Research Activity

- Quantum Mechanics, Atomic Modeling and Machine Learning for Fuel Cell, Hydrogen and Energy Materials- 

As global warming caused by carbon dioxide and depletion of petroleum energy have become global issues, the importance of renewable energy such as biomass, hydrogen, and fuel cells as an alternative to fossil fuels have been emphasized. 

<H2-powered energy society>

One of the major barriers to develop renewable energy technology is the low efficiency of materials such as catalyst and oxygen/proton conductor in the biomass conversion, hydrogen production, fuel cell, solar cell, and battery operation. Note that the existing material development has been conducted experimentally through time consuming trial-and-error design, synthesis, and evaluation at the particle level. The essential part in the development of high-performance materials is to properly tailor the physical and chemical properties of materials. However, a detailed understanding of how to control the properties of such materials is still lacking, despite its importance in designing and developing novel and cost-effective materials. This is in large part due to the difficulty of direct characterization. Alternatively, computational methods such as quantum mechanics-based Density Functional Theory (DFT is ab-initio calculation, which predicts material properties based solely on physical laws without experimental values) and atomistic modeling approaches have emerged as the powerful and flexible means to unravel the fundamental principles of energy and environmental materials, which may allow the new finding of the breakthrough materials.

<Computational design of nanocatalysts for energy applications>

- Fundamental Study on the Nanocatalysis of Multi-metallic Systems - 

Alloying effect can be attributed to: i) the existence of unique mixed-metal surface sites [the so called ensemble (geometric) effect]; ii) electronic state changes due to metal-metal interactions [the so called ligand (electronic) effect]; and iii) strain caused by lattice mismatch between the alloy components [the so called strain effect]. In addition, the presence of low-coordination surface atoms and preferential exposure of specific facets [(111), (100), (110)] in association with the size and shape of nanoparticle catalysts [the so called shape-size-facet effect] can be another important factor for modifying the catalytic activity. Therefore, a clear understanding of these alloying effects is essential for the successful development of effective catalysts.

By using spin-polarized density functional theory calculations, we have elucidated the role of heteronuclear interactions in determining the selective H2 formation from HCOOH decomposition on bimetallic Pdshell/Mcore (M = late transition FCC metal (Rh, Pt, Ir, Cu, Au, Ag)) catalysts. We found that the catalysis of HCOOH decomposition strongly depends on the variation of surface charge polarization (ligand effect) and lattice distance (strain effect), which are caused by the heteronuclear interactions between surface Pd and core M atoms. In particular, the contraction of surface Pd–Pd bond distance and the increase in electron density in surface Pd atoms in comparison to the pure Pd case are responsible for the enhancement of the selectivity to H2 formation via HCOOH decomposition. Our calculations also unraveled that the d band center location and the density of states for the d band (particularly dz2, dyz, and dxz) near the Fermi level are the important indicators that explain the impact of strain and ligand effects in catalysis, respectively. That is, the surface lattice contraction (expansion) leads to the downshift (upshift) of d band centers in comparison to the pure Pd case, while the electronic charge increase (decrease) in surface Pd atoms results in the depletion (augmentation) of the density of states for dz2, dyz, and dxz orbitals. Our study highlights the importance of properly tailoring the surface lattice distance (d band center) and surface charge polarization (the density of states for dz2, dyz, and dxz orbitals near the Fermi level) by tuning the heteronuclear interactions in bimetallic Pdshell/Mcore catalysts for enhancing the catalysis of HCOOH decomposition toward H2 production, as well as other chemical reactions.

We unravel the beneficial role of the Zn ensemble (in particular, an a single Zn atom) in the sixfold-coordinated kinked (Cu-vacant) site of the stepped Cu(2 1 1) surface for enhancing the reactivity and durability of catalyst in the CH3OH production from CO2 and H2. For such purpose, by using the density functional theory (DFT) and microkinetic modeling methods, we systematically calculate the catalytic properties (activation energy barrier, turn of frequency (TOF), and rate constant), physical properties (cohesive and formation energy) and electronic structures (local density of state, and local charge distribution) of the different defective Cu sites [such as the stepped, kinked, Zn-substituted stepped Cu(2 1 1) surfaces] and the different Zn ensembles [dimer, and linear ensemble]. First, our DFT calculations exhibit that the Zn atoms at the sevenfold-coordinated site of the Cu(2 1 1) surface tend to be isolated and acts as the modifier to suppress the loss of Cu atoms from the stepped Cu(2 1 1) surface. Second, we find that the catalysis of CH3OH synthesis strongly depends on the type of defects at the Cu(2 1 1) surface. In particular, the single Zn atom-substituted (sevenfold-coordinated) stepped site in the Cu(2 1 1) surface is found to have the superior catalytic activity (TOF = 3.07 × 10−5 s−1 @ P = 75 bar and T = 523 K) toward the CH3OH formation compared to the traditionally-known active Cu(2 1 1) surface (TOF = 2.73 × 10−7 s−1). In contrast, the sixfold-coordinated kinked site is determined to largely slow down the rate of CH3OH production (TOF = 3.34 × 10−15 s−1). The increased catalysis in the Zn-associated stepped site is related to the significant enhancement of the surface affinity toward the adsorbate having the oxygen moiety (especially, HCOO), which leads to the large reduction of the activation energy barrier in the initial energy-demanding CO2 hydrogenation reaction and in turn the improved catalysis of CH3OH synthesis. 

- Ammonia as Hydrogen Carrier - 

Using the spin-polarized density functional theory (DFT) calculations, we examined the electrochemical N2 reduction (N2RR) toward NH3 production on the hetero RuM (M = 3d transition metals) double atom catalysts supported on the defective graphene by means of the analysis on the geometric ensemble structure, the N2RR mechanism, the decoupling of strain, dopant and configurational effects and the d-orbital resolved density of state (ORDOS) (dz2, dxz, dyz, dxy, and dx2−y2) on the hetero double atoms. In addition, we computationally screened the novel catalysts by exploring the 4d, 5d and p block metals as the hetero M metals in RuM system. First, we found the significantly enhanced N2RR activity of the inclined pentagon M (Fe, Mn, and Sc) double atom catalysts (RuFe has highest activity) compared to the homo Ru2 double atom catalyst. Our DFT calculations on the interplay of strain, dopant and configurational effects in the inclined pentagon M (Fe, Mn, and Sc) double atom catalysts predicted that (1) the dopant effect was the promoter to improve N2RR activity for RuSc and RuMn, (2) the tensile strain (RuSc) tended to reduce the NH3 productivity via N2RR, while the effect of compressive strain (RuFe, RuMn) was insignificant, (3) the dopant-support interaction induced the unique inclined pentagon M double atom ensemble structure, which leads to the large reduction of the N2RR activity for the hetero RuSc double atom but the activity increase for the hetero RuFe and RuMn cases. Finally, our DFT calculation on the analysis of the p−d (dz2, dxz, dyz, dxy, and dx2−y2) orbital overlap identified the key d orbitals in determining the descriptor (NH2 adsorption energy) for representing N2RR. That is, the orbitals (dz2, dxz , dyz) having the orientation toward z direction in the horizontal Ru2 double atom played an important role in determining NH2 adsorption process, while for the inclined pentagon M double atoms (RuFe, RuSc, RuMn), the dxz and dxy orbitals were found to be essential for the modification of NH2 adsorption energy. Finally, a descriptor based DFT search additionally discovered the promising hetero RuOs and RuIr double atom catalysts. This study highlights that the dopant engineering of hetero double atom catalysts supported on the defective graphene can significantly modify the electrochemical reactivity, particularly by the dopant type and geometric ensemble structure.  

We have engineered the MXene supports to boost the single and homo double atoms of Fe, Ru, and Os for efficient NH3 production via electrochemical nitrogen reduction reaction (N2RR) using DFT calculations. We designed the different MXene surfaces which are composed of nine early transition metals [M2CO2 (M = Cr, Hf, Mo, Nb, Ta, Ti, V, W, Zr)] and examined the activity/stability of single and homo double atoms by calculating the free energy diagram of N2RR, dissolution potential, and agglomeration energy. First, we found that the NH2 adsorption energy is the activity descriptor for representing the NH3 productivity and the density of state near the Fermi level of the single Ru atom is the important factor in determining N2RR activity. Next, our DFT calculation on the descriptor-based computational search for the novel MXene-based catalysts showed that among the chemically and electrochemically stable candidate catalysts, the homo double Ru2/Mo2CO2 catalyst showed the highest NH3 productivity with the high N2RR selectivity over hydrogen evolution reaction. In addition, the best Ru2/Mo2CO2 catalyst exhibited the intermediate density of state near the Fermi level, leading to the optimal descriptor value (NH2 adsorption strength) for NH3 production and in turn the reduction of overpotential for the electrochemical NH3 production. More fundamentally, we identified that the electron density near the Fermi level of these single or double atoms is closely correlated with the electron structure of the cationic metal atoms constituting MXene supports. Our study highlights the rational design of single and homo double atom catalysts by tuning the property of MXene supports, which provides insight into the key factors in enhancing NH3 production at ambient conditions.

- Renewable Hydrogen Production from "unused" biomass - 

There is currently no theoretical study on the hydrogenation of xylose to xylitol on a catalyst's surface, limiting proper understanding of the reaction mechanisms and the design of effective catalysts. In this study, DFT techniques were used for the first time to investigate the mechanisms of xylose to xylitol conversion on five notable transition metal (TM) surfaces: Ru(0001), Pt(111), Pd(111), Rh(111), and Ni(111). Two transition state (TS) paths were investigated: TS Path A and TS Path B. The TS Path B, which was further subdivided into TS Path B1 and B2, was proposed to be the minimum energy path (MEP) for the reaction process. According to our computational results, the MEP for this reaction begins with the structural rearrangement of cyclic xylose into its acyclic form prior to step-wise hydrogenation. The rate-determining step (RDS) on Ru(0001), Pt(111), Pd(111), and Ni(111) was discovered to be the ring-opening process via C–O bond scission of cyclic xylose. On Rh(111), however, the RDS was found to be the first hydrogenation stage, leading to the hydrogenation intermediate. Furthermore, based on the RDS barrier, our results revealed that the activities of the tested TM surfaces follow the trend: Ru(0001) > Rh(111) ≥ Ni(111) > Pd(111) > Pt(111). This result demonstrates the higher activity of Ru(0001) compared to other surfaces used for xylose hydrogenation. It correlates with experimental trends in relation to Ru(0001) superiority and provides the basis for understanding the theoretical design of economical and more active catalysts for xylitol production.

<Development of catalyst from the unused C5/C6 compounds  for H2 production>

Shorter chain alcohols, as opposed to longer ones, are beneficial as biomass feedstock for chemicals and fuels, including hydrogen production. More so, it has been demonstrated that carbon–carbon rather than carbon–oxygen bond-cleaving activity determines the product selectivity of a metal catalyst for higher oxygenates reforming. In this report, we investigate the direct C2–C3 bond-cleaving activity of xylitol via first-principles, periodic density functional theory calculations to identify the differences in activities between single-crystal catalysts (SCCs) and single-atom catalysts (SACs). A comparison of the kinetic barriers revealed that xylitol's C–C bond scission appears to be a near-impossible task on SCCs. However, SACs demonstrated higher performance. For example, Ir1/MgO and Ir1/MgO_Ovac (having surface oxygen vacancy) yielded ∼72% and 54% decrease, respectively, in Gibb’s free activation energy compared to Ir (111) at the xylitol reforming operating temperature of 473 K. Furthermore, electronic structure calculations revealed an up-shift in the DOS for the surface M1 atoms in all investigated SACs compared to the surface atoms of their respective SCCs, resulting in M1 higher d-band center and stronger adsorbate (s) binding. This study highlights the importance of SACs for boosting the atom efficiency of costly metals while also offering a new strategy for tuning the activity of catalytic reactions. 

- Machine (deep) Learning for Material Discovery- 

<Machine learning and DFT/MD based design of catalysts>

- Electricity generation by using fuel cell - 

Despite recent efforts on replacing a noble Pt to less expensive catalysts (such as Pt-Ni and Pt-Co alloys) for improving oxygen reduction reaction (ORR) for PEMFC (polymer electrolyte membrane fuel cell) application, the performance and stability of a noble Pt catalyst still remains superior. We have proposed the systematic procedure for designing the Ir3M (M = 3d, 4 d, 5 d transition metal) nanoalloy as Pt alternatives with enhanced ORR activity and stability using density functional theory (DFT). First, we computationally optimized the surface occupied/unoccupied d states and lattice distance of the thermodynamically-stable Ir3M nanoalloy in order to achieve the wanted oxygen affinity for promoting ORR. In the next screening process, the nanoalloy prone to the segregation of inside M atom toward the surface layer was excluded, leading ultimately to the potential candidates such as the pure Ir monolayer on the top of Ir3Cr, Ir3V, Ir3Re, and Ir3Tc alloy cores. The detail mechanism on the enhanced activity in Ir-M alloy was also examined. The design principle of alloy catalysts used in this study can be further extended to the screening of catalytic materials for the application to the next-level electrochemical reaction.

- CH4 and CO2 utilization - 

Catalytic descriptors were studied to design optimum catalysts for the oxidative coupling of methane (OCM) by combining density functional theory (DFT) calculations and actual reaction experiments. SrTiO3 perovskite catalysts, selected for OCM, were modified using metal dopants, and their electronic structures were calculated using the DFT method. The CH3 adsorption energy Eads(CH3) and the oxygen vacancy formation energy Ef(vac) exhibited volcano-type correlations with the C2+ selectivity and O2-consumption for the formation of COx, respectively. The optimum catalytic activity, represented by the C2+ selectivity, was obtained for Eads(CH3) = −2.0 to −1.5 eV, indicating that overly strong adsorption of methyl radicals (or easily dissociated Csingle bondH bonds of methane) and relatively insufficient oxygen supplementation to the catalyst surface improve deep oxidation to CO and CO2. Praseodymium (Pr)- and neodymium (Nd)-doped SrTiO3 catalysts confirm the DFT-predicted optimum electronic structure of the OCM catalysts. 

- PEMFC multi-scale modeling  - 


- Recent Sponsored Research Projects-