Structure Design and Performance Tuning of Nanomaterials for

Article pubs.acs.org/accounts

Structure Design and Performance Tuning of Nanomaterials for Electrochemical Energy Conversion and Storage Published as part of the Accounts of Chemical Research special issue “Nanoelectrochemistry”. Tian Sheng,† Yue-Feng Xu,† Yan-Xia Jiang, Ling Huang, Na Tian, Zhi-You Zhou, Ian Broadwell, and Shi-Gang Sun* State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China CONSPECTUS: The performance of nanomaterials in electrochemical energy conversion (fuel cells) and storage (secondary batteries) strongly depends on the nature of their surfaces. Designing the structure of electrode materials is the key approach to achieving better performance. Metal or metal oxide nanocrystals (NCs) with high-energy surfaces and open surface structures have attained significant attention in the past decade since such features possess intrinsically exceptional properties. However, they are thermodynamically metastable, resulting in a huge challenge in their shape-controlled synthesis. The tuning of material structure, design, and performance on the nanoscale for electrochemical energy conversion and storage has attracted extended attention over the past few years. In this Account, recent progress made in shape-controlled synthesis of nanomaterials with high-energy surfaces and open surface structures using both electrochemical methods and surfactant-based wet chemical route are reviewed. In fuel cells, the most important catalytic materials are Pt and Pd and their NCs with high-energy surfaces of convex or concave morphology. These exhibit remarkable activity toward electrooxidation of small organic molecules, such as formic acid, methanol, and ethanol and so on. In practical applications, the successful synthesis of Pt NCs with high-energy surfaces of small sizes (sub-10 nm) realized a superior high mass activity. The electrocatalytic performances have been further boosted by synergetic effects in bimetallic systems, either through surface decoration using foreign metal atoms or by alloying in which the high-index facet structure is preserved and the electronic structure of the NCs is altered. The intrinsic relationship of high electrocatalytic performance dependent on open structure and high-energy surface is also valid for (metal) oxide nanomaterials used in Li ion batteries (LIB). It is essential for the anode nanomaterials to have optimized structures to keep them more stable during the charge/discharge processes for reducing damaging volume expansion via intercalation and subsequent reduced battery lifetime. In the case of cathodes, tuning the surface structure of nanomaterials should be one of the most beneficial strategies to enhance the capacity and rate performance. In addition, metal oxides with unique defective structure of high catalytic activity and carbon materials of porous structure for facilitating fast Li+ diffusion paths and efficiently trapping polysulfide are most important approached and employed in Li−O2 battery and Li−S battery, respectively. In summary, significant progress has already been made in the electrocatalytic field, and likely emerging techniques based on NCs enclosed with high-energy surfaces and high-index facets could provide a promising platform to investigate the surface structure− catalytic functionality at nanoscale, thus shedding light on the rational design of practical catalysts with high activity, selectivity, and durability for energy conversion and storage. new types of energy resources. The fields of catalysis, electrocatalysis, photocatalysis, fuel cells, and batteries are all examples of where nanotechnology is impacting current science, and their performances are strongly determined by nanomaterials.1

1. INTRODUCTION Electrochemical energy conversion (fuel cells) and storage (secondary batteries) are playing important roles in today’s society for solving the shortage of fossil fuels and environmental pollution. In comparison with traditional materials, nanomaterials owing to their exceptional physicochemical properties are amainstay in chemical industries, environmental protection, and © 2016 American Chemical Society

Received: September 26, 2016 Published: October 14, 2016 2569

DOI: 10.1021/acs.accounts.6b00485 Acc. Chem. Res. 2016, 49, 2569−2577

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Accounts of Chemical Research

that generates high-index facets. By changing the precursor, trapezohedral (TPH) NCs can be synthesized with {522} facets.5 By replacing the H2SO4 solution with deep eutectic solvents (DESs), the shape of THH Pt NCs can be altered from convex to concave enclosed by {910} and {10,1,0} facets,6,7 which can be viewed as a cube with the centers of each face “pushed in” to form square pyramid-shaped depressions. Despite the high specific activity, the low mass activity of the above Pt NCs greater than 20 nm makes it not optimal in comparison with commercial Pt/C catalysts of size ∼3 nm. We have synthesized high-index faceted Pt NCs supported on carbon black (HIF-Pt/C) with a size of 2−10 nm, comparable to that of commercial catalysts.8 The key for decreasing size is the employment of insoluble Cs2PtCl6 dispersed on carbon black as precursors (Figure 2a). The border atoms are clearly resolved, and small facets can be clearly observed, illustrating that the surface is composed of low-coordinated Pt atoms, which break the C−C bond in ethanol effectively (Figure 2b,c). Another approach is based on shape transformation, in which cubic Pt NCs of 10 nm size bounded by {100} facets are synthesized first, and then, using SWP treatment, such cubes can be converted into THH Pt NCs in the size range 6−20 nm enclosed by {310} facets (Figure 2d−f).9 On the basis of the shape transformation approach, by replacing Pt nanocubes with Pt nanoparticles of ∼3 nm as crystal seeds supported on graphene, particles of sub-10 nm THH Pt{210} were obtained (Figure 2g,h).10 It is reassuring that this method can be extended to activate commercial Pt/C catalysts. The particle size of Pt/C catalysts increases from 3.1 to 6.3 nm after SWP treatment, and the facets were identified as {320}. Although the specific activity decreases, the mass activity for ethanol electrooxidation increases significantly by 53% (Figure 2i). More importantly, after 1000 potential cycles, only 28% activity is lost in comparison with the 51% loss of activity of the pristine Pt/C. By applying the SWP method, THH Pd NCs with {730} facets (Figure 1b,e), TPH Pd NCs with {311} facets and concave

With particle sizes being reduced to nanoscale, the surface-to-volume ratio proportionally increases and small-size effects become more pronounced. Understanding the structure−catalytic functionality at nanoscales is the key to shedding light on rational design of practical catalysts with high activity and durability for energy conversion and storage. The common structural features of high performance electrode nanomaterials consist of open surface structures. The performance of nanocrystals (NCs) used as catalysts depends strongly on surface structure, and thus, an electrocatalyst in fuel cells with a high density of active sites is desired to help resolve the globally diminishing supply of Pt-group metals. Open structure surfaces also bring benefits to the charging/discharging process in lithium ion batteries (LIB) by providing parallel channels for faster intercalation of Li+ ions through surfaces than other crystal planes of compact atomic arrangements.1,2 The synthesis of electrode nanomaterials with open structure is a significant challenge because nanomaterials of high-energy surface are thermodynamically unstable, resulting in their rapid transformation under normal growth conditions to nanomaterials of low surface energy. Crystal growth habit, governed by thermodynamics, usually prefers NCs enclosed by low energy facets.3

2. METAL NANOMATERIALS 2.1. Structure-Controlled Synthesis by Electrochemistry

Platinum is the most important element in fuel cells, and many pure and applied investigations have been performed. The first high-energy surface Pt NCs of tetrahexahedral shape with {730} facets (THH Pt{730}) have been made by electrochemical methods, which is based on a cube with each face capped by a square-based pyramid (Figure 1a,d).4 Through periodic oxygen adsorption/desorption at high/low potentials in electrochemical square-wave potential (SWP), the growth of THH NCs is fulfilled. The dynamical oxygen adsorption/desorption process induces the place-exchange between oxygen and Pt surface atoms

Figure 1. (a−c) SEM images of Pt,4 Pd,11 and Rh13 THH NCs, respectively. The inset figures show the corresponding high-magnification SEM images and atomic models of {hk0} high-index planes. (d) Steady-state current as a function of electrode potential for ethanol electrooxidation.4 (e) CVs of Pd THH NCs and commercial Pd black catalysts for ethanol electrooxidation.11 (f) LSVs of Rh THH NCs and commercial Rh black catalysts for ethanol electrooxidation.13 Reproduced with permission from refs 4, 11, and 13. Copyright 2007 the American Association for the Advancement of Science. Copyright 2010 American Chemical Society. Copyright 2014 Wiley-VCH. 2570


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Figure 2. (a) Synthesis process of HIF-Pt/C.8 (b) Aberration-corrected high resolution TEM (HRTEM) image of HIF-Pt/C.8 (c) Steady-state cyclic voltammograms of HIF-Pt/C and commercial Pt/C for ethanol electrooxidation.8 (d) Shape transformation from Pt nanocubes to THH NCs.9 (e) HRTEM images of Pt THH NCs/CNTs oriented along the [001] direction.9 (f) Cyclic voltammograms of Pt THH Pt NCs/CNTs for ethanol electrooxidation.9 (g) Growth processes of Pt THH NCs from Pt nanoparticles.10 (h) TEM images of sub-10 nm Pt THH NCs supported on graphene and size distribution.10 (i) Cyclic voltammograms of THH-Pt/G, Pt/G, and commercial Pt/C for ethanol electrooxidation.10 Reproduced with permission from refs 8−10. Copyright 2010 Wiley-VCH. Copyright 2012 Elsevier. Copyright 2016 American Chemical Society.

by electrochemical methods.15 Zheng et al. reported the synthesis of concave Pt NCs having {411} facets using amine as capping agents.16 Concave THH Pt NCs enclosed by {510}, {720}, and {830} facets were synthesized by reduction of K2PtCl4 with NaBH4 in the presence of Na2H2P2O7 and KBr. The sluggish reduction rate is thought to be vital for the kinetically controlled growth of concave Pt NCs.17 Concave Pd NCs with {310} facets can be directly synthesized by reducing H2PdCl4 with ascorbic acid. Other Pd NCs with {730} facets can be obtained with other precursors.18,19 The electrocatalytic activities of concave Pd NCs are much higher than that for commercial Pd black catalysts.18 Using template directed overgrowth following by etching away the core, Xia et al. reported Pt octahedral nanocages having advantages of a large surface area and a uniform surface structure for oxygen reduction reaction.20 The numerical values of both the mass activity and specific-area activity at 0.9 V vs RHE were higher than those of commercial Pt/C catalysts (Table 1).

hexoctahedral (HOH) Pd NCs enclosed by {321} facets were also prepared.11,12 Rhodium with much higher surface energy than platinum and palladium can be furthermore prepared with THH NCs enclosed by {830} facets (Figure 1c,f).13 On the basis of electrochemical shape transformation, the method of electrochemical milling and faceting (ECMF) was developed for reducing the size of NCs without remarkable loss of metal for in situ regeneration of deactivated catalysts.14 After the SWP treatment of heavy and mild ECMF, large Pd NCs (35 nm) of low-index facets supported on TiO2 nanotube array can be milled into small NCs (7 nm) enclosed by {210}, {410}, {211}, and {311} facets. 2.2. Structure-Controlled Synthesis by Surfactant-Based Wet-Chemistry

In wet-chemical routes, the role of oxygen species in electrochemical methods is achieved by functional molecules such as surfactants, capping agents, and additives. The functional molecules can dramatically change the original surface energy by binding with surface atoms. In general, the low coordinated sites can be better stabilized by the capping agents, slowing the growth rate toward high-energy surface direction and preserving the high-energy surface. An interesting point worth mentioning is wet-chemical methods usually generate a concave THH crystal shape. Currently this method has not yet succeeded in creating convex crystals for Pt or Pd, whereas both shapes are accessible

3. ALLOY NANOMATERIALS 3.1. Electrocatalysts

Surface decoration and solid solution are two common types of bimetallic materials (Figure 3a). Surface decoration can be done 2571


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Accounts of Chemical Research Table 1. Comparison of the Specific Area Activity (mA·cm−2) at the Peak Current on the NCs with High Index Facets in the Electrooxidation of Ethanol and Formic Acid specific-area activity (mA·cm−2)

facets

surface energy but also for consideration of matching at least two elements both in size and in thermodynamic stability. Since pure Pt is easily poisoned by adsorbed CO in formic acid electroxidation, which undergoes a dual-path mechanism including the active path to form CO2 directly and the poisoning path to form CO, the decoration of foreign atoms, such Bi and Au, has been identified to avoid poisoning.21,22 On the bare THH Pt NCs, the peak current density in the positive-going potential scan is much smaller than that in the negative-going one due to the CO poisoning. Once the NCs are decorated with Bi, the oxidation current increases significantly, accompanied by a negative shift of onset potential (Figure 3d).23 The reduced hysteresis of CV profiles in the positive- and negative-going scans clearly indicates the absence of CO poisoning at high Bi coverages. Au can be modified on THH Pt NCs through the initial submonolayer Cu UPD onto the Pt surface and then the galvanic charge displacement of Cu with Au.24,25 At Au coverage of 0.72, the peak current density is 20 times greater than that without Au, but once the coverage is increased above 0.72, the activity decreases (Figure 3e). The enhanced yield of CO2 arising from Au is proposed to alter the reaction path from the poisoning path to the active path. With a small amount of Pt being alloyed in THH Pd{10,3,0}, the electroactivity toward formic acid can be boosted significantly with bigger peak currents and lower onset potentials.26 The THH PdPt NCs with 10% Pt content shows the highest peak current, 3.1 times that measured on THH Pd NCs and 6.2 times that on commercial Pd black (Figure 3b). PtRu catalysts are widely used in direct methanol fuel cells as anode materials in which surface Pt sites facilitate C−H bond

ref

Ethanol Pt HIF/C Pt Pt PtRh PtRh Pt Pt PdPt Pd PtBi PtAu Pt

3.5 4.3 8.4 5.4 4.2 3.3 2.9 2.7 Formic Acid 70 22.4 25.8 4.6 0.21, 0.23

{730} {110}, {210}, etc. {310} {100} {311} {830} {311} {830}

4 8 9 9 31 31 31 31

{10,3,0} {10,3,0} {730} {730} {730}

26 26 23 25 23, 25

in two ways. The first is direct electrodeposition of the target atoms by applying a potential scanning or a constant potential in a solution containing foreign metal ions. The second is carried out through underpotential deposition (UDP) of monolayer or submonolayer Cu on the surface first, and then the Cu is displaced with target metals. Alloying is another strategy, adding foreign atoms into the crystal lattice. Synthesizing alloy NCs of high-energy facets is extremely challenging not only for high

Figure 3. (a) Illustrations of surface decoration and alloying of NCs. (b) Current−potential curves of THH PdPt NCs, THH Pd, and commercial Pd black for formic acid electrooxidation.26 (c) CVs of THH PtRh, TPH PtRh, THH Pt, TPH Pt, and commercial Pt/C catalysts for ethanol electrooxidation.31 (d) CVs of THH Pt/Bi for formic acid electrooxidation.23 (e) Positive segments of CVs of THH Pt/Au for formic acid electrooxidation.25 (f) Positive segments of CVs of THH Pt/Ru for methanol electrooxidation.29 Reproduced with permission from refs 23, 25, 26, and 29. Copyright 2011 American Chemical Society. Copyright 2012 and 2013 Royal Society of Chemistry. Copyright 2012 American Chemical Society. 2572


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3D nanoarchitectured SnSbCo alloy electrode can deliver a capacity as high as 650 mAh/g after 150 cycles with a capacity retention of 79.1%, and demonstrate also exceptional electrochemical performance at a high rate of 15 A/g (Figure 4c,d). The enhanced electrochemical performance is attributed to the unique 3D nanoribbon array structure, which can facilitate the transport of electron and electrolyte and give enough room to buffer the volume change.33

breaking and Ru sites promote oxygen-species formation at low potentials, called the bifunctional mechanism.27,28 The enhancement was experimentally found to be structure sensitive by using Pt single crystal planes with the activity order of (111) > (110) > (100).28 As Ru coverage increases on THH Pt NCs, both the oxidation onset potentials and peak potentials are negatively shifted (Figure 3f).29 Furthermore, the enhancement effect of Ru on THH Pt NCs is more apparent than that on Pt black and the commercial PtRu catalysts, because the Ru decoration on a normal Pt NC will consume most of the highly active step sites, which is not in favor of methanol dehydrogenation. The addition of Rh to Pt has been reported to enhance the cleavage of C−C bonds in ethanol electrooxidation.30 Alloy THH PtRh{830} and TPH PtRh{311} NCs were also synthesized.31 According to the peak current density, the activity increases in the order of TPH PtRh > THH PtRh > TPH Pt > THH Pt > Pt/C (Figure 3c). The high-index faceted alloys are found to have greater ability to break the C−C bond to form CO2 than that of the pure Pt NCs.

4. OXIDE NANOMATERIALS 4.1. Transition Metal Oxide

Transition metal oxides (TMOs) have been widely studied as anodes of Li-ion batteries since the report by Tarascon et al.34 MnO, NiO, Co3O4, and Fe3O4 as anodes can deliver capacities several times higher than commercial graphite.35 However, the low electron conductivities and the big volume change of the TMOs during charge/discharge processes lead to fast capacity fading.36 Many efforts have been made to overcome these two shortcomings and enhance electrochemical performances. This has been achieved through synthesizing materials with appropriate nanostructures. MnO/C nanotubes with porous structures were synthesized through a hydrothermal method followed by thermal annealing.35 The porous MnO/C nanotubes could release a capacity of 763.3 mAh/g after 100 cycles at 100 mA/g and 618.3 mAh/g after 200 cycles at 500 mA/g. Considering both the structure stability and tap density of anodes, hierarchical micro- or nanostructures have attracted significant attention in TMOs.37,38 Anode of Co3O4 octahedra enclosed by {111} facets synthesized via wet-chemical methods followed by heating treatment exhibited excellent long cycle performance and excellent rate capability, which could maintain a reversible capacity as high as 955.5 mAh/g after 200 cycles at 100 mA/g.39 The (111) surface on Co3O4 appears an open structure in comparison with (100) and (110) planes, facilitating Li+ diffusion.

3.2. Anodes of Lithium Ion Batteries

Metal alloys exhibit high performances as anodes of LIBs. Although pure metallic anodes are promising because of their high specific capacity and safety characteristics, such as Sn (991 mAh/g), Sb (660 mAh/g), etc.,32 they suffer from tremendous volume expansion owing to their special alloying mechanism during charge/discharge processes. As a consequence, the active materials drop easily from current collector, leading to fast capacity fading. Adding some buffer elements into pure metal is an efficient way to mitigate the volume expansion, and design of different nanostructures could further improve the electrochemical properties, such as Sn-based anodes of hollow structure, core−shell structure, etc. Ke et al. prepared a novel nanoarchitectured SnSbCo alloy electrode by direct electrodeposition on Cu nanoribbon arrays (Figure 4a,b).33 This unique

4.2. Layered Oxides

Layered oxide materials (LiNixCoyMn(1−x−y)O2, LNCM) are most promising cathodes for next generation LIBs thanks to their higher working voltage and theoretical capacity.40 Increasing the diffusion channels of Li+ and the electron conductivity of cathode are efficient strategies to improve the rate performance of layered oxide materials.2 Wei et al. demonstrated that the rate capacity of layered materials could be significantly increased by tuning the surface structure of materials,40 and the high-energy {010} planes could provide more diffusion channels for Li+ and thus boost the rate capability (Figure 5a,b). At a 6 C rate, the capacity was 186 mAh/g after 50 cycles (Figure 5c). The results demonstrate that the performances of Li-rich cathode materials could be enhanced by exposing the active {010} planes. Along this direction, Fu et al. have synthesized single-crystal LNCM hexagonal nanobricks with increased sidewall thickness along the [001] direction.41 The as-prepared material with high percentage of exposed {010} facets exhibited excellent cycle performance at high rate, associated with fast and efficient lithium intercalation/deintercalation. Figure 4. (a) Schematic diagram showing the fabrication of 3D nanoarchitectured Sn−Sb−Co alloy.33 (b) SEM images of the 3D SnSbCo electrode.33 (c) Plots of specific capacity versus cycle number of the 3D SnSbCo electrodes at 6.0 and 23.0 C.33 (d) Schematic diagram showing the advantages of the 3D nanoarchitectured electrode.33 Reproduced with permission from ref 33. Copyright 2013 Royal Society of Chemistry.

5. NANOMATERIALS USED IN Li−O2 AND Li−S BATTERIES Li−O2 and Li−S batteries hold much higher theoretical energy densities of 3505 Wh/kg and 2600 Wh/kg, respectively, than those of LIBs. Metal oxides have been extensively explored as cathode catalysts for Li−O2 batteries because of their high 2573


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utilization, poor cycle performance, and inferior charge/ discharge efficiency caused by poor electronic conductivity of sulfur, intrinsic polysulfide shuttling, and Li corrosion.46 To enhance electrochemical performance of Li−S batteries, graphene-based carbon materials and porous carbon materials were extensively studied as carriers to load the sulfur, because of their high electronic conductivity and large specific surface area. In sulfur/carbon composites, tuning the structure of carbon materials can facilitate fast diffusion paths for Li+ or efficiently trap the polysulfide with porous structures, resulting in restraining the polysulfide shuttling and increasing sulfur utilization. Nazar’s group reported nanostructured polymermodified mesoporous carbon/sulfur composites with enhanced utilization of active sulfur materials, which was attributed to the entrapment of carbon framework (Figure 6a).47 The external polymer surface coating on the composite could further help hinder diffusion of polysulfide. Chen et al. synthesized ordered mesoporous carbon of large internal surface area, high pore volume, and bimodal mesopore structure (Figure 6b).48 Such an ordered mesoporous carbon carrier provides a short diffusion pathway for both electrons and ions, leading to the highest initial discharge capacity, excellent rate capability and long electrochemical stability (Figure 6c). An ultrahigh sulfur loading of 88.9 wt % was realized by Xu et al. through preparation of sulfur/ porous graphitic carbon composites (Figure 6d).49 Thanks to the porous graphitic carbon using specific surface area, pore volume, and electronic conductivity, and a fast electronic and ionic diffusion pathway, the material provides long cycle performance (Figure 6e). Apart from tuning the structure of carbon materials, Xin et al. successfully synthesized metastable sulfur allotropes S2−4 via confining them in carbon micropores (Figure 6f).50 These confined small S2−4 molecules presented high Li electroactivity and could essentially solve the critical problem of polysulfide dissolution in conventional Li−S batteries.

Figure 5. (a) Schematic illustration of growing two kinds of nanoplates and the microstructure of their surfaces.40 (b) SEM characterization of the crystal habit-tuned nanoplate materials of HTN-LNMO.40 (c) Stabilized discharge voltage profiles of HTN-LNMO cycled at different rates: 6, 3, 1, 0.5, 0.1 C from bottom to top.40 Reproduced with permission from ref 40. Copyright 2010 Wiley-VCH.

abundance, low cost, and high catalytic ability for both the oxygen reduction and evolution reactions. Generally, the porous structure of metal oxide or its dispersion on carbon materials can benefit the transportation of oxygen and electrolyte. Two kinds of unique defective structures, perovskite and pyrochlore are widely applied in Li−O2 batteries.42,43 Oh and Nazar reported that bismuth and lead ruthenium pyrochlore oxides with extended pyrochlore structures could yield discharge capacities over 10000 mAh/g and reduce significantly the overpotentials in Li−O2 battery.44 Pyrochlore is a generic term for materials having a chemical formula A2B2X6Z1−δm which crystallizes in the cubic space group Fd3m ̅ . The good catalytic behavior of bismuth and lead ruthenium pyrochlore oxides is ascribed to their significant concentration of surface active sites afforded by their high surface area, intrinsic variable redox states, and facile electron transport. Perovskite-based ABO3 oxides having high electronic/ionic conductivity and high electrochemical stability can serve as excellent cathode catalysts of Li−O2 batteries due to the unique defective structure and disorder-free channels of oxygen vacancies, providing thus many active sites for catalytic reactions.45 Zhang et al. prepared three-dimensionally ordered macroporous LaFeO3 through a rational and facile strategy.43 The Li−O2 battery using the LaFeO3 catalyst exhibited enhanced specific capacity, rate capability, and cycle stability. It is evident that the high electronic conductivity and substantial oxygen vacancies of perovskite structure facilitated the oxygen reduction and evolution reactions. Recently, rechargeable Li−S batteries have attracted extensive attention because they are closer to commercialization relative to Li−O2 battery. However, Li−S batteries suffer from low sulfur

6. SUMMARY AND PERSPECTIVES The study of nanomaterials with high-energy surface has already opened a new exciting avenue to design exceptional properties for electrochemical energy conversion and storage. This Account emphasizes the remarkable progress made mainly in the last 5 years on the synthesis of metal (alloy) and metal oxide nanomaterials with high-energy surface and open surface structures. Despite the significant progress, future challenges remain: (1) NCs enclosed with high-energy surfaces and well-defined atomic arrangement could provide a promising platform to investigate the surface structure−catalytic functionality at nanoscales, which requires methods that could precisely and continuously control the surface structure of NCs. (2) In situ or operando technologies are powerful for deeply unveiling the origin of high performance of nanomaterials with open structures and therefore guiding rational design and synthesis of functional materials at atomic levels in electrochemical energy conversion and storage. (3) Besides the aforementioned noble metal based catalysts in fuel cells, some other transition metals, Ru and Cu, are also important in Fischer−Tropsch reaction and CO2 reduction. How to extend the synthesis of platinum-group metal (PGM) nanocrystals with high-energy surfaces to those is a challenge that should be solved in future. (4) For bimetallic systems, systematically tuning the surface structure of catalysts to change the properties can make them more effective practically. 2574


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Figure 6. (a) Schematic diagram of sulfur confined within the interconnected pore structure of CMK-3.47 (b) Schematic diagrams of the optimized loading of sulfur in OMC carbon.48 (c) Cycling curves of the OMC/S-60, OMC/S-70, and comparative FDU/S-60 nanocomposite cathode at 0.1C.48 (d) Porous graphitic carbon and sulfur/porous graphitic carbon composite.49 (e) Overall discharge capacities of 60.8%S/PC-AB, 88.9%S/PC-AB, and 88.9%S/PC-BP2000 electrodes at 0.5 C.49 (f) Schematic diagram of confining S2−4 in carbon micropores.50 (g) GDC voltage profiles of S/(CNT@ MPC) at 0.1 C.50 Reproduced with permission from refs 47−50. Copyright 2009 Nature publishing group. Copyright 2010 Elsevier. Copyright 2013 American Chemical Society. Copyright 2012 American Chemical Society.

(5) Not only the nanomaterials for LIBs mentioned above but also new materials for sodium ion or other battery systems would attract more attention. The design of bifunctional catalyst in Li−O2 battery and the protection of Li anode in both Li−O2 and Li−S battery should be more attractive. (6) Lastly, the exceptional properties of nanomaterials with high-energy surfaces have been mostly demonstrated in the laboratory, but it is vital to explore synthesis technologies for upscaling to large-scale production. Future demand of such materials will not be met by today’s limited manufacturing technologies.

■

University Belfast in 2014. His research is to simulate electrocatalytic reactions using density functional theory. Yue-Feng Xu received his B.S. degree from Xiamen University in 2012 and currently is a Ph. D. candidate at Xiamen University. His research focuses mainly on transition metal oxide electrode materials for lithium oxygen batteries. Yan-Xia Jiang received her Ph.D. degree from Jilin University in 1999 and is now a professor at Xiamen University. Her current research interests include surface electrochemistry, electrocatalysis, and spectroelectrochemistry. Ling Huang received his Ph.D. degree from Xiamen University in 1997 and is now a professor at Xiamen University. His research interests include lithium ion batteries and interfacial in situ characterizations.

AUTHOR INFORMATION

Corresponding Author

Na Tian received her Ph.D. degree from Xiamen University in 2007 and is an associate professor in Xiamen University. Her research interests focus on the synthesis of metal NCs with high-energy surfaces.

*E-mail: [email protected]. Author Contributions †

Zhi-You Zhou received his Ph.D. degree from Xiamen University in 2004 and is a professor in Xiamen University. His research interests include electrochemical in situ FTIRS and non-noble metal electrocatalysts in fuel cells.

T.F. and Y.-F.X. contributed equally to this work.

Funding

This work is supported by NSFC (Grants 21321062, 21573183, 21229301, and 21361140374).

Ian Broadwell received his Ph.D. from the University of Hull in 2004 and is a visiting researcher at the University of Xiamen. His research interests include nanomaterials for implantable fuel cell applications and practical measurement systems.

Notes

The authors declare no competing financial interest. Biographies

Shi-Gang Sun received Doctorat d’Etat in 1986 from Université Pierre et Marie Curie (Paris VI), France, and is currently a professor at Xiamen University. He is Member of Chinese Academy of Sciences, Fellow of

Tian Sheng received his B.S. degree from East China University of Science and Technology in 2011 and Ph.D. degree from Queen’s 2575


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the Royal Society of Chemistry, U.K., and Fellow of the International Society of Electrochemistry. His research interests include electrocatalysis, spectroelectrochemistry, and chemical power sources.

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REFERENCES

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