Energy Conversion Reactions Research Articles

Reactivity mapping of nanoscale defect chemistry under electrochemical reaction conditions

Electrocatalysts often show increased conversion at nanoscale chemical or topographic surface inhomogeneities, resulting in spatially heterogeneous reactivity. Identifying reacting species locally with nanometer precision during chemical conversion is one of the biggest quests in electrochemical surface science to advance (electro)catalysis and related fields. Here, we demonstrate that electrochemical tip-enhanced Raman spectroscopy can be used for combined topography and reactivity imaging of electro-active surface sites under reaction conditions. We map the electrochemical oxidation of Au nanodefects, a showcase energy conversion and corrosion reaction, with a chemical spatial sensitivity of about 10 nm. The results indicate the reversible, concurrent formation of spatially separated Au2O3 and Au2O species at defect-terrace and protrusion sites on the defect, respectively. Active-site chemical nano-imaging under realistic working conditions is expected to be pivotal in a broad range of disciplines where quasi-atomistic reactivity understanding could enable strategic engineering of active sites to rationally tune (electro)chemical device properties.

Intramolecular Electron Transfer Governs Photoinduced Hydrogen Evolution by Nickel-Substituted Rubredoxin: Resolving Elementary Steps in Solar Fuel Generation.

The field of solar fuels is a rapidly growing area of research, though low overall efficiencies continue to preclude large-scale implementation. To resolve the elementary processes involved in light-driven energy storage and identify key factors contributing to efficiency losses, systematic investigation and optimization are necessary. In this work, a ruthenium chromophore is directly attached to a model hydrogenase enzyme, nickel-substituted rubredoxin, to construct a molecular system capable of photoinduced hydrogen evolution. Time-resolved absorption and emission spectroscopy reveal direct, rapid intramolecular electron transfer (ET) between the two metal centers to generate a charge-separated state that persists for ∼1 μs, though this species is not productive for hydrogen evolution. Investigation of the photochemical behavior under catalytic conditions in conjunction with thermochemical analyses suggests that ET to the catalytic nickel site from the reductively quenched ruthenium center is the rate-determining step. By eliminating the need for three components to diffuse together, direct mechanistic information about catalysis is obtained in a time-resolved manner. This approach is generalizable to study the activity and intramolecular charge transfer properties of a wide range of photosensitizers and catalysts, with applicability toward diverse energy conversion reactions.

High electrocatalytic performance of bimetallic sulfides dodecahedral nanocages (CoxM1-x)9S8/M/N–C (M=Ni, Cu) for triiodide reduction reaction and oxygen evolution reaction

Electrocatalysis is the key process for many important energy conversion reaction, such as water splitting and CO2 reduction. The development of high efficiency electrocatalysts has attracted more attention in the field of electrochemistry and photovoltaic cell. Here, a simple cation exchange method is used to prepare the bimetallic sulfides ((CoxM1-x)9S8/M/N–C, M = Ni or Cu) dodecahedral nanocages (DNCs), in which (CoxM1-x)9S8 nanoparticles are coated with N-doped carbon shell. Bimetallic sulfides exhibit enhanced electrocatalytic property for triiodide reduction reaction and oxygen evolution reaction (OER) owing to favorable compositional features and well-designed architectures. The catalytic activities extremely depend on the atom ratio of Co:M and the component. Remarkably, (Co0.75Ni0.25)9S8/N–C DNCs deliver the best electrocatalytic activity for the triiodide reduction reaction using as the low-cost Pt-free counter electrode (CE) in dye-sensitized solar cells (DSSCs). Furthermore, (Co0.64Ni0.36)9S8/Ni/N–C-1 DNCs as an OER catalyst exhibit excellent electrocatalytic property in the light of the low overpotential (326 mV) at the current density of 10 mA cm −2. This notion and expedient method of construction can be broadened to synthesize other bimetallic sulfides with preferable electrocatalytic performance.

Highly efficient decomposition of ammonia using high-entropy alloy catalysts

Ammonia represents a promising liquid fuel for hydrogen storage, but its large-scale application is limited by the need for precious metal ruthenium (Ru) as catalyst. Here we report on highly efficient ammonia decomposition using novel high-entropy alloy (HEA) catalysts made of earth abundant elements. Quinary CoMoFeNiCu nanoparticles are synthesized in a single solid-solution phase with robust control over the Co/Mo atomic ratio, including those ratios considered to be immiscible according to the Co-Mo bimetallic phase diagram. These HEA nanoparticles demonstrate substantially enhanced catalytic activity and stability for ammonia decomposition, with improvement factors achieving >20 versus Ru catalysts. Catalytic activity of HEA nanoparticles is robustly tunable by varying the Co/Mo ratio, allowing for the optimization of surface property to maximize the reactivity under different reaction conditions. Our work highlights the great potential of HEAs for catalyzing chemical transformation and energy conversion reactions.

Plasmonic Hot-Carrier-Mediated Solar Energy Conversion and Tunablephotochemical Reactions

The detailed balance between photon-absorption and energy loss constrains the efficiency of conventional solar cells to the Shockley-Queisser limit. Recent developments of hot-carrier science and technology have stimulated the design of solar cells utilizing hot-carriers, but the efficiency is limited. In this presentation, a theoretical model for hot-carrier solar cells is discussed. Our theoretical model indicates that this cell can significantly improve the absorption of solar radiation without reducing open-circuit voltage. However, a significant fraction of the hot-carriers have energies below the Schottky barrier, which makes the cell suffer low internal quantum efficiency. Despite the limitation in solar energy conversion, the plasmonic hot-carrier can be applied to drive the photochemistry which is impossible in previous thought. Examples of hot-carrier mediated photochemical reaction and how the plasmonic hot-carriers can be employed to tune the chemical reactions will also be introduced. Our non-adiabatic molecular dynamics simulations and time-dependent density functional calculations found the hot-carriers generated from the nanoparticle can transfer to the anti-bonding state of the attached molecule and drive the photochemical reaction. We also found chemical reaction is tunable if the molecule is placed in the center of the plasmonic dimer. The reaction rate can be either suppressed or enhanced depending on the geometry. Thus, our work demonstrates the possibility of tunable photochemistry via plasmonic hot-carriers.

Reduced graphene oxide‐based materials for electrochemical energy conversion reactions

AbstractThere have been ever‐growing demands to develop advanced electrocatalysts for renewable energy conversion over the past decade. As a promising platform for advanced electrocatalysts, reduced graphene oxide (rGO) has attracted substantial research interests in a variety of electrochemical energy conversion reactions. Its versatile utility is mainly attributed to unique physical and chemical properties, such as high specific surface area, tunable electronic structure, and the feasibility of structural modification and functionalization. Here, a comprehensive discussion is provided upon recent advances in the material preparation, characterization, and the catalytic activity of rGO‐based electrocatalysts for various electrochemical energy conversion reactions (water splitting, CO2 reduction reaction, N2 reduction reaction, and O2 reduction reaction). Major advantages of rGO and the related challenges for enhancing their catalytic performance are addressed.

Influence of Mesityl and Thiophene Peripheral Substituents on Surface Attachment, Redox Chemistry, and ORR Activity of Molecular Iron Porphyrin Catalysts on Electrodes.

Two iron porphyrin complexes with either mesityl (FeTMP) or thiophene (FeT3ThP) peripheral substituents were attached to basal pyrolytic graphite and Ag electrodes via different immobilization methods. By combining cyclic voltammetry and in-operando surface-enhanced Raman spectroscopy along with MD simulations and DFT calculations, their respective surface attachment, redox chemistry and activity toward electrocatalytic oxygen reduction was investigated. For both porphyrin complexes, it could be shown that catalytic activity is restricted to the first (few) molecular layer(s), although electrodes covered with thiophene-substituted complexes showed a better capability to consume the oxygen at a given overpotential even in thicker films. The spectroscopic data and simulations suggest that both porphyrin complexes attach to a Ag electrode surface in a way that maximum planarity and minimum distance between the catalytic iron site and the electrode is achieved. However, due to the distinctive design of the FeT3ThP complex, the thiophene rings are capable of occupying a conformation, via rotation around the bonding axis to the porphyrin, in which all four sulfur atoms can coordinate to the Ag surface. This effect creates a dense and planar surface coverage of the porphyrin on the electrode facilitating a fast (multi) electron transfer via several covalent Ag-S bonds. In contrast, bulky mesityl groups as peripheral substituents, which have been initially introduced to prevent aggregation and improve catalytic behavior in solution, exert a negative effect on the overall electrocatalytic performance in the immobilized state as a less dense coverageand less stable interactions with the surface are formed. Our results underline the importance of rationally designed heterogenized molecular catalysts to achieve optimal turnover, which not onlystrictly applies to the here discussed oxygen reduction reaction but eventually holds also true for other energy conversion reactions such as carbon dioxide reduction.

The electronic structure underlying electrocatalysis of two‐dimensional materials

AbstractThe understanding and development of advanced electrochemical catalysts have attracted intensive studies and achieved tremendous progress in the past decades. Two‐dimensional (2D) materials, such as doped graphene and atomically‐thin transition metal compounds, have shown great promise as electrocatalysts for various renewable energy conversion and storage reactions. Their further developments require improved understanding of the catalytic mechanisms at atomic level. Currently, most of the understandings are based on the formation free energies of the intermediates, which are determined by their binding strengths with the catalyst, usually calculated from density functional theory (DFT). These energies/binding strengths have been used as descriptor to describe the activity of many catalysts. However, it remains less explored why different catalysts have different binding strengths and what are the underlying factors controlling them, requiring studies going beyond atomic level to electronic level. This review aims to provide such links, focusing on 2D electrocatalysts for hydrogen evolution reaction (HER), oxygen reduction reaction/oxygen evolution reaction (ORR/OER), CO2 reduction reaction (CO2R) and nitrogen reduction reaction (NRR). We also discuss some of the significant issues that need to be addressed in DFT calculations, including the effects of varying charge and fixed potential of the catalyst, the passivation of active sites, and the solvation effects.This article is categorized under: Structure and Mechanism > Reaction Mechanisms and Catalysis Structure and Mechanism > Computational Materials Science

Graphite-Conjugated Acids Reveal a Molecular Framework for Proton-Coupled Electron Transfer at Electrode Surfaces.

Proton-coupled electron-transfer (PCET) steps play a key role in energy conversion reactions. Molecular PCET reactions are well-described by “square schemes” in which the overall thermochemistry of the reaction is broken into its constituent proton-transfer and electron-transfer components. Although this description has been essential for understanding molecular PCET, no such framework exists for PCET reactions that take place at electrode surfaces. Herein, we develop a molecular square scheme framework for interfacial PCET by investigating the electrochemistry of molecularly well-defined acid/base sites conjugated to graphitic electrodes. Using cyclic voltammetry, we first demonstrate that, irrespective of the redox properties of the corresponding molecular analogue, proton transfer to graphite-conjugated acid/base sites is coupled to electron transfer. We then show that the thermochemistry of surface PCET events can be described by the pKa of the molecular analogue and the potential of zero free charge (zero-field reduction potential) of the electrode. This work provides a general framework for analyzing and predicting the thermochemistry of interfacial PCET reactions.

Metallomacrocycle-Driven Atomically Dispersed M-N x Sites on Carbon Nanostructures for Electrocatlyic Energy Conversion Reactions

Developing highly active, durable, and low-cost electrocatalysts is of crucial importance to making renewable energy conversion devices, such as fuel cells and electrolyzers, economically viable. Hence, tremendous efforts have been geared to develop non-precious metal electrocatalysts that can replace expensive precious metal catalysts for energy conversion reactions. Among various classes of non-precious metal electrocatalysts, transition metal and nitrogen codoped carbon (M–N/C) catalysts have demonstrated excellent catalytic activity, which even rivals that of precious metal catalysts. In this presentation, we present our recent efforts toward high-performance M–N/C catalysts for energy conversion reactions, including oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) [1-7]. We have developed a general “silica-protective-layer-assisted” approach that can preferentially generate the catalytically active M–N x sites in M–N/C catalysts while suppressing the formation of catalytically less active or inactive large metallic particles [4,6,7]. The catalyst preparation consisted of adsorption of a metallomacrocyle precursor on a carbon support, low-temperature annealing, silica layer overcoating, high-temperature pyrolysis, and silica layer etching, which yielded a carbon coated with thin layer of metallomacrocyclic carbon catalysts [4]. The prepared catalysts contained a higher density of active M–N x sites compared to the catalytic prepared without the silica coating step. This preparation method is generally applicable to various carbon supports and metallomacrocyclic compounds. The prepared Fe-containing catalysts showed very high ORR activity and excellent stability in alkaline media. Importantly, an alkaline anion exchange membrane fuel cell with a CNT/PC-based cathode exhibited record high current and power densities among NPMC-based AEMFCs. Furthermore, using metallomacrocycle precursors with Co, Ni, and Pt metal centers, catalysts showing excellent performances toward HER [7], CO2 reduction reaction [8] were developed. References Y. Cheon et al., Scientific Reports 3, 2715 (2013).J. Han et al., Angew. Chem. Int. Ed. 54, 12622 (2015).Y. Cheon et al., Adv. Energy Mater. 6, 1501794 (2016).J. Sa et al., J. Am. Chem. Soc. 138, 15046 (2016).H. Kim et al., ACS Appl. Mater. Interfaces 9, 9567 (2017).Woo et al., Chem. Mater. 30, 6684 (2018).J. Sa et al., ACS Catal. 9, 83 (2019).J. Sa et al., manuscript in preparation (2019).

MoS2/NiS2 Composites for Efficient Hydrogen Evolution Both in Acidic and Alkaline Media

The growing concern about global warming, environmental pollution and energy security has raised the demand for clean energy resources in place of fossil fuel. Cost-efficient generation of hydrogen from water-splitting through electrocatalysis holds tremendous promise for clean energy (1, 2). Central to the electrocatalysis are efficient and robust electrocatalysts composed of earth-abundant elements, which are urgently needed for realizing low-cost and high-performance energy conversion devices. Transition metal compounds (TMCs) are a group of attractive noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER). Earth-abundant MoS2 has emerged as a promising hydrogen evolution reaction (HER) catalyst with high activity and durability, but it is kinetically retarded in alkaline media . Extensive efforts have been devoted to improving the HER activity of MoS2 in acid (3), but little attention has been paid to simultaneously enhancing its HER performance both in acid and base. This work demonstrates a dramatic enhancement of HER kinetics both in acidic and alkaline media by hybridizing vertical MoS2 with another earth-abundant material, nickel disulfide (NiS2). Field-emission SEM image shows that the MoS2/NiS2 composite is composed of a film-like structure of NiS2 cover on the edge surfaces of the MoS2 sheets (Figure 1a). As shown in Figure 1b, the XRD peaks observed at 2θ=14.1°, 32.9° and 58.8° of the MoS2/NiS2 sample can be indexed to 2H-MoS2 crystal (JCPDS No. 75-1539), and peaks at 2θ=31.4°, 45.1° and 53.4° indexed to (200), (220) and (311) facets of the NiS2, respectively (JCPDS No. 88-1709). The synthesized MoS2/NiS2 composite exhibits a much lower HER overpotential both in acidic and alkaline media than that of bare MoS2 and NiS2 catalysts (Figure 1c and d). Figure caption Figure 1. (a) FESEM image of the MoS2/NiS2 composite. (b) XRD patterns of the NiS2 and MoS2/NiS2 samples, together with the standard pattern of 2H MoS2 (JCPDS 75-1539) and NiS2 (JCPDS 88-1709). (c) Polarization curves of bare MoS2, NiS2 and MoS2/NiS2 composite catalysts in 0.5 M H2SO4 solution at a scan rate of 5 mV/s. (d) Polarization curves of bare MoS2, NiS2 and MoS2/NiS2 composite catalysts in 1 M KOH solution at a scan rate of 5 mV/s. References D. Strmcnik et al., Improving the hydrogen oxidation reaction rate by promotion of hydroxyl adsorption. Nat. Chem. 5, 300-306 (2013).Y. Jiao, Y. Zheng, M. T. Jaroniec, S. Z. Qiao, Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 44, 2060-2086 (2015).J. Hu et al., Engineering stepped edge surface structures of MoS2 sheet stacks to accelerate the hydrogen evolution reaction. Energy Environ. Sci. 10, 593-603 (2017). Figure 1

Heterogeneous Ni-Fe-P integrated with nickel foam as an efficient and durable electrocatalyst for water oxidation

Transition metal phosphides represent a class of promising electrocatalysts for electrochemical water oxidization and other energy conversion reactions. In this work, we report a novel seal strategy, followed by a second step phosphorization treatment, produces Ni-Fe-P heterostructure directly grown on nickel foam that shows highly catalytic activity for electrochemical water oxidation. The as-prepared catalyst exhibits an excellent electrocatalytic activity, manifested by a current density of 20 mA cm−2 with a very low overpotential (260 mV) for oxygen evolution reaction along with an extremely small Tafel slope of 27 mV decade−1 and a very excellent durability of 120 h in alkaline solution. The excellent performance with exceptional durability owe much to the synergistic effect between the Ni-Fe-P heterostructure and the outer oxidized layer. Remarkably, a current density of 10 mA cm−2 was reached at a low cell voltage of 1.63 V when the original electrode and surface oxidized electrode are used as cathode and anode, respectively.

Efficient Visible‐Light‐Driven Hydrogen Generation on g‐C3N4 Coupled with Iron Phosphide

AbstractTransition metal phosphides as promising noble metal free cocatalysts are gaining increasing interest for hydrogen generation and other energy conversion reactions. The present study reports a new Fe2P/g‐C3N4 hybrid for efficient photocatalytic hydrogen generation by water splitting under visible light irradiation. The H2 production rate obtained using this system is approximately 15 times higher than that obtained using pure g‐C3N4, and is comparable with that of Pt/g‐C3N4 under the same conditions. According to detailed studies using UV/Vis diffuse reflectance and photoluminescence spectroscopies as well as photoelectrochemical measurements, the reason for the high efficiency of the Fe2P/g‐C3N4 hybrid is due to a highly effective separation and low recombination rate of photogenerated electrons and holes rather than absorption under visible light. The present study reports a promising non‐toxic photocatalyst with low cost and high natural abundance for improving photocatalytic H2 generation.

Modular Design of Noble-Metal-Free Mixed Metal Oxide Electrocatalysts for Complete Water Splitting.

Electrocatalytic water splitting into H2 and O2 is a key technology for carbon-neutral energy. Here, we report a modular materials design leading to noble metal-free composite electrocatalysts, which combine high electrical conductivity, high OER and HER reactivity and high durability. The scalable bottom-up fabrication allows the stable deposition of mixed metal oxide nanostructures with different functionalities on copper foam electrodes. The composite catalyst shows sustained OER and HER activity in 0.1 m aqueous KOH over prolonged periods (t>10 h) at low overpotentials (OER: ≈300 mV; HER: ≈100 mV) and high faradaic efficiencies (OER: ≈100 %, HER: ≈98 %). The new synthetic concept will enable the development of multifunctional, mixed metal oxide composites as high-performance electrocatalysts for challenging energy conversion and storage reactions.

A Biochemical Nickel(I) State Supports Nucleophilic Alkyl Addition: A Roadmap for Methyl Reactivity in Acetyl Coenzyme A Synthase.

Nickel-containing enzymes such as methyl coenzyme M reductase (MCR) and carbon monoxide dehydrogenase/acetyl coenzyme A synthase (CODH/ACS) play a critical role in global energy conversion reactions, with significant contributions to carbon-centered processes. These enzymes are implied to cycle through a series of nickel-based organometallic intermediates during catalysis, though identification of these intermediates remains challenging. In this work, we have developed and characterized a nickel-containing metalloprotein that models the methyl-bound organometallic intermediates proposed in the native enzymes. Using a nickel(I)-substituted azurin mutant, we demonstrate that alkyl binding occurs via nucleophilic addition of methyl iodide as a methyl donor. The paramagnetic NiIII-CH3 species initially generated can be rapidly reduced to a high-spin NiII-CH3 species in the presence of exogenous reducing agent, following a reaction sequence analogous to that proposed for ACS. These two distinct bioorganometallic species have been characterized by optical, EPR, XAS, and MCD spectroscopy, and the overall mechanism describing methyl reactivity with nickel azurin has been quantitatively modeled using global kinetic simulations. A comparison between the nickel azurin protein system and existing ACS model compounds is presented. NiIII-CH3 Az is only the second example of two-electron addition of methyl iodide to a NiI center to give an isolable species and the first to be formed in a biologically relevant system. These results highlight the divergent reactivity of nickel across the two intermediates, with implications for likely reaction mechanisms and catalytically relevant states in the native ACS enzyme.

A wireless and redox mediator-free Z-scheme twin reactor for the separate evolution of hydrogen and oxygen

A twin reactor was constructed for the separate evolution of hydrogen and oxygen during photocatalytic water splitting using a redox mediator free Z-scheme approach. In this work, the overall water splitting process was carried out in a single reactor with a Nafion membrane dividing into twin chambers where CuFeO2 and Bi20TiO32 are as hydrogen and oxygen photocatalyst powders respectively. Reduced graphene oxides were used as a collector extender for electron conduction during in a Z-scheme photocatalytic water splitting system. Branched copper wires were employed to enhance electron collection from the reduced graphene oxides in the twin reactor system used for water splitting under light illumination. The evolution rate of hydrogen and oxygen was 2.23 and 1.14 μmol/h, respectively in the twin reactor, which is close to the stoichiometric ratio of 2:1. As compared to a single reactor, photocatalytic water splitting with separate evolution of H2 and O2 using a twin reactor device is vital since it avoids the explosion potential and hydrogen-purification cost. This redox mediator-free device has the potential for a wider range of applications and reduced the resistance losses resulting from the wiring and the cost related to a mediator. Consequently, this twin reactor with a redox mediator-free approach provides a new opportunity for further improving the efficiency of solar energy conversion reactions.

MoS2/LDH Composites for Efficient Hydrogen Evolution in Alkaline Media

Hydrogen (H2), which has the highest gravimetric energy density and zero carbon content, is widely regarded as a promising energy carrier to fulfill our needs cleanly and sustainably in the future.(1-3) Central to the electrocatalysis are efficient and robust electrocatalysts composed of earth-abundant elements, which are urgently needed for realizing low-cost and high-performance energy conversion devices. Earth-abundant MoS2 has emerged as a promising hydrogen evolution reaction (HER) catalyst with high activity and durability. However, such a high HER performance is only limited to acidic media, the kinetic becomes rather sluggish in alkaline media. Besides the hydrogen (Had) adsorption energy, there should be a second descriptor for gauging the HER catalytic activity in alkaline media—the binding of hydroxyl species, because here H has to be discharged from water instead of from hydronium ions in acidic media. Here, we demonstrate a dramatic enhancement of HER kinetics in base by judiciously hybridizing vertical MoS2 sheets with another earth-abundant material, layered double hydroxide (LDH) (Figure 1a). The resultant MoS2/NiCo-LDH hybrid exhibits an extremely low HER overpotential of 78 mV at 10 mA/cm2 and a low Tafel slope of 76.6 mV/dec in 1 M KOH solution (Figure 1b). At the current density of 20 mA/cm2 or even higher, the MoS2/NiCo-LDH composite can operate without degradation for 48 hours (Figure 1c). Benefiting from the desirable structural characteristics, the MoS2/LDH interfaces synergistically favor the chemisorption of H (on MoS2) and OH (on LDH), and thus can effectively accelerate the water dissociation step and thus the overall HER catalysis (Figure 1a). This work not only brought forth a cost-effective and robust electrocatalyst, but more generally, it opened up new vistas for developing high performance electrocatalysts in unfavorable media recalcitrant to conventional catalyst design. Figure captions Figure 1. (a) TEM images of the MoS2/NiCo-LDH composite, showing their interfaces. Inset: Schematic illustration of the HER in MoS2/LDH interface in alkaline environment. The synergistic chemisorption of H (on MoS2) and OH (on layered double hydroxide) benefits the water dissociation step. (b) Polarization curves of the carbon fiber paper substrate, bare NiCo-LDH, MoS2 and MoS2/NiCo-LDH composite catalysts in 1 M KOH solution at a scan rate of 5 mV/s. (c) Polarization curves recorded from MoS2/NiCo-LDH composite at a scan rate of 5 mV/s before (solid curve) and after (dotted curve) the chronopotentiometry test at -20 mA/cm2 of 48 hours. Inset: chronopotentiometry responses (η ~ t) recorded from MoS2/NiCo-LDH composite at high current densities of -20 mA/cm2 and -50 mA/cm2. References D. Strmcnik et al., Improving the hydrogen oxidation reaction rate by promotion of hydroxyl adsorption. Nat. Chem. 5, 300-306 (2013).Y. Jiao, Y. Zheng, M. T. Jaroniec, S. Z. Qiao, Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 44, 2060-2086 (2015).J. Hu et al., Increasing stability and activity of core–shell catalysts by preferential segregation of oxide on edges and vertexes: oxygen reduction on Ti–Au@Pt/C. J. Am. Chem. Soc. 138, 9294-9300 (2016). Figure 1

Dispersive Single-Atom Metals Anchored on Functionalized Nanocarbons for Electrochemical Reactions.

The use of dispersive single-atom metals anchored on functionalized carbon nanomaterials as electrocatalysts for electrochemical energy conversion reactions represents a burgeoning area of research, due to their unique characteristics of low coordination number, uniform coordination environment, and maximum atomic utilization. Here we highlight the advanced synthetic methods, characterization techniques, and electrochemical applications for carbon-based single-atom metal catalysts, and provide illustrative correlations between molecular/electronic structures and specific catalytic activity for O2 reduction, water splitting, and other emerging reactions including CO2 reduction, H2O2 production, and N2 reduction. We also discuss fundamental principles for the future design of carbon-based single-atom metal catalysts for specific electrochemical reactions. In addition, we explore the challenges and opportunities that lie ahead in further work with carbon-based single-atom metal electrocatalysts.

Versatile electrocatalytic processes realized by Ni, Co and Fe alloyed core coordinated carbon shells

Through encapsulation of Ni, Co and Fe alloyed core inside graphitic carbon shells, a versatile composite catalyst can be obtained that demonstrates excellent activity towards electrochemical energy conversion reactions.

Heterogeneous Co–N/C Electrocatalysts with Controlled Cobalt Site Densities for the Hydrogen Evolution Reaction: Structure–Activity Correlations and Kinetic Insights

The development of active and stable non-precious-metal electrocatalysts for energy conversion reactions involving hydrogen and oxygen has been of pivotal importance for realizing a clean-energy-based society. As a class of non-precious-metal electrocatalysts, cobalt- and nitrogen-codoped carbon (Co–N/C) catalysts have shown promising activity for the hydrogen evolution reaction (HER). The further advancement of Co–N/C catalysts is, however, hindered by the poor understanding of their active sites; the typical preparation of Co–N/C catalysts involves high-temperature pyrolysis, yielding catalysts with a heterogeneous distribution of atomically dispersed Co–Nx sites and metallic Co nanoparticles encapsulated in graphitic carbon shells (Co@C). Further, kinetic insights into the HER on Co–N/C catalysts are lacking. In this work, we prepared a series of Co–N/C catalysts with controlled Co–Nx and Co@C site densities, which served as model catalysts for identifying the active sites for the HER. We found that th...