due to the strong adsorption of polysulfides by polar Co3O4, whereas Fe atoms and Co3O4 catalyze polysulfide conversion. Therefore, the LiS batteries with FCO-180 (When the hydrothermal temperature is 180 °C) sea urchinlike composites exhibited a high superior energy density (992.7 mAh g−1 at 0.2 C, after 100 cycles) and long-term cyclability (649.4 mAh g−1 at 1 C, 300 cycles) with high sulfur loading (75 wt%). This work confirms that the FCO-180 sea urchinlike increases not only the capacity of high-rate but also a generic and feasible strategy to construct practical Li–S batteries for emerging energy-storage applications.

The efficacy of lithium-sulfur (Li-S) batteries crucially hinges on the sulfur immobilization process, representing a pivotal avenue for bolstering their operational efficiency and durability. This dissertation primarily tackles the formidable challenge posed by the high solubility of polysulfides in electrolyte solutions. Quantum chemical computations were leveraged to scrutinize the interactions of MXene materials, graphene (Gr) oxide, and ionic liquids with polysulfides, yielding pivotal binding energy metrics. Comparative assessments were conducted with the objective of pinpointing MXene materials, with a specific focus on d-Ti3C2 materials, evincing augmented binding energies with polysulfides and ionic liquids demonstrating diminished binding energies. Moreover, a diverse array of Gr oxide materials was evaluated for their adsorption capabilities. Scrutiny of the computational outcomes unveiled an augmentation in the solubility of selectively screened d-Ti3C2 MXene and ionic liquids—vis à vis one or more of the five polysulfides. Therefore, the analysis encompasses an in-depth comparative assessment of the stability of polysulfide adsorption by d-Ti3C2 MXene materials, Gr oxide materials, and ionic liquids across diverse ranges.

Abstract The application of lithium–sulfur (Li–S) batteries is hindered by the insulating characteristic of sulfur and slow reaction kinetics of lithium polysulfides. Here, we propose a three-dimensionally ordered macroporous (3DOM) structured conductive polar Ta-doped TiO2 framework with supported Co active site (CoTa@TiO2) to enhance the conversion kinetics of polysulfides. The 3DOM structure serves as an efficient sulfur host for the active sulfur through abundant pores and adsorption site. At the same time, the macropores can buffer the volume expansion of sulfur and enlarged mass transfer. The strong electrostatic attraction between Ti–O bond and polysulfide also promotes the adsorption of polysulfides. Moreover, the doped-Ta improves the conductivity of TiO2 by narrowing the band gap, whereas the supported Co can accelerate the catalytic transformation. Benefited from advanced structural design and synergistic effect of Co and Ta doped TiO2, the Li–S cell with 3DOM CoTa@TiO2 cathode exhibits an impressive areal capacity of 3.4 mAh cm−2 under a high sulfur loading of 5.1 mg cm−2. This work provides an alternative strategy for the development of carbon-based cathode materials for Li–S batteries.

The Lithium sulfur (Li-S) battery has a great potential to replace lithium-ion batteries due to its high-energy density. However, the “shuttle effect” of polysulfide intermediates (Li2S8, Li2S6, Li2S4, etc.) from the cathode can lead to rapid capacity decay and low coulombic efficiency, thus limiting its further development. Anchoring polysulfide and inhibiting polysulfide migration in electrolytes is one of the focuses in Li-S battery. It is well known that polar metal oxides-manganese oxides (MnO2) are normally used as an effective inhibitor for its polysulfide inhibiting properties. Considering the natural 1D tunnel structure, MnO2 with three kinds of typical tunnel-type were screened to study the effects of the tunnel size on the adsorption capacity of polysulfide. We found that MnO2 with larger tunnel sizes has stronger chemisorption capacity of polysulfide. It promotes the conversion of polysulfide, and corresponding cathode exhibits better cycle reliability and rate performance in the cell comparison tests. This work should point out a new strategy for the cathode design of advanced Li-S battery by controlling the tunnel size.

Room-temperature sodium-sulfur batteries (RT-NaSBs) with high theoretical energy density and low cost are ideal candidates for next-generation stationary and large-scale energy storage. However, the dissolution of sodium polysulfide (NaPS) intermediates and their migration to the anode side give rise to the shuttle phenomenon that impedes the reaction kinetics leading to rapid capacity decay, poor coulombic efficiency, and severe loss of active material. Inhibiting the generation of long-chain NaPS or facilitating their adsorption via physical and chemical polysulfide trapping mechanisms is vital to enhancing the electrochemical performance of RT-NaSBs. This review provides a brief account of the polysulfide inhibition strategies employed in RT-NaSBs via physical and chemical adsorption processes via the electrode and interfacial engineering. Specifically, the sulfur immobilization and polysulfide trapping achieved by electrode engineering strategies and the interfacial engineering of the separator, functional interlayer, and electrolytes are discussed in detail in light of recent advances in RT-NaSBs. Additionally, the benefits of engineering the highly reactive Na anode interface in improving the stability of RT-NaSBs are also elucidated. Lastly, the future perspectives on designing high-performance RT-NaSBs for practical applications are briefly outlined.

Adsorption is a versatile purification technique to selectively separate different peptide fractions from a mixture using mild operation conditions. Porous carbons are ideally suited to separate ACE-inhibiting dipeptides by combining tailored size exclusion and polarity selectivity. The desired peptide fraction is mostly hydrophobic and very small and should adsorb inside hydrophobic micropores. The second topic of this thesis is linked to energy storage. The lithium-sulfur battery is a promising alternative to common lithium-ion batteries with theoretical capacities of up to 1672 mAh g−1 sulfur. The second aim of this thesis is to conduct an in-depth investigation of polysulfides interacting with selected carbon materials in a simplified battery electrolyte environment. The focus of this study is laid on the impact of surface polarity and pore size distribution of the carbon to develop a quantitative correlation between polysulfide retention and porosity metrics. Both, the enrichment of ACE-inhibitors and the retention of polysulfides rely on liquid phase adsorption in porous materials, linking the above mentioned topics. This thesis not only aims to develop an enrichment process or to find a superior battery cathode but also strives to explore structure-property relationships that are universally valid. Understanding the complex interplay of pore size and polarity leading to selective interactions between pore wall and the adsorbed species is given a high priority.

Les batteries lithium-soufre (Li-S) sont des technologies de batterie prometteuses pour répondre à la demande croissante de stockage d’énergie. En raison de leur densité d’énergie théorique élevée de 2500 Wh.kg-1 en poids et de 2800 Wh.L-1 en volume [1], elles ont le potentiel de stocker pratiquement 3 fois plus d’énergie que les batteries Li-ion. Cependant, plusieurs défis entravent leur développement commercial. Parmi eux, l’effet de « navette redox » est l’un des principaux inconvénients de la technologie. Cette navette redox consiste en un mouvement d’aller-retour des polysulfures (Li2Sx, 2 < x < 8), composés intermédiaires générés lors de la dissolution du soufre entre les électrodes, entraînant une faible utilisation de soufre actif, une perte d’efficacité coulombique et une rapide dégradation de la capacité électrochimique au cours du temps.Dans la littérature, de nombreuses stratégies ont été proposées pour réduire ce phénomène, allant de l'utilisation de couches passives protectrices du lithium métal (Li), à la fonctionnalisation des séparateurs d'électrolyte, en passant par la conception de nouvelles électrodes positives utilisant des matériaux dont la fonction principale est de capturer efficacement les polysulfures (carbone poreux, structures métallo-organiques, matériaux à base de métaux tels que des oxydes ou des hydroxydes, voire des matériaux sulfures par exemple) [2]. Parmi les solutions proposées, le MoS2 s'est révélé être un bon candidat pour interagir spécifiquement avec les polysulfures [3].Ce projet de thèse est dédié à la conception d'électrodes positives de batteries Li-S, à base de MoS2-Ketenblack (Mo-KB), pour résoudre le phénomène de « navette redox ». Il vise à mieux comprendre les paramètres jouant un rôle dans le mécanisme de capture des polysulfures afin de concevoir des électrodes positive de Mo-KB optimisées pour i) réduire la diffusion des polysulfures et ii) favoriser leur réduction en Li2S.Différents échantillons de Mo-KB ont été synthétisés en veillant à varier la morphologie, la teneur, et la longueur des feuillets de MoS2 afin de modifier, à la fois, le type et le nombre de sites actifs disponibles et d'étudier l'impact sur les interactions avec les polysulfures et les performances des batteries Li-S qui en résulte.Pour ce faire, une nouvelle méthodologie UV-Vis, utilisant une sonde in situ pour quantifier systématiquement l'adsorption des polysulfures par les matériaux synthétisés, a été développée. En effet, cette méthodologie limite les artefacts générés lors de l’utilisation d’une configuration plus répandue : mesure UV-Vis avec cuvette en quartz. La méthodologie in situ contribue ainsi à comprendre l'effet réel de la nature des adsorbants (MoS2, MoS2-Ketjenblack, silice) sur les phénomènes d'adsorption et comment cela peut modifier la chimie des polysulfures en solution (réactions de dismutation et spéciation). Enfin, les échantillons poreux de Mo-KB, préalablement imprégnées de soufre, ont été intégrées dans la formulation d'électrodes positives Li-S afin d’évaluer leur efficacité d’adsorption et de conversion des polysulfures dans un système réel, au sein de pile-bouton. Des mesures électrochimiques ont été menées afin d’évaluer quantitativement l’impact de ces matériaux sur les performances électrochimiques (capacité, efficacité faradique, puissance, durée de vie du cycle) au fil du temps.References1. Seh, Z. W., Sun, Y., Zhang, Q. & Cui, Y. Designing high-energy lithium-sulfur batteries. Chemical Society reviews 45, 5605–5634; 10.1039/c5cs00410a (2016).2. Chen, Y. et al. Advances in Lithium-Sulfur Batteries: From Academic Research to Commercial Viability. Advanced materials (Deerfield Beach, Fla.), e2003666; 10.1002/adma.202003666 (2021).3. Liu, Y., Cui, C., Liu, Y., Liu, W. & Wei, J. Application of MoS 2 in the cathode of lithium sulfur batteries. RSC Adv. 10, 7384–7395; 10.1039/C9RA09769D (2020)
Lithium-sulfur (Li-S) batteries are promising candidates for energy storage. Due to their high theoretical gravimetric and volumetric energy density of 2500 Wh.kg-1 and 2800 Wh.L-1 [1], they have the potential to practically store about 3 times more energy than Li-ion batteries. However, several challenges hinder their commercial development. Among those, the “shuttle-effect” is one of the major drawbacks and consists of a back-and-forth movement between electrodes of the dissolved intermediates polysulfides (Li2Sx, 2 < x < 8) giving rise to low active sulfur utilization, poor coulombic efficiency, and rapid capacity decay.In literature, many strategies have been proposed ranging from protective Li passive layers to electrolyte separator functionalization, and new positive electrode design using efficient polysulfides trapping materials (e.g. porous carbon, metal-organic frameworks, metal-based material such as oxides or hydroxides or even sulfides materials)2. Among them, MoS2 has proven to be a good adsorbent candidate to interact with polysulfide species3.This PhD project is dedicated to the design of supported MoS2-Ketenblack (Mo-KB) for Li-S positive electrode to tackle the “shuttle effect” phenomenon. We aimed to better understand the parameter playing a role on the polysulfide trapping mechanism to design an optimized Mo-KB electrode to i) mitigate polysulfide shuttling, and ii) favor their reduction into Li2S.Samples with MoS2 morphology, Mo loading, slab length variation were synthesized to modify the type and number of actives sites to study the impact on polysulfides interactions, and the resulting impact on the Li-S battery performances.To do so, we setup a new UV-Vis methodology using in situ probe to systematically quantify the polysulfides adsorption onto the developed materials. Indeed, this methodology limits the artefacts due to the setup compared to usual UV-Vis setup using a quartz cuvette and helps to understand the true effect of adsorbents nature (MoS2, MoS2-Ketjenblack, silica) on the adsorption phenomena and how it may modify the chemistry in solution of polysulfides (disproportionation and speciation). Finally, the sulfur impregnated porous Mo-KB powders were subsequently integrated into the formulation of sulfur-positive electrodes within a coin cell battery environment to assess their effectiveness as both PS trap and catalytic surface to convert polysulfides. The electrochemical measurements performed aimed to quantitatively determine whether it would enhance the electrochemical performance (capacity, faradic efficiency, power, cycle life) over time.References1. Seh, Z. W., Sun, Y., Zhang, Q. & Cui, Y. Designing high-energy lithium-sulfur batteries. Chemical Society reviews 45, 5605–5634; 10.1039/c5cs00410a (2016).2. Chen, Y. et al. Advances in Lithium-Sulfur Batteries: From Academic Research to Commercial Viability. Advanced materials (Deerfield Beach, Fla.), e2003666; 10.1002/adma.202003666 (2021).3. Liu, Y., Cui, C., Liu, Y., Liu, W. & Wei, J. Application of MoS 2 in the cathode of lithium sulfur batteries. RSC Adv. 10, 7384–7395; 10.1039/C9RA09769D (2020)

The vast majority of commercial gas separation membrane systems are polymeric because of processing feasibility and cost. However, polymeric membranes designed for gas separations have been known to have a trade-off between permeability and selectivity as shown in Robeson's upper bound curves. The search for membrane materials that transcend Robeson's upper bound has been the critical issue in research focused on membranes for gas separation in the past decade. To that end, many researchers have explored the idea of mixed matrix membranes (MMMs). These membranes combine a polymer matrix with inorganic molecular sieves such as zeolites. The ideal filler material in MMMs should have excellent properties as a gas adsorbent or a molecular sieve, good dispersion properties in the polymer matrix of submicron thickness, and should form high quality interfaces with the polymer matrix. In order to increase gas permeance and selectivity of polymeric membranes by fabricating MMMs, we have fabricated mixed matrix membranes using carbon nanotubes (CNTs) and nano-sized mesoporous silica. Mixed matrix membranes containing randomly oriented CNTs showed that addition of nanotubes to a polymer matrix could improve its selectivity properties as well as permeability by increasing diffusivity. Overall increases in permeance and diffusivity for all tested gases suggested that carbon nanotubes can provide high diffusivity tunnels in the CNT within the polymer matrix. This result agreed well with molecular simulation estimations. In order to prepare ordered CNTs membranes, we have developed a simple, fast, commercially attractive, and scalable orientation method. The oriented CNT membrane sample showed higher permeability by one order of magnitude than the value predicted by a Knudsen model. This CNT membrane showed higher selectivities for CO₂ over other gas molecules because of preferential interaction of CO₂ with the amine functionalized nanotubes, demonstrating practical applications in gas separations. Recently, mesoporous molecular sieves have been used in MMMs to enhance permeability or selectivity. However, due to their micrometer scale in particle size, the composite membrane was extremely brittle and tended to crack at higher silica loading. In this study, we have developed fabrication techniques to prepare MMMs containing mesoporous MCM-41 nanoparticles on the order of ~50 nm in size. This smaller nanoparticle lead to higher polymer/particle interfacial area and provides opportunity to synthesize higher loading of molecular sieves in polymer matrix up to ~80 vol%. At 80 vol% of nano-sized MCM-41 silica loading, the permeability of the membrane increased dramatically by 300 %. Despite these increases in permeability, the separation factor of the MMMs changed only slightly. Therefore, these nanoscale molecular sieves are more suitable for commercialization of MMMs with very thin selective layers than are micro-sized zeolites or molecular sieves.
Ph. D.

Adsorption is a versatile purification technique to selectively separate different peptide fractions from a mixture using mild operation conditions. Porous carbons are ideally suited to separate ACE-inhibiting dipeptides by combining tailored size exclusion and polarity selectivity. The desired peptide fraction is mostly hydrophobic and very small and should adsorb inside hydrophobic micropores. The second topic of this thesis is linked to energy storage. The lithium-sulfur battery is a promising alternative to common lithium-ion batteries with theoretical capacities of up to 1672 mAh g−1 sulfur. The second aim of this thesis is to conduct an in-depth investigation of polysulfides interacting with selected carbon materials in a simplified battery electrolyte environment. The focus of this study is laid on the impact of surface polarity and pore size distribution of the carbon to develop a quantitative correlation between polysulfide retention and porosity metrics. Both, the enrichment of ACE-inhibitors and the retention of polysulfides rely on liquid phase adsorption in porous materials, linking the above mentioned topics. This thesis not only aims to develop an enrichment process or to find a superior battery cathode but also strives to explore structure-property relationships that are universally valid. Understanding the complex interplay of pore size and polarity leading to selective interactions between pore wall and the adsorbed species is given a high priority.

碩士
逢甲大學
環境工程與科學學系
107
The purpose of this study was to compare with pure polysulfone membrane and modified polysulfone membrane by surface-initiated atom transfer radical polymerization(SI-ATRP) to use for boron removal boric acid. The membranes were prepared by electrospun of the chloromethylated polysulfone followed by SI-ATRP of glycidyl methacrylate(GMA) grafted from chloromethylated polysulfone(CMPSF). The membrane was characterized by ATR-FTIR, FE-SEM and porometer. The effects of initial boronconcentration, adsorption time and Regeneration on boron adsorption properties are syste studied. compare with pure polysulfone membrane and modified polysulfone membranes by SI-ATRP to use for Transparent Exopolymer Particles and Modified Fouling Index. The membranes were prepared by electrospun of the chloromethylated polysulfone followed by SI-ATRP of 2‐hydroxyethyl methacrylate (HEMA) and poly(ethylene glycol)monomethacrylate (PEGMA) grafted from chloromethylated polysulfone(CMPSF). The membranes were characterized by ATR-FTIR, FE-SEM, porometer and water flux. Exploring the correlation between the removal of TEP and the decrease in Modified Fouling Index(MFI) of different membranes, and using Nanoparticle Tracking Analysis (NTA) to find its relevance.