Cyclic Penta-Twinned Rhodium Nanobranches as Superior Catalysts

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Cyclic Penta-twinned Rhodium Nanobranches as Superior Catalysts for Ethanol Electro-oxidation Jiawei Zhang, Jinyu Ye, Qiyuan Fan, Yating Jiang, Yifan Zhu, Huiqi Li, Zhenming Cao, Qin Kuang, Jun Cheng, Jun Zheng, and Zhaoxiong Xie J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03080 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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Journal of the American Chemical Society

Cyclic Penta-twinned Rhodium Nanobranches as Superior Catalysts for Ethanol Electro-oxidation Jiawei Zhang †, Jinyu Ye †, Qiyuan Fan †, Yating Jiang †, Yifan Zhu †, Huiqi Li †, Zhenming Cao †

, Qin Kuang †, *, Jun Cheng †, Jun Zheng &, Zhaoxiong Xie †, ‡, *

†

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

‡

Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen

361005, China

&

Institute of Physical Science and Information Technology, Anhui University, Hefei 230601,

China

KEYWORDS: Rh, cyclic penta-twinned nanostructure, tetrahedron, crystal growth, ethanol electrocatalysis

ACS Paragon Plus Environment

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ABSTRACT

Developing active and durable electro-catalysts towards ethanol oxidation reaction (EOR) with high selectivity towards the C–C bond cleavage is an important issue for the commercialization of direct ethanol fuel cell. Unfortunately, current ethanol oxidation electro-catalysts (e.g. Pt, Pd) still suffer from poor selectivity for direct oxidation of ethanol to CO2, and rapid activity degradation. Here we report a facile route to the synthesis of a new kind of cyclic penta-twinned (CPT) Rh nanostructures that are self-supported nanobranches (NBs) built with 1-dimension CPT nanorods as subunits. Structurally, the as-prepared Rh NBs possess high percentage of open {100} facets with significant CPT-induced lattice strains. With these unique structural characteristics, the as-prepared CPT Rh NBs exhibit outstanding electro-catalytic performance towards EOR in alkaline solution. Most strikingly, the selectivity of complete conversion ethanol to CO2 on the CPT Rh NBs is measured to be as high as 14.5± 1.1 % at −0.15 V, far exceeding that for single-crystal tetrahedral nanocrystals, icosahedral nanocrystals, and commercial Rh black, as well as majority of reported values for Pt or Pd-based electro-catalysts. By combining with density functional theory calculation, the effects of different structural features of Rh on EOR are definitively elucidated. It was found that the large amount of open Rh (100) facets dominantly contribute to the outstanding activity and exceptionally high selectivity, while the additional tensile strain on (100) planes can further boost the catalytic activity by enhancing the adsorption strength and lowering the reaction barrier of dehydrogenation process of ethanol. As a proof of concept, the present work shows that rationally optimizing surface and electronic structure of electro-catalysts by simultaneously engineering their surface and bulk structures is a promising strategy to promote the performance of electro-catalysts.


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INTRODUCTION High-performance low temperature fuel cells, which can directly convert chemical energy into electrical energy without emitting harmful products, play essential roles in dealing with global energy and climate challenge.1-4 Among various fuel cells, direct ethanol fuel cell (DEFC) has attracted increasing interests, due to its high theoretical energy density (comparable to that of gasoline), easy storage and transportation, and renewability of the ethanol.5-10 Despite its great potential in the commercialization, the current development of DEFC is still impeded by ethanol’s sluggish kinetics and inefficient oxidation on electro-catalysts. In fact, the high selectivity towards complete oxidation of ethanol to CO2 is the key issue to harvest high energy density. However, on the platinum (Pt) or palladium (Pd) based electro-catalysts, which are the well-known best catalysts towards oxidation of small organic molecules, the selectivity of complete oxidation ethanol to CO2 is only about 1–7.5% or 2.5% at room temperature, respectively.11-13 Although some progresses have been made, there is limited breakthrough and development in improving the selectivity of electro-catalysts for ethanol oxidation reaction (EOR). Owing to the close relationship between the structure of electro-catalyst and performance of EOR, to achieve high CO2 selectivity, it would be important to synthesize novel electrocatalyst and figure out the effects of its structural features on EOR.14-15 Rhodium (Rh) has a significant effect on breaking of C–C bond of ethanol molecule during EOR, which means Rh electro-catalysts would benefit the high selectivity of oxidation ethanol to CO2.16-18 To optimize the catalytic activity and selectivity, rationally tuning surface and electronic structures of Rh electro-catalysts is highly desired.19-20 However, the related study is very sparse.21-25 Of note, it has been well demonstrated in recent years that the cyclic pentatwinned (CPT) nanostructures, a kind of unique and interesting crystal forms of face-centered


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cubic (fcc) metals, have unique electronic structure resulting from unusual intrinsic lattice strains besides well-defined surface structure, so they often display superior catalytic activity and selectivity to the single crystal (SC) counterparts in many important reactions (e.g., oxygen reduction

reaction,

small

molecules

electro-oxidation).26-27

Unfortunately,

the

CPT

nanostructures of Rh are rarely observed when compared with other fcc metals such as Au, Pt and Pd, because the Rh possesses extremely high cohesive energy, and the resulting elastic strain energy will be too large to be counterbalanced.28-29 Herein, we successfully synthesize a new kind of Rh nanostructures that are self-supported nanobranches (NBs) built with cyclic penta-twinned (CPT) nanorods as subunits via a facile solvothermal route. Owing to the large percentage of open (100) facets and significant CPTinduced lattice strain, the as-prepared CPT Rh NBs not only exhibit outstanding electro-catalytic activities and structural stability towards EOR in alkaline solution, but also exhibit unprecedented high selectivity of complete oxidation of ethanol to CO2. The estimated CO2 selectivity was 14.5 ± 1.1 % at −0.15 V, which is about 1.3, 2.2 and 3.4 times that of icosahedral Rh NCs, tetrahedral Rh NCs and Rh black, respectively. And it is also far higher than the previously reported data for monometallic Pt (1–7.5%) and Pd (2.5%) at room temperature. The theoretical calculation further reveals that the open (100) facets and the CPT-induced tensible strain play different but synergistic roles for such outstanding performance of the as-prepared CPT Rh NBs. The findings renewed our understanding of the structure-performance relationship of Rh-based electro-catalysts in EOR, which will open a new avenue to construct highperformance EOR electro-catalysts.

RESULTS AND DISCUSSION


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Figure 1. (A) TEM image, (B) HAADF-STEM image, and (C) XRD pattern of the as-prepared CPT Rh NBs. (D) High magnification TEM image of a typical individual CPT Rh NB and (D1−3) HRTEM images taken from different branches. (E) Schematic models of the CPT nanorods, and different orientations of cross-section of CPT nanorods with respect to the electron beam.

In a typical synthesis of CPT Rh NBs, RhCl3 was reduced by n-octylamine in the presence of polyvinylpyrrolidone (PVP) in solvothermal condition. Transmission electron microscopy (TEM) image and high-angle annular dark-field scanning transmission electron microscopy


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(HAADF-STEM) image show the CPT Rh NBs are of a multi-armed shape, in which nanorods of various lengths radiate from the center along different directions and the diameter of every nanorod is about 4.3 ± 0.6 nm (Figure 1A and B). The X-ray diffraction (XRD) analysis confirms that the products have a fcc structure, matching well with the standard metallic Rh diffraction pattern (JCPDS No. 05-0685) (Figure 1C). Interestingly, the diffraction peaks are broad, which is attributed to the ultrathin feature of nanorods constructing the CPT Rh NBs. Moreover, it is noticed that the (220) peak is relatively sharper compared to other peaks, which suggests the growth direction of nanorods should be along direction. Generally, for CPT nanorod, its growth direction is usually the direction, while the SC nanorods grow along direction.30-33 The growth along direction of the as-prepared Rh NBs implies their CPT nature. The CPT structure was further verified by high-resolution (HR) TEM. As shown in the Figure 1D and 1D1, the cross-section of an upright arm displays a typical fivefold twinned structure. Besides that, by surveying arms along different side views, two classical orientations of CPT nanorods corresponding to / and / are observed, respectively (Figure 1D2−3 and Figure 1E).32 Therefore, the unique 3D open structure of the as-prepared product is essentially consisted of 1D ultrathin CPT nanorod subunits growing along the direction, and bounded by {100} side facets and {111} facets at the ends (bottom in Figure 1E). In addition, due to the CPT nature of nanorod subunits, there is inevitably at least 2% tensile strain for the side {100} planes (Figure S1).33 The presence of tensile strain on as-prepared CPT nanorod may bring different electronic structure. X-ray photoelectron spectroscopy (XPS) analysis (Figure S2) reveals that both the metallic state (i.e., Rh0) and the oxidized state (i.e., Rh3+) are detected in the as-prepared CPT Rh NBs, and the fraction of Rh3+ was about 39.7%. The high oxidized state indicates that the CPT Rh NBs possess a large amount of twinned


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defects, being consistent with the previous reports that the twinned defect sites are easy to be oxidized by O2 in air. 27-28, 34 In the previous reports, it was demonstrated that the presence of Cl– ions in the growth solution was not good for the formation of well-defined Rh nanostructures, especially the twinned Rh nanocrystals (NCs), due to the possible oxidative etching reaction between seeds/nanoparticles and Cl–/O2 pair (dissolved in solution), and thus Rh precursors were strictly limited to chloride-free precursors.35-36 In contrast, the selection of RhCl3 as precursor is very crucial for the formation of well-shaped CPT Rh NBs in our synthetic route (Figure S3). Rhodium (III) acetylacetonate or rhodium (II) acetate dimer as precursor gave rise to the mixture of Rh nanoplates and icosahedra, or ill-shaped Rh branches.


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Figure 2. TEM images of the CPT Rh NB intermediates that were collected at different reaction time intervals: (A) 25 min, (B) 30 min, (C) 4 h, and (D) 6 h. (E) Growth schematic of the CPT Rh NBs. The insets in (A) and (B) show the corresponding HRTEM images of the intermediates. With the aim of clarifying the possible growth mechanism, the growth intermediates of CPT Rh NBs were firstly investigated by TEM. At the initial stage (i.e., about 25 min), uniform spherical particles with a narrow size distribution (3.8 ± 0.8 nm) were formed (Figure 2A). Further HRTEM analysis confirms that these spherical nanoparticles are actually icosahedra with


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multiple {111} twins (inset of Figure 2A). The appearance of icosahedra may be due to the fact that the multiply twinned icosahedra enclosed by {111} facets are thermodynamically stable at small sizes.36-37 As the reaction progressed to 30 min, multi-pod nanostructures were formed (Figure 2B). As the reaction continued, the arms grew longer and longer, and the number of arms also increased, eventually leading to the formation of 3D branch-like structures (Figure 2C−D). The result indicates that the CPT Rh NBs are thermodynamically stable under the present reaction condition. To our best knowledge, the CPT NBs are not observed before. One possible growth model is that CPT nanorods preferentially grow from the vertexes of an icosahedron (i.e., 5-fold axis () direction of the icosahedron, as demonstrated in Figure S4). Figure 2E illustrates the growth of 3D CPT Rh NBs. At the early growth stage, lots of thermodynamically stable icosahedral nuclei are formed. Considering the relatively high energy and high activity at these twinning boundary sites, the CPT arms start to grow from the vertices of icosahedral seeds along the direction during the subsequent growth.35, 38

Figure 3. Representative (A) TEM image, (B) XRD pattern and (C) HRTEM image of the products obtained by using n-octanol as the solvent in the synthetic process.


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The PVP mainly acted as dispersant in our system. If no PVP involved, Rh CPT NBs still can be produced but slightly aggregated (Figure S5). But the choice of n-octylamine is very crucial for the formation of well-shaped Rh NBs. If amine with shorter chain (for example, n-butylamine) was used, severely agglomerated CPT nanorods formed (Figure S6A). Instead, the freestanding CPT nanorods or hyperbranched Rh triangle nanoplates were formed if n-dodecylamine or noctadecylamine was used, respectively (Figure S6B and 6C). Particularly, when the solvent noctylamine was replaced by the n-octanol, many small NCs with good monodispersity (5.2 nm ± 0.7 nm) were formed (Figure 3A). The products are also fcc-structured Rh, according to XRD analysis (Figure 3B). The HRTEM image shows that these near-monodisperse NCs are actually nanosized SC tetrahedra, most of which present a triangle projection profile under electron beam (Figure 3C). In sharp contrast to the CPT Rh NBs, the XPS analysis shows that Rh SC tetrahedra are mainly in metallic state (i.e., Rh0, 90.0%), which is consistent to the previous reports (Figure S7).28 The transformation from CPT to SC nanostructures is a very interesting phenomenon, which has inspired great interest very recently, but the process is still not fully understood.28,

39-40

Thermodynamically, it is possible to achieve this transformation via regulating the surface free energy of the NCs, and the SC nanostructures would dominate in the products when lowering the surface energy of the NCs (see Supporting Information for detailed discussion).39-40 However, as revealed by diffuse-reflectance Fourier transform IR spectra (DRIFTS), there were no obvious signals of surface radicals (mainly PVP in our case) on the as-prepared CPT Rh NBs and SC tetrahedral Rh NCs (Figure S8). This implies that besides the well-known thermodynamic factor, other factors should be taken into consideration when interpreting the growth mechanism of CPT nanostructures.


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It was observed in our study that the reaction rate was faster in n-octanol than that in noctylamine (Figure S9). Compared to the clear and transparent reaction solution with the noctylamine as solvent, black products were clearly observed in n-octanol when the reaction was terminated at 25 min. The TEM analysis indicated that the products were almost SC Rh NCs with size of 4.5 ± 0.5 nm. In contrast, a tiny amount of twinned Rh icosahedral nanoparticles with similar size (3.8 ± 0.8 nm) were produced when n-octylamine was applied. Obviously, the low reduction rate favors the formation of CPT seeds at the initial NC nucleation stage, and thus final CPT Rh NBs. Similar phenomenon was also reported by Xia and coworkers.41 They found that the CPT icosahedral Pd NCs were formed at slow reduction rate, while the SC Pd NCs would be obtained at high reduction rate.41 Although the transformation was simply attributed to the reaction kinetics, the behind role of reaction kinetics in controlling nanostructures is still unclear. Recently, we found that the high reduction rate may result in high supersaturation of growth units, which leads to form crystallites with high surface-energy faces.42 Considering the higher surface energy of SC tetrahedral NCs than that of CPT icosahedral particles, we assume the high reduction rate of Rh precursors increases the supersaturation of growth units in the presence of n-octanol, which may in turn facilitate the formation of SC nuclei at the nucleation stage. Nevertheless, further studies are still needed to fully understand the transformation mechanism.


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Figure 4. (A) Positive scan curves of EOR from –0.80 to 0.00 V (scan rate: 50 mV·s–1), and (B) corresponding chronoamperometric test at –0.35 V on CPT Rh NBs, icosahedral Rh NCs, tetrahedral Rh NCs and commercial Rh black in 1.0 M NaOH + 1.0 M ethanol solution. Current densities are normalized by their loading masses. (C) Chronoamperometric durability test of CPT Rh NBs with periodic activation in fresh 1.0 M NaOH solution every 1200 s (as marked by the arrows) for up to 6 cycles. The electro-catalytic properties of CPT Rh NBs towards EOR were carried in an alkaline medium. The selection of alkaline solution was due to the higher activities, lower corrosive properties (allowing the use of a broader range of cheaper catalysts) for electro-catalysts when compared to using acidic solutions.43-44 The 5 nm of icosahedral Rh NCs (Figure S10 for TEM images) were used as benchmark catalyst because the icosahedron is a usual CPT nanostructure. At the same time, tetrahedral Rh NCs (Figure 3 for TEM image), commercial Rh black (average size was about 8 nm, Figure S11 for TEM image) were also used as the reference catalysts. CO


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stripping experiments were conducted to determinate electrochemically active surface areas (ECSAs) of the catalysts (Figure S12). The measured ECSAs are 51.3, 30.4, 39.8 and 17.8 m2·g– 1

, for CPT Rh NBs, tetrahedral Rh NCs, icosahedral Rh NCs and commercial Rh black,

respectively. Figure 4A shows the positive scan CV curves for electro-catalytic EOR on the catalysts in a solution containing 1.0 M NaOH and 1.0 M ethanol, in which two typical oxidation peaks were observed. The peak at 0.30 V is commonly attributed to the formation of acetaldehyde, acetic acid and CO2, while the second peak at 0.10 V is attributed to the formation of acetic acid.11,

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The CPT Rh NBs exhibited the highest electro-catalytic activity (185.3

mA·mg–1), which is 3.6, 6.0 and 7.3 times that of icosahedral Rh NCs (51.5 mA·mg–1), tetrahedral Rh NCs (31.0 mA·mg–1) and Rh black (25.3 mA·mg–1), respectively. Moreover, the CPT Rh NBs still yielded the highest electrocatalytic specific activities (0.36 mA·cm–2), which is 2.8, 3.6 and 2.6 times that of icosahedral Rh NCs (0.13 mA·cm–2), tetrahedral Rh NCs (0.10 mA·cm–2) and Rh black (0.14 mA·cm–2), respectively. Furthermore, the durability of the catalysts was studied by long-term chronoamperometric experiments recorded at –0.35 V for 1200 s (Figure 4B). The CPT Rh NBs always exhibited highest catalytic activity among tested electro-catalysts. For CPT Rh NBs, about 66.0% electrocatalytic activity still maintained after 1200 s, while only 21.2%, 10.0% and 23.1% retained for icosahedral Rh NCs, tetrahedral Rh NCs and commercial Rh black, respectively. In addition, as revealed by TEM imaging, the CPT Rh NBs still maintained their structures very well after harsh electro-catalytic measurements. In sharp contrast, a severe aggregation and/or melting occurred for the tetrahedral Rh NCs, and commercial Rh black (Figure S13). Importantly, due to their great carbonaceous intermediates-tolerance capability, the catalytic activity of CPT Rh NBs can be quickly recovered by switching the working electrolyte into clean 1 M NaOH solution and


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running several CV cycles from −0.85 V to 0.00 V (Figure 4C). And after consecutive 6 cycles chronoamperometric test (about 2 hours), from CV curves in fresh 1.0 M NaOH + 1.0 M ethanol solution, the electro-catalytic activity of 169.0 mA·mg–1 was still retained (only 8.8% loss) for CPT Rh NBs (Figure S14). To further confirm the great catalytic activity and stability of CPT Rh NBs, the state-of-the-art commercial Pt black for EOR at same experiment condition was also used as a reference. The CPT Rh NBs displayed superior catalytic mass activity than that of Pt black catalyst (Figure S15A). For chronoamperometric stability test over 7200 s, the electrocatalytic mass activity for CPT Rh NBs remained about 49.4 mA·mg–1, while the mass activity for Pt black degraded to nearly 0 (Figure S15B). Further XPS analysis showed the surface structure of CPT Rh NBs almost kept unchanged after chronoamperometric measurement over 7200s (Figure S16). These results confirm the great stability of CPT Rh NBs in working condition.


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Figure 5. In situ FTIR spectra of EOR on (A) CPT Rh NBs, (B) icosahedral Rh NCs, (C) tetrahedral Rh NCs and (D) commercial Rh black at different potentials (ES) varying from −0.75 to 0.00 V at an interval of 0.05 V in 1.0 M NaOH + 1.0 M ethanol solution. The reference potential (ER) was fixed at −0.85 V, 200 scans. Insets show the enlarged areas marked with dashed line boxes from 2400 to 2300 cm−1.


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It is generally accepted that the EOR follows a so-called dual-pathway (C1 and C2 pathways) mechanism.20, 45-46 For C1-pathway, C–C bond cleavage of ethanol takes place and finally forms CO2 by totally delivering 12 electrons, while only 4 or 2 electrons are produced when the incomplete oxidation of ethanol to acetaldehyde and/or acetic acid occurs for C2-pathway. To harvest high energy density, the C1-pathway is highly preferred. In order to acquire further insight into the superiority of the CPT Rh NBs and product distribution of EOR, in situ Fourier Transform Infrared (FTIR) spectrum technique was applied (Figure 5, Figure S17).47 According to previous literature, two upward IR bands at 1045 and 1085 cm−1 belong to the C–O stretching vibration of ethanol, and two strong downward peaks at 1551 and 1414 cm−1 can be attributed to the O–C–O asymmetric and symmetric stretching vibrational modes, respectively, of acetate ions (CH3COO−).12, 43-44, 48 The higher intensity and asymmetrical shape of the band at 1414 cm−1 than that of 1551 cm−1 is caused by the band at 1390 cm−1 for carbonate (CO32−) overlaps with the acetate band at 1414 cm−1.12 The appearance of CO32− indicates the formation of CO2 comes from the C−C bond cleavage of ethanol at high pH environment. Surprisingly, a very weak peak at 2343 cm−1 appeared at high potentials (for example, E = −0.10 V) for CPT Rh NBs (Figure 6A), while the peak was missing on other Rh nanocatalysts (Figure 5B−D). This band belongs to the O–C–O asymmetric stretching of CO2.12, 46, 48 The presence of this band indicates the local pH drops significantly below 6.3, because newly formed CO2 neutralizes most of NaOH in the thin-layer solution.48-49 This may imply the great C−C bond cleavage ability of CPT Rh NBs towards EOR, which results in producing a large amount of CO2.


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Figure 6. (A) Integrated in situ FTIR band intensities of CO (1840 cm−1) as a function of the electrode potential and (B) selectivity (η (CO2)) for complete ethanol oxidation to CO2 for CPT Rh NBs, icosahedral Rh NCs, tetrahedral Rh NCs and commercial Rh black electro-catalysts. Of note, for all Rh nanocatalysts, CO intermediate can be clearly observed at around 1800– 1840 cm−1.12 As marked by dash line in the Figure 5, its frequency is dependent on the potential, which is caused by the Stark effect.45 And as the potential increased, the intensity of CO band first increased and then diminished at high potential. This means the faster CO oxidation rate than the C–C bond breaking rate at high potential, where the CO can be immediately oxidized after its formation. Besides, the CPT Rh NBs always exhibited smaller band areas than that of tetrahedral Rh NCs and icosahedral Rh NCs at low potential (for example, E < – 0.4 V), which may indicate the great CO oxidation property of CPT Rh NBs (Figure 6A). More importantly, at high potential (for example, E > – 0.4 V), less steep of the CO areas change on CPT Rh NBs than that of tetrahedral Rh NCs and icosahedral Rh NCs was observed. It may be caused by that there may be still a large amount of CO formed from C–C bond cleavage of ethanol molecule on CPT Rh NBs at high potential, which counterbalances the fast CO consumption rate. It should be noted that although the commercial Rh black exhibited the lowest CO band intensity, it should not be ascribed to its great CO removal ability, but its inability in breaking C–C bond of ethanol molecule, which in turn results in little CO production.20 This conclusion was further supported


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by the results of CO2 selectivity, which was obtained by applying quantitative analytical IR method to determinate the relative concentration of products of EOR (see Figure S18-20 in the Supporting Information for detailed method).50 As shown in the Figure 6B, the CPT Rh NBs always showed highest CO2 selectivity in the potential region with high catalytic activity from −0.40 to 0 V. Specifically, at −0.15 V, the calculated CO2 selectivity was 14.5 ± 1.1 %, 11.2 ± 0.6%, 6.7 ± 0.6% and 4.2 ± 1.0% for CPT Rh NBs, icosahedral Rh NCs, tetrahedral Rh NCs and Rh black, respectively. These results highlight the great electro-catalytic performance of asprepared CPT Rh NBs towards EOR. And the value for CO2 selectivity on CPT Rh NBs is far higher than the previously reported data for monometallic Pt (1–7.5%) and Pd (2.5%) at room temperature.11-12 In the as-prepared CPT Rh NBs, both the activity, stability and C1 selectivity towards EOR are dramatically enhanced. The remarkable catalytic performance of the CPT Rh NBs can be ascribed to their several structural advantages. Firstly, the as-prepared CPT Rh NBs that are intrinsically the assemblies of 1D CPT nanorods possess very large specific surface areas, which can ensure the extensive contact between reactant and catalyst surfaces.30 Secondly, the 1D CPT nanorods possess a large percentage of {100} facets, whose surface free energy are higher than that of close packed {111} facets, which can accelerate the catalytic process and improve the C– C bond cleavage.44, 51 This conclusion was further confirmed by our density functional theory (DFT) results, because the open Rh (100) facets have stronger binding strength, lower dehydrogenation barrier and lower reaction barrier of C–C scission of ethanol compared to the close-packed Rh (111) facets (Table S1-S3). Thirdly, according to DFT results, it was found the CPT induced additional tensile lattice strains would strengthen the adsorption of ethanol due to shifting up the d-band center and lowering the reaction barrier of dehydrogenation process of


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ethanol, and thus further boosting the electrocatalytic activity (Figure S21, and Table S1).26, 52-53 Moreover, in addition to less vulnerable to dissolution and aggregation of 1D CPT nanorods, the self-supported 3D radial geometric characteristic could endow them highly accessible porosity, and their accessible catalytic sites are easy to be preserved during catalysis, therefore giving rise to the great catalytic stability in harsh electro-catalytic environment. Besides, the unique geometric characteristic of as-prepared CPT Rh NBs also endows them great thermal stability. By heating the samples at different temperatures in air for 1 h, it was found that the as-prepared CPT Rh NBs exhibited a good thermal stability at temperatures below 300 °C (Figure S22). CONCLUSIONS In summary, we successfully synthesized CPT Rh NBs via a facile solvothermal process. As the subunits were sub-5 nm CPT ultrathin nanorods, this unique 3D architecture nicely combined multiple advantages, including large surface area of 1D nanorods with high percentage of {100} facets, and high density of highly active twinned defect sites of CPT nanostructures. In addition, the overall 3D radial NB structure further endows the as-prepared NCs with interconnected open structures, self-supported and high porosity feature, which can efficiently preserve the high active sites during catalysis, meanwhile impede the aggregation. We find that the as-prepared CPT Rh NBs not only possess high catalytic activities, but also show great CO2 selectivity and structural stability towards EOR in alkaline environment. The theoretical calculation reveals that the open {100} facets play a dominant role in promoting the activity and selectivity towards EOR, while the additional tensile strain on Rh (100) facet cooperatively contributed to the improved catalytic activity by enhancing the adsorption strength and lowering the reaction barrier of dehydrogenation process of ethanol. In fact, this result is one of the first to definitely elucidate the effects of different structural features (i.e., the surface structure from exposed facets


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and the bulk structure from CPT) of electro-catalysts on EOR. Considering higher CO2 selectivity towards ethanol oxidation reaction, the CPT Rh NBs are anticipated to potentially produce higher energy density for direct ethanol fuel cell than other catalysts. More importantly, we offer significant implications for the rational design and construction of other highperformance electro-catalysts.

EXPERIMENTAL SECTION Chemicals: Rh black, n-octadecylamine (97%), n-dodecylamine (98+%), n-octylamine, (99%), n-hexylamine (99%), and n-butylamine (99%) were purchased from Alfa Aesar; Rhodium(III) chloride hydrate (38–41% Rh, RhCl3·xH2O) were purchased from J&K Scientific Ltd.; 99.999% H2, 99.99% N2 and mixed gas containing 4% CO+96% N2 were purchased from Linde Industrial Gases. 1-octanol (AR), perchloric acid (HClO4, AR), polyvinylpyrrolidone (PVP, K30), methanol (AR), acetone (AR), ethanol (AR) and sodium hydroxide (NaOH, AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. All reagents were used as received. Cyclic Penta-twinned Rhodium Nanobranches Synthesis: 5 mg of RhCl3·xH2O, 8 mg of PVP and 8 mL of n-octylamine were mixed together. The resulting solution was transferred to a Teflon-lined stainless-steel autoclave with a capacity of 25 mL at room temperature. Then the sealed vessel was heated to 200 °C for 6 h. After naturally cooled to room temperature, the products were collected by centrifugation and washed several times by ethanol for further use. Tetrahedral Rhodium Nanocrystals Synthesis: The synthetic protocol is the same as above method for the synthesis of CPT Rh NBs, except that the solvent n-octylamine was replaced with the equivalent volume of n-octanol. The products were collected via centrifugation and washed by a mixture of acetone and ethanol (volume ratio: 3:1) several times for further use.


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Instrumentation: TEM and HRTEM images were obtained using JEM 2100 with an acceleration voltage of 200 kV. HAADF-STEM image was obtained using FEI TECNAI F20 microscope operated at 200 kV. XRD pattern was obtained using a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation. XPS measurements were investigated on Escalab 250Xi XPS system. DRIFTS was conducted on a Nicolet 6700 spectrometer. Electrochemical in situ FTIR spectroscopy were performed on a Nexus 870 FTIR spectrometer (Nicolet) equipped with a liquid-nitrogen-cooled MCT-A detector, an EverGlo IR source, at a spectral resolution of 8 cm−1. Electrochemical Measurement: Before every experiment, the glassy carbon electrode (working electrode) with a 5 mm diameter was carefully polished and washed. The sample ink was prepared by dispersing electro-catalysts in ethanol, and final concentration is 1.0 mg·mL–1. Then 5.0 µL of the suspensions was deposited and dried on the working electrode. A Pt slice was served as the counter electrode. Before each electrocatalytic experiment, continuous potential cycling between –0.35 and 1.00 V (reference electrode: SCE) at 50 mV·s–1 in N2-saturated 0.1 M HClO4 solution was applied to clean the working electrode. CO-stripping measurements were conducted in 0.1 M HClO4 solution. Before measurements, CO gas was first bubbled for 15 min, then N2 was subsequently purged into solution for additional 5 min to get rid of the additional CO in the solution. The EOR was performed in 1.0 M NaOH and 1.0 M ethanol solution (reference electrode: Hg/HgO). Electrochemical in situ FTIR Spectroscopy Measurement: The spectra were recorded as relative change in reflectivity: ∆R RሺES ሻ-RሺER ሻ = R RሺER ሻ


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where R(ES) and R(ER) are the single-beam spectra collected at sample potential ES and reference potential ER, respectively. The following Equation (1) was used to evaluate the CO2 selectivity: η(CO2 ) =

൤CO3 ൨ / 2 2-

[CH3 COO- ] + ൤CO3 ൨ / 2 2-

(1)

where [CH3COO−] and [CO32−] are relative concentration (CR) by using quantitative analytical method, as described in Figure S18-20 in Supporting Information. Computational Methods: Vienna ab initio simulation package (VASP) was used for all planewave DFT calculations. The standard projector augmented wave (PAW) pseudopotentials were used to describe the core-electron interactions. The plane wave basis sets were cut off at 450 eV. The revised Perdew-Burke-Ernzerhof (RPBE) form of the generalized gradient approximation (GGA) was utilized to determine the electron exchange and correlation energies. The p(2 × 2) supercells were taken to modelled Rh (111) and Rh (100) surfaces and 2×2×1 Monkhorst-Pack k-point meshes was used. The 2% tensile strain on (100) plane was obtained from ideal CPT nanorod model (Figure S1). Due to the great larger percentage of side (100) surfaces than end (111) surfaces for CPT rod, we simplified the calculation model by only considering the strained (100) surfaces. All the models consist of four metal layers, separated by vacuum regions of 10 Å, sufficiently large to avoid interactions with their periodic images. For all calculations, the bottom two metal layers were fixed, while adsorbed species and the uppermost two metal layers were allowed to fully relax. All forces on atoms were less than 0.05 eV/Å, the structures were considered as reaching convergence. ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Theoretical discussion and DFT results, TEM analysis of Rh NCs obtained using different precursors, different solvents, and in the absence of PVP. DRIFTS, XPS of the CPT Rh NBs, tetrahedral Rh NCs. TEM and photograph of products obtained at 25 min using n-octanol and noctylamine, respectively. CO-stripping curves for the Rh NBs and commercial Rh black. TEM images and CV curves of catalysts for EOR. TEM images of the CPT Rh NBs after thermal stability. Scheme of in situ FTIRS cell, and calculation procedure for CO2 selectivity. DFT results. (PDF) AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (Q. Kuang), [email protected] (Z. X. Xie) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank the National Basic Research Program of China (No. 2015CB932301), the National Key Research and Development Program of China (2017YFA0206500 and 2017YFA0206801), the National Natural Science Foundation of China (No. 21333008, 21603178, 21671163, 21721001 and 21773190) and China Postdoctoral Science Foundation (2016M602066,


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2017T100468). We thank especially Prof. Zhiyou Zhou (Xiamen University, China) for his constructive advice and help in in situ FTIR spectroscopy measurement.


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Cyclic Penta-Twinned Rhodium Nanobranches as Superior Catalysts

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