Earth-abundant Materials Research Articles

Transition metal tellurides as emerging catalysts for electrochemical water splitting

Renewable energy powered electrochemical water splitting has been recognized as a sustainable and environmentally-friendly way to produce green hydrogen, which is an important vector to decarbonize the transport sector and hard-to-abate industry, able to contribute to achieving global carbon neutrality. For large-scale deployment of water electrolyzers, it is essential to develop efficient and durable electrocatalysts—one of key components determining the electrochemical performance, based on cheap and earth-abundant materials. To this end, transition metal tellurides (TMTs) have recently emerged as a promising alternative to the conventional platinum group metals for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). This review article provides a brief account of the latest development in TMT-based HER and OER catalysts, with a focus on various strategies developed to improve the catalytic performance, such as nanostructure engineering, composition engineering, and heterostructuring/hybridization. Perspectives of future research on TMT-based catalysts are also shortly outlined.

Systematically-optimized charge transport properties in cumulatively solution-coated zinc tin oxide and applications for complementary inverter circuits combining organic p-type transistors

Zinc-tin-oxide (ZTO) is considered as a promising alternative thin-film semiconductors due to its merits of Earth-abundant materials, non-toxicity, and solution-processable low-cost manufacturing. However, there is still insufficient effort to implement a ZTO-based practical applications such as integrated circuits due to the lack of desirable switching behavior for circuit operation. In this context, we systematically investigate a solution processed ZTO film through a comprehensive analysis including scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission line method (TLM), and X-ray photoelectron spectroscopy (XPS). We also fabricate the synthesized ZTO-based transistors, and characteristics of the transistors are evaluated depending on annealing temperature and number of coatings. Through the systematic investigation, this study obtains the suitable turn-on voltage of V TO = 0.15 V as well as uniform charge transport property of 2.14 cm 2 /V∙s ± 5.14%. Furthermore, we apply the ZTO transistor optimized for the circuit to the complementary inverter logic, and the measured inverter circuit provides an excellent voltage gain of 141.18 V/V and improved noise margin as high as 71.4% of half the supply voltage. • ZTO films were characterized by scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. • Thickness-dependent ZTO TFT characteristics were investigated. • Morphological and chemical composition variations by annealing temperature were studied. • A complementary inverter using ZTO, DNTT TFTs with a voltage gain of 141.18 V/V and a noise margin of 71.4% was demonstrated.

Low-Cost, High-Yield ZnO Nanostars Synthesis for Pseudocapacitor Applications.

Energy storage devices based on earth-abundant materials are key steps towards portable and sustainable technologies used in daily life. Pseudocapacitive devices, combining high power and high energy density features, are widely required, and transition metal oxides represent promising building materials owing to their excellent stability, abundance, and ease of synthesis. Here, we report an original ZnO-based nanostructure, named nanostars (NSs), obtained at high yields by chemical bath deposition (CBD) and applied as pseudocapacitors. The ZnO NSs appeared as bundles of crystalline ZnO nanostrips (30 nm thin and up to 12 µm long) with a six-point star shape, self-assembled onto a plane. X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence spectroscopy (PL) were used to confirm the crystal structure, shape, and defect-mediated radiation. The ZnO NSs, dispersed onto graphene paper, were tested for energy storage by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) analyses, showing a clear pseudocapacitor behavior. The energy storage mechanism was analyzed and related to oxygen vacancy defects at the surface. A proper evaluation of the charge stored on the ZnO NSs and the substrate allowed us to investigate the storage efficiency, measuring a maximum specific capacitance of 94 due to ZnO nanostars alone, with a marked diffusion-limited behavior. The obtained results demonstrate the promising efficacy of ZnO-based NSs as sustainable materials for pseudocapacitors.

Soft Template-Based Synthesis of Mesoporous Phosphorus- and Boron-Codoped NiFe-Based Alloys for Efficient Oxygen Evolution Reaction.

Controlling the morphology, composition, and crystalline phase of mesoporous nonnoble metal catalysts is essential for improving their performance. Herein, well-defined P- and B-codoped NiFe alloy mesoporous nanospheres (NiFeB-P MNs) with an adjustable Ni/Fe ratio and large mesopores (11nm) are synthesized via soft-template-based chemical reduction and a subsequent phosphine-vapor-based phosphidation process. Earth-abundant NiFe-based materials are considered promising electrocatalysts for the oxygen evolution reaction (OER) because of their low cost and high intrinsic catalytic activity. The resulting NiFeB-P MNs exhibit a low OER overpotential of 252mV at 10mA cm-2 , which is significantly smaller than that of B-doped NiFe MNs (274mV) and commercial RuO2 (269mV) in alkaline electrolytes. Thus, this work highlights the practicality of designing mesoporous nonnoble metal structures and the importance of incorporating P in metallic-B-based alloys to modify their electronic structure for enhancing their intrinsic activity.

Unveiling microscopic carrier loss mechanisms in 12% efficient Cu2ZnSnSe4 solar cells

Understanding carrier loss mechanisms at microscopic regions is imperative for the development of high-performance polycrystalline inorganic thin-film solar cells. Despite the progress achieved for kesterite, a promising environmentally benign and earth-abundant thin-film photovoltaic material, the microscopic carrier loss mechanisms and their impact on device performance remain largely unknown. Herein, we unveil these mechanisms in state-of-the-art Cu2ZnSnSe4 (CZTSe) solar cells using a framework that integrates multiple microscopic and macroscopic characterizations with three-dimensional device simulations. The results indicate the CZTSe films have a relatively long intragrain electron lifetime of 10–30 ns and small recombination losses through bandgap and/or electrostatic potential fluctuations. We identify that the effective minority carrier lifetime of CZTSe is dominated by a large grain boundary recombination velocity (~104 cm s−1), which is the major limiting factor of present device performance. These findings and the framework can greatly advance the research of kesterite and other emerging photovoltaic materials.

Lead-doped cubic tin sulfide thin films for solar cell applications

Cubic tin sulfide (SnS), an earth-abundant and non-toxic material, is a promising candidate for thin film solar cells. Lead(Pb)-doped (0.0–11.6 atomic percent (at.%)) cubic SnS thin films were prepared via chemical bath deposition to achieve improved structural, optical, and electrical properties. X-ray diffraction studies revealed that the films were polycrystalline and exhibited cubic crystal structure, decreased crystallite size, and increased lattice parameter, strain, and dislocation density on increasing Pb content. Field emission scanning electron microscopy images revealed increased grain size up to 2.5 at.% and decreased grain size beyond 2.5 at.% of Pb content. Optical absorption studies showed a decrease in the band gap from 1.73 to 1.64 eV upon increasing the Pb dopant concentration. Hall measurements revealed an increase in the hole mobility up to 2.5 at.% and decreased mobility beyond the 2.5 at.% of Pb dopant concentration. In conclusion, the prepared cubic SnS films have potential for application in solar cells because of the demonstrated reduced optical band gap and enhanced hole mobility resulting from Pb-doping.

Stibnite at modified Becke-Johnson exchange potential

The demand for cheaper, nontoxic and earth-abundant materials as absorbing layer for solar cell is immensely needed to replace scarce, toxic and expensive one. In this regard, chalcogenide materials have considerably attracted the attention of a lot of researchers because of showing a great potential for different applications. Stibnite (Sb2S3), a chalcogenide binary material is intensively investigated for exploiting its potential for different energy technologies being a less toxic, abundantly available, stable and efficient one, which are the fundamentals for sustainability as well as to realize the dream of green energy. In this study, a computational study of the structural, electronic and optical properties of the stibnite (Sb2S3) crystal structure is presented using the full potential (FP) linearized augmented plane wave (LAPW) method framed within the density functional theory (DFT). For the estimation of the exchange-correlation potential part, besides the local density approximation (LDA), Wu-Cohen parameterized generalized gradient approximation (WC-GGA) Perdew-Burke-Ernzerhof parameterized generalized gradient approximation (PBE-GGA), and Perdew-Burke-Ernzerhof generalized gradient approximation for solids and surfaces (PBEsol-GGA), the Trans-Blaha approach of the modified Becke-Johnson (TB-mBJ) potential is used to improve the fundamental band gap value. Moreover, these calculations are performed by involving spin-orbit coupling (SOC) contribution as well. Additionally, optical properties, such as imaginary and real parts of the dielectric function, optical conductivity absorption coefficient, refractive index, reflectivity, and electron energy loss function were investigated as well. The obtained results of the band gap energy and optical properties with TB-mBJ potential were found closer to the experimental data and endorse its potentiality for the photovoltaics applications.

Nanoscale TiO2 Coatings Improve the Stability of an Earth-Abundant Cobalt Oxide Catalyst during Acidic Water Oxidation.

The large-scale deployment of proton-exchange membrane water electrolyzers for high-throughput sustainable hydrogen production requires transition from precious noble metal anode electrocatalysts to low-cost earth-abundant materials. However, such materials are commonly insufficiently stable and/or catalytically inactive at low pH, and positive potentials required to maintain high rates of the anodic oxygen evolution reaction (OER). To address this, we explore the effects of a dielectric nanoscale-thin layer, constituted of amorphous TiO2, on the stability and electrocatalytic activity of nanostructured OER anodes based on low-cost Co3O4. We demonstrate a direct correlation between the OER performance and the thickness of the atomic layer deposited TiO2 layers. An optimal TiO2 layer thickness of 4.4 nm enhances the anode lifetime by a factor of ca. 3, achieving 80 h of continuous electrolysis at pH near zero, while preserving high OER catalytic activity of the bare Co3O4 surface. Thinner and thicker TiO2 layers decrease the stability and activity, respectively. This is attributed to the pitting of the TiO2 layer at the optimal thickness, which allows for access to the catalytically active Co3O4 surface while stabilizing it against corrosion. These insights provide directions for the engineering of active and stable composite earth-abundant materials for acidic water splitting for high-throughput hydrogen production.

(Energy Technology Division Graduate Student Award sponsored by BioLogic) Design Principles for High-performance and Durable Anion Exchange Membrane water Electrolyzers

Water electrolysis powered by renewable sources such as wind or solar produces clean hydrogen gas, which is used for many industrial processes and will be essential in a future clean energy economy. Alkaline water electrolysis (AWE) and proton exchange membrane (PEM) electrolysis are mature technologies at megawatt to gigawatt scale. AWE uses earth abundant electrode and catalyst materials submerged in caustic liquid electrolyte (usually KOH or NaOH) separated by a porous diaphragm. This configuration can suffer from gas crossover and shunt current limitations. PEM electrolyzers circumvent these barriers by using an ionically-conductive solid electrolyte membrane that reduces gas crossover and allows for pure water operation that eliminates shunt currents. However, these membranes create a locally acidic environment that necessitates the use of expensive platinum-group-metal (PGM) catalyst materials and hardware coatings. Anion exchange membrane (AEM) electrolysis is an emerging technology that has the potential to combine the benefits of liquid alkaline and PEM. AEM electrolyzers use an anion-selective membrane, maintaining the pure water operation of PEM but creating a locally-alkaline environment allowing for the use of non-PGM materials. However, AEM electrolysis is still immature. In particular, the hydroxide-conducting membrane and ionomer have not yet achieved sufficient stability in pure water to compete with PEM systems. The specific dominant degradation mechanism and location has also not been conclusively identified. Further, while non-PGM catalysts such as Ni- and Fe-based oxyhydroxide catalysts out-compete PGM catalysts in three-electrode KOH studies, this performance does not carry to the pure water electrolyzer configuration. In my talk I report AEM electrolyzer performance below 2 V at 1 A·cm-2 in pure water for both PGM and non-PGM anode catalysts. I will also discuss the specific degradation mechanism of the ionomer and membrane during electrolyzer operation with both catalysts. Understanding the differences in performance and durability with precious metal and earth abundant materials is key for AEMs to compete with current electrolyzer technologies. I compare the performance and stability of five Ni-, Co-, and Fe-based catalysts and compare to IrOx. XPS and cross-sectional SEM data show oxidation of the ionomer binder at the anode for all catalysts, which results in significant degradation and performance loss. I also show additional degradation is induced by introducing soluble Fe species, indicating separate mechanisms beyond ionomer degradation are present in the non-PGM systems. In addition to the fundamental insights into the dynamic behavior of non-PGM catalysts and oxidative durability of ionomeric polymer materials, these findings contribute to bringing low-cost AEM electrolyzer technology to scale. Figure 1

(Digital Presentation) Sol-Gel Based Synthesis Route for High Capacity Disordered Rocksalt (DRX) Oxyfluoride Cathodes

The cost and energy density of commercial Li-ion batteries (LIBs) are largely limited by the cathode. Currently, the LIB cathode market is dominated by intercalation type layered transition metal oxides of the general form LiNixMnyCo1-x-yO2 (NMC). As the LIB market continues to grow, there is an increasing risk of over-reliance on cobalt and nickel, which not only leads to increased cost and resource depletion, but also raises ethical concerns related to the cobalt mining industry. Hence, there is an urgent need to develop new cathodes based on earth abundant materials. Disordered rocksalt (DRX) oxides and oxyfluorides have emerged as a promising candidate for next generation LIB cathodes due to their high energy density and ability to utilize low-cost, earth abundant elements such as Mn and Ti. Studies have shown that anionic substitution of oxygen with fluorine enhances electrochemical performance and cycling stability of DRX cathodes. DRX cathodes are traditionally synthesized using high temperature solid-state and high energy mechanochemical routes. While these methods have yielded promising cathodes, they face challenges with fluorine substitution and process scalability. Keeping this in mind, the goal of this research is to develop alternate synthesis routes to prepare DRX oxyfluoride cathodes and identify key structure – property correlations to optimize cathode performance. Our team recently developed a sol-gel based approach to synthesize Li1.2MnxTi0.8-xO2-yFy (y=0-0.2; x=0.4-0.6) DRX oxides and oxyfluorides. This presentation will discuss recent development related to the synthesis, structure, and performance of these cathodes Acknowledgements This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) and is sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) through the Vehicle Technologies Office (VTO).

(Digital Presentation) OER Catalysts Supported on Layered Zirconium Phosphate Nanomaterials

Learning from nature, instead of using expensive catalysts such as platinum for splitting water, cheaper alternatives (such as cobalt, iron, or nickel) could be used for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Hydrogen solar fuel produced in artificial photosynthesis schemes for water splitting could be used in fuel cells. However, obtaining efficient catalysts based on earth-abundant materials is still difficult, particularly for the OER. Recent theory has shown that nanoscopic confinement of catalysts increases the stability and efficiency of catalysts for OER. We have designed and develop earth-abundant electrocatalysts for oxygen reduction and water splitting using nanostructured layered inorganic materials of zirconium phosphate (ZrP). We have demonstrated improved electrocatalytic activity of ZrP nanomaterials loaded with metal ions suitable for the OER of water splitting. Adsorbing Co or Ni catalysts on the ZrP nanoparticles surface proved to improved OER activity compared to intercalated catalysts. A comparison between adsorbed Co or Ni catalysts and those catalysts on exfoliated ZrP hexagonal nanoplatelets proved that those on exfoliated nanoplatelets were more active, with diminished overpotentials and reduced Tafel plot slopes as well as higher mass activities. More recently, comparison between Co and Ni catalyst on ZrP particles with different morphologies (hexagonal platelets, rods, cubes, and spheres) revealed that the more active Co catalysts are those on hexagonal ZrP platelets, whereas the best Ni catalysts are those on ZrP spheres. Finally, encouraging recent results with a cobalt porphyrin catalyst and bimetal catalysts will be presented, as well as recent XAS operando results. These results demonstrate that ZrP nanoparticles are suitable supports for electrocatalysis of the OER and water splitting.Sponsored by the NSF-PREM Center for Interfacial Electrochemistry for Energy Materials (CIE2M) grant DMR-1827622.

(Digital Presentation) Earth-Abundant Copper-Based Electrode Materials for Li-Ion Storage

Lithium-ion batteries (LIBs) find their use in almost all applications starting from electronic gadgets to grid energy storage and hybrid electric vehicles. In this direction, the development of high-capacity anodes is crucial to fulfil the increasing energy demand. To evaluate the Li+ ion storage performance, we have synthesized an earth-abundant copper-based electrode material, Cu3PS4, using the chemical vapor transport (CVT) method from the constituent elements Cu, P, and S in a 3:1:4 ratio. It has an enargite-type structure and has a layered-type morphology (Figure 1a). A high capacity of 970 mAh g-1 is obtained at 0.1 A g-1 current density. The electrode is amenable for high discharge rates up to 5 A g-1, and at a discharge rate of 1 A g-1, the cell can be cycled 5000 times with 90% capacity retention. The high electrochemical performance of the material is related to the conversion reaction as probed by in situ Raman spectroscopy (Figure 1b) and surface-controlled capacitive contribution due to the formation of a polymeric film over the electrode surface. A full cell with LiCoO2 cathode results in a large number of cycles with stable performance. Further improvement is achieved by forming a composite with an organic polymer, polybenzimidazole. The composite electrode delivers almost double the capacity of the pristine electrode. Other studies on the use of the electrode material to multivalent ion storage based on Mg2+, Zn2+, and Al3+ are in progress.Reference D. Tripathy and S. Sampath, J. Power Sources 2020, 478, 229066. Figure 1. (a) Unit cell structure and SEM image of Cu3PS4 and (b) in situ Raman spectra of the electrode after discharge. Figure 1

Flow-through Battery Electrode for Grid-Scale Energy Storage

In order to combat climate change by switching from fossil fuels to renewable sources of energy generation, there is a critical need for the development of new energy storage technologies. A robust network of energy storage facilities is essential for the stability of a grid powered primarily by wind and solar. Given the scale of the need for energy storage, it is necessary for the storage technology to be low-cost, safe, and recyclable at the end of its life. At Lawrence Berkeley National Lab, we have developed a technology capable of achieving these goals. Our system is a novel flow-battery that uses a combination of strategies to address each of the above criteria. We use an aqueous (non-flammable) electrolyte and earth-abundant materials to keep projected chemical, manufacturing, and operation costs relatively low and to reduce safety concerns. Our newly developed flow-through electrode design further reduces projected manufacturing and operation costs and simplifies material recovery/recycling. Based on our prototype, the chemical cost of storage is projected to be $59/kWh compared with $66/kWh and $154/kWh for lithium ion and vanadium redox flow batteries respectively. As energy storage facilities become larger and larger, these savings from low up-front costs and efficient recycling will become increasingly important for keeping the cost of renewable energy storage affordable.

(Digital Presentation) Water Oxidation in Acid with Earth-Abundant Electrocatalysts

Green hydrogen production from renewable energy can help overcome difficult energy challenges worldwide. It can help decarbonize hard-to-abate sectors such as steel, chemicals, and ammonia fertilizer. Polymer Electrolyte Membrane (PEM) water electrolyzer serves as a promising technology when coupled with the fluctuating renewable energies for hydrogen production, with the advantages of fast response, dynamic operation, and excellent overload capacity. However, the only technically feasible anode catalysts for PEM are built on scarce iridium and require more than 40 years of annual production for 1 TW-scale. The scarce metal iridium has become a bottleneck limiting the scale development of PEM water electrolysis technology. Therefore, identifying earth-abundant catalysts with high activity and acid tolerance is essential to realizing large-scale hydrogen production via water electrolysis. Various 3d transition metals have been investigated as potential OER catalysts. In 2019, we reported how manganese oxide shows exceptional stability among earth-abundant materials for oxygen evolution reaction (OER) in acid. At the time, non-precious metal catalysts had difficulty maintaining activity for more than a week at 10 mA cm-2. In comparison, manganese oxide (γ-MnO2) was able to electrolyze water for over 11 months at 10 mA cm-2. This was made possible by identifying a stable potential window for γ-MnO2 in which the OER can be catalyzed efficiently while simultaneously suppressing deactivation pathways. After that, a binary spinel oxide catalyst (Co2MnO4) was synthesized by mixing stable manganese with active cobalt using a thermal decomposition method. Cobalt oxides were shown to be active for the OER but unstable in acid. By incorporating Mn into the Co3O4 spinel lattice, the lifetime was extended by two orders of magnitude while maintaining the activity. The water electrolysis with Co2MnO4 in acid can be sustained over 1500 hours at 200 mA cm-2. The activation barrier of the obtained spinel Co2MnO4 is comparable to state-of-the-art iridium oxides, which are reported to have activation energies of 25-30 kJ mol-1 in acidic conditions. The catalyst dissolution was experimentally analyzed by spectroscopy methods. It is clarified that the tetrahedral Co is the preferable dissolution site and the dissolution rate is suppressed in Co2MnO4 compared to Co3O4, resulting in outstanding stability. The active working time of Co2MnO4 as a function of current density for the OER reaction and other earth-abundant electrocatalysts reported in the literature were summarized. The marked improvement of the activity and stability in acid makes cobalt manganese spinel materials an intriguing candidate for non-noble metal water electrolysis. Acknowledgments: This work was supported by the New Energy and Industrial Technology Development Organization (NEDO). Figure caption: Long-term stability of Co2MnO4 during the OER in acid. a, Time dependence of the electrochemical potential necessary to perform OER at 100, 200, 500 and 1,000 mA cm-2 geo in H2SO4 (pH 1) and H3PO4 (pH 1), respectively. The stabilities of Co3O4 and γ-MnO2 in H2SO4 (pH 1) are shown for comparison. All the experiments were performed at 25 °C using an FTO substrate except for curve IX, for which a Pt/Ti mesh was used. b, A comparison of the stability of Co2MnO4 with Co3O4 and other Earth-abundant OER catalysts reported in the literature12-28 (Please refer Nat Catal 5, 109–118 (2022)) (upwards and downwards triangles represent lifetime and operation time, respectively). The measurements were performed under acidic conditions. Data points with roman numerals (I to XI) correspond to curves I to XI in a. Diagonal lines indicate the total amount of charge transferred before deactivation. Only studies that report stability at pH < 3 were included in the literature survey. Figure 1

The Flexible and Transparent Film Heaters Based on Regenerated Cellulose and Carbon Nanotubes

The flexible and transparent film heaters (FTFHs) with the advantages of mechanical flexibility, portability, and excellent electrothermal performance, are key to the next generation portable, wearable heaters and thermal protection systems. However, the present flexible and transparent substrates of FTFHs are fabricated by typical plastic PET films, which suffer from poor interfacial adhesion with the thermally conductive materials. In this work, the transparent regenerated cellulose fibers made of completely dissolved in NMMO solution followed by regeneration process is presented to disperse and support carbon nanotubes (CNTs) by a vacuum-dewatering process. In the presence of cellulose fibers, these CNTs have strong hydrogen bonding properties in the dehydration-deposition process and thus respond to tight intertwining structures in fibrous composites. The resulting regenerated cellulose fibers exhibit high optical transparency of 88% (@550 nm) and good mechanical properties (30 MPa). Interestingly, the FTFHs show a rapid heating response, high generation temperature, and resistance stability for up to 2 h. The FTFHs made with earth-abundant, cost-effective, and recyclable materials, have excellent potential in the areas of green flexible and transparent film heaters.

Lithium inserted ZnSnN2 thin films for solar absorber: n to p-type conversion

ZnSnN2 is a non-toxic and earth-abundant photoabsorber material for flexible photovoltaic devices because of its excellent optoelectronic behavior. However, theoretical studies show that the alkaline-earth metallic (Li, Na, K, Rb, Cs, and Fr) dopants in ZnSnN2, particularly lithium (Li), display shallow-acceptor behavior and improve the performance of ZnSnN2 semiconductors. Orthorhombic phase structure with (002) preferred orientation was observed for Li-doped films and the lattice parameters agree well with reported standards. Secondary ion mass spectroscopy (SIMS) analysis revealed the incorporation of Li in Li:ZnSnN2 films. XPS, the density of states, and Born effective charge analysis revealed the chemical bonding states of Li–ZnSnN2. In contrast to the pristine n-type ZnSnN2, Li:ZnSnN2 thin films showed conductivity with p-type hole concentrations varying between 1.14 × 1020–9.47 × 1019 cm−3 and the highest mobility of 20.03 cm2V−1s−1. Therefore, we obtained p-type conductivity by substituting an organolithium reagent (C₄H₉Li) on the Zn site, which highlights that Li:ZnSnN2 can be effectively used as the photoanode layer for next-generation thin-film solar cell devices.

Primary amine–thiourea grafted graphene–based heterogeneous chiral catalysts for highly enantioselective Michael additions

Heterogeneous asymmetric catalysis based on earth-abundant carbon materials put together the advantages for the ease of separation, simple regeneration, and high stability of solid catalysts as well as acts as low-cost, environmentally friendly metal-free alternative to the metal-based catalysts. Two new bifunctional carbocatalysts [(S,S) and (R,R) GO-PATU] were prepared by anchoring enantiomerically pure organosilanes bearing primary amine and thiourea groups to the graphene oxide (GO) skeleton covalently via silane coupling reaction. The surface modification of GO was analyzed by a combination of Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron spectroscopy (TEM) techniques. The catalytic performances of graphene-based materials were investigated in the asymmetric Michael addition reactions of α,α-disubstituted-aldehydes to nitrostyrenes and N-substituted-maleimides. The effects of solvents and Brønsted basic or acidic additives were evaluated using both catalysts and good yields (up to 85%) and stereoselectivities (up to 95% ee) were obtained in dichloromethane at room temperature in the presence of l- and d-camphorsulfonic acid. Besides, recycling and reusing studies of the catalysts were successfully performed.

Synergistic modulation on atomic-level 2D/2D Ti3C2/Svac-ZnIn2S4 heterojunction for photocatalytic H2 production

The photocatalytic hydrogen evolution (PHE) offers a promising and sustainable route to fuels and feedstocks from solar energy. Ti3C2 MXene as a highly active and stable earth-abundant material has been employed in photocatalytic hydrogen production due to its excellent conductivity and effective activation of H2O molecules. Herein, a synergistic strategy, via introducing atomic-thickness Ti3C2 MXene to tune the defect structure of the ultrathin ZnIn2S4 nanosheets, is reported to improve the PHE activity. By using microstructure characterization and DFT calculation, we reveal the generation of sulfur vacancies (Svac) on the photocatalysts and the improvement of adsorption property for reactant H2O molecules at the surface of ultrathin 2D/2D Ti3C2/ Svac-ZnIn2S4 heterojunction. During the reaction, the synergies between sulfur vacancies and Ti3C2 MXene affect the absorption of light, the separation of carriers, and the reaction of H2O molecules, thus improving the PHE rate up to 1435.31 μmol·g−1·h−1 with 6.3 times higher than that of Svac-ZnIn2S4 nanosheets (229.16 μmol·g−1·h−1). This work stresses the great importance of synergistic modulation for promoting photocatalytic hydrogen evolution.

First-principles study on the structural, electronic, vibrational, and optical properties of the Ru-doped SnSe

SnSe has a tunable bandgap, high absorption coefficient (105/cm), eco-friendly, earth-abundant material, rendering itself a solid prospect for numerous applications. Doping can influence the electronic and optical properties of the α-SnSe. Herein, we present the effect of Ru doping on the electronic, vibrational, and optical properties of the RuxSn1−xSe alloy by employing the first-principles framework. The RuxSn1−xSe at x = 0.0625 has an indirect bandgap of 0.87 eV. The introduction of more Ru content in the RuxSn1−xSe at x = 0.125 results in the metallic characteristics of the α-SnSe. A high absorption coefficient is found for the Ru-doped α-SnSe. The phonon dispersions calculations of the RuxSn1−xSe at x = 0.0625 demonstrate good dynamical stability. Also, we have used the Bethe-Salpeter equation after running the non-self-consistent G0W0 calculations which indicate that RuxSn1−xSe at x = 0.0625 is an excellent absorber of light in the visible region and can be used for applications in solar energy conversion and optoelectronics.

Electrochemical swelling induced high material utilization of porous polymers in magnesium electrolytes

Magnesium rechargeable batteries are promising candidates for large-scale energy storage due to their -high safety, low material cost, and earthabundant materials. Many redox-active polymers have recently been reported to show excellent cycling stability with decent Mg-storage performance. However, compared to their Li counterparts, these polymers exhibit lower Mg-storage capacity, resulting in a low ratio of materials utilization (ηMg/ηLi) for the same polymer in two electrolyte systems (i.e., Mg vs. Li electrolytes). Herein, we present a sulfur-linked porous polymer, poly(hexaazatrinaphthalene sulfide) (PSHATN), which sets a record for material utilization of 98% among all polymer electrodes and delivers a reversible capacity of 196 mAh g−1 in Mg electrolytes. Based on electrochemical impedance spectroscopy and operando optical microscopy, we discover a strong correlation between specific capacity and degree of electrochemical swelling of polymers in both electrolytes. Importantly, the high ratio (ηMg/ηLi) of PSHATN is ascribed to sufficient electrochemical swelling due to its large pore volume and flexible polymer nature, in contrast to linear polymers and rigid covalent organic frameworks that swell less effectively. This work highlights the critical need for polymer swelling in promoting ion transport in redox polymers for high material utilization and offers important polymer structural design insights for multivalent ion-based energy storage.