Research

I. Overview of Our Research

    The coupled challenges of a doubling in the world’s energy needs by the year 2050 and the ever-increasing demands for “clean” energy sources have resulted in increased attention worldwide to the possibilities of a “hydrogen economy” as a long-term solution for securing energy future. While the hydrogen economy offers a compelling vision of an energy future for the world that is abundant, clean, flexible, and secure, significant scientific and technical challenges must be overcome to achieve its implementation.
    The key components for hydrogen-based energy cycles are integrated electrochemical energy devices such as fuel cells, water electrolyzers, and solar fuel systems. The performance of these energy conversion devices depends critically on the efficiency and stability of catalysts for electrochemical reactions on the electrodes of these devices. The reactions include the electrocatalytic oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR), which occur on the cathode and anode of a hydrogen fuel cell, respectively; and the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at the cathode and anode of a water electrolyzer, respectively. These reactions involve multi-electron transfers, and are kinetically demanding. Hence, noble metal-based catalysts such as Pt, Ru, or Ir with high reaction kinetics have been predominantly used for these reactions. However, noble metal-based catalysts commonly show low durability during long-term operation and are susceptible to poisoning; furthermore, their prohibitively high cost and scarcity have also been bottlenecks that impede the widespread use of fuel cells and water electrolyzers. Hence, the development of economic electrocatalysts with high activity and durability has been of utmost importance in this area of research.

그림-전체연구개요
    Our group has been involved in the following projects: ORR electrocatalysts for fuel cells, bifunctional OER-ORR electrocatalysts for regenerative fuel cells, HER electrocatalysts for water electrolyzers, and development of new synthetic routes to nanoporous materials for electrocatalytic applications. In the following, the detailed strategies and achievements in these projects will be delineated.

II. ORR electrocatalysts for fuel cells

    Polymer electrolyte fuel cells (PEFCs) with hydrogen fuel have been considered a promising power source of alternative energy because of their high efficiency, environmental benignity, the reusability of exhaust heat, and their wide applicability for mobile, transportation, and stationary applications. In PEFCs, electrocatalysts for the ORR at the cathode play a pivotal role in dictating their overall performance. Pt-based electrocatalysts have hitherto been predominantly used for the ORR; however, their low durability, very high cost, and scarcity has hampered the widespread deployment of PEFC systems. Hence, tremendous effort has been geared towards the development of highly active and stable, yet low-cost ORR electrocatalysts that are based on low-Pt or Pt-free compositions. In this project, we have been focused on the three classes of electrocatalysts: (i) metal-free carbon-based catalysts, (ii) non-precious metal M-N/C (M=Fe and/or Co) catalysts, and (iii) Pt-based catalysts.

II-1. Metal-free, carbon-based ORR catalysts [1-4]
    Over the last few years, various carbon nanostructures, including carbon nanotubes, graphene, nanoporous carbons, and carbon nitrides, that are doped with various heteroatoms have been exploited as electrocatalysts for the ORR in alkaline media. Despite rapid progress in doped nanocarbon-based catalysts, there still remains a multitude of challenges, including relatively lower ORR activity compared to Pt/C catalysts and only sporadic demonstration of these catalysts in alkaline fuel cell systems. Furthermore, understanding of the working principles that underpin the ORR activity observed with doped nanocarbons is still limited to predictions based on theoretical calculations.
    We developed an ionic liquid (IL)-driven, facile, scalable route to new carbon nanostructures that comprise pure CNT cores and heteroatom-doped carbon (HDC) sheath layers ([1] Angew. Chem. Int. Ed. 2014). The CNT/HDC nanostructures showed excellent electrocatalytic activity for the ORR, which is one of the best performances among all heteroatom-doped nanocarbon catalysts in alkaline media. The CNT/HDC nanostructures also exhibited superior long-term durability and poison tolerance compared to Pt/C. Furthermore, the CNT/HDC nanostructures showed very high current and power densities when employed as the cathode catalyst in an alkaline fuel cell.그림1

    In an effort to understand the fundamental aspects of doped nanocarbon ORR catalysts, we reported direct experimental evidence that the nanoscale work function of doped nanocarbons, measured by Kelvin probe force microscopy, is strongly correlated with the ORR activity and reaction kinetics of doped nanocarbon catalysts ([2] J. Am. Chem. Soc. 2014). The results gleaned from this work suggest that if the local work function of the carbons can be lowered in the presence of dopants, it is possible to boost their ORR activity. Furthermore, this work identified the work function of carbonaceous materials can be used as an activity descriptor for the ORR and, potentially, other electrocatalytic reactions.

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[1] Carbon Nanotubes/Heteroatom-Doped Carbon Core-Sheath Nanostructures as Highly Active, Metal-Free Oxygen Reduction Electrocatalysts for Alkaline Fuel Cells
Angew. Chem. Int. Ed. 53, 4102 (2014).
[2] Intrinsic Relationship between Enhanced Oxygen Reduction Reaction Activity and Nanoscale Work Function of Doped Carbons
J. Am. Chem. Soc. 136, 8875 (2014).
[3] An Ice-Templated, pH-Tunable Self-assembly Route to Hierarchically Porous Graphene Nanoscroll Networks
Nanoscale 6, 9734 (2014).
[4] Ordered Mesoporous Carbon Nitrides with Graphitic Frameworks as Metal-Free, Highly Durable, Methanol-Tolerant Oxygen Reduction Catalysts in an Acidic Medium
Langmuir 28, 991 (2012).

II-2. Non-precious metal M-N/C (M=Fe and/or Co) ORR catalysts [5-7]
    Among various classes of non-Pt catalysts, non-precious metal-based M-N/C (M=Fe and/or Co) catalysts have been considered as the most attractive candidates that can replace Pt-based catalysts in acidic media. Although synthetic optimization in recent years has led to improved activities and durability of M-N/C catalysts, their ORR activities are still fairly lower than Pt-based catalysts in acidic electrolytes. Furthermore, high-performance M-N/C catalysts commonly require complex preparatory steps and the use of toxic reactive gas such as ammonia. In addition, due to the high-temperature annealing step during the preparation of these catalysts, the identification of active site structure remains elusive.
    We developed a simple approach to scalable and highly reproducible synthesis of high-performance M-N/C catalysts – self-supported, transition metal-doped ordered mesoporous porphyrinic carbons (M-OMPCs; M=Fe and/or Co) – which exhibit Pt-like catalytic activity for the ORR ([5] Scientific Reports 2013). The FeCo-OMPC showed an extremely high electrocatalytic activity for ORR in acidic media, which is among the best-performing M-N/C catalysts, and even higher than the Pt/C catalyst. In addition, the FeCo-OMPC showed superior long-term durability and methanol-tolerance in ORR, compared to the Pt/C. Density functional theory (DFT) calculations suggested a weakening of the interaction between oxygen atom and FeCo-OMPC compared to Pt/C, thereby enhancing the ORR activity of FeCo-OMPC.

그림3

    As our effort toward identifying the active sites for M-N-C catalysts, we constructed archetypical hybrid catalysts by the reaction of an organometallic complex, [CoII(acac)2] (acac=acetylacetonate), with N-doped graphene-based materials at room temperature ([6] Angew. Chem. Int. Ed. 2015). Experimental characterization combined with theoretical calculations revealed that the cobalt-containing species is coordinated to heterocyclic groups in N-doped graphene as well as to its parental acac ligands. The hybrid material shows high electrocatalytic activity for the ORR in alkaline media, and superior durability and methanol tolerance to a Pt/C catalyst. Based on the chemical structures and ORR experiments, we could identify a new active species for the ORR: “Co-O4-N” structure.

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[5] Ordered Mesoporous Porphyrinic Carbons with Very High Electrocatalytic Activity for the Oxygen Reduction Reaction
Scientific Reports 3, 2715 (2013).
[6] Coordination Chemistry of [Co(acac)2] with N-Doped Graphene: Implications for Oxygen Reduction Reaction Reactivity of Organometallic Co-O4-N Species
Angew. Chem. Int. Ed. 54, 12622 (2015).
[7] Three-Dimensional Pillared Metallomacrocycle-Graphene Frameworks with Tunable Micro- and Mesoporosity
J. Mater. Chem. A 1, 8432 (2013).

II-3. Pt-based ORR catalysts [8-11]
    Pt-based catalysts constitute the best-performing ORR catalysts. Our efforts in this area have been (i) the preparation of Pt-based alloy nanostructures that can both reduce the amount of Pt and maximize the ORR activity and (ii) the development of new carbon support materials that can enhance both activity and durability of supported Pt catalysts.
    In the former approach, we exploited octahedral Pt-Ni skeletal nanostructures for the ORR ([8] ACS Nano 2015). In Pt-Ni hollow, skeletal nanostructures, the utilization efficiency of Pt can be maximized, the electronic effect induced by second metal (Ni) can be exploited, and the collision frequency of reactants can be enhanced via a nano-confinement effect. Accordingly, the hollow Pt-Ni skeletal nanostructures showed very high ORR activity in acidic media, ranking one of the best ORR activities among the Pt-based catalysts.

그림5

    As an alternative approach to enhancing the activity and durability of Pt-based catalysts, the development of new carbon support have been of wide interests. In this line, we developed a new carbon support composed of ordered mesoporous carbon and carbon nanotubes (OMC-CNT) ([9] Chem. Commun. 2012; [10] J. Mater. Chem. A 2013). The OMC-CNT nanocomposites combine the advantages of both the carbon entities: the OMCs can provide a large surface area, mesoporosity, and interconnected porous structure, whereas the CNTs can function as electrical connectors between adjacent OMC particles, thereby lowering the interfacial resistance. The OMC–CNT nanocomposites showed enhanced ORR activity and durability for the ORR, compared to OMC samples. Further, the OMC–CNT-based cathode demonstrated better PEFC single cell performance and durability than OMC-based cells.

[8] Skeletal Octahedral Nanoframe with Cartesian Coordinates via Geometrically Precise Nanoscale Phase Segregation in a Pt@Ni Core-Shell Nanocrystal
ACS Nano 9, 2856 (2015).
[9] Highly Interconnected Ordered Mesoporous Carbon – Carbon Nanotube Nanocomposites: Pt-free, Highly efficient, and Durable Counter Electrode for Dye-Sensitized Solar Cells
Chem. Commun. 48, 8057 (2012).
[10] Ordered Mesoporous Carbon-Carbon Nanotube Nanocomposites as Highly Conductive and Durable Cathode Catalyst Supports for Polymer Electrolyte Fuel Cells
J. Mater. Chem. A 1, 1270 (2013).
[11] Impact of Framework Structure of Ordered Mesoporous Carbons on the Performance of Supported Pt Catalysts for Oxygen Reduction Reaction
Carbon 72, 354 (2014).

III. Bifunctional OER-ORR electrocatalysts [12-15]

    Bifunctional oxygen electrocatalysis involving both oxygen evolution and reduction reactions (OER and ORR, respectively) are ubiquitous and play a pivotal role in energy conversion and storage devices, such as metal-air batteries and unitized regenerative fuel cells (URFCs). Similar to ORR catalysts for fuel cells, precious metals have thus far been the prevalent choice for bifunctional oxygen electrocatalysts. To replace expensive, scarce precious metal catalysts, non-precious metal-based bifunctional electrocatalysts, including carbons doped with heteroatoms (and transition metals) and metal oxides including perovskites, have been actively pursued. However, the realization of high catalytic activity for both reactions using these non-precious metal catalysts remains a challenge.
    We designed and prepared highly integrated, high-performance, bifunctional oxygen electrocatalysts composed of highly graphitic nanoshells embedded in mesoporous carbon (GNS/MC) ([12] Adv. Energy Mater. 2016). The GNS/MC exhibited very high oxygen electrode activity, which is one of the best performances among non-precious metal bifunctional oxygen electrocatalysts, and substantially outperforms Ir- and Pt-based catalysts. Moreover, the GNS/MC shows excellent durability for both OER and ORR. The presence of Ni and Fe moieties as well as nitrogen species was found to be critical in enhancing activity and durability. The GNS/MC air cathode-based cell exhibits superior performance in aqueous Na-air battery tests to Ir/C- and Pt/C-based batteries.

그림6
    We also explored mesoporous cobalt oxides ([13] J. Mater. Chem. A 2013) and mesoporous Ni-doped graphitic carbons ([14] Chem. Commun. 2015) as bifunctional oxygen electrocatalysts for the OER and ORR. These catalysts also demonstrated excellent activity, which is superior to or far with noble metal-based catalysts.
    As bifunctional OER-ORR catalysts, cost-effective, abundant, and active Co-based materials have emerged as promising electrocatalysts, for which identifying catalytically active structures under reaction conditions and unraveling the structure–activity relationships are of critical importance. In this context, we investigated the size-dependent (3–10 nm) structure and catalytic activity of bifunctional cobalt oxide nanoparticle (CoOx NP) catalysts for the OER and ORR ([15] ACS Catal 2015 Revised). The OER activity increased with decreasing NP size, which correlated to the enhanced oxidation state and larger surface area in smaller NPs, whereas the ORR activity was independent of NP size.

[12] Graphitic Nanoshell/Mesoporous Carbon Nanohybrids as Highly Efficient and Stable Bifunctional Oxygen Electrocatalysts for Rechargeable Aqueous Na-Air Batteries
Adv. Energy Mater. 6, 1501794 (2016).
[13] Ordered Mesoporous Co3O4 Spinels as Stable, Bifunctional, Noble Metal-Free Oxygen Electrocatalysts
J. Mater. Chem. A 1, 9992 (2013).
[14] Simple Coordination Complex-Derived Three Dimensional Mesoporous Graphene as an Efficient Bifunctional Oxygen Electrocatalyst
Chem. Commun. 51, 6773 (2015).
[15] Size-Dependent Activity Trends Combined with In Situ X-Ray Absorption Spectroscopy Reveal Insights into Cobalt Oxide/Carbon Nanotube Catalyzed Bifunctional Oxygen Electrocatalysis
ACS Catal. Accepted for Publication (2016).

IV. HER electrocatalysts for water electrolyzers [16-18]

    Hydrogen has been of pivotal importance as a clean energy carrier for the realization of a hydrogen economy. Current hydrogen production depends mainly on the steam reforming of hydrocarbons, or the partial oxidation of methane. Water splitting, when coupled with renewable energy sources such as solar energy, is considered an ideal method for hydrogen production, owing to its unparalleled capacity and carbon-neutral nature. For water electrolyzers, expensive Pt is the most effective catalyst for the HER. Hence, intensive efforts have been devoted to replacing Pt-based catalysts with non-precious metal catalysts, among which metal sulfides have emerged promising class of catalysts.
    In this endeavor, we investigated the nanoscale size effect in MoS2-based HER catalysts ([16] ACS Nano 2015). For this purpose, we developed a “confined nanospace” approach to generate MoS2 nanoplates with monolayer precision from one to four layers with the nearly constant basal plane size of 5 nm. This series of model catalysts revealed that the turnover frequency (TOF) of MoS2 nanoplates in the HER increases with decreased layer numbers. Particularly, the TOF value of monolayer MoS2 approached that of best-performing HER catalyst. We attributed the higher HER activity of smaller MoS2 to the higher degree of oxidation, higher Mo-S coordination number, formation of the 1T phase, and lower activation energy required to overcome transition state.

그림7
    We also identified the preferential impact of oxide layer in WS2-based HER catalysts ([17] Chem. Commun. 2015). The core–shell structured W18O49@WS2 nanorods (NRs) showed enhanced HER activity, compared to WS2 nanotubes (WS2 NTs). In W18O49@WS2 NRs, the oxide core facilitates electron transfer between the catalyst and the electrode interface without the aid of any additional conducting supports such as carbon fibers or graphene.

[16] Monolayer-Precision Synthesis of Molybdenum Sulfide Nanoparticles and Their Nanoscale Size Effects in the Hydrogen Evolution Reaction
ACS Nano 9, 3728 (2015).
[17] Impact of a Conductive Oxide Core in Tungsten Sulfide-Based Nanostructures on the Hydrogen Evolution Reaction
Chem. Commun. 51, 8334 (2015).
[18] Facet-Controlled Hollow Rh2S3 Hexagonal Nanoprisms as Highly Active and Structurally Robust Catalysts toward Hydrogen Evolution Reaction
Energy Environ. Sci. 9, 850 (2016).

V. Preparation and applications of new nanoporous materials [19-20]

    Nanoporous metal oxide materials are ubiquitous in the material sciences because of their numerous potential applications in various areas, including energy conversion and storage, drug delivery, and optoelectronics.
    In this area, we developed a novel synthetic strategy that exploits a metal−organic framework (MOF)-driven, self-templated route toward nanoporous metal oxides ([18] J. Am. Chem. Soc. 2013; [19] Highlighted in “Editor’s Choice” of Science). While the preparation of siliceous nanoporous materials is well-established, non-siliceous metal oxide-based nanoporous materials still present challenges. In our approach, an aliphatic ligand-based MOF is thermally converted to nanoporous metal oxides with highly nanocrystalline frameworks. With this approach, hierarchically nanoporous magnesia (MgO) and ceria (CeO2), which have potential applicability for catalysis and energy storage.

그림8

    We next demonstrated the preparation of nanoporous metal oxides with tunable oxidation states using manganese oxides as model systems, by exploiting the MOF-based transformative route ([20] J. Mater. Chem. A 2014).

[19] Nanoporous Metal Oxides with Tunable and Nanocrystalline Frameworks via Conversion of Metal-Organic Frameworks
J. Am. Chem. Soc. 135, 8940 (2013).
[20] A Transformative Route to Nanoporous Manganese Oxides of Controlled Oxidation States with Identical Textural Properties
J. Mater. Chem. A 2, 10435 (2014).