Overview of Our Research

Our research is aimed at providing materials chemistry solutions to address the problems of societal importance. Our current interests are mainly focused on the design of advanced materials for activating small molecule transformation reactions that are relevant to renewable energy conversion and commodity chemical production. We design new materials tailored at a molecular level in a function-oriented manner, utilize spectroscopy, microscopy, and theoretical calculations to characterize materials at an atomic level, and explore these materials in a variety of catalytic reactions to establish the structure–property relationship.
Our specific target reactions in the renewable energy conversion area include the oxygen reduction reaction (ORR) and fuel oxidation reactions for fuel cells and the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) for water electrolyzers. For commodity chemical production, we are interested in hydrogen peroxide production and chlorine evolution reaction. For these reactions, platinum-group metals (PGMs) such as Pt, Ru, or Ir have been used prevalently as high-performance catalysts to overcome sluggish kinetics. However, PGM-based materials suffer from prohibitively high cost and scarcity, and they show declining activity during long-term operation and are susceptible to poisoning. In this context, we have explored materials made of non- or low-precious metal compositions, including but not limited to heteroatom-doped carbons, transition metal and nitrogen codoped carbons, transition metal compounds, atomically dispersed precious metals, and multimetallic materials with hollow and intermetallic structures.

Carbon-Based Materials [1−7]

Heteroatom-doped carbon-based materials have emerged as promising alternatives to PGM-based catalysts owing to their appreciable activity, tunable selectivity, and facile preparation. In this area of research, we discovered that the nanoscale work function of doped nanocarbons is strongly correlated with the activity and reaction kinetics of doped nanocarbon catalysts for the ORR [3]. Based on this principle, we developed highly active carbon nanotube-based materials incorporating multiple heteroatom dopants for enhancing the activity of ORR [4]. We designed highly efficient H₂O₂ production nanoporous carbon electrocatalysts with tailored surface functional groups by combining O-doping strategy and edge site engineering [5] and identified their active sites promoting H₂O₂ production [6]. We also made efforts to design new ordered nanoporous carbon materials that are constructed with hollow graphene-like frameworks, which could be exploited as advanced support materials for electrocatalysis [7].

[1] (Review) Heteroatom-Doped Carbon-Based Oxygen Reduction Electrocatalysts with Tailored Four-Electron and Two-Electron Selectivity Chem. Commun. 57, 7350−7361 (2021).
[2] (Review) Catalyst Design, Measurement Guidelines, and Device Integration for H₂O₂ Electrosynthesis from Oxygen ReductionCell Rep. Phys. Sci. 3, 100987 (2022).
[3] Intrinsic Relationship between Enhanced Oxygen Reduction Reaction Activity and Nanoscale Work Function of Doped CarbonsJ. Am. Chem. Soc. 136, 8875−8878 (2014).
[4] Carbon Nanotubes/Heteroatom-Doped Carbon Core-Sheath Nanostructures as Highly Active, Metal-Free Oxygen Reduction Electrocatalysts for Alkaline Fuel CellsAngew. Chem. Int. Ed. 53, 4102−4106 (2014).
[5] Active Edge-Site-Rich Carbon Nanocatalysts with Enhanced Electron Transfer for Efficient Electrochemical Hydrogen Peroxide ProductionAngew. Chem. Int. Ed. 58, 1100−1105 (2019).
[6] Designing Highly Active Nanoporous Carbon H₂O₂ Production Electrocatalysts through Active Site IdentificationChem 7, 3114−3130 (2021).
[7] Ordered Mesoporous Carbons with Graphitic Tubular Frameworks by Dual Templating for Efficient Electrocatalysis and Energy StorageAngew. Chem. Int. Ed. 60, 1441−1449 (2021).

Transition Metal and Nitrogen Codoped Carbons [8−14]

The structure of active sites of enzymes involved in bioenergetic processes can inspire the design of active, stable, and cost-effective catalysts for renewable-energy technologies. For example, the structure of nature’s enzyme for the ORR, cytochrome c oxidase (CcO), has inspired materials chemists to design advanced catalytic materials comprising active Fe–N_x sites. In this area, our group developed new strategies that can preferentially generate transition metal-based M–N_x sites on carbon support (M–N/C; M=Fe, FeCo, and FeNi) [9,10], based on the nanocasting method using mesoporous silica templates [9] and the silica-protective-layer-assisted strategy [10]. The resulting catalysts exhibited high activity and durability for the ORR and demonstrated device-level performances in polymer electrolyte fuel cells, suggesting their promise as a potential replacement for Pt-based ORR catalysts. We next extended the developed strategies to M–N/C catalysts of other compositions to explore their reactivity in other important small molecule conversion reactions [11-14]. The M–N/C electrocatalysts comprising Co–N_x sites are highly effective for catalyzing the HER [11] and H₂O₂ production [12,13], whereas Ni–N/C shows high activity toward CO₂ reduction [14]. Notably, a combination of the H₂O₂-producing Co–N/C electrocatalyst with a photocatalyst and biocatalyst can convert biomass into value-added chemicals with the aid of only sunlight [12]. Also, combining the Co–N/C electrocatalyst with a photocatalyst and heterogeneous catalyst enables the production of industrially important propylene oxide in an environmentally-benign manner, using only sunlight and oxygen.

[8] (Review) Steering Catalytic Selectivity with Atomically Dispersed Metal Electrocatalysts for Renewable Energy Conversion and Commodity Chemical ProductionAcc. Chem. Res. 55, 2672−2684 (2022).
[9] Ordered Mesoporous Porphyrinic Carbons with Very High Electrocatalytic Activity for the Oxygen Reduction ReactionSci. Rep. 3, 2715 (2013).
[10] A General Approach to Preferential Formation of Active Fe–N_x Sites in Fe–N/C Electrocatalysts for Efficient Oxygen Reduction ReactionJ. Am. Chem. Soc. 138, 15046−15056 (2016).
[11] Heterogeneous Co–N/C Electrocatalysts with Controlled Cobalt Site Densities for the Hydrogen Evolution Reaction: Structure–Activity Correlations and Kinetic InsightsACS Catal. 9, 83−97 (2019).
[12] Unassisted Solar Lignin Valorisation Using a Compartmented Photo-Electro-Biochemical CellNat. Commun. 10, 5123 (2019).
[13] Direct Propylene Epoxidation with Oxygen Using a Photo-Electro-Heterogeneous Catalytic SystemNat. Catal. 5, 37−44 (2022).
[14] Thermal Transformation of Molecular Ni²⁺–N₄ Sites for Enhanced CO₂ Electroreduction ActivityACS Catal. 10, 10920−10931 (2020).

Transition Metal Compound Materials [15−21]

Transition metal compound-based materials, including transition metal oxides, chalcogenides, and carbides are an important class of non-precious metal catalysts. For the water electrolyzer that can store renewable electricity in the form of chemical bonding energy, transition metal compounds have demonstrated their promise in the anodic OER and cathodic HER. In this endeavor, we identified structural factors of transition metal compounds affecting the activity of these reactions to develop advanced catalytic materials. We unveiled the role of particle size and phase in spinels [15] and defect density in perovskites [16] in dictating the activity and stability of OER. We established the layer number [17] and edge site density [18] of transition metal chacogenides for the acidic HER. Toward developing highly active catalysts for the alkaline HER, we designed transition metal carbide-based electrocatalysts that can promote water dissociation reaction, which is additionally involved elementary step during the alkaline HER [20,21].

[15] Size-Dependent Activity Trends Combined with In Situ X-Ray Absorption Spectroscopy Reveal Insights into Cobalt Oxide/Carbon Nanotube Catalyzed Bifunctional Oxygen ElectrocatalysisACS Catal. 6, 4347−4355 (2016).
[16] Oxygen-Deficient Triple Perovskites as Highly Active and Durable Bifunctional Electrocatalysts for Oxygen Electrode ReactionsSci. Adv. 4, eaap9360 (2018).
[17] Monolayer-Precision Synthesis of Molybdenum Sulfide Nanoparticles and Their Nanoscale Size Effects in the Hydrogen Evolution ReactionACS Nano 9, 3728−3739 (2015).
[18] Monomeric MoS₄²⁻-Derived Polymeric Chains with Active Molecular Units for Efficient Hydrogen Evolution ReactionACS Catal. 10, 656−662 (2020).
[19] (Review) Nanoscale Electrocatalyst Design for Alkaline Hydrogen Evolution Reaction through Activity Descriptor IdentificationMater. Chem. Front. 5, 4042−4058 (2021).
[20] Ordered Mesoporous Metastable α-MoC_1−x with Enhanced Water Dissociation Capability for Boosting Alkaline Hydrogen Evolution ActivityAdv. Funct. Mater. 29, 1901217 (2019).
[21] Metastable Phase-Controlled Synthesis of Mesoporous Molybdenum Carbides for Efficient Alkaline Hydrogen EvolutionACS Catal. 12, 7415−7426 (2022).

Atomically Dispersed Precious Metals [22−28]

PGM-based catalysts are mainstays in modern chemical industries. However, precious metals, such as Pt, Ir, and Rh, are expensive and scarce. Furthermore, widely used catalysts in the form of nanoparticles can exploit only a small fraction of the total metal atoms. In this regard, atomically dispersed precious metals have become a new frontier in the field of catalysis. They can combine the advantages of both homogenous catalysts (high activity and selectivity and maximum metal utilization) and heterogeneous catalysts (facile recyclability and separation). Furthermore, they sometimes show unusual catalytic activity and selectivity for a diverse set of reactions, which have thus far been unobserved. In this line of efforts, we developed a generalized method of preparing atomically dispersed precious metals via a “trapping-and-immobilizing” strategy over a wide range of precious metal compositions, which allowed us to reveal their activity and selectivity trends for important energy conversion reactions including the ORR [23]. We also developed a gas-phase ligand exchange method for modulating the type of coordinated ligands in atomically dispersed precious metals, which can reversibly tune the activity and selectivity of atomically dispersed catalysts [24]. Also, we found that by downsizing volatile osmium to atomically dispersed species can afford a thermally stable osmium catalyst [25].
Another important direction in this area is the use of atomically dispersed precious metals for the chorine evolution reaction (CER), which is a key anodic reaction in the chlor-alkali process for Cl₂ production, on-site generation of ClO⁻, and Cl₂-mediated electrosynthesis. Over the last half-century, the CER catalysts has been dominated by mixed metal oxides, based on RuO₂ and IrO₂. However, they unavoidably suffer from selectivity problems with competitive oxygen evolution reaction. We have recently discovered that atomically dispersed Pt−N₄ catalysts (Pt−N/C) can serve as a new class of CER catalysts with very high activity and selectivity [27]. We have also uncovered that the Pt−N/C catalysts exhibit unprecedented potential-dependent switching of reaction kinetics and mechanism [28].

[22] (Review) Electrocatalyst Design for Promoting Two-Electron Oxygen Reduction Reaction: Isolation of Active Site AtomsCurr. Opin. Electrochem. 21, 109−116 (2020).
[23] A General Strategy to Atomically Dispersed Precious Metal Catalysts for Unravelling Their Catalytic Trends for Oxygen Reduction ReactionACS Nano 14, 1990−2001 (2020).
[24] Reversible Ligand Exchange in Atomically Dispersed Catalysts for Modulating the Activity and Selectivity of Oxygen Reduction ReactionAngew. Chem. Int. Ed. 60, 20528−20534 (2021).
[25] Boosting Thermal Stability of Volatile Os Catalysts by Downsizing to Atomically Dispersed SpeciesJACS Au 2, 1811−1817 (2022).
[26] (Review) Circumventing the OCl vs. OOH Scaling Relation in the Chlorine Evolution Reaction: Beyond Dimensionally Stable AnodesCurr. Opin. Electrochem. 34, 100979. (2022).
[27] Atomically Dispersed Pt−N₄ Sites as Efficient and Selective Electrocatalysts for the Chlorine Evolution ReactionNat. Commun. 11, 412 (2020).
[28] General Efficacy of Atomically Dispersed Pt Catalysts for the Chlorine Evolution Reaction: Potential-Dependent Switching of the Kinetics and MechanismACS Catal. 11, 12232−12246 (2021).

Multimetallic Nanoparticles with Hollow and Intermetallic Structures [29−35]

PGM-based materials have been the best-performing oxygen and hydrogen electrode catalysts. One of our directions in this area is the design of multimetallic nanoparticles based on hollow nanostructures in collaboration with Prof. Kwangyeol Lee at Korea Univ. Such nanostructures can substantially reduce the amount of PGMs while enhancing the catalytic activity. Pt-based hollow nanostructures demonstrated excellent activity for the ORR [30] and Ir- and Ru-based nanostructures exhibited enhanced activity for the OER [31,32]. Another approach is the formation of PGM-based intermetallic structures to leverage strain and ligand effects for boosting catalytic activity and stability. Pt-based nanowires, nanoporous structures, and nanoframes constructed with intermetallic structures could enhance catalytic activity while significantly improving the stability of catalysts [34,35].

[29] (Review) Hollow Nanoparticles as Emerging Electrocatalysts for Renewable Energy Conversion ReactionsChem. Soc. Rev. 47, 8173−8202 (2018).
[30] Octahedral Nanoframe with Cartesian Coordinates via Geometrically Precise Nanoscale Phase Segregation in a Pt@Ni Core-Shell NanocrystalACS Nano 9, 2856−2867 (2015).
[31] Iridium-Based Multimetallic Nanoframe@Nanoframe Structure: An Efficient and Robust Electrocatalyst toward Oxygen Evolution ReactionACS Nano 11, 5500−5509 (2017).
[32] Topotactic Transformations in an Icosahedral Nanocrystal to Form Efficient Water-Splitting CatalystsAdv. Mater. 31, 1805546 (2019).
[33] (Review) Recent Advances in Nanostructured Intermetallic Electrocatalysts for Renewable Energy Conversion ReactionsJ. Mater. Chem. A 8, 8195−8217 (2020).
[34] Activity Origin and Multifunctionality of Pt-Based Intermetallic Nanostructures for Efficient ElectrocatalysisACS Catal. 9, 11242−11254 (2019).
[35]  Intermetallic PtCu Nanoframes as Efficient Oxygen Reduction Electrocatalysts
Nano Lett. 20, 7413−7421 (2020).