Showing papers by "Kwang S. Kim published in 2020"

Multi-heteroatom-doped carbon from waste-yeast biomass for sustained water splitting

Abstract: Producing hydrogen in clean, affordable and safe manners without damaging the environment can help address the challenge of meeting a growing energy demand sustainably. Yeast biomass-derived materials—such as multi-heteroatoms (nitrogen, sulfur and phosphorus) doped carbon (MHC) catalysts from waste biomass—can help develop efficient, eco-friendly and economical catalysts to improve the sustainability of hydrogen production. Here we report hydrogen and oxygen production in 1 M potassium hydroxide using ruthenium single atoms (RuSAs) along with Ru nanoparticles (RuNPs) embedded in MHC (RuSAs + RuNPs@MHC) as a cathode and magnetite (Fe3O4) supported on MHC (Fe3O4@MHC) as an anode. The RuSAs + RuNPs@MHC catalyst outperforms the state-of-the-art commercial platinum on carbon catalyst for hydrogen evolution reaction in terms of overpotential, exchange current density, Tafel slope and durability. Furthermore, compared with industrially adopted catalysts (that is, iridium oxide), the Fe3O4@MHC catalyst displays outstanding oxygen evolution reaction activity. For whole water splitting, it requires a solar voltage of 1.74 V to drive ~ 30 mA, along with remarkable long-term stability in the presence (12 h) and absence (58 h) of outdoor-sunlight exposure, as a promising strategy towards a sustainable energy development. Cleaner hydrogen production can help energy sustainability. The use of yeast biomass-derived materials to develop efficient, eco-friendly and economical catalysts—compared with industrially adopted catalysts—is shown to improve hydrogen production as a strategy towards a sustainable energy system.

Graphene-nanoplatelets-supported NiFe-MOF: high-efficiency and ultra-stable oxygen electrodes for sustained alkaline anion exchange membrane water electrolysis

Abstract: Practical hydrogen production using high-efficiency, low-cost, and stable oxygen electrodes is crucial for a sustainable clean energy future. Herein we report a graphene-nanoplatelets-supported (Ni,Fe) metal–organic framework (MOF) as a superior and ultra-durable (>1000 h) anode for alkaline water electrolysis. The MOF on carbon-fiber paper electrodes requires an overpotential η = 220 mV to achieve a current density j = 10 mA cm−2 (η = 180 mV on nickel foam for j = 20 mA cm−2) with a Tafel slope of 51 mV per decade, high turnover frequency (1.22 s−1), high faradaic efficiency (99.1%), and long-term durability of >1000 h in continuous electrolysis. In an alkaline anion exchange membrane water electrolyzer (AAEMWE), it exhibits a record current density of 540 mA cm−2 at 1.85 V at 70 °C, outperforming the state-of-the-art Pt/C//IrO2. A breakthrough strategy introduced in membrane electrode assembly fabrication by extending the electrical contact with an aqueous electrolyte offers an additional OH− transport pathway to regenerate the original conductivity of the AAEMWE in continuous electrolysis, without any significant change in the pH of the electrolyte. These findings open up durable, high-performance AAEMWE and direct solar-to-fuel conversion, especially to replace high-cost proton exchange membrane water electrolysis that already works with ultra-pure water.

Machine learning-based high throughput screening for nitrogen fixation on boron-doped single atom catalysts

Abstract: Production of ammonia via electrochemical nitrogen reduction reaction (NRR) has recently attracted much attention due to its potential to play a vital role in producing fertilizers and other chemicals. High throughput screening of electrocatalysts for the NRR requires numerous calculations in the search space, making the computational cost a bottleneck for predicting eligible electrocatalysts. Here we used a deep neural network (DNN) to predict efficient electrocatalysts for the NRR among boron(B)-doped graphene single atom catalysts (SACs). This model can noticeably reduce the time of computation by removing non-efficient catalysts from screening. Also, the adsorption energy and free energy can be predicted by the feature-based light gradient boosting machine (LGBM) model. These features represent the geometrical structure as well as bonding characteristics. Among the catalysts evaluated, three candidates were identified as very promising catalysts, offering excellent selectivity over the hydrogen evolution reaction (HER). CrB3C1 exhibited a minimal overpotential of 0.13 V for the NRR. This study provides a new pathway for the rational design of catalysts for nitrogen fixation by employing the most important features involved in the NRR by using machine learning methods.

Immiscible bi-metal single-atoms driven synthesis of electrocatalysts having superb mass-activity and durability

Abstract: Properties of metal elements can be modified by alloying. Given that catalytic efficiencies are often maximized using metal single-atoms (SAs), two immiscible metals can improve the catalytic activity when they can be present as non-agglomerated dual SAs. Here, we report yet-unexplored synthesis of high-performance electrocatalysts utilizing immiscibility of Cu/Ru bimetal atoms. In the synthesized electrocatalyst (Cu/Ru@GN), both Cu-SAs and Ru-SAs are dispersed on N-doped graphitic-matrix (GN), while other Cu-SAs are bridged to Ru nanoparticles (NPs) surface via nitrogen. It shows superior mass-activity with outstanding catalytic performance and durability for hydrogen evolution reaction (HER), outperforming commercial Pt20wt%/C. Cu-SAs along with Ru-NPs enhance electric conduction or charge-transfer rate of GN-templates for fast kinetics. Owing to Cu-Ru immiscibility, the coordination of Cu-SAs by N atoms which bridge to Ru-NPs surface, introduces new active sites and brings about long-term stability (over 600 h in acidic media) by preventing aggregation of Ru-NPs.

Machine Learning for Predicting the Band Gaps of ABX3 Perovskites from Elemental Properties

Abstract: The band gap is an important parameter that determines light-harvesting capability of perovskite materials. It governs the performance of various optoelectronic devices such as solar cells, light-e...

Interface Engineering Driven Stabilization of Halide Perovskites against Moisture, Heat, and Light for Optoelectronic Applications

Abstract: DOI: 10.1002/aenm.202000768 conveniences have raised serious concerns over global warming.[1] In this regard, harnessing solar energy for the electricity would be an immediate and effective remedy to the menacing global warming issues. Solar energy is an endless green energy source that offers 1000 times of power that the entire planet requires, while photovoltaic technology provides an ideal and clean route to be pursued.[2] An electronic device that converts solar energy into electricity is known as a photovoltaic cell.[3] With power conversion efficiencies (PCEs) reaching steadily beyond 25.2%, just marginally smaller than those of Si solar cells, perovskite solar cells (PSCs) have grabbed intensive attractions in the field of photovoltaic research.[4] However, the commercial scaling of PSCs will greatly rely on paths to reduced fabrication cost, enhanced PCE, and improved stability.[5] In addition to the great success of PSCs, halide perovskites are also promising materials for various high-performance optoelectronic devices such as lightemitting diodes (LEDs),[6] scintillators,[7] and photo detectors[8] due to their excellent photoluminescence quantum yield (PLQY), tunable bandgap, solution processability, and charge transport properties.[9] The halide perovskites have a general formula ABX3, where “A” corresponds to monovalent organic or inorganic cations such as methylammonium (CH3NH3, MA+), formamidinium (NH2 = CHNH2, FA+) or Cs+, “B” can be divalent metal cations such as Pb2+ and Sn2+, and “X” can be anions of the VII A group (halides) of the periodic table, namely I, Br, or Cl.[10] These halide perovskites are salts that can easily take up moisture if packed improperly. Light exposure can easily break the fragile bonds in perovskites, create halogen vacancies or interstitial pairs allowing them to migrate, and convert any oxygen present inside the crystals into highly active superoxide.[11] Even under moderate temperature and light/heat exposure, the perovskites can react with most of the metals, with heat causing organic species to volatilize, whereas light promotes mobility of ions inside the crystal.[12] Furthermore, the fracture energy of the perovskites is extremely low in the order[13] <1 J m−2. On the contrary, the perovskites possess an extremely high thermal expansion coefficient of the order of nearly ten times in magnitude to that of glass. Eventually, thermal stresses Moisture, heat, and light instabilities of halide perovskites (HPs) represent a serious Achilles’ heel that must be overcome, to enable future advancements in perovskite-based optoelectronic devices such as solar cells and light-emitting diodes. The instabilities are attributed to the unavoidable fragile ionic bonding between cationic and anionic parts of HPs during their formation. Surface passivation of HPs by various surface-passivating materials has proven to be an attractive approach to stabilize perovskites against moisture, heat, and light, keeping intact their structural integrity and ionic bonding. Herein, the experimental and theoretical background for degradation mechanisms of HPs is reviewed along with various surface passivating materials to stabilize HPs. Finally, the existing challenges associated with thin-film and device fabrication and an outlook for improving the stability of perovskites in optoelectronics are presented

Efficient electron extraction of SnO 2 electron transport layer for lead halide perovskite solar cell

Abstract: SnO2 electron transport layer (ETL) has been spotlighted with its excellent electron extraction and stability over TiO2 ETL for perovskite solar cells (PSCs), rapidly approaching the highest power conversion efficiency. Thus, how to boost the performance of ETL is of utmost importance and of urgent need in developing more efficient PSCs. Here we elucidate the atomistic origin of efficient electron extraction and long stability of SnO2-based PSCs through the analysis of band alignment, carrier injection, and interfacial defects in the SnO2/MAPbI3(MA = CH3NH3+) interface using unprecedentedly high level of first-principles calculations at the PBE0 + spin-orbit-coupling + dispersion-correction level for all possible terminations and MA directions. We find that Sn-s orbital plays a crucial role in carrier injection and defect tolerance. SnO2/MAPbI3 shows favorable conduction band alignments at both MAI- and PbI2-terminations, which makes the solar cell performance of SnO2/MAPbI3 excel that of TiO2/MAPbI3. Different electron transfer mechanisms of dipole interaction and orbital hybridization at the MAI- and PbI2-terminations indicate that post-transition metal (sp valence) oxide ETLs would outperform transition metal (d valence) oxide ETLs for PSCs.

Atomic scale study of black phosphorus degradation

Abstract: Black phosphorus (BP) is a promising two-dimensional (2D) material for future electronic devices due to its unique properties of high carrier mobility and large band gap tunability. However, thinner crystalline BP is more readily degraded under ambient conditions. For BP-based electronic devices, degradation of the exfoliated BP is a key issue. However, the nanometer scale study of BP degradation is rare so far. Herein, we report an atomically resolved degradation process of the BP surface using atomic force microscopy under temperature- and humidity-controlled environments. The atomically resolved crystal surface of BP deteriorated due to surface etching after cleavage, and showed monolayer etching. The etching process is accelerated by applying a bias voltage to BP via a conductive tip. After the voltage-assisted BP etching, the BP etching product shows crystalline BP confirmed by Raman spectroscopy and atomic force microscopy. Our atomic scale study of BP will be useful for the future 2D-based electronic devices to overcome conventional silicon-based electronic devices.

Towards Universal Sparse Gaussian Process Potentials: Application to Lithium Diffusivity in Superionic Conducting Solid Electrolytes

Abstract: For machine learning of interatomic potentials the sparse Gaussian process regression formalism is introduced with a data-efficient adaptive sampling algorithm. This is applied for dozens of solid electrolytes. As a showcase, experimental melting and glass-crystallization temperatures are reproduced for Li7P3S11 and an unchartered infelicitous phase is revealed with much lower Li diffusivity which should be circumvented. By hierarchical combinations of the expert models universal potentials are generated, which pave the way for modeling large-scale complexity by a combinatorial approach.

Enhanced photoluminescence quantum yield of MAPbBr 3 nanocrystals by passivation using graphene

Abstract: Diminishing surface defect states in perovskite nanocrystals is a highly challenging subject for enhancing optoelectronic device performance. We synthesized organic/inorganic lead-halide perovskite MAPbBr3 (MA = methylammonium) clusters comprising nanocrystals with diameters ranging between 20–30 nm and characterized an enhanced photoluminescence (PL) quantum yield (as much as ~ 7 times) by encapsulating the MAPbBr3 with graphene (Gr). The optical properties of MAPbBr3 and Gr/MAPbBr3 were investigated by temperature-dependent micro-PL and time-resolved PL measurements. Density functional theory calculations show that the surface defect states in MAPbBr3 are removed and the optical band gap is reduced by a 0.15 eV by encapsulation with graphene due to partial restoration of lattice distortions.

A high performance N-doped graphene nanoribbon based spintronic device applicable with a wide range of adatoms

Abstract: Designing and fabricating nanosize spintronic devices is a crucial task to develop information technology of the future. However, most of the introduced spin filters suffer from several limitations including difficulty in manipulating the spin current, incapability in utilizing a wide range of dopants to provide magnetism, or obstacles in their experimental realization. Here, by employing first principles calculations, we introduce a structurally simple and functionally efficient spin filter device composed of a zigzag graphene nanoribbon (ZGNR) with an embedded nitrogenated divacancy. We show that the proposed system, possessing a robust ferromagnetic (FM) ordering, exhibits perfect half metallic behavior in the absence of frequently used transition metals (TMs). Our calculations also show that the suggested system is compatible with a wide range of adatoms including basic metals, metalloids, and TMs. It means that besides d electron magnetism originating from TMs, p electrons of incorporated elements of the main group can also cause half metallicity in the electronic structure of the introduced system. Our system exploiting the robustness of doping-induced FM ordering would be beneficial for promising multifunctional spin filter devices.

Accurate Description of Nuclear Quantum Effects with High-Order Perturbed Path Integrals (HOPPI).

Abstract: Imaginary time path-integral (PI) simulations that account for nuclear quantum effects (NQE) beyond the harmonic approximation are increasingly employed together with modern electronic-structure calculations. Existing PI methods are applicable to molecules, liquids, and solids; however, the computational cost of such simulations increases dramatically with decreasing temperature. To address this challenge, here, we propose to combine high-order PI factorization with perturbation theory (PT). Already for conventional second-order PI simulations, the PT ansatz increases the accuracy 2-fold compared to fourth-order schemes with the same settings. In turn, applying PT to high-order path integrals (HOPI) further improves the efficiency of simulations for molecular and condensed matter systems especially at low temperatures. We present results for bulk liquid water, the aspirin molecule, and the CH5+ molecule. Perturbed HOPI simulations remain both efficient and accurate down to 20 K and provide a convenient method to estimate the convergence of quantum-mechanical observables.

Rational design of metal–ligands for the conversion of CH4 and CO2 to acetates: role of acids and Lewis acids

Abstract: The capture, sequestration and utilization of carbon dioxide have attracted global attention in the environmental field, science and industry. The concurrent conversion of CO2 and CH4 is a reasonable solution to reduce greenhouse gases, but it remains a challenging research topic. Herein, we reveal the catalytic activity of bifunctional half-sandwich complexes (metal = Ru, Rh, and Ir) in the direct conversion of methane and carbon dioxide to acetic acid (AA) using density functional theory. The use of Ru/Rh/Ir metals with ethanediphosphine (PP), N-tosylethylenediamine (NNTs), and ethylene glycol (OO) ligands shows superb performance in lowering the free energy (ΔG) barrier (ΔG‡) for the conversion. To activate CH4 and CO2 molecules, we used extra additives to reduce the high energy barriers required by simple metal–ligand complexes. For CH4 activation, additives of AA and trifluoroacetic acid (TFA) were used to assist the proton abstraction in the Ru–NNTs/OO complexes, while no acid additives were used for the PP ligands. In the case of the PP ligands, the C–H activation occurs through a “concerted oxidative addition–reductive elimination” (OA–RE) process, whereas in the NNTs/OO ligands, C–H activation occurs through “cyclometallation deprotonation” (CMD) irrespective of the presence of acid additives. The reduction in ΔG‡ for CH4 activation in Ru–NNTs/OO is 5–10 kcal mol−1 using acid additives. In contrast, in Ru–PP, this ΔG‡ barrier is 20 kcal mol−1 without additives. For CO2 activation, AlCl3 (Lewis acid) was used to insert CO2 into the metal site of the Ru–NNTs/OO and Ru/Rh/Ir–PP complexes. The reduction in ΔG‡ using AlCl3 for CO2 activation in Ru–NNTs·AA/OO·2TFA and Ru/Rh/Ir–PP is 15–30 kcal mol−1. Then, the overall activation barrier is reduced to 20–27 kcal mol−1.

Gapless and Massive 1D Singlet Dispersion Channel in Infinite Spin-1/2 Ladders ---Infinite Quasi-1D Entanglement Perturbation Theory for Excitation

Abstract: We solve for the elementary excitation in infinite quasi-1D quantum lattices by extending the recently developed infinite quasi-1D entanglement perturbation theory. The wave function of an excited state is variationally determined by optimizing superposition of cluster operation, each of which is composed of simultaneous on-site operation inside a block of lattice sites, on the ground state in a form of plane wave. The excitation energy with respect to the wave number gives the spectra for an elementary excitation. Our method is artificial broadening free and is adaptive for various quasi-particle pictures. Using the triplet spectrum, the application to $\infty$-by-$N$ antiferromagnetic spin-$\frac{1}{2}$ ladders for $N=2, 4, 6, 8$, and $10$ confirms a previous report that there is a quantum dimensional transition, namely, the lattice transits from quasi-1D to 2D at a finite critical value $N_c=10$. The massless triplet dispersion at $\left( \pi, \pi \right)$ sees a vanishing gap. Our results detect the anomaly at $\left(\pi,0\right)$ in the triplet spectrum, agreeing well with the inelastic neutron scattering measurement of a macroscopic sample. Surprisingly, our results also reveal a gapless and massive 1D singlet dispersion channel that is much lower than the triplet excitation. We note, however, the dimensional transition is determined by the massless triplet dispersion.